High capacitance B/C/N composites for capacitor electrodes synthesized by a simple method

The B/C/N composites were synthesized by a very simple method, that is, carbonization at HTT = 800–1200 °C of the precursor prepared by drying a solution mixture of polyacrylamide and boric acid, followed by boiling in water to remove borate by-products. The amount of insoluble B species in the composite increased linearly from 4.8 to 18.6 mass% with raising HTT. The XRD and FT-IR revealed that turbostratic h-BN started to form at around 1000 °C as a by-product. By XPS, major B and N components in the composite were B–N bond, C–B–O type B, pyridinic N, pyrrolic N, and quaternary N. A fraction for B–N bond including h-BN in the total B or N components increased with raising HTT and it exceeded 50 at% between 900 and 1000 °C. It was suggested that in the composites formed at HTT > 1000 °C the amounts of h-BN increased, leading to reduction in other B and N components. The SBET was almost unchanged up to 1000 °C, 410–420 m2 g−1. Large and broad redox peaks arisen from plural reactions appeared in the cyclic voltammogram (CV) measured in 1 mol dm−3 H2SO4 for the composites formed at HTT ≤ 1000 °C. These peaks disappeared in 1 mol dm−3 solutions of Na2SO4 and Li2SO4. By comparing CV with that for C/N composite formed from PAA by the MgO template method, the pseudo-capacitance owing to reactions of B–N and C–B–O components with protons was found to be added to commonly observed pseudo-capacitance for nitrogen-doped carbons. The capacitances for the composites formed at 850–950 °C exceeded 300 F g−1 at 2 mV s−1 in the acid electrolyte and the retention at 50 mV s−1 was 78–80%. The shape of CV in the neutral electrolytes was trapezoid and the current density increased with lowering potential, suggesting adsorption and desorption of Na+ and Li+ ions. This was considered to be due to doped nitrogen, indicating the development of pseudo-capacitance. The capacitance per SBET was 0.33–0.74 F m−2 and 0.17–0.32 F m−2, larger for lower HTT, in the acid and neutral electrolytes, respectively.