Basic properties of a liquid tin anode solid oxide fuel cell

An unconventional high temperature fuel cell system, the liquid tin anode solid oxide fuel cell (LTA-SOFC), is discussed. A thermodynamic analysis of a solid oxide fuel cell with a liquid metal anode is developed. Pertinent thermochemical and thermophysical properties of liquid tin in particular are detailed. An experimental setup for analysis of LTA-SOFC anode kinetics is described, and data for a planar cell under hydrogen indicated an effective oxygen diffusion coefficient of 5.3 × 10⁻⁵ cm² s⁻¹ at 800 °C and 8.9 × 10⁻⁵ cm² s⁻¹ at 900 °C. This value is similar to previously reported literature values for liquid tin. The oxygen conductivity through the tin, calculated from measured diffusion coefficients and theoretical oxygen solubility limits, is found to be on the same order of that of yttria-stabilized zirconia (YSZ), a traditional SOFC electrolyte material. As such, the ohmic loss due to oxygen transport through the tin layer must be considered in practical system cell design since the tin layer will usually be at least as thick as the electrolyte.     

Ag decorated (Ba,Sr)(Co,Fe)O3 cathodes for solid oxide fuel cells prepared by electroless silver deposition

We reported nano-structured Ag modified Ba_0.5Sr_0.5Co_0.6Fe_0.4O_3−δ (Ag@BSCF) cathode for solid oxide fuel cells (SOFCs) that is prepared by vacuum assisted electroless deposition technique. We show that the concentration of Ag can be easily adjusted by tuning the deposition time without altering the perovskite structure of the pristine BSCF. The effect of Ag loading on the electrochemical performance of the material has been systematically studied by varying the Ag loading and the working condition (oxygen partial pressure). An optimized electrode performance is observed with an Ag loading of ∼2 wt%. We demonstrate that the presence of Ag significantly reduces the electrode ohmic resistance and enhances the catalytic O₂ reduction performance of the BSCF cathode.     

Fabrication of solid oxide fuel cell anode electrode by spray pyrolysis

Large triple phase boundaries (TPBs) and high gas diffusion capability are critical in enhancing the performance of a solid oxide fuel cell (SOFC). In this study, ultrasonic spray pyrolysis has been investigated to assess its capability in controlling the anode microstructure. Deposition of porous anode film of nickel and Ce_0.9Gd_0.1O_1.95 on a dense 8 mol.% yttria stabilized zirconia (YSZ) substrate was carried out. First, an ultrasonic atomization model was utilized to predict the deposited particle size. The model accurately estimated the deposited particle size based on the feed solution condition. Second, effects of various process parameters, which included the precursor solution feed rate, precursor solution concentration and deposition temperature, on the TPB formation and porosity were investigated. The deposition temperature and precursor solution concentration were the most critical parameters that influenced the morphology, porosity and particle size of the anode electrode. Ultrasonic spray pyrolysis achieved homogeneous distribution of constitutive elements within the deposited particles and demonstrated capability to control the particle size and porosity in the range of 2-17 μm and 21-52%, respectively.     

A Co–Fe alloy as alternative anode for solid oxide fuel cell

A new type of Ni-free anode material based on Co_0.5Fe_0.5 + SDC (Sm_0.2Ce_0.8O_1.9) was identified and evaluated using the La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM) electrolyte-supported cells. The maximum power densities with humidified hydrogen at 800 °C using Au mesh and Pt mesh as anode current collector were 1.07 and 1.20 W cm⁻², respectively, which were comparable to that of the cell with the Ni + SDC anode. The cell with the new anode exhibited reasonable performance stability at 800 °C in hydrogen fuel; furthermore, no noticeable change in cell performance with the Co_0.5Fe_0.5 alloy anode was observed upon ordering reaction in the alloy to form the ordered α′ phase at 650 °C.     

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.     

