Progress in Natural Science: Materials International 27 (2017) 139–143 Contents lists available at ScienceDirect Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi 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 hydro- gen 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.