Investigation of the effect of ion transition type on performance in solid oxide fuel cells fueled hydrogen and coal gas

Renewable energy will be a panacea for environmental difficulties due to the extensive usage of carbon-rich fuels as a main source of energy. As a result, hydrogen-fueled solid oxide fuel cell is a revolutionary clean technology that has a great contribution in solving the current energy and environmental-related challenges. Thus, a 3D model of hydrogen and coal gases fueled solid oxide fuel cell (H₂–SOFC) using different electrolytes has been developed and simulated using COMSOL commercial software to explore the performance of electrolyte supported SOFC. The performance of the developed model has been studied and characterized using different differential equations. Accordingly, it has been found that the performance of hydrogen-fueled oxide ion conducting electrolytes (SOFC–O) is lower than that of protonic conducting one (SOFC–H) at 800 °C. Furthermore, a numerical simulation has been conducted to investigate the result of temperature changes on SOFC performance at 400 °C, 600 °C, and 800 °C for proton-conducting SOFC and 800 °C and 1000 °C for oxygen-conducting SOFC. It has been demonstrated that SOFC–O shows a better performance at high temperatures compared with SOFC–H while SOFC–H can be an agreeable selection at medium temperatures. Therefore, this study reveals that the temperature augments the performance of both electrolytes, yet at higher working temperatures SOFC–H becomes more advantageous than SOFC–O to use hydrogen and coal gas as a primary fuel. Besides, the effect of channel height was also analyzed numerically and the finding disclosed that decreasing the channel height emerges in a curtly current path. Thus, it can be reasoned out that the performance of SOFC decreases when the channel height is increased.     

Stable LSM/LSTM bi-layer interconnect for flat-tubular solid oxide fuel cells

A bi-layer interconnect with La_0.8Sr_0.2MnO₃ and La_0.4Sr_0.6Ti_0.6Mn_0.4O₃ (LSM/LSTM) is applied to anode-supported button cells and flat-tubular cells. Using a button cell, SEM images and gas permeation tests confirm that the bi-layer possesses a dense microstructure. The area specific resistance (ASR) of the LSM/LSTM remains nearly constant under oxidizing/reducing atmospheres with varying gas concentrations. For comparison, an LSM/LST with the same thickness is prepared; an increase in the ASR is observed as the concentration of H₂ feed to the LST side decreases. The difference in the ASR of LSM/LST can be explained by exposure to a relatively high oxygen partial pressure and partial destruction of the interfacial LST layer region where oxygen diffuses from the LSM layer. Flat-tubular cells with the LSM/LSTM bi-layer interconnect achieve a maximum power density (MPD) of 463 mW cm^?2 using humidified H₂ fuel and air at 800 °C. With decreasing H₂ concentration in the fuel, the polarization resistance increases rather than the ohmic resistance, implying that the LSM/LSTM interconnect provides stable conduction property. In comparison with the conventional LSM/LST interconnect cell, it shows improved stability and performance as the concentration of H₂ in the fuel decreases.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.     

La-doped SrTiO3 interconnect materials for anode-supported flat-tubular solid oxide fuel cells

An interconnect layer in an anode-supported flat-tubular solid oxide fuel cell connects electrically unit cells and separates fuel from oxidant in the adjoining cells. Nano-sized La-doped SrTiO₃ for the interconnect is synthesized in this study by the Pechini method using citric acid. The materials with stoichiometric and Sr-deficient compositions are prepared and sintered in an oxidizing atmosphere. The synthesized fine powders exhibit high sinterability, leading to near-full densification. The Sr deficiency plays a crucial role in mechanical, electrical and thermal expansion properties. The interconnect is coated using the synthesized powder on a porous flat-tubular anode support by a screen printing process. The thin and dense layer is obtained after co-sintering in air, and the interconnect/anode interface remains intact upon reduction.     

