Development of solid oxide fuel cells based on a Ce(Gd)O2−x electrolyte film for intermediate temperature operation

Initial tests have been carried out with the fuel cell arrangement La_0.6Sr_0.4Co_0.2Fe_0.8O₃Ce_0.9Gd_0.1O_1.95Ni/YSZ, incorporating dense film (5–10 μm) Ce_0.9Gd_0.1O_1.95 electrolyte tape cast onto the supporting anode, to investigate the feasibility of intermediate temperature operation (500–700°C). A good open circuit voltage of approx. 0.8 V was obtained at 550°C using moist hydrogen as the fuel. Slightly lower open circuit voltages were found at higher temperatures, which may have been caused by minor gas leakage and the electronic conductivity of the electrolyte. Power outputs in excess of 100 mW/cm² were obtained at 650°C, and the cell resistance was 0.8Ω cm² at this temperature. This resistance, and the greater resistance at lower temperature, was predominantly due to the cathode according to AC impedance measurements. Experiments were also carried out at 600°C using direct methanol fuels at the anode; the maximum power output was approximately half of that obtained with hydrogen.     

Solid state protonic conductor NH4PO3–(NH4)2Mn(PO3)4 for intermediate temperature fuel cells

A new proton-conductive composite of NH₄PO₃–(NH₄)₂Mn(PO₃)₄ was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH₄)₂Mn(PO₃)₄ was stable as a supporting matrix for NH₄PO₃. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH₄PO₃ in both dry and wet atmospheres. A conductivity of 7 mS cm⁻¹ was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 10²–10⁵ Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte. Fuel cells using 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm² was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH₄PO₃–(NH₄)₂Mn(PO₃)₄ electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.     

Study of the reverse water gas shift (RWGS) reaction over Pt in a solid oxide fuel cell (SOFC) operating under open and closed-circuit conditions

The (electro-)kinetics of the reverse water gas shift (RWGS) reaction was studied in a solid oxide fuel cell (SOFC) of the type Pt/YSZ/Pt. The effect of imposed potentials, cell temperature (650–800 °C), H₂ (1–10 kPa) and CO₂ (1–10 kPa) partial pressures on the kinetics and mechanism of the catalytic and electrocatalytic RWGS reaction, were systematically examined. The apparent catalytic activation energy was found equal to 15.6 kcal/mol, while H₂ and CO₂ apparent reaction orders were equal to 0.5 and 0.7, respectively. At both open and closed circuit operation, the associative formate decomposition reaction mechanism was considered to describe kinetics. Under closed circuit operation, rate enhancement factor, |Λ|, values up to 10 were achieved. Finally, current density–voltage and current density–power density characteristics of the cell were recorded at various temperatures and gas mixtures of CO₂ and H₂. It was found that electrical power output of the cell was optimized by increasing temperature and decreasing CO₂/H₂ feed ratio. Maximum power density obtained was 9 mW/cm² (at 520 mV cell voltage and a current density of 17.3 mA/cm², at 800 °C and P_CO2/P_H2=0.16).     

Plasma-Sprayed YSZ/Ni–LSGM–LSCo Intermediate-Temperature Solid Oxide Fuel Cells

An intermediate-temperature solid oxide fuel cell based on YSZ/Ni anode, LSGM electrolyte, and lanthanum strontium cobaltite (LSCo) cathode coatings were sequentially deposited onto a porous Ni substrate by atmospheric plasma spraying (APS). The spray parameters for each coating are well selected. The sprayed YSZ/Ni anode having a novel nanostructure with advantageous triple phase boundaries after hydrogen reduction shows a good electrocatalytic activity for hydrogen oxidation reactions. Dense LSGM with a thickness of about 60 μm and a conductivity of about 0.053 S/cm at 800°C shows a good gas tightness and gives an open circuit voltage value >1 V. The sprayed LSCo cathode with a thickness of 10–20 μm and a porosity of about 25% keeps the right phase structure and good porous network microstructure for conducting electrons and negative oxygen ions after plasma spraying and heat treatment at about 1000°C for 1 h. A maximum output power density of the sprayed cell achieved 365 mW/cm² at 800°C, 250 mW/cm² at 750°C, and 180 mW/cm² at 700°C. The results show that the use of APS cell allowed the reduction of the operating temperature to below 750°C.     

