Green Hydrogen in Focus: A Review of Production Technologies, Policy Impact, and Market Developments

Abstract

This paper navigates the critical role of hydrogen in catalyzing a sustainable energy transformation. This review delves into hydrogen production methodologies, spotlighting green and blue hydrogen as pivotal for future energy systems because of their potential to significantly reduce greenhouse gas emissions. Through a comprehensive literature review and a bibliometric analysis, this study underscores the importance of technological advancements, policy support, and market incentives in promoting hydrogen as a key energy vector. It also explores the necessity of expanding renewable energy sources and international cooperation to secure a sustainable, low-carbon future. The analysis highlights the importance of scalable and cost-effective hydrogen production methods, such as solar-thermochemical and photo-electrochemical processes, and addresses the challenges posed by resource availability and geopolitical factors in establishing a hydrogen economy. This paper serves as a guide for policy and innovation toward achieving global sustainability goals, illustrating the essential role of hydrogen in the energy transition.

1. Introduction

1.1. Historical Context of Hydrogen as an Energy Carrier

Currently, the production of hydrogen predominantly relies on fossil-based raw materials, which serve as both the hydrogen source and the energy for its conversion. This is commonly executed through methods such as steam reforming of natural gas, a process that significantly contributes to emissions of CO₂. On average, the production of each ton of hydrogen via this method results in the release of nearly 12 tons of CO₂, amounting to an annual total of 830 million tons [1]. Considering this, the focus of energy research is shifting towards the development of sustainable hydrogen production technologies that harness renewable energy sources, especially solar energy.

The journey of hydrogen from a scientific curiosity to a sustainable energy carrier is a narrative enriched by centuries of discovery and technological evolution. Discovered by Henry Cavendish in 1766 and named by Antoine Lavoisier in 1783 using the Greek terms “hydro” (water) and “genes” (forming), the characteristics of hydrogen have fascinated the scientific community since its introduction [2].

The advent of the Industrial Revolution in the 19th century marked a pivotal turn in the exploration of hydrogen’s applications. The principle of the fuel cell, discovered by Sir William Grove in 1839, laid the foundational understanding of the electrochemical reactions in hydrogen fuel cells, a cornerstone for modern green hydrogen production [3].

As the 19th century unfolded, hydrogen ventured beyond laboratories into practical uses in industries such as metallurgy and lighting, its production and distribution scaling up with the burgeoning gas industry. Its role in air travel, notably in hydrogen-filled balloons and airships such as the Hindenburg, highlighted its utility, albeit not without risks [4].

The 20th century witnessed a dual narrative for hydrogen. The Haber–Bosch process catalyzed a revolution in hydrogen production through steam methane reforming, a key industrial achievement [5]. However, the Hindenburg disaster in 1937 cast a pall over hydrogen’s safety, leading to a temporary eclipse by safer alternatives such as helium.

The late 20th and early 21st centuries have seen a resurgence in hydrogen’s role as a clean energy carrier. The rise of green hydrogen, generated via electrolysis powered by renewable energy, symbolizes a proactive response to environmental challenges and the global pursuit of a sustainable, low-carbon energy future [6,7,8,9].

In the present era, green hydrogen is a focal point in the global dialogue on decarbonization and sustainable energy solutions. The convergence of environmental awareness and advances in renewable energy technology positions green hydrogen as a pivotal element in the effort to mitigate climate change and achieve a carbon-neutral future [10].

1.2. Recent Advances and Challenges in Hydrogen Production

The recent literature on hydrogen production methods and their challenges has been thoroughly reviewed, highlighting key technological advancements and areas for future research. In the paper [11], a detailed analysis of the latest developments in hydrogen production techniques is provided, emphasizing the potential of solar and wind energy sources and discussing the flexibility and economic advantages of distributed and semi-centralized reforming with minimal greenhouse gas emissions. Similarly, refs. [12,13] examine green hydrogen generation from hybrid renewable energy sources, focusing on the critical assessment of solar, wind, and hybrid-powered electrolysis technologies and identifying key challenges and opportunities for commercial-scale deployment.

Several studies focus on the production processes and potential of green hydrogen [14,15]. In the paper [14], the production of green hydrogen using renewable energy sources across several leading countries is explored, highlighting the promotion of environmental sustainability through the electrolysis process. They emphasize the significant potential for “green” hydrogen production. The study [15] analyzes the challenges and opportunities related to green and blue hydrogen, focusing on the hydrogen supply chain, technical challenges, and economic and geopolitical implications. It underscores the importance of international cooperation and measurable indicators for a successful transition to a hydrogen-based energy system. The paper [16] conducts a bibliometric review of green hydrogen generation technologies, highlighting advancements in electrolyzer technologies and pinpointing critical areas for future research and development. Their findings indicate significant interest in alkaline and PEM electrolyzers, with Europe leading in scientific output and capacity forecasts for 2030. In the paper [17], the evolution of green hydrogen research themes is examined, highlighting four major areas: hydrogen storage, hydrogen production, electrolysis, and the hydrogen economy. They discuss the influence of policy decisions in the United States, Europe, India, and China on the development and storage of green hydrogen.

Focusing on water electrolysis technologies, ref. [18] provides a comprehensive review, summarizing recent developments in electrode materials and the techno-commercial prospects of hydrogen production, identifying research gaps, and suggesting solutions for driving cost-effective green hydrogen production. In contrast, ref. [19] review hydrogen production pathways and associated technologies, emphasizing their interconnection and interdependence with other stages of the hydrogen-based energy system. They introduce an innovative model for a hydrogen cleanness index, discussing the importance of the production pathway’s cleanness.

Several other studies contribute to the understanding of hydrogen production from renewable energies. The potential of solar, wind, and biomass for producing green hydrogen, along with the technological advancements needed for enhanced efficiency and scalability, is highlighted in recent research [20]. Recent progress in photocatalytic hydrogen production, emphasizing novel photocatalysts and their applications in solar-driven hydrogen production, is highlighted in [21]. Various techniques for green hydrogen production, including potential applications and the technological and economic challenges that need to be addressed, are reviewed comprehensively in [22]. Other significant contributions include an in-depth analysis of water electrolysis as a hydrogen production method, focusing on current status, technological advancements, and future trends [23]. Various methods and efficiencies of biohydrogen production from biomass and waste materials are explored [24]. Advancements in hydrogen production from biomass gasification, emphasizing biomass’s importance as a renewable source, are reviewed [25]. The potential of algal biomass for hydrogen production, illustrating the versatility of biomass, is discussed in several studies [26,27,28]. Recent developments in water splitting for hydrogen production using novel catalysts are reviewed [29], as well as advancements in hydrogen production using microbial electrolysis cells [30].

