WO2018106690A1

WO2018106690A1 - Metal nanofoam synthesis via microwave process

Info

Publication number: WO2018106690A1
Application number: PCT/US2017/064715
Authority: WO
Inventors: Arumugam Manthiram; Karl Joseph KREDER III
Original assignee: Board Of Regents, The University Of Texas System
Priority date: 2016-12-08
Filing date: 2017-12-05
Publication date: 2018-06-14

Abstract

The present disclosure relates to metal nanofoams and methods of making them and to electrodes for use in electrochemical devices and electrochemical devices, such as batteries, ultracapacitors, fuel cells, electrochemical sensors, catalytic substrates, and substrates for gas phase catalysts, containing such metal nanofoams.

Description

METAL NANOFOAM SYNTHESIS VIA MICROWAVE PROCESS

STATEMENT OF GOVERNMENT INTEREST

The present invention was made with government support under Grant Number DE-SC0005397 awarded by the United States Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to metal nanofoams and methods of making them and to electrodes for use in electrochemical devices such as batteries, ultracapacitors, fuel cells, electrochemical sensors, catalytic substrates, and substrates for gas phase catalysts, containing such metal nanofoams.

BACKGROUND

Electrochemical devices function by exchanging energy between an external electrical circuit and a chemical. For instance, during use, a battery produces electrical current, which is simply free electrons moving through a conductor, such as a metal wire, by removing those electrons from a chemical in the battery. This electron removal occurs in a part of the battery referred to as an electrode.

If the battery is rechargeable the chemical, often referred to as an active material, can both lose and gain electrons depending on whether the battery is supplying a current or being charged by an outside current supplied to the battery. Inside the battery the active material also loses and gains charged elements or compounds, called ions, to compensate for the loss or gain of electrons. Typically the active material that loses an electron and an ion when the battery supplies current and is discharged is called the anode active material and the electrode where it is located is called the anode. At the same time the anode loses an electron and an ion, the other electrode in the battery, called the cathode, gains an electron and ion in the cathode active material.

If the battery is rechargeable, the process merely happens in reverse when a current is supplied to the battery to charge it; the cathode active material in the cathode loses an electron and an ion, while the anode active material, at the same time, gains an electron and an ion.

During both discharge and charge, the ions move within the battery through a material called the electrolyte that also contains the type of ions entering and leaving the cathode active material and anode active material. Typically, if the battery is rechargeable, it is named after the electrolyte ion, called the working ion, such as the lithium ion (Li⁺) in lithium ion batteries and the sodium ion (Na⁺) in sodium ion batteries. Alternately, if the working ion, such as Li⁺ or Na⁺ changes valence to 0 to form the corresponding metal at the anode, these batteries are referred to as lithium and sodium batteries, respectively.

If a battery is not rechargeable, then the electron and ion movement process can occur one time only and once the anode active material has lost an electron and an ion, or once the cathode active material has gained an electron and an ion, they cannot gain or lose them again, respectively.

Other electrochemical devices also contain active materials in one or more electrodes. For instance, fuel cells often contain a catalyst anode active material that forms an ion, such as hydrogen ion (H⁺), that travels to the cathode through an electrolyte, and an electron that travels through an external circuit, providing electrical energy, before it recombines with the H⁺ at the cathode. Although most fuel cells only have an active material at the anode, a fuel cell cathode may also contain an active material to catalyze the recombination.

Other electrochemical devices with active materials in one or more electrodes, including, in some devices, a reference electrode, include a variety of electrochemical sensors, such as gas detectors or medical sensors.

Still other such devices include capacitors, such as ultracapacitors (also called supercapacitors) that exhibit pseudocapacitance via a reduction/oxidation (redox) reaction of the active material when the electrode is used as a capacitor plate.

For all of these electrochemical devices, a molecule or ion in either liquid, gas, or solid phase diffuses to an active material in a battery, fuel cell, and catalyst, or to an inactive material in a capacitor or supercapacitor, in either case referred to as the electrode material. Additionally, the electrode material has an electrical pathway by which to conduct electrons out of the electrochemical device, such as a mechanically robust conducting material, for example a metal foil, carbon cloth, referred to as the current collector. The balance between allowing diffusion, while simultaneously maintaining good electrical conductivity in the electrode material presents a design challenge for many electrochemical devices. The challenge of maintaining conductivity can be further exacerbated when nanoscale, semiconducting, or insulating materials are used as the electrode material, which subsequently limits the overall thickness of the given electrode material on the current collector.

To date, attention has focused on very basic design features, such as mixing conducting particles and a binder with the electrode material and casting the mixture onto a metal foil current collector to form the electrode. Moreover, attention has also focused on solving other flagrant problems that result in near or complete failure of the device, such as delamination of active material films from current collectors, or lack of contact between the active material and conductor particles surrounding it due to contraction and expansion of the active material during use of the device. To promote enhanced electrical conductivity and adhesion when using a metal foil, it is frequently necessary to incorporate additional, inactive materials in the electrode, typically mixed with the electrode material, thereby decreasing the porosity and increasing the tortuosity of the electrode, which results in decreased ionic conductivity to the active material.

Alternately, the electrode structure, which determines the electrical conductivity to, ionic conductivity to, and degree of adhesion of active material, may be modified if materials other than metal foils are adopted as the substrate and current collector. Metal nanofoams may provide a robust, high surface area, electrically conducting substrate to which electrode materials can be applied, creating a thick porous electrode, which simultaneously allows diffusion and thus good ionic conductivity while also providing good electrical contact and thus electrical conductivity to the electrode. Thus, it is desirable to have high surface area metal substrates, such as metal foams or metal nanofoams, for use in batteries, electrodes, and capacitors. Unfortunately, the previously-reported syntheses of metal foams have been very tedious and require the use of many synthetic steps. Some reported transition metal nanofoam synthetic methods include dealloying, nanosmelting, and combustion synthesis. The reported transition metal nanofoam synthetic methods have limitations that would hinder their usage in large-scale synthesis for the large-scale applications of transition metal nanofoams in electrodes, batteries, and capacitors.

