ES2931987A1 - Procedure for obtaining nanostructured silver electrodes and nanostructured silver electrodes (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Procedure for obtaining nanostructured silver electrodes and nanostructured silver electrodes. The present invention refers to a procedure for obtaining nanostructured silver electrodes with a three-dimensional (3D) nanoporous structure, based on an antireplication-replication method, by using electrochemical manufacturing techniques. It also refers to a nanostructured silver electrode obtainable according to the above method and to a nanostructured silver electrode with high electrical capacities and also with chromoactive behavior. The present invention is applicable in the field of supercapacitors, and its use for the efficient storage of electrical energy, preferably in systems with renewable sources. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Procedure for obtaining nanostructured silver electrodes and nanostructured silver electrodes

Technical sector

The present invention refers to a procedure for obtaining nanostructured silver electrodes with a nanoporous structure at 30, which is based on an anti-replication-replication process, which is reproducible, has low economic cost and reduced environmental impact, as well as being easily industrially scalable. The invention also refers to a nanostructured silver electrode obtainable by the above procedure, and to a high performance nanostructured silver electrode for electric charge storage.

The invention is applicable in those sectors in which energy storage and conversion systems are manufactured, used or marketed, as well as in the energy production, storage or transport sectors.

Background of the invention

Today there is an extraordinary interest and high demand in the development of new electrical energy storage and conversion systems that are much more versatile, efficient and environmentally sustainable, as well as highly competitive. This is because energy storage is essential for the development of smart grids or SmartGrid. SmartGrids are smart grids capable of predictively integrating user behaviors and actions to eliminate energy losses, increase the quality and quantity of available energy and ensure that the energy supply remains stable over time. Cities consume 78% of all the energy produced worldwide (United Nations [retrieved on 2020-03-09].

Retrieved from the internet:

<https:l/www.un.orgleslclimatechangelcities-pollution.shtml>). This explains the need to establish innovative technologies that avoid energy losses and promote a more rational and efficient use of electrical energy, with its consequent sustainability with the environment, such as smart networks. In other words, SmartGrids allow single-family homes, neighborhood communities and businesses to become small-scale energy producers, regardless of their size. These sources contribute to the common network the energy that they do not consume, at the same time that they can act on specific occasions as consumers. It is in this bidirectional capacity where energy storage technologies play such a decisive role.

Along with SmartGrids, another important field of application of supercapacitors is their use in electric vehicles, where recharging of supercapacitors can be carried out through the kinetic energy of regenerative braking (Tesla [retrieved on 2020-10-05 ). Retrieved from the internet:

<https://www.hibridosyelectricos.com/articulo/sector/tesla-ha-comprado-mayores-fabricantessupercondesadores/20190214194700025631.html>).

The importance of developing new storage technologies is even more evident if one considers that the European Union has made a commitment by 2030 that 32% of the total energy generated be produced from renewable energy sources (European Union [ retrieved on 2020-03-09) . Retrieved from the internet:

<https://ec.europa.eu/clima/policieslstrategies/2030_es>). In order to meet this goal, it is necessary to develop efficient electrical energy storage and transportation systems that allow maximum performance to facilitate the reliable conversion of intermittent supplies provided by renewable energy. In particular, electrochemical energy storage devices (batteries and supercapacitors) turn out to be the most suitable to cover most of these needs, where both are, in many aspects, complementary, due to their different energy accumulation mechanisms. The main advantage of supercapacitors over batteries is that the former have very high power densities and are therefore capable of providing a large amount of energy for short periods (seconds or minutes) (Yan J., Fan Z., Sun W ., Ning G., Wei T., Zhang Q., Zhang R., Zhi L., Wei F. (2012) "Advanced Asymmetric Supercapacitors Based on Ni(OH)21Graphene and Porous Graphene Electrodes with High Energy Density ".Advanced

Functional Materials, 22, p: 2632-2641). However, they need to improve their power densities (being able to provide power for longer) to increase their competitiveness in the sector. For that reason, current research in the field of supercapacitors is aimed at improving the storage energy density of such devices. For this, one of the possibilities consists in increasing the specific capacity (F/g where F are Farads and g are grams) of the electrode. Another is to use electrolytes that operate in higher voltage ranges.

The energy storage mechanisms of supercapacitors can be of two types:

i) Non-faradaic: where electrostatic energy is stored by charge separation at a double layer interface between a conductive electrode and an electrolyte. This would correspond to the supercapacitors known as electrical double layer (EDLC).

ii) Faradaic: where, according to Faraday's law and related to the electrode potential, energy storage takes place as a consequence of an electronic transfer produced by changes in the oxidation state or chemical changes in electroactive materials. Generally, a Faradaic process is called pseudocapacitance, referring to the reversible process where redox-type reactions take place between the electrode material and the electrolyte. The type of supercapacitors that store electrical energy through it are called pseudocapacitors. The latter, compared to the operation of double-layer capacitors, have begun to arouse great interest in the scientific community due to their lower cost, their greater specific capacity, their greater energy density, and because their manufacture and operation is much more efficient. (Ranjit S. K., Suraj A. K., Ramesh J. D. (2018). "Overview of nanostructured metal oxides and pure nickel oxide (NiO) electrodes for supercapacitor : A review ". Journal of Alloys and Compounds, 734, p: 89-111).

