Carbon Monoxide-Tolerant Palladium-Based Electrocatalyst for Carbon Dioxide Reduction

Abstract

The utilization of carbon dioxide (CO2) for the production of fuels and valuable chemicals has gained increasing attention as a strategy for solving both global energy and environmental issues.[1] Among the various proposed methods, the electrochemical reduction of CO2 using electricity powered by renewable resources is attractive, and numerous electrode materials capable of converting CO2 to carbon monoxide (CO), formate, methanol, methane, and other hydrocarbons have been identified in the past few decades. However, there still remain several fundamental challenges in the electrocatalytic reduction of CO2, such as high overpotential and low Faradaic efficiency due to the competitive hydrogen evolution reaction. Recently, it was reported that a palladium (Pd) nanoparticle electrode can reduce CO2 to formate with high energy efficiency (with low overpotential) [1]. However, the electrode was also reported to be deactivated by adsorption of CO which is produced as a byproduct. In this study, to suppress the deactivation by CO, we focused on a Pd nanoparticle electrode modified covered with copper (Cu) because the binding energy of CO on Cu was reported to be weaker than that on Pd [3]. In addition, it is known that Cu monolayer can be formed on Pd support by using underpotential deposition (UPD) [4]. Thus, we prepared a Pd nanoparticle electrode covered with Cu monolayer (Cu/Pd electrode) and examined its CO2reduction activity and tolerance to CO. A Pd nanoparticle electrode was prepared by loading homogeneous catalyst ink onto a fluorine-doped tin oxide (FTO) electrode. The ink was composed of a mixture of Pd-supporting carbon black powder, Nafion, and isopropanol. The Pd-supporting carbon black powder was purchased from Premetek. UPD of Cu was carried out by holding the electrode potential at 0.35 V vs. reversible hydrogen electrode (RHE) for 50 s in a mixed solution of 50 mM sulfuric acid (H2SO4) and 50 mM copper sulfate (CuSO4). Electrolyses were performed in a gas-tight two-compartment electrochemical cell with a piece of anion exchange membrane as the separator. An aqueous solution of 0.5 M sodium hydrogen carbonate (NaHCO3) saturated with CO2was used as an electrolyte. Figure 1 shows chronoamperograms for CO2 reduction of Pd and Cu/Pd electrodes measured at -0.15 V vs RHE. In the initial stage, the current density of a Cu/Pd electrode was smaller than that of a Pd electrode. This is plausible because the surface area of Pd exposed to the electrolyte decreased by deposition of Cu. However, the Cu/Pd electrode was active for CO2 reduction even after deposition of Cu and production of formate was confirmed by analysis of the electrolyte using gas chromatograph (GC). Notably, the reduction current of the Cu/Pd electrode maintained more steadily than that of the Pd electrode during long-term electrolysis, suggesting that the electrodeposited Cu monolayer improved the tolerance to CO. To examine the effect of Cu layer on CO adsorption, CO2 reduction activity in the presence of CO was also investigate. When CO was introduced into the electrolyte by bubbling, the current density of both Pd and Cu/Pd electrodes decreased, however the degree of deactivation of the Cu/Pd electrode was less than that of the Pd electrode. These results indicate that the covering of Pd nanoparticle with Cu monolayer improves its tolerance to CO without losing CO2reduction activity.