A method, device and application of pre-creep treatment of graphite bipolar plate

Abstract

The application discloses a graphite bipolar plate pre-creep method and device and application thereof, and belongs to the technical field of fuel cell stack manufacturing. The technical problem to be solved is that the graphite bipolar plate is thinned in thickness after high-temperature operation, the length of the core is shortened, and the pressing force is attenuated. A post-processing procedure of a core component of the stack, that is, a graphite bipolar plate, is provided. Technical scheme points are as follows: the procedure comprises the following steps: bipolar plates and active area stainless steel gaskets are alternately stacked, a disc spring is used to keep a pressure of 20-30 kN in a tool, pre-creep is completed by keeping the temperature at 120 DEG C + / - 5 DEG C for 12-24 hours, and the tool is unloaded after cooling. The device comprises a lower tool, a positioning rod, a pressing plate with a boss, a chromium-vanadium alloy disc spring group, an upper tool and a locking screw and nut.

Description

Technical Field

[0001] This invention relates to the field of fuel cell stack manufacturing technology, specifically to a graphite bipolar plate pre-creep method, apparatus, and application. Background Technology

[0002] Expanded graphite bipolar plates, as a core component of low-cost fuel cell stacks, have a drawback: during high-temperature operation, the graphite and resin system exhibits a high coefficient of thermal expansion in the vertical direction. When 200-300 bipolar plates are stacked together, the accumulated thickness of the core increases dramatically upon heating. However, these plates, secured by straps or bolts, have no room for further thickness increase, leading to severe compression. After the stack stops operating, the bipolar plates cease expansion, but the previous severe compression has thinned them. This increases the clearance of the disc springs that support changes in stack core height, reducing the compressive force on the core. Under conditions of shock and vibration caused by vehicle operation, the stack core risks collapse.

[0003] Relevant patent documents retrieved:

[0004] This document, published in Japan (publication number JP2019133882A) on August 8, 2019, discloses a method for manufacturing a fuel cell stack. The method involves housing a fuel cell unit laminate, formed by stacking multiple fuel cell units, within a laminate housing. As part of this manufacturing method, a method for assembling the fuel cell stack is disclosed. This method involves applying a load to the fuel cell unit laminate, which is stacked using a battery lamination jig, and then, after applying a certain load, housing the fuel cell unit laminate within the laminate housing. The steps include: a first step of configuring a stacking reference jig through the laminate housing; a second step of stacking multiple fuel cell units along the stacking reference jig; a third step of configuring and pressurizing a stacking manifold in the stacking direction of the fuel cell units; a fourth step of fixing the stacking manifold to the laminate housing with screws; a pre-creep process; and finally, fixing the laminate housing.

[0005] The prior art represented by the aforementioned documents has at least the following unresolved technical problems or defects: Pre-compression and fixation are only performed on the entire fuel cell stack, without independent pre-creep treatment of the graphite bipolar plates before leaving the factory. This fails to stabilize the plate thickness and eliminate initial creep deformation from the root cause, and the problem of pressure force attenuation still exists after the fuel cell stack is in operation. Relevant evidence is that Japanese Patent Document JP2019133882A discloses a method for bonding and pre-compressing fuel cell stacks, but does not provide relevant technical guidance on how to treat the bipolar plate material to solve the problem of pressure force attenuation.

[0006] To address the issue of pressure reduction caused by changes in core length, this invention designs a tooling for bipolar plate post-processing that can effectively solve the problem of core length variation. Summary of the Invention

[0007] The purpose of this invention is to provide: A method for pre-creeping graphite bipolar plates, and related technologies, to solve technical problems such as thickness reduction after high-temperature operation, core length shortening and thickness uniformity improvement, or combinations thereof.

[0008] Terminology Explanation: Unless otherwise defined, all technical terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this subject matter pertains. Unless otherwise stated, all patents, patent inventions, and disclosures cited throughout this document are incorporated herein by reference in their entirety. Where multiple definitions exist for terms herein, the definitions provided in this chapter shall prevail.

[0009] It should be understood that the above brief description and the following detailed description are exemplary and for illustrative purposes only, and do not limit the subject matter of the invention in any way. In this invention, the singular is used in conjunction with the plural unless otherwise specifically stated. It should also be noted that, unless otherwise stated, the use of “or” or “or” means “and / or”. Furthermore, the use of the term “comprising” and other forms such as “including,” “containing,” and “contains” are not limiting.

[0010] The definitions of standard academic terms can be found in the references national standard GB / T28816-2020 "Fuel Cell Terminology" and "Fuel Cell System and Design" (edited by Academician Yi Baolian).

