Mechanistic insights into C2 and C3 product generation using Ni3Al and Ni3Ga electrocatalysts for CO2 reduction.

Thin films of Ni3Al and Ni3Ga on carbon solid supports have been shown to generate multi-carbon products in electrochemical CO2 reduction, an activity profile that, until recently, was ascribed exclusively to Cu-based catalysts. This catalytic behavior has introduced questions regarding the role of each metal, as well as other system components, during CO2 reduction. Here, the significance of electrode structure and solid support choice in determining higher- versus lower-order reduction products is explored, and the commonly invoked Fischer-Tropsch-type mechanism of CO2 reduction to multi-carbon products is indirectly probed. Electrochemical studies of both intermetallic and non-mixed Ni-Group 13 catalyst films suggest that intermetallic character is required to achieve C2 and C3 products irrespective of carbon support choice, negating the possibility of separate metal sites performing distinct yet complementary roles in CO2 reduction. Furthermore, Ni3Al and Ni3Ga were shown to be incapable of generating higher-order reduction products in D2O, suggesting a departure from accepted mechanisms for CO2 reduction on Cu. Additional routes to multi-carbon products may therefore be accessible when developing intermetallic catalysts for CO2 electroreduction.

Covalent organic frameworks comprising cobalt porphyrins for catalytic CO2 reduction in water.

Conversion of carbon dioxide (CO2) to carbon monoxide (CO) and other value-added carbon products is an important challenge for clean energy research. Here we report modular optimization of covalent organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000, with initial turnover frequency of 9400 hour(-1)) at pH 7 with an overpotential of -0.55 volts, equivalent to a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.