A N-o-sulphonic acid benzyl chitosan (NSBC) and N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO) based multi-functional polymer electrolyte membrane with enhanced water retention and conductivity

Abstract

To achieve good proton conductivity even under dehydrated conditions, water retention is a pervasive issue for polymer electrolyte membranes (PEMs). Herein, we report a green and easy preparation procedure for multifunctional PEM grafted with –PO₃H₂ and –SO₃H groups based on a functionalized biopolymer (N-o-sulphonic acid benzyl chitosan (NSBC)) and N,N-dimethylene phosphonic acid propylsilane graphene oxide (NMPSGO). Loading of NMPSGO in the NSBC matrix enhanced water retention, proton conductivity, stability and other desired properties of PEM relevant for DMFC applications. The reported NSBC/NMPSGO composite PEM was designed to promote internal self-humidification responsible for water retention properties, to promote proton conduction due to the presence of different acidic functional groups. It was hypothesized that strong hydrogen bonding between multi-functional groups due to the presence of inter-connected hydrophobic graphene sheets and organic polymer chains provides a hydrophobic–hydrophilic phase separation and suitable architecture for proton conducting channels. The most suitable PEM (NSBC/NMPSGO-8) exhibited 4.42% bound water content; 8.87 × 10⁻² S cm⁻¹ proton conductivity; 2.09 mequiv. per g ion-exchange capacity; and 16.93 × 10⁻⁷ cm² s⁻¹ methanol permeability. The proposed route offers a facile and generic strategy to design a variety of composite functional materials, in which both organic (NSBC) and inorganic (NMPSGO) were functionalized, with superior functional group molality, proton conductivity, water retention properties and stability.