Comparative performance analysis of a direct ammonia-fuelled anode supported flat tubular solid oxide fuel cell: A 3D numerical study

Solid oxide fuel cells that are designed in different geometrical structures (planar, tubular, flat-tubular, etc.) are dirt-free, quiet, and efficient cells that run using different fuels including contagions fuels. In this work, the performance of a 3D model of direct ammonia feed anode supported flat-tubular solid oxide fuel cell having six fuel supply channels was developed, investigated, and elucidated numerically in comparison with hydrogen fuels at different operating conditions using COMOSOL Multiphysics. The finding of this study is revealed that the performance of the developed model that is running with direct ammonia is better than hydrogen feed one using the same geometrical dimensions and operating parameters. It is also confirmed that direct ammonia feed anode supported flat-tubular solid oxide fuel cell has outstanding performance over the corresponding anode supported tubular solid oxide fuel cell using the same active cell surface area, gas channel length, and operating conditions. Parametric sweep analyses have been also performed on selected operating parameters and the outcomes revealed that the working temperature and the amount of reactant gases have a powerful impact on cell performance. Thus, ammonia is a green auspicious, and profitable candidate to use as a carbon-neutral fuel for anode supported flat-tubular solid oxide fuel cells in the near future.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

Experimental characterization and mechanistic modeling of carbon monoxide fueled solid oxide fuel cell

The paper presents an elementary reaction based solid oxide fuel cell (SOFC) model coupled with anodic elementary heterogeneous reactions and electrochemical charge transfer reactions for CO/CO₂ fuel based on an anode supported button cell. The model is calibrated and validated using experimental data obtained for various CO/CO₂ fuel compositions at 750, 800 and 850 °C. The comparison shows that the modeling results agree well with the experimental data. The effects of operating conditions on the cell performance and the detailed species concentration distribution are predicted. Then, the carbon deposition on the SOFC anode with CO/CO₂ fuel is experimentally measured and simulated using the elementary reaction model. The results indicate that lower temperature and lower operation voltage are helpful to reduce the possibilities of carbon deposition on Ni particle surfaces.     

Asymmetric behavior of solid oxide cells between fuel cell and electrolyzer operations

The electrochemical performance of solid oxide cells (SOCs) is investigated under both fuel cell and electrolyzer operations to understand their asymmetric behavior between the two operation modes. The current–voltage and electrochemical impedance characteristics of a hydrogen-electrode-supported cell are experimentally analyzed. Also, a numerical model is developed to reproduce the cell performance and to understand the internal resistances of the cell. Partial pressures of supplied gas and load current are varied to evaluate their effects on the cell performance. The gas partial pressures of hydrogen and steam supplied to the hydrogen electrode are kept equivalent so that the cell performance can be fairly compared between the two operation modes when the same current is applied. It is found that the origin of the asymmetry is mostly from the hydrogen electrode; both activation and concentration overpotentials show asymmetric behavior particularly at high current densities. A numerical experiment is also conducted by deliberately changing parameters in the model. Asymmetry in the activation overpotential is found to be originated from the non-identical charge-transfer coefficients in the Butler–Volmer equation and also from the non-uniform gas concentration formed in the hydrogen electrode under current-biased conditions. On the other hand, asymmetry in the concentration overpotential is associated with the non-equimolar counter diffusion of hydrogen and steam caused by the effect of Knudsen diffusion. Therefore, enhancing gas transport in the hydrogen electrode and reducing the contribution of Knudsen diffusion are effective approaches to reduce asymmetry not only in the concentration overpotential but also in the activation overpotential.     

Performance analysis and parametric study of a solid oxide fuel cell fueled by carbon monoxide

A theoretical model of the solid oxide fuel cell (SOFC) fueled by carbon monoxide is adopted and validated, in which the activation overpotential, concentration overpotential, and ohmic overpotential are regarded as the main sources of voltage losses. Based on the thermodynamic-electrochemical analysis, mathematical expressions of some performance parameters such as the cell potential, power output, efficiency, and entropy production rate are derived. The effects of microstructure parameters such as the electrode porosity, tortuosity, pore size, grain size, etc. on the electrochemical performance characteristics of the SOFC are revealed. Moreover, the effects of some operation conditions such as the current density, anode inlet gas molar fraction, operating temperature, and operating pressure on some important performance parameters of the SOFC are also discussed. It is found that there exist some optimal values of microstructure parameters and operating conditions at which the better performance can be expected. The results obtained in the paper may provide some theoretical guidance for the design and operation of practical SOFCs fueled by coal-derived gases.     

