Films of crystalline WO(3) nanosheets oriented perpendicular to tungsten substrates were grown by a surfactant-free hydrothermal method, followed by sintering. The films exhibit photoelectrochemical oxygen evolution at low overpotential.

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Electrochemical impedance spectroscopy was used to study the transport properties of the three-layer Dion−Jacobson phase HLa 2 Ti 2 TaO 10 in the temperature range of interest (250−475°C) for intermediate temperature fuel cells. The compound was prepared by proton exchange of RbLa 2 Ti 2 TaO 10 , which in turn was made by direct solid state synthesis or by an organic precursor-based method. When prepared by the precursor method, HLa 2 Ti 2 TaO 10 ·nH 2 O (n = 1−2) could be exfoliated by tetrabutylammonium hydroxide to produce rectangular sheets with ∼30 nm lateral dimensions. HLa 2 Ti 2 TaO 10 ·nH 2 O lost intercalated water at temperatures between 100 and 200°C, but X-ray diffraction patterns up to 500°C did not show evidence of collapse of the interlayer galleries that has been observed with the structurally similar compound HCa 2 Nb 3 O 10 . Under humid hydrogen atmosphere, the conductivity of HLa 2 Ti 2 TaO 10 followed Arrhenius behavior with an activation energy of 0.9 eV; the conductivity was in the range of 10 −9 to 10 −5 S cm −1 depending on the preparation conditions and temperature. Modification of the stoichiometry to produce A-site or B-site (vacancy or substitution) defects decreased the conductivity slightly. The conductivity was approximately 1 order of magnitude higher in humid hydrogen than in humid air atmospheres, suggesting that the dominant mechanism in the intermediate temperature range is electronic. A-site substitution (Sr 2+ for La 3+ ) beyond the Ruddlesden−Popper phase limit converted the layered pervoskite to a cubic perovskite Sr 2.5 □ 0.5 Ti 2 TaO 9 with 2 orders of magnitude higher conductivity than HLa 2 Ti 2 TaO 10 at 475°C. ■ INTRODUCTIONElectrochemical energy conversion technologies are becoming increasingly important as the global energy economy evolves toward higher efficiency and sustainability. The development of these technologies is to a large extent driven by the discovery of higher performing and lower cost materials. In the case of fuel cells, which are devices that electrochemically oxidize fuels to produce electrical energy, the current technology is dominated by low temperature (<150°C) and high temperature (>600°C ) devices that contain polymer and solid oxide electrolytes, respectively. 1 A gap exists in the intermediate temperature regime (250−450°C) where there are reasons to expect that fuel cells could be made more efficient and less expensive. The loading of precious metal anode and cathode catalysts could be substantially decreased because of the higher activity of catalysts at higher temperature. Also, carbon-containing species such as CO, which poison noble metal catalysts in polymer fuel cells, are easily oxidized at the anode at intermediate temperatures.However, the development of intermediate temperature fuel cell technology is hampered by the lack of electrolyte materials that have sufficiently high proton conductivity in the temperature range of interest. 2,3 Experiments on intermediate temperature fuel cells have employed ceramic proton conductors th...

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