Cobalt Polyoxometalate Research Articles

Poly(dehydroalanine)-Based Hydrogels as Efficient Soft Matter Matrices for Light-Driven Catalysis.

Soft matter integration of photosensitizers and catalysts provides promising solutions to developing sustainable materials for energy conversion. Particularly, hydrogels bring unique benefits, such as spatial control and 3D-accessibility of molecular units, as well as recyclability. Herein, the preparation of polyampholyte hydrogels based on poly(dehydroalanine) (PDha) is reported. Chemically crosslinked PDha with bis-epoxy poly(ethylene glycol) leads to a transparent, self-supporting hydrogel. Due to the ionizable groups on PDha, this 3D polymeric matrix can be anionic, cationic, or zwitterionic depending on the pH value, and its high density of dynamic charges has a potential for electrostatic attachment of charged molecules. The integration of the cationic molecular photosensitizer [Ru(bpy)3 ]2+ (bpy = 2,2'-bipyridine) is realized, which is a reversible process controlled by pH, leading to light harvesting hydrogels. They are further combined with either a thiomolybdate catalyst ([Mo3 S13 ]2- ) for hydrogen evolution reaction (HER)or a cobalt polyoxometalate catalyst (Co4 POM = [Co4 (H2 O)2 (PW9 O34 )2 ]10- ) for oxygen evolution reaction (OER). Under the optimized condition, the resulting hydrogels show catalytic activity in both cases upon visible light irradiation. In the case of OER, higher photosensitizer stability is observed compared to homogeneous systems, as the polymer environment seems to influence decomposition pathways.

High-performance BiVO4 photoanodes: elucidating the combined effects of Mo-doping and modification with cobalt polyoxometalate

Significant water oxidation reactivity enhancement in BiVO4 photoanodes is achieved by simultaneous bulk doping with molybdenum and surface-modification with a polyoxometalate water oxidation co-catalyst.

Embedding cobalt polyoxometalate in polypyrrole shell for improved photoelectrochemical performance of BiVO4 core

Bismuth vanadate (BiVO4), as an appropriate semiconducting material with a narrow band gap and high light-harvesting, suffers from a high charge recombination rate and low photoactivity. To address these issues, in this work, the embedded cobalt polyoxometalate (POM) within the network of polypyrrole (PPy) coated on BiVO4 has been proposed to promote charge injection and separation. The photoanode based on BiVO4 was fabricated by electrophoretic deposition (EPD) under the optimized conditions. Conducting a comprehensive and fundamental study, we also implemented experiments to achieve the best conditions for BiVO4 preparation and the creation and optimization of PPy heterojunction with and without POM. The photoelectrochemical and physical analysis results revealed that the simultaneous incorporation of PPy and POM makes BiVO4 experience an impressive current enhancement, a 900% increase relative to bare BiVO4. Moreover, the hole extraction efficiency was enhanced to 93.7% from 15.12% (BiVO4), and charge separation yielded 33.83% from 24.47% (BiVO4). This study inspires promising studies on heterojunction formation through inorganic/organic semiconductor hybridization, achieved by molecular water oxidation co-catalysts like POM.

Ultrafast black-color tunability of electrochromic dimming films using cobalt polyoxometalate-anchored nickel oxide nanoparticles

A black color tunability of an electrochromic (EC) dimming device was explored using cobalt polyoxometalate (CoPW)-anchored nickel oxide nanoparticles (CoPW@NiOx).

A Triad Photoanode for Visible Light‐Driven Water Oxidation via Immobilization of Molecular Polyoxometalate on Polymeric Carbon Nitride

