Patent Application
20210280874
The present invention relates to a fuel cell. In particular, the present invention relates to a proton exchange membrane fuel cell (PEMFC), and to a catalyst and electrolyte combination suitable for use in a proton exchange membrane fuel cell.
A fuel cell is a device that generates electricity via a chemical reaction.
A fuel cell has an anode, a cathode and an electrolyte. The reactions that produce electricity typically take place at the electrodes (i.e. at the anode and cathode). These reactions form electrons (e−) and electrically charged particles (ions). In operation, the electrolyte facilitates the transfer of ions from one electrode to the other, whilst the electrons transfer from one electrode to the other via a connected exterior electrical circuit.
In a proton exchange membrane fuel cell, protons (H+) and electrons (e−) are formed at the anode, the protons transfer through the electrolyte from the anode to the cathode, while the electrons transfer to the cathode via an external electrical circuit. In a proton exchange membrane fuel cell, a membrane located between the anode and the cathode acts to permit the transfer of protons from the anode to the cathode, but impedes the transfer of electrons or anions from the anode to the cathode. Such a membrane causes electrons to flow through the external electrical circuit rather than travelling through the electrolyte.
An example of a reaction that may take place at the cathode of a PEMFC is the oxygen reduction reaction (ORR), wherein oxygen (O2) is reduced to either H2O2 (from [O2]2−) in a 2-electron process or H2O (from 2[O]2−) in a 4-electron process. An example of a reaction that may take place at the anode of a PEMFC is the hydrogen oxidation reaction (HOR), wherein hydrogen (H2) is oxidised to form protons (2H+) and electrons (2e−).
A catalyst is generally needed to catalyse the reactions at the cathode and the anode. The catalysts used typically comprise precious metals such as platinum. The high cost of the catalysts has limited the application of fuel cells.
As a person skilled in the art will appreciate, developments in fuel cell technology may be made in relation to any of the components of a fuel cell, either alone or in combination with the other components. For example, developments may be made in relation to the anode, the cathode, the electrolyte and/or the membrane of a fuel cell independently or in conjunction with the other components of the fuel cell.
Traditionally, developments in respect of the cathode have been slower and more challenging due to the complex nature of the reactions that typically occur at the cathode. For example, the reduction of oxygen involves many intermediates and is therefore more complex in nature than some other reduction or oxidation reactions. In addition, unfavourable energy barriers typically make the oxygen reduction reaction relatively sluggish compared to the hydrogen oxidation reaction.
It would be advantageous to provide alternative fuel cells. It would also be advantageous if at least preferred embodiments of the present invention were to provide fuel cells that do not require the use of a precious metal catalyst (such as platinum) to catalyse the reduction reaction at the cathode.
In a first aspect, the present invention provides a fuel cell comprising:
The electrolyte enables the movement of protons (or other cations) from the anode to the cathode.
Advantageously, the inventor has found that fuel cells of the present invention can be prepared in which the 4-electron reduction pathway of O2 is favored over the 2-electron reduction pathway of O2, thus forming H2O rather than H2O2. The 4-electron reduction process is desirable from an environmental perspective as it produces water (H2O). Further, H2O2 is reactive and the presence of H2O2 can limit the durability of fuel cells.
In an embodiment, the non-precious metal catalyst is a Fe—N/C catalyst.
In an embodiment, the protic ionic liquid has the formula (A−) (BH+), wherein A− is the conjugate base of acid HA, and BH+ is the conjugate acid of base B, and the difference in pKa between HA and BH+ (ΔpKa) is between about 12 and about 30. In an embodiment, the ΔpKa is between about 15 and about 18. In an embodiment, the base B is an amine. In an embodiment, the base B is of formula (I):
wherein R1, R2 and R3 are each independently selected from the group consisting of hydrogen, optionally substituted alkyl (e.g. C1-6alkyl), optionally substituted alkenyl (e.g. C2-6alkenyl), and optionally substituted alkynyl (e.g. C2-6alkynyl);
and wherein R1, R2 and/or R3 are optionally joined to form a saturated or unsaturated optionally substituted heterocycle, a saturated or unsaturated optionally substituted heterobicycle or a saturated or unsaturated optionally substituted heterotricycle.
In an embodiment, the base B is a trialkyl amine. In an embodiment, the base B is diethylmethylamine.
In an embodiment, the conjugate base A− is the conjugate base of a strong acid. In an embodiment, the conjugate base A− is non-nucleophilic.
In an embodiment, the conjugate base A− is triflate (TfO−).
In an embodiment, the protic ionic liquid is [dema][TfO].
In an embodiment, the Fe—N/C catalyst is derived from reagents including p-phenylenediamine.
In an embodiment, the Fe—N/C catalyst is derived from reagents including ferric chloride.
In an embodiment, the Fe—N/C catalyst is derived from reagents including carbon black.
In an embodiment, the Fe—N/C catalyst is derived from p-phenylenediamine, ferric chloride and carbon black.
In a second aspect, the present invention provides the use of a non-precious metal catalyst in a fuel cell to effect the 4e reduction of O2 to H2O in an electrolyte comprising a protic ionic liquid.
In a third aspect, the present invention provides a method of reducing O2 to H2O comprising contacting O2 and a non-precious metal catalyst in an electrolyte comprising a protic ionic liquid.
