WO2023216004A1

WO2023216004A1 - Method for producing graphene-based materials and use thereof in the manufacture of electrodes for electrochemical capacitors

Info

Publication number: WO2023216004A1
Application number: PCT/CL2022/050051
Authority: WO
Inventors: Juan MATOS LALE; Po Shan POON; Oscar PINTO BURGOS; Julio URZÚA AHUMADA
Original assignee: Universidad de Concepción
Priority date: 2022-05-13
Filing date: 2022-05-13
Publication date: 2023-11-16

Abstract

The present invention relates to a method for producing a graphene-based material useful for storing electrical energy for the incorporation thereof into electrochemical capacitors. The method comprises the synthesis of the graphene-based material that comprises 5 sub-steps: feeding, activation/exfoliation, micro-nano separation, nano-decanting and nano-extraction. After this graphene-based material has been obtained, the electrode is manufactured in a process which is made up of 4 sub-steps: mixing, film preparation, filling with the electrolyte, and electrode cutting process. In light of the fact that the reported results regarding the manufactured electrodes show better electrochemical performance than those disclosed in the prior art, it can be concluded that the technological solution presented, which proposes the use of graphene-based materials produced from forestry waste, is a low-cost and eco-friendly solution for use thereof in the construction of electrodes for electrochemical capacitors.

Description

PROCESS FOR THE PRODUCTION OF GRAPHENE-BASED MATERIALS AND THEIR USE IN THE MANUFACTURE OF ELECTROCHEMICAL CAPACITOR ELECTRODES

TECHNICAL FIELD

The present application focuses on energy storage, using graphene-based materials as double-layer electrochemical capacitors, its production process, and its use in the electrical industry.

STATE OF THE ART

The demand for electrical energy worldwide has increased substantially in recent decades. It has been estimated [1] that approximately 28 TW of energy will be required in the year 2050. Reaching this global demand for electrical energy not only lies in developing clean and renewable energy production technologies, such as solar photovoltaics and wind, but also , inevitably requires efficient electrical energy storage devices.

In this context, the most used electrical energy storage system today is the battery. However, conventional batteries have important operational disadvantages that make them unviable for the new energy model that requires high energy density and high charge capacity [2]. For example, batteries can suffer mechanical expansion due to the generation of vapors when subjected to continuous and abrupt energy charges and discharges; They have low energy efficiency (65 - 75%), which decreases with high power consumption; as well as they have a short useful life, since they present losses of over 50% of their storage capacity over 1000 charge/discharge cycles, which can be equivalent to a useful life of no more than 5 years, depending on their use [3 ]. In addition to the above, batteries contain materials hazardous to the environment, which complicates their final disposal. This is how electrochemical capacitors, called supercapacitors or supercapacitors (SC), have shown great potential to solve the limitations of current batteries, being a complement and even a replacement for them [3].

Now, without neglecting the role of all the parts that make up a SC, the component of greatest interest is the electrode. This component is responsible for the accumulation of electrical charge. This property is conditioned by the material's ability to adsorb charges from the electrolyte used in the SC. Thus, the global market for electrodes for implementation in SC is growing rapidly to address intermittent renewable sources, short-term applications or rapid regeneration in hybrid and electric vehicles [4],

The charge storage in SC occurs through the generation of an electrostatic double layer (EDL) by the interaction of the ions of an electrolyte with the surface of the electrode that is characterized by having two specific properties, high porosity and conductivity [5 ]. In recent years, many efforts have been made to develop highly efficient SCs. Several approaches have been proposed, such as the development of nanoporous carbon (NC) materials with high surface area and hierarchical porosity to increase capacitance and reduce diffusion resistance through non-tortuous channels, thereby obtaining high-speed capabilities [ 6]. The high surface areas of NCs and the short distance between the ion and the electrode provide the new generation of SCs with higher capacitance than conventional devices, while being able to release charge faster than batteries [4], However, the main disadvantage of SCs is their low specific energy compared to batteries [4,6].

