Fast electrochemical response of PEDOT:PSS electrodes through large combined increases to ionic and electronic conductivities

Abstract

Conducting polymers serve as the active materials in flexible electrochromic displays, organic electrochemical transistors, supercapacitors, artificial muscle and other electrochemical devices. poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is often employed in these devices since it is widely available, suitable for spraying, printing or casting, and can be treated to obtain high electronic conductivity. However, the ionic resistance limits charging rate and switching speed in this high volumetric capacitance polymer. Here we show that treatments that increase electronic conductivity can also increase ionic conductivity – by a factor of 300. Symmetric PEDOT:PSS supercapacitors with electrode thicknesses of 4.7 µm charge 40 times faster due to the combination of improved ionic and electronic conductivity. In these devices, PEDOT:PSS was deposited on two sides of a porous polymer separator by spray coating, and then soaked in either methanol, methanol mixed with the ionic liquid EMITFSI (50%v/v), ethylene glycol, ethylene glycol mixed with EMITFSI (50%v/v), or dimethyl sulfoxide. Ionic resistance dictates the charging time in untreated films. Treatment with methanol combined with EMITFSI results in the highest ionic conductivity (0.02 S/m), combined with an electronic conductivity of 498 S/m and the highest cut-off frequency. In addition, these treatments increase volumetric capacitance. The increases are most likely due to the removal of insulating PSS by the polar solvents, resulting in the formation of an open polymer structure in which ions can move more freely. Improving the charge/discharge speed has implications on the response times of color displays, energy storage devices, neural recording devices and actuators.