In a fourth aspect, the present invention provides the reduction of O2 to H2O using a non-precious metal catalyst in an electrolyte comprising a protic ionic liquid.
In a fifth aspect, the present invention provides an electrode for use in a fuel cell comprising an electrolyte comprising a protic ionic liquid, the electrode having a non-precious metal catalyst on its surface.
In an embodiment of the second, third, fourth or fifth aspects, the non-precious metal catalyst is a Fe—N/C catalyst. In an embodiment, the protic ionic liquid has the formula (A−)(BH+), wherein A is the conjugate base of acid HA, and BH+ is the conjugate acid of base B, and the difference in pKa between HA and BH+ (ΔpKa) is between about 12 and about 30.
The present invention relates to developments concerning the cathode and electrolyte combination for a fuel cell. As a person skilled in the art will appreciate, conventional anodes and membranes (e.g. proton exchange membranes) may be used in the fuel cell of the present invention, provided they are compatible with the cathode and electrolyte combination.
Preferred embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings in which:
In a first aspect, the present invention provides a fuel cell comprising:
The fuel cell comprises an anode and cathode (i.e. electrodes). In use, the electrodes are electrically connected to an external electrical circuit and provide electrical power to the external circuit (i.e. do work). The electrodes may be formed of any material capable of functioning as an electrode. In the fuel cell, the electrode forms an electrical bridge or connection for electrons to move or be transported between the electrolyte and the external electrical circuit. Accordingly, to function as an electrode, the electrode must be able to conduct electricity. Preferably, the electrode is formed of materials which are stable in, and do not react to an appreciable extent with, the electrolyte in contact with the electrode or products formed by the reactions in the fuel cell. Preferably, the electrodes have minimal resistance to maximise efficiency. Typically, the electrodes are made of a metallic or otherwise conductive substrate (such as carbon, platinum, copper, nickel etc.). In embodiments that utilise gaseous components (e.g. hydrogen and/or oxygen), the electrodes typically comprise gas diffusion layers to allow the gases to diffuse to the electrolyte (e.g. hydrogen fuel at the anode and oxygen at the cathode).
In an embodiment, the cathode comprises glassy carbon.
In an embodiment, the anode comprises platinum.
The electrodes may be formed entirely of a given material or may comprise layers of different materials. For example, an anode may be formed of platinum, or may be formed of another metal and have a platinum coating. In embodiments that have a surface coating on the electrode, the surface coating may completely coat the surface of the electrode or may coat only a portion of it. For example, an anode may be completely coated in platinum or may have only a portion of the surface coated in platinum. In some embodiments, the electrode may be formed of a largely electrically non-conductive material with a coating of an electrically conductive material.
The electrolyte enables the movement of protons (or other cations) from the anode to the cathode. A single electrolyte may be in contact with both the anode and the cathode (with a proton exchange membrane in the electrolyte). However, in some embodiments, an electrolyte comprising a protic ionic liquid may be in contact with the non-precious metal catalyst in contact with the cathode, and another electrolyte may be in contact with the anode (and the catalyst on the anode), provided the combination of electrolytes enables the movement of protons (or other cations) from the anode to the cathode.
In some embodiments, the electrolyte comprising a protic ionic liquid in contact with the non-precious metal catalyst in contact with the cathode may comprise components in addition to the protic ionic liquid, such as a solvent (e.g. water). In some embodiments, the electrolyte is incorporated in a solid polymeric matrix comprising a polymeric material (e.g. sulfonated polyimides, polybenzimidazole, polyvinylidenefluoride (PVDF) etc.) to form a solid matrix comprising the electrolyte.
In an embodiment, the electrolyte comprises at least 20% v/v PIL, for example, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 95%, at least about 97%, at least about 98%, at least about 99%, at least about 99.5%, or at least about 99.9% v/v PIL.
An ionic liquid is a salt in the liquid state. Ionic liquids typically have a melting point below about 100° C. Protic ionic liquids are a class of ionic liquids. Protic ionic liquids are ionic liquids formed from an acid and a base, in which a proton is transferred from the acid to the base, thus forming a conjugate base and a conjugate acid.
The low vapour pressure, non-flammability, high thermal and electrochemical stability of PILs make PILs suitable for use in proton exchange membrane fuel cells (PEMFCs). These properties allow PILs to operate at higher temperatures than aqueous-based electrolytes without the need for humidification. Some commonly used proton exchange membranes, such as Nafion, rely on liquid water humidification of the membrane to transport protons. When such membranes are used with aqueous-based electrolytes, especially when operated at a temperature of about 80 to 90° C. (or above), humidification of the fuel gas is commonly used to avoid the membrane dehydrating, adding to the complexity and cost of operating such fuel cells. In some embodiments, the fuel cell of the present invention can be operated at temperatures higher than about 90° C. without humidification.
A protic ionic liquid may be described by the formula (A−)(BH+), wherein A− is the conjugate base of acid HA, and BH+ is the conjugate acid of base B.
In some embodiments, the difference in pKa between HA and BH+ (ΔpKa) is between about 12 and about 30, for example, between about 13 and about 25, between about 15 and about 25, between about 15 and about 20, between about 20 and about 25, between about 17 and about 20, between about 12 and about 18, between about 16 and about 19, between about 17 and about 19.
