Evaluation of a variable flow ejector for anode gas circulation in a 50-kW class SOFC

Solid oxide fuel cells (SOFC) are highly efficient in terms of converting hydrogen's chemical energy into electrical energy through electrochemical reactions and for generating power in the range of several kW to several tens of kW. A variable flow ejector equipped with an adjustable recirculating flow rate mechanism was designed for this investigation. A prototype was manufactured to control the circulation of anode exhaust gas for a 50-kW class SOFC system. The ejector performance was evaluated using SOFC simulator equipment that simulated the pressure and temperature environment of a 50-kW SOFC system. In the heating simulation experiment, the mass flow rate ratio of the driving gas to the suction gas could be controlled from 3.7 to 5.4 under conditions simulating 100% of the rated load operation and from 4 to 7.5 when simulating 30%–50% of the partial load conditions. A simple heat transfer model for the motive nozzle was used in the ejector analysis, and issues for improving the ejector recirculation performance in the high-temperature field were identified.     

Analysis of design parameters in anodic recirculation system based on ejector technology for PEM fuel cells: A new approach in designing

In this study, the anodic recirculation system (ARS) based on ejector technology in polymer electrolyte membrane PEM fuel cell is studied with employing a theoretical model. A practical method is presented for selecting or designing the ejector in an ARS, that offers the best selection or design. A comprehensive parametric study is performed on the design parameters of a PEM fuel cell stack and an ARS ejector. Four geometrical parameters consist of cell active area, cell number, nozzle throat diameter, and mixing chamber diameter in the design of ARS are intended. The effect of each contributes to the overall system performance parameters is studied. In this parametric study, the correlation between stack design parameters and ejector design parameters are studied. Eventually, based on the results, two dimensionless parameters are useful in the design process are proposed.     

Anode gas recirculation behavior of a fuel ejector in hybrid solid oxide fuel cell systems: Performance evaluation in three operational modes

In this paper, a theoretical model for the performance monitoring and fault detection of fuel ejectors in the hybrid solid oxide fuel cell (SOFC) system is proposed. The procedures of using the model to analyze ejector properties such as the primary mass flow rate, the secondary mass flow rate, the recirculation ratio and steam to carbon ratio (STCR) are introduced. Based on the model, the anode gas recirculation performances of a hybrid SOFC system are studied under various operating conditions. Results show that the model can be used to evaluate the performance of ejector not only in the critical mode but also in the subcritical and back flow modes, which is especially useful at SOFC off-design operating conditions such as start up, load changes and shut down.     

New theoretical model for convergent nozzle ejector in the proton exchange membrane fuel cell system

A new theoretical model for the convergent nozzle ejector in the anode recirculation line of the polymer electrolyte membrane (PEM) fuel cell system is established in this paper. A velocity function for analyzing the flow characteristics of the PEM ejector is proposed by employing a two-dimensional (2D) concave exponential curve. This treatment of velocity is an improvement compared to the conventional 1D “constant area mixing” or “constant pressure mixing” ejector theories. The computational fluid dynamics (CFD) technique together with the data regression and parameter identification methods are applied in the determination of the velocity function's exponent. Based on the model, the anode recirculation performances of a hybrid PEM system are studied under various stack currents. Results show that the model is capable of evaluating the performance of ejector in both the critical mode and subcritical mode.     

Weight analysis on geometric parameters of ejector under high back pressure condition of SOFC recirculation system

With advantages of no parasitic energy consumption, small size, and low noise, ejector is a promising choice for the solid oxide fuel cell (SOFC) anode gas recirculation system. However, it is difficult to design an ejector with good performance under the high back pressure condition of the SOFC system. In this paper, weight analysis on key geometric parameters of ejector is carried out based on the result of an experimentally validated ejector simulation model. Four main geometric parameters that have the most significant effect on ejector performance, namely the ejector diameter ratio (D_r), mixing chamber length (L_m), diffuser length (L_d), and nozzle outlet position (NXP), are analyzed in detail. The D_r has a decisive influence on the momentum exchange in the mixing phenomenon between the primary flow and secondary flow in the mixing chamber where the normal shock position changes accordingly. The L_m mainly affects the intensive mixing flow which leads the normal shock to appear prematurely. The L_d should be long enough for boosting back pressure and reducing the effect on the mixing process. The NXP has no effect on the normal shock position. The results show that the critical back pressure increases with the rise of normal shock position and the impact weight of L_m, L_d and NXP can be treated as 21∶10∶1 approximately and the D_r is thought to be a decisive factor. This weight analysis method will be helpful for designing ejectors used in the high back pressure condition of the SOFC anode recirculation system.     

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.     

