Solid Oxide Fuel Cell

Submitted by:

Pathan Mohsinkhan (E&TC)

Sachin Kanungo (Instru)

Gangamai College of Engineering, Nagaon

Abstract

Solid oxide fuel cells (SOFCs), which will be operated at reduced temperature, are becoming a frontier of R and D. These compact size SOFCs will fit well with intermittent loads, of which share in energy system is increasing today, whereas the “conventional SOFCs” will be effectively operated with stationary mode. For such compact size SOFCs, throttle down operation following intermittent loads will be profitable because low current density gives higher efficiency. SOFCs are not suitable for quick start up. It was estimated that the hot standby mode would be more acceptable than cold start mode from the viewpoint of heat loss. The merit of internal reforming will also be lost for the reduced operation temperature. Solid Oxide Fuel Cells are becoming the most revolutionary platforms in making a greener earth with low cost electricity production. Within its category SOFC is the cheapest, high temperature working and output efficient. The project contains the explanation of primary elements for SOFC like anode, cathode and electrolyte with 3D diagram and actual working of SOFC. Applications and merits/demerits are concisely discussed. Blooming new technologies within SOFC and improvements after its foundation and the new process stacks are defined. Bloom Energy Server, which is the power plant box containing Solid Oxide Fuel Cells is the revolutionary product in large scale applications is discussed.

Introduction:

Solid oxide fuel cells are a class of fuel cell characterized by the use of a solid oxide material as the electrolyte. In contrast to proton exchange membrane fuel cells (PEMFCs), which conduct positive hydrogen ions (protons) through a polymer electrolyte from the anode to the cathode, the SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the oxygen ions with hydrogen or carbon monoxide thus occurs on the anode side.

They operate at very high temperatures, typically between 500 and 1,000°C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.

Solid oxide fuel cells have a wide variety of applications from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. Theoretical efficiency of a SOFC device can exceed 60 percent. The higher operating temperature make SOFCs suitable candidates for application with heat engine energy recovery devices or combined heat and power, which further increases overall fuel efficiency.

Because of these high temperatures, light hydrocarbon fuels, such as methane, propane and butane can be internally reformed within the anode. SOFCs can also be fueled by externally reforming heavier hydrocarbons, such as gasoline, diesel, jet fuel (JP-8) or biofuels. Such reformates are mixtures of hydrogen, carbon monoxide, carbon dioxide, steam and methane, formed by reacting the hydrocarbon fuels with air or steam in a device upstream of the SOFC anode. SOFC power systems can increase efficiency by using the heat given off by the exothermic electrochemical oxidation within the fuel cell for endothermic steam reforming process.

Thermal expansion demands a uniform and well-regulated heating process at startup. SOFC stacks with planar geometry require on the order of an hour to be heated to light-off temperature. Micro-tubular fuel cell design geometries promise much faster start up times, typically on the order of minutes.

Unlike most other types of fuel cells, SOFCs can have multiple geometries. The planar fuel cell design geometry is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte is sandwiched in between the electrodes. SOFCs can also be made in tubular geometries where either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. The tubular design is advantageous because it is much easier to seal air from the fuel. The performance of the planar design is currently better than the performance of the tubular design however, because the planar design has a lower resistance comparatively. Other geometries of SOFCs include modified planar fuel cell designs (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are highly promising, because they share the advantages of both planar cells (low resistance) and tubular cells.

SOFC technology dominates competing fuel cell technologies because of the ability of SOFCs to use currently available fossil fuels, thus reducing operating costs. Other fuel cell technologies (e.g. molten carbonate, polymer electrolyte, phosphoric acid and alkali) require hydrogen as their fuel. Widespread use of such fuel cells would require a network of hydrogen suppliers, similar to our familiar gas stations.

High efficiency and fuel adaptability are not the only advantages of solid oxide fuel cells. SOFCs are attractive as energy sources because they are clean, reliable, and almost entirely nonpolluting. Because there are no moving parts and the cells are therefore vibration-free, the noise pollution associated with power generation is also eliminated.

WHAT IS SOLID OXIDE FUEL CELL?

A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material and, as the name implies, the SOFC has a solid oxide, or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiencies, long term stability, fuel flexibility, low emissions, and cost. The largest disadvantage is the high operating temperature which results in longer start up times and mechanical/chemical compatibility issues.

Solid oxide fuel cells are a class of fuel cell characterized by the use of a solid oxide material as the electrolyte. In contrast to proton exchange membrane fuel cells (PEMFCs), which conduct positive hydrogen ions (protons) through a polymer electrolyte from the anode to the cathode, the SOFCs use a solid oxide electrolyte to conduct negative oxygen ions from the cathode to the anode. The electrochemical oxidation of the oxygen ions with hydrogen or carbon monoxide thus occurs on the anode side. They operate at very high temperatures, typically between 500 and 1,000°C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells such as PEMFCs, and are not vulnerable to carbon monoxide catalyst poisoning. However, vulnerability to sulfur poisoning has been widely observed and the sulfur must be removed before entering the cell through the use of adsorbent beds or other means.

HOW DOES IT WORK?

A solid oxide fuel cell is made up of four layers, three of which are ceramics (hence the name). A single cell consisting of these four layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then connected in series to form what most people refer to as an "SOFC stack". The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 600 to 1,000°C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again

Anode

The ceramic [anode] layer must be very porous to allow the fuel to flow towards the electrolyte. Like the cathode, it must conduct electrons, with ionic conductivity a definite asset. The most common material used is a cermet made up of nickel mixed with the ceramic material that is used for the electrolyte in that particular cell, typically YSZ (yttria stabilized zirconia). The anode is commonly the thickest and strongest layer in each individual cell, because it has the smallest polarization losses, and is often the layer that provides the mechanical support. Electrochemically speaking, the anode’s job is to use the oxygen ions that diffuse through the electrolyte to oxidize the hydrogen fuel. The oxidation reaction between the oxygen ions and the hydrogen produces heat as well as water and electricity. If the fuel is a hydrocarbon, for example methane, another function of the anode is to act as a catalyst for steam reforming the fuel into hydrogen. This provides another operational benefit to the fuel cell stack because the reforming reaction is endothermic, which cools the stack internally

Electrolyte

The electrolyte is a dense layer of ceramic that conducts oxygen ions. Its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. The high operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. However, as the operating temperature approaches the lower limit for SOFCs at around 873 K, the electrolyte begins to have large ionic transport resistances and affect the performance. Popular electrolyte materials include yttria stabilized zirconia (YSZ) (often the 8% form Y8SZ) and gadolinium doped ceria (GDC) The electrolyte material has crucial influence on the cell performances. Detrimental reactions between YSZ electrolytes and modern cathodes such as LSCF have been found, and can be prevented by thin (<100 nm) ceria diffusion barriers.

If the conductivity for oxygen ions in SOFC can remain high even at lower temperature (current target in research ~773 K), material choice for SOFC will broaden and many existing problems can potentially be solved. Certain processing technique such as thin film deposition can help solve this problem with existing material by

- reducing the traveling distance of oxygen ions and electrolyte resistance as resistance is inversely proportional to conductor length;

- producing grain structures that are less resistive such as columnar grain structure;

- controlling the micro-structural nano-crystalline fine grains to achieve "fine-tuning" of electrical properties;

- Building composite with large interfacial areas as interfaces have shown to have extraordinary electrical properties.

Interconnect

Just as an internal combustion engine relies on several cylinders to provide enough power to be useful, so too must fuel cells be used in combination in order to generate enough voltage and current. This means that the cells need to be connected together and a mechanism for collection of electrical current needs to be provided, hence the need for interconnects. The interconnect functions as the electrical contact to the cathode while protecting it from the reducing atmosphere of the anode.

The high operating temperature of the cells combined with the severe environments means that interconnects must meet the most stringent requirements of all the cell components: 100% electrical conductivity, no porosity (to avoid mixing of fuel and oxygen), thermal expansion compatibility, and inertness with respect to the other fuel cell components. It will be exposed simultaneously to the reducing environment of the anode and the oxidizing atmosphere of the cathode.

For an YSZ SOFC operating at about 1000 C, the material of choice is LaCrO3 doped with a rare earth element (Ca, Mg, Sr, etc.) to improve its conductivity. Ca-doped yttrium chromate is also being considered because it has better thermal expansion compatibility, especially in reducing atmospheres [Chou]. Interconnects are applied to the anode by plasma spraying and then the entire cell is co-fired.

