Use of mesoporous graphite particles for electrochemical applications

Abstract

The present invention relates to the use of mesoporous graphitic particles having a loading of sintering-stable metal nanoparticles for fuel cells and further electrochemical applications, for example as constituent of layers in electrodes of fuel cells and batteries.

Claims

1. A process comprising conducting an electrochemical reaction in the presence of a catalyst, said catalyst comprising mesoporous graphitic particles having a loading of sintering-stable metal nanoparticles (M-HGS) in an interconnected 3D mesopore structure, wherein the mesoporous graphitic particles having a loading of sintering-stable metal nanoparticles (M-HGS) in an interconnected 3D mesopore structure are obtainable by a process comprising: (a) impregnating particles having a mesoporous base framework with a graphitizable organic compound to yield impregnated particles; (b) subjecting the impregnated particles to a high-temperature graphitization step in order to yield graphitized particles having graphitic framework in the mesoporous base framework; (c) subjecting the graphitized particles to a process for removing the mesoporous base framework in order to yield mesoporous graphitic particles having a mesoporous graphitic framework; (d) impregnating the mesoporous graphitic particles with a solution of a salt of a catalytically active metal to yield impregnated mesoporous graphitic particles; (e) subjecting the impregnated mesoporous graphitic particles to a hydrogenation step in order to yield metal-loaded impregnated mesoporous graphitic particles having catalytically active metal particles in mesopores of the metal-loaded impregnated mesoporous graphitic particles; and (f) calcining the metal-loaded impregnated mesoporous graphitic particles in a temperature range of from 600 C. to 1000 C. in order to obtain said mesoporous graphitic particles having a loading of sintering-stable metal nanoparticles (M-HGS) in mesopores in an interconnected 3D mesopore structure; and wherein said mesoporous graphitic particles having a loading of sintering-stable metal nanoparticles (M-HGS) in an interconnected 3D mesopore structure comprise a hollow sphere structure comprising a mesoporous graphitic shell having a layer thickness of from 20 nm to 50 nm and a hollow core having a diameter of from 60 nm to 440 nm, and the mesoporous graphitic shell is loaded with the catalytically active metal, and the catalytically active metal is selected from among Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Mo, Se, Sn, Pt, Ru, Pd, W, Ir, Os, Rh, Nb, Ta, Pb, Bi, Au, Ag, Sc, Y, and combinations thereof in from 5-50% by weight based on the total weight of the mesoporous graphitic particles having a loading of sintering-stable metal nanoparticles (M-HGS) in mesopores in an interconnected 3D mesopore structure.

2. The process as claimed in claim 1, wherein the catalyst is a constituent of an electrode in an electrochemical cell.

3. The process as claimed in claim 2, wherein the catalyst is an oxidation catalyst anode constituent of an electrochemical cell.

4. The process as claimed in claim 2, wherein the catalyst is a reduction catalyst cathode of an electrochemical cell.

5. The process as claimed in claim 1, wherein the catalyst is a layer constituent in a layer structure of a membrane-electrode assembly (MEA) of a fuel cell, electrochemical cell, or electrochemical reformer or is a constituent of an electrode layer of a battery.

6. The process as claimed in claim 1, wherein the catalytically active metal is selected from Pt with at least one of Fe, Co, Ni, Cu, Ru, Pd, Au, Ag, Sn, Mo, Mn, Y, and Sc.

7. A process comprising conducting an electrochemical reaction in the presence of mesoporous graphitic particles without a loading of sintering-stable metal nanoparticles (n-HGS), wherein the mesoporous graphitic particles without a loading of sintering-stable metal nanoparticles (M-HGS) are obtainable by a process comprising: (a) impregnating particles having a mesoporous base framework with a graphitizable organic compound to yield impregnated particles; (b) subjecting the impregnated particles to a high-temperature graphitization step in order to yield graphitized particles having graphitic framework in the mesoporous base framework; (c) subjecting the graphitized particles to a process for removing the mesoporous base framework in order to yield said mesoporous graphitic particles without a loading of sintering-stable metal nanoparticles (n-HGS); and wherein said mesoporous graphitic particles without a loading of sintering-stable metal nanoparticles (n-HGS) comprise a hollow sphere structure comprising a mesoporous graphitic shell having a layer thickness of from 20 nm to 50 nm and a hollow core having a diameter of from 60 nm to 440 nm.

8. The process according to claim 7, wherein the electrochemical reaction occurs in a fuel cell, at an anode or at a cathode.

9. Process according to claim 1, wherein the metal-loaded impregnated mesoporous graphitic particles obtained in step e) have catalytically active metal sites on the metal-loaded impregnated mesoporous graphitic particles.

