Keggin Structure Research Articles

Deeply reduced empty Keggin clusters [MoIVxMVI12−xO40−xpyx] (x = 3, 6; M = Mo, W; py = pyridine): synthesis, structures, and Lewis field catalysis

First MoIV3-Keggin anions [MoIVxMVI12−xO40−xpyx]8− (M = Mo, W; x = 3, 6) display unprecedented empty Keggin structure caused by the weak u3-O coordinating capability of the [MoIV3O4] triads and LCF-dependent hydrogen transfer catalysis.

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes.

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga3+ ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and 71Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2–3) a polyoxogallate structure is identified as either Ga30 or Ga32 in contradiction with the previously proposed Ga13 Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga3+ ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.

Encapsulation of silicotungstic acid into chromium (III) terephthalate metal–organic framework for high proton conductivity membranes

Heteropoly acids have attracted great attention as potential solid-state proton-conducting materials owing to their high proton conductivity of 10–2 S cm−1 at room temperature. However, the proton conductivity can be significantly reduced under operating conditions due to the leaching of water molecules out of the acid Keggin structure. In the present study, silicotungstic acid (SiWA) was encapsulated in the mesoporous structure of the chromium (III) terephthalate metal–organic framework (MIL-101) in an attempt to prevent water loss and preserve proton conductivity. This hybrid material was then shaped into a membrane with the aid of polyvinyl alcohol (PVA). This approach of SiWA encapsulation with PVA addition to form a membrane proved to be successful as the proton conductivity of the resulting membranes was found to be of 2.4 × 10–2 S cm−1 at 25 °C and 25% RH. Under low levels of hydration, when the membrane was dried at 80 °C, the proton conductivity still remained high (1.9 × 10–2 S cm−1). This improved proton conductivity is believed to be the result of the continuous channels for facile proton transport created through the immobilisation of the heteropoly acid within the MIL-101 framework.

Unique Adsorption Properties of Malachite Green on Interlayer Space of Cu-Al and Cu-Al-SiW12O40 Layered Double Hydroxides

Cu-Al layered double hydroxide (LDH) was intercalated with Keggin ion of polyoxometalate K4[a-SiW12O40] to form Cu-Al-SiW12O40 LDH. The obtained materials were analyzed by X-ray Diffraction (XRD), Fourier Transform Infra Red (FTIR) spectroscopy, and Brunaur-Emmett-Teller (BET) surface area analysis. Furthermore, the materials were used as adsorbents of malachite green from aqueous solution. Some variables for adsorption, such as: effect of adsorption times, malachite green concentration, and also adsorption temperature, were explored. The results showed that diffraction at 11.72° on Cu-Al LDH has interlayer distance of 7.56 Å. The intercalation of that LDH with [a-SiW12O40]4− ion resulted increasing interlayer distance to 12.10 Å. The surface area of material was also increased after intercalation from 46.2 m2/g to 89.02 m2/g. The adsorption of malachite green on Cu-Al and Cu-Al-SiW12O40 LDHs followed pseudo second order kinetic and isotherm Langmuir model with adsorption capacity of Cu-Al and Cu-Al-SiW12O40 LDHs was 55.866 mg/g and 149.253 mg/g, respectively. That adsorption capacity is equal with increasing interlayer space and surface area properties of material after intercalation. Thus, the adsorption of malachite green on Cu-Al and Cu-Al-SiW12O40 LDHs is unique and dominantly occurred on interlayer space of LDH as active site adsorption. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Phosphotungstic acid on activated carbon: A remarkable catalyst for 5-hydroxymethylfurfural production

Phophotungstic acid (HPW) supported on activated carbon (AC) was prepared and used for dehydration of fructose to 5-hydroxymethylfurfural (HMF), a biomass-derived precursor for chemicals and fuels. Different characterization techniques showed that Keggin structure of HPW was maintained after impregnation process by incipient wetness method with ethanol as solvent. The high mass content of the active phase (50 wt%) on the support resulted in accessible acid sites and high acid density, important for the dehydration reaction. The effects of reaction time, catalyst amount, and temperature were investigated. The catalytic system with dimethyl sulfoxide (DMSO) as solvent provided an efficient transformation of fructose to HMF, resulting in high HMF yields (99.3 %) and total fructose conversion after 30 min of reaction at 140 °C. Fructose conversion was not significantly affected by the catalyst amount. On the other hand, the increase in temperature resulted in greater and faster conversion. HMF yield was deeply affected by all reaction variables. Moreover, HPW/AC was reused five times without significant loss of activity. Additionally, a kinetic evaluation was performed and the activation energy of fructose conversion to HMF was 109.4 kJmol−1.

