CO2 Electrocatalytic Reduction at Gold and Copper Electrodes: Role of Particle Size and Surface Chemistry

Electrochemical CO₂ reduction holds the potential to produce energy-dense liquid fuels using renewable energy sources. Previous works have shown that Cu provides some of the highest yields for valuable products such as methane or ethylene, yet the mechanisms for selectivity are not well understood.  Of course, nature performs similar reactions using enzymes including nanocluster metal alloys and ligands with extraordinary selectivity.   Here we discuss the effects of various ligands at Cu, Au and Au-Cu alloy electrocatalysts.

From the literature, results of CO₂ electroreduction on bulk Cu, Au and Au-Cu alloys are known (1-3) as are the size effects of Cu and Au nanoparticles (4, 5), but the effect of chemical surface modification is still unknown, as is the size effect of gold/copper alloy nanoparticles. 

In this work we demonstrate the effect of chemical surface modification and the size effect of Au-Cu alloy nanoparticles with the aim of combining the two effects to improve CO conversion and hydrogenation.  The effect of surface chemistry is studied using planar electrodes modified by organic compounds.  Copper and gold foils are exposed to strongly adsorbed ligands such as alkylthiols and glutathione (GSH) and subsequently used as electrocatalysts.  As shown in Figure 1, where copper foil was modified with GSH, the adsorbed ligands strongly affect the product yields and selectivity.  At the optimal CO yield potential, the yield of CO on the GSH-modified copper was 2.4 times that of blank bulk copper. Although the yield of CH₄ has been decreased by a factor of 0.5 in exchange, the overall yield of gas product has been increased by a factor of 2 on the surface engineered electrode. 

The size effect of Au-Cu nanoparticles is investigated using nanoparticles synthesized via the Brust-Schiffrin method immobilized in various binder inks as electrocatalysts for CO₂reduction.  Ultimately, we consider the potential to break scaling relationships associated with conventional metal electrodes through the use of size control, alloys and ligands. 

References
1.            Y. Hori, A. Murata, K. Kikuchi and S. Suzuki, J. Chem. Soc., Chem. Commun., 728 (1987).
2.            Y. Hori, K. Kikuchi and S. Suzuki, Chemistry Letters, 1695 (1985).
3.            J. Christophe, T. Doneux and C. Buess-Herman, Electrocatalysis, 3, 139 (2012).
4.            D. R. Kauffman, D. Alfonso, C. Matranga, H. Qian and R. Jin, Journal of the American Chemical Society, 134, 10237 (2012).
5.           R. Reske, H. Mistry, F. Behafarid, B. Roldan Cuenya and P. Strasser, Journal of the American Chemical Society (2014).