Effect of oxide on carbon deposition behavior of CH4 fuel on Ni/ScSZ cermet anode in high temperature SOFCs

This work investigated the effect of oxide in Ni-zirconia cermets on the carbon deposition behavior in internal reforming SOFCs. Within 800–1000 °C, carbon deposition was found to decrease with increasing temperature on Ni/ScSZ cermet anodes at a low oxygen / carbon ratio (O / C = 0.03) during anodic oxidation of methane. On the contrary, an opposite trend was observed on Ni/YSZ under the same conditions, consisting with the temperature dependence of carbon deposition predicted by a thermodynamic equilibrium calculation. Results of temperature-programmed-reduction (TPR) of NiO mixed with YSZ or ScSZ indicated that interaction of Ni with ScSZ is stronger than that with YSZ. The stronger interaction was corroborated by observed tendency of inhibiting Ni agglomeration by both BET specific surface area analysis and SEM observation. It was also found that the dependence of CO₂ production rate monitored by GC on current density showed a similar dependence trend of the equilibrium CO₂ content on O / C ratio. A model in which H₂O_ad enrichment effects on Ni surface by anodic current depend on the interaction between Ni and the oxide in Ni cermet was proposed to explain the different carbon deposition behaviors between Ni/YSZ and Ni/ScSZ cermets.     

Titania doped YSZ for SOFC anode Ni-cermet

The influence of doping YSZ (8 mol%YttriaStabilizedZirconia) with 5 and 10 mol% TiO₂ on the electrical conductivity and the microstructure was examined for different YSZ-Ni cermets as a function of Ni-content. For the 5 mol%TiO₂-doped cermets the electrical conductivity was always better than that of the undoped, exhibiting a lower degradation rate at 1000 °C. Microstructure examination of the samples showed a lower tendency for agglomeration of the Ni-particles of the doped cermets compared to the Ni/YSZ cermet. The 10 mol%TiO₂-doped cermets exhibited lower conductivity values and this was attributed to the significantly higher porosity which resulted from a reaction taking place during sintering between the insoluble TiO₂ in YSZ and NiO. This reaction results in the formation of a NiO-TiO₂ spinel phase, which leads to the expansion of the samples causing microstructural defects. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

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 CH₄ 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.     

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.     

Fabrication of hollow thin films of yttria-stabilized zirconia by chemical vapor infiltration using NiO as oxygen source

Deposition of yttria-stabilized zirconia films on surface oxidized Ni wire substrate by chemical vapor infiltration (CVI) using ZrCl₄ and YCl₃ as metal sources and NiO as oxygen source were studied. The resultant films were cubic crystals of YSZ with a Y₂O₃ content of 1.0–3.7 mol%. The growth rate is larger than that obtained by conventional method of chemical vapor deposition (CVD), increased with the flow rate and decreased with diameter of NiO fiber. The growth rate above its thickness of 4 μm decreased with an increase in the oxidation temperature since the porosity of NiO wire might decrease with an increase in the oxidation temperature. Growth of YSZ films with the CVI method simultaneously involved CVD and electrochemical vapor deposition (EVD).     

Electrochemical cells with multilayer functional electrodes

Microstructural and chemical changes in a NiO-YSZ electrocatalytic electrode were studied. The microstructural changes in the NiO-YSZ electrocatalytic electrode after the cell operation was compared with the electrode quenched under the applied voltage to suppress the oxidation process. The reversible reduction of NiO into Ni and the formation of intergranular Ni layers at the NiO/YSZ interface were investigated. It was shown that in a compositional range of the NiO-YSZ electrodes from 1/3 to 2/3 the value of the ambipolar conductivity increased with increasing voltage applied to the electrochemical cell. The observed reversible increase in the value of ambipolar conductivity of the electrocatalytic electrode is described in frames of the model of reversible reduction of NiO into Ni under the conditions of cell operation.     

