Manufacture of popcorn graphite electrodes for electronics and bioelectronics

Abstract

An electrode comprising a self-supporting porous structure of expanded graphite and an electrical connector. Method for manufacturing a self-supporting porous structure of expanded graphite, said method comprising the following steps: preparing a suspension of expanded graphite particles in a liquid; sonicating said suspension of graphite particles by applying ultrasound, to obtain a homogenized suspension; and filtering said homogenized suspension under pressure to obtain an expanded porous structure which is at least partially and preferably substantially dry.

Description

Description Title: Fabrication of popcorn graphite electrodes for electronics and bioelectronics

[0001] Field of invention

[0002] The present invention relates in particular to the field of electrode materials for manufacturing catalytic electrodes and / or enzymatic bioelectrodes for use in glucose biosensors and fuel biocells. This invention relates in particular to the development of porous structures of microwave-expanded graphite ("popcorn graphite" paper) from expandable graphite flakes, and their use for preparing electrodes such as, for example, enzymatic bioelectrodes.

[0003] Description of the prior art

[0004] Bioelectrodes (bioanodes and biocathodes) are often formed by immobilizing enzymes and redox molecules on the electrode, which enable bioelectrocatalysis. Current methods do not use graphite for bioelectrode applications. Carbon nanotubes and graphene or graphene-related nanoscale materials are mainly used, and are more expensive.

[0005] Expandable graphite, originally called graphite salts, has been known since the beginning of the last century. This expandable graphite is obtained by heat treatment of mineral graphite with strong inorganic acids such as sulfuric acid and oxidants such as persulfuric acid or hydrogen peroxide [1-3]. These powerful agents transform the pore structure to increase the specific surface area of ​​the mineral graphite. These agents, including sulfuric acid, have the ability to intercalate between the graphene sheets during this treatment, which makes it possible to obtain expandable graphite, such as graphite sulfate C24(HSO4)(H2SO4)2. The expansion is initiated by heat treatment (> 200°C) or microwave (800W) which leads to the spontaneous evaporation of the agent and [partial exfoliation of the different graphene layers.This phenomenon immediately causes a considerable increase in the volume of the material, like the formation of popcorn from grains. This expanded graphite, or "popcorn", has been used for decades in composite materials as flame retardants [4] and as additives for heat and energy storage [5], Graphite. Expanded graphite can be processed into sheets by calendering processes or extruded into composites for various applications. Expanded graphite has regained the attention of scientists because it can be an excellent precursor for the large-scale synthesis of high-quality single-layer graphene [6] by calendering.

[0006] There are disclosures on the use of expanded graphite as a metal catalyst support in alcohol fuel cells for electrocatalytic reduction of oxygen and oxidation of alcohols [7, 8, 9], Expanded graphite composites are also used in the energy conversion sector for heat conduction, gas diffusion and electron collectors (called bipolar plates in hydrogen fuel cells). Expanded graphite is also mentioned as an anode material in a microbial (not enzymatic) fuel cell

[0010] ,

[0007] However, when it comes to enzymatic fuel cells, carbon nanotubes are the material of choice due to their advantageous structure (e.g., high surface area and attractive porous and conductive structure), inertness, and the possibility of forming self-supporting macroscopic electrodes in different forms: films, papers, and pellets [11, 12]. Thus, carbon nanotube-based enzymatic fuel cells produce 2 to 3 orders of magnitude more energy than similar graphene-based fuel cells [11, 13]. However, developments of carbon nanotube-based enzymatic fuel cells have remained, with few exceptions, at the academic level due to the cost of this material (see above) and its potential and / or presumed toxicity [14, 15]. Thus, as of January 2020,Companies that manufacture or import nanoscale materials such as carbon nanotubes (CNTs) will be subject to additional legal requirements under the REACH regulation (https: / / echa.europa.eu / regulations / nanomaterials). When less porous materials are used, for example, glassy carbon or graphene, these are generally limited in terms of catalytic efficiency / voltage.

