In Situ Polarization Imaging of Multi-Bubble Dynamics on Nickel-Based Electrodes During Hydrogen Evolution

Water electrolysis is a promising route for renewable energy conversion and green hydrogen production, yet hydrogen bubbles generated during the hydrogen evolution reaction (HER) can mask active sites, hinder electrolyte transport, and disturb the interfacial reaction environment, leading to distorted apparent catalytic performance. Here, we develop a prism-coupled, dual-camera synchronous polarization imaging system for in situ visualization and quantitative analysis of multi-bubble interfaces on Ni-based electrodes. By extracting degree of polarization (DOP) maps, we correlate the polarization response with bubble coverage and define an apparent coverage parameter, θ_DOP, to describe the overall interfacial state. The quantitative correspondence between θ_DOP and reference bubble coverage was directly validated up to approximately 50% coverage. Bubble evolution was strongly affected by scan rate, while increasing overpotential led to more intense and complex multi-bubble dynamics. Comparative measurements on bare Ni and NiMo-loaded Ni electrodes, including matched-current-density tests from 1 to 50 mA/cm², revealed distinct bubble morphologies and interfacial coverage evolution. This work demonstrates that dual-camera polarization imaging combined with θ_DOP provides an effective in situ approach for characterizing bubble generation, accumulation, and detachment at complex multi-bubble interfaces in Ni-based HER systems.