Patent Application
20130085061
The present invention relates generally to electrocatalysts. More particularly, the present invention relates to systems and methods for a multi-metallic electrocatalyst for fuel cells and metal-air batteries.
This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
As the global demand for energy has increased, alternative energy generation systems have become an increasingly important component in the worldwide energy generation scheme. Similarly, as the focus on air pollution and energy generation emissions has increased, much attention has focused on fuel cells as a clean and portable source of energy. A primary hindrance to widespread commercial use, however, is that fuel cells require a costly platinum (Pt) catalyst for operation. Therefore, reducing the amount of Pt required will increase the economic viability of fuel cells.
The foreground of sustainable energy is built on a renewable and environment-compatible scheme of chemical-electrical energy conversion. One of the key processes for such energy conversion is the electrocatalytic reduction of oxygen, which is the cathode reaction in fuel cells and metal-air batteries where an electrocatalyst is used to accelerate the course of ORR. Current electrocatalysts used for this reaction are typically in the form of dispersed Pt nanoparticles on amorphous high-surface-area carbon. Considering the high cost and limited availability of Pt, large-scale applications of these renewable energy technologies demand substantial improvement of the catalyst performance so that the amount of Pt needed can be significantly reduced. For example, a five-fold improvement of catalytic activity for the ORR is required for the commercial implementation of fuel cell technology in transportation.
The advancement of heterogeneous catalysis relies on the capability of altering material structures at the nanoscale. Such alteration is particularly important for the development of highly active electrocatalysts with uncompromised durability. This disclosure reports the design and synthesis of a Pt-bimetallic catalyst with multilayered Pt-skin surface that provides superior electrocatalytic performance for the oxygen reduction reaction (ORR). This novel structure was first established on extended thin film surfaces with tailored composition profiles and then implemented in nanocatalysts by organic solution synthesis. Electrochemical studies for the ORR demonstrated that, after elongated exposure to reaction conditions, the Pt-bimetallic catalyst with multilayered Pt-skin surfaces exhibits an improvement factor in activity of more than one order of magnitude versus conventional Pt catalysts. This substantially enhanced catalytic activity, as well as improved durability, indicate great potential toward improving the material properties by fine tuning of the nanoscale architecture.
Prior work on well-defined extended surfaces has shown that high catalytic activity for the ORR can be achieved on Pt-bimetallic alloys (Pt3M, M=Fe, Co, Ni, etc.), due to the altered electronic structure of the Pt topmost layer and hence reduced adsorption of oxygenated spectator species (e.g., OH−) on the surface. It was also found that in acidic electrochemical environment the non-noble 3d transition metals are dissolved from the near-surface layers, which leads to the formation of a Pt-skeleton surface. Moreover, the thermal treatment of Pt3M alloys in ultra high vacuum (UHV) has been shown to induce segregation of Pt and formation of distinctive topmost layer that was termed Pt-skin surface. However, the same treatment did not cause Pt to segregate over PtM alloys with high content (≦50%) of non-Pt elements. More recently, the surfacing of an ordered Pt(111)-skin over Pt3Ni(111) single crystal having 50% of Ni in the subsurface layer was further demonstrated. This unique nanosegregated composition profile was found to be responsible for the dramatically enhanced ORR activity.
Based on these findings, it could be envisioned that the most advantageous nanoscale architecture for a bimetallic electrocatalyst would correspond to the segregated Pt-skin composition profile established on extended surfaces. Much effort has been dedicated, but it still remains elusive, to finely tune the Pt-bimetallic nanostructure in order to achieve this desirable surface structure and composition profile. Major obstacles reside not only in the difficulty for manipulation of elemental distribution at the nanoscale, but also in the fundamental differences in atomic structures, electronic properties and catalytic performance between extended surfaces and confined nanomaterials. For example, in attempt to induce surface segregation, high-temperature (greater than 600 degrees Celsius) annealing is typically applied for Pt-based alloy nanocatalysts. While improvement in specific activity is obtained, such treatment usually causes particle sintering and loss of electrochemical surface area (ECSA). Besides that, the surface coordination of nanomaterials is quite different from that of bulk materials, i.e., the surface of nanoparticles is rich in corner and edge sites, which have smaller coordination number than the atoms on long range ordered terraces of extended surfaces. These low-coordination surface atoms are considered as preferential sites for the adsorption of oxygenated spectator species (e.g., OH−), and thus become blocked for adsorption of molecular oxygen and inactive for the ORR. Additionally, due to strong Pt—O interaction these low-coordination atoms are more vulnerable for migration and dissolution, resulting in poor durability and fast decay of the catalyst. The latter effect is even more pronounced in Pt-bimetallic systems, considering that more low-coordination sites are present on the skeleton surfaces formed after the depletion of non-precious metals from near-surface regions.
