Technology for Green Hydrogen Production: Desk Analysis

Abstract

The use of green hydrogen as a high-energy fuel of the future may be an opportunity to balance the unstable energy system, which still relies on renewable energy sources. This work is a comprehensive review of recent advancements in green hydrogen production. This review outlines the current energy consumption trends. It presents the tasks and challenges of the hydrogen economy towards green hydrogen, including production, purification, transportation, storage, and conversion into electricity. This work presents the main types of water electrolyzers: alkaline electrolyzers, proton exchange membrane electrolyzers, solid oxide electrolyzers, and anion exchange membrane electrolyzers. Despite the higher production costs of green hydrogen compared to grey hydrogen, this review suggests that as renewable energy technologies become cheaper and more efficient, the cost of green hydrogen is expected to decrease. The review highlights the need for cost-effective and efficient electrode materials for large-scale applications. It concludes by comparing the operating parameters and cost considerations of the different electrolyzer technologies. It sets targets for 2050 to improve the efficiency, durability, and scalability of electrolyzers. The review underscores the importance of ongoing research and development to address the limitations of current electrolyzer technology and to make green hydrogen production more competitive with fossil fuels.

1. Introduction

The current challenge for the global energy economy is to implement a large-scale, low-emission energy alternative to fossil fuels that is cheap, safe, and sustainable [1]. The key to progress in reducing pollutant emissions and combating energy poverty is the energy source and its price. Balancing greenhouse gas emissions and their removals will enable the global economy to transition to zero-emission activities, but requires a comprehensive approach and a long-term strategy based on innovative technologies as priority tasks [2,3,4].

Investment in the hydrogen economy (HE) will allow the use of hydrogen as an energy carrier that will help meet the growing energy demand due to the excessive development of civilization and depleting fossil fuel resources [5,6,7,8,9]. Due to the possibility of ecologically obtaining energy from renewable energy sources (RESs) for green hydrogen production, research and development (R&D) is being carried out in the search for new electrode materials capable of catalytic participation in reducing the energy barrier for obtaining green hydrogen through the emission-free process of electrolysis [6,9,10,11,12,13,14,15,16,17,18]. The current technologies used for green hydrogen production primarily involve four types of electrolyzers: alkaline electrolyzers (AEs) [12,16,17,18,19,20,21,22,23], proton exchange membrane (PEM) electrolyzers [13,16,24,25,26,27,28,29,30], solid oxide electrolyzers (SOEs) [14,16,31,32,33,34,35,36,37,38,39,40,41,42,43], and anion exchange membrane (AEM) electrolyzers [15,16,44,45,46,47,48,49,50,51,52]. In addition to these electrolyzer technologies, there are other innovative methods being researched and developed for green hydrogen production, including photoelectrochemical (PEC) water splitting, which uses solar energy directly to drive electrolysis [53,54,55,56,57,58,59,60]; biological hydrogen production, which involves using microorganisms to produce hydrogen through processes like fermentation or biophotolysis [60,61,62,63,64,65,66]; and high-temperature electrolysis (HTE), which operates similarly to SOEs and can use heat from nuclear reactors or concentrated solar power to improve efficiency [67,68,69,70,71]. The choice of technology for green hydrogen production depends on various factors, including the availability and cost of renewable energy, the scale of production, system efficiency, capital and operating costs, and the specific application or end-use of the hydrogen. As the demand for green hydrogen grows, it is expected that these technologies will continue to evolve and become more cost-effective and efficient [16,72,73].

A comprehensive analysis of recent scientific research on green hydrogen production technologies based on data from the Scopus database allowed us to determine the number of research papers published in this field. The bar chart illustrates the number of publications related to different green hydrogen production technologies from 2020 to 2024 (Figure 1).

These key points from the chart regarding the AE, PEM, SOE, and AEM technologies of green hydrogen production were formulated as objectives for the refined analysis-based Scopus database search. The number of publications on green hydrogen production technologies demonstrates some notable trends. The largest number of publications is observed for PEM electrolysis, with 255 in total. Research on this technology is significantly advanced, demonstrating its importance and interest in green hydrogen production. AEs are followed by a total of 221 publications and remain a major focus of research, demonstrating their continued influence in the field. There are also 112 publications in total for SOEs. Although the number is relatively small, it still shows some research attention, indicating its potential in specific applications. AEMs have the lowest number of publications, with 77 in total. Although less researched, this area may be gradually receiving more attention. Generally, PEM and AE technologies dominate the research on green hydrogen production, while SOE and AEM technologies are in relatively minor positions, but there is still room for research and development.

Based on the obtained results it can be stated that green hydrogen, produced through RESs like wind, solar, or hydroelectric power, is seen as a key component of the transition to a low-carbon economy. The trends in green hydrogen research suggest that the number of publications in this area has been growing, reflecting the increasing interest and investment in green hydrogen as a clean energy carrier. Some areas where research has been particularly active concern electrolysis technologies [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52], renewable energy integration [74,75,76], innovative production methods [53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71], or policy and economic analysis [16,72,73,74,75,76]. Research results on more efficient and economical electrolysis technologies have been reported, including PEM electrolysis and alkaline electrolysis. There has been research on how to better integrate green hydrogen production with RESs, including optimizing energy storage and grid integration [74,75,76]. Research results concerning the exploration of innovative methods such as PEC, solar-driven thermochemical processes, and biological hydrogen production have been also noted but to a lesser extent [53,54,55,56,57,58,59,60,61,62,63,64,65,66]. Finally, research into the economic viability of green hydrogen production, including cost analysis, market potential, and policy implications, has been conducted within the HE [16,72,73].

In this context, the main goal of this article is to provide a desk analysis and technical review of green hydrogen production technologies and their advantages as well as current limitations and challenges. This dual strategy aims to help the scientific community grasp the current relevance and technical intricacies of water electrolysis technologies implemented within the framework of HE in the years 2020–2024.

2. Background of Analysis

2.1. Energy Production from Fossil Fuels and Renewable Sources

The most popular and widely used fuels for energy production are coal, petroleum, natural gas, and oil [2,3,4]. Unfortunately, when these fossil fuels are burned, in addition to energy, they also release industrial gases that pose a potential threat to the natural environment [77]. Undesirable by-products of the combustion of fossil fuels include sulfur (IV) oxide, carbon (II) oxide, carbon (IV) oxide, nitrogen oxides, dust, and ashes, as well as hydrocarbons that have not been burned. The combustion process also emits heavy metals such as radium, thorium, and lead, which are contained as admixtures in fossil fuels. All emitted combustion products change the natural environment irreversibly, polluting the air, soil, and groundwater, and contributing to the accelerated death of plants and animals, an increase in the incidence of asthma, allergies, cardiovascular diseases, and cancer. The effects of air pollution in the form of smog, the ozone hole, acid rain, odors, and the greenhouse effect are particularly dangerous to human health and life [78,79].

Reducing the amount of pollution in the combustion process of fossil fuels can be ensured by the modernization of outdated industrial plants, waste-free or at least low-waste production, reuse of exhaust gases, or energy saving. Taking into account the fact that the Earth’s fossil fuel resources are slowly decreasing and the need to diversify energy to avoid an energy crisis, it is necessary to generate electricity from renewable sources at an affordable price [2,4]. Primary energy consumption by source on a global scale in 2022, as shown in Figure 2, was already 166,588.47 TWh [80]. Primary energy, assuming that it is unconverted energy available as a resource, was based on the substitution method. It includes energy needed by the end user in the form of electricity, transportation, and heating, as well as inefficiencies and energy lost in converting raw materials into a usable form. Primary energy consumption for fossil fuels including coal, oil, and gas amounted to as much as 137,236.67 TWh, which constitutes over 83% of the total consumption. The highest value of 52,969.59 TWh among depleting fossil fuels was achieved in the case of oil. Primary energy consumption for nuclear and renewables in 2022 was only 6702.34 TWh and 22,649.47 TWh, respectively. It should be noted that primary energy consumption for other renewables should be increased.

Electricity production from fossil fuels, nuclear, and renewables in the world from 1985 to 2023 is presented in Figure 3 [81]. Yearly electricity generation, capacity, emissions, import, and demand data for over 200 geographies are contained in this dataset.

The share of fossil fuels in global electricity production has not changed much over the last three decades, decreasing only from 64 to 61%, while the share of nuclear has decreased from 15 to 9% and the share of renewables has increased from 21 to 30%. Total electricity production in 2023 amounted to 29,479 TWh, of which 17,879 TWh was generated from fossil fuels, 2686 TWh from nuclear, and 8914 TWh from renewables. One can observe a global upward trend in the implementation of renewable technologies, which is partially offset by a decline in nuclear energy production.

Currently, approximately 96% of the total hydrogen production in the world is based on fossil fuels and only 4% on water electrolysis, of which approximately 1% uses energy from RESs for the electrolysis process, in which green hydrogen is obtained in a zero-emission way [82]. According to data published by the International Energy Agency, in 2021 the total global production of hydrogen was 94 million tonnes (Mt H₂), including 74 Mt H₂ of pure hydrogen production and around 20 Mt H₂ mixed with carbon-containing gases in methanol production and steel manufacturing, as shown in Figure 4 [82].

Emissions associated with hydrogen production based on fossil fuels were above 900 Mt CO₂. The majority of hydrogen is produced from natural gas without Carbon Capture, Utilization and Storage (CCUS) technologies, accounting for 62% of total hydrogen production [82]. This method, known as steam methane reforming (SMR), is the most common due to its cost-effectiveness and the widespread availability of natural gas. Approximately 18% of total hydrogen production is hydrogen produced in refineries as a by-product of naphtha reforming, which is then used in various refinery processes such as hydrocracking and desulfurization. Hydrogen production from coal represents 19% of the total production. This method, known as coal gasification, is less environmentally friendly than SMR due to higher carbon emissions. Less than 1% of crude oil was also used to produce hydrogen. Low-emission hydrogen production, which accounts for less than 1% of the total, is almost entirely from fossil fuels with CCUS. This indicates that while there is a push towards cleaner hydrogen production, it is still in its infancy. Hydrogen produced via water electrolysis, using electricity, is a very small fraction of the total, but it saw a significant increase of almost 20% from 2020. This growth reflects the increasing deployment of water electrolyzers, driven by the need for green hydrogen in various sectors, including transportation, industry, and power generation. The data shows a strong reliance on fossil fuels for hydrogen production, with natural gas being the dominant source. The shift towards cleaner production methods, such as water electrolysis powered by renewable energy, is growing but still represents a minor part of the overall hydrogen market. The increasing interest in hydrogen as a clean energy carrier and its potential role in decarbonizing various sectors underscores the need for further development and deployment of low-emission hydrogen production technologies. This includes scaling up electrolysis powered by renewable energy sources to produce green hydrogen, as well as enhancing CCUS technologies to reduce the carbon footprint of hydrogen production from fossil fuels.

Renewable energy plays a crucial role in reducing the production cost of green hydrogen using electricity generated from renewable sources, despite the RES barriers [3,4,5,6,7,8,9,10,11,12,74,75,76]. The integration of renewable energy into green hydrogen production significantly reduces costs by providing a low-cost and low-emission source of electricity, enabling scalability and flexibility, and leveraging policy support and technological advancements [74,75,76]. As the renewable energy sector continues to grow and mature, the production cost of green hydrogen is expected to decrease further, making it an increasingly viable alternative to fossil fuels in various applications.

2.2. Types of Hydrogen

A concept illustration of grey, blue, green, and pink hydrogen production is presented in Figure 5. Grey hydrogen is a term used to describe hydrogen that is produced from fossil fuels in high-emission processes, primarily through SMR (Figure 5a) [83].

The SMR process involves reacting natural gas (methane, CH₄) with steam (H₂O) to produce hydrogen (H₂) and carbon monoxide (CO) at a temperature of 700–1000 °C in the presence of an appropriate metal catalyst according to the following chemical Reaction (1) [87,88]:

CH₄ + 2H₂O → CO₂ + 4H₂.