High performance of direct methane-fuelled solid oxide fuel cell with samarium modified nickel-based anode

Natural gas is one of the most attractive fuels for solid oxide fuel cell (SOFC), while the anode activity for methane fuel has a great influence on the performance and stability of SOFC. Samarium is a good catalyst promoter for methane reforming. In this work, samarium is used to modify nickel catalyst, which results in small nickel oxide particles. The SmNi-YSZ (yttria-stabilized zirconia) anode has smaller particles and better interfacial contact between nickel and YSZ compared with conventional Ni-YSZ anode. The fine structure of SmNi-YSZ anode results in high activity for electrochemical oxidation of hydrogen and low polarization resistance of the cell. The performance of SmNi-YSZ anode cell with humidified methane as fuel is greatly improved, which is similar to that with hydrogen as fuel. The maximum power densities of SmNi-YSZ anode cell are 1.56 W cm⁻² for humidified hydrogen fuel and 1.54 W cm⁻² for humidified methane fuel at 800 °C. The maximum power density is increased by 221% when samarium is used to modify Ni-YSZ anode for humidified methane fuel at 650 °C. High cell performance results in good stability of SmNi-YSZ anode cell and the cell runs stably for more than 600 min for humidified methane fuel.     

Analysis of a perovskite-ceria functional layer-based solid oxide fuel cell

A fuel cell based on a functional layer of perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) composited samarium doped ceria (SDC) has been developed. The device achieves a peak power density of 640.4 mW cm^?2 with an open circuit voltage (OCV) of 1.04 V at 560 °C using hydrogen and air as the fuel and oxidant, respectively. A numerical model is applied to fit the experimental cell voltage. The kinetics of anodic and cathodic reactions are modeled based on the measurements obtained by electrochemical impedance spectroscopy (EIS). Modeling results are in well agreement with the experimental data. Mechanical stability of the cell is also examined by using analysis with field emission scanning electron microscope (FESEM) associated with energy dispersive spectroscopy (EDS) after testing the cell performance.     

Thermal stress modeling of anode supported micro-tubular solid oxide fuel cell

An anode-supported micro-tubular solid oxide fuel cell (SOFC) is analyzed by a two-dimensional axisymmetric numerical model, which is validated with the experimental I-V data. The temperature distribution generated by the thermo-electrochemical model is used to calculate the thermal stress field in the tubular SOFC. The results indicate that the current transport in the anode is the same at every investigated position. The stress of the micro-tubular cell occurs mainly because of the residual stress due to the mismatch between the coefficients of thermal expansion of the materials of the membrane electrode assembly. The micro-tubular cell can operate safely, but if there is an interfacial defect or a high enough tensile stress applied at the electrolyte, a failure can arise.     

Performance studies of solid oxide fuel cell cathodes in the presence of bare and cobalt coated E-brite alloy interconnects

The electrochemical performances of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) electrodes were studied by half-cell measurements in the absence of chromia-forming alloy, in the presence of bare and Co coated E-brite alloy interconnects, respectively. The surface and cross-section properties of the bare and Co coated E-brite alloys, and LSCF electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, and electron probe microanalysis (EPMA). The results showed a rapid degradation in LSCF performance when the bare E-brite alloy was used as interconnect. The growth of chromia scale on the E-brite alloy and the increase of Cr content throughout the LSCF electrode were observed. The uniform and dense Co coating process was developed to coat the E-brite alloy by using both electroless and electrodeposition methods. It was demonstrated that the Co layer effectively mitigates the Cr migration, leading to improved electrochemical stability of LSCF electrodes.     

