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Current Research on Green Ammonia (NH3) as a Potential Vector Energy for Power Storage and Engine Fuels: A Review

Rafael Estevez
Francisco J. López-Tenllado
Laura Aguado-Deblas
Felipa M. Bautista
Antonio A. Romero
Diego Luna
Departamento de Química Orgánica, Instituto Químico para la Energía y el Medioambiente (IQUEMA), Universidad de Córdoba, Campus de Excelencia Internacional Agroalimentario CeiA3, Edificio Marie Curie, E-14014 Córdoba, Spain
Authors to whom correspondence should be addressed.
Energies 2023, 16(14), 5451;
Submission received: 8 June 2023 / Revised: 10 July 2023 / Accepted: 16 July 2023 / Published: 18 July 2023
(This article belongs to the Section F: Electrical Engineering)


Considering the renewable electricity production using sustainable technologies, such as solar photovoltaics or wind turbines, it is essential to have systems that allow for storing the energy produced during the periods of lower consumption as well as the energy transportation through the distribution network. Despite hydrogen being considered a good candidate, it presents several problems related to its extremely low density, which requires the use of very high pressures to store it. In addition, its energy density in volumetric terms is still clearly lower than that of most liquid fuels. These facts have led to the consideration of ammonia as an alternative compound for energy storage or as a carrier. In this sense, this review deals with the evaluation of using green ammonia for different energetic purposes, such as an energy carrier vector, an electricity generator and E-fuel. In addition, this study has addressed the latest studies that propose the use of nitrogen-derived compounds, i.e., urea, hydrazine, ammonium nitrate, etc., as alternative fuels. In this study, the possibility of using other nitrogen-derived compounds, i.e., an update of the ecosystem surrounding green ammonia, has been assessed, from production to consumption, including storage, transportation, etc. Additionally, the future challenges in achieving a technical and economically viable energy transition have been determined.

1. Introduction

Governments and researchers are currently striving to reduce CO2 emissions due to the massive use of fossil fuels through the incorporation of innovative technologies to produce renewable energy. In the transportation sector, which largely contributes to these emissions (25% of the global emissions considering both transport and freight), there are four main options for achieving decarbonization, based on the availability of renewable electricity, obtained with low or zero CO2 emissions (Figure 1): (a) Renewable electricity with battery-electric propulsion vehicles (BEV); (b) green hydrogen, operating in fuel cells vehicles (FCEV) or directly in internal combustion engines (ICEV); (c) renewable carbon-based fuels, such as green methanol, green methane or green ammonia, obtained from the hydrogenation of CO2 or N2, with green H2 that would be used in both FCEV or ICEV engines and (d) maintaining the use of fossil fuels but with the capture and storage of the CO2 generated and subsequent transformation, or by direct air capture (DAC).
As can be seen, hydrogen is involved in most of these processes. Therefore, the rapid and increasing attention placed on hydrogen throughout the world is understandable, since it can be used in many potential applications in industry, transportation, power and manufacturing. In addition, hydrogen does not emit carbon dioxide in any of these uses [2,3,4,5,6,7,8,9]. Although, in a short period of time, the global hydrogen demand has increased from 70 million tons in 2019 to 120 million tons by 2024 [10], it is expected that, in 50 years, more than 500 million metric tons would be required for different sectors (Figure 2).
Nowadays, fossil fuels are the main feedstock for hydrogen production—in particular, natural gas or coal (Figure 3), resulting in a heavy amount of greenhouse gas emissions [11,12,13]. Therefore, it is an absolute priority to promote the production of renewable hydrogen, or green hydrogen, by renewable energies, the use of wind and solar energy being the most successful in this regard, so far [14,15,16,17,18,19], although it seems that one of the most promising solution would be H2 production through water electrolysis [20].
Independently of the method of hydrogen production, there is still a handicap for the use of this green hydrogen, and it is necessary to compress and liquify it for its storage and shipping due to the low volumetric energy content that it exhibits, making transportation costs difficult to bear. That is why the energy storage, in the form of different vectors, can be considered as the best solution for overcoming this handicap [21,22,23,24]. A promising option is the use of energy-dense molecules as chemical energy stored, especially if these molecules can also be used as fuels. Fuels obtained from power energy, the so-called “electric fuels” (E-Fuels), can be considered as green fuels since they are produced from green hydrogen. These E-fuels include power-to-liquids (PtL) and power-to-gas (PtG) fuels, together known as PtX fuels, and can be considered as a major stake in the future production of green H2 [1,25,26,27,28,29]. In this sense, E-fuels are gaining a lot of importance since they allow for taking advantage of the enormous internal combustion engine fleet existing worldwide, that currently comprises around 99% of the global light-duty vehicles, ocean transport and aviation [1]. Figure 4 shows several routes for the application of green hydrogen for obtaining ammonia, methanol, synthetic hydrocarbons or other Power-to-X fuels.
In this sense, the use of methanol, ammonia or other PtL fuels that offer similar infrastructure as current fossil diesel fuels could be economically more favorable than the use of hydrogen [30].
The emergence of ammonia and methanol as new target molecules contributing to decarbonization is clearly understandable. Indeed, NH3 can be obtained from atmospheric N2, whereas methanol or methane can be obtained from CO2. In the latter case, CO2 can be obtained from the atmosphere or be recovered from certain industrial processes, so it will no longer be a waste or a pollutant but rather a raw material for producing clean fuels, green chemicals or several renewable commodities. The chemical transformation of captured CO2 into fuels or chemicals represents an additional advantage type “two birds, one stone” [31]; the extent of the benefits of Carbon Capture and Utilization (CCU) is still uncertain because it is a very immature technology that still does not offer enough guarantees [29].
Nevertheless, the case of ammonia is completely different. In first place, the well-known ammonia synthesis procedure, since the Haber–Bosch process, has provided ammonia-based fertilizer to feed the world’s increasing population for a century; it is currently among the most globally important processes in the chemical industry, owing to the massive use of NH3 as a main source of fertilizers. This fact has contributed to an extensive infrastructure for its production, transportation and distribution around the world. Moreover, ammonia is a carbon-free fuel with a hydrogen content of 17.6 wt.%, and it can easily be stored at 25 °C and 10 bar of pressure after liquefaction. Recent studies have found that the greenhouse emissions from ammonia-driven engines are less than one-third of those of gasoline- or diesel-driven engines [27,32], increasing its potential as fuel. In addition, several theoretical predictions and experimental tests have shown that ammonia can be employed as a clean alternative fuel for CI engines with minimal modifications [33,34]. Considering all these factors, this review aims to collect and evaluate the different possibilities that ammonia and/or ammonia-derived compounds are able to bring us, even in the near-future, to achieve the energy transition. The framework of this review is structured as follows: Section 1 presents an introduction of this study; Section 2 collects the possibilities of ammonia as an energy carrier vector; Section 3 shows different research regarding ammonia as an electricity generator in cycle gas turbines, via combustion; Section 4 discusses the current possibilities of ammonia as E-fuel for transport; Section 5 presents several research works regarding nitrogen-based compounds that can be employed as fuel; Section 6 shows the current state of the art of green ammonia and, finally, the final section summarizes the future perspectives and puts on the table some concluding remarks.

2. Ammonia as an Energy Carrier Vector

As mentioned above, ammonia can be employed for hydrogen storage due to its high volumetric hydrogen density and high storage stability, making it able to be carried out at relatively low pressures. Thus, the energy stored in the ammonia molecule can be gathered either as fuel or decomposed to hydrogen to further be used in several applications [35].
The competitive advantages of NH3, with respect to the energy carriers most considered for hydrogen storage, i.e., liquid hydrogen, compressed hydrogen, liquid ammonia and methanol, are shown in Table 1. Ammonia is not only the cheapest fuel but also exhibits a higher volumetric energy density at room temperature than liquid hydrogen at −252.9 °C (12.7 and 8.49 MJ/L, respectively) and even compressed hydrogen (4.5 MJ/L) at room temperature and 69 MPa. In addition, the combustion heat exhibited by ammonia (11.2 MJ/L) is significantly higher than that of liquid hydrogen (8.58 MJ/L). Considering methanol, despite it exhibiting a higher energy density value (20.1 MJ/kg) than ammonia (18.6 MJ/kg), it has lower gravimetric and volumetric hydrogen contents than ammonia (12.5 wt.% and 99 kg-H2/m3 versus 17.8 wt.% and 121 kg-H2/m3 exhibited by ammonia).
In any case, although both molecules display strengths and weaknesses, they can be considered as the most suitable options for hydrogen storage. The main handicap of ammonia is associated with the hydrogen release step, where a high amount of energy is consumed in comparison with methanol (30.6 and 16.3 kJ/mol-H2, respectively). Regarding methanol, the main drawback would be related to its reformation process, which generates carbon monoxide (CO), being able to deactivate, by poisoning, most of the catalysts employed in fuel cells, shortening the lifetime of those systems. Perhaps the greatest competitive advantage of ammonia with respect to methanol and other potential vectors lies in the possibility of its immediate application on an industrial scale. Likewise, if we look at the capacity of NH3 for the alternative storage of renewable energy, there is little doubt of its potential, since ammonia exhibits the highest gravimetric densities of the e-fuels contemplated (Figure 5), according to the study of Kojima et al. [36].
According to the electrical power generation of NH3, by feeding it into fuel cells, NH3 can be catalytically cracked to produce H2 for fuel cell applications—in other words, polymer electrolyte membrane fuel cells (PEMFCs) and alkaline fuel cells (AFCs). It is necessary to carry out additional separation and purification processes of the hydrogen produced for any distribution system to supply green NH3 for H2 refilling stations, as the direct car on board ammonia cracking is considered impractical, since it requires temperatures higher than 500 °C for the production of hydrogen of enough purity, particularly for vehicle applications, demanding a thermal energy of 4.2 GJ/tonNH3 (including H2 loss) [37,38,39,40,41] (Figure 6).
Be that as it may, there is an important consensus regarding the convenience of using ammonia instead of hydrogen as a central energy storage and distribution tool. In fact, different ammonia types are under consideration. Thus, there is a “gray ammonia” obtained from hydrogen by the Haber–Bosch process, consisting in the steam methane reforming, a “blue ammonia” obtained analogously to gray ammonia, but including the capture and storage of CO2 emissions, and the true “green ammonia” obtained by using green hydrogen, produced by the electrolysis of water using renewable energies. It must be said that all of them, i.e., gray, blue and green ammonia, exhibit a better energy efficiency than liquid hydrogen [42,43,44]. In addition, Cesaro et al. [45] have stated that green ammonia could be available from USD 400/t in the year 2040, with the possibility of approaching a price near USD 300/t if either of these conditions are met: the cost reduction of electrolysers or a global green ammonia market obtained with more favorable renewable resources [45]. Furthermore, Protonic ceramic electrochemical cells (PCECs) have also been intensively studied for power generation energy storage as well as for sustainable chemical synthesis, including ammonia. In fact, to the best of our knowledge, four major PCEC reactor configurations have been designed and validated for ammonia synthesis, some of them integrating electrochemical/thermochemical and separation processes with ammonia synthesis, aiming to circumvent the thermodynamic limitations of ammonia synthesis that are typically encountered in a classical HB reactor. This integration pursues circumventing the thermodynamic limitations of ammonia synthesis and using renewable power for ammonia synthesis [46].
Likewise, several sources for obtaining and/or producing ammonia, contributing to a circular economy have also been studied. Thus, the catalytic hydrogenation of nitrates and/or nitrites, giving rise to the production of ammonia and nitrogen, has been extensively discussed in the following reviews, using mainly Pt- and Pd-based catalysts [47,48]. Furthermore, another important ammonia source would be its recovery from wastewater [49], as well as the optimization of methodologies for removing the biological anaerobic production of ammonia [50].
As a summary, Figure 7 shows a possible roadmap for a renewable energy economy based on ammonia as a fundamental element for the storage and distribution of renewable energy in massive industrial quantities.
Despite the options here seeming to be practical in the near future, it cannot be forgotten that the bottleneck is still the substitution of the H2 currently produced from natural gas by another of a renewable nature, this being mandatory to improve the efficiency of the electrolysis processes, since it does not exceed, at present, the 21%.

3. Ammonia as an Electricity Generator in Combined Cycle Gas Turbines

As mentioned above, several studies manifest that green ammonia can be postulated as the best carbon-free energy storage vector, including the provision of green electricity for the power sector [45,51].
The main drawback that ammonia exhibits is its low calorific value and maximum laminar burning velocity (SL). This fact suggests that the combustion energy obtained in the ammonia combustion is less intense than that obtained with other fuels. In addition, different factors such as the narrow combustion range that ammonia exhibits as well as its high auto-ignition point make it difficult to ignite, which is undesirable in fuel applications.
Nevertheless, ammonia is able to exhibit premixed laminar flame properties in blends with hydrogen and nitrogen, very similar to those of methane gas [52,53,54,55]. The use of ammonia/methane blends is being considered to achieve immediate CO2 emission reduction in those existing facilities running on natural gas. The main drawback is the NOx production in the firing conditions of such blends. In this sense, a great number of research works regarding this topic have been carried out in recent years [56,57,58,59,60,61,62,63,64,65,66,67,68]. Generally, it has been observed that the SL values of NH3/CH4/air flames increase almost linearly as the content of CH4 also increases. Furthermore, the enhancement is applicable from fuel-lean to fuel-rich conditions, due to the higher SL and reactivity of methane [69,70]. At stoichiometric conditions, a more significant change is observed with CH4 substitution proportion [69,70,71,72]. Regarding NOx emissions, a high percentage of ammonia substitution in NH3/CH4 blends promoted a decrease in the CO, CO2, and NOx emissions. Generally, these emissions depend on the equivalence ratio, although, in some conditions, they can result in lower NOx emissions, e.g., fuel-rich and pressured conditions [73,74].
Analogous to the ammonia/methane blends, many others have been tested as electricity generators via combustion procedures. Thus, the ammonia–air combustion performance by blending it either with dimethyl or diethyl ether also exhibits better results than that of pure ammonia [75,76,77,78,79,80,81]. Ammonia/syngas blends have also been evaluated with good results [82,83,84], as well as ammonia/methanol and ammonia/ethanol blends [85,86,87]. Even several blends of ammonia with different organic compounds, e.g., methylcyclohexane and toluene [88], dimethoxymethane [89], n-heptane and isooctane [90], or more complex gas mixtures, such as hydrogen, methane and propane [91], NH3/C3H8/air [92], NH3/CH4/H2S [63] and CH4/H2/NH3-air mixtures [93,94], have been used.
Very recently, the possibility of using ammonia in mixtures with finely pulverized coal to reduce emissions in power thermal plants or industrial furnaces, currently operating with coal, has also been under study. In this sense, Stocks et al. [95] studied the behavior of imported ammonia co-combustion in coal-fired power stations between Japan and Australia, finding that the co-combustion of ammonia, produced with steam methane reforming by Haber–Bosch technology, provided no net benefit for the combined country emissions. This fact can be explained because the GHG emissions produced in Australia in the ammonia production are analogous to the reductions found in Japan. In contrast, it is estimated that the co-firing of green ammonia would reduce the emissions in those countries by 43 MT/year in 2030. In this research line, in last 3–5 years, a high number of research works affording the study of emissions, co-firing characteristics, etc. have been carried out [96,97,98,99,100,101,102,103,104].

4. Green Ammonia as E-Fuel in the Transportation Sector

The possibility of using ammonia as power for the transportation sector or as a component for several heating applications has been known for decades. Considering the new scenario in which many efforts are being directed to reduce carbon dioxide emissions, the competitive possibilities of green NH3 clearly increase, since it is able to be employed as fuel for any transport vehicle (i.e., fossil free ships [105], the maritime industry [106]), either in fuel cell vehicles or internal combustion engine (ICE) vehicles (Figure 8) [107].

4.1. Green Ammonia as Fuel

Attempts to use ammonia as a fuel for transportation go back to relatively distant times, as shown in Figure 9, where it is indicated that ammonia-powered vehicles appeared at the beginning of the last century [108]. Thus, fueling internal combustion engines with ammonia was intended in 1905, through the hydrogen derived from ammonia. Likewise, in the 1940s, the difficulty of accessing fossil fuels because of the Second World War prompted the use of ammonia to power the internal combustion engine military vehicles [109]. Inside this historical context, an important milestone was also the use of ammonia in combustion engines of Belgium’s public bus system in 1942. To do so, a little modification of the engine systems was carried out, consisting of running liquid ammonia with a small amount of coal gas to improve the combustion [107]. Analogously, during the energetic crisis of the 1960s, ammonia was again a transitory alternative energy source for transportation. In this sense, it should be noted that ammonia can be used in internal combustion engines (ICE), compression (CI) or spark ignition (SI) systems, making it possible to take advantage of the current fleet of vehicles, since minor modifications to these engines would be enough to incorporate ammonia as fuel [109].
In addition, according to the comparative values of different representative parameters of the different fuels shown in Table 2, ammonia has a higher ignition energy than fossil fuels, due to the lower flammability limit of ammonia [110].