(1)

The SMR process produces high emissions of carbon dioxide (CO₂), a greenhouse gas, as a by-product of 9–12 kg CO₂/kg H₂. Although methane is a raw material that ensures high efficiency, resource sufficiency, and relatively low costs compared to other methods using fossil fuels, the SMR process requires significant energy inputs and, due to its emissivity, contributes to climate change and environmental concerns. Grey hydrogen is the most common form of hydrogen produced today, accounting for the majority of the hydrogen used globally. The production of grey hydrogen in the world already exceeds 55 Mt per year. Grey hydrogen is widely used in various industries, including chemical production, refining, and as a feedstock for the production of ammonia and methanol. To address the environmental impact of grey hydrogen production, there are efforts to develop and implement CCUS technologies [89,90]. When these technologies are applied to the SMR process, the resulting hydrogen is often referred to as blue hydrogen as shown in Figure 5b [84]. The goal of CCUS is to capture the CO₂ emissions from the SMR process and store them underground or use them in other applications, thereby reducing the net carbon footprint of hydrogen production. Implementing CCUS technologies for grey hydrogen production involves several challenges, such as high costs, energy penalties, transportation, and storage, among others [82]. The initial capital expenditure for CCUS infrastructure is significant. This includes the cost of capture technologies, transportation systems, and storage facilities. Additionally, the operational costs can be high due to the energy requirements for capturing and compressing CO₂. Capturing CO₂ from industrial processes typically requires a lot of energy, which can reduce the overall efficiency of the hydrogen production process. This energy penalty can increase the cost of hydrogen and may lead to higher greenhouse gas emissions if the additional energy comes from fossil fuels. Transporting large volumes of CO₂ to storage sites can be challenging and costly. It requires pipelines, ships, or trucks, and the routes must be carefully planned. Capturing CO₂ from the flue gases of hydrogen production facilities is technically challenging, especially if the concentration of CO₂ is low. The capture process must be highly efficient to be economically viable, and it must not significantly disrupt the existing industrial processes. Demonstrating CCUS at a pilot or demonstration scale does not guarantee that it can be effectively scaled up to meet the needs of large industrial hydrogen production. Scaling up requires overcoming engineering challenges and ensuring the economic viability of the process. As RESs and technologies advance, green hydrogen produced from electrolysis using renewable electricity may become more competitive. This could reduce the incentive for investing in CCUS for grey hydrogen production. Addressing these challenges requires a combination of technological innovation, policy support, investment in infrastructure, and public engagement to ensure that CCUS can play a role in decarbonizing hydrogen production and other industrial processes.

In contrast, green hydrogen is produced through processes that do not emit carbon dioxide, such as water electrolysis, which ensures the production of hydrogen of the highest purity and is the most promising, but unfortunately currently the most expensive method [5,6,9,10,11,91,92,93,94,95,96]. Therefore, new catalysts with high electroactivity are being developed [97]. Green hydrogen obtained electrolytically can be used to balance energy surpluses produced using electricity generated by RESs, and in the long run, it may become competitive with grey hydrogen produced from fossil fuels in high-emission processes [87,88,89]. Figure 5c illustrates a diagram of green hydrogen production [85]. The process begins with the generation of electricity from RESs. For example, solar photovoltaic panels convert sunlight into electricity, or wind turbines generate electricity from the kinetic energy of the wind. The renewable electricity is then used to power an electrolyzer. The hydrogen gas produced is collected and may undergo further purification processes to remove any remaining moisture or impurities, ensuring it is suitable for its intended use. Once purified, the hydrogen can be stored in high-pressure tanks, underground caverns, or other storage facilities. It can then be distributed to where it is needed, either through pipelines or by transporting it in compressed or liquefied form. Green hydrogen is considered a key component of a sustainable energy future because it can be used in various applications, including powering fuel cells in vehicles, heating buildings, or as a feedstock in industrial processes, without emitting pollutants or greenhouse gases. However, the production of green hydrogen is currently more expensive than hydrogen produced from fossil fuels, largely due to the higher costs of renewable electricity and electrolysis equipment. As technology advances and economies of scale are achieved, the cost of green hydrogen is expected to decrease, making it more competitive with other forms of hydrogen and energy carriers.

Table 1. The advantages of green hydrogen, produced from RESs through the electrolysis of water, over grey hydrogen, which is produced from fossil fuels, typically via SMR of natural gas.

Advantage	Grey Hydrogen [83,87,88,89,90]	Green Hydrogen [5,6,9,10,11,91,92,93,94,95,96,97,98]
Environmental benefits	Grey hydrogen production through SMR releases significant amounts of carbon dioxide, contributing to climate change and global warming.	A clean energy carrier with zero greenhouse gas emissions at the point of use, as it only produces water when combined with oxygen in a fuel cell or burned for energy.
Renewable and sustainable	Grey hydrogen relies on finite fossil fuel resources, which are subject to depletion and price fluctuations.	Derived from RESs, which are inexhaustible and do not deplete natural resources.
Energy security	Grey hydrogen production is often linked to the availability and cost of natural gas, which can vary due to geopolitical factors and market dynamics.	By using domestic RESs, green hydrogen can enhance energy security by reducing dependence on imported fossil fuels.
Versatility	Grey hydrogen is also versatile. Its use perpetuates dependence on fossil fuels and the associated environmental impacts.	Green hydrogen can be used in a wide range of applications, including transportation, power generation, industrial processes, and as a storage medium for renewable energy, providing flexibility in energy systems.
Economic opportunities	Grey hydrogen production is often part of established fossil fuel industries, with fewer opportunities for innovation and economic expansion in renewable energy technologies.	The production of green hydrogen can create new industries and jobs in renewable energy and related sectors, contributing to economic diversification and growth.
Health and air quality	The SMR process and the combustion of gray hydrogen can release NOx and other pollutants, contributing to poor air quality and health issues.	Green hydrogen does not produce air pollutants that can harm human health, such as nitrogen oxides (NOx), sulfur dioxide (SO2), or particulate matter (PM).
Technological innovation	Grey hydrogen technology is more mature and less likely to benefit from the same pace of innovation and cost reductions seen in renewable energy sectors.	Investment in green hydrogen supports the development and improvement of renewable energy technologies, electrolysis systems, and energy storage solutions, driving technological advancements.

presents a comparative analysis of green hydrogen and grey hydrogen, highlighting the advantages of green hydrogen in terms of environmental benefits, sustainability, energy security, versatility, economic opportunities, health, air quality, and technological innovation.

Green hydrogen is produced from RESs through the electrolysis of water, offering a clean energy carrier with zero greenhouse gas emissions at the point of use. In contrast, grey hydrogen, produced from fossil fuels via SMR, releases significant amounts of carbon dioxide, contributing to climate change. Green hydrogen’s production from inexhaustible RESs enhances energy security by reducing reliance on imported fossil fuels, which are subject to depletion and price fluctuations. Green hydrogen’s versatility allows it to be used in various applications, such as transportation, power generation, and industrial processes, and as a storage medium for renewable energy, potentially providing flexibility in energy systems. Moreover, green hydrogen does not produce air pollutants like NO_x, SO₂, or PM, which can harm human health and contribute to poor air quality. Investment in green hydrogen supports technological advancements in renewable energy technologies, electrolysis systems, and energy storage solutions. This contrasts with grey hydrogen technology, which is more mature and less likely to benefit from the same pace of innovation and cost reductions seen in renewable energy sectors. Green hydrogen is positioned as a more sustainable, environmentally friendly, and versatile energy source compared to grey hydrogen, with the potential to drive economic growth, improve air quality, and foster technological innovation in the energy sector.

Despite these advantages, the widespread adoption of green hydrogen is currently limited by its higher production costs compared to grey hydrogen. The global hydrogen generation market size was estimated at USD 170.14 billion in 2023, and is expected to grow at a compound annual growth rate of 9.3% from 2024 to 2030 [98]. Grey hydrogen, produced from natural gas through SMR, benefits from well-established industrial processes and typically has lower production costs, often in the range of USD 1 to USD 2 per kilogram, depending on the price of natural gas and the efficiency of the production process. In contrast, green hydrogen, produced via electrolysis powered by renewable energy, can cost between USD 3 and USD 6 per kilogram or more, depending on factors such as the cost of electricity, the capacity factor of the renewable energy source, and the cost of the electrolyzer. However, as renewable energy technologies become cheaper and more efficient, and as economies of scale are achieved in electrolysis equipment, the cost of green hydrogen is expected to decrease, making it a more competitive alternative to fossil fuel-based hydrogen. The transition from grey to green hydrogen aligns with the goals of sustainability, environmental protection, and reducing the anthropogenic impacts on the climate and ecosystems. As green hydrogen technologies mature and costs decrease, the potential for these environmental benefits to be realized on a large scale increases.

The latest concept of hydrogen production in the water electrolysis process assumes the use of nuclear energy (Figure 5d) [87]. Hydrogen produced by nuclear-powered water electrolysis is called pink hydrogen, or alternately violet, purple, or red. The production of pink hydrogen will ensure low emissions and target profitability, and will be able to complement production from RES and thus meet the growing energy demands of industry. The wide range of hydrogen colors confirms the belief that the potential of hydrogen fuel is huge and, more importantly, it will be developed even more strongly in the coming years.

2.3. Water Electrolysis Process

The production of green hydrogen through electrolysis involves the use of an electrolyzer, where water is split into its constituent hydrogen and oxygen gases by passing an electric current through it [95,99,100]. In the electrolyzer, water is fed into the system, and an electric current is applied to two electrodes—the anode and the cathode—which are separated by a membrane, as shown in Figure 6 [101]. At the cathode, electrons from the electric current cause water molecules to gain electrons (reduction) and form hydrogen gas. At the anode, water molecules lose electrons (oxidation) and produce oxygen gas and positively charged hydrogen ions. The hydrogen ions migrate through the membrane to the cathode, where they combine with electrons to form hydrogen gas. The efficiency of different electrolyzer technologies for green hydrogen production can vary depending on the design of the electrolyzer, the operating conditions, the energy source used, the electrical energy input, the conductivity of the electrolyte, and, to a large extent, the electrode materials used for the HER at the cathode and the OER at the anode.

Electrolyzers play a pivotal role in green hydrogen production, as they are the devices responsible for splitting water [16,51,67,69]. They aim to maximize the efficiency of the electrolysis process, minimizing energy losses and ensuring that a significant portion of the electrical energy input is converted into chemical energy in the form of hydrogen. Advanced electrode materials and electrolyzer designs help reduce overpotential, which is the additional voltage required beyond the thermodynamic minimum to drive the electrolysis reactions [16]. Lower overpotential means less energy is wasted as heat. Electrolyzers can be scaled up or down to meet different production capacities, making them suitable for both small-scale and large-scale hydrogen production facilities [16,62,72]. Modular electrolyzer systems allow for easy expansion or reduction in capacity by adding or removing modules, providing flexibility in response to changing demand or availability of renewable energy. The materials used in electrolyzers, including electrodes, electrolytes, and membranes, must be stable under operating conditions to ensure long-term durability and performance [16,24,43,57]. Regular maintenance and timely replacement of components can extend the lifetime of electrolyzers, ensuring consistent and reliable hydrogen production. Electrolyzers can be designed to operate flexibly, adjusting their power consumption based on the availability of renewable energy. This capability is crucial for maximizing the use of intermittent RESs like solar and wind [74,75,76]. Electrolyzers can also serve as a form of energy storage, converting excess renewable energy into hydrogen, which can be stored and used later when demand exceeds supply. Large-scale production and deployment of electrolyzers can benefit from economies of scale, reducing the cost per unit of hydrogen produced [16,72].

