Infiltrated multiscale porous cathode for proton-conducting solid oxide fuel cells

A Sm_0.5Sr_0.5CoO_3−δ (SSC)-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) composite cathode with multiscale porous structure was successfully fabricated through infiltration for proton-conducting solid oxide fuel cells (SOFCs). The multiscale porous SSC catalyst was coated on the BZCY cathode backbones. Single cells with such composite cathode demonstrated peak power densities of 0.289, 0.383, and 0.491 W cm⁻² at 600, 650, 700 °C, respectively. Cell polarization resistances were found to be as low as 0.388, 0.162, and 0.064 Ω cm² at 600, 650 and 700 °C, respectively. Compared with the infiltrated multiscale porous cathode, cells with screen-printed SSC-BZCY composite cathode showed much higher polarization resistance of 0.912 Ω cm² at 600 °C. This work has demonstrated a promising approach in fabricating high performance proton-conducting SOFCs.     

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.     

Electrochemical modeling of ammonia-fed solid oxide fuel cells based on proton conducting electrolyte

An electrochemical model was developed to study the NH₃-fed and H₂-fed solid oxide fuel cells based on proton conducting electrolyte (SOFC-H). The modeling results were consistent with experimental data in literature. It is found that there is little difference in working voltage and power density between the NH₃-fed and the H₂-fed SOFC-H with an electrolyte-support configuration due to an extremely high ohmic overpotential in the SOFC-H. With an anode-supported configuration, especially when a thin film electrolyte is used, the H₂-fed SOFC-H shows significantly higher voltage and power density than the NH₃-fed SOFC-H due to the significant difference in concentration overpotentials. The anode concentration overpotential of the NH₃-fed SOFC-H is found much higher than the H₂-fed SOFC-H, as the presence of N₂ gas dilutes the H₂ concentration and slows down the transport of H₂. More importantly, the cathode concentration overpotential is found very significant despite of the thin cathode used in the anode-supported configuration. In the SOFC-H, H₂O is produced in the cathode, which enables complete fuel utilization on one hand, but dilutes the concentration of O₂ and impedes the diffusion of O₂ to the reaction sites on the other hand. Thus, the cathode concentration overpotential is the limiting factor for the H₂-fed SOFC-H and an important voltage loss in the NH₃-fed SOFC-H. How to reduce the concentration overpotentials at both electrodes is identified crucial to develop high performance SOFC-H.     

Fibrous mixed conducting cathode with embedded ionic conducting particles for solid oxide fuel cells

The Sm_0.5Sr_0.5CoO_3−δ (SSC) fibers with embedded nano-Sm_0.2Ce_0.8O_1.9 (SDC) particles are fabricated by electrospinning process using commercial SDC nanopowders and an SSC precursor gel containing polyvinyl alcohol (PVA) and aqueous metal nitrate. After calcination at 800 °C, fibers with diameters ranged between 300 and 500 nm and well-developed SSC cubic-perovskite structure and SDC fluorite are successfully obtained. The calculated crystallite sizes of SSC and SDC are 20.78 and 45.35 nm, respectively. Over whole measured temperature ranges during the symmetrical cell test, the fiber composite cathode exhibits much lower polarization resistance than conventional powder composite cathodes. The polarization resistances are estimated to 0.06 and 1.23 Ω cm² for the fiber composites and 0.15 and 2.10 Ω cm² for the powder composites at 700 and 550 °C, respectively. The single cell with the fiber composite cathode shows much higher performances; its maximum power density is 380.5 mW cm⁻² at 550 °C and higher than 1278 mW cm⁻² at 700 °C.     

