PREPARATION PROCESS THEREOF

Description

The present invention relates to electrodes containing copper and copper oxides and a method for the preparation thereof. Said electrodes can be used to electrochemically reduce carbon dioxide to produce alcohols, such as ethanol and n-propanol. The alcohols thus produced can then be used as fuel carriers or carriers for the production of fuels, thus enhancing the value of carbon dioxide.

Furthermore, the present invention relates to a method for screening electrodes containing copper, also commercially available .

In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even if not expressly declared.

For the purposes of the present discussion the term "comprising" or "including" also comprises the term "consisting of" or "essentially consisting of" .

For the purposes of the present discussion the definitions of the ranges always comprise the extreme values unless otherwise specified.

The carbon dioxide in the air is a potential source of carbon for fuel production. By electro-reducing CO2, energy can be stored in chemical bonds. To make this process possible, it is necessary to develop efficient and CO2 selective electro-catalysts.

The literature shows that many metals have been tested as catalysts, but only copper has proved effective in converting CO2 to hydrocarbons (M. Gattrell, N. Gupta, A. Co, Journal of Electroanalyt ical Chemistry 594 (2006) , 1- 19) . Generally, copper cathodes produce discrete amounts of CH4 and C2H4 in aqueous electrolytes (Yoshio Hori, Akira Murata J. Chem. Soc. Faraday Trans. 1, 1989, 85 (8) , 2309- 2326) . However, copper tends to form a broad spectrum of products, the selectivity of which is rather low (Kendra P. Kuhl, Energy Environ. Sci. , 2012, 5, 7050-7059) . In addition, copper electrodes require a high overvoltage for CO2 reduction, and their performance decays rapidly due to electrode poisoning (Yoshio Hori, Akira Murata J. Chem. Soc. Faraday Trans. 1, 1989, 85 (8) , 2309-2326) . In order to increase the selectivity towards methanol formation, the reactivity of Cu (I) oxide was explored. In the article "Le et al. Journal of The Electrochemical Society, 158 (5) E45-E49 (2011)", the authors examined the catalytic activity of an electrode consisting of copper oxide obtained through different types of synthesis (electrodeposited, anodized and heat-treated) and found a 38% efficiency for methanol production. The authors point out that Cu (I) plays a specific role precisely because of its selectivity towards methanol: the methanol formation yields, in fact, vary over time and copper oxide is reduced in a process simultaneous with that of CO2. The stability of Cu ( I ) was evaluated in the article "Karl W. Frese, Jr. J. Electrochemical Society, Vol. 138, No. 11, November 1991". However, the author specifies that careful control of the experimental parameters (current, voltage, surface area) is necessary to achieve good electrode stability levels. It is also not clear in this article how thick the oxide must be to maintain the catalytic effects over a long period of time. Oxide thickness has been studied by Christina W. Li et al. J. Am. Chem. Soc. 2012, 134, 7231- 7234. The authors pointed out that the properties of CU2O on CuO electrodes depend on the characteristics of the initial oxide. Thick layers of CU2O (>3pm) , formed by high- temperature annealing, lead to electrodes characterized by high roughness, are highly efficient catalysts for CO2 reduction and are stable with respect to deactivation processes. In this work, the main product at high overvoltages is hydrocarbons with two carbon atoms, while at low potentials the formation of carbon monoxide and formates predominates. Other authors (R.A. Geioushy et al. Electrochimica Acta, 245, 2017, 456-462) showed the formation of ethanol as the main reduction product using CU2O electrodes supported on graphene. Another article by the same authors (R.A. Geioushy et . al. Journal of Electroanalyt ical Chemistry 785 (2017) , 138-143) describes the formation of n-propanol with Cu2O/ZnO electrodes supported on graphene.

The main criticality of copper-containing electrodes is the poor stability of the electrode itself. The article "M. Le et al. , Journal of the Electrochemical society, 158 (5) , E45-E49, 2011" explains, for example, how methanol production tends to decrease after 30 minutes of reduction, while the amount of methane increases. The authors attribute this change in composition to the reduction of the electrodes to metallic copper, which is catalytic for methane production. In the previously cited article by Christina Li (Christina W. Li et al. J. Am. Chem. Soc. 2012, 134, 7231-7234) the electrodes are defined as stable on deactivation, but are tested for up to 7 hours.

In general, the reduction tests were carried out for a maximum of one day, and no evidence is reported on the stability of the electrodes after prolonged periods of activity, nor on how to regenerate exhausted electrodes. With regard to the preparation of electrodes, few works deal with in situ electrochemical preparations: in general, copper electrodes are thermally oxidized or synthetic preparations by electrodeposition or powder/ink deposition are used. The Applicant has found a mode to prepare an electrode containing copper and copper oxides having a particular morphology. Said electrode is used to reduce carbon dioxide electrochemically, producing alcohols, preferably ethanol and n-propanol, which can be used as fuel carriers or carriers for fuel production. Other products are carbon monoxide, formate and acetate.

The special preparation process gives the electrode the morphology needed to reduce carbon dioxide efficiently and with improved yields.