[0008] There remains a need for very inexpensive porous carbon-based electrodes that are advantageously more efficient than most industrially available materials and / or present a substantially reduced toxicological risk. Technical description of the invention

[0009] It has thus been determined that the use of expanded graphite in the form of a self-supporting porous structure (in particular in the form of paper, sheets, disc plates or pellets) based on very cheap carbon is more effective than most materials available in the industry. The term "self-supporting" is equivalent to the term "self-supporting" and indicates that the material is in a form capable of ensuring its own span from one support point to another without intermediate support, in particular at room temperature and atmospheric pressure. The term "self-supporting structure" is therefore understood in macroscopic space and relates to a structure advantageously having at least one dimension greater than or equal to 5 mm. The porosity can be characterized by a specific surface area (BET N2) of at least 10 m 2 .g' 1 , preferably more than 15 m 2 .g' 1 and more preferably more than 50 m 2 .g'1 / . A high porosity is in fact preferable and this is advantageously greater than 100 m 2 .g' 1 , and more advantageously more than 200 m 2 .g' 1 for example around 220m 2 .g' 11 Particularly advantageously, the porous structure according to the invention is a hierarchical porous structure. The expression "hierarchical porous structure" describes the presence of multiple porosities in terms of pore size, with orders of magnitude ranging from micro- to meso- and finally to macropores, with interconnections, significant accessibility from the outside and a high specific surface area.

[0010] Also an object of the invention is an electrode comprising a self-supporting porous structure of expanded graphite and an electrical connector. This structure, or element, is advantageously in the form of a sheet or plate and can adopt various shapes suitable for its use as an electrode. This structure is thus generally of small dimensions, ranging from a few millimeters (for example 5 mm) to 1 or two decimeters. Thus to adapt to devices such as "button" batteries the structure can take the form of a pellet of 4 to 35 mm, preferably 4.5 to 30 mm, even more preferably 5 to 20 mm, for example around 10 mm. The thickness of the sheet, or plate, can vary for example from 1 to 10 mm.

[0011] The self-supporting porous structure comprises expanded graphite flakes. Advantageously, it is essentially made up of such flakes which are advantageously integral with each other and / or slightly compacted. Thus the structure can only comprise flakes of this material with possibly a very small quantity, for example in trace form, of other components, such as the liquid dispersion medium or sulfuric acid. It is however also possible to incorporate a binder into said structure but it may be devoid of it. Advantageously the flakes are not of nanometric dimension, but are generally of millimetric dimension or larger.

[0012] The electrode according to the invention can be advantageously used in electrochemical devices such as cells or batteries. Thus, it can advantageously comprise at least one catalyst for an electrochemical oxidation or reduction reaction. This catalyst can simply be adsorbed by said self-supporting porous structure of expanded graphite.

[0013] The percentage of expanded graphite present in the structure is preferably greater than 95% and more preferably approximately 99% (in practice, for example, approximately 80 pg of catalyst in approximately 6 mg of expanded graphite, i.e. approximately 100 times more graphite).

[0014] Preferably, the catalyst is chosen from the group consisting of metal catalysts, such as platinum, and biocatalysts, such as enzymes, and in particular enzymes for the oxidation of glucose or the reduction of oxygen. One or more additional compounds such as a compound facilitating the transfer of electrons between said catalyst and the electrode may also be part of the electrode. These compounds may include mediators and promoters of such transfers. Such compounds are known and those skilled in the art may select them depending on the catalyst used. For example, they may be: - aromatic or polyaromatic molecules such as naphthoquinone, 1,4-Naphthoquinone, 9,10-phenanthrenequinone, 1,10-phenanthroline-5,6-dione, phenazines, phenathiazines, tetrathiofulvalene, benzonquinone, anthraquinones, anthracenes, porphyrins; - inorganic complexes among a compound of osmium, ruthenium, iron and cobalt, - redox polymers (polymers with multiple redox centers based on transition metals such as osmium and ruthenium); - conductive polymers such as PEDOT, PEDOT:PSS, polyaniline, polypyrole; - polyoxometalates; or - nanoparticles such as gold nanoparticles.

[0015] The bioelectrocatalytic properties of these electrodes according to the invention are comparable to those of carbon nanotube (CNT) electrodes available in industry.

[0016] Another aspect of the invention relates to a method for manufacturing the self-supporting porous structure of expanded graphite, said method comprising the following steps: a) preparing a suspension of expanded graphite particles in a liquid; b) sonicating said suspension of graphite particles by applying ultrasound, to obtain a homogenized suspension; c) filtering said homogenized suspension under pressure to obtain a self-supporting porous structure of expanded graphite at least partially and preferably substantially, dry.

[0017] The advantages of this method or its variants described below include the following: - the use of inexpensive material (compared to current market standards) and widely available reagents; - the absence of nanomaterials that could pose greater toxicity risks and / or regulatory hurdles. Expanded graphite is inexpensive and is classified as a non-hazardous substance in accordance with Regulation (EC) No 1272 / 2008. - a rapid synthesis process to obtain a self-supporting element (structure) and even a ready-to-use functionalized electrode; and -obtaining significant macroporosity facilitating the immobilization of enzymes.

