Elementary fuel cell

Hence these three key points will determine the energy efficiency and the specific power of the elementary fuel cell an improvement in each component of the cell will increase the power density from 0.175 to 1.05 Wcm, that is, an increase by a factor of 6. As a consequence, for the fuel cell systems the weight and volume will be decreased by a similar factor, for a given power of the system, and presumably the overall cost will be diminished. The improvement in the components of the elementary fuel cell thus has a direct effect on the system technology and therefore on the overall cost. 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

Elementary fuel cell science The essential of a fuel cell is the electrolyte, a material which conducts electricity by the transport of an ionic species. On one side of the electrolyte membrane there is a source of the ion species at a particular chemical potential and on the other side a sink for the ions at a relatively lower chemical potential. 

For the elementary fuel cell, the cell voltage E(j) is the difference between the cathode potential Ec and the anode potential a ... 

Arthur D. Little has carried out cost structure studies for a variety of fuel cell technologies for a wide range of applications, including SOFC tubular, planar and PEM technologies. Because phenomena at many levels of abstraction have a significant impact on performance and cost, they have developed a multi-level system performance and cost modeling approach (see Figure 1-15). At the most elementary level, it includes fundamental chemical reachon/reactor models for the fuel processor and fuel cell as one-dimensional systems. 

Kreuer, K. D., Paddison, S. J., Spohr, E. and Schuster, M. 2004. Transport in proton conductors for fuel-cell applications Simulations, elementary reactions, and phenomenology. Chemical Reviews 104 4637-4678. 

Transport in Proton Conductors for Fuel-Cell Applications Simulations, Elementary Reactions, and Phenomenology Klaus-Dieter Kreuer, Stephen J. Paddison, Eckhard Spohr, and Michael Schuster pp 4637 - 4678 (Review) DOl 10.1021/cr020715f... 

Transport in Proton Conductors for Fuel-Cell Applications Simulations, Elementary Reactions, and Phenomenology... 

When it comes to the equilibration of water concentration gradients, the relevant transport coefficient is the chemical diffusion coefficient, Dwp. This parameter is related to the self-diffusion coefficient by the thermodynamic factor (see above) if the elementary transport mechanism is assumed to be the same. The hydration isotherm (see Figure 8) directly provides the driving force for chemical water diffusion. Under fuel-cell conditions, i.e., high degrees of hydration, the concentration of water in the membrane may change with only a small variation of the chemical potential of water. In the two-phase region (i.e., water contents of >14 water molecules... 

Various modeling approaches have been used for the catalyst layers, with different degrees of success. The approach taken usually depends on how the other parts of the fuel cell are being modeled and what the overall goal of the model is. Just as with membrane modeling, there are two main classes of models. There are the microscopic models, which include pore-level models as well as more detailed quantum models. The quantum models deal with detailed reaction mechanisms and elementary transfer reactions and transition states. They are beyond the scope of this review and are discussed elsewhere, along with the issues of the nature of the electro catalysts. 

The principles of the fuel cell are illustrated in Figure 1.1. The electrochemical cell consists of two electrodes, an anode and a cathode, which are electron conductors, separated by an electrolyte [e.g. a proton exchange membrane (PEM) in a PEMFC or in a DAFC], which is an ion conductor (as the result of proton migration and diffusion inside the PEM). An elementary electrochemical cell converts directly the chemical... 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

At an electrode potential of U = V, the ORR is running with a high reaction rate. This situation would correspond to a short-circuited fuel cell where all elementary reaction steps are highly exothermic. At an electrode potential of U=1.23 V, where the chemical potential of the electrons is shifted by 1.23 eV, both protonation and electron transfer steps are activated. Process (26) or (28) are therefore rate-limiting under these conditions. 

A fuel cell system consists of a fuel cell stack and auxiliary equipments, which produce electric energy (and heat) directly from the electrochemical oxidation of a fuel in an elementary electrochemical cell (Figure 9.1). 

Figure 5.30. Schematic of the catalyst layer geometry and its composition, exhibiting the different functional parts, a A sketch of the layer, used to construct a continuous model, b A one-dimensional transmission-line equivalent circuit where the elementary unit with protonic resistivity Rp, the charge transfer resistivity Rch and the double-layer capacitance Cj are highlighted [34], (Reprinted from Journal of Electroanalytical Chemistry, 475, Eikerling M, Komyshev AA. Electrochemical impedance of the cathode catalyst layer in polymer electrolyte fuel cells, 107-23, 1999, with permission from Elsevier.)...

In efforts to mimic the function of heterobimetallic hydrogenases and provide potential molecular catalysts for fuel cell technology the Fe,Ru-heterobimetallic hydride complex [Cp RuH(dppf)], dppf = l,l -bis(diphenylphosphino)ferrocene, has been reported [44b] to catalyze the elementary reaction H2 —> 2H + 2e . It was suggested [44b] that the crucial oxidation involves the ruthenium center and not... 

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]. 

A lot of attention has been paid to the electrochemical oxidation of H2 in the context of fuel-cell research [141]. Obviously, materials that adsorb H2 diss-ociatively should be the better catalysts, and this is borne out in practice. The best electrocatalyst is Pt (in acid), as indeed for the reverse reaction (Fig. 5.40), the elementary steps being simply [142] ... 

Wang, X., Lau, K.C., Turner, C.H., Dunlap, B.I. Kinetic Monte Carlo simulation of the elementary electrochemistry in a hydrogen-powered solid oxide fuel cell. J. Power Sources 2010, 195, 4177-84. 

Proton transfer could certainly be another full chapter in this book. With applications ranging from photosynthesis to fuel cells this is one of the most important elementary reactions and as such was and is intensively investigated. This section does not pretend to provide any coverage of this process, and is included here mainly as a reminder that this important reaction should be on the mind of a researcher in condensed phase chemical dynamics. It is also of interest to point out an interesting... 

The aim of the present work is to develop a model of corrosive dissolution oiPt binary nanocluster Pt Me (Me Cr, Fe, Co, Ni, Ru) in working environment of low temperature fuel cells on the basis of quantum-chemical methods application and deduction of physico-chemical peculiarities ofPr (with different structure and elementary content) surface destruction under the influence of H O, Cl, OH, HjO. ... 

Analysis of hydrodynamic equations for the flow in the fuel cell channel shows that this flow is incompressible [13]. In other words, the variation of pressure (total molar concentration) along the channel is small. Consider first the case of zero water flux through the membrane. Each oxygen molecule in the cathode channel is replaced with two water molecules. Pressure is proportional to the number of molecules per unit volume. To support constant pressure, the flow velocity in the channel must increase. The growth of velocity provides expansion of elementary fluid volume the expansion keeps pressure in this volume constant. 

Membrane operation in the fuel cell is affected by structinal characteristics and detailed microscopic mechanisms or proton transport, discussed above. However, at the level of macroscopic membrane performance in an operating fuel cell with fluxes of protons and water, only phenomenological approaches are feasible. Essentially, in this context, the membrane is considered as an effective, macrohomogeneous medium. All structures and processes are averaged over micro-to-mesoscopic domains, referred to as representative elementary volume elements (REVs). At the same time, these REVs are small compared to membrane thickness so that non-uniform distributions of water content and proton conductivities across the membrane could be studied. 

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