Apparent Exchange Current Density Research Articles

Electrodeposition of Sb(III) in alkaline solutions containing xylitol

The electrodeposition of antimony in alkaline solutions containing xylitol was investigated using cyclic voltammetry, linear sweep voltammetry and chronoamperometry. The antimony electrodeposition and the chemical stability of xylitol in alkaline solutions were studied by cyclic voltammetric technique. Apparent activation energy, apparent transfer coefficient and exchange current density were obtained by linear sweep voltammetric technique. Initial stages of antimony electrocrystallization were determined by chronoamperometry. Xylitol in alkaline solutions exhibits high chemical stability and there is no redox in solutions when the potential ranges from −1.20 V to +0.60 V (vs Hg/HgO). There is no other redox reaction but hydrolysis occurring on stainless steel in the potential range of −1.75 V to 1.25 V (vs Hg/HgO) while the xylitol decomposition maybe take place on antimony electrode when potential is more negative than −1.70 V (vs Hg/HgO). Cyclic voltammograms with different scan rates indicate that the antimony electrodeposition process is an electrocrystallization which is a completely irreversible electrode process. The apparent activation energy, apparent transfer coefficient and exchange current density were calculated to be 46.33 kJ/mol, 0.64 and 4.40×10−6 A/m2, respectively. The analyses of the chronoamperometric responses support the view of a three-dimensional growth under progressive nucleation. The average diffusion coefficient of antimony was calculated to be 1.53×10−6 cm2/s.

Copper molybdenum sulfide: a new efficient electrocatalyst for hydrogen production from water

A new inorganic solid state electrocatalyst for the hydrogen evolution reaction (HER) is reported. Highly crystalline layered ternary sulfide copper-molybdenum-sulfide (Cu2MoS4) was prepared by a simple precipitation method from CuI and [MoS4]2− precursors. In aqueous solution and over a wide pH range (pH 0 to 7), this Cu2MoS4 showed very good catalytic activity for HER with an overvoltage requirement of only ca. 135 mV and an apparent exchange current density of 0.040 mA cm−2 (Tafel slope of ca. 95 mV per decade was found irrespective of the pH value). This Cu2MoS4 catalyst was found to be stable during electrocatalytic hydrogen generation. Therefore, it represents an attractive alternative to platinum.

Lead electrodeposition from alkaline solutions containing xylitol

The electrodeposition of lead in alkaline solutions containing xylitol (1, 2, 3, 4, 5-pentahydroxypentane) was studied. The lead electrodeposition and the chemical stability of xylitol in alkaline solutions were investigated by cyclic voltammetry. Apparent activation energy, apparent transfer coefficient and exchange current density were obtained by linear sweep voltammetry. Initial stages of lead electrocrystallization were determined by chronoamperometry. Voltammograms of a AISI 316 stainless steel electrode in xylitol solution exhibit no current in the potential range of −1.3 V to 0.75 V (vs Hg/HgO), implying that xylitol is stable to oxidation and reduction. The apparent activation energy, apparent transfer coefficient and exchange current density were calculated to be 35.15 kJ/mol, 1.56 and 9.65×10 −5 A/m 2. Analysis of the chronoamperometric responses implies three-dimensional growth of nuclei, with the type of nucleation depending on overpotential.

Electrochemical-reduction Characteristics of Maleic Acid on a Porous Pb-Zn Electrode

A Pb-Zn coating was electroplated from methyisulfonic acid bath.The coating was then corroded by 10%H2SO4(volume fraction) solution to dissolve zinc.A porous Pb-Zn electrode was therefore successfully prepared.The Pb-Zn coating was composed of Pb(Fm3m) and Zn(P663/mmc).Electrochemical reduction characteristics of maleic acid(MA) on porous Pb-Zn and Pb electrodes were investigated by voltammetry,cathode polarization,chronocoulometry,and electrochemical impedance spectrum(EIS).As a result,MA was absorbed on the electrode surface,and then underwent irreversible electron-transfer reaction.The corresponding diffusion coefficients for MA in H2SO4 solution on a Pb and porous Pb-Zn electrode was 1.2×10-7 and 1.7×10-7 cm2/s,respectively.The apparent exchange current density(i0) of MA on a porous Pb-Zn electrode was steadier than that on a Pb electrode.The i0 of MA on the porous Pb-Zn electrode had a linear relationship with temperature in low temperature,and increased sharply above 50 ℃,while decreased rapidly when reached to 65 ℃.The best electrochemical-reduction temperature for MA on porous Pb-Zn electrode was 50～60 ℃.The catalytic activity of MA on porous Pb-Zn electrode was higher than that on lead electrode.