4.2. Green Ammonia as a Fuel for Spark Ignition (SI) Engines

The high-octane number value that ammonia exhibits (~130) can improve the combustion properties with a reduction in the engine knocking as well as any other undesirable combustion effects in the gasoline SI engines. Despite the high number of investigations carried out on the combustion process using ammonia, either as single or dual fuel, there is still an unresolved drawback: the incomplete combustion of the ammonia and the poor fuel energy conversion efficiency [109,111,112,113,114,115,116].
Recent research has shown the possibility of running common SI engines with pure ammonia, although the range of operation is still limited, due to the unfavorable properties of ammonia for premixed combustion [115,117,118], as aforementioned. The use of ammonia as a sole fuel in spark ignition would deteriorate the engine performance [116]. Therefore, a supporting fuel, e.g., H2, seems to be necessary to enlarge the operating range of ammonia [119,120,121,122,123,124,125,126]. Analogous to the use of H2, many other fuels are still under study, e.g., natural gas [127], gasoline [118,128], methanol [129], organic solvents such as ethanol [128], dimethyl ether [76], alkanes [90] or higher alcohols that reduce the toxicity and increase the energy content [130]. It must be taken into account that liquid solvents are preferable to simplify engine mechanics. In this regard, a series of suitable organic solvents have been identified to solubilize ammonia under the best conditions for its use as dual fuel (Table 3). As can be seen, different functional groups are compatible with ammonia for being used as dual fuel, although a constant can be observed: the number of carbon atoms of the compound must be lower than nine. In general, using these solvents, a minimum value of 10% ammonia in the dual fuel can be achieved [130].
In addition, by the use of any of these solvents, a pressurized tank for its storage, bearing additional costs, can be avoided. This is displayed in Figure 10.
In summary, upon increasing the ammonia percentage of the fuel blends, the overall energy and exergy efficiencies of the SI engines decrease. These reductions in the efficiencies are due to the lower energy density and lower heating values of ammonia compared to those of gasoline; however the reduction in CO2 emissions makes it worth using ammonia as a gasoline additive. Thus, ammonia alone, or, better, as a dual fuel option, has been demonstrated to be useful for SI-engines, mainly mixed with hydrogen or gasoline.

4.3. Green Ammonia as a Fuel for Compression Ignition (CI) Engines

At present, the number of CI engines operating around the world is much greater than that of spark ignition engines, i.e., urban transport, motorized vehicles, the marine industry and the electric power generation sector. In addition, the gas emissions that affect global warming and the atmospheric pollution emitted by diesel engines are also higher than those generated by spark ignition engines. In this sense, analogous to the use of ammonia in SI engines, it could be used with diesel or any other fuels in a dual-fuel model. As aforementioned, due to the low auto-ignition temperature of ammonia, the candidate fuel that will be blended with ammonia must preferably have a high cetane number to improve ignition characteristics.
In this respect, there are many research works about dual fuel diesel-gas ammonia [131,132,133,134,135,136,137,138,139,140,141] and diesel-aqueous ammonia [142] operating in CI engines. In general, the thermodynamic efficiency of the dual blend is reduced as the ammonia proportion increases, leading to higher energy consumption in comparison with fossil diesel [109,138,143]. Regarding the CO and HC emissions, a positive effect is observed with the addition of ammonia, since a reduction in these emissions is achieved. However, with respect to NOx emissions, two opposite effects have been found. Thus, whereas a low proportion of ammonia in the blend leads to high NOx emissions, a high proportion promotes the opposite behavior. As expected, the proportion of NOx emissions is an important issue during ammonia usage, closely depending on the ammonia amount introduced into the engine [140]. NOx emissions can be partially palliated by reducing the combustion temperatures.
Analogous to the use of diesel, many other organic compounds such as biodiesel [144], kerosene [145,146] and hydrocarbons such as n-heptane [147,148,149,150], dodecane [142,151,152], n-decane [153] or n-hexane [154] have been studied as potential fuels for CI engines in blends with ammonia. Regarding oxygenated compounds, dimethyl ether (DME) [80,109,143,148] and diethyl ether have also exhibited very good results as solvents for ammonia in dual mixtures [77,79,155].

4.4. Green Ammonia as a Fuel-Cell for Clean Engines

Nowadays, fuel cells are attracting a lot of attention in the transportation sector, since the electricity is obtained through a series of electrochemical reactions [156,157]. Moreover, fuel cells have the advantage that different green fuels can be employed, i.e., hydrogen, renewable methane, ammonia and methanol. Therefore, and considering parameters such as energy efficiency, power capacity and sensitivity to fuel impurities, fuel cells based on the proton exchange membrane, molten carbonate and solid oxide are found to be promising options for transport applications [158].
Despite green hydrogen appearing to be the ideal candidate to be used for fuel cell-based vehicles, its use entails a series of logistical difficulties (storage and transportation). In this regard, green ammonia could be a promising candidate for fuel cells, because its storage and transportation infrastructure are already available, making it an accessible fuel source that also exhibits a high capacity for an indirect hydrogen storage medium. At present, there are several technologies capable of achieving electricity from ammonia. In this respect, a great number of research works are being conducted on one of the most promising technologies based on ammonia fuel cell technologies, the solid oxide fuel cells (SOFCs) [159,160,161,162,163,164].
This technology allows for the direct use of ammonia as fuel, being cracked at elevated temperatures ranging between 500 and 1000 °C, either by direct or indirect methods. As shown in Figure 11, ammonia is first catalytically decomposed in N2 and H2, and then the hydrogen produced is electrochemically transformed into water and electrical power [165,166,167,168,169,170]. For indirect decomposition, the reactor must be in contact with the fuel cell, thus providing heat transfer between the two units [171]. For its part, in direct decomposition, the cracking reaction occurs in the same chamber as the anode, using the heat obtained in the same exothermic electrochemical reaction. That is, an additional decomposition reactor is not necessary [172]. However, direct decomposition requires a multifunctional electrode, which provides adequate multiple catalytic activity, both in the decomposition of ammonia and in the electrochemical reaction. One of the main advantages of the SOFCs is their versatility, since many different fuels can be employed for the bed. Thus, in addition to natural gas and hydrogen, many other fuels are able to be applicable as well, such as ammonia, dissolved urea (AdBlue), methane/steam and ethanol/water mixtures, which can directly be fed to the cell [173].
Moreover, the SOFCs are classified depending on the electrolyte applied, i.e., the oxygen anion conducting the electrolyte-based solid (SOFC-O) or the proton conducting electrolyte-based solid oxide fuel cells (SOFC-H) (Figure 12a,b). Each one presents important differential characteristics based on the effect of the electrolyte, electrocatalyst material and operating temperature [159].
In addition to SOFCs, alkaline fuel cells (AFCs) and membrane-based fuel cells (AAFCs) (Figure 12c) are under development. These fuel cells are included in the low-temperature ammonia fuel cells, being compatible with power devices of the small–medium range [175,176,177,178,179]. Nowadays, SOFCs, particularly SOFC-Hs, are the most promising form of the next-generation ammonia fuel cell technology. For its part, low-temperature ammonia fuel cells need to overcome several challenges before commercial application, such as its power density and stability. Nevertheless, these cells would palliate many of the limitations associated with the hydrogen economy, offering a clean and reliable energy source.

5. Nitrogen-Based Fuels: Compounds Derived from Green Ammonia as Fuels for Engines

As a complementary option for the use of ammonia as a free greenhouse emissions fuel, some derivatives of ammonia can also be considered. Currently, the existing bibliography on nitrogen-based fuels is much smaller than that based on ammonia, and in some cases, it refers to fuels for highly specialized applications such as rocket and space engines. However, this does not prevent the consideration of the potential possibilities that could open the use of nitrogen-based molecules in transport and the energy sector, highlighting their use as energy carriers [180].

5.1. Green Urea as Fuel

The use of urea may play an interesting role, since it is currently used to reduce the (NOx) levels produced in the combustion of nitrogenous compounds. In addition, urea is non-toxic, non-volatile, non-flammable and technically affordable [181]. Thus, urea has been described as a proper hydrogen supply that can be applied as an alternative clean source for the generation of electricity [182,183]. In this respect, the hydrogen production process by urea electrolysis is more efficient than that by water electrolysis because the power consumption in the former is considerably lower than that of water electrolysis [184]. On the other hand, urea can be applied in fuel cells to obtain green energy by using wastewater containing urea/urine as a fuel [185].
It is comprehensible that, at present, there are many studies in progress for enhancing the catalytic activity of Ni-based anodic catalysts for being applied in direct urea fuel cell oxidation (DUFC) [186,187,188,189,190,191,192,193,194,195,196,197]. Direct urea fuel cells (DUFC) are the most valuable and inexpensive method for simultaneously obtaining wastewater treatment (urine and urea-contaminated water) and electricity generation. This fact is not trivial, since untreated urea-rich wastewater, coming from different sources, as can be seen in Figure 13, has become a potential danger for ground water and air due to the decomposition of urea into toxic gas ammonia. Therefore, the transformation of urea into hydrogen by electrolytic procedures is really important for ecological protection and clean energy generation, especially if we compare this method with the pyrolysis one, where the urea starts to decompose at 333 K for NH3 and CO2 to be rapidly cracked over 406 K [195]. The number of studies related to urea as a fuel for internal combustion also continues to rise, mainly to achieve NOx emission levels satisfying vehicle exhaust regulations [198,199,200,201].

5.2. Green Hydrazine as a Fuel

Considering the properties, handling and storage issues, hydrazine can considered, together with ammonia, as the main alternative nitrogen-based fuel [143,180,202]. Under standard conditions, hydrazine is a colorless liquid, with density close to 1003.7 kg/m3, a boiling point of 114 °C and a freezing point of 1.5 °C. The fact that the freezing point of hydrazine is over 0 °C is a serious drawback that complicates its potential use as a fuel in a combustion engine, as it has to be used in blends with antifreezes, with mainly ammonia and water. In this way, according to Kopchikov et al. [202], a triple blend of 64% hydrazine, 10% ammonia and 26% water (by weight) diminished the freezing point from 1.5 to −54 °C [202]. Obviously, the main drawback of hydrazine is the high toxicity that it exhibits, it being employed mainly as a rocket propellant. Nevertheless, in the last decade, a lot of research has addressed the use of hydrazine in the field of fuel cells. Thus, low-temperature fuel cells that work with hydrazine, either in acid or basic media, represent an outstanding technology for the use of new, unconventional energy sources as fuel [203,204,205]. In this sense, several investigations are being carried out to achieve improvements in the catalytic electrooxidation of hydrazine to be used in direct hydrazine fuel cells (DHFC) [206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223].

5.3. Green Ammonium Nitrate as Fuel: Aqueous Solutions of Urea, Ammonium Nitrate and Their Mixtures (UAN) as Fuels

Nitrogen derivatives such as urea (U), ammonium nitrate (AN) and their mixtures (UAN) have also been proposed as fuels. These nontoxic and nonvolatile low-carbon nitrogen-based fuels have the advantage of containing, in the same solution, both the oxidizing species (i.e., nitrate ions) as well as the reducing species (i.e., ammonium ions and urea). Moreover, urea and ammonium nitrate are currently global commodities, manufactured mainly as fertilizers in tens of millions of tons annually [224].
For its part, the UAN mixture has become the most popular nitrogen fertilizer in the United States, even more so than liquid ammonia and urea [225]. Therefore, the production and handling of this commodity involves mature technologies. However, we cannot forget the explosive nature of the NA and its most recent accident in 2020, when 2750 tons of stored ammonium nitrate exploded in a warehouse at the harbor in Beirut (Lebanon).
The main application of aqueous UAN solutions is related to its excellent gas generator capacity, employed to drive stationary turbines. The intended reaction of aqueous UAN yields around 73.0% H2O, 21.6% N2 and 5.4% CO2 on a molar basis, according to Equation (1):
3NH4NO3 (aq) + NH2CONH2 (aq) + 5.56H2O (l) → 4N2 (g) + 13.56H2O (l) + CO2 (g)
This supposes a volumetric energy density of 4.44 MJL−1 such that the gravimetric energy density of UAN is 3.34 MJkg−1. The reaction occurs through the complex mechanism shown in Figure 14, where the dissociation of AN is an equilibrium reaction mediated by a proton transfer, which produces ammonia and nitric acid, constituting the initial step of AN decomposition. The homolysis of nitric acid is the rate-determining step in the combustion of aqueous UAN [224].
In this respect, several studies have shown the continuous combustion of clean aqueous UAN as a possible solution to be used for fuels. In fact, the thermal ignition of aqueous UAN could be performed in a preheated fuel inlet tube. In addition, the fuel combustion would take place continuously, without shutdown or uncontrolled thermal behaviors, and with an NOx emissions level below actual regulation [224,225,227].

5.4. Green Ammonium Nitrate as a Fuel: Aqueous Solutions of Ammonium Hydroxide and Ammonium Nitrate (AAN)

Aqueous solutions of ammonium hydroxide and ammonium nitrate (AAN) are defined as monofuels by the fact that they contain the oxidizer and the reducer in the same solution. Therefore, air is not needed as an external oxidizer for the combustion. Thus, whereas in the aqueous UAN case, urea is acting as the reducer, in the aqueous AAN case, ammonia in the form of ammonium hydroxide is the reducer that is able to react with the net oxidizing AN without the emission of any amount of CO2 [228], according to Equation (2):
3NH4NO3 (aq) + 2NH4OH (aq) + 3.9H2O → 4N2 (g) 14.9H2O (l)
Therefore, both nitrogen-based fuels could be employed in continuous processes (such as in stationary power generation [224], as well as in combustion processes (internal combustion engine). The combustion effluents of these fuels (AAN and UAN) do not contain carbon pollutants such as CO, VOCs or PM. Moreover, NOx emissions below regulatory standard levels have been obtained over UAN as fuel, and similar lower levels can be reached with AAN when combustion is carried out at a high pressure [229,230].
Likewise, similar to the UAN fuel, aqueous blends of UAN and AAN are also excellent gas generators to be used in stationary turbines [231,232]. In addition, they exhibit a volumetric energy density around 4 MJ per liter, equivalent to 50 MPa of compressed hydrogen or to 10 MPa of compressed natural gas (CNG) [201]. The main drawback of using ammonium nitrate is related to the operational and safety performance it exhibits [233]. That is why the safe use of ammonium nitrate is also the object of intense study, since it is incompatible with many materials [234].

5.5. Other Synthetic Nitrogen-Based Fuels

Recent research works have studied the use of aqueous ammonium hydroxide urea (AHU) as a synthetic nitrogen-based fuel in internal combustion, diesel, rocket engines or energy carriers [201,229,235]. This compound cannot be categorized as monofuel, because it contains only the reducer specie, so it requires an external oxidizer such as atmospheric oxygen to combust, as can be seen in Equation (3):
NH4OH (aq) + 0.22 NH2CONH2 (aq) + 0.3H2O (1) + 1.09O2 (g) → 0.72N2 (g) 3.25H2O (l) + 0.22CO2 (g)
It has been described that this fuel undergoes spontaneous thermal ignition above 400 °C, without any catalyst or a spark, in a closed pressurized chamber with the fuel/pure oxygen stoichiometric ratio at 20 bar [201]. Analogous to the abovementioned nitrogen-based fuels, AHU together with aqueous UAN and AAN can be considered an excellent gas generator in stationary turbines.
Similarly, ammonium formate has also been evaluated as an energy carrier [236]. Ammonium formate can be easily obtained by the reaction of ammonia with formic acid. In addition, it is solid under standard conditions, so the transportation and storage are both safe. By using an electrochemical cell, ammonium formate decomposes at 105 °C to an ionic liquid. Whereas ammonium formate is oxidized at the anode, hydrogen evolves at the cathode with a Faradaic efficiency ca. 100%. Then, ammonia evaporates before it can oxidize, a modular device being necessary to achieve the complete oxidation. In summary, this process constitutes an electrochemical fuel ionic liquid, where the electrolyte is the fuel and hydrogen is released with zero net-carbon emissions (Figure 15).
As can be seen, formic acid and ammonia can be generated in traditional or sustainable plants by feeding H2. Then, ammonium formate will be spontaneously formed by the combination of ammonia and formic acid. This ammonium formate can be decomposed in an electrochemical cell into hydrogen, nitrogen and carbon dioxide. In practice, the fuel cell reduces ammonium to hydrogen at the cathode and oxidizes formate to carbon dioxide at the anode [236].
In addition to this application, there are many other processes based on ammonium formate for its use as an energy carrier, mainly based in the catalytic reversible hydrogen storage–evolution process for hydrogen recovery. Thus, the ammonium bicarbonate/formate redox equilibrium system is feasible for reversible hydrogen storage and recovery (Figure 16).
In this respect, direct formate fuel cells (DFFC), which convert the chemical energy stored for ammonium formate directly into electricity, are recently attracting more attention, primarily because of the use of the carbon-neutral fuel and the low-cost electrocatalytic and membrane materials [237]. Despite the fact that high values of yield of formate and hydrogen have been reported over Pd supported over activated carbon (AC) nanocatalysts [238] and over Pd−Au alloy AC catalysts [239], a lot of effort is being directed to obtaining a better metal catalyst, i.e., one that is low-cost, fast, stable, selective, non-hazardous and capable of work in an aqueous medium [240,241,242], in order to achieve an efficient vehicle for hydrogen and energy storage based on the formate–bicarbonate cycle.
In summary, Table 4 collects the main parameters for the most important of the abovementioned fuels. As can be seen, hydrogen exhibits the highest specific energy (143 MJ/kg) but a poor volumetric energy density value (5.6 GJ/m3). This fact can be explained by the low energy density, even at the high pressures that hydrogen exhibits. Regarding the other parameters, a high fuel viscosity (μ) would promote an increase in the pumping costs or heating requirements. For its part, low octane number values (RON) indicate that a fuel is more suitable for CI engines, whereas high values make the fuel preferable for SI engine performances, as is the case with hydrogen and ammonia. Regarding the autoignition temperature values, the lower the value, the higher the fire risk, requiring more safety measurements. The adiabatic flame temperature (Tad) is a measure of the energy content of a fuel. High values of Tad usually imply an increase in the equipment costs, lower efficiencies and high pollutant emissions. High flame speed values (SL), such as in the case of hydrogen, improve engine efficiency, but they also reduce fuel safety. In contrast, the low flame speed (SL) value that ammonia exhibits results not only in lower efficiency but also in flame stability. In summary, since there is no perfect fuel, these properties should help in choosing the most suitable fuel for a specific application, where the selected fuel should balance the intended application and its economical−environmental cost.
Be that as it may, the potential of nitrogen derivatives regarding their use as fuels can be understood based on the enormous increase in publications on this subject in recent times. In fact, in addition to the aforementioned nitrogen-based fuels, many others such as aqueous ammonium carbonate [243,244], aqueous ammonium acetate [245], aqueous ammonium carbamate [224], aqueous urea [246,247,248], ammonium perchlorate [249,250,251,252,253], ammonia borane [254,255,256,257,258], dimethylamine borane [259,260] and ammonium dinitramide (ADN) [261,262,263,264] are objects of study.

