Potential fuel cell electrodes

The electrode polarization curve characteristics exhibited above are typical of those seen in many fuel cell electrodes. Thus a potential application of the ADM approximate solutions is determining the key electrochemical and mass transport parameters. 

Anodic reactions at Pt have been claimed to be dependent upon the surface state of the platinum. The Kolbe reaction is perhaps the best known case (for a review, see Conway and Vijh, 1967) for which a change in the surface composition has been held responsible and indeed necessary for the reaction to occur. Thus, at a low potential, < 0-8 V, acetate in aqueous solution is completely oxidized to carbon dioxide and water on pure platinum sites (i.e. we have in effect a fuel cell electrode). On raising the potential, PtO and adsorbed oxygen begin to cover the surface and oxygen evolution takes place in the range between 1-2- 1-8 V. A further increase in the... 

Figure 5.4. Nyquist plots obtained at a DC bias potential of 0.2 V versus Ag/AgCl for electrodes with various amounts of catalyst ink applied [2], (Reprinted from Electrochimica Acta, 50(12), Easton EB, Pickup PG. An electrochemical impedance spectroscopy study of fuel cell electrodes, 2469-7, 2005, with permission from Elsevier.)...

Figure 6.5. Impedance spectra for the oxygen reduction reaction at three different electrode potentials a 0.8 V b 0.7 V c 0.6 V. The microporous layer (loading 3.5 mg/cm2) of the electrode has varying PTFE content ( ) 10 ( ) 20 (A) 30 (+) 40 wt% [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

Basically, a fuel cell electrode can, thus, be seen as a highly dispersed interface between Pt and electrolyte (ionoiner or water). Due to the random composition, complex spatial distributions of electrode potential, reaction rates, and concentrations of reactants and water evolve under PEMFC operation. A subtle electrode theory has to establish the links between these distributions. 

In the given form, the Butler-Volmer equation is applicable rather broadly, for flat model electrodes, as well as for heterogeneous fuel cell electrodes. In the latter case, concentrations in Eq. (2.13) are local concentrations, established by mass transport and reaction in the random composite structure. At equilibrium,/f = 0, concentrations are uniform. These externally controlled equilibrium concentrations serve as the reference (superscript ref) for defining the equilibrium electrode potential via the Nernst equation. 

This is the original Butler-Volmer equation. It has, however, rather limited applicability. It should be used only when electrode potential and all concentrations are uniform. Such conditions are barely encountered in fuel cell electrodes. 

At zero current, the fuel cell electrodes provide the thermodynamic open circuit voltage Vceii = Voc- Connection of a load induces current I in the system and reduces Vceii by V I). The current drawn from the fuel cell thus costs some potential the thermodynamic voltage Vgc is the capital at our disposal. The value of Voc is given by Nernst equation [1] in this chapter, Voc is assumed to be constant, and we will focus on 6 V f). 

Finally, the most important concerns regarding alloys as substitutes for Pt in fuel cell electrodes include the potential leach and contamination of the electrolyte membrane with cations coming from the dissolution of the base-metal therefore, the design of new catalysts requires not only optimizing the catalytic activity but also analyzing the stabihty of the Pt and non-Pt elements under proton exchange fuel cell conditions. 

Applications that we have touched upon include supercapacitors, battery materials, sensors, biosensors, catalysts, fuel cell electrodes, electrochemical ion exchange switches and others. Composite catalysts for fuel cells is an area that is rapidly expanding. Use of nanomaterials in all these applications is also an ongoing pursuit. Potential uses in ceramics and optoelectronics are also areas of interest. 

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