Modifying the surface chemistry of carbon electrodes by introducing heteroatoms (such as O, N or S) has also been explored as a strategy to improve the performance of surface redox reactions (pseudocapacitance charging) [4,7] However, the synthesis of these advanced carbon materials often requires complex, long, expensive and some even toxic methodologies, which involve high carbonization temperatures and petrochemical precursors such as mineral coal, hydrocarbons, among others, which limit their widespread commercialization. An attempt to Developing more "green" and competitive SC corresponds to the use of NC derived from agricultural or forestry waste [8,9], which have been successfully used to develop SC of the EDL type. At the same time, the formulation of nanoporous carbonaceous materials from phenolic raw materials such as lignin, bark, or phenolic resins, have increased considerably within the scientific community since they provide capacitance values between 190-300 F g ¹ and a high stability that can reach over 90% efficiency even after more than 5000 charge/discharge cycles [10-12],

In this context, Chile has a unique opportunity for the development of optimal materials for the construction of electrodes with SC applications, mainly based on graphene, whose physical and chemical properties can be designed for such an application. An abundant carbon precursor to produce graphene-based materials at competitive costs are lignocellulosic materials or residues of forest origin such as pine, oak, mahogany, apamate, carob, among others; which is of clear interest in Chile given the high number of forest plantations that exist in the country, with the bark of Pinus Radiata being one of the wastes of greatest interest. Our group has reported that the properties of graphene-type materials produced from biomass waste can be designed to control the interactions of electrostatic and dispersive forces, which are necessary for the mobility of electronic charges [13]. This design would guarantee high electrical conductivity and accessible sites for electrical charge storage. The main and differentiating element of this patent consists of the formulation and production of graphene-based materials for the construction of electrodes and their application as electrical superconductors.

The best material obtained from graphene and presented in this patent is totally different from the documents reported in the literature because we have used as a precursor a biocarbon previously obtained from the pyrolysis of tannins extracted from forest materials or residues such as pine and oak bark. , mahogany, apamate, carob, among others. In summary, this biocarbon has been subjected to a simultaneous process of activation and thermochemical intercalation exfoliation in the presence of KOH at moderate temperatures. No There are reports to date of electrochemical capacitors prepared from graphene materials from biocarbons derived from the pyrolysis of tannins from forest residues ("tree barks").

For example, document US8784764B2 describes a method for the production of activated carbon for use as high energy density capacitors. This material is produced from an aqueous phase mixture of a non-lignocellulosic carbon precursor and an inorganic compound. This mixture is probably mineral coal in the presence of mineral coal's own inorganic impurities. This document does not describe forms of graphene as the component of interest in the capacitor, so it does not affect the present application.

Document US6060424 describes the production of activated carbon derived from lignocellulosic materials (wood) by chemical activation useful as double-layer devices for high energy density storage. Even though the precursor is lignocellulosic, the document does not describe the exfoliation processes by thermochemical intercalation of biocarbons, nor the production of graphene, so it does not affect the present application.

Document US8940145B1 describes the production of a supercapacitor electrode comprising a porous conductive substrate composed of a surface deposited of one or more metal oxides and a chemically reduced graphene oxide deposited as a second surface, thus providing the electrical double layer composition associated with the substratum. The layers of the supercapacitor are wound cylindrically. Even despite the good electrochemical performance of the device, this document is not focused on the production of graphene and is also characterized by having mixtures of conductive metal oxides, and consequently, it does not affect the present application.

Based on the background described, this patent application proposes “A process for the production of graphene-based materials and their use in the manufacture of electrochemical capacitor electrodes”, which is composed of two stages. The first related to the production of the graphene material and the second stage consists of the preparation of the electrode for electrochemical capacitor, using graphene-based material as raw material.

Likewise, this patent describes the development of eco-friendly and sustainable synthesis protocols for graphene-based materials, so that they can be used as electrodes for double-layer electrical capacitors for the storage of electrical energy.

References.

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Energy Convers. Manag. 51 (2010) 2901 -2912. https://doi.Org/10.1016/j.enconman.2010.06.031.

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Electrochemical Performance, ChemSusChem. 5 (2012) 818-841. https://doi.Org/10.1002/cssc.201100571.

[8] H. Lu, X.S. Zhao, Biomass-derived carbon electrode materials for supercapacitors, Sustain. Energy Fuels. 1 (2017) 1265-1281. https://doi.Org/10.1039/C7SE00099E.

[9] Z. Bi, Q. Kong, Y. Cao, G. Sun, F. Su, X. Wei, X. Li, A. Ahmad, L. Xie, C.-M. Chen, Biomass-derived porous carbon materials with different dimensions for supercapacitor electrodes: a review, J. Mater. Chem. A. 7 (2019) 16028-16045. https://doi.Org/10.1039/C9TA04436A.

