Physical and electrochemical characterization of activated carbons prepared from firwoods for supercapacitors

Activated carbons prepared from firwoods by means of a steam activation method at 900 °C for 1–7 h are demonstrated as promising materials for supercapacitors. The carbons exhibit high-power, low equivalent series resistance and highly reversible characteristics between −0.1 and 0.9 V in aqueous electrolytes. The pore structure of the carbons is systematically characterized by the t-plot method based on N2 adsorption isotherms. The adsorption equilibria of tannic acid, methylene blue, 4-chlorophenol and phenol from aqueous solutions on such carbons are perfectly fitted by the Langmuir equation. All the steam-activated carbons prepared at different activation times (tA) display ideal capacitive performance in aqueous media. This is attributed mainly to the development of mesopores (with an average pore diameter, Dp, between 2.68 and 3.04 nm), which depends strongly on tA. The average specific capacitance of a steam-activated carbon with a tA of 7 h, as estimated from cyclic voltammetic curves measured at 200 mV s−1, reaches 120 F g−1 between −0.1 and 0.9 V in acidic electrolytes. The capacitive characteristics of steam-activated carbons in NaNO3, H2SO4 and HNO3 can be roughly determined by the adsorption data of species with suitable molecular weights. The results indicate that the observed increase in double-layer capacitance arises mainly from the development of mesopores.