Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M12/00—Hybrid cells; Manufacture thereof
  • H01M12/04—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type
  • H01M12/06—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type with one metallic and one gaseous electrode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/24—Alkaline accumulators
  • H01M10/26—Selection of materials as electrolytes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M10/00—Secondary cells; Manufacture thereof
  • H01M10/34—Gastight accumulators
  • H01M10/345—Gastight metal hydride accumulators
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M12/00—Hybrid cells; Manufacture thereof
  • H01M12/08—Hybrid cells; Manufacture thereof composed of a half-cell of a fuel-cell type and a half-cell of the secondary-cell type
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
  • H01M4/383—Hydrogen absorbing alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/8605—Porous electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/8647—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites
  • H01M4/8657—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites layered
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/9075—Catalytic material supported on carriers, e.g. powder carriers
  • H01M4/9083—Catalytic material supported on carriers, e.g. powder carriers on carbon or graphite
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M6/00—Primary cells; Manufacture thereof
  • H01M6/30—Deferred-action cells
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04276—Arrangements for managing the electrolyte stream, e.g. heat exchange
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/08—Fuel cells with aqueous electrolytes
  • H01M8/083—Alkaline fuel cells
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
  • H01M2004/8689—Positive electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M2300/00—Electrolytes
  • H01M2300/0002—Aqueous electrolytes
  • H01M2300/0014—Alkaline electrolytes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
  • H01M4/90—Selection of catalytic material
  • H01M4/9016—Oxides, hydroxides or oxygenated metallic salts
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the present invention relates to water management in a metal-air battery or fuel cell containing an air electrode.
• the invention relates to the use of hygroscopic materials to control the humidity of the battery system.
• Fuel cells convert chemical energy of a fuel into electrical energy. Unlike batteries, the reactants are continuously fed from an external source. The most typical fuel cell reactions are the oxidation of hydrogen at the anode and the reduction of oxygen from air at the cathode.
• the air electrode is usually made from thin porous PTFE bonded carbon layers. Within the electrode a double pore structure exists. Hydrophobic pores are used to enable high rates of oxygen diffusion. A hydrophilic pore structure of narrow pores enables penetration of the electrolyte by capillary forces. The reduction of oxygen takes place on catalyst particles in the 3 -phase boundary within the electrode.
• a hydrophobic backing layer prevents any liquid penetration.
• gas interactions will occur between the interior and the exterior of the system.
• high humidity >45 % RH
• flooding of the electrode might occur. Drying out or flooding of the system results in increased ohmic resistance and subsequently a loss in the power density and efficiency of the fuel cell.
• the electrode can dry out completely causing irreversible system failure.
• a fuel cell is thus a more dynamic system; monitoring and adjusting the water balance can be done by peripheral systems.
• the metal-air battery system combines properties from both fuel cell and battery technology.
• An air electrode is used for the cathode. This enables an unlimited source of reactants for the cathode within a thin layer (300-700 ⁇ m).
• a metal with high energy density per weight and volume is used for the anode. Metals such as Zn, Al, Mg, Fe and Li are suitable anode materials. The benefit of the metal-air system is the high energy capacity.
• a rechargeable metal-air system is enabled by the development of bifunctional air electrodes and the use of rechargeable anode materials.
• a metal-air battery system is a partially open system where the air electrode interacts with the environment.
• a method to prevent dry-out or flooding caused by the humidity of air must thus be implemented under certain conditions. It has been shown that if the humidity is below 45% the battery may slowly dry-out and that if the humidity is above 45% the system may be flooded. The applications for this technology are thus limited by the influence of the humidity.
• Metal-air batteries are commercially available only as primary zinc-air button cell batteries. These batteries have a long shelf life due to the closed air access packaging. When in use the surrounding environmental conditions cause a slow deactivation of the battery. The lifetime is thus limited by the environmental influence. Due to these constraints only a small part of the button cell size battery market is available for these batteries. The main limitation for applying it in a larger share of the market is the limited current density and the low stand-by time available.
• Anode materials such as Zn, Al, Mg, Fe and Li have often been proposed in the literature for primary or refillable metal-air batteries.
• For rechargeable batteries there are difficulties in recharging such anodes due to shape changes and dendrite formation.
• An alternative approach is the use of metal- hydride materials as the anode.
• Metal- hydrides are used in rechargeable nickel / metal-hydride batteries with high stability (typically 500-600 cycles are shown).
• Another rechargeable anode material is Cd, however this material is somewhat restricted due to the environmental aspects.
• the prior art discloses primarily polymers and resins for use as water absorbing constituents in batteries and fuel cells. Use has also been made of metal oxides and carbon particles.
