Yttria-stabilized Zirconia Anode Research Articles

Highly Dispersed Ni over a YSZ Anode Anchored by SiO2 at High Temperature through Low-Temperature Chemical Vapor Deposition

Supported Ni on yttria stabilized zirconia (YSZ) is the most widely used anode for a solid oxide fuel cell (SOFC), where Ni particles are easy to agglomerate at a high temperature of 1400 °C and reductive hydrothermal environments. In this study, Ni particles are successfully dispersed on YSZ through strong interaction between Ni and SiO2. The thin and amorphous SiO2 films, grown by low-temperature chemical vapor deposition (CVD), become spherical particles of nickel silicates uniformly dispersed on the YSZ skeleton after sintering at 1400 °C. Also, the SiO2-anchored Ni particles supported on YSZ (Ni/YSZ-SiO2) are obtained via direct reduction. The electrochemical performance of the Ni/YSZ-SiO2 anode shows a higher power density of approximately 18% than an unmodified Ni/YSZ anode, which resulted from the longer effective TPB area assigned to the highly dispersed Ni particles. The microstructure containing SiO2-anchored Ni particles exhibits high stability under testing and reductive hydrothermal conditions. Accordingly, the method of growing a SiO2 layer via low-temperature CVD is considered as a probable route for the preparation of the Ni/YSZ electrode material with a well-defined microstructure.

ReaxFF reactive molecular dynamics study on electrochemistry of H2/CO hybrid fuel in Ni/YSZ anode

Fuel flexibility, e.g. using reformate or syngas fuel directly, is one of the most advantages of solid oxide fuel cells (SOFC). However, the electrochemical mechanism of the H2/CO mixture, which is common and key in hydrocarbon fuel-fed SOFC, is unclear and relatively rarely studied at present. This article makes Reactive Force-Field (ReaxFF) reactive molecular dynamics (RMD) simulations on the electrochemical co-oxidation of H2/CO hybrid fuel in Ni (nickel)/YSZ (yttria-stabilized zirconia) anode. The RMD simulations at different fuel gas components show that in the H2/CO/Ni/YSZ system the H2 conversion rate maintains the maximum (100%), and the conversion rate of CO improves compared with that of pure CO fuel in the case of 30%H2−70%CO. Two ways of CO2 generation are found: one is that the CO(Ni) species formed by the adsorption and activation of CO on the Ni surface is oxidized by the lattice oxygen; the other is the oxidation of CO(Ni) by spillover atomic oxygen, which is adsorbed on the Ni surface. It is worth noting that the effect of oxygen vacancies on the oxidation of CO is more significant than that of H2. A viewpoint is proposed to explain how H2 enhances the oxidation of CO. The results help understand the electrochemistry of H2/CO involved fuel in the context of SOFC operations.

Anodic polarization characteristics in hydrogen and methane oxidations at nickel-cobalt yttria-stabilized zirconia cermet relating to prolonged cell performance stability

The conventional solid oxide fuel cell (SOFC) anode, which consists of metallic Ni and yttria-stabilized zirconia (YSZ), is liable to deactivation when a carbon-containing gaseous fuel is directly supplied. To retard the degradation due to a direct supply of CH4, the Ni/YSZ has been modified with the incorporation of Co into the Ni matrix, forming a substitutional solid-solution alloy. As a result, the porous anode exhibits well-connected compositional grains owing to the Co3O4 precursor as a sintering aid. The 25 mol% Co incorporation producing the Ni0.75Co0.25/YSZ anode significantly improves the cell performance and prolonged stability. In addition, the Ni0.75Co0.25/YSZ anode provides the apparent activation energy of the charge transfer reaction to a lower value in the electrochemical oxidation of CH4 at the triple-phase boundary. However, the apparent activation energy decrease is not identified for H2. The Co alloying effect on catalyzing the electrochemical oxidation of CH4 is caused by suppressing the carbon deposition originating from the thermal dissociation of adsorbed CH4 on the Ni1-xCox alloy surface.

