WO2009050378A1

WO2009050378A1 - Method for texturing the electrolyte of a fuel cell

Info

Publication number: WO2009050378A1
Application number: PCT/FR2008/051716
Authority: WO
Inventors: Antoine Latour; Christophe Serbutoviez
Original assignee: Commissariat A L'energie Atomique
Priority date: 2007-10-09
Filing date: 2008-09-26
Publication date: 2009-04-23
Also published as: FR2922047A1

Abstract

The invention relates to a method for texturing the electrolyte (3) of a fuel cell that comprises depositing drops of an electrolyte solvent on the surface thereof using an inkjet technique.

Description

PROCESS FOR TEXTURING THE ELECTROLYTE OF A FUEL CELL

FIELD OF THE INVENTION

The present invention relates to the field of polymer membrane fuel cells.

There is provided a method for producing a three-dimensional electrolytic membrane in an easy manner. This configuration makes it possible to increase the surface of the interface between the electrolyte and the electrodes, and thus to increase the current delivered by the battery. This leads to an increase in the power of the fuel cell.

PRIOR STATE OF THE ART

Figure 1 illustrates the structure of a core of a proton exchange membrane fuel cell, as conventionally described in the prior art. The electrodes (anode and cathode) 2 are separated by an electronic insulating medium but protonic conductor, called electrolyte 3. Current collectors 1 transfer the electrons to the outer surface of the electrodes.

In the case of proton exchange membrane fuel cells, the electrolyte is generally a membrane consisting of a cationic exchange polymer, such as Nafion® (Dupont) or Hyflon® ^R (Solvay). operating in an alkaline medium, there are also anion exchange polymer membranes, such as the Solvay Morgane membrane.

The electrodes are the locus of the electrochemical reactions of oxidation of the fuel (anode) and reduction of the oxidant, in this case oxygen (cathode). These electrochemical reactions are kinetically favored by the presence of a catalyst constituting the electrodes. Several materials can be used depending on the type of reaction and fuel, but platinum proves to be the most effective catalyst for most reactions and fuels. Fuel cells are intended to be integrated in electric generators, the performance of the battery are evaluated in terms of power delivered by the battery core (watt). The performance of the battery core is, in turn, brought back to the electrode surface: it is called power density (watt per unit area). Power is defined as the product of the voltage across the battery (electromotive force) by the current (amperes) delivered by the battery. Thus, the performance of the stack is even higher than one, the other or both terms are high.

The electrochemical reactions occurring at the electrodes are operated with gaseous species (oxygen or hydrogen), liquid species (water, anodic fuel solution), electrons e ^"and protons H ^+. These reactions are illustrated below in the where the fuel is hydrogen and the oxidant oxygen:

Anode: H ₂ → 2H ⁺ + 2e ^"

= 0 V _{/ E} NH Cathode: O ₂ + 4H ⁺ + 4e ^"→ 2H ₂ EO _cathode ° = 1, 23 V _{/ W} NH

The overall reaction taking place at the terminals of the stack is therefore as follows:

H ₂ + Y ₂ O ₂ → H ₂ OE ° _eq = E ° _cathode -

= 1, 23 V

The electromotive force at the terminals of the battery is therefore 1.23V under standard conditions.

However, this voltage corresponds to a state of equilibrium, that is to say that no current flows through the cell. As soon as the system delivers a current, it is necessary to consider the kinetic aspect.

The kinetics of the anodic and cathodic reactions generate overvoltages which lead to a decrease in the potential of the cell. In addition, these overvoltages are a function of the current density delivered. Thus, when the battery delivers current, the potential across the electrodes is lower than the standard potential of 1.23 V.

Figure 2 clearly shows that it is difficult to work over a wide range of potential and that an increase in potential implies low current densities.

Based on this observation and for a given cell core (nature and quantity of fixed catalysts), one of the solutions to increase the power of the cell is to increase the surface S of these electrodes. Indeed, we have the following relations: P = U * I and I = i * S so P = U * i * S where P: power U: stack potential

I: current i: current density S: electrode surface.

It appears that the current is all the more important that the electrode surface is important.

However, it is very difficult to use large areas of flat electrodes. Indeed, the catalysts are expensive. In addition, there are often problems of "packaging" of the electrodes, which must fit on as small a surface as possible. This is particularly critical in the case of portable applications, for example for phones, mp3 players, or cameras.

