Effect of Pressure on the Charge Transfer Processes in Stable Free-Radical Organic Polymer Cathode Materials

Organic radical batteries (ORBs) comprise a relatively new technology that uses cathodes based on stable polymeric organic radical-based polymers. These materials show great promise as cathodes in batteries because the neutral, radical species (at left, Figure 1) is remarkably stable and the one-electron oxidation depicted is fully reversible. The rate at which charge can be pushed into or extracted from an electrode is determined by the timescales on which charges are able to move within an electrode and across an electrode-electrolyte, electrode-current collector interface.  This work focused on various test-cell pressures to determine how the charge/discharge processes based on the radical polymer’s redox interfacial and intramolecular mass-transfer processes would be influenced by the external pressure on the organic radical polymer electrode matrix. 

For the Redox process, reversible oxidation/reduction process for a stablized nitroxide radical PTMA, poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl,  the expansion-contraction of the polymer by the inclusion-exclusion of the solvated anions during charge-discharge can have a direct affect on the mass transfer mechanism with the electrode matrix.  Therefore, we have observed that the rate of the interfacial charge transfer may be affected by; nitroxide radical concentration, cathode capacity, pendant group chain length, cathode thickness and the current collector surface.

AC impedance measurement of the composite electrodes at pressure from 25 to 85 inch-lbs revealed a correlation between the overall electron transfer resistance of the composite electrode and the composition of the current collector surface; capacity of nitroxide polymer; and the charge-discharge rate limits.  These data suggest that the ionic conduction pathways both across the solvent-electrode interface and within the electrode matrix can be altered by pressure.

Specifically, we have observed a gradual decrease in charge transfer resistance with an increase in pressure to 65 in-lbs.  It is postulated that due to compression of the porous polymer structure variations possible in mass transfer process, i.e., anionic interfacial migration/diffusion, and intra-electrode diffusion pathways.  Then as we continue to increase pressure, that the inability for the polymer matrix to swell, causes an increase in charge transfer resistance, possible due minimal pathways for anion mass-transfer.

Overall, our results indicate that the interfacial electron transfer processes both within the electrode matrix, and across the electrolyte-electrode interface can dominate the charge/discharge performance of an organic radical battery system.  Therefore, the performance of organic cathode materials could be improved by suitably designing the interfacial structure for specific charge-discharge rate requirements to maximize interfacial charge transfer through distinct composition for maximal pressure, correct polymer expansion and maximized redox sites.

The detailed electrochemical investigation including cyclic voltammetry, AC impedance and spectroelectrochemical studies will be presented.

Acknowledgements:  This work is supported by a grant from the US Department of Energy Basic Energy Sciences Materials Division.  Alliance for Sustainable Energy, LLC, managing and operating contractor for the National Renewable Energy Laboratory under U.S. Department of Energy M&O Contract No., DE-AC36-08GO28308