WO2011026214A1 - Stabilisation cinétique d'hydrure de magnésium - Google Patents

Stabilisation cinétique d'hydrure de magnésium Download PDF

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WO2011026214A1
WO2011026214A1 PCT/CA2010/000869 CA2010000869W WO2011026214A1 WO 2011026214 A1 WO2011026214 A1 WO 2011026214A1 CA 2010000869 W CA2010000869 W CA 2010000869W WO 2011026214 A1 WO2011026214 A1 WO 2011026214A1
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hydrogen
desorption
magnesium
absorption
catalyst
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PCT/CA2010/000869
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David Mitlin
Beniamin Zahiri
Mohsen Danaie
Xuehai Tan
Erik Luber
Christopher Harrower
Babak Shalchi Amirkhiz
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The Governors Of The University Of Alberta
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Publication of WO2011026214A1 publication Critical patent/WO2011026214A1/fr

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/04—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of alkali metals, alkaline earth metals or magnesium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
    • B01J20/0211—Compounds of Ti, Zr, Hf
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
    • B01J20/0214—Compounds of V, Nb, Ta
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
    • B01J20/0225—Compounds of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt
    • B01J20/0229—Compounds of Fe
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/30—Processes for preparing, regenerating, or reactivating
    • B01J20/32—Impregnating or coating ; Solid sorbent compositions obtained from processes involving impregnating or coating
    • B01J20/3202—Impregnating or coating ; Solid sorbent compositions obtained from processes involving impregnating or coating characterised by the carrier, support or substrate used for impregnation or coating
    • B01J20/3204—Inorganic carriers, supports or substrates

Definitions

  • Magnesium-based thin films and nanostaictures are a subject of extensive research as they are becoming increasingly more utilized for optical hydrogen sensing, switchable mirrors and solar absorbers, and as model alloys for designing and
  • the interdiffusion would presumably result in the formation of a discontinuous layer of binary intermetallics, the oxidation of the underlying active metal, and a subsequent loss of hydrogen dissociation catalytic activity.
  • Subsequent studies utilized Ti or Fe underlayers to reduce the interdiffusion of the palladium with the base material and the consequent formation of intermetallics.
  • the base materials tested include pure Mg, a range of Mg-Ni alloys, Y and Mg-Al alloys.
  • MgAl alloys have attracted interest for their favorable hydrogen storage properties for more than 20 yrs. This relatively simple system, available commercially in ingot form, is attractive due to a combination of the relative low material cost and the environmentally benign nature of the alloy. In general, most studies found MgAl alloys promising however, the kinetics was still inadequate for the rapid low temperature desorption required of a commercially viable hydrogen storage material for automotive and portable hydrogen applications. Low temperature hydrogen absorption in MgAl thin films has been achieved with a single layer Pd, a bilayer Ti Pd, and Fe(Ti)/Pd catalysts. However, appreciable hydrogen desorption is only possible at temperatures too high for practical applications.
  • Binary Mg-Fe and Mg-Ti alloys are a subject extensive research since they possess significantly accelerated kinetics relative to other Mg-based systems. Both systems show good gravimetric and volumetric hydrogen densities that vary with the alloy content but can be equivalent to or even higher than that of pure Mg.
  • Mg-Fe Upon hydriding Mg-Fe system forms a combination of Mg 2 FeH 6 and MgH 2 , the ratio of the two phases depending on the composition. The pure Fe phase does not form a hydride itself. Mg 2 FeH 6 begins to desorb at an equivalent temperature as MgH 2 , about 300°C, and has a similar heat of formation (actual reported values vary). In the Mg-Ti alloys the hydrided staicture is poorly understood. It appears to be more complex than simply a mixture of the equilibrium MgH 2 and TiH 2 phases. This is supported by the known stability of binary TiH 2 , which has a heat of formation of -136 kJ/mol and therefore should not desorb at 300°C.
  • Mg-Fe and Mg-V - systems relative to other Mg-based alloys. Both Mg-Fe and Mg-V show good gravimetric and volumetric hydrogen densities that vary with the alloy content. At equilibrium, neither Fe nor V has appreciable solubility in Mg, nor do they form any intermediate phases .
  • Mg-Fe system forms a combination of Mg 2 FeH 6 and MgH 2 , the ratio of the two phases depending on the composition and synthesis method. The pure Fe phase does not form a hydride itself.
  • Mg 2 Fe3 ⁇ 4 has a similar heat of formation as MgH 2 (reports vary from 70 - 80 kj/mol ). Due to the need for Fe diffusion, the sorption cycling kinetics of Mg 2 FeH 6 are relatively slow. Even under rough vacuum neither Mg 2 FeH 6 nor MgH 2 normally show appreciable desorption below 300°C.
  • Mg-V powder composites display some of the fastest hydrogen sorption kinetics of any magnesium-based system.
  • the heat of formation for the most commonly reported form of vanadium hydride VH 0 5 is -35 to -42 kj/mol H.
  • VH 0 8i phase is -35 to -42 kj/mol H.
  • PCT plateau pressure - composition - temperature
  • magnesium is alloyed with another metal or metals for catalyzing the absorption and desorption of hydrogen by the magnesium.
  • a first embodiment comprises an alloy of magnesium, iron and titanium in which the iron and titanium forms a highly disperse amorphous or nanocrystalline phase.
  • a second embodiment comprises an alloy of magnesium, iron and vanadium in which the iron and vanadium forms a highly disperse amorphous or nanocrystalline phase, and the magnesium may comprise at least 50% of the alloy by atomic percentage.
