International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) Volume 4, No. 6, June 2015

ISSN: 2319-4413

Fuel Cells - Solid Oxide Fuel Cell Ramanjeet Kaur, Assistant Professor, Physics Department, R.S.D.College, Firozpur city, Punjab, India

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ABSTRACT The increasing demands of energy in the world require sustainable and highly efficient energy production. Fuel cells are one of the devices that offer many advantages over conventional energy sources. Fuel cell uses hydrogen (or hydrogen-rich fuel) and oxygen to create electricity by an electrochemical process. There are many types of fuel cells out of which Solid Oxide Fuel Cells (SOFC) are preferred over other because of their remarkable properties. Various types of SOFC include Tubular design solid oxide fuel cell, planar design solid oxide fuel cell, Electrolyte-supported and electrode supported cells. The basic components of SOFC are anode, cathode and electrolytes for which large number of materials has been discovered for better properties.

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Keywords Fuel cells, solid oxide fuel cells, anode, cathode, electrolyte, perovskites

1. INTRODUCTION Production and distribution of energy affects all sectors of the global economy. The increasing industrialization of the world requires sustainable, highly efficient energy production. Without a major technology advance, energy production will impact the quality of life on earth. For this reason, the application of the fuel cell technologies may be one of the most important technological advancement of the next decade. A fuel cell is an electrochemical device that produces electricity without combustion by combining hydrogen and oxygen to produce water and heat. The Fuel Cell was first developed by William Grove, a Welsh judge with intense scientific curiosity. In 1839, Grove was experimenting on electrolysis (the process by which water is split into hydrogen and oxygen by an electric current), when he observed that combining the same elements could also produce an electric current. Other scientists paid sporadic attention to fuel cells throughout the 19th century.

2. ADVANTAGES OVER CONVENTIONAL ENERGY SOURCES

Fuel cells produce energy through electrochemical conversion of the fuel. Therefore they produce zero or very low emissions, especially Green House Gases (GHGs) depending on the fuel used. Fuel cells produce power at efficiencies much higher than conventional power systems such as the internal combustion engine. This efficiency contributes to the environmental benefits of the fuel cell. Fuel cells have few moving parts and thus require minimal maintenance, reducing life cycle costs of energy production. Fuel cells operate efficiently at part load and in all size configurations. Fuel cells are modular in design, offering flexibility in size and efficiencies in manufacturing. Fuel cells can be utilized for combined heat and power purposes, further increasing the efficiency of energy production.

3. BASIC WORKING OF FUEL CELLS A fuel cell is a device that uses hydrogen (or hydrogenrich fuel) and oxygen to create electricity by an electrochemical process. A single fuel cell consists of an electrolyte sandwiched between two thin electrodes (a porous anode and cathode). While there are different fuel cell types, all work on the same principle: Hydrogen, or a hydrogen-rich fuel, is fed to the anode where a catalyst separates hydrogen's negatively charged electrons from positively charged ions (protons). At the cathode, oxygen combines with electrons and, in some cases, with species such as protons or water, resulting in water or hydroxide ions, respectively. The electrons from the anode side of the cell cannot pass through the membrane to the positively charged cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This movement of electrons is an electrical current. The amount of power produced by a fuel cell depends upon several factors, such as fuel cell type, cell size, the temperature at which it operates, and the pressure at which the gases are supplied to the cell. Still, a single fuel cell produces enough electricity for only the smallest applications. Therefore, individual fuel cells are typically combined in series into a fuel cell stack. A typical fuel cell stack may consist of hundreds of fuel cells.

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International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) Volume 4, No. 6, June 2015

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electrolyte used accelerate component breakdown and corrosion, decreasing cell life. PAFCs are 85 percent efficient when used for the cogeneration of electricity and heat, but less efficient at generating electricity alone (37 to 42 percent). This is only slightly more efficient than combustion-based power plants, which typically operate at 33 to 35 percent efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell. Fig.1 fuel cell B. Advantages of SOFC: A fuel cell can theoretically produce electrical energy for as long as fuel and oxidant are fed to the porous electrodes, but the degradation or malfunction of some of its components limits the practical life span of a fuel cell. Different fuels can be used, such as hydrogen, ethanol, methanol, or gaseous fossils fuels like natural gas. Solid or liquid fossil fuels need to be gasified first before they can be used as fuel. Oxygen or air can be used as oxidant [1].

