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Molecules 2020, 25, 1712 17 of 44 Furthermore, Sahu et al. [146] presented a Nafion composite membrane with mesoporous zirconium phosphate as the filler, prepared via a co-assembly method. The single cell testing was performed at 70 ◦C and at varying relative humidities, 100, 50, 31 and 18%. The difference between the composite membrane and pristine Nafion membrane increases with decreasing RH (via the maximum power density peaks). In terms of filler loading, the best performing was 5 wt. %, followed by 10 and 2.5. At 18% RH, the composite membrane produced a maximum power density of 353 mW cm−2, in comparison to pristine Nafion’s 224 mW cm−2 (both at 500 mA cm−2). Pineda-Delgado et al. [147] decided to study the behaviour and performance of Hafnium oxide Nafion composite membranes. The fabricated composite membranes displayed greater water uptake of 61% at 100 ◦C compared to 29% for recast Nafion. This improvement in water uptake led to better proton conductivity at 100 ◦C with 112 vs 82 mS cm−1, for the composite and recast Nafion respectively. Following from this, the authors decided to test their membranes in a single cell set up at operating temperatures of 30, 50, 80 and 100 ◦C. The recast Nafion achieved a greater maximum power density at 30 and 50 ◦C, but at 80 and 100 ◦C the composite membrane performed better. At 100 ◦C, the composite produced a maximum power density of 0.336 W cm−2 compared to 0.188 W cm−2 for the recast Nafion, at a voltage of 0.46 V. The performance of sulphonated silica Nafion composites where assessed where the filler was synthesised with a simple sol-gel calcination process [148]. Optimisation studies revealed that a 1% filler was the optimum loading, outperforming 0.5, 1.5% and recast Nafion. In-situ fuel cell testing under reduced humidity also confirmed the initial ex-situ results. The authors attributed the enhanced performance to efficient proton transport due to the well-defined phases in the membrane structure which was seen with TEM. One method to improve the dispersion of filler material within the polymer matrix is to swell the polymer membrane in a solution of the filler [149,150]. Xu et al. employed this technique by swelling the Nafion membrane with silica to achieve a composite membrane, in comparison to the traditional solution casting technique. They highlighted that this method maintains the ordered nanophase-separation structure of Nafion. This was shown in water uptake tests, where the swelled composite showed a higher water uptake but lower swelling, in comparison to the recast membrane. Fuel cell testing at 110 ◦C and 20% RH showed that the swelled composite produced a maximum power density of 113 mA cm−2, in comparison to 80 mW cm−2 for recast Nafion with no filler. The performance was explained due to the lower internal resistance of the composite membrane.Saccà et al. [151] introduced titanium oxide of different loadings (5, 10 and 15 wt. %) into Nafion for the purpose of operating fuel cells at a reduced humidity. SEM images revealed that the dispersion of filler throughout the cross-section of the membrane show that the lower filler loadings are better dispersed. The higher loading membranes showed the presence of filler agglomerates. Water uptake testing at different temperatures showed that there is a small drop initially when the filler material is introduced. In addition, the higher loading membranes are less influenced by the increasing temperatures. A similar trend was also observed for swelling, with the composite membranes having lower swelling percentages. However, excessive introduction of filler material can result in the membrane becoming stiffer and more fragile. Fuel cell testing revealed that the 10 wt. % composite membrane was the best performing, with it being closest in polarisation behaviour to recast Nafion. Saccà et al. [152] continued their work by studying the characteristics of Nafion-Titanium oxide membrane for PEMFCs operating at medium temperatures. Introduction of 3 wt. % of titania powder increased the water uptake from 20% for recast Nafion to 29%. The composite membrane outclassed commercial Nafion at all fuel cell operating temperatures (80, 90, 110 and 130 ◦C). At 110 ◦C (and 0.56 V), maximum power densities were 0.514 W cm−2 and 0.354 W cm−2 for the composite and commercial membrane respectively. As well as the better polarisation performance, the cell resistance of the composite membrane decreased with temperature up until 110 ◦C, where it starts to increase (0.106 Ω cm−2). This is in comparison to the commercial membrane whose resistance begins to increase after 100 ◦C (0.088 Ω cm−2). Interestingly, experiments with steam reforming fuel (withPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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