EP4128394A1 - Électrodes de carbone douées d'une activité électrocatalytique améliorée - Google Patents

Électrodes de carbone douées d'une activité électrocatalytique améliorée

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Publication number
EP4128394A1
EP4128394A1 EP21716238.7A EP21716238A EP4128394A1 EP 4128394 A1 EP4128394 A1 EP 4128394A1 EP 21716238 A EP21716238 A EP 21716238A EP 4128394 A1 EP4128394 A1 EP 4128394A1
Authority
EP
European Patent Office
Prior art keywords
plasma
electrode
electrodes
carbon
treated
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP21716238.7A
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German (de)
English (en)
Inventor
Sebastiano BELLANI
Francesco Bonaccorso
Leyla Najafi
Vittorio Pellegrini
Mirko PRATO
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Fondazione Istituto Italiano di Tecnologia
Original Assignee
Fondazione Istituto Italiano di Tecnologia
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Publication of EP4128394A1 publication Critical patent/EP4128394A1/fr
Pending legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/18—Regenerative fuel cells, e.g. redox flow batteries or secondary fuel cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/22—Electrodes
    • H01G11/30—Electrodes characterised by their material
    • H01G11/32—Carbon-based
    • H01G11/34—Carbon-based characterised by carbonisation or activation of carbon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G11/00—Hybrid capacitors, i.e. capacitors having different positive and negative electrodes; Electric double-layer [EDL] capacitors; Processes for the manufacture thereof or of parts thereof
    • H01G11/84—Processes for the manufacture of hybrid or EDL capacitors, or components thereof
    • H01G11/86—Processes for the manufacture of hybrid or EDL capacitors, or components thereof specially adapted for electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8647—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites
    • H01M4/8657—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites layered
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8663—Selection of inactive substances as ingredients for catalytic active masses, e.g. binders, fillers
    • H01M4/8668—Binders
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8663—Selection of inactive substances as ingredients for catalytic active masses, e.g. binders, fillers
    • H01M4/8673—Electrically conductive fillers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/88—Processes of manufacture
    • H01M4/8878—Treatment steps after deposition of the catalytic active composition or after shaping of the electrode being free-standing body
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/96—Carbon-based electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/02—Details
    • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
    • H01M8/0204—Non-porous and characterised by the material
    • H01M8/0213—Gas-impermeable carbon-containing materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/18—Regenerative fuel cells, e.g. redox flow batteries or secondary fuel cells
    • H01M8/184—Regeneration by electrochemical means
    • H01M8/188—Regeneration by electrochemical means by recharging of redox couples containing fluids; Redox flow type batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/13—Energy storage using capacitors
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to the field of batteries and, in particular, it relates to hierarchical carbon electrodes having improved catalytic activity, useful as electrodes for electrocatalytic devices such as fuel cells, metal-ion batteries, supercapacitors, water splitting systems (electrolyzers) and, in particular, for redox flow batteries ( RFB ).
  • the invention further relates to the process for the production of the aforesaid electrodes, and to the electrocatalytic devices that comprise them. Background of the invention
  • Redox flow batteries are now one of the most promising technologies for large scale energy storage. Unlike batteries contained in a single container, RFBs store their energy in the electrolyte with redox-active materials that fill external tanks. The electrolyte flows from these tanks towards the active surfaces of the electrodes, where oxidation-reduction reactions take place, possibly more quickly compared to metal-ion batteries (e.g. lithium Li, sodium Na and potassium K batteries, etc.). Accordingly, the total capacity of RFBs can be adapted to the industrial scale application by simply expanding the volume of the external tanks regardless of the power characteristics, which are defined by the dimensions and the number of cells in a module unit. Simultaneously, the power of the RFBs is defined by the sizing of their electrodes.
  • metal-ion batteries e.g. lithium Li, sodium Na and potassium K batteries, etc.
  • Graphitic materials in particular graphite felts (GF), are commonly used as electrodes both in commercial RFBs based on vanadium (i.e. VRFB, “Vanadium Redox-Flow Batteries”) and in RFBs based on zinc, thanks to the low production costs and the excellent electrical conductibility, electrochemical stability and porosity.
  • VRFB vanadium Redox-Flow Batteries
  • RFBs Graphite felts
  • ZIRFB Zinc- Iodine Redox-Flow Batteries
  • Plasma treatment which enables the activation and functionalization of the surface of the materials that constitute the electrodes, and also its change from hydrophilic to hydrophobic nature.
  • Plasma treatments can be performed in general with various gases, e.g. O2, water vapour, (compressed) air, reducing gases such as H2, or N2; gaseous noble metals such as Ar, He, Xe, Ne, and Kr; NH 3 ; fluorinated gases such as CF 4 and SFe; and other gases such as CO2 and ethylene.