There are currently several ways used to manufacture the materials that make up the electrodes that make up pseudocapacitors, including sol-gel technology, microwave treatments, electrodeposition techniques, hydrothermal techniques or the use of molds or templates. This last method of manufacturing presents notable advantages over the others, since it allows obtaining materials with a high specific surface area, giving rise to a greater contact interface between the electrolyte and the electrode, which translates into greater specific capacities on the part of the materials thus obtained (Yan Y., Wang T., Li X., Pang H., Xue H. (2017). "Noble metal-based materials in high-performance supercapacitors ". lnorganic Chemistry Frontiers, 4 (1), p: 33-51).

When manufacturing a pseudocapacitor, the most commonly used materials are conductive polymers or metal oxides. Thus, one of the most investigated metal oxides is RuO, because it is the one with the best conductivity and highest specific capacity. However, its large-scale application is limited by its high cost because it is a rare element in nature. For this reason, work has begun with other types of metal oxides that a priori appear to be very promising, such as metal oxides from noble metals. Among the most common noble metals used to form these metallic oxides, silver stands out, due to its excellent physicochemical properties (high capacitance, conductivity, thermal and chemical stability or cyclical stability, among others).

In the state of the art, precedents have been found for the production of silver electrodes for use as supercapacitor devices. Thus, in the patent document WO201316510 1A 1 a process for manufacturing a hybrid electrode of silver and graphene nanowires is described. In another example, patent CN102169103B describes a process for obtaining electrodes from solid silver mixed with silver chloride.

Also in the state of the art are a series of articles related to the manufacture of silver electrodes (Tang X., Tsujia M., Jiang P., Nishio M., Jang S-M., Yoon S-H. (2009). "Rapid and high-yield synthesis of si/ver nanowires using air-assisted po/yo/ method with chloride ions': Colloids Surf. A, 338, p: 33-39; Hu L., Kim H. S., Lee J-Y., Peumans P ., Cui Y. (2010) "Sca/ab/e coating and properties of transparent, flexible, si/ver nanowire e/ectrodes ", ACS Nano,4, p: 2955 2963; Choi D.Y., Kang H. W., Sung H. J. , Kim S. S. (2013) "Annealing - free, flexible si/see nanowire-polymer composite e/ectrodes via a continuous two-step spray- coating method", Nanoscale, 5, p: 977-983; Sawangphruk M., Suksomboon M., Kongsupornsak K., Khuntilo J., Srimuk P., Sanguansak Y., Klunbud P., Suktha P., Chiochan P., (2013) "High-performance supercapacitors based on si/see nanoparticle/e-po /yaniline- graphene nanocomposites coated on flexible carbon fiber paper'', J. Mater. Chem. A, 1 (24), p:9630).

However, in all the aforementioned cases, silver is usually found in powder form, so it is necessary to carry out an intermediate forming stage, in which the silver is combined with some other material to give it mechanical consistency and In this way, it is possible to configure the electrode itself for a real functional application, which greatly limits its use and prevents it from taking full advantage of its properties (Yuksela R., Alpugan E., Unalan H. E. (2018). "Coaxial si/ver nanowire/polypyrrole nanocomposite supercapacitors", Org Electron 52, p: 272-280), such as their excellent conductivity, low flammability, high wettability, etc. (Yuksel R., Coskun S., Kalay Y. E. (2016). "Flexible, silver nanowire network nickel hydroxide core-shell electrodes for supercapacitors ", J. Power Sources, 328, p: 167-173; K. Devarayan, J. Park, H-Y Kim (2017) "Facile green synthesis of si/see nanodendritelcellulose acetate thin film electrodes for flexible supercapacitors ", Carbohydr. Polym., 163, p: 153-161).

Therefore, based on all the consulted bibliography, it can be concluded that the techniques known to date to design and obtain electrodes for supercapacitors from noble metals are not effective and efficient enough, nor are they enough to produce electrodes with good benefits. This is due to the fact that current processes do not allow their application on a larger scale because the proposed operations are complex and the materials, compounds and technologies are difficult to use, either because of their cost, their scarce abundance or because they result in necks of bottle due to its specificity. This means that only low-volume productions can be undertaken, normally using specialized laboratory technology.

The known electrodes also present several problems due to their low performance even when they are made of noble materials, such as silver. Its electrical and energy storage capacities are not optimized enough to compensate for its use in industrial or consumer systems at the user level. Another added problem with electrodes is that it is not possible to know when they are damaged and stop working properly, unless analysis techniques are used using probes or electrical measurement devices that excite the electrode to check its electrical behavior. When arrays or groups of electrodes are formed or used in mass storage systems, it becomes even more difficult to determine the failure of a specific element, which has a very negative impact on the efficiency and maintenance and operating costs of the production plants. storage

The development of new nanostructured electrodes for supercapacitors will be decisive in the near future, since they will make it possible to convert renewable energy sources into practical and more efficient systems. The improvement of these electricity storage systems will favor their definitive implementation throughout the planet, providing storage systems on a real scale of the electrical network, with enormous growth prospects in the world market (IDTechEx [retrieved on 2020-03-11] Retrieved Internet <https//www.idtechex.com/fr/research-report/supercapacitors-applications-players-markets-2020-2040/661>).

