Mechanistic study of methanol electro-oxidation on nickel thin films
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Abstract
Self-terminated nickel electrodeposition was studied on gold microelectrodes in the presence of sulfate and chloride ions. The voltammograms in both electrolytes conditions revealed a sharp current spike that was correlated with the self-termination of Ni electrodeposition and the onset of the H2O reduction.
Thin-layers of nickel were electrodeposited onto polycrystalline gold electrodes using current pulses on electrolytes containing 50 mM NiSO4. The behavior of the potential transients was the same as previously reported by our group. The potential transients curves during the nickel electrodeposition suggest that the nucleation process occurs in a time scale at which the surface area determines the diffusion field, while the geometric area is the one defining the diffusion field during the growth of the nuclei.
Gold electrodes modified with electrodeposited nickel films and nickel wire were activated by continuous cycling (30 cycles) in alkaline media. The formation of two anodic peaks around 0.35 and 0.407 V vs. Ag|AgCl were observed, suggesting the formation of different NiOOH structures α/γ or β/β. The higher catalytic activity for methanol oxidation (EI = 136.2 mA cm−2) was obtained when the deposits that gave the higher β character. Therefore, we can infer that electrodes with higher pro- portions of β-NiOOH than γ-NiOOH are more active material for methanol oxidation.
Finally sampled current voltammetry technique was employed to complement the study of the activation of the nickel surface and its methanol oxidation mechanism, confirming that the highest activity for methanol oxidation was achieved when higher proportions of β-NiOOH were on the surface, however at these conditions the deposits also showed higher catalytic activity for oxygen evolution reaction.