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Abstract
Currently, Nafion – a proton exchange membrane based on polymer fluorosulfonic acid ionomer has become dominant in the market owing to its high ionic conductivity, physical and chemical stability at moderate temperatures.
However, it is strongly dependent on water content to maintain high proton conductivity. Therefore, it cannot be used in several applications such as metal hydride-air batteries in which water is detrimental. In this thesis, proton exchange membranes consisting of heteropoly acid and polyvinyl alcohol were investigated to achieve high proton conductive polymer electrolytes working at room temperature and low humidity conditions. The encapsulation of heteropoly acid into metal-organic framework MIL-101 was also studied to enhance proton conductivity.
First, heterophony acids as proton donors were shaped into a membrane by incorporating with polyvinyl alcohol which functions as structural support. At room temperature and 25 % relative humidity, the proton conductivity of SiWA/PVA membranes was 1.2 x 10-2 S.cm-1 which is close to the value of Nafion membrane at fully hydrated condition.
However, the proton conductivity of the SiWA/PVA membrane was reduced to 0.5 x 10-3 Scm-1 after being heated to 80 °C which suggests some leaching of heteropoly acid and water molecules during operation. To solve this problem, silicotungstic acid was encapsulated into mesoporous chromium (III) terephthalate metal-organic framework (MIL-101) in an attempt to prepare high proton conductivity membranes (MIL-101/SiWA/PVA), which can operate at room temperature and low humidity conditions. This novel method was proven successful as the proton conductivity increased to 2.4 x 10-2 S.cm-1 at 25 °C and 25 % Relative humidity. This result is two times higher than the proton conductivity of SiWA/PVA membrane. Under low level of hydration, when the MIL- 101/SiWA/PVA membranes were dried at 80 °C, the proton conductivity experienced a reduction from 2.4 x 10-2 to 1.9 x 10-2 S.cm-1 which is also higher than that of SiWA/PVA membranes at the same conditions. This means that by immobilising heteropoly acid (i.e. SiWA) in MIL-101, the proton conductivity can be improved. This is due to continuous channels provided by MIL-101 facile proton transport even at low level of hydration.