Insight into Mo₂C Nanoplates Growth by Means of Scanning Electrochemical Cell Microscopy for High‐Resolution Electrocatalytic Hydrogen Evolution Activity Mapping

Molybdenum carbides have emerged as promising catalysts for the hydrogen evolution reaction (HER). While numerous studies have investigated synthesis methods, structural properties, and their application, the understanding of their local electrochemical behavior and the correlation between particle size and activity remains elusive. This study addresses this gap by carrying out a comprehensive investigation of the HER activity of well‐defined morphologies and sizes of α‐Mo 2 C nanoplates, grown via chemical vapor deposition. Scanning electrochemical cell microscopy (SECCM) is employed for high‐resolution HER mapping on flakes with dimensions ranging from 1 μm to 40 μm in lateral size, using SECCM capillaries with ≈130 nm tip diameter. Our findings reveal a significant variability in the HER activity at the subparticle level, suggesting that the heterogeneous activity observed in pristine flakes larger than 10 μm is due to an addition of effects caused by the long‐term growth, such as step‐edge formation, Mo 2 C oxidation, and the presence of residual graphene. This study underscores the importance of local characterization of individual Mo 2 C nanoplates, shedding light on the impact of size‐dependence on the HER activity.