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Molecules 2020, 25, 1712 20 of 44 The OCV also ranged from 0.97 to 1.03 V, indicating low crossover. Similar to the zirconium oxide nanotube, the titanium oxide nanotube composite displayed greater current densities at lower voltages. A 100 h stability test at 0.5 V, 80 ◦C and 18% RH showed that the composite membrane’s maximum power density decreased from 470 to 442 mW cm−2, whereas Nafion 212 only managed to produce a maximum power density of 55 mW cm−2 which degraded to 22 mW cm−2 after 100 h, an impressive difference in performance. Jun et al. [157] then fabricated a Nafion composite with functionalised titanium oxide nanotubes and 3-mercaptopropyl-tri-methoxysilane (MPTMS) was used to functionalise the inorganic filler, to further improve proton conductivity. Nanotubes are a promising filler due to their high surface area and internal space, in addition to providing mechanical strength. In addition, the water uptake was greater, with 27.2 to 23.7%, for functionalised nanotubes to functionalised nanoparticles, respectively. Proton conductivity measurements at 120 ◦C and varying relative humidities show that the functionalised titania nanotubes exhibited greater conductivities than recast Nafion, at all humidities, with the deviation being greater at lower RH. A Nafion composite comprising of porous zirconium oxide nanotubes were fabricated by Ketpang et al. for the purpose of high temperature PEMFCs [158]. The tubular structure of the filler was used to improve water transport, which should result in improved water uptake and proton conductivity. The performance of these composite membranes was tested at 80 ◦C at varying relative humidities of 100, 50 and 18%. It was found that the addition of the filler resulted in improved power densities at 0.6 V, implying that the filler lowers the ohmic resistance. In addition, the composite membrane revealed greater current densities at low voltages (0.3 V), this was explained due to the more efficient back diffusion from the cathode to the anode, mitigating flooding. A further 200 h durability test of the membrane (with 1.5 wt. % of filler) at the same operating conditions (80 ◦C and 18% RH) displayed a small decrease in OCV, from 0.99 to 0.92 V after 200 h. The authors have shown that use of a porous nanotube morphology can improve water transport and have potential advantages in low relative humidity application. Research into composite membranes extended beyond of the use of Nafion to other proton-conducting polymers. Marani et al. decided to combine sulphonated poly(ether ether ketone) (SPEEK) with titania nanosheets (an alternate material structure) for the application in PEMFCs operating at temperatures of 140 ◦C [159]. The authors studied the effect of treating the composite membranes with either water or with acid prior to use in addition to the effect of inorganic filler loading. It was found that acid treated membranes (with the lower filler loading of 1.67%) had the greatest proton conductivity in comparison to pristine SPEEK, with values of 4.14 × 10−2 Scm−1 at 140 ◦C and at 100% relative humidity to 1.76 × 10−2 Scm−1, respectively. This is because the acid washing displaced the tetrabutylammonium (TBA+), which was used to create the stable suspension of Titania nanosheets. However, acid treated membranes with higher loading displayed a porous structure and extreme swelling indicating chemical instability and high degradation rate. Devrim et al. [160] fabricated a composite membrane with titanium oxide and sulphonated polysulfone as the polymer matrix. The degree of sulphonation of the polymer was varied and higher levels of sulphonation led to a higher water uptake, with a sulphonation degree of 15% providing a water uptake of 7%, compared to 33% for a sulphonation degree of 40%. Adding titanium oxide to the sulphonated polymer (40% sulphonation degree) resulted in a drop in water uptake, to 29%. The authors explain that this is because the introduction of the filler reduces the membranes’ free volume and ability to swell sufficiently. Proton conductivity values increased with increasing levels of sulphonation and temperature, with the composite of 40% sulphonated polysulfone/titanium oxide producing a conductivity of 0.098 S cm−1. Single cell tests at varying operating temperatures from 60 to 85 ◦C reveal that the pristine sulphonated polysulfone undergoes excessive swelling above 70 ◦C, leading to lower power output. The composite membrane outperforms the pristine reference membrane as the filler provides mechanical reinforcement to the membrane, preventing excessivePDF Image | Composite Polymers for Electrolyte Membrane Technologies
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