Methane steam reforming reaction in solid oxide fuel cells: Influence of electrochemical reaction and anode thickness

The influence of operation temperature, inlet gas composition, current density and the anode thickness on the methane steam reforming reaction over nickel yttria-stabilized zirconia anodes was experimentally studied in solid oxide fuel cells. The experimental results were analyzed using data fitting in Power-Law and Langmuir–Hinshelwood kinetic models. Similar trends of dependence of methane and steam partial pressures were observed in both models. The methane reaction order is positive. Negative influence of steam partial pressure on the methane steam reforming reaction rate are found. The electrochemical reaction and anode thickness affect the reforming kinetics parameters. The anodes thickness shows particular influences on the steam reaction order, and the activation energy when a current is produced. The model evaluation suggests that the two models are comparable and the extra parameters within the Langmuir–Hinshelwood kinetic model are contributing to the lower mean absolute percentage error and higher coefficient of determination R 2 . • Current promotes the methane steam reforming reaction rate. • Anode thickness influences the fitted activation energy. • More experimental data is helpful in getting a reliable rate expression. • Importance of studying effect of mass-transfer on the global MSR kinetics.

Performance of Metal Supported SOFCs Operated in Hydrocarbon Fuels and at Low (

Solid oxide cells (SOCs) are a key enabling technology for the required future renewable energy systems by providing an efficient link between the power, gas, and heat sectors. Specifically, solid oxide fuel cells (SOFCs) convert the chemical energy of a fuel into electricity and heat with high electrical efficiencies. Hydrogen is the obvious choice of fuel due to the only emissions being steam, however hydrocarbon fuels are an interesting and safe alternative to hydrogen, especially for mobile applications, facilitating easy integration into existing infrastructure. The state-of-the-art (SoA) Ni:YSZ (yttria-stabilized zirconia) composite fuel electrode is capable of direct oxidation of hydrocarbons or of reforming of hydrocarbon gas mixtures. Unfortunately, the material that facilitates the above reactions, nickel, is also an excellent carbon deposition catalyst. This is problematic, due to the associated volumetric changes and reduction in electrochemically active sites. Ultimately, the fuel electrode can delaminate from the electrolyte, causing a catastrophic failure of the cell.Alternative fuel electrode materials are the key to avoid this. A-site deficient lanthanum doped strontium titanate (La0.4Sr0.4Fe0.03Ni0.03Ti0.94O3, LSFNT) based anodes, coated with Ni:CGO (gadolinium-doped ceria) electrocatalyst are a promising alternative - while avoiding Ni as a major part of the backbone, the infiltrated Ni ensures electrocatalytic activity. The co-infiltrated CGO serves to improve the ionic conductivity. Integrating these anodes into a metal supported (MS) design ensures lower cost and mechanical robustness. Full cells are manufactured through scalable tape casting and screen printing technology, including the metal support. Possible pitfalls for the MS-cell include vulnerability to corrosion of the metal support and decreased reforming activity due to less electrocatalyst (Ni) compared to the SoA-cell.In this study, the reforming activity of methane in 3 different SOFCs is investigated: Cell 1 is with a SoA Ni:YSZ anode, cell 2 is with a LSFNT anode and cell 3 is with a LSFNT-FeCr anode. The investigation involves electrochemical characterization by impedance spectroscopy (EIS) and performance under applied current (I/V-curves) at temperatures 750˚C, 700˚C, 650˚C and 620˚C, so-called fingerprints, with special emphasis on the anode activity for reforming and water gas shift reaction. Furthermore, galvanostatic durability tests are conducted for the MS-cells (cell 2 and cell 3) with focus on the durability at low temperature (650˚C) and corrosion resistance at medium to high fuel utilization (ca. 50%). The degradation of the tested cells is examined by comparing the electrochemical performance before and after durability testing.

