ELECTROCATALYSTS WITH TAILORED LOCAL CHEMICAL ENVIRONMENT
20250332583 ยท 2025-10-30
Assignee
Inventors
- Xiangfeng Duan (Los Angeles, CA, US)
- Yu HUANG (Los Anglees, CA, US)
- Chengzhang WAN (Los Angeles, CA, US)
Cpc classification
B01J35/80
PERFORMING OPERATIONS; TRANSPORTING
B01J33/00
PERFORMING OPERATIONS; TRANSPORTING
International classification
B01J35/50
PERFORMING OPERATIONS; TRANSPORTING
B01J35/80
PERFORMING OPERATIONS; TRANSPORTING
B01J33/00
PERFORMING OPERATIONS; TRANSPORTING
Abstract
The present embodiments relate generally to electrochemical processes and more particularly to methods and apparatuses for high-performance alkaline water electrolysis and renewable fuel generation. One or more embodiments relate to a unique core-shell structure (A@BOxHy) in which the amorphous or nanoporous shell structure (BOxHy) can significantly enhance reaction kinetics and allow selective transport of certain feedstock while protecting the core catalysts (A) from competitive adsorption and morphology degradation, leading to both optimized activity and durability.
Claims
1. An apparatus for a unique core-shell structure (A@BOxHy) comprising: a core comprising A; and an amorphous or nanoporous shell structure (BOxHy) that is configured to enhance reaction kinetics and allow selective transport of certain feedstock while protecting catalysts in the core from competitive adsorption and morphology degradation.
2. The apparatus of claim 1, wherein the shell comprises amorphous hydrated oxides/hydroxides and the metal core comprises a precious metal.
3. The apparatus of claim 1, wherein B is one or more of Fe, Co, Ni, Ru, Rh, and Si.
4. The apparatus of claim 2, wherein B is one or more of Fe, Co, Ni, Ru, Rh, and Si.
5. The apparatus of claim 1, wherein A is Pt.
6. The apparatus of claim 3, wherein A is Pt.
7. The apparatus of claim 1, wherein the composition and structure of the shell is tuned for hydrogen transport rate through the shell.
8. The apparatus of claim 7, wherein tuning includes tailoring the local equilibrium pH at the surface of the core catalysts, leading to the desired proton concentration for pH-sensitive reactions including CO.sub.2 RR or NRR.
9. The apparatus of claim 1, wherein the core and shell structure together comprise a Ni(OH).sub.2-clothed bare-foot Pt-tetrapod core/shell nanostructure [Pt.sub.tet@Ni(OH).sub.2].
10. The apparatus of claim 9, wherein the Pt.sub.tet@Ni(OH).sub.2 nanostructure comprises a Pt nano-tetrapod (Pt.sub.tet) core as the HER catalyst and an amorphous Ni(OH).sub.2 shell as the water dissociation (WD) catalysts and proton permselective encapsulation.
11. The apparatus of claim 10, wherein a hydrogen-bond framework in the amorphous Ni(OH).sub.2 shell stabilizes the intermediate state of the WD step and facilitates water dissociation into OH.sup. and H.sup.+, with OH.sup. diffusing into the alkaline electrolyte and the H.sup.+ efficiently transporting to the Pt.sub.tet core through the amorphous Ni(OH).sub.2 shell following a barrierless cascade pathway.
12. The apparatus of claim 10, wherein the encapsulation by the amorphous Ni(OH).sub.2 shell efficiently rejects water impurity ions.
13. The apparatus of claim 12, wherein the water impurity ions include Cl.sup. and I.sup..
14. The apparatus of claim 10, wherein the encapsulation by the amorphous Ni(OH).sub.2 shell efficiently suppresses Pt atom leaching.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and other aspects and features of the present embodiments will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures, wherein:
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DETAILED DESCRIPTION
[0035] The present embodiments will now be described in detail with reference to the drawings, which are provided as illustrative examples of the embodiments so as to enable those skilled in the art to practice the embodiments and alternatives apparent to those skilled in the art. Notably, the figures and examples below are not meant to limit the scope of the present embodiments to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present embodiments can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present embodiments will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the present embodiments. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the present disclosure is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present embodiments encompass present and future known equivalents to the known components referred to herein by way of illustration.
[0036] Among other things, the present disclosure includes a recognition that, analogous to natural enzymes, an elaborated design of catalytic systems with a specifically tailored local chemical environment could substantially improve reaction kinetics, effectively combat catalyst poisoning effect and boost catalyst lifetime under unfavorable reaction conditions. According to certain aspects, therefore, embodiments relate to a unique design of Ni(OH).sub.2-clothed Pt-tetrapods with an amorphous Ni(OH).sub.2 shell as a water dissociation catalyst and a proton conductive layer to ensure abundant proton supply while isolating the Pt core from bulk alkaline electrolyte and rejecting undesired poisoning species. This design creates a favorable local chemical environment with efficient proton supply to the active Pt sites, resulting in acidic-like HER kinetics with a lowest Tafel slope of 27 mV/decade and a record-high specific activity and mass activity in alkaline electrolyte. The proton conductive Ni(OH).sub.2 shell effectively rejects impurity ions and retards the Oswald ripening, endowing a high tolerance to solution impurities and long-term durability that is difficult to achieve in the naked Pt-catalysts. The markedly improved alkaline HER activity and durability promise an attractive catalyst material for alkaline water electrolyzers and renewable chemical fuel generation.
