Chronopotentiometric Studies Research Articles

Green mediated sol-gel synthesis of MxCu1-xO (M = La,Ce x = 0.02–0.06) as an efficient catalyst for electrocatalytic oxygen evolution reaction

The imperative for advancing contemporary human society lies in the essential requirement to create energy generation systems that are sustainable, environmentally friendly, and exceptionally efficient. The development of efficient electrocatalysts, that are environmentally benign, economically viable and easily available, for alkaline oxygen evolution reaction is a significant research area in this regard. The present study focuses on a green mediated sol–gel auto-combustion process for the synthesis of lanthanum and cerium doped copper oxide nanoparticles with excellent electrocatalytic activity. Citric acid and polyols enriched lemon juice serves as the medium for the synthesis of various metal doped copper oxide nanoparticles. The extensive electrochemical studies using these catalysts reveals that, among the various doped samples, 5% lanthanum and 5% cerium doped catalysts exhibited the best performance in oxygen evolution reaction. Doping was found to enhance electrical conductivity and create oxygen vacancies, which enhanced the electrocatalytic activity of the catalysts. Specifically, the 5% lanthanum-doped copper oxide showed an overpotential of 340 mV, while the cerium-doped material exhibited an overpotential of 337 mV at a current density of 10 mA cm−2 in 1 M KOH. Moreover, the lanthanum-doped sample displayed a Tafel slope of 49 mV dec−1, whereas the cerium-doped sample had a Tafel slope of 60 mV dec−1 with better durability. Additionally, the chronopotentiometric studies revels the stability and long-term performance of LCO5 and CCO5, maintaining consistent potential of 1.58 V throughout 5 h at a current density of 10 mA cm−2. This study is conclusive evidence that the incorporation of rare earth elements improves the conductivity and overpotential of pristine copper oxide nanoparticles, making them highly effective in the oxygen evolution reactions.

Electrode reactions on the aluminum electrode in 1-butyl-3-methylimidazolium chloride chloroaluminate ionic liquid: A comprehensive study

Mechanisms of Al electrochemical deposition/dissolution in acidic AlCl3–1-butyl-3-methylimidazolium chloride (BMImCl) ionic liquids are studied. Stationary polarization curves (SPCs) are obtained in the anodic range of potentials. The anodic SPCs are characteristic of electrode processes, which are complicated by passivation phenomena. The peaks intensity of AlCl4−, Al2Cl7− ions were obtained by in-situ Raman spectroscopy during the aluminum electrode polarization and correlated with the behavior of the SPCs. Based on the results obtained, a mechanism of electrochemical oxidation of aluminum in AlCl3–BMImCl IL is proposed. According to the mechanism in question, the anodic process is complicated by the presence of a following chemical reaction involving AlCl3. Chronopotentiometric studies were carried out to determine diffusion coefficients of Al2Cl7− anion, which are unaffected by the change in the molar ratio of AlCl3 to BMImCl (N). The activation energy (Ea) for the Al2Cl7− diffusion coefficient is independent of N and equals 14.5 ± 0.4 kJ mol−1. Exchange current density (i0) at the Al | AlCl3–BMImCl interface was determined using impedance spectroscopy for the compositions with 1.1 ≤ N ≤ 2.0 at temperatures from 303 to 386 K. The i0 increases from 0.54 ± 0.13 to 2.73 ± 0.22 mA cm−2 with the concentration of the electrochemically active Al2Cl7− anion in the concentration range indicated above at 303 K. The data on i0 is used to calculate the transfer coefficient of cathodic reaction (α = 0.18 ± 0.02), the electrochemical reaction rate constant (ks= (1.1 ± 0.3)∙10−6 cm s−1 (at T = 303 K)) and the Ea for the ks (Eaks = 36.5 ± 0.9 kJ mol−1), which do not depend on the IL composition.

Reaction induced conformational change in polyindole: Polyindole/PVA film as biomimetic sensors of temperature and electrical energetic condition.

