YSZ Cermet Research Articles

An Experimental Evaluation of the Temperature Gradient in Solid Oxide Fuel Cells

The use of solid oxide fuel cells SOFCs 1,2 as efficient energyconverter devices rests on their high working temperature. These ceramic fuel cells generally operate between 800 and 1000°C, although a major research effort is put forward to reduce their operation below 800°C for reducing manufacturing costs and improving cell lifetime. Depending on the selected range of temperature, a SOFC system would allow for either cogeneration or bottoming cycle, which add significantly to the overall power-generation efficiency. The high operating temperature of SOFC affords, in principle, for the direct use of hydrocarbon fuels by means of their catalytic conversion to carbon monoxide and hydrogen. The most economical way to convert these fuels is to proceed through direct internal reforming DIR, as it is observed in tubular or planar designs, or through catalytic partial oxidation CPOX, which is at the basis of the functioning of single-chamber SOFC SC-SOFC. However, both DIR and CPOX may lead to high temperature variations along the fuel cell. In the case of direct reforming of methane, a significant cooling at the inlet of the fuel cell is expected due to the fast kinetics of the endothermic reforming reaction, especially over conventional Ni– yttria-stabilized zirconia YSZ cermet. 3 It then aggravates the tem

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

Abstract A solid oxide fuel cell with Sm0.2Ce0.8O1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm0.5Sr0.5CoO3 (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm−2 at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm−2 or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.

Microstructural effects on the electrical and mechanical properties of Ni–YSZ cermet for SOFC anode

The electrical and mechanical properties of Ni–YSZ cermet as the anode support of solid oxide fuel cell (SOFC) are determined by the metallic and ceramic components, respectively. We used YSZ and NiO commercial powders of the average particle size from 1 to 10 μm to fabricate Ni–YSZ cermets with different microstructures. The porosity of the cermets was also modified by the amount of carbon black addition. The distribution of each phase of cermets was analyzed with scanning electron microscopy combined with energy dispersive spectroscopy. The electrical conductivity and fracture strength of the Ni–YSZ cermets were investigated and interpreted in a view of percolation phenomena. The finer particles, either NiO or YSZ, were interlinked well by sintering and the electrical and mechanical properties of Ni–YSZ cermets were enhanced by the percolation of Ni and YSZ, respectively.

Freeze casting of porous Ni–YSZ cermets

Dual-phase porous Ni–YSZ cermets were fabricated via the freeze casting of a ceramic/camphene slurry. After removing the frozen camphene via sublimation at room temperature, the green samples were sintered for 3 h in air at various temperatures, ranging from 1100 to 1350 °C, and then reduced in an Ar–5% H 2 atmosphere at 700 °C for 3 h. The fabricated Ni–YSZ cermets showed 3-D pore channels formed by the replication of the entangled camphene dendrite network and small pores in the Ni–YSZ walls produced by partial sintering of the NiO–YSZ composite. Furthermore, the fabricated samples were found to possess reasonable electrical conductivities, thus rendering them suitable for use as the basic components of planar solid oxide fuel cells (SOFCs).

Oxide anode materials for solid oxide fuel cells

A major advantage of solid oxide fuel cells (SOFCs) over polymer electrolyte membrane (PEM) fuel cells is their tolerance for the type and purity of fuel. This fuel flexibility is due in large part to the high operating temperature of SOFCs, but also relies on the selection and development of appropriate materials — particularly for the anode where the fuel reaction occurs. This paper reviews the oxide materials being investigated as alternatives to the most commonly used nickel–YSZ cermet anodes for SOFCs. The majority of these oxides form the perovskite structure, which provides good flexibility in doping for control of the transport properties. However, oxides that form other crystal structures, such as the cubic fluorite structure, have also shown promise for use as SOFC anodes. In this paper, oxides are compared primarily in terms of their transport properties, but other properties relative to SOFC anode performance are also discussed.

