Synthesis and properties of dense nickel and cobalt zirconia cermet anodes for solid oxide fuel cells

Porous Ni/Zirconia cermets have been traditionally used as anodes in solid oxide fuel cell configurations. They show excellent catalytic activity towards hydrogen oxidation as well as a number of other attributes. However, they are prone to sintering during long term operation, thus causing a drop in the efficiency of the cell. This paper describes the fabrication and properties of a dense cermet which, we suggest, may act as an intermediate layer between the electrolyte and the porous anode and possibly reduce anode degradation. Co and Ni based cermet systems were investigated and a 30 vol % Co/zirconia system could be fabricated with less than 20% porosity after being sintered at 1300°C.     

A redox-stable efficient anode for solid-oxide fuel cells

Solid-oxide fuel cells (SOFCs) promise high efficiencies in a range of fuels. Unlike lower temperature variants, carbon monoxide is a fuel rather than a poison, and so hydrocarbon fuels can be used directly, through internal reforming or even direct oxidation. This provides a key entry strategy for fuel-cell technology into the current energy economy. Present development is mainly based on the yttria-stabilized zirconia (YSZ) electrolyte. The most commonly used anode materials are Ni/YSZ cermets, which display excellent catalytic properties for fuel oxidation and good current collection, but do exhibit disadvantages, such as low tolerance to sulphur and carbon deposition when using hydrocarbon fuels, and poor redox cycling causing volume instability. Here, we report a nickel-free SOFC anode, La0.75Sr0.25Cr0.5Mn0.5O3, with comparable electrochemical performance to Ni/YSZ cermets. The electrode polarization resistance approaches 0.2 Omega cm2 at 900 degrees C in 97% H2/3% H2O. Very good performance is achieved for methane oxidation without using excess steam. The anode is stable in both fuel and air conditions, and shows stable electrode performance in methane. Thus both redox stability and operation in low steam hydrocarbons have been demonstrated, overcoming two of the major limitations of the current generation of nickel zirconia cermet SOFC anodes.     

A review of anode materials development in solid oxide fuel cells

High temperature solid oxide fuel cell (SOFC) has prospect and potential to generate electricity from fossil fuels with high efficiency and very low greenhouse gas emissions as compared to traditional thermal power plants. In the last 10 years, there has been significant progress in the materials development and stack technologies in SOFC. The objective of this paper is to review the development of anode materials in SOFC from the viewpoint of materials microstructure and performance associated with the fabrication and optimization processes. Latest development and achievement in the Ni/Y₂O₃-ZrO₂ (Ni/YSZ) cermet anodes, alternative and conducting oxide anodes and anode-supported substrate materials are presented. Challenges and research trends based on the fundamental understanding of the materials science and engineering for the anode development for the commercially viable SOFC technologies are discussed.     

Modelling of the agglomeration of Ni-particles in anodes of solid oxide fuel cells

The degradation of anodes of solid oxide fuel cells (SOFC), which consist of a porous metal – solid electrolyte material is described by a two particle model. The model is based on two main assumptions. Firstly, the difference in metal particle diameter is the driving force for the observed coarsening of the larger metal particle during long term annealing. Secondly, surface diffusion of metal atoms on the particle surface is the dominant diffusion mechanism. Additionally, a function was introduced which considers the limited space for the growth of the nickel particles in the cermet material. The found analytical function for the growth kinetics was compared to experimental results for the growth of nickel particles in a nickel - yttria stabilised zirconia (YSZ) anode annealed at 1000°C up to 4000 h. The model describes the time dependence of the observed particle radii in an adequate way. The resultant surface diffusion coefficients for Ni are lower than results found in literature. Possible explanations are discussed. However, the result shows that the proposed mechanism – surface diffusion of nickel atoms - is fast enough to explain the found amount of Ni agglomeration in SOFC anodes and is therefore considered to be the dominant mechanism.     

Innovations for anodes – catalysis and direct oxidation

Operation of solid oxide fuel cells with standard Ni‐YSZ cermet anodes requires a partial reforming of any hydrocarbon fuel to avoid deactivation of the electrode. Direct oxidation of the fuel would simplify the system, but requires significant innovation in anode materials.     

