Ionic Polymer of Intrinsic Microporosity and Their Electrochemical Devices

A M Mahmudul Hasan¹, Joshua D. Marquez¹, Rupam Roy¹, Saptasree Bose², Kent Kirlikovali², Omar K. Farha², and Austin M. Evans¹

1Department of Chemistry, Butler Polymer Research Laboratory, University of Florida, 2Department of Chemistry, Northwestern University

Polymers of intrinsic microporosity (PIM) are desirable for electrochemical devices because they are synthetically tunable, easily processable, and rapidly accessible by ionic species. However, most PIMs are not redox-active. Here, I show a new class of ionic and redox-active PIM that takes advantage of viologen subunits to overcome this limitation.  The straightforward processing of this material allows us to produce high-functioning supercapacitor electrodes without the need for conductive additives or binders. Electrochemical impedance spectroscopy and galvanostatic charge-discharge cycling measurements performed on our viologen PIM and a non-porous viologen polymer reveal that the high energy and power density are both due to the ability of ions to rapidly access the ionic PIM. The high performance of this material is directly related to its large accessible surface area (200 m² g^‑1), as evaluated by porosimetry measurements.  In 0.1 M H₂SO₄ electrolyte, a pseudocapacitive energy density of 315 F g^-1 is observed, whereas in 0.1 M Na₂SO₄, a purely capacitive energy density of 250 F g^-1 is obtained. In both cases, this capacity is retained over 10,000 charge-discharge cycles. These findings motivated us to create a prototype symmetric two-electrode capacitor, which had an areal capacitance of 193 mF cm^-2 that was virtually unchanged after 2000 charging-discharging cycles. The constructed device also undergoes a reversible color change when exposed to two distinct redox potentials. The enhanced ion transport within viologen PIMs gives a rapid and highly efficient color change. This discovery implies that redox-active PIMs may also exhibit excellent performance in other electrochemical devices, including chemical sensors and ion-separation membranes.