Supercapacitors
Grafting of electroactive materials on carbons
Current supercapacitor technology is based on activated carbon electrodes in contact with an organic electrolyte. The charge is then stored in the pores and surface of the carbon. The energy density of electrochemical capacitors can be increased by chemically grafting electroactive molecules. For example, it was shown that grafting quinone with an appropriate redox potential onto porous carbon increased the charge storage properties of the negative electrode as a result of the quinone faradaic contributions. Our team is focusing on the positive electrode by grafting suitable electroactive molecules such as iron tris-phenanthroline and phenanthroline dione. New imaging methods with XPS is used to evaluate the efficiency and uniformity of surface modification using the Fe 2p signal. The grafting can be monitored using in-situ techniques (EQCM-D, ESPR and imaging ellipsometry). The modified carbon electrodes’ electrochemical activity is also evaluated in ionic liquids at room temperature, which extends the capacitor’s voltage above 3 V, well beyond the 1 V limit for aqueous electrolytes. The first tests are done with commercially available ionic liquids.
Increasing the maximum operational voltage of supercapacitors is an efficient means to increase energy, as its energy is dependent on ΔE . However, secondary reactions must be taken into account as the electrode potential approaches the electrolyte stability limit. Currently, limited means are available to study these reactions, despite their significance. Essential information can be obtained by identifying and quantifying the products formed at the electrode in real time by high resolution time-of-flight mass spectrometry. Based on the acquired knowledge, new ionic liquids can then be developed to increase the stability of supercapacitors operating at high voltages. The modification of ionic liquids with electroactive functionality will also be considered to combine stable electrolytes with increased energy density.
2
Electroactive ionic liquids
Electroactive ionic liquids are obtained by modifying the structure of ionic liquids with redox active components. These room-temperature liquid phases provide new opportunities to study and develop electron transfer reactions, partly owing to their behaviour which differs from that of conventional solute-in-solvent systems. Our research is focusing on the structure-property relationships of electroactive ionic liquids. This will provide significant findings that will be used for the rational design and development of new redox systems to increase supercapacitor charge storage. A unique property of ionic liquids is the very high concentration in the redox center (one redox center per ion pair). As the electron transfer kinetics and transport properties of the modified ions in ionic liquids and in solution are significantly different, new models which take into account the different transport properties and predict the properties.
The infrastructure offered by MAPLES allows us to study the reactions involved in electron transfer with redox ionic liquids by coupling electrochemical cells with in situ mass measurements by EQCM-D and product identification by mass spectrometry. The low vapor pressure of ionic liquids is ideal for monitoring chemical composition changes directly in the analysis chamber (XPS). Pioneering data on the redox activity of ionic liquids during electron transfer will be obtained via XPS imaging. As ionic liquids have an intrinsic ion transport limitation due to their high viscosity, their use in micro-supercapacitors (where diffusion is much faster) will be considered. Ultimately, we want to develop new devices by combining innovations in micro-supercapacitor electrode architecture with electroactive electrolytes.
Toward next-generation of supercapacitors
Organic electroactive materials. The foundation for the use of organic electroactive materials such as melanin in
pseudocapacitive energy storage systems lies in the synergy between the redox activity of the building blocks and the capacity of several of their functionalities to reversibly bind cations. XPS, XRD, AFM and AFM-IR are our principal tools to study structure-property relationships of melanin and other bio-inspired materials with electronic and ionic conduction properties. These relationships must be determined in order to improve the energy storage of biomaterials. This is achieved by investigating aggregate structures, chemical composition, and electrochemical and electronic/ionic contributions to foster the creation of new technologies featuring biocompatibility and biodegradability (e.g. for energy storage and biomedical devices).
Flexible and stretchable supercapacitors. New ways to manufacture devices like flexible and stretchable supercapacitors are necessary for the development of wearable consumer electronics, which are based on organic conducting polymers. We plan to build supercapacitor devices on elastomeric substrates such as polydimethyl siloxane, based on conducting polymer films for charge storage, and stretchable electrolytes such as polyacrylamide hydrogels. The composition of the films will be engineered to achieve the delicate balance between energy storage, electrical and mechanical properties. Real-time analysis of energy storage and delivery mechanisms under stress/strain conditions represents a significant challenge in the characterization of flexible supercapacitors. This will be achieved through AFM-IR measurements. Our activities will study material formulation for batteries and supercapacitors as well as electrical and electrochemical device characterization.