Method for preparing polymer membrane for polymer electrolyte water electrolyzer

Abstract

The object of the present invention is to provide a method for preparing a polymer membrane for a polymer electrolyte water electrolysis cell in order to achieve a membrane with high crossover inhibition and without showing a negative impact on the cell performance. According to the present invention, this object is achieved by a method for preparing an ionomer of an ion exchange membrane (anion or cation exchange), the ionomer of which contains a recombination catalyst to prevent gas crossover of species such as hydrogen and / or oxygen to the anode cell compartment and the cathode cell compartment of an electrochemical cell; the method comprises the following steps: a) providing an ionomer as a proton or anion exchange polymer; b) selecting a recombination catalyst from the noble metal family; c) providing the selected recombination catalyst in ionic form contained in a liquid metal salt solution; d) immersing an ion exchange membrane in the liquid metal salt solution, thereby exchanging at least a portion of the ion ionomer interface with the ionic form of the recombination catalyst; e) assembling the immersed ion exchange membrane in an electrochemical cell; and f) at least partially reducing the content of the ionic form of the recombination catalyst to the metallic form by forcing hydrogen to permeate through the ionomer of the ion exchange membrane. Therefore, the present invention proposes a new method to reduce Pt-ions by diffusing hydrogen through the PEM in PEWE cells. The proposed solution provides a way to achieve a membrane with high crossover suppression without negatively affecting the cell performance and in-situ reduction on the go.

Description

[0001] The invention relates to a method for preparing a polymer membrane used in a polymer electrolyte water electrolysis cell.

[0002] A polymer electrolyte water electrolyser (PEWE) is an electrochemical device in which the electrodes are separated by a solid proton-conducting polymer membrane. On the anode side, water is split into oxygen and protons. The protons are transported to the cathode under the influence of the electric field, where molecular hydrogen is evolved. A key issue for the safe operation of PEWE cells is the high hydrogen crossover, which can lead to explosive hydrogen-oxygen mixtures. Safety aspects are particularly important in the case of membranes and high pressure differences.

[0003] Water electrolysis has the potential to become a key technology in the energy transition from a conventional fossil-based energy system to a renewable, carbon-free energy system. The supply of electricity based on a large part of these "emerging renewables" is associated with a large discrepancy between supply and demand due to the intermittent nature of these primary energy sources. Water electrolysis is a clean and efficient process that offers great prospects for storing large amounts of excess electricity in the form of chemical energy ("power-to-gas"). It is also a key element for coupling different sectors (electricity, mobility, heating) through power-to-x concepts. As Figure 1 The prior art PEWE cell shown in comprises a positive electrode for oxygen evolution reaction (OER), a negative electrode for hydrogen evolution reaction (HER), and a separator that insulates the electrodes electronically and prevents cross-mixing of evolved gases while allowing proton transport. Prior art membrane water electrolyzers use proton exchange membranes and can operate at relatively high current densities (1-3 A / cm2) due to thin polymer electrolytes (~0.2 mm) with low ohmic resistance. 2 ) and pressure differential. OER is catalyzed by noble metals such as Ir and Ru due to their activity and stability in acidic media. The most common HER catalyst in PEWE is Pt.

[0004] According to Faraday's law, the rate of gas generation in a PEWE cell is proportional to the applied current density. Increasing the current density while maintaining the conversion efficiency by using thinner membranes is one way to increase the rate of H2 production per unit cell area in PEWE. Thinner membranes allow for higher current densities at a given cell voltage, thereby effectively reducing the investment cost of the electrolyzer stack. Current densities as high as 19 A / cm2 at a cell potential of 3 V have been reported using a 50 μm thick membrane. 2 The associated power density is about 50 W / cm 2 .

