Catalyst–Electrolyte Interactions in Aqueous Reline Solutions for Highly Selective Electrochemical CO₂ Reduction

Abstract

Achieving high product selectivities is one challenge that limits viability of electrochemical CO2 reduction (CO2R) to chemical feedstocks. Here, it was demonstrated how interactions between Ag foil cathodes and reline (choline chloride + urea) led to highly selective CO2R to CO with a faradaic efficiency of (96±8) % in 50 wt % aqueous reline at −0.884 V vs. the reversible hydrogen electrode (RHE), which is a 1.5‐fold improvement over CO2R in KHCO3. In reline the Ag foil was roughened by (i) dissolution of oxide layers followed by (ii) electrodeposition of Ag nanoparticles back on cathode. This surface restructuring exposed low‐coordinated Ag atoms, and subsequent adsorption of choline ions and urea at the catalyst surface limited proton availability in the double layer and stabilized key intermediates such as *COOH. These approaches could potentially be extended to other electrocatalytic metals and lower‐viscosity deep eutectic solvents to achieve higher‐current‐density CO2R in continuous‐flow cell electrolyzers.

Understanding the Effects of Anion Interactions with Ag Electrodes on Electrochemical CO2 Reduction in Choline Halide Electrolytes

“…Figure 1D shows these selectivities in ChI and ChCl are much higher than the best reports in the literature for Ag foils evaluated at potentials more negative than −1.0 V, in other catholytes such as 0.1 M KHCO 3 [36] (FE CO =87 %) and 0.1 M CsHCO 3 [22] (FE CO =80 %) (additional data for the literature comparisons are in Table S1). Our result‘s importance lies in the high total current densities (right vertical axes Figure 1A–C) we achieved at the more negative potentials without sacrificing CO selectivity, which usually drops significantly at high current densities in alkali salt catholytes due to HER [22,33,36–40] . Ultimately, this result suggests Ch‐based electrolytes may have potential to achieve fast reaction rates and thus require smaller electrode areas (or catalyst volumes) to convert a given flowrate of CO 2 .…”

“…The evolution of catalyst structures and morphology during CO 2 R due to interactions of catholyte halide ions and the electrode, and the subsequent effects on CO 2 R selectivity, has been described by several groups, including through in‐situ and operando studies in Roldan Cuenya's lab [28] . We have previously reported [33] that Ag surfaces are restructured in ChCl+urea solutions (reline) via dissolution of Ag oxide layers and electrodeposition of Ag nanoparticles back onto the electrode, which enhanced FE CO during CO 2 R. To understand if the differences in FE CO at −1.1 to −1.3 V vs. RHE (Figure 1) for the three ChX solutions were related to changes in the Ag foils′ surface structure, we immersed polished Ag foils in each of the CO 2 ‐saturated ChX solutions for 2 h. We then cathodized these treated foils at −1.1 V vs. RHE for 2 h, as described in our prior work [33] . Figure 4A shows that the concentration of Ag in the ChX solutions increased after immersion of the foils for 2 h, which could be due to the dissolution of Ag ions from AgX (where X could be Cl, Br, or I) formed at the Ag foil surface in all three ChX solutions.…”

“…At these electrochemical conditions, K + ions are likely only to reach the outer Helmholtz plane (OHP) [34] . In our earlier work, we revealed that the specific adsorption of Ch + ions limits the availability of protons at the DL layer and consequently suppresses hydrogen production [33] . Here in the three different ChX solutions, the higher FE CO values at potentials from −1.1 to −1.3 V vs. RHE in Figure 1A–C suggest the adsorption of Ch + ions enhanced at the more negative potentials.…”

“…Halide ions can also promote changes to the catalyst surface morphology, and this change can improve the CO 2 R to CO over Ag electrodes [32] . In our previous work, [33] we demonstrated that restructuring surfaces of Ag cathodes in aqueous choline chloride (ChCl, or C 5 H 14 NOCl)+urea solutions lead to improved CO 2 R selectivity for CO. Here, we extend the investigation of choline‐based electrolytes to test the hypothesis that anion type also affects the restructuring of the Ag electrode's surface and the pathways for CO 2 R. We choose to study choline cations (C 5 H 14 NO + , Ch + ) here as the Ch + ion is known to suppress the hydrogen evolution reaction (HER) compared to alkali cations in electrolytes lie KHCO 3 [34] because Ch + is larger and has a lower degree hydrolysis than alkali cations.…”

Investigation of Zn/Ni-Based Electrocatalysts for Electrochemical Conversion of CO2to SYNGAS

“…In recent years, several studies were done on various electrocatalysts, but yet, there are problems in Faradaic Efficiency (FE), Current Density (CD), Energy Efficiency, electrocatalyst deactivate, the internal resistance of electrocatalysts, and the potential for scalability to the large sizes without the loss of efficiency, because CO 2 is a thermodynamically stable molecule, it is fully oxidized [3][4][5][6][7][8][9][10][11][12]. A suitable electrocatalyst to reduce CO 2 is necessary to reach a low-cost process with acceptable selectivity and efficiency.…”