WO2021028779A1

WO2021028779A1 - Process for forming and regenerating a copper cathode for an electrochemical cell and electrochemical cell for the production of industrial products

Info

Publication number: WO2021028779A1
Application number: PCT/IB2020/057363
Authority: WO
Inventors: Alessandro NIORETTINI; Carlo Alberto Bignozzi; Stefano Caramori; Serena BERARDI
Original assignee: Laboratorio Audio S.N.C. Di Niorettini C. & Doro S.
Priority date: 2019-08-09
Filing date: 2020-08-04
Publication date: 2021-02-18
Also published as: EP4010513A1; IT201900014580A1

Abstract

A process for forming and regenerating a copper cathode (1) for an electrochemical cell (10) for the production of industrial products, for example syngas, comprises an operating step of anodising a copper substrate (2) in a solution containing chlorides and sulphates at atmospheric pressure and ambient temperature, to which an alternating wave electric potential of 0 mV to 1500 mV is applied, at a frequency of 100 Hz to 1500 Hz, for at least 1 minute, so that copper salts form and are deposited on the substrate (2). The process further comprises an electrochemical reduction step in a non-acidic working solution for the formation of surface catalytic nanostructures (4) on the substrate (2). An electrochemical cell (10) for the production of industrial products is also part of the present patent application.

Description

ocess for forming and regenerating a copper cathode for an electrochemical cell and electrochemical cell for the production of industrial products

Technical Field

The present invention relates to a process for forming and regenerating a copper cathode for electrochemical applications.

In addition, the invention relates to an electrochemical cell for the production of industrial products, for example syngas.

In particular, the invention falls within the field of electrochemical devices for the production of useful molecules from an energy and industrial point of view by exploiting redox reactions which occur on the surface of the electrodes so as to develop syngas, a fundamental precursor for the industrial synthesis of hydrocarbon fuels.

Prior Art

There is now ample evidence that carbon dioxide is the main anthropogenic greenhouse gas greatly responsible for current climate change. In a context where industry and energy are still firmly dependent on fossil fuels, the capture of this gas and the conversion thereof into an alternative source of raw material represents an interesting opportunity to re-use what is currently considered waste and a source of environmental pollution.

To overcome the traditional solutions aimed at the simple sequestration of environmentally harmful gas, appropriate fuel cells (or flow cells) have been developed in the electrochemical field, configured to implement virtuous cycles of carbon dioxide reuse as a fuel and as a possible storage system for non-continuous renewable energy (for example, solar energy). Generally, the electrochemical cells consist of an anode and a cathode immersed in an electrolyte used to transport ions resulting from the redox reactions occurring on the surface of the electrodes. In this way, the electrochemical cells are advantageously configured to convert electrical energy through such redox reactions, without any thermal combustion process occurring. In this regard, the electrochemical solution is of particular interest, as it focuses on reducing the gas on the surface of an appropriate catalytic cathodic substrate.

To date, in the electrochemical cells developed the electrodes may exhibit a nanostructure surface morphology to implement the efficiency of the redox exchange reactions and thus the final amperometric yield.

However, the main limitations of these electrochemical cells are varied, including: stringent reaction conditions; unfavourable costs of the necessary materials (for example gold, platinum or palladium); the conversion yields of the related processes are not always regular and optimal, as they can irretrievably suffer from the overwhelming competition of the water splitting reaction, which is kinetically favoured in an aqueous environment.

Summary

In this context, the technical task underlying the present invention is to propose a process for forming and regenerating a copper cathode for an electrochemical cell and an electrochemical cell for the production of industrial products, both capable of overcoming the drawbacks of the aforementioned prior art.

In particular, an object of the present invention is to provide a process which can be calibrated with the control of a few simple parameters (e.g., frequency and time intervals), which is functional for both forming and regenerating a copper cathode for an electrochemical cell.

Another object of the present invention is to provide a forming and regenerating process having an excellent CO₂ reduction efficiency in an aqueous environment.

In addition, an object of the present invention is to provide an electrochemical or electrolytic cell capable of operating in environmentally compatible conditions. Another object of the present invention is to provide an electrochemical or electrolytic cell produced with materials of simple availability, low cost and low environmental impact.

A further object of the present invention is to provide an electrochemical or electrolytic cell compatible with use in conjunction with a renewable energy source, such as photovoltaics, for the storage of electrical energy in the form of industrial products.

The specified technical task and the specified objects are substantially achieved by a process for forming and regenerating a copper cathode for an electrochemical cell and an electrochemical cell for the production of industrial products, interesting from an energy point of view (as they can be used to obtain hydrocarbons), which comprise the technical characteristics set out in the independent claims. The dependent claims correspond to further advantageous aspects of the invention. It should be noted that this summary introduces, in a simplified form, a selection of concepts which will be further elaborated in the detailed description given below.

