Effects of Doping Fecl3 on Hydrogen Storage Properties of Li-N-H System

Progress in Natural Science: Materials International 27 (2017) 139–143

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Progress in Natural Science: Materials International

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Original Research ﬀ E ects of doping FeCl3 on hydrogen storage properties of Li-N-H system MARK ⁎ Weijin Zhanga,b, Han Wanga,b, Hujun Caoc, Teng Hea, Jianping Guoa, Guotao Wua, , Ping Chena a Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b University of Chinese Academy of Sciences, Beijing 100049, China c Institute of Materials Research, Materials Technology, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany

ARTICLE INFO ABSTRACT

Keywords: The eﬀects of doping FeCl3 on the LiNH2−2LiH system were investigated systematically. FeCl3 was prior to react Hydrogen storage with LiH during ball milling their mixtures. The metallic Fe, which is generated from metathesis reaction Iron trichloride between FeCl3 and LiH, plays an important role on improving the dehydrogenation kinetics of LiNH2−2LiH Lithium amide system. The results indicated that the dehydrogenation peak and ending temperatures of the doped 1 mol% Reaction kinetics FeCl sample shifted to low temperatures, and the dehydrogenation active energy changed from 102.45 kJ/mol Solid solution 3 to 87.52 kJ/mol. While increasing the amount of FeCl , an excess of LiCl, the by-product of metathesis reaction Lithium imide-chloride 3 between FeCl3 and LiH, can stabilize LiNH2 and thus hinder hydrogen desorption. The dehydrogenation product is a new solid cubic phase solution of lithium imide-chloride. The high limit of the solid solution of LiCl

and Li2NH is near the molar ratio of 1:1.

1. Introduction reversibly store more than 5 wt% H2 within a temperature range of 100–300 °C. The desorption enthalpy ΔHdes and entropy ΔSdes of It is over one decade since lithium nitride (Li3N) was firstly Mg(NH2)2−2LiH are 38.9 kJ/mol H2 and 112 J/(K mol H2), which proposed as a hydrogen storage material in 2002 [1].Li3N can allow hydrogen desorption at 90 °C under 1 bar H2 equilibrium reversibly storage ~11.4 wt% H2 via the heterolytic splitting of H2 by pressure [6]. a two-step reaction(Eq. (1)) involving imide (Li NH), amide (LiNH ) 2 2 2LiNH + MgH → Li Mg(NH) +2H ↔ Mg(NH ) + 2LiH (2) and hydride (LiH), and it is considered as one of the most promising 2 2 2 2 2 2 2 candidates for onboard application. However, hydrogen desorptions Some additives were also introduced into Li-N-H system, and − from LiNH2 2LiH and LiNH2-LiH are highly endothermic with showed good effects on thermodynamics and kinetics [7–12].In enthalpy changes of 80.5 and 66.1 kJ/mol H2 respectively, and only general, the enthalpy change of one reversible system can be decreased the second step (LiNH2-LiH) is considered for hydrogen storage due to by two means: destabilization of the reactants or stabilization of the its more favorable dehydrogenation enthalpy. The operating tempera- products [13]. Lithium ternary nitrides are usually more stable than ture of LiNH2-LiH system at 1.0 bar of equilibrium H2 pressure is Li3N, so the formation of ternary nitrides may be an effective way to above 250 °C. Therefore many researchers have been devoted to lower the dehydrogenation enthalpy of Li-N-H system. Until now, improving thermodynamics and kinetics of Li-N-H system [2,3]. kinds of lithium ternary nitrides, such as Li3AlN2 (Eq. (3)) [7,8], Li GaN [8],Li FeN [9],Li MnN and Li VN [10], have been studied Li N+2H ↔ Li NH + LiH + H ↔ LiNH + 2LiH (1) 3 2 3 2 7 4 7 4 3 2 2 2 2 for hydrogen storage. Hydrogen bonded with N in imides/amides is positively charged δ LiNH + 2LiH + AlN ↔ Li AlN +2H (3) (H +) due to its poor electronegativity. Conversely, hydrogen in ionic 2 3 2 2 δ− metal hydrides is negatively charged (H ). According to the solid-solid Ichikawa et al. [14] found that the reaction kinetics of Li-N-H δ+ δ− reaction mechanism which means that H reacts with H to produce system could be improved by addition of a small amount of transition ff H2, a variety of amide-hydride systems with di erent hydrogen metals or their chlorides, such as Fe, Co, VCl3 and TiCl3. Among them, capacities and thermodynamic properties have been developed [4,5]. the addition of 1 mol.% TiCl3 showed the best performance. According When LiNH2 is substituted with Mg(NH2)2 or LiH is substituted with to their further studies [15], it was found that the nano-sized Ti and − MgH2, Mg(NH2)2 2LiH system is formed (Eq. (2)) [6]. It can TiO2 had similar effects as TiCl3, but the micro-sized additives had little

Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (G. Wu). http://dx.doi.org/10.1016/j.pnsc.2016.12.017 Received 31 October 2016; Accepted 30 November 2016 Available online 20 January 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). W. Zhang et al. Progress in Natural Science: Materials International 27 (2017) 139–143 effects; they concluded that the uniform distribution of nano-sized Ti species between LiNH2 and LiH played an important role for catalytic effects. The chemical bonding states of the titanium species in the mixture of LiNH2 and LiH were characterized by X-ray absorption spectroscopy (XAS), which showed TiCl3 and nano-TiO2 were quite different from their original profiles [16]. They suggested the existence of TiCl3·5NH3,Li2TiO3 and Li2TiO4. Ionic conductivity measurements + showed that the mobility of the Li between LiNH2 and LiH solid phases was enhanced by adding Li2TiO3 and Li2TiO4 [17,18]. The transition metal chlorides have been found to improve hydrogen storage properties of Li-N-H system, but real chemical states of transition metal chlorides after doping are not clear. As early as 1995, Parkin et al. [19] reported the formation of transition-metal nitrides from the reactions of lithium amide and transition metal chlorides.

However, the details about the reaction between LiNH2 and FeCl3 were little. The effects of some halides added into Li-N-H and Li-Mg-N-H – systems have also been investigated [20 23]. This work is initiated to Fig. 1. TPD curves of LiNH −2LiH on different ramping rates (1.5 °C/min red, 2 °C/min ff 2 investigate e ects of the addition of FeCl3 on hydrogen storage green, 2.5 °C/min blue and 3 °C/min cyan) and Kissinger linear fitting. properties of Li-N-H system. And the effects of by-product LiCl on Li-N-H system are also investigated.

2. Experimental

The starting materials of iron trichloride (FeCl3, Alfa, 98%), lithium amide (LiNH2, Alfa, 95%), lithium hydride (LiH, Alfa, 97%) and lithium chloride (LiCl, J & K chemicals, 99%) were used without further purification. The mechanical ball milling was carried out with a Retsch PM400 planetary mill. The thermal decomposition properties of the samples were tested on a home-made temperature programmed desorption and mass spectrometer(HPR20, Hiden) combined system (TPD-MS). Thermogravimetric-differential thermal analysis (TG-DTA) performed by STA 449 F3 was used to evaluate the amount of desorbed hydrogen and the heat change of reaction process. Powder X-ray diffraction (XRD) was measured using an X′Pert Pro diffractometer with Cu Kα radiation at 40 kV and 40 mA. FTIR spectra were measured with a Varian 3100 FTIR spectrophotometer (Excalibar Series) in Fig. 2. TPD curves of LiNH −2LiH-0.01FeCl at different ramping rates (1.5 °C/min DRIFT mode (diffuse reflectance infrared Fourier transform). The cell 2 3 red, 2 °C/min green, 2.5 °C/min blue and 3 °C/min cyan) and Kissinger linear fitting. parameters were calculated with the software of Materials Studio 4.3. All the sample handles were carried out in a glove box filled with T is the peak temperature, β is the ramping rate, E is the purified argon (O < 10 ppm, H O < 0.1 ppm). m a 2 2 activation energy and R is the gas constant. As shown in Figs. 1 and 2 2, there exists a good linear dependency of ln(β/Tm ) upon 1/Tm for both 3. Results and discussion samples. The slope of the fitting line is used to calculate the reaction

active energy (Ea). The activation energy of LiNH2−2LiH is 102.45 kJ/ The mixtures of LiNH2−2LiH and LiNH2−2LiH-0.01FeCl3 (mol mol. The addition of 0.01 mol FeCl3 leads that the activation energy ratio) were ball milled for 24 h at a rotation rate of 200 rpm, and then reduces to 87.52 kJ/mol. temperature programmed desorption tests of these samples were High-energy ball milling decreases particle and crystallite sizes of conducted with diﬀerent ramping rates of 1.5, 2, 2.5, 3 °C/min. reactants, brings about nanostructures and better mixing of LiNH2 and Hydrogen (m/z=2) and ammonia (m/z=15) were recorded by the on- LiH, and hence improves the dehydrogenation rate [24]. High-energy line mass spectrometer during the heating process. It can be seen in ball milling can also cause mechanical chemical reactions, which lead

