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Development of Ti1.02Cr2-x-yFexMny (0.6≤x≤0.75, y=0.25, 0.3) alloys for high hydrogen pressure metal hydride system
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Development of Ti1.02Cr2-x-yFexMny (0.6≤x≤0.75, y¼0.25, 0.3) alloys for high hydrogen pressure metal hydride system Jigang Li a, Xiaojing Jiang a,b, Guoling Li c, Xingguo Li a,* a
Beijing National Laboratory for Molecular Science (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Beijing National Laboratory for Molecular Science (BNLMS), Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China c Beijing National Laboratory for Molecular Science (BNLMS), College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China
article info
abstract
Article history:
To save compressor investment and promote operation efficiency of hydrogen refueling
Received 17 December 2018
station, the hydrogen storage alloys for high-pressure hydrogen metal hydride tank is
Received in revised form
developed. Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys with main structure of C14
24 March 2019
type Laves phase and low dehydrogenation enthalpy were prepared by plasma arc melting
Accepted 31 March 2019
and heat treatment. Pressure-composition-temperature measurements show that
Available online 3 May 2019
hydrogen desorption plateau pressures increase with Cr substituted by Fe increasing. The maximum and reversible hydrogen storage capacities are more than 1.85 and 1.65 wt% at
Keywords:
201 K respectively. The hydrogen desorption plateau slopes are all less than 0.5. The
Intermetallic
symmetry weakening of 2a sites may deteriorate the plateau slop characteristic. Ti1.02-
Hydrogen storage
Cr0.95Fe0.75Mn0.3 and Ti1.02Cr1.0Fe0.75Mn0.25 alloys are suitable for high pressure hybrid tank
Tix(CrFeMn)2 alloy
which can supply the effective hydrogen (more than 70 MPa) about 40.0, 44.2, 46.9 kg/m3
Heat treatment
with 45, 70, 90 MPa compressor, respectively.
Hydrogen refueling station
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction 30% of the total world emission of greenhouse gas is attributed to transportation [1] and a third is attributed to heat generation in buildings and industry [2]. Hydrogen, as the best energy carrier, has the advantages of cleanness and renewability. Combining with the highly effective fuel cell technology, it offers a potential way for sustainable development, which is applied for stationary power, portable power, and
transportation [3e8]. In contrast with traditional oil fuel, hydrogen fuel cell vehicle (HFCV) has the advantages of no pollution: the product of hydrogen converting to energy is only water; and high energy conversion efficiency: the efficiency of the direct process of electron transfer from oxygen to hydrogen can reach 50e60%, which is not limited by the Carnot efficiency (the efficiency of the thermal process of petrol is about 19e22%) [9]. In comparison to pure battery electric vehicle, hydrogen fuel cell vehicle (HFCV) has the
* Corresponding author. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.ijhydene.2019.03.241 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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advantages of short refueling time and long driving range [10e13]. The hydrogen storage capacity of HFCV is critical to the driving range. And the hydrogen storage capacity of hydrogen refueling station (HRS) plays an important role on the full-utilization hydrogen cost (Station capacity: 240, 480, 900 kg/day, full-utilization hydrogen cost: 2.35, 1.66, 1.12 $/kg) [14]. The volumetric energy density of hydrogen storage tank is lower relative to traditional liquid fuels, which is the challenge for the application [14]. The high volumetric density of a hydrogen storage system can save more space and promote driving range. The higher the pressure, the greater the volumetric hydrogen storage density. To this end, the pure highpressurized gas tank and high-pressure hydrogen-metal hydride tank are developed to increase the volumetric density [3,15e17]. With the increase of the vehicle tank pressure from 35 MPa to 70 MPa, the HRS pressure for refueling must increase at the same time [18]. The full-utilization hydrogen cost in HRS is about 1.12, 1.32, 1.55 dollar/kg (the station capacity: 900 kg/day) at 233 K precooling (the tank of HFCV must stay at 233 K-358 K to prevent degradation of the tank liner [19]) of the delivered pressure 35, 50 and 70 MPa respectively [14]. The cost of compressor system may be up to 65% of total HRS investment and it is difficult to reduce below 50% [19e22]. With the delivered pressure increasing from 35 to 70 MPa, the cost of compressor and booster compressor increases by linear, and the effective utilization capacity of HRS influences the hydrogen refueling cost strongly [12]. The best refueling strategy for 70 MPa tank is by the low-middle-high hydrogen pressure vessels, which can decrease the refueling cost dramatically due to the reducing of the partial compress process [23]. In order to reduce the cost of refueling, it is important to increase the capacity and utilization of 90 MPa hydrogen source. To decrease the cost for hydrogen compression (Investment for compressor and reduce the open and failure rate of compressor) and operation of HRS, increasing the effective gravimetric or volumetric capacity of the storage tank is a good method. The high-pressure hydrogen-metal hydride vessel can be used as the hydrogen storage tank and the compressor due to its stationary compression. The stationary compressor which has the advantages of the high purity hydrogen released, no moving part, no noise, high pressure only during operation status
[24e27]. The previous reports of compressor are summarized as listed in Table 1. From Table 1, it is easy to get a conclusion that there are no suitable hydrogen storage alloys which can supply the hydrogen above 70 MPa (may reach up to 90 MPa) at moderate temperature (for example, 338 K) and low cost in HRS. The high-pressure hydrogen-metal hydride vessel requires hydrogen storage alloy with a lower dissociation enthalpy and especially a higher hydrogen desorption plateau pressure than the existing AB5 [31], AB [38], AB2 type [39e41] alloys. Kojima et al. developed the Ti1.1CrMn alloy which the hydrogen absorption and desorption plateau pressure was 11 MPa at 296 K [41]. The hydrogen absorption and desorption plateau pressures increase with the Fe substitution for partial Mn of TixCrMn alloys, which is found by Guo et al. [42]. A series of Cr-rich Tix(CrFeMn)2 alloys with high hydrogen desorption plateau pressures were developed by Chen et al. [43e46], which were higher than 45 MPa at 318 K. The activation behavior for alloy hydrogen absorption was improved by the additions of rare earth metals (La, Ce, Ho) to the Ti1.02Cr1.1Fe0.6Mn0.3 alloy but the hydrogen absorption and desorption plateau pressures decreased [47]. The ZreFeeV alloys were prepared by arc melting, and the hydrogen desorption plateau pressure of Zr1.04Fe1.9V0.1 can be up to 40 MPa at 353 K [48]. The compositions of the alloys influence the hydrogen desorption plateau pressures and capacities, the hydrogen storage properties of the Cr-rich Tix(CrFeMn)2 alloy were studied in this paper. We have investigated the series alloys of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys which might be suitable for HRS of high pressure hydrogen refueling.
Sample preparation and characterization methods The hydrogen storage properties of the alloys in accordance with Fig. 1 process were investigated. The alloys were prepared by Ar plasma arc melting method in a water-cooled copper crucible. The purities of the raw materials are 99.7 wt % for Ti, 99.9 wt% for Cr, 99.9 wt% for Fe and 99.8 wt% for Mn. The high-purity inert gas (Ar99.999%) was filled into the furnace chamber about 0.85e0.9 atm. Based on the stoichiometry ratio of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3)
Table 1 e Summarized data of the metal hydride compressor with different stage. Compressor type
Single Double
Three
Alloys
La0.4Ce0.4Ca0.2Ni5 Mm0.7Ca0.2La0.1Ni4.95Al0.05 LaNi5/La0.5Ce0.5Ni5 La0.9Ce0.1Ni5/La0.8Ce0.2Ni5 LaCe0.15Ni5/AB2-x La0.4Y0.6Ni4.8Mn0.1Al0.1/La0.4Y0.6Ni4.8Al0.2 Mm0.4La0.6Ca0.2Ni5/Ti1.1Cr1.5Mn0.4V0.1 Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2/Ti0.8Zr0.2Cr0.95Fe0.95V0.1 La0.35Ce0.45Ca0.2Ni4.95Al0.05/Ti0.8Zr0.2Cr0.95Fe0.95V0.1 Ti0.85Zr0.15Mn1.33V0.3/Ti0.8Zr0.2Mn1.2Cr0.6V0.2/Ti0.9Zr0.1Mn1.4Cr0.4V0.2
Initial stage
Last stage
Feed gas (MPa)
Outer pressure (MPa)/T (K)
Outer pressure (MPa)/T (K)
1.9 4 0.4 0.4 1 2 4 4 4 0.2
e e 4/423 4/353 6/393 8/373 10/372 30/423 38.5/423 10.8/383
12/423 [28] 45/443 [29] 7.5/423 [30,31] 7.5/353 [32] 20/393 [33] 38/448 [34] 45/372 [29] 70/423 [35] 74.5/423 [36] 18.3/405 [37]
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Fig. 1 e Flow chart of studying on hydrogen storage properties.
alloys, 12 wt% excess of Mn was compensated for the loss of evaporation due to the high vapor pressure of Mn. The ingots melting time was about 1.5 min, then turned over and remelted about 1.5 min. The ingot was melted overall 3 times to achieve a high homogeneity. The mass accuracy is about ±0.015 g between the total theoretical chemical stoichiometric mass (8 g) and terminal cast ingot. Finally, all the alloys undergone the heat treatment at 1273 K for 2 h. Phase structures of all the samples were determined by Xray powder diffraction equipment (XRD, PANalytical X-Pert3 Powder type, CuKa radiation, scanning rate, 1.3 /min). On the basis of XRD patterns, lattice parameters were calculated by Maud software [49,50]. Metallographic microstructures and particle morphologies were analyzed by scanning electron microscopy (SEM, Hitachi, S4800). The samples for metallographic microstructure were prepared by grinding with sand
paper and polishing with diamond abrasion paste. The element distribution on the diamond polished surface was measured by energy dispersive X-ray spectroscopy (EDS). In order to eliminate the oxides surface, all the annealed samples were polished by stainless grinding head, and then transferred into the glove box or hold in the vacuum dryer, which all kept low water and oxygen conditions [3]. In the glove box, the samples were crushed into small particles which were enclosed into thin-wall furnace tube with a loading mass of 1 g. Hydrogen absorption/desorption properties were measured by the pressure-composition-temperature (PeCeT) instrument which was made in Japan by Suzuki Shokang. First, the thin-wall furnace chamber with samples was vacuumed to below 2.0 101 Pa for 2 h at room temperature; Second, hydrogen with the pressure about 9.8 MPa was filled into the reaction chamber and kept 12e24 h approximately at 201 K; At last, the hydrogen was evacuated from the reaction chamber for 15e30 min. To make the alloy hydrogen absorbing fully activated, the above-mentioned processes need to be repeated for about 3e5 cycles. Then, the hydrogen storage properties were measured at the low temperature 201e253 K, which was limited by the maximum operating hydrogen pressure of the equipment (about 10 MPa, the pressure must be higher than the desorption plateau pressure) in our lab.
