No abstract

Recently, the role of nanostructured materials in addressing the challenges in energy and natural resources has attracted wide attention. In particular, oriented nanostructures demonstrate promising properties for energy harvesting, conversion, and storage. In this Review, we highlight the synthesis and application of oriented nanostructures in a few key areas of energy technologies, namely photovoltaics, batteries, supercapacitors, and thermoelectrics. Although the applications differ from field to field, a common fundamental challenge is to improve the generation and transport of electrons and ions. We highlight the role of high surface area to maximize the surface activity and discuss the importance of optimum dimension and architecture, controlled pore channels, and alignment of the nanocrystalline phase to optimize the transport of electrons and ions. Finally, we discuss the challenges in attaining integrated architectures to achieve the desired performance. Brief background information is provided for the relevant technologies, but the emphasis is focused mainly on the nanoscale effects of mostly inorganic-based materials and devices.

The performance of several negative ͑fuel͒ and positive ͑air͒ electrode compositions for use in reversible solid oxide fuel cells capable of operating both as a fuel cell and as an electrolyzer was investigated in half-cell and full-cell tests. Negative electrode compositions studied were a nickel/zirconia cermet ͑Ni/YSZ͒ and lanthanum-substituted strontium titanate/ceria composite, whereas positive electrode compositions examined included mixed ion-and electron-conducting lanthanum strontium ferrite ͑LSF͒, lanthanum strontium copper ferrite ͑LSCuF͒, lanthanum strontium cobalt ferrite ͑LSCoF͒, and lanthanum strontium manganite ͑LSM͒. While titanate/ceria and Ni/YSZ electrodes performed similarly in the fuel cell mode in half-cell tests, losses associated with electrolysis were lower for the titanate/ceria electrode. Positive electrodes gave generally higher losses in the electrolysis mode when compared to the fuel cell mode. This behavior was most apparent for mixed-conducting LSCuF and LSCoF electrodes, and discernible but smaller for LSM; observations were consistent with expected trends in the interfacial oxygen vacancy concentration under anodic and cathodic polarization. Full-cell tests conducted for cells with a thin electrolyte ͑7 m YSZ͒ similarly showed higher polarization losses in the electrolysis than fuel cell direction.

The performances of anode-supported solid oxide fuel cells ͑SOFCs͒ utilizing a Mn-containing ferritic stainless steel ͑Crofer22 APU͒ as the cathode current collector were assessed. Three cathodes were considered: ͑La 0.8 Sr 0.2 ͒ 0.99 MnO 3 , ͑La 0.8 Sr 0.2 ͒ 0.99 FeO 3 , and ͑La 0.6 Sr 0.4 ͒ 0.98 Fe 0.8 Co 0.2 O 3 ͑all samples incorporated a Sm-doped CeO 2 interlayer between the cathode and the thin-film yttria-stablized zirconia electrolyte͒. Inclusion of the Fe-Cr alloy caused rapid degradation in all samples, which was attributed to solid-state reactivity at the cathode-Crofer interface, in addition to Cr volatilization from the alloy and subsequent condensation/ reaction within the cathode and at the cathode-electrolyte interface. In situ high-temperature X-ray diffraction was used to assess the cathode-Crofer reaction products. Preoxidation of the Crofer at 800°C for 500 h to form a protective ͑Mn,Cr͒ 3 O 4 spinel coating resulted in a marginal reduction of the cell degradation rate.A major impetus in Solid Oxide Fuel Cell ͑SOFC͒ research is the reduction of operating temperature ͑to the 650-800°C range͒ to enable the use of metallic interconnects, which are more economical, in terms of raw material costs and processing, relative to the traditionally used doped lanthanum chromite ceramic interconnect. Chromia-forming ferritic stainless steels are among the most promising alloy candidates due to their electrically conducting oxide scale, appropriate thermal expansion behavior, and low cost. [1][2][3][4] However, even at reduced cell temperatures, the application of these ferritic stainless steels is problematic. The chromia scales on the steels can grow to micrometers or even tens of micrometers after thousands of hours in the SOFC environment, leading to high electrical resistance, which causes unacceptably high degradation rates in stack performance. 2,5-11 In addition, Cr volatilization ͓primarily in the form of CrO 3 , CrO 2 ͑OH͒ 2 , and CrO 2 ͑OH͔͒ from the oxide scale and redeposition/reaction within the cathode and/or at the cathodeelectrolyte interface results in significantly reduced cathode activity, and subsequent performance degradation. The exact mechanisms of degradation are unclear, though Taniguchi et al. 12 observed Cr 2 O 3 deposition at the cathode-electrolyte interface when a La͑Sr͒MnO 3 cathode was in the presence of various Cr-containing alloys and compounds; the deposits are thought to increase diffusion and charge-transfer resistances at the interface. Similar results have also been reported for cells tested in the presence of other Cr-containing alloys. 10,[12][13][14][15][16]14 observed both Cr 2 O 3 and ͑Mn,Cr͒ 3 O 4 deposits at the yttria-stabilized zirconia ͑YSZ͒ electrolyte when using a ferritic stainless steel in conjunction with La͑Sr͒MnO 3 cathodes. To improve the scale electrical conductivity and surface/ interface stability of ferritic stainless steel interconnects, manganese has been used as an alloy addition in several newly developed ferritic compositions. 6,[17][18][19]...

Lattice expansion, phase stability, and dimensional stability of doped lanthanum chromites have been examined over a wide range of temperatures and oxygen partial pressures. Reduction of doped lanthanum chromite resulted in a linear expansion of the sample that was dependent on the acceptor (Sr, Ca) concentration, temperature, oxygen partial pressure, and oxygen content within the sample. Additional doping with aliovalent B-site additives significantly reduced lattice expansion in reducing environments. The lattice expansion in reducing environments was directly related to the loss of lattice oxygen and the simultaneous reduction of Cr4 to Cr3 to maintain electroneutrality.