Introduction to Green Hydrogen

Abstract

Published as part of Chemical Reviews special issue “Green Hydrogen”. Green hydrogen, produced through water electrolysis powered by renewable energy, is an essential component of future global energy systems. In this thematic issue of Chemical Reviews, we present a collection of reviews on some of the key research topics related to the design of components and understanding of the elementary processes in current and emerging water-electrolysis technologies. Green hydrogen is produced through water electrolysis powered by renewable energy sources, such as wind, solar, or hydropower, or possibly nuclear energy, resulting in low carbon emissions. (1) While the CO₂ equivalent per kilogram of hydrogen produced (kgCO₂e/kgH₂) depends on many factors and requires a lifecycle analysis to assess, the U.S. Department of Energy’s Section 45 V tax credit targets green hydrogen at below 0.45 kgCO₂e/kgH₂. (2) This requires minimizing emissions throughout the entire production process, including electricity use and upstream activities. These emissions are much lower than “gray” hydrogen from reforming natural gas (CH₄ + 2H₂O → CO₂ + 4H₂) with ∼10 kgCO₂e/kgH₂ and “blue” hydrogen using natural gas with carbon capture and ∼4 kgCO₂e/kgH₂. (3) Green hydrogen can dramatically reduce carbon-dioxide emissions associated with transportation and heavy industry. In transportation, hydrogen will be used where direct electrification is challenging, for example in aviation, shipping, and trucking. How the hydrogen is used will depend on the scale and cost of the (currently expensive) infrastructure to store and transport hydrogen. Pipelines are in principle cost-effective, but retrofitting existing natural-gas infrastructure is difficult (in part due to metals embrittlement discussed in this issue (4)). Hydrogen can also be combined with captured CO₂ in efficient thermochemical processes to produce hydrocarbons, such as methanol or synthetic aviation fuels. (5) These renewable fuels can displace conventional fossil fuels without requiring major infrastructure changes, but CO₂ capture (not discussed here) is an issue. (6) Hydrogen is also essential for the production and upgrading of biofuels, particularly synthetic fuels derived from renewable biomass. Substantial amounts of hydrogen─roughly ¹/₂H₂ per carbon atom─in the resulting fuel are used to remove heteroatoms via hydro-deoxygenation, hydro-desulfurization, and hydro-denitrogenation processes. (7) Green hydrogen is likely to serve important roles in the future fully renewable electric grid that must deal with intermittent wind and solar generation on the daily, seasonal, and decadal time scales. (8,9) When energy storage is needed to fill gaps in production over multiple days, electricity generation from fuel cells and stored hydrogen become economically compelling, especially as the costs of fuel cells and electrolyzers continue to rapidly decline. (10) The ability of green hydrogen electrolyzers to serve as dispatchable load will also play a large role in enabling a reliable, expanded, and fully renewable electric grid. When renewable energy production is high or demand low, electrolyzers ramp up to productively use surplus electricity, and during peak demand, electrolyzers turn down or off. The hydrogen produced from these intermittent electrolyzers will be buffered in storage facilities and piped for industrial uses in fertilizer synthesis via Haber–Bosch chemistry and future green-steel and synthetic fuels industries. Fertilizer synthesis alone accounts for 2% of global CO₂ emissions─most of which come from making the fossil hydrogen currently used (and thus readily displaced by green hydrogen). This also enables seasonal energy storage to ensure grid reliability and obviate the need for natural-gas peaker plants. Electric power markets will evolve to create profitable businesses around this range of grid services. Despite these advantages for green-hydrogen production, there remain many technical and scaling challenges. The U.S. DOE has established the Hydrogen Earthshot goal to reduce the cost of producing green hydrogen to $1 per kg by 2030, roughly an 80% reduction from the current ∼$5/kg. Storing and transporting hydrogen add more challenges that increase costs, as hydrogen gas has a low energy density compared to liquid hydrocarbons. Liquefaction of hydrogen requires energy-intensive processes. New infrastructure, including high-pressure tanks, possible cryogenic systems, and pipelines resistant to hydrogen embrittlement are needed. Transportation costs are historically expensive due to limited capacity, safety protocols, and energy losses from