Nitrogen-doped carbon dots anchored NiO/Co₃O₄ ultrathin nanosheets as advanced cathodes for hybrid supercapacitors

Abstract

Herein, we demonstrate an advanced cathode of nitrogen-doped carbon dots (NCDs) anchored NiO/Co₃O₄ ultrathin nanosheets for hybrid supercapacitors by a facile hydrothermal-calcination route. Owing to the well defined thin-plate structure and ternary composition, the optimized NiO/Co₃O₄/NCDs nanosheets demonstrate a high specific capacity of 976.3 C g⁻¹ (1775 F g⁻¹) at 1 A g⁻¹, and a splendid cycling stability of approximately 95.7% retention over 10,000 continuous cycles (15 A g⁻¹). In addition, a hybrid supercapacitor is constructed by using NiO/Co₃O₄/NCDs nanosheets as cathode and reduced graphene oxide (RGO) supported NCDs composites as anode. The obtained NiO/Co₃O₄/NCDs//RGO/NCDs hybrid supercapacitor delivers a maximum energy density of 41.6 Wh kg⁻¹, together with outstanding cycling stability (no decay after 10,000 cycles at 10 A g⁻¹). Therefore, the ultrathin sheet-like structured NiO/Co₃O₄/NCDs cathode presents a great potential for supercapacitor application.

Introduction

With the gradual consumption of fossil fuel, the development of energy storage devices has become global concerns [1], [2]. As one of the prime choices, supercapacitors have elicited a great scientific interest by virtue of their desirable power density and fast charge–discharge rate [3], [4], [5]. Whereas, the poor energy density of supercapacitors severely hinder their use in energy storage systems [6], [7], [8]. Accordingly, numerous efforts have been dedicated to improve the energy density of supercapacitors without decreasing the power density of them. More recently, hybrid supercapacitors consisting of a capacitive electrode and a battery-like electrode are able to deliver much higher energy density than conventional supercapacitors due to the larger specific capacities of the battery-type materials [9], [10], [11]. Transition metal oxides are demonstrated to be superior battery-type electrode materials on account of their naturtal abundance and environmental friendliness [12]. Among these materials, NiO with ultrahigh theoretical capacity and Co₃O₄ with long cycling life have elicited the extensive interest as redox-active electrodes for supercapacitors [13], [14], [15]. Compared with the corresponding individual constituents, the effective combination of NiO and Co₃O₄ could offer competitive advantages for the increment of electrochemical activity [16]. However, the major obstacles for NiO/Co₃O₄ composites are their low specific surface area and poor electrical conductivity owing to their semiconductor nature. These disadvantages would lead to a lower specific capacity of NiO/Co₃O₄ composites and severely confine their further development in supercapacitors.

In order to overcome the above obstacles, integrating conductive carbon and constructing nanostructured materials are considered as plausible methods to optimize the electrochemical properties of NiO/Co₃O₄ [1], [17]. Recently, carbon dots, as a new type of zero-dimensional carbonaceous materials, have become the focus in the field of basic research owing to their diverse physicochemical properties and beneficial attributes, such as low production cost, good dispersion in solvent, and excellent surface grafting ability [18], [19]. It is found that carbon dots can provide a favorable interface for the interaction between electrolyte and electrodes, thus improving the specific capacities and cycle stabilities of the electrode materials. The emergence of carbon dots have exhibit exciting prospects in the energy conversion and storage fields [20]. On the other hand, attributing to the higher specific surface area, more exposed active sites and larger degree of anisotropy, ultrathin two-dimensional nanostructures offer superior charge storage characteristics for the electrode materials and have attracted increasing interest in electrochemical energy storage systems [21], [22]. Unfortunately, few study has been reported on carbon dots anchored NiO/Co₃O₄ two-dimensional nanostructures for energy storage applications. Herein, we developed an advanced cathode of ultrathin NiO/Co₃O₄ nanosheets supported nitrogen-doped carbon dots (NCDs) for hybrid supercapacitors. The NiO/Co₃O₄/NCDs ultrathin nanosheets were successfully synthetized through a hydrothermal-calcination two-step strategy, which realize simultaneous boosts in specific capacity (976.3 C g⁻¹, 1 A g⁻¹) and cycle stability (95.7% retention, 10,000 cycles). Meanwhile, a corresponding hybrid supercapacitor device based on NiO/Co₃O₄/NCDs ultrathin nanosheets and reduced graphene oxide (RGO)/NCDs shows a high energy density of 41.6 Wh kg⁻¹ at 800 W kg⁻¹ with excellent cycling stability (no decay after 10,000 cycles at 10 A g⁻¹), suggesting the high application potential of NiO/Co₃O₄/NCDs ultrathin nanosheets for energy storage systems.