[0018] Preferably the expanded graphite particle dispersion has an expanded graphite particle concentration of between 0.5 mg.mL' 1 and 10 mg.mL' 1 , and preferably 1.5 mg.mL' 1 ± 0.5 mg.mL' 1 .

[0019] The liquid is chosen to allow, among other things, good dispersion. It may comprise, or consist essentially of, an organic compound selected from the group consisting of N-methyl-2-pyrrolidone (NMP), dichloromethane (DCM), acetonitrile (ACN), 1,3-dioxolane (DXL), N,N-dimethylformamide (DMF), and mixtures of two or more thereof, and preferably comprises or consists essentially of toluene.

[0020] It can also be an aqueous medium such as water, demineralized or not, or be aqueous-based and include a significant proportion of water associated with at least one other component.

[0021] The dispersion liquid may optionally include a surfactant and / or a polymer, in the solvent to improve dispersion and possibly reproducibility.

[0022] The preparation of the suspension can be carried out by any means, but the use of sonication is generally preferred. The frequencies used can range from 10 to 400 kHz, preferably from 30 to 50 kHz, for example around 40 kHz. The sonication time can be adjusted to adapt the final morphology, for example to improve the quality of the dispersion. Also, the sonication time can vary from a few minutes to a few hours. A period ranging from 0.5 to 3 hours, and especially from 2.5 to 3 hours, is preferred.

[0023] After sonication the homogenized dispersion is advantageously filtered by placing it on one side of a filter and applying pressure (e.g. on one or other side of the filter). Thus, most of the liquid can be extracted from the dispersion by applying a vacuum, for example of 1 to 3 mbar, on the surface of the filter on which the dispersion has not been placed. The filter can be a glass fiber, sintered glass, Teflon or paper filter. A simple paper filter and the application of a vacuum pump is sufficient to obtain, after suction of the liquid, self-supporting sheet or paper structures of popcorn graphite (PGP) of varying thickness which are then easily removed from the filter paper.

[0024] The expanded graphite structure thus obtained can be cut into different shapes and sizes, as required, and is advantageously connected to a metal wire contact, preferably insulated, to form an electrode according to the invention.

[0025] Expanded graphite can be obtained by carrying out a pre-existing expansion step of particles, or flakes, of expandable graphite. This expansion step can be carried out according to a typical synthesis process where flakes of expandable graphite are treated in a microwave oven in a dry state. Typically, the flask, or container, containing the expandable graphite particles is almost half full.

[0026] The method according to the invention has many advantages such as the simplicity of manufacturing which requires only a minimal number of steps. In fact, before formation of the structure by filtration, it may require only a single step: "one-pot" synthesis. It is also possible to use the dispersion step to obtain an electrode whose graphite has one or more compounds functionalizing the electrode. To do this, the catalyst and / or a compound facilitating the transfer of electrons between said catalyst and the electrode and / or any other additional compound desirable to improve the performance of the bioelectrode (for example to improve stability, reproducibility, the electroactive surface) can be associated with the dispersion liquid so as to quickly and simply obtain a functionalized material. Such a material is obviously also part of the objects of the invention.

[0027] Porosity and self-supporting properties can be advantageously controlled by a selection of parameters such as dispersion time and filtration pressure. Increasing macroporosity and / or hierarchical porosity can indeed improve bioelectrocatalytic and electrocatalytic performances compared to reference materials.

[0028] Another subject of the invention is a self-supporting expanded graphite material as described above and in particular a material obtained according to the process of the invention. This can be distinguished by one or more characteristics associated with its manufacturing process such as its macroporosity, its low density, its porosity, its absence of binder, the presence of traces of dispersion medium or sulfuric acid.

[0029] It is noted that the self-supporting porous structure of expanded graphite and the electrode according to the invention are of great interest, particularly for energy storage and harvesting devices such as batteries, supercapacitors or hydrogen fuel cells due to the greatly reduced cost of the material. This interest also extends to the field of energy storage and harvesting, electronics, bioelectronics, biosensors, and biocatalysts which use carbon-based electrodes / materials. According to a distinct aspect of the invention, electrodes, enzymatic or otherwise, of different shapes (inks, etc.) can be manufactured using the dispersions of “popcorn” or expanded graphite flakes.

