Hydrogen crossover

Figure 1.5 The slope of E ath versus log /orr through the fuel-cell-relevant potential range has an apparently constant value near RT/F (measured current density, here designated i, is corrected for hydrogen crossover current, designated i and the measured cell voltage is ir-corrected to provide the cathode potential E) [Neyerlin et al., 2006].

Figure 1. Competing degradation mechanisms in PEM fuel cells. (CO hydrogen crossover rate ECA electrochemical area a membrane ionic conductivity.)...

Figure 10. RH cycling results in steady degradation of membrane strain-to-break, but little effect on the limiting current due to hydrogen crossover.36...

Under theoretical cell voltage conditions, for both half-cell reactions (HOR and ORR) there is no net reaction. In other words, both half-electrochemical reactions are in equilibrium, and no net current passes through the external circuit. The cell voltage can be considered the OCV. At 25 °C, if the pressures of both H2 and 02 are 1 atm, the OCV should be 1.23 V. However, in reality the OCV is normally lower and an OCV of 1.23 V is never observed. This is due to the mixed potential at the cathode side, and hydrogen crossover from the anode side to the cathode side [22, 23], At 1.23 V, Pt is not stable so oxidation of Pt occurs ... 

Other properties, hydrogen crossover measured at 22°C, 100% RH and 50-psi delta pressure ... 

United Technologies Corporation Fuel Cells (UTCFC) has developed and applied a new and novel in-situ electrochemical technique to quantify hydrogen crossover in membranes. 

Bacterial cellulose has several properties that have been identified as useful for PEM fuel cell development, including thermal stability and low hydrogen crossover characteristics. 

Several properties of the cellulose phosphate membrane were evaluated, including hydrogen crossover, ion-exchange capacity and thermal stability. 

Several properties of cellulose phosphate have been evaluated. The material has good thermal stability and low hydrogen crossover, two requirements that are important to meet DOE fuel cell program targets. Further characterization of the material is required, especially the determination of proton conductivity. In addition, testing of the material in an MEA will allow the effect of acid stability and swelling properties of cellulose phosphate be evaluated under typical PEM fuel cell operating conditions. 

Hydrogen crossover measured at 65°C and 100% RH. This is not a routine test. 

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.)... 

FIGURE 21.34 Hydrogen crossover rate growth as a function of RH cycling and time period under test. (Courtesy of W.L. Gore Associates, Elkton, MD.)... 

In a PEFC, oxygen and hydrogen crossover is important because of the obvious performance loss, the development of a mixed potential, and even durability issues due to hydrogen peroxide generation platinum migration, and possible carbon corrosion [69]. Furthermore, crossover becomes increasingly important as the membranes used become thinner. Presented in this section are the parameters and governing equations to model this phenomenon. 

Structural changes in PEM and catalyst layers due to platinum oxidation or catalyst contamination under open-circuit conditions On/off cyclic operation under different humid conditions Effect of hygro-thermal cycle on membrane stresses Water uptake effect on cyclic stress and dimensional change, hydrogen crossover... 

Vilekar, S. A., Datta, R. (2010,). The effect of hydrogen crossover on open-circuit voltage in polymer electrolyte membrane fuel cells. Journal of Power Sources, 795(8), 2241—2247. 

In the deuterium-hydrogen crossover experiment, enyne 23 was subjected to a mixed atmosphere of D2 and H2 or an atmosphere of DH (Scheme 11). In both cases, no crossover products were observed, which is in agreement with hemolytic hydrogen activation. It also indicates that oxidative cyclization took place prior to hemolytic hydrogen activation. 

The standard reduction potential of this reaction is around 0.21 V, much lower than the standard reduction potential of oxygen (R. 2.6) therefore, a mixed potential between 0.21 and 1.18 V is produced. Since the oxygen reduction reaction and Pt oxidation reaction dominate at the cathode due to kinetic reasons, the mixed potential is near the higher end. The other reason is hydrogen crossover through the PEM from the anode to the cathode. This is like an internal current flow in the cell and thus leads the electrodes (mainly the cathode) away from the 0 current thermodynamic equilibrium conditions. Due to the slow kinetics of Reaction 2.6, this internal current flow significantly lowers the cathode potential. Detailed estimation is given later in this chapter. For these two reasons, the OCV of a PEMFC is typically between 0.95 and 1.0 V. 

Although hydrogen crossover and internal currents are equivalent, they physically have different effects in a fuel cell. The loss of electrons occurs after the electrochemical reaction has taken place and therefore the effect on both anode and cathode activation polarization would have the effect as depicted by Equation (III.33). Hydrogen that permeates through the membrane does not participate in the electrochemical reaction on the anode side, and in that case the total current resulting from the electrochemical reaction would be the same as the external current. However, hydrogen that permeates through the membrane to the cathode side may... 

A test station often has a power supply fliat can drive reactions to proceed in the direction opposite to the spontaneous direction. For example, a power supply can drive water electrolysis to occur, and can be used to measure methanol or hydrogen crossover rates. 

Figure 11.30. Hydrogen crossover rates through a Hyflon extraded membrane under OCV tested with different reactants. Electrode area = 25 cm Pt = 0.25 mg cm at either anode or cathode T = 70 °C RH = 50% for both anode and cathode membrane thickness = 50 pm ionomer equivalent weight = 870 g equiv [26]. (Reprinted from Journal of Power Sources, 171(1), Merlo L, Ghielmi A, Cirillo L, Gebert M, Arcella V, Resistance to peroxide degradation of H3rflon Ion membranes, 140-7, 2007, with permission from Elsevier.)...

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