Preparation and Characterization of Mixed‐Valent Nickel Oxide/Nickel Hexacyanoferrate Hybrid Films and Their Electrocatalytic Properties

Polynuclear mixed‐valent nickel oxide and nickel hexacyanoferrate hybrid film was prepared on glassy carbon electrode by multiple scan cyclic voltammetry. The film growth was monitored using electrochemical quartz crystal microbalance (EQCM). The cyclic voltammogram of the nickel hexacyanoferrate film is characterized by single redox couple whereas nickel oxide/nickel hexacyanoferrate hybrid film exhibits two redox couples. Cyclic voltammetric features suggest that the charge transfer process in both films resembles that of surface‐confined redox species. In stronger basic solution (pH ≥9), nickel hexacyanoferrate film was gradually converted into nickel oxide film during potentiodynamic cycling. The peak potential of nickel oxide redox couple moved into more negative side with increasing pH of contacting solution whereas the peak potential of nickel hexacyanoferrate redox couple remains the same. Electrocatalytic behavior of hybrid film coated electrodes toward ascorbic acid, hydrazine and hydroxylamine was investigated using cyclic voltammetry technique. Analytical application of nickel oxide/nickel hexacyanoferrate hybrid film electrode was tested in amperometry and flow injection analysis.     

Electrochemical preparation and characterization of dysprosium hexacyanoferrate modified electrode

An electroactive polynuclear inorganic compound of rare earth metal hexacyanoferrate, dysprosium hexacyanoferrate (DyHCF), was prepared by a procedure of electrochemical deposition on the surface of a glassy carbon electrode with a potential cycling procedure. The cyclic voltammogram of DyHCF exhibits two pairs of redox peaks with the formal potential of +210 and +362 mV (vs. SCE), respectively, at a scan rate of 10 mV/s in 0.2 mol/L KCl solution. The different electrochemical behaviors of DyHCF in various cation-containing supporting electrolytes were investigated by cyclic voltammetry. DyHCF was also characterized by scanning electron microscope (SEM), FTIR , XPS etc. techniques.     

The Solid State Electrochemistry of Dysprosium(III) Hexacyanoferrate(II)

A new electroactive polynuclear inorganic compound of rare earth metal hexacyanoferrate, dysprosium hexacyanoferrate (DyHCF), was prepared chemically and characterized using techniques of FTIR spectroscopy, thermogravimetric analysis (TGA), UV‐vis spectrometry and X‐ray photoelectron spectroscopy (XPS) etc. The cyclic voltammetric behavior of DyHCF mechanically attached to the surface of graphite electrode was well defined and exhibited a pair of redox peaks with the formal potential of 217 mV (vs. SCE) at a scan rate of 100 mV/s in 0.2 M NaCl solution and the redox peak currents increased linearly with the square root of the scan rates.     

Mechanistic study of the oxidation of -ascorbic acid by chloranil at the nitrobenzene water interface

The redox reaction between -ascorbic acid in water and chloranil in nitrobenzene has been studied by means of polarography with an ascending water electrode as well as cyclic voltammetry with a stationary interface. Through accurate measurement of the limiting currents, it has been suggested that the redox reaction should be a two-electron reaction rather than a one-electron reaction described previously. A spectrophotometric technique has also been used to observe that the redox reaction proceeds spontaneously under certain conditions even without electrochemical control. Based on these findings, it has been concluded that the present heterogeneous charge transfer reaction is the ion transfer of chloranil semiquinone radical, which is driven by the homogeneous electron transfer between ascorbic acid and chloranil in the aqueous phase.     

电化学石英晶体微天平研究六氰亚铁铜膜修饰电极及其离子效应

利用电化学石英晶体微天平（EQCM）手段，结合循环伏安法．计时电流法对六氰亚铁铜（CuHCF)膜修饰电极及其在不同水溶液中的离子交换机制进行了研究。结果表明；通过循环伏安法，在Pt电极上可以牢固地形成CuHCF膜．在氧化还原过程中，不仅是阳离子，阴离子也参与了在CuHCF膜中的传输。     

固相配位化学反应研究XXXIV. 室温下六氰合铁(II)及(III)酸钾 的固相酸碱反 应

本文研究了室温时K~3Fe(CN)~6,K~4Fe(CN)~6在酸碱条件下发生的固相配位化学反应。结果表明:K~3Fe(CN)~6与NaBH~4固相混合物室温下不反应,但加入固体氢氧化钠后,K~3Fe(CN)~6与NaBH~4的固相氧化还原反应在室温下很容易进行。K~4Fe(CN)~6与K~2S~2O~8的固相氧化还原反应在室温下能顺利进行,但当固体KOH存在时,反应明显受到抑制。K~3Fe(CN)~6与K~2C~2O~4.H~2O室温下无反应,但与H~2C~2O~4.2H~2O在室温时即发生固相取代反应。     

