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WO2012114108A1 - Catalyseur de réaction de réduction d'oxygène - Google Patents

Catalyseur de réaction de réduction d'oxygène Download PDF

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Publication number
WO2012114108A1
WO2012114108A1 PCT/GB2012/050403 GB2012050403W WO2012114108A1 WO 2012114108 A1 WO2012114108 A1 WO 2012114108A1 GB 2012050403 W GB2012050403 W GB 2012050403W WO 2012114108 A1 WO2012114108 A1 WO 2012114108A1
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Prior art keywords
oxygen
catalyst
graphene
fuel cell
electrode
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Inventor
Pagona PAPAKONSTANTINOU
Surbhi SHARMA
Santosh Kumar BIKKAROLLA
Abhijit Ganguly
James Davis
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Ulster University
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Ulster University
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    • 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
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/28—Per-compounds
    • C25B1/30—Peroxides
    • 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
    • H01M4/90—Selection of catalytic material
    • H01M4/9016—Oxides, hydroxides or oxygenated metallic salts
    • 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
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M2008/1095—Fuel cells with polymeric electrolytes
    • 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
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
    • 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

Definitions

  • the present invention relates to catalysts for use in, inter alia, fuel cells and metal /air batteries.
  • the invention relates to catalysts for the oxygen reduction reaction in fuel cells, and most particularly to non-metallic catalysts for the oxygen reduction reactions in fuel cells.
  • the invention also relates to fuel cells comprising said catalysts and to the use of said catalysts in the catalytic reduction of oxygen at the cathode of fuel cells.
  • a fuel cell is an electrochemical device which oxidises fuel at the anode and reduces oxygen from the air at the cathode.
  • the efficiency of the oxygen reduction reaction at the cathode is a crucial factor in the performance of the cell and for this reason this reaction is usually catalysed using the most efficient methods known. This is typically by employing catalysts such as platinum group metals and their alloys.
  • an ion exchange membrane is positioned between the anode and the cathode to form a proton-exchange membrane fuel cell (PEMFC).
  • PEMFC proton-exchange membrane fuel cell
  • ORR oxygen reduction reaction
  • Pt platinum
  • factors that can reduce the lifetime of PEMFCs including: (1) platinum-particle dissolution and sintering, (2) carbon support corrosion, and (3) CO deactivation of Pt surface area in the cathode.
  • the exploration of non-noble metal compounds, notably cobalt- and iron-based nitrogen-containing catalysts (Lefevre et al, Science, 2009 and Zhang et al, Chem. Matter, 2009) are also promising substitutes for the currently used Pt/C catalyst owing to their comparable catalytic activities toward ORR and much lower cost.
  • N-graphene and N-carbon nanotubes were clearly shown to catalyse a four- electron ORR process free from CO "poisoning" with a four or three fold higher electrocatalytic activity and better long term operational stability than that of commercially available Pt-based electrodes in alkaline electrolytes. Nevertheless, despite these emerging improvements in the activity of non metal catalysts, there is no clear understanding about the nature of the active sites and the mechanism that underpins the oxygen reduction reaction remains elusive.
  • other carbon materials doped with both lower electronegative atoms than carbon such as P-doped graphite layers (electronegativity of phosphorus: 2.19)[ Liu, Z. W. et al. Angew. Chem., Int. Ed.
  • N-doped graphene From a commercial point of view, the production of N-doped graphene reported so far is cumbersome and entails many steps.
  • N-doped graphene is produced by a chemical vapour deposition method that requires detachment from the substrate, purification in strong acids and subsequent doping in an ammonia atmosphere.
  • vapour deposition type processes are poorly suited for large scale production or for low cost manufacture.
  • An alternative process that allows production of catalysts using facile methods is thus highly desirable for high volume production.
  • catalysts comprising oxygen-doped graphene can address one or more of these issues and can exhibit excellent catalytic activity for the ORR.
  • These catalysts include highly oxygenated graphenes such as graphene oxide and those with a lower atomic percentage oxygen, such as reduced graphene oxides..
  • oxygen doped graphene catalysts can show improved stability and/or poisoning resistance and hence represent promising alternatives to those already known in the art.
  • the catalysts of the invention can possess excellent tolerance to the methanol crossover effect.
  • the present invention relates to the development by the inventors of new catalyst materials, catalyst layers, fuel cells and methods for the use of these.
