Patent Application
20250149601
With the increasing need for energy and rapid depletion of traditional fossil fuels, hydrogen gas (H2) has been considered as one of the most promising green energy resources. However, currently H2 is mostly produced by steam-methane reforming at high temperatures (700˜1000° C.), making it energy- and capital-consuming. Electrochemical water splitting (i.e., water electrolysis) represents an effective alternative, where H2 is produced at the cathode using electricity produced from a sustainable source such as wind, sun light, and hydraulics. An appropriate catalyst is needed to catalyze the hydrogen evolution reaction (HER) so as to decrease the overpotential and increase the current density. Platinum (Pt)-based nanoparticles have remained the catalysts of choice towards HER, yet the high cost and limited natural abundance has hampered the wide-spread applications.
Carbon-based nanocomposites are attracting particular attention as high-performance, low-cost electrocatalysts for electrochemical water splitting. In particular, carbon-based nanocomposites have been hailed as viable catalysts for fuel cells and water electrolyzers. These materials are typically prepared via the pyrolysis and hydrothermal processes. While these methods are rather simple and effective in sample synthesis, they are energy and time-consuming, and the slow heating ramp makes it difficult to produce a non-equilibrium phase in the samples, which is critical in regulating the electronic structure, and hence, the electrocatalytic activity. A range of effective strategies have emerged recently, such as carbothermal shock, flash joule heating, laser ablation, and laser scribing. Despite the progress, the toolbox for such sample synthesis has been limited, and the range of materials that can be produced and the extent of structural engineering remain narrow. Further development of effective protocols for the synthesis of materials with unprecedented structures and properties is of both fundamental and technological significance. Thus, there is a need for HER catalysts that are cheaper and more-environmentally friendly than currently available Pt-based catalysts.
The present disclosure provides a novel method for preparing nanocomposites using magnetic induction heating/rapid quenching (MIHRQ). The nanocomposites may be carbon-iron (Fe)-nickel (Ni) spinel oxide or ruthenium (Ru) nanocomposites. The Fe—Ni nanocomposites exhibit a mixing of the Ni and Fe phases and a Cl-rich surface, in contrast to the conventional nanocomposite prepared by prolonged heating and/or natural cooling to the ambient temperature.
The disclosed process includes rapid heating and quenching of Ni and Fe precursors (e.g., at a temperature rate of change of up to 103 K s−1), which impedes the Ni and Fe phase segregation in FeNi spinels and produces a chloride (Cl)-rich surface thereon. Both of these properties contribute to the remarkable catalytic activity of the catalyst composition. The present disclosure also provides examples illustrating the unique advantage of rapid heating/quenching in the structural engineering of functional nanocomposites to achieve high electrocatalytic performance towards important electrochemical reactions.
In electrochemical measurements, the disclosed nanocomposites display an outstanding electrocatalytic performance towards oxygen evolution reaction (OER), with an ultralow overpotential of about 260 mV to reach the high current density of 100 mA cm−2. The overpotential at this chosen current density was used to quantify and compare the OER activity—the lower the better. Comparison can also be made based on the overpotentials at other current densities (e.g., 10 mA cm−2, 50 mA cm−2, etc.).
This is due to the formation of a metastable structure that is optimal for the adsorption of key OER intermediates and eventual production of oxygen. In electrochemical water splitting, OER has been recognized as a major bottleneck that limits the overall performance because of complex reaction pathways and sluggish electron-transfer kinetics, and FeNi spinel oxides provide a viable alternative to the traditional, noble metal-based commercial catalysts, where manipulation of the occupation of the eg orbitals of the octahedral metals and/or metal-oxygen covalency represent the leading strategies for further enhancement of the OER activity. This may be achieved by engineering the spinel components using heterometal doping and introduction of oxygen vacancies. Phase segregation of Fe and Ni in the spinels is believed to be the leading cause of the apparent loss of the electrocatalytic activity. Moreover, such segregation is inevitable for samples prepared via a conventional thermal process as it is energetically favorable. In addition, residual heteroanions (e.g., Cl) adsorbed on or doped into the surface of Fe, cobalt (Co) and Ni (hydro/oxyhydro) oxides also play a significant role in OER electrocatalysis. Yet the impacts of such anion impurities have remained largely ignored, although most pyrolytically prepared spinel oxides are derived from iron and nickel chlorides. Thus, appropriate synthetic methods of the present disclosure allow for production of such metastable structures with reduced phase segregation and a remarkable concentration of anion impurities.
Ru provides a competitive alternative for platinum towards HER, a critical process in electrochemical water splitting. Ru costs about half of Pt and has emerged as a viable substitute, due to its similar bonding strength with hydrogen (˜65 kcal mol−1) to that of Pt—H, a critical parameter in dictating the HER activity. However, previous studies have indicated Ru's being less suitable as a catalyst when compared to Pt. In the well-known volcano-shaped plot of hydrogen adsorption Gibbs free energy (ΔGH*), Ru is actually situated on the left side, suggesting a somewhat strong adsorption of H that is unfavorable for H desorption from the catalyst surface. Computational studies based on density functional theory (DFT) have shown that H adsorption onto the top sites of Ru(0001) facet possesses an almost ideal ΔGH* of only −0.07 eV, in comparison to the adjacent hollow Ru3 sites that exhibit a far more negative ΔGH* of approximately −0.45 eV, suggesting that the latter is actually the most likely dominant binding sites, leading to a nonideal HER performance. In addition, effect of ruthenium crystallinity on the HER activity has also been shown as being ineffective in diminishing ΔGH* for optimal HER. It was previously observed that the ΔGH* on the hollow or bridge sites on most facets of hexagonal closed pack (hcp) Ru and face centered cubic (fcc) Ru all ranged from −0.5 to −0.7 eV, markedly greater than that on Pt(111) (approximately 0 eV).