AbstractDue to their availability, low cost, nontoxicity, and tunability, polymeric carbon nitrides (CNx) represent one of the most attractive materials classes for the development of fully sustainable photo(electro)catalytic systems for solar‐driven water splitting. However, the development of CNx‐based photoanodes for visible light‐driven water oxidation to dioxygen is rather challenging, particularly due to issues related to photoelectrode stability and effective coupling of the light absorber with water oxidation catalysts. Herein, a triadic photoanode comprising a porous TiO2 electron collector scaffold sensitized by CNx coupled to a molecular cobalt polyoxometalate (CoPOM = [Co4(H2O)2(PW9O34)2]10) catalyst is reported. Complete water oxidation to dioxygen under visible (λ > 420 nm) light irradiation is demonstrated, with photocurrents down to relatively low bias potentials (0.2 V vs RHE). Furthermore, polyethyleneimine (PEI), a cationic polymer is shown to act as an effective and non‐sacrificial electrostatic linker for immobilization of the anionic CoPOM onto the negatively charged surface of CNx. The optimized deposition of CoPOM using the PEI linker translates directly into improved efficiency of the transfer of photogenerated holes to water molecules and to enhanced oxygen evolution. This work thus provides important design rules for effective immobilization of POM‐based catalysts into soft‐matter photoelectrocatalytic architectures for light‐driven water oxidation.

Stability evaluation of earth‐abundant metal‐based polyoxometalate electrocatalysts for oxygen evolution reaction towards industrial PEM electrolysis at high current densities

AbstractWe investigated the cobalt polyoxometalate catalyst Ba8[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] in oxygen evolution reaction for large‐scale water electrolysis. The catalyst was characterized, yielding BET surfaces (8.37 m2/g), crystal water content (8.38%, 44 H2O), elemental analyses and single crystal structures (space group P1̅, a = 19.901(4) Å, b = 21.177(4) Å, c = 24.036(5) Å, α = 92.689(7)°, β = 108.73(7)°, γ = 117.137(6)°, Co9Na16O196.05P5W27, V = 8310(3) Å2 with z = 2; R2final = 0.001).The catalyst was integrated in an industrially applicable membrane electrode assembly and electrochemically characterized. Polarization studies revealed catalyst dissolution in situ, visible as a current density peak (32.2 mA/cm2, 2.2 V) with subsequent collapse (<5 mA/cm2). Galvanostatic experiments showed voltage increase from 2.5 to > 10 V at 10 mA/cm2 tracing back to acid‐mediated decomposition of the anionic POM oxide framework. We deduced insufficient thermodynamic as well as kinetic stability for industrial requirements in PEM water electrolysis.

Co/Ni-polyoxotungstate photocatalysts as precursor materials for electrocatalytic water oxidation.

An open-core cobalt polyoxometalate (POM) [(A-α-SiW9O34)Co4(OH)3(CH3COO)3]8−Co(1) and its isostructural Co/Ni-analogue [(A-α-SiW9O34)Co1.5Ni2.5(OH)3(CH3COO)3]8−CoNi(2) were synthesized and investigated for their photocatalytic and electrocatalytic performance. Co(1) shows high photocatalytic O2 yields, which are competitive with leading POM water oxidation catalysts (WOCs). Furthermore, Co(1) and CoNi(2) were employed as well-defined precursors for heterogeneous WOCs. Annealing at various temperatures afforded amorphous and crystalline CoWO4- and Co1.5Ni2.5WO4-related nanoparticles. CoWO4-related particles formed at 300 °C showed substantial electrocatalytic improvements and were superior to reference materials obtained from co-precipitation/annealing routes. Interestingly, no synergistic interactions between cobalt and nickel centers were observed for the mixed-metal POM precursor and the resulting tungstate catalysts. This stands in sharp contrast to a wide range of studies on various heterogeneous catalyst types which were notably improved through Co/Ni substitution. The results clearly demonstrate that readily accessible POMs are promising precursors for the convenient and low-temperature synthesis of amorphous heterogeneous water oxidation catalysts with enhanced performance compared to conventional approaches. This paves the way to tailoring polyoxometalates as molecular precursors with tuneable transition metal cores for high performance heterogeneous electrocatalysts. Our results furthermore illustrate the key influence of the synthetic history on the performance of oxide catalysts and highlight the dependence of synergistic metal interactions on the structural environment.

