The present invention relates to the field of hydrogen storage.
Increasing concerns regarding global reliance on fossil fuels have stimulated the search for renewable energy technologies. Hydrogen is an ideal clean energy carrier to replace carbon-based fuels. Since hydrogen can be produced from water and water is the only combustion product, it offers the potential for an ideal closed energy cycle without undesirable byproducts. Moreover, hydrogen boasts an exceptionally high gravimetric energy density (120-142 MJ kg−1), compared to other energy storage materials (e.g. 44.4 MJ kg−1 for gasoline, 0.17-1.8 MJ kg−1 for batteries). However, the transition from fossil fuels to hydrogen energy is not simple, particularly for transportation applications, which require ample storage density to minimize refueling needs. Use of solid-state hydrogen storage materials has been identified as among the most promising methods for hydrogen energy delivery. For Fuel Cell Electric Vehicle (FCEV) applications, pressurized H2 storage (700 bar) is the predominant technology, given the lack of safe and high capacity solid-state hydrogen storage materials. Metal hydrides such as magnesium hydride (MgH2) have the potential to fulfill these requirements due to their high hydrogen capacity, low cost, and outstanding reversibility. They also eliminate energy costs associated with liquefaction or compression which are required for compressed storage. Furthermore, unlike other solid-state storage options such as MOFs (metal-organic frameworks), hydrogen atoms are bound to metal crystalline lattice sites upon the formation of metal hydrides, enabling a high volumetric capacity and non-cryogenic operation. Among all options for metal hydride precursors, magnesium (Mg) has unique advantages in sustainability and cost, as it is an environmentally friendly and earth-abundant element.
However, there exist stubborn kinetic and thermodynamic barriers to practical use of Mg for hydrogen storage; critical obstacles include the thermodynamic stability of the hydride phase, necessitating high operating temperatures, as well as sluggish hydrogen sorption kinetics. In general, it is extremely difficult to simultaneously achieve high capacity and fast kinetics for any single material. Encouragingly, it has been widely established that additives such as transition metal dopants and carbon based materials enhance the kinetics of solid-state hydrides. However, this effect is typically counterbalanced by a loss of capacity; additives increase the dead mass in the system without contributing to active hydrogen storage. Promisingly, nanostructuring has been shown to alleviate these kinetic barriers and reduce thermodynamic stability by taking advantage of shorter hydrogen diffusion lengths and high surface area-to-volume ratios. Despite these piecemeal advances, no single hydrogen storage material has been capable of leveraging the power of nanostructuring, catalysis, and composite stability to realize suitable performance in the three key domains—capacity, kinetics, and reliability.
The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.
In the discussions that follow, various process steps may or may not be described using certain types of manufacturing equipment, along with certain process parameters. It is to be appreciated that other types of equipment can be used, with different process parameters employed, and that some of the steps may be performed in other manufacturing equipment without departing from the scope of this invention. Furthermore, different process parameters or manufacturing equipment could be substituted for those described herein without departing from the scope of the invention.
These and other details and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.
Here we report the synthesis of a hierarchically ordered multi-component composite with synthetic control across atomic (dopant), nano (Mg crystals), and mesoscopic (rGO encapsulating layer) length scales to address these entangled issues of kinetics and thermodynamics. The high reactivity of zero-valent Mg has restricted their preparation and use under controllable conditions. Nanosizing Mg and MgH2 radically improves their hydrogen sorption properties; however, nanostructuring also causes the materials to become more reactive. The synthetic methods for creating nanostructured materials have been mainly focused on mechanical milling and gas-phase condensation, resulting in irregular size distributions and deteriorative particles due to agglomeration. In such synthetic routes, the addition of transition metals or carbon-based materials meant to advance the kinetic or thermodynamic properties often decimates structural control, adding undesired structural degrees of freedom. Furthermore, Mg nanocrystals are extremely vulnerable to aggregation and oxidation and are highly pyrophoric, restricting their use to inert environments. Thus, the nanostructured Mg-based system requires an appropriate passivating matrix prior to safe implementation in vehicles.
