Temperature effects hydrogen crossover

This chapter mainly deals with the fundamentals of H2/air PEM fuel cells, including fuel cell reaction thermodynamics and kinetics, as well as a brief introduction to the single fuel cell and the fuel cell stack. The electrochemistry and reaction mechanisms of H2/air fuel cell reactions, including the anode HOR and the cathode ORR, are discussed in depth. Several concepts related to PEM fuel cell performance, such as fuel cell polarization curves, OCV, hydrogen crossover, and fuel cell efficiencies, are also introduced. With respect to fuel cell stmctures and components, the material properties and effects on fuel cell performance are also discussed. In addition, several important conditions for fuel cell operation, including temperature, pressure, RH, and gas stoichiometries and flow rates, and their effects on fuel cell operation, are also briefly presented. This chapter provides the requisite baseline knowledge for the remaining chapters. 

Temperature Effect on Membrane Conductivity and Hydrogen Crossover 132... 

Our recent study indicated that the fuel cell OCV decreased with increasing temperature, as shown in Fig. 4.3 [58]. It can be seen that both the theoretical and the measured OCV decreased when the temperature increased from 23 to 120 °C. This was mainly because of the effect of temperature on the fuel cell thermodynamics and hydrogen crossover, which will be addressed in detail in Chapters 6 and 7, respectively. 

Although in the literature, Z>pem for Nalion membranes has been formulated empirically as a function of temperature (T) [6], the effects of backpressure (P) and RH should also be considered when measuring hydrogen crossover in real situations. 

Reactions (6.1) and (6.II) form a local cell at the cathode, which depresses the cathode potential, resulting in the reduction of fuel cell OCV. This has been proven by our recent study in the temperature range of 23-120 °C [9] the effect of hydrogen crossover on fuel cell OCV will be discussed in detail in Chapter 8. In addition, it is also possible for the chemical reaction between the crossed hydrogen and the oxygen at the cathode Pt catalyst surface to produce hydrogen peroxide or water, 1/2 O2 + H2— H20 and O2 + H2— H202, which leads to a reduction in O2 concentration and OCV. However, the electrochemical reactions shown in Reactions (6.1) and (6.11) are the dominant reactions. 

As one of the important operating conditions for PEM fuel cells, temperature has a significant effect on hydrogen crossover. Our recent study [3] indicated that the hydrogen crossover rate increases with increasing temperature. As shown in Table 6.1, when the temperature increases from 80 to 120 °C, the hydrogen crossover rate increases from 2.04 x 10 molcm s at 80 C with 100% RH and 3.04 atm backpressme, to 2.69 x 10 molcm s at 100 °C and 3.05 x 10 molcm s at 120 °C, with the same RH and... 

Pressure can also have an important effect on hydrogen crossover. Table 6.1 shows that the hydrogen crossover rate increased with increasing backpressure at all the cell temperatures and RHs. As mentioned above, the total inlet pressure is the sum of PH2O mid Pn, and increasing the backpressure will cause... 

The same effect is observed for the substituted pyridyl-pyrazole and -imidazole systems. While 2-(pyrazol-l-yl)pyridine 24 gives a low spin iron(II) complex a continuous spin transition is observed centred just above room temperature in solid salts of [Fe (31)3]2+ and just below in solution [39]. Spin crossover occurs in the [Fe N6]2+ derivative of 2-(pyridin-2-yl)benzimidazole 32 (Dq(Ni2+)=1050 cm"1) but not in that of the 6-methyl-pyridyl system 33 (Dq(Ni2+)=1000 cm"1). Although the transition in salts of [Fe 323]2+ is strongly influenced by the nature of the anion and the extent of hydration, suggesting an influence of hydrogen-bonding, in all instances it is continuous [40]. 

The temperature dependence of this rate constant was measured by Al-Soufi et al. [1991], and is shown in Figure 6.17. It exhibits a low-temperature limit of rate constant kc = 8x 105 s 1 and a crossover temperature 7 C = 80K. In accordance with the discussion in Section 2.5, the crossover temperature is approximately the same for hydrogen and deuterium transfer, showing that the low-temperature limit appears when the low-frequency vibrations, whose masses are independent of tunneling mass, become quantal at Tisotope effect increases with decreasing temperature in the Arrhenius region by about two orders of magnitude and approaches a constant value kH/kD = 1.5 x 103 at T[Pg.174]

Kensy et al. [1993] showed that the zwitterion formation and subsequent cyclization due to hydrogen transfer take place in the lowest triplet excited state of diphenylamine and its methyl substituents. The crossover temperature is about 100 K, and the values of C(H) are 10 2-10 4 s 1 for various substituents. The H/D kinetic isotope effect at T [Pg.177]

Chippar P, Ju H (2013) Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells. Int J Hydrogen Energy 38 7704—7714... 

---