FR2814857A1 - Miniature fuel cell incorporating an oxygen and combustible electrodes enclosing a membrane made with a semi-conductor microporous support impregnated with an electrolytic polymer - Google Patents

Abstract

A miniature fuel cell made up of an oxygen electrode (8a) and a combustible electrode (8b) enclosing a membrane made up of a microporous support impregnated with an electrolytic polymer, the cell being fed by a source or air and source of combustible. The microporous support is made from a semi-conductor material. An Independent claim is included for the utilisation of such a fuel cell for feeding portable electronic devices.

Description

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 The present invention relates to the field of micro fuel cells used in particular in portable electronic devices, in automotive equipment and in telecommunications devices.

l'oxygène de l'air, à la cathode. L'eau est le seul produit généré à l'issue du procédé et les électrodes ne sont pas consommées. Pour les batteries, au contraire, les électrodes elles-mêmes constituent le matériau qui participe à la réaction chimique. Les batteries compactes au lithium conçues pour les appareils électroniques portables fournissent une tension de sortie de l'ordre de 3 à 4 volts, tandis qu'une cellule élémentaire de pile à combustible ne peut développer plus d'un volt. Grâce à une puissance et un volume spécifique plus élevés, la pile à combustible peut contenir plus d'énergie dans le même volume et la tension de sortie désirée peut être obtenue en mettant plusieurs cellules élémentaires en série. En outre, la pile à combustible est utilisable presque immédiatement après rechargement en combustible tandis que les batteries secondaires du type lithium-ion nécessitent une phase d'immobilisation pour le rechargement dépendant de leur technologie. Fuel cells transform the chemical energy stored in a fuel into electrical energy by an electrochemical process which consists in reacting a gas or a liquid in the presence of an electrolyte, electrodes and a catalyst. More specifically, the catalyst induces the release of the electrons from the fuel which circulate from one electrode to the other by an external circuit incorporating the charge while the protons pass through the electrolytic material separating the two electrodes.
Unlike a battery, a fuel cell does not discharge or does not need to be recharged: it works as long as a fuel and an oxidant supply the cell continuously, by external supply. In a standard fuel cell, hydrogen atoms lose their electron at the anode and combine with other electrons and

oxygen from the air at the cathode. Water is the only product generated at the end of the process and the electrodes are not consumed. For batteries, on the contrary, the electrodes themselves constitute the material which takes part in the chemical reaction. Compact lithium batteries designed for portable electronic devices provide an output voltage of the order of 3 to 4 volts, while a basic fuel cell cannot develop more than one volt. Thanks to a higher power and a specific volume, the fuel cell can contain more energy in the same volume and the desired output voltage can be obtained by putting several elementary cells in series. In addition, the fuel cell can be used almost immediately after recharging with fuel, while secondary batteries of the lithium-ion type require an immobilization phase for recharging depending on their technology.

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 The miniaturization of fuel cells must make it possible to conserve sufficient power and energy (ie a low internal impedance), to reach a volume and a mass compatible with their use, and must allow the implementation of an efficient fuel capable of be distributed in a mainstream distribution network. Finally, the micropile manufacturing process must include a limited number of operations compatible with a low production cost.

Nafion 117 43)-joue le rôle d'électrolyte et permet le transfert des protons. La structure solide souple de polymère est revêtue sur ses deux faces d'un ensemble constitué d'une succession de couches minces dont le rôle consiste à catalyser l'oxydation du combustible, à transférer les électrons vers l'anode ou la cathode, à transférer les protons vers la membrane, et à assurer une étanchéité maximale vis-à-vis des liquides (voir WO 98/31062). Les micropiles actuelles sont constituées d'un empilement de membranes et d'électrodes qui sont comprimées pour tenter de garantir l'étanchéité. The membranes of current micropiles consist of a first polymer generally in the form of a microporous film impregnated with a second polymer. The second polymer - typically

Nafion 117 43) - plays the role of electrolyte and allows the transfer of protons. The flexible solid polymer structure is coated on both sides with an assembly consisting of a succession of thin layers whose role is to catalyze the oxidation of the fuel, to transfer the electrons to the anode or the cathode, to transfer the protons towards the membrane, and to ensure maximum sealing against liquids (see WO 98/31062). Current micro batteries consist of a stack of membranes and electrodes which are compressed in an attempt to guarantee sealing.

 In these devices, the water contained in the membrane which is necessary for the transfer of protons, evaporates under the thermal constraints of operation or the environment. In addition, liquid fuel tends to filter through the membrane, which reduces the efficiency of the cell. Finally, the assembly techniques used to manufacture current micropiles are highly hybridized and require numerous manipulations.

 The battery proposed in the context of the present invention overcomes all of these drawbacks.

 The present invention relates to a battery comprising an oxygen electrode and a fuel electrode framing a membrane consisting of a microporous support impregnated with an electrolytic polymer, said battery being supplied by an air source and a

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 fuel source. The microporous support consists of a semiconductor.

 The semiconductor used as a solid support for the electrolytic membrane makes it possible to obtain quality surfaces for depositing the electrodes, which can thus constitute a very effective barrier to the water molecules included in the membrane and to the fuel molecules.