In an embodiment, the base B is an amine.
In an embodiment, the base B is a tertiary amine.
In an embodiment, the base B is a trialkyl amine, for example, triethylamine, dimethylethylamine, diethylmethylamine.
In an embodiment, the base B is of formula (I):
wherein R1, R2 and R3 are each independently selected from hydrogen, optionally substituted alkyl (e.g. C1-6alkyl), optionally substituted alkenyl (e.g. C2-6alkenyl), and optionally substituted alkynyl (e.g. C2-6alkynyl);
and wherein R1, R2 and/or R3 are optionally joined to form a saturated or unsaturated optionally substituted heterocycle, a saturated or unsaturated optionally substituted heterobicycle or a saturated or unsaturated optionally substituted heterotricycle.
In some embodiments, the base B is an imine. For example, the base B may include 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
In some embodiments, the conjugate base A− is the conjugate base of a strong acid. Strong acids typically have a pKa of less than 0, for example, between about 0 and about −20, between about 0 and about −15, between about 0 and about −10, between about 0 and about −5, between about −1 and about −15, between about −1 and about −10, between about −1 and about −5, between about −2 and about −15, between about −2 and about −10, between about −2 and about −5, between about −3 and about −10, between about −4 and about −10, between about −5 and about −10, between about −6 and about −8, between about −10 and about −20, between about −10 and about −15, between about −5 and about −15 or between about −12 and about −16.
The pKa values of common acids (and conjugate acids of common bases) are widely reported in the literature and are well measured. For example, Evans' pKa table (http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf) and the Bordwell pKa table (http://www.chem.wisc.edu/areas/reich/pkatable/index.htm) provide the pKa values of many common acids (and conjugate acids of common bases).
Examples of strong acids which may be useful for forming the protic ionic liquid for use in a fuel cell of the present invention include triflic acid (CF3SO3H, TfOH) and other fluorinated alkyl sulfonic acids (e.g. F9C4SO3H), methanesulfonic acid (CH3SO3H, MsOH), trifluoroacetic acid (CF3CO2H, TFA) and trifluoromethanesulfonimide ((CF3SO2)2NH, also known as bistriflimide or bis(trifluoromethane)sulfonimide).
Mixtures of two or more acids and/or two or more bases may be used to form the PIL. For example, a PIL may comprise more than one conjugate base (e.g. two or more of TfO−, F9C4SO3−, CF3CO2−, CH3SO3− or (CF3SO2)2N−), and/or more than one conjugate acid (e.g. two or more of triethylammonium, dimethylethylammonium or diethylmethylammonium). Such mixtures may be prepared by mixing two different PILs, or may be made by adding two or more acids with a base, two or more bases with an acid, or two or more acids with two or more bases.
Suitable acid HA and base B pairs may be selected based on their reported or predicted ΔpKa value. In such embodiments, the ΔpKa may be chosen to optimise the open circuit potential (OCP) of the protic ionic liquid derived from the acid HA and base B pair. It has been reported that the OCP may depend on the ΔpKa value of the protic ionic liquid. For example, a protic ionic liquid having ΔpKa value in the range of about 12 to about 16 or about 19 to about 21 would be expected to have an OCP in the range of about 0.7 to about 0.9 V. In contrast, a protic ionic liquid having ΔpKa value in the range of about 16 to about 19 would be expected to have an OCP in the range of about 0.9 to about 1.1 V. Accordingly, in some embodiments, the protic ionic liquid has an OCP in the range of about 0.7 to about 1.1 V, for example, between about 0.9 to about 1.1, 1.0 to about 1.1 V.
In some embodiments, the conjugate base A− is non-nucleophilic. A non-nucleophilic conjugate base is a conjugate base that is substantially non-nucleophilic (i.e. a poor nucleophile or compound that does not act as a nucleophile to an appreciable extent). Without wishing to be bound by theory, it is believed that a non-nucleophilic conjugate base A− is less likely to bind to a nucleophilic site of the non-precious metal catalyst (which may also be an active site of the catalyst). An example of a non-nucleophilic conjugate base is triflate (TfO−), which is usually obtained from triflic acid (TfOH). Other non-nucleophilic conjugate bases include sterically encumbered conjugate bases.
In an embodiment, the protic ionic liquid is diethylmethylammonium trifluoromethanesulfonate [dema][TfO]. Diethylmethylammonium trifluoromethanesulfonate [dema][TfO] exhibits a high and stable open circuit potential (OCP) (1.03 V) at 150° C. which is better than anhydrous phosphoric acid under same conditions. The better electrochemical performance of [dema][TfO] may be attributed to its intermediate N—H bond strength and ΔpKa. Diethylmethylammonium bis(trifluoromethylsulfonyl)imide, [dema][NTf2], which has similar bulk properties as [dema][TfO], exhibits lower OCP (0.70 V) than [dema][TfO] under same conditions, likely due to the relatively slower kinetics of ORR and HOR. The poor electrochemical activity for ORR and HOR of [dema][NTf2] is likely due to lower proton activity resulting from a theoretically stronger N—H bond. Due to the enhanced kinetics of ORR and HOR, [dema][TfO] is a suitable electrolyte for non-humidified fuel cells.
