First-principles investigation on the role of interstitial site preference on the hydrogen-induced disproportionation of ZrCo and its doped alloys
Graphical abstract
Introduction
Compared with traditional hydrogen storage methods in gaseous or liquid states, the solid-state hydrogen storage is typically characterized by the form of reversible metal hydrides, which has been verified to be more reliable for storing abundant hydrogen and its isotopes with high efficiency and compactness [1,2]. As a type of hydrogen storage material for replacing metal uranium (U), zirconium-cobalt (ZrCo) is not radioactive and pyrophoric at room temperature, and its hydrogen equilibrium pressure is fairly low (about 10−3 Pa). Especially at 673 K, such a pressure reaches up to 105 Pa with excellent hydrogen absorption and desorption kinetics [3]. Therefore, ZrCo is deemed as one of the promising materials for International Thermonuclear Experimental Reactor (ITER) to store and recover hydrogen isotopes (deuterium and tritium) [[4], [5], [6]]. However, in comparison with uranium, ZrCo exhibits poor thermal cycling performance under high temperature, which becomes the biggest obstacle for its practical applications [7]. During hydrogen absorption process, H atoms enter into the 3c and 12i sites, and consequently there is the occurrence of α-phase ZrCoHx (x = 0–1), the unit cell structure of which is characterized by the body-centered cubic structure (BCC) with no phase transition. By increasing the hydrogen amount till H/ZrCo >1, the lattice distortion degree is aggravated gradually, and in turn the heat generated by the exothermal reaction makes the hydride temperature rise. Under the coupling action between lattice distortion and high temperature, the mechanical stress combined with thermal stress lead to material phase transition, thereby generating β-phase ZrCoHx (x = 1–3) with orthorhombic crystal structure, where H may occupy 4c2, 8f1, and 8e sites. When there are ZrCo2 and ZrH2 phases with superior thermal stability being generated under the condition of repeated hydrogen absorption-desorption cycles [8,9], the disproportionation reaction occurs, which is given to:
In fact, this behavior is mainly induced by high temperature combined with equilibrium hydrogen pressure [10,11]. Due to the presence of ZrH2 phase with preferable thermal stability and the ZrCo2 phase without hydrogen absorption, the above-mentioned reaction is irreversible in nature under normal operating temperatures [12,13]. As a result, after multiple hydrogen charging and discharging cycles, the disproportionation reaction leads to a decrease in the amount of ZrCo alloys, which directly affect overall hydrogen storage performance. Konishi et al. [14] found that if disproportionation products were heated to above 773 K in vacuum for several hours, the single-phase ZrCo was produced by the reverse reaction. Contrarily, Besserer et al. [15] experimentally revealed that there was the disproportionation reaction occurring from room temperature to 673 K in the first 5 cycles, and its rate increased with increasing the number of cycles. After 7 thermal cycles, the disproportionation rate remained unchanged. Furthermore, the hydrogen storage capacity decreases significantly beyond 25 cycles. It is evident that about 25 times is completely insufficient for the number of hydrogen absorption and desorption cycles in the ITER project [5,16]. Therefore, it is of great importance to improve hydrogen storage performance of ZrCo alloys for promoting practical applications, especially by enhancing the anti-disproportionation ability.
In recent years, much work committed to conducting experimental studies on the partial substitution of Zr or Co by the third alloy, like as Ti, Hf, Cu, and Nb [[17], [18], [19], [20]], to restraint hydride disproportionation reaction occurrence. For example, Zhang et al. [19] used Ti, Ni, Sc, and Fe elements to partially replace Zr or Co, and prepared Zr0.8Ti0.2Co, ZrCo0.8Ni0.2, Zr0.8Sc0.2Co, and ZrCo0.8Fe0.2 by arc melting, respectively. Some parameters, such as lattice constant, phase structure, and thermal stability before and after the substitution, were measured availably. The disproportionation kinetics results showed that the Ti substitution could effectively improve hydride anti-disproportionation capability, while Fe, Sc, or Ni doping accelerated the disproportionation reaction. The disproportionation rate sequence was: ZrCo0.8Ni0.2 > Zr0.8Sc0.2Co > ZrCo0.8Fe0.2 > ZrCo. Besides, they believed that the unstable hydrogen position within the unit cell was the driving force for the disproportionation reaction, and the partial substitution of Zr or Co affected the anti-disproportionation performance by altering its stability. In addition, Peng et al. [20] focused on hydrogen storage properties of Zr(1-x)HfxCo (x = 0, 0.1, 0.2, and 0.3), demonstrating that when the temperature ranged from 673 to 823 K at approximately 200 kPa of hydrogen pressure, Zr0.7Hf0.3CoH3 possessed superior anti-disproportionation performance in comparison with ZrCoH3, because the plateau pressure in hydrogen absorption was higher and the corresponding hydride decomposition temperature was lower after the Hf doping. Recently, Weng et al. [21] conducted an investigation on the influence of Mn substitution for Co on the initial activation behavior and thermodynamics of ZrCo alloys. Wang et al. [22] employed the electroless plating method to reveal the catalyst effect of Pd coating on ZrCo hydriding kinetic property.