[0005] Pressurized PEWE can make subsequent gas compression redundant or reduce the workload of drying by mechanical compression. In particular, differential pressurized PEWE is attractive for high-purity hydrogen production and higher Faradaic efficiency compared to balanced pressure operation. The diffusion rate of hydrogen to the anode side depends largely on the thickness of the membrane and the applied partial pressure. Since the diffusion coefficient of hydrogen in the membrane is about twice that of oxygen, and most of the oxygen diffused to the cathode side is recombined on the Pt cathode catalyst layer, the hydrogen content in the oxygen content on the anode side is a major safety aspect in PEWE, especially at high partial pressures using membranes. The lower explosion limit of hydrogen in oxygen is 4%.

[0006] Gas crossover can be suppressed by impregnating platinum particles into the membrane. The permeated hydrogen and oxygen recombine into water on the surface of the platinum particles. To date, there are two methods for introducing recombination catalysts into PEMs:

[0007] 1) In the first step, the membrane is immersed in a solution containing platinum ions. The platinum ions are absorbed by the proton exchange membrane through an ion exchange process. In the second step, the membrane doped with platinum ions is placed in a solution containing a chemical reducing agent (such as N2H4 or NaBH4) to reduce the Pt ions to Pt. The Pt particles are preferentially reduced at the membrane-solution interface.

[0008] 2) An ionomer layer containing platinum particles was sprayed onto one membrane. A second membrane was hot pressed onto the first membrane with the platinum-containing layer facing the membrane. This produced a platinum intermediate layer between the two membranes. The performance was reported to be inferior to that using the original membrane.

[0009] Another approach has been disclosed by D. Bessarabov, "Membranes with Recombination Catalyst for Hydrogen Crossover Reduction: Water Electrolysis", ECS Trans 85(11) (2018) 17-25 and US patent application US 2008 / 0044720, "Membrane Electrode Assembly having Porous Electrode Layers, Manufacturing Method thereof, and Electrochemical Cell Comprising the same".

[0010] Bessarabov discloses the introduction of a recombination catalyst into a proton exchange membrane, whereby a platinum salt is used as a precursor. Here, the chemical reduction step to produce metallic Pt particles uses hydrazine (N2H4), namely in the form of its sodium borohydride (NaBH4). Unfortunately, this leads to a very uneven distribution of the Pt recombination catalyst over the thickness of the proton exchange membrane (see Bessarabov et al., 2001). Figure 3 , see also Figure 6 Bottom). These results were also confirmed during measurements carried out in the applicant's laboratory.

[0011] In addition, the above-mentioned U.S. patent application discloses a method for preparing a porous catalyst layer for an electrochemical cell (fuel cell, electrolytic cell) from a precursor consisting of two types of metal salts, which cannot be compared with the generation of metal catalyst particles uniformly distributed over the thickness of the ion exchange membrane. One type of metal cation is reduced to metal by the reducing agent BH4- to form an electrocatalyst. The other type is not reduced, but is easily dissolved and washed out to leave pores, which improves battery performance.

[0012] However, none of the prior art documents discloses a way of achieving a uniform distribution of the metal particles of the recombination catalyst from the noble metal group over the thickness of the ion exchange membrane.

[0013] It is therefore an object of the present invention to provide a method for producing a polymer membrane for use in a polymer electrolyte water electrolyzer in order to achieve a thin film which has a high crossover suppression and does not show a negative influence on the cell performance.

[0014] According to the invention, this object is achieved by a method for preparing an ionomer for an ion exchange membrane, the ionomer containing a recombination catalyst for preventing gas crossover of species such as hydrogen and / or oxygen to the anode cell compartment and the cathode cell compartment of an electrochemical cell; the method comprising the following steps:

[0015] a) providing an ionomer as a proton or anion exchange polymer;

[0016] b) selecting a recombination catalyst from the noble metal family;

[0017] c) providing a selected recombination catalyst in ionic form contained in a liquid metal salt solution;

[0018] d) immersing an ion exchange membrane in the liquid metal salt solution, thereby exchanging at least a portion of the ion exchange sites of the ion exchange membrane with the ionic form of the recombination catalyst;

[0019] e) assembling the impregnated ion exchange membrane into an electrochemical cell; and

[0020] f) at least partially reducing the ionic form of the recombination catalyst to a metallic form by forcing hydrogen to permeate through the ionomer of the ion exchange membrane.

