Thermo-elastic properties of SOFC/SOEC electrode materials determined from three-dimensional microstructural reconstructions

Thermo-mechanical properties of SOFC/SOEC electrodes have been computed by homogenization from three-dimensional reconstructions of their microstructure. The support and the functional layer of a Ni/YSZ supported cell and the oxygen electrode made of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) have been studied. Values of the effective Young's modulus obtained for the thick porous Ni/YSZ support are in good agreement with the experimental data (E = 35 GPa). Besides, for thin functional layers, the methodology supplies original values that are difficult to obtain by conventional experimental methods (E = 105 GPa for the Ni/YSZ Functional Layer and E = 55 GPa for the LSCF electrode). Furthermore, it has been shown that the effective elastic parameters are influenced by the following morphological parameters: the porosity and the formation factor that could also be related to the manufacturing process. Theses morphological parameters have however negligible effect on the effective thermal expansion.     

A symmetrical,planar SOFC design for NASA's high specific power density requirements

Solid oxide fuel cell (SOFC) systems for aircraft applications require an order of magnitude increase in specific power density (1.0 kW kg⁻¹) and long life. While significant research is underway to develop anode supported cells which operate at temperatures in the range of 650–800 °C, concerns about Cr-contamination from the metal interconnect may drive the operating temperature down further, to 750 °C and lower. Higher temperatures, 850–1000 °C, are more favorable in order to achieve specific power densities of 1.0 kW kg⁻¹. Since metal interconnects are not practical at these high temperatures and can account for up to 75% of the weight of the stack, NASA is pursuing a design that uses a thin, LaCrO₃-based ceramic interconnect that incorporates gas channels into the electrodes. The bi-electrode supported cell (BSC) uses porous YSZ scaffolds, on either side of a 10–20 μm electrolyte. The porous support regions are fabricated with graded porosity using the freeze-tape casting process which can be tailored for fuel and air flow. Removing gas channels from the interconnect simplifies the stack design and allows the ceramic interconnect to be kept thin, on the order of 50–100 μm. The YSZ electrode scaffolds are infiltrated with active electrode materials following the high-temperature sintering step. The NASA-BSC is symmetrical and CTE matched, providing balanced stresses and favorable mechanical properties for vibration and thermal cycling.     

Aqueous slurry processing of monolithic films for SOFC - YSZ, LSM and YSZ-NiO systems

The powder precursor for yttria stabilized zirconia (YSZ) was prepared by reverse strike co-precipitation method while those for lanthanum strontium manganite (LSM) and nickel oxide (NiO) were prepared by gel combustion method. The thermal decomposition and phase evolution behavior of these powder precursors was carried out by thermogravimetry and XRD. Phase pure cubic YSZ formed around 430 °C from the dried amorphous gel and exhibited a mass loss of 23%. Even though the as formed LSM and NiO precursors exhibited nano-crystallinity, they contained some amount of volatiles (up to 8%), elimination of which required a post heat treatment. The optimum calcination temperature for these powders to obtain sintered bodies with desired densities (viz. >95% T.D. for YSZ and 65-75% T.D. for LSM, YSZ-NiO) at the desired sintering temperatures (1350, 1400 and 1500 °C respectively), was found to vary in the range of 900 1350 °C. Fine YSZ powder with size (D₅₀) 0.7-1.2 μm was used in formation of the electrolyte film while YSZ, LSM and NiO powder with size (D₅₀) 3-5 μm along with carbon pore former (15 wt% in LSM) were used for formation of electrode films. The conditions for slurry formation for film casting were evaluated through surface charge and rheological studies. The study of the effect of pH of aqueous suspension on zeta potential showed that YSZ and NiO were charged to sufficient extent (>20 mV) in both acidic and alkaline media while LSM and pore former exhibited sufficient surface charging only in alkaline medium. The slurries for tape casting were formulated using a polyvinyl binder solution and the composition was optimized through rheological studies. Compositions were fixed to form slurries with desired amount of pseudo-plasticity that could exhibit controlled flow to form flexible films with desirable thickness. The process conditions were optimized to form flat sintered electrolyte films possessing about 95% T.D. and electrode films possessing 65-75% T.D. Sintered bodies of the electrolytes exhibited fine-grained microstructure while the electrodes exhibited composite structure of grains and inter-connected pores.     