The protic ionic liquid is in a liquid state when the fuel cell of the present invention is in use. As a person skilled in the art will appreciate, a fuel cell may be operated at various temperatures and protic ionic liquids may solidify at lower temperatures. The protic ionic liquid is a liquid at the operating temperature of the fuel cell. Typically, the protic ionic liquid has a melting point below about 100° C. (e.g. below about 80° C., below about 50° C. or below about 20° C.).
A proton exchange membrane fuel cell comprises a membrane disposed between the anode and the cathode. The membrane acts to permit protons (or other cations) to pass or conduct through the membrane while inhibiting or impeding the passage or conduction of electrons. This acts to inhibit or impede the direct passage of electrons through the electrolyte between the anode and cathode; instead they must pass through an external electrical circuit. The membrane typically also prevents the fuel and oxygen from passing through, maintaining the separation of the fuel and oxygen in the fuel cell. Alternatively, an additional membrane may be used for this purpose.
The membrane may, for example, be formed of a proton conducting polymer, such as Nafion (a sulfonated tetrafluoroethylene based fluoropolymer-copolymer).
A variety of proton exchange membranes are commercially available and may be used in the fuel cell of the present invention. Such membranes include Nafion membranes produced by DuPont (e.g. Nafion HP, Nafion 211, Nafion XL, Nafion 212, Nafion NE1035, Nafion 115, Nafion 117, Nafion 1110, Nafion N2100TX, Aciplex (produced by Asahi Chemical Company), Flemion (produced by Asahi Chemical Company), BAM (produced by Ballard Advanced Materials Corporation), and SEBS (Dais Analytic Corporation). In addition, membranes comprising polyvinylidenefluoride (PVDF) and its derivatives may also be used.
The fuel cell of the present invention comprises a non-precious metal catalyst in contact with the cathode.
As used herein, the term “precious metal catalyst” refers to a catalyst comprising a precious metal (Pt, Pd, Ag or Rh), and the term “non-precious metal catalyst” refers to a catalyst that does not comprise a precious metal (Pt, Pd, Ag or Rh) or does not comprise a precious metal (Pt, Pd, Ag or Rh) in any appreciable amount. The “non-precious metal catalyst” in contact with the cathode typically comprises less than 0.05 mg/cm2 of a precious metal (Pt, Pd, Ag or Rh).
The inventors have found that fuel cells can be prepared comprising a protic ionic liquid electrolyte and a non-precious metal catalyst in contact with the cathode. A limiting factor for many potential applications of fuel cells is the cost of the precious metal catalysts commonly used in fuel cells, and therefore it is advantageous to provide a fuel cell in which at least the catalyst in contact with the cathode is a non-precious metal catalyst.
The non-precious metal catalyst is capable of catalysing the reduction of O2 in the electrolyte comprising the protic ionic liquid. Preferably the catalyst is capable of catalysing the 4e reduction of oxygen to water in the electrolyte comprising the protic ionic liquid.
The inventor has found that by combining a non-precious metal catalyst on the cathode with an electrolyte comprising a PIL in contact with the non-precious metal catalyst, it is possible to prepare a fuel cell comprising a non-precious metal catalyst in contact with the cathode, in which the non-precious metal catalyst is capable of catalyzing the 4e− reduction of oxygen to water.
To the best of the inventor's knowledge, the 4-electron reduction of O2 in a protic ionic liquid has previously only been achieved using platinum catalysts. The inventor has now surprisingly found that a non-precious metal catalyst can be used to catalyse the oxygen reduction reaction in a PIL. The inventor has also surprisingly found that the combination of a non-precious metal catalyst and a PIL may be used to effect the 4e− reduction of O2 to H2O (via 2[O]2−) at a low overpotential.
The non-precious metal catalyst is present in the fuel cell to catalyse the reduction of oxygen. In an embodiment, the non-precious metal catalyst catalyses the reduction of oxygen from O2 to [O2]2− (which may form H2O2 upon addition of 2H+). In another embodiment, the non-precious metal catalyst catalyses the reduction of [O2]2− (or H2O2) to 2[O]2− (thus forming H2O upon addition of 2H+). In another embodiment, the non-precious metal catalyst catalyses the reduction of oxygen from O2 to 2[O]2− (thus forming 2H2O upon addition of 4H+).
In some embodiments, the non-precious metal catalyst is on the entire surface (e.g. covers the entire surface) of the cathode. In other embodiments, the non-precious metal catalyst is on just a portion (e.g. covers only a portion) of the surface of the cathode.
The catalyst may be brought into contact with the electrode by any means that results in the catalyst being in contact with the electrode. The catalyst may be formed on the surface of the electrode. More typically, the catalyst is pre-formed and then applied to the surface of the electrode. A composition comprising the catalyst in a solvent and/or a binder may be used to assist with forming a layer of the catalyst on the electrode.
The non-precious metal catalyst in contact with the cathode may be a Fe—N/C catalyst.
Accordingly, in an aspect, the present invention provides a fuel cell comprising:
Fe—N/C catalysts may be made using various known methods. Fe—N/C catalysts can also be made using a variety of different conditions and starting materials, which can give rise to Fe—N/C catalysts with differing properties (e.g. different tolerances to different PILs, temperature, catalytic turnover etc.).
