CATALYST NANOARCHITECTURES WITH HIGH ACTIVITY AND STABILITY
20180154346 ยท 2018-06-07
Inventors
Cpc classification
B01J35/33
PERFORMING OPERATIONS; TRANSPORTING
B82Y40/00
PERFORMING OPERATIONS; TRANSPORTING
B01J35/30
PERFORMING OPERATIONS; TRANSPORTING
B01J35/40
PERFORMING OPERATIONS; TRANSPORTING
B82Y30/00
PERFORMING OPERATIONS; TRANSPORTING
B01J37/0217
PERFORMING OPERATIONS; TRANSPORTING
C01P2002/72
CHEMISTRY; METALLURGY
B01J35/393
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
In some embodiments, a method may include forming a catalytic nanoarchitecture. The method may include heating a non-catalytic metal compound within a specified temperature range and atmosphere in the presence of a catalytic metal. In some embodiments, heating the non-catalytic metal may include heating within a hydrogen-containing atmosphere. The method may include transforming a first architecture of the non-catalytic metal to a second architecture. The second architecture may include openings in the second architecture. The method may include incorporating the catalytic metal into the openings in the second architecture such that the catalytic metal is integrated into the second architecture. In some embodiments, the method may include increasing a catalytic activity of the catalytic metal by integrating the catalytic metal into the second architecture.
Claims
1. A method of forming a catalytic structure, comprising: heating the combination of a non-catalytic metal compound that is within a first architecture, comprising a network composed of interconnected solid domains and porosity, and a catalytic metal within a specified temperature range and atmosphere, wherein the first architecture is formed from a non-catalytic metal moiety and nonmetallic moieties of the non-catalytic metal compound; transforming the first architecture composed of the non-catalytic metal compound and the catalytic metal to a second architecture comprising openings in the second architecture; resulting in interaction of the catalytic metal with the non-catalytic metal within an integrated network of domains within the second architecture; and increasing a catalytic activity or stability of the catalytic metal by interaction of the catalytic metal with the non-catalytic metal within the second architecture.
2. The method of claim 1, wherein the first and/or the second architecture comprises a two-dimensional architecture consisting of nanosheets with thicknesses of less than 20 nm and thickness to lateral dimensions (width or length) ratios of at least 4.
3. The method of claim 1, further comprising heating the combination of non-catalytic metal compound and a catalytic metal within a hydrogen-containing atmosphere.
4. The method of claim 1, further comprising heating the combination of non-catalytic metal compound and a catalytic metal within an ammonia-containing atmosphere.
5. The method of claim 1, wherein the second architecture does not comprise carbon.
6. The method of claim 1, wherein the network is composed of interconnected solid domains with at least one dimension of at least 100 nm.
7. The method of claim 1, wherein the network is composed of pores with dimensions of at least 2 nm.
8. The method of claim 1, wherein the specified temperature range ranges from below 700 degrees Celsius.
9. The method of claim 1, wherein the specified temperature range ranges from about 150 to about 350 degrees Celsius.
10. The method of claim 1, wherein the specified temperature range ranges from about 150 to about 250 degrees Celsius.
11. The method of claim 1, wherein the non-catalytic metal compound contains nickel, cobalt, iron, manganese, chromium, vanadium, copper, zinc, zirconium, niobium, molybdenum, indium, tin, tantalum, tungsten, aluminum or gallium, lead or bismuth.
12. The method of claim 1, wherein the non-catalytic metal compound comprises a combination of more than one transitional metal including nickel, cobalt, iron, manganese, chromium, vanadium, copper, zinc, zirconium, niobium, molybdenum, indium, tin, tantalum, tungsten, aluminum or gallium, lead or bismuth.
13. The method of claim 1, wherein the non-catalytic metal compound comprises a metal hydroxide.
14. The method of claim 1, wherein the non-catalytic metal compound comprises a nickel hydroxide or cobalt hydroxide.
15. The method of claim 1, wherein the non-catalytic metal compound comprises a metal oxide.
16. The method of claim 1, wherein the non-catalytic metal compound comprises a nickel oxide or cobalt oxide.
17. The method of claim 1, wherein the non-catalytic metal compound comprises a metal oxyhydroxide.
18. The method of claim 1, wherein the catalytic metal comprises platinum, iridium, osmium, gold, silver, palladium, rhodium, or ruthenium.
19. The method of claim 1, wherein the non-catalytic metal compound is combined with more than one catalytic metal comprising platinum, iridium, osmium, gold, silver, palladium, rhodium, or ruthenium.
20. The method of claim 1, further utilizing chemical leaching to remove unstable metals within the second architecture.
21. The method of claim 1, further utilizing a second temperature/atmosphere treatment step to modify the second architecture.
22. The method of claim 1, further utilizing an electrochemical treatment step to modify the second architecture.
23. A catalyst, comprising: a catalyst architecture formed at least in part from a non-catalytic metal compound, wherein the catalyst architecture is formed from a non-catalytic metal moiety and nonmetallic moieties of the non-catalytic metal compound; and a catalytic metal that is positioned in an integrated network of domains within the catalyst architecture such that a transport of reactants and removal of products to the active catalytic sites is allowed during use; wherein the catalytic metal interacts with the non-catalytic metal within the integrated network of domains within the catalyst architecture such that a catalytic activity and/or a catalytic stability of the catalytic metal increases by the interaction of the catalytic metal with the non-catalytic metal within the catalyst architecture.
24. The catalyst of claim 23, wherein catalyst architecture comprises an architecture consisting of nanosheets with thicknesses of less than 20 nm and thickness to lateral dimensions (width or length) ratios of at least 4.
25-37. (canceled)
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description of the preferred embodiments and upon reference to the accompanying drawings.
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[0057] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
[0058] The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word may is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The words include, including, and includes indicate open-ended relationships and therefore mean including, but not limited to. Similarly, the words have, having, and has also indicated open-ended relationships, and thus mean having, but not limited to. The terms first, second, third, and so forth as used herein are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.) unless such an ordering is otherwise explicitly indicated. For example, a third die electrically connected to the module substrate does not preclude scenarios in which a fourth die electrically connected to the module substrate is connected prior to the third die, unless otherwise specified. Similarly, a second feature does not require that a first feature be implemented prior to the second feature, unless otherwise specified.
[0059] Various components may be described as configured to perform a task or tasks. In such contexts, configured to is a broad recitation generally meaning having structure that performs the task or tasks during operation. As such, the component can be configured to perform the task even when the component is not currently performing that task (e.g., a set of electrical conductors may be configured to electrically connect a module to another module, even when the two modules are not connected). In some contexts, configured to may be a broad recitation of structure generally meaning having circuitry that performs the task or tasks during operation. As such, the component can be configured to perform the task even when the component is not currently on. In general, the circuitry that forms the structure corresponding to configured to may include hardware circuits.
[0060] Various components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase configured to. Reciting a component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. 112, paragraph six, interpretation for that component.
[0061] The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.
[0062] It is to be understood the present invention is not limited to particular devices or biological systems, which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used in this specification and the appended claims, the singular forms a, an, and the include singular and plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a linker includes one or more linkers.