• polymerisation has the disadvantage that it becomes difficult to lead trapped gas out of the electrolyte. Further, the method of polymerisation only reduces water loss from the electrolyte, which means that there is still too much water loss compared to the required lifetime of most battery or fuel cell applications.
• DE 19917812 is directed to a membrane-electrode unit for a self- humidifying fuel cell battery.
• this electrode unit also comprises hygroscopic particles, such as ZrO 2 , SiO 2 and/or TiO 2 which serve to retain the water.
• JP 2004152571 to Hyundai Motor Corp. discloses an electrode structure for a solid polymer fuel cell. It describes a layer made of carbon particles and fiuoroplastic having a moisture absorption rate of not less than 150cc/g.
• US patents 5,652,043 and 5,897,522 describe an open electrochemical cell.
• the cell comprises three layers: an insoluble anode layer and an insoluble cathode layer separated by an electrolyte layer that includes deliquescent material, an electroactive soluble material for ionic conductivity and a water-soluble polymer for adhering the layers together.
• These patents do not relate to batteries having an air electrode, i.e. metal-air batteries or fuel cells, but instead relate to "classical" batteries such as zinc- manganese batteries.
• the teaching of these patents is not relevant to metal-air batteries or fuel cells because insoluble electrodes cannot be used for the cathode, since it requires liquid and gas penetration into the three-phase boundary.
• An electrolyte with an adhering material as described in these patents also is not suitable for use in metal-air batteries or fuel cells because it limits the absorption of electrolyte into the air electrode, thus resulting in a low reaction rate.
• any gelling agents within the electrolyte will result in gas being trapped inside the electrolyte resulting in low surface area contact between the electrolyte and the electrodes.
• a metal-air cell with an exchangeable anode is disclosed.
• a conductive separator which consists of KOH, a polymeric material such as PVC or PEO, and a small addition OfCaCl 2 as hygroscopic agent.
• the separator has the form of a membrane or thick film with a thickness below 1 mm.
• the ionically conductive materials are thus integrated in a self-supporting solid structure and having such a solid structure in the electrolyte of a metal-air battery with an air electrode as described below would hinder gas exchange.
• US patent 5,302,475 discloses a rechargeable zinc cell comprising an aqueous alkaline electrolyte containing KOH and a defined combination of KF and K 2 CO 3 salts with the aim of reducing shape changes and dendrite formation of the zinc electrode which constitutes a problem for the stability of secondary zinc batteries. It is known to those skilled in the art that it is possible to reduce the solubility of zinc species formed during discharge if one exchanges a part of the OH " ions with other anions and the method described appears to be based on this effect. However, the method is not concerned with any kind of humidity management in a metal-air cell which is to be effective at the interface between the liquid electrolyte and the air, i.e. inside the pores of the air electrode.
• French patent 2 835 656 discloses an ionically conductive layer placed next to the air electrode of a metal-air battery which layer consists of an anion exchanger material containing mobile OH " ions.
• the anion exchanger material inhibits the migration of carbonate ions into the electrolyte and the formation of hardly soluble sodium carbonate. The presence of this layer does not affect the water balance of the metal-air battery.
• peripheral systems such as oxygen humidifier. This involves treatment of the air before it enters the system in order to control the humidity.
• the drawback with this method is the increased size and cost of the battery.
• An object of the present invention is to provide an alternative solution to the problem of water management in metal-air battery systems and fuel cells.
• a further object of the invention is to provide a passive state small battery system having an air electrode, which requires no peripherals (e.g. pumps, humidity controls, etc).
• Passive state refers to the idea that the system is stable without the need for external controls. The system is stable in that it is resistant to flooding or drying out caused by environmental interactions through the air electrode.
• the invention provides a metal-air battery or fuel cell comprising a metal or metal hydride anode, an aqueous liquid electrolyte containing an ion conducting material, and an air electrode which allows ingress and egress of oxygen and which contains one or more catalysts capable of evolution and/or reduction of oxygen, wherein the air electrode has both hydrophobic and hydrophilic pores, the hydrophilic pores are at least partially filled with aqueous liquid electrolyte and the air electrode and/or the electrolyte comprises hygroscopic material and OH ions, whereby water vapour exchange with the environment is limited.
• the presence of a hygroscopic material balances the system and stabilises it by preventing it from flooding or drying out.
• the hygroscopic material comprises CaBr 2 , K3PO4, CH 3 COOK, K 2 CO 3 , K 2 HPO 4 , KH 2 PO 4 , Na 2 SO 4 , MgSO 4 , P 4 Oi 0 , CaO, CaCl 2 , or combinations thereof.
• the OH ions may in the form of NaOH, KOH and/or LiOH, and are preferably in the form of NaOH or LiOH.