Results and discussion XRD analysis
Fig. 2 e XRD patterns of (a) Ti1.02Cr1.0Fe0.65Mn0.3 and (b) Ti1.02Cr1.0Fe0.7Mn0.3 alloys As cast.
XRD patterns of Ti1.02Cr1.0Fe0.65Mn0.3 (a) and Ti1.02Cr1.0Fe0.7Mn0.3 (b) alloys analyzed by Maud soft after plasma melting but without heat treatment (As cast) are shown in Fig. 2. The phase structure of Ti1.02Cr1.0Fe0.65Mn0.3 alloy (As cast) is determined as a single phase of C14-type Laves structure, and no secondary phase is detected from Fig. 2 (a). The phase structure of Ti1.02Cr1.0Fe0.7Mn0.3 alloy (As cast) is determined as vast majority phase of C14-type Laves structure, and secondary phase is observed which can be ascribed to CrMn [45] or TiMn [51,52] alloy phase in Fig. 2 (b). Fig. 3 and Fig. 4 show the XRD patterns of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys before
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Fig. 3 e XRD patterns of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys after heat treatment.
Fig. 4 e XRD patter of annealed Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys after PCT measurement.
and after hydrogen absorption and desorption cycles (at least ten cycles) respectively. We can see that the phase structures of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are determined as single phase of C14-type Laves structure, and the secondary phase is very few. It indicates that the partial substitution of Fe for Cr or Mn at B side does not change the phase structure. Lattice parameters of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys calculated by Rietveld method with the Maud software are listed in Table 2. The values of a, c, V of annealed Ti1.02Cr1.05Fe0.65Mn0.3 alloy increase from 0.48446 nm, 0.79423 nm, 0.16143 nm3 to 0.48461 nm, 0.79461 nm, 0.16161 nm3 respectively. The values of a, c, V of Ti1.02Cr1.0Fe0.7Mn0.3 alloy decrease from 0.48439 nm, 0.79387 nm, 0.16131 nm3 to 0.48430 nm,
0.79382 nm, 0.16124 nm3 respectively. As the Fe stoichiometry content increasing from 0.6 to 0.75, the value of a decreases from 0.48477 nm to 0.48403 nm, c decreases from 0.79491 nm to 0.79319 nm, and V reduces from 0.16178 to 0.16094 nm3, respectively. The unit cell volumes of Ti1.02Cr2-x-yFexMn0.3 (0.6 x 0.75) alloys shrink due to the smaller atomic radius of Fe relative to Cr [3,44]. Comparing Ti1.02Cr0.95Fe0.75Mn0.3 alloy with Ti1.02Cr1.0Fe0.75Mn0.25 alloy, the value of a almost has no change from 0.48403 nm to 0.48402 nm, c increase from 0.79319 nm to 0.79332 nm, and V expands very little from 0.16194 to 0.16096 nm3. After hydrogenation and dehydrogenation for ten cycles at least, the lattice parameters of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys all increase. It indicates that the hydrogenation and dehydrogenation processes result in the deformation of lattice cell [53].
Table 2 e Lattice parameters of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys calculated from XRD patterns. Alloy Ti1.02Cr1.1Fe0.6Mn0.3 Ti1.02Cr1.1Fe0.65Mn0.3 Ti1.02Cr1.05Fe0.67Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3 Ti1.02Cr0.95Fe0.75Mn0.3 Ti1.02Cr1.0Fe0.75Mn0.25 Ti1.02Cr1.05Fe0.65Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3
Status Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des As cast As cast
Lattice Parameters(nm) a c 0.48477 0.48488 0.48461 0.48482 0.48433 0.48483 0.48430 0.48446 0.48403 0.48439 0.48402 0.48435 0.48446 0.48439
0.79491 0.79508 0.79461 0.79470 0.79422 0.79468 0.79382 0.79405 0.79319 0.79366 0.79332 0.79373 0.79423 0.79387
a/c
Unit Cell Volume V/(nm3)
0.6098 0.6099 0.6099 0.6101 0.6098 0.6101 0.6101 0.6101 0.6102 0.6103 0.6101 0.6102 0.6100 0.6102
0.16178 0.16189 0.16161 0.16177 0.16134 0.16177 0.16124 0.16140 0.16094 0.16127 0.16096 0.16126 0.16143 0.16131
*Abs-Des represents the XRD determination after hydrogen absorption and desorption about ten cycles, As cast represents the XRD determination of the alloys which is without heat treatment and hydrogen absorption/desorption process.
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Fig. 5 e SEM of the alloys (a)-(f) Backscatter electron micrographs (a) Ti1.02Cr1.05Fe0.65Mn0.3 As cast, (b) Ti1.02Cr1.0Fe0.7Mn0.3, As cast; (c) Ti1.02Cr1.1Fe0.6Mn0.3, Annealed; (d) Ti1.02Cr1.05Fe0.65Mn0.3, Annealed; (e) Ti1.02Cr1.03Fe0.67Mn0.3, Annealed; (f) Ti1.02Cr1.0Fe0.7Mn0.3, Annealed; (g) Backscatter electron micrographs on the left and the EDS mapping on the right of annealed Ti1.02Cr1.0Fe0.7Mn0.3; (h) and (i) Particles secondary electron micrographs after ten cycles of hydrogen absorption and desorption cycles.
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SEM analysis Fig. 5(a)e(g) show the SEM micrographs of Ti1.02Cr1.7-xFexMn0.3 (0.6 x 0.7) alloys by backscatter electron, EDS mapping and particles micrographs of Ti1.02Cr1.0Fe0.7Mn0.3 alloy after hydrogenation and dehydrogenation cycles by secondary electron scanning. From Fig. 5(a) and (b), the secondary phase of the dark fields exists in the alloys before heat treatment (the secondary phase are indicated by black arrows). Coupled with XRD patterns, it should be noted that the secondary phase sometimes can be detected and sometimes can't be detected, which may be caused by unstable plasma arc of the manual operation. From Fig. 5(c)e(f), the metallographic microstructures of the annealed alloys are more homogeneous, which maintain the vast majority of C14 or single C14 phase corresponding to the XRD diagram [45]. From Fig. 5(g), the annealed Ti1.02Cr1.0Fe0.7Mn0.3 alloy displays homogeneous morphology structure from the EDS mapping. After PCT measurement (more than ten cycles of hydrogenation and dehydrogenaiton), the particle sizes in Fig. 5(h) are below 20 mm. From Fig. 5(i), the breaks (the partial locations are marked by white ring) appear on the particles surface, which is similar with previous studies of AB2-type alloy [48,51,54].
Hydrogen storage properties PCT measurement Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys were prepared by plasma arc melting and heat treatment at 1273 K for 2 h. The PCT properties of the series alloys were measured after activation by the above-mentioned method in section Sample preparation and characterization methods. PeC-T curves of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are shown in Fig. 6 and Fig. 7. From PeC-T curves, the hydrogen storage characteristics including hydrogen absorption plateau pressure Pa, hydrogen desorption plateau pressure Pd, maximum capacity HMax, reversible capacity HRe, plateau slope factor Sf and hysteresis factor Hf can be obtained. Sf and Hf can be calculated by equations (1) and (2) respectively at measuring temperatures [3]. △Ha, △Sa, and △Hd, △Sd represent the enthalpy and entropy of the hydrogenation and dehydrogenation process, respectively.
Sf ¼ dðlnPÞ=dðH wt%Þ
(1)
Hf ¼ lnðPa =Pd Þ
(2)
PCT of Ti1.02Cr1.7-xFexMn0.3 (x ¼ 0.65, 0.7) alloys (As cast). Fig. 6 show the PCT measurements of Ti1.02Cr1.7-xFexMn0.3 (x ¼ 0.65, 0.7) alloys (As cast). It should be noted that the hydrogen storage properties of the alloys are not so good, or even terrible. The capacity plateau broadenings of Ti1.02Cr1.7xFexMn0.3 (x ¼ 0.65, 0.7) alloys are all less than 0.9 wt%. The bad hydrogen storage characteristics may be caused by the existences of the secondary phase and elements heterogeneous distributions of main phase from the SEM micrographics.
