Photoelectrocatalytic Water Splitting: Significance of Cocatalysts, Electrolyte, and Interfaces

The efficiency of photoelectrocatalytic (PEC) water splitting is limited by the serious recombination of photogenerated charges, high overpotential, and sluggish kinetics of surface reaction. Herein we describe the recent progress on engineering the electrode-electrolyte and semi-conductor-cocatalyst interfaces with cocatalysts, electrolytes, and interfacial layers (interlayers) to increase the PEC efficiency. Introducing cocatalysts has been demonstrated to be the most efficient way to lower the reaction barrier and promote charge injection to the reactants. In addition, it has been found that electrolyte ions can influence the surface catalysis remarkably. Electrolyte cations on the surface can influence the water splitting and backward reactions, and anions may take part in the proton transfer processes, indicating that fine-tuning of the electrolyte parameters turns out to be an important strategy for enhancing the PEC efficiency. Moreover, careful modification of the interface between the cocatalysts and the semiconductor via suitable interlayers is critical for promoting charge separation and transfer, which can indirectly influence the surface catalysis. The mechanisms of surface catalysis are assumed to involve transfer of photogenerated holes to the surface active sites to form high-valent species, which then oxidize the water molecules. Many key scientific issues about the generation of photovoltage, the separation, storage, and transfer of carriers, the function of cocatalysts, the roles of electrolyte ions, and the influences of other parameters during PEC water splitting will be discussed in detail with some perspective views.