Electrode Kinetic Parameters Research Articles

The phase-shift method and correlation constants for determining adsorption isotherms of hydrogen at a palladium electrode interface

The Frumkin and Temkin adsorption isotherms ( θ vs. E) of over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER) and related electrode kinetic and thermodynamic parameters at a Pd/0.5 M H 2 SO 4 aqueous solution interface are determined using the phase-shift method and correlation constants. The Frumkin and Temkin adsorption isotherms ( θ vs. E), equilibrium constants ( K = 3.3 × 10 - 5 exp ( - 1.4 θ ) mol - 1 for the Frumkin and K = 3.3 × 10 - 4 exp ( - 6 θ ) mol - 1 for the Temkin adsorption isotherm), interaction parameters ( g = 1.4 for the Frumkin and g = 6 for the Temkin adsorption isotherm), rates of change of the standard free energy of OPD H with θ ( r = 3.5 kJ mol - 1 for g = 1.4 and r = 14.9 kJ mol - 1 for g = 6 ), and standard free energies ( 25.6 ⩽ Δ G θ 0 ⩽ 29.0 kJ mol - 1 for the Frumkin and 22.8 < Δ G θ 0 < 31.8 kJ mol - 1 for the Temkin adsorption isotherm) of OPD H are determined using the phase-shift method and correlation constants. At the intermediate values of θ , i.e., 0.2 < θ < 0.8 , the Temkin adsorption isotherm ( θ vs. E) correlating with the Frumkin or the Langmuir adsorption isotherm ( θ vs. E), and vice versa, is readily determined using the correlation constants. The phase-shift method and correlation constants are unique, useful, and effective ways to determine the Langmuir, Frumkin, and Temkin adsorption isotherms ( θ vs. E) of intermediates for sequential reactions and related electrode kinetic and thermodynamic parameters ( K, g, r, Δ G θ 0 ) at interfaces.

The electrode reactions of several Pb(II)-macrocyclic complexes and their adsorption on mercury electrodes

Voltocoulometric, chronocoulometric and cyclic voltammetric studies of the electrode reactions of the Pb(2,2,2) 2+, Pb(2,2,1) 2+ and Pb(2,2) 2+ complexes were described ((2,2)-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane, (2,2,1)-4,7,13,16,21-pentaoxa-1,10-12 diazabicyclo[5.8.8]tricosane, (2,2,2)-4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane). It was found that such Pb(L) 2+ complexes are adsorbed on the Hg electrode. The analysis of adsorption data for Pb(2,2,2) 2+ in the presence of excess of ligand (2,2,2) shows that the complex is adsorbed according to the competitive adsorption mechanism. Electroreduction processes of the studied complexes proceed via two parallel pathways: from the solution and from the adsorbed state. The electrode kinetic parameters of lead(II) complexes free of adsorption influence were found by analysis of the normal pulse voltocoulometric curves recorded with a 1 ms delay after application of the short potential pulse. From the analysis of kinetic parameters it follows that the electroreduction of Pb(L) 2+ is a two-step CE process with the rate determining step preceded by complex dissociation. The rate constants of the studied Pb(L) 2+/Pb(Hg) systems are considerably lower than the rate constant of noncomplexed Pb 2+ ion in aqueous solution. We have found that the reaction rate constants of the systems Pb(L) 2+/Pb(Hg) decrease linearly with increasing stability of the complexes. This dependence is also obeyed by cadmium and thallium complexes with the same ligands.

The phase-shift method for determining adsorption isotherms of hydrogen in electrochemical systems

