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A new strategy for remarkably improving anti-disproportionation performance and cycling stabilities of ZrCo-based hydrogen isotope storage alloys by Cu substitution and controlling cutoff desorption pressure
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 ) 2 8 2 4 2 e2 8 2 5 1
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A new strategy for remarkably improving antidisproportionation performance and cycling stabilities of ZrCo-based hydrogen isotope storage alloys by Cu substitution and controlling cutoff desorption pressure Zhaoqing Liang a, Xuezhang Xiao a,**, Zhendong Yao a, Huaqin Kou b,***, Wenhua Luo b,c, Changan Chen b,c, Lixin Chen a,b,d,* a
State Key Laboratory of Silicon Materials; School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b Institute of Materials, China Academy of Engineering Physics, Mianyang, 621907, PR China c State Key Laboratory of Surface Physics and Chemistry, Mianyang 621907, PR China d Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou, 310013, PR China
highlights Dehydriding path of ZrCo1-xCux-H (x ¼ 0e0.3) systems is altered as two-step desorption. A new middle hydride of ZrCoH0.8 phase with orthorhombic structure is discovered. Optimum cycling stable capacity of ZrCo-based alloy is increased to 1.21 wt % after Cu substitution. Cyclic stability of ZrCo0.8Cu0.2 alloy is enhanced by controlling cutoff desorption pressure.
article info
abstract
Article history:
ZrCo1-xCux (x ¼ 0e0.3) alloys for hydrogen isotope storage were prepared via induction
Received 6 July 2019
levitation melting. The effects of partial substitution of Cu for Co on the microstructure,
Received in revised form
hydrogen storage properties including hydriding-dehydriding kinetics, thermodynamic
4 September 2019
characteristics, cycling stability and related mechanism have been systematically inves-
Accepted 9 September 2019
tigated. It is found that all synthesized alloy ingots consist of ZrCo main phase, the grain
Available online 2 October 2019
size is further refined but the segregation of Cu element at grain boundary is more serious with the increase of Cu content. The pressure-composition isotherms for dehydrogenation
Keywords:
show that both ZrCoH3 and ZrCo0.9Cu0.1H3 hydrides undergo one-step desorption, while
ZrCo-based alloys
ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3 hydrides experience two-step desorption since a new
Cu partial substitution
middle hydride phase of ZrCoH0.8 with CrB-type orthorhombic structure is discovered
Hydrogen isotope storage
during their dehydrogenation. This result is proposed to be linked with the changed
Cycling stability
electronic structure and lattice distortion induced by Cu substitution. To compare their dehydriding kinetics, the apparent activation energies for hydrogen desorption of different
* Corresponding author. State Key Laboratory of Silicon Materials; School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Xiao), [email protected] (H. Kou), [email protected] (L. Chen). https://doi.org/10.1016/j.ijhydene.2019.09.077 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Anti-disproportionation performance
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hydride phases in the alloys are also calculated systematically, and the value of Ea for hydrogen desorption of ZrCoH3 phase is decreased from 118.02 kJ/mol to 95.90 kJ/mol, 77.98 kJ/mol and 78.65 kJ/mol, respectively. Prominent cycling stabilities of the Cusubstituted alloys are obtained and follow the trend: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 > ZrCo0.9Cu0.1 > ZrCo. Specifically, the optimum cycling stable capacity of ZrCo-based alloy is increased from 0.4 wt % to 1.21 wt %. Furthermore, the ZrCo0.8Cu0.2 alloy exhibits further enhanced cycling stability during the cycle by controlling cutoff desorption pressure at 0.253 bar, where only the first-step dehydrogenation happens. Therefore, a new strategy for improving anti-disproportionation performance of ZrCo-based alloys by controlling cutoff desorption pressure is also proposed, which is in favor of the enhanced cycling stability and further application of Cu-substituted ZrCo-based alloys for hydrogen isotope storage and delivery in the International Thermonuclear Experimental Reactor (ITER). © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Metal hydrides are considered as suitable hydrogen isotope storage materials to be applied to store and deliver tritium from tritium source to Tokamak vessel for subsequent reaction in International Thermonuclear Experimental Reactor (ITER) [1e3]. At present, uranium with favorable properties, such as low equilibrium pressure under room temperature, large hydrogen storage capacity and excellent hydrogenation kinetics, is used in storage, supply and recovery of tritium [4,5]. However, uranium still has some drawbacks including radiation, pyrophoricity and pulverization. Comparing to other promising hydrogen storage materials, ZrCo alloy shows incomparable advantages, such as low pyrophoricity, nonradiation, low equilibrium hydrogen absorption pressure under room temperature and prominent hydriding/dehydriding kinetics [6e8]. Despite these superior advantages, the most serious problem hindering the replacement of ZrCo for uranium is disproportionation reaction induced by hydrogen during hydrogen desorption, which leads to rapid decrease of valid hydrogen storage capacity of ZrCo alloy during cycles [9]. Based on previous research about disproportionation mechanism, it is known that disproportionation happens rapidly under high hydrogen pressure and temperature [10], while re-disproportionation happens under higher temperature [11,12]. As for modified method to restrain disproportionation from thermodynamics points, alloying modification including partial substitution of Zr with other elements, such as Hf [13e15], Ti [16e19] and Nb [20], is an effective method to retard disproportionation under high temperature. However, their hydrogen storage capacities still decline significantly during hydrogenation-dehydrogenation cycling. Recent studies showed that partial substitution of Co with Ni [21] or Fe [22,23] can improve cycling stability of ZrCo alloy to some extent. Both Ni and Fe elements are adjacent to Co element in the period table and have similar chemical properties with Co. Considering Cu is also adjacent to Co in the period table and chemical properties of these two elements are similar [24], the attempt of Cu substitution for Co in the ZrCo alloy deserves performing in order to improve its cycling stability. In this work, ZrCo1-xCux (x ¼ 0e0.3) alloys were prepared via
induction levitation melting, and the effects of partial substitution of Cu for Co on the microstructure and hydrogen storage properties were investigated in detail. Accordingly, the enthalpy change and apparent activation energy for dehydrogenation of ZrCo1-xCux-H systems were compared to explain their improved cycling stabilities. Usually, the dehydrogenation of ZrCo-based alloys in previous researches was conducted by dynamic vacuum, which shows less prominent cycling performance. Here, cycling life of ZrCo0.8Cu0.2 alloy under different cutoff desorption pressures (<0.001e0.253 bar) were investigated and compared, thus a new strategy for improving anti-disproportionation of ZrCo-based alloys by controlling cutoff desorption pressure was also proposed.
Experimental section ZrCo1-xCux (x ¼ 0, 0.1, 0.2, 0.3) alloy ingots were prepared respectively via induction levitation melting high purity zirconium (99.8%), cobalt (99.9%) and copper (99.95%) metal with corresponding stoichiometry in a water-cooled copper crucible under high purity argon atmosphere. Each ingot was turned over for re-melting four times to ensure homogeneity. For the initial activation, ZrCo1-xCux (x ¼ 0e0.3) ingots were respectively cut into pieces and vacuumed for 1 h at 500 C in a reactor. When the reactor temperature decreased to 100 C, about 1 MPa hydrogen was introduced into the reactor and maintained for several hours to complete the activation. After activation, the obtained alloy powders were dehydrogenated at 550 C for 1 h under dynamic vacuum environment. The constituent phases and compositions of the alloy samples were characterized by X-ray diffraction (XRD) using a Rigaku D/max-3B diffractometer with Cu Ka radiation. The microstructures of the alloys were observed by back scattered electron imaging (BSE, Hitachi S 3400-I), and elements distribution were described by energy dispersive X-ray spectrometer (EDS), respectively. The hydrogen absorption of ZrCo1-xCux (x ¼ 0e0.3) alloys was conducted in Sieverts-type equipment with ~1 bar high purity hydrogen at room temperature. The dehydrogenation pressure-composition isotherm (PCT) measurement for each
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sample was carried out on a self-made Sieverts-type equipment. During the measurement, adequate time should be given to ensure that hydrogen desorption at each step reaches equilibrium hydrogen pressure. Multiple hydrogen absorption-desorption cycles were conducted on each sample to compare cycling stability of ZrCo1-xCux (x ¼ 0e0.3) alloys. In favor of simulating practical application of ITER, each sample with similar weight was loaded into a stainless-steel reactor, and hydrogen absorption under high purity hydrogen of ~1 bar at room temperature and desorption under pre-evacuated reservoir (~1 L) at 380 C were conducted iteratively. Besides, the cycling under different conditions were carried out by controlling the cutoff desorption pressure. The specific method to control the cutoff desorption pressure is declared as follows. With consistent initial desorption condition (<0.001 bar), the cutoff desorption pressures for the same sample with different weight are different. Thus the cutoff desorption pressures can be controlled by controlling weight of samples. As we all know, each equilibrium pressure on the PCT curves corresponds to its own desorption capacity. After choosing a definitive point on the PCT curves as the cutoff desorption point, the experimented weight of samples can be calculated according to its known cutoff hydrogen pressure and desorption capacity. In order to find out the distinction of dehydriding kinetics of ZrCo1-xCux (x ¼ 0e0.3) alloys, differential scanning calorimetry (DSC) were carried out on a Netzsch STA449F3 analyzer at heating rates of 5, 8, 10 and 12 C/min in flowing pure argon gas.
