Process for manufacturing of LIG electrodes

Abstract

Process for manufacturing of LIG-electrodes suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors through carbonization of polymers by CO2 laser treatment under ambient conditions, LIG-electrodes obtainable by such process and their uses in the electrochemical field.

Description

[0001] PROCESS FOR MANUFACTURING OF LIG ELECTRODES

[0002] STATE OF THE ART

[0003] Carbonization of several / different polymers is applied in numerous sectors, in particular in the energy (manufacture of electrodes, fuel cells, batteries) and sensing (production of electrodes, for electrochemical sensors) field as well as for the preparation of membranes by separation and filtration.

[0004] Under appropriate conditions, the carbonization of polymers results in the formation of graphene layers characterized by high conductivity, resistance and surface area. Conventional carbonization processes are carried out in the furnace in a controlled atmosphere.

[0005] The use of laser pulse irradiation techniques (LIG, acronym for Laser Induced Graphene) for the carbonization of polyimides and polyetherimides is described in EP 3535214 and US 20170062821. Laser writing allows to obtain a spatially selective carbonization for the manufacture of electrically conductive carbonaceous patterns and tracks.

[0006] This process has been extended to synthetic polymeric substrates under ambient conditions, i.e. in oxygen-rich atmospheres and in the Supporting Information of article Chyan et al., ACS Nano 2018, 12, 3, 2176-2183 and can therefore be extended to further polyimide and polyetherimide substrates (e.g. substrates such as Kapton®).

[0007] At present, the laser carbonization process is used for the manufacture of graphenebased conductive tracks LIG, characterized by an electrical conductivity on the xy-plane (parallel to the polymeric substrate).

[0008] On the other hand, more complex is the manufacture of a LIG material, which can act as an electrode (LIG-electrode), i.e. the manufacture of a material characterized by an electrical conductivity that is not only planar (i.e. along the xy-axis, parallel to the substrate) but also transverse (i.e. along the z-plane, perpendicular to the substrate).

[0009] In fact, the LIG-electrodes require the further characteristic of electrical conductivity along the z-plane, that is, the plane perpendicular with respect to the polymeric substrate, and along the entire thickness of the LIG-electrode.

[0010] Furthermore, for the manufacture of LIG-electrodes it is always necessary a step of transfer / removal of said electrode from the substrate where the LIG carbonization took place and the subsequent integration / deposition of said LIG-electrode on a new substrate capable of guaranteeing the electrical properties necessary for the correct operation of the final electrode.

[0011] An example of this process is mould-casting of PDMS directly onto the LIG, where infiltration of the liquid PDMS into the pores of the LIG causes the removal of the PDMS via peel-off to also result in the removal of the LIG from the substrate.

[0012] Therefore, there is still a need to provide a new method that allows obtaining a LIG- electrode, usable either as such (LIG membrane) or transferred and deposited on another substrate (devices and sensors), in any case avoiding the additional step of removing the LIG from the polymeric substrate, and thus producing a self-supporting and flexible LIG and streamlining the process for forming of LIG materials, and specifically of LIG-electrodes.

[0013] SUMMARY OF THE INVENTION

[0014] In a first aspect thereof, the invention concerns a process for manufacturing of LIG- electrodes suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors comprising: a) providing a polymeric support selected from: i) at least one bulk polymer; or ii) nano- or micro-structured fibres deposited on the at least one bulk polymer; and having two faces, of which a first upper face and a second lower face; b) a first CO2 laser treatment on the first upper face of the support of step a); c) a second CO2 laser treatment on the second lower face of the support of step a); wherein:

[0015] - when the polymeric support of step a) is a support i), the thickness of the polymeric support is equal to or less than 50 pm, preferably from 25 pm to 50 pm, more preferably is 50 pm; and

[0016] - when the polymeric support of step a) is a support ii), the thickness of the nano- or micro-structured fibres is equal to or less than 15 pm, preferably from 5 pm to 15 pm, more preferably is 15 pm.

[0017] The invention also concerns a LIG-electrode suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors, preferably obtainable by the process of the invention, having two conductive faces (1,2) made of graphene-like material, having interposed the same graphene-like material interspersed with insulating polymeric material (3).

[0018] The invention further concerns the use of said LIG-electrodes, according to the invention or obtainable according to the method of the invention, in the energy, sensing, electronic and of flexible and / or wearable devices fields, preferably in the electrochemical field for the manufacture of electrodes, also flexible ones, usable in fuel-cell batteries, also wearable or in the sensing field, preferably in the manufacture of electrodes, also flexible ones, for electrochemical sensors, also wearable. DESCRIPTION OF THE DRAWINGS

[0019] Figures 1-3 illustrate schematic representations of LIG cross-sections obtained starting from supports of different thickness, of which Figures 1 and 2.1 represent comparative LIGs and Figure 3 the LIG of the invention.

[0020] Figure 1.1 A-B shows the optical microscope analysis of the cross-section of the LIG- electrode having a polymeric support with a thickness higher than 125 pm.

[0021] Figures 2.2 A-B show the morphologies of the sample with a polymeric support with a thickness of less than 25 pm, analysed by FESEM analysis.

[0022] Figures 2.3 A-B and 2.5A-D show the morphologies and cross-section of the LIG- electrode of the sample having a polymeric support with a thickness of less than 25 pm, analysed by FESEM analysis.