Due to the special oxidation-reduction reaction, copper oxides with Cu (I) and Cu (II) oxidation states are formed on the copper electrode, forming a uniform layer of spherical nanoparticles with a diameter ranging from 20 nm to 60 nm, distributed over the entire electrode. Such layers turn out to be nucleation centers of needle-shaped dendritic structures of around microns in size, which develop mainly in one direction. The dendritic structures unevenly cover the electrode surface. Alternatively, the spherical nanoparticles of the Cu (I) and Cu (II) oxide layers aggregate into clustered structures consisting of spheroidal aggregates of sub-micrometric dimensions.

In the present patent application, sub-micrometric dimensions means dimensions lower than or equal to 1 pm. In the present patent application, spheroidal aggregates means a set of particles whose shape is a spheroid. A spheroid is a three-dimensional surface obtained by rotation of an ellipse around one of its principal axes. There are three types of spheroids:

• if the ellipse is rotated about its major axis, a prolate spheroid is obtained;

* if the ellipse is rotated about its minor axis, an oblate spheroid is obtained; • if the generating ellipse is a circle, the resulting surface is a sphere.

An example of said dendritic morphology (morphology obtained with the main mode and therefore preferred) is shown in Figures 11 and 12. An example of the alternative (obtained with the secondary mode) cluster structure is shown in Figure 14. The electrodes thus obtained can be used in methods for the electrochemical reduction of CO2.

An object of the present invention is therefore an electrode containing copper and copper oxides with Cu (I) and Cu (II) oxidation state, wherein said oxides form a layer of spherical nanoparticles having a diameter ranging from 20 nm to 60 nm and are nucleation centers of needlelike dendritic structures that develop mainly following a direction .

A further object of the present invention is an electrode containing copper and copper oxides with Cu (I) and Cu(II) oxidation state, wherein said oxides form a layer of spherical nanoparticles having a diameter ranging from 20 nm to 60 nm, which aggregate in cluster structures made up of spheroidal aggregates of sub-micrometric dimensions.

A further object of the present invention is a redox process, at a temperature of at least 25°C, to prepare a copper-containing electrode, comprising the following stages : o immersing in an aqueous alkaline solution a copper- containing electrode acting as an anode, a reference electrode, a counter-electrode containing Group VIIIB metals or steel that acts as a cathode, and an ion exchange membrane; said membrane creating two separate compartments, an anode compartment where the electrode and the reference electrode are immersed, a cathode compartment where the counter-electrode is immersed; o applying and controlling a potential difference between anode and cathode in two modes: o -in the main mode, by applying a potential ramp with a scanning speed of 0.01 Volt/second until a potential between 1.2V - 2.5V vs Ref. is reached and maintaining said potential difference for a period of at least 3 minutes; o -in the secondary mode, by means of one or more voltammetric cycles starting from a minimum value equal to -1 V vs Ref. and increasing the maximum value up to 2.5V vs Ref. until a current of at least 400 mA is reached (8.5“10 ⁶ mA/m ² ) ; o if necessary flushing an inert into the aqueous alkaline solution both in the anodic zone and in the cathodic zone, preferably nitrogen.

The described and claimed electrode can be used to reduce carbon dioxide according to a process described below, forming alcohols, formate, acetate and carbon monoxide. The process for reducing carbon dioxide to form alcohols, formate, acetate and carbon monoxide comprises the following stages:

- providing an electrochemical cell including o an electrode containing copper and copper oxides as described and claimed in the present patent application, and acting as a cathode; o a counter-electrode acting as an anode, containing oxidation-resistant metals, preferably selected from the metals belonging to Group VIIIB of the Periodic Table of Elements, or steels, most preferably platinum; and o a reference electrode, o an ion exchange membrane, which creates two separate compartments, a cathode compartment where the electrode and the reference electrode are immersed, an anode compartment where the counter-electrode is immersed, said electrodes, counter-electrode and membrane being immersed in a basic aqueous solution containing alkali metals (known in the text as the electrolytic solution or electrolyte) ;

- flushing carbon dioxide in the cathodic zone until saturation;

- flushing an inert into the anodic zone, preferably nitrogen ;

- applying and controlling a potential difference between anode and cathode up to the reduction of at least part of the copper oxide (pre-reduct ion phase) and up to the reduction of CO2.

The process described and claimed in the present patent application allows to prepare new nano-structured electrodes, or to activate electrodes obtained by methods other than electrochemistry, and to discern a priori whether the oxide obtained is an efficient catalyst for CO2 reduction .

The process described and claimed in the present patent application allows to prepare the electrode directly in the electrochemical cell which will then be used for the carbon dioxide reduction process (in situ preparation) . Thanks to this process, the electrode has a particular morphology, as described above and illustrated for example in Figures 3, 11-14, which confers greater faradic efficiency to the cell and leads to higher yields in the carbon dioxide reduction process.

Anodizing of copper-containing electrodes in situ means that no further stages of synthesis and deposition, or even further annealing or cooling stages, are required before the test stage.

The electrodes obtained by the described and claimed process show good stability over time without altering the catalytic properties, but even improving them over time. The electrodes produced in this way are found to recover from any drop in performance during their operation in the carbon dioxide reduction process, for example by working at 50 mA (l.l“10 ⁶ mA/m ² ) for up to 4 days of use of the same electrode on consecutive days, showing larger catalytic surfaces and an increase in CO2 reduction products. Reducing at increasing currents from 25 to 150 mA ( 5.3 ■ 10 ⁵ -3.2 ■ 10 ⁶ mA/m ² ) for 5 consecutive days, the electro-catalytic system is optimized in terms of activation speed and selectivity towards alcohols.

