Probing the Local Reaction Environment During High Turnover Carbon Dioxide Reduction with Ag-Based Gas Diffusion Electrodes

Abstract

Discerning the influence of electrochemical reactions on the electrode microenvironment is an unavoidable topic for electrochemical reactions that involve the production of OH^- and the consumption of water. That is particularly true for the carbon dioxide reduction reaction (CO₂ RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH^- and H₂ O activity that in turn can possibly affect activity, stability, and selectivity of the CO₂ RR. We determine the local OH^- and H₂ O activity in close proximity to a CO₂ -converting Ag-based gas diffusion electrode (GDE) with product analysis using gas chromatography. A Pt nanosensor is positioned in the vicinity of the working GDE using shear-force-based scanning electrochemical microscopy (SECM) approach curves, which allows monitoring changes invoked by reactions proceeding within an otherwise inaccessible porous GDE by potentiodynamic measurements at the Pt-tip nanosensor. We show that high turnover HER/CO₂ RR at a GDE lead to modulations of the alkalinity of the local electrolyte, that resemble a 16 m KOH solution, variations that are in turn linked to the reaction selectivity.

Keywords: carbon dioxide reduction; electrocatalysis; gas diffusion electrodes; local pH gradient; silver.

Figure 1

Local H₂O and OH⁻ activities modulated by the competing HER and CO₂RR are monitored using a precisely positioned Pt nanoelectrode. The PtO reduction peak potential changes with the different ion activities established inside the three‐phase boundary of the GDE at varying reaction rates.

Figure 2

a) pH‐dependent CVs obtained using a Pt nanoelectrode (ø=700 nm) in KOH solutions with concentrations from 1 m up to 16 m. CVs (scan rate: 200 mV s⁻¹) were acquired between +600 and −1100 mV vs. Ag/AgCl/3 m KCl. b) Calibration curves derived from the PtO_red. peak position (E ${}_{\mathrm{PtO}{}_{\mathrm{red}.}}$ ) as a function of the KOH concentration. Linear fits (black lines) with different slopes (displayed in mV dec⁻¹) suggest distinct concentration dependencies between 1 and 4 mol L⁻¹ and between 8 and 16 mol L⁻¹ as suggested by the slopes of −56 mV dec⁻¹ and −220 mV dec⁻¹, respectively. Peak positions are averaged over 3 different calibration measurements as indicated by the red error bars.

Figure 3

a) Pt‐tip voltammograms in the potential range between −0.8 and 0.8 V vs. Ag/AgCl/3 m KCl at a scan rate of 200 mV s⁻¹ in 1 m KOH. The Pt‐sensor is approached to a distance of approximately 100 nm over the Ag‐GDE, which is polarized to potentials between −0.1 and −1.57 V vs. Ag/AgCl/3 m KCl invoking CO₂RR and HER. b) Zoom‐in to the voltammograms displayed in (a), highlighting the shift of E ${}_{\mathrm{PtO}{}_{\mathrm{red}.}}$ in dependence of an increased cathodic potential applied to the Ag‐GDE. c) Peak potentials of E ${}_{\mathrm{PtO}{}_{\mathrm{red}.}}$ from the voltammograms in (a) as a function of applied E _Ag‐GDE. Stars refer to measurements at −0.1 V vs. Ag/AgCl/3 m KCl, the lower one directly after the measurement at −1.69 V and the higher one after further biasing the GDE for 15 min to −0.1 V.

Figure 4

a) E _Ag‐GDE‐dependent FE displaying the formation of CO (orange) and H₂ (green) in dependence on the applied GDE potential. The individual measurements were averaged over 2 GC measurements per potential. b) Partial currents for H₂ (green) and CO (orange) formation at Ag‐GDE (0.2 cm² size) potentials between −1.0 and −1.8 V vs. Ag/AgCl/3 m KCl in 1 m KOH.