(6cc) Electrochemical Reduction of Carbon Dioxide with Highly Dispersed Metal Nanoparticles

The electrochemical conversion of carbon dioxide into hydrocarbons is an alternative route for synthesizing fuels and feedstocks that are typically derived from oil or natural gas. This route also represents a potential strategy to store electrical energy derived from intermittent sources of clean energy. Although metals in the form of polycrystalline foils are increasingly well-characterized as electrocatalysts for carbon dioxide reduction, their nanoscale counterparts remain poorly understood.  Highly dispersed nanoparticle catalysts are ideal for practical electrolyzers due to their high surface area and because they can be integrated into gas diffusion layers in membrane electrode assemblies. The unique ensembles of surface atoms on gold and copper nanoparticle catalysts dramatically alter their activity and stability during carbon dioxide reduction compared to polycrystalline foils.

Our experiments reveal that the primary route by which gold nanoparticles lose surface area during electrochemical carbon dioxide reduction is by forming dendrites on the electrode surface. Random walk simulations coupled with fractal theory establish that the mechanism by which the dendrites form involves diffusion of entire nanoparticles along the support, followed by their collision and fusion to form dendritic structures. Although the mobility of nanoparticles along a support is typically thought to be limited at room temperature, our results demonstrate that nanoparticle catalysts readily move along the support during electrochemical carbon dioxide reduction. We show that an in-depth understanding of the polarization-dependent surface chemistry of gold nanoparticles can be used to limit the dendritic assembly process, thereby increasing surface area for catalysis. We also extend our understanding of surface area loss in gold particles to explain the instability of copper nanoparticles during catalysis.

Both gold and copper nanoparticles exhibit unique activities and selectivities for electrochemical carbon dioxide reduction compared to the polycrystalline foils. We find that there is a continuum in the selectivity for hydrocarbon products between copper nanoparticle and foil electrodes. Detailed electrochemical analysis reveals that nanoscale copper metal particles reduce carbon dioxide to hydrocarbons through a mechanism involving intermediates that are distinct from those observed on polycrystalline foils. Our insights into the structural and chemical factors that influence the activity and stability of copper and gold nanoparticles have enabled us to design advanced catalytic networks for electrochemical carbon dioxide reduction.