The selection of electrode materials for green hydrogen production through electrolysis is critical for achieving high efficiency, durability, and cost-effectiveness. Key factors that should be considered when choosing electrode materials include catalytic activity, stability and durability, cost and availability, compatibility with electrolytes, scalability and manufacturing, environmental impact, and operational conditions [16,24,43,57,62,72]. The electrode material should have high catalytic activity for the HER and OER to ensure high current densities and fast reaction kinetics [13,14,15,18,20,24,25,26,27,28,29,30,50,97]. Low overpotential is desirable to minimize the energy required for electrolysis. Noble metals like Pt for HER [16,19,22] and Ir or ruthenium Ru for OER are known for their low overpotentials but are expensive [16,27,30]. The electrode material must be chemically stable in the electrolyte under operating conditions to prevent corrosion and degradation, and have good mechanical properties to withstand the stresses of operation, including changes in pressure and temperature [33,34,38,39,40,41,68]. The electrodes should maintain their performance over long periods without significant degradation. The cost of the electrode material is a significant factor, especially for large-scale applications [16,91]. Noble metals are expensive, so there is a push toward developing low-cost alternatives [16,17,18,20,29]. The availability of the material and the sustainability of its supply chain are also important considerations to ensure long-term viability and scalability. The electrode material must be also compatible with the electrolyte pH (acidic, neutral, or alkaline). The material should maintain a stable interface with the electrolyte to ensure consistent performance over time and be easily processable into the desired electrode shape and structure. The manufacturing process for the electrode material should be scalable to meet the demands of large-scale hydrogen production [16,72]. The environmental impact and toxicity of the electrode material and its manufacturing process are important considerations. The sustainability of the material, including its end-of-life recycling and disposal, should be considered [100]. Finally, the material should be able to withstand the operating temperature and pressure conditions of the electrolysis process, and be able to handle the desired current density without significant degradation [33,34,38,39,40,41,68]. Emerging trends in electrode materials include the use of nanomaterials, which can enhance catalytic activity and stability due to their high surface area and tunable properties [20,27,44,97]; two-dimensional (2D) materials like graphene [102] and transition metal dichalcogenides (TMDs), which have been investigated for their unique properties and potential for high performance [103]; and bio-inspired catalysts like enzyme-inspired catalysts and other bio-inspired materials, explored for their efficiency and sustainability [104].

Table 2. Components and materials of AEs, PEM electrolyzers, AEM electrolyzers, and SOEs technology, where PFSA—Perfluoroacidsulfonic, PTFE—Polytetrafluoroethylene, ETFE—Ethylene Tetrafluorethylene, PSF—poly(bisphenol-A sulfone), PSU—Polysulfone, YSZ–yttriastabilized zirconia, DVB–divinylbenzene, PPS—Polyphenylene sulfide, LSCF—La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ, LSM—(La_1−xSr_x)_1−yMnO₃ [16].

Component	AEs	PEM Electrolyzers	AEM Electrolyzers	SOEs
Electrolyte	KOH 5–7 mol L−1	PFSA membranes	DVB polymer support with KOH or NaHCO3 1 mol L−1	Yttria-stabilized Zirconia (YSZ)
Separator	ZrO2 stabilized with PPS mesh	Solid electrolyte	Solid electrolyte	Solid electrolyte
Electrode/catalyst (oxygen side)	Nickel-coated perforated stainless steel	Iridium oxide	High-surface-area Ni or NiFeCo alloys	Perovskite-type (e.g., LSCF, LSM)
Electrode/catalyst (hydrogen side)	Nickel-coated perforated stainless steel	Platinum nanoparticles on carbon black	High-surface-area Nickel	Ni/YSZ
Porous transport layer Anode	Nickel mesh (not always present)	Platinum coated sintered porous titanium	Nickel foam	Coarse Nickel-mesh or foam
Porous transport layer Cathode	Nickel mesh	Sintered porous titanium or carbon cloth	Nickel foam or carbon cloth	None
Bipolar plate anode	Nickel-coated stainless steel	Platinum-coated titanium	Nickel-coated stainless steel	None
Bipolar plate cathode	Nickel-coated stainless steel	Gold-coated titanium	Nickel-coated stainless steel	Cobalt-coated stainless steel
Frames and sealing	PSU, PTFE, EPDM	PTFE, PSU, ETFE	PTFE, Silicon	Ceramic glass

summarizes the key components and materials of various types of electrolyzers, such as AEs, PEM electrolyzers, AEM electrolyzers, and SOEs, used in the production of green hydrogen [16]. The cells colored grey represent conditions or components that show significant variation across manufacturers or R&D institutions. Based on the data obtained by the International Renewable Energy Agency depicted in Table 2 [16], it can be observed that AEM electrolyzers and SOEs are less mature at lab scale compared to other types, such as already commercial AEs or PEM electrolyzers, which might have more standardized or optimized components and conditions across different manufacturers. For AEM electrolyzers, the technology is relatively new and still under development, which could lead to a wide range of materials and operating conditions being explored by different institutions. AEM electrolyzers operate in an alkaline environment, which can offer advantages in terms of catalyst activity and durability but also presents challenges in terms of membrane stability and performance. SOEs operate at high temperatures, which creates challenges in material selection, thermal expansion mismatch, and durability. The significant variation in these technologies could indicate that they are still in the research phase, with many institutions experimenting with different approaches to improve performance, durability, and cost-effectiveness. This diversity of approaches is typical in the early stages of technology development and is essential for the eventual maturation and commercialization of these technologies.

Each type has specific materials and configurations for its electrolyte, separator, electrodes/catalysts, and bipolar plates (Table 2) [16]. For AE, the electrolyte is a solution of KOH, the separator is ZrO₂ stabilized with a PPS mesh, and the electrodes are nickel-coated stainless steel with a nickel mesh as the porous transport layer. The bipolar plates are made of nickel-coated stainless steel. PEM electrolyzers use PFSA membranes as the electrolyte, a solid electrolyte for the separator, and platinum nanoparticles on carbon black for the hydrogen side electrode. The oxygen side electrode is made of high-surface-area Ni or NiFeCo alloys, and the bipolar plates are nickel-coated steel or stainless steel. AEM electrolyzers employ DVB polymer support with either KOH or NaHCO₃ as the electrolyte, a solid electrolyte separator, and perovskite-type catalysts like LSCF or LSM for the oxygen side electrode. The hydrogen side uses high-surface-area nickel, and the bipolar plates are made of nickel-coated stainless steel. SOEs utilize yttria-stabilized zirconia (YSZ) as both the electrolyte and separator. The oxygen side electrode is made of perovskite-type materials, and the hydrogen side uses Ni/YSZ. The bipolar plates are constructed from nickel-coated stainless steel or cobalt-coated stainless steel.

The current limitations in electrolyzer technology that are being actively addressed by R&D strategies include many factors, such as efficiency, cost, durability and lifetime, scalability, energy source integration, hydrogen compression and storage, system integration and control [5,6,10,11,16,21,64,72]. While electrolyzers can convert electricity into hydrogen, the process is not yet as efficient as desired. Energy losses occur due to electrical resistance, heat generation, and inefficiencies in the electrochemical reactions. R&D is focused on improving the efficiency of the electrolysis process, which includes developing better catalysts, optimizing cell design, and improving the electrical conductivity of materials [21,86,92,105,106,107]. The cost of electrolyzers and their components, such as membranes, catalysts, and bipolar plates, is currently high [13,14,15,18,20,24,25,26,27,28,29,30,46,48,49]. This is partly due to the use of expensive materials like precious metals as catalysts. R&D is working on reducing costs by finding alternatives to these expensive materials, improving manufacturing processes, and scaling up production to benefit from economies of scale. Electrolyzers need to operate for thousands of hours under varying conditions to be economically viable (Table 2) [16]. However, degradation of components over time can lead to reduced efficiency and increased maintenance costs [33,34,38,39,40,41,68,108,109]. R&D is focused on enhancing the durability of electrolyzer components, including the development of more robust membranes, catalysts, and other materials that can withstand corrosive environments and maintain performance over extended periods. To meet the growing demand for green hydrogen, electrolyzers need to be scaled up effectively. This involves not only building larger electrolyzers but also ensuring that they can be manufactured and operated at scale without significant increases in cost or decreases in efficiency. R&D is exploring modular designs and standardization to facilitate scalability [72,110]. Electrolyzers are most environmentally beneficial when powered by RESs like wind and solar. However, the intermittent nature of these sources can pose challenges for continuous hydrogen production. R&D is investigating ways to better integrate electrolyzers with variable RESs, including energy storage solutions and smart grid technologies. After production, hydrogen needs to be compressed and stored for transportation or use. This step can account for a significant portion of the overall cost and energy requirements. R&D is exploring inexpensive and effective methods for hydrogen compression and storage, including new materials for storage tanks and innovative compression technologies. Optimizing the operation of electrolyzers within larger energy systems requires advanced control systems and integration strategies [31,106,109,111]. R&D is focused on developing smart control algorithms and integration solutions that can maximize the efficiency and flexibility of hydrogen production systems. By addressing these limitations, R&D strategies aim to make green hydrogen production more efficient, cost-effective, and scalable, thereby enabling it to compete with fossil fuels and other forms of energy storage in various applications.

3. Methodology

Writing the review article involved a systematic and structured approach to gather, analyze, and synthesize information from the Scopus database, which is a bibliographic database containing abstracts and citations for academic journal articles. For the most accurate and up-to-date information, direct access to the Scopus database was obtained on 17 August 2024 using an institutional login. To find the number of publications concerning green hydrogen production technologies from 2020 to 2024 using the Scopus database and technical reports, a step-by-step methodology for writing this article was used (Figure 7).

Step I focused on defining the scope and objectives, which were formulated in the Introduction Section. Specific aspects of the desk analysis topic were identified, including green hydrogen production technologies, their applications, and recent development trends. A clear objective of the review was defined to provide information, identify gaps, and highlight future research directions. Step II was to conduct a comprehensive literature search in the Scopus database to identify relevant articles, reviews, and reports. A list of keywords and phrases related to the topic was developed and used in the search. Criteria for including or excluding studies were defined based on factors such as publication date, relevance, and study type. In Step III, titles and abstracts of search results were screened to identify potentially relevant studies, and then the full texts of selected studies were reviewed to determine their eligibility for inclusion in the review. Relevant data from the selected studies, including key findings, methodologies, and conclusions were extracted. Step IV involved data analysis and synthesis, which included qualitative and quantitative analysis and critical appraisal. The findings and amount of numerical data from the selected studies were summarized and trends and gaps in the literature were identified. The quality and validity of the studies were assessed. Step V was about organizing the review article. A structure for the article was developed, including sections such as the Introduction, Background of Analysis, Methodology, Results and Discussion, Conclusions, and Future Directions. The Introduction Section provides background information on the topic and outlines the objectives of this review. The Background of Analysis Section presents issues related to energy production from fossil fuels and renewable sources, types of hydrogen, and the water electrolysis process. The Methodology Section explains how the review article was developed. The Results and Discussion Section discusses the results of the literature review by presenting the findings from the analysis, organizing them into themes or issues, and comparing and contrasting the different studies and findings. The discussion of the results involves interpreting the findings, discussing their implications, and highlighting any gaps or limitations in the current literature. The Conclusions Section summarizes the key points and conclusions drawn, and the Future Directions Section suggests areas for future research. Step VI involved writing a review article that used clear and concise language to present the findings and avoid jargon, defined technical terms for a wider audience, and properly cited all sources using a consistent citation style (MDPI and ACS Style). It also involves the use of visual aids in the form of tables, figures, and graphs to illustrate key points and data. The described scientific methodology was applied to produce a review article on green hydrogen production technologies that is comprehensive, well-organized, and contributes valuable insights to the field.

The specific results of a search on the Scopus database were dependent on the criteria used and the state of the database at the time of search. Desktop analysis was performed using a complex search string to identify relevant documents. The search bar was used to enter keywords, author names, article titles, and other relevant information. Scopus allowed advanced search options, including field-specific searches such as TITLE-ABS-KEY to search titles, abstracts, and keywords. Boolean operators (AND, OR, NOT) to combine or exclude terms for more precise results were used. After the initial search, results were refined by document type, publication year, subject area, source title, author, affiliation, and other criteria. Scopus offers an “Analyze Search Results” feature that provides insights into your search, such as the most cited documents, most productive authors, and most relevant publication sources. The search results could be exported to a citation manager or in various file formats for further analysis. Scopus also offers a plagiarism detection tool.

The search strings related to electrolyzers used to analyze documents containing keywords in titles, abstracts, or keywords published in 2020–2024 are presented with the results obtained in Figure 8. These pie charts illustrate the distribution of different types of publications for each search term from 2020 to 2024, indicating a predominant focus on articles across all categories. The graphics show trends in the types of publications related to electrolyzer research in this period. Articles dominate in all areas of search, representing the majority of publications. This indicates a strong focus on original research. Conference papers constitute the second most common type of publication, especially in the areas of “alkaline electrolyzer” and “proton exchange membrane electrolyzer”. Although less frequent than articles, reviews have a notable presence, especially in the field of “proton exchange membrane electrolyzer”. Book chapters and errata represent a small proportion of the total, indicating that these formats are less used for the dissemination of research in this field. Overall, there is a clear preference for publishing research articles, while conferences and reviews remain important, but to a lesser extent.

