Improved energy generation or energy storage system

Abstract

A proton exchange membrane fuel cell (PEMFC) comprising a porous membrane element formed from a porous silicon wafer is disclosed, wherein the porosity is at least partially coated with a precious metal. Alternatively, the porous silicon wafer may be sandwiched between sheets of paper, carbon, or graphite impregnated with a precious metal. A separator is constructed using MEMS technology. Additionally, a lithium-ion battery is disclosed having a cathode electrode; an anode electrode formed from a porous silicon substrate, wherein the pore surface of the porous silicon substrate is at least partially coated with a metal silicide; a separator element disposed between the cathode and the anode; and an electrolyte.

Description

Technology Field

[0001] In one aspect, the present disclosure relates to a proton exchange membrane fuel cell (PEMFC) and a method for forming the fuel cell, and more specifically, to a proton exchange membrane fuel cell comprising a new membrane formed of a porous silicon material and a method for forming a new porous silicon membrane for use in a proton exchange membrane fuel cell. In another aspect, the present disclosure relates to a lithium ion rechargeable battery having a positive electrode formed of a porous silicon substrate coated at least partially with a metal silicide. Background Technology

[0002] Fuel cells, particularly proton exchange membrane fuel cells (PEMFCs), are attractive due to their high theoretical efficiency and the pollution-free nature of their reaction byproducts.

[0003] In addition, due to its wide power range, PEMFCs are suitable for stationary applications such as high-power installations for electricity generation, as well as for electric vehicles or other devices requiring an autonomous power source (e.g., power generators, portable electronic devices, etc.).

[0004] Generally, PEMFCs operate by oxidizing fuel (e.g., hydrogen or methanol) at the anode, where protons move from the anode to the cathode through a proton exchange membrane. Electrons generated by the oxidation reaction are transferred back to the cathode through an external circuit, converting chemical energy into electrical energy and heat.

[0005] PEMFCs have many advantages, such as low sensitivity to carbon dioxide, a relatively low operating temperature enabling rapid startup, flexibility in use and thermal management, reduced electrode corrosion issues, and low electrolyte leakage.

[0006] However, PEMFCs also have disadvantages, such as high sensitivity to carbon monoxide, the inability to effectively utilize heat generated due to relatively low operating temperatures (below 100°C), and expensive precious metal catalysts (typically platinum-based).

[0007] Membranes useful for PEMFCs must be gas-impermeable and possess excellent mechanical properties and high proton conductivity. Additionally, membranes useful for PEMFCs should generally be thin, with a thickness of several microns. Finally, the membrane must be made of electrically and chemically stable materials.

[0008] Currently, membranes for PEMFCs are formed from perfluorosulfonate type ionomers (PFSA), such as DuPont's Nafion® and Solvay Specialty Polymers' Aquivion®. In these perfluorosulfonate type ionomers, the proton conductivity of the membrane is ensured by the -SO3H group (sulfonic acid function).

[0009] However, these membranes have disadvantages due to their permeability to methanol and hydrogen. Additionally, the mechanical properties of the membrane deteriorate beyond the optimal operating temperature (80°C). For example, these drawbacks pose a particular constraint in the automotive sector. In practice, this type of application requires PEMFCs that operate at -30 to 120°C in the presence of slightly humid gases (relative humidity 0 to 50%).

[0010] The performance of PEMFCs is also related to the following other problems:

[0011] The presence of carbon monoxide (CO) generally causes poisoning of catalysts. Hydrogen obtained by reforming hydrogen (fuel) typically contains trace amounts of carbon monoxide. The presence of CO reduces the efficiency of platinum-based catalysts that adsorb CO. Consequently, the performance of PEMFCs deteriorates. On the other hand, CO adsorption by platinum-based catalysts is preferred at low temperatures but is affected at high temperatures due to the negative entropy of the adsorption reaction. Therefore, tolerance to CO increases with temperature. Consequently, the performance degradation of PEMFCs due to CO poisoning can be significantly mitigated at high temperatures (approximately 140°C).

[0012] Considering that typical PEMFCs generate 40–50% of their energy in the form of heat, thermal management of PEMFCs is more complex at low temperatures. Therefore, when the cell operates at low temperatures, a large amount of energy must be consumed. Conversely, when the cell operates in the temperature range of 120–140°C, the heat generated by the cell enables the maintenance of the system temperature and requires a smaller cooling system. This is particularly important for applications in the automotive industry. Additionally, at temperatures above 100°C, the generated heat can be used for other purposes (e.g., heating in cogeneration mode).