The present disclosure provides a novel method based on magnetic induction heating and rapid quenching (MIHRQ) to synthesize Ru nanoparticles supported on carbon paper within seconds. Notably, in the ultrafast heating-quench process, metallic Ru nanoparticles were generated and deposited evenly on carbon paper by thermal decomposition of RuCl3 salt even in the ambient atmosphere. Because of the short heating duration, RuCl3 was incompletely decomposed, leading to residual Cl on the surface of Ru nanoparticles with the Cl content being controlled by adjusting magnetic current and heating time and directly related to the HER performance of the nanoparticles. The examples of the present disclosure confirmed in DFT studies where the Cl species influenced the electronic structure of metallic Ru and the adsorption configuration and energetics of H*. Among the series, the best sample was obtained with a magnetic current of about 300 A and heating time of about 6 s, which demonstrated an HER activity similar to that of commercial Pt/C in both alkaline and acidic media with a respective overpotential (η10) of −12 mV and −23 mV to reach the current density of 10 mA cm−2.
The Ru nanoparticles supported on carbon substrate are formed by MIHRQ within seconds and include a Cl-enriched surface that is unattainable via conventional thermal annealing. The Ru nanoparticles demonstrate remarkable HER activity in both acidic and alkaline media with an η10 of only −23 mV and −12 mV, respectively. Theoretical studies based on DFT showed that the excellent electrocatalytic activity can be accounted for by the metal-Cl species that facilitate charge transfer and shift of the d-band center. These results highlight the unique advantages of MIHRQ in rapid sample preparation where residual anion ligands play an important role in manipulating the electronic properties of the metal surfaces and hence the electrocatalytic activity.
According to one embodiment of the present disclosure, a method for making a catalyst composition is disclosed. The method includes placing a substrate with at least one precursor composition disposed thereon in contact with a ferromagnetic material and placing the substrate and the ferromagnetic material within an induction solenoid. The method further includes generating an alternating magnetic field within the induction solenoid upon energization by a power source supplying alternating current, thereby heating the substrate and the ferromagnetic material to a temperature of from about 200° C. to about 1,500° C. The method additionally includes rapidly cooling the substrate and the ferromagnetic material.
Implementations of the above embodiment may include one or more of the following features. According to one aspect of the above embodiment, rapidly cooling may further include quenching the substrate and the ferromagnetic material are submerged in a liquid having a temperature of from about −50° C. to about −100° C. The liquid may include an alcohol and dry ice. The precursor composition may include a solution of a nickel salt and iron salt. The nickel salt may be nickel chloride and the iron salt may be iron chloride. The substrate may be a carbon paper. The ferromagnetic material may be an iron sheet. The substrate with the at least one precursor composition may be disposed between two sheets of the ferromagnetic material. The alternating current may be from about 200 amps to about 600 amps. The alternating current may be about 300 amps. The alternating current may be supplied from about 3 seconds to about 12 seconds. The alternating current may be supplied for about 6 seconds. The precursor composition may include a ruthenium halide salt. The ruthenium halide salt may be ruthenium (iii) chloride. Rapidly cooling may form a plurality of nanoparticles, which may include ruthenium and a chloride-rich surface. Rapidly cooling may form a plurality of nanoparticles, which may include the ferromagnetic material and at least one heteroanion.
According to another embodiment of the present disclosure, a catalyst composition is disclosed. The catalyst composition includes a plurality of nanoparticles disposed on a carbon substrate, the plurality of nanoparticles having at least one ferromagnetic material and at least one heteroanion.
Implementations of the above embodiment may include one or more of the following features. According to one aspect of the above embodiment, the plurality of nanoparticles may have an average particle diameter from about 20 nm to about 100 nm. The heteroanion may be chloride. The at least one heteroanion may be present in an amount from about 1% to about 15% by weight of the catalyst composition. The catalyst composition may further include a plurality of nanospindles. The plurality of nanospindles may include a higher amount of chloride than the plurality of nanoparticles. The at least one ferromagnetic material may include iron and nickel. The plurality of nanoparticles may be spinels having oxygen. The spinels may have a formula of Fe3-xNixO4.
According to a further embodiment of the present disclosure, a catalyst composition is disclosed. The catalyst composition may include a plurality of nanoparticles disposed on a carbon substrate, the plurality of nanoparticles may include ruthenium and one or more heteroanions.
Implementations of the above embodiment may include one or more of the following features. According to one aspect of the above embodiment, the catalyst composition may include the plurality of nanoparticles having an average particle diameter from about 2 nm to about 10 nm. The heteroanion may be chloride. The heteroanion may be present in an amount from about 1% to about 15% by weight of the catalyst composition. The heteroanion may be primarily disposed on a surface of the plurality of nanoparticles. The catalyst composition has an HER overpotential of about −23 mv to reach 10 mA cm−2 in an acidic medium. The catalyst composition has an HER overpotential of about −12 mV to reach 10 mA cm−2 in an alkaline medium.
