Dataset complementary to supporting information, such as optical movies, COMSOL model, and Python codes to analyze the experimental and simulated data according to the manuscript submitted for publication. The movies correspond to cyclic voltammetry operando monitoring by optical microscopy of the reduction of water + KCl in the presence of NiCl2 at an ITO electrode or NiCl2 or MgCl2 at ITO electrode coated with Pt nanoparticles. The python function was used to extract the halo size around each nanoparticle from optical images, the python routines were used to postprocess the COMSOL simulation and evalaute the simulated halo size.