The present invention relates to a method for preparing a polymer membrane for a polymer electrolyte water electrolyser.
The polymer electrolyte water electrolyser (PEWE) is an electrochemical device where the electrodes are separated by a solid proton conducting polymer membrane. On the anode side water is split into oxygen and protons. Protons are transported to the cathode under the influence of the electric field, where molecular hydrogen is evolved. A key concern for the safe operation of PEWE cells is the high hydrogen gas crossover, which can lead to explosive hydrogen-oxygen gas mixtures. The safety aspect is especially important with thin membranes and high differential pressures.
Water electrolysis has a potential to be a key technology in the energy transition from the traditional fossil-based to the renewable, carbon-free systems. Power supply based on a significant share of these “new renewables” is associated with large discrepancies between supply and demand, owing to the intermittent nature of these primary energy sources. Water electrolysis is a clean and efficient process which offers strong prospects to store large amounts of excess electricity in form of chemical energy (‘power-to-gas’). It is also a key component in coupling different sectors (electricity, mobility, heating) through the power-to-x concept. A state of the art PEWE cell as shown in
The production rate of gases in the PEWE cell is directly proportional to the current density applied, according to Faraday's laws. Increasing the current density while maintaining conversion efficiency by using thinner membranes is an approach for increasing the rate of H2 production per unit cell area of the PEWE. Thinner membranes allow for higher current density at a given cell voltage, and thereby effectively decrease the investment costs for the electrolyzer stack. Current densities up to 19 A/cm2 have been reported using a 50 μm thick membrane, with a cell potential of 3 V. The associated power density is on the order of 50 W/cm2.
Pressurized PEWE could make subsequent compression of gases redundant or reduce the effort for drying with mechanical compression. Especially differential pressurized PEWE is attractive for high purity hydrogen generation and higher faradaic efficiency compared to balanced pressure operation. The diffusion rate of hydrogen to the anode side strongly depends on the thickness of the membrane and the partial pressure applied. Since the diffusion coefficient of hydrogen in the membrane is about twice as high as the diffusion coefficient of oxygen, and the majority of oxygen diffusing to the cathode side recombines 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 thin membranes. The lower explosion limit of hydrogen in oxygen is at 4%.
Gas crossover can be suppressed by impregnating platinum particles into the membrane. Permeated hydrogen and oxygen recombine on the surface of the platinum particles to water. So far, there are two methods to introduce the recombination catalysts in the PEM:
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 the US Patent application US 2008/0044720, “Membrane Electrode Assembly having Porous Electrode Layers, Manufacturing Method thereof, and Electrochemical Cell Comprising the same”.
Bessarabov discloses the introduction of a recombination catalyst in the proton exchange membrane thereby using platinum salt as a precursor. Here, the chemical reduction step to produce metallic Pt particles uses hydrazine (N2H4), i.e. in its form of sodium borohydride (NaBH4). Unfortunately, this leads to the formation of a very inhomogeneous distribution of Pt recombination catalyst across the thickness of the proton exchange membrane (cf. FIG. 3 in Bessarabov, also
Further, the US patent application mentioned above discloses a method to produce a porous catalyst layer for electrochemical cells (fuel cell, electrolyzer) from a precursor consisting of a two types of metal salt which cannot be compared to the generation of a homogenous distribution of metallic catalyst particles across the thickness of the ion exchange membrane. One type of metal cation is reduced by the reducing agent BH4- to the metal, forming the electrocatalyst. The other type is not reduced but is easily dissolved and washed out to leave behind porosity, which improves cell performance.
However, none of the prior art documents disclose a way to achieve a homogenous distribution of metallic particles of a recombination catalyst from the precious metals group across the thickness of the ion exchange membrane.
It is therefore the objective of the present invention to provide a method for preparing a polymer membrane for a polymer electrolyte water electrolyser in order to achieve thin membranes with high crossover suppression and without showing a negative impact on the cell performance.
This objective is achieved according to the present invention by a method to prepare 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; said method comprising the steps of:
a) providing the ionomer as a proton or an anion exchange polymer;
b) selecting the recombination catalyst from the precious metals group;
c) providing the selected recombination catalyst in an ionic form being comprised in a liquid metal salt solution;
d) immersing the ion exchange membrane into the liquid metal salt solution thereby exchanging at least a part of the ion exchange sites of the ion exchange membrane with 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 the metallic form by forcing hydrogen to permeate through the ionomer of the ion exchange membrane.
Therefore, the present invention presents a new reduction method for Pt-Ions by diffusing hydrogen through the PEM in a PEWE cell. The presented solution provides a way to achieve thin 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.
In order to achieve the high crossover suppression, the ionic form of the recombination catalyst is distributed homogenously over the whole cross section of the ionomer of the ion exchange membrane. Preferably, the electrochemical cell is a polymer electrolyte water electrolyzer.
Depending on the ionic characteristic of the ion exchange membrane, it is reasonable when the ionic form of the recombination catalyst is either a cationic form or an anionic form.
Preferably, the selected metal is platinum. Alternatively, palladium or silver can be used.
Preferred embodiments of the present invention are hereinafter described in more detail with reference to the attached drawings which depict in:
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.
In
The membranes (A=100 cm2, 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.
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 Ared=66.2 cm2.
Alternatively to the examples disclosed above with platinum, palladium or silver can be used as well.
Different from that,
Number | Date | Country | Kind |
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19175071.0 | May 2019 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/063331 | 5/13/2020 | WO |