[0013] Since conventional PFSA type membranes require a continuous supply of moisture, membrane humidification is essential at low temperatures. The additives required for humidification reduce system reliability and complicate the process. Humidification is necessary considering that the membrane's proton conductivity increases with the amount of water contained in the polymer matrix and with the amount of water outside the membrane (relative humidity). However, achieving and managing such humidification is more complex, and it requires more energy as the temperature increases.

[0014] Therefore, there is a need to develop a PEMFC membrane that can be used at low temperatures as well as high temperatures by using a gas with a low moisture content (relative humidity less than 50%).

[0015] Furthermore, the demand for high-capacity rechargeable batteries is strong and increasing every year. Many applications, such as aerospace, medical devices, portable electronic devices, and automotive applications, require batteries with high gravimetric and / or volumetric capacities. Lithium-ion electrode technology finds important applications in this field. However, to date, lithium-ion batteries employing graphite electrodes have a limitation in that their theoretical specific energy density is only 372 mAh / g.

[0016] Silicon is an attractive active electrode for use in lithium-ion battery materials due to its high electrochemical capacity. Silicon has a theoretical capacity of approximately 4,200 mAh / g, which is Li 4.4It corresponds to the Si phase. However, silicon is not widely used in commercial rechargeable lithium-ion batteries. One reason is that silicon undergoes significant volume changes during charge and discharge cycles. For example, silicon can swell up to 400% when charged to its theoretical capacity. Such a volume change can induce significant stress on the active material structure, leading to fracture and pulverization, loss of electrical and mechanical connections within the electrode, and capacity degradation.

[0017] Conventional lithium-ion secondary battery electrodes generally contain a polymer binder used to immobilize the active material on a carbon or graphite substrate.

[0018] However, most polymer binders are not elastic enough to accommodate the large expansion of some high-capacity materials. As a result, active material particles tend to separate from each other and from the current collector. Overall, there is a need to improve and apply high-capacity active materials to lithium-ion secondary battery electrodes to minimize the disadvantages described above.

[0019] US Patent Nos. 8,257,866 and 8,450,012 propose to solve the elasticity and expansion problems of prior art lithium-ion secondary battery electrode materials by providing an electrochemically active electrode material comprising a high surface area template containing a metal silicide and a high-capacity active material layer deposited on the template. The template is known to act as a mechanical support for the active material and / or an electrical conductor between the active material and the substrate. According to the inventors of the '866 and '012 patents, thanks to the high surface area of ​​the template, sufficient active material loading and electrode capacity per surface area can be provided even with a thin active material layer. Therefore, theoretically, the thickness of the active material layer can be kept sufficiently small below the fracture threshold to maintain structural integrity during battery cycling. The thickness and / or composition of the active layer can also be specifically profiled to reduce expansion near the substrate interface and preserve interface connectivity.

[0020] In one aspect, the present disclosure provides a PEMFC membrane that can be used in a wide temperature range and a wide relative humidity range.

[0021] More specifically, the present disclosure provides a new porous silicon wafer substrate material, a method for forming the new porous silicon wafer substrate material, and a method for using the new porous silicon wafer substrate material as a PEMFC membrane. More specifically, the present disclosure provides a method for forming a new porous silicon wafer for use as a PEMFC membrane separator using microelectromechanical system (MEMES) technology. According to the present disclosure, a silicon wafer is selectively masked using resist deposition and photolithography techniques, and a selected portion of the wafer is electrochemically etched to form pores or channels extending through the silicon wafer. The channels or pores are preferably substantially cylindrical and have a relatively high length-to-cross section diameter aspect ratio (e.g., 25:1, preferably 35:1, more preferably 50:1 or higher).

[0022] In one embodiment, the pore size, membrane selectivity, and ion conductivity are "tuned" by inorganic doping of the silicon wafer to allow only positively charged ions to pass through the membrane to the cathode when the membrane is used as a separation barrier in a PEMFC.

[0023] The present disclosure also provides a PEMFC in which a novel porous silicon wafer is used as a membrane material. More specifically, the present disclosure provides a PEMFC comprising a separator membrane element formed from a porous silicon wafer.

