Membrane crossover

Based on our experience, the strength of membrane/MEA, particularly strain-to-failure, measured ex situ has the potential to serve as a metric/indicator for PEMFC life prediction and accelerated testing when the failure mode is membrane crossover. It is an... 

Under most common DMFC operation conditions, the oxygen reduction process is under interfacial kinetics control and the methanol oxidation process is determined by the methanol flux across the membrane,. /crossover, (Eqs 59-61). The increase in cathode overpotential is therefore be given by... 

Eigure 5.32 shows the same as in Eigure 5.31 curves for the hundred times smaller oxygen-limiting current density on the anode side (yjj = 0.02 A cm ). This mimics the situation when in the R-domain oxygen penetrates to the anode side through the membrane (crossover). 

Methanol l/3/0F Skin/Eye irritant, Poisinous in large quantities CH4O + O2 CO2 + 2H2O Membrane Crossover (causes mixed potential, parasitic O2 consumption)... 

The concept of the reversed fuel cell, as shown schematically, consists of two parts. One is the already discussed direct oxidation fuel cell. The other consists of an electrochemical cell consisting of a membrane electrode assembly where the anode comprises Pt/C (or related) catalysts and the cathode, various metal catalysts on carbon. The membrane used is the new proton-conducting PEM-type membrane we developed, which minimizes crossover. 

Although ORR catalysts for DMFCs are mostly identical to those for the PEM fuel cell, one additional and serious drawback in the DMFC case is the methanol crossover from the anode to the cathode compartment of the membrane electrode assembly, giving rise to simultaneous methanol oxidation at the cathode. The... 

Therefore, one main drawback of the PEMFC configuration with a standard proton exchange membrane (such as Nafion) and a standard platinum gas diffusion cathode is the cathode depolarization caused by a mixed potential resulting from the methanol crossover through the mem-... 

All these attempts to improve the membrane characteristics by decreasing the methanol crossover and increasing their temperature resistance gave encouraging results. However, modification of the existing membranes (e.g., with barrier screens) without decreasing the electrical... 

Figure 15. Extent of methanol crossover through different ETFE proton-conducting membranes. Comparison with the behavior ofNafion 117 ( ).

Methanol crossover was also investigated as a function of the ionomeric membrane thickness " and it was found to decrease with... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

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. 

The emergence of commercial fuel cell cars will depend on developments in membrane technology, which are about one third of the fuel cell cost. Improvements are desired in fuel crossover from one side of a membrane to the other, the chemical and mechanical stability of the membrane, undesirable side reactions, contamination from fuel impurities and overall costs. 

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

For DMFC systems, Pt cathodes are also used as the catalyst of choice however, given Pt s ability to reduce oxygen and oxidize methanol, this lack of selectivity makes them sensitive to methanol crossover from anode to cathode via the membrane. This methanol crossover can have a depolarizing effect on cathode performance, reducing overall cathode activity. To combat this, an extensive effort has been made to identify and develop selective oxygen/reduction catalysts unaffected by MeOH crossover. 

The MeOH crossover rate is closely related to several factors, including membrane structure and morphology, membrane thickness, membrane acid content, and the cell operating parameters, such as temperature and MeOH feed concentration. The MeOH crossover rate decreases with an increase in the thickness and can be reduced greatly by using a membrane with sufficiently high Therefore, it may be advantageous to use either thicker... 

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