Sol-gel derived carbon ceramic electrode for the investigation of the electrochemical behavior and electrocatalytic activity of neodymium hexacyanoferrate

The electrochemical properties of an electroactive rare earth metal hexacyanoferrate, neodymium hexacyanoferrate (NdHCF) were studied by mechanically attaching NdHCF samples to the surface of carbon ceramic electrodes (CCEs) derived from sol-gel technique. The resulting modified electrodes exhibit well-defined redox responses with the formal potential of 0.241 V (versus SCE) at a scan rate of 20 mV s⁻¹ in 0.5 M KCl solution. The voltammetric characteristics of the NdHCF-modified CCEs in the presence of different alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) were investigated by voltammetry. The NdHCF-modified CCEs presented a good electrocatalytic activity towards the reduction of hydrogen peroxide (H₂O₂), and was used for amperometric detection of H₂O₂. In addition, the NdHCF-CCEs exhibited a distinct advantage of simple preparation, surface renewal, good stability and reproducibility.     

Rapid Separation of Tl+ and Pb2+ from Various Binary Cation Mixtures Using Dicyclohexano-18-crown-6 Incorporated into Emulsion Membranes

Abstract

Relative transport rates of metal cation nitrates (Na⁺, K⁺, Rb⁺, Cs⁺, Ag⁺, Tl⁺, Ca²⁺, Sr²⁺, Ba²⁺, and Pb²⁺) in a water-toluene-water emulsion membrane system were measured. The toluene component contained the surfactant Span 80 and the crown ether dicyclohexano-18-crown-6. The aqueous receiving phase contained Li₄P2O₇. When each metal cation was individually present in the aqueous source phase, metal extraction was complete within 10 min with the order of extraction being Tl⁺ > Cs⁺ > Ag⁺ > Rb⁺ > K⁺ ≥m Na⁺ and Pb⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ for uni-and bivalent cations, respectively. Significant extraction was found for all cations except Na⁺, K⁺, and Ba²⁺. Some metal ions were concentrated nearly 10-fold in a 10-min period. Relative transport rates were determined when binary cation mixtures of either Tl⁺ or Pb²⁺ were present at equal concentrations with each of the remaining metal ions in the source phase. Tl⁺, when present with either Na⁺, Cs⁺, or Rb⁺, was selectively extracted from the source phase. Complete and nearly exclusive extraction of Pb²⁺ was observed in the presence of all cations including Tl⁺. The enrichment ratios of Pb²⁺ in the binary mixtures were approximately 10 while those of the second cation were less than 0.5 except for Sr²⁺ which was 0.86. Corresponding separation factors for Pb²⁺ ranged from 1000 to > 6000.     

Preparation and characterization of osmium hexacyanoferrate films and their electrocatalytic properties

Osmium hexacyanoferrate films have been prepared using repeated cyclic voltammetry, and the deposition process and the films’ electrocatalytic properties in electrolytes containing various cations have been investigated. The cyclic voltammograms recorded the deposition of osmium hexacyanoferrate films directly from the mixing of Os³⁺ and Fe(CN)₆³⁻ ions from solutions containing various cations. An electrochemical quartz crystal microbalance, cyclic voltammetry, and UV-visible spectroscopy were used to study the growth mechanism of the osmium hexacyanoferrate films. The osmium hexacyanoferrate films showed a single redox couple, and the redox reactions included “electron transfer” and “proton transfer” with a formal potential that demonstrates a proton effect in acidic solutions up to a 12 M aqueous HCl solution. The electrochemical and electrochemical quartz crystal microbalance results indicate that the redox process was confined to the immobilized osmium hexacyanoferrate film. The electrocatalytic reduction of dopamine, epinephrine, norepinephrine, S₂O₃²⁻, and SO₅²⁻ by the osmium hexacyanoferrate films was performed. The preparation and electrochemical properties of co-deposited osmium(III) hexacyanoferrate and copper(II) hexacyanoferrate films were determined, and their two redox couples showed formal potentials that demonstrated a proton effect and an alkaline cation effect, respectively. Electrocatalytic reactions on the hybrid films were also investigated.     

