Tae-Hyun Yang

Hydrogen absorption and diffusion into and in palladium: ac-impedance analysis under impermeable boundary conditions

Abstract The hydrogen absorption reaction (har) and diffusion into and in palladium electrode has been investigated in 0.1 M NaOH solution under impermeable boundary conditions by using ac-impedance, open-circuit potential transient and current transient techniques. The ac-impedance measurements were carried out in the overpotential range of − 0.10 to 0.25 Vrhe. Measured impedance spectra were analyzed by using complex non-linear least squares (CNLS) fitting method on the basis of Faradaic admittance equations for hydrogen absorption under impermeable boundary conditions. From the occurrence of plateau region of the open-circuit potential transients and hydrogen content below 0.03 determined from the current transients it is suggested that a thin β-phase palladium hydride layer is formed beneath the electrode surface. The indirect to direct har transition in mode occurs at the overpotential of 0.08 Vrhe below which the direct har is predominant. From the hydrogen diffusivity reduced with decreasing overpotential, it is indicated that the thin β-phase palladium hydride layer acts as a barrier for hydrogen diffusion in the electrode. The formation of the thin β-phase palladium hydride layer between the electrode surface and subsurface accounts for the predominant direct har mode and hydrogen diffusion impeded by the phase boundary between α-and β-phase palladium hydride below 0.08 Vrhe.

Hydrogen transport through Pd electrode: current transient analysis

Abstract The hydrogen transport through a palladium electrode in the coexistence of two palladium hydride phases (α- and β-PdH) has been investigated by analysing current decay transients based on the modified McNabb and Fosters physical model of hydrogen trapping, considering interstitial sites in β-phase as reversible trap sites, supplemented by cyclic polarization curve and open-circuit potential transients. From the appearance of the three-staged current decay transients and hydrogen content in the electrode, it is inferred that the β-phase is formed just beneath the electrode surface during the hydrogen injection into the electrode at overpotentials between 0.08 and −0.02 V(rhe) as the β-PdH patches sporadically embedded in α-PdH matrix and below − 0.02 V(rhe) as the β-phase layer completely embedded in α-PdH matrix. During the hydrogen extraction from the electrode, it follows the hydrogen transport initially proceeds to decomposition of the complete β-phase layer into α-phase by “up-hill diffusion” of hydrogen from inner hydrogen-poor α-phase towards outer hydrogen-rich β-phase, accompanied by interface-controlled phase boundary movement, intermediately it proceeds to complete decomposition of the α-phase into Pd by simple diffusion through α-phase, finally followed by complete decomposition of the remaining sporadic β-PdH patches into α-PdH or Pd. The “up-hill diffusion” is accomplished by transferring interstitial hydrogen in the α-phase to hydrogen trapped in the β-phase. The stress gradient across the α β phase boundary developed during the hydrogen injection helps the hydrogen transport during the hydrogen extraction.

An investigation of the hydrogen absorption reaction into, and the hydrogen evolution reaction from, a Pd foil electrode

The hydrogen absorption reaction (har) into, and the hydrogen evolution reaction (her) from, Pd foil electrode have been investigated in 0.1 M NaOH solution by using electrochemical impedance spectroscopy. The generalised Faradaic admittance for the indirect har into, and her from, metal foil electrodes under the impermeable boundary conditions has been derived on the basis of Lim and Pyuns kinetic approach to har and her. The measured impedance spectra were analysed by using a complex non-linear least squares data-fitting method applied to the derived Faradaic admittance. Also, impedance spectra were theoretically calculated with different kinetic parameters for the har and her having physical significance in order to characterise the influence of each kinetic parameter on the change of the impedance spectra in the Pd foil electrode.

A multi-layer structured cathode for the PEMFC

Multi-layer structured cathodes for PEMFC were prepared by a spray-drying method to provide more efficient oxygen reduction in the cathode. The catalytic layer is, in general, composed of electrolyte for proton conduction and of Pt/C for both the electrochemical reaction and electron conduction. Although the presence of electrolyte is essential for proton conduction, the electrolyte phase retards the electron conduction through the catalytic layer because the electrolyte is electronically insulating. For the Pt/C part, vice versa is valid. In an attempt to develop a cathode possessing superior properties both in the proton and electron conduction, double catalytic electrolyte-rich and -poor layers were coated on the polymer electrolyte membrane (PEM). Performances of the double layered cathodes were evaluated from the current–voltage (I–V) characteristics of single cells. In addition, pressure drops across the fabricated cathodes were determined by using permeability measuring apparatus. From the experimental results, the rate of electrochemical reaction in the cathode was discussed in terms of proton transport and electronic conduction along with oxygen transport.

Theoretical analysis of hydrogen transport through an electrode at the coexistence of two hydrogen-poor and -rich phases based upon the concept of hydrogen trapping

Abstract The potentiostatic current transient equations for hydrogen transport through an electrode at the coexistence of two hydrogen-poor α and -rich β phases under impermeable boundary conditions were derived based on McNabb and Fosters physical model of hydrogen trapping, considering the interstitial sites in the β phase and phase boundaries between the α and β phases as two kinds of reversible and irreversible trap sites, respectively. The decay transients characterised by hydrogen transport through the electrode in the simultaneous presence of the reversible and irreversible traps show a linear relationship between reduced hydrogen flux and reduced time on the logarithmic scale with a slope of − 1/2 and then a steep exponential decay with reduced time, followed by a concave downward curve (three-staged transient). The decay transients for hydrogen transport through the electrode in the presence of both trap sites and the trapped hydrogen concentration profile across the electrode at various times were simulated as functions of such parameters as capture rate, release rate, irreversible trap strength and fraction of trap occupied in the electrode, in order to characterise the influence of each parameter on the decay transients and concentration profile.