evaporation. There are a number of realistic pathways to reach the $1 per kg hydrogen target, bolstered by the science and technology innovation discussed in this thematic issue, coupled with reduced production costs with scaling. (11) A basic calculation is simple. The thermoneutral voltage for liquid water electrolysis is 1.48 V; below this voltage requires extra heat input. Faraday’s law (and unit conversion) yields a minimum of 39.4 kW·h per kg of hydrogen. A modern electrolyzer runs between 1.7 and 2.0 V, depending on current density, and there is additional electrical power associated with balance of plant. A practical high-performance electrolyzer thus requires ∼50 kW·h per kg H₂. Therefore, electricity is needed at $0.01–$0.015 per kW·h so that the electricity cost is $0.50–$0.75 per kg H₂. This need for extremely low-cost electrical energy has been used to argue that the cost target is not possible, particularly by entities involved in competing energy technologies. Yet the economics of the electric grid is changing. In 2024 there were a record number of hours with negative electricity prices in the European, Texas, and California markets where intermittent renewable generation has high penetration. While markets will adapt to limit negative electric prices, renewables will be overbuilt to provide capacity when rates are high─this necessarily leads to periods when rates are low. The fraction of time for which rates will average $0.01 per kW·h is unknown but is likely to be substantial. A reasonable prediction is that electrolyzer systems can run with 30% capacity factor and access ∼$0.01 per kW·h electricity, i.e. during the middle of the day and/or when there is high wind. While current electrolyzer prices from US and European manufacturers are too high to run at 30% capacity factor (∼1000 $/kW combined price for alkaline stacks and balance of plant), (12) electrolyzer prices are dropping with manufacturing scale; similar, although a bit slower, to how battery and photovoltaic technologies have, (13) which both dropped ∼10× since 2010. If we then assume a reasonable capital cost of $200 per kW by 2030, and a 20-year lifetime running at 30% capacity factor, the amortized cost is $0.19 per kg of H₂. While there are additional maintenance, land, and financing costs, the sum of electric and equipment expenses can be below $1 per kg of hydrogen over a range of scenarios. The above simple analysis makes the case that the core needs for electrolysis are that next-generation technologies: be relatively inexpensive and thus not use large amounts of precious metals (although low loadings may be acceptable), have electrode, catalyst, and system designs that allow for intermittent operation without damage to the cell, be able to operate with high efficiency given the high fraction of the cost electrical energy is to the output hydrogen, even at ∼$0.01 per kW·h, and must be durable for at least a decade or two under these conditions (recognizing that electrode/stack reconditioning may be much less expensive than building entirely new systems and enable operation for much longer). The contributions in this thematic issue of Chemical Reviews reflect the interdisciplinary nature of the science and engineering research needed to overcome the challenges associated with green hydrogen and reach the cost levels where hydrogen is able to dramatically reduce carbon dioxide emissions from large segments of the global economy. Several of the reviews deal with solid-oxide-based electrochemical technology. For example, Ming Chen’s review provides analysis of solid oxide electrolysis cells (SOECs), which offer the highest efficiency (∼36 kWh/kg) among all technologies because they operate at temperatures near 800 °C, where thermodynamics favor efficient water splitting and the electrode kinetics are fast. (14) The review covers advances in SOEC component design─from electrodes to electrolytes─and discusses strategies for mitigating performance degradation over time, which is a major issue today with this technology. Challenges, such as stack scalability, cost, and system durability, are addressed, with recommendations for future research focused on enhancing reliability and reducing capital costs. Eranda Nikolla’s review focuses on solid-oxide cells (SOCs), particularly those that can switch between fuel-cell and electrolyzer modes. (15) SOCs are relevant in the context of green hydrogen because they can efficiently convert renewable electricity into hydrogen during times of surplus and generate power from hydrogen when energy demand peaks, both with much higher thermodynamic efficiency than possible in lower temperature (<100 °C) systems. The