Additives in proton exchange membranes for low- and high-temperature fuel cell applications: A review

Polymer electrolyte membranes, also known as proton exchange membranes (PEMs), are a type of semipermeable membrane that exhibits the property of conducting ions while impeding the mixing of reactant materials across the membrane. Due to the large potential and substantial number of applications of these materials, the development of proton exchange membranes (PEMs) has been in progress for the last few decades to successfully replace the commercial Nafion® membranes. In the course of this research, an alternate perspective of PEMs has been initiated with a desire to attain successful operations at higher working temperatures (120–200 °C) while retaining the physical properties, stability and high proton conductivity. Both low- and high-temperature PEMs have been fabricated by various processes, such as grafting, cross-linking, or combining polymer electrolytes with nanoparticles, additives and acid-base complexes by electrostatic interactions, or by employing layer-by-layer technologies. The current review suggests that the incorporation of additives such as plasticisers and fillers has proven potential to modify the physical and chemical properties of pristine and/or composite membranes. In many studies, additives have demonstrated a substantial role in ameliorating both the mechanical and electrical properties of PEMs to make them effective for fuel cell applications. It is notable that plasticiser additives are less desirable for the development of high-temperature PEMs, as their inherent highly hydrophilic properties may stiffen the membrane. Conversely, filler additives form an inorganic-organic composite with increased surface area to retain more bound water within the polymer matrices to overcome the drawbacks of ohmic losses at high operating temperatures.