Explanation of the invention

The present invention refers to a procedure for obtaining nanostructured silver electrodes, to an electrode obtainable by the above procedure and to a nanostructured silver electrode.

A first object is, therefore, a procedure for obtaining nanostructured silver electrodes that comprises the following stages:

a) Arrange an aluminum sheet, previously cleaned, degreased and electropolished, as anode in an electrolytic acid solution and anodize the aluminum sheet in a first phase of anodization under controlled conditions of voltage, temperature, pH, time, agitation and concentration of acid electrolyte to obtain an ordered nanoporous structure of anodic alumina. In this first anodizing phase, the voltage is between 15V and 300V, the temperature is between 0°C and 20°C, the pH measured at room temperature is between 0 and 7, the time is at least 0.5 h, stirring at least 40 rpm and electrolyte concentration at least 0.1 M.

b) Carry out a chemical attack to completely eliminate the nanoporous structure of anodic alumina grown in the first phase of anodization, by using an acid or alkaline solution.

c) Carry out a second phase of anodization, with the same conditions of voltage, temperature, pH, type and concentration of electrolyte as those of the first phase of anodization, for the time necessary to obtain the desired thickness of a second nanoporous ordered structure of anode alumina.

d) Widening the pores of the second ordered nanoporous structure of anodic alumina by contacting the ordered nanoporous structure with a hot solution of an inorganic acid.

e) Depositing a silver film on the second ordered nanoporous structure.

f) Introduce a solution of a polymer inside the pores of the second ordered nanoporous structure, to obtain an anti-replication mold and allow it to dry until solidified.

g) Perform at least one selective etching to remove the aluminum foil and the ordered nanoporous structure of alumina.

h) Depositing metallic silver on the polymer antireplication template, replicating the second ordered nanoporous alumina structure removed in step g).

i) Eliminate the antireplicative template from the polymer, by means of at least one chemical attack with an organic solvent, to obtain the nanostructured silver electrode.

In a preferred embodiment, the aluminum foil from step a) has a purity of at least 95%.

In another preferred embodiment, the previous process for cleaning the aluminum sheet in step a) is carried out by washing the aluminum sheet. Washing is carried out first with alcohols and later with distilled water with an immersion time in each of the liquids of at least 5 minutes at room temperature and using ultrasound.

In a more preferred embodiment, the previous process of cleaning and degreasing the aluminum sheet with alcohols in step a) is carried out using isopropanol first, and then ethanol.

In another preferred embodiment, the previous process of electropolishing the aluminum sheet in step a) is carried out by using an acid solution, with constant stirring of at least 40 rpm, with a minimum voltage of 10 V, for at least 5 min and at room temperature.

In another preferred embodiment, the electrolytic acid solution used for the anodizing of stages a) and c) is an aqueous solution of oxalic acid with a concentration of 0.3 M and a pH of 7.

In another preferred embodiment, the voltage of the two anodization phases is 40 V, the temperature at which both anodizations take place is 3°C and stirring is 60 rpm. The first phase lasts 24 h and the second phase lasts 1 h.

In a specific embodiment, the selective chemical attack of step b) consists of immersing the sample in an aqueous solution of CrQ3 (0.18 ^m ) and H3PQ4 (0.61 M) at room temperature for at least 8 h.

In another specific embodiment, the pore widening of step d) is promoted by contacting the second ordered nanoporous structure with an aqueous solution of H3PQ4 at 5% by weight for 35 minutes and at a temperature of 30 °C.

In a more specific embodiment, the promoted nanopore diameter is 70 ± 6 nm with a length of 1.8 ± 0.2 pm.

In another specific embodiment, the deposition of step e) is performed by silver sputtering or sputtering.

In a more specific embodiment, sputtering is performed by applying a current of 20 mA for a time of 270 seconds, in order to obtain a silver plasma and using argon as a purge gas. Operating in this way, a silver coating with a purity greater than 98% is obtained.

In another specific embodiment, the solution of step f) is a solution of polymethylmethacrylate in chloroform with a concentration of 6% by weight. Its introduction into the pores of the ordered nanoporous structure of alumina and drying are carried out at room temperature.

In another specific embodiment, in step g) two chemical attacks are carried out. In a first chemical attack, the aluminum foil is removed by placing it in contact with an aqueous solution of 2.2% CuCl2 and 11.4% HCI by weight at room temperature for between 10 and 20 minutes. In a second chemical attack, the ordered nanoporous structure of alumina is eliminated by placing it in contact with a 3 M NaOH solution at room temperature for between half an hour and 2 hours.