[0030] Also another object of the invention is the use of an electrode and / or the structure described here in a device for storing and / or recovering energy, such as a battery, a supercapacitor or a fuel or hydrogen biocell; an electronic or bioelectronic device; a biosensor; a biodetector; a bioreactor; or a biocatalyst. The electrode according to the invention is particularly suitable for the manufacture of a fuel biocell.

[0031] Description of figures

[0032] [Fig. 1] Figure 1 represents a self-supporting popcorn graphite paper (PGP) disc manufactured by the method described in Example 1 and obtained using a dispersion of PGP particles prepared with a sonication time of A) 2h30 min and B) 3h00, then filtered using a vacuum pump.

[0033] [Fig. 2] Figure 2 represents a self-supporting popcorn graphite paper (PGP) disc manufactured according to the method described in Example 1 and obtained using a dispersion of PGP particles prepared with a sonication time of 2h30 then filtered using a water pump.

[0034] [Fig. 3] Figure 3: shows the cyclic voltammograms for (black) a commercial buckypaper (cBP) carbon nanotube paper bioanode and (grey) a popcorn graphite paper (PGP) bioanode in the absence (straight line) and presence (dotted line) of 100 mmolL-1 of glucose in 0.1 M phosphate buffer, pH 7.4. Scan rate 5 mVs-1 described in Example 2 (bioanode)

[0035] [Fig. 4] Figure 4: represents the cyclic voltammograms for (black) a commercial buckypaper (cBP) biocathode and (grey) a popcorn graphite paper (PGP) biocathode (straight line) under argon and (dotted line) in 0.1 M phosphate buffer, pH 7.4 saturated with oxygen described in Example 2 (biocathode).

[0036] [Fig. 5] Figure 5: shows the calibration curves (current versus glucose concentrations) for the PGP-thin disc (triangle), PGP-wafer (round) and cBP (square) bioanodes described in Example 3.

[0037] [Fig. 6] Figure 6 shows the cyclic voltammograms for (black) a commercial buckypaper (cBP) electrode and (light gray) a wafer popcorn graphite (PGP) electrode and (dark gray) a thin disk popcorn graphite (PGP) electrode, under argon, described in Example 4

[0038] Example 1 - synthesis of a material according to the invention

[0039] 1 g of dry expandable graphite flakes from Sigma Aldrich, ref. 808121 with formula C24(HSO4)(H2SO4)2 are placed in a flask 800 ml and treated in a microwave oven at a power of 500 W for only 15 seconds. The environment must be controlled and safety procedures followed because heating a conductive material in a microwave oven can lead to flash formation and very high temperatures. Typically, the flask is almost half filled. 100 mg of expanded graphite in popcorn form is then dispersed in 70 ml of toluene for 2.5 or 3 hours by sonication at 40 kHz at room temperature to obtain a dispersion. After sonication, the suspension is filtered through a PTFE Mi II ipore filter (JHWP, porosity 0.45 μm, diameter = 46 mm) by applying a vacuum or strong depression using a vacuum pump or a water pump, which allows to obtain a self-supporting popcorn graphite paper (PGP). This is then easily removed from the filter paper, cut into discs.The disc is connected to a wire contact with electrical contact insulation at the base of the electrode.

[0040] The thickness and degree of regularity (the presence or absence of bumps) of the upper surface, i.e., the surface that has not been in contact with the filter paper during filtration, change depending on the degree of dispersion, which changes depending on the sonication time. The thickness (approx. 3 mm) and degree of expansion of the popcorn graphite paper obtained after 2h30min of sonication remain the same or very similar in the case of vacuum filtration using a membrane pump (Fig. 1A) or a water pump (Fig. 2). The thickness (approx. 2 mm) and density of the PGP paper obtained after 3h of dispersion is thinner and more compact than those obtained after 2h30min (Fig. 1B) and has been called PGP-thin disc, while the PGP obtained after 2h30 sonication has been called PGP-wafer. The influence of the filtration method is less important or even negligible.The surface exposed to the Teflon filter paper is very smooth and probably more compact and less permeable to the electrolyte or aqueous solutions used to modify the electrodes, while the top surface is rougher and probably more open. This surface was therefore selected for the immobilization of the catalysts. Example 2 - Effect of the use of a PGP on the bioelectrocatalytic performance by cyclic voltammetry of an electrode

[0041] The efficiency of popcorn graphite paper (PGP) electrodes for bioelectrocatalytic reactions was exemplified for two reactions via two distinct bioelectrocatalytic processes: (i) indirect electron transfer by mediation (MET: <Mediated Electron Transfer> in English) and (ii) direct electron transfer (DET:<Direct Electron Transfer> Expanded graphite paper (EGP) electrodes were modified by physical adsorption of flavin adenine dinucleotide-dependent glucose dehydrogenase (FAD-GDH), for glucose oxidation, or bilirubin oxidase (BOx), for oxygen reduction.

