Gregory W. Coffey

Competition Between Bulk and Surface Pathways in Mixed Ionic Electronic Conducting Oxygen Electrodes

In mixed ionic electronic conductors (MIECs) the electrochemistry of oxygen reduction and evolution is complicated by the competition between surface adsorption/desorption, interaction of these surface species with bulk vacancies and diffusion of these species to the MIEC/electrolyte interface. Charge transfer may occur by diffusion of oxygen adsorbates to the triple phase boundary or by diffusion of vacancies from the electrolyte with subsequent exchange with oxygen adsorbed on the porous MIEC wall. A model describing the competition between these two charge transfer pathways is developed. Key to the model is treatment of the boundary condition at the MIEC/electrolyte interface. The absolute potential across this interface is used to relate the surface overpotential of each pathway in terms of the other and terms arising from the concentration overpotentials. Results show the conditions under which one process may dominate over the other.

Electrochemical properties of lanthanum strontium aluminum ferrites for the oxygen reduction reaction

Abstract The oxygen reduction reaction was studied on the La 1− x Sr x Al y Fe 1− y O 3 ( x =0.2, y =0.1, 0.2, 0.3, 0.4) system by cyclic voltametry and electronic conductivity. Activation energies for the bulk and film conductivities were determined. Tafel analysis afforded the activation energies “from the temperature dependence of the exchange current densities” as well as the charge transfer coefficient. The electrical conductivity of bulk material was found to decrease with aluminum content. Formation of the materials into thin porous films further decreased the conductivity after correcting for porosity. Aluminum substitution substantially decreased the performance through influence of the pre-exponential factor in the Butler–Volmer formulation. Neither the activation energies nor the charge transfer coefficient for these materials varied significantly. Aluminum does not adversely influence the basic mechanism of oxygen reduction. It may occupy and block electrochemically active sites on the electrode surface, but it does not appear to decrease the intrinsic activity of available surface sites.

Oxygen Reduction Activity of Lanthanum Strontium Nickel Ferrite

The reduction of oxygen on nickel-doped lanthanum strontium ferrite was studied by current interrupt cyclic voltammetry. Nickel doped on the B site of the perovskite ranged from 0 to 40%. Nickel strongly influenced the sintering of the films. The minimum temperature at which a stable adherent robust film could be formed increased with nickel content. The electrochemical performance for reduction of oxygen was compared with nickel content. Undoped lanthanum strontium ferrite consistently showed greater activity than the doped materials. The data were further analyzed to obtain the exchange current density as a function of temperature and then further analyzed to obtain the activation energy and pre-exponential factor. These values did not correlate with nickel composition but did correlate with one another. The variation in performance was tentatively attributed to subtle variations in microstructure.

Electrical, Thermoelectric, and Structural Properties of La ( M x Fe1 − x ) O3 ( M = Mn , Ni , Cu )

Electrical, thermoelectric, and structural properties were studied in transition metal ion-substituted LaFeO 3 : La(Mn x Fe 1-x )O 3 , La(Ni x Fe 1-x )O 3 , and La(Cu x Fe 1-x )O 3 . Structural analysis showed that a continuous series of solid solutions with no intermediate phases are forming over a wide range (0 < x < 1) with substitutions of Mn and Ni, whereas the maximum Cu content is 30% from this study. The Ni-substituted LaFeO 3 specimens have substantially higher conductivity than those substituted with either Mn or Cu, measured in air from 100 to 1000°C. The Seebeck coefficient of La(Mn x Fe 1-x )O 3 and La(Cu x Fe 1-x )O 3 has a strong temperature dependence, indicating a thermally activated carrier formation. The activation energy for carrier formation in La(Cu x Fe 1-x )O 3 is greater than that in La(Mn x Fe 1-x )O 3 . Thermoelectric and electrical properties evidence conduction through polaron hopping in both Mn- and Cu-substituted LaFeO 3 , whereas the Ni-substituted LaFeO 3 shows metallic conductivity.

Nonstoichiometry and Transport Properties of Ca[sub 3]Co[sub 4±x]O[sub 9+δ] (x=0–0.4)

Nonstoichiometries of Ca3Co4O9+δ and transport properties of Ca3Co4±xO9+δ were investigated. At 1100°C, Ca3Co4O9+δ transformed to CaO and CoO. The reaction products offer a precise baseline for thermogravimetric analysis. At room temperature, δ in Ca3Co4O9+δ is 0.38, which decreases at T ~450°C, indicating the onset point of the formation of oxygen vacancies, and δ is ~0.20 at 900°C. Correspondingly, the average Co valence state is 3.19 at room temperature and 3.10 at 900°C. In contrast to conventional defect chemistry theory in p-type oxide conductors, the formation of oxygen vacancies in Ca3Co4O9+δ has a negligible impact on the carrier density of holes, indicating that oxygen vacancies and the redox couple responsible for hole carriers are in different layers. With control over the ratio of Ca/Co, the phase boundary for the misfit layered structure is between Ca3Co3.95O9+δ and Ca3Co4.05O9+δ. Beyond the phase boundary, the second phase is present, which effectively lowers the electrical conductivity while increasing the Seebeck coefficient.

Nonstoichiometry and Transport Properties of Ca3Co4 ± x O9 + δ ( x = 0 – 0.4 )

Nonstoichiometries of Ca3Co4O9+δ and transport properties of Ca3Co4±xO9+δ were investigated. At 1100°C, Ca3Co4O9+δ transformed to CaO and CoO. The reaction products offer a precise baseline for thermogravimetric analysis. At room temperature, δ in Ca3Co4O9+δ is 0.38, which decreases at T ~450°C, indicating the onset point of the formation of oxygen vacancies, and δ is ~0.20 at 900°C. Correspondingly, the average Co valence state is 3.19 at room temperature and 3.10 at 900°C. In contrast to conventional defect chemistry theory in p-type oxide conductors, the formation of oxygen vacancies in Ca3Co4O9+δ has a negligible impact on the carrier density of holes, indicating that oxygen vacancies and the redox couple responsible for hole carriers are in different layers. With control over the ratio of Ca/Co, the phase boundary for the misfit layered structure is between Ca3Co3.95O9+δ and Ca3Co4.05O9+δ. Beyond the phase boundary, the second phase is present, which effectively lowers the electrical conductivity while increasing the Seebeck coefficient.