Three-dimensional numerical simulation and experimental validation on ammonia and hydrogen fueled micro tubular solid oxide fuel cell performance

The main aim of this research is to investigate the performance of ammonia-powered microtubular solid oxide fuel cells in order to use ammonia as a possible candidate for eco-friendly and sustainable power generation systems. The performance of a direct ammonia-powered cell has been elucidated and validated with the experimental results of pure hydrogen gas at Ni?de Ömer Halisdemir University Prof. T. Nejat Veziro?lu Clean Energy Research Center. For both studies, the cathode electrode is supplied with atmospheric air. The performance of anode, electrolyte, and cathode-supported microtubular solid oxide fuel cells has been compared numerically. The findings confirmed that the peak possible power densities obtained numerically using direct ammonia, hydrogen and experimentally using pure hydrogen gas are is 628.92 mW/cm², 622.29 mW/cm^2, and, 589.28 mW/cm² respectively at the same geometrical dimensions, component materials, and operating parameters. Thus, the results of this study demonstrate that simultaneous experimental and numerical studies make a great contribution to minimizing biases due to literature data during model validation. The numerical simulation also indicates that the performance of cathode supported is superior to that of anode supported cells run with hydrogen and ammonia fuel. Likewise, parametric sweep analysis asserts that the working temperature has a greater effect than operating pressure on tubular cell performance. Therefore, the results of this study advise that ammonia will become a carbon-free alternative fuel for solid oxide fuel cells in the coming years.     

Numerical modelling of methane-powered micro-tubular, single-chamber solid oxide fuel cell

An experimentally validated, two-dimensional, axisymmetric, numerical model of micro-tubular, single-chamber solid oxide fuel cell (MT-SC-SOFC) has been developed. The model incorporates methane full combustion, steam reforming, dry reforming and water-gas shift reaction followed by electrochemical oxidation of produced hydrogen within the anode. On the cathode side, parasitic combustion of methane along with the electrochemical oxygen reduction is implemented. The results show that the poor performance of single-chamber SOFC as compared to the conventional (dual-chamber) SOFC (in case of micro-tubes) is due to the mass transport limitation on the anode side. The gas velocity inside the micro-tube is far too low when compared to the gas-chamber inlet velocity. The electronic current density is also non-uniform over the cell length, mainly due to the short length of the anode current collector located at the cell outlet. Furthermore, the higher temperature near the cell edges is due to the methane combustion (very close to the cell inlet) and current collection point (at the cell outlet). Both of these locations could be sensitive to the silver current collecting wire as silver may rupture due to cell overheating.     

Gas transport in hydrogen electrode of solid oxide regenerative fuel cells for power generation and hydrogen production

To further develop solid oxide regenerative fuel cell (SORFC) technology, the effect of gas diffusion in the hydrogen electrode on the performance of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) is investigated. The hydrogen electrode-supported cells are fabricated and tested under various operating conditions in both the power generation and hydrogen production modes. A transport model based on the dusty-gas model is developed to analyze the multi-component diffusion process in the porous media, and the transport parameters are obtained by applying the experimentally measured limiting current data to the model. The structural parameters of the porous electrode, such as porosity and tortuosity, are derived using the Chapman–Enskogg model and microstructural image analysis. The performance of an SOEC is strongly influenced by the gas diffusion limitation at the hydrogen electrode, and the limiting current density of an SOEC is substantially lower than that of an SOFC for the standard cell structure under normal operating conditions. The pore structure of the hydrogen electrode is optimized by using poly(methyl methacrylate) (PMMA), a pore-forming agent, and consequently, the hydrogen production rate of the SOEC is improved by a factor of greater than two under moderate humidity conditions.     