[0023] Figure 2.4 shows the XRD analysis of the LIG-electrode of the invention according to Example 1.

[0024] The analyses Figures 4A-B show the optical microscope analyses of the cross-section of the LIG-electrode of the invention according to Example 1.

[0025] Figure 5 shows the image of a mechanical flexibility test on the LIG-electrode of the invention according to Example 1.

[0026] Figure 6 shows the image of the LIG-electrode of the invention starting from a substrate ii) i.e. nano- or micro-structured fibres deposited on bulk SPEEK polymer according to Example 8.

[0027] Figure 7 shows the image of the samples according to Examples 1 and 8, analysed according to Example 4.

[0028] Figure 8 shows the exemplary diagram of the Greek cross geometry used in the electrical characterization of the LIG-electrodes of the invention according to Example 1 and 8.

[0029] Figure 9 shows the graphs of the average resistivity values for the samples of the invention according to Example 1.

[0030] Figure 10 shows the Raman spectroscopy analysis on the sample according to Example 1.

[0031] Figure 11 shows the BET analysis, specifically the isothermal curve, of the sample according to Example 1.

[0032] Figure 12 shows the BET analysis, the BJH curve, of the sample according to Example 1.

[0033] Figures 13A-C shows the morphologies of the sample of the invention according to Example 1, analysed by FESEM analysis.

[0034] Figure 14A-B show the morphologies of the sample according to Example 8, analysed by FESEM analysis.

[0035] Figure 15 shows the graphs of the average resistivity values for the samples of the invention according to Example 8.

[0036] Figure 16 RRDE measurements for samples of the invention according to Example 8.

[0037] Figure 17 shows the Raman spectroscopy analysis on the sample according to Example 8.

[0038] Figure 18 shows the graphs of the average resistivity values for the samples of the invention according to Example 8, starting from a polymeric substrate with a thickness higher than 125 pm.

[0039] Figure 19 shows the comparison measurements between the resistance along the xy- plane and along the z- direction for the samples of the invention according to Example 8, starting from a polymeric substrate with a thickness higher than 125 pm.

[0040] DETAILED DESCRIPTION OF THE INVENTION

[0041] In the present invention, by the following terms is intended:

[0042] “bulk polymer” polymer in continuous form, not micro- / nano-structured nor fibrous;

[0043] “graphene-like material” material comprising or consisting of graphene obtained by LIG carbonization.

[0044] “polymeric nano- or micro-structured fibres” fibres having a diameter comprised in the range from a few nanometres to a few microns (nano-structured fibres) and fibres having a diameter higher than 10 pm (micro-structured fibres). These fibres may be deposited and / or formed by electrospinning; and

[0045] “micro- / nano-structured or fibrous polymer” polymers having structures of sizes included in the range from a few nanometres to a few microns (nano-structured polymer) and polymers having structures characterized by sizes higher than 10 pm (micro-structured polymer).

[0046] It has now been found that it is possible to manufacture electrodes by applying laser- induced graphitization techniques (LIG-el ectrode s) characterized by electrical conductivity that is not only planar (i.e. along the xy-plane) but also along their thickness (i.e. along the z-axis) under ambient conditions. Said graphitization technique is applicable to bulk polymers, to polymeric nano- or micro-structured fibres, and to synthetic polymers after treatment with alkaline solutions before exposure to laser radiation. Furthermore, the LIG-electrodes thus obtained are self-supporting, that is, they do not require a substrate but can be used immediately after their production by the LIG technique, since they are characterized immediately, after their manufacture, by adequate electrical conductivity. Thus, the process of the invention eliminates the need to remove the once produced LIG-electrodes from the source substrate, e.g. Kapton® (i.e. an insulating polyimide film) and to transfer them onto another substrate. The process is thus faster, with less materials waste and more flexible than the prior art processes , since the LIG material produced can be used both as such, since it is self-supporting, and transferred / removed and deposited on a new substrate of choice.

[0047] In a first aspect thereof, the invention therefore has as its object a process for manufacturing of LIG-electrodes suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors comprising: a) providing a polymeric support selected from: i) at least one bulk polymer; or ii) nano- or micro-structured fibres deposited on the at least one bulk polymer; and having two faces, of which a first upper face and a second lower face; b) a first CO2 laser treatment on the first upper face of the support of step a); c) a second CO2 laser treatment on the second lower face of the support of step a); wherein:

[0048] - when the polymeric support of step a) is a support i), the thickness of the polymeric support is equal to or less than 50 pm, preferably from 25 pm to 50 pm, more preferably is 50 pm; and

[0049] - when the polymeric support of step a) is a support ii), the thickness of the nano- or micro-structured fibres is equal to or less than 15 pm, preferably from 5 pm to 15 pm, more preferably is 15 pm.

[0050] In step a) of the process, a polymeric support i) may be provided.

[0051] Said polymeric substrate can be a bulk polymer or bulk copolymer i) belonging to the class of thermoplastic polymers, such as:

[0052] - polyimide polymers, for example Kapton®;

[0053] - aliphatic polymers derived from alkenes, such as PE and PP;

[0054] - synthetic polymers derived from acrylonitrile such as polyacrylonitrile, acrylonitrile butadiene styrene (ABS), Nylon;

[0055] - Polymers with vinyl functionality, such as Polyvinyl alcohol (PVA) and polyvinyl chloride (PVC); - Fluorinated polymers, such as teflon, Polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP);

[0056] - Polyurethane polymers;

[0057] - Aromatic polymers derived from styrene;

[0058] - Polyesters, such as polycarbonate, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), poly (lactic acid) PLA;

[0059] - thermosetting polymers, such as melamine resins, thermosetting synthetic resins obtained by polycondensation of formaldehyde with melamine, urea-formaldehyde resins, thermosetting synthetic resins obtained by reaction of urea and formaldehyde; and

[0060] - Poly ether ether ketone sulphonate.