The invention relates to a process for forming and regenerating a copper cathode for an electrochemical cell for the production of industrial products, for example syngas.

Such process comprises the operating steps of: preparing a copper substrate defining an electrode; anodising the substrate in an electrolytic solution based on sulphates and chlorides for a period of at least 1 minute, at an AC electric potential varying between 0 mV (millivolt) and +2000 mV and with a frequency varying between 100 Hz (Hertz) and 1500 Hz. The anodisation is also carried out at atmospheric pressure and ambient temperature so that copper salts form and can be deposited on an active surface of the substrate; carrying out an electrochemical reduction of the anodised substrate in an electrolytic working solution having a non-acidic pH, so that catalytic neutral copper nanostructures having a variable density depending on the parameters used form on the active surface, so as to obtain cell current densities between about 50 mA/cm² (milliAmps/square centimetre) and 200 mA/cm² at an operating potential of about -1500 mV. Preferably, the surface density of the copper nanocubes varies between 1 - 2 nanocubes per mm² (square micrometre) and about ten nanocubes per mm² associated in agglomerates and columnar structures.

In other words, in the field of electrochemical carbon dioxide reduction, the nanostructuring of a catalytic surface makes it possible to significantly modify the efficiency and selectivity of the same material with respect to the different reaction products, as well as allow the exposure of an electrochemically active surface greater than a planar geometry or "bulk” interface.

In the specific case described above, the result of the process is a copper cathode having catalytic activity which is extremely sensitive to the interface structure on which the various reaction intermediates coordinate, so as to make the nanostructured material radically different from the initial material. Furthermore, the invention relates to an electrochemical cell for the production of industrial products, which comprises a box-like body having a containment volume for an electrolyte, preferably liquid, and inside which a membrane permeable to protons is placed so as to divide the containment volume into an anodic compartment and a cathodic compartment.

The anode is placed in the anodic compartment and at least partially immersed in the electrolyte, while the cathode is placed in the cathodic compartment and at least partially immersed in the electrolyte. The cathode is preferably obtained according to the above-mentioned method, such that the active surface of the copper substrate, placed in contact, during use, with said electrolyte or directly in contact with the gaseous carbon dioxide, has a nanostructured surface morphology with nanocubes having dimensions varying between 100 nm and 1000 nm and preferential crystallographic orientation according to Miller's indices (2,0,0). More precisely, the two-electrode electrochemical system described is particularly effective for the development of syngas, a fundamental precursor for the industrial synthesis of hydrocarbon fuels.

Advantageously, the development of nanostructured copper cathodes according to a fine morphology with a cubic structure, i.e., neutral copper nanocubes with sides varying between 100 nm and 1000 nm with crystallographic orientation (2,0,0) and with excellent catalytic properties, allows to obtain total current densities greater than 50 mA/cm² at an applied electric potential of -1500 mV with high carbon dioxide reduction yields even in an aqueous environment (in which the water reduction is normally kinetically preferred) using plate-type geometry electrodes.

Brief description of the drawings

Further characteristics and advantages of the present invention will become more apparent from the approximate and thus non-limiting description of a preferred, but not exclusive, embodiment of an electrochemical cell for the production of industrial products, as illustrated in the accompanying drawings, in which: figure 1 illustrates, in schematic view, a flow chart representative of the process for forming and regenerating a copper cathode for an electrochemical cell; figure 2 illustrates, with an SEM spectroscopy image, an active surface of an electrocleaned copper substrate by acidic treatment; figure 3 illustrates, with an SEM spectroscopy image, the surface morphology of the active surface following anodisation; figures 4a-4d illustrate, with an SEM spectroscopy image, different concentrations and dimensions of the nanocubes obtainable with the electrochemical reduction step; figure 5 illustrates, with an AFM spectroscopy image, the nanostructured substrate surface with nanocubes; figures 6a and 6b illustrate, with an SEM spectroscopy image, anodised “sponge"-type copper substrates;

- figure 7 illustrates, in schematic view, an electrochemical cell;

- figure 8 illustrates a profile of a chronoamperometry interspersed with interfacial regeneration cycles on a nanostructured copper substrate;

- figure 9 illustrates a histogram graph representative of the faradic efficiencies for the reduction of carbon dioxide in water at pH 7.4 as a function of the electric potential applied;

- figure 10 illustrates a graph representative of the linear scanning voltammetry in copper sponges (those nanostructured in red; the initial, superficially non-functionalised ones in black) in the presence of saturated aqueous carbon dioxide;

- figure 11 illustrates, in schematic view, a flow chart representative of a variant embodiment of the process illustrated in figure 1 ;

- figure 12 illustrates, with reference to a plate-type substrate, the graph of faradic efficiency at different operating potentials;

- figure 13 illustrates the graph representative of the comparison of the production efficiency for carbon monoxide between plate-type copper and nanostructured copper foam-type substrates;

- figures 14a, 14b illustrate, respectively with reference to a plate-type and a sponge-type substrate, the current density graphs at different operating potentials.

