Application of a corrosion-resistant oxide film and a bipolar plate for a fuel cell

Abstract

The present invention relates to corrosion-resistant oxide films and their application to bipolar plates for fuel cells. A corrosion-resistant oxide film for a proton exchange membrane fuel cell is described. The bipolar plates of a proton exchange membrane fuel cell are subjected to a highly acidic environment, which can deteriorate the bulk material and associated properties of the bipolar plates, resulting in a reduced proton exchange membrane fuel cell lifespan. Materials, structures, and techniques for improving the corrosion resistance of bipolar plate materials are disclosed. Such materials include a substrate having a surface portion that includes an Fe2O3 oxide layer having (110), (012), or (100) Fe2O3 surface facets, which is configured to impart corrosion-resistant properties to the substrate.

Description

Technical Field

[0001] The present disclosure generally relates to corrosion-resistant oxide membranes for proton exchange membrane fuel cells. More specifically, the present disclosure relates to bipolar plates of proton exchange membrane fuel cells having a corrosion-resistant iron oxide substrate surface structure and methods for producing the same. Background Art

[0002] Fuel cells and in particular proton exchange membrane fuel cells (PEMFCs) hold great promise as efficient, high power density, relatively lightweight, and zero-carbon emission energy sources for various applications. Such applications include, but are not limited to, transportation, stationary power generation, and portable power generation. In particular, in relation to automotive and other transportation-related applications, PEMFCs represent an environmentally friendly alternative to internal combustion engines for various vehicles.

[0003] PEMFCs operate based on proton transfer between an anode and a cathode. Key components of a PEMFC particularly include a proton exchange membrane through which protons are transferred and a membrane electrode assembly (MEA) in which the proton exchange membrane is included. A PEMFC also includes bipolar plates (BPPs) that connect and separate individual fuel cells in series to form a fuel cell stack. Among other functions, the BPPs provide the required voltage, facilitate the distribution of fuel gas and oxygen on the active surfaces of the MEAs, and conduct current from the anode of one cell to the cathode of the next cell in the stack. Given such functions, the BPPs not only need to have sufficient chemical inertness to resist degradation in the highly corrosive environment of the fuel cell but also need to have sufficient electrical conductivity to facilitate electron transfer in the oxygen reduction reaction of the fuel cell.

[0004] The BPPs can constitute 60 - 80% of the weight of a PEMFC stack and are one of the most expensive PEMFC components, typically accounting for 25% - 45% of the stack cost. Although other metals such as titanium and aluminum can be used, BPPs are typically made of stainless steel. Since the operation of PEMFCs generally takes place in a highly acidic environment, BPP materials with high corrosion resistance are required for long-term PEMFC operation. Processing techniques such as introducing conductive oxide and / or nitride coatings into stainless steel BPPs can help improve their lifespan in an acidic PEMFC environment. Although such techniques have potential, corrosion to form Fe 2 O 3 is inevitable when Fe metal is exposed to water and oxygen because the Fe metal is under the acidic operating conditions of the PEMFC environment. Therefore, materials, structures, and techniques for improving the corrosion resistance of BPP materials are needed. Summary of the Invention

[0005] In at least one embodiment, a corrosion-resistant substrate is disclosed. The substrate may include a body portion and a surface portion, the surface portion including an Fe 2 O 3 oxide layer having surface facets of (110), (012), or (100) Fe 2 O 3 configured to impart corrosion resistance to the substrate. In some embodiments, the Fe 2 O 3 oxide layer is 0.001 - 0.5 µm thick. Additionally, the Fe 2 O 3 oxide layer of the corrosion-resistant substrate may be characterized by a surface morphology having a first set of surface facets and a second set of surface facets, the first set of surface facets including (110), (012), or (100) Fe 2 O 3 surface facets, and the second set of surface facets including (001) or (101) Fe 2 O 3 surface facets such that the Fe 2 O 3 oxide layer consists primarily of the first set of surface facets. According to one or more embodiments, the surface portion of the corrosion-resistant substrate may include a protective coating containing MgO, Al 2 O 3 , TiO 2 , or ZrO 2 . In another embodiment, the surface portion of the corrosion-resistant substrate may include a protective coating formed of a ternary (or more) compound such as ABO x , where A is Mg, Al, Ti, or Zr, B is Zn, Sn, Cr, or Mo, and x is an integer in the range of 1 - 8.

[0006] In another embodiment, a bipolar plate (BPP) for a proton exchange membrane fuel cell (PEMFC) is disclosed. The BPP may include a corrosion-resistant substrate including a body portion and a surface portion, the surface portion including an Fe 2 O 3 oxide layer having surface facets of (110), (012), or (100) Fe 2 O 3 configured to impart corrosion resistance to the substrate. In some embodiments, the Fe 2 O 3 oxide layer is 0.001 - 0.5 µm thick. The Fe 2 O 3The oxide layer may be characterized by a surface morphology having a first set of surface facets and a second set of surface facets, the first set of surface facets including (110), (012), or (100) Fe 2 O 3 surface facets, and the second set of surface facets including (001) or (101) Fe 2 O 3 surface facets, such that the Fe 2 O 3 oxide layer consists predominantly of the first set of surface facets. The BPP substrate may consist of stainless steel. According to one or more embodiments, the surface portion of the BPP substrate may include a protective coating containing MgO, Al 2 O 3 , TiO 2 , or ZrO 2 . In another embodiment, the surface portion of the BPP substrate may include a protective coating formed from a ternary (or higher) compound such as ABO x , where A is Mg, Al, Ti, or Zr, B is Zn, Sn, Cr, or Mo, and x is an integer in the range of 1 - 8. In one or more embodiments, in the presence of 80 °C, pH 2 - 3, and approximately 0.1 ppm HF, the corrosion resistance of the surface portion of the BPP substrate is less than 1 µA cm -2 .

