Patent Application
20200270291
The present invention belongs to the field of electrocatalysts, more specifically to electrocatalysts derived from metal-organic frameworks. In particular, the present invention refers to an iron zeolitic imidazolate framework, the process for producing it, a graphite carbon nanocomposite, and iron nanoparticles, as well as the process of obtaining said nanocomposite from the iron zeolitic imidazolate framework. The present invention also refers to the use of the nanocomposite as a catalyst.
The electrochemistry of oxygen involves the reactions of oxygen reduction (ORR) and evolution (OER), which are the two most important reactions for electrochemical energy storage and conversion technologies, including fuel cells, metal batteries, and water electrolysis. For these electrochemical applications, highly active and stable electrocatalysts are needed for the ORR and OER. The noble metals are usually good electrocatalysts for these applications. For example, platinum-based nanocomposites are the most effective commercial electrocatalysts for the ORR, while precious nanocomposites based on ruthenium and iridium are commonly used in the OER process. However, the low stability, scarcity, and high cost of these noble metal-based oxygen electrocatalysts prevent their large-scale implementation. Therefore, it is urgent to develop highly effective and durable alternatives with a low cost, ideally with a bifunctional capacity for both the ORR and OER.
Over the past decade, a wide range of alternative materials, including nanocarbons, metal oxides, carbides/nitrides and their composites have been found to be electroactive for electrochemical oxygen processes. Among them, the nanocarbons have shown promising catalytic activity as well as stability. Furthermore, the catalytic properties could be improved by the introduction of heteroatoms, including nitrogen, sulphur, boron, etc. In particular, several nitrogen-doped graphitic nanocarbons, including carbon nanotubes (CNCDs), graphene, mesoporous carbon and its nanocomposites, have been described as possible alternatives to the platinum catalysts. It is believed that the improved embodiment is related to the modified electronic structure and the carbon defects induced by the heteroatoms. In a few cases, however, excellent activity and durability have been found that is comparable to that of the platinum/carbon catalysts.
Recently, metal-organic frameworks have emerged as a new platform for the synthesis of new nanocarbon compounds. As a subclass of metal-organic frameworks, zeolitic imidazolate frameworks, known as ZIFs from their English name, are excellent precursors for nanocarbon electrocatalysts in view of the existence of abundant carbon and nitrogen species. However, nanocomposites derived from metal-organic frameworks are mostly microporous and of poor graphitic grade, which is considered as unfavourable for the transport of ions and electrons. Although many nanocarbon derivatives of metal-organic frameworks have been investigated as electrocatalysts, most of them exhibit unsatisfactory electrochemical activity.
ZIFs are a type of metal-organic framework that topologically has the same morphology as zeolites. Zeolites are porous aluminosilicate minerals that are found in nature but are also produced industrially on a large scale due to their commercial interest as adsorbents and catalysts.
ZIFs are composed of transition metal ions that are tetrahedrally coordinated and connected by imidazolate ligands. ZIFs are said to have zeolite topologies since the metal-imidazole-metal angle is similar to the 145° Si—O—Si angle in zeolites. ZIFs are usually prepared by solvothermal or hydrothermal techniques, wherein the crystals grow slowly by heating a solution of a hydrated metal salt, an imidazolate, a solvent, and a base.
U.S. Pat. No. 8,314,245 B2 describes different zinc ZIFs obtained by heating a tetrahydrated zinc nitrate solution and imidazole or an imidazole derivative in a solvent at temperatures between 85 and 150° C. for 48 to 96 hours.
Bao Yu Xia et al. describe the so-called cobalt ZIF-67, obtained from a solution of cobalt nitrate hexahydrate and 2-methylimidazole in a mixture of 1:1 methanol:ethanol (Nature Energy, 2016, 1, 15006). Bao Yu Xia et al. also describe the use of this ZIF-67 as a precursor of an electrocatalyst based on nitrogen-doped carbon nanotube structures.
Zhao et al. describe a non-porous iron and imidazolate material with both tetrahedral and octahedral centres that has a structure that cannot be described as a zeolite (and therefore cannot be considered a zeolitic framework) which is mixed with a zinc ZIF-8 to obtain a catalyst (Chemical Science 2012, 3, 11, 3200-3205).
