Abstract: Photoelectrodes consisting of wide-bandgap metal oxides functionalized with chromophores and catalysts are a potential low-cost solution to generate solar fuels from aqueous solution. NiO is a common photocathode material used in these systems, yet it generally exhibits poor performance in aqueous conditions. Although the poor performance has been attributed to various surface species, the atomistic nature of these traps and role of water in dictating performance has not been resolved.
Here, using first-principles calculations, we find that adsorption of water to a defective NiO (111) surface can result in intra-bandgap surface electronic states that are associated with hydroxyl and oxygen moieties adjacent to nickel vacancies. Electrochemical measurements on mesoporous NiO exhibit surface localized faradaic events that give rise to a density of states (DoS). Data collected in solutions of acetonitrile and water at various ratios, and in water after surface passivation by target atomic deposition (TAD) of aluminum, yield large shifts in the DoS and in the open-circuit voltage, short-circuit current, and charge transfer resistance of dye-sensitized solar cell (DSSC) devices. Correlations between measured DoS and DSSC metrics in various solvent ratios and after TAD treatment suggest that surface states facilitate proton coupled charge transfer with the iodide/triiodide (I-/I3-) electrolyte and are a primary driver of the dark recombination current in devices. TAD prevents this charge transfer, dramatically improving aqueous DSSC characteristics. This work provides a framework to better understand and tune NiO performance for applications that require aqueous solution.