The influence of surrounding temperature and electrical energetic condition on the conformational movements (cooperative actuation) of polyindole is verified using a polyindole-coated polyvinyl alcohol (PIN/PVA) film. Chronopotentiometric studies revealsthat the consumed electrical energy during the reaction varies linearly with the change in working temperature. The influence of temperature on the reversible conformational movements of the polymer chain is related to the charge consumed during the reaction. The logarithmic dependence of reversible redox charge obtained from coulovoltammogram with inverse of temperature further proved the temperature sensing characteristics and the influence of temperature on the cooperative actuation of the PIN/PVA film. The conformational relaxation increases as the temperature increases through hosting higher number of counter anions with the solvent molecule. The extension of the redox reaction or charge consumption was found to decrease as the scan rate increases. The double logarithmic relation between the consumed redox charge and the scan rate has proved that the electrical energetic condition can influence the conformational movement or the extension of the redox reaction in a reversible manner. The results suggest that the PIN/PVA film can act as a biomimetic macro molecular sensor of working temperature and electrical energetic condition as biological muscles do.

NiFe Alloy Nanoparticles Tuning the Structure, Magnetism, and Application for Oxygen Evolution Reaction Catalysis

In this study, Ni-Fe alloy nanoparticles were prepared using the proteic sol–gel method, followed by a reduction in H2 at 500 and 700 °C, namely hereafter as NiFe-500 and NiFe-700, respectively. The morphological, structural, and magnetic properties were tuned via the thermal treatment in H2. The samples were studied using XPS, TEM, Mössbauer spectroscopy, DC magnetic measurements, and electrochemical measurements. Ritveld refinements showed that the sample NiFe-500 has FCC (face-centered cubic) and BCC (body-centered cubic) NiFe alloys, while the sample NiFe-700 has only FCC NiFe alloy. For both samples, magnetization measurements in the range of 300–900 K showed the presence of the Griffiths phase, indicating the formation of clusters of either Fe or Ni-Fe alloys rich in Fe. The sample NiFe-500 presented ferromagnetic (FM) transitions at 533, 700, and 834 K, assigned to the alloys Ni37Fe63-FCC, Ni46Fe54-FCC, and Ni55Fe45-FCC, respectively. In contrast, we could not observe the FM transition of the BCC Ni-Fe alloy because of limitations in our experimental setup (T ≤ 900 K). Meanwhile, three FM transitions were observed for the sample NiFe-700 at 480, 655, and 825 K, attributed to the alloys Ni34Fe66-FCC, Ni43Fe57-FCC, and Ni54Fe46-FCC, respectively. At 5 K, the samples NiFe-500 and NiFe-700 have saturation magnetizations of 164.2 and 173.6 emu g−1, respectively. For application in Oxygen Evolution Reaction catalysis, the samples NiFe-500 and NiFe-700 showed different overpotentials of 319 and 307 mV at 10 mA cm−2. These low overpotential values indicate a higher electrochemical activity of the FCC Ni-Fe alloy and, for both samples, a superior electrocatalytic activity in comparison to RuO2 e IrO2 conventional catalysts. Furthermore, the samples showed high electrochemical stability in chrono potentiometric studies for up to 15 h. This current work highlights that the Ni-Fe alloys produced via the proteic sol–gel and with a reduction in H2 methods can be promising for OER systems due to their good performance and low costs.

Ru tailored hydrous cobalt phosphate as a rational approach for high-performance alkaline oxygen evolution reaction

The water splitting is an appealing approach to fulfil the demand of energy without any global concerns. The oxygen evolution reaction is one of way to achieve unlimited energy from water but the low stability, high energy required to drive the reaction (overpotential), complicated and unscalable synthesis method of expensive catalysis restricts its real time implementation into modern society. Herein this report, we suggested Ru-tailored hydrous cobalt phosphate and its concentration effect as a dopant to improve the catalytic properties. Electrocatalytic properties were studied via linear sweep voltammetry, chrono-potentiometric studies at a 10 mA/cm2 for 20 h, electrochemical active surface area, turnover frequency, and Tafel slope. Cobalt phosphate showed the connected-nanorods morphology, which is further tunned by increasing Ru-doping concentration, achieving various features from agglomerated nanorods sensitized with uniform nanoparticles on the micro-flower shapes consisting of interconnected nanorods sensitized with nanoparticles. The good electrocatalytic OER performance of 5% Ru-doped hydrous cobalt phosphate was achieved, exhibiting low overpotential up to 310 mV (vs. RHE) at 10 mA/cm2 besides a small Tafel slope of 62 mV/dec in 1 M KOH. As well, stable O2 evolution for a period of 20 h was shown. DFT calculation reveals that the Ru can increase electrical conductivity and promote OER activity, observed from Bader charge analysis, electron density difference, and OER Gibbs free energy. The EIS plot, ECSA, and TOF estimation disclose that the increased electrical conductivity, the improved ECSA, and the high intrinsic activity are together responsible for the good OER activity for 5%Ru–CoPO.