6. Storage, Transportation, Safety, Production, Decomposition and Consumption of Green Ammonia

6.1. Storage and Transportation of Ammonia Fuels

Figure 17 shows a comparison between the mass and relative volume of some hydrogen storage materials. As can be seen, for an equal amount of hydrogen storage, metal amine salts are able to store an equal amount of hydrogen as metal hydrides in much less volume due to their higher densities. Furthermore, a high pressure of hydrogen storage is associated with the use of very heavy cylinders for the relatively little stored amount of gas. In addition, liquid hydrogen is also difficult to handle, due to the necessary very low temperature of −252 °C and the associated evaporation loss of hydrogen, about 2–3% per day [265].
Ammonia, for its part, has the infrastructure for its storage and transportation well established for many decades. Thus, liquid ammonia is generally transported around the world in many ways, i.e., ships, pipelines, trains and trucks. However, for automotive applications, the end user should avoid any contact with it because of its toxicity and high vapor pressure. In this sense, some additional steps such as the use of some metal ammine salts must be considered to diminish the potential risks [265].
In this sense, the solid storage of ammonia would try to solve these safety issues, since it offers a safe, reliable and cost-effective method for ammonia storage, with the ammonia easily thermally liberated for direct use in internal combustion engines or cracked to provide hydrogen for fuel cells [266].

6.2. Production of Ammonia Fuels

In order to reduce the carbon footprint in the current ammonia manufacture, different sustainable approaches for both hydrogen and ammonia production are under research [267,268,269,270]. The development of the low-temperature electrochemical synthesis of NH3 is being attempted, although to obtain practical applications, the production rates must be increased by at least one or two orders of magnitude [271,272,273]. Up to now, the best approximation achieved in reducing material and energy consumption is the Solid State Ammonia Synthesis (SSAS). This process is carried out in a solid electrolyte cell, in which steam is used as the source of hydrogen, as can be seen in Figure 18 [274].
The outstanding results obtained using this SSAS methodology have promoted the development of many studies on the electrochemical synthesis of ammonia at high, moderate and low temperatures. Nevertheless, the challenge is impressive, since, of the 60 million tons of H2 produced annually for industrial purposes, 95% come from fossil gas, oil and coal, and the rest come from H2O electrolysis. Thus, it would be necessary to increase the production of electricity from renewable sources to a high extent, as well as to improve the performance of the electrolyzer operated by the different technologies currently in use. Therefore, it is mandatory to implement a “solar ammonia refinery” [275] or “liquid sunshine” [276,277] to obtain green ammonia under a new and more profitable paradigm.
Figure 19 represents this Solar Ammonia Refinery concept. As can be seen, ammonia can be obtained from N2 and H2/H2O, which can be produced on-site and/or transported to the solar refinery, using electricity, heating and photons produced by solar energy.

6.3. Decomposition and Consumption of Green Ammonia

To close the cycle of ammonia as a transportable store of renewable sources of hydrogen, an additional step is necessary, which reverts the ammonia to hydrogen for its specific application. Figure 20 shows the feasible routes for the production and utilization of ammonia, constituting a circular and sustainable cycle called “ammonia economy”. In the first stage, ammonia is produced by energy sources through several processes, including pre-treatment, conversion and synthesis. Furthermore, the surplus electricity can also be converted to hydrogen, which is further converted to ammonia, leading to the application of power-to-ammonia. Then, ammonia is stored, transported and distributed for its utilization.
Therefore, the final step of this cycle requires a previous one in which the hydrogen is extracted in a pure form from ammonia [278,279,280], which is carried out through the cracking reaction, as can be seen in Equation (4):
2NH3 ⇔ N2 + 3H2;  ΔH298 = 46.19 kJ/mol
Hence, an amount of 46.19 kJ of external heat needs to be supplied to obtain 3 mol of hydrogen by the decomposition of 2 mol of ammonia. Such external heat can be obtained in different ways. One way is the use of waste-heat after fuel-consuming equipment. One part of waste-heat is spent for ammonia preheating, and the other part is supplied for a chemical reaction of ammonia decomposition. Such a way of using waste-heat is called thermochemical recuperation (TCR) [280,281,282].
In this respect, the limiting factor is the existence of an effective catalyst for decomposing ammonia into nitrogen and hydrogen. Thus, a lot of research is focused on the study of many different metal-supported catalysts for obtaining efficient ammonia decomposition, including different transition metals, Ru, Pd, Ni, Fe or Co [283,284], as well as bimetallic catalytic systems [285,286,287,288,289]. Likewise, the development of catalysts capable of acting in Haber−Bosch processes in both directions to promote ammonia synthesis and decomposition under mild conditions is also under study [290,291].

7. Summary and Future Perspectives

The introduction of renewable energies capable of reducing the carbon footprint, produced by energies from fossil sources, requires the development of carriers that can harvest, store and transport those primary and secondary energy resources as well as release the energy on the site of demand without GHG emission. In this respect, hydrogen is a good candidate because of its abundance, ubiquity, zero emission and high energy density. However, the direct application of hydrogen presents several technical bottlenecks, related mainly to methods of effectively storing and transporting this commodity, that seriously hinder its practical application. This comes from the extremely low density of hydrogen, even when compressed, in comparison with most liquid fuels. Another aspect that must be considered is its potentially explosive nature, in a wide concentration range.
Regarding this point, ammonia can play an essential role as a hydrogen carrier molecule because of its high hydrogen density, 50% higher than that of compressed or liquefied hydrogen, among other advantages previously mentioned. In addition, stored ammonia can be reversibly transformed into hydrogen over an adequate catalyst; it could also be directly used as a fuel to obtain power electricity or be used in a transport engine.
On the other hand, ammonia could be a very interesting fuel candidate because when it combusts under ideal conditions, the only emissions obtained are nitrogen and water, although nitrous oxides (NOx) can also be generated, mainly at high temperatures and pressures. Its widespread use as a fuel is also inhibited due to its injuriousness in a potential leakage. To overcome such challenges, several ammonia-blended fuels as well as different ammonia-derived compounds are being evaluated for their use as green fuels instead of ammonia. Among these derivatives, urea, ammonium nitrate, mixtures of urea/ammonium nitrate, hydrazine, etc. are currently being evaluated. The question is if the additional process for their obtention compensates for fuel properties clearly superior to ammonia. Be that as it may, these derivatives could play an important role as alternative fuels, diversifying the future energy portfolio.
Finally, if the climate goals proposed for 2050 are to be achieved, it is absolutely necessary that all world production of ammonia be carried out through the use of hydrogen obtained through renewable processes. Therefore, obtaining technically and economically viable routes to producing green ammonia is a mandatory priority, since it must not only replace the current industrial production dedicated to the production of fertilizers and other commodities but also attend a substantial part of the engine fuels, as well as different functions as a carrier of hydrogen.
In this sense, significant improvements must still be carried out in different fields, e.g., technical barriers, i.e., the optimization of green ammonia production, not only improving the yields but also diminishing the costs, mainly regarding its high capital and operational costs (CAPEX and OPEX). Likewise, substantial work is still required regarding system optimization, starting with new injection designs, optimized combustion promotors, a thorough understanding of kinetic chemistry, an optimal combination of design parameters and NOx emission control.

Author Contributions

This review article has been elaborated on by D.L. and R.E., who, in a general way, conceived and wrote the paper. However, all coauthors have also made substantive intellectual contributions to this study, providing substantial input to the conception and design of it. All of them have also been involved in drafting and revising the manuscript such that everyone has given final approval of the current version to be published in the Energies journal. All authors have read and agreed to the published version of the manuscript.


This work was funded by Spanish MCIN through the PID2019-104953RB-I00 Project, the TED2021-132224B-I00 Project funded by MCIN/AEI10.13039/501100011033 and the EU “NextGenerationEU/PRTR” and Junta de Andalucía and FEDER funds (P18-RT-4822). The authors are also thankful for the technical assistance of the staff of the Institute of Chemistry, Energy and Environment (QUIEMA) of the University of Córdoba.

Data Availability Statement

Not applicable.


The authors are thankful to Spanish MCIN through the PID2019-104953RB-I00 Project, to the TED2021-132224B-I00 Project funded by MCIN/AEI10.13039/501100011033 and the EU “NextGenerationEU/PRTR” and Junta de Andalucía and FEDER funds (P18-RT-4822). The authors are also thankful for the technical assistance of the staff of the Institute of Chemistry, Energy and Environment (QUIEMA) of the University of Córdoba.

Conflicts of Interest

The authors declare no conflict of interest.


