FR2789520A1 - Gas electrode production process, especially for proton exchange membrane fuel cell electrodes comprises chemically reducing platinum anions impregnated into heat treated carbon - Google Patents

Abstract

Membrane-type fuel cell gas electrode production involves impregnating heat treated carbon with a platinum anion solution and chemically reducing the platinum. Production of a gas electrode for a membrane-type fuel cell comprises subjecting carbon to heat treatment, and then impregnation with a solution of platinum anions prior to mixing the carbon with an ionic solution and deposition of the mixture on a carbonaceous support, the platinum being reduced by a chemical method. An Independent claim is also included for a PEM (proton exchange membrane) fuel cell having one or more electrodes produced by the above process. Preferred Features: Chemical reduction is carried out by electrode immersion in a molecular hydrogen saturated aqueous solution or by exposure to a gaseous hydrogen stream.

Description

PROCESS FOR THE PREPARATION OF POROUS ELECTRODES

GAS CONSUMPTION

  The present invention relates to a method for preparing porous electrodes for gas consumption, in particular

  membrane fuel cell electrodes.

  In the field of cells operating at low temperature, that is to say between ambient temperature and 200 ° C., 0o fuel cells with proton exchange membrane (PEMFC) appear, for the hydrogen / air couple, as well suited electrochemical generators to an application to electric vehicles. The advantage of the PEMFC technology lies in particular in the compactness and simplicity of the assembly: the bonding of two platinum carbon electrodes on either side of a membrane leads to

  cells whose thickness may not exceed 0.5 cm.

  In general, porous gas electrodes, in particular those used in fuel cells with a proton exchange membrane, consist of at least two layers: - a diffusion layer, deposited on a carbon fabric or a metallic grid, serving supply of current and mechanical support to the electrode and allowing good distribution of the gases towards the active layer; she may be

  made of PTFE and carbon powder.

  - an active layer where the actual electrochemical reaction takes place and generally consisting of carbon powder supporting the particles of catalyst, solid electrolyte (ionomer in the case of membrane batteries) or liquid, and possibly polytetrafluoroethylene (PTFE, facilitating the formation of

gas pores and serving as a binder).

  When using noble metals (platinum) or alloying these metals as catalysts, it is important to use the catalyst in the best possible way and this implies: - an optimal particle size from the activity point of view mass (3 to 5 nm for the reduction of 02 in an acid medium) - a thin active layer to reduce the limitations by the diffusion of gases and the ionic ohmic drop, therefore a high catalyst / (carbon + catalyst) mass ratio, at 40 % for example - good electronic and ionic percolations in the

active layer.

  To manufacture the electrodes, the active layer is usually produced by depositing on the diffusion layer a mixture of previously platinum carbon powder, electrolyte and optionally PTFE, before heating and pressing the assembly to obtain an electrode. This type of method generally provides electronic percolation between the carbon grains if the proportion of carbon is sufficient, but it does not ensure ionic percolation (catalyst particles not covered with electrolyte). In addition, the carbon powders supporting catalyst particles, which are commercially available, have particle sizes increasing rapidly with the mass ratio already mentioned and are no longer suitable when this ratio exceeds%. The alternative constituted by the in situ electrochemical deposition of the catalyst has the advantage of carrying out electronic and ionic percolations, but if the deposition is carried out from species in solution in the ionomer of the active layer or from cations of Pt (NH3) 2+ type, even preadorbed on carbon by surface ion exchange, the mass percentages obtained cannot exceed 5 to 10%. If the species come from an external electrolyte the deposit is very

inhomogeneous and not usable.

  The publication FR2744840 describes a process for the preparation of electrodes for membrane fuel cells, in which an electrochemical deposition is carried out in situ from species pre-impregnated on carbon, in particular from older PtC162-; the two percolations are then carried out and the mass percentages exceed 20% for sizes

from 2 to 5 nm.

  This method involving electrochemical reduction, however, requires skilled labor and, for very large electrodes, generators which

could be costly.

  The present invention proposes to replace the electrochemical reduction in situ by a simple chemical reduction in situ at ordinary temperature obtained by immersion of the electrode in an aqueous solution saturated with hydrogen; this chemical reduction can also be obtained, after constitution of a gas electrode and mounting of the gas electrode in a fuel cell or an electrolyser, by

  passage of hydrogen gas through said gas electrode.

  To this end, the present invention provides a method for preparing electrodes for membrane fuel cells and gas electrodes, by a mixture of carbon and an ionomer solution, depositing the mixture on a carbon support, characterized in what: - the carbon undergoes a heat treatment then is prepreg with a solution of platinum anions before mixing then deposit on the carbonaceous support to form an electrode, - and in that the reduction of the platinum is carried out chemically. 1o According to the invention, the reduction of platinum by chemical means is carried out by immersion of the electrode in a solution

  aqueous saturated with molecular hydrogen.

  According to the invention, the reducing agent is water (or an aqueous solution) at ordinary temperature saturated with molecular hydrogen. According to the invention, the reduction of platinum by chemical means can also be carried out by subjecting the gas electrode to a

flow of hydrogen gas.

  According to the invention, the prepreg solution is a solution comprising anions of the PtC162- type or any other

  anion with similar chemical characteristics.

  According to the invention, the prepreg solution contains isopropanol. According to the invention, the carbon is treated with hot CO2 and

  brutally cooled or treated under hot neutral gas.

  The invention also relates to a PEMFC battery comprising at least one electrode obtained by a method comprising

  previously stated characteristics.

  The present invention will be better understood on reading the

examples below.

