CO₂ Electroreduction to Ethylene: The Determining Role of the Cell and Electrode Design

A series of copper-based electrocatalysts, selected from the literature for their high activity in the CO₂RR to ethylene, is compared under the same conditions, revealing a large discrepancy with the optimal results reported in the literature. A further detailed analysis of a copper oxide nanosheet electrocatalyst under different reaction conditions (cell configuration, i.e., liquid- vs gas-phase, membrane type, i.e., proton exchange membrane (PEM) vs anion exchange membrane (AEM), type of electrolyte, CO₂ mass transport under flow-field-enhanced conditions, and current density) reveals the dramatic role of cell and electrode configurations in determining selectivity. The Faradaic efficiency (FE) for ethylene production on copper oxide nanosheet electrocatalysts can vary widely, ranging from 0% to nearly 70%, depending on reaction conditions. The discussion of these results highlights how the surface reactivity and conversion pathways critically depend on surface accessibility and the population of CO₂ and protons, which in turn depend on both surface characteristics beyond the presence of specific active sites and the rate of transport to the electroactive surface. This is a more critical factor in determining selectivity than the mere type of active sites. However, it is noted that this behavior cannot be described by adding mass transfer limitations to the intrinsic electrocatalyst activity, due to the complex interrelation between many aspects beyond the conventional engineering mass transport considerations. This is demonstrated, for example, by operating at high current densities where ethylene selectivity drastically improves. The results also shed light on and provide a different perspective on the literature results and mechanistic-derived conclusions.