Corrosion-resistant oxide films and application for bipolar fuel cell plates

Abstract

Corrosion-resistant substrate, including: a mass area; and a surface area containing Fe2O 3- includes oxide layer with (110), (012) or (100) Fe2O3 surface facets configured to impart corrosion resistance properties to the substrate; where the Fe2O 3- oxide layer is between 0.001 and 0.5 µm thick and the (110)-, (012)- and (100)-Fe2O3 surface facets cover more than 50% of the surface area of ​​the substrate; where the Fe2O 3- The oxide layer comprises a first surface facet group that includes (110), (012) or (100)-Fe2O3 surface facets, and a second surface facet group that includes (001) or (101)-Fe2O3 surface facets; where the Fe2O 3- The oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group.

Description

TECHNICAL AREA

[0001] The present disclosure is generally directed to corrosion-resistant oxide films for use in proton exchange membrane fuel cells. In particular, the present disclosure relates to bipolar plates of proton exchange membrane fuel cells with corrosion-resistant substrate surface structures made of iron oxide, and to methods for producing the same. BACKGROUND

[0002] Fuel cells, and in particular proton exchange membrane fuel cells (PEMFCs), hold great promise as energy sources offering high efficiency, high power density, relatively low weight, and zero carbon dioxide emissions for use in a wide range of applications. These applications include, but are not limited to, transportation, stationary power generation, and portable power generation. Particularly relevant for applications in automobiles and other transportation-related fields, PEMFCs represent an environmentally friendly alternative to internal combustion engines for a variety of vehicles.

[0003] Proton-exchange fuel cells (PEMFCs) operate on the basis of proton transfer between an anode and a cathode. Key components of PEMFCs include a proton exchange membrane (PEM), through which protons are transferred, and a membrane electrode assembly (MEA) that encloses the PEM. PEMFCs also include bipolar plates (BPPs), which connect and disconnect individual fuel cells in series to form a fuel cell stack. Among other functions, BPPs provide the necessary voltage, assist in distributing fuel / propellant gas and oxygen across the active surface of the MEA, and conduct electrical current from the anode of one cell to the cathode of the next within the stack.With regard to such functionality, BPPs must not only be sufficiently chemically inert to resist degradation in the highly corrosive environment of the fuel cell, but also sufficiently electrically conductive to enable electron transfer for the oxygen reduction reaction of the fuel cell.

[0004] BPPs can account for 60 to 80% of the weight of a PEMFC stack and are among the most expensive PEMFC components, often contributing between 25% and 45% of the stack cost. Although other metals, such as titanium and aluminum, can be used, BPPs are typically made of stainless steel. Because PEMFC operation typically takes place in highly acidic environments, BPP materials with high corrosion resistance are desirable for long-term PEMFC operation. Treatment techniques, such as the introduction of conductive oxide and / or nitride coatings for stainless steel BPPs, can help improve their lifetime in the acidic PEMFC environment. Despite the potential of such techniques, the corrosive formation of Fe₂O₃ is unavoidable when ferrous metal is exposed to water and oxygen, as occurs under the acidic operating conditions of the PEMFC environment.Therefore, materials, structures, and techniques for increasing the corrosion resistance of BPP materials are desirable. JP H09-125224A discloses a weather-resistant steel with a hematite coating. US 2004 / 0247978A1 describes a bipolar plate for a fuel cell comprising a metal substrate and a metallic coating formed on at least a portion of the surface of the metal substrate. US 2001 / 0013474A1 discloses metal-based anodes for aluminum production cells. JP 2003-42403A describes a furnace wall structure obtained by sequentially forming an iron oxide layer and a coating layer containing an oxide with a higher affinity for oxygen than the iron oxide layer on the surface of a steel wall. SUMMARY

[0005] In at least one embodiment, a corrosion-resistant substrate is disclosed. The substrate includes a bulk region and a surface region comprising an Fe₂O₃ oxide layer with (110), (12), or (100) Fe₂O₃ surface facets configured to confer corrosion resistance properties to the substrate. The Fe₂O 3- The oxide layer is between 0.001 and 0.5 µm thick, and the (110), (012), and (100)-Fe₂O₃ surface facets cover more than 50% of the substrate's surface area. Furthermore, the Fe₂O 3- The oxide layer of the corrosion-resistant substrate is characterized by a surface morphology with a first surface facet group that includes (110), (012) or (100)-Fe2O3 surface facets, and a second surface facet group that includes (001) or (101)-Fe2O3 surface facets, such that the Fe2O 3-The oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. According to one or more embodiments, the surface area of ​​the corrosion-resistant substrate can include a protective coating comprising MgO, Al₂O₃, TiO₂, or ZrO₂. In another embodiment, the surface area of ​​the corrosion-resistant substrate can include a protective coating composed of ternary (or higher) chemical compounds, such as, for example, ABO₄. x , is formed 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 to 8.