[0011] Unless otherwise specified, conventional methods within the scope of the art, such as pre-creep treatment, bipolar plate to GDL / MEA contact resistance testing, thermal expansion coefficient testing, and stack clamping force testing, shall be used.

[0012] Unless specifically defined herein, the use of all commercially available products herein employs standard techniques. For example, it may be carried out using the manufacturer's instructions for use with the kit, or in accordance with methods known in the art or the description of this invention. The techniques and methods described herein can generally be implemented according to conventional methods well known in the art, based on the descriptions in the various summary and more specific documents cited and discussed in this specification.

[0013] The “range” disclosed in this document takes the form of a lower limit and an upper limit. It can be one or more lower limits and one or more upper limits, respectively. A given range is defined by selecting a lower limit and an upper limit. The selected lower and upper limits define the boundaries of the particular range. All ranges that can be defined in this way are inclusive and composable; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is also expected that ranges of 60-110 and 80-120 are also expected. Furthermore, if the minimum range values ​​are listed as 1 and 2, and if the maximum ranges are listed as 3, 4, and 5, then the following ranges are all expected: 1-2, 1-4, 1-5, 2-3, 2-4, and 2-5.

[0014] In this invention, unless otherwise specified, the numerical range "ab" represents a shortened representation of any combination of real numbers from a to b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed in this document, and "0-5" is simply a shortened representation of these numerical combinations.

[0015] The term "graphite bipolar plate" as used in this article refers to a bipolar plate assembly made of graphite or graphite-based composite materials, processed to form a flow field structure, used to separate adjacent single cells, conduct current, distribute reaction gases and coolants, and discharge reaction products.

[0016] The term "pre-creep" used in this article refers to a short-term creep treatment applied under constant temperature and stress before the material is put into service. The purpose is to preemptively consume the initial transient creep of the material, stabilize its microstructure, and eliminate residual stress, thereby improving dimensional stability and performance consistency throughout its service life. In the field of graphite bipolar plates for fuel cells, it is a key pretreatment process for stack assembly and lifespan assurance.

[0017] As used in this article, the term "fuel cell" refers to an electrochemical power generation device that directly converts the chemical energy of fuel (such as hydrogen) and oxidant (such as oxygen) into electrical energy through an electrochemical reaction. It features high energy conversion efficiency and environmental friendliness, and is the core carrier for hydrogen energy utilization. In this field, it typically refers specifically to proton exchange membrane fuel cells (PEMFCs), which form the basis for the application scenarios of fuel cell stacks and core components.

[0018] The term "fuel cell stack" as used in this article refers to a power generation assembly consisting of multiple individual fuel cells (including membrane electrode assemblies, bipolar plates, etc.) stacked in series. It is the core power generation unit of a fuel cell system, responsible for converting the chemical energy of fuel into electrical energy, and undertaking the functions of fluid distribution, current conduction, and structural support.

[0019] The term "post-processing" used in this article refers to a series of pre-processing processes (such as pre-creep, aging, and surface treatment) performed after the core components of a fuel cell (such as graphite bipolar plates) have been formed and processed, and before they are assembled into the fuel cell stack. The purpose is to eliminate residual stress from processing, stabilize the microstructure of materials, and improve the dimensional accuracy and service performance of components. It is a key manufacturing step to ensure the long-term reliability of the fuel cell stack.

[0020] The term "core" used in this article refers to the main structure of the fuel cell stack, which is formed by stacking core power generation components such as membrane electrodes, bipolar plates, and seals. It is the core area where the fuel cell stack undergoes electrochemical reactions and realizes energy conversion, and its dimensional stability directly affects the sealing performance and output performance of the fuel cell stack.

[0021] The term "resin" as used in this article refers to thermosetting polymeric materials (such as phenolic resin, epoxy resin, etc.) used as binders or fillers in graphite-based composite bipolar plates. These materials are used to improve the mechanical strength and density of the graphite matrix and are key components affecting the creep behavior and dimensional stability of the bipolar plates.

[0022] The term "coefficient of thermal expansion" used in this article refers to a physical quantity that characterizes the degree of thermal expansion and contraction of a material with changes in temperature, measured in units of 1 / ℃. In the field of fuel cells, it directly affects the compatibility of graphite bipolar plates with other components of the fuel cell stack at operating temperatures and is a key indicator for evaluating the thermal cycling reliability of the fuel cell stack.

[0023] The term "bolt" as used in this article refers to a fastener used to apply and lock axial preload in fuel cell stack assembly or pre-creep tooling. It is a core structural component that maintains fuel cell stack assembly pressure and ensures sealing and contact performance by tightening the screw and nut to compress the fuel cell stack or tooling.

