Nanosheet assembled microspheres of metal (Zn, Ni, and Cu) indium sulfides for highly selective CO 2 electroreduction to methane

Herein, metal indium sulfides (ZnIn 2 S 4 , NiIn 2 S 4 , and CuInS 2 ) were synthesized using a hydrothermal method. Nanosheet-assembled microspheres were observed. According to cyclic voltammetry, CuInS 2 revealed 19 and 6-fold current density enhancement compared to ZnIn 2 S 4 and NiIn 2 S 4 , respectively. Also, linear sweep voltammetry results suggested a higher current density of CuInS 2 than the others. The lowest Tafel plot of CuInS 2 (189.15 mV dec −1 ) suggested the improvement of electrocatalytic activity compared to those of ZnIn 2 S 4 (282.53 mV dec −1 ) and NiIn 2 S 4 (247.32 mV dec −1 ). An H-type cell was used for selective electrochemical reduction of CO 2 into CH 4 . The gaseous and liquid products were analyzed by gas chromatography and 1 H NMR, respectively. A higher Faraday efficiency (FE) was observed in CuInS 2 (80.11%) compared to those in ZnIn 2 S 4 (67.78%) and NiIn 2 S 4 (75.31%) towards the electrocatalytic reduction of CO 2 into methane (CH 4 ) at −0.6 V vs. RHE. A higher turnover frequency (TOF) value was observed in CuInS 2 compared to those in the other catalysts. CuInS 2 demonstrated remarkable stability, with neither an obvious current drop nor large FE fluctuation for 20 h during electrochemical CO 2 reduction into CH 4 , indicating a superior electrocatalytic performance. The higher electrochemical performance of CuInS 2 is associated with a larger BET surface area/electrochemical surface area, rich structural design, and abundant active sites. This work indicates a promising strategy for developing a ternary electrocatalyst for highly selective electrochemical reduction of CO 2 to CH 4 .