Bio-based polymer-carbon composite formulation for electrodes and electrochemical energy conversion and storage devices

Abstract

Is provided a composite material comprising bio-based and bio-degradable and thermoplastic and insoluble polyesters, bio-based and bio-degradable and soluble or partially soluble polysaccharides and conductive carbon additives and optionally bio-based and natural poly-acids and / or bio-based and natural poly-bases and optionally other additives, and the process for the preparation thereof, wherein said material is used for the preparation of electrodes, to be used in electrochemical energy conversion devices and storage device comprising the electrode thereof.

Description

[0001] Bio-based polymer-carbon composite formulation for electrodes and electrochemical energy conversion and storage devices

[0002] The present invention refers to a bio-based polymer-carbon composite formulation which can be used to fabricate electrodes and electrochemical energy conversion and storage devices.

[0003] Prior art

[0004] The energy generated from renewable sources such as sun or wind is considered a key option for the growing energy demand. Additionally, along with large scale energy harvesting systems such as wind turbine or photovoltaic panels for grid applications, micro-generators are currently under development to be applied for consumer electronics and wearable devices. The intermittency of natural energy sources and consequent fluctuations in harvested and transformed energy raised the need of systems to ensure a regulated power supply. Among the different classes of systems used to address these issues electrochemical devices for energy conversion and storage through chemical reactions are one of the most practical and reliable solutions for the next future and already exploited and manufactured both for stationary (e.g. electric grid balance) and mobile (e.g. hybrid and electric vehicles -HEV and EV- and consumer electronics) applications.

[0005] The core of any electrochemical device is an electrochemical cell, which is mainly constituted by three fundamentals elements namely (i) electrodes, (ii) separator, (iii) electrolyte. The electrode usually consists of a current collector and an active material for the converting and / or store electrical energy through chemical processes. The separator is an electrically insulator which avoid short-circuit by electric contact between the two electrodes. The electrolyte contains the active material for balancing the charge accumulation and release at the electrodes (ions, redox molecules, additives etc.). The electrolyte can be formulated as a liquid, a solid or a gel. In the latter two cases it usually acts as both electrolyte and separator.

[0006] Depending on the materials used and chemical or physical charge accumulation and release process, electrochemical devices can be divided in two main categories, namely capacitors, which exploit a capacitive charge accumulation and release, and batteries, which take advantage from faradaic (redox) processes. While capacitors are usually able to work at high current rates and power, i.e. fast charge / discharge (minutes), with limited volumetric and mass energy density, batteries have a lower power supply and charge / discharge rates (hours) but can store much more energy. Batteries are divided in primary, not rechargeable (including alkaline, aluminium, Al-air, primary lithium, Li-air, Hg, NiOx, silver-oxide, Zn-air, Zn-C, zinc chloride) and secondary, rechargeable (including lead-acid, Li-ion including Li-polymer, Li-S and LFP, Ni-Cd, Ni-Fe, Ni-Zn, NiMH, Ag-Zn, Si- air, K-ion, Na-ion, flow batteries including V, Zn-Br, Zn-Ce and quinones, fuel cell). Due to the very limited energy density of classic capacitors many research efforts and commercial interests for grid, EV and consumer electronics applications are directed to supercapacitors (also known as ultracapacitors), which employ a high surface area electrode material and an ionic electrolyte. Depending on the electrode material, supercapacitors can be divided in Electric Double Layer Capacitors (EDLC), employing conductive carbon, and pseudo-capacitors, employing metal oxides and / or conductive polymers. Finally, in order to take advantage from both high capacitive charge / discharge rates and high faradaic energy density, Hybrid Electrochemical Capacitors (e.g. Li-, Na-, K-HEC) and Ultra Batteries (e.g. lead-acid), combining one capacitive electrode or part of that with faradaic electrodes. Such hybrid electrochemical devices have been already developed at a commercial scale.

[0007] Due to the exponential growth of demand of energy storage systems in the grid, automotive and consumer electronics sectors the problem of availability of materials, and especially critical metals, the overall life-cycle of the energy storage device, the high carbon footprint and, most importantly, the generation of huge volumes of electronic waste, the sustainability of conventional energy storage devices manufacturing represent a societal challenge. Metal components recycling and complex and dedicated waste treatment is required to overcome the problem associated to the necessity of increasing the production and availability of energy storage devices. Thus, many efforts are devoted to developing so called green energy storage devices, with the aim of reducing the use of metals and fossil-based components in the energy storage device, trying to substitute them with bio-based materials.

[0008] Prototypes and proof-of-concept devices based on natural concepts and biomimicry, using for example carbohydrates or other organic compounds as energy carriers, where redox enzymatic reactions allow to store the electrical energy, have been demonstrated. More advanced products have been developed by using biomasses to produce the fundamentals components of energy storage devices, including separators, binders, and electroactive electrode materials. Many the properties of biomass-derived materials such as renewability, low-cost, earth-abundance, multi-functional and mechanical properties are very attractive, especially in the view of replacing non-sustainable materials currently on stream. Among the most widely used bio-based materials for the preparation of energy storage devices surely cellulose, with its derivatives and composites is one of the most studied. Cellulose and paper have been successfully used as electrically insulating ion conducting separators thanks to their peculiar chemical and mechanical properties. Electrically conductive cellulose composites with metal or carbon additives and inks have been prepared to produce all cellulose-based devices. Although paper-making techniques for these bio-based composites are promising for a large-scale application still fundamental challenges remain for a practical application of paper and cellulose-based energy storage device. Main drawback of cellulose-based materials is their high hydrophilicity resulting in a high-water adsorption that reduces the overall lifetime of the electronic device and electrode materials. Consequently, while cellulose and its derivatives have been effectively used as separators, their use as replacement for conventional current collectors and electrodes is still intrinsically limited.

[0009] The combination of electrically conductive carbon additives with polymers is well known approach for making electrodes and composite film electrodes. Several carbon-polymer composite electrodes have been disclosed and proposed for their use in batteries, supercapacitors, fuel cells and a variety of electrochemical energy conversion and storage devices, but their wide application and market adoption is limited due to intrinsic shortcomings.

[0010] A first class of carbon polymer electrodes, known in the state of the art, is based on fossil-carbon based polymers (Fritsch et al., 2020, Batteries 2020, 6, 60) described a polymer-carbon composite-based current collector foil which has been proved to be effective in substituting aluminium foil in the fabrication of cathodes for lithium-ion batteries. They disclosed a composite based on poly(vinylidene fluoride) (PVDF) and carbon additives such as multi-walled carbon nanotubes (MWCNT). The composite is not bio-based and fluorinated polymers presents are not generally considered a fully sustainable or green option. Moreover, PVDF and its derivatives present a quite high density with respect to other polymers such as polyesters.

[0011] Moreover, PVDF and its derivatives present a quite high density with respect to other polymers such as polyesters. The high density of PVDF represents a potential drawback for the application in energy storage devices as it affects negatively the overall gravimetric and volumetric energy density of resulting electrodes and devices.

[0012] US patent n. US10991476 provides further improvement of PVDF-based carbon composite electrodes and describes the necessity to activate the electrode surface by polishing, sanding or plasma treatment for instance to improve the electrochemical performances of the electrode as the interactions with redox species or its capacitance. The need of post-processing surface treatment and activation for carbon-polymer electrodes, which is generally provided by wet or dry oxidative processes, is another general limit for the state-of-the-art formulations.

