Electrochemical characterizations of microtubular solid oxide fuel cells under a long-term testing at intermediate temperature operation

We report the long-term stability of a microtubular solid oxide fuel cell (SOFC) operable at ∼500 °C. The SOFC consists of NiO-Gd doped ceria (GDC) as the anode as well as the tubular support, GDC as an electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-GDC as the cathode. A single tubular cell with a diameter of approximately 1.8 mm and an effective electrode length of approximately 20 mm generated 150 mW cm⁻² and 340 mW cm⁻² at 500 °C and 550 °C, respectively, under the operation conditions of 0.7 V and humidified H₂ fuel flow. The cell exhibited good stability with a degradation rate of 0.25%/100 h under operation conditions of 200 mA and 0.75 V.     

Anode-supported micro tubular SOFCs for advanced ceramic reactor system

In this study, micro tubular SOFCs under 1 mm diameter have been fabricated and investigated at 450–550 °C operating temperature with H₂ fuel. The performance of the 0.8 mm diameter tubular SOFC was 110–350 mW cm⁻² at 450–550 °C operating temperatures. To maximize the performance of the cell as well as to optimize the geometry of tubular cells, a current collecting method used in the experiment was examined. A model was proposed to estimate the loss of performance for single cell due to the current collecting method as functions of anode tube length and thickness. The results showed that the losses of performance were calculated to be 0.8, 2.0, and 4.6% at 450, 500, and 550 °C operating temperatures, respectively, for the 0.8 mm diameter tubular SOFC with the length of 1.2 cm.     

Cube-type micro SOFC stacks using sub-millimeter tubular SOFCs

Fabrication and characterization of tubular SOFCs under sub-millimeter (0.8 mm), bundles and stacks for low temperature operation were shown. The materials used in this study were Gd doped CeO₂ (GDC) for electrolyte, NiO–GDC for anode and (La, Sr)(Co, Fe)O₃ (LSCF)–GDC for cathode, respectively, and LSCF for supports of the tubular cells for bundle fabrication. After applying a sealing layer and current collector for each bundle of five micro tubular SOFCs, each bundle was stacked vertically, to build a four-storey cube-type stack with volume of about 0.8 cm³. The performance of the stack was shown to be 3.6 V OCV and 2 W maximum output power under 500 °C operating temperature. Preliminary quick start-up test was also conducted at the condition of 3 min start-up time from 150 to 400 °C for 5 times, and the results showed no degradation of the performance during the test.     

Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature

Tubular SOFC systems appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. Our goal is to develop innovative processing method to fabricate new micro tubular SOFCs with sub-millimeter diameter and its stack module which enable to generate high volumetric power density. In this study, micro tubular SOFCs under 1 mm diameter have been successfully fabricated and tested in the intermediate temperature region (550 °C or under). The cell consists of NiO–Gd doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and (La, Sr)(Fe, Co)O₃ (LSCF)–GDC as a cathode. The single tubular cell with 0.8 mm diameter and 12 mm length generated over 70 mW at 550 °C with H₂ fuel, which indicates that the cell generated over 0.3 W cm⁻² at 550 °C.     

Fabrication and characterization of metal-supported solid oxide fuel cells

Metal-supported solid oxide fuel cells (SOFCs) are an acceptable approach to solving the serious problems of SOFC technology, such as sealing and mechanical strength. In this work, commercial stainless-steel plates, STS430, are used as supporting bodies for a metal-supported SOFC in order to decrease the number of fabrication steps. The metal support for a single-cell has a diameter of 28 mm, a thickness of 1 mm, and a channel width of 0.4 mm. A thin ceramic layer, composed of yttria-stabilized zirconia (YSZ) and NiO/YSZ, is attached to the metal support by using a cermet adhesive. La_0.8Sr_0.2Co_0.4Mn_0.6O₃ perovskite oxide serves as the cathode material because of its low impedance on the YSZ electrolyte, according to half-cell tests. The maximum power density of the cell is 0.09 W cm⁻² at 800 °C. The effects of temperature, oxygen partial pressure, and current collection by pastes are investigated. The oxygen reduction reaction at the cathode dominates the overall cell performance, according to experimental and numerical analyses.     