Novel gradient porous cathode for intermediate temperature solid oxide fuel cell

As a mixed ion electronic conducting oxide, PrBaCo₂O_5+δ is regarded as a promising solid oxide fuel cell cathode. To further improve PrBaCo₂O_5+δ cathode's oxygen reduction reaction activity, porosity graded PrBaCo₂O_5+δ-based cathode is prepared by screen printing technology. With a porous top and a comparative denser base, oxygen ions concentration and oxygen gas concentration in the cathode can be graded distributed. This concentration gradient works as driven force which can enhance the cathode catalytic activity. And distribution of relaxation time analysis is carried out to investigate cathodes performance optimization mechanism, the result shows that gradient porous PrBaCo₂O_5+δ cathode's area specific resistance value is much lower than the traditional homogeneous porosity PrBaCo₂O_5+δ cathode. The scaffold porosity modification promotes the cathode oxygen ions transfer processes without obvious impact on the cathode oxygen surface processes.     

Microstructural stability of SSC fibrous cathode with embedded SDC particles for solid oxide fuel cells operating on hydrogen

The research dealing with performance enhancement by Sm_0.5Sr_0.5CoO_3?δ (SSC) fibrous cathode with embedded Sm_0.2Ce_0.8O_1.9 (SDC) particles has been reported by our previous study. In this paper, in order to assess the feasibility and reliability of this fibrous SSC-SDC cathode as a practical cathode, we conducted the short-term stability test using a half-cell under cathodic polarization treatment to identify the degradation phenomena only at the cathode. The polarization treatment was conducted under the current density of 500 mA/cm². The stability test was carried out at 700 °C in air under the condition of open circuit. After the measurement, the resistance associated with charge transfer and surface catalytic reaction were increased by 159 and 77%, respectively. The reasons for this degradation can be considered common approaches of perovskites cathode materials such as grain growth and Sr-enrichment. Fortunately, the fibers maintain their original shape, porosity and interfacial adhesion to electrolyte.     

On the equation of electrical conductivity relaxation method to measure kinetic parameters of solid oxide fuel cell materials with a three-dimensional rectangular geometry

The electrical conductivity relaxation method has been widely used to measure kinetic properties of species transport process in solid oxide fuel cell materials using rectangular dense samples. It is found that the equation of ECR used in some studies is flawed. An improved equation is presented in this communication.     

Thermal stress and probability of failure analyses of functionally graded solid oxide fuel cells

Thermal stresses and probability of failure of a functionally graded solid oxide fuel cell (SOFC) are investigated using graded finite elements. Two types of anode-supported SOFCs with different cathode materials are considered: NiO-YSZ/YSZ/LSM and NiO-YSZ/YSZ/GDC-LSCF. Thermal stresses are significantly reduced in a functionally graded SOFC as compared with a conventional layered SOFC when they are subject to spatially uniform and non-uniform temperature loads. Stress discontinuities are observed across the interfaces between the electrodes and the electrolyte for the layered SOFC due to material discontinuity. The total probability of failure is also computed using the Weibull analysis. For the regions of graded electrodes, we considered the gradation of mechanical properties (such as Young’s modulus, the Poisson’s ratio, the thermal expansion coefficient) and Weibull parameters (such as the characteristic strength and the Weibull modulus). A functionally graded SOFC showed the least probability of failure based on the continuum mechanics approach used herein.     

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.     

A novel post-treatment to calcium cobaltite cathode for solid oxide fuel cells

The calcium cobaltite (CCO) cathodes are post-treated by dipping in the hydrogen peroxide (H₂O₂). The electrochemical properties are investigated by the electrochemical impedance spectra (EIS) and current-voltage test in the symmetrical cell and single cell, respectively. The phase structure and morphology of the cathodes are characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The experiment results show that the mesopores are created on the surface of the cathode particles and the pore channels of the cathode are cleaned up after leaching with 10 wt % H₂O₂, resulting in a remarkable decreasing of the area specific resistance (e.g. only 42.5% of that for the untreated cathode at 800 °C). The single cell with treated cathode is about 2 times the peak power density of the cell with untreated cathode, signifying the post-treating method may be promising.     