[10] C. Wang, T. Liu, Activated carbon materials derived from liquefied bark-phenol formaldehyde resins for high performance supercapacitors, RSC Adv. 6 (2016) 105540-105549.

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[13] A. Dasgupta, J. Matos, H. Muramatsu, Y. Ono, V. González, H. Liu, C. Rotella, K. Fujizawa, R. Cruz-Silva, Y. Hashimoto, M. Endo, K Kaneko, L. Radovic, M. Terrones. Nanostructured carbon materials for enhanced nitrobenzene adsorption: Physical vs. Chemical surface properties. Carbon 139 (2018) 833-844.

DESCRIPTION OF THE FIGURES

Figure 1. Production scheme of graphene-based materials, composed of activation (1 a) and exfoliation (1 b) sub-stages to obtain a mixture of micro- and nanometric carbons, which subsequently undergo phase separation. micro- and nanometric by filtration (1 c) followed by the decantation of the nanomethca phase in solution (1 d), and finally a second filtration for the extraction of the nanomethca phase (1 e) that corresponds to the graphene-based material.

Figure 2. Textural characterization of the graphene-based material. Figure (2a) shows the adsorbed amount of nitrogen (cm ³ g ^-1 , STP) at -196 °C versus the relative pressure (P/Po); Figure (2b) shows the pore volume distribution [dV/dW, (cm ³ g ^-1 nm ^-1 )] versus the pore width W (nm); Figure (2c) shows the adsorbed volume of accumulated nitrogen [Vcum (cm ³ g' ¹ )] versus the pore width W (nm). Figure 3. RAMAN spectra of the graphene-based material, showing the intensity (counts) versus the Raman shift (cm' ¹ ). The carbon and atomic oxygen content (At wt. %) inserted in the figure shows that the material is composed of graphene oxide with a C/O ratio = 7.3. The deconvolutions of the peak associated with the material defects, indicated as D1, D2, D3 and D4, are associated with the distortions of the graphite plane (G), which suggests the formation of isolated graphene sheets.

Figure 4. Electrochemical characterization of the graphene-based material. Figure (4a) shows the generated current density (A g ¹ ) versus the applied voltage (V); Figure (4b) shows capacitance accumulated by the electrochemical cell Cceii (F g' ¹ ) versus the applied voltage.

Figure 5. Electrochemical results of the electrode made with the graphene-based material. Figure (5a) shows the specific capacitance of the electrode (Ceiec), in (F g' ¹ ) versus the sweep rate of the applied voltage (mV s' ¹ ); Figure (5b) shows Ceiec (F g' ¹ ) versus current density (A g' ¹ ); Figure (5c) shows the Ragone graph where the energy density E (Wh kg' ¹ ) versus the power density P (W kg' ¹ ) is observed; Figure (5d) shows the percentage of electrode capacitance retention Cretention (%) versus the number of charge/discharge cycles applied.

DESCRIPTION OF THE INVENTION

This technology corresponds to the development of a material for its implementation as a storage and disposal device for electrical energy. The graphene-based materials were manufactured by simultaneous activation/exfoliation by the thermochemical intercalation method, using different weight ratios of KOH/biocarbon (between 0.3 - 3.6) to introduce modifications in the texture and surface chemistry of the graphene. graphene and improve electrochemical performance. In the following parts, we will refer to the graphene-based material obtained with the KOH/biocarbon weight ratio equal to 1.1.

A biocarbon derived as a byproduct of the pyrolysis of tannins extracted from pine bark was used as raw material. After the process of Simultaneous activation/exfoliation (Figure 1a-b) by thermochemical intercalation, a foamy material with cylindrical geometry and monolithic composition composed of a mixture of micro- and nanocarbons was obtained. This monolithic material was separated into two parts, first performing a filtration and washing in hot water (60 °C - 80 °C), using medium-sized Buchner funnels with a filtration pass. We have called this step “Micro-Nano separation” (Figure 1c). The micrometer-sized carbons are retained in the filter while the fraction of graphene-based material is located in the filter waters. This filtration water, containing fractions of graphene-based material, was allowed to decant slowly between 20 - 28 hours (Figure 1d), and subsequently filtered using commercial membrane filters between 0.1 - 0.45 micrometers of passage, which allow the separation of nanometric carbons, that is, those materials based on graphene. We have called this stage “Nano Extraction” (Figure 1e). The graphene-based materials were characterized in terms of their texture and porosimetry (Figure 2), Raman spectroscopy (Figure 3), and then used for the preparation of electrodes for their implementation as SC in aqueous acid medium (1 M H2SO4) and electrochemically analyzed by cyclic voltammetry (CV) methods (Figure 4), and energy efficiency (Figure 5) by making Ragone plots and verifying long-term capacitance retention was analyzed by consecutive cycles of galvanostatic charge/discharge (GCD).

