Mixed Ionic-Electronic Conductors Based on PEDOT:PolyDADMA and Organic Ionic Plastic Crystals

Mixed ionic-electronic conductors, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) are postulated to be the next generation materials in energy storage and electronic devices. Although many studies have aimed to enhance the electronic conductivity and mechanical properties of these materials, there has been little focus on ionic conductivity. In this work, blends based on PEDOT stabilized by the polyelectrolyte poly(diallyldimethylammonium) (PolyDADMA X) are reported, where the X anion is either chloride (Cl), bis(fluorosulfonyl)imide (FSI), bis(trifluoromethylsulfonyl)imide (TFSI), triflate (CF3SO3) or tosylate (Tos). Electronic conductivity values of 0.6 S cm−1 were achieved in films of PEDOT:PolyDADMA FSI (without any post-treatment), with an ionic conductivity of 5 × 10−6 S cm−1 at 70 °C. Organic ionic plastic crystals (OIPCs) based on the cation N-ethyl-N-methylpyrrolidinium (C2mpyr+) with similar anions were added to synergistically enhance both electronic and ionic conductivities. PEDOT:PolyDADMA X / [C2mpyr][X] composites (80/20 wt%) resulted in higher ionic conductivity values (e.g., 2 × 10−5 S cm−1 at 70 °C for PEDOT:PolyDADMA FSI/[C2mpyr][FSI]) and improved electrochemical performance versus the neat PEDOT:PolyDADMA X with no OIPC. Herein, new materials are presented and discussed including new PEDOT:PolyDADMA and organic ionic plastic crystal blends highlighting their promising properties for energy storage applications.


Characterization Methods
All the compounds were characterized by thermogravimetric analysis (TGA), performed using a TGA Q-500 thermobalance, with a standard furnace coupling and nitrogen flow of 50 cm 3 min -1 . Sample weight was between 1 and 15 mg. Then, the decomposition temperature, Td, was obtained from the maximum of the first derivate of the thermogram.
The organic salts were characterized by differential scanning calorimetry (DSC). The experiments were performed using a Perkin Elmer 8000 DSC equipped with an Intracooler II and calibrated with indium and tin standards. The heating rate was 10 ˚C min -1 in the temperature range of −70 to 225 °C and between 3 and 10 mg of sample was used every time. The measurements were performed by sealing the samples in aluminum pans. The samples were first heated from room temperature to 225 °C to erase thermal history, then cooling and finally a second heating was performed.
Compressive mode dynamic mechanical analysis (DMA, PerkinElmer DMA8000) was used to analyze the thermal behavior of the polymers when nothing was observed by DSC. The polymer was pressed in KBr die and dried at 70 ˚C under vacuum overnight to finally obtain pellets of around 2 mm of thickness and 13 mm of diameter. The temperature range of DMA was from 40 °C to 200 °C and the frequency was set at 1 Hz. The measurements were performed in a N2 filled glovebox with a H2O level lower than 100 ppm.
The ionic conductivities were measured by electrochemical impedance spectroscopy (EIS) using an Autolab 302N potentiostat galvanostat (Metrohm AG, Herisau, Switzerland) with the temperature controlled by a Microcell HC station. PEDOT based samples were measured making pellets of around 500 μm and 11 mm of diameter. Polyelectrolytes and organic salts were solvent cast on stainless steel electrodes. All the samples were dried at 70 °C under vacuum overnight. Afterward, the samples were sandwiched between two stainless steel electrodes (with a surface area of 0.5 cm 2 ). The plots were obtained by applying a 10 mV perturbation to an open circuit potential in the frequency range of 1 MHz to 0.1 Hz. Electronic conductor samples were analyzed considering the resistance of the electronic conduction negligible versus ionic resistance as previously done by McDonald et al. [1]. The activation energy (Ea) of the different materials was also studied in the linear region using the Arrhenius Equation (Eq. 1): where σ is the ionic conductivity, σ0 is the pre-exponential factor, Ea is the activation energy, R the universal gas constant and finally T is the absolute temperature.
The electronic sheet resistance were measured using a Jandel 4-Point Probe with the RM3000+ test unit. The samples were pellets with a thickness between 250 and 500 μm and 13 mm in diameter. Coatings with a thickness of around 50 μm were measured when they were possible to form. Electronic conductivity was calculated taking into account the thickness of the samples as follows the Eq. S2 where σelec. is the electronic conductivity, RS is the sheet resistance and t the thickness of the sample.
To evaluate the electrochemical performance of the system, cyclic voltammetry (CV) was carried out with an Autolab PGSTAT204 potentiostat in a conventional three electrode set up. A platinum wire was used as the counter electrode, Ag/AgCl as the reference electrode and glassy carbon as the working electrode. The samples were dissolved in the relevant solvent to finally drop cast a known quantity of material on top of the carbon electrode. The experiments were performed at 0.2 V s -1 .

Tan Delta
For [CF3SO3] − compounds, an equimolar quantity of 4-(Trifluoromethyl) benzaldehyde was added as standard to quantify 1 H and 19 F and thus, see if the ratio between the anion and the cation is one.   For PolyDADMA CF3SO3, 4-(Trifluoromethyl)benzaldehyde was in excess of around 1.2 over the cation and the anion. Traces of water was observed as well.