Performance of anodic recirculation by a variable flow ejector for a solid oxide fuel cell system under partial loads

An anode gas recycle (AGR) system driven by a variable flow rate ejector was developed for use in small-scale solid oxide fuel cell (SOFC) systems. The partial load conditions were simulated through recycling power generation experiments to clarify the fundamental characteristics of the variable flow ejector by using actual 1 kW-class SOFC equipment at the steady state. We achieved power generation in a range of recirculation ratios under partial load conditions of 62.5%–80% by controlling the recirculation characteristics with the developed ejector by using a needle. Results showed that the recirculation ratio can be controlled in the range of 0.595–0.694 by adjusting the driving energy with the ejector even at a partial load where the fuel gas flow rate of the ejector changes. Furthermore, the effect of the recirculation ratio on SOFC output was discussed based on the results of gas analyses and temperature measurements. As the recirculation ratio increased, the fuel concentration at the SOFC inlet decreased and the water vapor concentration increased. However, the effect of the recirculation ratio on the stack temperature and output power was proposed to be small. In addition, it was confirmed that the operation was performed under safe conditions where no carbon deposition occurred by circulating the steam generated inside the SOFC without an external water supply. Ejector characteristics during power generation experiments were lower than those at room temperature, which indicates that an ejector upstream pressure of approximately 20–170 kPa gauge pressure was required. Variations in the fluid properties of the driver gas in the ejector motive nozzle heated by the hot suction gas were found to degrade the performance of the ejector installed in the SOFC system, as compared with the results of simulation experiments at room temperature. Nevertheless, the recirculation ratio range required for operation could be satisfied by adjusting the flow velocity of the driving gas through needle control.     

Modelling and flow rate control methods for anode tail gas circulation intake system at SOFC

In the solid oxide fuel cell (SOFC) power generation system, the anode tail gas circulation intake system plays an important role in the entire system for efficient and stable operation. Currently, there are relatively few studies on the operating characteristics of claw pump, which works as the core component of the SOFC anode exhaust gas circulation intake system. Based on the prototype of 20 kW SOFC power generation system, this paper builds the emulation testbed for the anode exhaust gas circulation intake system and analyzes the working characteristics of the claw pump in the testbed firstly. The support vector regression (SVR) and Gauss process regression (GPR) methods are then used to establish the characteristics model of the claw pump based on the operation data. According to the verification results, GPR method owns higher prediction accuracy than SVR method in our case. Therefore, the GPR method is adopted to build the flow rate prediction model of the claw pump. Finally, the flow rate control method based on feedforward and model prediction (FMP) with GPR flow rate prediction model is proposed for the built anode tail gas circulation intake system. In experiments, the response time of the FMP method is within 22 s and the overshoot is less than 3.2%, whose performance is better than the traditional flow rate control method, such as PID or model predictive control.     

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.     

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.     

A CFD-based model of a planar SOFC for anode flow field design

This study presents a 3D CFD model of a planar SOFC with internal reforming for anode flow field design. The developed model reflects the influence of various factors on fuel cell performance including flow field design and kinetics of chemical and electrochemical reactions. The case study illustrates applications of the CFD model for planar SOFC with different anode flow field designs. Simulation results indicate the importance of the anode flow field design for planar SOFCs. The model is useful for optimization of fuel cell design and operating conditions.     

Porous Ni/8YSZ anode of SOFC fabricated by the plasma sprayed method

In the present study, porous electrode coating of Ni/8YSZ on the interconnector material was made by the plasma-spraying. By introducing the pore former into the composite powder, the porous structure of SOFC anode will be obtained. By using the plasma spraying technique for SOFC fabrication, we can avoid the thermal failure between the components of SOFC which made from the traditional sintering method at high temperature. In this study, two kinds of composite powders in the granulate form were prepared, one with the nano carbon as a pore former and the other without the carbon. The results showed that the porous structure of SOFC anode could be achieved by the plasma spraying technique. The porosity of the anode made from the composite powder with pore former was 40%. Without pore former the porosity in the anode coating after hydrogen reduction was almost 30%. These results suggest that this method exhibits the potential to manufacture the porous ceramic/metal composite anode of SOFC to achieve the larger triple phase boundary for fuel oxidation.     

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.     

A versatile salt evaporation reactor system for SOFC operando studies on anode contamination and degradation with impedance spectroscopy

The dependence of the degradation kinetics in Ni-CGO (cerium-gadolinium oxide) solid oxide fuel cell (SOFC) anodes upon salt evaporation is demonstrated operando with a custom built versatile reactor system. The system is based on evaporation and subsequent condensation of low concentration salt vapor aerosol mixtures representative of salt vapors typically present in biomass gasification processes. Fast changes in the charge transfer and ohmic resistance are observed in the anodes fuelled with a gas mixture containing a high KCl vapor concentration. Rapid condensation of salt vapors into the porous anode and partial delamination of the anode from the electrolyte surface because of salt deposits inside the porous anode is observed. The flexibility to produce vapor-aerosol mixtures with different concentrations and particle size distributions is proved, and suitability of these aerosols for anode testing in long term fuel cell test is evaluated.     

