WO2016078630A1

WO2016078630A1 - Electrochemical cell and method for production thereof

Info

Publication number: WO2016078630A1
Application number: PCT/DE2015/000500
Authority: WO
Inventors: Markus Stähler
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2014-11-21
Filing date: 2015-10-09
Publication date: 2016-05-26

Abstract

The invention relates to an electrochemical cell comprising at least one acid polymer membrane, as well as comprising at least one electrode and a current collector arranged adjacent thereto. According to the invention, between the electrode and the current collector, the surface of the electrode and/or of the current collector has at least one region which comprises at least one alkaline polymer. With the addition of water, the alkaline polymer increases the local pH value between the electrode and current collector, and reduces or prevents ideally the corrosion that would otherwise normally occur. In the method for producing an electrochemical cell comprising at least one acid polymer membrane, as well as at least one electrode and a current collector arranged adjacent thereto, according to the invention, a solution or dispersion comprising an alkaline polymer is applied on the surface of the current collector and/or the adjacent electrode, which is then dried and fixed such that at least one surface has regions comprising at least one alkaline polymer.

Description

Electrochemical cell and method of making the same

The invention relates to an electrochemical cell, in particular an electrochemical cell having a proton exchange membrane (PEM), in particular an acidic polymer electrolyte membrane. The invention further relates to a method for producing such an electrochemical cell, in particular a PEM cell.

State of the art

An electrochemical cell, such as a PEM fuel cell, typically includes a membrane electrode assembly (MEA) formed by a catalyst-coated PEM, also called CCM (Catalyst Coated Membrane), between two gas diffusion layers. The catalyst layers are also referred to as electrodes. The CCM is disposed between a pair of gas diffusion layers made of electrically conductive porous substrates such as woven and nonwoven carbon fiber structures. They have the task of distributing the substances needed for the operation of the cell homogeneously over the electrode and allowing the resulting substance to diffuse away. At the same time, since the electrons released or consumed in the electrodes have to be transported to or removed from the diffusion layer, the diffusion layers are also referred to as current collectors. Depending on the design of the electrochemical cell, the potentials of the electrodes and current collectors may be well below 1.3V (with respect to a normal hydrogen electrode (NHE), fuel cell design) or well above 1.3V (with respect to a NHE, electrolysis cell design ) lie.

In electrochemical cells comprising an acidic polymer membrane, the electrodes also typically contain a certain amount of an acidic polymer, thereby adversely lowering the pH of the operation of the electrochemical cell close to the electrode surface, at the boundary to the current collector, and causing acid damage Corrosion can occur. An important aspect of this corrosion is that the acidic polymer is present in the electrode as a solid. The acidic polymer consists of a backbone with chemically bonded negative end groups and electrostatically bonded positively charged ions, the protons. If you choose to operate a liquid whose molecules contain either a permanent electric dipole moment, such as water, or a induce such dipole moment in the molecule, the solvent molecules upon contact with the polymer attach themselves to the ionic groups and weaken the electrical interactions between the negative end groups attached to the polymer backbone and the protons. Due to the weakening, the protons can detach themselves somewhat from the negative ions. The further the proton moves away from the negative group, the greater the electrostatic energy, the increase of which then prevents further removal. The proton thus becomes slightly mobile and forms a shell around the polymer particle, in which many of the protons then remain (see FIG. 1). As a result of the higher proton concentration compared to the liquid, the pH in the casing is reduced, while outside the particle the pH value in the liquid can be markedly higher.

Considering now the electrode surface containing the acidic polymer and the contacted current collector, these are two rough surfaces which are contacted at the tips upon compression (see Figure 2). After contacting, there are direct connections between the acidic proton shells of the polymer particles in the electrode and the current collector, at which corrosion may preferentially occur.

Generally, it is observed that electrochemical cell components containing acidic polymer electrolyte membranes regularly corrode at the sites where they are in direct contact with the acidic polymers. The higher the potential and the lower the pH, the stronger these corrosion processes occur at the phase boundary between the electrode and the current collector. In order to reduce these corrosion processes, two different methods are proposed in the prior art, both of which produce a continuous, electrically conductive protective layer on the current collector.

