Modifying Catalyst-Electrolyte Interactions for Enhanced Electrochemical CO2 Reduction
Authors: Garg Sahil , Li Mengran , Ge Lei , Wang Geoff , Rufford Thomas Edward 
Published: May 2020

Electrochemical CO2 reduction (CO2R) could potentially be used with electricity from renewable sources to convert CO2 into various products such as CO, formic acid (HCOOH), methane (CH4), ethylene (C2H6), etc. The overall efficiency of the CO2R process is largely linked to the catalyst on which CO2 is being reduced. However, the local environment surrounding the catalyst is generally subjected to reaction-driven changes during CO2R which play a significant role in affecting the overall efficiency, product selectivity, and reaction rate. Therefore, the understanding of catalyst-electrolyte interactions during CO2R could be effective in further improving the performance of the catalyst. For example, the potential buffering effects of different alkali cations at the catalyst surface due to variations in cation hydrolysis can influence the local proton concentration and thus CO2R.1 The surface tethering of catalyst surface with functional additives such as glycine2, poly(acrylamine)3, thiol group4, etc. have been shown to improve product selectivity by affecting the binding strength with CO2R intermediates. In our recent work5, we uncovered that the interactions between a polycrystalline Ag foil and reline solution including (i) in-situ nano-structuring of Ag foil by electrodeposition of dissolute native Ag oxide layer, (ii) HER suppression by specifically adsorbed choline ions which restrict the protons availability at the interface, and (iii) stabilization of CO2R intermediates by hydrogen bonding with amino group of urea led to a remarkable CO selectivity of (96±8)% at - 0.884 V vs. RHE.
Anions of the electrolyte especially halide ions (Cl-, Br-, and I-) have been reported to affect both the selectivity and activity of CO2R.6, 7 Specifically adsorbed halide ions can modulate the coverage of adsorbed CO on the catalyst surface by stabilizing the intermediates.8 Moreover, halide ions can induce morphological changes of the catalyst surface during CO2R which have demonstrated to enhance the selectivity of CO over Ag catalysts.9 Inspired by the effect of halide ions, we are investigating the role of different halide ions such as Cl-, Br- and I- in choline based electrolytes over Ag foil for CO2R. Our initial experiments have shown that at less negative potentials (between – 1.0 to – 0.7 V vs RHE), I- exhibited the lowest selectivity for CO, and we attribute this result to I- having the highest specific adsorption at the Ag surface and adsorbed I- may restrict the active sites for CO2R. However, at larger negative potentials the electrostatic repulsion between the Ag surface and specifically halide ions increases causing the interactions of the catalyst-halide ions to be weakened and thus improving the CO selectivity. SEM characterization of the Ag foil after CO2R has confirmed the change in morphology of the Ag surface (more-rougher). Moreover, we performed several flow-cell CO2R experiments over Ag-based gas diffusion electrodes (GDEs) and could achieve a CO selectivity over 90% at 150 mA·cm-2 in all three electrolytes. Therefore, altering the interactions between the Ag catalyst and choline-based halide ions during CO2R could be a potential approach to enhance the catalytic activity of Ag-metal or Ag nanoparticles.

References

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