• the hygroscopic material and OH ions are in the electrolyte.
• the electrolyte comprises CaBr 2 as hygroscopic material and OH ions in the form of NaOH.
• the electrolyte may contain varying proportions of CaBr 2 and NaOH, but most preferably the weight ratio of CaBr 2 to NaOH is between 4: 1 and 1 :2. According to one embodiment the weight ratio is 1:1.
• the hygroscopic material and OH ions are in the air electrode.
• the air electrode comprises CaBr 2 as hygroscopic material and OH ions in the form of NaOH.
• the air electrode may contain varying proportions Of CaBr 2 and NaOH, but most preferably the weight ratio of CaBr 2 to NaOH is between 4: 1 and 1 :2. According to one embodiment the weight ratio is 1:1.
• hydrophilic pores are pores situated within an activated carbon or graphite material or a combination thereof, whilst the hydrophobic pores are rendered hydrophobic by a complete or partial coating of the walls of the pores with PTFE or other polymers such as polyolefms, e.g.
• PE polyethylene
• PP polypropylene
• PIB polyisobutylene
• thermoplastics such as polybutylene terephthalate (PBT ) or polyamides
• PVDF polyvinylidene fluoride
• silicone-based elastomers such as polydimethyl siloxane (PDMS) or rubber materials such as natural rubber (NR), ethylene propylene rubber (EPM) or ethylene propylene diene monomer rubber (EPDM), or combinations thereof.
• PDMS polydimethyl siloxane
• NR natural rubber
• EPM ethylene propylene rubber
• EPDM ethylene propylene diene monomer rubber
• the battery may be a button cell, a cylindrical cell or a prismatic cell.
• the battery may be a primary battery or a secondary battery, but is preferably a secondary battery.
• the invention provides the use of a hygroscopic material and OH ions in the air electrode and/or the electrolyte of a metal-air battery or fuel cell system to control the humidity of the system.
• the air electrode has both hydrophobic and hydrophilic pores and contains one or more catalysts capable of evolution and/or reduction of oxygen and the air electrode and/or the electrolyte contains OH ions and a hygroscopic material and the hydrophilic pores are at least partially filled with electrolyte.
• Other preferable features are as described above.
• the invention provides a method for controlling the humidity of a metal-air battery or fuel cell system comprising a metal or metal hydride anode, an aqueous liquid electrolyte and an air electrode that takes oxygen from the environment as cathode, which comprises providing hygroscopic material and OH ions in the air electrode and/or the electrolyte.
• the invention provides a method for the dry assembly of a metal-air battery or fuel cell comprising an air electrode that takes oxygen from the environment as cathode, a metal or metal hydride anode, and an electrolyte, said method comprising assembling the cathode, anode and a dry powder mixture of hygroscopic material and a source of OH ions to form the battery and allowing the powder mixture to self-activate by absorbing water from the air thereby forming an ionic conductive aqueous electrolyte.
• the metal-air battery or fuel cell is preferably as described above.
• the invention provides a method for the wet assembly of a metal-air battery or fuel cell comprising an air electrode that takes oxygen from the environment as cathode, a metal or metal hydride anode, and an electrolyte, which comprises the steps of: dissolving a hygroscopic powder in an aqueous solution containing OH ions to form an electrolyte solution; adjusting the pH of the electrolyte solution such that it is equivalent to an alkaline solution with a 2-12 M OH ; and assembling the cathode, anode and electrolyte solution to form the battery.
• the pH of the electrolyte solution is equivalent to an alkaline solution with a 4-10 M OH , most preferably 4 to 6.6 M OH .
• the metal-air battery or fuel cell is preferably as described above.
• the invention provides a method for reactivating a dry metal-air battery or fuel cell comprising an air electrode that takes oxygen from the environment as cathode, a metal or metal hydride anode, and an electrolyte comprising hygroscopic material and OH ions, said method comprising exposing the battery to a humid environment whereby the dry electrolyte self-activates by absorbing water from the air thereby forming an ionic conductive aqueous electrolyte.
• the metal-air battery or fuel cell is preferably as described above.
• the invention provides a metal-air battery or fuel cell comprising an air electrode that takes oxygen from the environment as cathode, a metal or metal hydride anode, and an electrolyte, wherein the electrolyte and/or the air electrode comprises CaBr 2 as hygroscopic material and NaOH as a source of OH ions.
• the electrolyte and/or the air electrode may contain varying proportions of CaBr 2 and NaOH, but most preferably the weight ratio Of CaBr 2 to NaOH is between 4:1 and 1:2. According to one embodiment the weight ratio is 1:1.
• Figure 1 shows the weight increase for the hygroscopic materials CH 3 COOK, K 2 CO 3 and CaBr 2 as a function of time.