Various embodiments of the present disclosure are described herein below with reference to the figures wherein:
The present disclosure is directed to a catalyst composition including a nanocomposite material having a carbon substrate and a plurality of nanoparticles, which may be Fe—Ni spinel nanoparticles or Ru nanoparticles, disposed on the carbon substrate. As used herein, the term “nanoparticle” denotes a particle having any shape and size from about 1 nm to about 100 nm. As used herein the terms “about”, “approximately”, and other relative terms, denote a range of +5% of the stated value.
The nanoparticles according to the present disclosure may be formed from a precursor composition disposed on a substrate placed between ferromagnetic material sheets, which allow for rapid induction heating. The heating process is followed by rapid cooling via quenching to form nanoparticles that are doped with heteroanions provided by the precursor composition.
Induction heating is a process of heating an electrically conductive material via magnetic induction, through heat generated in the material by Eddy currents. An induction heater may include an electromagnet and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic field passes through the material, generating Eddy currents therein. The Eddy currents encounter the resistance of the material thereby heating it by Joule heating. In ferromagnetic materials, such as Ni and Fe, heat may also be generated by magnetic hysteresis losses.
With reference to
The catalyst composition may be formed by contacting a precursor composition with a ferromagnetic material. In embodiments, the precursor composition may be a solution of salts including Ni and Fe salts, such as NiCl2 and FeCl3. Suitable solvents include any polar solvent, such as water. The precursor composition may also include urea.
The catalyst composition may also be formed by placing a precursor composition on a substrate. In embodiments, the precursor composition may include a catalytic metal halide salt, such as metal chlorides, metal fluorides, metal iodides, and the like. Catalytic metals of the halide salt may include platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), nickel (Ni), osmium (Os), manganese (Mn), iron (Fe). Suitable catalytic metal halide salts may include, but are not limited to, platinum (II) chloride (PtCl2), rhodium (III) chloride (RhCl3), iridium (III) chloride (IrCl3), ruthenium (III) chloride (RuCl3), and the like
The precursor composition may be a solution of the catalytic metal halide salt dissolved in any suitable solvent, which include, but is not limited to, acetone, alcohols, water, and other polar solvents.
The precursor composition may be placed on a substrate, such as carbon paper, using any liquid deposition technique, including drop casting, solution casting, and the like. The loaded substrate is then placed in contact with a ferromagnetic material, such as an Fe, Ni, and combinations thereof. The ferromagnetic material may be in the shape of a sheet. In embodiments, the ferromagnetic material may have a larger surface area than the substrate, such that the substrate is completely covered by the iron sheet. In further embodiments, the substrate may be placed between a pair of ferromagnetic sheets to form a composite sheet 20.
The composite sheet 20 may be suspended inside the induction coil 14 using a metal post 22, such as an iron nail, which is then attached to a holder or any other apparatus for securing the composite sheet 20 relative to the induction coil 14 without physical contact therebetween. The composite sheet 20 may be heated to a temperature of from about 200° C. to about 1,500° C. Various combinations of the induction coil 14, i.e., materials, thickness, shape, configuration, and operating parameters of the power source 18 may be adjusted to heat the sample to a desired temperature.
The composite sheet 20 may be disposed within the induction coil 14 using any suitable support structure. In embodiments, the composite sheet 20 may be suspended by one or more shafts 22. In further embodiments, the composite sheet 20 may be supported by a scaffold or a post (not shown). The shaft 22 may be operated by a submerging mechanism (e.g., clamp) configured to lower the composite sheet 20 into the cooling bath 24.
The system 10 also includes a cooling bath 24 disposed underneath the induction coil 14 to allow for the composite sheet 20 to be submerged (e.g., dropped) in the cooling bath 24 thereby quenching the composite sheet 20. The cooling bath 24 may be from about −50° C. to about −100° C. The cooling bath 24 may be any liquid mixture or composition configured to rapidly cool the heated composite sheet 20. In embodiments, the liquid mixture or composition may include a solvent, e.g., ethanol, and a cooling agent, e.g., dry ice (solid carbon dioxide).
Quenching is performed after the composite sheet 20 has been sufficiently heated to reach the desired temperature. Sufficient heating may be determined once a suitable parameter (e.g., time, temperature, current, etc.) reaches a threshold. The parameter may be measured using a corresponding sensor, such as a timer, a temperature sensor configured to measure the temperature of the composite sheet 20 during heating, a current sensor to monitor current output by the power source 18, etc. The sensor may be used in a control loop operating the submerging mechanism configured to quench the composite sheet 20 once the parameter reaches the threshold.
The rapid heating and quenching process includes increasing and lowering the temperature of the composite sheet 20 at a rate of from about 500 K s−1 to about 1,000 K s−1. This process as shown in
The rapid heating and quenching produce a nanoparticle surface that is rich with heteroanions, e.g., Cl, provided by the precursor salt with the core of the nanoparticle being primarily formed from the catalyst metal, e.g., Ru. A Cl-rich surface contributes to the relatively high catalytic HER activity of the nanoparticles of the catalyst composition in both acidic and alkaline media with an overpotential of only −23 mV and −12 mV, respectively, to reach the current density of 10 mA cm−2.