A study of the charge propagation in nanoparticles of Fe2O3 core-cobalt hexacyanoferrate shell by chronoamperometry and electrochemical impedance spectroscopy

A composite electrode comprised of graphite, Nujol, and nanoparticles of Fe₂O₃ core–cobalt hexacyanoferrate shell was prepared and the charge transport processes in the bulk of this composite were studied. The electrode/solution interface was assumed as a binary electrolyte whose charge transport occurred between redox sites of the nanoparticles present in the composite and counter cations present in the solution. Using cyclic voltammetry, the diffusion of counter cation in the shell was investigated. Using chronoamperometry, an effective diffusion coefficient and its dependency on the applied potential was obtained. In the Nyquist diagrams, different time constants were appeared with relation to different physical and electrochemical processes. Percolation of electron in the shell of the nanoparticles appeared at very high frequencies and exhibited the feature of a diffusion process with a transmissive boundary condition at interface of core–shell structure/graphite particles. The diffusion coefficients of electron and counter cation and the standard rate constants of each individual electrochemical reaction were obtained.     

Selective electrooxidation of uric acid in presence of ascorbic acid at a room temperature ionic liquid/nickel hexacyanoferarrate nanoparticles composite electrode

A novel amperometric sensor for the determination of uric acid was fabricated using room temperature ionic liquid and nickel hexacyanoferrate nanoparticle composite which was immobilized on paraffin wax impregnated graphite electrode. The nickel hexacyanoferrate nanoparticle was characterized by UV-vis, X-ray diffraction and field emission scanning electron microscopy. The electrochemical behavior of the modified electrode was investigated in detail by electrochemical impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. Various experimental parameters influencing the electrochemical behavior of the modified electrode were optimized by varying the supporting electrolyte, scan rate and pH. The apparent electron transfer rate constant (K(s)) and charge transfer coefficient (α) of the modified electrode were found to be 1.358(± 0.02)cm/s and 0.65, respectively from cyclic voltammetry. The sensor exhibited an excellent electrocatalytic activity towards the oxidation of uric acid. The interference from ascorbic acid was easily overcome by coating the modified electrode with PEDOT layer. Under optimal condition, the determination range for uric acid is from 1.0 × 10(-6)M to 2.6 × 10(-3)M and the detection limit was 3.3 × 10(-7)M (3σ). The proposed method has been used for the determination of uric acid in human urine samples.     

Catalytic effect of copper on the hexacyanoferrate(III)-cyanide redox reaction-I The uncatalysed and catalysed reactions

A kinetic study of the hexacyanoferrate(III)-cyanide redox reaction has been made in connection with development of a new catalytic method for copper. The reaction kinetics change with time from first- to second-order dependence with respect to hexacyanoferrate(III). The reaction is nearly inverse first-order with respect to hexacyanoferrate(II) and first-order with respect to cyanide. The reaction shows a strong positive primary salt effect, but a very small increase in the reaction rate with temperature is found. A parallel reaction proceeds with a first-order dependence with respect to hydroxide. A tentative mechanism is proposed for the first reaction, involving the formation of cyanogen radicals. The second reaction corresponds to the well-known decomposition of hexacyanoferrate(III) in alkaline medium. The catalysed reaction exhibits similar kinetics with respect to hexacyanoferrate(II) and (III) but is zero-order with respect to cyanide and hydroxide and first-order with respect to catalyst. The proposed mechanism involves two consecutive interactions of the hexacyanoferrate(III) with copper(I) and with copper(II) cyanide complexes respectively, followed by a 2-electron oxidation of a co-ordinatively bridging cyanide group.     

基于铁氰化镍的非标记型核酸适配体电化学传感器检测凝血酶

在玻碳电极（GCE）表面首先用增敏作用的多壁碳纳米管(MWCNTs)夹心于两层电沉积的铁氰化镍（NiHCF）氧化还原电化学探针之间,然后以金纳米粒子为固定核酸适配体的载体,构建了检测凝血酶的非标记型核酸适配体生物传感器。 利用扫描电子显微镜(SEM)对MWCNTs和NiHCF的形貌进行了表征。 利用电化学阻抗谱对传感器的组装过程进行了监测,用循环伏安法（CV）和差分脉冲伏安法（DPV）对传感器的电化学行为进行了研究。 以铁氰化镍为探针的传感器对凝血酶的检测在1.0 ng/L~1.0 mg/L范围内呈良好的线性关系,相关系数为0.998,检测限为0.2 ng/L(S/N=3)。     

Highly sensitive lable-free electrochemical aptasensor for thrombin detection with cobalt hexacyanoferrate as the electrochemical probe

