Contactless measurement of the photovoltage in BiVO4 photoelectrodes

Abstract

The power output of photoelectrochemical devices for solar energy-to-fuel conversion is determined by the photovoltage of the junction under illumination. In the presence of fast redox couples, the photovoltage can be obtained directly from current–voltage measurements of the device. However, for slow redox couples (H⁺/H₂, O₂/H₂O) used in solar fuel photoelectrodes, photovoltage measurements are not straightforward, due to the kinetic overpotentials during charge transfer. Here we show that the photovoltage of BiVO₄ electrodes in contact with fast electron donors KI, Na₂SO₃ or H₂O₂ or K₄Fe(CN)₆ can be measured in a contactless way with vibrating Kelvin probe surface photovoltage (SPV) spectroscopy. The photovoltage varies with illumination wavelength and intensity and matches the open circuit potential of the electrodes, obtained separately from electrochemical measurements. Plots of the photovoltage versus irradiance can be used to predict the oxidizing power of each electrode under zero applied bias. Except for K₄Fe(CN)₆, which causes shunting in the BiVO₄ electrode, photovoltage values correlate well with the built-in potential of each junction. The ability to obtain photovoltage information through contactless SPV measurements will be useful in the search for solid–liquid junctions with superior energy conversion properties.