  • plasma treatment with O2 creates reactive species, e.g. O 3 , O radicals, and ionic species such as 0 + , which can react with carbon surfaces.
  • reactive species e.g. O 3
  • O radicals e.g., O radicals
  • ionic species such as 0 +
  • These functional groups have been shown to be catalytically active for redox reactions in VRFB batteries.
  • surface changes to carbon materials can also take place during plasma treatments with O2, as a consequence of losses of C caused by the development of CO and/or CO2 [8]
  • Plasma treatments with O2 are also effective for cleaning the materials of the carbon electrodes from organic contaminations.
  • N2 plasma treatments with N2 create N atoms and radicals that form nitrogenous functionalities on the carbon surfaces [9], e.g. by introducing C-N bonds on graphitic surfaces [10], thus introducing pyridinic-N, pyrrolic-N, quaternary N, N-oxides of pyridinic-N and aminic N (more rarely graphitic N) [11]
  • These functionalities have been shown to be catalytically active for the redox reactions involved in VRFB batteries [12]
  • the valency of the N atoms with its 5 electrons contributes with a further charge to the bond of the graphene layers, improving the conductivity of carbon materials.
  • it can create structural defects, e.g. unsaturated C atoms, which react with the O2 present in the material of the electrode or with the O2 in the environment
  • O-doped carbon felt O-CF
  • N-CF N-doped carbon felt
  • N, O-CF N,0 co-doped carbon felt
  • the last electrode was made by initially treating CF with plasma treatment with O2 for 9 minutes, followed by plasma treatment with N2 for 1 minute.
  • the results indicate that the electrocatalysis kinetics of the redox process on the electrodes are in the order O- N-CF > O-CF >N-CF > untreated CF.
  • the N,O-CF show a much better electrochemical performance than CFs doped with a single atom, because of the synergistic effect of the co-doping.
  • the EE of the VRFB battery with N,O-CF was improved passing from 65% of the VRFB with untreated CF with plasma treatment, to 78% at a current density of 50 mA cm -2 , with excellent cyclic stability.
  • the surface of samples treated with plasma was characterized by the presence of pyrrolic and pyridinic N.
  • the felt treated with N2 plasma demonstrated better electrochemical performance with respect to an untreated felt.
  • the cell functioning with the sample treated with plasma with N2 demonstrated a loss of energy capacity, or capacity fade, that can most probably be attributed to the development of hydrogen at the negative electrode.
  • European patent No. EP2626936B1 describes the use of carbon material as an electrode in redox flow cells. More in particular, this patent describes how to prepare graphite and carbon materials intended for use in efficient redox flow batteries through activation with plasma treatments in an atmosphere containing oxygen.
  • functional groups containing oxygen act as active centres for a number of electrochemical reactions, and they increase the hydrophilicity of these surfaces too.
  • the activation of the carbon material can comprise a modification to the surface, in particular a hydrophilization of the carbon material.
  • the number of functional groups containing oxygen on the surface of the carbon material is increased by a factor of at least 2, at least 5 or at least 10 compared to material not treated with plasma treatment.
  • the functional groups containing oxygen preferably comprise at least one functional group selected from hydroxyl, carbonyl and carboxyl groups.
  • the working gas for the plasma treatment air, nitrogen, argon, carbon monoxide, carbon dioxide and/or helium and mixtures thereof can be used.
  • the working gas is generally mixed with a specific proportion of oxygen, e.g. in the range between 1 and 40% by volume and in particular in the range from 20 to 30% by volume.
  • the plasma treatment is carried out in a pressure range of the working gas between 1 and 500 kPa. Typical exposure times in the plasma treatment of carbon materials are in the range between 1 and 600 s and in particular in the range between 10 and 90 s, e.g. 30 s.
  • the publication of international patent application No. W02003/070998A1 describes the possibility of combining more than one precursor gas in a plasma source for the activation of surfaces with precision molecular coatings. More in particular, this publication describes a method for the deposition of ionized molecules on the surface of an object in a vacuum system. Such method comprises a surface plasma treatment of the object in the vacuum system and a step of deposition of ionized molecules on the surface of the object in a vacuum system.
  • the plasma treatment described produces dangling bonds on the surface.
  • the plasma treatment comprises the substitution of chemical groups on the surface.
  • the plasma treatment comprises the addition of chemical groups on the surface.
  • Such plasma treatment can be conducted with at least one of the following as working gases: O 2 , N 2 , N 2 O, He, Ar, NH 3 , CO 2 , CF 4 and air.
  • Plasma treatments with gas have also been proposed as a means of functionalization for electrodes already previously functionalized.