The first method is to coat the current collector with precious metals such as gold or platinum, which do not corrode themselves and thus protect the component from corrosion.

In the second method are used for the construction of the current collectors materials such as titanium, which either themselves cover with a protective oxide layer (passivation) or on which another conductive layer, such as a titanium nitride layer, is generated to the material of the Protect current collector from corrosion. The two aforementioned methods disadvantageously use expensive metals, such as platinum, gold or titanium, which thereby cause a large part of the cost of producing electrochemical cells. With the passivation method, there is also the risk that the passivation layer becomes increasingly thicker during operation of the electrochemical cell, as a result the cell resistance increases as a result and the efficiency of the cell decreases.

Task and solution

The object of the invention is to provide an electrochemical cell with an acidic polymer membrane, in which the hitherto customary corrosion between a current collector and an electrode which also comprises this polymer is reduced or even completely prevented.

Another object of the invention is to provide a method for producing such an electrochemical cell.

The objects of the invention are achieved by an electrochemical cell according to the main claim, as well as by a manufacturing method according to the independent claim. Advantageous embodiments of the electrochemical cell and / or the manufacturing process can be found in the respective dependent claims again.

Subject of the invention

The essential feature of the invention is the treatment of components of an electrochemical cell with a solution or a dispersion comprising at least one alkaline polymer. After drying and fixing of the alkaline polymer, at least individual regions thus arise on the surface of the component, which comprise at least one alkaline polymer and which increase the pH at the component surface upon contact with a liquid, in particular with water. In this way, the component is advantageously protected against corrosion, without the necessary material and electron transport is hindered.

The invention describes a method with which in particular increases the local pH at the contact points between a current collector and an electrode in an electrochemical cell with a polymer-electrolyte membrane by the deposition and fixation of at least one alkaline polymer and so the corrosion is reduced or prevented. For the purposes of the invention, an alkaline polymer is understood as meaning a polymer of a skeleton on whose side chains electrically positively charged, cationic groups are attached. The electrically negatively charged hydroxide counterions are responsible for the alkaline effect. Examples of suitable backbones are those based on polysulfone, crosslinked polystyrene (copolymer of styrene with divinylbenzene), cellulose, agarose, dextran, polyphenylene oxide or the like.

For the cationic groups attached via side chains, alkylammonium compounds such as aminoethyl-, diethylaminoethyl- (DEAE-), quaternary aminoethyl- (QAE-), 1,4-diazabicyclo [2.2.2] octane, imidazole groups, pyridium groups can be used become. In addition to the alkylammonium groups, it is also possible, for example, to use positively charged phosphonium groups or organometallic cationic groups. A small selection of other possible cationic groups suitable for the purposes of the invention can be found in J.R. Varcoe, P. Atanassov, D. Dekel, A. Herring, M. Hickner, P.A. Kohl, A. Kucernak, W. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy Environ. See, 2014, DOI: 10.1039 / C4EE01303D, which are to be regarded as co-disclosed.

In the context of the invention, both individual alkaline polymers and mixtures of alkaline polymers can be used for the treatment of components of an electrochemical cell.

Such alkaline polymers which are suitable for the purposes of the invention are frequently also used as anion exchangers, for example under the trade name LEWATIT® MonoPlus M 500 from Lanxess. Furthermore, diethylaminoethylcellulose (DEAE-cellulose), which is frequently used in ion chromatography, would also be considered suitable. However, the cellulose variants are probably not so stable in a fuel cell or electrolysis cell. Commercially available are also alkaline ionomer dispersions, for example the ionomer dispersion 12 from Acta or AS-4 from Tokuyama, in which the composition is unclear.

The alkaline polymer applied and fixed to the component of an electrochemical cell shows a similar behavior when contacted with a liquid whose molecules have a permanent electric dipole moment or at which electric dipole moments can be induced in comparison to the acidic polymer.

The polymer backbone of the alkaline polymer used according to the invention contains cationic groups bonded via side chains, for example quaternary ammonium groups. pen, and negatively charged ions, for example, hydroxide ions, which build up on contact with the liquid, a shell of hydroxide ions to the positively charged polymer backbone (see Figure 3). As a result of the deposition of the alkaline polymer on the surface of the current collector, there arise regions which advantageously reduce the acidic effect of the electrode on contact and increase the pH, as shown in FIG.

