Hydrophobic Biobinders for Cathode Formulations

Abstract

The present invention relates to the preparation of a hydrophobic double emulsion (hydrophobic bio-binder) that is incorporated into a bio-cathode ink to provide a cathode ink formulation for a hydrophobic bio-cathode.

Description

[Technical Field]

[0001] The present invention relates to the preparation of a hydrophobic double emulsion, also called a "hydrophobic biobinder," that is incorporated into biocathode inks to provide cathode ink formulations for hydrophobic biocathode to improve oxygen diffusion, avoid flooding without the use of fluorinated polymers, and maintain the same global biofuel cell including biodegradability.

[0002] In the following description, references in square brackets ([]) refer to the list of references at the end of this document. [Background technology]

[0003] Biological fuel cells (biofuel cells) offer an attractive means of providing environmentally friendly and sustainable power to electronic devices, particularly small, portable devices for applications such as healthcare, environmental monitoring, and bioterrorism defense. Given their ability to operate using abundant substrates in biological fluids and environmental effluents (i.e., glucose and oxygen) while exhibiting power densities often superior to those of microbial fuel cells, enzyme-based fuel cells offer an attractive proposition for augmenting or self-powering small, wearable, or implantable devices [1, 2, 3]. Furthermore, paper-based devices are gaining popularity as a proposition for these types of applications due to their low mass, small form factor, and flexibility, which allows them to conform to a variety of different surfaces.

[0004] One of the key issues associated with fuel cells is water management, which requires a delicate water balance scheme between system hydration and cathode flooding.

[0005] On the one hand, water is essential for the separator membrane to promote proton conductivity. On the other hand, if excess water fills the pores of the catalyst layer (CL) and gas diffusion layer (GDL), the electrode reaction becomes mass-transfer limited, leading to a rapid cell voltage drop and immediate cell performance degradation. Considering the fact that water is produced by the oxygen reduction reaction in the CL and transported from the anode to the cathode via electroosmotic drag, proper flooding prevention in cathode design and operation is essential to ensure high performance and long life of fuel cells.

[0006] During operation, the properties of the CL and GDL significantly affect the water balance within the fuel cell. Sinha et al. [4] investigated the significant influence of substructure and surface wettability on liquid water transport and interfacial dynamics in fuel cell GDLs at the pore level.

[0007] Although the importance of the GDL in preventing flooding has been studied extensively, the role of the C1N fuel cell water balance has never been deeply investigated either experimentally or numerically. The C1N can act as a watershed within the fuel cell to regulate the balance between liquid water and steam. Park et al. [5] numerically showed that different wettability in both the C1N and GDL can significantly affect the liquid water flow pattern and therefore the cell performance.

[0008] Various strategies to modify the cathode structure to improve the overall stack performance have been reported in the literature.

[0009] Qiu et al. [6] developed a novel cathode structure with a double CL, which includes a hydrophilic inner layer covered with a hydrophobic layer. This MEA with a double CL leads to better Pt utilization, reduces the interfacial resistance between the CL and GDL, improves mass transfer, and leads to a higher performance MEA. Wei et al. [7] designed a cathode structure with gradient Pt nanowires, which enhanced Pt utilization, promoted mass transport, and resulted in high performance. Deshmukh et al. [8] prepared an MEA with a micropatterned electrode structure to improve performance by reducing cathode flooding and facilitating reactant diffusion. The aforementioned studies focused on the structural design of CLs to improve MEA performance. Another approach to enhancing MEA performance and water management is to adjust the hydrophilicity and hydrophobicity of CLs or GDLs. [9] Oh et al.

[10] introduced poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) copolymer as a binder in CLs to increase their hydrophobicity and thereby prevent water flooding. Li et al.

[11] improved MEA performance by adding dimethyl silicone oil to CLs to increase their oxygen permeability and hydrophobicity, resulting in better oxygen diffusion and water management. Hiramitsu et al.

[12] designed a GDL with a smaller pore structure, allowing the CL to more effectively resist water flooding. In a previous study, Chi et al.