Further aims and advantages of the present invention will appear more clearly from the following description and from the accompanying figures, given purely by way of a nonlimiting example, which represent preferred embodiments of the present invention.

Figure 1 shows the diagram of an electrochemical cell used for CO2 reduction, where 1 is the potentiostat /galvanostat , 2 is the CO2 bubbling tube, 3 the N2 bubbling tube, 4 the cathode compartment, 5 the anode compartment, 6 the separation membrane, 7 the working electrode, 8 the reference electrode and 9 the counter-electrode. The anode and cathode compartments contain two different alkaline aqueous solutions.

Figure 2 shows anodizing curves obtained by applying an increasing potential of 1.2V, 1.4V, 1.6V, 1.8V and 2V vs Ag/AgCl, relative to example 2, i.e. , the main operating mode .

Figure 3 shows scanning electron microscope SEM images of the morphology of anodized copper wire at 5000X magnification, generated by anodizing at 2V vs Ag/AgCl potential, using the main operating mode and referring to Example 3.

Figure 4 shows the galvanostat ic curve E vs t of CO2 reduction and refers to Example 5.

Figure 5, Image A shows the full NMR spectrum of the electrolyte after galvanostat ic reduction to 50 mA for 6 hours, carried out in Example 6.

Figure 5, Image B shows the magnification of the area highlighted in the full spectrum of Figure 5, Image A. Figure 5, Images A and B refer to examples 5 and 6.

Figure 6 illustrates the faradic efficiencies obtained by processing data from NMR analyses for the formation of ethanol, acetic acid, formic acid and 1-propanol after galavanostat ic reduction of CO2 for 6 hours, referring to Example 6.

Figure 7 shows the characteristic trend of the cyclic voltammetry curves applied to an electrode containing copper and copper oxides CU/CU2O obtained by means of anodizing process, which proved to be an efficient catalyst for the reduction of carbon dioxide, referring to example 7.

Figure 8 shows the effect of the post-passivation treatment; it refers to comparative example 1 and shows the development of an anodizing curve in which the application of the potential was stopped immediately after the oxidation peak.

Figure 9 refers to example 8 and shows the cyclic voltammetry curves in the range -0.2V-1.5V vs Ag/AgCl to monitor the current carried by the electrodes in tests carried out on successive days.

Figure 10 shows the faradic efficiency for ethanol and propanol production in tests carried out on successive days with the same electrode, as described in example 8. Figures 11 and 12 illustrate scanning electron microscope SEM images of the anodized copper wire at 2V and 1.8V respectively using the main operating mode, at 5000X magnification. Figures 11 and 12 show the dendritic structures .

Figure 13 shows scanning electron microscope SEM images of the anodized copper wire at 1.4V using the main operating mode. Figure 13 shows the absence of dendritic structures. Figure 14 shows scanning electron microscope SEM images of the anodized copper wire using the secondary operating mode based on voltammetric cycles. Figure 14 refers to example 9.

Detailed description.

The Applicant now describes in detail the electrodes object of the present patent application.

The described and claimed electrode contains copper and copper oxides with Cu (I) and Cu (II) oxidation states. These oxides form a layer of spherical nanoparticles with a diameter ranging from 20 nm to 60 n. Spherical nanoparticles are nucleation centers of needle-shaped dendritic structures that develop mainly following a direction. This type of electrode, for example, is shown in Figures 11 and 12.

Alternatively, the spherical nanoparticles aggregate into clustered structures consisting of spheroidal aggregates of sub-micrometric dimensions. This type of electrode is shown in Figure 14.

The Applicant now describes in detail the process object of the present patent application also by reference to Figures 1-14.

With this process, the described and claimed electrodes can preferably be prepared.

In an electrolytic cell (Figure 1) filled with an aqueous alkaline solution, an electrode containing copper acting as an anode (7) , a reference electrode (8) , a counterelectrode acting as a cathode (9) containing metals which must not be oxidized, preferably selected from Group VIIIB metals or steel, more preferably platinum, and an ion exchange membrane (6) are immersed. Said membrane creates two separate zones: an anodic zone in which the copper electrode and the reference electrode are immersed, and a cathodic zone in which the counter-electrode is immersed. The reference electrode can be selected from a saturated calomel (SCE) , or an electrode containing silver and silver chloride (referred to as Ag/AgCl) .

A potential difference is applied and controlled between the anode and cathode in two alternative operating modes. In the main mode, a potential ramp is applied with a scanning speed of 0.01 Volt/second until a potential between 1.2 V - 2.5 V vs Ref. is reached and said potential difference is maintained for a period of at least 3 minutes .

In the secondary mode, the potential difference is applied by means of one or more voltammetric cycles, starting from a minimum value of -IV vs Ref. and increasing the maximum value up to 2.5V vs Ref. where Ref. indicates a reference electrode, increasing the number of cycles until a current of at least 400 mA (8.5“10 ⁶ mA/m ² ) is reached.

The preparation process is based on a redox reaction carried out at a temperature of at least 25°C and preferably not more than 30°C.

The aqueous alkaline solutions can be the same or different in the anode and cathode compartments.