  • a palladium-tantalum bilayer catalyst may be used to coat a hydrogen- absorbing metal or alloy to improve the rate of absorption or desorption.
  • a palladium-tantalum bilayer catalyst may be used to coat an alloy of the first or second embodiments.
  • Magnesium offers a good material to store hydrogen in a metal hydride form, but it does not take up and release hydrogen well. In order to remedy this deficiency it is desirable to alloy the magnesium with another metal or metals which better interacts with hydrogen, for example with a greater tendency to split hydrogen molecules.
  • iron and titanium are used together for this purpose, and in a second embodiment iron and vanadium are used together for this purpose. Iron, titanium and vanadium each interact easily with hydrogen. Titanium has a high affinity for hydrogen, so that if it is alloyed alone with magnesium then it will tend to absorb the magnesium itself and not pass it on to the magnesium. Iron has a low affinity for hydrogen, so that it will not absorb enough hydrogen to pass on to the magnesium.
  • both iron and magnesium are alloyed with the magnesium and the combination of iron and titanium has an intermediate strength of interaction with hydrogen which makes magnesium-iron-titanium alloy more effective for uptake and release of hydrogen than magnesium-iron or magnesium-titanium alloy.
  • Vanadium has relatively good performance compared when alloyed alone with magnesium as compared to for example iron alloyed alone with magnesium, but is expensive.
  • both iron and vanadium are alloyed with the magnesium. We have found the combination of iron and vanadium alloyed with magnesium to have performance comparable to and in at least some ways superior to vanadium alloyed alone with magnesium, while using less vanadium.
  • the staicture of the alloy is such that a nanocrystalline or amorphous mixture of a catalyst for the kinetic absorption and desorption of hydrogen, such as iron and titanium or iron and vanadium, is dispersed throughout the magnesium at a fine scale, ie nanoscale.
  • Nanoscale dispersion means that the typical size of features is on a scale of less than 100 nanometres. That is, at least one dimension of the dispersed particles is less than 100 nanometers. This fine dispersion improves the uptake and release of hydrogen, as it improves the typical proximity of the magnesium to the catalyst for the kinetic absorption and desorption of hydrogen.
  • the catalyst for the kinetic absorption and desorption of hydrogen may form a separate phase which dissociates hydrogen and transports it to the hydrogen-storing magnesium phase.
  • the magnesium itself may be in a hexagonal close packed crystal staicture with relatively few or no non- magnesium atoms within the crystal staicture.
  • a thin film magnesium-iron-titanium alloy is formed by co-deposition with a nanoscale dispersion of the catalyst for the kinetic absorption and desorption of hydrogen, not a lumpy (microscale) dispersion.
  • an additional catalyst is used on the surface, preferably a palladium bilayer catalyst applied to the surface of the alloy.
  • a thin film may have a thickness in the range of 10 nm to 10 microns.
  • the catalyst for the kinetic absorption and desorption of hydrogen comprises iron and titanium
  • iron and titanium are present in approximately equal quantities in terms of atomic percentage (plus or minus 5% of the FeTi total). This maximizes the proportion of iron and titanium that is present in the form of TiFe phase. However, some TiFe phase may be present if the atomic percent of iron is between 15 and 67 of the FeTi total. Thus an atomic percent of iron in the iron-titanium component of the alloy of between 15 and 67 may be used although an atomic percent of close to 50 is preferable.
  • the Mg-Fe-Ti forming the alloy may be co-deposited for example on a substrate using any suitable method including physical or chemical vapour deposition, sputtering, evaporation or electrochemical methods.
  • co-deposition the Mg, Fe, Ti are combined as fluxes during the deposition process.
  • Various techniques of co-deposition of metal fluxes are known in the art and may be used to yield a dispersion of Fe-Ti in Mg.
  • the catalyst for the kinetic absorption and desorption of hydrogen comprises iron and vanadium
  • the atomic percent of magnesium in the magnesium-iron-vanadium alloy is greater than 50 in order to have a high capacity of hydrogen storage as the hydrogen is stored in the magnesium.
  • the good results obtained may be due to a nanocrystalline CsCl-type Fe-V phase dispersed through the magnesium, or due to an amorphous Fe-V phase dispersed through the magnesium. If a nanocrystalline CsCl-type phase is responsible, it is likely that a sigma phase would also be effective as the atoms would not have a greatly different an environment in either phase.
  • the ratio of iron and vanadium should be suitable to form a phase that is effective to return the desired results.
  • a range of ratios of approximately 3 : 1 Fe/V through 1 :9 Fe/V in atomic percent may be suitable. These numbers are obtained from a Fe-V phase diagram.
  • a similar range of ratios may form a CsCl-type phase, or published documents such as for example
  • Mg-Fe-V at 200 degrees Celsius but temperatures approximately in the range of 100 degrees to 350 degrees Celsius would also be effective.
  • the Mg-Fe-V forming the alloy may be co-deposited for example on a substrate using any suitable method including physical or chemical vapour deposition, sputtering, evaporation or electrochemical methods.
  • co-deposition the Mg, Fe, V are combined as fluxes during the deposition process.
  • Various techniques of co-deposition of metal fluxes are known in the art and may be used to yield a dispersion of Fe-V in Mg.
  • a palladium-tantalum bilayer catalyst is used to enhance absorption and desorption of hydrogen to and from an underlying metal or alloy.
  • a nanoscale layer of palladium at the surface catalyzes the interaction with hydrogen, and an underlying nanoscale layer of tantalum protects the palladium and the underlying metal or alloy from interacting with each other. It is hypothesized that the tantalum layer may also promote better catalytic properties.
