Catalyst thin layer and method for fabricating the same

The catalyst thin layer consists of electronically conductive catalyst nano-particles embedded in a polymeric matrix. The ratio number of catalyst atoms/total number of atoms in the catalyst layer is comprised between 40% and 90%, more preferably between 50% and 60%.

The invention relates to a catalyst thin layer and a method for fabricating the same. The invention also concerns a catalytic electrode of a fuel cell comprising said catalyst thin layer.

BACKGROUND OF INVENTION

Catalyst thin layers are used in many applications to promote reactions. For example, they can be used in energetic systems, such as catalyst combustion systems or in sensor systems such as glucose, hydrogen or oxygen detectors, as well as in microsystems such as micro-electro-mechanical-systems (MEMs), LabOn-chips or micro fluidic systems. Catalyst thin layers are more specifically used in the fabrication of catalytic electrodes for fuel cell.

A basic structure of a fuel cell is schematically illustrated inFIG. 1. The fuel cell comprises an electrolytic material1, that is sandwiched between two electrodes, for example, between a porous anode2and a porous cathode3. An electrochemical reaction occurs between a fuel gas4and an oxidant gas5. A hydrogen cell uses hydrogen as fuel and oxygen (usually from air) as oxidant. Other fuels include hydrocarbons and alcohols, as for example, glucose in abiotic biofuel. Other oxidants include air, chlorine and chlorine dioxide. Fuel cell electrodes may be made of metal, nickel or carbon nanotubes, and are generally coated with a catalyst layer6for higher efficiency in ion generation and conductive transfer. The input fuel gas4and the oxidant gas5flow respectively to the anode2and to the cathode3through gas supply pathways in plates7. The input fuel gas4and the oxidant gas5are catalytically dissociated into ions and electrons in the anode2and in the cathode3.

In solid polymer electrolyte fuel cell also known as proton exchange membrane (PEM) fuel cell, a proton exchange membrane (PEM)1constitutes the electrolytic material (FIG. 1). This membrane is sandwiched between the two electrodes, preferably covered by catalyst layers6. The PEM1is proton permeable but constitutes an electrical insulator barrier. This barrier allows the transport of protons from the anode2to the cathode3through the PEM1but forces the electrons to travel around a conductive path to the cathode3.

Catalyst layers6are preferably formed on both surfaces of the PEM1to promote electrochemical reactions. The performance data of such a fuel cell depends critically on the quality of the interface between catalyst layers6and the PEM1.

In the prior art, catalyst layers6have been incorporated by hot pressing or by ink application directly onto the surface of the PEM1.

As illustrated inFIG. 2, patent EP-B-0600888 and patent publication U.S. Patent Application No. 2005/0064276 disclose a catalyst layer6on a PEM1comprising catalyst nano-particles8of platinum supported on carbon particles9obtained from a homogeneous ink preparation. The latter comprises supported platinum catalyst nano-particles8uniformly disperse in a proton conducting material also called ionomer10. Indeed, the carbon particles9of the above-mentioned catalyst layers, are ten to hundred times larger than catalyst metal nano-particles8. The catalytic sites where the gas reaction takes place are therefore relatively small and, the three-phase interface and the catalyst content are not efficient enough.

Moreover, EP-B-1137090 discloses a method for forming a catalyst layer consisting in sputtering a catalytic metal and a carbon source on a PEM1to form, as illustrating inFIG. 3, a nanophase of catalyst nano-particles8and nano-sized carbon particles9. Both catalyst nano-particles8and nano-sized carbon particles9have a preferred particle size of 2 to 10 nm.

In any case, the catalyst layer contains carbon particles, which have a bad conductivity i.e. a conductivity of less than 104S/m. Furthermore, the access to the catalytic sites might be difficult.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an efficient catalyst thin layer, with high electronic conductivity and improved access to its catalyst sites.

This is attained by a catalyst thin layer according to the appended claims. This is more particularly obtained by a catalyst thin layer consisting of electronically conductive catalyst nano-particles embedded in a polymeric matrix, with a ratio number of catalyst atoms/total number of atoms is comprised between 40% and 90%, more preferably, between 50% and 60%.