Results and discussion Microstructure XRD patterns of the ZrCo1-xCux (x ¼ 0e0.3) as-cast alloys are shown in Fig. 1. All samples are identified as ZrCo phase with CsCl-type cubic structure corresponding to JCPDS files (No.180436). With increasing the Cu content, the diffraction peaks of ZrCo phase gradually shift to smaller angle direction, which indicates the increased lattice volume of ZrCo phase after Co partly substituted by Cu due to instability of lattice matching
[25]. The specific lattice parameters and cell volumes of ZrCo phase in these alloys were calculated via Rietveld refinement of XRD patterns, as listed in Table 1. It can be seen that the variation tendency of lattice parameters and cell volumes of ZrCo phase is in accordance with its diffraction peaks shifting tendency. Fig. 2(aec) shows the BSE micrographs and EDS mapping for Zr, Co and Cu elements of the ZrCo1-xCux (x ¼ 0.1e0.3) as-cast alloys. From BSE micrographs, the bulge part is grain boundary so that the grain size can be confirmed that is about 20e40 mm for ZrCo0.9Cu0.1 alloy and 5-10 mm for ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys. It can be seen that the grain size decreases with the increase of Cu content in the alloy, and a few second phase distributed along grain boundary can be found in ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys. Moreover, the segregation of Cu element at grain boundary can be discovered according to the denser and lighter green spots around the grain boundary in the EDS mapping of ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys, and the segregation degree increases with the increase of Cu content. The second phase, which is abundant in Cu element, was identified as ZrCo2 phase through spot scanning. It should be point out that the content of second phase in the alloys is too little to be detected by XRD detector.
Hydriding kinetics Excellent hydriding kinetics of ZrCo alloy is significant for ensuring operation of recovery and delivery of hydrogen isotope [26]. Fig. 3 shows the hydriding kinetics curves of the activated ZrCo1-xCux (x ¼ 0e0.3) alloys at room temperature and XRD patterns of their hydrides after fully hydrogenation. In this work, superior hydrogen absorption kinetics is still maintained in spite of Cu substitution, and the saturated hydrogen absorption capacities of these alloys are listed in Table 2. ZrCo1-xCux-H systems is mainly consisted of ZrCoH3 phase. Nevertheless, a little ZrH2 phase exists in ZrCo0.7Cu0.3H system, which may be generated during activation.
Thermodynamic characteristics The effects of Co partly substituted by Cu on the thermodynamic characteristics of ZrCo1-xCux alloys have been
Fig. 1 e (a) XRD patterns of ZrCo1-xCux (x ¼ 0e0.3) as-cast alloys and (b) Rietveld refinement of XRD pattern of ZrCo0.8Cu0.2 alloy.
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Table 1 e Lattice parameters and cell volumes of ZrCo phase in the ZrCo1-xCux (x ¼ 0e0.3) as-cast alloys. Alloys Lattice parameters ( A) Cell volumes ( A3) ZrCo ZrCo0.9Cu0.1 ZrCo0.8Cu0.2 ZrCo0.7Cu0.3
3.1973 3.1999 3.2020 3.2153
32.69 32.77 32.83 33.24
investigated systematically. Fig. 4(aed) shows the PCT curves of hydrogen desorption at different temperature for ZrCo1xCux-H (x ¼ 0e0.3) systems. Comparing their PCT curves at 380 C, as shown in Fig. 4(e), the dehydrogenation platforms of ZrCo1-xCuxH3 become higher and more inclined with increasing the Cu content. Usually, the shape of PCT curves
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including width and slope reveals the stability of hydride phase [27]. In this work, a lower but unapparent plateau appears on the PCT curves of ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3, and their a solid-solution region and the width of platform are shortened. According to the previous study [28], each hydride phase has its own plateau. Therefore, the existence of two platforms for ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3 implies twostep dehydrogenation including two kinds of hydride phases, which suggests variation of thermodynamic characteristics. The higher platform on PCT curves corresponds to the first-step dehydrogenation and the lower platform corresponds to the second-step dehydrogenation. In consideration of these unapparent low plateau region, equilibrium pressure of them is set to be the pressure corresponding to half capacity of the second-step dehydrogenation. To improve the accuracy of experimental results, higher temperature (410 C) was
Fig. 2 e BSE micrographs of the ZrCo1-xCux alloys and their corresponding EDS mapping for Zr, Co and Cu elements: (a) x ¼ 0.1, (b) x ¼ 0.2, (c) x ¼ 0.3. The inset tables in picture (a), (b), (c) are the results of spot scanning at point A1, A2, A3 inside a grain and point B1, B2, B3 at grain boundary, respectively.
Fig. 3 e (a) Hydrogenation kinetics curves of the activated ZrCo1-xCux (x ¼ 0e0.3) alloys at room temperature and (b) XRD patterns of their corresponding hydrides.
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Table 2 e Thermodynamic characteristics for dehydrogenation of ZrCo1-xCux-H (x ¼ 0e0.3) systems and their maximum hydrogen storage capacities. Paa) (Pa)
Samples ZrCo ZrCo0.9Cu0.1 ZrCo0.8Cu0.2 PHc) PLd) ZrCo0.7Cu0.3 PH PL a) b) c) d)
320 C 9326 320 C 8472 320 C 11100 1325 350 C 27671 1877
350 C 19900 350 C 18780 350 C 26000 3250 380 C 57557 3802
380 C 50024 380 C 47546 380 C 53890 7194 410 C 115054 11059
DH (kJ$mol1 H2)
DS (kJ$K1$mol1 H2)
Cmaxb) (wt. %)
87.28
223.3
1.91
90.7
228.3
1.91
82.1 88.1
216.4 207.9
69 97.7
195.1 219.4
1.9
1.85
Pa is the equilibrium platform pressure for hydrogen desorption. Cmax is the maximum capacity of hydrogen absorption. PH is the equilibrium platform pressure for the first-step dehydrogenation. PL is the equilibrium platform pressure for the second-step dehydrogenation.
chosen to calculate the low equilibrium plateau pressure for ZrCo0.7Cu0.3 sample. On the basis of equilibrium plateau pressure for ZrCo1xCux-H (x ¼ 0e0.3) systems, the relationship between temperature and the equilibrium plateau pressure is described by the Van't Hoff plots, as displayed in Fig. 4(f). The equation between lnP and 1000/T has been established as: ln P ¼ DH=RT þ DS=R
(1)
where DH and DS refer to the dissociation enthalpy and entropy for dehydrogenation reactions of ZrCo1-xCux-H (x ¼ 0e0.3) systems. The specific thermal parameters including DH and DS are listed in Table 2, which are obtained
from the fitting curves of Van't Hoff plots. The calculated dissociation enthalpy of ZrCoeH system is 87.28 kJ/mol in this study, which is close to the previous research [25,29]. Comparing to ZrCoeH system, the dissociation enthalpy of ZrCo0.9Cu0.1-H system is slightly increased, which implies that ZrCo0.9Cu0.1H3 phase is more stable. For ZrCo0.8Cu0.2-H and ZrCo0.7Cu0.3-H systems, the dissociation enthalpy of first-step dehydrogenation is sharply decreased while that of secondstep dehydrogenation is sharply increased with the increasing Cu content. This variation trend of enthalpy demonstrates that hydrogen desorption becomes easier in firststep dehydrogenation but more difficult in second-step dehydrogenation when Cu substitution content exceeds a
Fig. 4 e (aee) Pressure-composition isotherms and (f) Van't Hoff plots for dehydrogenation of ZrCo1-xCux-H (x ¼ 0e0.3) systems.