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4. TYPES OF FUEL CELLS

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Today five types of fuel cells are commonly known, all named after the employed electrolyte material:  Solid polymer proton conductor fuel cell (SPFC),  Alkaline fuel cell (AFC),  Phosphoric acid fuel cell (PAFC),  Molten carbonate fuel cell (MCFC),  Solid oxide fuel cell (SOFC)

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A. Disadvantages of other fuel cells: 

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The disadvantage of AFC is that it is easily poisoned by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can affect the cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours. This is possibly the most significant obstacle in commercializing this fuel cell technology. The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive

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These cells can reach efficiencies of around 60 per cent .In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80-85 percent. Solid oxide fuel cells (SOFCs) use a hard, nonporous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. Solid oxide fuel cells operate at very high temperatures around 1,000ºC (1,830ºF). High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This allows SOFCs to use gases made from coal. Solid oxide fuel cells (SOFCs), based on an oxide ion conducting electrolyte; offer a clean, lowpollution technology to electrochemically generate electricity at high efficiencies.

Quiet, vibration-free operation of SOFCs also eliminates noise usually associated with conventional power generation systems. In addition, SOFCs offer the possibility of co-generation with gas turbine power systems to enable full exploitation of both electricity and heat, thereby enhancing the efficiency up to approximately 70% [2-6]. Furthermore, because of their high operation temperature (700-1000oC), some hydrocarbon fuels such as natural gas can be reformed within the cell stack eliminating the need for an expensive, external reformer.

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5. SOLID OXIDE FUEL CELLS (SOFC) A. Solid Oxide Fuel Cells (SOFC) Solid oxide fuel cells use a solid ceramic electrolyte instead of a liquid and operate at 800 to 1000°C. In solid oxide fuel cells, negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons. B. Basic principle of SOFC SOFCs essentially consist of two porous electrodes separated by a dense, oxide ion conducting electrolyte. Oxygen supplied at the cathode (air electrode) reacts with incoming electrons from the external circuit to form oxide ions, which migrate to the anode (fuel electrode) through the oxide ion conducting electrolyte. At the anode, oxide ions combine with H2 or CO in the fuel to form H2O (and/or CO2), liberating electrons. Electrons flow from the anode through the external circuit to the cathode. The overall cell reaction is simply oxidation of fuel. A schematic representation of an electrolyte supported solid oxide fuel cell is shown in figure 2. The cathode is typically an oxide that catalyzes the oxygen reduction reaction: 1/2O2 + 2e- ------->O2The anode catalyzes the oxidation of fuel, either hydrogen or reformed hydrocarbons: H2 + O2 --------> H2O

Fig.(2) Schematic representation of an electrolyte supported solid oxide fuel cell

6. TYPICAL DESIGNS OF SOLID OXIDE FUEL CELLS (SOFC) As described above, the voltage obtained from a single cell is about 1V. Therefore, it is necessary to arrange the cells in series and parallel in order to obtain the voltage and output power suitable for commercial use. SOFC has greater flexibility in its cell design compared with PAFC and MCFC because the components are all solid. Today, many developers have proposed many cell designs. Here two typical cell designs are discussed.

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A. Tubular design solid oxide fuel cell In the tubular cells, the cell components are deposited in the form of thin layers on a cathode tube [7-9] which can be fabricated by extrusion and sintering. Electrolyte material can be deposited in the form of about 40 um thick dense layers by electrochemical vapour deposition [10] or by plasma spraying; and by the similar methods anode material can be deposited. The interconnection strip along the length of the cell can be deposited by plasma spraying. Tubular cells perform satisfactorily for extended periods of time under a variety of operating conditions with less than 0.1% per 1000 hours performance degradation and have a power density at 1000oC of about 0.20-0.25 W/cm2. To date, the tubular design has progressed the most and power generation systems of up to 250 kW size have been produced and operated using such cells. Another example of a tubular-type cell is the transverse stripe type tubular cell, which was developed by Mitsubishi Heavy Industries, Ltd. In this type of cell, the single cells are arranged in series on one tube. Therefore, each tube can be regarded as a small stack. This type of cell has several advantages such as easy-sealing and mechanical strength due to its structure B. Planar design solid oxide fuel cell: Even though the tubular SOFCs have progressed the most, their electrical resistance is high, power densities (W/cm2 and W/cm3) low, and manufacturing costs high. The low power density (0.20 to 0.25 W/cm2) makes tubular SOFCs suitable only for stationary power generation and not very attractive for transportation applications. Planar SOFCs, in contrast, are capable of achieving very high power densities of up to about 2 W/cm2. In the planar design, in its most generic version, the cell components are configured as thin, flat plates. The interconnection, which is ribbed on both sides, forms gas flow channels and serves as a bipolar gas separator contacting the anode and the cathode of adjoining cells. The cells are fabricated by low-cost conventional ceramic processing techniques such as tape casting, slurry sintering, screen printing, or by plasma spraying [11]. Different organizations have developed several different variations of the planar design and use different manufacturing processes. However, the problems encountered while using planar design fuel cells are as follows:  The temperature becomes non-uniform, which causes thermal stress and eventually the mechanical breakdown of the cell.  It is difficult to seal the cell.  There is relatively high contact resistance between cells.