Electrochemical water electrolysis using electrodeposited (NiMo) coatings from a low concentration bath

Electrodeposited (NiMo) coatings are noted as benign alternative for Pt-free electro-catalysts in alkaline water electrolysis. Hitherto, many Ni-based alloy coatings have been developed using baths of high salts’ concentration. The present paper attempts to see the effect of reducing the metal salt concentrations of a (NiMo) bath on the electro-catalytic functionality of its alloy coatings towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Electro-catalytic performance was evaluated through cyclic voltammetry (CV) and chronopotentiometric (CP) studies and the findings were compared with those obtained from high concentration bath, with the same constituents. It was revealed that the electro-catalytic efficacy of (NiMo) coatings has a direct relation only with the alloy composition, rather than the concentration of the bath, which is due to the inherent induced co-deposition behavior of (NiMo) system, supported by Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray Spectroscopy (EDXS), and X-ray Diffraction (XRD) analyses.

Morphology-controlled synthesis of one-dimensional zinc molybdate nanorods for high-performance pseudocapacitor electrode application

Nanorod-shaped Zinc molybdate (ZnMoO4) materials have been prepared using a cetyl-trimethylammonium bromide (CTAB) template-assisted hydrothermal synthesis, followed by subsequent calcination process. The morphology and size of the ZnMoO4 can be altered by adjusting the concentration of CTAB. The optimal concentration of CTAB provides nanorod-shaped ZnMoO4, which accordingly determines the electrochemical features of prepared supercapacitor electrodes. The electrochemical properties were examined using cyclic voltammetric (CV) and chronopotentiometric (CP) techniques. The cyclic voltammetric studies confirm the pseudocapacitor mechanism with the specific capacitance of 779 Fg−1 at a scan rate of 5 mVs−1. Furthermore, the chronopotentiometric studies deliver the specific capacitance of 540 Fg−1 at a current density of 1 Ag−1. Besides, the ZnMoO4 demonstrated outstanding cyclic stability and the capacitance retention of about 90% was observed even after continuous 3000 cycles at a scan rate of 100 mVs−1. This endeavor demonstrated an important promising approach to synthesize ZnMoO4 nanorods with high specific capacitance, rate capability and superior cycle stability, which have attractive interest to be the candidate for electrode materials towards high-performance supercapacitor applications.

Treatment of Cyanide-Free Wastewater from Brass Electrodeposition with EDTA by Electrodialysis: Evaluation of Underlimiting and Overlimiting Operations.

Growing environmental concerns have led to the development of cleaner processes, such as the substitution of cyanide in electroplating industries and changes in the treatment of wastewaters. Hence, we evaluated the treatment of cyanide-free wastewater from the brass electroplating industry with EDTA as a complexing agent by electrodialysis, aimed at recovering water and concentrated solutions for reuse. The electrodialysis tests were performed in underlimiting and overlimiting conditions. The results suggested that intense water dissociation occurred at the cathodic side of the commercial anion-exchange membrane (HDX) during the overlimiting test. Consequently, the pH reduction at this membrane may have led to the reaction of protons with complexes of EDTA-metals and insoluble species. This allowed the migration of free Cu2+ and Zn2+ to the cation-exchange membrane as a result of the intense electric field and electroconvection. These overlimiting phenomena accounted for the improvement of the percent extraction and percent concentration, since in the electrodialysis stack employed herein, the concentrate compartments of cationic and anionic species were connected to the same reservoir. Chronopotentiometric studies showed that electroconvective vortices minimized fouling/scaling at both membranes. The electrodialysis in the overlimiting condition seemed to be more advantageous due to water dissociation and electroconvection.

A simple way to modify selectivity of sodium sensitive electrodes by using organic conductive crystals