Starting materials for making Fe—N/C catalysts typically include an iron source, a nitrogen source and a carbon source. For example, iron sources that may be used for making Fe—N/C catalysts include iron (II) or iron (III) salts such as FeCl3, Fe(NO3)3, Fe(acetylacetate)3 and FeSO4. Nitrogen sources that may be used for making Fe—N/C catalysts include p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, aniline and 2,4,6-tris(2-pyridyl)-s-triazine. Carbon sources that may be used for making Fe—N/C catalysts include amorphous carbon (e.g. carbon black). Other carbon sources may also be used.
The Fe—N/C catalyst is typically prepared by pyrolysing the starting materials. The temperature at which the pyrolysis reaction takes place can vary significantly depending on the starting materials that are to be employed in making the Fe—N/C catalyst. A person skilled in the art will be able to determine a suitable temperature at which to perform the pyrolysis reaction. The pyrolysis reaction typically takes place at above about 200° C., for example, between about 200 and about 2000° C., between about 200 and about 1500° C., between about 300 and about 1000° C., between about 500 and about 1000° C. or between about 800 and about 1000° C.
In some embodiments, the Fe—N/C catalyst is prepared by pyrolysing:
In an embodiment, the Fe—N/C catalyst is derived from p-phenylenediamine, ferric chloride and carbon black. In an embodiment, the Fe—N/C catalyst is prepared by pyrolysing p-phenylenediamine, ferric chloride and carbon black.
In some embodiments, the Fe—N/C catalyst has nanostructured Fe—N/C particles which are relatively uniform and well dispersed. In some embodiments, the Fe—N/C catalyst has a high proportion of pyridinic N and graphitic N sites compared to Fe3C and FeS sites. A person skilled in the art will be able to determine the proportion of such sites using known techniques. For example, the pyridinic N and graphitic N sites in an Fe—N/C catalyst may be identified and quantified using a combination of different techniques (e.g. XRD, scanning transmission electron microscopy (STEM), high resolution transmission electron microscopy (HRTEM) and XPS). FeS and Fe3C can be detected by using XRD and STEM/HRTEM. Peak intensities and peak positions in the XPS spectrum respectively provide information about the quantity of sites as well as elemental/chemical composition.
Compared to some Pt/C catalysts, the Fe—N/C catalyst may be easily produced and/or produced at lower cost, thus making it attractive for large scale commercial applications.
In operation, the fuel (e.g. H2 gas) is brought into contact with the anode, whilst oxygen (e.g. from air) is brought into contact with the cathode (e.g. by dissolution in the electrolyte).
At the anode, a chemical reaction oxidises the fuel to form protons and electrons. Typically a catalyst is in contact with the anode to catalyse this reaction. The catalyst in contact with the anode may be any catalyst capable of catalysing the reaction and may be a precious metal catalyst (e.g. a Pt catalyst) or a non-precious metal catalyst. The electrolyte in contact with the anode enables the movement of protons from the anode to the cathode.
At the cathode, the oxygen combines with the electrons that have travelled or been conducted though the external electrical circuit and the protons that have travelled or been conducted through the electrolyte from the anode thereby reducing oxygen to either H2O2 or H2O, preferably H2O. The non-precious metal catalyst (e.g. the Fe—N/C catalyst) catalyses this reaction (the reduction of the oxygen). Preferably, the oxygen is reduced to H2O as water is a relatively inert and environmentally safe product. Furthermore, the presence of H2O2 within the fuel cell can reduce the durability of the fuel cell.
Typically multiple fuel cells are combined in series (a fuel cell stack) to increase the voltage generated. Fuel cells may also be combined in parallel to increase the current delivery (i.e. increase the overall current output (Amps)).
Unless otherwise herein defined, the following terms will be understood to have the general meanings which follow. The terms referred to below have the general meanings which follow when the term is used alone and when the term is used in combination with other terms, unless otherwise indicated. Hence, for example, the definition of “alkyl” applies to “alkyl” as well as the “alkyl” portions of “arylC2-6alkyl”, “heteroarylC1-6alkyl” etc.
The term “alkyl” refers to a straight chain or branched chain saturated hydrocarbyl group. Preferred are C1-6alkyl and C1-4alkyl groups. The term “C1-6alkyl” refers to an alkyl group having 1 to 6 carbon atoms. Examples of C1-6alkyl include methyl (Me), ethyl (Et), propyl (Pr), isopropyl (i-Pr), butyl (Bu), isobutyl (i-Bu), sec-butyl (s-Bu), tert-butyl (t-Bu), pentyl, neopentyl, hexyl and the like. Unless the context requires otherwise, the term “alkyl” also encompasses alkyl groups containing one less hydrogen atom such that the group is attached via two positions, i.e. divalent.
The term “alkenyl” refers to a straight chain or branched chain hydrocarbyl group having at least one double bond of either E- or Z-stereochemistry where applicable. Preferred are C2-6alkenyl and C2-4alkenyl groups. The term “C2-6alkenyl” refers to an alkenyl group having 2 to 6 carbon atoms. Examples of C2-6alkenyl include vinyl, 1-propenyl, 1- and 2-butenyl and 2-methyl-2-propenyl. Unless the context requires otherwise, the term “alkenyl” also encompasses alkenyl groups containing one less hydrogen atom such that the group is attached via two positions, i.e. divalent.
