ES2994056B2 - SYNTHESIS PROCEDURE FOR A CARBONOSE MATERIAL AND ITS USE AS AN ELECTROCATALYST, ESPECIALLY FOR OXYGEN REDUCTION - Google Patents

Description

[0001] DESCRIPTION

[0003] PROCEDURE FOR SYNTHESIS OF A CARBONOSE MATERIAL AND ITS USE AS AN ELECTROCATALYST, ESPECIALLY FOR THE REDUCTION OF OXYGEN

[0005] FIELD OF THE INVENTION

[0007] The field of application of the present invention is the energy industry sector and, in particular, the present invention focuses on the use of biomass waste to obtain high-performance materials for industries that manufacture or use fuel cells.

[0009] STATE OF THE ART

[0011] The massive use of fossil fuels has led to an energy crisis that demands a shift towards a renewable energy economy. Although significant steps have been taken towards decarbonizing the energy system, there are major limitations to its implementation in sectors such as transportation, which is the second largest contributor to CO2 emissions and the primary driver of the increase in greenhouse gas emissions in recent years. The use of batteries in certain transportation sectors is not technologically viable, making the development of technology for the generation and consumption of renewable energy using green hydrogen essential. Beyond this sector, hydrogen is a potential energy carrier since it can be produced from renewable energy sources, stored for long periods, and subsequently used to generate electricity during periods of high energy demand. Fuel cells play a crucial role in this application.

[0013] In this regard, in the consumption of green hydrogen, the electrochemical route using fuel cells has attracted the most scientific interest due to the high efficiency of this technology compared to combustion. However, this is not a reality on a global scale due to the large amount of platinum required in the catalyst for the oxygen reduction reaction (ORR) at the cathode. Platinum nanoparticles supported on carbonaceous materials, generally carbon black, have shown the highest efficiency for ORR and the hydrogen oxidation reaction (HOR). However, ORR has slower kinetics and therefore requires a higher amount of Pt (between 0.2 and 0.4 mg Pt·cm<-2> at the cathode compared to 0.05 mg Pt·cm<-2> at the anode), so the biggest problem lies in the electrocatalyst for ORR. Furthermore, in commercial platinum catalysts, the agglomeration of metallic nanoparticles during catalyst use causes a significant reduction in catalytic activity over fuel cell operating time. The most developed fuel cells are low-temperature proton exchange polymer membrane fuel cells (PEMFCs), but in recent years, interest in and development of alkaline-mediated fuel cells (AAEMFCs) has increased considerably, as the activity of non-noble metal-based materials is superior in alkaline media [Z. Yang et al. J. Power Sources (2013, 236, 238-249); Z. Xueqiang et al. J. Energy Chem. (2016, 25, 967-984)].

[0015] A crucial parameter for ORR is the selectivity of the materials, as the oxygen reduction reaction can occur via two different mechanisms, yielding different products. The first mechanism involves reduction through the exchange of four electrons, resulting in the formation of water (H₂O). This is the desired reaction in fuel cells because it produces a greater amount of energy. On the other hand, if the reaction proceeds via a two-electron step, the product formed is hydrogen peroxide (H₂O₂). This is a product of interest to many industries, but in the context of fuel cell energy generation, its production must be minimized as it reduces device efficiency and causes corrosion of system components. While commercial platinum-based catalysts exhibit very high selectivity for the four-electron pathway, many materials exhibit mixed selectivity, where the hydrogen peroxide produced can block active sites for oxygen reduction and even damage cell components.

[0017] Due to the significant limitations of using platinum as an electrocatalyst for ORR, the scientific community is dedicating considerable effort to obtaining new materials to replace Pt in the cathode. This is being carried out primarily through two approaches:

[0018] 1. Metal-based catalysts: non-precious metals (e.g. iron, nickel, chromium, etc.) or based on low content of noble metals such as Pt, Pd or Au, usually forming alloys to decrease the content of these.

[0019] 2. Metal-free catalysts, generally carbonaceous materials doped with heteroatoms (e.g. nitrogen, sulfur, etc.).

[0021] Carbonaceous materials have unique characteristics that make them ideal for use in a wide range of electrochemical applications. These include high stability in strongly acidic or basic media, availability and low cost, and their versatile surface chemistry, which can be modified to introduce new active sites. Regarding the oxygen reduction reaction, metal-free carbonaceous materials have demonstrated great potential, exhibiting excellent catalytic activities, as exemplified by the materials patented in the invention by J. Quílez-Bermejo et al. (ES2725302A1). However, metal-free materials have limitations in other electrochemical reactions, such as those that occur in electrolyzers or other reactions of interest.

[0023] Furthermore, carbonaceous materials doped with nitrogen and metal sites have shown great potential in electrocatalysis, as the nitrogen species can interact with the metal species, reducing the size of the active phase and improving the catalytic activity towards the oxygen reduction reaction. This has been reported in numerous studies such as Dahuan Li et al. J. Solid State Electrochem. (2020, 24, 1105-1112), Guanghua Wang et al. Int. J. Hydrog. Energy (2016, 41(32), 14101-14110), and Yahao Li et al. J. Energy Chem. (2021, 55, 572-579), among many others, or the electrocatalyst patented in the invention by J. V. Armel-Marie et al. (CN107949941A). Furthermore, modifying the metallic species allows for a wide range of possibilities to encompass other electrochemical reactions of interest. The preparation of bimetallic species is of great interest because they can exhibit higher activities than the metals of which they are composed due to synergistic effects. An example of materials based on bimetallic alloys with iron species can be found in the work published by Fang Shen et al. Appl. Catal. B (2020, 278(5), 119327).