  • US patent application 20180108915A1 relates to an electrode for batteries with functionalized flow with conductive nanoparticles and then further treated with plasma treatment.
  • this application relates to a porous electrode for a liquid flow battery comprising 1) particulate fibres of a non-electrically conductive polymer in the form of a first porous substrate, wherein the first porous substrate is at least one from among paper, felt, mat and woven or non-woven fabrics and 2) electrically conductive carbon particulate incorporated into the pores of the first porous substrate adhering directly to the surface of the non-electrically conductive polymer particulate fibres of the first porous substrate.
  • the electrically conductive carbon particulate of the porous electrode may be at least one from among carbon particles, carbon flakes, carbon fibres, carbon dendrites, carbon nanotubes and branched carbon nanotubes.
  • the electrically conductive carbon particulate that includes particles, flakes, fibres, dendrites and the like may be graphene. Flakes of particulate include particulate with a length and width each of which is significantly greater than the thickness of the flakes. A flake includes particulate with a length/width ratio and width/thickness ratio each greater than 5, without a particular upper limit. The width and length of the flake may each be from about 0.001 micro to about 50 micron. In some examples, the electrically conductive carbon particulate can be treated on the surface.
  • the surface treatment can increase the wettability of the porous electrode to provide an anolyte or a catholyte or to provide or improve the electrochemical activity of the electrode with respect to the oxidation-reduction reactions associated with the chemical composition of a given anolyte or catholyte.
  • Surface treatments include at least one from among chemical treatments, thermal treatments and plasma treatments.
  • the electrically conductive carbon particulate has improved the electrochemical activity, produced by at least one chemical treatment, a thermal treatment and a plasma treatment.
  • the term “improved” means that the electrochemical activity of the electrically conductive carbon particulate is selectively increased after treatment with respect to the electrochemical activity of the electrically conductive carbon particulate before treatment.
  • the improved electrochemical activity can include at least one from among increased current density, reduced oxygen development and reduced hydrogen development at a determined potential.
  • the ammoxidative surface reactions of the pure GF with NH 3 /O 2 result in an effective co-doping of N and O mainly with functional groups of N and O significant for speeding up the kinetics of the redox reactions.
  • the O-N-GF displayed an initial voltage and energy efficiencies about 4-6% higher with respect to the O-GF electrode during operation with high current density (80-110 mA/cm 2 ). Such effect caused an improvement of about 1.4 times in the discharge energy capacity of the VRFB battery.
  • the inventors have now found an electrode that helps in overcoming the technical limits highlighted above for the known electrodes and, in particular, it provides an unexpected improvement in the performance levels of the electrode thanks to a plasma treatment in a single step, with a combination of precursor gases of O2 and N2.
  • the functionalization of the electrode obtained with such treatment shows a synergistic effect in terms of performance of the electrode when compared with what is obtained by a treatment with a precursor of the individual gas O2 or N2, or by consecutive treatment with a precursor of the aforesaid gases in sequence.
  • Plasma treatment with the combination of the two gases according to the present invention can be performed on commercial electrodes or on electrodes functionalized with carbon particles, as described in detail in the following.
  • a subject of the present invention is therefore an electrode made of activated carbon material as claimed in claim 1, which solves the technical problems highlighted above for the known electrodes, providing in particular a carbon electrode having an improved electrocatalytic activity.
  • a further subject of the present invention is a process for producing the aforesaid electrode as claimed in claim 6, the use thereof, an electrochemical cell and an electrocatalytic device that comprises it as respectively claimed in claims 13, 14 and 15.
  • FIG. 2 is an SEM (Scanning Electron Microscopy) image of a fibre representative of a graphite felt (GF) not treated with plasma treatment;
  • FIG. 3 is an SEM image of a fibre representative of a GF treated with O2 plasma at the pressure of 40 Pa;
  • - Figure 4 is an SEM image of a fibre representative of a GF treated with N2 plasma at the pressure of 40 Pa;
  • - Figure 5 is an SEM image of a fibre representative of a GF treated with 0 2 :N 2
  • FIG. 6 shows two SEM images of a fibre representative of a GF treated with C>2:N2 1:1 plasma at the pressure of 16 Pa at two different enlargements: at 10020x (Fig. 6a) and at 40009x (Fig. 6b);
  • FIG. 7 shows an SEM image of a fibre representative of a GF treated with 0 2 :N 2 1 :1 plasma at the pressure of 4 Pa at three different enlargements: at 4003x (Fig.
  • FIG. 8 shows cyclic voltammetry measurements for GF not treated (pristine) with plasma and GF treated with different gaseous plasmas for the anode region related to the h/l redox reaction (Fig. 8a) and for the cathode region related to the Zn 2 7Zn redox reaction.
