Influence of ionic speciation on electrocatalytic performance of polyaniline coated platinum electrode: Fe(III)/Fe(II) redox reaction

Porous-polyaniline coated Pt electrode (PANI/Pt) was electro-synthesized potentiodynamically in 0.1 M aniline + 0.5 M H₂SO₄ and morphologically characterized by scanning electron microscopy (SEM). Nature of predominant Fe-species in HCl and H₂SO₄ was checked by UV-vis spectrophotometry. Electrocatalysis of Fe(III)/Fe(II) reaction was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for three different solution compositions viz. (i) FeCl₃/FeCl₂ in 1 M HCl, (ii) FeCl₃/FeCl₂ in 0.5 M H₂SO₄ and (iii) Fe₂(SO₄)₃/FeSO₄ in 0.5 M H₂SO₄. For different thicknesses of PANI, the peak current increased irrespective of the nature of the Fe-species, but the polarity of the charge on the Fe-species showed great influence on reversibility of electrocatalysis by PANI/Pt. The Donnan interaction of the polyaniline modified electrode for the three compositions was investigated with respect to [Fe(CN)₆]³⁻/H₂[Fe(CN)₆]²⁻ which are believed to be the predominant species present in K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution in 0.5 M H₂SO₄. The electrocatalytic performance of PANI/Pt for Fe(III)/Fe(II) redox reaction was found superior in HCl compared to that in H₂SO₄.     

Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery

The Mn(II)/Mn(III) couple has been recognized as a potential anode for redox flow batteries to take the place of the V(IV)/V(V) in all-vanadium redox battery (VRB) and the Br₂/Br⁻ in sodium polysulfide/bromine (PSB) because it has higher standard electrode potential. In this study, the electrochemical behavior of the Mn(II)/Mn(III) couple on carbon felt and spectral pure graphite were investigated by cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, transient potential-step experiment, X-ray diffraction and charge-discharge experiments. Results show that the Mn(III) disproportionation reaction phenomena is obvious on the carbon felt electrode while it is weak on the graphite electrode owing to its fewer active sites. The reaction mechanism on carbon felt was discussed in detail. The reversibility of Mn(II)/Mn(III) is best when the sulfuric acid concentration is 5 M on the graphite electrode. Performance of a RFB employing Mn(II)/Mn(III) couple as anolyte active species and V(III)/V(II) as catholyte ones was evaluated with constant-current charge-discharge tests. The average columbic efficiency is 69.4% and the voltage efficiency is 90.4% at a current density of 20 mA cm⁻². The whole energy efficiency is 62.7% close to that of the all-vanadium battery and the average discharge voltage is about 14% higher than that of an all-vanadium battery. The preliminary exploration shows that the Mn(II)/Mn(III) couple is electrochemically promising for redox flow battery.     

The kinetics of the silver (I)/silver (II) couple at a platinum electrode in perchloric and nitric acids

The kinetics of the silver (I)/silver (II) couple have been studied in 3 M nitric acid and 3 M perchloric acid at a platinum electrode. It has been found that silver (II) can be produced in high current yields in a fast electrode reaction although the conversion of silver (I) to silver (II) is limited by the chemical reaction of silver (II) with water. It has also been shown that silver (I) can successfully be used as a catalyst or current carrier in the anodic oxidation of organic and inorganic substrates.     

Electrochemical oxidation of manganese(II) at a platinum electrode

Electrochemical oxidation of Mn²⁺ in sulphuric acid to form MnO₂ was studied using stationary and rotating platinum/platinum ring-disc electrodes. It appears that nucleation of MnO₂ is governed by an equilibrium involving a Mn(III) intermediate. Growth of MnO₂ involves the reduction of MnO₂ surfaces by Mn²⁺ ions in the solution to form MnOOH intermediates. The subsequent electrochemical oxidation of MnOOH releases a hydrogen ion and results in the formation of MnO₂. The rate constant of MnOOH oxidation to MnO₂ was estimated to be 40 s^–1. With a sufficient supply of Mn²⁺ ions, a layer of MnOOH is built up and the in-solid diffusion of hydrogen ions becomes the ratedetermining-step. With a low Mn²⁺ concentration, diffusion of Mn²⁺ ions from bulk electrolyte to the MnO₂/electrolyte interface is a factor controlling the growth of MnO₂. The activation energy and the pre-exponential term of the diffusion coefficient of Mn²⁺ in 0.5m sulphuric acid were determined to be 44.8 kJ mol^–1 and 100 cm² s^–1, respectively.     