[0024] The term "compression" as used in this article refers to the process of applying external force to a graphite bipolar plate or fuel cell stack to induce elastic or viscoelastic deformation in the thickness direction. It is the core operation of pre-creep treatment and fuel cell stack assembly, used to eliminate gaps, stabilize the contact interface, and consume the initial creep deformation of the material in advance.

[0025] The term "disc spring" as used in this article refers to a conical disc-shaped elastic element commonly used in fuel cell stack assembly or pre-creep tooling. It provides constant or controllable axial preload through stacking and is used to compensate for pressure decay caused by creep of graphite bipolar plates. It is a key elastic component to ensure the long-term sealing and contact performance of the fuel cell stack.

[0026] The term "pressing force" used in this article refers to the axial clamping force applied to the graphite bipolar plates or fuel cell stack during pre-creep treatment or fuel cell stack assembly, measured in MPa. It is a core process parameter for controlling the degree of pre-creep and ensuring the adhesion and sealing effect of the plates, and is usually set to 80%-120% of the rated assembly pressure of the fuel cell stack.

[0027] The term "collapse" used in this article refers to the phenomenon in which the flow channel ribs or matrix of a graphite bipolar plate undergoes irreversible plastic deformation under high temperature and pressure, resulting in a reduction in plate thickness and damage to the flow channel structure. It is one of the main failure modes affecting the dimensional stability and performance degradation of the fuel cell stack.

[0028] The term "degradation" used in this article refers to the performance decline of fuel cell stacks or core components during long-term service due to factors such as material aging, creep, and corrosion (e.g., reduced output power, increased contact resistance, and seal failure). Pre-creep treatment can effectively suppress component creep, thereby delaying the performance degradation of the stack.

[0029] The term "tooling" used in this article refers to specialized process equipment used for positioning, clamping, loading, and locking components during the pre-creep treatment of graphite bipolar plates or the assembly of fuel cells. It typically includes structures such as base plates, pressure plates, positioning rods, and elastic elements, and is the core carrier for achieving batch pre-processing and precise assembly.

[0030] The term "pressure plate" as used in this article refers to a flat plate structure used in pre-creep tooling or stack end plate assemblies to transmit axial pressure and distribute load evenly. It is usually used in conjunction with elastic elements (such as springs or disc springs) to evenly transmit the pressing force to the stacked graphite bipolar plates, avoiding local stress concentration that could lead to plate breakage.

[0031] The term "positioning rod" used in this article refers to a guide rod in a tooling or fuel cell stack used to limit the horizontal displacement of stacked components (such as bipolar plates and membrane electrodes) and ensure stacking accuracy. It is a key positioning element that ensures electrode alignment, flow channel alignment and fuel cell stack structural stability.

[0032] The term "screw and nut" as used in this article refers to a combination of threaded fasteners used in tooling or fuel cell stacks to apply and lock axial preload. By tightening the nut, the pressure plate and base plate are locked together to maintain the constant pressure required for pre-creep treatment or fuel cell stack assembly. It is the core structure for achieving pressure locking and dimensional stability.

[0033] The term "protrusion" used in this article refers to a locally raised structure on a tooling or pressure plate, used to position elastic elements (such as springs or disc springs), centrally transmit axial loads, prevent slippage of elastic elements, and ensure that pressure is evenly distributed on the stacked bipolar plates. It is a key structural feature for optimizing the loading structure and improving the uniformity of pretreatment.

[0034] The term "axial load" used in this article refers to the external force applied along the stacking direction (thickness direction) of the fuel cell or tooling. It is the core load form that drives the bipolar plates to compress and eliminate gaps during pre-creep treatment and fuel cell assembly, and directly affects the pre-creep effect and fuel cell sealing performance.

[0035] The term "disc" as used in this article refers to a separator (such as a metal disc or a non-metal disc) that is stacked alternately with graphite bipolar plates in pre-creep or fuel cell assembly. It is used to separate the bipolar plates, ensure uniform stress, and prevent adhesion or wear on the plate surfaces. It is an auxiliary structural component for achieving batch pre-processing.

[0036] The term "chromium-vanadium alloy" used in this article refers to a high-strength alloy steel with chromium and vanadium as the main alloying elements. It has excellent mechanical properties and fatigue resistance and is often used in tooling or fuel cell fasteners (such as screws and bolts), elastic elements (such as disc springs), and other structural components to ensure the structural reliability and stability under long-term loads.

[0037] In a first aspect, the present invention provides: a method for pre-creep treatment of graphite bipolar plates, comprising the steps of: (1) Graphite bipolar plates are positioned and stacked along the thickness direction to obtain stacked graphite bipolar plates; (2) Apply an elastic holding load to the stacked graphite bipolar plates obtained in step (1), and then heat treat them.

[0038] Preferably, the graphite bipolar plate is an expanded graphite bipolar plate.

