Achieving high-capacity aqueous supercapacitors via anion-doped construction of dual redox centers in Ni x Co 1- x SeO 3 .

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, Ni x Co 1- x SeO 3 (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) x N 2- x /NO 3 - , 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 .