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    • 专利摘要:
    The process of this invention comprises the development of a method of synthesis of carbonaceous materials for application as electrocatalysts of the oxygen reduction reaction that occurs in the cathode of the fuel cells. The process is based on the heat treatment at high temperatures of polyaniline or derivatives thereof that gives rise to carbonaceous materials with a high nitrogen content. The appropriate selection of the precursor material and the temperature of the heat treatment allows to obtain carbonaceous materials with an electrocatalytic activity in alkaline medium very remarkable and very close to that of commercial catalysts based on platinum. In addition, these catalysts are resistant to carbon monoxide or methanol poisoning and have high stability. Therefore, the procedure described demonstrates the possibility of preparing carbonaceous materials with high electrocatalytic activity by means of a quick, simple and very low cost synthesis. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2725302A1
    申请号:ES201830278
    申请日:2018-03-21
    公开日:2019-09-23
    发明作者:Bermejo Javier Quilez；Nunez Emilia Morallon；Amoros Diego Cazorla
    申请人:Universidad de Alicante；
    IPC主号:
    专利说明:

    [0001] APPLICATION AS ELECTROCATALIZERS AND MATERIAL OBTAINED BY MEANS
    [0002]
    [0003]
    [0004]
    [0005] FIELD OF THE INVENTION
    [0006] The field of application of the present invention is the industrial energy sector, and in particular, the present invention is encompassed in the works in which electrocatalysts are used and where chemical transformations derived from electron transfer occur.
    [0007]
    [0008] STATE OF THE TECHNIQUE
    [0009] The present invention defines a process for the preparation of carbonaceous materials with excellent electrocatalytic properties in oxygen reduction, a reaction that occurs in the cathode of low temperature fuel cells. The use of different carbonaceous materials has already been studied for the oxygen reduction reaction; however, few have achieved a catalytic activity similar to that of commercial platinum-based catalysts and, those who have achieved it, have failed to reduce the cost of commercial catalysts due to the use of high-cost reagents and procedures of synthesis consisting of many stages of high economic value. By the appropriate selection of the different variables in the synthesis stage and the precursor, this procedure can produce carbonaceous materials with excellent electrocatalytic activity in the oxygen reduction reaction under alkaline conditions.
    [0010]
    [0011] In this sense, fossil fuels have been widely studied and used during the last centuries with the aim of generating energy, covering most of the world's energy demand. This large consumption of fossil fuels has generated two serious problems. The first of these is that it is not about unlimited resources and, therefore, sooner or later they will be depleted and new sources of energy will be needed to replace them. The second problem, and the one that currently has a greater social impact, are the serious damages that these types of fuels cause in human health and the environment. Some of its most prominent consequences are carbon dioxide emissions, responsible for global warming, and sulfur oxides and nitrogen emissions, responsible for acid rain, photochemical smog, or others.
    [0012] Therefore, the search for new forms of energy generation that diminish the harmful effects generated by fossil fuels and increase the energy efficiency of the devices used is essential.
    [0013]
    [0014] In this sense, in recent decades, hydrogen-based energy systems have been proposed as great candidates to solve these problems. Since then, an extensive number of research papers have been carried out that have established the basis of hydrogen-based energy systems.
    [0015]
    [0016] Hydrogen stands out as an excellent energy carrier with unique properties: it is the lightest, most efficient and cleanest fuel to date. In addition, it can be used for the generation of electricity by electrochemical processes with efficiencies higher than the current ones in thermoelectric plants that use fossil fuels and with minimal pollution. Thanks to this property, fuel cells (FCs) have emerged as the main choice of future for the automotive industries and for the storage and use of renewable energy.
    [0017]
    [0018] These devices are power generation systems, in which the overall reaction that occurs in the battery is a consequence of the individual reactions that occur in the half-cells that compose it: the oxidation reaction of the fuel (usually hydrogen or light alcohols such as methanol or ethanol) in the anode and the reduction of the oxidizer in the cathode (oxygen or air). The semi-cells are separated by a selective ion exchange membrane.
    [0019]
    [0020] In fuel cells, unlike other energy sources such as batteries, the fuel is continuously fed by an external source. In these devices, which use H 2 , the operation consists of feeding the fuel in the anode, where with the help of a catalyst, it oxidizes giving rise to protons and electrons. These protons migrate through the selective membrane to the cathode where they recombine with reduced oxygen species (which acts as a oxidizer) to create, as a final product, water. The generated water is expelled from the battery with an excess flow of oxygen. On the other hand, the electrons generated in the anode migrate through current collectors to the cathode doing a useful job. The net result is the current of electrons through the outer circuit that gives rise to an electric current.