Ce(III)/Ce(IV) in methanesulfonic acid as the positive half cell of a redox flow battery

The characteristics of the Ce(III)/Ce(IV) redox couple in methanesulfonic acid were studied at a platinum disk electrode (0.125 cm²) over a wide range of electrolyte compositions and temperatures: cerium (III) methanesulfonate (0.1–1.2 mol dm⁻³), methanesulfonic acid (0.1–5.0 mol dm⁻³) and electrolyte temperatures (295–333 K). The cyclic voltammetry experiments indicated that the diffusion coefficient of Ce(III) ions was 0.5 × 10⁻⁶ cm² s⁻¹ and that the electrochemical kinetics for the oxidation of Ce(III) and the reduction of Ce(IV) was slow. The reversibility of the redox reaction depended on the electrolyte composition and improved at higher electrolyte temperatures. At higher methanesulfonic acid concentrations, the degree of oxygen evolution decreased by up to 50% when the acid concentration increased from 2 to 5 mol dm⁻³. The oxidation of Ce(III) and reduction of Ce(IV) were also investigated during a constant current batch electrolysis in a parallel plate zinc–cerium flow cell with a 3-dimensional platinised titanium mesh electrode. The current efficiencies over 4.5 h of the process Ce(III) to Ce(IV) and 3.3 h electrolysis of the reverse reaction Ce(IV) to Ce(III) were 94.0 and 97.6%, respectively. With a 2-dimensional, planar platinised titanium electrode (9 cm² area), the redox reaction of the Ce(III)/Ce(IV) system was under mass-transport control, while the reaction on the 3-dimensional mesh electrode was initially under charge-transfer control but became mass-transport controlled after 2.5–3 h of electrolysis. The effect of the side reactions (hydrogen and oxygen evolution) on the current efficiencies and the conversion of Ce(III) and Ce(IV) are discussed.     

A study of the Ce(III)/Ce(IV) redox couple for redox flow battery application

In this study, the electrochemical behavior of the Ce(III)/Ce(IV) redox couple in sulfuric acid medium with various concentrations and the influence of the operating temperature were investigated. A change of the concentration of sulfuric acid mainly produced the following two results. (1) With an increase of the concentration of sulfuric acid the redox peak currents decreased. (2) The peak potential separation for the redox reactions increased with rising concentration of sulfuric acid from 0.1 to 2 M and then decreased with further increase of the concentration. Elevated temperature was electrochemically favorable for Ce(III)/Ce(IV) couple, which caused an increase of the peak currents for the redox reactions and a decrease of the peak potentials separation. Constant-current electrolysis shows that the current efficiency was 73% for the oxidation process of Ce(III) and 78% for the reduction process at 298 K, and could be improved by elevating the temperature. The open-circuit voltage of the Ce-V cell, after full charging, remained constant at 1.870±0.005 V for more than 48 h, and is about 29% higher than that of the all-vanadium batteries. The coulombic efficiency was approximately 87%, showing that self-discharge of the Ce-V battery was small. The preliminary exploration shows that the Ce(III)/Ce(IV) couple is electrochemically promising for redox flow battery (RFB) application.     

Polymerization kinetics of methylmethacrylate by oxidation: Reduction system using cerium(IV)/lactic acid in aqueous medium

The kinetics of polymerization of methylmethacrylate initiated by cerium(IV)–lactic acid redox system was studied in an aqueous medium in the temperature range of 25–50°C. The rate of polymerization (R_p) and the rate of cerium(IV) disappearance have been measured. The effects of some water‐miscible organic solvents, cationic, anionic, nonionic surfactants, and complexing agents on the rate of polymerization were investigated. The temperature dependence of the rate was studied, and the activation parameters were computed using the Arrhenius and Eyring plots. The effects of inorganic and organic solvents on polymerization were also investigated. All of them depressed both the initial rate and limiting conversion. A mechanism consistent with the experimental data, involving cerium(IV)–lactic acid complex formation, which generates free radicals, is suggested. The chain termination step of the polymerization reaction is by mutual interaction of the growing macromolecules. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3498–3505, 2007     

A study of the Fe(III)/Fe(II)-triethanolamine complex redox couple for redox flow battery application

The electrochemical behavior of the Fe(III)/Fe(II)-triethanolamine(TEA) complex redox couple in alkaline medium and influence of the concentration of TEA were investigated. A change of the concentration of TEA mainly produces the following two results. (1) With an increase of the concentration of TEA, the solubility of the Fe(III)-TEA can be increased to 0.6 M, and the solubility of the Fe(II)-TEA is up to 0.4 M. (2) In high concentration of TEA with the ratio of TEA to NaOH ranging from 1 to 6, side reaction peaks on the cathodic main reaction of the Fe(III)-TEA complex at low scan rate can be minimized. The electrode process of Fe(III)-TEA/Fe(II)-TEA is electrochemically reversible with higher reaction rate constant than the uncomplexed species. Constant current charge-discharge shows that applying anodic active materials of relatively high concentrations facilitates the improvement of cell performance. The open-circuit voltage of the Fe-TEA/Br₂ cell with the Fe(III)-TEA of 0.4 M, after full charging, is nearly 2.0 V and is about 32% higher than that of the all-vanadium batteries, together with the energy efficiency of approximately 70%. The preliminary exploration shows that the Fe(III)-TEA/Fe(II)-TEA couple is electrochemically promising as negative redox couple for redox flow battery (RFB) application.     