 The use of a semiconductor also allows the implementation of processes using microtechnologies - for the machining and deposition of metallic layers - and conventional bounding techniques. Microtechnologies make it possible to produce multilayer metallic deposits of optimized thickness.

 The potential of collective microfabrication from wafers in semiconductor materials allows the implementation of a limited number of hybridization operations, compatible with low production costs.

 The micro cells according to the invention can be mass produced using automated means in the semiconductor industry. The size and geometric arrangement of the cells can be easily adapted. Finally, the micropiles thus manufactured can be directly integrated at the level of the electronics to be supplied.

 The use of an electrolytic membrane comprising a semiconductor support makes it possible to reduce the internal impedance of the cell thanks to a reduced membrane thickness. In addition, a perfect seal against the fuel and the water contained in the membrane is guaranteed, so that operation of the cell can be envisaged in the long term in a fluctuating environment.

 The semiconductor is preferably silicon. It is advantageously used in the form of wafers with standard dimensions 3 ′ or 5 ′.

 The semiconductor is oxidized to make it electrically insulating and porous over predetermined areas. The porosity is adapted to the size

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 molecular of the electrolytic polymer. It is impregnated with a conventional electrolytic polymer which ensures the diffusion of protons in the membrane. The electrolytic polymer is for example Nations 117 or a polymer of similar ionic conductivity.

 The electrodes are deposited on the surface of the semiconductor.

They advantageously consist of a metal conventionally used in electrochemical reactions, permeable to protons, preferably gold or Platinum or a conductive mask.

 The thickness of the catalyst / electrode assembly is optimized so as to ensure a good efficiency of the cell and to guarantee the sealing of the fuel. Sealing can be improved by retaining a membrane consisting of a stack of elementary membranes separated by metal layers permeable to protons and impermeable to fuel.

 On the hydrogen fuel side, the electrode is covered with a catalyst such as Platinum or Ruthenium and palladium. According to one embodiment, the catalyst material is deposited in a thin layer on the electrode. Advantageously, the catalyst material is deposited in several thin layers, the granular structure of which may be different. According to another embodiment, the fuel electrode is covered with a layer of porous silicon doped with palladium increasing the effective surface of catalysis.

 The fuel is preferably an alcohol, such as methanol diluted in water necessary for the chemical reaction. A low dilution rate guarantees high specific energy. Conversely, the low dilution rate is not favorable to limiting the poisoning of the catalyst by the CO generated by the chemical reaction.

 The cell according to the invention is advantageously provided with distributor exchangers for fuels, oxidants and the gases and energy generated by the electrochemical reactions. On the fuel side, the evacuation of gases, in particular CO2 and saturated CO, is designed according to the supply connections of an interchangeable tank.

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 Because of the low kinetics of the gases generated by the electrochemical reaction, and in order to limit the dimensions of the exchangers responsible for responding to the thermal management of the cell, a micropump of MEMS technology may be used. Likewise, a micropump which may be of similar technology will be advantageously used to manage the circulation of air and water. this contribution of active microsystems contributes to the miniaturization of the device.

 Given the moderate efficiency of the device (between 50 and 60%), the condensed part of the water generated by the reaction can be evaporated after passing through an exchanger near the active cells.

 The exchangers can advantageously consist of glass or silicon or carbon or a technical plastic.

 The batteries according to the invention can be provided with a reheating device in the event of freezing of the water contained in the membrane. This device is advantageously located at the periphery of the cells in the unthinned part of the oxidized silicon. It consists of a metallic film through which a weak current flows. This current can come from a secondary backup battery constantly maintained by the battery.

 The stack advantageously consists of a collective of elementary cells. The semiconductor support is worked from a standard wafer according to the desired geometry. It is oxidized and made porous in the areas concerned, so as to obtain the mechanical, thermal and electrical functionalities necessary for producing the cell. The collective of elementary cells is adapted in number to the power requirements. It can then be encapsulated in the exchanger-distributors described above.

 The electrolytic membrane consisting of a semiconductor support impregnated with an electrolytic polymer can be integrated into a bipolar or unipolar architecture.

 The battery according to the invention is in particular suitable for supplying portable electronic devices with low consumption known as nomads.

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The invention is illustrated by the following figures without being limited thereto. Figure 1 is a schematic representation illustrating a mobile phone in which is integrated a battery according to a possible embodiment of the invention.

Figure 2 is an exploded perspective view of the battery shown in Figure 1
Figure 3a is a schematic representation of the stack in sectional view.

 Figure 3b is a sectional view representation along line AA of Figure 3a.

 Figure 3c is a top view representation of the stack of Figure 3a.

 Figure 4a and 4b are enlarged views of the cross section of a cell according to the invention, at the electrolytic membrane.

Figure 5a is a view from below of the air / cathode distribution network
Figure 5b is a top view representation of the anode fuel distribution network.

 Figure 6 is a perspective representation of the assembled stack.

 The stack-referenced by 1-shown in the figures has a planar architecture and consists of an assembly which comprises a set 3c with membrane and electrodes and two elements 3a, 3b forming exchangers / distributors between which said set 3c is encapsulated.