It is therefore interesting to increase the surface area of the electrodes, without increasing the projected area of the electrode.

Figure 3 shows a schematic sectional view of a conventional stack core. In the case of fuel micro-cells, the membrane 3 has a thickness of approximately 40 μm, the electrodes 2 approximately 10 μm and the collectors 1 have a thickness of less than 1 μm.

In this conventional configuration, it therefore appears that the total surface of the electrode is equal to the projected surface or geometric surface of the electrode.

The technical problem to be solved by the present invention is therefore to increase the developed surface of the electrode, without increasing the surface projected on the electrode, and this in order to increase the current densities of the core. stack.

In other words, the present invention proposes to increase the ratio surface area developed on projected surface (also called form factor) to increase the performance of a fuel cell, while maintaining a small footprint. Several avenues have been explored to achieve this goal. In particular, Taylor et al. {"Nanoimprinted electrodes for micro-fuel cell applications", 2007, Journal of Power Sources 171 (1): 218-223) have proposed to increase the developed surface of fuel cell electrodes, thanks to the 3D pattern inlay in the electrolyte membrane or in the electrodes.

In practice, the incrustation process is carried out under pressure, using molds, and makes it possible to give the electrodes or the electrolytic membrane 3D texturing.

The present invention is part of the search for alternative technical solutions for increasing the developed surface of the electrodes of a fuel cell.

SUMMARY OF THE INVENTION

Thus, the invention relates to a method of texturing the electrolyte of a fuel cell comprising depositing, on the surface of the electrolyte membrane, drops of a solvent of the electrolyte.

The drops of solvent deposited on the surface are intended to "dig wells" and, through this particular texturing, to obtain a larger developed surface, resulting in a higher current and higher powers.

The solvent, by definition, must be chosen for its ability to locally attack and solubilize the electrolytic material in the presence. As mentioned above, the electrolyte is generally a polymeric membrane, the polymer may be a cationic exchange polymer or anion exchanger. The solvent is advantageously chosen from the group comprising water, ethanol, isopropanol, tetrahydrofuran (THF) or N-methyl-2-pyrrolidone (NMP).

In the case of Nafion® (Dupont), a commonly used cation exchange polymer, water, ethanol, or isopropanol are advantageously chosen. The elaboration of the electrolyte membrane may itself require different steps, such as the drying of the polymer mixture and / or an annealing step. Drying can be carried out at room temperature or in an oven at a relatively low temperature (below 50 ^° C.) for periods generally ranging from 15 minutes to 2 hours. The annealing is generally carried out at higher temperatures of between 60 ^° C. and 90 ^° C. for a period ranging from 2 to 6 hours. These stages, prior to the deposition of the solvent, are preferred because, for the solvent to act precisely, the electrolyte must not be in solution but partially or completely dried before texturing.

The most appropriate moment for depositing the drops of solvent, however, depends on the polymer / solvent pair in the presence and can take place before or after drying, and / or before or after annealing.

According to the invention, the drops are advantageously ejected or deposited by spraying on the surface of the electrolyte membrane.

It has been demonstrated by the inventors that the technique known as inkjet ("inkjet"), traditionally used for the deposition of ink on a material, was particularly well suited to the problem of the invention. Indeed, it allows to create a repetition of individualized patterns, whose structure and layout are perfectly controlled.

In practice, the solvent is placed in a print head comprising an ejection nozzle to be projected on the surface of the polymeric membrane. Thanks to the control of the dimensions of the nozzle (which typically varies from 20 to 80 μm), it is possible to control the size of the drops and thus the diameter of the wells formed by the impact of the drops (in this case microdroplets) of solvent on the surface of the polymer. It is clear that the wells formed on the surface of the membrane are substantially circular in shape.

Conventionally, the size of the drops is 1.5 to 2 times greater than the diameter of the nozzle. Advantageously and in practice, the wells formed on the surface of the electrolyte and texture have a diameter less than 100 microns.

In addition, the depth of these wells depends on the number of deposited drops, more precisely the number of repetitions of the deposit at the same point of impact, and possibly the parameters of the deposit (height, pressure, ...). In the case of inkjet, the ejection speed, controlled by the voltage applied to the head when it is of the piezoelectric type, is also a parameter to control.

In any case, it is possible to control the depth of the wells, which is advantageously greater than 5 μm for an electrolytic membrane which has a thickness generally of between 30 and 40 μm.