Preparation of ultra-active alumina of designed porous structure by successive hydrothermal and thermal treatments of porous anodic Al2O3 films

A porous anodic alumina film was prepared by the anodic oxidation of Al metal sheet in a thermostated and vigorously stirred bath of H₂SO₄ 15% (w/v) at a temperature of 25°C and a current density of 15 mA cm⁻². It had a geometric surface area of 33 cm², a surface density of pores 1.269×10¹¹ cm⁻² and the maximum limiting thickness and porosity achieved at these conditions which are 50.3 μm and 0.42, respectively. This oxide was tried in the catalytic test reaction of the decomposition of HCOOH at temperatures 270–390°C. Then, the oxide was treated hydrothermally in H₂O at 100°C for 5 h and tried in the same test reaction. The procedure of hydrothermal treatment and catalysis experiment was repeated 40 times. In all cases the oxide showed an almost exclusively dehydrative catalytic effect, 98–100%. Both the total activity of the alumina film with the aforementioned constant geometric surface area and its specific activity referred to the unit of oxide mass gave a maximum in the first and a minimum about the fourth hydrothermal treatment; then, they increased strongly with the order of hydrothermal treatment. Despite the decrease of the oxide mass during hydrothermal treatment, the final promotion of the total catalytic activity of oxide was 13.7–10.6 times that of non-treated oxide for temperatures 330–390°C. The corresponding promotion of specific activity was 31.5–24.5 times that of the non-treated oxide. The results of the present study showed that the successive hydrothermal and thermal treatments of porous anodic Al₂O₃ films produce more and more active alumina catalysts. In this way ultra-active alumina catalysts or supports can be prepared.     

Fabrication of electrolyte materials for solid oxide fuel cells by tape-casting

In this research, solid oxide fuel cell electrolytes were fabricated by aqueous tape-casting technique. The basic compositions for SOFC electrolyte systems were focused on yttria-stabilized zirconia (YSZ) system. The powders used in this study were from different sources. ZrO₂-based system doped with 3, 8, and 10 mol% of Y₂O₃, and 8YSZ electrolyte tape illustrated the desirable properties. The grain size of the sintered electrolyte tapes was in the range of 0.5–1 μm with 98–99% of theoretical density. Phase and crystal structure showed the pure cubic fluorite structure for 8–10 mol% YSZ and tetragonal phase for 3 mol% doped. The electrolyte tapes sintered at 1450 °C for 4 h had the highest ionic conductivity of 30.11 × 10⁻³ S/cm which was measured at 600 °C. The flexural strengths were in the range of 100–180 MPa for 8–10 mol% YSZ, and 400–680 MPa for 3 mol% YSZ.     

A Novel Method for Fabrication of Y2O3-Stabilized ZrO2 Electrolyte Films

Gas-tight Y₂O₃-stabilized ZrO₂ (YSZ) films were prepared on NiO–YSZ and NiO–SDC (Sm_0.2Ce_0.8O_1.9) anode substrates by a novel method. A cell, Ni–YSZ/YSZ(10 μm)/LSM–YSZ, was tested with humidified hydrogen as fuel and ambient air as oxidant. The maximum power densities of 1.64, 1.40, 1.06, and 0.60 W/cm² were obtained at 850°, 800°, 750°, and 700°C, respectively. With methane as fuel, a cell of Ni–SDC/YSZ (12 μm)/LSM–YSZ exhibited the maximum power densities of 1.14, 0.82, 0.49, and 0.28 W/cm² at 850°, 800°, 750°, and 700°C, respectively. The impedance results showed that the performance of the cell was controlled by the electrode polarization rather than the resistance of YSZ electrolyte film.     

