METAL-CARBON NANOSTRUCTURES AND METHOD OF MANUFACTURING THEREOF

TECHNICAL FIELD

The present application relates to the field of nanotechnology, and more specifically to carbon nanostructures.

BACKGROUND

Carbon dioxide is massively emitted into the atmosphere since the industrial era. Human-made CO₂is linked with global warming and its consequences. Reducing the emissions of this greenhouse gas is a challenge that can be achieved through several strategies. These strategies include, but are not limited to: the energy transition, to switch from fossil fuels to renewable energy sources [1]; associating energy storage systems to renewables to make them more reliable [2]; promoting collective transportation and electric vehicles [3]; protecting and multiplying the number of green spaces, natural carbon storage systems [4]; making systems and processes more energy efficient [5].

These strategies take time, money and political willingness to be fully implemented in the world. [6] [7] [8] [9].

SUMMARY

Extensive research has been done in the last decades on the CO₂conversion toward valuable products. CO₂is a highly stable and poorly reactive molecule. The process of converting it may thus be energy intensive. Different methods to convert CO₂exist, such as plasma [10] or photocatalytic [11] based techniques, but the electrocatalytic reduction of CO₂may be the most promising technology [12]. A catalyst is required to lower the energy intake to convert CO₂. Most of the transition metals used in the CO₂electroreduction have a tendency to produce carbon monoxide, CO [31]. CO has to be associated with dihydrogen to form syngas and go through the Fischer-Tropsch process before obtaining hydrocarbons, re-used as fuels [13]. Copper is an element able to directly produce complex hydrocarbons through the electrochemical process. However, copper as a catalyst may produce a large variety of hydrocarbons with low efficiency and low selectivity for each [14].

A recent study realized by Song et al. [15] presented a catalyst based on copper nanoparticles dispersed on a graphitic material where graphene was grown on a substrate, resulting in a single carbon structure once grown and not carbon particles or carbon powder that can be dispersed, producing ethanol with faradaic efficiency of 61% and a selectivity of 62% toward the carbon-based products (value extrapolated from the article's data). The synergistic effect between the carbon-based material and the copper nanoparticles is apparently responsible for the high electrocatalytic performance of the catalyst described in the research article. Despite the high efficiency of their catalyst, the reported current density is relatively low, around 2 mA·cm⁻²at −1.2 V vs RHE, making the catalyst not suitable for a commercial application. Commercial catalysts for CO₂reduction are expected to have current densities higher that 100 mA·cm⁻²for faradaic efficiencies reaching at least 50% [32].

The present study is using graphene nanoflakes (GNFs)[16]. GNFs have been proven to be an interesting material as carbon support for catalytic applications, thanks to outstanding crystallinity properties [17]. Different methods of functionalization and nanoparticles decoration have been developed and applied to the GNFs [18]-[20]. The wet chemistry method initially developed by Yeager et al. [21] is chosen in this study to disperse copper nanoparticles on the surface of the GNFs (Cu-GNFs) and thus synthesize an active catalyst toward the CO₂electroreduction.

Catalysts with various amount of copper and various chemical composition are synthesized and characterized in this study. Physical characterization aims to understand the structure and composition of the samples. The electrochemical characterization provides an insight on the electrocatalytic performance of the Cu-GNFs.

A broad aspect is a graphene powder composed of graphene nanoflakes including copper sulfide nanoparticles deposited thereon.

In some embodiments, the graphene nanoflakes may be composed of five to twenty stacked layers of graphene.

In some embodiments, the copper may compose at least 15% wt of the graphene nanoflakes.

Another broad aspect is an electrode at least partially coated with graphene nanoflakes, the graphene nanoflakes comprising copper sulfide nanoparticle deposits.

Another broad aspect is a method of manufacturing graphene nanoflakes with copper nanoparticle deposits. The method includes introducing graphene nanoflakes composed of stacked layers of graphene into a hydrophilic solvent, resulting in a mixture. The method includes dissolving a copper salt in the mixture. The method includes drying the mixture containing the dissolved copper salt. The method includes pyrolyzing the dried mixture containing the dissolved copper salt to yield the graphene nanoflakes with copper nanoparticle deposits.

In some embodiments, the introduced graphene nanoflakes may be composed of five to twenty stacked layers of graphene.

In some embodiments, the hydrophilic solvent may be a mixture of water and ethanol.

In some embodiments, the pyrolysis may be performed at a temperature above 500° C.

In some embodiments, the mass of copper introduced to the hydrophilic solvent may be at least 20 wt %.

In some embodiments, the copper salt may be CuSO₄and the copper nanoparticle deposits may include copper sulfide.

Another broad aspect are graphene nanoflakes with copper nanoparticle deposits manufactured in accordance with the method as defined herein.

In some embodiments, the graphene particles may be graphene nanoflakes.

A broad aspect is carbon powder composed of carbon particles comprising metallic particles deposited thereon, wherein said metallic particles include, but not limited to copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; iridium dihydride; iron carbonyl; one or more manganese complexes; one or more rhodium complexes; one or more iron complexes; one or more copper complexes; bismuth; one or more bismuth complexes; cobalt oxide; platinum, one or more platinum complexes; one or more ruthenium complexes; one or more rhenium complexes; one or more osmium complexes; lead; lead oxide; mercury; and/or an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

A broad aspect is a graphene powder composed of graphene nanoflakes comprising metallic particles deposited thereon, wherein the metallic particles include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; iridium dihydride; iron carbonyl; one or more manganese complexes; one or more rhodium complexes; one or more iron complexes; one or more copper complexes; bismuth; one or more bismuth complexes; cobalt oxide; one or more ruthenium complexes; one or more rhenium complexes; one or more osmium complexes; lead; lead oxide; mercury; and an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

In some embodiments, the metallic particles may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; iridium dihydride; iron carbonyl; one or more manganese complexes; one or more rhodium complexes; one or more copper complexes; bismuth; one or more bismuth complexes; cobalt oxide; one or more ruthenium complexes; one or more rhenium complexes; one or more osmium complexes; lead; lead oxide; mercury; and an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

In some embodiments, the metallic particles may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; and alloy of copper and tin.

