The electrochemical redox processes in methacrylate-based polymer electrolytes II. - Study on microelectrodes

The electrochemical behaviour of ferrocene was studied in different gel polymer electrolytes based on methyl, ethyl and 2-ethoxyethyl methacrylate and compared to the liquid aprotic solution (propylene carbonate). Voltammetric and chronoamperometric measurements on microelectrodes were conducted in order to describe the qualitative as well as quantitative behaviour of ferrocene in different conditions. Heterogeneous electron-transfer rate constants and diffusion coefficients of ferrocene in polymer electrolytes were estimated to be 1.1-7.8 × 10⁻³ cm s⁻¹ and 4-13 × 10⁻⁸ cm² s⁻¹ depending on the electrolyte composition. The influence of the polymer polarity, ferrocene concentration and level of polymer cross-linkage on the kinetics of ferrocene oxidation and its transport was discussed. The electrolytes with poly(2-ethoxyethyl methacrylate) exhibit the highest ionic conductivity (2-4 × 10⁻⁴ S cm⁻¹) as well as diffusion coefficient of ferrocene (1.3 × 10⁻⁷ cm² s⁻¹) in their structure.     

A study of the electrochemical behaviour of electrodes in operating solid-state supercapacitors

The electrochemical behaviour of electrodes and of complete solid-state supercapacitors has been studied by cyclic voltammetry (CV) and galvanostatic charge/discharge (CD) measurements using two independent electrochemical equipments. The first one controlled the execution of the test and recorded the voltage and current values of the complete supercapacitor while the other one recorded the potential changes of the single electrodes. In this work, two different types of capacitors were studied: (a) a symmetric supercapacitor using carbon electrodes, and (b) a hybrid (asymmetric) supercapacitor with ruthenium oxide/carbon in the positive electrode and carbon in the negative electrode. The studies evidenced that in the symmetric capacitors the positive electrode controlled the capacitive performance and an optimal mass ratio from 1.2:1 to 1.3:1 between the positive and the negative electrodes was found in the investigated conditions. For the hybrid supercapacitor it was observed that the ruthenium-based positive electrode influenced the capacitive performance of carbon-based negative electrode and that an accurate balance of carbon loading in the negative electrode was necessary.     

Electrochemical studies of ferrocene in a lithium ion conducting organic carbonate electrolyte

We carried out a detailed study of the kinetics of oxidation of ferrocene (Fc) to ferrocenium ion (Fc⁺) in the non-aqueous lithium ion conducting electrolyte composed of a solution of 1 M LiPF₆ in 1:1 EC:EMC solvent mixture. This study using cyclic (CV) and rotating disk electrode (RDE) voltammetry showed that the Fc⁰/Fc⁺ redox couple is reversible in this highly concentrated electrolyte. The ferrocene and ferrocenium ion diffusion coefficients (D) were calculated from these results. In addition, the electron transfer rate constant (k⁰) and the exchange current density for the oxidation of ferrocene were determined. A comparison of the kinetic data obtained from the two electrochemical techniques appears to show that the data from the RDE experiments are more reliable because they are collected under strict mass transport control. A Tafel slope of c.a. 79 mV/decade and a transfer coefficient α of 0.3 obtained from analysis of the RDE data for ferrocene oxidation suggest that the structure of the activated complex is closer to that of the oxidized specie due to strong interactions with the carbonate solvents. The experiments reported here are relevant to the study of redox reagents for the chemical overcharge protection of Li-ion batteries.     

The role of gel electrolyte composition in the kinetics and performance of dye-sensitized solar cells

Gel-type polymer electrolytes based on the copolymer poly(ethylene oxide-co-epichlorohydrin) and the plasticizer γ-butyrolactone (GBL) were optimized and applied in dye-sensitized solar cells. The plasticizer added to the electrolyte allowed the dissolution of a higher concentration of salt, reaching conductivity values close to 1 mS cm⁻¹ for the sample prepared with 30 wt% of LiI. Raman spectroscopy confirmed polyiodide formation in the electrolyte when the salt concentration exceeds 7.5 wt%, introducing a significant contribution of electronic conductivity in the electrolyte. The devices were characterized under AM 1.5 conditions and the I-V curves were fitted using a two diode equation. Increasing the concentration of LiI-I₂ accelerates dye cation regeneration as measured by transient absorption spectroscopy; however, it also contributes to an increase in the dark current of the cell by one order of magnitude. The best performance was achieved for the solar cell prepared with the electrolyte containing 20 wt% of LiI, with efficiencies of 3.26% and 3.49% at 100 and 10 mW cm⁻² of irradiation, respectively.     

Comparative study on electrochemical techniques for determination of hydrogen diffusion coefficients in metal hydride electrodes

The constant potential discharge technique, constant current discharge technique and electrochemical impedance spectroscopy were employed to determine the hydrogen diffusion coefficient in a metal hydride electrode of MlNi_3.65Co_0.75Mn_0.4Al_0.2 alloy with various depth of discharge (DOD) at room temperature and with a specific DOD at various temperatures. The results were compared and the advantages and disadvantages of these techniques were also discussed.     