中温固体氧化物燃料电池阳极的研究

以NiO和Ce_0.8Gd_0.2O_1.9(CGO)为原料, 通过静压成型, 在1450℃高温焙烧, 并于700℃用80%He气稀释的H₂还原后, 制成了Ni-CGO中温固体氧化物燃料电池(SOFC)阳极, 测定了阳极的孔特性, 用SEM观察了阳极的微观形貌, 通过XRD衍射图谱表征了阳极材料还原前后的晶相变化, 用EDS分析了阳极的元素组成与分布, 测试了阳极的电导率和燃料电池性能. 研究结果表明, 所制备的Ni-CGO阳极孔径主要在1～2μm, 孔隙率随NiO含量的增加而增大, 最大可达到30%. 通过SEM观察可知金属相与CGO陶瓷相融合良好, 阳极与电解质结合紧密, 用20%的H₂气体700℃可将NiO彻底还原成金属Ni, 但是CGO晶相没有变化, 还原后的阳极电导率随NiO量减少而降低, NiO质量比为40%时是电导率的阈值; 用Ni-CGO为阳极, CGO为电解质, LSCF为阴极制备的中温SOFC功率密度650℃可达0.14W/cm².     

Multi-scale solid oxide fuel cell materials modeling

Performance and degradation of fuel cell components are discussed in a multi-scale framework in this paper. Electrochemical reactions in a solid oxide fuel cell occur simultaneously as charge and gas pass through the anode, electrolyte, and cathode to produce electric power. Since fuel cells typically operate at high temperatures for long periods of time, performance degradation due to aging of the fuel cell materials should be examined. This phenomenon is treated in a multi-scale framework by considering how microstructure evolution affects the performance at the macro-scale. Mass and charge conservation equations and electrochemical kinetic equations are solved to predict the overall cell performance using the local properties calculated at the micro-scale. Separately, the microstructures assigned to the macroscopic integration points are evolved via the Cahn–Hilliard equation using an experimentally calibrated kinetic parameter. The effective properties of the evolving microstructure are obtained by homogenization and incorporated in the macro-scale calculation. The proposed model is applied to a solid oxide fuel cell system with a nickel/yttria stabilized zirconia (Ni/YSZ) cermet anode. Our model predicts performance degradation after extended hours of operation related to nickel particle coarsening and the resulting decrease in triple phase boundary (TPB) density of the anode material. The investigation of the microstructural effects on TPB density suggests that using Ni and YSZ particles of similar size may retard performance degradation due to anode aging.     

Infiltrated NiCo Alloy Nanoparticle Decorated Perovskite Oxide: A Highly Active,Stable, and Antisintering Anode for Direct‐Ammonia Solid Oxide Fuel Cells

Direct ammonia solid oxide fuel cell (DA‐SOFC) is superior to low‐temperature direct ammonia fuel cell using anion exchange membrane because of much improved anode reaction kinetics at elevated temperature. However, significant performance degradation due to severe sintering of conventional nickel cermet anode under operating conditions is a big challenge for realizing its practical use. Herein, a high‐performance anode based on La_0.55Sr_0.30TiO_3?δ (LST) perovskite substrate with its surface decorated with in situ exsolved and strongly coupled NiCo alloy nanoparticles (NPs) is designed and fabricated for DA‐SOFCs, exhibiting superior catalytic activity for NH₃ decomposition reaction due to balanced NH₃ adsorption and N₂ desorption processes. An electrolyte‐supported single cell with infiltrated NiCo/LST on Sm_0.2Ce_0.8O_1.9 scaffold anode delivers a maximum power density of 361 mW cm^?2 at 800 °C in NH₃ fuel, superior to similar SOFCs with Ni or Co NP‐decorated LST based anodes (161 and 98 mW cm^?2). Furthermore, the SOFC with this newly developed anode displays favorable operational stability without obvious performance degradation at 700 °C for a test period of ≈120 h, attributed to its high antisintering capability. This study provides some strategies to develop highly active, stable, and antisintering perovskite‐based nanocomposite for DA‐SOFCs, facilitating the practical use of this technology.     