[0024] Carbonaceous materials used as active phase supports for electrocatalysis can be of very diverse origins. One of the most widely used is carbon black, which is produced by the incomplete combustion of petroleum products. However, biomass waste is considered a resource of high interest by the European Commission because it allows for the production of high-performance materials, fuels, high-value-added chemicals (levulinic acid, gamma-valerolactone, etc.), and energy. Furthermore, its use reduces dependence on non-renewable sources and promotes sustainable management of natural resources. The use of carbonaceous materials obtained from biomass waste has been extensively studied for electrochemical applications, especially for use in supercapacitors.

[0026] Regarding the production of high-performance carbonaceous materials from biomass waste, the precursor must undergo carbonization treatment to achieve adequate electrical conductivity. Furthermore, to convert biomass precursors into high-performance carbonaceous supports, they are typically subjected to an activation process to generate porosity. This results in a support where the active sites can be distributed homogeneously, leading to superior catalytic properties in terms of activity and durability.

[0028] Hydrothermal carbonization has emerged as a process of great interest for obtaining carbon materials from biomass waste. It consists of a thermochemical conversion that uses temperature and pressure to convert carbon-rich compounds into carbon materials. Hydrothermal carbonization yields three distinct fractions:

[0029] ● solid phase, where the carbon-rich precursors are found,

[0030] ● liquid phase, from which some products of interest can be extracted,

[0031] ● gaseous phase, which is generally difficult to utilize.

[0033] On the other hand, the activation treatment to develop porosity in the carbonaceous material from carbonization can be carried out using physical or chemical methods. Chemical activation is more efficient because it is performed at lower temperatures and occurs more quickly, resulting in higher yields and greater porosity development. Chemical activation consists of impregnating the carbonaceous precursor with an activating agent and subjecting it to heat treatment. Some of the most common activating agents are sodium hydroxide (NaOH), potassium hydroxide (KOH), zinc chloride (ZnCl₂), or phosphoric acid (H₃PO₄). However, chemical activation also has drawbacks, such as the environmental impact of the washing stage to remove the activating agent residue and the corrosion generated during the activation process.

[0035] Although numerous studies have focused on preparing carbonaceous materials for electrocatalytic applications using biomass waste as a precursor, all these procedures require catalyst washing in acidic media, water, or even organic solvents. Some studies achieve or come close to achieving the stated objective in terms of catalytic activity and selectivity for the oxygen reduction reaction in basic media. However, these washing processes have a significant economic and environmental cost, hindering the industrial scale-up of these materials. Furthermore, more than one high-temperature heat treatment is typically applied during the preparation process.

[0037] The article “Biomass-derived porous heteroatom-doped carbon spheres as a high-performance catalyst for the oxygen reduction reaction” by Guangua Wang et al. refers to the synthesis of electrocatalysts for fuel cells using biomass waste as a precursor for the carbonaceous material. The results obtained for catalytic activity and selectivity are comparable to those of the commercial platinum-based catalyst. However, this study differs from the present invention by using 1 M sulfuric acid washes and three thermal treatments: (1) 900 °C for 60 minutes in a N₂ atmosphere, (2) 900 °C for 165 minutes in a N₂ atmosphere, and (3) 900 °C for 15 minutes in an NH₃ atmosphere.

[0039] Also known is the information published in the article “A novel honeycomb Fe-N-C composition derived from wheat flour as an efficiency catalyst for the oxygen reduction reaction” by Dahuan Li et al., which refers to the preparation of catalysts based on iron supported on a nitrogen-doped carbonaceous material for the oxygen reduction reaction using wheat flour as a biomass precursor. In this work, similar activity to the commercial catalyst based on platinum nanoparticles is obtained. However, SiO₂ is used as the template material. This must be subsequently removed using a hydrofluoric acid solution and then washed several times with distilled water. Furthermore, the thermal treatment of the preparation consists of two stages at 800°C for 2 hours each.

[0041] Also known is the material patented in the invention by Hao Xiaoqiong et al. (CN110474057A), which refers to a material obtained by hydrothermal treatment, high-temperature pyrolysis, and subsequent washing with an acidic solution based on hydrochloric acid, sulfuric acid, and nitric acid, where the metallic precursor consists of chloride salts of different transition metals. This material exhibits excellent activity in the oxygen reduction reaction. The present invention differs from the process described in this invention in that it does not require a washing step after the high-temperature heat treatment.

[0043] Furthermore, the information published in the article “Boost oxygen reduction reaction performance by tuning the active sites in Fe-N-P-C catalysts” by Yahao Li et al. is also known, which refers to the preparation of catalysts doped with nitrogen and phosphorus heteroatoms and highly dispersed iron species. The material presented in this document achieves catalytic activities similar to a commercial catalyst with 30 wt% Pt. However, the procedure differs from the present invention in that it uses a temperature of 1000 °C for calcination and requires a washing step to remove the acidic residues caused by the H₃PO₄ used for the synthesis.

