Tape Casting of Anode Supports for Solid Oxide Fuel Cells at Forschungszentrum Jülich

This contribution describes the development of tape casting for solid oxide fuel cells (SOFCs) anode supports starting with the characterization of the powders and ending with manufacturing of cells for stack testing. After casting the support, full cells were prepared by screen printing and sintering of the functional layers. The results of single‐cell and stack tests of the novel SOFC will be discussed. The new cell showed excellent electrochemical performance in single‐cell tests with more than 1.5 A/cm² (800°C, 0.7 V). Furthermore, stack tests showed no significant difference from earlier standard cells when operated at 800°C with a current density of 0.5 A/cm².     

Dense 8 mol% yttria-stabilized zirconia electrolyte by DLP stereolithography

Solid Oxide Fuel Cells (SOFCs) are environmentally efficient energy conversion devices, but are partially limited by the complicated fabrication procedure. In this work, dense 8 mol% yttria-stabilized zirconia (8YSZ) ceramics were successfully realized through a DLP (digital light processing) stereolithography method and the electrolyte self-supported fuel cell was also tested at 800 °C. The sintering behavior of the as-printed planar samples were investigated and a fully dense ceramic can be achieved at 1450 °C. The total conductivity of the sintered 8YSZ can reach 2.18 × 10⁻² S cm⁻¹ at a test temperature of 800 °C, which is acceptable for practical application. For the electrolyte self-supported fuel cell test, a power density of 114.3 mW cm⁻² can be achieved when Ni-8YSZ cermet and La_0.8Sr_0.2MnO₃ (LSM) were used as anode and cathode. It was demonstrated that 3D printing is a promising processing technique to build up electrolyte self-supported SOFCs with desired structure for the future development.     

Slip casting combined with colloidal spray coating in fabrication of tubular anode-supported solid oxide fuel cells

A simple and cost-effective slip casting technique was successfully developed to fabricate NiO–YSZ anode substrates for tubular anode-supported single SOFCs. An YSZ electrolyte film was coated on the anode substrates by colloidal spray coating technique. A single cell, NiO–YSZ/YSZ (20 μm)/LSM–YSZ, using the tubular anode supports with YSZ coating, was assembled and tested to demonstrate the feasibility of the techniques applied. Using humidified hydrogen (75 ml/min) as fuel and ambient air as oxidant, the maximum power densities of the cell were 760 mW/cm² and 907 mW/cm² at 800 °C and 850 °C, respectively. The observed OCV was closed to the theoretical value and the SEM results revealed that the microstructure of the anode fabricated by slip casting is porous while the electrolyte film coated by colloidal spray coating is dense.     

Fabrication and Operation of Tubular Segmented‐In‐Series (SIS) Solid Oxide Fuel Cells (SOFC)

A tubular segmented‐in‐series (SIS) solid oxide fuel cell (SOFC) sub module for intermediate temperature (700–800 °C) operation was fabricated and operated in this study. For this purpose, we fabricated porous ceramic supports of 3 YSZ through an extrusion process and analyzed the basic properties of the ceramic support, such as visible microstructure, porosity, and mechanical strength, respectively. After that, we fabricated a tubular SIS SOFC single cell by using dip coating and vacuum slurry coating method in the case of electrode and electrolyte, and obtained at 800 °C a performance of about 400 mW cm^–2. To make a sub module for tubular SIS SOFC, ten tubular SIS SOFC single cells with an effective electrode area of 1.1 cm² were coated onto the surface of the prepared ceramic support and were connected in series by using Ag + glass interconnect between each single cell. The ten‐cell sub module of tubular SIS SOFC showed in 3% humidified H₂ and air at 800 °C a maximum power of ca. 390 mW cm^–2.     