[0037] By way of background, water electrolysis is of increasing interest for converting intermittent renewable electricity (e.g. from solar cells and windmills) into high purity hydrogen. The electrochemical water-splitting reaction is comprised of two half-reactions: the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). In general, the HER and OER display distinct kinetics in acidic and basic electrolytes and often require costly platinum group metal (PGM) catalysts. Platinum (Pt) is regarded as the best element to catalyze HER for its optimal hydrogen binding energy (HBE). (Cao, Z. et al. Platinum-nickel alloy excavated nano-multipods with hexagonal close-packed structure and superior activity towards hydrogen evolution reaction. Nature Communications 8, 15131, (2017); Danilovic, N. et al. Enhancing the alkaline hydrogen evolution reaction activity through the bifunctionality of Ni (OH).sub.2/metal catalysts. Angewandte Chemie 124, 12663-12666, (2012); Yin, H. et al. Ultrathin platinum nanowires grown on single-layered nickel hydroxide with high hydrogen evolution activity. Nature communications 6, 6430, (2015); Zhao, Z. et al. Surface-engineered PtNi-O nanostructure with record-high performance for electrocatalytic hydrogen evolution reaction. Journal of the American Chemical Society 140, 9046-9050, (2018); Subbaraman, R. et al. Trends in activity for the water electrolyser reactions on 3d M (Ni, Co, Fe, Mn) hydr (oxy) oxide catalysts. Nature materials 11, 550, (2012); Liu, Z. et al. Aqueous Synthesis of Ultrathin Platinum/Non-Noble Metal Alloy Nanowires for Enhanced Hydrogen Evolution Activity. Angewandte Chemie 130, 11852-11856, (2018))
[0038] In particular, the Pt catalysts feature a rather small overpotential for HER in the acidic condition where the cathodic HER is usually regarded as a trivial challenge. However, for the complete water electrolysis, the anodic OER in the acidic condition is considerably more challenging and often features large overpotential and limited durability even with the most advanced design of PGM catalysts (Reier, T., Nong, H. N., Teschner, D., Schlogl, R. & Strasser, P. Electrocatalytic Oxygen Evolution Reaction in Acidic EnvironmentsReaction Mechanisms and Catalysts. Advanced Energy Materials 7, 1601275, (2017)). On the other hand, the OER in the alkaline condition is much more friendly and can be readily facilitated with non-precious metal (e.g. Ni, Fe, Co, etc.) oxide/hydroxide catalysts, offering considerable kinetic and cost benefits (Suen, N.-T. et al. Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chemical Society Reviews 46, 337-365, (2017)). With the continued development of anion exchange membranes (AEMs) of lower resistance and lower hydrogen diffusivity (Schalenbach, M. et al. Acidic or Alkaline? Towards a New Perspective on the Efficiency of Water Electrolysis. Journal of The Electrochemical Society 163, F3197-F3208, (2016)), alkaline electrolysis is becoming an increasingly attractive alternative for commercial electrolyzers (Leng, Y. et al. Solid-State Water Electrolysis with an Alkaline Membrane. Journal of the American Chemical Society 134, 9054-9057, (2012); Abbasi, R. et al. A Roadmap to Low-Cost Hydrogen with Hydroxide Exchange Membrane Electrolyzers. Advanced Materials 0, 1805876, (2019)).
[0039] However, the HER kinetics in the alkaline condition is considerably slower. Even with the Pt catalysts, the HER rate is orders of magnitude lower than that in the acidic electrolyte because of the sluggish water dissociation step and the poor proton supply rate (
[0040] In general, the local chemical environment plays a fundamental role in determining the reaction pathway and kinetics on the catalytic surface. A practical water electrolysis requires the concerted supply of the reactants and removal of the products under specific operating conditions to achieve high activity, tolerance to impurities in water, and long lifetime. To this end, a comprehensive approach that integrates electrocatalytic active site design with rational strategies to manipulate nanoscale mass/charge transport, ion separation, or structural evolution is highly desired for designing high-performing electrocatalysts that can facilitate efficient electron transfer and chemical transformations under practical conditions. This is analogous to the natural enzymes where precisely tailored micro-environment works in concert with the active sites to ensure superior activity, selectivity, and durability under practical solution conditions. Such an elaborate design is particularly important for alkaline water electrolysis where the local chemical environment near the active Pt sites in alkaline electrolytes is far more complex than that in an acidic electrolyte due to the limited proton supply rate, competitive adsorption of positively charged alkali metal cations (vs. protons) or other undesirable strong binding impurities (e.g., Cl.sup.) that could poison the catalytic sites.
