Formation of NiO/YSZ functional anode layers of solid oxide fuel cells by magnetron sputtering

The decrease in the polarization resistance of the anode of solid-oxide fuel cells (SOFCs) due to the formation of an additional NiO/(ZrO₂ + 10 mol % Y₂O₃) (YSZ) functional layer was studied. NiO/YSZ films with different NiO contents were deposited by reactive magnetron sputtering of Ni and Zr–Y targets. The elemental and phase composition of the films was adjusted by regulating oxygen flow rate during the sputtering. The resulting films were studied by scanning electron microscopy and X-ray diffractometry. Comparative tests of planar SOFCs with a NiO/YSZ anode support, NiO/YSZ functional nanostructured anode layer, YSZ electrolyte, and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were performed. It was shown that the formation of a NiO/YSZ functional nanostructured anode leads to a 15–25% increase in the maximum power density of fuel cells in the working temperature range 500–800°C. The NiO/YSZ nanostructured anode layers lead not only to a reduction of the polarization resistance of the anode, but also to the formation of denser electrolyte films during subsequent magnetron sputtering of electrolyte.     

Yttria stabilized zirconia solid electrolyte surface modification with ZrO<Subscript>2</Subscript>, Y<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript> + 9 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> films

The surface of ceramic electrolyte ZrO₂ + 9 mol % Y₂O₃, hereinafter referred to as YSZ (abbreviated yttria stabilized zirconia), was modified with 0.1 to 0.2 μm oxide films of ZrO₂, Y₂O₃, and YSZ (same composition as substrate) by dip coating in alcohol solutions of the relevant salts and further annealing. The results of scanning electronic microscopy and X-ray diffraction evidence epitaxial film growth. By means of impedance spectroscopy at the temperatures of 500 to 600°C, the effect of YZS electrolyte surface modification with ZrO₂, Y₂O₃, and YSZ films to the polarization resistance of silver electrode was studied.     

Calorimetric analysis of dysprosia and dysprosia-doped zirconia ceramics

Dysprosia (Dy₂O₃) and dysprosia-doped ZrO₂ (~10?wt% Dy₂O₃) samples were subjected to calorimetric and thermal analyses to understand the effect of dysprosia doping on the thermal properties of ZrO₂ and to compare the thermal conductivity of dysprosia-doped ZrO₂ (DySZ) to standard 7YSZ (7?wt% Y₂O₃ in ZrO₂). All doped samples were plasma sprayed and subsequently sintered to ensure material densification (reduced porosity for bulk property analysis) and sufficient diffusion of constituents throughout the samples. Differential scanning calorimetry was used to measure the specific heat capacity values for powder and sintered samples as a function of temperature. The thermal conductivities of sintered samples were measured using laser flash techniques. The results showed that the addition of dysprosia to ZrO₂ has lowered both the specific heat capacity and thermal diffusivity when compared to the standard 7YSZ. The resulting thermal conductivity of DySZ was 75% lower than that of 7YSZ under the sintered condition.     

Magnetron formation of Ni/YSZ anodes of solid oxide fuel cells

Physico-chemical and structural properties of nanocomposite NiO/ZrO₂:Y₂O₃ (NiO/YSZ) films applied using the reactive magnetron deposition technique are studied for application as anodes of solid oxide fuel cells. The effect of oxygen consumption and magnetron power on the discharge parameters is determined to find the optimum conditions of reactive deposition. The conditions for deposition of NiO/YSZ films, under which the deposition rate is maximum (12 μm/h), are found and the volume content of Ni is within the range of 40–50%. Ni-YSZ films reduced in a hydrogen atmosphere at the temperature of 800°C have a nanoporous structure. However, massive nickel agglomerates are formed in the course of reduction on the film surface; their amount grows at an increase in Ni content in the film. Solid oxide fuel cells with YSZ supporting electrolyte and a LaSrMnO₃ cathode are manufactured to study electrochemical properties of NiO/YSZ films. It is shown that fuel cells with a nanocomposite NiO/YSZ anode applied using a magnetron sputtering technique have the maximum power density twice higher than in the case of fuel cells with an anode formed using the high-temperature sintering technique owing to a more developed gas-anode-electrolyte three-phase boundary.     