Cathode

The cathode, or air electrode, is a thin porous layer on the electrolyte where oxygen reduction takes place. The overall reaction is written in Kröger-Vink Notation as follows:

Cathode materials must be, at minimum, electronically conductive. Currently, lanthanum strontium manganite (LSM) is the cathode material of choice for commercial use because of its compatibility with doped zirconia electrolytes. Mechanically, it has similar coefficient of thermal

expansion to YSZ and thus limits stresses built up because of CTE mismatch. Unfortunately, LSM is a poor ionic conductor, and so the electrochemically active reaction is limited to the triple phase boundary (TPB) where the electrolyte, air and electrode meet. LSM works well as a cathode at high temperatures, but its performance quickly falls as the operating temperature is lowered below 800°C. In order to increase the reaction zone beyond the TPB, a potential cathode material must be able to conduct both electrons and oxygen ions. Composite cathodes consisting of LSM YSZ have been used to increase this triple phase boundary length. Mixed ionic/electronic conducting (MIEC) ceramics, such as the perovskite LSCF, are also being researched for use in intermediate temperature SOFCs as they are more active and can makeup for the increase in the activation energy of reaction.

The charge carrier in the SOFC is the oxygen ion (O2-). At the cathode, the oxygen molecules from the air are split into oxygen ions with the addition of four electrons. The oxygen ions are conducted through the electrolyte and combine with hydrogen at the anode, releasing four electrons. The electrons travel an external circuit providing electric power and producing by-product heat.

Anode Reaction: 2 H2 + 2 O2- => 2 H2O + 4 e-

Cathode Reaction: O2 + 4 e- => 2 O2-

Overall Cell Reaction: 2 H2 + O2 => 2 H2O

The operating efficiency in generating electricity is among the highest of the fuel cells at about 60%. Furthermore, the high operating temperature allows cogeneration applications to create high-pressure steam that can be used in many applications. Combining a high-temperature fuel cell with a turbine into a hybrid fuel cell further increases the overall efficiency of generating electricity with a potential of an efficiency of more than 70%.

Fig: Solid oxide fuel cell (SOFC) stacks

Stack construction  Stack construction is concerned with the preparatory steps for material-connected joints between the metallic parts of the SOFC (casing, cover, interconnects, end connectors).

Operation

SOFCs operate at extremely high temperatures (600ºC–1000ºC) resulting in a significant time required to reach operating temperature and responding slowly to changes in electricity demand. It is therefore considered to be a leading candidate for high-power applications including industrial and large-scale central-electricity generating-stations.

The very high operating temperature of the SOFC has both advantages and disadvantages. The high temperature enables them to tolerate relatively impure fuels, such as those obtained from the gasification of coal or gasses from industrial process and other sources. However, the high temperatures require more expensive materials of construction.

Solid Oxide Electrolyser Cell:

A solid oxide electrolyser cell (SOEC) is a solid oxide fuel cell set in regenerative mode for the electrolysis of water with a solid oxide, or ceramic, electrolyte to produce oxygen and hydrogen gas. A regenerative fuel cell (mode) or reverse fuel cell (RFC) is a fuel cell run in reverse mode, which consumes electricity and chemical B to produce chemical A.

RESEARCH:

Research is going now in the direction of lower-temperature SOFC (600°C) in order to decrease the materials cost, which will enable the use of metallic materials with better mechanical properties and thermal conductivity.

Research is currently underway to improve the fuel flexibility of SOFCs. While stable operation has been achieved on a variety of hydrocarbon fuels, these cells typically rely on external fuel processing. For the case of natural gas, the fuel is either externally or internally reformed and the sulfur compounds are removed. These processes add to the cost and complexity of SOFC systems. Work is underway at a number of institutions to improve the stability of anode materials for hydrocarbon oxidation and, therefore, relax the requirements for fuel processing and decrease SOFC balance of plant costs.

Research is also going on in reducing start-up time to be able to implement SOFCs in mobile applications. Due to their fuel flexibility they may run on partially reformed diesel, and this makes SOFCs interesting as auxiliary power units (APU) in refrigerated trucks.

Specifically, Delphi Automotive Systems are developing an SOFC that will power auxiliary units in automobiles, while BMW has recently stopped a similar project. A high-temperature SOFC will generate all of the needed electricity to allow the engine to be smaller and more efficient. The SOFC would run on the same gasoline or diesel as the engine and would keep the air conditioning unit and other necessary electrical systems running while the engine shuts off when not needed (e.g., at a stop light).

Rolls-Royce is developing solid-oxide fuel cells produced by screen printing onto inexpensive ceramic materials. Rolls-Royce Fuel Cell Systems Ltd is developing a SOFC gas turbine hybrid system fueled by natural gas for power generation applications on the order of a megawatt (e.g. Futuregen).

