Hydrogen storage properties of Zr1-xTixCo intermetallic compound

The intermetallic compound Zr1-xTixCo was prepared and its suitability for hydrogen storage was investigated.The alloys obtained by magnetic levitation melting with the composition of Zr1-xTixCo (x=0, 0.1, 0.2 and 0.3, at.%) show single cubic phase by X-ray diffraction.A single sloping plateau was observed on each isothermal, and pressure-composition-temperature (PCT) measurement results show that the equilibrium hydrogen desorption pressure of Zr1-xTixCo alloy increases with increasing Ti content.The desorption temperatures for supplying 100 kPa hydrogen are about 665, 642, 621 and 614 K for ZrCo, Zr0.9Ti0.1Co, Zr0.8Ti0.2Co and Zr0.7Ti0.3Co alloy, respectively.Repeated hydrogen absorption and desorption cycles do not generate separated ZrCo, TiCo and ZrH2 phases, indicating that alloys have good thermal and hydrogen stabilization.     

Parameter optimization of ZrCo–H system for durable tritium storage and delivery system of ITER

Cycling stability of ZrCo–H system is extremely important for the long-term operation of the storage and delivery system (SDS) in ITER. Herein, the optimal cycling operation parameters were systematically investigated. It indicates that various parameters, such as hydrogen pressure, temperature, composition, and stoichiometric ratio of H atoms, will all affect the cycling performance of the ZrCo–H system significantly. The decline rate of the hydrogen capacity of the ZrCo–H system is positively correlated with the hydrogen pressure. The experimental result shows that 54% of hydrogen capacity decreases under 28.1 kPa hydrogen pressure, while 30% of attenuation is obtained when the pressure is decreased to 8.1 kPa after 14 cycles. In terms of temperature, the lowest cycling attenuation can be maintained at about 25% after 14 cycles when the dehydrogenation temperature at 550 °C. The effects of doping elements, Hf and Ti, on the cycling stability of ZrCo–H system are also compared. The Zr_0.8Ti_0.2Co sample exhibits higher cycling capacity than ZrCo and Zr_0.8Hf_0.2Co samples. The extremely excellent behavior can be achieved when all ZrCo alloys are continuously evacuated during the hydrogen release process, and the attenuation of only 1.1% is observed for Zr_0.8Ti_0.2Co after 15 cycles. Besides, the cycling attenuation is related to residual stoichiometric ratio of H atoms in ZrCo alloy during the cycling test. When the residual H atoms proportion exceeds 1 in ZrCo during dehydrogenation, hydrogen cycling capacity hardly fades. The XRD results reveal that the disproportionation of ZrCo is directly associated with the cycling degradation, yielding the more stable products of ZrCo₂ and ZrH₂, However, the disproportionation can be avoided during the cycling process by controlling the stoichiometric ratio of H atoms remained in ZrCo above 1. This study demonstrates that the cycling performance of ZrCo can be substantially improved when the operation parameters are properly adjusted, which provides a significant important reference for durable running of SDS in ITER.     

New insights into the anti-disproportionation mechanism of ZrCo alloying with Ti,Hf, Sc,Cu, and Fe elements

First-principles calculations were performed to investigate the effects of alloying substitutions (i.e. Ti, Hf, Sc, Cu and Fe) on the anti-disproportionation ability of ZrCo alloy. For the first time, we revealed the disproportionation mechanism from the energy point of view and provided a new theoretical method to predict whether the substitution element has the ability to enhance the anti-disproportionation performance of ZrCo alloy. Based on the hydrogen atom occupancy behavior in ZrCoH₃ and the results of our calculation of binding energy, the hydrogen atom migration model during hydrogenation and theoretical computational model were established. Through structural optimization, a series of stable 2 × 1 × 2 ZrCoH₃ supercells were obtained, which contain four hydrogen atoms occupying 8e site and various substitution atoms with different amounts except for pure system. The binding energy of hydrogen atom in the 8e site and activation energy of diffusing from 8e site to 4c₂ site of these ZrCoH₃ supercells were calculated. The results showed that the substitution of Ti and Hf increased the binding energy of hydrogen atom in the 8e site, while the substitution of Fe, Cu and Sc decreased the binding energy of hydrogen atom in the 8e site. Meanwhile, both of Ti and Hf substitution reduced the activation energy of diffusing from 8e site to 4c₂ site, while all of Fe, Cu and Sc substitution increased the activation energy of diffusing from 8e site to 4c₂ site. These results indicated that hydrogen-induced disproportionation was the inherent property of ZrCo alloy, and element substitution can restrain or accelerate disproportionation by affecting both the binding energy and activation energy. With simultaneous consideration of the binding energy and activation energy, the effect of these alloying substitutions on the anti-disproportionation ability could be ascertained and the results were in good agreement with the previous theoretical and experimental results.     

