RU2577860C1 - Method for protection from oxidation of bipolar plates and current collectors of electrolysers and fuel elements with solid polymer electrolyte - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to a method of protection against oxidation bipolar plates of fuel elements and current collectors of electrolytic cells with solid polymer electrolyte (TPE), involving preliminary processing of metal substrate applied on treated metal substrate of electroconductive coating of noble metals by magnetron-ion depositing. Method is characterised by that is applied on the treated substrate electroconductive coating layer with fixation of each layer pulse implantation of oxygen ions or inert gas.

EFFECT: technical result is production of stable coating with service life, in 4 times exceeding the obtained at the prototype, and preserving current-conducting properties.

8 cl, 3 dwg, 1 tbl, 16 ex

Description

Technical field

The invention relates to the field of chemical current sources, and in particular to methods of creating protective coatings for metal current collectors (in the case of electrolyzers) and bipolar plates (in the case of fuel cells - TE) with a solid polymer electrolyte (TPE). During electrolysis, current collectors made, as a rule, of porous titanium, are constantly exposed to aggressive environments of oxygen, ozone, and hydrogen, which leads to the formation of oxide films on the oxygen collector current (anode), resulting in increased electrical resistance, decreased electrical conductivity and performance electrolyzer. On the hydrogen collector (cathode) of the current as a result of hydrogenation of the surface of porous titanium, its corrosion cracking occurs. When operating in such harsh conditions with constant humidity, current collectors and bipolar plates need reliable corrosion protection.

The main requirements for corrosion protective coatings are low electrical contact resistance, high electrical conductivity, good mechanical strength, uniform application over the entire surface area to create electrical contact, low cost of materials and production costs.

For plants with TPE, the most important criterion is the chemical resistance of the coating, the inability to use metals that change the oxidation state during operation and evaporate, which leads to poisoning of the membrane and catalyst.

Given all these requirements, Pt, Pd, Ir and their alloys have ideal protective properties.

State of the art

Currently, there are many different ways to create protective coatings - galvanic and thermal recovery, ion implantation, physical vapor deposition (PVD spraying methods), chemical vapor deposition (CVD spraying).

The prior art method for protecting metal substrates (US patent US No. 6887613 for the invention, publ. 03.05.2005). Previously, the oxide layer passivating the surface was removed from the metal surface by chemical etching or machining. A polymer coating mixed with conductive particles of gold, platinum, palladium, nickel and others was applied to the surface of the substrate. The polymer is selected for its compatibility with the metal substrate — epoxies, silicones, polyphenols, fluoropolymers, etc. The coating was applied with a thin film using electrophoretic deposition; with a brush; powder spray. The coating has good anti-corrosion properties.

The disadvantage of this method is the high electrical resistance of the layer due to the presence of the polymer component.

The prior art method of protection (see US patent US No. 7632592 for the invention, publ. 15.12.2009), which proposed the creation of an anti-corrosion coating on bipolar plates using the kinetic (cold) process of spraying a powder of platinum, palladium, rhodium, ruthenium and their alloys. Spraying was carried out with a gun using compressed gas, for example helium, which is supplied to the gun at high pressure. The velocity of the powder particles is 500-1500 m / s. Accelerated particles remain in a solid and relatively cold state. In the process, their oxidation and fusion do not occur; the average layer thickness is 10 nm. The adhesion of particles to a substrate depends on a sufficient amount of energy - with insufficient energy, weak adhesion of particles is observed, at very high energies, deformation of the particles and the substrate occurs, and a high degree of local heating is created.

The prior art method of protecting metal substrates (see US patent US No. 7700212 for an invention, publ. 04/20/2010). Previously, the surface of the substrate was roughened to improve adhesion to the coating material. Two coating layers were applied: 1 - stainless steel, a layer thickness of 0.1 μm to 2 μm, 2 - a coating layer of gold, platinum, palladium, ruthenium, rhodium and their alloys, with a thickness of not more than 10 nm. The layers were deposited using thermal spraying using a gun, from the spray nozzle of which a stream of molten particles was ejected, which formed a chemical bond with the metal surface, and coating was also possible using the PVD method (physical vapor deposition). The presence of 1 layer reduces the corrosion rate and reduces manufacturing costs, but its presence also leads to a disadvantage - a passive layer of chromium oxide is formed from stainless steel, which leads to a significant increase in the contact resistance of the anticorrosion coating.

