COPPER AND ANTIMONY BASED MATERIAL AND ELECTRODE FOR THE SELECTIVE CONVERSION OF CARBON DIOXIDE TO CARBON MONOXIDE

Abstract

An electrocatalyst material comprising cuprous oxide and antimony, the process for the production thereof and its use in the electrochemical reduction of CO2 to CO with high selectivity and efficiency are described.

Description

FIELD OF THE INVENTION

The present invention relates to a copper and antimony based material, and an electrode obtained from this material, useful for the electrochemical reduction of carbon dioxide to carbon monoxide with high efficiency and selectivity.

STATE OF THE ART

Massive emissions of carbon dioxide (CO₂), also known as carbonic anhydride, due to the burning of fossil fuels, have been recognized as responsible for global climate change. To tackle this problem, strategies such as CO₂ capture and storage are being studied, with the aim of slowing or even stopping the accumulation of CO₂ in the atmosphere. The transformation of captured CO₂ into additional chemicals, fuels or other products is of paramount importance to achieve a sustainable carbon cycle and to store energy in the long term. Among the different technologies for CO₂ transformation, electrochemical conversion is considered particularly interesting since it can use energy obtained from renewable sources. This technology, although very promising, is of non-immediate applicability due to the high stability of the CO₂ molecule, the slow kinetics and the complex mechanisms of the CO₂ reduction reaction.

CO₂ reduction can occur according to several proton-coupled electron transfer processes. CO₂ reduction reactions for the production of compounds containing a single carbon atom and the electrochemical evolution of H₂ are reported below as R1-R5, together with their standard potentials:

Values of E⁰ are reported under standard conditions (1 atm and 25° C.) with respect to the reversible hydrogen electrode (RHE) in aqueous media. Unless otherwise stated, all potentials in this description refer to the RHE.

Among the numerous products of CO₂ reduction, formic acid (HCOOH) and carbon monoxide (CO) are the only economically viable products that have been obtained so far with relevant productivity. CO is highly desired in the industrial sector, since its mixture with hydrogen (H₂), i.e., synthetic gas or syngas, can be converted into hydrocarbons through the Fischer-Tropsch process.

Since, however, the values of the standard potentials of the above reactions are similar, the result of the process is usually a mixture of products, which is difficult or not easy to use industrially. In addition, the parasitic reaction of hydrogen evolution usually occurs in higher yield than the reduction of CO₂ in aqueous electrolyte.

Therefore, electrode materials are required that can provide high CO₂ conversion efficiency and at the same time high selectivity towards a specific reaction product, in particular towards CO; materials of this kind are generally known in electrochemistry as electrocatalysts.

According to experimental and theoretical studies, gold (Au), silver (Ag) and palladium (Pd) are considered the best metal electrocatalysts to convert CO₂ into CO; however, these metals cannot be used on an industrial scale for this purpose due to their high cost and low availability.

In addition to the previous materials, the electrocatalytic properties, in CO₂ reduction, of metals such as copper (Cu), zinc (Zn), tin (Sn), indium (In) and bismuth (Bi) have been studied. Cu alone has no good selectivity for any product; Zn has sufficient, but not optimal, selectivity for CO production; Sn, In and Bi are selective for HCOOH production.

In some papers, the properties as electrocatalysts of compositions other than single metals are discussed.

Patent Application US 2019/0127866 A1 describes an electrocatalyst material for converting CO₂ to ethanol, comprising nanoparticles of copper or alloys thereof supported by nanometer-sized tips (“nanospikes”) of carbon doped with nitrogen, boron or phosphorus. Copper alloys indicated as useful by this document are all those of the element with one or more elements selected from those in the Groups 3-15 of the periodic table. Alloys indicated as preferred are those between copper and an element selected from Ni, Co, Zn, In, Ag and Sn. The electrocatalysts of this document exhibit higher selectivity for CO₂ electroreduction than H₂ evolution with high faradic efficiency in ethanol production, with a yield in this compound of at least 60% of the mixture; other species, such as carbon monoxide, are thus produced with yields not exceeding 40%. In addition to the fact that a mixture of products is produced, the preparation of the doped carbon nanospikes makes the process not straightforward.

