While numerous efforts are produced towards the design of sustainable and efficient nano-catalysts of hydrogen evolution reaction (HER), there is a need for the operando observation and quantification of gas nanobubbles (NBs) formation involved in this electrochemical reaction. It is achieved herein through interference reflection microscopy (IRM) coupled to electrochemistry and optical modelling. Besides analyzing the geometry and growth rate of individual NBs at single nanocatalysts, the toolbox offered by super-localization and quantitative label-free optical microscopy allows analyzing the geometry (contact angle and footprint with surface) of individual NBs and their growth rate. It turns out that after few seconds, NBs are steadily growing while they are fully covering the Pt NPs that allowed their nucleation and their pinning on the electrode surface. It then raises relevant questions related to gas evolution catalysts as for example: does the evaluation of NB growth at single nano-catalyst really reflect its electrochemical activity? 2

By shortening solid-state diffusion times, the nanoscale size reduction of dielectric materials -such as ionic crystalshas fueled synthetic efforts towards their use as nanoparticles, NPs, in electrochemical storage and conversion cells. Meanwhile, there is a lack of strategies able to image the dynamics of such conversion, operando and at the single NP level. It is achieved here by optical microscopy for a model dielectric ionic nanocrystal, a silver halide NP. Rather than the classical core-shrinking mechanism often used to rationalize the complete electrochemical conversion and charge storage in NPs, an alternative mechanism is proposed here. Owing to its poor conductivity, the NP conversion proceeds to completion through the formation of multiple inclusions. The super-localization of NP during such heterogeneous multiple-step conversion suggests the local release of ions, which propels the NP towards reacting sites enabling its full conversion.