On the other hand, substantial attempts focused on hydrogen storage properties of ZrCo and its doped compounds from a perspective of microscopic scale using theoretical calculations. Chattaraj et al. [23] calculated structural, electronic, and thermodynamic properties of ZrCo and ZrCoH3 by the first-principles method, highlighting the role of Zr and Co on the bonding with H in resulting hydrides. Yang et al. [24,25] used the density functional theory to evaluate the anti-disproportionation ability of ZrCoH3 and its doped alloys from the cell level. The calculation results showed that the 4c2 and 8f1 sites were more energy-efficient than the 8e site, but none of the three positions occupied by H were as stable as ZrH2. Besides, the disproportionation ability of ZrCo after being replaced by various alloy elements was assessed comprehensively, by considering hydrogen diffusion process, 8e site character, and Zr–H bonding length.
Most experimental and theoretical studies indicated that disproportionation occurrence was likely due to the hydrogen specific interstitial site in the ZrCo unit cell, which had been partially confirmed by neutron diffraction experiments [[26], [27], [28]]. It was discovered from those experiments that the occurrence of disproportionation was attributed to the occupancy of H at the 8e site in ZrCoH3. The disproportionation reaction was also found to be dominated by the hydrogen storage capacity and the corresponding Zr–H bonding length. In other words, there was better anti-disproportionation ability when the hydrogen storage capacity was lower and the bonding length was longer in the 8e site.
In fact, most studies concentrated on investigating hydrogen storage properties associated with ZrCo and its doped alloys. However, very few of the models attempted to investigate the effect of hydrogen storage capacity on disproportionation performance in each interstitial site. Compared with previous studies, what the nicest feature the present study has is that it takes into account the effects of interstitial site and hydrogen storage content on ZrCo anti-disproportionation ability.
In this work, the first-principles calculation based on the density functional theory is performed to explore the influences of hydrogen storage content on formation enthalpy, lattice constant, and total density of states in each position, and in turn thermodynamic and lattice stabilities are evaluated comprehensively. In particular, the crystal performance when H occupies in the 8e site is analyzed in details to promote an understanding of hydrogen storage behavior. Furthermore, the Zr0.875Ti0.125Co and ZrCo0.875Rh0.125 compounds are formed by partial substitution of Zr and Co by Ti and Rh, and then resultant disproportionation performance is studied.
Section snippets
Modeling
In general, the first-principles calculations based on quantum mechanics are taken as an effective tool for accurately obtaining electronic structures by directly solving the Schrödinger equation without relying on any empirical and semi-empirical parameters [29]. In this work, crystal properties of ZrCo and its doping hydrides were investigated by the density functional theory (DFT), which was achieved through Cambridge Serial Total Energy Package (CASTEP) program in Material Studios software.
Results and discussion
For the analytical purpose, the 3c, 12i, 4c2, 8f1, and 8e sites that are occupied by the hydrogen are considered, which are represented by H(3c), H(12i), H(4c2), H(8f1), and H(8e), respectively. Also, ZrCoH(x, y) is used to represent the ZrCo hydride formed by H atoms occupying the y interstitial site, and the amount of hydrogen is x atom/f.u..
Conclusions
This study has conducted first-principles calculations to investigate the stability of ZrCo hydrides when H occupies each interstitial site, and then the anti-disproportionation performance of resultant hydrides when H is located at the 8e site is evaluated comprehensively. Besides, the bonding properties and TDOS of hydrides after the Ti/Rh doping are examined as well.
The computational results show that when H occupies the 3c or 8e site, the hydride crystal formation enthalpy is larger than
Acknowledgements
The authors gratefully acknowledge the financial support of the Venture & Innovation Support Program for Chongqing Overseas Returnees under grant number cx2018078, the Natural Science Foundation Project of CQ CSTC under grant number cstc2018jcyjAX0581, the Fundamental Research Funds for the Central Universities under grant number XDJK2018B002, and the Special Foundation for Distinguished Talents from Institute of Materials of CAEP.
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