METHOD FOR PREPARING A POLYMER MEMBRANE FOR A POLYMER ELECTROLYTE WATER ELECTROLYSER

Abstract

A method of preparing an ionomer of an ion exchange membrane with a recombination catalyst to prevent gas crossover of species, such as hydrogen and/or oxygen, to anodic and cathodic cell compartments of an electrochemical cell. An ionomer of an ion exchange membrane is prepared with a recombination catalyst. The ionomer is a proton or anion exchange polymer and the recombination catalyst, selected from the precious metals group, is provided in ionic form in a liquid metal salt solution. The ion exchange membrane is immersed into the liquid metal salt solution to exchange ionic ionomer ports with the ionic form of the recombination catalyst. The membrane is then assembled in the electrochemical cell and the ionic form of the recombination catalyst is at least partly reduced to metallic form by forcing hydrogen to permeate through the ionomer of the ion exchange membrane.

Claims

1-5. (canceled)

6. A method of preparing an ionomer of an ion exchange membrane with a recombination catalyst to prevent gas crossover of species to anodic and cathodic cell compartments of an electrochemical cell, the method comprising the steps of: a) providing the ionomer of the ion exchange membrane as a proton or an anion exchange polymer; b) selecting the recombination catalyst from the precious metals group; c) providing the recombination catalyst selected in step b) in an ionic form in a liquid metal salt solution; d) immersing the ion exchange membrane into the liquid metal salt solution to thereby exchange at least a part of the ion exchange sites the ionic form of the recombination catalyst; e) assembling the immersed ion exchange membrane in the electrochemical cell; and f) at least partially reducing the ionic form of the recombination catalyst into a metallic form by forcing hydrogen to permeate through the ionomer of the ion exchange membrane.

7. The method according to claim 6, which comprises forming the ion exchange membrane to prevent a gas crossover of hydrogen and/or oxygen.

8. The method according to claim 6, which comprises distributing the ionic form of the recombination catalyst homogenously over an entire cross section of the ionomer of the ion exchange membrane.

9. The method according to claim 6, wherein the electrochemical cell is a polymer electrolyte water electrolyzer.

10. The method according to claim 6, which comprises providing the ionic form of the recombination catalyst either in a cationic form or in an anionic form.

11. The method according to claim 6, wherein step b) comprises selecting platinum as the recombination catalyst.

Description

[0019] Preferred embodiments of the present invention are hereinafter described in more detail with reference to the attached drawings which depict in:

[0020] FIG. 1 schematically the known principle of a PEWE cell and the electrochemical reactions taking place therein;

[0021] FIG. 2 a transmission electron microscopy (TEM) image of the cross-section of a hydrogen reduced platinum PEM (H2-Pt N212);

[0022] FIG. 3 the hydrogen content in oxygen in the anode compartment of a PEWE cell using a H2-Pt N212 and a pristine membrane (N212) at different current densities;

[0023] FIG. 4 polarization curves for PEWE cells using N212 (black), and H.sub.2-Pt N212 (light grey), respectively, at 60° C. and ambient pressure;

[0024] FIG. 5 HFR for PEWE cells using N212 (black), and H.sub.2-Pt N212 (light grey), respectively, at 60° C. and ambient pressure; and

[0025] FIG. 6 TEM image of proton exchange membrane wherein the ionic platinum content has been reduced with hydrogen (top) and according to the prior art with hydrazine (bottom).

[0026] The present invention presents a viable and efficient way of introducing recombination catalyst particles homogenously over the whole cross-section of a polymer electrolyte membrane (PEM) without a second external reduction step. PEMs are immersed into a recombination catalyst precursor ion containing solution. The recombination catalyst precursor ion-doped PEM is exposed to hydrogen gas from one side. The hydrogen diffusing through the membrane reduces the recombination catalyst precursor ions to metallic particles.

