Priit Möller

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).

Influence of Surface Charge Density on the Electrochemically Derived Surface Roughness of Bi Electrodes

Electrical double-layer properties of variously treated Bi(001) and Bi(111) electrodes were studied in terms of the Debye length dependent roughness theory (i.e., nonlinear Poisson-Boltzmann theory), recently developed by Daikhin et al. The geometrical roughness factor, root mean square (rms) departure of the surface from flatness, the medium lateral correlation length values have been obtained. According to the experimental data and results of theoretical calculations, the surface roughness increases in the order of Bi electrodes: electrochemically polished single-crystal Bi(111) plane < cut at the temperature of liquid nitrogen Bi(111) plane < electrochemically etched Bi(111) plane < chemically etched Bi(111) plane < chemically etched Bi(001) plane. This order of the electrode surface roughness is in agreement with the ex situ AFM data, i.e., with the rms roughness R ms values. At small electrolyte concentrations (c el ≤ 0.01 M), the deviation of experimental roughness function vs. inverse Debye length curves from the theoretically calculated ones has been observed, which is explained by the influence of energetic inhomogeneity of the polycrystalline surfaces, i.e., the nonequal surface charge density values at the various homogeneous regions exposed at the macropolycrystalline electrode surface, on the interfacial capacitance values.

The electrochemical activity of two binary alloy catalysts toward oxygen reduction reaction in 0.1 M KOH

The platinum group metals (Pt, Ir and Ru) and the carbide-derived carbon support with the very high specific surface area were used to synthesise the low noble metal loading Pt-C, IrPt-C and RuPt-C alloy catalysts. The alloying of the platinum group metals in the studied catalysts was proved by the several independent physical characterization methods like: the X-ray diffraction, time of flight secondary ion mass-spectrometry, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy. The electrocatalytic activity toward oxygen reduction reaction of the synthesised catalysts in an alkaline solution was studied and compared with the commercially available Pt-Vulcan. The combined and detail approach using the transmission electron microscopy and inductively coupled plasma mass spectrometry for estimation of the surface area of metal particles is provided. The noticeably higher calculated mass corrected and specific kinetic current density values for Pt-C catalyst were established. For IrPt-C and RuPt-C alloy catalysts, mass corrected current density values are comparable with the commercial Pt-Vulcan. The specific kinetic current density values increase in the following sequence: RuPt-C < IrPt-C < Pt-Vulcan < Pt-C.

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.