PCT of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys. Heat treatment can improve the hydrogen storage properties of the alloys [45,55e57]. Fig. 7 displays the PCT measurements of the annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys. From the PeCeT curves in Fig. 7(a)e(e), hydrogen storage characteristics of annealed Ti1.02Cr1.7-xFexMn0.3 (0.65 x 0.75) alloys are obtained as listed in Table 3. From Table 3, with Fe stoichiometry content increasing from 0.6 to 0.75, the hydrogen desorption plateau pressures increase from 0.79 to 1.65 MPa at 201 K and 4.4e8.00 MPa at 233 K. Van't Hoff plots of ln P vs 1/T for dehydrogenation following the linear relation are displayed in Fig. 8(a). Good linear fits are obtained in all cases as shown in Fig. 8(a) and the values of R-square are more than 0.998 from Table 4. Through the linear Van't Hoff relation, the thermodynamics of Ti1.02Cr2x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are calculated, which are listed in Table 4. From the Table 4, the values of △Hd and △Sd for dehydrogenation of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are in the range from 18.5 to 20.5 kJ/mol H2 and 115.5 to 112.0 J mol1K1, respectively. The values of △Hd obtained from the PCT measurement at 201e253 K are a little higher than the △Hd values of TixCr2.0-yzFeyMnz alloys (1 x 1.1, 0.60 y 0.65, 0.2 z 0.4) in the range of 14e17 kJ/mol H2 at 243e263 K [43e45,47]. From the calculated values of △Hd and △Sd, hydrogen desorption plateau pressure at 338 K can be predicted, which are
Fig. 6 e PCT of Ti1.02Cr1.7-xFexMn0.3 (x ¼ 0.65, 0.7) alloys (As cast) (a) Ti1.02Cr1.05Fe0.65Mn0.3, (b) Ti1.02Cr1.0Fe0.7Mn0.3.
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Fig. 7 e PCT of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys (Annealed) (a) Ti1.02Cr1.1Fe0.6Mn0.3, (b) Ti1.02Cr1.05Fe0.65Mn0.3, (c) Ti1.02Cr1.03Fe0.67Mn0.3, (d) Ti1.02Cr1.0Fe0.7Mn0.3, (e) Ti1.02Cr0.95Fe0.75Mn0.3, (f) Ti1.02Cr1.0Fe0.75Mn0.25.
summarized in Table 4. With Fe stoichiometry content increasing from 0.6 to 0.75, hydrogen desorption plateau pressures at 338 K increase from 107 MPa to 165 MPa. Comparing with Chen et al.’ results of the Ti1.02Cr1.1Fe0.6Mn0.3 alloy (Hydrogen desorption plateau pressure is about 60e70 MPa at 338 K) [45], hydrogen desorption plateau pressure predicted by linear Van't Hoff equation at 338 K is too high and may deviate from the actual. We choose the moderate temperature of 338 K to predict, and consider it can save
energy cost for heating the tank to release the high-pressure hydrogen and pre-cooling to refuel. If the temperature is lower than 338 K, the hydrogen absorption of the alloys at 273 K may not be completed of the pressure 45 MPa in current station. The Van't Hoff differential equation can be expressed by equation (3).
dlnP DH ¼ dT RT2
(3)
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Table 3 e Hydrogen storage characteristics of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys. Alloy
Ti1.02Cr1.1Fe0.6Mn0.3 Ti1.02Cr1.05Fe0.65Mn0.3 Ti1.02Cr1.03Fe0.67Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3 Ti1.02Cr0.95Fe0.75Mn0.3 Ti1.02Cr1.0Fe0.75Mn0.25 Ti1.02Cr1.05Fe0.65Mnþ 0.3 Ti1.02Cr1.0Fe0.7Mnþ 0.3 a b
Hf
Sf
201 K
233 K
201 K
Pd (MPa) 233
201 K
HRe (wt%) 233 K
HMax (wt%) 201 K
1.19 1.26 1.26 1.40 1.28 1.28 e e
0.202 0.226 0.315a 0.233 0.348 0.253 e e
0.79 0.93 1.00 1.21 1.65 1.61 1.2 1.56
4.40 4.92 5.85b 6.42 7.70 8.00 e e
1.69 1.72 1.69 1.70 1.66 1.69 1.2 1.01
1.61 1.63 1.56a 1.58 1.59 1.61 e e
1.83 1.83 1.83 1.85 1.84 1.85 1.33 1.05
Represents that the value was measured at 229 K. Represents that the value was calculated from experimental data on the both sides closest the temperature 233 K,þRepresents that the alloy didn't undergo the heat treatment, - Represents that the value was not measured or was measured invalid.
The linear Van't Hoff equation can be expressed by equa tion (4), which is based on the assumption of that DH and DS are independent with temperature.
lnP ¼
DH DS þ RT R
(4)
Based on the above-calculated data, the linear Van't Hoff equation is not appropriate for the alloys to predict the hydrogen desorption plateau pressure. The dehydrogenation DH of Tix(CrFeMn)2 alloys can be expressed by equation (5) based on the metallurgical thermodynamics. Da2 3 Da1 2 T2 T21 þ T2 T31 DH ¼ DH0 þ Da0 ðT2 T1 Þ þ 2 3 1 J=ðmol H2 Þ Da3 T1 2 T1
(5)
Where DH is a constant, Da0 , Da1 , Da2 , Da3 are the coefficients for the Tx (x ¼ 1, 2, 3, 1) factors, respectively. In general, the DH can be considered as a constant at higher temperature (above 273 K). With the temperature decreasing, the influence of temperature on DH can't be neglected, and the monomial including the factors of Tx (x ¼ 1, 2, 3) in the polynomial can still be ignored, but the monomial including the factors of Tx (x ¼ 1) can't be ignored. Finally, the non-linear Van't Hoff equation can be expressed by equation (6). 0
lnP ¼ a þ
b c þ T T2
(6)
For non-linear relation, DH and DS for dehydrogenation of hydrogen storage alloys can be calculated by equations (7) and (8), respectively. 2c DH ¼ R b þ T
(7)
c DS ¼ R a 2 T
(8)
Non-linear Van't Hoff plots of ln Pd vs 1/T of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are displayed in Fig. 8(b). Good non-linear fits are obtained in all cases, and the values of the R-square are more than 0.998 (see in Table 4). Through the non-linear Van't Hoff equation, the thermodynamics of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are calculated as listed in Table 4. From the Table 4, it can be seen that the values of △Hd and △Sd for dehydrogenation of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are in the range from 13.5 to 17 kJ/mol H2 and 98e108.0 J mol1K1, respectively. From the calculated values of △Hd and △Sd, hydrogen desorption plateau pressure at 338 K can be predicted as listed in Table 4. When Fe stoichiometry contents increase from 0.6 to 0.75, hydrogen desorption plateau pressure predicted according to non-linear relation increases from 72 MPa to 100 MPa which are consistent with the experimental data of the Ti1.02Cr1.1Fe0.6Mn0.3 alloy at 338 K measured by Chen et al. [45].
Fig. 8 e Hydrogen desorption Van't Hoff plots of annealed Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys.
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Table 4 e Enthalpy, entropy and hydrogen desorption plateau pressure at 338 K of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys. Samples
Ti1.02Cr1.1Fe0.6Mn0.3 Ti1.02Cr1.05Fe0.65Mn0.3 Ti1.02Cr1.03Fe0.67Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3 Ti1.02Cr0.95Fe0.75Mn0.3 Ti1.02Cr1.0Fe0.75Mn0.25
DH (kJ/mol H2)
DS (J/(molH2K))
Pd (MPa)
R-Square
Linear
PN
Linear
PN
Linear
PN
Linear
PN
20.2 19.8 19.3 19.9 18.7 19.0
15.6 15.9 17.2 14.9 14.0 13.6
117.6 117.1 115.5 119.8 116.4 117.9
100.1 103.1 107.4 101.1 98.6 97.6
107 114 111 152 155 166
72 82 91 96 99 100
0.999 0.999 0.998 0.998 0.999 0.999
0.999 0.999 0.998 0.998 0.999 0.999
PN represents the Polynomial fitting.
As shown in Table 3, the maximum and reversible hydrogen capacities can reach to 1.85 and 1.72 wt% at 201 k respectively, and the reversible hydrogen capacities are more than 1.55 wt% at 233 K for Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys, which are little better than Ti1.02Cr1.1Mn0.3Fe0.6RE0.03 (RE ¼ La, Ce, Ho) prepared by induction melting and no heat treatment [47]. The maximum and reversible hydrogen capacities have no obvious change with the Fe stoichiometry content increasing from 0.6 to 0.75 at B side in the range of 201e233 K. Plotting unit cell volumes and desorption plateau pressures of annealed Ti1.02Cr1.7-xFexMn0.3 (0.65 x 0.75) alloys at 201 K vs Fe stoichiometry contents (in Fig. 9) comes to the conclusion that the hydrogen desorption pressure increase with Fe stoichiometry content increasing. It may be caused by the unit cell volume contraction because of the smaller radius of Fe than Cr. The tendency is consistent with the reports by Li et al. [3] of (TiZr0.1)1.1Cr1.7-xFexMn0.3 (x ¼ 0.2e0.6), Chen et al. [44] of TiCr1.9-xFexMn0.1 (0.4 x 0.6) and Abdul et al. [58] of Ti250.5xV40Cr350.5x Fex (x ¼ 0, 2, 5, 6) alloys. From the values of Sf in Table 3, we can see that all the samples have excellent plateau property due to its value less than 0.5. Comparing Ti1.02Cr0.95Fe0.75Mn0.3 alloy with Ti1.02Cr1.0Fe0.75Mn0.25 alloy, not only the maximum capacities are very close which is about 1.84 and 1.85 wt% at 201 K, but also the
Fig. 9 e Unit cell volume and plateau pressure at 201 K as a function of Fe atom stoichiometry content of Ti1.02Cr2-xyFexMny (0.6 ≤ x ≤ 0.75) alloys.
reversible capacities of 1.66, 1.69 wt% are close. However, the Sf value of Ti1.02Cr0.95Fe0.75Mn0.3 alloy (0.348) is more than that of Ti1.02Cr1.0Fe0.75Mn0.25 alloy (0.253) at 233 K. In C14 Laves phase, Ti atoms are located at 4f sites, Cr atoms are located at 2a and 6 h sites, and Fe, Mn atoms are located at 6 h site when the sum of Fe and Mn stoichiometry content is less than 1 [59], which is shown in Fig. 10. If the sum of Fe and Mn stoichiometry content is more than 1 such as Ti1.02Cr0.95Fe0.75Mn0.3 alloy, the Fe or Mn atoms may be located at 2a sites partially, which may weaken the symmetry of Cr, resulting in the increase of Sf. In a similar way, the partial super-stoichiometry Ti may occupy 2a sites which can weaken the symmetry of Cr. Hence, the Sf properties deteriorate, which is in agreement with the experimental data (the Sf value of Sf increase from 0.35 to 1.16 as Ti super-stoichiometry increasing from 1.0 to 1.1 at A side) by Chen et al. [43].
