An effective strategy of constructing multi-metallic oxides of ZnO/CoNiO2/CoO/C microflowers for improved supercapacitive performance

Abstract

In this work, a new ZnO/CoNiO2/CoO/C metal oxides composite is prepared by cost-effective hydrothermal method coupled with annealing process under N2 atmosphere. Notably, the oxidation-defect annealing environment is conducive to both morphology and component of the composite, which flower-like ZnO/CoNiO2/CoO/C is obtained. Benefited from good chemical stability of ZnO, high energy capacity of CoNiO2 and CoO and good conductivity of C, the as-prepared sample shows promising electrochemical behavior, including the specific capacity of 1435 C·g−1 at 1 A·g−1, capacity retention of 87.3% at 20 A·g−1, and cycling stability of 90.5% for 3000 cycles at 5 A·g−1, respectively. Furthermore, the prepared ZnO/CoNiO2/CoO/C/NF//AC aqueous hybrid supercapacitors device delivers the best specific energy of 55.9 W·h·kg−1 at 850 W·kg−1. The results reflect that the as-prepared ZnO/CoNiO2/CoO/C microflowers are considered as high performance electrode materials for supercapacitor, and the strategy mentioned in this paper is benefit to prepare mixed metal oxides composite for energy conversion and storage.

1. Introduction

Nowadays, the concept of sustainable and environmental-friendly energy has been deeply rooted in people's minds due to well-known reasons [1e4]. How to store and use of these energies well is as important as themselves, and supercapacitors (SCs) can be competent for this job because of long lifespan, high power density, non-pollution, and so on [5e7]. Electrode material is an important part of SCs, which has great influence on their electrochemical performance. Transition metal oxides and their composites have the advantages of high theoretical specific capacitance, various morphologies, and easy preparation and have been widely studied as electrode materials for SCs. Moreover, binary metal oxides (such as CoMoO4, MnCo2O4, ZnCo2O4, and so on) are also prepared to enhance electrochemical performance by potential synergistic effect of different metal ions [8e11]. In the meanwhile, the core-shell structure or physically mixed composites of metal oxides are designed in order to obtain higher properties by synergistic effect arising from multicomponent of metal oxides. Although the electrochemical performance of transition metal oxides can be improved by composite strategy, it is undeniable that there are still some problems that baffle the development of transition metal oxides in electrode materials. In this case, time-consuming and unwieldy multi-step preparation processes are usually needed. For example, NiCo2O4@CoMoO4 arrays were prepared by a two-step hydrothermal method [12]; however, NiCo2S4@CoMoO4 composite was successfully obtained by a three-step hydrothermal method [13]. In addition, the poor conductivity of metal oxides and their composites are also need to be considered.

In the past few years, ternary transition metal oxides (TTMOs) were designed and prepared for positive electrode of SCs [14e18]. It is easy to understand that the possible synergistic effects among multi-metal ions in TTMOs can lead to better supercapacitive behavior than that of single or binary metal oxides [19,20]. For example, Lee et al. [21] prepared ZneNieCo oxide@NiMoO4 composite, and they found that the specific surface area (SSA) of sample can be enhanced by introducing Zn and their electric conductivity can be improved by Ni doping. Typically, two main factors need to be paid attention to the control of structures and components of TTMOs, which play significant roles in their electrochemical performance. First, three dimensional and hierarchical structure with large surface area is hard to achieve due to complex reaction composition and rational design is needed. Second, whether TTMOs is single phase or a metal oxides mixture depends on different preparation processes, which is rarely reported. In our previous work, ZneNieCo TTMOs with spinel cubic structure has been prepared [22]. It is questionable why ZneNieCo oxide is spinel cubic structure and how it becomes a mixture of single or binary metal oxides (such as NiCo2O4, ZnCo2O4, or ZnO, CoO, NiO, etc.). In this work, a controllable preparation of mixed transition metal oxides composite was proposed and an oxidation-deficient annealing process was crucial. Simultaneously, the electric conductivity of the composite can be upgraded by C doping through glucose carbonization. Accordingly, the as-prepared ZnO/CoNiO2/CoO/C composite showed modified electrochemical performance and can be used as a candidate for electrode of supercapacitors.