According to the invention, at least one alkaline polymer is arranged between the affected components, in particular between an electrode and a current collector arranged adjacent thereto. Advantageously, either the current collector or the electrode can be mixed with this alkaline polymer. Also, treatment of both the current collector and the electrode may be considered. It is important that the arrangement of the alkaline polymer on the surface affected as little as possible for the cell transport gas transport and the necessary electrical contact between the electrode and the current collector can be made by pressing. In the case of the arrangement of the at least one alkaline polymer on the surface, it is customary not to speak of a layer in the strict sense, and to that extent also not of a porous layer. Rather, only very little material is deposited on the surface by the method according to the invention, so that the alkaline polymer is usually arranged there in an incoherent manner only in the form of individual regions.

In a further embodiment of the invention, the treatment of a component surface can be carried out with a dispersion consisting of at least one alkaline polymer and an electrically conductive solid, for example carbon black. In such a case as thin as possible layers can be applied in the range of 0.5 to 20 pm.

The electrical conductivity of such a thin layer comprising the alkaline polymer can be ensured inter alia by adding at least one electrically conductive substance to the alkaline polymer. As a suitable electrically conductive additive, for example, carbon black, graphite or titanium powder can be used. After the treatment of a component of an electrochemical cell with a solution or a dispersion comprising at least one alkaline polymer, and after subsequent drying and fixing of the polymer, the area arranged on the surface may contain the electrically conductive substance in an amount of up to 90% by weight. % exhibit.

For example, if carbon black is added to the coating as an electrically conductive substance, the same carbon black as could be used in the electrodes could be used. This usually has a particle size of around 50 nm. It is also conceivable, however, that significantly larger particles can be used, for example graphite having a particle size of up to 50 or 100 μm. The larger the particles are selected, the larger the pores are formed and the lesser the transport resistance.

The at least one alkaline polymer is applied with or without an electrically conductive additive according to the invention in the form of a solution or a dispersion on one of the component or both components of an electrochemical cell. In particular, spraying, doctoring, slot casting or dip coating may be mentioned as application methods suitable for this purpose.

In one embodiment of the process according to the invention, for example, a mixture of about 70% by weight of carbon black and about 30% by weight of an alkaline polymer in a suitable solvent is first mixed and then finely dispersed. For dispersion, an ultrasonic disperser or a high-speed disperser could be used. A ball mill would be conceivable. Depending on the coating method used, the solids content of the dispersion can be adjusted to the required viscosity. A lot of solid means a high viscosity, little solid means a low viscosity. For example, a typical doctoring process requires a viscosity of about 200 to 500 mPas, a spraying process rather a viscosity in the range of 1 to 100 mPas.

After the solution or dispersion has been applied to the component, the thin layer or discontinuously present regions are dried and fixed so that they can no longer be detached from the component. This process step can be carried out, for example, by a sintering process.

In such a sintering process, the melting point, or the glass transition temperature of the respective alkaline polymer should be achieved, that is typically carried out in the range between 50 ° C and 200 ° C (Nafion® with acidic group, for example, has a glass transition point of approx. 130 ° C). The glass transition temperature is the temperature in which completely or partially amorphous polymers change from the liquid or rubber-elastic, flexible state to the glassy or hard-elastic, brittle state. Since the concrete polymer composition of commercial ionomer dispersions is often unknown, the glass transition temperature for the sintering process may have to be determined beforehand in each case, for example by means of DSC (differential scanning calorimetry).

It is important in the case of dispersion or solution preparation that the alkaline polymer, if it has been mixed with an electrically conductive substance, does not form a continuous film in the layer during drying, but is present as a mixture with the electrically conductive particles, thus ensuring mass transfer and the electrical conductivity of the layer can be ensured. The exact method of preparation of the dispersion and the drying process can be developed individually depending on the polymer used and the conductive particles.