Co-precipitation synthesis of nickel cobalt hexacyanoferrate for binder-free high-performance supercapacitor electrodes

Prussian blue analogue with a typical metal-organic framework has been widely used as an electrode material in supercapacitor. In this work, nickel cobalt hexacyanoferrate (Ni₂CoHCF) was grown on nickel foam directly using a simple co-precipitation method. The as-prepared Ni₂CoHCF was tested by transmission electron microscope, scanning electron microscope, X-ray diffraction and X-ray electron energy spectrum. The results showed that Ni₂CoHCF has a unique open face-centered cubic structure. The Ni₂CoHCF was used to set an asymmetric supercapacitor directly. A series of electrochemical tests showed that Ni₂CoHCF had an excellent electrochemical performance. The specific capacitance of the supercapacitor was 585 C g⁻¹ (1300.0 F g⁻¹, 162.5 mAh g⁻¹) at the current density of 0.5 A g⁻¹. After 2000 cycles, it still maintained 85.57% of its initial specific capacitance at the current density of 10 A g⁻¹. The energy density was 30.59 Wh kg⁻¹ at the power density of 378.7 W kg⁻¹. The results show that the supercapacitor constructed by Ni₂CoHCF as an electrode material has high-current charge-discharge capacity, high energy density and long cycle life.     

Hydrothermal synthesis of nano-sized MnO2 supported on attapulgite electrode materials for supercapacitors

Among the electrode materials of supercapacitors, transition metal oxides have been widely used because of their low price, high theoretical capacitance and good cycle stability, and MnO₂ is one of the typical representative materials. However, the actual specific capacitance of MnO₂ is low because of its poor conductivity, easy agglomeration in the preparation process and large volume change in the process of repeated charge and discharge. Attapulgite can not only provide a large specific surface area for transition metal oxide materials, but also provide a skeleton on which nano-sized materials can be grown or dispersed. Therefore, the electrochemical performance of electrode materials can be improved by designing nanostructures and compounding a variety of materials with different properties. Herein, a new type of composites electrode material is prepared by simple one-step hydrothermal method. As an electrode material, the ATP-MnO₂ composites exhibited a high specific capacitance of 138.2 F/g at a current density of 0.5 A/g, which was 13.4% higher than that of pure MnO₂ nanoflowers. Under the current density of 3 A/g, the capacitance retention of ATP-MnO₂ composites was 89.4% after 5000 cycles.     

Parametric study for electrode microstructure influence on SOFC performance

A solid oxide fuel cell (SOFC) is a clean and high-efficiency energy conversion device, which undergoes improvement of performance continuously. The transport of gas species and charges proceed in the porous electrodes. The porous electrodes are also responsible for the removal of exhaust gases. In this paper, a fully coupled 3D single-channel multiphysics computational fluid dynamics (CFD) model was developed based on the finite element method (FEM). The governing equations for momentum, species, charges, and heat transport were solved by a segregated solver. The impact of decreased ionic, electronic, and pore phase tortuosity on the SOFC performance such as fuel utilization, current density, activation overpotential and temperature distribution are analyzed and compared with the base case. In addition to the tortuosity investigation, the volume fraction of the electronic phase in the active layer and the support layer is also investigated using a parametric sweep study. Of all the decreased tortuosity cases, there is an increase in ionic current density and temperature compared with the base case. Except for a decreased pore tortuosity, all other cases led to an increase of electronic current density compared with the base case. The consumption of hydrogen increased for all cases compared with the base case. The activation overpotential increased with decreased electronic phase and pore phase tortuosity, while a decrease of ionic phase tortuosity caused a decrease. Finally, when decreasing all phase tortuosity, both current density, temperature, activation overpotential, and hydrogen consumption increased. For the parametric sweep, there is an optimum electronic phase volume fraction value. This work allows for a better understanding of the relationship between the microstructure and performance of SOFCs. Meanwhile, it provides theoretical guidance for a better porous electrode design.     

