FORM 2
THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10; rule 13]
TITLE: “A METHOD FOR IMPROVING PHYSICAL PROPERTIES OF A BIPOLAR
PLATE”
Name and Address of the Applicant:
TATA MOTORS PASSENGER VEHICLES LIMITED of Floor 3, 4, Plot-18, Nanavati Mahalaya, Mudhana Shetty Marg, BSE, Fort, Mumbai, Mumbai City, Maharashtra, 400001 India
Nationality: Indian
The following specification particularly describes the invention and the manner in which it is to be performed.

TECHNICAL FIELD
Present disclosure, in general, relates to a field of material sciences and manufacturing. Particularly, but not exclusively, the present disclosure relates to manufacturing of bipolar plates. Further, embodiments of the present disclosure discloses a method for improving physical properties such as thermal and electrical conductivity of the bipolar plate.
BACKGROUND OF THE DISCLOSURE
Generally, fuel cells produce energy by combining hydrogen and oxygen. Typically, multiple fuel cells are stacked in series to form a fuel cell stack. In the fuel cell stack, one side of a flow field plate serves as anode for one fuel cell while the opposite side of the flow field plate serves as the cathode for an adjacent fuel cell. As each flow field plate serves as both an anode and a cathode, the flow field plate is also known as a bipolar plate.
The bipolar plates serve a number of functions in the fuel cell. The bipolar plates facilitate distribution of fuel gas and air uniformly over an active area of a membrane electrode assembly (MEA). The bipolar plates conduct electric current from cell to cell and/or to an external load. As such, the bipolar plates should have low bulk electrical resistance and low contact resistance. Additionally, the bipolar plates may also facilitate heat removal from the active area of the fuel cell helping to maintain proper operation temperature and therefore require good thermal conductivity. Further, the bipolar plates should be adapted to prevent leakage of gases and coolant and be resistant to chemical corrosion. Moreover, bipolar plates should have good mechanical properties including suitable flexural strength as well as a suitable thermal expansion coefficient.
Conventionally, fuel cell manufacturers have used graphite bipolar plates, which are electrically-conducting and resistant to corrosion in the fuel cell environment. However, graphite plates are brittle, and therefore, difficult to machine. This adds to the cost of the bipolar plates and volumetric power density of the fuel cell stack. While the use of metal bipolar plates is advantageous, metals such as titanium and stainless steel, which can be easily machined, are easily attacked by ions in a fuel cell environment. While stainless steel exhibits a fair corrosion resistance to ions, the corrosion rate increases with increase in the ion leach out rate. Additionally, due to the compression of the stack, the bipolar plate is subjected to fatigue loading, due to which grain orientation of the bipolar plate changes over time. Furthermore, during manufacturing of stainless

steel bipolar plates, metallurgical changes occur such as a grain boundary defects, lattice changing effects, orientation of grain, grain size, which lead to generations of cracks, which is undesired.
With advancements in technology, surface coating methods have been developed for increasing the corrosion resistance and enhancing physical properties of the substrate (stainless steel) used for manufacturing bipolar plates. For example, the surface of a stainless steel substrate is deposited or coated with at least one of gold, an electrically-conductive polymer, carbon and the like. However, such surface coating techniques are costly due to the high cost materials and process steps involved, which in-turn increase the cost of manufacturing the bipolar plates, which is again undesired.
Present disclosure is directed to overcome one or more limitations stated above or any other limitations associated with the known arts.
SUMMARY OF THE DISCLOSURE
One or more shortcomings of the prior art are overcome by a method as claimed and additional advantages are provided through the method as claimed in the present disclosure. Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.
In one non-limiting embodiment of the present disclosure a method of improving physical properties of a bipolar plate is disclosed. The method includes steps of heating, the bipolar plate to a first predefined temperature for a first predefined time. Heating of the bipolar plate facilitates formation of new crystal grains. The heated bipolar plate is then rapid cooled in a first cooling medium from the first predefined temperature to a second predefined temperature for a second predefined time. The rapid cooling aids in converting an austenite structure of the bipolar plate into a soft martensite structure. Further, the bipolar plate is superimpose cooled in a second cooling medium from the second predefined temperature to a third predefined temperature at a predefined cooling rate for a third predefined time. The structure of the bipolar plate after superimpose cooling converts from the soft martensite structure into a hard martensite structure. Additionally, the bipolar plate is tempered at a fourth predefined temperature for a fourth predefined time. The tempering of the bipolar plate after cooling provides a strain hardening effect. Thus, the method