Special description

A first possible embodiment of the invention provides for coating the current collector of an electrochemical cell with a dispersion of an alkaline polymer and an electrically conductive substance, for example electrically conductive carbon black, graphite or titanium powder. Suitable basic polymers are alkaline polymers consisting of a backbone, for example based on a styrene-divinylbenzene copolymer, which contains side chains of quaternary ammonium compounds which are responsible for the alkaline action. Such polymers are also used as anion exchangers, for example under the trade name LEWATIT® MonoPlus M 500 from Lanxess. Furthermore, it is also possible to use commercially available alkaline ionomer dispersions in which the exact composition is not known, such as, for example, the ionomer 12 from Acta or AS-4 from Tokuyama. An alternative embodiment is, for example, spraying the current collector of an electrochemical cell with an alkaline polymer solution, then drying the solution and then sintering the polymer.

Theoretically, treatment of the electrode may be used instead of a current collector. However, when coating the electrode, it must be remembered that the alkaline polymer does not neutralize too much of the acidic polymer needed in the electrode for proton transport. Otherwise, the electrode could be detrimental to performance to lose. The alkaline polymer solution should therefore not penetrate into the electrode. However, it would be quite conceivable to coat the electrode with a highly viscous dispersion of carbon black and alkaline polymer, wherein the yield strength of the dispersion could prevent penetration into the electrode. In this case, you could also treat the electrode, and a treatment of the current collector would be no longer necessary because with the treatment of the electrode, the pH would already be set to neutral.

1st embodiment:

10 g of the commercially available ionomer dispersion AS-4 from Tokuyama are diluted with 30 g of 1-propanol and then sprayed with a spray device directly onto a carbon substrate, which has the function of a current collector of an electrochemical cell. After the spraying process, the carbon substrate is dried until the liquid content in the layer is less than 3% by weight. After drying, the weight per unit area of the polymer applied is determined by weighing.

These process steps, ie the spraying of a polymer solution onto the component and subsequent drying and weighing can be repeated until a surface weight of 0.1 mg / cm ^{2 is} reached. Depending on the objective, the basis weight may be between 0.01 and 1 mg / cm ² .

After reaching the desired alkaline polymer occupancy, the treated carbon substrate is heated for 15 minutes to temperatures above the corresponding glass transition temperature of the polymer. The glass transition temperature must first be determined experimentally, for example by means of DSC.

2nd embodiment

A mixture of 1 g of carbon black (Vulcan® XC72 from Cabot) and 7.5 g of commercial ionomer dispersion AS-4 from Tokuyama are first stirred in a reactor with a mixer and cooled to 5 ° C. Subsequently, the mixture is dispersed for 2 minutes with an ultrasonic finger (diameter 6 mm) at an amplitude of 30% in the pulse method (0.5 s on / 0.5 s off). The dispersion is subsequently stirred under reduced pressure until the 1-propanol present in the ionomer dispersion has evaporated to such an extent that the dispersion has a solids content of about 25% by weight and a viscosity of between 200 and 1000 mPas. The dispersion is then knife-coated onto a carbon nonwoven with a gap of 100 μm at a feed rate of 0.3 m / min. Subsequently, the layer is dried at 80 ° C until the residual moisture of the layer is less than 3 wt .-%. For the subsequent sintering process, the coated nonwoven is heated to the glass transition temperature.

The alkaline polymer, upon ingestion of water, regularly increases the pH directly between the electrode and the current collector, thereby reducing acid-induced corrosion on the current collector. By raising the pH to neutral levels (around pH 7), less noble and thus less expensive materials can be used to construct the current collectors, such as graphite.

In fuel cell operation, the water is produced in the cathode, where, according to the invention, the corrosion by the alkaline polymer should also be reduced. In the case of the electrolysis operation, the electrodes are supplied with water. The water passes through the diffusion layers to the electrode, where it is decomposed electrochemically. Optionally, the reaction with water can already take place during assembly, if moist components are used. Then the beneficial alkaline effect could already be noticeable.

FIGS. 1 to 4 show schematically the structure of the alkaline polymer or of the alkaline polymer mixture used according to the invention and the effects on the electrode / current collector interface.

