FR2843896A1 - Porous substrate containing a metallic phase for the production of fuel cell electrodes and connections for micro-electronic components has controlled concentration varying with depth - Google Patents

Abstract

A porous substrate (4) contains a metallic phase (2) extending in the substrate from one of its surfaces. The concentration of the metallic phase varies as a function of the depth in the substrate, the depth being computed from the surface of the substrate. An Independent claim is also included for a method for the fabrication of this porous substrate containing a metallic phase. The metallic phase is deposited in the porous substrate (4) by plasma spraying using a target made of the metallic phase. The plasma is formed at low pressure of 0.1-10 Pa and at a high frequency of 1-100 MHz. Conditions for formation of the plasma comprises the time for deposition of the metallic phase, the values of pressure and frequency, and the polarization of the target.

Description

POROUS SUBSTRATE CONTAINING A METAL PHASE A

  CONCENTRATION GRADIENT AND METHOD OF MANUFACTURING THE SUBSTRATE

DESCRIPTION

TECHNICAL AREA

  The present invention relates to a substrate

  porous containing a metallic phase.

  The latter may be a catalytic metal phase (a catalyst).

  In this case, the invention applies in particular to the manufacture of fuel cell electrodes ("fuel cells") of all shapes and sizes, comprising mono-metallic or multi-metal catalysts, for uses in various energy sources based on fuel cells

  hydrogen-air, methanol-air or hydrogen-oxygen.

  The power of these sources can range from a few milliwatts, for use in portable devices, to a few megawatts, for use in static installations, through intermediate powers, for

use in vehicles.

  More generally, the invention applies to the manufacture of any system comprising a support

  porous, provided with a catalytic active phase.

  Nevertheless, in the present invention, the metal phase is not necessarily catalytic and the invention also applies, in the field of microelectronics, to the manufacture of electrical interconnections.

  STATE OF THE PRIOR ART STATE OF THE PRIOR ART

  It is known to form metal deposits, having catalytic properties, on porous substrates or substrates, by chemical impregnation techniques, from salts or

  of organometallic in the liquid phase.

  These techniques have the particular disadvantage of using solvents which are

pollutants.

  As far as known fuel cells are concerned, it is known that the electrodes of the PEMFC type cells, ie proton exchange membrane fuel cells ("Cells"). , are obtained by successively depositing, on a carbon fabric, two layers of composition and

function determined.

  The first layer, called the "diffusion layer", contains carbon and a hydrophobic agent. It is generally deposited with the intention of limiting the surface porosity of the support and of obtaining a surface state that is suitable for

deposit of the second layer.

  The latter, called the "active layer", contains the catalyst and a hydrophobic agent. It has a uniform thickness and allows the catalyst to be in the vicinity of the electrolyte electrode interface. These two layers are generally in the form of inks whose viscosity is greater or less depending on whether they are deposited by coating or spraying on the carbon support or on the electrolyte. The catalyst contained in the active layer is in the form of small diameter particles, approximately equal to 2.0 nm, which is deposited on carbon. The concentration of the catalyst in the active layer is therefore homogeneous throughout the thickness of the

this layer.

    For example, it is known to deposit, on a porous substrate of carbon of 360 microns in thickness, a platinum-containing layer with a thickness of about thirty microns on the substrate and whose platinum concentration is

  homogeneous throughout the thickness of this layer.

  However, numerous studies of fuel cell electrodes and modeling have shown that the active amount of platinum varies according to the

current density.

  When the latter is low, the entire thickness of platinum is used, then

  At high current density, because of the competition between reagent supply phenomena and ionic resistance of an electrode, only a few micrometers of the platinum layer at the electrode-electrolyte interface are used. .

  In such electrodes, the use of the catalyst is not optimal: about 90% of the platinum

is not used.

SUMMARY OF THE INVENTION

  The present invention aims to

  remedy the previous disadvantages.

  In the present invention, the concentration of the metal phase contained in a porous substrate is varied according to the depth: this metallic phase, which may be a catalytic metal phase or an active phase, is not homogeneous but

  admits, on the contrary, a concentration gradient.