4. Results and Discussion

4.1. Tasks and Challenges of Hydrogen Economy towards Green Hydrogen

The main tasks of the HE towards green hydrogen are presented in Figure 9 [99,112,113,114]. R&D research conducted within the functional stages of the HE concerns green hydrogen production (stage I), green hydrogen purification and transportation (stage II), green hydrogen storage (stage III), and conversion of chemical energy of green hydrogen into electricity using fuel cells (stage IV) [8,9,91]. Stage I primarily involves the development of low-emission or emission-free methods of hydrogen production [92,93,94,95]. A concept of green hydrogen production through water electrolysis supporting net zero emissions in the future is shown in Figure 9a [99]. Currently, mainly prototype solutions and technologies are used in hydrogen energy. Their costs must be significantly reduced in widespread use and mass production for hydrogen energy to be competitive with conventional fossil fuel-based energy [91]. Hydrogen can be transferred using gas pipelines as in the case of natural gas (stage II) [115,116,117,118,119], or transported under pressure using tankers (stage III) [105,110,120,121,122,123].

An exemplary pipeline in modern style for green hydrogen long-distance transport is shown in Figure 9b [112]. Current challenges related to the HE focus on the search for new methods of green hydrogen storage (stage III) [96,106,124,125,126] as Figure 9c shows [113], and converting green hydrogen (stage IV) [124,125,126] by fuel cells (Figure 9d) [114]. Hydrogen storage is the most difficult technical barrier to overcome, hindering the implementation of hydrogen technologies on an industrial scale. Presently, expensive pressure vessels and less durable cryogenic tanks are used to store hydrogen. Addressing the challenges of storage efficiency, safety, and cost for large-scale hydrogen applications is crucial for the widespread adoption of hydrogen as a clean energy carrier. Several innovative solutions and strategies are being developed and implemented to overcome these challenges [96,105,106,110,120,121]. The compressed hydrogen storage strategy under development is based on high-pressure tanks and metal hydrides. The liquid hydrogen storage strategy is based on cryogenic storage and insulated tanks. Vacuum-insulated cryogenic tanks are used to minimize heat transfer and boil-off. The underground storage strategy uses salt caverns and geological formations. The chemical storage strategy includes Liquid Organic Hydrogen Carriers (LOHC) and ammonia [127]. A solid-state storage strategy is developed based on Metal–Organic Frameworks (MOFs) and chemical hydrides [96,106,120,128]. Safety considerations concern leak detection and monitoring, fire and explosion prevention, and regulatory compliance. Advanced sensors and monitoring systems are crucial for detecting hydrogen leaks and ensuring the safety of storage facilities. Hydrogen’s wide flammability range and high diffusion rate require stringent safety measures, including inert gas purging and explosion-proof designs. Adherence to international standards and regulations for hydrogen storage and handling is essential to ensure safety and gain public acceptance. Cost reduction strategies are focused on economies of scale, R&D, and policy and incentives [105,110,121]. Large-scale production and storage of hydrogen can benefit from economies of scale, reducing the cost per unit of hydrogen stored. A combination of technological innovations, material advancements, and policy support is needed to address the challenges of storage efficiency, safety, and cost for large-scale hydrogen applications. As the hydrogen economy grows, these solutions will play a critical role in ensuring the reliable and safe storage of hydrogen for various applications.

In stage III, it is necessary to develop innovative methods for storing hydrogen, especially for transport applications [105,120,124]. Progress in technologies for obtaining materials for hydrogen storage is a factor determining the economic success of using hydrogen to power cars. Therefore, R&D research is being conducted on the safe storage of hydrogen in the crystalline structures of metals and their alloys [96,106,120,123,124,129,130,131]. The most important indicator for the use of hydrogen in fuel cells in stage IV is the level of its purity requiring the 5.0 standard. Even the smallest traces of hydrogen contamination may damage the efficiency and durability of the fuel cell system, especially in hydrogen vehicles. Due to the need to reduce emissions in each economic sector, not only the purity of hydrogen but also the origin of the energy used in the hydrogen production process becomes particularly important.

The HE represents a significant shift towards a more sustainable and cleaner energy future. Hydrogen, as an energy carrier, has the potential to revolutionize energy markets by providing a versatile and clean alternative to fossil fuels. Some key insights regarding the HE in terms of energy markets include the following [8,16,74,75,76]:

• Diverse Applications: Hydrogen can be used in various sectors, including transportation, power generation, industrial processes, and heating. This versatility makes it a potential game-changer for decarbonizing energy-intensive industries and sectors that are difficult to electrify.
• Energy Storage and Grid Balancing: Hydrogen can play a crucial role in energy storage, especially for intermittent renewable energy sources like solar and wind. Excess electricity can be used to produce hydrogen through electrolysis, which can then be stored and used to generate power when demand is high or renewable sources are not available.
• Market Growth and Investment: The hydrogen economy is gaining momentum, with significant investments from governments and private sectors worldwide. This includes funding for research and development, infrastructure, and commercial projects. The growth of the hydrogen market is expected to accelerate as technology matures and costs decrease.
• Technological Advancements: Advances in hydrogen production, storage, and utilization technologies are critical for the success of the hydrogen economy. Efficiency improvements and cost reductions in electrolysis, fuel cells, and hydrogen storage are key areas of focus.
• Green vs. Grey Hydrogen: Currently, most hydrogen is produced from natural gas (grey hydrogen), which emits carbon dioxide. The transition to green hydrogen, produced through the electrolysis of water using renewable energy, is essential for the hydrogen economy to be truly sustainable. The cost competitiveness of green hydrogen is improving, but it still faces challenges in terms of scale and infrastructure.
• Infrastructure Development: Building a hydrogen economy requires significant infrastructure development, including production facilities, storage and transportation systems, and refueling stations. This infrastructure must be safe, efficient, and integrated with existing energy systems.
• Policy and Regulation: Government policies and regulations play a crucial role in shaping the hydrogen economy. This includes setting standards, providing incentives for investment and adoption, and creating a supportive regulatory environment for the development of hydrogen technologies and infrastructure.
• Global Collaboration: The hydrogen economy is a global challenge and opportunity. International collaboration is essential for sharing knowledge, harmonizing standards, and creating a global market for hydrogen and hydrogen-based products.
• Economic Impact: The hydrogen economy has the potential to create new industries and jobs while transforming existing ones. It can drive economic growth and provide a competitive edge for early adopters and leaders in hydrogen technology.
• Environmental Benefits: One of the primary motivations for the hydrogen economy is its potential to reduce greenhouse gas emissions and combat climate change. Achieving these environmental benefits requires a concerted effort to scale up green hydrogen production and integrate it into the energy system.

The HE offers a promising pathway to a more sustainable energy future. However, realizing its potential will require overcoming technical, economic, and policy challenges. With continued innovation, investment, and international cooperation, hydrogen can play a pivotal role in transforming energy markets and contributing to a cleaner, more resilient energy system, as was reported recently [74,75,76].

The integration of combined heat and power (CHP) and power-to-heat (P2H) technologies into multi-energy microgrids (MEMGs) represents a sophisticated approach to enhancing energy efficiency and reducing environmental impact [74]. MEMGs, which coordinate the production and consumption of electricity, gas, and heat, offer a promising solution for leveraging renewable energy sources while ensuring reliability and flexibility in energy supply. However, the inherent uncertainties associated with renewable energy generation and variable load demands pose significant challenges to the efficient operation of MEMGs. The proposed two-stage robust operation method for electricity–gas–heat integrated MEMGs represents a significant advancement in the field of multi-energy systems. By effectively addressing uncertainties and coordinating multiple energy carriers, the method ensures a reliable, cost-effective, and environmentally friendly energy supply. The integration of P2HH units and the implementation of a carbon trading mechanism further enhance the sustainability and economic viability of MEMGs. As energy markets continue to evolve, such innovative approaches will play a crucial role in shaping the future of the energy sector. Figure 10 illustrates an integrated energy system that combines various energy sources and loads [74].

The Distribution System connects with the power grid, wind turbines (WT), and traditional power plants (TP). It supplies electricity to meet the electricity load. The Natural Gas System connects gas sources to the natural gas load. It includes components like gas turbines (GT) and microturbines (MR). The Hydrogen System involves an electrolyzer (EL) to produce hydrogen, hydrogen storage (HST), and utilization in hydrogen engines (HE) or hydrogen loads. The Heat System, combined with CHP plants, provides heat to the heat load. In Energy Conversions, electricity and natural gas can be converted and utilized across these systems for efficient energy management. This integration allows for the optimized use of different energy sources to meet varying loads effectively.

The integration of variable RESs into power systems necessitates the development of sophisticated energy storage systems (ESSs) and advanced control strategies to ensure reliability and efficiency [75]. The intermittent nature of RESs, such as wind and solar, introduces significant uncertainties that must be addressed for effective grid management. In this context, the development of a two-stage model for ESSs that incorporates distributionally robust model predictive control (DR-MPC) is a significant advancement. This approach not only respects the nonanticipativity of multistage dispatch but also enables the ESSs to operate in multiple selective modes, thereby optimizing its power interval scheduling. The proposed two-stage model for ESSs and the distributionally robust model predictive control scheme represent significant advancements in the field of energy storage and power system management. By addressing the challenges posed by the uncertainties of RESs and providing a flexible and adaptive framework for ESS operation, this approach can contribute to the development of more reliable, efficient, and sustainable energy systems. The novel algorithm for solving distributionally robust optimization problems further enhances the practical applicability of this framework, demonstrating its potential for widespread adoption in energy markets.

The integration of hydrogen fuel cell vehicles (HFCVs) into transportation networks (TNs) represents a significant step towards decarbonizing the transportation sector. HFCVs, which use hydrogen as a fuel to produce electricity through fuel cells, offer a promising alternative to traditional internal combustion engine vehicles and complement electric vehicles (EVs) in the transition to sustainable transportation [76]. This research proposes a novel approach to the coordinated operation of a networked hydrogen-power-transportation system with distributed hydrogen supplies, aiming to maximize the synergistic effects and overall profits. The coordinated operation of a networked hydrogen-power-transportation system with distributed hydrogen supplies represents a significant advancement in the decarbonization of transportation networks. By integrating hydrogen transport delays, refueling/charging demand, and supply process constraints, the proposed model ensures efficient and profitable operation. The adoption of data-driven robust chance-constrained programming enhances the system’s resilience to uncertainties, making it a practical and effective solution for sustainable transportation. As energy markets and transportation networks continue to evolve, such innovative approaches will play a crucial role in shaping the future of the transportation sector.

4.2. Current Technologies Used for Green Hydrogen Production

4.2.1. Alkaline Electrolyzers

AEs are the most mature and commonly used electrolyzers for industrial-scale hydrogen production [12,16,19,20,21,22,23,132,133,134,135]. Figure 11a shows an alkaline electrolyzer and water isolated [136]. The operating principles of AEs are schematically illustrated in Figure 11b [134]. The operating parameters of AEs are summarized in

Table 3. Operating parameters of AE, where LHV is lower heating value [16].