Cobalt polyoxometalate on N-doped carbon layer to boost photoelectrochemical water oxidation of BiVO4

• Ternary BiVO 4 -N/C-CoPOM photoanode is constructed. • High photocurrent density of 3.30 mA cm −2 and low onset potential are achieved. • Much higher applied bias photon-to-current efficiency and fill factor are achieved after depositing CoPOM. • Charge injection efficiency is greatly improved after depositing CoPOM. Photoelectrochemical (PEC) water splitting can convert solar power into clean energy carrier of hydrogen, which is a promising strategy to address the global energy crisis and environmental issues. At present, the sluggish kinetics of the separation and migration of the photo-excited electrons and holes for water oxidation is the bottleneck for the high solar-to-hydrogen efficiency in this field. Herein, we exploit cobalt polyoxometalate (CoPOM) as a novel molecular cocatalyst locating at N-doped carbon (N/C) to boost the charge separation and injection of BiVO 4 photoanodes for PEC water oxidation. Comprehensive results reveal that the uniformly distributed CoPOM specially improves the hole injection efficiency, leading to the lower onset potential of water oxidation and confirming the improved the kinetics of BiVO 4 photoanode for PEC water splitting. A high maximum applied bias photon-to-current efficiency of 1.22% is achieved at a low potential of 0.63 V vs . reversible hydrogen electrode (RHE) for BiVO 4 -N/C-CoPOM, and the photocurrent density of increases up to 1.93 mA cm −2 at 0.63 V vs . RHE and 3.30 mA cm −2 at 1.23 V vs . RHE, which are 11.4 and 5.4 times higher than that of the pristine BiVO 4, respectively. Particularly, the significantly improved PEC performance in the low bias range indicates CoPOM can effectively reduce the water oxidation energy barrier. We believe this research provides a novel molecular cocatalyst of POM for the improvement of PEC performance of photoanode, and expands the scope for the design of efficient cocatalysts for solar energy-conversion devices.

PVIM-Co5POM/MNC composite as a flexible electrode for the ultrasensitive and highly selective non-enzymatic electrochemical detection of cholesterol.

To address the issues allied with cardiovascular diseases and the increased prevalence of hypertension, a novel electrochemical sensor was developed based on poly(ionic liquid) [PVIM]-cobalt polyoxometalate (Co5POM) supported on carbonaceous materials (MNCs) for the highly selective and ultrasensitive non-enzymatic detection of cholesterol. The novel composite exhibits excellent selectivity and sensitivity towards the non-enzymatic electrochemical detection of cholesterol with a low detection limit of 1 fM (1 × 10-15 M), a response time of ∼5 s, and a wide detection range of 1 fM-5 mM with two linear ranges of 1 fM-200 nM and 0.5 μM-5 mM with sensitivities of 210 and 64 μA μM-1 cm-2, respectively, even in the presence of ascorbic acid, glucose and uric acid. The proposed sensor was tested with a human blood serum sample at physiological pH in a physiologically relevant concentration range. A novel flexible sensor was developed with PVIM-Co5POM/MNC coated over Whatman filter paper and this shows a high sensitivity with a wide detection range of 1 fM-5 mM.

Interface Engineering of Hematite with Nacre-like Catalytic Multilayers for Solar Water Oxidation.

An efficient water oxidation photoanode based on hematite has been designed and fabricated by tailored assembly of graphene oxide (GO) nanosheets and cobalt polyoxometalate (Co-POM) water oxidation catalysts into a nacre-like multilayer architecture on a hematite photoanode. The deposition of catalytic multilayers provides a high photocatalytic efficiency and photoelectrochemical stability to underlying hematite photoanodes. Compared to the bare counterpart, the catalytic multilayer electrode exhibits a significantly higher photocurrent density and large cathodic shift in onset potential (∼369 mV) even at neutral pH conditions due to the improved charge transport and catalytic efficiency from the rational and precise assembly of GO and Co-POM. Unexpectedly, the polymeric base layer deposited prior to the catalytic multilayers improves the performance even more by facilitating the transfer of photogenerated holes for water oxidation through modification of the flat band potential of the underlying photoelectrode. This approach utilizing polymeric base and catalytic multilayers provides an insight into the design of highly efficient photoelectrodes and devices for artificial photosynthesis.