  Thus, unlike the chemical impregnation techniques mentioned above and known techniques for manufacturing fuel cell electrodes, the present invention lends itself to optimizing the efficiency of the catalytic phase. The invention indeed makes it possible to control the gradient or "profile" of concentration of the active phase, which makes it possible to reduce the cost of manufacture of the fuel cell electrodes by not having

  the catalyst only where it is useful.

  More generally, the present invention aims at optimizing the efficiency of a metallic phase

formed on a porous substrate.

  Specifically, the subject of the present invention is a porous substrate containing a metal phase, this metal phase extending into the

  substrate from a surface thereof, which substrate is characterized in that the concentration of the metal phase varies as a function of the depth in the substrate, this depth being counted from the surface of the substrate.

  The metallic phase can extend into the

  substrate on a thickness less than or equal to 10m.

  The concentration of the metallic phase

  can be a decreasing function of depth.

  According to a particular embodiment of the substrate which is the subject of the invention, the metallic phase is catalytic. In this case, according to a particular application of the invention, the substrate may be for forming an electrode of a fuel cell. According to another particular application of

  In the invention, which does not use a catalytic metal phase, the substrate may be intended to form an electrical connection of a microelectronic component.

  The present invention also relates to a method of manufacturing the porous substrate containing a metal phase, in which a metal phase is formed in the substrate which extends in this substrate from a surface thereof, and one

  The formation conditions of the metal phase are adjusted to vary the concentration of this metal phase as a function of the depth in the substrate, this depth being counted from the surface of the substrate.

  According to a preferred embodiment of the method which is the subject of the invention, the metal phase is formed in the porous substrate by a plasma sputtering deposition technique, using a target made

of the metallic phase.

  The plasma is preferably formed at low pressure, this pressure being in the range of 0.1 Pa to 10 Pa, and at high frequency, this frequency being in the range of 1MHz to 100MHz. Preferably, training conditions

  The metal phase comprises the deposition time of this metal phase, the value of the pressure, the value of the frequency and the polarization ("bias") of the target.

BR VE DESCRIPTION OF THE DRAWINGS

  The present invention will be better understood at

  the reading of the description of exemplary embodiments given below, for information only and

  in no way limiting, with reference to the accompanying drawings, in which: - Figure 1 is a schematic sectional view of a porous substrate containing a metal phase according to the invention; - Figure 2 shows two different profiles of concentration; platinum (curves I and II), obtained according to the invention in a porous carbon substrate, and - FIG. 3 schematically illustrates the

  electrochemical performances of electrodes respectively having these two different profiles, by voltage-current density curves (curves IA and IIA) and power-density curves of current (curves IB and IB).

  EXPOSURE OF PARTICULAR RESTORATION MODES

  An example of a porous substrate according to the invention is schematically shown in section in FIG. 1. This substrate contains a metal phase 2 and this metal phase extends over a thickness E of the porous substrate 4, starting from the surface 6 of the latter, which is flat in the example. The total thickness of the substrate is denoted H. According to the invention, the concentration of the metallic phase varies according to

depth in the substrate.

  FIG. 1 shows a reference defined by an origin 0 and by three axes X, Y and Z which are perpendicular to each other. The plane X O Y 15 corresponds to the surface 6 of the substrate 4 and the depth in the substrate is counted according to the axis

  Z, from the surface 6 of the substrate.

  In some applications of the invention, particularly when the porous substrate containing the metal phase is to form a fuel cell electrode, a catalytic phase is used,

  for example platinum, as a metallic phase.

  In addition, the thickness E is then of the order

  from 2 μm to 3 μm and the concentration of the metal phase decreases as the depth Z increases.

  However, in other applications of

  According to the invention, the thickness E may be greater than 3 μm and the concentration of the metal phase may vary differently depending on Z. This concentration may for example increase as the depth Z increases.

  Moreover, in other applications, the

  metal phase may not be catalytic.