  • the nanoscale layers may be made using any suitable method including physical or chemical vapour deposition or electrochemical methods. In particular sputtering, evaporation or electroplating may be used.
  • the palladium and tantalum layers may be present on one or both sides of a thin film.
  • Fig. 1 is a cross sectional side view of a magnesium alloy thin film having a bilayer catalyst on the upper surface of the thin film;
  • Fig. 2A is a graph of absorption behaviour of Mg- 15at.%Fe- 15at.%Ti alloy at 200 °C over cycles 1 - 107;
  • Fig. 2B is a graph of desorption behaviour of Mg- 15at.%Fe- 15at.%Ti alloy at 200 °C over cycles 1 - 107;
  • Fig. 3 is a graph of the indexed XRD pattern of the post-cycling, steady- state sorbed microstaicture of the Mg-10at.%Fe-10Ti alloy;
  • Fig. 4A is a graph of the 6 th cycle absorption behaviour of thin films for three different values of Fe and Ti content
  • Fig. 4B is a graph of the 6 1 cycle desorption behaviour of thin films for three different values of Fe and Ti content
  • Fig. 5A is a graph of the time to absorption as a function of cycle number for three different values of Fe and Ti content
  • Fig. 5B is a graph of the time to desorption as a function of cycle number for three different values of Fe and Ti content
  • Fig. 6A is a graph of pressure as a function of hydrogen content for absorption by Mg-10at.%Fe-10Ti at three different temperatures;
  • Fig. 6B is a graph of absorption plateau pressure as a function of temperature for Mg-10at.%Fe-10Ti;
  • Fig. 6C is a graph of pressure as a function of hydrogen content for desorption by Mg-10at.%Fe-10Ti at three different temperatures;
  • Fig. 6D is a graph of desorption plateau pressure as a function of temperature for Mg-10at.%Fe-10Ti;
  • Fig. 7A is an SEM micrograph of hydrogen absorbed Mg-13at.%Fe-7V thin film flakes after 105 absorption/desorption cycles.
  • Fig. 7B is a cross sectional SEM micrograph of a sorbed flake surface revealing the Ta/Pb bilayer catalyst that has peeled off from both sides of the intact film during cycling;
  • Fig. 8A is a graph of absorption curves for Mg-20at.%Fe at 200°C, over cycles 1-4;
  • Fig. 8B is a graph of desorption curves for Mg-20at.%Fe at 200°C, over cycles 1-4;
  • Fig. 9A is a graph of absorption curves for Mg-20at.%V at 200°C, over cycles 1-107;
  • Fig. 9B is a graph of desorption curves for Mg-20at.%V at 200°C, over cycles 1-107;
  • Fig. 1 OA is a graph of absorption curves for Mg-10at.%Fe-10at.%V at
  • Fig. 10B is a graph of desorption curves for Mg-10at.%Fe-10at.%V at
  • Fig. 1 1 A is a graph of absorption curves for three Mg-Fe-V ternary alloys and for Mg-20at.%V, over cycle 80;
  • Fig. 1 IB is a graph of desorption curves for three Mg-Fe-V ternary alloys and for Mg-20at.%V, over cycle 80;
  • Fig. 12A is a graph comparing the time to absorb 80 weight % of the maximum measured capacity for several different alloys, as a function of sorption cycle number;
  • Fig. 12B is a graph comparing the time to desorb 80 weight % of the maximum measured capacity for several different alloys, as a function of sorption cycle number;
  • Fig. 13 A is a graph showing pressure-composition isotherm absorption data for Mg-15at.%Fe-15V at 200°C, 230°C and 260°C;
  • Fig. 13B is a graph showing pressure-composition isotherm desorption data for Mg-15at.%Fe-15V at 200°C, 230°C and 260°C;
  • Fig. 14 is a graph showing the indexed X-Ray diffraction pattern of Mg-
  • Fig. 15A is an SEM micrograph of a cross section of a Pd/Nb catalyzed film flake after removal from the wafer and testing;
  • Fig. 15B is a plan view SEM micrograph of the top surface of the flake shown in Fig. 7A;
  • Fig. 16A is a graph of the cycled kinetics of magnesium with Pd catalyst layers;
  • Fig. 16B is a graph of the XRD pattern of the magnesium with Pd catalyst layers whose kinetics are shown in Fig. 8A, after the last sorption cycle (partial desorption);
  • Fig. 17A is a graph of the cycled kinetics of magnesium with Pd/Fe bi- layer catalysts
  • Fig. 17B is a graph of the XRD pattern of the magnesium with Pd/Fe bi- layer catalysts whose kinetics are shown in Fig. 17A, after the last cycle (absorption);
  • Fig. 18A is a graph of the cycled kinetics of magnesium with Pd/Ta bi- layer catalysts
  • Fig. 18B is a graph of the first and second desorptions of Fig. 18A;
  • Fig. 18C is a graph of the XRD pattern of the magnesium with Pd/Ta bilayer catalysts of Fig. 18A, after the last sorption cycle (absorption);
  • Fig. 19A is a graph of the cycled kinetics of magnesium with Pd/Nb bilayer catalysts
  • Fig. 19B is a graph of the first and second desorptions of Fig. 19A;
  • Fig. 19C is a graph of the XRD pattern of the magnesium with Pd/Nb bilayer catalysts of Fig. 19A, after the last sorption cycle (absorption);
  • Fig. 20A is a graph of the cycled kinetics of magnesium with Pd/Ti bilayer catalysts
  • Fig. 20B is a graph of the XRD pattern of the magnesium with Pd/Ti bilayer catalysts of Fig. 20A after the last sorption cycle (desorption);
  • Fig. 21 A is a graph of the time to absorb 5wt.% hydrogen (4wt.% for
  • Fig. 2 IB is a graph of the time to desorb 5wt.% hydrogen (4wt.% for
  • Fig. 22 is a series of graphs showing the neutron reflectivity curves of a 27 nm thick Mg 0.7 A1 0.3 film prepared on a Si(100) wafer with 10 nm Ta buffer layer and capped with a (5 nmTa/5 nm Pd) bilayer: (a) before hydrogen absorption, (b) after hydrogen absorption, measured at 25 °C, and (c) after annealing of 1 h at 100 °C, where open circles represent experimental data, the solid lines are fits, and the insets show the corresponding SLD profile;
  • Fig. 23 is a graph showing the desorption characteristics of a Mg 0 . 7 Al 0 . 3 H v film capped with a 10 nm Pd single catalyst layer (open circles), and a (5 nm Ta/5 nm Pd) catalyst bilayer (solid dots); and
  • Fig. 24 is a series of graphs showing XRD patterns of a 27 nm thick
  • a thin film 10 is shown (not to scale) having magnesium 12 with catalyst for the kinetic absorption and desorption of hydrogen 14, such as iron and titanium or iron and vanadium, dispersed throughout the magnesium.