  • the invention thus provides a catalyst for the oxygen reduction reaction at the cathode of a fuel cell, wherein the catalyst comprises oxygen-doped graphene.
  • the catalyst of the invention is metal-free.
  • the invention provides a catalyst as herein defined, wherein the oxygen-doped graphene is in the form of oxygen-doped graphene nanosheets or oxygen-doped graphene nanoflakes.
  • the graphene nanoflakes are optionally and preferably vertically aligned (as defined herein).
  • the invention provides a catalyst layer comprising a catalyst as herein defined.
  • the invention provides an electrode comprising a catalyst layer as herein defined.
  • the invention provides a fuel cell comprising an electrode as herein defined.
  • the fuel cell is preferably a proton exchange membrane fuel cell.
  • the invention provides a method of reducing oxygen, said method comprising exposing oxygen to a catalyst as herein defined.
  • the method may correspondingly also be performed by exposing oxygen to the catalyst layer or electrode of the invention or by supplying oxygen to the cathode of the fuel cell of the invention.
  • the invention provides for the use of oxygen-doped graphene as a catalyst for the oxygen reduction reaction at the cathode of a fuel cell.
  • the present invention relates to catalysts, their products, uses, methods of formation and other aspects wherein a doped graphene is utilised.
  • graphene is used herein to refer to a single atom thick planar sheet of sp 2 -bonded carbon atoms which are positioned in a honeycomb crystal lattice.
  • oxygen-doped graphene is used to refer to graphene, onto which oxygen atoms have been chemically bound. This is particularly by means of at least one covalent bond such as one or more covalent single-bonds or a covalent double-bond.
  • the oxygen can be in any chemical form, such as epoxy, hydroxy, carboxyl or carboxylic acid groups.
  • the oxygen doping is preferably not solely in the form of adsorbed oxygen or oxygen held by non-bonded electrostatic interactions. Methods for the production of oxygen doped graphene are well known and documented in the art and are further described in the Examples herein.
  • dopant is used to describe an impurity element which is inserted into a substance in order to alter the properties of the substance.
  • Oxygen doping of graphene is a key aspect of the present invention and oxygen doped graphene is intended herein in any instance where the dopant is not specified.
  • Other doping or co-doping materials may also be used in addition to oxygen in all embodiments.
  • any oxygen doped graphene described herein may optionally also comprise at least one additional dopant (co-dopant, additional dopant or second dopant). Each additional dopant may independently be added before, simultaneously with or after the oxygen dopant. Many of these other doping or co-doping materials are known in the art and some are indicated herein.
  • Suitable co-doping materials include all dopants indicated herein such as nitrogen, chlorine and fluorine. Doping with one or more elements more electronegative than carbon is particularly desirable and thus co-doping with oxygen plus at least one additional dopant more electronegative than carbon forms a preferred embodiment. Catalyst
  • the catalyst of the invention comprises, and optionally consists of, oxygen- doped graphene (with an optional co-dopant).
  • the oxygen-doped graphene can take any form but is preferably in the form of oxygen-doped graphene nanosheets or oxygen-doped graphene nanoflakes.
  • the oxygen doped graphene may optionally also be doped with other elements as indicated herein.
  • the catalyst of the invention as indicated herein in all respects may be employed in any of the indicated forms in any aspect of the invention.
  • graphene is in the form of nanoflakes
  • these may be positioned perpendicular to a surface in a formation often termed "vertically aligned". This term is used in the art and herein to indicate the position of nanoflakes such that one edge of each flake is against a surface. This may be the surface upon which the flakes were formed.
  • An example of a suitable surface is a carbon surface, such as carbon clothpaper.
  • the graphene nanoflakes referred to herein may be vertically aligned oxygen doped graphene nanoflakes (as defined herein).
  • the graphene is in the form of nanosheets, these may comprise multiple layers.
  • a preferred number of layers is in the range 1 to 10 (e.g 2 to 10) and more preferably 1 to 4 (e.g. 2 to 4).
  • the oxygen content of the oxygen-doped graphene in all embodiments herein is preferably in the range 3.8 to 40 atom% (e.g. 15 to 40 atomic %, such as 15 to 35 at%, more preferably 4 to 35 atom% (e.g. 20 to 35 atomic %, such as20 to 30 at%).
