Behavior of manganite electrodes in contact with LSGM electrolyte: the nature of low electrochemical activity

The electrochemical cells with electrodes based on La_0.8Sr_0.2MnO₃ (LSM) and supporting solid electrolytes La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 (LSGM) and Ce_0.80Sm_0.20O_1.90 (SDC) were studied comparatively. Characteristics of LSM electrodes and composite electrodes comprising a mixture of LSM and electrolytes of different origins [LSGM, SDC, and Zr_0.82Sc_0.18O_1.91 (SSZ) in the mass ratio of 1:1] were analyzed. It was shown that: 1) the electrode polarization conductivity and the ohmic resistance of the cells with the LSM–LSGM composite electrodes on the LSGM and SDC electrolytes had very similar values, while they were largely different from all the other electrodes, 2) the electrochemical activity of the electrodes on the SDC electrolyte was much higher than on the LSGM electrolyte, and 3) the ohmic resistance of the cells with the SDC electrolyte corresponded to the electrolyte resistance, whereas, the ohmic resistance of the cells with the LSGM electrolyte was much larger than the electrolyte resistance. The obtained results are due to the interaction between the LSM and LSM-containing electrodes with the LSGM electrolyte during sintering, leading to the formation of a product with a very low conductivity.     

Electrochemical behavior of composite cathodes in contact with solid electrolyte La<Subscript>10</Subscript>Ge<Subscript>6</Subscript>O<Subscript>27</Subscript>

The cathodic overvoltage of composite cathodes 50 wt % La_0.8Sr_0.2MnO₃ (LSM) + 50 wt % La₁₀Ge₆O₂₇ (LGO) (further on, LSM-LGO), LSM-SSZ (Zr_0.835Sc_0.165O_2?δ), Ag-Pd-LGO, and Ag-Pd-SSZ in contact with the LGO electrolyte is measured. The temperature dependences of the polarization conductivity and the working-current densities of the same composite cathodes are investigated. The study is performed at 700–900°C. A comparison with the SSZ electrolyte is conducted. The chemical interaction in the LSM-LGO composition is studied. It is demonstrated that the interaction of lanthanum-strontium manganite with lanthanum germanate occurs with the dissolution of the initial phases in one another and with the formation of fresh phases at elevated temperatures. Coefficients of linear thermal expansion of the LGO and SSZ electrolytes and the LSM, LSM-LGO, and LSM-SSZ electrode materials are compared at 40–900°C. Most of the studied electrodes in contact with the LGO electrolyte demonstrate thermomechanical stability and high electrochemical activity.     

Improved sintering and electrical properties of La-doped CeO<Subscript>2</Subscript> buffer layer for intermediate temperature solid oxide fuel cells using doped LaGaO<Subscript>3</Subscript> film prepared by screen printing process

Effects of a sintering agent for La-doped ceria (LDC) as a buffer layer to prevent a chemical reaction between Ni in anode and Sr- and Mg-doped lanthanum gallate (LSGM) electrolyte during sintering were studied for improving sintering and electrical properties. Electrochemical performance of anode-supported solid oxide fuel cells (SOFCs) using LDC and LSGM films prepared by screen printing and co-sintering (1,350 °C) was also investigated. The prepared cell with dense LDC (ca. 17 μm) and LSGM electrolyte (ca. 60 μm) films showed an open circuit voltage close to the theoretical value of 1.10 V and a high maximum power density (0.831 W cm^–2) at 700 °C. The addition of 1 wt.% LSGM to porous LDC buffer layer was effective for improving the sintering density and electrical conductivity, resulting in the high power density due to the decreased internal resistance loss.     

Gas diffusion hindrances on Ni cermet anode in contact with Zr<Subscript>0.84</Subscript>Y<Subscript>0.16</Subscript>O<Subscript>1.92</Subscript> solid electrolyte

