The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO₂ Reduction Conditions: An Experimental and Theoretical Study

A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO₂ reduction (CO₂R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO₂R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO₂R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO₂R. On Ni-MoS₂, a limitation is the kinetic barrier for the proton-electron transfer to CO. These theoretical and experimental results demonstrate that selective CO₂R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers.