Anode supported solid oxide fuel cells (SOFC) by electrophoretic deposition

Solid oxide fuel cells (SOFC), with its ability to use hydrocarbon fuels and capability to offer highest efficiency, have attracted great attention in India in recent years as an alternative energy generation system for future. But a great deal of problems associated with SOFC is needed to be solved before it can find commercial application. The relatively high operating temperature of 800-1000 °C of SOFC imposes a stringent requirement on materials that significantly increases the cost of SOFC technology. Reducing the operating temperature of an SOFC to below 800 °C can reduce degradation of cell components, improve flexibility in cell design, and lower the material and manufacturing cost by the use of cheap and readily available materials such as ferritic stainless steel. The operating temperature can be reduced by two possible approaches: (i) developing alternative electrolyte materials with high ionic conductivity at lower temperature, and (ii) developing much thinner and denser electrolyte layer such that the ohmic losses are minimised.In this work we report the use of inexpensive Electrophoretic deposition (EPD) technique in making about 10 micron thin and dense YSZ electrolyte on NiO-YSZ substrate. The effect of different operating parameters such as applied voltage, deposition time etc have been optimised during deposition from YSZ suspension in acetylacetone. The YSZ/NiO-YSZ bi-layers were then co-sintered at 1450 °C for 5 h. The single SOFC cells were then fabricated by brush painting LSM:YSZ (50:50) paste on the electrolyte layer followed by sintering at 1200 °C for 2 h. The single SOFC cell when tested using H₂ as fuel and ambient air as oxidant exhibited an open circuit voltage (OCV) of 1.03 V and the peak power density of about 624 mW/cm² at 800 °C.     

Self-propagating room temperature synthesis of nanopowders for solid oxide fuel cells (SOFC)

Ceria based solid solutions are promising ceramic electrolytes for SOFC which will be able to function at intermediate temperatures. In accordance with common trends to decrease the production and operating costs of new generation of SOFC materials the reaction based on metathetical pathway is described, whereby solid solution powders of rare earth doped ceria in the nanoscale range were obtained at room temperature. By simple hand mixing of reactants the reaction was enhanced, propagating afterwards by itself. Nanometric ceria powders doped with Y or Nd, as well as co doped with both cations were synthesized. Composition of Ce_1 − xMe_xO_2 − y ranged from x = 0–0.25. The reaction course is described in detail and the properties of the obtained powders are presented.     

Direct-methane solid oxide fuel cell (SOFC) with Ni-SDC anode-supported cell

The performance of a Ni-SDC anode-supported cell operating with a dry CH₄ feed stream and the effectiveness of exposing the anode to H₂ as a method of removing carbon deposits are evaluated. This has involved the continuous monitoring of the outlet gas composition during CH₄ operation and H₂ exposure. A degradation rate in the cell voltage (～1.33 mV h^?1) is observed during 100 h operation with dry CH₄. Carbon is detected in the Ni-SDC anode after the stability test but only in the portion of the anode closest to the fuel channel. No carbon is detected at the electrolyte-anode interface, which is the likely reason that the cell performance remains relatively stable. The information obtained from SEM and gas outlet composition analyses can be explained by a process whereby most of the CH₄ that reacts decomposes into H₂ and C in the Ni-SDC anode near the fuel channel. H₂ then makes its way to the anode-electrolyte interface where it is electrochemically oxidized to H₂O which can also react with any C that may have formed, leaving behind C primarily at the fuel channel. When an aged cell is exposed to H₂, carbon-containing gases (CO, CH₄ and CO₂) are released, indicating that some carbon has been removed from the anode. Examination of the anode after the test shows that some carbon still remains after this treatment.     

A transient analysis of a micro-tubular solid oxide fuel cell (SOFC)

A two-dimensional, axisymmetric transient computational fluid dynamics model is developed for an intermediate temperature micro-tubular solid oxide fuel cell (SOFC), which incorporates mass, species, momentum, energy, ionic and electronic charge conservation. In our model we also take into account internal current leak which is a common problem with ceria based electrolytes. The current density response of the SOFC as a result of step changes in voltage is investigated. Time scales associated with mass transfer and heat transfer are distinguished in our analysis while discussing the effect of each phenomenon on the overall dynamic response. It is found that the dynamic response is controlled by the heat transfer. Dynamic behavior of the SOFC as a result of failure in fuel supply is also investigated, and it is found that the external current drops to zero in less than 1 s.     

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.     