[0024] In one embodiment, the pores of the porous silicon wafer are substantially cylindrical through holes. Preferably, the cylindrical through holes have a length-to-diameter aspect ratio of 25:1, preferably 35:1, more preferably 50:1 or greater.

[0025] In another embodiment, the pore surface of the porous silicon wafer is treated to improve surface ion conductivity. For example, the surface of the pores can be modified by the deposition of a precious metal catalyst, preferably platinum.

[0026] The present disclosure also provides a PEMFC comprising an electrical assembly including a cathode (positive electrode) and an anode (negative electrode) electrodes respectively located in the fuel cell, and a proton exchange membrane formed of a porous silicon wafer sandwiched between two porous sheets coated with a precious metal catalyst.

[0027] In one embodiment of the PEMFC, the catalyst comprises a precious metal, preferably platinum.

[0028] In addition, in the case of a lithium-ion secondary battery, to overcome the problems of the prior art as described above, a high surface area porous silicon substrate material for forming an anode electrode for a lithium-ion secondary battery is provided. More specifically, according to the present disclosure, the silicon substrate material undergoes electrochemical etching to form an interconnected nanostructure or through-holes or pores through the silicon substrate material. Subsequently, an electrochemically active material such as a metal silicide is formed on the pore surface of the silicon substrate material by depositing a suitable metal, such as titanium, tungsten, or cobalt, on the porous silicon substrate material, and various deposition techniques are provided, including but not limited to chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), thermal CVD, electroplating, electroless plating, and / or solution deposition techniques, and the metal coating on the porous silicon substrate material is converted into a corresponding metal silicide by heating.

[0029] The produced substrate is a porous silicon substrate comprising a metallurgically bonded surface layer of metal silicide on the walls of a porous structure, which can be advantageously used as an electrode in a lithium-ion secondary battery.

[0030] Although the generated porous substrate material may have slightly lower efficiency per charge compared to conventional carbon or graphite-based electrodes used in lithium-ion secondary batteries, the porous structure offers several significant advantages. First, the porous structure allows protons to travel through the electrode matrix for a longer period of time. As a result, expansion during charging cycles is significantly reduced. Therefore, the substrate is less likely to form dendrites or fractures during charging cycles. Additionally, when used as an anode, the overall performance can be further improved by making the anode much larger than the cathode.

[0031] The present disclosure also provides a lithium-ion battery comprising: a cathode electrode; an anode electrode formed of a porous silicon substrate in which the pore surface of the porous silicon substrate is at least partially coated with a metal silicide; a separator element disposed between the cathode and the anode; and an electrolyte. The silicon substrate may comprise single-crystal silicon, polycrystalline silicon, or amorphous silicon. It is preferable that the pores have a length-to-diameter aspect ratio of 25:1, preferably 35:1, more preferably 50:1 or greater, and the electrolyte comprises a conventional lithium salt electrolyte such as lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4) in an organic solvent such as vinylene carbonate, 1,3-propane sultone, 2-propylmethanesulfate, cyclohexylbenzene, t-amylbenzene, or adiponitride, which are given as examples.

[0032] In one embodiment, the metal silicide coating is selected from the group consisting of titanium silicide (TiSi2), cobalt silicide (CoSi2), and tungsten silicide (WSi2), which are given as examples.

[0033] Additionally, the present disclosure provides an electrode for a lithium-ion battery comprising an anode electrode formed of a substrate in which the surface region of the pores is at least partially coated with a metal silicide. The silicon substrate may comprise single-crystal silicon, polycrystalline silicon, or amorphous silicon, and the pores have a length-to-diameter aspect ratio of 25:1, preferably 35:1, more preferably 50:1 or higher, and the metal silicide is preferably selected from the group consisting of titanium silicide (TiSi2), cobalt silicide (CoSi2), and tungsten silicide (WSi2), which are given as examples. Brief explanation of the drawing