There is still a need to find new, more efficient electrocatalysts that can be equated with the expensive platinum catalysts.
The present invention provides a new carbon and iron nanocomposite with excellent electrocatalytic behaviour. The inventors of the present invention have obtained an electrocatalyst from an iron zeolitic framework not described to date. The inventors have also found an advantageous process to obtain said iron zeolitic framework, a precursor of a nanocomposite with excellent electrocatalytic activity. Said advantageous process is cleaner and more environmentally friendly as it does not use solvents and therefore does not generate waste.
In addition, the process to obtain the nanocomposite of the present invention from the zeolitic framework of the present invention is fast and economical, since it is carried out at lower temperatures and in shorter timeframes than for other state-of-the-art processes.
Therefore, as a first aspect, the present invention refers to a zeolitic framework comprising the general structure A-B-A wherein A is iron, and B is a compound of formula I
wherein R1, R2 and R3 are independently hydrogen, C1-4 alkyl, halo, cyano, or nitro, wherein when R2 and R3 are C1-4 alkyl, R2 and R3 may be (are optionally) joined together to form a ring comprising 3 to 7 carbon atoms.
In a preferred embodiment, the zeolitic framework of the first aspect is isolated. Preferably, the zeolitic framework of the first aspect of the present invention has a purity of at least 80%, preferably at least 85%, more preferably of at least 90%, and even more preferably of at least 95%. Preferably, the zeolitic framework of the first aspect is isolated and has a purity of at least 99%. Preferably, the zeolitic framework of the first aspect is isolated and has a purity of 100%.
In a preferred embodiment of the first aspect of the present invention, the compound of formula I is imidazolate or 2-methylimidazolate. More preferably, the compound of formula I is 2-methylimidazolate.
In a preferred embodiment, the zeolitic framework of the first aspect has a SOD (sodalite) zeolitic topology. In a preferred embodiment, the zeolitic framework of the first aspect has the crystallographic structure of the ZIF-8.
In a preferred embodiment of the first aspect, the zeolitic framework is isolated, has a purity of at least 95%, comprises the general structure A-B-A wherein A is iron, and B is 2-methylimidazole, has a SOD zeolitic topology, and the crystallographic structure of the ZIF-8.
In a preferred embodiment of the first aspect, the zeolitic framework has a micropore volume, calculated by means of adsorption assays, greater than 0.15 cm3·g−1, preferably greater than 0.3 cm3·g−1.
In a preferred embodiment of the first aspect, the zeolitic framework has a BET area greater than 100 m2/g, preferably greater than 200 m2/g, and more preferably greater than 400 m2/g, calculated by adsorption assays.
A second aspect of the present invention refers to a process for obtaining the zeolitic framework of the first aspect, comprising the following steps:
a. mixing ferrocene and a compound of formula I as described in the first aspect, preferably 2-methylimidazole, in the presence of a template ligand,
b. heating the sealed mixture of step (a) to a temperature of between 80 and 250 ° C. for at least 12 hours, preferably for at least 24 hours.
As used here, the term “template ligand” refers to a compound that is not incorporated in the zeolitic framework structure and influences the reaction kinetics between ferrocene and the compound of formula I, which is preferably 2-methylimidazole. In a preferred embodiment of the process of the second aspect, the template ligand is solid at room temperature (25° C.). Preferably, the template ligand is an aromatic heterocycle. More preferably, the template ligand is an aromatic heterocycle wherein the heteroatom is nitrogen. Even more preferably, the template ligand is a pyridine, a pyridine derivative, or an imidazole derivative. More preferably, the template ligand is a bipyridine, a bipyridine derivative, or a benzimidazole derivative. In a preferred embodiment of the process of the second aspect, the template ligand is 4.4-bipyridine. In a preferred embodiment of the process if the second aspect, the template ligand is 2-methylbenzimidazole.
In another preferred embodiment of the process of the second aspect, before heating the mixture of step (a), said mixture is sealed in a container under vacuum. Preferably, the vacuum is at least 10−2 mbar, preferably at least 10−3 mbar. Preferably, the mixture of step (a) is prepared in the absence of a solvent.