[0013] Alternatively, a general method adopted to improve the electrochemical performances and reactivity of polymer carbon electrodes is represented by pre-processing treatments of active carbon materials as disclosed by US patent n. US4758473 for graphite carbonbased thermoplastic composite electrode. Usually, these pre- or post-processing treatments of carbon materials and carbon electrodes are aimed to generate superficial reactive chemical moieties as oxygen or nitrogen containing functional groups. While these moieties are beneficial for the chemical reactivity and possibly for the electrochemical performances, they represent structural defects which can negatively affects the electrical conductivity increasing local and overall electrical resistance.

[0014] UK patent n. GB1315617A discloses formulations to optimize mechanical properties and dispersion of conductive fillers in the polymer matrix.

[0015] US patent N. US7008991 disclose a composition employing to different thermoplastic components to improve electrical properties such as conductivity and antistatic properties without affecting mechanical strength and fluidity.

[0016] Chinese patent n. CN113861558A reports the results of efforts during the last decades in improving the formulations by using different polymers and additives without adding relevant functional properties while modifying the composition.

[0017] Bio-polymers (bio-based, bio-degradable or both) have been considered as viable option for their use in composite electrode or conductive components in bio-based or biodegradable electronics to decrease their carbon footprint on environment and human health (Feig et al., 2018, ACS Cent. Sci., 4, 3, 337-348). The role of bio-polymers, which are electronically insulating, in the composite electrode is usually just structural, i.e. to provide good mechanical properties such as strength and flexibility. A first class of bio-based polymers used to make composite electrodes is represented by polysaccharides, and particularly cellulose and its derivatives among all polymers used. A recent example is reported by Pal et al., 2022, Electrochimica Acta, 415, 140239 using cellulose-carbon composite electrodes for the fabrication of a supercapacitor. The preparation of such composites is generally limited to solution-based techniques and mainly water-based processing. The general affinity for polar solvents as water or other organic solvents used for electrolytes, and in many cases the quite high solubility of such polymers in these solvents, is an intrinsic limit for the stability of polysaccharide-based carbon composite electrodes, especially over time or prolonged operation cycles as charge and discharge in energy storage devices and limit their practical application and industrial adoption.

[0018] A different class of structural polymer components for bio-based composite electrodes is represented by bio-based and bio-degradable thermoplastic polymers, which are generally highly hydrophobic and water resistant and can be processed in organic solvent or even in absence of solvents by melting. This class include thermoplastic starch mixtures (TPS), poly-lactic acid (PLA) and poly-hydroxyalkanoates (PHAs). Recently, de Freitas et al., 2022, ECS Advances, 1, 036504 described a TPS-graphite composite electrode, while Omar et al., 2021, RSC Adv., ,11, 16557 reported on PLA-based composite especially for 3D-printing electrode manufacturing. Main application for such bio-based thermoplastic electrodes have been proposed for electrochemical sensing.

[0019] Polyhydroxyalkanoates (PHAs), a very interesting class of thermoplastic bio-based and bio-degradable polyesters, have been proposed as well to be used in electrochemical energy conversion and storage devices. The use as matrix for polymer composite electrolytes in electrochemical energy storage devices has been disclosed by US patent n. US5266422 and European Patent n. EP3888108.

[0020] More recently PHAs have been reported in some research papers as matrix for electrically conductive polymer composites when blended with PLA (Bozo et al., 2021, ACS Appl. Mater. Interfaces 2021, 13, 41, 49301-49312) or as binder for active materials used in lithium-ion electrodes (Xie et al., 2023, ACS Appl. Polym. Mater. 2023, 5, 1, 1192- 1200). Luckarift et al., 2012, ACS Appl. Mater. Interfaces, 4, 2082-2087 and Buscemi et al., 2022, ACS Appl. Mater. Interfaces 14, 23, 26631-26641 reported on the use of PHAs as matrix for binary composite with conductive carbon fillers as carbon nanofibers and carbon nanotubes to fabricate hybrid bio-anodes with microbial cells suitable for bioelectrochemical systems.

[0021] While bio-based thermoplastic polymer-carbon composite electrodes present a much better stability than polysaccharides one and can be fabricated and processed both by solvent and solvent-less techniques, their wettability in conventional polar solvents and electrolytes is very limited, and, consequently, their electrochemical performances need to be further improved for practical applications, so that the same aforementioned surface treatments for conventional fossil-carbon based polymer composites are usually required.

[0022] US 2022200003 discloses a composite material comprising thermoplastic polymers, ethylene-propylene rubber, thermoplastic elastomers such as polyurethane resins, polyester resins, polyimide resins, polyamide resins, and / or epoxy resins, methylcellulose, ethylcellulose, carboxymethylcellulose (CMC) and a conductive assistants such as carbon blacks, acetylene black and ketjen black, natural graphite, scale graphite, flake graphite, and earthy graphite, artificial graphite, carbon whiskers, carbon fibers further comprising metal such as copper, nickel, aluminum, silver, gold powders, metal fibers, and conductive ceramic materials and dispersants, polycarboxylic acids, and additives such as sodium polyacrylate, compounds having a vinylpyrrolidone structure, sodium alginates which is used as a coating to on supporting film such as metal foil. The composite material leads to obtain coatings having the purpose of adhering on a surface.

[0023] EP3809502 discloses a composite material comprising thermoplastic polymers polysaccharides, conductive assistants, metal and also polycarboxylic acids, and conductive polymers being used for coating metal foils.

[0024] Technical problem

[0025] There is a strong need for green energy storage devices that are lightweight and have good mechanical properties for new generation flexible consumer electronics, while addressing the problem of critical raw materials in the device and the problem of the generation of electronic waste.

[0026] It is strongly desirable to improve the overall product life cycle and overcome the limits of recycling exhausted devices, by providing bio-based and bio-degradable materials for all the energy storage device fundamental components. A technical solution which addresses all the aforementioned problems is still missing. None of the documents of the state of the art addressed neither the problem of using fossil-carbon based polymer nor the problem of the need of electrode or electroactive material or surface treatment to improve electrochemical performances and to obtain free standing film / sheet / foil.

[0027] The above technical problem is solved by providing bio-based electrodes alternative to conventional fossil-based polymer-carbon composite electrodes.

[0028] The bio-based electrodes of the present invention are in the form of free-standing film / sheet / foil being an alternative to coated metal foils. characterised in a novel mixture of biopolymers in the composite formulation with optimal thermo-mechanical properties and ensuring a good electrical conductivity and electrochemical performances.

[0029] Flexible polymer-carbon composite film represents a promising alternative to metals to develop metal-free current collectors and electrodes. They are mainly based on a hydrophobic thermoplastic polymer matrix, which is electrically insulating but ensures good mechanical properties to the film, and carbon additives, which provide electrical conductivity. Other additives as plasticizer are also typically added.

[0030] They represent a very good cheap, flexible, and lightweight alternative to metal-based electrodes. A key technical advantage is their inertness towards corrosion especially in aqueous environments which strongly affects metal electrodes both during electrode processing and manufacturing and during electrochemical processes. Indeed, to avoid corrosion and degradation of metal current collectors and electrodes often complex and expensive surface treatments are applied or noble or expensive metals are used in specific applications.

