WO2013190164A1

WO2013190164A1 - Solid oxide electrolyte membrane supported on doped silicon ribs for uses in micro solid-oxide fuel cells

Info

Publication number: WO2013190164A1
Application number: PCT/ES2013/070406
Authority: WO
Inventors: Iñigo GARBAYO SENOSIAIN; María de les Neus SABATÉ VIZCARRA; Marc SALLERAS FREIXES; Albert TARANCÓN RUBIO; Alex MORATA GARCÍA
Original assignee: Consejo Superior De Investigaciones Científicas (Csic); Fundació Institut De Recerca En Energia De Catalunya (Irec)
Priority date: 2012-06-21
Filing date: 2013-06-21
Publication date: 2013-12-27
Also published as: ES2446465B1; ES2446465A1

Abstract

The invention relates to a solid oxide fuel cell comprising: (a) a substrate having at least one cavity for forming a membrane; (b) an electrolyte membrane based on a thin layer of a solid oxide of more than 5 nm but less than 5 μm in thickness, covering the cavity formed in the substrate; (c) a network of doped silicon ribs crossing the cavity just below the electrolyte membrane, such as to serve as a support for the electrolyte, said silicon ribs defining individual electrolyte membranes, the size of which is always greater than the thickness of the ribs and which together form the large-surface electrolyte membrane; and (d) two fine layers acting as electrodes, deposited on each side of the electrolyte membrane. The invention also relates to the method for producing said fuel cell.

Description

SOLID OXIDE ELECTROLYTIC MEMBRANE SUPPORTED ON DOPED SILICON NERVES FOR APPLICATIONS IN MICRO BATTERIES OF SOLID OXIDE FUEL

DESCRIPTION

Technical sector

The invention is located within the area of microelectronics, more specifically in the manufacture of microsystems and the energy production sector. The invention relates to solid oxide micro fuel cells, in particular to increasing the effective area of self-supporting electrolytic membranes.

State of the art

The proliferation of portable electronic devices in daily life (including mobile phones, laptops ...) requires the search for new energy sources compatible with this type of device. From the point of view of functionality, the integration of a power source in the same device is a very appropriate solution. This implies the manufacture of systems of small size, high energy density, and compatible with the other components of the device. The development of these energy sources for portable devices has become a tremendously active research field in recent years.

Within all the different alternatives, the micro-batteries and the micro fuel cells appear to be the most viable to manufacture, due to their high lifetime, high energy density and integration capacity.

In the face of micro-batteries, nowadays developed and marketed as an energy source for portable devices, micro fuel cells have recently received great interest from the community scientific Although the concept has been known for decades (for large-scale energy production), now the goal has been to develop them on a small scale, for applications in the low power regime. Advantages such as its high energy density, the emission of non-polluting waste (water) and the possibility of avoiding possible moving parts (micro motors, micro turbines ...) make micro fuel cells really attractive.

The principle of action of a fuel cell is based on two oxidation and reduction reactions that occur on both sides of an electrolytic membrane. This membrane, known as electrolyte, acts as a barrier for electrons that are exchanged in redox reactions, forcing them to travel an external circuit and thus generating the electric current. On the contrary, the electrolyte must allow the passage of certain ions through it, in order to complete the ion exchange between the two reactions.

The different types of fuel cell differ basically in the material that the electrolyte is made of and, as a consequence, in the ionic species that are exchanged through it during the process. Among all types, the most promising to be miniaturized and thus be integrated into portable devices are polymeric electrolytic membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC). In the case of PEMFCs, the electrolyte is made of a protonic conductive polymer (H ⁺ ), while in an SOFC the electrolyte is a ceramic with ionic conductor properties (0 ^{2 ~} ). The reactions that occur on each side of the electrolyte are the reduction of oxygen to oxide ions in the cathode (O2 + 2e ^~ 20 ^{2 ~} ) and the oxidation of fuel (¾, for example) to protons in the anode (¾ 2H ⁺ + 2e ^~ ). The electrons generated in this reaction travel through an external circuit until they reach the cathode, thus closing the electronic exchange in the reaction and generating the electric current. Protons (H ⁺ ) and ions (0 ^{2 ~} ) combine well in the cathode (in PEMFCs), or in the anode (SOFC) forming ¾0 as a residue.

