William E. O'Grady

In Situ X‐Ray Absorption Near‐Edge Structure Evidence for Quadrivalent Nickel in Nickel Battery Electrodes

The oxidation state of nickel in a nickel oxide battery has been examined using X-ray absorption spectroscopy. Chemically synthesized {beta}-NiOOH and BaNiO{sub 3} were used as reference standards for the Ni(III) and Ni(IV) oxidation states of nickel, respectively. The shift of the Ni K-edge absorption in both a completely charged nickel oxide electrode (NOE) and a 1,000% overcharged nickel oxide electrode corresponds to a Ni oxidation state of +3.5. This oxidation state clearly indicates the presence of Ni{sup +4} in the charged electrodes.

In Situ X‐Ray Absorption Spectroscopic Studies of Nickel Oxide Electrodes

In situ transmission x-ray absorption spectroscopy was used to investigate changes in the structure of Ni(OH)2 as it was cycled as a nonsintered nickel oxide electrode in concentrated alkali. Results were obtained for dry as-prepared electrodes and electrodes that were charged or discharged. Changes were observed in the (0001) plane of the Ni(OH)2 as a function of cycling. In the charged material there was a contraction in the Ni-O and Ni-Ni distances. The EXAFS results and the XANES spectra strongly indicate that the Ni-O coordination is symmetrical in Ni(OH)2, whereas the coordination is asymmetric in the charged material. The nickel oxide electrode, also known as the nickel or nickel hydroxide electrode (NOE), is used in batteries, fuel cells, and electrolyzers. Other possible uses are in electrochromic and switching devices. Because of its many applications it has been the subject of numerous investigations and reviews (1-4). In spite of this, the electrode processes remain an enigma. Much of this confusion is due to the amorphous nature of the reactants and products. Extended x-ray absorption fine structure (EXAFS) technique can be used to study the average local structure of systems that do not possess long range order. A previous paper has presented preliminary results of an in situ EXAFS study of a nickel oxide electrode (5). This work has shown that the EXAFS spectroscopy can be used to study electrochemical systems under working conditions (in situ). The present paper is a more extensive report of the previous work which includes analysis of higher shells and x-ray absorption near edge structure (XANES).

Chloride Ingress into Aluminum Prior to Pitting Corrosion An Investigation by XANES and XPS

Two distinct chloride (Cl - ) species were detected on and/or in the passive oxides of polycrystalline Al samples, which were anodically polarized below the stable pitting potential in Cl - -containing solutions. Chloride was found to be present as an adsorbed specie at the surface of the Al oxide, as well as an incorporated specie within the passive oxide. The two species of Cl - were recorded by X-ray absorption near edge structure (XANES), using both an electron yield detector and an X-ray fluorescence detector, and by X-ray photoelectron spectroscopy (XPS). Electron yield XANES and XPS results indicate that adsorbed Cl - migrates from the solution/Al oxide interface into the passive Al oxide film, prior to stable pit initiation. Cl - migration occurs once a critical anodic potential or critical adsorbed Cl - concentration is reached. The migration of Cl - is followed by a loss of oxidized Al from the passivating film, as determined by XPS, and can be attributed to (i) metastable pitting events or (ii) oxide dissolution. The ingress of Cl - into the oxide appears to be a key factor for the onset of metastable pitting or passive film dissolution.

Fabrication of a high surface area boron-doped diamond coated metal mesh for electrochemical applications

An electrode for electrochemical uses is made of a conductive metal mesh coated with boron-doped diamond. The electrode may be used in electrochemical reactions either as a cathode or as an anode, or can be used with an alternating current.

In situ far-infrared evidence for a potential dependence of silver–water interactions

The influence of the adsorption of fluoride on the bonding of water to a silver electrode surface has been studied by observing the ν(Ag–O) stretching vibration in the far-IR. The experiments were conducted using potential-difference external-reflectance spectroscopy with synchrotron radiation as the far-IR source. It was found that the adsorption of fluoride ions at potentials positive of the potential of zero charge (p.z.c.) decreased the coverage of water adsorbed onto the silver electrode surface. At potentials negative of the p.z.c., water molecules were found to be bound to the silver surface through the oxygen atom.