In another specific embodiment, the deposition of metallic silver in stage h) is carried out using the electrodeposition technique at a voltage between 1.5 and 2 V, at a temperature between 20 and 30 °C, for a time of 1 and 6 h. , and with an electrolyte with a silver content of 28.7 g/I. In another specific embodiment, the chemical attack of step i) is with chloroform at 99.2% purity, for a time of at least 1 h and is carried out at room temperature.

Another aspect to which the invention refers is a nanostructured silver electrode obtainable by means of the above process. The electrode obtainable after applying the procedure is a composition greater than 98% silver by weight that has the following characteristics: - an electrical energy storage capacity of at least 0.5 F/cm2,

- a maximum leakage current of 0.07 mA,

- an energy density of at least 385.87 pWh/ cm2,

- a power density of at least 3.82 LW cm2,

- a coulombic efficiency of at least 97.5%, and

- a chromoactive behaviour.

For the purposes of this invention and its description, a material with chromoactive behavior is one that responds to an external stimulus by changing its optical properties, such that the material significantly modifies its visual appearance; that is, its absorptivity, reflectivity, and transmissivity properties change enough to cause a change in its color, reflectivity, or transparency.

A third aspect to which the invention refers is a nanostructured silver electrode with a composition greater than 98% silver by weight with the following characteristics:

- an electrical energy storage capacity of at least 0.5 F/ cm2,

- a maximum leakage current of 0.07 mA,

- an energy density of at least 385.87 pWh/ cm2,

- a power density of at least 3.82 pW/ cm2,

- a coulombic efficiency of at least 97.5%, and

- a chromoactive behaviour.

In a preferred embodiment, the nanostructured silver electrode comprises a plurality of nanopores with a diameter of 70 nm ± 6 nm and a length of 1.8 ± 0.2 pm.

In any aspect of the invention, the nanoporous structure is responsible for the high electrical energy storage capacity of the silver electrode, as well as the excellent properties for its use in the field of electrical energy storage and conversion.

The invention provides a new procedure for obtaining nanostructured electrodes and describes a new type of electrode that allows the development of energy storage systems that are much more efficient and effective than those currently existing, especially if the technology traditionally used in generation, storage and distribution is considered. from renewable energy sources. Furthermore, although in recent years there have been great developments in the improvement of supercapacitors, they have not yet made the necessary quantum leap for their real implementation in the market. However, the technological development proposed in this invention represents an extraordinary advance to develop electrodes for supercapacitors, since it takes advantage of the benefits offered by nanotechnology to develop a product with excellent qualities for use in supercapacitors, through an easily scalable, reproducible and with low economic cost.

One of the advantages of the procedure is that it is simple and cheap, since it is carried out using an anti-replication-replication process based on electrochemical methods with high levels of performance, reproducibility and production capacity, when compared to other methods used to manufacture this procedure. type of electrodes, such as sol-gel technology.

On the other hand, the procedure can be easily implemented at an industrial level, by simply retrofitting the facilities that industries such as steel currently have, such as those used in galvanizing processes. This makes it an easily scalable procedure with minimal investment.

In addition, it is an environmentally friendly procedure, since it allows working at room temperature, the working voltages used do not present high values and the synthesis times are short.

Another advantage of the procedure is that it allows an easy-to-handle noble metal electrode to be obtained, by completing simple stages, without the need to use other types of materials for its shaping, allowing to take advantage of the enormous benefits that noble metals have. to form electrodes for supercapacitors.

The invention also provides a new silver nanostructured electrode that, unlike other alternatives, presents a high nanostructural development, which turns out to be a key factor for it to be able to store a large amount of electrical energy.

The electrode also presents high working stability in cyclical conditions and in basic environments, something that does not occur with other types of materials, which lose stability over time. In addition, it presents high electrical storage capacities.

Another advantage provided by the electrode is that it exhibits a chromoactive behavior, which greatly facilitates its use, since, by means of the visual observation of a simple color change of the material, it is possible to know if the supercapacitor device is charged or discharged and if it works correctly.

The electrode has many qualities and high performance for its use as an electrode for supercapacitors (high capacity, low leakage current and low self-discharge value, high energy and power density values, as well as high coulombic efficiency), making it a very versatile and interesting product, compared to others on the market.

The field of application of the present invention is very broad and includes, by way of example, the field of the accumulation and discharge of electrical energy applied to a plurality of sectors (automotive, transport, etc.) or the manufacturing processes of electrodes, accumulators, batteries or capacitors. In general, the invention is applicable to those sectors in which energy storage and conversion systems are manufactured, used or marketed, as well as in the energy production, storage or transport sectors.

Throughout the description and claims, the word "comprises" and its variants are not intended to exclude other technical features, components or steps. For those skilled in the art, other objects, advantages, and features of the invention will emerge in part from the description and in part from the practice of the invention. The following examples, graphs, figures or schematics are provided by way of illustration, and are not intended to be limiting of the present invention.

Brief description of the drawings

Fig. 1 shows a scanning electron microscope (SEM) image of the nanostructured silver electrode, where a well-defined nanoporous structure can be seen.