DETAILED DESCRIPTION
Definitions
[0063] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art.
[0064] The term connected as used herein generally refers to pieces which may be joined or linked together.
[0065] The term coupled as used herein generally refers to pieces which may be used operatively with each other or joined or linked together, with or without one or more intervening members.
[0066] The term directly as used herein generally refers to one structure in physical contact with another structure, or, when used in reference to a procedure, means that one process affects another process or structure without the involvement of an intermediate step or component.
EMBODIMENTS
[0067] In some embodiments, a set of catalyst nanoarchitectures or 2D nanoframes (NFs), based on Ni.sub.xPt.sub.y structure are employed as ORR catalysts. The 2D-NFs ORR catalysts were synthesized by (i) formation of Ni(OH).sub.2 nanosheets; (ii) deposition of Pt on Ni(OH).sub.2, and (iii) heat treatment to form NiPt nanoframes. This approach provides a carbon-free high surface area framework structure with through-connected porosity and electronic conductivity and an active PtNi alloy catalyst with high activity and stability.
[0068] In some embodiments, different treatments at relative mild temperature and chemical environments were determined to significantly alter the architecture and atomic-level structure which impacted the electrochemical activity and stability. In some embodiments, a method may include forming a catalytic two-dimensional structure. The nanosheets were treated at different temperature/atmosphere treatments which were determined to significantly alter the atomic-level structure and influenced the electrochemical activity. The temperature-atmosphere treatment condition may result in the formation a catalyst architecture composed of both a network of interconnected solid domains (crystalline and/or amorphous) and a network of interconnected pores. In some embodiments, a method may include heating the combination of a non-catalytic metal compound that is within a first architecture and a catalytic metal within a specified temperature range and atmosphere. The first architecture may include a network composed of interconnected solid domains and porosity. In some embodiments, the first architecture may be formed from non-catalytic metal moieties and nonmetallic moieties of the non-catalytic metal compound. As such the first architecture may be formed from something other than metal (excluding for example minor amounts of nonmetallic moieties which have reacted with the surface).
[0069] In some embodiments, a metallic carbon-free high surface area framework structure with through-connected porosity may be formed. In some embodiments, the NiPt nanoframes may be 8 times higher specific activity compared with commercial Pt/C. The high activity is attributed to the formation NiPt alloys or solid solutions. In some embodiments, the carbon free materials are more resistant to corrosion and have improved stability.
[0070] In some embodiments, the method may include heating a non-catalytic metal within a specified temperature range in the presence of a catalytic metal. Non-catalytic metals may include nickel, cobalt, iron, manganese, chromium, vanadium, copper, zinc, zirconium, niobium, molybdenum, indium, tin, tantalum, tungsten, aluminum or gallium, lead or bismuth.
[0071] In some embodiments, hierarchical NiPt 2D nanoframes were formed through temperature/atmosphere treatment of Pt-decorated Ni(OH).sub.2 nanosheets. In some embodiments, the first and/or the second architecture comprises a two-dimensional architecture consisting of nanosheets with thicknesses of less than 20 nm. In some embodiments, the first and/or the second architecture comprises a two-dimensional architecture consisting of nanosheets with a thickness to lateral dimensions (width or length) ratios of greater than or equal to 4.
[0072] In some embodiments, the method may include transforming a first architecture of the non-catalytic metal to a second architecture. The second architecture may include openings in the second architecture. The method may include incorporating the catalytic metal into the openings in the second architecture such that the catalytic metal is integrated into the second architecture. This approach demonstrates a metallic carbon-free high surface area framework structure with through-connected porosity, electronic conductivity, and active catalytic metal-non-catalytic metal solid domains. In some embodiments, the method may include increasing a catalytic activity of the catalytic metal by integrating the catalytic metal into the second architecture. In some embodiments, the method may include increasing the catalytic stability of the catalytic metal by integrating the catalytic metal into the second architecture. In some embodiments, the network is composed of interconnected solid domains with at least one dimension of at least 100 nm. In some embodiments, the network is composed of pores with dimensions of at least 2 nm.
[0073] In some embodiments, a method may include heating the non-catalytic metal compound within a reactive atmosphere (i.e., relative to the non-catalytic metal). The reactive material may include a hydrogen-containing atmosphere.
[0074] In some embodiments, the second architecture comprises more openings that the first architecture. In some embodiments, the first and/or the second architecture comprises a two-dimensional architecture. In some embodiments, the second architecture does not comprise carbon.
[0075] In some embodiments, mild temperature treatments are needed to maintain the 2D framework and enable direct PtNi interaction and avoid Pt diffusion below surface region. The NiPt 2D NF catalyst heated to 200 C. showed a specific activity around 4.61 mA cm.sup.2.sub.Pt which is almost one order of magnitude higher than commercial Pt/C (0.57 mA cm.sup.2.sub.Pt). In some embodiments, the specified temperature range ranges up to 700 degrees Celsius. In some embodiments, the specified temperature range ranges from about 150 to about 350 degrees Celsius. The specified temperature range may range from about 150 to about 250 degrees Celsius. In some embodiments, if the non-catalytic metal compound first architecture is heated beyond the prescribed temperature range the first architecture may collapse.
[0076] In some embodiments, the non-catalytic metal compound may include nickel, cobalt, iron, manganese, chromium, vanadium, copper, zinc, zirconium, niobium, molybdenum, indium, tin, tantalum, tungsten, aluminum or gallium, lead or bismuth. In some embodiments, the non-catalytic metal compound may include one or more transitional metals. In some embodiments, the non-catalytic metal compound may include a metal hydroxide. In some embodiments, the non-catalytic metal compound may include a nickel hydroxide. In some embodiments, the non-catalytic metal compound may include a cobalt hydroxide. In some embodiments, the non-catalytic metal compound may include a metal oxide (e.g., nickel oxide, cobalt oxide). In some embodiments, the non-catalytic metal compound may include a metal oxyhydroxide. In some embodiments, the catalytic metal may include one or more of platinum, iridium, osmium, gold, silver, palladium, rhodium, or ruthenium. In some embodiments, the catalytic metal may include one or more metals including platinum, iridium, osmium, gold, silver, palladium, rhodium, or ruthenium.
[0077] In some embodiments, a catalyst may include a two-dimensional architecture formed of a non-catalytic metal compound and a catalytic metal. The catalyst may include a catalytic metal incorporated into openings in the second architecture such that the catalytic metal is integrated into the second architecture. The catalytic activity of the catalytic metal may be increased relative to an unincorporated catalytic metal. The catalytic stability of the catalytic metal may be increased relative to an unincorporated catalytic metal.
[0078] In some embodiments, the method may include chemical leaching (e.g., chemical leaching in nitric acid (e.g., 0.05 M HNO.sub.3)). Chemical leaching may remove unstable non-noble metals within the structure.
[0079] In some embodiments, the method may include using a second temperature-atmosphere treatment step to modify the second architecture.
[0080] In some embodiments, the method may include utilizing an electrochemical treatment step to modify the second architecture.
