Solar Hydrogen Production

This Special Issue on solar hydrogen production focuses on innovative approaches and emerging technologies to transform solar energy into H2 or derivative energy carriers via water splitting pathways; those discussed include photoelectrochemical, photocatalytic, and thermochemical processes. The articles published in this edition range from fundamental materials research to integration in operational components and photoreactors, and experimentation. Sustainable and, particularly, solar-driven hydrogen production is an important topic of global interest because it can enable a shift from fossil fuels towards sustainable (solar) fuels. Because of the inherent variability of solar energy (and other renewables), cost-effective conversion and storage solutions are necessary in order to realize a truly sustainable energy future. Hydrogen is attractive as an energy vector because of its high mass-specific energy density, its inconsequential emissions upon combustion, limited reliance on precious metals and raw material mining (e.g. lithium),[1] and its capability to be utilized in a variety of applications. For example, it can be used as (seasonal) energy storage solution at the utility and residential scale and as fuel in fuel cell electric vehicles and heavy duty transport such as rail and shipping. Hydrogen can even be flexibly blended with natural gas for cleaner power production, or act as feedstock for chemical process industry. In 2017, Japan adopted a “Basic Hydrogen Strategy” aimed at decreasing the cost of hydrogen production and competing with gasoline and liquified natural gas.[2] In March 2020, the European Union launched the European Clean Hydrogen Alliance to install at least 40 GW of renewable hydrogen electrolysers by 2030.[3] Hydrogen has also been promoted in June 2021 by the U.S. Department of Energy's H2 Earthshot Program with the aim of producing $1/kg H2 in 1 decade.[4] To achieve this goal, large technological advancements must be made. Currently, about 75% of the world's H2 is produced from natural gas[5] (followed by coal) via steam methane reforming and the water gas shift reaction (CH4 + 2H2O → CO2 + 4H2). This is not a long-term, sustainable strategy because of the reliance on fossil fuels, but it can be coupled with CO2 sequestration to produce carbon free hydrogen, or so called “blue hydrogen”.[6] Storage of renewable energy as “green hydrogen” can be achieved via a variety of technologies and other renewable feedstocks such as biomass and water.[6] The focus of this Special Issue is specific to water splitting pathways that focus on driving the reversible chemical reaction 2H2O ↔ 2H2 + O2 using solar energy. This endothermic reaction may be driven directly by photons (photoelectrochemical[7, 8] and photocatalytic[8, 9]), renewable electrons (high and low temperature electrolysis),[10] renewable thermal energy (thermochemical processes)[11] or a combination of these. This Special Issue has research articles focused on photocatalytic, photoelectrochemical and thermochemical, as well as hybrid processes aimed at utilizing solar energy to drive the reforming of biomass derivatives or methane, which can offset a large percentage of the CO2 emissions because of the reduced heating demands. Articles ente.202100525, ente.202100469, ente.202100356, ente.202100302, ente.202100265, ente.202100259, ente.202100188, ente.202100161, and ente.202000950 focus on photocatalytic, ente.202100570, ente.202100461, ente.202100457, and ente.202100181 on photocatalytic and photoelectrochemical, and ente.202100515, ente.202100491, ente.202100473, ente.202100222, ente.202100220, and ente.202000925 focus on thermochemical. We hope you enjoy this Special Issue on Solar Hydrogen Production and are grateful to all the authors and editorial staff at Energy Technology that made this possible. Sincerely, Jonathan R. Scheffe, Sophia Haussener, Greta R. Patzke Jonathan Scheffe is an Associate Professor in the Department of Mechanical and Aerospace Engineering at the University of Florida. Prof. Scheffe is Principle Investigator of the Renewable Energy Conversion Laboratory that is focused on research in the area of conversion and storage of solar energy. Applications include the production of renewable fuels/electricity, H2 production and fuel reforming. He has co-authored more than 40 peer received publications in the field of solar thermal energy conversion. Sophia Haussener is an Associate Professor heading the Laboratory of Renewable Energy Science and Engineering at the Ecole Polytechnique Fédérale de Lausanne (EPFL). Her research is focused on providing multi-scale design guidelines for thermal, thermochemical, electrochemical and photoelectrochemical energy conversion reactors and processes through multi-physics modeling and demonstration. She is a co-founder of the startup SoHHytec, the former chair of ASME's Solar Energy Division, and member of multiple scientific advisory boards promoting energy conversion and solar fuels. Greta R. Patzke is a Full Professor in the Department of Chemistry at the University of Zurich. Her research includes the synthesis and monitoring of nanomaterials and composites for energy and environmental applications. This encompasses a wide range of molecular, nanostructured and solid transition metal-based catalysts for artificial photosynthesis. She is a board member of the UZH Research Priority Program “Light to Chemical Energy Conversion”, and she serves on various other panels for sustainable energy research.