An ion-selective membrane is an essential element in the construction of ion-selective electrodes. In this work, some investigations concerned on influence of an ion-selective membrane composition on sodium-selective electrode properties are presented. Several coated-disc sodium-selective electrodes were prepared with the use of sodium ionophore III or sodium ionophore VI, or mixture of these two selective carriers. Membranes with the same compositions were also used to obtain sensors with solid contact transducer layer consisted of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its sodium radical salt (NaTCNQ). In other polymeric membranes, a whole or some part of ionophore was replaced by NaTCNQ in order to assess the effect of its presence on sodium single-piece electrode characteristics. All kinds of prepared electrodes were tested using potentiometric and chronopotentiometric studies, and significant impact of membrane composition on electrode parameters was observed. Among investigated sensors, TCNQ/NaTCNQ-contacted all-solid-state electrodes exhibited the best analytical and electrical performance due to the presence of the intermediate layer, which simplifies the ion-to-electron transduction process between the ion-sensing membrane and the glassy carbon electrode. Exemplary, these electrodes with sodium ionophore III had a close-to-Nernstian slope (58.63 mV/pNa) in the range from 10−5–10−1-M NaCl and revealed detection limit of 10–5.2 M. However, the mixing of sodium ionophores III and VI in one polymeric membrane led to improved sensitivity and limit of detection (58.79 mV/pNa and 10–5.4 M, respectively). The highest capacitance observed for electrodes with TCNQ/NaTCNQ based on intermediate layer was 139 μF.

Electrochemical behavior and electrowinning of uranium(IV) from FLiNaK molten salt

The electrochemical behaviors and electrowinning of uranium from LiF–NaF–KF (46.5–11.5–42.0 mol%, FLiNaK) molten salt were investigated. Cyclic voltammetric and chronopotentiometric studies revealed the presence of two steps during the electro-reduction of UF4 on W electode. U(IV) undergoes a one-electron reduction to form U(III) followed by a three-electron reduction to form U(0). The diffusion coefficient (D) of U(IV) ion was determined to be 1.33 × 10−6 cm2 s−1 at 550 °C. Based on the electrochemical behaviors of U(IV) in FLiNaK melt, pulse current electrolysis was carried out using Ni electrode. The black bulks obtained as electrolysis products were characterized to contain metallic U. Consecutive electrolysis experiments demonstrated up to 74.7% U can be separated from the fluoride melt.

Nanocomposite of hexagonal β-Ni(OH)2/multiwalled carbon nanotubes as high performance electrode for hybrid supercapacitors

β-Ni(OH)2 and its composite with multiwalled carbon nanotubes (MWCNTs) were synthesized by hydrothermal process. Their electrochemical properties such as specific capacitance, energy density, power density, coulombic efficiency and charge-discharge cycles were investigated by Cyclic voltammetry (CV), Chronopotentiometry (CP) and Electrochemical impedance spectroscopy (EIS) techniques. The materials were analyzed for their textural and structural properties by different analytical techniques such as Powder X-ray diffraction (PXRD), Brunauer-Emmett-Teller (BET) surface area, Scanning electron microscopy (SEM) and Energy dispersive X-ray spectroscopy (EDS), Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and Raman spectroscopy. The diffraction peaks obtained from PXRD studies and the TEM images of the materials inferred the formation of hexagonal nanostructured β- Ni(OH)2 in both pure and composite materials. The composite material exhibited better electrochemical supercapacitance performance than pure β-Ni(OH)2. Their specific capacitance values were found to be 1882 F g−1 and 819 F g−1 respectively at a scan rate of 2 mVs−1. The presence of MWCNTs enhanced the specific capacitance value of β-Ni(OH)2 nearly by 57% at 2 mVs−1. Chronopotentiometric studies conducted at a current density of 5 A/g indicated that the composite material was stable up to 5000 charge-discharge cycles. Another interesting observation made is that the composite material exhibited 100% coulombic efficiency where as it was only 84% for β-Ni(OH)2 at 5000th cycle. The composite material gave an energy density of 40 Wh/kg which was nearly 4 times that of β-Ni(OH)2.

Electrochemistry of Ionophore-Based Ion-Selective Electrodes: Response Limits, Slope and Potential Stability

