Pseudocapacitance Induced by Undissolved Redox Couples

We discussed the undissolved redox couples in Section 3.2.1. However, those discussions mainly focus on a single redox process. In practice, several important requirements for redox couples should be mentioned here (1) the redox couple should exhibit multiple reversible redox states and remain stable over a large potential window (2) the redox material should be very electrically conductive and have the accessible structure needed to support performance (normally, high conductivity allows rapid distribution of charge within the structure) and (3) a high diffusion rate of charge balancing ions (e.g., metal oxides) as protons within the material matrix. 

Cyclic voltammograms of pseudocapacitance for ferricyanide and ferrocyanide couple free in solution (solid line), compared to ferrocyanide bound to a collector with PVP (dashed Hne). (Source Conway, B. E., V. Birss, and J. Wojtowicz. 1997. Journal of Power Sources, 66, 1-14. With permission.) 

The attractiveness of cheap nickel oxide maferials led to sfudies of different crystal and nano morphologies that offer improved performance and reversibility via improvements to surface area. Table 3.2 illusfrafes the variability possible for fhis pseudocapacitive material, highlighfing the importance of optimizing internal diffusion and conductivity by controlling morphology and choosing the correct crystal structure. This table also 

However, the cycle life is a major limitafion because the material phase may be changed over time through faradic processes, leading to a much shorter cycle life than that possible with double-layer capacitance. This statement holds true for many other redox materials including transition metal candidates, providing variable results among studies of the same active material. 

---