Specifically, the methods typically require the preparation of a nanoscale template, usually a pearlite structure, which takes a non-trivial amount of time to prepare. There are usually at least three subsequent steps, depending on the specific method, to produce the transition metal nanofoam. Dealloying methods are limited to producing more expensive noble metal frameworks, such as silver and gold. Nanosmelting methods are very simple, but do not create macroporous materials, which are useful in some applications, nor are these materials phase-pure metals, which may impact their function in some applications. The combustion synthesis methods use a multi-step synthetic route using hazardous azide chemistry to produce precursors. Most of the precursors used in the combustion synthesis method were originally investigated for use as contact explosives, further highlighting the limitations of the combustion synthesis method. Finally, the combustion synthesis method precursors are typically combusted in an inert atmosphere at high pressures (300-1000 psi), generating highly toxic gas byproducts.

There have also been prior reports on self-assembled Nickel/Gold/Platinum/Palladium (Ni/Au/Pt/Pd) nanochains via room-temperature chemical methods, but the size of less than 20 nm of the self-assembled Ni/Au/Pt/Pd nanochains is structurally inadequate for most applications.

SUMMARY

The present disclosure relates to metal nanofoams and methods of making them and to electrodes for use in electrochemical devices and electrochemical devices, such as batteries, ultracapacitors, fuel cells, electrochemical sensors, catalytic substrates, and substrates for gas phase catalysts, containing such transition metal nanofoams. The method may include to a single-step microwave-assisted solvothermal (MW-ST) method for synthesizing porous metal materials, such as metal nanofoams, with high surface areas. The metal may be a transition metal or a non-transition metal.

The present disclosure particularly provides a method of forming a metal nanofoam by dissolving a metal salt in a reaction solvent and heating the reaction solvent in a microwave reactor to form a reaction product including a metal nanofoam.

The method may also include the following additional features, which may be combined with one another unless clearly mutually exclusive:

i) the method may further include rinsing the reaction product with a washing solvent

ii) the metal in the metal nanofoam may include scandium, ytterbium, titanium, zirconium, hafnium, vanadium, neodymium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, mercury, germanium, indium, tin, antimony, thallium, lead, bismuth, and any combinations thereof;

iii) the metal in the metal nanofoam may include a transition metal, including cobalt, iron, platinum, palladium, nickel, copper, gold, silver, and any combinations thereof;

iv) the metal nanofoam may include a nickel nanofoam, a copper nanofoam, a silver nanofoam, and any combinations thereof;

v) the metal salt may include a metal ion and a counterion;

vi) the counterion may include acetate, nitrate, sulfate, chloride, flouride, bromide, oxylate, citrate, ammonium, isopropoxide, carbonate, acetylacetonate, phosphate, tetrafluorob orate, hexafluorophosphate, hydroxide, oxide, and any combinations thereof;

vii) the metal salt may include nickel acetate, copper acetate, silver acetate, and any combinations thereof;

viii) the metal salt may include nickel acetate monohydrate, nickel acetate polyhydrate, copper acetate monohydrate, copper acetate polyhydrate, silver acetate monohydrate, silver acetate polyhydrate, and any combinations thereof;

ix) the reaction solution may be heated in the microwave reactor at a power of 800 watts or greater;

x) the reaction solution may be heated to a temperature of 250 °C. or greater; xi) the reaction solution may be heated in the microwave reactor for less than three minutes; xii) the reaction solution may be heated in the microwave reactor for a predetermined period of time after a pressure spike is observed in the microwave reactor;

xiii) the predetermined period of time may be one minute or less;

xiv) the reaction solvent may be an organic solvent;

xv) the metal nanofoam may be heated after formation in a furnace to a temperature of less than 650 °C for less than 48 hours in the presence of an inert gas; xvi) the metal nanofoam may be heated after formation in a furnace to a temperature less than 650 °C for less than 48 hours in the presence of a combination of a reducing gas and an inert gas;

xvii) the washing solvent may be an organic solvent;

xviii) the method may also include drying the metal nanofoam at a temperature of at least 50 °C;

xix) the method may further include adding a substrate onto which the metal nanofoam forms with the reaction solvent in microwave reactor;

xx) the substrate may be a metal foil, a metal microfoam, or a carbon-based cloth;

xix) the method may further include compressing all or part of the metal nanofoam to form a compressed metal foil.

The present disclosure further includes a metal nanofoam. The metal nanofoam includes any metal nanofoam formed using any of the above methods. The metal nanofoam, whether formed using any of the above methods or differently formed, may include pores and a network of fine structural elements fused at joints, wherein the network of fine structural elements comprise a transition metal and are elongated with a width of between 100 nm and 500 nm or generally spherical with a diameter of between 10 nm and 200 nm, and wherein the transition metal nanofoam has a surface area of at least 1.0 m²/g.

The metal nanofoam may have any of the following additional features, which may be combined with one another unless clearly mutually exclusive:

i) the metal may include scandium, ytterbium, titanium, zirconium, hafnium, vanadium, neodymium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, mercury, germanium, indium, tin, antimony, thallium, lead, bismuth, and any combinations thereof;

ii) the metal may be a transition metal, including cobalt, iron, platinum, palladium, nickel, copper, gold, silver, and any combinations thereof;

iii) the metal nanofoam may include nickel nanofoam, copper nanofoam, silver nanofoam, and any combinations thereof.

