The efficiency of electrochemical reaction in polymer electrolyte fuel cell (PEFC) depends highly on operating conditions. Of those conditions, temperature has been expected to play an important role on the performance of a PEFC and durability of the membrane electrode assembly (MEA), especially due to its limitation at higher temperature [1]. One of the outcomes of high-temperature operation is membrane degradation that can increase fuel crossover. Great amount of studies have focused on effects of temperature incorporating other parameters such as relative humidity or total pressure gradient between electrodes on either proton exchange membrane (PEM) or MEA [2,3]. In the current study, hydrogen crossover through a Nafion membrane and a Nafion membrane with catalyst layer has been compared from the points of various cell temperatures and total pressure difference.
Experimental
Hydrogen crossover was measured using 3 MEAs (containing NR-211, NR-212, and N-115) at Pt loading of 0.36 mg/cm2 (anode) and 0.38 mg/cm2 (cathode) with the equal catalyst layer thickness (10 μm) and the 4 Nafion membranes (NR-211, NR-212, N-115, and N-117). The parallel flow channels with active surface area of 4.0 cm2 were used. Gas crossover was tested at cell temperature during 59 oC – 94 oC and atmospheric pressure with the desired fixed inlet relative humidity (70 % – 79 %). Hydrogen and nitrogen were supplied to the supply side of the cell to control a hydrogen partial pressure and only nitrogen was supplied to the permeate side. The gas was collected from the cell outlet for gas chromatography analysis (GC-12A, Shimadzu).
A pristine PEM was exposed to heat for 6 min at 120 oC and 140 oC for comparison of the MEA that experienced heat treatment during MEA fabrication.
An MEA (containing NR-212) at the identical catalyst layer properties was exposed to inlet pressure difference at 0.29 – 74.9 kPa and hydrogen crossover was measured for investigating effects of the total pressure difference.
Results and Discussion
Hydrogen gradient between the anode and cathode drives the crossover as follows:
NH(M) = kpH(M) ΔpH (1)
where kpH(M) represents the permeance of hydrogen andΔpH represents the difference in the hydrogen partial pressures at both sides. Measured hydrogen crossover flux NH(M) proved that the hydrogen permeation through the PEM and MEA was highly temperature dependent as shown in Fig. 1. Hydrogen crossover fluxes measured by linear sweep voltammetry and gas chromatography were in good agreement (at 60 oC, H2 partial pressure difference at 50 kPa). The apparent activation energy of permeance, kpH(M) was 40 kJ/mol and 18 kJ/mol for MEA and PEM, respectively as shown in Fig. 2. The apparent activation energy of the H2 permeance was remarkedly higher than the value expected for ordinary diffusion. It suggests that the polymer structure determines the H2 permeance and the total pressure is not likely to affect the H2 permeance.
Measurements using PEM and MEA (with several PEM thicknesses) showed that increasing the PEM thickness in MEA resulted in more resistance (1/kpH(M)) to hydrogen crossover, as expected. It should be noted that PEM and PEM in MEA showed different crossover resistance per unit thickness. The resistance of the bulk layer per thickness was calculated as more than three times higher than that of the skin layer per thickness. Heat treatment of PEM did not affect the crossover considerably (Fig. 3.).
Total gas permeation flux driven by the total pressure difference was calculated by subtracting contribution of partial pressure difference from the measured H2 flux, and plotted against the total pressure difference in Fig. 4. The total gas permeation flux increased cconsiderably arouund DP = 75 kPa regardless of the partial pressure difference. It suggests the membrane was fractured and no H2 permeation due to the total pressure difference was observed.
Conclusions
Increasing cell temperature activated hydrogen permeation (18–40 kJ/mol). Unlike a pristine PEM, an MEA containing the same membrane indicated ca. 2 times faster crossover. Contribution of molecular diffusion (effect of total pressure) to H2 permation was little . Contribution of the total pressure difference to the H2 permeation was not observed, but a great total pressure difference (75 kPa) fractured the membrane. At the higher moisture content of the membrane, the H2 permeance was the higher though the H2 permeance was higher than the minimum at extremely low moisture content in case of MEAs.
References
[1] T. Li et al., ACS Omega 5, 17628–17636 (2020).
[2] J. Zhang et al., J. Pow. Sour. 163, 532–537 (2006).
[3] X. Cheng et al., J. Pow. Sour. 167, 25–31 (2007).