Young-Gi Yoon

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.

Hydrogen transport through plasma enhanced chemical vapour-deposited TiO2 film-palladium bilayer by ac-impedance spectroscopy

Abstract Hydrogen adsorption-absorption reaction and transport through TiO 2 film were investigated by using ac -impedance spectroscopy and modulation method as a function of applied cathodic potential and applied anodic potential. The hydrogen diffusivity in the oxide film and kinetic parameters related to hydrogen adsorption and absorption reactions were evaluated by using complex non-linear least squares (CNLS) fitting method. The diffusivity of hydrogen in the oxide film and the concentration of hydrogen in the each layer were determined from the measured phase shift between the input hydrogen concentration and the hydrogen flux at the hydrogen exit side of the membrane specimen and from the measured steady-state hydrogen flux, respectively. The kinetic parameters were discussed in terms of hydrogen adsorption and absorption reaction on/into the oxide surface. Also, variation in hydrogen diffusivity in the oxide film was explained in terms of the hydrogen content in the oxide film.

Hydrogen transport through nickel hydroxide film: current transient analysis

Hydrogen transport through electro-deposited nickel hydroxide films containing various fractions of cobalt hydroxide has been investigated by analysing build-up and decay current transients under the application of various voltage steps from 70 to 400 mV. The build-up and decay transients of hydrogen flux into and from the electrode film with reversible trap under the impermeable boundary condition were numerically simulated on the basis of the McNabb and Fosters physical model of hydrogen trapping. The simulated transients were compared to the measured data. The occurrence of current plateau upon hydrogen extraction from the completely hydrogen-injected films indicated that the hydrogen transport through the film proceeds by the movement of NiOOH/Ni(OH)2 phase boundary. The velocity and mobility of the phase boundary movement upon hydrogen extraction were determined to be orders of 10−6cm s−1 and 10−5cm s−1 V−1 in magnitude, respectively. It is concluded that both the velocity and mobility of the NiOOH/Ni(OH)2 phase boundary movement is raised by the Co(OH)2 incorporation into the Ni(OH)2 film. From the appearance of build-up and decay current transient curves, it is suggested that Co(OH)2 incorporation into the Ni(OH)2 film enhances the release of the trapped hydrogen.

Hydrogen permeation through PECVD (plasma-enhanced chemical vapor deposition) TiO2 film on Pd by the time lag method

Abstract Hydrogen transport through a PECVD (plasma-enhanced chemical vapour deposition) TiO 2 film on a Pd substrate was investigated using the time lag method and a.c. impedance spectroscopy. The permeation experiment was carried out on a Pd/TiO 2 film bilayer specimen by applying a constant cathodic current density of 80 μA cm −2 to the Pd side and anodic potentials from −300 to 300 mV vs. SCE (saturated calomel electrode) to the TiO 2 side. A.c. impedance measurements were made at the TiO 2 side before and after permeation. The diffusivity of hydrogen in the TiO 2 film was determined from the time lag to be (3.9−5.2) × 10 −14 cm 2 s −1 in the applied anodic potential range from −300 to 300 mV (SCE). The relatively low value of hydrogen diffusivity in the oxide film was explained in terms of hydrogen trapping in the film. Hydrogen injection into the TiO 2 film greatly raised the capacitance of the oxide film estimated from the measured impedance in the lower measuring frequency range. The result suggested that trapped hydrogen forms deep donor levels in the band gap of the oxide film.

Hydrogen transport through metals determined by electrochemical methods

Abstract The topics related to electrochemical hydrogen absorption reaction into the metal electrode are reviewed, with particular attention to palladium as a model system. Faradaic admittance expressions involving hydrogen transport through the metal in the low hydrogen concentration range are reviewed under the appropriate boundary conditions experimentally accessible. It was shown that direct hydrogen absorption into the Pd foil without adsorbed intermediate state of hydrogen does occur at certain hydrogen overpotentials. The nature of the subsurface hydride just beneath the metal surface is discussed together with its role in hydrogen transport through Pd metal. Transport of hydrogen through the metal in the presence of two coexisting phases of hydrogen-rich and -deficient phases is discussed in terms of stress generation at the interface of two hydride phases as well as the movement of the boundary between two phases. In addition, the effects of hysteresis and microstructure on the hydrogen absorption are dealt with. Finally, hydrogen transport through the metal covered by the oxide film is reviewed, considering the electric field across the oxide film and the electrochemical equilibrium at the interface between oxide film and underlying metal. Mathematical expressions concerning hydrogen transport through an oxide/metal bilayer are introduced for the steady state and transient states. Interaction between absorbed hydrogen and the anion of the oxide is discussed in detail together with the location of charge transfer in the metal/oxide composite.

