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Energies 2020, 13, 6101 2 of 14 The recent approach for PEM development has shifted from more proton conductivity to more proton conductance (in other words, from less resistivity to less areal specific resistance) at low relative humidity and higher temperature. Thus, the development of thinner PEMs is crucial to minimize areal specific resistance by the trade-off between less thickness and mechanical/chemical stability. It directly results in a significant decrease in Ohmic losses which is an increase in stack power density and a decrease in material cost [10]. Mechanical reinforcement of thin PEMs (ca. 10–20 μm) is one of the approaches to overcome less mechanical/chemical stability by thinning the thickness. The reinforcement could be achieved by the development of porous substrate-reinforced composite membranes. It could be done only by filling high-conductivity ionomers into porous substrates or by forming a three-layered structure (ionomer/ionomer filled substrate/ionomer) [12–20]. Perfluorosuflonic acid (PFSA) ionomers are still the most frequently used material even though less expensive hydrocarbon membranes have been intensively developed [7,21–23] Among many reasons for the use of PFSA ionomers, the main one would be the good stability against mechanical and chemical stress occurring during fuel cell or water electrolysis operation. Since the less volume of PFSA is used compared to non-reinforced PFSA membranes, benefits can be obtained in terms of material cost [12,24,25]. To prepare reinforced composite membranes, PFSA ionomer dispersion normally in a mixed solvent of water and alcohols and poly(tetrafluoroethylene) (PTFE) porous substrate is often used due to high proton conductivity of the ionomers and polymeric compatibility with PFSA, respectively [26–28]. Nevertheless, the preparation process of porous PTFE-reinforced PFSA composite membranes is very difficult since not all the hydrophilic ionomer dispersions are compatible with hydrophobic PTFE porous substrates. The fabrication of the composite membranes with an incomplete filling of PFSA ionomers into the substrates causes them to lose mechanical strength and chemical stability as well as gas permeability [29]. However, few studies to discuss the effect of materials for hydrophilic treatment of porous substrates for the preparation of porous PTFE-reinforced PFSA composite membranes have been reported. The hydrophilization of the hydrophobic material surface could be attained by oxygen plasma, UV radiation, grafting, surface oxidation by strong acids, hydrolysis, coating, or lamination. Hydrophilization treatment with plasma or UV radiation is effective because it directly exposes energy to the hydrophobic material surface. However, the hydrophilization using plasma has a limitation in that the process must be performed in a vacuum state, and the one using long-term or strong UV radiation may damage the material surface to irreversibly change the properties of materials. In addition, there is a limit to completely hydrophilize the inner pores of porous substrates. Similarly, lamination is not good for materials with complicated structure. The coating is lower than the plasma and UV radiation in terms of durability, but it is much simpler than the aforementioned methods and inexpensive. Thus, it is frequently used in the industrial hydrophilization process [30]. Initially, the increase in hydrophilicity of PTFE surfaces was obtained by surfactants, but there are too many parameters to be considered for good wettability [31,32]. Recently, biomimetic materials have been deposited on porous hydrophobic microfiltration/ultrafiltration membranes for better water flux from the oil-in-water emulsion and protein wastewater [33]. It is found that the pyrogallol moiety in gallic acid (GA) with amino-terminated substances (ATS) such as siloxane generated a similar mussel-inspired adhesive coating via Michael addition/Schiff base reactions in alkaline conditions [34–36] Herein, an approach to increase the wettability of hydrophobic PTFE substrates is investigated by using the nature-born materials, i.e., mussel-inspired silicified polysiloxane adhesive materials, to overcome the aforementioned incompatibility between hydrophobic PTFE substrates and hydrophilic ionomer dispersions. Hydrophilization on porous PTFE substrates (porosity 40-90%) from the polymerization of GA with respect to the ATS, i.e., 3-aminopropyltriethoxysilane (APTES), N-[3-(trimethoxysilyl)propyl]ethylenediamine (TMPEDA), and (3-trimethoxysilylpropyl)diethylenetriamine (TMPDETA) is carried out, not on microfiltration (MF) or ultrafiltration (UF) which is less porous (porosity <40%) than PTFE. The properties of composite membranes using the porous substrates hydrophilically treated by GA and one of the amino-terminatedPDF Image | Composite Membranes Using Hydrophilized Porous Substrates
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