• the powders have been left in ambient atmosphere for about 3000 hours.
• the experiment was performed at a mean temperature of 25 0 C and mean relative humidity of 30 %.
• Figure 2 shows the open circuit potential for cells with two different aqueous electrolytes, KOH and NaOH, as a function of time. The experiment was carried out at 25 0 C and 30 % humidity.
• Figure 3 shows the increase in weight for the dry mixture of NaOH (20 %) and hygroscopic material CaBr 2 (80 %), compared to the weight loss in a standard electrolyte, KOH (6M). During testing the relative humidity was in the range 20-30 % and the temperature 25 0 C.
• Figure 4 shows the weight loss with time for the wet mixtures of 0.5 g CaBr 2 in 10.0 g KOH and 0.5 g CaBr 2 in 10.0 g NaOH. During the experiment the samples were left in ambient air. The temperature was in the range 20 - 30 0 C and the mean humidity was 30 %.
• Figure 5 shows the weight and open circuit potential of a button cell with electrolyte of wet powders, a 50:50 mixture Of CaBr 2 and NaOH dissolved in water, as a function of time. The experiment was performed at two different temperature and humidity intervals:
• Figure 6 shows weight and open circuit potential of a button cell with electrolyte of dry powders, a 50:50 mixture Of K 3 PO 4 and NaOH, as a function of time. The experiment was performed at two different temperature and humidity intervals:
• Figure 7 shows the potential of a wet assembled button cell with a 50:50 powder mixture of NaOH and CaBr 2 dissolved in water as electrolyte. The experiment was performed at low current, 0.16 mA for 1000 hours
• Figure 8 shows the polarisation curve for an air electrode with a 50:50 mixture Of CaBr 2 and NaOH in water as catalyst.
• Figure 9a shows a prismatic cell design
• Figure 9b shows a part of the cell in larger scale, illustrating the battery case (91), Zn- electrode (92) and air electrode with current collector (93).
• Figure 10 shows a button cell design, illustrating the battery case (101), Zn electrode (102), air electrode (103) and opening for air access (104).
• the present invention relates to the introduction of a hygroscopic material to the alkaline electrolyte and/or the air electrode of a metal-air battery or fuel cell. Surprisingly such a battery shows a better stability when exposed to changes of the water vapour content of the environment.
• the invention makes it possible to keep the water within the system even in surroundings of low humidity and also prevents flooding of the system in surroundings of high humidity.
• the hygroscopic material will reactivate the electrode if exposed to low humidity over long time periods.
• metal-air battery system is intended to include any battery or fuel cell which contains an air electrode that takes oxygen from the environment as cathode.
• the anode electrode may be a metal, such as Cd, Al, Li, Fe and Mg, or a metal hydride.
• metals in metal hydride materials are the AB5 or AB 2 structure types where the "AB x " designation refers to the ratio of A elements and B elements.
• A is a combination of La, Ce, Pr and Nd
• A can be Ti, Zr or a combination of Ti and Zr.
• B is typically a combination of Ni, Mn, Co, Al and Fe.
• Typical hygroscopic material are CaBr 2 , K 3 PO 4 , CH 3 COOK, K 2 CO 3 , K 2 HPO 4 , KH 2 PO 4 , Na 2 SO 4 , MgSO 4 , P 4 OiO, CaO, CaCl 2 , but any hygroscopic material may be used in the context of this invention.
• Hygroscopic materials readily absorb and retain water. By measuring the weight increase of a powder when left in ambient air, the water absorption can be measured. Dry CaBr 2 powder was saturated with water after two days in ambient atmosphere and the increase in weight was almost 50%. K 3 PO 4 and CH 3 COOK also showed similar properties.
• electrolytes containing an ion conducting material such as OH ions which are preferably in the form of NaOH, KOH and/or LiOH. Also described are electrolytes having both water retaining properties and sufficient electrochemical properties for use in a metal-air battery or fuel cell.
• the above mentioned hygroscopic powders have the desired property of water retention, but the electrochemical activity and conductivity in solutions of these powders are poor.
• OH ions preferably in the form of NaOH, KOH and/or LiOH, are mixed with the hygroscopic material. The combined effect of these properties is higher with NaOH than with KOH.
• air electrode sometimes also called gas diffusion electrode.
• the most common air electrodes are based on a combination of a pore forming material, a binding agent and a catalyst or a combination of catalysts and possibly some other additives.
• the pore forming material usually consists of an activated carbon material or graphite (both preferably with a BET surface area of more than 100 m 2 -g ⁇ ) or a combination thereof. Hydrophilic pores are present within these materials and they may also serve as a support for the catalyst(s).