Ceres Power Ltd. has developed a low cost and low temperature (500-600 degrees) SOFC stack using cerium gadolinium oxide (CGO) in place of current industry standard ceramic, yttrium stabilized zirconium (YSZ), which allows the use of stainless steel to support the ceramic.

Solid Cell Inc. has developed a unique, low cost cell architecture that combines properties of planar and tubular designs, along with Cr-free cermets interconnect.

APPLICATIONS:

Using planar SOFCs, stationary power generation systems of from 1-kW to 25-kW size have been fabricated and tested by several organizations. Several hundred 1-kW size combined heat and power units for residential application were field tested by Sulzer Hexis; however, their cost and performance degradation was high and stack lifetime too short. With improved sealing materials and sealing concepts, planar SOFC prototype systems in the 1- to 5-kW sizes have recently been developed and are being tested by various organizations with greater success. Using tubular (cylindrical) SOFCs, Siemens/ Westinghouse fabricated a 100-kW atmospheric power generation system (Figure 4). The system was successfully operated for two years in the Netherlands on desulfurized natural gas without any detectable performance degradation. It provided up to 108 kW of ac electricity at an efficiency of 46% to the Dutch grid and approximately 85 kW of hot water for the local district heating system. At the conclusion of the operation in The Netherlands, the system was moved to a German utility site in Essen, Germany,

where it operated successfully for another 4,000 hours. After replacing some cells, the system was then installed and operated in Italy, for over two years, again with very stable performance. Siemens/Westinghouse tubular cells have also been used to fabricate and field test over a dozen 5-kW size combined heat and power units, each about the size of a refrigerator. These units gave excellent performance and performance stability on a variety of hydrocarbon fuels. However, at present, their cost is high; future such units are expected to use higher power density alternate tubular geometry cells to drive down the cost.

Another application of SOFC systems is in the transportation sector. The polymer electrolyte membrane fuel cell is generally regarded as the fuel cell of choice for transportation applications. These fuel cells require pure hydrogen, with no carbon monoxide, as the fuel to operate successfully. However, presently no hydrogen infrastructure exists, and on-board reformer systems to produce hydrogen from existing fuel base (gasoline, diesel) are technically challenging, complex, and expensive. Furthermore, it is difficult to eliminate the carbon monoxide entirely from the reformate stream. In contrast, SOFCs can use carbon monoxide along with hydrogen as fuel, and their higher operating temperature and availability of water on the anode side makes on-cell or in-stack reformation of hydrocarbon fuels feasible. Also, no noble metal catalysts are used in SOFCs reducing cost of the cells. The initial application of SOFCs in the transportation sector will be for on-board auxiliary power units. Such auxiliary power units, operating on existing fuel base, will supply the ever increasing electrical power demands of luxury automobiles, recreational vehicles, and heavy-duty trucks. Delphi Corporation has developed a 5-kW auxiliary power unit using anode-supported planar SOFCs. This unit is intended to operate on gasoline or diesel, which is reformed through catalytic partial oxidation. The building blocks of such an auxiliary power unit consist of an SOFC stack, fuel reformation system, waste energy recovery system, thermal management system, process air supply system, control system, and power electronics and energy storage (battery) system. Delphi has reduced the mass and volume in successive generation auxiliary power units to meet the stringent automotive requirements; the remaining issues of start up time and tolerance to thermal cycling are presently being worked on.

SOFCs are of high interest to the military because they can be established on-site in remote locations, are quiet, and non-polluting. Moreover, the use of fuel cells could significantly reduce deployment costs: 70% by weight of the material that the military moves is nothing but fuel. Stationary installations would be the primary or auxiliary power sources for such facilities as homes, office buildings, industrial sites, ports, and military installations. They are well suited for mini-power-grid applications at places like universities and military bases. SOFC technology is ideal for such an expansion, since much of the anticipated demand is expected to come from growing economies with minimal infrastructure. SOFCs can be positioned on-site, even in remote areas; on-site location makes it possible to match power generation to the electrical demands of the site.