The electrochemical behavior of Ni cermet electrode with CeO_{2 ? x} additive in contact with YSZ electrode was studied by means of impedance spectroscopy in H₂, H₂O, CO₂, CO, He, and Ar gas media of various composition within the temperature range of 700 to 950°C. Near the equilibrium potential, the electrochemical impedance spectra of the studied electrodes indicate to three stages of electrode reaction. The polarization conductivity of the low-frequency stage of electrode reaction (σ_lf) is characterized with the following regularities: (a) temperature dependence of σ_lf has a positive slope in Arrhenius coordinates; (b) σ_lf increases upon replacement of gas mixture with lower mutual diffusion coefficient by mixture with higher mutual diffusion coefficient, while polarization conductivity values of other stages remain practically invariable; (c) concentration relationships of 1/σ_lfrecorded for constant activity of oxygen in the gas phase are linear in the 1/σ_lf vs. 1/P _{CO 2} (P _CO) coordinates; (d) no low-frequency stage of the electrode reaction is observed upon electrochemical inflow (outflow) of the gas reagents (reaction products) to (from) the test electrodes (current passing through closely pressed specimens and central specimen impedance measurement); and (e) no change in the gas flow rate affects σ_lf value. The observed regularities were explained by assuming the gas diffusion nature of the low-frequency stage of the electrode reaction. The gas diffusion layer thickness was estimated.     

Al-doped La<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> as oxide anode for solid oxide fuel cells using dry C<Subscript>3</Subscript>H<Subscript>8</Subscript> fuel

Direct hydrocarbon type solid oxide fuel cells are attractive from simple gas feed process and also high energy conversion efficiency. In this study, La_0.5Sr_0.5MnO₃ (LSM55) perovskite oxide was studied as oxide anode for direct hydrocarbon type solid oxide fuel cell (SOFC). Although reasonable power density like 1 W/cm² and open circuit voltage (OCV) (1.1 V) at 1273 K was exhibited when H₂ was used as fuel, the power density as well as OCV of the cell using LSM55 for anode was significantly decreased when dry C₃H₈ was used for fuel. After power generation measurement, LSM55 phase was decomposed to MnO and La₂MnO₄. Effects of various dopants to Mn site in LSM55 were studied and it was found that partial substitution of Mn in LSM55 with other cation, especially transition metal, is effective for increasing maximum power density. In particular, reasonable high power density can be achieved on the cell using Ni-doped LSM55 for anode. On the other hand, Al substitution is effective for increasing stability against reduction and so, dopant effects of Al were studied in more details for dry C₃H₈ fuel. The power density as well as OCV increased with increasing Al content and the highest power density was achieved at x = 0.4 in La_0.5Sr_0.5Mn_1 ? x Al _x O₃. Among the examined composition, it was found that the cell using La_0.5Sr_0.5Mn_0.6Al_0.4O₃ anode shows the largest power density (0.2 W/cm²) at 1173 K and high OCV (1.01 V) against dry C₃H₈ fuel.     

Oxygen evolution on Ti/Co<Subscript>3</Subscript>O<Subscript>4</Subscript>-coated electrodes in alkaline solution

The electrochemical performances of Co₃O₄ nanopowders, obtained by the sol-gel method, were investigated and compared with those of commercial Co₃O₄ powders, for oxygen evolution reaction in alkaline solution. The active oxide powder was mixed with teflon and assembled on Ti substrate to form thin catalyst film. Cyclic voltammetry, polarization curves, and electrochemical impedance spectroscopy were used to assess the mechanism of oxygen evolution reaction, chemical structure, and morphology of the catalyst. Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May 2006.     

Options for adjustment of microstructure and conductivity of cathodic substrates of La(Sr)MnO<Subscript>3</Subscript>

The electrodes of solid-oxide fuel cells (SOFCs) must be characterized by high conductivity to decrease ohmic losses and sufficient porosity to provide high gas diffusion rate. In the cases, when the SOFC electrodes are substrates, they must be synthesized at the temperature above the temperature of formation of their solid-electrolyte coating. Herewith, manufacturing of supporting electrodes with the required micro-structure is rather complicated. The present paper studies the effect of the method of manufacturing of the initial La_0.6Sr_0.4MnO₃ (LSM) powders, their degree of dispersion, introduction of sintering additives and pore agents on their microstructure, conductivity, and possibility of adjusting the temperature of SOFC cathodic substrate formation at which the required characteristics are reached. It is shown that sintering of cathodic substrates to the relative density of 65–70% can be carried out at the temperatures from 1050 to 1350–1400°C, which would allow obtaining electrolyte films of powders with different sintering ability on such substrates. The average pore size in cathodic substrates can be varied in the range of 0.4 to 2.5 μm by using the initial LSM powder with different dispersion degree and by employing graphite as a pore agent. At 900°C, conductivity of cathodic substrates of LSM grows at an increase in their relative density from 50% to 70% approximately from 50 to 100 S/cm and weakly depends on the dispersion degree of the initial powders.     