The aqueous alkaline solutions can contain oxides or carbonates of alkali metals. The latter can preferably be selected from lithium (Li) , sodium (Na) , potassium (K) , rubidium (Rb) , caesium (Cs) and Francium (Er) ; more preferably potassium. More preferably, they can be selected from KHCO ₃ or KOH.

In the main operating mode, a potential AV between 1.5V and 2.5V vs Ref. is preferably achieved, more preferably between 2V vs Ref and 2.5V vs Ref. , even more preferably 2V vs. Ref. Preferably said potentials are maintained for at least 5 minutes, more preferably for at least 10 minutes so as to complete the oxidation. As mentioned, the AV applied to each stage of the ramp is 0.01V, equal to 0. OlV/s .

The minimum time applied during the main operating mode is necessary to complete the post-passivation stage of the electrode, which is important for the formation of the morphologies shown in the present description and for improving the selectivity to alcohols.

In the secondary operating mode, it is preferable to start from a minimum potential of -1.15V vs Ref. increasing the maximum value up to 2.5V vs Ref.; even more preferable to start from a minimum potential of-1.25V vs Ref. increasing the maximum value up to 1.8V vs Ref. ; even more preferable to start from a minimum value of -1.25V vs Ref. increasing the maximum value up to 0.5V vs Ref.

Once prepared, the described and claimed electrodes are subjected to a test stage to assess which ones show adequate selectivity to alcohols in the carbon dioxide electrochemical reduction process. This test is carried out by electrochemically reducing CO2 as a function of the anodizing potential and evaluating the reduction products obtained. It has been observed that electrodes obtained at high maximum potentials (1.8V-2V vs. Ref. ) show greater selectivity towards alcohols.

When a potential difference is applied between the anode and cathode, part of the copper metal is oxidized, releasing Cu ²⁺ and Cu ⁺ ions. The electrode surface is covered with CuO, while part of said ions remain in the electrolyte .

Part of the copper is oxidized to Cu ⁺ and part to Cu ²⁺ , and part of the Cu ²⁺ passes in solution in the electrolyte. When the electrode is used for CO2 reduction, a first stage occurs in which the Cu ²⁺ ions in solution are deposited on the cathode, reducing to CU2O, and simultaneously the CuO phase previously formed in the anodizing stage is reduced to CU2O, thus regenerating the morphology shown in the SEM photographs shown for example in Figures 3 and 11-12. The reduction of carbon dioxide leads to the formation of carbon monoxide, formic acid, acetic acid, ethanol and n- propanol .

Preferably in the main operating mode, after the desired maximum potential has been reached and maintained for at least 3 minutes, voltammetry tests are carried out consisting of a linear scan in cyclic voltammetry (CV) current. The purpose of said tests is to assess the formation of copper oxide on the electrode. A linear current scan allows to reduce any CuO formed on the electrode or dissolved in the electrolyte and to estimate in advance the potential that will be reached during the galvanostat ic reduction of carbon dioxide. This allows to measure the efficiency of the electrode.

It is necessary to increase the number of active sites of the electrode, making an efficient oxide. In order to verify the performance of the electrode, the current reached during the voltammetric cycles was monitored: a higher current corresponds to a greater number of active sites. Furthermore, an anodic peak with onset approximately OV vs Ref. and a cathodic peak with onset approximately - 0. IV vs Ref. must be present in the CV scans. The current intensity must reach 400mA (8.5“10 ⁶ mA/m ² ) . Such a position and shape of the peaks is a fundamental and surprising characteristic for establishing a priori whether an efficient catalyst has formed during the oxidation stages. The process for preparing copper-containing electrodes described and claimed in the present patent application can also be applied to commercial electrodes or electrodes produced according to state-of-the-art methods, in order to increase their selectivity to alcohols in the carbon dioxide reduction process, and thereby increase their faradic efficiency.

Hence said process is used as a method for testing appropriately pre-selected electrodes (screening stage) . For example, starting from electrodes prepared by means of the thermal oxidation technique of copper, known in the state of the art, and applying the process described and claimed in the present patent application, it is possible to promote the formation of ethanol and 1-propanol as CO2 reduction products and thus evaluate the efficiency of the electrode .

A further object of the present patent application is an electrode containing copper and copper oxides with Cu (I) and Cu (II) oxidation state, said electrode being obtainable with a redox process, at a temperature of at least 25°C, said process comprising the following stages: o immersing in an aqueous alkaline solution a copper- containing electrode acting as an anode, a reference electrode, a counter-electrode containing Group VIIIB metals or steel that acts as a cathode, and an ion exchange membrane; said membrane creating two separate compartments, an anode compartment where the working electrode and the reference electrode are immersed, a cathode compartment where the counterelectrode is immersed; o applying and controlling a potential difference between anode and cathode by one of these two mode: o in the main mode, applying a potential ramp with a scanning speed of 0.01 Volt/second until a potential between 1.2 V - 2.5 V vs Ref. is reached and maintaining said potential difference for a period of at least 3 minutes; or o in the secondary mode, applying one or more voltammetric cycles starting from a minimum value equal to -1 V vs Ref. and increasing the maximum value up to 2.5 V vs Ref. until a current of at least 400 mA is reached (8.5“10 ⁶ mA/m ² ) ; o if necessary, flushing an inert into the aqueous alkaline solution in both the anode and cathode compartments, preferably nitrogen.