Phase transition of doped LaFeO3 anode in reducing atmosphere and their power generation property in intermediate temperature solid oxide fuel cell

In general, transition metal-doped La_0.6Sr_0.4FeO₃ (LSF) has been used as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) because of its high mixed electronic−ionic conductivity and catalytic properties. Recently, some research groups have been investigating the doped LSF as an anode material. In this study, we evaluated the influence of dopant in LSF on anodic properties of LSF in SOFCs. Whereas Mn-doped LSF showed typical perovskite oxide structure even after reduction in hydrogen at high temperature, the LSF and Co-doped LSF exhibited phase transition partially to LaSrFeO₄ and exsolution of metal particles after reduction. The phase transition and metal exsolution occurred at temperature higher than 1008 K in a reducing atmosphere. Despite the partial phase transition, the cell using Co-doped LSF anode exhibited fairly high power density of 1.33 W/cm² at 1173 K with the lowest polarization resistance. These results may originate from the high oxygen-ion conductivity of LaSrFeO₄–La(Sr)Fe(Co)O₃ and the high hydrogen oxidation property of the Co–Fe particles on ceramic anode surface.     

Comparison of solid oxide fuel cell anode coatings prepared from different feedstock powders by atmospheric plasma spray method

Atmospheric plasma spray (APS) deposition of a high-performance anode coating, which is essential for obtaining high power density from a solid oxide fuel cell (SOFC), is developed. A conventional, micron-sized, nickel-coated graphite – yttria stabilized zirconia (YSZ) – graphite blend feedstock leads to a non-uniform layered coating microstructure due to the difference in the physical and thermo-physical properties of the components. In this research, new types of feedstock material received from a spray-drying method, which includes nano-components of NiO and YSZ (300 nm), are used. The microstructure and mechanical properties of a coating containing a nano composite that is prepared from spray-dried powders are evaluated and compared with those of a coating prepared from blended powder feedstock. The coating microstructures are characterized for uniformity, mechanical properties and electrical conductivity. The coatings prepared from spray-dried powders are better as they provide larger three-phase boundaries for hydrogen oxidation and are expected to have lower polarization losses in SOFC anode applications than those of coatings prepared from blended feedstock.     

Investigation of the electrochemical active thickness of solid oxide fuel cell anode

Determination of the electrochemical active thickness (EAT) is of paramount importance for optimizing the solid oxide fuel cell (SOFC) electrode. However, very different EAT values are reported in the previous literatures. This paper aims to systematically study the EAT of SOFC anode numerically. An SOFC model coupling electrochemical reactions with transport of gas, electron and ion is developed. The microstructure features of the electrode are modeled based on the percolation theory and coordinate number theory. Parametric analysis is performed to examine the effects of various operating conditions and microstructures on EAT. Results indicate that EAT increases with decreasing exchange current density (or decreasing TPB length) and increasing effective ionic conductivity. In addition to the numerical simulations, theoretical analysis is conducted including various losses in the electrode, which clearly shows that the EAT highly depends on the ratio of concentration related activation loss R_act,con to ohmic loss R_ohmic. The theoretical analysis explains very well the different EATs reported in the literature and is different from the common understanding that the EAT is controlled mainly by the ionic conductivity of electrode.     

Carbon monoxide-fueled solid oxide fuel cell

This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm² generated power up to 16 W. Open circuit voltages for various CO/CO₂ mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm⁻² were achieved at 1 A cm⁻² and 850 °C. This performance compared well with 0.74 W cm⁻² measured for pure H₂ in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm⁻²) power output remained unchanged over 375 h of continuous operation at 850 °C. In addition, a 50-cell planar SOFC stack was operated at 800 °C on 95% CO (balance CO₂), which generated 1176 W of total power at a power density of 224 mW cm⁻². The results demonstrate that CO is a viable primary fuel for SOFCs.     

Optimizing strontium titanate anode in solid oxide fuel cells by ytterbium doping