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Fig. 5 e XRD curves of the dehydrogenated samples of ZrCo1-xCux-H (x ¼ 0e0.3) systems desorbed to different cutoff pressure corresponding to site P1 (a), P2 (b) and P3 (c) on the desorption PCT curves (Fig. 4(e)).
certain value. It should be related with the lattice distortion [30] of ZrCo1-xCux-H systems induced by Cu substitution.
Dehydriding phases To find out the detailed process of hydrogen desorption for Cu-substituted samples, phase constituent at each of the sites marked as P1, P2 and P3 on the PCT curves (Fig. 4(e)) are measured by XRD respectively, as shown in Fig. 5(aec). The corresponding phase compositions at sites of P1, P2 and P3 are listed in Table 3. It is worth noting that the diffraction peaks of ZrCoH3 phase in Fig. 5(a) shift to larger angle slightly as a results of lattice shrinkage after desorbing some hydrogen atoms, compared with its standard diffraction peaks (No. 330416). Obviously, only one kind of hydride phase (ZrCoH3) exists in the desorption PCT curves of ZrCo and ZrCo0.9Cu0.1 alloys. However, except for ZrCoH3 hydride, a new middle hydride with CrB-type orthorhombic structure [31,32] is discovered in the PCT curves of ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples, which corresponds with their dual plateau characteristics. Referred from ICSD files (No.1619859), the lattice parameters of the middle hydride phase are listed as a ¼ 0.33319(3) nm, b ¼ 1.0301(1) nm and c ¼ 0.44111(4) nm. In addition, single middle hydride phase can be obtained when their cutoff desorption pressure is controlled to be 0.253 bar at site P2 for ZrCo1-xCux (x ¼ 0.2, 0.3) alloys, as displayed Fig. 5(b). Hence, the hydrogen storage capacity of middle hydride phase is calculated to be approximately 0.8H (f. u.) on the basis of its dehydrogenated capacity at site P2, and its chemical formula is marked as ZrCoH0.8. In brief, ZrCoH3 and ZrCo0.9Cu0.1H3 alloys just undergo one-step dehydrogenation while ZrCo1xCuxH3 (x ¼ 0.2, 0.3) alloys experience two-step dehydrogenation, which is expressed as follows:
Step1: ZrCoH3/ZrCo þ H2
(2)
Step1: ZrCo0.9Cu0.1H3/ZrCo0.9Cu0.1þH2
(3)
Step1: ZrCo0.8Cu0.2H3/ZrCo0.8Cu0.2H0.8þH2 ZrCo0.8Cu0.2H0.8/ZrCo0.8Cu0.2þH2
Step2:
Step1: ZrCo0.7Cu0.3H3/ZrCo0.7Cu0.3H0.8þH2 ZrCo0.7Cu0.3H0.8/ZrCo0.7Cu0.3þH2
Step2:
(4)
(5)
Furthermore, comparing the dehydrogenated phases of ZrCo1-xCuxH3 (x ¼ 0.2, 0.3) at site P3, the existence of ZrCo0.7Cu0.3H0.8 phase illustrates that it is more stable than ZrCo0.8Cu0.2H0.8 phase, which accords with their calculated results of dissociation enthalpy. Hence, ZrCoH0.8 phase becomes more stable thermodynamically with the increase of Cu substitution content.
Cycling stability The effect of Co partly substituted by Cu on the cycling stability of ZrCo1-xCux (x ¼ 0.1e0.3) alloys was further investigated. Hydrogenation-dehydrogenation cycles of ZrCo1-xCuxH (x ¼ 0e0.3) systems were conducted for 50 times as before [20], and the hydrogen desorption capacity of each cycle was obtained, as shown in Fig. 6(a). Obviously, the hydrogen desorption capacities of all samples decline monotonously at different rates. After 50 cycles, the dehydrogenation capacity retention of ZrCo1-xCux-H (x ¼ 0e0.3) systems are 22.2%, 47.5%, 77.6%, 74.9%, respectively, and the final hydrogen desorption capacity follows the trend: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 > ZrCo0.9Cu0.1 > ZrCo. Compared with others’ research [20,21,23], the cycling stability, which represents anti-
Table 3 e Phase constituent of the dehydrogenated samples of ZrCo1-xCux-H (x ¼ 0e0.3) systems desorbed to different cutoff pressure. Samples
Phase constituent of the dehydrogenated samples at different cutoff pressure P1
ZrCo ZrCo0.9Cu0.1 ZrCo0.8Cu0.2 ZrCo0.7Cu0.3
ZrCoH3, ZrCoH3, ZrCoH3, ZrCoH3,
ZrCo, ZrCo2 ZrCo ZrCoH0.8 ZrCoH0.8, ZrH2
P2
P3
ZrCo, ZrCo2, ZrCoH3 ZrCo, ZrCoH3, ZrCo2 ZrCoH0.8 ZrCoH0.8, ZrH2
ZrCo, ZrCo2 ZrCo, ZrCo2 ZrCo ZrCo, ZrCoH0.8, ZrH2
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Fig. 6 e (a) Cycling dehydrogenation capacity curves of ZrCo1-xCux (x ¼ 0e0.3) alloys at 380 C and their corresponding XRD patterns after (b) 1st and (c) 50th dehydrogenation.
disproportionation property during the cycles, is significantly enhanced for Cu-substituted samples, and ZrCo0.8Cu0.2 sample shows the optimal cycling stability. Moreover, the phase composition of ZrCo1-xCux-H (x ¼ 0e0.3) systems after the 1st and 50th dehydrogenation was characterized by XRD, as shown in Fig. 6(b and c). It can be found that both ZrCo and ZrCo2 phases exist in the resultants of ZrCo1-xCux (x ¼ 0, 0.1) samples after the first desorption. Differently, ZrCoH3 phase still exists in the dehydrogenated resultants of ZrCo0.9Cu0.1 sample, implying that its thermodynamic stability has been improved in ZrCo0.9Cu0.1 sample, which is also verified from the dehydrogenation PCT curves
above. Main phase of ZrCoH0.8 along with a little ZrCo phase is detected after the first dehydrogenation when the replaced content of Cu reaches 20 at. %. It indicates that the main path of dehydrogenation during cycles is the first-step desorption for ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys, which is determined by their cutoff desorption pressure. After 50th dehydrogenation, ZrCo2 and ZrH2 phases coexist in all samples, which reveals that the decline of hydrogen desorption capacity is exactly ascribed to the generation of ZrCo2 and ZrH2 phases from disproportionation reaction. Except for disproportionation phase, a little ZrCo phase with worsen crystallinity exists in ZrCo sample, and Cu-substituted samples consist of
Fig. 7 e (aec) DSC curves of ZrCo1-xCux-H (x ¼ 0.1e0.3) systems at different heating rates and (d) their Kissinger plots for the first-step dehydrogenation. (eeg) DSC curves of ZrCo0.8Cu0.2H0.8 as well as ZrCo0.7Cu0.3H0.8 and (h) their Kissinger plots for the second-step dehydrogenation.
