ES2342811A1 - Electrocatalizers for protonic exchange membrane fuel batteries (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Electrocatalysts for proton exchange membrane fuel cells. Process for obtaining a mesoporous carbon material (smc) comprising: the addition of colloidal silica to a polymeric carbon precursor; thermal curing; the carbonization of the compound obtained; washing the carbonized compound with hydrofluoric acid (hf) or naoh; and the oxidation of the washed material. Furthermore, the invention relates to a catalyst for polymer electrolyte fuel cells based on the use of the new mesoporous carbonaceous support. (Machine-translation by Google Translate, not legally binding)

Description

Electrocatalysts for fuel cells of proton exchange membrane.

The present invention relates to a catalyst for polymeric electrolyte fuel cells based on the use of a new carbonaceous support that allows greater accessibility of the active phase of the catalyst, improving So its efficiency. In addition, the present invention also relates to the procedure for obtaining said support.

Prior art

The global energy sector is in a transition period. The progressive decrease in reserves of fossil fuels and the environmental problems associated with their combustion forces to look for new energy alternatives that secure and diversify the energy supply. In this context, hydrogen emerges as a new energy vector, and opens a new era in the transport sector as it allows to use renewable energy, fossil and nuclear for this purpose, to while reducing CO2 emissions. Therefore, the hydrogen technologies constitute an important alternative to fossil fuels. Among them, the most promising is the use of hydrogen through fuel cells. its development is seen as one of the main means of future for combat the environmental pressure to which the dependence on fossil fuels, and also as one of the Solutions to your exhaustion. In this sense, the stacks of fuel are proposed as an alternative for both applications portable as stationary and, fundamentally, to replace Internal combustion engines.

Among the different types of fuel cells, polymer electrolyte (PEMFC) and direct methanol (DMFC) are the most promising candidates for portable and stationary applications due to their low weight, low operating temperature (55-95ºC) , and that allow a quick start (Wee et al ., J. Power Sources 165 (2007) 667). However, despite the great progress made in the development of this technology, it is still necessary to reduce its cost and solve some technological problems to make possible its commercialization and use on a large scale.

One of the components that mainly contributes to raising the cost of this technology is the catalyst. To reduce its cost it is necessary to reduce the amount of metal, and increase its durability and stability. Normally, platinum-based catalysts supported on carbon blacks, namely Vulcan XC-72 (R), are used. The use of the Vulcan XC-72 (R) is based on its high electrical conductivity and its mainly mesoporous structure. However, it also contains a relatively high amount of micropores in its structure (30%) that decrease the effectiveness of the catalyst. The metallic particles that are deposited within the micropores have a lower electrocatalytic activity, or may even not show activity, due to the difficult access of the reagents and products through these micropores. On the other hand, micropores that are smaller than metal particles could become blocked further worsening the diffusion of reagents and products (Liu et al ., J. Power Sources 155 (2006) 95).

In view of the above, new carbonaceous materials with mesoporous structure have begun to be used as a support for electrocatalysts. In recent years, the greatest activity of supported catalysts in nanofibers and carbon nanotubes has been demonstrated (Tang et al ., J. Colloid Interf. Sci. 269 (2004) 26; Paoleti et al ., J. Power Sources 183 (2008) 84), xerogels and carbon aerogels (Marie et al ., J. Non-Cryst. Solids 350 (2004) 88; Job et al ., Ener. Convers. Manage 49 (2008) 2461), and materials ordered mesoporous carbon (OMCs) (Calvillo et al ., J. Power Sources 169 (2007) 59; Ding et al ., Electrochim. Acta 50 (2005) 3131).

Carbon materials synthesized by the nanomolding method (OMCs) have aroused great interest since they have an ordered structure, which is an inverse replica of the mesoporous silicas used as molds, in addition to a very high surface area and a large pore volume . The greatest limitation of the use of ordered mesoporous silicas as molds is the little flexibility they offer when controlling the pore size of carbon materials, which is determined by the thickness of the walls of their pores (approximately 1- 5 nm) (Gierszal et al ., Carbon 45 (2007) 2171).

