Structured Materials for Photoelectrochemical Water Splitting

Efficient and economical photoelectrochemical water splitting requires innovation on several fronts. Tandem solar absorbers could increase the overall efficiency of a water splitting device, but economic considerations motivate research that employs cheap materials combinations. The need to manage simultaneously light absorption, photogenerated carrier collection, ion transport, catalysis, and gas collection drives efforts toward structuring solar absorber and catalyst materials. This chapter divides the subject of structured solar materials into two principal sections. The first section investigates the motivations, benefits, and drawbacks of structuring materials for photoelectrochemical water splitting. We introduce the fundamental elements of light absorption, photogenerated carrier collection, photovoltage, electrochemical transport, and catalytic behavior. For each of these elements, we discuss the figures of merit, the critical length scales associated with each process and the way in which these length scales must be balanced for efficient generation of solar fuels. This discussion assumes a working knowledge of the fundamentals of semiconductor-liquid junctions; for more details the reader is encouraged to consult review articles. The second section of this chapter reviews recent approaches for generating structured semiconductor light absorbers and structured absorber-catalyst composites. This literature review emphasizes the insights gained in the last six years that are specifically related to photoelectrochemical water splitting, rather than to general photoelectrochemistry or photovoltaic applications. This chapter concludes with perspectives and an outlook for future efforts aimed at solar water splitting using structured materials. The realization of a practical, efficient, and useful water splitting device requires significant new developments in materials synthesis as well as deeper understanding of the relevant chemistry and physics. This chapter is intended to motivate such developments.