[0061] Further synthetic polymers or copolymers employable according to the invention comprise polyvinyl chloride, polyvinyl alcohol, nylon 6,6, polymethylmethacrylates, ureaformaldehyde resins, melamine-formaldehyde resins, Teflon, polylactate, polyurethanes, high- density polyesters, fluorinated ethylene propylene copolymers, acrylonitrile butadiene styrene, polyacrylonitrile, polyvinylidene fluoride.

[0062] Further polymers usable are polyethylene, polyethylene terephthalate, polyethylene naphthalate, polystyrene, polypropylene, polycarbonates.

[0063] If used, the above further synthetic polymers or copolymers are pre-treated by treatment with alkaline solutions.

[0064] If used, said polymers can be pretreated by means of films or membranes of polymers can be prepared by evaporation under ambient conditions of solutions of the polymers in suitable solvents, in concentrations typically included between 10 and 20 % by weight.

[0065] In step a) of the process, a polymeric support ii) can be provided made of nano- or microstructured fibres deposited on at least one bulk polymer, made for example of the same polymers as the polymeric support i).

[0066] Said fibres can be prepared and / or deposited by electrospinning, for example by solutions of the polymers in suitable solvents, in concentrations typically included between 10 and 20% by weight, according to known techniques.

[0067] Preferably, the at least one polymer of substrate i) or ii) comprises a polyimide, preferably it is made of an insulating polyimide film, more preferably it is Kapton®.

[0068] When the support of step a) is a support i) the thickness is equal to or less than 50 pm, preferably from 50 pm to 25 pm, more preferably is 50 pm.

[0069] When the support of step a) is a support ii), the thickness of the nano- or micro- structured fibres deposited is equal to or less than 15 pm, preferably from 5 pm to 15 pm, more preferably is 15 pm.

[0070] In fact, thicknesses of the support, for example of the support i), for example of the Kapton® sheet, higher than those included in said ranges, result in two LIG layers separated by a substrate layer, for example of insulating polyimide film, for example of Kapton®, electrically insulating interposed between them, thus not allowing their use as electrodes, specifically in sensor and electrochemical applications, for example in fuel cells. Moreover, in the case of a too thick support, the mechanical properties of the final product would be worse.

[0071] If, on the other hand, the support, for example support i), were of a thickness less than the ranges indicated above, when subjected to laser treatment, the entire polymeric substrate would be converted into LIG material, and this would lead to embrittlement of the structure until the electrode itself breaks.

[0072] The support in a thickness comprised in the ranges of the invention ensures the LIG at the same time mechanical support, stability, mechanical strength and flexibility as will be evident from the attached experimental part.

[0073] In the case instead of a support ii), the thickness of the nano- or micro-structured fibres equal to or less than 15 pm guarantees simplicity in the process of depositing the nano- or micro-structured fibres on the at least one bulk polymer, a short deposition time, with consequent limitation of the costs and times of deposition and for obtaining the final support ii).

[0074] In an advantageous and preferred aspect, step a) of the process of the invention provides a support i), made of at least one bulk polymer having a thickness equal to or less than 50 pm, preferably from 25 pm to 50 pm, more preferably of 50 pm.

[0075] In another advantageous and preferred aspect, step a) of the process of the invention provides a support ii) made of nano- or micro-structured fibres deposited on the at least one bulk polymer, wherein the fibres have a thickness equal to or less than 15 pm, preferably from 5 pm to 15 pm, more preferably of 15 pm.

[0076] The above range of thicknesses allows the conversion of LIG fibres, favouring the formation of a LIG material with hierarchical porosity and therefore high surface area.

[0077] The first CO2 laser treatment of step b) is carried out on the first upper face of the support of step a) under ambient conditions, i.e. in the presence of air at a temperature of about 20- 25 °C.

[0078] The second CO2 laser treatment of step c) is carried out on the second lower face of the support of step a) under ambient conditions, i.e. in the presence of air at a temperature of about 20-25 °C.

[0079] Preferably, said steps b) and c) take place respectively on the first and second face of the substrate of step a) in correspondence with the same region of the substrate.

[0080] By means of steps b) and c) the partial conversion of the insulating polymeric substrate of step a) into a LIG material is made possible, so as to obtain a final product, i.e. a LIG- electrode, comprising a polymeric substrate and a graphene-like material having electrical conductivity characteristics in the direction of the z-plane, i.e. in the direction perpendicular to the plane of the substrate.

[0081] In a preferred embodiment, the source of the CO2 laser employed in step b) and c) is capable of emitting a maximum nominal power of 30W.

[0082] The spot diameter of the CO2 laser on the sample of step b) and c) is 270pm.

[0083] The CO2 laser is provided with a DPI variable from 1 to 1000, a frequency variable from 1 to 25 kHz and a speed variable from 1 to 1000 mm / s.