Preparation of nanostructure mixed copper-zinc oxide via co-precipitation rout for dye-sensitized solar cells: The influence of blocking layer and Co(II)/Co(III) complex redox shuttle

Mixed copper-zinc oxide nanostructures (average size 43 nm) were effectively fabricated via co-precipitation route. Field-emission scanning electron microscope (FESEM), powder X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FT-IR) and UV–vis diffuse reflectance spectrum (DRS) were used to characterize the properties of the oxides. At the optimized condition, copper-zinc oxide nanostructures were used for fabrication of working electrodes by doctor blade technique on the fluorine-doped tin oxide (FTO) in dye sensitized solar cells. Their photovoltaic behavior were compared with standard using D35 dye and an electrolyte containing [Co(bpy)₃](PF₆)₂, [Co(pby)₃](PF₆)₃, LiClO₄, and 4-tert-butylpyridine (TBP). The ranges of short-circuit current (J_sc) from 0.13 to 0.30 (mA/cm²), open-circuit voltage (V_oc) from 0.20 to 0.51 V, and fill factor from 0.34 to 0.29 were obtained for the DSSCs made using the working electrodes. A titania blocking layer on the copper-zinc oxide surface improve both the open-circuit voltage (V_oc), short-circuit current (J_sc) and the power-conversion efficiency is consequently enhanced by a factor of approximately five.     

The integral capacitance,kinetics and mechanisms of the Cu/Cu(II) system in sulphuric acid media

Galvanostatic investigation has been carried out on the Cu/Cu(II) system 2 M H₂SO₄ + 0.7 M CuSO₄, at 298 K. The pseudo-capacitance (integral capacitance) has been extracted as a function of overvoltage from the portion of the charging curve prior to plateaux. It has been found that the pseudo-capacitance, for both anodic and cathodic processes depended upon the current density and time. On the basis of the above findings the reaction mechanism has been suggested to be Cu?Cu(I)_ad^v + ve^?, Cu(I)_ad^v?Cu(I) + (1 ? v)e^?, Cu(I)?Cu(II)e^?.     

Electrochromism of Crystal Violet Lactone in the presence of Fe(III)/Fe(II) redox pair

Selective interaction between Crystal Violet Lactone and Fe³⁺/Fe²⁺ in methanol leads to a reversible ionochromic colour change. This interaction can be controlled electrochemically, in order to achieve reversible colour changes with high contrast between colourless and dark blue solutions. The presented system is proposed as an alternative electrochromic solution.     

The role of platinum oxide in the electrode system Pt(O2)/yttria-stabilized zirconia

The existence and role of platinum oxide in the solid state electrode system Pt(O₂)/yttria-stabilized zirconia is discussed. Covering and porous model-type Pt film electrodes on YSZ single crystals are investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and in situ scanning photoelectron microscopy. The formation of Pt oxide and its amount strongly depend on the experimental conditions, such as temperature, oxygen partial pressure, and oxygen flux towards the electrode during anodic polarization. Electrode activation and deactivation processes can be explained by formation and decomposition of Pt oxide, which is reducing or inhibiting the oxygen exchange rate.     

On the kinetics and mechanism of the Cd(II)/Cd(Hg) system in acid media

The kinetics and mechanism have been investigated in the Cd(II)/Cd(Hg) system in 2 M NaClO₄ + 1 M HClO₄, at 25 ± 0.5°C. The galvanostatic single-pulse method has been used. The value of double-layer capacity, exchange current density, as also the α value, k_s, and the partial reaction order of each of the component which takes place in the reaction, have been determined on the grounds of different kind of anions in the solution. The two-step single-electron exchange mechanism has been suggested, as also the one-step exchange reaction in combination with the reaction of disproportionation.     

Electrocatalysis of the charge-transfer process for the Sb(III)/Sb(V) couple at a platinum electrode in concentrated HCl

Electrocatalysis of the electrochemical oxidation and reduction of Sb(V) and Sb(III) by adsorbed I⁻ and I₂ at a rotating Pt disc electrode was investigated in 12 M HCl. The heterogeneous rate constant in the absence of adsorbed halogen was increased by 200x for a maximum surface coverage by halogen. This large rate increase cannot be predicted on the basis of an electrostatic argument and a mechanism based on electron-transfer bridging is proposed.     

