Electrified carbon/iodide interface for hybrid capacitors

Electrochemical capacitors (ECs) are unique energy storage devices capable of delivering high power compared to rechargeable batteries e.g., lithium-ion batteries. In addition, due to the mainly physical charge storage, ECs can be charged and discharged millions of times without substantial performance loss. Therefore, ECs find a number of applications, for example in regenerative braking of a hybrid vehicle where they save fuel up to 30% as estimated by the manufacturers. The spectrum of ECs applications could be further enhanced by improving the energy performance of these devices. For this purpose, project titled Charge transfer at the electrified carbon/iodide interface and its application for the development of high energy hybrid electrochemical capacitors is proposed. In this work, hybrid ECs will be developed where one electrode operates as battery-type (very high capacity) and the other as capacitor-type one. These hybrid ECs will utilize safe, environmentally friendly and high conductivity aqueous electrolytes with benefit to dissolve redox active species such as iodides in order to get additional charge storage by faradaic processes. The basic research target of the project is to investigate the charge transfer at the carbon/iodide interface which depends on whether the iodide species are introduced in carbon pores by physical methods or under electrochemical polarization. In situ electrochemical techniques, Resonance Raman Spectroscopy and Mssbauer Spectroscopy will be used to investigate the interactions of polyiodide species confined within the carbon materials. The optimized high capacity positive carbon electrodes will be combined with capacitive negative electrodes to develop hybrid ECs while using highly concentrated neutral aqueous salt solutions in order to prevent deleterious oxidation of carbon materials during long-term operation at wide temperature range. The evolution of oxygenated functional groups on carbon electrode surface during aging tests will be investigated by thermogravimetry, mass spectrometry and electrochemical techniques. The final deliverable of the project will be a better understanding of carbon/iodide interface under electrochemical treatments and of polyiodide species formed under confinement in pores or upon interaction with carbon surface, which might be different from the compounds known until now. This knowledge will enable to develop new electrode and electrolyte materials for obtaining high energy and power hybrid ECs in environmentally friendly and low cost aqueous electrolyte.

The Lise Meitner project M 2576-N37 (awarded to Dr. Qamar Abbas as project leader, PI) carried out at the Graz University of Technology from 1st March 2019 to 28th Feb 2021 was focused on the investigations of iodine/carbon interface by electrochemical and physicochemical methods. In the framework of this project, new research directions were discovered and new collaborations were established. The main focus of this project was tracking the charge transfer effects on carbon structural parameters and the use of iodine stored in the nanoporous carbon as a high capacity electrode for energy storage devices. Iodine was electrodeposited in the positive carbon electrode from the oxidation of iodides in eco-friendly aqueous electrolytes and thereby, sustainable hybrid supercapacitor devices were realized. Iodine/carbon based electrodes were prepared from commonly available activated nanoporous carbon and concentrated aqueous electrolytes with iodide additives that made this system one of the most cost-effective technology to store and deliver energy at fast rates. We discovered that charge is stored via iodine nanocrystals in the small pores of carbon electrode and thus it displays battery-like behavior. With the help of Raman spectroscopy and complementary techniques, changes in the carbon structural parameters such as D- and G-bands were evaluated. In addition, the formation of polyiodides (conversion of iodine to triiodide and pentaiodide) that resulted in loss of charge was also investigated. Following important areas of research were developed in this project: Iodine electrodeposition inside carbon nanopores and polyiodide species were identified. Changes in structural parameters of carbon electrodes were investigated. Charging mechanism and charge compensation behavior of carbon electrodes in iodide-based aqueous electrolyte was investigated. Concentrated aqueous electrolytes were developed for suppressing parasitic reactions of carbon electrode. Hybrid supercapacitors in eco-friendly aqueous electrolytes with efficient power delivery were developed. Following electrochemical methods were implemented: in situ Raman spectroscopy, small angle X-ray scattering methods, electrochemical quartz crystal microbalance, UV-Visible spectroscopy, thermogravimetry, combined with electrochemical methods such as cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. Following selected publications highlight the research outcomes: C. Prehal, H. Fitzek, G. Kothleitner, V. Presser, B. Gollas, S. Freunberger, Q. Abbas, "Persistent and reversible solid iodine electrodeposition in nanoporous carbons" Nature Communications 11, 4838 (2020). Q. Abbas, P. Nürnberg, R. Ricco, F. Carraro, B. Gollas, M. Schönhoff, "Less Water, Naked Choline, and Solid Iodine for Superior Ecofriendly Hybrid Energy Storage" Advanced Energy and Sustainability Research 2 (2021) 2100115, https://doi.org/10.1002/aesr.202100115 F. Barzegar, V. Pavlenko, M. Zahid, A. Bello, X. Xia, N. Manyala, K. Ozoemena, Q. Abbas, "Tuning the nanoporous structure of carbons derived from the composite of cross-linked polymers for charge storage applications" ACS Applied Energy Materials 4 (2021) 1763-1773.