[0021] Therefore, the present invention proposes a new method to reduce Pt-ions by diffusing hydrogen through the PEM in PEWE cells. The proposed solution provides a way to achieve a thin film with high crossover suppression without negatively affecting the cell performance and in-situ reduction by uniform distribution on the fly.

[0022] To achieve high crossover suppression, the ionic form of the recombination catalyst is uniformly distributed over the entire cross-section of the ionomer of the ion exchange membrane.Preferably, the electrochemical cell is a polymer electrolyte water electrolysis cell.

[0023] Depending on the ionic characteristics of the ion exchange membrane, it is reasonable when the ionic form of the recombination catalyst is a cationic form or an anionic form.

[0024] Preferably, the metal selected is platinum. Alternatively, palladium or silver may be used.

[0025] Preferred embodiments of the present invention are described in more detail below with reference to the accompanying drawings, wherein:

[0026] exist Figure 1 The known principle of the PEWE battery and the electrochemical reactions occurring therein are schematically depicted;

[0027] exist Figure 2 Transmission electron microscopy (TEM) images of a cross section of a hydrogen-reduced platinum PEM (H2-Pt N212) are depicted in FIG;

[0028] exist Figure 3 The hydrogen content in oxygen in the anode compartment of PEWE cells using H2-Pt N212 and pristine membrane (N212) at different current densities is depicted in FIG.

[0029] exist Figure 4 The polarization curves of PEWE cells using N212 (black) and H2-Pt N212 (light gray) at 60 °C and ambient pressure are depicted in the figure.

[0030] exist Figure 5 The HFR of PEWE cells using N212 (black) and H2-Pt N212 (light gray) at 60 °C and ambient pressure are depicted; and

[0031] exist Figure 6 Depicted in FIG are TEM images of a proton exchange membrane in which the ionic platinum content is reduced with hydrogen (top) and in which the ionic platinum content is reduced with hydrazine according to the prior art (bottom).

[0032] The present invention proposes a feasible and effective way to introduce recombination catalyst particles uniformly over the entire cross-section of a polymer electrolyte membrane (PEM) without a second external reduction step. The PEM is immersed in a solution containing recombination catalyst precursor ions. The recombinant catalyst precursor ion-doped PEM is exposed to hydrogen from one side. The hydrogen diffused through the membrane reduces the recombination catalyst precursor ions to metal particles.

[0033] Figure 1 A cross-section of a PEWE cell and a schematic diagram of the electrochemical reactions occurring at the anode and cathode catalysts comprising the overall water splitting reaction are shown (BBP = bipolar plate, PTL = porous transport layer, RHE = reference hydrogen electrode).

[0034] Figure 2 A transmission electron microscope (TEM) image of a cross section of a hydrogen-reduced platinum PEM (H2-Pt N212) introduced in this study is shown. The Pt particles are uniformly distributed throughout the membrane and the particle size range is between 1 and 120 nm. The Pt-impregnated PEM was assembled into the electrolytic cell system described above. In detail, a transmission electron microscope image of a Pt-impregnated membrane via H2-reduction and the number of counted particles in size distribution over a 10 μm membrane cross-sectional area are shown.

[0035] Figure 3 The hydrogen content in oxygen in the anode compartment of a PEWE cell using H2-Pt N212 and a pristine membrane (N212) at different current densities is shown. In detail, the hydrogen content in oxygen at ambient cathode pressure, 5 bar, 10 bar and ambient anode pressure, and 60 ° C for pristine N212 (black) and Pt-doped N212 obtained by reduction with hydrogen (H2-Pt N212, light gray). The data show that the hydrogen fraction decreases with increasing current density due to the increased rate of the oxygen evolution reaction. An increase in pressure on the cathode side leads to a higher hydrogen permeation rate and increases the hydrogen fraction in oxygen. Compared with the original N212, for the H2-Pt N212 membrane, the hydrogen content is significantly reduced over the entire current density range at all cathode pressures.