Ba0.5Sr0.5Zn0.2Fe0.8O3−δ Perovskite Oxide as a Novel Cathode for Intermediate-Temperature Solid-Oxide Fuel Cells

A cobalt-free perovskite oxide, Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF), was investigated as a novel cathode material for intermediate-temperature solid-oxide fuel cells (IT-SOFCs). Room-temperature nonstoichiometry of BSZF was as high as 0.412, which was favorable for oxygen–reduction reaction. At 700°C, the polarization resistances were as low as 0.22 and 0.13 Ω·cm², respectively, for pure and impregnated BSZF cathodes under open-circuit conditions, suggesting that oxygen-deficient BSZF was a promising cathode candidate for IT-SOFCs.     

Production of high porosity nanoparticles of cerium oxide in clay

Cerium oxide nanoparticles modified montmorillonite was obtained by interaction of a clay with (NH₄)₂Ce(NO₃)₆. The mean size of cerium oxide nanoparticles in clay was at 3.5 nm. The product was an amorphous solid and showed high permanent porosity and stability at high temperatures. The amorphous structure of the sample was proven by X-ray diffraction and electronic diffraction. The porous structure was studied by means of chemisorption and it was shown that samples calcined at 550 °C had S_BET = 239 m²/g; micropore volume = 0.1839 cm³/g; average pore diameter = 3.07 nm.     

Low temperature synthesis of MgxAl2(1−x)Ti(1+x)O5 films by sol–gel processing

Aluminium titanate films thicker than 0.5 μm have been synthesized by sol–gel methods. The films have been deposited via repetitive dip-coating on silicon wafers and their thermal stability has been tested as a function of the annealing time and temperature. The sol–gel approach has allowed the formation of the aluminium titanate phase at temperatures (700 °C) much lower than those necessary for solid-state reactions (1450 °C). Magnesium oxide has been used to improve the thermal stability of the films at high temperatures. The behavior of samples prepared with two different Mg content, i.e. Mg_0.2Al_1.6Ti_0.8O₅ and Mg_0.6Al_0.8Ti_1.6O₅, has been studied. The films have proven to be stable at 1150 °C, for up to 90 h. X-ray photoelectron spectroscopy has shown that after firing at 500 °C the surface chemical composition of the films is in accordance with the nominal one, whilst at higher annealing temperatures some differences, attributed to diffusion effects, have been observed.     

Kinetics of asphaltenes conversion during hydrotreating of Maya crude

The kinetics of asphaltene conversion was studied during the hydrotreating of Maya heavy crude oil. Experimental tests were conducted in a pilot plant at the following reaction conditions: total pressure of 70–100 kg/cm², liquid hourly space-velocity (LHSV) of 0.33–1.5 h⁻¹, and reaction temperature of 380–420 °C at a constant hydrogen-to-oil ratio of 5000 ft³/bbl. A commercial NiMo/Al₂O₃ catalyst was used in all experiments. Asphaltenes were precipitated from Maya crude and from hydrotreated products in a Parr Batch Reactor at 25 kg/cm² and 60 °C with n-heptane as solvent. Asphaltene hydrocracking data were used to estimate reaction orders and activation energy using a power-law model, and the average absolute error between experimental and calculated concentrations of asphaltenes was found to be less than 5%.     

Phase transitions of antiferroelectric lead zirconate thin films in high electric field

Phase transitions in antiferroelectric lead zirconate thin films were studied at room temperature and at 77 K. The lead zirconate films were prepared on Pt coated Si substrates by a reactive magnetron co-sputtering mthod followed by a rapid thermal annealing process at 700 °C. An electric field induced antiferroelectric — ferroelectric phase transition was observed at room temperature with a maximum polarization value of 70 μC/cm². The average field required to induce the ferroelectric state and that for the reversion to the antiferroelectric state were 294 kV/cm and 179 kV/cm respectively. At 77 K a metastable ferroelectric — ferroelectric transition was observed with a maximum polarization of 58 μC/cm². These transitions were found to be coincident with those of lead zirconate single crystals.     