Recent studies show that solid oxide micro fuel cells (SOFCs) have important advantages compared to other micro batteries, since they can generate high efficiency in energy conversion, high energy density and have the ability to run on different types of fuel. (including hydrocarbons). In addition, the typical high operating temperature of SOFCs that could be considered a problem in the face of miniaturization, can be reduced when working with SOFCs, thus reducing the energy consumption derived from working at such high temperatures. For this, it is necessary to drastically reduce the thickness of the electrolyte, but also integrate the device into structures with low thermal inertia. In this sense, on the one hand it is essential to use thin layer deposition techniques that allow reducing the thickness of the electrolyte below 1 μπι. On the other hand, it is also very important to develop support structures that contribute as little as possible to the loss of battery efficiency and that are compatible with the typical materials used in a SOFC. In this sense, the use of manufacturing processes associated with micro electronic technology is really promising, due to the high reproducibility of these processes and the possibility of reducing the size of the different components of the battery to the micro scale.

The development of SOFC has focused mainly on the manufacture of fine electrolytes to reduce ionic resistance and thus reduce the operating temperature of the battery. The most frequently used materials as electrolytes in both SOFC and SOFC are ytria stabilized zirconia (YSZ) and ceria doped with gadolinium (CGO). The most promising results in the development of SOFCs have been obtained in systems whose design is based on electrolytic membranes of one of these two materials, self-supported on micro platforms based on silicon or glass. Self-supported membranes with a really high area-thickness ratio (up to 1000: 1) have been obtained, although the fact that the ceramic layers from which the membranes are made have a thickness of less than 0.5 μπι creates a problem in the maximum area that can be obtained, this being always less than lxl rom ² . Membranes with larger areas usually suffer breakage, which causes leaks between the two sides of the electrolyte by short-circuiting anode and cathode. This limitation in the maximum area translates into a limitation in the maximum power that can be obtained in a single device.

If you want to improve the maximum power generated by device, two possibilities appear. On the one hand, the possibility of stacking a series of devices and connecting them to each other. On the other hand, and regardless of the first option (since both can be combined), it is interesting to think about the possibility of developing larger areas in a single device. In this sense, only a few papers have been submitted recently. In them, it is possible to manufacture membranes with larger areas (from 1 mm ² to 1 cm ² ) supported on metal meshes that are deposited on the electrolytic ceramic layer before releasing the self-supported membrane.

The invention presented here proposes a new approach to the objective of obtaining self-supported membranes of large area, with the aim of improving the maximum power achievable in a single SOFC thus improving its energy density.

Description of the invention Brief Description of the Invention

The invention consists in the development and manufacturing of large surface solid oxide micro fuel cells. The micro fuel cells are based on self-supported membranes on silicon-based micro platforms. The process includes the manufacture of electrolytic membranes based on thin ceramic layers and the inclusion of electrodes on both sides of the membrane.

A unique aspect of the invention is the manufacture of the electrolytic membrane. This membrane is manufactured supported on the silicon platform, in whose center a silicon-free area is defined, where the membrane is located.

The electrolytic membrane can be made of any electrolytic material typically used in SOFC, for example YSZ or CGO. The material is deposited by means of any thin layer deposition technique, including pulsed laser deposition (PLD), chemical vapor vapor deposition (CVD), sputtering, evaporation ... and can comprise a thickness range between 5 nm and the 5 μπι. Another unique aspect of the invention is the use of silicon ribs as a support for electrolytic membranes, since they have larger areas than usual. During the manufacturing process of the silicon platform that supports the membranes, a series of silicon ribs is defined by crossing the silicon-free zone for the membrane. These nerves act as a support for the ceramic membrane, thus allowing the silicon-free zone to be larger than normal. Using a silicon doping process, certain areas (the doped nerves) of the attack with the main chemical agents used to etch the silicon are prevented. Thus, it is possible not to selectively record the desired areas, which will act as support nerves. The membrane may have some dimensions of between 500x500 μπι and 50x50 mm, taking into account that part of this area will be occupied by the nerves.