Ni–YSZ SOFC anodes—Minimization of carbon deposition

An advantage of solid oxide fuel cells (SOFC) over some other types of fuel cells is the extended range of fuels they can use. Besides hydrogen, hydrocarbons are often used. A problem that may arise on a prolonged operation of cells using hydrocarbons is the formation of carbon deposits on the anodes. These deposits deteriorate the cell performance by blocking the access of reactants to the anode surface and into its pores, thus changing the micromorphology of the anode. The behavior of anodic material during the extended contact with such fuels must be tested and if needed modifications must be made in its composition in order to reduce the rate of carbon deposition. Ni–YSZ cermet materials prepared by different processes (sol–gel and combustion synthesis) with variations in composition and presence of dopants were tested by exposing these materials to methane at elevated temperatures. The thermal dissociation of methane, which leads to carbon deposits on the cermet surface, was studied by the analysis of the gas phase using mass spectrometry and gas chromatography. The influence of anode composition and its microstructure on carbon deposition was studied as well as the influence of some dopants. The amount of the deposited carbon was determined by temperature programmed oxidation.

Electrodeposition of Cu into a Highly Porous Ni ∕ YSZ Cermet

The electrochemical deposition of Cu into and thick, highly porous Ni/yttria-stabilized zirconia (YSZ) cermets was investigated. An electrochemical cell in which the electrolyte solution was allowed to flow through a porous substrate was used to eliminate mass-transfer limitations and to determine the conditions for which the potential drop in the electrolyte solution was minimized and a uniform Cu layer was produced throughout the porous substrate. The conditions determined from these experiments were then used to electrodeposit Cu throughout a thin, porous cermet anode layer on a solid oxide fuel cell (SOFC) using a standard nonflow-through setup. This SOFC was found to exhibit stable operation while using methane as the fuel.

SOFC-anodes, proof for a finite-length type Gerischer impedance?

The impedance of a symmetric cell with Ni/Ti-doped YSZ cermet anodes was measured as function of ambient ( P H 2 , P H 2 O) and temperature. The impedances showed identical shapes with a minor dispersive contribution in the high frequency region and a dominating dispersion down to 0.01 Hz. The characteristic shape of this dispersion was clearly in between a finite-length Warburg (FLW) and a Gerischer impedance. Analysis of the dispersion in the Bode representation, after subtraction of the high frequency contribution, showed a clear relation with the Gerischer impedance. Assuming a finite-length constraint led to quite reasonable modelling of the data. The parameter set of this finite-length Gerischer showed a consistent dependence on P H 2 , P H 2 O. A qualitative interpretation of this modified Gerischer is presented.

Study of the Ni–NiAl2O4–YSZ cermet for its possible application as an anode in solid oxide fuel cells

Nanocrystalline Ni–NiAl2O4–YSZ cermet with a possible application as anode in solid oxide fuel cells (SOFCs)has been developed. The powders were prepared by using an alternativesolid-state method that includes the use of nickel acetylacetonate as aninorganic precursor to obtain a highly porous material after sintering at1400 °C and oxide reduction () at 800 °C for 8 h in a tubular reactor furnace using10% H2/N2. Eight sampleswith 45% Ni and 55% Al2O3–YSZ in concentrations of Al2O3 oxides from 10 to 80 wt% of were mixed to obtain the cermets. The obtained material wascompressed using unidirectional axial pressing and calcinations from room temperature to800 °C. Good results were registered using a heating rate of1 °C min−1 and a special ramp to avoid anode cracking. Thermal expansion, electricalconductivity, and structural characterization by thermo-mechanical analyser (TMA)techniques/methods, the four-point probe method for conductivity, scanning electronmicroscopy (SEM), x-ray energy dispersive spectroscopy (EDS), x-ray diffraction(XRD), and the Rietveld method were carried out. Cermets in the range 5.5 to 11%Al2O3 present a crystal size around 200 nm. An inversion degree(I) in theNiAl2O4 spinel structureof the cermets Ni–NiAl2O4–YSZ was found after the sintering and reduction processes. Good electrical conductivity andthermal expansion coefficient were obtained for the cermet with 12 wt% of spinel structureformation.

FABRICATION AND PROPERTIES OF Ni-YSZ AND Ni-Co-YSZ CERMETS

Ni -YSZ and Ni - Co -YSZ cermets were fabricated by hot-press-sintering from nanocomposite powders. The structure, morphology, relative density, hardness, specific heat capacity and thermal conductivity were measured. Results show that the crystalline grains are much homogeneous. Their size distribution is quite narrow and between 0.2–0.4 μm (200–400 nm). The Ni - Co -YSZ cermet reaches a maximum relative density (96.2%) at 1350°C and a maximum Rockwell hardness (70.5) at 1400°C. The specific heat capacity of Ni -YSZ cermet is larger than that of Ni - Co -YSZ cermet. The thermal conductivity of Ni -YSZ cermet is smaller than that of Ni - Co -YSZ cermet.