Description

(1) In addition, the structure of the particles having a hollow core and a porous shell makes it possible to improve the mass transfer in the catalyst layer which usually contains, as 3 phases, the empty volume (P1: gas and water transport), HGS-based catalyst particles (P2: electric power and heat transport and also catalytic activity) and membrane polymer (P3: proton transport). Gas and liquid can diffuse not only through the intesticies between the spheres but additionally through the interior of the spheres and thus overall better and be transported to/from the reaction sites.

(2) Electron transport from sphere to sphere can occur via the sphere shell, while gas transport can occur internally through the porous shells and the hollow spaces from sphere to adjacent sphere. The ion or charge transport can in this case occur in the polymer network surrounding the spheres, thus making it possible in the case of close packing of spheres, unlike the case of the solid standard materials, for complete flooding or filling of the intesticies between the spheres with polymer network to occur, which leads to an increase in the ionic conductivity and the mechanical stability of the catalyst layer. Ultimately, improved supply of ions to the catalyst particles (resulting in better catalyst utilization) and thus an increase in the performance of the catalyst layer results therefrom.

(3) At the same time, when HGS particles (n-HGS) which have not been coated with catalyst metal are used, an improvement in the gas diffusion into and through the catalytically inactive intermediate layer which is usually present in the gas diffusion electrode (GDE) of a fuel cell, also referred to as microporous layer (MPL), is made easier and standard MPL structures having PTFE as binder can be used here. The porosity or permeability of the shell of n-HGS particles also allows the hollow space of the sphere to be coated on the inside with liquid Teflon (THV or similar, soluble PTFE-like polymers), which leads to an improvement in water transport through the particles. If a coating or a binder having hydrophobic properties is appropriately selected, the use in the MPLs of only an additive which, both in the interior of the spheres and also outside around them, can serve as agent for hydrophobicizing the surface, is therefore possible.

(4) The inventors have discovered that the use according to the invention of the M-HGS particles leads to a higher power even from thicker catalyst layers (d.sub.CL>50 m), which is not possible using the material known from the prior art. Correspondingly, it is possible to reduce the thickness of the catalyst layer and consequently the amount of catalyst at a power which remains the same compared to the prior art.

(5) Owing to the size of the M-HGS particles and the performance even of relatively thick catalyst layers, it is possible, when the polymer network is appropriately selected, to leave out the MPL on the gas diffusion layer (GDL) and apply the catalyst layer in the polymer network directly to the GDL in order to produce a GDE which consists of only two instead of the usual three sublayers. Here, the pores which are close to the surface of the GDL and face the catalyst layer can be partially filled by the catalyst layer itself, and firm adhesion of the catalyst layer to the GDL can be achieved in this way.

(6) Low temperature or intermediate-temperature polymer electrolyte membrane fuel cells (LT- and IT-PEMFC) and high-temperature variants containing phosphoric acid (HT-PEMFC), the HGS particles can likewise be used according to the invention as anode or as cathode in direct methanol fuel cells (DMFC), which compared to the material known in the prior art leads to improved transport of oxygen, water and also of methanol and carbon dioxide (anode-side product) and at the same time gives improved corrosion resistance.

(7) The materials according to the invention can likewise be used in electrolysis for the cathode (H.sub.2 side) or in an electrochemical methanol reformer for the cathode (H.sub.2 side) or for anode (methanol side) and cathode (H.sub.2 side).

(8) The n-HGS particles according to the invention are in principle also suitable as constituents of lithium ion batteries and lithium-sulfur batteries, especially of the electrodes. The smaller particle diameter and the high electrical conductivity firstly makes it possible for them to be used as conductivity additive, and the particles can also be used as constituents of the coating on anode or cathode. Especially in the case of lithium ion batteries, silicon-based anodes are being used in the research stage; owing to the high theoretical capacity of silicon of 4000 mAh/g, these are subject of intensive research. However, owing to the volume expansion of silicon on incorporation of the lithium atoms in the crystal lattice, mechanical stresses occur and these destroy the Si crystallites. When M-HGS particles or Si-HGS particles having silicon crystallites present in the hollow core are used, lithium battery anodes which have increased cycling stability compared to the Si composites in anodes known from the prior art are possible.

(9) Furthermore, the use of both the n-HGS particles and the M-HGS particles for metal-air batteries or metal-air fuel cells both in primary cells and secondary cells is possible according to the invention. Here, the HGS particles can be used to increase the catalytic activity or mass transfer in the air electrode.

(10) It can be seen from all the above that the HGS particles can be used according to the invention for many applications in the field of electrochemistry or electrochemical energy transformers. This use is made possible, in particular, by the properties of the HGS materials in respect of the high specific surface area, hollow nature and porosity (sphere interior, sphere shell), good electrical conductivity, good thermal conductivity, excellent corrosion resistance and good wettability. Furthermore, the pseudo-monodispersity at an average sphere diameter which can be set and a narrow particle size distribution is advantageous. In addition, an increased mechanical strength resulting from the configuration of the particles as hollow spheres is advantageous, especially during application of layers and pressing operations, since the particles do not break up under a mechanical load.