[0042] Glucose oxidation is of interest for glucose biosensing or as a bioelectrocatalytic reaction at the bioanode in a biofuel cell, while oxygen reduction is of great interest as a bioelectrocatalytic reaction at the biocathode. The glucose oxidation bioelectrocatalytic reaction is carried out by TEM with an immobilized redox molecule as an electron transfer mediator. The oxygen reduction bioelectrocatalytic reaction is achieved by a simple DET reaction between BOx and the PGP electrode, for example, by taking advantage of the presence of oxygen-reactive sites on the edges of the expanded graphite. No additional surface functionalization, such as immobilization of a DET-promoting molecule, was used. Anode: Oxidation of glucose.

[0043] An anode was fabricated by cutting the PGP obtained as previously described into a 10 mm diameter disc. Its upper surface was incubated with a 40 pL drop of a redox mediator, 1,10-phenanthroline-5,6-dione (PLQ), prepared at a concentration of 5 mMnOlL 1 in a solution of demineralized water and 50% (v / v) acetone. After drying, 80 pL of enzyme (FAD-GDH) dissolved in a solution of 0.1 molL' 1 TP (phosphate buffer, pH 7.4) at a concentration of 5 mg mL' 1, were deposited dropwise onto the PLQ-exposed part of the electrode. After an incubation of approximately 2 hours, when the electrode surface appeared visibly dry, the electrode was rinsed with TP to remove weakly adsorbed material. It should be noted that the method is based on that described in application WO2018115710A1 which deals with the use of PLQ as a redox mediator with FAD-GDH on carbon nanotubes. The electrical connector and the silicone paste insulating layer were attached to the back of the bioelectrode and a half-cell electrochemical setup was constructed with reference and counter electrodes. Cyclic voltammograms were obtained in the absence (0 mmolL'1 of glucose) and in the presence of 100 mmolL'1 of glucose in a 0.1 molL'1 TP solution.As shown in Figure 3, the PGP electrode shows an improvement in bioelectrocatalytic performance by a factor well above 100% compared to a commercial bioelectrode based on discs (diameter 10mm) of buckypaper nanotubes. of carbon (cBP, NTL-12218). From CV (cyclic voltammetry) data the average catalytic current at 0.5 V against Ag / AgCl for PGP samples is 2100 pA ± 75 pA versus 880 pA ± 48 pA for cBP samples, which equates to a percentage increase of 138%. The onset of glucose oxidation currents appears to start at similar potentials for all electrodes, but the catalytic slope is considerably steeper for the PGP bioelectrode, which is an indication of significantly improved bioelectrocatalysis (e.g., faster catalytic rate). The maximum glucose oxidation current of about 3500 A is very significantly higher than the maximum current of about 900 A observed at cBP.Higher currents at lower potentials are of interest for voltage performance in fuel cells, but also for glucose biosensing where efficient catalytic reactions at lower potentials, or over a range of potentials, are desirable (e.g. to avoid electrochemical interferences such as oxidation reactions at positive potentials).

[0044] The high rate of glucose oxidation at different potentials is advantageous for glucose biosensing, e.g., to minimize interferences (e.g., oxidation reactions at positive potentials). The observed catalytic current due to glucose oxidation is highlighted with arrows. Cathode: Reduction of oxygen.

[0045] Cathodic performance was investigated with the enzyme bilirubin oxidase (BOx) that was deposited onto the electrodes from a 0.1 M TP buffer solution pH 7.4 containing 5 mg / ml of the enzyme. After incubation with 80 μl of solution for approximately 3 hours, the surface was visibly dry. A half-cell electrochemical setup was constructed with an Ag / AgCl reference electrode and a counter electrode (Pt wire). Cyclic voltammograms were first recorded under saturated argon atmosphere and then in the presence of oxygen-saturated 0.1 M TP, pH 7.4. Figure 4 presents these cyclic voltammograms, which show the performance of bioelectrocatalytic oxygen reduction by arrows.