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ZrCoH0.8 main phase, which implies that the thermodynamic characteristics of ZrCo1-xCux-H (x ¼ 0.1e0.3) systems become consistent after multiple cycles. Moreover, the separate existence of ZrCoH0.8 phase, rather than ZrCo phase, as shown in Fig. 6(c), demonstrates that only first-step dehydrogenation happens afterwards for Cu-substituted alloys when desorbing hydrogen. With regard to the reason for generation of ZrCoH0.8 phase, it can be inferred from previous research [33,34] that partial substitution of Cu for Co can alter electronic structure of ZrCo alloy to a certain extent, thus ZrCoH3 hydride tends to form a new metastable dehydrogenated phase of ZrCoH0.8 preferentially during hydrogen desorption.
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Therefore, the decreased Ea for hydrogen desorption can guarantee that dehydrogenation goes on rapidly during the cycles, which is beneficial for restraining disproportionation reaction. Here, anti-disproportionation performance can be significantly improved during the first-step dehydrogenation with lower Ea for both ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples, which favors to their enhanced cycling stability. Furthermore, dissociation enthalpy of ZrCoH0.8 phase for both ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples can be further verified by calculating the integral area of endothermic peak from DSC curves, and the values are close to the results obtained from Van’ Hoff plots.
Improving anti-disproportionation property Dehydriding kinetics To further verify dehydrogenation paths and compare hydrogen desorption kinetics of ZrCo1-xCux-H (x ¼ 0.1e0.3) systems with that of ZrCoeH system, DSC measurements were conducted on different samples, and their apparent activation energy (Ea) of hydrogen desorption is calculated by Kissinger method [35], which can be described as follows: lnðb=T2p Þ ¼ Ea=RTp þ ln ðAR=Ea Þ
(6)
Where b, Tp, A and R represent heating rate, peak temperature related to the maximum desorption rate, the pre-exponential factor and gas constant, respectively. The values of Tp for ZrCo1-xCux-H (x ¼ 0.1e0.3) systems are obtained from the DSC curves at different heating rates (5, 8, 10, 12 K/min), as shown in Fig. 7(aec). Since the value of Tp for the second-step dehydrogenation in Fig. 7(b and c) is ambiguous, single phase of ZrCo0.8Cu0.2H0.8 and ZrCo0.7Cu0.3H0.8 are prepared to confirm the value of Tp by DSC analysis, as displayed in Fig. 7(e and f). Based on good linear relationship between ln(b/T2p) and 1/Tp shown in their Kissinger plots (Fig. 7(d, h)), the values of Ea for different samples are calculated and listed in Table 4. With the increase of Cu substitution content, Ea for hydrogen desorption of ZrCoH3 phase decreased from 118.02 kJ/mol to 95.90 kJ/ mol, 77.98 kJ/mol and 78.65 kJ/mol, respectively. However, Ea for hydrogen desorption of ZrCo0.8Cu0.2H0.8 and ZrCo0.7Cu0.3H0.8 phases are calculated to be 103.93 kJ/mol and 136.35 kJ/ mol, respectively. Phase and structural modification during disproportionation involving migration and rearrangement of atoms usually requires enormous energy [36]. Since disproportionation is just a side reaction during hydrogen desorption of ZrCo alloy, it can be reasonably inferred that the value of Ea kinetically for disproportionation reaction may be much larger than that for dehydrogenation (118.02 kJ/mol).
Table 4 e Apparent activation energy for the dehydrogenation of ZrCo1-xCux-H (x ¼ 0e0.3) systems. Samples
Ea (kJ/mol)
ZrCo ZrCo0.9Cu0.1
ZrCo0.8Cu0.2 ZrCo0.7Cu0.3
118.02 [37] 95.90 Ea1 (kJ/mol)
Ea2 (kJ/mol)
77.98 78.65
103.93 136.35
To further confirm that anti-disproportionation performance is greatly improved during the first-step dehydrogenation with lower Ea, cycling life of ZrCo0.8Cu0.2 alloy, which shows optimal cycling stability, under different cutoff desorption pressures were investigated. Three types of cycling capacity curves are obtained, as displayed in Fig. 8(a). And the initial cutoff desorption pressures of these cycling curves named C1, C2, C3 are respectively controlled at point S1 (0.253 bar), S2 (0.117 bar), S3 (<0.001 bar) on the hydrogen desorption PCT curve, as shown in the inset Fig. 8 (Sa). Specifica lly, the red cycling curve C2 is same to the curve shown in Fig. 5(a), whose initial cutoff desorption pressure (0.117 bar) reaches the second plateau region. Since the blue cycling curve C3 is obtained by carrying on dynamic vacuum during hydrogen desorption, its recorded value is hydrogen absorption capacity. According to the hydrogen desorption PCT curve in Fig. 8 (Sa), only first-step dehydrogenation happened during cycle C1 and the second-step dehydrogenation happened incompletely during cycle C2 while both two-step dehydrogenation happened completely during cycle C3. By comparison, the cycling capacities of cycle C3 are decreased sharply and continuously and the cycling capacities of cycle C2 are decreased slowly while almost no attenuation of capacities occurs during cycle C1. Hence, the cutoff desorption pressure for cycle C1 can be considered constant during the cycles, and it is reasonable to declare that the cutoff desorption pressure is controlled at 0.253 bar. The capacity decline is ascribed to disproportionation reaction, which can be proved again from the XRD patterns shown in Fig. 8(b). Hence, the antidisproportionation performance of ZrCo0.8Cu0.2 sample during cycle C1 is further improved. In other words, the antidisproportionation performance is significantly enhanced during the first-step dehydrogenation with lower Ea (77.98 kJ/ mol), while the disproportionation during cycle C2 and C3 mainly takes place in the second-step dehydrogenation with higher Ea (103.93 kJ/mol). As far as their enhanced cycling capacities concerned, it is ascribed to that the primary reaction of dehydrogenation for ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples is the first-step desorption, where anti-disproportionation property is significantly enhanced. And the stable cycling capacities of Cusubstituted samples is due to that only first-step dehydrogenation happens after multiple cycles when desorbing hydrogen, which has been confirmed by XRD analysis in Fig. 6(c) above.
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Fig. 8 e (a) Cycling capacity curves of ZrCo0.8Cu0.2 alloy desorbed to different cutoff pressure corresponding to point S1, S2 and S3 on the hydrogen desorption PCT curve in the inset (Sa) and (b) XRD patterns of these samples after 30th dehydrogenation.
In this work, not only the reason for enhanced cycling stability of Cu-substituted alloys is systematically declared, but also a new enhanced strategy of anti-disproportionation property is put forward that restricting the dehydrogenation step by controlling the cutoff desorption pressure.
Conclusions In this work, the influence of partial substitution of Co with Cu on the microstructure and hydrogen storage properties of ZrCo1-xCux (x ¼ 0.1e0.3) alloy have been systematically investigated. It is found that the grain size is further refined and Cu element segregates more significantly at the grain boundary with the increase of Cu content in the alloys. The hydrides of ZrCoH3 and ZrCo0.9Cu0.1H3 undergo one-step dehydrogenation, while ZrCo1-xCuxH3 (x ¼ 0.2, 0.3) hydrides experience two-step dehydrogenation since a new middle hydride phase of CrB-type ZrCoH0.8 with orthorhombic structure is discovered during dehydrogenation. With partial substitution of Cu for Co, the anti-disproportionation property is greatly improved, and the excellent cycling stability is obtained following the tendency: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 >ZrCo0.9Cu0.1 > ZrCo, which is attributed to their altered dehydrogenated thermodynamics and kinetics. Specifically, the optimum cycling stable capacity of ZrCo-based alloy is increased to 1.21 wt %. Above all, the disproportionation reaction of Cu-substituted alloys can be almost restrained by controlling cutoff desorption pressure, which is in favor of the cycling stability and further application of ZrCo-based alloys for the storage and delivery of hydrogen isotopes in ITER.
