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Challenges and Opportunities in SOFC technology
In the past several years, SOFC technology has received much attention in the developed countries like Japan and USA and in Europe. Development effort in this area has expanded significantly as evident by the increase in number of conferences and publications in SOFC. The widening interest in this technology arises in part, from the continuing need to develop cleaner and more efficient means of converting energy sources into useful forms. Recent advances in ceramic technology, especially in synthesizing fine powders, engineering material compositions, tailoring composition/property relationship, and processing intricate structures have also contributed to the increased interest in SOFC.
But the SOFC technology, at present, is still in its development stage, and commercial production of fuel cells are yet to be up to expectation for a variety of reasons. Several technical challenges remains to be resolved before the fuel cell becomes a full –fledged, practical power system. Hence, in order to implement a cost efficient commercialization, it is crucial for India to identify its position in the SOFC research.
Materials Selection and Processing
An SOFC essentially consists of two porous electrodes separated by a dense, oxide ion conducting electrolyte. The operating principle of such a cell is already illustrated in Figure 1. 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 hydrogen (and/or carbon monoxide) in the fuel to form water (and/or carbon dioxide), liberating electrons. Electrons (electricity) flow from the anode through the external circuit to the cathode.
The materials for the cell components are selected based on suitable electrical conducting properties required of these components to perform their intended cell functions; adequate chemical and structural stability at high temperatures encountered during cell operation as well as during cell fabrication; minimal reactivity and interdiffusion among different components; and matching thermal expansion among different components.
Although the operating concept of SOFCs is rather simple, the selection of materials for the individual components presents enormous challenges. Each material must have the electrical properties required to perform its function in the cell. There must be enough chemical and structural stability to endure fabrication and operation at high temperatures. The fuel cell needs to run at high temperatures in order to achieve sufficiently high current densities and power output; operation at up to 1000 oC is possible using the most common electrolyte material, yttria-stabilized zirconia (YSZ). Reactivity and interdiffusion between the components must be as low as possible. The thermal expansion coefficients of the components must be as close to one another as possible in order to minimize thermal stresses which could lead to cracking and mechanical failure. The air side of the cell must operate in an oxidizing atmosphere and the fuel side must operate in a reducing atmosphere. The temperature and atmosphere requirements drive the materials selection for all the other components.
In order for SOFCs to reach their commercial potential, the materials and processing must also be cost-effective. Initially, the first successful demonstration of SOFC used platinum as both the cathode and anode, but fortunately less expensive alternatives are available today.
Cathode
The cathode must meet all the above requirements and be porous in order to allow oxygen molecules to reach the electrode/electrolyte interface. In some designs (e.g. tubular) the cathode contributes over 90% of the cell’s weight and therefore provides structural support for the cell.
Today the most commonly used cathode material is lanthanum manganite (LaMnO3), a p-type perovskite. Typically, it is doped with rare earth elements (eg. Sr, Ce, Pr) to enhance its conductivity. Most often it is doped with strontium and referred to as LSM
(La1-xSrxMnO3). The conductivity of these perovskites is all electronic (no ionic conductivity), a desirable feature since the electrons from the open circuit flow back through the cell via the cathode to reduce the oxygen molecules, forcing the oxygen ions through the electrolyte. In addition to being compatible with YSZ electrolytes, it has the further advantage of having adequate functionality at intermediate fuel cell temperatures (about 700 oC), allowing it to be used with alternative electrolyte compositions. Any reduction in operating temperature reduces operating costs and expands the materials selection, creating an opportunity for additional cost savings.
Fabrication of LSM depends on cell design. For example, at Siemens Westinghouse, USA, a tubular cell design is being developed [1]. The cell is constructed by extruding a cathode tube and building the rest of the cell around it. At NexTech Materials,USA, where several planar cell designs are being investigated, the cathode is designed as the bottom supporting layer, and fabricated with tape casting techniques using nanoscale particles [2]. In both cases, the challenge is to sinter the cathode adequately, often by co-sintering with the other components, while maintaining sufficient interconnected porosity.