[0007] In yet another embodiment, a method of producing a corrosion - resistant substrate is disclosed. According to at least one embodiment, the method may include cleaning a stainless - steel substrate with an organic solvent and electrochemically oxidizing the stainless - steel substrate to form a corrosion - resistant surface portion, the corrosion - resistant surface portion including an Fe 2 O 3 oxide layer having (110), (012), or (100) Fe 2 O 3 surface facets. In another embodiment, the method may include growing an Fe 2 O 3 oxide film on the stainless - steel substrate by using a solution - based method. In one example, hydrolysis may be carried out in a water bath at 80 - 100 °C for different aging times of 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 minutes. In other examples, hydrolysis is carried out in a water bath at 25 - 100 °C for an aging time of 1 - 120 minutes. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 Shows a schematic depiction of a proton - exchange membrane fuel cell including a bipolar plate according to one or more embodiments;

[0009] Figure 2 Shows a perspective view of a non - limiting example of a bipolar plate having a surface portion comprising a corrosion - resistant iron oxide structure according to one or more embodiments;

[0010] Figures 3A - 3E Depicts the atomic structure of importance to the morphology of the α - Fe 2 O 3 iron oxide surface;

[0011] Figures 4A - 4E Depicts the atomic structure of an iron oxide surface containing - OH termini;

[0012] Figure 5 Shows a graph depicting the surface energy of an iron oxide surface as a function of increasing - OH termini;

[0013] Figure 6A and 6B Shows when different U values are applied to Fe in density functional theory (DFT) calculations, the 2 O 3 lattice parameter of Fe c and a graph of the calculated band gap;

[0014] Figures 7A - 7D Shows a graph of the density of states (DOS) of Fe 2 O 3 at different U values; and

[0015] Figure 8A and 8B Shows an example of an X - ray powder diffraction (XRD) graph of two different Fe 2 O 3 samples. DETAILED DESCRIPTION

[0016] Embodiments of the present disclosure are described below. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Thus, the specific structural and functional details disclosed herein should not be construed as limiting, but rather as a representative basis for teaching one skilled in the art to use the invention in various ways. As will be understood by those of ordinary skill in the art, the various features shown and described with reference to any one figure may be combined with features shown in one or more other figures to produce embodiments not explicitly shown or described. Combinations of the shown features provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desirable for a particular application or implementation.

[0017] The description of a group or class of materials suitable for a given purpose in connection with one or more embodiments means that a mixture of any two or more members of the group or class is suitable. A description of ingredients in chemical terms means the ingredients when added to any combination recited in the specification and does not necessarily preclude chemical interactions between the ingredients of the mixture once mixed.

[0018] Except where otherwise expressly indicated, all numerical quantities representing dimensions or material properties in this specification are to be understood as being modified by the word "about" when describing the broadest scope of the disclosure.

[0019] The first definition of an acronym or other abbreviation applies to all subsequent uses of the same abbreviation in this document and, as necessary with appropriate modification, to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurements of properties are determined by the same techniques as those cited previously or subsequently for the same property.

[0020] Reference is made in detail to the compositions, embodiments, and methods of embodiments known to the inventors. However, it is to be understood that the disclosed embodiments are merely examples of the invention, which may be embodied in various and alternative forms. Accordingly, the specific details disclosed herein should not be construed as limiting, but merely as a representative basis for teaching one skilled in the art to use the invention in various ways.

[0021] The terms "substantially" or "about" may be used herein to describe the disclosed or claimed embodiments. The terms "substantially" or "about" may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such cases, "substantially" or "about" may mean that the value or relative characteristic being modified is within ±0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or 10% of the stated value or relative characteristic.

[0022] Corrosion is a natural process that converts refined metals into chemically more stable forms such as metal (one or more) oxides, (one or more) hydroxides, (one or more) sulfides, and / or other salts. The conversion is manifested as the gradual destruction of the metallic material, which is caused by the electrochemical oxidation of the metal reacting with an oxidant such as oxygen or sulfate. Corrosion can be caused by exposing the metal substrate to moisture in the air, solutions with a relatively low pH, various chemicals such as acids, microorganisms, elevated temperatures, and / or other factors. Especially in an acidic environment, corrosion begins at the interface between the bulk metallic material (e.g., steel) and the solution (e.g., ions dissolved in water or the water surface layer, which react to degrade the bulk material).

[0023] Due to the highly acidic operating environment, corrosion-resistant metals, metal surfaces, treatments, and coatings are particularly useful in proton exchange membrane fuel cells (PEMFCs). PEMFCs typically operate under acidic conditions where the pH is usually 2 - 4. The operating temperature range within a PEMFC stack is approximately 60 °C - 85 °C. These factors and others contribute to the highly corrosive operating environment of PEMFCs. For example, between startup and shutdown, there is a low voltage within the PEMFC stack, and during PEMFC operation, fluoride ions are released due to the degradation of the polymer membrane. Additionally, during startup and shutdown, H 2 and O 2 both exist at the anode, which results in a high cathode potential that causes cathode corrosion. Given such conditions, PEMFCs require durable components that can withstand the corrosive operating environment.

