Solvent lignin was mixed with urea (Fisher Scientific Inc. USA) in
a mass ratio of 1:2 first. Proper amount of deionized water was
added, while keeping stirring till the urea was dissolved. Then the
mixture was placed into an oven at 110 C to dry. The dried powder
was transferred into a steel crucible and heated for 1 h at 400 C
with a heating rate of 10 C min1 in a muffle furnace (1100 box
furnace, Lindberg/Blue M, Thermo Scientific Inc.) under the protection of nitrogen (flow rate was 96 ml min1). After the precarbonized material was cooled to room temperature, it was
mixed with potassium hydroxide (KOH, Fisher Scientific Inc. USA)
in a mass ratio of 1:3 in a steel crucible that contained 30 mL
deionized water. The crucible was placed in an oven and dried at
110 C for 24 h. Then, the crucible was transferred into a muffle
furnace and activated at 800 C for 1 h with a heating rate of
10 C min1 under N2 protection (flow rate was 96 ml min1). Afterwards, the carbonized solids were washed with 30 mL
0.1 mol L1 HCl (Fisher Scientific Inc. USA) at 110 C for 1 h in a
60 mL polytetrafluoroethylene (PTFE) autoclave to remove the residual KOH and impurities. Finally, the carbon solid was washed
with deionized water until the pH stabilized at 7. The material was
then dried at 105 C for 12 h in an oven. The carbonized samples
were denoted as L-U.