Moreover, the displacement of the inkjet device or the membrane makes it possible to perfectly control the disposition of the formed wells, in particular the distance between them and their alignment. The wells can thus be arranged according to a predetermined distribution.

By controlling all these parameters, in particular the diameter, the depth and the number of wells, the method according to the invention makes it possible to obtain a textured electrolyte having a form factor (ratio between the developed surface and the geometrical surface). between 1, 5 and 2.

The texturing of the electrolyte is only one step in the process of developing a fuel cell core. It allows the texturing of the electrodes subsequently deposited on the surface of the electrolyte membrane, then that of the current collectors. The increase of the surface of the current collectors thus allows a better collection of the current.

After texturing, the developed surface is greater than the geometric surface of the chip, resulting in an increase in the current delivered by the battery core and an improvement in the power of the fuel cell.

This method therefore offers a simple technical solution, inexpensive and reproducible.

EXAMPLE OF CARRYING OUT THE INVENTION

The way in which the invention can be realized and the advantages which result therefrom will emerge more clearly from the following exemplary embodiment, given by way of non-limiting indication, in support of the appended figures.

FIG. 1 represents a diagrammatic sectional view of a proton exchange membrane fuel cell core of the prior art. Figure 2 illustrates an example of curve E (potential) = f (i) (current density). FIG. 3 represents a diagrammatic sectional view of the membrane-electrode assembly (containing the catalyst) of a battery of the prior art.

FIG. 4 schematically represents the method allowing the texturing of the electrolyte (A) according to the invention and a section of the electrode-membrane assembly without (B) and with (C) texturing.

As illustrated in Figure 4A, the polymer serving as electrolyte membrane 3 in a heart of a fuel cell, for example Nafion ^® (Dupont), is placed under the print head of an ink jet device, example Altadrop device "of Altatech. the 40 .mu.m Nafion ^® are deposited on an Si substrate and dried 15 minutes at room temperature.

The chosen solvent, for example isopropanol, is placed in the print head and is ejected perpendicularly to the surface of the polymer, 800 μm high, using a 60 μm diameter nozzle, applying 60 V on the piezoelectric head.

The impact of the drops of solvent on the surface of the polymer forms wells of diameter of the order of 80 microns and depth of the order of 25 microns.

The offset of the position of the print head or the polymer makes it possible to form wells on the entire surface of the electrolyte, in the form of a square mesh of 150 μm of pitch. Obviously, other meshes are possible (diamond, triangle, ...) with variable density distributions according to the desired structure.

The repetition of the projection at the same point of impact makes it possible to increase the depth of the well which can reach several tens of μm, compared with the conventional thickness of the electrolyte membrane of the order of 30 to 40 μm.

The electrodes 2 containing the catalyst and then the current collectors 1 are then deposited on the surface of the membrane and adopt the same texturing.

FIG. 4B illustrates the fact that, without texturing and in a conventional manner, the developed surface of the assembly is equal to its projected area. On the other hand, after texturing (Figure 4C), the developed surface is much larger than the projected area, the form factor can reach a value of 2.

Claims

1. A method of texturing a fuel cell electrolyte (3) comprising the deposition of drops, on its surface, of a solvent of said electrolyte, said deposit being produced by the ink jet technique.

2. A method of texturing a fuel cell electrolyte according to claim 1, characterized in that the electrolyte is of polymeric nature and the solvent is selected from the group comprising: water, ethanol, isopropanol , tetrahydrofuran (THF) or

N-methyl-2-pyrrolidone (NMP).

3. Fuel cell electrolyte obtained by the method of claim 1 or 2 being in the form of a membrane with, on at least one of its surfaces, at least one well of substantially circular shape.

4. Fuel cell electrolyte according to claim 3, characterized in that the wells have a diameter less than 100 microns.

5. fuel cell electrolyte according to claim 3 or 4, characterized in that the wells have a depth greater than 5 microns.

6. Fuel cell electrolyte according to one of claims 3 to 5, characterized in that the wells are arranged in a predetermined distribution.

7. Fuel cell electrolyte according to one of claims 3 to 6, characterized in that it has a form factor between 1.5 and 2.

A method of preparing a fuel cell core comprising the steps of:

* texturing of the electrolytic membrane (3) according to the method of claim 1 or 2;

depositing electrodes (2) on the textured membrane and then collectors (1).