Renewable energy-to-hydrogen technologies are reviewed extensively from technical, economic, ecological, and social perspectives, summarizing the current states of five technologies and emphasizing developments in the literature. Information gaps, research directions, opportunities, and potential improvements related to efficiency, greenhouse gas emissions, manufacturing costs, hydrogen-based mobility, and public acceptability are identified, highlighting the urgency of incorporating renewable energy into hydrogen production processes [12].

The critical role of hydrogen in achieving carbon peaking and carbon neutrality within power systems that heavily rely on renewable energy sources is examined. The comprehensive analysis focuses on hydrogen energy’s advantages and disadvantages, particularly its production, storage, and various applications in modern power systems [31,32,33].

This paper aims to provide a comprehensive review of the current state of green hydrogen research and its multifaceted applications. It contributes to the scientific literature by offering an in-depth analysis of the latest advancements in green hydrogen production technologies, examining the policies that influence its market dynamics, and evaluating global research trends through bibliometric analysis. The novelty of this work lies in its holistic approach, combining technological, policy, and market perspectives to present a unified overview of green hydrogen’s potential and challenges. The necessity for this review is underscored by the urgent need for sustainable energy solutions and the growing recognition of green hydrogen as a critical component in achieving global climate goals. By synthesizing existing research and identifying gaps, this paper aims to guide future research and policymaking toward the effective integration of green hydrogen into the global energy landscape.

2. Hydrogen Production Techniques

Hydrogen, predominantly found in a chemically bound state on Earth, requires extraction from its compounds to be harnessed as an energy carrier. The extraction techniques fall into two primary categories: water splitting and extraction from organic, hydrocarbon-based compounds. Water, abundant on Earth and yielding only hydrogen and oxygen upon splitting, emerges as a particularly appealing source. However, extraction from hydrocarbon compounds invariably involves CO₂ emissions unless the carbon is effectively captured and stored [34].

Today, a staggering 95 percent of globally produced hydrogen is obtained from fossil resources [17,35]. Termed as “gray” hydrogen, this method is marked by significant CO₂ emissions, rendering it an unsustainable choice for a hydrogen-driven economy. “Blue” hydrogen emerges as an alternative, where greenhouse gases generated during hydrogen production are captured for storage [36,37]. Nonetheless, it falls short of being entirely CO₂-neutral because of the emissions released during fossil resource extraction. Additionally, some regions are exploring nuclear energy for hydrogen production, a method fraught with challenges such as public acceptance, radioactive waste management, and dependence on finite resources.

In contrast, “green” hydrogen, produced solely from water, renewable biomass, and renewable energy sources, is the cornerstone of a sustainable hydrogen economy. Therefore, the development of green hydrogen production methods, especially water splitting through electrochemical and thermochemical techniques, is an area of significant research interest [38].

To bridge the gap until renewable energy capacities are fully realized, minimizing emissions from current hydrogen production methods is also vital. This interim solution involves enhancing the efficiency of fossil fuel usage and adapting these methods for biomass feedstock.

The decline in renewable energy costs in recent years points towards a promising future where, with dedicated research and supportive policies, renewable green hydrogen has the potential to become cost-competitive. This evolution in hydrogen production technologies marks a critical step towards achieving a sustainable and low-carbon energy future.

2.1. Water Splitting Techniques in Hydrogen Production

Electrolysis currently stands as the most advanced and economically feasible approach for producing green hydrogen from water [39,40,41]. Soon, there are plans to expand electrolysis capacities significantly, enabling the generation of hydrogen using electricity from renewable energy sources [18,42,43,44]. Thermochemical processes, however, present an exciting potential for greater efficiency and more effective land use. These processes use high-temperature heat instead of electricity for water splitting, which can be efficiently generated using concentrated solar thermal energy [45,46,47]. This approach is particularly suitable for regions abundant in sunlight. With several pilot plants already in operation, thermochemical methods are poised to play a crucial role in hydrogen production in the long term [48].

In addition to these, photo-electrochemical and photocatalytic processes represent emerging technologies in hydrogen production [49,50,51]. These methods directly harness solar radiation for water splitting and are in the early stages of research and development. With their advancement, they could provide a more direct and efficient way to utilize solar energy for hydrogen production. The continuous exploration and development of these innovative water-splitting technologies are essential for expanding the range and efficiency of green hydrogen production methods [49].

Water electrolysis, the method of dividing water into hydrogen and oxygen through electrical current, stands as a key method for converting electrical energy into chemical energy. This technology is one of the oldest known electrochemical processes [39,52,53]. Presently, there are three primary electrolysis technologies of significance:

Alkaline Electrolysis (AEL): This technology has been in large-scale use for about a century. It is well-established and has a proven track record of reliability and maturity.

Proton Exchange Membrane Electrolysis (PEMEL): A newer technology that has been commercially available for several years now. It offers advantages over AEL, such as improved dynamic behavior and higher purity of the product gases under partial loads, leading to a growing market presence.

High-Temperature Electrolysis (HTEL): Still in the developmental stage and currently operational only in pilot plants. Operating at temperatures between 700 to 1000 °C [54], HTEL potentially offers significantly higher efficiency, indicating considerable future potential.

Although a well-established method, electrolysis has predominantly been utilized in scenarios where electricity is inexpensively available or where small quantities of high-purity hydrogen are needed; currently, electrolysis accounts for only about five percent of the world’s annual hydrogen production [55]. However, the prospect of producing CO₂-free hydrogen using renewable electricity sources is expected to drive a substantial increase in electrolysis capacities globally in the coming years.

2.1.1. Alkaline Electrolysis

Several review papers have extensively discussed the advancements, challenges, and future directions of alkaline water electrolysis in the literature [55,56,57,58,59]. The history of large-scale alkaline electrolysis was in Norway, where Norsk Hydro developed an electrolyzer facility that was initially piloted in 1927 and fully operational by 1928 [60]. In this technology, potassium hydroxide (KOH) serves as the electrolyte, providing hydroxide ions (OH⁻) for charge transfer. A diaphragm between the electrodes prevents the mixing of hydrogen and oxygen, thus averting the risk of explosive mixtures. The hydroxide ions migrate through the diaphragm towards the anode, with potassium hydroxide circulated continuously from adjacent tanks for degassing. Despite its established status, alkaline electrolyzers face challenges in dynamic load responsiveness because of their relatively slow electrolyte circulation [61,62]. This type of electrolysis is characterized by low investment costs, longevity, and minimal reliance on critical raw materials [63].

In 1960, Egypt established its first plant for producing renewable ammonia, which also stands as potentially the nation’s initial venture into synthetic ammonia production [64,65]. Situated near Aswan and adjacent to a hydroelectric facility on the Aswan River, this facility was strategically placed to capitalize on the hydroelectric power for ammonia synthesis. The impetus for the plant’s creation was to ensure national food security at a time when Egypt had not yet harnessed natural gas for commercial use, which occurred a decade later, starting in 1970 [64]. With an output ranging from 400–500 tons of NH₃ per day, or 140–175 kilotons annually, the Aswan facility became the world’s most significant renewable ammonia production site. Initially, De Nora electrolyzers were employed, but these were supplanted by Brown Boveri’s technology in 1977 [64]. Known as the KIMA plant, this renewable ammonia producer remained active well into the 2000s [65]. Nonetheless, a more recent Aswan-based ammonia facility focused on urea production using natural gas emerged, leading to the decommissioning of the KIMA plant in 2019 [64].