 This battery 1 can be-as illustrated in Figure 1-a battery integrated in the housing B of a mobile phone. By way of example, it may be able to produce a supply at 2 Volts with a power of 2 Watts.

 As illustrated more precisely in FIGS. 2,3a to 3c, as well as 4a and 4b, the assembly 3c is constituted by an anode 8a and a cathode 8b between which a membrane 4 is interposed.

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 The membrane 4 and the metallizations 8a, 8b which define the anode 8a and the cathode 8b have, on one side and the other of the assembly 3c, a plurality of recesses which define on the membrane 4 a plurality of cells 5 of electrochemical exchanges.

 The membrane 4 consists of a "wafer" made of an oxidized semiconductor material (oxidized silicon for example), which has been made porous in the zones which correspond to the cells 5. This porosity is directed to obtain parallel channels between them.

 To this end, this membrane 4 is previously treated by masking and etching techniques conventionally known in themselves for semi-co-inducing materials, in order to make the recesses which correspond to the cells 5, as well as at the level of the zones which correspond to these cells 5, a plurality of micro-channels 11 which pass through the membrane 4 and make it porous, metallizations corresponding to the electrodes 8a and 8b then being deposited respectively on one side and the other of the membrane 4.

In the example illustrated in FIG. 4a, the porous semiconductor impregnated with electrolytic polymer is covered with a layer constituted by an electrode-catalyst assembly 10
As a variant, as illustrated in FIG. 4b, the membrane 4 is constituted at the level of the cells 5 of a set of elementary membranes 12 separated by metal layers 13, the whole being traversed by microchannels 11 which provide.

 With reference to FIGS. 2 and 3a to 3c, as well as to FIGS. 5a, 5b, it is understood that the cells 5 of the membrane 4 are, at the level of the anode 8a, supplied with fuel by means of 6a diffusion which are formed in the element 3a and which lead the fuel stored in a cartridge 2 to the cells 5.

 This cartridge 2 is for example in a cylindrical form. It is received in a receptacle provided for this purpose on the exchanger / distributor element 3a.

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 At the cathode 8b, the cells 5 of the membrane are supplied with air via diffusion channels 6b which extend through the element 3b and open on the one hand into the cells 5 and on the other hand part outside of element 3b.

 The fuel is in particular a methanol / water mixture. The released protons diffuse into the membrane 4 of the elementary cells 5, while the electrons circulate towards the cathode 8b.

 CO2, water and water vapor are released in each cell 5.

 The released CO2 is evacuated in the exchanger 3a. The water formed at the cathode 8b can be evacuated by evaporation at the level of exchangers 15 provided for this purpose at the element 3b. At the level of the element 3a, it can be recycled in the distribution circuit 6, a micropump 9 ensuring for this purpose the pumping of this water helped by the kinetics of the water vapor resulting from the exothermic oxidation reaction.

 The vertical position of use of the electronic device-illustrated by the arrows in the figures-favors the collection in the lower part of the cells thus minimizing the negative impact of isolation of the presence of water on the active surface of the cell .

 As can be seen more particularly in FIG. 3b, as well as in FIGS. 5a, 5b and 6, there are provided on the element 3b forming exchanger / distributor metal tabs 16a, 16b which extend through said element 3b and which make it possible to ensure the connection of the anode 8a and the cathode 8b on a printed circuit card 17 which is for example a card which manages the power supply of the mobile telephone B.

 Preferably, the electrodes are made of a highly conductive metal.

Claims (12)

 1. Fuel cell comprising an oxygen electrode (8a) and a fuel electrode (8b) framing a membrane (11) consisting of a microporous support impregnated with an electrolytic polymer, said cell being supplied by an air source and a fuel source, characterized in that the microporous support is made of a semiconductor material. 2. Battery according to claim 1, characterized in that the microporous support is made of oxidized silicon. 3. Battery according to claim 1 or 2, characterized in that the electrolytic polymer is Nations 117 or an equivalent polymer 4. Battery according to one of the preceding claims, characterized in that the electrodes (8a and 8b) consist of platinum, gold or a conductive mask obtained by techniques of deposition of thin layers. 5. Battery according to one of claims 1 to 3, characterized in that the electrodes are made of a highly conductive metal. 6. Battery according to one of the preceding claims, characterized in that the electrodes (8a and 8b) are covered with a catalyst consisting of Platinum or Platinum / Ruthenium. 7. Battery according to one of the preceding claims, characterized in that the fuel is an alcohol, such as methanol. 8. Battery according to claim 6, characterized in that the fuel is methanol diluted in water. <Desc / Clms Page number 10>  9. Stack according to one of the preceding claims, characterized in that said stack is provided with gas and heat exchanger-distributors (3a, 3b). 10. Battery according to one of the preceding claims, characterized in that it consists of a collective of elementary cells. 11. Battery according to one of the preceding claims, characterized in that the membrane consists of a stack of elementary membranes separated by metallic layers of the Palladium type permeable to H2 protons and impermeable to Methanol to improve the sealing of the membrane with Methanol. 12. Use of a battery according to one of the preceding claims for powering portable electronic devices.