Electrocatalytic Reduction of CO₂at Au Nanoparticle Electrodes: Effects of Interfacial Chemistry on Reduction Behavior

Abstract

Nanoscale Au electrocatalysts demonstrate the extraordinary ability to reduce CO 2 at low overpotentials with high selectivity to CO. Here, we investigate the role of surface chemistry on CO 2 reduction behavior using Au 25 and 5 nm Au nanoparticles. Onset potentials for CO 2 reduction at Au 25 nanoparticles in Nafion binders are shifted anodically by 190 mV while the hydrogen evolution reaction is shifted cathodically by 300 mV relative to Au foil. The net effect of this beneficial separation in onset potentials is relatively high Faradayic efficiencies for CO (90% at 0.8 V versus RHE) at high current densities. Experimental results show Faradayic efficiencies for CO are greatest using electrodes made with Nafion-immobilized Au 25 nanoparticles. Likewise, CO 2 reduction onset potential shifts are greater for smaller nanoparticles and when Nafion binders are used instead of (sulfonate-free) polyvinylidene fluoride. X-ray photoelectron spectroscopy analysis reveals Au nanoparticles may react with the sulfonates of Nafion binders. The results suggest sulfonate interfaces may alter the binding energies of key species or lead to favorable reconstructions, either of which ultimately results in remarkable improvements in Faradayic efficiencies relative to Au foil electrodes. The electrochemical reduction of CO 2 holds promise to generate energy-dense fuels using only atmospheric CO 2 , water and solar or wind energy. In recent works, several types of wet-synthesized metal nanoclusters (Au, Ag, Pt, Pd, Cu) have been demonstrated in electrochemical sensors 1-3 or electrolytic cells 3-5 with remarkable advantages over conventional metal (foil) electrodes. Recent works using Au 25 nanoclusters as electrochemical sensors demonstrate nanomolar sensitivity for species such as dopamine, ascorbic acid, uric acid, iodide or nitrites.6-8 Likewise composite electrodes with Au, Ag and Cu nanoclusters have shown desirable current-potential behaviors in CO 2 and O 2 reduction reactions including significantly lower onset potentials relative to their foil analogs (>100 mV). The underlying nature of activity enhancements associated with the structure or surface chemistry of composite nanocluster electrodes is not well established. As with enzymatic mechanisms, it is possible that structural effects associated with the ligated surface may facilitate specific reactant or product interactions. It is also possible that both kinetic and thermodynamic benefits originate from unique properties associated with under-coordinated (or ligated) metal atoms that result in improved binding energies or reduced transition state energies. In general, literature reports suggest decreasing particle dimensions increases catalytic activity; 9,10 however, trends in the electrochemical behavior associated with adsorbed ligands are less clear.11,12 Recent work by Liu et al. 13 shows alkyl-terminated Pt nanoparticles (2.85 nm) exhibit semiconducting behavior while phenyl-terminated Pt nanoparticles of the same size demonstrate metallic behavior and are more a...

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