Active layer/membrane arrangement for a hydrogen production device and assembly comprising said arrangement suitable for a porous current collector and method for producing the arrangement

Abstract

An active layer/membrane assembly to be incorporated into a hydrogen production device comprises an active layer in contact with a membrane capable of exchanging ions, the active layer comprising catalyst particles and particles referred to as support particles, wherein the size of the support particles is greater than the thickness of the active layer, so that the support particles emerge from the active layer, at the surface opposite the surface in contact with the membrane. A unit comprising the assembly and a porous current collector, the assembly and the collector having a complementarity of surface finish is provided. A process for manufacturing the assembly is also provided.

Claims

1. An active layer/membrane assembly intended to be incorporated into a hydrogen production device, said assembly comprising: a collector; an active layer in contact with a membrane capable of exchanging ions, said active layer comprising catalyst particles; wherein the catalyst particles are particles based on iridium, or on iridium oxide; and support particles in contact with the active layer; wherein the support particles are titanium particles; wherein at least one of the support particles is large enough to be partially incorporated in said active layer so that a portion of the at least one of said support particles emerges from a surface of said active layer in contact with the collector while a remaining portion of the least one of the support particles is within said active layer, wherein a size of the at least one support particle is larger than a thickness of said active layer; wherein the surface of said active layer in contact with the collector is opposite to a surface of said active layer in contact with said membrane.

2. The active layer/membrane assembly as claimed in claim 1, wherein the support particles have a size of greater than 1.2 micron, the thickness of the active layer being of the order of a micron.

3. The active layer/membrane assembly as claimed in claim 1, wherein the membrane capable of exchanging ions comprises an ionomer.

4. The active layer/membrane assembly as claimed in claim 1, wherein the degree of catalyst loading of said active layer is less than around 0.3 mg/cm.sup.2.

5. A unit comprising an active layer/membrane assembly as claimed in claim 1, said collector having pores with a porosity such that the support particles emerging from said active layer penetrate at the surface into the pores of said collector.

6. The unit as claimed in claim 5, wherein said collector is a porous titanium material.

7. The unit as claimed in claim 5, wherein said collector is an assembly of grids.

8. A process for manufacturing an assembly as claimed in claim 1, comprising: the deposition of an active layer and particles referred to as support particles onto a membrane capable of exchanging ions; contacting the active layer and particles with a collector on a side opposite the membrane capable of exchanging ions; wherein the active layer comprises catalyst particles based on iridium, or on iridium oxide; wherein the support particles comprise titanium particles; wherein at least one of the support particles is large enough to be partially incorporated in said active layer so that a portion of the at least one of said support particles emerges from a surface of said active layer in contact with the collector while a remaining portion of the least one of the support particles is within said active layer, wherein a size of the at least one support particle is larger than a thickness of said active layer.

9. The process for manufacturing an assembly as claimed in claim 8, wherein the production of said deposited active layer and particles comprises the production of an ink added to said membrane capable of exchanging ions, said ink comprising a powder of said catalyst particles and said support particles, an ionomer, and a mixture of water and alcohol.

10. The process for manufacturing an assembly as claimed in claim 9, comprising: the spraying of the ink onto a polytetrafluoroethene support; the transfer by hot pressing onto said membrane capable of exchanging ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other advantages will appear on reading the following description, given non-limitingly, and by virtue of the appended figures, among which:

(2) FIG. 1 illustrates a hydrogen production device according to the known art;

(3) FIG. 2 illustrates an active layer/membrane assembly according to the invention;

(4) FIG. 3 shows a comparison of the cell voltages as a function of the current densities for a cell comprising an assembly with an active layer without support particles and with an active layer according to the invention (identical loadings of noble metals);

(5) FIGS. 4a, 4b and 4c respectively illustrate the change in the cell voltage of the anodes as a function of the iridium oxide loading for current densities of 0.12 A.Math.cm.sup.2 and 1 A.Math.cm.sup.2 and the change in the ohmic resistance as a function of the iridium oxide loading at 80 C.;

(6) FIGS. 5a and 5b illustrate the change in the cell voltage over time for current densities of 0.04 A.Math.cm.sup.2 and 1 A.Math.cm.sup.2 and for anodes with or without titanium particles;

(7) FIG. 6 illustrates the average values of the thickness of the active layer as a function of the degree of iridium oxide loading for anodes with or without titanium support particles;

(8) FIGS. 7a and 7b illustrate the change in the cell voltage of an MEA as a function of the current density and the change over time, respectively with an assembly produced dry and with a pre-hydrated assembly.

