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Molecules 2020, 25, 1712 5 of 44 Also, in PEMWE, the commercial Nafion membrane is commonly used as a solid electrolyte due to its chemical and thermal stability, good proton conductivity, and mechanical strength [36]. A drawback of membranes made of Nafion is that they are known to lose water, and thus ionic conductivity, at temperatures above 100 ◦C [37], which prohibits them from being used for higher temperature water electrolysis. However, high operating temperature offers several advantages from enhanced electrode kinetics and reduced overpotentials [38]. Moreover, higher operation pressure would also be favourable for PEM electrolysis since it would reduce the gas pressurization constraints for storage purposes [39]. Hence, it would be important to develop membranes that can sustain high performance at higher operating temperature and pressure [40]. To overcome the drawbacks of Nafion membranes, novel membranes have been developed and can be classified into three main categories, namely; (i) polymeric, (ii) ceramic, and (iii) composite membranes. Among these three categories, composite membranes have generated great attention recently. A composite (or hybrid) material can be defined as a material that includes two or more blended compounds on the molecular scale [41]. The use of filler material mixed into the Nafion (or an alternative ionomer) can aid in providing additional properties such as mechanical reinforcement, chemical resistance and proton conductivity. For example, hydrophilic fillers would result in increased membrane water uptake, ideal for low relative humidity (RH) operation. These filler materials can also be functionalised to provide secondary functionalities (e.g., sulphonating a hydrophilic filler) or boost the functionality it already has. Another example is the use of cerium oxide as a radical scavenger to slow down membrane degradation [42]. This review paper aims to provide a summary and analysis of the published work focusing on the development of composite membranes to improve the performance of the DMFC, modify operating conditions and enhance durability for PEMFC and PEMWE. 2. Composite Membranes for DMFC In this part, the range of composite membranes that have been developed to improve the performance of the DMFC at low temperature with reduced methanol crossover and low cost is reviewed. Two categories of composite membrane materials are considered in the literature; modified Nafion; and non-perfluorinated polymers. 2.1. Composite Nafion-Based Membrane Composite Nafion membranes can be loaded with organic and inorganic fillers that have been used predominantly to increase proton conductivity and to act as a barrier to methanol crossover [43,44]. The following sub-sections discuss the latest developments in the organic, inorganic and carbon nano-material filler-based membrane. 2.1.1. Organic Fillers Organic materials are commonly used as fillers in the polymeric composite membrane for fuel cells. They supply reinforcement and allow higher stability of the polymer matrix while making it more cost-effective. One of the most commonly applied organic filler is polytetrafluoroethylene (PTFE). PTFE is highly hydrophobic and although it is not suitable alone for membrane application for fuel cells [45], it can be used as a reinforcement of Nafion membrane due to its chemical stability, corrosion resistance and mechanical strength [46]. Few papers focused on testing DMFC performance using Nafion/PTFE membranes. Lin et al. [47] conducted a study on the application of this composite membrane, the authors investigated the effect of this polymer on conductivity, methanol crossover, and cell performance, and compared them with that of commercial Nafion membrane. Experimental data indicated that introducing PTFE into the Nafion polymer reduced both methanol diffusion and methanol electro-osmosis crossover in the membrane. The comparison between Nafion 117 and Nafion/PTFE was performed in a DMFC at 70 ◦C: Nafion/PTFE membrane was able to operate inPDF Image | Composite Polymers for Electrolyte Membrane Technologies
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