  1. Brynolf, S.; Hansson, J.; Anderson, J.E.; Skov, I.R.; Wallington, T.J.; Grahn, M.; Korberg, A.D.; Malmgren, E.; Taljegard, M.J. Review of electrofuel feasibility-Prospects for road, ocean, and air transport. Prog. Energy 2022, 4, 042007. [Google Scholar] [CrossRef]
  2. Van Renssen, S. The hydrogen solution? Nat. Clim. Chang. 2020, 10, 799–801. [Google Scholar] [CrossRef]
  3. Kovač, A.; Paranos, M.; Marciuš, D. Hydrogen in energy transition: A review. Int. J. Hydrogen Energy 2021, 46, 10016–10035. [Google Scholar] [CrossRef]
  4. Cheng, W.; Lee, S. How green are the national hydrogen strategies? Sustainability 2022, 14, 1930. [Google Scholar] [CrossRef]
  5. Lebrouhi, B.; Djoupo, J.; Lamrani, B.; Benabdelaziz, K.; Kousksou, T. Global hydrogen development-A technological and geopolitical overview. Int. J. Hydrogen Energy 2022, 47, 7016–7048. [Google Scholar] [CrossRef]
  6. Nnabuife, S.G.; Ugbeh-Johnson, J.; Okeke, N.E.; Ogbonnaya, C. Present and projected developments in hydrogen production: A technological review. Carbon Capture Sci. Technol. 2022, 3, 100042. [Google Scholar] [CrossRef]
  7. Ishaq, H.; Dincer, I.; Crawford, C. A review on hydrogen production and utilization: Challenges and opportunities. Int. J. Hydrogen Energy 2022, 47, 26238–26264. [Google Scholar] [CrossRef]
  8. Hong, X.; Thaore, V.B.; Karimi, I.A.; Farooq, S.; Wang, X.; Usadi, A.K.; Chapman, B.R.; Johnson, R.A. Techno-enviro-economic analyses of hydrogen supply chains with an ASEAN case study. Int. J. Hydrogen Energy 2021, 46, 32914–32928. [Google Scholar] [CrossRef]
  9. Capurso, T.; Stefanizzi, M.; Torresi, M.; Camporeale, S. Perspective of the role of hydrogen in the 21st century energy transition. Energy Convers. Manag. 2022, 251, 114898. [Google Scholar] [CrossRef]
  10. Osman, A.I.; Mehta, N.; Elgarahy, A.M.; Hefny, M.; Al-Hinai, A.; Al-Muhtaseb, A.A.H.; Rooney, D.W. Hydrogen production, storage, utilisation and environmental impacts: A review. Environ. Chem. Lett 2022, 20, 153–188. [Google Scholar] [CrossRef]
  11. Megía, P.J.; Vizcaíno, A.J.; Calles, J.A.; Carrero, A. Hydrogen production technologies: From fossil fuels toward renewable sources. A mini review. Energy Fuels 2021, 35, 16403–16415. [Google Scholar] [CrossRef]
  12. Martino, M.; Ruocco, C.; Meloni, E.; Pullumbi, P.; Palma, V. Main hydrogen production processes: An overview. Catalysts 2021, 11, 547. [Google Scholar] [CrossRef]
  13. Longden, T.; Beck, F.J.; Jotzo, F.; Andrews, R.; Prasad, M. ‘Clean’hydrogen?–Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen. Appl. Energy 2022, 306, 118145. [Google Scholar] [CrossRef]
  14. Guilbert, D.; Vitale, G. Hydrogen as a clean and sustainable energy vector for global transition from fossil-based to zero-carbon. Clean Technol. 2021, 3, 881–909. [Google Scholar] [CrossRef]
  15. Xiang, H.; Ch, P.; Nawaz, M.A.; Chupradit, S.; Fatima, A.; Sadiq, M. Integration and economic viability of fueling the future with green hydrogen: An integration of its determinants from renewable economics. Int. J. Hydrogen Energy 2021, 46, 38145–38162. [Google Scholar] [CrossRef]
  16. Liu, W.; Wan, Y.; Xiong, Y.; Gao, P. Green hydrogen standard in China: Standard and evaluation of low-carbon hydrogen, clean hydrogen, and renewable hydrogen. Int. J. Hydrogen Energy 2022, 47, 24584–24591. [Google Scholar] [CrossRef]
  17. Dong, H.; Wu, Y.; Zhou, J.; Chen, W. Optimal selection for wind power coupled hydrogen energy storage from a risk perspective, considering the participation of multi-stakeholder. J. Clean. Prod. 2022, 356, 131853. [Google Scholar] [CrossRef]
  18. Li, L.; Feng, L.; Manier, H.; Manier, M.-A. Life cycle optimization for hydrogen supply chain network design. Int. J. Hydrogen Energy 2022, in press. [Google Scholar] [CrossRef]
  19. Alkawsi, G.; Baashar, Y.; Abbas, U.D.; Alkahtani, A.A.; Tiong, S.K. Review of renewable energy-based charging infrastructure for electric vehicles. Appl. Sci. 2021, 11, 3847. [Google Scholar] [CrossRef]
  20. Rego de Vasconcelos, B.; Lavoie, J.-M. Recent advances in power-to-X technology for the production of fuels and chemicals. Front. Chem. 2019, 7, 392. [Google Scholar] [CrossRef] [Green Version]
  21. Cano, M.H.; Agbossou, K.; Kelouwani, S.; Dubé, Y. Experimental evaluation of a power management system for a hybrid renewable energy system with hydrogen production. Renew. Energy 2017, 113, 1086–1098. [Google Scholar] [CrossRef]
  22. Dadak, A.; Mehrpooya, M.; Kasaeian, A. Design and development of an innovative integrated structure for the production and storage of energy and hydrogen utilizing renewable energy. Sustain. Energy Technol. Assess. 2021, 45, 101123. [Google Scholar] [CrossRef]
  23. Hannan, M.; Wali, S.; Ker, P.; Abd Rahman, M.; Mansor, M.; Ramachandaramurthy, V.; Muttaqi, K.; Mahlia, T.; Dong, Z. Battery energy-storage system: A review of technologies, optimization objectives, constraints, approaches, and outstanding issues. J. Energy Storage 2021, 42, 103023. [Google Scholar] [CrossRef]
  24. de Siqueira, L.M.S.; Peng, W. Control strategy to smooth wind power output using battery energy storage system: A review. J. Energy Storage 2021, 35, 102252. [Google Scholar] [CrossRef]
  25. Jurczyk, M.; Węcel, D.; Uchman, W.; Skorek-Osikowska, A. Assessment of operational performance for an integrated ‘power to synthetic natural gas’ system. Energies 2022, 15, 74. [Google Scholar] [CrossRef]
  26. Skov, I.R.; Schneider, N. Incentive structures for power-to-X and e-fuel pathways for transport in EU and member states. Energy Policy 2022, 168, 113121. [Google Scholar] [CrossRef]
  27. Oh, S.; Park, C.; Kim, S.; Kim, Y.; Choi, Y.; Kim, C. Natural gas–ammonia dual-fuel combustion in spark-ignited engine with various air–fuel ratios and split ratios of ammonia under part load condition. Fuel 2021, 290, 120095. [Google Scholar] [CrossRef]
  28. Galimova, T.; Ram, M.; Bogdanov, D.; Fasihi, M.; Khalili, S.; Gulagi, A.; Karjunen, H.; Mensah, T.N.O.; Breyer, C. Global demand analysis for carbon dioxide as raw material from key industrial sources and direct air capture to produce renewable electricity-based fuels and chemicals. J. Clean. Prod. 2022, 373, 133920. [Google Scholar] [CrossRef]
  29. Desport, L.; Selosse, S. An overview of CO2 capture and utilization in energy models. Resour. Conserv. Recycl. 2022, 180, 106150. [Google Scholar] [CrossRef]
  30. Breuer, J.L.; Scholten, J.; Koj, J.C.; Schorn, F.; Fiebrandt, M.; Samsun, R.C.; Albus, R.; Görner, K.; Stolten, D.; Peters, R. An Overview of Promising Alternative Fuels for Road, Rail, Air, and Inland Waterway Transport in Germany. Energies 2022, 15, 1443. [Google Scholar] [CrossRef]
  31. Roy, S.; Cherevotan, A.; Peter, S.C. Thermochemical CO2 hydrogenation to single carbon products: Scientific and technological challenges. ACS Energy Lett. 2018, 3, 1938–1966. [Google Scholar] [CrossRef] [Green Version]
  32. Angeles, D.A.; Tan, R.R.; Aviso, K.B.; Are, K.R.A.G.; Razon, L.F. Fuzzy optimization of the automotive ammonia fuel cycle. J. Clean. Prod. 2018, 186, 877–882. [Google Scholar] [CrossRef]
  33. Liu, R.; Ting, D.S.-K.; Checkel, M.D. Ammonia as a Fuel for SI Engine; SAE Technical Paper 0148-7191; SAE International: Warrendale, PA, USA, 2003. [Google Scholar]
  34. Westlye, F.R.; Ivarsson, A.; Schramm, J. Experimental investigation of nitrogen based emissions from an ammonia fueled SI-engine. Fuel 2013, 111, 239–247. [Google Scholar] [CrossRef]
  35. Aziz, M.; Wijayanta, A.T.; Nandiyanto, A.B.D. Ammonia as effective hydrogen storage: A review on production, storage and utilization. Energies 2020, 13, 3062. [Google Scholar] [CrossRef]
  36. Hasan, M.H.; Mahlia, T.M.I.; Mofijur, M.; Rizwanul Fattah, I.; Handayani, F.; Ong, H.C.; Silitonga, A. A comprehensive review on the recent development of ammonia as a renewable energy carrier. Energies 2021, 14, 3732. [Google Scholar] [CrossRef]
  37. Chatterjee, S.; Parsapur, R.K.; Huang, K.-W. Limitations of ammonia as a hydrogen energy carrier for the transportation sector. ACS Energy Lett. 2021, 6, 4390–4394. [Google Scholar] [CrossRef]
  38. Sánchez, A.; Castellano, E.; Martín, M.; Vega, P. Evaluating ammonia as green fuel for power generation: A thermo-chemical perspective. Appl. Energy 2021, 293, 116956. [Google Scholar] [CrossRef]
  39. Ikäheimo, J.; Kiviluoma, J.; Weiss, R.; Holttinen, H. Power-to-ammonia in future North European 100% renewable power and heat system. Int. J. Hydrogen Energy 2018, 43, 17295–17308. [Google Scholar] [CrossRef]
  40. Wu, S.; Salmon, N.; Li, M.M.-J.; Bañares-Alcántara, R.; Tsang, S.C.E. Energy decarbonization via green H2 or NH3? ACS Energy Lett. 2022, 7, 1021–1033. [Google Scholar] [CrossRef]
  41. Kojima, Y.; Yamaguchi, M. Ammonia as a hydrogen energy carrier. Int. J. Hydrogen Energy 2022, 47, 22832–22839. [Google Scholar] [CrossRef]
  42. Salmon, N.; Bañares-Alcántara, R. Green ammonia as a spatial energy vector: A review. Sustain. Energy Fuels 2021, 5, 2814–2839. [Google Scholar] [CrossRef]
  43. Morlanés, N.; Katikaneni, S.P.; Paglieri, S.N.; Harale, A.; Solami, B.; Sarathy, S.M.; Gascon, J. A technological roadmap to the ammonia energy economy: Current state and missing technologies. Chem. Eng. J. 2021, 408, 127310. [Google Scholar] [CrossRef]
  44. Del Pozo, C.A.; Cloete, S. Techno-economic assessment of blue and green ammonia as energy carriers in a low-carbon future. Energy Convers. Manag. 2022, 255, 115312. [Google Scholar] [CrossRef]
  45. Cesaro, Z.; Ives, M.; Nayak-Luke, R.; Mason, M.; Bañares-Alcántara, R. Ammonia to power: Forecasting the levelized cost of electricity from green ammonia in large-scale power plants. Appl. Energy 2021, 282, 116009. [Google Scholar] [CrossRef]
  46. Liu, F.; Ding, D.; Duan, C. Protonic ceramic electrochemical cells for synthesizing sustainable chemicals and fuels. Adv. Sci. 2023, 10, 2206478. [Google Scholar] [CrossRef]
  47. Huo, X.; Van Hoomissen, D.J.; Liu, J.; Vyas, S.; Strathmann, T.J. Hydrogenation of aqueous nitrate and nitrite with ruthenium catalysts. Appl. Catal. B Environ. 2017, 211, 188–198. [Google Scholar] [CrossRef] [Green Version]
  48. Boasiako, C.A.; Zhou, Z.; Huo, X.; Ye, T. Development of Pd-based catalysts for hydrogenation of nitrite and nitrate in water: A review. J. Hazard. Mater. 2023, 446, 130661. [Google Scholar] [CrossRef]
  49. Desai, P.D.; Turley, M.; Robinson, R.; Zimmerman, W.B. Hot microbubble injection in thin liquid film layers for ammonia separation from ammonia rich-wastewater. Chem. Eng. Process.-Process Intensif. 2022, 180, 108693. [Google Scholar] [CrossRef]
  50. De la Rubia, M.Á.; Walker, M.; Heaven, S.; Banks, C.J.; Borja, R. Preliminary trials of in situ ammonia stripping from source segregated domestic food waste digestate using biogas: Effect of temperature and flow rate. Bioresour. Technol. 2010, 101, 9486–9492. [Google Scholar] [CrossRef] [Green Version]
  51. Wen, D.; Aziz, M. Design and analysis of biomass-to-ammonia-to-power as an energy storage method in a renewable multi-generation system. Energy Convers. Manag. 2022, 261, 115611. [Google Scholar] [CrossRef]
  52. Wiseman, S.; Rieth, M.; Gruber, A.; Dawson, J.R.; Chen, J.H. A comparison of the blow-out behavior of turbulent premixed ammonia/hydrogen/nitrogen-air and methane–air flames. Proc. Combust. Inst. 2021, 38, 2869–2876. [Google Scholar] [CrossRef]
  53. Vigueras-Zuniga, M.-O.; Tejeda-del-Cueto, M.-E.; Vasquez-Santacruz, J.-A.; Herrera-May, A.-L.; Valera-Medina, A. Numerical predictions of a swirl combustor using complex chemistry fueled with ammonia/hydrogen blends. Energies 2020, 13, 288. [Google Scholar] [CrossRef] [Green Version]
  54. Wang, B.; Yang, C.; Wang, H.; Hu, D.; Wang, Y. Effect of Diesel-Ignited Ammonia/Hydrogen mixture fuel combustion on engine combustion and emission performance. Fuel 2023, 331, 125865. [Google Scholar] [CrossRef]
  55. Chai, W.S.; Bao, Y.; Jin, P.; Tang, G.; Zhou, L. A review on ammonia, ammonia-hydrogen and ammonia-methane fuels. Renew. Sustain. Energy Rev. 2021, 147, 111254. [Google Scholar] [CrossRef]
  56. Zhang, M.; An, Z.; Wei, X.; Wang, J.; Huang, Z.; Tan, H. Emission analysis of the CH4/NH3/air co-firing fuels in a model combustor. Fuel 2021, 291, 120135. [Google Scholar] [CrossRef]
  57. Ji, L.; Wang, J.; Hu, G.; Mao, R.; Zhang, W.; Huang, Z. Experimental study on structure and blow-off characteristics of NH3/CH4 co-firing flames in a swirl combustor. Fuel 2022, 314, 123027. [Google Scholar] [CrossRef]
  58. Elbaz, A.M.; Albalawi, A.M.; Wang, S.; Roberts, W.L. Stability and characteristics of NH3/CH4/air flames in a combustor fired by a double swirl stabilized burner. Proc. Combust. Inst. 2022, 39, 4205–4213. [Google Scholar] [CrossRef]
  59. Sun, J.; Yang, Q.; Zhao, N.; Chen, M.; Zheng, H. Numerically study of CH4/NH3 combustion characteristics in an industrial gas turbine combustor based on a reduced mechanism. Fuel 2022, 327, 124897. [Google Scholar] [CrossRef]
  60. Elbaz, A.M.; Wang, S.; Guiberti, T.F.; Roberts, W.L. Review on the recent advances on ammonia combustion from the fundamentals to the applications. Fuel Commun. 2022, 10, 100053. [Google Scholar] [CrossRef]
  61. He, C.; Jiang, J.; Sun, M.; Yu, Y.; Liu, K.; Zhang, B. Analysis of the NH3 blended ratio on the impinging flame structure in non-premixed CH4/NH3/air combustion. Fuel 2022, 330, 125559. [Google Scholar] [CrossRef]
  62. Okafor, E.C.; Somarathne, K.K.A.; Ratthanan, R.; Hayakawa, A.; Kudo, T.; Kurata, O.; Iki, N.; Tsujimura, T.; Furutani, H.; Kobayashi, H. Control of NOx and other emissions in micro gas turbine combustors fuelled with mixtures of methane and ammonia. Combust. Flame 2020, 211, 406–416. [Google Scholar] [CrossRef]
  63. Zhu, R.; Han, X.; Zhang, Z.; He, Y.; Wang, Z. Experimental and kinetic study on laminar burning velocities of NH3/CH4/H2S/air flames. Fuel 2023, 332, 126174. [Google Scholar] [CrossRef]
  64. Mikulčić, H.; Baleta, J.; Wang, X.; Wang, J.; Qi, F.; Wang, F. Numerical simulation of ammonia/methane/air combustion using reduced chemical kinetics models. Int. J. Hydrogen Energy 2021, 46, 23548–23563. [Google Scholar] [CrossRef]
  65. Xiao, H.; Howard, M.; Valera-Medina, A.; Dooley, S.; Bowen, P. Reduced chemical mechanisms for ammonia/methane co-firing for gas turbine applications. Energy Procedia 2017, 105, 1483–1488. [Google Scholar] [CrossRef]
  66. Ayaz, S.K.; Altuntas, O.; Caliskan, H. Enhanced life cycle modelling of a micro gas turbine fuelled with various fuels for sustainable electricity production. Renew. Sustain. Energy Rev. 2021, 149, 111323. [Google Scholar] [CrossRef]
  67. Kurata, O.; Iki, N.; Matsunuma, T.; Inoue, T.; Tsujimura, T.; Furutani, H.; Kobayashi, H.; Hayakawa, A. Performances and emission characteristics of NH3–air and NH3CH4–air combustion gas-turbine power generations. Proc. Combust. Inst. 2017, 36, 3351–3359. [Google Scholar] [CrossRef]
  68. Mong, G.R.; Chiong, M.-C.; Chong, C.T.; Ng, J.-H.; Mashruk, S.; Tran, M.-V.; Lee, K.M.; Samiran, N.A.; Wong, K.Y.; Valera-Medina, A. Fuel-lean ammonia/biogas combustion characteristics under the reacting swirl flow conditions. Fuel 2023, 331, 125983. [Google Scholar] [CrossRef]
  69. Han, X.; Wang, Z.; Costa, M.; Sun, Z.; He, Y.; Cen, K. Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames. Combust. Flame 2019, 206, 214–226. [Google Scholar] [CrossRef]
  70. Xiao, H.; Valera-Medina, A.; Bowen, P.J. Study on premixed combustion characteristics of co-firing ammonia/methane fuels. Energy 2017, 140, 125–135. [Google Scholar] [CrossRef]
  71. Shu, T.; Xue, Y.; Zhou, Z.; Ren, Z. An experimental study of laminar ammonia/methane/air premixed flames using expanding spherical flames. Fuel 2021, 290, 120003. [Google Scholar] [CrossRef]
  72. Okafor, E.C.; Naito, Y.; Colson, S.; Ichikawa, A.; Kudo, T.; Hayakawa, A.; Kobayashi, H. Experimental and numerical study of the laminar burning velocity of CH4–NH3–air premixed flames. Combust. Flame 2018, 187, 185–198. [Google Scholar] [CrossRef]