The prior art method of protection (see US patent US No. 7803476 for the invention, publ. 09/28/2010)., Which proposed the creation of ultrathin coatings of noble metal Pt, Pd, Os, Ru, Ro, Ir and their alloys, thickness the coating is from 2 to 10 nm, preferably even a monatomic layer with a thickness of 0.3 to 0.5 nm (thickness equal to the diameter of the coating atom). Previously, a layer of non-metal having good porosity — coal, graphite mixed with a polymer, or metal — aluminum, titanium, and stainless steel was applied to a bipolar plate. Metal coatings were applied by electron beam sputtering, electrochemical deposition, magnetron-ion sputtering.

The advantages of this method include: elimination of the etching stage of the substrate to remove oxides, low contact resistance, minimal cost.

Disadvantages - in the presence of a non-metallic layer, the electrical contact resistance increases due to differences in surface energies and other molecular and physical interactions; mixing of the first and second layers is possible, as a result, base metals may be exposed to oxidation.

The prior art method of protecting a metal substrate (see US patent US No. 7150918 for invention, publ. 12/19/2006), including: processing a metal substrate to remove oxides from its surface, applying an electrically conductive corrosion-resistant metal coating of noble metals, applying an electrically conductive corrosion resistant polymer coating.

The disadvantage of this method is the high electrical resistance in the presence of a significant amount of a binder polymer, in the case of an insufficient amount of a binder polymer, the electrically conductive soot particles are washed out of the polymer coating.

The prior art method of protecting bipolar plates and current collectors against corrosion is a prototype (see US patent US No. 8785080 for an invention, publ. 07.22.2014), including:

- treating the substrate in boiling deionized water, or heat treatment at a temperature above 400 ° C, or soaking in boiling deionized water to form a passive oxide layer with a thickness of 0.5 nm to 30 nm,

- deposition of a conductive metal coating (Pt, Ru, Ir) on a passive oxide layer with a thickness of 0.1 nm to 50 nm. The coating was applied by magnetron-ion sputtering, electron beam evaporation, or ion deposition.

The presence of a passive oxide layer increases the corrosion resistance of the metal coating, however, and leads to disadvantages - a non-conductive oxide layer sharply affects the conductive properties of the coatings.

Disclosure of invention

The technical result of the claimed invention is to increase the resistance of the coating to oxidation, increase the corrosion resistance and service life and preserve the conductive properties inherent in non-oxidized metal.

The technical result is achieved by the fact that the method of protection against oxidation of bipolar plates of fuel cells and current collectors of electrolytic cells with solid polymer electrolyte (TPE) is that the metal substrate is pretreated, the conductive coating of the noble metals is applied to the treated metal substrate by magnetron-ion sputtering, wherein the electrically conductive coating is applied layer by layer with the fastening of each layer by pulse implantation of oxygen ions or an inert gas.

In a preferred embodiment, platinum, or palladium, or iridium, or a mixture thereof, is used as the noble metal. Pulse ion implantation is performed with a gradual decrease in ion energy and dose. The total coating thickness is from 1 to 500 nm. Sequentially sprayed layers have a thickness of 1 to 50 nm. Argon, or neon, or xenon, or krypton is used as an inert gas. The energy of implanted ions is from 2 to 15 keV, and the dose of implanted ions is up to 10 ¹⁵ ions / cm ² .

Brief Description of the Drawings

The features and essence of the claimed invention are explained in the following detailed description, illustrated by drawings and a table, which shows the following.

In FIG. 1 - distribution of platinum and titanium atoms displaced as a result of argon implantation (calculation by the SRIM program).

In FIG. 2 is a section of a titanium substrate with deposited platinum prior to implantation of argon, where

1 - titanium substrate;

2 - a layer of platinum;

3 - pores in the platinum layer.