[13] added polytetrafluoroethylene (PTFE) to the CL to balance the hydrophobic and hydrophilic properties, thereby improving water management.

[0010] For biofuel cells, water management and oxygen mass transfer are also essential to ensure optimal performance and long life. Controlled hydrophobicity of the cathode can be achieved by adding a hydrophobic binder emulsion into its composition.

[0011] Elsewhere, bioenzymatic fuel cells have the potential to be biodegradable and biobased due to their biobased catalysts. This environmental parameter is particularly important from an application perspective and constrains the formulation of materials used in their compositions. To date, no biodegradable or bio-derived materials have been used as hydrophobicity enhancers in the biofuel cell field.

[0012] Currently, global agricultural developments and food security issues have raised many issues, including the need for improved post-harvest preservation of agricultural products, especially fresh fruits and vegetables. Consequently, much research has been conducted into various methods of food preservation to ensure quality during transportation. Some research aims to create protective films that are environmentally friendly and safe for consumers, to increase value from utilizing local raw materials, and to promote the export potential of fresh fruits and vegetables.

[0013] Various natural polymeric materials, including polysaccharides (chitosan, cellulose, starch, and their derivatives), proteins (gelatin and soy protein), and lipids (beeswax, candelilla wax, and carnauba wax), have been used to develop edible emulsion-based coatings, either alone or in place of synthetic polymers [14, 15]. Chitosan, a natural polycationic polysaccharide, has been widely used as a coating material due to its excellent film-forming ability and antibacterial activity [16, 17], but its poor flexibility and ductility have significantly limited its application. Carboxymethylchitosan (CCS), an amphoteric derivative of chitosan, has improved water solubility and retains other properties of chitosan, which has been widely applied in food packaging [18-20]. However, most coating materials derived from water-soluble polymers exhibit high hydrophilicity, which severely limits their application in humid environments. Lipid compounds, such as waxy materials (beeswax, candelilla wax, and carnauba wax), have high water-insolubility capabilities. Unfortunately, coating films formed with pure wax have poor mechanical strength. When polysaccharides such as cellulose and its derivatives are applied and mixed with the wax component, films with reduced water vapor permeability and water swelling can be formed [21, 22]. Therefore, to overcome the drawbacks of coatings made from a single component, it is of great significance to prepare edible coatings consisting of CCS, cellulose, and beeswax by the Pickering emulsion method. It was.

[0014] In biofuel cells, catalysis is based on enzymatic reactions. Immobilization of enzymes, with or without redox mediators, offers many advantages, such as improved electron transfer and long-term stability. However, immobilization can also affect the stability and / or activity of the enzyme. The stability of immobilized enzymes depends on the nature and strength of the electrode bond, the immobilization conditions, the degree of confinement, and the conditions under which the enzymatic reaction occurs at the electrode during operation. Each immobilization method has its advantages and disadvantages. Therefore, the immobilization process must be carefully selected to avoid denaturation of the enzymes and the loss of the structural freedom required for their activity.

[0015] The main immobilization techniques are physical adsorption, apoenzyme reconstitution, encapsulation, covalent grafting, immobilization by cross-linking, and immobilization in hydrogels.

[0016] One strategy for enzyme immobilization and stabilization has been the use of micellar polymers. Enzymes in solution are typically active for hours to days. This lifetime can be extended to 7–20 days by entrapment in hydrogels and attachment to electrode surfaces. However, researchers at Saint Louis University extended the lifetime of active enzymes on electrode surfaces to over a year by immobilizing them within hydrophobically modified micellar polymers.

[23]

[0017] Micellar polymers such as Nafion™ and chitosan can be hydrophobically modified to tailor the micellar pore or pocket structure to an optimal size for enzyme immobilization, while simultaneously ensuring a hydrophobic and buffered pH microenvironment for optimal enzyme activity. This strategy has been shown to increase the enzyme's enzymatic activity by up to 2.5-fold in chitosan, as well as increase the enzyme's active lifetime.

[24] Hydrophobically modified Nafion™ membranes can also be used as separators between electrodes to ensure a hydrophobic environment for the cathode.

