Physical Science Research Frontiers 2016 54 Development of H – conductive oxyhydrides Efficiency of the hydrogen transport in solids is a key to determining the performance of electrochemical devices such as fuel cells and batteries. Indeed, active studies on proton (H + ) conduction in oxides and other systems have been carried out. In contrast, hydrogen can also accept one electron to form a hydride ion (H – ). Hydride ions are also attractive for use as charge carriers because they are similar in size to oxide and fluoride ions, which are suitable for fast ionic conduction, while they also exhibit strong reducing properties owing to their standard H − /H 2 redox potential (−2.3 V), which is comparable to that of Mg/Mg 2+ (−2.4 V). Hydride ion conductors may therefore be applied in energy storage/conversion devices with high energy densities. However, pure H – conduction has been verified only for a few hydrides of alkaline earth metals such as BaH 2 [1]. Unfortunately, utilization of the hydrides is difficult because of their structural inflexibility, which makes it difficult to control the lattice structure to create smooth transport pathways as well as the conducting hydride ion content. We have considered oxyhydrides, where hydride ions and oxide ions share anion sublattices, as candidate hydride conductors equipped with flexible anion sublattices. A further complication in achieving pure H − conduction in an oxide framework structure is the difficulty in inhibiting electron conduction. It is well known that hydride ions act as electron donors in oxides, transferring electrons from hydride ions to the lattice. This causes the conduction of electrons accompanied by a characteristic change in the hydrogen charge from H – to H + . Indeed, the perovskite and mayenite-type oxyhydrides are dominated by electron conduction caused by the dissociation of hydride ions into electrons and protons [2,3]. In the present study, we attempted to synthesize a series of K 2 NiF 4 -type oxyhydrides, La 2- x-y Sr x+y LiH 1- x + y O 3- y (0≤ x ≤1, 0≤ y ≤ 2, 0≤ x + y ≤ 2), which are equipped with cation sublattices featuring cations more electron-donating than H – and anion sublattices that exhibit flexibility in the storage of H – , O 2– , and vacancies [4]. The oxyhydrides La 2- x - y Sr x + y LiH 1- x + y O 3- y were synthesized by a solid-state reaction under high pressure and temperature using a cubic anvil cell. Synchrotron X-ray diffraction (SXRD) measurements in this study were performed at SPring-8 BL02B2 . A Debye-Scherrer diffraction camera was used for measurements at 298 K. The SXRD pattern of La 2 LiHO 3 ( x = y = 0) was assigned to the K 2 NiF 4 -type structure with an orthorhombic Immm symmetry. Figure 1 shows SXRD patterns of solid solutions between La 2 LiHO 3 and Sr 2 LiH 3 O, represented as La 2 -y Sr y LiH 1+ y O 3- y ( x =0, 0 ≤ y ≤ 2). Here, the H:O ratio changes accordingly as La is substituted with Sr, maintaining the simple A 2 BX 4 composition ( A :La, Sr; B :Li; X :O, H). In La 2- x Sr x LiH 1- x O 3 (0 ≤ x ≤ 1, y = 0) and La 1- x Sr1+ x LiH 2- x O 2 (0 ≤ x ≤ 1, y = 1), Regarding the Sr- substituted series of La 2 -y Sr y LiH 1+ y O 3- y , the diffraction peaks continuously shifted to lower angles with increasing y and the lattice symmetry changed from Immm ( y < 1) to I 4/ mmm ( y ≥ 1). The compositions and structures of La 2 -y Sr y LiH 1+ y O 3- y ( y = 0, 1, 2) were determined by X-ray and neutron Rietveld analyses. Figure 2 shows the determined crystal structures of La 2 -y Sr y LiH 1+ y O 3- y . In La 2 LiHO 3 , the two apical sites of the Li X 6 octahedra are occupied only by O 2– , while the four in-plane apexes are orderly occupied by O 2– and H – in an orderly manner. These results indicate that the highly charged cations, i.e., La 3+ and Sr 2+ , require highly charged anions around them. LaSrLiH 2 O 2 is composed of tetragonal (LiH 2 ) – and (LaSrO 2 ) + layers alternately stacked along the c axis. A further increase in the hydride content up to Sr 2 LiH 3 O results in the formation of (Sr 2 HO) + layers. A Rietveld analysis for the anion deficient series, La 1- x Sr1+ x LiH 2- x O 2 ( x > 0, y = 1), was also carried out. As a result, it was clarified that vacancies were introduced at the LiH 4 plane with increasing x . The ionic conductivities of La 2- x - y Sr x + y LiH 1- x + y O 3- y were examined by impedance measurement. For the vacancy- free composition La 2 -y Sr y LiH 1+ y O 3- y ( x = 0), the conductivity increases with increasing H – content, with the highest conductivity of 3.2 × 10 –5 S cm –1 at 573 K being observed for Sr 2 LiH 3 O ( y = 2). The introduction of hydride ions into the anion sites of the K 2 NiF 4 structure improved the ionic conductivity, suggesting that the primary charge carriers are these hydride ions. The conduction is further facilitated by the introduction of vacancies, as can be seen for both La 2- x