[0039] Preferably, in step (1), an isolation pad is provided between adjacent graphite bipolar plates; More preferably, the size of the insulating pad matches the size of the active region of the graphite bipolar plate; More preferably, the insulating gasket is a stainless steel gasket with a thickness of 40-80μm, for example: 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or any range derived thereof.

[0040] Preferably, the number of stacked graphite bipolar plates in step (1) is 20-40, for example: 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, or any range derived thereof.

[0041] Preferably, in step (2), the magnitude of the elastic retaining load is 20-30kN, for example: 20kN, 22kN, 25kN, 28kN, 30kN, or any range derived therefrom.

[0042] In some specific embodiments, the elastic holding load can be provided by a disc spring assembly; More preferably, the disc spring assembly consists of multiple sets of conical annular disc springs made of chromium-vanadium alloy; More preferably, the number of disc spring assemblies is 3-6, for example: 3, 4, 5, 6, or any range derived therefrom. More preferably, the number of disc spring assemblies is 4.

[0043] More preferably, the disc spring assembly contains 3-6 disc springs, for example: 3, 4, 5, 6, or any range derived thereof; More preferably, the disc spring assembly contains four disc springs.

[0044] Preferably, in step (2), the heat treatment is a high-temperature heat preservation treatment; More preferably, the temperature of the high-temperature insulation treatment is 110-140℃, for example; 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, or any range derived therefrom; More preferably, the temperature of the high-temperature insulation treatment is 115-125℃; Most preferably, the temperature of the high-temperature insulation treatment is 120°C; More preferably, the heat preservation time is 12-24h, for example: 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h, or any range derived thereof.

[0045] More preferably, after the above heat treatment is completed, a cooling step is also included; More preferably, the cooling temperature is: cooling to below 50°C.

[0046] In some specific implementations, the average thickness of the graphite bipolar plate treated by the above method is reduced by 15-20 μm, and the thickness uniformity is improved.

[0047] Based on further solutions to the technical problems of the present invention, or simultaneous solutions to multiple technical problems, the preferred solution in the technical solution provided in the first aspect of the present invention includes: The first preferred embodiment: the number of disc spring groups is 3-6, for example: 3 groups, 4 groups, 5 groups, 6 groups, or any range derived therefrom; preferably, the number of disc spring groups is 4; the number of disc springs in each disc spring group is 3-6, for example: 3, 4, 5, 6, or any range derived therefrom; preferably, the number of disc springs in each disc spring group is 4. This technical solution, based on solving the technical problem of "alleviating the thinning of the core and shortening of the reactor core length after high-temperature operation, and improving thickness consistency," further solves the technical problem of "further alleviating the thinning of the core and shortening of the reactor core length after high-temperature operation, and improving thickness consistency."

[0048] The second preferred embodiment: the high-temperature insulation treatment temperature is 110-140℃, for example; 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, or any range derived thereof; preferably, the high-temperature insulation treatment temperature is 115-125℃; more preferably, the high-temperature insulation treatment temperature is 120℃. This technical solution, while addressing the technical problem of "alleviating the thinning of the core and shortening of the reactor core length after high-temperature operation, and improving thickness consistency," further addresses the technical problem of "further alleviating the thinning of the core and shortening of the reactor core length after high-temperature operation, and improving thickness consistency."

[0049] Secondly, the present invention provides: an apparatus for pre-creep treatment of graphite bipolar plates, comprising: A positioning and stacking mechanism is used to position and stack graphite bipolar plates along the thickness direction. An elastic load application mechanism is used to apply an elastic holding load to the stacked graphite bipolar plates; A heat treatment mechanism for heat-treating graphite bipolar plates under elastic holding load.

[0050] Preferably, the positioning and stacking mechanism includes a lower tooling, a positioning rod, and a pressure plate; In some specific embodiments, the lower tooling is located at the bottom of the device and serves as a bearing reference. In some specific embodiments, the positioning rod is vertically installed on the lower tooling and is used for positioning and guiding the bipolar plate and the gasket; In some specific embodiments, the pressure plate is disposed above the stacked bipolar plates, and a boss is provided on the upper surface of the pressure plate; Preferably, the number of protrusions is 3-6, for example: 3, 4, 5, 6, or any range derived therefrom; More preferably, the number of bosses is four.

[0051] Preferably, the elastic load application mechanism includes a disc spring assembly, an upper tooling, a locking screw, and a nut.

[0052] In some specific embodiments, the disc spring assembly is mounted on the pressure plate; Preferably, the disc spring assembly is mounted on a boss on the upper surface of the pressure plate to provide a continuous axial clamping force; In some specific implementations, the number of disc spring assemblies corresponds one-to-one with the number of bosses; Preferably, the number of disc spring assemblies is 3-6, for example: 3, 4, 5, 6, or any range derived therefrom. More preferably, the number of disc spring assemblies is 4.

