(254g) Design Principles for Oxygen Evolution Catalysts on Autonomous Water Splitting Particles

The development of approaches that harness solar energy to split water, thereby producing sustainable hydrogen fuel, is appealing as an environmentally sustainable alternative to fossil fuels.  Among various approaches, photocatalytic processes that use energetic charge carriers produced from photon absorption by semiconductors to drive hydrogen and oxygen evolution are considered a leading potential technology.  A recent technoeconomic analysis by the Department of Energy has identified an autonomous photocatalytic approach, where all necessary functionality exists in a single particle, as being the cheapest process design.

Realization of autonomous water structures requires the design of hetero-structured materials consisting of semiconductors that absorb a significant fraction of the solar spectrum and co-catalysts that efficiently accept charge carriers and have a low inherent overpotential to drive the oxygen (OER) and hydrogen (HER) evolution half reactions. Significant focus has been placed on understanding universal characteristics necessary for driving OER and HER efficiently using electrocatalysis. The binding energy of critical reaction intermediates on catalysts have been related to the required overpotential to drive the reactions, providing a design criterion for optimum HER and OER based on reaction kinetics. However, in autonomous photocatalytic systems, the maximum provided overpotential for each half reaction is defined by the difference between the semiconductor valence or conduction band and the OER and HER evolution potentials, respectively. Furthermore, charge transport across semiconductor-co-catalyst interfaces can cause a loss of overpotential depending on the direction and magnitude of the interfacial band bending.  

In this work we examine trends in oxygen evolution rates in autonomous photocatalytic systems for co-catalysts that have previously been identified as optimal in electrocatalytic environments.  In addition, we compare how the electronic structure of the semiconductor impacts the co-catalysts performance. Electronic structure measurements based on photoelectron spectroscopy provide mechanistic insight into why the performance of co-catalysts in autonomous water splitting systems cannot be directly correlated to the electrocatalytic performance. Finally we suggest design principles, based on the inherent electrocatalytic activity and co-catalyst electronic structure, for optimal OER co-catalysts in autonomous water splitting systems.