Doped silicon nerves have a thickness between 1 and 50 μπι and a width between 1 and 200 μπι. Being crosslinked, they define a series of singular membranes that, together, form the membrane of large area. These singular membranes can have different geometries, depending on the design of the network of nerves, including the circular, square, hexagonal, triangular geometry ... The dimensions of these singular membranes must always be greater than the width intended for the silicon ribs that define them. The geometry of the large area membrane can also vary depending on the design of the nerve network, in order to optimize the distribution of the singular membranes. In this sense, large area membranes can comprise series of singular membranes between 2x2 and 50x50 membranes. On both sides of the electrolytic membrane supported on the nerves, the electrodes are deposited (anode and cathode). These electrodes can be made of any metal, for example Pt, Ag, Ni ... but also of ceramic materials or cermets. The electrodes can be deposited using the same thin layer deposition technique as for the electrolyte, or a different one depending on the material chosen as the electrode.

Another particular aspect of the invention is the use of doped silicon ribs also as current collectors for one of the electrodes, as well as as support ribs. On one side of the membrane, when the electrode is deposited it is in contact with the silicon ribs in addition to the electrolyte. This fact is used to collect the electrode current through the nerves, since they have a high electronic conductivity as they are made of silicon doped Several very significant advantages derive from the fact of using these nerves as current collectors:

1) The resistance derived from the conductivity ba along the plane of some electrodes in thin layer format is reduced.

2) The need to add more components to the micro stack is avoided if the collection of electrons through the electrodes is not good enough.

3) It allows the electrical contacts of both electrodes to be made from the same side of the silicon platform, through the use of buried doped silicon tracks that contact the network of nerves in the membrane and the contact point. Likewise, the invention contemplates the possibility of adding an extra metallic current collector on the opposite electrode, which is not in contact with the doped silicon. This current collector would be formed by a network of metal tracks in the form of a mesh. The metal tracks would be deposited on the electrode, in the same areas as the doped silicon nerves. In this way, it would be possible to have current collectors for both electrodes while maintaining the total active area of the battery. The dimensions of the metal tracks are therefore limited by the dimensions of the ribs, always being as wide as possible and following the architecture of the rib network. The thickness of the metal collector could vary between 50 nm and 5 μπι. Another particular aspect of the invention consists in the inclusion of a micro heater to locally heat the electrolytic membrane. In case of not having an external heat source in the device, the invention includes the possibility of implementing a resistive type micro heater based on metal tracks forming a coil. The heater is located on the silicon ribs, so it does not imply any loss of effective membrane area. The heater is passivated with layers dielectrics that prevent electrical leakage. The dimensions of the metal tracks are always determined by the dimensions of the ribs, so that the metal tracks of the heater are always narrower than the ribs. The thickness of the heater tracks can vary between 10 nm and 2 μπι and can be made of various materials, including metals typically used to manufacture micro heaters in micro electronics (Pt, Au, W ...).

This element is particularly useful considering that the operating temperatures of SOFCs are between 400 and 600 ° C. The implementation of a micro heater allows the active part of the micro stack (the electrolytic membrane, plus the two electrodes) to reach the corresponding working temperature while the support platform is practically maintained at room temperature. This fact greatly simplifies the maneuverability of the device, also making it easier to seal the platform with the interconnects. When sealing at room temperature, the risks of rupture are drastically reduced due to the different thermal expansion of the materials involved.

Brief description of the content of the figures

Figure 1. (a) Scheme of a solid oxide micro fuel cell according to the invention, (b) Scheme of a solid oxide micro fuel cell according to the invention, including a micro heater.

Figure 2. Scheme of the manufacturing process for obtaining a solid oxide micro fuel cell according to the invention (cross sections).

