BiVO<sub>4</sub>: a new champion material for solar water splitting

Roel van de Krol^a

Proceedings of International Conference on New Advances in Materials Research for Solar Fuels Production (SolarFuel13)

Granada, Spain, 2013 June 13th - 14th

Organizer: Dunwei Wang

Invited Speaker, Roel van de Krol, presentation 041
Publication date: 31st March 2013

Transition metal oxides are promising candidates for the conversion of solar energy to chemical fuels. They combine reasonable semiconducting properties with excellent chemical stability and low cost. As such, they can bridge the gap between conventional PV semiconductors and molecular/biological systems. Although exciting progress has been made in the past 5 years on e.g. Fe₂O₃ and Cu₂O, further improvement of these binary oxides is severely hampered by intrinsic materials limitations. To expand our database of photo-electrode candidates, research efforts are currently shifting towards ternary or more complex oxides. We report on a promising ternary metal oxide, bismuth vanadate (BiVO₄), which can be prepared by a low-cost and simple spray pyrolysis technique. We will show how photo-electrochemical measurements can be used to determine the performance-limiting factors in this material. This provide valuable guidelines to improve the photoresponse, and several approaches to do this will be discussed. Specifically, the slow surface reaction kinetics can be enhanced by applying a cobalt phosphate water oxidation co-catalyst, and the poor conductivity of the material can be improved by doping with tungsten. Moreover, the charge carrier separation efficiency can be improved by introducing a gradient in the dopant concentration, effectively creating a distributed n⁺-n homojunction. These improvements have resulted in AM1.5 photocurrents of 3.6 mA/cm² at 1.23 V vs RHE, which is the highest photocurrent ever reported for a metal oxide photoanode. We will conclude by showing a stand-alone water splitting device based on a BiVO₄ photoanode and an amorphous silicon tandem cell with a solar-to-hydrogen (STH) efficiency of 4.9%.

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