review highlights significant degradation of electrocatalytic materials under dynamic conditions, such as switching between modes or handling multiple fuel types, and identifies the bottlenecks in catalytic performance due to effects such as sintering, poisoning, and undergoing phase transitions, and provides guidelines for designing stable, efficient electrode catalysts. Other reviews discuss materials and processes for low temperature electrolysis systems that rely on liquid water and either alkaline or acidic electrolytes. Paul Kempler’s review focuses on the role of gas bubbles generated during the hydrogen and oxygen evolution reactions in electrolysis. (16) Gas evolution impacts the overall efficiency of electrolyzers by blocking active catalytic sites, increasing electrical resistance, and disrupting fluid dynamics at the electrode surface. The review discusses techniques for characterizing gas evolution in real time and optimizing electrode designs to enhance gas removal at high current densities. Effective bubble management is particularly essential for liquid alkaline electrolysis technology that has been historically limited to lower current densities (<0.5 A cm^–2) and constant current operation, but where innovation could enable dynamic operation and higher currents (∼1–2 A cm^–2) at high efficiency. The review identifies key strategies, such as tailoring electrode surfaces and structure to promote gas transport and bubble detachment. Managing gas evolution directly impacts the operational efficiency of electrolyzers, particularly at the high currents needed to reduce capital expense. Dirk Henkensmeier’s review focuses on another key component for alkaline water electrolysis─the separator and/or membrane between the anode and cathode. (17) The review traces the historical evolution from asbestos-based diaphragms to polymeric composite diaphragms and the possibility to employ anion exchange membranes (AEMs), which offer improved conductivity and safety but are not yet fully proven stable for commercial application. Challenges in balancing the trade-offs between conductivity, membrane lifetime, and operational costs are discussed highlighting recent developments in ion-solvating membranes and improvements in AEM durability in strong hot alkaline solutions through chemical structure design. Thinner separators with lower ionic resistance that can effectively separate the evolved hydrogen and oxygen under varying load are central to lower the capital cost of the liquid alkaline electrolysis through higher current density while maintaining high efficiency. Travis Jones’s review provides a detailed review of oxygen evolution reaction (OER) catalysts and their fundamental mechanisms, focusing on the challenges involved in understanding and modeling multiple electron transfers and bond-forming/breaking steps. (18) The review traces the evolution of OER models, from early phenomenological approaches to modern ab initio simulations that incorporate electric fields, solvent effects, and explicit reaction kinetics. Jones explores how these models compare with experiment and identifies areas where key work is needed to bridge theory and practice. Because the OER is one of the major sources of inefficiency in low-temperature water-electrolysis technology, these understandings are central to designing electrodes with better activity and durability without resorting to high mass loading of expensive precious metals (in the case of proton-exchange membrane, PEM, electrolysis technology). Laurie King’s review addresses an important challenge facing PEM electrolysis technology, the use of precious metals. (19) PEM electrolyzers offer perhaps the most-compelling performance metrics, but remain more expensive. Emerging alkaline membrane electrolyzer systems also tend to use precious metal cathodes. (20) The review highlights alternative catalyst materials, including molybdenum disulfides (MoS₂), nickel–molybdenum alloys, phosphides, and carbides, with a focus on their stability and performance in both acidic and alkaline electrolytes, reducing dependence on expensive precious metals, such as platinum, used in hydrogen evolution reaction catalysts. King also evaluates metrics for measuring HER activity and stability, offering insights into best practices for catalyst testing under real-world conditions. Jeffrey Dick’s review focuses on single-entity electrocatalysis, an emerging field that investigates the catalytic behavior of individual nanoparticles, atomic structures, or molecules. (21) Traditional studies measure the performance of catalysts as ensembles, where the activity of many particles is averaged. Dick’s review highlights