Various embodiments of the present invention relate to compositions and methods for preparing a platinum alloy having enhanced catalytic properties.
One embodiment of the present disclosure comprises a bimetallic alloy having enhanced electrocatalytic properties. The bimetallic alloy comprises a PtNi substrate having a surface layer, a near-surface layer, and an inner layer, where the surface layer comprises a nickel-depleted composition, such that the surface layer comprises a platinum skin having at least one atomic layer of platinum.
A second embodiment of the present disclosure comprises a method of preparing a bimetallic alloy having enhanced electrocatalytic properties. The method of preparing the bimetallic alloy comprises the steps of synthesizing a PtNi nanoparticle, depleting a surface of the PtNi nanoparticle of nickel to create a platinum skeleton, and smoothing a surface of the platinum skeleton by annealing and creating the PtNi nanoparticles with multilayered platinum skin surfaces.
A third embodiment of the present disclosure comprises a method of preparing a platinum-nickel electrocatalyst. The method of preparing the platinum-nickel electrocatalyst comprises the steps of synthesizing a PtNi nanoparticle, depleting a surface of the PtNi nanoparticle of nickel by exposing the PtNi nanoparticle to an acidic environment to create a platinum skin, smoothing the surface of the platinum skin by annealing the PtNi nanoparticle, and incorporating the PtNi nanoparticle on high surface area carbon by colloidal deposition.
These and other features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below.
a)-(c) and
a)-(b) are representative transmission electron microscopy (TEM) images for the as-synthesized PtNi nanoparticles;
a)-(e) illustrate microscopic characterizations and theoretical simulations of nanostructure evolution in the PtNi/C catalysts.
a)-(d) illustrate in situ X-ray absorption and electrochemical studies of the PtNi/C catalysts.
a)-(e) summarize electrochemical durability studies obtained by RDE before and after 4,000 potential cycles between 0.6 and 1.1 V for the Pt/C and PtNi/C catalysts in 0.1M HClO4 at 0.95 V and 60 degrees Celsisus.
a)-(b) illustrate representative CO stripping curves (solid lines) recorded for electrochemical oxidation of adsorbed CO monolayer obtained from RDE in hanging meniscus configuration in 0.1M HCl04 on (a) 400 degrees Celsius annealed 3 ML Pt/PtNi film and (b) acid treated/annealed PtNi/C. The scans (dashed lines) for blank CV are also shown for comparison. The charge calculated by integration of the area under the CO stripping peak was used to estimate ECSA in order to diminish the underestimation from Hupd due to the altered electronic/adsorption properties of Pt-skin surface.
a)-(c) illustrate EDX spectra for the as-prepared, acid leached, and acid leached/annealed PtNi/C catalysts.
a)-(b) illustrate statistical results of the particle sizes for the as-prepared and acid leached/annealed PtNi/C catalysts. Particle sizes were obtained by counting 100 nanoparticles from representative TEM images.
a)-(b) illustrate EDX spectra for the acid leached and acid leached/annealed PtNi/carbon catalysts after the durability studies (4,000 cycles between 0.6 and 1.1 V at 60 degrees Celsius).
a) is a schematic of a cubo-octahedral particle viewed along [110];
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.
Embodiments of the present invention relate to fine tuning the Pt-bimetallic nanostructure to achieve the advantageous Pt-skin surface structure and composition profile established on extended surfaces as described in U.S. Pat. No. 7,871,738 B2, which is fully incorporated herein by reference. The present invention relates to the composition and synthesis of Pt-bimetallic catalysts with Pt terminated surfaces. Some embodiments of the present invention demonstrate an advanced Pt-bimetallic catalyst that achieves high catalytic activity and superior durability for the ORR. The catalyst contains a unique nanoscale architecture with a PtNi core of 50% Ni and a multilayered Pt-skin surfaces. This particular composition was explored in Chao Wang et al., Correlation Between Surface Chemistry and Electrocatalytic Properties of Monodisperse Ptx Nil-x Nanoparticles, 21 Adv. Funct. Mater. 147 (2011), which is hereby incorporated by reference in its entirety. In one embodiment, the catalyst has critical parameters such as particle size, thermal treatment, particle sintering, alloy composition, and elemental composition profile that define the catalyst's structure
In a method of synthesis in accordance with one embodiment, the initial step involves the synthesis of monodisperse and homogeneous PtNi followed by acid treatment and intentional depletion of Ni from the surface, producing a several layers thick Pt-skeleton type of surface structure. The final portion of the process is the thermal treatment (annealing) aimed to induce the transition from Pt-skeleton into Pt-skin type of structure by surface relaxation and restructuring. For that purpose, in one embodiment, PtNi nanoparticles are synthesized by simultaneous reduction of platinum acetylacetonate, Pt(acac)2 and nickel acetonate, Ni(ac)2 in an organic solution with an average particle size of 5 nm and the ratio between Pt and Ni was set to be 1:1.