  • hydrogen 14 such as iron and titanium or iron and vanadium
  • the palladium and tantalum layer can be on any suitable surface or portion thereof of the thin film.
  • the geometry of the samples was a 1.5 ⁇ Mg-Fe-Ti films with a 7.5nm
  • the films had compositions Mg-10at.%Fe-10Ti, Mg-15at.%Fe-15Ti and Mg-20at.%Fe-20Ti.
  • the Mg-Fe-Ti films were co-sputtered either onto a Si (100) substrate covered by native oxide layer, or onto same wafer but coated with a hardened (so as not to outgas in the chamber) photoresist. Depositions of the catalysts and of the bulk Mg were performed sequentially without any interaiption. We used Ar gas with a purity of 99.999% at a sputtering pressure of 5 ⁇ 10 ⁇ 3 mbar, with a maximum base pressure of
  • Deposition was performed using a DC-magnetron co-sputtering system (AJA InternationalTM 1 ). The substrate temperature was maintained near ambient.
  • Deposition was done in a sputter-up configuration with continuous substrate rotation. Film thickness and deposition rates were obtained through the use of crystal deposition rate monitor held at the substrate plane. A separate series of experiments involving ex- situ film thickness measurements versus deposition parameters were used to cross check the thickness/rate accuracies. The deposition rates were the following: Mg 3; Pd 1.7; Ta 0.3 A/sec; Fe and Ti varied to adjust for stoichiometry.
  • Figures 2A and 2B shows the absorption and desorption behavior for the
  • the initial activation period may be due to a variety of microstaictural factors.
  • the as-synthesized films were composed of supersaturated solid solutions of Fe and Ti in Mg, with the Mg having a strong [0001] fibre texture.
  • the film will decompose into an equilibrium two-phase mixture of magnesium (a-MgH 2 in sorbed state) and FeTi.
  • a-MgH 2 in sorbed state At 200°C neither the Fe nor the Ti have any appreciable solubility in magnesium.
  • microstaicture may not be rapidly achievable from a solid solution. Most likely it rather evolves during several initial cycles.
  • One hypothesis is that until the minority FeTi phase fully precipitates the kinetics remain sluggish.
  • the activation period may be attributed to the interdiffusion of the two elements to make a catalytic Ta-Pd alloy, or to the ultimate formation of a tantalum hydride phase.
  • Fig. 3 shows the indexed XRD pattern of the post-cycling, steady-state sorbed microstaicture of the Mg-10at.%Fe-10Ti alloy.
  • the broad peak centered at 2 ⁇ ⁇ 18.5° is due to the quartz mounting slide used to support the powders.
  • the most prominent peaks may be unambiguously indexed to belong to a-MgH 2 phase, with no detectable variation of the lattice parameter from the literature-reported values.
  • the Mg 6 Pd phase is cubic with the space group F-43m (216) and a lattice parameter of 20.108A. Three of the most intense Mg 6 Pd peaks, (224), (066) and (446) are labeled in the figure.
  • Mg 6 Pd forms despite the presence of a Ta underlayer is an interesting result.
  • thermal effects drive the interdiffusion of Mg and Pd, and the subsequent formation of Mg 6 Pd and MgO.
  • similar interediffusion may occur.
  • Pd and Ta have appreciable mutual solubility at 200°C ( ⁇ 9at.%Pd in Ta, and ⁇ 15at.%Ta in Pd) the formation of Mg 6 Pd after multiple cycles is feasible. It is not possible to conclusively identify or negate the presence of ⁇ -TaHo s since its XRD peaks overlap those of Mg 6 Pd.
  • Fig. 4 compares the steady-state (6th cycle) absorption (Fig. 4A) and the desorption (Fig. 4B) behavior of the films as a function of Fe/Ti content.
  • the alloys display quite similar kinetics.
  • a comparison of the absorption and desorption curves implies analogous microstaictures and sorption enhancement mechanisms for the three alloys.
