Devising a Polyoxometalate-Based Functional Material as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

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, [{CuII(2,2'-bpy)(H2O)2}][{CoIIWVI12O40}{CuII(2,2'-bpy)(H2O)}{CuII(2,2'-bpy)}]·2H2O (1). The bis(aqua)-mono(bipyridine) Cu(II)-complex fragment {CuII(2,2'-bpy)(H2O)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/cm2 is found to be 520 mV with a Faradaic efficiency of 81%.

Evolution of metal organic frameworks as electrocatalysts for water oxidation.

In the last two decades, metal organic frameworks (MOFs) have been extensively investigated to develop heterogeneous electrocatalysts for water oxidation (WO). The scope of reticular synthesis, enormous surface area and accessible internal volume of MOFs make them promising candidates for catalysis. However, low electrical conductivity, slow mass transport and lack of stability restrict the scope of MOF-based WO. In recent times, various material designing approaches, e.g., the introduction of mixed metal and multi-metal systems, ligand engineering, guest@MOF composite formation, preparation of thin films, MOF composite formation with conducting carbon-based materials, metal oxides, polymers and layered compounds, etc. have emerged as an effective means to counteract the aforementioned limitations. This feature article critically discusses the common MOF-based material designing strategies with respect to electrochemical WO and provides a platform to understand the potential of MOFs to prepare a sophisticated hybrid electrocatalyst for WO.

Efficient Electrocatalytic Water Oxidation by Fe(salen)-MOF Composite: Effect of Modified Microenvironment.

An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)(OH) + H4[SiW12O40]·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 SiW12 (i.e., H4[SiW12O40]) 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 SiW12 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 SiW12 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.

Designing UiO-66-Based Superprotonic Conductor with the Highest Metal-Organic Framework Based Proton Conductivity.

Metal-organic framework (MOF) based proton conductors have received immense importance recently. The present study endeavors to design two post synthetically modified UiO-66-based MOFs and examines the effects of their structural differences on their proton conductivity. UiO-66-NH2 is modified by reaction with sultones to prepare two homologous compounds, that is, PSM 1 and PSM 2, with SO3H functionalization in comparable extent (Zr:S = 2:1) in both. However, the pendant alkyl chain holding the -SO3H group is of different length. PSM 2 has longer alkyl chain attachment than PSM 1. This difference in the length of side arms results in a huge difference in proton conductivity of the two compounds. PSM 1 is observed to have the highest MOF-based proton conductivity (1.64 × 10-1 S cm-1) at 80 °C, which is comparable to commercially available Nafion, while PSM 2 shows significantly lower conductivity (4.6 × 10-3 S cm-1). Again, the activation energy for proton conduction is one of the lowest among all MOF-based proton conductors in the case of PSM 1, while PSM 2 requires larger activation energy (almost 3 times). This profound effect of variation of the chain length of the side arm by one carbon atom in the case of PSM 1 and PSM 2 was rather surprising and never documented before. This effect of the length of the side arm can be very useful to understand the 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 S cm-1 at 80 °C, which is the highest reported value to 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 analysis and theoretical p Ka calculation of models of functional sites along with relevant NBO analyses.

Cytochrome c peroxidase activity of heme bound amyloid ß peptides.

Heme bound amyloid ß (Aß) peptides, which have been associated with Alzheimer's disease (AD), can catalytically oxidize ferrocytochrome c (Cyt c(II)) in the presence of hydrogen peroxide (H2O2). The rate of catalytic oxidation of Cyt(II) c has been found to be dependent on several factors, such as concentration of heme(III)-Aß, Cyt(II) c, H2O2, pH, ionic strength of the solution, and peptide chain length of Aß. The above features resemble the naturally occurring enzyme cytochrome c peroxidase (CCP) which is known to catalytically oxidize Cyt(II) c in the presence of H2O2. In the absence of heme(III)-Aß, the oxidation of Cyt(II) c is not catalytic. Thus, heme-Aß complex behaves as CCP.