Tubular solid oxide fuel cells fabricated by a novel freeze casting method

Freeze casting is an established method for fabricating porous ceramic structures with controlled porosity and pore geometries. Herein, we developed a novel freeze casting and freeze drying process to fabricate tubular anode supports for solid oxide fuel cells (SOFCs). Freeze casting was performed by injecting aqueous anode slurry to a dual-purpose freeze casting and freeze drying mold wrapped with peripheral coils for flowing a coolant. With the use of an ice barrier layer, proper control of the experimental setup, and adjustments in the drying temperature profile, complete drying of the individual anode tubes was achieved in 4 hours. The freeze-cast anode tubes contained radially aligned columnar pore channels, thus significantly enhancing the gaseous diffusion. SOFC single cells with conventional Ni/yttria-stabilized zirconia/strontium-doped lanthanum manganite materials were prepared by dip coating the thin functional layers onto the anode support. Single cell tests showed that the concentration polarization was low owing to the highly porous anode support with directional pores. With H₂/N₂ (1:1) fuel, maximum power densities of 0.47, 0.36, and 0.27 W/cm² were recorded at 800°C, 750°C, and 700°C, respectively. Our results demonstrate the feasibility of using freeze casting to obtain tubular SOFCs with desired microstructures and fast turn-around times.     

Anode-supported SOFCs based on Sm0.2Ce0.8O2−δ electrolyte thin-films fabricated by co-pressing using microwave combustion synthesized powders

Decreasing the electrolyte thickness is an effective approach to improve solid oxide fuel cells (SOFCs) performance for intermediate-temperature applications. Sm_0.2Ce_0.8O_2−δ (SDC) powders with low apparent density of 32±0.3 mg cm⁻³ are synthesized by microwave combustion method, and SDC electrolyte films as thin as ~10 μm are fabricated by co-pressing the powders onto a porous NiO–SDC anode substrate. Dense SDC electrolyte thin films with grain size of 300–800 nm are achieved at a low co-firing temperature of 1200 °C. Single cells based on SDC thin films show peak power densities of 0.86 W cm⁻² at 650 °C using 3 vol% humidified H₂ as fuel and ambient air as oxidant. Both the thin thickness of electrolyte films and ultra-fine grained anode structure make contributions to the improved cell performance.     

Preparation and Electrochemical Properties of Sr‐doped K2NiF4‐type Cathode Material Pr1.7Sr0.3CuO4 for IT‐SOFCs

A novel K₂NiF₄‐type oxide Pr_1.7Sr_0.3CuO₄ (PSCu) is studied to obtain its electrochemical properties as the cathode for intermediate‐temperature solid oxide fuel cells (IT‐SOFCs). The PSCu cathode powder and Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte powder were synthesized by sol‐gel method and glycine‐nitrate method, respectively. The crystal structure of PSCu powder and PSCu‐SDC composite powder were identified with X‐ray diffraction (XRD). It is shown that PSCu belongs to tetragonal K₂NiF₄‐type and has good chemical compatibility with SDC. The thermal expansion coefficient (TEC) of PSCu is close to that of SDC. The conductivity of PSCu tested with four‐probe method exhibits a semiconductor‐pseudometal transformation at 400–450 °C, where the maximum conductivity of 103.6 S cm⁻¹ is obtained. The polarization test indicates the area specific resistance (ASR) of PSCu decreases with increasing temperature, reaching 0.11 Ω cm² at 800 °C. The activation energy of oxygen reduction reaction during 600–800 °C is 1.19 eV. The single fuel cell performance test reveals the open circuit voltage (OCV) and resistivity of PSCu reduce with increasing temperature, but the power density ascends with increasing temperature. The maximal power density is 243 mW cm⁻² at 800 °C, and the corresponding current density and OCV are 633 mA cm⁻² and 0.77 V, respectively.     

Electrochemical Performance of a Ni and YSZ Composite Synthesised by Ultrasonic Spray Pyrolysis as an Anode for SOFCs

The electrochemical performance of an anode material for a solid oxide fuel cell (SOFC) depends highly on microstructure in addition to composition. In this study, a NiO–yttria‐stabilised zirconia (NiO–YSZ) composite with a highly dispersed microstructure and large pore volume/surface area has been synthesised by ultrasonic spray pyrolysis (USP) and its electrochemical characteristics has been investigated. For comparison, the electrochemical performance of a conventional NiO–YSZ is also evaluated. The power density of the zirconia electrolyte‐supported SOFC with the synthesised anode is ∼392 mW cm^–2 at 900 °C and that of the SOFC with the conventional NiO–YSZ anode is ∼315 mW cm^–2. The improvement is ∼24%. This result demonstrates that the synthesised NiO–YSZ is a potential alternative anode material for SOFCs fabricated with a zirconia solid electrolyte.     