[0046] Catalytic currents were higher for PGP electrodes (-535 pA ± 38 pA) compared to -390 pA ± 11 pA for cBP electrodes at 0.1 V. This represents an approximate percentage increase of 137%. It is important to note that PGP-wafer (Fig. 1A and Fig. 2, sonication for 2.5 h) and PGP-thin disk (Fig. 1 B, sonication for 3 h) electrodes showed similar results. similar for cathodic reduction based on our experiments, with the PGP wafer values ​​showing poorer reproducibility. As with the bioanode, the reaction initiation potential is similar, but the catalytic slope is steeper, indicating a faster catalysis rate and the possibility of obtaining higher oxygen reduction currents at higher potentials (which is of interest for biocathodes). We also observe a much higher background capacitive current for PGP compared to cBP in the voltage range of 0.7 V to 0.1 V during CV, which is consistent with the improved performance of the bioelectrode as a supercapacitor or biosupercapacitor. Example 3: Biological detection using chronoamperometry

[0047] To better understand the usefulness of these PGP electrodes for bioelectrochemical applications, chronoamperometric tests were performed for conventional glucose detection. The PGP-thin disc and cBP electrodes were incubated with 50 μl of 5 mmolL' 1 1,10-phenanthroline-5,6-dione dissolved in 50% acetone / FhO solution. After a few minutes, once dry, 100 μl of FAD-GDH enzyme (5 mg m*--1 ) were immobilized on the electrodes and allowed to dry overnight. The electrodes were rinsed with TP to remove loosely adsorbed material and were then tested in a 3-electrode cell (one cell as written in Example 2) using the chronoamperometry technique. The electrodes were subjected to a constant potential of 0.4 V against Ag / AgCl for 1800 s to obtain a stable background current before the addition of any glucose from a stock solution. The addition of glucose resulted in an initial concentration of 50 p.molL'1 , and was followed by successive additions. The bioelectrode was allowed to stabilize for 3 minutes after each glucose addition. The system was constantly stirred to maintain solution homogeneity and glucose transport.

[0048] The results obtained for PGP thin discs, PGP pancake discs and cBP bioelectrodes of identical diameters and prepared as described above are shown in Figure 5. All electrodes exhibit practically the same performance in terms of sensitivity (64.7 mA mol' 1 cm' 2 for PGP-Galette electrodes, 62.6 mA mol' 1 cm' 2 for GP thin disks and 63.6 mA mol' 1 cm' 2 for cBP) and linear range (between approximately 50 pmol and 20 mmol with R values 2 between 0.9910 and 0.9923). These values ​​are excellent for glucose biosensor applications that perfectly cover the human glucose concentration range (2-12 mmolL' 1 in extreme cases, and about 5 ± 2 mmolL' 1for healthy people). The main differences between these electrodes are in the maximum current, which represents the maximum catalytic rate (or velocity) of the bioelectrocatalytic electrode. This is reflected by the calculated Michaelis Menten apparent constant (KMapp) which is 26.7 mmolL-1 for PGP thin discs, 45 mmolL-1 for PGP wafer, and 40.7 mmolL-1 for cBP bioelectrodes. These values ​​are slightly lower than the constant of the free enzyme in solution (50 mmolL-1 ) reported by the supplier (Sekisui Diagnostics, UK) and represent excellent affinities of the enzyme for the substrate and the mediator. Another parameter of the KMapp is the diffusion kinetics of the substrate.Since the same enzyme and mediator are used to prepare the electrodes, it is very likely that the diffusion of glucose to the enzyme is reduced for the PGP-thin disk bioelectrode and almost optimal for the PGP-wafer bioelectrode. Example 4: Effect on the electrocatalytic performance of platinum catalysts by cyclic voltammetry

[0049] The good properties of this material for electrocatalysis were demonstrated in a comparative study using the standard catalyst platinum. PGP electrodes and Pt-modified cBP electrodes were evaluated using cyclic voltammetry measurements to understand their ability to consume H2 in an argon-saturated environment. In a typical measurement, a 4% Pt solution in graphite dispersed in acetone was used. 10Opl of the solution was deposited on the upper surface of all electrodes (diameter = 10 mm) and left to dry overnight. The electrical contacts were then fixed on the back side of the electrode and then insulated using a silicone paste. Measurements were performed using a 3-electrode cell with Pt as the counter electrode, Ag / AgCl (sat. KCl) as the reference electrode, and 0.5M H2SO4 as the electrolyte. The electrodes were first cycled for 20 times at 20mV s -1under argon, followed by the introduction of H2 gas via a Parker Biaston proton generator. After 10 minutes of H2 gas flow at 150 cc / min, the hydrogen flow was replaced by Argon and the electrodes were cycled 100 times at 100 mV / s. After this Pt nanoparticle activation step, the final CV measurements were performed at 20 mV / s. The results obtained are shown in Figure 6. They clearly indicate that the electrocatalytic performances of PGP-wafer and PGP-thin-disk electrodes are superior to those of cBP. In this case, PGP thin-disk electrodes are slightly better than PGP thick (wafer) electrodes. It could be suggested that hydrogen diffusion is less restricted than glucose diffusion and that the compact morphology of PGP-thin disc causes a denser accumulation of Pt particles which are more accessible to the catalytic reaction.

