ABSTRACT
We report the formation of a stable neutral diboron diradical simply by coordination of an aromatic dinitrogen compound to an ortho-phenyldiborane. This process is reversible upon addition of pyridine. The diradical species is stable above 200 °C. Computations are consistent with an open-shell triplet diradical with a very small open-shell singlet-triplet energy gap that is indicative of the electronic disjointness of the two radical sites. This opens a new way of generating stable radicals with fascinating electronic properties useful for a large variety of applications.
ABSTRACT
Thermally stable, ordered mesoporous thin films of 8 mol % yttria-stabilized zirconia (YSZ) were prepared by solution-phase coassembly of chloride salt precursors with an amphiphilic diblock copolymer using an evaporation-induced self-assembly process. The resulting material is of high quality and exhibits a well-defined three-dimensional network of pores averaging 24 nm in diameter after annealing at 600 °C for several hours. The wall structure is polycrystalline, with grains in the size range of 7 to 10 nm. Using impedance spectroscopy, the total electrical conductivity was measured between 200 and 500 °C under ambient atmosphere as well as in dry atmosphere for oxygen partial pressures ranging from 1 to 10(-4) bar. Similar to bulk YSZ, a constant ionic conductivity is observed over the whole oxygen partial pressure range investigated. In dry atmosphere, the sol-gel derived films have a much higher conductivity, with different activation energies for low and high temperatures. Overall, the results indicate a strong influence of the surface on the transport properties in cubic fluorite-type YSZ with high surface-to-volume ratio. A qualitative defect model which includes surface effects (annihilation of oxygen vacancies as a result of water adsorption) is proposed to explain the behavior and sensitivity of the conductivity to variations in the surrounding atmosphere.
ABSTRACT
The present study compares the physico-chemical properties of non-aqueous liquid electrolytes based on NaPF6, NaClO4 and NaCF3SO3 salts in the binary mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The ionic conductivity of the electrolytes is determined as a function of salt concentration and temperature. It is found that the electrolytes containing NaClO4 and NaPF6 exhibit ionic conductivities ranging from 5 mS cm(-1) to 7 mS cm(-1) at ambient temperature. The electrochemical stability window of the different electrolytes is studied by linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements with respect to a variety of working electrodes (WE) such as glassy carbon (GC), graphite and a carbon gas diffusion layer (GDL). Electrolytes containing NaPF6 and NaClO4 are found to be electrochemically stable with respect to GC and GDL electrodes up to 4.5 V vs. Na/Na(+), with some side reactions starting from around 3.0 V for the latter salt. The results further show that aluminium is preferred over different steels as a cathode current collector. Copper is stable up to a potential of 3.5 V vs. Na/Na(+). In view of practical Na-ion battery systems, the electrolytes are electrochemically tested with Na0.7CoO2 as a positive electrode. It is inferred that the electrolyte NaPF6-EC : DMC is favorable for the formation of a stable surface film and the reversibility of the above cathode material.
