Hydrogen, Volume 6, Issue 2 (June 2025) – 15 articles

Green hydrogen has emerged as a critical pillar of sustainable energy transitions, with its potential as a carbon-free fuel to decarbonize hard-to-electrify sectors while bolstering energy security. This study examines the trajectory of green hydrogen and renewable energy research within extended BRICS nations (the five core BRICS plus recent entrants) using bibliometric analysis, aiming to map publication trends, thematic focus, and collaborative networks from 2005 to 2024. A comprehensive dataset of 292 publications (2005–2024) was retrieved from Scopus. The results reveal a rapidly growing body of literature with an accelerating output in recent years and substantial citation impact. China leads in both publication volume and influence, followed by India and Russia, attesting to robust national research initiatives. Approximately one-quarter of the studies involve international co-authorship, underlining active collaboration among these countries and beyond. The novelty of this study lies in conducting the first comprehensive bibliometric analysis of green hydrogen and renewable energy research within the extended BRICS, highlighting publication trends, collaboration patterns, and thematic evolution to inform future research and policy development. These bibliometric insights offer valuable guidance for policymakers and industry stakeholders by highlighting core strengths (hydrogen production technologies) and pinpointing gaps where capacity-building is needed. Full article

International projects study the safety aspects of the storage and long-distance transportation of liquid hydrogen at large scales. Catalytic recombiners, which are today key elements of hydrogen risk mitigation in nuclear power plants, could become an efficient safety device to prevent flammable gas mixtures after liquid hydrogen leakages in closed rooms. This study tackles fundamental questions about the operational behavior of typical recombiner catalysts related to the conditions of the start-up and the termination of the catalytic reaction. For this purpose, small-scale catalyst sheets with coatings containing either platinum or palladium as active materials were exposed to gas mixtures of air and hydrogen of up to 4 vol.% at temperatures between −50 °C and 20 °C. Both platinum and palladium showed variation to performance and had stochastic results. Overall, the initialized platinum catalyst was better than the palladium. The experimental results show that the transfer of the recombiner technology from its current application is not easily possible. Full article

Hydrogen and some of its derivatives (such as e-methanol, e-methane, and e-ammonia) are promising energy carriers that have the potential to replace conventional fuels, thereby eliminating their harmful environmental impacts. An innovative use of hydrogen as a zero-emission fuel is forming weakly ionized plasma by seeding the combustion products of hydrogen with a small amount of an alkali metal vapor (cesium or potassium). This formed plasma can be used as a working fluid in supersonic open-cycle magnetohydrodynamic (OCMHD) power generators. In these OCMHD generators, direct-current (DC) electricity is generated straightforwardly without rotary turbogenerators. In the current study, we quantitatively and qualitatively explore the levels of electric conductivity and the resultant volumetric electric output power density in a typical OCMHD supersonic channel, where thermal equilibrium plasma is accelerated at a Mach number of two (Mach 2) while being subject to a strong applied magnetic field (applied magnetic-field flux density) of five teslas (5 T), and a temperature of 2300 K (2026.85 °C). We varied the total pressure of the pre-ionization seeded gas mixture between 1/16 atm and 16 atm. We also varied the seed level between 0.0625% and 16% (pre-ionization mole fraction). We also varied the seed type between cesium and potassium. We also varied the oxidizer type between air (oxygen–nitrogen mixture, 21–79% by mole) and pure oxygen. Our results suggest that the ideal power density can reach exceptional levels beyond 1000 MW/m³ (or 1 kW/cm³) provided that the total absolute pressure can be reduced to about 0.1 atm only and cesium is used for seeding rather than potassium. Under atmospheric air–hydrogen combustion (1 atm total absolute pressure) and 1% mole fraction of seed alkali metal vapor, the theoretical volumetric power density is 410.828 MW/m³ in the case of cesium and 104.486 MW/m³ in the case of potassium. The power density can be enhanced using any of the following techniques: (1) reducing the total pressure, (2) using cesium instead of potassium for seeding, and (3) using air instead of oxygen as an oxidizer (if the temperature is unchanged). A seed level between 1% and 4% (pre-ionization mole fraction) is recommended. Much lower or much higher seed levels may harm the OCMHD performance. The seed level that maximizes the electric power is not necessarily the same seed level that maximizes the electric conductivity, and this is due to additional thermochemical changes caused by the additive seed. For example, in the case of potassium seeding and air combustion, the electric conductivity is maximized with about 6% seed mole fraction, while the output power is maximized at a lower potassium level of about 5%. We also present a comprehensive set of computed thermochemical properties of the seeded combustion gases, such as the molecular weight and the speed of sound. Full article