[0050] Conclusion

[0051] The performance of these bioelectrodes is compared to that of commonly used carbon nanotube-based "buckypaper" electrodes, which are currently considered the most or one of the most suitable materials for biofuel cell applications.

[0052] For the expanded graphite paper according to the invention, the bioelectrocatalytic current density can be increased approximately twofold for the biocathode and the bioanode compared to the equivalent bioelectrode prepared from commercially available multi-walled carbon nanotube paper ('commercial buckypaper'). Also, some electrocatalytic performances (e.g., H2 conversion) and capacitive performances are improved compared to the reference systems. For example, the surface of the Pt electrode can be more active (see the oxidation and reduction peaks in Fig. 6).

[0053] Besides the advantage of catalytic current production which in turn is linked to the improvement of energy performance in a biofuel cell the low cost of the precursor material makes this invention attractive for industrial processes, e.g. ~50 € / kg for expandable graphite vs. ~50,000 € / kg for multi-walled carbon nanotubes. The improvement in bioelectrocatalytic efficiency compared to buckypaper based on carbon nanotubes carbon is related to several factors such as favorable pore architecture, immobilization of the biocatalyst (redox enzyme and redox molecule or catalytic promoter), electron transfer and mass transport of the substrate (e.g., oxygen, glucose) and ions. Publications (1) McCreery, RL Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108 (7), 2646-2687. Traitement et ingénierie des nanotubes de carbone (2) Futaba, D. N. ; Hata, K. ; Yamada, T. ; Hiraoka, T. ; Hayamizu, Y. ; Kakudate, Y. ; Tanaike, O. ; Hatori, H. ; Yumura, M. ; lijima, S. Shape-Engineerable and Highly Densely Packed Single- Walled Carbon Nanotubes and Their Application as Super-Capacitor Electrodes. Nat. Mater. 2006, 5 (12), 987-994. (3) Kobashi, K. ; Yoon, H. ; Ata, S. ; Yamada, T. ; Futaba, D. N. ; Hata, K. Designing Neat and Composite Carbon Nanotube Materials by Porosimetric Characterization. Nanoscale Res. Lett. 2017, 12. (4) Whitby, R. L. D. ; Fukuda, T. ; Maekawa, T. ; James, S. L. ; Mikhalovsky, S. V. Geometric Control and Tuneable Pore Size Distribution of Buckypaper and Buckydiscs. Carbon N. Y. 2008, 46, 949-956. (5) Armeniades, Constantine D. ; Barrera, Enrique V. ; Kim, J. D. Polymer / carbon-Nanotube Interpenetrating Networks And Process For Making The Same. US20110201764A1 , 2011. Processing and engineering of expanded graphite (popcorn graphite) (6) RA Mercuri, G. Getz, RA Greinke, RA Howard, no. US5149518A (Ed.: UCT Corp), Ultra-thin pure flexible graphite calendered sheet and method of manufacture Graftech Inc, United States, 1990. Expanded graphite catalytic electrodes (7) M. Chen, B. Lou, Z. Ni, B. Xu, PtCo nanoparticles supported on expanded graphite as electrocatalyst for direct methanol fuel cell, Electrochimica Acta 165 (2015) 105-109. (8) A. Bhattacharya, A. Hazra, S. Chatterjee, P. Sen, S. Laha, I. Basumallick, Expanded graphite as an electrode material for an alcohol fuel cell, Journal of Power Sources 136(2) (2004) 208-210. Expandable graphite used as bipolar plates in hydrogen fuel cells (9) Exemple : S. Dhakate, S. Sharma, M. Borah, R. Mathur, T. Dhami, Expanded graphitebased electrically conductive composites as bipolar plate for PEM fuel cell, International Journal of Hydrogen Energy 33(23) (2008) 7146-7152. Graphite expansé dans les piles à biocarburant microbiennes (10) M. Kim, Y.E. Song, S. Li, J.R. Kim, Microwave-treated Expandable Graphite Granule for Enhancing the Bioelectricity Generation of Microbial Fuel Cells, Journal of Electrochemical Science and Technology 12(3) (2021) 297-301. Bioélectrodes à base de nanotubes de carbone (11) Gross, A. ; Hammond, J. ; Holzinger, M. ; Cosnier, S. Flotation Assembly of Large-Area Ultrathin MWCNT Nanofilms for Construction of Bioelectrodes. Nanomaterials 2017, 7 (10), 342. (12) Gross, A. J. ; Holzinger, M. ; Cosnier, S. Buckypaper Bioelectrodes : Emerging Materials for Implantable and Wearable Biofuel Cells. Energy Environ. Sci. 2018, 11 (7), 1670-1687. (13) Lalaoui, N.; Le Goff, A.; Holzinger, M.; Cosnier, S. Fully oriented bilirubin oxidase on porphyrin-functionalized carbon nanotube electrodes for the electrocatalytic reduction of oxygen. Chemistry: Eur. J., 2015, 21 (47), 16868-73. (14) Gross, AJ; Chen, X.; Giroud, F.; Abreu, C.; Le Goff, A.; Holzinger, M.; Cosnier, S. A High Power Buckypaper Biofuel Cell: Exploiting 1,10-Phenanthroline-5, 6-dione with FAD-Dependent Dehydrogenase for Catalytically-Powerful Glucose Oxidation. ACS Catal, 2017, 7 (7), 4408-4416. (15) Lalaoui, N.; Holzinger, M.; Le Goff, A.; Cosnier, S. Diazonium Functionalization of Carbon Nanotubes for Specific Orientation of Multicopper Oxidases: Control of Electron Entry Points and Oxygen Diffusion to the Enzyme. Chemistry: Eur. J., 2016, 22 (30), 10494-500. Other porous carbon materials for bioelectrodes (16) Tsujimura, S. ; Murata, K. ; Akatsuka, W. Exceptionally High Glucose Current on a Hierarchically Structured Porous Carbon Electrode with "Wired" Flavin Adenine Dinucleotide- Dependent Glucose Dehydrogenase. J. Am. Chem. Soc. 2014, 136, 14432-14437. (17) Funabashi, H. ; Takeuchi, S. ; Tsujimura, S. hierarchical meso / macro-porous carbon fabricated from dual MgO templates for direct electron transfer enzymatic electrodes. Sci. Rep. 2017, 7, 45147.