  • the CV curves were acquired with a potential measurement rate of 2 mV s 1 ;
  • FIG. 9 shows in the form of histograms the chemical composition of GFs not treated (pristine) and treated with the different gaseous plasmas indicated, as the elementary composition (Fig. 9a), as distribution of the functionalities of oxygen on the surface of the electrodes (Fig. 9b), and as distribution of the functionalities of nitrogen on the surface of the electrodes (Fig. 9c), taken from the analysis of the XPS spectra C 1s, N 1s and O 1s of the different electrodes.
  • the present invention relates to an electrode made of carbon material activated by a plasma treatment characterized in that said carbon material was activated by exposure to an electrical discharge in the atmosphere of a combination of precursors of gaseous N2 and O2.
  • the present electrodes can be applied as electrodes for electrocatalytic devices, such as fuel cells, metal-ion batteries, supercapacitors, water splitting systems and, preferably, as electrodes in redox flow batteries (RFBs).
  • electrocatalytic devices such as fuel cells, metal-ion batteries, supercapacitors, water splitting systems and, preferably, as electrodes in redox flow batteries (RFBs).
  • RFBs redox flow batteries
  • the carbon material of the present electrodes can, for example, be any graphite material, preferably a graphite felt (GF) based on rayon or polyacrylonitrile (PAN) as precursors.
  • the starting material for making the present electrodes may be, for example, a commercial graphite felt electrode, or another suitable electrode typically used in RFBs.
  • the combination of the gaseous precursors of O2 and N2 can be for example used in a weight ratio in the range comprised between about 0.05:0.95 and 0.95:0.05, and preferably such weight ratio is about 1:1.
  • the carbon material of the electrode was functionalized, on at least a part of its surface, with electrically conducting carbon particles, in particular two- dimensional (2D) carbon particles, such as, for example, graphene shavings or flakes (single or multiple layers of graphene) or graphene derivatives, and preferably graphene flakes.
  • electrically conducting carbon particles in particular two- dimensional (2D) carbon particles, such as, for example, graphene shavings or flakes (single or multiple layers of graphene) or graphene derivatives, and preferably graphene flakes.
  • graphene derivatives is meant, for example, reduced graphene oxide.
  • Such graphene flakes are preferably obtained through “wet-jet milling” exfoliation as described in Italian patent application No. IT102015000077259 in the name of the Applicant, the description of which is incorporated herein by reference.
  • the invention also relates to the process for producing the aforesaid electrode, comprising the steps of providing a piece of carbon material, and subjecting it to activation through plasma treatment by exposure to an electric discharge in the atmosphere of a combination of precursors of gaseous N2 and O2, preferably at a pressure of the combination of gases ranging between about 4 and about 100 Pa, more preferably at a pressure between about 4 and about 40 Pa, and even more preferably at a pressure of about 4, or about 16, or about 40 Pa.
  • the exposure to electrical discharge for the generation of plasma can be performed according to any known method, and preferably with an inductively coupled radiofrequency reactor, wherein said electrical discharge can be for example of power comprised between about 20 and about 500 W, more preferably between about 50 and about 200 W, and more preferably at a power of about 100 W. Because of its nature, such treatment is very quick, typically it has a duration that varies between about 10 seconds and 60 minutes, and on average it has a duration of about 10 minutes. According to a preferred embodiment of the process of the invention, the process further comprises, prior to the activation of the carbon material with plasma treatment, a functionalization step of the material itself, on at least part of the surface thereof, with conductive carbon particles, e.g.
  • a polymer agent is preferably used selected for example from the group consisting of polyvinyldenfluoride (PVDF), copolymers of fluoropolymers based on tetrafluoreoethylene sulfonate (e.g.
  • polymer binding agent is polyvinyldenfluoride.
  • the binding agent can also be selected from the group consisting of styrene butadiene rubbers (SBR), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyamide imide (PAI), sodium carboxymethyl cellulose, multipolymer acrylonitrile (LA133), polytetrafluoroethylene (PTFE).
  • SBR styrene butadiene rubbers
  • PAA polyacrylic acid
  • PVA polyvinyl alcohol
  • PEG polyethylene glycol
  • PAI polyamide imide
  • LA133 multipolymer acrylonitrile
  • PTFE polytetrafluoroethylene
  • the binding agent can be added for example in an amount comprised between 1 and 50% by weight with respect to the total weight of the dispersion of conductive particles, preferably in an amount comprised between 1 and 30% by weight.
  • the binding agent is PVDF and it is added in an amount of about 10% by weight with respect to the total weight of the dispersion of conductive particles.
  • the subsequent drying of the electrode functionalized with the conductive particles before the plasma treatment can for example be performed by heating to a temperature comprised between 50 and 200°C, preferably about 150°C, in vacuum conditions for a certain period of time, typically between 1 and 24 hours, preferably for about 12 hours.