However, alkaline water electrolysis presents several challenges that can impact its adoption and efficiency. Alkaline electrolysis can present disadvantages when integrated with fluctuating power sources, such as decreased gas purity under partial loads and degradation issues [42]. Moreover, one significant barrier is the high initial investment needed to set up the system, which may deter potential users. Additionally, the process’s efficiency declines in colder temperatures, rendering it less effective in cooler climates or seasons [66]. Another critical requirement is the need for high-quality, pure water; the presence of impurities can significantly reduce the process’s effectiveness [40]. Moreover, hydrogen is generated at a relatively slow pace, potentially delaying the completion of tasks reliant on hydrogen production. Finally, to maintain optimal performance, the system necessitates regular maintenance, which can lead to increased operational costs over time [40,67].

2.1.2. Proton Exchange Membrane (PEM) Electrolysis

Proton exchange membrane (PEM) water electrolysis is widely regarded as one of the most advanced and efficient methods for producing high-purity hydrogen from renewable energy sources, including wind, solar photovoltaic, and hydropower. Several papers have discussed the potential of PEM technology, highlighting its advantages, such as high current density, greater energy efficiency, and its ability to integrate seamlessly with intermittent renewable energy sources. These studies underscore the critical role of PEM electrolysis in advancing clean hydrogen production and supporting the transition to a sustainable energy future [18,68,69,70,71].

PEM electrolysis offers several benefits, including the ability to achieve high current densities, which contributes to a more compact system design and enables rapid response times [72,73,74]. This technology is capable of producing hydrogen at a greater rate, with an exceptional purity level of 99.99%. Additionally, PEM electrolysis is noted for its high energy efficiency, ranging between 80–90%, and its capability for high dynamic operation [72]. However, there are challenges associated with PEM electrolysis. It is a relatively new and only partially established technology, facing obstacles such as the high cost of components. The acidic environment required for its operation can lead to low durability of the system. Furthermore, while commercialization is on the horizon, it is still considered to be in the near term, indicating that wider market adoption has yet to be fully realized.

2.1.3. High-Temperature Electrolysis (HTEL)

High-temperature co-electrolysis of CO₂ and steam presents a promising avenue for CO₂ utilization, leveraging solid oxide electrolysis cells (SOECs) functioning at 500 to 900 °C to convert CO₂ into CO and, with the addition of steam, produce hydrogen that forms syngas [75,76,77,78]. This syngas can then be transformed into hydrocarbon fuels and chemicals, offering a pathway to efficiencies beyond what is achievable with lower-temperature electrolysis. However, the development of high-temperature electrolyzers such as SOECs faces hurdles, including the need for components to exhibit chemical and physical stability at elevated temperatures, in both reducing and oxidizing environments and under varying ionic concentrations [75]. These challenges necessitate materials that maintain their integrity and conductivity in extreme conditions. Despite these obstacles, advancements in materials and design are making strides toward mitigating cell degradation, a critical factor for the commercial success of SOEC technology. Durability tests have shown promising results, with minimal degradation over 1000 h at low current densities when proper gas cleaning is implemented, although higher densities still pose challenges. Addressing these issues is key to unlocking the full potential of high-temperature electrolysis for sustainable hydrogen production [76].

One of the promising aspects of HTEL is its potential for integration with thermal power plants or industrial processes where waste heat is available. This can lead to more efficient use of energy and a reduction in overall emissions.

HTEL also offers the advantage of co-electrolysis, allowing for the direct production of synthesis gas. By feeding a mix of water and carbon dioxide to the cathode, synthesis gas, a combination of hydrogen and carbon monoxide, is produced [75]. This gas can subsequently be transformed into liquid hydrocarbon compounds, such as via the Fischer–Tropsch (FT) process, presenting an opportunity for carbon utilization and alignment with circular economy principles [79].

The prospects for HTEL are viewed with optimism, as it is anticipated to achieve the most cost-effective hydrogen production among various electrolysis methods. However, with any innovative technology, there are uncertainties, and ongoing advancements will play a crucial role in providing more accurate predictions. The potential for lowering costs, along with its efficiency, makes HTEL a promising candidate for advancing hydrogen production technologies. Figure 1 offers a prospective analysis of the investment costs for different electrolysis methods. The chart suggests a downward trend in the investment costs for HTEL, PEM electrolysis, and Alkaline Electrolysis over the next decade [80]. This expected reduction in costs is likely due to technological improvements, increased production scale, and greater efficiency.

The anticipated trajectory suggests an increasingly cost-competitive environment for electrolysis technologies. It highlights the prospective affordability of High-Temperature Electrolysis (HTEL), even with its present higher initial costs. This shift in cost-effectiveness is crucial for the advancement of hydrogen production, particularly for green hydrogen, indicating a trend towards more economically viable sustainable production techniques.

3. Renewable Energy for Hydrogen Production

Hydrogen production methods are often categorized by the source of energy used in the production process, which can imply the environmental impact of the hydrogen generated. Figure 2 shows a breakdown based on color designation.

The production of hydrogen, a potential cornerstone for clean energy, can be achieved through various methods, each distinguished by its environmental impact and energy source. Numerous studies have delineated hydrogen production methods by categorizing them based on their source, employing a color-coding system to distinguish between the various techniques [36,81,82,83,84]. Green hydrogen, the most eco-friendly variety, is generated via the electrolysis of water, employing electricity sourced from renewable resources such as solar, wind, hydro, and biomass, and is characterized by its zero-carbon emission process. Moreover, integrating green hydrogen with other green technologies, such as green buildings, can create a synergistic energy ecosystem where renewable hydrogen powers energy-efficient infrastructures, enhancing overall sustainability and reducing carbon footprints [85].

Blue hydrogen, on the other hand, is derived from natural gas using processes such as steam methane reforming or autothermal reforming with the added step of carbon capture and storage (CCS) to sequester the CO₂ produced, thereby reducing its carbon footprint [86].

Grey hydrogen, currently the most common form, is derived from fossil fuels such as natural gas or coal without CO₂ capture, thus contributing to greenhouse gas emissions [87]. Turquoise hydrogen represents an emerging technology that employs methane pyrolysis to split methane into hydrogen and solid carbon, a process that yields no CO₂ emissions but is not yet widely implemented [88]. Yellow hydrogen typically refers to hydrogen created through electrolysis, where the electricity comes from a combination of renewable and non-renewable sources, including nuclear power [89]. Similarly, pink or red hydrogen is produced by electrolysis as well, but specifically with electricity supplied by nuclear energy.