[0034] FIG. 1 is a schematic flowchart illustrating the formation of a porous silicon wafer useful as a film in a PEMFC according to one embodiment of the present disclosure. FIGS. 2a to 2h are cross-sectional views showing silicon wafers at various stages of the process of FIG. 1. FIG. 3 is a perspective view illustrating the formation of a porous silicon wafer useful as a porous membrane in a PEMFC according to a second embodiment of the present disclosure, similar to FIG. 1. FIGS. 4a to 4k are cross-sectional views showing silicon wafers at various stages of the process of FIG. 3. FIGS. 5a to 5d are schematic cross-sectional views illustrating the formation of a porous silicon wafer produced according to another embodiment of the present disclosure. FIG. 6 is a schematic diagram of a PEMFC according to the present disclosure. FIG. 7 is a schematic block diagram illustrating a process for producing an electrode material according to one embodiment of the present disclosure. FIGS. 8a and FIGS. 8b are cross-sectional views of an electrode material at various production stages according to the present disclosure. FIG. 9 is a schematic block diagram of a process for producing an electrode material according to another embodiment of the present disclosure. FIG. 10 is a schematic block diagram of another process for manufacturing an electrode material according to the present disclosure. FIG. 11 is a cross-sectional view of a secondary battery made according to the present disclosure. FIG. 12 is a schematic block diagram of another process for producing an electrode material according to the present disclosure. FIG. 13 is a cross-sectional view of a secondary battery according to the present disclosure. FIG. 14 is a perspective view of a battery made according to the present disclosure. Specific details for implementing the invention

[0035] Further features and benefits of the present disclosure will be apparent from the following detailed description, wherein similar numbers denote similar parts, and the terms 'top' and 'bottom', 'left' and 'right' are used in a relative rather than an absolute sense to facilitate explanation and to describe the relative positions of elements. The terms may be used interchangeably.

[0036] A mode for carrying out the present disclosure will be described in detail below with reference to the drawings.

[0037] First embodiment

[0038] FIGS. 1 and FIGS. 2a through 2h are schematic and cross-sectional views illustrating the steps of manufacturing a porous silicon wafer according to one embodiment of the present disclosure. In the drawings, the cross-sectional dimensions of the pores are enlarged for clarity in the horizontal direction of the drawings.

[0039] Referring to FIGS. 1 and FIGS. 2a through 2h, as shown in FIG. 2a, starting from a silicon wafer (10), a dielectric material is deposited in step 100 to form a hard mask on the front and back surfaces of the wafer (10). In this case, on each side of the wafer, a 50 nm layer of SiO2 (12a, 12b) is first deposited, followed by a 300 nm layer of SiNx (14a, 14b).

[0040] Next, in step 102, the front mask (14a) is rotated on the front of the wafer and patterned with a photoresist (16), and a polymer material (18) is rotated on the back of the wafer. The pattern (16) defines a hard mask etch used for a deep anisotropic etch. An alignment element (not shown) for a subsequent back etch is also formed in this step (102).

[0041] FIG. 2c illustrates a cross-section of the wafer after pad hardmask etching (step 104). Here, dry etching (plasma) is used to control the edges of the hardmask to ensure uniform edge erosion during KOH etching.

[0042] As illustrated in FIG. 2d, the front side of the wafer is rotated with a polymer (20) in step 106 to protect the pattern on the front side while the pad structure on the back side is patterned in 22 in step 108. Alternatively, a back hard mask may be deposited after the patterning of the front side. The back pattern (22) is aligned with the marks (not shown) formed on the front side of the wafer to ensure that the marks formed on the front side of the wafer are aligned.

[0043] After patterning the back pad structure in step 108, dry etching (plasma) is used in step 110 to etch the dielectric while controlling the edge shape. This is illustrated in FIG. 2e.

[0044] FIG. 2e illustrates a nitride (PAD) etch of a back pad structure aligned with a front pattern. Following this step, a resist strip and a wafer cleaning step (112) prepare a wet etch of the feature.

[0045] FIG. 2f illustrates the configuration of the wafer after the resist strip and before potassium hydroxide (KOH) or other anisotropic etching in step 114. Wet etching is preferred so that both sides can be etched simultaneously to ensure the same etch depth on both sides. However, plasma etching can be used to etch each side independently. Open areas (24) depicted by the etching of the dielectric are shown on each side of the wafer.

[0046] The next step 116 is to locally etch silicon to create a region (26) for defining a thinner silicon region for forming a porous silicon material in the subsequent step 118 described below. This step may use a tool etch, but it is preferable to perform it using a simple open bath etch. FIG. 2g shows a wafer after anisotropic wet etch (116).

