Patent Application
20230357289
The present invention relates to a compound for splitting water by means of an electrolysis process. More specifically, the compound is useful for producing hydrogen and for producing oxygen. The invention also relates to an electrode, the methods for preparing the electrode, a battery (or electrochemical cell), the method for preparing the compound and uses thereof.
The development of highly active, versatile and durable electrocatalytic materials for the production of clean and sustainable energy is at the forefront of the technological challenges facing humanity.
Electrochemical technologies largely depend on rare and endangered chemical elements, such as platinum, iridium or ruthenium, whose scarcity is creating a bottleneck for future technological progress. Therefore, the use of more abundant and greener sources is highly desirable.
In addition, these elements often only work well for one type of reaction. For example, the best-known catalyst for water reduction is platinum, whereas iridium and ruthenium are used for water oxidation.
Since the surface requirements of the catalyst during water reduction and oxidation are completely different, the development of bifunctional catalysts for water splitting, which in turn should be compatible with operation under acidic conditions and prevent corrosion, is therefore extremely difficult.
Polyoxometalate clusters have been applied as electrocatalysts for both water reduction and oxidation, although they are mainly restricted to use at neutral and basic pH, especially in the water oxidation reaction.
Therefore, it is still necessary to design new bifunctional electrocatalysts that can be used at acidic pH, thus allowing their use at industrial scale, preferably with the use of proton exchange membranes that present a high current density together with the possibility of producing high purity hydrogen gas.
The authors of the present invention have designed a new polyoxometalate (POM)-based compound capable of performing kinetically fast water oxidation or reduction under strong acidic conditions, for example at pH below 1, when on an electrode. In addition, it allows electrolysis under acidic conditions, being much more stable than other electrodes.
Therefore, in a first aspect, the invention refers to a compound (I) comprising
In a second aspect, the invention refers to an electrode comprising a compound (I) such as described above.
In a third aspect, the invention refers to the preparation of the electrodes comprising a compound (I) as described above.
In a fourth aspect, the invention refers to a battery (or electrochemical cell) comprising an electrode such as described in the second aspect of the invention.
In a fifth aspect, the invention refers to a method for preparing the compound of the first aspect of the invention.
Another aspect of the invention refers to the use.
In a first aspect, the invention refers to a compound (I) comprising
In a particular embodiment, the metal of the polyoxymethalate in compound (I) is selected from vanadium, tungsten, molybdenum, niobium, tantalum, zirconium and titanium.
In a particular embodiment, the positively charged counterion in compound (I) is selected from sodium, potassium and lithium.
In a particular embodiment, the ligand head in compound (I) is a linear or branched, cyclic or non-cyclic hydrocarbon chain having between 1 to 12 carbon atoms, with more than 2 hydroxyl groups.
In a particular embodiment, the ligand head in compound (I) is a group of formula (AlqOH)3Alq, wherein each Alq can be the same or different and is independently selected from a linear or branched, cyclic or non-cyclic hydrocarbon chain having between 1 to 12 carbon atoms.
In a particular embodiment, each Alq in compound (I) can be the same or different and is independently selected from a linear or branched non-cyclic hydrocarbon chain having between 1 to 4 carbon atoms.
In a particular embodiment, the tail in compound (I) is selected from an amino group and phosphonium.
In a particular embodiment, the ligand in compound (I) has the formula tris(hydroxyalkyl(C1-C4)aminoalkyl(C1-C4).
In a particular embodiment, compound (I) is Zn(H2O)m[(AlqOH)3AlqNH2]l[MXOY],
In a particular embodiment, the compound of formula (I) is Na4 (H2O)12[(CH2OH)3CNH3]2[V10O28]·4H2O.
In a second aspect, the invention refers to an electrode comprising compound (I) as described above.
In a particular embodiment, the electrode further comprises a carbon source. In a more particular embodiment, the carbon source is selected from a carbon nanotube, black carbon and a carbon nanofibre, preferably a multi-walled nanotube.