Characteristics of SOFC single cells with anode active layer via tape casting and co-firing

SOFC (solid oxide fuel cell) single cells with anode active layers of various thicknesses were fabricated successfully via tape casting and co-firing in order to improve their electrochemical performance and long-term stability. The mercury porosimeter and the gas permeability were measured so as to examine the effects of the anode active layer while under a gaseous flow. It was found that the anode active layers affected the microstructural characteristics as a result of the pore distribution and the gas permeation behavior. The anode active layers improved the cell performance by increasing the number of active sites in the anode. The thickness of the anode active layer was optimized at 20 μm in this work through a combination of the power density, the ohmic ASR (area specific resistance), and the cell ASR. SOFCs with the optimized active layer showed good electrochemical performance at 600–700 °C in hydrogen or hydrocarbon fuel (methane) and excellent long-term stability for 500 h.     

Advances in the development of titanates for anodes in SOFC

Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices with the advantage that a wide variety of hydrocarbon fuels can be used directly. Recently, the field of research on anodic materials of SOFC has advanced rapidly, with special emphasis on the development of materials with resistance to H₂S as well as to the formation of coke. Therefore, it is crucial to design new anodic materials with higher catalytic activity, stability, tolerance to coke deposition and to sulfur poisoning. Due to their stability in redox conditions, the titanates are among the most studied perovskites. Strontium titanate (SrTiO₃) is a good electronic conductor at low partial pressures of oxygen and during redox cycles and presents excellent dimensional and chemical stabilities as well as sulfur tolerance. However, for application as the anode in a SOFC, it must be doped to improve important properties such as the conductivity and power density. This article describes the progress in the knowledge of titanates with perovskite structure with potential application as anodes for SOFC.     

Effect of composition of NiO/YSZ anode on the polarization characteristics of SOFC fabricated by atmospheric plasma spraying

Atmospheric plasma spraying is regarded as one of the most promising processing methods for the production of solid oxide fuel cells (SOFCs) because of its fast deposition rate and cost-effective characteristic compared with other film formation processes. Thermally sprayed ceramic deposits are characterized by lamellar structure resulting from the insufficient filling and incomplete wetting of molten liquid on previously formed rough coating surfaces. There are large voids (especially larger than 10 μm), nonbonded interface, and vertical cracks (in particular, in individual ceramic lamella which are in the submicrometer size range). The bonding at the interface between flattened particles is much less than the apparent total interface area. The microstructure of plasma-sprayed coating can be significantly influenced by the feedstock.     

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.     

Quantification of SOFC anode microstructure based on dual beam FIB-SEM technique

The three-dimensional microstructure of an SOFC anode is quantified using a dual beam focused ion beam scanning electron microscopy (FIB-SEM) system equipped with an energy dispersive X-ray spectroscopy (EDX) unit. The microstructure of the Ni-YSZ anode is virtually reconstructed in a computational field using a series of acquired two-dimensional SEM images. The three-phase boundary (TPB) density and tortuosity factors are carefully evaluated by applying two different evaluation methods to each parameter. The TPB density is estimated by a volume expansion method and a centroid method, while the tortuosity factors are evaluated by a random walk calculation and a lattice Boltzmann method (LBM). Estimates of each parameter obtained by the two methods are in good agreement with each other, thereby validating the reliability of the analysis methods proposed in this study.     

Evaluation of cost reduction potential for 1 kW class SOFC stack production: Implications for SOFC technology scenario

Recently, a commercial version of a residential solid oxide fuel cell (SOFC) system with a flat tubular cell has been developed. However, the system cost still remains very high, which is a barrier to its widespread use. In this study, the potential for cost reductions in SOFC stack production was investigated in order to contribute to the viability of the widespread use of such residential SOFC systems in future. A cost analysis of 700 W SOFC stack production based on a process integration modeling was conducted. The present bottom–up approach enabled us to perform a sensitivity analysis with a variety of parameters in terms of cell design, the production process and cell performance. This allowed us to investigate the effects of these factors on the production cost, thereby revealing the quantitative impact of each technological improvement on the cost reduction potential. The present analysis also revealed innovation pathways which could result in technology scenarios where residential SOFC systems could reach a break-even point in comparison with the baseload electricity cost. The analysis of the cost reduction potential presented here provides a useful viewpoint for developing a research strategy for state-of-the-art SOFC technology.     

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.     

Synthesis of matrix-type NiO–SDC composite particles by spray pyrolysis with acid addition for development of SOFC cermet anode

Matrix-type NiO–SDC composite particles were synthesized by spray pyrolysis from the starting solution containing citric acid without pre-heat treatment. Matrix-type composite particles synthesized in this study were spherical shape with high-dispersed state of NiO and SDC. The calcined matrix-type NiO–SDC composite particles at 500 and 1000 °C showed the high performance of SOFC anode. From the electrochemical characterization, the matrix-type structure was effective to reduce the ohmic loss, and the calcination treatment for the matrix-type composite particles would reduce the anodic polarization. It was found that the addition of citric acid into the starting solution for spray pyrolysis led to the high-dispersed matrix-type NiO–SDC composite particles with spherical shape, which showed the high performance anode, without any pre-heat treatment of the starting solution.     

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.     

Internal reforming SOFC running on biogas

Direct feeding of biogas to SOFC, which is derived from municipal organic wastes, has been investigated as a carbon-neutral renewable energy system. CH₄/CO₂ ratio in the actual biogas fluctuated between 1.4 and 1.9 indicating biogas composition is strongly affected by the kinds of organic wastes and the operational conditions of methane fermentation. Using anode-supported button cells, stable operation of biogas-fueled SOFC was achieved with the internal reforming mode at 800 °C. Cell voltage above 0.8 V was recorded over 800 h at 200 mA cm⁻². It has been revealed that air addition to actual biogas reduced the risk of carbon formation and led to more stable operation without compromising cell voltage due to the lowering of anodic overvoltage.     

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.     

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.