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An organic cathode material based on a copolymer of poly(3,4-ethylenedioxythiophene) containing pyridine and hydroquinone functionalities is described as a proton trap technology. Utilizing the quinone to hydroquinone redox conversion, this technology leads to electrode materials compatible with lithium and sodium cycling chemistries. These materials have high inherent potentials that in combination with lithium give a reversible output voltage of above 3.5 V (vs Li0/+) without relying on lithiation of the material, something that is not showed for quinones previously. Key to success stems from coupling an intrapolymeric proton transfer, realized by an incorporated pyridine proton donor/acceptor functionality, with the hydroquinone redox reactions. Trapping of protons in the cathode material effectively decouples the quinone redox chemistry from the cycling chemistry of the anode, which makes the material insensitive to the nature of the electrolyte cation and hence compatible with several anode materials. Furthermore, the conducting polymer backbone allows assembly without any additives for electronic conductivity. The concept is demonstrated by electrochemical characterization in several electrolytes and finally by employing the proton trap material as the cathode in lithium and sodium batteries. These findings represent a new concept for enabling high potential organic materials for the next generation of energy storage systems.

A conducting redox polymer based on PEDOT with hydroquinone and pyridine pendant groups is reported and characterized as a proton trap material. The proton trap functionality, where protons are transferred from the hydroquinone to the pyridine sites, allows for utilization of the inherently high redox potential of the hydroquinone pendant group (3.3 V versus Li^0/+) and sustains this reaction by trapping the protons within the polymer, resulting in proton cycling in an aprotic electrolyte. By disconnecting the cycling ion of the anode from the cathode, the choice of anode and electrolyte can be extensively varied and the proton trap copolymer can be used as cathode material for all-organic or metal-organic batteries. In this study, a stable and nonvolatile ionic liquid was introduced as electrolyte media, leading to enhanced cycling stability of the proton trap compared to cycling in acetonitrile, which is attributed to the decreased basicity of the solvent. Various in situ methods allowed for in-depth characterization of the polymer’s properties based on its electronic transitions (UV–vis), temperature-dependent conductivity (bipotentiostatic CV-measurements), and mass change (EQCM) during the redox cycle. Furthermore, FTIR combined with quantum chemical calculations indicate that hydrogen bonding interactions are present for all the hydroquinone and quinone states, explaining the reversible behavior of the copolymer in aprotic electrolytes, both in three-electrode setup and in battery devices. These results demonstrate the proton trap concept as an interesting strategy for high potential organic energy storage materials.

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.