In FIG. 3 is a section of a titanium substrate with sprayed platinum after implantation of argon, where:

1 - titanium substrate;

4 - intermediate titanium-platinum layer;

5 - platinum coating.

The table shows the characteristics of all examples of implementation of the claimed invention and prototype.

The implementation and examples of implementation of the invention

The method of magnetron-ion sputtering is based on a process based on the formation of a ring-shaped plasma above the surface of the cathode (target) as a result of collision of electrons with gas molecules (usually argon). Positive gas ions generated in the discharge, when a negative potential is applied to the substrate, are accelerated in an electric field and knock out atoms (or ions) of the target material, which are deposited on the surface of the substrate, forming a film on its surface.

The advantages of magnetron-ion sputtering are:

- high spraying speed of the deposited substance at low operating voltages (400-800 V) and at low working gas pressures (5 · 10 ^-1 -10 Pa);

- the ability to control over a wide range of spraying and deposition of sprayed matter;

- low degree of contamination of deposited coatings;

- the possibility of simultaneous sputtering of targets from different materials and, as a consequence, the ability to obtain coatings of complex (multicomponent) composition.

- relative ease of implementation;

- low cost;

- ease of scaling.

At the same time, the resulting coating is characterized by the presence of porosity, has low strength and insufficiently good adhesion to the substrate material due to the low kinetic energy of atomized atoms (ions), which is about 1-20 eV. This level of energy does not allow the penetration of atoms of the sprayed material into the surface layers of the substrate material and ensures the creation of an intermediate layer with high affinity for the substrate and coating material, high corrosion resistance and relatively low resistance even when an oxide surface film is formed.

In the framework of the claimed invention, the task of increasing the resistance and preserving the conductive properties of the electrodes and protective coatings of structural materials is solved by exposing the coating and the substrate to a stream of accelerated ions that move the coating material and the substrate at the atomic level, leading to the interpenetration of the substrate material and the coating, resulting in erosion of the interface between the coating and the substrate with the formation of the phase of the intermediate composition.

The type of accelerated ions and their energy are selected depending on the coating material, its thickness and the substrate material in such a way as to cause the movement of the coating and substrate atoms and their mixing at the phase boundary with minimal spraying of the coating material. Selection is made using appropriate calculations.

In FIG. Figure 1 shows the calculated data on the movement of atoms of a coating consisting of 50A thick platinum and atoms of a substrate consisting of titanium under the influence of argon ions with an energy of 10 keV. Ions with lower energies at the level of 1-2 keV do not reach the phase boundary and will not provide efficient atomic mixing for such a system at the phase boundary. However, at energies above 10 keV, a significant spraying of the platinum coating occurs, which negatively affects the product’s life.

Thus, in the case of a single-layer coating with a large thickness and high energy required for the implantable ions to penetrate to the phase boundary, the coating atoms are sputtered and precious metals are lost, in the case of a small coating thickness with optimal ion energy, the coating atoms penetrate the substrate material, mixing the material substrates and coatings; and an increase in coating strength. However, such a small (1-10 nm) coating thickness does not provide a long product life. In order to increase the strength of the coating, its life and reduce losses during sputtering, pulsed ion implantation is performed by layer-by-layer (thickness of each layer 1-50 nm) coating with a gradual decrease in ion energy and dose. The reduction in energy and dose makes it possible to practically eliminate losses during spraying, but allows to provide the required adhesion of the applied layers to a substrate on which the same metal has already been deposited (lack of phase separation) increases their uniformity. All this also helps to increase the resource. It should be noted that films with a thickness of 1 nm do not give a significant (required for current collectors) increase in product life, and the proposed method significantly increases their cost. Films with a thickness of more than 500 nm should also be considered economically unprofitable, because the consumption of platinum group metals increases significantly, and the life of the product as a whole (electrolyzer) begins to be limited by other factors.