In some embodiments, the metallic particles may include metallic nanoparticles.

In some embodiments, the graphene nanoflakes may be composed of five to twenty stacked layers of graphene.

In some embodiments, the copper may compose at least 15% wt of the graphene nanoflakes.

Another In some embodiments, the electrode may include a gas diffusion layer and a binding polymer, the binding polymer binding the graphene powder to the gas diffusion layer.

Another broad aspect is a method of manufacturing carbon particles with metallic particle deposits. The method includes introducing carbon particles into a hydrophilic solvent, resulting in a mixture; dissolving a metal salt in the mixture; drying the mixture containing the dissolved metal salt; and pyrolyzing the dried mixture containing the dissolved metal salt to yield the carbon particles with metal particle deposits.

In some embodiments, the metal salt may include one or more of copper salt and tin salt.

In some embodiments, the metal salt may be copper salt.

In some embodiments, the copper salt may be CuSO4 and the copper particle deposits may include copper sulfide, copper oxide and/or metallic copper.

In some embodiments, the mass of copper introduced into the hydrophilic solvent may be at least 20 wt %.

In some embodiments, the carbon particles may be graphene nanoflakes, and the introduced carbon particles may be introduced graphene nanoflakes composed of stacked layers of graphene.

In some embodiments, the introduced graphene nanoflakes may be composed of five to twenty stacked layers of graphene.

In some embodiments, the hydrophilic solvent may be a mixture of water and ethanol.

In some embodiments, the pyrolysis may be performed at a temperature above 500° C.

Another broad aspect are carbon particles with metal particle deposits manufactured in accordance with the method as defined herein.

Another broad aspect is a method of producing a fuel from carbon dioxide comprising performing a carbon dioxide electroreduction using a cathode comprising carbon powder, the carbon powder composed of carbon particles with metallic particles deposited on the carbon particles, wherein a product of the carbon dioxide electroreduction is the fuel.

In some embodiments, the metallic particles may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; iridium dihydride; iron carbonyl; one or more manganese complexes; one or more rhodium complexes; one or more iron complexes; one or more copper complexes; bismuth; one or more bismuth complexes; cobalt oxide; one or more ruthenium complexes; one or more rhenium complexes; one or more osmium complexes; lead; lead oxide; mercury; and an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

In some embodiments, the metallic particles may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; and alloy of copper and tin.

In some embodiments, the metallic particles may include metallic nanoparticles.

In some embodiments, the fuel that is produced from the electroreduction may include at least one of: formic acid, n-propanol and acetate.

In some embodiments, water may act as a proton or hydroxide donor during the electroreduction.

In some embodiments, the cathode may include a liquid comprising the graphene powder, a solvent and a binding polymer, the liquid deposited on a gas diffusion layer.

In some embodiments, the carbon particles may be graphene nanoflakes.

In some embodiments, the method further includes collecting the fuel following the electroreduction.

Another broad aspect is an electrolytic cell for producing a fuel by electro-reducing carbon dioxide. The cell includes a carbon dioxide inlet; a proton or hydroxide donor inlet; a cathode connected to the carbon dioxide inlet, wherein the cathode comprises a gas diffusion layer, a binding polymer and carbon powder, the carbon powder comprising carbon particles with metallic particles deposited on the carbon particles, wherein the binding polymer binds the carbon powder to the gas diffusion layer; an anode comprising a gas diffusion layer, the anode connected to the proton or hydroxide donor inlet; a first current collector plate that is positioned closer to the cathode than to the anode; a second current collector plate that is positioned closer to the anode than to the cathode; a first separator plate in proximity with the first current collector plate; a second separator plate in proximity with the second current collector plate; an ion exchange medium between the cathode and the anode; a by-product outlet in communication with the anode; and a fuel outlet in communication with the cathode.

In some embodiments, the proton or hydroxide donor may be water or an organic solvent.

In some embodiments, the metallic particles may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; iridium dihydride; iron carbonyl; one or more manganese complexes; one or more rhodium complexes; one or more iron complexes; one or more copper complexes; bismuth; one or more bismuth complexes; cobalt oxide; one or more ruthenium complexes; one or more rhenium complexes; one or more osmium complexes; lead; lead oxide; mercury; and an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

In some embodiments, the metallic particles of the graphene powder may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; iridium dihydride; iron carbonyl; one or more manganese complexes; one or more rhodium complexes; one or more copper complexes; bismuth; one or more bismuth complexes; cobalt oxide; one or more ruthenium complexes; one or more rhenium complexes; one or more osmium complexes; lead; lead oxide; mercury; and an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

In some embodiments, the metallic particles may include at least one of copper; copper oxide; copper sulfide; tin; tin sulfide; tin oxide; and alloy of copper and tin.

In some embodiments, the carbon powder may be graphene power, and the carbon particles may be graphene nanoflakes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which:

FIG. 1 are images taken by a TEM micrograph of Cu-GNF2 with a scale of 200 nm (left) and 5 nm (right);

FIG. 2 is a graph of an EDS spectrum on area containing copper nanoparticles and area containing only GNFs;

FIG. 3 is a graph of an XPS Survey for Cu-GNF2;

FIG. 4 are graphs of high-resolution XPS peaks of Carbon C1s, Nitrogen N1s, Oxygen O1s and Copper Cu2p for Cu-GNF2;

FIG. 5 is an X-ray diffractogram of Cu-GNF3;

FIG. 6 is a graph of LSVs of Cu-GNF2 under N2 bubbling and CO2 bubbling;

FIG. 7 is a graph of faradaic efficiency toward formate and n-propanol at −0.6 and −0.9 V vs RHE for the Cu-GNFs;

FIG. 8 is a graph of faradaic efficiency toward formate and n-propanol at −0.6, −0.7, −0.8 and −0.9 V vs RHE for Cu-GNF30;

FIG. 9 is a drawing of the molecular structure of graphitic material showing exemplary different nitrogen species found in graphitic material by X-Ray Photoelectron Spectroscopy;

FIG. 10 is a graph showing the Faradaic Efficiency of iron decorated graphene nanoflakes, prepared using an exemplary thermal plasma technique;

FIG. 11 is a graph showing the Faradaic Efficiency of copper decorated graphene nanoflakes, prepared using an exemplary thermal plasma technique;

FIG. 12 is a graph showing the Faradaic Efficiency of copper decorated commercial graphene, prepared using an exemplary wet chemistry method;

FIG. 13 is a graph showing the Faradaic Efficiency of tin decorated graphene nanoflakes, prepared using an exemplary wet chemistry method;

FIG. 14 is a graph showing the Faradaic Efficiency of bare graphene nanoflakes;

FIG. 15 is a drawing of a cross-sectional rendering of an exemplary electrolyser cell; and

FIG. 16 is a drawing of an exemplary system for electro-reducing carbon dioxide.