A further object of the present patent application is an electrode containing copper and copper oxides in the Cu (I) and Cu (II) oxidation state, in which said oxides form a layer of spherical nanoparticles having a diameter ranging from 20 nm to 60 nm, said particles being nucleation centers of needle-like dendritic structures which develop mainly following a direction; said electrode obtainable by a redox process, at a temperature of at least 25°C, said process comprising the following stages: o immersing in an aqueous alkaline solution a copper- containing electrode acting as an anode, a reference electrode, a counter-electrode containing Group VIIIB metals or steel that acts as a cathode, and an ion exchange membrane; said membrane creating two separate compartments, an anode compartment where the electrode and the reference electrode are immersed, a cathode compartment where the counter-electrode is immersed; o applying and controlling a potential difference between anode and cathode by means of the following mode (main mode) : o applying a potential ramp with a scanning speed of 0.01 Volt/second until a potential between 1.2V-2.5V vs Ref. is reached and maintaining said potential difference for a period of at least 3 minutes ; o if necessary, flushing an inert into the aqueous alkaline solution in both the anode and cathode compartments, preferably nitrogen.

A further object of the present patent application is an electrode containing copper and copper oxides having Cu (I) and Cu (II) oxidation states, in which said oxides form a layer of spherical nanoparticles having a diameter ranging from 20 nm to 60 nm, which aggregate into cluster structures consisting of spheroidal aggregates of submicrometric dimensions, said electrode being obtainable by a redox process, at a temperature of at least 25°C, said process comprising the following stages: o immersing in an aqueous alkaline solution a copper- containing electrode acting as an anode, a reference electrode, a counter-electrode containing Group VIIIB metals or steel that acts as a cathode, and an ion exchange membrane; said membrane creating two separate compartments, an anode compartment where the electrode and the reference electrode are immersed, a cathode compartment where the counter-electrode is immersed; o applying and controlling a potential difference between anode and cathode by means of the following mode (secondary mode) : o applying by means of one or more voltammetric cycles starting from a minimum value equal to - 1 V vs Ref. and increasing the maximum value up to 2.5 V vs Ref. until a current of at least 400 mA is reached (8.5“10 ⁶ mA/m ² ) ; o if necessary, flushing an inert into the aqueous alkaline solution in both the anode and cathode compartments, preferably nitrogen.

The Applicant has also developed a method for selecting electrodes described and claimed in the present patent application. According to this selection method, the following stages are comprised: -providing an electrochemical cell containing an aqueous solution (electrolytic solution) in which the electrode described and claimed in the present patent application is positioned;

-flushing carbon dioxide to saturate the electrolytic solution and proceeding to apply the potential.

As mentioned above, the screening phase of the electrodes described and claimed in the present patent application, is carried out with CV measurements: in the CV scans, an anodic peak with onset about 0V vs Ref. and a cathodic peak with onset about -0.1V vs Ref. must be present. The current intensity must reach 400mA (8.5“10 ⁶ mA/m ² ) . Such a position and shape of the peaks is a diriment and surprising characteristic for establishing a priori whether an efficient catalyst has been formed during the oxidation stages .

The methodology allows to understand a priori whether a good copper oxide catalyst for CO2 reduction is present on the electrode.

The pre-testing of the electrode by means of voltammetry cycles allows to avoid lengthy reduction tests by establishing a priori whether the oxide formed is an efficient catalyst. When copper-containing electrodes are prepared with the state-of-the-art techniques, they are not very stable during the electrochemical reduction of carbon dioxide. The electrodes according described and claimed in the present patent application appear to be particularly stable with respect to what is known in the state of the art; indeed, it has been observed that said electrodes show stable performance during the electrochemical reduction of carbon dioxide. The results so far show a working stability of 6 hours of the electrode in CO2 reduction with tests carried out on several consecutive days. Furthermore, one of the advantages of the electrodes and processes described an claimed in the present patent application lies in the fact that if, during the operation of the electrodes in CO2 reduction, the current performance decreases, it is sufficient to proceed with a few voltammetry cycles to restore or even improve the initial performance of the electrode .

Once prepared, the described and claimed electrodes are subjected to a test stage to evaluate which ones show adequate selectivity to alcohols in the carbon dioxide electrochemical reduction process according to a galvanostat ic mode. Said analysis method comprises the following stages: -providing an electrochemical cell containing an aqueous solution (electrolytic solution) in which the electrode described and claimed in the present patent application is positioned;

-flushing carbon dioxide to saturate the electrolytic solution and proceeding to apply the potential.

Preferably the carbon dioxide oxidation time can vary from

1 to 24 hours, even more preferably 6 hours. Preferably the applied current can vary from 50mA (l.l“10 ⁶ mA/m ² ) to 200mA (4.2“10 ⁶ mA/m ² ) , more preferably from 100mA (2.1 “IO ⁶ mA/m ² ) to 150mA (3.2 “IO ⁶ mA/m ² ) .

Gas-phase reduction products were determined by gas chromatography and liquid-phase reduction products were determined and quantified by NMR spectroscopy. The hydrogen derives from a parasitic hydrolysis reaction.

