Membrane cell voltage distribution

In reality, this behavior is only observed in the limit of small jg. At currents o 1 A cm-2 that are relevant for fuel cell operation, the electro-osmotic coupling between proton and water fluxes causes nonuniform water distributions in PEMs, which lead to nonlinear effects in r/p M- These deviations result in a critical current density, p at which the increase in r/pp j causes the cell voltage to decrease dramatically. It is thus crucial to develop membrane models that can predicton the basis of experimental data on structure and transport properties. 

Figure 12. Temperature distribution (in K) in tbe middie of the membrane (EW < 1000) for a straight-channel PEM fuel cell (cell voltage = 0.6 V, average current density = 1.42A/cm2) with a cell temperature of 80 °C.

The effect of inlet stoichiometry on transport characteristics and performance of PEFC was also investigated by Pasaogullari and Wang. In Figure 24 the local current density distributions along the flow direction are displayed at a cell voltage of 0.65 V. As explained earlier, the membrane is hydrated much faster in lower flow rates, and therefore, the performance peak is seen earlier in lower stoichio-... 

Figure 29. Current density distributions in a fully humidified PEFC using 30 jum membrane (EW < 1000) under various cell voltages.

In a diaphragm cell operating at 2.3 kA/m2, for example, the percentages of the total cell voltage attributable to these three components are 67 percent thermodynamic, 23 percent IR, and 10 percent overvoltage. Typical voltage distribution in a membrane cell is given in Table 26.9. 

TABLE 26.9 Typical Voltage Distribution in a Membrane Cell at 5 kA/m2, 90°C, 34 Weight % NaOH, DSA Anode, 200 g/L NaClAnolyte, and Activated Cathode... 

Electrochemical experiments to measure cell voltage and current efficiency are simple, direct, and usefiil for the investigation of membrane performance. Visual and microscopic observations are indispensable. Visual inspection is the principal method used for determining the soundness of the membrane surface. SEM is quite useful for the inspection of morphology of the surface or cross-sections. It can be used to identify deposits of impurities. XRD and X-ray fluorescence spectroscopy (XRF) are useful for the semiquantitative determination of impurity accumulation and distribution [126]. Table 4.8.10 [ 132] sununarizes the reconunended methods for detecting the physical and chemical damage to membranes. 

FIGURE 10.2.7. Energy distribution in a membrane chlor-alkali cell (MGC-26) operating at 5kAm . (Energy consumption 2,607 kW hr ton" of CI2 Cell voltage 3.35 V Current efficiency 97%.)... 

Figure 31.7 Liquid water saturation distributions at time instants of 0, 0.01, 0.1, 1.0, 1.25, and 1.75 s, in transient of changing the RH from 100 to 50% for both sides (a) and time evolutions of cell voltage and membrane resistance (b) [18].

In addition, the numerical models can be used in order to understand the overall effect of CO poisoning on other transport phenomena, such as liquid water transport. In a study by Wang and Chu [24], they developed a transient, one-dimensional, two-phase numerical model of the electrolyte membrane and anode and cathode catalyst layers. Their model was used to look into the effect of CO poisoning on the water distribution in the catalyst layers and the electrolyte membrane. With 100% H2 (i.e., the hydrogen feed was not dilute), 10 ppm CO level, and a cell voltage of 0.6 V, they investigated the liquid water saturation in the catalyst layers and the water content in... 

FIGURE 7.4 100-Cell stack voltage distribution model computations for one anomalous bus plate (either stainless steel, nickel, or aluminum) and one copper bus plate, at 300 A. Model curves correspond to inlet, middle, and outlet locations. (Reprinted from Journal of Power Sources, 152, Kim, G. S. et al. Electrical coupling in proton exchange membrane fuel cell stacks. 210-217. Copyright (2005), with permission from Elsevier.)... 

The performance of a plant is determined by the electrolyzer, the cell voltages, and the current efficiency of the membrane. It is essential to design an electrolyzer with an homogeneous electrolyte concentration, temperature, and current density distribution across the whole area of the membrane. 

Voltage distribution/V estimated reversible cell voltage overpotentials electrolyte iR membrane iR total... 

As we have mentioned earlier, one of the fuel cell voltage losses is the mass transfer loss or concentration loss caused by lower reactant gas concentration distribution at the reaction sites. Mass transport establishes reactant gas concentration distributions in gas supply channels and in the electrodes of a fuel cell, and hence in the distribution of local current densities. The gas supply rates to the anode-membrane and cathode-membrane interface must be sufficient enough to meet the gas consumption rate given by the electrochemical reaction rates. Any insufficient supply of gas to reaction sites may cause sluggishness in the reactions and cause mass transfer loss and reduction in fuel cell output voltage. 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

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