A simple and easy one-step fabrication of thin BaZr0.1Ce0.7Y0.2O3−δ electrolyte membrane for solid oxide fuel cells

A green BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) electrolyte layer was deposited on porous anode substrate (BZCY:NiO = 35:65, in weight ratio) by a suspension spray. In this process, the suspension was prepared by directly ball-milling the mixed BaCO₃, CeO₂, ZrO₂ and Y₂O₃ powders in ethanol for 24 h. Then the bi-layers were co-sintered at 1400 °C for 5 h in air to obtain dense and uniform electrolyte membrane in the thickness of 10 μm. With Nd_0.7Sr_0.3MnO_3−δ cathode, a fuel cell was assembled. It was tested from 600 °C to 700 °C using humid hydrogen as fuel and air as oxidant. The cell at 700 °C exhibited 1.02 V for open circuit voltage (OCV), 450 mW/cm² for peak output and 0.18 Ω cm² for electrode polarizations under open circuit conditions, respectively. The results indicate that it is feasible to fabricate thin electrolyte membrane for solid oxide fuel cells (SOFCs) by this simple, cost-effective and efficient technique.     

Analysis of chemical interactions between stabilized zirconia and perovskites

LaMnO₃-based perovskites are used as cathode materials in solid oxide fuel cells (SOFC). A major aspect for their applicability is their chemical inertness in connection with the electrolyte material YSZ (Zr_0.85Y_0.15O_1.93) against zirconate formations. Perovskites with the composition La_y-x(Sr, Ca)_xMn_1-uCo_uO₃ (y = 1.0 and 0.95; x = 0– 0.2 and 1; u = 0 and 0.2) were investigated with regard to their reactivity with YSZ at different reaction times and temperatures. Powder mixtures and double-layer reaction couples were used for the investigations. XRD phase analyses, SEM/EDX and EPMA were applied for the characterization of the annealed samples. La-deficient perovskites (y = 0.95) partially substituted by Sr and Ca improve the chemical compatibility of perovskite compositions towards YSZ. Sr-containing perovskites were found to have a higher reactivity than Ca perovskites for La₂Zr₂O₇ formation. On the other hand enhanced Ca diffusion into YSZ was observed. Co substitution on Mn lattice sites decreased the chemical compatibility, especially for Sr containing perovskites.     

Mid-temperature solid oxide fuel cells with thin film ZrO<Subscript>2</Subscript>: Y<Subscript>2</Subscript>O<Subscript>3</Subscript> electrolyte

Data on the mid-temperature solid-oxide fuel cells (SOFC) with thin-film ZrO₂-Y₂O₃ (YSZ) electrolyte are shown. Such a fuel cell comprises a carrying Ni-YSZ anode, a YSZ electrolyte 3–5 μm thick formed by vacuum ion-plasma methods, and a LaSrMnO₃ cathode. It is shown that the use of a combined method of YSZ electrolyte deposition, which involves the magnetron deposition of a 0.5–1.5-μm thick sublayer and its pulse electron-beam processing allows a dense nanostructured electrolyte film to be formed and the SOFC working temperature to be lowered down as the result of a decrease in both the solid electrolyte Ohmic resistance and the Faradaic resistance to charge transfer. SOFC are studied by the methods of voltammentry and impedance spectroscopy. The maximum power density of the SOFC under study is 250 and 600 mW/cm⁻² at temperatures of 650 and 800°C, respectively.     

Cellulose-precursor synthesis of nanocrystalline Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>2−δ</Subscript> for SOFC anodes

Developments of intermediate-temperature solid oxide fuel cells (IT SOFCs) require novel anode materials with a high electrochemical activity at 800–1070 K. The polarization of cermet anodes, made of nickel, ceria and yttria-stabilized zirconia (YSZ) and applied onto a YSZ solid electrolyte, can be significantly reduced by catalytically active ceria additions, the relative role of which increases with decreasing temperature. Further improvement is observed when using Ce_0.8Gd_0.2O_2– (CGO) having a high oxygen ionic conductivity instead of undoped ceria, owing to enlargement of the electrochemical reaction zone. Nanocrystalline CGO powders with grain sizes of 8–35 nm were thus synthesized via the cellulose-precursor technique and introduced into Ni–CGO–YSZ cermets, and tested in contact with a (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3– (LSGM) electrolyte at 873–1073 K. The results showed that the anode performance can be enhanced by additional surface activation, in particular by impregnation with a Ce-containing solution, and also by incorporation of YSZ, which probably acts as a cermet-stabilizing component. The overpotential of the surface-modified Ni–CGO (25 wt%–75 wt%) anode in a 10% H₂/90% N₂ atmosphere was approximately 110 mV at 1073 K with a current density of 200 mA/cm².Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