of the present disclosure provides microstructural changes in the bipolar plate to improve the physical properties.
In an embodiment, the bipolar plate is made of stainless steel.
In an embodiment, the first predefined temperature is in a range of 300°C to 350°C and the first predefined time is in a range of 20 mins to 30 mins.
In an embodiment, the second predefined temperature is in the range of -160°C to -185°C and the second predefined time is in the range of 7 hours to 10 hours.
In an embodiment, the third predefined temperature is in the range of -250°C to -265°C and the third predefined time is in the range of 8 hours to 12 hours.
In an embodiment, the fourth predefined temperature is in the range of 200°C to 250°C and the fourth predefined time is in the range of 20 mins to 30 mins.
In an embodiment, the predefined cooling rate is in a range of -10°C/20 mins to -40°C/20 mins.
In an embodiment, the first cooling medium is a nitrogen medium and the second cooling medium is at least one of a helium or an argon medium.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The novel features and characteristics of the disclosure are set forth in the appended claims. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiments when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Fig. 1 is a flowchart illustrating a method of improving physical properties of a bipolar plate, according to an exemplary embodiment of the present disclosure.
Fig. 2 is a graphical representation of the method of improving physical properties of the bipolar plate of Fig. 1.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the system and method illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which forms the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that, the conception and specific embodiments disclosed may be readily utilized as a basis for modifying other methods, materials, and processes for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that, such equivalent method do not depart from the scope of the disclosure as set forth in the appended claims. The novel features which are believed to be characteristics of the disclosure, to its composition and method, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.
In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusions, such that a method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such method. In other words, one or more elements in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the method.
Reference will now be made to the exemplary embodiments of the disclosure, as illustrated in the accompanying drawings. Wherever possible, same numerals have been used to refer to the same or like parts. The following paragraphs describe the present disclosure with reference to Figs. 1-2.
Fig. 1 is a flowchart depicting a method for improving physical properties of a bipolar plate. In an embodiment, the bipolar plate or a current collector plate [hereafter referred to as bipolar plate] may be employed in a fuel cell. The bipolar plate in the fuel cells may be configured to uniformly distribute fuel gas and air, conduct electrical current from cell to cell, remove heat from the active area, prevent leakage of gases and coolant. In an embodiment, the physical properties of the bipolar plate that are improved may be including but not limited to thermal conductivity, electrical conductivity, corrosion resistance, hardness and the like. In an exemplary embodiment of the present disclosure, the bipolar plate may be manufactured by steel. As an example, the steel may be stainless steel such as SS316 stainless steel. However, this should not be construed as a limitation as the bipolar plate may be made of any other material including metals and non-metals suitable for operation in the fuel cells. In an embodiment, the bipolar plate may formed or machined in a required shape and profile and may be subjected to a method of the present disclosure for improving the physical properties. In another embodiment, the material which may be used for manufacturing the bipolar plate may also be subjected to the method of the present

disclosure for improving the physical properties. The method is now described with reference to the flowchart blocks illustrated in Fig. 1.
At block 101, the method may include heating the bipolar plate. The bipolar plate may be heated to a first predefined temperature [as seen in Fig. 2]. In an embodiment, heating may be facilitated within a heater or may be heated in an open environment by suitable means. In an embodiment, the first predefined temperature may be in a range of 300°C to 350°C. Further, the bipolar plate is maintained at the first predefined temperature for a first predefined time [as seen in Fig. 2]. In an embodiment, the first predefined time may be in the range of 20 mins to 30 mins. Upon heating to the first predefined temperature and maintaining the bipolar plate at the first predefined temperature for the first predefined time, may cause the bipolar plate to undergo microstructural changes where new crystal grains may be formed in the matrix of the bipolar plate.
At block 102, the method may include rapid cooling of the bipolar plate. In an embodiment, the rapid cooling may refer to subjecting the hot bipolar plate at a low temperature such that the temperature of the bipolar plate decreases in a rapid rate. The rapid cooling may include cooling the bipolar plate in a first cooling medium from the first predefined temperature to a second predefined temperature. Further, the rapid cooling may include maintaining the bipolar plate at the second predefined temperature for a second predefined time [as seen in Fig. 2]. In an embodiment, the first cooling medium may be but not limiting to nitrogen. As an example, the bipolar plate may be positioned in a surrounding maintained at the second predefined temperature having nitrogen medium for rapid cooling of the bipolar plate from the first predefined temperature to the second predefined temperature. In an another embodiment the second predefined temperature may be in the range of -160°C to -185°C. Further, the second predefined time may be in the range of 7 hours to 10 hours. The rapid cooling of the bipolar plate as described may result in microstructural changes such that an austenite structure of the bipolar plate may convert into a soft martensite structure.
Further, referring to block 103, the method may include superimpose cooling of the bipolar plate. In an embodiment, superimpose cooling may be inferred as a secondary cooling which may be carried out after rapid cooling at higher cooling parameters to improve the cooling effects. In an embodiment, the bipolar plate may be subjected to superimpose cooling after rapid cooling [as