Application in high-pressure hydrogen metal hydride tank In order to save the cost for hydrogen refueling station, it plays an important role to increase the effective capacity, which can reduce energy consumption of the compressor and promote the utilization of high-pressure hydrogen. Refueling the hydrogen to the 70 MPa tank of vehicle, the pressure of HRS must be higher than 70 MPa (the higher pressure, the faster
Fig. 10 e Crystal structure of C14 type Laves phase of Ti(CrFeMn)2.
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Fig. 11 e Schematic of refueling hydrogen from HRS to HFCV.
refueling rate), and needs to reach up to 90 MPa. The highest Pd at 338 K of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys may be up to 100 MPa, which can be applied in HRS for refueling to the 70 MPa HFCV. The maximum volume fraction of hydrogen storage alloys enclosed into the high pressure hydrogen metal hydride tank should not be exceed 50% of total inner volume [48,60], and other volume is filled with pure pressurized hydrogen gas. The gravimetric and volumetric densities are not calculated, because of the values are impacted by the design of the 90 MPa high-pressure metal hydride tank obviously. Comparing with the 35 [61,62] and 70 MPa [62,63] tank for HFCV, the volume of the 90 MPa tank is not considered for calculating the hydrogen storage capacity (volumetric density, kg/m3) due to the data lacking of the tank for HRS. For the high-pressure hydrogen gas, the volumetric density does not change with the pressure increasing as linear relation. Taking account of several mass density expression about pressurized hydrogen gas [64e66], the gas compression factor Z from the NIST database is introduced to calculate the mass density. Z and hydrogen mass can be calculated by equations (9) and (10), respectively. z¼1þ
m¼
aP T
2PV ZRT
(9)
(10)
Where a is the gas compression factor coefficients, which is equal to 1.9155 106 K/Pa. V is the volume of pure pressurized hydrogen gas. The process of hydrogen refueling to HFCV from HRS is shown in Fig. 11. The heat from the tank surface is the chief heat exchange channel, and the internal heat transfer system is also designed. The temperature for hydrogen storage through alloy absorption in HRS is 273 K, which is cooled through the low temperature heat conducting oil (cooled by refrigerator). The heat transfer coefficient for heat exchange system is 1000 W/m2/K. When the temperature increases to 338 K (electric heating heat conduction medium), the pressure of the hydrogen storage tank will be in the range of 70 and 90 MPa. The volume expansion of hydrogen storage alloy after hydrogen absorption is 25% [67]. So the volume of pure pressurized hydrogen gas is by the total volume minus the 125 percentages volume of hydrogen storage alloy. The alloy volumetric fraction x in the tank impacts the hydrogen storage capacity obviously in the reasonable ranges. The maximum hydrogen pressure of the most HRS in China can be up to 45 MPa, and the hydrogen pressure needs to be further increased. So the 45, 70, 90 MPa hydrogen pressures supplied by the compressor in HRS are considered in the next calculations. Fig. 12 shows the HRS hydrogen gravimetric density (a) and capacity increase percentages (b) of different x. From Fig. 12 (a), the hydrogen storage capacity increases at the any same x (0 < x 0.5) with the hydrogen pressure (supplied by compressor) increasing from 45 MPa to 90 MPa. When hydrogen pressure supplied by compressor is 45, 70, 90 MPa, the hydrogen storage capacity increases from 30.1, 41.4, 48.6 to 57.8 to 62.0, 64.7 kg/m3 as x increasing from 0 to 0.5 (in Table 5), respectively. Comparing with pure pressurized gas tank, the maximum increase percentages of hydrogen capacity by high pressure hydrogen metal hydride tank are 91.8, 49.9, 33.2% at the different supplying pressures of 45, 70, 90 MPa, respectively as shown in Fig. 12(b). It should be noted that the effect on hydrogen capacity increase is weakened with the hydrogen pressure increasing supplied by compressor. Fig. 13 shows effective hydrogen capacities for supplying high pressure more than 70 MPa at different alloy volumetric fractions. When the hydrogen pressure supplied by the compressor is 45 MPa at 273 K, the pure pressurized tank can't offer the high pressure hydrogen more than 70 MPa; when the
Fig. 12 e HRS hydrogen capacity vs Volumetric fraction of hydrogen storage alloy (a) HRS hydrogen capacity, (b) HRS hydrogen capacity density increase percentage.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 5 0 8 7 e1 5 0 9 9
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higher than 45 MPa supplied by compressor dramatically decreased, for which the compression cost will be saved. Therefore, due to the increase of effective hydrogen capacity, it offers a potential way to reduce the frequency of opening the compressor which can reduce the accident rate.
Conclusion
Fig. 13 e HRS effective hydrogen capacity for 70 MPa HFCV vs volumetric fraction of hydrogen storage alloy.
volume fraction x of hydrogen storage alloy is enclosed into the tank below 0.1, the tank still can't supply for the high pressure hydrogen more than 70 MPa (below the horizontal line in Fig. 13); as x increasing from 0.1 to 0.5, the tank can offer the high pressure hydrogen more than 70 MPa at 338 K from 3.6 kg/m3 (40 Nm3) to 40.0 kg/m3 (448 Nm3). When the hydrogen pressures supplied by the compressor are 70, 90 MPa, the maximum effective hydrogen capacity increases from 5.7 kg/m3 (64 Nm3), 12.9 kg/m3 (155 Nm3) to 44.2 kg/m3 (495 Nm3), 46.9 kg/m3 (525 Nm3) as x increasing from 0 to 0.5 (see in Table 5), respectively. In comparison to the pure pressurized gas tank, the maximum increase percentages of effective capacity by the high-pressure hydrogen metal hydride tank can be up to 677 and 262% at the hydrogen pressure supplied by the compressor about 70 and 90 MPa at 273 K respectively. Comparing with the current metal-hydride compressor including single, double-stage and even threestage compressor, product hydrogen with the higher pressure of 90 MPa can be obtained at lower temperature at 338 K by the alloys in this work, which is beneficial for saving the thermal energy cost to release hydrogen. Furthermore, the solid state hydrogen occupy the volume of the tank, which can be absorbed under the pressure of 45 MPa. The remainder volume of the tank filled by gaseous hydrogen can be reduced up to 75%. It means that the hydrogen with the pressure
Table 5 e Hydrogen storage capacity and effective hydrogen storage capacity of x ¼ 0 and 0.5. Hydrogen pressure by compressor
x ¼ 0 (kg/ m3) Cp
Ceff-p
Cp
Ceff-p
45 70 90
30.1 41.4 48.6
0 5.7 12.9
57.8 62.0 64.7
40 44.2 46.9
x ¼ 0.5 (kg/m3)
* Cp, Ceff-p represent the hydrogen storage capacity and effective hydrogen storage capacity, respectively.
In this paper, hydrogen storage properties of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys have been studied. The main phase structure of all samples before and after heat treatment at 1273 k for 2 h is C14 type Laves phase, and the little secondary phase is reduced or even eliminated completely after heat treatment. The parameters of cell lattice decrease and hydrogen desorption plateau pressure increase with Fe stoichiometry content increasing. As Fe stoichiometry in the range between 0.6 and 0.75, hydrogen desorption plateau slopes are all less than 0.5 at 233 K. Comparing with Ti1.02Cr0.95Fe0.75Mn0.3 alloy, Ti1.02Cr1.0Fe0.75Mn0.25 alloy has better hydrogen desorption plateau slope, which is beneficial for promoting the reversible hydrogen storage capacity and operation in the HRS. The reversible hydrogen storage capacities can remain 1.55 wt% when the temperature is higher than 233 K. The Ti1.02Cr0.95Fe0.75Mn0.3 and Ti1.02Cr1.0Fe0.75Mn0.25 alloys, whose △Hd are about 14 kJ/mol H2, are suitable for high-hydrogen pressure metal hydride tank. In comparison to the pure pressurized gas tank, the effective hydrogen capacities (more than 70 MPa) are promoted by the high pressure metal hydride tank at different pressure supplied by compressor, but the maximum increase percentages of effective capacity decrease with the hydrogen supplying pressure increasing.
Acknowledgement The authors acknowledge Beijing Municipal Commission of Science and Technology (Z17110000091702), National Natural Science Foundation of China (No. 51431001, 51771002, U1607126, 21321001 and 51804174) and Natural Science Foundation of Shandong Province (ZR2017BEE010).
