Nanoporous Covalent Organic Framework and Polybenzimidazole Composites for Proton Exchange Membranes

Abstract

Fabrication and design of proton conduction nanochannels within the solid electrolyte materials is pivotal and challenging in order to develop an efficient proton exchange membrane (PEM) for the use in fuel cells. To address this, we have synthesized a melamine-based Schiff base network type porous covalent organic framework (MCOF) and impregnated phosphoric acid (H3PO4) as the electrolyte 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 strong membrane, mixed matrix membranes (MMMs) were fabricated using P@MCOF as nanofillers and [2,2′-(m-phenylene)-5,5′-benzimidazole] or m-PBI as the membrane forming polymer matrix. Formation of acid base pair occurred in the m-PBI-P@MCOF nanocomposite membrane owing to H-bonding interactions between the filler and polymer. Also, the acidic functionalities in the pores of P@MCOF provides abundant sites for labile proton transport, which enables uninterrupted proton conduction ion channels with low energy barrier in the nanocomposite membranes. Furthermore, all the composite membranes were immersed and loaded with phosphoric acid (PA) to increase electrolyte contents in the resulting MMM-based PEM. Superior proton conductivity, excellent thermal, thermo-mechanical and tensile strength, improved acid (PA) holding efficiency, and improved chemical stability of 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 °C is 0.309 S cm–1, a five-fold increment with respect to pristine m-PBI proton conductivity (0.061 S cm–1) under the identical experimental condition. This work clearly illustrates the nature of H-bonded interactions between the nanofillers and polymers which efficiently enhanced proton conduction along with chemical and mechanical durability in the MMM materials.