[0041] Surface decoration has been recognized as an effective approach for tailoring the local chemical environment near the active sites. For example, the decoration of crystalline NiO or NiS.sub.x particles on Pt surface has been reported to improve HER specific activity (SA, activity normalized by electrochemical surface area: ECSA) in the alkaline electrolyte (Wang, P. et al. Precise tuning in platinum-nickel/nickel sulfide interface nanowires for synergistic hydrogen evolution catalysis. Nature Communications 8, 14580, (2017); Wang, P., Jiang, K., Wang, G., Yao, J. & Huang, X. Phase and Interface Engineering of Platinum-Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution. Angewandte Chemie International Edition 55, 12859-12863, (2016)), which has been mostly attributed to enhanced water dissociation kinetics facilitated by the Ni species. However, such crystalline Ni species are less permeable to protons and partly block the surface active sites (see
[0042] Additionally, the protons generated from Ni catalyzed water dissociation could be rapidly consumed within 1 nm (see notes below) through reassociation with the abundant hydroxyl in alkaline electrolyte. Thus, only a small fraction of Pt sites in close proximity of the decorated Ni species can benefit from the improved water dissociation kinetics on decorated Ni species, while most Pt sites farther away from Ni species are barely impacted (see
[0043] To address the above and other challenges, the present embodiments relate to a core-shell nanostructure that is able to create a local chemical environment that can provide efficient proton (H.sup.+) supply to the Pt active sites and greatly boost HER performance in alkaline medium. In a particular example, the present embodiments include a Ni(OH).sub.2-clothed Pt-tetrapod in which the proton conductive amorphous Ni(OH).sub.2 tailors a local chemical environment for optimum HER in bulk alkaline electrolyte. For example, the Ni(OH).sub.2-clothed Pt-tetrapod ([Pt.sub.tet@Ni(OH).sub.2]) structure offers an ideal geometry for isolating most of the Pt surface sites from the bulk alkaline electrolyte and rejecting undesired poisoning species while allowing the less encapsulated feet to make robust electrical contacts with the carbon support for efficient electron transport to the catalytic sites. The amorphous Ni(OH).sub.2 shell functions as an effective water dissociation catalyst and a low-barrier proton conductive layer to ensure efficient proton supply to the interfacial Pt sites, creating a proton-enriched local environment and fundamentally altering the HER to kinetics to the acidic-like Tafel-step limited pathway. The proton conductive Ni(OH).sub.2 shell effectively rejects impurity ions and retards Oswald ripening process, endowing a high tolerance to water impurities and long-term durability not attainable in the naked Pt-catalysts.
[0044] It should be noted that although the present embodiments will be described with respect to a useful example of Pt.sub.tet@Ni(OH).sub.2, those skilled in the art will appreciate that the example can be extended to a more general composition of A@BO.sub.xH.sub.y), where A can be a precious metal such as Pt, Au, etc., and B can be Fe, Co, Ni, Ru, Rh, Si, etc. Those skilled in the art will understand how to extend the principles of the present embodiments to such other alternatives after being taught by the present examples.
[0045] An example core-shell nanostructure according to embodiments is shown in
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[0047] Additional aspects of the above and other characterizations are as follows. Pt.sub.tet@Ni(OH).sub.2 nanoparticles were prepared through a facile one-pot synthesis as described in more detail below. Transmission electron microscopy (TEM) studies such as the example shown in
[0048] The Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) signal of Pt.sub.tet@Ni(OH).sub.2 such as that shown in
[0049] EXAFS wavelet transform (WT) analysis is powerful for discriminating the backscattering atoms. As shown in
[0050] Since it is difficult to directly evaluate the proton permeability through the amorphous Ni(OH).sub.2 layer, the present Applicants have compared cyclic voltammetry (CV) characteristics of Pt.sub.tet@Ni(OH).sub.2 and the naked Pt.sub.tet in the alkaline electrolyte to evaluate the proton accessibility of the Ni(OH).sub.2 encapsulated Pt core in Pt.sub.tet@Ni(OH).sub.2 according to embodiments, as illustrated in
[0051] In the above characterizations, the naked Pt.sub.tet was obtained by completely etching the Ni(OH).sub.2 shell in the acidic electrolyte (see
[0052] Most Pt catalysts typically exhibit notably different CV in acidic or alkaline electrolytes with highly distinct behavior in the hydrogen desorption region (H.sub.upd) region due to distinct local proton concentration and different hydrogen adsorption/desorption potential (Sheng, W. et al. Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy. Nature Communications 6, 5848, (2015); Wang, X., Xu, C., Jaroniec, M., Zheng, Y. & Qiao, S.-Z. Anomalous hydrogen evolution behavior in high-pH environment induced by locally generated hydronium ions. Nature Communications 10, 4876, (2019)). Indeed, the naked Pt.sub.tet shows a rather different CV behavior in the alkaline vs. acidic conditions, as illustrated in
[0053] The efficient proton supply could lead to considerably improved HER kinetics. To this end, the present Applicants have conducted linear scan voltammetry (LSV) and compared the HER polarization curves (normalized by hydrogen desorption area) of Pt.sub.tet@Ni(OH).sub.2, naked Pt.sub.tet, and Pt/C in pH 14 and pH 0 as shown in
[0054] These specific HER kinetics can be highlighted by the Tafel slope analysis. The HER reaction follows three basic steps, with distinct rate-determining steps (rds) and Tafel slopes: [0055] Volmer step: H.sub.3O.sup.++e.sup..fwdarw.H.sub.ad+H.sub.2O (acidic electrolyte) [0056] Or: H.sub.2O+e.sup..fwdarw.H.sub.ad+OH.sup. (alkaline electrolyte) [0057] Heyrovsky step: H.sub.ad+H.sub.3O.sup.++e.sup..fwdarw.H.sub.2T+H.sub.2O (acidic electrolyte) [0058] Or: H.sub.ad+H.sub.2O+e.sup..fwdarw.H.sub.2T+OH.sup. (alkaline electrolyte) [0059] Tafel step: 2H.sub.ad.fwdarw.H.sub.2T (both acidic and alkaline electrolyte)
[0060] Depending on the electrolyte conditions, one can expect distinct reaction kinetics. In the acidic electrolyte, the sufficient H.sup.+ supply ensures a Tafel-step limited HER pathway, with the Tafel step as the rds and an ideal Tafel slope of 29.6 mV/dec. In the alkaline electrolyte where the H.sup.+ is supplied by water dissociation, the HER reaction typically follows Volmer- or Heyrovsky-step limited pathway, with the Volmer step or Heyrovsky step as the rds, and ideal Tafel slopes of 118.4 and 39.5 mV/dec, respectively.
