Understanding of redox behavior of Ni–YSZ cermets

The oxidation of Ni–YSZ cermet as well the reduction of re-oxidized Ni–YSZ cermet was investigated by using temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR) and scanning electron microscope (SEM). The scanning electron microscope (SEM) photographs and temperature-programmed reduction (TPR) profiles indicated that the sintering of smaller nickel oxide crystallites to larger aggregates occurred concurrently with the formation of smaller nickel oxide crystallites from the oxidation of nickel at 800 °C, and the sintering of smaller nickel oxide crystallites at 600 °C was slower than that at 800 °C. The SEM results showed that each Ni particle was separated into a lot of smaller NiO particles during oxidation. The TPO profiles showed that two kinds of nickel particles exist in the anode reduced at 800 and 600 °C, one with high activity towards oxidation for the nickel crystallites directly from reduction, and another one with low activity towards oxidation for the sintered nickel particles. The Ni–YSZ anodes reduced at higher temperature showed higher re-oxidation temperature than the one reduced at lower temperature because of the accelerated passivating and sintering of the smaller nickel particles at higher temperature. The re-oxidation profiles were almost unchanged during redox cycling at 600 °C, whereas the re-oxidation peak temperature decreased during redox cycling at 800 °C, indicating that the primary nickel grains split to smaller ones upon cyclic reduction at higher temperature.     

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.     

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.     

Composite (La,Sr)MnO3–YSZ cathode for SOFC

(La, Sr)MnO₃ (LSM)–Y doped ZrO₂ (YSZ) composite was prepared using YSZ colloidal suspension (initial YSZ particle size ∼100 nm), YSZ and LSM polymer precursors on dense substrates at 800 °C annealing temperature. The results of a symmetrical LSM–YSZ composite cell test showed the area specific resistance for overpotential of 0.14 Ω cm² at 800 °C, which indicated that the LSM–YSZ composite could be a potential candidate for cathode in SOFCs. The performance of the cell with the LSM–YSZ composite cathode and Ni-YSZ anode was investigated and the power density of about 0.26 W cm^− 2 was obtained at 850 °C using hydrogen fuel.     

Fe alloying effect on the performance of the Ni anode in hydrogen fuel

Composite materials of YSZ (yttria-stabilized zirconia) with various Ni–Fe alloys were synthesized and evaluated as the solid oxide fuel cell (SOFC) anode using a 200-µm thick YSZ electrolyte as support and YSZ +La_0.8Sr_0.2MnO₃ (LSM) as cathode. The single cell with the YSZ + Ni_0.75Fe_0.25 anode exhibited the highest performance among all the investigated cells, e.g. a peak power density of 403, 337, 218 and 112 mW/cm² was achieved with H₂ fuel at 900, 850, 800 and 750 °C, respectively. The composite anode with the Ni_0.75Fe_0.25 alloy also had the lowest polarization resistance of 0.55 Ω·cm² at 800 °C among all the alloy compositions, indicating that this specific alloy offered a better anode composition than pure Ni. The possible mechanism for the improved performance of Ni with the Fe alloying addition towards H₂ oxidation was discussed.     

Effect of starting powder on screen-printed YSZ films used as electrolyte in SOFCs

Screen-printing technology was developed to fabricate dense YSZ electrolyte films onto NiO–YSZ porous anode substrates. A single fuel cell of Ni-YSZ/YSZ (31 μm)/LSM-YSZ was successfully prepared by screen-printing technology. Using humidified hydrogen as fuel and ambient air as oxidant, the fuel cell provided the maximum power densities of 0.18, 0.33, 0.58, 0.97 and 1.3 W/cm² at 650, 700, 750, 800 and 850 °C, respectively. The properties of the starting YSZ powder exerted a significant effect on the characteristics of the screen-printed YSZ electrolyte films. The aggregates of the powder could be partially broken by ball milling. The YSZ powder with a small particle size and a narrow particle size distribution helped to obtain dense YSZ films. The films prepared from the YSZ powder with high aggregates were very porous, which resulted in a low open circuit voltage, a high ohmic resistance, a high polarization resistance and thus a poor cell performance.     

Spray pyrolyzed NiO–C nanocomposite as an anode material for the lithium-ion battery with enhanced capacity retention

NiO–C nanocomposite was prepared by a spray pyrolysis method using a mixture of Ni(NO₃)₂ and citric acid solution at 600 °C. The microstructure and morphology of the NiO–C composite were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) mapping, and thermogravimetric analysis (TGA). The results showed that the NiO nanoparticles were surrounded by amorphous carbon. Electrochemical tests demonstrated that the NiO–C nanocomposites exhibited better capacity retention (382 mAh g^− 1 for 50 cycles) than that of pure NiO (141 mAh g^− 1 for 50 cycles), which was also prepared by spray pyrolysis using only Ni(NO₃)₂ as precursor. The enhanced capacity retention can be mainly attributed to the NiO–C composite structure, composed of NiO nanoparticles surrounded by carbon, which can accommodate the volume changes during charge–discharge and improve the electrical conductivity between the NiO nanoparticles.     