Synergetic catalyst effect of Ni/Pd dual metal coating accelerating hydrogen storage properties of ZrCo alloy

Aimed at enhancing the hydrogen absorption/desorption performances of ZrCo system, Ni/Pd dual metal coating is employed on ZrCo alloy combined with the electroless plating and displacement plating. The effects of Ni/Pd dual metal coating on the microstructure, hydrogen storage performance of ZrCo alloys were investigated systematically. The results show that Ni/Pd dual metal coating deposits on the surface of ZrCo sample successfully with the thickness of 500 nm. The hydrogen absorption kinetic property is substantially enhanced for ZrCo alloy after Ni/Pd dual metal coating, which is owing to the catalytic effect of Ni/Pd coating. Further, the activation energies (E_a) for hydrogen absorption and desorption are calculated using the Arrhenius Equation and Kissinger method, respectively. Compared with the bare ZrCo, the activation energies of the Ni/Pd coated samples for hydriding/dehydriding process decrease which facilitate the hydrogenation/dehydrogenation reaction. This work introduces a rational approach by building new catalytic coating on the hydrogen storage materials to improve the hydriding/dehydriding kinetic performance.     

An analogy of interstitial site occupancy and hydrogen induced disproportionation of Zr1−xTixCo ternary alloys

The hydrogen induced disproportionation behavior of Ti-substituted ZrCo alloys was investigated to explore their suitability for International Thermonuclear Experimental Reactor (ITER) Storage and Delivery System (SDS). The isothermal disproportionation studies on Ti-substituted alloys were carried out in conditions simulating ITER SDS i.e. 750 K temperature and 100 kPa hydrogen pressure. It was observed that the rate of disproportionation of Ti-substituted ZrCo alloys was found to vary as ZrCo > Zr_0.9Ti_0.1Co > Zr_0.7Ti_0.3Co > Zr_0.8Ti_0.2Co. X-ray diffraction measurements revealed the formation of TiCoH phase along with Ti-substituted ZrCo₂ and ZrH₂ phases as a result of disproportionation reaction of alloys. Neutron diffraction measurements on deuterides indicated that deuterium occupancy in 8e site and the corresponding Zr-D distance provide the primary driving force for disproportionation of alloys to take place. A plausible potential energy profile based on thermodynamic and kinetic considerations was proposed to explain the disproportionation mechanism of alloys.     

Effects of Mo substitution on the kinetic and thermodynamic characteristics of ZrCo1-xMox (x = 0–0.2) alloys for hydrogen storage

In this study, ZrCo_1-xMo_x (x = 0, 0.05, 0.1, 0.15, 0.2) alloys were prepared via vacuum arc-melting method. The effects of substituting Co with Mo on the structure, initial activation behaviors, and thermodynamic properties of the afore-mentioned alloys were systematically investigated. The results showed that ZrCo_1-xMo_x (x = 0, 0.05, 0.1, 0.15) alloys exhibited a single ZrCo phase and their corresponding hydrides, a ZrCoH₃ phase. Furthermore, ZrCo_0.8Mo_0.2 alloy consisted of ZrCo phase and a trace of ZrMo₂ phase, and the hydride contained ZrCoH₃ and ZrH phases. As the Mo content was increased, the initial activation period decreased significantly from 19277 s for ZrCo to 576 s for ZrCo_0.8Mo_0.2, which was closely related to the catalytic effect of ZrMo₂. The plateau width of pressure composition temperature curves were shortened, and the equilibrium pressures of hydrogen desorption decreased slightly as Mo content increased. Additionally, the experiments showed that the anti-disproportionation performance was greatly improved by Mo substitution. The extent of disproportionation decreased from 64.28% for ZrCo to 24.11% for ZrCo_0.8Mo_0.2. The positive effect of Mo substitution on improving the anti-disproportionation property of ZrCo alloy was attributed to the reduction of hydrogen atom in 8f₂ and 8e sites, which decreased the driving force of the disproportionation reaction.     

Hydrogen-induced disproportionation characteristics of Zr(1 − x)Hf(x)Co(x = 0, 0.1, 0.2 and 0.3) alloys

Doping hafnium to partially substitute zirconium in ZrCo is a promising strategy to improve the ability to resist hydrogen-induced disproportionation. Herein, Zr(1 ? x)Hf(x)Co(x = 0,0.1,0.2, and 0.3) alloys were fabricated by arc melting and the effect of hafnium substitution ratio and temperature on their hydrogen-induced disproportionation was studied. Additionally, the disproportionated products were characterized by XRD, DSC and TDS. Results showed that disproportionation rate and the extent of disproportionation decreased with hafnium substitution ratio increasing from 0 to 30% and increased with temperature increasing from 400 °C to 550 °C. It was exciting that Zr0.7Hf0.3Co alloy had much better ability of anti-disproportionation than ZrCo in hydrogen pressure of about 200 kPa when temperature increasing from 400 °C to 550 °C, which was practical for tritium application.     