A wet-chemical route for the preparation of Ni–BaCe0.9Y0.1O3 − δ cermet anodes for IT-SOFCs

Nano-sized BaCe_0.9Y_0.1O_3 ? δ (BCY10) protonic conductor powders were used to prepare Ni-BCY10 cermets for anode-supported intermediate temperature solid oxide fuel cells. A new wet-chemical route was developed starting from Ni nitrates as precursors for NiO. BCY10 powders were suspended in a Ni nitrate aqueous solution that was evaporated to allow NiO precipitation on the BCY grains, obtaining NiO-BCY10 cermets. To obtain the final Ni-BCY10 anodes, pellets were reduced in dry H₂ at 700 °C. The structural and microstructural properties of the pellets were investigated using X-ray diffraction analysis and field emission scanning electron microscopy. A homogeneous dispersion of perovskite and nickel phases was observed. The chemical stability of the anodes was evaluated under wet H₂ and CO₂ atmosphere at 700 °C. The electrical properties of the Ni-BCY10 pellets were evaluated using electrochemical impedance spectroscopy measurements. The Ni-BCY10 cermet electrodes showed large electronic conductivity, demonstrating percolation through the Ni particles, and low area specific resistance at the BCY10 interface. These characteristics make the cermet suitable for application in BCY-based protonic fuel cells. The developed chemical route offers a simple and low-cost procedure to obtain promising high performance anodes.     

Kinetics of oxidation and reduction of Ni/YSZ cermets

A cyclic reduction and oxidation of Ni/YSZ-cermet anodes for Solid Oxide Fuel Cells (SOFC) resulted in an increase of the polarization resistance. Therefore, investigations concerning kinetics of oxidation/reduction and the impact of redox cycles on the mi-crostructure of Ni/YSZ bulk ceramics were made. The reaction process of the basic system Ni/NiO was compared with cermet bulk samples and the influence of NiO and YSZ particle sizes and sintering temperatures on kinetics and microstructure was studied using thermo-gravimetry and dilatometry. The investigations on bulk ceramics indicated that no length change occurred during reduction, whereas reoxidation led to an increase in the length of the samples which strongly depended on the microstructure. It was shown that bulk samples sintered at temperatures below 1300 °C can withstand redox cycles much better than those sintered at higher temperatures. Furthermore, it was found that by decreasing the NiO particle size and using a NiO/YSZ particle size ratio of aproximately 3:2, a smaller length increase after reoxidation was achieved. An increase of the polarization resistance could be ascribed to the formation of cracks within the bulk sample which interrupt current paths and therefore reduce the amount of the active triple phase boundary. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Thermal stability of atmospheric plasma sprayed (Ni, Cu, Co)–YSZ and Ni–Cu–Co–YSZ anodes cermets for SOFC application

This paper reports the results of the thermal stability study of M–YSZ (M=Ni, Cu, Co, Cu–Co, Ni–Cu–Co) SOFC anode cermets materials at different heat treatment temperatures. The cermets were elaborated by atmospheric plasma spraying (APS) with optimized conditions and then submitted to heat treatments at 800 and 1000 °C, respectively. Scanning electron microscopy and X-ray diffraction were used to characterize, respectively, the morphology and structure of obtained films, before and after thermal annealing. A correlation was made between the differential scanning calorimetry analysis results and those obtained by X-ray diffraction. Focusing, in this study, on YSZ matrix thermal stability, the obtained results were as follows: the monometallic cermets with the lowest amount of metal and the trimetallic one exhibited a good stability at 800 °C, whereas at 1000 °C, all considered cermets were unstable with a eutectoid decomposition of YSZ matrix.     

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.     