A linear relationship between the behavior ( - ϕ vs. E) of the phase shift ( 0 ∘ ⩽ - ϕ ⩽ 90 ∘ ) for the optimum intermediate frequency and that ( θ vs. E) of the fractional surface coverage ( 1 ⩾ θ ⩾ 0 ) of H for the cathodic H 2 evolution reaction (HER), i.e., the phase-shift method, in electrochemical systems has been suggested and verified using cyclic voltametric, linear sweep voltametric or differential pulse voltametric, and ac impedance techniques. The Langmuir and Frumkin adsorption isotherms of under-potentially deposited hydrogen (UPD H) or over-potentially deposited hydrogen (OPD H) for the cathodic HER at noble and transition-metal (Pt, Ir, Rh, Re, Pd, Au, Pt–Rh alloy, Ni)/aqueous electrolyte interfaces have been studied using the phase-shift method. At the interfaces, the fractional surface coverage ( θ ), equilibrium constant ( K), and standard free energy ( Δ G ads 0 , Δ G θ 0 ) of H (UPD H, OPD H) for the cathodic HER are determined using the phase-shift method. The applicability of the Temkin adsorption isotherm ( θ vs. E) of OPD H for the cathodic HER at noble and transition-metal (Ir, Re, Ni)/aqueous electrolyte interfaces also has been studied using the phase-shift method. The phase-shift method is a simple and efficient tool for determining the adsorption, electrode kinetic, and thermodynamic parameters ( θ , K, Δ G ads 0 , Δ G θ 0 ) of H (UPD H, OPD H) for the cathodic HER in electrochemical systems. The phase-shift method can be effectively used as a new electrochemical method to determine adsorption isotherms (Langmuir, Frumkin, Temkin) of intermediates for sequential reactions in electrochemical systems.

Carbon nanotubes as a secondary support of a catalyst layer in a gas diffusion electrode for metal air batteries

In this paper, we report the use of binary carbon supports (carbon nanotubes (CNTs) and active carbon) as a catalyst layer for fabricating gas diffusion electrodes. The electrocatalytic properties for the oxygen reduction reaction (ORR) were evaluated by polarization curves and electrochemical impedance spectroscopy (EIS) in an alkaline electrolyte. The binary-support electrode exhibits better performance than the single-support electrode, and the best performance is obtained when the mass ratio of carbon nanotubes and active carbon is 50:50. The results from the electrode kinetic parameters indicate that the introduction of carbon nanotubes as a secondary support provides high accessible surface area, good electronic conductivity, and fast ORR kinetics. Furthermore, the effect of CNT support on the electrocatalytic properties of Pt nanoparticles for binary-support electrodes was also investigated by different loading-reduction methods. The electrocatalytic activity of the binary-support electrodes is improved dramatically by Pt loading on CNT carbon support, even at very low Pt loading. Additionally, the EIS analysis results indicate that the process of ORR may be controlled by diffusion of oxygen in the electrode thin film for binary-support electrodes with or without Pt catalyst.

Electrochemical study of silver thiosulphate reduction in the absence and presence of ultrasound

The electrochemical reduction of silver thiosulphate was studied potentiostatically on platinum electrodes in the absence and presence of ultrasound (20 kHz). This system is irreversible and the reaction is both diffusion and kinetically controlled. The slowest step is the kinetic reaction especially the chemisorption of ions at the electrode surface. Ultrasound greatly improves the mass transport, which can be explained by changing from diffusion to mainly convection. This paper reports the effect of ultrasound upon electrode kinetic and mass-transport parameters at various RDE rotation speeds and ultrasonic intensities. It was found that the heterogeneous rate constant ( k f) is improved in the presence of ultrasound due to the increase in the formal or standard heterogeneous rate constant ( k 0) (approximately by 10-fold under sonication).

Electrochemistry and electrocatalytic activities of superoxide dismutases at gold electrodes modified with a self-assembled monolayer.

In this article, the electrochemical properties and electrocatalytic activity of three kinds of superoxide dismutases (SODs), that is, bovine erythrocyte copper-zinc superoxide dismutase (Cu/Zn-SOD), iron superoxide dismutase from Escherichia coli (Fe-SOD), and manganese superoxide dismutase from E. coli (Mn-SOD), in the SOD family were studied. It was revealed that the direct electron transfer of the three kinds of SODs could be efficiently promoted by a self-assembled monolayer (SAM) of 3-mercaptopropionic acid (MPA) confined on a gold electrode. The electrochemical properties of the SODs at the MPA-SAM electrode vary with the sort of SOD with respect to the formal potential, reversibility of electrode reactions, kinetic parameters, and pH dependence, suggesting different mechanisms for the electrode reactions of the individual SODs. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SODs via a redox cycle of their active metals substantially offered a flexible electrochemical route to determination of O(2)(*)(-) where O(2)(*)(-) can be sensed with the SOD-based biosensors in both anodic and cathodic polarizations. Such an intrinsic feature of the SOD-based biosensors successfully enabled a sensitive determination scheme for O(2)(*)(-) free from the interference from some coexisting electroactive species, such as ascorbic acid (AA) and uric acid (UA). Further potential applications for in vivo determination of O(2)(*)(-) is also suggested.