Green synthesis of nickel oxide coupled copper hexacyanoferrate (NiO2–CuHCF) nanocomposites for efficient and highly stable natural sunlight-driven photocatalytic degradation of wastewater pollutants

Metal hexacyanoferrates are found to be the promising candidates utilized to degrade toxic dyes from waste water under natural sunlight irradiation. Coupling of metal hexacyanoferrate with metal oxide enhances the degradation efficacy. In present work, nickel oxide coupled copper hexacyanoferrate nanocomposites (NiO₂–CuHCF) are synthesized by employing facile co-precipitation route. XRD reveals the incorporation of NiO₂ with CuHCF. SEM confirms the sheet like structures of NiO₂–CuHCF nanocomposite. TEM demonstrates the even distribution of elements present in the NiO₂–CuHCF nanocomposite. XPS shows the active participation of constituents having different valence states. The UV–Vis (DRS) is employed to measure the optical properties. The band gap of nickel oxide coupled copper hexacyanoferrate nanocomposite (NiO₂–CuHCF) decreases in comparison with pure copper hexacyanoferrate (CuHCF) from 1.23 to 1.14 eV. 97% degradation of Methyl blue (MB) is observed in just 110 min by NiO₂–CuHCF nanocomposite when exposed to natural sunlight. Moreover, it also retains 92% photocatalytic efficiency over 6 cycles which shows its excellent stability and reusability. The improved photocatalytic performance of NiO₂–CuHCF nanocomposite is attributed to their sheet resembling morphologies which may provide adequate active sites responsible to transport more charge carriers and also reduce charge recombination. In general, this research may open new avenue to use CuHCF with the addition of NiO₂ in future as an efficient degradation agent at industrial level.     

Electrochemistry-electrospray-mass spectrometry study of cesium uptake in nickel hexacyanoferrate films

The electrochemically-controlled uptake and release of Cs⁺ by nickel hexacyanoferrate thin-film electrodes that were prepared with different counter cations (Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺) were examined using a thin-layer flow cell coupled on-line with electrospray-mass spectrometry (ES-MS). A potential step technique was used to control the uptake and release of the cations by the films. Performance of the different films in terms of Cs⁺ capacity, Cs⁺ uptake selectivity versus the other alkali metal ions, film stability, and the effect of a saline matrix on Cs⁺ uptake were examined. These results show that the electrochemistry-mass spectrometry (EC-MS) combination provides rapid, Cs⁺ specific information concerning the films that is similar to or complementary to that information which might be obtained using conventional electrochemical approaches. Information that cannot be obtained electrochemically (e.g. isotopic distributions of metal ions accumulated and released) might also be gathered. Any number of electrode reactions involving ion uptake and release might be studied using this approach.     

Effect of cations in solution on the oxidation of methanol on the surface of platinum electrode

The effect of metal cations in solution on the oxidation of methanol on the electrode surface of platinum is a neglected aspect to direct methanol fuel cell (DMFC). In this paper, a smooth platinum electrode absorbing metal cations as the working electrode was applied to investigate the methanol oxidation with the cyclic voltammetry (CV) in 1.0 mol L⁻¹ H₂SO₄. From the analysis of experiment, it is found that the cations, Li⁺, Ce⁴⁺, Mn²⁺, Ni²⁺, Cu²⁺, have some negative effect on the catalytic oxidation of methanol on the surface of platinum. The degree of the effect from different cations was analyzed.     