An electrochemical method for measurement of mass transport in polymer membranes using acetaminophen as a model system

An electrochemical approach has been used to model and measure acetaminophen transport through a microporous polycarbonate membrane. The dynamic response of a membrane covered planar electrode system was investigated to derive a simplified middle point formula, the time at which amperometric current reaches half steady state value, that provided reliable solute diffusion coefficients. Experimental values may be noise-contaminated; when this noise is significant, the diffusion coefficient by this method needs to be refined. Here a direct simulation method with a bipartite mathematical expression for current transient was used, which fitted calculated current transients to experimental data. A best fit was reached by minimising the standard deviation between the simulated and experimental current profiles. Mathematically, the simulated curve was linear operated, i.e. shifted and stretched vertically, as well as shifted and stretched horizontally to coincide with the experimental curve. The single point formula, bipartite expression and simulation approach allowed extraction of accurate diffusion values from a previously reported approximated method. The computation approach is not only valid for membrane covered electrodes, but is suitable for other experimental set ups where a one-dimensional Fickian diffusion model is valid, for example, for axial diffusion through cylindrical geometries.     

Liquid–liquid electro-organo-synthetic processes in a carbon nanofibre membrane microreactor: Triple phase boundary effects in the absence of intentionally added electrolyte

An amphiphilic carbon nanofibre membrane electrode (ca. 50 nm fibre diameter, 50–100 μm membrane thickness) is employed as an active working electrode and separator between an aqueous electrolyte phase (with reference and counter electrode) and an immiscible organic acetonitrile phase (containing only the redox active material). Potential control is achieved with a reference and counter electrode located in the aqueous electrolyte phase, but the electrolysis is conducted in the organic acetonitrile phase in the absence of intentionally added supporting electrolyte. For the one-electron oxidation of n-butylferrocene coupled to perchlorate anion transfer from aqueous to organic phase effective electrolysis is demonstrated with an apparent mass transfer coefficient of m = 4 × 10⁻⁵ m s⁻¹ and electrolysis of typically 1 mg n-butylferrocene in a 100 μL volume. For the two-electron reduction of tetraethyl-ethylenetetracarboxylate the apparent mass transfer coefficient m = 4 × 10⁻⁶ m s⁻¹ is lower due to a less extended triple phase boundary reaction zone in the carbon nanofibre membrane. Nevertheless, effective electrolysis of up to 6 mg tetraethyl-ethylenetetracarboxylate in a 100 μL volume is demonstrated. Deuterated products are formed in the presence of D₂O electrolyte media. The triple phase boundary dominated mechanism and future microreactor design improvements are discussed.     

The electrochemical performance of a La-Mg-Ni-Co-Mn metal hydride electrode alloy in the temperature range of −20 to 30 °C

The effect of temperature on the overall electrochemical properties of La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 hydrogen storage alloy has been studied systematically. The results show that temperature has a striking effect on the overall electrochemical properties, especially the electrochemical kinetic performance. The maximum discharge capacity and the high rate dischargeability (HRD) of La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 alloy electrode both decrease with decreasing test temperature, mainly due to the slower hydrogen transfer in the bulk of the alloy and the lower electrocatalytic activity at lower temperatures. Detailed studies on the temperature effect on the polarization resistance (R_D), the exchange current density (I₀), the limiting current density (I_L) and the hydrogen diffusion coefficient (D), indicate that the diffusion of hydrogen in the bulk for La-Mg-Ni-Co system hydrogen storage alloy electrodes is the rate-determining factor for the discharge process of the alloy electrode for the temperature over 10 °C and the charge-transfer reaction is rate-determining step at lower temperature.     

The role of composition, morphology and crystalline structure in the electrochemical behaviour of TiNx thin films for dry electrode sensor materials

A morphological, structural and electrochemical study of titanium nitride (TiN_x) thin films, obtained by DC reactive sputtering on titanium substrates, was carried out for a wide range of compositions (0 < x < 1.34) aiming a selection of the best coatings for dry biomedical electrodes. The films displayed a columnar-type structure, with morphologies strongly dependent on the composition: a compact and smooth surface was found for the Ti-rich films, (x < 1), whereas the N-rich films, (x ≥ 1) displayed a rough and porous structure. The electrochemical study of the TiN_x films was performed in synthetic sweat, aiming at simulating the contact with the skin. The voltammetric analysis showed anodic currents higher for TiN_x films than for titanium for low and medium polarization potentials, whereas for potentials beyond 2 V the blocking behaviour of the TiN_x films allowed them to display lower current values. The passive dissolution currents in the sub-μA/cm² range and the charge transfer resistances of the order of the MΩ proved the excellent stability of all films in sweat conditions. Finally, the electrochemical noise analysis showed that the near-stoichiometric and N-rich films display the lowest noise, being therefore the most suitable for electrode applications, where signals in the microvolt range, such as the electroencephalographic (EEG) signals, are to be monitored.     

