Effect of NiO addition on oxygen reduction reaction at lanthanum strontium cobalt ferrite cathode for solid oxide fuel cell

Lanthanum strontium cobalt ferrite (LSCF), a perovskite's family member, has gained much attention as the electrocatalyst for the oxygen reduction reaction (ORR) in intermediate temperature solid oxide fuel cells. However, it still needs a strategy to improve its catalytic activity. In this work, NiO is primarily investigated as a possible synergistic catalyst for ORR on the LSCF surface. The effect of NiO particles on the effective oxygen chemical surface exchange coefficient is revealed with the electrical conductivity relaxation (ECR) technique. At 800 °C, the coefficient is increased from 3.48 × 10^?5 to 6.65 × 10^?5 cm s^?1 and 6.9 × 10^?4 cm s^?1 when NiO particles are deposited using the sputter and drop coating methods, respectively. Adding 5 wt % NiO to LSCF reduces the area specific interfacial polarization resistance, for example from 0.108 to 0.082 Ω cm² at 700 °C, as demonstrated by impedance spectroscopy on symmetric cells using samaria-doped ceria as the electrolyte. Adding NiO can also improve the performance of anode supported button cells, increasing the peak power density from 0.731 to 1.031 W cm^?2 at 800 °C. On the whole, the increased oxygen surface exchange rate together with the reduced electrode resistance and improved power density, exhibit that NiO is a potential additive to enhance the LSCF catalytic activity.     

Performance and degradation of metal-supported solid oxide fuel cells with impregnated electrodes

Metal-supported solid oxide fuel cells (MS-SOFCs) containing porous 430L stainless steel supports, YSZ electrolytes and porous YSZ cathode backbones are fabricated by tape casting, laminating and co-firing in a reducing atmosphere. Nano-scale Ni and La_0.6Sr_0.4Fe_0.9Sc_0.1O_3−δ (LSFSc) coatings are impregnated onto the internal surfaces of porous 430L and YSZ, acting as the anode and the cathode catalysts, respectively. The resulting MS-SOFCs exhibit maximum power densities of 193, 418, 636 and 907 mW cm⁻² at 650, 700, 750 and 800 °C, respectively. Nevertheless, a continuous degradation in the fuel cell performance is observed at 650 °C and 0.7 V during a 200-h durability measurement. Possible degradation mechanisms were discussed in detail.     

High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers

Metal-supported solid oxide fuel cells are believed to have commercial advantages compared to conventional anode (Ni-YSZ) supported cells, with the metal-supported cells having lower material costs, increased tolerance to mechanical and thermal stresses, and lower operational temperatures. The implementation of a metallic support has been challenged by the need to revise the cell fabrication route, as well as electrode microstructures and material choices, to compete with the energy output and stability of full ceramic cells.The metal-supported SOFC design developed at Risø DTU has been improved, and an electrochemical performance beyond the state-of-the-art anode-supported SOFC is demonstrated possible, by introducing a CGO barrier layer in combination with Sr-doped lanthanum cobalt oxide (LSC) cathode. Area specific resistances (ASR) down to 0.27 Ω cm², corresponding to a maximum power density of 1.14 W cm⁻² at 650 °C and 0.6 V, were obtained on cells with barrier layers fabricated by magnetron sputtering. The performance is dependent on the density of the barrier layer, indicating Sr²⁺ diffusion is occurring at the intermediate SOFC temperatures. The optimized design further demonstrate improved durability with steady degradation rates of 0.9% kh⁻¹ in cell voltage for up to 3000 h galvanostatic testing at 650 °C and 0.25 A cm⁻².     

A free-cobalt barium ferrite cathode with improved resistance against CO2 and water vapor for protonic ceramic fuel cells