[0061] The Pt/C, naked Pt.sub.tet, and Pt.sub.tet@Ni(OH).sub.2 show a low and comparable Tafel slope of around 18-19 mV/dec at pH 0 in
[0062] It is interesting to note from
[0063] Although the WD kinetics of transition metal hydroxides have been suggested to explain the enhanced HER activity of Pt/transition metal oxide catalysts, a direct measurement of the enhanced WD steps on electrocatalyst is challenging due to the difficulties to decouple with subsequent hydrogen production steps. To directly elucidate the role of the amorphous Ni(OH).sub.2 shell as an efficient WD catalyst, the present Applicants tested the WD activity of the Ni(OH).sub.2 shell by using bipolar membrane (BPM) electrolysis. The BPM electrolysis is conducted in an H-cell where a combination of AEM and PEM is used to separate the acidic HER half-cell (pH=0) and alkaline OER half-cell (pH=14).
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[0065] As shown, the Pt.sub.tet@Ni(OH).sub.2 or Pt.sub.tet were uniformly dispersed at the interface between the PEM and AEM as the WD catalysts. The standard potential required to drive WD is 0.83 V (Oener, S. Z., Foster, M. J. & Boettcher, S. W. Accelerating water dissociation in bipolar membranes and for electrocatalysis. Science 369, 1099-1103, (2020); Tufa, R. A. et al. Bipolar Membrane and Interface Materials for Electrochemical Energy Systems. ACS Applied Energy Materials 4, 7419-7439, (2021)), above which the WD current increases exponentially until reaching the mass transport limit. The WD polarization curves reveal that the pure BPM with no catalyst, with the naked Pt.sub.tet core or with crystalline Ni(OH).sub.2 nanoplates require an overpotential of 1.64 V, 0.97 V, and 0.64 V, respectively, to reach the 50 mA/cm.sup.2, while that with the amorphous Ni(OH).sub.2 shell (in Pt.sub.tet@Ni(OH).sub.2) only needs 0.18 V overpotential to reach 50 mA/cm.sup.2, demonstrating a much faster WD kinetics, and experimentally confirming the exceptional WD catalytic activity of the amorphous Ni(OH).sub.2.
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[0067] More particularly, as shown in
[0068] The proton conductive Ni(OH).sub.2 shell could help isolate the active Pt sites from the bulk electrolyte environment and thus improve the catalytic tolerance to water impurities with competitive adsorption or capability of etching. For example, halide anions (e.g., Cl.sup.) represent common impurity anions that can strongly bind with Pt sites and partly suppress the catalytic activity. In this regard, the amorphous Ni(OH).sub.2 shell may help block halide anions from the active Pt sites due to the lack of transport path and Donnan exclusion effect (Sarkar, S., SenGupta, A. K. & Prakash, P. The Donnan Membrane Principle: Opportunities for Sustainable Engineered Processes and Materials. Environmental Science & Technology 44, 1161-1166, (2010)). To this end, evaluated was the Cl.sup. and I.sup. tolerance of the Pt.sub.tet@Ni(OH).sub.2 and the naked Pt.sub.tet catalysts. Notably, the Pt.sub.tet@Ni(OH).sub.2 maintains essentially the same HER current level in the presence of 0.5 M Cl.sup. or 0.25 M I.sup. in electrolyte, while the Pt.sub.tet shows a substantial current drop by 26% and 52%, respectively (see
[0069] Additionally, the Ni(OH).sub.2 encapsulation can also prevent the Pt surface from dissolution, leaching, ripening, or aggregation. Chronopotentiometry (CP) tests were conducted to evaluate the durability of the Ptt.sub.et@Ni(OH).sub.2 catalysts at a constant current of 10 mA/cm.sup.2 (normalized by electrode geometrical area). These CP studies show that Pt.sub.tet@Ni(OH).sub.2 coreshell catalysts exhibit only a 30 mV overpotential increase over the 10-hour continuous test as shown in
[0070] It is apparent that the potential degradation decreases substantially with the increasing Pt loading. This is not surprising since higher loading usually comes with more active surface sites, which reduces the average catalytic current per active site and thus reduces the chemical stress or slows the surface degradation process. Indeed, the present Pt.sub.tet@Ni(OH).sub.2 catalysts with four different loading amounts show a similar trend with a notably smaller potential degradation at the higher catalyst loading levels (see
[0071] In summary, the present embodiments relate to a unique design of Ni(OH).sub.2-clothed Pt-tetrapod core-shell nanostructure, in which the amorphous Ni(OH).sub.2 shell functions as a water dissociation catalyst and proton conductive shell to isolate the catalytic Pt surface from the bulk alkaline electrolyte while ensuring efficient proton supply to Pt sites. It delivers an acidic-like HER kinetics in bulk alkaline electrolyte, with the lowest Tafel slope and the highest alkaline HER activity among all Pt-based catalysts reported to date. Moreover, the encapsulation of the catalytic surface by the proton conductive shell considerably slows the dissolution/diffusion of Pt atoms from catalytic surfaces and suppresses the undesirable poisoning effect from impurity ions, thus ensuring high structural stability and activity durability that is difficult to achieve in the naked Pt-catalyst designs. The markedly improved alkaline HER activity and presents an attractive catalyst material for alkaline water electrolyzers and renewable chemical fuel generation. Additionally, the demonstrated capability to fundamentally modify the reaction kinetics by tailoring the local chemical environment may be expanded as a general strategy for the design of a new generation of electrocatalysts with a favorable reaction environment and high selectivity or durability for a wide range of fundamentally and technologically important electrochemical reactions.