[0006] In another embodiment, a bipolar plate (BPP) for a proton exchange membrane fuel cell (PEMFC) is disclosed. The BPP includes a corrosion-resistant substrate comprising a mass region and a surface region containing an Fe₂O₂. 3-The oxide layer comprises (110), (012), or (100)-Fe2O3 surface facets configured to impart corrosion resistance properties to the substrate. The Fe2O 3- The oxide layer is between 0.001 and 0.5 µm thick. The Fe₂O 3-The oxide layer of the corrosion-resistant substrate is characterized by a surface morphology comprising a first surface facet group including (110), (012), or (100) Fe₂O₃ surface facets, and a second surface facet group including (001) or (101) Fe₂O₃ surface facets, such that the Fe₂O₃ oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. The BPP substrate can be made of stainless steel. According to one or more embodiments, the surface area of ​​the BPP substrate can include a protective coating comprising MgO, Al₂O₃, TiO₂, or ZrO₂. In another embodiment, the surface area of ​​the BPP substrate can enclose a protective coating consisting of ternary (or higher) chemical compounds, 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 to 8. In one or more of the embodiments, the corrosion resistance of the surface area of ​​the BPP substrate is less than 1 µA cm. -2 at 80°C, a pH value between 2 and 3 and in the presence of approximately 0.1 ppm HF.

[0007] In other embodiments, methods for producing corrosion-resistant substrates are disclosed. The method includes cleaning a stainless steel substrate with an organic solvent and electrochemically oxidizing the stainless steel substrate to create a corrosion-resistant surface area containing Fe₂O₂. 3- to form an oxide layer between 0.001 and 0.5 µm thick with (110)-, (012)- or (100)-Fe2O3 surface facets covering more than 50% of the substrate's surface area, wherein the Fe2O 3-The oxide layer comprises a first surface facet group including (110), (012) or (100)-Fe2O3 surface facets, and a second surface facet group including (001)- or (101)-Fe2O3 surface facets; wherein the Fe2O 3- The oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. In another embodiment, the process can include the growth of the Fe₂O₃ oxide film on a stainless steel substrate by applying a solution-based method. In one example, hydrolysis can be carried out at 80 to 100°C in a water bath with varying 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 at 25 to 100°C in a water bath with an aging time between 1 and 120 minutes. BRIEF DESCRIPTION OF THE DRAWINGS The Fig. Figure 1 shows a schematic representation of a proton exchange membrane fuel cell that includes a bipolar plate according to one or more embodiments; The Fig. Figure 2 shows a perspective view of a non-restrictive example of a bipolar plate with a surface area that includes a corrosion-resistant iron oxide structure according to one or more embodiments; The Fig. 3A to 3E represent morphologically significant atomic structures of α-Fe2O3 iron oxide surfaces; The Fig. 4A to 4E represent atomic structures of iron oxide surfaces that include an -OH termination; The Fig. Figure 5 shows a graph representing the surface energy of iron oxide surfaces as a function of increasing -OH termination; The Fig. 6A and Fig.Figure 6B shows graphs in which the lattice parameter c and the calculated band gap of Fe2O3 are plotted when different U values ​​are applied to Fe in the density functional theory (DFT) calculations; The Fig. Figures 7A to 7D show graphs of the density of states (DOS) of Fe2O3 at different U values; and The Fig. 8A and Fig. Figure 8B shows examples of powder X-ray diffraction (XRD) graphs for two different Fe2O3 samples. DETAILED DESCRIPTION

[0008] This document describes embodiments of the present disclosure. It is understood, however, that the disclosed embodiments are merely examples and that other embodiments may take different and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of specific components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for instructing a person skilled in the art to apply the present invention in different ways.As persons with the usual technical knowledge will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to provide embodiments not expressly illustrated or described. The combinations of features that are illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of this disclosure may be desirable for specific applications or implementations.

[0009] The description of a group or class of materials as suitable for a particular purpose in combination with one or more of the embodiments implies that mixtures of any two or more members of the group or class are also suitable. The description of constituents in chemical terms refers to the constituents at the time of their addition to any combination specified in the description and does not necessarily exclude chemical interactions among constituents of the mixture after they have been mixed.

[0010] Unless expressly stated otherwise, all numerical quantities in this description that indicate dimensions or material properties are to be understood as modified by the word "approximately" in the description of the broadest scope of the present disclosure.

[0011] The first definition of an acronym or other abbreviation applies to all subsequent uses of the same abbreviation herein and, by analogy, to normal grammatical variations of the abbreviation defined at the beginning. Unless expressly stated otherwise, the measurement of a property is determined by the same method as previously or subsequently specified as a reference for the same property.

[0012] Reference is made to specific compositions, embodiments, and methods of embodiments known to the inventors. However, it should be understood that disclosed embodiments are merely examples of the present invention, which may be embodied in various and alternative forms. Therefore, the specific details disclosed herein should not be interpreted as limiting, but merely as representative guidelines to instruct a person skilled in the art on how to apply the present invention in different ways.

[0013] The terms “essentially” or “approximately” may be used herein to describe disclosed or claimed embodiments. The terms “essentially” or “approximately” may modify a value or relative characteristic disclosed or claimed in the present disclosure. In such cases, “essentially” or “approximately” may mean that the value or relative characteristic it modifies is within ± 0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or 10% of the value or relative characteristic.

[0014] Corrosion is a natural process that transforms a refined metal into a chemically more stable form, such as the metal's oxide(s), hydroxide(s), sulfide(s), and / or other salts. This transformation represents a gradual degradation of the metal material, caused by electrochemical oxidation upon reaction with an oxidizing agent, such as oxygen or sulfates. Corrosion can be induced by exposure of the metal substrate to atmospheric moisture, a solution with a relatively low pH, various chemical substances such as acids, microbes, elevated temperatures, and / or other factors. Particularly in acidic environments, corrosion begins at the interface between the bulk of a metal material (e.g., steel) and a solution (e.g., ions dissolved in water or a surface layer of water, which react to decompose the bulk material).