Another object of the invention is to provide a method for fabricating such a catalyst thin layer.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 4, catalyst thin layer11consists of electronically conductive catalyst nano-particles12embedded in a polymeric matrix13. The catalyst thin layer11has a preferred thickness less than or equal to 2 μm. The catalyst nano-particles12are embedded in the polymeric matrix13without any other support and ensure the electronic conduction in the catalyst layer11through the percolation mechanism. The suppression of carbon particles9according to the prior art, enhances the electronic conduction since catalyst nano-particles12have a better conductivity than carbon particles9. However, at low catalyst atomic percentage, he when the ratio number of catalyst atoms/total number of atoms is low, the electronically conductive catalyst nano-particles12are dispersed in the polymeric matrix13. They are too far from one another to be reached by electrons provided, for example, by electrochemical reactions. On the other hand, at high catalyst atomic percentage, the electronically conductive catalyst nano-particles12might form a bulk. In this case, the specific surface, i.e. the total surface area per unit of mass, of the catalyst nano-particles12would drop down. Such an aggregation thus involves the reduction of the catalytic effect. Indeed, a high specific surface involves increased contact area between the electronically conductive catalyst nano-particles12and reactants. To efficiently contribute to electrochemical reactions and simultaneously allow electron conduction, the catalyst atomic percentage of the catalyst thin layer11must be comprised between 40% and 90%, preferably between 50% and 60%. As illustrated inFIGS. 4 and 5, the electronically conductive catalyst nano-particles12then form clustered networks14in the polymeric matrix13. These clustered networks14create an electronic link between nano-particles12. This enhances the electron conduction (FIG. 5, white arrow) and thereby improves the electronic conductivity of the catalyst thin layer11.

The particle size of the electronically conductive catalyst nano-particles12is preferably in the range of 3 to 10 nm.

A preferred electronically conductive catalyst is a metal. More particularly, the electronically conductive catalyst may be pure platinum (Pt) or an alloy of Pt and at least another metal, for example, gold (Au), rhodium (Rh), iridium (Ir), ruthenium (Ru), tin (Sn), bismuth (Bi) and molybdenum (Mo). It also may be a mixture of Pt and at least one of the metals above-mentioned. A platinum (Pt) alloy, for example, a Pt—Ru—Bi alloy, is preferably used if carbon dioxide is a by-product of the electrochemical reaction, for example, in abiotic biofuel cell using glucose or methanol as fuel.

The polymeric matrix13contributes to strengthening the binding force between the electronically conductive catalyst nano-particles12and promotes the formation of the clustered networks14. The polymeric matrix13may be polyolefin, polyfluorocarbon and organometallic polymer or a ionomer.

In a specific embodiment, a Pt catalyst thin layer11is coated on a substrate15by a vacuum process, advantageously, by physical and chemical vapor deposition (PVD and CVD). The electronically conductive catalyst nano-particles12and a precursor of the polymeric matrix13are then simultaneously applied on the substrate15. The latter may be Nafion™ (E. I. DuPont), Flemion™ (Asahi Glass Co.), fluorine-free polymer such as polyethylene and polypropylene, sulfonated polyetherketones or polyarylketones, ceramic materials or even electronically conductive materials, such as a glassy carbon electrode.

Vacuum processes are particularly suitable to realize the catalytic thin layer11because they allow the control of the distribution of catalyst nano-particles12in a given polymeric matrix13for high volume fractions of catalyst nano-particles12. More particularly, the Pt catalyst thin layer11is applied on the substrate15by a combination of physical and chemical vapor deposition (PVD and CVD). This process involves placing the substrate15to be coated in a vacuum chamber and contacting the substrate15with suitable plasma. The properties of the ultimate Pt catalyst thin layer11can be controlled by adjusting the composition of the plasma and by varying process parameters, such as pressure and specific sequences of cleaning and etching. Plasma can be generated either by applying RF energy or by applying pulsed DC biased power to the substrate15in the presence of a gaseous precursor of the polymeric matrix13. More preferably, the precursor of the polymeric matrix13is selected from the group consisting of hydrocarbon, fluorocarbon and organometallic.

For example, a substrate15formed by a glassy carbon electrode of 5 mm of diameter has been coated by RF PVD. A catalytic metal target, for example a catalytic Pt target, has been physically sputtering with a radio frequency plasma under a gas stream of gaseous ethane and an inert gas or a mixture of inert gases. Gaseous ethane then constitutes the precursor of a polyolefin matrix constituting the polymer matrix13. The sputtering conditions may be varied according to the size of the electronically conductive catalyst nano-particles12.

The glassy carbon electrode constituting the substrate15has previously be pre-treated as follows. The glassy carbon electrode is polished with diamond paste down to 1 μm and washed for 15 mm in three successive ultrasonic baths of acetone, ethanol-water (1-1) and water. The plasma is created by RF powered electrode (power between 10 and 800 W, preferably 100 W). The chamber is preferably pumped down to vacuum at 1 mTorr or less, and then a gas pressure is maintained between 1 mTorr to 1000 mTorr (preferably 100 mTorr) with a gas stream of gaseous ethane and inert gas, preferably argon (Ar). The gas flow rate is maintained at 0.5 sccm (standard cubic centimeter per minute) for gaseous ethane and at 45 sccm for argon. By maintaining these conditions during a deposition time of 30 mm, a Pt catalyst thin layer11with a catalyst atomic percentage of 54% is obtained. Different amounts of catalytic Pt loading may be obtained by varying the plasma power. The sputtering conditions are adjusted to form a Pt catalyst thin layer11with the desired thickness and with nano-particles of a given size. The sputtering may be carried out either in one step process or in several steps.