Structural and electrical properties of ZnO-doped 8 mol% yttria-stabilized zirconia

8 mol% Yttria-stabilized zirconia (8YSZ) powder was prepared by coprecipitation. ZnO (0.5, 1.0, 2.0, 5.0, 10.0 wt.%) was added to the YSZ powder through a mechanical mixing method. The densification , microstructure and electrical properties of the YSZ ceramics sintered at 1300 °C for 2 h, were investigated. It was found that the small addition of ZnO was effective in reducing the sintering temperature and promoting the densification rate of the ceramics. The 5.0 wt.% ZnO-doped YSZ has ∼ 96% relative density, as compared to ∼ 89% relative density for the undoped sample. The total conductivity of 8YSZ was evidently increased by doping small amount of ZnO. For the 0.5 wt.% doped sample, the total conductivity of 2.89 × 10^− 2 Ω^− 1 cm^− 1 and an increase of 120% in conductivity were observed at 800 °C, as compared to that of the undoped one. We also found that the grain boundary (GB) conductivity could be improved by small addition of ZnO. At intermediate temperature (∼ 300 °C), the maximum enhancement of GB conductivity was observed with 5.0 wt% ZnO dopant. Finally, the volume percentage of GB in the ceramics was estimated by the brick layer model. The possible mechanism related to the improved GB conduction of the YSZ due to the ZnO additions was discussed.     

Improvement of Cu–CeO2 anodes for SOFCs running on ethanol fuels

Ethanol is considered to be an attractive green fuel for solid oxide fuel cells (SOFCs) due to several advantages. In this paper, we presented recent progress of our group in Cu–CeO₂ anodes for SOFCs with ethanol steam as a fuel. Cu–CeO₂–ScSZ (scandia stabilized zirconia)anodes with different ratios of copper versus ceria were fabricated and the impedance spectra of symmetric cells were measured to optimize the anode composition. Area specific resistance (ASR) of these anodes was examined to prove the thermal stability of them, and possible reasons for degradation were analyzed. Furthermore, a Ni–ScSZ interlayer was added between Cu–CeO₂–YSZ (yttria stabilized zirconia) anode and ScSZ electrolyte to improve the anode performance, and the three-layer structure was fabricated by acid leaching of nickel and wet impregnation method. The maximum power density of the single cell reached 604 mW cm^? 2 and 408 mW cm^? 2 at 800 °C in hydrogen and ethanol steam respectively, and the cell obtained stable output in ethanol steam over an operation period of 50 h.     

Synthesis of nickel nanoparticles supported on hollow samaria-doped ceria particles via the solution-spray plasma technique: Anode catalysts for SOFCs

Aiming at SOFC anode applications, we have synthesized nanometer-sized nickel catalysts supported on hollow spherical particles of samaria-doped ceria (Ni/SDC) by spraying a mixed solution of nickel, samarium, and cerium nitrates into an atmospheric pressure plasma. The as-prepared particles consisted of SDC (average diameter d_SDC = ca. 0.8 µm) and uniformly dispersed nanometer-sized NiO particles. When reduced in H₂ at 800 °C or 1000 °C, Ni nanoparticles (average diameter d_Ni = 34 nm) were found to be embedded uniformly into the SDC surface.     

Performance of La-doped strontium titanate (LST) anode on LaGaO3-based SOFC

Ni-containing anode is currently used with many electrolytes of solid oxide fuel cells (SOFCs). However, Ni is easily oxidized and deteriorates the LaGaO₃-based electrolyte. A La-doped SrTiO₃ (LST, La_0.2Sr_0.8TiO₃) is a candidate as an anode material to solve the Ni poisoning problem in LaGaO₃-based SOFC. In this study, a single-phase LST and an LST-Gd_0.2Ce_0.8O_2 ? δ (GDC) composite were tested as the possible anodes on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 ? δ (LSGM) electrolyte. In order to further improve the anodic performance, Ni was impregnated into the LST-GDC composite anode. The performance was examined from 600 °C to 800 °C by measuring impedance of the electrolyte-supported, symmetric (anode/electrolyte/anode) cells. A polarization resistance (R_p) of LST-GDC anode was much reduced from that of LST anode. When Ni was impregnated into LST-GDC composite, the R_p value was further reduced to ~ 10% of the single-phase LST anode, and it was 1 Ωcm² at 800 °C in 97% H₂ + 3% H₂O atmosphere. A single cell with Ni-impregnated LST-GDC as an anode, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ (BSCF) as a cathode and LSGM as an electrolyte exhibited the maximum power density of 275 mW/cm² at 800 °C, increased from ~ 60 mW/cm² for the cell using the LST-GDC as an anode. Thus, LST-GDC composite is promising as a component of anode.     