[0015] Due to the highly acidic operating environment, corrosion-resistant metals, metal surfaces, treatments, and coatings are particularly beneficial in proton exchange membrane fuel cells (PEMFCs). PEMFCs typically operate under acidic conditions, with a pH usually between 2 and 4. Operating temperatures within a PEMFC stack range from approximately 60 to 85°C. These and other factors contribute to the highly corrosive operating environment of PEMFCs. For example, low voltages exist within the PEMFC stack between startup and shutdown, and fluorine ions are released during PEMFC operation as a result of polymer membrane degradation. Furthermore, both H₂ and O₂ are present at the anode during startup and shutdown, creating a high cathodic potential that leads to cathodic corrosion.Given such conditions, PEMFCs require durable components that are able to withstand the corrosive operating environment.

[0016] Earlier fuel cell systems often used graphite for the bipolar plate (BPP) of the PEMFC because graphite can achieve high chemical stability and conductivity within the PEMFC environment. However, graphite is both brittle and expensive. Stainless steel is now generally considered one of the best candidates for BPPs due to its excellent mechanical stability, electrical and thermal conductivity, and relatively simple manufacturing process. Stainless steel is, of course, the generic term for a number of different steel compositions. Stainless steels typically include at least 10% chromium (Cr), which can form a stable chromium oxide surface layer known to prevent staining of the metal surface. SS304 and SS316 are two of the most common stainless steel compositions. SS304 contains 18% Cr and 8% nickel (Ni). SS316 contains 16% Cr, 10% Ni, and 2% molybdenum (Mo).Depending on the specific application, the composition of stainless steel can be varied. Such variations result in distinct mechanical stability, corrosion resistance, and magnetic properties. As is known, apart from iron (Fe), chromium (Cr), nickel (Ni), and molybdenum (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%).

[0017] Although less so than with pure iron, stainless steel is still susceptible to corrosion. Corrosion of stainless steel occurs when the metal is exposed to and reacts with water / air and various impurities on its surfaces. When iron is exposed to water and oxygen, it forms rust, typically characterized by the formation of reddish oxide. Rust comprises the oxidized forms of iron—that is, hydrated iron(III) oxides (especially Fe₂O₃·xH₂O) and iron(III) oxide hydroxides (especially FeO(OH)₂ and Fe(OH)₃). In acidic environments, the formation of such iron oxide complexes can be accelerated, and some of these oxides may dissolve further in solution.

[0018] When iron-based oxide films form on stainless steel BPPs, they significantly affect the contact resistance and electrical conductivity of the PEMFC. Most oxides are insulators, for example, and therefore negatively impact the electrical conductivity of the bulk material. This is particularly problematic in the case of BPPs designed to be highly electrically conductive. The formation of insulating oxide film layers on BPPs reduces electron transfer, which can lead to decreased PEMFC power output. Furthermore, corrosion films can grow over time, resulting in increased contact resistance. Significantly, if the products of such corrosion can be ionized (e.g., Fe), the situation becomes even more critical. 2+ or Fe 3+An acidic solution containing such ions can be transported to other fuel cell components. Within PEMFC stacks, for example, iron dissolution can poison the platinum catalyst, leading to reduced reaction rates of H₂ and O₂ adsorption, H₂O formation, and poor fuel cell efficiency.

[0019] Numerous efforts to prevent or slow down metal corrosion have led to the development of various types of coatings. Examples include applied coatings such as paint, plating or electroplating, enamel, reactive coatings including corrosion inhibitors such as chromates, phosphates, conductive polymers, surfactant-like chemicals designed to suppress electrochemical reactions between the environment and the metal substrate, anodized surfaces, and biofilm coatings. In the case of BPPs used in PEMFC stacks, corrosion resistance can be conferred by treatment techniques such as the introduction of conductive oxide and / or nitride coatings onto the stainless steel.Despite such techniques, the corrosive formation of Fe2O3 is unavoidable when Fe metal is exposed to water and oxygen, as is the case under the acidic operating conditions of the PEMFC environment.

[0020] As described in the present disclosure, some Fe₂O₃ species formed on the surface of stainless steel—and in other metal compositions consisting mainly of Fe—are more resistant to metal dissolution. Density functional theory (DFT) calculations, based on the fundamental principles, can be used to determine the relevant energetics of Fe₂O₃ oxide surfaces. In accordance with the present disclosure, the formation of corrosion-resistant Fe₂O₃ surfaces can be carefully controlled and transferred to stainless steel BPPs by precisely tuning the narrow range of Fe₂O₃ surface energies that are sensitive to synthesis conditions and the local environment.

[0021] A non-restrictive example of a proton exchange membrane fuel cell is in the Fig. Figure 1 shows a core component of the PEMFC 10: a membrane electrode assembly (MEA) 12, which supports the electrochemical reaction within the stack. The MEA 12 includes subcomponents such as electrodes, catalysts, and proton exchange membranes. In addition to the MEA 12, the PEMFC 10 typically includes other components such as current collectors 14, gas diffusion layer(s) 16, cuffs 18, and at least one bipolar plate (BPP) 20.

