Date Published: December 27, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Jinghuang Lin, Henan Jia, Haoyan Liang, Shulin Chen, Yifei Cai, Junlei Qi, Chaoqun Qu, Jian Cao, Weidong Fei, Jicai Feng.
NiO is a promising electrode material for supercapacitors. Herein, the novel vertically standing nanosized NiO encapsulated in graphene layers (G@NiO) are rationally designed and synthesized as nanosheet arrays. This unique vertical standing structure of G@NiO nanosheet arrays can enlarge the accessible surface area with electrolytes, and has the benefits of short ion diffusion path and good charge transport. Further, an interconnected graphene conductive network acts as binder to encapsulate the nanosized NiO particles as core–shell structure, which can promote the charge transport and maintain the structural stability. Consequently, the optimized G@NiO hybrid electrodes exhibit a remarkably enhanced specific capacity up to 1073 C g−1 and excellent cycling stability. This study provides a facial strategy to design and construct high‐performance metal oxides for energy storage.
Supercapacitors, including electrical double‐layer capacitors (EDLCs) and pseudocapacitors, have become a research focus recently owing to their desirable properties, such as fast charge and discharge, high power densities, excellent cycling performance and safety.1, 2, 3, 4, 5 Compared with carbon nanomaterials for EDLCs, pseudocapacitor materials, especially transition metal oxides (TMOs), can provide higher specific capacity and energy density due to the efficient reversible redox reaction.6, 7, 8, 9 Among various TMOs, nickel oxide (NiO) has attracted great interest owing to the merits of low cost, high theoretical capacity and environmental friendliness.10, 11, 12, 13 Unfortunately, in many case, NiO often suffers from low specific capacity, inferior rate performance and poor cycling stability. It is mainly constrained by the poor conductivity, limited electroactive sites and structural instability during charge–discharge process.11, 12, 13
For comparison, the PECVD process was maintained for 0.5, 1, 2, and 3 min to obtain different samples (briefly named as G@NiO‐0.5, G@NiO‐1, G@NiO‐2, and G@NiO‐3). Figure2 shows the typical scanning electron microscope (SEM) images of NiO and G@NiO‐1. As shown in Figure 2a,b, pristine NiO nanosheets could be synthesized on substrates. Notably, the nanosheets have a smooth surface with about 30 nm thickness and about 2 µm height (Figure 2b,c), where such high surface–volume ratio can provide sufficient surface areas with electrolytes. With the plasma treatment for 1 min, the resultant G@NiO‐1 nanosheets exhibit more flexible outer surface, as shown in Figure 2d,e. However, the vertical nanosheet morphology was still maintained and the height of G@NiO‐1 was also about 2 µm (Figure 2f). With extend the plasma exposing time, it can be found that the whole nanosheet morphology was maintained (Figure S2, Supporting Information).
In conclusion, we provide a new approach to boost the electrochemical performance of NiO by constructing the unique structure of G@NiO composites without organic binders, where nanosized NiO particles are in situ encapsulated by high‐quality graphene layers as the vertical‐standing nanosheet structure. The optimized G@NiO hybrid electrodes shows an improved capacity of as high as 1073 C g−1, almost three times higher than that of the counterpart in pristine NiO, good rate capability, and excellent cycling performance. Further, the constructed G@NiO‐1//NGH device shows a maximum energy density up to 52.6 W h kg−1 at a power density of 800 W kg−1, and good cycling performance. In principle, the strategy in our study feasibly offers insight into the in situ forming graphene on nanosized TMOs particles directly as electrodes for maximizing their electrochemical performances, which may help to accelerate development of TMOs as electrode for high‐performance supercapacitors.
The authors declare no conflict of interest.