[0024] Early fuel cell systems often used graphite for the bipolar plates (BPPs) of PEMFCs because graphite can achieve high chemical stability and conductivity in a PEMFC environment. However, graphite is both brittle and costly. Due to the excellent mechanical stability, electrical conductivity, and thermal conductivity of stainless steel, and its relatively easy manufacturability, stainless steel is now generally considered one of the best candidate materials for BPPs. Of course, stainless steel is a general name for many different steel compositions. Stainless steel typically contains at least 10% chromium (Cr), which can form a stable chromium oxide surface layer known to prevent "rusting" of the metal surface. SS304 and SS316 are two of the most commonly used stainless steel compositions. SS304 contains 18% Cr and 8% nickel (Ni). SS316 contains 16% Cr, 10% Ni, and 2% molybdenum (Mo). Depending on the nature of its specific application, the stainless steel composition can be varied. Such variations result in different mechanical stabilities, corrosion resistances, and magnetic properties. It is known that in addition to iron (Fe), Cr, Ni, and Mo, other elements in stainless steel include: carbon (~0.03%), manganese (1 - 2%), silicon (0.5 - 2%), nitrogen (0.01 - 0.1%), copper (0.5 - 2%), and cobalt (<0.5%).

[0025] Although not as prone as pure Fe, stainless steel is still susceptible to corrosion. Stainless steel corrosion occurs when the metal is exposed to water / air and various contaminants on the metal surface and reacts with them. When Fe is exposed to water and oxygen, it results in the formation of rust, which is typically characterized by the formation of a red oxide. Rust includes the oxidized forms of Fe, namely hydrated iron(III) oxide (specifically, Fe 2 O 3 ·xH 2 O) and iron(III) oxyhydroxide (specifically, FeO(OH) and Fe(OH) 3)。In an acidic environment, the formation of such iron oxide complexes may be accelerated, and some of these oxides may further dissolve into the solution.

[0026] When an iron-based oxide film forms on stainless steel BPP, the contact resistance and conductivity of the PEMFC will be significantly affected. For example, most oxides are insulators and thus have a negative impact on the conductivity of the bulk material. This is particularly problematic when the BPP is designed to be highly conductive of electrons in nature. The formation of an insulating oxide film layer on the BPP reduces the transfer of electrons, which can lead to reduced PEMFC output performance. Additionally, the corrosion film can grow over time, resulting in a greater contact resistance. Notably, if the products of such corrosion can be ionized (e.g., Fe 2+ or Fe 3+ ), then an acidic solution containing such ions can be transported to other fuel cell components. Inside a PEMFC stack, the dissolution of Fe, for example, can poison the Pt catalyst, which results in a reduced reaction rate of H 2 and O 2 adsorption, the formation of H 2 O, and poor fuel cell efficiency.

[0027] In many efforts to prevent or slow down metal corrosion, various types of coatings have been developed. Examples include applied coatings such as paints, plating, enamels, reactive coatings that contain corrosion inhibitors such as chromates, phosphates, conductive polymers, surfactant-like chemicals designed to inhibit the electrochemical reaction between the environment and the metal substrate, anodized surfaces, and biofilm coatings. In the case of the BPP used in a PEMFC stack, corrosion resistance can be imparted by using processing techniques such as introducing conductive oxide and / or nitride coatings to stainless steel. Despite these techniques, however, when Fe metal is exposed to water and oxygen, the formation of Fe 2 O 3 corrosion is inevitable because the Fe metal is under the acidic operating conditions of the PEMFC environment.

[0028] As described in the present disclosure, Fe 2 O 3 formed on some surfaces in stainless steel - and in other mainly Fe-based metal compositions - is more resistant to metal dissolution. First-principles density functional theory (DFT) calculations can be used to determine the relevant Fe 2 O 3 oxide surface energy. According to the present disclosure, by adjusting a narrow range of Fe 2 O 3 surface energy that is sensitive to synthesis conditions and the local environment, the corrosion-resistant Fe 2 O 3Formation of the surface and imparting it to stainless steel BPP.

[0029] Figure 1 Non-limiting embodiments of a proton exchange membrane fuel cell are depicted. The core component of PEMFC 10 is the membrane electrode assembly (MEA) 12, which facilitates the electrochemical reactions within the stack. MEA 12 includes sub-components such as electrodes, catalysts, and a proton exchange membrane. In addition to MEA 12, PEMFC 10 typically also includes other components such as current collectors 14, gas diffusion layer(s) 16, gaskets 18, and at least one bipolar plate (BPP) 20.

[0030] The BPP 20 is implemented in a PEMFC stack to distribute gases, collect current, and separate the individual cells within the stack from each other. BPP 20 also provides additional functions such as removing reaction products and water as well as thermal management within PEMFC 10. BPP 20 thus forms a critical part of PEMFC 10. BPP 20 is also both a relatively expensive component and a common cause of degradation in the PEMFC system. For example, the BPP can constitute approximately 60 - 80% of the stack weight, approximately 50% of the stack volume, and approximately 25 - 45% of the stack cost of PEMFC 10. BPP 20 presents another material challenge because BPP 20 also needs to be sufficiently conductive to facilitate electron transfer for the oxygen reduction reaction. Thus, the BPP 20 material should be both conductive and chemically inert with respect to reactions with ions present in the PEMFC 10 environment.