  • the three alloys possess different hydrogen capacities. Assuming that a- MgH 2 is the only hydrogen storing phase present in the microstaicture and neglecting the catalyst layers, the theoretical hydrogen capacities of the Mg-10at.%Fe-10Ti, Mg- 15at.%Fe-15Ti, Mg-20at.%Fe-20Ti alloys are 5.1, 4.1 and 3.2 wt.%. Comparing these values to Fig. 5 and allowing for some (less than 10%) capacity reduction due to the presence of the catalysts layers we can conclude that the system is quite close to being fully sorbed.
  • Fig. 5 compares the time to absorption (5A) and time to desorption (5B) as a function of cycle number and alloy content. There is a noticeable induction period during the first several cycles of absorption. Interestingly at steady-state the higher FeTi content alloy (Mg-20at.%Fe-20Ti) actually absorbs hydrogen at a slower rate than the lower content alloys. The absorption times observed for Mg-15at.%Fe-15Ti alloys are the fastest ever reported for a relatively thick (1.5 micrometer) Mg-based film. Fig. 5B indicates that the optimum desorption performance, both in terms of the rates and in terms of the stabilities, is achieved in the Mg-15at.%Fe-15Ti alloy. The lower Fe/Ti content film has slower kinetics, while the higher alloy content film begins to display some kinetic degradation after about 20 cycles.
  • the heats of intermetallic formation are -31 for FeTi and -28 kJ/mol for Fe 2 Ti .
  • the main hydrogen-storing phase is a-MgH2
  • no net reduction of its heat of formation is possible by the formation of TiFe or TiFe 2 upon desorption.
  • ternary hydride Mg 2 FeH 6 and no evidence that the a-MgH 2 possesses a fundamentally different staicture (i.e. destablized due to alloying)
  • we have to conclude that rapid sorption behavior of the films is due to better kinetics. This conclusion, at least for the Mg-10at.%Fe-10Ti alloy, is supported by the pressure - composition isotherm absorption and desorption data shown in Fig. 6.
  • Fig. 6A shows pressure-composition isotherm absorption data for the Mg-10at.%Fe-10Ti alloy, with one curve at each of 200°C, 230°C and 260°C.
  • Fig. 6B graphs the plateau pressures for these curves v. the temperatures.
  • Figs. 6C and 6D are analogous to Figs. 6A and 6B respectively but show data for desorption rather than absorption.
  • the calculated enthalpies agree with 72 - 79 kJ/mol H 2 values commonly reported in literature for the Mg to a-MgH2 transformation.
  • the x-ray spectaim also shows a broad hump centered near (01 1) reflection of CsCl-type Fe-V phase.
  • a densely distributed nanoscale Fe-V acts both as a potent hydrogen dissociation catalyst and a heterogeneous nucleation site.
  • bi-metallic Fe-V catalysts substantially improve hydrogenation kinetics of magnesium, in some cases even above the
  • Thin film alloys are useful for understanding and improving bulk hydrogen storage materials, being amiable to fast and accurate synthesis via a variety of techniques, and suffering less from contamination issues compared to milled powders.
  • Fe-V catalytic additions may also have use for tremendously enhancing the performance of Mg-based thin film devices such as hydrogen sensors, switchable mirrors and solar absorbers.
  • the geometry of the samples was a 1.5 ⁇ Mg-Fe-V films with a 7.5nm
  • the films had compositions Mg-13at.%Fe-7V, Mg-10at.%Fe-10V, Mg-10at.%Fe-20V, Mg-15at.%Fe- 15V, Mg-20at.%V and Mg-20at.%Fe.
  • Magnesium and magnesium oxide are known to have poor activity towards hydrogen dissociation, which is the first step in the absorption process. Because of this, Pd catalyst films are normally deposited on the fresh
  • these catalysts consist of bi-layers, consisting of Pd on an oxide or a metallic support.
  • This intermediate layer serves the critical role of reducing the highly deleterious interdiffusion between the Pd and the underlying hydrogen storing material.
  • thermal effects drive the interdiffusion of Mg and Pd, and the subsequent formation of Mg 6 Pd and MgO.
  • tantalum is chosen as the intermediate layer because we have found it to be effective in preventing elevated temperature interdiffusion of Pd and the underlying Mg during hydrogen sorption, as detailed in the fourth study detailed below.
  • Film thickness and deposition rates were obtained through the use of crystal deposition rate monitor held at the substrate plane. A separate series of experiments involving ex-situ film thickness measurements versus deposition parameters were used to cross check the thickness/rate accuracies. The deposition rates were the following: Mg 3; Pd 1.7; Ta 0.3 A/sec; Fe and V varied to adjust for stoichiometry.
  • the photoresist was washed away using acetone allowing the films to be fully released from the Si wafer. Release from the substrate allowed the films to be treated as free flakes, in turn allowing for both accurate volumetric sorption testing and XRD analysis. During the release step the films developed cracks
  • the ultimate geometry of the films was that of powder-like flakes that were 1.5 micrometers thick with the bi-layer catalysts coating the top and the bottom flake surfaces.
  • Figs. 7A and 7B show SEM micrographs of the Mg-13at.%Fe-7V films after they have undergone 105 absorption/desorption cycles.
  • Figure 7A shows the film flakes in plan view while Figure 7B shows a film cross-section.
  • Figure 7A indicates that even after extensive cycling the film flakes largely remain intact, with some finer sub- micron powder particles being present as well.
  • Figure 7B highlights that during sorption cycling the films remain relatively intact through-thickness as well.
  • Figure 7B also indicates that much of the catalyst bi-layer on both sides of the film (arrowed) peels away during sorption cycling.