Economic feasibility of solid oxide fuel cell (SOFC) for power generation in Iran

Supply utilities needed in site with the lowest cost and emission and highest efficiency are considered one of the main concerns of the process industry’s owners that requires doing more research in this area. In this research, during a case study, first comprehensive site of producing utility is optimized, and then the energy and environmental analysis of fuel cell system is taken place. Then fuel cell integration with utility site during two scenarios of entire supply of steam generated by HRSG of gas turbine and entire supply of gas turbine generated power was evaluated. According to the evaluation done, if the target is the entire supply of steam generated by HRSG of gas turbine, selecting solid oxide fuel cell (SOFC) system is economically and environmentally more affordable. So if the target is preheating, selecting the system is based on entire supply of steam generated by the boilers, and the number of eight SOFC systems will be required by which a power about 22 MW can be produced.     

Ni-samaria-doped ceria (Ni-SDC) anode-supported solid oxide fuel cell (SOFC) operating with CO

The performance of nickel-samaria-doped ceria (Ni-SDC) anode-supported cell with CO-CO₂ feed was evaluated. The aim of this work is to examine carbon formation on the Ni-SDC anode when feeding with CO under conditions when carbon deposition is thermodynamically favoured. Electrochemical tests were conducted at intermediate temperatures (550–700 °C) using 20 and 40% CO concentrations. Cell operating with 40% CO at 600–700 °C provided maximum power densities of 239–270 mW cm^?2, 1.5 times smaller than that achieved with humidified H₂. Much lower maximum power densities were attained with 20% CO (50–88 mW cm^?2). Some degradation was observed during the 6 h galvanostatic operation at 0.1 A cm^?2 with 40% CO fuel at 550 °C which is believed due to the accumulation of carbon at the anode. The degradation in cell potential occurred at a rate of 4.5 mV h^?1, but it did not lead to cell collapse. EDX mapping at the cross-section of the anode revealed that carbon formed in the Ni-SDC cell was primarily deposited in the anode section close to the fuel entry point. Carbon was not detected at the electrolyte-anode interface and the middle of the anode, allowing the cell to continue operation with CO fuel without a catastrophic failure.     

Electrolytes for solid oxide fuel cells

The high operating temperature of solid oxide fuel cells (SOFCs), as compared to polymer electrolyte membrane fuel cells (PEMFCs), improves tolerance to impurities in the fuel, but also creates challenges in the development of suitable materials for the various fuel cell components. In response to these challenges, intermediate temperature solid oxide fuel cells (IT-SOFCs) are being developed to reduce high-temperature material requirements, which will extend useful lifetime, improve durability and reduce cost, while maintaining good fuel flexibility. A major challenge in reducing the operating temperature of SOFCs is the development of solid electrolyte materials with sufficient conductivity to maintain acceptably low ohmic losses during operation. In this paper, solid electrolytes being developed for solid oxide fuel cells, including zirconia-, ceria- and lanthanum gallate-based materials, are reviewed and compared. The focus is on the conductivity, but other issues, such as compatibility with electrode materials, are also discussed.     

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.     

Seal glass for solid oxide fuel cells

Seal glass plays a crucial role in solid oxide fuel cell performance and durability. In this review paper, overall composition-structure-property relations of seal glasses are discussed from bulk glass behavior, interfacial interaction, and sealing ability point of view. A seal glass should have a combination of desired thermal, chemical, mechanical, and electrical properties in order to seal cell components and stacks and prevent gas leakage. It must be stable for ∼40,000 h at 500-1000 °C in oxidizing and reducing atmospheres and withstand ∼10,000 thermal cycles between room temperature and cell operating temperature. A SrO-La₂O₃-Al₂O₃-SiO₂ based seal glass shows the promise to meet all the desired thermophysical properties, long-term stability, and thermal cycling resistance. In this paper, the most recent advances in the field are discussed along with this glass. Future seal glass research directions for solid oxide fuel cells are also analyzed.     

Suitable operational strategy for power interchange operation using multiple residential SOFC (solid oxide fuel cell) cogeneration systems

A suitable operational strategy for a power interchange operation using multiple residential solid oxide fuel cell (SOFC) cogeneration systems for saving energy is investigated by an optimization approach based on mixed-integer linear programming. In this power interchange operation, electricity generated by residential SOFC cogeneration systems is shared among households in a housing complex without allowing a reverse power flow to a commercial electric power system in order to increase electric load factors of the system. For an SOFC cogeneration system operated continuously with the minimum output, two types of operational strategies for the power interchange operation are adopted: an operation to meet the total demand for electricity in intended households by the electricity output of SOFC cogeneration systems and an operation to meet the demand for hot water in each household by the hot water output of the SOFC cogeneration system. To clarify a theoretical limit of the energy-saving effects of the two strategies, this study numerically analyzes optimal operation patterns for 20 households on three representative days. The results show that the former operational strategy, which takes advantage of the high electricity generating efficiency of the SOFC, is more suitable for saving energy as compared to the latter strategy.     

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.     