• the binder is used to increase the mechanical stability of the electrode and it causes the pores to become hydrophobic.
• the most common binder is PTFE, but other polymer materials may also be used. Suitable materials are PE, PP, PIB, thermoplastics such as PBT or polyamides, PVDF, silicone-based elastomers such as PDMS or rubber materials such as NR, EPM or EPDM, or combinations thereof.
• a catalyst or a catalyst combination is generally introduced to improve the reaction rate of the oxygen reduction reaction.
• catalytically active metals or oxygen- containing metal salts are used such as Pt, Pd, Ag, Co, Fe, MnO 2 , KMnO 4 , MnSO 4 , SnO 2 , Fe 2 O 3 , CoO, Co 3 O 4 , or others.
• a combination of more than one catalytically active material may also be used.
• additional catalysts or catalyst combinations capable of evolving oxygen may be used additionally.
• Such catalysts are materials like WC, TiC, CoWO 4 , FeWO 4 , NiS, WS 2 , La 2 O 3 , Ag 2 O, Ag, spinels (spinels are a group of oxides of general formula AB 2 O 4 , where A represents a divalent metal ion such as magnesium, iron, nickel, manganese and/or zinc and B represents trivalent metal ions such as aluminium, iron, chromium and/or manganese) and perovskites (perovskites are a group of oxides of general formula AXO 3 , where A is a divalent metal ion such as cerium, calcium, sodium, strontium, lead and/or various rare earth metals, and X is a tetrahedral metal ion such as titanium, niobium and/or iron where all members of this group have the same basic structure with the XO3 atoms forming a framework of interconnected octahedrons).
• spinels are a group of oxide
• an air electrode consists of two or more separate layers with different properties and a metal mesh (generally made from nickel or stainless steel) which is used as a current collector and which helps to stabilize the electrode.
• GDL gas diffusion layer
• Both the reduction and the oxygen evolution reaction take place in one or more layers closely bonded to this layer.
• GDL gas diffusion layer
• a layer which allows oxygen and electrolyte penetration to the reaction zone is required.
• This layer with a double pore structure of both hydrophobic and hydrophilic pores is called the active layer (AL).
• AL active layer
• OEL oxygen evolution layer
• an air electrode is created in a three-step process. Firstly one has to perform the mixing process for every layer separately. For this purpose the pore forming materials, the catalysts, the binding materials and other additives are mixed under the influence of mechanical, thermal or mechanical and thermal energy. In this process the materials have to be well distributed. If the mixture contains a hydrophobic binding agent then this binder forms a three dimensional network connecting the powders into an agglomerate. The mixture or the agglomerate is then extruded and/or calendared into a layer. Secondly, layers with different properties are combined by calendaring and/or pressing. Thirdly, the current collector is pressed or calendared into the combined layers.
• a dry process all of the ingredients of a layer are mixed together in the form of dry powders.
• a “wet” process one or more solvents are added at the beginning or during the mixing process or alternatively one or more ingredients may be used in the form of a dispersion or suspension.
• the solvent(s) have to be removed directly after the mixing process or in a later state of the production process by some kind of heating or drying process.
• the amount of binding agent is preferably from 5 to 50 % by weight of the composition of the layer.
• the rest of the active layer preferably consists of a high surface area carbon and/or graphite material and possibly some other additives.
• the amount of binding agent is preferably between from 5 to 50 % by weight of the composition of the layer and the amount of catalyst(s) is preferably from 2 to 30 % by weight of the composition of the layer.
• the rest of the active layer preferably consists of a high surface area carbon and/or graphite material and possibly some other additives.
• the amount of binding agent is preferably from 2 to 15 % by weight of the composition of the layer and the amount of catalyst(s) is preferably from 25 to 65 % by weight of the composition of the layer.
• the rest of the oxygen evolution layer preferably consists of a high surface area carbon and/or graphite material and possibly some other additives.
• Every layer may be produced using either the dry or the wet process.
• PTFE usually PTFE with a particle size below 1 mm is used as a binder and an additional pore forming aid such as ammonium bicarbonate may be employed to create the GDL or the OEL.
• an additional pore forming aid such as ammonium bicarbonate may be employed to create the GDL or the OEL.
• a suspension of PTFE in water may be employed to create the OEL.
• An active layer was prepared using 15 wt.-% PTFE as a powder with a particle size below 1 mm (Lawrence Industries), 70 wt.-% high surface area carbon (XC 500, Cabot) and 15 wt.-% of manganese sulfate (MnSO 4 , Prolabo) as a catalyst.
