Advanced in-Situ Characterization of Carbon-Based Supercapacitors

Abstract

Dynamic development of supercapacitors technology especially in terms of electrode materials design requires a novel and more in-depth approach for their investigation. Apart from numerous materials synthesis and characterization methods proposed to date, there is a need for their investigation during device operation, in order to recognize the major aspects of charge accumulation and ageing phenomena as well as the performance failure. Conventional electrochemical techniques used allow the typical parameters (capacitance, resistance) to be determined, however, the mechanism of performance degradation requires a novel insight on the overall chemistry in the system. On one hand, the cycle life of electrochemical capacitors (especially in Electric Double Layer ones) is by definition unlimited, as there is no structural change of the electrode material and charge is accumulated only on the electrostatic manner. On the other hand, several additional processes occurring during device operation cause that cycle life is somewhat limited. This study is majorly focused on the employment of in-situ techniques such as Raman spectroscopy or Quartz Crystal Microbalance (EQCM) for determination of charge storage phenomena and recognition of ageing factors in activated carbon-based supercapacitors. In-situ Raman investigation for activated carbon electrodes operating in neutral aqueous media like Li2SO4 or LiNO3 solutions indicated that there is a mild oxidation of positive electrode during cycling (vibration modes from oxygen-based functionalities found) whereas the surface chemistry of negative electrode is rather stable. Extended voltage, i.e. above 1.4V caused serious oxidation of the positive electrode and hydrogen storage in negative one followed by its further recombination. EQCM study confirmed significant frequency/mass variation on positive side, whereas negative electrode remained almost constant. More interesting results were obtained for carbon electrodes operating in redox active electrolytes, like KI or KBr solutions. It has been confirmed that iodide anion undergoes several redox processes and strongly interacts with activated carbon surface; formation of <C>-I bond as well as polymeric forms of iodine/iodide species (triiodides, pentaiodides, etc.) have been observed. Moreover, oxidation of carbon surface has been identified near to iodide/iodine redox activity potentials. EQCM study confirmed the presence of various iodine specimen in the electrolyte solution with strong dependence of the potential and polarization-exposure time. Carbon ‘corrosion’ has been observed especially for more concentrated iodide solutions, however, we have proved that IO3 - anion does not contribute significantly in this process; it has significant influence on the cyclability. In case of bromide-based solutions, it has been observed that bromide has similar affinity to carbon surface as iodide, but typical <C>-Br bonds have not been found to date. Finally, all aforementioned considerations will be discussed in direct regard of cyclability issues of carbon electrodes, supported by typical electrochemical investigations.