Preparation of functional layers for anode-supported solid oxide fuel cells by the reverse roll coating process

The roll coating technique represents a novel method for applying functional layers to solid oxide fuel cells (SOFCs). This fast process is already used for mass production in other branches of industry and offers a high degree of automation. It was utilized for coating specially developed anode (NiO + 8YSZ, 8YSZ: 8 mol% yttria-stabilized zirconia) and electrolyte (8YSZ) suspensions on green and pre-sintered tape-cast anode supports (NiO + 8YSZ). The layers formed were co-fired in a single step at 1400 °C for 5 h. As a result, the electrolyte exhibited a thickness of 14-18 μm and sufficient gas tightness. Complete cells with a screen-printed and sintered La_0.65Sr_0.3MnO_3−δ (LSM)/8YSZ cathode yielded a current density of 0.9-1.1 A cm⁻² at 800 °C and 0.7 V, which is lower than the performance of non-co-fired slip-cast or screen-printed Jülich standard cells with thinner anode and electrolyte layers. The contribution of the cell components to the total area-specific resistance (ASR) was calculated by analyzing the distribution function of the relaxation times (DRTs) of measured electrochemical impedance spectra (EIS) and indicates the potential improvement in the cell performance achievable by reducing the thickness of the roll-coated layers. The results show that the anode-supported planar half-cells can be fabricated cost-effectively by combining roll coating with subsequent co-firing.     

Behavior of 3 mol% yttria-stabilized tetragonal zirconia polycrystal film prepared by slurry spin coating

A dense and uniform 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3YSZ) electrolyte film of 6 μm in thickness was fabricated by slurry spin coating on a porous NiO/3YSZ anode substrate. Composite cathodes of La_0.7Sr_0.3MnO₃ impregnated with Sm_0.2Ce_0.8O_1.9 were fabricated on the 3YSZ films. A single cell produced in this way was tested at 700, 750 and 800 °C with hydrogen as fuel and stationary air as oxidant. Test results revealed an open-circuit voltage of 1.04 V at 800 °C, and maximum power density of 551, 895 and 1143 mW cm⁻² at 700, 750 and 800 °C, respectively. Impedance spectra results demonstrated that the cell performance was determined by the polarization resistance of the cathode.     

Investigation of shrinkage behavior of Ni-Fe bimetallic anode tube support and the densification of electrolyte using co-sintering temperature

NiO-Fe₂O₃/gadolinium-doped CeO₂ (GDC), NiO-Fe₂O₃/yttria-stabilized ZrO₂ (YSZ) anode supported fuel cells were fabricated at co-sintering temperatures of anode-electrolyte from 1250 °C to 1400 °C. The volumetric shrinkage of the anode-electrolytes and the porosity of the anode tube were studied systematically at different temperatures. 1300 °C is the marginal temperature to obtain sufficient electrocatalytic activity of electrodes, and a higher temperature is needed to suppress gas leakage through the scandia-stabilized zirconia (ScSZ) electrolyte. At each co-sintering temperature from 1250 °C to 1400 °C, the porosity of NiO-Fe₂O₃/GDC anode tubes is nearly 10% higher than that of NiO-Fe₂O₃/YSZ anode tubes. SEM results exhibited the anode-supported electrolyte tends to be more dense as co-sintering temperature increasing to 1400 °C from 1250 °C. However, the high co-sintering temperature of 1400 °C will result in low porosity of anode which negatively affected the power density.     