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Development of Ti1.02Cr2-x-yFexMny (0.6≤x≤0.75, y¼0.25, 0.3) alloys for high hydrogen pressure metal hydride system Jigang Li a, Xiaojing Jiang a,b, Guoling Li c, Xingguo Li a,* a
Beijing National Laboratory for Molecular Science (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b Beijing National Laboratory for Molecular Science (BNLMS), Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China c Beijing National Laboratory for Molecular Science (BNLMS), College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China
article info
abstract
Article history:
To save compressor investment and promote operation efficiency of hydrogen refueling
Received 17 December 2018
station, the hydrogen storage alloys for high-pressure hydrogen metal hydride tank is
Received in revised form
developed. Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys with main structure of C14
24 March 2019
type Laves phase and low dehydrogenation enthalpy were prepared by plasma arc melting
Accepted 31 March 2019
and heat treatment. Pressure-composition-temperature measurements show that
Available online 3 May 2019
hydrogen desorption plateau pressures increase with Cr substituted by Fe increasing. The maximum and reversible hydrogen storage capacities are more than 1.85 and 1.65 wt% at
Keywords:
201 K respectively. The hydrogen desorption plateau slopes are all less than 0.5. The
Intermetallic
symmetry weakening of 2a sites may deteriorate the plateau slop characteristic. Ti1.02-
Hydrogen storage
Cr0.95Fe0.75Mn0.3 and Ti1.02Cr1.0Fe0.75Mn0.25 alloys are suitable for high pressure hybrid tank
Tix(CrFeMn)2 alloy
which can supply the effective hydrogen (more than 70 MPa) about 40.0, 44.2, 46.9 kg/m3
Heat treatment
with 45, 70, 90 MPa compressor, respectively.
Hydrogen refueling station
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction 30% of the total world emission of greenhouse gas is attributed to transportation [1] and a third is attributed to heat generation in buildings and industry [2]. Hydrogen, as the best energy carrier, has the advantages of cleanness and renewability. Combining with the highly effective fuel cell technology, it offers a potential way for sustainable development, which is applied for stationary power, portable power, and
transportation [3e8]. In contrast with traditional oil fuel, hydrogen fuel cell vehicle (HFCV) has the advantages of no pollution: the product of hydrogen converting to energy is only water; and high energy conversion efficiency: the efficiency of the direct process of electron transfer from oxygen to hydrogen can reach 50e60%, which is not limited by the Carnot efficiency (the efficiency of the thermal process of petrol is about 19e22%) [9]. In comparison to pure battery electric vehicle, hydrogen fuel cell vehicle (HFCV) has the
* Corresponding author. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.ijhydene.2019.03.241 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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advantages of short refueling time and long driving range [10e13]. The hydrogen storage capacity of HFCV is critical to the driving range. And the hydrogen storage capacity of hydrogen refueling station (HRS) plays an important role on the full-utilization hydrogen cost (Station capacity: 240, 480, 900 kg/day, full-utilization hydrogen cost: 2.35, 1.66, 1.12 $/kg) [14]. The volumetric energy density of hydrogen storage tank is lower relative to traditional liquid fuels, which is the challenge for the application [14]. The high volumetric density of a hydrogen storage system can save more space and promote driving range. The higher the pressure, the greater the volumetric hydrogen storage density. To this end, the pure highpressurized gas tank and high-pressure hydrogen-metal hydride tank are developed to increase the volumetric density [3,15e17]. With the increase of the vehicle tank pressure from 35 MPa to 70 MPa, the HRS pressure for refueling must increase at the same time [18]. The full-utilization hydrogen cost in HRS is about 1.12, 1.32, 1.55 dollar/kg (the station capacity: 900 kg/day) at 233 K precooling (the tank of HFCV must stay at 233 K-358 K to prevent degradation of the tank liner [19]) of the delivered pressure 35, 50 and 70 MPa respectively [14]. The cost of compressor system may be up to 65% of total HRS investment and it is difficult to reduce below 50% [19e22]. With the delivered pressure increasing from 35 to 70 MPa, the cost of compressor and booster compressor increases by linear, and the effective utilization capacity of HRS influences the hydrogen refueling cost strongly [12]. The best refueling strategy for 70 MPa tank is by the low-middle-high hydrogen pressure vessels, which can decrease the refueling cost dramatically due to the reducing of the partial compress process [23]. In order to reduce the cost of refueling, it is important to increase the capacity and utilization of 90 MPa hydrogen source. To decrease the cost for hydrogen compression (Investment for compressor and reduce the open and failure rate of compressor) and operation of HRS, increasing the effective gravimetric or volumetric capacity of the storage tank is a good method. The high-pressure hydrogen-metal hydride vessel can be used as the hydrogen storage tank and the compressor due to its stationary compression. The stationary compressor which has the advantages of the high purity hydrogen released, no moving part, no noise, high pressure only during operation status
[24e27]. The previous reports of compressor are summarized as listed in Table 1. From Table 1, it is easy to get a conclusion that there are no suitable hydrogen storage alloys which can supply the hydrogen above 70 MPa (may reach up to 90 MPa) at moderate temperature (for example, 338 K) and low cost in HRS. The high-pressure hydrogen-metal hydride vessel requires hydrogen storage alloy with a lower dissociation enthalpy and especially a higher hydrogen desorption plateau pressure than the existing AB5 [31], AB [38], AB2 type [39e41] alloys. Kojima et al. developed the Ti1.1CrMn alloy which the hydrogen absorption and desorption plateau pressure was 11 MPa at 296 K [41]. The hydrogen absorption and desorption plateau pressures increase with the Fe substitution for partial Mn of TixCrMn alloys, which is found by Guo et al. [42]. A series of Cr-rich Tix(CrFeMn)2 alloys with high hydrogen desorption plateau pressures were developed by Chen et al. [43e46], which were higher than 45 MPa at 318 K. The activation behavior for alloy hydrogen absorption was improved by the additions of rare earth metals (La, Ce, Ho) to the Ti1.02Cr1.1Fe0.6Mn0.3 alloy but the hydrogen absorption and desorption plateau pressures decreased [47]. The ZreFeeV alloys were prepared by arc melting, and the hydrogen desorption plateau pressure of Zr1.04Fe1.9V0.1 can be up to 40 MPa at 353 K [48]. The compositions of the alloys influence the hydrogen desorption plateau pressures and capacities, the hydrogen storage properties of the Cr-rich Tix(CrFeMn)2 alloy were studied in this paper. We have investigated the series alloys of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys which might be suitable for HRS of high pressure hydrogen refueling.
Sample preparation and characterization methods The hydrogen storage properties of the alloys in accordance with Fig. 1 process were investigated. The alloys were prepared by Ar plasma arc melting method in a water-cooled copper crucible. The purities of the raw materials are 99.7 wt % for Ti, 99.9 wt% for Cr, 99.9 wt% for Fe and 99.8 wt% for Mn. The high-purity inert gas (Ar99.999%) was filled into the furnace chamber about 0.85e0.9 atm. Based on the stoichiometry ratio of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3)
Table 1 e Summarized data of the metal hydride compressor with different stage. Compressor type
Single Double
Three
Alloys
La0.4Ce0.4Ca0.2Ni5 Mm0.7Ca0.2La0.1Ni4.95Al0.05 LaNi5/La0.5Ce0.5Ni5 La0.9Ce0.1Ni5/La0.8Ce0.2Ni5 LaCe0.15Ni5/AB2-x La0.4Y0.6Ni4.8Mn0.1Al0.1/La0.4Y0.6Ni4.8Al0.2 Mm0.4La0.6Ca0.2Ni5/Ti1.1Cr1.5Mn0.4V0.1 Ti0.95Zr0.05Cr0.8Mn0.8V0.2Ni0.2/Ti0.8Zr0.2Cr0.95Fe0.95V0.1 La0.35Ce0.45Ca0.2Ni4.95Al0.05/Ti0.8Zr0.2Cr0.95Fe0.95V0.1 Ti0.85Zr0.15Mn1.33V0.3/Ti0.8Zr0.2Mn1.2Cr0.6V0.2/Ti0.9Zr0.1Mn1.4Cr0.4V0.2
Initial stage
Last stage
Feed gas (MPa)
Outer pressure (MPa)/T (K)
Outer pressure (MPa)/T (K)
1.9 4 0.4 0.4 1 2 4 4 4 0.2
e e 4/423 4/353 6/393 8/373 10/372 30/423 38.5/423 10.8/383
12/423 [28] 45/443 [29] 7.5/423 [30,31] 7.5/353 [32] 20/393 [33] 38/448 [34] 45/372 [29] 70/423 [35] 74.5/423 [36] 18.3/405 [37]
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Fig. 1 e Flow chart of studying on hydrogen storage properties.
alloys, 12 wt% excess of Mn was compensated for the loss of evaporation due to the high vapor pressure of Mn. The ingots melting time was about 1.5 min, then turned over and remelted about 1.5 min. The ingot was melted overall 3 times to achieve a high homogeneity. The mass accuracy is about ±0.015 g between the total theoretical chemical stoichiometric mass (8 g) and terminal cast ingot. Finally, all the alloys undergone the heat treatment at 1273 K for 2 h. Phase structures of all the samples were determined by Xray powder diffraction equipment (XRD, PANalytical X-Pert3 Powder type, CuKa radiation, scanning rate, 1.3 /min). On the basis of XRD patterns, lattice parameters were calculated by Maud software [49,50]. Metallographic microstructures and particle morphologies were analyzed by scanning electron microscopy (SEM, Hitachi, S4800). The samples for metallographic microstructure were prepared by grinding with sand
paper and polishing with diamond abrasion paste. The element distribution on the diamond polished surface was measured by energy dispersive X-ray spectroscopy (EDS). In order to eliminate the oxides surface, all the annealed samples were polished by stainless grinding head, and then transferred into the glove box or hold in the vacuum dryer, which all kept low water and oxygen conditions [3]. In the glove box, the samples were crushed into small particles which were enclosed into thin-wall furnace tube with a loading mass of 1 g. Hydrogen absorption/desorption properties were measured by the pressure-composition-temperature (PeCeT) instrument which was made in Japan by Suzuki Shokang. First, the thin-wall furnace chamber with samples was vacuumed to below 2.0 101 Pa for 2 h at room temperature; Second, hydrogen with the pressure about 9.8 MPa was filled into the reaction chamber and kept 12e24 h approximately at 201 K; At last, the hydrogen was evacuated from the reaction chamber for 15e30 min. To make the alloy hydrogen absorbing fully activated, the above-mentioned processes need to be repeated for about 3e5 cycles. Then, the hydrogen storage properties were measured at the low temperature 201e253 K, which was limited by the maximum operating hydrogen pressure of the equipment (about 10 MPa, the pressure must be higher than the desorption plateau pressure) in our lab.