[25] However, fluoropolymers are not a sustainable solution, given their extreme persistence or release associated with their production, use, and disposal.

[26] Furthermore, this hydrophobic modification involves an expensive and lengthy freeze-drying procedure to produce the polymer. In addition, such hydrophobic modification renders the polymer completely hydrophobic, without allowing for controlled tuning of this property.

[0018] Therefore, there remains a need to provide a biodegradable hydrophobic composition for biofuel cell cathodes that does not have the drawbacks of the prior art. Summary of the Invention

[0019] Thus, the inventors addressed the dual problem of cathode water flooding and fine-tuning the hydrophobic / hydrophilic ratio of the cathode layer, while maintaining the overall biodegradability of the fuel cell through the use of bio-based products.

[0020] To this end, we have developed a double emulsion consisting of a hydrogel biopolymer, e.g., chitosan, as the first emulsion and a bio-based hydrophobic wax, e.g., beeswax, as the second emulsion, and then added this double emulsion blend to a bioactive ink composition as the main binder for realizing a biocathode for a bioenzymatic fuel cell.

[0021] Thus, the present inventors have provided a biodegradable hydrophobic composition for biofuel cell cathodes with a tunable hydrophilic / hydrophobic ratio.

[0022] Therefore, an object of the present invention is to provide a hydrogel biopolymer as a first emulsion. and a mixture of a bio-based hydrophobic wax as a second emulsion, and a bioactive ink.

[0023] "Hydrogel" biopolymer means a three-dimensional network of hydrophilic polymers that can swell and retain liquid when placed in water.

[0024] "Bio-based" means sustainable products or materials made entirely or essentially from materials of natural origin, i.e., plant or animal origin.

[0025] Polymer means a macromolecule made up of repeating subunits, monomers. For the purposes of this invention, this definition excludes fluoropolymers, as they are not sustainable materials.

[0026] "Ink" means a fluid or viscous substance composed of at least a pigment and a binder used in printing.

[0027] "Bioactive ink" means a fluid or viscous substance used for printing, consisting of at least a pigment, a binder and a biological catalyst. The term bioactive in this case refers to the possibility of catalyzing oxidation-reduction reactions by incorporating a biological catalyst, such as an enzyme.

[0028] According to certain embodiments of the present invention, the cathode ink formulation comprises: (a) a double emulsion formulation comprising a hydrogel bio-based polymer as a first emulsion with a bio-based hydrophobic wax as a second emulsion; (b) a bioactive ink; The bioactive ink is added to the double emulsion formulation.

[0029] In the cathode ink formulation of the present invention, the polymer of the hydrogel biopolymer can be selected from the group consisting of chitosan, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, poly(acrylic acid).

[0030] According to the present invention, the bio-derived hydrophobic wax may be selected from the group consisting of beeswax, carnauba wax, candelilla wax, sunflower wax, berry wax, bayberry fruit wax, lanolin, cetyl esters wax (cetyl palmitate) and other vegetable waxes.

[0031] "Vegetable wax" means a wax of plant origin.

[0032] According to the present invention, the bioactive ink can include mesoporous carbon, water, at least one polymer, and a multi-copper enzyme. Preferably, the bioactive ink is added after the formation of the double emulsion, since it contains enzymes and high molecular weight polymers (e.g., about 250,000-3,000,000 Da) that are not fully compatible with the high shear rate process used to make the emulsion.

[0033] According to the present invention, the mesoporous carbon of the bioactive ink can be selected from the group consisting of carbon nanofibers, carbon black, and carbon nanotubes.

[0034] According to the present invention, at least one polymer of the bioactive ink is selected from the group consisting of chitosan, hydroxypropyl cellulose, hydroxypropyl methylcellulose, carbo The polymer may be selected from the group consisting of hydroxymethylcellulose, poly(acrylic acid).

[0035] According to the present invention, the multi-copper enzyme of the bioactive ink may be selected from the group consisting of bilirubin oxidase and laccase.