• the binding agent, the pore forming agent and the catalyst were mixed together in a single-shaft rotary mixer at 1000 rpm to form a homogeneous mixture. The mixture was then heated to a temperature of 90 0 C. When the powder mixture reached the required temperature the powder was milled at 10000 rpm for 1 hour and an agglomerate was formed. This agglomerate was pressed into a brick of about 2 mm thickness and calendared into a sheet of about 0.5 mm thickness.
• a mixture of 25 wt.-% PTFE with a particle size below 1 mm (Lawrence Industries) and 75 wt.-% of ammonium bicarbonate with a particle size below 10 ⁇ m (Aldrich) was mixed at a maximum temperature of 40 0 C for 2 hours at 1500 rpm in a single-shaft rotary mixer.
• An agglomerate was formed that was pressed into a brick of about 2 mm thickness and calendared into a sheet of about 1 mm thickness.
• the two layers were then calendared together to a total thickness of 0.8 mm.
• a nickel mesh current collector was pressed into the electrode at 8O 0 C and 70 bars.
• the electrode was then dried at 7O 0 C for 8 hours to create the hydrophobic porosity of the GDL and to remove the ammonium bicarbonate.
• An active layer was prepared using 15 wt.-% PTFE as a suspension containing 60 wt.-% PTFE dispersed in water (Aldrich), 65 wt.-% high surface area carbon (XC 500, Cabot) and 20 wt.-% of manganese sulfate (MnSO 4 , Prolabo) as catalysts.
• the high surface area carbon was mixed with both catalysts in water.
• a PTFE suspension was mixed with water. Then, the PTFE suspension was added to the carbon suspension and the materials were mixed and agglomerated to form a slurry.
• the slurry was then mixed in an ultrasonic bath for 30 minutes and subsequently dried at 300 0 C for 3 hours to remove any surfactants.
• the dried mixture was then agglomerated and a hydrogen treated naphtha with low boiling point (Shellsol D40, Shell Chemicals) was added to form a paste and the paste was then calendared into a thin layer of about 0.8 mm thickness to form the active layer (AL).
• a hydrogen treated naphtha with low boiling point Shellsol D40, Shell Chemicals
• a hydrophobic layer (GDL) was produced by the same method. In this layer only high surface area carbon (65 wt.-%) and PTFE (35 wt.-%) were used and the final thickness was 0.8 mm.
• One embodiment of the invention includes the use of hygroscopic material(s) and a source of OH ions in the air electrode.
• the hygroscopic material(s) can be introduced into the powder mixture of the active layer.
• a powder including the hygroscopic material(s) already mentioned and the ion conducting powder(s) such as KOH, NaOH and/or LiOH may be mixed with the other powder ingredients.
• the hygroscopic powders should be thoroughly dried before they are mixed together with the other ingredients of the AL according to a procedure similar to the dry process described in assembly example 1.
• the total amount of hygroscopic material and alkaline hydroxide as well as their ratio may be freely varied according to the environmental conditions (range of relative humidity and temperature) to which a metal-air cell with an air electrode is intended to be exposed during operation.
• the total amount of hygroscopic material and alkaline hydroxide together shall not exceed 20 wt.-% so as not to weaken the mechanical stability of the air electrode.
• a separate layer which consists mainly of hygroscopic material(s) and a source of OH ions.
• a powder including the hygroscopic material(s) already mentioned and the ion conducting powder(s) such as KOH, NaOH and/or LiOH may be mixed with a binder.
• the binder material is PTFE and the concentration range of the binder is between 5 wt- % and 20 wt.-% to establish a sufficiently high mechanical stability for handling without reducing the ionic conductivity of the final electrode in which such a separate layer is to be introduced.
• This separate layer is pressed or calendared in a first step and then pressed or calendared together with an active layer in a second step and possibly subjected to some kind of heat treatment afterwards which will accelerate the migration of the components of the separate layer into the active layer.
• the resulting active layer may have similar properties compared to an active layer which has been created with the above mentioned method of directly mixing all the ingredients.
• the hygroscopic materials in both above mentioned embodiments absorb water resulting in a wetting of the active layer of the air electrode. A three phase boundary is thus obtained within the air electrode that can withstand changes to the humidity of the environment.
• a dry powder mixture of hygroscopic material and a source of OH ions is placed in the cell in place of the electrolyte.
• This dry mixture will self activate by absorbing water from air, thereby forming an ionic conductive aqueous electrolyte. This allows a dry assembly of a metal-air battery. It also shows that a dry cell can be reactivated by exposure to a humid environment.
• a pre-wetted electrolyte is used. This involves dissolving a hygroscopic powder in an aqueous solution containing OH ions, preferably an aqueous solution of NaOH, KOH or LiOH.
• the pH of this solution can be varied.