Direct reforming of Methane–Ammonia mixed fuel on Ni–YSZ anode of solid oxide fuel cells

To control the temperature distribution in the Ni–YSZ (yttria-stabilized zirconia) anode of solid oxide fuel cells (SOFCs) by efficiently utilizing the heat generated by electrochemical reactions, the supply of methane–ammonia mixed fuel is proposed. The reaction characteristics of reforming/decomposition of the mixed fuel on a Ni–YSZ catalyst are experimentally investigated. A mixture gas of methane, steam, ammonia, and balance argon is supplied to a packed bed catalyst placed in a quartz tube in an electric furnace. The crushed Ni–YSZ anode of SOFCs is used as the catalyst. The exhaust gas composition is analyzed by gas chromatography and the streamwise temperature distribution of the catalyst bed is measured by an infrared camera. It is found that ammonia decomposition preferentially proceeds and steam methane reforming becomes active after sufficient ammonia has been consumed. A low-temperature region is formed by steam methane reforming owing to its strongly endothermic nature. Its position moves downstream while its magnitude decreases as the ammonia concentration in the fuel increases. This shows that the local temperature distribution can be controlled by tuning the ratio of methane to ammonia in the mixed fuel. It is also found that, at a certain mixture ratio, the mixed fuel realizes a hydrogen production rate higher than that for only methane or ammonia.

Porous NiO–YSZ and Ni–YSZ composites fabricated using NiO–YSZ composite nanopowders and WO3 additive

NiO–YSZ composite nanopowders were synthesized through a hydrothermal process without an additional mixing of NiO and YSZ nanopowders. Porous Ni–yttria stabilized zirconia (YSZ) composites, Ni–YSZ composites, with high porosity (25.0–32.8%) were fabricated by reducing NiO-YSZ composites in (94% Ar + 6% H2) gas. The electrical conductivity (14,221 cm−1 at 30 °C) and bending strength (130 MPa) of Ni–YSZ composites were enhanced by the addition of WO3 (0.1 wt% WO3) to NiO–YSZ composite nanopowders. Furthermore, the Y2(WO4)3 with a negative coefficient of thermal expansion (CTE) reduced the CTE of Ni–YSZ composites. The CTE values of 0 and 0.6 wt% WO3–added Ni–YSZ composites were 13.8 × 10−6 and 12.4 × 10−6 °C−1, respectively. We demonstrate that the fabrication of NiO–YSZ composite nanopowders and the addition of WO3 to the NiO–YSZ composite nanopowders are highly effective methods for the fabrication of high-quality Ni–YSZ anode materials in solid oxide fuel cells.

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd0.1Ce0.9O2 (GDC) barrier layer and a Sm0.5Sr0.5CoO3 (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm2 at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm2 current density and 700 °C.

Densification behavior of yttria-stabilized zirconia powders for solid oxide fuel cell electrolytes

Yttria-stabilized zirconia (YSZ) is the most common electrolyte material for solid oxide fuel cells. Herein, we conducted a comparative study on the densification behavior of three different kinds of commercial 8 mol% YSZ powders: (i) TZ-8Y (Tosoh, Japan), (ii) MELox 8Y (MEL Chemicals, UK), and (iii) YSZ-HT (Huatsing Power, China). The comparison was made on both the self-supporting pellets and thin-film electrolytes coated onto a NiO–YSZ anode support. For the pellets, MELox 8Y showed the highest densification at lower sintering temperatures with 93% and 96% of the theoretical density at 1250 and 1300 °C, respectively. Although YSZ-HT showed a higher sintering rate than TZ-8Y, a sintering temperature of 1350 °C was required for both the powders to reach 95% of the theoretical density. For the thin-film electrolytes, on the other hand, YSZ-HT showed the highest sintering rate with a dense microstructure at a co-sintering temperature of 1250 °C. Our results indicate that besides the average particle size, other factors such as particle size distribution and post-processing play a significant role in determining the sintering rate and densification behavior of the YSZ powders. Additionally, a close match in the sintering shrinkage of the electrolyte and anode support is important for facilitating the densification of the thin-film electrolytes.