The graphene-based materials presented high surface areas, greater than 900 m ² g ^-1 (Figure 2a) and a high total pore volume of 0.455 cm ³ g ^-1 (Figure 2b), indicating that their porosity is mainly constituted by supermicropores. with pore diameter values less than 1 nm (Figure 2b). At the same time, a porosimetry composition characterized by a mixture of 76% and 24% micro- and mesopores, respectively (Figure 2c), was achieved, which guarantees the correct diffusion of electrolyte ions through the mesoporous structure for its storage in micropores. Advantageously, the RAMAN spectrum (Figure 3) shows that the material obtained is an amorphous material whose intensity in the vibration bands corresponding to the defects (D1, D2, D3, and D4) and the graphitic mode (G), which allows us to conclude that the present material is graphene oxide. Additionally, it was possible to verify by X-ray photoelectron spectroscopy that the surface of this material is made up of 88.8% and 12.2% carbon and oxygen atoms, respectively. That is, for every 1 oxygen atom, this material has 7.3 carbon atoms for each oxygen atom.

The process to make the electrochemical capacitor includes a stage of synthesis of the graphene-based material, and a second stage of making the electrode, which are detailed below:

1.- Synthesis of the graphene-based material: which is in turn composed of 5 sub-stages.

(a) Feed: Mix and grind, in the absence of solvent, a biocarbon and KOH in a KOH/Biocarbon mass ratio between 0.3 and 3.6, until obtaining a paste; wherein said biocarbon is obtained from the pyrolysis of lignocellulosic materials or waste; Said lignocellulosic materials or residues are obtained without limitation from pine, oak, mahogany, apamate, carob bark; or more specifically said biocarbon is produced by pyrolysis of tannins extracted from pine bark: Pinus Radiata.

(b) Activation/Exfoliation: Introduce the paste obtained in (a) into a reactor inside an oven and hermetically close it under a nitrogen flow (between 80 - 120 mL/min); purge the system for 10 - 30 min at room temperature; heat at heating rates between 8 - 12 °C/min, until reaching a temperature between 600 - 800 °C for 0.5 - 1.5 h; cool the system slowly (3

- 5 h) until room temperature; remove the exfoliated paste from the oven;

(c) Filtration-1, Micro-Nano Separation: separate the material obtained in (b) into two parts, performing a first filtration of particles larger than 100 nm in particle size, and washing in hot distilled water (60 °C - 80 °C), (Figure 1 c); dry the solid material obtained at a temperature between 80 - 120 °C for 1

- 3 hours; collect the filtration water, which is used for the next stage;

(d) Nano Settlement: transfer the filtration water obtained in (c) to a settling funnel, filter nanomechanical particles smaller than 100 nm in particle size, and rest slowly between 20 - 28 h; (e) Filtration-2, Nano Extraction: filter the decanted liquid from step (d) using a filtration system with membrane filters between 0.1 - 0.45 micrometers pitch; This configuration allows the retention of graphene-based material; dry said material in an electric oven under static air between 80 - 120 °C for 1 - 3 h.

2.- Manufacturing of the electrode: composed of 4 sub-stages:

(f) Mixing: Mix the finely granulated graphene-based material from stage 1, with polytetrafluoroethylene as a binder and carbon black, in a weight ratio equal to 85:10:5, using N-methylpyrrolidone as a solvent; subject this mixture to vigorous stirring between 125 - 175 rpm for 10 - 30 minutes;

(g) Film preparation: deploy the mixture obtained in (f) with a glass rod on an inert support, according to Dr. Blade's technique; then, verify the quality of the film obtained, through observation in an optical microscope. Check that the film has no fractures and is homogeneous;

(h) Filling the Electrolyte: Subject the film obtained in (g) to a bath with an aqueous solution of the electrolyte H2SO41 M between 6 - 8 days;

(i) Cutting process: from the graphene-based material film obtained in (h), cut discs of 5 mm diameter, with a mass to surface area ratio between 8-10 mg cm ^-2 . These discs are called "electrodes".