    [0021] It is a fact that FCs are not yet a reality on a world scale. This is because, unlike fossil fuels, they are not fully optimized, that is, they have a series of limitations that impair the performance and efficiency of batteries. The main limiting factor of FCs is the use of platinum as a catalyst to accelerate electrode reactions (fuel oxidation and oxygen reduction). There are three reasons that explain the problem of using platinum as an electrocatalyst. In the first place, the low abundance of metal in nature derives from its high cost and, consequently, from the pile. In fact, the value of platinum is so high that it reaches a third of the total cost of the battery. On the other hand, if fuel cells were implanted in all vehicles, there would not be enough platinum considering the quantities currently used. In addition, one of the main problems is that the agglomeration of platinum nanoparticles during use, generates a significant reduction of their catalytic activity with working time. Finally, it is also known that small traces of carbon monoxide or methanol can completely poison the catalyst, rendering it useless. Finally, it should be noted that the oxygen reduction reaction is the main limiting factor of fuel cells because more than 90% of platinum is in the cathode because it is a slow reaction. Consequently, many lines of research are focused on the advancement and improvement of the cathode of fuel cells, which can generate a substantial difference in the development and commercialization of these devices.
    [0022]
    [0023] The oxygen reduction reaction can occur by two different mechanisms:
    [0024] 1. Through four electrons, which is the desired reaction in the fuel cells since in addition to producing a greater amount of energy, it prevents the formation of harmful by-products for the battery components (H 2 O 2 ).
    [0025] 2. Through a mechanism of two consecutive stages of two electrons, the first with the formation of hydrogen peroxide and the subsequent reduction of this to water. However, the H 2 O 2 reduction reaction occurs in parallel, blocking active sites for oxygen reduction, reducing the speed of the process.
    [0026]
    [0027] Due to the serious problems of the use of platinum as an electrocatalyst for the oxygen reduction reaction, an important research effort is being made in the search for new materials capable of replacing or removing this metal from the cathode. This work is mainly produced through two approaches:
    [0028] 1. Use of other metals of greater abundance in nature and, therefore, lower cost.
    [0029] 2. Development of catalysts exempt from the presence of metal, where there being no metal content radically reduces the cost of the electrode.
    [0030]
    [0031] There are several alternative research lines that seek to replace or reduce the platinum content with non-precious alternative metals, which can generate similar or superior characteristics, with a lower cost and greater stability and availability. There have been great advances in the development of catalysts based on low-cost non-precious metals for the oxygen reduction reaction, reaching values similar to that of the platinum catalyst. The search for more stable and active electrocatalysts in both acidic and alkaline media is of great importance. As a consequence, the development of platinum, silver, palladium or gold catalysts controlling both the size and structure and distribution of the nanoparticles have resulted in improvements with respect to spherical nanoparticles. On the other hand, numerous studies have focused attention on bimetallic catalysts based on platinum and noble metals such as palladium and gold and also with other metals such as cobalt, nickel, iron, magnesium, chromium and vanadium in order to reduce the cost and improve the electrocatalytic activity of the oxygen reduction reaction. The limitations of this first group are directly related to the use of metal and the problems that these entail:
    [0032] • The use of metals continues to result in a high cost of the catalyst.
    [0033] • The nanoparticles of the metals agglomerate producing a loss of catalytic activity and the consequent low durability. Within this group there are quite a few materials that have been previously patented.
    [0034]
    [0035] Another alternative is the use of an innovative class of metal-free catalysts, which, because it does not contain metals, should reduce the cost of these cathodes for the oxygen reduction reaction on a large scale and can even increase the efficiency of the batteries. made out of fuel. Unfortunately, metal-free catalysts do not, today, meet the objective of cost reduction, due to the use of very expensive precursors, such as graphene, and because the synthesis procedure involves many, long and expensive stages that They raise the price of the electrocatalyst to values close to those of commercial platinum. To our knowledge, some problems of the platinum catalyst have not been eliminated by means of metal-free catalysts, either due to the high price of the precursor material, the multiple and expensive stages, which end up raising the cost of the process to values similar to those of commercial catalyst or because its activity is very far from that of platinum.
    [0036] Within this group of metal-free catalysts, carbonaceous materials stand out, being widely studied for this reaction since they present a surface chemistry that can be modified to introduce new active sites. The physical, chemical, optical and electronic properties of these carbonaceous materials vary according to the allotropic form and will depend largely on the surface and morphological composition of the materials. These properties can be modified by functionalizing the carbonaceous materials with the introduction of heteroatoms, thus increasing the utility in a greater number of applications. Doped with heteroatoms is equivalent to the introduction of atoms other than carbon. Carbon materials doped with heteroatoms such as nitrogen have a very interesting behavior as catalysts in different reactions.
    [0037]
    [0038] Taking into account the possibilities of introducing nitrogen atoms in carbonaceous materials, the chemical-physical changes they generate and the variety of nanostructures of carbonaceous materials that exist, we can find numerous studies on the incorporation of these heteroatoms and their impact on the properties catalytic for the oxygen reduction reaction.
    [0039]
    [0040] These materials can be synthesized by various methods:
    [0041] 1. Carbonization of a material rich in nitrogen.
    [0042] 2. Reaction between a nitrogen compound and a porous carbonaceous material.
    [0043] 3. Hydrothermal carbonization of a suitable precursor.