This section offers a comprehensive review of the strides made in integrating renewable energy technologies such as photovoltaics (PV) and wind into hydrogen production systems. This segment of the paper will present the existing body of work on this subject, underscoring the progress in energy storage solutions designed to address the challenge of intermittency inherent in renewable sources. By examining the nexus of renewable integration and hydrogen production, the section will showcase the critical advancements and innovative approaches that are propelling the energy sector towards a sustainable future.

Figure 3 shows the projected growth of various renewable energy technologies from 2016 to 2028 [90]. It shows historical data and future projections for the installed capacity of solar PV, wind, hydropower, bioenergy, and the percentage of wind and PV energy.

The bars represent the total gigawatt capacity of each technology, while the line indicates the percentage of wind and PV in the total renewable mix, demonstrating a significant increase over time. The growth of solar PV is particularly prominent, highlighting its expanding role in global energy production.

The pursuit of a sustainable energy future has positioned the integration of renewable energy sources into hydrogen production as a key strategy. This method not only adheres to global decarbonization initiatives but also leverages the expansive potential of clean energy to satisfy the escalating demand for hydrogen as a versatile and environmentally friendly energy vector. This section explores the role of various renewable energy sources in hydrogen production, examining the current landscape, potential capacity, and operational systems. The focus will be on solar and wind power, leaders in the renewable domain, assessing their efficiency and scalability for hydrogen generation. The contributions of hydropower, biomass, and geothermal energy are also scrutinized, each presenting distinct advantages and challenges within the hydrogen production framework. This comprehensive review aims to shed light on the advancements and practical applications of these renewable energies, elucidating their combined influence on the evolving hydrogen economy.

Numerous studies have analyzed the economic aspects of hydrogen production technologies, revealing that the levelized cost of hydrogen (LCOH) produced with photovoltaic (PV) generated electricity falls between $5 and $7 per kg [91,92,93,94,95,96]. In contrast, hydrogen derived from wind energy exhibits a cost range of $5 to $9 per kg [97,98,99,100,101,102], with costs associated with proton exchange membrane (PEM) electrolysis typically being higher. The efficiency of electricity and hydrogen generation from these renewable sources is influenced by several factors, including meteorological conditions (e.g., solar irradiance, wavelength, wind velocity, and tower elevation), the type of electrolyzer used, and the geographic location of the production site.

Furthermore, analyzing energy performance in green hydrogen production ensures efficient technology deployment and informs strategic decisions, helping to advance cost-effective and scalable hydrogen solutions for achieving long-term sustainability goals [103].

3.1. Solar Energy in Hydrogen Production

Solar energy presents a versatile avenue for hydrogen production, which can be harnessed through PV systems that directly transform sunlight into electricity or via solar thermal methods, including concentrated solar power (CSP) systems such as parabolic troughs or central receiver and tower systems [104,105]. These CSP systems are particularly effective in regions with high direct normal radiation (DNI), where they can efficiently generate the high-temperature heat required for thermochemical reactions. The potential for solar energy in hydrogen production is substantial, given the vast, untapped solar resources available globally. This capacity for renewable hydrogen generation positions solar energy as a cornerstone in the transition to sustainable energy practices.

The success of solar thermal technologies for hydrogen production depends heavily on the location-specific availability of direct sunlight. Annual solar radiation totals and their temporal distribution are vital metrics for evaluating a region’s capability for hydrogen production. Regions with high DNI, as depicted in Figure 4 [106], are well-suited for CSP systems, which are integral for electrolytic and thermochemical hydrogen production methods. Prime CSP locations worldwide, including areas in Chile, Namibia, and North Africa, receive exceptionally high DNI, making them ideal for hydrogen production initiatives.

In contrast, Europe faces more variability in solar resources throughout the year. The Middle East and North Africa (MENA) region stands out for its combination of high DNI and available land, positioning it as a premier region for hydrogen production via CSP. This is also true for other regions with high solar potential, such as Australia and the southwestern USA. Looking forward, leveraging both CSP and PV systems is projected to enhance green hydrogen production, furthering the goal of a sustainable energy future.

PV systems distinguish themselves from Concentrated Solar Power (CSP) by their ability to effectively generate energy in regions with lower DNI levels. PV technology is uniquely capable of utilizing both direct and diffuse solar radiation, making it suitable for locations such as Northern Europe, where diffuse radiation marginally surpasses direct radiation in contributing to total solar radiation. Solar radiation levels across the globe exhibit considerable variation, reflecting the diverse climatic and geographical landscapes of different regions, as shown in Figure 5 [106].

In equatorial areas, characterized by their proximity to the equator, solar radiation levels are exceptionally high because of the sun’s direct angle of incidence, often exceeding 2200 kWh/m²/year. Moving towards the temperate zones, which include much of the United States, Southern Europe, and parts of China and Japan, the annual solar radiation values range between 1500 to 2000 kWh/m²/year, offering substantial potential for solar energy generation [106]. In contrast, regions situated at higher latitudes, such as Northern Europe and parts of Canada, experience lower levels of solar irradiance, typically between 900 to 1200 kWh/m²/year, because of the sun’s oblique angle and shorter daylight hours, especially during winter months. Despite these variations, advancements in photovoltaic technology continue to enhance the efficiency of solar energy systems, making solar power a viable and increasingly attractive option for renewable energy production across a wide array of geographical settings.

This adaptability of PV systems to various geographic and climatic conditions, including areas with less intense sunlight or higher cloud cover, enhances their suitability for a broad range of regions. Coupled with decreasing costs and advancements in PV technology, this versatility strengthens its role in the renewable energy sector and green hydrogen production. The global solar PV power generation is set to continue its upward trajectory, with significant additions expected in the coming years [59]. The recent achievement of installing solar panels capable of generating 1 TW of electricity showcases a milestone in renewable energy. According to the International Energy Agency (IEA), the solar PV sector will remain a key driver of global renewable capacity expansion. The year 2023 saw solar accounting for 286 gigawatts (GW), and this figure is projected to increase to almost 310 GW in 2024 [107]. This growth is propelled by factors such as lower module prices, increasing adoption of distributed PV systems, and policies that encourage large-scale deployment. These developments indicate a robust and growing contribution of solar PV to the global energy mix, reinforcing its central role in the transition towards more sustainable energy systems.

The rapid expansion of PV technology reflects its potential for large-scale deployment in the shift towards renewable energy, including for green hydrogen production. The technology’s adaptability, combined with improving efficiency and decreasing costs, cements its role as a sustainable energy solution on a global scale.