Cathodic materials for intermediate-temperature solid oxide fuel cells based on praseodymium nickelates-cobaltites

A unique combination of methods (TPD of O₂, thermogravimetry, isotopic heteroexchange of oxygen in different modes) was used to carry out detailed studies of oxygen mobility and reactivity in mixed praseodymium nickelates-cobaltites (PrNi_{1 ? x} Co _x O_{3 + δ}) and their composites with doped cerium dioxide (Ce_0.9Y_0.1O_{2 ? δ}) as promising cathodic materials stable towards the effect of CO₂ in the intermediate-temperature region. It is shown that in the case of composites of PrNi_{1 ? x} Co _x O_3+δ-Ce_0.9Y_0.1O_{2 ? δ} synthesized using the Pechini method and ultrasonic treatment, stabilization of the disordered cubic perovskite phase due to redistribution of cations between the phases provides high oxygen mobility. Preliminary results on tests of cathodic materials of this type supported on planar NiO/YSZ anodes (H.C. Starck) with a thin layer of YSZ electrolyte and a buffer Ce_0.9Y_0.1O_{2 ? δ} layer showed that power density of up to 0.4 W/cm² was reached in the region of medium (600–700°C) temperatures, which was close to typical values for fuel cells of this type with cathodes based on strontium-doped perovskites and their composites with electrolytes.     

Feasibility of simultaneous NO reduction and electricity generation in SOFCs with V2O5 or Cu added LSCF-GDC cathodes

Solid oxide fuel cell (SOFC) units were constructed with Ni-GDC (gadolinia-doped ceria) as the anode, YSZ (yttria-stabilized zirconia) as the electrolyte, and V₂O₅ or Cu added La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF)-GDC composites as the cathodes, respectively. The electrochemical NO reduction occurs over either V₂O₅ or Cu added LSCF-GDC cathode without or with the presence of oxygen. The maximum power density decreases with decreasing O₂ concentration. When the O₂ concentration is 3% or larger, simultaneous NO reduction and electricity generation in the SOFCs can be not only feasible but also beneficial via a synergistic effect.     

Preparation of YSZ thin films for intermediate temperature solid oxide fuel cells by dip-coating method

By studying the effects of different solvents, dispersants and solid loading amount on the suspension stability from sedimentation and viscosity experiments, a simple, effective and highly stable YSZ particle suspension based on MEK/EtOH was developed for dip-coating anode supported electrolyte films for intermediate temperature solid oxide fuel cells (IT-SOFCs). The morphologies of the prepared YSZ thin films from different dip-coating times were studied by scanning electron microscopy (SEM), and a film with a thickness of 16 μm by twice dip-coating was determined to be homogeneous, crack-free and well adherent to the anode substrate. The single cell assembled with this film presents an open-circuit voltage (OCV) of 1.01 V, and a maximum power density of 262 mW cm⁻² under H₂ as the fuel at 800 °C.     

Synthesis and properties of CaZrO<Subscript>3</Subscript> films on YSZ electrolyte surface

CaZrO₃ films are studied that were obtained on ceramic supports of solid electrolyte of ZrO₂ + 9 mol % Y₂O₃ (YSZ, yttria stabilized zirconia) from alcohol solutions of zirconium oxychloride and calcium nitrate using the method of dipping with the following drying and annealing. The thickness and morphology of films depend on the concentration of the film-forming solution. Vickers microhardness of the CaZrO₃ films was determined. The impedance spectroscopy method was used to study conductivity of films at the temperature of 400–600°C by comparison of impedance spectra of clean supports and supports with a film coating.     