Influence of microstructure on nano-mechanical properties of single planar solid oxide fuel cell in pre- and post-reduced conditions

The present work investigates, both the macro- and nano-mechanical properties of all the three component layers e.g., anode, cathode and electrolyte of a planar single solid oxide fuel cell (SOFC). The flexural fracture strength experiments in three point bending mode are employed in both pre- and post-reduced conditions to study the macro-mechanical failure behavior of the single cell. Further, the nanoindentation technique is utilized in both pre- and post-reduced conditions to evaluate the nanomechanical properties e.g. nanohardness, Young’s modulus, mean contact pressure, relative stiffness and relative spring back at scale in both pre- and post-reduced conditions. The nanohardness and Young’s modulus of the pre-reduced anode are considerably degraded after reduction as NiO gets converted to Ni. However, as expected; those of the pre-reduced electrolyte and cathode are only slightly decreased after reduction because there are no chemical conversions involved. Further, the experimentally obtained data of nanomechanical properties, is explained with the application of the well established Weibull statistics as the microstructures with characteristically present pores and defects are highly heterogeneous in nature. The characteristic values of the various nanomechanical properties are analyzed using Weibull distribution for the anode, electrolyte and cathode layers of the SOFC in both pre- and post-reduced conditions.     

Anodes for Direct Oxidation of Dry Hydrocarbons in a Solid‐Oxide Fuel Cell

The manufacture of fuel cells that can operate directly on various hydrocarbon fuels, without the need for reforming, has the potential of greatly speeding the application of fuel cells for transportation and distributed‐power applications. This paper will briefly review the literature in this area and describe recent developments in solid‐oxide fuel cells (SOFCs) that demonstrate that direct‐oxidation fuel cells are possible with Cu‐based anodes. A new method for synthesizing thin‐electrolyte, anode‐supported cells is described that is based on tape casting with graphite pore formers (see Figure), followed by impregnation with aqueous solutions of Cu(NO₃)₂ and Ce(NO₃)₃. The performance of model SOFCs for direct conversion of n‐butane and methane is shown. Finally, future developments that are needed for this technology to be commercialized are discussed.     

Properties of Ni/YSZ porous cermets prepared by electroless coating technique for SOFC anode application

A conceptual nickel–yttria stabilized zirconia (Ni/YSZ) cermet has been prepared by coating YSZ particles with metallic nickel using electroless coating technique. Concentration of nickel was varied between 5 and 60 vol%. Bulk samples were prepared by uniaxial pressing followed by sintering in the temperature range 1,200–1,350 °C with a soaking time of 2–6 h. Thorough investigation on the electrical characteristics, thermal expansion behavior of the samples have been performed as a function of Ni-content in the cermet. Thermal expansion behavior and conductivity measurement results suggest that the samples prepared by this technique are suitable for solid oxide fuel cell (SOFC) anode application at Ni concentration as low as 20 vol%.     

Oxidation behaviour of ferrous alloys used as interconnecting material in solid oxide fuel cells

Under operating conditions in the solid oxide fuel cell (SOFC), metallic interconnect plates form electrically insulating or poor-conducting oxide scales (e.g. Cr₂O₃, Al₂O₃) at their surface which increase the contact resistance from one fuel cell membrane to the next. In order to minimize electric losses in a fuel cell stack, the formation of oxide scales on the interconnect surface must either be prevented or the oxide scale formed must have sufficient electrical conductivity. In the present work, investigations were carried out on the corrosion behaviour of different FeCrAl and FeCrMn alloys, some of which were coated with nickel (Ni). Information about ageing of these alloys on the anode side of the fuel cell was obtained by means of contact resistance measurements and scanning electron microscopy. The results reveal that FeCrMn(LaTi) alloys and Ni-coated interconnects exhibit low ageing rates and are thus suitable for use on the anode side of SOFCs.     

Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2Ce0.8O2 for a SOFC

A Ni/scandia-stabilized zirconia (ScSZ) cermet anode was modified by coating with nano-sized gadolinium-doped ceria (GDC, Gd_0.2Ce_0.8O₂) within the pores of the anode for a solid oxide fuel cell (SOFC). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the anode characterizations. Open circuit voltages (OCVs) increased from 1.027 to 1.078 V, and the maximum power densities increased from 238 to 825 mW/cm², as the operating temperature of a SOFC with 2.0 wt.%GDC-coated Ni/ScSZ anode was increased from 700 to 850 °C in humidified hydrogen. The coating of nano-sized Gd_0.2Ce_0.8O₂ particle within the pores of the porous Ni/ScSZ anode significantly improved the performance of anode supported cell. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibited far greater impedances than the cell with 2.0 wt.%GDC-coated Ni/ScSZ anode. Consequently, 2.0 wt.%GDC-coated Ni/ScSZ anode could be used as a novel anode material for a SOFC due to better electrochemical performance.     