  • graphene oxide may be comprised in the oxygen-doped graphene and in that case the oxygen content will most commonly be 10 to 40 atom%, preferably 15 to 40 or 20 to 35 atom %.
  • the oxygen-doped graphene may comprise at least one form of "reduced" graphene oxide, such as electrochemically reduced graphene oxide (ERGO) and/or nitrogen- reduced graphene oxide (NRGO). Where at least one reduced graphene oxide forms 50% or more of the oxygen-doped graphene, the oxygen content will typically be in the range of 3.5 to 25 at% (e.g. 3.8 to 25 at%), preferably 3.8 to 18 at% (e.g. 4 to 14 at%).
  • ERGO electrochemically reduced graphene oxide
  • NRGO nitrogen- reduced graphene oxide
  • the oxygen atomic concentration decreases from 27.7 at% in GO to 11.6at% in NRGO, (as experimentally determined in by the inventors) and to levels between 5-4 at% in ERGO according to the reported literature (see examples for citations).
  • Graphene nanosheets produced by ionic liquid assisted grinding possess low levels of naturally absorbed oxygen (e.g. up to around 3.5 at%), inherited from the starting graphite. Such low oxygen level is very difficult to remove completely even with high temperature vacuum annealing at very high temperatures of 1000 °C.
  • oxygen-doped graphenes used in the present invention such as GO and reduced GO, including NRGO and ERGO, differ from “graphene” in a number of ways. These include the oxygen content and bonding configurations of the remaining oxygen and also the surface area of the product.
  • ERGO produced by electrochemical reduction of GO possess a high surface area which is evident by the large capacitive and Faraday loop in figure 1. Without being bound by theory, it is thought that the observed increase in current and onset potential observed for the ERGO can be attributed to the remnant structural defects induced by harsh oxidation of the GO and probably "wrinkles" induced by the electrochemical removal of oxygen resulting in large surface area.
  • Highly preferred oxygen-doped graphenes for use in the present invention thus include GO, and reduced GO, particularly NRGO and ERGO.
  • the catalyst of the invention may further comprise a second, preferably non-metallic, heteroatom dopant.
  • the second dopant is not oxygen and is referred to herein as a co-dopant, additional dopant or second dopant.
  • second dopants include nitrogen, chlorine and fluorine. Nitrogen is particularly preferred as a second dopant.
  • nitrogen is present as an additional dopant, it is preferable that the nitrogen is covalently bound, such as by incorporation into the crystal lattice of the oxygen-doped graphene in substitutional (also known quartenary) or pyridine-like sites. Methods for the formation of nitrogen doped graphenes are known in the art.
  • the second dopant may optionally be used in any aspect of the invention. More than one "second" dopant may be present.
  • a suitable level of second or additional dopant will depend upon the nature of that dopant and the level of oxygen doping and any other dopants present.
  • the total doping level will typically be 5 to 50 atomic % (e.g. 10 to 40 at% or 15 to 40 at%), more preferably 15 to 35 (e.g.10 to 35, 15 to 35 or 20 to 35 atomic %, such as 15 to 30 at% or 20 to 30 at%).
  • an additional dopant may be present at levels of 0.1 to 40 at%, preferably 1 to 35 at%, more preferably 2 to 30 at% (e.g. 5 to 30, 2 to 10 or 5 to 15 at%).
  • the level of oxygen doping (measured as atomic %) will be higher than the combined level of all other dopant materials.
  • Nitrogen reduced graphene oxide forms one preferred embodiment of the present invention and the oxygen doped graphene may thus consist in whole or in part (e.g. 2% to 99 (such as 2% to 20%)), or 2% to at least 11%) of NRGO.
  • the oxygen doped graphene comprises or consists of GO, NRGO, ERGO and mixtures thereof.
  • the catalyst is metal-free. This term is well understood in the art, but could be taken, for example, to mean that there is no metal present at a level greater than 1 at%, preferably no greater than 0.5 or 0.2 at%, and more preferably no greater than 0.1 or 0.01 at%. A level of 0.001 to 0.1 at% might be typical.
  • the catalyst of the invention can form part of cathode catalyst layer in the proton exchange membrane assembly in PEM fuel cells.
  • the catalyst e.g. graphene oxide and/or a reduced graphene oxide
  • the cathode will typically comprise oxygen doped graphene deposited on Toray paper.
