Carbon spheres with ultra-hydrophilic surface and ordered pores on the borderline between micropores and mesopores for improving supercapacitive performance

The unique porous carbon (named AUSH) with superior porosity, wettability, and uniform spherical morphology was successfully prepared using a thermal pretreatment of glucose-derived spherical hydrochar with urea followed by KOH activation. Thermal pretreatment of hydrochar with urea induces nitrogen-containing functional groups and increases sp3-hybridized carbon to improve the surface wettability of hydrochar with KOH solution to facilitate the activation process and thus obtaining the borderline microporous-mesoporous structure. The highly porous AUSH possesses a high surface area (2775.9 m2 g−1) and large pore volume (1.89 cm3 g−1) with narrow pore size distribution (~2 nm). This borderline microporous-mesoporous structure exhibits an ultra-hydrophilic surface to increase the electrochemically active surface area in aqueous electrolytes and thus benefits the charge-storage capacity. The AUSH has higher specific capacitance (354 F g−1 at 1 A g−1), rate capability (264 F g−1 at 50 A g−1), and cycle-life stability in 6 M KOH than other KOH-activated porous carbon materials. The enhanced supercapacitive characteristics are largely attributed to the spherical morphology, hydrophilic surface, and well-developed porosity that bridges the gap in attainable pores between micropores and mesopores for facilitating the ion transport and surface-controlled redox kinetics.