Electrochemical studies and self diffusion coefficients in cyclic ammonium based ionic liquids with allyl substituents

Several cyclic ammonium-based ionic liquids (ILs) with allyl substituent are synthesized, these allyl substituent ILs have high ionic conductivity (up to 4.72 mS cm⁻¹ at 30 °C) and wide electrochemical window of 5 V. The electrochemical behaviors of two organometallic redox couples Fc/Fc⁺ (ferrocene/ferrocenium) and Cc/Cc⁺ (cobaltocene/cobaltocenium) have been studied in these ILs, the calculated Stokes–Einstein product (Dη T⁻¹) of ferrocene in ILs is larger than that of cobaltocenium in ILs. The self-diffusion coefficients of cation and anion in these ILs are studied using pulsed gradient spin-echo NMR technique. There are very few reports where electrochemically derived diffusion coefficients and self diffusion coefficients are available for comparison, so a new key concept in electrochemistry could be developed in this paper.     

An EQCM study of polyethyleneglycol 8000 adsorption and its coadsorption with Cl ions on Pt in perchloric acid solutions

The adsorption of the additive polyethyleneglycol 8000 (PEG8) and its coadsorption with Cl⁻ ions was investigated by cyclic voltammetry and linear potential scans in conjunction with simultaneous measurements of the frequency change of an electrochemical quartz crystal microbalance (EQCM). Data obtained from the studies of EQCM for solutions of HClO₄ containing PEG8, shows the formation of a peak (IcI) in the potential range from 0.2 to 0.4 V during the cathodic potential scan, which is due to the adsorption of PEG8 onto Pt. Analysis of simultaneously recorded massograms and voltammograms revealed that the adsorption of PEG8 occurs via a non-Faradaic process, and that no adsorption of PEG8 is observed at the open circuit potential. As the concentration of PEG8 in the solution was increased over the range  M, the degree of coating by PEG8 on the Pt surface increased to 0.21. The presence of Cl⁻ ions in the solution inhibited PEG8 adsorption, and the degree of inhibition gradually increased with increasing Cl⁻ concentration.     

Nickel foam-supported porous Ni(OH)2/NiOOH composite film as advanced pseudocapacitor material

A porous net-like β-Ni(OH)₂/γ-NiOOH composite film is prepared by a chemical bath deposition. The as-prepared porous composite film shows a highly porous structure built up by many interconnected nanoflakes with a thickness of about 20 nm. The pseudocapacitive behavior of the porous composite film is investigated by cyclic voltammograms (CV) and galvanostatic charge–discharge tests in 1 M KOH. The porous β-Ni(OH)₂/γ-NiOOH composite film exhibits a noticeable pseudocapacitance with 1420 F g⁻¹ at 2 A g⁻¹ and 1098 F g⁻¹ at 40 A g⁻¹, respectively, much higher than those of the dense Ni(OH)₂ film (897 F g⁻¹ at 2 A g⁻¹ and 401 at 40 A g⁻¹). The porous architecture is responsible for the enhancement of the electrochemical properties, and it increases electrochemical reaction area, shortens ions diffusion paths and relaxes volume change caused by the electrochemical reactions.     

Effects of viscosity-dependent diffusion in the analysis of rotating disk electrode data

This paper demonstrates the application of a modified Levich equation for chemical systems with varying viscosity. A commonly used technique to analyze rotating disc electrode (RDE) experiments is to fit the data to the Levich equation assuming a constant effective diffusion coefficient which may be valid for conditions where the viscosity does not vary significantly (less than an order of magnitude). However, most diffusion coefficient models (e.g. Stokes–Einstein) show an inverse relationship with viscosity which consequently indicates that a constant effective diffusion coefficient may result in poorer model-to-data agreement. Here, data are presented for a series of RDE experiments for the electrodissolution of Cu in phosphoric acid, water and glycerin based baths. Viscosity changes of greater than one order of magnitude allow for testing the assumption of a constant effective diffusion coefficient. The collected data, as well as data published elsewhere, can be explained by a modified Levich equation which takes into account the viscosity dependence of the diffusion coefficient.     