[0081] Further understanding of the composition, synthetic conditions, treatment parameters, atomic arrangement, and the influence of these parameters on the activity and stability provides a pathway to develop carbon-free catalyst nanoarchitecture electrocatalysts with high activity, extended durability, and low cost.
[0082] The development of nanostructured catalysts with high activity, long-term stability and low cost is a key unsolved challenge for electrochemical energy conversion and storage devices including fuel cells, metal air batteries, and electrolyzers. Additional uses of the catalysts described herein may include catalyst for chemical reactions, electrochemical catalysts, oxygen reduction, oxygen evolution, hydrogen evolution, carbon dioxide conversion reactions, and other catalytic reactions, etc. Operation at higher temperatures is also desirable. Methods described herein may invention has the potential to address all of these issues.
EXAMPLES
[0083] Having now described the invention, the same will be more readily understood through reference to the following example(s), which are provided by way of illustration, and are not intended to be limiting of the present invention.
Example 1
[0084] Potassium hexachloroplatinate (K.sub.2PtCl.sub.6, 99.98%) was obtained from Sigma-Aldrich. Nickel nitrate hexahydrate (Ni(NO.sub.3).sub.2.6H.sub.2O, 98%) and urea (N.sub.2COH.sub.4, 99.3%), were purchased from Alfa Aesar. Ethyleneglycol, isopropanol (HPLC grade), and ACS grade water (18 M-cm) were obtained from VWR Analytical. All electrochemical measurements were carried out in 0.1 M HClO.sub.4 prepared with 70% HClO.sub.4 (Veritas Doubly Distilled) (0.000001% Cr). All reagents were used without further purification.
[0085] The -Ni(OH).sub.2 nanosheets were synthesized using a hydrothermal process. To prepare the -Ni(OH).sub.2 nanosheets, 1.06 g (3.64 mmol) of Ni(NO.sub.3).sub.2.6H.sub.2O was combined with 0.6597 g (10.98 mmol) of urea in 20 mL of ultrapure water (18 M-cm) and 25 mL of ethyleneglycol (EG). The solution was then transferred into a 50 mL Telfon-lined autoclave reactor (Pan Instruments). The reaction vessel was placed in a 120 C. pre-heated oven and allowed to react for 4.5 h without active stirring. After the reaction time, the autoclave was removed from the oven and allowed to cool at room temperature. The light green powder was recovered by centrifugation (3000 RPM, 3 min, Thermo, Sorvall ST16) and subsequently rinsed five times with water and two times with isopropanol using the same centrifuge conditions. Following the rinsing steps, the powder was dried under vacuum at 60 C. overnight. Based on analysis discussed in the following paragraphs, the dried powder was notated as -Ni(OH).sub.2 nanosheets.
[0086] The -Ni(OH).sub.2 nanosheets were decorated with Pt. To deposit Pt on the -Ni(OH).sub.2 nanosheets, 0.10382 g of the prepared Ni(OH).sub.2 was dispersed in 25 mL of ultrapure water, and subsequently 0.02730 g of K.sub.2PtCl.sub.6, corresponding to 20 wt. % of metallic Pt vs -Ni(OH).sub.2, was added. The slurry was then transferred to the autoclave, stirred 12 h, and then heated at 80 C. for 4 days without stirring. Finally, a grey powder was recovered, rinsed, and dried as described above.
[0087] The 2D Ni(OH).sub.2@Pt catalyst was thermally treated at 200 or 300 C. for 20 minutes under 100 mL/min flowing Ar/H.sub.2 (95/5 vol. %) using a ramp rate of 10 C./min starting from room temperature. The synthesized samples were notated as -Ni(OH).sub.2, Ni(OH).sub.2@Pt, NiPt-200, and NiPt-300, for the as-prepared precursor nanosheets, nanosheets after platinum deposition, and samples after thermal treatment at 200 or 300 C. respectively. Samples were also characterized after electrochemical conditioning (AEC) which consisted of cyclic voltammetry (CV), carbon monoxide (CO)-stripping, and linear voltammetry over the ORR potential region. Details of the specific experimental parameters used for each of these steps is described in the following paragraphs. The sample were labeled with the suffix AEC to denote the characterization after the electrochemical treatment steps: Ni(OH).sub.2@Pt-AEC, NiPt-200-AEC and NiPt-300-AEC.
Physical and Structural Characterization
[0088] The catalyst composition was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) carried out by Galbraith Laboratories Inc. (Knoxville, Tenn.). From ICP analysis, the as-prepared NiPt-200 catalyst was determined to be 25.55.1 wt % Pt, and the NiPt-300 catalyst was determined to be 29.91.3 wt % Pt. Powder X-Ray diffraction (XRD) measurements were conducted using a Bruker AXS D8 Advance powder X-Ray diffractometer with a Cu Ka (=1.5406 ) radiation source, operating at 40 kV and 25 mA and a high resolution energy dispersive 1D Linxeye XE detector. The scan range of 2 was 5<2<85 with a 0.01 increment. Brunauer-Emmett-Teller (BET) surface areas were obtained from nitrogen adsorption/desorption isotherms measured using a Micromeritics ASAP 2020 surface area and porosimetry analyzer. Samples were degassed at 120 C. for 16 h prior to characterization.
[0089] The structure, morphology, crystallinity, and elemental distribution of the catalysts were determined by high-resolution electron microscopy (HR-TEM) and selected area electron diffraction (SAED) using a 2010-F (200 kV, JEOL). SAED measurements were calibrated using the lattice distance from 2D-Ni-nanoframes obtained by heating -Ni(OH).sub.2 nanosheets at 300 C. under H.sub.2/Ar.
[0090] Cs-corrected scanning transmission electron microscopy (STEM) was carried out by using a JEM-ARM200F (200 kV, JEOL) equipped with an energy dispersed spectrometer (EDAX Silicon Drift detector). High angle annular dark field STEM (HAADF-STEM) was obtained with a convergence angle of 26 mrad and a collection semi-angles from 50 to 180 mrad. The probe size used was about 0.09 nm with the probe current of 22 pA.
Rotating Disk Electrochemical Characterization.
