Characterization of metal-supported axial injection plasma sprayed solid oxide fuel cells with aqueous suspension plasma sprayed electrolyte layers

A method for manufacturing metal-supported SOFCs with atmospheric plasma spraying (APS) is presented, making use of aqueous suspension feedstock for the electrolyte layer and dry powder feedstock for the anode and cathode layers. The cathode layer was deposited first directly onto a metal support, in order to minimize contact resistance, and to allow the introduction of added porosity. The electrolyte layers produced by suspension plasma spraying (SPS) were characterized in terms of thickness, permeability, and microstructure, and the impact of substrate morphology on electrolyte properties was investigated. Fuel cells produced by APS were electrochemically tested at temperatures ranging from 650 to 750 °C. The substrate morphology had little effect on open circuit voltage, but substrates with finer porosity resulted in lower kinetic losses in the fuel cell polarization.     

Study on GDC-LSGM composite electrolytes for intermediate-temperature solid oxide fuel cells

The electrolyte materials Ce_0.9Gd_0.1O_1.9 (GDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) were synthesized by means of glycine-nitrate processes, respectively, then GDC-LSGM composite electrolytes were prepared by mixing GDC and LSGM. The GDC and LSGM powders were mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as GL9505, GL9010 and GL8515. Their structures and ionic conductivities were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and AC impedance spectroscopy. The grain sizes of GDC-LSGM composites could be increased distinctly and the grain boundary resistance could be significantly decreased by small addition of LSGM. The experimental results show that the GDC-LSGM composites exhibit excellent ionic conductivity and could significantly enhance the fuel cell performances. The open circuit voltages are higher in the cell with composite electrolytes than in the cell with single GDC as electrolyte at the working temperature. Among these electrolytes, GL9505 has the highest ionic conductivity and the maximum power density.     

Thermodynamic analysis of ammonia fed solid oxide fuel cells: Comparison between proton-conducting electrolyte and oxygen ion-conducting electrolyte

A thermodynamic analysis has been performed to compare the theoretical performance of ammonia fed solid oxide fuel cells (SOFCs) based on proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). It is found that the ammonia fed SOFC-H is superior to SOFC-O in terms of theoretical maximum efficiency. For example, at a fuel utilization of 80% and an oxygen utilization of 20%, the efficiency of ammonia fed SOFC-H is 11% higher than that of SOFC-O. The difference between SOFC-H and SOFC-O becomes more significant at higher fuel utilizations and higher temperatures. This is because an SOFC-H has a higher hydrogen partial pressure and a lower steam partial pressure than an SOFC-O. In addition, an increase in oxygen utilization is found to increase the efficiency of ammonia fed SOFCs due to an increase in oxygen molar fraction and a reduction in steam molar fraction. With further development of new ceramics with high proton conductivity and effective fabrication of thin film electrolyte, the SOFC based on proton-conducting electrolyte is expected to be a promising approach to convert ammonia fuel into electricity.     

Preparation and property-performance relationships in samarium-doped ceria nanopowders for solid oxide fuel cell electrolytes

In a systematic study, Samarium doped ceria (SDC) nanopowders, Sm_xCe_1−xO_2−x/2 (x = 0.1, 0.2 or 0.3), were prepared by a low temperature citrate complexation route. The synthesis and crystallisation of the SDC powders were followed by thermochemical techniques (TGA/DTA), X-ray diffraction, elemental analysis, specific surface area determination (BET) and electron microscopy (SEM and TEM). Mean crystallite sizes were found to be around 10 nm for all compositions calcined at 500 °C. Dense electrolyte bodies were prepared at 1300 °C, 1400 °C and 1450 °C using two sintering times, 4 h or 6 h. Densities of 91-97% of theoretical were obtained, with a marked improvement in density on going from 1300 °C to higher sintering temperatures. Grain size analysis was conducted using SEM. Grain size distributions were related to %Sm and sintering conditions. Impedance spectroscopy was used to determine the total, bulk and grain boundary conductivities, the related activation energies and enthalpies of defect association and ion migration. Sintering at 1400 °C/6 h or 1450 °C/4 h gave superior grain structure and conductivity, with oversintering occurring after more severe treatments. At 600 °C the highest total ionic conductivity was 1.81 × 10⁻² S cm⁻¹ for Sm_0.2Ce_0.8O_1.9. The relationships between chemical composition, sintering parameters, grain structure and electrochemical performance are discussed.     

