Development Of Solid Oxide Fuel Cells Research Articles

ChemInform Abstract: Low‐Temperature Solid‐Oxide Fuel Cells

AbstractReview: 50 refs.

Status of Solid Oxide Fuel Cell Development at Elcogen

Elcogen is a manufacturer of solid oxide fuel cell (SOFC) unit cells and stacks. The competitive advantage of Elcogen products is their performance characteristics at reduced temperatures. The unit cell structure is optimized for operating temperatures 600 – 700°C and is made of a material system from anode to cathode of NiO/YSZ – YSZ – GDC – LSC. Elcogen tailors the unit cells with different thicknesses and shapes according to the customer requirements. Elcogen stacks are optimized for stationary applications in which both the operation temperature and pressure drop can be reduced due to stack characteristics enabling cost savings because of reduced material costs both in stack and system level, and increased system efficiencies due to reduced air compression losses and enhanced Nernst potential. The article describes the performance characteristics of Elcogen unit cells and stacks. As an example, the area specific resistance of the unit cells has been determined to be 0.17 Ω.cm2 at 700°C, U fuel = U air = 20%, and the degradation rate below 5 mΩ.cm2.kh-1 in 10,000 hours test. The average stack voltage is measured to be 0.97V at 30A, T ave = 700°C, and V air = 2.2 and V H2 = 1.0 lN/min/cell. The pressure drop for the air and fuel at the same conditions is less than 5mbar. With the encoring technical results and customer feedback, Elcogen is constantly increasing its unit cell and stack production capacity.

Protective Coatings for Metallic Components in Solid Oxide Fuel Cell Systems

To make solid-oxide fuel cell (SOFC) systems more manufacturable and reduce system costs, SOFC developers, wherever possible, have substituted lower cost stainless steel into the stack design, including replacing ceramic interconnects within the stack and high-cost nickel-based superalloys in balance-of-plant components. However, for successful implementation of these steels, protective coatings are necessary to protect the air-facing metal surfaces from high temperature oxidation and to minimize chromium volatilization from the metal, because chromium volatiles poison the cathode and degrade cell performance. NexTech Materials has developed a cost-effective coating technology to improve the high temperature performance of stainless steel components. Coatings include an electrically conductive, manganese cobaltite (MCO) coating tailored for the cathode active-area of metallic interconnects and a complementary diffusion-based aluminide coating for the non-active, seal-area of interconnects and also readily amenable to balance of plant components. In this paper, the suitability of this coating technology for SOFC applications will be demonstrated. Microstructural analysis in combination with long-term electrical area-specific resistance (ASR), oxidation and chromium volatility testing will illustrate the stability and functionality of the coatings under application specific conditions. Stack-level testing will demonstrate the successful scale-up of the technology.

Development of a Novel Co-fired SOFC at Murata

Murata Manufacturing Co., Ltd. has been developing planar-type solid oxide fuel cells (SOFCs) based on new design concept. The cell is fabricated by a single-step co-firing process and consists of an electrolyte, electrodes, current collectors, gas-separators with manifolds, and gas-flow channels. The membrane electrode assembly (MEA) is sandwiched between ceramic gas-separators with via-hole electrodes providing electrical paths between cells. Murata’s new concept stack achieved volume power density higher than 0.5kW/L. By arranging gas manifolds in the central part of the cell, the temperature distribution in plane of the cell was moderated while keeping high volume power density. As a result, the stack operated stably at high efficiency. A long-term operation has been carried out at 750°C over 8000h under galvanostatic condition. The cell power degradation rate was less than 0.20%/kh after 2000h operation. Cr poisoning in the LSCF cathode after durability test was not observed. That is one reason of high durability of the cell.

Status of Solid Oxide Fuel Cell Development at Elcogen

Elcogen is a manufacturing company of solid oxide fuel cell (SOFC) and solid oxide electrolysis (SOE) unit cells and stacks. The competitive advance of Elcogen unit cells and stacks are their performance characteristics at reduced operating temperatures between 600°C to 700°C. The unit cells and stacks are based on conventional material solutions and manufacturing processes, which together with reduced operating temperature, provide cost benefits both for Elcogen’s products and system integrators.