Thermodynamics of lithium,potassium and thallium electrode in propyléne carbonate

Standard potentials (E°) of Li, K and Tl electrodes in propylene carbonate (PC) have been determined, at 25°, on the basis of the “solvent-independent” half-reaction: Ferricinium⁺ +e ?Ferrocene. In addition, estimates of ΔG°, ΔH°, and ΔS° for the formation of Li⁺, K⁺, and Tl⁺ in PC have been obtained from temperature-dependence studies. The observed values of E° are in the order Tl > Li > K as compared to the order Tl > K > Li in water. The decrease in entropy, which would serve as a measure of the order-promoting ability of an ion, follows the sequence Li > K > Tl. From a consideration of the crystallographic radii of the cations involved this trend would suggest that such ordering is primarily a result of electrostatic solvation in the immediate vicinity of the ions. The somewhat large decrease in entropy observed in the case of Li⁺ reflects very extensive solvation of this particular cation. The free energies of transfer for the three cations from water (D = 78·54) to PC (D = 64·4) are calculated to be +1·81 (Li⁺), ?2·42 (K⁺), and ?2·88 (Tl⁺) kcal/mole. Although an approximately linear relationship exists between these free energies of transfer and the reciprocals of crystallographic radii of the ions, both the magnitudes and the signs of these free energies are not readily understood in terms of the Born equation. The relatively large free energy decrease associated with the transfer of Tl⁺ might be attributed to a fairly strong solvation encountered by this cation due to its empty 5f orbital, aided by the possibility that the solvating carbonyl oxygen of PC is more accessible to the cations in general.     

Stabilized heteropolyacid/Nafion composite membranes for elevated temperature/low relative humidity PEFC operation

Organic/inorganic composite membranes with different inorganic heteropolyacid (HPA) additives maintain sufficient proton conductivities for atmospheric pressure elevated temperature (>100 °C) polymer electrolyte fuel cell (PEFC) operation. However, membrane and membrane electrode assembly (MEA) processing is severely curtailed because of the solubility of the HPA additives in aqueous media. Composite membranes with the HPA (phosphotungstic acid; PTA) additive rendered insoluble by ion exchanging protons with larger cations such as Cs⁺, NH₄⁺, Rb⁺ and Tl⁺ were fabricated. The additive loss in aqueous media was lowered from nearly 100% (unmodified HPA) to about 5% (modified HPA). The membranes were robust, and demonstrated low H₂ crossover currents of around 2 mA/cm² for a 28 μm thick membrane. All membranes were evaluated at high temperatures and low relative humidities in an operating fuel cell. The conductivities of the composite membranes at 120 °C and 35% relative humidity were on the order of 1.6 × 10⁻² S/cm.     

Study of the potentiometric response of the doped spinel Li1.05Al0.02Mn1.98O4 for the optimization of a selective lithium ion sensor

In this paper, we studied the development of a selective lithium ion sensor constituted of a carbon paste electrode modified (CPEM) with an aluminum-doped spinel-type manganese oxide (Li_1.05Al_0.02Mn_1.98O₄) for investigating the influence of a doping ion in the sensor response. Experimental parameters, such as influence of the lithium concentration in the activation of the sensor by cyclic voltammetry, pH of the carrier solution and selectivity for Li⁺ against other alkali and alkaline-earth ions were investigated. The sensor response to lithium ions was linear in the concentration range 5.62 × 10⁻⁵ to 1.62 × 10⁻³ mol L⁻¹ with a slope 100.1 mV/decade over a wide pH 10 (Tris buffer) and detection limit of 2.75 × 10⁻⁵ mol L⁻¹, without interference of other alkali and alkaline-earth metals, demonstrating that the Al³⁺ doping increases the structure stability and improves the potentiometric response and sensitivity of the sensor. The super-Nernstian response of the sensor in pH 10 can be explained by mixed potential arising from two equilibria (redox and ion-exchange) in the spinel-type manganese oxide.     

Electrochemical and spectroscopic studies of metal hexacyanometalate films—III. Equilibrium and kinetics studies of cupric hexacyanoferrate

The potential response of cupric hexacyanoferrate (CuHCF) thin films on glassy carbon substrate was studied by cyclic voltammetry in potassium and ammonium ions. The shift in potential with ammonium ion concentration is Nernstian with a difference of +0.70 V in 1 M NH⁺₄ with respect to 1 M K⁺. Equilibrium spectroelectrochemical studies have shown that the mole fraction of CuHCF is a reflection of film activity. Kinetic studies have indicated that the redox reaction is faster in ammonium ion than in potassium ion. Similarities and differences in the equilibrium and kinetic response of the film in both electrolytes are discussed.     

