Investigations on Pr1.6Sr0.4NiO4-YSZ-Ag composite cathode for solid oxide fuel cells

Ag-network was successfully deposited by VA-EP (vacuum assisted electroless plating) method on Pr_1.6Sr_0.4NiO₄-YSZ cathode to form (1−x) wt% Pr_1.6Sr_0.4NiO₄−x wt% YSZ-Ag (x=0, 10, 20, 30, 40) (abbr. PYx-Ag) composite cathode. XRD results suggested that there was a good chemical stability between Pr_1.6Sr_0.4NiO₄ and YSZ at temperatures below 1050 °C. PY20-Ag cathode exhibited higher exchange current density, lower overpotential and ASR (Area Specific Resistance) than PY20 cathode. At 650 °C, the ASR of PY20-Ag cathode was 2.5 Ω cm², which was only about 42% of that of PY20, 5.9 Ω cm². PY20-Ag can be a promising candidate for SOFC cathode.     

NiO+YSZ anode substrate for screen-printing fabrication of YSZ electrolyte film in solid oxide fuel cell

Anode substrate has a great effect on screen-printing fabrication of yttria-stabilized zirconia (YSZ) electrolyte film and cell performance. In this work, NiO+YSZ anode substrate was prepared by a conventional ceramic sintering method, on which dense YSZ electrolyte film was successfully fabricated by screen-printing method. Microstructure of the anode substrate and cell performance were investigated. The optimal amount of addition of starch to the anode substrate was 20 wt%. The optimal temperature for pre-sintering of NiO powder was 800 °C. A single cell with the NiO powder pre-sintered at 800 °C exhibited the highest power density of 0.95 W cm⁻² at 700 °C.     

Highly dispersed anodes for solid oxide fuel cells using NiO/YSZ/BZY triple-phase composite powders prepared by spray pyrolysis

NiO/Y₂O₃-stabilized ZrO₂ (YSZ)/Y-doped BaZrO₃ (BZY) triple-phase composite powders were prepared by spray pyrolysis and evaluated for Ni/YSZ/BZY cermet anodes, which are considered effective for dry CH₄ operation in solid oxide fuel cells. The structure of the particles in these powders was fine crystal fragments, and the individual material phases were clearly separated and highly dispersed within the particles. The Ni/YSZ/BZY cermet anodes fabricated with these composite powders maintained a fine electrode microstructure equivalent to that in a simple Ni/YSZ cermet anode manufactured using a composite powder prepared by spray pyrolysis. Furthermore, the addition of BZY improved the anode performance in humidified H₂ and dry CH₄ operation.     

Novel layered perovskite GdBaCuFeO5+x as a potential cathode for proton-conducting solid oxide fuel cells

A layered perovskite GdBaCuFeO_5+x (GBCuF) was developed as a cathode material for intermediate-temperature solid oxide fuel cells based on a proton-conducting electrolyte of stable BaZr_0.1Ce_0.7Y_0.2O_3?δ (BZCY). The X-ray diffraction results showed that GBCuF was chemically compatible with BZCY after co-fired at 1,000 °C for 10 h. The thermal expansion coefficient of GBCuF, which showed a reasonably reduced value (15.1?×?10^?6 K^?1), was much closer to that of BZCY than the cobalt-containing conductor. The button cells of Ni–BZCY/BZCY/GBCuF were fabricated and tested from 500 to 700 °C with humidified H₂ (～3 % H₂O) as a fuel and ambient oxygen as the oxidant. A high open-circuit potential of 1.04 V, maximum power density of 414 mW cm^?2, and a low electrode polarization resistance of 0.21 Ω cm² were achieved at 700 °C, with calculated activation energy (E _a) of 128 kJ mol^?1 for the GBCuF cathode. The experimental results indicated that the layered perovskite GBCuF is a good candidate for cathode material.     

Performance characteristics of composite film electrolytes for intermediate-temperature solid oxide fuel cells

This paper reports on the estimated performance of a cell with a three-layer electrolyte, consisting of one gadolinia-doped ceria (GDC) layer, one yttria-stabilised zirconia (YSZ) electronblocking layer and one CGO-YSZ solid solution interlayer, the latter being used to avoid solid-state reaction and interdiffusion between YSZ and GDC, in comparison to a cell with a double-layer YSZ-CGO composite electrolyte. For a constant temperature and overall cell oxygen potential as boundary conditions, the open circuit voltage, the voltage under operating conditions and the oxygen potential profile inside the electrolyte are related to the ionic and electronic transport properties of the materials involved and are calculated as a function of the thickness of the layers involved and the relative positions of the YSZ and GDC layers. Thermodynamic stability of the electrolyte is shown to depend upon the transport properties of the materials and primarily the electronic conductivity of the air-side layers. To determine the particular ionic and electronic contributions for conduction of the materials involved, conductivity was measured as a function of the oxygen partial pressure and temperature, using the standard four point d.c. method. Based on the calculations, performed the conditions are discussed under which a functionally graded composite electrolyte YSZ-CGO can be effective for intermediate-temperature solid oxide fuel cells (SOFCs). Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999. TMR Grant Holder     