Electrolyte
Once the molecular oxygen has been converted to oxygen ions it must migrate through the electrolyte to the fuel side of the cell. In order for such migration to occur, the electrolyte must possess a high ionic conductivity and no electrical conductivity. It must be fully dense to prevent short circuiting of reacting gases through it and it should also be as thin as possible to minimize resistive losses in the cell. As with the other materials, it must be chemically, thermally, and structurally stable across a wide temperature range.
There are several candidate materials: YSZ, doped cerium oxide, and doped bismuth oxide. Of these, the first two are the most promising. Bismuth oxide-based materials have a high oxygen ion conductivity and lower operating temperature (less than 800 C), but do not offer enough crystalline stability at high temperature to be broadly useful [3]. Although it has been found by several researchers that LaGaO3 based oxides and gadolinium doped ceria (GDC) exhibits high oxide ion conductivity to be useful as electrolytes in intermediate temperature SOFCs, development of other electrolyte materials will increase the flexibility of SOFC. It should also be ensured that the electrolyte materials do not react with the cathode materials to produce undesirable insulating phases at the SOFC operating temperature.
YSZ has emerged as the most suitable electrolyte material. Yttria serves the dual purpose of stabilizing zirconia into the cubic structure at high temperatures and also providing oxygen vacancies at the rate of one vacancy per mole of dopant. A typical dopant level is 10 mol% yttria [1].
A thin, dense film of electrolyte (< 40 microns thick) needs to be applied to the cathode substrate. Several methods currently used for thin electrolyte film deposition vary greatly with respect to methodology and result. These methods include the physical vapor deposition (PVD) such as sputtering, pulsed laser deposition, molecular beam epitaxy (MBE), chemical vapor deposition (CVD) or electrochemical vapor deposition methods (EVD), combustion chemical vapor deposition (CCVD) and plasma technologies. However, the PVD and CVD deposition methods ordinarily require sophisticated and expensive equipment, making them either undesirable or impracticable for implementation in manufacturing environment. The electrochemical vapor deposition (EVD) method, for example, offers high purity and a high level of process control and enables depositing a dense film onto a porous substrate. In this process, oxygen is passed through the inside of the cathode tube while chlorides of zirconium and yttrium are passed along the outside. They react at the tube surface to form YSZ and, because the reaction comes to the surface from both sides, the porosity is closed off. Once the porosity is closed off, the electrolyte deposition continues, but now the oxygen diffuses through the growing YSZ layer to react with the chlorides, thereby ensuring a highly dense electrolyte layer. The process, while effective, is expensive and capital-intensive [1]. Hence, development of techniques for deposition of thin films of these solid electrolytes at low cost still remains to be a challenge to the development of commercial SOFCs.
Alternative electrolyte deposition methods that show promise are spray coating and dip coating followed by sintering. Colloidal suspensions of YSZ are applied in thin layers of at least 20 microns, using nanosize (5-10 nm) particles in order to meet the critical requirement of low porosity. Recently, we demonstrated deposition of thin and dense YSZ electrolyte of about 10 micron on NiO-YSZ substrate for SOFC application, by electrophoretic deposition [4]. Since these colloidal routes are low cost, it is thought that through careful engineering of the particle size distribution and dispersions, these deposition methods are likely to replace electrochemical deposition [3].
Cerium oxide has also been considered as a possible electrolyte. Its advantage is that it has high ionic conductivity in air but can operate effectively at much lower temperatures (under 700 oC); this temperature range significantly broadens the choice of materials for the other components, which can be made of much less expensive and more readily available materials. The problem is that this electrolyte is susceptible to reduction on the anode (fuel) side. At low operating temperatures (500-700 C) grain boundary resistance is a significant impediment to ionic conductivity. Efforts must be directed to develop compositions which address these problems [5].