Another type of mold, which allows controlling the pore size of the carbonaceous materials above 5 nm, is colloidal silica (Han and Hydeon, Carbon 37 (1999) 1645). These molds consist of a colloidal solution of spherical silica particles, the diameter of which can be adjusted by varying the preparation conditions. The use of commercial colloidal silicas represents an alternative to the use of porous silicas to synthesize mesoporous carbon materials with uniform spherical pores of any diameter above 5 nm. The carbon materials obtained by this method have high surface areas and large pore volumes, which are generally higher than those obtained by the nanomolding technique (Han et al ., Chem. Mater. 12 (2000) 3337). Thus, prior synthesis of the siliceous mold is not required and in addition, the pore diameter of the carbonaceous material can be easily controlled using an appropriate colloidal silica, which is available in a wide range of particle sizes (Joo et al ., J. Electroceram. 17 (2006) 713), or varying the synthesis conditions such as pH or molar ratio between carbon precursor and silica (Joo et al ., Catal. Today 111 (2006) 171). These materials have been used as a support for electrocatalysts from Pt, PtRu (Kim et al ., J. Power Sources 145 (2005) 139) or PtCr (Choi et al ., J. Power Sources 156 (2005) 466). By using these carbonaceous materials as support, better results are obtained than with commercial catalysts, which is attributed to the high surface area that this type of materials presents, which combined with its mesoporous structure, have a positive effect on the dispersion of the metal and the diffusion of reagents and products to and from metal particles.

However, the electronic conductivity of electrodes during electrochemical reactions when using these electrocatalysts is not high due, among other factors, to the low conductivity of the carbonaceous material. This causes older ohmic losses in the fuel cell and therefore decreases its effectiveness

Description of the invention

The present invention provides a procedure for preparing a mesoporous carbon material (SMC) as electrocatalyst support for fuel cells proton exchange membrane (PEMFC or DMFC). This material has its surface chemistry modified by treatments simple oxidation, both gas phase or liquid phase, to create surface oxygenated groups and improve interaction metal support.

The excellent textural properties of the carbonaceous materials obtained through the use of a colloidal silica, such as the high surface area and the large pore volume, facilitate the access of the gas to the metal particles, increasing the active catalytic area since they facilitate the contact between gas, catalyst and electrolyte, which is where electrochemical reactions take place (O'Hayre et al ., J. Electrochem Soc. 152 (2005) A439; Sasikumar et al ., J. Power Sources 132 (2004) 11 ). On the other hand, the surface oxygenated groups, created during oxidation treatments, allow to improve the electronic transfer from the metal particles to the support during electrochemical reactions, thus improving the electronic conductivity in the electrodes. Both effects result in lower polarization losses in the fuel cell during operation, thus improving its energy efficiency.

A first aspect of the present invention is refers to a procedure for obtaining a carbon material mesoporous (SMC) comprising the following steps:

to.

adding colloidal silica to a polymeric carbon precursor, before or after its polymerization depending on the precursor, preferably the Colloidal silica addiction is carried out after the polymerization of the precursor;

b.

thermal curing of the mixture obtained in step (a);

C.

carbonized compound obtained in step (b);

d.

washing of the carbonized compound in the step (c) with hydrofluoric acid (HF) or NaOH, for the removal of the silica and get the porosity of the carbonaceous material.

and.

oxidation of the material obtained in the step (d), in the liquid or gaseous phase, is preferably carried out in liquid phase.

By "polymeric carbon precursor" is refers in the present invention to a carbonizable compound that will give rise to the carbonaceous base of the prepared material. This precursor can be, among others, a furanic resin or a mixture of resorcinol and formaldehyde, preferably a furanic resin which is composed of furfuryl alcohol and formaldehyde.

By "colloidal silica" is understood in this description of a suspension in water of SiO2 particles, with an average diameter that can range from 5 to 100 nanometers. The Selection of the particle size of the suspension determines the pore diameter of the carbon materials obtained.

The polymerization of the carbon precursor of procedure of the invention is carried out by any method known to a person skilled in the art.

Before or after polymerizing the precursor of carbon, colloidal silica is added, and the compound formed After subjecting the mixture to thermal curing, it is carbonized to a temperature that can be equal to or greater than about 600 ° C.

The thermal curing of the compound obtained in the step (a) is carried out so that solidification of the same. The conditions of said curing will depend on the precursor Carbon polymer used. In this way, the temperature of curing usually varies between room temperature and up approximately 120 ° C.

Washing the carbonized compound of step (c), described above, with hydrofluoric acid (HF) or sodium hydroxide (NaOH) would result in the mesoporosity of the material after the colloidal silica was removed by said washing. The carbon material produced by this technique is designated as SMC ( Silica sol-Mediated synthesized Carbon ).

One of the most relevant steps to get the Mesoporous carbon material of the invention is the oxidation of the material obtained after washing described above. This oxidation process is carried out by using a oxidizing agent, in liquid or gas phase, which preferably it can be, but not limited to oxygen, ozone, a halogen, permanganate hypochlorite, chlorate, nitric acid or peroxides. Preferably the oxidizing agent is nitric acid.

A preferred embodiment of the process of obtaining the SMC support also includes the step of: drying, washing with water and re-dry the compound obtained before the process of oxidation.

The SMC support obtained through the procedure described above can be used for the manufacture of an electrocatalyst.