Development of a Novel Co-fired SOFC at Murata

Murata Manufacturing Co., Ltd. has been developing planar type solid oxide fuel cells (SOFCs) based on a new design concept. The cell is fabricated by a single-step co-firing process (1). In this study, we report the results of a new type of cell with an improved cooling ability to remove heat of the electrochemical reactions. By arranging the gas manifolds in the central line of the cell, the temperature distribution in the plane of the cell was moderated while keeping a high volume power density. As a result, the stack operated stably and achieved a high power efficiency. A long-term operation has been carried out at 750ºC over 8000 h under galvanostatic conditions. The cell power degradation rate was less than 0.20 %/kh after 2000 h operation. Cr-poisoning in the LSCF cathode after the durability test was not observed, which led to the high durability of the cell.

ChemInform Abstract: Enhancing SOFC Cathode Performance by Surface Modification Through Infiltration

AbstractReview: 264 refs.

(Invited) Reversible Solid Oxide Fuel Cell Development at Versa Power Systems

Versa Power Systems (VPS) is a developer of Solid Oxide Fuel Cell (SOFC) technology focused on SOFC stack development for commercial applications. In recent years VPS has been developing reversible SOFC (RSOFC) materials systems with a view to future commercial development of RSOFC stacks. VPS has demonstrated significant technical improvements in RSOFC materials technology in a relatively short timeframe, and has begun to test these at the stack level. This paper will provide an update of RSOFC activities including cell and stack testing in both steady-state electrolysis operation and operating with RSOFC cycles (fuel cell and electrolysis cycling).

(Invited) Reversible Solid Oxide Fuel Cell Development at Versa Power Systems

Versa Power Systems (VPS) is a developer of Solid Oxide Fuel Cell (SOFC) technology focused on SOFC stack development for commercial applications. In recent years VPS has been developing reversible SOFC (RSOFC) materials systems with a view to future commercial development of RSOFC stacks. VPS has demonstrated significant technical improvements in RSOFC materials technology in a relatively short timeframe, and has begun to test these at the stack level. This paper will provide an update of RSOFC activities including cell and stack testing in both steady-state electrolysis operation and operating with RSOFC cycles (fuel cell and electrolysis cycling).

Preparation and characterization of the superlattice (Sm-doped ceria/yttria-stabilized zirconia)N electrolyte film

As is well known, a solid oxide fuel cell(SOFC) is a device that can convert chemical energy directly into electrical energy. The possibility to grow thin films of oxide materials is of great importance in the development of SOFC. Recently, it is expected that oxide heterostructures, with almost ideal interfaces, should lead to the interesting artificial materials with some novel properties. So we have prepared superlattice electrolyte thin films.On the MgO single-crystal substrates, multilayer epitaxial thin-film heterostructures of 20 mol% samarium-oxide-doped ceria (Ce0.8Sm0.2O2-, SDC) and 8 mol% yttria-stabilized zirconia (Y2O3: ZrO2YSZ) have been deposited in turn, using the pulsed laser deposition (PLD) technique. Five different superlattices (SDC/YSZ)N (N=3, 5, 10, 20, 30) films are fabricated, keeping the total thickness constant (300 nm), but with a different number of hetero-interfaces. The choice of coupling SDC and YSZ aims to have both layers in the superlattices made of oxygen-ion conductors with compatible crystallographic features. On one hand, we have to remove any potential contribution of the deposition substrate to the total conductivity, and the superlattices may be grown on 110-oriented MgO single-crystalline wafers. On the other hand, the YSZ electrolyte film requires that the temperature should be higher, and SDC as the electrolyte leakage; both should be combined with each other, so as to have complementary advantages. In this way, not only the temperature of SOFCs is reduced, but also the leakage avoided. Because YSZ and SDC both belong to cubic fluorite structure and have similar lattice parameters, the lattice constant of YSZ is 5.14 , and the lattice constant of SDC is 5.44 . Compatible crystal properties, oxygen ion conductivity, and great matching defects can lead to a semi-coherent interface between the ZrO2 and CeO2 layers. The interface effect is obvious. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and AC impedance mearurements are used to characterize the surface morphology, phase structure and electrical properties of the electrolyte film. Results show that the superlattice (SDC/YSZ)N electrolyte films will form obviously interface and better superlattice structure. This kind of thin films do not show crack, with no element diffusion at the interfaces, growing uniformly, densely and smoothly. Electrochemical measurements show a sizable increase in conductivity with increasing number of SDC/YSZ interfaces. So it is an ideal low-temperature fuel cell electrolyte materials.