Reduction and Reoxidation Processes of NiO∕YSZ Composite for Solid Oxide Fuel Cell Anodes

Solid oxide fuel cells (SOFCs) are an emerging technology in hydrogen-based energy production, thanks to their high performance, high power density, high efficiency, and reduced emissions over conventional power generation technologies. For these reasons, a great attention has been addressed in these years to SOFCs materials and technologies. An important issue related to the utilization of SOFCs as power generators is the capability of SOFCs materials to resist to thermal cycles in different atmospheres. The present work proposes an experimental investigation on the reduction process of NiO∕YSZ anode into Ni∕YSZ cermet, which is the best candidate for anode material in SOFCs, its reoxidation into NiO∕YSZ and the following rereduction into Ni∕YSZ. Anodes with different NiO∕YSZ ratios were analyzed through different physical and chemical techniques, such as scanning electron microscopy (SEM) and porosity measurements. The reduction, reoxidation and rereduction behaviors were studied by thermogravimetric analysis (TGA) in a unique long experiment, at different temperatures in the range of 700–800°C. The kinetics of the processes was studied and thermodynamic parameters such as activation energy were also calculated and correlated to the compositions and microstructure of the materials. The study clears up the effect of anode composition and microstructure on the reduction, reoxidation, and rereduction processes.

Microstructural control of Ni–YSZ cermet anode for planar thin-film SOFCs

A Ni–Y2O3-stabilized ZrO2 (Ni–YSZ) cermet anode was fabricated for solid oxide fuel cells (SOFCs) by conventional ceramic processing using NiO–YSZ composite particles. Microstructures of the anode were carefully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Ni–YSZ cermet anode consists of fine YSZ connections with a continuous pore structure, which provides paths for the conduction of oxygen ions on the surface of the Ni network, as well as that of electrons and gaseous species. No amorphous phases were present at the interface between Ni and YSZ, and there was an orientation relationship between the Ni and YSZ grains, (111)Ni//(111)YSZ. The cermet anode showed high electrical performance at 800°C. These results indicate that the electrochemical activity of the Ni–YSZ cermet anode is enhanced with the present microstructure.

Materials and reaction mechanisms at anode/electrolyte interfaces for SOFCs

The present paper reviews anodic reaction mechanisms of porous cermet and model anodes at metal/oxide interfaces in solid oxide fuel cells (SOFCs). Some analytical results, electrochemical methods, and reaction models were presented at Ni–YSZ cermets and well defined model anodes. Isotope labeling/secondary ion mass spectrometry (SIMS) analysis techniques were applied to determine the oxygen surface reactivity of oxide electrolytes in reducing atmospheres. The technique was also applied to determine the catalytic activity of metal/oxide interfaces for CH 4 decomposition and reactivity with the reformed gases at the mesh or stripe shaped anodes on different oxides. Observed SIMS images and the electrochemical analyses were compared at the model anode/electrolyte interfaces.

Microstructural control of Ni–YSZ cermet anode for planer thin-film solid oxide fuel cells

Ni–Y 2O 3-stabilized ZrO 2 (Ni–YSZ) cermet anode was fabricated for solid oxide fuel cells (SOFCs) by conventional ceramic processing using NiO–YSZ composite particles. Microstructures of the anode were carefully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Ni–YSZ cermet anode was consisting of fine YSZ connections, as the conducting pass of oxygen ions, on the surface of Ni network, as that of electrons, with continuous pore structure and as that of gaseous species. No amorphous phases were present at the interface between Ni and YSZ, and there was an orientation relationship between Ni and YSZ grains, (111) Ni//(111) YSZ. The cermet anode showed a high electrical performance at 800 °C. These results indicated that the electrochemical activity of the Ni–YSZ cermet anode was enhanced with the present microstructure.

Electrical behavior of nickel coated YSZ cermet prepared by electroless coating technique

Nickel–yttria stabilized zirconia (Ni/YSZ) cermet has been prepared by coating YSZ particles with metallic nickel using electroless coating technique. Concentration of nickel was varied between 7.23 and 64.99 wt%. Bulk samples were prepared using these nickel coated YSZ powders by uniaxial pressing followed by sintering in the temperature range 1200–1350 °C with a soaking time of 2–6 h. A thorough investigation on the electrical characteristics of the samples has been performed and an attempt has been made to study the effect of starting YSZ particle size, matrix density on the temperature dependence of conductivity of the cermet. Samples prepared by this technique shows metallic conductivity at a Ni concentration as low as 27.04 wt%. A detailed microstructural investigation of the samples is also reported.

