Achieving high-capacity aqueous supercapacitors via anion-doped construction of dual redox centers in NiCo_1-SeO₃

In asymmetric supercapacitors, transition metal selenates are promising electrodes, but their capacity are limited by a single redox center. To further enhance the performance of transition metal selenates, NiCo_1-SeO₃ (NCSeO) doped with N and Cl was prepared on nickel-plated carbon cloth (NCSeO-NCl-NiCC). During electrochemical reactions, NCSeO can be converted to M(OH)₂ (M = Ni/Co) and OH^- is replaced by N and Cl. Two redox centers, M(OH)₂/MOOH and M(OH)N_2-/NO₃^-, are formed during charging and discharging, which is attributed to the increased capacity of the NCSeO-NCl-NiCC electrode. On NCSeO, the substitution of Cl facilitates the regulation of the electronic structure and enhances the stability of N-doping. The optimised electrode exhibits a high capacity of 417 mA h g^-1 at 1 A g^-1 and an impressive rate capability of 235 mA h g^-1 at 50 A g^-1. Asymmetric supercapacitors with this design have an ultra-high energy density of 73.6 W h kg^-1, as well as an excellent rate and cycling performance with a capacitance retention of 97.8% after 20 000 cycles at a current density of 20 A g^-1.