[0036] exist Figure 4 In Figure 2, the PEWE polarization curves at ambient temperature and at 60 °C are shown. As the current density increases, the H2-Pt N212 cell shows slightly better performance, and as shown in Figure 5 The high frequency resistance (HFR) shown in the figure differs by an average of 14 mΩcm 2 This may be due to the change in clamping pressure during the cell assembly process. In addition, the recombination of hydrogen and oxygen into water on the platinum particles in the membrane may lead to an increase in the conductivity of the membrane.

[0037] Platinum ion doping of membranes

[0038] The membrane ( A = 100 cm 2 , Nafion N212, DuPont) was immersed in 1 M NaCl solution at 60° C. for 2 h. After rinsing with DI water, the membrane was transferred to a 50 mL sealable cylinder containing 1 mM (NH 3 ) 4 PtCl 2 for Pt-doping and kept at 80° C. for 24 h.

[0039] Platinum ion reduction in the membrane

[0040] The Pt-doped membrane was assembled into a PEWE cell with a gas diffusion layer (GDL) on each side. Liquid water was circulated in one compartment to wet the membrane, and 5 bar of hydrogen pressure was applied to the other compartment. The portion of the membrane exposed to hydrogen had A red = 66.2 cm 2 area.

[0041] As an alternative to the above disclosed embodiments using platinum, palladium or silver may also be used.

[0042] Figure 6 TEM image showing the distribution of platinum over the thickness of the ion exchange membrane. As already discussed above, the method according to Bessarov using hydrazine as reducing agent leads to a very inhomogeneous distribution of the Pt recombination catalyst (black spots in the TEM image) over the thickness of the proton exchange membrane (see Figure 6 Bottom). The platinum particles are disadvantageously accumulated relatively close to the surface of the ion exchange membrane rather than being evenly distributed through the thickness of the ion exchange membrane.

[0043] In contrast, Figure 6 The top shows the result after reducing the ionic form of the metal catalyst by using hydrogen (H2) according to the present invention. The metal Pt particles are evenly distributed over the thickness of the ion exchange membrane. Thus, the solution according to the present invention provides a way to achieve thin ion exchange membranes with high crossover suppression without negatively affecting the battery performance and in-situ reduction on the go by even distribution.

Claims

1. A method for preparing an ionomer for an ion exchange membrane, the ionomer for the ion exchange membrane containing a recombination catalyst to prevent gas crossover of species to an anode cell compartment and a cathode cell compartment of an electrochemical cell; the method comprising the steps of: a) providing an ionomer as a proton or anion exchange polymer; b) selecting a recombination catalyst from the noble metal family; c) providing a selected recombination catalyst in ionic form contained in a liquid metal salt solution; d) immersing an ion exchange membrane in the liquid metal salt solution, thereby exchanging at least a portion of the ion exchange sites with the ionic form of the recombination catalyst; e) assembling the immersed ion exchange membrane in an electrochemical cell; and f) at least partially reducing the ionic form of the recombination catalyst to the metallic form by forcing hydrogen to permeate through the ionomer of the ion exchange membrane.

2. The method of claim 1, wherein the ionic form of the recombination catalyst is uniformly distributed throughout the cross-section of the ionomer of the ion exchange membrane.

3. The method of claim 1 or 2, wherein the electrochemical cell is a polymer electrolyte water electrolysis cell.

4. The method according to claim 1 or 2, wherein the ionic form of the recombination catalyst is a cationic form or an anionic form.

5. The method of claim 1 or 2, wherein the selected metal is platinum.

6. The method of claim 1, wherein the species is hydrogen and / or oxygen.

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