In HT-PEFCs

At the cell level, the above-described mechanisms lead to the following consequences. The acid composition is a function of current density and cell temperature due to the delicate balance between internal water production and removal. Hence the conductivity changes as a function of current density, which can be observed experimentally during load changes in HT-PEFC operation [14], As the phosphoric acid takes up water, the volume of the membrane increases, that is, it swells as shown by synchrotron radiography experiments [53]. The resulting situation is depicted in Figure 29.2. At present, there is no model for an HT-PEFC which also... 

Unlike Pt and Pt alloys, NPMCs do not suffer from the specific adsorption of anions. As shown in Fig. 8.26a, the ORR performance of a PANI-Fe-C catalyst in the kinetic range of the steady-state RDE polarization curves is virtually independent of the solution concentration of H3PO4. In the case of a Pt/C catalyst (Fig. 8.26b), a continuous ORR activity loss is observed with an increase in H3PO4 concentration due to the chemisorption of phosphate ions. Thanks to their resistance to anions (tme also of (bi)sulfate in sulfuric acid solutions), NPMCs represent an attractive alternative to Pt-based catalysts at the HT-PEFC cathode. 

Figure 18.18 In-plane views of the HT-PEFC MEA at different current densities j (a) 0mA(OCV before) (b) 140mAcm (c) 300 mAcm ...

As shown in the preceding sections, the application of the presented imaging methods is not restricted to studies of the water management of PEFCs and DMFCs. Neutron and synchrotron X-ray imaging were also successfully applied to study the media distribution and CO2 bubble formation in DMFCs and the variation of membrane properties in an operating HT-PEFC. 

Expected values were determined in a process analysis for a lOkW HT-PEFC combined with autothermal reforming of dodecane (C12H26) at a mixture formation with H2O C = 1.9 and... 

Figure 22.22 shows a flow sheet diagram for a 10 kW HT-PEFC system based on kerosene for a process analysis under realistic conditions verified by experiments. Kerosene was heated in the hquid phase only to 400 K to avoid undesired prereactions. A heat exchanger for air preheating was omitted. Experimental results showed that a certain heat exchange occurs within the metallic construction with no special heat exchanger areas. Water was heated, evaporated, and superheated to 798 K. The mixture of water, air, and fuel led to a mixing temperature of 623 K. 

In comparison with the PEFC, the HT-PEFC requires a description of the electrochemistry with modification to higher tolerance against carbon monoxide (CO) and a simpler approach to fluid flow because of the absence of liquid water. The CO tolerance requires special submodels that account for the reversible decrease in catalyst activity if the fuel is reformate gas. Compared with the SOFC, the HT-PEFC requires different electrochemical parameters because of the very different catalysts and operating temperatures and the use of H+ instead of 0 as charge carrier. Thermomechanical stress is less important because of the much more moderate operating temperature. 

Currently, there are two major types of PEFC operated at elevated temperatures one with polysulfonic acid (Nafion)-based membranes and the other with polybenzimidazole (PBl)-phosphoric acid-based membranes. For Nafion-type membrane PEFCs with a typical operating temperature of about 110-120°C, already existing models can be readily used. A detailed discussion of these models tcan be found in review articles in the literature [9-12]. This chapter focuses on the HT-PEFC type based on PBl-phosphoric acid membranes, which type can be referred to as a phosphoric acid fuel ceU (PAFC) with a polymer membrane. The classical PAFC... 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

In the following sections, recent model approaches for HT-PEFCs are reviewed. A more detailed discussion about the role of the electrolyte is presented in Section 29.4 without claiming to be a perfect explanation. In Section 29.5, a very basic approach to modeling the polarization curve of an HT-PEFC is discussed using the example of a Celtec MEA from BASF. The intention is to demonstrate the consequences of the unique behavior of the electrolyte, which is discussed in Section 29.4. 

The influence of CO poisoning at the anode of an HT-PEFC was investigated by Bergmann et ul. [28]. The dynamic, nonisothermal model takes the catalyst layer as a two-dimensional plane between the membrane and gas diffusion layer into account. The effects of CO and hydrogen adsorption with respect to temperature and time are discussed in detail. The CO poisoning is analyzed with polarization curves for different CO concentrations and dynamic CO pulses. The analysis of fuel-cell performance under the influence of CO shows a nonlinear behavior. The presence of water at the anode is explicitly considered to take part in the electrooxidation of CO. The investigation of the current response to a CO pulse of 1.31% at the anode inlet showed a reversible recovery time of 20 min. 

Lobato et al. used an artificial neural network approach to describe the polarization curve ofan HT-PEFC [35]. Four different neural network types were applied. Special attention was paid to describe the influence ofthe PTFF content in the gas diffusion layer. For this purpose, the tortuosity was used as a model parameter and the results showed good agreement with experimental polarization curves. 

HT-PEFCs and PEFCs mainly differ in the nature of the polymer membrane and electrolyte. In an HT-PEFC, a combination of PBf, phosphoric acid, and water is used, whereas in a PEFC, Nafion and water are employed. Although phosphoric acid has been used for a longer time in PAFCs, many aspects of this substance remain unknown. The most important facts are summarized below to give an overview of this fairly complex matter which is discussed in greater detail in a separate chapter of this book [47]. 

In order to obtain a detailed model of the conductivity for an operating HT-PEFC, the above-described effects of phosphoric acid and water should be combined with the interaction of the membrane material, the dynamic water production at the catalyst sites, and the dynamic water removal due to the gas flow in the channels. A fully featured model with atomistic resolution seems to be impossible at present. Recent modeling approaches can describe the proton conductivity mechanism at a single concentration with atomistic resolution [50, 51]. 

In the previous section, the underlying principles of the behavior of the electrolyte were laid out. The main intention of this section is to discuss the consequences following from these considerations on the parameters of fuel-cell models and mediate a critical discussion. For this purpose, the widely used standard approach is applied, where the polarization curve of a HT-PEFC is described by the following... 

In order to design a basic flow sheet, aU operating conditions must be defined for chemical reactors and for the fuel cell. Different operating temperatures require a set of different heat exchangers. During the first phase of process analysis, the exchangers are not coimected to each other. An HT-PEFC system based on jet fuels as the energy carrier is analyzed here as an example. The thermodynamic conditions must be defined for each flow line ... 

In order to analyze the processes in a fuel-ceU system, all units with a positive change in Gibbs energy and those with a negative change were summarized in two composite curves. Figure 32.3 shows the composite curves for a lOkWei HT-PEFC system based on C12H26 as a model fuel for kerosene. At 298 K, the black curve... 

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