In a preferred embodiment of the process of the second aspect, the molar ratio of the template ligand:formula I compound in the mixture of step (a) is 0. In another preferred embodiment of the process of the second aspect, the molar ratio of the template ligand:formula I compound in the mixture of step (a) is greater than 0.1 (≥1:10), preferably greater than 0.5 (≥5:10), more preferably greater than 1 (≥1:1).
In a preferred embodiment of the process of the second aspect, the molar ratio 4.4-bipyridine:2-methylimidazole in the mixture of step (a) is at least 1 (≥1:1). In a preferred embodiment of the process of the second aspect, the molar ratio 2-methylbenzimidazole:2-methylimidazole in the mixture of step (a) is at least 1 (≥1:1).
In another preferred embodiment of the process of the second aspect, step (b) is carried out at a temperature between 110 and 200° C., preferably step (b) is carried out at a temperature between 140 and 160° C.
In another preferred embodiment of the process of the second aspect, step (b) lasts between 2 and 6 days, and, preferably, step (b) lasts between 3.5 and 4.5 days.
In a preferred embodiment, the process of the second aspect of the present invention comprises the following steps:
In a third aspect, the present invention refers to a nanocomposite comprising:
Preferably, the nanocomposite of the third aspect has a current density in the hydrogen evolution reaction (HER) less than −300 mA/cm2 in KOH 1M, preferably less than −430 mA/cm2 in KOH 1M, and more preferably less than −500 mA/cm2 in KOH 1M.
The current density of the nanocomposite of this invention was analysed in the HER reaction at −0.75 V vs RHE and the OER reaction at 1.8 V vs RHE.
The current density of a nanocomposite can be calculated in the HER reaction or the OER reaction, and in different media, so that the current density for the same nanocomposite for the same reaction is not the same depending on the medium in which it is calculated. When the current density in the OER reaction is calculated in KOH 0.1 M, the nanocomposite of the third aspect of this invention has a current density greater than 50 mA/cm2, preferably greater than 100 mA/cm2, and more preferably greater than 180 mA/cm2. When the current density in the HER reaction is calculated in KOH 0.1 M, the nanocomposite of the third aspect of the present invention has a current density less than −100 mA/cm2, preferably has a current density less than −140 mA/cm2, and more preferably has a current density less than −200 mA/cm2. When the current density in the HER reaction is calculated in H2SO4 1 M, the nanocomposite of the third aspect of the present invention has a current density less than −100 mA/cm2, preferably has a current density of less than −200 mA/cm2, and more preferably has a current density of less than −250 mA/cm2. When the current density in the HER reaction is calculated in H2SO4 0.5 M, the nanocomposite of the third aspect of the present invention has a current density less than −100 mA/cm2, preferably has a current density of less than −140 mA/cm2, and more preferably has a current density of less than −200 mA/cm2. When the current density in the HER reaction is calculated in a buffer at pH 7, the nanocomposite of the third aspect of this invention has a current density less than −20 mA/cm2, preferably it has a current density less than −25 mA/cm2, and more preferably it has a current density less than −40 mA/cm2.
In a preferred embodiment, the nanocomposite of the third aspect of the present invention has an initial hydrogen evolution reaction (HER) of more than −0.5 V (vs RHE in KOH 1M), or more than −0.42 V (vs RHE in KOH 0.1 M), or more than −0.62 V (vs RHE in H2SO4 1 M), or more than −0.75 V (vs RHE in H2SO4 0.5 M), or more than −0.85 V (vs RHE in a buffer solution of pH 7). In a more preferred embodiment, the nanocomposite of the third aspect of the present invention has an initial hydrogen evolution reaction (HER) of more than −0.45 V (vs RHE in KOH 1M), or more than −0.35 V (vs RHE in KOH 0.1 M), or more than −0.57 V (vs RHE in H2SO4 1 M), or more than −0.70 V (vs RHE in H2SO4 0.5 M), or more than −0.80 V (vs RHE in pH 7 buffer solution). In an even more preferred embodiment, the nanocomposite of the third aspect of the present invention has an initial hydrogen evolution reaction (HER) of more than −0.40 V (vs RHE in KOH 1M), or more than −0.32 V (vs RHE in KOH 0.1 M), or more than −0.53 V (vs RHE in H2SO4 1 M), or more than −0.67 V (vs RHE in H2SO4 0.5 M), or more than −0.78 V (vs RHE in buffer solution pH 7).
