Effective anions

Abstract

The researchers employed an Au electrocatalyst with CO₂-to-CO selectivity to investigate the differences when using the potassium salts of several inorganic and carboxylate anions in place of bicarbonate electrolyte. The generated species (H₂ and CO) were monitored online using in situ differential electrochemical mass spectrometry. Compared to bicarbonate, the inorganic anions perchlorate, sulfate and chloride lead to a reduced CO₂ reduction rate but also higher selectivity for CO over H₂ as they are poor proton donors at the operating pH. Interestingly, the performance with all three inorganic anions is roughly similar. In contrast, the series of carboxylate anions tested (propionate, acetate, formate and trifluoroacetate) offer a range of performances depending on the electron-withdrawing or -donating property of the attached moiety (pictured). Molecular dynamics simulations are used to identify anion adsorption energy as a descriptor for CO₂ reduction kinetics, with increasing kinetics correlated to decreasing adsorption energy. The observed CO generation is therefore a balance between the rate of CO₂ reduction (which is controlled by anion adsorption) and selectivity for CO₂ reduction over H₂ evolution (which is affected by the basicity of the anion). Of all the tested species, propionate is found to be optimum due to both weak adsorption at the surface and low proton availability, outperforming bicarbonate for CO selectivity at a similar rate.

Disentangling all the contributions to the overall activity and selectivity of these complex electrocatalytic systems is of paramount importance but remains challenging. This story highlights the need to include electrolyte anion identity, along with cations, in the list of parameters that need to be controlled, optimized and ultimately understood to progress the field.