Facile and low-cost fabrication of nanostructured NiCo₂O₄ spinel with high specific capacitance and excellent cycle stability

Abstract

Nanostructured NiCo₂O₄ spinel was fabricated with a facile, low-cost coprecipitation process. The ethylene glycol was applied to serve as a stabilizer to govern over the nucleation, growth, and agglomeration processes. The influence of the calcination time was also studied on the coprecipitation reaction. Both the reaction conditions have statistically significant effects on the size distribution, crystallinity, electronic conductivity and the electrochemical performances of the products. The optimized, 5–10 nm NiCo₂O₄ spinel, calcinated at 375 °C for 8 h, had excellent crystallinity and electric conductivity. It showed a superior-specific capacitance of 671 F g⁻¹ at a current density of 1 A g⁻¹. The delay in the specific capacitance was insignificant, after a 7000-cycle test, revealing the high stability of the product. The simplicity and practicability of the modified coprecipitation process may facilitate the successful commercial applications in electrochemical capacitors for nanostructured NiCo₂O₄ spinel.

Introduction

Electrochemical capacitors (ECs) are power devices that can be fully charged or discharged in seconds, which have an important role in complementing or replacing batteries and load-leveling; as a consequence, their energy density is lower than in batteries. Future generations of ECs are expected to come close to current Li-ion batteries in energy density, maintaining their high power density. The RuO₂ pseudocapacitor has the highest specific capacitance (1580 F g⁻¹), but it is prohibitive in price. Efforts to develop more practical pseudocapacitive materials are now quite active, and more are being made worldwide to replace ruthenium oxide by other nanostructured transition metal oxides. The focus seems to be, however, on achieving high surface areas with low ‘matrix’ resistivity [1], [2], [3], [4], [5].

Spinel nickel cobaltite (NiCo₂O₄) is a low-cost, multiple oxidation states transition metal oxide. It has been reported that nickel cobaltite possesses a much better electronic conductivity, at least two orders of magnitude higher, and a better electrochemical activity than that of NiO and Co₃O₄ [6], [7], [8]. In particular, the NiCo₂O₄ pseudocapacitor shows a high specific capacitance value, which has attracted much attention as electrode material [6], [9], [10], [11], [12], [13], [14], [15], [16], [17]. More specifically, spinel nickel cobaltite aerogels exhibit an extremely high specific capacitance of 1400 F g⁻¹ from an epoxide-driven sol–gel process as reported by Wei et al. [6]. Hence, NiCo₂O₄ with an ultrahigh specific capacitance is a promising low-cost candidate for RuO₂. Nevertheless, a realistic assessment for successful commercial applications needs to surmount many barriers including cost and scale-up issues which are inherent in aforementioned nanotechnology processes [2]. Therefore, it is one of the key issues to develop facile and cost-effective fabrication processes, facilitating successful commercial applications of nanostructured NiCo₂O₄ spinel.

Recently, several methods have been developed to synthesize NiCo₂O₄, including coprecipitation of oxalates, hydroxide carbonates, and metal hydroxides [12], [18], [19], [20], [21], nanocasting [22], cryochemical, spray pyrolysis, and thermal decomposition of common salts [19], [23], [24], sol–gel [6], [7], [25], combustion [8], hydrothermal [11], [26]. Among them, coprecipitation offers a convenient method for the preparation of mixed metal salts and incorporates a high degree of mixing, which can yield powders with high surface area, small crystallite size and high conductivity [19]. However, coprecipitation reactions involve the simultaneous occurrence of nucleation, growth, coarsening, and/or agglomeration processes. In some cases, reactions carried out in polyalcohols such as ethylene glycol (EG) tend to yield more monodispersed products [27]. Such polyols effectively act as bidentate chelating agents for the solvated metal cations or stabililizing agents for the products [28]. Since coprecipitation of nanostructured NiCo₂O₄ spinel carrys out at room temperature or lower [19], EG can effectively act as bidentate chelating agents for the solvated metal cations and, in some cases, also serve as stabililizing agents once the nanoparticles are precipitated. In addition, EG as a very weak acid (pK_a ≈ 15) can act as a buffering solvent of NaOH [29], which may decrease the concentrate of OH⁻ and give some control over the formation rate of nanostructured NiCo₂O₄.

On the other hand, the coprecipitation reaction of NiCo₂O₄ must be subjected to further calcination for producing crystalline oxide [19]. The calcination will, however, almost invariably lead to agglomeration even if it is at relatively low temperatures (<400 °C) due to its low thermal stability and high surface area. Consequently, it is imperative to consider the calcination time, as well as the calcination temperature. A number of studies have detailed the effect of the temperature on the fabrication of nanostructured NiCo₂O₄ during calcination or postannealing process [6], [7], [8], [14], [19]. Given the consideration of the production of phase pure NiCo₂O₄ with both high surface area and conductivity, the temperature of 375 °C was identified as being the most suitable temperature [19], [22]. However, the research on the time of calcination process received less attention.

In this work, based on the above consideration, we modified the coprecipitation of nanostructured NiCo₂O₄ by addition of EG to govern over the coprecipitation processes and researched the effect of the time of the calcination process. We achieved an optimized, 5–10 nm NiCo₂O₄ spinel with superior electrochemical performances, good crystallinity and high conductivity. It showed a high specific capacitance of 671 F g⁻¹ under a mass loading of 0.6 mg cm⁻¹ at a current density of 1 A g⁻¹ with a potential window of 0–0.45 V in 1 M NaOH solution. The excellent cycle stability of the nanostructured NiCo₂O₄ spinel was also demonstrated. The modified coprecipitation, undergoing a facile and cost-effective fabrication process, facilitates successful commercial applications of nanostructured NiCo₂O₄ spinel.