The amount of hetereanion present on the surface of the catalyst metal nanoparticles depends on the amount and duration of heat applied to the composite sheet 20. The heteroanion may be present in an amount from about 1% to about 15% by weight of the catalyst composition. The heteroanion is present primarily on the surface of the nanoparticles, which may be from about 80% to about 100% of the total amount of the heteroanion. In embodiments, the power source 18 may be energized to supply a current from about 100 amps to about 600 amps, and in embodiments from about 200 amps to about 400 amps, and in further embodiments about 300 amps. The current may be applied from about 1 second to about 20 seconds, and in embodiments from about 3 seconds to about 12 seconds, and in further embodiments about 6 seconds.
In embodiments, where Fe and Ni compounds are used as precursor materials, the catalyst composition includes a plurality of FeNi oxide spinel nanoparticles having a formula of Fe3-xNixO4. The nanoparticles are disposed on the carbon substrate and may have an average particle diameter from about 20 nm to 100 nm. The catalyst composition also includes a plurality of nanospindles or nanocrystals having an average particle diameter from about 1 nm to about 5 nm. The catalyst composition also includes one or more heteroanion, e.g., Cl, disposed on the surface of the nanoparticles and within and/or on the surface of the nanospindles. The catalyst composition may include Fe from about 20% to about 40% by weight of the composition, oxygen (O) from about 50% to about 60% by weight of the composition, Ni from about 5% to about 15% by weight of the composition, and Cl from about 1% to about 15% by weight of the composition. In embodiments, Fe, Ni, O, and Cl may be present at a ratio from about 1.9:1:4.9:1.1 to about 4.1:1:5.8:0.2.
The rapid temperature changes during heating and cooling impede phase segregation of Ni and Fe resulting in atomic mixing of Ni and Fe. The rapid heating and quenching also produces a surface rich with heteroanions, e.g., Cl, provided by the precursor salt. Lack of phase segregation and Cl-rich surface contribute to the relatively high catalytic activity needing only 260 mV to reach the high current density of 100 mA cm−2 as described in the Examples section below.
The catalyst compositions according to the present disclosure may be used in hydrogen evolution reaction (HER), a water splitting electrolysis reaction. The rate of hydrogen generation from the HER according to present disclosure may be affected by the pH and temperature at which HER is carried out. Accordingly, the HER may be carried out at a pH from about 0 (0.5M H2SO4 to 14 (1 M KOH), from about 9 to about 13, and in embodiments from about 10 to about 12. The HER may also be carried at a temperature from about 22° C. and 100° C., in embodiments from about 30° C. to about 80° C., and in further embodiments, from about 40° C. to about 60° C. HER may be carried with any suitable water; however, certain impurities present in the water may affect the rate of hydrogen generation.
The method for hydrogen generation according to the present disclosure includes providing a catalyst composition according to the present disclosure and exposing the catalyst composition to a hydrogen containing compound such as water or an aqueous solution. Exposure to the compound may be carried by placing the catalyst composition in a liquid container.
The hydrogen containing compound may be an aqueous alkaline medium, which may be prepared by dissolving an alkaline compound including alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, and tetraalkylammonium hydroxides such as tetramethylammonium hydroxide and tetraethylammonium hydroxide. Suitable solvents include pure water or water that is mixed with various water-miscible solvents including alcohols such as methyl and ethyl alcohols, dimethylformamide, dimethylacetamide, ethyleneglycol, diethyleneglycol and the like. The aqueous alkaline medium may include from about 1% by to about 30% by weight of the alkaline compound dissolved therein. The generated hydrogen may be collected or syphoned for later use. In further embodiments, the generated hydrogen may be used directly with any system and or apparatus that utilizes hydrogen as a source of fuel, such as a fuel cell.
The following Examples illustrate embodiments of the present disclosure. These Examples are intended to be illustrative only and are not intended to limit the scope of the present disclosure. Also, parts and percentages are by weight unless otherwise indicated. As used herein, “room temperature” or “ambient temperature” refers to a temperature from about 20° C. to about 25° C.
This example describes synthesis of FeNi spinel.
An MIHRQ apparatus as shown in
Carbon paper (source: Toray Industries, Inc.) was cut into 1 cm×1.5 cm pieces. A solution was prepared by dissolving 40 mg of NiCl2·6H2O, 10 mg of FeCl3 and 0.8 g of urea into 10 ml of water (supplied with a Barnstead Nanopure Water System, 18.2 MΩ cm). 100 μL of the solution was dropcast onto the carbon paper, which was then dried at ambient temperature and sandwiched between two iron sheets of 2.5 cm×2.5 cm×0.1 cm.
An iron nail was inserted into the center of the iron sheets and clamped to hold the assembly, which was placed in the center of the induction solenoid. A high frequency current at about 30 kHz was passed through the solenoid, producing a strong magnetic field, which instantly generated a strong Eddy current in the iron sheets, and thus, heating the sample rapidly to a high temperature.
The induction current and time was varied to control the heating temperature. A solenoid current of 200 A for a heating time of 4 s generated a temperature of about 200-300° C., which barely changed the color of the iron sheets (
After a period of heating (e.g., less than one minute), the sample was dropped into the quenching solution of an ethanol-dry ice at about −78° C., which was placed underneath the induction coil for rapid quenching. Control samples were removed from the solenoid and cooled down naturally under ambient conditions. This offered an additional control of the materials structures.