Determination of the Langmuir adsorption isotherms of under- and over-potentially deposited hydrogen for the cathodic H 2 evolution reaction at poly-Ir/aqueous electrolyte interfaces using the phase-shift method

The Langmuir adsorption isotherms of under-potentially deposited hydrogen (UPD H) and over-potentially deposited hydrogen (OPD H) for the cathodic H 2 evolution reaction (HER) at poly-Ir/0.5 M H 2SO 4 and 0.05 M KOH aqueous electrolyte interfaces have been studied using cyclic voltammetric, differential pulse voltammetric, and ac impedance techniques. The behavior of the lagged phase shift (0°⩽− φ⩽90°) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1⩾ θ⩾0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. A linear relationship between the phase-shift profile (− φ vs. E) for the optimum intermediate frequency and the Langmuir adsorption isotherm ( θ vs. E) of H (UPD H, OPD H), i.e., the phase-shift method, can be used as a new electrochemical method to determine or estimate the fractional surface coverage ( θ), equilibrium constant ( K), and standard free energy (Δ G ads 0) of H (UPD H, OPD H) for the cathodic HER at the interfaces. At the poly-Ir/0.5 M H 2SO 4 aqueous electrolyte interface, K and Δ G ads 0 of the OPD H for the cathodic HER are 9.9×10 −5 and 22.8 kJ/mol , respectively. At the poly-Ir/0.05 M KOH aqueous electrolyte interface, K and Δ G ads 0 of the UPD H for the cathodic HER are 3.9 and −3.4 kJ/mol , respectively. At the poly-Ir/0.05 M KOH aqueous electrolyte interface, K and Δ G ads 0 of the OPD H for the cathodic HER are 5.8×10 −4 and 18.5 kJ/mol , respectively. The two different Langmuir adsorption isotherms for H (UPD H, OPD H) correspond to two different adsorption sites of H (UPD H, OPD H) on the poly-Ir electrode surface. The two different adsorption sites of UPD H and OPD H act as two distinguishable electroadsorbed H species. The phase-shift method is a simple and efficient tool for determining the adsorption, electrode kinetic, and thermodynamic parameters ( θ, K, Δ G ads 0) of H (UPD H, OPD H) for the cathodic HER at the interfaces.

An experimental and modeling study of isothermal charge/discharge behavior of commercial Ni–MH cells

In this study, a previously developed nickel–metal hydride (Ni–MH) battery model is applied in conjunction with experimental characterization. Important geometric parameters, including the active surface area and micro-diffusion length for both electrodes, are measured and incorporated in the model. The kinetic parameters of the oxygen evolution reaction are also characterized using constant potential experiments. Two separate equilibrium equations for the Ni electrode, one for charge and the other for discharge, are determined to provide a better description of the electrode hysteresis effect, and their use results in better agreement of simulation results with experimental data on both charge and discharge. The Ni electrode kinetic parameters are re-calibrated for the battery studied. The Ni–MH cell model coupled with the updated electrochemical properties is then used to simulate a wide range of experimental discharge and charge curves with satisfactory agreement. The experimentally validated model is used to predict and compare various charge algorithms so as to provide guidelines for application-specific optimization.

