The concerns over energy crisis and climate change caused by the excessive consumption of fossil fuels and the associated emission of greenhouse gases, have driven many countries to develop policies for an energy transition into zero-carbon energy sources, essentially aiming at decarbonizing their energy systems. Among various renewable sources, hydrogen has been hailed as an ideal alternative to provide secure, cost-effective, and non-polluting energy. In recent years, there has been noteworthy progress in hydrogen electrochemical conversion and utilization techniques and devices, including water electrolyzers, proton-exchange membrane fuel cells (PEMFCs), solid oxide fuel cells (SOFCs), etc. Each of these technologies has yielded a series of encouraging advances. To showcase these recent advances, Frontiers in Energy is launching a special issue titled “electrochemical conversion and utilization of hydrogen energy”.
In this special issue, eight feature articles, either from invited or selected thematic articles, have been compiled to collectively illustrate the typical advancements in this field. Among those, the five review papers provide invaluable insights into various aspects, including oxygen reduction reaction catalysts, full water electrolysis catalysts, and hydrogen evolution reaction catalysts, with the goal of achieving efficient, stable, and inexpensive targets. The other three papers address the construction of stable energy conversion devices from perspectives such as photocatalytic seawater decomposition devices, SOFC components, and operating conditions.
Electrochemical technology for water electrolysis to produce hydrogen is the key to obtaining green hydrogen energy and solving the energy crisis. Various newly proposed materials, such as bimetallic nanoparticles, two-dimensional (2D) nanomaterials, metal-organic framework (MOF), and high-entropy materials (HEMs) are attracting significant attention in water electrolysis. Chen et al. (this issue) prepared highly dispersed PtCo bimetallic nanoparticles (PtCo/MXene) loaded on MXene using a stepwise reduction strategy, displaying a good hydrogen evolution performance and stability under acidic conditions. Fang et al. (this issue) reviewed effective modification strategies for 2D molybdenum disulfide (MoS2), designing and developing low-cost, high-activity MoS2 catalysts by reducing the charge transfer potential barrier, increasing the density of active sites, and optimizing surface hydrophilicity.
Water electrolysis consists of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and the development of bifunctional catalysts for full water electrolysis is significant. Shang et al. (this issue) introduced Se elements into Co–Fe MOF by adopting the diffusion method, further evaluated their electrochemical performance, and found that Se doping helps to improve the bifunctional catalytic performance for water electrolysis. Sha et al. (this issue) summarized the synthesis strategies of HEM in water electrolysis, and explored its catalytic mechanism and stability as well as its application in hydrogen production by electrolyzing water. Additionally, they also elaborated on the application prospects of HEM to overcome the difficulties in the preparation and application of HEM catalysts.
In addition to electrochemical path for water electrolysis, the photolysis of water to produce hydrogen is also a key research direction. The study of its devices is crucial for commercial development. Li et al. (this issue) focused on the cutting-edge design and system integration of photothermal-photocatalytic devices, reviewing the latest research progress in seawater steam decomposition and industrial implementation over the past few decades. They also discussed the system structure for seawater hydrogen production and the challenges that have not yet been resolved.
Aside from water electrolysis for hydrogen production, the utilization of hydrogen energy also plays a crucial role in the future sustainable energy system. The corresponding electrochemical technologies, mainly including PEMFCs and SOFCs. Oxygen reduction reaction (ORR), is one of the key reactions of the PEMFC, and Pt-based catalysts are important for the ORR reaction. In order to optimize the behavior of Pt-based catalysts, morphological control is pivotal. Accordingly, Chen et al. (this issue) reviewed this phenomenon and emphasized the basic physicochemical changes behind morphology control, such as lattice strain and changes in the number of active sites. They then summarized the morphology of the catalysts, including different dimensions, different pore diameters, and different particle sizes. Finally, they proposed the prospects of artificial intelligence and machine learning in the control of Pt catalyst morphology in the future.
The design of electrochemical technology for hydrogen energy conversion devices is related to the possibility of commercial application. In SOFCs, to deal with the oxidation of metal interconnects and the Cr poisoning caused by oxidation, Cao et al. (this issue) use (La0.75Sr0.25)0.95MnO3–δ (LSM) and Mn1.5Co1.5O4 (MCO) to manufacture interconnector coatings. They found that the MCO coating had the best electrochemical performance, thermal cycling stability, and Cr diffusion inhibition performance. In addition to the concerns in materials, some other operating factors such as gas composition, flow rate, current, and temperature should also be considered to facilitate the further realization. Yu et al. (this issue) studied the impact of effective area on battery performance based on a three-dimensional (3D) electrochemical-thermal multiphysics numerical model. They deeply understood and explained the impact of gas composition, flow rate, current and temperature on this process, and improved the performance and durability of SOFC by optimizing and adjusting the above factors.
We hope that this special issue can provide readers with valuable information on electrochemical conversion and utilization of hydrogen energy. We are extremely grateful for the contributions of all authors and the review work of peer reviewers. Special thanks also go to the Editorial Office of Frontiers in Energy for the tireless efforts and strong support in bringing this special issue into production.
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