Effect of polarization on the electrode behavior and microstructure of (La,Sr)MnO<Subscript>3</Subscript> electrodes of solid oxide fuel cells

The electrode behavior and microstructure of freshly prepared (La_0.8Sr_0.2)_0.9MnO₃ (LSM) electrodes were investigated under various polarization conditions. The original, large agglomerates in freshly prepared LSM electrodes were broken down into sphere-shaped grains when exposed to cathodic or anodic current passage of 200 mA cm^–2 at 800 °C in air for 3 h. Microstructural changes under cathodic polarization could be related to the pronounced diffusion and migration of oxygen vacancies and Mn ions on the LSM surface and lattice expansion, while lattice shrinkage under oxidation conditions most likely contributes to the structural changes under anodic polarization. Such morphological changes were irreversible and were found to be beneficial to the performance of freshly prepared LSM electrodes. Freshly prepared LSM electrodes behaved very differently with respect to the cathodic and anodic current passage treatment.     

Sintering behavior and ionic conductivity of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> with a small amount of MnO<Subscript>2</Subscript> doping

A 20% GdO_1.5 doped ceria solid solution with a small amount of MnO₂ doping (≤5% molar ratio) was prepared via the mixed oxide method from high-purity commercial powders with grain size around 0.2–0.5 μm. X-ray diffraction analysis indicated that all the samples exhibited the fluorite structure, and no new phase was found. The data from dilatometeric measurements and scanning electron microscopy observations revealed that 1% Mn doping reduced the sintering temperature by over 150 °C, and enhanced the densification and grain growth. Mn doping has little effect on grain interior conductivity, but a marked deterioration in grain boundary behavior is observed. This leads to a lower total conductivity in comparison with the undoped Ce_0.8Gd_0.2O_2–δ. Therefore, for solid oxide fuel cells (SOFCs) with Mn-containing compounds as electrodes, optimization of electrode fabrication conditions is needed to prevent the formation of a lower conductivity layer at the electrode/electrolyte interface since Mn will diffuse from the electrode side to the electrolyte during fabrication and operation of SOFCs. Electronic Publication     

Electrochemical Methods for Studying Transport Properties and the Type of Ionic Conduction in Solid Electrolytes Based on CaPr<Subscript>2</Subscript>S<Subscript>4</Subscript> and Ca(Ba)Sm<Subscript>2</Subscript>S<Subscript>4</Subscript>

The theoretical feasibility of sulfide ion transport in phases based on MeLn₂S₄ (Me = Ca, Ba; Ln = Sm, Pr) is considered. Regions of solid solutions on the basis of ternary compounds are determined. A systematic investigation of phases is performed with the application of a variety of electrochemical methods, specifically, the conductimetry, emf in chemical concentration cells with and without transport, and Tubandt methods.__________Translated from Elektrokhimiya, Vol. 41, No. 6, 2005, pp. 707–713.Original Russian Text Copyright © 2005 by Ushakova, Kalinina, Fominykh, Yurlov, Murin.Published on the basis of a report delivered at the VII Meeting on Fundamental Problems in Solid-State Ionics (Chernogolovka-2004).     

Development of nano IrO<Subscript>2</Subscript> composite-reinforced nickel–phosphorous electrodes for hydrogen evolution reaction

Electroless and electroplated nickel electrodes are extensively used for hydrogen evolution reaction (HER). In the present work, TiO₂-supported IrO₂ mixed oxide composite was prepared and used to reinforce Ni–P electroless plates to be used as catalytic electrodes for HER. The electrodes exhibited high electrocatalytic activity when the electrodes were used for HER. All the parameters including particle size of the catalyst, surface roughness, and surface active sites were studied. The particle size of the IrO₂ catalyst in the mixed oxide was found to have high influence on the catalytic activity of the electrodes. Low overpotential as low as 70 mV at a current density of 200 mA cm⁻² was achieved with the mixed oxide-reinforced Ni–P electrodes.     