Voltammetric behaviour of the copper(II)—thiourea system in sulphuric acid medium at platinum and glassy carbon electrodes

Thiourea, a levelling agent used in copper electrorefining baths, is the focus of a cyclic and rotating disc or ring-disc electrode voltammetric study. Thiourea adsorption, its interaction with Cu²⁺ and formation of Cu(Tu) _n ⁺ complexes are part of the multistep electrode mechanism proposed. Results at platinum and glassy carbon electrodes are applicable to copper electrodes in electrorefining.     

Indium(III)-hexacyanoferrate(III, II) as an inorganic material analogous to redox polymers for modification of electrode surfaces

Polynuclear indium(III)-hexacyanoferrate(III, II) films were prepared electrochemically on a glassy carbon substrate. The deposition was performed by cycling the potential between 0.85 and 0.0 V vs sce in a fresh 0.5 mM InCl₃/0.5 mM K₃Fe(CN)₆/0.5 M KCl mixture at pH 2. During the negative scans, the cationic In³⁺ species interact with the simultaneously generated anionic Fe(CN)⁴⁻₆ to yield sparingly soluble mixed-valent coatings on the electrode surfaces. Different thicknesses, typically corresponding to 1–100 nmol cm⁻², could be obtained by varying the cycling times. The modified electrode exhibits a single set of well-defined and highly reversible voltammetric peaks (related to the hexacyanoferrate redox transitions) in K⁺-containing electrolytes. Electrochemical charging is accompained by the cation motion, leading to the cation storage in the reduced state and its release upon oxidation. The results are consistent with high ionic conductivity of potassium in the film. Experiments in the other alkali metal electrolytes show that the cataion transport, necessary for the charge compensation during redox transitions, cannot be simply related to the size of a cation and the zeolitic structure of indium hexacyanoferrate.     

The free energy of transfer of Pb(II) ion and electrode kinetics of the Pb(II)/Pb(Hg) system in dimethyl sulfoxide (DMSO) and DMSO-water mixtures

The electrode kinetics of the Pb(II)/Pb(Hg) system has been studied in DMSO and DMSO-water mixtures using the faradaic impedance measurements. With the increase of the mole fraction of the organic component in the mixture rate constant decreases steadily from 2 cm s^?1 in pure water to 5 × 10^?2 cm s^?1 in DMSO in rough proportion to the coverage of the electrode by DMSO. The change of the kinetics with the solvent composition was discussed in the frame of existing models.Basing on the determined potentials of the Pb(II)/Pb(Hg) electrode expressed vs the ferrocinium ion/ferrocene electrode potential, the free energy of transfer of Pb(II) from water to DMSO and its mixtures was calculated and discussed.     

Electrochemical redox response of iron (II)/(III) ions implanted in permeable,polymeric electrode caotings

Polymer films with imbedded iron (II)/(III) ions as redox centres were prepared on electrodes with the method of glow- discharge polymerisation. The films were permeable to acidic electrolytes but diffusion of the redox centres was inhibited by the high degree of cross-linking of the matrix, leading to high stability of the electrochemical response. In hydrochloric acid all the present redox centres could be oxidized/reduced reversibly via electron exchange processes between neighbouring sites. This electronhopping mechanism is discussed in detail.     

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to:

Potentials (vs nhe) and species are given for 1.0 M HClO₄ solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.     

Impedance spectroscopy of the Ti(IV)/Ti(III) redox couple in the molten LiCl-KCl eutectic melt at 470°C

The electrochemical oxidation of Ti³⁺ ions in LiCl-KCl eutectic melt at 470°C occurs in two different ways according to the current densities. In our experimental conditions (concentration of Ti³⁺ ions 0.11 mol kg^–1) for potentials lower than –0.48 V (vs chlorine electrode) and current densities lower than 10 mA cm^–2 soluble Ti⁴⁺ ions are formed, whereas, for higher potentials solid K₂ TiCl₆ precipitates at the electrode resulting in a partial passivation of the electrode. Both reactions have been studied by means of impedance measurements which have shown the reversibility of the electrochemical reaction (k ₀=0.25 cm s^–1; =0.3) and the porosity of the K₂ TiCl₆ layer electrochemically formed. The thickness of this layer has been estimated from the amount of electricity involved in the reaction and from the resistance and the capacitance characterising the deposit, measured at high frequencies. This comparison shows that the deposit is composed of a very thin (0.1 m) barrier layer covered by a very thick (70 m) porous external layer.Paper presented at the EUCHEM Conference on Molten Salts 1988, St Andrews, Scotland, 6 July 1988.