Reducing the reaction between boron-containing sealing glass-ceramics and lanthanum-containing cathode: Effect of Bi2O3

The volatile boron species from sealing glass-ceramics have a significant poisoning effect on the electrochemical activity of Solid Oxide Fuel Cell (SOFC) cathodes. The reaction between boron-containing sealing glass-ceramics and cathodes thus presents a challenge for the development of SOFC. Here we report for the first time that the addition of La2O3 can significantly reduce the boron volatility from sealing glass-ceramics and consequently the formation of LaBO3 in reaction couple between glass and (La,Sr)(Co,Fe)O3 (LSCF) cathode. In particular, the boron volatility can be reduced by about 6 times with the addition of 4 mol.% La2O3 (3.7 × 10−3 vs. 0.6 × 10−3%). The addition of La2O3 condenses the structure of glasses and glass-ceramics, contributing to the decrease in boron volatility from glass-ceramics. In addition, La2O3 dopant favors the formation of boron-containing phase (La5SiBO13) in glass-ceramics, which dramatically reduces the boron volatility. The reported results provide an effective approach for solving the sealing challenge.

A top-down strategy for the synthesis of mesoporous Ba0.5Sr0.5Co0.8Fe0.2O3−δ as a cathode precursor for buffer layer-free deposition on stabilized zirconia electrolyte with a superior electrochemical performance

We develop a facile and effective top-down method for the fabrication of mesoporous Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) oxide with a high specific surface area (∼25 m2 g−1). The original BSCF is first synthesized by the simple EDTA–citric acid complexing method, and then treated in H2O2 to obtain the mesoporous BSCF. The structure and morphology of as-prepared BSCF power is systematically characterized by N2 adsorption/desorption isotherms, XRD, TEM, SEM and ICP techniques. A possible mechanism for the creation of mesoporous BSCF is proposed, in which Ba2+ and Sr2+ dissolve selectively from partial BSCF particles during the catalytic decomposition of H2O2. The electrochemical properties are investigated by the EIS and I–V test in the symmetrical cell and integrated single cell configurations, respectively. The interfacial reaction between BSCF electrode and YSZ electrolyte was suppressed successfully by using the BSCF with high specific surface area to decrease the sintering temperature (800 °C), thus the electrode exhibits high oxygen reduction reaction activity. The solid oxide fuel cell (SOFC) achieves an exciting peak power density of ∼1800 mW cm−2 at 800 °C, signifying the mesoporous BSCF, together with the preparation method, has a good application prospect in the development of SOFCs.

Electrochemical Simulation of Planar Solid Oxide Fuel Cells with Detailed Microstructural Modeling

A quasi-two-dimensional physically-based model for the description of transport and reaction in planar solid oxide fuel cells (SOFC) is presented in this study. Electrochemistry and transport phenomena in the cell are locally described in 2D using mass conservation equations and well-established global electro-kinetics, coupled with the 1D representation of gas channels in both co-flow and counter-flow configurations. The key feature of the model consists in the numerical reconstruction, through packing algorithms, of the three-dimensional microstructure of each porous layer for an accurate evaluation of the effective properties. Coupling of a detailed microstructural modeling into the cell-level electrochemical model allows the prediction of the polarization behavior from the knowledge of operating conditions and powder characteristics, thus eliminating the need for empirical correlations and adjusted parameters, which is typically the weak point of existing cell-level models. The framework is used for the simulation of a short stack of anode-supported cells with LSM-based cathode and 1.5mm thick anode support, developed and tested by Forschungszentrum Jülich. The effective properties of each layer are calculated and compared with available experimental data. A good agreement is also reached when comparing simulated and experimental polarization curves under different operating conditions without fitting any parameter. Simulations show that at 800°C the activation resistance in the cathode functional layer is the main contribution to the cell overpotential. In addition, the model suggests that gas concentration effects at the anode play an important role on the global electrochemical response. The study shows that quantitative predictions can be obtained using this integrated approach, making it an attractive tool to assist the SOFC development.