[0031] The metallic surface of BPP 20, which may comprise stainless steel, can include coatings such as graphitic-like coatings or protective oxide and / or nitride coatings to increase the corrosion resistance of BPP 20. The BPP 20 surface can thus include elements such as Fe, Cr, Ni, Mo, Mn, Si, P, C, S, or combinations thereof. Alternative coatings include Ti alloys, doped TiO x , TiN, CrN, or ZrN. In addition, even in the case of using such coatings, materials, structures, and techniques for improving the corrosion resistance of BPP materials (such as stainless steel) in an erosive corrosion environment such as that of PEMFC 10 are desirable. According to the embodiments described herein, materials, structures, and techniques for improving the corrosion resistance of stainless steel BPP are disclosed.

[0032] Non-limiting embodiments of a PEMFC bipolar plate (BPP) 20 are depicted in Figure 2Among them, BPP 20 represents a non - limiting example of a substrate having a solid body or body portion 22 and a surface portion 24. The body portion 22 can be formed of a metal such as stainless steel. Alternatively, the body portion 22 can be formed of graphite, steel, aluminum, copper, an alloy of two or more metals, a combination thereof, or a composite material. According to one or more embodiments, the surface portion 24 can include a corrosion - resistant iron oxide film structure. The entire area of the surface portion 24 can include the iron oxide film structure. Alternatively, the surface portion 24 can include one or more sub - portions that do not contain the iron oxide film structure. In one embodiment, the entire surface portion 24 includes the iron oxide film structure. The surface portion 24 can further include a protective coating such as a binary oxide coating applied at the nanoscale to the microscale (a few nm to 100 µm) and applied on top of the corrosion - resistant iron oxide film structure. Such binary oxide coating materials include, but are not limited to, MgO, Al 2 O 3 , TiO 2 and ZrO 2 . These oxide coating materials can be undoped and / or partially doped with nitrogen, carbon, or fluorine to further enhance the resulting conductivity. Additionally, the surface portion 24 can include a protective coating formed of a ternary (or higher) compound such as ABO x , where A is Mg, Al, Ti, or Zr, B is Zn, Sn, Cr, or Mo, and x is an integer in the range of 1 - 8. According to some embodiments, the protective coating can include conductive nitrides and / or carbides.

[0033] The thickness of the surface portion 24 and its associated corrosion - resistant iron oxide film layer can be adjusted according to the specific application and can be in the range of a few nm to about 1 µm. The thickness of the iron oxide film layer itself can also be in the range of about 1 nm to about 1 µm. Non - limiting examples of such thicknesses can be about 0.1 - 0.8 µm, 0.2 - 0.6 µm, or 0.3 - 0.5 µm. In one or more embodiments, the thickness of the corrosion - resistant iron oxide film can be 1 nm - 0.5 µm. In other embodiments, the thickness of the corrosion - resistant iron oxide film can be 150 nm - 0.3 µm.

[0034] α - Fe 2 O 3 The characteristics of the iron oxide surface can lie in their morphological atomic structure. Such surface facet structures can be defined by the following Miller indices: (001), (110), (100), (101), and (012). Each of these morphological α - Fe 2 O 3 surfaces is depicted in Figures 3A - 3E . Figure 3A Depicts the (001) Fe 2 O3 The atomic structure of Figure 3B depicts (110) Fe 2 O 3 The atomic structure of Figure 3C depicts (100) Fe 2 O 3 The atomic structure of Figure 3D depicts (101) Fe 2 O 3 The atomic structure. Finally, Figure 3E depicts (012) Fe 2 O 3 The atomic structure. The bulk region of Fe 2 O 3 is composed of FeO 6 octahedra, while the surface region is coordinatively unsaturated. For example, as shown in Figure 3B and 3C , both the (110) and (012) iron oxide surfaces are capped with FeO 5 polyhedra. For example, as shown in Figure 3D , (101) iron oxide has both FeO 4 and FeO 5 surface units. The (100) and (001) iron oxides are even more coordinatively unsaturated at their surfaces than the (101) iron oxide. Figure 3C shows that (100) iron oxide is capped with FeO 4 , and Figure 3A shows that (001) iron oxide is capped with FeO 3 .

[0035] Figures 3A - 3E The density functional theory (DFT) surface energy of the iron oxide Fe 2 O 3 surface shown in 2 O 3 can be calculated by applying the generalized gradient approximation (GGA) scheme. Such surface energy calculations can be performed using the Vienna Ab-initio Simulation Package (VASP) software. The surface energy γ is the amount of energy required to cleave a bulk sample and create two surfaces exposed to vacuum. A lower surface energy represents a more energetically favorable state, while a higher surface energy represents a more energetically unfavorable state. The DFT surface energy (γ) of Fe

[0036] γ = ( E 0，厚片 – n · E 0，本体 ) / (2 A ) (1),

[0037] Among them E 0，厚片 is Figures 3A - 3E the total internal DFT energy of the specific Fe 2 O 3 thick sheet, E 0，本体 is the bulk Fe of each formula unit 2 O 3 internal DFT energy, n is the number of formula units in the thick sheet structure, and A is the surface area of a specific facet from the thick sheet structure. It is expected that the role of the water-oxide interface is roughly the same for Figures 3A - 3E all the iron oxide surfaces shown in Figures 3A - 3E The various Fe 2 O 3 surface calculated DFT surface energies are shown in Table 1 below.

[0038] Table 1

[0039] The various Fe 2 O 3 surface calculated DFT surface energies

[0040] .

[0041] As shown in Table 1, (012) Fe 2 O 3 is the most energetically favorable surface facet structure, followed by (001) and (110) Fe 2 O 3 . (101) and (100) Fe 2 O 3 exhibit the highest DFT surface energies and are thus less favorable than other iron oxide surface structures.

