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Molecules 2020, 25, 1712 24 of 44 that these functional groups are easily accessible. The inclusion of filler materials can also improve the mechanical strength of the composite membrane. Kumar et al. [179] prepared a GO/Nafion membrane for PEMFCs operation. Addition of GO in 2, 4, and 6% loading to recast Nafion led to a subsequent increase in water up from 21.1 to 27.9, 37.2 and 36.1% respectively. Additionally, IEC changed from 0.891 to 1.21, 1.38 and 1.26 meq g−1 respectively. The authors argued that there is an optimum quantity of filler and any addition would result in increased membrane stiffness and subsequently reduced water uptake. Fuel cell tests at 100 and 25% RH show that the 4% GO composite membrane outperformed the reference recast Nafion by nearly 4 times (212 mW cm−2 to 56 mW cm−2). Sahu et al. [180] instead functionalised graphene with sulfonic acid groups inside a Nafion matrix for low relative humidity operation. This is interesting as the use of graphene oxide as a filler is due to its abundant oxygen containing functional groups, which make it more hydrophilic. This is in comparison to graphene, which is hydrophobic and hard to disperse in water, however the sulfonation procedure would have reduced the hydrophobicity of the graphene filler. This is shown in the water uptake and IEC tests. Recast Nafion has a water uptake of 20.1%, the addition of graphene slightly increases it to 21.4%. However, the introduction of sulphonated graphene, in 0.5, 1 and 1.5% loading results in improved water uptakes of 24.5, 27.3 and 29.2% respectively. The IEC values are: 0.88, 0.89, 0.92, 0.96 and 0.95 meq g−1 respectively. A similar trend was also observed with the proton conductivity, with the 1% sulphonated graphene having the best performance, which is also hinted at by it having the highest IEC. Fuel cell testing at 70 ◦C and 20% RH revealed that the composite membrane with sulphonated graphene (1%) produced a maximum power density of 300 mW cm−2, whereas recast Nafion and Nafion-graphene (1%) produced peak power densities of 220 mW and 246 mW cm−2 respectively. Lee et al. [181] prepared Nafion/GO and a novel Pt on graphene/Nafion composite membranes for low humidity PEMFCs. The idea behind using platinum on graphene as a filler is to use platinum as a reaction site to produce water and “self-humidify” the membrane. Water uptake experiments showed that the GO composite membrane outperformed the pristine Nafion sample. In comparison, the Pt/Graphene filler led to a drop in water uptake. The authors explained that this is because of the less hydrophilic nature of platinum as well as the GO being reduced to graphene in the synthesis step. However, the Pt/Graphene membrane had a greater proton conductivity compared to the other two membranes, which was explained via the electronic tunnel effect. The GO composite had a lower proton conductivity due to the filler impeding the ionic pathways, but this issue was resolved when the loading was greater than 3%, resulting in an increase in proton conductivity. The GO/Nafion membrane was tested at 80 ◦C and under a range of RH. At 40% RH, the peak power densities of the membranes with different GO loadings were all around 0.5-0.6 W cm−2. On the other hand, the Pt/Graphene membrane gave disappointing current output under anhydrous conditions, with peak current densities of 0.27, 0.36 and 0.14 A cm−2 for 0.5, 3 and 4% loading respectively. The authors followed up this work with designing a composite membrane with platinum on graphene in addition to silicon dioxide to improve the “self-humidifying” capabilities of the membrane by using the silica to retain the water produced by the platinum-graphene [182]. The water uptake and proton conductivity of these novel membranes increased with increasing silica content. Maximum water uptake of 30% was achieved with 3% Pt-G and 3% silica content. Fuel cell experiments showed that the addition of silica improved the polarisation curve. However, performance dropped with too much silica at low RH, which the authors explain is possibly due to the filler blocking the ionic pathways. Filler optimisation was concluded by the authors, as increases in Pt-G loading also resulted in a drop in performance. Yang et al. [183] fabricated a composite membrane with platinum deposited on titania, which is then incorporated with graphene oxide into a Nafion polymer matrix. The composite membranes displayed a better IEC than recast Nafion, with increased until 20% GO is reached, where the IEC began to decrease beyond that. The proton conductivity followed a similar trend to the IEC, which alsoPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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