seen in Fig. 2]. The superimpose cooling may include cooling the bipolar plate in a second cooling medium from the second predefined temperature to a third predefined temperature at a predefined cooling rate and maintaining the third predefined temperature for a third predefined time. Further, the superimpose cooling may include maintaining the bipolar plate at the third predefined temperature for a third predefined time. In an embodiment, the second cooling medium may be including but not limiting to helium or argon. As an example, the bipolar plate may be positioned in a surrounding having helium medium for superimpose cooling of the bipolar plate from the second predefined temperature to the third predefined temperature. In another embodiment, the bipolar plate may be positioned in a surrounding having argon medium for superimpose cooling of the bipolar plate from the second predefined temperature to the third predefined temperature. In an another embodiment the third predefined temperature may be in the range of -250°C to -265°C. Further, the third predefined time may be in the range of 8 hours to 12 hours and the predefined cooling rate may be in a range of -10°C/20 mins to -40°C/20 mins.
The superimpose cooling of the bipolar plate as described may result in microstructural changes such that the soft martensite structure of the bipolar plate attained during rapid cooling may convert into a hard martensite structure. The martensite structure results in reduced distance between atoms which helps in free flow of electrons, thereby improving thermal conductivity of the bipolar plate.
In an embodiment, the superimpose cooling may improve the thermal and electrical properties of the Bipolar plate, which improves the performance of a stack in the fuel cell. Further, the superimpose cooling may eliminate the metallurgical defects, which may be created during rolling and manufacturing of the bipolar plate, and also provides a balanced and refined microstructure which improves the stability of the stack of the fuel cell. Additionally, the superimpose cooling of the bipolar plate may eliminate high cost process such as a gold platting and carbon coating, which in-turn helps to develop low cost fuel cell stack.
In an embodiment, the superimpose cooling may reduce the chemical erosion and corrosion of bipolar plate, which improves the life and efficiency of the stack.
At block 104, the bipolar plate after the superimpose cooling may be subjected to a tempering process. Tempering process may be inferred as “heating it to a high temperature, though below the melting point, then cooling it” for improving characteristics of the metal. In an embodiment,

tempering process of the method may include heating the bipolar plate from the third predefined temperature to a fourth predefined temperature. In an embodiment, the fourth predefined temperature may be in the range of 200°C to 250°C and the fourth predefined time may be in the range of 20 mins to 30 mins [as seen in Fig. 2]. After maintaining the fourth predefined temperature for the fourth predefined time, the bipolar plate is cooled to an ambient temperature by at least one of natural cooling or forced cooling. Upon tempering, the bipolar plate may exhibit increased toughness or hardness with reduced brittleness and reduced internal stresses.
In an embodiment, the rapid cooling and the superimpose cooling of the bipolar plate may be conducted in a cooling chamber which may be capable of receiving and storing the first cooling medium and the second cooling medium.
It should be noted that in an exemplary embodiment, as seen in the Figs. 1-2 the features, steps, connections and process should not be construed as a limitation as the method may include any other type of features, steps, connections and may work with other process or any other combinations for improving the properties of the bipolar plate.
Experiments:
Further embodiments of the present disclosure will be now described with experiments conducted to compare the base material of the bipolar plate and a bipolar plate coated with surface coating with the bipolar plate manufactured by the method as disclosed in the present disclosure.