[0084] After exposing to the CO2 laser writing the first upper face of the substrate, preferably of the polyimide sheet, more preferably of Kapton®, it is possible to observe a reduction of about 20pm in the thickness of the polymeric layer, thus leaving about 30pm of substrate, preferably Kapton®, remaining.

[0085] A possibility of controlling the mechanical properties of the LIG is provided by the set parameters of laser writing, to be modified with the intention of reducing the thermal energy transmitted to the sample and not making the layer of LIG material obtained at the end of the second step too brittle or even to break.

[0086] Preferably, between step b) and step c) there is a minimum waiting time of 1 minute.

[0087] This waiting time can in fact increase the repeatability of the process.

[0088] The parameters of steps b) and c) are: power emitted as a percentage of the maximum nominal power, density of dots (Dots Per Inch - DPI) irradiated by the laser, frequency, scanning speed of the source at the focal dot, dot delay (waiting time between reaching a new position on the sample and the emission of laser radiation); and number of repetitions of the process.

[0089] The density of dots (DPI) and the power emitted are the parameters that have the greatest influence on the mechanical properties of the LIG obtained.

[0090] Advantageously, at least one of the DPI value and the emitted power value of step b) is higher than that of step c).

[0091] Preferably the DPI value in steps b) and c) is included from 230 to 310, preferably it is 300 DPI in step b) and 250 DPI in step c).

[0092] Preferably the value of power emitted in steps b) and c) is comprised from 7 to 30%, preferably it is of 25% in step b) and of 25% in step c).

[0093] In a preferred embodiment, the frequency in steps b) and c) is comprised from 3.5 to 5 kHz, preferably it is 4 kHz.

[0094] Preferably, the scanning speed at the focal dot in steps b) and c) is comprised from 100 to 240 mm / s, preferably it is 125 mm / s.

[0095] Advantageously, the dot delay in steps b) and c) is 14 ps and has an influence on the dissipation of the heat accumulated by the material during the process.

[0096] Preferably, the number of repetitions of the process is 1, in order to speed up the preparation of the electrode as much as possible.

[0097] Optimization of all parameters brought the advantage of minimizing the resistance across the plane (along the z-axis), while maintaining the desired mechanical properties (flexibility).

[0098] The process of the invention, compared to the carbonization / graphitization technique in the furnace in a controlled atmosphere, allows the creation of LIG-electrodes written directly by laser, transforming the material in a localized way only in the areas that interact with the laser, without the need for technical gases and suitable equipment to contain and control them.

[0099] In a second aspect thereof, the invention further concerns a LIG-el ectrode, preferably obtainable by the process of the invention, suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors.

[0100] Said LIG-electrode has two conductive faces (1, 2) made of graphene-like material, having interposed the same graphene-like material interspersed with insulating polymeric material (3).

[0101] The LIG, therefore, comprises portions (3) of non-conductive insulating polymeric material, having variable shape and sizes. The portions of polymeric material (3) provide mechanical support and flexibility to the LIG obtained and, at the same time, do not affect the electrical conductivity properties of the final LIG material thus obtained, as will be evident from the attached experimental part. Said islands are incorporated in the central area of the faces of graphic material (1, 2) and are visible at a sectional cut of the LIG material. The presence of conductive LIG (1, 2) interspersed with non-conductive islands (3) is necessary in order to obtain a LIG-electrode suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors. It is necessary, in fact, that the two faces of the conductive LIG (1,2) are electrically connected, to ensure electrical conductivity, while sporadic islands (3) of non-conductive polymer remain within the LIG, to maintain its mechanical properties.

[0102] The LIG of the invention, preferably obtained according to the method of the invention, is a material having a hierarchical pore distribution, i.e. a material comprising multiple order porous structures, i.e. comprising both micro- and meso-pores.

[0103] Preferably, the pore distribution in the LIG of the invention is as follows:

[0104] - pores having a diameter of less than 2 nm and

[0105] - pores having a diameter included between 2 and 50 nm.

[0106] Still advantageously, the surface area of the LIG of the invention is included in a range from 50 m2 / g to 350 m2 / g, preferably it is 350 m2 / g as measured by BET (Brunauer-Emmett- Teller).

[0107] Also thanks to these porosity characteristics, together with the conductivity, and the specific geometry, i.e. the presence of portions of polymeric material (3), i.e. the islands that ensure electronic mobility and therefore provide the conductivity properties to the LIG of the invention, without affecting its mechanical properties of resistance and flexibility, said LIG is suitable for use as an electrode, for example for fuel cells, since a high surface area of the electrode for fuel cells is an important parameter for a good deposition of the catalyst in the fuel cells, while hierarchical porosity is advantageous in fuel cells since the presence of porous structures with different sizes (having hierarchical porosity) can allow the modulation and optimization of the diffusion of all those species, such as for example ions and oxygen molecules, which must reach the catalytic site.

[0108] Specifically, larger pores (meso-pores) (and therefore with lower capillary resistance) could be dedicated to the removal of the water produced, while smaller pores (micro-pores) (with higher capillary resistance) would guarantee the transport of the reactive gases to the catalytic sites.

[0109] Therefore, the invention further concerns the use of said LIG-electrodes, according to the invention or obtainable according to the method of the invention, in the energy, sensing, electronic and of flexible and / or wearable devices fields, preferably in the electrochemical field for the manufacture of electrodes, also flexible ones, usable in fuel-cells, in batteries, or in the sensing field, preferably in the manufacture of electrodes, also flexible ones, or for electrochemical sensors, also wearable.

