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Molecules 2020, 25, 1712 16 of 44 A composite membrane with Nafion and a filler consisting of silica nanoparticles with surface modified fluoroalkyl functionalities was presented in [139]. It was noted that although the silica nanoparticles are hydrophobic, the water uptake of the membrane was not negatively affected. In addition, the composite membrane showed thermal stability up to temperatures of 240 ◦C. Proton conductivity tests revealed that the composite membrane with 5 wt. % silica nanoparticles with a ratio of [Nafion/ (Si80F) 0.7] had the highest conductivity at 0.083 S cm−1 at 135 ◦C. Following from this, a single cell test was performed with the composite membrane at 85 ◦C. The composite membrane displayed a better power density compared to the recast Nafion, when the oxidant is air and oxygen (Under air: 0.38 vs 0.27, under oxygen: 0.48 vs 0.35 W cm−2 composite and recast Nafion respectively). It would be interesting to see the behaviour of the membrane at elevated temperatures. Following on from their work, Griffin et al. [140] fabricated and characterised a composite membrane with sulphonated zirconia dispersed in a Nafion matrix. The idea behind this is that functionalising the zirconia with sulphonic groups would boost the proton conductivity of the membrane. Proton conductivity tests at 120 ◦C and under anhydrous conditions revealed that the membrane had a conductivity of 3 × 10−3 S cm−1. This makes the membrane ideal for fuel cell applications at intermediate temperatures and under dry condition. Saccà et al. [141] studied the influence of zirconium oxide as a filler material at different loadings of 5, 10 and 20% for Nafion composites. Recast Nafion membranes had a water uptake of 20%. The addition of zirconium oxide led to an increase in water uptake to 24, 24, and 30% for loadings of 5, 10 and 20% respectively. Fuel cell testing of the membranes in a single cell at operating temperatures of 80, and 110 ◦C show that at 80 ◦C, addition of 5% of filler makes very little difference in performance compared to recast Nafion. However, the membrane with 20% filler had a much lower potential, potentially due to excessive water uptake at 80 ◦C. Composite MEAs with 10% filler produced higher polarisation compared to Nafion at both temperatures with a maximum power density of 400 mW cm−2 was achieved at 130 ◦C, 85%, and 0.5–0.6 V. D’Epifanio et al. [142] took this one step further and sulphonated the zirconium oxide. Water uptake experiments at 25 ◦C with varying relative humidity revealed that both composite membranes outperformed recast Nafion at all relative humidities (30 to 100%). Polarisation curves at 70 ◦C and at three different RH (65, 83 and 100%), show that the composite membrane produced better current densities at all voltage ranges, with current densities of 1015 mA cm−2 vs 680 mA cm−2 at 0.6 V, respectively. The difference between the two polarisation curves was emphasized during the ohmic and mass transport region, showing that the filler reduced ohmic resistance and improved water diffusion. A final test at 30% RH showed even greater differences with current densities of 930 mA cm−2 vs only 200 mA cm−2. Alberti et al. [143] attempted to improve the proton conductivity and stability of membranes at elevated temperatures and studied the effect of doping Nafion with zirconium phosphate. However, they found that the conductivity decreases with increasing filler loadings. In addition, the authors explained that the difference in proton conductivity between Nafion and their composite membrane is mostly at lower relative humidities and higher filler loadings. On the other hand, Sahu et al. embedded silica nanoparticles into Nafion via a sol-gel method [144]. Single cell tests at 100 ◦C and at 100% RH showed the composite membrane (doped 10 wt. % silica) produced a peak power density of 350 mW cm−2. Moreover, composite membranes with 15 wt. % experience large mass transport losses due to flooding. Costamagna et al. [145] then prepared zirconium phosphate Nafion composite membranes via the impregnation of Nafion 115 and recast Nafion for high temperature PEMFC use. The composite membrane from Nafion 115 produced a current density of 1000 mA cm−2 at 0.45 V and at an operating temperature of 130 ◦C, which is much better compared to 250 mA cm−2 pristine Nafion. In addition, the cell fabricated from recast Nafion reached current densities of 1500 mA cm−2 at the same operating conditions.PDF Image | Composite Polymers for Electrolyte Membrane Technologies
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