Composition dependence of methanol oxidation activity in nickel-cobalt hydroxides and oxides: An optimization toward highly active electrodes

Non-precious metal electrodes, Ni and Co hydroxides and oxides, have been recently found active towards electro-oxidation of methanol in alkaline. In this article, we present a first and complete study on composition dependence of Ni-Co hydroxides and oxides for methanol electro-oxidation. Ni-Co hydroxide electrodes were prepared by co-electrodeposition on stainless steel mesh (SSM). The atomic ratio of Ni/Ni + Co in Ni-Co hydroxides was controlled by adjusting the ratio of precursor concentration. Ni-Co oxide electrodes were further obtained by annealing the Ni-Co hydroxides. The morphology factors of Ni-Co hydroxides and oxides were revealed by measuring double layer capacitance using cyclic voltammetry (CV). Methanol oxidation reaction (MOR) performance of these Ni-Co hydroxides and oxide electrodes was investigated by CV, and electrochemical impedance spectroscopy (EIS) techniques at room temperature (RT, ∼25 °C). It is found that the MOR performance of Ni-Co hydroxides increased with the increase of Ni content, while the performance of Ni-Co oxide electrodes presented a volcano plot. The highest MOR performance, the smallest charge transfer resistance and Tafel slope were found at the atomic composition of 46% Ni. Such an enhancement probably was due to the synergistic effect of co-existing Ni and Co in the spinel structure. In contrast, the electrode with the mixture of Ni oxide and Co oxide was unable to reach such a high activity. The function of Ni in Ni-Co hydroxides and oxides was attributed to facilitating the methanol oxidation, and in low potential it presented high absorption of intermediate products.