Effect of viscosity and applied potential on oscillations at a Pt-Pt dual-electrode in a ferricyanide system

This paper outlines the effect of viscosity and applied potential on oscillations occurring at two platinum electrodes placed proximal to each other. Potential oscillations taking place on the primary electrode (WE1) under galvanostatic control in the ferricyanide system are affected by the solution viscosity as it modifies the convective feedback mechanism necessary for oscillations. Measured transition times correlate with those calculated using Sand equation thus allowing the estimation of current density windows for periodic oscillation for different solution viscosities for pre-determined transition times. Current oscillations on the secondary working electrode (WE2) - under potentiostatic control and induced by the potential oscillations on WE1 - can be tuned through the applied potential. At higher potentials the reaction is oxidation of [Fe(CN)₆]⁴⁻ and the coupling is primarily through the transfer of [Fe(CN)₆]⁴⁻ from WE1 to WE2 via H₂ evolution whilst at more cathodic potentials the reduction of [Fe(CN)₆]³⁻ takes place at both WE1 and WE2 and the convective feedback from WE1 refreshes the surface of both electrodes simultaneously.     

Electrochemical behavior of antimony and electrodeposition of Mg–Li–Sb alloys from chloride melts

The electrochemical behavior of Sb(III) ions was investigated in LiCl–KCl molten salt at 673 K. The reaction mechanism and transport parameters of electroactive species were determined by transient electrochemical techniques (such as cyclic voltammetry, square wave voltammetry, chronopotentiometry and chronoamperometry) at a molybdenum electrode. The results showed that electrochemical reduction of Sb(III) in LiCl–KCl melts occurred in a reaction step with an exchange of three electrons. A voltammogram with a different scan rate in LiCl–KCl containing 1.45 × 10⁻⁴ mol cm⁻³ SbCl₃ showed that the deposition/dissolution reaction of Sb(III) ions was not completely reversible. The diffusion coefficient of Sb(III) ions was 1.65(±0.01) × 10⁻⁵ cm⁻² s⁻¹ at 673 K. The electroreduction of Sb(III) ions at an Al electrode was also studied by cyclic voltammetry and open circuit chronopotentiometry in the temperature range of 668–742 K. The redox potential of Sb(III)/Sb at an Al electrode was observed at the more positive potentials values than those at an inert electrode. This potential shift due to the formation of intermetallic compound with Al electrode. AlSb alloys were prepared in LiCl–KCl–SbCl₃ melts at 742 K by potentiostatic electrolysis at an Al electrode. The activity of Sb and the Gibbs energy of AlSb formation were also calculated. Mg–Li–Sb alloys were obtained by galvanostatic electrolysis at 673 K and the electrochemical codeposition of Mg, Li and Sb was investigated on a molybdenum electrode in LiCl–KCl containing MgCl₂ and SbCl₃ melts at 673 K by cyclic voltammetry, chronopotentiometry and chronoamperometry. Cyclic voltammograms, chronopotentiometry and chronoamperometry measurements indicated that the electrochemical codeposition of Mg, Li and Sb metal occurred at current densities lower than −0.466 A cm⁻² or at an applied potential more negative than −2.350 V vs. Ag/AgCl. X-ray diffraction (XRD) suggested that Mg₃Sb₂ and Li₃Sb were formed in Mg–Li–Sb alloys. The distribution of Sb element in Mg–Li–Sb alloys from the analysis of scan electron micrograph (SEM) and energy dispersive spectrometry (EDS) indicated that Sb metal showed a distribution which resembled an interlaced network.     