Improvement of plasma-sprayed YSZ electrolytes for solid oxide fuel cells by alumina addition

Atmospheric plasma spray is a fast and economical process for deposition of yttria-stabilized zirconia (YSZ) electrolyte for solid oxide fuel cells. YSZ powders have been used to prepare plasma-sprayed thin ceramic films on the metallic substrate employing plasma spray technology at atmospheric pressure. Alumina doping was employed to improve the structural characteristics and electrical properties of YSZ. The effect of alumina addition from 1 to 5 wt.% on the properties of plasma-sprayed YSZ films was investigated. It was found that the gas permeability of the Al-doped YSZ electrolyte layer reached a level of 8.6 × 10⁻⁷ cm⁴ gf⁻¹ s⁻¹, which is a necessary value for the practical operation of solid oxide fuel cells. Alumina doping considerably increased the ionic conductivity of plasma-sprayed YSZ. The open circuit voltage of the alumina-doped YSZ coating was approximately equal to the theoretical value for dense YSZ material.     

Performance of a double-layer CGO/YSZ electrolyte for solid oxide fuel cells

The use of a double-layer ceria-gadolinia (CGO) - yttria-stabilized zirconia (YSZ) electrolyte has been suggested as an alternative for efficient intermediate temperature operation of Solid Oxide Fuel Cells (SOFC). CGO offers the advantage of high ionic conductivity and good chemical compatibility with Co-containing cathode perovskite materials, while YSZ serves as an electron blocking layer. The main problem for the applicability of such a composite film still remains the formation of a poorly conductive solid solution phase at the CGO/YSZ interface. The microstructure and the elemental distribution of this solid solution phase were examined with the aid of electronic probe microanalysis. Powders with the same composition were synthesized in order to examine their crystal structure and electrical properties, with the objective to propose a suitable gradation at the interface in order to improve the feasibility of CGO/YSZ two- layer composite electrolyte films. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

Performance and durability of Ni-coated YSZ anodes for intermediate temperature solid oxide fuel cells

NiO-coated YSZ composite powders were synthesized through the Pechini process in order to improve the performance and durability of SOFC anodes. Their microstructures and electrical properties have been investigated with thermal and redox cycling tests. The coverage of NiO crystals on the YSZ surface could be modulated by controlling the composition of the reaction mixture and the ratio of NiO and YSZ. Ni–YSZ electrodes were manufactured by sintering the die-pressed NiO–YSZ pellet at 1400 °C for 3 h, followed by reducing it to 800 °C under hydrogen atmosphere. The anode made from NiO/YSZ composite powder, which has a high homogeneity and plenty of contact sites between Ni and YSZ, has an excellent tolerance against thermal and redox cycling. The maximum power density of a single cell made from NiO/YSZ composite powder was 0.56 W cm^− 2 at 800 °C in reactive gases of humidified hydrogen and air. It can be concluded that the functional NiO/YSZ composite powder will suppress the degradation of anodes and enhance the long-term and redox stability of the unit cell at elevated temperatures.     

(La0.75Sr0.25)(Cr0.5Mn0.5)O3/YSZ composite anodes for methane oxidation reaction in solid oxide fuel cells

The synthesis and performance of (La_0.75Sr_0.25)(Cr_0.5Mn_0.5)O₃/Y₂O₃–ZrO₂ (LSCM/YSZ) composites are investigated as alternative anodes for the direct utilization of methane (i.e., natural gas) in solid oxide fuel cells. Addition of YSZ phase greatly improves the adhesion and reduces the electrode polarization resistance of the LSCM/YSZ composite anodes. LSCM/YSZ composite anodes show reasonably good performance for the methane oxidation reaction in wet CH₄ and the best electrode performance was achieved for the composite with LSCM contents of 50–60 wt.% with polarization resistances of 2–3 Ω cm² in 97% CH₄/3% H₂O at 850 °C. The electrode impedance for the methane oxidation in wet CH₄ on the LSCM/YSZ composite anodes was characterized by three separable arcs and the electrode behavior could be explained based on the ALS model for the reaction on the MIEC electrode. The results indicate that electrocatalytic activity of the LSCM/YSZ composite anodes for the methane oxidation is likely limited by the oxygen vacancy diffusion in the substituted lanthanum chromite-based materials.     