A series of POM-based compounds constructed by piperazine and morpholine derivatives: Characterization, selective photocatalytic and electrochemical/fluorescence sensing properties

Through tunning the N and O coordintion donors in piperazine and morpholine derivatives, six POM-based compounds were obtained, namely [CuII(tep)][HCr(OH)6Mo6O18]·H2O (1), [CoII(tep)2][H4(γ-Mo8O26)] (2), [AgI2(tem)4][H2PW12O40]2·2H2O (3), [M(tem)2][H2(γ-Mo8O26)] (M ​= ​CuII 4, CoII 5, ZnII 6) (tep ​= ​1-(2-(4H-1,2,4-Triazol-4-yl)ethyl)piperazine, tem ​= ​4-(2-4H-1,2,4-Triazol-4-yl)ethyl)morpholine). Compounds 1–6 show 0D, 1D, 2D and 3D structures, respectively. In compound 1, two Anderson type anions hang on either side of the copper dimer [Cu2(tep)2]4+, forming a discrete structure. In compound 2, the N donor in piperazine coordinates with a Co atom, inducing a 3D framework. Compond 3 exhibits a 0D structure containing discrete Keggin anions and binuclear [Ag2(tem)4]2+ clusters. Compounds 4, 5 and 6 are isostructural, showing 3D grid structures with 2D POM-based layers linked by metal-organic lines. Compounds 2, 4, 5 and 6 contain Mo–N bonds to enhance structural stability. Tunning N and O coordintion donors in tep and tem ligands is reasonable to construct different POM-based compounds. Furthermore, the electrochemical and selective photocatalytic properties of the title compounds were also studied. Compound 3 can act as a fluorescence sensor for sensing Hg2+.

Synthesis, physical and photo-electrochemical characterizations of a new hybrid host-guest complex [Cu12(C2N3H2)8Cl][(PW12O40)

Through careful choice of reduction conductions, a new hybrid host-guest compound [Cu12(C2N3H2)8Cl][(PW12O40)] was synthesized hydrothermally in the presence of 1, 2, 4 triazole as structure-directing agent. The material has been characterized by single-crystal X-ray diffraction, chemical analysis, FT-IR spectroscopy, thermal analysis and UV–Vis diffuse reflectance. The compound based on a two-dimensional (2D) supramolecular porous coordination polymer [Cu12 (C2N3H2)8Cl]3+ can accommodate in its cavities the Keggin polyoxotungstate anions, with host-guest interactions crystallizing in the monoclinic system (Space Group: C2/m). The metal organic species and Keggin anions are connected via hydrogen bonds and Cu…O short contacts giving rise to a three-dimensional architecture. The optical study shows a direct transition (1.60 eV) assigned to the O2−: 2p → Cu+: 4 s transition. The transport properties revealed a semiconductor comportment; the thermal variation of log (conductivity) versus 1000/T was fitted by a least square method to a straight line whose slope gave an activation energy of 54 meV. The chrono-amperometry and photo-electrochemistry, carried out in Na2SO4 (0.5 M) electrolyte, indicate n-type conduction with a flat band potential of +0.10 VSCE, making [Cu12(C2N3H2)8Cl][(PW12O40)] a promising solar material.