A novel design of a proton exchange membrane electrolyzer is presented. In contrast to previous designs, the flow field plates are round and oriented horizontally with the feed water entering from a central hole and spreading evenly outward over the anode flow field in radial, interdigitated flow channels. The cathode flow field consists of a spiral channel with an outlet hole near the outside of the bipolar plate. This results in anode and cathode flow channels that run perpendicular to avoid shear stresses. The novel sealing concept requires only o-rings, which press against the electrolyte membrane and are countered by circular gaskets that are placed over the flow channels to prevent the membrane from penetrating the channels, which makes for a much more economical sealing concept compared to prior designs using custom-made gaskets. Hydrogen leaves the electrolyzer through a vertical outward pipe placed off-center on top of the electrolyzer. The electrolyzer stack is housed in a cylinder to capture the oxygen and water vapor, which is then guided into a heat exchanger section, located underneath the electrolyzer partition. The function of the heat exchanger is to preheat the incoming fresh water and condense the escape water, thus improving the efficiency. It also serves as internal phase separator in that a level sensor controls the water level and triggers a recirculation pump for the condensate, while the oxygen outlet is located above the water level and can be connected to a vacuum pump to allow for electrolyzer operation at sub-ambient pressure to further increase efficiency and/or reduce the iridium loading. Full article

Green hydrogen is gaining recognition as a viable substitute for fossil fuels, presenting a sustainable solution for global decarbonization. While significant progress has been made in hydrogen production, storage, and utilization, there remains a crucial need to assess its economic viability and integration into current energy systems and to reduce its emission footprint. This review delves into the prospects and challenges of green hydrogen deployment into the renewable energy mix, with a particular focus on cost reduction approaches, storage limitations, transportation, scalability, advancements in electrolysis, and diverse sectoral applications. By analyzing recent technological developments and policy frameworks, this review contributes a thorough evaluation of green hydrogen’s viability to achieve net-zero emissions. Furthermore, this review enhances understanding of the role of green hydrogen in climate change mitigation by identifying major scalability barriers and proffering actionable solutions, assessing life cycle emission reductions, and examining key policy measures required for large-scale adoption. Our analysis emphasizes the importance of advancing green hydrogen storage solutions, increasing the efficiency of electrolysis processes, reducing costs, and implementing stronger policy measures to support large-scale adoption. Our findings and results demonstrate that green hydrogen has 66–95% potential of reducing global warming when integrated with other renewables. Its widespread adoption will drastically reduce anticipated climate mitigation costs of $10.0–15.7 trillion in the next decades, with progress in electrolysis technology, cost efficiency, and various industrial applications. Our recommendation for future studies emphasizes improved catalyst durability, material enhancements for electrolyzer, integration of green hydrogen into hybrid renewable energy networks, and establishment of globally coordinated policies to accelerate its deployment. By bridging the divide between technological advancements and practical implementation, this research provides valuable guidance for scientists, policymakers, and industry stakeholders striving for a sustainable energy transition. Full article