The electrode described and claimed can be used to reduce carbon dioxide according to a process described below with which gas phase products such as carbon monoxide and liquid phase products such as ethanol, n-propanol, acetate and formate can be obtained. Said process comprises several stages .

In a first stage, an electrochemical cell must be provided which includes:

• an electrode containing copper and copper oxides in the Cu ( I ) and Cu (II) oxidation state as described and claimed in the present patent application, and acting as a cathode;

• a counter-electrode acting as an anode, containing metals which must not oxidize, preferably selected from the metals belonging to Group VIIIB of the Periodic Table of Elements, or steels, most preferably platinum; and

• a reference electrode,

• an ion-exchange membrane.

The membrane creates two separate compartments, a cathode compartment in which the electrode and reference electrode are immersed, and an anode compartment in which the counter-electrode is immersed.

The electrodes containing copper and copper oxides in the Cu ( I ) and Cu (II) oxidation state, the counter-electrode and the membrane are immersed in an aqueous solution containing alkali metals (known in the text as electrolytic solution) .

Carbon dioxide is flushed into the cathode compartment until saturation; an inert is flushed into the anode compartment, preferably the inert is nitrogen.

A potential difference is applied and controlled between anode and cathode until at least part of the copper oxide Cu (II) is reduced (pre-reduct ion stage) and until the CO2 is reduced.

The reduction process can be carried out at a temperature of at least 25°C and preferably not more than 30°C.

The aqueous solutions can be the same or different in the anode and cathode compartments. The purpose of a different solution is to have a higher conductivity in the compartment where water oxidation occurs.

The aqueous alkaline solutions can contain oxides or carbonates of alkali metals. The latter can preferably be selected from lithium (Li) , sodium (Na) , potassium (K) , rubidium (Rb) , caesium (Cs) and francium (Fr) ; more preferably potassium. More preferably, they can be selected from KHCO3 or KOH.

Among the metals that must not oxidize, the preferred metals are those belonging to Group VIIIB of the Periodic Table of Elements, preferably selected from Fe, Rb, Os, Co. Rh, Ir, Ni, Pd, Pt; even more preferred is platinum. Some examples are given below for a better understanding of the invention and of the scope of application despite not constituting in any way a limitation of the scope of the present invention.

EXAMPLES

The anodizing and CO2 electrochemical reduction tests were carried out in an H-type cell with two compartments separated by a membrane as shown in Figure 1. The cell contains an aqueous solution of KHCO3 at a concentration of IM. The Nafion 115 membrane separates the cell into two compartments in each of which are immersed in the aqueous alkaline solution respectively copper-containing working electrode and silver-silver chloride (Ag/AgCl) reference electrode, and counter-electrode which is a platinum or steel filament. A 3-electrode configuration was therefore used .

The working electrode acts as an anode during the preparation or anodizing stage, and as a cathode during the carbon dioxide reduction process.

Thus, the counter-electrode acts as a cathode during the electrode preparation stage and as an anode during the carbon dioxide reduction process.

During the CO2 reduction process, CO2 is flushed in the cathode compartment to saturate the solution, while N2 is flushed in the anode compartment to degas the solution and dilute any oxygen formed during the process.

The carbon dioxide reduction process is carried out in KHCO3 electrolyte at IM concentration applying a potential in the range 1.2-2V vs Ag/AgCl for a time in the range 10- 13 minutes. The desired potential value is reached by applying a potential ramp with a scanning rate of O .OlV/s starting at -0.15V vs Ag/AgCl and maintaining the final potential for at least 10 minutes to achieve complete oxidation of the surface. The graphs of the anodizing curves are shown in Figure 2.

In the CO2 reduction process, the reduction products were evaluated as a function of the anodizing potential, and it was established that working electrodes obtained at high potential values ranging from 1.8V to 2V demonstrate greater selectivity towards alcohols. However, when the working electrodes are prepared at low potentials below 1.8V, the main product is formic acid.

To verify the performance of the working electrode, the current reached in the voltammetric cycles was monitored: a higher current corresponds to a greater number of active sites on the electrode surface. Furthermore, an anodic peak with onset approximately OV vs Ag/AgCl and a cathodic peak with onset approximately -0.1V vs Ag/AgCl must be present in the cyclic voltammetry CV scans. The current intensity must reach 400mA (8.5“10 ⁶ mA/m ² ) . Such a position and shape of the peaks is a diriment and surprising characteristic for establishing a priori whether an efficient catalyst has been formed during the oxidation stages.

The electrodes were tested as catalysts for CO2 reduction in galvanostat ic mode for a time of 6 hours continuously. The currents applied were 50, 100 and 150 mA (l.l“10 ⁶ , 2.1 “IO ⁶ and 3.2 “IO ⁶ mA/m ² ) . The reduction products in the gas phase were determined by gas chromatography and are mainly H2 (from the parasitic hydrolysis reaction) and, to a much lesser extent, CO, ethylene and ethane. The liquid phase products were determined and quantified by NMR spectroscopy and found to be formate, ethanol, propanol and acetate. The percentage faradic efficiencies for products obtained with 50mA applied current after 6 hours of reduction were 4.7% for formate, 4.1% for acetate, 2.4% for ethanol and 2.4% for n-propanol. The efficiencies decrease linearly as the applied current increases.

Example 1 : experimental set up Figure 1.