Entropy generation analysis in a monolithic-type solid oxide fuel cell (SOFC)

The aim of the paper is to investigate possible improvements in the geometry design of a monolithic solid oxide fuel cells (SOFCs) through analysis of the entropy generation terms. The different contributions to the local rate of entropy generation are calculated using a computational fluid dynamic (CFD) model of the fuel cell, accounting for energy transfer, fluid dynamics, current transfer, chemical reactions and electrochemistry. The fuel cell geometry is then modified to reduce the main sources of irreversibility and increase its efficiency.     

The hybrid solid oxide fuel cell (SOFC) and gas turbine (GT) systems steady state modeling

Solid Oxide Fuel Cells (SOFCs) are of great interest nowadays. The feature of SOFCs makes them suitable for hybrid systems because they work high operating temperature and when combined with conventional turbine power plants offer high cycle efficiencies. In this work a hybrid solid oxide fuel cell and gas turbine power system model is developed. Two models have been developed based on simple thermodynamic expressions. The simple models are used in the preliminary part of the study and a more realistic based on the performance maps. A comparative study of the simulated configurations, based on an energy analysis is used to perform a parametric study of the overall hybrid system efficiency. Some important observations are made by means of a sensitivity study of the whole cycle for the selected configuration. The results of the selected model were compared to an earlier model from an available literature.     

Numerical simulation of intermediate-temperature direct-internal-reforming planar solid oxide fuel cell

A numerical model for an anode-supported intermediate-temperature direct-internal-reforming planar solid oxide fuel cell (SOFC) was developed. In this model, the volume-averaging method is applied to the flow passages in the SOFC by assuming that a porous material is inserted in the passages as a current collector. This treatment reduces the computational time and cost by avoiding a full three-dimensional simulation while maintaining the ability to solve the flow and pressure fields in the streamwise and spanwise directions. In this model, quasi-three-dimensional multicomponent gas flow fields, the temperature field, and the electric potential/current fields were simultaneously solved. The steam-reforming reaction using methane, the water-gas shift reaction, and the electrochemical reactions of hydrogen and carbon monoxide were taken into account. It was found that the endothermic steam-reforming reaction led to a reduction in the local temperature near the inlet and limited the electrochemical reaction rates therein. Computational results indicated that the local temperature and current density distributions can be controlled by tuning the pre-reforming rate. It was also found that a small amount of heat loss from the sidewall can cause significant nonuniformity in the flow and thermal fields in the spanwise direction.     

Effects of physical properties and operating parameters on numerically developed flat-tube solid oxide fuel cell performance

Fuel cells are promising as a clean energy conversion device in the era of global warming threat. Solid oxide fuel cells stand out among other fuel cell types because they are especially feasible for high-temperature applications. Besides the operating parameters, which are frequently encountered problems in SOFCs, physical parameters also directly affect cell performance. For this reason, careful examination of the effects of the parameters while designing the SOFC will contribute to the increase of the maximum power to be provided from the cell in applications. In this study, a solid oxide fuel cell (SOFC) created in flat-tube geometry is numerically modeled. The effect of the electrode and electrolyte layers on the performance is investigated parametrically on the created geometry. In addition, the effect of temperature on cell power is investigated comparatively by making analyzes at different temperatures for each case. In the analyzes, performance values are investigated for electrode layer thicknesses of 0.75 mm, 0.5 mm, and 0.25 mm, and electrolyte layer thicknesses of 1.25 mm, 1 mm, and 0.75 mm, respectively. As a result of the study, the highest cell power is obtained at 0.25 mm of the anode layer thickness. The maximum predicted average cell power of the flat tubular solid oxide fuel cell (FT-SOFC) is approximately 68.2 mW/cm² for the operating temperature of 1273 K. In the study, the effect of electrolyte conductivity on the performance of the developed cell is also investigated. It is observed that the cell with a conductivity value of 100 S/m at 1073 K operating temperature has the best performance. In addition, in the last part of the study, the performance of SOFC under non-uniform operating temperature conditions is also examined and a comparison is made for this situation, which is frequently experienced in practice. The results show that small changes in fuel cell operating temperature affect the cell performance in positive and negative directions depending on the increasing and decreasing temperature values.     