Acknowledgments This work is supported by the National Key Research and Development Project (2017YFE0301505) and the National Natural Science Foundation of China (51671173 and 51571179).
references
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A new strategy for remarkably improving antidisproportionation performance and cycling stabilities of ZrCo-based hydrogen isotope storage alloys by Cu substitution and controlling cutoff desorption pressure Zhaoqing Liang a, Xuezhang Xiao a,**, Zhendong Yao a, Huaqin Kou b,***, Wenhua Luo b,c, Changan Chen b,c, Lixin Chen a,b,d,* a
State Key Laboratory of Silicon Materials; School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b Institute of Materials, China Academy of Engineering Physics, Mianyang, 621907, PR China c State Key Laboratory of Surface Physics and Chemistry, Mianyang 621907, PR China d Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou, 310013, PR China
highlights Dehydriding path of ZrCo1-xCux-H (x ¼ 0e0.3) systems is altered as two-step desorption. A new middle hydride of ZrCoH0.8 phase with orthorhombic structure is discovered. Optimum cycling stable capacity of ZrCo-based alloy is increased to 1.21 wt % after Cu substitution. Cyclic stability of ZrCo0.8Cu0.2 alloy is enhanced by controlling cutoff desorption pressure.
article info
abstract
Article history:
ZrCo1-xCux (x ¼ 0e0.3) alloys for hydrogen isotope storage were prepared via induction
Received 6 July 2019
levitation melting. The effects of partial substitution of Cu for Co on the microstructure,
Received in revised form
hydrogen storage properties including hydriding-dehydriding kinetics, thermodynamic
4 September 2019
characteristics, cycling stability and related mechanism have been systematically inves-
Accepted 9 September 2019
tigated. It is found that all synthesized alloy ingots consist of ZrCo main phase, the grain
Available online 2 October 2019
size is further refined but the segregation of Cu element at grain boundary is more serious with the increase of Cu content. The pressure-composition isotherms for dehydrogenation
Keywords:
show that both ZrCoH3 and ZrCo0.9Cu0.1H3 hydrides undergo one-step desorption, while
ZrCo-based alloys
ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3 hydrides experience two-step desorption since a new
Cu partial substitution
middle hydride phase of ZrCoH0.8 with CrB-type orthorhombic structure is discovered
Hydrogen isotope storage
during their dehydrogenation. This result is proposed to be linked with the changed
Cycling stability
electronic structure and lattice distortion induced by Cu substitution. To compare their dehydriding kinetics, the apparent activation energies for hydrogen desorption of different
* Corresponding author. State Key Laboratory of Silicon Materials; School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Xiao), [email protected] (H. Kou), [email protected] (L. Chen). https://doi.org/10.1016/j.ijhydene.2019.09.077 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Anti-disproportionation performance
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hydride phases in the alloys are also calculated systematically, and the value of Ea for hydrogen desorption of ZrCoH3 phase is decreased from 118.02 kJ/mol to 95.90 kJ/mol, 77.98 kJ/mol and 78.65 kJ/mol, respectively. Prominent cycling stabilities of the Cusubstituted alloys are obtained and follow the trend: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 > ZrCo0.9Cu0.1 > ZrCo. Specifically, the optimum cycling stable capacity of ZrCo-based alloy is increased from 0.4 wt % to 1.21 wt %. Furthermore, the ZrCo0.8Cu0.2 alloy exhibits further enhanced cycling stability during the cycle by controlling cutoff desorption pressure at 0.253 bar, where only the first-step dehydrogenation happens. Therefore, a new strategy for improving anti-disproportionation performance of ZrCo-based alloys by controlling cutoff desorption pressure is also proposed, which is in favor of the enhanced cycling stability and further application of Cu-substituted ZrCo-based alloys for hydrogen isotope storage and delivery in the International Thermonuclear Experimental Reactor (ITER). © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Metal hydrides are considered as suitable hydrogen isotope storage materials to be applied to store and deliver tritium from tritium source to Tokamak vessel for subsequent reaction in International Thermonuclear Experimental Reactor (ITER) [1e3]. At present, uranium with favorable properties, such as low equilibrium pressure under room temperature, large hydrogen storage capacity and excellent hydrogenation kinetics, is used in storage, supply and recovery of tritium [4,5]. However, uranium still has some drawbacks including radiation, pyrophoricity and pulverization. Comparing to other promising hydrogen storage materials, ZrCo alloy shows incomparable advantages, such as low pyrophoricity, nonradiation, low equilibrium hydrogen absorption pressure under room temperature and prominent hydriding/dehydriding kinetics [6e8]. Despite these superior advantages, the most serious problem hindering the replacement of ZrCo for uranium is disproportionation reaction induced by hydrogen during hydrogen desorption, which leads to rapid decrease of valid hydrogen storage capacity of ZrCo alloy during cycles [9]. Based on previous research about disproportionation mechanism, it is known that disproportionation happens rapidly under high hydrogen pressure and temperature [10], while re-disproportionation happens under higher temperature [11,12]. As for modified method to restrain disproportionation from thermodynamics points, alloying modification including partial substitution of Zr with other elements, such as Hf [13e15], Ti [16e19] and Nb [20], is an effective method to retard disproportionation under high temperature. However, their hydrogen storage capacities still decline significantly during hydrogenation-dehydrogenation cycling. Recent studies showed that partial substitution of Co with Ni [21] or Fe [22,23] can improve cycling stability of ZrCo alloy to some extent. Both Ni and Fe elements are adjacent to Co element in the period table and have similar chemical properties with Co. Considering Cu is also adjacent to Co in the period table and chemical properties of these two elements are similar [24], the attempt of Cu substitution for Co in the ZrCo alloy deserves performing in order to improve its cycling stability. In this work, ZrCo1-xCux (x ¼ 0e0.3) alloys were prepared via
induction levitation melting, and the effects of partial substitution of Cu for Co on the microstructure and hydrogen storage properties were investigated in detail. Accordingly, the enthalpy change and apparent activation energy for dehydrogenation of ZrCo1-xCux-H systems were compared to explain their improved cycling stabilities. Usually, the dehydrogenation of ZrCo-based alloys in previous researches was conducted by dynamic vacuum, which shows less prominent cycling performance. Here, cycling life of ZrCo0.8Cu0.2 alloy under different cutoff desorption pressures (<0.001e0.253 bar) were investigated and compared, thus a new strategy for improving anti-disproportionation of ZrCo-based alloys by controlling cutoff desorption pressure was also proposed.
Experimental section ZrCo1-xCux (x ¼ 0, 0.1, 0.2, 0.3) alloy ingots were prepared respectively via induction levitation melting high purity zirconium (99.8%), cobalt (99.9%) and copper (99.95%) metal with corresponding stoichiometry in a water-cooled copper crucible under high purity argon atmosphere. Each ingot was turned over for re-melting four times to ensure homogeneity. For the initial activation, ZrCo1-xCux (x ¼ 0e0.3) ingots were respectively cut into pieces and vacuumed for 1 h at 500 C in a reactor. When the reactor temperature decreased to 100 C, about 1 MPa hydrogen was introduced into the reactor and maintained for several hours to complete the activation. After activation, the obtained alloy powders were dehydrogenated at 550 C for 1 h under dynamic vacuum environment. The constituent phases and compositions of the alloy samples were characterized by X-ray diffraction (XRD) using a Rigaku D/max-3B diffractometer with Cu Ka radiation. The microstructures of the alloys were observed by back scattered electron imaging (BSE, Hitachi S 3400-I), and elements distribution were described by energy dispersive X-ray spectrometer (EDS), respectively. The hydrogen absorption of ZrCo1-xCux (x ¼ 0e0.3) alloys was conducted in Sieverts-type equipment with ~1 bar high purity hydrogen at room temperature. The dehydrogenation pressure-composition isotherm (PCT) measurement for each
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sample was carried out on a self-made Sieverts-type equipment. During the measurement, adequate time should be given to ensure that hydrogen desorption at each step reaches equilibrium hydrogen pressure. Multiple hydrogen absorption-desorption cycles were conducted on each sample to compare cycling stability of ZrCo1-xCux (x ¼ 0e0.3) alloys. In favor of simulating practical application of ITER, each sample with similar weight was loaded into a stainless-steel reactor, and hydrogen absorption under high purity hydrogen of ~1 bar at room temperature and desorption under pre-evacuated reservoir (~1 L) at 380 C were conducted iteratively. Besides, the cycling under different conditions were carried out by controlling the cutoff desorption pressure. The specific method to control the cutoff desorption pressure is declared as follows. With consistent initial desorption condition (<0.001 bar), the cutoff desorption pressures for the same sample with different weight are different. Thus the cutoff desorption pressures can be controlled by controlling weight of samples. As we all know, each equilibrium pressure on the PCT curves corresponds to its own desorption capacity. After choosing a definitive point on the PCT curves as the cutoff desorption point, the experimented weight of samples can be calculated according to its known cutoff hydrogen pressure and desorption capacity. In order to find out the distinction of dehydriding kinetics of ZrCo1-xCux (x ¼ 0e0.3) alloys, differential scanning calorimetry (DSC) were carried out on a Netzsch STA449F3 analyzer at heating rates of 5, 8, 10 and 12 C/min in flowing pure argon gas.
