PNNL

Abstract

Hematite photoanodes are promising for the oxygen evolution reaction, however, their high overpotential (0.5-0.6 V)1 for water oxidation and limited photocurrent make them economically unviable at present. The work needed to orient dipoles at an electrode surface may be an overlooked contribution to the overpotential, especially regarding dipoles of water, the electron source in the oxygen evolution reaction (OER). Here, we employ second harmonic amplitude and phase measurements to quantify the number of net-aligned Stern layer water molecules and the work associated with water flipping, on hematite, an earth abundant OER semiconductor associated with a high overpotential. At zero applied bias, the pH-dependent potentials for Stern layer water molecule flipping exhibit Nernstian behavior. At positive applied potentials and pH 13, approximately one to two monolayers of water molecules points the oxygen atoms towards the electrode, favorable for the OER. The work associated with water flipping matches the cohesive energy of liquid water (44 kJ mol-1) and the OER current density is highest. This current is negligible at pH 5, where the work approaches 100 kJ mol-1. Our findings suggest a causal relationship between the need for Stern layer water flipping and the OER overpotential, opening avenues for lowing the latter.