Electroless immobilization and electrochemical characteristics of nickel hexacyanoruthenate film at an aluminum substrate

The surface of an aluminum (Al) electrode was modified with a thin film of nickel hexacyanoruthenate (NiHCR) as a novel electrode material. The modification procedure of Al surface, includes two consecutive procedures: (i) the electroless deposition of metallic nickel on the Al electrode surface from NiCl₂ solution, and (ii) the chemical transformation of deposited nickel to nickel hexacyanoruthenate films in solution of 20 mM K₃[Ru(CN)₆] + 0.5 M KNO₃. Cyclic voltammogram of the modified Al electrode showed a well-defined redox reaction due to [Ni^IIRu^III/II(CN)₆]^1−/2− system. The effects of different supporting electrolytes and solution pH were studied on the electrochemical characteristics of the modified electrode. The diffusion coefficients of K⁺ and Na⁺ cations in the film (D), the transfer coefficient (α), and the charge transfer rate constant at the modifying film/electrode interface (k_s), were calculated in the presence of both K⁺ and Na⁺ cations. The stability of the modified electrode was investigated under various experimental conditions.     

Sticking probability of CN radicals

The sticking probability, s, of CN(X²Σ⁺) radicals which were the precursor of the formation of amorphous carbon nitride films with high [N]/([N]+[C]) ratios (≤ 0.5) was re-evaluated. CN(X²Σ⁺) radicals were generated from the decomposition of BrCN with the microwave discharge flow of Ar of the pressure of 0.2-0.4 Torr. The number density of CN(X²Σ⁺), n_CN(X), was evaluated from the intensity of the CN(A²Π_i-X²Σ⁺) laser-induced fluorescence spectrum calibrated against Rayleigh scattering intensity of Ar. The weight of the C and N components of films, w, was evaluated from the compositional analysis for the deposited films using Rutherford back scattering and elastic recoil detection analysis. The [N]/([N]+[C]) ratios of films were 0.4-0.5. Based on n_CN(X), w, and the flow speed measured by a time-resolved emission, s was evaluated both under the desiccated and H₂O-added conditions as (8.5 ± 2.1) × 10^− 2 − (6.1 ± 1.2) × 10^− 2 and (11.4 ± 1.3) × 10^− 2 − (7.4 ± 1.8) × 10^− 2, respectively. The variation of s under various experimental conditions was discussed based on the electron densities in the reaction region.     

Ionic liquids and plastic crystals based on tertiary sulfonium and bis(fluorosulfonyl)imide

A new series of low-melting, low-viscosity, hydrophobic ionic liquids based on relatively small tertiary sulfonium cations ([R¹R²R³S]⁺, wherein R¹, R², R³ = CH₃ or C₂H₅, R³ = CH₂CH₂OCH₃, CH₂CH₂COOCH₃, CH₂CH₂CN) and bis(fluorosulfonyl)imide (FSI⁻) anion have been prepared and characterized. The important physicochemical and electrochemical properties of these salts, such as melting point, glass transition, viscosity, density, ionic conductivity, thermal and electrochemical stability, have been determined. The influence of structure variation in the tertiary sulfonium cations on the above physicochemical properties is discussed. Among these new salts, some of them show the desirable properties including low-melting points, low viscosities, and high conductivities, to be selected as potential candidates as electrolytes in energy devices, and two salts are ionic plastic crystals.     