[0110] EXPERIMENTAL PART

[0111] The invention is illustrated below with specific reference to Kapton® (thickness 50 pm) and Sulphonated poly (ether ether ketone) - SPEEK (thickness 25 pm). The techniques reported below are also applicable to other polymers, with similar results.

[0112] Example 1 - preparation of the LIG of the invention having a bulk polymer i) as

[0113] A 50 pm Kapton® sheet was subjected to carbonization induced by CO2 laser writing in continuous mode. In particular, a commercial CO2 (Laser Scriber MIC-LS0002, k=10.6 pm) laser capable of delivering a maximum power equal to 30W and characterized by a writing speed of 500 cps was used for the carbonization process. The LIGs connected to each other locally in a direction perpendicular to the plane of the substrate (Kapton®) were obtained by laser writing having the following parameters:

[0114] 1) “Front” parameters:

[0115] - Power: 25% (7.5 W)

[0116] - DPI: 300

[0117] Frequency: 4 kHz

[0118] Scanning speed at focal dot: 125 mm / s

[0119] - Dot delay: 14 ps

[0120] - Repetitions: 1

[0121] 2) “Back” parameters:

[0122] - Power: 25% (7.5 W)

[0123] - DPI: 250

[0124] Frequency: 4 kHz

[0125] Speed: 125 mm / s

[0126] - Dot delay: 14 ps

[0127] - Repetitions: 1

[0128] A LIG-electrode having 50 pm support i) made of Kapton® and two LIG layers (1, 2) connected to each other locally in a direction (along the z-plane) perpendicular to the plane of the substrate (3) was thus obtained. The electrode thus obtained has regions (islands) of unconverted polymeric Kapton® substrate (3) that are interposed between the two LIG layers (1, 2).

[0129] Overall, the material appears to be conductive both along the xy-plane and through it (z-plane) thanks to the LIG interconnections between the two surface layers, but it retains the mechanical properties of flexibility of the source Kapton® thanks to the regions (islands) of unconverted polymer that lie between the two LIG layers.

[0130] Example 2 - influence of the substrate thickness in the LIG of the invention according to Example 1

[0131] To demonstrate the importance of the initial thickness of the “bulk” polymeric substrate i) of step a), 3 samples were provided and treated according to the process of the invention of Example 1 :

[0132] 1) a substrate having a thickness greater than 125 pm (comparative);

[0133] 2) a substrate having a thickness of less than 25 pm (comparative); and

[0134] 3) a substrate having a thickness equal to 50 pm (of the invention).

[0135] In the first case, the bulk polymer tested, i.e. Kapton® having a thickness greater than 50pm, specifically greater than 125 pm, was found to be too thick and therefore the two LIG layers were separated by a layer of electrically insulating Kapton® not converted into LIG (Figure 1).

[0136] The remaining Kapton® thickness between the two LIG layers is clearly visible in Figure 1.1 and it is shown as a continuous layer and with a thickness of about 70 pm. Said thickness is too high to allow the passage of electrons, as will be shown below in Comparative Example 13.

[0137] In the second case, Kapton® or Sulphonated poly (ether ether ketone) - SPEEK, having a thickness of less than 50p, specifically of 25pm, was found to be too thin and therefore the entire substrate was found to be converted to LIG in a direction perpendicular to the plane of the sheet (Figures 2.1-2.2A-B).

[0138] The material obtained (Figures 2.1-2.2A-B) was almost completely converted into LIG after laser writing on only one side.

[0139] Traces of material not converted into LIG are visible (darker areas with white light reflections). The remaining material (15-20 pm in Figure 2.5 A-D) does not form a continuous layer (Figure 2.3A-B) suitable for undergoing a second laser writing process. It is also not flexible and is extremely brittle, thus totally losing the desired mechanical properties.

[0140] In the third case, in which a polymeric substrate, specifically Kapton®, is used in the thickness range of the invention, the two LIG layers are locally connected through the plane of the sheet, but there is still some Kapton® not converted into LIG which provides mechanical support and allows a certain degree of flexibility to be maintained (Figures 3, 4A-B and 5).

[0141] Also evident in Figure 4A are the islands (3) of substrate (in white and indicated with a dashed white arrow in Figures 4 A and 4B), specifically Kapton®, not converted and interposed between the two LIG layers (1, 2) (in black and indicated with a black arrow in Figures 4A and 4B).

[0142] The XRD analysis reported in Figure 2.4 shows, thanks to the identification of the planes (100) and (002) characteristic of the graphitic materials, that the LIG of the invention starting from the substrate having thickness in the range of the invention shows the trace of the remaining Kapton® islands that overlaps the LIG carbon (legend “LIG-el ectrode” in Figure 2.4), while by obtaining a powder scraped from the electrode, it can be confirmed that the surface is composed only of graphitic carbon LIG material without Kapton® contributions (legend “LIG_powder electrode” in Figure 2.4).

[0143] As regards instead the substrate ii), made of polymeric nano- or micro-structured fibres deposited by electrospinning on the bulk SPEEK polymer, a thickness of the fibres of less than 15 pm was tested on a substrate of less than 25 pm and the conversion of the micro-nano- structured fibres deposited on the polymer in LIG material is reported in Figures 6A-C. The sample was found to maintain good mechanical properties during pre and post laser treatment, comparable to those of the individual pre-conversion polymers, thus resulting in a material suitable to be used as an electrode, as will be evident from the following Examples 8-12.