Microstructural control of Ni-YSZ cermet anode for planer thin-film solid oxide fuel cells

Ni-Y₂O₃-stabilized ZrO₂ (Ni-YSZ) cermet anode was fabricated for solid oxide fuel cells (SOFCs) by conventional ceramic processing using NiO-YSZ composite particles. Microstructures of the anode were carefully characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Ni-YSZ cermet anode was consisting of fine YSZ connections, as the conducting pass of oxygen ions, on the surface of Ni network, as that of electrons, with continuous pore structure and as that of gaseous species. No amorphous phases were present at the interface between Ni and YSZ, and there was an orientation relationship between Ni and YSZ grains, (111)_Ni//(111)_YSZ. The cermet anode showed a high electrical performance at 800 °C. These results indicated that the electrochemical activity of the Ni-YSZ cermet anode was enhanced with the present microstructure.     

Synthesis and electrochemical performances of LiNiCuZn oxides as anode and cathode catalyst for low temperature solid oxide fuel cell

Low temperature solid oxide fuel cell (LTSOFC, 300-600 degrees C) is developed with advantages compared to conventional SOFC (800-1000 degrees C). The electrodes with good catalytic activity, high electronic and ionic conductivity are required to achieve high power output. In this work, a LiNiCuZn oxides as anode and cathode catalyst is prepared by slurry method. The structure and morphology of the prepared LiNiCuZn oxides are characterized by X-ray diffraction and field emission scanning electron microscopy. The LiNiCuZn oxides prepared by slurry method are nano Li0.28Ni0.72O, ZnO and CuO compound. The nano-crystallites are congregated to form ball-shape particles with diameter of 800-1000 nm. The LiNiCuZn oxides electrodes exhibits high ion conductivity and low polarization resistance to hydrogen oxidation reaction and oxygen reduction reaction at low temperature. The LTSOFC using the LiNiCuZn oxides electrodes demonstrates good cell performance of 1000 mW cm(-2) when it operates at 470 degrees C. It is considered that nano-composite would be an effective way to develop catalyst for LTSOFC.     

Ni/TiO2（110）表面甲烷重整反应生成CO和H2反应机理的第一性原理研究（英文）

基于密度泛函理论(DFT)计算,本文研究了Ni/TiO2(110)表面甲烷重整反应的机理,揭示了固体氧化物燃料电池中TiO2基阳极较传统ZrO2或者CeO2基阳极材料具有良好抗积碳性能的重要原因.本文对六种不同的甲烷重整反应路径(干燥和湿润的气氛环境)进行了详细研究,阐明了TiO2,Ni/TiO2界面和水分子在甲烷重整反应中的作用以及Ni/TiO2基阳极抗积碳性能的来源.经过计算发现,在干燥和湿润的环境下,碳原子和界面的TiO2晶格氧反应生成CO,以及后续水分子吸附和解离在界面的氧空位上并提供反应所需O原子是甲烷重整反应的主要路径(C-O路径),而水分子直接参与C原子或者CH基团的氧化反应则要困难很多.值得注意的是,在研究的六种反应路径中,CO从反应表面的脱附都非常困难,需要约2.3 eV的能量才能使得其脱附.因而造成大量表面反应活性位点被占据,这是目前很多阳极材料不具备抗积碳性能的一个重要原因.然而,在湿润环境中,水分子的吸附放热大大降低了整个反应体系所需能量,尤其是本文中水分子在TiO2表面的快速解离吸附更是大大降低了整个反应体系的能量.进一步研究发现,水分子在Ni,YSZ和CeO2表面的吸附解离要比在TiO2表面困难很多.这也是TiO2基阳极材料具有较好抗积碳性能的一个重要原因.本研究对于指导合成碳氢燃料气氛下具有优异抗积碳性能的固体氧化物燃料电池阳极材料具有重要的意义.     

