Enhancement of the capacitance of rich-mixed-valence Co-Ni bimetal phosphide by oxygen doping for advanced hybrid supercapacitors

ABSTRACT Tailoring of the electrode microstructure and its active components for the development of high-performance supercapacitors was rather indispensable to advance current technologies. Therefore, we fabricated a novel O-doped Co-Ni nanophosphide (O-CoxNiyP) with a porous structure. Our approach included the synthesis of 3D Co-Ni precursors with tunable Co/Ni ratio utilizing a phosphorization treatment. Our characterization results indicated that incorporating the optimized O content into CoxNiyP could tune its electronic structure and yield mixed valences of metals and P, which was beneficial to enhance electrochemical reactivity. The O-Co1Ni4P-based electrode with the 1:4 Co:Ni molar ratio manifested the best performance as demonstrated by its specific capacity, which was equivalent to 717.1 C/g (at 1 A/g). This value was significantly higher than those of other O-CoxNiyP family electrodes. The cycling stability of the electrode containing O-Co1Ni4P as an active material was found to be excellent: it demonstrated a 95.1% capacity retention after 5000 10 A/g cycles. We also assembled an asymmetric supercapacitor containing O-Co1Ni4P as a positive electrode material and commercial activated carbon as a negative electrode material. The device demonstrated very high energy density (47.5 Wh/kg) and 90.3% capacity retention after 10000 cycles. These results established that our anion-doping method represented a novel strategy for improving the electrochemical performance of transition metal-based material. These discoveries were useful for energy storage applications.