Crossover current density

Figure 9. Top HOR (less O2 crossover) current density distribution with respect to length scales of a local H2-starved region determines where H2 depletes. Bottom carbon corrosion current distribution with respect to length scales of a local H2-starved region shows how a fully starved region develops. The cell operates on neat H2/air at s 1.5 A/cm2 (80 °C, 150 kPaabs, 100% RHin).

Figure 11. Calculated length scales with respect to applied current density for a cell operating on neat H2/air (80 °C, 150 kPaabs, 100% RHin). The solid line represents the length scale beyond which Fl2 depletes. The long and short dashed lines denote the length scales beyond which the maximum carbon corrosion current density would exceed 10% and 50% of O2 crossover current density, respectively.

Narayanan and coworkers by measuring the COj content in the cathode exit stream with an on-line CO2 analyzer, and later by researchers at Los Alamos National Lab. It was observed that the crossover rate decreases with increasing current density, and increases with temperature. The trend of decreasing crossover at higher current densities was explained as arising from the increased utilization of methanol at the anode. When the methanol crossover rate is expressed as a parasitic crossover current density, values corresponding to 200 mA/cm and 350 mA/cm were observed at 60 C and 90 °C, respectively, when compared at an applied current density of 150 mA/cm. Thus, it is readily apparent that with Nafion 117-based systems the methanol... 

However, when the crossover rates are compared for the various fuels in terms of the parasitic current density experienced at the cathode, the values observed for TMM are more than double than that of methanol when comparing equimolar solutions under open circuit conditions. The observed crossover current densities at the cathode for different concentrations of TMM is illustrated... 

Fig. 1.48 Crossover current density of trimethoxymethane as a function of concentration (0.25 M 0.5 M, and 1.0 M solutions) in liquid-feed direct oxidation fuel cells at 90 C.

In order to evaluate the impact of the relative amount of fuel crossover upon the cathode performance, and thus the overall cell voltage, it is necessary to compare the crossover current densities, rather than on a molar basis. In Fig. 1.50, the crossover current densities are compared for ail three fuels, dimethoxymethane, trimethoxymethane. and methanol, of varying concentrations. When the behavior of TMM is contrasted with that of methanol, it is apparent that a 1.0 M methanol solution displays similar crossover characteristics compared with a - 0.3 M solution of trimethoxymethane. This trend is understandable in that one trimethoxymethane molecule is converted to four carbon dioxide molecule in an overall 20 electron electro-oxidative... 

Fig. 1.50 Comparison of the crossover current densities of dimethoxymethane (DMM), trimethoxymethane (TMM) and methanol in direct oxidation fuel cells under applied load.

Fig. 1.61 Methanol crossover current density observed in a 4" x 6"-size fuel cell at 90 C. 

Fig. 1.63 Comparison of the crossover rates of 1.0 M trimethoxymethane and 1.0 M methanol (crossover current density, mA cm ) in a 4" x 6 size fuel cell at 90 C.

Fig. 1.83 Methanol crossover rates, expressed as crossover current densities, as a function of temperature for MEA 2 in a 1 x 1 size direct methanol fuel cell.

The methanol crossover rates measured in a 1 x 1 -size DMFC containing another PSSA-PVDF membrane (MEA 3) were even lower than those for the previous sample reported above, corresponding to greater than a 95% reduction in the methanol crossover observed. The crossover current densities were calculated from the determination of the COg content of the cathode stream of an operating cell while under OCV conditions and as a function of temperature, as shown in Fig 1.86. A comparison of the methanol crossover current densities for systems utilizing PSSA-PVDF and Nafion -117... 

Fig. 1.102 A comparison of methanol crossover current densities observed with direct methanol fuel cell operating with Nafion -117 and PSSA-PVDF (USC-MEA 10) electrolyte membranes at various temperatures..

Fig. 1.103 Methanol crossover current densities as a function of applied current density measured for an operating fuel cell containing a PSSA-PVDF membrane (USC-MEA 10) at 60 °C. 