The term “alkynyl” refers to a straight chain or branched chain hydrocarbyl group having at least one triple bond. Preferred are C2-6alkynyl and C2-4alkynyl groups. The term “C2-6alkynyl” refers to an alkynyl group having 2 to 6 carbon atoms. Examples of C2-6alkynyl include ethynyl, 1-propynyl, 1- and 2-butynyl, 2-pentynyl, 3-pentynyl, 4-pentynyl, 2-hexynyl, 3-hexynyl, 4-hexynyl and 5-hexynyl and the like. Unless the context indicates otherwise, the term “alkynyl” also encompasses alkynyl groups containing one less hydrogen atom such that the group is attached via two positions, i.e. divalent.
The terms “hydroxy” and “hydroxyl” refer to the group —OH.
The term “alkoxy” refers to an alkyl group as defined above covalently bound via an O linkage, such as methoxy, ethoxy, propoxy, isoproxy, butoxy, tert-butoxy and pentoxy. Preferred are C1-6alkoxy, C1-4alkoxy and C1-3alkoxy groups.
The term “carboxylate” or “carboxyl” refers to the group —COO or —COOH.
The term “ester” refers to a carboxyl group having the hydrogen replaced with, for example, an alkyl group (“alkylester” or “alkylcarbonyl”), an aryl or aralkyl group (“arylester” or “aralkylester”) and so on. CO2C1-3alkyl groups are preferred, such as for example, methylester (—CO2Me), ethylester (—CO2Et) and propylester (—CO2Pr) and reverse esters thereof (e.g. —OC(O)Me, —OC(O)Et and —OC(O)Pr).
The term “amino” refers to the group —NH2.
The term “substituted amino” or “secondary amino” refers to an amino group having a hydrogen replaced with, for example, an alkyl group (“alkylamino”), an aryl or aralkyl group (“arylamino”, “aralkylamino”) and so on. C1-3alkylamino groups are preferred, such as for example, methylamino (—NHMe), ethylamino (—NHEt) and propylamino (—NHPr).
The term “disubstituted amino” or “tertiary amino” refers to an amino group having the two hydrogens replaced with, for example, an alkyl group, which may be the same or different (“di(alkyl)amino”), an aryl and alkyl group (“aryl(alkyl)amino”) and so on. Di(C1-3alkyl)amino groups are preferred, such as, for example, dimethylamino (—NMe2), diethylamino (—NEt2), dipropylamino (—NPr2) and variations thereof (e.g. —N(Me) (Et) and so on).
The term “acyl” or “aldehyde” refers to the group —C(═O)H.
The term “substituted acyl” or “ketone” refers to an acyl group having the hydrogen replaced with, for example, an alkyl group (“alkylacyl” or “alkylketone”), an aryl group (“arylketone”), an aralkyl group (“aralkylketone”) and so on. C1-3alkylacyl groups are preferred.
The term “amido” or “amide” refers to the group —C(O)NH2.
The term “aminoacyl” refers to the group —NHC(O)H.
The term “substituted amido” or “substituted amide” refers to an amido group having a hydrogen replaced with, for example, an alkyl group (“alkylamido” or “alkylamide”), an aryl (“arylamido”), aralkyl group (“aralkylamido”) and so on. C1-3alkylamide groups are preferred, such as, for example, methylamide (—C(O)NHMe), ethylamide (—C(O)NHEt) and propylamide (—C(O)NHPr) and reverse amides thereof (e.g. —NHC(O)Me, —NHC(O)Et and —NHC(O)Pr).
The term “disubstituted amido” or “disubstituted amide” refers to an amido group having the two hydrogens replaced with, for example, an alkyl group (“di(alkyl)amido” or “di(alkyl)amide”), an aralkyl and alkyl group (“alkyl(aralkyl)amido”) and so on. Di(C1-3alkyl)amide groups are preferred, such as, for example, dimethylamide (—C(O)NMe2), diethylamide (—C(O)NEt2) and dipropylamide (—C(O)NPr2) and variations thereof (e.g. —C(O)N(Me)Et and so on) and reverse amides thereof.
Unless otherwise defined, the term “optionally substituted” as used herein indicates a group may or may not be substituted with 1, 2, 3, 4 or more groups, preferably 1, 2 or 3 groups, more preferably 1 or 2 groups, independently selected from the group consisting of alkyl (e.g. C1-6alkyl), alkenyl (e.g. C2-6alkenyl), alkynyl (e.g. C2-6alkynyl), cycloalkyl (e.g. C3-8cycloalkyl), hydroxyl, oxo, alkoxy (e.g. C1-6alkoxy), aryloxy, arylC1-6alkoxy, halo, haloC1-6alkyl (such as —CF3 and —CHF2), haloC1-6alkoxy (such as —OCF3 and —OCHF2), carboxyl, esters, cyano, nitro, amino, substituted amino, disubstituted amino, acyl, ketones, amides, aminoacyl, substituted amides, disubstituted amides, aryl, arylC1-6alkyl, heterocyclylC1-6alkyl, arylC2-6alkenyl, heterocyclylC2-6alkenyl, arylC2-6alkynyl, heterocyclylC2-6alkynyl, heteroarylC1-6alkyl, heteroarylC2-6alkenyl, heteroarylC2-6alkynyl, heterocyclyl and heteroaryl, wherein each alkyl, alkenyl, alkynyl, cycloalkyl, aryl and heterocyclyl and groups containing them may be further optionally substituted. Optional substituents in the case of heterocycles containing N may also include but are not limited to C1-6alkyl i.e. N—C1-6alkyl.