[0144] Example 3 - flexibility tests of the LIG of the invention according to Example 1

[0145] Manual flexibility tests were carried out and are represented in Figure 5 together with optical microscope analysis of the section of the material obtained and represented in Figures 4A-B, they allowed to confirm that the configurations and parameters used according to Example 1 and 2 allowed to obtain a LIG able to guarantee in terms of flexibility good mechanical properties, comparable to those intrinsic to the unconverted polymer.

[0146] These results demonstrate the fundamental role of the islands (3) of substrate, specifically Kapton®, not converted and interposed between the two LIG layers (1, 2), i.e. the ability to maintain the properties of mechanical strength and flexibility unchanged, as highlighted in Figure 5 on a 10 mm x 5 mm-sized sample.

[0147] Example 4 - resistivity and electrical conductivity tests of the LIG of the invention according to Example 1

[0148] An analysis of resistivity and electrical conductivity was carried out on the LIG- electrodes of the invention obtained according to Example 1. Figure 8 represents the exemplary diagram of the Greek cross geometry used in the electrical characterization of the LIG layers independently of each other. The measurement requires the use of four probes, according to the Van der Pauw procedure, and can characterize the layer resistance (intrinsic resistivity of the material per unit of thickness) of the square region at the intersection of the arms of the cross.

[0149] Van der Pauw's method allows to determine the layer resistance of the square area at the intersection of the arms of the Greek cross, as explained in the previous section. The measurement consists of forcing current values through the sample and of measuring a potential difference, from which the resistance is obtained through the slope of the straight line in the figure below.

[0150] The electrical resistivity analyses were carried out on a sufficiently large sample to individually characterize the LIG obtained with the parameters used for each of the two sides of the Kapton® sheet. Figure 7 shows the samples analysed, where “F” (front) corresponds to the LIG obtained with the set of parameters 1) of Example 1, used for the first exposure to the laser, while “B” (back) corresponds to the set of parameters 2) of Example 1, used for the second exposure to the laser.

[0151] Ten nominally identical samples in Figure 7 and all obtained according to Example 1 were measured, and the average electrical resistivity is about 0.23 Q*cm which corresponds to an electrical conductivity of 4.4 S / cm with an 8% uncertainty on the average value. The analysis is reported in the graph in Figure 9.

[0152] The electrical conductivity in the direction perpendicular to the substrate (along the z- axis) was instead verified with two-contact measurements and appears to be lower than the conductivity along the (x, y) plane due to the reduced local contact regions between the two LIG layers (1, 2), alternated with islands (3) of polymeric and electrically insulating Kapton® as highlighted in Figure 3 and in the optical microscope images (Figures 4A-B). Although the internal structuring of the sample obtained according to Example 1 (Figure 3 and Figures 4A- B) makes it difficult to control and quantify the contact area between the two LIG layers, sufficient conductivity has been highlighted and confirmed also in the (x, y) plane and the electrical resistance measured in the direction perpendicular to the sheet is equal to about 100 .

[0153] These measurements therefore confirm that a conductive LIG-electrode is obtained, also by means of the method of the invention, also reproducibly on various samples, both along the xy-plane and through it (z-plane). Example 5 - LIG Raman spectroscopy of the invention according to Example 1

[0154] A qualitative analysis was carried out by Raman spectroscopy on the LIG-electrodes of the invention obtained according to Example 1, in order to analyse the composition thereof.

[0155] Figure 10 represents the spectrum obtained in which the 3 characteristic peaks of the graphene materials are present (as reported in literature peak “D” due to lattice defects, peak “G” uniquely characterizes the presence of the hexagonal lattice in graphitic and graphene materials, and peak “2D” characteristic of graphene is very sensitive to the number of layers).

[0156] From the ratios between the integrated intensities (indicated with I) of the peaks it is possible to analyse how: a value of — « 1 implies that the material obtained is defective, i.e. IG characterized by lattice structural defects (such as interruptions at the edges of the graphene flakes, the presence of 5 or 7-element rings instead of 6 carbon atoms of the ideal graphene) or by substitutional defects (for example the doping of the LIG with elements coming from the starting polymer, for example nitrogen from Kapton); while a value of — « in a range of 0.8 - 1 or of 0.9 represents the “few-lay ers IG graphene” having an average size of a crystallite of La« 17.4 nm

[0157] In the case of the analysis of Figure 10 it is evident how the value is in the range between 0.8 - 1 confirming the efficiency of the transformation of the polymeric material into graphenelike material, when subjected to the process of the invention, according to Example 1.

[0158] Example 6 - BET (Brunauer-Emmett-Teller) analysis for calculation of the surface area and pore size of the LIG of the invention according to Example 1

[0159] The method is based on the isothermal adsorption and desorption of nitrogen close to the condensation temperature (about 77 K) on the surface of the material. From this analysis it is possible to obtain the surface area and pore size of the LIG of the invention according to Example 1.

[0160] The isothermal curve of Figure 11 is that of both a micro- and meso-porous material (pores < 2 nm and between 2 and 50 nm, respectively, according to the IUPAC classification). The surface area obtained is about 350 m2 / g; therefore, confirming itself as a LIG suitable for use as an electrode for fuel cells.