Results and discussion Microstructure XRD patterns of the ZrCo1-xCux (x ¼ 0e0.3) as-cast alloys are shown in Fig. 1. All samples are identified as ZrCo phase with CsCl-type cubic structure corresponding to JCPDS files (No.180436). With increasing the Cu content, the diffraction peaks of ZrCo phase gradually shift to smaller angle direction, which indicates the increased lattice volume of ZrCo phase after Co partly substituted by Cu due to instability of lattice matching
[25]. The specific lattice parameters and cell volumes of ZrCo phase in these alloys were calculated via Rietveld refinement of XRD patterns, as listed in Table 1. It can be seen that the variation tendency of lattice parameters and cell volumes of ZrCo phase is in accordance with its diffraction peaks shifting tendency. Fig. 2(aec) shows the BSE micrographs and EDS mapping for Zr, Co and Cu elements of the ZrCo1-xCux (x ¼ 0.1e0.3) as-cast alloys. From BSE micrographs, the bulge part is grain boundary so that the grain size can be confirmed that is about 20e40 mm for ZrCo0.9Cu0.1 alloy and 5-10 mm for ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys. It can be seen that the grain size decreases with the increase of Cu content in the alloy, and a few second phase distributed along grain boundary can be found in ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys. Moreover, the segregation of Cu element at grain boundary can be discovered according to the denser and lighter green spots around the grain boundary in the EDS mapping of ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys, and the segregation degree increases with the increase of Cu content. The second phase, which is abundant in Cu element, was identified as ZrCo2 phase through spot scanning. It should be point out that the content of second phase in the alloys is too little to be detected by XRD detector.
Hydriding kinetics Excellent hydriding kinetics of ZrCo alloy is significant for ensuring operation of recovery and delivery of hydrogen isotope [26]. Fig. 3 shows the hydriding kinetics curves of the activated ZrCo1-xCux (x ¼ 0e0.3) alloys at room temperature and XRD patterns of their hydrides after fully hydrogenation. In this work, superior hydrogen absorption kinetics is still maintained in spite of Cu substitution, and the saturated hydrogen absorption capacities of these alloys are listed in Table 2. ZrCo1-xCux-H systems is mainly consisted of ZrCoH3 phase. Nevertheless, a little ZrH2 phase exists in ZrCo0.7Cu0.3H system, which may be generated during activation.
Thermodynamic characteristics The effects of Co partly substituted by Cu on the thermodynamic characteristics of ZrCo1-xCux alloys have been
Fig. 1 e (a) XRD patterns of ZrCo1-xCux (x ¼ 0e0.3) as-cast alloys and (b) Rietveld refinement of XRD pattern of ZrCo0.8Cu0.2 alloy.
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Table 1 e Lattice parameters and cell volumes of ZrCo phase in the ZrCo1-xCux (x ¼ 0e0.3) as-cast alloys. Alloys Lattice parameters ( A) Cell volumes ( A3) ZrCo ZrCo0.9Cu0.1 ZrCo0.8Cu0.2 ZrCo0.7Cu0.3
3.1973 3.1999 3.2020 3.2153
32.69 32.77 32.83 33.24
investigated systematically. Fig. 4(aed) shows the PCT curves of hydrogen desorption at different temperature for ZrCo1xCux-H (x ¼ 0e0.3) systems. Comparing their PCT curves at 380 C, as shown in Fig. 4(e), the dehydrogenation platforms of ZrCo1-xCuxH3 become higher and more inclined with increasing the Cu content. Usually, the shape of PCT curves
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including width and slope reveals the stability of hydride phase [27]. In this work, a lower but unapparent plateau appears on the PCT curves of ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3, and their a solid-solution region and the width of platform are shortened. According to the previous study [28], each hydride phase has its own plateau. Therefore, the existence of two platforms for ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3 implies twostep dehydrogenation including two kinds of hydride phases, which suggests variation of thermodynamic characteristics. The higher platform on PCT curves corresponds to the first-step dehydrogenation and the lower platform corresponds to the second-step dehydrogenation. In consideration of these unapparent low plateau region, equilibrium pressure of them is set to be the pressure corresponding to half capacity of the second-step dehydrogenation. To improve the accuracy of experimental results, higher temperature (410 C) was
Fig. 2 e BSE micrographs of the ZrCo1-xCux alloys and their corresponding EDS mapping for Zr, Co and Cu elements: (a) x ¼ 0.1, (b) x ¼ 0.2, (c) x ¼ 0.3. The inset tables in picture (a), (b), (c) are the results of spot scanning at point A1, A2, A3 inside a grain and point B1, B2, B3 at grain boundary, respectively.
Fig. 3 e (a) Hydrogenation kinetics curves of the activated ZrCo1-xCux (x ¼ 0e0.3) alloys at room temperature and (b) XRD patterns of their corresponding hydrides.
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Table 2 e Thermodynamic characteristics for dehydrogenation of ZrCo1-xCux-H (x ¼ 0e0.3) systems and their maximum hydrogen storage capacities. Paa) (Pa)
Samples ZrCo ZrCo0.9Cu0.1 ZrCo0.8Cu0.2 PHc) PLd) ZrCo0.7Cu0.3 PH PL a) b) c) d)
320 C 9326 320 C 8472 320 C 11100 1325 350 C 27671 1877
350 C 19900 350 C 18780 350 C 26000 3250 380 C 57557 3802
380 C 50024 380 C 47546 380 C 53890 7194 410 C 115054 11059
DH (kJ$mol1 H2)
DS (kJ$K1$mol1 H2)
Cmaxb) (wt. %)
87.28
223.3
1.91
90.7
228.3
1.91
82.1 88.1
216.4 207.9
69 97.7
195.1 219.4
1.9
1.85
Pa is the equilibrium platform pressure for hydrogen desorption. Cmax is the maximum capacity of hydrogen absorption. PH is the equilibrium platform pressure for the first-step dehydrogenation. PL is the equilibrium platform pressure for the second-step dehydrogenation.
chosen to calculate the low equilibrium plateau pressure for ZrCo0.7Cu0.3 sample. On the basis of equilibrium plateau pressure for ZrCo1xCux-H (x ¼ 0e0.3) systems, the relationship between temperature and the equilibrium plateau pressure is described by the Van't Hoff plots, as displayed in Fig. 4(f). The equation between lnP and 1000/T has been established as: ln P ¼ DH=RT þ DS=R
(1)
where DH and DS refer to the dissociation enthalpy and entropy for dehydrogenation reactions of ZrCo1-xCux-H (x ¼ 0e0.3) systems. The specific thermal parameters including DH and DS are listed in Table 2, which are obtained
from the fitting curves of Van't Hoff plots. The calculated dissociation enthalpy of ZrCoeH system is 87.28 kJ/mol in this study, which is close to the previous research [25,29]. Comparing to ZrCoeH system, the dissociation enthalpy of ZrCo0.9Cu0.1-H system is slightly increased, which implies that ZrCo0.9Cu0.1H3 phase is more stable. For ZrCo0.8Cu0.2-H and ZrCo0.7Cu0.3-H systems, the dissociation enthalpy of first-step dehydrogenation is sharply decreased while that of secondstep dehydrogenation is sharply increased with the increasing Cu content. This variation trend of enthalpy demonstrates that hydrogen desorption becomes easier in firststep dehydrogenation but more difficult in second-step dehydrogenation when Cu substitution content exceeds a
Fig. 4 e (aee) Pressure-composition isotherms and (f) Van't Hoff plots for dehydrogenation of ZrCo1-xCux-H (x ¼ 0e0.3) systems.