a) and 3(b) show representative transmission electron microscopy (TEM) images of the as-synthesized (as set forth in the Examples) PtNi nanoparticles prepared with a molar ratio of 1:2 between the Pt and Ni precursors. The nanoparticle size is confirmed to be approximately 5 nm with a very narrow size distribution, as evidenced by the formation of various types of super lattices after drying the nanoparticle suspension (in hexane) under ambient conditions. The shape of particles are revealed from TEM images, and it is verified that cubooctahedral particles are present. The final composition of the examples below was characterized by energy-dispersive X-ray spectroscopy (EDX), which confirmed an atomic ratio of Pt/Ni≈1/1 (
The as-synthesized nanoparticles are incorporated into carbon black (˜900 m2/g) via a colloidal-deposition method and the organic surfactants are efficiently removed by thermal treatment. Such as-prepared PtNi/C catalyst are firstly treated by acid to dissolve the surface Ni atoms (
The applied treatments, in particular annealing at the moderate temperature (400 degrees Celsius) did not induce agglomeration of the catalyst particles. Both the as-prepared and treated catalysts show monodisprse particle size distribution. The consistent control in particle size has enabled the systematic study of electrocatalytic properties of the bimetallic catalysts with particle size effect excluded. Additionally, X-ray diffraction (XRD) analysis was used to characterize the crystal structure of the nanoparticles. Compared with the commercial Pt/C catalyst (Tanaka, ˜6 nm), both the as-prepared and acid treated/annealed PtNi/C systems show a face-centered cubic (fcc) pattern with noticeable shifts (e.g., ˜1 degree for (111) peak) toward high angle (
The nanostructures and composition profiles of the PtNi/C catalysts were characterized by atomically resolved aberration-corrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) in combination with energy dispersive x-ray spectroscopy (EDX).
The microscopic characterizations strongly point towards surface restructuring in the bimetallic catalyst upon annealing. This was additionally depicted by atomistic simulations of the nanostructure evolution, which was subject to the acid and annealing treatments (
To gain further insight into the nanostructure evolution, especially the correlation of surface structures to their electrochemical properties, in situ X-ray absorption near edge structure (XANES) studies of the nanocatalysts were conducted.
These results have achieved the desirable nanoscale architecture established on PtNi supported Pt films, i.e., multilayered Pt-skin over a particle core with 50% of Ni. Considering what was revealed from the studies on extended surfaces, the obtained nanocatalyst should show superior catalytic performance for the ORR, which was examined by RDE measurements.
Moreover, the developed Pt-bimetallic catalyst with the unique nanoscale architecture does not only show enhanced catalytic activity, but also improved catalyst durability for the ORR.
Thus, for some embodiments it is assured that the multilayered Pt-skin formation has indeed provided complete protection of the Ni inside the catalyst and enabled the sustained high catalytic activity under fuel cell operating conditions. Based on that, in addition to diminished number of vulnerable undercoordinated Pt surface atoms after annealing, multilayered Pt-skin formation is also thick enough to protect subsurface Ni from dissolution that otherwise occurs through the place-exchange mechanism, (
Examples started with Pt thin films of controlled thickness deposited over PtNi substrate to explore the correlation between the surface composition profile and catalytic performance. These findings were then applied for guiding the synthesis of nanocatalysts with the optimized structure. The outcome of such effort is an advanced Pt-bimetallic catalyst with altered nanoscale architecture that is highly active and durable for the ORR.
Pt films were deposited at room temperature on PtNi substrates (6 mm in diameter) which were set 125 mm away from DC sputter magnetrons in 4 mTorr Argon gas (base vacuum 1×10−7 Torr). Pt source rate (0.32 Å/sec) was determined by quartz crystal microbalance and an exposure duration of 7.0 sec was calibrated for the nominal thickness of approximately 2.2-2.3 Å for a monolayer of Pt. The film thickness was derived from exposure time of computer controlled shutters during sputtering.