The disclosure further includes an electrode including any metal nanofoam described above. The disclosure further includes an electrochemical device containing any such electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color.

Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

A more complete and thorough understanding of some embodiments and advantages of the invention may be acquired by referring to the following description taken in conjunction with the accompanying drawings, which are not to scale.

FIG. 1 is a schematic drawing of a cross-section of a coin cell battery containing a metal nanofoam material electrode.

FIG. 2 is a schematic drawing of cross-section of a hydrogen fuel cell containing a metal nanofoam material electrode

FIG. 3 is a schematic drawing of a cross-section of a sensor containing a metal nanofoam material electrode.

FIG. 4 is a schematic drawing of a cross-section of an ultracapacitor containing a metal nanofoam material electrode.

FIG. 5 is a plot of pressure, temperature, and power versus time of the Anton-

Paar microwave reactor during the synthesis of nickel metal nanofoam.

FIG. 6 is a plot of pressure, temperature, and power versus time of the Anton-

Paar microwave reactor during the synthesis of silver metal nanofoam.

FIG. 7 is a plot of pressure, temperature, and power versus time of the Anton-

Paar microwave reactor during the synthesis of copper metal nanofoam. FIG. 8 is a series of photographs of nanofoams. The upper image is a photograph of copper nanofoam. The middle image is a photograph of silver nanofoam. The lower image is a photograph of nickel nanofoam.

FIG. 9 is a series of low magnification and high magnification scanning electron microscope (SEM) micrographs. The upper left panel is a low magnification SEM micrograph of copper nanofoam. The lower left panel is a high magnification SEM micrograph of copper nanofoam. The upper middle panel is a low magnification SEM micrograph of silver nanofoam. The lower middle panel is a high magnification SEM micrograph of silver nanofoam. The upper right panel is a low magnification SEM micrograph of nickel nanofoam. The lower right panel is a high magnification SEM micrograph of nickel nanofoam.

FIG. 10 is a series of SEM micrographs of copper nanofoams formed from different concentrations of copper(II) acetate (Cu(OAc)₂). The upper left panel is a micrograph of copper nanofoam formed at 0.01 M copper(II) acetate. The upper right panel is a micrograph of copper nanofoam formed at 0.02 M copper(II) acetate. The lower left panel is a micrograph of copper nanofoam formed at 0.04 M copper(II) acetate. The lower right panel is a micrograph of copper nanofoam formed at 0.06 M copper(II) acetate.

FIG. 11 is a plot of X-ray diffraction patterns of nickel nanofoam (upper plot), silver nanofoam (middle plot), and copper nanofoam (lower plot).

FIG. 12 is a high-resolution transmission electron microscopy (URTEM) micrograph of copper nanofoam with interplanar spacing in the bulk corresponding to the Cu (111) and (200) planes and at the surface corresponding to the copper(II) oxide (CuO) (100) and (100) planes.

FIG. 13 is a high-resolution transmission electron microscopy (URTEM) micrograph of nickel nanofoam with the interplanar spacings in the bulk corresponding to the Ni (111) and (200) planes and at the surface corresponding to the nickel(II) oxide NiO (111) planes.

FIG. 14 is a high-resolution transmission electron microscopy (URTEM) micrograph of silver nanofoam with interplanar spacing corresponding to the (111) plane. FIG. 15 is a plot of Brunauer, Emmett, and Teller (BET) analyzer surface area of the copper nanofoam, silver nanofoam, and nickel nanofoam compared to those of commercial copper foam, silver foam (prepared by a templating method), nickel- nickel-oxide foam (prepared by combustion synthesis), and nickel foam (prepared by a templating method).

FIG. 16 is a series of SEM micrographs of nickel nanofoam after 5 min of additional heating in the microwave reactor shown at low magnification (left panel) and high magnification (right panel).

FIG. 17 is a series of SEM micrographs of copper nanofoam (left panel), silver nanofoam (middle panel), and the nickel nanofoam of FIG. 16 (right panel) after heating in an argon atmosphere at 600 °C for 4 h.

DETAILED DESCRIPTION

The present disclosure relates to a method for producing a porous metal material, such as a metal nanofoam. The present disclosure further relates to electrodes for use in electrochemical devices and electrochemical devices, such as batteries, ultracapacitors, fuel cells, electrochemical sensors, catalytic substrates, and substrates for gas phase catalysts containing a porous metal material formed using the methods of the present disclosure.

Transition Metal Nanofoams and Methods of Forming Transition Metal Nanofoams

A transition metal nanofoam of the present disclosure may be formed using a

MW-ST process in which a metal salt is dissolved in a reaction solvent to form a reaction solution, which is then heated in a microwave reactor without stirring to form a reaction product. The reaction product may be subsequently rinsed with a washing solvent to provide the metal nanofoam.

The metal nanofoam may include a transition metal, such as a Group 3-12 metal, particularly such as scandium, ytterbium, titanium, zirconium, hafnium, vanadium, neodymium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, or mercury, or a any combinations thereof. The metal nanofoam may also include a non-transition metal, such as a Group 13, 14, or 15 metal, such as germanium, indium, tin, antimony, thallium, lead, or bismuth, or any combinations thereof.

The metal nanofoam may include any combinations of different transition metals or transition metal groups and different non-transition metals or non-transition metal groups, and any combinations of transition metals and transition metal groups and non-transition metals and non-transition metal groups, including any combinations of those listed above.

The metal salt used in forming the metal nanofoam may include a metal salt including a metal ion and a counterion. The metal ion may be a transition metal ion, such as an ion of scandium, ytterbium, titanium, zirconium, hafnium, vanadium, neodymium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, or mercury, or any combinations thereof. The ion may also be a non-transition metal ion, such as an ion of germanium, indium, tin, antimony, thallium, lead, or bismuth, or any combinations thereof.