Design and performance analysis of a de-coupled solid oxide fuel cell gas turbine hybrid

Hybrid solid oxide fuel cells (SOFC) cycles of varying complexity are widely studied for their potential efficiency, carbon recovery and co-production of chemicals. This study introduces an alternative de-coupled fuel cell-gas turbine hybrid arrangement that retains the high efficiency thermal integration of a topping cycle without the high temperature heat exchanger of a bottoming cycle. The system utilizes a solid-state oxygen transport membrane to divert 30%–50% of the oxygen from the turbine working fluid to the intermediate temperature SOFC. Thermodynamic modeling delineates design trade-offs and identifies a flexible operating regime with peak fuel-to-electric efficiency of 75%. Co-production of electricity and high purity hydrogen result in net energy conversion efficiencies greater than 80%. The potential to retrofit existing turbine systems, particularly micro-turbines and stand-by ‘peaker’ plants, with minimal impact to compressor stability or transient response is a promising pathway to hybrid fuel cell/turbine development that does not require turbomachinery modification.     

Development of paper-structured catalyst for application to direct internal reforming solid oxide fuel cell fueled by biogas

A flexible paper-structured catalyst (PSC) that can be applied to the anode of a solid oxide fuel cell (SOFC) was examined for its potential to enable direct internal reforming (DIR) operation. The catalytic activity of three types of Ni-loaded PSCs: (a) without the dispersion of support oxide particles in the fiber network (PSC-A), (b) with the dispersion of (Mg,Al)O derived from hydrotalcite (PSCB), and (c) with the dispersion of (Ce,Zr)O_2-δ (PSCC), for dry reforming of CH₄ was evaluated at operating temperatures of 650–800 °C. Among the PSCs, PSC-C exhibited the highest CH₄ conversion with the lowest degradation rate. The electrochemical performance of an electrolyte-supported cell (ESC) was evaluated under the flow of simulated biogas at 750 °C for cases without and with the PSCs on the anode. The application of the PSCs improved the cell performance. In particular, PSC-C had a remarkably positive effect on stabilizing DIRSOFC operation fueled by biogas.     

A high performance direct carbon solid oxide fuel cell fueled by Ca-loaded activated carbon

Ca-loaded activated carbon is developed as fuel for direct carbon solid oxide fuel cells (DC-SOFCs), operating without any carrier gas and liquid medium. Ca is loaded on activated carbon through impregnation technique in the form of CaO, which exhibits excellent catalytic activity and significantly promotes the output performance of DC-SOFCs. DC-SOFCs fueled by activated carbon with different Ca loading content (0, 1, 3, 5 and 7 wt. %) are tested and the performances are compared with the DC-SOFC running on the conventional Fe-loaded activated carbon. It is found that the performance of the DC-SOFC with 5 wt. % (373 mW cm^?2) and 7 wt. % (378 mW cm^?2) Ca-loaded activated carbon is significantly higher than that of the cells operated on 5 wt. % Fe-loaded activated carbon, 1 wt. % and 3 wt. % Ca-loaded activated carbon. The discharging time and fuel utilization of the DC-SOFC with 5 wt. % Ca-loaded activated carbon are also the optimal ones among all the cells. The microstructure, element distribution and carbon conversion rate of the Ca-loaded carbon, the impedance spectra of the corresponding DC-SOFCs are measured. The reasons for the reduced fuel utilization of 7 wt. % Ca-loaded carbon fuel are analyzed and the advantage of Ca-loaded carbon for DC-SOFCs is demonstrated in detail.