Electrocatalytic activity of anodic biofilm responses to pH changes in microbial fuel cells

This study investigates the effects of anodic pH on electricity generation in microbial fuel cells (MFCs) and the intrinsic reasons behind them. In a two-chamber MFC, the maximum power density is 1170 ± 58 mW m −2 at pH 9.0, which is 29% and 89% higher than those working at pH 7.0 and 5.0, respectively. Electrochemical measurements reveal that pH affects the electron transfer kinetics of anodic biofilms. The apparent electron transfer rate constant ( k app ) and exchange current density ( i 0) are greater whereas the charge transfer resistance ( R ct) is smaller at pH 9.0 than at other conditions. Scanning electron microscopy verifies that alkaline conditions benefit biofilm formation in MFCs. These results demonstrate that electrochemical interactions between bacteria and electrodes in MFCs are greatly enhanced under alkaline conditions, which can be one of the important reasons for the improved MFC output.

Anodic Dissolution Behavior of Magnesium in Hydrophobic Ionic Liquids

Anodic dissolution behavior of magnesium (Mg) metal was examined in an ionic liquid bath containing Mg(II) ions as Mg(Tf2N)2. A set of sampled-current voltammograms obtained by a potential-step method revealed that the Mg2+/Mg0 redox potential in the ionic liquid was +1.41 V vs. Li+/Li0, and was little affected by residual water in the ionic liquid. It was found, in contrast, that the residual water increased faradaic current density for the anodic dissolution of Mg and also the apparent exchange current density of the Mg2+/Mg0 redox.

Preparation of the Ni/NiCo<sub>2</sub>O<sub>4</sub> Composite Electrode and Its Properties toward the Oxygen Evolution Reaction in Alkaline Media

A NiCo2O4 spinel composite oxide was obtained by co-precipitation. A Ni/NiCo2O4 composite coating was prepared by composite electrodeposition in a Ni plating solution by mixing with NiCo2O4 powder. The best plating conditions for the composite electrodeposition were investigated by changing factors such as the plating bath pH and the cathode current density jk. The surface morphology, the NiCo2O4 content, and the structure of the Ni/NiCo2O4 composite coating were characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). As a result, the NiCo2O4 content in the Ni/NiCo2O4 composite coating was found to be the highest (30.6%, w) for a bath pH of 6.2 and a cathodic current density jk of 100 mA·cm - 2 . In 5 mol·L - 1 KOH solution the electrocatalytic properties toward the oxygen evolution reaction (OER) of the Ni/NiCo2O4 composite electrode was studied using cyclic voltammetry, electrochemical steady-state polarization, and electrochemical impendence spectroscopy (EIS). Compared with the Ni electrode, the electrocatalytic properties for the OER of the Ni/NiCo2O4 composite electrode improved greatly and the apparent activation energy was reduced to 53.2 kJ·mol - 1 . The apparent exchange current density of the Ni/NiCo2O4 composite electrode toward the OER was 7 times as high as that of a nickel electrode. An electrochemical impendence spectroscopy analysis indicated that the oxygen evolution reaction of the Ni/NiCo2O4 composite electrode was co-controlled by an electrochemical step and a diffusion step. Constant potential electrolysis over extended periods for the OER shows that the stability of the Ni/NiCo2O4 composite electrode is good.