Fig. 2 shows the compositional analysis performed on the nanostructured silver electrode, using X-ray spectroscopy (EDX). The ordinates represent the counts (C) and the abscissas the energy (E) in keV. The compositional analysis carried out on the sample makes it possible to verify that, indeed, the electrode is made of silver.

Fig. 3 shows a graph corresponding to the cyclic voltammetry performed on the nanostructured silver electrode, in which the ordinate axis corresponds to the current intensity (mA) and the abscissa axis corresponds to the applied voltage (V). In the recorded voltammograms, the potential was varied from -0.5 to 0.8 V with different sweep rates (from 10 mV/s to 70 mV/s).

Fig. 4 shows a graph corresponding to the chronopotentiometry performed on the nanostructured silver electrode 25 to obtain the charge and discharge cycles. The ordinate axis represents the applied potential (V) and the time (s) on the abscissa axis. The range of potential selected was from -0.5 to 0.7 V, in accordance with the work window observed in cyclic voltammetry, and passing a current intensity of 5 mA.

Fig. 5 shows a graph corresponding to the chronoamperometry performed on the nanostructured silver electrode to thereby obtain the leakage current. The current intensity (A) is represented on the ordinate axis and the time (s) on the abscissa axis. The voltage used to carry out this measurement was 0.7 V for 2 h.

Fig. 6 shows a graph corresponding to the self-discharge profile of the nanostructured silver electrode, in which the ordinate axis corresponds to the percentage of electric potential retention of said electrode (%) and the abscissa axis to time (s). . In this study, the electrode was charged at 0.7 V for 10 min, and then left open-circuited for 5 h.

Fig. 7 shows a graph corresponding to the color change experienced by the nanostructured silver electrode during a measurement carried out using cyclic voltammetry. The measurement was made with a sweep rate of 70 mV/s, obtained frame by frame, associating the change in luminance for each voltage value with respect to a reference electrode, in which the left ordinate axis represents the luminance (RGB), the current intensity (mA) is represented on the right ordinate axis and the applied voltage (V) is represented on the abscissa axis. In addition, the graph shows, by means of a dashed line, the luminance corresponding to the charging process (LC), which decreases in value as the nanostructured silver electrode is charged, which implies that it is acquiring a darker and darker in color until finally remaining black. Also shown by a dotted line 15 is the luminance corresponding to the discharge process (LO), which increases as the nanostructured silver electrode discharges, which implies that it acquires an increasingly lighter color until it remains finally light gray. Therefore, the change in surface color experienced by the nanostructured silver electrode during the charge and discharge process of the capacitor constitutes irrefutable proof of its correct operation.

Preferred embodiment of the invention

For a better understanding of the present invention, the following preferred embodiment examples are described, described in detail, which must be understood without limiting the scope of the invention.

EXAMPLE 1.

In this example, a procedure for obtaining nanostructured silver electrodes is described. The procedure included the following stages:

a) An aluminum foil was provided as an anode and was anodized in a first anodizing stage.

The aluminum sheet was of high purity (Al 99.999%) of 2.5 x 2.5 cm2 and 0.5 mm thick.

It had previously been cleaned and electropolished to prepare its surface. For this, it was submerged in a container, first with isopropanol for five minutes. Next, it was submerged for another 5 minutes in ethanol, while an ultrasonic bath was applied. Finally it became clear with distilled water (18.2 MO^cm ^-1 . All this was done in order to eliminate any surface contaminating substance found on the sheet.

After this cleaning, an electropolishing process (electrochemical polishing) was carried out to eliminate surface roughness. This process was carried out in an electrochemical cell, in which the aluminum foil is arranged as an anode. The electrolyte used for this process consisting of a cold mixture (5°C) of perchloric acid and ethanol (25/75) was poured into said electrochemical cell. The voltage used for this electropolishing was 20 V for a period of 5 min, with constant stirring at 60-80 rpm and all of this was carried out at room temperature.

Once prepared, the aluminum sheet was arranged as anode and the first phase of electrochemical anodization began to obtain a nanoporous structure of anodic alumina. The first anodization was carried out using an aqueous solution of oxalic acid of 0.3 M concentration as electrolyte, at a temperature between 1 and 3° C and with a voltage of 40 V, applied between the sample and a platinum counter electrode for 24 h. with constant stirring at 60-80 rpm.

b) A chemical attack was carried out to completely eliminate the nanoporous structure of anodic alumina grown in the first phase.

After the first anodization, the aluminum oxide layer grown was selectively etched by immersing the sample in a solution of chromium trioxide (CrO ³ ) and orthophosphoric acid (H ³ PO ⁴ ), in water, with concentrations of 0.18 M and 0.61 M, respectively, for 24 h at room temperature.

c) A second phase of anodization was carried out to obtain a second nanoporous ordered structure of anodic alumina. The second anodization was carried out under the same conditions as the first anodization, except that, in this case, the time used was 1 h.

d) The widening of the pores of the nanoporous ordered structure of anodic alumina was carried out.