[0031] The Free-standing nano-sized carbon films are extremely difficult to prepare since they need complex and specific manufacturing protocols.

[0032] One aspect is to afford a composite having an electrically conductivity high enough to be used as an electrode. Usually a low percentages of carbon additives in bio-based polymer matrix composites / (i.e. < 1% w / w) result in sheet resistance higher than 100- 1000 Q / cm2' which is not suitable for practical use in electronics or electrical devices. On the other hand, using higher percentages of carbon additives present problems of mixing and homogeneity and can lead to composite with poor mechanical properties. The multi-functional bio-based electrodes of the present invention presents both the advantages of carbon electrode being electrochemical stability and inertness toward undesired reaction as corrosion.

[0033] Within the composite material provided by the present invention, the combination of polymers, in particular polyesters, with polysaccharides gives to the material improved stability, due to the adhesion between said components, together with improved electrochemical activity. Said combination exerts a synergistic effect since donate superficial electrochemical reactivity to the composite material.

[0034] The adhesion provided to the final composite material by combining said two different classes of biopolymers is higher than the one obtained by the single components alone as binder to metal or pure carbon surfaces and / or electrodes .

[0035] In the final composite material, the polyester-polysaccharide adhesion is higher than the polyester-metal and polysaccharide-metal adhesion characterizing the coatings known in the art.

[0036] Moreover, the multi-functional bio-based electrodes of the present invention has superficial chemical modifications with reactive chemical functional groups to improve electrochemical performances, preferably in aqueous systems, without negatively affecting the electrical conductivity by creating defects in the carbon structure and or in the surface with oxidative treatments.

[0037] Polymer-carbon composites have been reported to be highly desirable for the use as current collectors, electrodes or bipolar electrodes in electrochemical energy conversion and storage devices as batteries and supercapacitors.

[0038] The aim of the present invention is to overcome the drawbacks of the carbon-polymer composite electrodes known in the art, by providing a multi-functional bio-based formulation which offers both the advantage of carbon electrode electrochemical stability and inertness toward undesired reaction as corrosion, and the advantage of a superficial chemical modification with reactive chemical functional groups to improve electrochemical performances, especially in but not limiting to aqueous systems, without negatively affecting the electrical conductivity by creating defects in the carbon structure and or in the surface with oxidative treatments. In the present invention, the functionalities of different biopolymers are combined to provide structural stability and good adhesion in a single formulation for different carbon materials, useful also in the fabrication of electrochemical energy conversion and storage devices.

[0039] Contrary to coated electrodes known in the art, wherein the stability is driven by the metallic surface on which the coating is deposited, in the composite material of the present invention, wherein metal are absent, the structural stability and good adhesion is obtained by the purposive selection of components.

[0040] Object of the invention

[0041] The above technical problem is therefore solved by providing a free-standing composite material comprising i) at least one bio-based and bio-degradable and thermoplastic and insoluble polymer ii) at least one bio-based and bio-degradable and soluble or partially soluble polysaccharides iii) at least one electrically conductive carbon additive optionally further comprising iv) at least one bio-based and natural poly-acid and / or bio-based and natural polybase, and / or v) Further additives wherein the polymer in i) are insoluble and the polysaccharides in ii) are soluble or partially soluble in water and polar solvents.

[0042] Another object of the present invention is the process for the preparation thereof.

[0043] Another object of the present invention is the use of the above composite material for the preparation of electrodes.

[0044] Another object of the present invention are electrodes comprising said composite material.

[0045] Further object of the present invention are electrochemical energy conversion devices and storage device comprising the electrode thereof. Further features of the present invention would be clear from the following detailed description with reference to the experimental examples provided and the attached drawings.

[0046] Brief description of the figures

[0047] Figure 1 shows cyclic voltammetry of a CR2032 supercapacitor in 10.000 consecutive cycles (10mV / s, IM Na2SO4). The first cycle is represented by the dashed line.

[0048] Figure 2 shows capacitance % retention over 10.000 charge and discharge cycles.

[0049] Figure 3 shows galvanostatic charge-discharge profiles at different current densities.

[0050] Figure 4 shows cyclic voltammetry (10mV / s) of caffeic acid (A) and chlorogenic acid (B) at 0.5mM concentration in IM KCI aqueous solution.

[0051] Figure 5 shows exemplificative configurations of the energy storage device

[0052] Figure 6 shows measurement of Equivalent Series Resistance (ESR) of the different devices and comparison by electrochemical impedance spectroscopy.

[0053] Detailed description of the invention

[0054] Within the meaning of the present invention free-standing electrode means an object in the form of film or sheet which is used directly as means of conduction of electricity to provide current for capacitive or faradaic energy storage, without the need to be attached to a surface or supported by an additional film or electrode like metal electrode foils.

[0055] Within the meaning of the present invention, biopolymers are totally or partially biobased polymers, preferably thermoplastic polymers and elastomers, preferably also biodegradable which can be selected from the group consisting of epoxies, alkyd resins, natural oil polyols (NOP), polyurethanes (PUR), cellulose acetate (CA) and cellulose esters, polyethylene terephthalate (PET), Polytrimethylene terephthalate (PTT), Polybutylene terephthalate (PBT), Polycyclohexanedimethylene terephthalate (PCT) Polyethylene isosorbide terephthalate (PEIT) and terephthalate co-polymers, Polyethylene furanoate (PEF) and furane-based polymers, Ethylene propylene diene monomer rubber (EPDM), Polyethylene (PE), Polypropylene (PP), Polyethylene adipate (PEA), Polybutylene succinate (PBS), Poly(butylene adipate-co-terephthalate) (PBAT), Polyamides (PA), Polyvinyl chloride (PVC), Aliphatic polycarbonates (PC), Polythioalkanoates (PTA), bio-based polyesters, unsaturated polyester resins (UPR), Starch mixtures, thermoplastic starches (TPS), thermoplastic elastomers (TPE), Polylactic acid (PLA), Polyhydroxyalkanoates (PHA). The can derive from organic waste as raw material, as agri-industrial or food-industry waste.

[0056] Within the meaning of the present invention, polyester means a chemical entity having repeating units, i.e. monomers, chemically bounded together through an ester bond. The polyester can be a homo-polymer, with identical repeating monomers or a copolymer, with at least two different repeating monomers.

[0057] Within the meaning of the present invention, bio-based polyester means a polyester which is bio-derived, and also bio-degradable.