In a preferred embodiment, the nanocomposite of the third aspect of the present invention has an initial oxygen evolution reaction (OER) of less than 1.75 V (vs RHE in KOH 1M or KOH 0.1 M). Preferably, the nanocomposite of the third aspect of this invention has an initial oxygen evolution reaction (OER) of less than 1.70 V (vs RHE in KOH 1M or KOH 0.1 M). More preferably, the nanocomposite of the third aspect of the present invention has an initial oxygen evolution reaction (OER) of less than 1.65 V (vs RHE in KOH 1M or KOH 0.1 M).
In a preferred embodiment, the nanocomposite of the third aspect of the present invention has a Tafel slope of less than 57 mV per decade (in KOH 1M) or less than 68 mV per decade (in KOH 0.1M). Preferably, the nanocomposite of the third aspect of the present invention has a Tafel slope of less than 47 mV per decade (in KOH 1M) or less than 58 mV per decade (in KOH 0.1M). More preferably, the nanocomposite of the third aspect of the present invention has a Tafel slope of less than 40 mV per decade (in KOH 1 M) or less than 50 mV per decade (in KOH 0.1M).
In a preferred embodiment, the nanocomposite of the third aspect of the present invention has a pore size of 0.5 to 15 nm, preferably from 1 to 10 nm, and more preferably from 3 to 5 nm, calculated by adsorption assays.
In a preferred embodiment, the nanocomposite of the third aspect of the present invention has a pore volume of 0.1 to 2 cm3 g−1, preferably from 0.5 to 1.5 cm3 g−1, and more preferably from 0.9 to 1.1 cm3 g−1, calculated by adsorption assays.
In a preferred embodiment, the nanocomposite of the third aspect of the present invention has a micropore volume of 0.01 to 1 cm3 g−1, preferably from 0.05 to 0.5 cm3 g−1, more preferably from 0.09 to 0.11 cm3 g−1, calculated by adsorption assays.
In a preferred embodiment of the third aspect of the present invention, the zeolitic framework has a BET area greater than 100 m2/g, preferably greater than 200 m2/g, and more preferably greater than 400 m2/g, calculated by adsorption assays. In a preferred embodiment, the nanocomposite of the third aspect of the present invention has a BET area between 100 and 1,200 m2/g, preferably between 200 and 800 m2/g, and more preferably between 400 and 600 m2/g, calculated by adsorption assays.
In a preferred embodiment, the nanocomposite of the third aspect comprises a graphite carbon matrix and between 0.3 and 2% by weight of iron nanoparticles with respect to the total weight of the nanocomposite, wherein said iron nanoparticles have a diameter of between 5 and 45 nm, wherein said nanocomposite comprises between 80 and 94% by weight of carbon, between 5 and 15% by weight of oxygen, and between 0.5 and 3% by weight of nitrogen, with respect to the total weight of nanocomposite, wherein said nanocomposite has a current density in the hydrogen evolution reaction (HER) less than −430 mA/cm2 in KOH 1M and a current density in the OER reaction greater than 230 mA/cm2 in KOH 1M, and wherein said nanocomposite has a BET area greater than 200 m2/g and a micropore volume of 0.05 to 0.5 cm3 g−1.
A fourth aspect of the present invention refers to a process for obtaining a nanocomposite according to the third aspect, comprising the following steps:
a. obtaining a zeolitic framework comprising the general structure A-B-A wherein A is iron, and B is a compound of formula I according to the first aspect, through a process according to the second aspect, and
b. heating the zeolitic framework obtained in step (a) at a temperature between 500 and 900° C., preferably between 600 and 800° C., more preferably between 680 and 720° C., during at least 1 hour, preferably for at least 2 hours, more preferably for at least 3 hours.
In a preferred embodiment of the process of the fourth aspect, during step (b) the zeolitic framework obtained in step (a) is heated at a temperature between 500 and 900° C. for between 2 and 5 hours, preferably for between 3 and 4 hours.