Nanostructured Cu-CGO anodes fabricated using a microwave-assisted glycine–nitrate process

This work reports a study of nanostructured copper-doped gadolinium cermet (Cu-CGO) composite anodes prepared via conventional synthesis (CS) and microwave-synthesis (MS) involving the glycine–nitrate process (GNP). A detailed investigation on the mechanical properties, electrical conductivity and electrochemical performance of prepared Cu_0.5(Ce_0.9Gd_0.1)_0.5O_2−δ anodes is included. The prepared samples were characterized by techniques, such as XRD, EDX, SEM and electrical characterizations. After reduction in 10% H₂ and 90% N₂, the DC conductivities of the Cu-CGO anodes prepared via CS-GNP and MS-GNP are found to be 5.43×10³ and 1.09×10⁴ S cm⁻¹ at 700 °C, respectively. The electrochemical performances of the spin-coated anode symmetrical cells sintered at 700 °C are evaluated at cell operating temperatures of 600, 700 and 800 °C. The lowest area specific resistance (ASR) values for the Cu-CGO/CGO/Cu-CGO symmetrical cells prepared via the MS-GNP route at operating temperatures of 600, 700 and 800 °C are found to be 0.34, 0.71 and 1.10 Ω cm², respectively. The as-prepared (via MS-GNP) Cu-CGO anode exhibits excellent electrical and electrochemical performance consistent with the uniform nanostructured morphology compared with the anode prepared via CS-GNP.     

Fabrication and characterization of Ni-supported solid oxide fuel cell

Metal-supported solid oxide fuel cells (SOFCs) are promising due to their good mechanical and thermal properties. Stainless steels are often used as a supporting metal. In this study, the possible use of Ni as a supporting metal was tested. Ni is also a good model of supporting metal due to its lack of Cr which poisons Ni anode.YSZ electrolyte and Ni-YSZ anode were coated on a porous Ni support to fabricate Ni-supported SOFC. A porous Ni support in thick-film form (t ~ 150 µm) was prepared by the mold casting of Ni powders. An anode and an electrolyte layer were sequentially coated on the Ni support using screen printing and tape casting methods, respectively. The Ni-supported cell (t ~ 200 µm) was sintered at 1400 °C in a reducing atmosphere and the performance was evaluated at 800 °C with a La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathode. The unit cell showed an open circuit voltage (OCV) of 0.96 V and a peak power density of 470 mW/cm² at 800 °C.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

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.     

Effect of calcination temperatures on the electrochemical performances of nickel oxide/reduction graphene oxide (NiO/RGO) composites synthesized by hydrothermal method

A series of NiO/RGO composites based on NiO nanoparticles anchored on layered RGO surfaces were proposed by the same hydrothermal method combined with different calcination temperatures (250, 300, 400 and 500 °C). The effects of calcination temperatures on the capacitive behaviors have been discussed by investigating the components, morphologies, surface conditions of the NiO/RGO composites. The specific capacitance values of NiO/RGO composites calcined at 250, 300, 400 and 500 °C are 950, 553, 375 and 205 F/g at the current density of 1 A/g and the corresponding capacitance retention are 91.3%, 83.9%, 71.9% and 67.3% after 1000 cycles at the current density of 10 A/g. The results suggest the calcination temperature plays an important role in the electrochemical performances of NiO/RGO composites and the electrochemical performances were deteriorated with the increasing calcination temperatures.     

Magnetic properties of Ni/NiO nanocomposites synthesized by one step solution combustion method

Ni/NiO nanocomposites were synthesized using solution combustion method and characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and carbon, hydrogen, nitrogen (CHN) analyser. The Ni or NiO content in Ni/NiO nanocomposites vary with the quantity of HNO₃ used for the synthesis. Magnetic coercivity (H_c) of Ni/NiO nanocomposites is found to be 413 Oe which can be used in magnetic applications. A feeble exchange bias of 7 Oe is seen from the NiO rich Ni/NiO.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.     

Impedance spectroscopy study of the sintering of yttria-stabilized zirconia/magnesia composites

In the present study, the sintering of (ZrO₂:8 mol%Y₂O₃)_1 ? m–(MgO)_m, YSZ–mMg composites, with m in the 0–30 mol% range, has been investigated by impedance spectroscopy (IS), dilatometry, and X-ray diffraction. Impedance diagrams were collected at 400 °C after heating the green compacts up to a selected sintering temperature, which was increased stepwise from 800 to 1400 °C. The combined experimental results revealed that the samples can be separated in two categories: below and above the solubility limit of MgO in the YSZ (m ~ 10). Moreover, important microstructural features associated with both the sintering process and solid solution formation of YSZ–mMgO samples were correlated to the electrical properties inferred by IS.     

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.