[0045] Finally, the findings published in the article “Synthesis of unused-wood derived C-Fe-N catalysts for oxygen reduction reaction by heteroatom doping during hydrothermal carbonization and subsequent carbonization in nitrogen atmosphere” by Y. Goto et al. are also known, where they refer to the preparation of iron-based electrocatalysts supported on a carbonaceous material using biomass as a precursor. In this work, the materials are prepared by a single high-temperature heat treatment. However, the kinetic parameters they exhibit for the oxygen reduction reaction are not comparable to those obtained with commercial platinum-based catalysts. Furthermore, the final material must be washed with an aqueous solution of hydrochloric acid, H₂O, and finally, ethanol.

[0047] Taking into account all the aspects discussed, although the preparation of electrocatalysts based on highly dispersed metals supported on nitrogen-doped carbonaceous materials prepared from biomass waste has been studied previously, it is anticipated that the present invention differs from all of them in two fundamental aspects:

[0048] 1. Conventional activating agents that require subsequent washing steps are not used. In this way, this invention is able to provide a simpler synthesis method than those used in the prior art of these materials.

[0049] 2. The chemical activation of the carbonaceous precursor and the incorporation of the active sites is carried out during the same heat treatment, using just the right amount of metal precursor and nitrogen precursor.

[0051] Given the existing background in the state of the art, it can be observed that none of the known strategies have solved the problem of obtaining low-cost biomass-based catalysts with sufficient catalytic activity and durability to replace current platinum catalysts in ORR and whose synthesis can be easily extended to an industrial scale.

[0053] DESCRIPTION OF THE INVENTION

[0055] The present invention discloses a process for synthesizing low-cost, high-performance electrocatalysts for use in electrochemical reactions such as the oxygen reduction reaction. Materials based on iron atoms and other non-noble metals highly dispersed in nitrogen-doped carbonaceous materials have been extensively studied in recent years. However, although some materials have achieved catalytic activities close to those of commercial platinum-based catalysts, the synthesis methods require several steps, including washing processes, which increase the cost of the materials and are major contributing factors to environmental degradation and resource depletion. By appropriately selecting metal and nitrogen precursors, materials have been synthesized based on biomass waste, a non-noble metal such as iron, and functionalized with nitrogen groups in a single high-temperature heat treatment. Furthermore, the materials of the present invention do not require washing steps and achieve activities similar to commercial catalysts in the oxygen reduction reaction in alkaline media.

[0057] The present invention provides a process for preparing electrocatalysts that allows the use of the material obtained directly after heat treatment, as conventional activating agents such as KOH or H₃PO₄ are not used. In this way, the costly washing process is unnecessary, thus considerably reducing the final price and environmental impact of the catalyst. Instead of using the aforementioned activating agents, the materials of the present invention were prepared using other compounds, such as iron(II) oxalate (FeC₂O₄·2H₂O; Sigma Aldrich) and dicyandiamide as a nitrogen precursor (C₂H₄N₄; Wako Pure Chemical Industries). In this way, the active sites for electrocatalytic application are incorporated into the carbonaceous material while simultaneously developing porosity within the material. To achieve this, a mixture of biomass, a metal precursor, and a nitrogen precursor is placed in an autoclave and subjected to hydrothermal treatment, for example, for 24 hours at 200 °C. The resulting solid-phase carbon is then treated, for example, in an inert nitrogen atmosphere at 900 °C for 30 minutes. The decomposition of FeC₂O₄ and C₂H₄N₄ essentially yields CO₂ and N₂, respectively, and other species that become strongly incorporated into the carbonaceous material. Since the prepared material contains no residues of unwanted activating agents, subsequent washing steps are unnecessary, and the functionalization and activation of the carbonaceous material are achieved in a single step.

[0059] The object of the present invention relates to a process for synthesizing a carbonaceous powder material comprising:

[0060] ● a first stage of hydrothermal carbonization treatment of biomass together with nitrogen precursors and precursors of a transition metal other than the platinum group metals, obtaining a solid carbonate,

[0061] ● a second stage of heat treatment in an inert atmosphere of the solid carbonized material obtained previously, obtaining a carbonaceous powder material.

[0062] In the present invention, the expression “platinum group metals” refers to iridium, osmium, palladium, platinum, rhodium, and ruthenium.

[0063] Transition metals can be, for example, iron, cobalt, nickel, and preferably iron.

[0065] The transition metal precursor can be a metallic compound, for example, an inorganic salt or an organic salt of the metal, such as oxalate.

[0067] The iron precursor can be an iron compound, for example, an inorganic salt or an organic iron salt, such as iron(II) oxalate.

[0069] The nitrogen precursor can be a nitrogen compound, such as urea, melamine, or dicyandiamide. According to particular embodiments, the nitrogen precursor is dicyandiamide (C<2>H<4>N<4>).

[0071] The ratio in which the metallic precursor and the nitrogen precursor are mixed in the first stage is between 1:10 and 1:30 by mass of metallic precursor: nitrogen precursor.

[0073] According to particular embodiments, the ratio in which the metallic precursor and the nitrogen precursor are mixed in the first stage is between 1:10 and 1:20 by mass of metallic precursor: nitrogen precursor.

[0075] The proportion of water used in the first stage relative to the sum of the masses of the metal precursor and the nitrogen precursor is sufficient to achieve liquid-vapor equilibrium pressure during hydrothermal treatment. This is typically achieved by filling the reactor to approximately half its volume.

[0077] The first stage of hydrothermal carbonization treatment is carried out in an autoclave at a temperature between 150 ºC and 280 ºC, according to particular realizations, between 180 ºC and 220 ºC.