Synthesis and characterization of a Ni/Ba2In0.6Ti1.4O5.7□0.3 cermet for SOFC application

Ni-based cermets were prepared and reduced from mixtures of NiO and Ba₂In_0.6Ti_1.4O_5.7□_0.3. A cermet containing 18.7 vol.% of Ni exhibits promising characteristics: 40% of open porosity and a lower DC resistivity than a Ni/YSZ cermet with a larger Ni content (30 vol.%). Its thermal expansion coefficient is 11.4 × 10^− 6 K^− 1 whereas that measured for Ba₂In_0.6Ti_1.4O_5.7□_0.3 is 9.9 × 10^− 6 K^− 1. Electrical measurements vs. the Ni content have shown that the percolation threshold corresponds to 15.7 vol.% of Ni. By using saccharose as a pore former, the porosity of the electrode can be tuned. It is shown that the pore size is controlled by the particle size distribution of the pore former.     

A Ni/YSZ composite containing Ce0.9Ca0.1O2−δ particles as an anode for SOFCs

A composite material (hereafter referred to as NYC) containing Ni, Y₂O₃-stabilized ZrO₂ (YSZ) and Ce_0.9Ca_0.1O_2−δ (CC10) particles was prepared and used as the anode of solid oxide fuel cells (SOFCs). The performance of NYC was better than that of conventional Ni/YSZ anodes in terms of anodic overpotential and interface impedance. The additional CC10 particles improved the anode properties. XRD results suggest that a solid solution of YSZ and CC10 was produced. From impedance measurements, it is concluded that the solid solution exhibits substantial electronic conduction. Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ anodes exhibited the best properties over the experimental temperature range. A SOFC with an anode of Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ provided the maximum power density and current density. Addition of CC10 with an average particle size of 0.3 μm was more advantageous than that with an average size of 3 μm.     

Redox kinetics of NiO/YSZ for chemical-looping combustion and the effect of support on reducibility

This paper explores the reaction kinetics of NiO supported on YSZ (Yttria Stabilized Zirconia) as an oxygen carrier for chemical looping combustion. Nickel particles with size less than 1 μm mixed with YSZ nano-powders are used to prepare the solid mixture, with 45% mol of NiO. Redox reactivity and oxygen carrying capacity are measured in a laboratory scale fixed bed reactor in the temperature range 500–1000 °C with different concentrations of the reactive gasses. Samples are subjected to repeated redox cycles using synthetic air (O₂+Ar) for oxidation, and H₂/H₂O/Ar mixtures for reduction. NiO/YSZ demonstrates superb cyclic regenerability starting with the 2nd cycle, with full utilization of its oxygen carrying capacity. Compared to pure nickel, pronounced improvement is achieved in the kinetics and oxygen utilization. Full reduction is achieved, and the presence of H₂O does not affect the reduction rate. Reactivity is also determined as a function of conversion. Global models of redox conversion are developed, in which surface chemistry and solid diffusion are considered. Oxidation exhibits the characteristics of a shrinking-core model with internal reactions at the Ni/NiO interface being the rate limiting step, and it is weakly temperature dependent. Reduction with H₂ generally exhibits surface chemistry limitation (adsorption-desorption), with surface product formation being the rate limiting step. YSZ significantly enhances ionic transport during oxidation and reduction. Reaction rate dependencies on conversion during the two steps suggest an optimal range for the oxygen carrying capacity of the material.     

Synthesis of hierarchical flower-like NiO and the influence of surfactant

The NiO nanoflowers were prepared by a facile surfactant assisted hydrothermal method using Ni(NO₃)₂–6H₂O or NiCl₂–6H₂O as precursor compound. The microstructure of the samples was characterized by SEM and XRD. The gas sensing properties of the NiO nanoflowers toward ethanol was also investigated. The results show that surfactant plays a key role in the synthesis of flower-like NiO. The NiO nanoflowers show excellent sensing performances to ethanol gas. This morphology holds substantial promise for applying NiO as a potential gas sensing material for future sensor application.     