The metal ion may include any combinations of different transition metal ions or transition metal group ions and different non-transition metal ions or non-transition metal group ions, and any combinations of transition metal ions and transition metal group ions and non-transition metal ions and non-transition metal group ions, including any combinations of those listed above.

Most transition metals and some other metals may exist in multiple valence states. As a result, the metal ion may have any charge corresponding to a valence state in which the metal may exist. The metal salt may include an ion of any particular metal with only one charge, or substantially only one charge, or it may include ions of any particular metal with more than one charge. For example, the metal ion may be all or substantially all nickel(II) ion (Ni²⁺) or all or substantially all nickel(III) ion (Ni³⁺), or it may be a combination of Ni²⁺ and Ni³⁺. Similarly, the metal ion may be all or substantially all copper(II) ion (Cu²⁺) or all or substantially all copper(I) ion (Cu⁺), or it may be a combination of Cu²⁺ and Cu⁺. If the metal is silver, then the metal ion will be all silver ion (Ag⁺) because silver has only one valence state. The metal ion may also change charge during formation of the metal nanofoam, depending on formation conditions, such as available alternative counterions. For example, Ni³⁺ may change to Ni²⁺ if at all possible because the 2+ charge corresponds to a lower energy and thus preferred valence state.

The counterion in the metal salt may include acetate, nitrate, sulfate, chloride, flouride, bromide, oxylate, citrate, ammonium, isopropoxide, carbonate, acetyl acetonate, phosphate, tetrafluorob orate, hexafluorophosphate, hydroxide, or oxide, or any combinations thereof. The same metal ions may be present with more than one type of counterion. For example, Ni²⁺ may be present with a different counterion than Ni³⁺, resulting in a combination of different metal salts used in the formation of the metal nanofoam. Ni²⁺ may also be present with two different counterions, also resulting in a combination of different metal salts used in the formation of the metal nanofoam. In addition, the same counterion may be present with more than one metal ion. For example, acetate may be present with both Ni²⁺ and Cu²⁺, also resulting in a combination of different metal salts used in the formation of the metal nanofoam.

Nickel acetate, copper acetate, and silver acetate, or a combination thereof may be particularly useful transition metal salts. Nickel acetate monohydrate, nickel acetate polyhydrate, copper acetate monohydrate, copper acetate polyhydrate, silver acetate monohydrate, and silver acetate polyhydrate, or a combination thereof may also be particularly useful transition metal salts.

The reaction solvent may be any solvent able to dissolve all of the metal salts that will form the metal nanofoam. For example, the reaction solvent may include an organic solvent, such as a polyethylene glycol (PEG), particularly tetraethylene glycol (TEG), triethylene glygol (tEG), diethylene glycol (DEG), and ethylene glycol (EG), or any combinations thereof. The reaction solvent may be aged over one or more activated molecular sieves prior to use. Dissolution of the metal salts may be facilitated, for example by stirring or ultra- soni cation. Typically the temperature remains at room temperature, or between 20 °C and 40 °C, while the metal salts are being dissolved in the reaction solvent.

The concentration of the metal salt in the reaction solution affects the ultimate metal nanofoam structure, with smaller fine structural elements being formed in the metal nanofoam as metal salt concentration decreases. However, concentrations between 0.01-0.08 M may be suitable to produce a metal nanofoam.

The reaction solution is then heated in a microwave reactor with or without stirring or significant agitation, such as agitation that causes the metal to form a compacted ball instead of a nanofoam or any other agitation or stirring that prevents or disrupts nanofoam formation. The microwave reactor may be configured to heat the reaction solution as fast as possible. For example, the microwave reactor may be operated at a power of 700 watts or greater and the reaction solution may be heated to a temperature of 250 °C or greater, or 300 °C or greater. The microwave reactor may be operated for a predetermined period of time, such as less than three minutes, or the pressure in the microwave reactor may be monitored and the microwave reactor may be turned off a predetermined period of time, such as one minute or less, after a pressure spike is observed. The pressure spike may be a pressure increase of at least 10% in a time period of less then 1 minute. The pressure spike may correspond with a power spike, such as a spike of at least 100 watts or at least 150 watts.

The metal nanofoam may occur in two distinct steps. The first step may be the reduction of the metal salt to the transition metal, which produces metal nanoparticles. The metal nanoparticles may subsequently undergo growth as the metal salt precursors are expended. As the temperature is increased further, a rapid second step may take place, in which the metal nanoparticles agglomerate and fuse into metal nanofoam. The transition between the first step of forming metal nanoparticles and the second step of forming the metal nanofoam is indicated by the pressure in the microwave reactor.

If a substrate is added to the microwave reactor with the reaction solution, then the metal nanofoam may form on the substrate. Suitable substrates include a metal foil, a metal microfoam, a carbon-based cloth, or a hybrid substrate combining one or more of these materials.

The metal nanofoam may be washed with a washing solvent, such as an organic solvent, particularly acetone. The metal nanofoam may also be dried at an elevated temperature, such as at least 50°C, or at least 100 °C.

After formation, the nanofoam may be heated, for example in a furnace, to a temperature of less than 650 °C for less than 48 hours in the presence of an inert gas, such as helium, nitrogen, argon, neon, krypton, xenon, and any combinations thereof. Alternatively, after formation, the nanofoam may be heated, for example in a furnace, to a temperature of less than 650 °C for less than 48 hours in the presence of a combination of a reducing gas, such as hydrogen, ammonia, or carbon dioxide, and any combinations thereof, and an inert gas, such as helium, nitrogen, argon, neon, krypton, xenon, and any combinations thereof.

All or part of the metal nanofoam may be compressed to form a compressed metal foil.