• the pH of the solution should be equivalent to alkaline solution with a concentration of 2-12 M OH , more preferably 4-10 M and most preferably 4-6.6 M.
• the electrolyte is filled into the battery case before the case is closed or sealed.
• the active layer (optionally with an oxygen evolution layer) is wetted with an aqueous mixture of hygroscopic material(s) and a source of OH ions.
• a powder including the hygroscopic material(s) already mentioned and the ion conducting powder(s) like KOH, NaOH and/or LiOH may be dissolved in water.
• the resulting wet mixture may be introduced into the active layer (and optionally into the oxygen evolution layer if this is present) by means of dip coating, spraying or soaking the mixture into the porous layer before combining the active layer with the gas diffusion layer and the assembly of the electrode. Additional electrolyte may be filled into the battery case before the case is closed or sealed.
• Electrolytes of various compositions of hygroscopic material and OH " source may be used in accordance with the invention. Tests done on different compositions of a mixture of NaOH and CaBr 2 powder showed that a very good humidity control in a metal-air cell operating at moderate conditions (i.e. a temperature range between 20 and 30 0 C and a relative humidity between 20 and 50 %) was obtained for a 1:1 mixture regarding the weight of NaOH and CaBr 2 in an aqueous electrolyte (which may be prepared by mixing 1O g CaBr 2 powder, 1O g NaOH powder and 15 ml of water).
• the ratio Of CaBr 2 to NaOH may be adjusted to the environmental conditions (range of relative humidity and temperature) to which a metal-air cell with an air electrode is intended to be exposed during operation because the magnitude of the levelling effect of the hygroscopic material upon the water balance depends on its concentration.
• the weight ratio of CaBr 2 to NaOH in an electrolyte is between 4:1 and 1:2.
• An embodiment of the invention provides the use of the modified electrolyte in button cells.
• the anode for a button cell can be prepared from Zn powder with binding agents and gelling agents, and can be mixed with KOH or with some hygroscopic material.
• One method of production is described in patent application WO 2005/004260.
• the air electrode can be prepared from PTFE and activated carbon as described in patent application WO 2005/038967.
• a membrane separates the two electrodes.
• Set-up of the battery is showed in Figure 10 which shows a cross-sectional view of a button cell design.
• the button cell has a battery case 101 which surrounds the cell.
• the electrolyte resides in the space between the electrodes 102 and 103 and also inside the pores of the zinc electrode 102 and partly inside the pores of the air electrode 103.
• a further embodiment of the invention provides a button cell with modified electrolyte that can operate at low humidity without drying.
• the water balance in a battery with standard electrolyte is significantly influenced by the temperature and humidity of its surroundings. Introduction of hygroscopic material to the system causes the electrolyte to be less sensitive to fluctuations in the external temperature and humidity.
• An embodiment of the invention provides a button cell with modified electrolyte that can operate for long periods of time both at open circuit potential (OCP) and under polarisation. This can be done without covering up the air access, contrary to what is customary in currently available commercial metal-air batteries.
• OCP open circuit potential
• Another embodiment provides that larger holes for oxygen access are used than would be possible without humidity management. This enables the battery to operate at higher currents without oxygen diffusion limitations and increased dry out rates.
• primary button cell size 675
• Zn-air batteries show diffusion limitation at about 30 mA due to the limited oxygen access. Opening the air access with a hole of 4.9 mm in diameter gives a current of more than 150 mA without diffusion limitations.
• FIG. 9a shows a cross-sectional view of a prismatic cell in which a zinc electrode 92 runs along the length of the cell.
• Fig 9b shows an enlarged view of one end of the cell shown in Fig 9a.
• Fig 9b shows battery casing 91 surrounding the cell, zinc electrode 92 inside the casing together with the air electrode and current collector 93.
• the electrolyte resides in the space between the electrodes 92 and 93 and also inside the pores of the zinc electrode 92 and partly inside the pores of the air electrode 93.
• the upper part of the casing 91 contains holes for the air access (not shown in the sectional view). The invention is further illustrated by the following examples.
• the water retaining properties were tested for the following hygroscopic salts: CH 3 COOK, K 2 CO 3 and CaBr 2 .
• CH 3 COOK, K 2 CO 3 and CaBr 2 1.000 g of each of the three powders was weighed out and placed in open glass containers. The samples were left at ambient conditions and weight measurements were performed once a day. The humidity and temperature were logged continuously. After 1-3 days it was observed that the powder slowly turned into a liquid as water was absorbed. The ability of the different powders to absorb water from the atmosphere is shown in Figure 1. The curves show the increase in weight as a function of time for each of the powders. It was observed that CaBr 2 absorbed water from the atmosphere, and after two days the powder was completely dissolved in water. The increase in weight was almost 50%. A similar result was obtained with CH 3 COOK and K 2 CO 3 , but it takes 10 days to have a similar increase in weight.