Anode
The anode (the fuel electrode) must meet most of the same requirements as the cathode for electrical conductivity, thermal expansion compatibility and porosity, and must function in a reducing atmosphere. The reducing conditions combined with electrical conductivity requirements make metals attractive candidate materials.
Most development has focused on nickel owing to its abundance and affordability. However, its thermal expansion (13.3 x 10-6/ oC compared with 10 x 10-6/ oC for YSZ) is too high to pair it in pure form with YSZ; moreover, it tends to sinter and close off its porosity at operation temperatures. These problems have been solved by making the anode out of a Ni-YSZ composite. The YSZ provides structural support for separated Ni particles, preventing them from sintering together while matching the thermal expansions. Adhesion of the anode to the electrolyte is also improved [1].
Anodes are applied to the fuel cell through powder technology processes. Either a slurry of Ni is applied over the cell and then YSZ is deposited by electrochemical vapor deposition, or a Ni-YSZ slurry is applied and sintered. More recently NiO-YSZ slurries have been used, the NiO being reduced to particulate Ni in the firing process. In order to maintain porosity, pore formers such as starch, carbon, or thermosetting resins are added. These burn out during firing and leave pores behind.
There are problems with this approach. First, the process tends to form tortuous porosity pathways that reduce the transport efficiency of reacting gasses through the anode. Second, there is an increased likelihood of cracking on firing because of the thinness of the interior solid structure left behind. Third, there are environmental issues associated with the burning of the pore formers.
For these reasons, recent research is investigating the possibility of a freeze-drying approach to forming porous structures without the use of fillers. The slurry is applied through a simple dipping process and then freeze-dried; the resulting ice is then sublimed out of the unfired structure. The resulting pore structure, neatly aligned because of the way water crystallizes, allows efficient flow of gases to and from the electrolyte/anode interface. The fineness of the pore structure is easily controlled by adjusting the solids content (and therefore water content) of the slurry [6].
Other choices of material are under investigation as well. Although Ni-YSZ is currently the anode material of choice and the freeze-drying process solves most of the associated problems, nickel still has a disadvantage: it catalyzes the formation of graphite from hydrocarbons. The deposition of graphite residues on the interior surfaces of the anode reduces its usefulness by destroying one of the main advantages of SOFCs, namely their ability to use unreformed fuel sources.
Cu-cerium oxide anodes are being studied as a possible alternative. Copper is an excellent electrical conductor but a poor catalyst of hydrocarbons; cerium oxide is used as the matrix in part because of its high activity of hydrocarbon oxidation. A composite of the two thus has the advantage of being compatible with cerium oxide electrolyte fuel cells. Initial results using a wide range of hydrocarbon fuels are promising [7].
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 a SOFC based on YSZ electrolyte, and 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 chromite is also being considered because it has better thermal expansion compatibility, especially in reducing atmospheres [8]. Interconnects are presently applied to the anode by expensive techniques like plasma spraying and then the entire cell is co-fired.
Any reduction in component costs (either raw materials or processing) directly translates into improved energy affordability. Presently, the SOFCs are designed to operate at high temperature (800 to 1000C) which imposes stringent requirements on materials that significantly increase the cost of SOFC technology. Materials with high thermal stability and corrosion resistance, such as doped LaCrO3 and sometimes metallic Ni, Fe alloys are also used as interconnect materials. The strong economic incentive to use traditional metals for the interconnect is driving the development of intermediate and low temperature SOFCs. Reducing the operating temperature to below 800 oC can reduce degradation of cell components, improve flexibility in cell design, and lower material and manufacturing cost by use of cheap and readily available materials. At operating temperatures in the 900-1000 oC range, interconnects made of such nickel base alloys as Inconel 600 are possible [9]. At or below 800 oC, ferritic steels can be used. At even lower temperatures (below 700 oC), it becomes possible to use stainless steels, which are comparatively inexpensive and readily available [5]. However, the electrolyte conductivity and electrode kinetics drop significantly with lowered operating temperature. This can be overcome by lowering the electrolyte resistance (i.e. ohmic losses across the electrolyte) either by decreasing the electrolyte thickness or using alternative materials of higher ionic conductivity at lower temperatures.