  • Figs. 8A, 8B, 9A, 9B, 10A, 10B, 1 1A and 1 IB show the absorption and desorption behavior for the Mg-Fe, Mg-V and Mg-Fe-V, tested at 200°C. The roughness of the desorption curves is due to instaimental noise.
  • Fig. 8A shows the hydrogen absorption and Fig. 8B the hydrogen desorption results for the Mg-20at.%Fe.
  • the alloy is able to sorb over 4wt.% hydrogen in less than 10 minutes.
  • the first desorption is however fairly slow, requiring over an hour to release 2.5wt.% hydrogen.
  • cycle 2 the capacity of the system degraded. Testing was concluded after hydrogenation cycle 4.
  • Figures 9A-1 IB indicate that in the binary Mg-V and in all ternary Mg-
  • Fig. 9A shows the absorption and Fig. 9B the desorption for Mg-20at.%V.
  • Fig. 10A shows the absorption and Fig. 10B the desorption for Mg-10at.%Fe-10at.%V.
  • the kinetics of Mg-Fe-V and Mg-V alloys may be qualitatively compared to what was measured in pure Mg films.
  • the pure Mg samples had identical preparation, identical dimensions and identical bi-layer Pd-Ta catalysts. They were tested at 250°C with absorption(desorption) pressures of 2.5 and 0.05 bar, respectively. The post- activation period times to absorb(desorb) were on the order of 40 minutes and 2 hours.
  • Figs. 12A and 12B compare the Mg-Fe-V and the Mg-V alloys, showing the time to absorb 80wt.% of the measured hydrogen absorption capacity (Fig. 12A) and the time to desorb 80wt.% of the measured hydrogen desorption capacity (Fig. 12B).
  • the absorption data highlights the activation period present for all alloys during the initial cycles. More importantly it highlights a key difference in the hydrogenation behavior between the binary Mg-V alloy and the ternary Mg-Fe-V: Starting at about 40 sorption cycles the kinetics of Mg-20at.%V begin to display some degradation. Conversely the absorption kinetics of the Mg-Fe-V alloys remain constant over the 100+ cycles of testing.
  • the desorption data in general shows more experimental scatter making a clear interpretation of the trends more difficult.
  • the kinetics are also markedly slower than for absorption, though still very fast relative to other Mg-based systems.
  • Mg-20at.%V there does seem to be a trend of prolonged (80+) cycling leading to some degradation of the desorption kinetics.
  • the Mg-10at.%Fe-20V may degrade analogously to the Mg- 20at.%V alloy. In the remaining Mg-Fe-V alloys the data points to either very minor kinetic degradation or to none at all.
  • Figs. 13 A and 13B shows the pressure - composition - isotherm plots for the Mg-15at.%V-15at.%Fe alloy for absorption (Fig. 4A) and desorption (Fig. 4B).
  • Fig. 4A absorption
  • Fig. 4B desorption
  • the calculated enthalpy for hydride formation is -71 kJ/mol H 2 while the enthalpy for hydride decomposition is 73 kJ/mol H 2 .
  • the calculated entropy is in the 130 J/Kmol H 2 range.
  • Figure 14 shows the indexed XRD pattern of the hydrogentated and desorbed Mg-10at.%Fe-10at%V films.
  • the top curve shows the XRD pattern after absorption and the bottom curve shows the XRD pattern after desorption.
  • the samples which were in loose flake form, were analyzed after undergoing over 100 sorption cycles. Hence the microstaicture may be considered as "steady-state".
  • the most prominent peaks are unambiguously indexed to belong to a-MgH 2 phase, with no detectable variation of the lattice parameter from the literature-reported values.
  • a simulation was ain to predict the peaks belonging to Mg 2 FeH 6 , with the results clearly showing it not being present.
  • the hump may be caused by an amorphous phase (being peaked at the average near-neighbor distance), a nanocrystalline phase or a mixture of both.
  • microstaicture It should also act as a heterogeneous nucleation site for both magnesium hydride and for metallic magnesium. That would explain the observed non-sigmoidal shape of the absorption and desorption curves, since copious nucleation events would occur.
  • elemental Fe and V have a negative heat of mixing and consequently reduced diffusivities in Mg, a phase consisting of both elements should be more resistant to microstaictural coarsening relative to pure V or Fe phases. This may explain the unique prolonged cyclic stability of the ternary system.
  • NbH 0 5 and TiH 2 are formed during testing.
  • Basic thermodynamic analysis indicates that NbH 0 5 and TiH 2 should be stable both during absorption and during desorption. We believe that this is why Nb and Ti are the most effective intermediate layers: The elements form stable hydrides at the Mg surfaces preventing complete Pd-Mg interdiffusion and/or acting as hydrogen catalysts and pumps.
  • the samples consisted of 1.5 mm Mg films coated with bilayer 7.5 nmPd/7.5 nm Fe (or Ti or Nb or Ta) catalysts on both the top and the bottom Mg surfaces.
  • the transition metal served as an intermediate layer between the Mg and the Pd.
  • the films were sputtered onto a nominally room temperature 4 inch Si(100) substrate that was coated with a hardened (so as not to outgas in the chamber) photoresist. Inside the sputter system the thin films stack had following sequence: vacuum/7.5 nm Pd/7.5 nm transition metal/1.5 mm Mg/7.5 nm transition metal/7.5 nm Pd/photoresist/Si wafer.
  • the photoresist was washed away using acetone allowing the films to be fully released from the Si wafer. Release from the substrate allowed the films to be treated as free powders, in turn allowing for both accurate volumetric sorption testing and XRD analysis. During the release step the films developed cracks
  • Fig. 15A shows a SEM micrograph of a cross section of the Pd/Nb catalyzed Mg thin film flake after removal from the wafer and prior to testing.