With reference to the drawings, they serve solely to illustrate embodiments of the invention with the aim of better clarifying, in combination with the description, the inventive principles underlying the invention.

Detailed description

The present invention relates to a process for forming and regenerating a copper cathode for an electrochemical cell and an electrochemical cell for the production of industrial products, for example syngas. With reference to the figures, an electrochemical cell is generically indicated with the number 10, while a process for forming and regenerating a cathode is indicated with the number 500.

The other numerical references refer to technical features of the invention which, barring indications otherwise or evident structural incompatibilities, the person skilled in the art will know how to apply to all the variant embodiments described.

Any modifications or variants which, in the light of the description, are evident to the person skilled in the art, must be considered to fall within the scope of protection established by the present invention, according to considerations of technical equivalence. In the present description, the reported electric working potentials have Volts (SCE) as the unit of measurement, i.e., they are electric potentials calculated with respect to the “Saturated Calomel Electrode” reference electrode. Therefore, for simplicity's sake, the unit of measurement of the electric potentials will be shown only with the symbol “V”. In addition, in the present detailed description, the term "copper cathode” means both a substrate made internally of copper metal and a substrate made of a material other than copper, for example carbon fibres or sponges, on which a sufficiently thick copper layer, i.e., having a minimum thickness of about 500 nanometres, has been deposited (by conventional electrochemical or physical deposition methods).

Therefore, as analysed by the Applicant, the nanostructuring of a copper cathode is extendable to a substrate made internally of copper metal or to any conductive substrate on which effective copper deposition has been carried out. Figure 1 schematically illustrates a process 500 for forming and regenerating a copper cathode 1 for an electrochemical cell 10 for the production of industrial products, for example syngas.

In detail, the process comprises the operating steps of:

(step 501) preparing a copper substrate 2 capable of defining an electrode, both during the steps of the same process and during the use of the electrochemical cell 10 (i.e., when the completed cathode 1 is installed in the electrochemical cell 10);

(step 502) anodising the substrate 2 in an electrolytic solution based on sulphates and chlorides for a period of at least 1 minute, preferably between 1 minute and 10 minutes, at an AC electric potential varying between 0 mV and +2000 mV, at a frequency varying between 100 Hz and 1500 Hz and at atmospheric pressure and ambient temperature so that copper salts form and are deposited on an active surface 3 of the substrate

2;

(step 503) carrying out an electrochemical reduction of the anodised substrate 2 in an electrolytic working solution having a non-acidic pH so that catalytic neutral copper (Cu⁰) nanostructures 4 form on the active surface 3 in the form of nanocubes, where different crystalline orientations can be observed, preferably towards those of the type (2,0,0), with sides varying between 100 nm and 1000 nm and a surface density varying depending on the time spent during the anodisation step.

Preferably, the surface density of the copper nanocubes varies between 1 - 2 nanocubes per mm² (square micrometre) and about ten nanocubes per mm² associated in agglomerates and columnar structures, as can be seen in the accompanying figures (Fig. 4a-4d). Advantageously, the process can be calibrated by controlling the few simple variable parameters which occur during the carrying out thereof, i.e., the working frequency, the time interval of the surface electrochemical reactions, the electrolyte used during the steps and the electric working voltage (minimum and maximum). According to one aspect of the invention, the substrate 2 prepared at the beginning of the process may be a substantially two-dimensional element, such as a plate, or a three-dimensional structure, such as a sponge or a foam advantageous for the scalability characteristic thereof.

Preferably, as seen in figure 2, the cathode 1 is made through a copper plate-type substrate 2 having a degree of purity greater than 99% and a thickness of about 0.127 mm. Alternatively, through the same electrochemical treatment methods described above, it is also possible to functionalise structures with a different macroscopic morphology with respect to the first ones, for example copper sponges normally present on the market in various degrees of porosity and in different thicknesses (figure 6a). The use of such porous three-dimensional structures allows to extend the extension of the active surface 3 and potentially favours the formation of surface depressions necessary to ensure local pH variations favourable for the CO₂ reduction reactions. As can be seen in figure 6b, the pulsed anodic treatment effectively shows the appearance of cubically oriented nanostructures. According to one aspect of the invention, the step of anodising the substrate 2 is carried out in an electrolytic aqueous solution comprising potassium sulphate (K₂SO₄) at a concentration of about 0.10 M and potassium chloride (KCI) at a concentration of about 0.01 M, which is usually known as “Derived Oxide Treatment”.

According to another aspect of the invention, the step of anodising the substrate 2 is carried out with the application of a slightly oxidising AC electric potential of the square wave type.