Sr x LiH 1- x O 3 ( y = 0) and La 1- x Sr 1+ x LiH 2- x O 2 ( y = 1), reaching 2.1 × 10 –4 S cm –1 for La 0.6 Sr 1.4 LiH 1.6 O 2 at 590 K. To further identify the nature of charge carriers, an all- Fig. 1. Comparison of the synchrotron X-ray diffraction patterns for La 2- x - y Sr x + y LiH 1- x + y O 3- y ( x = 0, 0 ≤ y ≤ 2). Intensity (arb. units ) 24 20 16 12 8 4 2 (degree) ( λ = 0.6 Å) θ 002 013 103 110 011 101 004 112 105 114 006 ( Immm ) ( I 4/ mmm ) y = 2 y = 1.4 y = 1 y = 0.5 y = 0 : Unknown : SrLiH 3 015 020 200 002 011 004 013 110 112 006 015 114 020 10.8 10.0 011 101 004 004 011 (i) (i) Research Frontiers 2016 55 solid-state Ti/La 2 LiHO 3 /TiH 2 cell was constructed using La 2 LiHO 3 as the solid electrolyte, and the galvanostatic discharge reaction was examined. Figure 3(a) shows the discharge curve of the cell, displaying a constant discharge current of 0.5 m A at 573 K. The cell showed an initial open circuit voltage of 0.28 V, which is consistent with the theoretical value calculated from the standard Gibbs energy of formation of TiH 2 . During the electrochemical reaction, the cell voltage dropped rapidly from 0.28 V to 0.06 V, and then decreased gradually to 0.0 V. This steep drop-off in the first reaction step corresponds to an increase in the hydride ion content at the anode in accordance with the constant current discharge reaction Ti + x H − → TiH x + x e − , where the reaction at the cathode is as follows: TiH 2 + x e − → TiH 2- x + x H − . These discharge reactions were confirmed by observation of the phases that appeared following the reaction. Figure 3(b) shows the synchrotron X-ray diffraction patterns for the cathode, electrolyte, and anode, both before and after the reaction. The absence of any variation in the diffraction patterns of the electrolyte indicates that the La 2 LiHO 3 electrolyte is stable when in contact with the Ti and TiH 2 electrodes during the reaction. The phase changes detected for the cathode and anode materials are consistent with those expected from the Ti-H phase diagram [5], where the δ -TiH 2 phase releases hydrogen and is transformed into a -Ti through a two- phase ( a -TiH b + δ -TiH 2- a ) coexistence region. In the case of the cathode, additional diffraction peaks corresponding to the a -Ti phase were detected. In addition, the signals corresponded to a shift of TiH 2 to a higher angle, thus indicating that lattice shrinkage takes place with the release of hydrogen from TiH 2 . In the case of the anode, peaks corresponding to the δ -TiH 2 phase were detected. These results indicate that during the electrochemical reaction, hydride ions are released from the TiH 2 cathode and diffuse into the Ti anode through the La 2 LiHO 3 . In conclusion, pure H – conduction was realized in the La 2- x - y Sr x + y LiH 1- x + y O 3- y system. The present successful construction of an all-solid-state electrochemical cell exhibiting H – diffusion confirms not only the capability of the oxyhydride to act as H – solid electrolyte but also the possibility of developing electrochemical solid devices based on H – conduction. References [1] M.C. Verbraeken et al. : Nat. Mater. 14 (2015) 95. [2] K. Hayashi et al. : Nature 419 (2003) 462. [3] Y Kobayashi et al. : Nat. Mater. 11 (2012) 507. [4] G. Kobayashi, Y. Hinuma, S. Matsuoka, A. Watanabe, M. Iqbal, M. Hirayama, M. Yonemura, T. Kamiyama, I. Tanaka R. Kanno: Science 351 (2016) 1314. [5] A. San-Martin, F.D. Manchester: Bulletin of Alloy Phase Diagrams 8 (1987) 30. Genki Kobayashi a, * and Ryoji Kanno b a Research Center of Integrative Molecular Science, Institute for Molecular Science b School of Materials and Chemical Technology, Tokyo Institute of Technology *Email: gkobayashi@ims.ac.jp Fig. 2. Crystal structures of La 2- x - y Sr x + y LiH 1- x + y O 3- y ( x = 0, y = 0, 1, 2). Fig. 3. All-solid-state hydride cell. (a) Discharge curve for a solid-state battery with the Ti/La 2 LiHO 3 / TiH 2 structure. The inset shows an illustration of the cell and the proposed electrochemical reaction. (b) X-ray diffraction patterns for the electrolyte (La 2 LiHO 3 ), cathode (TiH 2 + La 2 LiHO 3 ), and anode (Ti + La 2 LiHO 3 ) materials after the reaction. La Sr Li H – O Sr 2 LiH 3 O ( y = 2) LaSrLiH 2 O 2 ( y = 1) La 2 LiHO 3 ( y = 0) H (4 c ) H/O (4 e ) H (2 b ) O (4 i ) O (2 d ) O (4 e ) Li(H/O) 6 octahedra LiH 4 square LiH 2 linear 0.4 0.3 0.2 0.1 0.0 Voltage (V) 1000 800 600 400 200 0 Time (min) Ti / o -La 2 LiHO 3 / TiH 2 Intensity (arb. units ) 30 25 20 15 10 5 o -La 2 LiHO 3 + TiH 2 o -La 2 LiHO 3 + Ti after discharge after discharge α -Ti ( P 63/ mmc ) δ -TiH 2 ( Fm _ 3 m ) α -phase δ -phase 13.6 13.2 15.5 15.1 111 δ 200 δ 100 α 110 α o -La 2 LiHO 3 V La 2 LiHO 3 + TiH 2 La 2 LiHO 3 + Ti La 2 LiHO 3 H – e – TiH 2 + x e – TiH 2-x + x H – Ti + x H – TiH x + x e – (a) (b) 2 (degree) ( = 0.6 Å) θ λ 2 (degree) ( = 0.6 Å) θ λ
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