Effect of GDC addition method on the properties of LSM–YSZ composite cathode support for solid oxide fuel cells

Equal amounts of Gd_0.1Ce_0.9O_2−δ (GDC) were added to La_0.65Sr_0.3MnO_3−δ/(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM/YSZ) powder either by physical mixing or by sol–gel process, to produce a porous cathode support for solid oxide fuel cells (SOFCs). The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250 °C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396 mW cm⁻² at 750, 800, and 850 °C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by sol–gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs.     

Tailoring Electrochemical Property of Layered Perovskite Cathode by Cu‐doping for Proton‐Conducting IT‐SOFCs

Layered perovskite oxide YBaCuCoO_5+x (YBCC) was synthesized by an EDTA‐citrate complexation process and was investigated as a novel cathode for proton‐conducting intermediate temperature solid oxide fuel cells (IT‐SOFCs). The thermal expansion coefficient (TEC) of YBCC was 15.3 × 10⁻⁶ K⁻¹ and the electrical conductivity presented a semiconductor‐like behavior with the maximum value of 93.03 Scm⁻¹ at 800 °C. Based on the defect chemistry analysis, the electrical conductivity gradually decreases by the introduction of Cu into Co sites of YBaCo₂O_5+x and the conductor mechanism can transform from the metallic‐like behavior to the semiconductor‐like behavior. Thin proton‐conducting (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3–δ) BZCYYb electrolyte and NiO–BZCYYb anode functional layer were prepared over porous anode substrates composed of NiO–BZCYYb by a one‐step dry‐pressing/co‐firing process. Laboratory‐sized quad‐layer cells of NiO‐BZCYYb / NiO‐BZCYYb / BZCYYb / YBCC with a 20 μm‐thick BZCYYb electrolyte membrane exhibited the maximum power density as high as 435 mW cm⁻² with an open‐circuit potential (OCV) of 0.99 V and a low interfacial polarization resistance of 0.151 Ωcm² at 700 °C. The experimental results have indicated that the layered perovskite oxide YBCC can be a cathode candidate for utilization as proton‐conducting IT‐SOFCs.     

Characterization of Ba0.5Sr0.5Co0.7In0.1Fe0.2O3−δ as the Cathode Material for Proton‐conducting SOFCs

Ba_0.5Sr_0.5Co_0.7In_0.1Fe_0.2O_3−δ powders are successfully synthesized as the cathode materials for proton‐conducting solid oxide fuel cells (SOFCs). The prepared cells consisting of the structure of a BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7)‐NiO anode substrate, a BZCY7 electrolyte membrane and a cathode layer, are measured from 600 to 700 °C with humidified hydrogen (ca. 3% H₂O) as the fuel. The electrochemical results show that the cell exhibits a high power density which could obtain an open‐circuit potential of 0.986 V and a maximum power density of 400.84 mW cm⁻² at 700 °C. The polarization resistance measured at the open‐circuit condition is only 0.15 Ω cm² at 700 °C.     

Development and Fabrication of a New Concept Planar‐tubular Solid Oxide Fuel Cell (PT‐SOFC)

The paper reports a new concept of planar‐tubular solid oxide fuel cell (PT‐SOFC). Emphasis is on the fabrication of the required complex configuration of Ni‐yttria‐stabilised zirconia (YSZ) porous anode support by tert‐butyl alcohol (TBA) based gelcasting, particularly the effects of solid loading, amounts of monomers and dispersant on the rheological behaviour of suspension, the shrinkage of a wet gelcast green body upon drying, and the properties of final sample after sintering at 1350 °C and reduction from NiO‐YSZ to Ni‐YSZ. The results show that the gelcasting is a powerful method for preparation of the required complex configuration anode support. The anode support resulted from an optimised suspension with the solid loading of 25 vol% has uniform microstructure with 37% porosity, bending strength of 44 MPa and conductivity of 300 S cm^—1 at 700 °C, meeting the requirements for an anode support of SOFC. Based on the as‐prepared anode support, PT‐SOFC single cell of Ni‐YSZ/YSZ/LSCF has been fabricated by slurry coating and co‐sintering technique. The cell peak power density reaches 63, 106 and 141 mW cm^—2 at 700, 750 and 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