Stability of Channeled Ni–YSZ Cermets Produced from Self‐Assembled NiO–YSZ Directionally Solidified Eutectics

Channeled yttria‐stabilized cubic phase of the zirconia (Ni–YSZ) cermets are produced by reduction of laser‐assisted directionally solidified NiO–YSZ lamellar eutectics. The material is formed by ∼400 nm wide alternating lamellae of 40% porous Ni and YSZ, which serve as parallel channels for gas flow and electronic transport and for oxygen ion diffusion. The low‐energy interfaces formed between the metallic Ni particles and the YSZ prevent particle coarsening and impart long‐term stability to the anode at operating temperatures. The electrical conductivity and the pore size distribution present no degradation after 300 h at 900°C under a H2/N2 atmosphere. This stability is indicative of an improvement in comparison with conventional Ni–YSZ anodes for solid oxide fuel cells.

Identification of nickel sulfides on Ni–YSZ cermet exposed to H 2 fuel containing H 2S using Raman spectroscopy

Ni–YSZ cermet was exposed to hydrogen containing different concentrations of H 2S to identify the phases formed under various conditions using Raman spectroscopy and X-ray diffraction (XRD). For Ni–YSZ samples exposed to hydrogen containing 100 ppm H 2S at 727 °C for 5 days, thermodynamic calculations indicate that Ni–YSZ would be stable and XRD analysis was unable to detect any changes. However, the vibration modes of Ni 3S 2 were detected using Raman spectroscopy, suggesting that Raman spectroscopy could be a powerful tool for in situ study of sulfur poisoning of SOFC anodes. For Ni–YSZ cermet exposed to hydrogen containing 10% H 2S at 950 °C for 5 days, Ni was converted to nickel sulfide, and vibration modes of NiS were detected using Raman spectroscopy.

Processing microstructure property correlation of porous Ni–YSZ cermets anode for SOFC application

The present paper investigates microstructural properties and electrical conductivity of cermets prepared by a solid-state technique, a liquid-dispersion technique and a novel electroless coating technique. The Ni–YSZ processed through different techniques shows varying temperature-conductivity behaviour. The cermets synthesised by electroless coating were found to be electronically conducting with 20 vol% nickel, which is substantially lower than that normally reported. The conductivity of Ni–YSZ cermets was found highest for the samples prepared by an electroless coating technique and lowest for the samples prepared by a solid-state technique, the samples prepared from liquid-dispersion show an intermediate value for a constant nickel content. The variation in electrical conductivity has been well explained from the microstructure of the samples.

Performance and ageing of an anode-supported SOFC operated in single-chamber conditions

Anode-supported cells made of conventional materials were tested in single-chamber conditions under various CH 4/air gas mixtures. Methane-to-oxygen ratio ( R MIX) and nominal temperature between 600 and 800 °C both affect the performance of the cell. At a flow rate of 350 sccm, maximum values of power density (260 mW cm −2) and cell voltage (1.05 V) were obtained for R MIX = 2 at 800 °C. However, short term ageing experiments show that the stability of the cells depends on R MIX as well as the flow of current. Scanning electron micrographs (SEM) reveal some important changes in anode microstructure close to the fuel inlet that may be, assign to the volatilization of the nickel contained in the Ni–YSZ cermet.

Thin-film heterostructure solid oxide fuel cells

A micro thin-film solid oxide fuel cell (TFSOFC) has been designed based on thin-film deposition and microlithographic processes. The TFSOFC is composed of a thin-film electrolyte grown on a nickel foil substrate and a thin-film cathode deposited on the electrolyte. The Ni foil substrate is then processed into a porous anode by photolithographic patterning and etching to develop pores for gas transport into the fuel cell. A La0.5Sr0.5CoO3 (LSCO) thin-film cathode is then deposited on the electrolyte, and a porous NiO–YSZ cermet layer is added to the anode to improve the electrode performance. The TFSOFC has stably operated in a temperature ranges as low as 480–570 °C, significantly lower than bulk SOFC’s, and has yielded a maximum output power density of ∼110 mW/cm2 in that temperature range.