NiO/YSZ,anode-supported,thin-electrolyte,solid oxide fuel cells fabricated by gel casting

A simple and cost-effective gel-casting technique is developed and optimized to fabricate NiO/stabilized yttria–zirconia (YSZ) anode-supported solid oxide fuel cells (SOFCs). The effect of ammonium poly-(methacrylate) (PMAA) dispersant and pH on the zeta potential of YSZ and NiO particles and the viscosity of the NiO/YSZ slurries is studied in detail. The results show that the absolute zeta potential of YSZ and NiO particles reaches a maximum value at pH value ∼10 and the viscosity of the NiO/YSZ slurry is lowest when the PMAA dispersant content is 1.5 wt.%. A gel-cast NiO/YSZ anode-supported button cell with a spin-coated, thin, YSZ electrolyte film (∼9 μm) and a La_0.72Sr_0.18MnO_3−δ (LSM)/YSZ composite cathode gives a peak power output of 1.07 and 0.65 W cm⁻² at 900 and 800 °C under humidified hydrogen and air. The effect of a graphite pore-former in the gelation and microstructure of NiO/YSZ anode substrates is investigated.     

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFC) were successfully fabricated by a single step co-firing process. The cells comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ or scandia-stabilized zirconia (ScSZ) electrolyte, a (La_0.85Ca_0.15)_0.97MnO₃ (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells in the temperature range of 1300–1330 °C for 2 h. Cells were tested in the temperature range of 700–800 °C with humidified hydrogen as fuel and air as oxidant. Cell test results and polarization modeling showed that the polarization losses were dominated by the ohmic loss associated with the electrodes and the activation polarization of the cathode, and that the ohmic loss due to the ionic resistance of the electrolyte and the activation polarization of the anode were relatively insignificant. Ohmic resistance associated with the electrode was lowered by improving the electrical contact between the electrode and the current collector. Activation polarization of the cathode was reduced by the improvement of the microstructure of the cathode active layer and lowering the cell sintering temperature. The cell performance was further improved by increasing the porosity in the anode. As a result, the maximum power density of 1.5 W cm⁻² was achieved at 800 °C with humidified hydrogen and air.     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.     

Synthesis and electrochemical properties of LSM and LSF perovskites as anode materials for high temperature steam electrolysis

La_0.8Sr_0.2MnO₃ (LSM) and La_0.8Sr_0.2FeO₃ (LSF) perovskites used as the anode materials for high temperature steam electrolysis (HTSE) were synthesized by sol–gel self-propagating method. These two powders were mixed with yttria-stabilized zirconia (YSZ) powders, respectively to fabricate composite anodes of solid oxide electrolysis cells (SOECs). The LSM–YSZ and LSF–YSZ composite anodes were tested at 1073 K SOEC working temperature under electrolysis conditions, using cells with a YSZ electrolyte and a Pt counter electrode. Their electrochemical performances were compared and the possibilities of using as SOEC anodes were discussed.     

Fabrication and evaluation of anode and thin Y2O3-stabilized ZrO2 film by co-tape casting and co-firing technique

Co-tape casting and co-firing of supporting electrode and electrolyte layers could drastically increase productivity and reduce fabrication cost. In this study, Ni-YSZ anode supporting electrode and the YSZ electrolyte with the size of 6.5 cm × 6.5 cm have been successfully fabricated by co-tape casting and co-firing technique. The cell with 1.5 mm anode and 10 μm electrolyte is flat without warping, cracks or delaminations. The power density reaches 661, 856, 1085 mW cm⁻² at 0.7 V and 750, 800 and 850 °C, respectively. The EIS results demonstrate that the cathodic electrochemical resistance is 0.0680 Ω cm², about twice of the anode's which is 0.0359 Ω cm². SEM images show the dense YSZ film had a crack free of surface morphology. The anode and cathode layers are well-adhered to the YSZ electrolyte layer. The La_0.8 Sr_0.2 MnO_3−δ particles do not form a continuous network. Optimization of finer cathodic microstructure and anodic porosity are underway.     

