Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-N bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Lanthanum strontium cobalt ferrite (LSCF) nanofibers have been fabricated by the electrospinning method and used as the cathode of an intermediate-temperature solid oxide fuel cell (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte. The three-dimensional nanofiber network cathode has several advantages: (i) high porosity; (ii) high percolation; (iii) continuous pathway for charge transport; (iv) good thermal stability at the operating temperature; and (v) excellent scaffold for infiltration. The fuel cell with the monolithic LSCF nanofiber cathode exhibits a power density of 0.90 W cm À2 at 1.9 A cm À2 at 750 C. The electrochemical performance of the fuel cell has been further improved by infiltration of 20 wt% of gadolinia-doped ceria (GDC) into the LSCF nanofiber cathode. The fuel cell with the LSCF-20% GDC composite cathode shows a power density of 1.07 W cm À2 at 1.9 A cm À2 at 750 C. The results obtained show that one-dimensional nanostructures such as nanofibers hold great promise as electrode materials for intermediate-temperature SOFCs.

Cobalamins are of widespread importance in biology. Both of the cofactors essential for human metabolism, the organocobalamins coenzyme B and methylcobalamin, are highly photolabile, as are other alkylcobalamins. The alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and the arylcobalamin 4-ethylphenylcobalamin (EtPhCbl) with "atypical" Co-C-bonds to unsaturated carbons, were recently designed as metabolically inert cobalamins, classified as "antivitamins B". The further development of an ideal light-activated or "conditional" antivitamin B would require it to be readily converted by light into an active B vitamin form. Very photolabile "antivitamins B" would also represent particularly useful scaffolds for therapeutic light-activated reagents. Here, the photoactive arylcobalamin EtPhCbl and the remarkably photostable alkynylcobalamin PhEtyCbl are examined using femtosecond to picosecond UV-visible transient absorption spectroscopy. PhEtyCbl undergoes internal conversion to the ground state with near unit quantum yield on a time scale < 100 ps and an activation energy of 12.6 ± 1.4 kJ/mol. The arylcobalamin EtPhCbl forms an excited state with a ca. 247 ps lifetime. This excited state branches between internal conversion to the ground state and formation of a long-lived base-off species with a quantum yield of ∼9%. Anaerobic steady state photolysis of "light-sensitive" EtPhCbl results in the formation of cob(II)alamin, but only with quantum yield <1%. Hence, our studies suggest that suitably modified arylcobalamins may be a rational basis for the design of photoresponsive "antivitamins B".

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B, cyanocobalamin (CNCbl), in the αβ-band at 550 nm and the γ-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S excited state structure within 500 fs of excitation.

Changes in cell performance with operation time were investigated for commercial cells possessing a composite cathode modified by nanostructured La 0.6 Sr 0.4 CoO 3-δ (LSCo). The electrocatalyst was introduced into a porous scaffold composed of Sm 2 O 3 -doped CeO 2 (SDC) and La 1-x Sr x Co y Fe 1-y O 3-δ (LSCF) by solution infiltration, which decreased polarization resistance (R p ) of baseline cells by 20-35%. R p of both a baseline cell and an infiltrated cell increased as the operation progressed for 1,500 h, while obeying a parabolic power law and showing 65% of total increase in the initial 150 hours. The degradation rate of the infiltrated cell was statistically indistinguishable from that of the baseline cell, implying that there was no measurable negative effect of infiltrated electrocatalysts on the cells' time-dependent performance for the systems studied here. The stability of the infiltrated cathode was affirmed by multiple samples tested for 200 hours. AC impedance spectra analysis demonstrated that degradation of cathode activity would be the critical mechanism affecting long-term stable performance for both baseline cells and infiltrated cells.