Long-term solar water and CO2 splitting with photoelectrochemical BiOI–BiVO4 tandems

Photoelectrochemical (PEC) devices have been developed for direct solar fuel production but the limited stability of submerged light absorbers can hamper their commercial prospects.1,2 Here, we demonstrate photocathodes with an operational H2 evolution activity over weeks, by integrating a BiOI light absorber into a robust, oxide-based architecture with a graphite paste conductive encapsulant. In this case, the activity towards proton and CO2 reduction is mainly limited by catalyst degradation. We also introduce multiple-pixel devices as an innovative design principle for PEC systems, displaying superior photocurrents, onset biases and stability over corresponding conventional single-pixel devices. Accordingly, PEC tandem devices comprising multiple-pixel BiOI photocathodes and BiVO4 photoanodes can sustain bias-free water splitting for 240 h, while devices with a Cu92In8 alloy catalyst demonstrate unassisted syngas production from CO2.