Temperature distribution PEMFC

The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

Chang M H and Cheng C H (2005), Non destructive inverse methods for determination of irregular internal temperatures distribution in PEMFCs , J. Power Sources, 142, 200-210. 

Understanding. As electrochemical reactors with gradients of species, temperature, and potential in all dimensions and over a broad range of scales from nanometers to meters, fuel cells are complex devices. Changing a single parameter (e.g., the gas humidity in a PEMFC) can result in effects on the scale of reaction kinetics and other parameters, all the way to temperature distribution in a fuel ceU stack. This multitude of effects, as well as their consequences as felt in important parameters such as efficiency or power density, is difficult or impossible to comprehend without modeling approaches. 

Daino et al. (2011) report studies of water transport and thermal profile in the through-plane direction of a PEMFC GDL. Direct optical and infrared access to both cathode and anode GDLs are provided in a typical 50-cm test section. Dynamic visualization (pixel resolution of about 0.6 J,m at 28 Hz) of liquid water transport in the gas distribution channels and gas-diffusion layers is described, and the underlying transport processes are discussed. The temperature distributions across the anode and cathode GDLs are also measured with a high-resolution infrared camera with a pixel resolution of 5 Tm at 30 Hz. 

Thermal management of PEMFC is key to ensure high cell performance and efficiency. The irreversibility of electrochemical reactions and joule heating are the most important factors causing heat generation inside PEM fuel cells. The temperature distribution in the cell has a strong impact on the cell performance. It influenees the water distribution by means of condensation and affects the multi-component gas diffusion transport characteristics through thermo capillary forces and thermal buoyancy. 

Figure 5 shows the MRI visualization of the transversal water content distribution in the membrane in an operational PEMFC with variation of the current density.29 The cell temperature and relative humidify were 70°C and 92%, respectively. The vertical width of the images is about 1.0 cm across the gas channels, which is in the central part of the GDL. The horizontal width of the images is 600 xm. The anode is on the left side of each figure. Figure 6 shows one-dimensional water content profiles in the membrane that were obtained from MRI visualization results at variation of the relative humidity and current density. The horizontal axis and vertical axis respectively indicate the through-plane position of the... 

Fig. 5.20 Temperature (K) distribution of the six cell cross-flow PEMFC stack with different air inlet velocities inxy section plane (a) 5 m/s (b) 3 m/s (c) 1 m/s (after Liu et al., 2006). (Reprinted from Journal of Power Sources, Vol. 160, Liu, Z., Mao, Z., Wang, C., Zhuge, W., and Zhang, Y., Numerical simulation of min PEMFC stack , pp. 1111-1122, Copyright 2006, with permission from Elsevier.)...

To tackle the problem outlined above and obtain information on the structure and composition of fuel cell catalysts under relevant conditions, a number of authors have proposed in situ XRD or XAS cells where samples were (1) subjected to a controlled gas atmosphere (H2, CO, etc.) at specified temperatures [17,157-160], (2) characterized in model electrochemical cells filled with liquid electrolytes [160-164], or (3) studied in operating PEMFCs and DMECs [165-169]. Both XRD [17] and XAS [158] measurements confirm that Pt and Ru oxides are reduced upon heating at 373 to 423 K in a hydrogen atmosphere. On the contrary, Roth et al. [158] have shown that in a CO atmosphere, ruthenium oxides remain relatively stable, their susceptibility to reduction depending on the Pt-to-Ru site distribution. It has been suggested that Pt in contact with Ru acts as a catalyst for the reduction of ruthenium oxides and strengthens the Ru-CO bond, favoring it over Ru-0. Reduction also occurs in electrochemical cells... 

J. P. Meyers, Fundamental Issues in Subzero PEMFC Startup and Operation, presentation at 2005 DOE Workshop on Fuel Cell Operations at Subfreezing Temperatures, February 1-2, 2005, Phoenix, Arizona, http //www.eere.energy. gov/hydrogenandfuelcells/pdfs/03 meyers distribution.pdf... 

PEMFCs are characterized by low operative temperature (80-100°C), high current density, compactness, fast start-up and suitability for discontinuous operation [21]. These features make PEMFCs the most promising and attractive candidate for a wide variety of power applications ranging from portable/micropower and transport to large-scale stationary power systems for buildings and distributed generation [22], as shown in Fig. 2.6. 

Unlike most other types of fuel eells, PEMFCs use a quasi-solid eleetrolyte, whieh is based on a polymer baekbone with side-ehains possessing aeid-based groups. The numerous advantages of fliis family of eleetrolytes make the PEM fuel eell partieularly attractive for smaller-scale terrestrial applieations sueh as transportation, home-based distributed power, and portable power applieations. The distinguishing features of PEMFCs inelude relatively low-temperature (under 90 °C) operation, high power density, a eompact system, and ease in handling liquid fuel. 

Figure 18.10. Histograms of the particle size distribution of the PtZr02/C catalyst before (a) and after (b) the potential cycling test [24], (Reprinted from Electrochemistry Communications, 9(1), Liu G, Zhang H, Zhai Y, Zhang Y, Xu D, Shao Z, Pt4Zr02/C cathode catalyst for improved durability in high temperature PEMFC based on H3PO4 doped PBI, 135-41, 2007, with permission from Elsevier.)...

Made et al. (2005) designed an electrochemical cell for in situ micro-Raman measurements on the polymer membrane of an operating PEMFC. The method is applicable to studies of both the distribution of water and membrane structure in the working cell environment. It was shown that a hydration profile with a lower water content at the anode forms when current is applied to the ceU. In addition, the overall liquid water content in the membrane decreases with increasing current, possibly as a result of an increase in cell temperature. 

Several recent studies of carbon corrosion in a membrane electrode assembly of a PEMFC system found the process to be much more complicated than in aqueous solutions. The corrosion rate depends, among other factors, on the type of carbon, operating potential, temperature, humidity, and uniformity of fuel distribution. Generally, three types of carbon corrosion were identified (FuUer and Gray 2006). 

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