Influence of sintering support design on the properties of NiO-YSZ anode support micro-tubes

The effects of the sintering support design on the microstructural and mechanical properties of NiO/YSZ anode support micro-tubes are investigated in this study. For this purpose, a number of micro-tubes of the same geometrical properties are fabricated via extrusion method from the same extrusion paste. The micro-tubes are then sintered at the same temperature on sintering plates with different C- and V-type sintering channels designed. The micro-tubes after sintering are found to have similar geometric properties. However, the microstructural investigations and three point bending tests indicate that the microstructural properties of the micro-tubes are strongly influenced by the sintering plate design. As a result, the flexural strength of the micro-tubes are found to be different. Among the cases considered, the micro-tubes of V2 (half of the micro-tube is initially in the V-type channel) and V4 (whole micro-tube is initially in the V-type channel) design provide relatively fine microstructure and thus relatively higher flexural strength values at a reliability of 80%. The flexural strengths of these micro-tubes are also found to be more consistent. © 2017 Elsevier Ltd and Techna Group S.r.l

Glass fiber reinforced sealants for solid oxide fuel cells

Novel sealants for solid oxide fuel cells are developed by addition of glass fiber into glass-ceramic as a reinforcement material. Various sealants including three different fiberglass types and four different structural designs are fabricated. The mechanical and sealing performances of the sealants are investigated via tensile and short stack leakage tests, respectively. The tensile tests reveal that the fracture strength of the sealants varies depending on the type and number of the glass fiber used. In general, the sealants having relatively high number of glass fiber layers exhibit relatively low joining strength. The best bonding strength values are obtained from the sealants having a structure where a single glass fiber layer is sandwiched between two glass-ceramic layers. The sealing performance tests are performed for the sealants showing the highest and lowest fracture strengths in the tensile tests as well as for the sealant without glass fiber addition as a base case for a comparison. The results indicate that it is possible to obtain a gas-tight sealing at high temperatures under all pressures studied, whereas leakage occurs at room temperature for all cases considered. However, the sealing performance is found to be related with the mechanical strength of the sealants. © 2019 Hydrogen Energy Publications LL

Effects of lamination conditions on the performance of anode-supported solid oxide fuel cells

14th International Symposium on Solid Oxide Fuel Cells, SOFC 2015; held as part of the Electrochemical Society, ECS Conference on Electrochemical Energy Conversion and Storage -- 26 July 2015 through 31 July 2015 -- -- 113202Solid oxide fuel cells (SOFCs) operate at high temperatures and produce electricity and heat energy from fuels in an electrochemical way. Membrane electrode assembly (MEA) composed of a dense electrolyte coated with two porous electrodes on each side is generally constructed on the electrolyte. However, high operation temperatures are required to obtain acceptable performance values due to high electrolyte resistance as a result of the use of thick electrolyte layer as a mechanical support for MEA. Alternatively, the cells can be fabricated as anode-supported where the anode is responsible for supporting the cell mechanically. By doing so, the operation temperature can be lowered due to reduced electrolyte thickness. In this study, the effects of isostatic press parameters i.e. temperature and pressure for NiO/YSZ anode support on the anode-supported cell performance and the microstructure of the anode support are investigated. The experimental results reveal that the cell performance is strongly influenced by the pressing parameters. In this aspect, the optimum pressing temperature and pressure are found to be 50°C and 40 MPa, respectively. © The Electrochemical Society

Transport phenomena of convergent and divergent serpentine flow fields for PEMFC

Reactive species and water transport are crucial for the proton exchange membrane fuel cell operation and performance, and for this, effective flow field design can facilitate the desired transport characteristics of species. From this motivation, the conventional single serpentine flow field pattern is modified by convergent and divergent design concepts and the complex transport phenomena of the newly developed flow field designs are investigated by a numerical approach. For the numerical analyses, an experimentally validated mathematical model is developed to predict the current density, oxygen mass transport, water concentration and pressure distribution. The different configurations of modified convergent and divergent serpentine flow fields are then numerically solved and the results are compared with the conventional serpentine flow field pattern. The transport of reactive species and water concentration are analyzed from the different perspectives including cathode domains and surfaces with a quantitative formulation of the transport species. The numerical results reveals that the modified convergent serpentine flow fields yield to a uniform current density due to the lower mass fraction of water concentration over the reaction zone facilitating better oxygen mass transport and also higher channel pressure distribution along the flow field comparing the conventional and divergent type serpentine flow fields. © 2018 Elsevier LtdFirat University Scientific Research Projects Management UnitThis research supported by Nigde Omer Halisdemir University Scientific Research Projects Coordination Unit under a project number of FEB2016/31 -YULTEP is gratefully acknowledged

Numerical Investigation of Convergent and Divergent Parallel Flow Fields for PEMFCs

In this study, the classical parallel flow field pattern is modified by a convergent and divergent design concept for proton exchange membrane fuel cells (PEMFCs). The modification is achieved by varying the channel depth with different constant inclination gradients from the inlet to the outlet, along with the bipolar plate width which creates either a convergent or divergent flow field depending on the position of the inlet and outlet selected. The numerical model is solved to predict the cell performance including the mass transport, water vapor concentration and, pressure and current density distribution for the flow fields. The numerical results reveal that the modified convergent parallel flow fields have a better mass transport and water removal characteristics than those in the conventional one, whereas mal-distribution of species and excessive water concentration occur in the conventional and divergent parallel flow fields. Introducing convergent parallel flow field design concept not only minimizes these issues but the cell power density is also improved by a maximum of 16%, which can show a new window in PEMFC flow field design concept. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimFirat University Scientific Research Projects Management UnitThis research supported by Nigde Omer Halisdemir University Scientific Research Projects Coordination Unit under a project number of FEB2016/31-YULTEP is gratefully acknowledged