The metal nanofoam thus produced may contain pores surrounded by fine structural elements of metal with nanometer dimensions and which may contain multiple crystallites. These fine structural members may be elongated and have a width of between 100 nm and 500 nm, between 100 nm and 300 nm, between 100 nm and 400 nm, between 150 nm and 300 nm, between 150 nm and 400 nm, between 150 nm and 500 nm, between 200 nm and 300 nm, between 200 nm and 400 nm, or between 200 nm and 500 nm. The fine structural elements may be generally spherical and have a diameter of between 10 nm and 200 nm, between 10 nm and 100 nm, between 50 nm and 200 nm, or between 50 nm and 100 nm. The mechanical robustness of the metal nanofoam may inversely correlate with the size of the fine structural elements. The tendency for form elongated fine structural members or generally spherical fine structural members may depends on the metal(s) in the metal nanofoam.

The fine structural elements may form the metal nanofoam by being fused into a network at joints. These joints may have a width at least 50% greater or at least 80% greater than the width of the smallest fine structural element being joined at the joint.

The metal nanofoam may contain other structural elements of metal larger than the fine structural elements. These other structural elements may also be welded into the network.

If the metal nanofoam is heated in the microwave reactor for more extended periods of time after the pressure spike, the fine structural elements may thicken. In addition larger deposits may be formed within the nanofoam. Heating the metal nanofoam in the microwave reactor or otherwise to 600 °C may cause thickening of the fine structural elements, for example by as much as 5%, 10%, 15% or 20%. The metal nanofoam may retain its pores even when the fine structural elements thicken, although the pore volume may decrease, for example by 50% or more.

The metal nanofoam may contain a surface coating, such as a metal oxide, which may be between 1 nm and 8 nm thick. The metal nanofoam may also lack a surface coating.

The metal nanofoam may have a surface area of at least 1 m²/g, at least 3.0 m²/g, at least 4.0 m²/g, at least 5.0 m²/g, or at least 8.0 m²/g, as measured with a BET analyzer. The surface area may depend on the metal(s) in the transition metal nanofoam as well as the concentration of metal salt and heating rate in the microwave reactor.

The metal nanofoam may be formed on and attached to a substrate, such as a metal foil, a metal microfoam, a carbon-based cloth, or a hybrid substrate combining one or more of these materials. The metal nanofoam may also or alternatively be attached to a compressed metal foil substrate formed by compressing all or part of the metal nanofoam.

Electrodes and Electrochemical Devices

Metal nanofoams disclosed herein may be formed into electrodes and used in electrochemical devices, such as batteries, ultracapacitors, fuel cells, electrochemical sensors, catalytic substrates, and substrates for gas phase catalysts.

An electrode may include an electrode material, such as a cathode material or an anode material deposited on the metal nanofoam. The electrode may also include a substrate attached to the metal nanofoam as described herein. The substrate may function as a current collector, or the electrode may have a separate current collector, such as additional metal foil. The current collector may also be a hybrid of the metal nanofoam substrate and a separate current collector component. The electrode may contain or lack binder. The electrode may also contain or lack conductivity enhancers. Given the conductivity of the metal nanofoam, conductivity enhancers are particularly likely to be unnecessary and thus be lacking in the electrode. Conductivity enhancers not integrally formed with the electode material are particularly likely to be unnecessary and thus be lacking in the electrode. Any of a variety of electrochemical devices may use at least one electrode containing the metal nanofoam. Four specific types of electrochemical devices are discussed below.

FIG. 1 is a schematic drawing of a coin cell 100 containing a cathode formed from electrode 110 which may contain the metal nanofoam, a separator 120, electrolyte 130, anode 140, cathode can 150, and anode cap 160 sealed with gasket 170. A metal nanofoam may be used in other battery formats as well, such as jelly- rolls, multi-cell batteries, and prismatic cells. Higher porosity electrodes have high ionic conductivity, which, in turn, provides higher power density at the expense of lower energy density and vice versa.

Batteries may range from simple electrochemical cells and simple combinations of cells in parallel or in series to complex batteries with monitoring, shutoff, and interface systems, including even computer systems. Batteries may be rechargeable (secondary) or non-rechargeable (primary) batteries. Primary batteries with electrode 110 containing the metal nanofoam can include, but are not limited to, lead acid, zinc-carbon, zinc-chlorine, alkaline, such as Zn-Mn0₂, nickel oxyhydroxide, such as Zn-Mn0₂/NiO_x, lithium, such as Li-FeS₂, Li-CuO, Li-Mn0₂, Li-(CF)_n, Li-Cr0₂, Zn-0₂, Zn-Au, Ag/Zn-Ag₂0, and Mg-Mn0₂ batteries. Rechargeable batteries with electrode 110 containing the metal nanofoam may include, lithium-ion batteries such as lithium-lithium metal phosphate batteries, including lithium iron phosphate, lithium manganese phosphate, lithium iron/manganese phosphate batteries, and lithium iron/cobalt phosphate batteries; lithium-lithium metal oxide batteries such as lithium manganese spinel batteries, and lithium-lithium cobalt oxide batteries, nickel cadmium batteries, nickel metal hydride batteries, nickel iron batteries, lead-acid batteries, nickel zinc batteries, silver zinc batteries, lithium-polymer batteries, lithium-sulfur batteries, and lithium-air batteries. Rechargeable batteries with electrode 110 containing the metal nanofoam may also include sodium-ion batteries, such as sodium-sulfur batteries and sodium-air batteries.

Batteries using electrode 110 containing the metal nanofoam as the cathode or anode or both may otherwise have the same architecture as with prior cathodes or anodes. The prior cathode or anode is simply replaced with electrode 110 containing the metal nanofoam. Batteries using electrode 110 containing the metal nanofoam may be used to power any device. For instance, they may be used in consumer electronics, such as computers and phones, power tools, cars and other conveyances, and even in grid energy storage systems.