An experimental study of electroreduction of CO2 to HCOOH on SnO2/C in presence of alkali metal cations (Li+, Na+, K+, Rb+ and Cs+) and anions (HCO3−, Cl−, Br− and I−)

It is well-known that the electrolytes can influence the electrochemical reduction of carbon dioxide (ERCO₂) in aqueous media. In this work, we explore the effects of alkali metal cations and anions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, HCO₃⁻, Cl⁻, Br⁻, I⁻) on the current density and product selectivity for the ERCO₂ into formic acid (HCOOH) on the SnO₂/carbon paper (SnO₂/C) electrode. Results of the ERCO₂ experiments show that for the cations, the promotion effects on current density and faradaic efficiencies (FEs) are in the order of Li⁺ < Na⁺ < K⁺ < Cs⁺ < Rb⁺. For the anions, the current density values are in the order of NaHCO₃ < NaCl < NaBr < NaI and KHCO₃ < KCl ≈ KI < KBr, respectively, and that on the FEs for the formation of the HCOOH (FE_HCOOH) is HCO₃⁻ < Cl⁻ < Br⁻ < I⁻. Based on this result, the effects of alkali metal cations and anions on ERCO₂ are discussed.     

Modification of poly acrylic acid using calix[4]arene derivative for the adsorption of toxic heavy metals

Poly acrylic acid (PAA) was grafted with p‐t‐butyl calix[4]arene diamine (distal cone) (2) to adsorb toxic heavy metal and alkali metal cations. The grafting method includes the amidation reaction of PAA with calixarene diamine derivative 2 which was carried out in N,N‐dimethylformamide (DMF) and N–methyl‐2‐pyrrolidone (NMP) as solvents. The modified PAAs (PAA‐C1 and PAA‐C2) were characterized by FTIR, ¹H‐NMR, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). PAA‐C1 and PAA‐C2 were used to evaluate the sorption properties of some toxic heavy metal cations (Co²⁺, Cu²⁺, Cd²⁺, Hg²⁺), alkali metal cations (Na⁺, K⁺, Cs⁺), and Ag⁺. Results showed that the modified PAAs were good sorbents for heavy metal and alkali metal cations. The main goal of this project is to design hydrophobically modified PAA that is suitable for ion selective membranes and chemical sensor devices for adsorption of toxic heavy metals. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Ionic liquid electrolytes based on multi-methoxyethyl substituted ammoniums and perfluorinated sulfonimides: Preparation, characterization, and properties

New functionalized ionic liquids (ILs), comprised of multi-methoxyethyl substituted quaternary ammonium cations (i.e. [N(CH₂CH₂OCH₃)_4−n(R)_n]⁺; n = 1, R = CH₃OCH₂CH₂; n = 1, R = CH₃, CH₂CH₃; n = 2, R = CH₃CH₂), and two representative perfluorinated sulfonimide anions (i.e. bis(fluorosulfonyl)imide (FSI⁻) and bis(trifluoromethanesulfonyl)imide (TFSI⁻)), were prepared. Their fundamental properties, including phase transition, thermal stability, viscosity, density, specific conductivity and electrochemical window, were extensively characterized. These multi-ether functionalized ionic liquids exhibit good capability of dissolving lithium salts. Their binary electrolytes containing high concentration of the corresponding lithium salt ([Li⁺] >1.6 mol kg⁻¹) show Li⁺ ion transference number (tLi⁺) as high as 0.6-0.7. Their electrochemical stability allows Li deposition/stripping realized at room temperature. The desired properties of these multi-ether functionalized ionic liquids make them potential electrolytes for Li (or Li-ion) batteries.     