Carbon monoxide-fueled solid oxide fuel cell

This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm² generated power up to 16 W. Open circuit voltages for various CO/CO₂ mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm⁻² were achieved at 1 A cm⁻² and 850 °C. This performance compared well with 0.74 W cm⁻² measured for pure H₂ in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm⁻²) power output remained unchanged over 375 h of continuous operation at 850 °C. In addition, a 50-cell planar SOFC stack was operated at 800 °C on 95% CO (balance CO₂), which generated 1176 W of total power at a power density of 224 mW cm⁻². The results demonstrate that CO is a viable primary fuel for SOFCs.     

The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode suported solid oxide fuel cell (SOFC)

Tubular solid oxide fuel cells (SOFCs) have many desirable advantages compared to other SOFC applications. Recently, micro-tubular SOFCs were studied to apply them into APU systems for future vehicles. In this study, electrochemical properties of the micro-tubular SOFCs (1.6 mm O.D.) have been characterized. Electrochemical analysis showed excellent performance with a maximum power density of 1.3 W/cm² at 550 °C. The impedance information gained at cell operating temperatures of 450, 500, and 550 °C showed individual cell ohmic resistances of 1.0, 0.6, and 0.2 Ω respectively. Within the operating temperature range of 450-550 °C, the ceria based micro-tubular SOFCs (cathode length: 8 mm) were found to have power densities ranging between 0.263 and 1.310 W/cm². The mechanical properties of the tubes were also analyzed through internal burst testing and monotonic compressive loading on a c-ring test specimen. The two testing techniques are compared and related, and maximum hoop stress values are reported for each of the fabrication parameters. This study showed feasible electrochemical properties and mechanical strength of micro-tubular SOFC for APU applications.     

Solid oxide fuel cell with compositionally graded cathode functional layer deposited by pressure assisted dual-suspension spraying

In this work, the benefit of compositionally grading a cathode functional layer (CFL) for solid oxide fuel cells (SOFCs) is explored. Cells are prepared wherein either a standard cathode functional layer (SCFL) or a linearly compositionally graded cathode functional layer (CGCFL) is placed between the cell electrolyte and cathode current collecting regions. The electrochemical performance of these cells is compared with a SOFC cell containing no CFL. All cells are fabricated using a pressurized dual-suspension spraying system. Electrolytes, cathode functional layer, and cathode current collecting materials are deposited on a powder compacted anode support. SEM and EDAX area maps are taken to study the resulting micro-structures and to verify that the desired CFL profiles are produced. The EDAX area map verifies that a compositionally graded CFL and a SCFL are obtained. The cells are analyzed using impedance spectroscopy to evaluate the electrochemical performances of each cell. The open circuit voltage (OCV) and peak power densities of all three cells are 1.04 V with 80 mW cm⁻², 1.12 V with 108 mW cm⁻², and 1.08 V with 193 mW cm⁻² at 850 °C for the SCFL cell, the cell without a CFL, and the compositionally graded CFL cell respectively. The results show that this approach is a viable means for producing SOFC functional layers with unique composition and interfacial properties.     

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La_0.7Sr_0.3MnO₃ (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm_0.2Ce_0.8O_1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm⁻² at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm⁻² was derived from the cell with LSM/YSZ cathode, using 100 ml min⁻¹ oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.     