The article “Achieving highly selective electrocatalytic CO₂ reduction by tuning CuO—Sb₂O₃ nanocomposites”, Y. Li et al., ACS Sustainable Chem. Eng. 2020, 8, 12, 4948-4954, describes an electrocatalyst material comprising a mixture of carbon in a finely divided form (“carbon black”) and powders of a mixed oxide of copper(II) (CuO) and antimony(III) (Sb₂O₃). The purpose of this study is to identify the best conditions for converting CO₂ to CO. The materials in this paper are produced by dissolving soluble Cu(II) and Sb(III) salts in a suspension of carbon black in ethanol, adding a base (KOH) to the suspension and allowing the system to react for 6 hours at a temperature of 80° C. obtained with an oil bath; the precipitate obtained is then washed with water and ethanol and finally dried. The mixture of powders thus obtained is then distributed on a carbon paper obtaining electrodes. In the section “Results and discussion” of the article, it is confirmed that copper oxide is in the form of CuO (i.e., copper is in oxidation state (II)) and that antimony oxide is in the form of Sb₂O₃ (i.e., antimony is in oxidation state (III)), by X-ray diffraction analysis (XRD, FIG. 1a of the article) showing the presence of the characteristic peaks of CuO and Sb₂O₃, by X-ray photoelectron spectroscopy (XPS, FIG. 1b) and by Raman spectroscopy (FIG. 1c). As shown in the article (see FIG. 3b), the best results are obtained with the molar ratio Cu:Sb 10:1, with which faradic yields of approximately 10% for HCOOH, 10% for H₂ and 80% for CO are obtained, while the authors report that as the Sb content increases, the CO yield drops rapidly. The results obtained with the best material of this article are already interesting, but still not optimal both as CO yield and as selectivity towards this compound (a mixture of three products is obtained).

The object of the present invention is to overcome the problems of the prior art, and in particular to provide an electrocatalyst material which allows to obtain in the electrochemical reduction reaction of CO₂ a CO yield and a selectivity towards this compound higher than with the electrocatalysts of the prior art. Another object of the invention is to make available a cost-effective process for large-scale production of this electrocatalyst.

SUMMARY OF THE INVENTION

These objects are achieved with the present invention, which in a first aspect relates to an electrocatalyst material comprising copper(I) oxide (Cu₂O) containing antimony, wherein the amount of antimony is between 5% to 30% by weight.

This material is used in a finely divided form to produce electrodes for the electrochemical reduction of CO₂, wherein said material is combined with an electroconductive material.

In a second aspect thereof, the invention relates to a process for the production of the electrocatalyst material, comprising the following steps:

• a) dissolving a copper(II) salt and an antimony(III) salt in a solvent selected from ethanol, ethylene glycol, acetylacetone, diethylamine, ethylenediamine, oleylamine, N,N-dimethylformamide, mixtures of these solvents with each other, with water or with aqueous solutions of D-glucose, hydrazine hydrate, amino acids or sodium carboxymethylcellulose, obtaining a solution;
• b) heating the solution in a microwave oven at a temperature between 180 and 230° C. for a time between 1 and 10 minutes;
• c) separating the precipitate from the solution and its drying.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail in the following with reference to the figures, in which:

- FIG. 1 shows photomicrographs obtained by field effect scanning electron microscope (FESEM) of various materials of the invention and three comparison materials;

- FIG. 2 shows results of X-ray diffraction (XRD) of powder samples of materials of the invention having different compositions and three comparison materials;

- FIG. 3 shows spectra obtained by X-ray photoelectron spectroscopy (XPS) for Cu and Sb on a sample of the invention;

- FIG. 4 represents in a schematic form an electrolytic cell used to carry out the CO₂ reduction tests reported in the Examples section;

- FIG. 5 shows graphs representative of the faradic efficiency in the conversion of CO₂ to CO obtained with a material of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found that copper(I) oxide (Cu₂O, cuprous oxide) containing antimony in an amount between 5 and 30% by weight, when used to produce an electrode, enables the electrochemical reduction of CO₂ to CO to be achieved with higher values of faradic efficiency and selectivity than known materials. The compounds of the invention enable these results to be obtained by employing copper and antimony, which are inexpensive and widely available components.