A highly sensitive label-free electrochemical aptasensor has been constructed for the electrochemical detection of thrombin (TB), where two layers of cobalt hexacyanoferrate (CoHCF) redox probes sandwiched with carbon nanotubes–Nafion were directly immobilized on the electrode surface by electrodeposition. Through the strong interaction between CN^? (CoHCF) and gold nanoparticles (GNPs), GNPs were assembled on the CoHCF-modified electrode for the immobilization of thiolated thrombin aptamers (TBA). In the presence of target TB, TBA on the electrode surface could catch TB to form TBA–TB complex, which made a barrier for the electron transfer, resulting in a greater decrease in CoHCF redox probe signals. Thus, the proposed aptasensor showed a high sensitivity and a much wider linearity to TB in the range of 1.0 pg/mL?～?1.0 μg/mL with a detection limit of 0.28 pg/mL.     

酶的光谱电化学研究（Ⅰ）：辣根过氧化物酶的电化学性质

利用薄层光谱电化学技术研究了辣根过氧化物酶(HRP)及其化合物的氧化还原过程。指出HRP可在固体电极上进行直接电子传递,该电极反应不是酶中二硫键的还原,而是血红素辅基中心金属离子的氧化态转变。测定了HRP(Fe~(3+)/Fe~(2+))电对的标准氧化还原电位和电化学动力学参数,讨论了HRP氧化性中间物的电化学性质。     

A Novel Electrochemical Method for Enhancing the Conductivity of Polyaniline Solid Matrices in Pre-Formed Films

A fast, simple and environmentally friendly new electrochemical method capable of enhancing the conductivity of a preformed polyaniline film has been found. Utilizing this method of electrochemical forcing pre-treatment at a certain effective voltage, a polyaniline solid matrix can be made more conductive. For example, the conductivity of a preformed polyaniline film (as thick as 10 μm) can be easily enhanced by about an order of magnitude within a pretreatment time of only ca. 5 min. The UV-Vis-NIR and ESR spectroscopic evidences indicated that the charge carriers in the polyaniline matrix are more delocalized after such electrochemical pretreatments. The results of CV studies indicated that the resultant polyaniline film has higher charge transport efficiency and a greater redox rate. Such phenomenon may be linked with a possible backbone conformational change, as induced by this novel electrochemical pretreatment, within the solid matrix ofpolyaniline film.     

EXAFS structural studies of electrodeposited Co and Ni hexacyanoferrate films

XAS (EXAFS and XANES), XPS and IR spectroscopies were used to extract redox compositional and structural information on films of electrodeposited Co and Ni hexacyanoferrates whose redox state was manipulated electrochemically. The X-ray methods provided direct information on the metal species and IR provided indirect information via the behaviour of the ligand vibration. XPS responses showed that the electrochemical response of Co hexacyanoferrate is attributable to Co (except for a small amount of Fe^II oxidation at very positive potentials), and of Ni hexacyanoferrate to Fe; XANES edge shifts confirm these deductions. Local structure around the metal atoms was extracted from EXAFS data in terms of M′–N, M′–C and M′–Fe (M′ = Co or Ni) distances and the associated Debye-Waller factors as functions of film charge state. For Co hexacyanoferrate, the redox variation of static disorder was consistent with a molecular model involving discrete Co^II and Co^III sites, whose populations respond to potential, but not with a solid-state model.     

Dynamics of ion exchange between self-assembled redox polyelectrolyte multilayer modified electrode and liquid electrolyte

A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.     

Functionalized mesoporous silica films as a matrix for anchoring electrochemically active guests

Mesoporous silica thin films were shown to be an appropriate matrix for immobilization of discrete electroactive moieties, yielding uniform transparent thin film electrodes with defined texture and enhanced electrochemical activity. The mesoporous silica films prepared on conducting FTO-coated glass substrate were postsynthetically functionalized. Alkoxysilanes were used as precursors for subsequent grafting via ionic or covalent bonds of representative electroactive species, such as polyoxometalate PMo12O(40)3-, hexacyanoferrate(III), and ferrocene. The electrochemically active concentration within the silica-based composite electrodes achieves 90, 260, and 60 micromol cm(-3) for polyoxometalate, hexacyanoferrate(III), and ferrocene, respectively. The amount of molecules involved in the charge-transfer sequence is proportional to the film thickness and comparable to the total amount of embedded guests. Thus, eventually the whole bulk volume of the modified silica films is electrochemically accessible. Immobilization in the chemically modified silica matrix alters the redox potential of the electroactive molecules. Electron exchange between the adjacent redox centers (electron hopping) is proposed as a possible charge propagation pathway through the insulating silica matrix, which is supported by the fact that the high charge uptake is observed also for the hybrid electrodes with the covalently anchored redox guests.     