Enhanced long-term durability of proton exchange membrane fuel cell cathode by employing Pt/TiO 2/C catalysts

Pt/TiO 2/C catalysts are employed as the cathode catalysts for proton exchange membrane fuel cell (PEMFC). The comparative studies on the Pt/C and Pt/TiO 2/C catalysts are conducted with the physical and electrochemical techniques. After the accelerating aging test (AAT), the remaining electrochemical active surface area (EAS) of the Pt/TiO 2/C catalysts is 75.6%, which is larger than that of the Pt/C catalysts (42.6%). The apparent exchange current density ( i app 0 ) of the oxygen reduction reaction (ORR) at the Pt/C catalysts decreases from 3.02 × 10 −9 to 1.32 × 10 −11 A cm −2 after the AAT. And the value of i app 0 of the ORR at the Pt/TiO 2/C catalysts is 2.88 × 10 −9 A cm −2 before the AAT and 2.51 × 10 −9 A cm −2 after the AAT. Furthermore, the output performance degradation of the PEMFC using the Pt/TiO 2/C cathode catalysts is also less than that using the Pt/C catalysts. The particle size of the Pt/C catalysts increases significantly from 5.3 to 26.5 nm after the AAT. The mean particle size of the Pt/TiO 2/C catalysts is 7.3 nm before the AAT and 9.2 nm after the AAT. It can be concluded that the long-term durability of the Pt/TiO 2/C catalysts in a PEMFC is much better than that of the Pt/C catalysts.

EIS-assisted performance analysis of non-noble metal electrocatalyst (Fe–N/C)-based PEM fuel cells in the temperature range of 23–80 °C

A carbon-supported non-noble metal catalyst, Fe–N/C, was used as the cathode catalyst to construct membrane electrolyte assemblies (MEAs) for a proton exchange membrane (PEM) fuel cell. The performance of such a fuel cell was then tested and diagnosed using electrochemical impedance spectroscopy (EIS) in the temperature range of 23–80 °C. Based on the EIS measurements, individual resistances, such as charger transfer resistance and membrane resistance, were obtained and used to simulate polarization curves (current–voltage ( I– V) curves). A close agreement between the simulated I– V curves and the measured curves demonstrates consistency between the polarization and EIS data. The temperature-dependent parameters obtained via EIS, such as apparent exchange current densities and electrolyte membrane conductivities, were also used to acquire activation energies for both the oxygen reduction reaction (ORR) catalyzed by an Fe–N/C catalyst and the proton transport process across the electrolyte membrane. In addition, the maximum power densities for such a fuel cell were also analyzed.

Electrocatalysis of oxygen reduction by lanthanum-strontium manganate

The electrocatalysis of the oxygen reduction reaction by lanthanum-strontium manganate La0.5Sr0.5MnO3 (LSM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium. From the ring-disc data and other kinetic parameters it was concluded that the oxygen reduction occurs by dissociative chemisorption at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2− in this case) on the electrocatalyst has been observed. The apparent exchange current density for oxygen reduction on LSM has been found to be 2 × 10−7 A cm−2, while the corresponding Tafel slope is 0.100 V per decade. The possible reaction mechanism for electroreduction of oxygen on this oxide catalyst has been discussed.

Diffusion–convection/electrochemical model studies on polybenzimidazole (PBI) fuel cell based on AC impedance technique

A steady state, two dimensional mathematical model on PBI fuel cells based on an alternative current(AC) impedance technique is presented. The model mainly accounts for the mass transport and polarization phenomena of the cathode side, and for the first time, the AC impedance technique is combined with the mathematical model to offer a model with two very important electrochemical parameters – internal resistance and apparent area exchange current density. A GUI (graphic user interface) based PBI fuel cell model software is developed to make the model more applicable. O 2 and H 2O distributions are calculated and polarization curves are predicted. The simulated results compare well with experimental data.

Effect of denitrifying bacteria on the electrochemical reaction of activated carbon fiber in electrochemical biofilm system

An electrochemical-activated denitrifying biofilm system consisting of activated carbon fiber electrodes immobilized with denitrifying bacteria film as cathode was studied. A revised model for an electrochemical-activated denitrifying biofilm was developed and validated by electrochemical analysis of cathodal polarization curves and nitrate consumption rate. The cathodal polarization curve and nitrate consumption rate were introduced to verify the rate of electrochemical reaction and the activity of denitrifying bacteria, respectively. It was shown that the denitrification process effectively strengthened the electrochemical reaction while the electron also intensified denitrification activity. Electron was transferred between electrochemical process and biological process not only by hydrogen molecule but also by new produced active hydrogen atom. Additionally, a parameter of apparent exchange current density was deprived from the cathodal polarization curve with high overpotential, and a new bio-effect current density was defined through statistical analysis, which was linearly dependent to the activity of denitrification bacteria. Activated carbon fiber (ACF) electrode was also found to be more suitable to the electrochemical denitrifying system compared with graphite and platinum.

