ABSTRACT
The need for clean energy during the rapid growth of technology and global population has been made more urgent due to the exhaustion of fossil fuels, natural gas and increasing environmental damage caused by these fuel sources. Recently, proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC) and vanadium redox flow battery (VRFB) technologies have received considerable research interest due to their promise as environmentally benign electrochemical energy conversion devices. The proton exchange membrane (PEM) is crucial for the above-mentioned devices. In spite of extensive modifications made on state-of-the-art perfluorinated polymeric PEM materials such as Nafion® or Flemion®, which have decent physical and chemical stability—along with high proton conductivity under a wide range of relative humidity conditions and moderate operating temperatures—their high cost, limited operating temperature (0–80°C), insufficient durability and high fuel permeability pose serious challenges. In DMFCs and VRFBs, the Nafion membrane reduces cell performance through methanol and vanadium ion permeability, respectively. Extensive research efforts have been devoted in developing a modified Nafion or alternate hydrocarbon-based polymer nanocomposite membranes as promising alternative PEM. Since the early 2000s, sulfonated poly(arylene ether)s (SPAES, e.g., Sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(phenylene oxide) (SPPO), and sulfonated polyimides (SPI) have gained attention in moderate-temperature fuel cells, whereas polybenzimidazoles (PBIs) were investigated for high operating temperature PEMFCs. Sulfonated poly(arylene ether sulfone) membranes are reported to possess outstanding proton conductivity and good fuel cell performance compared to Nafion membranes, but they suffer from high swelling, that increases with the increasing degree of sulfonation, which is essentially responsible for the high proton conductivity. Since the development of nanomaterials, research has been recently triggered to develop novel, high-performance PEMs, in an attempt to take full advantage of the fascinating properties of these materials, such as high specific area, barrier effect, and good thermo-mechanical stability. Understandably, the dispersion of these nanomaterials inside the polymer matrix, which greatly affects the performance of the resultant materials, is always a major concern during the preparation process. Incorporation of inorganic nanomaterials such as (i) ZrO2, SiO2, TiO2, P2O5 and Zeolite nanoparticles, (ii) TiO2 nanotubes and nanowires, and (iii) 2D layered GO, MoS2 (Transition metal dichalcogenides), into the structure of the PFSA or hydrocarbon-based sulfonated polymers has been demonstrated to result in composite membranes with promising proton conductivity at high temperatures and low relative humidity. Modified Nafion and alternate polymer nanocomposite PEMs will be expected to render proton conductivity, reduced methanol crossover and vanadium ion permeability comparable to commercial Nafion as well as to pristine sulfonated hydrocarbon polymer membranes. Polymer nanocomposite PEMs are also expected to be comparable with Nafion in terms of their chemical, mechanical and thermal stability.