Example Methods
[0072] Chemicals. Platinum(II) acetylacetonate [Pt(acac).sub.2, Pt 48.0%], nickel(II) acetylacetonate [Ni(acac).sub.2, 95%], glucose, tungsten(0) hexacarbonyl (W(CO).sub.6, 97%), oleylamine (>98%), 1-octadecene (ODE, >90%), nickel(II) nitrate hexahydrate [Ni(NO.sub.3).sub.2.Math.6H.sub.2O], Cetrimonium bromide ([(C.sub.16H.sub.33)N(CH.sub.3).sub.3]Br and Nafion 117 solution (5%) were purchased from Sigma Aldrich. Commercial Pt/C catalyst (10 wt % Pt, and particle size 2 nm) was purchased from Alfa Aesar. Ethanol (200 proof) was obtained from Decon Labs, Inc. Potassium hydroxide (KOH) was purchased from Fisher Chemical. All the above reagents were used as received without further purification. Carbon black (Vulcan XC-72) was received from Cabot Corporation and was annealed for 2 hours under Ar gas environment at 400 C. before being used. The deionized water (18 M/cm) was obtained from an ultra-pure purification system (Milli-Q advantage A10). The Naftion 117 (PEM) and the Fumasep Fas-50 (AEM) were purchased from the Fuel cell Store.
[0073] Synthesis. In a 30 mL glass vial, 20 mg Pt(acac).sub.2, 25.6 mg Ni(acac).sub.2, 32 mg W(CO).sub.6 and 135 mg glucose were dissolved in a mixture of 3 mL oleylamine and 2 mL octadecene. The mixture was sonicated for 1 hour and the resulting homogenous solution was kept at 80 C. for 2.5 hours and then heated to 140 C. for another 8 hours. After the reaction, the precipitate was centrifuged out at 12100 r.p.m. and washed by ethanol/hexane (25 mL/5 mL) three times. The final product was suspended in 10 mL cyclohexane. In a 30 mL glass vial, 30 mg carbon black (the carbon black was annealed under Ar at 200 C. for 1 hour before use) was sonicated in 15 mL ethanol for 1 hour. 5 mL Pt.sub.tet@Ni(OH).sub.2 hexane solution was then added into the carbon black/ethanol solution and the mixture was sonicated for another 1 hour. The catalysts were centrifuged out at 12100 r.p.m. and washed with cyclohexane/ethanol solution three times, followed by being dried in the vacuum oven for 1 hour. The Pt.sub.tet@Ni(OH).sub.2/C were then annealed in the air at 200 C. for 2 hours to fully remove the surface remaining ligands. The Pt yield is about 40%, on the scale of 6 mgPt/batch. It has been scaled up to 120 mgPt per batch. The crystalline Ni(OH).sub.2 nanoplates were synthesized via adding 0.5 mL 30 wt % ammonia water drop by drop into 100 mL Ni(NO).sub.3 solution (10 g/L) with 0.25 g CTAB as the surfactant.
[0074] Characterizations. Transmission electron microscopy (TEM) images were taken on an FEI T12 operated at 120 kV. Atomic resolution high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and X-ray energy dispersive spectroscopy (EDS) mapping were taken on FEI Titan Cubed Themis G2 300 at 200 kV and JEOL Grand ARM 300CF TEM/STEM with double spherical aberration-correctors operated at 300 kV. Samples for TEM measurements were prepared by dropping 10-20 L nanoparticles dispersion in hexane on a carbon-coated copper grid (Ladd Research, Williston, VT). Powder X-ray diffraction patterns (PXRD) were collected on a Panalytical X'Pert Pro X-ray Powder Diffractometer with Cu-K radiation. The composition of catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP AES, Shimadzu ICPE-9000) as well as SEM-EDS (JEOL JSM-6700F FE-SEM). X-ray photoelectron spectroscopy (XPS) tests were done with Kratos AXIS Ultra DLD spectrometer.
[0075] X-ray adsorption data analysis. Ni K-edge and Pt L3-edge X-ray absorption spectra were acquired under ambient conditions in fluorescence and transmission modes at beamline 1W2B of the Beijing Synchrotron Radiation Facility(BSRF), using a Si (111) double-crystal monochromator. The storage ring of BSRF was operated at 2.5 GeV with a maximum current of 250 mA in top-up mode. While the energy was calibrated using Ni/Pt foil, the incident, transmitted and fluorescent X-ray intensities were monitored by using standard ion chambers and Lytle-type detector, respectively.
