One-pot solvothermal synthesis of ternary Ni-Co-P micro/nano-structured materials for high performance aqueous asymmetric supercapacitors

- The Ni-Co-P ternary materials were synthesized by one-pot solvothermal method.
- The performance of Ni-Co-P materials was tuned by varying Ni/Co ratios.
- The Ni8-Co1-P showed superior synergistic effect of Ni and Co redox species.
- The AC//Ni8-Co1-P capacitors were optimized with electrode mass ratio.

The ternary Ni-Co-P micro/nano-structured materials were synthesized via a facile one-pot solvothermal strategy and further served as positive electrode materials for supercapacitors. X-ray diffraction (XRD) showed that the optimal Ni8-Co1-P materials exhibited the two major phases of M12P5 and M2P (M = Ni, Co). X-ray photoelectron spectroscopy (XPS) revealed the binding energy shifts of P2p, Ni2p and Co2p of Ni8-Co1-P materials compared with the bare Ni-P and Co-P, indicating the formation of ternary materials. Transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) mapping exhibited the micro/nano-structured morphology with rich surface nanowires (5-10 nm in diameter) and the consistent distribution of Ni, Co and P elements of the Ni8-Co1-P materials respectively. Nitrogen sorption measurements illustrated the mesoporous structure of the Ni8-Co1-P materials with the specific surface area of 257 m2 g−1. Due to the richer Ni, Co surface electroactive sites and the stronger synergistic effect of Ni and Co redox species, the Ni8-Co1-P materials showed superior specific capacitance, rate capability and charge transfer kinetics (1448 F g−1 at 1 A g−1, 1173 F g−1 at 16 A g−1, 333 Ω) than the Ni-Co-P with other Ni/Co ratios (1:0, 16:1, 4:1, 2:1, 0:1). The mass ratio optimized activated carbon (AC)//Ni8-Co1-P asymmetric capacitor (m+/m− = 1:2) exhibited high energy and power densities (22.8 Wh kg−1, 4.32 kW kg−1) together with excellent cycle life (no decay after 5000 cycles at 4 A g−1). The high performance makes the ternary Ni-Co-P micro/nano-structured materials promising electrode materials for supercapacitors.