(Invited) Chemical and Photochemical Transformations of Bismuth Vanadate and Catalyst Integration for Stable Photoanodes

Monday, 29 May 2017: 11:20
Churchill C2 (Hilton New Orleans Riverside)
G. Liu, I. D. Sharp, and F. M. Toma (Lawrence Berkeley National Laboratory)
Artificial photosynthesis relies on the availability of stable semiconductors that efficiently harness the solar radiation and convert it into chemical energy for the sustainable production of solar fuels. Because of the thermodynamic and kinetic challenges associated with the oxygen production from water and of the harsh conditions in which this reaction is performed, much attention has been devoted to the study of stable n-type metal oxide semiconductors. Among available candidates for photoanodes, bismuth vanadate (BiVO4) is a promising, widely utilized metal oxide material for water splitting catalysis. In this work, we present a detailed study on the corrosion and photocorrosion of BiVO4 through analysis of changes in morphology, bulk and surface composition as a function of the photoelectrochemical perforomance and stability. We find that BiVO4 degradation is accelerated, in decreasing order, by light, increasing pH and applied bias. Electron microscopy and in-situ electrochemical atomic force microscopy indicate that degradation initiates at and propagates from surfaces and grain boundaries at solid/liquid interfaces. While from a thermodynamic perspective, it is expected that BiVO4 self-passivates via formation of a chemically stable and less catalytically active bismuth oxide at its surface, we observe the contemporary presence of Bi and V ion in the electrolyte solution, and conclude that self-passivation does not prevent corrosion. By using computational methods, we surmise that the degradation of this material cannot be only described by theromodynamic considerations, and that kinetic limitations must be taken into account in this process. In addition, our finding suggest that the accumulation of photogenerated holes at the surface accelerate degradation. Using this material as a model example, we have recently described high-throughput methodologies, which demonstrated that specific compositions and loadings of mixed-metal oxide catalyst coatings can significantly improve the performance of BiVO4-based photoanodes. In order to further understand the role of interface property in catalyst/BiVO4 assemblies, it is critical to employ a deposition method that is able to produce diverse compositions and interfacial structures of the catalysts. Photoanodic-electrochemical deposition (PED) is a straightforward method to achieve this goal since the growth of catalysts can be easily controlled by deposition variables such as potential, current and time. Here we will also show how PED of different catalyst compositions based on Ce-containing catalysts can be applied for the investigation of stabilities of new functional materials under operating conditions, where kinetic factors should be considered in the search for stable and visible-light-absorbing novel semiconductors for solar-to-fuel conversion.