Simplifying the Evaluation of Graphene Modified Electrode Performance Using Rotating Disk Electrode Voltammetry

Graphene modified electrodes have been fabricated by electrodeposition from an aqueous graphene oxide solution onto conducting Pt, Au, glassy carbon, and indium tin dioxide substrates. Detailed investigations of the electrochemistry of the [Ru­(NH₃)₆]^3+/2+ and [Fe­(CN)₆]^3‑/4‑ and hydroquinone and uric acid oxidation processes have been undertaken at glassy carbon and graphene modified glassy carbon electrodes using transient cyclic voltammetry at a stationary electrode and near steady-state voltammetry at a rotating disk electrode. Comparisons of the data with simulation suggest that the transient voltammetric characteristics at graphene modified electrodes contain a significant contribution from thin layer and surface confined processes. Consequently, interpretations based solely on mass transport by semi-infinite linear diffusion may result in incorrect conclusions on the activity of the graphene modified electrode. In contrast, steady-state voltammetry at a rotating disk electrode affords a much simpler method for the evaluation of the performance of graphene modified electrode since the relative importance of the thin layer and surface confined processes are substantially diminished and mass transport is dominated by convection. Application of the rotated electrode approach with carbon nanotube modified electrodes also should lead to simplification of data analysis in this environment

Polyoxometalate-Promoted Electrocatalytic CO₂ Reduction at Nanostructured Silver in Dimethylformamide

Electrochemical reduction of CO₂ is a promising method to convert CO₂ into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons, and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo₁₂O₄₀]^3– (PMo). Metallic Ag is formed upon reduction of Ag⁺. Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag–PMo nanocomposite for CO₂ reduction was evaluated in a CO₂ saturated dimethylformamide medium containing 0.1 M [<i>n</i>-Bu₄N]­PF₆ and 0.5% (v/v) added H₂O. The results show that this Ag–PMo nanocomposite can catalyze the reduction of CO₂ to CO with an onset potential of −1.70 V versus Fc^0/+, which is only 0.29 V more negative than the estimated reversible potential (−1.41 V) for this process and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec^–1 suggests that rapid formation of *CO₂^•– is followed by the rate-determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO₂ (and presumably CO₂^•–) and hence promotes the formation of CO₂^•–

PdCu@Pd Nanocube with Pt-like Activity for Hydrogen Evolution Reaction

The electronic properties of metal surfaces can be modulated to weaken the binding energy of adsorbed H-intermediates on the catalyst surface, thus enhancing catalytic activity for the hydrogen evolution reaction (HER). Here we first prepare PdCu alloy nanocubes (NCs) by coreduction of Cu­(acac)₂ (acac = acetylacetonate) and Na₂PdCl₄ in the presence of oleylamine (OAm) and trioctylphosphine (TOP). The PdCu NC coated glassy carbon electrode is then anodized at a constant potential of 0.51 V vs Ag/AgCl at room temperature in 0.5 M H₂SO₄ solution for 10 s, which converts PdCu NCs into core@shell PdCu@Pd NCs that show much enhanced Pt-like activity for the HER and much more robust durability. The improvements in surface property and HER activity are rationalized based on strain and ligand effects that enhance the activity of the edge-exposed Pd atoms on core@shell PdCu@Pd structure. This work opens up a new perspective for simultaneously reducing metal Pd cost and achieving excellent performance toward the HER

Voltammetric Determination of the Reversible Potentials for [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]^10– over the pH Range of 2–12: Electrolyte Dependence and Implications for Water Oxidation Catalysis

Voltammetric studies of the Ru-containing polyoxometalate water oxidation molecular catalyst [{Ru₄O₄(OH)₂(H₂O)₄}­(γ-SiW₁₀O₃₆)₂]^10– ([<b>1</b>(γ-SiW₁₀O₃₆)₂]^10– where <b>1</b> represents the {Ru₄O₄(OH)₂(H₂O)₄} core and <b>1</b>(0) stands for its initial form with all ruthenium centers in the oxidation state IV) have been carried out in aqueous media over a wide range of pH (2–12 using Britton–Robinson buffer) and ionic strength. Well-defined voltammograms in buffered media are only obtained when Frumkin double layer effects are suppressed by the presence of a sufficient concentration of additional supporting electrolyte (LiNO₃, NaNO₃, KNO₃, Ca­(NO₃)₂, Mg­(NO₃)₂, MgSO₄, or Na₂SO₄). A combination of data derived from dc cyclic, rotating disk electrode, and Fourier transformed large amplitude ac voltammetry allow the assignment of two processes related to reduction of the framework and the complete series of Ru^III/IV and Ru^IV/V redox processes and also provide their reversible potentials. Analysis of these data reveals that K⁺ has a significantly stronger interaction with <b>1</b>(1) (the number inside bracket stands for the number of electrons removed from <b>1</b>(0)) than found for the other cations investigated, and hence its presence significantly alters the pH dependence of the <b>1</b>(0)/<b>1</b>(1) reversible potential. Comparison of experimental data with theory developed in terms of equilibrium constants for process <b>1</b>(0)/<b>1</b>(1) reveals that both H⁺ and K⁺ interact competitively with both <b>1</b>(0) and <b>1</b>(1). Importantly, reversible potential data reveal that (i) proton transfer does not necessarily need to be coupled to all electron transfer steps to achieve catalytic oxidation of water, (ii) the four-electron oxidized form, <b>1</b>(4), is capable of oxidizing water under all conditions studied, and (iii) under some conditions, the three-electron oxidized form, <b>1</b>(3), also exhibits considerable catalytic activity