Electrode polarization measurements in the Fe|SrCe 0.95Yb 0.05O 2.975|Au proton conducting solid electrolyte cell

The effect of hydrogen partial pressure (1.3–5.8 kPa) and cell temperature (600–800 °C) on the kinetics and mechanism of the charge transfer electrode reaction taking place at the three phase boundary H 2–Fe–SCY (SCY = SrCe 0.95Yb 0.05O 2.975), was thoroughly examined by performing electrode polarization measurements. A three electrode single chamber proton conducting solid electrolyte cell of the type Fe–SCY–Au, was used in order to conduct the electrode kinetic studies. The steady-state current–overpotential characteristics were analysed with the high field approximations of the Butler–Volmer equation, by taking into account the presence of limiting currents. Both, apparent exchange current density, I o, and anodic/cathodic charge transfer coefficients ( α a / α c), were calculated. Limiting currents, I l, were observed in all reaction conditions. The apparent reaction order, q, was found in most cases, within experimental error, close to 0.5 suggesting a possible reaction model, where a competition exists between charge transfer and mass transport of hydrogen ad-atoms or protons along the electrode/solid electrolyte interface.

Single PEMFC Design and Validation for High-Temperature MEA Testing and Diagnosis up to 300°C

A single proton exchange membrane fuel cell (PEMFC) and associated hardware were designed for high-temperature membrane electrode assembly (MEA) testing and diagnosis in the range of . The fuel cell heating strategy and its design considerations with respect to heat balance are also discussed in this paper. The fuel cell performance of PEMEAS -doped polybenzimidazole (PBI)-based MEAs and the corresponding ac impedance diagnosis demonstrated that this single cell and its associated hardware designs are feasible for high-temperature MEA testing. The apparent exchange current densities for the cathodic oxygen reduction reaction as a function of temperature were obtained through ac impedance measurement. Rapid performance degradation was observed at extreme temperatures, such as , suggesting an accelerated fuel cell testing method. AC impedance results showed that the activity loss of the catalyst/catalyst layer was responsible for this degradation.

Electrocatalytic Studies of Oxygen Reduction by Lanthanum Barium Manganate

The electrocatalysis of the oxygen reduction reaction by Lanthanum Barium Manganate, (La0.5Ba0.5MnO3) (LBM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium. From the ring-disc data and other kinetic parameters it has been assumed that the oxygen reduction occurs by dissociative chemisorption at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2 - in this case) has been observed on this electrocatalyst. The apparent exchange current density value for oxygen reduction on LBM has been found to be 4 x 10-8 Acm-2, while the corresponding Tafel slope is 0.115 V per decade. The possible reaction mechanism for electroreduction of oxygen on this oxide catalyst has been discussed.

PEM fuel cell reaction kinetics in the temperature range of 23–120 °C

The performance of a Nafion 112 based proton exchange membrane (PEM) fuel cell was tested at a temperature range from 23 °C to 120 °C. The fuel cell polarization curves were divided into two different ranges based on current density, namely, <0.4 A/cm 2 and >0.4 A/cm 2, respectively. These two ranges were treated separately with respect to electrode kinetics and mass transfer. In the high current density range, a linear increase in membrane electrode assembly (MEA) power density with increasing temperature was observed, indicating the advantages of high temperature operation. Simulation based on electrode reaction kinetic theory, experimental polarization curves, and measured cathodic apparent exchange current densities all gave temperature dependent apparent exchange current densities. Both the calculated partial pressures of O 2 and H 2 gas in the feed streams and the measured electrochemical Pt surface areas (EPSAs) decrease with increasing temperature. They were also used to obtain the intrinsic exchange current densities. A monotonic increase of the intrinsic exchange current densities with increasing temperature in the range of 23–120 °C was observed, suggesting that increasing the temperature does promote intrinsic kinetics of fuel cell reactions. There are two sets of cathode apparent exchange current densities obtained, one set is for the low current density range, and the other is for the high current density range. The different values of cathode current densities in the two current density ranges can be attributed to the different states of the cathode Pt catalyst surface. In the low current density range, the cathode catalyst surface is a Pt/PtO, and in the high current density range, the catalyst surface becomes pure Pt.