DETAILED DESCRIPTION

(9) Within the context of the present invention, the Applicant has demonstrated the influence of adapting the surface finish of the active layer to that of the current collector. For this, the Applicant tested active layers based on iridium oxide catalyst particles and on titanium support particles. The Applicant was able to observe that the effectiveness of the electrodes was linked to the distribution of the size of the titanium support particles.

(10) The Applicant thus discovered that when the particles are incorporated into the active layer, they prevent the reactants from accessing the catalyst while the large particles go past this active layer, and thereby make it possible to improve the electrical contacts with the current collector.

(11) The Applicant proposes to then exploit a matching of size of support particles with a current collector capable of developing a complementarity of surface finish between the emerging catalyst support particles and said current collector.

(12) In order to demonstrate these phenomena, the Applicant has analyzed, by SEM imaging, the surface of a porous titanium material used as current collector and the surface of an anode based on IrO.sub.2 catalyst alone without titanium particles after a thousand hours of operation.

(13) It clearly appears that the surface of the anode adapts to the morphology of the porous titanium material due to the swelling of the Nafion membrane into the pores of the current collector.

(14) However, in view of the size of the particles of the porous titanium material (greater than 200 m), the electrical contacts between the active layer and the porous material are not satisfactory. Indeed, the deformation of the anode active layer is limited, of the order of 10-15 m: only 5% of the surface of the pores of the current collector is used for electrical conduction.

(15) This is why, according to the present invention, the Applicant proposes the following assembly: active layer/membrane, having a very good complementarity in terms of surface, with the current collector as illustrated in FIG. 2. The membrane 200 is in contact with the active layer, said layer containing support particles 300a of large enough size so that they can emerge from the thickness of said active layer and catalyst particles 300b of small size.

(16) The active layer is intended to be in contact with the porous current collector 310, the particles 300a being sized in order to be partially incorporated into the pores of said current collector.

(17) Advantageously but non-limitingly, the current collector may be a porous titanium material, it being possible for said porous titanium material to be manufactured by pressing at high temperature (sintering) of spherical titanium particles of identical sizes.

(18) The active layer is composed of catalyst particles having a particle size which may be of the order of 10-50 nm.

(19) The active layer also comprises titanium particles having a size typically of greater than a micron: which may be of the order of 1.2 micron to 80 m, and which may preferably be between 1.2 m and 10 m.

(20) According to the invention, it is possible to obtain good performances with a low degree of loading with expensive IrO.sub.2 particles.

(21) Indeed, since the diameters of the titanium support particles are greater than the thickness of the active layer, they may be incorporated into the porosities of the current collector. The active layer/current collector electrical contacts are improved, which reduces the cell voltage at high current density. Furthermore, these large titanium particles may act as a relay of the current collector, which makes it possible to improve the conduction in the thickness of the active layer.

(22) On the contrary, if the diameters of the titanium particles are smaller than the thickness of the active layer, some of the titanium particles may be covered by iridium oxide. In operation, the access of the water is blocked by the titanium particles, which means that the iridium oxide particles located underneath are less active than those in contact with the current collector. The active surface is reduced and the cell voltage at low current density increases.

(23) The Applicant carried out comparative tests for an assembly intended to be used in an electrolytic cell:

(24) in the case of an active layer comprising IrO.sub.2 particles; and

(25) in the case of an active layer comprising IrO.sub.2 particles and titanium support particles, according to the invention, said support particles emerging from the active layer,

(26) in order to validate the solution presented in the present application.

