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 CO.sub.2 to CO with high selectivity and efficiency are described.

Claims

1. An electrocatalyst material consisting of copper(I) oxide (Cu.sub.2O) 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 CO.sub.2 to CO, comprising the use of an electrode of claim 4 at a potential between -0.69 V to -1.09 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention will be described in detail in the following with reference to the figures, in which:

[0021] - FIG. 1 shows photomicrographs obtained by field effect scanning electron microscope (FESEM) of various materials of the invention and three comparison materials;

[0022] - FIG. 2 shows results of X-ray diffraction (XRD) of powder samples of materials of the invention having different compositions and three comparison materials;

[0023] - FIG. 3 shows spectra obtained by X-ray photoelectron spectroscopy (XPS) for Cu and Sb on a sample of the invention;

[0024] - FIG. 4 represents in a schematic form an electrolytic cell used to carry out the CO.sub.2 reduction tests reported in the Examples section;

[0025] - FIG. 5 shows graphs representative of the faradic efficiency in the conversion of CO.sub.2 to CO obtained with a material of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The inventors have found that copper(I) oxide (Cu.sub.2O, cuprous oxide) containing antimony in an amount between 5 and 30% by weight, when used to produce an electrode, enables the electrochemical reduction of CO.sub.2 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.

[0027] 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 Cu.sub.2O doped with 0.75% molar Sb.

[0028] The materials of the invention will generally be referred to in the following by the notation Cu.sub.2O/Sb, regardless of the specific composition.

[0029] The Cu.sub.2O/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.

[0030] 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:

TABLE-US-00001 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.

[0031] 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 .Math.m), composed of tightly packed nanoparticles. For concentrations higher than 26.4%, Sb-rich particles and the formation of an isolated phase consisting of crystalline Sb.sub.2O.sub.3 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.

[0032] 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 Cu.sub.2O 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 Sb.sub.2O.sub.3 phase appear instead, although with low intensity.

[0033] 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 Sb.sup.3+ ions, as highlighted by the intense peaks relative to Sb 3d.sub.5/2 and Sb 3d.sub.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.sup.+. 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.sup.+ ion.

[0034] 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 CO.sub.2 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.

[0035] 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.

[0036] 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 Cu.sub.2O 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.

[0037] 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 Cu.sup.2+ ion of the starting copper salt to Cu.sup.+ ion present in the Cu.sub.2O 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.sup.+ from Cu.sup.2+ in the given solution.

[0038] 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.

[0039] 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 Cu.sup.2+ ion of the starting copper salt and the formation of the Cu.sub.2O phase.

[0040] The invention will be further described in the experimental section below.

Materials, Instrumentation and Methods

[0041] The following precursors were used in the preparation of the samples: [0042] copper(II) acetate, Cu(OAc).sub.2.Math.xH.sub.2O (Sigma-Aldrich, catalogue No. 66923-66-8 degree of hydration, ~1), 98% purity; [0043] antimony(III) acetate, Sb(OAc).sub.3, (Sigma-Aldrich, catalogue No. 6923-52-0), 99.99% purity; [0044] ethylene glycol (Sigma-Aldrich, catalogue No. 107-21-1), 99.8% purity; [0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] High-resolution (HR) XPS analyses were performed with a PHI 5000 VersaProbe instrument (Physical Electronics) using monochromatic Al Kα (1486.6 eV) radiation.

[0050] Analyses of gaseous products derived from CO.sub.2 electroreduction were performed in real time with an INFICON Fusion® microgascromatograph (.Math.GC) 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

[0051] This example relates to the synthesis of the materials of the invention.

[0052] 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).

TABLE-US-00002 Sample Amount of precursor (mg) Sb content (% by weight) Cu(OAc).sub.2.Math.xH.sub.2O Sb(OAc).sub.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 /

[0053] The indicated amounts of precursors were dissolved in 40 ml of ethylene glycol and 5 ml of double distilled H.sub.2O (resistivity about 18 MΩ.Math.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 H.sub.2O and subsequently once with ethanol. Each powder sample was finally dried under vacuum at 60° C. overnight.

[0054] 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

[0055] This example relates to the production of electrodes for electrochemical CO.sub.2 reduction using the materials of the invention (samples A-H) and the three comparison materials (samples Cu, Sb and NI).

[0056] Each electrode was prepared by mixing 10 mg of sample A-H, Cu, Sb or NI, 1 mg of carbon black from acetylene, 90 .Math.l of Nafion® 117 solution and 320 .Math.l 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 cm.sup.2. 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.sup.-2 The electrodes thus obtained are referred to in the following by the abbreviations E.sub.x, where the subscript × corresponds to the sample A-H, Cu, Sb or NIused for its production.

EXAMPLE 3

[0057] This example refers to the measurement of the CO.sub.2 reduction efficiency of the electrodes prepared in the previous Example.

[0058] 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 KHCO.sub.3 was used as the electrolyte solution. In this configuration, gaseous CO.sub.2 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 (.Math.GC). The inlet of the .Math.GC 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 (.Math.GC). During the chronoamperometric measurements, the electrolytes on both sides of the anode and cathode were static, while a constant CO.sub.2 flow rate of 15 ml/min was maintained to saturate the cathode electrolyte and to bring the gaseous products to the .Math.GC. 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).

[0059] 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:

[00001]$\text{FE}\left(\%\right)=\text{nNF/Q x 100}$

[0060] where n is the number of electrons transferred in the faradic process (for the reduction of CO.sub.2 to CO and to H.sub.2, 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.

[0061] The results of the tests at two potential values are shown in Table 2.

TABLE-US-00003 Electrode Potential -0.79 V Potential -0.99 V FE.sub.co (%) FE.sub.H2 (%) FE.sub.co (%) FE.sub.H2 (%) E.sub.Cu 9.5 90 8.5 85 E.sub.A 87 14 73 26 E.sub.B 85 13 84 15 E.sub.c 90 8.5 81 18 E.sub.D 90 8 92 7 E.sub.E 91 8.5 90 8 E.sub.F 90 10.5 89 9.5 E.sub.G 89 10 85 14 E.sub.H 83.8 16.5 68.5 33 E.sub.NI 55 43 62 37 E.sub.Sb 0 63 0 83

[0062] As can be seen from the test results, the E.sub.Sb electrode does not produce CO at either test potential. The Cu electrode has poor selectivity for CO, with FE.sub.co values below 10%. The comparison E.sub.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.sub.A-E.sub.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

[0063] This example relates to the measurement of CO.sub.2 reduction with an electrode of the invention at various potentials.

[0064] The E.sub.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 H.sub.2 over time was evaluated during tests lasting between one and two hours.

[0065] 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 H.sub.2, taken when the reduction process has reached steady state, are reported at all evaluated potentials.

[0066] 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 .Math.GC. Then, the FE values stabilize, indicating the stable performance of the electrode. The E.sub.Delectrode shows very good performance in the conversion of CO.sub.2 to CO (FE.sub.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.sub.H2 values remain low (≤ 9%) from -0.69 V to -1.09 V. No other gas phase products other than CO and H.sub.2 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 H.sub.2 measured in all tests is around 100%.

COMMENTARY ON THE RESULTS

[0067] As demonstrated in the tests described above, the electrocatalyst materials of the invention catalyze the electrochemical reduction of CO.sub.2 with high selectivity toward CO. The materials of the invention then offer further advantages.

[0068] 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.

[0069] 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.