According to a further aspect of the invention, the step of anodising the substrate 2 is carried out between 0 mV and +1500 mV, preferably between 0 mV and +1000 mV, and/or at a frequency which can vary between 100 Hz and 1500 Hz, preferably at a frequency of about 1000 Hz for a period of about 5 minutes. Optionally, the frequency value and duration of the anodisation period can be varied to obtain similar results. Preferably, the anodisation of the substrate 2 is carried out in a singlecompartment cell without the ion-permeable separation membrane and in the presence of a metal platinum metal counter-electrode. Even more preferably, the square wave electrolysis occurs with the combination of the aforesaid parameters so as to obtain the surface morphology of the anodised cathode 1 illustrated in figure 3.

Preferably, during the anodisation step, if the substrate 2 is of the plate type, it is placed on a horizontal supporting surface so that the active surface 3 is turned upwards for the deposit of the copper salts. In other words, for the full duration of this treatment step, the plate-type substrate 2 is kept in a horizontal position so as to favour the permanence on the active surface 3 of the copper salts formed during the process conducted entirely in ambient atmosphere and at ambient temperature.

According to one aspect of the invention, the electrochemical reduction step of the substrate 2 is carried out in an electrolytic working solution comprising a buffer solution of bicarbonate (buffer solution with CO₂) and potassium bicarbonate (KHCOa) at a concentration of about 0.50 M.

According to another aspect of the invention, the electrochemical reduction step of the substrate 2 is performed in an electrolytic working solution having a pH equal to about 7.4, obtained following bubbling of carbon dioxide gas (CO₂ gas) into the same electrolytic working solution. Preferably, the anodised substrate 2 is subjected to a first electrochemical reduction directly in the reaction cell, i.e., the electrochemical cell 10, under the electrical CO₂ reduction conditions and potentials, i.e., preferably with negative potentials varying from -200 mV to -1600 mV. Even more preferably, the electrochemical reduction will be carried out with a combination of the parameters described above. The final morphology of the nanostructured active surface 3 will be determined by the conditions under which it was decided to perform the previous anodisation step. In other words, as can be seen in figures 4a-4d, the surface density and dimensions of the nanocubes will depend on the anodisation parameters and, in particular, on the duration of the working time interval. Preferably, the surface density of the copper nanocubes varies between 1 -2 nanocubes per mm² (square micrometre) and about ten nanocubes per mm² associated in agglomerates and columnar structures.

In addition, figure 5 illustrates a further image of the nanostructured active surface 3 of the cathode 1 obtained by AFM spectroscopy. In the specific case illustrated in figure 5, although the large irregularity of the substrate 2 made the investigation difficult, it was possible to identify three-dimensional structures with dimensions in the order of 50-100 nm of closely interconnected nanocubes, confirming the morphological indications obtained by SEM microscopy (figures 4a-4d). According to one aspect of the invention, the process comprises a preliminary step of electrocleaning (step 504) the active surface 3, preferably to be carried out prior to the anodising step, in which the copper substrate 2 is immersed in an acidic electrolytic mixture at ambient temperature and with no inert atmosphere. According to another aspect of the invention, the preliminary electrocleaning step provides that the acidic electrolyte mixture used preferably contains 85% phosphoric acid (H₃PO₄).

According to a further aspect of the invention, the preliminary electrocleaning step uses a titanium counter-electrode to which an electric potential of about +4000 mV is applied for a time interval of about 5 minutes. In this way, the active surface 3 of the substrate 2 is completely free from any particulates or molecules capable of inhibiting the deposition of copper salts for the formation of the catalytic nanostructures 4.

According to one aspect of the invention, the process comprises a step of cleaning (step 505) the anodised substrate 2 carried out, preferably, following the anodising step and prior to the electrolytic reduction step. During this step, the anodised substrate 2 is immersed in a potassium bicarbonate (KHCO₃) mixture having a concentration equal to about 0.50 M. Generally, during this cleaning, a chromatic change of the treated active surface 3 is observed, which varies from a light white/yellow to a deep yellow/orange colour. Under these conditions it is advisable to proceed quickly to the next processing step of the process, i.e., the electrochemical reduction, as the substrate 2 has a marked sensitivity to oxidising in the atmosphere. According to one aspect of the invention, the process comprises a preliminary step of purification (step 506) of the potassium bicarbonate mixture used for the step of cleaning the anodised substrate 2.

Preferably, the preliminary cleaning step is an electrolysis with two electrodes, preferably made of titanium, maintained at an electric potential of about -2000 mV so as to eliminate any unwanted metal species. In other words, the purification of the electrolyte is preferably carried out, as metal cation impurities (especially Fe(ll), Zn(ll) and Pb(ll)) may be present inside the electrolyte. These impurities can lead to the inhibition of the copper catalytic nanostructures 4, i.e., nanocubes, following their deposition on the surface of the cathode 1 . According to one aspect of the invention, each electrolytic solution or mixture used during the process is preferably prepared from salts with a high degree of purity (i.e., with values usually indicated as “99+%”) and/or low-conductivity deionised water, e.g., MilliQ® water.