[0078] The first stage of hydrothermal carbonization treatment is carried out for a period of between 8 and 24 hours, depending on particular realizations, between 12 and 24 hours, and depending on additional particular realizations, for 24 hours.

[0080] According to specific embodiments, the first stage of hydrothermal carbonization treatment is carried out in an autoclave at a temperature between 150°C and 280°C, for a period of between 8 and 24 hours. According to further specific embodiments, the hydrothermal carbonization treatment is carried out in an autoclave at a temperature of 200°C for 24 hours.

[0082] The second stage of heat treatment in an inert atmosphere is carried out under N<2> or argon, preferably under N<2.>

[0084] The second heat treatment stage is carried out, according to particular embodiments, using an inert gas flow of between 50 and 200 mL min<-1>, preferably between 80 and 130 mL min<-1>, and according to additional particular embodiments, 100 mL min<-1> of inert gas.

[0086] The second heat treatment stage is carried out, according to particular embodiments, using a heating ramp of between 3 and 7 °C·min<-1>, more preferably 5 °C·min<-1> to the target temperature between 700 and 950 °C, where it is maintained for a time between 1 and 30 minutes.

[0088] The second heat treatment stage comprises raising the temperature to a target temperature between 700 and 950 °C, preferably between 800 and 950 °C, and more preferably between 850 and 950 °C, especially preferably up to 900 °C.

[0090] The result of the second heat treatment stage is the decomposition of the metallic precursor and the nitrogen precursor, giving rise to species that are strongly incorporated into the carbonaceous material.

[0092] According to particular realizations, the autoclave is a Teflon autoclave with a stainless steel jacket.

[0093] According to additional particular embodiments, the process comprises a prior step of grinding biomass, for example, to a particle size between 0.2 and 2.5 mm, preferably between 0.3 and 1.4 mm, and more preferably between 0.4 and 1 mm.

[0095] Biomass can be, for example, a waste such as almond shells, olive pits, peach pits, wood, etc.

[0097] According to specific realizations, the preliminary stage comprises:

[0098] a) Grinding the biomass residue to obtain a particle size between 0.4 and 1 mm;

[0099] b) Introduction of the biomass residue, preferably almond shell, the nitrogen precursor, preferably dicyandiamide and the metallic precursor, preferably iron (II) oxalate), into a Teflon autoclave with a stainless steel jacket.

[0101] According to additional particular realizations, the first stage comprises:

[0102] a) heating the autoclave containing the biomass and the nitrogen and metallic precursors, in an oven to a temperature of 200 ºC;

[0103] b) maintaining said temperature constant for 24h;

[0104] c) cooling to room temperature.

[0106] The first stage may also include,

[0107] - transfer of the solid phase of the product obtained to a ceramic container (such as a boat), where it is distributed homogeneously;

[0108] - introduction of the ceramic container into a hermetically sealed oven;

[0109] - purge the oven with inert gas.

[0111] According to specific realizations, the second stage comprises:

[0112] a) establishing an inert gas flow, preferably N<2> of 100 mL·min<-1>; b) gradually heating the oven at 5 ºC·min<-1> to a temperature of 900 ºC;

[0113] c) maintaining said temperature constant for 30 min;

[0114] d) Cooling to room temperature while maintaining an inert gas atmosphere; e) Obtaining the carbonaceous material in powder form.

[0116] According to specific realizations, the procedure comprises the following steps:

[0117] a) introducing a previously ground biomass precursor with a particle size between 0.4 and 1.0 mm into an autoclave, together with a metallic precursor (such as iron oxalate) and a nitrogen precursor (such as dicyandiamide), where they are homogeneously distributed using H<2>O as a solvent;

[0118] b) introduction of the autoclave into an oven at 200 ºC for 24 hours;

[0119] c) introduction of the obtained solid into a hermetically sealed tubular furnace;

[0120] d) heating the oven gradually (5ºC·min<-1>) to a temperature of at least 900 ºC and with a flow of an inert gas, such as nitrogen, of 100 mL min<-1>;

[0121] e) maintaining said temperature constant for 30 minutes;

[0122] f) cooling to ambient temperature while maintaining a controlled inert atmosphere; and obtaining the carbonaceous material in powder form.

[0124] According to additional concrete realizations, the procedure comprises:

[0125] a) Grinding the biomass residue to obtain a particle size between 0.4 and 1 mm;

[0126] b) Introduction of the biomass residue (almond shell), the nitrogen precursor (dicyandiamide) and the metallic precursor (iron (II) oxalate) into a Teflon autoclave with a stainless steel jacket;

[0127] c) Heating the autoclave in an oven to a temperature of 200 ºC;

[0128] d) Maintaining said temperature constant for 24h;

[0129] e) Cooling to room temperature;

[0130] f) Transfer of the solid phase of the product obtained to a small boat, where it is distributed homogeneously;

[0131] g) Introduction of the ceramic container into a hermetically sealed oven;

[0132] h) Purging the furnace with inert gas;

[0133] i) Establishment of an N<2> flow of 100 mL·min<-1>;

[0134] j) Heating the oven gradually at 5 °C·min<-1> to a temperature of 900 °C; k) Maintaining this temperature constant for 30 min;

[0135] j) Cooling to room temperature while maintaining an inert atmosphere of N<2> gas; l) Obtaining the carbonaceous material in powder form.

