Designing an efficient oxygen evolution reaction (OER) electrocatalyst is a challenging task because molecular oxygen is essential to sustain life. Herein, we have synthesized and structurally characterized a {BiKeeping in mind that the {Bi(OH 2 ) 2 } 3+ species on the surface of the POM has the potential to act as an active site for electrocatalytic water oxidation, we encapsulated the functionalized Keggin POM anion [Bi(OH 2 ) 2 SiW 11 O 39 ] 5− inside the cavities of a well-known zeolitic-type framework material ZIF-8, a zinc imidazole-containing metal−organic framework, because compound K 5 1•13H 2 O per se is water-soluble and unstable in the oxidation window of water oxidation. The resulting host−guesttype composite material H 5 [Bi(H 2 O) 2 SiW 11 O 39 ]@ZIF8 (H 5 1@ZIF8) functions as an efficient electrocatalyst for water oxidation. Detailed electrochemical analyses have established that the title OER catalyst, H 5 1@ZIF8, works in a wide pH window, ranging from alkaline pH 13.0 to neutral pH 7.0 through an acidic pH 4.0. Diverse controlled experiments have been performed to infer that the bismuth−aqua complex is the functional site responding to the OER, with a catalytic turnover frequency of 1.24, 5.90, and 0.93 s −1 of pH 13.0, pH 7.0, and pH 4.0 solutions, respectively, at the respective overpotentials of 375.0 mV [vs the reversible hydrogen electrode (RHE)], 585.21 mV (vs the RHE), and 830.10 mV (vs the RHE).
Two copper-based barrel-shaped polyoxometalates (POMs), namely, [{H 3 O} 4 {Na 6 (H 2 O) 22 }][{Cu I (H 2 O) 3 } 2 {Cu II (H 2 O)} 3 {B-α-Bi III W VI 9 O 33 } 2 ]•7H 2 O (NaCu-POM) and Li 4have been synthesized and structurally characterized. The single-crystal X-ray diffraction analyses of NaCu-POM and LiCu-POM reveal the presence of penta-and hexa-nuclear copper wheels per formula units, respectively; these copper wheels are sandwiched between two lacunary Keggin anions {B-α-Bi III W VI 9 O 33 } 9− (BiW 9 ) to form the barrel-shaped title POM compounds. In both the compounds NaCu-POM and LiCu-POM, the mixed-valent copper centers are present in their respective pentaand hexa-nuclear copper wheels, established by X-ray photoelectron spectroscopy (XPS) as well as by bond valence sum (BVS) calculations. Compound LiCu-POM additionally shows the presence of a sulfhydryl ligand (SH − ), coordinated to one of the copper centers of its {Cu 6 }-wheel, that is expected to be generated from the in situ reduction of sulfate anion present in the concerned reaction mixture (lithium-ion in ammonia solution may be the reducing agent). Interestingly, the title compounds, NaCu-POM and LiCu-POM exhibit an efficient electrocatalytic hydrogen evolution reaction (HER) by reducing water at neutral pH. Detailed electrochemical studies including controlled experiments indicate that the active sites for this electrocatalysis are the W(VI) centers of the title compounds, not the copper centers. However, a relevant tri-lacunary Keggin cluster anion {P V W VI 9 O 33 } 7− (devoid of copper ion) does not show comparable HER as shown by the title compounds. The intra-cluster cooperative interactions of the mixed-valent copper centers (Cu II /Cu I ) with the tungsten centers (W 6+ ) make the overall system electrocatalytically active toward water reduction to molecular hydrogen at neutral pH. High Faradaic efficiencies (89 and 92%) and turnover frequencies (1.598 s −1 and 1.117 s −1 ) make the title compounds NaCu-POM and LiCu-POM efficient catalysts toward electrochemical water reduction to molecular hydrogen.
When a polyoxometalate cluster surface is grafted with WVI–(OH)2 functionality, the surface-modified POM turns into an efficient & stable electrocatalyst for hydrogen evolution reaction by water reduction. We performed detailed kinetic studies for this electrocatalysis.
Two polyoxovanadate compounds, [Na7(H2O)14][H8VIV18O42(VVO4)]·N2H4·7H2O and [Na5(H2O)16][H8VIV18O42(Cl)]·4N2H4·6H2O, each having fully reduced host cage and accommodating eight acidic protons per formula unit, exhibit moderate proton conductivity.
A fully reduced polyoxovanadate compound [Na6(H2O)24][H8VIV15O36(CO3)]·3N2H4·10H2O (1) with CO32− encapsulation in its internal cavity (from dissolved aerial CO2 in the synthesis reaction mixture) is reported. Compound 1 crystals, on exposure of HCl vapor, excludes carbonate as a CO2 gas that can be reacted with a Grignard reagent to produce triphenylcarbinol as a major product.
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