Porous NiO/Ag composite film for electrochemical capacitor application

A highly porous NiO/Ag composite film is prepared by the combination of chemical bath deposition and silver mirror reaction. The as-prepared NiO/Ag composite film has an interconnecting reticular morphology made up of NiO flakes with highly dispersed Ag nanoparticles of about 6 nm. The pseudocapacitive behavior of the NiO/Ag composite film is investigated by cyclic voltammograms (CV) and galvanostatic charge–discharge tests in 1 M KOH. The NiO/Ag composite film exhibits weaker polarization, higher specific capacitance and better cycling performance as compared to the unmodified porous NiO film. The specific capacitance of the porous NiO/Ag composite film is 330 F g⁻¹ at 2 A g⁻¹ and 281 F g⁻¹ at 40 A g⁻¹, respectively, much higher than that of the unmodified porous NiO film (261 F g⁻¹ at 2 A g⁻¹ and 191 F g⁻¹ at 40 A g⁻¹). The enhancement of pseudocapacitive properties is due to highly dispersed Ag nanoparticles in the composite film, which improves the electric conductivity of the film electrode.     

Diffusion regimes at nanoelectrode ensembles in different ionic liquids

The electrochemical and diffusion behaviour of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles (NEEs) in comparison with millimetre sized gold (Au-macro) and glassy carbon (GC) disk electrodes. The redox probes are neutral ferrocene (Fc), the ferrocenylmethyltrimetylammonium cation (FA⁺) and the ferrocenylmonocarboxylate anion (FcCOO⁻). The ILs are the dicyanamide, [N(CN)₂] or bis(trifluoromethylsulfonyl)amide), [N(Tf)₂] salts of the following cations: 1-butyl-3-methylimidazolium, [BMIm], 1-butyl-3-methylpyrrolidonium, [BMPy], or tris(n-hexyl)tetradecylphosphonium [P_14,666]. These ILs are characterized by different viscosities, ranging from 32 to 277 cP. The cyclic voltammetric behaviour of the redox probes is reversible and diffusion controlled at GC electrodes. Diffusion coefficients (D) calculated by the Randles-Sevcik equation scales inversely with the IL viscosity, ranging from 2 × 10⁻⁸ to 3 × 10⁻⁷ cm² s⁻¹. Ionic solutes, namely FA⁺ and FcCOO⁻, present slightly lower D values than neutral Fc. At the Au-macro the electrochemical behaviour of the redox probes is diffusion controlled in the ILs containing the [N(Tf)₂] anion, while it involves relevant adsorption processes in the [N(CN)₂] containing electrolyte. For this reason the diffusion at gold NEEs is studied only in the former ILs.The CVs of the redox probes at the NEEs are peak shaped at low scan rate (v), while they are sigmoidally shaped at high v, but with some shift between forward and backward patterns. This is indicative of the occurrence of a total overlap (TO) diffusion condition when v is low which becomes a mixed diffusion layers (MDL) regime, with only a partial overlapping of individual diffusion layers, at high v values. In the most viscous IL, namely [P_14,666] [N(Tf)₂], at v higher than 0.8 V s⁻¹, a plateau current independent on the scan rate is achieved, indicating the tendency to reach the pure radial regime in this IL. The v values at which the transition between TO and MDL is observed scales directly with D and inversely with the IL viscosity. This behaviour is interpreted on the basis of the dependence of individual diffusion layers at each nanoelectrode on redox probe/IL interaction which fits with existing theoretical models very recently developed for nanoelectrode arrays.     

Probing anodic reaction kinetics and interfacial mass transport of a direct formic acid fuel cell using a nanostructured palladium-gold alloy microelectrode