Hydrogen-rich gas production for solid oxide fuel cell (SOFC) via partial oxidation of butanol: Thermodynamic analysis

Butanol partial oxidation for hydrogen-rich gas production has been studied by Gibbs free energy minimization method. The optimum conditions for hydrogen-rich gas production are identified: reaction temperatures between 1115 and 1200 K and oxygen-to-butanol molar ratios between 1.6 and 1.7 at 1 atm. Under the optimal conditions, complete conversion of butanol, 93.07%–96.56% yield of hydrogen and 94.02%–97.55% yield of carbon monoxide could be achieved in the absence of coke formation. The butanol partial oxidation with O₂ is suitable for providing hydrogen-rich fuels for Solid Oxide Fuel Cell (SOFC). Higher pressures have a negative effect, but inert gases have a positive effect, on the hydrogen yield. Coke tends to form at lower temperatures and lower oxygen-to-butanol molar ratios.     

Decentralized generation of electricity with solid oxide fuel cells from centrally converted biomass

A thermodynamic evaluation of different energy conversion chains based on centralized biomass gasification and decentralized heat and power production by a solid oxide fuel cell (SOFC) has been performed. Three different chains have been evaluated, the main difference between the chains is the secondary fuel produced via biomass gasification. The secondary fuels considered are hydrogen, synthetic natural gas (SNG) and syngas. These fuels are assumed to be distributed through a transport and distribution grid to the micro-combined heat and power (μ-CHP) systems based on a SOFC and a heat pump.     

Novel nano-network cathodes for solid oxide fuel cells

A novel nano-network of Sm_0.5Sr_0.5CoO_3−δ (SSC) is successfully fabricated as the cathodes for intermediate-temperature solid oxide fuel cells (SOFCs) operated at 500–600 °C. The cathode is composed of SSC nanowires formed from nanobeads of less than 50 nm thus exhibiting high surface area and porosity, forming straight path for oxygen ion and electron transportation, resulting in high three-phase boundaries, and consequently showing remarkably high electrode performance. An anode-supported cell with the nano-network cathode demonstrates a peak power density of 0.44 W cm⁻² at 500 °C and displays exceptional performance with cell operating time. The result suggests a new direction to significantly improve the SOFC performance.     

Alternative anode materials for solid oxide fuel cells

The electrolyte of a solid oxide fuel cell (SOFC) is an O²⁻-ion conductor. The anode must oxidize the fuel with O²⁻ ions received from the electrolyte and it must deliver electrons of the fuel chemisorption reaction to a current collector. Cells operating on H₂ and CO generally use a porous Ni/electrolyte cermet that supports a thin, dense electrolyte. Ni acts as both the electronic conductor and the catalyst for splitting the H₂ bond; the oxidation of H₂ to H₂O occurs at the Ni/electrolyte/H₂ triple-phase boundary (TPB). The CO is oxidized at the oxide component of the cermet, which may be the electrolyte, yttria-stabilized zirconia, or a mixed oxide-ion/electron conductor (MIEC). The MIEC is commonly a Gd-doped ceria. The design and fabrication of these anodes are evaluated. Use of natural gas as the fuel requires another strategy, and MIECs are being explored for this application. The several constraints on these MIECs are outlined, and preliminary results of this on-going investigation are reviewed.     

Thin films for micro solid oxide fuel cells

Thin film deposition as applied to micro solid oxide fuel cell (μSOFC) fabrication is an emerging and highly active field of research that is attracting greater attention. This paper reviews thin film (thickness ≤1 μm) deposition techniques and components relevant to SOFCs including current research on nanocrystalline thin film electrolyte and thin-film-based model electrodes. Calculations showing the geometric limits of μSOFCs and first results towards fabrication of μSOFCs are also discussed.     

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFC) were successfully fabricated by a single step co-firing process. The cells comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ or scandia-stabilized zirconia (ScSZ) electrolyte, a (La_0.85Ca_0.15)_0.97MnO₃ (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells in the temperature range of 1300–1330 °C for 2 h. Cells were tested in the temperature range of 700–800 °C with humidified hydrogen as fuel and air as oxidant. Cell test results and polarization modeling showed that the polarization losses were dominated by the ohmic loss associated with the electrodes and the activation polarization of the cathode, and that the ohmic loss due to the ionic resistance of the electrolyte and the activation polarization of the anode were relatively insignificant. Ohmic resistance associated with the electrode was lowered by improving the electrical contact between the electrode and the current collector. Activation polarization of the cathode was reduced by the improvement of the microstructure of the cathode active layer and lowering the cell sintering temperature. The cell performance was further improved by increasing the porosity in the anode. As a result, the maximum power density of 1.5 W cm⁻² was achieved at 800 °C with humidified hydrogen and air.