However, unlike CSP, electricity production from PV lacks the capability for thermal storage shifting. Thus, even in optimal locations such as the MENA Region, a PV installation may not surpass 2000 full-load hours annually, as shown in [107]. The global distribution of PV full load hours (FVL) map [108] serves as a guide for determining regions where PV systems could yield maximum efficiency and contribute effectively to the renewable energy mix in green hydrogen production. The map visualizes FLH globally, a measure indicative of a solar panel’s annual energy output. Areas with intense colors, such as red and orange, typically located in regions such as MENA, reveal a higher potential for solar energy generation, marked by an abundance of FLH. Conversely, cooler colors such as blues represent lower FLH, suggesting limited solar potential, often seen in higher latitudes. This distribution underscores the geographic disparities in solar energy viability and emphasizes the importance of strategic placement for optimizing solar energy systems. Directly using photovoltaic-generated electricity for electrolysis can lead to increased hydrogen production costs because of the absence of thermal storage. However, hybrid systems that combine PV with CSP technologies enhance the solar energy potential, particularly in regions with high direct solar irradiance. These hybrid systems capitalize on CSP’s thermal storage capabilities and PV’s ability to convert both direct and diffuse sunlight, offering a comprehensive solution to harness solar energy more effectively for hydrogen production.

With the enormous potential of solar energy, short-term strategies for producing green hydrogen are focusing on using alkaline or PEM electrolysis powered by either CSP systems or a combination of PV and CSP technologies. Looking ahead, solar-thermochemical processes and high-temperature electrolysis, which utilize both the electrical and thermal energy from CSP, are seen as viable long-term solutions. These approaches are set to capitalize on the abundant solar resources for green hydrogen production, marking a critical step in the global shift to a sustainable energy paradigm.

3.2. Wind Energy in Hydrogen Production

Wind energy represents a significant and widely accessible resource for renewable power generation on a global scale. The Global Wind Energy Council’s 2023 report showcases a significant year-on-year growth of 9% in wind energy installations, bringing the global capacity to 906 GW. With a robust forecast predicting a steady increase, including an unprecedented surge expected to surpass 100 GW of new capacity in 2023, the council envisions a vibrant future for wind energy [109]. The report projects that by 2030, the cumulative installations will see an increase of 13% more than previously anticipated, indicating an optimistic outlook for the sector’s expansion and its role in sustainable energy transitions [109]. The top five markets for new wind installations in 2022 included China, the USA, Brazil, Germany, and Sweden, accounting for 71% of the total global wind installation.

Figure 6 depicts the global wind power class map [110] that shows the wind energy potential at different locations around the world. The color-coded scheme indicates the strength of wind power classes from low to high, with the deep purple areas signifying the highest wind power potential.

The map illustrates that some regions, particularly those offshore and in certain onshore areas, have significant wind energy potential, indicating their suitability for wind power development. These visual data are crucial for identifying optimal locations for establishing wind farms and harnessing wind as a hydrogen production source. The substantial potential of wind energy, particularly in coastal and offshore regions, emphasizes its pivotal role in the global shift towards renewable energy sources. Integrating both onshore and offshore wind energy into the energy mix is essential for diminishing reliance on fossil fuels and meeting international climate objectives. Moreover, the scalability of wind energy and its decreasing cost profile render it an increasingly viable option for large-scale renewable energy production. This growth in wind energy not only aligns with energy diversification strategies but also supports the broader transition to a more sustainable and resilient energy future.

The countries leading the way in wind energy capacity are [109]:

• United States: The US has the largest onshore wind farm capacity, with the Alta Wind Energy Centre in California alone having a capacity of 1548 MW. Texas produces about a quarter of US wind power [109].
• Germany: Germany boasts the highest installed wind capacity in Europe, exceeding 64 GW. The Gode Windfarms and the Nordsee One Offshore Wind farm are among the largest offshore wind farms in the country [109].
• India: With an installed capacity of 42 GW, India’s Muppandal wind farm in Tamil Nadu and the Jaisalmer Wind Park in Rajasthan are among the largest onshore wind farms in the world [109].
• Spain: Around 20% of Spain’s electricity is generated from wind, contributing to its 29 GW of installed capacity. The country is also a key player in global wind manufacturing [109].
• United Kingdom: The UK has a significant offshore wind capacity, with six of the ten highest-capacity offshore wind projects in the world located in its waters. The Hornsea One wind farm is currently the world’s largest [109].
• Brazil: With more than 19 GW, Brazil has the largest wind capacity in South America, competing closely with hydroelectric power for the country’s second place in energy generation [109].
• France: France is aiming to significantly increase its wind generation capacity and streamline its wind construction processes. It currently has an installed wind capacity of 18.7 GW [109].
• Canada: Wind power constitutes about 5% of Canada’s energy supply, with the Rivière-du-Moulin project in Quebec being the largest wind farm in the country at a capacity of 300 MW [109].
• Italy: Italy reached an installed wind generation capacity of 12.7 GW in 2021. The country’s wind industry is largely concentrated in the south and on its islands [109].

Wind energy has been extensively discussed in the literature as a promising source for hydrogen production, with both offshore and onshore wind energy showing significant potential. The economic analysis of wind-powered hydrogen production systems highlights the feasibility of using wind energy to generate green hydrogen, a key component in the transition to a decarbonized energy system. Studies such as those by [111,112,113,114,115]. They have explored the techno-economic aspects of integrating wind farms with hydrogen production facilities, noting that offshore wind farms offer higher capacity factors and can produce hydrogen more economically due to the steadier wind resources available at sea. Moreover, the integration of wind energy with water electrolysis systems, as discussed in [115], underscores the potential of hydrogen as a storage medium for renewable energy, addressing intermittency issues and enhancing energy security. These reviews collectively emphasize the critical role of wind energy in advancing green hydrogen technologies and contributing to the global effort to reduce carbon emissions.

3.3. Geothermal Energy in Hydrogen Production

Geothermal energy’s stable supply of heat and electricity positions it as a key player in the creation of green hydrogen. Found in geothermally active regions, it supports consistent power generation, which is crucial for hydrogen production, and the Earth’s surface radiates a considerable amount of heat, totaling approximately 46 ± 3 terawatts [116], which is contributed to by various geological processes, including the mantle’s absorption of heat from the core, its own cooling, and the radiogenic heating due to the decay of radioactive elements. When this geothermal potential is translated into theoretical energy availability, it amounts to roughly 385,000 TWhth/a [117]. However, practical applications for electricity generation can harness only a fraction of this energy. Current estimates suggest that about 240 GWel can be viably utilized for electric power generation, offering a significant resource for stable and continuous energy production needed for systems such as those used in green hydrogen production [117].

The growth in geothermal capacity, reaching 15.96 GW for electricity and 107 GW for heating and cooling, signifies its expanding role [117]. By providing reliable energy, geothermal plants could supply the necessary power for electrolyzers to split water into hydrogen, contributing to a sustainable and clean energy future. This is particularly important for green hydrogen production, which relies on renewable energy sources to minimize its carbon footprint.