[0053] Preferably, the disc spring assembly contains 3-6 disc springs, for example: 3, 4, 5, 6, or any range derived thereof; More preferably, the disc spring assembly contains four disc springs.

[0054] Preferably, the disc spring is in the shape of a conical ring.

[0055] Preferably, the disc spring is made of chromium-vanadium alloy.

[0056] In some specific embodiments, the upper tooling cover is located above the disc spring assembly; In some specific embodiments, the locking screw passes through the lower fixture, the pressure plate, and the upper fixture, and is locked by a nut to maintain pressure.

[0057] Preferably, the device further includes an isolation pad; More preferably, the insulating pad is disposed between adjacent graphite bipolar plates and matches the active region; More preferably, the insulating gasket is a stainless steel gasket.

[0058] Preferably, a bipolar plate and a corresponding stainless steel gasket can be accommodated between the pressure plate and the lower tooling; More preferably, the number of the bipolar plates and corresponding stainless steel gaskets is 20-40 pieces, for example: 20 pieces, 22 pieces, 24 pieces, 26 pieces, 28 pieces, 30 pieces, 32 pieces, 34 pieces, 36 pieces, 38 pieces, 40 pieces, or any range derived therefrom; The thickness of the stainless steel gasket is 40-80μm, for example: 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm, 75μm, 80μm, or any range derived thereof.

[0059] Preferably, the heat treatment mechanism includes an oven; More preferably, the temperature control range of the oven is 110-140℃, for example; 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, or any range derived therefrom; More preferably, the temperature control range of the oven is 120±5℃.

[0060] Thirdly, the present invention provides the application of the method and / or the apparatus in the post-processing of expanded graphite bipolar plates in fuel cells.

[0061] Preferably, the post-processing is the final pre-processing step before fuel cell stack assembly.

[0062] Preferably, the application is to improve the thickness uniformity of graphite bipolar plates, suppress the attenuation of pressing force after the fuel cell stack is in operation, and prevent the fuel cell stack core from collapsing.

[0063] Preferably, the application is that the method and / or the apparatus are suitable for graphite bipolar plates in automotive proton exchange membrane fuel cell stacks.

[0064] Fourthly, the present invention provides: a graphite bipolar plate for a proton exchange membrane fuel cell stack for vehicles prepared by the method and / or the apparatus described herein.

[0065] Fifthly, the present invention provides a fuel cell stack comprising a graphite bipolar plate obtained by the method and / or the apparatus described herein.

[0066] In this invention, embodiments 1-4 at least support the protection range of "the temperature of the high-temperature insulation treatment is 110-140℃".

[0067] The statement "the temperature of the high-temperature insulation treatment is 110-140℃" is derived from the foregoing explanation and / or the "setting temperature of 120℃" in Example 1; the "temperature of high-temperature pre-creep is 110℃" in Example 2; and the "temperature of high-temperature pre-creep is 140℃" in Example 3. Therefore, those skilled in the art can reasonably infer that "the temperature of the high-temperature insulation treatment is 110-140℃," its subordinate concepts, its essentially equivalent technical means, and technical means that can replace it within the scope of conventional technical means and common knowledge based on the existing technical level should all fall within the protection scope of this invention.

[0068] Examples 1-4 of this invention at least support the protection scope of "application in the post-processing of expanded graphite bipolar plates in fuel cells".

[0069] The term "application in the post-processing of expanded graphite bipolar plates for fuel cells" is a generalization derived from the foregoing explanation and / or the corresponding technical features in Examples 1-4, such as "improving the thickness uniformity of graphite bipolar plates, suppressing the attenuation of pressing force after stack operation, and preventing stack core collapse," and "the device is suitable for graphite bipolar plates used in automotive proton exchange membrane fuel cell stacks." Therefore, those skilled in the art can reasonably infer that "application in the post-processing of expanded graphite bipolar plates for fuel cells," its subordinate concepts, its essentially equivalent technical means, and technical means that can replace it within the scope of conventional and common knowledge based on existing technology should all fall within the protection scope of this invention.

[0070] The beneficial effects of this invention are as follows: The present invention has at least the following beneficial effects: 1. The thickness uniformity of bipolar plates is significantly improved, with an average reduction of 15-20 μm, resulting in a substantial improvement in dimensional stability; 2. Initial creep is consumed in advance, and the core length is stable after the stack is in operation, with no risk of collapse; 3. The pressing force remains constant and does not decrease, ensuring reliable fuel cell stack sealing and stable contact resistance; 4. The process can be batch processed and is suitable for industrial production, serving as a final processing step for bipolar plates.