The contribution is devoted to the electrochemical issues related to the response span, slope, and potential stability of ionophore-based ion-selective electrodes (ISEs). The discussion is primarily focused on those research topics in this area which have been studied by the late R.P. Buck.Theoretical and experimental efforts aimed at the improvement of the lower detection limit of the ISE response constitute the current mainstream in the ISE theory and practice. This is mostly due to high environmental relevance of the trace level analysis. The galvanostatic polarization of ISEs, as the most flexible approach to the low detection problem, was proposed first by R.P. Buck et al [1]. Since then, the method was largely improved and modified in different ways. We recently proposed tuned galvanostatic polarization [2, 3] which proved to be suitable also for ISEs with crystalline membranes [4]. The idea of the method will be discussed and illustrated with several applications.It will be shown that the upper detection limit of ISEs (which is mostly of industrial relevance) can also be improved in a very similar way. Thus the so-called anion interference (or cation interference in the case of anion-sensitive ISEs): the effect studied originally by R.P. Buck in 1970s [5], can be overpassed successfully. Furthermore, simple theoretical modeling of the trans-membrane fluxes effect in the ISE potentiometric response allows for a unified description of both lower and upper limits of the ISEs response, together with the response slope. Theoretical conclusions on the origin of slightly sub-Nernstian slopes typically observed for real-world ISEs, and on the relation between the slope and the response span, will be supported with the direct experimental evidence.When studying the so-called solid contact ISEs (those without internal solution), R.P. Buck suggested that a reversible and fast RedOx reaction at the interface between ionically-conducting membrane and electronically-conducting substrate is a prerequisite for the electrode potential stability over time [6]. This idea is widely recognized among the ISE researchers community. However, our recent studies showed that this RedOx reaction is the necessary, but not the sufficient condition, and low diffusional polarizability at the interface is also needed for the solid contact ISEs stability [7, 8].Besides the experimental facts which can be treated positively in the frames of the existing concepts of the ISE response mechanism, some peculiar observations will be presented. These evidences obtained by impedance and chronopotentiometric studies appear challenging and require further thorough studies. E. Lindner, R.E. Gyurcsanyi, R.P. Buck, Electroanalysis, 1999, 11 695.M.A. Peshkova, T. Sokalski, K.N. Mikhelson, A. Lewenstam, Anal. Chem., 2008, 80 9181.M.A. Peshkova, K. N. Mikhelson, Electrochimica Acta, 2013, 110, 829.G. Lisak, T. Sokalski, J. Bobacka, L. Harju, K. Mikhelson, A. Lewenstam, Anal. Chim. Acta, 2011, 707, 1.J.H. Boles, R.P. Buck Anal. Chem. 1973, 45, 2057.R.P. Buck, V.R. Shepard, Anal. Chem. 1974, 46, 2097.E.N. Samsonova, V.M. Lutov, K.N. Mikhelson, J Solid State Electrochem. 2009, 13, 69.N.M. Ivanova, I.V. Podeshvo, M.Ya. Goikhman, A.V. Yakimanskii, K.N. Mikhelson, Sensors & Actuators B, 2013, 186, 589.

Chronopotentiometric and electroanalytical studies of Ni(II) selective polyaniline Zr(IV) molybdophosphate ion exchange membrane electrode

Polyaniline Zr(IV) molybdophosphate (PAZMP) was synthesized by sol–gel method and its ion exchange capacity (IEC) was determined as 2.50meqg−1. On the basis of distribution co-efficient (Kd values) the material was selective for Ni(II). The Ni(II) selective membrane was prepared (IEC 0.65meqg−1) to fabricate Ni(II) selective membrane electrode with its linear potential response in the concentration range of 1×10−7M–1×10−1M. The morphology and composition of the prepared PAZMP membrane were ascertained by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) respectively. The ion exchange mechanism through membrane was determined by chronopotentiometric studies using Nernst-Planck equation at 30°C, 45°C and 60°C. As a result physical parameters such as self-diffusion coefficient (D0), energy of activation (Ea) and entropy of activation (ΔS°) were calculated. The analytical utility of this electrode was established by employing it as an indicator electrode in potentiometric titration.

Functionalized chitosan based nano-filter membranes for pH-controlled separation of amino acids

pH-controlled separation of AAs was achieved by electro-membrane cell (EMC) with three compartments (central, comp. 1 and comp. 2). EMC is based on the principles of electrodialysis (ED) using indigenous cation-exchange and anion-exchange nanofilter (C-NF and A-NF, respectively). Water permeabilities (1.1–1.5mh−1bar−1) and molecular cut-off (MWCO) values (400–500Da) for C-NF and A-NF, were suggested their nanofilter (NF) nature. Electrochemical properties of these membranes confirmed their suitability for EMC. Chronopotentiometric studies revealed the electro-transport of positively charged lysine (LYS+) and negatively charged glutamic acid (GLU−) across C-NF and A-NF, respectively, at pH: 6.1 under applied voltage. While glycine at its pI (pH: 6.1) remained immobile. Thus, separation of AAs (ternary mixture) by iso-electric point (pI) focusing of one component was proposed. Under optimized experimental conditions (i.e. at 4.0V constant applied voltage, pH: 6.1 for 0.05M equi-molar LYS–GLU–GLY mixture), about 4.0kWhkg−1 of AA separated energy consumption, 82% current efficiency and 90% product recovery showed the economic and technical feasibility of EMC for industrial exploitation.