[0136] Preferably, one or more of the following conditions are used in the procedure: - the autoclave used in the procedure is made of stainless steel with a Teflon jacket, the ceramic container is made of quartz, and the oven used is tubular.

[0137] - The autoclave is placed in a conventional oven where it is heated to 200°C for a period of between 8 and 24 hours, for example, 24 hours. - The resulting product is transferred to a ceramic container, where it is distributed homogeneously.

[0138] - the temperature in the furnace during hydrothermal treatment is controlled by a thermocouple located at a point near the center of a ceramic container where the sample is located,

[0139] - the purge is carried out at a flow rate of between 50 and 200 mL·min<-1> of pure nitrogen, for example, 100 mL·min<-1> of pure nitrogen and the purge time is variable depending on the size of the tube used.

[0140] - The oven heating in the second stage is carried out gradually with a ramp of 5 °C·min<-1> up to the maximum temperature of between 850 and 950 °C, for example, 900 °C. - The temperature obtained in the second stage, after the gradual heating, is kept constant for a time of between 1 and 30 minutes, for example, 30 minutes.

[0141] The carbonaceous material is obtained after treatment without subsequent washing steps. In this way, the prepared electrocatalysts have specific surface areas greater than 500 m²·g.

[0143] The present invention also describes, outside the scope of the claims, a carbonaceous material obtained from the process defined above. According to specific embodiments, it is a powdered material with dispersed iron species, doped with nitrogen, and with a specific surface area greater than 500 m²/g.

[0145] The present invention also describes, outside the scope of the claims, the use of the carbonaceous material obtained by the process defined above as an electrocatalyst in electrochemical reactions, preferably as an electrocatalyst in the oxygen reduction reaction, in the cathode of the alkaline polymeric membrane fuel cell.

[0146] The present invention provides a simpler and less expensive synthesis procedure compared to those existing to date. Due to their excellent catalytic activity, durability, and low cost, the materials of the present invention are potential candidates to replace commercial Pt catalysts in the alkaline oxygen reduction reaction, thereby reducing the overall cost of fuel cells. Furthermore, this synthesis method is versatile, allowing the study of other electrochemical reactions of interest through modification of the metal precursor or the introduction of other metals to form alloys.

[0148] The invention provides a low-cost method for synthesizing electrocatalysts from biomass waste, abundant transition metals (specifically iron (Fe)), and nitrogen heteroatoms without requiring more than one high-temperature stage or washing stages that increase the cost of the process. To this end, the present invention presents a novel approach that has not been previously explored.

[0150] This synthesis procedure, for which it is not necessary to use washing stages or more than one stage at high temperature, allows obtaining carbonaceous materials in powder form with excellent catalytic activity and very high selectivity for the formation of H<2>O by the desired pathway of transfer of 4 electrons per oxygen molecule, so they are usable as electrocatalysts of fuel cells in alkaline medium.

[0152] The kinetic parameters obtained by this catalyst for the oxygen reduction reaction in alkaline medium consist of a reaction start potential of 0.92 V vs. RHE, a half-wave potential of 0.79 V, a limiting current density similar to that of a platinum-based electrocatalyst, and a number of electrons at useful fuel cell potentials (0.6–1.0 V) very close to 4, corresponding to the formation of practically H₂O as the only product. This avoids the formation of byproducts (H₂O₂) that limit energy efficiency and can be detrimental to fuel cell operation. These materials, as previously mentioned, are therefore presented as candidates to replace commercial platinum-based catalysts, and the synthesis procedure reduces the cost compared to other known preparation strategies.

[0153] Regarding the synthesis of materials, it is important to consider three factors: a) The temperature of the heat treatment;

[0154] b) The use of a nitrogen precursor and a metallic precursor, in particular embodiments iron, and;

[0155] c) The use of biomass as a precursor of carbonaceous material.

[0157] To illustrate the relevance of the final temperature in the heat treatment, a detailed study of electrocatalysts synthesized at different temperatures was carried out. During this study, it was observed that increasing the temperature has a significant impact on catalytic activity and selectivity. Catalytic activity values comparable to the commercial platinum nanoparticle-based catalyst were only achieved by the samples prepared at the highest temperature.

[0159] In the present invention, it has been demonstrated that the role played by the nitrogen precursor in obtaining the present carbonaceous materials is fundamental to achieving high performance. While the development of porosity is mainly due, in the case of using iron, to the use of FeC<2>O<4>, only the materials prepared in the presence of dicyandiamide used as a nitrogen precursor achieved an activity and selectivity for the oxygen reduction reaction similar to that of the commercial platinum catalyst.

[0161] This study has also demonstrated that the excellent results obtained are related to the use of biomass as a precursor for the carbonaceous material. It has been observed that the catalytic activity of materials prepared using the present synthesis method with carbon black instead of almond shell results in lower performance.

[0163] Therefore, by controlling these fundamental factors, it is possible to prepare electrocatalysts from biomass waste that exhibit excellent catalytic activity and at a lower cost and environmental impact than the other previously studied materials and much lower than the commercial platinum-based catalyst.

[0164] Furthermore, the synthesis process of the present invention has the significant advantage of being simple, and the materials obtained after heat treatment contain no residues from the activation process and can be used without subsequent washing and filtration steps. Therefore, the process of the present invention is very simple and economical, and can be scaled up to an industrial level.

[0166] This procedure stands out for its simplicity, low cost and versatility to extend to other electrochemical reactions of interest, the following being its main advantages:

[0167] ● Uses a biomass residue, specifically almond shell, as a precursor to carbonaceous material.