We have shown that nanostructuring of Mg improves the hydrogen absorption/desorption rates over comparable bulk Mg, approaching activation energies of some transition metal catalyzed bulk Mg crystals. Moreover, the interface between graphene layers and Mg nanocrystals further enhances kinetics, a result we attribute to local strain fields. Also, these gas-selective reduced graphene oxide (rGO) encapsulating layers were found to provide remarkable protection of Mg nanocrystals from oxidation, preserving the zero-valent Mg state in air. Further, the literature has established that either alloying Mg with transition metals or incorporation of these transition metals as a dopant considerably enhances hydrogen absorption/desorption properties, although this has been challenging to integrate with metal hydride nanocrystal synthesis in a controlled fashion. Motivated by our previous work and the doping effects of these transition metals, we aimed to encapsulate the transition metal doped Mg crystals by rGO layers to provide an atomically thin protecting layer that prevents oxidation of the encased zero-valent metals, a phenomenon which is attributed to the high hydrogen-selectivity of GO/rGO sheets. This exceptional oxidative stability removes the potential risk of explosion from nanostructured Mg systems in hydrogen storage applications, while simultaneously minimizing dead mass in the system (the rGO layers occupy only up to 2 wt. % theoretically). Moreover, the rGO layers have a beneficial effect on hydrogen sorption of encapsulated Mg crystals, and it is expected that such hydriding/dehydriding properties would be further enhanced by the addition of a transition metal dopant, producing dual-channel doping which couples externally (rGO layer) and internally (transition metal) (
3d-Transition Metal Doped Mg Crystals.
Transition metal doped Mg crystals encapsulated by rGO layers were prepared by modifying previously reported methods via a solution-based, one-pot synthesis, whereas most other studies achieved the material doping using either mechanical milling or gas condensation methods; these approaches are subject to a critical vulnerability in the aspect of lack of structural control or difficulty to implement in a large-scale synthesis of nanocrystalline matter. In this synthetic procedure, the Mg precursor, transition metal precursor, and GO are simultaneously reduced in a one-pot to form zero-valent Mg and transition metals encapsulated by rGO sheets as previously depicted. A series of canonical 3d-transition metals—Ti, Cr, Mn, Fe, Co, and Ni—were studied as candidate dopants, and doping concentrations were maintained at 5 mol. % in Mg to isolate the effect of varying the transition metal. Representative TEM images of the Ni-doped rGO-Mg nanocomposites are shown in
Hydrogen absorption properties of a series of the doped composites were examined (
To investigate hydrogen sorption properties of these systems, hydrogen absorption/desorption tests were performed as a function of temperature at a H2 pressure of 15 bar/0 bar, respectively (
Upon hydriding, most of the crystalline Mg phase was converted to MgH2, based on analysis of the relative XRD peak intensities while the hydride phase expected from the Mg—Ni alloy—Mg2NiH4—was not detected. We conclude that in our composite the Mg—Ni alloy participates in hydrogen sorption catalytically, but does not contribute meaningfully to active hydrogen storage. To explore the reversibility and stability of this performance, a cycle test was performed at 125° C./300° C. for 30 cycles (
Thermodynamics of Ni-Doped rGO-Mg for Hydrogen Absorption/Desorption.
To quantitatively understand the thermodynamic properties of the dual-doped composites, pressure-composition-temperature (PCT) measurements were performed at three different temperatures for each absorption/desorption (
Structural Analysis of Ni-Doped rGO-Mg Composites.