[0161] The surface area measured by BET method is expressed as m2 / g. In particular, in this specific case, it has been verified that, for each g of sample, the material exposes an area of 350 m2. Said surface area is the consequence of a good porosity of the final LIG sample, which is a micro and meso-porous sample, i.e. characterized by pores having sizes of less than 2 nm and by pores having sizes included between 2 and 50 nm.

[0162] Figure 12 represents a curve obtained according to the Barret- Joy ner-Halenda method, i.e. a mathematical procedure, through which the distribution of the diameter of the pores is obtained starting from the Kelvin equation, which connects the radius r with the relative pressure (P / PO), at which the condensation of nitrogen occurs inside the pores.

[0163] The BJH curve of Figure 12 highlights the pore size and confirms the presence of a hierarchical porosity; therefore, also in this case, confirming the LIG as a material suitable for use as an electrode for fuel cells.

[0164] Example 7 - FE-SEM analysis of the LIG of the invention according to Example 1

[0165] FE-SEM images of Figures 13A-C of the surface of the LIG obtained according to Example 1, highlight a periodic roughness induced by laser writing, and the presence of very high porosity at different magnifications (in accordance with the BET of Example 6).

[0166] Example 8 - preparation of the LIG of the invention having nano- or micro- structured fibres deposited on the at least one bulk polvmer ii) as polymeric support

[0167] The nano- or micro-structured nanofibers were prepared and deposited on the bulk polymer ii) by electrospinning, for example by solutions of the polymers in suitable solvents, in concentrations typically included from 10 to 20% by weight, according to known techniques. Subsequently, the aforesaid support ii) was subjected to carbonization induced by laser CO2 writing in continuous mode, according to the procedure and parameters of Example 1.

[0168] Thus, a LIG-electrode having support ii) made of Kapton® with deposited nano- or micro-structured fibres having a thickness of the aforesaid nano- or micro-structured fibres equal to or less than 15 pm was obtained.

[0169] Example 9 - FE-SEM analysis of the LIG of the invention according to Example 8

[0170] FE-SEM images of Figures 14A-B of the surface of the LIG obtained according to Example 8, highlight a periodic roughness induced by laser writing. In addition, morphological characterization (FESEM) allows to highlight the preservation of the nanostructure even after the Laser CO2 process of conversion of the substrate having nano- or micro-structured fibres deposited on the at least one bulk polymer in LIG-NFs and having a thickness of the aforesaid nano- or micro-structured fibres equal to or less than 15 pm.

[0171] Example 10 - resistivity and electrical conductivity tests of the LIG of the invention A resistivity and electrical conductivity analysis was performed on the LIG-el ectrode s obtained according to Example 8, by means of the same procedure as Example 4.

[0172] The electrical characterization (I-V) for the samples obtained according to Example 8 reported in Figure 15 (where LIG-NFs_l and LIG-NFs_2 are two identical repetitions to verify the reproducibility of the measurement), showed a low resistance equal to (74.3 ± O.2)f2 typical of a material with excellent electrical conductivity properties. These measurements therefore confirm that a conductive LIG-electrode is obtained, also by means of the method of the invention, also in a reproducible manner and on various samples, both along the xy-plane and through it (z-plane) starting from a substrate ii).

[0173] Example 11 - electrocatalysis tests (RRDE) of the LIG of the invention according to Example 8

[0174] RRDE measurements were carried out through a CH Instrument 760D electrochemical workstation and an ALS RRDE-3 A rotating ring electrode apparatus. During all measurements, the material, developed as an electrode for Fuel Cell and catalyst for the oxygen reduction reaction, is fixed on the working electrode (a BioLogic glassy carbon disc / Pt ring, area 0.13 cm2).

[0175] For comparison purposes, commercial Pt / C (Sigma-Aldrich) was used as the reference catalyst. A Pt wire is used as the counter electrode and Ag / AgCl as the reference electrode. The electrolyte used is a 0.1 M aqueous solution of KOH saturated with O2 and the experiments were carried out at room temperature (about 20-25 °C). For the measurements of the rotating disc electrode (RRDE), the disc electrode was scanned from +0.2 to -0.8 V with respect to the Ag / AgCl reference, with a speed of 5 mV / s and a fixed rotation speed of 2500 rpm, while the ring electrode was maintained at a fixed potential of 0.2V. When scanning the potential of the disc at a fixed rotation speed, the current of the disc and ring electrode was measured. The current of the disc is linked to the four-electron ORR current, while the current of the ring is associated with the two-electron paths. In particular, the ring electrode is made of platinum, and a high potential value is applied to it. The percentage of HCE' and the number of electrons exchanged were calculated with the following Equations (I) and (II), which show that the number of exchanged electrons increases as the current of the disc increases: wherein IR and ID are the currents of the ring and disc respectively, n is the number of electrons exchanged, and N is the collection efficiency of the current of the Pt ring.

[0176] The aforesaid measurement was carried out in order to evaluate the electro-catalytic properties of the LIG-NFs according to Example 8 and applied, for example as a catalytic layer, in the cathode electrode in order to be able to replace platinum (Pt), considered as the ideal catalyst for the ORR (oxygen reduction reaction).

[0177] In Figure 16 it is possible to determine how the LIG nanofibers (LIG-NFs, specifically two identical samples LIG-NFs_l and LIG NFs_2 of nanofibers transformed into LIG material) guarantee a number of exchanged electrons equal to about 4, having performances comparable to those of Pt.