For multiple deposition of coating layers, treatment with ions of higher energy is advisable only after applying the first layer with a thickness of 1-10 nm, and when processing subsequent layers with a thickness of 10-50 nm, argon ions with an energy of 3-5 keV are sufficient for compaction. The implantation of oxygen ions during the deposition of the first coating layers, along with the solution of the above problems, makes it possible to create a corrosion-resistant oxide film on the surface doped with coating atoms.

Example 1 (prototype).

Samples of titanium foil VT1-0 brand with an area of 1 cm ² , a thickness of 0.1 mm and porous titanium grade TPP-7 with an area of 7 cm ² are placed in an oven and kept at a temperature of 450 ° C for 20 minutes.

The samples are alternately clamped into a frame and installed in a special sample holder of a MIR-1 magnetron-ion sputter with a removable platinum target. The camera is closed. The mechanical pump is turned on and air is pumped out of the chamber to a pressure of ~ 10 ^-2 Torr. The chamber air is shut off and the diffusion pump is opened and its heating is turned on. After about 30 minutes, the diffusion pump goes into operation. Open the pumping chamber through a diffusion pump. After reaching a pressure of 6 × 10 ^-5 Torr open the argon inlet into the chamber. Argon pressure set argon pressure of 3 × 10 ^-3 Torr. By smoothly increasing the voltage at the cathode, a discharge is ignited, a discharge power of 100 W is set, a bias voltage is applied. Open the shutter between the target and the holder and begin counting the processing time. During processing, the pressure in the chamber and the discharge current are monitored. After 10 minutes of treatment, the discharge is turned off, the rotation is turned off, the argon supply is shut off. After 30 minutes, the pumping chamber was shut off. Turn off the heating of the diffusion pump and after cooling, turn off the mechanical pump. The camera is opened to the atmosphere and the extraction of the frame with the sample. The thickness of the sprayed coating was 40 nm.

The obtained coated materials can be used in electrochemical cells, primarily in electrolyzers with a solid polymer electrolyte, as cathode and anode materials (current collector, bipolar plates). The maximum problems are caused by anode materials (intense oxidation), in connection with this, life tests were carried out when they were used as anodes (that is, with a positive potential).

A current lead is welded to the obtained titanium foil sample by the spot welding method and placed as a test electrode in a three-electrode cell. A 10 cm ² Pt foil is used as a counter electrode; a standard silver chloride electrode connected to the cell through a capillary is used as a reference electrode. As an electrolyte, a solution of 1M H ₂ SO ₄ in water is used. The measurements are carried out using the AZRIVK 10-0.05A-6 V device (manufactured by Booster LLC, St. Petersburg) in the galvanostatic mode, i.e. A positive DC potential is applied to the electrode under study, which is necessary to achieve a current value of 50 mA. The tests consist in measuring the change in potential needed to achieve a given current over time. If the potential is exceeded above 3.2 V, the electrode resource is considered exhausted. The resulting sample has a resource of 2 hours 15 minutes.

Examples 2-16 of the implementation of the claimed invention.

Example 2

Samples of titanium foil grade VT1-0 with an area of 1 cm ² , a thickness of 0.1 mm and porous titanium grade TPP-7 with an area of 7 cm ^{2 are} boiled in isopropyl alcohol for 15 minutes. Then the alcohol is drained and the samples are boiled 2 times for 15 minutes in deionized water with a change of water between boils. Samples are heated in a solution of 15% hydrochloric acid to 70 ° C and kept at this temperature for 20 minutes. Then the acid is drained and the samples are boiled 3 times for 20 minutes in deionized water with a change of water between boils.

The samples are alternately placed in a MIR-1 magnetron-ion sputter with a platinum target and a platinum coating is applied. The magnetron current is 0.1 A, the magnetron voltage is 420 V, the gas is argon with a residual pressure of 0.86 Pa. After 15 minutes of spraying, a coating with a thickness of 60 nm is obtained. The resulting coating is exposed to the flow of argon ions by plasma pulsed ion implantation.

Implantation is carried out in a stream of argon ions with a maximum ion energy of 10 keV; the average energy is 5 keV. The dose during the exposure was 2 * 10 ¹⁴ ions / cm ² . A cross-sectional view of the coating after implantation is shown in FIG. 3.