DETAILED DESCRIPTION

The present disclosure relates to carbon powder composed of carbon particles with metallic particles deposited thereon that act as a catalyst in the electroreduction of carbon dioxide into a fuel, such as formic acid, acetate and/or n-propanol. Exemplary carbon particles may be graphene nanoflakes. Exemplary metallic particles may be copper, tin, copper sulfate, an alloy of copper and tin, or a metal, metal oxide, metal sulfide or metal alloy that can be used as a catalyst to produce formic acid from carbon dioxide as described herein. The disclosure also relates to an electroreduction cell, where the cathode of the cell includes the carbon powder as described herein.

The carbon powder are small particles (e.g. micro or nano particles) that can then be bound to, e.g., a porous substrate, to form an electrode used, e.g., for the electroreduction of carbon dioxide.

The disclosure further relates to graphene powder composed of graphene nanoflakes with metal particle deposits as explained herein. Moreover, an electrode may include the graphene powder.

The disclosure further relates to a method of manufacturing carbon particles with metallic particles deposited thereon through pyrolysis as explained herein.

As such, the carbon powder can be used as a cost-effective, power-effective, efficient solution to transform carbon dioxide into a usable fuel, therefore reducing carbon dioxide emissions.

The cells and system described herein can be employed or added to machines, factories or installations that emit carbon dioxide. After the capture of the emissions and, in some cases, the isolating of the carbon dioxide, the cells and the system can be used to convert the carbon dioxide emissions into a fuel that can be used by the same machines, factories or installations, or stored and/or transported for later use.

In some examples, the cells and system can be used as standalone equipment, where reserves of carbon dioxide (e.g. stored in liquid, solid or gaseous form) can be provided for transformation by the cell or system.

Definitions

Transmission Electron Microscopy (TEM): TEM is a technique based on the interaction between electrons and the samples to create a picture, with a resolution down to the atom. An electron beam is generated and focused on the sample, thin enough to allow electrons being transmitted. The transmitted electrons are detected and create the picture based on the difference of thickness of the sample. The energy of the transmitted being low, the operation is realized under an ultra-high vacuum (10-7 to 10-9 kPa). The analyzed sample has to be thin enough, at most hundreds of nanometers thick, without any volatile products degassing in the chamber and support the ultra-high vacuum. The sample is generally mounted on a 5 mm copper grid covered by an ultra-thin carbon film.

X-Ray Photoelectron Spectroscopy (XPS): XPS is a technique to determine the elemental composition at the surface of a sample. Samples are placed under ultra-high vacuum (10-9 to 10-10 kPa) and are irradiated by monochromatic x-rays. Under the energy of the x-ray photons, electrons from the core shells of surface atoms are extracted. These photoelectrons are typically coming from the first 10 nanometers of the sample thickness, but most of them are actually extracted from the first atomic layers. The binding energy of these photoelectrons is then measured to establish a spectrum of energies. The peak position is specific for each element while the peak intensity is used to calculate the atomic percentage of the elements. Closer analysis of the peaks can also help determine the local bonding of one element.

Neutron Activation Analysis (NAA): NAA is used to determine the bulk composition of a sample. Samples are irradiated with thermal neutrons, generated by enriched uranium in a water-cooled nuclear reactor. A small fraction of the atoms in the sample capture a thermal neutron to generate an unstable isotope, or radioisotope, which will decay rapidly. The decay of the radioisotope to a ground state emits specific gamma rays, whose energy depends on the nature of the element. The intensity of the peaks also enables the determination of the bulk composition in weight percentage. Some elements are not detected at all by NAA for several reasons (H, He, Li, Be, B, C, N, O). The samples do not need any particular preparation and can be activated in solid or liquid state.

X-Ray Diffraction (XRD): X-rays can be diffracted by the crystalline structure of a sample when their wavelength is close to the interspacing of the crystalline planes. This property is used to determine the unknown crystalline structure of a sample by analyzing its diffraction scan. Monochromatic x-rays are focused and irradiate the sample at a varying angle. A constructive interference is created, and thus detected, with specific conditions described by Bragg's law:

nλ=2d sin θ

where λ is the wavelength of the incident x-ray beam, d Is the interspacing of the crystalline planes and θ is the diffraction angle. The sample can be analyzed under the form of a powder, packed to provide a flat surface to the x-ray beam.

Gas Chromatography (GC): GC is used to determine the composition of a gas in volume percentage. A sample of the analyzed gas is injected in a column containing a solid stationary phase and flushed by a carrier gas, typically an inert gas such as helium. The different components of the analyzed gas have different rates of progression due to molecular interaction with the stationary phase, and thus come out of the column at different times. For gases overlapping at the exit, it is possible to vary the carrier gas flow rate, column length, or temperature of the operation. Many different detectors can be used at the exit of the column to quantify the different gaseous components, including flame ionization or thermal conductivity detectors. The GC is usually calibrated with gases of interest which have a known concentration.

Nuclear Magnetic Resonance (NMR): NMR is a technique to determine molecular structures which can be used for quantification. The molecules are placed in an intense magnetic field, where the protons nuclei absorb the radiofrequencies (RF). The RF frequency is shifted during the absorption, and this shift is dependent on the chemical environment of the protons. The chemical environment includes the molecules themselves as well as the solvent used for the analysis. The measurement of the difference between the initial and shifted frequency provides the identification of the molecules, while the area under the peaks gives information on the quantification. For quantification purposes, calibration curves are produced from products with known concentration.