[0022] The BPP 20 is implemented in a PEMFC stack for gas distribution, power generation, and separating individual cells within the stack. The BPP 20 also provides additional functions, such as the removal of reaction products and water, as well as thermal management within the PEMFC 10. The BPP 20 is therefore an essential component of the PEMFC 10. However, the BPP 20 is also a relatively expensive component and a common cause of PEMFC system degradation. For example, BPPs can account for approximately 60 to 80% of the stack weight of the PEMFC 10, about 50% of the stack volume, and approximately 25-45% of the stack cost. The BPP 20 presents another material challenge, as it must also be sufficiently electrically conductive to enable electron transfer for the oxygen reduction reaction.Therefore, the material of the BPP 20 should be both electrically conductive and chemically inert to reactions with ions present in the PEMFC 10 environment.

[0023] The metal surface of the BPP 20, which may include stainless steel, can incorporate a coating such as a graphite-like coating or a protective oxide and / or nitride coating to increase the corrosion resistance of the BPP 20. The BPP 20 surface can thus incorporate elements such as Fe, Cr, Ni, Mo, Mn, Si, P, C, S, or a combination thereof. Alternative coatings include a Ti alloy or doped TiO₂. x, TiN, CrN or ZrN. However, even when such coatings are used, in an aggressively corrosive environment such as that of PEMFC 10, materials, structures, and techniques for increasing the corrosion resistance of BPP materials (e.g., stainless steel) are desirable. According to the embodiments set forth herein, materials, structures, and techniques for increasing the corrosion resistance of stainless steel BPPs are disclosed.

[0024] A non-restrictive example of a bipolar plate (BPP) 20 of the PEMFC is in the Fig.Figure 2 shows BPP 20 as a non-restrictive example of a substrate comprising a solid body or mass region 22 and a surface region 24. The mass region 22 may be formed of a metal, such as stainless steel. Alternatively, the mass region 22 may be formed of graphite, steel, aluminum, copper, an alloy of two or more metals, a combination thereof, or a composite material. The surface region 24 includes a corrosion-resistant iron oxide film structure according to one or more of the embodiments. The entire surface region 24 may include the iron oxide film structure. Alternatively, the surface region 24 may include one or more sub-region(s) that is / are free of the iron oxide film structure. In one embodiment, the entire surface region 24 includes the iron oxide film structure.Surface area 24 can further include protective coatings, such as binary oxide coatings applied to the top surface of the corrosion-resistant iron oxide film structure in the nano to micron range (a few nm to 100 µm). Such binary oxide coating materials include, but are not limited to, MgO, Al₂O₃, TiO₂, and ZrO₂. These oxide coating materials can be undoped and / or partially doped with nitrogen, carbon, or fluorine to further enhance the resulting electrical conductivity. Furthermore, surface area 24 can include protective coatings composed of ternary (or higher) chemical compounds, such as ABO. x , are formed 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 to 8. In accordance with some embodiments, the protective coating may include conductive nitrides and / or carbides.

[0025] The thickness of the surface area 24 and its associated corrosion-resistant iron oxide film layer can be adapted according to the specific application and can range from a few nm to approximately 1 µm. The thickness of the iron oxide film layer itself is between 1 nm and 0.5 µm. Non-limiting examples of such thicknesses are approximately 0.1, 0.2, or 0.3 to 0.5 µm. In other embodiments, the thickness of the corrosion-resistant iron oxide film can range between 150 nm and 0.3 µm.

[0026] Surfaces of α-Fe₂O₃ iron oxide can be characterized by their atomic morphological structures. Such surface facet structures can be defined by the following Miller indices: (001), (110), (100), (101), and (012). Each of these α-Fe₂O₃ surface morphologies is described in the Fig. 3A to 3E are shown. Fig. Figure 3A illustrates the atomic structure of (001) Fe2O3. Fig.Figure 3B illustrates the atomic structure of (110)-Fe2O3. Fig. Figure 3C illustrates the atomic structure of (100)-Fe2O3. Fig. 3D illustrates the atomic structure of (101)-Fe2O3. Finally, the Fig. 3E the atomic structure of (012)-Fe2O3. The bulk region of Fe2O3 is composed of FeO6 octahedra, while the surface region is undercoordinated. As shown in the Fig. 3B and Fig. As shown in Figure 3C, for example, the (110) and (012) iron oxide surfaces are both terminated with FeO5 polyhedra. As shown in the Fig. Shown in 3D, (101) iron oxide has both FeO4 and FeO5 surface units. (100) and (001) iron oxides are even more undercoordinated at their surface than (101) iron oxide. Fig. 3C shows that (100)-iron oxide is terminated with FeO4, and the Fig. Figure 3A shows that (001)-iron oxide is terminated with FeO3.

[0027] The density functional theory (DFT) surface energy of the iron oxide Fe2O3 surfaces, which are described in the Fig. The surface energy shown in Figures 3A to 3E can be calculated by applying the generalized gradient approximation (GGA) scheme. The Vienna Ab initio Simulation Package (VASP) software can be used to perform such surface energy calculations. The surface energy, γ, is the amount of energy required to split a bulk sample, thereby creating two vacuum-exposed surfaces. A lower surface energy represents an energetically more favorable state, and a higher surface energy represents a less energetically favorable state. The DFT surface energy Fe₂O₃ (γ) can be calculated based on the following equation (1): γ=(E0,Plate−n⋅E0,Mass) / (2A) where R 0,Platte The total internal DFT energy of the specific Fe2O3 plate is, as in the Fig.3A to 3E shown, E 0,Masse is the internal DFT energy of mass Fe2O3 per formula unit, n is the number of formula units in the plate structure, and A is the surface area of ​​a specific facet of the plate structure. The effect of the water-oxide interface is expected to be approximately the same for all iron oxide surfaces in the Fig. 3A to 3E are shown. Examples of calculated DFT surface energies of the various in the Fig. The Fe2O3 surfaces shown in 3A to 3E are shown in Table 1 below. Table 1 Calculated DFT surface energies of various Fe2O3 surfaces α-Fe 2 O 3 -Surface facets (001) (110) (100) (101) (012) Surface energy, γ(J / m²) 2 ) 0,82 0,92 1,21 1,06 0,63