Electrochemical dissolution of Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> in acid solutions

The present study deals with the electrochemical reductive dissolution of Mn₃O₄, which was added to carbon-paste electroactive electrodes (CPEEs) in acid solutions. It was found that in the experimental conditions the thermodynamically stable form of manganese was . Kinetic features of the electrochemical reductive dissolution of Mn₃O₄, which was realized under potential cycling conditions (+1.0 V→−0.7 V→+1.0 V), were determined by the electrode polarization direction. It was shown that the cathodic reduction of Mn₃O₄ was accomplished in three stages. Manganese was dissolved in the supporting solution only at the third stage. The first two stages involved solid-phase reactions. The anodic cycling stage included an active dissolution of Mn₃O₄ and the lower manganese oxide (MnO) accumulated on the electrode surface during the cathodic reduction.     

Comparative Study of the Oxygen Storage Capacity of Pt/M<Subscript>x</Subscript>O<Subscript>y</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Systems

The oxygen storage capacity of 1% Pt/15% M_xO_y/Al₂O₃ systems containing a rare-earth or an alkaline-earth metal oxide or TiO₂ as the oxygen-storing component was studied. Oxygen storage capacity was evaluated as the amount of C₃H₈ reacting at 400°C with oxygen that was taken up by the catalyst during oxidative treatment. The systems containing a rare-earth metal oxide or TiO₂ possess the highest oxygen storage capacity among the catalysts examined (80 and 75 µmol C₃H₈/g Cat, respectively). Of the BaO and SrO systems, the latter is of interest, although its oxygen storage capacity (∼27 µmol C₃H₈/g Cat) is somewhat lower than the oxygen storage capacity of any rare-earth metal oxide or the TiO₂ system.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 585–589.Original Russian Text Copyright © 2005 by Sinel’nikov, Tolkachev, Stakheev.     

Influence of pressure on the electrical and electrochemical behaviour of high-temperature steam electrolyser La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Co<Subscript>0.2</Subscript>Fe<Subscript>0.8</Subscript>O<Subscript>3</Subscript> anode

This paper is dedicated to the impact of pressure on the electrochemical behaviour of LSCF (La_1-xSr_xCo_yFe_1-yO_3-δ) anode in high-temperature electrolysers. This study was carried out with symmetrical cells associating LSCF electrodes to a 3YSZ (yttrium-stabilised zirconia) electrolyte. Impedance spectroscopy measurements were performed using a three-electrode configuration, at temperature as high as 700 to 800 °C, in a pressure range from 1 to 30 bar. A clear improvement in terms of electrode resistance decrease is highlighted, mainly due to faster oxygen adsorption/desorption kinetics and a better supply of gas to electrochemical reaction sites. Other assumptions were considered and analysed, such as the impact of pressure on LSCF electrical conductivity and on the mechanical contacts. Thus, three contributions were determined as limiting steps at low pressure, up to 5 bar, whilst for higher pressure, the optimised conditions in operation are reached. This study completes a previous one related to a modelling approach.     

Electrochemical behavior of europium perovskites (Ca<Subscript>0.6</Subscript>Eu<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>) in alkaline aqueous media

In this paper, the electrochemical behavior of europium perovskites (Ca_0.6Eu_0.4MnO₃) prepared by a gel combustion method followed by a thermal treatment performed at 1073 and 1473 K is compared with the pristine oxide CaMnO₃ (T = 1073 K) obtained by the same method. The experiments were performed in alkaline aqueous media via open-circuit potential, cyclic voltammetry, chronopotentiometry, and impedance spectroscopy. The data show that the electrode roughness is inversely proportional to the oxide particle size. The chronopotentiometric curves show also that the presence of europium is advantageous due to the increase of the electrode roughness for the oxides formed at the same temperature (1073 K) or by increasing the charge per unit area for the oxide formed at 1473 K. The impedance spectra, which were obtained in the capacitive behavior domain, reflect the porous morphology of the electrode surfaces according to the theory developed by Levie. This feature is particularly evident for the electrodes with an average particle diameter of 90 and 60 nm, corresponding respectively to the oxides CaMnO₃ and Ca_0.6Eu_0.4MnO₃, both formed at 1073 K.     