Kinetic study of the hexacyanoferrate (III) oxidation of dihydroxyfumaric acid in acid media

The kinetics of the hexacyanoferrate (III) oxidation of dihydroxyfumaric acid to hexacyanoferrate (II) and diketosuccinic acid was looked into within the 0.04 to 5.3 M HCl acidity range under different temperatures, ionic strengths, and solvent permittivity conditions. The kinetic effect of alkali metal ions, transition metal impurities, and substrate concentrations have also been analyzed. The observed inhibition effect brought about by addition of the reaction product, hexacyanoferrate (II), is a sign of a complex mechanism. The rate constants remained essentially unchanged up to 1 M HCl, diminished between 1.0 and 3.0 M HCl, and rose above 3.0 M HCl. Depending on the medium acidity, three mechanisms can be put forward, which involve different kinetically active forms. At low acidity, the rate-determining step involves a radical cation and both the neutral and the anion substrate forms are equally reactive ( k 1 = k 2 = 2.18 +/- 0.05 M (-1) s (-1), k -1 = 0.2 +/- 0.03). When the medium acidity is boosted, the rate-determining step involves the neutral dihydroxyfumaric acid and two hexacyanoferrate (III) forms. In the intermediate region the rate constant diminished with rising [H (+)] ( k' 1 = 0.141 +/- 0.01 and k' 2 = 6.80 +/- 0.05). Specific catalytic effect by binding of alkali metal ions to oxidant has not been observed. In all instances it was assessed that the substrate decomposition is slow compared to the redox reaction.     

Electrostatic and electrophilic catalysis in the reductive cleavage of alkyl aryl ethers. The influence of ion pairing on the regioselectivity

Electrophilic and electrostatic catalysis have been identified as distinct contributions that affect the reactivity of radical anions in the reductive cleavage of alkyl aryl ethers. Two modes of mesolytic scission of these radical anions are possible: homolytic (dealkylation, a thermodynamically favored but kinetically forbidden process) and heterolytic (dealkoxylation). From our studies (alkali metal reductions, electrochemical studies, use of substrates with a preformed positive charge in certain positions of their structure) it can be concluded that the heterolytic scission is very much dependent on the electrophilic assistance by the counterion and it is only observed in contact ionic pairs with unsaturated cations (electrophilic catalysis). On the other hand, the homolytic scission is observed in solvent-separated ionic pairs, and it is especially efficient when the pair has a controlled topology with a tetralkylammonium cation (saturated cation) near the oxygen atom. The effect of the cation has, in this case, electrostatic origin (electrostatic catalysis), probably lowering the barrier of the intramolecular pi-sigma electron transfer process and thus reducing the kinetic control of the reaction in such a way that the thermodynamically more favorable process is produced.     

Overcoming Diffusion Limitation of Faradaic Processes: Property-Performance Relationships of 2D Conductive Metal-Organic Framework Cu3(HHTP)2 for Reversible Lithium-Ion Storage

Faradaic reactions including charge transfer are often accompanied with diffusion limitation inside the bulk. Conductive two-dimensional frameworks (2D MOFs) with a fast ion transport can combine both—charge transfer and fast diffusion inside their porous structure. To study remaining diffusion limitations caused by particle morphology, different synthesis routes of Cu-2,3,6,7,10,11-hexahydroxytriphenylene (Cu₃(HHTP)₂), a copper-based 2D MOF, are used to obtain flake- and rod-like MOF particles. Both morphologies are systematically characterized and evaluated for redox-active Li⁺ ion storage. The redox mechanism is investigated by means of X-ray absorption spectroscopy, FTIR spectroscopy and in situ XRD. Both types are compared regarding kinetic properties for Li⁺ ion storage via cyclic voltammetry and impedance spectroscopy. A significant influence of particle morphology for 2D MOFs on kinetic aspects of electrochemical Li⁺ ion storage can be observed. This study opens the path for optimization of redox active porous structures to overcome diffusion limitations of Faradaic processes.     

The Thermodynamics of Insertion Electrochemical Electrodes—A Team Play of Electrons and Ions across Two Separate Interfaces

Insertion electrochemical electrodes exhibit simultaneous electron and ion transfer, with the two transfers proceeding across different interfaces. Herein the thermodynamics of the overall electrochemical electrode reaction is discussed with respect to the thermodynamics of these two charge‐transfer equilibria. This Minireview includes insertion electrochemical systems where the redox centers are in a solid phase and the ions are transferred between that phase and a solution, and also systems where the redox centers are in a liquid phase that is immiscible with another liquid phase and ions are transferred between the two liquid phases. The Minireview is intended to spark similar studies on battery materials to improve their performance.