In the present application, by “carbon particles”, it is meant small graphene carbon structures, such as graphene nanoflakes, carbon nanotubes, black carbon, graphene oxide, reduced graphene oxide, etc.

In the present application, by “carbon powder”, it is meant a powder that includes carbon particles with metallic particles deposited thereon.

In the pre sent application, by “graphene powder”, it is meant a carbon powder with metallic particles deposited thereon, where the carbon particles are or include graphene nanoflakes.

EXPERIMENTAL METHODS

The present exemplary method relates to the preparation and testing of an exemplary catalyst of graphene powder that is composed of graphene nanoflakes with metal particles deposited thereon. The exemplary graphene powder may be used for the electroreduction of carbon dioxide into a fuel, such as formic acid. However, it will be understood that the graphene powder may have other applications that acting as a catalyst in the electroreduction of carbon dioxide such that a fuel (e.g. formic acid) be produced.

Even though the present study is directed to graphene nanoflakes, it will be understood that, in some examples, other carbon particles (e.g. carbon nanotubes, graphene oxide, etc.) with metal particles deposited thereon may also be used to electro-reduce carbon dioxide to produce a fuel without departing from the present teachings.

Experiment 1

Catalyst Preparation

The exemplary catalysts consist of copper nanoparticles dispersed on stacked-graphene nanoparticles, the graphene nanoflakes (GNFs) and are referred to as Cu-GNFs. GNFs are grown by thermal plasma method, following Pristavita's procedure [22]. Methane is injected in an ICP torch where molecules are decomposed into carbon and hydrogen atoms by the argon thermal plasma (10,000 K). The ICP torch is connected to a water-cooled axisymmetric reactor with conical expansion. The well controlled flow lines of the plasma allow the homogeneous nucleation and growth of the well-crystallized GNFs downstream of the ICP torch in a temperature window from 3,700 to 4,900 K [23]. The GNFs are then deposited on the walls and bottom plate of the reactor due to the thermophoretic forces. A small flow of nitrogen (0.1 slpm) is also added during the growth procedure along with methane, leading to a nitrogen content up to 2 at % on the surface of the GNFs. The argon thermal plasma is shut down and the nanoparticles are collected from the reactor. The GNFs consist of 5 to 20 layers of stacked graphene typically measuring 100 by 100 nm.

The addition of the copper nanoparticles to the GNFs is done by wet chemistry method. GNFs are dispersed in a 1:1 mixture of water and ethanol. Various amounts of copper sulfate salt (CuSO₄) purchased from Sigma Aldrich © is dissolved in the water-ethanol solvent. The amount of salt added to the mixture is calculated for the final catalyst to contain copper amount from 20 to 50 wt %. The mixture is stirred until complete dissolution of the copper sulfate salt, and then dried in an oven at 80° C. for 24 h. The dried solid is collected before undergoing in a pyrolysis step. The pyrolysis step is performed in a tubular quartz furnace at 700° C. for an hour under a flow of 542 sccm of argon. The mass of the catalysts is controlled before and after pyrolysis. It can be noted that the GNFs do not go under decomposition at the temperature used during the pyrolysis due to their good crystallinity. The experimental conditions of the different batches are summarized in Table 1.

TABLE 1

Experimental conditions of the wet chemistry method for catalyst synthesis.

1:1
Expected mass
Catalyst
Catalyst

Name of
GNFs
CuSO₄
Water-Ethanol
content of Cu
mass before
mass after

sample
mass (mg)
mass (mg)
volume (mL)
in catalyst (wt %)
pyrolysis (mg)
pyrolysis (mg)

Cu-GNF1
50
31
100
20
83.3
54.5

Cu-GNF2
50
54
100
30
115.0
70.5

Cu-GNF3
50
84
100
40
141.8
73.6

Cu-GNF4
50
126
100
50
182.3
84.4

Physical Characterization

The catalysts are also analyzed in terms of their structure and composition. Transmission Electron Microscopy (TEM) is used for observing the catalysts at an atomic level and especially the copper nanoparticles for size distribution determination. The TEM is a FEI Tecnai G2 F20 200 kV Cryo-STEM. The overall copper content in the different catalysts is determined by Neutron Activation Analysis (NAA) on the SLOWPOKE nuclear reactor. The SLOWPOKE nuclear reactor is coupled with an Ortec GEM30185-P germanium semiconductor gamma-ray detector, an Ortec DSPEC Pro™ multichannel analyzer, a Sartorius precision balance and the EPAA analysis software [24]. The elemental composition of the samples is analyzed by X-ray Photoelectron Spectroscopy (XPS) on a Scientific K-Alpha XPS from Thermo Scientific. An aluminum x-ray source is used on 400 μm size area. X-Ray Diffraction (XRD) is employed to determine the crystalline phase of the copper nanoparticles in the samples and performed on a Brucker D8 Discovery X-Ray Diffractometer with a VANTEC detector, and having a copper Kα source.

Electrochemical Characterization

The electrochemical characterization of the Cu-GNFs catalysts is realized in a custom made three-electrode set-up with different configurations for gas and electrolyte sampling. The counter electrode is a platinum wire separated to the other two electrodes by a Nafion membrane, to avoid the oxidation of the products generated at the cathode. The reference electrode is silver/silver chloride (Ag/AgCl) containing saturated solution. The working electrode is a 1 cm by 1 cm carbon cloth piece covered by 1 mg·cm⁻²catalyst. Nitrogen and carbon dioxide gases are bubbled through a sintered quartz sparger in the electrolyte, a 0.1 M potassium bicarbonate (KHCO₃) solution. The electrolyte is prepared by dissolving the KHCO₃salt purchased at Sigma Aldrich O into deionized water. Potentials are converted from Ag/AgCl to the reverse hydrogen electrode (RHE) using Nernst equation (eq. 1):

E
_RHE
=E
_Ag/AgCl
+E
_Ag/AgCl
⁰+0.059·pH (eq. 1)

where E_/AgCl⁰is the standard potential for Ag/AgCl electrode and is equal to 0.1976 V. The pH of the potassium bicarbonate solution is 6.8.

