No abstract

Electroreduction of carbon dioxide (CO(2))--a key component of artificial photosynthesis--has largely been stymied by the impractically high overpotentials necessary to drive the process. We report an electrocatalytic system that reduces CO(2) to carbon monoxide (CO) at overpotentials below 0.2 volt. The system relies on an ionic liquid electrolyte to lower the energy of the (CO(2))(-) intermediate, most likely by complexation, and thereby lower the initial reduction barrier. The silver cathode then catalyzes formation of the final products. Formation of gaseous CO is first observed at an applied voltage of 1.5 volts, just slightly above the minimum (i.e., equilibrium) voltage of 1.33 volts. The system continued producing CO for at least 7 hours at Faradaic efficiencies greater than 96%.

This Perspective highlights recent efforts and opportunities in the heterogeneous electrochemical conversion of carbon dioxide to help address the global issues of climate change and sustainable energy production. Recent research has shown that the electrochemical reduction of CO 2 can produce a variety of organic compounds such as formic acid, carbon monoxide, methane, and ethylene with high current efficiency. These products can be used as feedstocks for chemical synthesis or converted into hydrocarbon fuels. This process is of interest (i) for the recycling of CO 2 as an energy carrier, thereby reducing its accumulation in the atmosphere, (ii) for the production of renewable hydrocarbon fuels from CO 2 , water, and renewable electricity for use as transportation fuels, and (iii) as a convenient means of storing electrical energy in chemical form to level the electrical output from intermittent energy sources such as wind and solar. Accomplishments to date in this field of study have been encouraging, yet substantial advances in catalyst, electrolyte, and reactor design are needed for CO 2 utilization via electrochemical conversion to become a technology that can help address climate change and shift society to renewable energy sources.

Electrochemical conversion of CO holds promise for utilization of CO as a carbon feedstock and for storage of intermittent renewable energy. Presently Cu is the only metallic electrocatalyst known to reduce CO to appreciable amounts of hydrocarbons, but often a wide range of products such as CO, HCOO, and H are formed as well. Better catalysts that exhibit high activity and especially high selectivity for specific products are needed. Here a range of bimetallic Cu-Pd catalysts with ordered, disordered, and phase-separated atomic arrangements (Cu:Pd = 1:1), as well as two additional disordered arrangements (Cu3Pd and CuPd3 with Cu:Pd = 3:1 and 1:3), are studied to determine key factors needed to achieve high selectivity for C1 or C2 chemicals in CO reduction. When compared with the disordered and phase-separated CuPd catalysts, the ordered CuPd catalyst exhibits the highest selectivity for C1 products (>80%). The phase-separated CuPd and Cu3Pd achieve higher selectivity (>60%) for C2 chemicals than CuPd3 and ordered CuPd, which suggests that the probability of dimerization of C1 intermediates is higher on surfaces with neighboring Cu atoms. Based on surface valence band spectra, geometric effects rather than electronic effects seem to be key in determining the selectivity of bimetallic Cu-Pd catalysts. These results imply that selectivities to different products can be tuned by geometric arrangements. This insight may benefit the design of catalytic surfaces that further improve activity and selectivity for CO reduction.