Karen E. Swider

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.

The Chemical State of Sulfur in Carbon‐Supported Fuel‐Cell Electrodes

Vulcan carbon is the favored support for fuel-cell electrocatalysts, but as-received it contains high levels of sulfur (ca. 5000 ppm or greater) which could potentially poison the fuel-cell electrochemistry. The chemical state of the sulfur in platinum/carbon electrocatalysts has been examined by measuring its oxidation state via x-ray photoelectron spectroscopy at different points during the preparation of a mock fuel-cell electrode. Also monitored were the presence of sulfate in the aqueous wash from the electrocatalysts and the cyclic voltammetry of the electrocatalysts after each preparation step. Our studies indicate that the platinum catalytically oxidizes some of the covalent sulfur in the vulcan carbon to sulfate when water, heat, and strong physical contact between Pt and C are all present. These conditions are attained during the preparation of typical fuel-cell electrodes. Most of the zero-valent sulfur remains in the carbon after treatment, however, and appears not to be initially in contact with the Pt. This remaining unoxidized sulfur may be a source of poisoning to the Pt electrocatalyst with long-term electrochemical use, particularly at the fuel-cell cathode.

Aerogels as a tool to study the electrical properties of ruthenium dioxide

RuO2 is a well-known oxidation catalyst for electrolytic generation of Cl2. Although bulk RuO2 is a metallic conductor, studies of mixed-oxide aerogels containing RuO2 confirm that the surface of RuO2 is a hydrous oxide and a mixed conductor. Our previous work showed that at room temperature, protonic and electronic conductivity occurs in mixed phase aerogels of RuO2+TiO2 (designated as (Ru–Ti)Ox). Impedance measurements up to 340°C demonstrate that electron hopping and/or semiconduction also occurs in (Ru–Ti)Ox aerogels. Electron hopping may occur between Ru3+ and Ru4+ ions in the ruthenia, while n-type semiconduction may be introduced into the titania by Ru3+ defects. The multiplicity of charge-transfer mechanisms measured on the surfaces of RuO2-based aerogels is indicative of the complexity of the catalytic and electrocatalytic chemistry performed on RuO2.

Effect of re-wetting on silica aerogel structure: a SANS study

Small angle neutron scattering (SANS) has been used to study (1) the ability to refill the pores of a silica aerogel (ρ=0.3 g/cm3) with water and (2) any effect of the water and the re-wetting process on the aerogel structure on the 1 nm to 100 nm scale. A sample filled with a D2O:H2O solution which was chosen to match the scattering properties of silica shows no scattering at length scales <40 nm down to the detection limit (~2 nm) of the experiment. This lack of scattering demonstrates that the mesopores of the silica aerogel were successfully filled with the contrast-matching fluid. SANS spectra for samples filled with air or D2O are similar, indicating that the aerogel structure as detected by this technique is not affected by the reintroduction of water into the pores. Results suggest that some aerogels may be suited for applications in aqueous environments such as catalysis or chemical sensing.

Characterization of multi-phase aerogels by contrast-matching SANS

Contrast-matching small-angle neutron scattering (SANS) has been used to independently characterize the structures of two-phase aerogels consisting of a metal oxide supported on silica aerogel. Ru oxide was formed on silica aerogel by oxidation at 400°C of ruthenocene, impregnated from the vapor phase, or Ru(III)acetylacetonate, impregnated via an acetone solution. Fe oxide was formed by vapor-phase impregnation of ferrocene, followed by reduction at 400°C and exposure to ambient air. The Ru oxide forms particles several tens of nanometers in diameter in the aerogel mesopores. The Fe oxide forms a coating which mimics the structure of the support. The silica aerogel structure is not affected by the deposition and heat treatment processes.

Structure of Ru–Ti oxide aerogels: a SANS study

Small-angle neutron scattering (SANS) has been used to characterize Ru–Ti oxide aerogels annealed under various atmospheres and temperatures. The pores were filled with H2O/D2O solutions that match the scattering properties of either RuO2 or TiO2, therefore scattering from the remaining unmatched phase is measured independently. The structure of the RuO2 and TiO2 components of the aerogel detected by SANS depends more on annealing temperature than annealing atmosphere. The results are consistent with a branched, polymeric network made up of 8–16 nm TiO2 particles and <5-nm RuO2 particles. The SANS results are supported by TEM. This study demonstrates that contrast-matching SANS is complementary to other techniques for characterizing the structure of two-phase aerogels.