Electroactive polymer-based electrochemical capacitors using poly(benzimidazo-benzophenanthroline) and its pyridine derivative poly(4-aza-benzimidazo-benzophenanthroline) as cathode materials with ionic liquid electrolyte

A novel processing technique was used to solution cast films of poly(benzimidazo benzophenanthroline), (BBL), and the novel ladder polymer poly(4-aza-benzimidazo benzophenanthroline) (Py-BBL), which were used as cathode materials in Type IV electroactive polymer-based electrochemical capacitors (EPECs). This new processing technique involves co-casting the polymer from solution with a room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIBTI). The new processing technique gave polymer films with superior transport properties and electrochemical stabilities, did not require a break-in period, and yielded higher charge capacity than the standard films. Co-cast films of BBL and Py-BBL were each incorporated into separate Type IV EPECs using poly(3,4-propylene dioxythiophene) (PProDOT) as the anode material. It was found that the PProDOT/BBL capacitors store, on average, about 50% more energy than a comparable PProDOT/Py-BBL EPEC. While PProDOT/BBL films have an energy density advantage at rates (power densities) less than 0.01 kW kg−1, PProDOT/Py-BBL EPECs are capable of delivering higher energy than the BBL EPECs at rates greater than 0.01 kW kg−1 (550 s per cycle). In fact, PProDOT/Py-BBL devices delivered more than ten times the energy density of PProDOT/BBL devices at 0.5 kW kg−1 (50 s per cycle). The PProDOT/Py-BBL EPECs were cycled for 10,000 cycles at 65% depth of discharge and maintained 96% of the initial energy and power density, whereas the PProDOT/BBL EPECs were cycled under the same conditions and lost more than 35% of the initial energy and power density after only 2300 cycles. ▸ A new processing technique gives polymer films with superior transport properties. ▸ Using this technique, Type IV capacitors were constructed. ▸ BBL-based capacitors store slightly more energy than a comparable Py-BBL device at low rates. ▸ The Py-BBL devices deliver much higher energy at higher rates. ▸ The Py-BBL devices were found to last least five times as long as the BBL devices.