    [0044]
    [0045] Of the three methods mentioned above, the first one can be performed using nitrogen-containing polymers in its polymer chain, with the advantage that the starting compound has a defined chemical structure and can be synthesized by different chemical processes.
    [0046]
    [0047] Among these materials, very interesting precursors for the synthesis of carbonaceous materials are conductive polymers with high nitrogen content (such as, for example, polyaniline and polypyrrole) since they can be obtained by chemical and electrochemical methods. In addition, it is known that its carbonization leads to materials with a high nitrogen content and with interesting properties for different applications. This is because nitrogen has a similar atomic size but a different electronic configuration than carbon. Consequently, by doping a carbonaceous material with nitrogenous groups the electronic properties of the material are modified making them possible form active sites for different reactions such as the oxygen reduction reaction.
    [0048]
    [0049] Polyaniline (PANI) has been studied as a precursor of carbonaceous materials with high nitrogen content for use as a support of metal catalysts for the oxygen reduction reaction. However, it is also possible to find some works in which PANI has been used as a precursor to nitrogen-rich carbonaceous materials in order to be used as metal-free catalysts. All these investigations can be classified into two different groups:
    [0050] 1. Carbonaceous materials in a predefined structure, where PANI is polymerized in the preliminary structure (support) and subsequently carbonized.
    [0051] 2. Direct heat treatment of PANI in an inert atmosphere at low temperatures.
    [0052]
    [0053] However, there is an important technical aspect that is not resolved and that is that the catalytic activity and / or the number of electrons of all these materials is still too low, indicating that the reaction does not produce water as the final product and prevents Take your production on a large scale. Therefore, the electrocatalysts published above are not sufficiently electrocatalytic due to the large generation of products detrimental to the optimal functioning of the cell such as hydrogen peroxide. However, some studies manage to comply, or are close to it, with the objective of catalytic activity and the number of electrons. All of them are obtained from aniline supported by a preliminary slaughter structure where it is polymerized and subsequently heat treated. The preliminary structure generates a further stage in the synthesis process, increases the number of reagents and increases the duration of catalyst production, so the final manufacturing cost is definitely raised.
    [0054]
    [0055] Alternative methods have been sought to generate carbonaceous materials from PANI with high catalytic activity and low production of H 2 O 2 through a single stage, in order to reduce production costs, although significant results have not been achieved. For this, the synthesis stage chosen was a thermal treatment of PANI powder. In this process an oxidizing atmosphere was used during the heat treatment instead of an inert atmosphere. Although improvements were made in the catalytic activity and in the number of electrons and very valuable information about the nature of the active sites in the oxygen reduction reaction, the catalytic activity of the samples obtained was still far from that obtained by the commercial catalyst based on platinum, and by Both the technical problem remains unresolved, nor does it seem obvious to obtain a positive result based on this development.
    [0056]
    [0057] In addition, there are antecedents such as the article “ Effect of carbonization conditions of polyaniline on its catalytic activity towards ORR. Some insights about the nature of the active sites ” by Quílez-Bermejo J et al ; where reference is made to the synthesis of electrocatalysts for fuel cells starting from PANI. This document differs from the present invention by the use of temperatures below 800 ° C. The low temperatures of the heat treatment do not allow to reach a degree of structural order as advanced as that obtained at temperatures above 1100 ° C. This structural order directly affects the electrical conductivity and chemical composition and, consequently, the catalytic activity of the carbonaceous materials. The temperature parameter used is the fundamental aspect of this invention.
    [0058]
    [0059] While there are a large number of jobs that perform thermal treatments of polymers containing aniline in their monomer units, those that have applied temperatures above 1100 ° C are very scarce. The lack of studies at these temperature values is mainly due to the fact that conventional furnaces usually work at temperatures below 1000 ° C, so these are the ones that are frequently used. In cases where temperatures above this value have been used (for example, Catalysts, 51034-1045, 2015; Carbon, 102, 346-356, 2016), the results found of activity are lower than those of lower temperatures . However, there is a possibility that these results are influenced by the presence of high-activity metal impurities. In the first case, the polymerization of the aniline is carried out with iron chloride (111), so that traces of this metal may remain after the preparation of the carbonaceous material that can explain the great catalytic activity obtained at temperatures below 1100 ° C . However, at higher temperatures the iron species can react with the carbonaceous material forming substances of low activity. In the second case, the authors indicate that they cannot rule out the presence of metallic impurities generated during the preparation of the graphene oxide used to support the polyaniline.
    [0060]
    [0061] Also known in the article " Facile Synthesis of Nitrogen and Sulfur Dualdoped Hierarchical Micro / mesoporous Carbon Foams as Efficient Metal-free Electrocatalysts for Oxygen Reduction Reaction " by Jiang Shu et al ; which, although not subject to study, refers to the preparation of suitable PANI-based composite materials for application as electrocatalysts in fuel cells. This document, again, differs from the present invention in the use of temperatures below 1000 ° C in heat treatments, while in the present invention higher temperatures are required to achieve the desired catalytic activity and therefore with this antecedent the previously highlighted objective problem is not solved. In addition, this work uses a template or slaughter material where PANI is deposited and subsequently carbonized. This slaughter structure increases the manufacturing cost, not only because of the cost of more reagents, but also because of the increase in time spent in the synthesis of carbonaceous materials, which means that it is not a feasible solution.
