Extended high-temperature fuel cells

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Staudt, R., Boyer, J., and Elter, J.F., Development, design, and performance of high temperature fuel cell technology, in Extended Abstracts for 2005 Fuel Cell Seminar, Palm Springs, CA, November 14-18, 2005, p. 207. 

In high-temperature fuel cells, the activation-related losses are often much less significant, and hence the characteristic concave portion of the V-I curve is hard to distinguish. In addition, as transport-related losses play a more important role, the convex portion of the curve often extends further to the left. 

The low cost and excellent oxidation and thermal stability of phosphoric acid doped polybenzimidazole (PBI) prompted researchers at Case Western University (Samms et a/., 1996 Wang et /., 1996a,b,c) to develop this membrane as a polymer electrolyte for DMFCs. After investigation of the thermal stability of PBI doped with phosphoric acid up to 600°C, it was concluded that this membrane is adequate for use as PEM in a high temperature fuel cell. These studies may also be extended to other polymers like the polybenzimidazobenzophenanthrolines (PBIPAs) which exhibit excellent thermal and mechanical properties (Zhou and Lu, 1994). 

Apart from the electrochemical kinetics, a major issue for one-dimensional models in the transverse direction is transport in porous structures, most often the transport of mass but also that of heat and charge. Whereas in high-temperature systems this includes the transport of species in gas mixtures in the voids and the transport of charge in the solid and of heat in both phases, in low-temperature fuel cells containing liquid water, the description in the voids extends to the high complexity of a two-phase system, where phase changes of water provide a complex coupling to the temperature of the soUd phase. 

Features of ab-PBI are a Tg of 485°C, low coefficient of friction, high wear resistance properties, very high limiting oxygen index (LOI), very high heat deflection temperature (HDT), and extended high-temperature mechanical performance and excellent chemical resistance. Indeed, ab-PBI offers high heat resistance and mechanical property retention over 300°C. Potential uses are PBI areas mentioned earlier and in membranes in fuel cells, which operate at elevated temperatures. 

Since doped zirconia allows one to extend the oxide electrochemistry up to very high temperatures and since it can serve as a fuel cell electrolyte and even as a heating element in high temperature furnaces, we will briefly formalize the structure element transport in zirconia, which is the basis for all of this. Let us introduce the SE fluxes in their usual form. We know that only oxygen ions and electronic defects contribute to the electrical transport (/ = 02, e, h )... 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

This chapter deals primarily with reactions in aqueous media, but the principles can be extended to fuel cells and other batteries like the high-temperature batteries with exotic nonaqueous electrolytes. 

A competitive cell is likely to have in its V/I curve (Figure 6.5) a small IR drop (thin electrolyte film) and an extended current range before concentration polarisation sets in. In the SOFC, moreover, an expensive platinum catalyst is avoided due to high reaction rates at high temperature. Engineering for mass production is very important to the achievement of low capital cost, for example the Rolls-Royce fuel cell (Section 4.3). Its compact stack arrangement is described in US patent 2003/0,096,147 Al, an improved version of the previous patents. 

Endoh, E., Kawazoe, H., and Honmura, S., Highly durable MEA for PEMEC under high temperature and low humidity conditions, in Extended Abstracts of 2006 Fuel Cell Seminar, Honolulu, HI, November 13-18, 2006, p. 284. 

J.W. Patterson, "Extended Abstracts, ERDA Workshop on High Temperature Solid Oxide Fuel Cells", H. Isaacs (editor), Brookhaven National Laboratory, (May 1977)... 

After extended operation of an STR PEM fuel cell with the same membrane electrode assembly (> 2500 h), autonomous oscillations were observed under conditions where the STR PEM fuel cell exhibited 5 steady states [23]. An example of the oscillations is shown in Figure 3.11.These oscillations have periods of 10 -10 s and show characteristics of a capacitively coupled switch. The oscillations transition very rapidly (<10s) between high and low states with an overshoot on the rise and undershoot and recovery on the fall. The period, magnitude and on/off times for these oscillations varied with temperature, and load resistance. Benziger and co-workers have suggested that these unusual dynamics are associated with mechanical relaxations of the polymer membrane driven by changing water content, but the detailed physical processes causing these unusual dynamics are not yet understood. 

From all that has been said above, it can be concluded that polymer electrolyte membrane fuel cells, working at elevated temperatures, are highly promising. Many difficulties must still be overcome in order to develop models, which will function in a stable and reliable manner, and for extended periods of time. At present, about 90% of all publications on fuel cells are concerned precisely with the attempts to overcome these difficulties. Most of the publications deal with research into new varieties of membrane materials. Some results of these works are described in the reviews on elevated-temperature-polymer electrolyte fuel cells (Zhang et al., 2006 Shao et al., 2007). 

As an outlook to further improvements of catalyst kinetics and durability in low-and high-temperature polymer electrolyte fuel cells, several possibilities are currently under investigation [73] (1) extended large-scale Pt and Pt-alloy surfaces [70] (2) extended nanostructured Pt and Pt-aUoy films [74] (3) de-alloyed Pt-alloy nanoparticles [75] (4) precious metal free catalyst as described by Lefevre et al. [76], e.g., Fe/N/C catalysts and (5) additives to the electrolyte which modify both adsorption properties of anions and spectator species and also the solubility of oxygen [77]. The latter approach is specific to fuel cells using phosphoric acid as electrolyte. 

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