[0047] Then, the silicon wafer thinned or contoured in step 116 is electrochemically etched in step 118 by applying a uniform electric field across the wafer while immersing the wafer in an etchant such as dimethylformamide (DMF) / dimethylsulfoxide (DMSO) / HF etchant in an electrochemical immersion cell to form through holes or pores (28) through the thinned section (26) as illustrated in FIG. 2h. The growth of clearly defined cylindrical micropores or through holes can be controlled by controlling etching conditions, namely the etching current density, etchant concentration, temperature, silicon doping, etc., which is incorporated herein by reference to Chapter 1 of Santos et al., Electrochemically Engineered Nanoporous Material, Springer Series in Materials Science 220 (2015).

[0048] The generated pores have a high aspect ratio of length to cross-sectional diameter, and generally, the length-to-diameter aspect ratio is 25:1, preferably 35:1, more preferably 50:1 or higher. The resulting structure illustrated in FIG. 2h comprises a porous silicon wafer (30) having substantially cylindrical through-holes or pores (28) with a length of 180 μm and a diameter of 1.6 μm, i.e., an aspect ratio of 112.5:1, which is very effective for use as a membrane in a PEMFC to be described below. Then, the surface of the generated porous silicon wafer (30) can be coated with a precious metal in step (120), and the generated precious metal-coated porous wafer (30) can be incorporated as a membrane in a PEMFC as described below.

[0049] Second embodiment

[0050] FIGS. 3 and 4 illustrate a second embodiment of the present disclosure. Process steps 200 to 216 of FIG. 3 and cross-sectional views of FIGS. 4a to 4g are identical to process steps 100 to 116 of FIG. 1 and cross-sectional views of FIGS. 2a to 2g described above.

[0051] However, referring to FIG. 4h, once the contour etching step (216) is completed, a photolithography resist step (220) is applied to the front of the contour wafer, followed by a thin metal layer (40) on the back of the contour wafer by sputtering in step 218. The metal layer (40) on the back of the wafer improves electrical contact with the wafer, while the resist (42) applied in the photolithography step 220 limits the formation of porous silicon in the thinned region (26) of silicon in the subsequent etching step described below.

[0052] As shown in FIG. 4i, electrochemical etching (step 222) is used to form porous silicon (44) within an area not protected by the resist (42).

[0053] After the porous silicon formation (step 222), in step 224, the front surface is protected by rotating the photoresist (46) (see FIG. 4j), and a wet etch (step 226) is used to remove a thin metal (40) from the back surface. Then, the front resist (46) is stripped in the resist stripping step (228). FIG. 4k illustrates the configuration after the metal etch and photoresist strip. Then, in step 230, the pores can be coated with a precious metal. Optionally, an additional process such as atomic layer deposition can be used to coat the surface or diameter of the pores with a precious metal before the stripping step (228). The resulting porous silicon wafer can be incorporated into the membrane of a PEMFC as described below.

[0054] Third embodiment

[0055] FIGS. 5 and FIGS. 6 illustrate a third embodiment of the present disclosure. This process begins by covering one side of a silicon wafer (400) with a resist layer (402) and the opposite side with a sacrificial metal layer (404) formed of a precious metal, for example, platinum (see step FIG. 5a). The resist layer (402) is patterned in step (502) and etched in step (504) to expose a selected surface (406) on one side of the wafer (400) (Fig. 5b). Then, the resist-covered and patterned wafer is electrochemically etched by applying a uniform electric field across the metal layer (404) and the substrate wafer (400) when the wafer is immersed in an electrochemical cell containing an etchant such as HF and hydrogen peroxide (H2O2) in step (506), thereby creating substantially uniform pores (408) in the metal layer (404) through the exposed portion of the substrate (400) (Fig. 5c). As before, the growth of clearly defined cylindrical micropores with two holes can be controlled by controlling the etching conditions, namely the etching current density, etching concentration, temperature, silicon doping, etc., again following the teachings of Santos et al. Alternatively, the formation of micropores or through holes can be controlled by covering a selected portion of the silicon wafer with a nanoporous anodic alumina mask. Self-ordered nanoporous anodic alumina is an alumina-based nanoporous matrix characterized by a dense arrangement of cells arranged in a hexagonal pattern, in which cylindrical nanopores grow vertically on a base aluminum substrate at its center.Nanoporous anodic alumina can be produced by electrochemical anodization of aluminum, which also follows the teaching of Santos et al., which is incorporated into this document for reference. The resist layer (402) and the sacrificial metal layer (404) can be removed in step (508) to leave a porous silicon wafer having a section (405) having substantially cylindrical through-holes or pores (408) (Fig. 5), which can then be coated with a noble metal catalyst coating, and the resulting porous silicon substrate can be incorporated as a membrane of a PEMFC as described below.

