title: Activity, Selectivity and Stability of Colloidal Silver Catalysts for Electrochemical CO₂ Reduction Reaction
Creators: name: Hu, Huifang
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 540 Chemistry
subject: 570 Life sciences; biology

description: Electrochemical CO2 reduction reaction (denoted hereinafter CO2RR) using renewable electric power from carbon-free sources shows high potential to transform CO2 into value-added commodity chemicals, such as carbon monoxide and formic acid. The ultimate goal of research activities in this field is to achieve a commercial scale of CO2 conversion, which requires not only the development of effective catalysts but relies also on the design of appropriate electrochemical reactors to reach the high target product partial current density with long-term operation stability. There are still challenges towards industrial application, such as CO2 mass transport limitations, and severe system stability issues due to flooding, and precipitation phenomena occurring in the gas diffusion electrodes (GDEs) during operation.
Colloidal catalysts are typically prepared by reducing metal precursors in the presence of capping agents which are used to control the size distribution of the colloidal particles, their shape and composition. These well-defined colloidal catalysts have already showed excellent electrocatalytic performance for CO2RR. However, most of research has prioritized the activity and selectivity of catalysts, while the stability received less attention, especially in the presence of surfactants. In addition, the studies on the residual capping agents that may block the active site from colloidal synthesis processes are few for CO2RR.
For this PhD project, various colloidal Ag catalysts with different sizes, morphologies and capping agents were employed for electrochemical CO2RR. The capping agents (i.e. polyvinylpyrrolidone, PVP) were demonstrated to severely affect the CO2RR performance on Ag catalysts due to its coverage of active site. Electrochemical looping has been proved herein to be a suitable and effective method for surfactant removal in particular when using H-type electrolysis cells where the catalysts are exposed to a liquid electrolyte solution; this leads the Ag catalysts to perform close 100% of faradaic efficiency for CO. More importantly, the excess capping agents (e.g. PVP) not only changed the degradation pathway of Ag catalysts but they also reduced the stability of gas-fed electrolyzer systems. Ag catalysts with distinct sizes, morphologies and capping agents showed dissimilar degradation pathways for CO2RR when using these gas-fed flow cells. However, the loss of CO2RR performance could be in many cases regained after removing precipitate formed on the GDEs. It is further demonstrated herein that cracks often appearing as defects in the gas-diffusion layers (GDLs) and the removal of excess PVP from the catalyst inks could effectively mitigate flooding phenomena and prolong the life-time of the electrolyzer systems.
In conclusion, to advance electrochemical CO2RR towards commercial scale, durability is currently more reliant upon the reactor and GDE design rather than the catalyst itself in this stage. The influence of capping agents, especially the excess surfactant, should not be neglected.