A right-angular configuration for the single-chamber solid oxide fuel cell

A right-angular configuration for the single-chamber solid oxide fuel cell (SC-SOFC) has been proposed and operated successfully in a methane-oxygen mixture (CH₄:O₂ = 2:1) with a total flow rate of 300 mL min⁻¹. It was fabricated by attaching the Ni/yttria-stabilized zirconia (YSZ) anode and the Sm_0.2Ce_0.8O_1.9-impregnated La_0.7Sr_0.3MnO₃ (LSM) cathode on two mutually perpendicular planes of a YSZ electrolyte substrate. Effect of the relative position of the electrodes on the ohmic resistance has been investigated. It is shown that the cell exhibits the smallest ohmic resistance when the two electrodes are symmetrically located on the two planes. Compared with the conventional coplanar SC-SOFC, this configuration can make full use of the edge area of the electrolyte substrate and shorten the conductive channel of oxygen ion, leading to a remarkable reduction in ohmic resistance, an elevation of the open-circuit voltage, and, ultimately, an improved performance. The simple stack, consisting of two right-angular cells connected in series on an electrolyte, generated an open-circuit voltage 1.4 V at 700 °C and a maximum power 14.9 mW at 800 °C.     

Fabrication and evaluation of a Ni/La0.75Sr0.25Cr0.5Fe0.5O3−δ co-impregnated yttria-stabilized zirconia anode for single-chamber solid oxide fuel cells

In this study, an anode-supported solid oxide fuel cell (SOFC) has been prepared using a porous yttria-stabilized zirconia (YSZ) anode matrix. The anode was prepared by impregnating the sintered porous YSZ matrix with a nitrate aqueous containing La³⁺, Sr²⁺, Cr³⁺, Fe³⁺, Ni²⁺ and urea. The formed anode exhibited high surface area and porosity, and had fast path for the transportation of oxygen ion and electron, as well as resulting in high three-phase boundaries (TPBs). Single-chamber fuel cell test was conducted in a methane-oxygen gas mixture using an YSZ membrane as the electrolyte and La_0.8Sr_0.2MnO_3−δ (LSM) as the cathode. The influences of environmental temperature and gas composition on the cell performance were also investigated. Under the optimized gas composition (CH₄/O₂ = 2/1) and furnace temperature (800 °C) conditions, a maximum power density of 214 mW cm⁻² was achieved. The test results demonstrated good cell stability and indicated that the perovskite oxide-based anodes were quite robust with redox cycling.     

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 Gd_0.1Ce_0.9O₂ (GDC) barrier layer and a Sm_0.5Sr_0.5CoO₃ (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.     

Anode-supported solid oxide fuel cell based on dense electrolyte membrane fabricated by filter-coating

A simple and cost-effective technique, filter-coating, has been developed to fabricate dense electrolyte membranes. Eight mole percent yttria-stabilized zirconia (YSZ) electrolyte membrane as thin as 7 μm was prepared by filter-coating on a porous substrate. The thickness of the YSZ film was uniform, and could be readily controlled by the concentration of the YSZ suspension and the rate of the suspension deposition. The YSZ electrolyte film was dense and was well bonded to the Ni-YSZ anode substrate. An anode-supported solid oxide fuel cell (SOFC) with a YSZ electrolyte film and a La_0.85Sr_0.15MnO₃ (LSM) + YSZ cathode was fabricated and its performance was evaluated between 700 and 850 °C with humidified hydrogen as the fuel and ambient air as the oxidant. An open circuit voltage (OCV) of 1.09 V was observed at 800 °C, which was close to the theoretical value, and the maximum power density measured was 1050 mW cm⁻². The results demonstrate that the dense YSZ film fabricated by filter-coating is suitable for application to SOFCs.     