the experimental challenges of studying single entities, such as the development of new measurement tools and techniques capable of probing individual particles in real-time. Case studies discussed in the review include hydrogen evolution, carbon-dioxide reduction, and hydrazine oxidation, each demonstrating how single-entity studies can uncover new insights into catalyst behavior that ensemble measurements cannot capture. This approach is useful in future catalyst design and targeted improvements in the catalyst nanopowders and nanostructures used in electrolyzer electrodes. Returning to foundational understanding, Yuanyue Liu’s review explores the potential of atomistic modeling to accelerate the discovery of high-performance catalysts for green hydrogen production. (22) This review focuses on addressing the complexity of electrochemical interfaces, which involve interactions between dynamic catalysts, electrolyte ions, solvent molecules and electrode surfaces. Liu discusses techniques to model solvation effects, electrode potentials, reaction kinetics, and pH. The review also highlights computational spectroscopy methods, which can bridge the gap between theory and experiment by offering molecular-level insights into mechanisms. Aliaksandr Bandarenka’s review provides an in-depth review of the electrical double layer (EDL)─the critical region of molecular dimensions that forms at the interface between the electrode and the electrolyte, where ions accumulate, large electric fields exist, and solvent and other molecular species have properties very different than those of their bulk counterparts. (23) The review explores experimental techniques and theoretical models used to assess double-layer capacitance and ion interactions at electrochemical interfaces. Bandarenka emphasizes the influence of electrode composition, electrolyte chemistry, temperature, and pressure on EDL behavior, providing insights for optimizing catalysts and improving the efficiency and durability of electrochemical systems. Finally, Haiyang Yu and Zhiliang Zhang’s review addresses perhaps one of the most critical issues for green hydrogen systems: hydrogen embrittlement, a phenomenon in which hydrogen atoms penetrate metallic structures, degrading their mechanical strength and causing cracks or fractures under stress. (4) This degradation is particularly dangerous for pipelines, tanks, and equipment used in hydrogen transport, storage, and processing. With green hydrogen production expected to scale rapidly, hydrogen embrittlement poses a significant threat to infrastructure reliability and safety particularly for any repurposed infrastructure from the natural gas industry. The review discusses how hydrogen interacts with different metals─steels, nickel alloys, and aluminum alloys─commonly used in energy infrastructure. Yu and Zhang explore the behavior of high-entropy alloys, the use of additive manufacturing to produce materials resistant to embrittlement, and the need for AI-driven predictive models to assess material performance and forecast failures. Reliable and predictable materials are central to hydrogen pipelines and storage facilities deployment at scale. The reviews presented in this thematic issue of Chemical Reviews collectively address some of the key science and engineering challenges associated with green-hydrogen production. From fundamental research on catalysts and materials to applied studies on electrolysis systems and gas-evolution, these contributions reflect some, but certainly not all, of the breadth of innovation required to realize the potential of green hydrogen. With work, green hydrogen is poised to be a cornerstone in the global energy transition. Shannon W. Boettcher is the Vermeulen Professor in the Departments of Chemical and Biomolecular Engineering and Chemistry at the University of California, Berkeley, and the Deputy Director of the Energy Storage and Distributed Resources Division at Lawrence Berkeley National Laboratory. From 2010–2023, he was a Professor of Chemistry at the University of Oregon, where he founded the Oregon Center for Electrochemistry and, along with Paul Kempler, the Nation’s first master’s program in Electrochemical Technology. His research includes mechanistic studies of interfacial and transport processes in electrochemical systems and applying the insights in the design of next-generation materials, components, and devices for electrochemical technology, including for hydrogen production by water electrolysis. In 2023 he was named the Blavatnik National Award Laureate in Chemistry. This article references 23 other publications. This article has not yet been cited by other publications.