Electrochemically Driven Water-Oxidation Catalysis Beginning with Six Exemplary Cobalt Polyoxometalates: Is It Molecular, Homogeneous Catalysis or Electrode-Bound, Heterogeneous CoO x Catalysis?

A series of six exemplary cobalt-polyoxometalate (Co-POM) precatalysts have been examined to determine if they are molecular water-oxidation catalysts (WOCatalysts) or if, instead, they actually form heterogeneous, electrode-bound CoO x as the true WOCatalyst under electrochemically driven water-oxidation catalysis (WOCatalysis) conditions. Specifically, WOCatalysis derived from the following six Co-POMs has been examined at pH 5.8, 8.0, and 9.0: [Co4(H2O)2(PW9O34)2]10- (Co4P2W18), [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16- (Co9P5W27), [ ββ-Co4(H2O)2(P2W15O56)2]16- (Co4P4W30), [Co(H2O)PW11O39]5- (CoPW11), [α1-Co(H2O)P2W17O61]8- (α1-CoP2W17), and [α2-Co(H2O)P2W17O61]8- (α2-CoP2W17). The amount of Co(II)aq in 500 μM solutions of each Co-POM was measured after 3 h of aging as well as from t = 0 for pH = 5.8 and 8.0 by μM sensitive Co(II)aq-induced 31P NMR line broadening and at pH = 9.0 by cathodic stripping. The amount of detectable Co(II)aq after 3 h for the six Co-POMs ranges from ∼0.25 to ∼90% of the total cobalt initially present in the Co-POM. For 12 out of 18 total Co-POM and different pH cases, the amount Co(II)aq detected after 3 h forms heterogeneous CoO x able to account for ≥100% of the observed WOCatalysis activity. However, under 0.1 M NaPi, pH 5.8 conditions for CoPW11 and α1-CoP2W17 where ∼1.5% and 0.25% Co(II)aq is detectable, the measured Co(II)aq cannot account for the observed WOCatalysis. The implication is that these two Co-POMs are primarily molecular, Co-POM-based, WOCatalysts under electrochemically driven, pH 5.8, phosphate-buffer conditions. Even for the single most stable Co-POM, α1-CoP2W17, CoO x is still an estimated ∼76× faster WOCatalyst at pH = 5.8 and an estimated ∼740× faster WOCatalyst at pH = 8.

Self-Assembled Supramolecular Hybrid of Carbon Nanodots and Polyoxometalates for Visible-Light-Driven Water Oxidation.

Water splitting is considered the most attractive pursuit in the field of solar energy conversion. In this study, we report the synthesis and application of a supramolecular hybrid of carbon nanodot (CD) and cobalt polyoxometalate (Co-POM) to solar water oxidation. The self-assembly of the alginate-based CD and Co-POM led to the formation of a spherical hybrid of CD/Co-POM. Owing to the facile transfer of photogenerated holes from CD under visible light irradiation, the hybrid donor-acceptor type of CD/Co-POM enabled the rapid scavenging of holes and accumulation of a long-lived oxidation state of Co-POM for efficient solar water oxidation, outperforming conventional [Ru(bpy)3]2+-based systems. We believe that this study offers new insights into the development of CD-based nanocomposites with various photocatalytic and optoelectronic applications.

Electrochemical Water Oxidation Catalysis Beginning with Co(II) Polyoxometalates: The Case of the Precatalyst Co4V2W18O6810–

The question is addressed of whether the cobalt polyoxometalate (Co-POM) precatalyst Co4V2W18O6810– (hereafter Co4V2W18) is a stable, homogeneous water oxidation catalyst under electrochemically driven conditions and in 0.1 M pH 5.8 and 8.0 NaPi buffer as well as pH 9.0 sodium borate (NaB) buffer. This question is of considerable interest since Co4V2W18 has been reported to be highly stable and a 200-fold faster water oxidation catalyst than its P congener Co4P2W18O6810– (hereafter Co4V2W18), for reasons that were not specified. The nature of the true water oxidation catalyst with Co4V2W18 as the starting material is of further fundamental interest because a recent report reveals that the 51V NMR peak at ca. −507 ppm assigned by others to Co4V2W18 and used to argue for its solution stability is, instead, correctly assigned to the highly stable cis-V2W4O194–, in turn raising the question of the true stability of Co4V2W18 under water oxidation catalysis conditions. A battery of physical methods is used to a...