  • Such a catalyst layer will preferably be effective in the catalysis of the oxygen reduction reaction, particularly when exposed to oxygen, most preferably in a fuel cell.
  • the catalysts and catalyst layers of the invention may be used to coat an electrode.
  • Any electrode is suitable, although preferable electrodes include glassy carbon electrodes, graphite felt and carbon fibers.
  • Such an electrode will preferably be effective in generating charge by means of the oxygen reduction reaction, particularly when exposed to oxygen, most particularly in a fuel cell.
  • the catalyst of the invention is capable of catalysing the reduction of oxygen.
  • the reduction of oxygen may be achieved by exposing oxygen to the catalysts, catalyst layers or electrodes of the invention as defined herein.
  • the oxygen may be in any suitable form, such as pure oxygen gas or preferably as an oxygen-containing gas mixture, most preferably air (which may include dry or partially dried or treated air).
  • the catalyst is effective for catalysing the oxygen reduction reaction at a cathode, particularly the cathode of a fuel cell.
  • the method of the invention may therefore also be achieved by supplying oxygen (in any appropriate form) at the cathode of a fuel cell (such as a cathode and fuel cell as herein described).
  • ORR oxygen reduction reaction
  • the catalysts of the invention may catalyse the ORR by any process and may catalyse (or be capable of catalysing) the ORR by a four- electron process.
  • this will optionally be a method for the reduction of oxygen by a four-electron process.
  • the catalysts, catalyst layers electrodes and fuel cells of the present invention may be used in the reduction of oxygen by a four-electron process.
  • the ORR may take place by means of a two-electron process. All aspects of the invention will thus potentially be provided in the form of the four- electron reduction process and/or in the form of the two electron process with GO and/or reduced GO, such as ERGO and/or NRGO. Thus, in the method of the invention, this will optionally be a method for the reduction of oxygen by a two- electron process.
  • the catalysts, catalyst layers electrodes and fuel cells of the present invention may be used in the reduction of oxygen by a two- electron process.
  • the present invention provides of the use of any of the oxygen doped graphenes, catalysts, catalyst layers or electrochemical cells in the production of hydrogen peroxide.
  • the invention provides for a method of the production of hydrogen peroxide comprising use of any or the materials or catalysts mentioned herein. Such a method will typically be an electrochemical method.
  • the catalysts, catalyst layers and electrodes of the invention may be incorporated into a fuel cell.
  • a fuel cell is an electrochemical cell which converts a source fuel and an oxidant (typically molecular oxygen) into an electric current.
  • a fuel cell comprises two electrodes, an anode and a cathode, each of which is in contact with at least one electrolyte.
  • the electrolyte may be in solid or liquid form and can further comprise an ion-permeable membrane, such as a proton exchange membrane.
  • the source fuel is supplied to the anode, where it is oxidised to form ions and electrons.
  • the electrolyte allows the passage of ions (or of secondary ions).
  • the electrons are forced to travel through an external circuit, generating the electric current.
  • electrons and ions then recombine, together with another reagent, commonly oxygen, at the cathode to generate a product.
  • the fuel cells of the invention can be any known type of fuel cell in the art.
  • a proton exchange membrane fuel cell is particularly preferred.
  • example fuels which can be supplied to the anode of the fuel cells of the invention include hydrocarbon liquids or gasses, hydrogen gas, sugars, alcohols or mixtures thereof.
  • Preferred fuels include light alcohols (e.g. CI to C5 alcohols) such as ethanol or methanol, light hydrocarbons (such as CI to CIO hydrocarbons), glucose, sucrose, hydrogen gas or mixtures thereof.
  • Most preferred fuels include hydrogen gas and methanol.
  • the fuel cells of the invention may contain at least one fuel.
  • this will be at least one fuel selected from ethanol, methanol, light hydrocarbons (such as CI to CIO hydrocarbons), glucose, sucrose, hydrogen gas or mixtures thereof.
  • Preferred fuels which may be present in the cell include hydrogen gas and methanol.
  • the fuel cells of the invention are suitable for use in a number of end applications, such as stationary or portable power sources, generation of current for electric vehicles, methods for allowing use of alternative fuels, such as in the transport industry, and portable charging for small electronic devices including, for example, phones (or other communication devices), cameras, music, game and/or video players (or other entertainment devices), navigation devices, prosthetic devices and/or portable computers.