Energy storage in ultrathin solid oxide fuel cells

The power output of hydrogen fuel cells quickly decreases to zero if the fuel supply is interrupted. We demonstrate thin film solid oxide fuel cells with nanostructured vanadium oxide anodes that generate power for significantly longer time than reference porous platinum anode thin film solid oxide fuel cells when the fuel supply is interrupted. The charge storage mechanism was investigated quantitatively with likely identified contributions from the oxidation of the vanadium oxide anode, its hydrogen storage properties, and different oxygen concentration at the electrodes. Fuel cells capable of storing charge even for short periods of time could contribute to ultraminiaturization of power sources for mobile energy.     

Effect of characteristics of (Sm,Ce)O2 powder on the fabrication and performance of anode-supported solid oxide fuel cells

Effect of characteristics of Sm_0.2Ce_0.8O_1.9 (SDC) powder as a function of calcination temperature on the fabrication of dense and flat anode-supported SDC thin electrolyte cells has been studied. The results show that the calcination temperature has a significant effect on the particle size, degree of agglomeration, and sintering profiles of the SDC powder. The characteristics of SDC powders have a significant effect on the structure integrity and flatness of the SDC electrolyte film/anode substrate bilayer cells. The SDC electrolyte layer delaminates from the anode substrate for the SDC powder calcined at 600 °C and the bilayer cell concaves towards the SDC electrolyte layer for the SDC powder calcined at 800 °C. When the calcinations temperature increased to 1000 °C, strongly bonded SDC electrolyte film/anode substrate bilayer structures were achieved. An open-circuit voltage (OCV) of 0.82–0.84 V and maximum power density of ～1 W cm^?2 were obtained at 600 °C using hydrogen as fuel and stationary air as the oxidant. The results indicate that the matching of the onset sintering temperature and maximum sintering rate temperature is most critical for the development of a dense and flat Ni/SDC supported SDC thin electrolyte cells for intermediate temperature solid oxide fuel cells.     

固体氧化物燃料电池LaxSr2-3x/2Fe1.5Ni0.1Mo0.4O6-δ阳极性能研究

本研究利用固相反应法合成了一系列镧取代LaxSr2-3x/2Fe1.5Ni0.1Mo0.4O6-δ(LaxSFNM,x=0,0.1,0.2,0.3,0.4)钙钛矿陶瓷材料,并研究其作为固体氧化物燃料电池阳极的电化学性能。X射线衍射(XRD)测试表明合成的粉末具有立方钙钛矿结构。在高温下利用氢气还原LaxSFNM样品,发现其晶粒表面析出纳米尺度的Fe-Ni合金颗粒,并且偏析纳米颗粒的密度随着La^3+掺杂量的增加而显著降低。在对称电池阻抗测试中,随着La^3+掺杂量的增加,阳极极化阻抗逐渐降低,掺入量为0.3时阻抗达到最小值。La0.3SFNM对称电池在750℃下极化阻抗仅为0.16W·cm^2,进一步增加掺杂量时,La0.4SFNM对称电池极化阻抗增加至0.17W·cm^2。La0.3SFNM材料良好的电极反应催化活性源于适当分布的Fe-Ni合金纳米偏析颗粒与LaxSFNM陶瓷基体的共同作用。利用流延法制备一系列以LaxSFNM为阳极、SmBa0.5Sr0.5Co2O6为阴极、LSGM为电解质的单电池,使用氢气作为燃料时,La^3+掺杂量x=0.3的单电池表现出最高的功率密度,在750、650和550℃时峰值功率密度可达1.26、0.90和0.52W·cm^-2。上述结果表明,La0.3Sr1.55Fe1.5Ni0.1Mo0.4O6-δ可以用作高性能SOFC阳极催化剂。     

Electrode materials: a challenge for the exploitation of protonic solid oxide fuel cells

Abstract

High temperature proton conductor (HTPC) oxides are attracting extensive attention as electrolyte materials alternative to oxygen-ion conductors for use in solid oxide fuel cells (SOFCs) operating at intermediate temperatures (400–700 °C). The need to lower the operating temperature is dictated by cost reduction for SOFC pervasive use. The major stake for the deployment of this technology is the availability of electrodes able to limit polarization losses at the reduced operation temperature. This review aims to comprehensively describe the state-of-the-art anode and cathode materials that have so far been tested with HTPC oxide electrolytes, offering guidelines and possible strategies to speed up the development of protonic SOFCs.