La0.4Ce0.6O1.8–La0.8Sr0.2MnO3–8 mol% yttria-stabilized zirconia composite cathode for anode-supported solid oxide fuel cells

The composite cathodes of La_0.4Ce_0.6O_1.8 (LDC)–La_0.8Sr_0.2MnO₃ (LSM)–8 mol% yttria-stabilized zirconia (YSZ) with different LDC contents were investigated for anode-supported solid oxide fuel cells with thin film YSZ electrolyte. The oxygen temperature-programmed desorption profiles of the LDC–LSM–YSZ composites indicate that the addition of LDC increases surface oxygen vacancies. The cell performance was improved largely after the addition of LDC, and the best cell performance was achieved on the cells with the composite cathodes containing 10 wt% or 15 wt% LDC. The electrode polarization resistance was reduced significantly after the addition of LDC. At 800 °C and 650 °C, the polarization resistances of the cell with a 10 wt% LDC composite cathode are 70% and 40% of those of the cell with a LSM–YSZ composite cathode, respectively. The impedance spectra show that the processes associated with the dissociative adsorption of oxygen and diffusion of oxygen intermediates and/or oxygen ions on LSM surface and transfer of oxygen species at triple phase boundaries are accelerated after the addition of LDC.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.     

Electrochemical characteristics of an La0.6Sr0.4Co0.2Fe0.8O3–La0.8Sr0.2MnO3 multi-layer composite cathode for intermediate-temperature solid oxide fuel cells

An La_0.6Sr_0.4Co_0.2Fe_0.8O₃–La_0.8Sr_0.2MnO₃ (LSCF–LSM) multi-layer composite cathode for solid oxide fuel cells (SOFCs) was prepared on an yttria-stabilized zirconia (YSZ) electrolyte by the screen-printing technique. Its cathodic polarization curves and electrochemical impedance spectra were measured and the results were compared with those for a conventional LSM/LSM–YSZ cathode. While the LSCF–LSM multi-layer composite cathode exhibited a cathodic overpotential lower than 0.13 V at 750 °C at a current density of 0.4 A cm⁻², the overpotential for the conventional LSM–YSZ cathode was about 0.2 V. The electrochemical impedance spectra revealed a better electrochemical performance of the LSCF–LSM multi-layer composite cathode than that of the conventional LSM/LSM–YSZ cathode; e.g., the polarization resistance value of the multi-layer composite cathode was 0.25 Ω cm² at 800 °C, nearly 40% lower than that of LSM/LSM–YSZ at the same temperature. In addition, an encouraging output power from an YSZ-supported cell using an LSCF–LSM multi-layer composite cathode was obtained.     

Low temperature processed composite cathodes for Solid-oxide fuel Cells

Low temperature processed composite cathodes for solid-state fuel cell (SOFC) have been developed, consisting of La_0.6Sr_0.4Ti_0.1Fe_0.9O₃ and Ag, as low as 800 °C process temperature. Using micro-tubular design, the performances of the cathodes have been investigated at the operating temperatures of 650 and 700 °C. The cell consists of NiO-Y stabilized zirconia (YSZ) as an anode (support tube), Sc stabilized zirconia (ScSZ) as an electrolyte, Gd doped ceria (GDC) for an inter-layer between the electrolyte and the cathodes. The single performance has varied depending upon the types of cathodes, and the composite cathode with 50 wt% La_0.6Sr_0.4Ti_0.1Fe_0.9O₃ and 50 wt% Ag has shown comparable performance to the cell with standard LSCF-GDC cathode.     

Direct carbon solid oxide Fuel Cell—a potential high performance battery

Tubular cone-shaped Ni-based anode-supported solid oxide fuel cells (SOFCs), with yttria-stabilized zirconia (YSZ) electrolyte and La_0.8Sr_0.2MnO₃ (LSM) cathode, were investigated with Fe catalyst-loaded activated carbon directly filled in as fuel. Three identical single cells were operated at different current and it turned out that larger current resulted in shorter operation life and smaller carbon utilization. A 3-cell-stack, with the segmented cone-shaped cells connected in series, was assembled and tested. A peak power density of 465 mW cm⁻² and a volumetric power density of 710 mW cm⁻³ were achieved at 850 °C. The degradation performance was analyzed according to the electrochemical characterization and SEM-EDX measurement. Based on the experimental results, the potential of developing such direct carbon SOFC into a high performance battery was proposed.     

