Gunnar Nurk

Optimization of the Cathode Composition for the Intermediate-Temperature SOFC

Electrochemical impedance, cyclic voltammetry, and chronoamperometry have been used for characterization of the medium-temperature half-cells Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖La 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 1), Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖Pr 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 2), and Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖Gd 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 3) at various electrode potentials and temperatures. Analysis of the impedance data shows that the kinetically mixed process is probable, characterized by the slow electron transfer to an adsorbed and thereafter dissociated oxygen atom O a d s , as well as by slow mass transfer of electroactive species inside the cathode or O a d s at the internal surface of the porous cathode. The total polarization resistance increases with rising the atom mass of the A-site cation in the porous perovskite structure. The values of activation energy decrease slightly with increasing negative polarization and in the order of half-cells Sys 3 > Sys 2 > Sys 1. The transfer coefficient α c over 0.5 indicates the deviation of the mainly charge-transfer-limited process toward the mass-transfer-limited process in the porous cathode with decreasing temperature. The electrochemical characteristics of half-cells Sys 2 and Sys 1 are stable under repetitive potential and thermocycling during long operation times (t > 1200 h for Sys 2 and t > 4800 h for Sys 1).

Adsorption kinetics of uracil on bismuth single crystal planes

The electrochemical impedance method has been used for the quantitative study of uracil adsorption kinetics at the bismuth single crystal plane j aqueous Na2SO4 solution interface. Analysis of impedance data demonstrates that the adsorption process of uracil is complicated and limited by the mixed kinetics, i.e. by the slow adsorption and diffusion steps. Analysis of the Cole � /Cole and other dependences indicates that, at small frequencies and higher uracil concentrations, two-dimensional association of the uracil molecules is possible. Nonlinear regression analysis has been used for fitting the experimental complex plane (Z ƒ, Z ?) plots. The classical Frumkin � /Melik-Gaikazyan circuit in the region of higher ac frequencies and lower uracil concentrations (curacil 5/2 � /10 � 3 M), and the modified Frumkin � /Melik-Gaikazyan equivalent circuit (taking into account the inhomogeneous semi-infinite diffusion) at curacil ]/2 � /10 � 3 M fit with a reasonable accuracy the experimental Z ƒ, Z ?-plots. It was found that the values of the double layer capacitance, partial charge transfer resistance, adsorption capacitance and Warburg-like diffusion impedance depend noticeably on the electrode potential and concentration of uracil. Other more complicated equivalent circuits have been used for fitting the experimental impedance data. # 2003 Elsevier B.V. All rights reserved.

Melt-electrospinning as a method to improve the dissolution and physical stability of a poorly water-soluble drug

&NA; The present study introduces a modified melt‐electrospinning (MES) method for fabricating the melt‐electrospun fibers (MSFs) of a poorly water‐soluble drug and carrier polymer. The MES of poorly water‐soluble model drug indomethacin (IND) and hydrophilic carrier polymer, Soluplus® (SOL) were prepared at a 1:3 drug‐polymer weight ratio. Water was used as an external plasticizer to regulate a MES processing temperature and to improve fiber formation. The fiber size, surface morphology, physical solid state, drug‐polymer (carrier) interactions, thermal and chemical stability and dissolution behavior of MSFs were investigated. Solid state nuclear magnetic resonance spectroscopy (NMR) was used to measure T1(1H), and the domain size of IND in MSFs (25–100 nm) was calculated from these results. Solid‐state and thermal analysis confirmed the presence of amorphous solid dispersions of IND and SOL. IND was found to be chemically stable during an entire MES process. Only small drug content variability of different MSF batches was detected with high performace liquid chromatography (HPLC). Given findings were verified with the liquid NMR spectroscopy. The dissolution of MSFs was significantly faster than that of physical mixtures (PMs) or pure drug. The enhanced dissolution of MSFs was caused by high surface area, amorphous state of the drug and solubilizing properties of the carrier polymer (SOL). Graphical abstract Figure. No caption available.

Influence of humidified synthetic air feeding conditions on the stoichiometry of (La1-xSrx)yCoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes under applied potential measured by electrochemical in situ high-temperature XRD method

The solid oxide fuel cell symmetrical cells with porous (La1-xSrx)yCoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ electrodes for intermediate temperature applications have been studied under electrochemical polarization and synthetic air + H2O vapor (so called moisturized cathode gas) feeding conditions using high-temperature electrochemical in situ X-ray diffraction method, chronoamperometry, cyclic voltammetry, and impedance spectroscopy methods. Changes in the lattice parameters and electrochemical activity of La0.6Sr0.4CoO3−δ, (La0.6Sr0.4)1.01CoO3−δ, (La0.6Sr0.4)0.99CoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ were calculated depending on the temperature (T), electrode potential (E), and oxygen partial pressure (pO2) applied. Influence of H2O vapor in synthetic air on (La1-xSrx)yCoO3−δ parameters was irreversible and continued expansion of (La1-xSrx)yCoO3−δ lattice observed in H2O vapor + synthetic air feeding conditions has been observed continuously after changing wet cathode gas to dry oxygen. Nearly, reversible behavior of La0.6Sr0.4Co0.2Fe0.8O3−δ lattice has been established and for La0.6Sr0.4Co0.2Fe0.8O3−δ, the cell volume and polarization resistance started to decrease after changing humidified synthetic air to dry synthetic air in the cathode compartment. For the slightly cationic deficient (La0.6Sr0.4)0.99CoO3−δ, the cathode structure is more stable and the electroreduction of the oxygen was faster. Detailed comparison of experimental data demonstrates that the dependence of the crystallographic parameters on the electrode potential and temperature applied is in a good agreement with the electrochemical impedance spectroscopy data, indicating that the electrocatalytic activity of the cathode decreases with the rise of Fe ion concentration in B-position of LSCF cathode.