Oxidizing methanol at cubic-hexagonal junction of NiO-ZnO at low onset potential

Abstract

A bimetallic, low-cost electrocatalyst consisting of non-noble nickel (Ni) and zinc (Zn) metal was synthesized for the effective oxidation of methanol (CH3OH). The formation of cubic NiO (nickel oxide) and hexagonal ZnO (zinc oxide) was confirmed by various spectrophotometric analyses. NiO-ZnO modified with -COOH functionalized MWCNT, NiO-ZnO-f-MWCNT exhibited significant methanol oxidation reaction (MOR) activity without any substantial oxygen evolution reaction (OER). At a lower onset potential value of 1.33 V versus RHE (reversible hydrogen electrode), the catalyst NiO-ZnO-f-MWCNT demonstrated a high current density of 3.88 x 103 Am-2 in 1 M KOH and 1 M CH3OH. The catalyst NiO-ZnO-f-MWCNT, with the lowest Tafel slope value compared to the other synthesized catalysts, was found to be stable without significantly losing current density, for about 500 cycles and 25000s. Density functional theory (DFT) calculation unveiled that all the steps involved during the interaction of CH3OH and the catalyst surface for oxidation of CH3OH were exergonic indicating a thermodynamically feasible and favourable process of MOR. The electrocatalytic activity was found to be dependent on the concentration of both Ni and Zn. An increase in Ni concentration hampered the MOR by favouring the OER.