Characterizing CO₂ Reduction Catalysts on Gas Diffusion Electrodes: Comparing Activity, Selectivity, and Stability of Transition Metal Catalysts

Abstract

Continued advancements in the electrochemical reduction of CO 2 (CO 2 RR) have emphasized that reactivity, selectivity, and stability are not explicit material properties but combined effects of the catalyst, double-layer, reaction environment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm –2 , which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO 2 RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and 50 mA cm –2 , respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral and alkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm –2 . We hope the collective work provides a resource for researchers setting up CO 2 RR experiments for the first time.

Hydrodynamics Change Tafel Slopes in Electrochemical CO2 Reduction on Copper

“…Gas diffusion electrodes and gas sparging have been investigated to address the issues of low CO 2 solubility (34 mM) and low mass transport in aqueous CO 2 reduction . Gas diffusion assemblies enhance CO 2 transport to the catalyst by decreasing the diffusion path length of CO 2 to the electrode surface, but they inherently create a distribution of mass transport conditions and local electrochemical environments that impede their utilization for studying catalyst mechanisms. , …”

“…12 Gas diffusion assemblies enhance CO 2 transport to the catalyst by decreasing the diffusion path length of CO 2 to the electrode surface, but they inherently create a distribution of mass transport conditions and local electrochemical environments that impede their utilization for studying catalyst mechanisms. 13,14 H-cells, in which CO 2 is typically bubbled across the electrode surface, are widely adopted for mechanistic studies. Their limitations in elucidating intrinsic catalyst behavior include the documented sensitivity of product distribution to the bubble rate and size.…”

Direct observation of the local microenvironment in inhomogeneous CO2 reduction gas diffusion electrodes via versatile pOH imaging

“…Our results are consistent with copper GDEs previously reported in literature. 15,18,57 H 2 production dominates for low current density magnitudes. As the magnitude of the current density increases, we observe more CO 2 R products, and the selectivity shifts towards C 2+ products.…”

Hydrodynamics Determine Tafel Slopes in Electrochemical CO2 Reduction on Copper

“…12 Gas diffusion assemblies enhance CO2 transport to the catalyst by decreasing the diffusion path length of CO2 to the electrode surface, but inherently create a distribution of mass transport conditions and local electrochemical environments that impede their utilization for studying catalyst mechanisms. 13,14 H-cells, in which CO2 is typically bubbled across the electrode surface, are widely adopted for mechanistic studies.…”