Proton-Controlled Electron Injection in MoS₂ During Hydrogen Evolution Revealed by Time-Resolved Spectroelectrochemistry

Monolayer MoS₂ is an effective electrocatalyst for the hydrogen evolution reaction (HER). Despite significant efforts to optimize the active sites, its catalytic performance still falls short of theoretical predictions. One key factor that has often been overlooked is the electron injection from the conductive substrate into the MoS₂. The charge transfer behavior at the substrate-MoS₂ interface is nonclassical, exhibiting a liquid-gated electron injection behavior, the underlying mechanism of which remain under debate. To investigate this, we employ nanosecond time-resolved spectroelectrochemistry to probe the electron injection dynamics into monolayer MoS₂ under operando HER conditions. Simultaneously, transient current measurements provide insights into the electron density at the substrate. By combining the electron density obtained from the MoS₂ through spectroelectrochemical analysis with the electron density at the conductive substrate derived from transient current measurements, we explore the electron injection dynamics and characterize the current density potential (J-E) behavior at the substrate-MoS₂ interface. Our findings show that the electron injection barrier and capability correlate strongly with proton concentration in the electrolyte. This relationship likely reflects the electron concentration-dependent conductivity of MoS₂, where higher proton concentrations lead to fewer stray electrons before injection begins.