Parameter	2020	Target 2050	R&D Focus
Nominal current density	0.2–0.8 A cm−2	>2 A cm−2	Diaphragm
Voltage range (limits)	1.4–3 V	<1.7 V	Catalysts
Operating temperature	70–90 °C	>90 °C	Diaphragm, frames, balance of plant components
Cell pressure	<30 bar	>70 bar	Diaphragm, cell, frames
Load range	15–100%	5–300%	Diaphragm
Hydrogen purity	99.9–99.9998%	>99.9999%	Diaphragm
Voltage efficiency (LHV)	50–68%	>70%	Catalysts, temperature
Electrical efficiency (stack)	47–66 kWh kg−1 H2	<42 kWh kg−1 H2	Diaphragm, catalysts
Electrical efficiency (system)	50–78 kWh kg−1 H2	<45 kWh kg−1 H2	Balance of plant
Lifetime (stack)	60,000 h	100,000 h	Electrodes
Stack unit size	1 MW	10 MW	Electrodes
Electrode area	10,000–30,000 cm2	30,000 cm2	Electrodes
Cold start (to nominal load)	<50 min	<30 min	Insulation (design)
Capital costs (stack) minimum 1 MW	USD 270/kW	<USD 100/kW	Electrodes
Capital costs (system) minimum 10 MW	USD 500–1000/kW	<USD 200/kW	Balance of plant

. AEs are characterized by their straightforward design and ease of production, with electrode areas up to 3 m². They utilize a concentrated KOH solution as the electrolyte, zirconium dioxide-based diaphragms, and nickel-coated stainless-steel electrodes. The OH⁻ ion serves as the ionic charge carrier, facilitating the electrochemical reaction. The electrochemical reactions in an AE involve the transfer of electrons and the exchange of ions at the electrodes. At the anode, hydroxide ions react and form water and oxygen gas. The half-reaction releases electrons to the anode, which then flow through an external circuit to the cathode as described in Reaction (2):

$$\mathrm{Anode} \mathrm{reaction} \left(\mathrm{Oxidation}\right):2O{H}^{\_}\to H_{2}O+{\displaystyle \frac{1}{2}}{O}_2+2e^{-}.$$

(2)

At the cathode, the protons that have been produced at the anode, along with electrons from the external circuit, combine to form hydrogen gas. The electrons are supplied to the cathode by the external circuit, completing the electrical circuit according to the half-reaction given in Reaction (3):

Cathode reaction (Reduction): 4H⁺ + 4e⁻ → 2H₂.

(3)

Combining the two half-reactions gives the overall Reaction (4), which shows that water is split into hydrogen and oxygen gases:

Overall reaction: 2H₂O → 2H₂ + O₂.

(4)

In AEs, the alkaline electrolyte facilitates the movement of ions between the electrodes. The hydroxide ions from the electrolyte participate in the reactions at the electrodes. At the anode, hydroxide ions are oxidized to form oxygen gas and water, while at the cathode, they combine with the protons and electrons to form water and hydrogen gas.

Table 3 outlines the operating parameters and cost considerations for the AE technology used for green hydrogen production [16]. In 2020, AE operated with a nominal current density range of 0.2 to 0.8 A cm⁻², a voltage range of 1.4 to 3 V, and a temperature range of 70 to 90 °C. The cell pressure was below 30 bar, with a load range of 15 to 100% and hydrogen purity between 99.9 and 99.9998%. The voltage efficiency based on the lower heating value (LHV) was 50 to 68%, with electrical efficiency for the stack and system at 47 to 66 kWh kg⁻¹ H₂ and 50 to 78 kWh kg⁻¹ H₂, respectively. The stack lifetime was approximately 60,000 h. By 2050, the targets for AE include a nominal current density exceeding 2 A cm⁻², a voltage below 1.7 V, an operating temperature above 90 °C, and a cell pressure greater than 70 bar. The load range is expected to be 5 to 300%, with hydrogen purity exceeding 99.9999%. The voltage efficiency is targeted to be over 70%, with electrical efficiency for the stack and system reducing to less than 42 kWh kg⁻¹ H₂ and 45 kWh kg⁻¹ H₂, respectively. The stack lifetime is projected to increase to 100,000 h. R&D efforts are focused on various components, including the diaphragm, catalysts, cell frames, balance of plant components, and electrodes. The minimum size for a stack unit is 1 MW, with an electrode area of 10,000 to 30,000 cm². The time to reach nominal load from a cold start should be less than 50 min for a 1 MW unit and less than 30 min for a 10 MW unit. Capital costs for the stack in 2020 were USD 270/kW for a minimum 1 MW unit and less than USD 100/kW for a 10 MW unit. For the system, capital costs were between USD 500 and 1000/kW for a minimum 10 MW unit, with a target to reduce this to less than USD 200/kW.

AEs are reliable and have a long operational lifetime, but they typically have lower efficiency and energy density compared to other types of electrolyzers [12,16,19,20,21,22,23,132,133,134,135]. They are known for their robustness and typically operate at lower current densities. However, their design limitations include the intermixing of hydrogen and oxygen gases due to the permeability of the diaphragm, which can be mitigated by using thicker diaphragms or adding spacers, but at the cost of increased resistance and reduced efficiency. Advancements in alkaline electrolyzer technology have seen the introduction of zerogap electrodes, thinner diaphragms, and improved electrocatalysts, which have helped to close the performance gap with PEM technology. Despite these advancements, alkaline systems are known for their reliability and long lifespan. The operation of alkaline electrolyzers involves the recirculation of the KOH electrolyte through the stack, which introduces a pressure drop and requires specific pumping solutions. This recirculation can negatively impact efficiency, with power consumption for pumping typically less than 0.1% of the stack’s power consumption but varying significantly among manufacturers. Some systems operate without the need for pumping peripherals. The produced gases must be separated from the alkaline solution in gas-water separators located above the stack. The water column within these separators acts as a buffer for load changes, and the water management system controls the filling level of each separator. Water permeation through the diaphragm is a factor that must be managed, and a mixing pipe is used to balance the OH⁻ charges consumed or produced during the electrochemical reaction. AEs can operate at high pressure by maintaining both sides of the stack at high pressure within a high-pressure vessel. This configuration requires more robust cell frames and a balance of plant materials, which can increase capital expenditure [16]. However, the need to balance charges between the anode and cathode complicates the operation of the stack at differential pressures compared to PEM technology.

4.2.2. Proton Exchange Membrane Electrolyzers

PEM electrolyzers use a solid polymer electrolyte to conduct protons from the anode to the cathode while keeping the electronic conductivity low to prevent short-circuiting [13,16,24,25,26,27,28,29,30,111,134,137,138,139,140,141]. The electrochemical reactions in PEM electrolyzers are similar to those in AEs but are facilitated by the proton exchange membrane, which plays a crucial role. The membrane also serves to keep the hydrogen and oxygen gases separated to prevent recombination, which is important for safety reasons. A minimalist and photorealistic image of a PEM electrolyzer, showcasing technological sophistication and commercial potential, is shown in Figure 12a [142]. Figure 12b illustrates the operating principles of PEMs [134].

In a PEM electrolyzer, the oxidation reaction occurs at the anode, in which water is oxidized to produce oxygen gas, protons, and electrons in the half-reaction represented as Reaction (5):

Anode reaction (Oxidation): 2H₂O → O₂ + 4H⁺ + 4e⁻.

(5)

Reaction (5) releases electrons to the anode, which then flow through an external circuit to the cathode.

At the cathode, the reduction reaction takes place, where the protons that have been produced at the anode, along with electrons from the external circuit, combine to form hydrogen gas. In the half-reaction, the electrons are supplied to the cathode by the external circuit, completing the electrical circuit as described in Reaction (6):

Cathode reaction (Reduction): 4H⁺ + 4e⁻ → 2H₂.

(6)

Reaction (7) represents the overall electrolysis reaction showing the water splitting into hydrogen and oxygen gases as a result of combining the two half-reactions:

Overall reaction: 2H₂O → 2H₂ + O₂.

(7)

Table 4. Operating parameters of PEMs, where MEA—membrane electrode assembly, PTLs—porous transport layers, BPs—bipolar plates [21].

Parameter	2020	Target 2050	R&D Focus
Nominal current density	1–2 A cm−2	4–6 A cm−2	Design, membrane
Voltage range (limits)	1.4–2.5 V	<1.7 V	Catalyst, membrane
Operating temperature	50–80 °C	80 °C	Effect on durability
Cell pressure	<30 bar	>70 bar	Membrane, reconversion catalysts
Load range	5–120%	5–300%	Membrane
Hydrogen purity	99.9–99.9999%	Same	Membrane
Voltage efficiency (LHV)	50–68%	>80%	Catalysts
Electrical efficiency (stack)	47–66 kWh kg−1 H2	<42 kWh kg−1 H2	Catalysts/membrane
Electrical efficiency (system)	50–83 kWh kg−1 H2	<45 kWh kg−1 H2	Balance of plant
Lifetime (stack)	50,000–80,000 h	100,000–120,000 h	Membrane, catalysts, PTLs
Stack unit size	1 MW	10 MW	MEA, PTL
Electrode area	1500 cm2	>10,000 cm2	MEA, PTL
Cold start (to nominal load)	<20 min	<5 min	Insulation (design)
Capital costs (stack) minimum 1 MW	USD 400/kW	<USD 100/kW	MEA, PTLs, BPs
Capital costs (system) minimum 10 MW	USD 700–1400/kW	<USD 200/kW	Rectifier, water purification

presents the operating parameters of PEM electrolyzers, a key technology for green hydrogen production, as outlined by the International Renewable Energy Agency in their 2020 report [16]. Table 4 compares the nominal current density, voltage range, operating temperature, and cell pressure between the year 2020 and the target for 2050, indicating a significant improvement in efficiency and operational parameters by the latter year. In 2020, PEM electrolyzers operated with a nominal current density of 1–2 A cm⁻², a voltage range of 1.4–2.5 V, a temperature between 50 and 80 °C, and a cell pressure below 30 bar. By 2050, the target is to achieve a higher current density of 4–6 A cm⁻², a reduced voltage of less than 1.7 V, an increased operating temperature of 80 °C, and a higher cell pressure exceeding 70 bar. These advancements are expected to enhance the durability and efficiency of the electrolyzers.

R&D efforts are focused on various aspects such as design improvements, membrane and catalyst technologies, and the impact on durability (Table 4). Specifically, the R&D aims to improve the membrane, reconversion catalysts, and the balance of plant components like the membrane electrode assembly (MEA) and platinum group metal (PGM) catalysts.

Table 4 also outlines the load range, hydrogen purity, voltage efficiency, electrical efficiency, and lifetime of the electrolyzer stack and system. In 2020, the load range was 5–120%, with hydrogen purity varying from 99.9 to 99.9999%. The voltage efficiency was 50–68%, and the electrical efficiency ranged from 47 to 66 kWh kg⁻¹ H₂ for the stack and 50–83 kWh kg⁻¹ H₂ for the system. The stack lifetime was between 50,000 and 80,000 h, with a unit size of 1 MW and an electrode area of 1500 cm². By 2050, the load range is expected to increase to 5–300%, with hydrogen purity maintained, voltage efficiency exceeding 80%, and electrical efficiency dropping below 42 kWh kg⁻¹ H₂ for the stack and 45 kWh kg⁻¹ H₂ for the system. The stack lifetime is projected to extend to 100,000–120,000 h, with larger unit sizes of 10 MW and an electrode area greater than 10,000 cm².

Capital costs for the electrolyzer stack and system are also highlighted in Table 4. In 2020, the minimum capital cost for a 1 MW stack was USD 400/kW, with a target of less than 5 min for a cold start to nominal load. For a 10 MW system, the capital costs ranged from USD 700–1400/kW. Table 4 sets future targets of less than USD 100/kW for the stack and less than USD 200/kW for the system, indicating a significant reduction in costs by 2050. Further insights and developments in PEM electrolyzer technology are discussed more specifically in [16].

PEM electrolyzers operate at low temperatures and can respond more quickly to changes in power supply, making them well-suited for integration with variable RESs like solar and wind [13,16,24,25,26,27,28,29,30,111,134,137,138,139,140,141]. They are more efficient and have a higher power density than AEs, but they can be more expensive due to the use of precious metal catalysts such as platinum and the need for high-purity deionized water. The efficiency of PEM electrolyzers is generally higher than AEs. PEM electrolyzers offer faster response times, higher current densities, and the ability to operate at higher pressures, which can lead to higher efficiencies and potentially lower compression costs for the green hydrogen produced. Despite their higher cost compared to alkaline electrolyzers, PEMs offer a compact and straightforward design that is sensitive to water impurities. The technology is rapidly advancing, with electrode areas increasing and the promise of future large-scale MW stack units.