Therefore, another aspect of the present invention refers to a procedure to obtain a electrocatalyst comprising:

to.

impregnation or deposition of a metal in the mesoporous carbon material (SCM), obtained according to procedure described above; Y

b.

optionally metal reduction supported in SCM, if necessary, depending on the method used in step (a). For example metal reduction would be necessary if impregnation of incipient moisture is performed of SCM with a solution of a metal salt.

The metal for obtaining a Electrocatalyst is preferably selected from Pt or Pt combinations with at least one other metal that can be selected between, but not limited to Ru, Cr, Re, Os, Mo, Sn, Cr, Ni, Rh, Go, W, Co or Pd. Therefore, they can be bimetallic or trimetallic. Preferably the metal is Pt or PtRu.

Another aspect of the present invention relates to to an electrocatalyst obtainable by the procedure above described

Another aspect of the present invention is refers to the use of the electrocatalyst of the invention for the manufacture of an MEA (membrane-electrode assembly) for a proton exchange membrane fuel cell (PEMFC or DMFC).

There are different productive sectors within from the fields of the chemical industry, energy production and the environmental care to which the different ones can be applied aspects of the present invention. Among them, companies stand out working in the manufacture of catalysts and in the sector of transport, where battery technology is being developed of fuel to replace internal combustion engines conventional.

Throughout the description and the claims the word "comprises" and its variants not they intend to exclude other technical characteristics, additives, components or steps. For those skilled in the art, other objects, advantages and features of the invention will be partly detached of the description and in part of the practice of the invention. The The following examples are provided by way of illustration, and are not It is intended to be limiting of the present invention.

Examples

The following examples represent a more detailed description of the invention that highlights the effectiveness of the electrocatalyst of the present invention.

Example 1

This example illustrates the synthesis of Platinum-based catalysts supported on materials Mesoporous carbon obtained from a colloidal silica. This catalyst was used to produce energy in a cell of PEM type polymer electrolyte fuel.

The catalyst was prepared as described in continuation:

15 ml of a furanic resin (furfuryl alcohol and formaldehyde) were taken and 15 ml of acetone was added, to reduce its viscosity and facilitate subsequent mixing with the colloidal silica suspension. Subsequently, 0.5 ml of HNO3 (65%) was added to polymerize the resin. Once a homogeneous solution was achieved, 30 ml of the commercial solution of Ludox SM-30 colloidal silica was added and the mixture was kept under stirring until a gel ( composite ) was formed. Next, the composite was subjected to thermal curing first at room temperature for 72 h, and then at 108 ° C in an oven for 24 h.

Once cured, the composite is ground, to ensure that the carbonization process is carried out in a homogeneous way, and is introduced into a 68 cm long and 2.5 cm diameter kanthal reactor, where it is carbonized at 700ºC in atmosphere of nitrogen (100 ml / min) for 3 h. Finally, a wash with 100 ml of HF was carried out at room temperature for 24 h, to remove the colloidal silica and obtain the carbonaceous material. After treatment with HF, the carbon material was filtered and washed with plenty of distilled water until a pH = 7 of the filtrate was obtained. It was then dried in an oven at 108 ° C overnight.

In order to carry out the oxidation of the support, introduced in 100 ml of a solution of 2M HNO3 with stirring keep going. After 0.5 h, the support was filtered and washed with abundant distilled water until a pH = 7 filtrate is obtained. TO It was then dried in an oven at 108 ° C overnight.

Once the support was prepared, the active phase (platinum) by moisture impregnation method incipient. To prepare 1 g of catalyst, 531 mg was dissolved of H 2 PtCl 6 · 6H 2 O in 8 ml of a solution of ethanol: 1: 1 water (v / v). Said solution was added dropwise on 1 g of support and mixed well using a glass rod until the support was wet. At that time it was introduced in an oven at 60 ° C to dry it. This process was repeated until impregnate the support with all the solution.

Once the support was impregnated, it was introduced in a quartz reactor with an internal diameter of 17 mm and 425 mm of length and a stream of nitrogen was passed to reach 300 ° C. Once this temperature was reached, a current of H2 was passed (50 ml / min) for 2 h to reduce the precursor of Pt to its state metal. The cooling of the system was carried out again in nitrogen atmosphere Catalysts were prepared with 20% wt. of Pt to be able to compare them with the commercial catalyst (E-TEK), which contains the same percentage of platinum.

Example 2

Once the catalyst was prepared, the MEA to later test it in a fuel cell.

The MEA was prepared as described in continuation:

The prepared catalyst was tested at the anode of a PEM fuel cell. To do this, before preparing the MEA was it is necessary to prepare the electrode with the catalyst based on Pt / SMC.