Recent anode advances in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.     

Preparation of honeycomb porous solid oxide fuel cell cathodes by breath figures method

Honeycomb porous LSM/YSZ composite cathodes are prepared using the breath figures (BFs) method with nontoxic and easily available water droplets as templates. They were characterized by scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). Experimental results indicate that 1) ambient temperature, relative humidity, and thickness of the slurry have significant influence on the morphology of porous membrane; 2) the membrane with a thickness of 35 μm prepared at 35 °C and relative humidity of 70-75% shows the lowest polarization resistance between the LSM/YSZ composite cathode and YSZ electrolyte; and 3) the honeycomb structure of composite cathodes is favorable for lowering the low-frequency resistance concerning diffusion processes at relatively low operating temperatures of 650 and 700 °C.     

Silver (Ag) as anode and cathode current collectors in high temperature planar solid oxide fuel cells

Planar electrolyte supported solid oxide fuel cells were operated at 900 °C with humidified H₂ for 200 h using silver mesh and paste for cathode current collection. Continuous potentiostatic tests at 0.7 V appeared to induce migration of Ag towards electrode-electrolyte interphase, while continuous OCV tests caused no mass transport. Similar SOFCs fueled by coal syngas at 850 °C using Ag for both anode and cathode current collection indicated little, if any, Ag migration; providing the possibility of employing Ag for 100 h laboratory scale tests using coal-derived syngas. Use of high temperature steam, carbon dioxide and carbon monoxide did not result in the formation of silver carbonates.     

Cobalt and molybdenum co-doped Ca2Fe2O5 cathode for solid oxide fuel cell

Recently, Brownmillerite oxides Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) (M = transition metal such as Co, Mo), have been drawing attention as they possess mixed ionic and electronic conductivity. Fe site of parent Ca₂Fe₂O₅ (CFO) structure is partially substituted by Co and/or Mo as well as CoMo co-doping and tested as cathodes in SOFC. Physical characterizations such as X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM), and Brunauer–Emmett–Teller (BET) have been carried out to assess the phase formation, microstructure, presence of constituent elements, particle size, and surface area of the cathode, respectively. The Co doped CFO cathodes have better percolation, large surface area, and extended triple phase boundary. Further, the doped CFO cathodes exhibited chemical compatibility with other cell components during fabrication and cell testing as evident from SEM micrographs. The Ca₂Fe_2-xM_xO₅ (0 ≤ x ≤ 0.2) oxides show a semiconductor behaviour having sufficient electrical conductivity values in the SOFCs operating temperature 600–800 °C range. The best electrical conductivity, 0.47 S/cm at 800 °C and the corresponding activation energy of 0.17 eV is exhibited by Ca₂Fe_1.8Co_0.2O₅ (CFCO), whereas Ca₂Fe_1.8Mo_0.2O₅ (CFMO) and Ca₂Fe_1.8Mo_0.1Co_0.1O₅ (CFMCO) cathode shows electrical conductivity 0.11 S/cm and 0.15 S/cm at 800 °C, respectively. CFMO performed better with SDC than YSZ electrolyte between 600 and 700 °C although the lowest area specific resistance (ASR) of 1.28 Ω cm² at 800 °C is observed for CFMO with YSZ electrolyte. Similarly, CFMCO provided low ASR at lower temperature with SDC than that with YSZ electrolyte but exhibited lowest ASR of 0.41 Ω cm² at 800 °C with YSZ. The CFCO cathode shows lower ASR with YSZ than that with SDC for all the temperature and provided lowest value of ASR 0.21 Ω cm² at 800 °C. CFCO cathode has been tested in 900 μm thick electrolyte (SDC/YSZ) supported solid oxide fuel cell (SOFC) CFCO-SDC/SDC/NiO-SDC and CFCO-YSZ/YSZ/NiO-YSZ provided maximum power densities of 171 and 506 mW/cm² (i-R corrected) at 800 °C, respectively.     

