Hydrogen-fluorine exchange in NaBH4-NaBF4

Area of Science:

• Materials Science
• Inorganic Chemistry
• Solid-State Chemistry

Background:

• Sodium borohydride (NaBH4) is a promising material for hydrogen storage.
• Understanding hydrogen-fluorine exchange is crucial for developing advanced energy materials.
• The NaBH4-NaBF4 system offers potential for improved hydrogen storage properties.

Purpose of the Study:

• To investigate the hydrogen-fluorine exchange mechanism in the NaBH4-NaBF4 system.
• To characterize the novel compounds formed during the exchange.
• To evaluate the impact of this exchange on hydrogen storage capacity and cycling stability.

Main Methods:

• In situ synchrotron radiation powder X-ray diffraction (SR-PXD).
• Solid-state (19)F Magic Angle Spinning Nuclear Magnetic Resonance ((19)F MAS NMR).
• Fourier Transform Infrared Spectroscopy (FT-IR).
• Density Functional Theory (DFT) calculations.
• Thermogravimetric Analysis (TGA).
• Sievert's method for hydrogen storage capacity measurement.

Main Results:

• Formation of a new rock salt type compound, NaBF2H2, via fluorine substitution in NaBH4-NaBF4 at 200-215 °C.
• Identification of a BF2H2(-) complex ion using NMR and FT-IR, distinct from BF4(-).
• The NaBH4-NaBF4 composite decomposes at lower temperatures (~300 °C) than NaBH4 alone (476 °C).
• Enhanced reversible hydrogen storage capacity (30% after 3 cycles) for NaBH4-NaBF4 compared to NaBH4 (8%).
• Sodium fluoride (NaF) additive facilitates hydrogen uptake and improves material stability during cycling.

Conclusions:

• The NaBH4-NaBF4 system undergoes hydrogen-fluorine exchange, forming NaBF2H2 and a BF2H2(-) complex ion.
• Fluorine substitution leads to lower decomposition temperatures and improved reversible hydrogen storage capacity.
• The formation of stable byproducts like NaF and Na2B12H12 influences long-term stability.
• Additives like NaF can enhance the practical performance of NaBH4 for hydrogen storage applications.