Electrochemical measurements: after sub-stage (i), the electrochemical characteristics of the graphene-based materials were analyzed in a potentiostat/galvanostat, using a system of two symmetrical Swagelok cell-type electrodes, at 25 °C. The traditional three-electrode system was also used in the cyclic voltammetry (CV) study (Figure 4a) for a better estimation of the pseudocapacitive behavior of the material in the cell (Figure 4b). The normalized results of the capacitance of the fabricated electrode as a function of scanning speed and current density are shown in Figure 5a and Figure 5b, respectively. These electrodes are useful in electrical energy storage processes, according to the graph of Ragone (Figure 5c) and to the stability in the retention of the capacitance (Figure 5d), where after 10000 galvanostatic charge/discharge cycles.

In conclusion, the technological solution presented proposes the use of low-cost and eco-friendly graphene-based materials for incorporation into electrochemical capacitors.

APPLICATION EXAMPLES

Example 1. Preparation of electrodes as electrical capacitors.

The process to make an electrical capacitor included a synthesis of graphene-based materials by activation/exfoliation by thermochemical intercalation and a synthesis of the electrode, which are detailed below: 1 Synthesis of the graphene-based material: composed of 5 sub -stages.

1 .1 Food. A solid paste is prepared by mechanically mixing a biochar followed by KOH for 8 - 12 min. The mixture is prepared at different KOH/Biocarbon mass ratios between 0.3 - 3.6.

1.2.- Activation/Exfoliation: The paste is introduced into a sample holder in the center of a quartz tubular reactor and inside an electric oven. It is closed hermetically and under a nitrogen flow between 80 - 120 mL/min, the system is purged for 10 - 30 min, at room temperature, and then heated between 8 - 12 °C min until the final temperature between 600 - 800 ° C and is maintained between 0.5 - 1.5 h. After that, the system is allowed to cool for 3 - 5 h, and then the sample is removed from the tubular furnace and weighed.

1 .3.- Filtration- 1, Micro-Nano Separation: the material obtained was separated into two parts, performing a first filtration and washing in hot water between 60°C - 80°C with a volume between 500 - 800 mL of water distilled, using aliquots of 80 - 120 mL each. The solid material obtained on the filter is dried between 80 - 120°C for 1 - 3 h in an electric oven. The collected filtration water is used for the next stage.

1.4.- Nano Settlement: The filtration water containing graphene fractions was transferred to a settling funnel, and allowed to rest for 20 - 28 h. This process allows us to observe the progressive decantation of the nanometric carbon phases based on graphene. 1.5.- Filtration-2, Nano Extraction: The decanted liquid is filtered using commercial membrane filters between 0.1 - 0.45 pm passage. The retained graphene-based material is dried in an oven between 80

- 120 °C for 1 - 3 h. These working conditions allow obtaining graphene synthesis yields between 10 - 35%.

2.- Synthesis of the electrode based on graphene material. It is made up of 4 sub-stages:

2.1. Mixed. The graphene-based material obtained from stage 1, polytetrafluoroethylene, and carbon black are mixed in a weight ratio equal to 85:10:5, suspended in the solvent N-methylpyrrolidone. This mixture is subjected to vigorous stirring between 125 - 175 rpm, and between 10 - 30 minutes;

2.2. Film preparation: the previous mixture is placed (sub-stage 2.1.) on an inert support and deployed with a glass rod according to the Dr. Blade technique. Then, verify the quality of the film obtained, through observation in an optical microscope. Check that the film has no fractures and is homogeneous;

23. Electrolyte filling: the film obtained in sub-stage 2.2. is subjected to a bath with an aqueous solution of the electrolyte H2SO41 M between 6

- 8 days;

2.4. Cutting process: the film obtained from sub-stage 2.3 is removed and 5 mm diameter discs are cut, using a mass to surface ratio between 8 -10 mg cnr ² . These discs are called “electrodes”.

Example 2. Evaluation of graphene-based material as electrodes for the storage of electrical energy.

1 . Cyclic voltammetry and cell capacitance.