Figure 3. Diagram of a large surface electrolytic membrane supported on nerves of doped silicon with different geometries of the singular membranes (top view): (a) hexagonal, (b) square, (c) circular. In (a) a scheme of a membrane is also represented Large surface electrolytic including a micro heater (top view).

Figure 4. Optical microscope images of electrolytic membranes according to the scheme shown in Figure 3 (a).

Detailed description of the invention

The present invention consists in the use of doped silicon ribs for the manufacture of large surface solid oxide micro fuel cells. Figure 1 (a) shows a diagram of a complete SOFC supported on a silicon platform {substrate), where the different components of the device can be distinguished. The silicon ribs located below the fine electrolyte allow the manufacture of membranes with larger areas. The porous anode and the porous cathode are deposited on both sides of the electrolyte completing the battery. The porous anode also covers the silicon nerves, establishing contact with them and thus allowing the collection of the current generated at the anode through them {contact of the anode). The electrical connections of both electrodes are, therefore, on the cathode side. Figure 1 (b) also shows a scheme of a SOFC according to the invention, but in this case including a micro heater to locally heat the membrane, while the rest of the device {substrate) is kept at room temperature. The metal tracks of the heater are deposited on the silicon ribs coated on both sides with insulating dielectric layers. The heater contacts are also placed on the cathode side, as are the anode and cathode contacts. Figure 2 shows the manufacturing process for obtaining the device described in Figure 1 (b). The manufacture of the device as described in Figure 1 (a) is similar, only that steps 2 (d) and 2 (e) They must skip. The different components of the device are marked with numbers so that they can be easily identified. The following table (table 1) shows what each number corresponds to.

Table 1. Description of reference numbers

Each of the process steps is described below: Figure 2 (a): Isotropic silicon doping. The areas (2) where you wish to dop the silicon substrate (1) are defined by photolithography. These areas correspond to the future nerves of doped silicon. The silicon ribs have a width wl and are separated from each other by a distance w2. The ribs have a circular section due to semi ^¬ isotropic doping process.

Figure 2 (b): Deposit of the dielectric layers. A thin layer of silicon dioxide (3) is grown by heat treatment and a layer of silicon nitride {4) is subsequently deposited by CVD. These layers act as insulating layers to avoid short circuits between anode, cathode and heater in the final device. Figure 2 (c): Opening contacts for the anode current collection. Photolithography defines squares of dimensions w3, where the dielectric layers 3 and 4 are removed by reactive ionic etching (RIE). In this step, an area of the doped silicon nerves is released from its passivating layers. The anode current collection will therefore be made through these rectangular contacts from the cathode side. Figure 2 (d): Metal micro heater tank. The tracks where the metal is deposited are defined by photolithography (5). Through a lift-off process, the metal remains only in the defined areas, giving rise to the coil-shaped heater. In the same process, a metallic layer is also deposited on the area destined to contact the silicon ribs (anode current collection). Thus, a good electrical contact with the doped silicon is ensured. The metal tracks of the heater have a thickness ti and a width w4.

Figure 2 (e): Isolation of the micro heater. A dielectric layer is deposited on the entire substrate, except in the areas destined for the electrical contacts for the anode and the heater {5a and 5b, respectively). First, the dielectric layer is deposited on the entire substrate, and subsequently, by a photolithography process, it is selectively removed from the desired areas (contacts).

Figure 2 (f): Engraving by reactive ions from the back side of the substrate. The layers of silicon nitride and silicon oxide are removed in certain areas of the back side of the substrate. Thus, the areas where the silicon will be subsequently engraved to create the membranes are defined. The defined area will have a width w5. Figure 2 (g): Wet etching of silicon. The silicon substrate (1) and the silicon oxide layer (3) of the upper face (which will be exposed from the back side after etching the silicon) are removed by wet etching with KOH and HF respectively, made from the face back of the substrate. The silicon nitride layer on the back side acts as a mask allowing the etching of silicon only in the desired areas. These zones define large surface silicon nitride membranes supported on the silicon ribs, which will act as a substrate during the electrolyte deposition in the next step. Doped silicon is selective when taxed with KOH, so the nerves are not recorded during this step. Due to the anisotropic etching of silicon, the width of the w6 membrane will always be smaller than the width defined during the etching of the silicon nitride on the back side in the previous step (w5).