In this way, all the solutions which come into contact with the nanostructured cathode 1 , or more specifically with the active surface 3, during the forming or regenerating processing and during the use of the cathode 1 itself when placed in the electrochemical cell 10, are also sufficiently pure so as not to compromise the nanostructuring and/or functionalisation of the active surface 3. Thus, according to a preferred embodiment of the invention illustrated in figure 1 , the steps of the process for forming and regenerating a copper cathode for an electrochemical cell are schematically summarised below: preparing a copper substrate 2 defining an electrode; preliminary cleaning said substrate 2 by using an acidic electrolytic mixture (figure 2, figure 6a); anodising the substrate 2 in an electrolytic solution of sulphates and chlorides at atmospheric pressure and ambient temperature with an electric potential with a square wave at a predefined frequency and for a preset time interval according to the density and dimension to be obtained for the surface nanocubes (figure 3); preparing a mixture of potassium bicarbonate; electrochemically purifying said potassium bicarbonate mixture; electrochemically reducing the copper of the substrate 2 so as to definitively form the fine surface morphology comprising copper nanocubes having predefined density and dimensions (figures 4a-4d, 5, 6b). Preferably, the surface density of the copper nanocubes varies between 1 -

2 nanocubes per mm² (square micrometre) and about ten nanocubes per mm² associated in agglomerates and columnar structures.

Advantageously, all the steps of the aforementioned process can be carried out in the same cell, changing the electrolyte and possibly the counter- electrode (anode) necessary for the specific step.

In this way, the process can be implemented initially for forming the nanostructured copper cathode 1 and superficially functionalised with the nanocubes, and subsequently with electrolyte substitution, for the surface regeneration of the cathode 1 . In accordance with a further aspect of the invention illustrated in figure 11 , the process comprises a surface deposit step (step 507) following the electrochemical reduction step (step 503) in which at least the active surface

3 of the substrate 2 is functionalised with one or more carbon dioxide reduction co-catalyst elements. In this way, the surface deposition (or, possibly, even a partial inclusion within the copper interface of the cathode 1 ) of metal materials other than copper (even those metal materials which usually do not have particular catalytic characteristics with respect to the electrochemical carbon dioxide reduction reaction) is capable of bringing benefits to the catalysis process which develops on the active surface 3 of the substrate 2.

According to one aspect of the invention, the surface deposition step involves depositing one or more reduction co-catalyst elements with a density varying between 10 C/cm² [coulomb per square centimetre] and 60 C/cm². The reduction co-catalyst elements may be some metal materials such as, for example, indium, tin, zinc, cadmium, gold, and silver. Advantageously, preferring one co-catalyst element over another allows an increase in selectivity with respect to carbon monoxide or other products of interest.

Preferably, in the case where metal indium is deposited on the copper substrate 2, the optimal copper-indium ratio is between 30 C/cm² and 40 C/cm². Advantageously, the deposit of metal indium allows to maximise the faradic yield and selectivity (with values close to 100%) of the cathode 1 with respect to syngas.

The deposit of the reduction co-catalyst elements may be by standard physical-chemical methods, for example electrodeposition, vacuum thermal evaporation or magnetron sputtering.

In the exemplary case of metal indium deposition, the surface functionalisation of the cathode 1 involves preparing an acidic aqueous solution containing indium salts, for example indium nitrate or sulphate at a concentration of 0.04 M and citric acid at a concentration of 0.5 M.

Preferably, moreover, during the electrolysis it is advisable to mechanically stir the same solution. Advantageously, the electrodeposition can be conducted on a standard mono-compartment cell configured with two electrodes in which the anode is preferably made of metal indium (or, alternatively, they can also be used with other inert metals such as platinum, multi-metal oxide electrodes, or catalytic metal oxides for the development of oxygen).

An operating example illustrated in figures 12, 13, 14a, 14b, shows the results of metal indium deposition on the substrate 2. More precisely, as illustrated in figure 12, such surface functionalisation of a plate-type substrate 2 allows to increase the selectivity of the cathode 1 against carbon monoxide up to average values equal to about 70% of the total faradic efficiency (considering a voltage value equal to about -1400mV vs SCE). Similarly, as illustrated in figure 13, the surface functionalisation of a copper nanostructured sponge (foam) substrate 2 confirms that carbon monoxide is one of the main products of the reduction of carbon dioxide on the surface of the cathode 1 . In fact, figure 13 shows the comparison of the production efficiency for the carbon monoxide between copper substrates that are sponge-type nanostructured and functionalised with indium as co-catalyst and copper plate-type substrates which have received the same functionalisation treatment. In other words, the ordinate axis of graph 13 shows, at different operating potentials, the actual production of carbon monoxide expressed as the faradic efficiency multiplied by the current density.