Stationary SOFC power generation is no longer just a hope for the future. Siemens Westinghouse has tested several prototype tubular systems, with excellent results. A plant in the Netherlands has been operational for two years and an earlier prototype installation has been operating for 8 years. The fuel cells have been through over 100 thermal cycles and the voltage degradation during the test time has been minimal less than 0.1%/thousand hours. Siemens Westinghouse expects to have its first fully operational tubular fuel cell plant in place by October 2003 [Siemens]. Meanwhile, in Australia, Ceramic Fuel Cells, Ltd. has been operating prototype planar fuel cell plants since 2001 and expects to be ready with market-entry products in 2003 [CFCL].

In the transportation sector, SOFCs are likely to find applications in both trucks and automobiles. In diesel trucks, they will probably be used as auxiliary power units to run electrical systems like air conditioning and on-board electronics. Such units would preclude the need to leave diesel trucks running at rest stops, thereby leading to a savings in diesel fuel expenditures and a significant reduction in both diesel exhaust and truck noise. Meanwhile, automobile manufacturers have invested at least $4.5 billion in fuel cell research (not all SOFC) [ceramic]. There are an estimated 600 million vehicles worldwide, 75% of which are personal automobiles, and the number is expected to grow by 30% in the next 10 years [SECA]. With more stringent environmental restrictions in the United States and European Union, automobile manufacturers

are under growing time pressure to bring non-polluting cars to the marketplace. SOFCs are attractive prospects because of their ability to use readily available, inexpensive fuels.

Bloom Energy Server:The Bloom Energy Server is a solid oxide fuel cell made by Bloom Energy, of

Sunnyvale, California, that uses liquid or gaseous hydrocarbons (such as gasoline, diesel or propane produced from fossil or bio sources) to generate electricity on the site where it will be used; Bloom Energy representatives assert that it is at least as efficient as a traditional large-scale coal power station. According to the company, a single cell (one 100mm × 100mm metal alloy plate between two ceramic layers) generates 25 watts.

Fig: Bloom Energy Server

The Bloom Energy Server uses thin white ceramic plates (100mm × 100mm) which are made from sintered modified zirconia (an oxide derived from common beach sand). Each ceramic plate is coated with a green Nickel oxide -based ink on one side (anode) and another black (probably Lanthanum strontium manganite) ink on the other side (cathode). According to the San Jose Mercury News, "Bloom's secret technology apparently lies in the proprietary green ink that acts as the anode and the black ink that acts as the cathode--" but in fact these materials are widely known in the field of solid oxide fuel cells. Wired reports that the secret ingredient may be yttria-stabilized zirconia based upon a 2006 patent filing (7,572,530) that was granted to Bloom in 2009; but this material is also one of the most common elecrolyte materials in the field. To save money, the Bloom Energy Server uses inexpensive metal alloy plates for electric conductance between the two ceramic fast ion conductor plates. In lower temperature fuel cells, platinum is required at the cathode.

Advantages of Solid Oxide Fuel Cells

One advantage of SOFC is that hydrogen and carbon dioxide are used as fuel in the cell. This means that SOFC can use many common hydrocarbon fuels such as natural gas, diesel, gasoline and alcohol without the need to reform the fuel into pure hydrogen. In other fuel cells, such as the polymer electrolyte fuel cell, which are fueled with pure hydrogen, the carbon dioxide is a poison.

SOFC have a potentially lower cost due to the absence of precious metals, compared to proton exchange membrane and phosphoric acid fuel cells which use platinum as a catalyst. Some other fuel cell types use liquid electrolytes, similar to battery acid that can have a corrosive effect on components. Since SOFC use one piece solid state ceramic cells, they are easier to maintain due to the lack of this corrosion.

Disadvantages of Solid Oxide Fuel Cell

A major disadvantage of the SOFC, as a result of the high heat, is that it “places considerable constraints on the materials which can be used for interconnections”. Another disadvantage of running the cell at such a high temperature is that other unwanted reactions may occur inside the fuel cell. It is common for carbon dust, graphite, to build up on the anode, preventing the fuel from reaching the catalyst.

ConclusionForty years have passed since the first successful demonstration of a solid oxide fuel cell.

Through ingenuity, materials science, extensive research, and commitment to developing alternative energy sources, that seed of an idea has germinated and is about to bloom into a viable, robust energy alternative. Materials development will certainly continue to make SOFCs increasingly affordable, efficient, and reliable.

SummaryThe challenge in successfully commercializing SOFCs offering high power densities and

long term durability requires reduction of costs associated with the cells and the balance-of-plant. Additionally, for transportation auxiliary power unit applications, ability for rapid start up and thermal cycling needs to be developed.

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