Sl. Test Type Base material Coated bipolar Bipolar plate
no plate manufactured by
the method of
Fig. 1.
1 Micro hardness (Vickers test) 258 264 263
2 Grain structure distribution Fairly uniform Fine uniform structure Fine uniform structure
3 Grain size 6-7 3-4 4-5
4 Intermediate structure% 2-4% 4-5% 5-6%

5 Other phase Carbide Fine carbide Fine carbide
twinned grains worked grain worked grain
6 Thermal conductivity (W/mK) 14.70 22.4 20.6
7 Electrical conductivity (at 25°C) in (S/m) 1.45x106 1.90x106 1.75x106
Table 1
Referring to Table 1, VickersTM hardness test has been carried out on the base plate, the coated bipolar plate and the bipolar manufactured by the method of the present disclosure and the results have been recorded in the Table 1. The bipolar plate manufactured by the method of the present disclosure exhibits similar hardness when compared to the coated bipolar plate. Further, the grain structure distribution, the grain size and intermediate structure % of both the bipolar plate manufactured by the method of the present disclosure and the coated bipolar plate are also similar. Additionally, other physical parameters such as phase, thermal conductivity and electrical conductivity of both the bipolar plate manufactured by the method of the present disclosure and the coated bipolar plate are similar. In view of the experimental results showcased in Table 1, the bipolar plate manufactured by the method of the present disclosure exhibits enhanced physical properties when compared to the base material of the bipolar plate and similar physical properties when compared to the coated bipolar plate, thereby facilitating low cost manufacturing of the bipolar plate with improved physical properties without requirement of surface coating.
In an embodiment, the method facilitates in release of micro sized carbide particles within the steel material which increases the thermal deformation resistance and abrasion resistance of the bipolar plate.
In an embodiment, the method enables the bipolar plate to exhibit change in microstructure and grain boundary and formation of hard martensite structure which increases corrosion resistance, erosion efficiency, the electrical properties and mechanical properties of the bipolar plate.
In an embodiment, the method ensures that the grain orientation of the bipolar plate does not change even when the compressive forces acting on the bipolar plate is high. Further, the

metallurgical changes which occur during manufacturing of the bipolar plates, such as, grain boundary defects, lattice changing effects, orientation of grain, grain size, partial conversion on gamma phase of austenite into beta phase of austenite, which may be responsible for crack generation are eliminated.
In an embodiment, the method facilitates improvement in properties of the bipolar plate at low cost.
It should be imperative that the method and any other elements described in the above detailed description should not be considered as a limitation with respect to the figures. Rather, variation to such method should be considered within the scope of the detailed description.
Equivalents:
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or

“an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.
While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope.

Referral Numerals:

Reference Number Description
101-102 Flow chart blocks
101 Heating stage
102 Rapid cooling stage
103 Superimpose cooling stage
104 Tempering stage

We Claim:
1. A method for improving physical properties of a bipolar plate, the method comprising:
heating, the bipolar plate to a first predefined temperature for a first predefined time;
rapid cooling, the bipolar plate in a first cooling medium from the first predefined temperature to a second predefined temperature for a second predefined time;
superimpose cooling, the bipolar plate in a second cooling medium from the second predefined temperature to a third predefined temperature at a predefined cooling rate for a third predefined time; and
tempering, the bipolar plate at a fourth predefined temperature for a fourth predefined time.
2. The method as claimed in claim 1, wherein the bipolar plate is made of stainless steel.
3. The method as claimed in claim 1, wherein the first predefined temperature is in a range of 300°C to 350°C and the first predefined time is in a range of 20 mins to 30 mins.
4. The method as claimed in claim 1, wherein the second predefined temperature is in the range of -160°C to -185°C and the second predefined time is in the range of 7 hours to 10 hours.
5. The method as claimed in claim 1, wherein the third predefined temperature is in the range of -250°C to -265°C and the third predefined time is in the range of 8 hours to 12 hours.
6. The method as claimed in claim 1, wherein the fourth predefined temperature is in the range of 200°C to 250°C and the fourth predefined time is in the range of 20 mins to 30 mins.
7. The method as claimed in claim 1, wherein the predefined cooling rate is in a range of -10°C/20 mins to -40°C/20 mins.
8. The method as claimed in claim 1, wherein the first cooling medium is a nitrogen medium.

9. The method as claimed in claim 1, wherein the second cooling medium is a helium medium.