A material similar to that of the present invention has been described in the paper “Optimal synthesis of antimony-doped cuprous oxides for photoelectrochemical applications”, Dae Yun et al., Thin Solid Films 671 (2019) 120-126. However, this paper is directed to the study of the influence of Sb concentration on the structural, electrical and photoelectrochemical properties of cuprous oxide thin films for the purpose of photoelectrochemical water splitting; besides, this study reports materials in which the amount of Sb reaches at most up to 1% in moles, and indicates as a preferred material for the mentioned purpose Cu₂O doped with 0.75% molar Sb.

The materials of the invention will generally be referred to in the following by the notation Cu₂O/Sb, regardless of the specific composition.

The Cu₂O/Sb materials of the invention have a Sb content between 5 and 30% by weight; preferred are the materials having a Sb content between 17.2 and 23.9% by weight.

The materials of the invention are obtained and used in powder form. The morphology of these powders is uniform and homogeneous at least up to the Sb concentration of 26.4%. FIG. 1 shows images obtained by field effect scanning electron microscope (FESEM) of samples of the invention with increasing Sb content (FIG. 1(b) to 1(i)) and, for comparison, of three samples produced following the same method as the samples of the invention but containing only copper (FIG. 1(a)), only antimony (FIG. 1(k)), and a sample not of the invention containing an amount of antimony of 36% (FIG. 1(j)); in particular, the weight percentage amount of Sb in the samples of the invention prepared as described in Example 1, determined by chemical analysis, is as follows:

FIG. 1(b): 5.2;
FIG. 1(c): 9.4;
FIG. 1(d): 13.6;
FIG. 1(e): 17.2;
FIG. 1(f): 20.1;
FIG. 1(g): 23.9;
FIG. 1(h): 25.2
FIG. 1(i): 26.4.

As can be seen in the images, the materials of the invention with a Sb content of up to 26.4% by weight have a similar morphology to one another, and comprise powders in the form of essentially spherical particles with very narrow size distribution (all particles have a size of about 5 µm), composed of tightly packed nanoparticles. For concentrations higher than 26.4%, Sb-rich particles and the formation of an isolated phase consisting of crystalline Sb₂O₃ are observed (octahedral particles in FIG. 1(j), to be compared with the image of pure antimony oxide in FIG. 1(k)). Energy dispersive X-ray spectroscopy (EDX) analysis indicates that Sb is uniformly distributed in the samples of the invention.

XRD analysis confirms that the material is essentially copper oxide. In FIG. 2 are shown, from top to bottom, the diffractograms for the sample containing only copper (diffractogram indicated with (Cu)), of the samples of the invention with increasing concentration of antimony (diffractograms from A to H), and of the sample containing 36% by weight of antimony (diffractogram indicated with (NI), which stands for “not of the invention”), respectively. As can be seen in the figure, in the samples of the invention up to a Sb content of 26.4% by weight, only peaks attributable to the Cu₂O phase are present (with decreasing intensity as the Sb content increases); in the sample with a Sb content of 36.0% by weight, peaks attributable to the Sb₂O₃ phase appear instead, although with low intensity.

The composition is also confirmed by high-resolution (HR) XPS spectroscopy. FIG. 3 shows the typical spectra of the sample containing 17.2% by weight of Sb. From the XPS measurement (FIG. 3a) it appears that antimony is present in the sample in the form of Sb³⁺ ions, as highlighted by the intense peaks relative to Sb 3d_5/2 and Sb 3d_3/2 centred at 530.06 eV and 539.45 eV, respectively. FIG. 3b shows instead the region of the XPS spectrum corresponding to the Cu 2p doublet; since the Cu 2p peak is difficult to deconvolve due to the overlap of numerous peaks, the Auger CuLMM region is also acquired (inset in FIG. 3b). The kinetic energy of the peak is 916.8 eV, which corresponds to Cu⁺. The modified Auger parameter is about 1848.8 eV, which correlates with an average oxidation state of Cu(I). It is therefore evident that copper is present in the samples in the form of Cu⁺ ion.