[0056] The precious metal catalyst may be platinum black, platinum-on-carbon and / or other complex precious metal materials (e.g., silver, gold, rhodium, iridium, palladium, ruthenium and osmium).

[0057] Now, referring to Fig. 6, the PEMFC is assembled as follows:

[0058] The porous silicon membrane formed as described above can be integrated into a PEMFC module (700) as schematically illustrated in FIG. 6. The PEMFC module (700) includes the porous silicon membrane (702) formed as described above and is sandwiched between an anode or negative electrode (704) and a cathode or positive electrode (706). The anode / membrane / cathode sandwich is, in turn, sandwiched between a hydrogen gas flow channel or plate assembly (708) on the anode side and an oxidant (oxygen or air) on a flow channel or plate assembly (710) on the cathode side. The assembly is fixed together in a case (not shown) comprising a fitting for flowing oxygen-hydrogen gas and oxide, a sump and drain (not shown) for discharging water formed by the reaction of hydrogen gas and oxide, and an electrical circuit (712) including electrodes (714, 716) coupled across the payload / source (718).

[0059] When in operation, gaseous hydrogen fuel is delivered to the anode side of the fuel cell through the hydrogen gas flow assembly (708), and oxygen gas (oxygen or air) is delivered to the cathode side of the cell through the oxidant gas flow assembly (710). At the anode (704), a platinum catalyst separates hydrogen into positive hydrogen ions (protons) and negatively charged electrons. A porous silicon membrane (702) allows only positively charged ions to pass to the cathode. At the cathode (706), the electrons and positively charged hydrogen ions combine with oxygen to form water, which is then collected at the bottom of the cell and removed.

[0060] Various modifications may be made to the above disclosure. For example, as mentioned above, the precious metal catalyst may be coated directly into the pores of the porous silicon substrate film, or the porous silicon substrate film may be sandwiched between porous paper or carbon or graphite sheets impregnated with precious metals. Additionally, other hydrogen fuel sources, such as methanol and chemical hydrogen compounds, may be used.

[0061] Now, referring to FIGS. 7 to 14, the improved lithium-ion secondary battery according to the present disclosure is formed as follows.

[0062] In particular, referring to FIG. 7, starting with a thin single-crystal silicon wafer (10) with a thickness of 50 to 200 millimeters, the wafer (1010) is electrochemically etched by applying a uniform electric field across the wafer while immersing the wafer in an etchant such as dimethylformamide (DMF) / dimethyl sulfoxide (DMSO) / hydrogen fluoride (HF) etchant in an electrochemical immersion cell during the electrochemical etching step (1012), thereby forming micron-sized through-holes or pores (1016) in the wafer as illustrated in FIG. 8a. The growth of clearly defined cylindrical micropores or through-holes can be controlled by controlling etching conditions, namely the etching current density, etchant concentration, temperature, silicon doping, etc., which is incorporated herein by reference to Chapter 1 of Santos et al., Electrochemically Engineered Nanoporous Material, Springer Series in Materials Science 220 (2015). The growth of clearly defined cylindrical micropores or through holes can be controlled by controlling etching conditions, namely etching current density, etchant concentration, temperature, silicon doping, etc., and this is incorporated here with reference to Chapter 1 of Santos et al., Electrochemically Engineered Nanoporous Material, Springer Series in Materials Science 220 (2015).

[0063] The generated pores have a high aspect ratio of length to cross-sectional diameter, and generally, the length to diameter aspect ratio is 25:1, preferably 35:1, more preferably 50:1 or higher. The resulting structure illustrated in FIG. 8a comprises a porous silicon wafer (1018) having substantially cylindrical through-holes or pores (1016) that are highly effective for use as electrodes in a lithium-ion battery, for example, having a length of 180 μm and a diameter of 1.6 μm, i.e., an aspect ratio of 112.5:1, which is highly effective for use as electrodes in a lithium-ion battery as described below. Then, the walls of the generated porous silicon wafer (1018) are coated with a metal such as titanium, tungsten, or cobalt in step 1020, and the metal-coated porous silicon wafer is heat-treated in step 1022 to convert the deposited metal into the corresponding metal silicide (1025). As a result, a porous silicon substrate material (1024) is produced in which the walls of the pores of the material are coated with a thin layer of metal silicide material (1026) (Fig. 8a).