One of advantages of solid oxide fuel cells (SOFCs) is able to utilize various hydrocarbon fuels. Whereas, the classical Ni anode suffers severe carbon deposition especially operated under CH₄. Strontium titanate (SrTiO₃) perovskite anodes with strong carbon deposition resistance and good structural stability have been extensively investigated. In this work, Sr_0.88Y_0.08-xYb_xTiO₃ and Sr_0.88Y_0.08Ti_1-xYb_xO₃ are synthesized by Yb³⁺ doping in A-site and B-site of Sr_0.88Y_0.08TiO₃ perovskite, respectively. XRD results confirm that the SrTiO₃ cubic perovskite phase is formed in all the samples. Among the Yb³⁺ doping samples, Sr_0.88Y_0.06Yb_0.02TiO₃ exhibits the lowest thermal expansion coefficient (11.48 × 10⁻⁶/K), indicating the best compatibility with the electrolyte. The ionic conductivity of Sr_0.88Y_0.08TiO₃ can be improved by proper Yb³⁺ doping both in A-site and B-site, and the Sr_0.88Y_0.06Yb_0.02TiO₃ sample has the highest ionic conductivity among all the samples. The maximum power density of SOFC with Sr_0.88Y_0.06Yb_0.02TiO₃ anode is 87 mW/cm² under CH₄ at 800 °C, which is much higher than that with Sr_0.88Y_0.08TiO₃ and Ni anode. This can be related to its high electrocatalytic activity to CH₄ oxidation. In addition, SOFC with the Sr_0.88Y_0.06Yb_0.02TiO₃ anode shows a superior stability operated under CH₄ due to the strong carbon deposition resistances.     

Quantitative assessment of anode contribution to cell degradation under various polarization conditions using industrial size planar solid oxide fuel cells

Quantitative assessment of anode contribution to cell performance has been investigated under various polarization in three stack repeating units made of 10 cm × 10 cm planar anode-supported solid oxide fuel cells. The measurement is performed in-situ by inserting an ultra-thin Pt probe at the anode/electrolyte interface. The results reveal that the polarization resistance accounts for more than 90% of the total cell resistance when the cell is operated under the activation and concentration polarizations, respectively. The anode almost has no effect on cell degradation when the cell is operated under activation polarization. However, the anode has an obvious contribution to the cell degradation when the cell is operated under ohmic and concentration polarization, where the anode voltage increases by 23.36%/100 h and 512.28%/100 h, respectively. The anode operated under concentration polarization has twenty times of contribution to cell degradation than that of the ohmic polarization. However, when the cell is operated under the ohmic polarization, the main degradation comes from the ohmic resistance caused by the anode.     

Solid oxide fuel cell bi-layer anode with gadolinia-doped ceria for utilization of solid carbon fuel

Pyrolytic carbon was used as fuel in a solid oxide fuel cell (SOFC) with a yttria-stabilized zirconia (YSZ) electrolyte and a bi-layer anode composed of nickel oxide gadolinia-doped ceria (NiO-GDC) and NiO-YSZ. The common problems of bulk shrinkage and emergent porosity in the YSZ layer adjacent to the GDC/YSZ interface were avoided by using an interlayer of porous NiO-YSZ as a buffer anode layer between the electrolyte and the NiO-GDC primary anode. Cells were fabricated from commercially available component powders so that unconventional production methods suggested in the literature were avoided, that is, the necessity of glycine-nitrate combustion synthesis, specialty multicomponent oxide powders, sputtering, or chemical vapor deposition. The easily-fabricated cell was successfully utilized with hydrogen and propane fuels as well as carbon deposited on the anode during the cyclic operation with the propane. A cell of similar construction could be used in the exhaust stream of a diesel engine to capture and utilize soot for secondary power generation and decreased particulate pollution without the need for filter regeneration.     

Effect of anode structure on performance of cone-shaped solid oxide fuel cells fabricated by phase inversion

Anode-supported cone-shaped tubular solid oxide fuel cells (SOFCs) are successfully fabricated by a phase inversion method. During processing, the two opposite sides of each cone-shaped anode tube are in different conditions--one side is in contact with coagulant (the corresponding surface is named as “W-surface”), while the other is isolated from coagulate (I-surface). Single SOFCs are made with YSZ electrolyte membrane coated on either W-surface or I-surface. Compared to the cell with YSZ membrane on W-surface, the cell on I-surface exhibits better performance, giving a maximum power density of 350 mW cm⁻² at 800 °C, using wet hydrogen as fuel and ambient air as oxidant. AC impedance test results are consistent with the performance. The sectional and surface structures of the SOFCs were examined by SEM and the relationship between SOFC performance and anode structure is analyzed. Structure of anodes fabricated at different phase inversion temperature is also investigated.     