FIG. 2 is a schematic drawings of a fuel cell 200 with electrode 110 containing the metal nanofoam as its anode. Fuel cell 200 further includes liquid electrolyte 210, solid electrolyte/separator 220, cathode 230, fuel chamber 240 through which fuel fluid flows as indicated by the arrows, exhaust chamber 250, through which exhaust fluid flows as indicated by the arrows, and electrical connectors 260.

The fuel cell in FIG. 2 is a simple design often used for hydrogen fuel cells.

Other fuel cell designs, which may be more complex or simpler, are also compatible with electrode 110 containing the metal nanofoam. Any suitable fuel, such as hydrogen gas or a hydrocarbon gas, is compatible with electrode 110 containing the metal nanofoam. Reversible fuel cells are compatible with electrode 110 containing the metal nanofoam, which may also be used as a cathode in such cells. Fuel cells may further include monitoring and regulation components, including computers, as well as air flow control units. A fuel cell may include more than one individual cell. Fuel cells of the present disclosure may be used in, but not limited to, cars and other conveyances, portable electronics, and stationary energy-supply sources.

FIG. 3 is a simple electrochemical sensor 300 employing electrode 110 containing the metal nanofoam as a working electrode. The senor also contains counter electrode 310, reference electrode 320, leads 330, and sample container/protective housing 340. Electrochemical sensors using electrode 110 containing the metal nanofoam may be simpler than depicted; for instance they may lack a reference electrode, or they may be more complicated; for instance they may include displays, alarms, computers, and other elements. Electrode 110 containing the metal nanofoam may be particularly useful in sensors that make repeated measurements over time. Electrochemical sensors using electrode 110 containing the metal nanofoam may include any sensor in which an analyte or other material undergoes an electrochemical reaction with active material 60. For instance, the sensors may include, but not limited to, medical sensors and environmental sensors, such as gas sensors. FIG. 4 is an ultracapacitor 400 employing reference electrode 110 containing the metal nanofoam as a first plate. Ultracapacitor 400 further includes second plate 410, separator 420, dielectric 430, and electrical connectors 440. An active material undergoes an electrochemical reaction with dielectric 430 near first plate 110 to form a pseudocapacitance layer. Although only one plate is shown as using electrode 110 containing the metal nanofoam in FIG. 4, both plates may use electrode 110 containing the metal nanofoam and exhibit pseudocapacitance, particularly if the electrodes 110 containing the metal nanofoam have different active materials. Ultracapacitors using electrode 110 containing the metal nanofoam may be used in any of a wide variety of applications, particularly applications where quick energy delivery is needed, such a power tools.

EXAMPLES

The following examples illustrate aspects of the invention; no example is intended to encompass the invention as a whole. Furthermore, although some examples may present discrete embodiments of the invention, aspects of such examples may be combined with other variations of the invention as described above or in different examples unless such combinations would be clearly inoperable to one of skill in the art.

Example 1 - Synthesis of Transition metal nanofoams

Metal nanofoams of copper, silver, and nickel were synthesized by dissolving 200 mg of copper(II) acetate monohydrate (Alfa-Aesar), silver(I) acetate (Fischer Scientific, laboratory grade), or nickel(II) acetate tetrahydrate (Arcos Organics, 99+%) in 12 mL of tetraethylene glycol (TEG) (Sigma-Aldrich 99%). The TEG was dried prior to use by storing over 4A molecular sieves (Fisher Scientific) (Williams, D. B. G.; Lawton, M. J. Org. Chem. 2010, 75, 8351-8354). Dissolution of the metal acetates was expedited with approximately 10 min of ultra-sonication, creating translucent blue, green, and clear solutions, respectively, for the copper, nickel, and silver solutions. The reactions were subsequently transferred to 20 mL borosilicate vessels and heated as fast as possible in an Anton-Paar Monowave microwave reactor, with a max power of 850 W, to 300 °C. The reactions were not stirred while in the microwave reactor as stirring would cause the reactants to become compacted into a ball of metal. The reactions were stopped approximately 1 min after the observation of a pressure spike. The resultant reaction products were then washed three times with acetone and dried in an oven at 100 °C.

The transition between the first step of forming metal nanoparticles and the second step of forming transition metal nanofoam can be observed in the pressure and power of the microwave reactor, as shown in FIGS. 5-8. Specifically, for the nickel nanofoam (FIG. 5), the microwave reactor was at 0 bar relative pressure until approximately 175 s into the reaction, at which point there was an approximately 150 W spike in the power, required to maintain the temperature, as well as an approximately 4 bar increase in pressure. The temperature, pressure, and power of the microwave reactor for the silver nanofoam and copper nanofoam are shown in FIG. 6 and FIG. 7, respectively. Only metal nanoparticles were formed when the microwave reactor was stopped prior to the power/pressure spike, whereas transition metal nanofoams were produced when the microwave reactor was stopped subsequent to the spike.

Example 2 - Characterization of the Transition metal nanofoams

The metal nanofoams formed in Example 1 were further characterized. The metal nanofoams existed as free-standing cotton-like structures, as shown in FIG. 8. Scanning electron microscopy (SEM) images as well as photographs of the synthesized metal nanofoams are presented in FIG. 9. The SEM images reveal that the nickel nanofoam had the finest structure with generally spherical fine structural elements having a diameter of less than 100 nm. The silver nanofoam had the next finest structure with its elongated fine structural elements having widths between 150 nm and 300 nm. The copper nanofoam had the coarsest structure with its fine structural elements having widths between 200 nm and 400 nm. Unlike the nickel nanofoam, the copper nanofoam and silver nanofoam fine structural elements did not have a well-defined shape.

