Synthesis And Assessment Of Self-supported Nickel Phosphide（Ni₂P） Array Flexible Electrode For Supercapacitor

With high power density, long lifespan, low cost, free maintenance and eco-friendliness, supercapacitors have been widely used in solar energy system, wind power generation, new-energy vehicles, smart power grids, urban rail transit and so on. The development of classical electric double-layer capacitors have been limited owing to the fact that the specific surface areas of active materials have touched the threshold value. Fortunately, pseudo-capacitors offer fast charge storage and transfer through Faraday reactions. This type of supercapacitors not only enhance the specific capacitance of the electrode, but also keep the fast charge-discharge property and long lifespan. These changes provide a new method to develop the new generation of supercapacitors with high specific capacitance.Nickel phosphide（Ni₂P）, as a kind of excellent pseudo-capacitor active material, has similar reaction mechanism with Ni（OH）2. Due to the high mass percentage of Ni element（79.1%） and high conductivity, Ni₂ P has higher theoretical specific capacitance and excellent electrochemical properties than Ni（OH）2. In this work, Ni（OH）2 arrays have grown on carbon cloth through chemical bath deposition（noted Ni（OH）2/CC）, and subsequently they are low temperature phosphatized to obtain Ni₂ P phase（noted Ni₂P/CC）. The reversible specific capacitance of the Ni₂P/CC electrode is 1121 F·g-1 at the current density of 1 A·g-1. When the current density reaches 5 A·g-1, the specific capacitance stays on 545 F·g-1. Lifespan test presents a stable cycle performance with the retention of 70% of the original capacity after 2000 cycles at 1 A·g-1. Comparing with Ni（OH）2/CC electrode, CV curves indicate that Ni₂P/CC is much more electrochemical active. In addition, electrochemical impedance spectroscopy（EIS） further proves that the charge transfer resistance of Ni₂ P is remarkably lower than that of Ni（OH）2. That is attributed to the high electronic conductivity of Ni₂ P.The full capacitors have been assembled with Ni₂P/CC as the positive electrode and commercial active carbon as the negative electrode. The electrochemical data deduced from CV curve have been calculated to point out the optimized mass ratio（0.5⁰.6） of the two kinds of electrochemical active materials in full capacitors. The CV curve of full capacitor shows that this asymmetric supercapacitor has higher potential windows up to 1.7 V, and the original array morphology remains after 3000 charge-discharge cycle. The flexible all-solid-state supercapacitors have been assembled with the same electrodes except PVA-KOH as gel electrolyte. The flexible supercapacitors have been bent at angles from 0° to 120° without obvious electrochemical performance decay. The specific capacitance of Ni₂P/CC reaches 263 F·g-1 at current density of 1 A·g-1 and 108 F·g-1 at 10 A·g-1.