Crossover of fuel

The first limitation is related to interference of the anode and the cathode. The finite permeability of the Nation membrane to fuel and oxygen results in crossover of fuel from the anode to the cathode, and oxygen crossover in the opposite direction. This may have a significant influence on electrode kinetics. 

SECM has been recently used by Bath et al. in the investigation of elec-troosmotic convective flow (10). Electroosmosis occurs in porous membranes employed in fuel cells, representing an important practical issue in the parasitic crossover of fuel (e.g., methanol) and in the flooding/drying of the fuel cell electrodes (29,30). Electroosmotic flow across skin, a naturally occurring ion-selective membrane, is also of interest in transdermal drug delivery, as discussed in Sec. III.B.2. 

Another OCV loss is caused by the crossover of fuel through the electrolyte. Ideally the electrolyte allows the transport of only ions. In reality, however, some fuel permeates across the membrane from the anode to the cathode. In addition, some direct transfer of electrons across the membranes can occur and cause electronic short. A fuel loss due to crossover leads to a current loss. The current loss associated with an electrical short is generally small (ca. few milli-amperes) relative to the typieal operating current of a fuel cell, and therefore is not a significant source of current inefficiency. However, these effects have a significant effect on the OCV of the cell. This is particularly true of a low-temperature cell, in which activation losses are considerable [126]. 

In DAFC, main hurdle to be over come is the development of membrane such that it does not allow crossover of fuel (methanol, ethanol) through the membrane restricting the fuel oxidation at cathode and minimizing the over voltage losses. 

Similarly, in the development of solid oxide fuel cells (SOFCs), it is well recognized that the microstructures of the component layers of the fuel cells have a tremendous influence on the properties of the components and on the performance of the fuel cells, beyond the influence of the component material compositions alone. For example, large electrochemically active surface areas are required to obtain a high performance from fuel cell electrodes, while a dense, defect-free electrolyte layer is needed to achieve high efficiency of fuel utilization and to prevent crossover and combustion of fuel. 

R Liu, G. Lu, and G. Y. Wang. Low crossover of methanol and water through thin membranes in direct methanol fuel cells. Journal of the Electrochemical Society 153 (2006) A543-A553. 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

The generation of heat always accompanies the operation of a fuel cell. The heat is due to inefficiencies in the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel, and electrical heating of interconnection resistances. Spatial temperature variation can occur if any of these heat-generating processes occur preferentially in different parts of the fuel cell stack. For example, non-uniform distribution of fuel across the surfaces of electrodes, different resistances between the interconnections in a stack, and variations among... 

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. 

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