[0076] XAS analysis was performed according to standard procedures using the ATHENA and ARTEMIS modules implemented in the IFEFFIT software package (Ravel, B. & Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. Journal of Synchrotron Radiation 12, 537-541, (2005)). The EXAFS signal was first obtained by background-subtraction and normalization, then the x(k) data were Fourier transformed to real (R) space using a Hanning window. To obtain the quantitative structural parameters around the central atoms, a least-squares curve-fitting analysis of the EXAFS (k) data was carried out based on the EXAFS equation in R-space. The structural models were constructed based on the crystal structures of Ni(OH).sub.2, with the scattering amplitudes, phase shifts, and photoelectron mean free path for all paths calculated with the ab initio code FEFF 8.5 (Rehr, J. J. & Albers, R. C. Theoretical approaches to x-ray absorption fine structure. Reviews of Modern Physics 72, 621-654, (2000)).
[0077] Electrochemical Measurements. To obtain a homogeneous catalyst ink, 1 mg of dried Pt.sub.tet@Ni(OH).sub.2/C was mixed with 1 mL ethanol and sonicated for 5 minutes. Then, 10 L (20 L for stability test) of Nafion (5 wt %) was added to the solution. After sonication, 20 L of the homogeneous ink was dropped onto a 5 mm diameter glassy carbon electrode (0.196 cm.sup.2, Pine Research Instrumentation). The ink was dried under ambient air before electrochemical testing.
[0078] All electrochemical tests were carried out in a three-electrode cell from Pine Research Instrumentation. The working electrode was a glassy carbon rotating disk electrode (RDE) coated with corresponding catalysts. The reference electrode was a Hg/HgO electrode from CH Instrument and was calibrated in 1.0 M KOH with saturated H.sub.2. A graphite rod was used as the counter electrode. Cyclic voltammetry was conducted in 1.0 M KOH and 1.0 M HClO.sub.4 between 50 mV to 1100 mV vs. RHE at a sweep rate of 100 mV/s. The polarization curves were tested between 200 mV to 100 mV vs. RHE at a sweep rate of 5 mV/s in 1.0 M KOH and 1.0 M HClO4 with a Pt loading of 5.1 g/cm.sup.2 for Pt/C and 5.6 g/cm.sup.2 for Pt.sub.tet@Ni(OH).sub.2 and Pt.sub.tet, under a rotation speed of 1600 r.p.m. The solution resistances were measured via impedance test. ECSA was measured through the hydrogen desorption region in N.sub.2 saturated 1 M KOH. The bipolar membrane test was conducted in the H-cell. The BPM was fabricated by wet pressing the Nafion 117 PEM and the Fumasep Fas-30 AEM and removing all the bubbles in between two films. When desired, the WD catalysts were pre-deposited on the PEM and the press together with AEM to form BPM with sandwiched WD-CL. The stability test was performed with chronopotentiometry under 10 mA/cm.sup.2 in Ar purged KOH for 10 hours.
TABLE-US-00001 TABLE 1 Comparison of the Tafel slope of Pt.sub.tet@Ni(OH).sub.2 in this work with the state- of-the-art literature. Tafel slope Pontential range Material (mV/dec) (mV vs. RHE) Electrolyte Ref 1 Pt.sub.tet@Ni(OH).sub.2 25 10 to 20 1.0M KOH This Pt.sub.tet 40 10 to 20 work .sup.a 102 50 to 100 Pt/C 75 100 to 160 183 0 to 50 2 PtNiO 40 10 to 20 1.0M KOH .sup.4.sup.b 3 SANiPt 37 10 to 20 1.0M KOH .sup.43.sup.b 4 Ni.sub.3NPt 36.5 0 to 50 mV 1.0M KOH 44 Pt/C 50.6 0 to 50 mV 5 Pt.sub.3.21Ni@Ti.sub.3C.sub.2 38.5 20 to 20 mV 0.1M KOH 45 Pt/C 39.5 20 to 20 mV 6 1.9 nm Pt island 69 10 to 20 mV 1.0M KOH 46 7 D-PtNi/C 55 5 to 25 mV 1.0M KOH 47 Pt/C 56 5 to 25 mV 8 NtS.sub.2/PtNi NWs 20 10 to 20 mV 0.1M HClO.sub.4 48 38 10 to 20 mV 0.1M KOH .sup.aThe Pt.sub.tet@Ni(OH).sub.2 coreshell catalysts show a Tafel slope notably smaller than those of naked Pt.sub.tet, commercial Pt/C, and all previously reported surface decorated Pt catalysts alkaline HER, demonstrating that this full encapsulation of Pt surface with proton permeable Ni(OH).sub.2 shell effectively isolates the Pt surface from the alkaline electrolyte and create a lower local pH on Pt surface, and thus fundamentally altering the HER pathway to acidic-like kinetics with the Tafel step limited pathway, achieving a much lower Tafel slope (<30 mV/dec) and much higher activity. .sup.bThe Tafel slope is obtained from the recalculation from the original data in a different potential range compared with the value reported in the published version.
TABLE-US-00002 TABLE 2 Ni K-edge EXAFS curve fitting parameters..sup.a E.sub.0 R, sample path N R () .sup.2 (.sup.2) (eV) % Ni foil.sup.b NiNi 12 2.48 0.008 0.5 0.01 Ni(OH).sub.2.sup.c NiO 6 2.06 0.008 7.8 0.6 NiNi 6 3.11 0.008 NiPt.sub.tet(OH).sub.2.sup.c NiO 5.5 2.07 0.009 5.4 0.3 NiNi 5.0 3.05 0.010 NiPt 1.0 3.10 0.010 .sup.aN, coordination number; R, the distance between absorber and backscatter atoms; 62, Debye-Waller factor to account for both thermal and structural disorders; AEO, inner potential correction; R factor (%) indicates the goodness of the fit. Error bounds (accuracies) that characterize the structural parameters obtained by EXAFS spectroscopy were estimated as N 20%; R 1%; .sup.2 20%; E0 20%. S.sub.0.sup.2 was determined from Ni foil fitting. Bold numbers indicate fixed coordination number (N) according to the crystal structure. .sup.bFitting range: 3.2 k (/) 11.0 and 1.0 R () 3.0. .sup.cFitting range: 2.5 k (/) 10.4 and 1.0 R () 3.4.