The anodic reaction kinetics and interfacial mass transport of a direct polymer electrolyte membrane formic acid fuel cell have been investigated in an all solid-state electrochemical cell using a highly active nanostructured palladium-gold alloy microelectrode as an in situ probe. Well-defined “S-shaped” steady-state cyclic voltammograms exhibiting current-rising region at lower overpotentials and limiting current region at higher overpotentials have been first obtained for the electrochemical oxidation of formic acid at varying temperature. The “S-shaped” steady state polarization curves and chronoamperometric curves enable convenient measurements of the anodic reaction kinetics and interfacial mass transport of formic acid under real polymer electrolyte membrane conditions. It is encouragingly found that formic acid can be directly oxidized to CO₂ with the first electron transfer being the likely rate-determining step and the formation of surface poison can be neglected. The exchange current density for the electrooxidation of formic acid is on the order of magnitude of 10⁻⁷ A cm⁻² in the temperature range of 20-60 °C. The permeability and diffusion coefficient of formic acid through a Nafion^® 117 membrane are of the order of magnitude of 10⁻⁹ mol cm⁻¹ s⁻¹ and 10⁻⁶ cm² s⁻¹, respectively. The combination of a nanostructured microelectrode and an all solid-state electrochemical cell offers a versatile approach to evaluate potential electrocatalysts for fuel cells and electrochemical sensors employing polymer electrolyte membranes.     

Investigation on V(IV)/V(V) species in a vanadium redox flow battery

Stokes radii of V(IV) and V(V) species in concentrated sulfuric acid solutions were determined from their diffusion limited current densities on a rotating platinum disk electrode and the solution viscosity. In addition, V(IV) and V(V) species were estimated based on their solubility, UV-Vis spectra, and cyclic voltammetric data. The possible ion-pair formation of V(IV) cation with SO₄²⁻ and/or HSO₄⁻ and the spontaneous polymerization of V(V) at a low H₂SO₄ concentration were discussed.     

Electrochemical performance of Li3V2(PO4)3/C cathode materials using stearic acid as a carbon source

The Li₃V₂(PO₄)₃/C cathode materials are synthesized by a simple solid-state reaction process using stearic acid as both reduction agent and carbon source. Scanning electron microscopy and transmission electron microscopy observations show that the Li₃V₂(PO₄)₃/C composite synthesized at 700 °C has uniform particle size distribution and fine carbon coating. The Li₃V₂(PO₄)₃/C shows a high initial discharge capacity of 130.6 and 124.4 mAh g⁻¹ between 3.0 and 4.3 V, and 185.9 and 140.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.1 and 5 C, respectively. Even at a charge–discharge rate of 15 C, the Li₃V₂(PO₄)₃/C still can deliver a discharge capacity of 103.3 and 112.1 mAh g⁻¹ in the potential region of 3.0–4.3 V and 3.0–4.8 V, respectively. Based on the analysis of cyclic voltammograms and electrochemical impedance spectra, the apparent diffusion coefficients of Li ions in the composites are in the region of 1.09 × 10⁻⁹ and 4.95 × 10⁻⁸ cm² s⁻¹.     

Nucleation of Sn and Sn-Cu alloys on Pt during electrodeposition from Sn-citrate and Sn-Cu-citrate solutions

The initial stages of Sn and Sn-Cu electrodeposition from Sn-citrate and Sn-Cu-citrate solutions on Pt were studied using both current-controlled and potential-controlled electrochemical techniques. For both Sn-citrate and Sn-Cu-citrate solutions, when the current density is controlled to lower than 15 mA/cm², potentials remain almost constant which is appropriate to plate dense and uniform films. When the current density is controlled to between 25 and 35 mA/cm², potentials drop quickly initially, followed by a gradual increase to a constant value. When current density is controlled to higher than 50 mA/cm², potential oscillation happens, and significant hydrogen evolution prevents the formation of dense and continuous Sn and Sn-Cu films. A constant transition time constant indicates a diffusion-controlled process. The diffusion coefficient calculated from the Sand equation is about 3.8 × 10⁻⁶ cm²/s for the Sn-citrate solution and 4.1 × 10⁻⁶ cm²/s for the Sn-Cu-citrate solution. The morphology of both Sn and Sn-Cu deposits plated under different potentials was examined by atomic force microscopy (AFM) and the distribution of each element were analyzed using Auger imaging. Analysis of both the electrochemical results at −0.72, −1.1 and −1.5 V and AFM images for both Sn and Sn-Cu deposits at −1.1 and −1.5 V suggested progressive nucleation controlled by diffusion for both Sn and Sn-Cu electrodeposition. Tin reacted with Pt to form PtSn₄, and co-deposited with Cu to form Cu₆Sn₅ during nucleation, with more Sn forming at higher applied potentials.     

