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Molecules 2020, 25, 1712 7 of 44 interaction between the sulphonate groups of Nafion and polypyrrole, decreased the pore volume of Nafion membrane which led to low methanol permeability. However, the electrostatic interaction between the polypyrrole chains and sulphonate groups of Nafion also decreased the proton conductivity and therefore increased membrane and cell resistances. So, the benefit of the reduced methanol crossover was neutralized particularly when DMFC worked at high current densities. Polyaniline is a good conductive polymer that can improve both methanol oxidation and the stability of the catalyst; it can also be included in the Nafion structure through both electrochemical and chemical modification to improve its properties. Wang et al. [65] and Escudero-Cid et al. [66] assembled a composite membrane of Nafion/polyanilina and carried out DMFC performance tests including polarization curve and durability tests showing that both the ionic conductivity and methanol permeability of the Nafion membrane containing PANI decreased when compared with Nafion membrane. In particular, Wang et al. indicated that the performance of the fuel cell increased using the modified membrane especially at high methanol concentration (maximum power output at 6 M) while the power output using Nafion pristine membrane was reduced with increasing methanol concentration. Moreover, it was noted that the PANI composite membrane performed better than that with polypyrrole [67]. It is important to highlight here that the reviewed papers; investigating the use of conductive polymers, do not consider the change in the electronic conductivity of the membrane (short circuit current) in the composite membrane. This should be considered in any future work on these materials. In recent years, two types of sulphonate fillers, sulphonate poly arylene ether ketone (SPAEK), and sulphonate poly ether ether ketone (SPEEK) have been developed and used to modify the Nafion membrane in DMFCs. Both SPAEK and SPEEK have good attributes: high proton conductivity and methanol resistance for SPAEK [68,69]; good mechanical properties, proton conductivity and good processing capacity of SPEEK polymers [70,71]. Regarding the behaviour in a methanol fuel cell [72,73], an increase by at least 30% in OCV and by 10% in highest power density were observed. The positive results suggest that these membranes could be taken into account for use in future DMFC application once durability is deeply investigated. 2.1.2. Inorganic Fillers The implementation of inorganic particles into Nafion helps enhance the thermal stability and proton conductivity of composite membranes making them more attractive and appropriate than bare Nafion membranes [74]. This section focuses on silica, metal oxides, montmorillonite and zeolite fluorinated composite membranes; materials that have received considerable attention due to their chemical and thermal properties. Silica materials have attracted a considerable attention because they possess high surface area and high chemical stability [75]. Generally, they are prepared using different precursors such as alkoxy silanes (like tetraethyl orthosilicate (TEOS)), sodium metasilicate and fumed silica [76]. The addition of silica compounds into polymer membrane is believed to lower methanol crossover [77,78]. In this context, Ren et al. [79] prepared a composite membrane modifying Nafion polymer with tetraethyl orthosilicate (TEOS) and sulfonic TEOS using the casting method. They investigated the influence of silica into Nafion matrix and the changes in proton conductivity, methanol permeability and performance. The results showed that the proton conductivity of these composite membranes was lower than that of commercial Nafion membranes due to the hydrophobic side chain of the TEOS that reduced the water content of the membrane. However, the methanol permeability was also reduced. DMFC single cell tests were carried out at both 1 M and 5 M and at 75 ◦C. The polarisation curves depicted how the silica composite membrane could achieve better performance than the Nafion when using high methanol concentration because although the proton conductivity of the composite membrane decreased, the methanol permeability also reduced. Works are necessary to increase proton conductivity of those composite membranes.PDF Image | Composite Polymers for Electrolyte Membrane Technologies
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