    [0062]
    [0063] Finally, it is also known what is disclosed in the article “ Metal-free nitrogen-doped carbon nanoribons as highly efficient electrocatalysts for oxygen reduction reaction” by Huang Jinzhen et al; where reference is made to the preparation of what they call a doped carbon nanocint with nitrogen (N-doped carbon nanoribbon NCNR) using a melamine fiber coated with PANI which they call PANI @ MF using heat treatments with temperatures below 1000 ° C. This document differs from the present invention in the use of a structure of sacrifice such as melamine fibers, in which they are subsequently coated with PANI, as in the work of Jiang Shu et al commented above, the differences with the present invention are: the use of temperatures below 1000 ° C and the use of sacrificial structures, which raise the cost by increasing the number of reagents and by the need for another stage of synthesis.
    [0064]
    [0065] In the work presented by Quílez-Bermejo et al it is clearly shown that temperatures below 800 ° C are not sufficient to achieve a catalytic activity close to that obtained by platinum (more than 0.2V difference in the potential for initiation of reaction ) due to the low structural order and electrical conductivity obtained by the low temperatures.
    [0066]
    [0067] On the other hand, the work of Jiang Shu et al and Huang Jinzhen et al in addition to employing temperatures below 1000 ° C, use sacrifice structures that raise the cost of the catalysts and increase the synthesis time.
    [0068]
    [0069] Taking these aspects into account, although the heat treatment of polymers containing aniline in their monomer units is known within the state of the art, it is advanced that the present invention differs from all of them in two fundamental aspects:
    [0070] 1. The temperature of the heat treatment of polymers containing aniline in their monomer unit is greater than 1100 ° C to obtain carbonaceous materials with an improved structural order and electrical conductivity and catalytic activity.
    [0071] 2. Sacrificial structures that increase the number of synthesis stages and manufacturing cost are not used. In this way, this invention is able to provide a method of synthesis of excellent electrocatalysts more easily.
    [0072]
    [0073] Given the background in the state of the art, it can be seen that none of the known and indicated methodologies solves the technical problem of obtaining a carbonaceous material with a high catalytic activity capable of replacing the current platinum catalysts.
    [0074]
    [0075] DESCRIPTION OF THE INVENTION
    [0076] By means of the present invention a simpler synthesis process is achieved in comparison with those existing to date and with excellent catalytic activity in the reaction of oxygen reduction in alkaline medium. These materials, due to their excellent catalytic properties, are presented as great candidates to replace the platinum catalyst in alkaline medium and, consequently, reduce the cost of the fuel cell. In addition, the materials synthesized by this invention show greater stability than commercial catalysts and resistance to methanol poisoning, which opens its application to the direct oxidation methanol fuel cell.
    [0077]
    [0078] The invention relates to a low cost synthesis method of metal free catalysts for the oxygen reduction reaction from a nitrogen rich polymer such as PANI or copolymers containing aniline in their monomer units and without the need for use supports or other materials that increase the cost of the product. With the same objective, the present invention shows a new working route just studied previously for the synthesis in a stage of carbonaceous materials with excellent catalytic activity: the thermal treatment of PANI at elevated temperatures.
    [0079]
    [0080] This synthesis procedure, for which the use of template or slaughter materials is not necessary, allows to obtain carbonaceous powdered materials with excellent catalytic activity and minimum generation of hydrogen peroxide, so that they can be used as electrocatalysts of the reduction reaction of oxygen that happens in the cathode of the fuel cells in alkaline medium.
    [0081] The kinetic parameters obtained by this catalyst consist of a 0.94V reaction initiation potential, 0.85V mid-wave potential, a limit current density equal to that of a platinum-based electrocatalyst, 5.8 mA -cm-2, and a number of electrons in the useful potentials of the fuel cell (0.6 - 1.0 V vs. RHE) very close to four, corresponding to a reduction in oxygen through a mechanism of four electrons, which implies the total reduction to water, thus avoiding the formation of harmful by-products for the optimal functioning of the battery. These materials, as previously advanced, due to their excellent catalytic properties, are presented as great candidates to replace the platinum catalyst in alkaline medium and, consequently, reduce the cost of the fuel cell
    [0082]
    [0083] Therefore, the object of the present invention relates to a synthesis process. This synthesis of carbonaceous materials is made from a procedure that does not use any other template or slaughter material and is based on:
    [0084] • A pretreatment to avoid any contamination of the atmosphere for which a purge is performed.