Measurement of hydrogen uptake by palladium using a quartz crystal microbalance

In this communication it is shown that these large frequency shifts can be attributed to stresses arising in the metal as the lattice expands due to the hydrogen (deuterium) charging. We also want to alert others using this technique of the possible role stresses can play in these measurements

Chlorine Evolution and Reduction Processes at Oriented Single‐Crystal RuO2 Electrodes

RuO/sub 2/ single-crystal electrodes have been used to study the mechanism of the chlorine evolution/reduction reactions. RuO/sub 2/ (110) and (101) surfaces show the same anodic Tafel slopes equal to 40 mV/decade. Significant differences in the electrocatalytic behavior of these two surfaces were found for the chlorine reduction reactions. Two cathodic voltammetric peaks at E = 780 and E = 220 mV vs. SCE reference were observed for the RuO/sub 2/ (110) electrodes. The peak at E = 780 mV is completely suppressed in case of the RuO/sub 2/ (101) electrodes. It is suggested that the natural oxygen sites that are available only on the ideal (110) surfaces provide active centers for the formation of surface O.Cl/sup +/ groups that are intermediates in the Krishtalik mechanisms. The cathodic LSV peak at E = 780 mV observed only for the RuO/sub 2/ (110) surfaces indicates, then, the reaction path with chloronium intermediates. The influence of the surface ruthenium oxide rearrangement processes on catalytic activity of the electrode and possible changes in the reaction mechanism are discussed in detail.

XANES study of underpotential deposited copper on carbon-supported platinum

Abstract Underpotential deposited (UPD) copper on carbon-supported platinum was investigated in-situ in 0.5 M H2SO4 at 0.05 V (SCE) by XANES (X-ray Absorption Near Edge Structure). By taking XANES spectra at the Cu K-edge and the Pt LIII-edge it was possible to determine the valence state of the copper and observe modifications in the electronic structure of the platinum on adsorption of copper. The XANES for UPD copper shows that the adsorbed copper had an oxidation state close to Cu+. XANES features indicate strongly a tetrahedral coordination for the adsorbed Cu species. A reduction in intensity of the white line in the platinum XANES is consistent with a partial filling of empty Pt d-band vacancies on adsorption of copper. Thus, UPD species can modify the electronic structure of a platinum catalyst. The adsorbed Cu+ species are apparently associated with HSO−4 ions.

Chloride Uptake by Oxide Covered Aluminum as Determined by X‐Ray Photoelectron and X‐Ray Absorption Spectroscopy

The uptake of chloride by aluminum polarized at potentials below (less positive than) the pitting potential in 0.1 M NaCl solutions was studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The XPS chloride spectra showed that two distinct sets of doublets are present. One doublet is related to chloride on the surface and the second is related to chloride incorporated in the oxide film. In the case of XAS, deconvolution of the spectrum obtained for samples polarized below the pitting potential also showed the presence of chloride in the near surface region and in the oxide film. The important point of this work is that the observed chloride was present in two different chemical environments as determined with both XAS and XPS.

Redox behavior of the nickel oxide electrode system: quartz crystal microbalance studies

The electrochemical reactions occurring in the nickel oxide electrode system have been studied with the quartz crystal microbalance. Following cathodic deposition from nickel sulfate solutions, α-nickel hydroxide films were cycled in 1.0M alkali metal hydroxide solutions. The oxidation process produced frequency decreases in these solutions, indicating a corresponding increase in mass in the electrode layer. Upon subsequent reduction, a return to the initial frequency value was observed. These shifts ranged from a relatively small change for lithium hydroxide to progressively larger shifts as the cation size increased. It is clear from these results that alkali metal cations are being taken up into the electrode structure during oxidation. A mechanism based upon a net 1.7 electron oxidation has been proposed, involving predominant oxidation of nickel from the +2 to the +4 state as indicated by recent EXAFS studies. The mechanism is consistent with the quartz crystal microbalance data provided that alkali metal cations enter, and water molecules leave, the electrode during oxidation. β-Nickel hydroxide films were prepared by contacting α-Ni(OH)2 films with hot 8 M KOH. Cycling these films in 1.0 M alkali hydroxide solutions gave frequency changes which were positive for the lighter cations and negative for the heavier ones, in contrast to the results for α-Ni(OH)2 films. Cation uptake during oxidation is also occurring in this case; however, a small but significant amount of water is also expelled, producing the observed frequency response. The mechanism in this case also involves some oxidation to the +4 state, as shown by XANES studies.