Results and discussion XRD analysis
Fig. 2 e XRD patterns of (a) Ti1.02Cr1.0Fe0.65Mn0.3 and (b) Ti1.02Cr1.0Fe0.7Mn0.3 alloys As cast.
XRD patterns of Ti1.02Cr1.0Fe0.65Mn0.3 (a) and Ti1.02Cr1.0Fe0.7Mn0.3 (b) alloys analyzed by Maud soft after plasma melting but without heat treatment (As cast) are shown in Fig. 2. The phase structure of Ti1.02Cr1.0Fe0.65Mn0.3 alloy (As cast) is determined as a single phase of C14-type Laves structure, and no secondary phase is detected from Fig. 2 (a). The phase structure of Ti1.02Cr1.0Fe0.7Mn0.3 alloy (As cast) is determined as vast majority phase of C14-type Laves structure, and secondary phase is observed which can be ascribed to CrMn [45] or TiMn [51,52] alloy phase in Fig. 2 (b). Fig. 3 and Fig. 4 show the XRD patterns of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys before
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Fig. 3 e XRD patterns of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys after heat treatment.
Fig. 4 e XRD patter of annealed Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys after PCT measurement.
and after hydrogen absorption and desorption cycles (at least ten cycles) respectively. We can see that the phase structures of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are determined as single phase of C14-type Laves structure, and the secondary phase is very few. It indicates that the partial substitution of Fe for Cr or Mn at B side does not change the phase structure. Lattice parameters of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys calculated by Rietveld method with the Maud software are listed in Table 2. The values of a, c, V of annealed Ti1.02Cr1.05Fe0.65Mn0.3 alloy increase from 0.48446 nm, 0.79423 nm, 0.16143 nm3 to 0.48461 nm, 0.79461 nm, 0.16161 nm3 respectively. The values of a, c, V of Ti1.02Cr1.0Fe0.7Mn0.3 alloy decrease from 0.48439 nm, 0.79387 nm, 0.16131 nm3 to 0.48430 nm,
0.79382 nm, 0.16124 nm3 respectively. As the Fe stoichiometry content increasing from 0.6 to 0.75, the value of a decreases from 0.48477 nm to 0.48403 nm, c decreases from 0.79491 nm to 0.79319 nm, and V reduces from 0.16178 to 0.16094 nm3, respectively. The unit cell volumes of Ti1.02Cr2-x-yFexMn0.3 (0.6 x 0.75) alloys shrink due to the smaller atomic radius of Fe relative to Cr [3,44]. Comparing Ti1.02Cr0.95Fe0.75Mn0.3 alloy with Ti1.02Cr1.0Fe0.75Mn0.25 alloy, the value of a almost has no change from 0.48403 nm to 0.48402 nm, c increase from 0.79319 nm to 0.79332 nm, and V expands very little from 0.16194 to 0.16096 nm3. After hydrogenation and dehydrogenation for ten cycles at least, the lattice parameters of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys all increase. It indicates that the hydrogenation and dehydrogenation processes result in the deformation of lattice cell [53].
Table 2 e Lattice parameters of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys calculated from XRD patterns. Alloy Ti1.02Cr1.1Fe0.6Mn0.3 Ti1.02Cr1.1Fe0.65Mn0.3 Ti1.02Cr1.05Fe0.67Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3 Ti1.02Cr0.95Fe0.75Mn0.3 Ti1.02Cr1.0Fe0.75Mn0.25 Ti1.02Cr1.05Fe0.65Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3
Status Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des Annealed Abs-Des As cast As cast
Lattice Parameters(nm) a c 0.48477 0.48488 0.48461 0.48482 0.48433 0.48483 0.48430 0.48446 0.48403 0.48439 0.48402 0.48435 0.48446 0.48439
0.79491 0.79508 0.79461 0.79470 0.79422 0.79468 0.79382 0.79405 0.79319 0.79366 0.79332 0.79373 0.79423 0.79387
a/c
Unit Cell Volume V/(nm3)
0.6098 0.6099 0.6099 0.6101 0.6098 0.6101 0.6101 0.6101 0.6102 0.6103 0.6101 0.6102 0.6100 0.6102
0.16178 0.16189 0.16161 0.16177 0.16134 0.16177 0.16124 0.16140 0.16094 0.16127 0.16096 0.16126 0.16143 0.16131
*Abs-Des represents the XRD determination after hydrogen absorption and desorption about ten cycles, As cast represents the XRD determination of the alloys which is without heat treatment and hydrogen absorption/desorption process.
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Fig. 5 e SEM of the alloys (a)-(f) Backscatter electron micrographs (a) Ti1.02Cr1.05Fe0.65Mn0.3 As cast, (b) Ti1.02Cr1.0Fe0.7Mn0.3, As cast; (c) Ti1.02Cr1.1Fe0.6Mn0.3, Annealed; (d) Ti1.02Cr1.05Fe0.65Mn0.3, Annealed; (e) Ti1.02Cr1.03Fe0.67Mn0.3, Annealed; (f) Ti1.02Cr1.0Fe0.7Mn0.3, Annealed; (g) Backscatter electron micrographs on the left and the EDS mapping on the right of annealed Ti1.02Cr1.0Fe0.7Mn0.3; (h) and (i) Particles secondary electron micrographs after ten cycles of hydrogen absorption and desorption cycles.
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SEM analysis Fig. 5(a)e(g) show the SEM micrographs of Ti1.02Cr1.7-xFexMn0.3 (0.6 x 0.7) alloys by backscatter electron, EDS mapping and particles micrographs of Ti1.02Cr1.0Fe0.7Mn0.3 alloy after hydrogenation and dehydrogenation cycles by secondary electron scanning. From Fig. 5(a) and (b), the secondary phase of the dark fields exists in the alloys before heat treatment (the secondary phase are indicated by black arrows). Coupled with XRD patterns, it should be noted that the secondary phase sometimes can be detected and sometimes can't be detected, which may be caused by unstable plasma arc of the manual operation. From Fig. 5(c)e(f), the metallographic microstructures of the annealed alloys are more homogeneous, which maintain the vast majority of C14 or single C14 phase corresponding to the XRD diagram [45]. From Fig. 5(g), the annealed Ti1.02Cr1.0Fe0.7Mn0.3 alloy displays homogeneous morphology structure from the EDS mapping. After PCT measurement (more than ten cycles of hydrogenation and dehydrogenaiton), the particle sizes in Fig. 5(h) are below 20 mm. From Fig. 5(i), the breaks (the partial locations are marked by white ring) appear on the particles surface, which is similar with previous studies of AB2-type alloy [48,51,54].
Hydrogen storage properties PCT measurement Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys were prepared by plasma arc melting and heat treatment at 1273 K for 2 h. The PCT properties of the series alloys were measured after activation by the above-mentioned method in section Sample preparation and characterization methods. PeC-T curves of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are shown in Fig. 6 and Fig. 7. From PeC-T curves, the hydrogen storage characteristics including hydrogen absorption plateau pressure Pa, hydrogen desorption plateau pressure Pd, maximum capacity HMax, reversible capacity HRe, plateau slope factor Sf and hysteresis factor Hf can be obtained. Sf and Hf can be calculated by equations (1) and (2) respectively at measuring temperatures [3]. △Ha, △Sa, and △Hd, △Sd represent the enthalpy and entropy of the hydrogenation and dehydrogenation process, respectively.
Sf ¼ dðlnPÞ=dðH wt%Þ
(1)
Hf ¼ lnðPa =Pd Þ
(2)
PCT of Ti1.02Cr1.7-xFexMn0.3 (x ¼ 0.65, 0.7) alloys (As cast). Fig. 6 show the PCT measurements of Ti1.02Cr1.7-xFexMn0.3 (x ¼ 0.65, 0.7) alloys (As cast). It should be noted that the hydrogen storage properties of the alloys are not so good, or even terrible. The capacity plateau broadenings of Ti1.02Cr1.7xFexMn0.3 (x ¼ 0.65, 0.7) alloys are all less than 0.9 wt%. The bad hydrogen storage characteristics may be caused by the existences of the secondary phase and elements heterogeneous distributions of main phase from the SEM micrographics.