Performance improvement of bioethanol-fuelled solid oxide fuel cell system by using pervaporation

This work proposes an improvement in performance with respect to the electrical efficiency of a bioethanol-fuelled Solid Oxide Fuel Cell (SOFC) system by replacing a conventional distillation column by a pervaporation unit in the bioethanol purification process. The simulation study indicates that the membrane separation factor has a significant influence on the electrical power and heat energy required to generate a feed of 25 mol% ethanol in water to the reformer. The values of overall electrical efficiency of the SOFC systems with a distillation column and with a pervaporation unit are compared under the thermally self-sufficient condition (Q_net = 0) which offers their maximum electrical efficiency. At the base case, the SOFC system with a pervaporation unit provides an electrical efficiency of 42% compared with 34% achieved from the system with a distillation unit, indicating a significant improvement by using a pervaporation unit. An increase in ethanol recovery can further improve the overall electrical efficiency. The study also reveals that further improvement of the membrane selectivity can slightly enhance the overall efficiency of the SOFC system. Finally, an economic analysis of a bioethanol-fuelled SOFC system with pervaporation is suggested as the basis for further development.     

Novel materials for solid oxide fuel cell technologies: A literature review

This study aims to review novel materials for solid oxide fuel cell (SOFC) applications covered in literature. Thence, it was found that current SOFC operating conditions lead to issues, such as carbon surface deposition, sulfur poisoning and quick component degradation at high temperatures, which make it unsuitable for a few applications. Therefore, many researches are focused on cell performance enhancement through replacing the materials being used in order to improve properties and/or reduce operating temperatures. Most modifications in the anode aim to avoid some issues concerning conventionally used Ni-based materials, such as carbon deposition and sulfur poisoning, besides enhancing catalytic activity, once this component is directly exposed to the fuel. It was also found literature about the cathode with the aim of developing a material with enhanced properties in a wider temperature range, which has been compared to the currently used one: LSM perovskite (La_1-xSr_xMnO₃). Novel electrolyte materials can have ionic or protonic conductivity, thus performance degradation must be avoided at several operating conditions. In order to enhance its electrochemical performance, different materials for electrodes (cathode and anode) and electrolytes have been assessed herein.     

Fundamental mechanisms limiting solid oxide fuel cell durability

The fundamental issues associated with solid oxide fuel cell (SOFC) durability have been reviewed with an emphasis on general features in SOFCs and respective anode and cathode related phenomena. As general features, physicochemical properties and cell performance degradation/failure are correlated and bridged by the electrode reaction mechanisms. Particular emphasis is placed on the elemental behaviour of gaseous impurities and the possible role of liquids formed from gaseous substances. The lifetime of a state-of-the-art Ni cermet anodes is limited by a variety of microstructural changes, which mainly result from material transport-, deactivation- and thermomechanical mechanisms. Anode degradation can mainly be influenced by processing, conceptual and operating parameters. Designing a redox stable anode is currently one of the biggest challenges for small scale SOFC systems. Degradation mechanisms of different cathode materials are reviewed with a focus on the intrinsic degradation of doped lanthanum manganites (e.g. LSM) and doped lanthanum ferro-cobaltites (LSCF). Manganese-based perovskites can be regarded to be sufficiently stable, while for the better performing LSCF cathodes some intrinsic degradation was detected. New materials that are supposed to combine a better stability and high performance are also shortly mentioned.     

Fuel ejector design and simulation model for anodic recirculation SOFC system

In this paper, a new modeling technique for fuel ejectors with high entrainment ratio, low pressure increment and over heated working gases in an anodic recirculation solid oxide fuel cell (SOFC) system is presented. By utilizing the thermodynamic, fluid dynamic principles and chemical constraints inside ejectors and employing a two-dimensional function to compute fluid velocity, the developed model involves no more than nine algebraic equations and this is very simple compared to all existing models. The detailed procedures for fuel ejector design and simulation are provided and its effectiveness is verified through simulation and compared with testing results. It shows that the proposed model is more accurate than presently available models, and therefore can be better used for ejector design and performance simulations. The ejector performances for both situations of stand-alone and integrated into the SOFC system are also studied.     

Performance analysis of solid oxide fuel cell using reformed fuel

Fuelling SOFC with reformed fuel can be beneficial due to it being cheaper compared to pure hydrogen. A biomass fuel can be easily modeled as a reformed fuel, as it can be converted into H₂ and CO using gasification or biodegradation, the main composition of product from a reformer. Hence in this study it is assumed that feed to the fuel cell contains only H₂ and CO. A closed parametric model is formulated. Performance is analyzed with changes in temperature, pressure and fuel ratio; considering the possible voltage losses, like ohmic, activation, mass transfer and fuel crossover. Performance curves consisting of operating voltage, fuel utilization, efficiency, power density and current density are developed for both pure hydrogen and mixture of CO and H₂. Variations of open circuit voltage with temperature, power density with current density, operating voltage with current density and maximum power density with fuel utilization are also evaluated.     

Performance investigation on a coaxial-nozzle ejector for PEMFC hydrogen recirculation system

The ejector driven by the high-pressure gas potential energy from the hydrogen storage tank can reliably recirculate the unconsumed hydrogen in the proton exchange membrane fuel cell (PEMFC) system. However, the fixed-geometry ejector cannot maintain consistently high performance among the whole power output range in the PEMFC system due to its shortage of limited operating range. In this paper, a coaxial two-nozzle ejector, satisfying the requirements of the PEMFC system under different power outputs, is developed for hydrogen recirculation. The proposed ejector is investigated numerically based on an experimentally verified simulation model to reveal the flow distribution and predict its performance. The simulation results show that the proposed ejector can work in a wide power range of 17.90–84.00 kW within a suitable supply hydrogen pressure range of 4–7 bar. More importantly, the ejector can not only maintain a recirculation ratio above 0.9 in the wide output power range but a high recirculation ratio greater than 2.0 in the low power output. The proposed ejector broadens the working range of a single ejector used in the PEMFC system, which significantly promotes the development of fuel cell being widely adopted in automobiles.     