Removal of Iron(II) Using Ni/Al Layered Double Hydroxide Intercalated with Keggin Ion

Layered double hydroxide (LDH) Ni/Al-NO3 was synthesized using a coprecipitation method under base condition following with intercalation using Keggin ion [a-SiW12O40]4- to form Ni/Al-[a-SiW12O40] LDH. The LDHs were characterized using XRD, FTIR, BET, and pHpzc analyses. Furthermore, LDHs were applied as adsorbent of iron(II) from aqueous solution. The adsorption process was studied through the effect of adsorption time, the concentration of iron(II), and temperature adsorption. The results show the interlayer distance of LDHs was increased from 7.408 Å to 10.533 Å after intercalation process. The adsorption of iron(II) on LDHs showed that adsorption of iron(II) on both LDHs follows pseudo first-order kinetic model with R2 value is close to one. The adsorption process was spontaneous, with adsorption capacity up to 36.496 mg g-1.

Fabrication of Fe-POMs as Visible-light-active Heterogeneous Photocatalyst

Visible-light-active Fe-POMs was fabricated via precipitating Fe3+ with Keggin type polyoxometalates (H3PW12O40, H4SiW12O40 or H3PMo12O40) under solvothermal condition. The as-prepared Fe-POMs were denoted as FePW, FeSiW and FePMo, respectively. Among the three kinds of Fe-POMs, FePMo displayed the highest visible light absorption, the largest specific surface area, the most sensitive photocurrent response and the smallest charge transfer resistance, which were all beneficial for heterogeneous photocatalysis. The efficiency for Cr(VI) reduction was ca. 88% by FePMo after 50 min visible light irradiation. The estimated rate constant(0.042 min−1) was ca. 2.5 and 1.8 times that by FePW and FeSiW, respectively. FTIR spectra indicated that the Keggin structure of PMo12O403− was maintained in FePMo. Mechanism study indicated that the photogenerated electrons in LUMO and the holes in HOMO were thermodynamically feasible for Cr(VI) reduction and H2O oxidation, respectively. Using FePMo as an optimized photocatalyst, good stability was also observed after 5 cyclic runs in both photocatalytic performance and XRD structure.

Lacunary Keggin-type hetero polyoxometalate, K7PMo2W9O39, supported on nano ZnO as an efficient Photocatalyst for degradation of phenol in water solution

In the present research, Lacunary Keggin-type heteropolyoxometalate, (K7PMo2W9O39) supported on ZnO nanoparticles was prepared by the impregnation method. Nanoparticle characteristics and the remaining Keggin structure in the nanocomposites were confirmed by FTIR and XRD analyses. The photocatalytic activity of prepared K7PMo2W9O39/ZnO for degradation of phenol under UV light was investigated. H2O2 was used as oxidant in the photocatalytic degradation process of phenol. The results indicated that synthesized nanophotocatalyst could be considered an appropriate heterogonous photocatalyst in the removing of organic pollutant from aqueous solutions and heterogenization of Lacunary Keggin-type heteropolyoxometalate on ZnO nanoparticles resulted in the improved light absorption intensity and decreased band gap of nanocomposites. Degradation of phenol in the presence of the K7PMo2W9O39/ZnO could lead to the disappearance approximately 93% of phenol after 60 min. But degradation for the same experiment performed in the presence of the K7PMo2W9O39 or /ZnO was less than 60% in the same time.

Interaction between dopamine and the [HPW12O40]2−Keggin ion–an X-ray and NMR study

A novel complex between the [HPW12O40]2− Keggin ion and the biomolecule dopamine (C8H11NO2), [HPW12O40(dop)2] • 4H2O (1) was crystallized and structurally characterized by single crystal X-ray diffraction and infrared spectrometry. Dopamine interacts via hydrogen bonding between its ammonium group and a WO oxygen atom in [HPW12O40]2−. NMR analyses (1H DOSY) suggests that the [HPW12O40(dop)2] complex dissociates in D2O.