PCT of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys. Heat treatment can improve the hydrogen storage properties of the alloys [45,55e57]. Fig. 7 displays the PCT measurements of the annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys. From the PeCeT curves in Fig. 7(a)e(e), hydrogen storage characteristics of annealed Ti1.02Cr1.7-xFexMn0.3 (0.65 x 0.75) alloys are obtained as listed in Table 3. From Table 3, with Fe stoichiometry content increasing from 0.6 to 0.75, the hydrogen desorption plateau pressures increase from 0.79 to 1.65 MPa at 201 K and 4.4e8.00 MPa at 233 K. Van't Hoff plots of ln P vs 1/T for dehydrogenation following the linear relation are displayed in Fig. 8(a). Good linear fits are obtained in all cases as shown in Fig. 8(a) and the values of R-square are more than 0.998 from Table 4. Through the linear Van't Hoff relation, the thermodynamics of Ti1.02Cr2x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are calculated, which are listed in Table 4. From the Table 4, the values of △Hd and △Sd for dehydrogenation of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are in the range from 18.5 to 20.5 kJ/mol H2 and 115.5 to 112.0 J mol1K1, respectively. The values of △Hd obtained from the PCT measurement at 201e253 K are a little higher than the △Hd values of TixCr2.0-yzFeyMnz alloys (1 x 1.1, 0.60 y 0.65, 0.2 z 0.4) in the range of 14e17 kJ/mol H2 at 243e263 K [43e45,47]. From the calculated values of △Hd and △Sd, hydrogen desorption plateau pressure at 338 K can be predicted, which are
Fig. 6 e PCT of Ti1.02Cr1.7-xFexMn0.3 (x ¼ 0.65, 0.7) alloys (As cast) (a) Ti1.02Cr1.05Fe0.65Mn0.3, (b) Ti1.02Cr1.0Fe0.7Mn0.3.
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Fig. 7 e PCT of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys (Annealed) (a) Ti1.02Cr1.1Fe0.6Mn0.3, (b) Ti1.02Cr1.05Fe0.65Mn0.3, (c) Ti1.02Cr1.03Fe0.67Mn0.3, (d) Ti1.02Cr1.0Fe0.7Mn0.3, (e) Ti1.02Cr0.95Fe0.75Mn0.3, (f) Ti1.02Cr1.0Fe0.75Mn0.25.
summarized in Table 4. With Fe stoichiometry content increasing from 0.6 to 0.75, hydrogen desorption plateau pressures at 338 K increase from 107 MPa to 165 MPa. Comparing with Chen et al.’ results of the Ti1.02Cr1.1Fe0.6Mn0.3 alloy (Hydrogen desorption plateau pressure is about 60e70 MPa at 338 K) [45], hydrogen desorption plateau pressure predicted by linear Van't Hoff equation at 338 K is too high and may deviate from the actual. We choose the moderate temperature of 338 K to predict, and consider it can save
energy cost for heating the tank to release the high-pressure hydrogen and pre-cooling to refuel. If the temperature is lower than 338 K, the hydrogen absorption of the alloys at 273 K may not be completed of the pressure 45 MPa in current station. The Van't Hoff differential equation can be expressed by equation (3).
dlnP DH ¼ dT RT2
(3)
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Table 3 e Hydrogen storage characteristics of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys. Alloy
Ti1.02Cr1.1Fe0.6Mn0.3 Ti1.02Cr1.05Fe0.65Mn0.3 Ti1.02Cr1.03Fe0.67Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3 Ti1.02Cr0.95Fe0.75Mn0.3 Ti1.02Cr1.0Fe0.75Mn0.25 Ti1.02Cr1.05Fe0.65Mnþ 0.3 Ti1.02Cr1.0Fe0.7Mnþ 0.3 a b
Hf
Sf
201 K
233 K
201 K
Pd (MPa) 233
201 K
HRe (wt%) 233 K
HMax (wt%) 201 K
1.19 1.26 1.26 1.40 1.28 1.28 e e
0.202 0.226 0.315a 0.233 0.348 0.253 e e
0.79 0.93 1.00 1.21 1.65 1.61 1.2 1.56
4.40 4.92 5.85b 6.42 7.70 8.00 e e
1.69 1.72 1.69 1.70 1.66 1.69 1.2 1.01
1.61 1.63 1.56a 1.58 1.59 1.61 e e
1.83 1.83 1.83 1.85 1.84 1.85 1.33 1.05
Represents that the value was measured at 229 K. Represents that the value was calculated from experimental data on the both sides closest the temperature 233 K,þRepresents that the alloy didn't undergo the heat treatment, - Represents that the value was not measured or was measured invalid.
The linear Van't Hoff equation can be expressed by equa tion (4), which is based on the assumption of that DH and DS are independent with temperature.
lnP ¼
DH DS þ RT R
(4)
Based on the above-calculated data, the linear Van't Hoff equation is not appropriate for the alloys to predict the hydrogen desorption plateau pressure. The dehydrogenation DH of Tix(CrFeMn)2 alloys can be expressed by equation (5) based on the metallurgical thermodynamics. Da2 3 Da1 2 T2 T21 þ T2 T31 DH ¼ DH0 þ Da0 ðT2 T1 Þ þ 2 3 1 J=ðmol H2 Þ Da3 T1 2 T1
(5)
Where DH is a constant, Da0 , Da1 , Da2 , Da3 are the coefficients for the Tx (x ¼ 1, 2, 3, 1) factors, respectively. In general, the DH can be considered as a constant at higher temperature (above 273 K). With the temperature decreasing, the influence of temperature on DH can't be neglected, and the monomial including the factors of Tx (x ¼ 1, 2, 3) in the polynomial can still be ignored, but the monomial including the factors of Tx (x ¼ 1) can't be ignored. Finally, the non-linear Van't Hoff equation can be expressed by equation (6). 0
lnP ¼ a þ
b c þ T T2
(6)
For non-linear relation, DH and DS for dehydrogenation of hydrogen storage alloys can be calculated by equations (7) and (8), respectively. 2c DH ¼ R b þ T
(7)
c DS ¼ R a 2 T
(8)
Non-linear Van't Hoff plots of ln Pd vs 1/T of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are displayed in Fig. 8(b). Good non-linear fits are obtained in all cases, and the values of the R-square are more than 0.998 (see in Table 4). Through the non-linear Van't Hoff equation, the thermodynamics of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are calculated as listed in Table 4. From the Table 4, it can be seen that the values of △Hd and △Sd for dehydrogenation of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys are in the range from 13.5 to 17 kJ/mol H2 and 98e108.0 J mol1K1, respectively. From the calculated values of △Hd and △Sd, hydrogen desorption plateau pressure at 338 K can be predicted as listed in Table 4. When Fe stoichiometry contents increase from 0.6 to 0.75, hydrogen desorption plateau pressure predicted according to non-linear relation increases from 72 MPa to 100 MPa which are consistent with the experimental data of the Ti1.02Cr1.1Fe0.6Mn0.3 alloy at 338 K measured by Chen et al. [45].
Fig. 8 e Hydrogen desorption Van't Hoff plots of annealed Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys.
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Table 4 e Enthalpy, entropy and hydrogen desorption plateau pressure at 338 K of Ti1.02Cr2-x-yFexMny (0.6 ≤ x ≤ 0.75, y ¼ 0.25, 0.3) alloys. Samples
Ti1.02Cr1.1Fe0.6Mn0.3 Ti1.02Cr1.05Fe0.65Mn0.3 Ti1.02Cr1.03Fe0.67Mn0.3 Ti1.02Cr1.0Fe0.7Mn0.3 Ti1.02Cr0.95Fe0.75Mn0.3 Ti1.02Cr1.0Fe0.75Mn0.25
DH (kJ/mol H2)
DS (J/(molH2K))
Pd (MPa)
R-Square
Linear
PN
Linear
PN
Linear
PN
Linear
PN
20.2 19.8 19.3 19.9 18.7 19.0
15.6 15.9 17.2 14.9 14.0 13.6
117.6 117.1 115.5 119.8 116.4 117.9
100.1 103.1 107.4 101.1 98.6 97.6
107 114 111 152 155 166
72 82 91 96 99 100
0.999 0.999 0.998 0.998 0.999 0.999
0.999 0.999 0.998 0.998 0.999 0.999
PN represents the Polynomial fitting.
As shown in Table 3, the maximum and reversible hydrogen capacities can reach to 1.85 and 1.72 wt% at 201 k respectively, and the reversible hydrogen capacities are more than 1.55 wt% at 233 K for Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys, which are little better than Ti1.02Cr1.1Mn0.3Fe0.6RE0.03 (RE ¼ La, Ce, Ho) prepared by induction melting and no heat treatment [47]. The maximum and reversible hydrogen capacities have no obvious change with the Fe stoichiometry content increasing from 0.6 to 0.75 at B side in the range of 201e233 K. Plotting unit cell volumes and desorption plateau pressures of annealed Ti1.02Cr1.7-xFexMn0.3 (0.65 x 0.75) alloys at 201 K vs Fe stoichiometry contents (in Fig. 9) comes to the conclusion that the hydrogen desorption pressure increase with Fe stoichiometry content increasing. It may be caused by the unit cell volume contraction because of the smaller radius of Fe than Cr. The tendency is consistent with the reports by Li et al. [3] of (TiZr0.1)1.1Cr1.7-xFexMn0.3 (x ¼ 0.2e0.6), Chen et al. [44] of TiCr1.9-xFexMn0.1 (0.4 x 0.6) and Abdul et al. [58] of Ti250.5xV40Cr350.5x Fex (x ¼ 0, 2, 5, 6) alloys. From the values of Sf in Table 3, we can see that all the samples have excellent plateau property due to its value less than 0.5. Comparing Ti1.02Cr0.95Fe0.75Mn0.3 alloy with Ti1.02Cr1.0Fe0.75Mn0.25 alloy, not only the maximum capacities are very close which is about 1.84 and 1.85 wt% at 201 K, but also the
Fig. 9 e Unit cell volume and plateau pressure at 201 K as a function of Fe atom stoichiometry content of Ti1.02Cr2-xyFexMny (0.6 ≤ x ≤ 0.75) alloys.
reversible capacities of 1.66, 1.69 wt% are close. However, the Sf value of Ti1.02Cr0.95Fe0.75Mn0.3 alloy (0.348) is more than that of Ti1.02Cr1.0Fe0.75Mn0.25 alloy (0.253) at 233 K. In C14 Laves phase, Ti atoms are located at 4f sites, Cr atoms are located at 2a and 6 h sites, and Fe, Mn atoms are located at 6 h site when the sum of Fe and Mn stoichiometry content is less than 1 [59], which is shown in Fig. 10. If the sum of Fe and Mn stoichiometry content is more than 1 such as Ti1.02Cr0.95Fe0.75Mn0.3 alloy, the Fe or Mn atoms may be located at 2a sites partially, which may weaken the symmetry of Cr, resulting in the increase of Sf. In a similar way, the partial super-stoichiometry Ti may occupy 2a sites which can weaken the symmetry of Cr. Hence, the Sf properties deteriorate, which is in agreement with the experimental data (the Sf value of Sf increase from 0.35 to 1.16 as Ti super-stoichiometry increasing from 1.0 to 1.1 at A side) by Chen et al. [43].