[0058] Within the meaning of the present invention, bio-derived or bio-based refers to a chemical substance or material which is produced starting from natural and renewable resources as opposed to fossil-carbon based substances and materials; when referred to polymers said terms are thus used for a carbon-based polymeric material which can be extracted or produced by man-made synthetic processes, i.e. not involving living organisms, such as polymerization of bio-derived monomers, and natural synthetic processes, i.e., involving living organisms such as plants or microorganisms, as in the case of bacterial digestion, based on natural sources as raw materials. Examples of bio-based polyesters are selected from the group consisting of: polyhydroxyalkanoates (PHA), polyglycolide (PGA), polylactide or polylactic acid (PLA), polylactide-co-glycolide (PLGA), polycaprolactone (PCL), polydioxanone (PDO), bio-derived aliphatic co-polymers, such as polyethyleneadipate (PEA) and polybutylenesuccinate (PBS), polyethylenefuranoate (PEF), bio-derived terephthalate co-polymers, such as polyethyleneterephtalate (PET), polytrimethyleneterephtalate (PTT), and polybutyleneterephtalate (PBT). Polyhydroxyalkanoates include a number of poly-hydroxyacids in the form of homopolymers and co-polymers. Exemplary PHAs include poly(3-hydroxybutyrate), poly(4- hydroxybutyrate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), poly(3- hydroxybutyrate-co-3-hydroxyhexanoate), poly(3-hydroxybutyrate-co-3- hydroxyheptanoate), poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), poly(3- hydroxybutyrate-co-3-hydroxypropionate, or poly(3-hydroxybutyrate-4- hydroxyvalerate) and other homo -polymers and co-polymers of 3-hydroxyacids, 4- hydroxyacids, and 5-hydroxyacids. Within the meaning of the present invention, suitable polyesters are selected from the group consisting of: polyhydroxyalkanoates (PHA), polyglycolide (PGA), polylactide (PLA), polylactideglycolide (PLGA), polycaprolactone (PCL), polydioxanone (PDO), bio-derived aliphatic copolymers, such as polyethyleneadipate (PEA) and polybutylenesuccinate (PBS), polyethylenefuranoate (PEF), bio-derived terephthalate co-polymers, such as polyethyleneterephtalate (PET), polytrimethyleneterephtalate (PTT), and polybutyleneterephtalate (PBT), homo -polymers, co-polymers and mixtures thereof.

[0059] Within the meaning of the present invention, polysaccharide means a chemical entity having repeating units, i.e. monomers, chemically bounded together through glycosidic bond. Monomers include monosaccharides units which can be bonded together or to other chemical structures such as lipids or proteins. The polysaccharide can be a homopolymer or homo-polysaccharide, with the same repeating monosaccharide unit or a copolymer or hetero-polysaccharide, with at least two different repeating monomers. Within the meaning of the present invention, the bio-based or natural polysaccharide means that it has a natural origin as opposed to fossil-carbon origin, being the polymer of its constituent monomers bio-derived, as disclosed in Mohammed et al. Journal of Polymers and the Environment 2021, 29, 2359. Polysaccharides constituent monomers include glucose, fructose, galactose, mannose, arabinose, xylose but also some monosaccharide derivatives as amino-derivatives such as glucosamine or galactosamine, acetylated derivatives as N-acetylneuraminic acid or N-acetylmuramic acid, and acids as glucuronic and iduronic acids. Depending on the specific chemical structure, natural polysaccharides can be classified as linear or branched, and, depending on chemical functional groups as neutral, positively (cationic) or negatively (anionic) charged. Polysaccharides suitable for the present invention include plant-based, including but not limiting to, cellulose and its derivatives as methyl-, ethyl- propyl-, carboxymethyl-, carboxyethyl-cellulose, cellulose acetate and ethyl-acetate, hemicelluloses, starches, pectins, gums as Acacia, Arabic, Guar, Karaya, Tara, Fenugreek, Locust bean, Tragacanth, and Cassia tora gum, Arabinoxylan, Glucuronoarabinoxylan, Arabinogalactan, Glucuronoxylan, Costus glucans, Xylan, Glucomannan, Galactoglucomannan, Galacturonan, Xyloglucan, Rhamnogalacturonan, Inulin, beta-Glucan; animal-based including but not limiting to Glycogen, Hyaluronan, Chitin, Chitosan, Heparin, Chondroitin sulfate, Keratan sulfate, Dermatan sulfate; algae-based such as Alginate, Agar, Carrageenan, Fucoidan, Ulvan, Laminarin, Angelan, Porphyran, Spirulan, Agarose, Rhodymenan. Within the meaning of the present invention, electrically conductive carbon additive is constituted mainly by carbon in its different allotropic forms; the function of carbon additives is primarily to provide electrical conductivity to the bio-based polymer-carbon composite, high enough to be effectively used as electrode or current collector in electrochemical energy conversion and storage devices. Micro- and nano-sized carbon materials are used as conductive additive, preferably in the form of powders. Carbon micro- and nano-structures are able to conduce electrons upon applying an external electrical field. Low concentrations of carbon materials have been used in bio-based polymer composite in order to increase the electrical conductivity, along with thermal and mechanical properties improvement, of the originally insulating bio-based polymer. On the other hand, free standing nano-sized carbon films have been widely studied, in order to fabricate highly conductive, metal-like carbon sheet. Carbon micro- and nanostructured materials used to produce electrically conductive sheets include graphite, activated carbon (AC), activated carbon fibers, carbide derived carbon (CDC), zeolite templated carbon (ZTC), template porous carbon, carbon nanofibers (CNF), single walled (SWCNT) and multiwalled carbon nanotubes (MWCNT), fullerenes, onion-like carbon (OLC), carbon aerogels, graphene, graphene oxide (GO) and reduced graphene oxide (RGO), amorphous carbon and mixture thereof. Beside imparting electrical conductivity to the composite, are at the same time the active energy storage materials, as in the case of supercapacitors. Carbon additives maybe used in relatively high percentage to afford a composite that has an electrically conductivity high enough to be used as an electrode. Low percentages of carbon additives in bio-based polymer matrix composites, i.e. < 1% w / w, usually resulting in sheet resistance higher than 100-1000 Q / cm2, not suitable for practical use in electronics or electrical devices. Higher percentages of carbon additives usually present problems of mixing and homogeneity in bio-based polymer matrix composites and can lead to composite with poor mechanical properties. On the other hand, although free standing binder-free nanosized carbon materials, such as thin films, are highly electrically conductive and suitable to replace metal parts in electronic devices, they are extremely difficult to prepare in terms of complex and specific manufacturing protocols and large-scale operations. Moreover, specialty nano-sized carbon materials, that is in these cases are used as the only constituent, and thus used in large quantity, lead to high costs.

[0060] Within the meaning of the present invention, additives are compounds improving electrochemical performances in general or specific energy conversion and storage devices, they provide heteroatoms other than carbon (e.g. oxygen, nitrogen, phosphorous, sulphur) with organic molecules containing functional groups which are conventionally generated by chemical or physical treatments of the electrode surface, or the carbon material, as reported in the background description. They provide additional reactive sites without affecting the structure of the electrode or the carbon material, thus preserving the structural order and consequently its electrical conductivity. Additives provide an additional function when the electrode is contacted with redox molecules that undergoes through acid or base-catalysed electro-reduction or electro-oxidation. Such redox molecules can be added or dissolved in the electrolyte, or adsorbed to the electrode surface, or even incorporated in the electrode formulation. Another function of these additives is to chemically interact and coordinate metal-ions (such as copper, nickel, cadmium, zinc, manganese, cobalt, aluminium ions) as in the ionic form that can be found in aqueous solution, as wastewater of industrial or municipal origin. Such metal-ions when adsorbed onto the electrode surface can provide additional functions to the electrode, including acting as additional reactive or catalytic centres, in the ionic form or reduced in their neutral charge state, providing additional ionic exchange thus increasing specific capacitance, or further modified to provide pseudocapacitive behaviour, as for instance upon oxidation to relevant metal oxides.

