We have investigated the decomposition path and reversibility of Ca(BH4)2 and Ca(BH4)2 + MgH2 composite using X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and Raman spectroscopy. Formation of CaB6 during dehydrogenation of both systems was confirmed for the first time. CaB6 appears as broad peaks in X-ray diffraction data, but Raman spectroscopy unambiguously captures the existence of CaB6. Reversibility of catalyzed Ca(BH4)2 was previously reported, and here we demonstrate reversibility of Ca(BH4)2 + MgH2 composite. Dehydrogenated product of Ca(BH4)2 + MgH2 is composed of CaH2, CaB6, and Mg. About 60% reversibility was achieved after rehydrogenation for 24 h under 90 bar of hydrogen pressure at 350 °C even without the help of catalysts, which makes a good contrast with the case of pure Ca(BH4)2 where almost negligible rehydrogenation occurs under the same conditions. To understand the difference, the role of Mg in rehydrogenation is worth further investigation. Formation of CaB6 seems critical in the reversibility of Ca(BH4)2 containing systems; the case of other borohydrides is compared.
O3-type layered oxide
materials are considered to be a highly suitable
cathode for sodium-ion batteries (NIBs) due to their appreciable specific
capacity and energy density. However, rapid capacity fading caused
by serious structural changes and interfacial degradation hampers
their use. A novel Sn-modified O3-type layered NaNi
1/3
Fe
1/3
Mn
1/3
O
2
cathode is presented, with
improved high-voltage stability through simultaneous bulk Sn doping
and surface coating in a scalable one-step process. The bulk substitution
of Sn
4+
stabilizes the crystal structure by alleviating
the irreversible phase transition and lattice structure degradation
and increases the observed average voltage. In the meantime, the nanolayer
Sn/Na/O composite on the surface effectively inhibits surface parasitic
reactions and improves the interfacial stability during cycling. A
series of Sn-modified materials are reported. An 8%-Sn-modified NaNi
1/3
Fe
1/3
Mn
1/3
O
2
cathode exhibits
a doubling in capacity retention increase after 150 cycles in the
wide voltage range of 2.0–4.1 V
vs
Na/Na
+
compared to none, and 81% capacity retention is observed
after 200 cycles in a full cell
vs
hard carbon. This
work offers a facile process to simultaneously stabilize the bulk
structure and interface for the O3-type layered cathodes for sodium-ion
batteries and raises the possibility of similar effective strategies
to be employed for other energy storage materials.
KZn(BH4)Cl2, synthesized for the first time, contains a heteroleptic complex anion [Zn(BH4)Cl2]–, extending the structural diversity of metal borohydrides. In‐situ synchrotron powder diffraction, NMR and Raman spectroscopy were used to characterize KZn(BH4)Cl2 and to evaluate the mechanism for its thermal decomposition. The title compound decomposes at a significantly lower temperature than KBH4 and may be used for inspiration for the design of novel hydrogen storage materials. Combining different ligands in modified metal borohydrides is proposed as a way to adjust stability with respect to hydrogen desorption.
Hydrogen back pressure remarkably promotes the formation
of metal boride during the dehydrogenation of 4LiBH4 +
YH3, 6LiBH4 + CeH2 and 6LiBH4 + CaH2 composites, which seems to be a general
phenomenon in LiBH4-based reactive hydride composites that
enables mutual destabilization between LiBH4 and metal
hydride. The formation of metal boride plays a crucial role in the
reversible hydrogen storage properties of these composites. The dependence
of the dehydrogenation behavior on hydrogen back pressure might be
associated with the microstructural evolution of the dehydrogenation
products formed by a solid−liquid reaction.
In this paper we present amorphous chromium(III) hydride gels that show promise as reversible room temperature hydrogen storage materials with potential for exploitation in mobile applications. The material uses hydride ligands as a light weight structural feature to link chromium(III) metal centres together which act as binding sites for further dihydrogen molecules via the Kubas interaction, the mode of hydrogen binding confirmed by high pressure Raman spectroscopy. The best material possesses a reversible gravimetric storage of 5.08 wt% at 160 bar and 25 °C while the volumetric density of 78 kgH2 m(-3) compares favourably to the DOE ultimate system goal of 70 kg m(-3). The enthalpy of hydrogen adsorption is +0.37 kJ mol(-1) H2 as measured directly at 40 °C using an isothermal calorimeter coupled directly to a Sieverts gas sorption apparatus. These data support a mechanism confirmed by computations in which the deformation enthalpy required to open up binding sites is almost exactly equal and opposite to the enthalpy of hydrogen binding to the Kubas sites, and suggests that this material can be used in on-board applications without a heat management system.
A detailed investigation of the decomposition
reactions and decay
in the hydrogen storage capacity during repeated hydrogen release
and uptake cycles for the reactive composite LiBH4–Al
(2:3) is presented. Furthermore, the influence of a titanium boride,
TiB2, additive is investigated. The study combines information
from multiple techniques: in situ synchrotron radiation powder X-ray
diffraction, Sieverts measurements, simultaneous thermogravimetric
analysis, differential scanning calorimetry and mass spectroscopy,
solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR),
and Raman spectroscopy. The decomposition of LiBH4–Al
results in the formation of LiAl, AlB2, and Li2B12H12 via several reactions and intermediate
compounds. The TiB2 additive appears to have a limited
effect on the decomposition pathway of the samples, but seems to facilitate
formation of intermediate species at lower temperatures compared to
the sample without additive. Solid solutions of Li
x
Al1–x
B2 or Al1–x
B2 are observed during
decomposition and from Rietveld refinement the composition of the
solid solution is estimated to be Li0.22Al0.78B2. The intercalation of Li in the AlB2 structure
is further investigated by 11B and 27Al MAS
NMR spectra of the LiH-AlB2 and AlB2 samples
(presented in Supporting Information).
Hydrogen release and uptake for LiBH4–Al reveals
a significant loss in the hydrogen storage capacity, that is, after
four cycles a capacity of about 45% remains, and after 10 cycles,
the capacity is degraded to approximately 15% of the theoretically
available hydrogen content. This capacity loss may be due to the formation
of Li2B12H12, as observed by 11B MAS NMR and Raman spectroscopy. Formation of Li2B12H12 has previously been observed during
the decomposition of LiBH4, but it has not been reported
earlier in the LiBH4–Al (2:3) system.
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