Cu-M-chalcogenides as catalysts for electrochemical CO2 reduction

• Main Author: Goh, Daniel Yong Yi
• Other Authors: Lydia Helena Wong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2025
• Subjects: Engineering; Chalcogenide; Electrocatalysis; CO2 reduction
• Online Access: https://hdl.handle.net/10356/182494
• Institution: Nanyang Technological University

Electrochemical CO2 reduction (CO2RR) is gaining attention in research as a promising way to produce chemical feedstocks. CO2 can be reduced to several products, such as HCOO−, CO, as well as reduced products such as CH4, C2H4 and C2H5OH. CO and C2H4 in particular are valuable products as CO is a useful intermediate for the production of synthetic kerosene or green methanol, while C2H4 is useful to make plastics. However, achieving high selectivity and a high current density with low-cost catalysts to these products is challenging. For example, the state-of-the-art Ag and Au catalysts for CO2 reduction to CO are expensive, while Cu catalyst for CO2 reduction to C2H4 is not selective. Scaling up to high current density is also challenging on these catalysts due to the competing H2 evolution reaction (HER). Understanding the underlying mechanisms for the catalyst to bind to CO2 and relevant intermediates is important to achieve these aims. As copper is the only metal known to produce reduced products, but at the same time is also capable of producing HCOO− and CO, it is the most flexible starting point for the construction of more complex catalysts that favour certain reaction products. For example, oxide-derived copper has been shown to favour liquid products, while various bimetallic catalysts consisting of copper and another metallic element have been tried for increasing copper’s selectivity to CO, CH4 or C2H4. On another hand, sulfide-derived catalysts have been shown to produce almost exclusively HCOO− and attempts to use this method for other products has not been very successful. This is despite calculations showing that sulfur can have an effect in stabilizing *COOH relative to *CO to tune selectivity towards CO. Some particularly successful studies have shown that CdS produces CO and is effectively the only sulfide-derived monometallic catalyst thus far to do so. In contrast to two-element catalysts which have been extensively researched, three-element catalysts are comparatively rare in the literature. This thesis focuses on Cu-M-S/Se as possible three-element catalysts to achieve selective CO2 reduction, preferably to CO or even reduced products. In the first work, nine Cu-M-S bimetallics (M=In, Sn, Sb, Bi, Ga, Ge, Ag, Co, Fe) are tested. It is found that the Cu-Sb-S system is able to selectively produce CO and the sulfoselenide Cu-Sb-S/Se is then tested. It is also found that the sulfide-derived version is better than the sulfoselenide-derived one. Other elements (including In, Sn, as well as other metals such as Ag, Co, Fe, Ga, Ge) as the second metal are not able to do so, and this is linked to the ability of the metal sulfide to reduce during electrochemical CO2 reduction to remove excess sulfur, indicating that small amounts of sulfur doping is able to boost CO selectivity while the usual HCOO− tuning effect is minimized as this effect seems to be greater when sulfur is present in larger amounts. In the second work, an in-depth study of the Cu-Sb-S system for electrochemical CO2 reduction is carried out. Three phases, skinnerite (SK, Cu3SbS3), tetrahedrite (TH, Cu12Sb4S13) and chalcostibite (CS, CuSbS2) are synthesized and tested for electrochemical CO2 reduction. The Cu-Sb-S catalysts are found to have high CO selectivities of 50-80%. This is in contrast to the individual control samples of CuSx and SbSx that demonstrate a preference towards the formate product. The Cu-Sb-S catalysts are found to lose most of the sulfur during reduction, with post-reduction sulfur content ranging from 2-15% found in EDX and XPS. The differences in CO selectivity between the SK, TH and CS phases are attributed to crystallinity in XRD where a higher amount of sulfur induces a drop in crystallinity in the TH sample, and phase segregation caused by a higher Cu at% resulting in S-doped Cu that produces excess HCOO− and H2 by-products. DFT calculations indicate that the substitution of Sb sites with sulfur improves *COOH binding relative to *CO, breaking scaling relations and facilitating subsequent CO (g) formation. It is found that the TH phase results in the highest CO selectivity of 80% at −1.0 V RHE with 37.6 mA cm−2 geometric partial current density. In the third work, the Cu-Sb/Bi-S system is tested to elucidate the role of the second metal. It is found that Sb promotes CO while Bi promotes HCOO−, where the difference likely stems from the miscibility of the second metal with Cu as well as elemental loss. The results indicate that: (1) Sb alloys well with Cu, which weakens the binding of Cu to form CO. (2) Bi does not alloy very well with Cu, leading to loss of Bi, increasing the amount of sulfur-doped Cu which then results in HCOO− as well as H2 formation. These results point to the limitations of sulfur in three-element catalysts in electrochemical CO2 reduction, where recommendations are made in the conclusion that: (1) for electrochemical CO2 reduction, trimetallics are better while (2) for sulfides, photoelectrochemical CO2 reduction is better.