Low-temperature solid-oxide fuel cells

Abstract

Improvement of Thermal Reliability and Power Density of SOFCs By Preparing Nano LSC Particles Between GDC Electrolyte and Lscf Cathode

1.Introduction Solid oxide fuel cells (SOFCs) have received much attention as promising electrochemical power generation devices because of their high electrical efficiency and environment adaptability. Development of SOFCs is now focused on lowering the operating temperature from the conventional range of 750-900°C to an intermediate range of 500-700ºC, allowing a wider range of materials for SOFC components and improvement of thermal reliability. Preparing the Yttlia Stabilized Zirconia (YSZ) and Gadolinia-doped ceria (GDC)) bilayer electrolytes are effective for reducing operating temperature [1]. In the present work, NiO-YSZ anode supported cells with YSZ/GDC bilayer electrolytes and (La,Sr) (Co,Fe) O3 (LSCF) thin film cathodes were fabricated by pulsed laser deposition (PLD) and screen printing. However, one thermal cycle test of a cell between 200°C and 600°C, with heating and cooling rates of 400°C /h, resulted in detachment of the LSCF thin film and thus reduced durability. To improve the adhesion between the LSCF and GDC, (La,Sr)CoO3(LSC), nano particles were deposited on the GDC thin films by PLD. This step was expected to increase contact area between electrolyte and cathode. We investigated the influence of the LSC nano particle on thermal reliability and I-V characteristic of SOFCs. 2.Experimental Details NiO-YSZ cermet (NiO:YSZ＝60:40 wt%, 500 μm thick) was used as the anode substrate, YSZ (8 mol% Y2O3) and GDC (10 mol% GdO2) as electrolyte materials, and La0.6Sr0.4Co0.2Fe0.8 (LSCF) and La0.6Sr0.4Co (LSC) as cathode materials. YSZ (8 μm thick) and LSCF (30 μm thick) films were prepared by screen printing and GDC (3 μm thick) was deposited by PLD. Two sets of unit cells were prepared: one set was with LSC nano particles deposited between GDC and LSCF thin films by PLD (NiO-YSZ / YSZ / GDC / LSC particle / LSCF) and the other set was without LSC nano particles (NiO-YSZ / YSZ / GDC / LSCF).Thermal cycle test between 200 and 600°C with heating and cooling rates of 400° C /h were performed. Power densities and I-V characteristics were measured. 3.Results (a) Deposition of LSC nano particles by PLD on the GDC thin film. Figure 1 shows a scanning electron microscopy image of the surface of a GDC thin film. Many LSC nano particles (50-nm diameter) were deposited on the dense GDC thin film by PLD, which increased the area of contact between the GDC and LSCF thin films. (b) Thermal cycle test of NiO-YSZ anode supported cells with or without LSC nano particles. Two types of unit cells, (1) NiO-YSZ / YSZ / GDC / LSC particle / LSCF and (2) NiO-YSZ / YSZ / GDC / LSCF were prepared by screen printing and PLD. Photographs of unit cells after one thermal cycle are shown in Fig. 2. It is evident that adding the LSC nano particles succeeded in suppressing detachment of the LSCF thin film. Spike effect resulted in high adhesion between LSCF and GDC. The unit cell with LSC nano particles survived 50 thermal cycles and yielded higher thermal reliability. (c) I-V characteristics of NiO-YSZ anode supported cells with or without LSC nano particles. Figure 3 shows the I-V characteristics of unit cells with and without LSC nano particles. The maximum power densities of unit cells with or without LSC nano particles were 0.5 and 0.3 W·cm-2, respectively. Cell performances were found to be better when LSC nano particles were deposited, because of increase in reactive sites.[1] T. Mukai et al., Journal of Fuel Cell Science and Technology , 10(2013) 061006-1-6.