For optionally substituted “alkyl”, “alkenyl” and “alkynyl”, the optional substituent or substituents are preferably selected from amino, substituted amino, disubstituted amino, aryl, halo (e.g. F, Cl, Br, I), heterocyclyl, C1-6alkoxy, hydroxyl, oxo, aryloxy, carboxyl, carboxylate and esters. Each of these optional substituents may also be optionally substituted with any of the optional substituents referred to above.
The present invention is further described below by reference to the following non-limiting Examples.
Diethylmethylamine (97%), trifluoromethanesulfonic acid (TfOH) (98%), p-phenylenediamine (p-PD) (>99%), Nafion solution (5 wt %), potassium chloride (KCl) (>99%) and Pt/C (10 wt % Pt) were purchased from Sigma Aldrich. Conductive carbon black (Super C-65) was purchased from Timcal. Potassium ferricyanide (K3[Fe(CN)6]), ammonium peroxydisulfate (APS) (98%), and hydrogen peroxide (H2O2) (30%) were purchased from Chem Supply. Ferric chloride (FeCl3) (97%) was purchased from Strem Chemicals and hydrochloric acid (HCl) (32%) was purchased from RCI Labscan. Oxygen (high purity) and argon (high purity) were purchased from Air Liquide.
The PIL, [dema][TfO], was synthesized and characterized using the procedure described previously (A. Khan, X. Lu, L. Aldous, C. Zhao, J. Phys. Chem. C 2013, 117, 18334-18342; X. Lu, G. Burrell, F. Separovic, C. Zhao, J. Phys. Chem. B 2012, 116, 9160-9170; C. Zhao, G. Burrell, A. A. J. Torriero, F. Separovic, N. F. Dunlop, D. R. MacFarlane, A. M. Bond, J. Phys. Chem. B 2008, 112, 6923-6936).
Briefly, equimolar amounts (1:1 mol/mol) of Brønsted acid and base were added dropwise simultaneously while stirring vigorously to dissipate the heat of the exothermic reaction. Prior to each experiment, [dema][TfO] was further dried at 80° C. under vacuum for 48 hours. After drying, Karl Fischer titrations were performed using a Metrohm 831 KF coulometer (Herisau, Switzerland) to confirm the water content in the PILs were below 200 ppm. The viscosity and density of [dema][TfO] were recorded using Anton Paar Lovis 2000 ME Microviscometer (MEP Instruments Pty Ltd, Australia) at a temperature of 25° C.
The Fe—N/C based catalyst was synthesized by pyrolysing polymerised p-phenylenediamine, ferric chloride and carbon black (Y. Zhu, B. Zhang, X. Liu, D.-W. Wang, D. S. Su, Angew. Chem. Int. Ed. 2014, 53, 10673-10677).
Briefly, acid treated carbon black was dispersed in p-phenylenediamine in 0.5 M HCl followed by slow addition of aqueous solution of ammonium peroxydisulfate while keeping the temperature below 10° C. An aqueous solution of FeCl3 (1:4 molar ratio of FeCl3 to phenylenediamine) was slowly added to the suspension. The mixture was continuously stirred for 24 hours after which the solvent was removed using a rotary evaporator. The precursor was heat-treated at 900° C. under argon atmosphere for 1 hour, pre-leached with 1 M HCl at 80° C. for 8 hours to remove unstable and inactive species and thoroughly rinsed with ultra-pure water. Finally, the catalyst was again heat-treated at 900° C. under argon atmosphere for 3 hours to obtain the Fe—N/C catalyst.
Scanning electron microscope (SEM) images of Fe—N/C were recorded using FEI Nova NanoSEM 230 with a Bruker Energy Dispersive X-Ray (EDX) system operated on 10 kV accelerating voltage. Transmission electron microscopy (TEM) was performed on a Philips CM 200 at 200 kV. The sample was prepared by drop casting ethanol suspensions onto copper grids. The XRD samples were prepared by drop casting the ethanol suspension onto the glass substrates while XPS measurements were carried out on a Thermo ESCALAB250i X-ray Photoelectron Spectrometer employing a monochromatic Al K α x-ray source (energy 1,486.68 eV).
All the electrochemical measurements were performed on a CHI 760D bipotentiostat (CH Instruments) in a custom built electrochemical cell. For all electrochemical experiments, a rotating ring disk electrode (RRDE) with a glassy carbon (GC) disk (0.125 cm2) and Pt ring (0.188 cm2) (ALS Co., Ltd, Japan) was employed as working electrode. A constant flow of either oxygen or argon was maintained over the solution through a gas bubbler providing constant overpressure to prevent atmospheric contamination. The collection efficiency of the RRDE electrode was 41% (estimated using 5 mM K3[Fe(CN)6] in 1.0 M KCl solution). Pt wire and Ag wire were employed as counter and quasi reference electrodes, respectively. The Ag QRE was found to be relatively stable.