PEM systems are characterized by their simplicity, requiring only circulation pumps, heat exchangers, and pressure control at the anode side. The cathode side involves a gas separator, de-oxygenation component, gas dryer, and final compressor step. These systems offer design flexibility, including atmospheric, differential, and balanced pressure operations, which can reduce costs and maintenance. Balanced pressure operation keeps the anode and cathode at the same pressure, while atmospheric pressure operation maintains a constant pressure below one standard atmosphere. Differential pressure operation, which can range from 30 to 70 bar, requires a thicker membrane for stability and may need an additional catalyst to reconvert hydrogen that permeates back to water, potentially reducing efficiency.

PEM electrolyzers are efficient and robust systems for hydrogen production, with the potential for large-scale applications. They operate under high-pressure differentials and require specialized materials to withstand harsh conditions. Despite their higher cost and sensitivity to impurities, PEMs offer a simple design with operational flexibility, making them a promising technology for the future of hydrogen production.

4.2.3. Solid Oxide Electrolyzers

SOEs operate at high temperatures, typically between 700 and 850 °C, which allows for the efficient electrolysis of water vapor to produce hydrogen using a solid oxide or ceramic electrolyte [14,16,31,32,33,34,35,36,37,38,39,40,41,42,43,70,71,108,109,134,143,144]. They can achieve higher efficiency by using some of the heat energy in addition to electrical energy, which is particularly useful when integrated with industrial processes that produce excess heat. The efficiency of SOEs is high, potentially up to 85% or more on an HHV basis, due to their ability to utilize high-temperature heat. SOEs operate at very high temperatures, which allows for the integration of waste heat from industrial processes or solar thermal energy, improving overall efficiency. They are still in the development stage and are not yet widely used commercially. Figure 13a shows the world’s largest SOE installed at the NASA facility in California [145]. This 4 MW unit will be 20–25% more efficient than same-sized commercially available alkaline or PEM electrolyzers. A schematic illustration of SOE operating principles is presented in Figure 13b [134].

The electrochemical reactions in an SOE occur at the interfaces between the electrodes and the electrolyte. At the cathode, the reduction reaction occurs, in which water molecules from the cathode side are reduced to form hydrogen gas and oxygen ions. The O²⁻ ions then migrate through the solid oxide electrolyte towards the anode. The half-reaction is represented as Reaction (8):

Cathode reaction (Reduction): H₂O + 2e⁻ → H₂ + O²⁻.

(8)

At the anode, the oxygen ions reaching the anode release their electrons to the anode, completing the electrical circuit. These OH⁻ ions then combine to form oxygen gas. The half-reaction related to oxidation at the anode is represented by Reaction (9):

Anode reaction (Oxidation): 4O²⁻ → 2O₂ + 8e⁻.

(9)

Reaction (10) shows the overall electrolysis reaction in an SOE combining the reactions at the cathode and anode:

Overall reaction: 2H₂O → 2H₂ + O₂.

(10)

This reaction shows that water is split into hydrogen and oxygen gases.

Table 5. Operating parameters of SOEs [16].

Parameter	2020	Target 2050	R&D Focus
Nominal current density	0.3–1 A cm−2	>2 cm−2	Electrolyte, electrodes
Voltage range (limits)	1.0–1.5 V	<1.48 V	Catalysts
Operating temperature	700–850 °C	<600 °C	Electrolyte
Cell pressure	1 bar	>20 bar	Electrolyte, electrodes
Load range	30–125%	0–200%	Electrolyte, electrodes
Hydrogen purity	99.9%	>99.9999%	Electrolyte, electrodes
Voltage efficiency (LHV)	75–85%	>85%	Catalysts
Electrical efficiency (stack)	35–50 kWh kg−1 H2	<35 kWh kg−1 H2	Electrolyte, electrodes
Electrical efficiency (system)	40–50 kWh kg−1 H2	<40 kWh kg−1 H2	Balance of plant
Lifetime (stack)	<20,000 h	80,000 h	All
Stack unit size	5 kW	200 kW	All
Electrode area	200 cm2	500 cm2	All
Cold start (to nominal load)	>600 min	<300 min	Insulation (design)
Capital costs (stack) minimum 1 MW	>USD 2000/kW	<USD 200/kW	Electrolyte, electrodes
Capital costs (system) minimum 10 MW	Unknown	<USD 300/kW	All

outlines the specifications and targets for SOEs for the production of green hydrogen, comparing the parameters in 2020 with the projected targets for 2050 [16].

Based on key parameters, it can be stated that nominal current density is expected to increase from 0.3 to 1 A cm⁻² in 2020 to over 2 cm⁻² by 2050. The voltage range is to be reduced from an upper limit of 1.5 V in 2020 to less than 1.48 V by 2050. The aim is to reduce the operating temperature from 700 to 850 °C to less than 600 °C. Cell pressure is to increase from 1 bar to over 20 bar. The load range is to be expanded from 30 to 125% to 0–200%. Hydrogen purity is to be improved from 99.9% to over 99.9999%. Voltage efficiency (LHV) and electrical efficiency (stack and system) both target improvements of more than 85% and reductions in energy consumption to less than 35 kWh kg⁻¹ H₂ and less than 40 kWh kg⁻¹ H₂, respectively. Stack life will increase significantly from less than 20,000 h to 80,000 h. Stack unit size is expected to be increased from 5 kW to 200 kW. R&D focus areas are identified for each parameter, with a particular emphasis on electrolytes, electrodes, catalysts, and the balance of plant. Additionally, Table 5 notes the importance of reducing capital costs for both the stack and the system, with targets of less than USD 200/kW for the stack and less than USD 300/kW for the system for a minimum size of 1 MW and 10 MW, respectively.

Traditionally, SOEs use an oxygen-ion-conducting electrolyte with oxygen ions moving through the electrolyte from the cathode to the anode (O-SOEs), where they combine with electrons and hydrogen to form water and release hydrogen gas. However, recently there has been increasing research interest in an alternative prototype operating based on proton-conducting electrolyte solid oxide electrolyzers (H-SOEs) instead of O-SOEs, which could lower the operating temperature to approximately 400–700 °C and even below 400 °C due to the high proton mobility [67,146,147,148,149,150,151,152,153,154]. In H-SOEs, protons are the charge carriers that move through the electrolyte from the anode to the cathode, where they combine with electrons from the electrical circuit to form hydrogen gas.

Proton-conducting electrolytes in H-SOEs are typically made from materials that can conduct hydrogen ions at elevated temperatures [152,153,154]. These materials are often ceramic oxides that have been doped to create protonic defects, which allow for proton conduction. Recently, 3D printing technology has been proposed for the production of ceramic oxides for H-SOEs [155]. Figure 14 presents some of the most commonly used materials for proton-conducting electrolytes in H-SOEs.

In H-SOEs, perovskite oxides with the general formula ABO₃ are mainly used, where A and B represent two different cations, and O represents oxygen anions [152,153,154,155,156,157,158,159,160]. A larger A-site cation is often an alkali metal, alkaline earth metal, or rare earth element, and a smaller B-site cation is typically a transition metal. The perovskite structure is characterized by a cubic or pseudocubic arrangement of oxygen anions, with the A cations occupying the larger interstitial sites and the B cations occupying the centers of the oxygen octahedra. The oxygen anions typically form a cubic close-packed (ccp) or face-centered cubic (fcc) arrangement, and the A and B cations occupy specific sites within this arrangement. There are several methods to improve the proton conductivity and chemical stability of perovskite oxides for H-SOEs by introducing defects into the crystal structure and modifying the electronic and ionic transport properties, including doping, codoping, hydrogenation, surface modification, grain boundary engineering, and the use of composite materials. Doping the A-site with acceptor dopants like alkaline earth metals can create oxygen vacancies, facilitating proton transport via the Grotthuss mechanism [161]. Codoping the A and B sites can further improve proton conductivity and stability by balancing oxygen vacancies and excess ions. Hydrogenation treatment introduces hydroxyl groups into the perovskite structure, acting as proton donors and enhancing conductivity. Surface modification with catalytic materials like Pt or Pd can improve proton exchange kinetics, while grain boundary engineering through sintering and additives can reduce resistance and enhance conductivity. Microstructural engineering, such as creating composite materials with other proton conductors, can provide additional proton transport pathways. To enhance the chemical stability of the perovskite oxides, the choice of dopants and the doping level are critical. Dopants that do not significantly disrupt the perovskite structure and maintain a high tolerance factor are preferred. Additionally, doping strategies that minimize the formation of secondary phases and maintain structural integrity under operating conditions are essential for achieving high chemical stability.

Perovskite oxides, such as those based on barium zirconate (BaZrO₃) and barium cerate (BaCeO₃), exhibit high proton conductivity at intermediate temperatures, which is crucial for efficient electrolysis [152,153,154,155,156,157,158,159,160]. The electrolyte must be stable under the reducing conditions at the cathode and the oxidizing conditions at the anode. Perovskite oxides are generally stable in a wide range of oxygen partial pressures. For efficient electrolysis, the electrolyte should have low electronic conductivity to minimize ohmic losses and ensure that the current is carried predominantly by protons. The properties of perovskite oxides can be tailored by doping the A or B sites with different elements. The electrolyte material must be easily sintered to form a dense layer that prevents gas leakage and has sufficient mechanical strength to withstand thermal cycling and operating conditions. The cost and availability of the materials used in the perovskite structure are important considerations for commercial viability. Researchers are exploring alternative cations that are less expensive and more abundant. The electrolyte must be compatible with the electrodes to form stable interfaces that allow for efficient charge transfer.

Research into perovskite oxides for proton-conducting electrolytes in H-SOEs is an active area of study [152,153]. Scientists are exploring new compositions, doping strategies, and processing techniques to further enhance their proton conductivity and stability under operating conditions. The goal is to develop materials that can improve the efficiency and cost-effectiveness of hydrogen production through electrolysis. The choice of material depends on the desired operating temperature, chemical stability, mechanical properties, and cost. Advances in materials science, processing techniques, and understanding of proton conduction mechanisms are expected to further enhance the viability of these materials for large-scale hydrogen production [161].

Brownmillerite oxides are another class of materials that have garnered interest for their potential use as proton-conducting electrolytes in H-SOEs (Figure 14). These oxides have a general formula of A₂B₂O₅, where A and B are typically metal cations, with A being larger and occupying eight-coordinated sites, and B being smaller and occupying four- and six-coordinated sites [152,153,156,162,163,164]. A is typically an alkali metal, alkaline earth metal, or rare earth element, and B is a transition metal. The structure of brownmillerite oxides consists of layers of corner-shared BO₆ octahedra and BO₄ tetrahedra, with the A cations located between these layers. Doping of brownmillerite oxides can occur at the A-site (acceptor doping) or B-site (donor doping). A-site doping with alkaline earth metals creates oxygen vacancies, facilitating proton transport via the Grotthuss mechanism [161]. B-site doping with trivalent cations enhances proton uptake and conductivity. Codoping at both sites optimizes the concentration of oxygen vacancies and excess ions, improving conductivity and stability. Hydrogenation of brownmillerite oxides introduces hydroxyl groups into the structure, acting as proton donors and enhancing conductivity. Surface modification with catalytic materials like Pt or Pd improves proton exchange kinetics. Grain boundary engineering reduces resistance, and microstructural engineering, such as creating composite materials with other proton conductors, provides additional transport pathways. Brownmillerite oxides must have good sinterability and adequate mechanical strength to withstand thermal cycling and operating stresses [152,153,156]. The electrolyte must be compatible with the electrode materials to ensure good contact and minimize interfacial resistance. This includes thermal expansion match and chemical compatibility. The cost and availability of the constituent elements are important considerations for the commercial viability of brownmillerite oxides as electrolyte materials. Examples of brownmillerite oxides that have been recently investigated for proton conductivity include materials such as Ca₂Fe₂O5 [162], Ba₂In₂O₅(H₂O)_x (x = 0.30 and 0.92) [163], and Ba₂In_1.8Si_0.2O_5.1 [164]. These materials have shown promising proton conductivity at intermediate temperatures, making them potential candidates for use in H-SOEs.

To enhance the chemical stability of brownmillerite oxides, the choice of dopants and the doping level are critical. Dopants that do not significantly disrupt the brownmillerite structure and maintain a high tolerance factor are preferred. Additionally, doping strategies that minimize the formation of secondary phases and maintain structural integrity under operating conditions are essential for achieving high chemical stability. Research into brownmillerite oxides for proton-conducting electrolytes is ongoing, with efforts focused on optimizing their composition, microstructure, and processing conditions to enhance their performance and stability [162,163,164]. The goal is to develop materials that can operate efficiently at lower temperatures, thereby reducing the cost and improving the durability of H-SOEs for green hydrogen production.