Additives for Bottom-up Copper Plating from an Alkaline Complexed Electrolyte

An alkaline, tartrate-complexed copper electrolyte containing additives that provide bottom-up fill is described. Bottom-up fill is achieved using a mixture of two additives: bis-(3-sulfopropyl) disulfide (SPS) and polyethyleneimine (PEI). Chronopotentiometric studies indicate that, unlike in conventional acidic electrolytes, SPS acts as a ‘suppressor’ and PEI acts as an ‘anti-suppressor’ in the alkaline medium. Partial-fill experiments on patterned structures confirm a SPS-PEI interaction leading to bottom-up fill from the tartrate-complexed copper electrolyte.

An Electrochemical Investigation of Fe(II) Dissolved in a Cryolite Melt

Chronopotentiometric studies were made on a cryolite melt containing 3.0 wt pct Al2O3 and 0.466 wt pct Fe(II) at 1293 K (1020 °C). The diffusion coefficient calculated from the time of the principal chronopotentiometric transition decreased as the current density was increased, and at the same time, a second subsequent transition appeared. The diffusion coefficient calculated from this second transition was constant at 5.44 × 10−5 cm2 s−1. The results were interpreted to show that Fe(II) in the solution exists in two forms. Fe is deposited reversibly from an active form; its exchange current density must be >1 A cm−2. Deposition from the other form is irreversible, and it occurs directly only at high overpotentials, leading to the second transition. The equilibrium constant [active]/[inactive] = 5.4. When the equilibrium is displaced by electrolysis of the active form, the inactive form decomposes to replenish it with a rate constant of 0.9 s−1. The Tafel curve for the direct deposition of the inactive form shows a slope of 113 mV/decade, which is interpreted as n = 2 and a symmetry factor ≈1. The exchange current density is approximately 0.3 μA cm−2. The active and inactive forms are identified tentatively as FeF 3 − and FeF 5 3− , respectively.

Chronopotentiometric Investigation of Borohydride Oxidation at a Gold Electrode

Sodium borohydride presents interesting options for electrochemical power generation acting either as a hydrogen source or an anodic fuel for direct borohydride fuel cells. Though there have been several papers concerning electrochemical determination of relevant thermodynamic and kinetic parameters to the borohydride system, there are many basic aspects that have not yet been systematically studied. This paper reports chronopotentiometric studies of the electro-oxidation of at a gold electrode in 2 M NaOH solutions at temperatures ranging from 25 to . Gold displays a rather good oxidation activity, and the overall oxidation process is shown to be irreversible involving a number of exchanged electrons close to the theoretically expected value of 8. For the specific potential and concentration ranges observed, the results suggest that the rate-determining step is an irreversible, diffusion-controlled, one-electron oxidation step for which several key kinetic parameters (α, , and ) are calculated.

Properties of Ca 1− xHo xMnO 3 perovskite-type electrodes

In this work, Ca 1− x Ho x MnO 3 ( x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K. The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn 4+/Mn 3+ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn 4+/Mn 3+ redox couple and the other to the Mn 7+/Mn 4+ and Mn 6+/Mn 4+ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Ho-substituted electrodes present higher current density when compared with CaMnO 3 electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho.

Surface Homogeneity of Anion Exchange Membranes: A Chronopotentiometric Study in the Overlimiting Current Range

Electrotransport of sodium chloride near and through the ASV anion exchange membrane was first investigated. Chronopotentiometric and current-voltage characteristics results have shown that the ASV membrane acts as a totally conducting plane with respect to the transport of NaCl electrolyte. SEM and AFM images contributed to confirm the overall homogeneous surface of the membrane. Further chronopotentiometric studies of the membrane were evaluated in the presence of different alkaline chloride solutions in order to explore the influence of alkali co-ions on the transport phenomena. Membrane characterization led to determine the transport number of chloride counterion in the membrane. It is reported in this work that chronopotentiometry using the Sand equation toward the homogeneous ion exchange membrane is a simple and efficient method for determination of the diffusion coefficient of the electrolytes in the bulk solution. Discussions on the transport properties of the electrolyte solutions in relation with the hydrated ion sizes allowed us to verify the diffusion coefficient of the electrolytes determined by means of chronopotentiometric method.

Chronopotentiometric Studies on the Passivation of Industrial Copper Anode at Varying Current Densities and Electrolyte Concentrations

Chronopotentiometric Studies on the Passivation of Industrial Copper Anode at Varying Current Densities and Electrolyte Concentrations