[0168] ● The equipment needed to carry out the synthesis procedure is affordable for any laboratory or industry.

[0169] ● The catalysts are synthesized in powder form, making them easy to handle and disperse in aqueous medium, which facilitates their shaping as electrodes.

[0170] ● The cost of the prepared materials is much lower than that of current commercial platinum-based catalysts, without compromising their excellent electrocatalytic activity. ● The prepared electrocatalysts are stable and exhibit excellent durability after numerous reaction cycles.

[0171] ● The development of porosity and the functionalization of materials is carried out in a single step, without the need to use conventional activating agents.

[0172] ● The metallic precursor used is incorporated in just the right amount to avoid costly subsequent washing stages that would compromise the environmental impact and cost of the procedure.

[0173] ● In the oxygen reduction reaction in alkaline medium, the materials act by means of a 4 electron mechanism, so that they produce H<2>O as the only product and the production of intermediates that decrease the energy density of the device and can deteriorate the fuel cell are avoided.

[0175] Therefore, the present invention represents a significant advance compared to other previously known processes, as it allows obtaining low-cost carbonaceous materials with excellent electrocatalytic activity and selectivity for the oxygen reduction reaction through a synthesis process that uses a single high-temperature heat treatment stage and does not require subsequent washing steps. Previously, washing steps with water, acidic solutions, or organic solvents were required, which negatively impacted cost and the environment. Furthermore, their excellent durability makes them suitable for replacing platinum in low-temperature fuel cells, and the ease of scaling up their synthesis makes this process very attractive for industrial use.

[0177] BRIEF DESCRIPTION OF THE FIGURES

[0179] Figure 1:(a) Number of electrons transferred and (b) linear sweep voltammetry curves in 0.1 M KOH solution saturated in O<2>a 5 mV·s<-1>y 1600 rpm for the electrocatalysts obtained at different temperatures.

[0181] Figure 2:(a) Number of electrons transferred and (b) linear sweep voltammetry curves in 0.1 M KOH solution saturated in O<2>a 5 mV·s<-1>y 1600 rpm for the carbonaceous materials obtained by modifying the use of dicyandiamide.

[0183] Figure 3:(a) Number of electrons transferred and (b) linear sweep voltammetry curves in 0.1 M KOH solution saturated in O<2>a 5 mV·s<-1>y 1600 rpm for carbonaceous materials obtained with biomass and Vulcan carbon black.

[0185] Figure 4: Durability study showing (a) number of electrons transferred and (b) linear sweep voltammetry curves of the fresh and used catalyst after 200 cycling cycles (between 0.0-1.0 V at 50 mV·s<-1> and 1600 rpm in 0.1 M KOH solution saturated in O<2>).

[0187] EXAMPLES

[0188] The described procedure shows the preparation of catalysts based on dispersed transition metals, specifically iron, on nitrogen-doped carbonaceous materials that act as high-performance electrocatalysts for the oxygen reaction in an alkaline medium.

[0190] The synthesis procedure is carried out in a 50 mL stainless steel autoclave with a Teflon jacket and subsequent heat treatment in a tubular furnace in the presence of an inert nitrogen gas atmosphere. The purge stream was maintained for one hour prior to heat treatment.

[0191] The following describes the common conditions for all examples presented in the present invention:

[0192] a) The precursors used (0.5 mmol of FeC<2>O<4>, 1 gram of dicyandiamide and 2 grams of almond shell) are introduced into a 50 mL Teflon autoclave with a stainless steel jacket where they are homogeneously mixed in the presence of 16 mL of H<2>O. b) The hydrothermal treatment is carried out in an oven for 24 hours at a temperature of 200 °C.

[0193] c) Once the hydrothermal treatment is completed, the autoclave is gradually cooled to room temperature and the solid phase of the product is obtained.

[0194] d) For heat treatment, the materials are placed in a quartz ceramic container, which is 15 cm long and 3 cm wide.

[0195] e) The oven used is a hermetically sealed tubular oven. Its diameter is 70 mm, its length is 1500 mm and its volume is 3 L.

[0196] f) The ceramic container is placed inside the oven and the temperature is controlled by using a thermocouple inserted at a point close to the center of the ceramic container.

[0197] g) Prior to heat treatment, a purge is performed with the inert gas to ensure an inert atmosphere for the heat treatment. The flow rate used for this procedure is 100 mL·min<-1>. The gas used was nitrogen, although other inert gases may be used.

[0198] h) The heat treatment of the material obtained from hydrothermal carbonization is carried out at a temperature of 900 °C, where it is maintained for a time of 30 minutes with a gradual heating of 5 °C·min<-1> with an N<2> flow of 100 mL min<-1>. The time at maximum temperature is chosen to ensure the complete reaction at the target temperature.

[0199] i) Once the heat treatment is complete, the material is cooled to room temperature while maintaining an inert nitrogen atmosphere at all times. This is done to prevent unwanted transformations of the material due to external factors.

[0201] In this way, as previously mentioned, a carbonaceous powder material with a specific surface area greater than 500 m²/g is obtained. The carbonaceous material obtained shows excellent catalytic activity for the oxygen reduction reaction. The selectivity shown for the reaction is high towards the formation of H₂O, with a reduction in the percentage of H₂O produced to values comparable to the commercial platinum-based catalyst within the useful range of fuel cells. Furthermore, the stability of the material is comparable to that shown by commercial catalysts.

