Abstract:
A system and a method for suppressing the build up of metal carbonates in the electrolyte, using a porous cell separator is used to allow the use of different electrolyte compositions around the anode (anolyte) and the cathode (catholyte). This cell configuration enables the oxygen cathode to operate in a molten hydroxide electrolyte, and the carbon anode to operate in mixed carbonate-hydroxide melt, so that most of the advantages of using a molten hydroxide electrolyte will be retained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Provisional Patent Application Ser. No. 60/581,387, filed Jun. 22, 2004, entitled A MEANS OF STABILIZING THE ELECTROLYTE IN A DIRECT CARBON-AIR FUEL CELL BASED ON A MOLTEN METAL HYDROXIDE ELECTROLYTE, the teaching of which are expressly incorporated herein by reference. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND 
     The present invention relates in general to a carbon-air fuel cell, and more particularly, to a system and a method for stabilizing a molten metal hydroxide electrolyte in a direct carbon-air fuel cell. 
     The use of a molten metal hydroxide as an electrolyte in a direct carbon-air fuel cell has several distinct advantages over the use of a molten metal carbonate. These advantages include higher electrical conductivity, higher electrochemical activity of carbon and oxygen electrodes, lower operating temperatures (500° C. versus 750°) and consequently use of less expensive materials for cell container. Thus, an inexpensive ultra-low carbon steel material can be used to fabricate the cell container. Furthermore, when this material is doped with 1-2% titanium, a surface oxide containing a degenerative semiconductor with stable electrical conductivity and enhanced corrosion stability is formed. Moreover, this material was found to possess excellent catalytic properties for oxygen reduction. Therefore, it can be used to fabricate both cell containers and cathodes. At the lower operating temperature of the molten metal hydroxide, the dominant product of carbon oxidation is CO 2 . This allows four electrons to be exchanged per one carbon atom. In a molten carbonate system at temperature higher than 750° C., the dominant product is CO where only two electrons are exchanged per one carbon atom. This allows the fuel cell using molten metal hydroxides to have a higher operating voltage than a fuel cell using molten metal carbonate electrolytes. Furthermore, in a molten hydroxide electrolyte the electrochemical activity of the oxygen/air cathode is exceptionally high. That is not the case with molten carbonate electrolyte. This enables a very simple cell design in which the oxygen electrode is not a complex high surface area structure as is used in traditional fuel cell designs. Thus, a solid carbon rod or plate immersed into molten metal hydroxide electrolyte can serve as the anode fuel. The carbon fuel can also be in form of chunks and other particulate type of carbon material. The molten hydroxide is contained in the cell container which can also act as the air cathode. The shape of the cell container can be either cylindrical or prismatic. The air cathode is fed with oxygen by introducing air into the molten electrolyte via a gas bubbler located at the bottom of the cell. The simple cell design of the carbon-air fuel cell with a molten hydroxide electrolyte makes the cost of this cell substantially lower than the cost of the cell with a carbonate electrolyte. 
     However, these advantages have, in the past come at a price, primarily due to the lack of invariance of the molten hydroxide electrolyte caused by its reaction with carbon dioxide produced at the anode resulting in the formation of carbonate salt. The carbonate salt adversely affects the cell operation and in the course of time lessens its efficiency. 
     A need therefore exists for a system and a method that reduces or eliminates the carbonization of a molten metal hydroxide electrolyte in a carbon-air fuel cell. 
     BRIEF SUMMARY 
     A system and a method for suppressing the build up of metal carbonates in the electrolyte are provided. A porous cell separator is used to allow the use of different electrolyte compositions around the anode (anolyte) and the cathode (catholyte). This cell configuration enables the oxygen cathode to operate in a molten hydroxide electrolyte, and the carbon anode to operate in mixed carbonate-hydroxide melt, so that most of the advantages of using a molten hydroxide electrolyte will be retained. In brief, oxygen is reduced at the cathode in a hydroxide environment producing hydroxyl ions. Hydroxyl ions thus formed are transported from the catholyte through a porous cell separator into the anolyte where they react with carbon dioxide produced at the anode, thus forming carbonate ions. The reaction between carbon dioxide and hydroxyl ions leads to an increase in carbonate salt concentration in the anolyte. When the carbonate salt concentration reaches a certain level, carbonate ions start taking part in the anodic oxidation of carbon producing gaseous carbon dioxide that escapes the molten electrolyte thus preventing further carbonate salt build up, rendering the electrolyte invariant. 
     The two-compartment cell design using a porous cell diaphragm separator allows the compositions of the anolyte and the catholyte to be different from each other; and consequently, the hydroxyl ions in the catholyte from being reacted with carbon dioxide, and, once the anolyte composition has come to an equilibrium the carbonization of the electrolyte at the carbon anode is effectively suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
         FIG. 1  shows a first embodiment of a direct carbon-air fuel cell based on a molten metal hydroxide electrolyte; 
         FIG. 2  shows a second embodiment of a direct carbon-air fuel cell based on a molten metal hydroxide electrolyte; and 
         FIG. 3  shows a third embodiment of a direct carbon-air fuel cell based on a molten metal hydroxide electrolyte. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a first embodiment of a direct carbon-air fuel cell based on a molten metal hydroxide electrolyte. As shown, the direct carbon-air fuel cell includes a carbon anode  12 , an oxygen cathode  13 , and a porous separator  11  placed between the carbon anode  12  and the oxygen cathode  13 . By placing the porous separator  11  between the carbon anode  12  and the oxygen cathode  13 , the cell is divided into two compartments  14  and  15 , in which the electrolyte near the carbon anode  12 , that is, the anolyte  14 , is separated from the electrolyte near the oxygen cathode, namely, the catholyte  15 . To allow a certain type of ions to transport through the separator  11 , the separator  11  includes a plurality of small pores and is preferably fabricated from porous ceramic, metal, glass material or a combination thereof. The pores of the separator  11  are formed so small that the dominant means of ions transport through the separator  11  is migration under the influence of an electric field between the carbon anode  12  and the oxygen cathode  13 , while other means of ion transport such as diffusion and convection are hindered by the small pore size. This enables the compositions of the anolyte in the compartment  14  and the catholyte in the compartment  15  to be different. Thus, under the influence of an electric field between the carbon anode  12  and the oxygen cathode  13 , negatively charged ions such as OH −  and CO 3   2−  are transported from the cathode  13  towards the carbon anode  12 , and positively charged ions such as Na + , Li +  and K +  are transported from the anolyte in the compartment  14  towards the catholyte in the compartment  15 . Since the carbonate ions cannot be transported in the opposite direction, the carbonization of the catholyte does not take place in the compartment  15 . This means that the cathodic reduction of oxygen takes place in a hydroxide electrolyte environment. An oxygen containing gas, either dry or humid, is introduced into the catholyte compartment  15  and its reduction at the cathode  13  takes place according to the equation:
 
