Extremely stable bare hematite photoanode for solar water splitting

Highlights: • Optimization of bare hematite photoanodes using a design of experiments approach. • Hematite ultrathin films of ca. 18.8 nm are deposited by spray pyrolysis. • Optimized photoanode reaches a photocurrent of ca. 0.94 mA cm⁻² at 1.45 V_RHE. • Record-breaking stability over 1000 h under 1-sun AM 1.5 G illumination. Photoelectrodes that are efficient, highly stable, made from low cost materials and easily prepared using inexpensive techniques are required for commercially viable solar photoelectrochemical (PEC) water-splitting technology. Hematite is one of few materials that is being considered for this application. In this work, bare hematite thin films prepared by spray pyrolysis were systematically optimized following a design of experiments approach. A response surface methodology was applied to factors: (i) sprayed volume of solution; (ii) temperature of the glass substrate during the deposition; and (iii) time gap between sprays and the optimized operating conditions obtained were v=42 mL, T=425 °C and t=35 s. The optimized hematite photoelectrode showed a photocurrent density of ca. 0.94 mA cm⁻² at 1.45 V_RHE, without dopants or co-catalysts, which is remarkable for a thin film of ca. 19 nm. The stability of this photoelectrode was assessed over 1000 h of PEC operation under 1-sun of simulated sunlight. A record-breaking result was obtained with no evidences of hematite film degradation neither of current density loss. These results open the door to turn PEC cells into a competitive technology in the solar fuel economy.