Improvement of anode performance by surface modification for solid oxide fuel cell running on hydrocarbon fuel

A Ni/ yttria-stabilized zirconia (YSZ) cermet anode was modified by coating with samaria-doped ceria (SDC, Sm_0.2Ce_0.8O₂) sol within the pores of the anode for a solid oxide fuel cell (SOFC) running on hydrocarbon fuel. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the triple phase boundary (TPB), which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition and nickel sintering.The maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30 μm, and cathode; La_0.85Sr_0.15MnO₃) with the modified anode was about 0.3 W/cm² at 700 °C in the mixture of methane (25%) and air (75%) as the fuel and air as the oxidant. The cell was operated for 500 h without significant degradation of cell performance.     

Fabrication and characterization of Ni/ScSZ cermet anodes for IT-SOFCs

In this study the fabrication and characterization of Ni/10ScSZ (Ni/10 mol% Sc₂O₃-90 mol% ZrO₂) and Ni/10Sc1CeSZ (Ni/10 mol% Sc₂O₃-1 mol% CeO₂-89 mol% ZrO₂) cermet anode films was studied and compared. Both 10ScSZ and 10Sc1CeSZ electrolyte powders showed tetragonal and cubic phases at room temperature, respectively. The NiO/10ScSZ and NiO/10Sc1CeSZ composites with 10-60 vol% of Ni content were prepared by mixing as-received commercial powders of NiO, 10ScSZ and 10Sc1CeSZ followed by ink preparation. Samples were sintered for 1 h at temperatures of 1250-1350 °C. All the cermet films were then reduced under a mixture of hydrogen (10%) and nitrogen (90%) at 800 °C for 2 h. The effect of Ni content and sinter temperature on the DC electrical conductivity were investigated, and the results showed a sharp change in conductivity at around 30 vol% Ni, corresponding to continuity/discontinuity of the Ni-Ni contact network, and the conductivity increased as the sinter temperature increased from 1250 to 1350 °C. An acceptable electrical conductivity at 700 °C for these cermet films was obtained at >40 vol% Ni, consistent with behaviour reported for more conventional Ni/YSZ cermets. The effect of sinter temperature on the microstructure and porosity of Ni/10Sc1CeSZ and Ni/10ScSZ cermet films was also investigated. This revealed that the porosity of the cermet films with the same Ni content decreased as the sinter temperature increased and that, for a given sinter temperature, the porosity of the cermet films increased with Ni content. The porosities of 40Ni/60ScCeSZ (40 vol% Ni/60 vol% 10Sc1CeSZ) and 40Ni/60ScSZ (40 vol% Ni/60 vol% 10ScSZ) anodes sintered at 1250, 1300 and 1350 °C for 1 h were in the range of 30-45%. Electrochemical measurement of symmetrical cells using an 8YSZ electrolyte at 700 °C revealed that the lowest electrode polarization resistance of 40Ni/60ScCeSZ and 40Ni/60ScSZ anodes was obtained at sinter temperatures of 1350 °C and 1300 °C respectively. Carbon deposition over 40Ni/60ScCeSZ, 40Ni/60ScSZ and 40Ni/60YSZ catalysts was evaluated at 700 °C for 1 h at S/C = 0.8 and the results showed that the ratio of deposited carbon was lower in the case of Ni/10ScSZ and Ni/10Sc1CeSZ compared to Ni/YSZ (0.35). Overall, Ni/10Sc1CeSZ and Ni/10ScSZ cermets having 40 vol% Ni were found to be optimum, with the 40Ni/60ScCeSZ cermet proving to be better than 40Ni/60ScSZ cermet in terms of both electronic conductivity and electrode polarization resistance, with both materials exhibiting improved tolerance towards carbon deposition compared to Ni/YSZ.