    [0085] • An inert atmosphere heat treatment of polyaniline or copolymers containing aniline as a monomer at temperatures above 1100 ° C, from which carbonaceous materials with a high nitrogen content are derived. For this, a flow at 100-200 mL / min is used during the pyrolysis, with a heating ramp used of 5 ° C / min to the target temperature where it is maintained for one hour, in order to obtain a constant mass value which indicates complete carbonization at the target temperature
    [0086]
    [0087] Specifically, the method object of the invention comprises the following steps:
    [0088] a) Introduction of a powder precursor consisting of a nitrogen-rich copolymer in which the aniline is maintained as a monomer unit in a small ship where it is distributed homogeneously;
    [0089] b) Introduction of the boat in a hermetically sealed oven;
    [0090] c) Purging the oven with inert gas;
    [0091] d) Heating the oven gradually to a temperature above 1100 ° C; e) Maintenance of said constant temperature;
    [0092] f) Gradual cooling to room temperature, maintaining until now the controlled atmosphere; Y
    [0093] g) Obtaining the carbonaceous material synthesized in powder.
    [0094] Specifically, the vessel used in the procedure is quartz and the oven is tubular.
    [0095]
    [0096] In addition, the temperature in the oven is controlled by means of a thermocouple introduced at a point near the center of said vessel.
    [0097]
    [0098] The purge is carried out at a flow rate of at least 100mL / min, preferably between 100-200 mL / min; and the purge time is variable depending on the size of the oven, preferably around 60 minutes. Preferably the inert gas used is pure argon.
    [0099]
    [0100] The oven is gradually heated with a ramp of 5 ° C / min until the maximum temperature is obtained, which is a temperature above 1100 ° C.
    [0101]
    [0102] The flow used in the gradual heating of the heat treatment is between 100 and 200 mL / min.
    [0103]
    [0104] The temperature obtained after gradual heating remains constant for a period between 20 and 60 minutes.
    [0105]
    [0106] In this way, a carbonaceous material is obtained whose composition in atomic percentage is at least 90% in C; between 1 and 3% in N; less than 2% in H; and between 4 and 6% in O. These components are combined in such a way that 100% of the percentage results from their sum.
    [0107]
    [0108] It is emphasized that for the heat treatment there are two factors that must be taken into account, which are the temperature of the treatment and the precursor used.
    [0109]
    [0110] To illustrate the importance of the final temperature of the heat treatment, an exhaustive study of the carbon materials synthesized at different temperatures has been carried out.
    [0111]
    [0112] In this study a clear trend is observed in which the increase in temperature generates important changes in the catalytic activity, reaching values as high in the samples obtained at high temperatures as those obtained by the commercial catalyst, based on platinum nanoparticles.
    [0113] Another aspect demonstrated by this invention is the fundamental role that the precursor plays in obtaining carbonaceous materials with excellent properties in the oxygen reduction reaction.
    [0114] While the improvement in catalytic activity with increasing temperature is appreciable for other nitrogen-rich polymers, only PANI and derivatives thereof produce an activity with values close to those of the commercial platinum catalyst.
    [0115]
    [0116] Thus, by controlling these two fundamental factors, it is possible to synthesize metal-free electrocatalysts for the oxygen reduction reaction with an excellent activity towards the formation of water (H 2 O), without production of H 2 O 2 and with a radically cost lower than that obtained by the commercial platinum-based catalyst.
    [0117]
    [0118] Another favorable aspect of these electrocatalysts lies in their simple synthesis, in which simply by taking PANI as a precursor and performing a heat treatment at high temperatures in a controlled inert atmosphere, we get carbonaceous materials with a catalytic activity equivalent to that obtained by commercial catalysts, based in platinum In addition, its simple and economical synthesis facilitates its possible production on an industrial scale.
    [0119]
    [0120] This method stands out for its simplicity, versatility and reduced cost, the following being its main advantages:
    [0121] • Uses affordable materials for any laboratory or industry and does not require special equipment.
    [0122] • The synthesized carbonaceous materials are very easy to handle, dispersing without difficulty in aqueous medium and at room temperature. Fact that facilitates its conformation as electrodes for application in a fuel cell.
    [0123] • The cost of these materials is radically lower when compared to the currently marketable catalysts, without losing the excellent electrocatalytic activity they present.
    [0124] • The stability obtained with the use of these materials is very advantageous, which would provide a longer working time of the electrodes in the fuel cell or in the metal-air batteries.
    [0125] • The synthesized carbonaceous materials have resistance against methanol poisoning which also makes them candidates to act on methanol fuel cells.
    [0126] • The synthesized carbonaceous materials have the great advantage of being synthesized in a single stage, without the use of template or slaughter materials, whereby it is possible to obtain catalysts with excellent activity and with a very low manufacturing cost.
    [0127] • The oxygen reduction, using the carbonaceous materials described in this invention as electrocatalysts, is carried out through a four-electron mechanism and therefore as a reaction product is water. This fact implies a minimum production of reaction intermediates that decrease the energy density of the device and can also damage the fuel cell.
    [0128]
    [0129] Therefore, this invention involves a technical leap against what was previously known, since with a synthesis step it is possible to obtain low-cost carbonaceous materials with excellent catalytic activity in the oxygen reduction reaction and excellent selectivity in this reaction to the water formation, done only previously using expensive reagents or with long and expensive stages that increase the final product cost.
