Polyoxometalate-based metal-organic frameworks for boosting electrochemical capacitor performance

Polyoxometalate-based metal–organic frameworks (POM-based MOFs) are extended structures that simultaneously possess the virtues of POMs and MOFs. They have been attracting immense attention because of their diverse architectures and charming topologies, and also due to their probable application prospects in the areas of catalysis, sorption and separation, magnetism, proton conduction, lithium-ion batteries, supercapacitors, electrochemistry, medicine, and so on. In particular, the strong acidity, multielectron redox and photoactivity of POMs combined with the high surface area and adjustable pore size of MOFs make them excellent heterogeneous catalysts in various kinds of organic reactions. This short review firstly presents a concise introduction related to POMs, MOFs and POM-based MOFs, followed by the synthetic strategies of POM-based MOFs. The structural features and catalysis properties of representative POM-based MOFs have been classified and summarized according to different catalytic reactions, including acidity, oxidation, photocatalysis, electrocatalysis and other types of catalysis. The challenges present research situations, and prospects for the preparation and catalytic properties of POM-based MOFs are discussed.

Polyoxometalate-based metal–organic frameworks for heterogeneous catalysis

Different transition metal (TM) units are introduced into a trivacant Keggin cluster to form three sandwich polytungstate derivatives, (H2en)[{K(H2O)0.5}2{K2(H2O)3}{Ni(H2O)(en)2}2{Ni4(H2O)2(PW9O34)...

Multinuclear Transition Metal Sandwich-Type Polytungstate Derivatives for Enhanced Electrochemical Energy Storage and Bifunctional Electrocatalysis Performances.

Mo-Based crystalline polyoxometalate-based metal–organic frameworks (POMOFs), namely, [CuIH2(C12H12N6)(PMo12O40)]·[(C6H15N)(H2O)2] (1) and [CuII2(C12H12N6)4(PMoVI9MoV3O39)] (2) (C12H12N6, 1,4-bis(triazol-1-ylmethyl) benzene, abbreviation btx) as promising capacitor electrode materials were synthesized by a hydrothermal reaction. Compound 1 consisted of two-dimensional (2D) lattice structures with free triethylamine (abbreviation, TEA) molecules and H2O molecules, and compound 2 showed a 3D host–guest structure, in which 1D polyoxometalate (POM) chains were encapsulated into a 3D Cu(II)-btx metal–organic framework (MOF). The compound 1-based electrode showed much higher specific capacitance (249.0 F g−1 at 3 A g−1) than the 2-based one (154.5 F g−1 at 3 A g−1). Moreover, the specific capacitance of the 1-based electrode was not only higher than those of the majority of the reported POMOF materials as supercapacitors, but also higher than those of most state-of-the-art MOF-based and POM-based supercapacitor electrode materials. This superior capacitance performance of the 1-based electrode could be attributed to the high redox capacity and excellent electronic conductivity. More importantly, this work may open a new avenue for optimizing the performance of POMOF-based capacitor electrode materials.

Mo-Based crystal POMOFs with a high electrochemical capacitor performance.

We successfully synthesized two kinetically stable covalent polymer chains and ethylenediamine modified MXene-like-{AlW12O40} compounds. According to the energy storage research results, these materials showed high capacitance performance and cycle stability. The capacitance values of the 1-CC (carbon cloth) and 2-CC materials are 478.41 and 625.99 F g−1 (1.0 A g−1), and the capacitance retention is 95.71% and 97.62% after 5000 cycles, respectively. The symmetric water system supercapacitor device was assembled with two 2-CC and achieved an energy density of 6.32 W h kg−1 at 237 W kg−1. Compounds 1 and 2 were used to test the new sensitive current hydrogen peroxide sensor, displaying linear ranges of 1.20–3.20 mM (1-GCE: glassy carbon electrode) and 19.95 μM to 0.90 mM (2-GCE) with a detection limit of 0.93 μM and 0.86 μM, respectively. In addition, the photocatalysis mechanisms of compounds 1 and 2 were studied to provide a powerful basis for the photocatalytic mechanism of this series of compounds.

Covalent conductive polymer chain and organic ligand ethylenediamine modified MXene-like-{AlW12O40} compounds for fully symmetric supercapacitors, electrochemical sensors and photocatalysis mechanisms

In this work we synthesized two novel isostructural twin hybrids Comp1: [H(C10H10N2)Cu2][PMo12O40] & Comp2: [H(C10H10N2)Cu2][PW12O40], based on the Keggin ions (PMo12O40–3 & PW12O40–3), Cu(I) cation, and 4,4′-bipyridine, by in situ hydrothermal reduction of Cu, facilitated through extensive standardizations of synthetic pH conditions Both compounds crystallized in monoclinic P21/c space group with similar lattice parameters and crystal structures The structural similarity prompted us to explore comparative catalytic properties of the hybrids, to understand the relative role of the POM species in the activity While characterization techniques like powder X-ray diffraction (XRD), single-crystal XRD, IR, adsorption studies, etc confirmed the identical structural hierarchy in the twin polyoxometalate-based metal organic frameworks (POMOFs), critical analyses through X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and magnetic property studies elucidated the electronic 

Two Keggin-Based Isostructural POMOF Hybrids: Synthesis, Crystal Structure, and Catalytic Properties

Electrochemical CO2 reduction reaction (eCO2RR) is performed on two intermetallic compounds formed by copper and gallium metals (CuGa2 and Cu9Ga4). Among them, CuGa2 selectively converts CO2 to methanol with remarkable Faradaic efficiency of 77.26% at an extremely low potential of −0.3 V vs RHE. The high performance of CuGa2 compared to Cu9Ga4 is driven by its unique 2D structure, which retains surface and subsurface oxide species (Ga2O3) even in the reduction atmosphere. The Ga2O3 species is mapped by X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption fine structure (XAFS) techniques and electrochemical measurements. The eCO2RR selectivity to methanol are decreased at higher potential due to the lattice expansion caused by the reduction of the Ga2O3, which is probed by in situ XAFS, quasi in situ powder X‐ray diffraction, and ex situ XPS measurements. The mechanism of the formation of methanol is visualized by in situ infrared (IR) spectroscopy and the source of the carbon of methanol at the molecular level is confirmed from the isotope‐labeling experiments in presence of 13CO2. Finally, to minimize the mass transport limitations and improve the overall eCO2RR performance, a poly(tetrafluoroethylene)‐based gas diffusion electrode is used in the flow cell configuration.

Kaja Sai Manoj

Papers

Two Keggin-Based Isostructural POMOF Hybrids: Synthesis, Crystal Structure, and Catalytic Properties

Structure‐Tailored Surface Oxide on Cu–Ga Intermetallics Enhances CO2 Reduction Selectivity to Methanol at Ultralow Potential