[0091] The electrochemical measurements were conducted at constant temperature (298 K) in a three electrode cell using a rotating disk electrode (RDE) or rotating ring disk electrode (RRDE) configuration with an Autolab PGSTAT128N bipotentiostat and rotation control (Pine Instruments). A Pt mesh and a freshly prepared reversible hydrogen electrode (RHE) were used as counter and reference electrodes respectively. The potential of the RHE electrode was confirmed by measuring the potential versus a commercial silver/silver chloride reference electrode (0.199 V.sub.NHE). The catalyst was suspended within an ink that was prepared by combining the catalyst, Nafion suspension (Aldrich, 5 wt %, 1100 g equivalent weight), isopropanol, and ultrapure water. The catalyst ink was prepared by mixing 3 mg of the catalyst with 3 mL of a stock solution consisting of 7.96 mL ultra-pure water (18.2 M), 2 mL of isopropanol, and 40 L of 5 wt % Nafion solution (Sigma-Aldrich). The same ink formulation was used for all catalysts. The catalyst inks were sonicated in an ultrasonic bath (Fisher, 40 kHz) for 5 minutes while maintaining the temperature around 25 C. The catalyst ink was deposited onto a minor polished glassy carbon electrode (0.196 cm.sup.2) which was used as the working electrode. To control the catalyst loading for the activity tests, ink volumes of 20 uL (Pt/C), 18 uL (Ni(OH).sub.2:Pt), and 18 uL (NiPt-200 and NiPt-300) were deposited onto the glassy carbon electrode. After depositing the ink, the working electrode was dried at room temperature within a fume hood. The uniformity of the thickness and distribution of the catalyst layer on the glassy carbon working electrodes used for RDE and RRDE tests are critical factors that affect the accurate measurement of electrochemical catalytic activity since non-uniform film thicknesses and distributions can influence mass transport processes within the catalyst layer. For films used for activity measurements, the current within the potential region of 0.2 V.sub.RHE<E<0.7 V.sub.RHE obtained from linear voltammetry ORR measurements was within 10% of the theoretical diffusion limited current of 5.7 mA cm.sup.2.sub.geo, which is a reasonable metric used as part of assessing film quality. Variations in measured activities for the same catalyst are attributed to differences in film thickness and distribution of the catalyst film on the glassy carbon working electrode, and specific and mass activities are reported as averages with associated standard deviations of multiple measurements. The Pt loading was optimized based on the maximum specific and mass activity for each catalyst in order to mitigate sources of error during the determination of specific activities. The resulting Pt loading for Pt/C was L.sub.Pt=16 g.sub.Pt cm.sup.2.sub.geo. For the unsupported catalysts, the resulting Pt loadings were 31 g cm.sup.2.sub.geo (Ni(OH).sub.2:Pt) and 35 g cm.sup.2.sub.geo (NiPt-200 and NiPt-300). The unsupported NiPt-200 and NiPt-300 catalysts have lower ECSA.sub.Pt values (5.3-7.4 m.sup.2 g.sup.1.sub.Pt) compared with Pt/C (72.8 m.sup.2 g.sup.1.sub.Pt) and required higher loadings on the electrode compared with Pt/C.
[0092] The electrolyte used for RDE and RRDE tests was 0.1 M HClO.sub.4 (70 wt %, Fisher, ultrapure) in ultrapure water. After preparation, the electrodes were immersed in deaerated 0.1 M HClO.sub.4 under potential control at 0.1 V. IR-compensation and background subtraction were utilized to obtain electrochemical parameters. The catalysts were conditioned by cycling 40 times between 0.05 and 1.2 V.sub.RHE at a scan rate 50 mV s.sup.1 within Argon-purged 0.1 M HClO.sub.4 in order to produce a stable electrode surface. The hydrogen underpotential deposition (H.sub.upd) charge over the H desorption region of the 40.sup.th cycle was integrated to obtained the Pt electrochemical surface area (ECSA.sub.Pt-Hupd) using a specific charge of 210 C cm.sup.2. For the H.sub.upd charge for NiPt-300, the potential window used for analysis was restricted between 0.08 and 0.4 V in order to avoid the hydrogen evolution reaction (HER) feature, as described in the following paragraphs. CO stripping voltammetry was also carried out to complement the ECSA.sub.Pt-Hupd obtained by H.sub.upd. The electrochemical surface area obtained from CO stripping, ECSA.sub.Pt,CO, was calculated using a scan rate of 20 mV s.sup.1 and using 420 C cm.sup.2 as the charge corresponding to a monolayer of adsorbed CO.
[0093] For evaluation of the oxygen reduction activity, CVs were obtained from 0.05 to 1.05 V.sub.RHE using a scan rate 20 mV s.sup.1 and a rotation rate of 1600 rpm in oxygen-saturated 0.1 M HClO.sub.4. The ECSA.sub.Pt used for the calculation of the ORR specific activity was determined from the ECSA.sub.Pt,Hupd obtained from the third cycle of the CV measured after the ORR activity testing. The voltammetric curves in the ORR region were corrected by background subtraction of the CVs obtained under Ar to account for the capacitive contribution. Tafel plots were obtained at 25 C. from the positive scan direction at 1600 rpm, and Tafel slopes were determined using a voltage range of 0.9 to 1.0 V.sub.RHE. Carbon-supported Pt nanoparticles (Pt/C, Etek, 20 wt % Pt) were used as a reference for the CVs, CO stripping, and ORR activity tests.
[0094] The H.sub.2O.sub.2 production was evaluated using a rotating ring disk electrode (RRDE) configuration (disk area of 0.237 cm.sup.2). The disk potential was scanned between 0.05 and 1.05 V.sub.RHE at 20 mV s.sup.1 using a rotation rate of 1600 rpm while the ring was potentiostated at 1.4 V.sub.RHE. The collection efficiency was calculated using a deaerated electrolyte KOH 0.1 M (Alfa Aesar) with 10 mmol L.sup.1 K.sub.3Fe(CN).sub.6 (Alfa Aesar). A Pt wire and mercury-mercury oxide electrode (E.sub.Hg/HgO=0.131 V vs SCE; E=0.009 V), were used as counter and reference electrode respectively, however all the potentials are reported with respect to RHE (E.sub.RHE=E.sub.Hg/HgO-measured+0.059 pH+E.sub.Hg/HgO; E.sub.RHE-corrected=0.898 V) For the collection efficiency experiment, the disk potential was scanned at 20 mV s.sup.1 while the ring was potentiostated at 1.5 V.sub.RHE. The collection efficiency was determined according to the equation N=I.sub.R/I.sub.D, where I.sub.R and I.sub.D correspond to the ring and disk currents respectively. The average collection efficiency obtained at 1600 rpm was 0.430.05.
[0095] The accelerated stability test was carried out using the RDE configuration described above within an O.sub.2-saturated 0.1 M HClO.sub.4 electrolyte, without stirring by cycling the electrode over 1000 cycles from 0.6 to 1.3 V.sub.RHE at scan rate of 100 mV s.sup.1 at 25 C. In order to increase the detection of small amounts of Pt within the electrolyte using ex-situ ICP-MS, for the accelerated stability tests the Pt loading was increased to 22 g cm.sup.2.sub.geo (Pt/C), 43 g cm.sup.2.sub.geo (NiPt-200) and 58 g cm.sup.2.sub.geo (NiPt-300). Prior to the evaluation, the surface of the commercial catalysts Pt/C was cleaned and the NiPt catalysts were electrochemical dealloyed (activation step) by cycling from 0.05 V to 1.2 V at a scan rate of 100 mV s.sup.1 (60 scans) and 20 mV s.sup.1 (20 scans) within an Ar-saturated 0.1 M HClO.sub.4 electrolyte. In addition, after the activation step, the electrolyte was replaced with a fresh electrolyte solution, which was used for the stability tests. During the stability testing, every 200 scans the potential was swept between 0.05 and 1.2 V.sub.RHE in Ar-saturated electrolyte to determine the electrochemical surface area (ECSA.sub.Pt,H-upd). The ECSA.sub.Pt,H-upd was determined from the electrochemical double layer-corrected charge obtained from the H desorption peak, using the details described above. The normalized remaining ECSA.sub.Pt was determined according to the equation (Q.sub.x/Q.sub.1*100) where Q.sub.1 is the charge at the beginning of the test (after the activation step) and Q.sub.x the charge determined at the corresponding number of scans during the stability test (i.e. 200, 400, etc.). The Pt content of the electrode and electrolyte after accelerated stability testing were determined using ICP-MS measurements (Galbraith Laboratories).