The triple conducting (electron, oxygen-ion and proton) cathode is promising for protonic ceramic fuel cells (PCFCs). Heavy doping of Fe can impel BaZr_0.9Y_0.1O_3-δ to cross the electronic percolation threshold, forming the triple-conducting BaFe_0.8Zr_0.1Y_0.1O_3-δ (BFZY) material. The partial A-site substitution with La is also designed to further optimize the properties of BFZY. Herein, a systematic study on Ba_0.95La_0.05Fe_0.8Zr_0.1Y_0.1O_3-δ (BLFZY) cathode is conducted for searching potential free-cobalt PCFC cathodes with good tolerance for both CO₂ and water vapor. The XRD result indicates that BLFZY maintains stable cubic perovskite structure. The comprehensive analysis based on XRD, TG and TPD-CO₂ reveals that the BLFZY cathode exhibits an excellent tolerance for CO₂ and water vapor. The La doping also has a positive influence on the thermal expansion coefficient, electrical conductivity, and electrocatalytic activity. The anode-supported single cell with single-phase BLFZY cathode achieves a peak power density of 305 mW cm^?2 and polarization resistance of 0.492 Ω cm² at 600 °C and maintains steady power output in the short-term durability test. This indicates that the BLFZY material is a good cathode candidate for PCFCs.     

Effect of reduced graphene oxide addition on cathode functional layer performance in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) operating at high temperatures are highly efficient electrochemical devices since they convert the chemical energy of a fuel directly into heat and electrical energy. The electrochemical performance of an SOFC is significantly influenced by the materials and microstructure of the electrodes since the electrochemical reactions in SOFCs take place at three/triple phase boundaries (TPBs) within the electrodes. In this study, graphene in the form of reduced graphene oxide (rGO) is added to cathode functional layer (CFL) to improve the cell performance by utilizing the high electrical properties of graphene. Various cells are prepared by varying the rGO content in CFL slurry (1–5 wt %), the number of screen printing (1–3) and the cathode sintering temperature (900–1100 °C). The electrochemical behavior of the cells is evaluated by electrochemical performance and impedance tests. It is observed that there is a ∼26% increase in the peak performance of the cell coated with single layer CFL having 1 wt % graphene and 1050 °C sintering temperature, compared to that of the reference cell.     

Development of lanthanum strontium cobalt ferrite perovskite electrodes of solid oxide fuel cells – A review

There is an enormous driving force in solid oxide fuel cells (SOFCs) to reduce the operating temperatures from high temperatures (800–1000 °C) to intermediate and low temperatures (400–800 °C) in order to increase the durability, improve thermal compatibility and thermal cycle capability, and reduce the fabrication and materials costs. One of the grand challenges is the development of cathode materials for intermediate and low temperature SOFCs with high activity and stability for the O₂ reduction reaction (ORR), high structural stability as well as high tolerance toward contaminants like chromium, sulfur and boron. Lanthanum strontium cobalt ferrite (LSCF) perovskite is the most popular and representative mixed ionic and electronic conducting (MIEC) electrode material for SOFCs. LSCF-based materials are characterized by high MIEC properties, good structural stability and high electrochemical activity for ORR, and have played a unique role in the development of SOFCs technologies. However, there appears no comprehensive review on the development and understanding of this most important MIEC electrode material in SOFCs despite its unique position in SOFCs. The objective of this article is to provide a critical and comprehensive review in the structure and defect chemistry, the electrical and ionic conductivity, and relationship between the performance, intrinsic and extrinsic factors of LSCF-based electrode materials in SOFCs. The challenges, strategies and prospect of LSCF-based electrodes for intermediate and low temperature SOFCs are discussed. Finally, the development of LSCF-based electrodes for metal-supported SOFCs and solid oxide electrolysis cells (SOECs) is also briefly reviewed.     

CaO effect on the electrochemical performance of lanthanum strontium cobalt ferrite cathode for intermediate-temperature solid oxide fuel cell

The oxygen reduction reaction (ORR) on lanthanum strontium cobalt ferrite (LSCF) catalyst is critical for intermediate temperature solid oxide fuel cells (SOFCs). The reaction rate can be effectively improved by addition various nanoparticles including electrocatalysts such as Pd, Ag and mixed electronic-ionic conductors and electrolytes like samaria doped ceria (SDC). This work shows that ORR rate can also be improved by CaO, which is neither catalyst nor conductor. The CaO nanoparticles are deposited to porous LSCF electrodes using the infiltrating technique. No obvious reaction between CaO and LSCF is detected with X-ray diffraction analysis, indicating that CaO is chemically compatible with LSCF in the intermediate-temperature SOFC operation conditions. Impedance spectrum analysis demonstrates that the CaO nanoparticles can effectively reduce the interfacial polarization resistances for both single phase LSCF electrodes and LSCF-SDC composite electrodes. In addition, CaO nanoparticles can improve the peak power densities and reduce the total electrode resistances of single cells consisting of NiO-SDC anodes, SDC electrolytes, and LSCF based cathodes. Further, CaO can increase the oxygen surface exchange coefficient as demonstrated with electrical conductivity relaxation measurement. The improving factor is comparable to those for Rh and Pd catalysts, suggesting it is effective to increase ORR rate by infiltrating CaO nanoparticles.     