The resulting sample is tested in a three-electrode cell, the process is similar to that shown in example 1. The resulting sample has a resource of 4 hours. For comparison, the data on the resource of titanium foil with the initial deposited platinum film (60 nm) without argon implantation is 1 hour.

Examples 3-7.

The process is similar to that shown in example 2, but vary the implantation dose, ion energy and coating thickness. The implantation dose, ion energy, coating thickness, as well as the operating life of the obtained samples are shown in table 1.

Example 8

The process is similar to that shown in example 2 and differs in that samples with a sprayed layer thickness of up to 15 nm are processed in a krypton stream with a maximum ion energy of 10 keV and a dose of 6 * 10 ¹⁴ ions / cm ² . The resulting sample has a resource of 1 hour 20 minutes. According to electron microscopy, the platinum layer thickness was reduced to a value of 0-4 nm, but a titanium layer with platinum atoms embedded in it was formed.

Example 9

The process is similar to that described in example 2 and differs in that samples with a thickness of the deposited layer of 10 nm are treated in a stream of argon ions with a maximum ion energy of 10 keV and a dose of 6 * 10 ¹⁴ ions / cm ² . After applying the second layer with a thickness of 10 nm, the argon ion is treated with an energy of 5 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and then spraying is repeated 4 times with a new layer thickness of 15 nm, and each subsequent layer is treated in an ion stream argon with an ion energy of 3 keV and a dose of 8 * 10 ¹³ ion / cm ² . The resulting sample has a resource of 8 hours 55 minutes.

Example 10

The process is similar to that described in example 2 and differs in that the samples with a thickness of the deposited layer of 10 nm are treated in a stream of oxygen ions with a maximum ion energy of 10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² . After applying a second layer with a thickness of 10 nm, the argon ion is treated with an energy of 5 keV and a dose of 1 * 10 ¹⁴ ion / cm ² , and then spraying is repeated 4 times with a new layer thickness of 15 nm, with each subsequent layer being treated in an argon ion stream with an ion energy of 5 keV and a dose of 8 * 10 ¹³ ion / cm ² (so that there is no sputtering!). The resulting sample has a resource of 9 hours 10 minutes.

Example 11

The process is similar to that shown in example 2 and differs in that the samples are placed in a MIR-1 magnetron-ion sputter with an iridium target and an iridium coating is applied. The magnetron current is 0.1 A, the magnetron voltage is 440 V, the gas is argon with a residual pressure of 0.71 Pa. The spraying speed provides the formation of a coating with a thickness of 60 nm in 18 minutes. The resulting coating is exposed to the flow of argon ions by plasma pulsed ion implantation.

Samples with a thickness of the first deposited layer of 10 nm are treated in a stream of argon ions with a maximum ion energy of 10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² . After applying a second layer with a thickness of 10 nm, the argon ions are treated with an energy of 5-10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and then spraying is repeated 4 times with a new layer thickness of 15 nm, each subsequent layer is treated in a stream argon ions with an ion energy of 3 keV and a dose of 8 * 10 ¹³ ion / cm ² . The resulting sample has a resource of 8 hours 35 minutes.

Example 12

The process is similar to that shown in example 2 and differs in that the samples are placed in a MIR-1 magnetron-ion sputter with a target made of a platinum-iridium alloy (PlI-30 alloy according to GOST 13498-79), and a coating consisting of platinum and iridium is applied. The magnetron current is 0.1 A, the magnetron voltage is 440 V, the gas is argon with a residual pressure of 0.69 Pa. The spraying speed provides the formation of a coating with a thickness of 60 nm in 18 minutes. The resulting coating is exposed to the flow of argon ions by plasma pulsed ion implantation.

Samples with a thickness of the deposited layer of 10 nm are treated in a stream of argon ions with a maximum ion energy of 10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and then spraying is repeated 5 times with a new layer thickness of 10 nm. After applying the second layer, processing is carried out in a stream of argon ions with an energy of 5-10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and each subsequent layer is treated in a stream of argon ions with an energy of ions of 3 keV and a dose of 8 * 10 ¹³ ion / cm ² . The resulting sample has a resource of 8 hours 45 minutes.