This paper provides a system-level and dimensional analysis of green hydrogen, assessing its realistic deployment potential within broader energy transitions. While green hydrogen—produced via electrolysis using renewable electricity—is often promoted as a versatile decarbonization solution for industry, mobility, and civil applications, its practical implementation is constrained by high energy consumption, conversion inefficiencies, and complex supply chain requirements. This study highlights typical energy demands across key sectors and evaluates the scale of the renewable infrastructure needed to support them, offering quantitative insight into the feasibility of large-scale hydrogen integration. It also reflects current technological maturity, noting that many promising solutions remain far from industrial readiness. Finally, the paper underscores the importance of targeted policies and bankable investment models to foster the development of hydrogen ecosystems, emphasizing that its role should be framed within a selective, evidence-based strategy that focuses on high-impact applications. The analysis identifies key dimensional challenges, including the magnitude of renewable energy capacities required for sector-wide hydrogen integration and the scale of infrastructure investments needed to bridge current gaps. Full article

This study comparatively evaluates the performance of ABO₃ perovskite materials (A = La, Ca, Sr; B = Mn, Fe) as oxygen carriers in three-step Chemical Looping Hydrogen (CLH) technology, focusing on redox behavior, oxygen transport capacity, hydrogen production, and selectivity under controlled pulse-mode conditions. The redox behavior of the materials is analyzed in relation to their defect chemistry. Perovskites such as (La_1−xCa_x)MnO₃, (La_1−xSr_x)MnO₃, and (La_0.6Ca_0.4)(Mn_1−xFe_x)O₃ were synthesized via wet chemical methods and tested in chemical looping cycles. Doping A-site cations with Ca or Sr enhanced oxygen delivery capacity by more than 100% upon reduction with CH₄ when dopant content (x) increased from 0 to 0.5. However, H₂ selectivity decreased from 52% to 2.5% for (La_1−xCa_x)MnO₃ and from 46% to 14% for (La_1−xSr_x)MnO₃ under the same conditions. In contrast, substituting Mn with Fe significantly improved hydrogen production, particularly in LaFeO₃, which exhibited the highest hydrogen selectivity and yield. At 1000 °C, LaFeO₃ produced nearly 10 mmol H₂ g⁻¹, with 80% generated during the reduction step at 99.9% selectivity and the remaining 20% during the water-splitting step at 100% selectivity. These results are linked to the extent of B-site cation reduction reactions (i) B⁴⁺ → B³⁺, which facilitates complete fuel oxidation and (ii) B³⁺ → B²⁺, which leads to partial fuel oxidation. The reverse of (ii) also contributes to H₂ production during water splitting. Additionally, the study assesses the materials’ microstructure and stability over prolonged cycles. The findings highlight Fe-based perovskites, particularly LaFeO₃, as promising candidates for CLH applications, emphasizing the need for structural and compositional optimization to enhance hydrogen production efficiency. Full article

As hydrogen emerges as a pivotal energy carrier in the global transition towards net-zero emissions, addressing its technological and regulatory challenges is essential for large-scale deployment. The widespread adoption of hydrogen technologies requires extensive research, technical advancements, validation, testing, and certification to ensure their efficiency, reliability, and safety across various applications, including industrial processes, power generation, and transportation. This study provides an overview of key enabling technologies for green hydrogen production and distribution, highlighting the critical challenges that must be overcome to facilitate their widespread adoption. It examines key hydrogen use cases across multiple sectors, analysing their associated technical and infrastructural challenges. The technological advancements required to improve hydrogen production, storage, transportation, and end-use applications are discussed. The development of state-of-the-art testing and validation facilities is also assessed, as these are vital for ensuring safety, performance, and regulatory compliance. This work also reviews some of the ongoing academic and industrial initiatives in the UK aimed at promoting technological innovation, advancing hydrogen expertise, and developing world-class testing infrastructures. This study emphasises the need for stronger, more integrated collaboration between universities, industries, and certifying bodies for building a strong network that promotes knowledge sharing, standardisation, and innovation for expanding hydrogen solutions and creating a sustainable hydrogen economy. Full article

