Transition metal phosphides for high efficient and long cycle life metal-air batteries

Abstract

An electrochemical cell and method of use, including an anode of metal, an air permeable cathode, an electrolyte between the anode and the cathode, and a transition metal phosphide catalyst on the cathode or between the cathode and the electrolyte. Also, a method of generating electrical current with an electrochemical cell by introducing a transition metal phosphide catalyst on a cathode side of the electrochemical cell. The catalyst can be in the form of any suitable nanostructure, such as molybdenum phosphide nanoflakes.

Claims

1. An electrochemical cell, comprising: an anode comprising metal; a cathode comprising an air flow; an electrolyte disposed between the anode and the cathode, the electrolyte configured for oxygen reduction reaction and oxygen evolution reaction and comprising a combination of an ionic liquid and dimethyl sulfoxide (DMSO); and a catalyst in combination with the cathode, wherein the catalyst comprises a nanosized tri-molybdenum phosphide catalyst selected from nanoparticles, nanoflakes, nanosheets, nanoribbons, and combinations thereof.

2. The electrochemical cell according to claim 1, wherein the anode consists essentially of the metal.

3. The electrochemical cell according to claim 2, wherein the metal of the anode is selected from lithium, sodium, potassium, calcium, magnesium, zinc, and aluminum.

4. The electrochemical cell according to claim 1, wherein the metal of the anode is lithium.

5. The electrochemical cell according to claim 1, wherein the cathode is coated with the catalyst.

6. The electrochemical cell according to claim 1, wherein the cathode comprises an air-permeable porous structure.

7. The electrochemical cell according to claim 6, wherein the porous structure is electrically-conductive.

8. The electrochemical cell according to claim 1, wherein the catalyst is disposed between the cathode and the electrolyte.

9. The electrochemical cell according to claim 1, further comprising a reference electrode disposed in contact with the electrolyte.

10. The electrochemical cell according to claim 1, wherein the catalyst comprises Mo.sub.3P.

11. The electrochemical cell according to claim 1, wherein the ionic liquid includes an anion and a cation selected from imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium, choline, sulfonium, prolinate or methioninate cations.

12. The electrochemical cell of claim 11, wherein the cation comprises imidazolium of the formula: ##STR00002## wherein R.sub.2 is hydrogen, and each of R.sub.1 and R.sub.3 is independently a linear or branched C.sub.1-C.sub.4 alkyl.

13. The electrochemical cell of claim 11, wherein the anion is selected from the group consisting of C.sub.1-C.sub.6 alkylsulfate, tosylate, methanesulfonate, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, tetrafluoroborate, triflate, halide, carbamate, sulfamate, and combinations thereof, wherein the electrolyte is substantially free of water.

14. The electrochemical cell according to claim 11, wherein the ionic liquid comprises 1-ethyl-3-methylimidazolium tetrafluoroborate.

15. A method of generating an electrical potential, comprising: providing an electrochemical cell according to claim 1; contacting the cathode to oxygen; allowing the metal of the anode to be oxidized to metal ions; and allowing the oxygen to be reduced at a surface of the transition metal dichalcogenide to form one or more metal oxides with the metal ions, thereby generating the electrical potential between the anode and the cathode.

16. The electrochemical cell according to claim 1, wherein the catalyst improves formation of superoxides in the electrochemical cell over peroxides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a cross-section representation of a Na-air (O.sub.2) battery according to one embodiment of this invention.

(2) FIG. 2 shows a partial sectional representation of a Na-air (O.sub.2) battery according to one embodiment of this invention.

(3) FIGS. 3A-F summarize test results according to embodiments of this invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present invention provides energy storage systems incorporating transition metal catalysts, such as nanostructured transition metal phosphide catalysts (TMPs). The catalysts of this invention provide improved electrocatalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are two basic reactions during battery discharge and charge processes, respectively.

(5) FIGS. 1 and 2 illustrate incorporation of the catalyst into battery systems according to embodiments of this invention. FIG. 1 shows a cross-section of a Na-air (O.sub.2) electrochemical cell 20, such as a metal-air battery, according to one embodiment of this invention. The cell 20 includes an anode 22, a cathode 24, and an electrolyte 26 disposed between the anode 22 and the cathode 24. The cathode provides an air flow 28, and is desirable formed of an air-permeable porous material, such as a carbon material. The porous material can also be electrically conductive. As shown in FIG. 1, the cell 20 can also include an optional reference electrode 25 in contact with the electrolyte.

(6) The invention provides a catalyst, such as working with electrolytes as a co-catalyst system. As shown in FIG. 1, the catalyst 30 is disposed on the cathode side, such as between the cathode 24 and the electrolyte 26. In embodiments of this invention, the catalyst is coated on the cathode 24, such as coated on the porous structure. The catalyst 30 is disposed between the sodium-based electrolyte 26 and the air-permeable porous structure 24, which allows oxygen to contact the catalyst 30 and the electrolyte 26.

(7) FIG. 2 is an exemplary schematic of a continuous air flow Na-air battery cell 40 according to one embodiment of this invention. The cell includes an anode 42, a cathode 44, and electrode 46. The cell 40 also includes an anode current collector 52, a cathode current collector 54, a cell guard membrane 56, and an anode protection layer 58 In this embodiment the air electrode (cathode) 44 is porous for air stream 48 and coated with the nanostructured catalyst of this invention. Various and alternative sizes (1 to 1000 nm), amounts, shapes, and configurations are available for the battery, the electrodes, and the catalyst material, depending on need.