  73. Somarathne, K.D.K.A.; Okafor, E.C.; Sugawara, D.; Hayakawa, A.; Kobayashi, H. Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure. Proc. Combust. Inst. 2021, 38, 5163–5170. [Google Scholar] [CrossRef]
  74. Okafor, E.C.; Yamashita, H.; Hayakawa, A.; Somarathne, K.K.A.; Kudo, T.; Tsujimura, T.; Uchida, M.; Ito, S.; Kobayashi, H. Flame stability and emissions characteristics of liquid ammonia spray co-fired with methane in a single stage swirl combustor. Fuel 2021, 287, 119433. [Google Scholar] [CrossRef]
  75. Yin, G.; Li, J.; Zhou, M.; Li, J.; Wang, C.; Hu, E.; Huang, Z. Experimental and kinetic study on laminar flame speeds of ammonia/dimethyl ether/air under high temperature and elevated pressure. Combust. Flame 2022, 238, 111915. [Google Scholar] [CrossRef]
  76. Issayev, G.; Giri, B.R.; Elbaz, A.M.; Shrestha, K.P.; Mauss, F.; Roberts, W.L.; Farooq, A. Ignition delay time and laminar flame speed measurements of ammonia blended with dimethyl ether: A promising low carbon fuel blend. Renew. Energy 2022, 181, 1353–1370. [Google Scholar] [CrossRef]
  77. Shrestha, K.P.; Giri, B.R.; Elbaz, A.M.; Issayev, G.; Roberts, W.L.; Seidel, L.; Mauss, F.; Farooq, A. A detailed chemical insights into the kinetics of diethyl ether enhancing ammonia combustion and the importance of NOx recycling mechanism. Fuel Commun. 2022, 10, 100051. [Google Scholar] [CrossRef]
  78. Meng, X.; Zhang, M.; Zhao, C.; Tian, H.; Tian, J.; Long, W.; Bi, M. Study of combustion and NO chemical reaction mechanism in ammonia blended with DME. Fuel 2022, 319, 123832. [Google Scholar] [CrossRef]
  79. Issayev, G.; Giri, B.R.; Elbaz, A.M.; Shrestha, K.P.; Mauss, F.; Roberts, W.L.; Farooq, A. Combustion behavior of ammonia blended with diethyl ether. Proc. Combust. Inst. 2021, 38, 499–506. [Google Scholar] [CrossRef]
  80. Cai, T.; Zhao, D. Enhancing and assessing ammonia-air combustion performance by blending with dimethyl ether. Renew. Sustain. Energy Rev. 2022, 156, 112003. [Google Scholar] [CrossRef]
  81. Chen, J.; Gou, X. Experimental and kinetic study on the extinction characteristics of ammonia-dimethyl ether diffusion flame. Fuel 2023, 334, 126743. [Google Scholar] [CrossRef]
  82. Mei, B.; Ma, S.; Zhang, Y.; Zhang, X.; Li, W.; Li, Y. Exploration on laminar flame propagation of ammonia and syngas mixtures up to 10 atm. Combust. Flame 2020, 220, 368–377. [Google Scholar] [CrossRef]
  83. Zhou, S.; Yang, W.; Tan, H.; An, Q.; Wang, J.; Dai, H.; Wang, X.; Wang, X.; Deng, S. Experimental and kinetic modeling study on NH3/syngas/air and NH3/bio-syngas/air premixed laminar flames at elevated temperature. Combust. Flame 2021, 233, 111594. [Google Scholar] [CrossRef]
  84. Wang, S.; Wang, Z.; Elbaz, A.M.; Han, X.; He, Y.; Costa, M.; Konnov, A.A.; Roberts, W.L. Experimental study and kinetic analysis of the laminar burning velocity of NH3/syngas/air, NH3/CO/air and NH3/H2/air premixed flames at elevated pressures. Combust. Flame 2020, 221, 270–287. [Google Scholar] [CrossRef]
  85. Xu, H.; Wang, J.; Zhang, C.; Dai, L.; He, Z.; Wang, Q. Numerical study on laminar burning velocity of ammonia flame with methanol addition. Int. J. Hydrogen Energy 2022, 47, 28152–28164. [Google Scholar] [CrossRef]
  86. Giri, B.R.; Shrestha, K.P.; Mai, T.V.T.; Mauss, F.; Huynh, L.K. A theoretical kinetic study of the reactions of NH2 radicals with methanol and ethanol and their implications in kinetic modeling. Int. J. Chem. Kinet. 2023, 55, 3–14. [Google Scholar] [CrossRef]
  87. Ronan, P.; Pierre, B.; Christine, M.-R.; Guillaume, D.; Fabien, H. Laminar flame speed of ethanol/ammonia blends–An experimental and kinetic study. Fuel Commun. 2022, 10, 100052. [Google Scholar] [CrossRef]
  88. Lavadera, M.L.; Pelucchi, M.; Konnov, A.A. The influence of ammonia on the laminar burning velocities of methylcyclohexane and toluene: An experimental and kinetic modeling study. Combust. Flame 2022, 237, 111839. [Google Scholar] [CrossRef]
  89. Elbaz, A.M.; Giri, B.R.; Issayev, G.; Shrestha, K.P.; Mauss, F.; Farooq, A.; Roberts, W.L. Experimental and kinetic modeling study of laminar flame speed of dimethoxymethane and ammonia blends. Energy Fuels 2020, 34, 14726–14740. [Google Scholar] [CrossRef]
  90. Lubrano Lavadera, M.; Han, X.; Konnov, A.A. Comparative effect of ammonia addition on the laminar burning velocities of methane, n-heptane, and iso-octane. Energy Fuels 2020, 35, 7156–7168. [Google Scholar] [CrossRef]
  91. Cellek, M.S. The decreasing effect of ammonia enrichment on the combustion emission of hydrogen, methane, and propane fuels. Int. J. Hydrogen Energy 2022, 47, 19916–19934. [Google Scholar] [CrossRef]
  92. Wang, Z.; Ji, C.; Wang, D.; Zhang, T.; Zhai, Y.; Wang, S. Experimental and numerical study on laminar burning velocity and premixed combustion characteristics of NH3/C3H8/air mixtures. Fuel 2023, 331, 125936. [Google Scholar] [CrossRef]
  93. Berwal, P.; Kumar, S. Laminar burning velocity measurement of CH4/H2/NH3-air premixed flames at high mixture temperatures. Fuel 2023, 331, 125809. [Google Scholar] [CrossRef]
  94. Wang, S.; Elbaz, A.M.; Wang, G.; Wang, Z.; Roberts, W.L. Turbulent flame speed of NH3/CH4/H2/H2O/air-mixtures: Effects of elevated pressure and Lewis number. Combust. Flame 2023, 247, 112488. [Google Scholar] [CrossRef]
  95. Stocks, M.; Fazeli, R.; Hughes, L.; Beck, F.J. Global emissions implications from co-combusting ammonia in coal fired power stations: An analysis of the Japan-Australia supply chain. J. Clean. Prod. 2022, 336, 130092. [Google Scholar] [CrossRef]
  96. Trencher, G.; Downie, C.; Hasegawa, K.; Asuka, J. Divestment trends in Japan’s international coal businesses. Renew. Sustain. Energy Rev. 2020, 124, 109779. [Google Scholar] [CrossRef]
  97. Xu, Y.; Wang, H.; Liu, X.; Zhu, J.; Xu, J.; Xu, M. Mitigating CO2 emission in pulverized coal-fired power plant via co-firing ammonia: A simulation study of flue gas streams and exergy efficiency. Energy Convers. Manag. 2022, 256, 115328. [Google Scholar] [CrossRef]
  98. Wang, X.; Fan, W.; Chen, J.; Feng, G.; Zhang, X. Experimental study on effects of air-staged strategy and NH3 co-firing ratios on NO formation characteristics in ammonia/coal co-firing process. Fuel 2023, 332, 126217. [Google Scholar] [CrossRef]
  99. Cardoso, J.S.; Silva, V.; Chavando, J.A.M.; Eusébio, D.; Hall, M.J. Numerical modelling of the coal phase-out through ammonia and biomass co-firing in a pilot-scale fluidized bed reactor. Fuel Commun. 2022, 10, 100055. [Google Scholar] [CrossRef]
  100. Chen, P.; Jiang, B.; Wang, H.; Gu, M.; Fang, Y.; Wang, P. Experimental and theoretical calculations study on heterogeneous reduction of NO by char/NH3 in the reduction zone of ammonia co-firing with pulverized coal: Influence of mineral Fe. Fuel 2022, 310, 122374. [Google Scholar] [CrossRef]
  101. Chen, C.; Wang, Z.; Zhu, R.; Tan, J.; He, Y.; Cen, K. Co-firing characteristics and fuel-N transformation of ammonia/pulverized coal binary fuel. Fuel 2023, 337, 126857. [Google Scholar] [CrossRef]
  102. Ma, P.; Huang, Q.; Wu, Z.; Lyu, J.; Li, S. Optical diagnostics on coal ignition and gas-phase combustion in co-firing ammonia with pulverized coal on a two-stage flat flame burner. Proc. Combust. Inst. 2022, 39, 3457–3466. [Google Scholar] [CrossRef]
  103. Lim, H.; Park, Y.; Lee, Y.; Lee, Y.; Chae, T.; Lee, J.; Yang, W.; Kim, J. Impact of ammonium sulfate and kaolin on ash deposition during co-firing of straw pellets and pulverized coal. Korean J. Chem. Eng. 2022, 39, 2089–2098. [Google Scholar] [CrossRef]
  104. Chen, L.; Wang, C.; Wang, W. Effect of ammonia co-firing on heat transfer, safety, and economy of coal-fired boilers. Fuel 2023, 334, 126649. [Google Scholar] [CrossRef]
  105. Korberg, A.D.; Brynolf, S.; Grahn, M.; Skov, I.R. Techno-economic assessment of advanced fuels and propulsion systems in future fossil-free ships. Renew. Sustain. Energy Rev. 2021, 142, 110861. [Google Scholar] [CrossRef]
  106. Mallouppas, G.; Ioannou, C.; Yfantis, E.A. A Review of the Latest Trends in the Use of Green Ammonia as an Energy Carrier in Maritime Industry. Energies 2022, 15, 1453. [Google Scholar] [CrossRef]
  107. Tsang, S.; Ayvali, T.; Van Vrijaldenhoven, T. The Position of Ammonia in Decarbonising Maritime Industry: An Overview and Perspectives: Part I: Technological advantages and the momentum towards ammonia-propelled shipping. Johns. Matthey Technol. Rev. 2021, 65, 275–290. [Google Scholar]
  108. Cardoso, J.S.; Silva, V.; Rocha, R.C.; Hall, M.J.; Costa, M.; Eusébio, D. Ammonia as an energy vector: Current and future prospects for low-carbon fuel applications in internal combustion engines. J. Clean. Prod. 2021, 296, 126562. [Google Scholar] [CrossRef]
  109. Dimitriou, P.; Javaid, R. A review of ammonia as a compression ignition engine fuel. Int. J. Hydrogen Energy 2020, 45, 7098–7118. [Google Scholar] [CrossRef]
  110. Yapicioglu, A.; Dincer, I. Performance assesment of hydrogen and ammonia combustion with various fuels for power generators. Int. J. Hydrogen Energy 2018, 43, 21037–21048. [Google Scholar] [CrossRef]
  111. Chiong, M.-C.; Chong, C.T.; Ng, J.-H.; Mashruk, S.; Chong, W.W.F.; Samiran, N.A.; Mong, G.R.; Valera-Medina, A. Advancements of combustion technologies in the ammonia-fuelled engines. Energy Convers. Manag. 2021, 244, 114460. [Google Scholar] [CrossRef]
  112. Kurien, C.; Varma, P.S.; Mittal, M. Effect of ammonia energy fractions on combustion stability and engine characteristics of gaseous (ammonia/methane) fuelled spark ignition engine. Int. J. Hydrogen Energy 2023, 48, 1391–1400. [Google Scholar] [CrossRef]
  113. Wang, Y.; Zhou, X.; Liu, L. Feasibility study of hydrogen jet flame ignition of ammonia fuel in marine low speed engine. Int. J. Hydrogen Energy 2023, 48, 327–336. [Google Scholar] [CrossRef]
  114. Liu, Z.; Zhou, L.; Zhong, L.; Liu, P.; Wei, H. Experimental Investigation on the Combustion Characteristics of NH3/H2/air by the Spark Ignition and Turbulent Jet Ignition. Combust. Sci. Technol. 2022, 1–22. [Google Scholar] [CrossRef]
  115. Mercier, A.; Mounaïm-Rousselle, C.; Brequigny, P.; Bouriot, J.; Dumand, C. Improvement of SI engine combustion with ammonia as fuel: Effect of ammonia dissociation prior to combustion. Fuel Commun. 2022, 11, 100058. [Google Scholar] [CrossRef]
  116. Ezzat, M.; Dincer, I. Development and assessment of a new hybrid vehicle with ammonia and hydrogen. Appl. Energy 2018, 219, 226–239. [Google Scholar] [CrossRef]
  117. Lhuillier, C.; Brequigny, P.; Contino, F.; Mounaïm-Rousselle, C. Experimental investigation on ammonia combustion behavior in a spark-ignition engine by means of laminar and turbulent expanding flames. Proc. Combust. Inst. 2021, 38, 5859–5868. [Google Scholar] [CrossRef]
  118. Liu, S.; Lin, Z.; Zhang, H.; Lei, N.; Qi, Y.; Wang, Z. Impact of ammonia addition on knock resistance and combustion performance in a gasoline engine with high compression ratio. Energy 2023, 262, 125458. [Google Scholar] [CrossRef]
  119. Dinesh, M.; Pandey, J.K.; Kumar, G. Study of performance, combustion, and NOx emission behavior of an SI engine fuelled with ammonia/hydrogen blends at various compression ratio. Int. J. Hydrogen Energy 2022, 47, 25391–25403. [Google Scholar] [CrossRef]
  120. Zhu, R.; Fang, X.; Xu, C.; Zhao, M.; Zhang, H.; Davy, M. Pulsating one-dimensional detonation in ammonia-hydrogen–air mixtures. Int. J. Hydrogen Energy 2022, 47, 21517–21536. [Google Scholar] [CrossRef]
  121. Mounaïm-Rousselle, C.; Bréquigny, P.; Dumand, C.; Houillé, S. Operating limits for ammonia fuel spark-ignition engine. Energies 2021, 14, 4141. [Google Scholar] [CrossRef]
  122. Mounaïm-Rousselle, C.; Brequigny, P. Ammonia as fuel for low-carbon spark-ignition engines of tomorrow’s passenger cars. Front. Mech. Eng. 2020, 6, 70. [Google Scholar] [CrossRef]
  123. Zhang, H.; Li, G.; Long, Y.; Zhang, Z.; Wei, W.; Zhou, M.; Belal, B.Y. Numerical study on combustion and emission characteristics of a spark-ignition ammonia engine added with hydrogen-rich gas from exhaust-fuel reforming. Fuel 2023, 332, 125939. [Google Scholar] [CrossRef]
  124. Yang, W.; Dinesh, K.R.; Luo, K.; Thévenin, D. Direct numerical simulations of auto-igniting mixing layers in ammonia and ammonia-hydrogen combustion under engine-relevant conditions. Int. J. Hydrogen Energy 2022, 47, 38055–38074. [Google Scholar] [CrossRef]
  125. Xin, G.; Ji, C.; Wang, S.; Meng, H.; Chang, K.; Yang, J. Effect of different volume fractions of ammonia on the combustion and emission characteristics of the hydrogen-fueled engine. Int. J. Hydrogen Energy 2022, 47, 16297–16308. [Google Scholar] [CrossRef]
  126. Liu, Z.; Zhou, L.; Zhong, L.; Wei, H. Enhanced combustion of ammonia engine based on novel air-assisted pre-chamber turbulent jet ignition. Energy Convers. Manag. 2023, 276, 116526. [Google Scholar] [CrossRef]
  127. Oh, S.; Park, C.; Ahn, M.; Jang, H.; Kim, S. Experimental approach for reducing nitrogen oxides emissions from ammonia–natural gas dual-fuel spark-ignition engine. Fuel 2023, 332, 126065. [Google Scholar] [CrossRef]
  128. Salek, F.; Babaie, M.; Shakeri, A.; Hosseini, S.V.; Bodisco, T.; Zare, A. Numerical study of engine performance and emissions for port injection of ammonia into a gasoline\ethanol dual-fuel spark ignition engine. Appl. Sci. 2021, 11, 1441. [Google Scholar] [CrossRef]
  129. Zheng, K.; Ning, X.; Huo, X.; Chen, M.; Wang, X.; Wang, J. Explosion Characteristics of NH3/CH3OH/Air Mixtures. Energy Fuels 2022, 36, 12737–12749. [Google Scholar] [CrossRef]
  130. Rehbein, M.; Meier, C.; Eilts, P.; Scholl, S. Mixtures of ammonia and organic solvents as alternative fuel for internal combustion engines. Energy Fuels 2019, 33, 10331–10342. [Google Scholar] [CrossRef]
  131. Yousefi, A.; Guo, H.; Dev, S.; Lafrance, S.; Liko, B. A study on split diesel injection on thermal efficiency and emissions of an ammonia/diesel dual-fuel engine. Fuel 2022, 316, 123412. [Google Scholar] [CrossRef]
  132. Kane, S.P.; Northrop, W.F. Thermochemical recuperation to enable efficient ammonia-diesel dual-fuel combustion in a compression ignition engine. Energies 2021, 14, 7540. [Google Scholar] [CrossRef]
  133. Zhang, Y.; Zhou, W.; Liang, Y.; Yu, L.; Lu, X. An experimental and detailed kinetic modeling study of the auto-ignition of NH3/diesel mixtures: Part 1-NH3 substitution ratio from 20% to 90%. Combust. Flame 2023, 251, 112391. [Google Scholar] [CrossRef]
  134. Li, T.; Zhou, X.; Wang, N.; Wang, X.; Chen, R.; Li, S.; Yi, P. A comparison between low-and high-pressure injection dual-fuel modes of diesel-pilot-ignition ammonia combustion engines. J. Energy Inst. 2022, 102, 362–373. [Google Scholar] [CrossRef]
  135. Zhou, X.; Li, T.; Wang, N.; Wang, X.; Chen, R.; Li, S. Pilot diesel-ignited ammonia dual fuel low-speed marine engines: A comparative analysis of ammonia premixed and high-pressure spray combustion modes with CFD simulation. Renew. Sustain. Energy Rev. 2023, 173, 113108. [Google Scholar] [CrossRef]
  136. Nadimi, E.; Przybyła, G.; Lewandowski, M.T.; Adamczyk, W. Effects of ammonia on combustion, emissions, and performance of the ammonia/diesel dual-fuel compression ignition engine. J. Energy Inst. 2023, 107, 101158. [Google Scholar] [CrossRef]
  137. Zhu, J.; Zhou, D.; Yang, W.; Qian, Y.; Mao, Y.; Lu, X. Investigation on the potential of using carbon-free ammonia in large two-stroke marine engines by dual-fuel combustion strategy. Energy 2023, 263, 125748. [Google Scholar] [CrossRef]
  138. Zhang, Z.; Long, W.; Dong, P.; Tian, H.; Tian, J.; Li, B.; Wang, Y. Performance characteristics of a two-stroke low speed engine applying ammonia/diesel dual direct injection strategy. Fuel 2023, 332, 126086. [Google Scholar] [CrossRef]
  139. Kuta, K.; Przybyła, G.; Kurzydym, D.; Żmudka, Z. Experimental and numerical investigation of dual-fuel CI ammonia engine emissions and after-treatment with V2O5/SiO2–TiO2 SCR. Fuel 2023, 334, 126523. [Google Scholar] [CrossRef]
