Effects of Fe-doping induced by valence modulation engineering on the nickel hydroxyfluoride cathode of hybrid supercapacitors

Abstract

Nickel hydroxyfluoride (NHF) with an elevated theoretical specific capacity has attracted increasing interest for use as an electrode material but its deficient electrical conductivity and inadequate number of active sites have hampered its wide applications. Herein, small amounts of Fe were doped into NHF by a novel single-step solvothermal method. The experimental findings revealed that doping of Fe into NHF accelerated the electron transfer at the Ni sites and promoted the generation of high valence Ni^(3+δ)+ for stimulating a two-step reaction. The presence of highly active Ni^(3+δ)+ enriched the redox process and effectively enhanced energy storage. At a current density of 1 A g⁻¹, the optimal doped sample NHF-0.03 displayed a maximum specific capacity of 640 C g⁻¹. The assembled NHF-0.03//AC asymmetric supercapacitor exhibited a maximum power density of 750 W kg⁻¹ and a maximum energy density of 55.5 W h kg⁻¹. After 10 000 charge/discharge cycles, the supercapacitor device preserved 77% of its original capacity at a high current density of 15 A g⁻¹, indicating good cycling stability. Overall, the proposed innovative doping strategy looks promising for the enhancement and refinement of materials based on hydroxyfluoride for advanced supercapacitor electrodes.