A Fuel‐Flexible Alkaline Direct Liquid Fuel Cell

We constructed a fuel‐flexible fuel cell consisting of an alkaline anion exchange membrane, palladium anode, and platinum cathode. When an alcohol fuel was used with potassium hydroxide added to the fuel stream and oxygen was the oxidant, the following maximum power densities were achieved at 60 °C: ethanol (128 mW cm⁻²), 1‐propanol (101 mW cm⁻²), 2‐propanol (40 mW cm⁻²), ethylene glycol (117 mW cm⁻²), glycerol (78 mW cm⁻²), and propylene glycol (75 mW cm⁻²). We also observed a maximum power density of 302 mW cm⁻² when potassium formate was used as the fuel under the same conditions. However, when potassium hydroxide was removed from the fuel stream, the maximum power density with ethanol decreased to 9 mW cm⁻² (using oxygen as oxidant), while with formate it only decreased to 120 mW cm⁻² (using air as the oxidant). Variations in the performance of each fuel are discussed. This fuel‐flexible fuel cell configuration is promising for a number of alcohol fuels. It is especially promising with potassium formate, since it does not require hydroxide added to the fuel stream for efficient operation.     

Co‐synthesis of Sm0.5Sr0.5CoO3‐Sm0.2Ce0.8O1.9 Composite Cathode with Enhanced Electrochemical Property for Intermediate Temperature SOFCs

A 70 wt.% Sm_0.5Sr_0.5CoO₃ – 30 wt.% Sm_0.2Ce_0.8O_1.9 (SSC–SDC73) composite cathode was co‐synthesized by a facile one‐step sol–gel method, which showed lower polarization resistance and overpotential than those of physically mixed SSC–SDC73 cathode. The polarization resistance of co‐synthesized SSC–SDC73 cathode at 800 °C was as low as 0.03 Ω cm² in air. Scanning electron microscopy (SEM) images showed that the enhanced electrochemical property was mainly attributed to the smaller grains and good dispersion of SSC and SDC phases within the composite cathode, leading to an increase in three‐phase boundary length. The dependence of polarization resistance with oxygen partial pressure indicated that the rate‐limiting step for oxygen reduction reaction was the dissociation of molecular oxygen to atomic oxygen process. An anode supported fuel cell with a co‐synthesized SSC–SDC73 cathode exhibited a peak power density of 924 mW cm⁻² at 800 °C. Our results suggested that co‐synthesized composite was a promising cathode for intermediate temperature solid oxide fuel cells (IT‐SOFCs).     

New Stack Design of Micro‐tubular SOFCs for Portable Power Sources

Micro‐tubular solid oxide fuel cells (SOFCs) have high thermal stability and higher volumetric power density, which are considered to be ideal features for portable power sources and auxiliary power units for automobile. Here, we report a new stack design using anode supported micro‐tubular SOFCs with 2 mm diameter using Gd doped CeO₂ (GDC) electrolyte, NiO‐GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)‐GDC cathode. The new stack consists of three bundles with five tubular cells, sealing layers and interconnects and fuel manifolds. The performance of the stack whose volume is 1 cm³ was shown to be 2.8 V OCV and maximum power output of 1.5 W at 500 °C, applying air only by natural convection. The results also showed strong dependence of the fuel flow rates on the stack performance, which was correlated to the gas diffusion limitation.     

Preparation of BaZr0.1Ce0.7Y0.2O3–δ Based Solid Oxide Fuel Cells with Anode Functional Layers by Tape Casting

Solid oxide fuel cells (SOFCs) based on the proton conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte were prepared and tested in 500–700 °C using humidified H₂ as fuel (100 cm³ min^–1 with 3% H₂O) and dry O₂ (50 cm³ min^–1) as oxidant. Thin NiO‐BZCY anode functional layers (AFL) with 0, 5, 10 and 15 wt.% carbon pore former were inserted between the NiO‐BZCY anode and BZCY electrolyte to enhance the cell performance. The anode/AFL/BZCY half cells were prepared by tape casting and co‐sintering (1,300 °C/8 h), while the Sm_0.5Sr_0.5CoO_3–δ (SSC) cathodes were prepared by thermal spray deposition. Well adhered planar SOFCs were obtained and the test results indicated that the SOFC with an AFL containing 10 wt.% pore former content showed the best performance: area specific resistance as low as 0.39 Ω cm² and peak power density as high as 0.863 W cm^–2 were obtained at 700 °C. High open circuit voltages ranging from 1.00 to 1.12 V in 700–500 °C also indicated negligible leakage of fuel gas through the electrolyte.     