To closely scrutinize the interaction between rGO layers and the Ni-doped Mg crystals, as well as the oxidation state of Mg and Ni metals in the composite along with the incorporation and distribution of Ni within Mg crystals, X-ray absorption near-edge structure (XANES) measurements were performed. Both Mg K- and L-edge spectra confirm the presence of zero-valent Mg metal—a characteristic K-edge peak shoulder located at 1303 eV and a unique sharp L-edge peak protruding at 49.8 eV (
To provide additional insights into the nature of the Mg—Ni nano-alloys and their elevated-temperature evolution, molecular dynamics (MD) vapor deposition simulation methods were used to computationally synthesize Mg-5% Ni crystals at low (300K) and high (600K) temperatures (
In-Situ X-Ray Absorption Near-Edge Structure (XANES) Upon Hydriding.
To elucidate structural changes during hydriding, in-situ XANES measurements were performed under low (1 bar) H2 pressure (
Kinetic Analysis for Hydrogen Absorption and Desorption.
While the enhanced thermodynamics by rGO-encapsulation was confirmed by PCT measurements, the kinetic enhancements associated with the Ni-doped Mg composite materials were quantified by calculating activation energies (ΔE) for absorption/desorption. This was done by fitting the measured rate of hydrogen absorption/desorption to an Arrhenius law at each composition. Interestingly, this results in reaction rates and barriers that change as the reaction progresses, rather than a single barrier for the entire process (
To gain mechanistic insight into
To reach these conclusions, we first analyzed the absorption kinetics of undoped rGO-Mg, for which the energy barrier is initially high and decreases as absorption proceeds (blue line in
Unlike undoped rGO-Mg, the barrier of ˜45-55 kJ/mol for Ni-doped rGO-Mg remains relatively consistent throughout the absorption reaction (blue lines in
Compared to hydriding, dehydriding is less enhanced by the addition of Ni; accordingly, for brevity we do not discuss details of the process here. However, hypothesized dehydriding mechanisms based on the calculated rates and ΔE in
Significantly, our proposed mechanisms suggest that the enhanced absorption and desorption kinetics result from at least two synergistic chemomechanical factors: nanoconfinement favors incomplete MgH2 formation to introduce additional near-surface diffusion pathways, whereas Ni-doping changes the nature and concentration of these pathways, catalyzes H2 dissociation, and exerts favorable stresses on the particle core. Accordingly, “inside-out” doping (i.e., Ni-dopants and rGO encapsulation) appears to have enabled an entirely new path toward optimizing Mg as a hydrogen storage material.
We have demonstrated robust, environmentally stable Mg nanocrystals with Ni as a dopant for a high-performance hydrogen storage material. Among a series of 3d-transition metal dopants, Ni stands out as a high performing additive whose functionality is connected to the formation of a Mg—Ni nano-alloy phase. The thermodynamic and kinetic barriers to hydrogen absorption/desorption are significantly improved with a synergistic effect of nanosizing, rGO encapsulation and Ni doping, notably without sacrificing the high hydrogen sorption capacity of the composite (6.5 wt % of H2 at the system level). The Ni dopants are found to localize primarily near the surface, likely promoting the dissociation of H2 molecules and facilitating subsequent migration of H atoms. As reported previously, the use of encapsulating rGO layers can selectively sieve H2 molecules on the surface, preventing the penetration of other gas molecules such as O2. Leveraging these complementary functionalities, the Ni-doped rGO-Mg composites achieve remarkably high performance in both capacity and transport kinetics with excellent air stability. Potentially, other 3d-transition metals could similarly act as high performing catalysts in a stable and reproducible way, pending formation of nano-alloy phases under controlled conditions. The composite material presented in this work elucidates the mechanism by which this “inside-out” doping system participates in both thermodynamics and kinetics of hydrogen storage materials and provides a new platform for practical use of hydrogen storage for mobile applications.
Materials.