[0042] In Fe-based metals such as stainless steel, rust forms according to the following reaction. Specifically, when exposed to water and air, Fe 2 O 3 forms Fe 2 O 3 · x H 2 O as follows:

[0043] Fe (固) → Fe 2+ (水溶液) + 2e - ( A , oxidation half-reaction)

[0044] O 2(气) + 2H 2 O (液) + 4e - → 4OH - (水溶液) ( B , reduction half-reaction)

[0045] 2Fe (固) + O 2(气) + 2H 2 O (液) → 2Fe 2+ (水溶液) + 4OH - (水溶液) (2 A + B )

[0046] Fe 2+ (水溶液) + 2OH - (水溶液) → Fe(OH) 2(固) (intermediate state)

[0047] 4Fe(OH) 2(固) + O 2(气) + x H 2 O (液) → 2Fe 2 O 3 ·( x +4)H 2 O (固) (reacting further with H 2 O and O 2 )

[0048] Considering the above reaction process for the formation of rust in iron-based metals such as stainless steel, the atomic structure of the surface of iron oxide Fe 2 O 3 can be characterized by -OH capping. Figures 4A - 4E Non-limiting examples of the atomic structure of the iron oxide surface containing -OH termini are depicted in Figure 4A The atomic structure of (001) Fe 2 O 3 containing -OH termini is depicted. Figure 4B The atomic structure of (110) Fe 2 O 3 containing -OH termini is depicted. Figure 4C The atomic structure of (100) Fe 2 O 3 containing -OH termini is depicted.Figure 4D depicts the atomic structure of (101) Fe containing -OH terminations 2 O 3 Finally, Figure 4E depicts the atomic structure of (012) Fe containing -OH terminations 2 O 3 of it.

[0049] Figure 5 shows a graph depicting the surface energy of the iron oxide surface as a function of increasing -OH terminations. This graph shows the changes in particle morphology and the affected surface energy when -OH terminations are introduced Figures 3A - 3E to the Fe 2 O 3 surface facets shown and when increasing therein. The calculations supporting Figure 5 the graphs are carried out as described above in conjunction with Table 1. Thus, the density functional theory (DFT) surface energy of the iron oxide Fe 2 O 3 surface is calculated by applying the generalized gradient approximation (GGA) scheme. The particle shape can be inferred from the calculated surface energy using the known Wulff construction. Consistent with the experimental results, Figure 5 shows that both cubic and hexagonal shapes of Fe 2 O 3 are observed. Figure 5 Further shows that under dry conditions, Fe 2 O 3 is cubic (or quasi-cubic) in shape and is dominated by (110) and (012) Fe 2 O 3 surface facets. The increased -OH terminations can be produced by wetting conditions (where H 2 O is present) and / or acidic conditions (where available protons (H + +) have reacted with surface oxygen atoms in Fe 2 O 3 . By controlling the nucleation time, Fe 2 O 3 particles of different sizes and morphologies can be obtained. As Figure 5 shown, the particle shape of the -OH capped Fe 2 O 3 system is hexagonal, with the (001) surface facet being dominant. This further explains why the most experimentally studied Fe 2 O 3 surface is (001)Fe 2 O 3 .

[0050] As shown in FIGS. 3-5 and the related disclosures, first-principles DFT calculations within the generalized gradient approximation (GGA) can accurately reproduce the Fe 2 O 3 morphology observed in experiments. However, more advanced computational methods can account for the error of over-delocalization of electrons present in the ordinary GGA method.

[0051] Figure 6A and 6B show plots of the lattice parameters and the calculated band gaps of the Fe U O 2 O 3 bulk structure when different c values are applied in the DFT calculations. For example, as shown, when U Fe = 8 eV is applied, both the lattice parameter c ( Figure 6A , ~13.7 Å) and the experimental band gap ( Figure 6B , ~2.0 eV) can be obtained. As U Fe increases in the DFT calculations, both the lattice parameter and the band gap approach the experimental values. Generally, U Fe = 4 eV has traditionally been used for DFT calculations, which is compatible with the experimental formation energy of iron oxide. However, as Figure 6A and 6B show, considering the near agreement between the experimental and calculated lattice parameters and band gaps when U Fe = 8 eV, introducing U Fe = 8 eV can more accurately reflect the actual Fe 2 O 3 system.

[0052] Figures 7A - 7D show plots of the density of states (DOS) of Fe 2 O 3 at different U values. As shown, the band gap appears as the U value increases. The band gap (E g ) is defined as the distance between the Fermi level E F (x = 0) and the conduction band (where x is the positive number in the plot of FIG. 7). For example, Figure 6B and 7A -7D show that pure GGA and U = 3 have no band gap, while higher U values such as U Fe= 6 and 8 show that the distance between the occupied and unoccupied states (close to x = 0) (i.e., the bandgap) has increased.

[0053] Considering the above parameters for accurately representing the Fe 2 O 3 system - both in terms of structure (particle shape) and in terms of electrons (density of states) - the energy required to remove Fe atoms in each of the (101), (001), (110), (012), and (100) Fe 2 O 3 systems can be determined by calculation. Table 2 below illustrates the formation of the lowest Fe surface vacancies on the above various Fe 2 O 3 surfaces. As shown in Table 2, it is most difficult to remove Fe from the (110), (012), and (100) Fe 2 O 3 surfaces, and it is relatively easier to remove Fe from the (101) and (001) Fe 2 O 3 surfaces.