(27) More specifically, the Applicant studied the change in the cell voltage as a function of the current density, this being in the precise case of a size of the iridium oxide particles of around 15 nm and a size of the titanium support particles of between 1.24 m and 40 m.

(28) FIG. 3 shows a comparison of the polarization curves of MEAs with or without titanium particles at 80 C. More specifically, and in order to better demonstrate the effects of the addition of the catalyst support on the performance and take into account all of the MEAs tested (both with anodes of pure iridium oxide and those with the supported catalyst), the change in the cell voltage at low and high current densities was plotted on this FIG. 3 as a function of the iridium oxide loading. A lower cell voltage appears with the presence of support particles in the active layer.

(29) FIGS. 4a, 4b and 4c show that the addition of a catalyst support has a positive effect on the entire current density range; the cell voltage is reduced by the addition of the support:

(30) at low current densities (FIG. 4a) for anodes having loadings of less than 0.25 mg.Math.cm.sup.2 IrO.sub.2 (the origin of this threshold loading is explained hereinbelow);

(31) at high current densities (FIG. 4b) for anodes having loadings of less than 0.5 mg.Math.cm.sup.2 IrO.sub.2.

(32) For low loadings below the limit threshold of 0.5 mg.Math.cm.sup.2 IrO.sub.2, there are not enough iridium oxide particles to obtain a continuous active layer and the contact resistance between the electrode and the current collector is high (FIG. 4c).

(33) The titanium particles acting as conductive support material make it possible to improve the electron conduction (FIG. 4c) and therefore to reduce the cell voltage at high current density (FIG. 4b).

(34) Furthermore, a greater number of catalyst particles may be passed through by a current which automatically increases the number of active sites available for the oxygen-generating reaction and therefore makes it possible to reduce the cell voltage at low current density (FIG. 4a).

(35) Above this threshold value of 0.5 mg.Math.cm.sup.2 IrO.sub.2, the addition of titanium particles has a negative effect on the performance at high current density (FIG. 4b) which is explained by the increase in the ohmic resistance (FIG. 4c).

(36) The Applicant also monitored the change in the cell voltage as a function of time for the two types of active layers tested. FIGS. 5a and 5b thus illustrate the change in the voltage over time respectively for current densities of 0.04 A.Math.cm.sup.2 and 1 A.Math.cm.sup.2 for anodes with or without titanium particles.

(37) The table below then makes it possible to define the degradation slopes for current densities of 0.04 A.Math.cm.sup.2 and 1 A.Math.cm.sup.2 of two MEAs with or without titanium particles:

(38) TABLE-US-00001 Current density 0.04 A .Math. cm.sup.2 1 A .Math. cm.sup.2 Anode loading Degradation Degradation (mg .Math. cm.sup.2 IrO.sub.2) (V .Math. h.sup.1) (V .Math. h.sup.1) Pure IrO.sub.2 0.32 3.6 10.sup.5 11 10.sup.5 IrO.sub.2/Ti 0.12 .sup.1 10.sup.5 2 10.sup.5

(39) Thus, smaller slopes are advantageously obtained with an active layer of the invention.

(40) The Applicant also made observations using an electron microscope, in order to compare said observations and the results presented above.

(41) Selection of a Particle Size

(42) Particular attention should be paid to the size of the titanium particles since it determines their actions within the active layer: if the diameters of the titanium particles are greater than the thickness of the electrode, these particles jut out from the active layer and will be able to be incorporated into the porosities of the current collector. The active layer/current collector electrical contacts are improved (FIG. 4c) which reduces the cell voltage at higher current density (FIG. 4b). Furthermore, these large titanium particles act as a relay of the current collector which makes it possible to improve the conduction in the thickness of the active layer. The effect of these particles is therefore two-fold: improving the surface conduction and the volume conduction; if the diameters of the titanium particles are smaller than the thickness of the electrode, some of the titanium particles may be covered by iridium oxide. The access of the water is blocked by the titanium particles, which means that the iridium oxide particles located underneath are less active than those in contact with the current collector; the available active surface is reduced and the cell voltage at low current density increases (FIG. 4a).

