Metal-halide perovskites have been widely investigated in the photovoltaic sector due to their promising optoelectronic properties and inexpensive fabrication techniques based on solution processing. Less work has been done in using halide perovskites for the generation of solar fuels, such as hydrogen. Photoelectrochemical (PEC) water splitting is an emerging technique that uses semiconductors submerged in aqueous solution to directly split water into oxygen and hydrogen using sunlight. Unfortunately, lead halide perovskites are extremely moisture sensitive and need to be protected from high humidity environments, severely limiting their use for direct PEC water splitting applications.

The first example of a halide perovskite absorber used as a photoanode was reported by Da et al. in 2015, where the CH₃NH₃PbI₃ perovskite surface was passivated with a thin Ni layer [1]. However the promising initial photocurrent of 12.5 mAcm^-2 gradually dropped to 2.5 mAcm^-2 after only 15 minutes. The highest stabilities of halide perovskite based photoelectrodes reported so far have been achieved by covering the perovskite film with Field’s metal (a Bi-In-Sn alloy) to give a lifetime of up to 6 hours for methylammonium lead iodide (MAPbI₃) [2] and up to 7 hours for (CsFAMA) triple cation mixed halide perovskite  [3]. However, Field’s metal is expensive due to the high indium content (~50wt%).

Clearly, less expensive and more effective methods are needed for utilising the inexpensive, tuneable and photo-efficient halide perovskites in PEC water splitting. We produced an effective encapsulation technique for halide perovskite materials using inexpensive metal free encapsulation layers. This straightforward approach has proven outstandingly effective, and we report stabilities of over 19 hours of continuous water-splitting with a halide perovskite material submerged in aqueous solution. We have used an all-inorganic CsPbBr₃ perovskite that produces open circuit voltages above 1.45 V [4].

Our encapsulated CsPbBr₃ based photoanodes were very stable across a wide range of pH (3-13) with positive photocurrents that rise steeply from ∼ 0.4 V_RHE. These photoanodes were easily fabricated under ambient conditions using only solution-based processes such as spray pyrolysis and spin coating within 3 hours. Moreover, we demonstrate the versatility of our approach by functionalising the photoanode surface with an Ir-based water oxidation catalyst (WOC) which lowers the onset potential of the functionalised photoanode by  ̴100 mV by way of improved charge transport and reaction kinetics. Photovoltages of up to 1.38 V were measured in aqueous solution at pH 12.5, and for the first time, we quantify the Faradaic efficiency for O₂ evolution by a halide perovskite-based photoanode, which even under non-optimised laboratory conditions exceeded 80%.

Our work demonstrates how an inexpensive encapsulation technique can successfully be used to create stable halide perovskite-based photoanodes for aqueous PEC cells for solar energy conversion.