The interaction of compound (I) with the carbon source influences the usefulness of the electrode of the invention. Thus, when the compound (I) is simply physically mixed with the carbon source in the electrode of the invention, then the electrode is useful for the production of oxygen, as demonstrated in example 2. When the carbon source is a multi-walled nanotube, the best results in the production of oxygen are obtained, as demonstrated by the higher current density when compared to the current density when the carbon source is black carbon.
The hypothesis that can explain this reaction is that the cation is stabilised by the ligand and in this configuration the cation activates the oxygen atom of the water, and finally the oxidation of water takes place.
Therefore, in a particular embodiment, the invention relates to an electrode comprising a physical mixture of the compound of formula (I) and a carbon source. Preferably the carbon source is a carbon nanotube, more preferably a multi-walled nanotube. In another embodiment the invention relates to the use of this electrode for the production of oxygen or for the oxidation of water.
However, when compound (I) is assembled (assembly) or coating the surface of the carbon source, the electrode is useful for the production of hydrogen, as demonstrated in example 4.
The hypothesis that can explain the generation of hydrogen due to this interaction of compound (I) with the carbon source is that the ligand acts as a sponge, supporting polyoxometalate in the attraction of hydrogen atoms and finally the reduction of water takes place.
In a particular embodiment, the invention relates to an electrode comprising compound (I) coating the surface of a carbon source. In another embodiment the invention relates to the use of this electrode for the production of hydrogen or for the reduction of water.
In another embodiment, the invention relates to a method for preparing an electrode for the production of oxygen, comprising:
The grinding of step a) can be carried out in a mortar or similar equipment, and is intended to obtain particles of smaller size from the carbon source. The person skilled in the art may select different oils available in the art to prepare the suspension, such as a mineral oil.
In another embodiment, the invention relates to a method for preparing an electrode for the production of hydrogen, comprising:
In step b) of this process, the addition of the solution may be carried out dropwise or by means of an addition equipment in which the amount added over time can be controlled to ensure assembly of compound (I) on the carbon surface. The resuspension of the solid described in step d) can be carried out with materials generally employed for this purpose and known to the skilled person in the art, such as polymers, copolymers, oils, in some cases using also an alcoholic or glycolic solvent or mixtures thereof.
In a particular embodiment, the carbon source used in the methods of the invention is selected from carbon nanotubes, black carbon or carbon nanofibres.
In a fourth aspect, the invention refers to a battery comprising an electrode as described in the third aspect of the invention.
In a particular embodiment, the electrolyte of the battery of the invention has an acidic pH, preferably between 0.05 and 5.
As demonstrated in example 6, a battery prepared with electrodes formed by a compound of formula (I), wherein the electrolyte has a very acidic pH, for example below pH=1, is much more stable than a battery formed by electrocatalysts which are reference in the prior art, such as Pt/C and Ir/C. This constitutes a significant advantage of the electrodes of the invention, since they allow working for longer periods at a strong acidic pH.
In a fifth aspect the invention refers to the preparation of a compound (I), comprising:
Step b) of this process may require heating the mixture between 25° C. and 100° C. under stirring. The process may require a final filtration to remove by-products precipitating in the reaction medium, in order to obtain the final compound by evaporation of the water from the solution. This evaporation process can be carried out at atmospheric conditions or under reduced pressure.
The following examples serve to illustrate the invention and should not be considered a limitation thereof.
Sodium metavanadate (96%) and HCl (37%) were purchased from Alfa Aesar. Ultrapure (>99%) tris (hydroxymethyl) aminomethane (TRIS) was obtained from Molekula.
All these reagents and solvents were used as received. Carbon nanotubes (CNT), multi-walled nanotubes, were purchased from Pyrograf Products Inc., USA. XC-72 black carbon (BC) was purchased from FuelCellStore, mineral oil (Nujol) was purchased from Sigma Aldrich. Carbon supported Ir (Ir/C) (20% Ir) was obtained from Premeter co. Carbon supported Platinum (Pt/C) (20% Pt) and HISPEC 3000 was purchased from Alfa Aesar.
