Uniaxially oriented carbon monoliths as supercapacitor electrodes

Cylindrical carbon monoliths of 7 mm in diameter and certain heights (1, 2, 3, 4 and 5 mm) are studied as model electrodes for supercapacitors. The monoliths show a narrow microporous structure with average micropore size of 0.73 nm and specific surface area of 1086 m2 g−1. The monoliths show straight walls and channels, both arranged along the cylinder axis. The former account for a remarkable electrical conductivity (6.5 S cm−1 at room temperature). The latter allow a rapid ionic transport between the electrolyte bulk and the carbon walls and account for a high specific capacitance at high current density. The cell capacitance and resistance increase linearly with the monolith height according to C = (1.78 ± 0.06)h and ESR = (0.08 ± 0.01)h + (1.67 ± 0.04), respectively. The contribution of the electrolyte resistance, monolith resistance and monolith/collector resistance to ESR is discussed. The cell response time or constant time increases with the monolith height but according to a power dependence, τ = (4.5 ± 0.2)h(1.61 ± 0.03). The carbon of the monoliths show in KOH electrolyte a specific capacitance of 150 F g−1 and a capacitance per surface area of 14 μF cm−2.