Qualitative Analysis of the Frumkin Adsorption Isotherm of the Over-Potentially Deposited Hydrogen at the Poly-Ni/KOH Aqueous Electrolyte Interface Using the Phase-Shift Method

The phase-shift method of the Frumkin adsorption isotherm of the over-potentially deposited hydrogen (OPD H) for the cathodic evolution reaction (HER) at the poly-Ni/0.05 M KOH aqueous electrolyte interface has been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift for the optimum intermediate frequency corresponds well to that of the fractional surface coverage at the interface. The phase-shift method, i.e., the phase-shift profile vs. for the optimum intermediate frequency, can be used as a new method to estimate and analyze the Frumkin adsorption isotherm (θ vs. of the OPD H for the cathodic HER at the interface. At the poly-Ni/0.05 M KOH aqueous electrolyte interface, the rate of change of the standard free energy of the OPD H with θ, the interaction parameter for the Frumkin adsorption isotherm, the equilibrium constant for the OPD H with θ, and the standard free energy of the OPD H with θ are 24.8 kJ/mol, 10, and respectively. The electrode kinetic and thermodynamic parameters depend strongly on θ © 2002 The Electrochemical Society. All rights reserved.

Rare earth cuprates as electrocatalysts for methanol oxidation

A series of rare earth cuprates with overall composition Ln 2− x M x Cu 1− y M y ′O 4− δ (where Ln=La and Nd; M=Sr, Ca and Ba; M′=Ru and Sb: 0.0≤ x≤0.4 and y=0.1) have been tested as anode electrocatalysts for methanol oxidation. The evaluation of electrode kinetic parameters was made galvanostatically. The catalyst characterization was carried out by specific conductivity measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Iodometry. These materials exhibit significant activity for methanol oxidation at higher potentials. The linear correlation between Cu(3+) content and methanol oxidation activity suggests that the active sites for adsorption of methanol is Cu(3+). The methanol oxidation onset potential depends on the ease of Cu(2+)→Cu(3+) oxidation reaction. These materials show better tolerance towards the poisoning by the intermediates of methanol oxidation compared to that of conventional noble metal electrocatalysts (supported and bulk). The lattice oxygen in these oxides could be considered as active oxygen to remove CO intermediates of methanol oxidation reaction.

Direct Electron Transfer for Heme Proteins Assembled on Nanocrystalline TiO2 Film

Nanosized TiO2 is now an attractive biocompatiable material widely used in toothpaste and cosmetics. Due to its unique physiochemical properties and its inclination to selectively combine with some groups of biomolecules, nanosized TiO2 has been proposed as a promising interface for the immobilization of biomolecules. In this article, heme proteins involving cytochrome c, myoglobin and hemoglobin were attempted to be assembled onto a nanocrystalline TiO2 film, their electrochemical behaviors on such interface were characterized with cyclic voltammetry. It was proved that TiO2 film could not only offer a friendly platform to assemble protein molecules, but also enhance the electron transfer process between protein molecules and the electrode. The electrode kinetic parameters of each hemoprotein on such electrode as well as the effect of scan rate on the electrochemical behavior of each protein are discussed in details.

Analysis on the Langmuir Adsorption Isotherm of the Over-Potentially Deposited Hydrogen (OPD H) at the Polycrystalline Au|Acidic Aqueous Electrolyte Interface Using the Phase-Shift Method

The Frumkin adsorption isotherm of the over-potentially deposited hydrogen (OPD H) for the cathodic evolution reaction (HER) at the poly-Ni|0.05M KOH aqueous electrolyte interface has been studied using the phase-shift method. The behavior of the phase shift for the optimum intermediate frequency corresponds well to that of the fractional surface coverage at the interface. The phase-shift method, i.e., the Phase-shift profile for the optimum intermediate frequency, can be used as a new method to estimate the Frumkin adsorption isotherm of the OPD H for the cathodic HER at the interface. At the poly-Ni|0.05M KOH aqueous electrolyte interface, the rate (r) of change of the standard free energy of the OPD H with , the interaction parameter (g) for the Frumkin adsorption isotherm, the equilibrium constant (K) for the OPD H with , and the standard free energy of the OPD H with are . The electrode kinetic parameters depend strongly on