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Fig. 5 e XRD curves of the dehydrogenated samples of ZrCo1-xCux-H (x ¼ 0e0.3) systems desorbed to different cutoff pressure corresponding to site P1 (a), P2 (b) and P3 (c) on the desorption PCT curves (Fig. 4(e)).
certain value. It should be related with the lattice distortion [30] of ZrCo1-xCux-H systems induced by Cu substitution.
Dehydriding phases To find out the detailed process of hydrogen desorption for Cu-substituted samples, phase constituent at each of the sites marked as P1, P2 and P3 on the PCT curves (Fig. 4(e)) are measured by XRD respectively, as shown in Fig. 5(aec). The corresponding phase compositions at sites of P1, P2 and P3 are listed in Table 3. It is worth noting that the diffraction peaks of ZrCoH3 phase in Fig. 5(a) shift to larger angle slightly as a results of lattice shrinkage after desorbing some hydrogen atoms, compared with its standard diffraction peaks (No. 330416). Obviously, only one kind of hydride phase (ZrCoH3) exists in the desorption PCT curves of ZrCo and ZrCo0.9Cu0.1 alloys. However, except for ZrCoH3 hydride, a new middle hydride with CrB-type orthorhombic structure [31,32] is discovered in the PCT curves of ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples, which corresponds with their dual plateau characteristics. Referred from ICSD files (No.1619859), the lattice parameters of the middle hydride phase are listed as a ¼ 0.33319(3) nm, b ¼ 1.0301(1) nm and c ¼ 0.44111(4) nm. In addition, single middle hydride phase can be obtained when their cutoff desorption pressure is controlled to be 0.253 bar at site P2 for ZrCo1-xCux (x ¼ 0.2, 0.3) alloys, as displayed Fig. 5(b). Hence, the hydrogen storage capacity of middle hydride phase is calculated to be approximately 0.8H (f. u.) on the basis of its dehydrogenated capacity at site P2, and its chemical formula is marked as ZrCoH0.8. In brief, ZrCoH3 and ZrCo0.9Cu0.1H3 alloys just undergo one-step dehydrogenation while ZrCo1xCuxH3 (x ¼ 0.2, 0.3) alloys experience two-step dehydrogenation, which is expressed as follows:
Step1: ZrCoH3/ZrCo þ H2
(2)
Step1: ZrCo0.9Cu0.1H3/ZrCo0.9Cu0.1þH2
(3)
Step1: ZrCo0.8Cu0.2H3/ZrCo0.8Cu0.2H0.8þH2 ZrCo0.8Cu0.2H0.8/ZrCo0.8Cu0.2þH2
Step2:
Step1: ZrCo0.7Cu0.3H3/ZrCo0.7Cu0.3H0.8þH2 ZrCo0.7Cu0.3H0.8/ZrCo0.7Cu0.3þH2
Step2:
(4)
(5)
Furthermore, comparing the dehydrogenated phases of ZrCo1-xCuxH3 (x ¼ 0.2, 0.3) at site P3, the existence of ZrCo0.7Cu0.3H0.8 phase illustrates that it is more stable than ZrCo0.8Cu0.2H0.8 phase, which accords with their calculated results of dissociation enthalpy. Hence, ZrCoH0.8 phase becomes more stable thermodynamically with the increase of Cu substitution content.
Cycling stability The effect of Co partly substituted by Cu on the cycling stability of ZrCo1-xCux (x ¼ 0.1e0.3) alloys was further investigated. Hydrogenation-dehydrogenation cycles of ZrCo1-xCuxH (x ¼ 0e0.3) systems were conducted for 50 times as before [20], and the hydrogen desorption capacity of each cycle was obtained, as shown in Fig. 6(a). Obviously, the hydrogen desorption capacities of all samples decline monotonously at different rates. After 50 cycles, the dehydrogenation capacity retention of ZrCo1-xCux-H (x ¼ 0e0.3) systems are 22.2%, 47.5%, 77.6%, 74.9%, respectively, and the final hydrogen desorption capacity follows the trend: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 > ZrCo0.9Cu0.1 > ZrCo. Compared with others’ research [20,21,23], the cycling stability, which represents anti-
Table 3 e Phase constituent of the dehydrogenated samples of ZrCo1-xCux-H (x ¼ 0e0.3) systems desorbed to different cutoff pressure. Samples
Phase constituent of the dehydrogenated samples at different cutoff pressure P1
ZrCo ZrCo0.9Cu0.1 ZrCo0.8Cu0.2 ZrCo0.7Cu0.3
ZrCoH3, ZrCoH3, ZrCoH3, ZrCoH3,
ZrCo, ZrCo2 ZrCo ZrCoH0.8 ZrCoH0.8, ZrH2
P2
P3
ZrCo, ZrCo2, ZrCoH3 ZrCo, ZrCoH3, ZrCo2 ZrCoH0.8 ZrCoH0.8, ZrH2
ZrCo, ZrCo2 ZrCo, ZrCo2 ZrCo ZrCo, ZrCoH0.8, ZrH2
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Fig. 6 e (a) Cycling dehydrogenation capacity curves of ZrCo1-xCux (x ¼ 0e0.3) alloys at 380 C and their corresponding XRD patterns after (b) 1st and (c) 50th dehydrogenation.
disproportionation property during the cycles, is significantly enhanced for Cu-substituted samples, and ZrCo0.8Cu0.2 sample shows the optimal cycling stability. Moreover, the phase composition of ZrCo1-xCux-H (x ¼ 0e0.3) systems after the 1st and 50th dehydrogenation was characterized by XRD, as shown in Fig. 6(b and c). It can be found that both ZrCo and ZrCo2 phases exist in the resultants of ZrCo1-xCux (x ¼ 0, 0.1) samples after the first desorption. Differently, ZrCoH3 phase still exists in the dehydrogenated resultants of ZrCo0.9Cu0.1 sample, implying that its thermodynamic stability has been improved in ZrCo0.9Cu0.1 sample, which is also verified from the dehydrogenation PCT curves
above. Main phase of ZrCoH0.8 along with a little ZrCo phase is detected after the first dehydrogenation when the replaced content of Cu reaches 20 at. %. It indicates that the main path of dehydrogenation during cycles is the first-step desorption for ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 alloys, which is determined by their cutoff desorption pressure. After 50th dehydrogenation, ZrCo2 and ZrH2 phases coexist in all samples, which reveals that the decline of hydrogen desorption capacity is exactly ascribed to the generation of ZrCo2 and ZrH2 phases from disproportionation reaction. Except for disproportionation phase, a little ZrCo phase with worsen crystallinity exists in ZrCo sample, and Cu-substituted samples consist of
Fig. 7 e (aec) DSC curves of ZrCo1-xCux-H (x ¼ 0.1e0.3) systems at different heating rates and (d) their Kissinger plots for the first-step dehydrogenation. (eeg) DSC curves of ZrCo0.8Cu0.2H0.8 as well as ZrCo0.7Cu0.3H0.8 and (h) their Kissinger plots for the second-step dehydrogenation.