[0054] Table 2

[0055] Various Fe 2 O 3 Calculated DFT surface vacancy formation of Fe on the surfaces

[0056] <![CDATA[α-Fe 2 O 3 Surface facet]]> <![CDATA[Δ E 空位,Fe,表面 > Surface end (101) 1.457 <![CDATA[FeO 4 and FeO 5 > (001) 2.728 <![CDATA[FeO 3 > (110) 3.057 <![CDATA[FeO 5 > (012) 4.331 <![CDATA[FeO 5 > (100) 5.987 <![CDATA[FeO 4 >

[0057] The energy associated with the DFT surface vacancy formation of Fe is determined based on the following equation (2):

[0058] Δ E 空位,Fe,表面 = E 0,w / Fe空位 – ( E 0,原始 + µ Fe ) (2),

[0059] where E 0,w / Fe空位 is the internal DFT energy of the thick slab of Fe 2 O 3 from which one surface Fe atom has been removed, E 0,原始 is the internal DFT energy of the original thick slab, and µ Fe is the energy of the bulk bccThe chemical potential of Fe determined for Fe metal. The higher DFT vacancy formation energy indicates an increased difficulty associated with removing Fe atoms from the system.

[0060] According to the data recorded in Table 2, if Fe 2 O 3 films can grow together with (110), (012), and (100) surface facets, then Fe dissolution may be more difficult compared to Fe 2 O 3 films dominated by (101) and (001) surface facets. As Figure 5 has been shown, cubic Fe 2 O 3 is mainly dominated by (110) and (012) surfaces, while hexagonal Fe 2 O 3 mainly includes (001) and (110) planes. The preparation and selection of Fe 2 O 3 surfaces including increased resistance to Fe dissolution can be used to appropriately identify and utilize corrosion-resistant oxide films within bipolar plates (BPPs) of PEMFC stacks. And although stainless steel corrodes to produce Fe 2 O 3 , the application of highly tolerant Fe 2 O 3 surfaces can prevent (or at least slow down) further Fe ion dissolution in the acidic operating environment of PEMFCs. Stainless steel BPPs with protective Fe 2 O 3 surface layers including dissolution-resistant (110), (012), and (100) facets and with a thickness in the range of a few nm to about 1 µm can minimize and / or inhibit dissolution reactions that lead to the formation of species such as free radicals that can trigger polymer membrane degradation and catalytic deterioration in PEMFCs. By using a more stable Fe 2 O 3 surface within the BPP, for example, Pt catalyst poisoning triggered by Fe dissolution can be inhibited, thereby increasing the potential lifespan of PEMFCs.

[0061] In one or more embodiments, stainless steel with a corrosion-resistant surface oxide layer containing (110), (012), and / or (100) Fe 2 O 3 surface facets is used for PEMFC BPPs. In other embodiments, stainless steel with a corrosion-resistant surface oxide layer containing (001) Fe 2 O 3Stainless steel with a corrosion-resistant surface oxide layer of surface facets is used for PEMFC BPP. Other beneficial surface facets may include lattice plane families of (110), (012), (100), and / or (001), such as (006), (113), (024), (116), (122), (213), (300), etc. As described above, the thickness of the corrosion-resistant oxide layer can be adjusted according to the needs of specific applications and can be in the range of about 1 nm to about 1 µm. Non-limiting examples of such thicknesses can be about 0.1 - 0.8 µm, 0.2 - 0.6 µm, or 0.3 - 0.5 µm. In certain embodiments, the thickness can be 1 nm - 0.5 µm. In other embodiments, the thickness of the corrosion-resistant iron oxide film can be 150 nm - 0.3 µm.

[0062] According to certain embodiments, a stainless steel substrate having a corrosion-resistant surface oxide layer defined by a surface morphology comprising a first surface facet group and a second surface facet group is disclosed, the first surface facet group including (110), (012), and (100) Fe 2 O 3 surfaces, and the second surface facet group including (001) and (101) Fe 2 O 3 surfaces. In some embodiments, the first surface facet group may include only (110), (012), or (100) Fe 2 O 3 surfaces. In other embodiments, the first surface facet group may include two or more of (110), (012), or (100) Fe 2 O 3 surface structures. Similarly, in some embodiments, the second surface facet group may include only (001) or (101) Fe 2 O 3 surfaces. In other embodiments, the second surface facet group may include both (001) and (101) Fe 2 O 3 surface structures. In certain embodiments, the Fe 2 O 3 oxide layer mainly comprises Fe of the first surface facet group 2 O 3Surface Structure. In some embodiments, the stainless steel substrate may include a corrosion-resistant surface oxide layer characterized by a surface morphology comprising 70% - 90% of a first surface facet group and 10% - 30% of a second surface facet group. Thus, according to certain embodiments, the ratio of the first surface facet group to the second surface facet group may be in the range of 9:1 - 7:3. According to other embodiments, the stainless steel substrate may include a corrosion-resistant surface oxide layer characterized by a surface morphology comprising greater than 90% of the first surface facet group. In at least another embodiment, the corrosion-resistant surface oxide layer may be characterized by a surface morphology comprising greater than 95% of the first surface facet group. Other embodiments may include a corrosion-resistant surface oxide layer characterized by a surface morphology comprising 100% of the first surface facet group. Some embodiments may include amorphous FeO x Surface structure, where x is in the range of 1 - 2. It may also include additional metal impurities. Such metals include, but are not limited to, Cr, Ni, Co, Mn, and Si.