Since the electrocatalyst materials of the invention are poor electrical conductors per se, they are used in combination with conductive materials for the production of electrodes for CO₂ reduction. Preferably, the conductive material is in turn in the form of powders or other finely divided form. A carbon-based material is generally used for this purpose, thanks to its low catalytic activity, for example carbon black, graphite, graphene, carbon nanotubes or mixtures thereof; the preferred conductive material is carbon black. The electrocatalyst material of the invention and the conductive material are used in weight ratios between 9:1 and 19:1. For the production of the electrode, the mixture between the electrocatalyst material of the invention and the conductive material is distributed on a support, which may in turn be conductive or non-conductive. Examples of preferred supports are conductive carbon paper, conductive carbon cloth and metal mesh. Stabilization of the powder mixture on the support can be achieved with ionomers, i.e., ion conductive polymers, which form a containing and conductive film on the powders.

In a second aspect thereof, the invention relates to a process for the production of the electrocatalyst material, which consists of steps a) to c) above.

Step a) consists in dissolving a copper(II) salt and an antimony(III) salt in a solvent selected from ethanol, ethylene glycol, acetylacetone, diethylamine, ethylenediamine, oleylamine, N,N-dimethylformamide, mixtures of these solvents with each other, with water or with aqueous solutions of D-glucose, hydrazine hydrate, amino acids and sodium carboxymethylcellulose. The most suitable salts for the purposes of the invention are acetates, sulfates and nitrates of both metals. The starting salts are weighed to obtain the desired weight ratio of Cu:Sb, and thus the desired weight ratio of Cu₂O to Sb; the calculations necessary to determine the quantities to be used of the starting salts, given a desired final composition, are of simple executability for the average chemist.

The solution thus formed is heated in a microwave oven, within a sealed container of suitable material (e.g., Teflon) at a temperature between 180 and 230° C. for a time between 1 and 10 minutes. In addition to causing the metal salts to react to form the final material, microwave heating in the presence of the aforementioned solvents results in the reduction of the Cu²⁺ ion of the starting copper salt to Cu⁺ ion present in the Cu₂O oxide. In the case of ethylene glycol, glycol functions as both a solvent and a reducing agent, and increasing temperature can increase its reducing capacity. Normally a temperature between 180° C. and 230° C. is suitable for the formation of Cu⁺ from Cu²⁺ in the given solution.

Finally, the precipitate formed in the microwave heating is separated from the liquid phase, e.g., by filtration or centrifugation, washed with ethanol, and dried, e.g., by treatment in an oven at a temperature between 50 and 100° C. under vacuum or in an inert atmosphere.

The process of the invention differs from that of the article by Li et al. cited above in that microwave heating is used instead of conventional heating, that as said results in the reduction of the Cu²⁺ ion of the starting copper salt and the formation of the Cu₂O phase.

The invention will be further described in the experimental section below.

Materials, Instrumentation and Methods

The following precursors were used in the preparation of the samples:

• copper(II) acetate, Cu(OAc)₂·xH₂O (Sigma-Aldrich, catalogue No. 66923-66-8 degree of hydration, ~1), 98% purity;
• antimony(III) acetate, Sb(OAc)₃, (Sigma-Aldrich, catalogue No. 6923-52-0), 99.99% purity;
• ethylene glycol (Sigma-Aldrich, catalogue No. 107-21-1), 99.8% purity;
• Nafion® 117 solution (Sigma-Aldrich, catalogue no. 31175-20-9; Nafion is a registered trademark of E. I. du Pont de Nemours and Company), purity: ~ 5% in a mixture of lower aliphatic alcohols and water.

Chemical composition analyses of the samples were performed by inductively coupled plasma optical emission spectroscopy (ICP-OES, iCAP 7600 DUO instrument, Thermo Fisher Scientific); each analysis was performed by dissolving 5.0 mg of the sample in 10.0 ml of an aqueous solution with 10% aqua regia.

Electron microscope images and energy dispersive X-ray spectroscopy (EDX) analyses were obtained with a FESEM Supra 40 (Zeiss) equipped with a detector (Oxford Instruments Si(Li)) for energy dispersive X-ray spectroscopy (EDX) analyses.