[0061] Additional additives can be poly-acids,

[0062] Within the meaning of the present invention poly-acids are organic molecules presenting two or more functional groups able to generate a local change in pH in polar solvents as water, as poly-carboxylic acids.

[0063] Poly-acids can be bio-based or "natural", meaning that they have a natural origin, opposed to fossil-carbon origin, which means that are extracted or synthesized from natural sources such as plants or microorganisms.

[0064] More preferably poly-acids are molecules containing two carboxylic groups including Oxalic, Malonic, Succinic, Glutaric, Adipic, Pimelic, Sebacic acid, unsaturated or aromatic molecules including maleic, fumaric, phtalic, isophtalic, terephtalic, furan-dicarboxylic acid, tricarboxylic acids as citric, aconitic, trimellitic, trimesic acid, polycarboxylic acids as 1,2,3,4-Butanetetracarboxylic Acid (BTCA), Pyromellitic acid, Mellitic, Ethylenetetraacetic acid (EDTA), and more preferably naturally occurring molecules as citric, tartaric, malic and aspartic acid. Within the meaning of the present invention, poly-acids are monomeric units and polymers of carboxylic acids, with the proviso that as polyacrylic and polyacrylates are excluded.

[0065] The composite material of the present invention comprises three components: i) at least one bio-based and bio-degradable and thermoplastic and insoluble polymer ii) at least one bio-based and bio-degradable and soluble or partially soluble polysaccharides iii) at least one electrically conductive carbon additive wherein the polymer in i) are insoluble and the polysaccharides in ii) are soluble or partially soluble in water and polar solvents.

[0066] It has been designed after extensive research efforts to overcome the aforementioned drawbacks of polymer-carbon composite electrodes at the state of the art.

[0067] Preferably polymer i) is selected from the group consisting of: polyesters, epoxies, alkyd resins, natural oil polyols, polyurethanes, cellulose, furane-based polymers, furane-based polymers, starch mixtures, thermoplastic elastomers, ethylene propylene diene monomer rubber.

[0068] Preferably cellulose is selected from the group consisting of cellulose acetate, cellulose esters.

[0069] Preferably starch is thermoplastic starch (TPS).

[0070] Preferably polyester is a co-polymer of two or more monomers independently selected from the group consisting of 3-hydroxyacids, 4-hydroxyacids, 5-hydroxyacids, glycolic acid, lactic acid, capro lactone and suitable bio-derived monomers to form an ester bond, including bio- derived poly-carboxylic acids, containing in their molecular structure at least two carboxylic functional groups, and bio-derived poly- functional monomers, containing in their molecular structure at least two functional groups able to form an ester bond if reacted with carboxylic functional groups, as for instance diols and polyols.

[0071] Preferably polyester is selected from the group consisting of polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polycyclohexanedimethylene terephthalate, Polyethylene isosorbide terephthalate, terephthalate co-polymers, polyethylene furanoate, polyethylene, polypropylene, polyethylene adipate, polybutylene succinate, Poly(butylene adipate-co-terephthalate) (PBAT), Polyamides (PA), Polyvinyl chloride (PVC), Aliphatic polycarbonates (PC), Polythioalkanoates (PTA), bio-based polyesters, unsaturated polyester resins (UPR), Polylactic acid (PLA), Polyhydroxyalkanoates (PHA).

[0072] Preferably polyester i) is selected from the group consisting of polyhydroxyalkanoates and polylactic acid.

[0073] More preferably is selected from the group consisting of poly(3-hydroxybutyrate), poly(3-hydroxyvalerate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and mixtures thereof.

[0074] Preferably, polyester i) is a mixture of PHAs and PLA.

[0075] Polymers i)provides strength, flexibility and more in general structural properties to the carbon-polymer composite; provides chemical resistance to polar solvent, including water, thus avoiding dissolution of the composite when contacted for short or prolonged times with a solution or a liquid electrolyte, as in the case of an electrochemical device; provides chemical interactions with other polymers used as polysaccharides.

[0076] More preferably polyhydroxyalkanoate is selected from the group consisting of homopolymers of 3-hydroxyacids, 4-hydroxyacids, 5-hydroxyacids, co-polymers and mixture thereof.

[0077] Preferably polyester i) is a co-polymer of two or more monomers independently selected from the group consisting of 3-hydroxyacids, 4-hydroxyacids, 5-hydroxyacids, glycolic acid, lactic acid, capro lactone and suitable bio-derived monomers to form an ester bond, including bio- derived poly-carboxylic acids, containing in their molecular structure at least two carboxylic functional groups, and bio-derived poly- functional monomers, containing in their molecular structure at least two functional groups able to form an ester bond if reacted with carboxylic functional groups, as for instance diols and polyols.

[0078] More preferably is selected from the group consisting of poly(3-hydroxybutyrate), poly(3-hydroxyvalerate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and mixtures thereof.

[0079] Preferably, polyester i) is a mixture of PHAs and PLA. Preferably polysaccharide ii) is polysaccharides extracted from natural sources such as animals, plants and algae or synthesized by micro-organisms more preferably is cellulose and its derivatives.

[0080] More preferably polysaccharide ii) is are synthesized by -organisms as Extracellular Polymer Substances (EPS), preferably selected from the group consisting of Curdlan, Scleroglucan, Pullulan, Dextran, Xanthan, Levan, Gellan, Emulsan, Schizophyllan, Glucuronan, Succinoglycan, Nigeran.

[0081] More preferably cellulose is bacterial cellulose, micro- and nano-cellulose and their derivatives.

[0082] Even more preferably cellulose is selected from the group consisting of cellulose, cellulose derivatives, xanthan, CarboxyMethylCellulose or a mixture thereof.

[0083] Most preferably cellulose is carboxy-methyl-cellulose (CMC) and xanthan.

[0084] Preferably polysaccharide ii) is Xanthan, stable over a wide range of pH, temperatures and high saline concentrations in polar solvents as water.

[0085] Polysaccharide ii) increases the hydrophilicity of the composite and more in general its wettability in polar solvent including water. Such improved interaction of the solid polymer-carbon composite is highly desirable when such composite is used as electrode in liquid electrolyte because is generally associated to better electrochemical response in charge transfer processes and ion adsorption or desorption. Polysaccharide ii) is improve the electrochemical response of a bio-based polymer-carbon composite electrode without affecting its structural stability and its electrical conductivity.

[0086] Preferably, carbon additive iii) is present in a percentage higher than 5% and lower than 95% in weight on the total weight of the final composite, and preferably between 15 and 75%, and more preferably between 25 and 40%.

[0087] The percentage of carbon additive iii) can vary on the carbon additive iv) used and its intrinsic electrical conductivity, its percolation limit in the composite material and on the final value of electrical conductivity desired for the composite material.

[0088] The resulting composite material combines the advantages of a good electrical conductivity, i.e. sheet resistance < 100 Q / cm2 and bulk conductivity > 10 S / cm, a good mixing and homogeneity of the final composite material, a good mechanical resistance to have a free standing material, such as a free standing composite film or thin sheet, suitable for practical use in an electrochemical energy storage cell and device, and a lower cost due to the cheaper polymer matrix use.

