Electrode/Electrolyte Interface with Various Redox Couples

Typical electrostatic attraction of ions is at the origin of capacitance, however, some faradaic reactions may occur at electrode/electrolyte interface called pseudocapacitive phenomena. Such effects can be initiated from electrode but also from solutions with electroactive species (iodides, bromides, vanadium compounds,  di-hydroxybenzenes etc.). Redox reactions are strongly affected by thermodynamics; Pourbaix diagrams indicate range of potential values where faradaic reactions take place. In typical two-electrode capacitor positive and negative electrode quite often operate not equally. One electrode can work in the very narrow range of potentials, e.g. iodides, bromides, di-hydroxybenzenes (0.1-0.4V) giving very high capacitance values (>1000 F/g) whereas the second one operates in the wide voltage limit but supplying moderate capacitance (ca. 100 F/g). Unfortunately electrode with smaller capacitance will affect a total capacitor performance. Selection of two various and well matched pseudocapacitive materials enables to optimize system. Asymmetric configuration with two different redox couples offers interesting capacitor performance with good cycling.

A few examples of pseudocapacitive phenomena originated from electroactive species present in electrolyte will be shown. Reactions with cerium-based autocatalytic redox shuttle for brominated species oxidizing malonic acid will be also presented as example of oscillating systems. They can serve as great source of pseudocapacitance and may significantly enhance total capacitance of the system. Detailed physicochemical and electrochemical characterization of electrode materials supported by modelling will reflect the role of texture, structure, conductivity of materials as well as concentration of species in electrolytic solutions on capacitor performance. 

Acknowledgements

The authors would like to acknowledge a financial support from Swiss-Polish project INGEC.