Mathematical modeling of ammonia-fed solid oxide fuel cells with different electrolytes

An electrochemical model was developed to study the ammonia (NH₃)-fed solid oxide fuel cells with proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). Different from previous thermodynamic analysis, the present study reveals that the actual performance of the NH₃-fed SOFC-H is considerably lower than the SOFC-O, mainly due to higher ohmic overpotential of the SOFC-H electrolyte. More analyses have been performed to study the separate overpotentials of the NH₃-fed SOFC-H and SOFC-O. Compared with the NH₃-fed SOFC-H, the SOFC-O has higher anode concentration overpotential and lower cathode concentration overpotential. The effects of temperature and electrode porosity on concentration overpotentials have also been studied in order to identify possible methods for improvement of SOFC performance. This study reveals that the use of different electrolytes not only causes different ion conduction characteristics at the electrolyte, but also significantly influences the concentration overpotentials at the electrodes. The model developed in this article can be extended to 2D and 3D models for further design optimization.     

An error in solid oxide fuel cell stack modeling

This paper points out an error in the literature and analyzes its effect on electrochemical models of solid oxide fuel cell stacks. A correction is presented.     

Implications of electronic short circuiting in plasma sprayed solid oxide fuel cells on electrode performance evaluation by electrochemical impedance spectroscopy

Electronic short circuiting of the electrolyte in a solid oxide fuel cell (SOFC) arising from flaws in the plasma spray fabrication process has been found to have a significant effect on the perceived performance of the electrodes, as evaluated by electrochemical impedance spectroscopy (EIS). The presence of a short circuit has been found to lead to the underestimation of the electrode polarization resistance (R_p) and hence an overestimation of electrode performance. The effect is particularly noticeable when electrolyte resistance is relatively high, for example during low to intermediate temperature operation, leading to an obvious deviation from the expected Arrhenius-type temperature dependence of R_p. A method is developed for determining the real electrode performance from measurements of various cell properties, and strategies for eliminating the occurrence of short circuiting in plasma sprayed cells are identified.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

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.     

Air plasma spray processing and electrochemical characterization of Cu-SDC coatings for use in solid oxide fuel cell anodes

Air plasma spraying has been used to produce porous composite anodes based on Ce_0.8Sm_0.2O_1.9 (SDC) and Cu for use in solid oxide fuel cells (SOFCs). Preliminarily, a range of plasma conditions has been examined for the production of composite coatings from pre-mixed SDC and CuO powders. Plasma gas compositions were varied to obtain a range of plasma temperatures. After reduction in H₂, coatings were characterized for composition and microstructure using EDX and SEM. As a result of these tests, symmetrical sintered electrolyte-supported anode-anode cells were fabricated by air plasma spraying of the anodes, followed by in situ reduction of the CuO to Cu. Full cells deposited on SS430 porous substrates were then produced in one integrated process. Fine CuO and SDC powders have been used to produce homogeneously mixed anode coatings with higher surface area microstructures, resulting in area-specific polarization resistances of 4.8 Ω cm² in impedance tests in hydrogen at 712 °C.     

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.     

Performance of a Ni/Al2O3 cermet-supported tubular solid oxide fuel cell operating with biomass-based syngas through supercritical water

Thermochemical gasification of biomass through the supercritical water gasification (SCWG) has high gasification efficiency at lower temperatures and can deal directly with wet biomass without drying. Besides, solid oxide fuel cells (SOFCs) appear to be an important technology in the future as they can operate at a high efficiency. Therefore, the combination of biomass gasification through supercritical water with SOFC represents one of the most potential applications for highly efficient utilization of biomass.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

Low temperature assisted chemical coprecipitation synthesis of 8YSZ plasma sprayable powder for solid oxide fuel cells

Plasma spraying is one of the potential manufacturing technologies widely used in tubular solid oxide fuel cells (SOFC) fabrication. The plasma spray technology requires powders with good flowability and large particle size (5-200 μm). A simple, low temperature assisted chemical process was used for the preparation of plasma grade yttria stabilized zirconia powder without any agglomeration process. The powder was characterized by X-ray diffractometry, particle size analysis, scanning electron microscopy and Raman spectroscopy. The powder exhibited cubic phase, good flowability and blocky angular shape. The 8YSZ powder was plasma sprayed and the coatings after sintering showed gas tightness (gas leak rate ∼ 1 × 10⁻⁶ mbar l s⁻¹ cm²). This was substantiated by the presence of densely packed grains as seen in the surface FESEM image of the sintered plasma sprayed 8YSZ free form. Conductivity values in par with the values reported in literature were obtained for plasma sprayed 8YSZ coating.     