Viscosity effects in square wave polarography polyethylene glycols—Cd2+, Tl+—system

In both the dc polarography and in the square wave polarography, the Stokes—Einstein—Ilkovi? equation connecting polarographic current and viscosity of the solution, is valid only in the case of monomeric solutions. In solutions of polymers, the polarographic diffusion current is higher than it would be expected from the macro-viscosity. The phenomenon is explained by the fact of micro-viscosity which is lower than macro one and controls the diffusion of small species like inorganic cations. They do not “see” long chains and coils of polymers. Paper discusses the phenomenon from different points of view: electrochemical (filling the gap in general application of square wave polarography) and physicochemical (diffusion of cations as a means of investigation of the structure of polymeric solutions). Conclusions of the paper are also important for radiation chemistry where discussion of diffusion of small species cannot be done in terms of macro-viscosity.     

Preparation and electrochemical characterization of MnOOH nanowire–graphene oxide

MnOOH nanowire–graphene oxide composites are prepared by hydrothermal reaction in distilled water or 5% ammonia aqueous solution at 130 °C with MnO₂–graphene oxide composites which are synthesized by a redox reaction between KMnO₄ and graphene oxide. Powder X-ray diffraction (XRD) analyses and energy dispersive X-ray analyses (EDAX) show MnO₂ is deoxidized to MnOOH on graphene oxide through hydrothermal reaction without any extra reductants. The electrochemical capacitance of MnOOH nanowire–graphene oxide composites prepared in 5% ammonia aqueous solution is 76 F g⁻¹ at current density of 0.1 A g⁻¹. Moreover, electrochemical impedance spectroscopy (EIS) suggests the electrochemical resistance of MnOOH nanowire–graphene oxide composites is reduced when hydrothermal reaction is conducted in ammonia aqueous solution. The relationship between the electrochemical capacitance and the structure of MnOOH nanowire–graphene oxide composites is characterized by cyclic voltammetry (CV) and field emission scanning electron microscopy (FESEM). The results indicate the electrochemical performance of MnOOH nanowire–graphene oxide composites strongly depends on their morphology.     

Rheological study of mixing in molten polymers: 1-mixing of low viscous additives

A new mixing device has been designed and developed in order to investigate complex mixing situations encountered in polymer blends and formulations. This mixing device called here rheo-mixer has been adapted on a classical rheometer and calibrated in terms of shear/stress that both permanent and dynamic regimes may be quantitatively run. The ability of the mixer to bring interesting information about complex polymeric systems was demonstrated by following the process of incorporation of a miscible liquid or partially miscible into a high viscosity polymer. As examples, EVA/diethyl 2-hexyl phtalate, poly(ε-caprolactone)/ε-caprolactone, and EVA/ε-caprolactone systems were tested.It was demonstrated that the viscosity ratio is not the predominant parameter for mixing process control because the mixing is mainly controlled by the diffusion for these low viscosity ratio systems (λ<10⁻⁵) as miscible systems (EVA/DOP and PCL/ε-CL) and partially miscible systems (EVA/ε-CL) are concerned. Actually, the mixing efficiency of these fluids is poor because the shear migration diminishes the rate of mixing due to a decrease of the deformation in the high viscous media. The mixing by thick striation is only effective when the viscosity ratio becomes, by the diffusion process, typically higher than 10⁻³. Furthermore, the discussion based on the mutual diffusion coefficient agrees well with the experimental finding, at least qualitatively.     

