Tubular Nickel Hydroxide Embedded in Zeolitic Cobalt Oxide for Methanol Oxidation Reaction

Abstract

Non-noble metal-based electrocatalysts have been designed for effective electrochemical oxidation of methanol in basic medium. The catalyst consisting of nickel (Ni) and cobalt (Co) metals was synthesized with a zeolitic imidazolate framework (ZIF) template approach. Such a method leads to the decoration of cylindrically shaped nickel hydroxides, Ni(OH)2, in nanodimen-sions within the zeolitic crystals of spinel cobalt oxides, Co3O4. The mixed metal hydroxide and oxide, Ni(OH)2-Co3O4, represented good activity toward electrochemical oxidation of methanol in 1 M KOH at a low onset potential. When combined with a carboxylate-functionalized multiwalled carbon nanotube (COOH-MWCNT), the same material, Ni(OH)2-Co3O4, exhibited superior MOR (methanol oxidation reaction) activity, giving a peak current density of 4.2 Amg-1 at similar conditions. The MWCNT-modified catalyst, Ni(OH)2-Co3O4-MWCNT, also showed high stability up to 500 cycles and 25000 s without a significant loss in the current density. The linear dependency of the current density against the square root of the scan rate indicated a diffusion-controlled MOR process. The decrease in onset potential with increasing scan rate also predicted a kinetically favorable MOR process. Fourier transform infrared and Raman analyses suggested that the MOR mechanism proceeded through the adsorption of methanol (CH3OH) on the catalyst surface, and its deprotonation formed the methoxide ion (CH3O-) which in the later course decomposed to CO2 and H2O. The Raman study also showed that, during the electrochemical oxidation process, Co2+ species in Co3O4 transformed to CoOOH and thereby favored the MOR.