A review of additional modifications of additives through hydrophilic functional groups for the application of proton exchange membranes in fuel cells

The challenges related to the proton exchange membrane (PEM) of proton exchange membrane fuel cells and direct methanol fuel cells are high swelling, loss of mechanical structure strength and high fuel crossover at elevated temperatures that cause low efficiency. The single polymer membrane may replace the commercial membrane with its dimensional stability and low swelling but it shows low proton conductivity. Hydrophilic additives such as polymers and nanomaterials increase the strength of the single polymer membrane internal channels through interaction and lockdown the swelling while maintaining proton conduction through the Grotthuss and Vehicular methods. However, these additives still have limited capability of enhancing attributes of PEM such as limited water absorption, proton conductivity and loose interaction with the single polymer membrane due to their limited or weak functional groups. Hydrophilic group functionalization such as sulfonic groups, phosphonic groups, amino acid groups, carboxylic groups, zwitterionic groups, silane groups and acid doping with additives further strengthen the walls of channels, prevent swelling and capture more water molecules that increase proton conduction. This review article describes the different types of functionalized additives, their application in the membrane of fuel cells, further improvements in these modified additives, application methods and future prospects.