Characterization of NiO–yttria stabilised zirconia (YSZ) hollow fibres for use as SOFC anodes

NiO–yttria stabilised zirconia (YSZ) hollow fibres with varying NiO content and a desired microstructure were prepared using a phase inversion technique and sintering. By controlling the fabrication parameters, microstructures with predominately finger-like pores near the inner and outer surfaces and a denser central layer with sponge-like pores were produced, for use as substrates for anode-supported hollow fibre solid oxide fuel cells (HF-SOFC). The NiO–YSZ fibres were reduced to Ni–YSZ at 250–700 °C in hydrogen flowing at 20 cm³ min^? 1 to produce Ni–YSZ hollow fibres, the mechanical and electrical properties of which were determined subsequently, reduction to Ni being verified by X-ray diffraction. The effects of NiO concentration and sintering temperature of the fibre precursors on the conductivity, strength and porosity of the reduced hollow fibres were investigated to assess their suitability for use as anode substrates. As expected, increasing Ni concentration increased electrical conductivities and decreased mechanical strength. Sintering temperature had a critical effect in producing axially conductive hollow fibres of sufficient mechanical strength for use as SOFC anodes. The hollow fibres retained their initial microstructure through the reduction process, though ca. 41% volume contraction is predicted on reduction of NiO to Ni, producing increased porosity in the reduced fibres. The mean porosity of the Ni–YSZ hollow fibres was ca. 60% and ca. 40% after sintered at 1250 °C and 1400 °C, respectively. The mean pore sizes for all the fibres after reduction varied between ca. 0.3 and 1 µm. The hollow fibres produced with 60% NiO, of length ca. 300 mm, electrical conductivities of ca. (1–2.25) × 10⁵ S m^? 1 and a porosity of ca. 43% are being used currently to construct and test the electrical behaviour of an anode-supported HF-SOFC.     

Performance of a tubular solid oxide fuel cell with model kerosene reformate gas

A performance of an anode-supported tubular Ni–8YSZ/Ni–ScSZ/ScSZ/GDC/LSC cell was investigated at 650–750 °C by feeding model kerosene reformate gas (H₂, H₂O, CO, CO₂, and CH₄) to a Ni–8YSZ/Ni–ScSZ anode. Variations of gas composition were observed not only between inlet and outlet of anode to estimate the degree of internal reforming, but also during current input by online quadrupole mass spectrometry and Fourier-transform infrared spectrometry.The electrochemical performance of the cell was independent of reforming temperature of kerosene, i.e. gas composition (in particular CH₄ concentration) at moderate anode gas flow rates. At open-circuit states, 10% or less methane in the kerosene-reformed gas was readily converted by steam or CO₂ over the Ni–8YSZ/Ni–ScSZ electrode so that gas compositions could almost follow the thermodynamic equilibrium at 650–750 °C. This suggests that the internal reforming should proceed almost completely over the Ni anode. Consumption of H₂ and CO and production of CO₂ were observed during current input. I–V characteristics remained constant at 650 °C as long as anodic W/F was more than 0.2 kg mmol^− 1 s. It was demonstrated that a catalytic activity of an anode electrode for hydrocarbons will be important for SOFCs with liquid fuels such as kerosene in order not to deteriorate cell performance.     

Effects of powder sizes and reduction parameters on the strength of Ni–YSZ anodes

In order for solid oxide fuel cells to survive the mechanical loading associated with residual manufacturing stresses, assembly, thermal mismatches, ion activity gradients, or operational loading, one or more components of the cell must provide sufficient mechanical strength. In anode-supported electrolyte designs, the anode layer is called upon to provide the necessary mechanical strength, in addition to fulfilling its electrical and electrochemical roles. To investigate how the starting powder sizes and how the reduction process parameters influenced the strength of NiO(Ni)–YSZ anode laminates, concentric ring-on-ring, biaxial flexure experiments were performed. Two composite microstructures and two reduction processes were examined. One specimen was obtained from powders with only fine (≈ 2 μm) NiO and YSZ particles, while the other had a bi-modal distribution of coarse and fine particles of NiO (11 μm and 5 μm) and YSZ (4 μm and 1 μm). One reduction process introduces forming gas at room temperature, while the other process introduced forming gas only after the specimen reached its reduction temperature (600 or 800 °C). The anodes containing coarse and fine particles had slower reduction rates, poorly connected microstructures, and had 35–40% lower biaxial flexure strengths than anodes with only fine starting powders. The temperature at which forming gas was introduced had a significant impact on the microstructural evolution and thus also on the mechanical properties. Although introducing forming gas at room temperature led to more complete and faster reductions, the resulting microstructures were poorly connected, and the reduced laminates had almost 30% less strength than laminates that were reduced at constant temperature.     

