Charge Transfer Kinetics and Thermodynamics Control the Energy Conversion Efficiency of a Gallium Phosphide Solar Hydrogen Photocathode

P-type gallium phosphide (GaP) photocathodes for hydrogen evolution from water have a theoretical energy conversion efficiency of 12% based on the 2.4 eV optical band gap of the material. The performance of actual GaP photocathodes is much lower, for reasons not entirely clear. Here we use vibrating Kelvin probe surface photovoltage (VKP-SPV), open circuit potential (OCP) measurements, and photoelectrochemical (PEC) experiments to evaluate the kinetic and thermodynamic factors that control energy conversion with GaP photocathodes for the hydrogen evolution reaction (HER). We find that the open circuit photovoltage of the bare GaP-H₂O junction is limited by recombination at surface states and that an CdS overlayer increases both photovoltage and photocurrent due to formation of a n-p-junction. An optimized GaP/CdS/Pt photocathode drives hydrogen evolution with a quantum efficiency of 62% at 400 nm and 0.0 V RHE and an open circuit photovoltage of 0.43 V at 250 mW cm^-2. The Pt cocatalyst increases the photocurrent due to improve HER kinetics but reduces the photovoltage by promoting recombination. Added hydrogen or oxygen gas raise or lower the photovoltage by modifying the electrostatic barrier (band bending) in GaP. This shows that the GaP/CdS junction is not "buried" but behaves like a Schottky junction whose charge separating properties are controlled by the electrochemical potential of the electrolyte. The dynamic junction properties need to be considered in the design of optimized hydrogen evolution photoelectrodes and photocatalysts. Additionally, the work reveals that PEC or OCP measurements tend to underestimate the photovoltage because they do not account for changes in the electrochemical potential at the electrode-liquid contact. In contrast, the VKP-SPV method provides the open circuit photovoltage value directly. By combining the photovoltage data with OCP data, the minority carrier electrochemical potential at the electrode-liquid contact can be measured in a contactless way. This provides an improved understanding of illuminated photoelectrodes for the production of solar fuels.