MOF based proton conductors have received immense importance recently. The present study endeavors to design two post-synthetically modified UiO-66 based MOFs and study the effects of their structural differences on their proton conductivity. UiO-66-NH 2 is modified by reaction with sultones to prepare two homologous compounds i.e., PSM 1 and PSM 2, which have SO 3 H groups in comparable extent (Zr:S ≈ 2: 1) in both. But the pendant alkyl chain holding the -SO3H group is of different length. PSM 2 has longer alkyl chain attachment than that of PSM 1. This difference in length of side arm results in huge difference in proton conducting behavior of the two compounds. PSM 1 is observed to have highest MOF based proton conductivity (1.64 × 10 -1 Scm -1 ) at 80 °C, which is comparable to commercially available Nafion while PSM 2 shows significantly lower conductivity. Again, the activation energy for proton conductivity is one of the lowest among all MOF based proton conductors in case of PSM 1 while, PSM 2 requires larger activation energy (almost three times).This profound effect of variation of chain length of side arm by 1 carbon atom in case of PSM 1 and PSM 2 was rather surprising and never documented before. This effect of length of side arm can be very useful to understand proton conduction mechanism of MOF based compounds and also to design better proton conductors. Besides, PSM 1 showed proton conductivity as high as 1.64 × 10 -1 Scm -1 at 80 °C temperature, which is the highest reported value till date among all MOF based systems. The lability of the -SO3H proton of the post synthetically modified UiO-66 MOFs has theoretically been determined by molecular electrostatic potential (MEP) analysis and theoretical pK a calculation of models of functional sites along with relevant NBO analyses.

An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)(OH) + H 4 [SiW 12 O 40 ]•HCl)@ZIF-8) for oxygen evolution reaction (OER) at the neutral pH, was developed by coencapsulation of Fe-salen (i.e., Fe(salen)Cl) and SiW 12 (i.e., H 4 [SiW 12 O 40 ]) inside the cavity of zeolitic imidazolate framework-8 (ZIF-8) material by an in situ synthesis. Here ZIF-8 functions as a host, Fe-salen as the active catalyst, and SiW 12 helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s −1 ) and high Faradaic efficiency (∼94%) make the concerned composite an efficient catalyst toward water oxidation. This is the first report of one of the simplest known metal complexes, Fe-salen, to perfrom electrocatalytic OER as a heterogeneous catalyst in the neutral pH. This work also highlights the benefits of coencapsulation of the Keggin polyoxometalate (POM) along with the active catalyst Fe-salen species. Encapsulation of SiW 12 results in (i) faster formation of FSWZ-8 composite, (ii) higher loading of Fe-salen, and, most importantly, (iii) lowering of required overpotential for electrochemical OER by more than 150 mV. The Keggin POMs, located as discrete molecular oxides inside the cavity of ZIF-8 as well as on the surface of ZIF-8, facilitate electrical charge conduction in the ZIF-8 matrix and lower the overall charge-transfer resistance.

Controlled incorporation of missing-linker defect sites could be achieved in four sister metal−organic frameworks (MOFs) of MOF-808, namely, MOF-808 A−D via modulated synthesis. An impressive enhancement in proton conductivity is observed in these sister MOFs (increase from 10 −3 to 10 −1 S cm −1 ) as compared to a highly crystalline, low-in-defect variant of MOF-808. MOF-808 C, having optimized defect density, shows a proton conductivity of 2.6 × 10 −1 S cm −1 at 80 °C and 98% relative humidity−the highest value reported to date for pure MOF-based proton conductors without extensive structural modifications. The introduction of defects induces super-protonic conductivity by modifying different properties, like porosity, water sorptivity, and acidity. Furthermore, an assembly of this super proton-conducting MOF with Pt�a versatile hydrogen evolution reaction (HER) catalyst, has been studied with the objective to influence the catalytic activity. This MOFcatalyst assembly was found to have lower overpotential requirements, owing to an increase in local acidity and efficient proton management around the catalyst.

Hydrogen is the solution to all the problems associated with the energy crisis. Generating hydrogen from water splitting is one of the greener approaches, but it requires an efficient catalyst that is economical for the bulk production of hydrogen. The transition metal-aqua coordination complexes, which are otherwise inactive/unstable for electrochemical hydrogen evolution reaction (HER) activity, can efficiently be utilized for the same by attaching these metal-aqua species on a stable support. With a similar approach, we have synthesized and structurally characterized a two-dimensional polyoxometalate (POM)-copper complex hybrid that supports a copper(II)-aqua-bypyridine complex with a molecular formula of the overall system, [{Cu II . The bis(aqua)-mono(bipyridine) Cu(II)-complex fragment {Cu II (2,2′bpy)(H 2 O) 2 } 2+ , attached to the two-dimensional POM-Cu-complex support, acts as an active catalytic center that catalyzes the electrochemical HER. The electrochemical studies done for this work enabled us to understand the role of compound 1 as an electrocatalyst for the HER in near-neutral medium (pH 4.8), under buffered conditions (acetate buffer). Through detailed electrochemical experiments including controlled ones, we understand that compound 1 follows a proton-coupled electron transfer (PCET) pathway with one proton and one electron involvement in the HER. The overpotential required to achieve a current density of 1 mA/cm 2 is found to be 520 mV with a Faradaic efficiency of 81%.

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.