  • the bi-layer catalyst although not discernable in the figure, coats the Mg film conformally, with the Nb being in contact with the Mg.
  • Fig. 15B shows a plan-view SEM micrograph of the top flake surface revealing the morphology of the Mg grains.
  • the Mg is microcrystalline with columnar grain morphology.
  • the substrate temperature was maintained near ambient.
  • Deposition was done in a sputter-up configuration with continuous substrate rotation. Film thickness and deposition rates were obtained through the use of crystal deposition rate monitor held at the substrate plane. A separate series of experiments involving ex-situ film thickness measurements versus deposition parameters were used to cross check the thickness/rate accuracies.
  • the deposition rates were the following: Mg 3; Pd 1.7; Ta 0.3; Ti 0.4; Nb 0.4; and Fe 0.7A/s.
  • Fig. 16A shows the absorption and desorption data of the Mg films with the baseline 15 nm Pd capping layers.
  • absorption was at 2.5 bar hydrogen and desorption was at 0.05 bar hydrogen.
  • the testing temperature was 250 °C.
  • the kinetics are very slow, agreeing with the commonly reported observation that pure magnesium requires temperatures in excess for 300 °C for appreciable sorption.
  • the absorption time for the first cycle was 2.5 h. While the magnesium would have taken on more hydrogen if held for longer times the point was to demonstrate the sluggishness of the baseline reaction.
  • the base material microstaicture is a mixture of magnesium that displays peaks of the highest intensity and some a-MgH 2 . No Pd peaks were detected. Instead clear and relatively intense Mg 6 Pd intermetallic peaks were present, indicating almost complete (within the detection limits of the XRD analysis) reaction of the Pd with the Mg at elevated temperatures.
  • Figs. 17A-20B show the sorption and the XRD data for the Pd/Fe, Pd/Ta,
  • FIGs. 21A and 2 IB provide a comparison of the absorption (Fig. 21 A) and desorption (Fig. 2 IB) times for each of these systems as a function of sorption cycle number.
  • Pd/Ti is marked as A
  • Pd/Ta is marked as B
  • Pd/Nb is marked as C
  • Pd/Fe is markedly different from the case when single-phase Pd was used.
  • Fig. 17A demonstrates that the initial absorption/de sorption behavior of Mg with the Pd/Fe bi-layer catalysts is markedly different from the case when single-phase Pd was used.
  • the Pd Fe sample absorbs over 4 wt.% hydrogen in 10 min.
  • the first desorption cycle is also quite encouraging: 12 niin to fully desorb.
  • the subsequent sorption cycle is much less impressive, with the required time to absorb 4 wt.% hydrogen being an hour.
  • Subsequent desorption is not possible, with the sample losing only about 0.5wt .% before the kinetics become very sluggish. For this reason there is only one data point for the Pd/Fe graph D in Fig. 2 IB, which essentially coincides with the corresponding data point for the Pd/Nb graph C and so is not visible in the figure.
  • phase diagrams for Mg-Fe, Mg-Pd and Fe-Pd Magnesium and Fe are virtually immiscible and do not form any intermediate phases. Magnesium and Pd form a series of intermetallics with the Mg 6 Pd being the stable phase on the Mg-rich side.
  • the Fe-Pd phase diagram consists of an a-Fe phase, with negligible solubility for Pd at 250 °C, in equilibrium with ordered Fe-Pd having the prototype AuCu staicture. At higher Pd compositions the ordered FePd 3 phase is formed (prototype AuCu 3 ).
  • Figs. 18 A - 18C show the cycling and the XRD data for the Pd/Ta bilayer samples.
  • Fig. 18A shows the cycling data in its entirety while Fig. 18B highlights the first and the second desorption cycle.
  • the initial absorption cycle is quite slow, taking 5.5 h to absorb 5 wt.% hydrogen. However the first desorption is extremely fast, where 5 wt.% is released in 6 min.
  • the second absorption cycle becomes faster (2 h) while the second desorption cycle becomes slower (also 2 h).
  • the XRD data, shown in Fig. 18C, from the sorbed specimen indicates that a-MgH 2 coexists with remaining Mg and with Mg 6 Pd.
  • Figs. 19A- 19C show the results for the Pd/Nb system, which behaves similarly to Pd/Ta.
  • Fig. 19A shows the cycling data and
  • Fig. 18B shows the first and second desopriton cycles from the cycling data, while
  • Fig. 18C shows the XRD data for the system.
  • the first absorption cycle is slow, taking over slightly over 2 h to reach 5 wt.% hydrogen content.
  • First desorption is very rapid, achieving fully metallic state in just 13 min.
  • the second desorption cycle is slower while the second absorption cycle becomes faster.
  • the XRD pattern shows the presence of a- MgH 2 , Mg, Mg 6 Pd and NbH 0 5 phases.
  • Niobium and Pd have nearly 20 at.% mutual solubility and thus allow for the formation of Mg 6 Pd.
  • Fig. 20A shows cycling data and Fig. 20B XRD data for the the Pd/Ti system.
  • the initial time to absorb 5 wt.% hydrogen is 14 h.
  • the time to desorb is even longer: 16 h.
  • the time to absorb is 4 h, while the time to desorb is 3 h.
  • the XRD pattern obtained after the last desorption cycle indicates the presence of Ti 2 Pd 3 intermetallics as well as TiH 2 (really a substoichiometric TiHi in addition to metallic Mg.