[0089] Preferably, carbon additive iii) is bio-based, i.e. derived from agricultural residues such as fruit husk, organic waste, sugars, lingo-cellulosic by-products, hemp, etc. Said residues are converted with high temperature treatments into ordered carbon structures with good electrical conductivity. Such treatments include carbonization to convert bio-based raw materials into carbon material, activation to increase the surface area and porosity of carbon materials and consequently their specific capacitance (F / g), and graphitization to increase the graphitic fraction of carbon materials to improve their electrical conductivity.

[0090] Carbon additive iii) has hierarchical porosity, meaning with a wide pore size distribution which is beneficial in terms of capacitive electric charges storage and ion adsorption.

[0091] Preferably carbon additive iii) are selected from the group consisting of carbon nanotubes, nanofibers, nanosheets, graphene and their derivatives.

[0092] Carbon additive iii) may be synthesized by chemical vapor deposition methods using biogas or bio-based gases such as methane, ethylene etc. as chemical feedstock and carbon sources. When bio-based carbon additives are used the final composite is fully bio-based and optionally fully bio-degradable when the bio-based polymer matrix is also bio-degradable.

[0093] Optionally the composite material comprising three components further comprising a fourth component being iv) at least one bio-based and natural poly-acid and / or bio-based and natural polybase, which can be contacted with the other component of the composite material prior to or during the preparation of the composite.

[0094] Preferably poly-acids are monomeric units and polymers of carboxylic acids, with the proviso that as polyacrylic and polyacrilates are excluded.

[0095] More preferably poly-acids are molecules containing two carboxylic groups including Oxalic, Malonic, Succinic, Glutaric, Adipic, Pimelic, Sebacic acid, unsaturated or aromatic molecules including maleic, fumaric, phtalic, isophtalic, terephtalic, furan-dicarboxylic acid, tricarboxylic acids as citric, aconitic, trimellitic, trimesic acid, polycarboxylic acids as 1,2,3,4-Butanetetracarboxylic Acid (BTCA), Pyromellitic acid, Mellitic, Ethylenetetraacetic acid (EDTA), and more preferably naturally occurring molecules as citric, tartaric, malic and aspartic acid.

[0096] Preferably poly-bases are natural molecules containing at least two basic functional groups with oxygen or nitrogen atoms.

[0097] More preferably poly-bases are selected from the group consisting of: basic polyalcohols, polycarboxylates, polyamines, alcohol-amines.

[0098] Exemplary preferred natural poly-bases are: di- tri- and tetra-amines as 1,3-diamino- propane, putresceine, cadaverine, spermidine, norspermidine, spermine, thermospermine, norspermine, their derivatives as polyamine alkaloids such as di- / tri- / tetra-amine alkaloids, polyamine siderophores, polyaminosterols.

[0099] The composite material comprising three components has been achieved after an intensive experimental work as it is technically difficult to obtain a homogenous composite electrode starting from these different classes of materials with different properties, i.e. different solubility in organic and aqueous solvents, different wettability toward water and different thermal properties.

[0100] Nonetheless, the composite material comprising four components provides an unprecedented synergic effect: to provide a multi-functional bio-based formulation which offers both the advantage of carbon electrode electrochemical stability and inertness toward undesired reaction as corrosion, and the advantage of a superficial chemical modification with reactive chemical functional groups to improve electrochemical performances, especially in but not limiting to aqueous systems, without negatively affecting the electrical conductivity by creating defects in the carbon structure and or in the surface with oxidative treatments.

[0101] The composite material may also comprise further additives v) to improve electrochemical performances in general or specific energy conversion and storage devices. Additives v) can be selected from any material, substance, molecule undergoing to redox reactions, preferably reversible redox reactions, to be used to convert or store energy electrochemically.

[0102] Preferably additives are selected from the group consisting of: antioxidants present in natural sources like plants or produced by micro-organisms, such as quinones, catechols and their derivatives; metal-ions (such as copper, nickel, cadmium, zinc, manganese, cobalt, aluminium ions) adsorbed onto the electrode surface; plasticizer or compatibilizer.

[0103] The composite material is a free-standing material.

[0104] In the free-standing composite material, the components are mixed together to form a unique block, or are disposed in different layers (multi-layer).

[0105] Preferably the free-standing composite material can be in the form of flexible film, foil, or sheet, to be directly used as electrode and / or current collector in an electrochemical energy conversion and storage device.

[0106] The free-standing composite material can be used for the preparation of electrodes wherein the components are mixed together to form a unique block, or in the electrode the different components are disposed in different layers.

[0107] In one multi-layer embodiment, a firs layer a) consisting of components i) +iii) and layer b) consisting of components ii) + iii) + iv).

[0108] In another multi-layer embodiment a first layer a) consisting of components i) + iii) and layer b) consisting of components ii) + iv).

[0109] The compositive material can be prepared by combining the components i) to v) by different methos and can be prepared in different final shapes depending on the specific electrochemical energy storage cell and device configuration. Preferably, the composite material is prepared in the form of foil or sheet by a method comprising the following steps: a) dissolving the at least one bio-based and bio-degradable and thermoplastic and insoluble polymer i) in a suitable solvent b) adding to the solution as obtained in a) the at least one bio-based and biodegradable and soluble or partially soluble polysaccharides ii) if the solvent is able to dissolve both the polyester and the polysaccharides is or adding ii) to the solution as obtained in a) in the form of finely suspended solids, or dissolving ii) in water and adding to form a micro-emulsion with the solution as obtained in a) c) adding the at least one electrically conductive carbon additive iii) d) undergoing the solution as obtained at the end of step c) to a processing step to obtain the desired form.

[0110] Preferably, in step d) processing is carried out by a technique selected from the group consisting of casting, spin-coating, blade or slot-die coating, spray-drying.

[0111] Preferably, in step c) the electrically conductive carbon additive can be added in the form of powder and finely suspended solids in the polyester solution.

[0112] Preferably in step c) components iv) and / or v) are added.

[0113] Preferably, when the composite material is in the form of a multilayer the polymers i) and the polysaccharides ii) are dissolved in different immiscible solvents.

[0114] Preferably a first layer is prepared by dissolving polymer i) in a suitable solvent, adding the carbon additive iii) and optionally further additives v) to obtain a suspension and a layer is allowed to be formed on a surface upon solvent evaporation.

[0115] Preferably a second layer by dissolving the polysaccharide ii) in water or another suitable solvent in which the first layer is insoluble, adding carbon additive iii) and optionally further additives v) to obtaining a suspension and then the second layer is allowed to be formed on the first layer upon water or solvent evaporation.

[0116] Preferably, the bio-based polyester is PHAs.

[0117] Preferably polymer in i) is polyester PHA and it is dissolved in a suitable solvent in a concentration ranging from 0.1 to 20 % w / w with respect to the weight of solvent, preferably between 1 and 5% w / w. Preferably the carbon additive iii) is added in a concentration from 1 to 95 % w / w with respect to the total mass of the PHA.