Calcium-containing cathodic material for solid oxide fuel cells

Using the citrate sol-gel method, a new complex oxide Ca_0.75Y_0.25Co_0.15Mn_0.85O_2.92 is synthesized. It is shown that this compound is crystallized in the rhombically distorted version of perovskite structure (a = 0.53397(8), b = 0.7470(1), c = 0.52810(6) nm). The phase is characterized by a low coefficient of thermal expansion (CTE) (13.8 ppm K^?1) and high electric conductivity (135 S/cm at 900°C). The chemical reaction between Ca_0.75Y_0.25Co_0.15Mn_0.85O_2.92 and the YSZ and GDC electrolyte materials is studied. The material is highly reactive and reacts with YSZ and GDC at 900°C and 1070°C, respectively. It is concluded that Ca_0.75Y_0.25Co_0.15Mn_0.85O_2.92 is a promising cathodic material for solid oxide fuel cells, provided a reliable protection SDC sublayer is formed between the cathode and the YSZ membrane.     

Composite cathode materials Ag-Ba0.5Sr0.5Co0.8Fe0.2O3 for solid oxide fuel cells

Silver-Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) cathodes were prepared in two ways. In the first method, Ag-BSCF composite powder was prepared in ethanol solution, where Ag nanoparticles serving as a component in the preparation of Ag-BSCF composite cathodes had been previously obtained via one-step synthesis in absolute ethanol using a neutral polymer (polyvinylpyrrolidone). To the best of our knowledge, this is the first study to use a Ag sol obtained by the above method for preparation of Ag-BSCF composite powder. Then, a paste containing this powder was screen-printed on a Sm_0.2Ce_0.8O_1.9 electrolyte and sintered at 1,000 °C. In the second technique, an aqueous solution of AgNO₃ was added to a previously sintered BSCF cathode, which was then sintered again at 800 °C. The oxygen reduction reaction at the quasi-point BSCF cathode on the Sm_0.2Ce_0.8O_1.9 electrolyte was tested by electrochemical impedance spectroscopy at different oxygen concentrations in three electrode setup. The continuous decrease of polarization resistance was observed under polarization ?0.5 V at 600 °C. The comparative studies of both obtained composite Ag-BSCF materials were performed in hydrogen-oxygen IT-SOFC involving samaria-doped ceria as an electrolyte and Ni-Gd_0.2Ce_0.8O_1.9 anode. In both cases, the addition of silver to the cathode caused an increase in current and power density compared with an IT-SOFC built with the same components but involving a monophase BSFC cathode material.     

Remarkable Enhancement of O2 Activation on Yttrium‐Stabilized Zirconia Surface in a Dual Catalyst Bed

Yttrium‐stabilized zirconia (YSZ) has been extensively studied as an electrolyte material for solid oxide fuel cells (SOFC) but its performance in heterogeneous catalysis is also the object of a growing number of publications. In both applications, oxygen activation on the YSZ surface remains the step that hinders utilization at moderate temperature. It was demonstrated by oxygen isotope exchange that a dual catalyst bed system consisting of two successive LaMnO₃ and YSZ beds without intimate contact drastically enhances oxygen activation on the YSZ surface at 698 K. It can be concluded that LaMnO₃ activates the triplet ground‐state of molecular oxygen into a low‐lying singlet state, thereby facilitating the activation of the O₂ molecule on the YSZ oxygen vacancy sites. This phenomenon is shown to improve the catalytic activity of the LaMnO₃‐Pd/YSZ system for the partial oxidation of methane.     

Impedance Study of the Conductivity of Solid Oxide Electrolyte Films SrZr<Subscript>0.95</Subscript>Y<Subscript>0.05</Subscript>O<Subscript>3–δ</Subscript> and CaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3–δ</Subscript>

Information on the across-plane conductivity of films of solid-oxide electrolytes SrZr_0.95Y_0.05O_3–δ and CaZr_0.9Y_0.1O_3–δ deposited on ion-conducting supports is acquired by the impedance method. It is shown that the support/film interface and the intergrain boundaries considerably affect the across-plane charge transfer in the film. The effect of the crystallographic orientation of the YSZ support on the microstructure and conductivity of the CaZr_0.9Y_0.1O_3–δ electrolyte film is demonstrated.     