The electrochemical measurements of the material were made using the system of two symmetrical Swagelok cell-type electrodes, at 25 °C in the study of cyclic voltammetry (CV) and the cell capacitance. Figure 4a shows the results obtained from the current density observed at different scanning speeds between 5 and 300 mV s ^-1 using a potential window between 0 - 0.9 V for the graphene-based material. From the current density data at each scan speed, the cell capacitance values (Cceii, F g ^-1 ) are obtained and are plotted in Figure 4b as a function of the scan speed.

2. Electrode capacitance, Ragone plots and electrode stability. This configuration of two symmetrical Swagelok cell-type electrodes allows the specific capacitance of the electrode (Ceiec) to be directly obtained from the data obtained in the cell (Cceii), given that Ceiec = 4 Cceii. In this way, the normalized results of the electrode capacitance as a function of the scanning speed and current density are shown in Figure 5a and Figure 5b, respectively. It can be seen that using a scanning speed of 5 mV s ^-1 , the maximum Ceiec observed by the electrode is close to 157 F g ¹ (Figure 5a) while using current densities of 0.2 A g ^-1 , A maximum Ceiec of 194 F g ^-1 was observed.

To demonstrate that the manufactured electrodes are useful in the storage of electrical energy, a Ragone-type graph was made (Figure 5c) which shows that using current densities of 1 A g ¹ , comparable to that of some commercial electronic devices, we obtain energy densities between 3.8 - 4.0 Wh kg ¹ using power densities between 210 - 220 W kg ¹ . At the same time, the work stability in the long-term capacitance retention of the graphene-based material was verified (Figure 5d), where after 10,000 galvanostatic charge/discharge cycles, the material retained between 76 - 78% of its original specific capacitance.

In this way, the electrochemical performances observed, both in specific capacitance, current density and cyclability, show that the elaborated electrodes are clearly superior to the majority of electrochemical capacitors reported in publications [2-12] and/or in documents US8784764B2 , US6060424, US8940145B1, indicated above.

In conclusion, based on this background, it can be concluded that the present invention provides an extremely useful, simple, economical, eco-friendly, and scalable process for the production of graphene-based materials for 5 their use as electrodes in electrochemical capacitors for the storage of electrical energy.

Claims

1.- A process to prepare a potential material for the construction of electrodes for the storage of electrical energy, CHARACTERIZED because it includes at least the following stages:

(a) Feed: mix a biocarbon obtained from the pyrolysis of lignocellulosic materials or waste, with KOH at different KOH/Biocarbon mass ratios between 0.3 - 3.6; to obtain a mixture;

(b) Activation/Exfoliation: subject the mixture obtained in stage (a) to a nitrogen flow between 80 - 120 mL/min at a final temperature between 600 - 800 °C;

(c) Filtration-1, Micro-Nanol Separation: subject the material obtained in step (b) to successive filtration processes of micrometric material, with filtration of particles greater than 100 nm in particle size;

(d) Nano Decantation: decant and filter the nanometric material with filtration of particles less than 100 nm in particle size;

(e) Filtration-2, Nano Extraction: filter the decanted liquid from step (d) using a filtration system with membrane filters between 0.1 - 0.45 micrometers of passage, and then dry the material obtained between 80 - 120°C for 1 - 3 h; These working conditions allow obtaining graphene synthesis yields between 10 - 35%.

2. The process to prepare a potential material for the construction of electrodes for the storage of electrical energy according to claim 1, CHARACTERIZED because said biocarbon is produced by pyrolysis of lignocellulosic materials or waste, where said lignocellulosic materials or waste are obtained without limitation from pine bark, oak, mahogany, apamate, carob.

3. The process to prepare a potential material for the construction of electrodes for the storage of electrical energy according to claim 1 -2, CHARACTERIZED because said biocarbon is produced by pyrolysis of tannins extracted from pine bark: Pinus Radiata.

4. A process to make an electrode for the storage of electrical energy, CHARACTERIZED because it includes the stages of:

(f) Mix the material obtained according to claims 1-3, polytetrafluoroethylene and carbon black, in a weight ratio equal to 85:10:5;

(g) Spread the mixture obtained in (f) with a glass rod on an inert surface;

(h) Subject the film obtained in (g) to a bath of aqueous solution of the electrolyte H2SO4 1 M, between 6 - 8 days;

(i) Cut 5 mm diameter discs, meaning electrodes, with a mass to surface area ratio between 8 -10 mg cm-2.