Figs. 1 and 2 that H2 ﬁrstly appears around 136 °C. NH3 is undetect- that the added FeCl3 cannot keep its initial chemical state. The XRD able in the range of experimental temperatures by means of mass patterns of LiNH2−2LiH-0.01FeCl3 after BM and TPD in Fig. 3(a) and spectrometer. Comparing the plots of samples with and without 1 mol. (b) indicate that metallic Fe formed by ball milling and remains during

% FeCl3, the starting dehydrogenation temperature is not changed but the TPD test. Fe particles formed by mechanical methods have a good the peak and ending temperatures decrease about 18.4 °C and 50.0 °C thermal conductivity and a physical dispersion effect. Hence Fe when the ramping rate is 2 °C/min, respectively. With the increase of particles may have a good catalytic effect on the dehydrogenation ramping rates, the hydrogen desorption peak moves to higher tem- reaction, which leads to the decrease of active energy. For comparison, peratures for both samples. The peak temperatures of pristine sample XRD patterns of ball milled FeCl3−3LiH and FeCl3−3LiNH2 samples are 235.0 °C, 239.8 °C, 244.9 °C and 248.6 °C at the ramping rates of are also showed in Fig. 3(c) and (d). The mixture of FeCl3 and LiH with 1.5, 2, 2.5 and 3 °C/min respectively. Furthermore the peak tempera- a molar ratio of 1:3 was ball milled for 6 h at a rotation rate of 200 rpm. tures of the doped 1 mol% FeCl3 sample are 218.2 °C, 224.3 °C, After ball milling, the gas pressure in the jar increases and the released 229.0 °C and 233.2 °C at the ramping rates of 1.5, 2, 2.5, and 3 °C/ gas is identified to be H2 by MS. The solid products are identified as min respectively. The Kissinger's method has been used to measure the LiCl and metallic Fe by XRD(Fig. 3(c)). So the reaction of FeCl3 and activation energy (Ea) for these samples (Eq. (4)). LiH produces LiCl, Fe and H2 (Eq. (5)). 2 → dln(β/TdTm )/[ (1/m )] = −Ea/R (4) 2FeCl3 + 6LiH 6LiCl + 2Fe + 3H2 (5)

140 W. Zhang et al. Progress in Natural Science: Materials International 27 (2017) 139–143

ture of LiNH2−2LiH-0.5LiCl presents at 255 °C, which is about 15 °C higher than that of LiNH2−2LiH-0.167FeCl3, and after then the dehydrogenation of LiNH2−2LiH-0.5LiCl does not accomplish even at the end of TPD. LiNH2 can react with LiCl to form a new compound, Li4(NH2)3Cl, which is more stable than LiNH2 [25], so the dehydrogenation of LiNH2−2LiH-0.5LiCl becomes more diﬃcult. The half-peak widths of dehydrogenation curves of LiNH2−2LiH, LiNH2−2LiH- 0.01FeCl3, LiNH2−2LiH-0.167FeCl3, LiNH2−2LiH-0.5LiCl are 82.0, 60.6, 77.6 and 100.3 °C, respectively. So Fe particles do not change

the starting dehydrogenation temperature of LiNH2−2LiH system, and however accelerate the kinetics of dehydrogenation. With the help of Fe particles, the maximum dehydrogenation rate is attained at a lower temperature, and the dehydrogenation can accomplish within a shorter time when the eﬀects of LiCl is eliminated. The dehydrogenation

products of ball milled LiNH2−2LiH-0.01FeCl3 include Li2NH, the remained LiH, Fe and the impurity of Li2O, which are identiﬁed by the XRD (Fig. 3(b)). However, metallic Fe and a new cubic phase of

