Realization of a photoelectrochemical cascade for the generation of methanol, a liquid solar fuel

Biochemical networks use reaction cascades to achieve selectivity, suggesting a route to improve selectivity of photoelectrochemical carbon dioxide reduction (PEC CO2R). We report the co-design and implementation of a two-step photoelectrochemical cascade to a liquid solar fuel: reduction of CO2 to CO and subsequent reduction of CO to methanol. The potentials required were generated using custom-made III-V-based three-terminal tandem (3TT) solar cells. Cobalt phthalocyanine immobilized on multi-walled carbon nanotubes catalyzed both reactions. The champion integrated photocathode produced methanol with 3.8% Faradaic efficiency (FE), with tested photocathodes having 0.7-3.8% methanol FE. Methanol production ceased when the 3TT subcell driving CO reduction was deactivated, supporting the assignment of a cascade mechanism. The low methanol yield is attributed to insufficient CO flux to, and CO2 depletion at, the methanol-producing subcell. Prospects for increasing product yields are discussed. This work develops the basic science principles underlying the PEC CO2R cascade, demonstrates the co-design of a 3TT-based photoelectrode to produce carbon-based fuels, and finally discusses routes for the further development of this concept.