Super-hydrophilic microporous biochar from biowaste for supercapacitor application.

[Display omitted] • Super-hydrophilic microporous biochar prepared by a simple and safe method from biowastes. • Detailed interpretation on the surface characteristics for exploring the charge storage mechanism. • High and durable charging/discharging capacitance as supercapacitor electrode. Super-hydrophilic microporous biochar materials, for supercapacitor application, are prepared through a simple and safe method from pomelo peel. Zinc nitrate and urea are used as activator, porogen and nitrogen source, which generate gases moderately and avoid the risk of explosion in calcination. This method endows the biochar products with abundant micropores and extraordinary large N and O doping content (>20% in atomic ratio). Such large doping content leads to super-hydrophilicity, which in turn makes the micropores available for charge storage. In addition, under a moderate calcination condition (700 ℃, 2 h), the biochar material gains large conductivity, without diminishing the super-hydrophilicity. Theoretical analysis on characterizations demonstrates that, the higher ratio of sp2 C and larger proportion of quaternary N (N-Q) in this material (CNO700) account for its much higher conductivity. The synergetic effect of large pore volume (0.69 cm3·g−1), high N and O doping content, proper ratio of N species and large conductivity (0.713 S·m−1) makes the material (CNO700) exhibit high and durable specific capacitance, i.e., 391.0 F·g−1 at 0.5 A·g−1, and cycling stably for >25,000 times at 10.0 A·g−1 with good capacitance retention. [ABSTRACT FROM AUTHOR]