[0028] As shown in Table 1, (012)-Fe2O3 is the energetically most favorable surface facet structure, followed by (001)- and (110)-Fe2O3. (101)- and (100)-Fe2O3 exhibit the highest DFT surface energies and are therefore less advantageous than the other iron oxide surface structures.

[0029] In iron-based metals, such as stainless steel, rust forms according to the following reactions. Specifically, Fe₂O₃ forms Fe₂O₃·xH₂O upon exposure to water and air, as described below: Fe (s) → Fe 2+ (aq.) + 2e - (A, Oxidation half-reaction) O 2(g) + 2H2O (1) + 4e - → 4OH - (aq.) (B, Reduction half-reaction) 2Fe (s) + O 2(g) + 2H2O (l) → 2Fe 2+ (aq.) + 4OH - (aq.) (2A + B) Fe 2+ (aq.) + 2OH - (aq.) → Fe(OH) 2(s)(Intermediate state) 4Fe(OH) 2(s) + O 2(g) + xH2O (I) → 2Fe2O3·(x+4)H2O (s) (further reaction with H2O and O2)

[0030] Given the above reaction pathways for the formation of rust in iron-based metals, such as stainless steel, the atomic structure of iron oxide (Fe₂O₃) surfaces can be characterized by -OH termination. Non-restrictive examples of the atomic structures of iron oxide surfaces that include -OH termination are presented in the Fig. 4A to 4E are shown. Fig. 4A represents the atomic structure of (001)-Fe2O3, which includes an -OH termination. Fig. Figure 4B represents the atomic structure of (110)-Fe2O3, which includes an -OH termination. Fig. 4C represents the atomic structure of (100)-Fe2O3, which includes an -OH termination. Fig.Figure 4D represents the atomic structure of (101)-Fe2O3, which includes an -OH termination. Finally, the Fig. 4E shows the atomic structure of (012)-Fe2O3, which includes an -OH termination.

[0031] A graph showing the surface energy of iron oxide surfaces as a function of increasing -OH termination is in the Fig. Figure 5 shows changes in particle morphologies and affected surface energies when an -OH termination is introduced into the Fig. The Fe2O3 surface facets shown in sections 3A to 3E are introduced and increased. The calculations that form the basis of the diagram are described below. Fig.The five supporting structures were carried out as described in conjunction with Table 1 above. Accordingly, the density functional theory (DFT) surface energies of the iron oxide Fe₂O₃ surfaces were calculated by applying the generalized gradient approximation (GGA) scheme. The particle shape can be derived from the calculated surface energies using the well-known Wulff construction. In accordance with experimental results, the Fig. 5, that both cubic and hexagonal forms of Fe2O3 can be observed. Fig. Figure 5 further shows that under dry conditions, Fe₂O₃ has a cubic (or pseudo-cubic) shape and is dominated by (110)- and (012)-Fe₂O₃ surface facets. Increased -OH terminations can result from humid conditions—where H₂O is present—and / or acidic conditions—where available protons (H₂) +) have reacted with surface oxygen atoms in Fe2O3. Fe2O3 particles of different sizes and morphologies can be obtained by controlling the nucleation time. As in the Fig. As shown in Figure 5, the particle shape of -OH-terminated Fe2O3 systems is hexagonal, with the (001) surface facet dominating. This further explains why the most experimentally studied Fe2O3 surface is (001)-Fe2O3.

[0032] As shown by the Fig. As demonstrated in Figures 3-5 and the associated disclosure, DFT calculations based on the fundamental principles within a generalized gradient approximation (GGA) can accurately reproduce the Fe2O3 morphology observed in the experiment. However, more advanced computational methods can account for the over-delocalization error of electrons present in the simple GGA approach.

[0033] Graphs showing the lattice parameter c and the calculated band gap of an Fe2O3 bulk structure, where different U-values ​​are applied to Fe in the DFT calculations, are in the Fig. 6A and Fig. 6B shown. For example, as shown in the figures, if U Fe = 8 eV is applied, both the grid parameter c ( Fig. 6A, ~13.7 Å) as well as the experimental band gap ( Fig. 6B, ~2.0 eV) are recorded. To the extent that the value for U Fe As the value increases in the DFT calculations, both the lattice parameter and the band gap approach the experimental values. Typically, U is conventionally used for the DFT calculation. Fe = 4 eV is used, which is adapted to the experimental formation energy of iron oxides. As shown by the Fig. 6A and Fig. However, as demonstrated in 6B, the inclusion of U Fe= 8 eV reflects the actual Fe2O3 system more accurately when considering the close agreement between the experimental and calculated lattice parameters and band gap when U Fe = 8 eV.