  • end applications such as stationary or portable power sources, generation of current for electric vehicles, methods for allowing use of alternative fuels, such as in the transport industry, and portable charging for small electronic devices including, for example, phones (or other communication devices), cameras, music, game and/or video players (or other entertainment devices), navigation devices, prosthetic devices and/or portable computers.
  • the catalysts of the invention can be prepared by any method known in the art.
  • Graphene Oxide was produced using a modified Hummer's process (.Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270)
  • the starting material Graphite powder (product: 78391) had a particle size ⁇ 20 ⁇ .
  • the vertically aligned multilayer graphene nanoflakes (MGNFs), which terminate on a few graphene layers ( ⁇ 2 nm thick edges) using microwave plasma enhanced chemical vapour deposition have been previously reported (Shang N.; Papakonstantinou, P.; etal , Adv. Funct.
  • Oxygen/Nitrogen moieties can be introduced in graphene nanoflakes by treatment in acids and/or plasmas (e.g by treatment in plasmas induced by Electron cyclotron resonance as reported by Iyer, GRS,
  • Figure 1 (a) TEM image of the triple-layered Graphene Oxide (GO) with the corresponding cross-sectional profile (bottom inset), and the SAED pattern (top inset). The arrows indicate the sheet separation (0.48 nm).
  • Figure 2. XRD spectra for pristine graphite and graphene oxide.
  • FIG. 5 TGA and corresponding mass loss rate results for GO and pristine graphite, (b) Comparison of GO and Pt/C mass loss rate.
  • Figure 6. (a). Comparison of GO response in N 2 and 0 2 , 0.1M KOH saturated solution, (b) Comparison of Pt/C response in N 2 and 0 2 saturated solution
  • Figure 7. Comparison of the performance of traditional Pt/C cathode and Graphene oxide catalysts in the presence of methanol.
  • Figure 14 (a) CV curves of oxygen reduction on the GO, ERGO and NRGO electrodes in an 02-saturated 0.1M KOH solution at a scan rate of 100 mV/ s using a mass loading of 0.02mg; ERGOs have been subjected to 7 and 22 reducing cycles, (b) RDE curves for oxygen reduction on the GO, ERGOs, NRGO and Pt/C electrodes in an 02-saturated 0.1 M KOH solution at a scan rate of 10 mV /s.
  • Figure 15 (a) CV curves of oxygen reduction on the GO, ERGO and NRGO electrodes in an 02-saturated 0.1M KOH solution at a scan rate of 100 mV/ s using a mass loading of 0.02mg; ERGOs have been subjected to 7 and 22 reducing cycles, (b) RDE curves for oxygen reduction on the GO, ERGOs, NRGO and Pt/C electrodes in an 02-saturated
  • FIG. 18 (a) CV curves of oxygen reduction on the graphene produced by ionic liquid assisted grinding and ERGO an 0 2 -saturated 0.1M KOH solution at a scan rate of 100 mV/ s using a mass loading of 0.02mg; ERGOs have been subjected to 22 reducing cycles, (b) RDE curves for oxygen reduction on the graphene and ERGO in 0 2 -saturated 0.1 M KOH solution at a scan rate of 10 mV /s. Examples
  • Highly oxidised GO was produced using a modified Hummer's process.
  • Graphite product: 78391 with particle size ⁇ 20 ⁇ was purchased from Fluka. All other chemical and reagents were purchased from Aldrich.
  • a mixture of 2.5 g of Graphite and 1.9 g of NaN0 3 was placed in a flask cooled in an ice bath. 85 mL of H 2 S0 4 was added to the mixture and stirred until homogenized. Solution of 11.25 g of KMn0 4 in distilled water was gradually added to the solution while stirring. After 2 hours, the solution was removed from the ice bath, and further stirred for 5 days. Brown-coloured viscous slurry was obtained.
  • the slurry was added to 500 mL aqueous solution of 5 wt% H 2 S0 4 over 1 hour while being continuously stirred. The mixture was stirred for further 2 hours. After that, 10 ml of H 2 0 2 . (30 wt% aqueous solution) was added to the mixture and stirred for further 2 hours. This mixture was then left to settle overnight. The resultant mixture was filtered and further purified by dispersing in 500 mL aqueous solution of 3 wt% H 2 S0 4 and 0.5 wt% H 2 0 2 . After two days of precipitation, supernatant solution was removed. This process was repeated five times. The solid obtained after the rigorous cleaning process was rinsed using copious amounts of distilled water and dried in oven, as reported in literature. Appropriate amounts of the solid was dispersed in water by
  • TEM Transmission electron microscopy
  • JEOL 21 OOF which has a point resolution of 0.19nm.