O 2 +2H 2 O+4 e   − →4OH −   (1).
 
     The hydroxyl ions formed according Equation (1) migrate through the porous separator  11  into the anolyte compartment  14  leaving the composition of the catholyte unchanged. In the anolyte compartment  14 , the hydroxyl ions react at the carbon anode  12  according to the equation:
 
C+6OH − →CO 3   2− +3H 2 O+4 e   −   (2)
 
     The concentration of carbonate ions formed according to Equation (2) increases over time and when it reaches a certain level, carbonate ions start reacting at the carbon anode  12  according to the equation:
 
C+2CO 3   2− →3CO 2 +4 e   −   (3)
 
     The increase in concentration of carbonate ions is stopped by the reaction according to Equation (3); and subsequently, gaseous carbon dioxide is released. Thus, anodic carbon oxidation takes place in a mixed carbonate-hydroxide electrolyte and therefore the performance of the carbon anode  12  is better than in pure carbonate melts. 
     The catholyte in the compartment  15  comprises metal hydroxides. A single metal hydroxide or a combination of metal hydroxides may be used, in particular, mixtures of low melting alkali and/or alkaline earth hydroxides are preferably used. Commonly used hydroxides are eutectic mixtures of LiOH, KOH, and/or NaOH. 
     The anolyte in the compartment  14  may also comprise metal hydroxides as the catholyte does since hydroxides will convert into carbonates up to a certain level during the course of the cell operation. Alternatively, the anolyte can initially comprise metal carbonates since hydroxyl ions are transported through the porous separator  11  to the anode compartment  14 . A single metal carbonate or a combination of metal carbonates may be used at cell startup, in particular, mixtures of low melting alkali and/or alkaline earth carbonates can be preferably used. Commonly used carbonates are eutectic mixtures of Li 2 CO 3 , Na 2 CO 3  and/or K 2 CO 3  that should match with the mixtures of alkali and/or alkaline earth hydroxide in the catholyte. 
     The porous separator  11  can comprise a porous planar plate, porous tubular plate or other similar structures. The porous separator  11  is constructed to be capable of transporting hydroxyl and metal ions between the anode compartment  14  and the cathode compartment  15 . The separator  11  can comprise a non-reactive metal oxide such as ZrO 2 , Al 2 O 3 , LiAlO 2 , MgO, mullite, kaolite, rare earth oxides or other similar materials. Corrosion resistant metals such as nickel and its alloys, stainless steels, titanium doped mild steel and similar materials can also be used to fabricate the porous separator  11 . 
     The oxygen cathode  13  in this embodiment serves also as the cell housing/container as well as the cathode current collector. The cathode  13  can be selected from any non porous, electrically conducting material which is chemically stable in molten hydroxide electrolyte and electrochemically active for oxygen reduction, such as mild steel doped with titanium, nickel doped with titanium, stainless steel, Inconel 600, Permalloy 80, and other nickel based alloys. In order to increase the cathode surface area the cell housing/cathode  13  may have a two layer structure in which the inner layer facing the molten electrolyte is made in form of a mesh, grid, felt, screen, sintered frit or other similar structure. The cathode  13  is in contact with an oxygen-containing gas while concurrently in contact with molten hydroxide in the compartment  15 . The combination of a molten hydroxide in the presence of gaseous oxygen creates a very corrosive environment and thus the cathode  13  can beneficially contain a corrosion resistant and electrically conducting metal such as nickel, stainless steel, a corrosion resistant alloy, conductive oxides, such as NiO, LiCoO 2 , LiFeO 2 , Li x Ni 1-x O or other similar material. In the two layer-structure of the cathode  13  the outer layer is nonporous and is made of corrosion resistant materials such as mild steel doped with titanium, nickel doped with titanium, stainless steel, Inconel 600, Permalloy 80, and other nickel based alloys the same material as the inner layer. 
