Sulfur Modified Copper Nitride for Electrochemical CO2 Reduction Reaction

Abstract

Metallic copper has been attracted attention as efficient catalyst for electrochemical CO2 reduction reaction (CO2RR). However, the activity of the catalyst with a single adsorption site is limited by the imbalance of the intermediate binding energy, so-called “scaling relationship”. [1] Therefore, it is crucial to break the scaling relationship between reaction intermediates in order to increase product selectivity and lower overpotential. One of the candidates to overcome this scaling relationship is metal sulfide. According to the computational study, since sulfur atoms work as an adsorption site, metal sulfide is expected to be a suitable catalyst for CO2RR with multiple adsorption sites.[2] In addition, surface modification by sulfur is also effective approach to enhance product selectivity in CO2RR. Increase of formate (HCOO-) selectivity by sulfur introduction on Cu has been reported in some previous reports.[3]-[5] On the other hand, the repulsion between the oxygen lone pair electrons of the CO2 molecule and the electronic clouds of the surface sulfur atoms become an obstacle on CO2RR.[6] A potential solution to this problem is the construction of sulfide-nitride composite. Because of the difference in electronegativity between nitrogen and metal or sulfur, it is considered that the electronic structure of the metal sulfide is altered to reduce the repulsion between oxygen and sulfur.[7] Based on these backgrounds, it is expected that the CO2RR activity of copper nitride (Cu3N) will be enhanced by sulfur introduction.Here, sulfur modified Cu3N with different amount of sulfur were synthesized by reflux method with some modification of previously reported method.[8] X-ray diffraction pattern and SEM-EDX elemental analysis result indicate that the sulfur ratio increases and copper sulfide (Cu2S) crystal phase appears with the amount of sulfur source precursor. CO2RR activity was evaluated in 0.1M KHCO3 electrolyte, and volcano trend was identified between the amount of sulfur and methane selectivity. Among all the catalysts including pure Cu3N and Cu2S which were synthesized by the same method, 4.2 mol% sulfur containing Cu3N showed the highest methane faradaic efficiency (41.4 %). This faradaic efficiency is more than 10 times higher than that of Cu3N (0.48 %) and Cu2S (1.3 %). This finding offers valuable insights into the sulfur role on the modified catalyst.