Bifunctional phosphorization synthesis of mesoporous networked Ni-Co-P/phosphorus doped carbon for ultra-stable asymmetric supercapacitors

Highlights: • A core-shell structure constructed by Ni-Co-P NSs network grown on P doped CSs are designed, noted as P-CSs@Ni-Co-P NSs. • The optimized P-CSs@Ni₁-Co₂-P//AC device shows notable cycling stability without any decay after 20000 cycles at 6 A g^-1. • Investagation of charge kinetics indicates that capacitive behavior is dominant at high-rate charge/discharge process. -- Abstract: Low-cost and resourceful transition metal phosphides (TMPs), as a new kind of pseudocapacitive materials, have received considerable interest for electrochemical energy storage/conversion. Combination of TMPs and carbon nanostructures is supposed to greatly improve the charge storage capability due to the synergistic effect. In this work, nickel cobalt phosphides nanosheets supported by P doped C spheres (P-CSs@Ni-Co-P NSs) composites with different Ni/Co molar ratios (1:0, 1:1, 1:2, 1:4 and 0:1) were prepared by in-situ bifunctional phosphorization of the as-synthesized CSs@Ni-Co-OH NSs precursor. The optimized 3D networked mesoporous P-CSs@Ni₁-Co₂-P NSs electrode exhibits a remarkable specific capacitance of 1040.3 F g⁻¹ at 1 A g⁻¹ and excellent rate capability with a capacitance retention of 49.2% even at 30 A g⁻¹, which are much higher than those of A-CSs@Ni₁-Co₂-O NSs (870.5 F g⁻¹ at 1 A g⁻¹, 37.4% at 30 A g⁻¹) and Ni₁-Co₂-P NSs (500.5 F g⁻¹ at 1 A g⁻¹, 40.0% at 30 A g⁻¹). The improved supercapacitive performances are attributed to the 3D mesoporous nanosheets network and bifunctional phosphorization treatment which lead to enhancement of conductivity. Meanwhile, Ni-Co bimetallic phosphide with abundant oxidation states facilitates redox charge transfer efficiently. Furthermore, P-CSs@Ni₁-Co₂-P NSs assembled asymmetric supercapacitors (ASC) achieve outstanding cycling stability with no decay of capacitance after 20000 cycles. Additionally, the charge storage kinetic of the device is further investigated, indicating that fast surface-control capacitive behavior is dominant at high current densities.