[0071] 5. Compared with the prior art, the present invention has better technical effects in terms of thickness deviation, change of disc spring gap after 100 hours of operation of the fuel cell stack, and impact test at -30℃.

[0072] According to experimental tests, the present invention reduces the thickness deviation from ±25 micrometers to ±20 micrometers.

[0073] According to experimental tests, the present invention ensures that the disc spring gap change is less than 1 mm after the fuel cell stack has been running for 100 hours.

[0074] According to experimental tests, this invention increases the impact test at -30℃ from 20G to 25G. Attached Figure Description

[0075] Figure 1 This is a schematic diagram of the pre-creep method of the present invention; Figure 2 This is a schematic diagram of the overall structure of the pre-creep device of the present invention; Figure 3 This is a schematic diagram of a stainless steel gasket structure. Figure 4 This is a schematic diagram of the assembly of the pressure plate and disc spring assembly; Figure 5 This is a cross-sectional diagram of the tooling in the locked state. Detailed Implementation

[0076] The following non-limiting embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the present invention, but do not limit the invention in any way. The following content is merely an exemplary description of the scope of protection claimed by the present invention, and those skilled in the art can make various changes and modifications to the present invention based on the disclosed content, and such changes should also fall within the scope of protection claimed by the present invention.

[0077] The present invention will be further described below by way of specific embodiments. Unless otherwise specified, all instruments, devices, equipment, reagents, products, etc., used in the embodiments of the present invention are obtained through conventional commercial means.

[0078] Specific raw material information is shown in Table 1: Table 1. Raw Material Information

[0079] Example 1 A method for post-creep treatment of graphite bipolar plates, comprising the following specific steps: (1) Material preparation Take 25 expanded graphite bipolar plates and stainless steel gaskets with a thickness of 60μm, the size of which is completely consistent with the active area of ​​the bipolar plates.

[0080] (2) Tooling stacking Install positioning rods on the lower fixture, and alternately place bipolar plates and stainless steel shims in sequence, stacking a total of 25 layers to ensure alignment without offset.

[0081] (3) Install pressure plate and disc spring Place a pressure plate on the topmost bipolar plate, aligning the pressure plate's positioning hole with the positioning rod; install four chromium-vanadium alloy disc springs on each of the four protrusions of the pressure plate, for a total of 16 springs; and cover with the upper tooling.

[0082] (4) Pressurize and lock Move the tooling to the manual press, apply 25kN axial pressure, keep the pressure constant, and evenly tighten the screws and nuts around the perimeter so that the disc springs can continuously provide clamping force.

[0083] (5) High-temperature pre-creep Place the entire tooling into an oven, set the temperature to 120℃, and keep it warm for 18 hours.

[0084] (6) Cooling and unloading After the heat preservation is completed, allow it to cool naturally. Once the tooling temperature drops below 45°C, move it to the press and apply slight pressure. Loosen the nuts, release the pressure, open the tooling, and remove the bipolar plate that has completed pre-creep.

[0085] Testing revealed that the average thickness of the bipolar plates was reduced by 18 μm, and the thickness range decreased from 30 μm to 15 μm, demonstrating excellent consistency. The impact test met the standard of -30℃, 25G (GB / T 36288-2018 Safety Requirements for Fuel Cell Stacks in Fuel Cell Electric Vehicles).

[0086] After the fuel cell stack operated for 10 hours, the disc spring clearance changed by 1.0 mm.

[0087] Example 2 Compared with Example 1, the only difference is that the high-temperature pre-creep temperature is 100°C, and everything else is the same.

[0088] Testing revealed that the average thickness of the bipolar plate decreased by 8 μm, and the thickness range decreased from 30 μm to 25 μm, demonstrating excellent consistency. The impact test met the standard of -30℃, 25G. After the fuel cell stack operated for 10 hours, the average change in disc spring clearance was 1.2 mm.

[0089] Example 3 Compared with Example 1, the only difference is that the high-temperature pre-creep temperature is 140°C, and all other aspects are the same.

[0090] Testing revealed that the average thickness of the bipolar plate was reduced by 18 μm, and the thickness range decreased from 30 μm to 12 μm, demonstrating excellent consistency. The impact test met the standard of -30℃, 25G. After the fuel cell stack operated for 10 hours, the disc spring clearance changed by 0.9 mm.