Improvement of the stability of NiO–YSZ anode material for solid oxide fuel cell

Improvement of long-term stability of 40vol.%NiO–60vol.% yttria-stabilized zirconia (YSZ) anode material in reducing atmosphere and under exposure to thermal shock through the modification of vacancy concentration and pore shape has been investigated for a solid oxide fuel cell. We varied the amount of Y₂O₃ additives from 8 to 10 mol% in YSZ and the type of carbon pore former, from plated activated carbon to spherical carbon black, to improve the strength and the stability of porous NiO–YSZ anode materials. Modifications by varying the amount of Y₂O₃ additives and carbon pore former result in a highly stable anode, even upon exposure to a reducing atmosphere for 1,200 h. In particular, the strengths of the new anode materials are markedly improved at the same porosity level. Higher strengths do not degrade during a longtime durability test in a reducing atmosphere or upon thermal shock testing. The relatively smaller degradation of electrical conductivity of the new anode material is discussed in terms of the possibility of suppression of the disconnectivity of Ni phases during operation of a solid oxide fuel cell.     

Cathodes based on (La,Sr)MnO<Subscript>3</Subscript> modified with PrO<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>

The electrochemical characteristics of composite cathodes made of (La, Sr) MnO₃-(Zr, Sc)O₂ (LSM-SSZ), modified with PrO_{2 − x} additive, and designed for application in solid oxide fuel cells at moderately high temperatures were studied. The relationship between activity of catalytically modified composite LSM-SSZ cathodes and dispersity of electrocatalyst was revealed. The boundaries of the temperature range with the maximum dispersity of electrocatalyst and electrochemical activity of cathodes were found. The composite LSM-SSZ cathodes modified with PrO_{2 − x} were shown inert with respect to oxidation reactions of hydrocarbon fuel (methane) and highly active electrochemically with respect to oxygen reaction in non-equilibrium gas mixture of CH₄ and O₂. In cells with (Ce, Sm)O₂ (SDC) and (Zr, Y)O₂ (YSZ) electrolytes, their overvoltage is below 80 mV at the current density about 0.5 A/cm² and temperature of 600°C. These electrodes can be used as cathodes in single-chamber fuel cells. Long-term experiments were carried out to study time stability of characteristics of the said composite cathodes. The studied electrodes show parabolic or damped exponential time curves of polarization resistance if contacting with YSZ or SDC electrolyte, respectively. According to the forecast based on the experimental regularities, the polarization resistance of LSM-SSZ cathodes in 10,000 h will not exceed 0.4 or 0.13 Ohm cm², respectively, if YSZ or SDC electrolyte is used.     

Rational Design of Lower‐Temperature Solid Oxide Fuel Cell Cathodes via Nanotailoring of Co‐Assembled Composite Structures

A novel in situ co‐assembled nanocomposite LSM‐Bi_1.6Er_0.4O₃ (ESB) (icn‐LSMESB) was obtained by conjugated wet‐chemical synthesis. It showed an enhancement of the cathode polarization at 600 °C by >140 times relative to conventional LSM‐Y_0.08Zr_0.84O_1.92 (YSZ) cathodes and exceptional solid oxide fuel cell (SOFC) performance of >2 W cm^?2 below 750 °C. This demonstrates that this novel cost‐effective and broadly applicable process provides new opportunities for performance enhancement of energy storage and conversion devices by nanotailoring of composite electrodes.     

Chemically stable anode-supported solid oxide fuel cells based on Y-doped barium zirconate thin films having improved performance

Anode-supported solid oxide fuel cells (SOFCs) based on thin BaZr_0.8Y_0.2O_3 ? δ (BZY) electrolyte films were fabricated by pulsed laser deposition (PLD) on sintered NiO–BZY composite anodes. After in situ reduction of NiO to Ni, the anode substrates became porous, while retaining good adhesion with the electrolyte. A slurry-coated composite cathode made of La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ (LSCF) and BaCe_0.9Yb_0.1O_3 ? δ (BCYb), specifically developed for proton conducting electrolytes, was used to assemble fuel cell prototypes. Depositing by PLD 100 nm thick LSCF porous films onto the BZY thin films was essential to improve the cathode/electrolyte adhesion. A power density output of 110 mW/cm² at 600 °C, the largest reported value for an anode-supported fuel cell based on BZY at this temperature, was achieved. Electrochemical impedance spectroscopy (EIS) measurements were used to investigate the different contributions to the total polarization losses.