Application in high-pressure hydrogen metal hydride tank In order to save the cost for hydrogen refueling station, it plays an important role to increase the effective capacity, which can reduce energy consumption of the compressor and promote the utilization of high-pressure hydrogen. Refueling the hydrogen to the 70 MPa tank of vehicle, the pressure of HRS must be higher than 70 MPa (the higher pressure, the faster
Fig. 10 e Crystal structure of C14 type Laves phase of Ti(CrFeMn)2.
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Fig. 11 e Schematic of refueling hydrogen from HRS to HFCV.
refueling rate), and needs to reach up to 90 MPa. The highest Pd at 338 K of annealed Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys may be up to 100 MPa, which can be applied in HRS for refueling to the 70 MPa HFCV. The maximum volume fraction of hydrogen storage alloys enclosed into the high pressure hydrogen metal hydride tank should not be exceed 50% of total inner volume [48,60], and other volume is filled with pure pressurized hydrogen gas. The gravimetric and volumetric densities are not calculated, because of the values are impacted by the design of the 90 MPa high-pressure metal hydride tank obviously. Comparing with the 35 [61,62] and 70 MPa [62,63] tank for HFCV, the volume of the 90 MPa tank is not considered for calculating the hydrogen storage capacity (volumetric density, kg/m3) due to the data lacking of the tank for HRS. For the high-pressure hydrogen gas, the volumetric density does not change with the pressure increasing as linear relation. Taking account of several mass density expression about pressurized hydrogen gas [64e66], the gas compression factor Z from the NIST database is introduced to calculate the mass density. Z and hydrogen mass can be calculated by equations (9) and (10), respectively. z¼1þ
m¼
aP T
2PV ZRT
(9)
(10)
Where a is the gas compression factor coefficients, which is equal to 1.9155 106 K/Pa. V is the volume of pure pressurized hydrogen gas. The process of hydrogen refueling to HFCV from HRS is shown in Fig. 11. The heat from the tank surface is the chief heat exchange channel, and the internal heat transfer system is also designed. The temperature for hydrogen storage through alloy absorption in HRS is 273 K, which is cooled through the low temperature heat conducting oil (cooled by refrigerator). The heat transfer coefficient for heat exchange system is 1000 W/m2/K. When the temperature increases to 338 K (electric heating heat conduction medium), the pressure of the hydrogen storage tank will be in the range of 70 and 90 MPa. The volume expansion of hydrogen storage alloy after hydrogen absorption is 25% [67]. So the volume of pure pressurized hydrogen gas is by the total volume minus the 125 percentages volume of hydrogen storage alloy. The alloy volumetric fraction x in the tank impacts the hydrogen storage capacity obviously in the reasonable ranges. The maximum hydrogen pressure of the most HRS in China can be up to 45 MPa, and the hydrogen pressure needs to be further increased. So the 45, 70, 90 MPa hydrogen pressures supplied by the compressor in HRS are considered in the next calculations. Fig. 12 shows the HRS hydrogen gravimetric density (a) and capacity increase percentages (b) of different x. From Fig. 12 (a), the hydrogen storage capacity increases at the any same x (0 < x 0.5) with the hydrogen pressure (supplied by compressor) increasing from 45 MPa to 90 MPa. When hydrogen pressure supplied by compressor is 45, 70, 90 MPa, the hydrogen storage capacity increases from 30.1, 41.4, 48.6 to 57.8 to 62.0, 64.7 kg/m3 as x increasing from 0 to 0.5 (in Table 5), respectively. Comparing with pure pressurized gas tank, the maximum increase percentages of hydrogen capacity by high pressure hydrogen metal hydride tank are 91.8, 49.9, 33.2% at the different supplying pressures of 45, 70, 90 MPa, respectively as shown in Fig. 12(b). It should be noted that the effect on hydrogen capacity increase is weakened with the hydrogen pressure increasing supplied by compressor. Fig. 13 shows effective hydrogen capacities for supplying high pressure more than 70 MPa at different alloy volumetric fractions. When the hydrogen pressure supplied by the compressor is 45 MPa at 273 K, the pure pressurized tank can't offer the high pressure hydrogen more than 70 MPa; when the
Fig. 12 e HRS hydrogen capacity vs Volumetric fraction of hydrogen storage alloy (a) HRS hydrogen capacity, (b) HRS hydrogen capacity density increase percentage.
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higher than 45 MPa supplied by compressor dramatically decreased, for which the compression cost will be saved. Therefore, due to the increase of effective hydrogen capacity, it offers a potential way to reduce the frequency of opening the compressor which can reduce the accident rate.
Conclusion
Fig. 13 e HRS effective hydrogen capacity for 70 MPa HFCV vs volumetric fraction of hydrogen storage alloy.
volume fraction x of hydrogen storage alloy is enclosed into the tank below 0.1, the tank still can't supply for the high pressure hydrogen more than 70 MPa (below the horizontal line in Fig. 13); as x increasing from 0.1 to 0.5, the tank can offer the high pressure hydrogen more than 70 MPa at 338 K from 3.6 kg/m3 (40 Nm3) to 40.0 kg/m3 (448 Nm3). When the hydrogen pressures supplied by the compressor are 70, 90 MPa, the maximum effective hydrogen capacity increases from 5.7 kg/m3 (64 Nm3), 12.9 kg/m3 (155 Nm3) to 44.2 kg/m3 (495 Nm3), 46.9 kg/m3 (525 Nm3) as x increasing from 0 to 0.5 (see in Table 5), respectively. In comparison to the pure pressurized gas tank, the maximum increase percentages of effective capacity by the high-pressure hydrogen metal hydride tank can be up to 677 and 262% at the hydrogen pressure supplied by the compressor about 70 and 90 MPa at 273 K respectively. Comparing with the current metal-hydride compressor including single, double-stage and even threestage compressor, product hydrogen with the higher pressure of 90 MPa can be obtained at lower temperature at 338 K by the alloys in this work, which is beneficial for saving the thermal energy cost to release hydrogen. Furthermore, the solid state hydrogen occupy the volume of the tank, which can be absorbed under the pressure of 45 MPa. The remainder volume of the tank filled by gaseous hydrogen can be reduced up to 75%. It means that the hydrogen with the pressure
Table 5 e Hydrogen storage capacity and effective hydrogen storage capacity of x ¼ 0 and 0.5. Hydrogen pressure by compressor
x ¼ 0 (kg/ m3) Cp
Ceff-p
Cp
Ceff-p
45 70 90
30.1 41.4 48.6
0 5.7 12.9
57.8 62.0 64.7
40 44.2 46.9
x ¼ 0.5 (kg/m3)
* Cp, Ceff-p represent the hydrogen storage capacity and effective hydrogen storage capacity, respectively.
In this paper, hydrogen storage properties of Ti1.02Cr2-x-yFexMny (0.6 x 0.75, y ¼ 0.25, 0.3) alloys have been studied. The main phase structure of all samples before and after heat treatment at 1273 k for 2 h is C14 type Laves phase, and the little secondary phase is reduced or even eliminated completely after heat treatment. The parameters of cell lattice decrease and hydrogen desorption plateau pressure increase with Fe stoichiometry content increasing. As Fe stoichiometry in the range between 0.6 and 0.75, hydrogen desorption plateau slopes are all less than 0.5 at 233 K. Comparing with Ti1.02Cr0.95Fe0.75Mn0.3 alloy, Ti1.02Cr1.0Fe0.75Mn0.25 alloy has better hydrogen desorption plateau slope, which is beneficial for promoting the reversible hydrogen storage capacity and operation in the HRS. The reversible hydrogen storage capacities can remain 1.55 wt% when the temperature is higher than 233 K. The Ti1.02Cr0.95Fe0.75Mn0.3 and Ti1.02Cr1.0Fe0.75Mn0.25 alloys, whose △Hd are about 14 kJ/mol H2, are suitable for high-hydrogen pressure metal hydride tank. In comparison to the pure pressurized gas tank, the effective hydrogen capacities (more than 70 MPa) are promoted by the high pressure metal hydride tank at different pressure supplied by compressor, but the maximum increase percentages of effective capacity decrease with the hydrogen supplying pressure increasing.
Acknowledgement The authors acknowledge Beijing Municipal Commission of Science and Technology (Z17110000091702), National Natural Science Foundation of China (No. 51431001, 51771002, U1607126, 21321001 and 51804174) and Natural Science Foundation of Shandong Province (ZR2017BEE010).
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