Ni1−x−yMgxAlyO–ScSZ anodes for solid oxide fuel cells

Three types of cermets based on NiO–ScSZ (A), Ni_0.9Mg_0.1O–ScSZ (AMg) and Ni_0.9Mg_0.095Al_0.005O–ScSZ (AMgAl) were applied as SOFC anodes. Humidified H₂ and simulated biogas (CH₄:CO₂ = 6:4) were directly supplied to the anode side of SOFC single cell. Catalytic activities for the reforming and the electrochemical reactions were tested in a typical electrochemical measurement setup. When hydrogen (3% H₂O) was supplied as a fuel, the three anodes showed almost the same voltage losses (anodic overvoltages) of ca. 40 mV at 400 mA cm^− 2 at 1000 °C. However, supplying the simulated biogas, AMg and AMgAl showed smaller losses of 25 and 29 mV, respectively, than those in supplying hydrogen, whereas A showed the loss of more than 40 mV. Through this study, it was revealed that when the biogas is selected as a fuel, the electrochemical efficiency of the internal reforming SOFC is enhanced by using AMg or AMgAl as anode materials instead of A. Although the higher performances of AMg and AMgAl mainly result from the stability of small Ni particles against sintering, in addition to this effect, basic (Ni,Mg)O solid solution or MgO existing in the electrocatalysts contributes to further activity enhancement.     

Chemical vapor deposition of multi-walled carbon nanotubes from nickel/yttria-stabilized zirconia catalysts

Multi-walled carbon nanotubes (MWNT) were produced by chemical vapor deposition using yttria-stabilized zirconia/nickel (YSZ/Ni) catalysts. The catalysts were obtained by a liquid mixture technique that resulted in fine dispersed nanoparticles of NiO supported in the YSZ matrix. High quality MWNT having smooth walls, few defects, and low amounts of by-products such as amorphous carbon were obtained, even from catalysts with large Ni concentrations (>50 wt. %). By adjusting the experimental parameters, such as flux of the carbon precursor (ethylene) and Ni concentration, both the MWNT morphology and the process yield could be controlled. The resulting YSZ/Ni/MWNT composites can be interesting due to their mixed ionic-electronic transport properties, which could be useful in electrochemical applications. PACS 61.46.Fg; 81.15.Gh; 82.45.Jn     

In-situ neutron diffraction study of phase stress evolutions in a Ni-based porous anode solid oxide fuel cells under uniaxial load

Ni/YSZ porous composite is used widely as anode for solid oxide fuel cells. In this study, neutron diffraction patterns were recorded in-situ while a bulk porous Ni–NiO–YSZ anode was loaded under uniaxial compression. Single peak refinement was used to calculate the lattice strains of each phase in the composite, and the local stress state of each phase was derived from the measured lattice strains and the corresponding diffraction elastic constants. An internal triaxial stress state was observed to develop in the bulk of the specimen under plastic deformation, specifically in the Ni phase. Meanwhile, the NiO and YSZ phases are deforming elastically even in the macroscopically plastic regime of deformation. The von Mises equivalent stress was used to quantify the phase stress evolution. A significant stress concentration induced by the presence of pores becomes manifest in all three phase components. The reduction of stress concentration factor in Ni above the yield point of the composite can be attributed to a gradual change of the grains–pores morphology during the plastic deformation.