  So that this new proposed method of chemical reduction in situ gives results equivalent to electrochemical reduction in situ (example: diameter of particles close to 3 nm and mass ratios metal / (carbon + metal) equal to or greater than 30% in 0o case of platinum), the procedure is as follows: - pretreatment (for 30 to 60 min) of the carbon powder (eg Vulcan): heating the powder to 930 C under CO2 (cleaning, "activation") followed by '' rapid cooling under C02 (to avoid excessive functionalization during slow cooling) or under neutral gas; - impregnation of the carbon powder (preferably for several days) with a mixture containing water, isopropanol (to improve wetting of the carbon), recent solution of an anionic compound of the metal (ex: H2PtCI6), ionomer solution ( ex: Nation) and / or suspension of PTFE; - chemical reduction is then obtained by simple immersion of the electrode (for 2 or 3 hours) (eg fabric + carbon diffusion layer + PTFE) carrying an active layer of impregnated carbon + PTFE and / or ionomer in water or an aqueous solution containing dissolved hydrogen; chemical reduction can also be obtained, after producing a gas electrode and mounting it in a fuel cell or an electrolyser, by passing hydrogen

gas in the gas electrode.

  The process provides an unpolluted catalyst, does not require highly skilled labor and is applicable to very large areas at minimal cost. The electrochemical percolation is ensured by the fact that all the metallic compound is prepreg / adsorbed on the carbon; the ionic percolation, which was not obvious a priori, is obtained by the fact that the carbon powder is prepreg with the ionomer solution (case of PEMFC and DMFC) and that the reducing hydrogen does not arrive on the carbon carrying the compound to be reduced than where the aqueous solution

maybe in contact with him.

  IO Example of the usual fabrication of an electrode A pretreatment of carbon powders is first carried out by heat treatment. This treatment consists of a gradual rise in temperature to 930 C, followed by a rapid return to room temperature under CO2 to avoid excessive carbon functionalization. To the carbon thus treated is added a solution of chloroplatinic acid, then after stirring, isopropanol in order to obtain good wetting of the carbon and of the water. In the case of a fuel cell electrode with an exchange membrane

  of protons a solution of Nation and a suspension of PTFE.

  On the other hand, in the case of an electrode with liquid electrolyte,

  only a suspension of PIFE is added.

  The whole is then stirred for several hours.

  This preparation is then deposited, for example by spray, on a diffusion layer which is obtained by pressing and by heating at 350 ° C. for at least two hours of

  mixture of carbon black and PTFE.

  The electrode thus formed is then annealed at 150 ° C., then rehydrated, before immersion at ordinary temperature and for a few hours in water (or a solution

acid), saturated with hydrogen.

Example 1:

  Preparation of a small electrode (a few cm2) with an active layer 5 to 6 grm thick (0.15 to 0.2 mg Pt / cm2) 1) 0.0250 g of Vulcan XC72 carbon activated at 950 C for min (loss of mass between 27 and 32%) and removed from the oven quickly under CO2 to obtain a sudden drop in

temperature.

  2) Addition of 0.6 ml of H2PtCl6,6 H2O on the treated carbon.

  3) Ultrasonic agitation for at least 30 min.

  4) Add between 1.5 ml and 2 ml of isopropanol.

  ) Agitation of the solution for 30 min by ultrasound.

  6) Adding between 1 and 1.5 ml of ultrapure H20.

  7) Magnetic stirring at least 24 hours.

  8) Addition between 40 and 60% of Nafion by mass compared to carbon.

  9) Agitation at least 30 min with ultrasound.

  ) Spray deposition (for example) of this solution on a previously prepared diffusion layer (consisting of

carbon black and PITFE).

  11) Heating of the assembly at 150 C for 1/4 hour.

  12) Rinsing with water 13) Immersion in a saturated water (or acid) solution

into molecular hydrogen.

Example 2

  For larger quantities of carbon powder, care must be taken: 1) When processing under CO2, to. spread the carbon inside the oven so that all of the powder

reacts with CO2.

  2) For the prepreg stage, shake the entire volume of

  the solution under the same conditions as above.

  3) Flatten the diffusion layer (or possibly deposit an intermediate diffusion layer of a few

  grm less porous and more planar thickness).

  Switching to electrodes of several dm2 poses no problem during the chemical reduction of the platinum salt by hydrogen in solution and at ordinary temperature or by

hydrogen gas.

Claims (6)

  1) Process for the preparation of electrodes for membrane fuel cells and of gas electrodes, by a mixture of carbon and an ionomer solution, deposition of the mixture on a carbon support, characterized in that: - the carbon undergoes a heat treatment then is pre-impregnated with a solution of platinum anions before mixing then depositing on the carbon support to constitute 0o an electrode, - and in that the reduction of the platinum is carried out chemically. 2) Method according to claim 1, characterized in that the reduction of platinum by chemical means is carried out by immersion of the electrode in a saturated aqueous solution into molecular hydrogen.   3) Method according to claim 2, characterized in that the reducing agent is water (or an aqueous solution) to   ordinary temperature saturated with molecular hydrogen.   4) Process according to claim 1, characterized in that the reduction of platinum by chemical means is carried out in   subjecting the gas electrode to a flow of hydrogen gas.   ) Method according to any one of the claims   above, characterized in that the prepreg solution is a solution comprising anions of type PtC162.   6) Method according to any one of claims   previous, characterized in that the solution of   prepreg contains isopropanol. o10   7) Method according to any one of claims   above, characterized in that the carbon is treated with C02 when hot and brutally cooled or treated under neutral gas hot.   8) PEMFC battery comprising at least one electrode according to one   any of the preceding claims.