Kinetics of sulfide ion adsorption at copper amalgam electrode

Convolutive voltammetry was employed in studies of the kinetics of sulfide ion adsorption at copper amalgam and mercury electrodes. Measurements were carried out for low electrode coverages to minimize the effect of the lateral interaction energy, and the results were extrapolated to zero coverage. In the case of a Cu amalgam electrode (7.4×10 −4 mol dm −3 of Cu) kinetic and thermodynamic parameters of the adsorption/desorption process were determined in solutions of different pH (3.6 to 12.8). For pH<7, the standard rate constant of the process was independent of the pH and equal to 92±7 s −1 , but in basic solutions it decreased with an increase in the pH down to 11±2 for pH 10 and to 3.1±0.5 s −1 for pH 12.8. The electrosorption valency and the activation barrier symmetry-factor were practically independent of the pH and equal to −1.32±0.03 and 0.65±0.02, respectively. Study of the effect of Cu concentration revealed the formation of 1:1 (CuS) compound at the interface. It was also found that both H + and Cu participate in the activated complex. Adsorption of S 2− on pure mercury is much faster than on the Cu(Hg) electrode, however the determination of kinetic parameters in this case was obscured by a kinetically controlled transition between the two dimensional nuclei of the HgS condensed phase and its dispersed forms.

The Phase‐Shift Method for the Frumkin Adsorption Isotherms at the Pd / H 2 SO 4 and KOH Solution Interfaces

The phase‐shift method for the Frumkin adsorption isotherms at the Pd/acidic and alkaline (KOH) solution interfaces has been studied using an ac impedance method. The phase shift (φ) depends on the cathode potential (E < 0) and is inversely proportional to the fractional surface coverage (θ) of the adsorbed hydrogen atom . The phase‐shift profile (φ vs. E) at the intermediate frequencies (ca. 130 Hz for the 0.1 M solution and ca. 63 Hz for the 0.1 M KOH solution) can be used as an experimental method to estimate or plot the Frumkin adsorption isotherm (θ vs. E). The rates (r) of change of the free energy of adsorption with θ at the Pd/0.1 M and 0.1 M KOH solution interfaces are 7.4 and 49.6 kJ/mol, respectively. The equilibrium constant (K) and the standard free energy (ΔGθ) of at the Pd/0.1 M solution interface are and 35.9 > ΔGθ > 28.5 kJ/mol, respectively. The equilibrium constant (K) and the standard free energy (ΔGθ) of at the Pd/0.1 M KOH solution interface are and 83.8 > ΔGθ > 34.2 kJ/mol, respectively. The electrode kinetic parameters (r, K, ΔGθ) depend on θ (0 < θ < 1).

Linear and non-linear analysis using the Oldham–Zoski steady-state equation for determining heterogeneous electrode kinetics at microdisk electrodes and digital simulation of the microdisk geometry with the fast quasi-explicit finite difference method

Abstract A procedure which utilizes a linearized version of the Oldham–Zoski current-potential equation has been developed for the evaluation of electrode kinetic parameters from near-steady state voltammograms obtained at microdisk electrodes. The inherent advantage of the method is its mathematical simplicity relative to the other procedures already available in the literature. The procedure is applied to synthetic data obtained under near-steady state conditions from the fast quasi-explicit finite difference simulation of quasi-reversible electron transfer at a microdisk electrode. A procedure using non-linear optimization based on the Marquardt algorithm is also applied to the simulated data. Near-steady state voltammograms showing the effect of a potential dependent charge transfer coefficient are presented and analyzed. The robustness of the analysis is tested with voltammograms containing a random noise component and the near-steady state terms inherently present in experimental data. Application of the Oldham–Zoski equation to the simulated voltammograms enables the fidelity of both methods to be established over a wide range of heterogeneous kinetic conditions.

Modeling of Proton Exchange Membrane Fuel Cell Performance with an Empirical Equation

An empirical equation was shown to fit the experimental cell potential vs. current density data for proton exchange membrane fuel cells (PEMFCs), at several temperatures, pressures, and oxygen compositions in the cathode gas mixture. The exponential term compensates for the mass‐transport regions of the vs. plot; i.e., the increase in slope of the pseudolinear region and the subsequent rapid fall‐off of the cell potential with increasing current density. As has been previously shown, the terms and yield the electrode kinetic parameters for oxygen reduction in the PEMFC and represents the resistance, predominantly ohmic and, to a small extent, the charge‐transfer resistance of the electro‐oxidation of hydrogen. The exponential term characterizes the mass‐transport region of the vs. plot. The parameter has more pronounced effects than the parameter in this region. A physicochemical interpretation of these parameters is needed.