The catalysts are dispersed into an ink prior deposition on the working electrode. The catalyst ink consists of 5 mg of catalyst, 1 mL of ethanol and 4 μL Nafion 5 wt % solution purchased at Sigma Aldrich O. The ink is sonicated before series of 4 deposition of 50 μL, for a total volume of 200 μL, is deposited on the electrodes. The electrodes are dried for 24 h in an oven at 80° C.

Linear Sweep Voltammetry (LSV) and Chrono-Amperometry (CA) are used to characterize the catalysts. LSV consists in recording the current density for a range of potential, while CA consist in holding a potential for a certain duration and measuring the current density. For both electrochemical analysis, the catalysts go through a conditioning step under nitrogen bubbling in order to remove impurities on the catalyst surface. The conditioning step consists of repeated sweeps from 0.0 to −1.4 V vs RHE at a scan rate of 100 mV·s⁻¹until there is no variation from a voltammogram to the next one. LSVs are then recorded in electrolytes saturated with nitrogen and then carbon dioxide from 0.0 to −1.4 V vs RHE at a scan rate of 20 mV·s⁻¹. Carbon dioxide is bubbled for 30 min in order to reach the saturation concentration in the electrolyte, estimated at 2.66×10⁻²mol·kg-1, using Henry's law (eq. 2) with Sechenov equation (eq. 3) [25].

e
^p
=e
^k
^H
C (eq. 2)

where p is the partial pressure of the gas, k_His the Henry's constant at 20° C. and C is the gas concentration in pure water.

$\begin{matrix} \ln (\frac{z_{B}^{*}}{z_{B}}) = k_{s y} y & (eq . 3) \end{matrix}$

where z_B* and z_Bcorrespond to either the concentration, molarity or molality among other parameters of CO₂in pure water and electrolyte, respectively.

CA is performed by holding potentials of −0.6 and −0.9 V vs RHE for 2 hours whereas the current density is recorded. Gas and liquid products are generated during the CA and are quantified by Nuclear Magnetic Resonance (NMR) and Gas Chromatography (GC). NMR is performed on a Bruker Advance III HD 600 MHz machine. NMR samples were prepared by mixing 500 μL of liquid analyte, 50 μL of a 12 mM DMSO solution in a 0.1 M KHCO₃solution and 100 μL of D₂O. A preliminary calibration is performed on the following five products: formate, methanol, acetate, ethanol and n-propanol. A calibration is done on hydrogen, methane, carbon monoxide, carbon dioxide, ethylene and ethene standard gases.

The quantification of the gaseous and liquids products generated during the CO₂electroreduction is used to calculate the Faradaic Efficiency (FE) shown in eq. 4.

$\begin{matrix} F E_{i} = \frac{n_{i} z_{i} F}{It} & (eq . 4) \end{matrix}$

where n_iis the number of moles of the product i, z_ithe number of electrons necessary to produce one molecule of the product i, F the Faraday constant, I the current passed through the working electrode and t the duration of the CA.

Results and Discussion

Structure and Composition of the Cu-GNFs

The Cu-GNFs are analysed by TEM, where both GNFs and copper nanoparticles are observed, appearing as the darkest particles on FIG. 1. The flake shape of the stacked graphene is typical for these nanoparticles. Copper nanoparticles seem to have a wide size distribution, from a few tens of nanometers to several hundreds of nanometers for the largest particles. They are not homogenously dispersed on the GNFs, with areas containing more copper than others. The composition of areas containing copper and areas containing only the GNFs is analyzed by Energy Dispersive Spectrometry (EDS). The analysis reveals the presence of Carbon, Oxygen, Copper and Sulfur. Areas containing only GNFs are mostly composed of carbon, with traces of copper, due to the TEM support grid, made in copper and covered by a thin layer of carbon. The copper nanoparticles are thus composed of copper and sulfur, suggesting the presence of copper sulfide.

The elemental composition of the surface of the Cu-GNFs reveal the presence of carbon, nitrogen, oxygen and copper as seen on the XPS survey from FIG. 1. High-resolution peaks of the different elements are recorded and analyzed to determine the chemical state of the elements through deconvolution (FIG. 2). Carbon is mostly in sp2 orbital configuration, corresponding to the graphitic form of carbon, with small amounts of carbon bonded to nitrogen and oxygen. Nitrogen is found is small amounts under the four different forms typically found in graphitic structure: pyridinic, pyrrolic, quaternary and oxidized nitrogen. Additional information of these chemical states for nitrogen are provided in the Annexe. Nitrogen is coming from the small flow of nitrogen added during the GNF growth in the thermal plasma reactor. It has been proved in the literature that nitrogen in its pyridinic form promotes the CO2 reduction [26]. Oxygen is present in the samples for several reasons: contamination in the thermal plasma reactor during the GNFs growth, contact of the powder with air, the wet chemistry method and the oxidation of the copper nanoparticles. It can also be noted that Cu-GNF1 contains some adsorbed humidity on its surface.

Sulfur was detected by EDS during the TEM analysis but no signal is detected by XPS at the typical binding energy of 162 eV for sulfur S 2p. XPS is a characterization technique analyzing the extreme surface of samples. The absence of sulfur detected by XPS can mainly be explained by the presence of sulfur in the core of the nanoparticles. The copper nanoparticles are synthesized from CuSO₄. During the pyrolysis step, while the SO₄groups are evaporated by the temperature of 700° C., some sulfur may be trapped inside the core of the nanoparticles to form copper sulfide.

The actual copper content in the bulk of the samples is determined by NAA. The amount of copper in the catalysts is 17.6, 26.6, 41.4 and 52.6 for Cu-GNF1, 2, 3 and 4 respectively. These values show that the synthesis process delivers the expected amount of copper with an error between −12% and +5%.

