Nanoporous Covalent Organic Framework and Polybenzimidazole Composites for Proton Exchange Membranes

Fabricationand design of proton conduction nanochannelswithinthe solid electrolyte materials is pivotal and challenging in orderto develop an efficient proton exchange membrane (PEM) for the usein fuel cells. To address this, we have synthesized a melamine-basedSchiff base network type porous covalent organic framework (MCOF)and impregnated phosphoric acid (H3PO4) as theelectrolyte into the pores of the MCOF via the vacuum-assisted method.Unfortunately, a stable membrane did not form from H3PO4-loaded MCOF (P@MCOF), and hence, in order to make a strongmembrane, mixed matrix membranes (MMMs) were fabricated using P@MCOFas nanofillers and [2,2 & PRIME;-(m-phenylene)-5,5 & PRIME;-benzimidazole]or m-PBI as the membrane forming polymer matrix.Formation of acid base pair occurred in the m-PBI-P@MCOFnanocomposite membrane owing to H-bonding interactions between thefiller and polymer. Also, the acidic functionalities in the poresof P@MCOF provides abundant sites for labile proton transport, whichenables uninterrupted proton conduction ion channels with low energybarrier in the nanocomposite membranes. Furthermore, all the compositemembranes were immersed and loaded with phosphoric acid (PA) to increaseelectrolyte contents in the resulting MMM-based PEM. Superior protonconductivity, excellent thermal, thermo-mechanical and tensile strength,improved acid (PA) holding efficiency, and improved chemical stabilityof these PEMs, obtained from MMMs of m-PBI-P@MCOF,were observed in comparison with the PEM of pristine m-PBI. The proton conductivity of m-PBI-P@MCOF-10%membrane at 180 & DEG;C is 0.309 S cm(-1), a five-foldincrement with respect to pristine m-PBI proton conductivity(0.061 S cm(-1)) under the identical experimentalcondition. This work clearly illustrates the nature of H-bonded interactionsbetween the nanofillers and polymers which efficiently enhanced protonconduction along with chemical and mechanical durability in the MMMmaterials.

Automated summary

The fabrication and design of proton conduction nanochannels within solid electrolyte materials is crucial for the development of efficient proton exchange membranes in fuel cells. In this study, a melamine-based Schiff base network type porous covalent organic framework (MCOF) was synthesized and impregnated with phosphoric acid (H3PO4) to create a stable membrane. Mixed matrix membranes (MMMs) were also fabricated using P@MCOF as nanofillers and [2,2'-(m-phenylene)-5,5'-benzimidazole] or m-PBI as the membrane forming polymer matrix. The resulting MMM-based PEM showed superior proton conductivity, excellent thermal and mechanical properties, improved acid holding efficiency, and enhanced chemical stability.