PVP-Assisted Synthesis of Self-Supported Ni₂P@Carbon for High-Performance Supercapacitor

Highly conductive and stable electrode materials are usually the focus of high-performance supercapacitors. In this work, a unique design of Ni₂P@carbon self-supported composite nanowires directly grown on Ni foam was applied for a supercapacitor. The Co₃O₄ nanowire array was first synthesized on the Ni foam substrate, and the resulting Ni₂P@carbon nanocomposite was obtained by hydrothermally coating Co₃O₄ with the Ni-ethylene glycol complex followed by gaseous phosphorization. We have discovered that the molecular weight of surfactant polyvinylpyrrolidone (PVP) used in the hydrothermal step, as well as the temperature for phosphorization, played very important roles in determining the electrochemical properties of the samples. Specifically, the sample synthesized using PVP with 10 k molecular weight and phosphorized at 300°C demonstrated the best supercapacitive performance among the different samples, with the highest capacitance and most stable cyclic retention. When an asymmetric supercapacitor (ASC) was assembled with this Ni₂P@carbon sample as the cathode and activated carbon (AC) as the anode, the ASC device showed excellent capacitances of 3.7 and 1.6 F cm^-2 at 2 and 50 mA cm^-2, respectively, and it kept a high capacitance of 1.2 F cm^-2 after 5000 cycles at a current rate of 25 mA cm^-2. In addition, the ASC could reach a high energy density of about 122.8 Wh kg^-1 at a power density of 0.15 kW kg^-1 and 53.3 Wh kg^-1 at the highest power density of 3.78 kW kg^-1. Additionally, this device also had the ability to power up 16 red LEDs effortlessly, making it a strong candidate in electrochemical energy storage for practical usage.