An experimental study of ammonia decomposition rates over cheap metal catalysts for solid oxide fuel cell anode

Solid oxide fuel cell (SOFC) is an electrochemical device for power generation with high efficiency and low emission. Ammonia is a low-cost and carbon-free hydrogen carrier that can be directly used as a fuel for SOFC. To further improve the performance and stability of SOFC fueled by ammonia (NH₃–SOFC), the design of NH₃–SOFC anode for efficient and stable utilization of NH₃ is critical. In this paper, the decomposition rates of NH₃ over four kinds of cheap metal catalysts (nickel, iron, copper and 304 stainless steel) were tested based on metal flakes with known fixed dimensions, and the empirical correlations of the decomposition rate over different catalysts were derived. These correlations are independent of catalyst structure parameters and only related to the catalyst material and the decomposition temperature, which are important basis for realizing the oriented design of NH₃–SOFC anode.     

Three dimensional stress analysis of solid oxide fuel cell anode micro structure

One of the most common problems in solid oxide fuel cells (SOFCs) is the delamination and thus the degradation of electrode/electrolyte interface which occurs in the consequences of the stresses generated within the different layers of the cell. Nowadays, the modeling of this problem under certain conditions is one of the main issues for the researchers. The structural and thermo-physical properties of the cell materials (i.e. porosity, density, Young's modulus etc.) are usually assumed to be homogenous in the mathematical modeling of solid oxide fuel cells at macro-scale. However, during the real operation, the stresses created in the multiphase porous layers might be very different than those at macro-scale. Therefore, micro-level modeling is required for an accurate estimation of the real stresses and the performance of SOFCs. This study presents a microstructural characterization and a finite element analysis of the delamination and the degradation of porous solid oxide fuel cell anode and electrode/electrolyte interface under various operating temperatures, compressing forces and material compositions by using the synthetically generated microstructures. A multi physics computational package (COMSOL) is employed to calculate the Von Misses stresses in the anode microstructures. The maximum thermal stress in the electrode/electrolyte interface and three phase boundaries is found to exceed the yield strength at 900 °C while 800 °C is estimated as a critical temperature for the delamination and micro cracks due to thermal stress generated. The thermal stress decreases in the grain boundaries with increasing content of one of the phases (either Ni or YSZ) and the porosity of the electrode. A clamping load higher than 5 kg cm⁻² is also found to exceed the shear stress limit.     

Vacuum-assisted electroless copper plating on Ni/(Sm,Ce)O2 anodes for intermediate temperature solid oxide fuel cells

Cu is incorporated by vacuum-assisted electroless plating into porous Ni/Sm_0.2Ce_0.8O_1.9 (Ni/SDC) anodes as the active anodes for the oxidation reaction of hydrogen and methane of intermediate temperature solid oxide fuel cells (IT-SOFCs). The scanning electron microscopy (SEM) observation indicates the formation of a uniformly distributed nano-structured Cu network within the porous Ni/SDC microstructure. The maximum power density of the cell with the Cu electroless-plated Ni/SDC anodes is 0.84 and 0.54 W cm⁻² in dry H₂ and dry CH₄ at 600 °C, respectively, enhanced by ∼30% as compared to the cell with conventional Ni/SDC anodes. The increase in the performance of the cell with the Cu electroless-plated Ni/SDC anodes is most likely attributed to the enlarged effective three-phase boundaries (TPBs) by interconnecting the isolated Ni and/or SDC particles with the electroless-plated Cu network and the formation of TPBs at the Cu/SDC interface due to the activation of SDC surface by the Cu deposition. The stability test shows that cell degradation in dry methane due to carbon deposition is significantly reduced by the electroless copper plating.