In Figure 1, the presence of an electrical double layer of an acidic polymer as found in your acidic polymer membrane is illustrated. The nucleus is a negatively charged backbone, which is surrounded by positively charged side chains (Figure 1, top). When a liquid, in particular water, is added, the respectively opposite charges are forced apart so that an ion shell is built up (FIG. 1, bottom).

FIG. 2 shows schematically how acidic polymers, which can be found not only in the acidic polymer membrane but also in the adjacent electrodes, behave with corresponding ion sheaths on the surface of the electrode in contact with a current collector. The positively charged ion sheaths come into direct contact with the current collector and cause there the adverse corrosion phenomena.

FIG. 3 shows, analogously to FIG. 1, the electrical double layer for an alkaline polymer used according to the invention, as it results from the positively charged skeleton and the negatively charged quaternary side chains (FIG. 3, top). When adding a liquid, especially water, the respective opposite charges pushed apart, so that here also a corresponding ion shell builds (Figure 3 below).

FIG. 4 shows a situation similar to that in FIG. 2, with the difference that according to the invention the surface of the current collector has at least one region with at least one alkaline polymer. Ideally, the surface-mounted alkaline polymer with its ionic envelope also prevents direct contact of the ion shell of the acidic polymer with the current collector. As a result, the pH is increased locally and thus possible corrosion phenomena avoided or completely prevented.

Claims

P a n t a n s p r e c h e

Electrochemical cell having at least one acidic polymer membrane, as well as having at least one electrode and a current collector arranged adjacently thereto, characterized

in that, between the electrode and the current collector, the surface of the electrode and / or of the current collector has at least one region comprising at least one alkaline polymer or an alkaline polymer mixture.

An electrochemical cell according to claim 1, wherein said region comprises at least one alkaline polymer having a polysulfone, cross-linked polystyrene, cellulose, agarose, dextran or polyphenylene oxide backbone.

An electrochemical cell according to any one of claims 1 or 2, wherein said region comprises at least one side-chain alkaline polymer having quaternary ammonium compounds, phosphonium groups or even organometallic cationic groups.

An electrochemical cell according to any one of claims 1 to 3, wherein said region additionally comprises at least one electrically conductive substance.

Electrochemical cell according to the preceding claim 4, wherein the region comprises carbon black, graphite and / or titanium powder as the electrically conductive substance.

An electrochemical cell according to any one of claims 4 to 5, wherein the region comprises the electroconductive substance in an amount of up to 90% by weight.

Electrochemical cell according to one of claims 4 to 6, wherein the additional electrically conductive material has a maximum average particle size of 100 μηι, advantageously of a maximum of 50 μητι, and particularly advantageously has a maximum of 50 nm.

Electrochemical cell according to one of claims 1 to 7, wherein the region on a surface forms a continuous layer with a layer thickness between 0.5 and 20 μητι.

9. A method for producing an electrochemical cell having at least one acidic polymer membrane, and at least one electrode and a current collector arranged adjacent thereto,

characterized,

- That on the surface of the current collector and / or the adjacently arranged electrode, a solution or dispersion comprising at least one alkaline polymer is applied, and

- That then the solution or dispersion is dried and fixed, so that at least one surface has at least one region comprising at least one Al kaiisches polymer or an alkaline polymer mixture.

10. The method of claim 9, wherein a solution or dispersion is used which comprises at least one alkaline polymer having a skeleton based on polysulfone, crosslinked polystyrene, cellulose, agarose, dextran or from poly- phenylene oxide. 11. The method according to any one of claims 9 to 10, wherein a solution or dispersion is used which comprises at least one alkaline polymer having side chains, the quaternary ammonium compounds, phosphonium groups or organometallic cationic groups.

12. The method according to any one of claims 9 to 11, wherein the application of the solution or the dispersion by means of spraying, knife coating, slot casting or by means of a dip coating takes place.

13. The method according to any one of claims 9 to 12, wherein for fixing the applied solution or dispersion, this is sintered together with the electrode and / or the current collector. 14. The method according to any one of claims 9 to 13, wherein the solution or dispersion additionally comprises at least one electrically conductive substance.

15. The method according to the preceding claim 14, wherein the solution or dispersion comprises carbon black, graphite and / or titanium powder as an additional electrically conductive substance.