Investigation of aluminosilicate as a solid oxide fuel cell refractory

Aluminosilicate represents a potential low cost alternative to alumina for solid oxide fuel cell (SOFC) refractory applications. The objectives of this investigation are to study: (1) changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) deposition of aluminosilicate vapors on yttria stabilized zirconia (YSZ) and nickel, and (3) effects of aluminosilicate vapors on SOFC electrochemical performance. Thermal treatment of aluminosilicate under high temperature SOFC conditions is shown to result in increased mullite concentrations at the surface due to diffusion of silicon from the bulk. Water vapor accelerates the rate of surface diffusion resulting in a more uniform distribution of silicon. The high temperature condensation of volatile gases released from aluminosilicate preferentially deposit on YSZ rather than nickel. Silicon vapor deposited on YSZ consists primarily of aluminum rich clusters enclosed in an amorphous siliceous layer. Increased concentrations of silicon are observed in enlarged grain boundaries indicating separation of YSZ grains by insulating glassy phase. The presence of aluminosilicate powder in the hot zone of a fuel line supplying humidified hydrogen to an SOFC anode impeded peak performance and accelerated degradation. Energy dispersive X-ray spectroscopy detected concentrations of silicon at the interface between the electrolyte and anode interlayer above impurity levels.     

Modeling of methane fed solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen ion conducting electrolyte

An electrochemical model was developed to study the methane (CH₄) fed solid oxide fuel cell (SOFC) using proton conducting electrolyte (SOFC-H) and oxygen ion conducting electrolyte (SOFC-O). Both the internal methane steam reforming (MSR) and water gas shift (WGS) reactions are considered in the model. Previous study has shown that the CH₄ fed SOFC-H had significantly better performance than the SOFC-O. However, the present study reveals that the actual performance of the CH₄ fed SOFC-H is considerably lower than the SOFC-O, partly due to higher ohmic overpotential of SOFC-H. It is also found that the CH₄ fed SOFC-H has considerably higher cathode concentration overpotential and lower anode concentration overpotential than the SOFC-O. The anode concentration overpotentials of the CH₄ fed SOFC-H and SOFC-O are found to decrease with increasing temperature, which is different from previous analyses on the H₂ fed SOFC. Therefore, high temperature is desirable for increasing the potential of the CH₄ fed SOFC. It is also found that there exist optimal electrode porosities that minimize the electrode total overpotentials. The analyses provided in this paper signify the difference between the CH₄ fed SOFC-H and SOFC-O. The model developed in this paper can be extended to 2D or 3D models to study the performance of practical SOFC systems.     

Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC)

A parametric analysis is carried out to study the effects of the operating conditions on the performance and operation of a micro-tubular solid oxide fuel cell. The computational fluid dynamics model incorporates mass, momentum, species and energy balances along with ionic and electronic charge transfers. Effects of temperature, fuel flow rate, fuel composition, anode pressure and cathode pressure on fuel cell performance are investigated. Polarization curves are compared to allow an understanding of the effects of different operating conditions on the performance of the fuel cell. Effects of anode flow rate on fuel cell efficiency and fuel utilization are also investigated. Moreover, influence of operating temperature on the internal electronic current leaks is outlined. Temperature distributions, current density profiles and hydrogen mole fraction profiles are also utilized to have a better understanding of the spatial effects of operating parameters. It is predicted that at 550 °C, for an output current demand of 0.53 A cm⁻², fuel cell needs to generate 0.65 A cm⁻² ionic current density where the difference in these values is attributed to internal current leaks. On the other hand for temperatures lower than 500 °C, the effect of electronic leakage currents are not significant.     

Fuel cell system modeling for solid oxide fuel cell/gas turbine hybrid power plants, Part I: Modeling and simulation framework

A sustainable future power supply requires high fuel-to-electricity conversion efficiencies even in small-scale power plants. A promising technology to reach this goal is a hybrid power plant in which a gas turbine (GT) is coupled with a solid oxide fuel cell (SOFC). This paper presents a dynamic model of a pressurized SOFC system consisting of the fuel cell stack with combustion zone and balance-of-plant components such as desulphurization, humidification, reformer, ejector and heat exchangers. The model includes thermal coupling between the different components. A number of control loops for fuel and air flows as well as power management are integrated in order to keep the system within the desired operation window. Models and controls are implemented in a MATLAB/SIMULINK environment. Different hybrid cycles proposed earlier are discussed and a preferred cycle is developed. Simulation results show the prospects of the developed modeling and control system.     

