The rapid development of novel energy technologies has decreased renewable electricity prices significantly over the past decade. This foreseen cheap electricity has motivated significant research interest in the development of electrified pathways for chemical and fuel production. Compared to traditional chemical processes driven by fossil energy, electrochemical processes are often more environmentally friendly, can operate under relatively mild conditions, and can also be coupled with renewable electricity sources at remote locations. Recently, efforts have been devoted to the development of CO2 electrolysis devices that can be operated at industrially relevant rates; however, limited progress has been made, especially for valuable C2+ products. In this presentation, I will present our recent work on nanoporous copper as a CO2 reduction catalyst and its integration into a microfluidic CO2 flow cell electrolyzer. The CO2 electrolyzer exhibited a current density of 653 mA/cm2 with a C2+ product selectivity of ~62% at an applied potential of -0.67 V (vs. reversible hydrogen electrode). The highly porous electrode structure facilitated rapid gas transport across the electrode-electrolyte interface at high current densities. Further investigations on electrolyte effects revealed that the surface pH value was substantially different from the pH of bulk electrolyte, especially for non-buffering near-neutral electrolytes when operating at high currents.
In addition to CO2 electrolysis, CO electrolysis has also been reported to yield enhanced multi-carbon (C2+) Faradaic efficiencies up to ~55% but only at low reaction rates. This is due to the low solubility of CO in aqueous electrolytes and operation in batch-type reactors. In a recent study, we constructed a high-performance CO flow electrolyzer with a well-controlled electrode-electrolyte interface that can reach total current densities up to 1 A/cm2 together with improved C2+ selectivities. Computational transport modelling and isotopic C18O reduction experiments suggest that the enhanced activity is due to a higher surface pH under CO reduction conditions, which facilitated the production of acetate. At optimal operating conditions, we achieved a C2+ Faradaic efficiency of ~91% with a C2+ partial current density over 630 mA/cm2. Further investigations show that maintaining an efficient triple-phase boundary at the electrode-electrolyte interface is the most critical challenge to achieving a stable CO/CO2 electrolysis process at high rates.