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It is necessary to have mechanical strength because the cell at the bottom of the stack has to support the weight of the stacked cells.

Monolithic-type and MOLB-type are considered to be variation of the planar-type cell. C. Electrolyte-supported and electrode supported cell The relative thickness for each component depends on cell structure, electrolyte-supported or electrode supported SOFCs. In terms of processing technique, the electrode-supported cells are more demanding than electrolyte-supported cells. However, electrode supported structures are now more widely used. The main advantage of electrode supported structures is that it provides thinner electrolyte and thus lower electrolyte ohmic resistance, which enables the operation of SOFC at low temperatures, especially for anode supported planar structure [12].

7. COMPONENTS OF SOFC The main components of SOFC are: anode, cathode and electrolyte. These components and their characteristics are discussed below: A. Cathode The cathode is the SOFC electrode where electrochemical reduction of oxygen occurs. For this, the cathode must have:  adequate porosity (approximately 30-40%) to allow oxygen diffusion;  chemical compatibility with the other contacting components (usually the electrolyte and interconnect) under operating conditions;  a thermal expansion coefficient(TEC) matching those of the another components;  chemical and microstructure stability under an oxidizing atmosphere during fabrication and operation;  low cost and relatively simple fabrication procedure;  high catalytic activity for the oxygen reduction reaction;  large TPB;  adhesion to electrolyte surface and  high electronic and ionic conductivity. Lanthanum strontium manganite (LSM), Lanthanum ferrite perovskites(LSF),Cobalt containing perovskites, La1-ySryNiO4+x, (La,Sr)n+1(Fe,Co)nO3n+1 and Ordered Double Perovskites are used as cathode materials.

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B. Electrolyte The electrolyte is the component of the cell responsible for conducting ions between the electrodes, for the separation of the reacting gases and for the internal electronic conduction blocking, forcing the electrons to flow through the external circuit. Without significant ion conduction, no current would pass through the cell and only a potential difference would be detected. There are three types of electrolytes that differ by the ion transport mechanism: anionic, protonic and mixed ionic. However, most of the high temperature fuel cells operate via oxygen ion (O2-) conduction from the air electrode to the fuel electrode. This conduction occurs because of the presence of oxygen ions vacancies, so the crystallites forming the electrolyte must have unoccupied anionic sites. The energy required for the oxide ion migration from one site to the neighboring unoccupied equivalent site must be small. For satisfactory performance, the electrolyte must meet some requirements that limit the choice of the material. These include:  Thermodynamic stability from room temperature to working temperature  Stability in both reducing and oxidizing atmospheres at the working temperature  High ionic conductivity In addition to the properties above, the followings are also important:  Sufficient mechanical strength  Chemical compatibility between electrode materials  Matching of the thermal expansion coefficient with electrode materials Of course, low cost of raw materials, ease in processing, low toxicity, and sufficient quantity of natural resources are important in the practical context. The latter three requirements and the practical requirements are also valid for electrode and interconnect materials. The large number of oxygen ion electrolytes that have been investigated can be grouped into a small number of structure types: fluorite-based systems (doped bismuth oxide, zirconia, ceria, pyrochlore);[13] perovskite and related intergrowth structures (lanthanum gallate, brownmillerites, BiMeVOX); [14] LaMOX and apatites. C. Anode Anode is that electrode where oxidation of fuel takes place by combining with oxygen ions. Material used for anodes should fill the following requirements:  thermodynamic stability under reducing conditions,  sufficient catalytic activity for hydrogen or methane oxidation,  high electronic and ionic conductivity,

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thermal expansion coefficients (TECs) which are compatible to other cell components, no adverse chemical reactions with the electrolyte or interconnect for prolonged operation times at high temperatures and in reducing atmospheres.

