Logo: to the web site of Uppsala University

Floods, droughts and unpredictable weather could be the new reality for millions of people in a near future, unless we drastically decrease our greenhouse gas emissions to prevent the global average temperature from increasing even further. Material innovations will most certainly be essential for many of the technical solutions needed in order to tackle environmental issues. One major challenge is how to deal with the massive energy demand, following the average lifestyle of today, in a way that is both reliable and sustainable. Renewable energy sources have a varying output over time, hence cannot meet the demand for electricity by themselves. To buffer between demand and production, new ways to store the renewably produced energy are crucial. From a life cycle aspect conventional battery types are far from sustainable, and, with the increasing number of electronic devices for numerous applications, we need new options.

This thesis explores conducting redox polymers (CRPs), which can be utilized as organic cathode materials in high potential energy storage. Hydroquinone (HQ) was applied as the capacity carrying pendant group, and by the introduction of a proton trap functionality the high reduction potential of quinone-proton cycling was achieved also in aprotic electrolytes. The high reduction potential allows for redox matching with the polymer backbone, crucial for CRPs to work as energy storage materials without any additives, and this was studied by in situ conductance with IDA. In situ EQCM was applied in order to examine the cycling chemistry, and the constant mass uptake during the full oxidation cycle (and reverse during the reduction cycle) indicated uptake of charge compensating ions. Further, the proton trap functionality and its effectiveness were investigated by compositional variation, FTIR and variation of electrolyte. In situ UV/Vis was applied in order to study the electronic transitions of the bandgap, the charge carriers and the pendant group redox conversion.

The results presented introduce a new route for utilizing protonated forms of quinones as capacity carriers in aprotic media, by incorporating a proton trap in the material. The battery prototypes point to the versatility of the proton trap materials, having reversible proton cycling also when the electrolyte contains metal salts. With dual-ion type batteries the cycling chemistry of the anode is disconnected from the cathode, which allows for free choice of anode material.

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.

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.

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.