Sustainable solar hydrogen production: from photoelectrochemical cells to PV-electrolyzers and back again

Sustainable hydrogen production could, in principle, be accomplished along several different routes, where some of the most promising approaches involve utilization of solar energy. Photoelectrochemical cells (PEC-cells) and PV-electrolyzers for solar hydrogen production are here analyzed and compared. The analysis is performed by theoretically designing a number of intermediate devices, successively going from PEC-cells to PV-electrolyzers. The main physical processes: absorption, charge carrier separation, charge carrier transport, and catalysis are analyzed in the different devices. This demonstrates how the two concepts are related, and how one could easily be transformed and converted into the other. The awareness of the close relationship between PEC-cells and PV-electrolyzers is not as widely recognized as it should be. Traditionally, these two approaches have often been considered as fundamentally different, and are far too seldom analyzed in the same context. We argue that the different device designs for solar hydrogen production are best seen as essentially equivalent approaches, and as topological variations of the same basic theme, and can in many cases be unified under the acronym photo driven catalytic (PDC) devices. We further argue that much is to gain by acknowledging the similarities between PEC water splitting and PV-electrolysis, and that one concept alone should not be considered without also considering the other. The analysis and discussion presented could potentially lead to an increased fruitful crossbreeding of the accumulated knowledge in the respective sub-discipline, and aid in realizing solar hydrogen production as a sustainable and economically compatible energy alternative.