Li2+x(NH)Clx with the lattice parameters a=5.1855 can be found in the dehydrogenation products of ball milled LiNH2−2LiH-0.167FeCl3 (Fig. 3(e)). Anderson et al. [25] prepared lithium imide-chloride, − − Fig. 3. XRD patterns of several samples: (a) LiNH2 2LiH-0.01FeCl3-BM; (b) LiNH2 2LiH- 3Li2NH-LiCl by heating the mixture of Li2NH and LiCl with the molar − − − 0.01FeCl3-TPD; (c) FeCl3 3LiH-BM; (d) FeCl3 3LiNH2-BM; (e) LiNH2 2LiH-0.167FeCl3- ratio of 3:1 at 400 °C, and they did not obtained a pure phase but the TPD. face centered cubic solid solutions with the lattice parameters from a=5.13 to 5.21. The mixture of FeCl −3LiNH was also ball milled under the same 3 2 The desorbed hydrogen contents of above four samples were conditions. However, the released gases include NH ,N,H and the 3 2 2 measured by simultaneous TG-DTA measurements at a ramping rate solid products are LiCl, Fe N and metallic Fe( Fig. 3(d)). During ball 3 of 2 °C/min from 35 °C to 500 °C. As shown in Fig. 5, the weight losses milling, the metathesis reaction of FeCl and LiNH may firstly form 3 2 of these four samples are 4.70, 4.51, 2.51 and 3.01 wt%, respectively, LiCl and Fe(NH ) , and subsequently the unstable Fe(NH ) decom- 2 3 2 3 which are correspondence to 0.92, 0.90, 0.83 and 0.91 mol H . TPD- poses to Fe N, Fe, NH ,N,H. As discussed above, for LiNH −2LiH- 2 3 3 2 2 2 MS and TG measurements illustrate that the reaction of LiNH and 0.01FeCl sample the iron element is always in a metallic not a nitride 2 3 2LiH can release about 1 mol H according to Eq. (1) and the release of state, so FeCl is prior to react with LiH not LiNH . 2 3 2 NH is restrained under our experimental conditions. The starting, The effects of FeCl as well as LiCl were further studied by 3 3 peak and ending dehydrogenation temperatures of all samples can be increasing the amount of these additives. The mixtures of derived from TG curves, which are consistent with the results of TPD- LiNH −2LiH-0.167FeCl and LiNH −2LiH-0.5LiCl were prepared 2 3 2 MS measurements. Because the H signals detected by MS are more under the same conditions as LiNH −2LiH-0.01FeCl . TPD tests of 2 2 3 sensitive than the weight losses, there are slight differences between the samples were conducted at a ramping rate of 2 °C/min (Fig. 4). It these two kinds of methods. The dehydrogenation reaction of can be found that the addition of 0.167 mol FeCl increases the 3 LiNH −2LiH is endothermic, and however the exothermic process at dehydrogenation temperature. The starting dehydrogenation tempera- 2 130 °C is found before the endothermic dehydrogenation of LiNH - ture is postponed to 168 °C, but the peak and ending temperatures are 2 2LiH-0.167FeCl and LiNH -2LiH-0.5LiCl. This may be related to the changed little comparing with those of LiNH −2LiH. The increase of 3 2 2 formation of Li (NH ) Cl. This exothermic event at about 130 °C was dehydrogenation temperature may be caused by by-product, LiCl. The 4 2 3 also found in DSC measurements of LiNH -AlCl system by Albanesi addition of 0.167 mol FeCl means the formation of 0.5 mol LiCl from 2 3 3 et al. [22], and they contributed it to the formation of Li (NH ) Cl-type the reaction of FeCl and LiH. According to the amount of LiCl, 4 2 3 3 phase. LiNH −2LiH-0.5LiCl was tested under the same conditions. The 2 In order to verify this supposition, the ball milled 3LiNH -LiCl starting dehydrogenation temperature of LiNH −2LiH-0.5LiCl is 2 2 sample was prepared under the same condition and then treated at 170 °C similar as that of LiNH2−2LiH-0.167FeCl3. The peak tempera- 200 °C for 2 h. The XRD pattern of 3LiNH2-LiCl sample after heat treatment in Fig. 6 suggests the hexagonal phase of Li4(NH2)3Cl is

Fig. 4. TPD curves of LiNH2−2LiH samples with diﬀerent additives at a ramping rate of

2 °C/min. Fig. 5. TG-DTA curves of LiNH2−2LiH samples with diﬀerent additives.

