Tuning pore characteristics of porous carbon monoliths prepared from rubber wood waste treated with H3PO4 or NaOH and their potential as supercapacitor electrode materials

The facile preparation of porous carbon monolith electrodes as a supercapacitor was demonstrated using rubber wood wastes treated with NaOH or H3PO4 as starting materials. Different concentrations of treating solutions were used (5 and 10% w/v). The effect of the pore structures of the porous carbon electrodes on their electrochemical performance and which parameters played important roles in the specific capacitance were also studied. The results revealed that the H3PO4-activated samples had microporous characteristics and a specific surface area of 605–693 m2/g, while the NaOH-activated samples exhibited mesoporous characteristics with a developed mesopore volume of 0.27 cm3/g. The specific surface area tended to increase with an increased H3PO4 concentration but decreased with an increased NaOH concentration. As confirmed by X-ray photoelectron spectrophotometry, oxygenated functional groups (OGF) were detected in all samples. Notwithstanding, a pseudo-Faradaic reaction as a result of OGF occurred only with the reference carbon and H3PO4-activated samples, while NaOH-activated samples with OGF exhibited ideal electrical double-layer capacitor behavior. Measured at 1 mV/s in 1 M H2SO4 solution, a carbon electrode prepared from 10% w/v H3PO4-treated rubber wood exhibited the maximum gravimetric capacitance of 129 F/g, a volumetric capacitance of 104 F/cm3, an energy density of 14.2 Wh/kg, and a power density of 500 W/kg. The results also showed that the mesopore volume and the average pore size rather than the specific surface area played important roles in the specific capacitance, but only when the percentage difference in the specific surface area between the two samples did not exceed 41%.