Solubility and diffusion coefficient of oxygen in 1-ethyl-1-methylpyrrolidinium fluorohydrogenate room temperature ionic liquid at 298–373 K

Solubility (C) and diffusion coefficient (D) of oxygen in 1-ethyl-1-methylpyrrolidinium fluorohydrogenate (EMPyr(FH)_1.7F) room temperature ionic liquid (RTIL) have been determined at 298–373 K by the combination of linear-sweep voltammetry and hydrodynamic chronocoulometry with a rotating disc electrode (RDE). The solubility was 0.409 mmol dm⁻³ at 298 K and decreased with an increase in temperature. The solubility was compared with other RTILs and the difference was explained by the free volume estimated from the observed molar volume and the calculated molar volume. The diffusion coefficient was 1.59 × 10⁻⁵ cm² s⁻¹ at 298 K. The activation energy of diffusion was estimated to be 24.5 kJ mol⁻¹ from the slope of the Arrhenius plot. The permeability of oxygen (C × D) was compared with those in other RTILs, 0.5 M H₂SO₄ aqueous solution and Nafion^® in water to discuss the oxygen cross-over in fuel cell application.     

Nafion/lead oxide–manganese oxide combined catalyst for use as a highly efficient alkaline air electrode in zinc–air battery

In this study, we report the application of an inexpensive and easily prepared lead oxide–manganese oxide catalyst combined with Nafion (designated as Nf/PbMnO_x) as a highly efficient air-cathode for a zinc–air battery. We verify the mechanistic study of the reduction of O₂ for Nf/PbMnO_x in alkaline aqueous solution using rotating ring/disk electrode voltammetry, and also an electrochemical approach using a wall-jet screen-printed ring disk electrode. The presence of Nf/PbMnO_x shows great catalytic activity for the disproportionation reaction of HO₂⁻ to O₂ and OH⁻ with an overall 4e⁻ reduction of O₂ in the first reduction reaction. The 4e⁻ reduction of O₂ was eventually achieved at the Nf/PbMnO_x through evidence from the slope of Koutecky–Levich plots. With these inherent features, we then fabricated the zinc–air battery with the Nf/PbMnO_x catalyst and examine the performance for a practical application with air cathodes.     

Nano-CuO coated LiCoO2: Synthesis, improved cycling stability and good performance at high rates

LiCoO₂ nanoparticles were prepared through a sol–gel method, and then appropriate amount of CuO nanoparticles were deposited onto their surfaces to improve their cycling performances. It is found that the CuO coated sample has the capacity retention higher than 90% at the rates below 30 C after 10 cycles, which also has the highest capacities of 135 and 123 mAh g⁻¹ at 40 C (5480 mA g⁻¹) and 50 C (6850 mA g⁻¹), respectively. The cyclic voltammograms result reveals that the CuO coating reduces the polarization and improves the electrochemical activity of cathode. In addition, experimental results indicate that CuO coating plays an important role in reducing the charge transfer resistance of the cell during cycling, which has been demonstrated by the electrochemistry impedance spectroscopy analysis. The above data indicate the potential application of CuO coated LiCoO₂ in high power field.     

Electrochemical behavior of poly(3-hexylthiophene). Controlling factors of electric current in electrochemical oxidation of poly(3-hexylthiophene)s in a solution

Electrochemical response of regio-random and regio-controlled poly(3-hexylthiophene), P3HexTh, was investigated by cyclic voltammetry. P3HexTh underwent electrochemical oxidation at about 0.4 V vs. Ag⁺/Ag in a THF solution, and the peak anode electric current, i_pa, was proportional to the sweeping rate v; i_pa=const×v^1/2. These data indicated that diffusion of the P3HexTh molecule in the solution was important to determine i_pa. Application of a Matsuda's equation with assumptions gave a diffusion coefficient, D, of about 1×10⁻⁷ cm² s⁻¹ at molecular weight of about 5000, and the D value steeply decreased with increase in the molecular weight.     

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.