By the end of 2023, the worldwide capacity for geothermal power generation reached 16,355 MW [117]. Leading this are the United States, Indonesia, and the Philippines, among other countries, showcasing significant contributions to the geothermal landscape. These nations exemplify the global commitment to harnessing the Earth’s heat for clean, renewable energy, indicating a diversified and growing reliance on geothermal resources across different regions of the world. Figure 7 shows the world map highlighting the global distribution of geothermal resources [118].

The map highlights geothermal power capacity distributed across 31 countries and seven tectonic plates, with notable capacities such as 8508 MW on the Eurasian Plate, 4876 MW on the North American Plate, and 1105 MW on the Australian Plate. Additionally, regions like the African Plate (980 MW) and the South American Plate (81 MW) also show significant geothermal activity. This global distribution underscores the potential for leveraging geothermal energy in various tectonically active areas, providing a stable and reliable energy source. This is particularly beneficial for sustainable projects like green hydrogen production, which rely on continuous energy supplies. The concentration of geothermal resources in these regions emphasizes the strategic importance of geothermal energy in achieving a sustainable and secure energy future.

3.4. Hydropower in Hydrogen Production

As of the end of 2023, the global installed capacity of hydropower is reported to be over 34 GW, with hydropower providing over 15% of the world’s electricity [119]. This capacity includes more than 10 GW of pumped storage, marking the first time since 2016 that such a substantial amount of capacity has been brought online within a single year. Furthermore, there are currently 590 GW worth of hydropower projects at various stages of development. However, to meet net-zero targets by 2050, there remains a substantial gap in capacity, highlighting the need for increased investment and sustainable practice regulations [119].

Hydropower plays a vital part in the worldwide energy mix, providing a clean and renewable source of electricity that can help offset the intermittent nature of other renewables, such as wind and solar power. With built-in energy storage capabilities and fast response times, hydropower is uniquely positioned to balance fluctuations in the power grid and assist in stabilizing the energy supply [120,121]. However, the repercussions of climate change, such as changing precipitation patterns and diminishing snowmelt, pose risks to the reliability and predictability of hydropower. Despite these obstacles, hydropower continues to play an essential role in efforts to augment renewable energy consumption and diminish reliance on fossil fuel sources. In addition, aging hydropower infrastructure requires modernization to align with current power system demands. This includes retrofitting existing plants to enhance their performance and environmental footprint. Innovation is essential for sustainable hydropower to remain cost-effective, and solutions are being explored to harness energy from smaller reservoirs and plants that can operate with minimal water level differences without the need for large dams.

The European Union supports hydropower research and innovation, focusing on projects that demonstrate the potential of hydropower technology while reducing its environmental impact. The EU aims to fund efficient retrofitting of plants and enhance the sustainability of hydropower operations [122]. There are ongoing initiatives to explore the utilization of small-scale hydropower opportunities, which could be a game-changer in terms of accessibility and environmental impact.

In the realm of sustainable energy solutions, hydropower has been identified as a pivotal resource for hydrogen production, offering a promising avenue to reduce reliance on carbon-based fuels and their associated pollution. A handful of studies have embarked on planning and developing comprehensive action plans for countries such as Slovakia, Turkey, and Nepal [123,124,125], spotlighting the strategic use of their hydroelectric potential. These studies aim to meticulously craft hydrogen maps for several regions within the examined countries, detailing the prospects of leveraging hydroelectric energy using electrolyzers. Such initiatives underscore the significant contribution that hydroelectric energy can make towards sustainable hydrogen production. By tapping into abundant and renewable hydro resources, these efforts are set to furnish policymakers with valuable insights, enabling them to harness hydropower for hydrogen generation effectively. The anticipated outcomes of these studies promise not only to bolster the energy security of the involved nations but also to pave the way for a cleaner, more sustainable energy landscape by mitigating the environmental impact of traditional fossil fuels.

3.5. Biomass in Hydrogen Production

Bioenergy refers to renewable energy derived from biological sources, which can include agricultural byproducts, wood, waste, and other organic materials. Bioenergy refers to renewable energy derived from biological sources, which can include agricultural byproducts, wood, waste, and other organic materials. Globally, bioenergy contributes to the overall renewable energy share, which is growing as part of efforts to lower greenhouse gas emissions and advance toward a more sustainable energy infrastructure. The global total primary energy supply was approximately 606.5 exajoules, with renewables, including biomass, accounting for 13.8% of this total [126].

Energy crops currently play a minor role in global biomass for energy, with traditional uses such as cooking and heating dominating in Africa and Asia. However, countries such as Brazil, India, and China see biomass as a key to supplementing their energy needs. It’s estimated that biomass could meet 20% of the world’s energy demand by 2050, with significant land expansion needed, raising environmental concerns [126]. Sustainable sourcing from agricultural residues and waste can support hydrogen production without impacting the food supply, underscoring the careful balance needed between energy goals and ecological preservation.

Biomass is a significant source of hydrogen production, employing processes such as fermentation, where microorganisms break down organic matter to produce hydrogen, or bio photolysis, using solar energy to split water into hydrogen and oxygen. There are also electrochemical processes for hydrogen generation from biomass. The transformation of biomass into hydrogen is recognized as a feasible approach due to biomass being abundant, environmentally friendly, and renewable. Various research articles such as [27,127,128,129,130,131,132,133] offer detailed insights into the methods for obtaining hydrogen from biomass, encompassing thermochemical techniques such as gasification and pyrolysis alongside biological strategies such as bio photolysis, fermentation, and the water-gas shift reaction. While the production methods show promise, the economic and environmental impacts are key considerations for the commercial viability of these technologies. Harnessing biomass for energy represents a strategic approach to enhancing the renewable energy portfolio while managing waste and residues sustainably. It holds promise for hydrogen production, emerging as a renewable source that embodies circular economy ideals. This method not only contributes to a diversified energy supply but also plays a crucial role in mitigating greenhouse gas emissions, underscoring the importance of biomass in achieving a greener, more sustainable energy landscape.

Integrating technology transfer in green hydrogen production is crucial for accelerating advancements, fostering international cooperation, and addressing resource and geopolitical challenges. Mechanisms such as international collaborations, joint research and development, capacity building, and financial support can facilitate the widespread adoption of advanced hydrogen technologies, ensuring a balanced global supply and driving the transition towards a sustainable, low-carbon future [134].

4. Bibliometric Analysis

Bibliometric analysis represents a methodological approach that quantitatively assesses the landscape of the scientific and scholarly literature. This analytical framework employs statistical methods to explore the intricacies of patterns, trends, and interconnections present in a body of publications, which predominantly includes scholarly articles, academic books, and other forms of research output. The primary objective of bibliometric analysis lies in extracting meaningful insights regarding the productivity, impact, and broader influence of research endeavors. Additionally, it facilitates a deeper understanding of the interplay among authors, academic journals, and various research domains.

