Competitive enhancement of CO2 reduction reactions versus hydrogen evolution for high surface area electrodes: A comparative study for Cu and Ag nanomesh

Abstract

High surface area electrodes are an attractive option for electrocatalytic CO2 reduction reaction (CO2RR) on the pathway towards commercialization of this compelling technology. The intrinsic large quantity of catalytic surface sites opens the potential for high currents per geometric electrode area at low overpotentials, if mass transport can be ensured. However, CO2RR from aqueous solutions is limited by the solubility of similar to 0.034 M CO2 in water at 1 bar of CO2 partial pressure. In this work we studied electrochemical CO2RR on 3 mu m thin Cu and Ag nanomesh electrodes with similar to 40-50x enhanced surface area compared to planar electrodes in aqueous bicarbonate solutions. The regular 3D-nanowire networks or nanomeshes were fabricated by metal plating in 3D nanoporous anodized aluminum oxide templates The obtained partial currents for hydrogen evolution reaction (HER) and CO2RR matched very well with that modelled by mass transport including the active CO2 depletion by carbonate equilibria in the alkalinized diffusion layer, limiting the maximum (diffusion limited) partial current density for pure CO2RR to 9 mA/cm(2). Interestingly, it was found that the high surface area of the nanomesh electrodes primarily promoted the more catalytic reaction, i.e., with the lowest overpotential. On the Cu nanomesh the HER was preferentially enhanced with a reduction in overpotential of similar to 450 mV, moving the potential even outside the window of some of the CO2RR products found at planar electrodes. On the Ag nanomesh, on the other hand, the CO2RR to CO was preferred over HER where the 250 mV lowering of the overpotential for the same current density results in an increased energy efficiency for CO2RR. These findings highlight the need to investigate whether the desired CO2RR or the competing HER will be enhanced by high surface area electrodes in relation to the nano-architecture and the catalysts nature as an important step forward towards upscaling the CO2RR electrolysis technology.