  • the electrodes of the invention have different functionalizations based on nitrogen and oxygen on their surface.
  • they comprise pyridinic nitrogen, pyrrolic nitrogen, quaternary nitrogen, N-oxides of pyridinic nitrogen and of aminic nitrogen.
  • functionalizations of oxygen they comprise phenol, carboxyl and carbonyl groups and aliphatic hydrocarbons.
  • the electrodes of the invention have displayed certain atomic contents in the range of 75-95% for carbon, 0.1-5% for nitrogen and 5- 20% for oxygen, with the sum of said atomic content for C, O and N being equal to 100%; preferably, the present electrodes have an atomic content of 80-90% carbon, 12-17% oxygen and 0.3-2% nitrogen.
  • XPS X-ray Photoeletron Spectroscopy
  • the atomic content may be attributed to different functional groups.
  • the total atomic content for oxygen is comprised between 5 and 20%, to which total content can contribute in a varying way, to the extent indicated above, the percentages of atomic content for the various functional groups containing oxygen.
  • the atomic content of N can be attributed to pyridinic N (bond energy between 397.8 and 398.2 eV) for less than 0.1%, to pyrrolic N (bond energy between 400.0 and 400.4 eV) for a range of 0.05-1.00 %, quaternary N (graphitic N) (bond energy between 401.0 and 401.6 eV) for a range of 0.05-1.00%, N oxides (bond energy between 402.2 and 402.5 eV) for a range of 0.01-1.00%.
  • the atomic content of nitrogen can be attributed to pyridinic N for less than 0.01%, pyrrolic N for a range of 0.15-0.40% (more preferably in a range of 0.20- 0.35%), quaternary N for a content in the range of 0.05-0.30%, nitrogen oxides for a content in the range of 0.01-0.70%.
  • the electrodes of the invention From the XPS spectrum of C 1s, the electrodes of the invention have displayed for carbon an atomic content in the range 75-95%.
  • the present electrodes display high catalytic activity towards the redox reactions that take place in electrochemical devices, in particular in RFBs.
  • the electrodes of this invention have a larger electrochemically accessible surface area, as well as high wettability with the electrolyte, i.e. high hydrophilicity for aqueous and polar electrolytes.
  • the electrodes of the invention are carbon based electrodes, preferably graphitic, the electrodes of the invention further have high electric conductivity (i.e. low electrical resistivity).
  • composition of the combined gases of the plasma treatment used for the production of the electrodes of this invention can be appropriately varied to select the functional groups most desired to be incorporated into the material of the electrode for the purpose of modulating the electrochemical performance thereof.
  • RFBs aqueous redox flow batteries
  • VRFBs vanadium redox flow batteries
  • ZIRFBs zinc-iodine redox flow batteries
  • the GFs were treated with multiple combined gas plasma (O2 and N2 plasma, with a composition of 1:1 by weight) and, as a comparison, also with single gas plasma (O2 or N2) and sequential gas plasma (O2 plasma followed by N2 plasma or N2 plasma followed by O2 plasma).
  • the gaseous plasma treatments of the GFs were performed in a radiofrequency (RF) reactor inductively coupled (13.56 MHz) to a 100 W power and a working gas pressure (multiple or single) comprised between 4 and 40 Pa, i.e. 4, 16 and 40 Pa, (background gas pressure fixed at 0.2 Pa) for a time of 10 min.
  • RF radiofrequency
  • polar electrolytes e.g. aqueous electrolytes, including those used in VRFB batteries.
  • N2 introduces N-based functionalities onto the surface of the materials of the electrodes.
  • the N-based functionalities are N-pyridinic, N-pyrrolic, N-quaternary, N-oxides of N- pyridinic and N-aminic functionalities (more rarely N graphitic), and are catalytically active for the redox reactions that take place in the VRFBs.
  • the five valence electrons of the N atoms contribute to the additional charge of the bond of the graphene layers, improving the electrical conductivity of the carbon materials.
  • the plasma treatment with N2 creates structural defects, e.g. unsaturated C atoms, which can later react with O2 present in the material of the electrode or with environmental O2. Obtaining the subsequent gaseous plasma with different plasma, the effects of the different gases can be combined and possibly controlled to obtain an excellent functionalization ratio based on O and N, as well as an attachment of the morphology that cannot be obtained with O 2 plasma only.
  • Peristaltic pumps (Masterflex L/S ® series) were used to pump the electrolyte into the cell hardware in a one-directional way.
  • the electrochemical measurements of the VRFB batteries were performed with a potentiostat/galvanostat (VMP3, Biologic).