Synthesis Approach
[0096] The NiPt 2D nanoframes were synthesized by (i) formation of Ni(OH).sub.2 nanosheets; (ii) deposition of Pt onto the Ni(OH).sub.2 nanosheets, and (iii) controlled temperature/atmosphere treatments (either 200 C. or 300 C. under H.sub.2/Ar, 5/95 vol %) as summarized in
Scanning Electron Microscopy and Transmission Electron Microscopy Characterization
[0097] Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-prepared Pt-decorated Ni(OH).sub.2 (notated as Ni(OH).sub.2@Pt), 200 C.-treated (NiPt-200) and 300 C.-treated (NiPt-300) are shown in
[0098] The sample was heated under reducing conditions (H.sub.2/Ar) to promote direct PtNi interaction and transform the Ni(OH).sub.2 into metallic Ni (Ni(OH).sub.2+H.sub.2.fwdarw.Ni+2H.sub.2O). As shown in
High Resolution Scanning Transmission Electron Microscopy Characterization
[0099] To further understand the morphology and structural changes upon thermal treatments, scanning transmission electron microscopy (STEM) was used to characterize the as-prepared Ni(OH).sub.2@Pt, NiPt-200, and NiPt-300 samples (
[0100] Energy dispersive X-ray spectroscopy (EDS) elemental mapping further supports that Pt nanoparticles are no longer observed within the heated samples, and Pt and Ni are well distributed within the solid network (
X-Ray Diffraction Characterization
[0101] X-ray diffraction (XRD) (
[0102] After thermal treatment, significant structural modifications were observed in the XRD patterns. The thermally treated samples showed the three main XRD peaks corresponding to Pt-fcc, and diffraction peaks consistent with a metallic Ni-fcc phase. The presence of a metallic Ni-phase was consistent with the sample developing magnetic properties (
Characterization after Electrochemical Conditioning
[0103] Electrochemical or acid treatments are commonly used as a dealloying step in the synthesis of Pt alloys to remove the unstable non-noble metals. Ni is known to dissolve or passivate under acidic conditions. Based on the expected transformations within the electrochemical steps and to better correlate the structure with the oxygen reduction activity results, the catalyst structure was also characterized after electrochemical conditioning (notated as AEC), Ni(OH).sub.2@Pt-AEC, NiPt-200-AEC and NiPt-300-AEC, which comprised cyclic voltammetry (CV), carbon monoxide (CO)-stripping and linear voltammetry over the ORR potential region.
[0104] Shown in
[0105] For NiPt200-AEC and NiPt300-AEC, the 2D framework and the interconnected network of crystallites and pores remained after electrochemical conditioning, as shown by HAADF-STEM images (
[0106] In addition to Ni interaction, Pt lattice strain due to surface tension could affect the Pt lattice spacing since prior work has shown that unit cell parameter shifts of 0.03 were observed upon decreasing the size of nanoparticles from 28 nm to 2 nm. Based on the microscopy which shows PtNi interaction (
[0107] Based on the characterization of Ni and Pt within the structure using high resolution microscopy images, mapping, and line scans, an approximate representation of the atomic structure of the catalysts after electrochemical treatment is presented in
Electrochemical Characterization Using Cyclic Voltammetry and Carbon Monoxide Oxidation
[0108] Cyclic Voltammetry.
[0109] Electrochemical characterization of the catalysts first consisted of cyclic voltammetry and electrochemical CO oxidation (
[0110] After the electrochemical activation (40 scans), the voltammograms (
[0111] For the NiPt-200 and NiPt-300 samples, the potentials for oxide formation and reduction were not significantly shifted compared with Pt/C. Shifting of oxide formation/reduction potential to more positive potentials has been generally correlated with improved ORR activity, however this is not always the case. In this case, the oxide reduction potential remains similar to that of Pt/C, however XRD (
[0112] Electrochemical Carbon Monoxide Oxidation.
[0113] Electrochemical CO oxidation (CO stripping) was also used to characterize the catalyst surface since CO stripping is a surface sensitive reaction that depends on surface conformation, composition, particle size and shape. Shown in
[0114] The Pt electrochemical surface area (ECSA.sub.Pt) estimated by the CO-stripping peak integration was 1.7 f 0.2 m.sup.2 g.sup.1 for Ni(OH).sub.2@Pt and then increased after heating to 7.4 f 1.8 and 5.30.6 m.sup.2 g.sup.1 for NiPt-200 and NiPt-300, respectively. The experimental Q.sub.CO/2Q.sub.H ratios were 0.97 and 1.04 for NiPt-200 and NiPt-300 respectively. The Q.sub.CO/2Q.sub.H ratios are near unity and are consistent with a Pt-skeleton arrangement (1) rather that a Pt-skin configuration which showed a Q.sub.CO/2Q.sub.H ratio of 1.5. The microscopy analysis (
Electrochemical Oxygen Reduction Reaction Activity
[0115] Following analysis of the structure and surface of the 2D nanoframes, the electrochemical ORR activity was determined using a rotating disc electrode (RDE) configuration in O.sub.2-saturated 0.1 M HClO.sub.4 (
[0116] The enhancement of the specific activity of NiPt-300 compared with NiPt-200 may be due to a combination of several contributions including (i) complete transformation of Ni(OH).sub.2 within the NiPt-300 structure (
[0117] Regarding the Pt mass activities of the catalysts (
[0118] Tafel slopes at 25 C. of 653.7 and 633.6 mV dec.sup.1 determined for the NiPt-200 and NiPt-300 nanoframes respectively (
[0119] The ORR is a complex reaction which proceeds in either in a four-electron process to H.sub.2O or a two-electron process to H.sub.2O.sub.2. To evaluate the generation of H.sub.2O.sub.2, rotating ring disc electrode (RRDE) experiments of the NiPt-200, NiPt-300, and Pt/C catalysts were obtained (
Electrocatalyst Stability Testing.
[0120] In general, the stability of Pt-based PEMFC ORR catalysts has been shown to be related to (i) active catalyst material degradation due to Pt dissolution, agglomeration, Oswald ripening, and particle detachment, and (ii) corrosion of the support material. Although carbon is one of the most widely used catalyst support materials for Pt for PEMFC applications, carbon corrosion occurs under operating conditions which compromises the catalyst lifetime. Carbon is thermodynamically unstable above 0.2 V.sub.RHE in aqueous electrochemical environments, and the corrosion of carbon can also be accelerated by the presence of Pt.