Element peak
Cu-GNF1
Cu-GNF2
Cu-GNF3
Cu-GNF4

C 1 s
94.6
95.0
96.3
86.3

N 1 s
0.7
1.6
0.7
0.2

O 1 s
4.1
2.5
2.3
9.5

Cu 2p
0.6
0.9
0.7
4.0

XPS is used to determine the elemental composition at the surface of the Cu-GNF catalysts. Copper in the catalyst samples have two chemical states which are located at 954.0 and 952.0 eV. The peak located at 954.0 eV corresponds without any doubt to copper oxide CuO due to its peak location and satellite shape [27]. The identification of the chemical state from the peak at 952.0 eV can be more difficult, because metallic copper, copper oxide Cu₂O and copper sulfide Cu₂S present very similar signal in XPS. However, the presence of sulfur in the core of the nanoparticles, the analysis of the elemental composition (Table 2) and the relative amounts (Table 3) from copper and oxygen tend to support the presence of metallic copper in the samples. It can be noted that the samples Cu-GNF2 and 3 contain nearly half of the overall surfacic copper in metallic form while Cu-GNF1 and 4 have mostly oxidized copper. XPS being a surface analysis, it is possible that the copper nanoparticles in Cu-GNF2 and 3 have a thinner oxide layer compared to the ones in Cu-GNF1 and 4.

TABLE 2

Relative amounts of Carbon, Nitrogen, Oxygen and Copper

states from deconvolution of the high-resolution peaks.

Element peak
Cu-GNF1
Cu-GNF2
Cu-GNF3
Cu-GNF4

C 1s
284.4 ± 0.1 eV
Carbon sp2
76.8
91.8
89.9
91.3

286.0 ± 0.1 eV
C—O
3.9
6.4
7.3
7.5

286.9 ± 0.1 eV
C—N
3.9
1.8
2.7
1.2

288.5 ± 0.1 eV
O—C═O
15.4
0.0
0.0
0.0

N 1s
398.3 ± 0.3 eV
Pyridinic
14.3
44.4
36.4
22.7

399.6 ± 0.2 eV
Pyrrolic
21.4
11.1
24.2
45.5

400.9 ± 0.3 eV
Quaternary
28.6
33.3
27.3
22.7

402.3 ± 0.3 eV
N—O
35.7
11.1
12.1
9.1

O 1s
529.7 ± 0.1 eV
Cu—O
14.9
11.2
9.1
44.2

531.2 ± 0.1 eV
C═O
23.6
68.7
56.6
43.2

533.0 ± 0.2 eV
C—O
14.8
16.5
22.9
10.4

534.4 ± 0.1 eV
N—O
46.7
3.6
11.4
2.3

535.3 ± 0.1 eV
OH/H₂O
42.3
0.0
0.0
0.0

Cu 2p
932.4 ± 0.1 eV
Cu(0)/Cu(I)
19.6
46.9
45.5
13.0

3/2
934.2 ± 0.2 eV
Cu(II)
80.4
53.1
54.5
87.0

Cu 2p
952.0 ± 0.2 eV
Cu(0)/Cu(I)
19.8
46.4
43.5
25.6

1/2
954.0 ± 0.2 eV
Cu(II)
80.2
53.6
56.5
74.4

The Cu-GNFs are analyzed by XRD to investigate the crystalline structure of the copper nanoparticles as seen on FIG. 3. Indeed, the previous techniques used (TEM, EDS and XPS) show the possibility to have metallic copper, copper oxide and/or copper sulfide in the samples. The typical peaks for the GNFs are observed on the diffractograms, with a broad peak at 25° and peaks located at 42°, 72° and 88° [16] which are overlapping with the peaks associated to copper. The most intense peaks observed on the diffractograms are from metallic copper, located at 43°, 50°, 74°, and 90° [28]. The least intense peaks include phases of copper oxide and sulfide, whose relative amounts are depicted in Table 3.

TABLE 3

Percentage of crystalline phase containing

copper in the Cu-GNF samples.

Samples
Cu metal (%)
Cu₂S (%)
CuO (%)
Cu₂O (%)

Cu-GNF1
43.1
—
40.5
16.4

Cu-GNF2
53.6
46.4
—
—

Cu-GNF3
70.4
29.6
—
—

Cu-GNF4
88.3
—
11.7
—

It can be noted that the samples Cu-GNF2 and 3 contain copper sulfide while Cu-GNF1 and 4 contain copper oxide while metallic copper is present in all the samples, with increasing percentage. Comparing with XPS results, the samples containing solely copper sulfide also contained the lowest amount of oxygen, and the lowest amount of copper bonded to oxygen. The presence of copper oxide by XPS on samples Cu-GNF2 and 3 which was not detected by XRD can be explained by a thin and amorphous oxide layer on these samples. XRD also show an increasing amount of metallic copper in the samples synthesized with increasing amount of copper sulfate. This analysis suggests that changing the amount of copper sulfate while keeping the other synthesis parameters constant has a great impact on the composition of the samples, and possibly on the performance of the catalysts.

Electrochemical Characterization of the Catalysts

The Cu-GNF samples are tested through the electroreduction of CO₂by linear sweep voltammetry (LSV) first, under nitrogen and CO₂bubbling, as seen on FIG. 6. The onset potential and current density for each catalyst is reported in Table 4. The onset potential is typically defined as the potential where the corrected current density is no longer null. In this case, the onset potential is defined as the potential where the current density is reaching a value more negative with CO₂bubbling compared to nitrogen bubbling. The current density reported in Table 4 is measured at −0.6 and −0.9 V vs RHE, potential at which chronoamperometry (CA) is ran.

The catalysts all have a higher current density under the CO₂bubbling compared to nitrogen bubbling, suggesting an activity toward the CO₂reduction. The analysis of the LSVs provides a screening technique to compare the electrochemical performance of the different catalysts, and thus selecting the best one. Cu-GNF2 has the less negative onset potential among the catalysts, with the highest values of current density at −0.6 and −0.9 V vs RHE.

TABLE 4

Onset potential and Current density values from LSVs

at −0.6 and −0.9 V vs RHE for the catalysts.

j (mA · cm⁻²) from LSV

Onset potential
@ −0.6 V
@ −0.9 V

Catalyst
(V vs RHE)
vs RHE
vs RHE

Cu-GNF1
−0.44
−5.5
−14.6

Cu-GNF2
−0.40
−5.4
−15.9

Cu-GNF3
−0.53
−3.1
−9.4

Cu-GNF4
−0.48
−5.1
−12.9

Each catalyst undergoes CA for the two potentials of −0.6 and −0.9 V vs RHE during two hours. The liquid products are identified and quantified by NMR. Among the different products tested, formate and n-propanol were the major products detected, with a few traces of the other products, close to the detection limits. Due to the low amount of these products, they are not counted for the calculation of the faradaic efficiency, represented on FIG. 7 for the different catalysts. In all of the conditions, formate is generated with more than 99% selectivity among liquid products, while Cu-GNF2 exhibits the best performance in terms of faradaic efficiency. The efficiency is reaching up to 40% toward formate at the potential of −0.6 V vs RHE.

