Jeong-Nam Han

Analysis of anodic current transient and beam deflection transient simultaneously measured from Pd foil electrode pre-charged with hydrogen

Abstract Anodic current and beam deflection transients were measured simultaneously on a Pd foil electrode pre-charged with hydrogen at −0.02 and 0.09 V(RHE) in 0.1 M NaOH solution as a function of the hydrogen discharging potential. From the analysis of the anodic current transient measured, it is recognized that when the preceding potential jump is small enough to be below the transition discharging potential and this is followed by subjecting the electrode surface to a constant discharging potential, the hydrogen concentration corresponding to the discharging potential is not fixed at the electrode surface, but the change in surface concentration with time is then determined by the Butler–Volmer equation. From the hydrogen concentration profile transient simulated under the two constraints at the electrode surface depending on the discharging potential, we calculated numerically the transient of the deflection in the tensile direction caused by a smaller molar volume of α-PdH δ near the surface region than that in the inner region of the electrode during the hydrogen extraction. By comparison of the measured transient with that calculated, the movement of the maximum deflection in tensile and compressive directions was discussed in terms of the positive gradient of the molar volume towards the inner direction and the negative gradient due to the formation of a PdOH phase on the electrode surface, respectively.

Analysis of stresses generated during hydrogen transport through a Pd foil electrode under potential sweep conditions

In the present work, the stresses generated during cyclic voltammetric measurements on a Pd foil electrode in 0.1 M NaOH solution have been analysed by using a laser beam deflection technique combined with cyclic voltammetry. From the linear relationship between the anodic peak current density and the scan rate on a logarithmic scale, it is recognised that the hydrogen concentration at the electrode surface is determined by the applied potential above the transition scan rate, whereas the change in the hydrogen concentration at the electrode surface with time is specified by the Butler–Volmer equation below the transition scan rate. The deflection transient measured simultaneously with the cyclic voltammogram shows that in the high scan rate range, the compressive deflection increases to a maximum value and then is completely relaxed. In the low scan rate range, however, the deflection transient is characterised by the occurrence of a maximum compressive deflection, a transition of compressive to tensile deflection, a maximum tensile deflection and finally a complete decay of the tensile deflection in sequence. From the hydrogen concentration profile transient simulated under the two constraints at the electrode surface, we simulated the deflection transients as a function of the scan rate. From the coincidence of the calculated deflection transient with that measured, the movement of the deflection in the compressive and tensile directions can be accounted for in terms of the difference in the molar volume of -PdH across the whole electrode, developed during hydrogen diffusion.

Analysis of stresses generated during hydrogen extraction from and injection into Ni(OH)2/NiOOH film electrode

Abstract Stresses generated during the hydrogen extraction from and injection into a Ni(OH) 2 /NiOOH film electrode in 0.1 M KOH solution are analysed by means of a laser beam deflection technique combined with potentiostatic current transient and electrochemical quartz crystal microbalance (EQCM) techniques. From the measured values of the film thickness, the elapsed time for phase boundary movement (PBM) and the potential step given between the hydrogen extraction/injection potential and the plateau potential, the velocity and the mobility of PBM are determined to be 2.3×10 −6 –5.4×10 −6 cm s −1 and 3.5×10 −10 –2.6×10 −9 cm 2 s −1 V −1 , respectively. From the mass change measured by means of EQCM, it is suggested that, during hydrogen extraction, the PBM continues until the insertion of K + ions begins to occur simultaneously with the desertion of such neutral species as H 2 O molecules or KOH molecules. During hydrogen injection, the PBM is accompanied by the extraction of K + ions, followed by the insertion of neutral species. From the mass change transient and the deflection transient, simultaneously measured with the current transient, it is suggested that the sharp rise of tensile and compressive deflections in the initial stage is traced back to the PBM. The following relaxation of tensile and compressive deflections is attributed to the extraction/insertion of neutral species. Most of the stresses developed during the hydrogen extraction and injection originate mainly from the PBM, not the extraction/insertion of K + ions and neutral species.

