Synergistic effect of CO2 and H2O co-activation of Zhundong coal at a low burn-off rate on high performance supercapacitor

This study presents porous activated carbons, prepared by the physical activation of high-alkali Zhundong coal with CO2, H2O, or CO2/H2O, as supercapacitor electrode materials. The synergistic effect between CO2 and H2O activation on the specific capacitance, morphology, pore and physicochemical structure, and supercapacitor performance of activated carbons was systematically investigated for the first time. A hierarchical porous structure is formed in carbon co-activated by CO2 and H2O at 800 °C (TCC/H-800), which also has a rich graphite microcrystalline structure, long-range ordered graphite network, abundant oxygen- and nitrogen-containing functional groups and high char yield (about 70 wt%), all of which are desirable properties. Moreover, the TCC/H-800 electrode shows better electrochemical performance than electrodes based on carbon activated by CO2 or H2O alone, and has an ideal coulombic efficiency (98.95% after 2000 cycles at 5 A g−1) and a good specific capacitance. The symmetrical TCC/H-800 supercapacitor reaches a 155.1 F g−1 specific capacitance at 0.5 A g−1. Meanwhile, its energy density is 21.54 Wh kg−1 at 500.00 W kg−1, with 92.97% of the initial discharge capacity retains after 10,000 cycles. Coal-based porous carbon activated by CO2 and H2O synergistically has the advantages of low losses and low pollution.