In a preferred embodiment of the process of the fourth aspect, before step (b), the zeolitic framework of step (a) is introduced in a solvent, and an inert atmosphere is created, preferably with nitrogen. The solvent is preferably acetonitrile.
In a fifth aspect, the present invention refers to the nanocomposite obtained by the process according to the fourth aspect of the invention.
In a sixth aspect, the present invention refers to the use of the nanocomposite according to the third or fifth aspect, as a catalyst. The nanocomposite of the present invention is preferably used as a catalyst in proton exchange membrane fuel cells or PEMFC from the term in English.
The following figures form part of the present description and describe exemplary embodiments of the claimed invention. The skilled artisan will, in light of these figures and the description herein, be able to practice the invention without undue experimentation.
The following examples illustrate the present invention and demonstrate the advantageous properties of the nanocomposites of the present invention, as well as the method of the present invention. In view of the above description and the examples below, one of ordinary skill in the art will be able to practice the invention as claimed without undue experimentation. The foregoing will be better understood with reference to the following examples that detail certain procedures for the preparation of embodiments of the present invention. All references made to these examples are for the purposes of illustration. The following examples should not be considered exhaustive, but merely illustrative of only a few of the many embodiments contemplated by the present invention.
Ferrocene (30 mg, 0.16 mmol), 4.4-bipyridine (50 mg, 0.32 mmol) and 2-methylimidazole (20 mg, 0.24 mmol) are used for the iron ZIF synthesis. These three solids are mixed and sealed under vacuum in a tube. The mixture is heated to 150° C. for 4 days to obtain yellow crystals suitable for single-crystal X-ray diffraction (
In an alternative synthesis process, ferrocene, 2-methylbenzimidazole (as a template ligand), and 2-methylimidazole are used, and essentially the same steps are followed.
The crystallographic studies at 120 K reveal that the yellow crystals are isostructural with the ZIF-8 (a=17.1794 Å), with the spatial group 1-43 m. The metallic centres, Fe(II), are located in a tetrahedral coordination environment, connected by N—C—N bridges created by the ligands 2-methylimidazole, as shown in
The N—C—N bridges between the iron centres allow a magnetic exchange, and the Fe(II) tetrahedral environment that provides S=2 for each metallic centre enables the appearance of magnetic sorting. As we can see in
For the synthesis of the nanocomposite, the iron ZIF was introduced into a vessel with acetonitrile to avoid contact with oxygen in the atmosphere. The inert atmosphere of nitrogen was created, and the ramp was made, in which it is heated to 700° C. for 3.5 h, with a ramp up and down of 2° C./min. Once heated, the nanocomposite obtained from the heating is washed with a solution of nitric acid 0.5 M for 6 h to eliminate the excess metal.
X-ray measurements (XRPD) confirm the presence of small traces of iron nanoparticles in the nanocomposite, showing the characteristic peaks of metallic iron and graphite carbon (
On the other hand, the images of the high-resolution transmission electron microscope (HRTEM) show that the structure of the nanocomposite consists of a graphitised carbon matrix, with iron nanoparticles of approximately 10 to 30 nm in size, as can be seen in
The scanning electron microscope (SEM) images of the nanocomposite show how, after heating, the nanocomposite loses the geometric structure observed in the ZIF. Furthermore, a structure can be seen with different layers of graphene and many “dimples” that correspond to the pores that provide the high specific area to the nanocomposite.
The X-ray spectroscopy (XRS) measurements show that the nanocomposite has a percentage of 90.7, 8.2, and 1.1 percent of atomic carbon, oxygen, and nitrogen, respectively (Table 1), demonstrating that there is nitrogen doping. In said measurement, iron is not detected as it is a superficial measurement, and the nanoparticles are surrounded by several layers of carbon as can be seen in HRTEM images. With regard to nitrogen, we can see in
The nitrogen doping is very important in this type of composites, as it induces electronic interaction with nearby carbon/metal atoms to provide active catalytic areas and also produces structural defects in the carbon nanoforms to form oxygen adsorption sites. Finally,
The inductively coupled plasma atomic emission spectroscopy (ICP-OES) analyses indicate that the nanocomposite contains 0.79% of iron by weight.