[0091] Example 4 A device for pre-creep post-treatment of graphite bipolar plates, comprising a tooling body and a manual press. The overall position and connection relationships are as follows: The lower fixture is located at the bottom of the device; the positioning rod is vertically set above the lower fixture; the bipolar plates and gaskets are alternately stacked horizontally above the lower fixture and inside the positioning rod; the pressure plate is horizontally set above the stacked body; the disc spring assembly is vertically set above the pressure plate boss; the upper fixture is horizontally set above the disc spring assembly; the screw passes through the upper fixture, pressure plate and lower fixture from top to bottom, and is locked at the upper end by a nut.

[0092] The lower tooling is located at the bottom of the entire device. It is a rectangular rigid base plate with screw holes at the four corners and a bipolar plate bearing surface in the center.

[0093] The positioning rods are vertically fixed at the four corners of the lower fixture and extend upwards to provide radial positioning for the bipolar plates, gaskets, and pressure plates, ensuring concentric stacking.

[0094] The stacking cavity is formed between the lower tooling and the pressure plate to accommodate 20-40 bipolar plates and stainless steel gaskets. The gaskets are sandwiched between adjacent bipolar plates and only cover the active area.

[0095] The pressure plate is horizontally positioned above the stacked bipolar plates and slides in conjunction with the positioning rod. The upper surface of the pressure plate has four upward-protruding bosses at its four corners, which are used to position the disc springs and prevent slippage.

[0096] The disc spring assembly consists of four groups, each containing four conical annular disc springs made of chromium vanadium alloy. Each group of disc springs is vertically stacked on a boss, located between the pressure plate and the upper tooling, providing axial elastic clamping force.

[0097] The upper tooling horizontal cover is located above the disc spring assembly. It is a rigid cover plate that works with the positioning rod to press the disc spring downwards.

[0098] The screw and nut are connected by the screw, which vertically passes through the corresponding through holes of the lower tooling, pressure plate, and upper tooling; the nut is screwed onto the upper end of the screw, and after tightening, the entire structure is fixed, maintaining a constant pressure of 20-30kN.

[0099] Comparative Example 1 The only difference from Example 1 is that the pre-creep temperature is set to 70°C; all other aspects are the same.

[0100] Tests showed that the average thickness variation of the bipolar plate was approximately 2 μm, the thickness range remained constant at ±30 μm, and the disc spring gap changed by approximately 2 mm after 10 hours of operation.

[0101] Comparative Example 2 The only difference from Example 1 is that the pre-creep temperature is set to 150°C; all other aspects are the same.

[0102] Testing showed that the average thickness of the bipolar plates decreased by 20 μm, and the thickness range decreased from 30 μm to ±15 μm. After 10 hours of operation, the average change in disc spring clearance was 0.8 mm. However, some areas of the bipolar plates exhibited poor appearance due to localized blistering. Comparative Example 3 The only difference from Example 1 is that the applied axial pressure is set to 15kN; all other aspects are the same.

[0103] Testing revealed that the average thickness of the bipolar plates decreased by 8 μm, the thickness range decreased from 30 μm to ±22 μm, and the average change in disc spring clearance after 10 hours of operation was 1.4 mm. However, the bipolar plates exhibited some localized blistering and other aesthetic defects.

[0104] Comparative Example 4 The only difference from Example 1 is that the applied axial pressure is set to 35kN; all other aspects are the same.

[0105] Testing revealed that the average thickness of the bipolar plate decreased by 20 μm, and the change in disc spring clearance after 10 hours of operation was less than 0.9 mm. However, there is a risk of deformation to the tooling and bipolar plate.

[0106] Vibration, shock and thermal cycling tests: For specific testing requirements, please refer to GB / T 36288-2018 Safety Requirements for Fuel Cell Stacks in Fuel Cell Electric Vehicles.

[0107] Verification of technical effectiveness and / or analysis of technical problem solving (1) Regarding the technical problem of "thickness reduction and core length shortening after high-temperature operation": This invention pre-creepes the graphite bipolar plate for 12-24 hours at a constant pressure of 20-30KN at 120±5℃, so that the bipolar plate completes viscoelastic deformation and structural stabilization before the fuel cell assembly. According to actual measurements, the average thickness of the bipolar plate can be controlled to be reduced by 15-20μm, and the thickness difference is significantly reduced. This eliminates the irreversible thickness decay caused by thermal expansion-compression-cooling rebound during actual operation of the fuel cell, thereby avoiding core length shortening and solving the dimensional drift problem after long-term operation of the fuel cell.

[0108] (2) Regarding the technical problem of "pressure force attenuation and disc spring compensation failure": the pre-creep process ensures that the bipolar plate has completed the main creep deformation before service, and no significant thickness reduction occurs after the stack is assembled. The disc spring is always within the effective compression stroke and can continuously provide stable axial compensation force. Verification results show that after the bipolar plates treated by the present invention are assembled into a fuel cell stack, the core pressing force retention rate before and after thermal cycling is greatly improved, and the disc spring compensation gap does not increase significantly, thus completely solving the defect of rapid attenuation of pressing force in the traditional process.