[0027] FIG. 1 shows a schematic representation of the cross section of a PEWE cell and the electrochemical reactions taking place at the anode and cathode catalyst, including the overall water splitting reaction (BBP=Bipolar plate, PTL=Porous transport layer, RHE=Reference hydrogen electrode).

[0028] FIG. 2 shows a transmission electron microscopy (TEM) image of the cross-section of a hydrogen reduced platinum PEM (H.sub.2-Pt N212) introduced in the present study. The Pt particles are homogenously distributed over the whole membrane and the particles diameter ranges between 1 and 120 nm. Pt impregnated PEMs were assembled into an above described electrolyzer system. In detail, the transmission electron microscopy image of the Pt-impregnated membrane via H.sub.2-reduction and the number of counted particles with size distribution over a 10 μm membrane cross sectional area are shown.

[0029] FIG. 3 shows the hydrogen content in oxygen in the anode compartment of a PEWE cell using a H.sub.2-Pt N212 and a pristine membrane (N212) at different current densities. In detail, the content of hydrogen in oxygen at ambient cathodic pressure, 5 bar, 10 bar, and ambient anodic pressures respectively, and 60° C. for pristine N212 (black), and Pt doped N212 obtained by reduction with hydrogen (H.sub.2-Pt N212, light grey). The data shows a decreasing hydrogen fraction with increasing current density as the rate of the oxygen evolution reaction is increasing. Pressure increase on the cathode side leads to a higher hydrogen permeation rate and increases the fraction of hydrogen in oxygen. In comparison to pristine N212, the hydrogen content was significantly reduced for the H.sub.2-Pt N212 membranes over the whole current density range at all cathode pressures.

[0030] In FIG. 4, PEWE polarization curves are shown at ambient temperature and at 60° C. The H.sub.2-Pt N212 cell shows slightly better performance with increasing current density and the high frequency resistance (HFR) as shown in FIG. 5 differs on average by a value of 14 mΩ cm.sup.2. This could be due to variations in clamping pressure during cell assembly. Furthermore, recombination of hydrogen and oxygen to water on the platinum particles in the membrane could cause a conductivity increase of the membrane.

Platinum-Ion Doping of the Membranes

[0031] The membranes (A=100 cm.sup.2, Nafion N212, DuPont) were immersed in a 1 M NaCl solution for 2 h at 60° C. After rinsing with DI water, the membranes were transferred into a 50 mL sealable cylinder containing 1 mM (NH3)4PtCl2 for Pt-doping and were maintained therein for 24 h at 80° C.

Platinum-Ion Reduction in Membranes

[0032] The Pt-doped membranes were assembled into a PEWE cell with a gas diffusion layer (GDL) on each side. Liquid water was circulated in one compartment to humidify the membrane and a hydrogen pressure of 5 bar was applied to the other compartment. The part of the membrane which was exposed to the hydrogen had an area of A.sub.red=66.2 cm.sup.2.

[0033] Alternatively to the examples disclosed above with platinum, palladium or silver can be used as well.

[0034] FIG. 6 shows TEM images of the distribution of platinum across the thickness of a ion exchange membrane. As discussed already above, the method according to Bessarov using hydrazine as reducing agent leads to the formation of a very inhomogeneous distribution of Pt recombination catalyst (black dots in the TEM images) across the thickness of the proton exchange membrane (see FIG. 6 bottom). Platinum particles accumulate disadvantageously rather close to the surface of the ion exchange membrane than homogeneously distributed across the thickness of the ion exchange membrane.

[0035] Different from that, FIG. 6, top, shows the result after the reduction of the ionic form of the metallic catalyst by the use of hydrogen (H.sub.2) according to the present invention. The metallic Pt particles are homogeneously distributed across the thickness of the ion exchange membrane. Therefore, the solution according to the present invention provides a way to achieve thin ion exchange membranes with high crossover suppression, without a negative impact on the cell performance and a reduction in-situ on the flight by the homogenous distribution.