[0034] Graphs of the density of states (DOS) of Fe2O3 at different U-values ​​are in the Fig. Figures 7A to 7D are shown. As shown, the band gap widens as the U-values ​​increase. 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 a positive number in the graphs of the Fig. 7 is defined. Fig. 6B and 7A to 7D show, for example, that pure GGA and U = 3 have no band gap, while higher U values, such as U Fe Equations 6 and 8 show that the distance (near x = 0) between the occupied and unoccupied states (i.e., the band gap) has increased.

[0035] Considering the parameters discussed above for the accurate representation of the Fe₂O₃ system—both structurally (particle shapes) and electronically (density of states)—the energy required to remove an Fe atom from each system of (101), (001), (110), (012), and (100)-Fe₂O₃ can be determined by calculation. Table 2 below shows the lowest Fe vacancy formation on the surface of the various Fe₂O₃ surfaces described above. As shown in Table 2, removing Fe from the (110), (012), and (100)-Fe₂O₃ surfaces would be most difficult, and removing Fe from the (101) and (001)-Fe₂O₃ surfaces would be comparatively easier. Table 2 Calculated formation of Fe vacancies in DFT surfaces of different Fe2O3 surfaces α-Fe2O3 surface facet ΔE Vak,Fe,Oberfläche Surface termination (101) 1,457 FeO4 and FeO5 (001) 2,728 FeO3 (110) 3,057 FeO5 (012) 4,331 FeO5 (100) 5,987 FeO4

[0036] The energy associated with the formation of Fe vacancies in DFT surfaces was determined on the basis of the following equation (2): ΔEVak,Fe,surface=E0,w / Fevacancy−(E0,original+μFe) where R 0,w / FeVakanz The internal DFT energy of plate Fe2O3, where one of the surface Fe atoms has been removed, is E 0,ursprünglich the internal DFT energy of the original disk is and µ Fe The chemical potential of Fe is determined using bcc or body-centered cubic Fe bulk metal. Higher energies for the formation of DFT vacancies indicate increased difficulty in removing an Fe atom from the system.

[0037] According to the data recorded in Table 2, if the Fe2O3 film with (110), (012), and (100) surface facets can be grown, Fe resolution may be more difficult compared to the Fe2O3 film dominated by (101) and (001) surface facets. As already mentioned in the Fig.As shown in Figure 5, cubic Fe₂O₃ is largely dominated by (110) and (012) surfaces, while hexagonal Fe₂O₃ primarily includes (001) and (110) surfaces. The fabrication and selection of Fe₂O₃ surfaces exhibiting increased resistance to Fe dissolution can be used to properly identify and apply corrosion-resistant oxide films for use within the bipolar plates (BPPs) of PEMFC stacks. And although stainless steel corrodes to Fe₂O₃, the application of highly resistant Fe₂O₃ surfaces can prevent (or at least slow down) further Fe ion dissolution in the acidic operating environment of PEMFCs.BPPs made of stainless steel with protective Fe₂O₃ surface layers comprising dissolution-resistant (110), (012), and (100) facets, ranging in thickness from a few nm to approximately 1 µm, can minimize and / or suppress dissolution reactions that lead to the formation of species—such as radicals—which can trigger degradation of the polymer membrane and catalysis in PEMFCs. For example, poisoning of a Pt catalyst initiated by Fe dissolution can be suppressed by using more stable Fe₂O₃ surfaces within the BPP, thereby increasing the potential lifetime of PEMFCs.

[0038] In one or more embodiments, stainless steel with a corrosion-resistant surface oxide layer, comprising a first surface facet group including (110), (012), and / or (100) Fe₂O₃ surface facets, and a second surface facet group including (001) or (101) Fe₂O₃ surface facets, is used for a PEMFC-BPP. Other advantageous surface facets may include a family of lattice planes 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 requirements of a specific application and ranges from 1 nm to 0.5 µm. Non-restrictive examples of such thicknesses could be 0.1, 0.2, or 0.3 to 0.5 µm. In other embodiments, the thickness of the corrosion-resistant iron oxide film can range between 150 nm and 0.3 µm.

[0039] According to certain embodiments, a stainless steel substrate with a corrosion-resistant surface oxide layer is defined by a surface morphology that discloses a first surface facet group comprising (110), (012), and (100)-Fe₂O₃ surfaces, and a second surface facet group comprising (001) and (101)-Fe₂O₃ surfaces. In some embodiments, the first surface facet group may comprise exclusively (110), (012), or (100)-Fe₂O₃ surfaces. In other embodiments, the first surface facet group may comprise two or more of the (110), (012), and (100)-Fe₂O₃ surface structures. Similarly, in some embodiments, the second surface facet group may comprise exclusively (001) or (101)-Fe₂O₃ surfaces. In other embodiments, the second surface facet group can include both (001) and (101) Fe2O3 surface structures.The Fe₂O₃ oxide layer predominantly comprises Fe₂O₃ surface structures of the first surface facet group. The stainless steel substrate encloses a corrosion-resistant surface oxide layer characterized by a surface morphology comprising 70% to 90% of the first surface facet group and 10% to 30% of the second surface facet group. Thus, the ratio of the first surface facet group to the second surface facet group can range from 9:1 to 7:3. Some embodiments may incorporate an amorphous FeO. x - Include surface structure, where x ranges from 1 to 2. Other metal impurities may also be included. Such metals include, but are not limited to, Cr, Ni, Co, Mn, and Si.

