The effect of various electrolyte cations on electrochemical performance of polypyrrole/RGO based supercapacitors

In this work, polypyrrole/graphene doped by p-toluenesulfonic is prepared as an active material for supercapacitors, and its capacitance performance is investigated in various aqueous electrolytes including HCl, LiCl, NaCl, and KCl with a concentration of 3 M, respectively. A rising trend of capacitance is observed according to the cationic mobility (Li⁺ < Na⁺ < K⁺ < H⁺), which is due to its effect on the ionic conductivity, efficient ion/charge diffusion/exchange and relaxation time. On the other hand, long-term cycling stability is in the following order: KCl < NaCl < LiCl < HCl, corresponding to the decreasing tendency of cation size (K⁺ > Na⁺ > Li⁺ > H⁺). The reason can be attributed to the fact that the insertion/de-insertion of large size cation brings a significant doping level decrease and an over-oxidation increase during the charging-discharging cycles. Hence, we not only obtain good capacitance performance (280.3 F g^-1 at 5 mV s^-1), superior rate capability (225.8 F g^-1 at 500 mV s^-1) and high cycling stability (92.0% capacitance retention after 10000 cycles at 1 A g^-1) by employing 3 M HCl as an electrolyte, but also reveal that the electrolyte cations have a significant effect on the supercapacitors' electrochemical performance.