141 W. Zhang et al. Progress in Natural Science: Materials International 27 (2017) 139–143

– Fig. 6. XRD patterns of 3LiNH2-LiCl samples after ball milling and heating at 200 °C as Fig. 8. XRD patterns of xLi2NH-1LiCl (x=0.5, 1 6) and pure Li2NH samples. well as ball milled LiNH2. mixtures with different ratios, which were firstly ball milled for 24 h at a 200 rpm rate and then were thermally decomposed from 35 °C to 500 °C in the argon flow. As shown in Fig. 8, when the molar ratio of

Li2NH-LiCl is larger than 1:1, only the solid solution phase of Li2+x(NH)Clx can be found in the thermal decomposition product. The lattice parameters decrease linearly with the ratio of Li2NH and LiCl in the range of 1–2.5 in Fig. 9. However, thermal decomposition

products of 2LiNH2-LiCl and LiNH2-LiCl samples include two phases, LiCl and the solid solution of Li2+x(NH)Clx. Especially for ball milled 2LiNH2-LiCl sample, the peak intensities belong to LiCl phase are quite weak, which suggests the high limit of the solid solution of LiCl and

Li2NH is near the molar ratio of 1:1. The solid solution of lithium imide-chlorides has an anti-CaF2 type structure as similar as Li5NCl2 [29].NH2− and Cl− are disorderly located at 4a site of a face-centered cubic close packed structure, while Li+ ions are distributed at the

tetrahedral voids (8c site). The de/rehydrogenation process of LiNH2- LiH-xLiCl includes lithium amide-chlorides and lithium imide-chlor- ﬀ Fig. 7. FTIR spectra of 3LiNH2-LiCl samples after ball milling and heating at 200 °C as ides, and more investigations about the e ects of LiCl on hydrogen well as ball milled LiNH2. storage properties of Li-N-H system will be shown in the further work.

4. Conclusions obtained. After ball milling, the XRD pattern of 3LiNH2-LiCl sample suggests that a new cubic phase of Li amide-chloride forms. The ball The eﬀects of addition of FeCl3 on hydrogen storage properties of milling most possibly leads to the formation of Lix+y(NH2)xCly by the Li-N-H system were studied systematically. Chemical reactions of mechanochemical reaction between LiNH2 and LiCl, and then the FeCl3 and LiH, FeCl3 and LiNH2 can occur during ball milling to form hexagonal phase of Li4(NH2)3Cl forms by thermal treatment. Meantime, FTIR spectra in Fig. 7 also identify the phase change. The metallic Fe and iron nitride, respectively. However, FeCl3 is prior to react with LiH when it is doped into LiNH2−2LiH composite by ball symmetrical and asymmetrical stretching vibrations of NH2 bonds of −1 milling. Metallic Fe plays an important role in improving the dehy- LiNH2 are present at 3312 and 3258 cm . However, the spectrum of ball milled 3LiNH2-LiCl sample shows N-H vibrations at 3246, 3298 and 3369 cm−1. The vibration of 3369 cm−1 is correlated to N-H vibrations of cation-coordinated NH3 molecules [26]. After heat treatment at 200 °C, the characteristic vibrations of hexagonal phase −1 Li4(NH2)3Cl at 3244, 3260, 3302, 3312 cm are emerged [25]. Anderson et al. [25] reported that the reaction rate of LiNH2 and LiCl was very slowly below 300 °C, and the hexagonal phase of

Li4(NH2)3Cl was formed by heating at 400 °C for 1 h. If the heating time was extended to 12 h, the body centered cubic phase of

Li4(NH2)3Cl was obtained, which is isostructural with Li4BH4(NH2)3 [27].NH2 vibrations of ball milled 3LiNH2-LiCl sample are very close to those of the body centered cubic phase of Li4(NH2)3Cl and Li4BH4(NH2)3 in FTIR spectra [25,28]. Due to the post-milling treatment, the phase change of Li4(NH2)3Cl and the dehydrogenation properties of LiCl doped LiNH2−2LiH in our investigation are also quite diﬀerent from Anderson's results. In order to conﬁrm the component ratios of the solid solution, lithium imide-chlorides were synthesized from a series of LiNH2-LiCl Fig. 9. Relationship of the cell parameters varying with the ratio of Li2NH-LiCl.

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