Electrochemical reaction of 5 V cathode LiNi0.4Mn1.6O4

LiNi_0.4Mn_1.6O₄ was prepared under air and oxygen atmospheres using fine Mn₃O₄ particle and large MnO₂ particle at various temperatures. The sample prepared from Mn₃O₄ at 750 °C under air or oxygen atmosphere exhibited an ideal electrochemical behavior, which was based on three redox couples of Mn³⁺/Mn⁴⁺, Ni²⁺/Ni³⁺, and Ni³⁺/Ni⁴⁺. On the other hand, the sample prepared from MnO₂ had a larger capacity at 4 V plateau and smaller one at 5 V plateau. However, when using oxygen atmosphere, the sample exhibited more ideal behavior, which is similar to the sample prepared from Mn₃O₄. This means that some defects exist in LiNi_0.4Mn_1.6O₄, depending on preparation conditions. In order to confirm this point, the chemical composition and the valence state of Mn of the prepared sample was analyzed. From these results, it can be said that electrochemical reactions of LiNi_0.4Mn_1.6O₄ are well explained based on three redox couples of Mn³⁺/Mn⁴⁺, Ni²⁺/Ni³⁺, and Ni³⁺/Ni⁴⁺ by considering a presence of oxygen defects.     

Fabrication of LSM-SDC composite cathodes for intermediate-temperature solid oxide fuel cells

Microstructure, interfacial resistance, and activation energy for composite cathodes consisting of 50 wt% (La_0.85Sr_0.15)_0.9MnO_3-δ (LSM) and 50 wt% Sm_0.2Ce_0.8O_1.90 (SDC) were studied for intermediate-temperature solid oxide fuel cells based on SDC electrolytes. Microstructure and interfacial resistance were greatly influenced by the characteristics of starting powder and temperatures sintering the electrodes. Optimum sintering temperatures were 1100 and 950 °C, respectively, for electrodes with SDC prepared using oxalate coprecipitation technique (OCP) and glycine-nitrate process (GNP). Area-specific resistances determined using impedance spectroscopy were 0.47 and 0.92 Ω cm² at 800 °C for LSM-SDC/OCP and LSM-SDC/GNP, respectively. The high electrochemical performance is attributed to small grain size, high porosity, and high in-plane electrical conductivity of composite cathode, demonstrating the dramatic effects of microstructure on electrode performance. To increase the electrode performance, it is critical to enhance the diffusion rate of oxygen species.     

Hydrogen oxidation reaction at the Ni/YSZ anode of solid oxide fuel cells from first principles

By means of ab initio simulations we here provide a comprehensive scenario for hydrogen oxidation reactions at the Ni/zirconia anode of solid oxide fuel cells. The simulations have also revealed that in the presence of water chemisorbed at the oxide surface, the active region for H oxidation actually extends beyond the metal/zirconia interface unraveling the role of water partial pressure in the decrease of the polarization resistance observed experimentally.     

Mixed conductivity and oxygen permeability of doped Pr2NiO4-based oxide

Pr₂NiO₄-based oxide was studied as a new mixed electronic and oxide ionic conductor for the oxygen permeation membrane. It was found that Pr₂NiO₄ doped with Cu and Fe for Ni site exhibits the relatively high oxygen permeation rate. Doping second cation to Ni site is effective for improving the oxygen permeation rate and the trivalent cation seems to be effective for increasing the oxygen permeation rate. Among the examined cation, the highest oxygen permeation rate was obtained by doping 5 mol% Fe. The oxygen permeation rate was also significantly affected by the surface catalyst and the highest oxygen permeation rate of 80 μmol·min^− 1·cm^− 2 at 1273 K was achieved by using La_0.1Sr_0.9Co_0.9Fe_0.1O₃ for the surface catalyst. Since the electrical conductivity slightly decreased with decreasing P_O2 and it dropped significantly at P_O2 = 10^− 19 atm, chemical stability of Pr₂NiO₄-based oxide seems to be reasonably high. Application of this new mixed conductor for the oxygen permeation membrane under the CH₄ partial oxidation was also studied and it was confirmed that the oxygen permeation rate much improved under the CH₄ oxidation condition and this Pr₂NiO₄ can be used for the oxygen permeation membrane for the CH₄ partial oxidation.     

