Applied Materials Today
Moss-like nickel-cobalt phosphide nanostructures for highly flexible all-solid-state hybrid supercapacitors with excellent electrochemical performances
Introduction
With the development of renewable and environmental friendly energy sources, it is an ever urgent demand to seek high-power and long-life energy storage devices to balance the power grid. [1], [2], [3], [4], [5], [6] Besides, the smart electronics are highly desirable for their wearability or flexibility, which demands appropriate flexible energy storage devices with high energy and power densities to sustain. [7], [8], [9], [10], [11], [12] Supercapacitors (SCs), which possess high power density, long cycle life and superior security than batteries as well as high energy densities than conventional electrostatic and electrolytic capacitors, have been testified to be one of the most promising energy storage devices. [10, [13], [14], [15]] However, there are still many challenges in the development of SCs. On one hand, the activated carbon as the active materials of currently commercially available supercapacitor usually suffer from lower energy densities, on account of there is only a pure physical charge accumulation occurring at the interfaces between the active materials and electrolytes. [16], [17], [18] On the other hand, traditional SCs using cellulose diaphragms and liquid electrolytes have the risks of electrolyte leakage under deformation in application. [19], [20], [21] Thus, it prompts considerable research interests to hunt high-performance electrode materials for flexible SCs.
Many ways have been employed to further enhance the electrode performance, such as enlarging the surface areas of materials, searching for new materials and introducing faradic redox reactions. [22], [23], [24] Transition metal compounds have attracted extensive attentions on account of their high performance, low cost and environmental friendliness. [16, 25] Typically, the nickel/cobalt based materials have received considerable research interests for their lower prices, abundant natural resources, multiple oxidation states and highly reversible redox reactions. [15, [26], [27], [28], [29]] However, the performance of solitary nickel/cobalt compounds generally suffers from pitiable rate property and low capacity retention during chronic charge-discharge process. [5, 30] It seems to be an effective way to ameliorate the situation by forming multicomponent nickel-cobalt compound owing to the synergistic effect of nickel and cobalt altering the electronic architecture of the metal sites and providing richer redox reactions and structure stability. [31], [32], [33] Although the reasonable combination of NiCo-based materials shows promising potential in the application of SCs electrodes, the lower conductivity and unsatisfactory specific capacitances still are the barrier. The transition metal phosphides are newly type promising electrode materials for supercapacitors by virtue of their high intrinsic conductivity rooting from their metallic bond, which can provide a large number of free electron. [5, 34, 35] There are not only covalent bonds but metallic bonds between the Ni-P and Co-P in nickel cobalt phosphide. [36, 37] The covalent bonds of Ni-P and Co-P can store charges through faradaic redox reactions to provide capacity while the metallic bonds offer free electrons to enhance the electrical conductivity, which will be responsible for the favorable conductivity and excellent rate capability. [35, 38] For instance, Chen et al. [6] designed a hollow urchin-like Ni-Co-P/POx bimetallic phosphides/phosphate hybrid nanomaterials via a CVD route, which exhibited a high specific capacity of 647 C g−1 at 1A g−1 with the capacitance retention of 94% after 1000 cycles. Chen et al. [39] synthesized a NiCoP composites, which displayed the specific capacitance of 194 mAh g−1 at 1 A g−1 with about 87% retention at 10 A g−1. Although the previous reports have indicated the potential of nickel cobalt bimetallic phosphides, the barrier of low specific capacitances and lower cycling stability still exist. Besides, it has been confirmed by our previous works that the flexibility and electrochemical performance of the electrodes can be further optimized by directly depositing the materials on current collector. [40, 41]
Therefore, in this work, moss-like Ni-Co-P nanosheet arrays are directly loaded on carbon fabrics by phosphatizing the precursor of Ni-Co bimetallic hydroxides that are directly grown on carbon fabrics via electrodeposition. The Ni-Co-P-3 exhibits high specific capacity of 213.1 mAh g−1 at 1 A g−1, and holds extraordinary rate performance (about 86.0 % of the initial capacity at 20 A g−1). These remarkable electrochemical properties are ascribed to the intrinsical merits of these moss-like Ni-Co-P nanosheet arrays, whcih are confirmed by first principle calculations. Moreover, the corresponding all-solid-state hybrid supercapacitors are assembled by directly using the Ni-Co-P-3 nanosheet arrays as the positive electrode, which deliver high energy density (48.4 Wh kg−1 at 811.2 W kg−1), excellent cyclic stability (88.8 % retention after 8000 cycle) and robust mechanical flexibility (no obvious decay during bending 1000 times at different angles).
Section snippets
Experimental Section
All the reagents in this work are analytical grade without additional purification. Nickel nitrate hexahydrate (Ni(NO3)2•6H2O) and polyvinyl alcohol (PVA) are purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China), and other reagents are purchased from Sinopharma. The carbon fabrics (WOS 1002, 360 μm thick, 125 g m−2) are purchased from CeTech Co., Ltd.
Result and Discussion
The synthesis process of Ni-Co-P nanosheet arrays is schematically depicted in Fig. 1a. Firstly, the NixCoy(OH)2 nanosheet arrays are directly deposited on the carbon fabrics by co-electrodeposition. During the electrodeposition process of nickel-cobalt hydroxides, NO3− in solution is reduced on the cathodic surfaces, generating abundant OH− ions nearby. The OH− ions react simultaneously with Ni2+ or Co2+ in solution to form uniform Ni(OH)2 or Co(OH)2 nanosheets, which can be confirmed by the
Conclusions
We are rationally designed and synthesized moss-like Ni-Co-P nanosheet arrays on carbon fabrics by phosphatized NixCoy(OH)2 under argon atmosphere, which are directly served as the positive electrode for high performance all-solid-state flexible supercapacitors. The as-assembled supercapacitors possess high specific capacitance of 136 F g−1 at 1 A g−1, excellent energy density of 48.4 Wh kg−1 at 811.2 W kg−1, and satisfactory cycling stability (about 88.8 % after 8000 cycles). Besides, the
Acknowledgements
The work was financially by the National Natural Science Foundation of China (Grant No. 51672109), Natural Science Foundation of Shandong Province for Excellent Young Scholars (Grant No. ZR2016JL015).
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