The phase composition of each sample was determined by X-ray diffraction (XRD) with a diffractometer (PANalytical X’Pert Pro equipped with an X’Celerator detector) that uses Cu Kα radiation (λ = 1.54178 Å) generated at 40 kV and 30 mA. XRD diffractograms were recorded in the 20 25-80° range with a step (20) of 0.017° and a counting time of 0.45 seconds.

High-resolution (HR) XPS analyses were performed with a PHI 5000 VersaProbe instrument (Physical Electronics) using monochromatic Al Kα (1486.6 eV) radiation.

Analyses of gaseous products derived from CO₂ electroreduction were performed in real time with an INFICON Fusion® microgascromatograph (µGC) equipped with two channels with a 10 m Rt-Molsieve 5A column and an 8 m Rt-Q-Bond column, respectively, and thermal conductivity microdetectors (micro-TCD).

EXAMPLE 1

This example relates to the synthesis of the materials of the invention.

Seven samples of materials of the invention with different Sb contents were prepared using copper acetate and antimony acetate as precursors, used in the amounts shown in Table 1. The samples of the invention are indicated as A-H. For comparison, a sample from copper acetate alone (sample referred to as “Cu” in the table), a sample from antimony acetate alone (sample “Sb”), and a sample of mixed Cu/Sb composition not of the invention (sample “NI”) were also produced in the identical manner described below. The last column of the table shows the values of Sb content in each of the samples of the invention, obtained by ICP-OES analysis (the data for the Cu and Sb samples are not shown because naturally in these two cases the analysis for the determination of the percentage content of Sb was not carried out).

Sample	Amount of precursor (mg)	Sb content (% by weight)
Cu(OAc)2·xH2O	Sb(OAc)3
Cu	900	0	/
A	900	164	5.2
B	900	246	9.4
C	900	295	13.6
D	900	328	17.2
E	900	410	20.1
F	900	470	23.9
G	900	492	25.2
H	900	600	26.4
NI	900	820	36.0
Sb	0	900	/

The indicated amounts of precursors were dissolved in 40 ml of ethylene glycol and 5 ml of double distilled H₂O (resistivity about 18 MΩ·cm). Each solution was then transferred to a Teflon container (volume 100 mL). The Teflon container was sealed, placed in a microwave oven (Milestone, STARTSynth, HPR-1000-10S segment with temperature and pressure control), heated to 220° C. and then maintained at this temperature by powering the oven with a maximum power of 900 W for a total irradiation time of 2 minutes. After cooling to room temperature, the suspended product in each container was separated by centrifugation and washed twice with double-distilled H₂O and subsequently once with ethanol. Each powder sample was finally dried under vacuum at 60° C. overnight.

In addition to ICP-OES analysis, the samples of the invention were examined by scanning electron microscopy and EDX analysis to determine the morphology (also for Cu and Sb samples) and the antimony distribution, by X-ray diffraction to determine the crystal structure (also for Cu and Sb samples) and by XPS to determine the oxidation state of Cu and Sb; the results of the three analyses have been discussed above with reference to FIGS. 1, 2 and 3 respectively.

EXAMPLE 2

This example relates to the production of electrodes for electrochemical CO₂ reduction using the materials of the invention (samples A-H) and the three comparison materials (samples Cu, Sb and NI).

Each electrode was prepared by mixing 10 mg of sample A-H, Cu, Sb or NI, 1 mg of carbon black from acetylene, 90 µl of Nafion® 117 solution and 320 µl of isopropanol. Each mixture was sonicated for 30 minutes until a uniform suspension was obtained. Each suspension was then used to coat a carbon paper covered with a gas permeable layer (GDL; SIGRACET 28BC, SGL Technologies); the geometric area of each electrode was 1.5 cm². The obtained electrode was dried at 60° C. overnight to evaporate the solvents. The electrocatalyst loading on each electrode was approximately 3.0 mg cm^-2 The electrodes thus obtained are referred to in the following by the abbreviations E_x, where the subscript × corresponds to the sample A-H, Cu, Sb or NIused for its production.