Table 1.11 Comparison of methanol crossover rates expressed as crossover current densities (mA cm ) of various MEAs studied containing PSSA-PVDF membranes.

The methanol crossover rates present in operating fuel cells was measured by analyzing the CO2 content present in the cathode exit stream. This was accomplished by utilizing an on-line analyzer, which measures the CO2 volume percent in the cathode stream. Before each measurement, the instrument was calibrated with gases of known COg content. The way in which the fuel crossover current density was calculated from the carbon dioxide content detected in the cathode stream is shown below. 

FIGURE 21.27 Hydrogen crossover current density under the long-term RH cycling comparison between Nafion cast membrane and reinforced membrane. (From Choudhury, B., Material challenges in proton exchange membrane fuel cells. International Symposium on Material Issues in a Hydrogen Economy, November 12-15, Richmond, VA, 2007.)... 

It has been pointed out [35] that the crossover current density in Eq. 1.5 is not a constant, but it is rather sensitive function of the cell current density, decreasing at higher cell current due to the lowering of methanol crossover. This can be explained by resorting to the methanol concentration profiles in the anode, as shown in Fig. 1.9. 

H3PO4 doped PBI membranes (A = 2.5) 100 pm thick exhibit methanol crossover current densities less than 10 mA.cm [403, 410], while the PBI/PVPA composite commercial membrane Celtec-V shows crossover current densities higher than 100 mA.cm at 90 °C in 1 M methanol [201]. 

The relationship between the OCV and the H2 crossover current density is plotted in Figure 2.4. It can be seen that most of the OCV loss occurs in the extremely low crossover current density region (e.g., <10 mA cm ). 

When a fuel cell is at the open circuit, its voltage is also far lower than the thermodynamic voltage, mainly due to the crossover of H2 through the PEM from the anode to the cathode. At an H2 crossover current density of as low as 2 mA cm, the OCV drops from 1.18 V to around 1.04 V. But once the fuel cell is in operation, the voltage drop due to H2 crossover becomes unimportant. 

If we assume that the O2 solubility in Nafion is linearly proportional to its pressure, and the parameters for H2 are the same as those for O2, Table 3.15 lists the O2 and H2 crossover current densities at different pressures. 

Although the crossover current densities are not significant (for example, at O2/H2 pressures of 100/200 bars, the crossover current density is 21.2 mA cm 2), potential formation of explosive gas mixtures must be considered and avoided. The last two columns in Table 3.15 list the ratios of O2 or H2 crossover current density to the electrolysis current density (ECD) at 200 and 400 mA cm , respectively. For a mixture consisting of H2 and O2, the combustion range is around 4-94% vol. H2 (Wikipedia). Table 3.15 shows that the maximum H2 pressure within an electrolyzer cannot be higher than about 35 and 70 bars at 200 and 400 mA cm electrolysis current densities, respectively, in order for the H2 vol.% to be lower than 4%. 

O2 and Hj Limiting Crossover Current Density (CD) at Different Pressures and Ratio of Oj and H2 Limiting Crossover CD to Electrolysis CD... 

H2 crossover shows how much H2 crosses through the PEM to the cathode side. The US DOE target for the H2 crossover current density is less than 2 mA cm. For a pinhole-free PEM the H2 crossover current density is around 0.82, 0.66,0.33,0.19,0.13 and 0.09 mA cm" for Nafion membranes with a thickness... 

The H2 crossover rate can be determined similarly with the setup discussed in the ECSA section. One side is fed with humidified H2 and the other side with humidified N2. The N2 side is the working electrode and the H2 side is the counter electrode. The voltage of the N2 side can be set at around 0.5 V or scanned between 0.3 and 0.5 V. The H2 that crosses through the PEM to the N2 side will be quickly oxidized. The observed current density is the crossover current density. The amount of H2 can be calculated by Equation 4.19. 

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