The conductivity (σ) of several Pt catalyst thin layers11with respectively 9, 40, 47, 54, 58, 68 and 78 catalyst atomic percentages of Pt have been measured. As shown onFIG. 6, the conductivity increases rapidly between 20% and 40% and remains practically constant about 50%, where it reaches a maximum value of 105S/m.

The roughness (R), which is representative of the specific surface area of a Pt catalyst thin layer11, may also be evaluated by means of any known technique, for example, hydrogen-adsorption/desorption coulometry. A electrochemical three-electrodes cell is used to perform this technique (working, auxiliary and reference electrodes). The working electrode is a rotating disk electrode (RDE). Experiments are carried out in 1M sulphuric acid solution at room temperature. The electrochemical cell is deaerated by inert gas, for example nitrogen (N2) bubbling in the solution for 30 min. The inert gas stream is then held above the solution and ten voltammetric cycles at 0.1V·s−1(−0.05 to +1.5V/NHE) are applied to clean the Pt catalyst thin layer11surface. A voltammogram is then monitored in the same potential range. The specific surface area of platinum is determined by integrating the current density vs time curve under the hydrogen-desorption peak. A roughness factor R (m2Pt/m2geometric) can be therefore deduced from the coulometric charge under this peak using the well-known relationship of 200 μC.·cm−2of platinum.FIG. 7, graphically illustrates the roughness factor R vs. Pt catalyst atomic percentage. A peak of roughness of about 170 m2Pt/m2is observed at about 53% of Pt.

So, by selection of the catalyst atomic percentage of Pt in the range of 40% to 90%, more preferably, between 50% and 60%, combines the enhance effects due to a high specific surface area of Pt and to a high conductivity.

The catalyst thin layer11can also be obtained by conventional processes of serigraphy, enduction, spin coating or dip coating of a ink or a paste. The latter are generally prepared by blending the electronically conductive catalyst nano-particles12and the precursor of the polymer matrix13to form the ink or the paste, next applying the said ink or paste on the substrate15and then polymerizing the precursor, for example, by thermal treatment.

The catalyst thin layer11is advantageously used in a catalytic electrode of a fuel cell and, more particularly, of a PEM fuel cell. The latter comprises an electronically conductive layer16covered at least partially by the catalyst thin layer11. Very thin catalytic electrodes, having a thickness less or equal to 2 μm, can be provided with the catalyst thin layer11of the present invention.

The catalyst thin layer11is also particularly well suited for a use in a membrane-electrode assembly (MEA). The substrate15constituting the PEM1, and the polymeric matrix13can be made of the same material, for example Nafion™. The catalyst thin layer11, preferably metal catalyst thin layer, more preferably Pt catalyst thin layer, can be coated on both sides of the PEM1. The latter is then sandwiched between a first electrode and a second electrode, for example, a conductive carbon cloth or a carbon paper. Next, electrodes are hot pressed to form a complete MEA.

The catalyst thin layer11can be coated only on one side of the PEM1or on both.

The resulting MEA may be included in different kinds of fuel cells, such as solid alkaline fuel cell (SAFC) or solid oxide fuel cell (SOFC). More preferably, the resulting MEA is used in a PEM fuel cell.

The catalyst thin layer11of the present invention is particularly suitable for systems needing a very thin catalyst layer, i.e. with a thickness less than or equal to 2 μm, like three dimensional fuel cells or microsystems.

In a specific embodiment, illustrating inFIG. 8, a three dimensional fuel cell comprising catalyst thin layers11, is formed on a patterned ceramic layer17, advantageously porous. The pattern17preferably comprises an alternation of ribs separated by grooves. The dimensions of the pattern are generally in the range of 100 μm to 200 μm. The three dimensional fuel cell comprises a stack of the successively following layers provided on the patterned ceramic layer17:

a first electronically conductive layer16, preferably a gold layer,

a first catalyst thin layer11according to the invention, preferably metal catalyst thin layer, more preferably Pt catalyst thin layer, with a thickness of 2 μm,

a PEM1, preferably a Nafion™ layer,

a second catalyst thin layer11, preferably metal catalyst thin layer, more preferably Pt catalyst thin layer, with a thickness of 2 μm, and

a second electronically conductive layer16, preferably a gold layer.

The stack forms a series of undulations, according to the pattern formed on the surface of the ceramic layer17.

Classically a dielectric18, for example silicon oxide, is provided at the periphery of the above-described stack, between the first and the second electronically conductive layers16, preferably forming metallic collectors constituting the terminals of the fuel cell.

Advantageously, the direct coating of electronically conductive catalyst nano-particles12on a substrate15, more particularly on a PEM1, reduces the amount of catalyst required in the catalyst thin layer11, reduces the thickness of the catalyst thin layer11and improves the efficiency of the gas reactions.

The catalyst thin layer11can also be used in sensor systems such as hydrogen, oxygen or glucose detectors and also in energetic systems such as catalyst combustion systems.