    [0130]
    [0131] In addition, its resistance to methanol or carbon monoxide poisoning and its high durability make these materials very attractive electrocatalysts from an industrial point of view and great candidates to replace the platinum of low temperature fuel cells.
    [0132]
    [0133] BRIEF DESCRIPTION OF THE FIGURES
    [0134] Figure 1: Representation of the yields obtained in the thermal treatments of PANI at different temperatures.
    [0135]
    [0136] Figure 2: (a) Yield of H 2 O 2 and (b) curves for linear sweep voltammetry carbonaceous materials in 0.1M KOH solution saturated in O 2 to 5 mV s-1 and 1600 rpm.
    [0137]
    [0138] Figure 3: Study of stability and methanol poisoning of the treated sample at 1100 ° C under nitrogen atmosphere (blue line) and the commercial catalyst based on platinum (black line).
    [0139]
    [0140] Figure 4: (a) Performance in H 2 O 2 and (b) linear scan voltammetry curves for thermally treated PANI at 1100 ° C (blue line) and a piperazine-based copolymer and aniline in a 5: 1 ratio (green line), in 0.1M KOH solution saturated in O 2 at 5mV s "1 and 1600 rpm.
    [0141]
    [0142] Figure 5: (a) Performance in H 2 O 2 and (b) linear scanning voltammetry curves for PANI heat treated at 1100 ° C in nitrogen (blue line) and PANI treated at 1100 ° C in argon atmosphere ( orange line), in 0.1M KOH solution saturated in O 2 at 5mV s-1 and 1600rpm.
    [0143]
    [0144] Figure 6: (a) Performance in H 2 O 2 and (b) linear scan voltammetry curves for PANI heat treated at 1100 ° C in nitrogen with a flow of 100mL / min (blue line) and PANI treated at 1100 ° C in the same atmosphere with a flow of 200mL / min (brown line), in 0.1M KOH solution saturated in O 2 at 5mV s-1 and 1600rpm.
    [0145]
    [0146] DETAILED DESCRIPTION
    [0147] The described procedure shows the synthesis of carbonaceous materials doped with nitrogen that act as electrocatalysts in the oxygen reduction reaction with high yield.
    [0148]
    [0149] Said treatment is carried out in a horizontal tubular furnace by gradual heating from room temperature to the desired temperature to subsequently keep that temperature constant over time. The heat treatments were performed in the presence of an inert atmosphere. Prior to treatment, a purge was maintained for one hour for all gas streams.
    [0150]
    [0151] In this sense, the elements common to all the examples presented here are described below to illustrate the invention.
    [0152]
    [0153] The precursor used in all the examples, shown below, is introduced into a quartz boat where it is distributed homogeneously. The quartz vessel has an extension of 10cm in length and 1cm in width.
    [0154]
    [0155] A hermetically sealed tubular furnace is chosen in this study as a model of contribution and temperature control for heat treatments. Its diameter is 4cm, while it covers 60cm in length.
    [0156] The vessel is introduced inside the oven and the temperature is controlled by the use of a thermocouple, introduced 1cm from the center of the vessel, that is, at a point near the center of the vessel.
    [0157]
    [0158] Prior to the heat treatment, a one-hour purge with the inert gas is carried out previously in order to accurately control the atmosphere used in the heat treatments and minimize the likelihood of impurities in the heat treatment atmosphere. For this purge the flow used is 100mL / min. The inert gas used was argon, although other inert gases could be used.
    [0159]
    [0160] The heat treatment of the PANI or derivatives thereof at temperatures above 1100 ° C was carried out with a gradual heating of 5 ° C / min to the maximum target temperature where it remains constant for one hour. The flow in the gradual heating is preferably between 100 and 200 mL / min. The time in which the temperature remains constant is chosen due to thermogravimetric studies that showed that after twenty minutes at that temperature there were no mass changes. In order to ensure these constant values, it is decided to use a time three times greater, for a total of one hour at target temperature, that is, the time is between 20 and 60 minutes.
    [0161]
    [0162] Once the heat treatment is finished, cooling occurs gradually to room temperature, keeping the controlled atmosphere up to this point. This is done in order to avoid any alteration of the material due to external factors not associated with the atmosphere and temperature used in the carbonization process.
    [0163]
    [0164] In this way, as previously advanced, a carbonaceous material is obtained whose composition in atomic percentage is at least 90% in C; between 1 and 3% in N; less than 2% in H; and between 4 and 6% in O. This carbonaceous material obtained has an excellent catalytic activity in the oxygen reduction reaction, in which the reaction initiation potential, the mid-wave potential and the limit current density are similar or superior to that of the commercial platinum catalyst, also showing a selective electrocatalysis made a direct reduction to water, where the H 2 O 2 is reduced to values close to 0% in the useful range of the fuel cells. In addition, it has high resistance to methanol poisoning and its stability significantly exceeds that of the commercial platinum-based catalyst.