In one synthesis of PtNi nanoparticles, 0.67 mmol of Ni(ac)20.4H2O was dissolved in 20 mL of diphenyl either in the presence of 0.4 mL of oleylamine and 0.4 mL of oleic acid. 0.33 mmol of 1,2-tetradodecanediol was added to and the formed solution was heated to 80 degrees Celsius under Ar flow. After a transparent solution formed, the temperature was further raised to 200 degrees Celsius, where 0.33 mmol of Pt(acac)2 dissolved in 1.5 mL of dichlorobenzene was injected. The solution was kept at this temperature for 1 hour and then cooled down to room temperature. The amount of 60 mL of ethanol was added to precipitate the nanoparticles and the product was collected by centrifuge (6000 rpm, 6 minutes). The obtained nanoparticles were further washed by ethanol for two times and then dispersed in hexane. The as-synthesized PtNi nanoparticles were deposited on high surface area carbon (˜900 m2/g) by mixing the nanoparticles with carbon black (Tanaka, KK) in hexane or chloroform with a 1:1 ratio in mass. This mixture was sonicated for 1 h and then dried under nitrogen flow. The organic surfactants were removed by thermal treatment at approximately 150-200 degrees Celsius in an oxygenated atmosphere. The obtained catalyst is denoted as “as-prepared PtNi/C”. For the acid treatment, approximately 10 mg of the as-prepared PtNi/C catalyst was mixed with 20 ml of 0.1 M HClO4 electrolyte that has been used in electrochemical measurements. After overnight exposure to acidic environment, the product was collected by centrifuge and washed three times by deionized water. Such nanoparticles are named as “acid treated PtNi/C”. The acid treated PtNi/C was further annealed at 400 degrees Celsius to reduce low-coordinated surface sites, and the obtained catalyst is termed as “acid treated/annealed PtNi/C”.
TEM images were collected on a Philips EM 30 (200 kV) equipped with EDX functionality. XRD patterns were collected on a Rigaku RTP 300 RC machine. STEM and elemental analysis were carried out on JEOL 2200FS TEM/STEM with a CEOS aberration (probe) corrector. The microscope was operated at 200 kV in HAADF-STEM mode equipped with a Bruker-AXS X-Flash 5030 silicon drift detector. The probe size was approximately 0.7 Å and the probe current was approximately 30 pA during HAADF-STEM imaging. When accumulating EDX data, the probe current was increased to approximately 280 pA and the probe size was approximately 2 Å. The presented EDX data were confirmed to be from “e-beam damage-free” particles by comparing STEM images before and after EDX acquisition.
X-ray fluorescence spectra of at the Ni K and Pt L3 edges were acquired at bending magnet beamline 12-BM-B at the Advanced Photon Source (APS), Argonne National Laboratory. The incident radiation was filtered by a Si(111) double-crystal monochromator (energy resolution ΔE/E=14.1×10−5) with a double mirror system for focusing and harmonic rejection. All the data were taken in fluorescence mode using a 13-element Germanium array detector (Canberra) which was aligned with the polarization of the X-ray beam to minimize the elastic scattering intensity. Co and Ge filters (of 6 absorption length in thickness) were applied in front of the detector to further reduce the elastic scattering intensity for the Ni K and Pt L3 edges, respectively. The Ni K and Pt L3 edge spectra were calibrated by defining the zero crossing point of the second derivative of the XANES spectra for Ni and Pt reference foils as 8333 eV and 11564 eV, respectively. The background was subtracted using the AUTOBK algorithm and data reduction was performed using Athena from the IFEFFIT software suite. A scheme of the home-made in situ electrochemical cell and setup at beamline is illustrated in
The electrochemical measurements were conducted in a three-compartment electrochemical cell with a rotational disc electrode (RDE, 6 mm in diameter) setup (Pine) and a Autolab 302 potentiostat. A saturated Ag/AgCl electrode and a Pt wire were used as reference and counter electrodes, respectively. 0.1 M HClO4 was used as electrolyte. The catalysts were deposited on glassy carbon electrode substrate and dried in Ar atmosphere without using the ionomer. The loading was controlled to be 12 μgpt/cm2disk for PtNi/C nanocatalysts. All the potentials given in the discussion were against reversible hydrogen electrode (RHE), and the readout currents are recorded with ohmic iR drop correction during the measurements.
