Significant effect of cations on polypyrrole cycle stability

The specific capacitance of PPy films decreased by 8% in 1 M HCl solution after the 5000 cycles. However, the specific capacitance of PPy films decreased by 75% in 1 M NaCl solution. The cations in working electrolyte had an important effect on the capacitance decay of PPy films. The 0/C ratio of PPy films in 1 M HCl showed a small difference with that in 1 M NaCl. The results of electrochemical quartz crystal microbalance (EQCM) showed that the mass change of PPy films in 1 M HCl solution was about twice as that 1 M NaCl solution, which was attributed to the extensive participation of the cations in PPy doping/dedoping process. The participation of the cations can induce the formation of salts in PPy matrix. The results of scanning electron microscope showed the thickness of PPy films increased by 15% and 60% in 1 M NaCl solution after 2000 cycles and 5000 cycles, respectively. The excessive accumulation of salts in PPy matrix had to decrease of PPy free volume and result in the capacitance decrease of PPy films in 1 M NaCl solution. Therefore, the cation in working electrolyte should have a small ion radius considering the long cycle life of PPy electrode.