[0121] During fuel cell start-up and shut-down protocols, hydrogen starvation at the anode can occur and lead to elevated voltages of 1.3 V.sub.RHE to as high as 1.8 V.sub.RHE at the cathode. Voltages above 1.0 V.sub.RHE can lead to significant catalyst due to enhanced corrosion of the carbon support as well as increased rates of Pt dissolution. Determining the stability of PtNi catalysts over cycles which involve potentials well above 1.0 V.sub.RHE is of significant interest to enable catalysts with long-term durability over not only load cycles but also start-up/shut down cycles.
[0122] To evaluate the stability of the 2D nanoframe catalysts within the elevated potential region encountered during start-up/shut down where typical carbon-based materials significantly degrade, an accelerated stability test consisting of 1000 cycles between 0.6 and 1.3 V.sub.RHE was used. The use of the upper potential limit of 1.3 V.sub.RHE represents a significantly more severe durability test than using an upper potential limit of 1.0 V.sub.RHE since increased catalyst degradation occurs a potentials greater than 1.0 V.sub.RHE due to enhanced carbon corrosion, higher rates of Pt dissolution, and increased Oswald ripening.
[0123] Shown in
[0124] For the accelerated stability tests, the NiPt 2D nanoframe catalysts showed significantly better stabilities than Pt/C catalysts, with 116.37.3 and 121.931.8% of initial the ECSA.sub.Pt after 1000 cycles for the NiPt-200 and NiPt-300 catalysts respectively. The NiPt-200 and NiPt-300 2D nanoframes show comparable ECSA.sub.Pt after the 1000 cycles, within the margin of experimental error. The NiPt 2D nanoframe catalysts exhibited an initial increase in ECSA.sub.Pt after the first 200 cycles and then showed relatively stable values over the remaining 800 cycles. The increase in the ECSA.sub.Pt for the initial 200 cycles between 0.6 and 1.3 V.sub.RHE is attributed to exposure of more Pt active sites during electrochemical cycling due to Ni-leaching, pore formation, and increase of surface roughness.
[0125] The high stabilities of the NiPt-200 and NiPt-300 catalysts that showed 116-122% of initial ECSA.sub.Pt after 1000 cycles supports that the unsupported metallic 2D nanoframes show excellent durability when cycled to elevated voltages.
[0126] The specific activities of the catalysts after the accelerated stability testing were also evaluated (
[0127] Platinum dissolution occurs over the potential region of oxygen reduction, and Pt dissolution rates depend on a number of factors including particle size, support interactions, upper potential limit, lower potential limit, electrolyte, scan rate, number of cycles, temperature, and catalyst loading.
[0128] To evaluate the contribution of Pt dissolution to the catalyst degradation over the accelerated stability tests, ICP-MS measurements of Pt remaining within the electrode and dissolved within the electrolyte after cycling were performed (Table 1). For Pt/C after the accelerated stability tests, only 49.611.0 wt % of Pt remained within the electrode, while a significant percentage of Pt (50.411.0%) was dissolved within the electrolyte. In contrast, for the NiPt nanoframes Pt remained predominantly within the electrodes and a small amount of Pt was dissolved within the electrolyte. The NiPt 2D nanoframes showed 89.83.5 wt % and 96.01.6 wt % of Pt remained within the electrode after the stability tests for NiPt-200 and NiPt-300 catalysts respectively. Pt dissolution within the electrolyte over the same conditions was determined to be 10.23.5% and 4.01.6% wt Pt for the NiPt-200 and NiPt-300 catalysts respectively. In particular, the NiPt-300 catalysts showed a very low amount of dissolved Pt compared with Pt/C. The specific amount of Pt dissolved within the electrolyte was normalized over the total number of cycles and for the geometric area to allow for comparison. For Pt/C, Pt dissolution was determined from the accelerated stability tests to be 19.53.8 ng cm.sup.2 cycle.sup.1. Over the stability tests, NiPt-200 and NiPt-300 catalysts showed Pt dissolution rates of 3.61.0 and 2.11.0 ng cm.sup.2 cycle.sup.1 which were significantly lower (up to 9 lower for NiPt-300) compared to Pt/C tested under the same parameters. For the NiPt nanoframes it is possible that the initial increase in ECSA.sub.Pt over the initial 200 cycles of the stability test may involve a small amount of Pt dissolution which occurs along with partial removal of Ni. It is possible that either (i) dissolution is relatively steady over the stability cycles or (ii) there is a higher dissolution rate over the initial 200 cycles of the stability tests followed by a lower dissolution rate for subsequent cycles.
TABLE-US-00001 TABLE 1 Comparison of Pt content within the electrode and electrolyte after stability testing for Pt/C, NiPt-200, and NiPt-300 catalysts; Pt content was determined by ICP-MS analysis as discussed in the text. After accelerated stability testing Mass Pt (g), Mass Pt (g), % Pt (wt %), % Pt (wt %), Pt dissolution Catalyst remaining in dissolved in remaining in dissolved in rate Material electrode electrolyte electrode electrolyte (ng.sub.Pt cm.sup.2 cycle.sup.1) PtC 3.78 0.93 3.82 0.74 49.6 11.0 50.4 11.0 19.5 3.8 NiPt-200 6.34 0.68 0.71 0.20 89.8 3.5 10.2 3.5 3.6 1.0 NiPt-300 9.85 0.64 0.42 0.20 96.0 1.6 4.0 1.6 2.1 1.0
[0129] The higher stability of the NiPt 2D nanoframe architecture compared with carbon-supported Pt and PtNi alloys is attributed to (i) the carbon-free, unsupported 3D structural morphology that eliminates carbon corrosion, and (ii) the specific Pt environment within the integrated NiPt structure that lowers Pt dissolution and influences Pt migration and agglomeration which are issues common for carbon-supported materials. In addition to the absence of carbon corrosion, the surface structure of the NiPt nanoframes may contribute to enhanced stability.
[0130] In addition to high stabilities at elevated potentials, the NiPt-200 and NiPt-300 2D nanoframe catalysts show high specific activities The PtNi 2D nanoframes provide high combined activity and stability, particularly for significant and important elevated voltage region where carbon-supported catalysts, even with very high activities can suffer significant degradation in performance.
[0131] This example demonstrates the synthesis, structure, and electrochemical properties of metallic 2D nanoframes which consist of a hierarchical 2D framework composed of a highly catalytically active PtNi alloy phase and an interconnected pore network. A key aspect of the 2D nanoframe architecture is that the PtNi alloy catalyst is integrated within a 3D porous network rather than having isolated particles within a carbon support matrix. The combination of the highly active PtNi alloy and 3D network results in metallic 2D nanoframes that exhibit high ORR specific activities and high stabilities compared with Pt/C and function as ORR electrocatalysts without the addition of carbon within the electrode matrix.
Example 2
Material Synthesis
[0132] Chemicals.
[0133] Iridium chloride hydrate (IrCl.sub.3, 99.8%), nickel nitrate hexahydrate (Ni(NO.sub.3).sub.2. 6H.sub.2O, 98%) and urea (N.sub.2COH.sub.4, 99.3%), were obtained from Alfa Aesar. Ethylene glycol, isopropanol (HPLC grade), and ACS grade water (18 M-cm) were obtained from VWR Analytical. All reagents were used without further purification.