The analysis of the green hydrogen research domain utilized keywords that highlighted several key themes. The primary keywords included:

Hydrogen—Central to the research domain.

Green Hydrogen—Emphasizes the focus on sustainable production.

Hydrogen Production—Pertains to methods and technologies.

Hydrogen Storage—Involves the storage and transportation of hydrogen.

Hydrogen Energy—Covers the applications of hydrogen in energy systems.

Three main clusters emerged:

Production Methods: Keywords such as “water electrolysis”, “photocatalysis”, and “electrocatalysis”.

Applications: Keywords such as “fuel cell”, “energy storage”, and “renewable energy”.

Interdisciplinary Approaches: Keywords such as “density functional theory”, “machine learning”, and “nanoparticles”.

These keywords highlight the diverse and interconnected research areas within green hydrogen production, storage, and application, underscoring the technological, economic, and interdisciplinary aspects of this field.

A significant increase in related publications since 2008 is observed, marking an escalating focus on this interdisciplinary domain. Notably, countries such as China and the USA have emerged as key contributors, reinforcing their roles in spearheading sustainable and dependable energy solutions on a global platform. The principal scientific domains within this research landscape extend across engineering, energy storage, and renewable energy sectors. Through keyword analysis, central themes emerge, including hydrogen storage, electrolysis, advanced battery technologies, integration with the power grid, and the resilience of energy systems. Figure 8 presents a time series analysis of annual scientific production from the year 2000 through January 2024.

As shown in Figure 8 exhibits a gradual increase in the number of published articles over the years, with a notable spike in 2023, where publications surged to 19,822 articles. This is not an anomaly if compared to the surrounding years because, in 2024, only publications from January were counted as 2075. Assuming the same rate of publication for the entire year of 2024, the number approaches 25,000, keeping the ascendent trajectory that proves the enormous scientific interest. The overall upward trend underscores the expanding nature of scientific inquiry and publication in this field.

Figure 9 shows the countries that are leading in green hydrogen research, as measured by total citations (TC). These data reveal a geographic diversity among the highest contributors, with representation from Europe, North America, and Asia, underscoring the global interest in green hydrogen as a field of research.

The country with the highest total citations is China, with a remarkable count of 18,611 citations, which reflects its dominant position in green hydrogen research. Germany follows as the second most prolific contributor with 4006 citations, and the USA is ranked third with 3540 citations. Notably, the average citations per article for these countries indicate the relative impact of their research articles, with China at 1.80, Germany at 5.30, and the USA at 3.00.

Beyond these leaders, Italy, Korea, and India show substantial contributions, with total citations of 3392, 3269, and 3146, respectively, and higher average citations per article, particularly Italy, with 6.40. Australia and Canada also make significant contributions, with total citations of 2347 and 2053 and even higher average article citations of 5.90 and 5.60, respectively. The United Kingdom follows with a total citation count of 1932 and an average of 4.80 citations per article.

A notable outlier in terms of average citations per article is Cyprus, with 1732 total citations. Despite a lower total citation count, the average citation per article for Cyprus is an extraordinary 157.50, suggesting that while the volume of research output is smaller, the impact as measured by citations per article is significantly higher.

These data reflect not only the quantitative contributions of each country to the field of green hydrogen but also qualitatively, through the lens of citation impact per publication. The varying average citations per article across these countries may point to differences in research focus, publication practices, or the influence of specific high-impact articles within the field.

Figure 10 visualizes a bibliographic network representing the collaborative dynamics between countries in the realm of green hydrogen research.

The size of each node, labeled with the name of a country, corresponds to the total number of articles (TP) published by that country. The larger the node, the greater the number of publications, with China notably having the largest node, indicating it as the most prolific publisher in this field. The links, or edges, between nodes, illustrate the existence of co-authored papers between the paired countries. This denotes the presence of collaborative research efforts across national boundaries. The thickness of the lines between nodes is indicative of the volume of collaboration. Thicker lines suggest a higher level of collaborative output, while thinner lines indicate fewer co-authored publications.

Based on the visualization, it is apparent that China is not only the largest contributor in terms of published articles but also central to international collaboration, as depicted by the numerous and thick connecting lines. Other countries such as the USA, Australia, Germany, and the United Kingdom also show significant node sizes and multiple connections, suggesting their active roles in producing research as well as engaging in international partnerships.

The network also demonstrates the interconnectedness of research across continents, highlighting the global effort to advance the field of green hydrogen. Countries with smaller nodes, such as Malaysia, Egypt, and Turkey, while contributing fewer articles, are still connected within the network, suggesting participation in international research collaboration.

Such a figure is critical for understanding the landscape of global research collaboration and can help identify key players and potential opportunities for future partnerships in green hydrogen research.

Figure 11 presents a keyword co-occurrence network, offering a graphical depiction of the frequency at which keyword pairs co-occur within a collection of documents. This analytical approach is crucial for elucidating the intellectual framework and thematic focuses prevalent in a specific research domain. Specifically, for green hydrogen research, the network serves to pinpoint core themes and emergent trends through the examination of term interconnectedness in scholarly articles. This visualization aids in uncovering the collaborative and thematic landscape shaping the field of green hydrogen.

The visualization reveals “hydrogen” as the central and most prominent term, underscoring its foundational role in the research domain. Surrounding this core term are significant keywords such as “green hydrogen”, “hydrogen production”, “hydrogen storage”, and “hydrogen energy”, each forming distinct clusters that represent the primary focus areas within the field.

Node size indicates the frequency of each keyword’s appearance, with larger nodes denoting terms that occur more frequently. The proximity of nodes to one another and the thickness of connecting lines indicate the strength of the relationship between keywords: thicker lines reflect a higher frequency of co-occurrence, suggesting a closer or more extensively explored relationship in the literature.

Three main clusters emerge from the figure, inferred through color coding and connection density:

The first cluster centers around hydrogen production aspects, highlighted by keywords such as “water electrolysis”, “photocatalysis”, and “electrocatalysis”, pointing to a focus on the technological methods of generating hydrogen.

The second cluster emphasizes hydrogen’s application, with terms such as “fuel cell”, “energy storage”, and “renewable energy”, showcasing interest in hydrogen’s storage, delivery, and utilization within energy systems.

The third cluster likely involves the integration of advanced computational methods and materials science, with keywords such as “density functional theory”, “machine learning”, and “nanoparticles”, indicating an interdisciplinary approach that merges theoretical modeling, optimization through machine learning, and the development of novel materials.

This co-occurrence network offers a visual and analytical synthesis of the themes pervading green hydrogen research, illuminating both established research areas and nascent trends. This synthesis aids in pinpointing densely explored domains and those warranting further exploration.

Figure 12 represents a factorial analysis biplot used to visualize key thematic relationships within a dataset of literature titles related to green hydrogen production.

