Boron-doped Nanodiamond as an Electrode Material for Aqueous Electric Double-layer Capacitors

Herein, a conductive boron-doped nanodiamond (BDND) particle is prepared as an electrode material for an aqueous electric double-layer capacitor with high power and energy densities. The BDND is obtained by depositing a boron-doped diamond (BDD) on a nanodiamond particle substrate with a primary particle size of 4.7 nm via microwave plasma-assisted chemical vapor deposition, followed by heat treatment in air. The BDND comprises BDD and sp² carbon components, and exhibits a conductivity above 10^-2 S cm^-1 and a specific surface area of 650 m² g^-1. Cyclic voltammetry measurements recorded in 1 M H₂SO₄ at a BDND electrode in a two-electrode system shows a capacitance of 15.1 F g^-1 and a wide potential window (cell voltage) of 1.8 V, which is much larger than that obtained at an activated carbon electrode, i.e., 0.8 V. Furthermore, the cell voltage of the BDND electrode reaches 2.8 V when using saturated NaClO₄ as electrolyte. The energy and power densities per unit weight of the BDND for charging-discharging in 1 M H₂SO₄ at the BDND electrode cell are 10 Wh kg^-1 and 10⁴ W kg^-1, respectively, and the energy and power densities per unit volume of the BDND layer are 3-4 mWh cm^-3 and 10 W cm^-3, respectively. Therefore, the BDND is a promising candidate for the development of a compact aqueous EDLC device with high energy and power densities.