Analysis of the compressive and tensile stresses generation/relaxation during hydrogen ingress into and egress from Pd foil electrode

Abstract The build up/decay of compressive and tensile stresses has been analyzed as a function of the hydrogen charging potential during hydrogen ingress into and egress from a palladium (Pd) foil electrode in the presence of a single phase (α-Pd phase) and a mixture of two phases (α-Pd and β-Pd phases) in 0.1 M NaOH solution, by using a laser beam deflection technique, combined with a current transient technique. The transient of the hydrogen concentration profile across the electrode is derived from the compressive and tensile deflection transients, measured simultaneously with cathodic and anodic charge transients during hydrogen ingress into the fresh electrode and hydrogen egress from the precharged electrode, respectively. The time to the maximum compressive and tensile deflections measured during hydrogen ingress into and egress from the electrode, respectively, is assigned to specific hydrogen concentration profiles which are characterized by the `break-through time indicating in a first approximation the arrival of hydrogen at the opposite side of the interface. From the value of the time to the maximum deflection, hydrogen diffusivity in the Pd foil electrode was determined to be 4×10−8 to 5×10−7 cm2 s−1. It is indicated that the elastic tensile stress field, adjacent to the dislocation developed around the β-Pd phase formed in the Pd foil electrode, introduces the additional trap sites for hydrogen. Larger hydrogen diffusivity determined during hydrogen discharging than hydrogen charging is discussed in terms of how much the trap sites are filled with hydrogen during hydrogen charging. Stresses exerted due to local molar volume change across the electrode during hydrogen ingress into and egress from a single α-Pd phase are always exceeded by those stresses exerted across the electrode during hydrogen ingress into and egress from a mixture of α-Pd and β-Pd phases.

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.

Analysis of open-circuit potential transient and laser beam deflection transient simultaneously measured from Pd foil electrode pre-charged with hydrogen

Abstract In the present work open-circuit potential (OCP) transient and laser beam deflection transient were measured simultaneously from the impermeable Pd foil electrode pre-charged with hydrogen at −0.08–0.04 V (reversible hydrogen electrode (RHE)) in 0.1 M NaOH solution as a function of the pre-charging potential of hydrogen. The OCP is determined by a mixing of the potentials of two simultaneous reactions of anodic hydrogen oxidation MHads+OH−=M+H2O+eα and cathodic hydrogen underpotential deposition M+H2O+eβ=MHads+OH− below 0.15 V (RHE) and oxygen reduction 1/2O2+H2O+2eβ=2OH− above 0.15 V (RHE), coupled by a common corrosion rate. From the coincidence between the measured OCP transient and calculated corrosion potential transient, it is indicated that the plot of hydrogen oxidation rate versus reduced time calculated corresponds to the measured OCP transient and thus both transients are closely coupled each other. This means that the hydrogen self-discharge from the electrode proceeds with the rate of hydrogen oxidation at the electrode surface, calculated based upon the mixed potential theory from the measured Evans–Hoar diagram. By taking the value of self-discharge rate on one surface determined as a function of time and adopting the impermeable constraint on the other surface as the boundary condition for hydrogen diffusion through the electrode, hydrogen concentration profile across the electrode has been derived with time from the measured tensile deflection transient.

Characterisation of Pd electrode surface modified by phase transformation-induced plastic deformation using fractal geometry

The surface change of a Pd electrode developed by plastic deformation due to the phase transformation of α-PdH to β-PdH has been characterised by fractal geometry. The β-PdH phase formed in the matrix electrode of α-PdH phase causes the plastic deformation of the electrode and hence increases the surface roughness as well as the surface area. The fractal dimension of the Pd electrode surface modified by the plastic deformation was determined from the diffusion-limited peak current density during linear sweep voltammetric measurements. The surface fractal dimension obtained is increased from 1.97 to 2.09 with increasing fraction of β-PdH phase in the matrix electrode from zero to unity.