To carry out this widening of pores, said structure was placed in contact with an aqueous solution of H ³ PO ⁴ at 5% by weight and at a temperature of 30 °C for 35 minutes.

e) A silver film was deposited on the second nanoporous ordered structure of anodic alumina.

This deposition was done by silver sputtering or sputtering. Cathode spraying or sputtering was performed by applying a current of 20 mA on the silver target for a time of 270 seconds, in order to obtain a silver plasma and using argon as a purge gas. Operating in this way, a silver coating with a purity greater than 98% was obtained.

f) A solution of a polymer was introduced into the pores of the second ordered nanoporous structure, to obtain an antireplicative template.

The polymer used was composed of a solution of polymethylmethacrylate (PMMA) made up of 50 ml of chloroform and 3 grams of PMMA and its introduction into the pores of the nanoporous alumina membrane was carried out by pipetting said solution onto the surface of the sample, at room temperature. After that, it was subsequently dried, which was also carried out at room temperature.

g) An etch was performed to remove both the aluminum foil and the nanoporous alumina ordered structure.

The removal of the aluminum foil was carried out by placing it in contact with a solution of 200 ml of HCI and 14.4 g of CuCh in 400 ml of ultrapure water, at room temperature for 15 minutes. Then, the nanoporous alumina membrane was removed by contacting it with a 3M NaOH solution at room temperature for 1 hour.

h) Electroplating of metallic silver was carried out on the polymer mold, replicating the nanoporous structure of the alumina membrane previously removed.

Electroplating of metallic silver was carried out at a voltage of 1.7 V and for a time of 6 h. Although others could be used, a commercial electrolyte from Alfa Aesar with a silver content of 28.7 g/I was used. The electrolytic cell used for the procedure consisted of a working electrode, which was the sample itself, and a platinum counter electrode.

i) The polymer anti-replication template was removed by chemical attack with an organic solvent to obtain the nanostructured silver electrode.

This stage was carried out by immersing the sample in a chloroform solution with a concentration of 99.2 % and at room temperature. The time taken was that necessary for the polymeric template to be completely removed and the nanostructured silver electrode to be completely released, at least 1 h.

EXAMPLE 2

In this example an electrode obtainable by applying the procedure indicated above is described. The procedure allowed obtaining a nanostructured silver electrode with a composition of at least 99% pure silver. The electrode had a well-developed nanostructure and exceptional electronic and optical qualities, making it possible to use it in electrical storage systems. Thus, it had an electrical energy storage capacity of at least 0.5 F/cm2, a maximum leakage current of 0.07 mA, an energy density of at least 385.87 pWh/cm2, a power density of at least 3.82 pW/cm2 , a coulombic efficiency of at least 97.5%, and a chromoactive behaviour.

The physicochemical characterization of the material was carried out using the scanning electron microscopy (SEM) technique, with which it was possible to obtain information on the micro/nanostructure and the homogeneity of the electrode (see Fig. 1). For this, the JEOL JSM 5600 scanning electron microscope was used. This equipment also allows for an X-ray spectroscopic analysis (EDX) with which information about the elemental composition of the electrode was obtained (see Fig. 2).

The electrical behavior of the electrode was evaluated through the use of cyclic voltammetry, which consists in the application of a triangular potential whose amplitude will vary between two established limits (0.8 / - 0.5 V), increasing and decreasing it constantly (using an Ag/AgCI reference electrode (KCI 3M) to measure it), and applying different sweep rates (from 10 mV/s to 70 mV/s), while continuously measuring the current intensity. The data obtained by this technique are collected in a graph called voltammogram, which represents the variation of the current intensity against the applied voltage, at different sweep rates (see Fig. 3). The presence of very marked peaks, both at positive and negative voltages, are characteristic of pseudocapacitor type electrodes, due to the oxidation and reduction processes that take place during the charge and discharge cycles of these supercapacitors. In the case of silver, it presents two different oxidation states; in the first case, silver goes from oxidation state zero to one, while in the second process, it goes from one to two:

2Ag 2OH-~ Ag ² Ü H ² O 2e-(-0.46 V)

^Ag2O2OH- ~ ^2AgOH2O2e -(-0.80V)

As a consequence of this, the silver voltammogram shows two positive peaks, corresponding to oxidation processes, and two other negative peaks, corresponding to reduction processes.

The electrolyte used to carry out the measurements was KOH 5M.

The evaluation of the electrical storage capacity (F/cm2) of the electrode was carried out using the chronopotentiometry technique. This is an electrochemical technique in which a continuous electric current is supplied to the system and the electrostatic potential difference originating in the working electrode is measured with respect to an Ag/AgCI reference electrode (KCI 3M), function of time. By reversing the polarity of the electric current, charging and discharging cycles of the electrode can be obtained. With this technique, the capacity of the electrode of the invention can be calculated using the following formula:

C = I • t / A • AV (F/cm2)

where t is the duration of the discharge process for each cycle in seconds, 1 is the applied current in amperes, tiV is the range of the voltage interval that is present during the charge and discharge curves in volts, and A is the exposed area of the sample during the development of the process, in cm2.