• the example shows the amount of water absorbed into the hygroscopic powders.
• the example also shows that after the water is absorbed the formed liquid is in a stable state.
• the example shows that NaOH is a more stable electrolyte in metal-air batteries than KOH.
• the example shows that the cell with aqueous KOH dries out with time.
• the example also shows that the weight of the CaBr 2 and NaOH mixture remains stable after equilibrium of the water balance is reached.
• Example 4 The previous examples have demonstrated the water retaining properties of hygroscopic material. It has also been shown that NaOH is more stable than KOH with respect to the open circuit potential when combined with a hygroscopic material. In this example the water retaining properties of CaBr 2 in a KOH solution and CaBr 2 in a NaOH solution are tested.
• the anode was prepared from Zn powder mixed with KOH solution and pasted into a button cell battery case.
• the air electrode was prepared according to assembly example 2 in the detailed description of the invention. A standard polypropylene separator soaked with electrolyte was used. The assembly is illustrated in figure 10 and described above.
• the electrolyte consists of 57 wt% of a powder mixture with a 1:1 weight ratio Of CaBr 2 and NaOH and 43 wt% of H 2 O. The experiment was performed in a climate chamber to be able to control the temperature and humidity.
• the temperature was 25 0 C and the relative humidity 15 %.
• the right axis and dotted line of Figure 5 shows the OCP of the cell.
• the OCP increases as excess water in the electrolyte is vaporising. After about 20 hours it is at 1.4 V and stays stable for the rest of the experiment.
• This example shows the influence of the external conditions on full cells for other hygroscopic material.
• a dry mixture of 0.1 g K3PO4 powder and 0.1 g NaOH powder was used as electrolyte.
• the anode was prepared from Zn powder mixed with KOH solution and pasted into a button cell battery case.
• the air electrode was prepared according to assembly example 2 in the detailed description of the invention.
• a standard polypropylene battery separator was used.
• Figure 6 gives the weight increase and the OCP for the cell as a function of time. During the first 24 hours of the experiment the temperature was 80 0 C and the relative humidity 15 %. The weight of the cell increases in this period because the electrolyte powder mixture is taking up water from the atmosphere.
• the following 24 hours of the experiment was carried out under ambient conditions, 25 0 C and 25 % humidity.
• the weight of the cell is still increasing but at a slower rate.
• the external conditions are adjusted to 80 0 C and 15 % relative humidity for 24 hours and then 25 0 C and 15 % relative humidity.
• the weight of the cell stabilizes over this period and is not influenced by the change in the external condition.
• the OPC is low and increases as the cell is being activated by the liquefying of the electrolyte.
• a stable OPC of 1.4 V is obtained after about 20 hours and stays stables throughout the duration of the experiment and is hence not influenced by the external climate.
• the experiment confirms that hygroscopic powder in combination with NaOH powder prevents drying out of the cell.
• the experiment also shows that this is valid for high temperature and low humidity.
• the experiment shows that not only CaBr 2 but also other water absorbing materials like K 3 PO 4 can be used to retain water within the system of the cell.
• the potential of a button cell was measured for 1000 hours at a current of 0.16 mA, 0.92 mA/cm 2 of exposed air area.
• the anode of the cell was prepared from 1 gram of a powder mixture of Zn and Carbopol 940 (Noveon) and made into a paste by mixing in KOH.
• the anode material was pasted into the battery case and a polypropylene separator wetted with electrolyte was placed on top.
• the electrolyte was prepared from 57 wt% of a powder mixture with a 1 : 1 weight ratio of CaBr 2 and NaOH and 43 wt% of water.
• the air electrode was prepared according to assembly example 2 in the detailed description of the invention and was placed on top of the membrane.
• the assembly of the battery is illustrated in Figure 10 and described above.
• Figure 7 shows the potential in a wet assembled button cell with hygroscopic material as a function of time. The potential remains stable at 1.25 V.
• the experiment shows a cell with wet electrolyte of NaOH and CaBr 2 does not dry out throughout the period of the experiment, 1000 hours.
• the air electrode was prepared according to assembly example 2 in the detailed description of the invention; a Ni-mesh was used as counter electrode and a Zn-wire as reference electrode.
• the electrolyte was prepared from 10 g CaBr 2 and 10 g NaOH dissolved in 15 ml H 2 O. The test was carried out at ambient temperature and relative humidity and the current and potential was logged through the 40 hours experiment.
• Figure 8 shows the polarisation curve of the experiment. The experiment shows that high ionic activity for the air electrode is obtained with the new electrolyte

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