Current Research on Green Ammonia (NH 3 ) as a Potential Vector Energy for Power Storage and Engine Fuels: A Review

: 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.


Introduction
Governments and researchers are currently striving to reduce CO 2 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 CO 2 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 carbonbased fuels, such as green methanol, green methane or green ammonia, obtained from the hydrogenation of CO 2 or N 2 , with green H 2 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 CO 2 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).   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).   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]. 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 H 2 [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, NH 3 can be obtained from atmospheric N 2 , whereas methanol or methane can be obtained from CO 2 . In the latter case, CO 2 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 CO 2 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].
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 wellknown 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 combus- Nevertheless, the case of ammonia is completely different. In first place, the wellknown 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 NH 3 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.

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 NH 3, 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-H 2 /m 3 versus 17.8 wt.% and 121 kg-H 2 /m 3 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-H 2, 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 NH 3 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 NH 3 , by feeding it into fuel cells, NH 3 can be catalytically cracked to produce H 2 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 NH 3 for H 2 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/ton NH3 (including H 2 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 CO 2 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 Energies 2023, 16, 5451 6 of 33 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]. 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  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, Figure 6. Reasonable route for the energy employment of NH 3 as a green hydrogen energy vector [37]. 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%.

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 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 H 2 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%.

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 (S L ). 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 CO 2 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 S L values of NH 3 /CH 4 /air flames increase almost linearly as the content of CH 4 also increases. Furthermore, the enhancement is applicable from fuel-lean to fuel-rich conditions, due to the higher S L and reactivity of methane [69,70]. At stoichiometric conditions, a more significant change is observed with CH 4 substitution proportion [69][70][71][72]. Regarding NOx emissions, a high percentage of ammonia substitution in NH 3 /CH 4 blends promoted a decrease in the CO, CO 2 , 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].
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].

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 NH 3 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].

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].

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]. 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].

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., H 2 , seems to be necessary to enlarge the operating range of ammonia [119][120][121][122][123][124][125][126]. Analogous to the use of H 2 , 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]. Table 3. Organic solvents especially suitable for use as ammonia dual fuels in internal combustion engines (ICE) [130].

Organic Solvent Functional Group Carbon Atoms Number
Alkanes 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. 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 CO 2 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.

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.

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 cellbased 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 N 2 and H 2 , 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]. tained 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 lowtemperature 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, Figure 11. Solid-oxide fuel cells operating in external or internal cracking modes [174].  (Figure 12a,b). Each one presents important differential characteristics based on the effect of the electrolyte, electrocatalyst material and operating temperature [159].

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].

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]. 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 lowtemperature 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.

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].

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 Energies 2023, 16, 5451 14 of 33 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 ureacontaminated 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 NH 3 and CO 2 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]. 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 ureacontaminated 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].

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/m 3 , 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 Figure 13. Schematic representation of a typical urea electrolysis unit and its applications.

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/m 3 , 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].

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% H 2 O, 21.6% N 2 and 5.4% CO 2 on a molar basis, according to Equation (1): 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]. 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].

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): 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].

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 CO 2 [228], according to Equation (2): 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].

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): 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 nitrogenbased 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). 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).  As can be seen, formic acid and ammonia can be generated in traditional or sustainable plants by feeding H 2 . 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).
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/m 3 ). 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 (T ad ) is a measure of the energy content of a fuel. High values of T ad usually imply an increase in the equipment costs, lower efficiencies and high pollutant emissions. High flame speed values (S L ), such as in the case of hydrogen, improve engine efficiency, but they also reduce fuel safety. In contrast, the low flame speed (S L ) 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. 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]. 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].

Storage and Transportation of Ammonia Fuels
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].

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]. 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].

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 NH 3 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 H 2 produced annually for industrial purposes, 95% come from fossil gas, oil and coal, and the rest come from H 2 O 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 N 2 and H 2 /H 2 O, which can be produced on-site and/or transported to the solar refinery, using electricity, heating and photons produced by solar energy. 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.

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

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) 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].

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 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) 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].

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 Energies 2023, 16, 5451 22 of 33 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.
Funding: 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 "NextGen-erationEU/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.