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Molecules 2020, 25, 1712 28 of 44 Dai et al. [200] developed novel composite membranes consisting of carbon dots of different sizes and with varying levels of hydrophilicity within a matrix of polyvinylpyrrolidone (PVP) and polyethersulfone (PES). AFM and TEM characterization showed that carbon dots with a size of 2–5 nm showed no aggregation and good uniformity. Single cell tests at 150 ◦C and under anhydrous conditions revealed that the composite membrane had a higher peak power density in comparison to pristine PES-PVP, 166 to 113 mW cm−2, respectively. The idea of altering the hydrophilicity of the filler material is an interesting technique to improving the performance of the composite membrane.Ahmed et al. prepared a chitosan membrane with sulphonated multiwall carbon nanotube filler [201]. As chitosan has a lower proton conductivity than Nafion there is a greater need for using fillers to improve its proton conductivity. The mechanical strength and proton conductivity increased but water uptake decreased, and the authors explain that this is due to the decrease in -NH2 functional groups. It would be interesting to see how these membranes perform in a fuel cell in comparison to pristine chitosan and Nafion. 3.3. Acids and Ionic Liquids Fillers Ionic liquids have been extensively used in fuel cells operating at higher temperatures due to their high thermal degradation temperature. For example, Choi et al. fabricated two types of composite membranes doped with phosphotungstic acid, one with 1100EW Nafion and the other with 750EW [202]. At 120 ◦C, both the composite membranes performed better than the reference Nafion MEA, achieving a voltage of 0.51 and 0.55 V (1100EW and 750EW) at 400 mA cm−2, compared to 0.47 V for the reference Nafion. In addition, the ohmic resistance was smaller than that of the reference Nafion, at 0.32, 0.21 and 0.13 Ω cm−2 for Nafion, 1100EW composite and 750EW composite respectively. Lee et al. designed a composite membrane of a sulphonated polymer doped with fluorohydrogenate ionic liquid [203]. The ionic liquid was used due to their high thermal stability as the application was geared towards intermediate temperature operation with dry conditions. Single cell testing at 130 ◦C revealed an OCV of 1 V for 5 h. In addition, the ionic conductivity of the prepared composite membrane increased with temperature, from 11.3 mS cm−2 at 25 ◦C to 34.7 mS cm−2 at 130 ◦C. Ramani et al. [204] introduced heteropolyacids into Nafion for PEMFCs operating at higher temperatures and reduced relative humidity. Additives studied included phosphotungstic (PTA), silicotungstic (STA), phosphomolybdic acid (PMA) and silicomolybdic acid (SMA). Water uptake results revealed that there is no significant different between recast Nafion and the Nafion/PTA composite membrane at a range of relative humidities. Single cell tests at 120 ◦C and 35% RH of Nafion/PTA, Nafion/STA and Nafion/SMA showed relative performance, with Nafion/PTA and Nafion/STA reaching a maximum current density of around 800 mA cm−2. The same authors prepared a composite membrane of Nafion with a heteropolyacid (HPA), phosphotungstic acid (PTA) [205]. The MEA was “stabilized” via high temperature heat treatment (200 ◦C at 30 atm). In order to allow the membrane to not disintegrate and to prevent the HPA from dissolving, the MEA was ion exchanged in caesium carbonate, swapping the protons for much larger caesium ions. TGA experiments showed that this stabilized membrane degraded at higher temperatures in comparison to its proton exchanged counterpart. Fuel cell testing at 120 ◦C and at 35% RH showed that both the stabilized and reference membrane have similar polarisation behaviour. However, the specific area resistance was lower for the stabilized membrane and the authors explained that this is because of the lower contact resistance from the high temperature heat treatment. The work was followed by looking into the effect of extent of ion exchange. Composite membranes with 2, 1, and 0 protons left after substitution were prepared [206]. Weight loss measurements to assess the stability of PTA in Nafion were performed, with increasing proton substitution leading to less weight loss after protonation. Pristine PTA had a weight loss of around 27%, which decreased to less than 5% for the PTA modified to have its protons removed. Water uptake experiments interestingly showed that there is no difference between pristine PTA composite and the substituted protons. In addition, membranes were also ion exchanged using different cations,PDF Image | Composite Polymers for Electrolyte Membrane Technologies
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