Acoustic oscillations in cryogenic systems can either be imposed intentionally, as in pulse-tube cryocoolers, or occur spontaneously due to Taconis-type thermoacoustic instabilities. To predict the propagation of sound waves in ducts with sudden changes in cross-sectional areas, minor losses associated with such transitions in oscillatory flows must be known. However, the current modeling approaches usually rely on correlations for minor loss coefficients obtained in steady flows, which may not accurately represent minor losses in sound waves. In this study, high-fidelity computational fluid dynamics simulations are undertaken for acoustic oscillations at transitions between tubes of different diameters filled with cryogenic hydrogen. The variable parameters include the tube diameter ratios, temperatures (80 K and 30 K), and acoustic impedances corresponding to standing and traveling waves. Computational simulation results are compared with reduced-order acoustic models to develop corrections for minor loss coefficients that describe transition losses in sound waves more precisely. The present findings can improve the accuracy of design calculations for acoustic cryocoolers and predictions of Taconis instabilities. Full article

Hydrogen energy is essential in the transition to sustainable transportation planning, providing a clean and efficient alternative to traditional fossil fuels. As a versatile energy carrier, hydrogen facilitates the decarbonization of diverse transportation modes, including passenger vehicles, heavy-duty trucks, trains, and maritime vessels. To justify and clarify the role of hydrogen energy in sustainable transportation planning, this study conducts a comprehensive techno-economic and environmental assessment of hydrogen production in the USA, Europe, and China. Utilizing the Shlaer–Mellor method for policy modeling, the analysis highlights regional differences and offers actionable insights to inform strategic decisions and policy frameworks for advancing hydrogen adoption. Hydrogen production potential was assessed from solar and biomass resources, with results showing that solar-based hydrogen production is significantly more efficient, producing 704 tons/yr/km², compared to 5.7 tons/yr/km² from biomass. A Monte Carlo simulation was conducted to project emissions and market share for hydrogen and gasoline vehicles from 2024 to 2050. The results indicate that hydrogen vehicles could achieve near-zero emissions and capture approximately 30% of the market by 2050, while gasoline vehicles will decline to a 60% market share with higher emissions. Furthermore, hydrogen production using solar energy in the USA yields a per capita output of 330,513 kg/yr, compared to 6079 kg/yr from biomass. The study concludes that hydrogen, particularly from renewable sources, holds significant potential for reducing greenhouse gas emissions, with policy frameworks in the USA, Europe, and China focused on addressing energy dependence, air pollution, and technological development in the transportation sector. Full article

This study explores hydrogen’s potential as a sustainable energy source for Brunei, given the nation’s reliance on fossil fuels and associated environmental concerns. Specifically, it evaluates two hydrogen production technologies; steam methane reforming (SMR) and alkaline water electrolysis (AWE), through a techno-economic framework that assesses life cycle cost (LCC), efficiency, scalability, and environmental impact. SMR, the most widely used technique, is cost-effective but carbon-intensive, producing considerable carbon dioxide emissions unless combined with carbon capture to yield “blue hydrogen”. On the other hand, AWE, particularly when powered by renewable energy, offers a cleaner alternative despite challenges in efficiency and cost. The assessment revealed that AWE has a significantly higher LCC than SMR, making AWE the more economically viable hydrogen production method in the long term. A sensitivity analysis was also conducted to determine the main cost factors affecting the LCC, providing insights into the long term viability of each technology from an operational and financial standpoint. AWE’s economic viability is mostly driven by the high electricity and feedstock costs, while SMR relies heavily on feedstock costs. However, Environmental Impact Analysis (EIA) indicates that AWE produces significantly higher carbon dioxide emissions than SMR, which emits approximately 9100 metric tons of carbon dioxide annually. Nevertheless, findings suggest that AWE remains the more sustainable option due to its higher LCC costs and compatibility with renewable energy, especially in regions with access to low-cost renewable electricity. Full article

