(Invited) Thermally-Enhanced Photo-Electrochemical Water Splitting

Abstract

Transition-metal-oxide semiconductors are promising photoanodes for solar water splitting due to their excellent chemical stability and appropriate bandgaps. However, in absorbers such as BiVO4, TiO2, α-Fe2O3, and WO3, charge carriers localize as small polarons or become trapped, leading to low minority carrier mobilities. This limits the minority carrier collection efficiency in the quasi-neutral region of the light absorber, and lowers the overall photoactivity. In this work, we demonstrate that modestly elevating the temperature activates minority carrier hopping in monoclinic BiVO4, significantly enhancing the saturation photocurrent without a substantial anodic shift of the onset potential, and is an attractive alternative to employing complex passivation layers and nanostructured templates towards achieving the theoretical photocurrent density. Specifically, using a Mo:BiVO4/SnO2/Si tandem photoanode/photovoltaic, increasing the absolute temperature by 11% from 10 to 42 °C elevates the saturation photocurrent from 1.8 to 4.0 mA cm−2. This strong temperature enhancement, 3.8% K−1, is 5 times greater than that in α-Fe2O3. Concurrently, the onset potential shifts slightly from 0.02 V to 0.08 V versus the reversible hydrogen electrode (or equivalently, from −1.22 V to −1.13 V versus the equilibrium potential of oxygen evolution). Our observation contrasts with the prevailing understanding that the energy conversion efficiency generally decreases with temperature as a result of reduced photovoltage. Thermally-activating minority carrier transport represents a general pathway towards enhancing the photoactivity of light absorbers where hopping conduction limits the minority carrier collection in the quasi-neutral region.