The role of tape thickness on mechanical properties and performance of electrolyte supports in solid oxide fuel cells

The change in the mechanical and electrochemical performance of tape cast YSZ electrolyte as a function of doctor blade gap is investigated in this study via mechanical, microstructural and electrochemical analyses. Three point bending tests indicate that the flexural strength of the electrolyte can be improved by reducing the doctor blade gap. This can be elucidated by the microstructural observations, which reveal an increased number of small grains and reduced grain size with decreasing the doctor blade height, confirming the sintering shrinkage. The impedance results reveal that the electrical resistance of the electrolyte increases with decreasing the doctor blade gap due to reduced grain size. Similar behavior is also observed in the performance results. Therefore, the doctor blade gap can be considered as a critical parameter for the mechanical and electrochemical performance of tape cast YSZ electrolyte in solid oxide fuel cells, since it significantly affects the electrolyte microstructure. © 2018 Elsevier Ltd and Techna Group S.r.l

Effects of solid loading on joining and thermal cycling performance of glass-ceramic sealing pastes for solid oxide fuel cells

The variation of the joining performance of glass-ceramic sealants in the form of a paste as a function of the solid powder content in the sealing paste after the formation and a number of thermal cycles are experimentally studied. Three different sealing pastes having 40, 50 and 60 wt % solid loadings are prepared and tested for this purpose. The pastes are applied between two metallic interconnector plates and subjected to a glass formation step for the joining. The fracture strengths of 24 samples prepared for each case are determined via tensile tests. Similarly, the mechanical performances of the sealants after 3, 6 and 9 thermal cycles are also obtained. The results reveal that the joining strength tends to increase with the amount of solid powder content in the paste. This can be attributed to increased number of crystalline phases in the sealants with increasing the solid loading. The thermal cycles, on the other hand, are shown to have an adverse effect on the joining performance regardless of the solid loading. However, the rate of decrease in the fracture strengths is found to decrease with the solid powder contents in the pastes. This can be elucidated by the amount of glassy phases in the sealants, which can be expected to increase with the solid loading and provide self-healing ability. The microstructures of the fracture surfaces of all samples are also investigated by a scanning electron microscopy. The obtained images confirm the tensile test results. © 2019 Elsevier Ltd and Techna Group S.r.l

Geometric optimization of an ejector for a 4 kW SOFC system with anode off-gas recycle

One of the important energy saving tools used in solid oxide fuel cell (SOFC) system is the anode off-gas recycling (AGR) via an ejector which allows the recirculation of the unused fuels in the anode exhaust gas including hot steam which is essential for the elimination of the carbon deposition and the initiation of the reactions in the reformer. In an ejector system developed for the SOFCs, the steam to carbon ratio (STCR) and entrainment ratio are the crucial parameters for the determination of the ejector performance. These parameters can be engineered by modifying the geometric dimensions and operation conditions. This study focuses on the determination of the maximum STCR value and entrainment ratio via numerical geometric analyses for a micro combined heat and power (µ-CHP) system based on 4 kW SOFC, utilizing methane. A detailed numerical procedure for designing an ejector is provided and the ejector performance is investigated for different critical dimensions (throat diameter, nozzle exit angle and nozzle position etc.). The results show that the nozzle position and the nozzle exit angle significantly affect STCR and the entrainment ratio. When the nozzle position increases and nozzle exit angle decreases, the entrainment ratio and STCR is found to increase. The entrainment ratio and STCR are determined as around 7.3 and 2.7, respectively for a specific design created in the study. © 2018 Hydrogen Energy Publications LLCThe equipment and materials support of this research by TUBITAK under a project number of 213M030 is gratefully acknowledged

Influence of doctor blade gap on the properties of tape cast NiO/YSZ anode supports for solid oxide fuel cells

In this study, various tape cast NiO/YSZ anode support layers with similar geometric properties are fabricated by varying the doctor blade from 100 µm to 200 µm with an increment of 25 µm. The mechanical properties of the anode support layers are investigated by three point bending tests of 30 samples for each doctor blade gap. The reliability curves of the flexural strength data are also obtained via two-parameter Weibull distribution method. The effects of the doctor blade gap on the microstructure and the electrochemical performance of the anode support layers are determined via SEM investigations and single cell performance-impedance tests, respectively. The apparent porosities of the samples are also measured by Archimedes’ principle. The results indicate that the doctor blade gap or the resultant tape thickness influences the microstructure of tape cast NiO/YSZ anode supports significantly, yielding different mechanical and electrochemical characteristics. At a reliability level of 70%, the highest flexural strength of 110.20 MPa is obtained from the anode support layer with a doctor blade gap of 175 µm and the 16 cm2 active area cell with this anode support layer also exhibits the highest peak performance of 0.483 W/cm2 at an operating temperature of 800 °C. Thus, a doctor blade gap of 175 µm is found to have such a microstructure that provides not only better mechanical strength but also higher electrochemical performance. © 2018 Elsevier Ltd and Techna Group S.r.l