TABLE-US-00003 TABLE 3 Pt L3-edge EXAFS curve fitting parameters..sup.a E.sub.0 R, sample path N R () .sup.2 (.sup.2) (eV) % Pt foil.sup.b PtPt 12 2.76 0.004 6.8 0.01 PtO.sub.2.sup.c PtO1 6 2.02 0.005 4.2 0.02 PtPt 6 3.10 0.006 PtO2 12 3.70 0.010 PtPt.sub.tet@Ni(OH).sub.2.sup.b PtPt 8.9 2.70 0.004 5.0 0.2 PtNi 2.0 3.12 0.005 .sup.aN, coordination number; R, the distance between absorber and backscatter atoms; .sup.2, Debye-Waller factor to account for both thermal and structural disorders; E0, inner potential correction; R factor (%) indicates the goodness of the fit. Error bounds (accuracies) that characterize the structural parameters obtained by EXAFS spectroscopy were estimated as N 20%; R 1%; 2 20%; E0 20%. S02 was determined from Pt foil fitting and fixed. Bold numbers indicate fixed coordination number (N) according to the crystal structure. .sup.bFitting range: 3.0 k (/) 11.0 and 1.2 R () 3.2. .sup.cFitting range: 3.0 k (/) 11.0 and 1.2 R () 3.6.
[0079] Table 4Comparison of the coordination number and bond length fitted from EXAFS and simulated NiO.sub.xH.sub.y model. Our model is in good accordance with the experimental results and further confirms the accuracy and the robustness of the following simulation results based on this model. The slight deviation of coordination number (N) should be ascribed to much smaller numbers of atoms of the modeled motif compared with the real structure.
TABLE-US-00004 Sample Path N.sub.XANES N.sub.theory R.sub.XANES () R.sub.theory () Pt@NixOy NiO 5.50 5.17 2.07 2.08 NiNi 5.00 4.50 3.05 3.04 NiPt 1.00 2.17 3.10 3.10 PtPt 8.90 8.00 2.70 2.80 PtNi 2.00 1.67 3.12 3.14
TABLE-US-00005 TABLE 5 | G.sub.H.sub. G.sub.H (pH 0) G.sub.H(pH 14) Pt(111) fcc 0.09 eV 0.09 eV 0.74 eV Pt(111) atop 0.26 eV 0.26 eV 0.57 eV Pt@NixOy atop 0.14 eV 0.14 eV 0.69 eV 0.04-0.41 eV (proton supply rate corrected)
indicates data missing or illegible when filed
TABLE-US-00006 TABLE 6 Comparison of HER performance of the Pt.sub.tet@Ni(OH).sub.2 in this work with state- of-the-art catalysts. Mass activity (normalized by Pt loading) and specific activity (normalized by ECSA) were compared..sup.a Mass activity Specific activity ECSA at 70 mV vs. at 70 mV vs. (m.sup.2/g) in Catalysts RHE (mA/g.sub.Pt) RHE (mA/cm ) 1M KOH Reference Pt/C 0.75 0.98 76.4 This work Pt.sub.tet@Ni(OH).sub.2 13.4 27.7 48.4 This work SANi-JPtNWs 11.8 10.7 106 43.sup. PtNiO octahedra 7.23 14.8 48.8 4 Pt NWs/ 0.679 2.48 27.3 3 SL-Ni(OH).sub.2 NiO.sub.x/Pt.sub.3Ni 2.59 NA NA 18.sup.b Pt.sub.3Ni.sub.3NWs Pt.sub.3Ni.sub.2NWs/S C 2.48 NA NA 17.sup.b Pt.sub.3.6NiS NWs 4.37 NA NA 6 D-PtNi/C 1.03 4.26 24.2 47.sup. 1.9 nm Pt-island 7.7 14.7 52 46.sup.c on Ni PtNi5-0.3 2.36 11.8 20 51.sup. Pt SAsNi/NiO ~10.7 NA NA 53.sup.b .sup.aGenerally, overpotential, mass activity, and specific activity are three major descriptors when comparing the Pt catalysts' HER performance. However, the overpotential largely depends on the Pt-loading which varies substantially among literature, making it difficult to use overpotential as the activity descriptor without considering the Pt-loading. To alleviate the effect of Pt-loading, it is more appropriate to use the mass activity at certain overpotential (e.g. 70 mV vs. RHE) or the overpotential achieved at a given current density normalized by ECSA for comparing the practical HER activity of different noble metal catalysts in literature. Additionally, specific activity can provide a better description of the intrinsic HER activity of a given catalyst surface. .sup.bIn these cases, the mass activity values were recalculated based on the reported mass loading and the electrode geometric surface area normalized HER polarization curve provided in the literature. .sup.cThe electrochemistry characterizations are conducted in 0.1M KOH in these cases, otherwise in 1M KOH if not specified.