  • TEM samples were prepared on Holey carbon-coated Cu 300 mesh grids.
  • GO nanosheets were drop dried under infrared lamp to prepare thin films on Si substrates.
  • Figure 2 shows XRD patterns of GO and graphite, for comparison purposes.
  • the starting pristine graphite, PG exhibits atomically flat pristine graphene sheets with a well-known van der Waals thickness of -0.337 nm, estimated by using Bragg' s equation for the (002) peak located at -26.4°.
  • the increased d-spacing of GO sheets is due to the presence of abundant O- moieties on both sides of the graphene sheet causing an atomic-scale roughness on the graphene sheet.
  • Raman Spectroscopy of GO Raman spectroscopy is a powerful probe of both phonon dispersion and electron phonon coupling and serves as a sensitive probe of the extent of chemical modification of graphene.
  • the GO sample shows a prominent D peak with intensity comparable to G peak in sharp contrast to the small D peak of PG, indicative of significant structural disorder due to the O- incorporation.
  • the sharp increase in the D peak suggesting increase in the in-plane disorder, leads to an increase in ID/IG ratio (from -0.26 for PG to 0.93 for GO) and hence a decrease in in-plane crystal or domain size from -17 nm (PG) to -4.7 nm (GO).
  • Steep decrease in intensity and broadening of the D' peak for GO means loss of 3-D order along the crystallographic axis after oxidation.
  • the G peak of GO is shifted to higher wavenumbers with respect to that of graphite. Similar upward shifting of the G band has been observed in heavily oxidised carbon nanotubes and is most likely related to the emergence of a Raman active band overlapped with G band. This band becomes active due to phonon confinement caused by the defects.
  • Figure 5a shows the residual mass percentage and differential curves as a function of temperature of pristine graphite as GO, obtained from TGA analysis.
  • the GO starts to lose mass upon heating even below 100 °C, which is associated with elimination of loosely bound or adsorbed water and gas molecules.
  • First major mass loss can be observed along with an exothermic signal of mass loss rate (dW/dT, I ) around 200 °C, presumably due to reduction of the edge-plane O-moieties, yielding CO, C0 2 , and steam as by-products of the reduction process.
  • the GO sample is compared with Pt/CB which showed a residual Pt of 21.65%
  • Figure 8 compares the specific current response for GO and Pt/CB in the presence of 2M methanol introduced after 1000s of the initial scan time. Due to the oxidation of methanol a sudden jump in the current is observed upon introduction of methanol in Pt/C. The behaviour of GO was observed to be fairly independent of the presence of methanol. It exhibited a small initial (negative) increase in current and quickly regained its stable performance.
  • the graphene oxide was synthesized from natural graphite powder based on the Hummers method.
  • the electrochemical reduction of GO experiment was carried out in N 2 saturated 0.1 M Na 2 S0 4 in the potential range 0 to -1.5 V (vs. Ag/AgCl) at a scan rate of 50 mV/s for up to 40 cycles to obtain ERGO/GCE.
  • Alternative electrochemical reduction procedures are also effective on reducing GO (Peng, X. Y.; Liu, X. X.; Diamond, D.; Lau, K. T.; " Synthesis of electrochemically-reduced graphene oxide film with controllable size and thickness and its use in supercapacitor", Carbon, 2011, 49. 3488-3496) .
  • Nitrogen doped Reduced Graphene Oxide prepared by a hydrothermal approach was used for comparison purposes. Briefly 50 mg of Graphite oxide was dispersed in 40ml of DI water and exfoliated by ultrasoni cation for 60min. In a typical the GO solution was mixed with 50 ⁇ 1 of ammonia (28% wt in H 2 0, Sigma Aldrich) and 700 ⁇ 1 of hydrazine (35% wt in H 2 0, Sigma Aldrich). The weight ratio of hydrazine to GO was 50: 10. The mixture was stirred for 15min and transferred to 200ml of Teflon lined vessel for hydrothermal reaction at 160C for three hours. The resultant product was cleaned with DI water and NrGO was collected by
  • the preparation method of the working electrodes containing investigated catalysts is as follows. Glassy carbon electrode (GCE) was polished with alumina powders, then rinsed thoroughly, and finally dried with blowing N 2 . In short, 5 mg of catalyst powder was dispersed in 1 ml of DMF mixed solvent with 50 ⁇ of Nafion solution (5 wt%, Sigma- Aldrich), then the mixture was ultrasonicated for at least 60min to generate a homogeneous ink. Next, a 4 ⁇ .