Pt films of various thicknesses, i.e., approximately 1-7 atomic monolayers (ML), were deposited in vacuum by physical vapor sputtering on PtNi (Pt:Ni=1:1) substrate and then transferred to electrochemical cell for further characterizations. The as-sputtered Pt films consisted of randomly distributed Pt nanoclusters (˜1 nm), which simulate the Ni depleted Pt-skeleton structures. The film thickness was varied to explore the dependence of catalytic activity on the surface depletion depth in Pt-skeleton type of surfaces. The choice of PtNi substrate was based on previous results from nanosegregated extended surfaces and most recent findings related to composition dependent electrochemical properties of PtxNi1-x nanoparticles, which had confirmed the superior catalytic properties of systems with 50% of Ni in subsurface layers.
Since the as-sputtered skeleton-type of surfaces have abundant low-coordination sites that are detrimental to the ORR, thermal treatment was applied to investigate potential surface restructuring and further catalytic improvement. A moderate temperature of approximately 400 degrees Celsius was chosen as it was determined to be optimal for Pt-bimetallic nanocatalysts. In
Additional proof of the transition towards Pt-skin is provided by the measured boost in specific activity for the ORR (
The ECSA estimated from integrated Hupd charge was found to be substantially smaller than that obtained from electrochemical oxidation of adsorbed CO monolayer (
According to the geometry of the cubo-octahedron, the thickness profile along <001> direction while crossing the two edge-on (001) surfaces (shown as the shadowed arrow in
where x≦D/2. The diameter of the particle D is defined as the interval between surface (001) and (001), y is the thickness, and x is the distance to the center along <001>. The line profiles calculated in this way does not count the contrast between Pt and Ni, and thus can be viewed as the intensity profiles for homogeneous alloy nanoparticles.
Both intensity and composition line profiles revealed a Pt-overlayer thickness of approximately 2 atomic layers in the acid treated/annealed PtNi/C catalyst particles (
The amount of Ni in the particle can be written as
NNi=NtotalCNi,overall
where Ntotal and CNi, overall are the total number of atoms and overall ratio of Ni in the particle. EDX analysis, indicates that CNi, overall is about 27%. Since the Pt-skin does not contribute any Ni, NNi can also be expressed as
NNi=NcenterCNi, center.
The numbers of atoms (Ntotal and Ncenter) are proportional to the volumes (Vtotal and Vcenter), respectively. For fcc crystals each unit cell contains 4 atoms, represented by the following:
Ncenter=4Vcenter/αPtNi3
N
total4Vskin/αPt3+4Vcenter/αPtNi3
where αPt and αPtNi are the lattice parameter for the unit cells of Pt and PtNi, respectively. Due to the small difference between αPt and αPtNi, for simplicity, it is assumed that αPt=αPtNi=α, and write:
N
center=4Vcenter/α3
N
total=4Vskin/α3+4Vcenter/αi3=4Vtotal/α3.
Combining the above equations provides the following:
This result is also consistent with that obtained from atomistic simulation in the following.
In the simulation of particle structure, a perfect cubo-octahedral PtNi alloy nanoparticle (containing 4033 atoms) with a face-centered cubic (fcc) lattice and a diameter of 4.99 nm was constructed at first. In this Pt50Ni50 model particle, 2017 Pt atoms and 2016 Ni atoms were randomly distributed. Surface atoms in this cubo-octahedral nanoparticle refer to those atoms with coordination number less than or equal to 9. This definition was based on the fact that the atoms inside a cubo-octahedral particle of fcc phase have a coordination number of 12 while the atoms on the surface have coordination numbers of 9 on {111} facets, 8 on {100} facets, 7 at edges, and 6 at vertices.
To simulate the acid leaching process, all the Ni surface atoms were iteratively removed from the particle until the coordination number of all the remaining Ni atoms were larger than 9. With leaching out Ni surface atoms, the Pt surface atoms have lowering coordination numbers. The low-coordinated Pt atoms were relocated to the high-coordinated vacancies left by the removed Ni atoms for minimizing the total energy of the PtNi particle. In the resultant particle (
The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.
This application claims priority to U.S. Provisional Patent Application No. 61/541,943, filed on Sep. 30, 2011 and is incorporated herein by reference in its entirety.
The United States Government claims certain rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the U.S. Department of Energy and UChicago Argonne, LLC, representing Argonne National Laboratory.
| Number | Date | Country | |
|---|---|---|---|
| 61541943 | Sep 2011 | US |
