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.

A sole inorganic framework material [Li(H2O)4][{CuI(H2O)1.5} {CuII(H2O)3}2{WVI 12O36(OH)6}]·N2·H2S·3H2O (1) consisting of a hydroxylated polyoxometalate (POM) anion, {WVI 12O36(OH)6}6–, a mixed-valent Cu(II)– and Cu(I)–aqua cationic complex species, [{CuI(H2O)1.5}{CuII(H2O)3}2]5+, a Li(I)-aqua complex cation, and three solvent molecules, has been synthesized and structurally characterized. During its synthesis, the POM cluster anion gets functionalized with six hydroxyl groups, i.e., six WVI–OH groups per cluster unit. Moreover, structural and spectral analyses have shown the presence of H2S and N2 molecules in the concerned crystal lattice, formed from “sulfate-reducing ammonium oxidation (SRAO)”. Compound 1 functions as a bifunctional electrocatalyst exhibiting oxygen evolution reaction (OER) by water oxidation and hydrogen evolution reaction (HER) by water reduction at the neutral pH. We could identify that the hydroxylated POM anion and copper-aqua complex cations are the functional sites for HER and OER, respectively. The overpotential, required to achieve a current density of 1 mA/cm2 in the case of HER (water reduction), is found to be 443 mV with a Faradaic efficiency of 92% and a turnover frequency of 4.66 s–1. In the case of OER (water oxidation), the overpotential needed to achieve a current density of 1 mA/cm2 is obtained to be 418 mV with a Faradaic efficiency of 88% and turnover frequency of 2.81 s–1. Diverse electrochemical controlled experiments have been performed to conclude that the title POM-based material functions as a true bifunctional catalyst for electrocatalytic HER as well as OER at the neutral pH without catalyst reconstruction.

Cobalt formate dihydrate, Co(HCOO)[Formula: see text]H2O (Co-MOF), a known crystalline substance, is a three-dimensional coordination polymer. The compound crystallizes in a monoclinic space group [Formula: see text]. In this work, we are reporting a unique synthesis method to prepare this MOF without using any formate salt or formic acid. The source of formate in compound Co-MOF is dimethylformamide (DMF) solvent used in the relevant synthesis. This metal–organic framework (MOF) containing compound, which can be described as a functional MOF, has recently been exploited for hydrogen isotope separation. This functional material has never been used as an electrocatalyst. In this work, we have demonstrated that this water insoluble compound Co-MOF can be used as heterogeneous electrocatalyst for oxygen evolution reaction (OER) by water oxidation at neutral pH. An overpotential of 275.5[Formula: see text]mV versus RHE was required to attain a current density of 1[Formula: see text]mAcm[Formula: see text]. The relevant Faradic efficiency (89.93%) and the turn over frequency (TOF) of 0.277[Formula: see text]s[Formula: see text] make the material an efficient electrocatalyst for water oxidation to molecular oxygen at neutral pH.