Anion Effect on CO2 Electrochemical Reduction on Cu

Abstract

As cost of renewable energies continues to decrease, CO2 electrochemical reduction (CER) to value added fuels and chemicals is now a foreseeable possibility. Multi-carbon (C2+) products, such as ethanol and ethylene are especially promising due to their high economic potential. However, to date, all catalysts that produce C2+ products at an industrially relevant current density contain a significant amount of copper, which is intrinsically challenging in CER selectivity and stability. To improve performance, significant research effort was made to improve copper catalysts via techniques such as morphology engineering and incorporation of various organic and metallic additives. Nonetheless, CER is well known to be a delicate system, and catalyst optimization by itself is insufficient in achieving performance. Instead, other CER components such as catalyst support, reagent gas feed, and electrolyte are all essential contributors to a successful system. For example, electrolyte pH and cation have both been shown to exert significant effects on CER in both theorical and experimental studies. Nonetheless, comparatively, electrolyte anion receives much less literature attention, and its effect is insufficiently understood. Although CER literature typically chooses from a small pool of anion type, namely HCO3-, SO42-, Cl-, Br-, or I-, their selection is often poorly justified and lacks a rational basis. Therefore, this thesis reports how anion affects selectivity and stability of CER on polycrystalline copper. Via modelling, experiments, and in-situ measurements, the investigation reveals that anion, via salting out effect, modulates CO2 solubility and availability, which in turn affect catalyst performance. Specifically, copper stability is especially affected by anion. Altering anion type and anion concentration causes a multi-fold variance in stability time (halides > bicarbonate > sulfate; low concentration > high concentration). On the other hand, at high current densities (100 – 250 mA/cm2), CER selectivity is much less sensitive to changes in anion, only showing a difference in performance under extreme anion conditions. By linking anion, CO2 availability, and CER performance, this thesis provides a rational basis for future anion selection, and highlights CO2 availability’s importance in CER processes.