Greg W. Coffey

Electrode Performance in Reversible Solid Oxide Fuel Cells

Electrolysis has long been used to dissociate water into its constituents of oxygen and hydrogen. Various electrolyzers have been developed and are commercially available today, including those based on proton exchange membranes, molten carbonate, phosphoric acid, alkaline, and solid oxide technology. 1-5 Some of these are reversible systems capable of operating both as a fuel cell and as an electrolyzer, although fuel cell and electrolyzer functions are carried out in separate subsystems. A reversible fuel cell can take advantage of excess electrical grid capacity during off-peak hours to produce hydrogen fuel, to be utilized later during periods of high electrical demand. The power unit fuel cell is sized for the peaking load in a practical reversible fuel cell, whereas the electrolyzer is rated at a power that can produce sufficient hydrogen to recharge the hydrogen storage capacity over the remaining hours of the day. If energy conversion, electrical to chemical and chemical to electrical, can occur in the same device with reasonable efficiencies, there could be significant overall cost benefits. For solid oxide electrolysis cells SOEC to be of commercial interest, the cost of the hydrogen produced must be competitive with that of other means of production. The cost of electricity is a significant factor in steam electrolysis, comprising 75% to 95% of that of electrolysis-derived hydrogen according to performance and cost

Ni/YSZ Anode Interactions with Impurities in Coal Gas

Performance of solid oxide fuel cell (SOFC) with nickel/zirconia anodes on synthetic coal gas in the presence of low levels of phosphorus, arsenic, selenium, sulfur, hydrogen chloride, and antimony impurities were evaluated. The presence of phosphorus and arsenic led to the slow and irreversible SOFC degradation due to the formation of secondary phases with nickel, particularly close to the gas inlet. Phosphorus and antimony surface adsorption layers were identified as well. Hydrogen chloride and sulfur interactions with the nickel were limited to the surface adsorption only, whereas selenium exposure also led to the formation of nickel selenide for highly polarized cells.

Silver nanorod arrays for photocathode applications

We explore the use of plasmonic Ag nanorod arrays featuring enhanced photoemission as high-brightness photocathode material. Silver nanorod arrays are synthesized by the direct current electrodeposition method and their dimensionality, uniformity, crystallinity, and oxide/impurity content are characterized. The yielded arrays exhibit greatly enhanced two-photon photoemission under 400 nm femtosecond pulsed laser excitation. Plasmonic field enhancement in the array produces photoemission hot spots that are mapped using photoemission electron microscopy. The relative photoemission enhancement of nanorod hot spots relative to that of a flat Ag thin film is found to range between 102 and 3 × 103.

Nonstoichiometry and Transport Properties of Ca3Co4±xO9+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.