Electrochemical and Spectroelectrochemical Studies on Metallophthalocyanine‐Oxygen Interactions in Nonaqueous Solutions

Redox chemistries of cobalt(II) and iron(II) phthalocyanines, as well as interactions of the reduced phthalocyanines with oxygen, have been studied using spectroelectrochemical and other transient electrochemical techniques in nonaqueous solutions including pyridine, dimethylsulfoxide, and dichloromethane. Results indicate that metal‐ligand charge transfer (MLCT) bands are affected significantly by solvent molecules. The broad MLCT band observed from reduced cobalt phthalocyanine in the spectral region between the Soret and Q bands is shown to consist of two different transitions, generated at the first and second reduction potentials, respectively. The phthalocyanines undergo quasi‐reversible electron transfer reactions as characterized by electrode kinetic parameters. The results of studies on interactions of reduced cobalt phthalocyanine with oxygen indicate that the catalytic activities of phthalocyanines are achieved through a conventional regenerative catalytic mechanism (EC′).

Physicochemical and electrochemical characterization of active films of LaNiO 3 for use as anode in alkaline water electrolysis

Thin films of LaNiO 3 were prepared on Ni by the sequential coating method, and the effects of film preparation variables, such as concentration of the mixed metal nitrates solution, temperature and oxide loading, were examined for the electrocatalytic activity of the film regarding oxygen evolution in concentrated KOH solution. The oxide film with a loading of ~6 mg cm −2 was found to be the best electocatalyst among the electrodes prepared in situ. The activity of this film electrode was better than those prepared by othermethods, following sequential coatings of solution or mixed oxide coatings. Electrode kinetic parameters, such as Tafel slope, reaction order and activation energy, were found to be ~65 mV decade −1, ~1.2 and 18.4 kcal, respectively. Cyclic voltammetric studies, prior to the onset of oxygen evolution, exhibited two characteristic peaks corresponding to a diffusion-controlled surface redox process: Ni 2+ → Ni 3+. Based on these results, a probable mechanism for oxygen evolution on the oxide film electrode is suggested.

Analysis of proton exchange membrane fuel cell performance with alternate membranes

Renewed interest in proton exchange membrane fuel cell technology for space and terrestrial (particularly electric vehicles) was stimulated by the demonstration, in the mid 1980s, of high energy efficiencies and high power densities. One of the most vital components of the PEMFC is the proton conducting membrane. In this paper, an analysis is made of the performances of PEMFCs with Dupont's Nafion ®, Dow's experimental, and Asahi Chemical's Aciplex-S ® membranes. Attempts were also made to draw correlations between the PEMFC performances with the three types of membranes and their physico-chemical characteristics. Practically identical levels of performances (energy efficiency, power density, and lifetime) were achieved in PEMFCs with the Dow and the Aciplex-S ® membranes and these performances were better than in the PEMFCs with the Nafion ®-115 membrane. The electrode kinetic parameters for oxygen reduction are better for the PEMFCs with the Aciplex-S ® and Nafion ® membranes than with the Dow membranes. The PEMFCs with the Aciplex-S ® and Dow membranes have nearly the same internal resistances which are considerably lower than for the PEMFC with the Nafion ® membrane. The desired membrane characteristics to obtain high levels of performance are low equivalent weight and high water content.

High performance electrodes with very low platinum loading for polymer electrolyte fuel cells

Research and development on low platinum loading electrode is very important in reducing the capital cost of fuel stack and conserving limited Pt resources. This paper presents the advances made at the Centre for Electrochemical and Energy Research (CEER), SPIC Science Foundation, India on low platinum loading (0.1 mg Pt cm −2) electrodes. The performance of the electrode fabricated at the centre with 0.1 mg Pt cm −2 was found to be comparable with the state-of-the-art electrode with 0.4 mg Pt cm −2. The performance of the electrode was evaluated from the measurement of cell potential-current density plots. The electrode-kinetic parameters for oxygen reduction was evaluated from the results. The electrochemically active Pt surface area was measured by cyclic voltammetry.