A Model for Extracting Fundamental Kinetic Rates of SOFC Cathode Materials from Oxygen Isotope Exchange Experiments

Losses from cathode polarization in solid oxide fuel cells (SOFC) limit their performance at intermediate temperatures. A fundamental understanding of the oxygen reduction mechanisms on SOFC cathodes is essential for the development of lower temperature SOFCs with high performance. In order to separate the various contributions to cathode polarization, different techniques and experiments have been carried out to gain an understanding of the kinetics occurring at the gas-solid interface. Isotope exchange is a powerful tool for analyzing the oxygen exchange process (1, 2, 3, 4, 5), however, most isotope tracer experiments are limited in their scope. Experiments typically take one of two approaches, a focus on the kinetics of oxygen adsorption on the surface (SSIKTA), or the diffusion of oxygen through the lattice (SIMS). For SOFC cathodes, we are concerned with both the dissociation of oxygen on the surface and the ability of oxygen atoms to incorporate into and diffuse through the lattice. In this study, we attempt to determine the ORR mechanism and extract fundamental kinetics rates for La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3-x (LSM) using isothermal isotope exchange (IIE) (6,7). Studies of isotope exchange can greatly contribute to the elucidation of the interactions between molecular oxygen and the cathode surface. Isotope exchange can help characterize the dissociation of oxygen molecules at the oxide surface as well as the conduction of oxygen ions within the oxide. IIE experiments will be conducted at various temperatures and oxygen partial pressures.To extract fundamental kinetic rates from in-situ isotope exchange experiments on LSM and LSCF, a model has been developed based on a two-step reaction mechanism across the heterogeneous gas-solid interface. The coupled reactions can be shown in two elementary steps: dissociative adsorption and incorporation. The relation between the fundamental kinetics rates and surface exchange coefficient (kex) (8, 9) will also be linked. Acknowledgments This work was supported by the Department of Energy under contract DEFE0009084. The authors would like to thank Dr. Dongxia Liu for reviewing our model.ReferencesM.W. den Otter, B.A. Boukamp, H.J.M. Bouwmeester, Solid State Ionics 139 (2001) 89–94Henny J. M. Bouwmeester,* Chunlin Song, Jianjun Zhu, Jianxin Yi, Martin van Sint Annaland and Bernard A. Boukamp, Phys. Chem. Chem. Phys., 2009, 11, 9640–9643S. Lacombe, H. Zanthoff, and C. Mirodatos, Jounal of Catalysis, 155, 106-116(1995)M.W. den Otter, B.A. Boukamp, H.J.M. Bouwmeester, Solid State Ionics 139 (2001) 89–94Schohn L. Shannon and James G. Goodwin, Jr.*, Chem. Rev. 1995, 677-695C.C. Kan, E.D. Wachsman, Solid State Ionics 181 (2010) 338–347C.C. Kan, H. H. Kan, F. M. Van Assche IV, E. N. Armstrong, and E. D. Wachsman, Journal of The Electrochemical Society, 155 (10) B985-B993 (2008E. N. Armstrong, K. L. Duncan, D. J. Oh, J. F. Weaver, and E. D. Wachsman, Journal of The Electrochemical Society, 158 (5) B492-B499 (2011)S. Wang, P.A.W. van der Heide, C. Chavez, A.J. Jacobson, S.B. Adler, Solid State Ionics 156 (2003) 201–208

Structural and electrical characterisation of nanostructure electrodes for SOFCs

This paper reports the effects of sintering temperature on structure, particle size and conductivity of electrodes (Sn0.2Zn0.8Fe0.2O & Sn0.8Zn0.2Fe0.2O). The electrode material was prepared by the chemical method combining a solid state reaction. Structural analyses were performed using X-ray diffraction and scanning electron microscopy. The particle size of the material obtained using Scherrer's formula was 50–60 nm and the nanostructure's surface was studied using electrochemical characterisations tools. Electrical conductivity was determined using the 4-probe DC method, which was compared with the 4-probe AC method. These results suggest a promising substitute for the conventional electrodes of solid oxide fuel cells (SOFCs). It is known that a sintering temperature above 1000 °C causes an increase in density and a reduction of porosity. Therefore, we optimised the sintering temperature at 1000 °C and obtained electrical conductivity of about 5 S cm−1. Thus, this electrode could play a vital role in the development of high performance SOFCs at intermediate temperatures.

