Nanoporous Ag for Electrocatalytic CO2 Reduction

Feng Jiao
Assistant Professor, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, E-mail: jiao@udel.edu
Reduction in greenhouse CO2 emissions from fossil fuel utilization is critical for human society. Ideally, one would like to convert CO2 produced in power plants, refineries and petrochemical plants to fuels or other chemicals through renewable energy utilization. This desired solution imposes major technological challenges because CO2 is a fully oxidized and thermodynamically stable molecule. A suitable catalyst for CO2 reduction is essential to achieve a cost effective process with high efficiency and selectivity. In the past two decades, electrocatalytic CO2 reduction has attracted much attention because the required electricity may be obtained at a low cost from renewable energy sources, such as wind, solar, and wave. Researchers have identified several potential catalysts that are able to reduce CO2 electrochemically in aqueous electrolytes. For example, Hori et al. have shown that at a potential of about -0.7 V vs. the reversible hydrogen electrode (RHE) a polycrystalline gold electrocatalyst can deliver a current at 5.0 mA/cm2 with an efficiency of 87% toward CO production, whereas polycrystalline copper exhibits a poor selectivity and requires a potential of about -1.0 V (vs. RHE) to achieve the same current density (i.e. CO2 reduction reaction rate). However, gold is not suitable for large scale applications due to its low abundance and high cost. Searching for abundant catalysts with high selectivity is crucial for commercializing CO2 reduction processes by reducing costs associated with catalyst fabrication and product separation.
The selective conversion of CO2 to CO is a promising route for clean energy. The CO product can be used as feedstock in the Fischer-Tropsch process, a well-known and well-characterized process that has been used in industry to produce chemicals and synthetic fuels from syngas (CO + H2) for many decades. By coupling the catalytic reduction of CO2 to CO with the Fischer-Tropsch process to produce synthetic fuels and industrial chemicals, the estimated maximum reduction of atmospheric CO2 emissions is 40%.
Silver is an attractive CO2 reduction electrocatalyst, because it is able to reduce CO2 to CO with a good selectivity (~81%) and it also costs much less than other precious metal catalysts. In this talk, we will present a nanoporous silver (np-Ag) catalyst, which is able to reduce CO2 electrochemically to CO in a highly efficient and selective way. Not only the porous structure creates an extremely large surface area for catalytic reaction, but also the curved internal surface generates a large number of highly active step sites for CO2 conversion, resulting in an exceptional activity that is over three orders of magnitude higher than that of the polycrystalline counterpart at a moderate overpotential of < 500 mV. More importantly, such a remarkable activity for CO2 electroreduction has been achieved with a CO Faradaic efficiency of
92%.
Reference:
Lu, Q.#, Rosen, J.#, Zhou, Y., Hutchings, G. S., Kimmel, Y. C., Chen, J. G., & Jiao, F.* A selective and efficient electrocatalyst for carbon dioxide reduction. Nature Communications 5:3242 (2014).