[0118] Preferably additives v) are added in a concentration from 2 to 40 % w / w and more preferably between 5 and 35% w / w

[0119] Preferably the solvent able to dissolve PHA, is selected from the group consisting of fluorinated organic solvents, such as l,l,l,3,3,3-hexafluoro-2-propanol, chlorinated organic solvents, such as chloroform, dichloromethane, 1,2-dichloroethane, or acetone and 1,2-propylene carbonate, or non-halogenated solvents, such as methyl-t-butyl ether, isoamyl propionate, isoamyl valerate and propyl butyrate, bio-based solvents such as bio-based linear and cyclic carbonates, acetone, butanol, propanol, glycerol, cyrene, dimethyl isosorbide, gamma-valerolactone, 2-methyltetrahydrofuran, ethyl-lactate and mixtures thereof.

[0120] Preferably in step a) and b) solution undergoes to mechanically stirring for an amount of time necessary to obtain a homogeneous slurry, varying the time and the vigorousness of the stirring depending on the viscosity of the system.

[0121] Preferably the slurry is then casted onto a flat surface which is optionally heated to let the solvent evaporate thus originating a free-standing bio-based polymer composite film ready to be used (ready-to-use) in the fabrication of the electrochemical energy storage cell.

[0122] Preferably, the solvent is recovered by condensation to be reused in the preparation procedure.

[0123] Preferably the polysaccharide ii) and carbon additive iii) can be added in the form of solid or by the addition of a second solution of the polysaccharide ii) in water or another solvent which is immiscible with the solvent used for PHA.

[0124] Preferably the final mass of the polysaccharide ii) with respect to polyester i) in the composite material in the form of a film ranges from 0.01% and 20% w / w and more preferably from 0.1% and 10% w / w.

[0125] Preferably and optionally in step a) and b) the solvent is absent by taking advantage of the thermoplastic nature of the polymer

[0126] Therefore, in step a) the polymer is is heated up to melting and in step b) mixed with polysaccharide, and in step c) mixed with other components, and then cooled to fabricate the final composite material in a so-called dry process.

[0127] Preferably when polyester i) is PHA can be heated to its melting temperature, in the range of 140-190 °C, or to a lower temperature with the concomitant application of a mechanical pressure to induce PHA melting, to homogenize the final composite, the PHA matrix, when heated and optionally pressed to melt, acts as a viscous dispersing matrix for the conductive additives, and can be manipulated, homogenized and cooled to room temperature in the desired form, preferably in the form of a film, foil or sheet or different shaped containers which can be used for preparation of desired electrode and / or current collector, such as rod or bar.

[0128] Preferably the mass percentage of the carbon additive iii) with respect to bio-polymer matrix can be selected according to the description above for the solvent casting techniques.

[0129] Preferably the process is carried out in absence of solvents, and processing is carried out by hand / wet lay-up methods, extrusion including film cast extrusion, pultrusion, welding, compounding, lamination, compression, injection and vacuum bag moulding, and hot press.

[0130] Preferably, when the thermoplastic nature of the polysaccharide ii) is used to avoid the use of organic solvents the polysaccharide ii) and carbon additive iii) can be added to the formulation as solids or in the form of aqueous solution or suspension, promoting also reactive extrusion or melting processing.

[0131] Preferably, when a multilayer composite is fabricated, the first composite material layer can be fabricated by the thermal solvent-less techniques and the second polysaccharide- layer, optionally containing other desired carbon fillers and optional additives, can be formed by an aqueous suspension and then processing such suspension to obtain a homogenous composite second layer which is allowed to be formed on the first layer upon water evaporation.

[0132] Further additives v) e can be added at any step of preparation, in the solution-based and dry processing by considering their solubility properties.

[0133] Preferably, when poly-carboxylic acids or poly-amines are added are mixed with polysaccharides considering their similar chemical and physical properties. Preferably, when redox natural molecules as quinones and catechols are used, they can be mixed with polysaccharides by taking advantage of their polar functional groups or they can be previously adsorbed onto the carbon fillers, by means of impregnation and drying, by taking advantage of their aromatic functional moieties.

[0134] Optionally, the method can also comprise steps of pre-treatment of carbon additive iii) or the addition of coupling agent or plasticizers with the aim to increase the compatibility between the carbon additive iii) and the bio-based polymers, and the homogeneity and mechanical properties of the final material. Coupling agents and plasticizers can optionally be used to improve the interfacial adhesion between the bio-based polymers and the carbon additive iii).

[0135] Preferably agents are selected to preserve the green nature, biodegradability, compostability and / or biocompatibility of the final bio-composite. Examples of such agents include but are not limited to glycerol, citric acid, and other polyols or polycarboxylic acids, such agents can be the same optional additives used in the formulation.

[0136] The composite material of the present invention is used forthe preparation of electrodes (anodes or cathodes).

[0137] Another object of the present invention are electrodes comprising said composite material.

[0138] Further object of the present invention are electrochemical energy conversion devices and storage device comprising the electrode thereof, exemplificative configurations are shown in figure 5 wherein: a. Layer i)+iii) / separator / Layer i)+iii) b. Layer i)+iii) / gel or solid electrolyte / Layer i)+iii) c. Layer i)+iii) / energy storage active material +ii)+iv) / separator / energy storage active material +ii)+iv) / Layer i)+iii) d. Layer i)+iii) / energy storage active material +ii)+iv) / gel or solid electrolyte / energy storage active material +ii)+iv) / Layer i)+iii) e. i)+ii)+iii)+iv)+ energy storage active material / separator / i)+ii)+iii)+iv)+ energy storage active material f. i)+ii)+iii)+iv)+ energy storage active material / separator / gel or solid electrolyte / i)+ii)+iii)+iv)+ energy storage active material The energy storage material can differ between the two electrodes, namely anode and cathode (e.g. metal-ion batteries, asymmetric supercapacitor, hybrid devices).

[0139] Possible configurations of the energy storage device are shown in figure 5

[0140] Examples

[0141] Example 1

[0142] 100 mg of Poly-3-hydroxybutyrate (PHA) and 100 mg of poly-lactic acid (PLA) were dissolved in 10 mL of dichloromethane upon vigorous stirring and gentle heating for 30 minutes. Then 20 mg of xanthan gum, 50 mg of natural graphite powder, 50 mg of Carbon Nano-fibers - CNF (vapor-phase grown, graphitized) and 400 mg of activated carbon (from coconut shells) were added to the viscous solution and the resulting mixture was stirred upon gentle heating for 1 hour and then poured into a glass petri dish. The solvent was evaporated at room temperature and a homogeneous freestanding film was obtained. The resulting film was cut and directly used as working electrode for electrochemical characterization in three and two electrode configuration experiments. The electrode integrates all the components in a unique block.

[0143] A similar procedure was adopted to obtain a multilayer capacitive electrode. PHA, PLA, Carbon Nano-fibers and graphite were dissolved, mixed and dried as reported above to obtain a film current collector. A mixture of xanthan gum, citric acid, carbon black and activated carbon (0.7:0.3:5:85 ratios in weight) was suspended in water and the resulting slurry was deposited onto the current collector by doctor blade. The resulting electrode was dried and cut into desired shapes for the electrochemical experiments. The electrode is a multi-layered composite electrode integrating the different components in two layers ( layer a = i) +iii); layer b = ii) + iii) + iv).

[0144] A similar procedure was repeated by depositing the layer made of mixture of xanthan gum, carbon black and activated carbon (10:5:85 ratios in weight), onto a PHA / PLA / CNF / graphite foil, prepared as described above, and onto a commercial aluminium foil and a commercial pure graphite foil. A tape-test with a commercial adhesive tape was performed to compare the adhesion of the xanthan-based layer onto the different three surfaces.