indicates data missing or illegible when filed
[0080] Amorphous nickel hydroxide proton sieve tailors local chemical environment on Pt surface for high-performance alkaline hydrogen evolution reaction
Notes
[0081] 1. The estimation of H.sup.+ diffusion distance: The OH.sup. concentration in the pH 14 electrolyte is 1 mol/L, which equals 1 OH.sup. in 1.66 nm.sup.3. An H.sup.+ generated from the dissociation of an H.sub.2O molecule on the Ni species can only diffuse away for about 1.18 nm (the cubic root of 1.66 nm.sup.3) before encountering and reassociating with another OH.sup. from the bulk alkaline electrolyte. Based on this consideration, the diffusion distance of a free hydronium (H.sub.3O+) is estimated to be around 1.18 nm. This simple estimate suggests that on the Pt surface that is partially decorated with crystalline Ni species, only a small fraction of Pt atoms that are located within 1.18 nm from the Ni species can benefit from the enhanced water dissociation.
[0082] 2. Tafel slope extraction: The quantification and comparison of the Tafel slope is a nontrivial matter and requires extra caution to ensure a fair and consistent comparison. There are two commonly used methods to extract the Tafel slope from the HER polarization curve: (1) Butler-Volmer (B-V) fitting of the HER/HOR kinetic current in the H.sub.2 purged electrolyte; (2) linear fitting of the overpotential vs. logarithm of the HER kinetic current in the N.sub.2 purged electrolyte. Both methods have been used in different literature. For the BV fitting of Pt HER/HOR kinetic current under the H.sub.2 purge, there are three possible ideal models depending on the exact rds: [0083] i) Volmer (rds)-Tafel pathway, with the cathodic HER current
ii) Volmer (rds)-Heyrovsky pathway, with the cathodic HER current
[0085] where i0 is the exchange current density, ana are the symmetric factors) and are set to 0.5, R is the molar gas constant, T is the temperature and is the potential. Through extensive studies with different types of HER catalysts, it was found that the BV fitting only works well for Pt/C with a moderate alkaline HER activity and standard Volmer (rds)-Tafel mechanism (see
[0086] Due to the above challenges associated with BV fitting under H.sub.2 purge, it has been chosen to use the linear fitting of the N.sub.2 purged HER curve, which has been widely used by other literature (e.g. Ledezma-Yanez, I. et al. Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes. Nature Energy 2, 17031 (2017); Monteiro, M. C. O., Goyal, A., Moerland, P. & Koper, M. T. M. Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media. ACS Catalysis 11, 14328-14335 (2021)) to extract the Tafel slope for the following reasons: (i) Under the N.sub.2 purging and high rotating speed (1600 r.p.m.), the HOR branch is removed and thus linear fitting near the onset overpotential is not significantly affected by the HOR signal. There is a clearly linear region in the Tafel plot where the Tafel slope can be extracted. (ii) The Tafel slopes of different catalysts were extracted at similar current densities to ensure a similar mass transfer effect.
[0087] Lastly, it is recognized there could be some level of uncertainties associated with the Tafel slope derivations. Nonetheless, a careful analysis reveals a substantially lower Tafel slope (27 mV/decade) is achieved in Pt.sub.tet@Ni(OH).sub.2 in the alkaline electrolyte when compared with all other references (>36 mV/decade), which is fully validated by internal control samples tested and evaluated with the exactly the same method. The difference is clearly beyond derivation uncertainties. Furthermore, it is important to note that the present materials also deliver a record-high specific activity and a record-high mass activity, which is a directly measured number with less ambiguity. Together, comprehensive evaluation and analysis robustly demonstrated that the present design has enabled a superior HER catalyst in the alkaline electrolyte.
[0088] The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are illustrative, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively associated such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as associated with each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being operably connected, or operably coupled, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being operably coupleable, to each other to achieve the desired functionality. Specific examples of operably coupleable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components.
[0089] With respect to the use of plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
[0090] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as open terms (e.g., the term including should be interpreted as including but not limited to, the term having should be interpreted as having at least, the term includes should be interpreted as includes but is not limited to, etc.).
[0091] Although the figures and description may illustrate a specific order of method steps, the order of such steps may differ from what is depicted and described, unless specified differently above. Also, two or more steps may be performed concurrently or with partial concurrence, unless specified differently above. Such variation may depend, for example, on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Likewise, software implementations of the described methods could be accomplished with standard programming techniques with rule-based logic and other logic to accomplish the various connection steps, processing steps, comparison steps, and decision steps.
[0092] It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation, no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases at least one and one or more to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles a or an limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases one or more or at least one and indefinite articles such as a or an (e.g., a and/or an should typically be interpreted to mean at least one or one or more); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of two recitations, without other modifiers, typically means at least two recitations, or two or more recitations).
[0093] Furthermore, in those instances where a convention analogous to at least one of A, B, and C, etc. is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., a system having at least one of A, B, and C would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to at least one of A, B, or C, etc. is used, in general, such a construction is intended in the sense one having skill in the art would understand the convention (e.g., a system having at least one of A, B, or C would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase A or B will be understood to include the possibilities of A or B or A and B.
[0094] Further, unless otherwise noted, the use of the words approximate, about, around, substantially, etc., mean plus or minus ten percent. Although the present embodiments have been particularly described with reference to preferred examples thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the present disclosure. It is intended that the appended claims encompass such changes and modifications.