[0134] Synthesis of -Ni(OH).sub.2 Nanosheets.
[0135] The -Ni(OH).sub.2 nanosheets were synthesized using a microwave-assisted hydrothermal process. To prepare the -Ni(OH).sub.2 nanosheets, 6.0 g (20.6 mmol) of Ni(NO.sub.3).sub.2.6H.sub.2O was combined with 4.92 g (82 mmol) of urea in 18 mL of ultrapure water (18 M-cm) and 126 mL of ethylene glycol (EG). The solution was then divided into six aliquots (24 mL each) which were placed in a Discover SP Microware Reactor and then treated under variable microwave radiation power using a controlled temperature of 120 C. which was maintained for 13 minutes under active stirring. The obtained light green powder was recovered by centrifugation (1690 RCF, 3 min, Thermo, Sorvall ST16) and subsequently rinsed five times with water and two times with isopropanol using the same centrifuge conditions. Following the rinsing steps, the powder was dried under vacuum at 60 C. overnight.
[0136] Synthesis of NiO Nanosheets.
[0137] To prepare NiO nanosheets, 0.18 g of Ni(OH).sub.2 nanosheets were heated within a muffle furnace from room temperature to 300 C. for 90 minutes in ambient air using a ramp rate of 20 C./min. After the 90 minute dwell time, the sample was allowed to cool to room temperature, and then taken out from furnace.
[0138] Iridium-Decorated NiO Nanosheets (NiO:Ir).
[0139] The NiO nanosheets were decorated with Ir. To deposit Ir on the NiO nanosheets, 0.5 g of the prepared NiO was dispersed in 50 mL of ultrapure water using an ultrasonic bath. In a second beaker, 0.197 g of IrCl.sub.3.xH.sub.2O (53.29 wt. % Ir), corresponding to 20.8 wt % of metallic Ir vs NiO, was dissolved in 50 mL of ultrapure water and then added to the NiO slurry. Additional water was then added to obtain a total volume of 125 mL. The suspension was separated into 25 mL aliquots which were transferred to the microwave reactor and then treated under variable microwave radiation power using a controlled temperature of 120 C. which was maintained for 13 minutes under active stirring. This process was repeated for each aliquot. Finally, a gray powder was recovered, rinsed, and dried as described above. The Ir-decorated NiO nanosheets were notated as NiO:Ir.
[0140] 2D NiIr Nanoframes.
[0141] The NiO:Ir catalyst was thermally treated at either 200 C. or 300 C. for 23 min under 120 mL min.sup.1 flowing Ar/H.sub.2 (95/5 vol %) using a ramp rate of 20 C./min starting from room temperature. After 23 minutes dwell time, the reactor was removed from the oven and allowed to cool under Ar/H.sub.2 flow until room temperature. The synthesized samples were notated as NiIr-200, and NiIr-300 for the samples heated to either 200 C. or 300 C. respectively.
[0142] Chemical Leaching.
[0143] In order to chemically leach out unstable nickel from the material, 0.2 g of the as-prepared NiIr nanoframes was dispersed in 100 mL of 0.05 M HNO.sub.3. The solution was bubbled with Ar for 20 min then heated to 80 C. for 2 hours under stirring and under continuous Ar flow. After the reaction, the suspension was allowed to cool to room temperature. The chemically leached material was the recovered, rinsed, and dried as described above. The synthesized samples were notated as NiIr-200-CL, and NiIr-300-CL respectively. The wt % Ir and wt % Ni of NiIr-200-CL was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) carried out by Galbraith Laboratories, Inc. (Knoxville, Tenn.). From ICP analysis, the NiIr-200-CL catalyst was determined to be 65.51.1 wt % Ir and 18.00.6% Ni. For the NiIr-300-CL catalyst, the Ir and Ni content was determined from EDS analysis to be 51.014 wt % Ir and 23.07.4 wt % Ni.
Physical and Structural Characterization.
[0144] Powder X-ray diffraction (XRD) measurements were conducted using a Bruker AXS D8 Advance powder X-ray diffractometer with a Cu Ka (=1.5406 ) radiation source, operating at 40 kV and 25 mA and a high resolution energy dispersive 1D Linxeye XE detector. The scan range of 2 was 5<2<85 with a 0.01 increment. Brunauer-Emmett-Teller (BET) surface areas were obtained from nitrogen adsorption/desorption isotherms measured using a Micromeritics ASAP 2020 surface area and porosimetry analyzer. Samples were degassed under vacuum at 120 C. for 16 h prior to characterization. Cumulative pore volumes and mean pore diameters were calculated from the adsorption isotherm representing the volume in pores between 1.7 and 300 nm using the Barrett-Joyner-Halenda (BJH) method (Micromeritics Microactive software, version 4.02).
[0145] The morphology and elemental distribution of the catalysts were determined by scanning electron microscopy (SEM) using a SEM FEI-Helios Nanolab 400. Cs-corrected scanning transmission electron microscopy (STEM) was carried out by using a JEM-ARM200F (200 kV, JEOL) equipped with an energy dispersive spectrometer (EDAX Silicon Drift detector). High angle annular dark field STEM (HAADF-STEM) was obtained with a convergence angle of 26 mrad and collection semiangles from 50 to 180 mrad. The probe size used was about 0.09 nm with the probe current of 22 pA.
Rotating Disk Electrochemical Characterization.
[0146] The electrochemical measurements were conducted at constant temperature (298 K) in a three-electrode cell using a thin-film rotating disk electrode (TF-RDE) configuration with an Autolab PGSTAT128N bipotentiostat and rotation control (Pine Instruments). Catalyst inks were prepared by combining a specific mass of the acid-leached catalysts (NiIr-200-CL or NiIr-300-CL) with a specific volume of a stock solution to yield a catalyst concentration of 0.35 mg.sub.cat/mL. The stock solution was prepared by mixing 0.4 mL of Nafion suspension (Aldrich, 5 wt %, 1100 g equivalent weight), 20 mL of isopropanol, and 79.6 mL ultra-pure water (18 M-cm). The inks were sonicated (Fisher, 40 kHz) in an ice-bath for 20 minutes, and then treated with a probe sonicator for 20 seconds. The ink was immersed in a controlled temperature bath (25 C.) for 1 minute while maintaining the agitation and then used immediately. For comparison with the synthesized catalysts, inks were also prepared with IrO.sub.2 (Product number 43396) obtained from Alfa Aesar. For the IrO.sub.2 the catalyst ink consisted of 0.4 mg.sub.cat/mL (84 wt % Ir). The inks were applied to a polished Au working electrode (0.196 cm.sup.2 geometric area) while under rotation (700 rpm). The electrode loading was controlled by depositing a specific volume of the ink onto the Au working electrode. For the NiIr-200-CL or NiIr-300-CL catalysts, 15 uL was deposited onto the Au electrode corresponding to a loading of 17.5 g Ir cm.sub.geo.sup.2 for NiIr-200-CL and 15.3 g Ir cm.sub.geo.sup.2 for NiIr-300-CL. After depositing the inks on the rotating Au electrode, the inks were maintained under rotation and allowed to dry under ambient conditions.
