Electrochemical supercapacitors are charge storage devices that deliver energy densities approaching that of conventional batteries while being charged/discharged in a matter of seconds. Conjugated polymers are attractive candidates for supercapacitor electrodes as they are not only pseudocapactitive, but they switch very rapidly between redox states (seconds or less) due to their high conductivity relative to many inorganic counterparts. Currently the Reynolds group is focused on designing and developing solution processable conjugated polymers that are highly electroactive over broad voltage range and that are compatible with a wide range of solvents and electrode materials.
We synthesize new materials to study how polymer repeat unit and pendant side-chains impacts materials properties such as capacitance and conductivity.
We have demonstrated that symmetrical supercapacitors incorporating dioxythiophene-based polymers can reach operating voltages > 1.0 V, rapid charged/discharge rates (2 seconds or less), and remarkable stability (400,000 continuous redox cycles).1 Using the small form factor of these devices, we have worked to increase device current and operating voltage by constructing serial and parallel laminated modules.2
Left: device lifetime of a Type I supercapacitor with electrochemically polymerized PEDOT as the pseudocapacitive material and an ionic liquid as the device electrolyte monitored over 400 000 cycles with minimal loss of capacitance. Middle: schematic of a laminated device modules that can be operated in series or parallel to extend either device voltage or device capacitance. Right: comparison of charge/discharge behavior of a single Type I device (black curve) with two device device modules (serial device: blue curve, parallel device: red curve).
The Soluble and Capacitive PxEy Polymer Family:
Copolymers combining 3,4-propylenedioxythiophene (ProDOT) units functionalized with solubilizing side chains and unfunctionalized 3,4-ethylenedioxythiophene (EDOT) units have been explored as a route toward solution processable active layers for polymer-based supercapacitors. 3
By increasing the EDOT content, the onset of oxidation is lowered and the electroactive window expanded. PE2, for example, exhibits an ideal potential independent current response above -0.5 V as shown below (blue). Symmetrical supercapacitor cells incorporating PE2 exhibit a pseudocapacitive charge/discharge repsonse up to a remarkable 1.6 V.4
Left: Cyclic voltammograms of various PxEy copolymers showing a decrease in onset of oxidation and broadening of current density over the scanned potential window. Right: Charge density versus voltage for symmetric supercapacitor incorporating PE2 electrodes. Highly rectangular (capacitive) current density is maintained up to a 1.6 V charging window.
By appending ester-containing side chains to PE and PE2 we can use standard polymerization and purification techniques but then chemically modify the polymer post-polymerization to yield a water-soluble material. After film deposition, the polymer can be rendered solvent resistant by treating with a mild acid. This side chain defunctionalization affords polymers that are water soluble, highly pseudocapacitive, and compatible with aqueous and organic electrolytes.5 Due to the high conductivity and effective ion transport properties, these films can be charged and discharged at 10 V s-1without compromising the capacitance.
Flexible, composite electrodes composed of a high surface area carbon nanotube textile and the PE2 copolymer result in devices with a fourfold increase in capacitance compared to bare textile electrodes and the ability of maintaining their performance even while being bent to a 0.8 mm radius.6
1. Österholm, A. M.; Shen, D. E.; Dyer, A. L.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2013, 5 (24), 13432–13440.
2. Liu, D. Y.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2010, 2 (12), 3586–3593.
3. Ponder, J. F., Jr.; Österholm, A. M.; Reynolds, J. R. Macromolecules 2016, 49 (6), 2106–2111.
4. Österholm, A. M.; Ponder, J. F., Jr.; Kerszulis, J. A.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2016, 8 (21), 13492–13498.
5. Ponder, J. F., Jr.; Österholm, A. M.; Reynolds, J. R. Chem. Mater. 2017, 29 (10), 4385–4392.
6. Lang, A. W.; Ponder, J. F., Jr.; Österholm, A. M.; Kennard, N. J.; Bulloch, R. H.; Reynolds, J. R. J. Mater. Chem. A 2017, 5, 23887–23897.