Photoelectrochemical cells promise sustainable production of hydrogen fuel from water using sunlight, mimicking natural photosynthetic processes. However, finding light-absorbing semiconductor photoelectrode materials that are suitably efficient and also durable under operation has proven challenging. This is largely because electrode durability and efficiency are extremely sensitive to the chemistry at the interface between liquid water and the semiconductor, which is often extraordinarily difficult to probe under operating conditions. In this talk, I will illustrate how ab initio methods have been applied to understand the complex chemistry active at the interface of water with III-V semiconductor photocathodes (GaP, InP, and GaInP₂), which offer among the highest solar-to-hydrogen conversion efficiencies but suffer from durability shortcomings. I will show how large-scale ab initio molecular dynamics simulations have assisted in determining the dependence of the available chemical pathways on the surface structure and composition of these materials. I will also show how dynamics simulations can be coupled to many-body perturbation theory to directly probe the link between macroscopic optoelectronic properties and local interfacial chemistry, which is necessary for devising meaningful interface optimization strategies. Finally, I will discuss how combining X-ray photoelectron spectroscopy (XPS) simulations with near-ambient-pressure XPS experiments can identify interfacial chemical speciation under operating conditions. Key differences between InP and GaP will be highlighted, including how different chemistry and mass transport behavior at the interface with water impact relative stability and efficiency. Specific implications for understanding the performance and durability of III-V photocathodes will be discussed, along with some general insights for engineering photoelectrochemical interfaces.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.