Charge Transfer\nKinetics and Thermodynamics Control\nthe Energy Conversion Efficiency of a Gallium Phosphide Solar Hydrogen\nPhotocathode

P-type gallium phosphide (GaP) photocathodes for hydrogen\nevolution\nfrom water have a theoretical energy conversion efficiency of 12%\nbased on the 2.4 eV optical band gap of the material. The performance\nof actual GaP photocathodes is much lower, for reasons not entirely\nclear. Here we use vibrating Kelvin probe surface photovoltage (VKP-SPV),\nopen circuit potential (OCP) measurements, and photoelectrochemical\n(PEC) experiments to evaluate the kinetic and thermodynamic factors\nthat control energy conversion with GaP photocathodes for the hydrogen\nevolution reaction (HER). We find that the open circuit photovoltage\nof the bare GaP-H<sub>2</sub>O junction is limited by recombination\nat surface states and that an CdS overlayer increases both photovoltage\nand photocurrent due to formation of a n-p-junction. An optimized\nGaP/CdS/Pt photocathode drives hydrogen evolution with a quantum efficiency\nof 62% at 400 nm and 0.0 V RHE and an open circuit photovoltage of\n0.43 V at 250 mW cm<sup>–2</sup>. The Pt cocatalyst increases\nthe photocurrent due to improve HER kinetics but reduces the photovoltage\nby promoting recombination. Added hydrogen or oxygen gas raise or\nlower the photovoltage by modifying the electrostatic barrier (band\nbending) in GaP. This shows that the GaP/CdS junction is not “buried”\nbut behaves like a Schottky junction whose charge separating properties\nare controlled by the electrochemical potential of the electrolyte.\nThe dynamic junction properties need to be considered in the design\nof optimized hydrogen evolution photoelectrodes and photocatalysts.\nAdditionally, the work reveals that PEC or OCP measurements tend to\nunderestimate the photovoltage because they do not account for changes\nin the electrochemical potential at the electrode-liquid contact.\nIn contrast, the VKP-SPV method provides the open circuit photovoltage\nvalue directly. By combining the photovoltage data with OCP data,\nthe minority carrier electrochemical potential at the electrode-liquid\ncontact can be measured in a contactless way. This provides an improved\nunderstanding of illuminated photoelectrodes for the production of\nsolar fuels.