A series of samples were prepared with the MIHRQ setup at a controlled induction current (X=100-600 A) for a select period of time (Y=2-16 s), and referred to hereinafter as FeNiO-X-Y, FeNiO-250-4 and FeNiO-250-16. Control samples were prepared in the same manner except for cooling under ambient conditions, and referred to hereinafter as FeNiONC-X-Y, e.g., FeNiONC-250-4.
This Example describes structural characterization of samples of Example 1.
Structural characterization was carried out using STEM experiments, which were conducted with a transmission electron microscope equipped with an X-FEG field-emission source, operated at 200 keV. HAADF-STEM imaging and EDS analysis was also performed on the samples, which were first sonicated, dispersed in ethanol, and then deposited onto copper grids for TEM characterization. SEM studies were carried out on FEI Quanta 3D FEG dual beam instrument. XPS measurements were performed with a Phi 5400/XPS instrument equipped with an Al Kα source operated at 350 W and 10-9 Torr. XRD patterns were acquired with a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=0.15418 nm).
STEM measurements of the NiFeO-250-4 sample and of the NiFeO-250-4 sample were obtained at different preparation stages are shown in
HAADF-STEM images were also obtained and show formation of a number of nanoparticles having an average particle diameter from about 20 nm to 100 nm in irregular shapes as shown in
Furthermore, energy-dispersive X-ray spectroscopy (EDS)-based elemental mapping studies also demonstrated an even distribution of Fe and Ni within the lattice, as shown in
The atomic ratio of Fe:Ni:O:Cl in the nanospindles was estimated to be 1.9:1:4.9:1.1, while the overall ratio was close to 4.1:1:5.8:0.22, indicating that the spinel nanoparticles were Fe-rich oxide, while the nanospindles represented an intermediate phase between the precursors (metal chlorides) and the final spinel crystal. Furthermore, the Fe:Ni ratio was higher than the feeding ratio, which was likely due to Ni not being fully converted into Ni oxide and being washed away during the rapid quenching process.
For FeNiO-250-16, which was prepared via a longer heating time, the Fe:Ni:O:Cl ratio was estimated to be 6.6:1:8.6:0.028 (Table 1), indicative of the formation of a Fe-rich structure that was almost free of Cl. The nanospindle features, with the unique chlorine rich surface, were produced with a short heating time and rapid quenching process.
The control sample, FeNiONC-250-4 that was produced by similar heating but natural cooling in the ambient, exhibited a different morphology as shown in
The material were further characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements.
Further oxidation states and structural insights were obtained by XAS measurements. Fe and Ni K-edge XAS data was collected from the CLS@APS Sector 20-BM beamline at the Advanced Photon Source (operating at 7.0 GeV) in Argonne National Labs, Chicago, IL, USA. Samples were enclosed within Kapton tape and measured in fluorescence mode simultaneously with each elements foil reference. All measurements were conducted at room temperature and ambient pressure. EXAFS data was transformed and normalized into k- and R-space using the Athena program following conventional procedures. A k weighting of 2 was used to obtain all FT-EXAFS spectra. The k-range used for each sample is as follows for Fe: 3.1-9.2 Å−1 for FeNiO-250-4, 2.1-9.1 Å−1 for FeNiONC-250-4, 3.3-12.7 Å−1 for FeNiO-250-16. For Ni the k-range used was as follows: 3.0-8.9 Å−1 for FeNiO-250-4, 2.9-12.2 Å−1 for FeNiONC-250-4, 2.6-14.4 Å−1 for FeNiO-250-16. The R-range used for Fe is as follows: 1.0-3.7 Å for FeNiO-250-4, 1.0-3.6 Å for FeNiONC-250-4, 1.0-3.4 Å for FeNiO-250-16. The R-range used for Ni is as follows: 1.0-3.5 Å for FeNiO-250-4, 1.0-3.0 Å for FeNiONC-250-4, 1.0-3.0 Å for FeNiO-250-16. Self-consistent multiple-scattering calculations were performed using the FEFF6 program to obtain the scattering amplitudes and phase-shift functions used to fit various scattering paths with the Artemis program. In the fitting of each sample the E0 values were correlated together to minimize the number of independent values, allowing reliable fitting results to be obtained. The σ2 values were also correlated for some samples.
Further insights into the bonding configurations of the metal centers were obtained from the EXAFS results. Fitting of the FT-EXAFS data of
Results from these measurements show that prolonged heating and slow cooling facilitated the O and Cl loss for the spinel samples. Prolonged heating also promoted phase segregation of Ni into rock salt NiO or metallic form. With a deliberate control of the heating time and cooling rate, two key non-equilibrium features of the FeNiO spinel nanoparticles can be achieved, minimal Fe—Ni phase segregation, and formation of a Cl-rich surface, both critical in OER electrocatalysis (detailed below).
This Example describes electrochemical analysis of samples of Example 1.