Example 13

The process is similar to that shown in example 2 and differs in that the samples are placed in a MIR-1 magnetron-ion sputter with a palladium target and a palladium coating is applied. The magnetron current is 0.1 A, the magnetron voltage is 420 V, the gas is argon with a residual pressure of 0.92 Pa. After 17 minutes of spraying, a coating with a thickness of 60 nm is obtained. Samples with a thickness of the deposited first layer of 10 nm are treated in a stream of argon ions with a maximum ion energy of 10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² . After applying a second layer with a thickness of 10 nm, the argon ions are treated with an energy of 5-10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and then spraying is repeated 4 times with a new layer thickness of 15 nm, each subsequent layer is treated in a stream argon ions with an ion energy of 3 keV and a dose of 8 * 10 ¹³ ion / cm ² . The resulting sample has a resource of 3 hours 20 minutes.

Example 14

The process is similar to that described in example 2 and differs in that the samples are placed in a MIR-1 magnetron-ion sputtering device with a target consisting of platinum, comprising 30% carbon, and a coating consisting of platinum and carbon is applied. The magnetron current is 0.1 A, the magnetron voltage is 420 V, the gas is argon with a residual pressure of 0.92 Pa. After 20 minutes of spraying, a coating with a thickness of 80 nm is obtained. Samples with a sprayed layer thickness of 60 nm are treated in a stream of argon ions with a maximum ion energy of 10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and then spraying is repeated 5 times with a new layer thickness of 10 nm. After applying the second layer, processing is carried out in a stream of argon ions with an energy of 5-10 keV and a dose of 2 * 10 ¹⁴ ion / cm ² , and each subsequent layer is treated in a stream of argon ions with an energy of ions of 3 keV and a dose of 8 * 10 ¹³ ion / cm ² . The resulting sample has a resource of 4 hours 30 minutes.

Example 15

The process is similar to that shown in example 9 and differs in that 13 layers are deposited, the thickness of the first and second is 30 nm, the subsequent is 50 nm, the ion energy is successively reduced from 15 to 3 keV, the implantation dose is from 5 · 10 ¹⁴ to 8 · 10 ¹³ ion / cm ² . The resulting sample has a resource of 8 hours 50 minutes.

Example 16

The process is similar to that shown in example 9 and differs in that the thickness of the first layer is 30 nm, of the next six layers of 50 nm, the implantation dose is from 2 · 10 ¹⁴ to 8 · 10 ¹³ ion / cm ² . The resulting sample has a resource of 9 hours 05 minutes.

Thus, the claimed method of protection against oxidation of bipolar plates of TE and current collectors of electrolyzers with TPE allows to obtain a stable coating with a service life 4 times higher than that obtained from the prototype, and preserving the conductive properties.

Claims (8)

1. A method of protecting against oxidation of bipolar plates of fuel cells and current collectors of electrolytic cells with solid polymer electrolyte (TPE), which consists in pretreating a metal substrate, applying a noble metal electroconductive coating to a treated metal substrate by magnetron-ion sputtering, characterized in that it is applied to the treated substrate is an electrically conductive coating layer by layer with the fastening of each layer by pulse implantation of oxygen ions or an inert gas. 2. The protection method according to claim 1, characterized in that platinum, or palladium, or iridium, or a mixture thereof is used as the noble metals. 3. The protection method according to claim 1, characterized in that the pulse implantation of ions is carried out with a gradual decrease in ion energy and dose. 4. The protection method according to claim 1, characterized in that the total coating thickness is from 1 to 500 nm. 5. The protection method according to claim 1, characterized in that the successively sprayed layers have a thickness of from 1 to 50 nm. 6. The protection method according to claim 1, characterized in that argon, or neon, or xenon, or krypton is used as an inert gas. 7. The protection method according to claim 1, characterized in that the energy of the implanted ions is from 2 to 15 keV. 8. The method of protection according to claim 1, characterized in that the dose of implantable ions is up to 10 ¹⁵ ions / cm ² .

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