In addition to perovskite and brownmillerite oxides, several other types of oxides have been explored for their potential as proton-conducting electrolytes in H-SOEs (Figure 14). These materials include inter alia NASICON (Na Super Ionic Conductor)-type oxides with the general formula AM₂(XO₄)₃, where the symbols A and M represent different cations that occupy specific sites within the crystal structure [165,166,167]. A is a monovalent cation, which can be a proton in the case of proton-conducting electrolytes. M is a divalent cation, often a transition metal such as Zr, Ti, Ge, or Hf. X is a trivalent cation, such as P or S. O is oxygen and xH₂O represents the presence of water molecules that can contribute to proton conductivity through the formation of hydroxyl groups within the structure. The structure of NASICON-type oxides is based on a three-dimensional network of MO₆ octahedra (where M is the divalent cation) and XO₄ tetrahedra (where X is the trivalent cation), which creates a framework with interstitial sites that can accommodate the monovalent cations (A). These materials are known for their ability to conduct ions (such as Na+ or Li+) through these interstitial sites, making them useful for applications that require fast ion transport.

The NASICON structure can accommodate protonic defects, such as hydroxyl groups or water molecules, which are essential for proton conductivity. The presence of these defects can facilitate proton transport through mechanisms such as the Grotthuss mechanism [161]. NASICON-type oxides can absorb water from the atmosphere or an operating environment, leading to the formation of hydroxyl groups or water molecules within the structure. This hydration can enhance proton conductivity. Under humid conditions and at intermediate temperatures (~100–300 °C), NASICON-type oxides can exhibit significant proton conductivity. The proton conductivity is often attributed to the mobility of protonic defects within the crystal structure. NASICON-type oxides can exhibit good chemical stability under the operating conditions of SOEs, which is crucial for the long-term performance and durability of the electrolyzer. One of the most well-known NASICON-type oxides is Na₃Zr₂Si₂PO₁₂, where A = Na, M = Zr, and X = Si, P [165]. Another recent example of a proton-conducting NASICON-type oxide with superionic conduction is Li₃Zr₂Si₂PO₁₂ [165,166] and proton conductor NASICON-structure Li_1+xCd_x/2Zr_2−x/2(PO₄)₃ [167]. Doping can occur at the A-site (acceptor doping) or M-site (donor doping). A-site doping involves introducing alkaline earth metals to create oxygen vacancies, facilitating proton transport via the Grotthuss mechanism [161]. M-site doping includes adding trivalent cations to enhance proton uptake and conductivity. Codoping, combining both strategies, optimizes oxygen vacancies and excess ions for improved conductivity and stability. Incorporation of proton donors, such as through hydrogenation, introduces hydroxyl groups into the structure, acting as proton donors. Surface modification with catalytic materials enhances proton exchange kinetics. Microstructural engineering, including grain boundary engineering, reduces resistance and improves conductivity. Composite electrolytes, combining NASICON-type oxides with other proton-conducting materials, provide additional transport pathways for protons.

To enhance chemical stability, the choice of dopants and the doping level are critical. Dopants that do not significantly disrupt the NASICON structure and maintain a high tolerance factor are preferred. Additionally, doping strategies that minimize the formation of secondary phases and maintain structural integrity under operating conditions are essential for achieving high chemical stability. Research into NASICON-type oxides for proton-conducting electrolytes in H-SOEs is an active area of study. Scientists are exploring new compositions, doping strategies, and processing techniques to further enhance their proton conductivity and stability under operating conditions [165,166,167]. The goal is to develop materials that can improve the efficiency and cost-effectiveness of green hydrogen production through electrolysis.

Each of these oxide families has unique structural features and properties that can be tailored through doping and processing to enhance proton conductivity [67,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167]. The choice of material depends on various factors, including conductivity, stability, cost, and compatibility with other cell components. The use of H-SOEs offers several potential advantages as lower operating temperatures, higher efficiency, better integration with certain fuels, and simplified system design [165,166,167]. Proton-conducting electrolytes can operate at lower temperatures than oxygen-ion-conducting electrolytes, which can reduce thermal stresses and potentially allow for the use of less expensive materials. The proton-conducting mechanism can be more efficient because it avoids the need for the electrolyte to transport bulky oxygen ions, which can lead to lower energy losses. H-SOEs can be designed to co-electrolyze water and carbon dioxide, producing syngas as a mixture of hydrogen and carbon monoxide, which is a valuable feedstock for the production of synthetic fuels and chemicals. In H-SOEs, the direct production of hydrogen at the cathode can simplify the gas separation process, as there is no need to separate hydrogen from water vapor as in the case of oxygen-ion-conducting electrolyzers. Despite these advantages, H-SOEs also face challenges, such as finding materials that can withstand the harsh conditions of high-temperature operation while maintaining proton conductivity. Research and development in this area are ongoing with the aim of developing materials that offer higher conductivity, operate at lower temperatures, and are more durable and cost-effective [67,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167]. This research is critical for the advancement of clean energy technologies and reducing greenhouse gas emissions.

To sum up, SOEs are high-temperature devices offering several advantages [14,16,31,32,33,34,35,36,37,38,39,40,41,42,43,67,70,71,108,109,134,143,144,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167]. The elevated temperatures facilitate favorable chemical reactions, allowing for the use of inexpensive nickel electrodes and a reduction in electricity demand due to the utilization of waste heat, which can lead to apparent efficiencies exceeding 100%. Additionally, SOEs have the potential to operate in reverse as fuel cells and can facilitate the co-electrolysis of CO₂ and water to produce syngas, a critical feedstock for the chemical industry [31,36,38,40,42,146,150,162]. Despite these benefits, SOEs face challenges such as accelerated degradation and reduced lifespan due to thermo-chemical cycling, particularly during shutdowns and ramping periods. Other issues include difficulties with sealing at high differential pressures, electrode contamination from sealants like silica, and other contaminants from system components. Currently, SOEs are deployed at a small scale, with some demonstration projects reaching 1 MW. SOEs can be integrated with heat-generating technologies to enhance system efficiency, as the energy demand for water electrolysis decreases with increasing temperature due to Joule heating. This allows for the use of external heat sources, such as industrial waste heat or concentrated solar power, to drive the water-splitting reaction. A notable renewable application is the coupling of SOEs with concentrated solar power, which can provide both the electricity and heat required for the electrolysis process. SOEs offer benefits like using inexpensive electrodes and waste heat for increased efficiency, but face challenges in scaling up and maintaining performance due to material degradation, sealing difficulties, and system integration complexities.

4.2.4. Anion Exchange Membrane Electrolyzers

AEM electrolyzers are a newer technology that aims to combine the advantages of AEs and PEM electrolyzers [15,16,44,45,46,47,48,49,50,51,52,107,134,168,169,170,171,172,173]. AEM electrolyzers use a solid polymer electrolyte that conducts hydroxide ions from the cathode to the anode and can operate with less pure water than PEM electrolyzers. Figure 15a presents the AEM Flex 120, with a configuration from 70 to 480 kW, which enables the streamlined launch of green hydrogen in pilot projects from industrial process heat to refueling [173]. This modular AEM ensures reactivity to intermittent renewable energy, built-in redundancy, and easy scalability. A diagram of the AEM operating principles is shown in Figure 15b [134].

An AEM electrolyzer uses an anion exchange membrane as the electrolyte, which selectively conducts anions such as OH⁻ ions while blocking cations. The AEM electrolyzer operates similarly to a PEM electrolyzer but with the charge carriers reversed. In an AEM electrolyzer, anions migrate from the cathode to the anode, while in a PEM electrolyzer, cations (protons) migrate from the anode to the cathode.

In an AEM electrolyzer, hydroxide ions and electrons react to water and oxygen gas at the anode. The half-reaction is represented as Reaction (11):

$$\mathrm{Anode} \mathrm{reaction} \left(\mathrm{Oxidation}\right):2O{H}^{-}\to H_{2}O+{\displaystyle \frac{1}{2}}{O}_2+2e^{-}.$$

(11)

At the cathode, water molecules are reduced to form hydrogen gas and hydroxide ions. The half-reaction is described by Reaction (12):

Cathode reaction (Reduction): 2H₂O + 2e⁻ → H₂ + 2OH⁻.

(12)

The electrons are supplied to the cathode by the external circuit, completing the electrical circuit. Combining the two half-reactions leads to Reaction (13) that represents the overall electrolysis reaction, showing that water is split into hydrogen and oxygen gases (Reaction (13)):

Overall reaction: 2H₂O → 2H₂ + O₂.

(13)

In an AEM electrolyzer, the anion exchange membrane plays a crucial role by allowing hydroxide ions to pass through while blocking other ions and gases. This ensures that the hydrogen produced at the cathode is pure and that the oxygen produced at the anode is kept separate, which is important for safety and efficiency.

Table 6. Operating parameters of AEMs, where MEA—membrane electrode assembly [16].

Parameter	2020	Target 2050	R&D Focus
Nominal current density	0.2–2 A cm−2	>2 A cm−2	Membrane, reconversion
Voltage range (limits)	1.4–2.0 V	<2 V	Catalyst
Operating temperature	40–60 °C	80 °C	Effect on durability
Cell pressure	<35 bar	>70 bar	Membrane
Load range	5–100%	5–200%	Membrane
Hydrogen purity	99.9–99.999%	>99.9999%	Membrane
Voltage efficiency (LHV)	52–67%	>75%	Catalysts
Electrical efficiency (stack)	51.5–66 kWh kg−1 H2	<42 kWh kg−1 H2	Catalysts/membrane
Electrical efficiency (system)	57–69 kWh kg−1 H2	<45 kWh kg−1 H2	Balance of plant
Lifetime (stack)	>5000 h	100,000 h	Membrane, electrodes
Stack unit size	2.5 kW	2 MW	MEA
Electrode area	<300 cm2	1000 cm2	MEA
Cold start (to nominal load)	<20 min	<5 min	Insulation (design)
Capital costs (stack) minimum 1 MW	Unknown	<USD 100/kW	MEA
Capital costs (system) minimum 10 MW	Unknown	<USD 200/kW	Rectifier

presents operating parameters for AEM technology and provides a comparison between the current (2020) and target (2050) operating conditions for AEM electrolyzers, highlighting the significant improvements needed to meet the climate objective [16]. In 2020, AEM electrolyzers operated with a nominal current density range of 0.2 to 2 A cm⁻², a voltage range of 1.4 to 2.0 V, and a temperature between 40 and 60 °C. The cell pressure was below 35 bar, with a load range of 5 to 100%, producing hydrogen with a purity between 99.9 and 99.999%. The voltage efficiency based on the LHV was 52 to 67%, and the electrical efficiency for the stack and system was 51.5 to 66 kWh kg⁻¹ H₂ and 57 to 69 kWh kg⁻¹ H₂, respectively. The stack lifetime exceeded 5000 h, with a unit size of 2.5 kW. By 2050, the target operating parameters aim to increase the nominal current density to over 2 A cm⁻², reduce the voltage to less than 2 V, and operate at a higher temperature of 80 °C and pressure above 70 bar. The load range is expected to expand to 5 to 200%, with hydrogen purity exceeding 99.9999%. The voltage efficiency is targeted to be over 75%, with electrical efficiency reducing the energy consumption to less than 42 kWh kg⁻¹ H₂ for the stack and less than 45 kWh kg⁻¹ H₂ for the system. The stack lifetime should reach 100,000 h, and the unit size is projected to be 2 MW.

R&D efforts are focused on improving the membrane, catalysts, durability, and the balance of plant components. The electrode area for a cold start to nominal load should be less than 300 cm² for a time under 20 min or less than 1000 cm² for under 5 min. The capital costs for a stack with a minimum size of 1 MW are currently unknown but are expected to be less than USD 100/kW, while the system costs for a minimum of 10 MW are also unknown but targeted to be less than USD 200/kW. The need for substantial advancements in AEM technology to achieve the desired cost reductions and efficiency improvements for green hydrogen production, which is crucial for meeting the stringent climate goals set for 2050 is emphasized [16].

