The Pseudo-capacitive Effect

Since the time scales for establishing local H2 starvation events are on the order of seconds or 10 s of seconds,11,14 pseudo-capacitive effects will not be important. 

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The pseudo-capacitive effect can be incorporated in the coupled kinetic and transport model through Eqs. (19) and (20). Here we choose to illustrate the effect through the kinetic model for simplicity. With considering the pseudo-capacitive current density, the kinetic model becomes... 

Wang, J. G., Y. Yang, Z. H. Huang, and F. Kang. 2013. Effect of temperature on the pseudo-capacitive behavior of freestanding Mn02 carbon nanofibers composites electrodes in mild electrolyte. Journal of Power Sources 224 86-92. 

Lee, M. T W. T. Tsai, H. F. Cheng, I. W. Sun, and J. K. Chang. 2013. Effect of electrolyte temperature on the pseudo-capacitive behavior of manganese oxide in Af-butyl-A -methylpyrrolidinium-dicyanamide ionic liquid. Journal of Power Sources 233 28-33. 

Frackowiak s group coated PPy onto the multi-walled carbon nanotubes (MWNTs) via the chemical oxidative polymerization [18] or electropolymerization [19] to obtain the nanotubular composite materials for supercapacitors. Its SC reached 170 F/g in 1.0 mol/L H SO aqueous solution, about twice that given either by the nanotubes (80 F/g) or by the pure PPy (90 F/g). The author also claimed that a further treatment of the nanotubular materials, such as an oxidative treatment of the nanotubes or the deposition of PPy, was profitable for the enhancement of capacitance through pseudo-effects, however probably with a limited durability. However, in the cases the pseudo-capacitance by PPy was thought to be insufficiently utilized because of the thick and rigid structure and less entanglement of the MWNTs. 

Such a Fermi level shift can result in effects which can easily be confused with capacitive effects . These effects are called by the electrochemists, pseudo-capacitive effects. In solid state electrochemistry they are sometimes also described as an adsorption with partial transfer. To illustrate the point, let us consider the schematic situation depicted in Fig.6, It is familiar to electrochemists. Without entering into the details of the relevant surface levels and densities, we can say, from a thermodynamical viewpoint, that the electrode measures the chemical activity of 0 atoms in a perturbed layer located at the phase boundary. The electrode potential variations are related to the 0-chemical-activity-variations by formula (22). Extending the hypotheses, here, the 0 atoms are supposed to be soluble in the electronic conductor but the direct exchange of oxide ions is regarded as impossible. 

The number of studies which utilize ionic liquid electrol54e in redox capacitor system is still small, probably due to the difficulty to reproduce the pseudo-capacitive reaction in ionic liquid media. While the principle of pseudo-capacitance of conductive polymer electrodes permits to utilize ionic liquid electrolytes, high viscosity and rather inactive ions of ionic liquid may make their pseudo-capacitive reaction slow. The combination of nanostmctured conductive polymer electrode and ionic liquid electrolyte is expected to be effective [27]. It is far difficult that ionic liquids are utilized in transition metal-based redox capacitors where proton frequently participates in the reaction mechanisms. Some anions such as thiocyanate have been reported to provide pseudo-capacitance of manganese oxide [28]. The pseudo-capacitance of hydrous ruthenium oxide is based on the adsorption of proton on the electrode surface and thus requires proton in electrolyte. Therefore ionic liquids having proton have been attempted to be utilized with ruthenium oxide electrode [29]. Recent report that 1,3-substituted imidazolium cations such as EMI promote pseudo-capacitive reaction of mthenium oxide is interesting on the viewpoint of the establishment of the pseudo-capacitive system based on chemical nature of ionic liquids [30]. 

Chemical capacitance. When the mechanism involves significant involvement of the bulk, accumulation of reactive intermediates not only involves surface species but oxidation and reduction of the bulk. This can be detected as an anomalously high effective capacitance, often referred to as a chemical (or pseudo) capacitance. This capacitance can be as large as 0.1 — 1 F/cm and thus easily detected by current-interruption or impedance techniques. Thus, capacitance is a strong indicator (independent of resistance) as to what degree the interface, surface, and/or bulk are playing in the... 

Sharma et al. reported a novel and cost-effective fabrication of the CNT/ PPy nanocomposite on the poly(4-styrenesulfonic acid) (PSS)-dispersed MWNTs [79]. PSS not only stabilized the MWNT suspension but also provided charged groups to facilitate an ordered and uniform growth of pseudo-capacitive materials around the MWNTs through electrostatic attractions. The in situ oxidation of Py with KMnO yielded molecular-level dispersion of the MnO in PPy matrix that improved the electronic conductivity, mechanical stability, and pseudo-capacitance. The MWNT-PSS/PPy MnOj ternary nanocomposite exhibited a high SC of 268 F/g at 5 mV/s and only 7% faded in the specific capacity at 100 mV/s and 10% faded in the same after 5000 CV cycles (Figure 8.9). 

O- and N-enriched nanocarbons demonstrate high capacitance values in KOH or H2SO4 due to the contribution of pseudo-faradic reactions. Another positive effect of doping is the broadening of the potential stability window. Composites of these carbons with carbon nanotubes withstand remarkable capacitance values at high current load. Hence, the N- and O-doped carbons open the opportunity of developing high-performance supercapacitors in aqueous electrolytes. 

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