Electrochemical behavior of Ln0.6Sr0.4Co0.2Fe0.8O3−δ (Ln=Ce, Gd, Sm, Dy) materials used as cathode of IT-SOFC

The perovskite-type compounds Ln_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (Ln=Ce, Sm, Gd, Dy) used as the cathodes of intermediate temperature solid oxide fuel cell (IT-SOFC) were studied. The cells consisted of anode supported Sm-doped-ceria electrolyte bi-layer and cathode with 0.65 cm² effective area. Open-circuit voltage (OCV), V–I and P–I curves of the cells were measured over a temperature range from 400 to 800 °C, using H₂–3%H₂O as fuel and air as oxidant. Polarization potential of electrodes were measured with asymmetry three-electrode method during cell discharging. The results indicated that, Dy-SCF material cathode behaved with high catalytic activity for oxygen dissociation at low temperatures. For each cell with a particular cathode, there was a transition temperature, at which OCV of the cell reached the highest value. When temperature was higher than the transition temperature, OCV of the cell increases with decreasing temperature, whereas as temperature was lower than that, OCV decreased with lowering temperature.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia powder from sucrose derived organic precursors

An organic precursor synthesis of 8 mol% yttria stabilized zirconia (YSZ) powder from Zr–Y composite nitrate solution and sucrose has been studied. Oxidation of sucrose in Zr–Y composite nitrate solution containing excess nitric acid in situ generates hydroxy carboxylic acids that forms a white sol which showed peaks at 1640 cm⁻¹ and 1363 cm⁻¹ in IR spectrum corresponding to hydroxy carboxylic acid complexes of Zr and Y. Precursor mass obtained by drying the sol on calcinations at 600 °C produced loosely agglomerated particles of cubic YSZ. Deagglomerated YSZ contain submicron particles with D₅₀ value of 0.5 μm and the particles are aggregates of nanocrystallites of nearly 10 nm size. Compacts prepared by pressing the YSZ powder sintered to 96.7% TD at 1450 °C. The sintered YSZ ceramic showed an average grain size of 2.2 μm.     

Combinatorial investigation of Pt–Ru–M as anode electrocatalyst for direct methanol fuel cell

The addition of Au/TiO₂ and zeolites as active components to PtRu/C electrode in DMFC was investigated by using combinatorial high-throughput-screening test. Addition of Au/TiO₂ to PtRu/C electrode, especially in the ratio of PtRu/C: Au/TiO₂ 9:1, 8:2, 7:3, were effective to improve the performance of direct methanol fuel cell. The electrochemical properties of the prepared electrodes were compared using cyclic voltammetry, impedance spectroscopy and a single cell performance test of a direct methanol fuel cell (DMFC). The adsorbed CO on Pt might be easily oxidized on the surface of Au/TiO₂ by interaction between PtRu/C and Au/TiO₂. The addition of the solid acid proton conducting materials (ZSM-5) on PtRu/C anode leads to the high temperature operation. The cell performance was maintained over the cell temperature 120 °C (maximum current density was 200 mA/cm² at 160 °C) by the addition of ZSM-5 as proton conducting materials.     

Fabrication and characterisation of CuInSe2/Si(1 0 0) thin films by the stacked elemental layer (SEL) technique

Thin films of (Cu/In/Se) were fabricated by evaporated elemental layers of Cu, In and Se on Si (1 0 0) and on glass substrates at T_S = 250 °C. Films with phase chalcopyrite structure and strong (1 1 2) preferred orientation were produced. EDX showed uniform compositional properties of the films over a substrate area of 1 cm². The optical energy band gap of 0.984 eV was obtained and photoluminescence measurements have been carried out in as-deposited polycrystalline Cu/In/Se thin films deposited onto (1 0 0) oriented Si wafers doped with 10¹⁵ cm⁻³ of boron. The PL spectra of CuInSe₂ show emission peaks at 0.87 eV ranging from 0.75 to 0.98 eV. The broad emission band is ascribed to donor–acceptor pair (DAP) transition.     