Electrochemical measurements were carried out with a CHI 700e electrochemical workstation in a three electrodes configuration. The prepared carbon paper (i.e., sample of Example 1) was fixed onto a graphite electrode holder, with an exposed surface area of 1 cm2. A platinum wire was used as the counter electrode and an Ag/AgCl in saturated KCl as the reference electrode. The reference electrode was calibrated against a reversible hydrogen electrode (RHE) and all potentials in the present study were referenced to this RHE.
The FeNiO-250-4 sample that possessed a unique Fe—Ni oxide spinel with Cl-rich surface nanospindles exhibited a remarkably high activity towards OER. As shown in the polarization curves of
Notably, the FeNiO-250-4 sample represented the optimal condition for OER and also showed excellent stability. At the applied potential of 1.53 V, over 80% of the initial current was retained even after 10 hours of continuous operation as shown in the inset of
To unravel the mechanistic origin of the remarkable OER activity observed above with FeNiO-250-4, slab models of a crystal surface were built by chlorine substitution of the surface oxygen atom originally located between Feoct (octahedral site) and Nitd (tetrahedral site) in NiFe2O4(100) (
Spin-polarized density functional theory (DFT) calculations were carried out using the VASP (Vienna Ab-Initio Simulation Package) code. Projector-augmented wave (PAW) method with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was used in all calculations. On-site Coulomb interactions were corrected within the DFT+U framework based on Dudarev's approximation. Ueff=4.20 and 6.40 for Fe and Ni, respectively, were used. Plane-wave basis set with a 400 eV energy cutoff provides a balance of accuracy and computational cost. Either quasi-Newton scheme or conjugate gradient algorithm implemented in VASP was used to relax structure until forces are converged to less than −0.3 eV Å−1 on unconstrained atoms and self-consistent convergence until 10−5 eV. The Gaussian smearing with a σ value of 0.5 was used to minimize entropy contribution to free energy. The bulk structure of NiFe2O4 was taken from JCPDS (JCPDS Card No. 10-0325) and optimized in (4×4×1) k-point grid sampling of the surface Brillouin zone. The optimized lattice constant of 8.37 Å was found in close agreement with the experimental value of 8.35 Å.
A supercell consisting of five layers of NiFe2O4 with an exposed (100) surface was constructed from the optimized bulk structure. A vacuum space of 14 Å in z-direction was inserted between the slabs, and the atoms in the top three layers were allowed to relax while those in the bottom two layers were fixed at the corresponding bulk position during structural optimization.
Free energy calculations indicate that adsorption of OH favors the Nitd sites over the Feoct sites on the surface, consistent with the XPS results (
A two-site (*- #) model was adopted to study the OER mechanism. Generally, a single active site mechanism, shown in eq. S1-S4, has been widely used in analyzing oxygen evolution reaction (OER) catalyzed by an oxide catalyst, such as Ni, Co, Fe spinels.
*+OH→*—OH (S1)
*—OH+OH→*—O+H2O (S2)
*—O+OH→*—OOH (S3)
*—OOH+OH→O2+*+H2O (S4)
On the NiFe2O4 catalyst, Ni was considered as the active site for OER and the potential limiting step was *—O→*—OOH (eq. S3). This step has a reaction free energy of 2.0 eV, corresponding to a thermodynamic overpotential of approximately 770 mV. Based on the scaling relationship between the binding energies of *OH and *OOH, an overpotential less than 0.4 V cannot be achieved by following the single metal site mechanism. Therefore, the mechanism based on a single metal site was not believed to contribute to the high activity observed in the present study.
A mechanism involving two adjacent metal sites, i.e., *- #, was adopted. According to this mechanism, shown in eq. S5-S8, OER starts by OH binding on the first metal site forming HO*—#(ΔG1∘, eq. S5). This is followed by a second OH binding at the neighboring metal site forming HO* #OH (ΔG2∘, eq. S6). A stepwise reaction of HO* #OH with OH (ΔG3, eq. S7 and ΔG4∘, eq. S8) releases O2 and H2O and completes the cycle.
*-#+4(OH−h+)→HO*—#+3(OH−+h+)−ΔG1∘ (S5)
HO*—#+3(OH−+h+)→HO*—#OH+2(OH−+h+)ΔG2∘ (S6)
HO*—#OH+2(OH−+h+)→HO*#O+H2O+(OH−+h+)ΔG3∘ (S7)
HO*—#O+H2O+(OH−+h+)→*-#+O2+2H2OΔG4∘ (S8)
The reaction free energies of these steps were calculated according to eq. S5-S8. Based on the calculated reaction free energies, the reaction free energy profile shown in
The reaction free energy expressions for the reactions described in eq. S5-S8 are:
ΔG1∘=μ(HO*—#)+3μ(OH−+h+)−μ(*-#)−4μ(OH−+h+) (S9)
ΔG2∘=μ(HO*—#OH)+2μ(OH−+h+)−μ(HO*—#)−3μ(OH−+h+) (S10)
ΔG3∘=μ(HO*—#O)+μ(H2O)+μ(OH−+h+)−μ(HO*—#OH)−2μ(OH−+h+) (S11)
ΔG4∘=4.92−(ΔG1∘+ΔG2∘+ΔG3∘) (S12)
Since PBE significantly overestimates μ(O2(g)), ΔG4∘ in eq. S12 was computed on the basis of the experimental reaction free energy of 4.92 eV for 2H2O(l)→2H2(g)+O2(g). The chemical potential of OH, i.e., μ(OH−+h+), was computed using an approach based on CHE. Free energies of all intermediates were determined using G∘=Eelect∘−TS+ZPE. Eelect∘ was obtained from DFT calculations, whereas the contributions of TS and ZPE were computed from frequency calculations in which adsorbate together with the atoms in the topmost layer were allowed to move.
