Nickel-dimethylglyoxime complex modified graphite and carbon paste electrodes: preparation and catalytic activity towards methanol/ethanol oxidation

Nickel-dimethylglyoxime complex (abbreviated as Ni(II)(DMG)2) modified carbon paste and graphite electrodes were prepared by mixing Ni(II)(DMG)2 with graphite paste, and coating Ni(II)(DMG)2 to the graphite surface. It is necessary to cycle the electrode potential to a high value (e.g. 0.8 V versus SCE) for the preparation of the modified electrodes. The electrochemical reaction was originally assumed to be a one-electron process converting Ni(II)(DMG)2 to [(DMG)2(H2O)Ni(III)ONi(III)(OH)(DMG)2]−. [(DMG)2(H2O)Ni(III)ONi(III)(OH)(DMG)2]− showed a strong catalytic activity toward electro-oxidation of methanol and ethanol. The electrocatalytic oxidation currents consistently increase with the increase in Ni(II)(DMG)2 loading, OH−, and alcohol concentrations. Rotating disk electrode results obtained with a Ni(II)(DMG)2 coated graphite disk electrode showed that the electrocatalytic oxidation of alcohol is a 4-electron process producing formate anion (methanol oxidation) or acetate anion (ethanol oxidation). A mechanism for the electrocatalytic oxidation of methanol/ethanol was proposed, and a rate-determining step was also discussed.