Catalytic Degradation of Chitosan by Supported Heteropoly Acids in Heterogeneous Systems

Several kinds of composite materials with phosphotungstic acid (PTA) as the catalyst were prepared with activated carbon as support, and their structures were characterized. According to the Box–Behnken central combination principle, the mathematical model of the heterogeneous system is established. Based on the single-factor experiments, the reaction temperature, the reaction time, the amount of hydrogen peroxide and the loading capacity of PTA were selected as the influencing factors to study the catalyzed oxidation of hydrogen peroxide and degradation of high molecular weight chitosan. The results of IR showed that the catalyst had a Keggin structure. The results of the mercury intrusion test showed that the pore structure of the supported PTA catalyst did not change significantly, and with the increase of PTA loading, the porosity and pore volume decreased regularly, which indicated that PTA molecules had been absorbed and filled into the pore of activated carbon. The results of Response Surface Design (RSD) showed that the optimum reaction conditions of supported PTA catalysts for oxidative degradation of high molecular weight chitosan by hydrogen peroxide were as follows: reaction temperature was 70 ℃, reaction time was 3.0 h, the ratio of hydrogen peroxide to chitosan was 2.4 and the catalyst loading was 30%. Under these conditions, the yield and molecular weight of water-soluble chitosan were 62.8% and 1290 Da, respectively. The supported PTA catalyst maintained high catalytic activity after three reuses, which indicated that the supported PTA catalyst had excellent catalytic activity and stable performance compared with the PTA catalyst.

Preparation of phosphotungstic acid/SiC and their photocatalytic activity for rhodamine B

Four kinds of x wt.% phosphotungstic acid/silicon carbide (PTA/SiC) photocatalysts were prepared by impregnation method with SiC, 12-phosphotungstic acid (PTA) as raw materials. The structure of samples was characterised by X-ray diffraction, ultraviolet–visible diffuse reflectance spectra, scanning electron microscope. The photocatalytic activity of samples was evaluated by the degradation of rhodamine B (RhB) under a xenon lamp (λ≥420 nm). The results indicate that the photocatalysts not only maintained the SiC structure but also kept the Keggin structure of PTA. The 20 wt.% PTA/SiC exhibited the best photocatalytic degradation efficiency reaching up to 77% for 180 min irradiation, and the photo-degradation rate constant was determined to be 7.82 × 10−3 min−1. In the presence of H2O2, the photocatalytic degradation rate of 20 wt.% PTA/SiC composites was increased to 87%, and the photodegradation reaction rate constant was 7.77 × 10−3 min−1. According to free radical trapping experiments, it is proved that hydroxyl radical (·OH) plays an important role in photocatalysis.

Adsorption Behaviour and Kinetics of Zearalenone on Hydroxyl-Fe-Al-Intercalated Montmorillonite

Pristine montmorillonite (Mont) was used as raw materials to prepare hydroxyl-Fe-pillared Mont, hydroxyl-Al-pillared Mont, and hydroxyl-Fe-Al-pillared Mont composites. By varying the OH/Fe and Fe/Al molar ratios during the preparation of the pillared Mont, the adsorption capacity of zearalenone (ZEA) and the kinetics were elucidated. The characterization of X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy reveals the adsorption mechanism of pristine and modified Mont. The results indicated that the ZEA adsorption capacity is Mont (0.05 mg/g) &lt;&lt; 1.5OH/Fe-Mont (0.28 mg/g) &lt;&lt; OH/Al-Mont (0.51 mg/g) &lt; 0.5Fe/Al-Mont (0.56 mg/g) in the condition of pH = 8 and 37°C, in which both 0.5Fe/Al-Mont and OH/Al-Mont reached maximum adsorption capacity and 1.5OH/Fe-Mont attained 5 times the capacity of Mont. Adsorption isotherm studies revealed that Freundlich adsorption isotherms best represented the experimental data. The kinetic data for ZEA adsorption revealed that the Mont adsorption capacity for ZEA equilibrates in 1 hour and is best described using the pseudo-second-order rate equation. The XRD analysis indicated that the amplification of Fe-dominant pillared Mont interlayer spacing is the main reason for the observed increases in the adsorption capacity of ZEA, while Al-dominant pillared Mont has a relatively stable Keggin structure; therefore, interlayer spacing is not the primary mechanism for changes in the adsorption capacity of both OH/Al-Mont and Al-dominant pillared Mont. An FT-IR analysis demonstrated that cationic exchange was the dominant mechanism that allowed ZEA and hydroxyl-Al ions to enter the Mont interlayers, while this cationic exchange mechanism was not the dominant mechanism used by hydroxyl-Fe entering the Mont layers.