  140. Rodríguez, C.G.; Lamas, M.I.; Rodríguez, J.d.D.; Abbas, A. Possibilities of ammonia as both fuel and NOx reductant in marine engines: A numerical study. J. Mar. Sci. Eng. 2022, 10, 43. [Google Scholar] [CrossRef]
  141. Liu, L.; Wu, Y.; Wang, Y.; Wu, J.; Fu, S. Exploration of environmentally friendly marine power technology-ammonia/diesel stratified injection. J. Clean. Prod. 2022, 380, 135014. [Google Scholar] [CrossRef]
  142. Frost, J.; Tall, A.; Sheriff, A.M.; Schönborn, A.; Hellier, P. An experimental and modelling study of dual fuel aqueous ammonia and diesel combustion in a single cylinder compression ignition engine. Int. J. Hydrogen Energy 2021, 46, 35495–35510. [Google Scholar] [CrossRef]
  143. Kurien, C.; Mittal, M. Review on the production and utilization of green ammonia as an alternate fuel in dual-fuel compression ignition engines. Energy Convers. Manag. 2022, 251, 114990. [Google Scholar] [CrossRef]
  144. Sivasubramanian, R.; Sajin, J.; Omanakuttan Pillai, G. Effect of ammonia to reduce emission from biodiesel fuelled diesel engine. Int. J. Ambient Energy 2022, 43, 661–665. [Google Scholar] [CrossRef]
  145. Zhou, D.; Tay, K.L.; Tu, Y.; Li, J.; Yang, W.; Zhao, D. A numerical investigation on the injection timing of boot injection rate-shapes in a kerosene-diesel engine with a clustered dynamic adaptive chemistry method. Appl. Energy 2018, 220, 117–126. [Google Scholar] [CrossRef]
  146. Pham, Q.; Park, S.; Agarwal, A.K.; Park, S. Review of dual-fuel combustion in the compression-ignition engine: Spray, combustion, and emission. Energy 2022, 250, 123778. [Google Scholar] [CrossRef]
  147. Xu, L.; Chang, Y.; Treacy, M.; Zhou, Y.; Jia, M.; Bai, X.-S. A skeletal chemical kinetic mechanism for ammonia/n-heptane combustion. Fuel 2023, 331, 125830. [Google Scholar] [CrossRef]
  148. Førby, N.; Thomsen, T.B.; Cordtz, R.F.; Bræstrup, F.; Schramm, J. Ignition and combustion study of premixed ammonia using GDI pilot injection in CI engine. Fuel 2023, 331, 125768. [Google Scholar] [CrossRef]
  149. Dong, S.; Wang, B.; Jiang, Z.; Li, Y.; Gao, W.; Wang, Z.; Cheng, X.; Curran, H.J. An experimental and kinetic modeling study of ammonia/n-heptane blends. Combust. Flame 2022, 246, 112428. [Google Scholar] [CrossRef]
  150. Yu, L.; Zhou, W.; Feng, Y.; Wang, W.; Zhu, J.; Qian, Y.; Lu, X. The effect of ammonia addition on the low-temperature autoignition of n-heptane: An experimental and modeling study. Combust. Flame 2020, 217, 4–11. [Google Scholar] [CrossRef]
  151. Xu, S.; Li, G.; Zhou, M.; Yu, W.; Zhang, Z.; Hou, D.; Yu, F. Experimental and kinetic studies of extinction limits of counterflow cool and hot diffusion flames of ammonia/n-dodecane. Combust. Flame 2022, 245, 112316. [Google Scholar] [CrossRef]
  152. Wüthrich, S.; Cartier, P.; Süess, P.; Schneider, B.; Obrecht, P.; Herrmann, K. Optical investigation and thermodynamic analysis of premixed ammonia dual-fuel combustion initiated by dodecane pilot fuel. Fuel Commun. 2022, 12, 100074. [Google Scholar] [CrossRef]
  153. Wang, C.; Feng, Y.; Liu, Z.; Wang, Y.; Fang, J.; Qin, J.; Shao, J.; Huang, H. Assessment of thermodynamic performance and CO2 emission reduction for a supersonic precooled turbine engine cycle fueled with a new green fuel of ammonia. Energy 2022, 261, 125272. [Google Scholar] [CrossRef]
  154. Li, S.; Li, T.; Wang, N.; Zhou, X.; Chen, R.; Yi, P. An investigation on near-field and far-field characteristics of superheated ammonia spray. Fuel 2022, 324, 124683. [Google Scholar] [CrossRef]
  155. Gross, C.W.; Kong, S.-C. Performance characteristics of a compression-ignition engine using direct-injection ammonia–DME mixtures. Fuel 2013, 103, 1069–1079. [Google Scholar] [CrossRef]
  156. İnci, M.; Büyük, M.; Demir, M.H.; İlbey, G. A review and research on fuel cell electric vehicles: Topologies, power electronic converters, energy management methods, technical challenges, marketing and future aspects. Renew. Sustain. Energy Rev. 2021, 137, 110648. [Google Scholar] [CrossRef]
  157. Wang, G.; Yu, Y.; Liu, H.; Gong, C.; Wen, S.; Wang, X.; Tu, Z. Progress on design and development of polymer electrolyte membrane fuel cell systems for vehicle applications: A review. Fuel Process. Technol. 2018, 179, 203–228. [Google Scholar] [CrossRef]
  158. Ogungbemi, E.; Wilberforce, T.; Ijaodola, O.; Thompson, J.; Olabi, A. Selection of proton exchange membrane fuel cell for transportation. Int. J. Hydrogen Energy 2021, 46, 30625–30640. [Google Scholar] [CrossRef]
  159. Jeerh, G.; Zhang, M.; Tao, S. Recent progress in ammonia fuel cells and their potential applications. J. Mater. Chem. A 2021, 9, 727–752. [Google Scholar] [CrossRef]
  160. Mukelabai, M.D.; Gillard, J.M.; Patchigolla, K. A novel integration of a green power-to-ammonia to power system: Reversible solid oxide fuel cell for hydrogen and power production coupled with an ammonia synthesis unit. Int. J. Hydrogen Energy 2021, 46, 18546–18556. [Google Scholar] [CrossRef]
  161. Wen, D.; Aziz, M. Techno-economic analyses of power-to-ammonia-to-power and biomass-to-ammonia-to-power pathways for carbon neutrality scenario. Appl. Energy 2022, 319, 119272. [Google Scholar] [CrossRef]
  162. Wang, B.; Li, T.; Gong, F.; Othman, M.H.D.; Xiao, R. Ammonia as a green energy carrier: Electrochemical synthesis and direct ammonia fuel cell-a comprehensive review. Fuel Process. Technol. 2022, 235, 107380. [Google Scholar] [CrossRef]
  163. Al-Hamed, K.H.; Dincer, I. A novel ammonia solid oxide fuel cell-based powering system with on-board hydrogen production for clean locomotives. Energy 2021, 220, 119771. [Google Scholar] [CrossRef]
  164. Hauch, A.; Küngas, R.; Blennow, P.; Hansen, A.B.; Hansen, J.B.; Mathiesen, B.V.; Mogensen, M.B. Recent advances in solid oxide cell technology for electrolysis. Science 2020, 370, eaba6118. [Google Scholar] [CrossRef] [PubMed]
  165. Jantakananuruk, N.; Page, J.R.; Armstrong, C.D.; Persky, J.; Datta, R.; Teixeira, A.R. Integrated thermal reforming and electro-oxidation in ammonia-fueled tubular solid oxide fuel cells toward autothermal operation. J. Power Sources 2022, 548, 231999. [Google Scholar] [CrossRef]
  166. Mao, X.; Sang, J.; Xi, C.; Liu, Z.; Yang, J.; Guan, W.; Wang, J.; Xia, C.; Singhal, S.C. Performance evaluation of ammonia-fueled flat-tube solid oxide fuel cells with different build-in catalysts. Int. J. Hydrogen Energy 2022, 47, 23324–23334. [Google Scholar] [CrossRef]
  167. Xu, K.; Zhu, F.; Hou, M.; Li, C.; Zhang, H.; Chen, Y. Activating and stabilizing the surface of anode for high-performing direct-ammonia solid oxide fuel cells. Nano Res. 2023, 16, 2454–2462. [Google Scholar] [CrossRef]
  168. Pan, Y.; Zhang, H.; Xu, K.; Zhou, Y.; Zhao, B.; Yuan, W.; Sasaki, K.; Choi, Y.; Chen, Y.; Liu, M. A high-performance and durable direct NH3 tubular protonic ceramic fuel cell integrated with an internal catalyst layer. Appl. Catal. B Environ. 2022, 306, 121071. [Google Scholar] [CrossRef]
  169. Asmare, M.; İlbaş, M. Direct ammonia fueled solid oxide fuel cells: A comprehensive review on challenges, opportunities and future outlooks. Int. J. Energy Technol. 2020, 2, 70–91. [Google Scholar] [CrossRef]
  170. Welander, M.M.; Hu, B.; Belko, S.; Lee, K.X.; Dubey, P.K.; Robinson, I.; Singh, P.; Tucker, M.C. Direct utilization of gaseous fuels in metal supported solid oxide fuel cells. Int. J. Hydrogen Energy 2023, 48, 1533–1539. [Google Scholar] [CrossRef]
  171. Tan, W.C.; Iwai, H.; Kishimoto, M.; Brus, G.; Szmyd, J.S.; Yoshida, H. Numerical analysis on effect of aspect ratio of planar solid oxide fuel cell fueled with decomposed ammonia. J. Power Sources 2018, 384, 367–378. [Google Scholar] [CrossRef]
  172. Kishimoto, M.; Muroyama, H.; Suzuki, S.; Saito, M.; Koide, T.; Takahashi, Y.; Horiuchi, T.; Yamasaki, H.; Matsumoto, S.; Kubo, H. Development of 1 kW-class Ammonia-fueled Solid Oxide Fuel Cell Stack. Fuel Cells 2020, 20, 80–88. [Google Scholar] [CrossRef]
  173. Siddiqui, O.; Dincer, I. A review and comparative assessment of direct ammonia fuel cells. Therm. Sci. Eng. Prog. 2018, 5, 568–578. [Google Scholar] [CrossRef]
  174. Rathore, S.S.; Biswas, S.; Fini, D.; Kulkarni, A.P.; Giddey, S. Direct ammonia solid-oxide fuel cells: A review of progress and prospects. Int. J. Hydrogen Energy 2021, 46, 35365–35384. [Google Scholar] [CrossRef]
  175. Luo, Y.; Liang, S.; Wang, X.; Lin, B.; Chen, C.; Jiang, L. Facile Synthesis and High-Value Utilization of Ammonia. Chin. J. Chem. 2022, 40, 953–964. [Google Scholar] [CrossRef]
  176. Zhang, M.; Zou, P.; Jeerh, G.; Chen, S.; Shields, J.; Wang, H.; Tao, S. Electricity generation from ammonia in landfill leachate by an alkaline membrane fuel cell based on precious-metal-free electrodes. ACS Sustain. Chem. Eng. 2020, 8, 12817–12824. [Google Scholar] [CrossRef]
  177. Dekel, D.R. Review of cell performance in anion exchange membrane fuel cells. J. Power Sources 2018, 375, 158–169. [Google Scholar] [CrossRef]
  178. Hren, M.; Božič, M.; Fakin, D.; Kleinschek, K.S.; Gorgieva, S. Alkaline membrane fuel cells: Anion exchange membranes and fuels. Sustain. Energy Fuels 2021, 5, 604–637. [Google Scholar] [CrossRef]
  179. Medvedev, J.J.; Tobolovskaya, Y.; Medvedeva, X.V.; Tatarchuk, S.W.; Li, F.; Klinkova, A. Pathways of ammonia electrooxidation on nickel hydroxide anodes and an alternative route towards recycled fertilizers. Green Chem. 2022, 24, 1578–1589. [Google Scholar] [CrossRef]
  180. Elishav, O.; Mosevitzky Lis, B.; Miller, E.M.; Arent, D.J.; Valera-Medina, A.; Grinberg Dana, A.; Shter, G.E.; Grader, G.S. Progress and prospective of nitrogen-based alternative fuels. Chem. Rev. 2020, 120, 5352–5436. [Google Scholar] [CrossRef]
  181. Nhiem, L.T. An alternative approach for utilizing urea as fuel source. Braz. J. Chem. Eng. 2022, 1–6. [Google Scholar] [CrossRef]
  182. Wang, D.; Chen, C.; Wang, S. Defect engineering for advanced electrocatalytic conversion of nitrogen-containing molecules. Sci. China Chem. 2023, 66, 1052–1072. [Google Scholar] [CrossRef]
  183. Liu, X.; Qin, H.; Ye, Z.; Yao, D.; Miao, W.; Mao, S. Interconnected Mn-doped Ni (OH) 2 nanosheet layer for bifunctional urea oxidation and hydrogen evolution: The relation between current drop and urea concentration during the long-term operation. ACS EST Eng. 2022, 2, 853–862. [Google Scholar] [CrossRef]
  184. Song, M.; Zhang, Z.; Li, Q.; Jin, W.; Wu, Z.; Fu, G.; Liu, X. Ni-foam supported Co(OH)F and Co–P nanoarrays for energy-efficient hydrogen production via urea electrolysis. J. Mater. Chem. A 2019, 7, 3697–3703. [Google Scholar] [CrossRef]
  185. Gnana kumar, G.; Farithkhan, A.; Manthiram, A. Direct urea fuel cells: Recent progress and critical challenges of urea oxidation electrocatalysis. Adv. Energy Sustain. Res. 2020, 1, 2000015. [Google Scholar] [CrossRef]
  186. Nangan, S.; Ding, Y.; Alhakemy, A.Z.; Liu, Y.; Wen, Z. Hybrid alkali-acid urea-nitrate fuel cell for degrading nitrogen-rich wastewater. Appl. Catal. B Environ. 2021, 286, 119892. [Google Scholar] [CrossRef]
  187. Zhang, Q.; Kazim, F.M.; Ma, S.; Qu, K.; Li, M.; Wang, Y.; Hu, H.; Cai, W.; Yang, Z. Nitrogen dopants in nickel nanoparticles embedded carbon nanotubes promote overall urea oxidation. Appl. Catal. B Environ. 2021, 280, 119436. [Google Scholar] [CrossRef]
  188. Sayed, E.T.; Abdelkareem, M.A.; Bahaa, A.; Eisa, T.; Alawadhi, H.; Al-Asheh, S.; Chae, K.-J.; Olabi, A. Synthesis and performance evaluation of various metal chalcogenides as active anodes for direct urea fuel cells. Renew. Sustain. Energy Rev. 2021, 150, 111470. [Google Scholar] [CrossRef]
  189. Putri, Y.M.T.A.; Gunlazuardi, J.; Yulizar, Y.; Wibowo, R.; Einaga, Y.; Ivandini, T.A. Recent progress in direct urea fuel cell. Open Chem. 2021, 19, 1116–1133. [Google Scholar] [CrossRef]
  190. Li, B.; Song, C.; Yan, J.; Ye, K.; Cheng, K.; Cao, D.; Wang, G. Effect of graphene on the performance of nickel foam-based CoNi nanosheet anode catalyzed direct urea-hydrogen peroxide fuel cell. Int. J. Hydrogen Energy 2020, 45, 10569–10579. [Google Scholar] [CrossRef]
  191. Wen, X. NiFe-LDH/MWCNTs/NF nanohybrids as a high-performance bifunctional electrocatalyst for overall urea electrolysis. Int. J. Hydrogen Energy 2020, 45, 14660–14668. [Google Scholar] [CrossRef]
  192. Chen, Z.; Wei, W.; Ni, B.-J. Transition metal chalcogenides as emerging electrocatalysts for urea electrolysis. Curr. Opin. Electrochem. 2022, 31, 100888. [Google Scholar] [CrossRef]
  193. Singh, R.K.; Rajavelu, K.; Montag, M.; Schechter, A. Advances in catalytic electrooxidation of urea: A review. Energy Technol. 2021, 9, 2100017. [Google Scholar] [CrossRef]
  194. Wu, T.-H.; Lin, Y.-C.; Hou, B.-W.; Liang, W.-Y. Nanostructured β– NiS catalyst for enhanced and stable electro–oxidation of urea. Catalysts 2020, 10, 1280. [Google Scholar] [CrossRef]
  195. Hu, X.; Zhu, J.; Li, J.; Wu, Q. Urea Electrooxidation: Current Development and Understanding of Ni-Based Catalysts. ChemElectroChem 2020, 7, 3211–3228. [Google Scholar] [CrossRef]
  196. Wang, S.; Zhao, L.; Li, J.; Tian, X.; Wu, X.; Feng, L. High valence state of Ni and Mo synergism in NiS2-MoS2 hetero-nanorods catalyst with layered surface structure for urea electrocatalysis. J. Energy Chem. 2022, 66, 483–492. [Google Scholar] [CrossRef]
  197. Xu, X.; Zhang, C.; Li, J.; Liu, H.; Su, G.; Shi, Z.; Huang, M. Redistributing interfacial charge density of Ni12P5/Ni3P via Fe doping for ultrafast urea oxidation catalysis at large current densities. Chem. Eng. J. 2023, 452, 139362. [Google Scholar] [CrossRef]
  198. Mehregan, M.; Moghiman, M. Experimental investigation of the distinct effects of nanoparticles addition and urea-SCR after-treatment system on NOx emissions in a blended-biodiesel fueled internal combustion engine. Fuel 2020, 262, 116609. [Google Scholar] [CrossRef]
  199. Demir, U.; Kozan, A.; Özer, S. Experimental investigation of the effect of urea addition to fuel on engine performance and emissions in diesel engines. Fuel 2022, 311, 122578. [Google Scholar] [CrossRef]
  200. Yang, B.-M. Effect of Exhaust Gas Flow and Back Pressure on Urea Dosing Unit and Pipe of SCR in Industrial Diesel Engine. Int. J. Mech. Eng. Robot. Res. 2022, 11, 106–113. [Google Scholar] [CrossRef]
  201. Elishav, O.; Lewin, D.R.; Shter, G.E.; Grader, G.S. The nitrogen economy: Economic feasibility analysis of nitrogen-based fuels as energy carriers. Appl. Energy 2017, 185, 183–188. [Google Scholar] [CrossRef]
  202. Kopchikov, V. Alternative fuels and assessment of their applicability in internal combustion engines. In IOP Conference Series: Materials Science and Engineering; IOP Publishing: Bristol, UK, 2020; p. 062015. [Google Scholar]
  203. Kim, J.D.; Choi, M.Y.; Choi, H.C. Graphene-oxide-supported Pt nanoparticles with high activity and stability for hydrazine electro-oxidation in a strong acidic solution. Appl. Surf. Sci. 2017, 420, 700–706. [Google Scholar] [CrossRef]
  204. Akbar, K.; Kim, J.H.; Lee, Z.; Kim, M.; Yi, Y.; Chun, S.-H. Superaerophobic graphene nano-hills for direct hydrazine fuel cells. NPG Asia Mater. 2017, 9, e378. [Google Scholar] [CrossRef]
  205. Serov, A.; Kwak, C. Direct hydrazine fuel cells: A review. Appl. Catal. B Environ. 2010, 98, 1–9. [Google Scholar] [CrossRef]
  206. Wang, J.; Guan, X.; Li, H.; Zeng, S.; Li, R.; Yao, Q.; Chen, H.; Zheng, Y.; Qu, K. Robust Ru-N metal-support interaction to promote self-powered H2 production assisted by hydrazine oxidation. Nano Energy 2022, 100, 107467. [Google Scholar] [CrossRef]
  207. Deng, J.; Li, X.; Imhanria, S.; Chen, K.; Deng, X.; Wang, W. Molybdenum carbide-nitrogen doped carbon composites as effective non-precious electrocatalyst for direct hydrazine fuel cell. Electrochim. Acta 2021, 384, 138417. [Google Scholar] [CrossRef]
  208. de Oliveira, D.C.; Silva, W.O.; Chatenet, M.; Lima, F.H. NiOx-Pt/C nanocomposites: Highly active electrocatalysts for the electrochemical oxidation of hydrazine. Appl. Catal. B Environ. 2017, 201, 22–28. [Google Scholar] [CrossRef]
