Electrocatalytic and Solar-Driven Reduction of Aqueous CO2 with Molecular Cobalt Phthalocyanine–Metal Oxide Hybrid Materials

Electrolytic and solar-driven reduction of CO2 to CO using heterogenized molecular catalysts are promising approaches toward the production of a key chemical feedstock, as well as mitigating CO2 emissions. Here, we report a molecular cobalt phthalocyanine catalyst bearing four phosphonic acid anchoring groups (CoPcP) that can be immobilized on metal oxide electrodes. A hybrid electrode with CoPcP on mesoporous TiO2 (mesoTiO2) converts CO2 to CO in aqueous electrolyte solution at a near-neutral pH (7.3) with high selectivity and a turnover number for CO (TONCO) of 1949 ± 5 after 2 h of controlled-potential electrolysis at −1.09 V against the standard hydrogen electrode (∼550 mV overpotential). In situ UV–visible spectroelectrochemical investigations alluded to a catalytic mechanism that involves non-rate-limiting CO2 binding to the doubly reduced catalyst. Finally, the integration of the mesoTiO2|CoPcP assembly with a p-type silicon (Si) photoelectrode allowed the construction of a benchmark precious-metal-free molecular photocathode that achieves a TONCO of 939 ± 132 with 66% selectivity for CO (CO/H2 = 1.9) under fully aqueous conditions. The electrocatalytic and photoelectrochemical (PEC) activities of CoPcP were compared to those of state of the art synthetic and enzymatic CO2 reduction catalysts, demonstrating the excellent performance of CoPcP and its suitability for integration in tandem PEC devices.