The structure, morphology and electrochemical impedance study of the hydrogen evolution reaction on the modified nickel electrodes

Composite layers of modified amorphous nickel were prepared by simultaneous electrodeposition of Ni and TiO2 on a Cu substrate from a solution containing TiO2 (anatase) particles suspended by stirring. Scanning electron microscopy, X-ray diffractometry, Auger spectroscopy and absorption spectroscopy, were used for physical and chemical characterization of the layers. Obtained deposits exhibit an amorphous structure of the Ni-P matrix in which the crystalline component, TiO2, is embedded. Additionally, the presence of non-stoichiometric oxide, Ti2O3, formed on a boundary of the TiO2 grain and nickel matrix in consequence of the reduction conditions during the electrodeposition, was revealed by auger electron spectroscopy (AES). The hydrogen evolution reaction (HER) was investigated on the Ni-P+TiO2 and compared with Ni-P electrode in 5 M KOH at 25°C using steady-state polarization and electrochemical impedance spectroscopy (EIS). In order to explain the electrochemical behaviour of the electrode materials, electrical equivalent circuits containing: (i) the constant-phase element (CPE), (ii) the porous electrode impedance, and (iii) two-CPE elements were compared and verified. The ac impedance behaviour of the electrodes may be well described using the two-CPE or porous electrode model in case of the Ni-P+TiO2, and a simple CPE model for the Ni-P. The results obtained from the EIS and steady-state measurements allowed for the determination of the mechanism and kinetics of the HER. It has been found that an increase in electrochemical activity of the Ni-P+TiO2 electrode is due to both the increase in the real surface area and the presence of titanium oxides TiO2 and Ti2O3, as compared with the Ni-P electrode.