Gas crossover

The bipolar plate material of the PAFC is graphite. A portion of it has a carefully controlled porosity that sei ves as a resei voir for phosphoric acid and provides ffow channels for distribution of the fuel and oxidant. The plates are elec tronically conductive but impervious to gas crossover. 

PEFC The PEFC, like the SOFC, has a solid electrolyte. As a result, this cell exhibits excellent resistance to gas crossover. In contrast to the SOFC, the cell operates at a low 80°C. This results in a capability to bring the cell to its operating temperature quickly, but the rejected heat cannot be used for cogeneration or additional power. Test results have shown that the cell can operate at very high current densities compared to the other cells. However, heat and water management issues may limit the operating power density of a practical system. The PEFC tolerance for CO is in the low ppm level. 

Our evidence has also shown that defects generated and propagated during RH cycling condition do not contribute much to the gas crossover before the occurrence of the final mechanical breach. Figure 10 shows the RH cycling test results of MEAs cycled from 0 to 100% RH at 80°C. Multiple test runs were conducted, and the sample was taken out and mechanically tested after 200, 400, and... 

OCV represents the open circuit voltage, i.e. the voltage difference between the two current collectors, when no current flows. Under the assumption that no gas crossover from one electrode to the other takes place, and assuming that there is no electronic transport within the electrolyte, the Nemst equation can be employed to calculate the OCV ... 

As shown in Figure 3.13, the voltage losses can be analyzed from the polarization curve. Generally, the voltage losses consist of four parts (1) loss due to gas crossover this is represented by the open circuit voltage (OCV), which is lower than the thermodynamic voltage (2) loss due to activation resistance (3) loss due to ohmic resistance and (4) loss due to mass transport limitation. 

Masahiro Watanabe etal., 1998, Polymer Electrolyte Membranes Incorporated with Nano-meter Size Particles of Pt and/or Metal Oxides Experimental Analysis of the Self-Humidification and Suppression of Gas Crossover in Euel Cells. Journal of Physical Chemistry B, 102, 3129-3137. 

Alternatively, an additional layer constructed by using fine nickel powder, L1A102, and NiO is positioned between the anode and the electrolyte and filled with molten carbonate electrolyte. The purpose of this additional layer is to prevent gas crossover from one electrode to the other if cracks develop in the electrolyte structure. This bubble barrier layer serves as a reinforcement of the electrolyte matrix. This bubble pressure barrier (BPB) can be fabricated as an integral part of the anode structure. Typically, the pores of this barrier layer are smaller than the anode pores and provide ionic transport through the cell. ... 

M. Watanabe, H. Uchida and M. Emori, Polymer electrolyte membranes incorporated with nanometer-size particles of Pt and/or metal-oxides Experimental analysis of the self-humification and suppression of gas-crossover in fuel cell, J. Phys. Chem., B, 1998, 102, 3129-3137 M. Watanabe, H. Uchida, Y. Seki and M. Emori and P. Stonehart, Self-humidifying polymer electrolyte membranes for fuel cell, J. Electrochem. Soc., 1996, 143, 3847-3852 H. Uchida, Y. Mizuno and M. Watanabe, Suppression of methanol crossover in Pt-dispersed polymer electrolyte membrane for direct methanol fuel cell, Chem. Lett., 2000, 1268-1269 H. Uchida, Y. Ueno, H. Hagihara and M. Watanabe, Self-humidifying electrolyte membranes for fuel cells, preparation of highly dispersed Ti02 particles in Nafion 112, J. Electrochem. Soc., 2003, 150, A57-A62. 

To measure the gas crossover in HTMs, UTCFC developed a new in-situ electrochemical test. The test involves oxidizing the hydrogen that is transported across the membrane and measuring the limiting currents. Typical crossover limiting currents for two membranes of different thicknesses are depicted in Figure 1. 

Bacterial cellulose has several unique properties that potentially make it a valuable material for the development of PEM fuel cells (Reference 1) (1) it is an inexpensive and non-toxic natural resource (2) it has good chemical and mechanical stability (3) it is very hydrophilic and (4) it doesn t re-swell after drying. Additionally, its thermal stability and gas crossover characteristics are superior to Nation 117 , a material currently widely used as a proton conductive membrane in PEM fuel cells. 

There are various improvements that can be made to the presented model, some improvements could be accomphshed. Foremost among these possible future-work directions is the inclusion of nonisothermal effects. Such effects as ohmic heating could be very important, especially with resistive membranes or under low-humidity conditions. Also, as mentioned, a consensus needs to be reached as to how to model in detail Schroder s paradox and the mode transition region experiments are currently underway to examine this effect. Further detail is also required for understanding the membrane in relation to its properties and role in the catalyst layers. This includes water transport into and out of the membrane, as well as water production and electrochemical reaction. The membrane model can also be adapted to multiple dimensions for use in full 2-D and 3-D models. Finally, the membrane model can be altered to allow for the study of membrane degradation, such as pinhole formation and related failure mechanisms due to membrane mechanical effects, as well as chemical attack due to peroxide formation and gas crossover. 

Inaba M, Kinumoto T, Kiriake M, Umebayashi R, Tasaka A, Ogumi Z (2006) Gas crossover and membrane degradation in polymer electrolyte fuel cells. Electrochim Acta 51 5746-5753... 

Shim JY, Tsushima S, Hirai S (2008) Charactiaization and modeling of gas crossover and its impact on the membrane degradation of PEMFCs. ECS Trans 16 1705-1712... 

Nation membrane degradation is often monitored by changes in gas crossover rate or fluoride-ion emission rate (PER) during the in situ test. Often, the low humidity conditions were combined with OCV testing in order to accelerate hydrogen peroxide formation in the cathode [89, 90]. 

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