[0178] In addition, the percentage of hydrogen peroxide produced is less than 20%, confirming the good electro-catalytic properties of LIG-NFs capable of catalysing the direct oxygen reduction reaction.

[0179] Also in this case, it was possible to demonstrate that the LIG of the invention also starting from a support ii), is suitable for manufacturing electrodes usable in the electrochemical field, for example electro-catalytic sensing field.

[0180] Example 12 - LIG Raman spectroscopy of the invention according to Example 8

[0181] A qualitative analysis was carried out by Raman spectroscopy on the LIG-electrodes obtained according to Example 8, in order to analyse the composition thereof. Figure 17 represents the spectrum obtained in which the 3 characteristic peaks of the graphene materials are present (as per literature https https: / / doi.org / 10.1063 / 1.1674108), i.e. peak “D” due to lattice defects, peak “G” uniquely characterizes the presence of the hexagonal lattice in graphitic and graphene materials, and peak “2D” characteristic of graphene is very sensitive to the number of layers).

[0182] Figure 17 confirms the transformation efficiency of the nano- or micro-structured fibres deposited on a bulk polymer in graphene-like material, when subjected to the process of the invention, according to Example 8.

[0183] Example 13 - resistivity and electrical conductivity tests of a comparative LIG, obtained according to Example 1, starting from a substrate having a thickness greater than 125 pm.

[0184] An analysis of resistivity and electrical conductivity was carried out on the LIG- electrodes obtained, following the same procedure as in Example 4.

[0185] It is possible to verify (Figure 18) how the LIG-electrode obtained starting from a Kapton® with a thickness higher than 127 pm has an unconverted Kapton® layer between the two LIG faces. Said polymeric substrate has a thickness such as to affect the electrical conductivity along z. In fact, the results obtained show an electrically insulating behaviour when measuring the bulk resistance of the sample along the z-axis: 7?_sheet = 1.71*109± 1 14*1O9 / 2 In addition, measurements of comparison between the resistance along the xy-plane and along the z direction are shown in Figure 19, by means of the values shown in the upper-right box in Figure 19 that identify the electrical resistances obtained in units of Ohm (Q) and listed in the same order in which the samples are represented in the graph.

[0186] The aforesaid values show that the electrical conductivity (i.e. the inverse of the electrical resistance, which corresponds to the slope of the fitting line of the data measured in Figure 19) observed along the plane does not translate into an equally good conductivity through the plane, regardless of the type of electrical contacts used (point needle or clips applied to copper tape).

[0187] It can be concluded that the high Kapton® thickness (127pm) remaining between the two LIG layers does not allow the formation of a conductive electrode along the z-axis, therefore not making it an electrode usable in the electrochemical field, for example in fuel cells, or in the electrochemical sensing field.

Claims

CLAIMS1. A process for manufacturing of LIG-electrodes suitable for manufacturing electrodes in fuel cells, in batteries or as electrochemical sensors, comprising: a) providing a polymeric support selected from: i) at least one bulk polymer; and ii) nano- or micro-structured fibres deposited on the at least one bulk polymer; and having two faces, of which a first upper face and a second lower face; b) a first CO2 laser treatment on the first upper face of the support of step a); c) a second CO2 laser treatment on the second lower face of the support of step a); wherein:- when the polymeric support of step a) is a support i), the thickness of the polymeric support is equal to or less than 50 pm, preferably from 25 pm to 50 pm, more preferably is 50 pm; and- when the polymeric support of step a) is a support ii), the thickness of the nano- or micro-structured fibres deposited is equal to or less than 15 pm, preferably from 5 pm to 15 pm, more preferably is 15 pm.

2. The process according to claim 1, wherein the polymeric support of step a) is a support i).

3. The process according to claim 1, wherein the polymeric support of step a) is a support ii).

4. The process according to any one of claims 1-3, wherein the substrate of step a) comprises a polyimide, preferably it is made of an insulating polyimide film.

5. The process according to any one of claims 1-4, wherein the first CO2 laser treatment of step b) is carried out on the first upper face of the support of step a) under ambient conditions, in the presence of air at a temperature of about 20- 25 °C.

6. The process according to any one of claims 1-5, wherein the second CO2 laser treatment of step c) is carried out on the second lower face of the support of step a) under ambient conditions, in the presence of air at a temperature of about 20- 25 °C.

7. The process according to any one of claims 1-6, wherein steps b) and c) take place respectively on the first and second face of the substrate of step a) in correspondence with the same region of the substrate.

8. LIG-electrodes obtainable by the process according to any one of claims 1-7.

9. A LIG-electrode suitable for manufacturing electrodes in fuel cells, in batteries or aselectrochemical sensors, having two conductive faces (1,2) made of grapheme material, having interposed the same grapheme material interspersed with insulating polymeric material (3).

10. The LIG-electrode according to claim 9, having a hierarchical pore distribution comprising both micro- and meso-pores, preferably distributed between pores having a diameter of less than 2 nm and pores having a diameter between 2 and 50 nm.

11. The LIG-electrode according to claim 10, wherein the surface area is comprised in a range from 50 m2 / g to 350 m2 / g preferably it is 350 m2 / g, as measured by BET (B runauer-Emmett-T ell er) .

12. Use of LIG-electrode s according to any one of claims 8 or 9-11, in the energy, sensing, electronic and of flexible and / or wearable devices fields.

Images