For the study of the electrode, the potential range of -0.5 to 0.7 V was established as working voltage, in accordance with the work window observed in cyclic voltammetry, and at a current intensity of 5 mA. By being able to carry out a large number of charge and discharge cycles, this technique also makes it possible to study the operating stability and cyclical behavior of the electrode and, in this way, check that it does not degrade over time (see Fig. 4).

The analysis of the electrical behavior of the electrode consisted in the calculation of its power density, energy density and coulomb efficiency, based on the results obtained through the use of chronopotentiometry. To calculate the energy and power density, the following formulas or expressions were used.

On the one hand, the energy density is defined as follows:

where D ^e is the energy density (pWh/cm2), C is the storage capacity (F/cm2) determined by chronopotentiometry analysis, and AV is the potential difference between the limits established during cyclic voltammetry, expressed in volts.

And the power density was calculated as follows:

DP = (3600 • D ^e ) / At

where D ^p is the power density (pW/cm2), D ^e corresponds to the energy density (pWh/cm2), calculated previously, and At is the discharge time expressed in seconds.

Along with these two parameters, the coulombic efficiency of the electrode was also calculated. Coulombic efficiency is defined as the efficiency with which electrons are transferred to a system, taking into account the energy losses that occur during this process. This parameter is very important, since it offers information on the relationship between the electrical charge that is supplied to the capacitor and that which it returns during the discharge process. This parameter is calculated using the following expression:

1 = td / tc • 100

where tc and td are the charge time and discharge time respectively, measured from the charge and discharge curves of the nanostructured silver electrode.

Presenting high values for each of these three parameters is vital for any type of electrode that wants to be used for use in supercapacitor devices, since it is a reflection of its high performance for this application.

To measure the leakage current, use was made of chronoamperometry (see Fig. 5). In this way, analyzing the variation of the intensity of the current with time makes it possible to obtain the leakage current of the electrode. The importance of performing this measurement is that it allows one to determine if the charge is being stored correctly or current leakage is occurring in the electrode/supercapacitor device. This information is very necessary since thanks to it you can know if the electrode stores the charge optimally. The voltage applied to carry out this measurement was 0.7 V for 2 h. In Fig. 5 it can be seen how the leakage current decays rapidly at first, until it stabilizes at a value of -0.07 mA after maintaining the constant voltage for 2 h. This very low value of leakage current indicates an optimal behavior of the electrode, which could be attributed to the fact that it presents few secondary reactions, due to the low or null presence of impurities in the electrode.

The self-discharge profile of an electrode refers to the gradual decrease in voltage that occurs when the electrode is disconnected from the charging circuit, reflecting the loss of efficiency of the charging circuit, for a real electrical energy storage application. To carry out this measurement, the electrode was charged at 0.7 V for 10 min. and then it was left in open circuit for 5 h (see Figure 6).

Table 1 shows how the percentage of electrical potential retention of the electrode varies as a function of time, presenting high retention values, since, after 5 h of being in open circuit, the electrode retains 61% of the value of the electric potential of its initial charge.

Time (h) Potential retention (%)

0 100

0.5 86

1 62

1.5 62

2 62

2.5 62

3 62

3.5 62

4 62

4.5 62

5 61

Table 1. Potential retention percentage of the silver electrode of the invention as a function of time

Finally, an evaluation of the chromoactive behavior of the electrode was carried out.

Regarding the analysis of the chromoactive behavior of the electrode, the electric current applied to the electrode induces redox reactions that modify the energy bands in which the material interacts with visible light, giving rise to an observable color change. This effect has been quantified by means of an image treatment carried out through software. To do this, the change in luminance can be expressed from the RGB code (red - green - blue color code) in the form:

Luminance (%) = (0.2126R 0.7152G 0.0722B) • 100

which expresses how close the average color of the pixels is to white (100%) or black (0%). Specifically, the change in color of the electrode during cyclic voltammetry was analyzed frame by frame, associating the change in luminance for each voltage value with respect to the reference electrode (see Fig. 7).

Claims (19)