Sample Preparation for Microstructural Characterization of Ni–Yttria-Stabilized Zirconia Anodes

Microstructural characterization of Ni-yttria-stabilized zirconia (YSZ) anodes using secondary electron images has been limited by a lack of contrast between Ni and YSZ phases. This paper reports a sample preparation method for obtaining secondary electron images that allow the detection of Ni, YSZ, and pore phases together. Ni-YSZ anode samples were obtained by reducing NiO-YSZ samples prepared by using the mixed oxide method. Colloidal silica polishing and electrolytic etching were performed on the Ni-YSZ samples. The morphological change of the sample surface after each polishing process is examined. Key words: NiO-YSZ, Ni-YSZ, Microstructure, Polishing, Electrolytic Etching

Razvoj kristalnih faz in mikrostrukture LSCM perovskitnih materialov za SOFC anode, pripravljenih s karbonatno metodo koprecipitacije

Most SOFC development has been based on nickel yttria-stabilized zirconia anodes. Such materials have excellent catalytic properties for fuel oxidation, high electrical conductivity, good mechanical strength and an appropriate thermal expansion coefficient compatible with other cell components. Unfortunately, cermet anodes based on doped zirconia exhibit some disadvantages, e.g., the catalysing side reaction of carbon deposition during hydro-carbon fuel oxidation and a susceptibility to sulphur poisoning. Perovskite-type compounds based on lanthanum-strontium-manganese-chromium oxide (LSCM) can serve as an alternative material. Since the optimal perovskite composition is still not known, La1–xSrxMnyCr1.yO3± (x from 0 to 0.3 and y from 0.4 to 0.6) ceramics were prepared with the co-precipitation method. Crystalline phase formation was followed by X-ray powder diffraction and Rietveld refinement. Quantitative microstructure analysis of the samples sintered at various temperatures was performed on SEM micrographs using Axiovision 4.8 software.

In situ time-of-flight neutron imaging of NiO–YSZ anode support reduction under influence of stress

This article reports on in situ macroscopic scale imaging of NiO–YSZ (YSZ is yttria-stabilized zirconia) reduction under applied stress – a phase transition taking place in solid oxide electrochemical cells in a reducing atmosphere of a hydrogen/nitrogen mixture and at operation temperatures of up to 1073 K. This process is critical for the performance and lifetime of the cells. Energy-resolved neutron imaging was applied to observe the phase transition directly with time and spatial resolution. Two different approaches are presented for using this imaging technique for the investigation of chemical and physical processes requiring controlled atmosphere and elevated temperature. The first type of measurement is based on alternating stages of short-term partial chemical reaction and longer neutron image acquisition, and the second type is a real in situ neutron imaging experiment. Results of applying energy-resolved neutron imaging with both approaches to the NiO–YSZ reduction investigation indicate enhancement of the reduction rate due to applied stress, which is consistent with the results of the authors' previous research.

Modeling of gas transport with electrochemical reaction in nickel-yttria-stabilized zirconia anode during thermal cycling by Lattice Boltzmann method

Abstract This work reports an investigation of the impact of microstructure on the performance of solid oxide fuel cells (SOFC) composed of nickel yttria-stabilized zirconia (Ni YSZ). X-ray nano computed tomography (nano-CT) was used to obtain three-dimensional (3D) models of Ni-YSZ composite anode samples subjected to different thermal cycles. Key parameters, such as triple phase boundary (TPB) density, were calculated using 3D reconstructions. The electrochemical reaction occurring at active-TPB was modeled by the Lattice Boltzmann Method for simulation of multi-component mass transfer in porous anodes. The effect of different electrode geometries on the mass transfer and the electrochemical reaction in anodes was studied by TPB distributions measured by nano CT for samples subjected to different thermal cycles. The concentration polarization and the activation polarization were estimated respectively. The results demonstrate that a combined approach involving nano-CT experiments in conjunction with simulations of gas transport and electrochemical reactions using the Lattice Boltzmann method can be used to better understand the relationship between electrode microstructure and performance of nickel yttria-stabilized zirconia anodes.