Zr1-xCoNbx (x = 0-0.2)合金吸放氢性能及抗歧化机制研究

采用真空电弧熔炼法制备了Zr_1-xCoNb_x (x = 0,0.05,0.1,0.15,0.2)合金,研究了Nb掺杂对合金晶体结构、吸放氢及抗歧化性能的影响。XRD结果表明：Zr_1-xCoNb_x (x = 0-0.2)合金主相为ZrCo相,含有少量ZrCo₂杂相;其氢化物为ZrCoH₃和ZrCo₂相。Nb掺杂极大地提高了合金吸氢动力学性能,ZrCo吸氢反应活化时间为7690 s,Zr_0.8CoNb_0.2缩短至380 s。ZrCo吸氢反应活化能为44.88 kJ mol^_-1 H₂,Zr_0.8CoNb_0.2降低至32.73 kJ mol^_-1 H₂,有利于吸氢反应动力学性能。DSC测量结果表明：ZrCo放氢温度为597.15 K,Zr_0.8CoNb_0.2降低至541.36 K,放氢温度降低,有利于抗歧化性能。ZrCo合金放氢反应活化能为100.55 kJ mol^_-1 H₂,Zr_0.8CoNb_0.2降低至84.58 kJ mol^_-1 H₂。合金歧化程度随着Nb掺杂量增加而降低,798 K保温10 h,ZrCo歧化83.68%,Zr_0.8CoNb_0.2仅歧化8.71%,Nb掺杂降低8f₂和8e位置氢原子数量,减小岐化反应驱动力。     

Ti元素改性ZrCo合金在CO杂质气氛中吸氢行为及影响机制

采用EDS和XRD表征了Ti改性ZrCo合金的相结构及表面元素分布,采用飞行时间二次离子质谱(TOF-SIMS)以及程控升温热解脱附(TPD)方法研究少量CO(1.05%CO+98.95%H_2气氛,体积分数,)对Zr_(0.8)Ti_(0.2)Co合金氢化行为的影响及作用机制。结果表明:在纯氢环境下ZrCo合金和Zr_(0.8)Ti_(0.2)Co合金饱和吸氢时间分别少于2和4 min,饱和吸氢容量分别为1.8%和1.9%(质量分数)。而在含1.05%CO的氢中ZrCo合金和Zr_(0.8)Ti_(0.2)Co合金在2500 min内均未能达到吸氢饱和,吸氢容量分别下降到0.91%和0.48%,Ti改性导致ZrCo合金在CO杂质气氛中的吸氢动力学性能下降。实验表明,通过773 K、0.5 h热抽空处理可恢复至毒化前吸氢性能。     

First-principles investigation on the role of interstitial site preference on the hydrogen-induced disproportionation of ZrCo and its doped alloys

There are two phase structures involved in ZrCo hydrides (ZrCoH_x). When x ≤ 1, the α-phase hydride is generated when hydrogen atoms occupy the 3c and 12i sites. When 1 < x ≤ 3, three interstitial sites of 4c₂, 8f₁, and 8e are occupied by H, and in turn the β-phase hydride is formed. There is a disproportionation reaction in β-phase hydrides during hydrogen discharging process to produce the ZrH₂ phase with higher thermal stability, leading to inferior hydrogen storage performance. In this study, the influence of hydrogen storage capacity on thermodynamic and lattice stabilities of α- and β-phase hydrides for each occupancy position is investigated under the framework of the first-principles study. The results indicate that the binding energy in the 3c site is higher compared with the 12i site under the condition of identical hydrogen storage capacity. Similarly, the binding energy is the largest for the 8e site compared with the other two sites, indicating that there is the least energy released in the reaction process. Thus, the 8e site is proved as the most unfavorable site in β-phase ZrCo hydrides, which is due to its degraded thermodynamic stability. Also, comparisons of mechanical properties and total density of states for each site in two hydride phases are presented to demonstrate that compound lattice stability in the 8e site is the poorest, suggesting that it is more likely to produce disproportionation. Furthermore, the dependence of hydrogen storage performance of β-phase hydrides on Ti/Rh doping is examined as well. It is discovered that there is improved thermodynamic stability and lattice stability in the 8e site for Zr_0.875Ti_0.125Co after Zr is partially substituted by Ti, which significantly enhances the disproportionation resistance. In contrast, when Co is partially replaced by Rh, there is a deterioration in the thermodynamic stability of ZrCo_0.875Rh_0.125 in the 8e site, but its lattice stability is somewhat improved.