A study of the hydrogen absorption reaction into α- and β-LaNi5Hx porous electrodes by using electrochemical impedance spectroscopy

Abstract The hydrogen absorption reaction into α- and β-phase LaNi 5 H x porous electrodes in 6 M KOH solution has been investigated as a function of applied potential by analysing the a.c. impedance spectra complemented with cyclic polarization curves. The onset of the hydrogen evolution reaction (HER) can be accounted for in terms of the transition of the Warburg impedance to a capacitive loop at −0.980 V for the α-phase porous electrode and at −0.920 V for the β-phase porous electrode. The charge-transfer resistance, R ct , values from the β-LaNi 5 H x electrode are much smaller than those from the α-LaNi 5 H x electrode over the whole applied potential range. The α- to β-phase transition at −0.980 V and the β- to α-phase transition at −0.920 V are discussed in terms of the applied potential dependence of R ct for the hydrogen absorption reaction.

Hydrogen diffusion through palladium–gold alloy coatings electrodeposited on palladium substrate under permeable boundary condition

Abstract Hydrogen diffusion through the palladium–gold (Pd–Au) alloy coatings electrodeposited on Pd (Pd–Au/Pd bilayer symmetric electrode) has been investigated using ac-impedance spectroscopy combined with electrochemical hydrogen permeation method. From fitting the impedance spectra experimentally measured to those theoretically calculated on the basis of derived Faradaic admittance equation for the hydrogen absorption into and diffusion through the bilayer electrode under permeable boundary condition, it is concluded that Au atoms act as trap sites for the hydrogen diffusion through the Pd–Au layer on the one hand and increases the hydrogen overpotential on the Pd–Au layer surface on the other hand. From the comparison between the measured impedance spectra and those simulated as functions of the hydrogen solubility ratio at the Pd–Au/Pd interface and the hydrogen diffusivity in the Pd–Au layer it is deduced that Au atoms impede the hydrogen diffusion through the Pd–Au/Pd bilayer.

A study of the mechanism of oxygen reduction on bare palladium in 0.1 m liod solution using pt ring-pd disk electrode

Abstract The mechanism of oxygen reduction on a bare palladium surface was studied in 0.1 M LiOD solution using a rotating Pt ring-Pd disk electrode and programmed cyclic voltammetry. A polarization curve, obtained from linear sweep voltammetry (−0.1 → −0.7 V(SCE)) starting after the Pd disk had been prereduced at −0.5 V(SCE) for 300 s and then held at −0.1 V(SCE) for 120 s, showed a Tafel slope of −52 mV dec −1 . A polarization curve, obtained from the reverse sweep of cyclic voltammetry (−0.5 → −0.1 → −0.7 V(SCE)) started immediately after the electrochemical prereduction treatment, showed characteristics of reversible reaction under diffusional control. These results indicated that the spontaneous adsorption of oxygen species may possibly be avoided by using programmed cyclic voltammetry. From the analysis of Levich plots, an electron transfer number for the oxygen reduction reaction on bare palladium was determined to be about 2.9. The dependences of disk and ring currents on electrode rotation speeds were examined. On the basis of the experimental results, it is suggested that about one-third of oxygen molecules reacting at the bare palladium surface are reduced directly to OD − via the four-electron pathway and the rest of the molecules are reduced to deuterium peroxide via the two-electron pathway.

Performance evaluation of LaNi4.7Al0.3 and LaNi5 electrodes used as anodes in nickel/metal hydride secondary batteries by analysis of current transients

Abstract The performance of LaNi4.7Al0.3 and LaNi5 porous electrodes used as anodes in nickel/metal hydride secondary batteries has been evaluated by analysis of current decay transients. From the measured three-staged current decay transients for the hydrogen transport through both electrodes in the coexistence of two hydride phases, the discharge capacity and β-to α-phase transition time were determined. The optimum charging condition and velocity of α/β phase boundary movement are discussed with respect to the experimentally obtained discharge capacity and transition time. The potentiostatic current decay transient technique is more conveniently employed to establish the optimum charging condition, as compared with the usual galvanostatic charge-discharge technique.

Investigation of the hydrogen evolution reaction at a 10 wt % palladium-dispersed carbon electrode using electrochemical impedance spectroscopy

The hydrogen evolution reaction (h.e.r.) at a 10 wt % palladium-dispersed carbon (Pd/C) electrode in 0.1 m NaOH solution has been investigated with reference to that on carbon (Vulcan XC-72) and palladium foil electrodes by analysing the a.c.-impedance spectra combined with cyclic voltammograms. From the coincidence of the maximum charge transfer resistances and the minimum hydrogen evolution resistances for the h.e.r. at the respective electrode potential for the Pd/C, carbon and Pd foil electrodes, it is suggested that the h.e.r. at the Pd/C electrode takes place along with the absorption and diffusion of hydrogen above −1.10 V vs SCE, whereas the former dominates over the latter below −1.10V vs SCE. In the case of the Pd foil electrode the transition of absorption and diffusion to evolution occurs at −0.96V vs SCE. In contrast to the Pd/C and Pd foil electrodes the h.e.r. occurs strongly at the carbon electrode below −1.20V vs SCE. The hydrogen evolution overpotential on the Pd/C electrode is decreased by 0.10 V in comparison to the carbon electrode due to the larger electrochemical active area of the finely dispersed Pd particles.