The Electrochemical Conversion of Carbon Dioxide to Fuels on a Nanoporous Copper/M Catalyst

Introduction

The electrochemical conversion of CO₂ has been studied for many years, and copper is the only metal catalyst found to produce hydrocarbons¹. This work examines the behavior of a nanoporous copper/M electrode when used as a catalyst for the electroreduction of CO₂.

Materials and Methods

A copper/aluminum alloy was used as the starting material for producing a high surface area copper catalyst. The aluminum was removed through an etching procedure in strong base. The resulting nanoporous copper was crushed and mixed with Nafion, a binding agent, then casted over a copper foil substrate. After drying, transition metal (M) was galvanically displaced onto the copper to form nanoporous copper/M. Carbon dioxide electroreduction was performed by placing the catalyst in a specially designed flow cell. A bubbler in the cell was used to deliver CO₂ to saturate the electrolyte, 0.1 M KHCO₃, at a rate of 10 mL/min. The catalyst acts as the working electrode in this electrochemical cell. The product distribution from the electroreduction process was collected at a series of potentials. Gaseous products were identified and quantified using gas chromatography coupled with both mass spectrometry and a thermal conductivity detector. Liquid products were analyzed using nuclear magnetic resonance.

The surface morphology and composition of the catalyst was characterized using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). The SEM images show a rough surface that retains a porous structure before and after experiments. Depth profiling studies with XPS show that the transition metal (M) from the surface of the material migrates to the bulk after electrolysis.

Results and Discussion

Identification of reduction products reveal that the catalyst is selective toward C₂ species such as ethane and ethanol. This is a phenomenon that is also observed on nanoporous copper without the transition metal, while the addition of a transition metal changes the product selectivity. The only C₁ species produced were carbon monoxide and formic acid, while no methane and methanol was detected. The most interesting product observed was n-propanol, a C₃ species.

This work provides further insight into how selectivity of the CO₂ reaction can be altered by tuning the catalyst properties. The use of a nanoporous copper catalyst provides an increased surface area and introduces many step edges, which are sites that are considered to be especially electrochemically active². The role of the transition metal has yet to be determined, however future work will focus on optimizing the selectivity as a function of controlling the Cu/M (where M = transition metals) composition in the nanoporous copper framework.

References

1] Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695.

2] Billy, Coleman, Walz, Co, US 62/058,121.