When optimizing the anode build up, microstructure or material, one needs to understand the processes that are going on in the anode during operation. Among these processes are fuel gas diffusion, adsorption/desorption of fuel species, electrode reactions and charge transfer. A number of materials like porous Ni/YSZ cermet (YSZ: yttria stabilized zirconia); CeO2 (rare-earth doped) anode; Yttria-doped ceria (YDC) and Samaria-doped ceria (SDC), Ni/doped ceria; doped strontium titanate anodes(SrTiO3) La4Sr8Ti11Mn0.5Ga0.5O37.5, La0.4Sr0.6Ti1-xMnxO3-x ; doped LaCrO3 based materials; perovskite (La1-xSrx)Cr 0.5Mn0.5O3-x and double perovskite Sr2Mg1-xMnxMoO6-x, La0.8Sr0.2Cr0.97V0.03O3 (LSCV)–YSZ composite anode, (Ba/Sr/Ca/La)0.6M xNb1−xO3−δ (M: Mg, Ni, Mn, Cr, Fe, In, Sn) of the tetragonal tungsten bronze structure and mixed ionic-electronic conductors (MIEC) can be used as anode material.

8. CONCLUSION Fuel cells are the devices that help to meet the present need of cheap, conventional, and efficient energy sources. Fuel cell is the electrochemical device that produces electricity without combustion by combining hydrogen and oxygen to produce water and heat. Today five types of fuel cells are commonly known which include solid polymer proton conductor fuel cell (SPFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC). Solid Oxide Fuel Cells (SOFC) are used in preference to other cells because of their advantages over other. The basic components of SOFC are anode, cathode and electrolytes. The materials used for these components include Ni/YSZ cermet, doped ceria, doped strontium titanate, doped LaCrO3 based materials, perovskite, double perovskite and mixed ionic-electronic conductors for anodes; fluorite-based systems, perovskite and related intergrowth structures, LaMOX and apatites for electrolytes; Lanthanum strontium manganite (LSM), Lanthanum ferrite perovskites(LSF), Cobalt containing perovskites and Ordered Double Perovskites for cathode. All have some limitations and so the search for better materials continues.

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[2] S.D. Vora, “Proceedings of the Fourth European Solid Oxide Fuel Cell Forum” A.J. McEvoy (Ed.), Lucerne, Switzerland, vol.2, p. 175, 10–14 July, 2000. [3] J. Sukkel, “Proceedings of the Fourth European Solid Oxide Fuel Cell Forum” A.J. McEvoy (Ed.), Lucerne, Switzerland, vol.2, p. 159,10–14 July, 2000. [4] H. Yokoyama, A. Miyahara, S.E. Veyo, “Proceedings of the Fifth International Symposium on Solid Oxide Fuel Cells (SOFC-V)” U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Aachen,Germany, p. 94, 2–5 June 1997. [5] H. Mori, H. Omura, N. Hisatome, K. Ikeda, K. Tomida, “Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells (SOFC-VI)” S.C. Singhal, M. Dokiya (Eds.), Honolulu, Hawaii, p. 52, 17–22 October 1999. [6] K. Krist, K.J. Gleason, J.D. Wright, “Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells (SOFC-VI)” S.C. Singhal, M. Dokiya (Eds.), Honolulu, Hawaii, p. 107, 17–22 October 1999. [7] Singhal, S. C. and Kendall, K., “Solid Oxide Fuel Cells: Fundamentals, Design and Applications” Elsevier, Oxford, UK, 2003. [8] Singhal, S. C., Materials Research Bulletin, vol.25, no.3, p.16-21, 2000. [9] Singhal, S. C., Solid State Ionics, vol.152-153, p.405-410, 2002. [10] Pal, U. B. and Singhal. S. C., J. Electrochem. Soc., p.2937-41, Vol.137, 1990. [11] Kuo, L. J. H., Vora, S. D., and Singhal, S. C., J. Am. Ceram. Soc., vol.80, p.589-93, 1997. [12] Holtappels, P., Stimming, U., Handbook of fuel cells, Wiley & Sons, Chichester, 2003. [13] Moon, P. K.; Tuller, H., Solid State Ionics, vol.2830, p.470–474, 1988. [14] Abraham, F.; Boivin, J. C.; Mairesse, G.; Nowogrocki, G. Solid State Ionics, vol.40-41, p.934–937, 1990.

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