  • the electrolytes were previously prepared electrochemically from a 1 M solution of VOSO4 + 3 M of H2SO4.
  • the initial positive and negative electrolytes (respectively catholyte and anolyte) in the tanks were sized with a volume equal to 30 mL of the following solutions 1 M V0 2+ + 3 M of H 2 S0 4 and 1 M di V 3+ + 3 M of H 2 S0 4 , respectively, corresponding to a specific theoretical capacity of 13.4 Ah / L (calculated on the total volume of electrolyte, including both the catholyte and the anolyte).
  • the electrolytes were dispensed into the cell from the peristaltic pumps at a flow rate of 40 ml rnirr 1 . Purging with nitrogen into the anolyte tank was performed to prevent the oxidation of the charge from V 2+ to V 3+ in the presence of O2 when the battery is in a charged state.
  • the polarization curve was analysed to evaluate the kinetic activation polarization (kinetic losses) and the ohmic polarization (iR losses) inside the cells.
  • the polarization curves of the VRFBs were performed on completely charged cells (with 1 M V0 2+ + 3 M H 2 SO 4 as the catholyte and 1 M V 2+ + 3M H 2 SO 4 as the anolyte).
  • a constant current of 10 mA cm -2 was applied until the voltage of 1.7 V (charged state).
  • the cells were then discharged for 30 s at every applied current density (comprised between 1 and 200 mA cm -2 ).
  • the voltage measurements of the cells were mediated on 30 s of each current passage to provide a point on the polarization curve.
  • the high frequency resistance (at 15-30 kHz) of the VRFB was measured through electrochemical impedance spectroscopy (EIS), according to the previously reported protocols.
  • EIS electrochemical impedance spectroscopy
  • the magnitude of the voltage disturbance in AC alternating mode was set to 10 mV.
  • the iR losses were calculated from the product of the applied current (i) and the resistance measured through EIS (R).
  • Galvanostatic charge/discharge measurements were performed to evaluate the main Figure of Merit (FoM) of the VRFB - where in the present invention “figures of merit” refer to the parameters used to define the performance of the battery, i.e. : the coulombic efficiency (CE), which is the ratio between the electric charge passed from the cell during the discharge (Qdischarge) and that during charging (Qcharge); the voltage efficiency (VE), which is the ratio between the average voltages of the cells during charging and during discharging; EE, which is the product of CE and VE.
  • CE coulombic efficiency
  • VE voltage efficiency
  • EE which is the product of CE and VE.
  • the galvanostatic measurements CD of the individual cells of the VRFB battery were performed at different current densities, comprised between 25 and 200 mA cm 2 .
  • Figure 1 shows the polarization curves (after IR correction to isolate the kinetic activation polarizations, the proton exchange membrane and the electrolyte being the same for all the cells) obtained for VRFBs using GF before and after the different plasma treatments as electrodes (electrodes mentioned in the key of the figures as not treated with plasma and with the details of the gaseous plasma treatment, including the composition of the gas and the pressure of plasma).
  • the VRFB based on the GFs treated with combined gas plasma with 40 and 16 Pa as plasma pressure i.e. plasma of O2: N2 (1: 1) - 40 Pa, plasma of O2: N2 (1: 1) - 16 Pa
  • plasma pressure i.e. plasma of O2: N2 (1: 1) - 40 Pa
  • plasma of O2: N2 (1: 1) - 16 Pa shows the lowest kinetic polarizations of activation.
  • the kinetic activation polarizations increase because of the excessive attachment of the GF fibres by the reactive plasma species (by lowering the plasma pressure, the speed of the particles present in the plasma increases before impacting against the target sample).
  • the VRFBs based on GFs treated with N2 plasma at 40 Pa display polarizations of kinetic activation higher than those displayed by the VRFBs treated with O2 plasma at the same plasma pressure. Therefore, the plasma treatment with N2 is less effective for improving the catalytic activity of GFs towards the redox reactions of VRFBs with respect to O2 plasma treatment.
  • N2 plasma treatment after O2 plasma treatment further reduces the activation polarization losses, indicating a greater electrocatalytic activity towards redox reactions in the VRFB battery after sequential plasma treatments.
  • treatment with O2 plasma reduces the kinetic polarization of activation of the GF previously treated with N2 plasma.
  • neither of the sequential plasma treatments is effective for reducing the kinetic polarization as happens in the case of plasma treatment with multiple and combined gases at the same plasma pressure. This indicates that new effective and synergistic effects can be obtained which increase the catalytic activity of the GF towards VRFBs through the use of multiple gases during the plasma treatments.
  • the VRFBs based on GFs treated with plasma of mixtures C>2:N2 1:1 at 16 Pa display a VE of 96.3%, which is higher than those reported in literature on the subject.
