Local electrochemical techniques such as the scanning electrochemical microscopy (SECM), which use an ultramicroelectrode as a local probe, allows to characterize the topography and the local surface reactivity of a substrate. However, it should be mentioned that the SECM has largely been used in stationary conditions. Firstly, the development of in ac mode is presented from both experimental and theoretical point of views. The experiments were performed with the ferri/ferrocyanide redox mediator as a model system. Based on the analysis of the frequency-dependent collection efficiency, diffusion between the probe and the substrate is investigated, and analysis of time constants allows evaluation of the size of the sensing area under investigation. Secondly, SECM in ac mode was used for the characterization of the adsorption process during the hydrogen evolution reaction (HER) in sulfuric acid solution. It is shown that this technique allows quantitative analysis of the adsorption process, and the measurements of the differential capacitance as a function of frequency were obtained. The time constant for relaxation of adsorbed hydrogen was about 2 Hz, and the analysis of the Nyquist plot allows the direct evaluation of the charge involved. In addition, the comparison of the usual electrochemical impedance data and ac-SECM results obtained simultaneously allowed characterization of processes occurring at the surface and in solution.