To estimate the surface area of the nanocomposite, the porous texture of the nanocomposite was characterised by nitrogen adsorption assays (N2) at 77 K and carbon dioxide adsorption assays (CO2) at 273 K (
aData obtained from the N2 adsorption. Specific area calculated using the BET method. Area contributed by micropores Sμ and external area ST using the t-plot method.
bTotal Volume at P/P0 = 0.96.
c Data obtained from the CO2 adsorption. The micropore volume (<0.7 nm) calculated according to the DR method.
dMicropore volume calculated by N2 adsorption using the DR method.
eCalculated mesopore volume according to: Vmeso = VTotal − VμDR.
f Mesopore volume (Vmeso (P/P0)) calculated from the difference between the total (Vt) at P/P0 and the micropore volume (Vmicro).
The nitrogen isotherms show an IV type adsorption, whose values are illustrated in Table 4, showing a specific area of 463 m2g−1. The pore volume of the nanocomposite is 0.96 cm3g−1, indicating a distribution of micropores and mesopores of approximately 3 nm. For a better micropore study of less than 0.7 nm in size, CO2 adsorption measurements were made at 273 K. In this case, the measurements indicate a micropore volume of 0.12 cm3g−1 (
The electrocatalytic behaviour of the nanocomposite of the present invention was characterised by different electrochemical measurements in a typical 3-electrode cell. Different electrolytes with different concentrations (i.e. media with different pH) were used for said measurements, always using a stainless-steel sheet and an Ag/AgCl electrode as a counter-electrode and reference electrode respectively. Each working electrode used the different nanocomposites, embedded in nickel foam for the basic media and in carbon felt for the acid media, (to prevent the reaction between nickel foam and the acid) of an area of 0.2 cm2. The deposition of the nanocomposites was carried out by preparing a suspension of the material to be analysed with polyvinylidene difluoride (PVDF) and carbon black (ratio 80:10:10) in ethanol. Once it was deposited in the nickel foam or carbon felt, it was allowed to dry for two hat 80° C. Basic media (1 M and 0.1 M KOH), acid media (0.5 M H2SO4), and a neutral medium (phosphate buffer of pH 7) were used to study the electrocatalytic activity of the nanocomposite.
To measure their behaviour as an oxygen catalyst (OER), it was tested in two basic media (KOH 0.1 and 1 M). Linear voltammetry measurements were performed, showing the beginning of catalysis at 1.542 V and 1.588 V (vs RHE) for the 0.1 M and 1 M KOH media, respectively. As can be seen in
For a better characterisation of its catalytic behaviour other parameters were calculated, such as the overvoltage (η) obtained at different current densities (10 and 15 mA·cm−2); the current density (j) at an overvoltage of η=300 and 400 mV; and the Tafel slopes in the different media. In
The stability and durability of the nanocomposite of this invention were tested by means of a galvanostatic test applying continuous current densities of j=10 and 15 mA·cm−2, and by potentiometric tests applying an overvoltage of η=300 and 400 mV, for 1000 seconds in both cases. As can be seen in
Finally, the behaviour of the nanocomposite of the present invention was measured as a hydrogen catalyst (HER), tested in basic media (KOH 0.1 and 1 M), acid media (H2SO4 1 and 0.5 M), and in neutral medium (pH 7 phosphate buffer). Linear voltammetry measurements were performed, showing the initiation of catalysis always above the corresponding target measurement in that medium, as shown in
The zeolitic framework of the present invention is porous, as can be seen in
The ZIF of the present invention presents BET area values always greater than 400 m2/g and up to 1,200 m2/g after cleaning the pore by activation of the material.
The ZIF of the present invention presents micropore volume values between 0.3 and 0.6 cm3·g−1.
The above is a detailed description of particular embodiments of the invention. It will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. All of the embodiments disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| P201731106 | Sep 2017 | ES | national |
The present application is a continuation of and claims the benefit of application No. PCT/ES2018/070593 filed Sep. 12, 2018, which claims the benefit of application No. ES P201731106 filed Sep. 12, 2017, the entire disclosures of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/ES2018/070593 | Sep 2018 | US |
| Child | 16811345 | US |