EXAMPLE 3

This example refers to the measurement of the CO₂ reduction efficiency of the electrodes prepared in the previous Example.

Electrochemical measurements were performed with a cell having the configuration schematically shown in FIG. 4; the cell as a whole, 10, is shown in the figure enclosed by a discontinuous line. As shown in the figure, the cell has two compartments separated by an ion exchange membrane 11 (Nafion® N117 membrane, Sigma-Aldrich), and adopts a three-electrode configuration. Each compartment has a total volume of 10 ml and contains 7 ml of electrolyte, and thus 3 ml of headspace. The reference electrode, 12, is an Ag/AgCl electrode (1 mm, lossless LF-1) that is inserted into the cathode compartment. The counter electrode, 13, is a Pt foil (Goodfellow, 99.95%). The working electrode, i.e., the electrode of the invention, is shown in the figure as element 14. An aqueous solution of 0.1 M KHCO₃ was used as the electrolyte solution. In this configuration, gaseous CO₂ is fed into both half-cells from the lower part of the two compartments, while the mixture of products on which the results are evaluated is extracted from the cathode compartment (on the right in the figure); most of this mixture is sent to the separation and purification stage (performed with methods known in the field and not described in this text), while a fraction of the mixture is sent to the analysis. Chronoamperometric measurements were performed using a CHI760D electrochemical workstation (CH Instruments, Inc., USA). Gas phase products were analysed in real time with a microgascromatograph (µGC). The inlet of the µGC instrument was connected to the cathode side of the electrochemical cell through a GENIE filter, to remove humidity from the gas before it entered the analysis instrument (µGC). During the chronoamperometric measurements, the electrolytes on both sides of the anode and cathode were static, while a constant CO₂ flow rate of 15 ml/min was maintained to saturate the cathode electrolyte and to bring the gaseous products to the µGC. The tests were performed at different potentials between -0.79 V and -0.99 V. The potential was corrected by compensating for the ohmic potential drop, 85% of which was from the instrument (iR compensation).

Selectivity is described by the faradic efficiency (FE), which is the ratio of the amount of charge (coulomb, C) required to produce a certain amount of a product to the total charge consumed over the reaction time, and is expressed by the following equation:

$\text{FE}\left(\%\right)=\text{nNF/Q x 100}$

where n is the number of electrons transferred in the faradic process (for the reduction of CO₂ to CO and to H₂, n is 2 as shown in the reactions R1 and R5 above), N is the moles of a product generated in a specific reaction period, F is the faradic constant (96485.33 C/mol), and Q is the total charge in a specific reaction period.

The results of the tests at two potential values are shown in Table 2.

Electrode	Potential -0.79 V	Potential -0.99 V
FEco (%)	FEH2 (%)	FEco (%)	FEH2 (%)
ECu	9.5	90	8.5	85
EA	87	14	73	26
EB	85	13	84	15
Ec	90	8.5	81	18
ED	90	8	92	7
EE	91	8.5	90	8
EF	90	10.5	89	9.5
EG	89	10	85	14
EH	83.8	16.5	68.5	33
ENI	55	43	62	37
ESb	0	63	0	83

As can be seen from the test results, the E_Sb electrode does not produce CO at either test potential. The Cu electrode has poor selectivity for CO, with FE_co values below 10%. The comparison E_NI electrode shows poor selectivity values towards CO, probably because it is formed by a mixture containing only a small amount of active material together with a completely inactive material (antimony oxide). In contrast, the E_A-E_H electrodes of the invention exhibit high selectivity towards CO, with FEco above 80% for all A-H materials at -0.79 V. Among these materials, in particular, D and E show excellent selectivity values for CO, of at least 90% at both potentials.

EXAMPLE 4

This example relates to the measurement of CO₂ reduction with an electrode of the invention at various potentials.

The E_Delectrode, which gave the best results in Example 3, was tested at five different potential values ranging from -0.69 V to -1.09 V. In each test, the evolution of CO and H₂ over time was evaluated during tests lasting between one and two hours.

