Mass transport processes Fuel cell

Nanocasting can be considered as the best method to obtain a narrow pore size distribution. This is a very important feature because carbon supports with tunable properties allow the modeling of the system, in order to optimize the processes of methanol oxidation (adsorption and re-dissolution) and mass transport in fuel cells. 

The pores are for the transport of fuel cell reactants and product(s). Optimal porosity and pore size distribution can facilitate the mass transport process to minimize the fuel cell performance loss due to concentration overpotential. If some pores are more hydrophobic than others, what is the relative distribution Is the distribution of pore sizes and hydrophobicity within the allowable range ... 

Mass transport loss is defined as the loss in performance of the fuel cell due to limitations in mass transport processes. This performance loss is usually attributed to a reduction of the oxygen activity (associated to its partial pressure) at the electrode, in comparison to the oxygen partial pressure at the cell inlet. The accumulation of water in the transport pathways of the gaseous reactants can lead to increased mass transport losses and instability and can result in accelerated degradation. 

Mass transport processes are not strongly affected by temperature changes within the typical operating temperature and pressure ranges of most fuel cell types. 

Figure 11.13 illustrates a basic equivalent circuit to represent a general electrochemical reaction. Rs represents the electric resistance, which consists of the ionic, electronic, and contact resistances. Since the electronic resistance is typically much lower than the ionic resistances for a typical fuel cell MEA, the contribution of the electronic resistance to Rs is often negligible. Cj is the double-layer capacitance associated with the electrode-electrolyte interfaees. Since a fuel cell electrode is three-dimensional, the interfaces include not only Arose between Are surfaces of the electrodes and the membrane but also those between the catalysts and the ionomer within the electrodes. Ret is the resistanee associated with the charge transfer process and is called charge transfer resistanee. Z is called the Warburg impedance it deseribes the resistance arising from the mass transport processes. 

Eigure 14.11 shows isotherms of water desorption, that is, the dependence of water content V on the values of Ps/po for Nafion 117 membranes at temperatures of 20 and 80°C. These isotherms are of importance since, depending on mass-transport processes, the relative humidity in the gas chambers adjacent to the membranes can vary substantially. The corresponding changes in the membrane s water content influence the conductivity of the membrane and hence the fuel cell performance. From the figure it can be seen that a lowering of the Ps/po value from 1.0 to 0.8 at 80°C leads to a 20% decrease of the membrane s water content, which can substantially lower the performance of PEMFCs. 

The impedance spectra of Figs. B.13, B.14, B.15, and B.16 further clarify the previous points. By way of an example, at 650 C the ohmic resistance of the cell is around 10 mQ and clearly independent of the fuel composition, but total cell polarization increases as the N2-content increases with 20% nitrogen content in the fuel stream, total cell polarization is around 19 mQ, rising to around 24 mQ with 80% molar content. Moreover, as the N2 content increases, it is mass transport processes rather than electrochemical processes that clearly hmit fuel cell operation. 

During the operation of a polymer-electrolyte fuel cell, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. Only through fimdamental modeling, based on physical models developed from experimental observations, can the processes and operation of a fuel cell be truly understood. This review examines and discusses the various regions in a fuel cell and how they have been modeled. 

Mass and energy transport occur throughout all of the various sandwich layers. These processes, along with electrochemical kinetics, are key in describing how fuel cells function. In this section, thermal transport is not considered, and all of the models discussed are isothermal and at steady state. Some other assumptions include local equilibrium, well-mixed gas channels, and ideal-gas behavior. The section is outlined as follows. First, the general fundamental equations are presented. This is followed by an examination of the various models for the fuel-cell sandwich in terms of the layers shown in Figure 5. Finally, the interplay between the various layers and the results of sandwich models are discussed. 

A fundamental fuel cell model consists of five principles of conservation mass, momentum, species, charge, and thermal energy. These transport equations are then coupled with electrochemical processes through source terms to describe reaction kinetics and electro-osmotic drag in the polymer electrolyte. Such convection—diffusion—source equations can be summarized in the following general form... 

Figure 6.14. Cell Voltage vs. Cell Current profile of a hydrogen - oxygen fuel cell under idealized (dotted-dashed curve) and real conditions. Under real conditions the cell voltage suffers from a severe potential loss (overpotential) mainly due to the activation overpotential associated with the electroreduction process of molecular oxygen at the cathode of the fuel cell. Smaller contributions to the total overpotential losses (resistance loss and mass transport) are indicated.

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

In PEMFC systems, water is transported in both transversal and lateral direction in the cells. A polymer electrolyte membrane (PEM) separates the anode and the cathode compartments, however water is inherently transported between these two electrodes by absorption, desorption and diffusion of water in the membrane.5,6 In operational fuel cells, water is also transported by an electro-osmotic effect and thus transversal water content distribution in the membrane is determined as a result of coupled water transport processes including diffusion, electro-osmosis, pressure-driven convection and interfacial mass transfer. To establish water management method in PEMFCs, it is strongly needed to obtain fundamental understandings on water transport in the cells. 

The term electromembrane process is used to describe an entire family of processes that can be quite different in their basic concept and their application. However, they are all based on the same principle, which is the coupling of mass transport with an electrical current through an ion permselective membrane. Electromembrane processes can conveniently be divided into three types (1) Electromembrane separation processes that are used to remove ionic components such as salts or acids and bases from electrolyte solutions due to an externally applied electrical potential gradient. (2) Electromembrane synthesis processes that are used to produce certain compounds such as NaOH, and Cl2 from NaCL due to an externally applied electrical potential and an electrochemical electrode reaction. (3) Eletectromembrane energy conversion processes that are to convert chemical into electrical energy, as in the H2/02 fuel cell. 

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