Increasing the electrochemical activity of transition metal phosphates in lithium cells by treatment with intimate carbon: The case of titanium phosphate

Amorphous sodium-containing titanium phosphate (sample TiP), obtained by heating a disordered precursor at 300 °C under air, electrochemically reacts with lithium by means of non-aqueous lithium cells. Discharge curves of EC/DEC cells in the 3.5-1.5 V region, were different to that observed for NASICON titanium phosphates but the related capacity can be associated to the reduction of Ti⁴⁺ to Ti³⁺. Another sample, C-TiP, obtained by using nitrogen atmosphere during the thermal treatment, was able to provide 75% more capacity in this region. This sample contained carbon because of the incomplete pyrolysis of the surfactant present in the precursor. Carbon is supposed to stay occluded inside the pores considering the values of pores surface area, total conductivity and HRTEM measurements. These titanium phosphates were also tested as negative electrodes. Electrochemical impedance spectroscopy measurements suggest that both titanium phosphates are reduced to yield titanium, lithia and phosphorus at 0.0 V. In our opinion, the presence of carbon in C-TiP is the key factor for the enhanced reactivity in the 3.2-1.5 V region but is not enough to obtain a good electrochemical performance, in terms of cycling efficiency.     

The electrochemical hydrogenation of edible oils in a solid polymer electrolyte reactor. II. Hydrogenation selectivity studies

Soybean oil has been hydrogenated electrochemically in a solid polymer electrolyte (SPE) reactor at 60°C and 1 atm pressure. These experiments focused on identifying cathode designs and reactor operation conditions that improved fatty acid hydrogenation selectivities. Increasing oil mass transfer into and out of the Pd-black cathode catalyst layer (by increasing the porosity of the cathode carbon paper/cloth backing material, increasing the oil feed flow rate, and inserting a turbulence promoter into the oil feed flow channel) decreased the concentrations of stearic acid and linolenic acid in oil products [for example, an iodine value (IV) 98 oil contained 12.2% C_18:0 and 2.3% C_18:3]. When a second metal (Ni, Cd, Zn, Pb, Cr, Fe, Ag, Cu, or Co) was electrodeposited on a Pd-black powder cathode, substantial increases in the linolenate, linoleate, and oleate selectivities were observed. For example, a Pd/Co cathode was used to synthesize an IV 113 soybean oil with 5.3% stearic acid and 2.3% linolenic acid. The trans isomer content of soybean oil products was in the range of 6–9.5% (corresponding to specific isomerization indices of 0.15–0.40, depending on the product IV) and did not increase significantly for high fatty acid hydrogenation selectivity conditions.     

The effect of transition metal ions (M2+=Mn2+, Ni2+, Co2+, Cu2+) on the chemical synthesis polyaniline as counter electrodes in dye-sensitized solar cells

The effect of transition metal ions(M~(2+)=Mn~(2+),Ni~(2+),Co~(2+),Cu~(2+)) on the chemical synthesis of polyaniline(PANI) used as a platinum-free counter electrode(CE) in dye-sensitized solar cells(DSSCs) was investigated.PANI was synthesized by co-polymerization of aniline in the presence of different transition metal ions by using potassium dichromate in acidic medium. It was found that the ion doping of PANI showed a certain catalytic activity for the regeneration of traditional iodide/triiodide(I~-/I_3~-) redox couples. The power conversion efficiency(η) of PANI CEs doped with Mn~(2+),Ni~(2+),Co~(2+) (4.41%, 2.36% and 2.10%, respectively) were higher than 1.94%, the value measured for PANI CE without doping. Doping with Cu~(2+)decreased the power conversion efficiency of PANI CE(PANI-Cu~(2+) η = 1.41%). The electrical properties of the PANI, PANI-Ni~(2+), PANI-Co~(2+),PANI-Mn~(2+) and PANI-Cu~(2+) were studied by cyclic voltammetry(CV), impedance(EIS), and Tafel polarization curve. The experimental results confirmed that PANI was affected by the doping of different transition metal ions(M~(2+)=Mn~(2+),Ni~(2+),Co~(2+),Cu~(2+)). These results indicate a potential application of ion doped PANI as counter electrode in cost-effective DSSCs.     

The synergistic combination of bis-silane and CeO2·ZrO2 nanoparticles on the electrochemical behaviour of galvanised steel in NaCl solutions

Bis-1,2-[triethoxysilylpropyl]tetrasulfide silane films containing CeO₂·ZrO₂ nanoparticles were deposited by dip-coating on galvanised steel substrates. The morphological features of the coated substrates were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The anti-corrosion performance of the modified silane film applied on galvanised steel substrates was studied by electrochemical impedance spectroscopy (EIS). The ability of nanoparticles to mitigate localized corrosion activity at artificially induced defects was investigated via the scanning vibrating electrode technique (SVET) and by the scanning ion-selective electrode technique (SIET). The results showed that the addition of nanoparticles provides good corrosion protection of the galvanised steel substrates pre-treated with the modified silane solutions. The corrosion activity was reduced by more than one order of magnitude. Complementary d.c. experiments, using zinc electrodes exposed to NaCl solutions containing the nanoparticles were also performed in order to better understand the role of the nanoparticles. An ennoblement of the corrosion potential and polarisation of the anodic reactions could be detected.