Elevated temperature PEMFC

The main problem in ET-PEMFC operation is degradation of the membrane at the higher temperature. Marked water loss raises the ohmic resistance of the membrane, causes brittleness, and may give rise to crack formation. For this reason, most PEMFC research at present addresses the question of how to maintain the membrane in good working condition in an elevated-temperature PEMFC. 

The fuel cell is positioned within the magnet. Fuel gas (hydrogen) and oxidant gas (oxygen or air) are fed to the cell. For investigation of water transport in PEMFCs by MRI, it is preferable to operate PEMFCs in elevated temperature with humidity control, because both operating temperature and relative humidify (RH) inherently affects water transport in PEMFCs. Tsushima et al.27-29... 

Matos et al. 60] developed Nafion-titanate nanotube composites as the PEMFC electrolyte operating at elevated temperatures. The addition of 5-15 wt% nanotubes to the ionomer allowed the PEMFC performance essentially to be sustained up to 130 °C. The polarization curves of PEMFCs using composite electrolytes reflected a competing effect between an increase in water uptake due to the extremely large surface area of the nanotubes, and a decrease in the proton conductivity of the composites. 

In PEMFCs, carbon materials are exposed to conditions that favor their corrosion. These are the positive values of the electrode potential, the acidic environment (pHelevated temperature (333 to 363 K), but also the presence of electrocatalysts such as metal nanoparticles. Since the electrode potential, water content, and Pt mass fraction are higher at the cathode of a PEMFC, this may explain why stronger degradation of the carbon support is usually reported at this electrode [266]. The rate of corrosion of carbon in PEMFCs has been reported to increase with an increase in the relative humidity [97,255,256], but Borup et al. [273] arrived at the controversial conclusion that the rate of carbon corrosion increases with decreasing relative humidity. 

NAFION incorporated with Si02, Ti02, or Zr(HP04)2 nanoparticles exhibits enhanced performance in PEMFCs compared to fiUer-free NAFION at elevated temperatures under low RFl. The water transport properties, which were investigated by PFGSE NMR, spin—lattice relaxation Ti, and NMR spectroscopy, revealed at least two distinct water environments. The enhanced water uptake relative to fiUer-free NAFION was attributed to alteration of the pore structure of the membrane [80]. 

Many efforts have been made to find the proper alternative to Nafion at elevated temperature operation for PEMFCs, as Nafion degrades at temperatures higher than 110-130 °C. It was found that the organie-inorganic composite membrane can improve its own mechanical strength and thermal stability at... 

Song Y, Fenton JM, Kunz HR, BonvUle LJ, WiUiams MV. High-performance PEMFCs at elevated temperatures using Nation 112 membranes. J Electrochem Soc 2005 152 A539-44. 

There are two different strategies reported to overcome the influence of side reactimis and temperature effects in HT-PEMFCs during in situ CO stripping at elevated temperatures (for details, see [19]). The first one is based on the so-called reference CV. The reference CV simulates the actual CO stripping CV process. All parameters are kept identical compared to the CO stripping, but no CO adsorption is allowed to happen (N2 is used instead). Afterwards, the recorded reference CV is subtracted from the CO stripping CV, thus the effect of all side reactions is taken care of (cf. Fig. 14.4a, ECSAcv vs. ref. cv)-... 

An important aspect of the development of fuel cell stacks is to make them more compact, and a key to that is to develop thinner bipolar plates preferably from metal. Especially low temperature automotive PEMFC stacks have reached impressive power densities with metallic bipolar plates. However, because of the free phosphoric acid and the elevated temperature, research and demonstration of HT-PEMFCs has so far been done almost exclusively with plates of graphite and its composite materials. 

Carbon corrosion and platinum dissolution in the acidic electrolyte at elevated temperatures are well recognized from the early years of research on PAFCs and are definitely relevant to HT-PEMFCs based on the acid-doped FBI membranes. Both mechanisms are enhanced at higher temperatures and higher electrode potentials. This should be taken into account when platinum alloy catalysts are considered for the HT-PEMFC. More efforts are also needed to develop resistant support materials based on either structured carbons or non-carbon alternatives. 

Fuel cells of this variety are sometimes called high-temperature or midtemperature PEMFCs, but it is preferable to use the designation elevated-temperature PEMCFs (ET-PEMFCs), since in their application to soUd-oxide fuel cells, high-and midtemperature refer to different temperature ranges (see Chapter 8). 

In view of all these advantages, most of the research into PEMFCs concentrates on the elevated-temperature variant. At higher temperamres, the thermodynamic EMF value of a hydrogen-oxygen fuel cell is somewhat lower, but it can be seen from data reported by Zhang et al. (2006a) that the cell s OCV is practically unaffected. 

From all that has been said above, it can be concluded that PEMFCs working at elevated temperatures are highly promising. Many difficulties must still be overcome to develop models that 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 attempts to overcome these difficulties. Most of the publications deal with research into new varieties of membrane materials, and some of them are discussed in Chapter 13. One may also consult reviews on ET-PEMFCs (Zhang et al., 2006b Shao et al., 2007). 

In the Selective oxidation reactor a small amount of air (typically around 2%) is added to the fuel stream, which then passes over a precious metal catalyst. This catalyst preferentially absorbs the carbon monoxide, rather than the hydrogen, where it reacts with the oxygen in the air. In addition to the obvious problem of cost, these units need to be very carefully controlled. There is the presence of hydrogen, carbon monoxide, and oxygen, at an elevated temperature, with a noble metal catalyst. Measures must be taken to ensure that an explosive mixture is not produced. This is a special problem in cases where the flow rate of the gas is highly variable, such as with a PEMFC on a vehicle. 

A limiting factor in PEMFCs is the membrane that serves as a structural framework to support the electrodes and transport protons from the anode to the cathode. The limitations to large-scale commercial use include poor ionic conductivities at low humidities and/or elevated temperatures, a susceptibility to chemical degradation at elevated temperatures and finally, membrane cost. These factors can adversely affect fuel cell performance and tend to limit the conditions under which a fuel cell may be operated. For example, the conductivity of Nafion reaches up to 1(T S cm in its fully hydrated state but dramatically decreases with temperature above the boiling temperature of water because of the loss of absorbed water in the membranes. Consequently, the developments of new solid polymer electrolytes, which are cheap materials and possess sufficient electrochemical properties, have become one of the most important areas for research in PEMFC. 

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