From the free energy diagram of
With continued reference to
To understand the enhanced OER activity in Ni(OH)Fe2O4(Cl), charge density differences were tracked and compared the bond distances of the O* #OH species on Fe(OH)Fe2O4, Fe(OH)Fe2O4(Cl), Ni(OH)Fe2O4 and Ni(OH)Fe2O4(Cl). With reference to
Charge redistribution at the Fe1 and Fe2 sites was also reflected in part in the decrease of Bader charge of the O—O pair, which was −1.2|e|, −1.14|e|, −1.4|e| and −1.0|e| for the O—O pair adsorbed on Fe(OH)Fe2O4, Fe(OH)Fe2O4(Cl), Ni(OH)Fe2O4 and Ni(OH)Fe2O4(Cl), respectively. A decreased negative charge value indicates an increase of acidity of O*— #OH, which benefits the proton transfer from O*— #OH to the OH adsorbed on Nita or Feta (
Magnetic induction heating and rapid quenching was exploited for the rapid fabrication of metal oxide spinel nanostructures. Using NiCl2 and FeCl3 as the precursors, FeNi oxide spinels were obtained by heating at controlled currents within seconds and exhibited an even mixing of the Ni and Fe elements and a Cl-rich surface, in sharp contrast to samples prepared at prolonged heating and/or natural cooling in the ambient. The best sample, FeNiO-250-4 needed an overpotential of only 260 mV to reach the high current density of 100 mA cm−2 and exhibited significant stability in alkaline media. Such a remarkable activity was attributed to the unique metastable structure that facilitated the adsorption of key reaction intermediates and O—O coupling, a major limiting step in OER. Results from this study highlight the unique advantages of MIHRQ in the facile production of unprecedented material structures that are unattainable in conventional thermal processes for enhanced electrocatalytic performance and potential applications in the structural engineering of a diverse range of materials.
This example describes synthesis of Ru nanoparticle catalyst composition as shown in
Ruthenium (III) chloride (RuCl3) hydrate (RuCl3·XH2O, 35-40%, ACROS Organics), was dissolved in acetone (from Fisher chemicals) to form a solution at a concentration of about 20 mg mL−1. Carbon paper (from TGP-H-90, Toray) was thermally treated in a Muffle furnace at 500° C. in ambient atmosphere for about 1 h to increase surface wettability, cut into 1×2 cm2 pieces, and rinsed with acetone several times.
Approximately 100 μL of the solution of RuCl3 was first dropcast onto a piece of pretreated carbon paper and then dried at room temperature for 10 min. After drying in air for 30 min, the carbon paper was wrapped in graphite paper (0.01 mm thick) before being sandwiched between two iron sheets (2.5 cm×2.5 cm×0.01 mm) to prevent direct contact of the samples to the iron sheets to prevent Fe contamination at high temperature.
The assembly was placed into the middle of an induction coil of a magnetic induction heater (
A series of samples were prepared with the MIHRQ setup at a controlled induction current (X=200-600 A) for a select period of time (Y=3-12 s) and referred to hereinafter as Ru-X-Y, in particular Ru-200, Ru-300, Ru-400, Ru-600, which were heated for 6 seconds, Ru-300-S heated for 3 seconds, and Ru-300-L heated for 12 seconds.
After heating was completed, the sample was dropped into an ethanol-dry ice solution (−78° C.) placed underneath the induction coil for rapid quenching to cool the sample and to prevent oxidation in the air.
By virtue of the Joule effect, the iron sheets were heated up at an ultrafast rate (over 100 K s−1) owning to the Eddy current generated instantly by the magnetic field. As a thermal-radioactive material, carbon paper was heated up simultaneously, converting RuCl3 into Ru nanoparticles supported on carbon paper.
This Example describes structural characterization of samples of Example 4.
Structural characterization was carried out using TEM experiments, which were conducted with a Tecni G2 transmission electron microscope operated at about 200 keV. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Thermo Fisher K-alpha system, where the binding energy was calibrated against the C 1s binding energy. Raman measurements were conducted using a Horiba Jobin Yvon LabRAM ARAMIS automated scanning confocal Raman microscope under 532 nm excitation. X-ray absorption spectroscopy (XAS) measurements were carried out at 10 K on beamline 4-1 of the Stanford Synchrotron Radiation Lightsource using an Oxford liquid helium cryostat.
The structure of the samples was first characterized by TEM measurements.