Figure 2 (h): Deposit of the electrolyte layer. The ceramic electrolyte (7), with a thickness t2, is deposited by a thin layer deposition technique on the insulating layer (6).

Figure 2 (i): Dry etching of silicon nitride and other dielectric layers. Silicon nitride and other dielectric layers that acted as a substrate for electrolyte deposition are removed from the membrane area by etching with reactive ions from the back face of the substrate. The progression of the attack must be controlled so as not to over-attack the electrolytic layer.

Figure 2 (j): Deposit of the electrodes. The cathode (8) and anode (5) electrodes, with thicknesses t3 and t4 respectively, are deposited on both sides of the substrate and the membrane. The possible materials are multiple, including metals and ceramics or cermets. Also, these electrodes can be deposited by different thin layer deposition techniques. Figure 3 shows top views of different large surface electrolytic membranes. The support silicon ribs (2) define self-supporting singular membranes (1), with different geometries: hexagonal (a), square (b) or circular (c). In all three cases, a large surface membrane consisting of a set of 5x5 singular membranes is illustrated, although the number of singular membranes per large surface membrane may vary, forming larger or smaller membranes. The coil shape of the micro heater (3) can be seen in Figure 3 (a). The metal tracks of the heater follow the geometry of the ribs (2) on which they are supported.

Figure 4 shows optical microscope images of some of the large surface membranes manufactured in accordance with the manufacturing process detailed in Figure 2. The membranes correspond to the geometry detailed in Figure 3 (a), although in In this case a series of "secondary" (much finer) silicon nerves were added forming triangles within the hexagons, to ensure a good distribution of heat throughout the entire membrane.

Examples of embodiment of the invention

Example 1: Manufacture of a large surface YSZ membrane supported on nerves of doped silicon.

A 200 nm thick YSZ membrane was manufactured according to the manufacturing process detailed in Figure 2 (skipping steps d and d). First, a 300 nm silicon nitride pre-membrane supported on the network of doped silicon ribs was manufactured. The dimensions of the doped silicon nerves were defined as 85 μπι in width and 10 μπι in maximum thickness, and distributed so as to define singular hexagonal membranes with a side of 150 μπι. The large surface membrane, composed of a set of 7x7 singular membranes, had a total area of 2300x2300 μπι ² (see figure 3a).

The YSZ was deposited on said nitride pre-membrane by PLD. Subsequently, silicon nitride was removed by RIE from the back side of the membrane, thus releasing the self-supported YSZ membrane in the silicon nerves.

Example 2: Manufacture of a SOFC based on a large surface YSZ membrane supported on doped silicon ribs.

A large surface SOFC can be manufactured following the same steps as in example 1 but adding the electrode reservoir and current collectors.

On both sides of the YSZ membrane, platinum electrodes were deposited by sputtering, thus completing the fuel cell. The thickness of the electrode was defined as 80 nm, to form a porous layer by heat treatment of the platinum layers at 600 ° C.

On the anode side, doped silicon nerves were also used as current collectors. During the deposition process of the Pt layer, it was not only deposited on the YSZ membrane but also on the silicon ribs (see figure 2j, element 9). Thus, a good electrical contact between the electrode and the current collector was ensured. In addition, a contact was opened through the dielectric layers on the cathode side to be able to contact the doped silicon ribs (see Figure 2c). Therefore, the current collection of both electrodes was made from the same side of the cathode.

Example 3: Manufacture of a SOFC based on a large surface YSZ membrane supported on doped silicon ribs, including a buried micro heater.

A large surface SOFC with a micro heater integrated in the membrane can be manufactured following the same steps as in examples 1 and 2, but including the corresponding manufacturing steps with the tank and micro heater insulation (dye steps in figure 2).