  • the surface of the GF fibres still shows a smooth morphology (see Figure 3), which is similar to that observed for the native GF fibres. Therefore, the effects that originated from the O2 plasma are mainly chemical surface modifications, as already described in the state of the art. However, by increasing the power that generates the plasma, the energy of the species in the plasma could have an effect on the graphitic surfaces because of a concomitant evolution of CO and/or CO2.
  • N2 plasma increases the roughness of the surface of the fibres with respect to that of a GF not plasma treated (see Figure 4). This change to the surface morphology is caused by an incision process of the fibres through the formation of structural defects (unsaturated C atoms).
  • the Figures 6 and 7 show a GF treated with C>2:N2 1:1 mixtures at the pressure of 16 Pa and with C>2:N2 1:1 at the pressure of 4 Pa, respectively, therefore electrodes with lower pressures than that used to treat the sample shown in Figure 5 (i.e. 40 Pa).
  • Both the samples display significant modifications to the surface morphology, including the formation of cavities similar to craters having a diameter of hundreds of nanometres, and texturization of the surfaces on a lower scale than the dimensions of the craters (i.e. surface nano-texturization).
  • Such effects are more clearly pronounced in GFs treated with 0 2 :N 2 1:1 at the pressure of 4 Pa with respect to GFs treated with 0 2 :N 2 1:1 at the pressure of 16 Pa, in accordance with the expectations on the basis of what has been observed above.
  • An excessive incision of the GF reduces the electrochemical performance of the resulting VRFBs with respect to the optimal case based on GFs treated with 0 2 :N 2 1:1 at the pressure of 16 Pa.
  • the electrochemical activity of the electrodes can be evaluated from the analysis of the current density peaks of the redox reactions [15,16,17], the separation of the potentials of the current density peaks (DE) of the redox reactions [15,16,17], and from the corresponding ratios of the anode(cathode) and cathode(anode) current density peak ratios —I P A/I P C(I P C/I pa)— of the redox reactions in the region of the anode(cathode) current [15,16,17]
  • the CV measurements were performed with the same potenziostat/galvanostat used for the electrochemical characterization of the VRFBs (i.e.
  • the N2 plasma is significantly more effective than the O2 plasma for activating the catalytic activity of the electrode not treated for the redox reaction l 3 (o hBr)/! .
  • the plasma treatment with multiple and combined gases at the plasma pressure of 40 Pa i.e. 0 2 :N 2 1:1 plasma at the pressure of 40 Pa) further increases the catalytic activity of the GFs.
  • the difference DE of the redox reaction l 3_ (o Br )/l _ for GFs treated with plasma shows a slight increase with respect to the untreated GF.
  • I P A/I P C values measured for GFs treated with N2 plasma at 40 Pa (0.90) and with 0 2 :N 2 1:1 plasma at the pressure of 40 Pa (0.82) are significantly higher than those measured for GFs not treated with plasma (0.73).
  • gas plasma treatments increase the reversibility of the redox reaction l 3_ (o bBr)/! on the untreated GF.
  • the current density peaks increase after plasma treatments with N2 at 40 Pa and with C>2:N2 1:1 at 40 Pa.
  • Plasma treatment with O2 at the pressure of 40 Pa does not significantly change the catalytic activities of the GFs. Therefore, it is not suitable as treatment of the electrodes for ZIRFBs.
  • plasma treatment with C>2:N2 1:1 at 40 Pa is more effective than the plasma treatment with N2 at 40 Pa.
  • the difference DE of the redox reaction Zn 2 7Zn is slightly reduced after plasma treatment with N2 and with mixed gases with respect to untreated GFs.
  • the ratio Ipc/lp A is less than 0.5 for all the electrodes.
  • the atomic content values % detected for O are significantly higher than those reported in literature for GFs treated thermally for VRFBs (typically less than ⁇ 8%) [18,19]
  • the maximum of N as atomic content in % was detected for N 2 -40 Pa (1.67%), followed by O2: N2-I6 Pa and O2: N2-4 Pa (respectively 0.68% and 0.87%).
  • the present inventors maintain that all these O functionalities introduced onto the surface of the material of the electrodes thanks to the present plasma treatment with combined gases, can be configured as catalytic sites for the redox reactions that take place in electrochemical devices where such electrodes are used.
  • the plasma treatments according to the invention have introduced onto the surface of the electrodes also functional groups of N, i.e. pyrrolic N, N oxides and graphitic N, as illustrated in Figure 9c.
  • N i.e. pyrrolic N, N oxides and graphitic N
  • pyridinic groups were not observed.
  • these N functionalities can act as catalytic sites for redox reactions in electrochemical devices, as well as the O functionalities, with which synergistic catalytic effects can be caused.