[0040] The presence of different Fe₂O₃ surface facets can be verified using powder X-ray diffraction (XRD) or high-resolution transmission electron microscopy (HR-TEM). For example, the (012) peak for Fe₂O₃ is localized between 24 and 26° 2θ, (110) is localized between 35 and 38° 2θ, and (300) is localized between 62 and 65° 2θ when measured by XRD with a Cu-Kα source (λ = 1.54 Å). The relative relationship of different planes can further be quantified using the relative XRD height and / or the full width at half the maximum. Fe₂O₃ with more (110) and (012) fractions can be identified by HR-TEM with a d-distance of ~0.25 nm, whereby such structures can appear as a cubic or pseudocubic form. Of course, after more (001)-Fe₂O₃ has formed, Fe₂O₃ can appear as a hexagonal form.

[0041] Non-restrictive examples of powder X-ray diffraction (XRD) graphs for two different Fe2O3 samples are in the Fig. 8A and Fig. 8B shown. Fig. 8A and Fig. Figure 8B shows two examples of Fe2O3 with different XRD peak intensities. As described above, corrosion-resistant oxide films can mainly include (110), (012), and / or (100)-Fe2O3 surface facets and the lattice planes of their families (e.g., (024), (300), etc.). Fig. 8A and Fig. Figure 8B shows that the relative ratio of different planes in XRD patterns can differ, and this can be quantified either using the XRD height and / or the full width at half maximum. In this case, the two theta values ​​on the x-axis are given by using Cu as the X-ray source, specifically the Kα radiation from a source of 1.54 Å.

[0042] In one or more embodiments, BPPs made of stainless steel, utilizing the disclosed corrosion-resistant oxide surface layers, comprise at least 10 to 20% chromium (Cr) and 5 to 10% nickel (Ni). Other elements in the stainless steel may include, but are not limited to, molybdenum (~1 to 2%), carbon (~0.03%), manganese (1 to 2%), silicon (0.5 to 2%), nitrogen (0.01 to 0.1%), copper (0.5 to 2%), and cobalt (<0.5%), with the remainder being iron (Fe). Furthermore, a stable Cr oxide film (in addition to the specific Fe₂O₃ oxide films described herein) may be present on the BPP surface 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, MoO2, MoO3, MnO, Mn2O3, MnO2, SiO2, CuO, Co3O4, etc., may also be used to suppress corrosion of the BPP material.

[0043] Due to their composition, structure, and morphology, BPPs constructed from the Fe₂O₃ surfaces and coatings disclosed according to certain embodiments can exhibit a number of desirable properties. For example, BPPs constructed from the Fe₂O₃ surfaces and coatings disclosed herein can exhibit corrosion resistances of less than approximately 1 µA cm⁻¹. -2 at 80°C, at pH 2 to 3, in the presence of approximately 0.1 ppm HF in the solution. In other embodiments, the BPP oxide coatings can exhibit a corrosion current of at least less than approximately 0.5 to 10, 1 to 8, or 1.5 to 5 µA cm. -2 under the same operating conditions. The electrical conductivities of the BPP oxide coatings can be greater than 100 S / cm. -1The thickness of the coating layer can be optimized to achieve the target conductivity. In certain embodiments, the electrical conductivities of the BPP oxide coatings are between 100 and 150, 110 and 140, or 120 and 130 S / cm. -1 In other embodiments, the electrical conductivities of the BPP coatings are between 0.1 and 100, 1 and 80, or 10 and 50 S / cm. -1 The contact resistance of the interfaces between the stainless steel substrate and the given BPP coating can be less than approximately 0.01 ohms per cm. 2 In certain embodiments, the contact resistance of the interfaces is between 0.001 and 0.2, 0.005 and 0.1, or 0.01 and 0.05 ohms per cm. 2 .

[0044] Furthermore, a series of methods for growing corrosion-resistant oxide film layers are disclosed herein. In at least one embodiment, the method includes growing the Fe₂O₃ oxide film on the stainless steel using a solution-based process. Hydrolysis can be carried out at 80 to 100°C in a water bath with varying 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 of the embodiments, hydrolysis is carried out at 25 to 100°C in a water bath with an aging time of between 1 minute and 120 minutes. The reaction time can also vary from 2 to 96 hours and, according to some embodiments, can last between 24 and 72 hours.The presence of an iron-containing precursor, such as FeCl3, with an acid, such as HCl, HNO3, or H2SO4, can further contribute to controlling nucleation in various surface facet formations. The solution-based method can also include acid and / or base treatment with another type of oxidizing and / or reducing chemical agent.

[0045] An alternative method for growing the Fe₂O₃ oxide film(s) disclosed herein involves electrochemical processes. According to such a process, stainless steel can be polished and purified with an organic solvent, such as ethanol, and then electrochemically oxidized. The working electrode is typically stainless steel, with the counter and reference electrodes varying depending on the voltage windows. Typically, Pt foil and / or Ag / AgCl (with saturated KCl) can be used as counter and reference electrodes. The immersed electrolytic solution can be an acid with a varying concentration (e.g., 0.01 to 1 M sulfuric acid), with the exact pH (ranging from pH 1 to 13) being adjusted or neutralized as required.The electrochemical process may also include, but is not limited to, the use of acidic and / or basic solutions such as HCl, H2SO4, HClO4, NaOH and KOH.