It appears that increasing the overall amount of copper in the catalysts is not linked to an improvement in the electrocatalytic activity toward the CO₂reduction. Similarly, the increasing amount of metallic copper does not have any correlation with the catalyst's performance. Cu-GNF2 is the catalyst containing the highest amount of copper sulfide. Literature suggests that copper nanoparticles doped with sulfur are more selective toward formate generation [29], [30].

Cu-GNF2 is studied in more details due to its better performance compared to the other samples. CA is performed at −0.6, −0.7, −0.8 and −0.9 V vs RHE in order to find the potential where both faradaic efficiency and current density are maximized, as seen on FIG. 8. The highest efficiency toward formate generation is observed at −0.6 V vs RHE, representing a low overpotential of 0.38 V compared to the redox potential of the couple CO₂/HCOO— (−0.225 V). Hydrogen, CO₂and air are detected as gases, but H₂is the only gas produced through electrochemistry and counted for the calculation of the Faradaic Efficiency (FE). Hydrogen formation is a competitive process to the CO₂reduction and has to be minimized to favor formate generation. The overall Faradaic efficiency varies between 91% at −0.9V vs RHE and 96% at −0.6 V vs RHE. The FE is lower than 100% because of traces of products that are not counted in the calculation as well as electrical losses in the set-up.

Conclusion

Catalysts made of metallic copper and copper sulphide nanoparticles deposited on graphene nanoflakes are produced with an easy and scalable wet-chemistry method followed by pyrolysis step. One of the catalysts, Cu-GNF2, offers interesting electrochemical properties, with low overpotential, high current densities and faradaic efficiencies going up to 40% toward formate production. This specific catalyst is characterized by a high amount of copper sulphide amount in its composition. Industrial CO₂electrolyzer requires catalyst with higher current densities and lower energy consumption, which can be achieved by several strategies. These strategies include the optimization of the catalyst in a real electrolyzer instead of a three-electrode set-up with parameters such as catalyst loading, pressure of the gas, temperature of the electrolyte, the addition of ionic liquids to the catalysts.

Experiment 2

The second exemplary relates to a graphene powder that is composed of graphene nanoflakes with metal particles deposited thereon, where the metal particles are composed at least of copper, iron or tin.

The catalysts prepared in this exemplary study contain either copper, iron or tin metal, added to a carbon support, the graphene nanoflakes made in the laboratory or commercial graphene, with varying metal weight content. The catalysts are prepared through wet chemistry method, as described herein, and thermal plasma technique (see J.-L. Meunier, U. Legrand, D. Berk “Decoration of graphene nanoflakes with metal nanoparticles in a thermal plasma jet environment” ROI 2018-021 or U. Legrand, J.-L. Meunier, D. Berk, Iron functionalization on graphene nanoflakes using thermal plasma for catalyst applications, Applied Catalysis A, 528, 36-43, 2016). The electrochemical characterization and determination of the faradaic efficiency is performed as detailed herein.

TABLE 6

Experimental conditions for the different

metal - graphene support catalysts.

Preparation

Catalyst name
Technique
Metal content
Notes

Fe_p-GNF
Thermal plasma
10 to 40 wt %
—

Cu_p-GNF
Thermal Plasma
10 to 40 wt %
—

Cu-GNF
Wet chemistry
20 to 50 wt %
From initial report

Cu-G
Wet chemistry
20 to 40 wt %
G = commercial

graphene

Sn-GNF
Wet chemistry
10 to 40 wt %
—

GNF
No preparation
No metal
Bare graphene

powder used to

prepare other

catalysts

Results:

The results are presented in FIGS. 10 to 14, illustrating the faradaic efficiency of different electrodes comprising graphene powder composed of graphene nanoflakes with metal particles deposited thereon.

Discussion:

Bare graphene nanoflakes have been tested as catalysts and appear to have an overall faradaic efficiency of about 17% but with low current density (see FIG. 14). The addition of metal nanoparticles on the surface of the GNFs increases both faradaic efficiency and current density, in exception of iron.

The nature of the metal has an important influence on the overall performance of the catalysts. It appears that tin is related to a high faradaic efficiency with high current density and high selectivity. On the contrary, iron is relatively poorly active with respect to CO₂electroreduction, with a faradaic efficiency lower than 10%.

The method synthesis the catalyst may also impact the overall performance of the electrocatalytic CO₂reduction. For instance, catalysts containing copper nanoparticles prepared using a wet chemistry method have been observed to show a higher faradaic efficiency, current density and selectivity toward formic acid generation compared to the ones prepared through thermal plasma technique. This can be explained by a better control in the metal concentration with the wet chemistry method. Indeed, the addition of metal nanoparticles with the thermal plasma technique tends to provide lower metal concentration than expected, probably due to a higher metal loss rate during the process.

Commercial graphene has been tested as a carbon support in replacement of graphene nanoflakes. Both have very similar structures due to similar reactors used for the carbon nanoparticles synthesis. The main difference between these structures remain in the size of the nanoparticles (commercial graphene nanoparticles larger than GNFs) and in the composition at their surface. Indeed, a small amount of 2% of nitrogen is present on the surface of the GNFs. It can be seen that GNFs as carbon support offer higher faradaic efficiency, higher selectivity and lower current density for same copper concentration compared to the commercial graphene.

Other Metal Particles:

Even though Experiments 1 and 2 pertained to studying the effects of graphene with metal particle deposits, where the metal particle is selected from copper, tin and iron, it will be understood that other metals, metal oxides, metal sulfides, and metal alloys may be used without departing from the present teachings.