  • a hierarchical carbon electrode was prepared with a coating of graphene flakes on graphite fibres of a graphite felt (GF) available on the market (4.6 mm GFD, Sigracell ® ).
  • the graphene flakes were produced in the form of a dispersion in N- methyl-2-pyrrolidone (NMP) through the exfoliation of graphite with the jet-milling method described in Italian patent application No. IT102015000077259 in the name of the Applicant, incorporated herein by reference, as described below.
  • NMP N- methyl-2-pyrrolidone
  • NMP N-Methyl-2-pyrrolidone
  • graphite flakes (+100 mesh, Sigma Aldrich
  • a mechanical stirrer Eurostar digital Ika-Werke
  • the mixture was transferred into a processor consisting of five series of different perforated discs interconnected with one another by applying a pressure of 250 Mpa through a hydraulic piston. Two jet flows were created at the second disc, which is provided with two holes of 1 mm diameter. Therefore, the two jets collide between the second and the third disc, which consists of a nozzle with diameter 0.3 mm.
  • the turbulence of the solvent generates a shear force that causes the exfoliation of the graphite.
  • the dispersion as produced is cooled by a cooler, then collected in another container.
  • the sample is processed again various times (e.g. twice) in a WJM machine, passing consecutively through nozzles with a reduced diameter (e.g. 0.15 and 0.1 mm).
  • the SLG/FLG dispersion thus produced is concentrated or even dried in the form of powder with a rotary evaporator (Heidolph HEIVAP INDUSTRIAL, FKV Sri, Italy), setting the temperature of the bath to 80°C reducing the pressure to 5 mbar.
  • the SLG/FLG flakes are added to 1.5 L of dimethyl sulfoxide (DMSO) (Merck KGaA, Germany).
  • DMSO dimethyl sulfoxide
  • the mixture thus obtained is poured into five 300 mL Petri dishes made of aluminium and kept in a fridge for 1 hour at -15°C. Then, the Petri dishes are transferred into a freeze drying machine. Here the sublimation process is performed at a temperature of -10°C and a pressure of 0.1 mbar. After 50 minutes, the final freeze dried SLG/FLG powder is obtained.
  • the graphene flakes thus obtained are a mixture of graphene flakes in a single layer or in few layers (SLG/FLG).
  • the dispersion of such SLG/FLG graphene flakes was purified through ultracentrifugation at 1000 rpm for 30 minutes, and subsequently collecting the supernatant.
  • the dispersion of purified graphene flakes SLG/FLG was concentrated at 15 g L 1 through evaporation of NMP with a rotavapor (Heidolph, HEIVAP INDUSTRIAL, FKV Sri, Italy) at 60°C.
  • PVDF Polyvinyldenfluoride
  • GFs coated with SLG/FLG flakes were therefore prepared with a direct wet impregnation method.
  • the SLG/FLG flakes were impregnated in the GFs by filtering such GFs with the dispersion of SLG/FLG flakes as prepared.
  • 3 mL of the dispersion of SLG/FLG flakes and PVDF as prepared were infiltrated in a piece of GF with an area of 5 cm 2 .
  • Such electrodes were then vacuum dried at 150°C for 12 hours.
  • the hierarchical electrodes obtained display a larger surface area than that of the native GFs.
  • the VRFB that uses the GF/graphene electrodes plasma treated with 0 2 :N 2 1:1 at the pressure of 4 Pa shows the highest VE (e.g. 97.1% and 87.0% at 25 and at 100 mA cm -2 respectively) and EE (e.g. 93.1% and 84.9% at 25 and at 100 mA cm -2 respectively).
  • VE e.g. 97.1% and 87.0% at 25 and at 100 mA cm -2 respectively
  • EE e.g. 93.1% and 84.9% at 25 and at 100 mA cm -2 respectively.
  • the VRFBs that use GF/graphene treated with 0 2 :N 2 1:1 plasma at the pressure of 4 Pa show a VE of 97.1%.
  • coating the GF with SLG/FLG flakes can also protect the GF from excessive degradation induced by plasma treatments at low pressure, as already discussed for the pressure of 4 Pa.

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Abstract

La présente invention porte sur de nouvelles électrodes de carbone hiérarchiques douées d'une activité électrocatalytique améliorée, servant d'électrodes pour des dispositifs électrocatalytiques, tels que des piles à combustible, des batteries métal-ion, des supercondensateurs, des systèmes de décomposition d'eau et en particulier des batteries à flux redox, sur leur procédé de production et sur les dispositifs électrocatalytiques qui contiennent de telles électrodes.
EP21716238.7A 2020-04-03 2021-04-01 Électrodes de carbone douées d'une activité électrocatalytique améliorée Pending EP4128394A1 (fr)

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