     An oxygen-containing gas is introduced to the cell through the bubbler element at the cell bottom. The oxygen-containing gas comprises air, but can be any inert gas that contains oxygen or even pure oxygen. The oxygen containing gas may be humidified in order to reduce the corrosion of the cell components. In the cell with a parallel plate separator  11  molten catholyte and anolyte can flow through the cell. In that case an oxygen-containing gas can be introduced into the catholyte through the bubbler element located outside the cell and then the oxygen enriched catholyte can be introduced into the cell. 
     The carbon anode fuel  12  can be made of various carbon-containing materials such as coal, petroleum coke, coal coke, and gas carbon, as well as biomass carbon (charcoal) adb graphite. Since the carbon anode is at the same time the anode current collector its structure should be rigid enough. 
     A feature of the invention is that the separator  11  is impermeable to gas bubbles and/or completely wetted by the electrolyte to avoid direct chemical reaction of the carbon fuel with oxygen from the cathode compartment  15 . Furthermore, the porous separator  11  should be of sufficient thickness to assure mechanical strength. It is desirable that the porous separator  11  be thin enough to minimize resistive losses through the electrolyte. 
     In another embodiment as shown in  FIG. 2 , the carbon anode  12  is placed in a porous perforated metal basket  16 , which is then submersed into an electrolyte. The separator  11  is placed between the perforated metal basket  16  and the cathode  13  to separate the anolyte and the catholyte into two compartments  14  and  15 , respectively. The metal basket  16  serves as an anode current collector and the carbon anode  12  contained in the metal basket  16  can be in the form of chunks and other small carbon particles so that the anode compartment  14  is comprised of a slurry containing a mixture of carbon fuel particles and electrolyte (anolyte) and an anode current collector  16 . Thus, the use of a preformed single-piece carbon anode can be avoided. 
     The cathode compartment  15  is comprised of an oxygen cathode  13 , electrolyte (catholyte) and an oxygen-containing gas and is of the same structure and properties as the cathode  13  in the first embodiment. 
     A porous metal structure can serve as anode current collector  16 . The anode current collector  16  can comprise a mesh, grid, felt, screen, sintered frit or other similar electronically-conductive matrix that allows effective contact with and transport of the carbon fuel and electrolyte. In addition, the anode current collector  16  comprises any metal that cannot melt at the operating temperature of the cell and is stable against corrosion in the molten carbonate and hydroxide mixed electrolyte. Nickel and nickel based alloys can be preferentially employed as anode the current collector  16 . 
     In a third embodiment as shown in  FIG. 3 , a separator  31  is placed around a carbon anode  32  in a cell container  33  to divide the cell into two compartments  34  and  35 . In this embodiment, porous electric conductors  36  and  37  such as metal coatings, perforated or expanded metal sheets or like material are placed over both sides of the porous separator  31  in the compartments  34  and  35 , respectively. The porous electric conductor  36  is a basket that can accommodate chunks and other particulate type of carbon material. The electric conductor  37  placed in the compartment  35  serves as the cathode electrode. Similar to the embodiments as shown in  FIGS. 1 and 2 , ion transport based on diffusions and convection between the anolyte contained in the compartment  34  and the catholyte contained in the compartment  35  is hindered by the fine pores of the separator  31 . The transport of carbonate ions from the anolyte to catholyte is thus prevented. As a result, the composition of the catholyte remains unchanged, while reaction of the carbonate ions with the carbon anode  32  releases gaseous carbon dioxide. 
     In this embodiment the cell housing  33  and the cathode  37  are separate parts of the cell. The advantage of this cathode design is lower voltage losses across the catholyte. 
     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein, including various ways of preventing the carbonate ions from transporting towards the catholyte. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.