Voltammetric and Spectroscopic Studies of α- and β‑[PW₁₂O₄₀]^3– Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(<i>n</i>‑Bu₄N)₃][PW₁₂O₄₀] Salts

The structure of the Keggin-type β-[PW₁₂O₄₀]^3– (PW₁₂) polyoxometalate, with <i>n</i>-Bu₄N⁺ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [<i>n</i>-Bu₄N]­[PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W­(VI)/W­(V) processes converge into a single two-electron process if sufficient acid is present. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4– indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3–/β-[PW^VW^VI₁₁O₄₀]^4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index

Voltammetric and Spectroscopic Studies of α- and β‑[PW₁₂O₄₀]^3– Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(<i>n</i>‑Bu₄N)₃][PW₁₂O₄₀] Salts

The structure of the Keggin-type β-[PW₁₂O₄₀]^3– (PW₁₂) polyoxometalate, with <i>n</i>-Bu₄N⁺ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [<i>n</i>-Bu₄N]­[PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W­(VI)/W­(V) processes converge into a single two-electron process if sufficient acid is present. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4– indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3–/β-[PW^VW^VI₁₁O₄₀]^4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index

Voltammetric and Spectroscopic Studies of α- and β‑[PW₁₂O₄₀]^3– Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(<i>n</i>‑Bu₄N)₃][PW₁₂O₄₀] Salts

The structure of the Keggin-type β-[PW₁₂O₄₀]^3– (PW₁₂) polyoxometalate, with <i>n</i>-Bu₄N⁺ as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [<i>n</i>-Bu₄N]­[PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0–5 mM mimic experimental data exceptionally well. These simulation–experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W­(VI)/W­(V) processes converge into a single two-electron process if sufficient acid is present. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4– indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3–/β-[PW^VW^VI₁₁O₄₀]^4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index

Electrooxidation of Ethanol and Methanol Using the Molecular Catalyst [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]^10–

Highly efficient electrocatalytic oxidation of ethanol and methanol has been achieved using the ruthenium-containing polyoxometalate molecular catalyst, [{Ru₄­O₄­(OH)₂­(H₂O)₄}­(γ-SiW₁₀O₃₆)₂]^10– ([<b>1</b>(γ-SiW₁₀­O₃₆)₂]^10–). Voltammetric studies with dissolved and surface-confined forms of [<b>1</b>(γ-SiW₁₀O₃₆)₂]^10– suggest that the oxidized forms of <b>1</b> can act as active catalysts for alcohol oxidation in both aqueous (over a wide pH range covering acidic, neutral, and alkaline) and alcohol media. Under these conditions, the initial form of <b>1</b> also exhibits considerable reactivity, especially in neutral solution containing 1.0 M NaNO₃. To identify the oxidation products, preparative scale bulk electrolysis experiments were undertaken. The products detected by NMR, gas chromatography (GC), and GC-mass spectrometry from oxidation of ethanol are 1,1-diethoxyethane and ethyl acetate formed from condensation of acetaldehyde or acetic acid with excess ethanol. Similarly, the oxidation of methanol generates formaldehyde and formic acid which then condense with methanol to form dimethoxymethane and methyl formate, respectively. These results demonstrate that electrocatalytic oxidation of ethanol and methanol occurs via two- and four-electron oxidation processes to yield aldehydes and acids. The total faradaic efficiencies of electrocatalytic oxidation of both alcohols exceed 94%. The numbers of aldehyde and acid products per catalyst were also calculated and compared with the literature reported values. The results suggest that <b>1</b> is one of the most active molecular electrocatalysts for methanol and ethanol oxidation

Observation of Ferromagnetic Exchange, Spin Crossover, Reductively Induced Oxidation, and Field-Induced Slow Magnetic Relaxation in Monomeric Cobalt Nitroxides

The reaction of [Co^II(NO₃)₂]·6H₂O with the nitroxide radical, 4-dimethyl-2,2-di­(2-pyridyl) oxazolidine-<i>N</i>-oxide (L^•), produces the mononuclear transition-metal complex [Co^II(L^•)₂]­(NO₃)₂ (<b>1</b>), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co²⁺ ion with both ligands in the neutral radical form (L^•) forming a linear L^•···Co­(II)···L^• arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical–radical intramolecular ferromagnetic interactions stabilizing a <i>S</i> = ³/₂ ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that <b>1</b> has a positive <i>D</i> value and nonzero <i>E</i> values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*_NO orbitals with a small orbital overlap leading to radical–radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical–radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co^III(L^–)₂]­(BPh₄) (<b>2</b>) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on <b>1</b> and <b>2</b> revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues

Observation of Ferromagnetic Exchange, Spin Crossover, Reductively Induced Oxidation, and Field-Induced Slow Magnetic Relaxation in Monomeric Cobalt Nitroxides

The reaction of [Co^II(NO₃)₂]·6H₂O with the nitroxide radical, 4-dimethyl-2,2-di­(2-pyridyl) oxazolidine-<i>N</i>-oxide (L^•), produces the mononuclear transition-metal complex [Co^II(L^•)₂]­(NO₃)₂ (<b>1</b>), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co²⁺ ion with both ligands in the neutral radical form (L^•) forming a linear L^•···Co­(II)···L^• arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical–radical intramolecular ferromagnetic interactions stabilizing a <i>S</i> = ³/₂ ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that <b>1</b> has a positive <i>D</i> value and nonzero <i>E</i> values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*_NO orbitals with a small orbital overlap leading to radical–radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical–radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co^III(L^–)₂]­(BPh₄) (<b>2</b>) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on <b>1</b> and <b>2</b> revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues