Modified polyether sulfone incorporated with zeolitic imidazolate framework-8 porous electrolyte membrane for direct methanol fuel cell

Abstract

The evolution of research and development of the electrolyte membrane for direct methanol fuel cell (DMFC) has increased since the past few years, particularly on the fabrication of different membrane configurations. This happens due to the promising characteristics of DMFC, especially for portable applications in delivering a high-power supply and easy to handle system. However, the limitation of these types of membranes is their dense structure which may limit the transportation of protons. Nevertheless, the open porous structure is yet to be studied as a new electrolyte membrane due to its well-known drawback of the pores on methanol barrier properties. The main aim of this study was to investigate the potential application of a novel modified porous polyether sulfone (PES) incorporated with zeolitic imidazolate framework-8 (ZIF-8) in direct methanol fuel cell (DMFC). The porous PES membranes prepared by dissolving in different types of solvents (DMAc and NMP) were obtained via non-solvent induced phase separation (NIPS) technique at different solvent evaporation time (SET) from 0 to 5 minutes. The prepared membranes were characterized based on the effect of morphological studies on their physicochemical properties. Later, PES dope solutions containing different loading of cSMMs (0 to 5 wt. %) were cast on a glass plate at optimum SET and solvent type in order to prepare the modified porous PES (PES-cSMMs) membranes. ZIF-8 crystals with the 2-methylimidazole (HmIm)/Zn2+ ratio of 6 were grown inside the PES-cSMMs pores by in-situ growth technique via different methods, which are immersion, dead-end filtration and contra-diffusion in order to get fine growth of ZIF-8 crystals inside the pores of the PES-cSMMs flat sheet membranes. The prepared porous PES-cSMMs/ZIF-8 composite membranes were evaluated with respect to their proton conductivity, methanol permeability, morphology, mechanical and thermal properties, and DMFC performance in a single DMFC stack. From the scanning electron microscopy, the morphologies of porous PES membrane for both surface and cross-sectional images showed changes with respect to the SET. Besides, the pore size of PES membranes increased dramatically as the SET increased. The porous PES-cSMMs membranes were fabricated at SET of 3 minutes as a result of the optimum selectivity of PES-DMAc3MIN membranes. The PES-3 wt. % cSMMs exhibited higher selectivity as compared to Nafion® 117 owing to proton conductivity and methanol permeability values of 14.14 × 10-3 Scm-1 and 0.54 × 10-7 cm2s-1, respectively. However, higher loading of cSMMs deteriorated the proton transportations. The incorporation of ZIF-8 via in-situ growth inside the pores of porous PES-cSMMs with a complete rhombic dodecahedron was successfully obtained via immersion technique and was found to significantly improve the proton conductivity (19.5 × 10-3 Scm-1) and methanol barrier properties (0.04 × 10-7 cm2s-1) as well as the exhibited power density of 25.2 mWcm-2. Hence, the promising results obtained in this study have demonstrated the potential of the porous electrolyte membrane, like the porous PES-cSMMs/ZIF-8 membrane, which gives a warrant for further investigation in fuel cell applications, specifically DMFC.