Infrared spectra were measured using a Bruker Alpha FTIR spectrometer with a platinum ATR module. Milling was carried out using a Retsch MM400 high-energy ball mill instrument.
Electrochemical experiments were carried out in a typical three-electrode configuration with an AUTOLAB 302N electrochemical potentiostat using a glassy carbon electrode (GCE), a hydrogen electrode (RHE) and Pt wire as working, reference and counter electrode, respectively. Cyclic voltammetry was carried out in 1M H2SO4 saturated with nitrogen at a scan rate of 50 mV/s between −0.5 V and 1.8 V.
The electrocatalytic activity of the prepared catalyst electrodes was examined by polarisation curves using linear sweep voltammetry (LSV) at a scan rate of 5 mV/s conducted in a 1M H2SO4 solution. For the oxygen release reaction, the electrolyte was previously saturated with oxygen and the material was compared to commercial iridium on carbon (Ir/C), while for the hydrogen release reaction the electrolyte was saturated with hydrogen prior to the measurements, and the material was compared to commercial platinum on carbon (Pt/C) under the same conditions.
Oxygen production was detected with an Ocean Optics NeoFOX oxygen detection system equipped with a pre-calibrated FOXY probe inserted into the electrolyte.
Faradaic Efficiency (FE) was calculated using the following equation:
FE=experimental moles of O2 released (measured with NeoFOX)/theoretical moles of O2
Where the theoretical moles of 02 were calculated using the following equation:
Theoretical moles of O2 released=Q/nF
where Q is the charge produced (Coulombs), n is the number of electrons during the oxygen evolution reaction (4 electrons) and F is the Faraday constant. The experimental moles of oxygen released during the oxygen evolution reaction were measured by NeoFox.
In a 100 mL round bottom flask, 2.51 g of NaVO3 (20.6 mmol, 5 eq) were dissolved in 21 mL of water under stirring and heated to 85° C. After dissolving NaVO3, 6.18 mL (12.36 mmol, 3 eq) of an aqueous HCl solution (2M) were added dropwise until a pH of 4 was reached. The initial yellow solution turned dark red after addition of the acid. Then, 0.5 g of TRIS [(CH2OH)3CNH2] (4.12 mmol, 1 eq) were added to the solution and the reaction temperature was maintained at 85° C. for 6 hours with stirring. After that, the reaction was allowed to cool gradually to room temperature. The solution was filtered to remove the precipitate formed. The orange-red filtrate was allowed to evaporate slowly and after one day orange crystals of compound 1 were obtained and collected by filtration (10% yield of Na4(H2O)12[(CH2OH)3CNH3]2[V10O28]·4H2O (1)) (
The compound obtained was characterised by Raman (
The cyclic voltammogram (CV) of the CMPE obtained in acidic media showed that the observed changes in current with variations in the applied potential can only be assigned to 1, as both the electrode additives used (i.e., mineral oil, CNT and BC) and the TRIS ligand show no redox process under the same conditions.
The cyclic voltammogram under acidic conditions (pH<0.5) revealed current density values ten times higher for 1/CNT, when compared to 1/BC for the same amount of 1 (
In addition, an oxygen sensor was immersed in the electrolyte to confirm oxygen production when performing a linear sweep voltammogram from 1.0V to 1.9V. Indeed, there was an increase in density at 1.44V after approximately 45 seconds, which correlates with an increase in oxygen production as shown by the sensor. According to the calculations made, 5.09 micromoles of oxygen were generated, corresponding to a FE of 94.1% for the oxygen evolution.
Electrodes were prepared as described in example 2 of the present specification with the following compounds:
And the current density produced in the same potential range was compared and voltammograms were obtained for each of them.