A Fe—N/C catalyst “ink” was prepared by dispersing 9 mg of catalyst in isopropanol/water (0.2 mL, 3:1 v/v) followed by addition of Nafion (20 μL). The suspension was then sonicated for 30 minutes to get a homogenous “ink” of the catalyst. 5 μL of “ink” was drop casted to the GC disk and left to dry in air resulting in a catalyst loading of 1.8 mg cm2. In case of Pt/C and carbon black (after treatment with 6 M HCl for 24 hours to remove any impurities), the catalyst loading was 1.0 mg cm2. All rotating disk electrode (RDE) and RRDE voltammograms were 95% ohmic drop (iR drop) compensated using same potentiostat while cyclic voltammetric and amperometric measurements were recorded without any iR compensation. For RDE measurements, the disk electrode was scanned cathodically at a scan rate of 10 mV s at varying rotation rate from 100-1600 rpm. The number of electrons (n) was calculated from the slopes of the linear fits of Koutecky-Levich plots according to the Koutecky-Levich equation:
where j is the measured current density, jL and jK are diffusion limiting and kinetics current densities, n is the number of electrons transferred, w is the angular velocity of the disk (ω=2πf, f is the linear rotation speed), F is Faraday constant (96,485 C mol−1), ν is the kinematic viscosity of [dema][TfO] (0.225 cm2 s−1), D is the diffusion coefficient of O2 in [dema][TfO] (1.1±0.2×10−5 cm2 s−2), c is the concentration of O2 in [dema][TfO] (1.79±0.04×10−3 mol/L).
From the RRDE measurements, the number of electrons transferred and the percentage yield of H2O2 produced at the disk electrode during the ORR are calculated according to the following equations:
where iD is disk current, iR is ring current and N is the current collection efficiency of the Pt ring (0.41 determined from the reduction of K3[Fe(CN)6].
In this example, a Fe—N/C based catalyst was prepared and its catalytic activity in the oxygen reduction reaction (ORR) in a PIL ([dema][TfO]) was investigated and compared with a commercially available Pt/C catalyst.
To the best of the inventor's knowledge, this work represents the first non-precious metal based catalyst capable of catalysing the 4e− reduction of oxygen to water in a protic ionic liquid.
The inventor surprisingly found that Fe—N/C can catalyse the 4e− reduction of oxygen to water in [dema][TfO] with high catalytic activity—approaching that of Pt/C, and displayed a durability that was higher than Pt-based catalysts. These results demonstrate that such a system may be capable of reducing the cost associated with Pt-based catalysts and humidification of aqueous electrolytes, and may thus assist in the future large scale implementation of fuel cells based on this technology.
The PIL, [dema][TfO], was synthesized and characterized using the procedure described in Example 1. The Fe—N/C catalyst was prepared by pyrolysing p-phenylenediamine, ferric chloride and carbon black as described in Example 1. The SEM image (
In developing the present system, it was useful to determine the nature of the active sites of the Fe—N/C catalyst for improving the ORR electrocatalytic activity of the catalyst. In this regard, Fe3C and FeS sites are not known to be active sites for the ORR and are possibly removed during the acid leaching process (due to their poor stability in acid). In forming the Fe—N/C catalyst, pyrolysis at high temperature induces decomposition of polymerised p-phenylenediamine in the precursors containing P-pPD- or P-mPD-coated carbon and FeCl3 that leads to rearrangement of Fe, nitrogen and carbon to form FeN complexes bonded into carbon support. It is expected that the electrochemical activity of Fe—N/C is due to presence of Fe—N complexes in the catalyst.
More information concerning the chemical state of doped Fe—N/C catalyst was acquired by XPS. The XPS survey scan,
From the detailed S 2p spectrum (
The electrocatalytic activity of Fe—N/C was further studied using RDE voltammetry at rotation speeds ranging from 100 rpm to 1600 rpm in O2 saturated [dema][TfO],
The number of electrons transferred and the electron transfer mechanism for Fe—N/C and Pt/C were further investigated by the RDE measurements according to Koutecky-Levich equation.
To confirm that H2O is a major product of the ORR, RRDE measurements were carried out to quantify the yield of H2O2 during the ORR. To find out the potential where the oxidation of H2O2 in [dema][TfO] occurs, a CV in [dema][TfO] containing 50 mM H2O2 was carried out at a Pt electrode.
The stability of Fe—N/C in [dema][TfO] was investigated via a chronoamperometric method.
Tolerance to methanol is a notable factor when evaluating the performance of an ORR catalyst due to the poisoning of catalyst by methanol crossover. Therefore, the stability of Fe—N/C and Pt/C in the presence of methanol in O2 saturated [dema][TfO] was also examined.
These results indicate that the Fe—N/C catalyst has a significantly improved stability and methanol tolerance than the commercial Pt/C for ORR in [dema][TfO].
The results presented above in Example 2 demonstrate that a non-precious metal catalyst in contact with a cathode can effectively reduce oxygen to H2O via a 4 e reduction in a protic ionic liquid. Accordingly, and as will be apparent to a person skilled in the art, these results demonstrate that such a system can be used in a fuel cell comprising a protic ionic liquid.
It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.
In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016903329 | Aug 2016 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2017/050891 | 8/22/2017 | WO | 00 |
