Enhanced electrocatalytic CO formation from CO2 on nanostructured silver foam electrodes in ionic liquid/water mixtures

Abstract

We employ a novel type of nanostructured silver foam electrocatalyst to the electrochemical CO2 reduction reaction (CO2RR) in 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) ionic liquid. The Ag catalyst is prepared from an aqueous electroplating bath by means of an additive-assisted electrodeposition approach, yielding macro-porous Ag foams. The pore sidewalls are composed of Ag nanoneedles having diameters of ∼50–100 nm and lengths that can exceed 1 μm. We demonstrate that, when combined with the [Bmim][BF4] electrolyte, this Ag electrocatalyst significantly shifts the onset of CO2 electroreduction by ∼220 mV to less negative potentials as compared to a polished Ag foil electrode. The addition of moderate amounts of water (optimum water mole fraction of 20%) to the pure [Bmim][BF4] electrolyte leads to a further ∼120 mV decrease in the CO2RR overpotential. The combination of the Ag foam catalyst with the [Bmim][BF4]/water mixture results in a ∼300 mV wide potential window where CO is selectively produced with Faraday efficiencies >94%. The structural stability of the catalyst is supported by identical location scanning electron microscopy.