[0064] FIG. 9 illustrates an alternative embodiment of the present disclosure. The process begins with a silicon wafer (1030), and a thin metal layer (1032) is applied to the back surface of the wafer (1030), for example, by sputtering in step (1034). The metal layer (1032) on the back surface of the wafer improves electrical contact with the wafer. Electrochemical etching (step 1036) is used to form pores (1037) through the silicon wafer (1030). After forming porous silicon, the thin metal (1032) is removed from the back surface using wet etching (step 1038). Then, a porous silicon wafer similar to the porous silicon substrate shown in FIG. 8a is coated with metal in step 1040, and the metal is converted into silicide in a heating step (1042) similar to the first embodiment. As a result, a porous silicon substrate is produced in which the pore wall surface is coated with a metal silicide similar to the porous silicon substrate shown in Figure 8B.

[0065] FIG. 10 is a drawing illustrating a third embodiment of the present disclosure. This process begins by covering one side of a silicon wafer (1050) in step 1052 with a sacrificial metal layer (1054) formed of a precious metal, for example, platinum. Subsequently, electrochemical etching is performed by applying a uniform electric field across the metal layer (1054) and the substrate wafer (1050) when the silicon wafer (1050) is immersed in an electrochemical cell containing an etchant such as hydrogen fluoride (HF) and hydrogen peroxide (H2O2) in step 1056, thereby creating substantially uniform pores (1058) in the metal layer (1054) through the exposed portion of the silicon wafer substrate (1050). As before, the growth of clearly defined cylindrical micropores or through-holes can be controlled by controlling etching conditions (etching current density, etching concentration, temperature, silicon doping, etc.), again following the teachings of Santos et al. Alternatively, the formation of micropores or through-holes can be controlled by covering a selected portion of the silicon wafer with a nanoporous anodic alumina mask. Self-ordered nanoporous anodic alumina is an alumina-based nanoporous matrix characterized by a dense arrangement of cells arranged in a hexagonal pattern, in which cylindrical nanopores grow perpendicularly to the base aluminum substrate at its center. The sacrificial metal layer (54) can then be removed in step (58), which leaves a porous silicon wafer having a substantially cylindrical through-hole or pore with a length-to-diameter aspect ratio of 25:1, preferably 35:1, more preferably 50:1 or greater, i.e., a porous silicon substrate similar to the porous silicon substrate shown in FIG. 8a.Next, the porous silicon substrate is coated with metal in step 1058 and heated in step 1060 to convert the metal into a metal silicide, thereby producing a porous silicon substrate with the pore walls coated with a metal silicide similar to Fig. 8b.

[0066] The porous silicon wafer produced above is assembled into a lithium-ion battery as described below.

[0067] FIG. 11 illustrates a lithium-ion battery (1060) according to the present disclosure. The battery (1060) comprises a case (1062), an anode (1064) formed of a metal silicide-coated porous silicon substrate formed as described above, and a cathode (1066) formed of, for example, graphite, separated by a film or separator (1068).

[0068] The anode (1064) and cathode (1066) are each connected to external tabs (1070, 1072). A lithium-containing electrolyte (1074), for example, lithium cobalt oxide, is contained within the battery (1060). Both the anode and cathode allow lithium ions to move in and out of the structure through a process called intercalation or deintercalation. During discharge, cations move from the negative electrode (anode) to the positive electrode (cathode) to form a lithium compound through the electrolyte, and the electrodes flow in the same direction through the external circuit to form a lithium compound. When the battery is charging, the opposite phenomenon occurs, causing lithium ions and electrodes to move back to the negative electrode, moving to the negative electrode with a higher energy share.