Properties and Performance of Cation-Doped Ceria Electrolyte Materials in Solid Oxide Fuel Cell Applications

Cation-doped CeO₂ electrolyte has been evaluated in single-cell and short-stack tests in solid oxide fuel cell environments and applications. These results, along with conductivity measurements, indicate that an ionic transference number of ∼0.75 can be expected at 800°C. Single cells have shown a power density >350 mW/cm². Multicell stacks have demonstrated a peak performance of >100 mW/cm² at 700°C using metallic separators.     

Cathodic electro-deposition of low-temperature curable water-soluble polyepoxide coatings

Epoxidised phenol-novolac (EPN) type of polyepoxide resin was used for developing water-soluble cathodically electro-depositable coatings which could be self-curable without using any external/additional cross-linking agent. The self-curing of cathodically deposited films could be effected at a low temperature of 80°C in 30 min, as compared to 150–170°C in 30 min practised presently in many commercial cathodic electro-deposition (CED) installations. In this work, effect of varying molar ratio of secondary amines such as diethanol amine (DEtOA) and diethyl amine to epoxidised phenol-novolac (EPN) on the solubility of EPN–secondary amine adducts prepared at 80, 60 and 30°C for varying reaction times was investigated. Self-curing characteristics of EPN–DEtOA (1:2 moles) adduct at 30–160°C were studied.

Kinetics of film growth during CED was investigated by using aqueous solutions of acetic acid neutralised EPN–DEtOA (1:2 moles) adduct. Variables for kinetic studies were deposition time (30–600 s), applied voltage (5–250 V) and solution concentration (1–20% (w/w)). By carrying out CED of polyepoxide films, values of electro-deposition (ED) characteristics found were: ED yield as 5.89 mg/cm², Coulombic yield as 19.78 mg/C and dry film thickness (DFT) as 52.12 μm. Plots of DFT vs. deposition time (t), DFT vs. t^1/2 and current density vs. field strength were drawn to calculate the values of Coulombic efficiency of the process and film conductivity. Cathodically electro-deposited films were finally characterised for physical properties, chemical resistance and corrosion protection.     

Effects of LaNiO3 buffer layers on preferential orientation growth and properties of PbTiO3 thin films

(1 0 0)- and (1 0 1)-oriented PbTiO₃ (PT) thin films on conductive LaNiO₃ (LNO)-coated Si(1 1 1) substrates were prepared by a metal-organic decomposition method. It is found that the crystallization states of LNO thin films used as buffer layers have significantly effects on preferential orientation of PT thin films. PT thin films with (1 0 0) orientation could be obtained not only on the crystalline LNO (1 0 0) film, but also on the amorphous LNO thin film. The highly (1 0 0)-oriented PT films show high dielectric constant of 189.4 on LNO (1 0 0) films and 183.1 on amorphous LNO films. The PT capacitors fabricated on the LNO buffer layers display good P–E hysteresis loops. The remnant polarization (P_r) and coercive field (E_c) of PT films on amorphous, (1 0 0)- and (1 1 0)-oriented LNO films are 9.35 μC/cm² and 162.8 kV/cm, 10.03 μC/cm² and 163.3 kV/cm, 11.23 μC/cm² and 166.2 kV/cm, respectively.     

Foaming of Filled Polyurethanes for Fabrication of Porous Anode Supports for Intermediate Temperature-Solid Oxide Fuel Cells

The development of a tailored microstructure is considered essential for the development of anode-supported intermediate temperature-solid oxide fuel cells (IT-SOFCs) as it is expected to enhance kinetics at the triple phase boundary. In this study, the application of an in situ foaming technique for the preparation of Ni-yttria-stabilized zirconia (YSZ) cellular solids as an anode support for IT-SOFCs is presented. The cermet microstructure is dependent on the preparative route followed. The presence of contaminants in the commercial template precursor was found to be detrimental for electrical properties. A high-purity polyurethane was then formulated, and tailored Ni–YSZ foams were obtained with 87% porosity. The foam microstructure is characterized by a hierarchical architecture, with interconnected networks of Ni and YSZ particles, large pores related to the open cell structure, and a submicron porosity of the structural trabecular network, with a conductivity value of 80 S/cm at 700°C.