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.     

Nickel/gadolinium-doped ceria anode for direct ethanol solid oxide fuel cell

This report investigates the properties of nickel/gadolinium-doped ceria (Ni/GDC) as anode material for bio-ethanol fueled SOFC. The Ni/GDC cermets with 18 and 44 wt.% Ni were prepared by a hydrothermal method. Ethanol decomposition, steam reforming, and partial oxidation of ethanol were studied using a fixed-bed reactor at 1123 K. Carbon was formed only under dry ethanol for both catalysts. The addition of water or oxygen to the feed inhibited the formation of carbon. Ni/GDC was used as the anode current collector layer and as a catalytic layer in single cells tests. No deposits of carbon were detected in single cells with Ni/GDC catalytic layer after 50 h of continuous operation under direct (dry) ethanol. This result was attributed to the catalytic properties of the Ni/GDC layer and the operation mechanism of gradual internal reforming, in which the oxidation of hydrogen provides the steam for ethanol reforming, thus avoiding carbon deposition.     

A numerical study on the heat and mass transfer characteristics of metal-supported solid oxide fuel cells

The heat and mass transfer characteristics of solid oxide fuel cells (SOFCs) need to be considered when designing SOFCs because they heavily influence the performance and durability of the cells. The physical property models, the governing equations (mass, momentum, energy and species balance equations) and the electrochemical reaction models were calculated simultaneously in a 3-dimensional SOFC simulation. The current density-voltage (I-V) curves measured experimentally from a single SOFC were compared with the simulation data for code validation purposes. The error between the experimental data and the numerical results was less than 5% at operating temperatures from 700 °C to 850 °C. The current density and the mass transfer rate of an anode-supported SOFC were compared with those of a metal-supported SOFC. The metal-supported SOFC had a 17% lower average current density than the anode-supported SOFC because of the bonding layer, but it showed better thermal stability than the anode-supported SOFC because of its more uniform current density distribution. The current density, temperature and pressure drop of the metal-supported SOFC were investigated for several channel designs. A high current density was observed near the hydrogen inlet and at the intersection of the hydrogen and air channels. However, there was a low current density under the rib and at the cell edge because of an insufficient reactant diffusion flux. When the proper channel design was applied to the metal-supported SOFC, the average current density was increased by 45%.     

Study on Ce and Y co-doped BaFeO3-δ cubic perovskite as free-cobalt cathode for proton-conducting solid oxide fuel cells

The use of triple-conducting (electron, proton, oxide ion) cathodes is an effective strategy for significantly decreasing cathode polarization of proton-conducting SOFCs. In this study, a new triple-conducting BaFe_0.8Ce_0.1Y_0.1O_3-δ (BFCY) perovskite cathode is prepared by Pechini sol-gel process and its properties are evaluated comprehensively. High-temperature XRD measurement demonstrates that the codoping of Ce and Y can stabilize the cubic perovskite of BFCY in investigated temperature range from room temperature to 900 °C. BFCY material exhibits a moderate average thermal expansion coefficient of 22.08 × 10^?6 K^?1 smaller than cobalt-based cathode and good chemical compatibility with BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) electrolyte after the calcining treatment at 1000 °C. XPS analysis indicates the existence of Ce^3+/4+ and Fe^3+/4+ ions and abundant oxygen vacancies in BFCY powder surface. Thermal gravimetric analysis reveals that a larger number of oxygen deficiencies -are generated at elevated temperatures, which favors the catalytic activity on oxygen reduction. The maximum value of BFCY electrical conductivity remains at 1.55 S cm^?1 at 600 °C in humidified air. BCFY (BaCe_0.8Fe_0.1Y_0.1O_3-δ) material is introduced in order to construct BFCY-BCFY composite cathode with the good cathode/electrolyte interface adhesion. For the single cell with BFCY-BCFY composite cathode, the polarization resistance as low as 0.05 Ω cm² and peak power density as high as 750 mW cm^?2 are reached at 700 °C, respectively, demonstrating the great potential of BFCY oxides as proton-conducting SOFC cathode.     