A double perovskite decorated carbon-tolerant redox electrode for symmetrical SOFC

Double perovskite Sr₂MnMoO_5+δ (SMM) based hybrid catalyst is synthesized with SrMoO₄ (SM) nanoparticles' in-situ exsolution. And the super exchange model in double perovskite structure enable the excellent redox property of SMM based electrode. Electrolyte (Ce_0.8Sm_0.2O_1.9, SDC)-supported symmetrical solid oxide fuel cell (SOFC) is prepared with the structure as SMM/NiO-SDC||SDC||NiO-SDC/SMM. The NiO-SDC powders are prepared from gel-casting combined co-synthetic method and SMM hybrid catalyst is loaded on NiO-SDC scaffold by solution infiltration. In-situ exsolved SM particles provide additional oxygen reduction reaction (ORR) active sites on electrode surface which also contributes to the cell's cathodic performance. The peak power densities of 245 and 183 mW/cm² can be achieved by as-prepared electrode when operating in wet CH₄ and C₂H₅OH fuels at 800 °C, respectively. And the cell runs stably for over 320 h with hydrocarbon fuel, which reveals SMM/NiO-SDC electrode's high electrochemical activity and promising carbon deposition resistance.     

Functional materials for the IT-SOFC

The paper summarizes and discusses the basic properties of solid oxide fuel cell (SOFC) components (electrode materials and electrolyte) from the point of view of their essential functional parameters like chemical stability, transport, catalytic and thermomechanical properties under operational conditions in a SOFC. An interrelation between the defect structure of these materials related to oxygen nonstoichiometry and their electrical properties and catalytic activity was shown.     

Solvothermal synthesis of novel pod-like MnCo2O4.5 microstructures as high-performance electrode materials for supercapacitors

MnCo₂O_4.5 pod-like microstructures were successfully prepared through an initial solvothermal reaction in a mixed solvent containing water and ethanol, and combined with a subsequent calcinations treatment of the precursors in air. The total synthetic process was accomplished without any surfactant or template participation. The MnCo₂O_4.5 pods possessed a specific surface area as high as 73.7 m²/g and a mean pore size of 12.3 nm. The electrochemical performances were evaluated in a typical three-electrode system using 2 M of KOH aqueous electrolyte. The results demonstrated that such MnCo₂O_4.5 pods delivered a specific capacitance of 321 F/g at 1 A/g with a rate capability of 69.5% at 10 A/g. Moreover, the capacitance retention could reach 87% after 4000 cycles at 3 A/g, suggesting the excellent long-term cycling stability. Furthermore, the asymmetric device was fabricated by using MnCo₂O_4.5 porous pods as anode and active carbon as cathode. It could deliver a specific capacitance of 55.3 F g⁻¹ at 1 A g⁻¹ and an energy density of 19.65 W h kg⁻¹ at a power density of 810.64 W kg⁻¹. Such superior electrochemical behaviors indicate that the MnCo₂O_4.5 pods may be served as a promising electrode material for the practical applications of high-performance supercapacitors. The current synthesis is simple and cost-effective, and can be extended to the preparation of other binary metal oxides with excellent electrochemical properties.     

A numerical investigation of modeling an SOFC electrode as two finite layers

Modeling an electrode in solid oxide fuel cells (SOFCs) as two finite layers is numerically investigated. A simulation is conducted using the developed mathematical model, wherein an SOFC electrode is considered as a porous composite structure of electron- and ion-conducting particles. Moreover, an electrochemical reaction is considered to occur throughout the electrode. In other words, an electrode is treated as a reaction zone layer having triple phase boundaries (TPBs) scattered throughout the electrode, consistent with the micro modeling approach of treating electrodes. The model takes into account the transport of multi-component mixture in an electrode together with electrochemical reaction, the transport of electrons and ions in the respective electron-conducting and ion-conducting particles of the electrode. It is found that both the dimensionless electronic and ionic current densities remain constant with respective values of one and zero for most part of the anode before started to vary towards the end of the anode. Further, from the distributions of dimensionless electronic and ionic current densities in the anode, it can be deduced that the part of the anode (electrode) where the value of dimensionless electronic current density is one, can be considered as an electron-conducting (ion insulator) layer, referred to as the anode (electrode) backing layer; the part of the anode (electrode), where there is a variation in the electronic and ionic current densities and electrochemical reaction rate is most effective, can be considered as a mixed-conducting layer, referred to as the reaction zone layer. Finally, a parametric study is conducted to investigate the effect of key operating and design conditions on the thickness of the reaction zone layer in an SOFC anode.     

Progress of electrochemical capacitor electrode materials: A review

The electrode is the key part of the electrochemical capacitors (ECs), so the electrode materials are the most important factors to determine the properties of ECs. In this paper, the storage principles and characteristics of electrode materials, including carbon-based materials, transition metal oxides and conductive polymers for ECs are depicted briefly. Among them, more work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and organic electrolytes. But the composites of pseudocapacitive and carbonaceous materials are promising electrode materials for ECs because of their good electrical conductivity, low cost and high mass density.     