Lanthanum chromite as an anode material for solid oxide fuel cells

The electrochemical behavior of pure lanthanum chromite and strontium dopedlanthanum chromite was studied by impedance spectroscopy under H₂/H₂O, CO/CO₂ and CH₄/H₂O. Results show that the electrochemical oxidation of H₂ is faster than that of CO or CH₄. Strontium doping enhances the anodic activity of the material. The impedance diagrams are composed of two semi-circles. The high frequency one does not appear to be related to a chemical or electrochemical reaction. The low frequency one is linked to the nature and concentration of the electroactive species. Paper presented at the 4th Euroconference on Solid State Ionics in Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Preparation and photoluminescence properties of the Sr1.56Ba0.4SiO4:0.04Eu2+ phosphor

The Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ phosphors were prepared via a combustion reaction and following the calcination method at low temperature. The influences of the amount of the uncommonly used SrCl₂ flux, different calcination temperatures and time on the structure and the photoluminescence (PL) properties of the phosphors were investigated. Under the excitation of 450 nm blue light, the phosphor shows the intense broad emission band from 490 nm to 650 nm, and the emission peak is centered at 553 nm. The luminescence intensity of Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ was very sensitive to the crystallinity and morphology characteristics of the phosphor. The phosphor calcined at 950 °C for 3 h in 20%H₂/80%Ar atmosphere exhibits improved PL properties due to its high crystallinity and excellent morphology characteristics. The use of the SrCl₂ flux provides a novel way to improve the crystallinity of the silicates phosphors at low preparation temperature.     

Reaction mechanisms of the copper-manganese spinel cathode in a solid oxide fuel cell

Cu_1.25Mn_1.75O₄ spinel (CMO) was studied as a potential solid oxide fuel cell (SOFC) cathode material at intermediate temperatures. The reaction mechanism of a composite cathode consisting of Cu_1.25Mn_1.75O₄ and yttria-stabilized zirconia (YSZ) was investigated by impedance spectroscopy. The influence of the CMO/YSZ ratio, time exposed to current passage and temperature on the impedance spectra was examined. Activation energy of the corresponding processes was calculated to be near 1 eV and between 1.32 and 1.96 eV for the high and low frequency arcs in the impedance spectra. Comparison between CMO-YSZ and Sr-doped LaMnO₃ (LSM)-YSZ composite cathodes showed they had similar reaction mechanisms. The transport or transfer of oxygen intermediates or oxide ions between the catalyst and electrolyte was suggested to be the rate determining steps between 700 and 800 °C, whereas dissociative adsorption, mass transfer and surface diffusion were rate controlling between 600 and 700 °C.     

Soft chemical route and new solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are a promising technology for electric power generation in the 21st century. Recently, research are focusing on reduced temperature SOFCs. The fabrication of thin film electrolytes and electrode membranes for reduced temperatures by a soft chemical route is discussed. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

Simultaneous thermogravimetry - mass spectrometry for a solid oxide fuel cell cathode: (La0.7Sr0.3)0.9MnO3

A SOFC cathode related perovskite material, (La_0.7Sr_0.3)_0.9MnO₃, has been investigated by simultaneous thermogravimetry - mass spectrometry from room temperature to 1770 K. Water, carbon dioxide and oxygen were detected by mass spectrometry. Water and carbon dioxide evolution can be interpreted by assuming that prior to the thermogravimetry-mass spectrometry measurement about 0.5 % of the lanthanum component had reacted with carbon dioxide and water to form La₂(CO₃)₃*8H₂O, which dehydrated and decomposed via La₂O₂CO₃ into La₂O₃ and evolving H₂O and CO₂ during the present experiment. The observation that the lanthanum strontium manganite emitted oxygen in two stages can be ascribed to the two different oxygen sites in the perovskite lattice, that is, the oxygen excess and deficient regions.     

Ceria co-doped with calcium (Ca) and strontium (Sr): a potential candidate as a solid electrolyte for intermediate temperature solid oxide fuel cells

Co-doped samples of Ce_0.95?x Ca_0.05Sr _x O_1.95?x , where (x?=?0.00, 0.01, 0.02, and 0.03), have been prepared by auto-combustion method and characterized to explore their use as a solid electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Crystal structure, microstructure, and ionic conductivity have been characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy, respectively. All the compositions have been found to be single phase. Results show that the samples co-doped with Ca and Sr exhibit higher ionic conductivity than the samples singly doped with Ca in the intermediate temperature range. Ce_0.93Ca_0.05Sr_0.02O_2?δ exhibits maximum conductivity among all the compositions. This may be a potential candidate as a solid electrolyte for IT-SOFCs.