Differentiating electrochemically active regions of indium tin oxide electrodes for hydrogen evolution and reductive decomposition reactions. An in situ optical microscopy approach

Abstract

Indium tin oxide (ITO) is an important electrode material in electrochemical studies. However, it presents some chemical and electrochemical limitations, which are often underestimated. In this work the electrochemical behavior of ITO is investigated under opto-electrochemical monitoring in a scanning electrochemical cell microscopy, SECCM, configuration during the hydrogen evolution reaction (HER) in 5 mM of H2SO4, a concentration for which the ITO electrode remains stable. The in situ optical monitoring through interference reflection microscopy (IRM) allows discriminating the formation of H2 nanobubbles, NBs, and the formation of In(0) nanoparticles, NPs, issued from the HER and the ITO electrochemical reduction respectively. A segregation in electrode potential and space between NBs and NPs formation on the ITO electrode is observed, which is attributed to the intrinsic variation of the ITO conductivity. This variation is further evidenced by changing the droplet cell position on the ITO substrate and through SECM investigation. Besides the onset potential of the NPs formation is shifted toward less negative potentials when the conductivity of the ITO increases whereas the onset potential of the H2 formation remains mostly unchanged.