Multi-Regression Analysis of CO₂ Electroreduction Activities on Metal Sulfides

The electrochemical reduction of carbon dioxide (CO₂) is a promising approach toward the utilization of CO₂ as a carbon feedstock, which is desirable for mitigating global warming and closing the carbon cycle. Metal sulfides are particularly suited for the study of CO₂ reduction electrocatalysts because of their potential to break a scaling relationship due to their multicomponent characteristics, and understanding the properties that determine their electrocatalytic activity may also provide insights into prebiotic carbon fixation catalyzed by metal sulfide minerals. Here, we performed multi-regression analysis of electrochemical CO₂ reduction catalyzed by metal sulfides to gain insights into the activity-determining properties of these electrocatalytic materials. Among the 14 samples examined (Ag₂S, CdS, CoS₂, CuS, Fe₃S₄, In₂S_3, MnS, MoS₂, NiS₂, Ni₃S₂, SnS₂, TiS₂, WS₂, and ZnS), CdS and Ag₂S exhibited the highest CO production activity, with Faradaic efficiencies of 37.2 and 29.5%, respectively, in a 0.1 M KHCO₃ aqueous solution, whereas CuS maximized formate (HCOO^–) production, with a FE of 4.5%. Partial least squares and least absolute shrinkage and selection operator regression analyses revealed that CO production was largely affected by the structural parameter of metal–sulfur bond length; however, HCOO^– generation was influenced by the electronic parameters of the metal centers, particularly absolute electronegativity. These findings are distinct from those of metal electrodes, for which the generation of CO and HCOO^– is predominantly influenced by electronic parameters, demonstrating that CO₂ reduction on metal sulfides proceeds by a different mechanism compared to that of the corresponding metal electrodes. We anticipate that the key parameters identified through multi-regression analysis of metal sulfides will prompt further study of the factors influencing the specificity of these unique electrochemical CO₂ reduction catalysts.