Single-phase materials with mixed ionic and electronic conductivity underpin multiple solid-state electrochemical devices as promising electrodes. In particular, triple-conducting oxides that carry protons, oxygen ions, and electron holes simultaneously have...

In this work, electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis were used to evaluate the performance determining parameters for a solid-oxide fuel and electrolysis cell. The SOFC/EC was formulated with 8 mol% Yttria-stabilized zirconia (YSZ) electrolyte sandwiched between Ni-YSZ fuel electrode and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 -10mol% gadolinium doped ceria (LSCF6428-GDC10, 5:5) composite air electrode. To deconvolute the multistep elementary reactions at both of the electrodes, EIS was measured as a function of temperature (775 °C-850 °C), gas compositions, and gas flow rates (50-200 sccm). Concurrently, DRT analysis was employed to resolve frequency-dependent electrode reactions effectively. From the DRT analysis, multistep elementary reactions at fuel and air electrode were separated and corresponded to a transmission line equivalent circuit model with parameters (R , s el , . R and C p el p el , ., . ). Finally, the cell performance limiting factors such as apparent diffusivity (D app ~), surface reaction constant (k), and effective lengths (L eff ) of each electrode were determined.

In this work, relationship between defect equilibrium and thermodynamic quantities is investigated to elucidate mass and charge transport properties of La 0.5 Sr 0.5 FeO 3-δ (LSF55). The oxygen nonstoichiometry (δ) was measured as a function of oxygen partial pressure in 10 −19 ≤ (P O 2 /at m)≤ 0.21 and 750 ≤ (T/°C) ≤ 900 range. The δ − P O 2 − T relationships indicated an electronic n-p transition point at δ = 0.25 which moved to a higher P O 2 value with increasing temperature. The relative partial molar enthalpy ( H O ) and entropy ( S O ) of oxygen indicated that across the electronic stoichiometric point H O stabilized around −86.9 ± 3.6 kJ/mol in p-type and −402.2 ± 12.6 kJ/mol in n-type regimes, whereas S O values were kept changing because of the contribution from configuration entropy (S O(Con f ) ). From the DC 4-probe conductivity and electrical conductivity relaxation (ECR) measurement, the oxygen-ion conductivity at 900°C was 0.29 S.cm −1 with activation energy of 0.72 ± 0.04 eV and the oxygen self-diffusivity (D O ) for LSF55 with δ = 0.114 during oxidation increased from (8.27 ± 0.05) × 10 −8 cm 2 s −1 at 800°C to (2.55 ± 0.01) × 10 −7 cm 2 s −1 at 900°C with an activation energy of 1.22 ± 0.14 eV. Dilatometry measurements indicated an isothermal chemical expansion as function of P O 2 , which was explained on the basis of the relative change in mean ionic radius of transition metal cation Fe against δ. This model showed that transition metal cation existed as a mixture of high-spin and low-spin states and made a transition from low-spin to high-spin state with increasing δ.

An electrochemical NO 2 sensor is fabricated using yttria-stabilized zirconia as electrolyte and SnO 2 -NiO nanocomposites as sensing electrodes. The SnO 2 -NiO nanocomposites with varying molar ratio (Sn:Ni = 1:3, 1:1, 3:1) were synthesized by a citric acidassisted hydrothermal route and sintered over a YSZ-electrolyte to form the sensing electrode of the sensor. The phase structure and morphology of the electrodes were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) techniques. The NO 2 sensing performance of the fabricated sensor was measured at 700°C with varying NO 2 and O 2 concentrations. Furthermore, the electrochemical polarization curves were studied for the analysis of the sensing mechanism. It was found that the sensing response ( V), sensitivity, response/recovery dynamics as well as cross-sensitivity of these sensors were greatly dependent over the cationic molar ratio (Sn: Ni) of the nanocomposite sensing electrode. Moreover, the SnO 2 -NiO nanocomposites displayed high sensitivity, high stability and low cross-sensitivity; projecting it as a potential sensing electrode material for the electrochemical NO 2 sensors.

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