[0203] To investigate the electrocatalytic activity of the synthesized carbonaceous material, characterization was carried out using a rotating ring-disc electrode (RRDE). The working electrode, made of glassy carbon, had a diameter of 5.0 mm and a platinum ring. A graphite rod was used as the counter electrode, and a reversible hydrogen electrode (RHE) immersed in the working electrolyte served as the reference electrode. The optimal amount of electrocatalyst used was 120 µg, as this resulted in the highest limiting current density. The glassy carbon disc of the electrode was modified with a suspension of 1 mg·mL of the catalyst in an aqueous solution of 20% isopropanol and 0.02% Nafion®. The electrolyte used was 0.1 M KOH saturated in O₂. Figure 1a shows the production of H<2>O<2> as a number of electrons, which is calculated as n = (4 I<disk>)/(I<disk>+ I<ring>/N) where I<disk> and I<ring> are the currents measured in absolute value at the disk and ring electrodes, respectively, and N is the collection efficiency of the rotating electrode, which in our case has an experimentally determined value of 0.28. Figure 1b shows the linear sweep voltammetry curves at 5 mV·s<-1> and 1600 rpm.

[0205] Table 1 shows an extensive comparison of the parameters that allow comparison of the catalytic activity of the materials and the number of electrons involved in the oxygen reduction reaction of the different catalysts described in the state of the art.

[0207]

[0208]

[0211] Table 1

[0214] EXAMPLE 1: EFFECT OF HEAT TREATMENT TEMPERATURE ON THE

[0215] ELECTROCATALYTIC ACTIVITY.

[0217] The carbonaceous material prepared by the process of this invention was synthesized by heat treatment. This was carried out by gradual heating from 5 °C/min to a target temperature of 900 °C. The atmosphere used for the synthesis process was pure nitrogen. The mass of biomass precursor used for the synthesis was 2 grams. Furthermore, the carbonaceous material is obtained in the form of a fine powder that has a high capacity for dispersion in aqueous media, and the prepared suspensions remain stable over time, which facilitates the subsequent preparation of the electrodes. The yield obtained by this synthesis process was 11.5%.

[0219] Figure 1 shows the results obtained in the study with a rotating ring-disc electrode for two samples: the material of the present invention, prepared by heat treatment at 900 °C (Fe-N/AS_900), compared to a material treated at lower temperatures, specifically up to 800 °C (Fe-N/AS_800). For this second electrocatalyst treated at 800 °C, the yield obtained was 22.6%. When a higher temperature is used, a significant change occurs in the final materials. Specifically, when a temperature of at least 900 °C is used, the materials acquire catalytic activity comparable to the commercial platinum catalyst (Figure 1b). However, the sample prepared at 800 °C does not achieve the desired catalytic activity. Another aspect to consider is the selectivity of the prepared electrocatalysts. While the sample prepared at a higher temperature shows a behavior similar to the commercial catalyst with a number of electrons transferred in the reaction that corresponds to a hydrogen peroxide production of less than 5% in the useful range of the fuel cells (0.6-1.0V vs RHE), the material prepared at 800 ºC has a lower selectivity, which can impair the life of the fuel cells and decrease the energy efficiency of the device.

[0221] EXAMPLE 2: EFFECT OF NITROGEN PRECURSOR ON ELECTROCATALYTIC ACTIVITY.

[0223] Example 2 illustrates the need to use a nitrogen precursor. The synthesis of the carbonaceous material was carried out with a biomass precursor:dicyandiamide mass ratio of 2:1.

[0225] The incorporation of nitrogen heteroatoms was carried out by introducing dicyandiamide as a nitrogen precursor along with the metallic precursor in an autoclave with the biomass residue. A hydrothermal carbonization treatment was performed at 200 °C and subsequently a thermal treatment in a pure nitrogen atmosphere, resulting in carbonaceous materials with excellent electrocatalytic activity.

[0227] Carbonaceous materials prepared containing nitrogen heteroatoms exhibit high activity. For comparison, a sample prepared without dicyandiamide (Fe/AS_900), i.e., by simply adding the metal precursor to the biomass residue, was studied. Using this synthesis procedure, the yield obtained was 18.2%. The results are shown in Figure 2. These suggest that the presence of nitrogen in the carbonaceous material is absolutely essential for achieving excellent catalytic activity in the oxygen reduction reaction. The properties of the material prepared without the nitrogen precursor are insufficient to replace the commercial platinum-based catalyst.

[0229] Furthermore, the nitrogen precursor must be incorporated into the synthesis prior to hydrothermal treatment. For comparative purposes, another sample was studied in which the dicyandiamide was introduced after hydrothermal treatment (Fe-N(post.HTC)/AS_900). Using this synthesis procedure, the yield obtained was 21.5%. The carbonaceous material obtained by this modified synthesis method exhibits a high reaction initiation potential (0.88 V), but the kinetic parameters and selectivity are lower. Thus, it is demonstrated that the introduction of the nitrogen precursor prior to hydrothermal treatment is crucial to achieving excellent catalytic activities.

[0231] EXAMPLE 3: USE OF BIOMASS WASTE AS A PRECURSOR OF CARBONOSE MATERIAL.