[0063] Different Fe 2 O 3 The presence of surface facets can be confirmed by X-ray powder diffraction (XRD) or high-resolution transmission electron microscopy (HR-TEM). For example, when measured by XRD with a Cu-Kα source (λ = 1.54 Å), the (012) peak of Fe 2 O 3 is located at 2θ of 24 - 26°, the (110) is located at 2θ of 35 - 38°, and the (300) is located at 2θ of 62 - 65°. The relative ratio of different planes can be further quantified using relative XRD height and / or full width at half maximum. Fe with more (110) and (012) 2 O 3 can be identified using HR-TEM with a spacing of ~0.25 nm, where such a structure may appear cubic or quasi-cubic in shape. Of course, once more (001) Fe d is formed, then Fe 2 O 3 may appear hexagonal in shape. 2 O 3 2

[0064] Figure 8A and 8B show non-limiting examples of X-ray powder diffraction (XRD) curves of two different Fe 2 O 3 samples. Figure 8A and 8B show Fe with different XRD peak intensities 2 O 3Two examples. As described above, the corrosion-resistant oxide film may mainly include (110), (012), and / or (100) Fe 2 O 3 Surface facets and their lattice plane families (e.g., (024), (300), etc.). Figure 8A and 8B It shows that the relative ratios of different planes in the XRD pattern may be different, which can be quantified using the XRD height and / or the full width at half maximum. In this case, by using Cu as the X-ray source, especially the Kα radiation of a 1.54 Å source, the 2θ values on the x axis are given.

[0065] In one or more embodiments, the stainless steel BPP with the disclosed corrosion-resistant oxide surface layer contains at least 10 - 20% chromium (Cr) and 5 - 10% nickel (Ni). Other elements in the stainless steel may include but are not limited to molybdenum (~1 - 2%), carbon (~0.03%), manganese (1 - 2%), silicon (0.5 - 2%), nitrogen (0.01 - 0.1%), copper (0.5 - 2%), and cobalt (<0.5%), with the balance being (Fe). On the surface of the BPP, a stable Cr oxide film (in addition to the specific Fe 2 O 3 oxide film described herein) may additionally be present to slow down the corrosion of the BPP material. In some embodiments, other crystalline and / or amorphous metal oxides including but not limited to NiO, MoO 2 、MoO 3 、MnO、Mn 2 O 3 、MnO 2 、SiO 2 、CuO、Co 3 O 4 etc. can also be used to inhibit the corrosion of the BPP material.

[0066] Due to its composition, structure, and morphology, the BPP composed of Fe 2 O 3 surface and coating disclosed according to certain embodiments may include many desired properties. For example, in the case of 80 °C, pH = 2 - 3, and the presence of about 0.1 ppm HF in the solution, the BPP composed of Fe 2 O 3 surface and coating disclosed herein may exhibit a corrosion resistance of less than about 1 µA cm -2 . In other embodiments, the BPP oxide coating can achieve a corrosion current of at least less than about 0.5 - 10, 1 - 8, or 1.5 - 5 µA cm -2 under the same operating conditions. The conductivity of the BPP oxide coating can be greater than 100 S cm-1 , wherein the thickness of the coating can be optimized to achieve a target conductivity. In certain embodiments, the conductivity of the BPP oxide coating is 100-150, 110-140 or 120-130 S cm -1 . In other embodiments, the conductivity of the BPP coating is 0.1-100, 1-80 or 10-50 S cm -1 . The interfacial contact resistance between the stainless steel substrate and the given BPP coating can be less than about 0.01 ohm cm 2 . In certain embodiments, this interfacial contact resistance is 0.001-0.2, 0.005-0.1 or 0.01-0.05 ohm cm 2 .

[0067] Additionally, various methods for growing corrosion-resistant oxide film layers are disclosed herein. In at least one embodiment, the method includes growing Fe 2 O 3 oxide film on stainless steel by using a solution-based method. Hydrolysis can be carried out in a water bath at 80-100 °C for different aging times of 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes. In one or more embodiments, hydrolysis is carried out in a water bath at 25-100 °C for an aging time of 1-120 minutes. The reaction time can also be 2-96 hours and can last 24-72 hours according to some embodiments. Fe-containing precursors such as FeCl 3 in the presence of acids such as HCl, HNO 3 , H 2 SO 4 can additionally help control the nucleation of different surface facets formation. The solution-based method can further include acid and / or base treatment with other types of oxidation and / or reduction chemical reagents.

[0068] Alternative methods for growing (one or more) Fe 2 O 3 oxide film disclosed herein include electrochemical methods. According to such methods, stainless steel can be polished and cleaned with an organic solvent such as ethanol, and then electrochemically oxidized. The working electrode is usually stainless steel, and the counter electrode and reference electrode can vary according to the voltage window. Generally, Pt foil and / or Ag / AgCl (with saturated KCl) can be used as the counter electrode and reference electrode. The impregnated electrolyte solution can be an acid with different concentrations (e.g., 0.01-1 M sulfuric acid), where the exact pH (pH is 1-13) can be adjusted or neutralized as needed. The electrochemical method can further include using acidic and / or basic solutions, such as but not limited to HCl, H 2 SO4 , HClO 4 , NaOH and KOH.

[0069] Another method for growing one or more Fe 2 O 3 oxide films involves heat treatment. Stainless steel can be heat treated in a box furnace at a temperature of 150 - 900 °C in the presence of mild oxidants such as air or oxygen. The heat treatment method may further include adjusting the heating and / or cooling rate from 1 degree per minute to 10 degrees per minute. The cooling process can be accomplished by natural cooling or a rapid quenching step.