[0046] Another method for growing the Fe₂O₃ oxide film(s) disclosed herein involves heat treatment. Stainless steel can be heat-treated in a chamber furnace at temperatures ranging from 150 to 900°C in the presence of a mild oxidizing agent, such as air or oxygen. The heat treatment process can further include adjusting the heating and / or cooling rate to 1 to 10 degrees per minute. The cooling process can be accomplished by natural cooling or rapid quenching.

[0047] While the BPP of a PEMFC has been described as a suitable application for the corrosion-resistant iron oxides outlined above, the disclosed oxide layers may also be suitable for other applications. For example, the disclosed corrosion-resistant oxide films may be used as part of a surface area in other industrial applications requiring a chemically inert, conductive material, such as batteries, photovoltaics, consumer electronics, and / or where a conductive and inert oxide would otherwise be advantageous. Furthermore, a surface area of ​​an applicable device may enclose a relatively thin corrosion-resistant oxide film in such a way that the film is transparent. The material can thus function as a transparent conductive oxide film that can be used, for example, in photovoltaics.

Claims

[1] Corrosion-resistant substrate, comprising: a mass area; and a surface area containing Fe2O 3- includes oxide layer with (110), (012) or (100) Fe2O3 surface facets configured to impart corrosion resistance properties to the substrate; where the Fe2O 3- oxide layer is between 0.001 and 0.5 µm thick and the (110)-, (012)- and (100)-Fe2O3 surface facets cover more than 50% of the surface area of ​​the substrate; where the Fe2O 3- The oxide layer comprises a first surface facet group that includes (110), (012) or (100) Fe2O3 surface facets, and a second surface facet group that includes (001) or (101) Fe2O3 surface facets; where the Fe2O 3- The oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. [2] Substrate of claim 1, wherein the surface area includes a protective coating comprising MgO, Al2O3, TiO2 or ZrO2. [3] Substrate of claim 1, wherein the surface area further includes a protective coating with a structure described by: Subscription x , where A Mg, Al, Ti or Zr is, B Zn, Sn, Cr or Mo is and x is an integer in the range of 1 to 8. [4] Substrate of claim 1, wherein the mass range is stainless steel comprising between 10 and 20% chromium and 5 to 10% nickel. [5] Substrate of claim 1, wherein its electrical conductivity is greater than 100 S cm -1 is. [6] Substrate of claim 1, wherein the surface area has a corrosion resistance of less than 1 µA cm. -2 at 80 °C, a pH value between 2 and 3 and in the presence of 0.1 ppm HF. [7] Substrate of claim 1, wherein the Fe2O 3- The oxide layer is between 0.15 and 0.3 µm thick. [8] Bipolar plate for a proton exchange membrane fuel cell, comprising: a metal substrate with a mass range and a surface area that contains an Fe2O 3- includes oxide layer with (110), (012) or (100) Fe2O3 surface facets configured to impart corrosion resistance properties to the substrate; where the Fe2O 3- The oxide layer is between 0.001 and 0.5 µm thick and where the corrosion resistance of the surface area is less than 1 µA cm -2 at 80 °C, a pH range between 2 and 3 and in the presence of 0.1 ppm HF; where the Fe2O 3-The oxide layer comprises a first surface facet group that includes (110), (012) or (100) Fe2O3 surface facets, and a second surface facet group that includes (001) or (101) Fe2O3 surface facets; where the Fe2O 3- The oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. [9] Bipolar plate of claim 8, wherein the electrical conductivity of the metal substrate is greater than 100 S cm -1 is. [10] Bipolar plate of claim 8, wherein the mass range is stainless steel comprising between 10 and 20% chromium and 5 to 10% nickel. [11] Bipolar plate of claim 8, wherein the surface area further includes a protective coating comprising MgO, Al2O3, TiO2 or ZrO2. [12] Bipolar plate of claim 8, wherein the surface area further includes a protective coating with a structure described by: Subscription x , where A Mg, Al, Ti or Zr is, B Zn, Sn, Cr or Mo is and x is an integer in the range of 1 to 8. [13] Bipolar plate of claim 8, wherein the Fe2O 3- The oxide layer is between 0.15 and 0.3 µm thick. [14] Method for producing a corrosion-resistant stainless steel substrate with a mass range and a surface area comprising an Fe2O 3- includes an oxide layer with (110), (012) or (100) Fe2O3 surface facets, the method comprising the following: Cleaning a stainless steel substrate with an organic solvent; and Electrochemical oxidation of the stainless steel substrate to form a corrosion-resistant surface area containing Fe2O 3- oxide layer between 0.001 and 0.5 µm thick with (110)-, (012)- or (100)-Fe2O3 surface facets covering more than 50% of the substrate's surface area, wherein the Fe2O 3- The oxide layer comprises a first surface facet group including (110), (012) or (100)-Fe2O3 surface facets, and a second surface facet group including (001) or (101)-Fe2O3 surface facets; wherein the Fe2O 3- The oxide layer comprises between 70% and 90% of the first surface facet group and between 10% and 30% of the second surface facet group. [15] Method according to claim 14, further comprising the deposition of a protective coating comprising MgO, Al2O3, TiO2 or ZrO2 onto the surface area of ​​the substrate. [16] Method according to claim 14, further comprising the deposition of a protective coating having a structure described by: Subscription x , where A Mg, Al, Ti or Zr is, B Zn, Sn, Cr or Mo is and x is an integer in the range of 1 to 8.

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