The mechanical robustness of the metal nanofoams was inversely correlated to the size of the fine structural elements, with the copper nanofoam being the most robust and the nickel nanofoam being the most fragile. Additionally, all of the metal nanofoams appeared to be formed by fusing the fine structural elements into a network. The copper nanofoam and silver nanofoam were robustly joined with joints approximately 80% of the width of the joined fine structural elements. The nickel nanofoam was less well joined, with joints approximately 50% the width of the joined fine structural elements.

The colors of the metal nanofoams were reddish-brown, grayish-white, and black for copper, silver, and nickel, respectively. The colorings indicate metallic materials coated by a thin layer of the naturally occurring metal oxide.

The effect of precursor concentration on the size and structure of the copper nanofoam was explored, and the SEM micrographs are presented in FIG. 10. The size of the fine structural elements of the copper nanofoam decreased as the concentration of Cu-acetate decreased. Nanofoams were formed over a metal salt concentration range of 0.01 - 0.08 M, indicateing that the reaction is not very sensitive to precursor concentration.

The composition, crystal structure, and crystallite size of the metal nanofoams were assessed with x-ray diffraction (XRD) along with subsequent Rietveld refinement. The XRD patterns shown in FIG. 11 indicate that all of the metal nanofoams are pure metallic phases. The XRD pattern of the nickel nanofoam shows that it is composed of a major face-centered cubic (FCC) structure nickel phase and minor hexagonal close packed (HCP) structure nickel phase belonging to the Fm-3m and P6₃/mmc space groups, respectively. HCP nickel is typically a high pressure phase, but has also been known to form when nickel nanoparticles are made at low temperatures (Luo, X.; Chen, Y.; Yue, G. H.; Peng, D. L.; Luo, X. J. Alloys Compd. 2009, 476, 864-868). Furthermore, based on prior references of the synthesis of nickel nanoparticles, it is believed that the HCP nickel is concentrated on the surface while the FCC nickel constitutes the core of the nanoparticles (Zhou, W.; Zheng, K.; He, L.; Wang, R.; Guo, L.; Chen, C; Han, X.; Zhang, Z. Nemo Lett. 2008, 8, 1147- 1152).

XRD shows both the silver nanofoam and copper nanofoam are pure FCC metals with no impurities.

The metal oxide which visually appeared to form on the surface of the metal nanofoams is a small constituent because it did not appear in any of the XRD patterns. Further analysis of the metal nanofoams was carried out via high-resolution transmission electron microscopy (FIRTEM). The micrographs of the metal nanofoams showed that both the copper nanofoam (FIG. 12) and nickel nanofoam (FIG. 13) formed metal oxide layers on their surfaces of 5-8 nm and 1-3 nm, respectively, while the silver nanofoam (FIG. 14) did not have any appreciable metal oxide layer. The composition of the bulk fine structural elements was verified by measuring the interplanar spacing of the fine structural elements, while the composition of the surface layers of the transition metal nanofoams was determined to be CuO and NiO, respectively, for the copper nanofoam and nickel nanofoam.

The crystallite sizes of the metal nanofoams were determined with the

Williamson-Hall method in conjunction with PDXL 2 software's Rietveld refinement feature. The metal nanofoams crystallite sizes were found to be 29 nm, 27 nm, and 13 nm, respectively, for the silver nanofoam, copper nanofoam, and nickel nanofoam. Based on the crystallite analysis coupled with the SEM micrographs, it appears that the fine structural elements of the metal nanofoams contained multiple crystallites.

The surface areas of the metal nanofoams, shown in FIG. 15, were measured with a BET analyzer. The nickel nanofoam had a surface area of 8.6 m²/g. The copper nanofoam had a surface area of 3.9 m²/g. The silver nanofoam had a surface area of 3.0 m²/g. The surface areas of the metal nanofoams were compared with the previously reported non-microporous metal foams. The metal nanofoams had a surface area that was three orders of magnitude greater than that of the commercially available copper foam (E. A. Corporation). Additionally, the metal nanofoams also had a higher surface area when compared to those produced with a nanosmelting technique (0.5 m²/g) or a combustion synthesis method (3.5 m²/g) (Walsh, D. Arcelli, L.; Ikoma, T.; Tanaka, J.; Mann, S. Nat. Mater. 2003, 2, 386-390; Erri, P.; Nader, J.; Varma, A. Adv. Mater. 2008, 20, 1243-1245). The nickel nanofoam had a slightly lower surface area when compared to a templated inverse pearlite structured nickel foam with a surface area of 11.8 m²/g (Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957-7958).

The nickel nanofoam was subjected to 5 min of additional heating in the microwave reactor. SEM micrographs (FIG. 16) show thickened fine structural elements. The silver nanofoam and nickel nanofoam were heated in an argon atmosphere at 600 °C for 4 h. SEM micrographs (FIG. 17) show thickened fine structural elements for these nanofoams as well. Example 3 - Equipment and Methods

XRD patterns were collected on a Rigaku Miniflex 600 desktop X-ray diffractometer with copper K-alpha radiation (λ = 1.54184 A). The diffraction patterns were collected in continuous mode over a range of 10-80° 2Θ at a rate of 3° per minute. Subsequently, the average crystallite size was determined with Rietveld refinement on Rigaku PDXL 2 software. The morphology of the transition metal nanofoams was investigated with a JEOL JSM-5610 SEM. The samples for analysis were prepared by affixing a small transition metal nanofoam sample onto a carbon tape on a sample holder. The surface areas of the samples were determined with a Quantachrome autosorb iQ2 BET analyzer. Approximately 100 mg of each of the metal nanofoams was placed in a 10 mL sample holder, outgassed at 100 °C overnight, and subjected to a multi-point BET adsorption/desorption test. The surface areas of the metal nanofoams were then calculated with the multi-point BET method. The correlation coefficients for all of the BET surface area fits were greater than 0.999. The transmission electron microscopy (TEM) was carried out with a JEM- 21 OOF microscope with an accelerating voltage of 200 kV. The energy dispersive x- ray spectroscopy (EDS) was performed with the JEM-2100F microscope equipped with an Oxford Instruments EDS detector while operating in the scanning transmission electron microscopy mode. Although only exemplary embodiments of the disclosure are specifically described above, it will be appreciated that modifications and variations of these examples are possible without departing from the spirit and intended scope of the disclosure. For instance, numeric values expressed herein will be understood to include minor variations and thus embodiments "about" or "approximately" the expressed numeric value unless context, such as reporting as experimental data, makes clear that the number is intended to be a precise amount.