[0036] According to the present invention, the ratio of the second emulsion to the first emulsion in the double emulsion formulation can be between 70 / 30 and 99 / 1, preferably 98 / 2.

[0037] According to the invention, the bioactive ink can represent 1 to 20% of the total weight of the formulation, preferably 15%.

[0038] Another object of the present invention is a hydrophobic biocathode comprising the cathode ink formulation of the present invention.

[0039] Another object of the present invention is also a biofuel cell comprising a hydrophobic biocathode according to the invention.

[0040] Another object of the present invention is also a method for preparing the cathode ink formulation of the present invention, comprising: a) preparing a first emulsion of a hydrogel biopolymer; b) preparing a second emulsion of a bio-derived hydrophobic wax; c) mixing the first emulsion with the second emulsion, thereby obtaining a double emulsion blend; d) mixing the double emulsion formulation obtained in step c) with a bioactive ink.

[0041] Another object of the present invention is the use of a hydrophobic bio-binder in a biofuel cell, comprising or consisting of a double emulsion formulation, as defined herein, comprising a hydrogel bio-based polymer as a first emulsion together with a bio-based hydrophobic wax as a second emulsion, the double emulsion making the resulting layer sufficiently hydrophobic to allow for a high oxygen concentration near the catalytic center, while allowing for sufficient hydration to maintain good proton conductivity. [Brief explanation of the drawings]

[0042] [Figure 1] Polarization tests for four different cathode ink formulations are shown: (squares) A-BW E4 95 / 5 (plus) B-BW E5 90 / 10 (cross) C-BW E6 98 / 2 (circles) D-PTFE. [Figure 2] Schematic diagram of a hydrophobic biocathode containing (A) a double emulsion formulation (hydrophobic biobinder) used in accordance with the present invention, resulting in (B) the double emulsion formulation (cathode ink formulation) with the ink formulation added: (1) water, (2) hydrophobic bio-based wax (second emulsion), (3) hydrogel bio-based polymer (first emulsion), (4) water droplets, and (5) hydrophobic biocathode.

[0043] Example 1: Biodegradable Hydrophobic Composition for Biofuel Cell Cathode (Hydrophobic Biobinder (BIODINDER)) 1. Beeswax-based compound (double emulsion compound) A beeswax-based formulation was prepared using the following: -Bleached beeswax (CAS: 8012-89-3) - Chitosan (cas:9012-76-4) from crab shells dispersed in water (3 wt%). -Tween 20 (CAS: 9005-64-5) (same ratio as dried chitosan) -Glycerol (CAS: 56-81-5) (0.25 *(dry weight of chitosan)

[0044] Four different beeswax emulsions were prepared using different ratios of wax / chitosan (by weight: 0 / 100, 90 / 10, 95 / 5, 98 / 2).

[0045] For the preparation of the double emulsion formulation, beeswax was heated at 95°C until completely melted.

[0046] First emulsion: In parallel, chitosan dispersed in water, Tween 20 and glycerol were mixed and passed through an ULTRA-TURRAX® rotor-stator device at 25 000 rpm for 20 minutes.

[0047] Second emulsion: This preparation (first emulsion) and pre-melted beeswax were then mixed together and passed through an ULTRA-TURRAX® rotor-stator device at 25 000 rpm for 30 minutes.

[0048] This double emulsion formulation made the resulting layer hydrophobic enough to increase oxygen concentration near the catalyst center, while allowing it to be hydrated enough to maintain good proton conductivity.

[0049] 2. Contact angle measurement of beeswax-based formulations The four double emulsion formulations were coated onto glass at a controlled thickness of 200 μm and a squeegee speed of 30 mm / s.

[0050] The samples were then heated in an oven at 40°C until the layer was completely dry.

[0051] Contact angles were determined using an Ossila Contact Goniometer and associated software. 10 μL droplets of distilled water were dispensed using an electronic pipette. Six samples per beeswax emulsion were tested. Three contact angle measurements were performed per sample, and the left and right angles were averaged.

[0052] [Table 1]

[0053] Theory dictates that if the contact angle is between 0° and 90°, the system is hydrophilic. Conversely, if the contact angle is between 90° and 180°, the system is hydrophobic.

