A crosslinked conducting polymer with well-defined proton trap function for reversible proton cycling in aprotic environments

Abstract

In this paper, a well-defined proton trap material containing a hydroquinone unit flanked by two pyridine proton acceptors is presented. In combination with a terthiophene trimer, based on 3,4-ethylenedioxythiophene and 3,4-propylenedioxythiophene units, a conducting material with reversible redox properties is obtained. We apply post-deposition polymerization of the functionalized terthiophene trimer to provide a conducting polymer, which allows investigation of the electrochemical properties of the proton trap material. In situ studies concerning conductance measurements, mass uptake, electronic transitions and bonding vibrations indicate stable internal proton cycling between the hydroquinone and the pyridine functionality without affecting the conductivity or the doping process. The theoretical capacity of 42 mA h g⁻¹, based on the pendant group redox conversion, can be achieved in a three electrode setup by potential step charging (25 s) at 0.5 V vs. Fc^0/+ with subsequent discharging at 2C (0.5–0 V vs. Fc^0/+). The total theoretical capacity available, including the contribution from the backbone, is 84 mA h g⁻¹ and coin cell batteries with the conducting redox polymer as cathode material (without any additive) vs. lithium foil as anode showed a discharge capacity of 81 mA h g⁻¹ (97% of the theoretical capacity) already from the first cycle (2.5–3.8 V vs. Li^0/+ at 2C). The capacity was maintained during prolonged cycling and showed a capacity retention of 99% after 100 cycles and 98% after 200 cycles indicating high stability of this organic cathode material when applied in a battery configuration.