  By way of example, the invention applies to the formation of an electrical connection of a microelectronic component. Then, as a porous substrate, SiOX, CxFyHz, CHy and, for example, are used as phase

  metal, one uses for example Al, Cu, Pt, W.

  An example of a process for producing a porous substrate containing a metal phase whose concentration varies with

  depth, according to the invention.

  This metal phase, which may be catalytic, is deposited by a plasma spraying technique on the porous substrate and, during this deposition, the profile of the

  concentration of the metal phase.

  In Figure 1, there is shown very schematically a device for such a deposit. This device comprises a vacuum chamber 8 in which the porous substrate 4 is placed (on a support

not shown).

  The device also comprises means 10 for pumping the enclosure and means 12 for introducing, into this enclosure, a gas

plasmagene such as argon.

  The device further comprises a target 14, which is placed in the enclosure 8, facing the surface 6 of the porous substrate, and which consists of the metal phase to be deposited, as well as means 16

polarization of this target 14.

  In addition, this device comprises means 18 for creating the plasma in the chamber, from the plasma gas, as well as means for controlling the plasma.

  these means 18 for creating the plasma.

  In this example, in order to form the deposit, a low-pressure plasma is formed, of the order of 0.1 μP at 100 μA, and at high frequency, of the order of 100 MHz,

using argon.

  According to the polarization conditions of the target 14, the plasma ions are attracted by this target and, upon impact, cause the atoms of the target to be ejected. These atoms diffuse into the chamber and will settle on the substrate 4 to be covered. The device used, also called "reactor", has the advantage of providing an ion flux / metal atom flux ratio which is greater than unity, contrary to conventional devices of

  sputtering and magnetron sputtering.

  In this regard, reference is made to the following document: [1] A.-L. Thomann, J. P. Rozenbaum, P. Brault, C. Andreazza-Vignolle, P. Andreazza, "Pd nanoclusters grown by plasma sputtering deposition on amorphous substrates", Appl. Surf. Sci. 158, 172-183

(2000).

  The deposit obtained on the substrate, or support, plane 4 consists of aggregates ("clusters")

metal of the target 14.

  According to the invention, the deposition conditions are adjusted to vary the concentration of this metal as a function of the depth in the

porous substrate 4.

  These conditions are the deposition time of the metal, the value of the pressure in the enclosure 8, the value of the frequency from 1MHz to 100MHz and the voltage

polarization of the target 14.

  According to the operating conditions mentioned above, different sizes and different forms of metal aggregates are obtained and therefore, as indicated, a concentration gradient in

the porous substrate.

  It is specified that to control the depth reached by the aggregates in the substrate, it is possible to

  example control the deposition time of the metal.

  To control the distribution of aggregates according to this depth, we can control the time of

deposit, pressure.

  Battery electrodes can be made to

  fuel by the method mentioned above and to study the performance of these electrodes.

  By way of example, platinum is used as a catalytic phase and this platinum is deposited on a porous substrate which is formed by a flexible fabric of carbon, 360 gm thick, coated with a mixture of carbon and polytetrafluoroethylene, this mixture constituting a diffusion layer for a

PEMFC fuel cell.

  Such electrodes are associated with a polymer electrolyte membrane to form the core of a fuel cell. Obtaining a catalyst concentration gradient in the electrodes of a fuel cell optimizes efficiency

use of the catalyst.

  As already indicated, the active amount of catalyst, for example platinum, varies as a function of the current density J. When this density is low, the entire catalyst can be used throughout the thickness of the catalyst. active layer, whereas for a high current density, only the active phase located near the electrode / electrolyte interface is used, for reasons of supply of reagents and

  ionic resistance of the electrode.

  The present invention makes it possible to put a greater concentration of catalytic phase in the vicinity of the surface 6 that is placed on the side of the membrane, in the case of the application to manufacture

  an electrode for a fuel cell.

  Figure 2 shows concentration profiles, obtained for two conditions

  different plasma treatment.

