The electrochemical reduction of carbon dioxide (CO2RR) to value-added chemicals using excess intermittent electric power from renewable energy sources is considered a promising approach to mitigate global warming caused by anthropogenic CO2 emissions.
The product selectivity of the CO2RR can be controlled by the chemical nature and the morphology of the catalyst material. Among the various products of the CO2RR, the production of carbon monoxide (CO) is highly desirable because it can be used as feedstock in the Fischer–Tropsch synthesis to produce higher long-chain hydrocarbons and alcohols. Silver is well known as a promising catalyst material for CO production.
Most of the screening experiments to test the activity, selectivity, and stability of an electrocatalyst have been carried out in H-type cell configurations using aqueous electrolytes. However, the low solubility of CO2 in aqueous electrolytes under ambient conditions imposes severe mass transport limitations. This PhD thesis has addressed this challenge, by carrying out classical half-cell measurements in aqueous environments extended to a zero-gap gas-fed electrolyzer. The catalytic properties of two colloidal silver nanomaterials with different morphologies were studied (nanocubes and nanowires).
The electrocatalysts studied herein present high selectivity and activity towards CO formation, e.g., in the case of silver nanocubes, a partial current density of ~625 mA cm−2 and a faradaic efficiency of ~85% for CO were attained. Besides, it is particularly pointed out that the reaction environment plays an essential role in the product distribution of the reaction; formate is generated with higher selectivities and activities in a highly alkaline environment than in a weak one.
Furthermore, identical location scanning electron microscopy (IL-SEM) is herein demonstrated as a powerful technique to study the structural degradation of the electrocatalysts. By imaging the same spot on the catalyst before and after the CO2RR, it is possible to directly visualize changes of the catalyst morphology on a nm-length scale attributed to the electrolysis reaction. Limitations of this analysis technique are discussed based on surfactant-protected nanocatalysts.
Additionally, a new electrochemical surfactant removal method based on potentiostatic CO2RR electrolysis was developed to remove polyvinylpyrrolidone or PVP (the capping agent) from Ag nanowire and nanocube surfaces, resulting in a substantially improved selectivity towards CO formation.
Overall, the studies presented herein clearly demonstrate the importance of performing CO2RR under more realistic conditions to bring this process closer to what is needed for the scale-up of this reaction, which means that high faradaic efficiencies, partial current densities, and long stability are pursued