AEM electrolyzers are of interest due to their potential to use less expensive materials compared to PEM electrolyzers, as they can operate with non-noble metal catalysts and may allow for the use of a wider range of membrane materials. However, they face challenges such as the stability of the membrane and electrolyte in the highly alkaline environment and the management of the gas and water interfaces within the cell. R&D efforts in this area are ongoing to improve the performance and durability of AEM electrolyzers [174,175,176,177,178,179,180,181,182,183].

AEMs represent a nascent technology in the realm of electrolyzers, with only a handful of companies currently commercializing the product [173]. AEMs offer a promising synthesis of the milder conditions found in alkaline electrolyzers and the efficiency of PEM electrolyzers. This technology allows for the use of non-precious catalysts and titanium-free components, and like PEM, can operate under differential pressure. However, AEMs currently face challenges with chemical and mechanical stability, which results in inconsistent lifespan and performance that falls short of expectations. These issues are primarily due to low membrane conductivity, suboptimal electrode design, and sluggish catalyst kinetics. Efforts to enhance performance, such as adjusting membrane conductivity or introducing supporting electrolytes, may inadvertently compromise durability. The intrinsic lower conductivity of hydroxide ions compared to protons in PEM necessitates either thinner membranes or those with higher charge density for AEMs.

Despite these challenges, AEM electrolyzers share many design principles with PEM electrolyzers. Nonetheless, the immaturity of AEM technology means there is a dearth of information on the difficulties associated with high differential pressure operation. Improvements are anticipated in the robustness of AEM membranes, the purity of the gas produced, the ability to endure high-pressure differentials, and the expansion of the power range beyond what AEs can offer. However, AEM electrolyzers are currently constrained to a narrower power input range compared to PEM electrolyzers, with the limitation stemming not from the stack itself but from the sizing of the balance of plant [16].

5. Conclusions

Water electrolysis is a key process for producing green hydrogen. A comparative analysis of alkaline electrolyzers, PEM electrolyzers, solid oxide electrolyzers, and AEM electrolyzers evaluated based on several criteria, including the electrolyte used, electrode materials, efficiency, capital and operating costs, durability, flexibility, temperature range, scalability, environmental impact, and advantages and challenges revealed that each type of electrolyzer has its unique features, making them suitable for different applications (

Table 7. Comparative summary of electrolyzers used in the electrolysis of water to produce green hydrogen: criteria (grey color), alkaline electrolyzers (red color) [12,16,19,20,21,22,23,132,133,134,135], PEM electrolyzers (green color) [13,16,24,25,26,27,28,29,30,111,134,137,138,139,140,141], solid oxide electrolyzers (dark blue color) [14,16,31,32,33,34,35,36,37,38,39,40,41,42,43,67,70,71,108,109,134,143,144,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167] and AEM electrolyzers (light blue color) [15,16,44,45,46,47,48,49,50,51,52,107,134,168,169,170,171,172,173,176,177,178,179,180,181,182,183].

Criteria	Alkaline Electrolyzers	PEM Electrolyzers	Solid Oxide Electrolyzers	AEM Electrolyzers
Principle	Uses an alkaline electrolyte (e.g., KOH) to facilitate the electrolysis of water	Uses a solid polymer electrolyte membrane (e.g., Nafion) that selectively conducts protons	Operates at high temperatures (700–850 °C) using a solid oxide electrolyte that conducts oxygen ions	Similar to PEM electrolysis but uses an anion exchange membrane that conducts hydroxide ions
Electrolyte	Liquid alkaline solution	Solid polymer	Solid ceramic material, typically YSZ	Solid polymer membrane
Electrodes	Typically made of Ni or Ni-based alloys	Typically made of Pt or Pt-coated materials for the cathode and Ir or Ir oxide for the anode	Typically made of nickel-zirconia cermet for the cathode and LSM or LSC for the anode	Typically made of Ni or Ni-based alloys
Efficiency	Moderate	High	Very High	Moderate to High
Capital Cost	Low	High	Moderate to High	Low to Moderate
Operating Cost	Moderate	Moderate to High	Moderate to High	Low to Moderate
Durability	High	Moderate	Moderate	Moderate
Flexibility	Low	High	Low	Moderate
Temperature Range	Ambient	Ambient	High	Ambient
Scalability	High	High	Moderate	High
Environmental Impact	Low	Low	Low	Low
Advantages	Proven technology with a long history of industrial use Relatively low capital cost High efficiency at large scales	High efficiency due to low electrical resistance Fast response time, allowing for dynamic operation High current densities	High efficiency due to high operating temperatures, which can be used to utilize waste heat Can operate in reverse as a SOFC Potential for integration with high-temperature processes	Lower cost compared to PEM due to the use of less expensive materials Potential for higher efficiency due to lower electrical resistance Compatible with alkaline electrolytes, which are less corrosive
Challenges	Lower efficiency compared to some other methods Corrosive electrolyte requires robust materials Limited dynamic operation due to slower response times	High cost of materials, especially platinum and iridium Limited durability of membrane and electrodes Requires pure water to prevent membrane degradation	High operating temperatures require specialized materials Slow startup and shutdown processes Limited dynamic operation due to thermal stresses	Limited durability and stability of membranes and electrodes Lower current densities compared to PEM Requires high-purity water to prevent membrane degradation

). AEs use a liquid alkaline solution as the electrolyte and Ni-based electrodes. They are characterized by moderate efficiency, low capital and operating costs, high durability, and low environmental impact. However, they have lower efficiency compared to some other methods, require robust materials due to the corrosive electrolyte, and have limited dynamic operation [12,16,19,20,21,22,23,132,133,134,135]. PEM electrolyzers employ a solid polymer electrolyte membrane and use expensive materials like Pt and Ir for the electrodes. They offer high efficiency, fast response times, and high current densities but are challenged by the high cost of materials, limited durability, and the requirement for pure water to prevent membrane degradation [13,16,24,25,26,27,28,29,30,111,134,137,138,139,140,141]. SOEs operate at high temperatures, using a solid ceramic material as the electrolyte and specialized electrode materials. They boast high efficiency due to high operating temperatures, the ability to utilize waste heat, and the potential for integration with high-temperature processes. However, they require specialized materials, have slow startup and shutdown processes, and limited dynamic operation due to thermal stresses [14,16,31,32,33,34,35,36,37,38,39,40,41,42,43,67,70,71,108,109,134,143,144,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167]. AEM electrolyzers, similar to PEMs, use a solid polymer membrane but conduct hydroxide ions. They offer a lower cost due to less expensive materials, the potential for higher efficiency, and compatibility with less corrosive alkaline electrolytes. However, they suffer from limited durability and stability of membranes and electrodes, lower current densities, and require high-purity water to prevent membrane degradation [15,16,44,45,46,47,48,49,50,51,52,107,134,168,169,170,171,172,173,176,177,178,179,180,181,182,183].

The choice of technology for green hydrogen production is influenced by several key factors that vary based on the application and scale of green hydrogen production. The primary considerations concern the scale of production [16,72,91]. PEM electrolyzers are often preferred for small-scale applications due to their fast response times and high current densities, making them suitable for variable renewable energy sources. The ability to add or remove modules easily is crucial for scaling up or down based on demand or available renewable energy. AEs are commonly used due to their lower capital costs and proven technology. However, PEM electrolyzers are gaining popularity for their higher efficiency and energy density. The choice between wind, solar, hydro, or a combination thereof depends on the local availability and cost of these resources. SOEs or AEs are often considered for large-scale applications due to their potential for high efficiency and lower operational costs. The ability to integrate with large-scale renewable energy projects, such as wind farms or solar parks, is crucial for maximizing the use of renewable electricity [74,75,76].

The decreasing cost and increasing efficiency of renewable energy technologies are key drivers for the reduction in the cost of green hydrogen production [16,91,174]. Advances in electrolysis technologies, improvements in electrode and membrane materials, and innovative system designs are further contributing to this trend. Additionally, supportive policies, integration with RESs, and cost-reduction strategies are playing crucial roles in making green hydrogen a more competitive and sustainable energy carrier. As these trends continue, green hydrogen is expected to become increasingly viable for a wide range of applications, supporting the global transition to a low-carbon economy.

Ensuring a stable and continuous energy supply for electrolysis is crucial for the efficient and cost-effective production of green hydrogen. Key challenges concern the intermittency of RESs, grid integration and stability, energy storage, and management as well [74,75,76]. Wind and solar energy are inherently intermittent, depending on weather conditions and time of day. This variability can lead to fluctuations in power supply to electrolyzers. Seasonal changes can significantly affect the availability of renewable energy, particularly for solar and wind sources. The existing electricity grid may not be capable of handling large amounts of variable renewable energy, leading to curtailment or instability. Transporting large amounts of electricity from renewable sources to electrolysis plants can be challenging and costly, especially over long distances. Storing excess renewable energy for periods of low generation is challenging due to the limited capacity and high cost of current energy storage technologies. Energy conversion and storage processes can lead to significant efficiency losses, reducing the overall effectiveness of the energy supply for electrolysis.

Potential solutions foresee diversification of RESs, development of energy storage technologies, grid upgrades and flexibility, demand response and load management, policy and regulatory support, and R&D [9,16,72,73,91]. Ongoing research into new materials, energy storage technologies, and grid management systems can lead to breakthroughs that improve the stability and efficiency of the energy supply for electrolysis. Implementing pilot projects and demonstrations can provide valuable insights into the practical challenges and solutions for ensuring a stable energy supply for electrolysis. By addressing these challenges through a combination of technological innovations, infrastructure upgrades, and policy support, it is possible to ensure a stable and continuous energy supply for electrolysis, thereby facilitating the large-scale production of green hydrogen.

6. Future Directions

The future direction of green hydrogen production is pivotal for transitioning to a sustainable and carbon-neutral energy system [9,16,71,72,73,91]. Green hydrogen, produced through the electrolysis of water using RES such as solar, wind, or hydroelectric power, offers a clean alternative to traditional hydrogen production methods that rely on fossil fuels. The key future directions in green hydrogen production include efficiency improvements in electrolysis technology, integration with RES, scaling up production, reducing costs, developing end-use applications, international collaboration and policy support, and innovative business models, as well as environmental and social considerations [16,72,74,75,76,91].

Advancements in electrolyzer technology are crucial for reducing the cost and increasing the efficiency of green hydrogen production [143]. This includes the development of more durable and efficient electrolyzer materials that can operate at higher current densities and lower energy consumption. Enhancing the integration of green hydrogen production with variable RES is essential. This involves developing smart grid technologies and energy storage solutions that can manage the intermittent nature of renewable energy, ensuring a consistent supply of electricity for hydrogen production. Scaling up green hydrogen production from pilot and demonstration projects to large-scale commercial operations is necessary to meet the growing demand for clean energy. This includes building larger electrolysis plants and establishing infrastructure for hydrogen transport and storage. Reducing the cost of green hydrogen to be competitive with hydrogen produced from fossil fuels is a major goal. This can be achieved through technological advancements, economies of scale, and policy support mechanisms such as subsidies and carbon pricing. Expanding the use of green hydrogen in various sectors, including transportation, industry, and power generation, is crucial. This involves developing fuel cells for vehicles, using hydrogen in industrial processes as a feedstock or fuel, and incorporating hydrogen into the gas grid for heating and power generation. International collaboration on the research, development, and deployment of green hydrogen technologies is important for sharing knowledge and best practices [121,122].

Additionally, supportive policies such as renewable energy targets, carbon taxes, and funding for research and development can accelerate the adoption of green hydrogen [9,98]. Developing innovative business models that can finance the high upfront costs of green hydrogen production and infrastructure development is necessary. This includes public–private partnerships, green bonds, and other financial instruments that can attract investment. Ensuring that the production and use of green hydrogen are environmentally and socially sustainable is important. This includes assessing the lifecycle emissions of green hydrogen, ensuring water usage for electrolysis is sustainable, and considering the social impacts of hydrogen production and use. The future of green hydrogen production is promising, with the potential to play a significant role in decarbonizing the global economy. However, realizing this potential will require concerted efforts in technology development, infrastructure investment, and policy support as part of the further development of HE [8,73,147].