Development of 1 kW class solid oxide fuel cell stack using anode-supported planar cells

We have developed a 1 kW class solid oxide fuel cell (SOFC) stack composed of 50 anode-supported planar 120-mm-diameter SOFCs. Intermediate plates, which exhibited negligible deformation under operating conditions, were placed in the stack to cancel out the cumulative error related to the position and angle of the stack parts. The stack provided an electrical conversion efficiency of 54% (based on the lower heating value (LHV) of the methane used as a fuel) and an output of 1120 W when the fuel utilization, current density, and operating temperature were 67%, 0.28 A cm⁻², and 1073 K, respectively. The stack operated stably for almost 700 h.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

Preparation and performance of nanostructured porous thin cathode for low-temperature solid oxide fuel cells by spin-coating method

To reduce the cathode–electrolyte interfacial polarization resistance of low-temperature solid oxide fuel cells (SOFCs), a nanostructured porous thin cathode consisting of Sm_0.5Sr_0.5CoO₃ (SSC) and Ce_0.8Sm_0.2O_1.9 (SDC) was fabricated on an anode-supported electrolyte film using spin-coating technique. A suspension with nanosized cathode materials, volatilizable solvents and a soluble pore former was developed. The results indicated that the cell with the nanostructured porous thin cathode sintered at 950 °C showed relatively high maximum power density of 212 mW cm⁻² at 500 °C and 114 mW cm⁻² at 450 °C, and low interfacial polarization resistance of 0.79 Ω cm² at 500 °C and 2.81 Ω cm² at 450 °C. Hence, the nanostructured porous thin cathode is expected to be a promising cathode for low-temperature SOFCs.     

BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte-based solid oxide fuel cells with cobalt-free PrBaFe2O5+δ layered perovskite cathode

A new anode-supported SOFC material system Ni-BZCYYb|BZCYYb|PBFO is investigated, in which a cobalt-free layered perovskite oxide, PrBaFe₂O_5+δ (PBFO), is synthesized and employed as a novel cathode while the synthesized BZCYYb is used as an electrolyte. The cell is fabricated by a simple dry-pressing/co-sintering process. The cell is tested and characterized under intermediate temperature range from 600 to 700 °C with humified H₂ (∼3% H₂O) as fuel, ambient air as oxidant. The results show that the open-circuit potential of 1.006 V and maximal power density of 452 mW cm⁻² are achieved at 700 °C. The polarization resistance of the electrodes is 0.18 Ω cm² at 700 °C. Compared to BaZr_0.1Ce_0.7Y_0.1O_3−δ, the conductivity of co-doped barium zirconate-cerate BZCYYb is significantly improved. The ohmic resistance of single cell is 0.37 Ω cm² at 700 °C. The results indicate that the developed Ni-BZCYYb|BZCYYb|PBFO cell is a promising functional material system for SOFCs.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Thick-film electrolyte (thickness <20 μm)-supported solid oxide fuel cells

A novel design of solid oxide fuel cell (SOFC) which utilizes a thick film (<20 μm) as an electrolyte support is developed and tested. The sintered 16 μm-thick yttria-stabilized zirconia (YSZ) electrolyte film is mounted on a 1-mm thick YSZ ring by sintering the two pieces together. With this new configuration, it is possible to fabricate a thick (<20 μm) electrolyte-supported SOFC and measure the power density of the unit cell. With LSCF (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ) as a cathode and Ni–YSZ as a composite anode, the cell with a 16 μm-thick YSZ electrolyte achieves a high performance, i.e., a maximum power density of 590 mW cm⁻² at 800 °C. This value is comparable with that of most anode-supported SOFCs using YSZ electrolytes.     

Effect of unsintered gadolinium-doped ceria buffer layer on performance of metal-supported solid oxide fuel cells using unsintered barium strontium cobalt ferrite cathode