Bis(cyclopentadienyl) magnesium 99.99+% (Cp2Mg), Bis(cyclopentadienyl)titanium dichloride, 99+% (Titanocene dichloride) (Cp2TiCl2), Bis(cyclopentadienyl)chromium, min. 95%, sublimed (Chromocene) (Cp2Cr), Bis(cyclopentadienyl)manganese, 98+% (Manganocene) (Cp2Mn), Bis(cyclopentadienyl)cobalt(II), min. 98% (Cobaltocene) (Cp2Co), Bis(cyclopentadienyl)iron, 99% (Ferrocene) (Cp2Fe), Bis(cyclopentadienyl)nickel, 99% (Nickelocene) (Cp2Ni) were purchased from Strem Chemicals. Single layer graphene oxide was purchased from ACS Material, LLC. Lithium foil 99% was purchased from Alfa Aesar. Naphthalene 99% was purchased from Sigma Aldrich. Tetrahydrofuran (THF) was distilled before use.
Synthesis of 3d-Transition Metal Doped rGO-Mg.
A series of 3d-transition metal doped rGO-Mg composites were prepared in an argon glove box. Each composite was synthesized following the same procedure, varying only the transition metal incorporated. Lithium naphthalenide solutions were prepared by dissolving naphthalene (18.5 mmol, 2.52 g) in THF (120 mL), followed by the addition of Li metal (27.2 mmol, 0.189 g). GO (6.56 mg) was dispersed in THF (13.1 mL), sealed in a glove box and sonicated for 1.5 hours. Cp2Mg (15 mmol, 2.31 g) and each transition metal precursor (0.75 mmol, 0.028 g for Cp2Ni) were dissolved in THF (22.5 mL) and the solution was added into the GO solution and then stirred for 30 minutes. The combined solution was mixed with the lithium naphthalenide solution, then stirred for another 2 hours. The resultant solution was centrifuged for 20 minutes at 10,000 rpm and washed with THF twice (10,000 rpm, 20 minutes). The final product was completely dried under vacuum overnight.
Characterization and Instrumentation.
High resolution TEM images were obtained with JEOL 2100-F Field-Emission Analytical Transmission Electron operated at 200 kV and with Philips CM300FEG/UT at 300 kV. XRD patterns were obtained with a Bruker AXS D8 Discover GADDS X-Ray Diffractometer, using Co Kα radiation (λ=0.179 nm). Hydrogen absorption/desorption kinetic measurements were conducted using a HyEnergy Sieverts PCT Pro-2000 at 15 bar/0 bar of hydrogen at different temperatures. The PCT measurement was performed on the sample after running one absorption/desorption cycle. XANES measurements were performed on Beamline 8.0.1.3, 6.3.1.2, and 4.0.3 at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. The energy resolution was set to 0.1 eV and the experimental chamber had a base pressure of 1×10−8 torr. A reference sample was measured before and after all XANES measurements for energy calibration. The XANES spectra were recorded using total electron yield and total fluorescence yield detection modes. For in-situ XANES measurement, the cell was purged with nitrogen gas 12 hours prior to characterization. The temperature increased afterwards, simultaneously replacing nitrogen with hydrogen gas in the cell at a pressure of 1 bar; the TEY and TFY scans were performed successively until the temperature was equilibrated at 300° C. The hydrogen pressure was deliberately set to 1 bar-comparatively very low for conventional metal hydride studies, for the purpose of monitoring gradual phase conversion upon temperature ramping.
Md Simulation.
A previously developed and tested embedded atom method (EAM) interatomic potential was used in this MD model. The initial substrates were pure Mg in the [0001] orientation. The crystal growth was conducted at an adatom energy of 0.02 eV, a vapor flux ratio of Ni:Mg=5%, and a growth rate of 0.5 nm/ns. The atomic structures obtained after 4.0 ns of deposition are shown in
Kinetic Energy Barrier Calculations.
