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Molecules 2020, 25, 1712 26 of 44 Ibrahim et al. [187] studied the behaviour of GO composite membranes fabricated via solution casting with different thicknesses at intermediate operating temperatures. The composite membranes had improved mechanical strength and a higher water uptake in comparison to pristine Nafion. In-situ fuel cell testing of the membranes as MEAs revealed that the 30 μm composite membrane at 100 and 120 ◦C outperformed the 50 μm Nafion membrane at 80 ◦C. This is most likely due to the reduction in thickness and the GO filler retaining more water, hence reducing the drop in proton conductivity. Kumar et al. [188] sulphonated GO and incorporated it into the polymer matrix of SPEEK. The SGO improved the water of SPEEK from 57.58% to 60%. In addition, the composite membrane outperformed SPEEK at temperatures from 30 ◦C to 80 ◦C (at 100% RH) and at 80 ◦C (with varying RH from 30 to 50%). Fuel cell testing at 80 ◦C, 30% RH humidified hydrogen and dry oxygen showed that the composite membrane produced a maximum power density of 378 mW cm−2, a large increase in comparison to SPEEK which produced 250 mW cm−2. Sulphonated carbon nanotubes were used as a filler within a SPEEK matrix to offset the effect of high levels of sulphonation compromising the durability of the membrane [189]. The composite membrane had better proton conductivity and fuel cell performance compared to its pristine counterpart. A point of consideration is that the filler was functionalised to prevent the disruption of the proton transport channels, which is something that should be considered when incorporating a filler. Uregen et al. [190] fabricated a graphene oxide/polybenzimidazole membrane for the operation at high temperatures. The introduction of graphene oxide improved the proton conduction in comparison to pristine PBI as well as reducing the quantity of acid leaching (from 85% for pristine PBI, to 70% for the composite membrane). Fuel cell testing at 165 ◦C and with dry hydrogen and air revealed that the pristine PBI and GO composite membrane had maximum power densities of 0.31 and 0.38 W cm−2 respectively. However, the authors noted that there could potentially be degradation of the GO functional groups at operating temperatures above 165 ◦C. A 500 h durability test showed that the performance loss of the composite membrane was lower, at 3.8% in comparison to 8.3% for the PBI membrane. This could be due to the reduced hydrogen crossover and acid leaching. Xue et al. [191] decided to functionalise their graphite oxide, once again in a PBI polymer matrix for high temperature PEMFCs. Isocyanate functional groups were modified onto graphite oxide to improve the dispersion in water and organic media. This resulted in greater proton conductivity and less swelling. A similar study but with the GO sulphonated was studied by Xu et al. [192]. The proton conductivity of the membranes was increased from 0.023 S cm−1 for pristine PBI to 0.027 S cm−1 for GO/PBI and 0.052 S cm−1 for SGO/PBI. The respective activation energies for proton conduction fell from 16.1 kJ mol−1 to 11.4 kJ mol−1 to 9.3 kJ mol−1 respectively. Fuel cell testing at 175 ◦C and under anhydrous conditions with hydrogen and oxygen showed that the addition of GO or SGO result in an increase in maximum power density, from 0.22 for PBI, to 0.38 for GO/PBI and 0.6 W cm−2 for SGO/PBI. The same trend was observed under air. This work was followed by the same authors studying the same filler and polymer but this time functionalised the GO with an ionic liquid (1-(3-Aminopropyl)-3-methylimidazolium groups) [193]. The composite membrane had a higher proton conductivity in comparison to the reference PBI membrane. Fuel cell tests at 175 ◦C with dry inlet fuel showed that the addition of the ionic liquid improved peak power densities from 0.26 W cm−2 for PBI to 0.32 W cm−2 for the composite. The authors stated that this is due to the improved proton conduction within the composite membrane.Abouzari-Lotf et al. [194] designed a composite membrane for high temperature fuel cells by combining PBI that has been functionalized with 2,6-Pyridine with phosphonated grapene oxide. The use of the filler was in order to reduce the extent of acid leaching and to increase long term stability as increasing acid content can mechanically compromise the polymer. The addition of 1.5% phosphonated graphene oxide significantly increased the proton conductivity from 19.6 × 10−3 S cm−1 for pyridine PBI to 76.4 × 10−3 S cm−1 at 140 ◦C. Kannan et al. [195] presented a composite PBI membrane consisting of phosphonic acid functionalised multi-walled carbon nanotubes as the filler material. Proton conductivity tests revealed that the composite membrane achieved 0.11 S cm−1, whereas the pristine PBI produced a conductivityPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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