Electrochemical supercapacitors are charge storage devices that bridge the gap between conventional batteries (high energy, lower power) and capacitors (low energy, high power). Conjugated polymers are attractive candidates for this application as they are not only pseudocapactitive but they switch very rapidly between redox states (seconds or less) and transport ions through the bulk of the polymer, unlike some inorganic counter parts. Currently the Reynolds group is focused on developing and designing solution processable conjugated polymers that are highly electroactive and capacitive over broad voltage range and that are compatible with a wide range of solvents and electrode materials.
Upper left: synthetic approach for designing highly capacitive and highly soluble polymers.
Upper right: spray coating of supercapacitive polymers using an airbrush spray coater.
Lower left: : Comparison of the charge/discharge behavior of a supercapacitors with either soluble pseudocapacitive polymers as the active material (red) or electrochemically polymerized pseudocapacitive polymers (black).
Lower right: fast charge/discharge behavior of a polymer-based supercapacitor using salt water as the electrolyte.
In the Reynolds group we try to capitalize on our toolbox of synthetic methods and materials to allow for an understanding of how repeat unit structure influences ultimate device capacitance in both symmetrical (Type I) and unsymmetrical (Type III/IV) devices.1-6
Key past results include electropolymerized, poly(3,4-ethylenedioxythiphene) (PEDOT) with devices exhibiting competitive capacitances in thin film, light-weight structures that switch rapidly (2 seconds) for extended lifetimes (400,000 continuous switches).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
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).
Recently the focus of the Reynolds group has been in developing supercapacitive polymers that can be solution processed via e.g. spray-, blade-, or dip coated onto a variety of current collectors. This has been achieved by synthesizing a family of EDOT based co-polymers that contain alkoxy-functionlized ProDOT units as solubilizing groups.7
The soluble polymers developed in the Reynolds group can be incorporated into 3D, high surface area carbon electrodes. The focus is on improving the capacitance of different flexible carbon nanotube (CNT) substrates such as CNT mats6, CNT forests, and CNT fabrics. Devices using polymer-coated CNT fabrics allows for discharge times as low as 5 sec and a 100 % increase in mass capacitance compared uncoated CNT fabric devices.
Left: different carbon nanotube based materials that have been evaluated as 3D current collectors for polymer-based supercapacitors in the Reynolds group. Right: Effect of polymer loading on the charge/discharge current on devices using CNT fabric as the current collectors.
1. Osterholm A. M.; Shen, D. E.; Dyer, A. L.; Reynolds, J. R. Understanding the effects of electrochemical parameters on the areal capacitance of electroactive polymers, ACS Appl. Mater. Interfaces, 2013, 5, 13432.
2. Liu, D.; Reynolds, J. R. Dioxythiophene-based polymer electrodes for supercapacitor modules, ACS Appl. Mater. Interfaces 2010, 2, 3586.
3. Shen, D. E.; Estrada, L. A.; Österholm, A. M.; Salazar, D. H.; Dyer, A. L.; Reynolds, J. R. Understanding the effects of electrochemical parameters on the areal capacitance of electroactive polymers, J. Mater. Chem. A, 2014, 7509.
4. Estrada, L. A.; Liu, D. Y.; Salazar, D. H.; Dyer, A. L.; Reynolds, J. R. Poly[Bis-EDOT-Isoindigo: an electroactive polymer applied to electrochemical supercapacitors, Macromolecules 2012, 45, 8211.
5. Stenger-Smith, J. D.; Webber, C. K.; Anderson, N.; Chafin, A. P.; Zong, K.; Reynolds, J. R. Poly(3,4ethylenedioxythiophene)-based supercapacitors using ionic liquids as supporting electrolytes, J. Electrochem. Soc. 2002, 149, A973.
6. Ertas, M.; Walczak, R. M.; Das, R. K.; Rinzler, A. G.; Reynolds, J. R. Supercapacitors Based on Polymeric Dioxypyrroles and Single Walled Carbon Nanotubes, Chem. Mater. 2012, 24, 433.
7. Ponder, J. F.; Osterholm, A. M., Reynolds, J. R A PEDOT by Any Other Name: Designing a Soluble PEDOT Analog, to be submitted