High performance yttria-stabilized zirconia based intermediate temperature solid oxide fuel cells with double nano layer composite cathode

A composite double layer cathode of La_0.6Sr_0.4Co_0.8Fe_0.2O_3?δ/La_0.8Sr_0.2FeO_3?δ (LSCF/LSF) was successfully fabricated by infiltration method to accelerate the sluggish oxygen reduction reaction (ORR) processes. In this composite cathode, both LSF and LSCF layers are uniformly distributed on Yttria-stabilized Zirconia (YSZ) scaffold by optimizing the infiltrating solution components. LSF serves as a protective layer between LSCF and YSZ. The introduction of the LSCF exterior layer has greatly improved cell performance compared with the cell with sole LSF cathode. At 600 °C, the maximum power density of the cell with LSCF/LSF/YSZ composite cathode reaches up to 0.559 W cm^?2. The evolution of the cathode polarization resistance verifies that the ORR activity has been greatly enhanced. Therefore, the results indicate that the high cell performance at intermediate temperatures can be obtained by adopting the LSCF cathode into YSZ-based SOFCs using protective layer and that the infiltration method is a practical way for constructing electrode.     

Microstructure optimization of nickel/gadolinium-doped ceria anodes as key to significantly increasing power density of metal-supported solid oxide fuel cells

Metal-supported solid oxide fuel cells (MSCs) are promising candidates for mobile power generators like range extenders for battery electric vehicles due to their improved thermal conductivity and ruggedness. The limited space available in such vehicles heightens the need to achieve high power densities. In the present study, a significant increase in cell performance of the MSC concept of Plansee SE was demonstrated by means of systematic microstructure optimization of the complete cell architecture based on improved processing. Thickness and roughness of multi-layered Ni/GDC anode play a particularly important role in improving cell performance. After several optimization steps, a notable increase of current density from 1.29 A/cm² to 1.79 A/cm² at 700 °C and 0.7 V (+38%) was achieved. Additionally, lowering the anode roughness enables clear reduction of electrolyte thickness down to 2 μm, a starting point for the further enhancement of cell performance.     

Nanocrystalline doped lanthanum cobalt ferrite and lanthanum iron cobaltite-based composite cathode for significant augmentation of electrochemical performance in solid oxide fuel cell

Sr-doped lanthanum cobalt ferrite (La_0.54Sr_0.40Co_0.20Fe_0.80O_3−δ) and lanthanum iron cobaltite (La_0.54Sr_0.40Fe_0.20Co_0.80O_3−δ)-based mixed ionic and electronic conducting solid oxide fuel cell cathodes are synthesized by autocombustion technique. In order to examine the electrochemical activity including thermal matching with the adjacent cell components, a composite cathode comprising of both the ferritic and cobaltite system is prepared using mechanical mixing. Powder characterizations for cobaltite and ferritic-based perovskite revealed nanocrystallinity (15–30 nm) with particulate size ranging 50–100 nm. Anode-supported half cell with suitable doped ceria based interlayer on the top of the electrolyte and developed composite cathode augments the current density to 3.98 Acm⁻² at 0.7 V at 800 °C. The oxygen reduction reaction kinetics of such composite cathode shows high exchange current density of 1.16 Acm⁻² with relatively low electrode polarization of 0.02 Ωcm² at 800 °C. The electrochemical performance is clinically correlated with the cell microstructure exhibiting minimum SrO diffusion at the electrode-electrolyte interface.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

Study on the strontium segregation behavior of lanthanum strontium cobalt ferrite electrode under compression

Sr segregation is one of the key issues affecting the performance and durability of lanthanum strontium cobalt ferrite electrode (LSCF) for solid oxide cells (SOCs). In this study, we investigated the Sr segregation behavior in LSCF under compression with purpose of simulating the operating condition of LSCF electrode in an assembled SOC stack. The electrochemical performance of LSCF electrode in a symmetric cell was evaluated by impedance spectrometry. The Sr segregation behavior on the surface of LSCF electrodes was investigated by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). It was confirmed that the compression load would suppress the formation of SrO on the surface of LSCF during long-term holding in air at 800 °C, resulting in significantly improved durability compared with the LSCF electrode without compression. Furthermore, it was revealed that the type of materials (YSZ, GDC and Pt) in contact with LSCF would also affect the Sr segregation in LSCF. In particular, contacting with YSZ under compression would induce more SrO formation on the surface of LSCF than GDC and Pt.     