Efficient hydrogen storage is critical for advancing hydrogen-based technologies. This study investigates the effects of pressure and surface area on hydrogen storage in three carbon-based materials: graphite, graphene oxide, and reduced graphene oxide. Hydrogen adsorption–desorption experiments under pressures ranging from 1 to 9 bar revealed nonlinear storage capacity responses, with optimal performance at around 5 bar. The specific surface area plays a pivotal role, with reduced graphene oxide and exhibiting a surface area of 70.31 m²/g, outperforming graphene oxide (33.75 m²/g) and graphite (7.27 m²/g). Reduced graphene oxide achieved the highest hydrogen storage capacity, with 768 sccm and a 3 wt.% increase over the other materials. In assessing proton-exchange fuel cell performance, this study found that increased hydrogen storage correlates with enhanced power density, with reduced graphene oxide reaching a maximum of 0.082 W/cm², compared to 0.071 W/cm² for graphite and 0.017 W/cm² for graphene oxide. However, desorption rates impose temporal constraints on fuel cell operation. These findings enhance our understanding of pressure–surface interactions and underscore the balance between hydrogen storage capacity, surface area, and practical performance in carbon-based materials, offering valuable insights for hydrogen storage and fuel cell applications. Full article

Green hydrogen production is emerging as a crucial component in global decarbonization efforts. This review focuses on the role of computational approaches and artificial intelligence (AI) in optimizing green hydrogen technologies. Key approaches to improving electrolyzer efficiency and scalability include computational fluid dynamics (CFD), thermodynamic modeling, and machine learning (ML). As an instance, CFD has achieved over 95% accuracy in estimating flow distribution and polarization curves, but AI-driven optimization can lower operational expenses by up to 24%. Proton exchange membrane electrolyzers achieve efficiencies of 65–82% at 70–90 °C, but solid oxide electrolyzers reach up to 90% efficiency at temperatures ranging from 650 to 1000 °C. According to studies, combining renewable energy with hydrogen production reduces emissions and improves grid reliability, with curtailment rates of less than 1% for biomass-driven systems. This integration of computational approaches and renewable energy ensures a long-term transition to green hydrogen while also addressing energy security and environmental concerns. Full article

The operational performance of proton exchange membrane fuel cells (PEMFC) is highly influenced by temperature, making effective thermal management essential. However, the multivariate coupling between pumps and radiators presents significant control challenges. To address this issue, a dual DDPG-PID control strategy is proposed, integrating temperature and flow rate variations to enhance system stability and response. Simulation results demonstrate that the proposed method significantly reduces temperature control errors and improves response time compared to conventional PID-based strategies. Specifically, the D-DDPG PID achieves a temperature error reduction of up to 75.4% and shortens the average tuning time by up to 25.6% compared to PSO-PID. Furthermore, the strategy optimizes cooling system performance, demonstrating its effectiveness in PEMFC thermal management. Full article

The challenge of achieving net-zero CO₂ emissions will require a significant scaling up of the production of several raw materials that are critical for decarbonizing the global economy. In contrast, metal extraction processes utilize carbon as a reducing agent, which is oxidized to CO₂, resulting in considerable emissions and having a negative impact on climate change. In order to abate their emissions, extractive industries will have to go through a profound transformation, including switching to alternative climate-neutral energy and feedstock sources. This paper presents the authors’ perspectives for consideration in relation to the H₂ potential for direct reduction of oxide and sulfide ores. For each case scenario, the reduction of CO₂ emissions is analyzed, and a breakthrough route for H₂S decomposition is presented, which is a by-product of the direct reduction of sulfide ores with H₂. Electrified indirect-fired metallurgical kiln advantages are also presented, a solution that can substitute fossil fuel-based heating technologies, which is one of the main backbones of industrial processes currently applied to the extractive industries. Full article