Design of Flexible Metal–Organic Framework-Based Superprotonic Conductors and Their Fabrication with a Polymer into Proton Exchange Membranes

Abstract

In recent times, the deployment of metal–organic frameworks (MOFs) to develop efficient proton conductors has gained immense popularity in the arena of sustainable energy research due to the ease of structural and functional tunability in MOFs. In this work, we have focused on developing “flexible MOF”-based proton conductors with Fe-MIL-53-NH 2 and Fe-MIL-88B-NH 2 MOFs using postsynthetic modification (PSM) as the tool. Taking advantage of the porous nature of these frameworks, we have carried out PSM on the primary amine groups present on the MOFs and converted them to −NH(CH2CH2CH2SO3H) groups. The PSM increased the number of labile protons in the channels of the modified MOFs as well as the extent of H-bonded networks inside the framework. The modified Fe-MIL-53-NH 2 and Fe-MIL-88B-NH 2 MOFs, named hereafter as 53-S and 88B-S, respectively, showed proton conductivity of 1.298 × 10–2 and 1.687 × 10–2 S cm–1 at ∼80 °C and 98% relative humidity (RH), respectively. This reflects ∼10-fold and ∼5-fold increases in their proton conductivity than their respective parent MOFs. Since MOFs as such are difficult to make directly into flexible membranes, and these are essential for practical applications as proton conductors, we have incorporated 53-S and 88B-S as fillers into a robust imidazole-based polymer matrix, namely, OPBI [poly(4,4′-diphenylether-5,5′-bibenzimidazole)]. The resulting polymer–MOF mixed matrix membranes (MMMs) after doping with phosphoric acid (PA) performed as flexible proton exchange membranes (PEMs) above 100 °C under anhydrous conditions and were found to be much more efficient and stable than the pristine OPBI membrane (devoid of any filler loading). By optimizing the amount of filler loading in the membrane, we obtained the highest proton conductivity of 0.304 S cm–1 at 160 °C under anhydrous conditions.

Polymer-Grafted Graphene Oxide/Polybenzimidazole Nanocomposites for Efficient Proton-Conducting Membranes

“…The decrement in the interlayer spacing after polymer modification in the GO-BSPA surface is a direct consequence of H-bonding interaction between the primary amines, secondary amines, and carboxylic acid moiety present in the GO surface with the −CONH 2 functionalities of the pAAM, −NH and −SO 3 H groups of the pAMPS, and −SO 3 H functionalities present in the grafted pSPAK polymers, respectively which has resulted crouched molecular chains of polymer- g -GO samples (GOP2, GOP4, and GOP6) and hence the interlayer spacing decreased. The PXRD data of the OPBI membrane given in Figure B exhibit a broad amorphous signal in the 2θ (°) range between 20 and 35°. , Incorporation of GOP2 or GOP4 or GOP6 polymers as fillers (1 and 2.5 wt %) into the OPBI matrix resulted in totally altered PXRD patterns with appearance of three unique semicrystalline peaks in the 2θ value 10.30, 14.02, and 18.70° (marked with # in the Figure B) which are absent in both the GOP and pristine OPBI samples indicating the formation of an ordered structure into the MMM after incorporation of polymer-modified GO nanosheets. Also, with increasing GOP loading from 1 to 2.5 wt % in the OPBI, the intensity of the semicrystalline planes increases.…”

“…The thick fibrillar morphology can remarkably contribute toward improved electrolyte (phosphoric acid) uptake, proton conductivity, improved PA retention under saturated water vapor, and mechanical reinforcement in the mixed matrix PEMs. 39,55 TEM surface morphology of the MMMs displays the homogeneous dispersion of GOP all over the membrane surface. However, a flower-like self-aggregated network structure can be observed from the image in all the cases (Figures 4D−F and S19D−F).…”

“…This kind of morphology develops due to the involvement of extensive H-bonding network between the OPBI imidazole N–H and −N functionalities with the primary and secondary amines, −CO, and the hydrophilic −SO 3 H functionalities present in the polymer-modified GOP surface. The thick fibrillar morphology can remarkably contribute toward improved electrolyte (phosphoric acid) uptake, proton conductivity, improved PA retention under saturated water vapor, and mechanical reinforcement in the mixed matrix PEMs. , TEM surface morphology of the MMMs displays the homogeneous dispersion of GOP all over the membrane surface. However, a flower-like self-aggregated network structure can be observed from the image in all the cases (Figures D–F and S19D–F).…”

Nanoporous Covalent Organic Framework and Polybenzimidazole Composites for Proton Exchange Membranes

“…29 Very recently, we have reported mixed matrix PBI-supported membrane developed from synthetically modified MIL-53 and MIL-88B MOFs (53-S and 88B-S) as nanofillers which exhibited a proton conductivity of 0.304 S cm −1 at 160 °C under an anhydrous environment. 38 Unlike MOF, COF is intrinsically constructed by a strong covalent bonding interaction. 39 The covalently linked organic building blocks ensure excellent thermal stability to the COF materials and their exceptional compatibility with the polymer matrix.…”

Aliphatic Polybenzimidazoles: Synthesis, Characterization and High-Temperature Shape-Memory Performance

“…Due to these properties, PBIs appear to be attractive materials for use in various fields, such as aerospace, the petrochemical industry, electronic devices, gas separation, fuel cells, etc. [ 1 , 2 , 3 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ].…”