Organic Energy Storage Systems

To keep up with the power demands of an increasingly mobile generation, it is vital mobile electronic devices store sufficient electrical energy. Furthermore, the ability to inexpensively store large amounts of electrical energy is becoming increasingly important with the growing fraction of electricity generation from intermittent renewable sources such as wind and solar. As this fraction increases, problems associated with the mismatch between power supply from wind and solar and grid demand become more severe.

Various electrical energy storage devices already exist. Entering this domain, organic batteries for small devices as well as organic flow batteries for large units represent particularly promising and prospective technologies. It is anticipated the ERA Chair Team will contribute to this nascent field of science and technology.

In designing high-performance and sustainable batteries, the major technical barriers lie in developing new electrode materials that can outperform current electrode materials. Unlike conventional transition-metal-based inorganic electrodes, organic compounds, which can be obtained from earth abundant elements with minimum carbon footprint, are highly desirable for making sustainable electrodes. Thus, considerable efforts have been made to identify organic molecules and polymers that can reversibly react with lithium or sodium ions in rechargeable batteries.

Various redox-active organic materials (e.g., conducting polymers, carbonyls, organic-sulphurs, and biological cofactors) have been explored thus far; however, they still cannot rival state-of-the-art inorganic electrodes in terms of energy densities. The redox potentials rarely exceed 3 V resulting in a low-voltage cell which requires additional inactive parts in battery pack design to meet the voltage specification.

More importantly, most organic compounds do not contain lithium in their natural state. Therefore, their use as cathodes in lithium-ion batteries (LIBs) must be accompanied by lithium-containing anodes (e.g., lithium metal), which raises another difficult issue. Organic compounds with p-type redox activities and high redox potentials can provide new opportunities to create practically viable organic-based rechargeable LIBs. Unlike, n-type organic cathodes using alkali metal cations as charge carriers, p-type electrodes utilize molecular anions. Thus, they do not require a lithium-containing anode in LIBs, thereby presenting a significant advantage because conventional anode materials such as graphite can be used. Moreover, p-type cathodes are insensitive in their function to the selection of cations in the electrolyte, offering freedom in building battery systems using various types of cations (e.g., Li+, Na+, K+, Mg2+, and molecular ions) for the anodes.

Although research on p-type cathodes is in its infancy, recent studies have reported a few p-type radical polymers, graphite, and heterocyclic compounds containing sulphur or nitrogen atoms. Even though the feasibility of p-type organic cathodes was successfully demonstrated therein, these cathodes generally exhibited limited specific capacities of less than 100 mA h g−1. To achieve higher capacities in p-type cathodes, it is necessary to identify organic materials that are lighter and exhibit multi-electron redox capability. The requirements are substantial for storing photovoltaic or wind power place on a storage device, in order to provide power when it is needed and the wind is not blowing and the sun is not shining. Such batteries need to be able to discharge at full power for tens of hours whereas they are typically drained after tens of minutes.

Much more promising are flow batteries, in which the power capability and the energy storage capability can be sized independently, thus enabling tens of hours, or even days, of discharge at peak power. In a flow battery, the energy is stored in fluids held externally in arbitrarily large, inexpensive storage tanks. These fluids pass by the electrodes where they undergo an electrochemical redox reaction before flowing back out to the tanks. The electrodes, whose size is designed to match the required power delivery capacity, don’t participate in the reaction, although they may catalyse it.