Effect of process modification and presence of H2O2 in the synthesis of samaria-doped ceria powders for fuel cell applications

A fundamental step for a sustainable industrial development based on “H₂ Economy” is the implementation of fuel cell technology, in terms of new devices, materials and convenient processes for their production. Rare earth doped ceria oxides are suitable materials for the new generations of cells and their cost effective production becomes fundamental as the price of rare earths is increasing. In this view, our study investigates a modified method of co-precipitation of Ce_0.8Sm_0.2O_1.9−x (SDC) evaluating the effects of adding of H₂O₂ in the process. The parameters controlled were the molar ratio [H₂O₂]/[M³⁺], (M³⁺ = Ce³⁺, Sm³⁺ present in starting nitrate salts solutions) and the pH of precipitation; in some cases the precipitates were also treated under reflux at 373 K overnight. The powder catalysts, both as fresh precipitates and calcined oxides were analyzed via N₂ adsorption (BET), X-Ray diffraction (XRD) and temperature programmed reduction (TPR) techniques and their morphological, structural and redox properties were correlated with the synthesis parameters used. The electrical conductivity properties of these materials have also been investigated via electrochemical impedance spectroscopy (EIS) and the results compared with those of a commercial oxide. The synthesis approach was shown to be very versatile in the development of materials with properties exploitable for applications in catalysis and in intermediate temperature Solid Oxide Fuel Cell (IT-SOFC) systems.     

Preparation and characterisation of SOFC anodic materials based on Ce–Cu

Ce–Cu mixed oxide precursors with varing Ce:Cu atomic ratio have been prepared by freeze-drying and microemulsion coprecipitation methods. Nanostructured particles having different properties have been obtained. Physicochemical properties have been studied with X-ray diffraction, UV–vis spectroscopy, nitrogen adsorption–desorption, mercury intrusion porosimetry, ICP-AES, conductivity measurement and thermal expansion coefficient. All samples show fluorite structure with slight copper surface enrichment for samples having high copper content. Microemulsion method allows the introduction of a large quantity of copper into the cerium oxide structure, obtaining a nanostructured mixed oxide of high surface area. On the other hand, freeze-drying samples does not show evidence of copper incorporation to the lattice of cerium oxide. All materials have a thermal expansion coefficient similar to other components of SOFC.     

Plasma synthesis of ammonia in a tangled wire dielectric barrier discharge reactor: Effect of electrode materials

Plasma synthesis of NH₃ from N₂ and renewable H₂ under mild conditions is very attractive for decentralised sustainable green ammonia production using intermittent renewables. In this study, NH₃ synthesis was performed under ambient conditions in a dielectric barrier discharge (DBD) plasma reactor. Different tangled wire internal electrodes were employed to understand the influence of electrode materials on plasma ammonia synthesis. Compared with a rod electrode, a tangled wire electrode substantially enhanced the NH₃ concentration and reduced the energy cost for ammonia production, which can be attributed to the expanded surface area and the chemisorption properties of the tangled electrodes. The influence of the N₂/H₂ molar ratio and total flow rate on the reaction performance was also evaluated. The lowest energy cost (59.0 MJ mol⁻¹) for ammonia production was achieved using a Cu tangled electrode at a total flow rate of 250 ml min⁻¹ and a discharge power of 20 W. The electrical diagnostics of the plasma process showed that the tangled wire electrodes decreased the breakdown voltage of the DBD and enhanced charge deposition, which enhanced the NH₃ production. The reaction mechanism was discussed for the process optimisation of ammonia synthesis in a tangled wire DBD system.     

Development of a 700 W anode-supported micro-tubular SOFC stack for APU applications

A 700 W anode-supported micro-tubular solid-oxide fuel cell (SOFC) stack for use as an auxiliary power unit (APU) for an automobile is fabricated and characterized in this study. For this purpose, a single cell was initially designed via optimization of the current collecting method, the brazing method and the length of the tubular cell. Following this, a high-power single cell was fabricated that showed a cell performance of at 0.7 V and using H₂ (fuel utilization=45%) and air as fuel and oxidant gas, respectively. Additionally, a fuel manifold was designed by adopting a simulation method to supply fuel gas uniformly into a single unit cell. Finally, a 700 W anode-supported micro-tubular SOFC stack was constructed by stacking bundles of the single cells in a series of electrical connections using H₂ (fuel utilization=49%) and air as fuel and oxidant gas, respectively. The SOFC stack showed a high power density of ; moreover, due to the good thermo-mechanical properties of the micro-tubular SOFC stack, the start-up time could be reduced by 2 h, which corresponds to 6/min.     