Namely, metals, metal oxides, metal sulfides, and metal alloys that have been known to act as a catalyst to transform carbon dioxide to formic acid may be used. Examples may include but not limited to, for instance, copper, copper oxide, copper sulfide, tin, tin sulfide, tin oxide, iridium dihydride, iron carbonyl, one or more manganese complexes, one or more rhodium complexes, one or more iron complexes, one or more copper complexes, bismuth, one or more bismuth complexes, cobalt oxide, one or more ruthenium complexes, one or more rhenium complexes, one or more osmium complexes, lead, lead oxide, mercury, and or an alloy of one or more metals selected from copper, tin, bismuth, lead, mercury and iron.

Exemplary System for Performing Electroreduction of Carbon Dioxide:

Reference is now made to FIG. 15, illustrating an exemplary electrolysis cell 100 for performing the electroreduction of carbon dioxide into a fuel.

The exemplary cell 100 has two current collector plates 1, two separator plates 2, a sealant on each end (e.g. sealant gasket 3), an ion exchange medium (e.g. proton or anion exchange membrane 4), one or more gas diffusion layers 5, a first catalyst layer 6 laid on a first gas diffusion layer 5 that forms part of the cathode (5 and 6) and a second catalyst layer 7 laid on a second gas diffusion layer 5 that forms part of the anode (5 and 7).

The cell 100 has a carbon dioxide inlet 101 that is in communication with the cathode. The carbon dioxide inlet 101 may output carbon dioxide into a gas diffusion layer 5 that diffuses gas across the first catalyst layer 6.

The cell 100 has an ion donor inlet 102. In some examples, the proton or hydroxide donor may be water, but it will be understood that other substances may be used as a proton or ion donor (e.g. H₂, hydroxide salts). The ion donor inlet 102 is in communication with the anode. In some examples, the ion donor inlet 102 may lead to the second gas diffusion layer 5 that is in contact with the second catalyst layer 7.

The cell 100 also has a fuel outlet 103 which is directly or indirectly in communication with the cathode. The fuel (e.g. hydrogen, formic acid, acetate, n-propanol, etc.) is evacuated by the fuel outlet 103 once the fuel is produced. In some examples, the fuel travels through the gas diffusion layer 5 before reaching the fuel outlet 103.

The cell 100 has a by-product outlet 104 that is directly or indirectly in communication with the anode. The by-product outlet 104 is for evacuating the by-products of the reaction taking place with respect to the anode. Examples of by-products may be water and oxygen. In some examples, the by-products may travel through the gas diffusion layer 5 before reaching the by-product outlet 104.

In some examples, on the cathode side, the catalyst of metal-graphene nanoflakes (M-GNFs) is dispersed into an ink (e.g. powder, water/ethanol solvent and binding polymer such as Nafion™) which is deposited on the gas diffusion layer (GDL) 5 made of porous carbon paper and then dried out.

In some examples, the gas diffusion layer 5 may be made out of a porous material other than carbon paper, such as out of porous stainless steel or titanium. In some examples, a thin protective layer may be added over the gas diffusion layer 5, such as a thin protective layer of metal.

On the anode side, similar procedure is followed with a catalyst designed to break water molecules into hydrogen protons, electrons and oxygen. Exemplary catalysts on the anode side may be a precious metal/metal oxide, pure or dispersed onto a carbon support (e.g. iridium oxide (IrO₂), nickel, etc.)

The two GDLs with the catalyst may be hot pressed with an ion exchange membrane (e.g. polymer membrane 4) between the two. In some embodiments, a gap may be present between the GDL and the ion exchange membrane, with electrolyte flowing in this gap. The ion exchange membrane 4 allows ions to go through while electrically insulating the anode from the cathode and preventing products to diffuse between the two electrodes. The ion exchange membrane 4 can include Nafion™, or another proton or ion exchange membrane. The GDLs 5 with the catalyst layers and the ion exchange membrane 5 may represent a membrane electrode assembly (MEA).

The MEA may be pressed between two separator plates 2 and surrounded by a sealer (e.g. sealing gaskets 3) on both sides. The separator plates 2 have a flow pattern engraved on their surface in contact with the MEA for gas and liquid management in the electrolyser. The sealing gaskets 3 prevent leakage from the cell.

The current collector plates 1 deliver electricity to the electrolyzer cell 100. They allow electrons to go from the anode to the cathode.

In one example, at the anode, water, in liquid or vapor form, is injected at the inlet 102. Water molecules diffuse into the catalyst layer through the GDL 5 and produce dioxygen gas, hydrogen protons and electrons. The hydrogen protons migrate from the anode to the cathode 6 through the ion exchange membrane 4. At the cathode, pure or diluted CO₂(preferably pure CO₂) may be injected in the presence of water, enabling membrane humidification, electric conduction and liquid product transportation at the outlet of the electrolyser. The CO₂molecules diffuse to the catalyst layer and react with hydrogen protons and electrons to form fuel products (e.g. formic acid, n-propanol, acetic acid). A side reaction that may be competing with the CO₂electroreduction may result in the production of dihydrogen gas.

Reference is now made to FIG. 16, illustrating a three-dimensional rendering of a system 200 for electro-reducing carbon dioxide.

The electrolyzer cells 100 are connected in series and electrically insulated one from each other. The cells are pressed together between two end plates and form a stack, represented on FIG. 2 (middle part). The right part of the 3D drawing of FIG. 16 shows CO₂and water being stored in tanks before being injected inside the stack. The left part shows a separation unit at the outlet of the stack. Liquid and gas products may be separated and stored. CO₂and water at the outlet of the separation unit may be recirculated in the electrolyzer. The separation process unit may consist non-exclusively of a distillation column, gas separation membranes and compressors to eventually store gases products.

Although the invention has been described with reference to preferred embodiments, it is to be understood that modifications may be resorted to as will be apparent to those skilled in the art. Such modifications and variations are to be considered within the purview and scope of the present invention.

Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawing. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings.

Moreover, combinations of features and steps disclosed in the above detailed description, as well as in the experimental examples, may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

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	Number	Date	Country
Parent	62799191	Jan 2019	US
Child	16778772		US

METAL-CARBON NANOSTRUCTURES AND METHOD OF MANUFACTURING THEREOF

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