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.     

Microstructural analysis of a solid oxide fuel cell anode using focused ion beam techniques coupled with electrochemical simulation

Porous composite electrodes play a critical role in determining the performance and durability of solid oxide fuel cells, which are now emerging as a high efficiency, low emission energy conversion technology for a wide range of applications.In this paper we present work to combine experimental electrochemical and microstructural characterisation with electrochemical simulation to characterise a porous solid oxide fuel cell anode. Using a standard, electrolyte supported, screen printed Ni-YSZ anode, electrochemical impedance spectroscopy has been conducted in a symmetrical cell configuration. The electrode microstructure has been characterised using FIB tomography and the resulting microstructure has been used as the basis for electrochemical simulation. The outputs from this simulation have in turn been compared to the results of the electrochemical experiments.A sample of an SOFC anode of 6.68 μm × 5.04 μm × 1.50 μm in size was imaged in three dimensions using FIB tomography and the total triple phase boundary density was found to be 13 μm⁻². The extracted length-specific exchange current for hydrogen oxidation (97% H₂, 3% H₂O) at a Ni-YSZ anode was found to be 0.94 × 10⁻¹⁰, 2.14 × 10⁻¹⁰, and 12.2 × 10⁻¹⁰ A μm⁻¹ at 800, 900 and 1000 °C, respectively, consistent with equivalent literature data for length-specific exchange currents for hydrogen at geometrically defined nickel electrodes on YSZ electrolytes.     

Nickel-Zirconia cermet processing by mechanical alloying for solid oxide fuel cell anodes

This paper describes the development of a process based on high energy milling (or mechanical alloying—MA) of metallic Ni and YSZ at 40 vol% Ni composition for the preparation of solid oxide fuel cell anode material. The cermet powder is consolidated using the surface activated sintering (SAS) method. The cermet pellets possess microstructural characteristics that can potentially lead to higher electrocatalytic activity and fuel reforming capability. In addition to the development of a new processing method for this purpose, a further differential of this work is the addition of Cu in partial substitution of Ni as a means to prevent the formation of carbon on its surface and, hence, the anode’s degradation during service. The prepared powder samples are well dispersed and structured at the nanometric level, showing thin lamellar constituents. Suitable sintered pellets can be obtained from the powders with the required porosity and microstructure. The higher the energy delivered by MA the lower the initial sintering temperature. Activation energies are determined by stepwise isothermal dilatometry (SID) for Ni-YSZ and Ni/Cu-YSZ pellets, involving a 2-step sintering process. The Cu additive promotes sintering and leads to a refined microstructure.     

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.     

Design and characterization of an electronically controlled variable flow rate ejector for fuel cell applications

A variable flow ejector is presented to address the challenge of providing cost-effective recirculation of hydrogen in fuel cell systems. The ejector uses supersonic flow to provide sufficient pressure rise for the Ballard Mark 9 SSL stack used in the University of Delaware’s fuel cell hybrid buses. Details of geometry optimization via computational fluid dynamic simulation, control system design, electronic control implementation, and mechanical design are discussed. Results from testing in the final application are included, showing the ejector’s excellent performance compared to Ballard’s specifications for recirculation flow rate.     

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.     

Fabrication of anode support for solid oxide fuel cell using zirconium hydroxide as a pore former

A novel method of fabricating NiO-YSZ (yttria stabilized zirconia) anode substrates is developed using a composite pore former, i.e., PMMA (polymethyl-methacrylate) and carbon black or zirconium hydroxide Zr(OH)₄. By utilizing a composite pore former, both the shrinkage and porosity, which must be compatible with that of the electrolyte film and sufficient for the fuel supply and exhaust, are easily tailored. Carbon black and the inorganic pore former (Zr(OH)₄) affect the shrinkage of the anode substrate more effectively than its porosity, while the polymer spheres (PMMA) adjust the porosity more effectively. In particular, the successful use of zirconium hydroxide as a fine pore former, instead of carbon black, suggests that other zirconium or nickel compound derivatives may be used as pore formers.     

Hydrogen sulfide poisoning in solid oxide fuel cells under accelerated testing conditions

This study investigates the 0.2% hydrogen sulfide poisoning of Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The deterioration degrees and recovery extents of the cell current density, cell voltage and operation temperature are monitored. The results of impedance spectroscopy analysis show that hydrogen sulfide poisoning behavior may affect oxygen ion migration and gas diffusion and conversion on the anode side. Microstructural inspection reveals sulfur or sulfide formed on the anode-active area, which accounts for the immediate and severe cell power drop upon the injection of H₂S. The nickel sulfide in the anodic functional layer cannot be completely removed after long-term regeneration and thus may be a key factor in the permanent degradation of the cell.