[0109] (3) Regarding the technical problem of "core collapse and low reliability of fuel cell stack": This invention uses stainless steel gaskets matched with the active area for directional pressure application, so that the pressure is concentrated on the core working area of ​​the bipolar plate, achieving uniform creep across the entire area and avoiding rib deformation and matrix collapse caused by local stress concentration. After vibration, shock and thermal cycling tests, the pre-creeped bipolar plate showed no thickness collapse, no flow channel deformation and no sealing failure throughout the entire life cycle of the fuel cell stack, and the structural stability and operational reliability of the fuel cell stack were significantly improved.

[0110] (4) Regarding the effect of “improving thickness uniformity”: Under high temperature and constant pressure, the area with higher bipolar plate thickness has a large amount of plastic deformation, while the area with lower thickness has a small amount of deformation. After pre-creep, the overall thickness uniformity is improved, the thickness difference is reduced from 30μm to 8μm, the thickness difference between plates is controlled within a very small range, which greatly reduces the difficulty of stack assembly and improves contact uniformity and output stability.

[0111] In summary, this invention, through pre-creep technology, fundamentally solves a series of technical problems in fuel cell stack operation, such as thickness reduction, core shortening, pressurization force attenuation, and core collapse of expanded graphite bipolar plates. The technical effects are clear, repeatable, and verifiable, fully meeting the manufacturing requirements of long life and high reliability of fuel cell stacks.

[0112] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for pre-creep treatment of graphite bipolar plates, characterized in that, Including the following steps: (1) Graphite bipolar plates are positioned and stacked along the thickness direction to obtain stacked graphite bipolar plates; (2) Apply an elastic holding load to the stacked graphite bipolar plates obtained in step (1), and then heat treat them.

2. The method according to claim 1, characterized in that, In step (1), an isolation pad is provided between adjacent graphite bipolar plates; The isolation pad is matched to the size of the active area of ​​the graphite bipolar plate; The thickness of the isolation pad is 40-80μm; The number of stacked graphite bipolar plates is 20-40.

3. The method according to claim 1, characterized in that, In step (2), the magnitude of the elastic retaining load is 20-30kN; The elastic retaining load can be provided by the disc spring assembly; The number of disc spring assemblies is 3-6, and each disc spring assembly contains 3-6 disc springs.

4. The method according to claim 1, characterized in that, In step (2), the temperature of the high-temperature heat preservation treatment is 110-140℃ and the duration is 12-24h; After the heat treatment is completed, a cooling step is also included.

5. An apparatus designed according to the method of any one of claims 1-4, characterized in that, include: A positioning and stacking mechanism is used to position and stack graphite bipolar plates along the thickness direction. An elastic load application mechanism is used to apply an elastic holding load to the stacked graphite bipolar plates; A heat treatment mechanism for heat-treating graphite bipolar plates under elastic holding load.

6. The apparatus according to claim 5, characterized in that, The positioning and stacking mechanism includes a lower tooling, a positioning rod, and a pressure plate; The lower tooling is located at the bottom of the device and serves as a bearing reference. The positioning rod is vertically installed on the lower tooling and is used for positioning and guiding the bipolar plate and the gasket; The pressure plate is disposed above the stacked bipolar plates, and a boss is provided on the upper surface of the pressure plate; The number of protrusions is 3-6.

7. The apparatus according to claim 5, characterized in that, The elastic load application mechanism includes a disc spring assembly, an upper tooling, a locking screw, and a nut; The disc spring assembly is mounted on the pressure plate; The upper tooling cover is located above the disc spring assembly; The locking screw passes through the lower fixture, the pressure plate, and the upper fixture, and is locked by a nut to maintain pressure.

8. The apparatus according to claim 7, characterized in that, The disc spring assembly is mounted on the boss on the upper surface of the pressure plate to provide continuous axial clamping force; The number of disc spring assemblies corresponds one-to-one with the number of bosses; The number of disc spring assemblies is 3-6, and each disc spring assembly contains 3-6 disc springs.

9. The apparatus according to claim 5, characterized in that, The device also includes an isolation pad; The isolation pad is disposed between adjacent graphite bipolar plates and matches the active region; The pressure plate and the lower tooling can accommodate a bipolar plate and a corresponding stainless steel gasket. The number of bipolar plates and corresponding stainless steel gaskets is 20-40. The thickness of the stainless steel gasket is 40-80 μm; The heat treatment mechanism includes an oven.

10. The application of the method according to any one of claims 1-4 and / or the apparatus according to any one of claims 5-9 in the post-processing of expanded graphite bipolar plates in fuel cells.

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