  More precisely, the variations of the number N of atoms have been plotted, taking as unit 1015 atoms per cm 2, as a function of the depth Z, expressed in micrometers, for a condition in which 0, 012 mg of platinum per cm 2 is obtained. (curve I) and for a condition in which 0.096mg of

platinum per cm2 (curve II).

  It is further specified that these platinum concentrations in a porous carbon diffusion layer are obtained for an argon pressure of 0.04 Pa, a power of 5W for the high frequency generator and a polarization of the target equal to V = 500V.

  The increase of the platinum content

  is accompanied by a deep diffusion of platinum and not a surface accumulation which would have the effect of increasing the size of the catalyst particles and reduce their activity, including at low current density .

  Electrochemical tests are conducted in

  assembling cells with electrodes thus produced by plasma spraying, under the conditions mentioned above with reference to FIG. 2.

  The batteries are supplied with pure oxygen and hydrogen, at a pressure of 4x105Pa and at a

temperature of 80 C.

  Figure 3 aims to characterize the electrochemical performance of the electrodes thus obtained. In this FIG. 3, the curves IA and IB correspond to 0.012 mg / cm 2 of platinum (curve I of FIG. 2) while the curves IIA and IIB correspond to 0.096 mg / cm 2 of platinum (curve II

of Figure 2).

  It is pointed out that the curves IA and IIA are curves of the variations of the voltage U (in mV) as a function of the current density J (in mA / cm 2) and that the curves IB and IIB are curves of the variations of the power P = UJ (in mW / cm2) as a function of J (in mA / cm2). It is noted that at lower platinum content,

  the performances remain good up to about lA / cm2.

  As soon as the operating temperature increases, the performance also increases:

  700mA / cm2, the battery voltage varies from 0.4V to 0.5V.

  The voltage remains above 0.4V up to 1A / cm2, at

instead of 0.7A / cm2 at 80 C.

  The deposition of a concentration gradient catalytic phase on a porous sputtering support, in accordance with the invention, leads to quite accurate electrochemical performance in fuel cell tests using

electrodes obtained.

  In this case, the present invention is not limited to platinum electrodes: any type of catalytic metal phase is usable, namely a mono-metallic or multi-metallic phase. In addition, the substrate may be of another porous material than the porous carbon, for example a metal foam, a

  porous polymer or porous silica.

  In addition, deposits can be formed according to the invention over very large areas, which can be up to several m2. 30

Claims (10)

  A porous substrate (4) containing a metal phase (2), said metallic phase extending into the substrate from a surface thereof, said substrate being characterized in that the concentration of the metal phase varies depending on the depth in the substrate, this depth being   counted from the surface of the substrate.   Substrate according to claim 1, wherein the metal phase (2) extends into the substrate   (4) to a thickness of less than or equal to 10m.   The substrate of any one of claims 1 and 2, wherein the concentration of   the metallic phase (2) is a decreasing function depth.   The substrate of any one of claims 1 to 3, wherein the metal phase (2) is catalytic.   5. Substrate according to claim 4, this   wherein the substrate (4) is for forming an electrode of a fuel cell.   6. Substrate according to any of   Claims 1 to 3, this substrate (4) being intended to form an electrical connection of a microelectronic component.   7. Method of manufacturing a substrate   porous material (4) containing a metal phase (2) according to any one of claims 1 to 6, wherein a metal phase is formed in the substrate which extends into this substrate from a surface of   the latter, and the conditions of formation of the metal phase (2) are adjusted so as to vary the concentration of this metallic phase as a function of the depth in the substrate (4), this depth being counted from the surface of the substrate.   The process according to claim 7, wherein the metal phase (2) is formed in the porous substrate (4) by a plasma spray deposition technique using a target (14). made of the metallic phase.   9. The method of claim 8, wherein the plasma is formed at low pressure, this pressure being in the range of 0.1 Pa to 10 Pa, and high frequency, this frequency being in   the range from 1MHz to 100MHz.   The process of any one of claims 8 and 9, wherein the conditions of   formation of the metal phase (2) comprises the deposition time of this metallic phase, the value of the pressure, the value of the frequency and the polarization of the target (14). 30