Self-rising synthesis of Ni-SDC cermets as anodes for solid oxide fuel cells

Ni-SDC cermets have been obtained using a self-rising approach by two different ways, one-step direct synthesis (OS) and ball milling the separately prepared NiO and SDC powders (BM). The results showed that self-rising approach was an efficient way for the synthesis of porous materials composed of evenly distributed uniform size nanocrystals. The as-synthesized powders have been applied as anodes for solid oxide fuel cells, whose electrochemical properties have been systematically studied. Cells with anodes from the BM method showed better performance compared with those of the OS method, achieving a maximum power density of 400 mW cm⁻² at 600 °C.     

Synergistically enhancing CO2-tolerance and oxygen reduction reaction activity of cobalt-free dual-phase cathode for solid oxide fuel cells

High-performance cathodes with adequate CO₂ tolerance are vital for further development of intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, there is always a trade-off between CO₂ tolerance and oxygen reduction reaction (ORR) performance for single-phase cathodes. Here, we report a cobalt-free Ba_0.6La_0.4FeO_3-δ-Ce_0.8Sm_0.2O_2-δ (BLF-SDC) dual-phase cathode with excellent ORR activity and CO₂ tolerance. Introducing ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) into the Ba_0.6La_0.4FeO_3-δ (BLF) phase can boost ORR activity due to the extended active sites and enhanced oxygen surface exchange process with a polarization resistance of 0.121 Ω cm² for the BLF-30% SDC (weight ratio, BLF-30SDC) cathode at 700 °C. The CO₂ resistance of the BLF-30SDC composite cathode outperforms BLF cathode by three times at 600 °C. This stability enhancement is owing to low CO₂ adsorption of SDC, which is confirmed from thermodynamic calculation. This work indicates that dual-phase mixed conductors can be developed as highly active and stable cathodes for IT-SOFCs.     

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.     

Creep behaviour of porous metal supports for solid oxide fuel cells

The creep behaviour of porous iron–chromium alloy used as solid oxide fuel cell support was investigated, and the creep parameters are compared with those of dense strips of similar composition under different testing conditions. The creep parameters were determined using a thermo-mechanical analyser with applied stresses in the range from 1 to 15 MPa and temperatures between 650 and 800 °C. The Gibson–Ashby and Mueller models developed for uniaxial creep of open-cell foams were used to analyse the results. The influence of scale formation on creep behaviour was assessed by comparing the creep data for the samples tested in reducing and oxidising atmospheres. The influence of pre-oxidation on creep behaviour was also investigated. In-situ oxidation during creep experiments increases the strain rate while pre-oxidation of samples reduces it. Debonding of scales at high stress regime plays a significant role affecting the creep behaviour of the metal supports, in particular the stress exponent. The variation of the elastic modulus as function of temperature and oxidation conditions was also determined by a high temperature impulse excitation technique. Additionally nano-indentation testing was performed in the metal oxide interface to elucidate the mechanical properties of the oxide scales and qualitative information about the oxide scale-metal interfacial bonding.     

Layered perovskite GdBaCoFeO5+x as cathode for intermediate-temperature solid oxide fuel cells

A layered perovskite oxide, GdBaCoFeO_5+x (GBCF), was investigated as a novel cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). A laboratory-sized Sm_0.2Ce_0.8O_1.9 (SDC)-based tri-layer cell of NiO–SDC/SDC/GBCF was tested under intermediate-temperature conditions of 550–650 °C with humidified H₂ (∼3% H₂O) as a fuel and the static ambient air as oxidant. A maximal power density of 746 mW cm⁻² was achieved at 650 °C. The interfacial polarization resistance was as low as 0.42, 0.18 and 0.11 Ω cm² at 550, 600 and 650 °C, respectively. The experimental results indicate that the layered perovskite GBCF is a promising cathode candidate for IT-SOFCs.