Figures 14a and 14b illustrate the graphs of the variation in current density as the electric voltage changes, respectively, in the case of a plate-type substrate 2 and a sponge-type substrate 2. In particular, the aforementioned graphs show that the sponge-type substrates allow to reach current densities greater by a factor of ten with respect to similar plate-type substrates (about 300 mA/cm² at -1 .4 V vs SCE with respect to a value of about 30 mA/cm² at -1.4 V vs SCE), thus generating greater amounts of carbon monoxide than those generated by the similar plate-type substrates, more precisely by an amount about four or six times greater.

Figure 7 illustrates an electrochemical cell 10 for the production of industrial products, for example syngas. In particular, the electrochemical cell 10 for the CO₂ reduction comprises a box-like body 11 having a containment volume V in which an electrolyte 12 is contained, preferably in liquid form.

Moreover, a membrane 13 permeable to protons is placed inside the electrochemical cell 10, for example a polytetrafluoroethylene sulphonate Nation® membrane, capable of dividing the containment volume V into an anodic compartment 14 and a cathodic compartment 15. Advantageously, the membrane 13 is configured to prevent the oxidation reaction of the products present in the solution, that is, in the electrolyte 12 and deriving from the redox reactions on the surfaces of the electrodes. An anode 16 is placed inside the anodic compartment 14 and is at least partially immersed in the electrolyte 12, while the cathode 1 is placed inside the cathodic compartment 15 and is at least partially immersed in the electrolyte 12.

In particular, the cathode 1 used comprises a copper substrate 2 with an active surface 3, placed in contact, during use, with the electrolyte 12, having a fine morphology with a cubic structure, wherein the nanocubes have sides varying between 100 nm and 1000 nm, a variable surface density and preferably a prevailing crystallographic orientation of the type (2,0,0).

Preferably, the surface density of the copper nanocubes varies between 1 - 2 nanocubes per mm² (square micrometre) and about ten nanocubes per mm² associated in agglomerates and columnar structures.

Preferably, said cathode 1 is obtained by applying the process for forming and regenerating a copper cathode 1 for an electrochemical cell 10 described above.

Even more advantageously, the electrochemical cell 10 preferably operates also at ambient temperature and atmospheric pressure conditions.

According to one aspect of the invention, the working electrolyte 12 is an aqueous solution at ambient temperature and atmospheric pressure comprising potassium bicarbonate (KHCO₃) at a concentration of 0.50 M conditioned to the pH of the buffer solution by bubbling CO₂ . Preferably, such aqueous solution is previously subjected to an electrochemical purification.

Preferably, as can be seen in figure 7, the conditioning of the electrolyte 12 with carbon dioxide is carried out in a volume separate from the containment volume V of the box-like body 11 , and then brought into contact with the electrodes thanks to the aid of a pump.

Advantageously, the use of an external device for the carbon dioxide conditioning, thanks to the recirculation, allows the presence of gas on the surface of the electrodes to be kept constant so as to ensure constant operating conditions over time for the electrochemical cell 10. In other words, the use of these devices allows the main sources of carbon dioxide to be more simply interfaced with the electrochemical cell 10, since the operating conditions of the electrochemical cell 10 substantially coincide with the operation of these sources. For example, the electrochemical cell 10 could be installed directly with the outlet of incinerators or aluminium production plants or many other similar situations, since the gaseous mixture of carbon dioxide and water vapour is sufficient to act as an electrolyte for catalysis.

Even more advantageously, such a gas mixture allows the water solubility limit to be surpassed to obtain an increased reagent flow on the active surface 3 of the cathode 1 . According to another aspect of the invention, the electrochemical cell 10 comprises, as anode 16, a counter-electrode functionalised with carbon nanotubes or in iridium oxide or in platinum or titanium such as to ensure the development of oxygen in the respective anodic compartment 14. According to one aspect of the invention, the box-like body 11 comprises an inlet opening 18 and an extraction opening 19 for the electrolyte 12 for both the anodic compartment 14 and the cathodic compartment 15. In this way, the recirculation of the electrolyte 12 inside the electrochemical cell 10 is implemented to maintain the percentage of carbon dioxide therein substantially constant and, therefore, able to interact with the cathode 1. The box-like body 11 further comprises a first extraction mouth 20 for extracting the oxygen produced in the anodic compartment 14 and a second extraction mouth 21 for extracting the reaction gaseous products obtained in the cathodic compartment 15.