Enhancing SOFC cathode performance by surface modification through infiltration

Solid oxide fuel cells (SOFCs) have the potential to be one of the cleanest and most efficient energy technologies for direct conversion of chemical fuels to electricity. Economically competitive SOFC systems appear poised for commercialization, but widespread market penetration will require continuous innovation of materials and fabrication processes to enhance system lifetime and reduce cost. One early technical opportunity is minimization of resistance to the oxygen reduction reaction (ORR) at the cathode, which contributes the most to performance degradation and efficiency loss in the existing SOFCs, especially at temperatures <700 °C. Detailed study over the past 15 years has revealed the positive impact of catalyst infiltration on SOFC cathode performance, both in power density and durability metrics. However, realizable performance improvements rely upon strongly-coupled relationships in materials and morphology between the infiltrate and the backbone, and therefore efficacious systems cannot be simply generated with a set of simple heuristics. This article reviews recent progress in enhancing SOFC cathode performance by surface modification through a solution-based infiltration process, focusing on two backbone architectures – inherently functional and skeletal – infiltrated using wet-chemistry processes. An efficient cathode consists of a porous mixed-conducting backbone and an active coating catalyst; the porous backbone provides excellent ionic and electronic conductivity, while the infiltrated surface coating possesses high catalytic activity and stability. As available, performance comparisons are emphasized and reaction schematics for specific infiltrate/backbone systems are summarized. While significant progress has been achieved in enhancing surface catalytic activity and durability, the detailed mechanisms of performance enhancement are insufficiently understood to obtain critical insights and a scientific basis for rational design of more efficient catalysts and novel electrode architectures. Recent progress in characterization of surfaces and interfaces is briefly discussed with challenges and perspectives in surface modification of SOFC electrodes. Surface modification through infiltration is expected to play an increasingly important role in current and next-generation commercial SOFC development, and this review illustrates the sophisticated technical considerations required to inform judicious selection of an infiltrate for a given SOFC system.

Recent Progress of SOFC Combined Cycle System with Segmented-In-Series Tubular Type Cell Stack at MHI

Solid oxide fuel cell (SOFC) triple combined cycle system, integrated with gas turbine (GT) and steam turbine (ST), is an ultimate fossil fuel electric power generation system with very high efficiency. In 2008, Mitsubishi Heavy Industries, Ltd. (MHI) operated the 200 kW-class SOFC micro gas turbine (MGT) hybrid system under commission of New Energy and Industrial Technology Development Organization (NEDO) and we could demonstrate 52.1%-LHV net power generation efficiency. Based on the successful achievement, MHI developed a more compact 250 kW-class SOFC-MGT hybrid system for the purpose of utilization of a hybrid system in the NEDO project scheduled from fiscal 2011 to 2012. Furthermore, we have started a development of fundamental technologies of SOFC-GT-ST triple combined cycle system that aims to achieve more than 70% of power generation efficiency. In this paper, latest results on the development of SOFC and the systems are described.

Status of the Solid Oxide Fuel Cell Development at Topsoe Fuel Cell A/S and DTU Energy Conversion

Many years of close collaboration between Topsoe Fuel Cell A/S (TOFC) and Risø (to day DTU Energy Conversion) on SOFC development have ensured an efficient transfer of SOFC basic know how to industrial technology. The SOFC development in the consortium includes material development and manufacturing of materials, cells and stacks based on state of the art as well as innovative strategies. Today TOFC provides the SOFC technology platform: Cells, stacks, integrated multi stack module and PowerCore units that integrate stack modules with hot fuel processing units for high electrical efficiency. TOFC collaborates with integrator partners to develop, test and demonstrate possibilities and challenges in case of CHP, distributed generation, transportation application and electrolysis. Aiming at improved reliability, robustness and low material cost, TOFC has in the collaboration with DTU increased the efforts on development of next generation cells with metallic support including novel infiltrated nano-structured electrodes for operation in the temperature range 600-700 oC. Recently, record-breaking results have been obtained on cell level as well as on stack level.