Optimization of local CO2 concentration in catalyst layer for highly selective and efficient electrochemical conversion of carbon dioxide into multi-carbon products

Abstract

Electrochemical CO$_2$ reduction technology coupled with renewable energy has a great potential to reduce CO$_2$ emission and create an economic value at the same time. For successful commercialization of this technology, it is imperative in an economic point of view to maximize the selectivity toward industrially useful multi-carbon (C$_{2+}$) products which contain more than two carbons, such as ethylene, ethanol, and n-propanol, because these products have high global market demands and energy densities. In order to increase the selectivity for these multi-carbon products, past studies have been focused on engineering electrochemical environments and designing novel catalysts, i.e. pH, concentrations and species of cations and anions, molecular additives, etc.

Many recent researches have been increasingly employing a gas-diffusion electrode (GDE) in a flow electrolyzer to boost a production rate of C$_{2+}$ products to an industrially relevant scale, but fundamental understanding of the GDE system is still lacking. In this dissertation, we present the effect of local CO$_2$ concentration in a catalyst layer on the selectivity toward C$_{2+}$ products in the GDE-based flow electrolyzer. Local CO$_2$ concentration affects surface coverage of *CO$_2$, *CO, and *H on a Cu catalyst, and thus mechanistic pathways to electrochemically synthesize C$_{2+}$ products from CO$_2$. Guided by a mass-transport simulation modeling, we discovered three parameters that effectively modulate local CO$_2$ concentration: (1) the structure of a catalyst layer, (2) bulk partial pressure of CO$_2$ in a feed stream, and (3) a CO$_2$ feed flow rate. Conducting electrochemical analyses at industrially relevant current densities using a GDE-based flow electrolyzer in a near neutral electrolyte, we obtained a counter-intuitive result that the selectivity toward C$_{2+}$ products are not optimal in high local CO$_2$ concentration. Rather, modulating local CO$_2$ concentration to a moderate level enhances the C–C coupling reaction, and thus formation of C$_{2+}$ products. Utilizing reduced Cu$_2$O or oxide-derived Cu nanoparticles as a model catalyst, we demonstrated that in a moderate local CO$_2$ concentration, faradaic efficiency of C$_{2+}$ products and C$_{2+}$ partial current density increased up to 75.5% at 300 mA cm$^{-2}$ and 342 mA cm$^{-2}$ in 1.0 M KHCO$_3$, respectively.