[0147] All electrochemical measurements were carried out in 0.1 M HClO.sub.4 electrolyte prepared with 70% HClO.sub.4 (Veritas Doubly Distilled, GFS Chemicals) (0.000001% Cl.sup.) and ACS grade water (18 M-cm). For the electrochemical tests, the Au working electrode was placed in a three-electrode cell with the 0.1 M HClO.sub.4 electrolyte. A Pt mesh and a freshly prepared reversible hydrogen electrode (RHE) were used as counter and reference electrodes, respectively. The electrodes were immersed in Argon saturated 0.1 M HClO.sub.4 under potential control (0.1 V/RHE). The electrochemical surface area (ECSA) of metallic Ir was calculated by CO-stripping using a specific charge of 358 C cm.sup.2. The catalysts were electrochemically conditioned by cycling 60 times from 0.05-1.5 V/RHE at 100 mV/s.
[0148] For evaluating the oxygen evolution reaction activity, the electrode was then conditioned by cycling 10 times in the potential range 1.2-1.8 V.sub.RHE at 100 mV/s and 1600 rpm. Following this, the electrochemical activity was determined by linear sweep voltammetry (LSV) in the potential range of 1.2-1.8 V.sub.RHE using a scan rate of 20 mV s.sup.1 and a rotation rate of 1600 rpm. The mass-normalized OER activity was determined by the current at a specific voltage divided by the Ir mass on the electrode. Inspection of the Tafel plots for linear behavior within specific voltage ranges was used to evaluate that the electrode was not within a mass-transport limited regime within the specific voltage range. Following the measurements of the OER activity, an accelerated durability test was carried out by maintaining the electrode at a constant potential of 1.6 V.sub.RHE for 13.5 hours under a rotation rate of 1600 rpm. After the constant potential step was completed, the electrolyte was replaced, and LSV and cyclic voltammetry (CV) measurements were obtained as described above. During LSV and potentiostatic tests, to account for solution resistance the internal resistance was corrected by the potentiostat using a resistance of 25.
Synthesis Description
[0149] NiIr 2D nanoframes (or catalyst nanoarchitectures) were synthesized by (i) formation of Ni(OH).sub.2 nanosheets; (ii) thermal treatment in air to form NiO nanosheets, (iii) deposition of Ir onto the NiO nanosheets, and (iv) controlled temperature/atmosphere treatments (either 200 C. or 300 C.) under reducing conditions (H.sub.2/Ar, 5/95 vol %), and a chemical leaching step in nitric acid (0.05 M HNO.sub.3 at 80 C. during 2 h under Ar atmosphere) (
Scanning Electron Microscopy Characterization.
[0150] The Ni(OH).sub.2 nanosheets were synthesized using a microwave-assisted process and showed a nanosheet structure from the Scanning electron microscopy (SEM) image and the X-ray diffraction (XRD) pattern which was consistent with the -Ni(OH).sub.2 phase (
[0151] A particularly important attribute of this material is that the structure is an interconnected solid network (rather than separate particles) and an interconnected pore network as observed within the SEM images (
X-Ray Diffraction Characterization
[0152] The X-ray diffraction patterns (XRD) of NiO nanosheets as well as NiO:Ir and integrated 2D-NiIr structures treated at different temperatures (i.e. 200 and 300 C.), are shown in
Structure and Morphology from Scanning Electron Microscopy
[0153] Scanning transmission electron microscopy (STEM) was used to characterize the morphology and structural of the Mk catalysts. The STEM image of the NiIr-200 sample prior to chemical leaching shows the interconnected network of nanoparticles and lattice fringes of small domains. Both Z contrast imaging (Ir shows up brighter than Ni) and EDS mapping show a distribution of Ir and Ni within the structure at the atomic level (
Cyclic Voltammetry and Carbon Monoxide Stripping Analysis
[0154] The NiIr-200-CL and NiIr-300-CL catalysts were first electrochemically conditioned by cycling the electrodes from 0.05 to 1.5 V.sub.RHE for 60 cycles. The first CV scan of both NiIr-200-CL and NiIr-300-CL (
Oxygen Evolution Activity Measurements.
[0155] Following the electrochemical condition step and CV measurements, the materials were electrochemically tested for OER using a rotating disc electrode (RDE) configuration at 1600 rpm in 0.1 M HClO.sub.4. For reference, the catalysts were compared with the OER activity of a commercial IrO.sub.2 sample. The NiIr-200-CL exhibited significantly higher current than the NiIr-300-CL or commercial IrO.sub.2 material, with current normalized based on geometric area of the electrode disk (
TABLE-US-00002 TABLE 2 Comparison of oxygen evolution activity for NiIr-200-CL, NiIr-300-CL and IrO.sub.2. Initial After Durability Testing Final/ Loading (g.sub.Ir I.sub.geo.sup.1.55 V I.sub.m.sup.1.55 V I.sub.m.sup.1.50 V I.sub.geo.sup.1.55 V I.sub.m.sup.1.55 V I.sub.m.sup.1.50 V Initial Sample cm.sup.2.sub.geo) (mA cm.sup.2.sub.geo) (A g.sub.Ir.sup.1) (A g.sub.Ir.sup.1) (mA cm.sup.2.sub.geo) (A g.sub.Ir.sup.1) (A g.sub.Ir.sup.1) (%)* NiIr-200-CL 17.5 22.6 1290 180 14.32 820 120 66% NiIr-300-CL 15.3 11.8 550 112 2.45 210 10 27% IrO.sub.2-AA 17.2 2.34 136 28 1.95 113 22 83% *Final/initial mass activity at 1.55 V.sub.RHE, with the final value taken after the accelerated durability testing.
Evaluation of Oxygen Evolution Reaction Electrocatalyst Stability.
[0156] The stability of the catalyst is a particularly important parameter to evaluate since catalyst degradation will significantly reduce the lifetime of an electrolyzer which is a critical metric. An accelerated durability test (ADT) using a constant potential of 1.6 V for 13.5 hours was used. Shown in
[0157] For NiPt 2D nanoframes for ORR catalysts, heating to 300 C. resulted in higher activity than heating to 200 C. However, in the case of NiIr 2D nanoframes used for OER catalysts, higher temperature treatments resulted in lower activity and stability. It is possible that higher temperature treatment results in a metallic Ir-rich surface with subsurface Ni. The elemental composition (from EDS) showed higher amounts of Ni were present for 300 C. compared with 200 C. which may result from a protective Ir shell that stabilizes Ni from dissolution. However for the 200 C. sample, the specific interaction of Ir with Ni and the presence of higher amounts of oxide within the structure may contribute to higher OER activity and stability.
[0158] In this patent, certain U.S. patents, U.S. patent applications, and other materials (e.g., articles) have been incorporated by reference. The text of such U.S. patents, U.S. patent applications, and other materials is, however, only incorporated by reference to the extent that no conflict exists between such text and the other statements and drawings set forth herein. In the event of such conflict, then any such conflicting text in such incorporated by reference U.S. patents, U.S. patent applications, and other materials is specifically not incorporated by reference in this patent.
[0159] Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.