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ZrCoH0.8 main phase, which implies that the thermodynamic characteristics of ZrCo1-xCux-H (x ¼ 0.1e0.3) systems become consistent after multiple cycles. Moreover, the separate existence of ZrCoH0.8 phase, rather than ZrCo phase, as shown in Fig. 6(c), demonstrates that only first-step dehydrogenation happens afterwards for Cu-substituted alloys when desorbing hydrogen. With regard to the reason for generation of ZrCoH0.8 phase, it can be inferred from previous research [33,34] that partial substitution of Cu for Co can alter electronic structure of ZrCo alloy to a certain extent, thus ZrCoH3 hydride tends to form a new metastable dehydrogenated phase of ZrCoH0.8 preferentially during hydrogen desorption.
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Therefore, the decreased Ea for hydrogen desorption can guarantee that dehydrogenation goes on rapidly during the cycles, which is beneficial for restraining disproportionation reaction. Here, anti-disproportionation performance can be significantly improved during the first-step dehydrogenation with lower Ea for both ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples, which favors to their enhanced cycling stability. Furthermore, dissociation enthalpy of ZrCoH0.8 phase for both ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples can be further verified by calculating the integral area of endothermic peak from DSC curves, and the values are close to the results obtained from Van’ Hoff plots.
Improving anti-disproportionation property Dehydriding kinetics To further verify dehydrogenation paths and compare hydrogen desorption kinetics of ZrCo1-xCux-H (x ¼ 0.1e0.3) systems with that of ZrCoeH system, DSC measurements were conducted on different samples, and their apparent activation energy (Ea) of hydrogen desorption is calculated by Kissinger method [35], which can be described as follows: lnðb=T2p Þ ¼ Ea=RTp þ ln ðAR=Ea Þ
(6)
Where b, Tp, A and R represent heating rate, peak temperature related to the maximum desorption rate, the pre-exponential factor and gas constant, respectively. The values of Tp for ZrCo1-xCux-H (x ¼ 0.1e0.3) systems are obtained from the DSC curves at different heating rates (5, 8, 10, 12 K/min), as shown in Fig. 7(aec). Since the value of Tp for the second-step dehydrogenation in Fig. 7(b and c) is ambiguous, single phase of ZrCo0.8Cu0.2H0.8 and ZrCo0.7Cu0.3H0.8 are prepared to confirm the value of Tp by DSC analysis, as displayed in Fig. 7(e and f). Based on good linear relationship between ln(b/T2p) and 1/Tp shown in their Kissinger plots (Fig. 7(d, h)), the values of Ea for different samples are calculated and listed in Table 4. With the increase of Cu substitution content, Ea for hydrogen desorption of ZrCoH3 phase decreased from 118.02 kJ/mol to 95.90 kJ/ mol, 77.98 kJ/mol and 78.65 kJ/mol, respectively. However, Ea for hydrogen desorption of ZrCo0.8Cu0.2H0.8 and ZrCo0.7Cu0.3H0.8 phases are calculated to be 103.93 kJ/mol and 136.35 kJ/ mol, respectively. Phase and structural modification during disproportionation involving migration and rearrangement of atoms usually requires enormous energy [36]. Since disproportionation is just a side reaction during hydrogen desorption of ZrCo alloy, it can be reasonably inferred that the value of Ea kinetically for disproportionation reaction may be much larger than that for dehydrogenation (118.02 kJ/mol).
Table 4 e Apparent activation energy for the dehydrogenation of ZrCo1-xCux-H (x ¼ 0e0.3) systems. Samples
Ea (kJ/mol)
ZrCo ZrCo0.9Cu0.1
ZrCo0.8Cu0.2 ZrCo0.7Cu0.3
118.02 [37] 95.90 Ea1 (kJ/mol)
Ea2 (kJ/mol)
77.98 78.65
103.93 136.35
To further confirm that anti-disproportionation performance is greatly improved during the first-step dehydrogenation with lower Ea, cycling life of ZrCo0.8Cu0.2 alloy, which shows optimal cycling stability, under different cutoff desorption pressures were investigated. Three types of cycling capacity curves are obtained, as displayed in Fig. 8(a). And the initial cutoff desorption pressures of these cycling curves named C1, C2, C3 are respectively controlled at point S1 (0.253 bar), S2 (0.117 bar), S3 (<0.001 bar) on the hydrogen desorption PCT curve, as shown in the inset Fig. 8 (Sa). Specifica lly, the red cycling curve C2 is same to the curve shown in Fig. 5(a), whose initial cutoff desorption pressure (0.117 bar) reaches the second plateau region. Since the blue cycling curve C3 is obtained by carrying on dynamic vacuum during hydrogen desorption, its recorded value is hydrogen absorption capacity. According to the hydrogen desorption PCT curve in Fig. 8 (Sa), only first-step dehydrogenation happened during cycle C1 and the second-step dehydrogenation happened incompletely during cycle C2 while both two-step dehydrogenation happened completely during cycle C3. By comparison, the cycling capacities of cycle C3 are decreased sharply and continuously and the cycling capacities of cycle C2 are decreased slowly while almost no attenuation of capacities occurs during cycle C1. Hence, the cutoff desorption pressure for cycle C1 can be considered constant during the cycles, and it is reasonable to declare that the cutoff desorption pressure is controlled at 0.253 bar. The capacity decline is ascribed to disproportionation reaction, which can be proved again from the XRD patterns shown in Fig. 8(b). Hence, the antidisproportionation performance of ZrCo0.8Cu0.2 sample during cycle C1 is further improved. In other words, the antidisproportionation performance is significantly enhanced during the first-step dehydrogenation with lower Ea (77.98 kJ/ mol), while the disproportionation during cycle C2 and C3 mainly takes place in the second-step dehydrogenation with higher Ea (103.93 kJ/mol). As far as their enhanced cycling capacities concerned, it is ascribed to that the primary reaction of dehydrogenation for ZrCo0.8Cu0.2 and ZrCo0.7Cu0.3 samples is the first-step desorption, where anti-disproportionation property is significantly enhanced. And the stable cycling capacities of Cusubstituted samples is due to that only first-step dehydrogenation happens after multiple cycles when desorbing hydrogen, which has been confirmed by XRD analysis in Fig. 6(c) above.
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Fig. 8 e (a) Cycling capacity curves of ZrCo0.8Cu0.2 alloy desorbed to different cutoff pressure corresponding to point S1, S2 and S3 on the hydrogen desorption PCT curve in the inset (Sa) and (b) XRD patterns of these samples after 30th dehydrogenation.
In this work, not only the reason for enhanced cycling stability of Cu-substituted alloys is systematically declared, but also a new enhanced strategy of anti-disproportionation property is put forward that restricting the dehydrogenation step by controlling the cutoff desorption pressure.
Conclusions In this work, the influence of partial substitution of Co with Cu on the microstructure and hydrogen storage properties of ZrCo1-xCux (x ¼ 0.1e0.3) alloy have been systematically investigated. It is found that the grain size is further refined and Cu element segregates more significantly at the grain boundary with the increase of Cu content in the alloys. The hydrides of ZrCoH3 and ZrCo0.9Cu0.1H3 undergo one-step dehydrogenation, while ZrCo1-xCuxH3 (x ¼ 0.2, 0.3) hydrides experience two-step dehydrogenation since a new middle hydride phase of CrB-type ZrCoH0.8 with orthorhombic structure is discovered during dehydrogenation. With partial substitution of Cu for Co, the anti-disproportionation property is greatly improved, and the excellent cycling stability is obtained following the tendency: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 >ZrCo0.9Cu0.1 > ZrCo, which is attributed to their altered dehydrogenated thermodynamics and kinetics. Specifically, the optimum cycling stable capacity of ZrCo-based alloy is increased to 1.21 wt %. Above all, the disproportionation reaction of Cu-substituted alloys can be almost restrained by controlling cutoff desorption pressure, which is in favor of the cycling stability and further application of ZrCo-based alloys for the storage and delivery of hydrogen isotopes in ITER.
Acknowledgments This work is supported by the National Key Research and Development Project (2017YFE0301505) and the National Natural Science Foundation of China (51671173 and 51571179).
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