Efficient Hydrogen Generation From Hydrolysis of Sodium Borohydride in Seawater Catalyzed by Polyoxometalate Supported on Activated Carbon.

Phosphotungstic acid (HPW) as a polyoxometalate was selected as the active component of the catalyst. The activated carbon supported different percentage of HPW catalysts were prepared by impregnation and were characterized by X-ray diffraction (XRD), nitrogen adsorption, Fourier transform infrared (FTIR), and scanning electron microscope (SEM). The results showed that the HPW retained the original Keggin structure after being supported on activated carbon, the specific surface of the HPW/C was much bigger than that of pure HPW. The catalytic performance of HPW/C in the hydrogen generation reaction by hydrolysis of sodium borohydride in seawater and in deionized water were studied. 2.5 wt.% HPW/C showed the fastest hydrolysis reaction rate and the biggest volume of hydrogen generated. As for hydrolysis of sodium borohydride, catalytic effect of HPW/C was better in seawater than in distilled water. HPW dispersed on activated carbon is a real promising catalytic system for the development of hydrogen generation by hydrolysis of NaBH4 in seawater.

High dispersion of heteropolyacid nanoparticles on hydrothermally Cs-modified three-dimensionally ordered macroporous SiO2 with excellent selectivity in methacrolein oxidation

Heteropolyacid nanoparticles (NPs) were supported on Cs-modified three-dimensionally ordered macroporous (3DOM) SiO2 and used as the catalyst for the oxidation of methacrolein (MAL) to methacrylic acid (MAA). Hydrothermal treatment and incipient wetness impregnation were employed respectively for the Cs-modification. It was found that hydrothermal Cs-modification of 3DOM SiO2 promoted the dispersion of the supported heteropolyacid, which showed an average particle size of 5.2 nm, much smaller than that (17.6 nm) on the Cs-modified 3DOM SiO2 prepared by incipient wetness impregnation. The effects of hydrothermal treatment on the structure and catalytic performance of the catalyst were investigated. Results showed that the ion exchange between Cs+ and the surface silanol groups on 3DOM SiO2 was promoted with the increase of the hydrothermal temperature. Meanwhile, Cs-modification helped the heteropolyacid to retain intact Keggin structure, inhibiting the formation of MoO3. Highly dispersed heteropolyacid NPs with enhanced structural stability exhibited excellent selectivity to MAA in the oxidation of MAL.

Co-ion Effects in the Self-Assembly of Macroions: From Co-ions to Co-macroions and to the Unique Feature of Self-Recognition.

Macroions, as soluble ions with a size on the nanometer scale, show unique solution behavior different from those of simple ions and large colloidal suspensions. In macroionic solutions, the counterions are known to be important and well-explored. However, the role of co-ions (ions carrying the same type of charge as the macroions) is often ignored. Here, through experimental and simulation studies, we demonstrate the role of co-ions as a function of co-ion size on their interaction with the macroions (using {Mo72Fe30} and {SrPd12} as models) and the related self-assembly into blackberry-type structures in dilute solutions. Several regimes of unique co-ion effects are clearly identified: small ions (halides, oxoacid ions), subnanometer-scaled bulky ions (lacunary Keggin and dodecaborate ions), and those with sizes comparable to the macroions. Small co-ions have no observable effect on the self-assembly of fully hydrophilic {Mo72Fe30}, while due to hydrophobic interaction and intermolecular hydrogen bonds, the small co-ions show influences on the self-assembly of hydrophobic {SrPd12}. Subnanometer ions, a.k.a. "superchaotropic ions", are still too small to assemble into a blackberry by themselves, but they can coassemble with the macroions, showing a strong interaction with the macroionic system. When the co-ion size is comparable to that of the macroions, they assemble independently instead of assembling with the macroions, leading to the previously reported unique self-recognition phenomenon for macroions.