Cobalt Polyoxometalate Co4V2W18O68(10-): A Critical Investigation of Its Synthesis, Purity, and Observed (51)V Quadrupolar NMR.

The vanadium-containing cobalt polyoxometalate (Co-POM) Co4V2W18O68(10-) (hereafter Co4V2W18) has been reported to be a stable, homogeneous water-oxidation catalyst, one with a claimed record turnover frequency that is also reportedly 200-fold faster than its phosphorus congener, Co4P2W18O68(10-). The claimed superior water-oxidation catalysis activity of the vanadium congener, Co4V2W18, rests squarely on the reported synthesis of Co4V2W18, its purity, and its stability in both the solid-state and in solution. Attempts to repeat the preparation of Co4V2W18 by either of two literature syntheses, along with the other studies reported herein, led to the discovery of multiple, convoluted problems in the prior literature of Co4V2W18. The three most serious of those problems proved to be the prior misunderstanding of the quadrupolar (herein (51)V) NMR peak widths in complexes that also contain paramagnetic metals such as Co(II), the incorrect assignment of a -506.8 ppm (51)V NMR to Co4V2W18, and then the use of that -506.8 peak to argue for the stability of Co4V2W18 in solution. The results are reported in a somewhat historical, "story" fashion en route to elucidating and fully supporting the 11 insights and take-home messages listed in the Summary and Conclusions section.

Water Oxidation Catalysis Beginning with Co4(H2O)2(PW9O34)210– When Driven by the Chemical Oxidant Ruthenium(III)tris(2,2′-bipyridine): Stoichiometry, Kinetic, and Mechanistic Studies en Route to Identifying the True Catalyst

Stoichiometry and kinetics are reported for catalytic water oxidation to O2 beginning with the cobalt polyoxometalate Co4(H2O)2(PW9O34)210– (Co4POM) and the chemical oxidant ruthenium(III)tris(2,2′-bipyridine) (Ru(III)(bpy)33+). This specific water oxidation system was first reported in a 2010 Science paper (Yin et al. Science 2010, 328, 342). Under standard conditions employed herein of 1.0 μM Co4POM, 500 μM Ru(III)(bpy)33+, 100 μM Ru(II)(bpy)32+, pH 7.2, and 0.03 M sodium phosphate buffer, the highest O2 yields of 22% observed herein are seen when Ru(II)(bpy)32+ is added prior to the Ru(III)(bpy)33+ oxidant; hence, those conditions are employed in the present study. Measurement of the initial O2 evolution and Ru(III)(bpy)33+ reduction rates while varying the initial pH, [Ru(III)(bpy)33+], [Ru(II)(bpy)32+], and [Co4POM] indicate that the reaction follows the empirical rate law: −d[Ru(III)(bpy)33+]/dt = (k1 + k2)[Co4POM]soluble[Ru(III)(bpy)33+]/[H+], where the rate constants k1 ∼ 0.0014 s–1 and k2 ∼ 0.0044 s–1 correspond to the water oxidation and ligand oxidation reactions, and for O2 evolution, d[O2]/dt = (k1/4)[Co4POM]soluble[Ru(III)(bpy)33+]/[H+]. Overall, at least seven important insights result from the present studies: (i) Parallel WOC and Ru(III)(bpy)33+ self-oxidation reactions well documented in the prior literature limit the desired WOC and selectivity to O2 in the present system to ≤28%. (ii) The formation of a precipitate from ∼2 Ru(II)(bpy)32+/3 Co4(H2O)2(PW9O34)210– with a Ksp = (8 ± 7) × 10–25 (M5) greatly complicates the reaction and interpretation of the observed kinetics, but (iii) the best O2 yields are still when Ru(II)(bpy)32+ is preadded. (iv) CoOx is 2–11 times more active than Co4POM under the reaction conditions, but (v) Co4POM is still the dominant WOC under the Co4POM/Ru(III)(bpy)33+ and other reaction conditions employed. The present studies also (vi) confirm that the specific conditions matter greatly in determining the true WOC and (vii) allow one to begin to construct a plausible WOC mechanism for the Co4POM/Ru(III)(bpy)33+ system.