Claims

Claims 1. An electrode comprising a self-supporting porous structure of expanded graphite and an electrical connector.

2. The electrode of claim 1, wherein said self-supporting porous structure of expanded graphite comprises expanded graphite flakes.

3. The electrode according to any one of claims 1 or 2, wherein said electrode comprises at least one catalyst for an oxidation or reduction reaction.

4. The electrode of claim 3, wherein said catalyst is adsorbed by said self-supporting porous structure of expanded graphite.

5. The electrode according to any one of claims 3 or 4, wherein said catalyst is selected from the group consisting of metal catalysts, such as platinum, and biocatalysts, such as enzymes, and in particular enzymes for the oxidation of glucose or the reduction of oxygen.

6. Method for manufacturing a self-supporting porous structure of expanded graphite, said method comprising the following steps: a) preparing a suspension of expanded graphite particles in a liquid; b) sonicating said suspension of graphite particles by applying ultrasound, to obtain a homogenized suspension; c) filtering said homogenized suspension under application of a vacuum to obtain a self-supporting porous structure of expanded graphite which is at least partially and preferably substantially dry.

7. Method according to claim 6, wherein a step of expanding expandable graphite particles is carried out beforehand.

8. A method according to claim 6 or 7, wherein said expanded graphite particle dispersion has an expanded graphite particle concentration of between 0.5 mg.mL' 1 and 10 mg.mL ' 1 , and preferably 1.5 mg.mL-1 ± 0.5 mg.mL-1.

9. A method according to any one of claims 6 to 8, wherein said liquid is an organic compound selected from the group consisting of N-methyl-2-pyrrolidone, dichloromethane, acetonitrile, 1,3-dioxolane, dimethylformamide, and mixtures of two or more thereof, and preferably toluene.

10. Use of an electrode as described in claims 1 to 5 in an energy storage and / or recovery device, such as batteries, supercapacitors or fuel or hydrogen biocells, an electronic or bioelectronic device, a biosensor, a biodetector, a bioreactor, or a biocatalyst.

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