References
1. S.C. Singhal, “Science and technology of solid oxide fuel cels” MRS Bulletin, vol.25, No.3, 2000, p.16
2. NexTech Materials, http://www.nextechmaterials.com].
3. Jian-Hwa Liou, Po-Jou Liou, Tzer –Shin Sheu, “Physical properties and crystal chemistry of Bismuth oxide solutions,” Processing and characterization of electrochemical materials and devices. Proc. Symp Indianapolis, 2 April 1999, Ceram Trans. 1999, pp.3-10
4. L. Besra, S. Zha and M. Liu. Preparation of NiO-YSZ/YSZ Bi-layers for Solid Oxide Fuel Cells by Electrophoretic Deposition. J. Am Ceram.Soc. 160, 2006, pp. 207-214
5. J.M. Ralph, J.A. Kilner, B.C.H. Steele, “Improving Gd-doped ceria electrolyte for low temperature solid oxide fuel cells,” New Materials for batteries and fuel cells. Proc. Symp. San Francisco, 5-8 April 1999, pp.309
6. J.W.Moon, H-J. Hwang, M. Awano, K.Maeda, “Preparation of NiO-YSZ tubular support with radially aligned pore channels,” Materials letters, vol.57, No.8, 2003, pp.1428-1434
7. S. Park, John M. Vohs, Raymond J. Gorte, “Direct Oxidation of Hydrocarbons in a Solid-Oxide Fuel Cell,” Nature, Vol. 404, 16 March 2000, pp. 265-267.
8. Yeong-Shyung Chou, T. R. Armstrong, “Lattice Expansion Induced Stresses in Calcium-doped Yttrium Chromite Interconnect Materials under Reducing Environment,” Processing and Characterization of Electrochemical Materials and Devices. Proc. Symp. Indianapolis, 25-28 April 1999, pp 95-104. Ceram. Trans. 109.
9. Y. Matsuzaki, I. Yasuda, “Dependence of SOFC Cathode Degradation by Chromium-containing Alloy on Compositions of Electrodes,” Journal of the Electrochemical Society (USA), vol. 148, no. 2, Feb. 200, pp A126-A131,.
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September 25, 2009
How Does a Fuel Cell Work?
Figure 1 shows schematically how a solid oxide fuel cell works. The cell is constructed with two porous electrodes which sandwich an electrolyte. Air flows along the cathode (which is therefore also called the “air electrode”). When an oxygen molecule contacts the cathode/electrolyte interface, it catalytically acquires four electrons from the cathode and splits into two oxygen ions. The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter the anode (also called the “fuel electrode”). The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and — most importantly — electrons. The electrons transport through the anode to the external circuit and back to the cathode, providing a source of useful electrical energy in an external circuit.
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Figure 1. Operating concept of a SOFC (from: http://www.seca.doe.gov)
Two possible design configurations for SOFCs have emerged: a planar design (Figure 2) and a tubular design (Figure 3). In the planar design, the components are assembled in flat stacks, with air and fuel flowing through channels built into the cathode and anode. In the tubular design, components are assembled in the form of a hollow tube, with the cell constructed in layers around a tubular cathode; air flows through the inside of the tube and fuel flows around the exterior.