The anodizing and CO2 electrochemical reduction tests were carried out in an H-type cell with two compartments separated by a Nafion 115 membrane. A 3-electrode configuration was used, with the silver/silver chloride (Ag/AgCl) reference electrode and the copper-containing working electrode inserted into the cathode compartment and immersed in a 0.5M KHCO3 aqueous solution. The working electrodes used consist of wires (4 wires 150mm long and 0.025mm in diameter) or thin sheets (5*30mm ² ) of copper which is oxidized using the described and claimed process. The counter-electrode, consisting of a platinum or, alternatively, steel filament immersed in a IM KOH aqueous solution, is placed in the anode compartment. In order to saturate the solution in the cathode compartment, CO2 is flushed in at a flow rate of approximately 20 ml/min. The pH of the solution is initially 9 and drops to about 7 after saturation with CO2. In the anode compartment, N2 is flushed to degas the solution and to dilute any oxygen formed during the process. Measurements were made with an Autolab PGSTAT 20 potentiostat /galvanostat using the Nova program. The continuous monitoring of the potential difference between anode and cathode was carried out using an externally connected tester. All the measurements are carried out at temperatures above 25°C. The same cell used for this test will be used for subsequent carbon dioxide reduction tests.

Example 2 : preparation of working electrode by means of anodizing .

The anodizing of the copper filament /laminate is carried out in the same cell (Figure 1) used in example 1 and also used for the subsequent reduction tests. A copper filament is washed with dilute hydrochloric acid and then immersed in compartment 4 of the cell. The anodizing occurs in two stages: the first stage is a ramp of potential from -0.15V vs Ag/AgCl at a scanning rate of 0.01 V/s. The final ramp value is in the range 1.2-2V vs Ag/AgCl. The second anodizing stage is a potent iostat ic stage in which the maximum potential is applied for a period of 10-13 min. At this stage the cathode solution turns blue due to the Cu ²⁺ passed in solution. Figure 2 shows the trend of the current anodizing curves towards time, where the copper oxidation peak is present. The curves vary considerably as the maximum potential is reached. In particular, in the curves obtained at potentials greater than or equal to 1.8V vs Ag/AgCl, the current after the oxidation peak increases. It can be assumed that the initial oxide layer develops rapidly and then there is the formation of further oxidized low-resistive surfaces.

Example 3 : electrode morphology

In this example, the morphology of the working electrodes prepared in example 2 is analyzed.

The morphology of the electrodes is assessed by scanning electron microscopy with a secondary electron detector. From a morphological point of view, it can be observed that as the potential increases, the specific surface area and the anchoring of the oxide layer to the metal bulk increases .

The morphology of the electrode shows an oxide layer composed of spherical nanoparticles ranging in size from 2 to 60 nm, distributed throughout the sample. Spherical aggregates of sub-micrometric-dimension nanoparticles form on this layer and more or less uniformly cover the entire sample. Such formations appear to be nucleation centers of needle-shaped dendritic structures, which are around pm in size and develop mainly in one dimension. Such structures unevenly cover the electrode surface.

The degree of surface coverage and the size of these formations is greater the higher the anodizing potential. By way of example, figures 3 and 11 show the morphology of the CU2O sample obtained at 2V vs Ag/AgCl, while figure 12 shows that of the sample obtained at 1.8V vs Ag/ag/Cl.

Example 4 : Chemical and physical characterization of the electrodes prepared in Example 2.

X-ray diffraction analysis (XRD) , micro-Raman spectroscopy and hemispherical reflectance measurements (DRS) with a UV-Vis-NIR spectrophotometer were used to determine the composition of the electrodes in Example 2. Furthermore, the electrodes were tested in synchrotron light at the

ESCA-Microscopy line.

XRD analysis did not reveal the presence of any copper oxide in crystalline form.

Raman spectroscopy confirmed that the oxide formed is CU2O (bands at 147 cur ¹ , 214 cur ¹ , 411 cur ¹ , 501 cur ¹ , 642 cnr ¹ ) with a small percentage of CuO .

In the DRS UV Vis spectra there are bands associated with charge transfer between ligand (0 ² ~) and metal (Cu ²⁺ ) and between metal and ligand, the band associated with the surface plasmon resonance of copper nanoparticles and the band with a maximum at 750 nm associated with the d-d transition of Cu ²⁺ in octahedral coordination in CuO.

The presence of Cu (I) oxide on the surface of these electrodes was unequivocally demonstrated by ESCA spectroscopy measurements carried out in synchrotron light .

Example 5 : carbon dioxide reduction using the working electrodes produced in Example 2.

The CO2 reduction is carried out in situ, without changing the configuration of the electrochemical cell, using the same cell, experimental set up and working electrodes as in Examples 1 and 2. The tests are carried out in galvanostat ic mode, applying a current value in the range 25-150 mA ( 5.3 ■ 10 ⁵ -3.2 ■ 10 ⁶ mA/m ² ) for a period of 6 hours. Figure 4 shows the potential versus time curve of a reduction test carried out at 25 mA (5.3“10 ⁵ mA/m ² ) for the anodized electrode at 2V vs Ag/AgCl. At the end of each day, the electrolyte in the cathode compartment is discharged for analysis. The reduction products in the gas phase were determined by gas chromatography and are mainly H2 (from the parasitic hydrolysis reaction) and, to a much lesser extent, CO, ethylene and ethane. The liquid phase products were determined and quantified by NMR spectroscopy and found to be formate, ethanol, propanol and acetate.