  209. Ding, J.; Kannan, P.; Wang, P.; Ji, S.; Wang, H.; Liu, Q.; Gai, H.; Liu, F.; Wang, R. Synthesis of nitrogen-doped MnO/carbon network as an advanced catalyst for direct hydrazine fuel cells. J. Power Sources 2019, 413, 209–215. [Google Scholar] [CrossRef]
  210. Liu, X.; He, J.; Zhao, S.; Liu, Y.; Zhao, Z.; Luo, J.; Hu, G.; Sun, X.; Ding, Y. Self-powered H2 production with bifunctional hydrazine as sole consumable. Nat. Commun. 2018, 9, 4365. [Google Scholar] [CrossRef] [Green Version]
  211. Serov, A.; Padilla, M.; Roy, A.J.; Atanassov, P.; Sakamoto, T.; Asazawa, K.; Tanaka, H. Anode catalysts for direct hydrazine fuel cells: From laboratory test to an electric vehicle. Angew. Chem. 2014, 126, 10504–10507. [Google Scholar] [CrossRef]
  212. Khalafallah, D.; Zhi, M.; Hong, Z. Development trends on nickel-based electrocatalysts for direct hydrazine fuel cells. ChemCatChem 2021, 13, 81–110. [Google Scholar] [CrossRef]
  213. Tang, P.; Wen, H.; Wang, P. Hierarchically nanostructured Ni2Fe2N as an efficient electrocatalyst for hydrazine oxidation reaction. Chem. Eng. J. 2022, 431, 134123. [Google Scholar] [CrossRef]
  214. Jeong, J.; Choun, M.; Lee, J. Tree-Bark-Shaped N-Doped Porous Carbon Anode for Hydrazine Fuel Cells. Angew. Chem. Int. Ed. 2017, 56, 13513–13516. [Google Scholar] [CrossRef]
  215. Sun, Q.; Zhou, M.; Shen, Y.; Wang, L.; Ma, Y.; Li, Y.; Bo, X.; Wang, Z.; Zhao, C. Hierarchical nanoporous Ni(Cu) alloy anchored on amorphous NiFeP as efficient bifunctional electrocatalysts for hydrogen evolution and hydrazine oxidation. J. Catal. 2019, 373, 180–189. [Google Scholar] [CrossRef]
  216. Munde, A.; Sharma, P.; Dhawale, S.; Kadam, R.G.; Kumar, S.; Kale, H.B.; Filip, J.; Zboril, R.; Sathe, B.R.; Gawande, M.B. Interface Engineering of SRu-mC3N4 Heterostructures for Enhanced Electrochemical Hydrazine Oxidation Reactions. Catalysts 2022, 12, 1560. [Google Scholar] [CrossRef]
  217. Yu, Y.; Lee, S.J.; Theerthagiri, J.; Lee, Y.; Choi, M.Y. Architecting the AuPt alloys for hydrazine oxidation as an anolyte in fuel cell: Comparative analysis of hydrazine splitting and water splitting for energy-saving H2 generation. Appl. Catal. B Environ. 2022, 316, 121603. [Google Scholar] [CrossRef]
  218. Wang, Y.; Wang, Q.; Wan, L.-y.; Han, Y.; Hong, Y.; Huang, L.; Yang, X.; Wang, Y.; Zaghib, K.; Zhou, Z. KOH-doped polybenzimidazole membrane for direct hydrazine fuel cell. J. Colloid Interface Sci. 2020, 563, 27–32. [Google Scholar] [CrossRef] [PubMed]
  219. Cross, R.W.; Rondiya, S.R.; Dzade, N.Y. First-principles DFT insights into the adsorption of hydrazine on bimetallic β1-NiZn catalyst: Implications for direct hydrazine fuel cells. Appl. Surf. Sci. 2021, 536, 147648. [Google Scholar] [CrossRef]
  220. Sun, H.; Gao, L.; Kumar, A.; Cao, Z.; Chang, Z.; Liu, W.; Sun, X. Superaerophobic CoP Nanowire Arrays as a Highly Effective Anode Electrocatalyst for Direct Hydrazine Fuel Cells. ACS Appl. Energy Mater. 2022, 5, 9455–9462. [Google Scholar] [CrossRef]
  221. Crisafulli, R.; de Paula, D.F.; Zignani, S.C.; Spadaro, L.; Palella, A.; Boninelli, S.; Dias, J.A.; Linares, J.J. Promoting Effect of Cu on Pd Applied to the Hydrazine Electro-Oxidation and Direct Hydrazine Fuel Cells. Catalysts 2022, 12, 1639. [Google Scholar] [CrossRef]
  222. Hosseini, M.G.; Daneshvari-Esfahlan, V.; Aghajani, H.; Wolf, S.; Hacker, V. Palladium-nickel electrocatalysts on nitrogen-doped reduced graphene oxide nanosheets for direct hydrazine/hydrogen peroxide fuel cells. Catalysts 2021, 11, 1372. [Google Scholar] [CrossRef]
  223. Hosseini, M.G.; Daneshvari-Esfahlan, V.; Wolf, S.; Hacker, V. Cobalt-modified palladium nanocatalyst on nitrogen-doped reduced graphene oxide for direct hydrazine fuel cell. RSC Adv. 2021, 11, 39223–39232. [Google Scholar] [CrossRef] [PubMed]
  224. Grinberg Dana, A.; Shter, G.E.; Grader, G.S. Nitrogen-Based Alternative Fuels: Progress and Future Prospects. Energy Technol. 2016, 4, 7–18. [Google Scholar] [CrossRef] [Green Version]
  225. Goodman, S.M. The Impact of EU Anti-Dumping Duties on Urea Ammonium Nitrate Solution; Office of Industries, US International Trade Commission: Washington, DC, USA, 2020.
  226. Dana, A.G.; Shter, G.E.; Grader, G.S. Nitrogen-based alternative fuel: An environmentally friendly combustion approach. RSC Adv. 2014, 4, 10051–10059. [Google Scholar] [CrossRef]
  227. Grinberg Dana, A.; Shter, G.E.; Grader, G.S. Nitrogen-Based Alternative Fuel: Safety Considerations. Energy Technol. 2015, 3, 976–981. [Google Scholar] [CrossRef]
  228. Mosevitzky, B.; Shter, G.E.; Grader, G.S. Auto-ignition of a carbon-free aqueous ammonia/ammonium nitrate monofuel: A thermal and barometric analysis. Fuel Process. Technol. 2017, 159, 363–368. [Google Scholar] [CrossRef]
  229. Grinberg Dana, A.; Elishav, O.; Bardow, A.; Shter, G.E.; Grader, G.S. Nitrogen-based fuels: A power-to-fuel-to-power analysis. Angew. Chem. Int. Ed. 2016, 55, 8798–8805. [Google Scholar] [CrossRef] [Green Version]
  230. Mosevitzky, B.; Shter, G.E.; Grader, G.S. Effect of equivalence ratio on the thermal autoignition of aqueous ammonia ammonium nitrate monofuel. Combust. Flame 2018, 188, 142–149. [Google Scholar] [CrossRef]
  231. Mosevitzky, B.; Azoulay, R.; Naamat, L.; Shter, G.E.; Grader, G.S. Effects of water content and diluent pressure on the ignition of aqueous ammonia/ammonium nitrate and urea/ammonium nitrate fuels. Appl. Energy 2018, 224, 300–308. [Google Scholar] [CrossRef]
  232. Grinberg Dana, A.; Mosevitzky, B.; Tvil, G.; Epstein, M.; Shter, G.E.; Grader, G.S. Flow reactor combustion of aqueous urea ammonium nitrate fuel. Energy Fuels 2016, 30, 2474–2477. [Google Scholar] [CrossRef]
  233. Oluwoye, I.; Altarawneh, M.; Gore, J.; Dlugogorski, B.Z. Burning properties of redox crystals of ammonium nitrate and saccharides. Combust. Flame 2020, 213, 132–139. [Google Scholar] [CrossRef]
  234. Oluwoye, I.; Mosallanejad, S.; Soubans, G.; Altarawneh, M.; Gore, J.; Dlugogorski, B.Z. Thermal decomposition of ammonium nitrate on rust surface: Risk of low-temperature fire. Fire Saf. J. 2021, 120, 103063. [Google Scholar] [CrossRef]
  235. Hossain, F. Adaptation measures (AMs) and mitigation policies (MPs) to climate change and sustainable blue economy: A global perspective. J. Water Clim. Change 2021, 12, 1344–1369. [Google Scholar] [CrossRef]
  236. Schiffer, Z.J.; Biswas, S.; Manthiram, K. Ammonium Formate as a Safe, Energy-Dense Electrochemical Fuel Ionic Liquid. ACS Energy Lett. 2022, 7, 3260–3267. [Google Scholar] [CrossRef] [PubMed]
  237. An, L.; Chen, R. Direct formate fuel cells: A review. J. Power Sources 2016, 320, 127–139. [Google Scholar] [CrossRef]
  238. Su, J.; Yang, L.; Lu, M.; Lin, H. Highly efficient hydrogen storage system based on ammonium bicarbonate/formate redox equilibrium over palladium nanocatalysts. ChemSusChem 2015, 8, 813–816. [Google Scholar] [CrossRef]
  239. Nakajima, K.; Tominaga, M.; Waseda, M.; Miura, H.; Shishido, T. Highly efficient supported palladium–gold alloy catalysts for hydrogen storage based on ammonium bicarbonate/formate redox cycle. ACS Sustain. Chem. Eng. 2019, 7, 6522–6530. [Google Scholar] [CrossRef]
  240. Bahuguna, A.; Sasson, Y. Formate-Bicarbonate Cycle as a Vehicle for Hydrogen and Energy Storage. ChemSusChem 2021, 14, 1258–1283. [Google Scholar] [CrossRef]
  241. Russo, D.; Calabrese, M.; Marotta, R.; Andreozzi, R.; Di Benedetto, A. Thermodynamics of the cyclic formate/bicarbonate interconversion for hydrogen storage. Int. J. Hydrogen Energy 2022, 47, 31370–31380. [Google Scholar] [CrossRef]
  242. Dong, Z.; Mukhtar, A.; Ludwig, T.; Akhade, S.A.; Kang, S.; Wood, B.; Grubel, K.; Engelhard, M.; Autrey, T.; Lin, H. Efficient Pd on carbon catalyst for ammonium formate dehydrogenation: Effect of surface oxygen functional groups. Appl. Catal. B Environ. 2023, 321, 122015. [Google Scholar] [CrossRef]
  243. Lan, R.; Irvine, J.T.; Tao, S. Ammonia and related chemicals as potential indirect hydrogen storage materials. Int. J. Hydrogen Energy 2012, 37, 1482–1494. [Google Scholar] [CrossRef]
  244. Lee, K.R.; Song, D.; Park, S.B.; Han, J.-I. A direct ammonium carbonate fuel cell with an anion exchange membrane. Rsc Adv. 2014, 4, 5638–5641. [Google Scholar] [CrossRef]
  245. Lakshmi, K.S.; Ji, X.; Chen, T.-Y.; Vedhanarayanan, B.; Lin, T.-W. Pseudocapacitive and battery-type organic polymer electrodes for a 1.9 V hybrid supercapacitor with a record concentration of ammonium acetate. J. Power Sources 2021, 511, 230434. [Google Scholar] [CrossRef]
  246. Bashirnezhad, K.; Mehregan, M.; Kebriyaee, S.A. Experimental analysis of the influence of urea injection upon NOx emissions in internal combustion engines fueled with biodiesels. J. Energy Inst. 2016, 89, 115–120. [Google Scholar] [CrossRef]
  247. Mehregan, M.; Moghiman, M. Effects of nano-additives on pollutants emission and engine performance in a urea-SCR equipped diesel engine fueled with blended-biodiesel. Fuel 2018, 222, 402–406. [Google Scholar] [CrossRef]
  248. An, H.; Yang, W.; Li, J.; Zhou, D. Modeling analysis of urea direct injection on the NOx emission reduction of biodiesel fueled diesel engines. Energy Convers. Manag. 2015, 101, 442–449. [Google Scholar] [CrossRef]
  249. Xu, Y.; Wang, Y.; Zhong, Y.; Lei, G.; Li, Z.; Zhang, J.; Zhang, T. Transition metal complexes based on hypergolic anions for catalysis of ammonium perchlorate thermal decomposition. Energy Fuels 2020, 34, 14667–14675. [Google Scholar] [CrossRef]
  250. Li, R.; Li, X.; Tang, P.; Yang, B.; Xie, L.; Yang, G. Defect-engineered sp2 carbon as highly active catalyst and reactive fuel for combustion of ammonium perchlorate. Chem. Eng. J. 2021, 426, 131918. [Google Scholar] [CrossRef]
  251. Nagendra, K.; Vijay, C.; Ingole, M.; Ramakrishna, P. Combustion of ammonium perchlorate monopropellant: Role of heat loss. Combust. Flame 2019, 209, 363–375. [Google Scholar] [CrossRef]
  252. Li, R.; Li, X.; Yang, G.; Guo, C.; Zhang, L.; Hao, C.; Zhu, W. Defective-activated-carbon-encapsulated Co as a super reactive catalyst for combustion of ammonium perchlorate. Appl. Surf. Sci. 2023, 615, 156349. [Google Scholar] [CrossRef]
  253. Yang, L.; Xu, R.; Mi, Z.; Wan, Y.; Fu, X.; Jiang, L.; Jian, Y.; Li, J.; Zhang, G. Enhanced anti-migration performance of carbon nanotubes confined ferrocenyl compounds and their synergistic catalytic activity on the thermal decomposition of ammonium perchlorate. Mater. Today Chem. 2022, 26, 101168. [Google Scholar] [CrossRef]
  254. Demirci, U.B. Ammonia borane: An extensively studied, though not yet implemented, hydrogen carrier. Energies 2020, 13, 3071. [Google Scholar] [CrossRef]
  255. Baier, M.J.; Ramachandran, P.V.; Son, S.F. Characterization of the hypergolic ignition delay of ammonia borane. J. Propuls. Power 2019, 35, 182–189. [Google Scholar] [CrossRef]
  256. Navlani-García, M.; Salinas-Torres, D.; Cazorla-Amorós, D. Hydrolytic dehydrogenation of ammonia borane attained by Ru-based catalysts: An auspicious option to produce hydrogen from a solid hydrogen carrier molecule. Energies 2021, 14, 2199. [Google Scholar] [CrossRef]
  257. Liu, M.; Zhou, L.; Luo, X.; Wan, C.; Xu, L. Recent advances in noble metal catalysts for hydrogen production from ammonia borane. Catalysts 2020, 10, 788. [Google Scholar] [CrossRef]
  258. Salinas-Torres, D.; Navlani-García, M.; Kuwahara, Y.; Mori, K.; Yamashita, H. Non-noble metal doped perovskite as a promising catalyst for ammonia borane dehydrogenation. Catal. Today 2020, 351, 6–11. [Google Scholar] [CrossRef]
  259. Cai, H.-K.; Jiang, Z.-Y.; Xu, S.; Xu, Y.; Lu, P.; Dong, J. Polymer Hydrogel Supported Ni/Pd Alloys for Hydrogen Gas Production from Hydrolysis of Dimethylamine Borane with a Long Recyclable Lifetime. Polymers 2022, 14, 4647. [Google Scholar] [CrossRef] [PubMed]
  260. Karaboğa, S. Tungsten (VI) oxide supported rhodium (0) nanoparticles; highly efficient catalyst for H2 production from dimethylamine borane. Int. J. Hydrogen Energy 2021, 46, 17763–17775. [Google Scholar] [CrossRef]
  261. Shiota, K.; Izato, Y.-I.; Habu, H.; Miyake, A. Reactivity analysis of ammonium dinitramide binary mixtures based on ab initio calculations and thermal analysis. J. Therm. Anal. Calorim. 2019, 138, 2615–2622. [Google Scholar] [CrossRef]
  262. Chen, J.; Li, G.; Zhang, T.; Liu, Y.; Yang, R.; Chen, Y. Catalytic bed slenderness ratio and ADN/methanol ratio for decomposition and combustion characteristics within ammonium dinitramide (ADN)-based green aerospace thruster. Chin. J. Chem. Eng. 2019, 27, 1159–1165. [Google Scholar] [CrossRef]
  263. Li, H.-M.; Li, G.-X.; Li, L. Comparative investigation on combustion characteristics of ADN-based liquid propellants in inert gas and oxidizing gas atmospheres with resistive ignition method. Fuel 2023, 334, 126742. [Google Scholar] [CrossRef]
  264. Shen, J.; Yu, Y.; Liu, X.; Cao, J. Experimental research on microwave ignition and combustion characteristics of ADN-based liquid propellant. Micromachines 2022, 13, 510. [Google Scholar] [CrossRef]
  265. Klerke, A.; Christensen, C.H.; Nørskov, J.K.; Vegge, T. Ammonia for hydrogen storage: Challenges and opportunities. J. Mater. Chem. 2008, 18, 2304–2310. [Google Scholar] [CrossRef]
  266. Assumpção, M.H.; da Silva, S.G.; de Souza, R.F.; Buzzo, G.S.; Spinacé, E.V.; Neto, A.O.; Silva, J.C.M. Direct ammonia fuel cell performance using PtIr/C as anode electrocatalysts. Int. J. Hydrogen Energy 2014, 39, 5148–5152. [Google Scholar] [CrossRef]
  267. Ghavam, S.; Vahdati, M.; Wilson, I.G.; Styring, P. Sustainable ammonia production processes. Front. Energy Res. 2021, 9, 580808. [Google Scholar] [CrossRef]
  268. Saygin, D.; Blanco, H.; Boshell, F.; Cordonnier, J.; Rouwenhorst, K.; Lathwal, P.; Gielen, D. Ammonia Production from Clean Hydrogen and the Implications for Global Natural Gas Demand. Sustainability 2023, 15, 1623. [Google Scholar] [CrossRef]
  269. Liu, X.; Elgowainy, A.; Wang, M. Life cycle energy use and greenhouse gas emissions of ammonia production from renewable resources and industrial by-products. Green Chem. 2020, 22, 5751–5761. [Google Scholar] [CrossRef]
  270. Chisalita, D.-A.; Petrescu, L.; Cormos, C.-C. Environmental evaluation of european ammonia production considering various hydrogen supply chains. Renew. Sustain. Energy Rev. 2020, 130, 109964. [Google Scholar] [CrossRef]
  271. Hochman, G.; Goldman, A.S.; Felder, F.A.; Mayer, J.M.; Miller, A.J.; Holland, P.L.; Goldman, L.A.; Manocha, P.; Song, Z.; Aleti, S. Potential economic feasibility of direct electrochemical nitrogen reduction as a route to ammonia. ACS Sustain. Chem. Eng. 2020, 8, 8938–8948. [Google Scholar] [CrossRef]
  272. Xu, H.; Ithisuphalap, K.; Li, Y.; Mukherjee, S.; Lattimer, J.; Soloveichik, G.; Wu, G. Electrochemical ammonia synthesis through N2 and H2O under ambient conditions: Theory, practices, and challenges for catalysts and electrolytes. Nano Energy 2020, 69, 104469. [Google Scholar] [CrossRef]
  273. Liu, Q.; Xu, T.; Luo, Y.; Kong, Q.; Li, T.; Lu, S.; Alshehri, A.A.; Alzahrani, K.A.; Sun, X. Recent advances in strategies for highly selective electrocatalytic N2 reduction toward ambient NH3 synthesis. Curr. Opin. Electrochem. 2021, 29, 100766. [Google Scholar] [CrossRef]
  274. Garagounis, I.; Vourros, A.; Stoukides, D.; Dasopoulos, D.; Stoukides, M. Electrochemical synthesis of ammonia: Recent efforts and future outlook. Membranes 2019, 9, 112. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  275. Wang, L.; Xia, M.; Wang, H.; Huang, K.; Qian, C.; Maravelias, C.T.; Ozin, G.A. Greening ammonia toward the solar ammonia refinery. Joule 2018, 2, 1055–1074. [Google Scholar] [CrossRef] [Green Version]
  276. Service, R.F. Liquid sunshine. Science 2018, 361, 120–123. [Google Scholar] [CrossRef]
  277. MacFarlane, D.R.; Choi, J.; Suryanto, B.H.; Jalili, R.; Chatti, M.; Azofra, L.M.; Simonov, A.N. Liquefied sunshine: Transforming renewables into fertilizers and energy carriers with electromaterials. Adv. Mater. 2020, 32, 1904804. [Google Scholar] [CrossRef]