Water Oxidation Catalysis Beginning with 2.5 μM [Co4(H2O)2(PW9O34)2]10–: Investigation of the True Electrochemically Driven Catalyst at ≥600 mV Overpotential at a Glassy Carbon Electrode

Evidence for the true water oxidation catalyst (WOC) when beginning with the cobalt polyoxometalate [Co4(H2O)2(PW9O34)2]10– (Co4–POM) is investigated at deliberately chosen low polyoxometalate concentrations (2.5 μM) and high electrochemical potentials (≥1.3 V vs Ag/AgCl) in pH 5.8 and 8.0 sodium phosphate electrolyte at a glassy carbon working electrode—conditions which ostensibly favor Co4–POM catalysis if present. Multiple experiments argue against the dominant catalyst being CoOx formed exclusively from Co2+ dissociated from the parent POM. Measurement of [Co2+] in the Co4–POM solution and catalytic controls with the corresponding amount of Co(NO3)2 cannot account for the O2 generated from 2.5 μM [Co4(H2O)2(PW9O34)2]10– solutions. This result contrasts with our prior investigation of Co4–POM under higher concentration and lower potential conditions (i.e., 500 μM [Co4(H2O)2(PW9O34)2]10–, 1.1 V vs Ag/AgCl, as described in Stracke, J. J.; Finke, R. G. J. Am. Chem. Soc.2011, 133, 14872) and highlights the importance ofreactionconditions in governing the identity of the true, active WOC. Although electrochemical studies are consistent with Co4–POM being oxidized at the glassy carbon electrode, it is not yet possible to distinguish a Co4–POM catalyst from a CoOx catalyst formed via decomposition of Co4–POM. Controls with authentic CoOx indicate conversion of only 3.4% or 8.3% (at pH 8.0 and 5.8) of Co4–POM into a CoOx catalyst could account for the O2-generating activity, and HPLC quantification of the Co4–POM stability shows the postreaction Co4–POM concentration decreases by 2.7 ± 7.6% and 9.4 ± 5.1% at pH 8.0 and 5.8. Additionally, the [Co2+] in a 2.5 μM Co4–POM solution increases by 0.55 μM during 3 min of electrolysis—further evidence of the Co4-POM instability under oxidizing conditions. Overall, this study demonstrates the challenges of identifying the true WOC when examining micromolar amounts of a partially stable material and when nanomolar heterogeneous metal-oxide will account for the observed O2-generating activity.

Electrocatalytic Water Oxidation Beginning with the Cobalt Polyoxometalate [Co4(H2O)2(PW9O34)2]10–: Identification of Heterogeneous CoOx as the Dominant Catalyst

The question of "what is the true catalyst?" when beginning with the cobalt polyoxometalate (POM) [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) in electrochemical water oxidation catalysis is examined in pH 8.0 sodium phosphate buffer at a glassy carbon electrode. Is [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) a true water oxidation catalyst (WOC), or just a precatalyst? Electrochemical, kinetic, UV-vis, SEM, EDX, and other data provide four main lines of compelling evidence that, under the conditions used herein, the dominant WOC is actually heterogeneous CoO(x) and not homogeneous [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-).

Cobalt Polyoxometalate, [CoW12O40]5‐, as a New Reusable Catalyst for Addition of Trimethylsilyl Cyanide to Carbonyl Compounds.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Cobalt Polyoxometalate, [CoW12O40]5-, as a New Reusable Catalyst for Addition of Trimethylsilyl Cyanide to Carbonyl Compounds

Cobalt Polyoxometalate, [CoW₁₂O₄₀]^5-, as a New Reusable Catalyst for Addition of Trimethylsilyl Cyanide to Carbonyl Compounds

ChemInform Abstract: Cobalt Polyoxometalate, CoW12O405‐ as a New Reusable Catalyst for the Direct, Fast, and Efficient Acetylation of Alcohols and Phenols under Solventless Conditions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.