In this study, a Gd_0.1Ce_0.9O_1.95 (GDC) buffer layer and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode, fabricated without pre-sintering, are investigated (unsintered GDC and unsintered BSCF). The effect of the unsintered GDC buffer layer, including the thickness of the layer, on the performance of solid oxide fuel cells (SOFCs) using an unsintered BSCF cathode is studied. The maximum power density of the metal-supported SOFC using an unsintered BSCF cathode without a buffer layer is 0.81 W cm⁻², which is measured after 2 h of operation (97% H₂ and 3% H₂O at the anode and ambient air at the cathode), and it significantly decreases to 0.63 W cm⁻² after 50 h. At a relatively low temperature of 800 °C, SrZrO₃ and BaZrO₃, arising from interaction between BSCF and yttria-stabilized zirconia (YSZ), are detected after 50 h. Introducing a GDC interlayer between the cathode and electrolyte significantly increases the durability of the cell performance, supporting over 1000 h of cell usage with an unsintered GDC buffer layer. Comparable performance is obtained from the anode-supported cell when using an unsintered BSCF cathode with an unsintered GDC buffer layer (0.75 W cm⁻²) and sintered GDC buffer layer (0.82 W cm⁻²). When a sintered BSCF cathode is used, however, the performance increases to 1.23 W cm⁻². The adhesion between the BSCF cathode and the cell can be enhanced by an unsintered GDC buffer layer, but an increase in the layer thickness (1-6 μm) increases the area specific resistance (ASR) of the cell, and the overly thick buffer layer causes delamination of the BSCF cathode. Finally, the maximum power densities of the metal-supported SOFC using an unsintered BSCF cathode and unsintered GDC buffer layer are 0.78, 0.64, 0.45 and 0.31 W cm⁻² at 850, 800, 750 and 700 °C, respectively.     

Novel nano-network cathodes for solid oxide fuel cells

A novel nano-network of Sm_0.5Sr_0.5CoO_3−δ (SSC) is successfully fabricated as the cathodes for intermediate-temperature solid oxide fuel cells (SOFCs) operated at 500–600 °C. The cathode is composed of SSC nanowires formed from nanobeads of less than 50 nm thus exhibiting high surface area and porosity, forming straight path for oxygen ion and electron transportation, resulting in high three-phase boundaries, and consequently showing remarkably high electrode performance. An anode-supported cell with the nano-network cathode demonstrates a peak power density of 0.44 W cm⁻² at 500 °C and displays exceptional performance with cell operating time. The result suggests a new direction to significantly improve the SOFC performance.     

Proton conducting solid oxide fuel cells with layered PrBa0.5Sr0.5Co2O5+δ perovskite cathode

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate protonic conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) has advanced electrochemical properties. This research fully takes advantage of these advanced properties and develops a novel protonic ceramic membrane fuel cell (PCMFC) of Ni–BZCY7|BZCY7|PBSC. Experimental results show that the cell may achieve the open-circuit potential of 1.005 V, the maximal power density of 520 mW cm⁻², and a low electrode polarization resistance of 0.12 Ωcm² at 700 °C. Increasing operating temperature leads to the decrease of total cell resistance, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that PBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni–BZCY7|BZCY7|PBSC cell is a promising functional material system for SOFCs.     

Nano-structured cathodes based on La0.6Sr0.4Co0.2Fe0.8O3−δ for solid oxide fuel cells

Lanthanum-based iron- and cobalt-containing perovskite is a promising cathode material because of its electrocatalytic activity at a relatively low operating temperature in solid oxide fuel cells (SOFCs), i.e., 700-800 °C. To enhance the electrocatalytic reduction of oxidants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), nanocrystalline LSCF materials are successfully fabricated using a complexing method with chelants and inorganic nano dispersants. When inorganic dispersants are added to the synthesis process, the surface area of the LSCF powder increases from 18 to 88 m² g⁻¹, which results in higher electrocatalytic activity of the cathode. The performance of a unit cell of a SOFC with nanocrystalline LSCF powders synthesized with nano dispersants is increased by 60%, from 0.7 to 1.2 W cm⁻².     

A novel layered perovskite cathode for proton conducting solid oxide fuel cells

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate proton conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC) perovskite demonstrates advanced electrochemical properties based on doped ceria electrolyte. This research fully takes advantage of these advanced properties and develops novel protonic ceramic membrane fuel cells (PCMFCs) of Ni-BZCY7|BZCY7|SBSC. The results show that the open-circuit potential of 1.015 V and maximum power density of 533 mW cm⁻² are achieved at 700 °C. With temperature increase, the total cell resistance decreases, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that SBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni-BZCY7|BZCY7|SBSC cell is a promising functional material system for next generation SOFCs.