[0054] These results showed that the chitosan-only emulsions could be considered hydrophilic emulsions (average contact angle of 71°), whereas the results showed that all beeswax double emulsion formulations had contact angles above 90°, meaning that they could be considered hydrophobic.

[0055] Furthermore, the higher percentage of beeswax in the double emulsion formulation compared to chitosan The more hydrophobic the layer, the more hydrophobic it was. Indeed, the double emulsion "BW E5" with a beeswax / chitosan ratio of 90 / 10 had an average contact angle of 90°, the double emulsion "BW E4" with a beeswax / chitosan ratio of 95 / 5 had an average contact angle of 98°, and the double emulsion "BW E6" with a beeswax / chitosan ratio of 98 / 2 had an average contact angle of 100°.

[0056] 3. Cathode Ink Formulation Three beeswax double emulsions, "BW E4," "BW E5," and "BW E6," were used to formulate a cathode ink formulation.

[0057] Briefly, to formulate the cathode ink formulation, a double emulsion was prepared as described above before final mixing.

[0058] The mesoporous carbon was also pre-dispersed using a rotor-stator tool at high shear rates, which generally required at least 15 minutes of dispersion at room temperature until the particles were dispersed.

[0059] The graphite particles, carbon dispersed in a polymer (chitosan), double emulsion beeswax, and water were then mixed at low speed using a Dispermat for 30 minutes. The enzyme (bilirubin oxidase) and its promoter (hemin) were then added to this cathode ink formulation, and the system was stirred at room temperature for at least 1 hour.

[0060] Cathode formulations A, B, C, and D were composed of mesoporous carbon (3 wt%), water, the same proportions of polymer, and 1% (by weight) of bilirubin oxidase enzyme for each. The only difference was the hydrophobic component (double emulsion component), which accounted for 15% of the overall ink formulation.

[0061] The cathode ink formulation was ready to be printed onto a GDL (Gas Diffusion Layer) substrate using a coating table.

[0062] The exact percentages are given in the table below.

[0063] [Table 2]

[0064] A reference with PTFE was also tested for comparison with a completely hydrophobic polymer.

[0065] Example 2: Polarization Tests on Cathode Ink Formulations Four different fuel cells were constructed by assembling several layers of: an outer layer made of glued cellulose paper, with one opening dedicated to the ventilation of the cathode and two other openings for making contact with both the anode and the cathode; - a printed cathode made of cathode ink formulation (A, B, C, D) printed onto a GDL, the GDL being on the outer layer side; - a microfluidic layer that is paper, -Considered stable, 1.5 mg / cm 2 functionalized with glucose oxidase a reference anode buckypaper (functionalized side in contact with the paper); -GDL used as current collector for anode buckypaper, Another outer layer made of glued cellulose paper that closes the whole fuel cell (with two openings for making contact with both the cathode and anode sides).

[0066] Electrochemical tests were performed on these four different fuel cells, which were activated in a solution made up of a buffer solution (sodium phosphate NaPB 0.1 M pH 7), an electrolyte support (sodium sulfate NaSO 0.1 M), and an anode fuel (glucose 0.2 M).

[0067] The experiment was repeated five times.

[0068] Polarization tests were performed using a potentiostat and its associated software, EC-lab, with a protocol consisting of a 30-second pause followed by a discharge current of -1 μA / -5 μA / -10 μA...-500 μA.

[0069] The polarization test results (Figure 1) showed better results for formulations in which the beeswax ratio was more important. This trend may correlate with the contact angle results. Furthermore, after -150 μA, the potential was higher for formulation C (BW E6 98 / 2) compared to formulation D (PTFE), indicating that a tunable ratio between hydrophobic and hydrophilic properties performed better than a solely hydrophobic system, such as PTFE. Thus, the maximum efficiency threshold was for BW / CHIT ratios greater than 95 / 5. Additionally, the results demonstrated that a fully sustainable system could be used advantageously over molecules with extreme sustainability, such as fluoropolymers.

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