    [0165] With the carbonaceous material already synthesized and in order to study its electrocatalytic activity these materials were subjected to characterization via rotary disk-ring electrode (RRDE). As the working electrode, an electrode equipped with a glassy carbon disk (5.61mm in diameter) and a platinum ring were used, as a platinum wire as a counter electrode and as a reference electrode a reversible hydrogen electrode (RHE) submerged in the Work electrolyte The amount of catalyst employed was optimized, reaching the highest limit current density for 120 ^ g. Therefore, the vitreous carbon disc was modified with the samples using 120 ^ L of a suspension of 1mg / mL (20% isopropanol, 0.02% Nafion®), finally obtaining a catalyst load of 48mg / cm2 . The electrolyte used in the study consisted of 0.1M KOH. The production of by-product (H 2 O 2 ) formed and the catalytic activity of all the carbonaceous materials synthesized under the conditions previously shown are shown in Figure 2a and Figure 2b, respectively.
    [0166]
    [0167] Table 1 shows an extensive comparison of the parameters that allow comparing the catalytic activity of the materials and the number of electrons involved in the reaction of the different catalysts described in the state of the art.
    [0168]
    [0169]
    [0170]
    [0171]
    [0172]
    [0173] Table 1
    [0174]
    [0175] EXAMPLE 1: THERMAL TREATMENT OF POLYANILINE AND ITS ELECTROCATALYTIC STUDY IN THE OXYGEN REDUCTION REACTION.
    [0176] The PANI used to illustrate this invention was synthesized by chemical route (the synthesis procedure is explained in the description of this invention).
    [0177]
    [0178] The heat treatment was carried out from a gradual heating of 5 ° C / min, typical of conventional ovens up to temperatures between 600 and 1200 ° C. The atmosphere used in this study is pure nitrogen. The precursor mass (PANI) used in the heat treatment was 150mg.
    [0179]
    [0180] In addition, the carbonaceous material resulting from the carbonization of PANI has a high dispersion capacity in aqueous medium, which facilitates the formation of suspensions for the subsequent preparation of the catalyst. These suspensions remain stable for a long period of time, making the electrode conformation practical.
    [0181]
    [0182] The pyrolysis performance is an important parameter in the synthesis of materials from an economic-industrial point of view. Figure 1 shows the yields of carbonizations performed at different temperatures. It is observable as this performance decreases as the temperature of the heat treatment increases, reaching a stable value around 1100 ° C. At that time the pyrolysis yield reaches 28%. This yield can be considered high when compared with the pyrolysis of other precursors, which reach values below 10%. This result makes the synthesis of these materials a highly efficient experimental procedure.
    [0183] On the other hand, Figure 2 shows the results of the discoanillo rotary electrode study. When a temperature higher than 1000 ° C is used in the heat treatment, a significant change occurs in the final carbonaceous materials. Specifically, above 1100 ° C these materials acquire a catalytic activity almost identical to that of the commercial platinum catalyst (Figure 2b) both in reaction initiation potential and in limit current density, which makes them clear candidates to replace the platinum in the fuel cells. Another aspect to consider is the number of electrons, closely related to the hydrogen peroxide performance (Figure 2a), which has been reduced to values below 5% in the useful range of fuel cells (0.6 -1.0V vs RHE) which prevents the formation of harmful by-products for the life of fuel cells.
    [0184]
    [0185] From Figure 2 it is also possible to observe how increasing the temperature of the heat treatment to values greater than 1100 ° C generates an improvement in the catalytic activity of the synthesized material.
    [0186]
    [0187] EXAMPLE 2: STUDY OF STABILITY AND RESISTANCE TO METHANOL.
    [0188] Another objective of this invention is that the excellent catalytic activity shown by the synthesized materials remains stable for a long period of time. Therefore, in order to demonstrate the greater stability of the carbonaceous materials, a 0.65V vs RHE chronoamperometric study was carried out in 0.1M KOH of the PANI_1100 material and of a 20% Pt / Vulcan commercial catalyst by comparison. (Figure 3). In the stability study, the chronoamperometric test has been carried out for two hours, where there is a clear improvement in the stability of the material synthesized by this invention (95%) compared to the commercial platinum catalyst (87%). In order to also demonstrate its resistance to methanol, after two hours of stability study, MeOH was added to a concentration in 1M working solution. The synthesized catalyst continued to work without loss of its catalytic activity (Figure 3). However, when this study has been carried out on the platinum catalyst, it quickly becomes poisoned and loses all of its catalytic activity in the presence of methanol.
    [0189]
    [0190] The excellent catalytic activity obtained, its high stability and resistance to methanol poisoning make them a very viable option for use as electrocatalysts of the oxygen reduction reaction.
    [0191] EXAMPLE 3: THERMAL TREATMENT OF A PIPERAZINE-ANILINE COPOLYMER IN NITROGEN ATMOSPHERE.
    [0192] Example 3 illustrates the possibility of using copolymers containing aniline instead of PANI. The synthesis of the copolymer was also carried out according to the steps explained in the description of the synthesis of PANI. For this, the polymerization concentrations used in the synthesis were piperazine: 5: 1 aniline.
