Introduction At elevated temperatures under low humidification, the failure of the Nafion membrane is unavoidable and it leads to fuel/oxidant passing through the membrane yielding loss of fuel, undesired reaction of fuel and oxidant, and further damage to the membrane. The reaction of hydrogen and oxygen possibly results in formation of hydrogen peroxide. That causes degradation and pinhole formation and thus, membrane durability is affected at elevated temperatures. In the current study, the hydrogen crossover rate was measured under wide range of conditions.The hydrogen crossover flux, N H (M) [mol/(m2 s)], is expressed by Eq. (1) as a function of the difference in the partial pressure of hydrogen between the supply and permeate sides, Δp H, and the total pressure difference, ΔP, with the hydrogen permeance, k pH (M) [mol/(Pa m2 s)], and the gas permeation coefficient, k G (M) [mol/(Pa m2 s)], which are both functions of temperature, moisture content, and film thickness. N H (M)=k pH (M)Δp H+y H k G (M)ΔP (1)where y H represents the hydrogen mole fraction on the supply side. Experimental In the experiments, we used Nafion membranes of various thicknesses, NR-211 (25.4 µm), NR-212 (50.8 µm), and N-115 (127 µm), without any pre-treatment. Both the proton exchange membrane (PEM) and the membrane electrode assembly (MEA) samples were used in the testing cell. The catalyst layer thickness was 10 µm with a platinum loading of 0.35 mg/cm2. Our design featured a serpentine gas channel. Hydrogen diluted with nitrogen (H2:N2) and pure N2 were supplied to the supply side and the permeate side, respectively. To measure hydrogen permeation, micro gas chromatograph (Agilent 990 micro GC) connected to the outlet of the permeate side was employed. Results and Discussion Fig. 1(a) shows the N H (M) vs. Δp H profiles measured with NR-212. They displayed a linear tendency at ΔP = 0.5, 25, 50, and 75 kPa. k pH (M) was represented by the slope of N H (M) vs. Δp H. ΔP did not influence the total permeation flux up to 50 kPa as shown in Fig. 1(b). When ΔP increased above 50 kPa, N H (M) increased by approximately 10 %, which resulted from increase in the contribution of the total pressure difference, y H k G (M)ΔP, shown in Fig. 1(c). It means the flux was determined by the partial pressure difference; the total pressure difference did not remarkably contribute to the permeation flux.By modeling the membrane as a bulk layer sandwiched with skin layers, k pH (M) of PEM is expressed by Eq. (2),1/k pH (M)=δ(B)/P H (B)+2δ(S)/P H (S) (2)where the bulk layer thickness and the skin layer thickness are denoted by δ (B) and δ (S), respectively. P H (B) and P H (S) respectively represent the hydrogen permeability through bulk and skin layers. As shown in Fig. 2(a), PEM had the permeation resistance even at zero membrane thickness, which represents the permeation resistance of the thin skin layers. As shown in Fig. 2(b), however, MEA had no skin layers, and the permeation resistance, 1/ k pH (M), of PEM was approximately 1.5 times higher than that of MEA. It suggested the skin layers of PEM were fused with ionomer in catalyst layers (CLs) during hot pressing, which annihilated the skin layers.The slope in Fig. 2(a) gives P H (B) while the intercept gives the H2 permeance of the skin layer, k pH (S) = P H (S)/δ (S). They were found to be power-law functions of the moisture content at each temperature as shown in Figs. 3(a) and 3(b). P H (B) of PEM with and without CLs were substantially equal as shown in Fig. 3(a). The measured P H (B) was formulated as, P H (B) = (1.72×10– 14 mol Pa– 1m– 1 s– 1×λ 0.339) exp[–(22.8 kJ mol– 1/ R){1/T – 1/(353 K)}]Recalculated P H (B) is also shown in Fig. 3(a).The H2 permeance of the skin layer, k pH (S), was formulated as, k pH (S)= (2.71×10– 9 mol Pa– 1 m– 2s– 1×λ 0.858) exp[–(12.3 kJ mol– 1/R (1/T – 1/(353K)}]These equations reproduce the measured data accurately as shown in Figs. 3(a) and 3(b). Conclusions Hydrogen permeance through the bulk layer and the skin layer of PEM was obtained by measuring the permeation flux. The equations estimating them were determined as functions of temperature and moisture content of the membrane. Whereas the bulk layer showed similar behaviors both in PEM and MEA, the skin layer of MEA was not identified. To estimate the H2 crossover in an actual PEFC, the H2 permeance of the PEM should be measured using an MEA instead of a standalone PEM. Figure 1
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Jan 8, 2025
Wen Di Zhang + 1
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