Pr Doped Barium Cerate as the Cathode Material for Proton‐Conducting SOFCs

BaCe_0.8Pr_0.2O₃ (BCP20) and BaCe_0.6Pr_0.4O₃ (BCP40) powders are successfully synthesized through the Pechini method and used as the cathode materials for proton‐conducting solid state oxide fuel cells (SOFCs). The prepared cells consisting of the structure of a BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY7)‐NiO anode substrate, a BZCY7 anode functional layer, a BZCY7 electrolyte membrane, and a cathode layer, are measured from 600 to 700 °C with humidified hydrogen (∼3% H₂O) as the fuel and static air as the oxidant. The electricity results show that the cell with BCP40 cathode has a higher power density, which could obtain an open‐circuit potential of 0.99 V and a maximum power density of 378 mW cm^–2 at 700 °C. The polarization resistance measured at the open‐circuit condition of BCP40 is only 0.16 Ω cm² at 700 °C, which was less than BCP20.     

Preliminary Study on Direct Operation of LT‐SOFCs Based on GDC Electrolyte Film With DME Fuel

Direct operation of anode‐supported cone‐shaped tubular low temperature solid oxide fuel cells (LT‐SOFCs) based on gadolinia‐doped ceria (GDC) electrolyte film with dimethyl ether (DME) fuel was preliminarily investigated in this study. The single cell exhibited maximum power densities of 500 and 350 mW cm^–2 at 600 °C using moist hydrogen and DME as fuel, respectively. A durability test of the single NiO‐GDC/GDC/LSCF‐GDC cell was performed at a constant current of 0.1 A directly fuelled with DME for about 200 min at 600 °C. The results indicate that the single cell coking easily directly operated in DME fuel. EDX result shows a clear evidence of carbon deposition in the anode. Further studies are needed to develop the novel anti‐carbon anode materials, relate the carbon deposition with anode microstructure and cell‐operating condition.     

Paper‐Fibres Used as a Pore‐Former for Anode Substrate of Solid Oxide Fuel Cell

Paper‐fibres are studied for use as a pore‐former to produce gas channels in the anode substrates of solid oxide fuel cells (SOFCs). These fibres produce cylindrical pores within the anode substrate, which are different from the pores formed by the conventional pore‐formers such as wheat flour and graphite. The cylindrical pores make it easier to connect each other to form continuous pathways for rapid gas diffusion. Paper‐fibres can create more open porosity than the same amount of flour. The application of the paper‐fibres significantly improves the cell performance by enhancing the gas diffusion process. The anode‐supported YSZ film cells with 5 wt.‐% and 10 wt.‐% paper‐fibres exhibit maximum power densities of 0.72 and 1.06 W cm^–2, respectively, using hydrogen as fuel and ambient air as oxidant at 800 °C.     

Long‐Term Stability Studies of Anode‐Supported Microtubular Solid Oxide Fuel Cells

A long‐term stability study of an anode‐supported NiO/YSZ‐YSZ‐LSM/YSZ microtubular cell was performed, under low fuel utilization conditions, using pure humidified hydrogen as fuel at the anode side and air at the cathode side. A first galvanometric test was performed at 766 °C and 200 mA cm^–2, measuring a power output at 0.5 V of ∼250 mW cm^–2. During the test, some electrical contact breakdowns at the anode current collector caused sudden current shutdowns and start‐up events. In spite of this, the cell performance remains unchanged. After a period of 325 h, the cell temperature and the current density was raised to 873°C and 500 mA cm^–2, and the cell power output at 0.5 V was ∼600 mW cm^–2. Several partial reoxidation events due to disturbance in fuel supply occurred, but no apparent degradation was observed. On the contrary, a small increase in the cell output power of about 4%/1,000 h after 654 h under current load was obtained. The excellent cell aging behavior is discussed in connection to cell configuration. Finally, the experiment concluded when the cell suffered irreversible damage due to an accidental interruption of fuel supply, causing a full reoxidation of the anode support and cracking of the thin YSZ electrolyte.