Synthesis and electrochemical behavior of Fe-doped overstoichiometric LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>-based spinels

Ternary oxides of Li-Fe-Mn-O system with overstoichiometric spinel structure Li_1+xMn_1-yFe_yO₄, have been synthesized. The effect of composition and synthesis temperature on the electrochemical performance of the oxides as a material of positive electrodes of Li-ion batteries has been studied. The optimal temperature of synthesis was found to be 850 °C for the composition Li_1.2Mn_1.6Fe_0.4O₄. This oxide shows good reversibility at acceptable level of capacity near 105 mAh/g.Presented at the 3rd International Meeting "Advanced Batteries and Accumulators", June 16th–20th 2002, Brno, Czech Republic     

Structure and electrical properties of Pb(Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> thin films on LaNiO<Subscript>3</Subscript>—Coated thermally oxidized Si substrates

Pb(Zr_0.25Ti_0.75)O₃ (PZT25) thin films were prepared on LaNiO₃-coated thermally oxidized silicon substrates by chemical solution deposition method, where LaNiO₃ electrodes were also prepared by a chemical solution deposition technique. The dielectric constant and dielectric loss of the PZT25 thin films were 570 and 0.057, respectively. The remanent polarization and coercive field were 20.11 μC/cm² and 60.7 kV/cm, respectively. The PZT25 thin films on LaNiO₃-coated thermally oxidized silicon substrates showed improved fatigue characteristics compared with their counterparts on plantium-coated silicon substrates.     

Microstructure and Conduction of Composites Bi<Subscript>2</Subscript>CuO<Subscript>4</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> Near the Eutectic Melting Point

Microstructure and conduction of ceramic composites Bi₂CuO₄ + xBi₂O₃ (x = 5, 10, 15, 20 wt %) near the eutectic melting point (770°C) are studied. Bismuth oxide, initially randomly distributed over the ceramics bulk, after quenching from temperatures exceeding the eutectic melting point, becomes localized at triple junctions and grain boundaries in Bi₂CuO₄, which is caused by wetting grain boundaries and forming a liquid-channel structure. The jumpwise change in the composites’ conductivity near 730 and 770°C caused by polymorphic transformation of Bi₂O₃ and the eutectic melting with simultaneous formation of a liquid-channel structure. Transport numbers of the oxygen ion are measured at 770°C by coulomb-volumetric method. The conduction by oxygen ions increases in the composites with decreasing average size of Bi₂CuO₄ crystallites.__________Translated from Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 596–601.Original Russian Text Copyright © 2005 by Lyskov, Metlin, Belousov, Tret’yakov.     

LaNi<Subscript>0.5</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>-based sensor for sensitive determination of paracetamol

A novel electrochemical sensor based on LaNi_0.5Ti_0.5O₃/CoFe₂O₄ nanoparticle-modified electrode (LNT–CFO/GCE) for sensitive determination of paracetamol (PAR) was presented. Experimental conditions such as the concentration of LNT–CFO, pH value, and applied potential were investigated. Under the optimum conditions, the electrochemical performances of LNT–CFO/GCE have been researched on the oxidation of PAR. The electrochemical behaviors of PAR on LNT–CFO/GCE were investigated by cyclic voltammetry. The results showed that LNT–CFO/GCE exhibited excellent promotion to the oxidation of PAR. The over-potential of PAR decreased significantly on the modified electrode compared with that on bare GCE. Furthermore, the sensor exhibits good reproducibility, stability, and selectivity in PAR determination. Linear response was obtained in the range of 0.5 to 901 μM with a detection limit of 0.19 μM for PAR.     

New organic metal β-(DOET)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> · H<Subscript>2</Subscript>O: Synthesis and crystal structure

An organic metal β-(DOET)₂FSO₃ · H₂O based on a novel donor DOET (DOET is (1,4-dioxane-2,3-diyldithio)ethylenedithiotetrathiafulvalene) was synthesized and structurally characterized. Single crystals were prepared by electrochemical oxidation of DOET in the d.c. mode. The salt has a layered structure with the DOET^1/2+ radical cation layers parallel to the ab plane. The FSO ₃ ⁻ anion is equiprobably disordered over two positions relative to the center of symmetry (1/2, 0, 0). The temperature behavior of conductivity is characteristic of metals in the temperature range from 293 down to 96 K; below 96 K, an increase in resistance is observed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2387–2389, October, 2005.