Temperature field,H2 and H2O mass transfer in SOFC single cell: Electrode and electrolyte thickness effects

The temperature increment in electrodes and electrolyte of a fuel cell is mainly attributed to the chemical reaction and the irreversibilities. The aim of this work is to study the increasing temperature of a SOFC single cell under the influence of the electrode and electrolyte thicknesses for its type of heat source. The hydrogen and water field are also discussed according to anode thickness.     

An overview of electrode materials in microbial fuel cells

Electrode materials play an important role in the performance (e.g., power output) and cost of microbial fuel cells (MFCs), which use bacteria as the catalysts to oxidize organic (inorganic) matter and convert chemical energy into electricity. In this paper, the recent progress of anode/cathode materials and filling materials as three-dimensional electrodes for MFCs has been systematically reviewed, resulting in comprehensive insights into the characteristics, options, modifications, and evaluations of the electrode materials and their effects on different actual wastewater treatment. Some existing problems of electrode materials in current MFCs are summarized, and outlooks for future development are also suggested.     

A novel interconnector design of SOFC

In this study, a novel interconnector design is proposed, which is named as the X-type interconnector. The solid oxide fuel cell (SOFC) models are established for the conventional interconnector and the X-type interconnector. The results indicate that the design of the X-type interconnector is beneficial to the transport of gas in SOFC, which has a higher oxygen concentration under rib than that of the conventional interconnector. Furthermore, compared with the X-type interconnector, the potential difference in cathode is larger for the conventional interconnector, which indicates that the X-type interconnector reduces the current path and improves the performance of SOFC. For any porosity and conductivity of anode and cathode, the X-type interconnector is superior to the conventional interconnector. Moreover, when the cathode conductivity is smaller, the advantage of the X-type interconnector becomes more remarkable.     

Co-precipitation in aqueous medium of La0.8Sr0.2Ga0.8Mg0.2O3−δ via inorganic precursors

A simple and inexpensive co-precipitation route in aqueous medium is proposed to prepare La_0.8Sr_0.2Ga_0.8Mg₀₂O_3−δ ionic conductor (LSGM). Different synthetic procedures and operating parameters (i.e. nature and amount of the precipitating agents, HNO₃ addition and temperature) have been evaluated in order to underline their influence on the composition and microstructure of the final phase. Intermediate and final products were characterized by Thermal-Gravimetry, IR-spectroscopy, X-ray Powder Diffraction, Rietveld analysis and Scanning Electron Microscopy. The electrical properties were measured by Impedance Spectroscopy in the temperature range 250-800 °C. Slight variations of the synthetic procedure (such as precipitating agent amount or no HNO₃ addition) have a considerable and detrimental effect on the ions losses and the subsequent achievement of the final phase. The use NH₄OH as an alternative precipitating agent is dramatically disadvantageous. Ions losses during precipitation must be controlled (i) to avoid understoichiometry in the LSGM phase and (ii) to prevent the formation of large amounts of secondary phases. In fact, both affect the total electrical conductivity.The use of large excess of (NH₄)₂CO₃ precipitating agent and the addition of HNO₃ lead to the best material characterized by a rhombohedral structure, small amount of side phases, a relative density of 98% and a total conductivity of 6.44 × 10⁻² S cm⁻¹ at 800 °C and 1.13 × 10⁻² S cm⁻¹ at 600 °C.     

Taxonomies of SOFC material and manufacturing alternatives

Material and manufacturing alternatives for solid oxide fuel cells are listed and analyzed. Specifically, four categories of anode materials, five categories of cathode materials, four categories of electrolytes, and three categories of interconnect materials are presented. Design considerations including operating temperatures and compatibilities among stack materials are also highlighted. Similarly, stack manufacturing options are separated into seven categories and developed into process sequences based on the number and type of firing steps. This work is intended to facilitate material and manufacturing assessments through the consideration of the variety of alternatives prior to capital investment for wide-scale production.