Kinetic parameters characterizing the absorption and desorption processes (such as rate constants and energy barriers) are often obtained by fitting experimental data to simple kinetic models, such as e.g. the Johnson-Mehl-Avrami model. However, we have found that such models can fail to accurately fit the absorption/desorption data over the entire range of the reaction, making it difficult to extract reliable kinetic parameters from such an approach. For this reason, we instead obtain energy barriers by fitting the measured reaction rates (defined as the rate of change of absorbed weight of hydrogen) to an Arrhenius law at each stage of the reaction; i.e. we fit to an Arrhenius law for the rate r of the form r=f(x)*exp(−E(x)/kT), where the prefactor f(x) and energy barrier E(x) are assumed to be functions of the absorbed weight of hydrogen x. This approach allows one to extract effective energy barriers that are not biased by the underlying assumptions of any particular kinetic model, and moreover can provide evidence of changes in the reaction mechanism as absorption or desorption proceeds.
Rates were calculated by fitting the experimental data (shown in
Proposed Desorption Mechanisms for Undoped and Ni-Doped rGO-Mg
In this section we discuss the possible kinetic limitations during H2 desorption for undoped and Ni-doped rGO-Mg based on the rates in
For both undoped and Ni-doped rGO-Mg, multiple distinct dehydriding mechanisms can be identified from the kinetic analysis in
Following desorption from the near-surface region, there are a wide range of compositions that follow a characteristic nucleation-growth profile, with a barrier of 100 kJ/mol or higher for both the doped and undoped cases. It is reasonable to assume that this range is associated with the formation of crystalline Mg. The growth limitation in this range is likely related to the desorption of H2 from Mg (˜87-116 kJ/mol)9, 12-16, in agreement with our kinetics data in
A final benefit of Ni dopant is evident at higher temperatures in the final stages of dehydriding. Compared with the undoped sample, Ni-doped rGO-Mg exhibits a higher hydrogen release-to-uptake ratio (i.e., reversible hydrogen extraction) as the temperature increases. This is reflected in the increased effective energy barrier as dehydriding proceeds in the Ni-doped sample (
The invention consists of a composite of magnesium nanoparticles containing a metal catalyst all within a gas-selective polymer, which renders the nanomaterial air stable. Magnesium is one of the most promising inorganic materials for hydrogen storage. Magnesium hydride (MgH2) has a high hydrogen capacity of 7.6 weight %. The theoretical volumetric capacity of these composites is 55 g/L. This value is 180% greater than traditional compressed hydrogen gas cylinders (10,000 psi, 30 g/L). However, serious obstacles remain to the implementation of magnesium hydride for practical use. High bond formation enthalpy, slow hydrogen uptake and release kinetics, and high release temperatures renders magnesium hydride impractical for hydrogen storage. The Department of Energy has set ultimate temperature targets of 20 oc for absorption and 90 oc for desorption of hydrogen. In the present invention, we develop the synthetic methodology for metallic magnesium nanocomposites containing metal catalyst. Nanoscale metallic magnesium has a high surface area, short diffusion lengths for hydrogen, and reduced enthalpic barriers toward hydrogen molecules. By incorporating select metal catalyst dopants (for example titanium, palladium, etc.), hydriding may be catalyzed by the decrease in activation energy of H2 gas dissociation into hydrogen atoms on the metal surface. Additionally, other metal catalyst dopants (for example nickel, cobalt, copper, iron, etc.) may increase the kinetics of dehydrogenation due to an increase in the number of grain boundaries at the interface between metal hydride and the dopant metal, or strain induced within the metal hydride. We have currently doped our magnesium-polymer composites with titanium and nickel, achieving fast hydrogen absorption at room temperature. This is a dramatic improvement over other magnesium based systems which require temperatures in excess of 200 C. In addition, through inclusion of metal dopants we have reduced the time required for hydrogen desorption at 300 C.
This application claims priority to U.S. Provisional Application Ser. No. 62/445,610 filed Jan. 12, 2017, which application is incorporated herein by reference as if fully set forth in their entirety.
The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 between the U.S. Department of Energy and the Regents of the University of California for the management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention.
Number | Date | Country | |
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62445610 | Jan 2017 | US |