[0233] Example 3 illustrates the advantage of using biomass as a precursor for the carbonaceous material obtained through this synthesis procedure. The electrolyzer was also synthesized using a commercial carbon black (Vulcan XC-72R; Cabot Corporation). The same synthesis conditions used for the other materials were followed. The sample is named Fe-N/Vulcan_900. The yield obtained for this material was 92.7%.

[0234] The resulting powder was evaluated for the oxygen reduction reaction in an alkaline medium (Figure 3). The catalytic activity of this sample is considerably lower than that of the sample prepared with almond shells. These results indicate that the use of biomass waste is not only of interest due to its low cost and greater sustainability, but also because the nature of its composition makes it a material with better performance than conventional commercial supports such as carbon black for this synthesis procedure.

[0236] EXAMPLE 4: DURABILITY STUDY.

[0238] Another objective of the present invention is the preparation of robust carbonaceous materials that maintain excellent and stable electrocatalytic activity over a long period of time. Therefore, in order to demonstrate the excellent durability of the prepared carbonaceous materials, a study was conducted comparing the activity of a fresh sample with that of a sample subjected to a long-term durability test in the RRDE. This test consisted of cycling between potentials of 1.0 and 0.0 V vs. RHE at a sweep speed of 50 mV·s^-1, applying a rotation speed of 1600 rpm in 0.1 M KOH electrolyte saturated in O². The stability study (Figure 4) shows that the material synthesized by this invention is able to maintain the same catalytic activity after the durability experiment compared to a fresh sample, as the voltammetry curves and the representation of the number of electrons exchanged show no differences.

[0240] The excellent stability obtained makes these materials a very viable option for use in fuel cells as electrocatalysts in the cathode where the oxygen reduction reaction takes place in an alkaline medium, as they are robust materials that are not deactivated after a demanding durability test.

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Claims (13)

1. CLAIMS 1. A process for synthesizing a carbonaceous powder material comprising: ● a first stage of hydrothermal carbonization treatment of biomass together with nitrogen precursors and precursors of a transition metal other than the platinum group metals, obtaining a solid carbonate, ● a second stage of heat treatment in an inert atmosphere of the solid carbonized material obtained previously, obtaining a carbonaceous powder material, where the nitrogen precursor is dicyandiamide (C<2>H<4>N<4>) and the metallic precursor is iron(II) oxalate FeC<2>O<4>. 2. The process according to claim 1, wherein the hydrothermal carbonization treatment is carried out at a temperature between 150°C and 280°C, preferably between 180°C and 220°C and more preferably at 200°C. 3. The process according to claim 1, wherein the hydrothermal carbonization treatment is carried out for a period of between 8 and 24 hours, preferably between 12 and 24 hours, and more preferably for 24 hours. 4. The process according to claim 1, wherein the inert atmosphere heat treatment is carried out under N₂ or argon, preferably under N₂. 5. The process according to claim 1, wherein the inert atmosphere heat treatment is performed using an inert gas flow of between 50 and 200 mL min<-1>, preferably between 80 and 130 mL min<-1>, and more preferably 100 mL min<-1> of inert gas. 6. The process according to claim 1, wherein the inert atmosphere heat treatment is carried out using a heating ramp of between 3 and 7 °C·min<-1>, more preferably 5 °C·min<-1> to the target temperature between 700 and 950 °C, where it is maintained for a time between 1 and 30 minutes. 7. The process according to claim 1, comprising a prior step of grinding biomass, for example, to a particle size between 0.2 and 2.5 mm, preferably between 0.3 and 1.4 mm, and more preferably between 0.4 and 1 mm. 8. The process according to claim 1, wherein the biomass is a waste product, preferably almond shell. 9. The process according to claim 1, wherein the previous step comprises: a) Grinding the biomass residue to obtain a particle size between 0.4 and 1 mm; b) Introduction of the biomass residue, preferably almond shell, the nitrogen precursor dicyandiamide and the metallic precursor iron(II) oxalate, into a Teflon autoclave with a stainless steel jacket. 10. The process according to claim 1, wherein the first step comprises: a) heating the autoclave containing the biomass and the nitrogen and metallic precursors in an oven to a temperature of 200°C; b) maintaining said temperature constant for 24h; c) cooling to room temperature. 11. The process according to claim 1, wherein the second step comprises: - establishing an inert gas flow, preferably N<2> of 100 mL·min<-1>; - heating the oven gradually at 5 ºC·min<-1> to a temperature of 900 ºC; - maintaining this constant temperature for 30 min; - Cooling to room temperature while maintaining an inert gas atmosphere; - obtaining the carbonaceous material in powder form. 12. The process according to claim 1, comprising the following steps: a) introducing a previously ground biomass precursor with a particle size between 0.4 and 1.0 mm into an autoclave together with the metallic precursor (iron oxalate) and the nitrogen precursor (dicyandiamide), where they are homogeneously distributed using H<2>O as a solvent; b) placing the autoclave in an oven at 200°C; for 24 hours; c) introduction of the obtained solid in a hermetically sealed tubular oven d) heating the oven gradually (5ºC·min<-1>) to a temperature of at least 900 ºC and with a nitrogen flow of 100 mL min<-1>; e) maintaining said temperature constant for 30 minutes; f) cooling to room temperature while maintaining a controlled inert atmosphere; and obtaining the carbonaceous material in powder form. 13. The process according to claim 1, wherein the metal precursor and the nitrogen precursor are mixed in the first stage in a ratio of between 1:10 and 1:30 by mass of metal precursor:nitrogen precursor.