Pseudo-capacitance carbons

In some cases, the kinetics of the redox charge— discharge reactions can proceed almost as quickly and reversibly as EDL charging. Thin film redox electrodes, based on the lithium intercalation/inser-tion principle such as Li4Ti50i2, exhibit high reversibility and fast kinetics. The Ru02 materials deposited on carbon show pseudo-capacitive charge—... 

In this chapter, we will review the fundamental models that we developed to predict cathode carbon-support corrosion induced by local H2 starvation and start-stop in a PEM fuel cell, and show how we used them to understand experiments and provide guidelines for developing strategies to mitigate carbon corrosion. We will discuss the kinetic model,12 coupled kinetic and transport model,14 and pseudo-capacitance model15 sequentially in the three sections that follow. Given the measured electrode kinetics for the electrochemical reactions appearing in Fig. 1, we will describe a model, compare the model results with available experimental data, and then present... 

As shown in Fig. 14, the cathode potential changes abruptly across the H2/air-front. This fact warrants the inclusion of the pseudocapacitance into the previous steady-state kinetic model.12 It is clear that the electrode s pseudo-capacitance can supply protons in transient events and thereby reduce the cathode carbon-support corrosion rate in the case of fast moving H2/air- ronts. Figure 18... 

Therefore, for equal H2/air-front residence times, the pseudo-capacitive model would suggest lower rates of carbon-support oxidation, i.e., lower rates of C02 formation for the stop process if compared to the start process, which is consistent with on-line C02 measurements of the air exiting the cathode flow-field during H2/air-front start-stop events, as shown in Fig. 16. 

Figure 19. Predicted carbon loss distribution along anode flow-field channel over a complete H2/air-front start—stop cycle using the pseudo-capacitance model in comparison with one-dimensional, normalized mass activity from Fig. 17. The pseudo-capacitance value used in the model is obtained from AC-impedance measurements as described in references (42, 43).

Figure 20. The impact of a faster H2/air-front moving through die anode flow-field. 100% relative carbon loss is defined as the localized damage prediction when no pseudo-capacitance is considered in die model. Significantly less carbon corrosion is expected at the anode inlet region as the speed of H2/air-front increases but much less benefit at the anode outlet region.

A persistent question regarding carbon capacitance is related to the relative contributions of Faradaic ( pseudocapacitance ) and non-Faradaic (i.e., double-layer) processes [85,87,95,187], A practical issue that may help resolve the uncertainties regarding DL- and pseudo-capacitance is the relationship between the PZC (or the point of zero potential) [150] and the point of zero charge (or isoelectric point) of carbons [4], The former corresponds to the electrode potential at which the surface charge density is zero. The latter is the pH value for which the zeta potential (or electrophoretic mobility) and the net surface charge is zero. At a more fundamental level (see Figure 5.6), the discussion here focuses on the coupling of an externally imposed double layer (an electrically polarized interface) and a double layer formed spontaneously by preferential adsorp-tion/desorption of ions (an electrically relaxed interface). This issue has been discussed extensively (and authoritatively ) by Lyklema and coworkers [188-191] for amphifunctionally electrified... 

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. 

Mysyk, R., E. Raymnndo-Pinero, M. Anouti, D. Lemordant, and F. Beguin. 2010. Pseudo-capacitance of nanoporous carbons in pyrrolidinium-based protic ionic liquids. Electrochemistry Communications 12 414-417. 

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. 

By contrast, activated carbon, such as glassy carbon, has multiple surface functionalities which vary with provenance and pretreatment. Typically, these func-tionahties are oxygen-type and undergo reversible Faradaic reactions that manifest as a very large pseudo-capacitive component 400 pF cm" in the... 

For supercapacitor carbon electrodes, it will be further shown that (1) the developed surface area is responsible of an important electrical double-layer capacitance (2) both the oxygenated and nitrogenated functionalities may be involved in redox reactions with the electrolyte, which enhance capacitance through a pseudo-capacitive contribution. 

The most reported dopants which confer pseudo-capacitive properties to carbons are oxygen (Fig. 12.3) and nitrogen (Fig. 12.4). Some other studies described below have recently considered boron or phosphorous as pseudo-capacitive dopants. The nitrogenated and oxygenated functionalities can imdergo pseudo-faradic reactions, which can be pH-dependent or not, as presented in Fig. 12.10. The extent of the... 

Pseudo-capacitance Related with Reversible Hydrogen Electrosorption in Nanoporous Carbons . 

Egashira M, Matsuno Y, Yoshlmoto N, Morita M (2010) Pseudo-capacitance of composite electrode of ruthenium oxide with porous carbon in non-aqueous electrolyte containing imidazolium salt. J Power Sources 195 3036-3039... 

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