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 (CO2RR), which together with the competing hydrogen evolution reaction (HER) exert changes in the local OH− and H2O activity that in turn can possibly affect activity, stability, and selectivity of the CO2RR. We determine the local OH− and H2O 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/CO2RR 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.

Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media

“…However, in our case, at low current densities, and low pH, due to the lower mass transport limitation in the GDE configuration, the activity for CO (and corresponding production of OH − ) is not high enough to neutralize the protons diffusing towards the catalyst surface, favouring hydrogen production. On the other hand, at current densities above 50 mA cm −2 , especially for bulk pH 3 or 4, the local acidic environment can be rapidly neutralized 23 , 24 , enabling FEs for CO above 80%. We see that when starting from bulk pH 2, higher current densities are required to achieve this suppression of proton reduction.…”

A Metal–Organic Framework derived CuxOyCz Catalyst for Electrochemical CO2 Reduction and Impact of Local pH Change

“…Local changes of ion activities in close proximity to working GDEs surfaces were determined by positioning Pt ultramicroelectrodes (Pt‐UMEs) using shear‐force based SECM approach curves in about 100 nm distance from the GDE surface (see SI, Figure S28–S30). [ 18 , 19 , 20 ] Continuous water consumption during CO 2 RR as well as HER leads to locally generated OH − ions and induces a pH shift which modulates the CO 2 RR product selectivity. To evaluate if local pH changes are responsible for the observed change in the CO 2 RR selectivity, we used this approach to determine local OH − ions activity.…”

“…Local changes of ion activities in close proximity to working GDEs surfaces were determined by positioning Pt ultramicroelectrodes (Pt-UMEs) using shear-force based SECM approach curves in about 100 nm distance from the GDE surface (see SI, Figure S28-S30). [18][19][20] Continuous water consumption during CO 2 RR as well as HER leads to locally generated OH À ions and induces apHshift which modulates the CO 2 RR product selectivity.T oe valuate if local pH changes are responsible for the observed change in the CO 2 RR selectivity,weused this approach to determine local OH À ions activity.W ep ositioned aP t-disk nanoelectrode (< 1 mmd iameter) in close proximity to the PTFE/ HKUST@800-GDE/electrolyte.T he assessment of OH À ions and H 2 Oa ctivities is done by continuous cyclic voltammetry at the Pt-UME, as the peak potential of the PtO-reduction peak depends on the OH À /H 2 Oa ctivities according to the corresponding Nernst equation [Eq. (1)].…”

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure

“…In contrast, scanning probe techniques are powerful tools for investigating activity locally with high resolution. [19][20][21][22][23][24][25][26] Mayer et al 22 used a Pt ultramicroelectrode (Pt-UME) to detect formate, CO and H 2 during CO 2 RR on Sn/SnO x arrays. However, the tip-to-surface distance was determined using O 2 reduction diffusion limitation, which is only practical for probing flat electrodes.…”

“…25,26 Although these previous measurements were performed on non-permeable substrates, we could also demonstrate that probing the local hydroxide and water activities during the oxygen reduction reaction (ORR) and CO 2 RR on GDEs can be achieved with SECM by shear-force positioning of Pt nanoelectrodes at ~100 nm above the GDE surface. [23,24] This experimental approach allows for simultaneously deriving modulations of the local pH value in correlation with the current density, and the topography in situ and with high resolution.…”