1. Procedure for obtaining nanostructured silver electrodes comprising the following stages: a) arrange an aluminum sheet, previously cleaned and electropolished, as anode in an electrolytic acid solution and anodize the aluminum sheet in a first phase of anodization under controlled conditions of voltage, temperature, pH, time, agitation and acid electrolyte concentration to obtain an ordered nanoporous structure of anodic alumina, where in this first phase the voltage is between 15 V and 300 V, the temperature is between 0 °C and 20 °C, the pH measured at room temperature is between 0 and 7, the time is at least 0.5 h, the stirring at least 40 rpm, and the electrolyte concentration at least 0.1 M; b) carrying out a chemical attack to completely eliminate the nanoporous structure of anodic alumina grown in the first phase, by using an acid or alkaline solution; c) performing a second phase of anodization, with the same conditions of voltage, temperature, pH, type and concentration of electrolyte as those of the first phase of anodization, for the time necessary to obtain the desired thickness of a second nanoporous ordered structure of anode alumina; d) widening the pores of the second ordered nanoporous structure of anodic alumina by contacting the ordered nanoporous structure with a hot solution of an inorganic acid; e) depositing a silver film on the second ordered nanoporous structure; f) introducing a solution of a polymer inside the pores of the second nanoporous ordered structure, to obtain an anti-replication mold and allowing it to dry until solidified; g) performing at least one selective etching to remove the aluminum foil and the ordered nanoporous structure of alumina; h) depositing metallic silver on the polymer antireplication template, replicating the second ordered nanoporous alumina structure removed in step g), and i) removing the polymer antireplication template, by at least one chemical attack with an organic solvent, to obtain the nanostructured silver electrode. Process according to claim 1, characterized in that the aluminum foil from step a) has a purity of at least 95%. 3. Procedure according to claim 1, characterized in that the previous process of cleaning the aluminum sheet in step a) is carried out by washing the aluminum sheet, firstly with alcohols and later with distilled water, performing all the process at room temperature, through the use of ultrasound and with an immersion time in each of the liquids of at least 5 minutes. 4. Procedure according to claim 3, characterized in that the previous process for cleaning the aluminum sheet with alcohols in step a) is carried out by using isopropanol first, and then ethanol. 5. Procedure according to claim 1, characterized in that the previous process of electropolishing the aluminum sheet in step a) is carried out by using an acid solution, with constant stirring of at least 40 rpm, with a voltage 10 V minimum, for at least 5 min. and at room temperature. 6. Process according to claim 1, characterized in that the electrolytic acid solution used for the anodization of stages a) and c) is an aqueous solution of oxalic acid with a concentration of 0.3 M and a pH <7. 7. Process according to claim 1, characterized in that the voltage of the two anodization phases is 40 V, the temperature at which both anodizations take place is 3°C, the agitation is 60 rpm and that the first phase lasts 24 h and the second phase lasts 1 h. 8. Procedure according to claim 1, characterized in that the selective chemical attack of stage b) consists of immersing the sample in an aqueous solution of CrÜ ³ (0.18 M) and H3PÜ4 (0.61 M) at room temperature for at least 8 a.m. 9. Process according to claim 1, characterized in that the pore widening of step d) is promoted by contacting the second ordered nanoporous structure for 35 minutes with an aqueous solution of H ³ PO ⁴ at 5% by weight and at a temperature 30°C 10. Method according to claim 9, characterized in that the promoted nanopore diameter is 70 nm ± 6 nm with a length of 1.8 ± 0.2 pm. 11. Method according to claim 1, characterized in that the deposition of step e) is carried out by means of silver sputtering or cathodic spraying. 12. Method according to claim 11, characterized in that the sputtering is carried out by applying a current of 20 mA for a time of 270 seconds, in order to obtain a silver plasma and using argon as a purge gas, obtaining a silver coating. silver with a purity greater than 98%. 13. Process according to claim 1, characterized in that the solution of stage f) is a solution of polymethylmethacrylate in chloroform with a concentration of 6% by weight and in that the introduction thereof into the pores of the ordered nanoporous structure of alumina and drying is carried out at room temperature. 14. Procedure according to claim 1, characterized in that in step g) two chemical attacks are carried out, where in a first chemical attack the aluminum sheet is removed by placing it in contact with an aqueous solution of CuCh at 2.2% and HCI at 11.4 % by weight at room temperature between 10 and 20 minutes, and where in a second chemical attack the ordered nanoporous structure of alumina is eliminated by placing it in contact with a 3 M NaOH solution at room temperature between half an hour and 2 hours. 15. Method according to claim 1, characterized in that the metallic silver deposition of stage h) is carried out using the electrodeposition technique at a voltage between 1.5 and 2 V, at a temperature between 20 and 30 °C, for a time of 1 and 6 h, and with an electrolyte with a silver content of 28.7 g/l. 16. Procedure according to claim 1, characterized in that the chemical attack of stage i) is with 99.2% purity chloroform, for a period of at least 1 h and is carried out at room temperature. 17. Nanostructured silver electrode obtainable by the method of claim 1, characterized in that it is a composition greater than 98% silver by weight and that it has an electrical energy storage capacity of at least 0.5 F/cm2, a maximum current leakage current of 0.07 mA, an energy density of at least 385.87 ^Wh/cm2, a power density of at least 3.82 ^W/cm2, a coulomb efficiency of at least 97.5%, and chromoactive behavior. 18. Nanostructured silver electrode characterized in that it is a composition greater than 98% silver by weight and that it has an electrical energy storage capacity of at least 0.5 F/cm2, a maximum leakage current of 0.07 mA, a density of at least 385.87 ^Wh/cm2, a power density of at least 3.82 ^W/cm2, a coulombic efficiency of at least 97.5%, and chromoactive behavior. 19. Nanostructured silver electrode according to claim 18, characterized in that it comprises a size distribution of nanopores with a diameter of 70 nm ± 6 nm and a length of 1.8 ± 0.2 pm.