Claims

1. A method of forming a metal nanofoam, the method comprising:

dissolving a metal salt in a reaction solvent; and

heating the reaction solvent in a microwave reactor to form a reaction product including a metal nanofoam.

2. The method of claim 1, further comprising rinsing the reaction product with a washing solvent.

3. The method of claim 1, wherein the metal nanofoam comprises a metal selected from the group consisting of scandium, ytterbium, titanium, zirconium, hafnium, vanadium, neodymium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, mercury, germanium, indium, tin, antimony, thallium, lead, bismuth, and any combinations thereof.

4. The method of claim 1, wherein the metal nanofoam comprises a transition metal selected from the group consisting of cobalt, iron, platinum, palladium, nickel, copper, gold, silver, and any combinations thereof.

5. The method of claim 1, wherein the metal nanofoam is selected from the group consisting of a nickel nanofoam, a copper nanofoam, a silver nanofoam, and any combinations thereof.

6. The method of claim 1, wherein the metal salt comprises a metal ion and a counterion.

7. The method of claim 6, wherein the counterion is selected from the group consisting of acetate, nitrate, sulfate, chloride, flouride, bromide, oxylate, citrate, ammonium, isopropoxide, carbonate, acetylacetonate, phosphate, tetrafluorob orate, hexafluorophosphate, hydroxide, oxide, and any combinations thereof.

8. The method of claim 1, wherein the metal salt is selected from the group consisting of nickel acetate, copper acetate, silver acetate, and any combinations thereof.

9. The method of claim 1, wherein the metal salt is selected from the group consisting of nickel acetate monohydrate, nickel acetate polyhydrate, copper acetate monohydrate, copper acetate polyhydrate, silver acetate monohydrate, silver acetate polyhydrate, and any combinations thereof.

10. The method of claim 1, wherein the reaction solution is heated in the microwave reactor at a power of 800 watts or greater.

11. The method of claim 1, wherein the reaction solution is heated to a temperature of 250 °C. or greater.

12. The method of claim 1, wherein the reaction solution is heated in the microwave reactor for less than three minutes.

13. The method of claim 1, wherein the reaction solution is heated in the microwave reactor for a predetermined period of time after a pressure spike is observed in the microwave reactor.

14. The method of claim 1, wherein the predetermined period of time is one minute or less

15. The method of claim 1, wherein the reaction solvent is an organic solvent.

16. The method of claim 1, wherein the metal nanofoam is heated after formation in a furnace to a temperature of less than 650 °C for less than 48 hours in the presence of an inert gas.

17. The method of claim 1, wherein the metal nanofoam is heated after formation in a furnace to a temperature less than 650 °C for less than 48 hours in the presence of a combination of a reducing gas and an inert gas.

18. The method of claim 2 wherein the washing solvent is an organic solvent.

19. The method of claim 2, further comprising drying the metal nanofoam at a temperature of at least 50 °C.

20. The method of claim 1, further comprising adding a substrate onto which the metal nanofoam forms with the reaction solvent in microwave reactor.

21. The method of claim 20, wherein the substrate is a metal foil, a metal microfoam, or a carbon-based cloth.

22. The method of claim 1, further comprising compressing all or part of the metal nanofoam to form a compressed metal foil.

23. A metal nanofoam comprising pores and a network of fine structural elements fused at joints, wherein the network of fine structural elements comprise a transition metal and are elongated with a width of between 100 nm and 500 nm or generally spherical with a diameter of between 10 nm and 200 nm, and wherein the transition metal nanofoam has a surface area of at least 1.0 m²/g.

24. The metal nanofoam of claim 20, wherein the metal is selected from the group consisting of scandium, ytterbium, titanium, zirconium, hafnium, vanadium, neodymium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, mercury, germanium, indium, tin, antimony, thallium, lead, bismuth, and any combinations thereof.

25. The metal nanofoam of claim 20, wherein the metal a transition metal is selected from the group consisting of cobalt, iron, platinum, palladium, nickel, copper, gold, silver, and any combinations thereof.

26. The metal nanofoam of claim 20, wherein the metal nanofoam is selected from the group consisting of nickel nanofoam, copper nanofoam, silver nanofoam, and any combinations thereof.

27. An electrode comprising a metal nanofoam comprising pores and a network of fine structural elements fused at joints, wherein the network of fine structural elements comprise a metal and are elongated with a width of between 100 nm and 500 nm or generally spherical with a diameter of between 10 nm and 200 nm, and wherein the metal nanofoam has a surface area of at least of at least 1.0 m²/g.

28. An electrochemical device comprising an electrode comprising a metal nanofoam comprising pores and a network of fine structural elements fused at joints, wherein the network of fine structural elements comprise a metal and are elongated with a width of between 100 nm and 500 nm or generally spherical with a diameter of between 10 nm and 200 nm, and wherein the metal nanofoam has a surface area of at least of at least 1.0 m²/g.