In this phase, the solution is discolored and the Cu ²⁺ passed in solution (as described in example 2) is deposited on the surface of the electrode, helping to form the high surface area morphology.

Example 6 : NMR characterization of liquid CO2 reduction products .

The reaction products in the liquid phase were characterized by GC gas chromatography and ¹ H-NMR spectroscopy in solution. A standard pre-saturat ion sequence centered on the H2O signal and power of 57.7 dB was used to optimize the spectra. A glass capillary containing a ImM solution of phenol in deuterated dimethyl sulfoxide was also inserted into the NMR tube in order to calibrate the intensity of the signals in the different spectra against the same reference signal. The NMR spectra are shown as an example in Figure 5.

Figure 6 shows the graph of the formation efficiencies of the CO2 reduction products as a function of the anodizing potential with which the electrode was prepared. Galvanostat ic tests were carried out for a time of 6 hours at 25 mA (5.3“10 ⁵ mA/m ² ) , with the electrodes prepared according to example 2. The highest faradic efficiency towards alcohols is obtained using an electrode prepared at 2.0V vs Ag/AgCl: the resulting efficiencies of 2.09% for ethanol and 3.83% for n-propanol respectively

Example 7 : electrode testing stage

An efficient electrode is characterized by an adequate number of active sites and must therefore be covered by an efficient oxide. To verify the performance of the electrode, the current reached in voltammetric cycles GV was monitored: it is assumed that a higher current corresponds to a greater number of active sites. Furthermore, an anodic peak with onset approximately OV vs Ag/AgCl and a cathodic peak with onset approximately -0.1V vs Ag/AgCl must be present in the CV scans. The current intensity must reach 400mA (8.5“10 ⁶ mA/m ² ) . An example of such volt ammograms is shown in figure 7. Such a position and shape of the peaks is a diriment and surprising characteristic for establishing a priori whether an efficient catalyst has been formed during the oxidation stages .

Such testing methodology can be used to test electrodes obtained by applying other different preparation methods, such as thermal oxidation.

Comparative Example 1: anodizing curve description, test without post-passivation

The choice of high potential is not sufficient to ensure excellent alcohol yields and good catalytic surfaces of copper-based electrodes, but an adequate post-passivation stage is also required. As proof of this, anodizing at 2V vs Ag/AgCl was stopped immediately after completion of the passivation peak (Figure 8) . The results in terms of CO2 reduction products with the electrode thus obtained showed predominant formation of formic acid.

Example 8: stability tests of the electrodes of Example 2. Two different stability tests were carried out on the electrodes prepared in Examples 1 and 2 in order to test their service life. The results can be seen in Figure 9 and 10.

Stability Test 1

Duration tests were carried out to verify the stability over time of the 2V anodized electrodes prepared in Examples 1 and 2. The tests consisted of daily CO2 reduction tests in galvanostat ic mode at 50 mA (l.l“10 ⁶ mA/m ² ) by applying the current for a time of 6 hours each day; the tests were repeated for 4 consecutive days for a total of 24 hours of electrode work. The tests showed that the system activates with time: cyclic voltammetry tests in the range -0.2V -1.5V vs Ag/AgCl (figure 9) showed an increase over time in the current carried by the electrodes. Table 1 specifies the current values reached by the CV curves in Figure 9 at potential 1.2V and 1.5V vs. Ag/AgCl, further highlighting the increase in current over the following days.

Table 1 :

The faradic efficiency values for ethanol and propanol production are shown in Figure 10. It can be seen that the production of these alcohols also increases over the following days as the current carried by the electrodes increases .

Stability Test 2

The electrodes prepared with examples 1 and 2 were subjected to a daily stress test with duration tests for 6 hours each at increasing current. A different current was applied each day and kept constant for 6 hours. The currents tested were 25 mA, 50 mA, 75 mA, 100 mA and 150 mA. (5.3 - 10 ⁵ , 1-10 ⁶ , 1.6 - 10 ⁶ , 2.1 - 10 ⁶ , 3.2 - 10 ⁶ , mA/m ² ) .

Each test lasted 6 hours and the tests were carried out on consecutive days. The electrodes are stable and catalytically active towards CO2 reduction.

Example 9 : anodizing with alternative preparation

This example describes an alternative anodizing process, secondary to that described in example 2. The anodizing of the copper filament /laminate is carried out in the same cell (figure 1) used in example 1 and also used for the subsequent reduction tests. A copper filament is washed with diluted hydrochloric acid and then immersed in compartment 4 of the cell by means of one or more voltammetric cycles starting from a minimum value of -IV vs Ag/AgCl and increasing the maximum value up to 2.5V vs Ag/AgCl until a current of at least 400 mA (8.5“10 ⁶ mA/m ² ) is reached. The SEM images of the electrodes obtained with the procedure described are shown in image 14. Morphologically, the electrode consists of an oxide layer consisting of spherical nanoparticles ranging in size from 2 to 60 nm; the spherical nanoparticles aggregate into cluster structures consisting of spheroidal aggregates of sub-micrometric dimensions.