    [0193]
    [0194] The thermal treatment of this copolymer was carried out by gradual heating (5 ° / min) to a temperature of 1100 ° C in a pure nitrogen atmosphere in order to demonstrate that the copolymerization of different monomers, one of them being aniline, can also result in carbonaceous materials with excellent catalytic electroactivity.
    [0195]
    [0196] The carbonization yield of the copolymer is 18%, which makes PANI a more viable and effective precursor in the synthesis of electrocatalysts in the oxygen reduction reaction.
    [0197]
    [0198] Again, these nitrogen-rich carbonaceous materials show high activity (Figure 4), close to that of the commercial platinum catalyst and very similar to those obtained by heat treatment at 1100 ° C of aniline as the sole polymerization monomer.
    [0199]
    [0200] These results suggest that the presence of aniline in the polymer is decisive for obtaining excellent catalytic activity for the oxygen reduction reaction. However, the use of other monomers can lead to small variations in carbonization performance.
    [0201]
    [0202] EXAMPLE 4: THERMAL TREATMENT OF POLYANILINE AT 1100 ° C IN ARGÓN ATMOSPHERE.
    [0203] In the present example, the heat treatment in the presence of a heavy noble gas is aimed at reducing the probability of air entering and verifying that the high activity of the materials obtained under a nitrogen atmosphere is not due to the atmosphere used during the treatment. Thus, a gradual heating was carried out at 5 ° C / min to a temperature of 1100 ° C under an argon atmosphere. The heat treatment performance was identical to that obtained by nitrogen atmosphere.
    [0204]
    [0205] In this sense, the synthesized carbonaceous material was studied by means of a rotary disk-ring electrode. It is here that it was shown that, in fact, there are no differences between the use of both atmospheres (Figure 5) and, therefore, they maintain their inert property during the entire temperature range studied in the heat treatment. The excellent catalytic activity is still obtained from the thermal treatment of PANI and derivatives thereof in the presence of a pure argon atmosphere, which rules out any possible influence of the atmosphere on the resulting material and its properties.
    [0206]
    [0207] EXAMPLE 5: THERMAL POLYNILINE TREATMENT AT 1100 ° C WITH A HIGHER FLOW SPEED.
    [0208] The objective of example 5 is to illustrate that the mass / flow ratio does not influence the obtaining of excellent catalysts. For this example the same mass of 150mg of PANI and a flow of 200mL / min has been used. The nomenclature used to describe the material synthesized with a flow of 200mL / min was PANI_2N2_1100.
    [0209]
    [0210] The carbonization yield was 27% versus 28% of the heat treatment with a flow of 100mL / min. In other words, the mass / flow ratio does not modify the final pyrolysis performance.
    [0211]
    [0212] In the study of rotary disk-ring electrode shown in Figure 6, it can be observed that there are no differences between both samples and that, consequently, it can be said that the modification of the mass / flow ratio does not affect the catalytic activity final of the materials synthesized by this invention.
    权利要求:
    Claims (11)
    [1]
    1. - Method of synthesis of carbonaceous materials for application as electrocatalysts, where the use of template or slaughter materials is not required, and where a material usable as an electrocatalyst of the oxygen reduction reaction that occurs in the cathode is obtained of fuel cells, which includes the following steps:
    a) introduction of a powder precursor consisting of a nitrogen-rich copolymer in which the aniline is maintained as a monomer unit in a small ship where it is distributed homogeneously;
    b) introduction of the vessel into a hermetically sealed oven;
    c) purging the oven with inert gas;
    d) gradually heating the oven to a temperature above 1100 ° C; e) maintaining said constant temperature;
    f) gradual cooling to room temperature, keeping the controlled atmosphere up to this point; Y
    g) obtaining the carbonaceous material synthesized in powder.
    [2]
    2. - Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the vessel is made of quartz.
    [3]
    3. - Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the furnace is tubular.
    [4]
    4. - Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the temperature in the furnace is controlled by means of a thermocouple introduced at a point near the center of the vessel.
    [5]
    5. Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the purge is carried out at a flow of at least 100mL / min.
    [6]
    6. Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the purge time is 60 minutes.
    [7]
    7. Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the inert gas is pure argon.
    [8]
    8. - Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the heating of the furnace is carried out gradually with a ramp of 5 ° C / min.
    [9]
    9. - Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the flow used in the gradual heating of the heat treatment is between 100 and 200 mL / min.
    [10]
    10. - Method of synthesis of carbonaceous materials for application as electrocatalysts, according to claim 1, characterized in that the temperature obtained after gradual heating is kept constant for a time between 20 and 60 minutes.
    [11]
    11. - Carbonaceous material obtained from a process as defined in any of the preceding claims, characterized in that its composition in atomic percentage comprises at least 90% in C; between 1 and 3% in N; less than 2% in H; and between 4 and 6% in O.
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    WO2019180281A1|2019-09-26|
    ES2725302B2|2020-01-29|
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    PCT/ES2018/070728| WO2019180281A1|2018-03-21|2018-11-09|Method for synthesising carbonaceous materials for application as electrocatalysts and material obtained using said method|
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