[0145] A solution of KCI IM was used as electrolyte, with a Platinum rod as counter electrode and Ag / AgCI as reference electrode. For the two electrode characterization the composite electrode was directly used in a symmetric supercapacitor by assembling a CR2032 coin cell with a Na2SO4 IM electrolyte, using a commercial 40um thick paper separator. The stability and performance of the electrodes were tested in 10.000 consecutive ga Iva nostatic charge / discharge cycles at lA / g.

[0146] As shown figures 1 and 2 the composite electrodes are highly stable in aqueous electrolytes (IM Na2SO4) over 10.000 cycles of galvanostatic (1 A / g) charge and discharge between 0 and IV. The CV curves at lOmV / s were registered every 500 cycles. The capacitance (110 F / g) was retained at 95% after 10.000 cycles. The galvanostatic charge-discharge experiments were performed at different currents to provide evidence of the effective capacitive behaviour at high rates of charge and discharge. The adhesion test showed visually that a minor amount of material was onto the detached adhesive tape after its removal in the case of the PHA / PLA / CNF / graphite foil with respect to aluminium and graphite ones, confirming the improved adhesion of the present formulation due to the combination of polyesters and polysaccharides components.

[0147] 100 mg of Poly-3-hydroxybutyrate (PHA) and 100 mg of poly-lactic acid (PLA) were dissolved in 10 mL of dichloromethane upon vigorous stirring and gentle heating for 30 minutes. Then 50 mg of natural graphite powder, 50 mg of Carbon Nano-fibers (vaporphase grown, graphitized) were added to the viscous solution and the resulting mixture was stirred upon gentle heating for 1 hour and then poured into a glass petri dish. The solvent was evaporated at room temperature and a homogeneous free-standing film was obtained. A mixture of xanthan gum and citric acid (1:1 ratio in weight) was prepared in water and the resulting viscous solution was deposited onto the current collector by doctor blade. A similar experiment was also performed by adding xanthan gum and citric acid directly to the composite electrode in a one pot preparation with similar results. The electrode is a multi-layered composite electrode layer a = i) + iii); layer b = ii) + iv).

[0148] In figure 4 the composite electrode (solid black line) is compared with a benchmark state-of-the-art composite electrode (PHA+PLA, dashed line). Improvements in current densities in presence of redox molecules such as caffeic and chlorogenic acid are beneficial for their application in electroanalytical purposes as well as to improve energy storage, as for instance in redox electrolyte supercapacitors. The composite electrode containing xanthan gum (polysaccharide) and citric acid (poly-acid) is able to improve the electrochemical response of the exemplary selected molecules by providing a local acidic pH and acting as catalyst for the reduction and oxidation of such molecules, thus providing an higher exchange of electrons with the electrodes and thus an higher current density at the same molecule concentration with respect to the benchmark state-of-the- art composite electrode.

[0149] Example 2

[0150] 1.5 g of Poly-3-hydroxybutyrate (PHA) and 1.5 g of poly-lactic acid (PLA) were dissolved in 25 mL of dichloromethane upon vigorous stirring and gentle heating for 30 minutes. Carbon Nano-Fibers and graphite were then added and mixed and dried as reported above to obtain a free-standing film current collector. Different amounts of graphite e carbon nanofibers were used to obtain an overall polymer-carbon ratio of 64:33 % w / w. A mixture of xanthan gum, citric acid, carbon black and activated carbon (0.7:0.3:5:85 ratios in weight) was suspended in water and the resulting slurry was deposited onto the current collector by doctor blade. The resulting electrode was dried and cut into 15 mm dimeter disks to be directly used in a symmetric supercapacitor by assembling a CR2032 coin cell with a Na2SO4 IM electrolyte, using a commercial cellulose based 40um thick paper separator. The stability and performance of the electrodes were tested in 5.000 consecutive galvanostatic charge / discharge cycles at lA / g, showing a retention of +98% of initial capacitance. The Electrical Series Resistance (ESR) of the different devices were measured and compared by electrochemical impedance spectroscopy, showing a low resistance of 1.9 Ohm for the sample containing graphite and carbon nanofibers at 25% and 8% w / w ratio respectively.

[0151] The data are shown in the following table 1 and in figure 6.

[0152] Table 1

Claims

CLAIMS1. Composite material comprising: i) at least one bio-based and bio-degradable and thermoplastic polymer being insoluble in water and polar solvents ii) at least one bio-based and bio-degradable polysaccharides being soluble or partially soluble in water and polar solvents iii) at least one electrically conductive carbon additive2. Composite material of claim 1 further comprising iv) at least one bio-based and natural poly-acid and / or bio-based and natural polybase.

3. Composite material of claims 1 or 2 further comprising at least one further additive v) being material and / or substance and / or molecule undergoing to redox reactions, preferably reversible redox reactions.

4. Composite polymer according to claim 1 wherein the polymer in i) is selected from the group consisting of: polyesters, epoxies, alkyd resins, natural oil polyols, polyurethanes, cellulose, furane-based polymers, furane-based polymers, starch mixtures, thermoplastic elastomers, ethylene propylene diene monomer rubber.

5. Composite material of anyone of claims 1-4 wherein polysaccharide in ii) is extracted from natural sources or synthesized by micro-organisms.

6. Composite material according to claim 5 wherein polysaccharide is cellulose, cellulose derivatives, xanthan, CarboxyMethylCellulose or a mixture thereof.

7. Composite material according to claim 3 wherein additive v) is selected from the group consisting of antioxidants present in natural sources like plants or produced by micro-organisms, such as quinones, catechols and their derivatives; metal-ions such as copper, nickel, cadmium, zinc, manganese, cobalt, aluminium ions adsorbed onto the electrode surface; plasticizer or compatibilizer.

8. Composite material of anyone of the preceding claims wherein components i) to v) are mixed together to form a unique block, or are disposed in different layers.

9. Method for the preparation of the compositive material of anyone of claims 1-8 by a method comprising the following steps: a) dissolving the at least one bio-based and bio-degradable and thermoplastic and insoluble polyester i) in a suitable solvent b) adding to the solution as obtained in a) the at least one bio-based and biodegradable and soluble or partially soluble polysaccharides ii) if the solvent isable to dissolve both the polyester and the polysaccharides is or adding ii) to the solution as obtained in a) in the form of finely suspended solids, or dissolving ii) in water and adding to form a micro-emulsion with the solution as obtained in a) c) adding the at least one electrically conductive carbon additive iii) d) undergoing the solution as obtained at the end of step c) to a processing step to obtain the desired form.

10. Method according to claim 9 wherein the method is dry and in step a) and b) the solvent is absent and the polymer i) in step a) is heated up to melting, and mixed to polysaccharide in step b) and with other components iv) and / or v) in step c) and then cooled to obtain the final product composite material.

11. Electrodes comprising the composite material according to anyone of claims 1-8.

12. Electrode according to claim 11 consisting of a firs layer a) consisting of components i) and iii) and a second layer b) consisting of components ii) and iii) and iv); or a first layer a) consisting of components i) and iii) and a second layer b) consisting of components ii) and iv).

13. Electrochemical energy conversion device and / or storage device comprising the electrode of anyone of claims 11-12.

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