Sonia Dsoke

An XAS experimental approach to study low Pt content electrocatalysts operating in PEM fuel cells

We present an X-ray absorption spectroscopy (XAS) study of a low Pt content catalyst layer (Pt loading 0.1 mg cm(-2)) operating at the cathode of a proton exchange membrane fuel cell (PEMFC). This catalyst is based on the use of a mesoporous inorganic matrix as a support for the catalyst Pt nanoparticles. Due to the high Pt dilution, in situ measurements of its structural properties by XAS are challenging and suitable experimental strategies must be devised for this purpose. In particular, we show that accurate XAS in situ fluorescence measurements can be obtained using an optimized fuel cell, suitable protocols for alignment of a focused X-ray beam and an appropriate filter for the background signal of the other atomic species contained in the electrodes. Details, advantages and limitations of the XAS technique for in situ measurements are discussed. Analysis of the near-edge XAS and EXAFS (extended X-ray absorption fine structure) data, corroborated by a HRTEM (high-resolution transmission electron microscopy) study, shows that the Pt particles have a local structure compatible with that of bulk Pt (fcc) and coordination numbers match those expected for particles with typical sizes in the 1.5-2.0 nm range. Substantial changes in the oxidation state and in local atomic arrangement of the Pt particles are found for different applied potentials. The catalyst support, containing W atoms, exhibits a partial reduction upon PEMFC activation, thus mimicking the catalyst behavior. This indicates a possible role of the mesoporous matrix in favouring the oxygen reduction reaction (ORR) and stimulates further research on active catalyst supports.

High rate capability Li3V2¬xNix(PO4)3/C (x = 0, 0.05, and 0.1) cathodes for Li-ion asymmetric supercapacitors

Nanostructured Li3V2−xNix(PO4)3 (x = 0, 0.05, and 0.1) cathode materials, with a mean particle dimension ranging from 200 to 63 nm, are successfully synthesized with poly(acrylic acid) and D-(+)-glucose as carbon sources. All three samples show a monoclinic crystalline structure as confirmed by X-ray diffraction and Rietveld analysis. Ni-doping improves the specific capacity of Li3V2(PO4)3/C. Between 3.0 and 4.3 V vs. Li+/Li, all cathodes exhibit good rate capability, even at high C-rates. For these reasons, they are good candidates for high power and energy applications, in particular for the development of high energy density supercapacitors. Li3V1.95Ni0.05(PO4)3/C, because of its highest specific discharge capacity (93 mA h g−1 at 100 C) and capacity retention of 97% after 1000 cycles, is selected for building an asymmetric supercapacitor with activated carbon as the anode. At a power density of 2.8 kW L−1, the asymmetric system delivers 18.7 W h L−1, a value five orders of magnitude higher than that of the symmetric capacitor at the same power level.