  278. Makhloufi, C.; Kezibri, N. Large-scale decomposition of green ammonia for pure hydrogen production. Int. J. Hydrogen Energy 2021, 46, 34777–34787. [Google Scholar] [CrossRef]
  279. Huang, C.; Yu, Y.; Tang, X.; Liu, Z.; Zhang, J.; Ye, C.; Ye, Y.; Zhang, R. Hydrogen generation by ammonia decomposition over Co/CeO2 catalyst: Influence of support morphologies. Appl. Surf. Sci. 2020, 532, 147335. [Google Scholar] [CrossRef]
  280. Pashchenko, D.; Mustafin, R. Ammonia decomposition in the thermochemical waste-heat recuperation systems: A view from low and high heating value. Energy Convers. Manag. 2022, 251, 114959. [Google Scholar] [CrossRef]
  281. Pashchenko, D.; Mustafin, R.; Karpilov, I. Ammonia-fired chemically recuperated gas turbine: Thermodynamic analysis of cycle and recuperation system. Energy 2022, 252, 124081. [Google Scholar] [CrossRef]
  282. Pashchenko, D. Thermochemical waste-heat recuperation as on-board hydrogen production technology. Int. J. Hydrogen Energy 2021, 46, 28961–28968. [Google Scholar] [CrossRef]
  283. Itoh, N.; Kikuchi, Y.; Furusawa, T.; Sato, T. Tube-wall catalytic membrane reactor for hydrogen production by low-temperature ammonia decomposition. Int. J. Hydrogen Energy 2021, 46, 20257–20265. [Google Scholar] [CrossRef]
  284. Tang, H.; Wang, Y.; Zhang, W.; Liu, Z.; Li, L.; Han, W.; Li, Y. Catalytic activity of Ru supported on SmCeOx for ammonia decomposition: The effect of Sm doping. J. Solid State Chem. 2021, 295, 121946. [Google Scholar] [CrossRef]
  285. Kirste, K.G.; McAulay, K.; Bell, T.E.; Stoian, D.; Laassiri, S.; Daisley, A.; Hargreaves, J.S.; Mathisen, K.; Torrente-Murciano, L. COx-free hydrogen production from ammonia–mimicking the activity of Ru catalysts with unsupported Co-Re alloys. Appl. Catal. B Environ. 2021, 280, 119405. [Google Scholar] [CrossRef]
  286. El-Shafie, M.; Kambara, S.; Hayakawa, Y. Energy and exergy analysis of hydrogen production from ammonia decomposition systems using non-thermal plasma. Int. J. Hydrogen Energy 2021, 46, 29361–29375. [Google Scholar] [CrossRef]
  287. Sittichompoo, S.; Nozari, H.; Herreros, J.; Serhan, N.; da Silva, J.; York, A.; Millington, P.; Tsolakis, A. Exhaust energy recovery via catalytic ammonia decomposition to hydrogen for low carbon clean vehicles. Fuel 2021, 285, 119111. [Google Scholar] [CrossRef]
  288. Khan, W.U.; Alasiri, H.S.; Ali, S.A.; Hossain, M.M. Recent Advances in Bimetallic Catalysts for Hydrogen Production from Ammonia. Chem. Rec. 2022, 22, e202200030. [Google Scholar] [CrossRef]
  289. Rothan, Y.A.; Ali, F.F.; Issakhov, A.; Selim, M.M.; Li, Z. Optimization analysis of hydrogen production using ammonia decomposition. J. Mol. Liq. 2021, 335, 116190. [Google Scholar] [CrossRef]
  290. Fang, H.; Liu, D.; Luo, Y.; Zhou, Y.; Liang, S.; Wang, X.; Lin, B.; Jiang, L. Challenges and opportunities of Ru-based catalysts toward the synthesis and utilization of ammonia. ACS Catal. 2022, 12, 3938–3954. [Google Scholar] [CrossRef]
  291. Chang, F.; Gao, W.; Guo, J.; Chen, P. Emerging Materials and Methods toward Ammonia-Based Energy Storage and Conversion. Adv. Mater. 2021, 33, 2005721. [Google Scholar] [CrossRef]
Figure 1. Decarbonization options for transport, based on the use of renewable electricity or green hydrogen. BEV (battery electric vehicles); FCEV (fuel cell electric vehicles); HEV (hybrid electric vehicles); ICEV (internal combustion engine vehicles) or PHEV (plug-in hybrid electric vehicles) [1]. Figure extracted from Brynolf et al. (2022). Reproduced with the permission of Progress in Energy.
Figure 1. Decarbonization options for transport, based on the use of renewable electricity or green hydrogen. BEV (battery electric vehicles); FCEV (fuel cell electric vehicles); HEV (hybrid electric vehicles); ICEV (internal combustion engine vehicles) or PHEV (plug-in hybrid electric vehicles) [1]. Figure extracted from Brynolf et al. (2022). Reproduced with the permission of Progress in Energy.
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Figure 2. Forecasted H2 world demand (in million metric tons) by sectors, in the next 50 years, in a scenario of sustainable growth [6]. Figure extracted from Nnabuife et al. (2022). Reproduced with the permission of Elsevier.
Figure 2. Forecasted H2 world demand (in million metric tons) by sectors, in the next 50 years, in a scenario of sustainable growth [6]. Figure extracted from Nnabuife et al. (2022). Reproduced with the permission of Elsevier.
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Figure 3. Current contribution to hydrogen production by different sources.
Figure 3. Current contribution to hydrogen production by different sources.
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Figure 4. General scheme of different PtX fuel production pathways using electricity from renewable sources for the manufacturing of gaseous or liquid fuels.
Figure 4. General scheme of different PtX fuel production pathways using electricity from renewable sources for the manufacturing of gaseous or liquid fuels.
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Figure 5. Hydrogen densities in several potential hydrogen vectors [36]. Figure extracted from Hasan et al. (2021).
Figure 5. Hydrogen densities in several potential hydrogen vectors [36]. Figure extracted from Hasan et al. (2021).
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Figure 6. Reasonable route for the energy employment of NH3 as a green hydrogen energy vector [37]. Figure extracted from Chatterjee et al. (2021). Reproduced with the permission of ACS.
Figure 6. Reasonable route for the energy employment of NH3 as a green hydrogen energy vector [37]. Figure extracted from Chatterjee et al. (2021). Reproduced with the permission of ACS.
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Figure 7. Renewable energy roadmap where ammonia will play a fundamental role in storing and distributing large amounts of different renewable energies [43]. Figure extracted from Morlanes et al. (2021). Reproduced with the permission of Elsevier.
Figure 7. Renewable energy roadmap where ammonia will play a fundamental role in storing and distributing large amounts of different renewable energies [43]. Figure extracted from Morlanes et al. (2021). Reproduced with the permission of Elsevier.
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Figure 8. Green ammonia as fuel; two alternative propulsion system diagrams [107]. Figure extracted from Ayvali et al. (2021). Reproduced with the permission of Johnson Matthey Technology.
Figure 8. Green ammonia as fuel; two alternative propulsion system diagrams [107]. Figure extracted from Ayvali et al. (2021). Reproduced with the permission of Johnson Matthey Technology.
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Figure 9. Historical milestones in the attempts of using ammonia as fuel for transportation [108]. Figure extracted from Cardoso et al. (2021). Reproduced with the permission of Elsevier.
Figure 9. Historical milestones in the attempts of using ammonia as fuel for transportation [108]. Figure extracted from Cardoso et al. (2021). Reproduced with the permission of Elsevier.
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Figure 10. Concept of the complete production path of an alternative fuel for internal combustion engines (ICE) from purely renewable energy and resources (Adapted from Rehbein et al., 2019) [130].
Figure 10. Concept of the complete production path of an alternative fuel for internal combustion engines (ICE) from purely renewable energy and resources (Adapted from Rehbein et al., 2019) [130].
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Figure 11. Solid-oxide fuel cells operating in external or internal cracking modes [174]. Figure extracted from Rathore et al. (2021). Reproduced with the permission of Elsevier.
Figure 11. Solid-oxide fuel cells operating in external or internal cracking modes [174]. Figure extracted from Rathore et al. (2021). Reproduced with the permission of Elsevier.
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Figure 12. Schematic representation of: (a) oxygen direct fuel cell ammonia-fed (SOFC-O); (b) proton direct fuel cell ammonia-fed (SOFC-H) and (c) alkaline ammonia-fed fuel cells (AMFCs) [159]. Figure extracted from Wen et al. (2022). Reproduced with the permission of Elsevier.
Figure 12. Schematic representation of: (a) oxygen direct fuel cell ammonia-fed (SOFC-O); (b) proton direct fuel cell ammonia-fed (SOFC-H) and (c) alkaline ammonia-fed fuel cells (AMFCs) [159]. Figure extracted from Wen et al. (2022). Reproduced with the permission of Elsevier.
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Figure 13. Schematic representation of a typical urea electrolysis unit and its applications.
Figure 13. Schematic representation of a typical urea electrolysis unit and its applications.
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Figure 14. Main pathways of the combustion reaction of the UAN aqueous solution. Considering the reversible character of reactions, arrows indicate the main directions. The oxidation state of nitrogen is indicated in green numbers. (Adapted from Dana et al., 2016) [226].
Figure 14. Main pathways of the combustion reaction of the UAN aqueous solution. Considering the reversible character of reactions, arrows indicate the main directions. The oxidation state of nitrogen is indicated in green numbers. (Adapted from Dana et al., 2016) [226].
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Figure 15. Proposed model for hydrogen storage with ammonium formate (A) and schematic representation of an experimental H-cell (B) [236]. Figure extracted from Nakajima et al. (2019). Reproduced with the permission of ACS.
Figure 15. Proposed model for hydrogen storage with ammonium formate (A) and schematic representation of an experimental H-cell (B) [236]. Figure extracted from Nakajima et al. (2019). Reproduced with the permission of ACS.
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Figure 16. Scheme of Hydrogen Storage and Release in a Redox Cycle between Ammonium Bicarbonate and Formate.
Figure 16. Scheme of Hydrogen Storage and Release in a Redox Cycle between Ammonium Bicarbonate and Formate.
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Figure 17. Mass and volume of 10 kg hydrogen stored reversibly by eight different methods, based on the best obtained reversible densities reported in the literature without considering the space or weight of the containers [265]. Figure extracted from Saygin et al. (2023).
Figure 17. Mass and volume of 10 kg hydrogen stored reversibly by eight different methods, based on the best obtained reversible densities reported in the literature without considering the space or weight of the containers [265]. Figure extracted from Saygin et al. (2023).
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Figure 18. Schematic diagram of a solid-state H+ conducting cell, where NH3 is produced from H2O (steam) and N2 [274]. Figure extracted from MacFarlane et al. (2020). Reproduced with the permission of Wiley.
Figure 18. Schematic diagram of a solid-state H+ conducting cell, where NH3 is produced from H2O (steam) and N2 [274]. Figure extracted from MacFarlane et al. (2020). Reproduced with the permission of Wiley.
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Figure 19. Schematic diagram of the Solar Ammonia Refinery of the Future: Possible Pathways for Producing Ammonia from Renewable Energy [275]. Figure extracted from Wang et al. (2021). Reproduced with the permission of RSC.
Figure 19. Schematic diagram of the Solar Ammonia Refinery of the Future: Possible Pathways for Producing Ammonia from Renewable Energy [275]. Figure extracted from Wang et al. (2021). Reproduced with the permission of RSC.
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Figure 20. Schematic diagram for the production and utilization routes of ammonia in the energy sector “ammonia economy”.
Figure 20. Schematic diagram for the production and utilization routes of ammonia in the energy sector “ammonia economy”.
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Table 1. Comparison of the main useful properties of several potential hydrogen carrier molecules [35].
Table 1. Comparison of the main useful properties of several potential hydrogen carrier molecules [35].
PropertiesCompressed HydrogenLiquid HydrogenMethanolLiquid Ammonia
Storage methodCompressionLiquefactionAmbientLiquefaction
Temperature (°C)25−252.92525
Storage pressure (MPa)690.10.10.99
Density (kg/m3)3970.8792600
Explosive limit in air
(% Vol)
Gravimetric energy density (MJ/kg)12012020.118.6
Volumetric energy density (MJ/L)4.58.4915.812.7
Gravimetric hydrogen content (wt.%)10010012.517.8
Volumetric hydrogen content
Hydrogen releasePressure releaseEvaporationCatalytic
T > 200 °C
Catalytic decomposition
T > 400 °C
Energy to extract hydrogen (kJ/mol-H2)---0.90716.330.6
Table 2. Comparison of physicochemical properties between ammonia and the usual fuels currently used.
Table 2. Comparison of physicochemical properties between ammonia and the usual fuels currently used.
Lower Heating Value (MJ/kg)18.812050453742.527
Density (kg/m3)73080660850860700790
Autoignition temperature (°C)651520630254363370423
Flammability limit (Equivalence ratio)0.63–1.40.1–7.10.5–1.70.8–6.50.55–4.24
Maximum laminar flame speed (cm/s)72913790904348
Table 3. Organic solvents especially suitable for use as ammonia dual fuels in internal combustion engines (ICE) [130].
Table 3. Organic solvents especially suitable for use as ammonia dual fuels in internal combustion engines (ICE) [130].
Organic SolventFunctional GroupCarbon Atoms Number
Aldehydes R–C=O3–8
Carboxylic acidsR–COOH1–8
EthersR–C–O–C–R4–6, 8
Primary aminesR–NH23–8
Secondary aminesR–N(–H)–R3, 4, 6, 8
Table 4. Significant parameters in several nitrogen-based fuels, including ammonia in different rheological stats, as compared to H2: (a) Hydrogen at 700 bar; (b) NH3 At 10 bar and 0 °C; (c) NH3 at 50 bar and room temperature; (d) NH3 at −33 °C; (e) Hydrazine, N2H4; (f) Aqueous urea ammonium nitrate (UAN); (g) Aqueous ammonium hydroxide and ammonium nitrate (AAN).
Table 4. Significant parameters in several nitrogen-based fuels, including ammonia in different rheological stats, as compared to H2: (a) Hydrogen at 700 bar; (b) NH3 At 10 bar and 0 °C; (c) NH3 at 50 bar and room temperature; (d) NH3 at −33 °C; (e) Hydrazine, N2H4; (f) Aqueous urea ammonium nitrate (UAN); (g) Aqueous ammonium hydroxide and ammonium nitrate (AAN).
FuelH2 (a)NH3 (b)NH3 (c)NH3 (d)(e) N2H4(f) UAN(g) AAN
Phasegas liq. liq.liq.liq.
Energy density ρ (kg/m3)39.2639606682100413301106
Viscosity μ (mPa·s)0.0110.
Specific energy (MJ/kg)143------22.519.43.353.66
Vol. Energy Density (GJ/m3)5.6014.413.715.419.54.464.05
Octane Number (RON)>130130130 N/AN/AN/A
Flash point (°K)N/AN/AN/A 325N/AN/A
Autoignition temp. (AIT) (°K)810924924 438604586
Adiabatic flame temp. (Tad) °K251921072107 243313491334
Flame speed (SL) (m/s) N/AN/AN/A
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Estevez, R.; López-Tenllado, F.J.; Aguado-Deblas, L.; Bautista, F.M.; Romero, A.A.; Luna, D. Current Research on Green Ammonia (NH3) as a Potential Vector Energy for Power Storage and Engine Fuels: A Review. Energies 2023, 16, 5451.

AMA Style

Estevez R, López-Tenllado FJ, Aguado-Deblas L, Bautista FM, Romero AA, Luna D. Current Research on Green Ammonia (NH3) as a Potential Vector Energy for Power Storage and Engine Fuels: A Review. Energies. 2023; 16(14):5451.

Chicago/Turabian Style

Estevez, Rafael, Francisco J. López-Tenllado, Laura Aguado-Deblas, Felipa M. Bautista, Antonio A. Romero, and Diego Luna. 2023. "Current Research on Green Ammonia (NH3) as a Potential Vector Energy for Power Storage and Engine Fuels: A Review" Energies 16, no. 14: 5451.

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