A potentiometric and voltammetric sensor based on polypyrrole film with electrochemically induced recognition sites for detection of silver ion

Conducting polypyrrole membranes were deposited on glassy carbon electrodes by electropolymerizing pyrrole in the presence of Eriochrome Blue-Black B (EBB) as the counter anion. The electrodes were then subjected to several oxidation/reduction potential steps in pure silver nitrate solution for successive accumulation/stripping of silver species. This electrochemically mediated doping/templating generated selective recognition elements in the EBB/PPy film for silver ions. The resulting sensor exhibited a considerable enhancement in the potentiometric and voltammetric response characteristics: extending the linear dynamic range and lowering the detection limit. In the potentiometric mode, the sensor showed highly reproducible response with a Nernstian slope of 58.5 ± 0.3 mV per decade of Ag⁺ activity over a linear range spanning seven orders of magnitude (1 × 10⁻⁸ to 1 × 10⁻¹ M Ag⁺), with a detection limit of ∼6 × 10⁻⁹ M. The electrodes demonstrated high selectivity over a large number of cations including alkali, alkaline earth and several transition and heavy metal ions, and could be used over a wide pH range of 1-8.5. The EBB/PPy modified electrode was also used for preconcentration and differential pulse anodic stripping voltammetric (DPASV) measurements. The DPASV peak current was dependent on the concentration of Ag⁺ over the range 3 × 10⁻¹⁰ to 1 × 10⁻⁴ M. The presence of 1000-fold excess of Cd²⁺, Cu²⁺, Cr³⁺, Co²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Ni²⁺ and Pb²⁺ can be tolerated in the determination of silver ion.     

Electrochemical, EIS and AFM characterisation of biosensors: Trioxysilane sol-gel encapsulated glucose oxidase with two different redox mediators

Sol-gel encapsulated glucose oxidase (GOx) enzyme electrodes based on carbon film resistors with chemically deposited copper hexacyanoferrate (CuHCF) or poly(neutral red) (PNR), made by electrochemical polymerisation, as redox mediator have been developed and characterised using cyclic voltammetry, electrochemical impedance spectroscopy and atomic force microscopy. The sol-gel was prepared using three different trioxysilanes: 3-aminopropyl-triethoxysilane (APTOS), 3-glycidoxypropyl-trimethoxysilane (GOPMOS) and methyltrimethoxysilane (MTMOS), without alcohol addition, and alcohol formed during the hydrolysis of the precursor compounds was removed. The best sensitivity, ∼60 nA mM⁻¹, for glucose and limit of detection (2-40 μM, depending on the sol-gel precursor) were obtained when PNR was used as a mediator, but the linear range (50-600 μM) was two to four times lower than that at CuHCF mediated biosensors, using an operating potential of +0.05 V at CuHCF or −0.25 V versus saturated calomel electrode (SCE) at PNR mediated electrodes. The stability of the sensor depended on the sol-gel morphology and was 2 months testing the biosensor every day, while the storability was at least 4 months in the case of GOPMOS, the sensors being kept in buffer at +4 °C.     

Electrocatalytic oxidation of ascorbic acid by [Fe(CN)6] redox couple electrostatically trapped in cationic N,N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN)₆]⁴⁻), was electrostatically trapped in the cationic polymer film of N,N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN)₆]^3−/4−|PDMA|BDD) electrode. This [Fe(CN)₆]^3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ_Fe, within the polymer film. Diffusion coefficient (D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10⁻⁶ cm² s⁻¹. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10⁴ M⁻¹ s⁻¹. In the hydrodynamic amperometry using the [Fe(CN)₆]^3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm⁻² μM⁻¹.     

Electrochemical Recovery of Lithium Ions in the Aqueous Phase

Abstract

The electrochemical insertion of lithium ions into a Pt/λ-MnO₂ electrode was investigated in various metal chloride solutions. The Li⁺ insertion occurred effectively in LiCl solutions with higher concentration than 10 mmol/dm³, but it could hardly occur in a 0.1 mmol/dm³ LiCl solution. Alkaline earth metal ions showed a stronger inhibition effect against the Li⁺ insertion into the Pt/λ-MnO₂ electrode than alkali metal ions. However, since only Li⁺ ions were taken up from a mixed solution of lithium and alkaline earth metal chlorides, a high selectivity of the electrode for lithium ions was shown.

It was possible to recover lithium ions from geothermal water by this electrochemical method using the Pt/λ-MnO₂ electrode; the lithium uptake was 11 mg/g-MnO₂.