[0069] An advantage of the present disclosure is that the anode can be made physically larger, i.e., thicker, than the cathode. The porous structure with increased anode thickness allows more time for protons to move into the electrode matrix. Additionally, less lithium electrolyte is required for similar energy storage. Furthermore, because protons move more slowly to the anode, faster charge and discharge rates can be achieved without the risk of electrode breakage or fracture. Modifications may be made within the spirit and scope of the above disclosure. For example, while the production of the anode has been described as being formed from a single-crystal silicon wafer, a single-crystal silicon ribbon may be advantageously used to form the anode. Referring to FIG. 12, using a silicon ribbon (1080), a continuous process is allowed in which the ribbon passes through an electrochemical etching bath (1082) to form pores through the ribbon, then passes through a metal coating station (1084) and then through a heat treating station (1086) to form metal silicides on the surface of the pore walls. As a result, the porous silicon metal silicide-coated ribbon can be used to form a lithium-ion battery using standard roll manufacturing techniques. For example, referring to FIG. 13, a silicide-coated porous silicon ribbon anode electrode (1084) can be assembled in a stack with a cathode electrode (86) between separator sheets (1088). The electrodes (1084, 1086) and the separator sheet (88) are wound together in a jelly roll, and then the positive tab (1092) and the negative tab (1094) are inserted into a case (1090) extending from the jelly roll.Then, the tab can be welded to the exposed portion of the electrode (1084, 1086), the case (1090) filled with electrolyte, and the sealed case (1090). As a result, a high-capacity lithium-ion secondary battery is produced that includes a porous metal silicide coated with a porous silicon ribbon, which allows the anode material to repeat charging and discharging without adverse effects.

[0070] Other modifications are also possible. For example, polycrystalline silicon or amorphous silicon may be used instead of single-crystal silicon chips or single-crystal silicon ribbons. Additionally, while tungsten, cobalt, and titanium have been described as preferred metals for forming metallic silicides, other metals commonly used for forming metallic silicides, including silver (Ag), aluminum (Al), gold (Au), palladium (Pd), platinum (Pt), Zn, Cd, Hg, B, Ga, In, Th, C, Si, Ge, Sn, Pb, As, Sb, Bi, Se, and Te, may be preferred. Furthermore, while lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4) have been described as useful electrolytes, other electrolytes, including but not limited to lithium cobalt oxide (LiCoO2), are also commonly used in lithium-ion batteries.

Claims

Claim 1 A proton exchange membrane fuel cell (700), wherein the proton exchange membrane fuel cell (700) comprises a separator membrane element (702) formed from a contoured single-crystal, polycrystalline, or amorphous porous silicon wafer (30), wherein the porous silicon wafer (30) has a thinned section (26) having a plurality of through-hole pores (28) penetrating the membrane in the thickness direction, wherein the through-hole pores (28) have a diameter of microns and a length-to-diameter aspect ratio of 50:1 or greater, and the walls of the through-hole pores (28) are metal silicides of precious metals selected from the group consisting of silver, gold, platinum, rhodium, iridium, palladium, ruthenium, and osmium. Proton exchange membrane fuel cell (700) coated with a coating. Claim 2 A proton exchange membrane fuel cell (700) according to claim 1, wherein the porous silicon wafer is sandwiched between paper, carbon, or graphite sheets impregnated with a precious metal that may be selected from the group consisting of silver, gold, platinum, rhodium, iridium, palladium, ruthenium, and osmium. Claim 3 As a proton exchange membrane fuel cell (700), the proton exchange membrane fuel cell (700) comprises an electrical assembly including an anode (706) and a cathode (704) separated by a porous membrane (702), the porous membrane (702) comprises a porous silicon wafer (30) formed and contoured from single-crystal, polycrystalline, or amorphous porous silicon, the porous silicon wafer (30) comprises a thinned section (26) having a plurality of through-hole pores (28) penetrating the membrane in the thickness direction, the through-hole pores (28) have a diameter of micron size and a length-to-diameter aspect ratio of 50:1 or greater, and the walls of the through-hole pores (28) are coated with a metal silicide coating of a precious metal selected from the group consisting of silver, gold, platinum, rhodium, iridium, palladium, ruthenium, and osmium, proton Exchange membrane fuel cell (700). Claim 4 In paragraph 3, the porous silicon wafer is sandwiched between sheets of paper, carbon, or graphite impregnated with a precious metal selected from the group consisting of silver, gold, platinum, rhodium, iridium, palladium, ruthenium, and osmium, in a proton exchange membrane fuel cell (700). Claim 5 delete Claim 6 delete Claim 7 delete Claim 8 delete Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 delete Claim 13 delete Claim 14 delete Claim 15 delete

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