According to one aspect of the invention, the box-like body 11 is hermetically sealed so as to retain the gaseous products obtained in the anodic compartment 14 and in the cathodic compartment 15. In other words, the electrochemical cell 10 is developed in such a way as to ensure the seal of the gases produced for storage and the chemical analyses necessary for verifying the effectiveness of the electrolysis. Advantageously, the electrochemical cell is configurable to implement the steps of the process described above in order to determine the formation of a nanostructured copper cathode 1 or to regenerate a previously used nanostructured copper cathode 1 .

The reduction of carbon dioxide, in fact, tends to have a passivating action towards the electric currents developed between the anode 16 and the cathode 1 , which decrease over time. This is attributable to the adsorption of carbonate ions on the active surface 3 of the cathode 1. This phenomenon is easily avoided by imposing periodic regeneration cycles of the cathode 1 itself, corresponding to the application of open-circuit electric potential for limited periods of time, as can be best seen in figure 8, which illustrates the profile of a chronoamperometry interspersed with regeneration cycles of the active surface 3 of the substrate 2. Advantageously, the electrochemical cell 10 allows to obtain carbon dioxide reduction yields which, in combination with the parallel water splitting reaction, allow to surpass 60% of the faradic efficiency thanks to the development of syngas at the optimal potential of about -1500 mV.

As can be seen in figure 9, in fact, it has been noted that for each electric working potential applied (variable between -1100 mV and -1600 mV), the proportions of the reduction products obtained represent the optimal percentages for the composition of syngas (for the predominant synthesis of alkanes). In detail, the percentages of the reduction products comprise on average a percentage of carbon monoxide varying from 20% to 30%, of formic acid up to 10% and hydrogen over 60% (using the electrochemical cell 10 described above) and other minor products such as ethylene (at most 4%), methane (about 1%) and traces of many other molecules containing carbon at an oxidation state of less than four. Methane is present in minimal quantities due to the complexity of the processes of reducing carbon dioxide.

As previously mentioned, the parallel splitting reaction of the water molecule allows the remaining electric currents to be conveyed to the production of hydrogen, which is generated in the cathodic compartment 15 of the electrochemical cell 10. Instead, the parallel production of oxygen as a consequence of the oxidative process of water occurs at the anode 16. Using a copper plate-type substrate 2 (of the type shown above), it is possible to obtain total current densities in the order of 50 mA/cm² at a potential of -1500 mV. From the point of view of electrochemical performance, nanostructured copper sponges (e.g., according to the procedure described above) exhibit a four-factor increase in the cathodic current densities associated with the reduction of carbon dioxide and water. In figure 10, in fact, it can be seen that the maximum current density values are around 200 mA/cm² against about 50 mA/cm² of the initial non-nanostructured substrates.

According to a further aspect of the invention, the active surface 3 of the cathode 1 is functionalised with one or more co-catalyst elements reducing at least the carbon dioxide.

Preferably, some of the possible catalyst reduction elements are indium, tin, zinc, cadmium, gold and silver, which are selected based on the selectivity to be induced on the cathode 1 towards carbon monoxide or other products of interest.

Even more preferably, the catalyst reduction elements are deposited on at least the active surface 3 of the substrate 2 by standard physical-chemical methods, for example electrochemical reduction, vacuum thermal evaporation or magnetron sputtering.

In conclusion, some of the advantages of the present invention, and in particular of the electrochemical cell 10, are related to the fact of being able to operate under mild pressure conditions (about 1 bar), at ambient temperature (about 25°C) and, in the case of plate-type substrates, the considerable mechanical resistance which allows extended operation over time (up to about 12 h) and the possibility of regenerating the active surface 3 of the cathode 1 directly in the electrochemical cell 10 of the reaction, by simply replacing the electrolyte 12 and applying the corresponding electric potential values.

Claims

1. A process for forming and regenerating a copper cathode (1 ) for an electrochemical cell (10) for the production of industrial products, for example syngas, comprising the operating steps of: preparing a copper substrate (2) defining an electrode; anodising said substrate (2) in an electrolytic solution based on sulphates and chlorides for a period of at least 1 minute, preferably comprised between 1 minute and 10 minutes, at an AC electric potential varying between 0 mV and +2000 mV, a frequency varying between 100 Hz and 1500 Hz and atmospheric pressure and ambient temperature so that copper salts form and are deposited on an active surface (3) of said substrate (2); carrying out an electrochemical reduction of said anodised substrate (2) in an electrolytic working solution having a non-acidic pH, so that catalytic neutral copper nanostructures (4) form on said active surface (3) in the form of nanocubes, preferably with a crystallographic orientation of the type (2,0,0), with sides varying between 100 nm and 1000 nm and a surface density varying between 1 or 2 nanocubes per mm² and about ten nanocubes per mm² depending on the at least several parameters used during said process.

2. The process according to claim 1 , wherein said anodising step is performed in an electrolytic aqueous solution comprising potassium sulphate at a concentration of about 0.10 M and potassium chloride at a concentration of about 0.01 M.

