Proton Conductivity in {Mo 72 Fe 30 }‐Type Keplerate

Keplerate polyoxometalate compounds: a multifunctional nano-platform for advanced materials

Polyoxometalates (POMs) are robust, discrete, and structurally well-defined metal-oxide cluster anions that have stimulated research in broad fields of science. Keplerates, as porous giant POMs, serve as a multifunctional nano-platform exhibiting fascinating chemical properties stemming from the porous molecular structure, substantial interior space, delocalization of d-electrons over the large molecular surface, etc. Consequently, Keplerates have attracted significant attention from scientists in the fields of chemistry, physics, biology, and materials sciences. This work reviews recent research progress on Keplerates as nanocontainers, catalysts, and battery materials. Furthermore, current challenges and potential future research directions are discussed, providing a reference for the development and effective application of Keplerates and Keplerate-based materials.

Hybrid Membrane of Sulfonated Poly(aryl ether ketone sulfone) Modified by Molybdenum Clusters with Enhanced Proton Conductivity

Developing novel proton exchange membranes (PEMs) with low cost and superior performance to replace Nafion is of great significance. Polyoxometalate-doped sulfonated poly(aryl ether ketone sulfone) (SPAEKS) allows for the amalgamation of the advantages in each constituent, thereby achieving an optimized performance for the hybrid PEMs. Herein, the hybrid membranes by introducing 2MeIm-{Mo132} into SPAEKS are obtained. Excellent hydrophilic properties of 2MeIm-{Mo132} can help more water molecules be retained in the hybrid membrane, providing abundant carriers for proton transport and proton hopping sites to build successive hydrophilic channels, thus lowering the energy barrier, accelerating the proton migration, and significantly fostering the proton conductivity of hybrid membranes. Especially, SP-2MIMo132-5 exhibits an enhanced proton conductivity of 75 mS cm-1 at 80 °C, which is 82.9% higher than pristine SPAEKS membrane. Additionally, this membrane is suitable for application in proton exchange membrane fuel cells, and a maximum power density of 266.2 mW cm-2 can be achieved at 80 °C, which far exceeds that of pristine SPAEKS membrane (54.6 mW cm-2). This work demonstrates that polyoxometalate-based clusters can serve as excellent proton conduction sites, opening up the choice of proton conduction carriers in hybrid membrane design and providing a novel idea to manufacture high-performance PEMs.

Unravelling the Bi-Functional Electrocatalytic Properties of {Mo72Fe30} Polyoxometalate Nanostructures for Overall Water Splitting Using Scanning Electrochemical Microscope and Electrochemical Gating Methods

This study reports the use of Keplerate-type {Mo72Fe30} polyoxometalate (POMs) nanostructures as a bi-functional-electrocatalyst for HER and OER in an alkaline medium with a lower overpotential (135 mV for HER and 264 mV for OER), and excellent electrochemical stability. The bi-functional catalytic properties of {Mo72Fe30} POM are studied using a scanning electrochemical microscope (SECM) via current mapping using substrate generation and tip collection mode. Furthermore, the bipolar nature of the {Mo72Fe30} POM nano-electrocatalysts is studied using the electrochemical gating via simultaneous monitoring of the electrochemical (cell) and electrical ({Mo72Fe30} POM) signals. Next, a prototype water electrolyzer fabricated using {Mo72Fe30} POM electrocatalysts showed they can drive 10 mA cm-2 with a low cell voltage of 1.62 V in lab-scale test conditions. Notably, the {Mo72Fe30} POM electrolyzers' performance assessment based on recommended conditions for industrial aspects shows that they require a very low overpotential of 1.89 V to drive 500 mA cm-2, highlighting their promising candidature toward clean-hydrogen production.

Nanoblackberries of {W72Fe33} and {Mo72Fe30}: Electrocatalytic Water Reduction

The reversible self-assembly of a {Mo₇₂Fe₃₀} cluster into nanoblackberries in a dilute solution of the relevant crystalline compound [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)}(H₂O)₉₁]·150H₂O ({Mo₇₂Fe₃₀}_cryst) was demonstrated by Liu, Müller, and their co-workers as a landmark discovery in the area of polyoxometalate chemistry. We have described, in the present work, how these ∼2.5 nm nano-objects, {M₇₂Fe₃₀} (M = W, Mo) can be self-assembled into nanoblackberries irreversibly, leading to their solid-state isolation as the nanomaterials Fe₃[W₇₂Fe₃₀O₂₅₂(CH₃COO)₂(OH)₂₅(H₂O)₁₀₃]·180H₂O ({W₇₂Fe₃₃}_NM) and Na₂[Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₄(OH)₁₆(H₂O)₁₀₈]·180H₂O ({Mo₇₂Fe₃₀}_NM), respectively (NM stands for nanomaterial). The formulations of these one-pot-synthesized nanoblackberries of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM have been established by spectral analysis including Raman spectroscopy, elemental analysis including ICP metal analysis, volumetric analysis (for iron), microscopy techniques, and DLS studies. The thermal stability of the tungsten nanoblackberries {W₇₂Fe₃₃}_NM is much higher than that of its molybdenum analogue {Mo₇₂Fe₃₀}_NM. This might due to the extra three ferric (Fe³⁺) ions per {W₇₂Fe₃₀} cluster in {W₇₂Fe₃₃}_NM, which are not part of the {W₇₂Fe₃₀} cluster cage but are placed between two adjacent clusters (i.e., each cluster has six surrounding 0.5Fe³⁺) to form this self-assembly. The isolated blackberries behave like an inorganic acid, a water suspension of which shows pH values of 3.9 for {W₇₂Fe₃₃}_NM and 3.7 for {Mo₇₂Fe₃₀}_NM because of the deprotonation of the hydroxyl groups in them. We have demonstrated, for the first time, a meaningful application of these inexpensive and easily synthesized nanoblackberries by showing that they can act as electrocatalysts for the hydrogen evolution reaction (HER) by reducing water. We have performed detailed kinetic studies for the electrocatalytic water reduction catalyzed by {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM in a comparative study. The relevant turnover frequencies (TOFs) of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM (∼0.72 and ∼0.45 s^-1, respectively), the overpotential values of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM (527 and 767 mV, respectively at 1 mA cm^-2), and the relative stability issues of the catalysts indicate that {W₇₂Fe₃₃}_NM is reasonably superior to {Mo₇₂Fe₃₀}_NM. We have described a rationale of why {W₇₂Fe₃₃}_NM performs better than {Mo₇₂Fe₃₀}_NM in terms of catalytic activity and stability.