Figure 2. Configuration for a planar design SOFC (from: http://www.spice.or.jp/~fisher/sofc.html#descript)
Figure 3. Configuration for a tubular design SOFC (from: http://www.pg.siemens.com/en/fuelcells/sofc/tubular/index.cfm)
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September 24, 2009
Solid oxide fuel cell (SOFC)
Solid oxide fuel cell (SOFC)
Ceramic fuel cells or the solid oxide fuel cells (SOFC) is an all-solid-state energy conversion device that produces electricity by electrochemically combining fuel and oxidant gases across an ionic conducting ceramic. It consist of two electrodes (the anode and the cathode) separated by a solid electrolyte similar to the one shown in Fig 1. Fuel is fed to the anode, undergoes an oxidation reaction, and releases electrons to the external circuit. Oxidant is fed to the cathode, accepts electrons from the external circuit, and undergoes a reduction reaction. The electron flow (from the anode to the cathode) produces direct-current (DC) electricity. The solid electrolyte conducts ions between the two electrodes.
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Fig 4. Schematic diagram of reactions in SOFCs based on oxygen ion conductors.
SOFCs are among the several fuel cell technologies being developed for a broad spectrum of electric power generation applications. The key chacracteristics of this type of fuel cell is its ceramic electrolyte. The use of a solid electrolyte eliminates material corrosion and electrolyte management problems and permits unique cell design with performance improvement. The conductivity requirement for the ceramic electrolyte necessitates high operating temperatures (600 oC to 1000 oC). High operating temperature promotes rapid reaction kinetics, allowing reforming of hydrocabon fuels within the fuel cell, and produce high quality byproduct heat suitable for use in cogeneration or bottoming cycles. On the other hand, high operating temperatures impose stringent material and processing requirements. The present key technological challenges facing ceramic fuel cells is the development of a suitable material and fabrication processes to incorporate materials into required structures.
Two types of SOFCs are possible depending on the type of ion conducting electrolytes: (i) those based on oxygen ion conducting, or (ii) those based on proton conducting electrolytes. Fig 4 shows the reactions in an oxygen ion conductor SOFC.
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Molten carbonate fuel cells (MCFC)
Molten carbonate fuel cells (MCFC)
The electrolyte in a MCFC is an alkaline mixture of lithium and potassium carbonates, which is liquid at the operating temperature of 650ºC and is supported by a ceramic matrix. Both electrodes are nickelbased and a bipolar plate, which may also act as the current collector, is also required. The operation of an MCFC is fundamentally different to that of other fuel cells, involving net carbonate ion transfer across the electrolyte. This makes it uniquely tolerant to both CO and CO2, but the latter must be reintroduced to the cell with the oxidant. Hydrocarbon fuels, including coal-derived fuel-gas, may be reformed directly at the anode and an external reformer is not necessarily required, but sulphur tolerance remains a problem. The recycling of anode gas to the cathode places a limit of 5ppm on the sulphur content of fuel with current technology. The Molten Carbonate fuel cell (MCFC) has so far found almost no application in transport fields. However, for high power applications it could be quite an interesting technology and could , in the long term, also find its market in locomotive and ship propulsion units.
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Direct Methanol Fuel Cells (DMFC)
Direct Methanol Fuel Cells (DMFC)
The direct methanol fuel cell is a variant of the PEM fuel cellwhich uses methanol directly without prior reforming. The methanol is converted to carbon dioxide and hydrogen at the anode. The hydrogen then goes on to react with oxygen as in a standard PEM fuel cell.
Anode Reaction: CH3OH+ H2OCO2 + 6H+ + 6e-
Cathode Reaction: 3/2O2 + 6H+ + 6e- 3H2O
Cell Reaction: CH3OH+ 3/2O2CO2 + 2H2O
These cells are expected to operate at around 120°C, which is slightly higher than the standard PEM fuel cell, and give efficiencies of around 40 per cent. One drawback is that the low temperature conversion of methanol to hydrogen and carbon dioxide needs a larger quantity of platinum catalyst than in conventional PEM cells. This increased cost is, however, expected to be more than outweighed by the convenience of using liquid fuel and the ability to function without a reforming unit. The technology behind direct methanol fuel cells is still in the early stages of development but it has been successfully demonstrated powering mobile phones and laptop computers, potential target end uses in future years.
