Elsevier

Electrochimica Acta

Volume 306, 20 May 2019, Pages 9-17
Electrochimica Acta

Co3O4/ NiO bifunctional electrocatalyst for water splitting

https://doi.org/10.1016/j.electacta.2019.03.092Get rights and content

Abstract

The development of noble metal free and active bifunctional catalysts for water splitting in alkaline media is highly demanded but very challenging. Herein, synergetic effects developed between two nonprecious metal oxides, Co3O4 and NiO, are reported, with the resulting composite showing promising properties as a catalyst for alkaline water electrolysis. The activity of the composite material towards both the HER and the OER was enhanced and the dynamic potential decreased, as compared with its counterparts. Importantly, low Tafel slopes of 101 and 61 mVdec−1are found for the composite catalyst for OER and HER respectively. EIS measurements revealed a decreased impedance response of the composite dominated by the intermediate frequency relaxation, related to the adsorption of intermediates. Moreover, based on the structural features the improved catalytic activity of the composite is also due to high electroactive surface area, swift electron transfer kinetics, and excellent electrical chemical coupling between Co3O4 and NiO.

Introduction

Hydrogen gas is considered as a promising alternative green energy carrier since it allows zero carbon emission to the environment. It is a renewable energy vector and may offer high efficiency in energy conversion devices [1]. Among the means of production of hydrogen, electrochemical water splitting is known as a highly promising methodology that needs a thermodynamic potential of 1.23 V for the dissociation of water into oxygen and hydrogen [2]. The electrolysis efficiency is currently low due to high dynamic overpotentials associated with hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrode processes, as well as due to ohmic drops in the electrolyte [3]. It is, therefore, highly demanded to develop efficient catalysts capable of minimizing the overpotentials for water splitting [4]. In the last decades, extensive attempts have been devoted to the design of non-precious metal HER electrocatalysts and significant success have been achieved withthe exploration of efficient earth abundant transition metal-based catalysts including dichalcogenides [[5], [6], [7], [8], [9]], Ni-Mo alloys [[10], [11], [12]], and phosphides [[13], [14], [15], [16], [17], [18], [19]]. Also, many of these catalysts are found highly active in theacidic electrolyte and a few of them are highly efficient in alkaline solution. The first-row transition metal oxides, including nickel oxide and cobalt oxide, have been investigated as electrocatalysts for HER in alkaline electrolyte but showed poor HER activity. Moreover, metal-metal oxide/carbon composite (Ni-NiO/carbon nanotubes and Co-CoOx/nitrogen-doped carbon) produced by thermal decomposition have demonstrated improved HER performance in alkaline media due to the synergetic effects between metal, metal oxide and carbon materials [20,21]. These investigations on NiO andCoO showed that HER performance can be improved by coupling with either metal or metal oxide or carbon materials.

Additionally, the Co3O4 is among the active catalysts and has been studied extensively [22,23], but theoretical studies have shown that poor chemisorption of the oxygen-carrying species is responsible for the poor catalytic activity. Therefore, composite structures of cobalt oxide or metal doping approaches have been designed to enhance OER activity [[24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Several engineering strategies at the atomic or molecular level were investigated for improving the catalytic performance of cobalt oxide by providing the new physiochemical features and developing synergetic effects in order to enhance the OER kinetics [[34], [35], [36]].

In this work, we have developed a simple strategy by coupling Co3O4/NiO nanostructures via aqueous chemical growth method. The coupled nanostructures are used as an efficient electrocatalyst for water splitting in alkaline electrolyte. The objective is to enhance the conductivity via incorporation of Ni into cobalt oxide and surface area of Co3O4 nanostructures throughthe addition of NiO nanostructures and to develop synergetic effects between the two metal oxides that can lower the dynamic potential for both HER and OER processes. The enhanced performance of the composite catalyst towards both HER and OER results are encouraging and show a significant advancement related to the existing literature.

Section snippets

Synthesis of Co3O4/NiO composite nanostructures

The used reagents, namely cobalt nitrate hexahydrate, urea, nickel sulfate hexahydrate, absolute ethanol, hexamethylenetetramine, and potassium hydroxide, were of analytical grade. These chemicals were purchased from Sigma Aldrich. The Co3O4/NiO nanocomposite was synthesized in two steps. First, Ni(OH)2 nanostructures were prepared by aqueous chemical growth method [37] and then calcined at 500 °C for 3 h. The precursor solution used for NiO nanostructures, 0.1 M equimolar concentration of

Results and discussion

Fig. 1 shows the morphological and crystalline characterization by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively, of the pure NiO, Co3O4 and composite Co3O4/NiO nanostructured materials. The pristine NiO has nanoflake like structure embodied with some spherical nanoparticles as shown in Fig. 1 (a). The bundle of nanowires and nanoparticles like morphology of Co3O4 and some porosity is also visible as shown in Fig. 1 (b).

The composite material Co3O4/NiO has similar

Conclusions

In summary, a nanocomposite based on Co3O4/NiO is designed that has shown excellent electrochemical water splitting properties due to synergetic effects produced between metal-oxide-metal oxide materials. The interfacial chemistry of composite materials has increased the dissociation of water and adsorption of intermediate species. The electrochemical characterization shows that the nanocomposite has shown low dynamic potential compared to their counterparts for both HER and OER in alkaline

Acknowledgments

L. Amaral would like to thank Fundação para a Ciência e Tecnologia (FCT, Portugal) for postdoctoral research grant SFRH/BPD/97453/2013.

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