  • Fig. 21A compares the absorption behavior and Fig.
  • transition metal hydrides are sufficiently active towards hydrogen dissociation/reassociation and transport that the observed stable kinetic behavior is actually due the their presence on the Mg surface, rather than due to any remaining Pd.
  • Huot et al. for the case of MgH 2 - Nb powder composites. In both scenarios, however, the key is the hydride's thermodynamic stability.
  • Magnesium hydride (a-MgH2) with a heat of formation equal to -74 kJ/niol has a plateau pressure of 0.25 bar; an order of magnitude higher than the desorption pressure.
  • PdUo 6 has a heat of formation of -40 kJ/mol and a plateau pressure of 634 bar. This indicates that Pd will be in its metallic state throughout absorption and desorption.
  • FeH 0 5 has a positive heat of hydride formation of 10 kJ/mol and will remain metallic throughout the entire test as well. An ordered alloy of Fe and Pd should similarly not form hydrides at the test conditions.
  • Ta is the one outlier. Beta-TaH 0 5 has a heat of formation equal to -76 kJ/mol. Thermodynamically it should be a hydride during sorption and metallic during desorption. However in the case of Ta, the prolonged catalytic activity may be related to its elevated temperature stability. Metallic Ta may be utilized as a nano-scale barrier for hydrogen storing thin films, minimizing the interdiffusion of Mg and Al with the Pd catalyst. Alternatively the observed relatively stable cycling behavior may actually be due to the catalytic properties of Pd-Ta alloys that form during desorption. A theoretical study (Greely et al., "Alloy catalysts designed from first principles", Nature Materials 2004;3 :810) has predicted the Ta/Pd system to be very effective for hydrogen catalysis.
  • Pd/Ta, Pd/Nb and Pd/Ti catalysts remained active throughout multiple sorption cycles, reaching what appeared to be steady-state kinetics.
  • Mg catalyzed by Pd/Nb is able to sorb 5 wt.% hydrogen in approximately 40 min during cycles 5-8.
  • /Ta/Pd film staicture (a) before hydrogen absorption, (b) after hydrogen absorption, measured at 25 °C, and (c) fully desorbed, measured at 100 °C.
  • the chan ges in the film staicture can be best visualized by plotting the SLD profile, i.e., the SLD along the surface normal z of the film.
  • the SLD profiles corresponding to the fits are shown in Fig. 1 as insets. In all cases the model consisted of a Si substrate with a native Si0 2 layer, a Ta buffer layer, a MgAl layer, and a Ta/Pd bilayer.
  • the SLD of the MgAl film goes back up to 2.0 ⁇ 10 ⁇ 6 A ⁇ 2 [see part (c) of Fig. 22] proving that the hydrogen has been released.
  • the SLD of the desorbed film does not reach exactly the SLD value of the unsorbed film because the whole film staicture expands by about 15% due to the hydrogen absorption creating cracks and voids that result in a lower SLD of the layers.
  • the SLD of the Ta layer decreases during the desorption process further from 3.6 ⁇ 10 ⁇ 6 ⁇ ⁇ 2 to 3 ⁇ 10 ⁇ 6 ⁇ ⁇ 2 , which is an indication of a small amount of hydrogen being stored in the Ta layer after the annealing to 100 °C .
  • the SLD profiles shown in Fig. 22 prove that the Ta/Pd bilayer is still intact at the applied temperatures. In earlier experiments on MgAl films with single Pd catalyst layers we found that Pd diffuses into the MgAl layer.
  • Figure 23 shows the total hydrogen content y of the Mg 0.7 Al 0.3 H y film capped with a Ta/Pd bilayer (solid dots) as calculated from the SLD, plotted as a function of temperature.
  • Figure 24 shows the x-ray diffraction (XRD) results for an as-synthesized thin film (a), measured immediately after hydrogen absorption (b), stored at room temperature for 30 h after absorption (c), and annealed at 100 °C in argon d). It is the same film staicture as investigated with NR but it is not the identical film.
  • the XRD scan of the sample that was investigated with NR is displayed in part (e) of Fog. 24, measured after the annealing at 125 °C for 3 h. Because the films are strongly textured, not all possible reflections appear in the x-ray scan.
  • the as-synthesized microstaicture consists of a supersatured solid solution of Al in Mg.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Analytical Chemistry (AREA)
  • Organic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Hydrogen, Water And Hydrids (AREA)
  • Catalysts (AREA)

Abstract

L'invention porte sur : une matière d'absorption et désorption d'hydrogène formée par co-dépôt de magnésium avec un catalyseur pour l'absorption et la désorption cinétiques d'hydrogène; une matière d'absorption et désorption d'hydrogène constituée d'un alliage du magnésium avec un catalyseur pour l'absorption et la désorption cinétiques d'hydrogène, le catalyseur pour l'absorption et la désorption cinétiques d'hydrogène formant une phase amorphe ou nanocristalline dispersée dans le magnésium; une matière d'absorption et désorption d'hydrogène ayant une surface catalytique formée par un procédé comprenant les étapes consistant à déposer une couche de tantale sur la matière d'absorption et désorption d'hydrogène et déposer une couche de palladium sur la couche de tantale.
PCT/CA2010/000869 2009-09-01 2010-06-14 Stabilisation cinétique d'hydrure de magnésium WO2011026214A1 (fr)

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WO2017127800A1 (fr) * 2016-01-21 2017-07-27 Ih Ip Holdings Limited Procédés d'amélioration de rapport de charge de gaz d'hydrogène

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