Stochastic 3D modeling of complex three-phase microstructures in SOFC-electrodes with completely connected phases

A parametric stochastic 3D model for the description of complex three-phase microstructures is developed. Such materials occur for example in anodes of solid oxide fuel cells (SOFC) which consist of pores, nickel (Ni) and yttria-stabilized zirconia (YSZ). The model is constructed using tools from stochastic geometry. More precisely, we model the backbones of the three phases by a certain class of random geometric graphs called beta-skeletons. This allows us to reproduce complete connectivity of all three phases as observed in experimental image data of a pristine Ni–YSZ anode as well as the prediction of volume fractions by model parameters. Finally a slightly generalized version of this model enables a good fit to experimental image data with respect to transport relevant microstructure characteristics and the length of triple phase boundary. Model validation is performed by comparing effective transport properties from finite element (FE) simulations based on 3D-data from the stochastic model and from tomography of real Ni–YSZ anodes. Moreover, the virtual, but realistic Ni–YSZ microstructures can be used for investigating the quantitative influence of microstructure characteristics on various physical properties and consequently on the performance of the anode material.

Performance Investigation of Dual Layer Yttria-Stabilized Zirconia–Samaria-Doped Ceria Electrolyte for Intermediate Temperature Solid Oxide Fuel Cells

The performance of yttria-stabilized zirconia (YSZ)–samaria-doped ceria (SDC) dual layer electrolyte anode-supported solid oxide fuel cell (AS-SOFC) was investigated. Tape-casting, lamination, and co-sintering of the NiO–YSZ anode followed by wet powder spraying of the SDC buffer layer and BSCF cathode was proposed for fabrication of these cells as an effective means of reducing the number of sintering stages required. The AS-SOFC showed a significant fuel cell performance of ∼1.9 W cm−2 at 800 °C. The fuel cell performance varies significantly with the sintering temperature of the SDC buffer layer. An optimal buffer layer sintering temperature of 1350 °C occurs due to a balance between the YSZ–SDC contact and densification at low sintering temperature and reactions between YSZ and SDC at high sintering temperatures. At high sintering temperatures, the reactions between YSZ and SDC have a detrimental effect on the fuel cell performance resulting in no power at a sintering temperature of 1500 °C.

In-situ electron microscopy investigation of reduction-induced microstructural changes in NiO

The structural characteristics of NiO–YSZ (YSZ: Yttria-stabilized zirconia) composites and the redox stability of NiO have been explored by in-situ and high-resolution transmission electron microscopy (TEM). NiO has been reduced on alumina support while doing in-situ microscopy, and the observations on redox instability and microstructural changes have been extrapolated to understand the behavior of NiO/Ni–YSZ system. In the composite, the NiO and YSZ grains are strain free and no reaction product is observed at the grain boundaries after long term annealing. Under a low partial pressure of oxygen, NiO may begin to reduce to metallic Ni at temperatures as low as ~200°C. The volume change associated with such a reduction process is ~40.3%. Although metallic Ni nucleates in NiO coherently, such a huge volume contraction may lead to porosity generation and mechanical instability. Long term exposure of metallic Ni to high temperatures leads to coarsening, which reduces the available surface area. This in turn has a direct, negative consequence on the catalytic properties of the metallic Ni particles. The coarsened particles may even undergo faceting, and the facets may not be the most active catalytically. Synthesis of finely dispersed and percolatively connected Ni–YSZ anodes for SOFCs and core–shell or compositionally graded electrodes for batteries may be viable solutions to enhance the lifetime and structural integrity of such electrodes.

Effects of Manganese Oxide Addition on Coking Behavior of Ni/YSZ Anodes for SOFCs