The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode suported solid oxide fuel cell (SOFC)

Tubular solid oxide fuel cells (SOFCs) have many desirable advantages compared to other SOFC applications. Recently, micro-tubular SOFCs were studied to apply them into APU systems for future vehicles. In this study, electrochemical properties of the micro-tubular SOFCs (1.6 mm O.D.) have been characterized. Electrochemical analysis showed excellent performance with a maximum power density of 1.3 W/cm² at 550 °C. The impedance information gained at cell operating temperatures of 450, 500, and 550 °C showed individual cell ohmic resistances of 1.0, 0.6, and 0.2 Ω respectively. Within the operating temperature range of 450-550 °C, the ceria based micro-tubular SOFCs (cathode length: 8 mm) were found to have power densities ranging between 0.263 and 1.310 W/cm². The mechanical properties of the tubes were also analyzed through internal burst testing and monotonic compressive loading on a c-ring test specimen. The two testing techniques are compared and related, and maximum hoop stress values are reported for each of the fabrication parameters. This study showed feasible electrochemical properties and mechanical strength of micro-tubular SOFC for APU applications.     

Dependence of open-circuit potential and power density on electrolyte thickness in solid oxide fuel cells with mixed conducting electrolytes

A continuum-level electrochemical model previously developed by the authors [1] is used to investigate the dependence of open-circuit voltage (OCV), and maximum power density on electrolyte thickness for solid oxide fuel cells (SOFCs) with mixed conducting electrolytes. Experimental results confirm the models predictions that OCV decreases monotonically with decreasing electrolyte thickness due to increased permeation flux [1]. The model was further extended to show that there exists an optimal electrolyte thickness at which maximum power density occurs for mixed conducting electrolytes. As expected, for electrolyte thickness greater than optimal losses from ohmic overpotential reduce cell output. However, when the electrolyte thickness is lower than optimal losses from an increasing electronic “leakage” current reduce cell output.     

Effects of operating and design parameters on the performance of a solid oxide fuel cell–gas turbine system

We present a steady‐state thermodynamic model of a hybrid solid oxide fuel cell (SOFC)–gas turbine (GT) cycle developed using a commercial process simulation software, AspenPlus?. The hybrid cycle model incorporates a zero‐dimensional macro‐level SOFC model. A parametric study was carried out using the developed model to study the effects of system pressure, SOFC operating temperature, turbine inlet temperature, steam‐to‐carbon ratio, SOFC fuel utilization factor, and GT isentropic efficiency on the specific work output and efficiency of a generic hybrid cycle with and without anode recirculation. The results show that system pressure and SOFC operating temperature increase the cycle efficiency regardless of the presence of anode recirculation. On the other hand, the specific work decreases with operating temperature. Overall, the model can successfully capture the complex performance trends observed in hybrid cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Cycle analysis of an integrated solid oxide fuel cell and recuperative gas turbine with an air reheating system

Cycle simulation and analysis for two kinds of SOFC/GT hybrid systems were conducted with the help of the simulation tool: Aspen Custom Modeler. Two cycle schemes of recuperative heat exchanger (RHE) and exhaust gas recirculated (EGR) were described according to the air reheating method. The system performance with operating pressure, turbine inlet temperature and fuel cell load were studied based on the simulation results. Then the effects of oxygen utilization, fuel utilization, operating temperature and efficiencies of the gas turbine components on the system performance of the RHE cycle and the EGR cycle were discussed in detail. Simulation results indicated that the system optimum efficiency for the EGR air reheating cycle scheme was higher than that of the RHE cycle system. A higher pressure ratio would be available for the EGR cycle system in comparison with the RHE cycle. It was found that increasing fuel utilization or oxygen utilization would decrease fuel cell efficiency but improve the system efficiency for both of the RHE and EGR cycles. The efficiency of the RHE cycle hybrid system decreased as the fuel cell air inlet temperature increased. However, the system efficiency of EGR cycle increased with fuel cell air inlet temperature. The effect of turbine efficiency on the system efficiency was more obvious than the effect of the compressor and recuperator efficiencies among the gas turbine components. It was also indicated that improving the gas turbine component efficiencies for the RHE cycle increased system efficiency higher than that for the EGR cycle.     

Effect of pre-oxidation and environmental aging on the seal strength of a novel high-temperature solid oxide fuel cell (SOFC) sealing glass with metallic interconnect

A novel high-temperature alkaline-earth silicate sealing glass was developed for solid oxide fuel cell (SOFC) applications. The glass was used to join two ferritic stainless steel coupons for strength evaluation. The steel coupons were pre-oxidized at elevated temperatures to promote thick oxide layers to simulate long-term exposure conditions. In addition, seals to as-received metal coupons were also tested after aging in oxidizing or reducing environments to simulate the actual SOFC environment. Room temperature tensile testing showed strength degradation when using pre-oxidized coupons, and more extensive degradation after aging in air. Fracture surface and microstructural analysis confirmed that the cause of degradation was formation of SrCrO₄ at the outer sealing edges exposed to air.     