(8) Anodes of embodiments of this invention are formed of metal, and desirable consisting essentially of metal (meaning fully metal with only minor, insignificant other components/impurities). Exemplary metals include lithium, sodium, potassium, calcium, magnesium, zinc, and aluminum.

(9) Electrolytes of embodiments of this invention include any suitable salt, such as corresponding to the anode metal. For example, lithium salts are used to increase the lithium ion conductivity in the electrolyte, this is the case for other anode metals such as sodium, calcium, magnesium, zinc, and aluminum. The electrolyte can also include redox mediators, namely chemicals with electrochemical activity used to improve the activity of the reduction and oxidation reactions happening on the catalyst surface.

(10) Electrolytes of embodiments of this invention include any suitable ionic liquid. Exemplary ionic liquids include an anion and a cation selected from imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium, choline, sulfonium, prolinate or methioninate cations. As a further example, an exemplary imdazolium cation is of the formula:

(11) ##STR00001##
where each of R.sub.1, R.sub.2, and R.sub.3 is independently one of hydrogen, linear aliphatic C.sub.1-C.sub.6 group, branched aliphatic C.sub.1-C.sub.6 group, or cyclic aliphatic C.sub.1-C.sub.6 group. In one embodiment of this invention R.sub.2 is hydrogen, and each of R.sub.1 and R.sub.3 is independently a linear or branched C.sub.1-C.sub.4 alkyl. Exemplary anions include C.sub.1-C.sub.6 alkylsulfate, tosylate, methanesulfonate, bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, tetrafluoroborate, triflate, halide, carbamate, sulfamate, and combinations thereof. In one embodiment, the ionic liquid includes 1-ethyl-3-methylimidazolium tetrafluoroborate. The electrolyte desirably includes at least 90% of the ionic liquid, and preferably is substantially free of water or non-ionic liquid organic solvents.

(12) Catalysts of embodiments of this invention include a transition metal catalyst, and desirably a tri-transition metal catalyst. Exemplary catalysts include transition metal phosphide catalysts, such as, without limitation, Ti.sub.nP.sub.m, V.sub.nP.sub.m, Cr.sub.nP.sub.m, Zr.sub.nP.sub.m, Nb.sub.nP.sub.m, Mo.sub.nP.sub.m, Hf.sub.nP.sub.m, W.sub.nP.sub.m, Ta.sub.nP.sub.m, Tc.sub.nP.sub.m, and Re.sub.nP.sub.m, wherein each n and m is independently one of 1, 2, 3, 4 and 5.

(13) In embodiments of this invention, the catalyst comprises a plurality of nanoparticles. The nanoparticles have an average size between about 1 nm and 1000 nm, more between 1 nm and about 400 nm. Exemplary nanoparticle shapes include, without limitation, nanoflakes, nanosheets, nanoribbons, and combinations thereof.

(14) One exemplary catalyst of this invention includes molybdenum (Mo) terminated molybdenum phosphide nanoflakes (MoP NFs). The catalyst can be, without limitation, Mo.sub.3P, MoP, or MoP.sub.2. Experimental data has revealed the turn over frequency (TOF)—per atom activity—of MoP NFs is more than two orders of magnitude higher than noble metal catalysts such as gold (Au) and platinum (Pt) nanoparticles. The performance of this catalyst in sodium-oxygen (Na—O.sub.2) batteries has revealed that using MoP NFs on the cathode side and an ionic liquid/DMSO electrolyte of the cell make the formation of the sodium superoxide (NaO.sub.2) more favorable than sodium peroxide (Na.sub.2O.sub.2). As a result, the cell overpotential of 600 mV can be obtained, which is approximately two times lower than the state of the art existing system in the literature (1000 mV). The formation of the NaO.sub.2 as the product was confirmed by using in-situ differential electrochemical mass spectroscopy (DEMS) where the number of the electron per mole of O.sub.2 was calculated to be 1.07 during the charging process. Preliminary results also revealed that this system can work up to 100 cycles in a pure O.sub.2 environment.

(15) FIGS. 3A-F summarize testing using of a cell according to this invention. The electrolyte was 0.3M LiTFSI dissolved in DMSO:IL (75/25) with redox mediators as additives. The amount of Mo.sub.3P coating is 0.1 mg cm.sup.−2. FIG. 3A shows charge/discharge profiles over 1000 cycles at constant density of 500 mA/g and the constant specific capacity of 500 mAh/g. FIG. 3B shows changes in discharge specific capacity, charge specific capacity (lower dots), and the corresponding coulombic efficiency (upper dots) over 1000 cycles. FIG. 3C shows changes in polarization gap and energy efficiency over 1000 cycles. FIG. 3D shows discharge and charge potential values over 1000 cycles. The graph shows the stable discharge overpotential up to 600 cycles with respect to the electrochemical potential for Li.sub.2O.sub.2 formation, at 2.96V. FIG. 3E shows the polarization gap as a function of current density while the specific capacity is constant at 500 mAh/g. FIG. 3F shows charge/discharge profiles over 300 cycles at a constant density of 500 mA/g and the specific capacity of 1250 mAh/g.

(16) The invention thus provides transition metal catalysts for use in battery systems. The catalysts of this invention provide improved electrocatalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are two basic reactions during battery discharge and charge processes, respectively.

(17) The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.

(18) While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.