3. The process according to claim 1 or 2, wherein said anodising step is performed by applying an AC electric potential with a square wave, preferably between 0 mV and +1500 mV, preferably between 0 mV and +1000 mV, and/or at a frequency comprised between 800 Hz and 1200 Hz, preferably equal to 1000 Hz.

4. The process according to any preceding claim, wherein said step of carrying out an electrochemical reduction is performed in an electrolytic working solution comprising a buffer solution of bicarbonate and potassium bicarbonate at a concentration of about 0.50 M.

5. The process according to any preceding claim, wherein said step of carrying out an electrochemical reduction is performed in an electrolytic working solution having a pH equal to about 7.4, obtained following bubbling of carbon dioxide gas into said electrolytic working solution.

6. The process according to any preceding claim, comprising a preliminary step of electrocleaning said copper substrate (2) in an acidic electrolytic mixture at ambient temperature and with no inert atmosphere.

7. The process according to claim 6, wherein said preliminary electrocleaning is performed in an acidic electrolytic mixture containing 85% phosphoric acid by applying an electric potential of about +4000 mV for an interval of time of about 5 minutes.

8. The process according to any preceding claim, comprising a step of cleaning the anodised substrate (2) prior to the step of carrying out an electrochemical reduction; said cleaning step comprising the immersion of said anodised substrate (2) in a potassium bicarbonate mixture having a concentration equal to 0.50 M.

9. The process according to claim 8, comprising a preliminary purification step wherein said mixture is subjected to electrolysis with two electrodes, preferably made of titanium, maintained at an electric potential of about -2000 mV so as to eliminate any unwanted metal species.

10. The process according to any preceding claim, wherein each electrolytic solution or mixture used is prepared from salts with a high degree of purity and/or low-conductivity deionised water.

11. The process according to any preceding claim, wherein said step of preparing a substrate (2) comprises preparing a substantially two- dimensional plate-type substrate (2) or a three-dimensional sponge- or foam-type substrate (2).

12. The process according to claim 11 , wherein during said anodising step said plate-type substrate (2) is placed on a horizontal supporting surface so that said active surface (3) is turned upwards for the deposit of said copper salts.

13. The process according to any preceding claim, comprising a surface deposition step following said electrochemical reduction step wherein at least said active surface (3) of said substrate (2) is functionalised with one or more carbon dioxide reduction co-catalyst elements.

14. The process according to claim 13, wherein said surface deposition step involves depositing one or more reduction co-catalyst elements with a density varying between 10 C/cm² and 60 C/cm².

15. An electrochemical cell (10) for the production of industrial products, comprising: a box-like body (11 ) having a containment volume (V); a membrane (12) permeable to protons placed inside said box-like body (11) so as to divide said containment volume (V) into an anodic compartment (14) and a cathodic compartment (15); an electrolyte (13), preferably liquid, placed inside said containment volume (V); an anode (16) placed in said anodic compartment (14) and at least partially immersed in said electrolyte (13); a cathode (1 ) placed in said cathodic compartment (15) and at least partially immersed in said electrolyte (13); characterised in that said cathode (1 ) is obtained according to any one of claims 1 to 14 so as to comprise a copper substrate (2) having an active surface (3), placed in contact, during use, with said electrolyte (13), having a fine morphology with a cubic structure, wherein the nanocubes (4) have sides varying between 100 nm and 1000 nm, a surface density varying between 1 or 2 nanocubes per mm² and about ten nanocubes per mm² and preferably a crystallographic orientation of the type (2,0,0).

16. The electrochemical cell (10) according to claim 15, wherein said copper substrate (2) is made with a substantially two-dimensional plate-type substrate or as a three-dimensional sponge- or foam-type substrate.

17. The electrochemical cell (10) according to claim 15 or 16, wherein said box-like body (11) comprises an inlet opening (18) and an extraction opening (19) for the electrolyte (13), for both said anodic compartment (14) and said cathodic compartment (15), a first extraction mouth (20) for extracting the oxygen produced in said anodic compartment (14) and a second extraction mouth (21 ) for extracting the reaction products obtained in said cathodic compartment (15).

18. The electrochemical cell (10) according to any of claims 15 to 17, wherein said electrolyte (13) is a solution of potassium bicarbonate at a concentration of about 0.5 M at ambient temperature and atmospheric pressure.

19. The electrochemical cell (10) according to any of claims 15 to 18, wherein said box-like body (11) is hermetically sealed so as to retain the gaseous products obtained in said anodic compartment (14) and in said cathodic compartment (15).

20. The electrochemical cell (10) according to any of claims 15 to 19, wherein at least said active surface (3) of said cathode (1 ) is functionalised with one or more co-catalyst elements reducing at least the carbon dioxide.