Steady-state Electrochemical Methods Research Articles

Physicochemical Analysis of Chloride Diffusion and Adsorption in Water-saturated Concrete: Theory and Measurement

The theory of ionic diffusion in water-saturated concrete accompanying ion exchange with mobile or immobile adsorbed ions was constructed by using the general theory of diffusion and the condition of electrical neutrality. Analysis of the diffusion profile of chloride (Cl−) ions in concrete with the theory revealed that adsorbed Cl− ions in AFm and C-S-H are as mobile as free Cl− ions in the pore solution, so that the adsorption does not retard the ingress of Cl− ions. The existing test methods for determining the effective or apparent diffusion coefficient of Cl− ions were evaluated on the bases of the present theory and new experimental findings. It was revealed that steady-state electrochemical methods such as NTBUILD-355 and ASTM C1202 are not suitable for determining the diffusion coefficient, because the steady state methods expel preexisting diffusible ions which strongly affect the diffusion of Cl− ions. Moreover, the steady state methods overestimate the diffusion rate because the methods assume that adsorbed Cl− ions are immobile. The electrochemically-accelerated method NT BUILD 443 is also unsuitable for the diffusion test, because the acceleration is induced by the electro-osmotic flow of the external solution into concrete. The present diffusion theory necessitates the effective self-diffusion coefficient of not only Cl− ions but also all the other diffusible ions in concrete. A simple method of determining the effective self-diffusion coefficients of arbitrary ions in concrete from the diffusion profile of Cl− ions was presented.

Electrochemical co-reduction of Y(III) and Al(III) in a fluoride molten salt system and electrolytic preparation of Y–Al intermediate alloys

In this study, a molten salt co-reduction method was proposed for preparing Y–Al intermediate alloys and the electrochemical co-reduction behaviors of Y(III) and Al(III) and the reaction mechanism of intermetallic compound formation were investigated by transient electrochemical techniques. The results show that the reduction of Y(III) at the Mo electrode is a reversible electrochemical process with a single-step transfer of three electrons, which is controlled by the mass transfer rate. The diffusion coefficient of Y(III) in the fluoride salt at a temperature of 1323 K is 5.0238 × 10−3 cm2/s. Moreover, the thermodynamic properties associated with the formation of Y–Al intermetallic compounds were estimated using a steady-state electrochemical method. Y–Al intermediate alloy containing 92 wt% yttrium was prepared by constant current electrolysis at 1323 K in the LiF–YF3–AlF3–Y2O3 (6 wt%)–Al2O3 (1 wt%) system at a cathodic current density of 8 A/cm2 for 2 h. The Y–Al intermediate alloy is mainly composed of α-Y2Al and Y phases. The development and application of this innovative technology have solved major technical problems, such as a long production process, high energy consumption, and serious segregation of alloy elements at this stage.

Electrochemical behavior of Dy(III) on bismuth film electrode in eutectic LiCl-KCl melts

An electrochemical study of dysprosium (III) on Bi film electrode was carried out in eutectic LiCl-KCl melts by transient and steady state electrochemical techniques. The results of transient electrochemical techniques showed that the reduction of Dy (III) appears at a more positive value than the one detected on W electrode owing to the formation of Bi-Dy intermetallic compounds through electrochemical deposition of Dy on bismuth film electrode. The thermodynamic properties of the formation for Dy-Bi intermetallics were estimated by a steady state electrochemical method in a temperature range of 713–803 K. Furthermore, the electrochemical preparation of Bi-Dy alloys was conducted by galvanostatic electrolysis at different current intensities. The Bi-Dy alloys, characterized by scanning electron microscopy equipped with energy dispersive spectrometry and X-ray diffraction, are comprised of DyBi3/5 and DyBi phases.

Ultrafast Intramolecular Exciton Splitting Dynamics in Isolated Low-Band-Gap Polymers and Their Implications in Photovoltaic Materials Design

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.

