Redox Couple Behavior on Lithiated Nickel Oxide Electrodes

Abstract

The rotating electrode technique has been used to study the kinetics of various redox couples [Fe(CN)64-/Fe(CN)63-, Fe2+/Fe 3+, Cr2+/Cr 3+, and quinone/hydroquinone] on mosaic lithiated nickel oxide electrodes in sulfate solutions. At potentials cathodic to the flatband potential, the redox reactions are very inhibited. At potentials anodic to the flatband potential, the oxidation of the ferrocyanide ion exhibits a transfer coefficient of ~ : 1/2 with an apparent standard rate constant approximately two orders of magnitude smaller than on Pt. The other couples exhibit abnormal transfer coefficients under these conditions, probably because of specific adsorption on the oxide electrode surface. The kinetics of various redox reactions have been examined on lithiated mosaic nickel oxide electrodes with the objective of comparing the behavior of these couples on this compound semiconductor with that on metals. With NiO(Li) electrodes, the heterogeneous electron transfer is expected to involve rather localized valency states within the electrode phase [i.e., Ni 2+ (3d s) and Ni ~+ (3d 7) ] rather than the wide bands * Electrochemical Society Active Member. ~Present address: Fritz-Haber ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 35.8.11.2 Downloaded on 2015-06-26 to IP Vol. 121, No. 3 REDOX COUPLE BEHAVIOR ON NiO ELECTRODES 319 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 35.8.11.2 Downloaded on 2015-06-26 to IP

Anodic Breakdown of Passive Films on Metals

“…Figure 8 shows the anodic polarization curves of iron (20) and nickel (21) in sulfuric acid solution and the estimated band structure in the passive film at the flatband potential, where the band structure was estimated from the flatband potential [EFB ~ +0.43 VNHS for the passive film on iron (22) and EFI~ ~ +0.8 VNHE for NiO (23) at pH _--0.7] and the critical potential of transpassive dissolution (ETa, +1.7 VNHE for iron and ETp ~-~ 1.0 VNHE for nickel at pH --_ 0.7) by taking into account the previous data on the bandgap [eg ~ 1.6 eV for the passive film on iron (16) and ~g ~ 1.8 eV for NiO (24)] and the Fermi level [eF located at 0.3 eV below the conduction band for iron oxide (25)]. Figure 8 shows the anodic polarization curves of iron (20) and nickel (21) in sulfuric acid solution and the estimated band structure in the passive film at the flatband potential, where the band structure was estimated from the flatband potential [EFB ~ +0.43 VNHS for the passive film on iron (22) and EFI~ ~ +0.8 VNHE for NiO (23) at pH _--0.7] and the critical potential of transpassive dissolution (ETa, +1.7 VNHE for iron and ETp ~-~ 1.0 VNHE for nickel at pH --_ 0.7) by taking into account the previous data on the bandgap [eg ~ 1.6 eV for the passive film on iron (16) and ~g ~ 1.8 eV for NiO (24)] and the Fermi level [eF located at 0.3 eV below the conduction band for iron oxide (25)].…”

“…It follows that the passive potential region, where -~H is potential-independent and hence gives rise to potential-independent dissolution of the passive film, is determined by the band structure of electron levels in the film. Figure 8 shows the anodic polarization curves of iron (20) and nickel (21) in sulfuric acid solution and the estimated band structure in the passive film at the flatband potential, where the band structure was estimated from the flatband potential [EFB ~ +0.43 VNHS for the passive film on iron (22) and EFI~ ~ +0.8 VNHE for NiO (23) at pH _--0.7] and the critical potential of transpassive dissolution (ETa, +1.7 VNHE for iron and ETp ~-~ 1.0 VNHE for nickel at pH --_ 0.7) by taking into account the previous data on the bandgap [eg ~ 1.6 eV for the passive film on iron (16) and ~g ~ 1.8 eV for NiO (24)] and the Fermi level [eF located at 0.3 eV below the conduction band for iron oxide (25)]. As can be seen in Fig.…”

Electrocatalysis on non-metallic surfaces

“…This material is a p-type semiconductor and hence processes such as 0 -eduction would be expected to be inhibited. Examination of redox couples such as ferriferrocyanide with the rotating disk technique do indicate such cathodic inhibition in acid solutions with the anode branch under combined kinetic and diffusion control [87]. Differ-ential capacitance measurements in acid solutions indicate that most of the electrode potential change occurs across the space charge region within the NiO(Li) at electrode potentials cathodic to 0.8V vs. standard hydrogen electrode (SHE) [88].…”

“…Differ-ential capacitance measurements in acid solutions indicate that most of the electrode potential change occurs across the space charge region within the NiO(Li) at electrode potentials cathodic to 0.8V vs. standard hydrogen electrode (SHE) [88]. In alkaline solutions, however, the ferri-ferrocyanide couple is no longer cathodlcally inhibited and hoth the cathodic and anodic branches are essentially diffusion controlled even at high rotation rates [87].…”

The electron factor in catalysis on metals electrocatalysis on non-metallic surfaces

“…17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Qatalysis; el cc troca ta 1 ys i s ; surface physics and chemistry; electrochemistry; fuel cells; electrodes 18. AVAILABILITY…”