In Situ Infrared Spectroscopy Reveals Persistent Alkalinity near Electrode Surfaces during CO2 Electroreduction

Abstract

Over the past decade, electrochemical carbon dioxide reduction has become a thriving area of research with the aim of converting electricity to renewable chemicals and fuels. Recent advances through catalyst development have significantly improved selectivity and activity. However, drawing potential dependent structure-activity relationships has been complicated, not only due to the ill-defined and intricate morphological and mesoscopic structure of electrocatalysts, but also by immense concentration gradients existing between the electrode surface and bulk solution. In this work, by using in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and computational modeling, we explicitly show that commonly used strong phosphate buffers cannot sustain the interfacial pH during CO₂ electroreduction on copper electrodes at relatively low current densities, <10 mA/cm². The pH near the electrode surface was observed to be as much as 5 pH units higher compared to bulk solution in 0.2 M phosphate buffer at potentials relevant to the formation of hydrocarbons (-1 V vs RHE), even on smooth polycrystalline copper electrodes. Drastically increasing the buffer capacity did not stand out as a viable solution for the problem as the concurrent production of hydrogen increased dramatically, which resulted in a breakdown of the buffer in a narrow potential range. These unforeseen results imply that most of the studies, if not all, on electrochemical CO₂ reduction to hydrocarbons in CO₂ saturated aqueous solutions were evaluated under mass transport limitations on copper electrodes. We underscore that the large concentration gradients on electrodes with high local current density (e.g., nanostructured) have important implications on the selectivity, activity, and kinetic analysis, and any attempt to draw structure-activity relationships must rule out mass transport effects.

Figure 1

(a) AFM image of sputtered Cu film on to Ge. (b) Schematic representation of the spectroelectrochemical cell used for in situ SEIRAS measurements. Reference electrode (RE) is Agl/AgCl and counter electrode (CE) is a graphite rod. (c) pH dependent SEIRA spectra of phosphate solutions indicating the dominating species. (d) Equilibrium of phosphate species constructed by absorption in intensity in SEIRA spectrum. (e) Potential dependent changes in the concentrations of phosphate species near the electrode surface.

Figure 2

(a) Schematic representation of buffer reactions, pH gradient, and probed area SEIRA. (b) Experimentally measured and (c) simulated cathode surface pH as a function of phosphate buffer concentration and current density. (d) Current vs potential curve for different electrolyte concentrations (e) Experimentally measured cathode surface pH as a function of potential. All solutions are composed of equimolar H₂PO₄^– and HPO₄^2– mixtures. Shaded areas in b, c, e represent the buffering region that are relevant to local pH measurements.

Figure 3

(a) Experimentally measured and (b) simulated cathode surface concentrations of CO_2(aq) as a function of phosphate buffer concentration and current density. (c) Experimentally measured changes in CO₂ concentration near the electrode surface as a function of potential. All solutions are composed of equimolar H₂PO₄^– and HPO₄^2– mixtures. CO₂ concentrations are normalized to initial equilibrium concentrations.

Figure 4

SEM image of the (a) sputtered and (b) nanowire copper electrodes. Partial current density of H₂ and CO₂ ethylene and methane as a function of potential and phosphate buffer concentration for (c) sputtered and (d) nanowire copper electrode.