Preparation of boron–nitrogen co-doped diamond/boron-doped diamond electrodes for aqueous capacitors with AC line-filtering performance

Boron–nitrogen co-doped diamond (BNDD) nanostructures are grown in situ on a boron-doped diamond (BDD) film, and the effects of B and N doping on the BNDD-layer morphology and composition are systematically investigated. A competitive control mechanism of BNDD etching and growth under the synergistic action of the B and N atoms is revealed and, hence, a controllable preparation process for porous BNDD films and BNDD nanosheets on BDD is proposed. In an NaCl aqueous solution, the positive-potential window of the BNDD electrode reaches 0–1.2 V, the negative-potential window reaches −1.6–0 V, and the working voltage of the constructed capacitor reaches 2.8 V. The proposed capacitor exhibits a high power density of 21,458.33 μW cm−2 at 0.02 μW h cm−2 energy density, and a high energy density of 0.42 μWh cm−2 at 1224.22 μW cm−2 power density. Furthermore, the BNDD-based capacitor demonstrates excellent stability, with 104.8% capacitance retention and 101.2% Coulombic efficiency after 50,000 cycles. The capacitor filters alternating-current (AC) signals in a wide frequency range of 50 Hz–1 MHz, with an output-signal voltage loss of 0.17–0.48 V. The proposed aqueous BNDD-based capacitor has considerable application potential in the filtering of broadband and high-frequency AC signals.