It was observed that in the case of the electrode prepared with the compound obtained in example 1 of the patent application, when positive potentials slightly above 1.2 volts are applied, the electrode (1/CNT) is able to catalyse the oxidation of water producing oxygen, with the current density (mA/cm2) obtained being proportional to the amount of oxygen produced. On the contrary, in the case of the other two electrodes prepared, it is necessary to apply much higher potentials to produce the same current density. Thus, when a potential of 1.8 volts is applied, the amount of oxygen produced by the compound obtained in example 1 is 20 times higher than that produced by the other two electrodes (
It is also demonstrated that the simultaneous presence of sodium and tris in the same compound is indispensable to obtain electrodes with higher electrocatalytic activity for the oxidation of water (oxygen production) at a lower applied potential.
4.1. Preparation of 1@CNT.
100 mg of 1 were dissolved in 1.5 ml of ionised water and added to a suspension of short CNT (5 mg in 1.5 ml of acetone) dropwise under sonication. The mixture was stirred at room temperature for 3 days and then filtered through a polytetrafluoroethylene (PTFE) membrane filter, washed repeatedly and with plenty of water to collect a yellowish black solid (20 mg). By high-resolution transmission electron microscopy (HR-TEM) it is observed that an amorphous material is assembled on the surface of the CNT, both on the external and internal side (
4.2. Preparation of the HER Electrode.
10 mg of the material (1 @ CNT) were carefully dispersed in 2 mL isopropanol with 40 μL of Nafion solution (1%) and sonicated for 15 min. Then, 200 μl of the suspension were deposited dropwise onto the glassy carbon electrode to form a film, with the final load being 1 mg of the material on the GCE (surface area=1 cm2). The film thickness was optimised by adjusting the volume and/or the concentration of the graphene suspension. The electrode was allowed to dry before electrochemical measurements.
The same procedure was carried out with BC replacing the carbon nanotubes (CNT).
The change in the structure between compound 1 and the CNT obtained in example 4.1. versus that obtained in example 2.1. led to a change in electrochemical behaviour. In this way, the electrode prepared in 4.2. no longer showed activity in oxygen production and showed activity in hydrogen production (
Compared to other electrodes for hydrogen production, the electrode prepared in this example showed an excellent performance (Table 2).
Electrodes were prepared as described in example 3 of the present specification with the following compounds:
And voltammograms were obtained for each of them.
It was observed that in the case of the electrode prepared with the compound obtained in example 1 of the present specification, when negative potentials below 0 volts are applied, the electrode (1@CNT) is able to catalyse the reduction of water producing hydrogen, with the current density (mA/cm2) obtained in absolute value being proportional to the amount of hydrogen produced. On the contrary, in the case of the other two electrodes prepared, it is necessary to apply much higher and negative potentials to produce the same current density. Thus, when a potential of negative 0.2 volts is applied, the amount of hydrogen produced by the compound obtained in example 1 is more than 50 times greater than that produced by the other two electrodes, with the amount produced by (TRIS)6[V10O28]@CNT being negligible at that potential (
It is also demonstrated that both the presence of sodium and the presence of tris are necessary to obtain electrode compounds with higher electrocatalytic activity for the reduction of water (hydrogen production) at a lower applied potential.
To evaluate the behaviour of the electrocatalysts in the context of water splitting, a typical water splitting cell (two-electrode system) was built using 1/CNT at the positive electrode and 1@CNT at the negative electrode. The chronoamperometry test was carried out in a 1M H2SO4 solution applying a potential of 3.1 V for 120 s. This behaviour was compared with the behaviour shown by the reference standards for Pt/C//Ir/C water splitting carried out under the same conditions, applying a voltage of 1.8 V.
As shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| P202030912 | Sep 2020 | ES | national |
This application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Patent Application No. PCT/ES2021/070649, filed Sep. 7, 2021, which claims priority to Spanish Patent Application No. P202030912, filed Sep. 8, 2020, the contents of which are each hereby incorporated by reference in their respective entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2021/070649 | 9/7/2021 | WO |