Oxygen reduction at a manganate electrocatalyst in KOH solutions

The electrocatalysis of the oxygen reduction reaction by lanthanum calcium manganate, (La0.5Ca0.5MnO3) (LCM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium. From the ring-disc data and other kinetic parameters it has been assumed that the oxygen reduction occurs by dissociative chemisorption at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2- in this case) has been observed on this electrocatalyst. The apparent exchange current density value for oxygen reduction on LCM has been found to be 5 <FONT FACE=Symbol>´</FONT> 10-8 A cm-2, while the corresponding Tafel slope is 0.115 V per decade. The possible reaction mechanism for electroreduction of oxygen on this oxide catalyst has been discussed.

Effect of palladium oxidation state on the kinetics and mechanism of the charge transfer reaction taking place at the Pd/YSZ interface

Electrode polarization and conductivity measurements were carried out at the Pd/YSZ interface and at conditions close to the Pd–PdO thermodynamic equilibrium. The steady-state current–overpotential characteristics were analyzed with a Butler–Volmer type of equation. Both, apparent exchange current density, I o, and anodic/cathodic charge transfer coefficients ( α a/ α c), were calculated. Based on the experimental results, it was concluded that the charge transfer at the electrode is rate-determining in the case of PdO during anodic operation and Pd during cathodic operation, while in the other case mass transport of adsorbed oxygen species along the electrode/solid electrolyte interface is in competition with the charge transfer process.

Electro-catalytic effect of manganese oxide on oxygen reduction at teflonbonded carbon electrode

Abstract Oxygen reduction(OR)on Teflon-bonded carbon electrodes with manganese oxide as catalyst in 6 mol/L KOH solution was investigated using AC impedance spectroscopy combined with other techniques. For OR at this electrode, the Tafel slope is −0.084 V/dec and the apparent exchange current density is (1.02–3.0) × 10 −7 A/cm 2. In the presence of manganese oxide on carbon electrode, the couple Mn 3+/Mn 4+ reacts with the O 2 adsorbed on carbon sites forming O 2 − radicals and acceletes the dismutation of O 2 −, which contributes to the catalytic effect of manganese oxide for OR reaction.

Effect of sp2-Bonded Nondiamond Carbon Impurity on the Response of Boron-Doped Polycrystalline Diamond Thin-Film Electrodes

The physical and electrochemical properties of boron-doped polycrystalline diamond thin-film electrodes, prepared with varying levels of -bonded nondiamond carbon impurity, were systematically investigated. This impurity was introduced through adjustment of the methane-to-hydrogen (C/H) source gas ratio used for the deposition. Volumetric gas ratios of 0.3, 0.5, 1, 2, 3, and 5% were employed. Proportional increases in the fraction of grain boundary, the extent of secondary nucleation, and the -bonded carbon impurity content resulted in increasing C/H ratio. Variations in the morphology and microstructure were monitored using atomic force microscopy (AFM) and Raman spectroscopy, respectively. The electrode response was assessed using and 4-tert-butylcatechol (4-tBC). All were 1 mM in concentration and dissolved in either 1 M KCl or 0.1 M While increased -bonded carbon content had little effect on the cyclic voltammetric peak separation and peak current for the first two redox systems, the impurity had a significant impact on the latter two, as decreased proportionally with increased -bonded carbon content. The effect of the impurity on the reduction of oxygen in 0.1 M and 0.1 M NaOH was also investigated. A direct correlation was found between the relative amount of the impurity, as estimated from Raman spectroscopy, and the overpotential for oxygen reduction. The greater the nondiamond content, the lower the kinetic overpotential for the reduction reaction. Tafel plots yielded an apparent exchange current density that increased and a transfer coefficient that decreased with the increased nondiamond carbon content. The results demonstrate that the grain boundaries, and the carbon impurity presumably residing there, can have a significant impact on the electrode reaction kinetics for certain redox systems and little influence for others. © 2004 The Electrochemical Society. All rights reserved.