AbstractSolid oxide fuel cells with Ni‐MnO/yttria‐stabilized‐zirconia (YSZ) tricomposite anode supports were fabricated with different MnO concentrations, and the coking tolerances and catalytic activities were investigated in wet CH4 atmosphere. Ni0.9(MnO)0.1/YSZ (10MnO) anode support cell exhibited a maximum power density of 210, 354, 505, and 620 mWcm−2 at 700, 750, 800, and 850 °C, respectively, in H2. Moreover, a maximum power density in wet CH4 reaches 504 mWcm−2 at 800 °C; while the Ni/YSZ cell showed poorer performances. The coking tolerance improved with an increase in their MnO content, and the 10MnO anode showed the highest tolerance. 10MnO exhibited stable performance for more than 40 h in wet CH4 without undergoing deactivation. Furthermore, it showed negligible coke formation of 0.0045 g of coke per catalyst, during testing under steam reforming‐like conditions at a steam‐to‐carbon (S/C) ratio of 1. Outlet gas chromatography analysis indicated that MnO suppresses CH4 cracking, while only minimally lowering the catalytic activity of steam reforming. Thus, it can be inferred that MnO promotes the adsorption of steam and oxygen on the reaction sites, owing to its high basicity and oxygen storage capacity. The increase in the local S/C and oxygen‐to‐carbon ratios suppresses CH4 cracking and promotes coke gasification.

Effect of composition on microstructure and polarization resistance of solid oxide fuel cell anode Ni-YSZ composites made by co-precipitation

Highly dispersed composite powders of NiO and yttria-stabilized zirconia (YSZ) were synthesized through a newly developed co-precipitation method, using an organic base solution that contains an anionic zirconium carbonate complex. It was found that a wide variation in composition can be achieved in this way. Subsequent investigation of the effect of composition on microstructure and polarization resistance of sintered Ni–YSZ anodes revealed a significant variation in microstructure and polarization resistance. An Ni content of 40% by volume produces the finest and most uniformly porous anode structure with the lowest polarization resistance of approximately 0.18Ωcm2 at 800°C in a 97% H2+3% H2O atmosphere.

Decomposition of carbon monoxide and carbon dioxide using cathode-supported dense yttria-stabilized zirconia electrolyte cell

CO, CO2 and CO–CO2 mixed gas were electrochemically decomposed at 800°C using a cell of Ni–YSZ (8mol% yttria-stabilized zirconia) cathode (5mm thick), dense YSZ electrolyte (40μm thick) and Ru–YSZ anode (30μm thick). The decomposition rate of CO gas (2CO→2C+O2) was in the range of 9–15% at an applied voltage of 1.0V and increased to 67–100% at 2.0–8.0V. Carbon was deposited significantly near the electrolyte interface in the cathode. In the case of CO2 gas decomposition (2CO2→2CO+O2, CO2→C+O2), CO2 gas of 6–8vol% was converted to CO (6–8vol%), O2 (2–9vol%) and C. Deposition of a small amount of C was observed in the cathode. The decomposition of CO–CO2 mixed gas produced O2 gas of 2–10vol% in the anode but carbon deposition hardly occurred. Carbon deposited in the cathode from CO decomposition was easily oxidized to CO gas again by CO2 mixed (C+CO2→2CO).

Slip-cast and hot-solution infiltrated porous yttria stabilized zirconia (YSZ) supported tubular fuel cells

Hot solution infiltration was investigated as a flexible and rapid method to incorporate anode and cathode components into fully sintered, porous ceramic tubular templates for use as solid oxide fuel cells (SOFC). Composed of either a porous 8 mol% yttria-stabilized zirconia (YSZ) or 5 wt% NiO–YSZ support structure, a thin Ni–YSZ anode functional layer and an outer ca. 10 μm dense YSZ electrolyte, closed end tubes were first hot solution (ca. 100 °C) infiltrated on the inside with NiO–SDC (Sm0.2Ce0.8O1.9) to serve as the anode. Cathodes were either LSM (nominally La0.8Sr0.2MnO3+δ) infiltrated into a thin porous YSZ layer on the outer electrolyte surface, or an LSCF–GDC composite (Gd0.1Ce0.9O1.95–La0.6Sr0.4Co0.2Fe0.8O3−δ) on a thin GDC buffer layer. Although hot solution infiltration of the Ni, Ce and Sm salts into the anode support structure did not result in complete penetration (with the Ni contents in the tube wall ranging between 4 and 10 vol.%), well-sealed full cells produced power densities as high as 275, 196 and 153 mW cm−2 at 800, 750 and 700 °C, respectively. Hot solution infiltration of active SOFC electrode materials is thus shown to be a very flexible approach for the evaluation of their performance.