The numerical investigation of a planar single chamber solid oxide fuel cell performance with a focus on the support types

Single chamber solid oxide fuel cells (SC-SOFCs) could be an alternative to the conventional dual chamber types since they do not need any sealant and electrolyte crack growth does not lead to failure in performance. However, the reduced reactant activity due to spectator species present at anode and cathode results in a significantly decreased performance. The focus of this paper is to present a comparative study on the performance of single-chamber anode-, cathode, and electrolyte-supported cells. Our results show that anode-supported cells offer significantly better performance compared to electrolyte-supported cells. The cathode-supported cells show a similar performance to anode-supported cell close to open circuit voltages, i.e. voltages above 0.92 V, after which the cell current density decreases due to lack of oxygen at the cathode catalyst layer. Finally, a time-dependent performance study of the cathode-supported cell concept is presented and discussed.     

Effect of O2 concentration on performance of solid oxide fuel cells with V2O5 or Cu added (LaSr)(CoFe)O3-(Ce,Gd)O2−x cathode with and without NO

A solid oxide fuel cell unit is constructed with Ni-(Ce,Gd)O_2−x (GDC) as the anode, yttria-stabilized zirconia as the electrolyte, and V₂O₅ or Cu added (LaSr)(CoFe)O₃-GDC as the cathode. The effect of the O₂ concentration on the open circuit voltage (OCV) is studied and a mass-transfer limited OCV is observed. The power density with Cu addition can be much higher than that with V₂O₅ addition but the effect of the O₂ concentration with Cu addition is larger than that with V₂O₅ addition. Without the presence of NO, both the power density and the OCV decrease with decreasing O₂ concentration. The OCV variation can be substantial with the variation of the flow rate, the O₂ concentration and the NO concentration. The presence of CO₂ can increase the OCV while that of NO can decrease the OCV; however, a synergistic effect can occur on the OCV when NO is present at a very low O₂ concentration which results in a sudden drop of the OCV.     

Detailed dynamic Solid Oxide Fuel Cell modeling for electrochemical impedance spectra simulation

This paper presents a detailed flexible mathematical model for planar solid oxide fuel cells (SOFCs), which allows the simulation of steady-state performance characteristics, i.e. voltage-current density (V-j) curves, and dynamic operation behavior, with a special capability of simulating electrochemical impedance spectroscopy (EIS). The model is based on physico-chemical governing equations coupled with a detailed multi-component gas diffusion mechanism (Dusty-Gas Model (DGM)) and a multi-step heterogeneous reaction mechanism implicitly accounting for the water-gas-shift (WGS), methane reforming and Boudouard reactions. Spatial discretization can be applied for 1D (button-cell approximation) up to quasi-3D (full size anode supported cell in cross-flow configuration) geometries and is resolved with the finite difference method (FDM). The model is built and implemented on the commercially available modeling and simulations platform gPROMS™. Different fuels based on hydrogen, methane and syngas with inert diluents are run. The model is applied to demonstrate a detailed analysis of the SOFC inherent losses and their attribution to the EIS. This is achieved by means of a step-by-step analysis of the involved transient processes such as gas conversion in the main gas chambers/channels, gas diffusion through the porous electrodes together with the heterogeneous reactions on the nickel catalyst, and the double-layer current within the electrochemical reaction zone. The model is an important tool for analyzing SOFC performance fundamentals as well as for design and optimization of materials’ and operational parameters.     

Comparative study on the electrochemical performance of coals and coal chars in a molten carbonate direct carbon fuel cell with a novel anode structure

Molten carbonate direct carbon fuel cells (MC-DCFCs) allow the efficient and clean use of coal. In this study, a novel anode structure is designed, and the performances of six coal-based fuels are investigated in MC-DCFC. The mechanisms of performance differences are investigated, as well as the effect of operating temperature on performance. The results reveal the fuel cell performance in the following order: meagre coal (109.8) ≈ bituminous coal (108.7) > bituminous coal char (98.1) > lignite coal (83.7) > lignite coal char (71.3) > meagre coal char (53.2) in mW cm^?2. Coal performs better because of its high carbon content, high volatile content, rich oxygen-containing functional groups, larger specific surface area, stronger thermal reactivity, and other factors. The electrochemical reactivity of coal fuel increased with higher reaction temperatures and varied throughout the temperature ranges. This study implies that using coal fuel to commercialize MC-DCFC could be a realistic alternative.     

Assessment of ammonia as energy carrier in the use with reversible solid oxide cells

Ammonia represents one of the most promising potential solutions as energy vector and hydrogen carrier, having a higher potential to transport energy than hydrogen itself in a pressurized form. Furthermore, solid oxide fuel cells (SOFCs) can directly be fed with ammonia, thus allowing for immediate electrical power and heat generation. This paper deals with the analysis of the dynamic behavior of commercial SOFCs when fueled with ammonia. Several measurements at different temperatures have been performed and performances are compared with hydrogen and a stoichiometrically equivalent mixture of H₂ and N₂ (3:1 M ratio). Higher temperature led to smaller drops in voltage for both fuels, thus providing higher efficiencies. Ammonia resulted slightly more performant (48% at 760 °C) than hydrogen (45% at 760 °C), in short stack tests. Moreover, different ammonia-to-air ratios have been investigated and the stack area-specific resistance has been studied in detail by comparing numerical modeling predictions and experimental values.