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Molecules 2020, 25, 1712 22 of 44 was that the membranes were very thin for composites, with film thicknesses of around 15 μm before acid doping and 22 μm after and therefore hydrogen crossover tests would be interesting to perform to understand the difference in crossover between pristine and composite membranes. Ooi et al. investigated improving the acid retention and oxidative stability of PBI membranes operating at increased temperatures [168]. This was achieved by preparing a composite membrane which composed of partially fluorinated PBI and a filler of cesium hydrogen sulfate-silicotungstic acid (CsHSO4–H4SiW12O40, CHS-WSiA). The synthesised composite exhibited greater acid retention rates, which was attributed to the fluorinated PBI and the filler material. This retention was examined in a fuel cell 24 h stability test, where a voltage of 0.614 V at a constant current of 0.2 A cm−2 was produced with no drop. A longer test would be interesting to validate the durability of the membrane. Devrim et al. [169] prepared a silica polybenzimidazole (PBI) composite membrane for high temperature PEMFCs. The silica nanoparticles improved the acid retention and the proton conductivity. Proton conductivity results measured at 140, 165 and 180 ◦C revealed that the composite membrane had greater conductivity than pristine PBI, 0.0675 to 0.0600, 0.0866 to 0.0765, 0.1027 to 0.0944 Scm−1 for PBI/SiO2 respectively at the three temperatures. The addition of silica also reduced the degree of acid leaching from 41.5 (for pristine PBI) to 36.3% due to increased covalent bonding between the inorganic filler and acid. Single cell testing was also performed at the three temperatures previously stated, under hydrogen and air at 1 atmosphere. At 140 ◦C the pristine PBI outperformed the composite but at the two higher temperatures the composite membrane produced a greater maximum power density. The best performance was from the composite membrane at 165 ◦C, producing a maximum power density of 0.24 to 0.2 Wcm−2 for the pristine PBI, at 0.6 V. The authors have shown the novelty of using inorganic filler to retain acid in PBI for high temperature PEMFC applications. Plackett et al. [170] tested laponite clay as a filler in PBI for high temperature fuel cells. Two sets of fillers were prepared, by functionalising the clay with an imidazole group and another with quaternary ammonium group. Water uptake results showed that no difference was made when the organic filler was introduced in the PBI matrix, but the composite membranes did experience less acid swelling. The composite membranes achieved an OCV of 1.02 V (at room temperature, 0.96 V at 125 ◦C and 0.91 V at 200 ◦C), which implies low or almost non-existent hydrogen crossover, which was confirmed in permeability tests. Aili et al. [171] doped silica with phosphotungstic acid for use as a filler in phosphoric acid doped polybenzimidazole for high temperature PEMFCs. This composite had a lower swelling rate due to its lower uptake of phosphoric acid. Durability testing at 200 ◦C revealed that the composite membrane had a decay rate of 27 μV h−1, whereas the membrane without the filler decayed at a rate of 129 μV h−1. Other inorganic materials are used as they have the potential to improve the durability of the membrane. Rodgers et al. [172] used platinum nanoparticles as a filler to remove radicals formed during fuel cell operation and therefore reduce degradation. Membranes with 0, 10, 30, and 50 mol % of platinum were prepared, and their performance was evaluated in a 100 h fuel cell test at 90 ◦C and 100% RH. The highest degradation (through fluoride emission) was observed for the 10 mol % platinum composite. The authors explain that this is because of the low distribution and density of the platinum particles throughout the membrane. Pearman et al. [173] studied the influence of cerium oxide as a radical scavenger in PEMFCs. Two forms of cerium oxide were used as fillers within a PFSA polymer structure, a synthesised version with 2–5 nm sizing, and a commercial version with 20–150 nm. The addition of cerium oxide resulted in a 50% reduction in OCV decay rate (from a 94 h test), from 0.9 mV hr−1 for pristine Nafion to an average of around 0.4 mV hr−1. However, the weight percentage of cerium oxide seemed to make no difference in the decay rate. Electron microscopy images show that less platinum particles were present in the composite membrane in comparison to the recast. A 500 h OCV hold test with pre and post-test polarisation curves, depicted in Figure 6, demonstrated that the composite membranes had a much smaller deviation in polarisation compared to the baseline Nafion membrane. The authors followed this work up by studying the proton conductivity of the composite membranes [174]. Unfortunately,PDF Image | Composite Polymers for Electrolyte Membrane Technologies
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