Three-dimensional hierarchical porous carbon derived from lignin for supercapacitors: Insight into the hydrothermal carbonization and activation.

Three-dimensional hierarchical porous carbon is prepared by utilizing enzymatic hydrolysis lignin as a carbon source via hydrothermal carbonization and activation. The complicated operational parameters including temperature, time, concentration and pH in the hydrothermal carbonization are systemically investigated. We employed the hydrochar as electrode for supercapacitors. Accordingly, we not only achieve a high-performance specific capacitance for supercapacitors but also rationalize the effects of hydrothermal conditions on the specific capacitance via various characterizations. The activation process of hydrochar is also studied by comparing various activators and the activator/hydrochar ratios. The obtained materials possess a three-dimensional interconnected hierarchical structure with rational pore size distribution and a specific surface area reach up to 1504 m2 g-1. Then the corresponding supercapacitors achieve a large specific capacitance of 324 F g-1 as the current density is 0.5 A g-1. These supercapacitors acquire an outstanding cycling stability with 99.7% capacitance retention after 5000 cycles. The assembled symmetrical supercapacitors also show a high energy density of 17.9 W h kg-1 and can maintain at 5.6 W h kg-1 even at an ultra-high power density of 50,400 W kg-1.