There has been continuous scientific interest in the prospect of direct photoelectrochemical (PEC) conversion of sunlight to hydrogen since the pioneering demonstration of water splitting with a TiO₂ photo-electrode reported by Fujishima and Honda in 1972.¹  In the 2010s there has been an increase in attention, some of which is driven by the prospect of grid-parity pricing for solar photovoltaic power and the resulting need, at high penetrations of intermittent sources into the electrical power grid, of large scale energy storage.^2–5  Hydrogen has been identified as a potential energy carrier for this purpose.⁶  Also, the release of fuel cell vehicles into the commercial market creates a potential market for renewable hydrogen in the transportation sector.^7–9