Visualizing the role of applied voltage in non-metal electrocatalysts

Understanding how applied voltage drives the electrocatalytic reaction at the nanoscale is a fundamental scientific problem, particularly in non-metallic electrocatalysts, due to their low intrinsic carrier concentration. Herein, using monolayer molybdenum disulfide (MoS₂) as a model system of non-metallic catalyst, the potential drops across the basal plane of MoS₂ (ΔV_sem) and the electric double layer (ΔV_edl) are decoupled quantitatively as a function of applied voltage through in-situ surface potential microscopy. We visualize the evolution of the band structure under liquid conditions and clarify the process of E_F keeping moving deep into E_c, revealing the formation process of the electrolyte gating effect. Additionally, electron transfer (ET) imaging reveals that the basal plane exhibits high ET activity, consistent with the results of surface potential measurements. The potential-dependent behavior of k_f and n_s in the ET reaction are further decoupled based on the measurements of ΔV_sem and ΔV_edl. Comparing the ET and hydrogen evolution reaction imaging results suggests that the low electrocatalytic activity of the basal plane is mainly due to the absence of active sites, rather than its electron transfer ability. This study fills an experimental gap in exploring driving forces for electrocatalysis at the nanoscale and addresses the long-standing issue of the inability to decouple charge transfer from catalytic processes.