(43) The cross sections of the MEAs produced by freeze-fracture made it possible to measure the thicknesses of the anode active layers. FIG. 6 presents the average values of these thicknesses as a function of the degree of iridium oxide loading for anodes with or without titanium particles.

(44) Particle size dispersion analyses carried out on the titanium powder showed that the diameter of the particles is greater than 1.24 m; this makes it possible to define a threshold loading value below which the effect of the titanium particles is solely positive.

(45) Thus, for iridium oxide loadings of less than 0.25 mg.Math.cm.sup.2 IrO.sub.2, the thickness of the IrO.sub.2/Ti anode active layers is less than this value of 1.24 m and all the titanium particles will jut out from the active layer and improve the surface and volume conduction of the electrode.

(46) It is interesting to note that this loading value has already been defined above following the study of the polarization curves.

(47) The Applicant has thus demonstrated that with an assembly according to the invention comprising an active layer with support particles of a large enough size to emerge from said active layer, optimal performances were obtained with the following advantages: a low degree of loading with expensive catalyst particles; a low cell voltage; a good conductivity; a good aging.

(48) Indeed, the aging of the anode active layer is characterized by the agglomeration and the increase of the size of the iridium oxide particles; the electrode thus loses its porosity and becomes dense throughout its thickness. It is interesting to note that the increase in the size of the particles is greater when they are in contact with the current collector.

(49) For anodes of pure iridium oxide having very low loading (0.32 mg/cm.sup.2), the diameter of the catalyst particles may increase up to 100 nm.

(50) Unlike the anodes of pure iridium oxide, no disparity in the increase of the size of the particles is observed for the anodes with titanium particles. The electrode has a structure that is still porous and shows that the size of the iridium oxide particles is around 25 nm; this indicates that all the particles have worked uniformly.

(51) The titanium particles thus act as a relay of the current collector.

(52) Comparison of a Non-Hydrated Assembly and a Pre-Hydrated Assembly Before Incorporation in a Cell:

(53) FIGS. 7a and 7b relate to tests carried out on assemblies that are not pre-hydrated and that are pre-hydrated during the creation. More specifically, FIG. 7a illustrates the change in the cell voltage of an MEA assembled dry in the cell as a function of the current density with an active layer based on IrO.sub.2/Ti. The cell voltage degrades rapidly over time. FIG. 7b illustrates the cell voltage at 1 A.Math.cm.sup.2 of an MEA with an IrO.sub.2/Ti anode after a hydration for 12 hours, showing the stability over time.

(54) FIGS. 7a and 7b thus demonstrate the importance of pre-hydrating the membrane before assembling it in the cell. When the MEA is assembled dry in the cell, the performances degrade very rapidly after only a few hours of tests (FIG. 7a). When the MEA is assembled in the cell after having been pre-hydrated (leaving in deionized water overnight), the performances remain stable over more than 200 hours of tests (FIG. 7b).

(55) It is important that the titanium particles can be reorganized within the volume of the active layer in order to become embedded in the porosities of the current collector so as to improve the electrical contacts. This reorganization is permitted owing to the fact that the membrane is sufficiently hydrated.

(56) If this is not so, titanium particles are present as an overthickness under the current collector. All of the current passes through these few points and causes the oxidation of the titanium particles; this increases the total ohmic resistance of the system and the performances degrade over time.

(57) Exemplary Embodiment of an Assembly According to the Invention:

(58) During a first operation an ink is produced. For this, the powders of catalyst and of support particles are dispersed in a solvent of deionized water and isopropanol and 10% by weight of ionomer that may be Nafion.

(59) During a second operation, the ink is sprayed onto a Teflon support and transferred by hot pressing onto a Nafion membrane corresponding to the ion-exchange membrane.

(60) It is thus possible to produce an anode composed of a catalytic active layer comprising iridium oxide particles and titanium particles pressed onto a Nafion membrane, and which will advantageously be brought into contact with a porous titanium material, the current collector.