The results of these tests are shown graphically in FIG. 5. In detail, FIG. 5(a) to 5(e) report tests performed at the following potentials: 5(a) -0.69 V; 5(b) -0.79 V; 5(c) -0.89 V; 5(d) -0.99 V; 5(e) -1.09 V. The tests at -0.79 V and -0.99 V are the same as those whose results have already been reported in the previous example. The results of these tests are provided in summary form in the graph in FIG. 5(f), in which the faradic efficiency values for CO and H₂, taken when the reduction process has reached steady state, are reported at all evaluated potentials.

As can be seen in the graphs (FIG. 5(a)-(e)), in each test there is an initial settling time between about 10 minutes (test at -0.99 V) and 20 minutes; this is attributed to stabilization of the electrode and filling of the headspace of the electrochemical cell and of tubes between the cell and the µGC. Then, the FE values stabilize, indicating the stable performance of the electrode. The E_Delectrode shows very good performance in the conversion of CO₂ to CO (FE_co > 80%) over the whole range of potentials explored, with values up to 90-92% at potentials from -0.79 V to -1.09 V. At more negative potentials (< -1.09 V), FEco falls below 90%. FE_H2 values remain low (≤ 9%) from -0.69 V to -1.09 V. No other gas phase products other than CO and H₂ were detected. Liquid products (e.g., HCOOH) were not quantified, but can be assumed to be present in very small or negligible amounts, since the total faradic efficiency for CO and H₂ measured in all tests is around 100%.

COMMENTARY ON THE RESULTS

As demonstrated in the tests described above, the electrocatalyst materials of the invention catalyze the electrochemical reduction of CO₂ with high selectivity toward CO. The materials of the invention then offer further advantages.

Firstly, antimony and copper, and the compounds thereof used as precursors in the process of the invention, are inexpensive materials; moreover, the production of these materials is simple and easily scalable at an industrial level, also because it does not employ toxic or harmful products; the invention therefore offers a technically viable and competitive alternative to the use of metals such as Au, Ag and Pd.

Since the materials of the invention are in powder form, they can be used in reactors with various configurations as a gas diffusion electrode (GDE) and different sizes.

Claims

1. An electrocatalyst material consisting of copper(I) oxide (Cu2O) containing antimony, wherein the amount of antimony is between 5% to 30% by weight.

2. The electrocatalyst material according to claim 1, wherein the amount of antimony is between 5.2% and 26.4% by weight.

3. The electrocatalyst material according to claim 2, wherein the amount of antimony is between 17.2% and 23.9% by weight.

4. An electrode comprising powder of an electrocatalyst material of claim 1 and a conductive material deposited on a support, in a weight ratio between electrocatalyst material and conductive material between 9:1 and 19:1.

5. The electrode according to claim 4, wherein the conductive material is in the form of powder.

6. The electrode according to claim 4, wherein the conductive material is carbon based.

7. The electrode according to claim 6, wherein the conductive material is chosen from carbon black, graphite, graphene, carbon nanotubes and mixtures thereof.

8. The electrode according to claim 4, wherein the support is selected from conductive carbon paper, conductive carbon cloth and metal mesh.

9. The electrode according to claim 4, wherein the powder of the electrocatalyst material and possibly of the conductive material are stabilized on the support with an ionomer.

10. A process for the production of the electrocatalyst material of claim 1, comprising the following steps: a) dissolving a copper(II) salt and an antimony(III) salt in a solvent selected from ethanol, ethylene glycol, acetylacetone, diethylamine, ethylenediamine, oleylamine, N,N-dimethylformamide, mixtures of these solvents with each other, with water or with aqueous solutions of D-glucose, hydrazine hydrate, amino acids or sodium carboxymethylcellulose, obtaining a solution;b) heating the solution in a microwave oven at a temperature between 180 and 230° C. for a time between 1 and 10 minutes;c) separating the precipitate from the solution and its drying.

11. The process according to claim 10, wherein the copper(II) salt is selected from acetate, sulfate and nitrate, and the antimony(III) salt is selected from acetate, sulfate and nitrate.

12. A method for the selective electrochemical reduction of CO2 to CO, comprising the use of an electrode of claim 4 at a potential between -0.69 V to -1.09 V.