Influence of reactional parameters in the adipic acid synthesis from cyclohexene using heterogeneous polyoxometalates

One-step oxidative cleavage of cyclohexene to adipic acid using H2O2 as oxidizing agent and heteropolysalts as heterogeneous catalysts was successfully performed, without the use of additives, such as organic or inorganic acids. The K3PW12O40, K3PMo12O40, Cs3PW12O40 and Cs3PMo12O40 heteropolysalts were synthesized by ion exchange of the respective heteropolyacids using cesium and potassium carbonates. The catalysts were thermally treated at 200 or 600 °C and characterized by Raman spectroscopy, thermogravimetric analysis, X-ray fluorescence (XRF) and photoelectronic X-ray spectroscopy (XPS). Raman results showed that the Keggin structure of the polyoxometalates was successfully obtained for both heteropolyacids and heteropolysalts and remained stable even after thermal treatment at 600 °C. XRF analysis confirmed the insertion of the cations (Cs+ and K+) into the heteropolysalt structures. All catalysts completely converted cyclohexene within 24 h and they were selective to adipic acid. The best catalyst, K3PW12O40 calcined at 600 °C (KPW-600), achieved the highest adipic acid yield (83 %). In this work, we associate the best activity of the KPW-600 heteropolysalt not only to its greater acidity, but also to the higher concentration of (W-O-W)/W species (identified by XPS). Both properties are fundamental for the oxidation reaction of cyclohexene to adipic acid under the experimental conditions used. The results showed that the presence of peroxide species is essential, mainly to favor oxidation reactions, such as the Baeyer-Villiger rearrangement reaction. A reaction scheme for the oxidation of cyclohexene to adipic acid with the best catalyst (KPW-600), was proposed based on our kinetic studies and literature results. In addition to the main route of adipic acid formation from the direct oxidation of cyclohexene, undesirable parallel reactions of the rearrangement of 1,2 - cyclohexanediol and allyl oxidation were also observed. The influence of reaction conditions was also investigated, as well as the reuse of the most promising catalyst.

Monoterpenes etherification reactions with alkyl alcohols over cesium partially exchanged Keggin heteropoly salts: effects of catalyst composition

In this work, cesium partially exchanged Keggin heteropolyacid (HPA) salts were prepared, characterized, and evaluated as solid catalysts in monoterpenes etherification reactions with alkyl alcohols. A comparison of the activity of soluble HPAs and their insoluble cesium salts showed that among three different Keggin anions the phosphotungstate was the most efficient catalyst. Assessments on the effects of the level of the protons exchange by cesium cations demonstrated that Cs2.5H0.5PW12O40 solid salt was the most active and selective phosphotungstate catalyst, converting β-pinene to α-terpinyl methyl ether. The influences of the main reaction parameters such as reaction temperature, time, catalyst load, substrate nature (i.e., alcohols and monoterpenes) were investigated. We have demonstrated that the simultaneous presence of the cesium ions and protons in the catalyst plays an essential role, being the 2.5–0.5 the optimum molar ratio. The Cs2.5H0.5PW12O40 salt was efficiently recovered and reused without loss of catalytic activity.

Factors affecting performance of electrochemical capacitors operating in Keggin-type silicotungstic acid electrolyte

The key factors influencing the application of Keggin-type silicotungstic acid electrolyte, H4SiW12O40 (SiW12), in electrochemical capacitors are considered here by means of electrochemical, N2 adsorption, and Differential Scanning Calorimetry analysis. A series of activated carbons has been selected in order to match the porous structure of the electrode allowing to accommodate ca. 1.1 nm in diameter SiW12O404− anions. It has been shown that for carbons with adequately wide pore size distribution, the electrical parameters of capacitor can be as high as for the standard acidic H2SO4 electrolyte but the system is characterized by better discharge dynamics and, consequently, by higher specific power. The observation can be attributed to high population and mobility of protons in the Keggin structure and to a great ionic (molar) conductivity of SiW12. It has been also shown that a water/methanol mixture can be used to permit application of the SiW12 electrolyte at low temperatures with a performance exceeding the H2SO4 counterpart. Additionally, the electrochemical behaviour of SiW12 has been compared to the characteristics of phosphate-coordinated analogue, H3PW12O40, namely by exploring the influence of the POMs redox processes on the systems’ charge storage properties and self-discharge rates.