When the magnetic current was increased to 300 A, the corresponding sample, Ru-300 (
The sample morphology was also readily manipulated by adjusting the heating time. When the heating time was reduced to 3s (Ru-300-S), only amorphous particles were produced on carbon (
Elemental mapping analysis based on electron energy loss spectroscopy (EELS) measurements also showed that Ru was mostly confined within the nanoparticles, with residual O and Cl (
For Ru-400 that was prepared at a lower magnetic current, in addition to the metallic Ru 3p3/2/3p1/2 pair at 461.2/483.4 eV, a small, second doublet was also resolved at 463.9/485.1 eV, signifying the formation of electron-deficient Ru likely in the forms of RuClx/RuOy species. These latter became more pronounced in Ru-300 (464.1/486.3 eV), with the corresponding metallic peaks at 461.6/483.8 eV. Ru-200 exhibited an even more prominent doublet for the RuClx/RuOy species though at a binding energy about 0.8 eV higher at 465.3/487.5 eV. The other doublet was deconvoluted at 462.7/484.9 eV, which were at least 1.1 eV higher than those of the other samples in the series but markedly lower than those of RuCl3 (464.1 eV for Ru 3p3/2), suggesting only partial decomposition of RuCl3 and the formation of amorphous Ru nanoclusters as observed in TEM measurements (
A similar trend was observed when the heating duration was increased at a fixed magnetic current. With reference to
Consistent results were obtained from the Cl 2p spectra. With reference to
The C 1s and O 1s spectra of the samples series also provide important insights into the structural changes during the ultrafast heating process. As seen in
With reference to
Raman measurements showed that abundant RuClx species were indeed formed in Ru-200 and Ru-300, but not Ru-400 or Ru-600. Raman spectra of
Further structural details of the samples were obtained from X-ray absorption spectroscopy (XAS) measurements.
The corresponding Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of
This Example describes electrochemical analysis of samples of Example 4.
Electrochemical measurements were carried out with a CHI 700E electrochemical workstation in a three-electrode configuration. The Ru nanoparticle carbon paper was fixed onto a graphite electrode holder, with an exposed surface area of 0.25 cm2. A graphite rod was used as the counter electrode and Ag/AgCl in saturated KCl as the reference electrode. The reference electrode was calibrated against a reversible hydrogen electrode (RHE) and all potentials in the present study were referenced to this RHE.
The obtained Ru nanoparticles possessed a remarkable HER activity in both acidic and alkaline media.
The Ru samples also exhibited outstanding HER activity in alkaline media.
In the Tafel plots of
Ru-300 also exhibited excellent stability in both acidic and alkaline media. In accelerated LSV tests for 2,000 cycles (
This Example describes theoretical and computational analysis of samples of Example 4.
First-principles computations were performed using Quantum ESPRESSO, an open-source plane-wave code. A 4×4-unit cell with 48 atoms was used to build a hexagonal Ru (10-11) slab supercell, where periodic image interactions were removed by setting a vacuum of 15 Å. Ru atoms of the bottom layer have been fixed during all relax calculations. A cutoff of 50 and 500 Ry for kinetics and charge density was chosen with the GBRV ultrasoft pseudopotential. The total energy of the Monkhorst-Pack 4×4×1 K-point grid in the supercell was calculated at the convergence level of 1 meV per atom. The smearing parameter was set at 0.01 Ry in the Marzari-Vanderbilt smearing for all calculations. For geometric relaxation, the convergence was 10−8 Ry of the electronic energy and 10−4 au for the total force. Density functional perturbation theory was employed to calculate the phonon frequency as inputs for entropy and zero-point energy.
Based on the above experimental studies, it is believed that Ru—Cl species played an important role in the HER process. To understand the remarkable HER activity of Ru nanoparticles with surface-enriched Cl, DFT calculations were conducted to unravel the fundamental mechanism. As shown in
With Cl atom adsorbed on Ru-101 in a tetradentate fashion (Ru-101-tCl), it was found that the ΔGH* slightly shifted to −0.51 eV. With two neighboring Cl (
To further investigate the mechanism of weakened H adsorption, the total density of states (DOS) and partial density of states (PDOS) of the d electrons were calculated to determine the electronic structure of the bulk and the surface atoms. As can be seen in
Further insights into the interactions between Ru nanoparticles and RuClx or Cl ligands were obtained by Bader charge analysis, as shown in
In summary, magnetic induction heating was exploited for the ultrafast and green (i.e., environmentally friendly) preparation of Ru nanoparticles supported on carbon paper. The samples could be prepared within seconds, and the rapid synthesis led to the formation of metal Cl species on the Ru nanoparticle surface. With this unique structural feature, the samples all exhibited apparent electrocatalytic activity towards HER in both acidic and alkaline media, and the best sample, Ru-300 needed a respective overpotential of only −23 and −12 mV to reach 10 mA cm−2, rivaling commercial Pt/C benchmark, along with excellent stability. Results from DFT calculations showed that the surface Cl species induced apparent electron transfer from the Ru nanoparticles, rendering the downshift of the Ru Ea and hence weakened H adsorption, a unique feature for enhanced HER activity, as observed experimentally. Results from the Examples of this disclosure highlight the unique significance of MIHRQ in the structural engineering of metal nanoparticles by heteroanion functionalization for enhancement of their electrocatalytic performance.
It will be appreciated that of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, or material.
The present application claims the benefit of and priority to U.S. Provisional Application No. 63/298,760, filed on Jan. 12, 2022, and U.S. Provisional Application No. 63/331,415, filed on Apr. 15, 2022. The entire disclosures of each of the foregoing applications are incorporated by reference herein.
This invention was made with Government support under Grant No. CHE-1900235 and CHE-2003685 awarded by the National Science Foundation. The Government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2023/010098 | 1/4/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63331415 | Apr 2022 | US | |
| 63298760 | Jan 2022 | US |
