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.
In the Reynolds group we synthesize new materials to study how the polymer repeat unit and pendant side-chains structure impacts materials properties like capacitance and conductivity. These materials are then incorporated into devices where processing conditions are taken into account in order to develop prototype supercapacitors.
We have demonstrated that symmetrical supercapacitors incorporating dioxythiophene-based materials can reach operating voltages > 1.0 V, rapid charged/discharge rates (2 seconds or less), and show 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 ranges by constructing serial and parallel laminated modules allowing a doubling of the device voltage or capacitance.2 Additionally, copolymers of electron-rich (donor) and electron-poor (acceptor) subunits were synthesized to explore high voltage window devices.3
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 supercapacitor materials.4
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.5
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 side chains post-polymerization to yield a water-soluble polymer enabling benign solvent processing. 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.6 Due to the high conductivity and effective ion transport properties, these films can be charged and discharged at 10 V s-1 without compromising the capacitance.
Left: Synthetic route toward organic soluble PE that can then be rendered water soluble for benign processing and solvent resistant after film deposition. Middle: Cyclic voltammograms of solvent resistant PE in various electrolytes showing capacitive charging/discharging. Right: Capacitance normalized to polymer film mass versus scan rate for thin PE films up to 10 V s-1 scan rates.
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.-7
Left: High surface area carbon nanotube textile electrode used for making flexible, aqueous supercapacitors. Middle: Current density versus voltage for symmetrical devices incorporating unmodified carbon nanotube textile (CNT-T) electrodes and the textile electrodes with 1.4 mg cm-2 of solvent resistant PE2. Right: Capacitance evolution with bending radius of CNT-T/SR-PE2 devices from 50 mm down to 0.8 mm along with capacity retention after 2000 bending cycles.
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. Estrada, L. A.; Liu, D. Y.; Salazar, D. H.; Dyer, A. L.; Reynolds, J. R. Macromolecules 2012, 45 (20), 8211–8220.
4. Ponder, J. F., Jr.; Österholm, A. M.; Reynolds, J. R. Macromolecules 2016, 49 (6), 2106–2111.
5. Österholm, A. M.; Ponder, J. F., Jr.; Kerszulis, J. A.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2016, 8 (21), 13492–13498.
6. Ponder, J. F., Jr.; Österholm, A. M.; Reynolds, J. R. Chem. Mater. 2017, 29 (10), 4385–4392.
7. 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.