Degradation behavior of anode-supported solid oxide fuel cell using LNF cathode as function of current load

We investigated the effect of current loading on the degradation behavior of an anode-supported solid oxide fuel cell (SOFC). The cell consisted of LaNi_0.6Fe_0.4O₃ (LNF), alumina-doped scandia stabilized zirconia (SASZ), and a Ni-SASZ cermet as the cathode, electrolyte, and anode, respectively. The test was carried out at 1073 K with constant loads of 0.3, 1.0, 1.5, and 2.3 A cm⁻². The degradation rate, defined by the voltage loss during a fixed period (about 1000 h), was faster at higher current densities. From an impedance analysis, the degradation depended mainly on increases in the cathodic resistance, while the anodic and ohmic resistances contributed very little. The cathode microstructures were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

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.     

Measurement of the temperature distribution in a large solid oxide fuel cell short stack

During the operation of solid oxide fuel cells (SOFCs), nonhomogeneous electrochemical reactions in both electrodes and boundary conditions may lead to a temperature gradient in the cell which may result in the development of thermal stresses causing the failure of the cell. Thus, in this study, effects of operating parameters (current density, flow configuration and cell size) on the temperature gradient of planar SOFCs are experimentally investigated. Two short stacks are fabricated using a small (16 cm² active area) and a large size (81 cm² active area) scandia alumina stabilized zirconia (ScAlSZ) based electrolyte supported cells fabricated via tape casting and screen printing routes and an experimental set up is devised to measure both the performance and the temperature distribution in short stacks. The temperature distribution is found to be uniform in the small short stack; however, a significant temperature gradient is measured in the large short stack. Temperature measurements in the large short stack show that the temperature close to inlet section is relatively higher than those of other locations for all cases due to the high concentrated fuel resulted in higher electrochemical reactions hence the generated heat. The operation current is found to significantly affect the temperature distribution in the anode gas channel. SEM analyses show the presence of small deformations on the anode surface of the large cell near to the inlet after high current operations.     

A study of solid oxide fuel cell stack failure by inducing abnormal behavior in a single cell test

It is well known that cell imbalance can lead to failure of batteries. Prior theoretical modeling has shown that similar failure can occur in solid oxide fuel cell (SOFC) stacks due to cell imbalance. Central to failure model for SOFC stacks is the abnormal operation of a cell with cell voltage becoming negative. For investigation of SOFC stack failure by simulating abnormal behavior in a single cell test, thin yttria-stabilized zirconia (YSZ) electrolyte, anode-supported cells were tested at 800 °C with hydrogen as fuel and air as oxidant with and without an applied DC bias. When under a DC bias with cell operating under a negative voltage, rapid degradation occurred characterized by increased cell resistance. Visual and microscopic examination revealed that delamination occurred along the electrolyte/anode interface. The present results show that anode-supported SOFC stacks with YSZ electrolyte are prone to catastrophic failure due to internal pressure buildup, provided cell imbalance occurs. The present results also suggest that the greater the number of cells in an SOFC stack, the greater is the propensity to catastrophic failure.     

Electrochemical performance of a solid oxide fuel cell with an LSCF cathode under different oxygen concentrations

A new performance study has been performed on a commercially available anode supported planar SOFC containing an LSCF cathode. The SOFC cell is tested at different temperatures and different cathode gas compositions. The temperature and cathode gas dependence on the electrochemical performance is studied using voltage-current density curves and impedance spectroscopy at different cell voltages. The cell tested shows excellent performance at all temperatures and is not limited by diffusion losses for the tested conditions. This new study indicates that the cell impedance spectroscopy is comprised of at least four semicircles of which two are partially dependent on the cathode gas conditions. It was found that historical effects play a role in the impedance spectra, showing some scatter in the ohmic resistance as a function of applied voltage. The cell ohmic resistance decreases as the temperature increases and as the cathode gas conditions are switched from air to O₂-He mixture. However, the cell ohmic resistance under pure O₂ was found to be higher than the O₂-He mixture. In virtually all IS data, the cell ohmic resistance showed a maximum value around 0.8 V. The cell ohmic ASR shows that interfacial resistances are a significant portion of the total ohmic resistance. The total electrode polarization decreases as the temperature increases and as the cathode gas conditions are switched from air to O₂-He mixture and to pure O₂. Finally, the peak frequency of the largest semicircle observed at high frequency shows a linear dependence on the applied voltage in most cases. This behavior is related to the charge transfer that occurs in the high frequency range and indicates that the electrochemical reactions are occurring at faster rates as more current flows through the cell.