  • the electrolyte (0.1 M KOH) was degassed by bubbling oxygen for 30 min.
  • the polarization curves were obtained by sweeping the potential from 0.2 to -1 V vs. Ag/AgCl at room temperature and 1600 rpm, with a sweep rate of 10 mV/ s. All the data were recorded after applying a number of potential sweeps until which were stable.
  • the electrochemical reduction experiment was carried out in N 2 saturated 0.1 M Na 2 S0 4 in the potential range 0 to -1.5 V (vs. Ag/AgCl) at a scan rate of 50 mV/s for up to 40 cycles to obtain ERGO/GCE.
  • the cyclic voltammograms ( Figure 10) of a GO modified glassy carbon electrode (GCE) in a potential range from 0.0 to -1.5 V shows a large cathodic current peak at 1.5 V with a starting potential of -0.9 V.
  • This large reduction current should be due to the reduction of the surface oxygen groups since the reduction of water to hydrogen occurs at more negative potentials (e.g., 1.5 V).
  • the reduction current at negative potentials decreases considerably and almost disappears after -20 potential scans. This demonstrates that the reduction of surface- oxygenated species at GO occurs quickly and irreversibly and the exfoliated GO could be reduced electrochemically at negative potentials.
  • graphene nanosheets produced by ionic liquid assisted grinding possess low levels of naturally absorbed oxygen (-3.5 at%), inherited from the starting graphite (Shang, N. et al. "Controllable selective exfoliation of high-quality graphene nanosheets and nanodots by ionic liquid assisted grinding”. Chem. Comm., 48, (2012) 1877-1879). Such low oxygen level is very difficult to be removed completely even with high temperature vacuum annealing at very high temperatures of 1000 C.
  • the level of oxygen in ERGO is slightly more than from exfoliated graphene.
  • the oxygen level in graphene is about ⁇ 3 at%.
  • the CI s spectrum Upon hydrothermal treatment of GO, the CI s spectrum exhibits a transformation from a double peak, characteristic of graphene oxide to a single sharp peak indicative of a trend to restore the sp2 bonding graphene character
  • the wide X ray spectrum of NRGO shows clearly the presence of nitrogen .
  • the N Is peak can be deconvoluted into two sub-peaks at 399.0 and 400.5 eV, which have been assigned to pyridine and pyrrolic N in the film.
  • a doping level of 6.25 at% nitrogen in the NrGO was obtained and the N binding configuration includes 55 % pyridinic N, 45% pyrrolic N.
  • the typical two-step pathway was observed for the GO electrode with onset potentials at around -0.28 and -0.65 V, indicating a successive two-electron reaction pathway, instead of the direct four-electron pathway seen for the commercial Pt/C electrode.
  • the electrocatalytic activities of the GO/GCE and ERGO/GCE electrodes were evaluated by CV and RDE voltammetries.
  • a catalyst with a higher electrocatalytic activity toward the ORR will demonstrate an earlier onset potential and a higher peak current. Accordingly, its RDE voltammogram should also show a sooner current drop with a positively shifted onset potential and a higher steady-state current.

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Abstract

La présente invention concerne un catalyseur pour la réaction de réduction d'oxygène telle qu'à la cathode d'une pile à combustible, d'une batterie métal/air ou dans la génération de peroxyde d'hydrogène. L'invention concerne en particulier des catalyseurs comprenant du graphène dopé à l'oxygène tel que l'oxyde de graphène et/ou l'oxyde de graphène réduit. Cette invention concerne également une couche de catalyseur comprenant un tel catalyseur, une électrode comprenant une telle couche de catalyseur, une pile à combustible comprenant une telle électrode et l'utilisation desdits catalyseurs dans la réduction catalytique d'oxygène à la cathode de piles à combustible.
PCT/GB2012/050403 2011-02-22 2012-02-22 Catalyseur de réaction de réduction d'oxygène Ceased WO2012114108A1 (fr)

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