A coil-shaped tungsten heater was deposited on the silicon nitride on the cathode side, defining W tracks on the silicon ribs (see figure 3a). The thickness of the heater tracks was set at 500 nm, while its width at 50 μπι. A 500 nm insulating layer of silicon oxide was subsequently deposited covering the heater to avoid short circuits with the electrodes or the current collector.

Once the insulating layer was deposited, the process was continued with the deposit of YSZ, as detailed in example 1.

Claims

RE IVI DICATIONS

1. A solid oxide fuel cell consisting of: a. a substrate with at least one cavity to form a membrane;

b. an electrolytic membrane based on a thin layer of a solid oxide more than 5 nm but less than 5 μπι thick, covering the cavity formed in the substrate;

cradle a network of doped silicon nerves crossing the cavity, just below the electrolytic membrane, so that they serve as support for the electrolyte; The silicon ribs determine singular electrolytic membranes of a size always greater than the thickness of the nerves, which together form the large-surface electrolytic membrane;

d. two thin layers that act as electrodes, deposited one on each side of the aforementioned electrolytic membrane.

2. A cell according to claim 1, wherein the doped silicon ribs have a width within the range between 1 and 200 μπι and a thickness between 1 and 50 μπι.

3. A cell according to claim 1 or 2, wherein the network of doped silicon ribs has a combination of ribs of different thicknesses.

4. A battery according to claim 1, wherein the unique electrolytic membranes have a polygonal design, including geometries such as hexagonal, triangular, circular or square.

5. A cell according to claim 1, wherein the network of doped silicon nerves defines singular electrolyte membranes with irregular shapes.

A battery according to claim 1, wherein the silicon nerve network defines a series of singular electrolytic membranes, forming a large surface electrolytic membrane of between 2x2 and 50x50 singular electrolytic membranes.

A cell according to any one of claims 1 to 6, wherein the doped silicon nerve network is used as a current collector for the electrode on the side of the electrolytic membrane where the cavity in the substrate is located.

8. A cell according to claim 7, wherein a current collector is added on the opposite side to the silicon ribs of the electrolytic membrane, having the same shape and size as the network of doped silicon ribs, or having a different shape but deposited corresponding to the network of silicon nerves.

9. A cell according to claim 7, wherein an area is released on the side of the electrolytic layer opposite to that of the silicon nerve network so as to allow electrical contact with the doped silicon network located in the membrane, through of a buried track made of the same doped silicon; the electrical contact for the opposite electrode being on the same side as this new contact.

10. A battery according to any one of claims 1 to 6, wherein a coil-shaped micro heater is manufactured buried between the doped silicon ribs and the electrolytic layer; having a thickness limited by the thickness of the doped silicon ribs, therefore between 1 and 50 μπι.

11. A battery according to claim 10, wherein the material of the micro heater is selected from a group that includes all metals.

12. A cell according to claim 10, wherein the thickness of the micro heater tracks is between 10 nm and 2 μπι.

13. A cell according to any one of claims 1 to 12, wherein the material of the electrolytic layer is selected from a group that includes yttria-stabilized zirconia, gadolinium-doped ceria and any other oxide ion conductor.

14. A battery according to any one of claims 1 to 12, wherein the electrolytic layer is manufactured by a method selected from a group that includes pulsed laser deposition, chemical vapor deposition, sputtering, evaporation or any other technique. deposit of solid oxides in a thin layer.

15. A cell according to any one of claims 1 to 12, wherein the material of the electrode layer is selected from a group consisting of porous metallic layers, cermets or mixed ionic-electronic conductive ceramic materials.

16. A cell according to any one of claims 1 to 12, wherein the electrode layer is manufactured by a method selected from a group that includes pulsed laser deposition, chemical vapor deposition, sputtering, evaporation or any other thin layer deposition technique.

17. A series of cells according to any one of claims 1 to 12, on a single substrate, or manufactured on different substrates and connected together forming a stack of solid oxide fuel cells.

18. A method of manufacturing a battery according to any one of the preceding claims, which includes photolithography processes, physical or chemical processing and dry or wet etching.