Hydrogen crossover in high-temperature PEM fuel cells

In this paper, hydrogen crossover was measured in an environment of high-temperature proton exchange membrane (PEM) fuel cells using a steady-state electrochemical method at various temperatures ( T) (80–120 °C), backpressures ( P) (1.0–3.0 atm), and relative humidities (RH) (25–100%). An H 2 crossover model based on an MEA consisting of five layers – anode gas diffusion layer, anode catalyst layer, proton exchange membrane (Nafion 112 or Nafion 117), cathode catalyst layer, and cathode gas diffusion layer – was constructed to obtain an expression for H 2 permeability coefficients as a function of measured H 2 crossover rates and controlled H 2 partial pressures. The model analysis suggests that the dominant factor in the overall H 2 crossover is the step of H 2 diffusing through the PEM. The H 2 permeability coefficients as a function of T, P, and RH obtained in this study show that the increases in both T and P could increase the H 2 permeability coefficient at any given RH. However, the effect of RH on the permeability coefficient seems to be more complicated. The T effect is much larger than that of P and RH. Through experimental data simulation an equation was obtained to describe the T dependencies of the H 2 permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H 2 crossover through both Nafion 112 and 117 membranes were also evaluated. Both Nafion 112 and Nafion 117 showed similar values of such parameters, suggesting that membrane thickness does not play a significant role in the H 2 crossover mechanism.

Electron hopping dynamics in monolayer-protected au cluster network polymer films by rotated disk electrode voltammetry.

Electrons are transported within polymeric films of alkanethiolate monolayer-protected Au clusters (MPCs) by electron hopping (self-exchange) between the metal cores. The surrounding monolayers, the molecular linkers that generate the network polymer film, or both, presumably serve as tunneling bridges in the electron transfers. This paper introduces a steady-state electrochemical method for measuring electron hopping rates in solvent-wetted and swollen, ionically conductive MPC films. The films are network polymer films of nanoparticles, coated on a rotated disk electrode that is contacted by a solution of a redox species (decamethylferrocene, CpFe). Controlling the electrode potential such that the film mediates oxidation of the redox probe can force control of the overall current onto the rate of electron hopping within the film, which is characterized as the apparent electron diffusion coefficient D(E). D(E) is translated into an apparent electron hopping rate k(ET) by a cubic lattice model. The experiment is applied to MPC network polymer films linked by alpha,omega-alkanedithiolates and by metal ion-carboxylate connections. We evaluate the dependencies of apparent hopping rate on CpFe concentration, film thickness, electrode potential relative to the CpFe formal potential, film-swelling solvent, and temperature. The apparent hopping rates are in the 10(4)-10(5) s(-)(1) range, which is slower than those for the same kind of MPC films, but in a dry (nonswollen) state measured by electronic conductivities.

Electrochemical excitation method in kinetic study of Ag 2S

A steady-state electrochemical method of sinusoidal excitation has been developed in which the chemical diffusion coefficient of an atomic species in a solid is obtained via the determination of the spatial distribution of the phase and amplitude of its chemical potential oscillation. It has been applied to Ag 2+δS to measure the chemical diffusion coefficient D̃ of Ag in α-Ag 2+δS at 168°C and in β-Ag 2+δS at 220°C, with a typical precision of 0.2%. The measurements were performed at frequencies from 0.02 to 0.16 Hz, with δ covering the whole nonstoichiometry range. The values of D̃ measured fell between 0.01 and 0.15 cm 2/s at 168°C and between 0.08 and 0.32 cm 2/s at 220°C, changing significantly with f but slightly with f. The component diffusion coefficient of Ag + was determined for β-Ag 2+δS to be in the range of 3.0 to 3.6×10 −5cm 2/s, decreasing with a frequency dependence with increasing δ.

Slow charge transfer associated with a fast equilibrated follow-up dimerization reaction

The influence of a fast reversible follow-up reaction on the global kinetic behaviour of an EC scheme is investigated when the C reaction is a dimerization involving the parent compound and the product of the first electron transfer. It is shown that even when the rate-determining step is the electron transfer, the C reaction exerts a strong influence on the apparent kinetics of the electron transfer, resulting in the apparent rate constant and transfer coefficient depending on the concentration. For large values of the equilibrium constant of the follow-up reaction, the initial electron wave splits into two half-electron waves, the locations of which depend on both the electron transfer rate constant and the equilibrium constant. The whole analysis is performed in the context of steady-state electrochemical methods, and the transposition of the results to transient methods is discussed. The analysis presented is exemplified by the case of Sn(IV) reduction in concentrated hydrochloric acid solutions.

Kinetic studies of mixed conducting solids

A steady-state electrochemical method is presented for measurement of the chemical diffusion coefficient, ionic, mobility, partial ionic conductivity, enhancement factor and related kinetic quantities of mixed elecronic and ionic kinetic parameters of the fast mixed conductor ßAg 2S. A general derivation and discussion is presented of the conditions for enhanced ionic diffusion in solids which may be as high as in liquids or gases.