A suspension of 6.35 mg of catalyst and 21.2 mg of a 10% Nafion solution wt was prepared. in 0.5 ml of ethanol, and deposited on a commercial gas diffusion electrode, GDL ELAT carbon ( gas diffusion layer ) from E-TEK. The gas diffusion layer consists of a sheet of carbon cloth on which a layer of carbon and Teflon has been deposited on one side. The amount of catalyst employed in the suspension was such that the content of the prepared electrode contained 0.5 mg Pt / cm2, so that it could be compared with the commercial gas diffusion electrode (E-TEK Inc.) based on Pt / carbon black (Pt, 20%, 0.5 mg / cm2).

The MEA ( Membrane-Elelctrode Assembly ) was prepared using the electrode prepared as an anode, and as a cathode a commercial gas diffusion electrode (E-TEK Inc.). The commercial gas diffusion electrode was based on carbon cloth "A", with a thickness of 350 µm. The electrode is hydrophobic due to the deposition of a carbon and Teflon layer on one of its faces, in which a Pt / carbon black catalyst (Pt, 20%, 0.5 mg / cm2) has been deposited. The catalyst has a specific surface area of 128 m2 / g and a particle size of Pt of 2 nm.

The MEA was prepared using the hot-pressing technique by means of a press whose metal plates have been kept at 120 ° C. The proton exchange polymeric membrane (Nafion® 115) and the electrodes were joined by applying a pressure of 20 kg / cm2 for 90 s.

Prior to its use in the preparation of the different MEAs, the Nafion® 115 polymer membrane (Du Pont Chemical) was pretreated with a solution of 3% H 2 O 2 at 80 ° C for 1 h to remove organic impurities and at then with a solution of H 2 SO 4 0.5 M at 80 ° C for 1 h. H 2 SO 4 was removed by repeated washed with boiling distilled water.

Example 3

The MEA prepared with the catalyst proposed in the patent was tested in a 1 cm2 PEM fuel cell of geometric area, working at room temperature and pressure atmospheric The reaction that takes place in the cell is the next: H_ {2} + O_ {2} \ rightarrow H2_ + heat + electricity.

The cell is equipped with four bubblers containing distilled water through which they are passed the gases to humidify them, both at the entrance and at the exit of the cell. At the entrance of the cell, the humidification of the gases allowed to maintain a good proton conductivity of the Nafion, while at the exit, allowed to control the passage adequate gases through the cell. Hydrogen flow and fed oxygen was controlled by rotameters and was 9 ml / min and 50 ml / min, respectively.

The cell used was formed by two electrodes, in which the anode represents both the electrode of reference as the measurement, while the cathode represents the working electrode

The behavior of the cell was determined by registering the polarization curves, V (voltage of the cell) vs j (current density), obtained by a potentiostat / galvanostat (AMEL 2049). The anode and cathode of the cell were attached to the potentiostat / galvanostat by means of a wire and a platinum ring, which allowed electrical contact with the gas diffusion membrane. The measurements were carried out in modality galvanostatic, selecting the negative polarity of the electrode of work with respect to the reference and detecting the potential of the cell by varying the intensity of it.

Claims (15)

1. Procedure for obtaining a material of mesoporous carbon (SMC) comprising:

to.

adding colloidal silica to a polymeric carbon precursor;

b.

thermal curing of the compound obtained in step (a);

C.

carbonized compound obtained in step (b);

d.

washing of the carbonized compound in the step (c) with hydrofluoric acid (HF) or NaOH.

and.

oxidation of the material obtained in the step (d).

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2. Method according to claim 1, where the polymerization of the carbon precursor is carried out before adding colloidal silica. 3. Procedure according to any of the claims 1 or 2, further comprising the steps of: drying, wash with water and re-dry the compound obtained in the step (d). 4. Procedure according to any of the claims 1 to 3, wherein the carbon precursor is a resin Furious. 5. Method according to claim 1, where the oxidation of step (e) is carried out with acid nitric. 6. Mesoporous carbon material (SMC) obtainable by the procedure according to any of the claims 1 to 5. 7. Procedure for obtaining a electrocatalyst comprising:

to.

the impregnation or deposition of a metal in the carbon material mesoporous (SCM) described in claim 6.

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8. Method according to claim 7, which It also includes:

b.

reduction of the metal supported in the mesoporous carbon material obtained in step (a).

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9. Procedure according to any of the claims 7 or 8, wherein the metal is Pt or PtRu. 10. Electrocatalyst obtainable by the method according to any of claims 7 to 9, which comprises a metal supported on a mesoporous carbonaceous material according to claim 6. 11. Use of the electrocatalyst according to the claim 10, for the manufacture of an MEA. 12. MEA comprising an electrocatalyst according to claim 10. 13. Use of MEA according to claim 12, for manufacturing a membrane fuel cell proton exchange 14. Fuel cell comprising MEA according to claim 12. 15. Use of the fuel cell according to claim 14, for obtaining energy.