Composition dependence of methanol oxidation activity in nickel–cobalt hydroxides and oxides: an optimization toward highly active electrodesby Shengnan Sun, Zhichuan J. Xu

Electrochimica Acta

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Year
2015
DOI
10.1016/j.electacta.2015.03.008
Subject
Electrochemistry / Chemical Engineering (all)

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

Author: Shengnan Sun Zhichuan J. Xu

PII: S0013-4686(15)00590-3

DOI: http://dx.doi.org/doi:10.1016/j.electacta.2015.03.008

Reference: EA 24534

To appear in: Electrochimica Acta

Received date: 5-2-2015

Revised date: 2-3-2015

Accepted date: 2-3-2015

Please cite this article as: Shengnan Sun, Zhichuan J.Xu, Composition dependence of methanol oxidation activity in nickel-cobalt hydroxides and oxides: an optimization toward highly active electrodes, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.03.008

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

Shengnan Suna, Zhichuan J. Xua,b* a

School of Materials Science and Engineering, Nanyang Technological University, Singapore; b

Energy Research Institute @ NTU, ERI@N, Interdisciplinary Graduate School, Nanyang Technological

University, Singapore. *E-mail: xuzc@ntu.edu.sg

Graphical Abstract

Abstract

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 coexisting 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.

Keywords:

Electro-deposition; Ni-Co hydroxides; oxides; methanol oxidation 1. Introduction

Using methanol as the proton resource for low temperature proton exchange membrane fuel cells is attractive because the storage of methanol is much less problematic as compared with hydrogen gas. This has encouraged an intensive effort on the development of methanol fuel cells. However, the existing efficient catalysts for MOR are mainly noble metals [1, 2], such as Pt and its alloys [3-7]. The high cost and the limited reserve of noble metals hindered the practical application of such a device. Recently, as alternatives to noble metals, some transitional metal oxides and hydroxides, such as NiO, Ni(OH)2, Co3O4 and MnOx have brought about the attention on their electro-catalytic activity towards MOR in alkaline media.[8-11] These oxides and hydroxides are usually used as supercapacitor materials due to their unique redox chemistry.[12-14] The redox chemistry of these cations may also result in the oxidation of methanol under proper conditions.

Among these catalysts, cobalt and nickel oxides and hydroxides exhibited greater potentials for

MOR due to their higher activities. This finding has inspired much effort to explore these materials in recent years. M. Jafarian’s group investigated Co(OH)2 for MOR and they found that the electrochemical generation and consumption of Co(IV) ions are related to MOR activities.[15]

As for oxides, some investigation showed that the combination of two oxides, Co3O4 and NiO could lower the overpotential and improve the stability of MOR over the pure Co3O4 and NiO.

For example, Wu et al. prepared porous Co3O4/NiO core/shell nanowire array electrodes for

MOR, which showed a higher electrochemical activity, lower overpotential and better stability over the pure Co3O4.[16] Some studies also demonstrated that ternary oxides with cobalt and nickel might enhance the MOR performance over their binary counterparts. For example,

NiCo2O4 was found to be much active for MOR than NiO, Co3O4 and Co3O4/NiO structures. Ding et al. ascribed such high MOR activity and stability of NiCo2O4 to its high electron conductivity and intriguing mesoporous structures.[17] The phenomenon that Ni induced conductivity improvement can be ascribed to Ni3+ in the octahedral site in the spinel structure.[18, 19] In the spinel structure, Ni ions are located at the octahedral sites and Co ions are equally distributed at the tetrahedral and octahedral site. In general, Ni ions are +2 state, and Co ions are +3 state in octahedral site and +2 state in tetrahedral.[19, 20] However, it is found that Ni3+ ions also exist in the octahedral with a little amount.[19] In addition, some reports show that the Ni doping in

Co3O4 can increase the conductivity to four-five orders of magnitude.[18] The relationship between the conductivity and Ni amount was investigated and it was found that the conductivity reaches the highest as the ratio of Ni / (Ni + Co) is close to 0.5.[19] More comparative studies can be found with regarding to the MOR performance on NiO, Co3O4, and NiCo2O4 electrodes.[2123] The conclusions are highly consistent that NiCo2O4 had a higher MOR activity than NiO and