The horizontal axis, labeled “Dim 1”, accounts for 39.26% of these data’s variance, suggesting it is the most significant pattern detected across the titles, while the vertical axis, “Dim 2” captures an additional 22.74%. The plot’s keywords, represented by varying sizes of bubbles, hint at their frequency of occurrence within the titles; larger bubbles denote more common terms. Notably, there are discernible clusters that emerge: terms such as “hydrogen”, “production”, “green”, “energy”, “system”, “fuel cell”, and “gas” are grouped together, indicating a concentration on the practical aspects of generating green hydrogen as a fuel source. Another closely related set of terms, including “electrolysis”, “photocatalytic”, “carbon”, “catalysts”, “oxidation”, “water”, “activity”, and “splitting”, points towards the technical processes and chemical reactions central to hydrogen production, particularly those involving water splitting and catalysis. Meanwhile, “evolution”, “reaction”, “alkaline”, and “oxygen” are clustered, likely relating to the chemical processes of hydrogen production through methods such as alkaline electrolysis. More scattered throughout are terms such as “review”, “analysis”, “study”, “effects”, “mechanism”, “molecular”, and “organic”, suggesting a diversity of research approaches, from review papers to studies focused on the molecular mechanisms and organic components of hydrogen production. Overall, the biplot serves as a succinct visual summary of the research landscape around green hydrogen production, illustrating the interconnectivity and relative prominence of various research topics within the field.

All in all, as a carbon-free energy source, hydrogen offers numerous benefits, such as environmental sustainability and high energy density, making it a viable option for future energy systems [135].

5. Conclusions, Roadmap, and Recommendations for Hydrogen Development

Hydrogen stands as a pivotal element in advancing towards a sustainable energy landscape, offering substantial reductions in greenhouse gas emissions. With its versatile applications, especially in transportation and energy storage, hydrogen addresses key challenges of the energy transition. It enables clean industrial processes and enhances the economy through broad infrastructure development. Ensuring investment security for hydrogen import and distribution infrastructure is vital for realizing its full potential in a greener economy.

Hydrogen production encompasses a variety of methods, each differing in processes, materials, and energy sources, affecting costs and emissions. Emphasis on green and, to a lesser degree, blue hydrogen is essential for meeting climate objectives. Blue hydrogen’s feasibility hinges on the effective deployment of carbon capture and storage (CCS) technologies and assurance of its permanent underground containment. Provided these prerequisites are met, blue hydrogen could act as an interim solution toward achieving lower-emission hydrogen production.

Shifting towards green hydrogen, generated from renewable sources, is critical for achieving long-term environmental and climate goals. This transition necessitates comprehensive advancements in technology, widespread infrastructure development, supportive policies, and market incentives. Ultimately, for a carbon-neutral energy system, green hydrogen needs to become the dominant form. With thorough technical and economic analyses suggesting the feasibility of large-scale, cost-effective green hydrogen production, success hinges on the ample availability of renewable resources such as solar, wind, biomass, hydropower, and geothermal energy.

Currently, 95% of hydrogen is “gray” hydrogen from fossil fuels, leading to significant CO₂ emissions. “Blue” hydrogen captures some emissions, while “green” hydrogen, derived from water and renewables, is the sustainable goal. Electrolysis, accounting for only 5% of global hydrogen production, is key for green hydrogen. The main methods are Alkaline Electrolysis (AEL), established for a century; Proton Exchange Membrane Electrolysis (PEMEL), newer with high efficiency but costly; and High-Temperature Electrolysis (HTEL), promising high efficiency at 700–1000 °C. AEL is cost-effective but slow and less efficient in cold climates, PEMEL offers rapid response and high purity but is expensive, and HTEL shows potential in integrating with thermal plants and CO₂ utilization, producing syngas from water and CO2. Future advancements in these technologies are expected to reduce costs and enhance green hydrogen production.

It was noted from the literature that the investment costs for Alkaline Electrolysis (AEL), Proton Exchange Membrane Electrolysis (PEMEL), and High-Temperature Electrolysis (HTEL) are projected to decrease significantly from 2020 to 2050. AEL, starting at approximately 2000 Euro/kW in 2020, is expected to reduce to 500 Euro/kW by 2040 and remain stable. PEMEL is projected to decrease from about 2250 Euro/kW in 2020 to 500 Euro/kW by 2040. Similarly, HTEL, initially around 2400 Euro/kW in 2020, is anticipated to fall to 500 Euro/kW by 2040. These reductions indicate a trend towards increased economic viability for these electrolysis technologies in the future.

Harnessing excess electricity from fluctuating renewable sources for electrolysis stands as a significant route for hydrogen production. Yet, the cost efficiency of hydrogen generation is closely tied to the plant’s annual operational intensity, with plants operating at higher full-load hours benefiting from lower production costs. Moreover, the anticipation of future competition for this excess energy from other adaptable demands, such as power-to-heat systems, underscores the need for a considerable scale-up in renewable energy generation capabilities to meet the diverse requirements sustainably.

The scalability of renewable energies is constrained by the availability of resources and geographical limitations. Consequently, countries endowed with vast renewable resources and land are ideal for establishing large-scale, centralized hydrogen production facilities. Additionally, geopolitical factors will play a pivotal role in ensuring supply security for nations reliant on imported hydrogen, necessitating careful consideration of international relations and trade agreements in the development of a hydrogen-based economy.

Solar-thermochemical processes are positioned as scalable and cost-effective for hydrogen production, with future potential also seen in photo-electrochemical methods due to their flexibility in location. The use of renewable biomass for hydrogen production may face limitations due to competition for biomass and land. As a result, the bulk of future global hydrogen is expected to be derived from water-splitting techniques, primarily through electrolysis. Alkaline and PEM electrolysis systems are at the forefront, with the former already available on a large scale. Advances in other methods are ongoing, with cost reductions anticipated from improved manufacturing processes.

In the short term, prioritizing the installation and testing of large-scale electrolysis systems is crucial for developing the hydrogen market, alongside the expansion of renewable energy sources and the creation of supply chains in potential hydrogen-exporting nations. International partnerships will be instrumental in this phase. Given the initial higher costs of green hydrogen relative to gray hydrogen, policy measures are needed to incentivize its adoption, such as implementing effective CO₂ pricing and setting minimum quotas for green hydrogen usage, ensuring regulations promote hydrogen use over less expensive, short-term alternatives.

In the near to medium term, the focus is on enhancing electrolyzers through material and process innovations, including automated manufacturing and integration with renewable energies. The medium to long-term outlook shifts towards developing solar-thermochemical, algae-based, and photo-electrochemical processes. Key research areas include scalability, cost reduction, minimizing greenhouse gas emissions, ensuring raw material availability, and examining interactions with existing technologies, aiming to optimize the overall efficiency and sustainability of hydrogen production methods.