The Direct Methanol fuel cell (DMFC) enables the direct use of methanol as fuel without a sophisticated reformer. However, the problem of methanol crossover needs to be solved. On a world-wide basis this technology is studied by a number of renowned research groups. Due to the direct internal conversion of the fuel the efficiency and the performance densities of a DMFC stack are significantly lower than that of a hydrogen powered PEM fuel cell stack, however, if compared to a complete system (PEM plus external reformer), this technology offers interesting potential. The introduction of this fuel cell type is closely related to the question of fuel since the toxicological characteristics of methanol put a real wide range implementation into question. Due to its similarity to the PEM, the total DMFC system faces the same problems in the area of water and thermal management.
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Phosphoric Acid Fuel Cells (PAFC)
The phosphoric acid fuel cell is currently the most commercially advanced fuel cell technology. As the name suggests, these cells use liquid phosphoric acid as the electrolyte, usually contained in a silicone carbide matrix. Phosphoric acid cells work at slightly higher temperatures than PEM or alkaline fuel cells – around 150 to 200°C, but still require platinum catalysts on the electrodes to promote reactivity. The anode and cathode reactions are the same as those in the PEM fuel cell with the cathode reaction occurring at a faster rate due to the higher operating temperature.
This increased temperature also imparts a slightly higher tolerance to impurities and phosphoric acid cells can function with 1-2 per cent carbon monoxide and a few ppm of sulphur in the reactant streams.
The efficiency of phosphoric acid cells is lower than that of other fuel cell systems, at around 40 per cent, and these systems also take longer to warm up than PEM cells. Despite these drawbacks, there are a number of advantages of this technology including simple construction, stability and low electrolyte volatility. Phosphoric cells have been used to power buses and a number of these units are in operation but it is unlikely that these cells will ever be used in private vehicles. A considerable research effort over the last 20 years has, however, resulted in phosphoric acid cells being successfully developed for stationary applications. There are currently a number of working units with outputs ranging from 0.2-20MW installed around the world providing power to hospitals, schools and small power stations.
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Proton Exchange Membrane fuel cell (PEMFC)
The Proton Exchange Membrane fuel cell (PEMFC) currently realises the highest operating performance densities and it is the technology option mostly used for mobile applications. In difference to the alkaline fuel cell, this fuel cell operates with a proton conducting solid polymer membrane as a solid electrolyte. Its main problems, besides the cost of the membrane is in the balance of plant which includes components such as air-compressors, humidification, water-management and thermal-management. The cost of PEM fuel cell technology is still far above that of an internal combustion engine and the path to a solution is not clearly charted. When selecting fuels for PEM fuel cells it is necessary to consider the fact that it is highly CO sensitive and thus requires complex and sophisticated reformers, if not fuelled by pure hydrogen.
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Alkaline Fuel Cell (AFC)
Alkaline Fuel Cell (AFC)
The Alkaline Fuel Cell (AFC) is a relatively simple device and was the first to be developed. An alkaline electrolyte, such as potassium hydroxide, is used with activated nickel or precious metal electrodes. The electrolyte has excellent electrochemical properties but reacts with carbon oxides, which reduces performance; the cell is usually limited to operation with pure hydrogen and air, with the CO2 removed by a soda lime scrubber.The Alkaline Fuel Cell (AFC) mainly suited for applications which focus on efficiency rather than power density due to its lower activation losses but limited current density, which are at best case values of 400 – 600 mA. The use of a potassium alkaline solution as an electrolyte creates some problems mainly with sealing but generally enables easier system integration due to the fact that cooling can be effected via the electrolyte making an additional cooling water circuit unnecessary and the water generated by the energy conversion process can be transported via the electrolyte. The disadvantages of this technology are primarily its CO2 sensitivity where currently a number of possibilities are studied to solve this problem at acceptable cost.
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Fig 3. Schematic of chemical reactions in different fuel cell types.
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