Effect of addition method of gadolinia-doped ceria-added FeCr gas diffusion layer on performance of direct-methane solid oxide fuel cells

A solid oxide fuel cell (SOFC) was set up with a porous disk of gadolinia-doped ceria (GDC)-added FeCr as a gas diffusion layer under direct-methane feeding. The addition of GDC was done by mixing GDC powder with FeCr powder before disk fabrication, or by coating GDC powder or impregnating GDC precursor to the surface of the porous FeCr disk. When GDC was added by mixing, the direct-methane SOFC (DM-SOFC) performance degraded very rapidly. When GDC was added by either powder coating or impregnating, the DM-SOFC performance can be relatively stable. Both the current density and the CO₂ selectivity with GDC addition by impregnating are larger than those by powder coating.     

Functionally-graded composite cathodes for durable and high performance solid oxide fuel cells

A double-layer dual-composite cathode is fabricated and has an ideal cathode microstructure with large electrochemical active sites and enhanced the durability in solid oxide fuel cells (SOFCs). The insertion of a yttria-stabilized zirconia (YSZ)-rich functional layer between the electrolyte and the electrode allows for a graded transition of the YSZ phase, which enhances ionic percolation and minimizes the thermal expansion coefficient mismatch. Electrochemical measurements reveal that the double-layer composite cathode exhibits improved cathodic performance and long-term stability compared with a single-layer composite cathode. A cell with a well-controlled cathode maintains nearly constant interfacial polarization resistance during an 80 h accelerated lifetime test.     

Characterization of electrochemical reaction and thermo-fluid flow in metal-supported solid oxide fuel cell stacks with various manifold designs

The metal-supported solid oxide fuel cell (SOFC), in which a metal plate is bonded to a ceramic cell, was recently introduced as a new fuel cell design. Metal-supported SOFCs do not suffer from gas leakage, because the metal plates are welded to the metallic interconnects, which also provides high mechanical strength. However, the bonding layer existing between the interconnect and the ceramic cell increases mass transfer resistance, resulting in decreased performance. To better understand and control the mass transfer rate, the manifold structure of the fuel cell stack as well as the channel design in each single cell should be studied. Using a numerical approach, physical property models, governing equations and electrochemical reaction models were calculated simultaneously. The experimentally measured current density–voltage curves were compared with the simulation data to validate the code. Current densities, temperatures and pressure distributions resulting from various manifold designs were presented as numerical results. The parallel manifold design displayed an average current density of 2820.1 A/m² and a relatively uniform current density distribution. The serpentine design yielded the highest average current density among the studied manifold designs, but the maximum pressure was 32 times higher than with the parallel design. Moreover, the large temperature difference observed with the serpentine design may result in a thermal expansion problem. The expanding manifold design yielded an average current density of 2885.9 A/m² and a maximum pressure of 6350 Pa. The pressure distribution with this manifold design was clearly related to the manifold structure. The tapering manifold design is the opposite of the expanding manifold; with this design, the average current density and maximum pressure were slightly lower than the expanding manifold. The dual-flow hybrid manifold design combines two different manifold structures: a serpentine hydrogen manifold and a parallel air manifold. The dual-flow hybrid design yielded an average current density of 2905.4 A/m² and a maximum pressure of 750 Pa.     

Factors of cathode current-collecting layer affecting cell performance inside solid oxide fuel cell stacks

Factors of cathode current-collecting layer (CCCL) affecting cell performance are studied by investigation of solid oxide fuel cell (SOFC) stacks with various (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) as CCCL in-suit. A larger real contact area between cathode and interconnect appears when the LSM is coated on cathode side as CCCL through characterization of a 2-cell stack. The result reveals that the real contact area depends on the surface roughness match (SRM) between CCCL and its neighboring components (active cathode and interconnect). A 6-cell stack using CCCLs with various levels of surface roughness is assembled and characterized further. The results show a higher electrical output performance of the stack repeating unit can be obtained when the surface roughness of the CCCL matches that of its neighboring components better, i.e. the surface roughness match (SRM) is the factor of cathode current collector affecting cell performance inside stack. Accordingly, the cell performance inside SOFC stack can be regulated by designing the SRM to its neighboring components.