Hydrogen permeation through palladium-nickel-hydroxide film bilayer

Abstract The hydrogen permeation through the bilayers of palladium substrate covered with a film of pure nickel hydroxide (Pdue5f8Ni(OH)2, and with a film of mixed hydroxide of nickel and cobalt (Pdue5f8Co(OH)2-incorporated Ni(OH)2) has been investigated by analyzing the measured build-up and decay permeation current transients. Apparent hydrogen diffusivity increased with rising fraction of Co(OH)2 in the Ni(OH)2 film, as determined from the build-up permeation current transients, suggests that, at a given hydrogen content, the amount of the hydrogen injected into the Ni(OH)2 grain interior is reduced by the Co(OH)2 incorporation. Apparent hydrogen diffusivity determined from the decay permeation current transient is approximately 10−11 cm2 s−1 and there is no significant difference in hydrogen diffusivity between pure and doped Ni(OH)2 films, irrespective of oxyhydroxide and hydroxide phases involved. This result suggests that the hydrogen transport through pure and doped Ni(OH)2 films is governed by hydrogen trapping at the trap sites present at the grain boundaries rather than those existing in the grain interior.

Impedance analysis of plasma-enhanced chemical-vapour-deposited TiO2 film in 0.1 M NaOH solution near the flat-band potential

Abstract The complex non-linear least-squares data-fitting method allowed us to quantitatively determine the circuit elements from the proposed equivalent circuit at a plasma-enhanced chemical-vapour-deposited TiO2 film-0.1 M NaOH solution interface. The substantial difference between the circuit elements determined from the classical and proposed equivalent circuits accounted for the occurrence of slow relaxation processes near the flat-band potential. The appearance of the minimum value of adsorption capacitance close to the flat-band potential has been considered with respect to hydrogen adsorption-absorption on the TiO2 film.

Determination of energy distribution of donor levels in anodically passivating TiO2 film

Abstract The present work is concerned with determination of energy distribution of donor levels by analysing a.c. impedance response from the anodically passivating TiO 2 films, based upon a new numerical method. The fresh and halide ion-incorporated anodic TiO 2 films were galvanostatically prepared on titanium substrate at 10 mA cm −2 to a formation potential of 50 V in deaerated 0.5 M H 2 SO 4 solution and deaerated 0.5 M H 2 SO 4 solution containing 0.5 M of Cl − or Br − respectively. Both a.c. impedance and photocurrent spectra were measured from the fresh and halide ion-doped anodic TiO 2 films to quantitatively determine the frequency dependence of donor concentration and energy distribution of deep donors formed by the halide ions. The new numerical method analysing the frequency dependence of donor concentration was proposed to determine the energy distribution of donor levels in the fresh and halide ion-doped anodic TiO 2 films. From the analysis of a.c. impedance response on the basis of the proposed new numerical method, it was suggested that donor levels are distributed continuously in the energy range of 0.55 to 0.67 eV below the conduction band edge. In addition, it was concluded that the donor concentration is reduced by the halide ion incorporation into the fresh anodic TiO 2 films in the measuring frequency range of 10 to 10 3 Hz, suggesting that the doped halide ions occupy oxygen vacancy sites and simultaneously form deep donor levels in the band gap of the anodic TiO 2 films.

Role of hydrogen in the photoresponse of passivating TiO2 film

Abstract Alternating current impedance and photocurrent measurements were made on anodically passivating TiO2 films in 0.1 M NaOH solution. The experimental results suggested that hydrogen injected into the passivating TiO2 films acts as a shallow donor, without modifying the bandgap structure of the films. Also, it is suggested that injected hydrogen increases the electric field strength and enhances the electron-hole recombination at the near-surface region of the films.

Relationship between interfacial reaction and adhesion at PVD TiO2 film-metal (Ti or Al) interfaces

Abstract The present work is concerned with the relationship between the interfacial reaction and adhesion at PVD TiO 2 film-metal (titanium or aluminium) interfaces as a function of substrate temperature using Auger electron spectroscopy, scanning electron microscopy and the scratch adhesion test. TiO 2 films were prepared by reactive r.f. magnetron sputtering on titanium and aluminium at substrate temperatures ranging from 313 to 523 K. Scanning electron microscopy suggested that hardly any compound formation occurred but an interdiffusion layer was formed at the TiO 2 -Ti interface. In contrast, Auger electron spectroscopy depth profiling and scanning electron microscopy revealed the formation of aluminium oxide at the TiO 2 -Al interface. The fracture mode of TiO 2 films on titanium changed predominantly from chipping below 388 K to cohesive type above 438 K. In contrast, the TiO 2 films on aluminium fractured in a fully adhesive manner only at substrate temperatures higher than 438 K. Variations in adhesion of the TiO 2 films on titanium and aluminium could be explained by the effect of substrate temperature on the interfacial reaction.