Mechanically activated carbonized rayon fibers as an electrochemical supercapacitor in aqueous solutions

[Display omitted] •The carbonized rayon fibers were mechanochemically activated.•High operating voltage of 2.1V achievable with aqueous electrolyte.•Designed aqueous carbon capacitor delivers a very high energy density of 31 Wh kg−1. The activated carbon cloth (ACC), obtained by chemical/physical activation of carbonized rayon fibers, was grinded in a ball mill and studied from the aspect of double layer capacitance. The changes in pore structure, morphology and acid/basic properties caused by ball milling were studied by means of N2 adsorption/desorption, Fourier-transformed infrared spectrometry, Boehm’s titration and Scanning Electron Microscopy. Both potentiodynamic and galvanostatic cycling were used to evaluate the double layer capacitance in three alkaline, acidic and neutral aqueous solutions (KOH, H2SO4 and Na2SO4). While double layer capacitance of original ACC was found to be negligible, ball milled material (ACCm) displayed capacitance in the range of supercapacitors. In order to explain this huge capacitance improvement, we found that ball milling substantially increased the concentration of lactone, phenolic and quinone groups on the surface. We suggest that these groups, through improved hydrophilicity, enable faster ion diffusion into carbon micropores. The energy density stored by double layer was highest in neutral sodium sulphate solution. Namely, operational voltage of ∼2V and double layer capacitance of 220Fg−1 at 1A g−1, enable the energy density of ACCm/Na2SO4/ACCm capacitor of 31.7 Wh kg−1 at 2000Wkg−1, much higher than that of commercial EDLC carbon capacitors. According to the here presented literature survey in a tabular form, the energy density of the studied sample is also higher from that of numerous thus far published aqueous carbon capacitors.