Structural and spectroscopic characterization of potassium fluoroborohydrides

Abstract

Metal borohydrides are intensively investigated in the context of energy storage due to their high hydrogen content. 1 Recently, 15 there has also been reported high Li-ion conductivity in several Li-containing borohydrides, such as ortho-LiBH 4 2 and LiCe(BH 4 ) 3 Cl, 3 which makes them interesting as solid electrolytes in Li-ion batteries. Partial substitution of the tetraborohydride anions (BH 4 -) with heavy halides (Cl -, Br -or I -) 20 has been successfully used to alter the properties of borohydrides, e.g. to stabilize the Li-conducting, high-temperature ortho-LiBH 4 phase at room temperature. [4][5][6][7][8] However, the inclusion of heavy halides has only a minor effect on the temperature of hydrogen desorption, 9,10 which must be significantly lowered for alkali-and 25 alkaline earth borohydrides to become viable hydrogen storage materials. For alanates, substitution of hydrogen with fluorine has shown a large effect on the thermal stability, 11 and substitution of F -anions into LiBH 4 has been recently shown theoretically to result in materials with favorable thermodynamics for onboard 30 hydrogen storage applications. 12 Despite the chemical similarity between hydrides and fluorides, there is only one report of confirmed fluorine substitution in borohydrides. 13 An earlier report only presumes the formation of a fluoroborohydride. 14 The present paper reports the results from ball milling of 35 mixtures of KBH 4 and KBF 4 in the molar ratios 3:1 and 1:1 (samples 1 and 2, respectively). The mixtures were milled for 10 hours at ambient temperature. The powder X-ray diffraction (PXD) patterns of the milled products resembled that for pure KBH 4 (CsClO 4 -type structure, space group Fm-3m), but with 40 appreciable shifts in the Bragg peak positions to lower angles, thus indicating expansion of the unit cells. This is consistent with substitution of larger BF 4 -tetrahedra for BH 4 -tetrahedra or larger fluorine atoms for hydrogen. No Bragg peaks for KBF 4 (orthorhombic, BaSO 4 -type structure) were observed. 45The PXD data were analysed using the Rietveld method. The Rietveld fits for 1 and 2 are shown in Figures 1 and 2, respectively. The KBH 4 structure 15 was taken as the starting point. The K and B sublattices together form a NaCl-type structure. The B positions are cubically coordinated by 8 half- Table 1. The unit cell parameters of 1 and 2 were 6.8174(2) Å and 6.9219(9) Å, respectively, compared to 6.7280(8) Å for pure KBH 4 . 15 Thus, the unit cell a axis increases 65 linearly with the amount of fluorine in the sample; this is consistent with the existence of a solid solution. Moreover, the refined compositions of the 3:1 and 1:1 phases are close to the nominal compositions (see Table 1). Refinements with the phase compositions fixed at the nominal compositions gave little or no 70 change in the R-factor (R wp = 4.86% for 1 and unchanged for 2). These findings point towards a complete incorporation of the fluoride into the KBH 4 phase; the nominal compositions are therefore used i...

Complex and liquid hydrides for energy storage

“…The hydrogen release from KBH 2.8 F 1.2 takes place 433°C lower than it does in pure KBH 4 [63]. However, some of the obtained materials, specifically NaB(H 1-x F x ) 4 , appear to have relatively low thermal stability, and the released hydrogen may be contaminated with significant amounts of boron fluorides and/or borane gases and may lead to the formation of significant amounts of M 2 B 12 H 12 [64][65][66][67].…”

“…However, thermodynamic stabilization is also possible, as seen for NaMgH 2 F [61]. The first complex hydride with fluorine substitution was prepared by hydrogenation of the system NaF-Al, which provided complex hydride Na 3 AlH 6-x F x [62,63] [65]. Both of these phases, which are isostructural with KBH 4 , are stable at room temperature.…”

Hydrogen–fluorine exchange in NaBH4–NaBF4

“…25 Experimental reports indicate fluorine substitution in studies of MgB 2 -LiF and LiBH 4 -TiF 3 and also for the KBH 4 -KBF 4 system. [29][30][31] In this paper, the physical, structural and hydrogen storage properties of the NaBH 4 -NaBF 4 composite are investigated using in situ synchrotron radiation powder X-ray diffraction (SR-PXD), solid-state magic-angle spinning (MAS) NMR spectroscopy, infrared spectroscopy (IR), simultaneous differential scanning calori-metry (DSC), thermogravimetric analysis (TGA), and Sieverts method. DFT calculations on isolated BF 4 x H x ions were also performed and spectral parameters were computed.…”

Synthesis and Characterization of Nitro‐, Trinitromethyl‐, and Fluorodinitromethyl‐Substituted Triazolyl‐ and Tetrazolyl‐trihydridoborate Anions

“…prompted us to evaluate the prospect of applying this approach to a more general synthesis of mono‐substituted borohydride anions, such as [H 3 BX] − (X=halogen or pseudo‐halogen). However, a literature search revealed that the reaction of equimolar amounts of KF and BH 3 ⋅ SMe 2 in refluxing dimethoxyethane had already previously been studied and resulted in a complete redistribution of ligands, yielding discrete phases of KBH 4 and KBF 4 . Therefore, no further experiments were carried out in this direction.…”