[0070] Although the BPP of PEMFC has been described as a suitable application for the corrosion-resistant iron oxides described above, the disclosed oxide layers are equally suitable for additional uses. For example, the disclosed corrosion-resistant oxide film can be used as part of the surface portion of other industrial applications that require a chemically inert conductive material, such as battery packs, photovoltaic systems, consumer electronics, and / or anywhere else where a conductive and inert oxide would be beneficial. Additionally, the surface portion of the applicable device can include a relatively thin corrosion-resistant oxide film such that the film is transparent. The material can thus be used as a transparent conductive oxide film, which can be used, for example, in photovoltaic systems.

[0071] Although the exemplary embodiments have been described above, it does not mean that these embodiments describe all possible forms of the invention. On the contrary, the words used in the specification are descriptive rather than restrictive, and it should be understood that various changes can be made without departing from the spirit and scope of the invention. Additionally, the features of the various embodiments can be combined to form further embodiments of the invention.

Claims

1. A corrosion-resistant substrate, which comprises: a body part; and A surface portion, the surface portion including Fe 2 O 3 An oxide layer, the Fe 2 O 3 oxide layer containing a first group of surface facets including (110), (012), or (100) Fe 2 O 3 and a second group of surface facets including (001) or (101) Fe 2 O 3 surface facets, the surface portion being configured to impart corrosion resistance to the substrate; Among them, Fe 2 O 3 The oxide layer is 0.001 - 0.5 μm thick, and the (110), (012), and (100) Fe 2 O 3 surface facets of the first surface facet group cover more than 50% of the surface portion of the substrate.

2. The substrate according to claim 1, wherein the surface portion further comprises a protective coating containing MgO, Al 2 O 3 , TiO 2 or ZrO 2 .

3. The substrate according to claim 1, wherein the surface part further comprises a protective coating having a structure described as follows: ABO x , wherein A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo, and x is an integer in the range of 1-8.

4. The substrate according to claim 1, wherein the body part is a stainless steel containing 10-20 wt% chromium and 5-10 wt% nickel.

5. The substrate according to claim 1, wherein its conductivity is greater than 100 S cm -1 .

6. The substrate according to claim 1, wherein the surface portion has a corrosion resistance of less than 1 μA cm -2 in the presence of 0.1 ppm HF at 80 °C and pH 2-3.

7. The substrate according to claim 1, wherein the Fe 2 O 3 oxide layer is 0.15 - 0.3 μm thick.

8. The substrate according to claim 1, wherein the Fe 2 O 3 oxide layer comprises 70%-90% of a first set of surface facets and 10%-30% of a second set of surface facets.

9. A bipolar plate for a proton exchange membrane fuel cell, which comprises: A metal substrate having a body portion and a surface portion, the surface portion including Fe 2 O 3 oxide layer, the Fe 2 O 3 oxide layer comprising a first surface facet group including (110), (012), or (100) Fe 2 O 3 surface facets and a second surface facet group including (001) or (101) Fe 2 O 3 surface facets, the surface portion being configured to impart corrosion resistance to the substrate wherein the Fe 2 O 3 oxide layer is 0.001 - 0.5 μm thick, and Wherein, in the presence of 0.1 ppm HF at 80 °C and pH 2-3, the corrosion resistance of the surface portion is less than 1 μAcm -2 .

10. The bipolar plate according to claim 9, wherein the electrical conductivity of the metal substrate is greater than 100 S cm -1 .

11. The bipolar plate according to claim 9, wherein the body part is a stainless steel containing 10-20 wt% chromium and 5-10 wt% nickel.

12. The bipolar plate according to claim 9, wherein the surface portion further comprises a protective coating containing MgO, Al 2 O 3 , TiO 2 or ZrO 2 .

13. The bipolar plate according to claim 9, wherein the surface part further comprises a protective coating having a structure described as follows: ABO x , wherein A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo, and x is an integer in the range of 1-8.

14. The bipolar plate according to claim 9, wherein the Fe 2 O 3 oxide layer is 0.15 - 0.3 μm thick.

15. The bipolar plate according to claim 9, wherein the Fe 2 O 3 oxide layer comprises 70%-90% of a first surface facet group and 10%-30% of a second surface facet group.

16. A method for producing a corrosion-resistant stainless steel substrate having a body portion and a surface portion, the surface portion comprising a Fe 2 O 3 oxide layer, the Fe 2 O 3 oxide layer comprising a first group of surface facets including (110), (012) or (100) Fe 2 O 3 surface facets and a second group of surface facets including (001) or (101) Fe 2 O 3 surface facets, the method comprises: cleaning the stainless steel substrate with an organic solvent; and Electrochemically oxidize the stainless steel substrate to form a corrosion-resistant surface portion including an Fe 2 O 3 oxide layer having a thickness of 0.001 - 0.5 μm, and the Fe 2 O 3 oxide layer includes a first group of surface facets including (110), (012), or (100) Fe 2 O 3 surface facets and a second group of surface facets including (001) or (101) Fe 2 O 3 surface facets, and the (110), (012), and (100) Fe 2 O 3 surface facets of the first group of surface facets cover more than 50% of the surface portion of the substrate.

17. The method according to claim 16, further comprising depositing a protective coating comprising MgO, Al 2 O 3 , TiO 2 or ZrO 2 on a surface portion of the substrate.

18. The method according to claim 16, further comprising depositing a protective coating having a structure described as follows: ABO x , wherein A is Mg, Al, Ti or Zr, B is Zn, Sn, Cr or Mo, and x is an integer in the range of 1-8.

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