Hydrogen bonding in sodium alanate: a muon spin rotation study

Area of Science:

• Materials Science
• Chemistry
• Physics

Background:

• Sodium alanate (NaAlH4) is a promising material for hydrogen storage.
• Understanding hydrogen release and uptake mechanisms is crucial for optimizing its performance.
• Titanium (Ti) doping is known to enhance the hydrogen cycling properties of NaAlH4.

Purpose of the Study:

• To investigate the role of hydrogen bonding in the hydrogen release mechanism of NaAlH4.
• To elucidate the effect of Ti doping on the dynamics of hydrogen atoms within NaAlH4.

Main Methods:

• Muon spin spectroscopy (μSR) was employed to probe the local environment and dynamics of muons within NaAlH4.
• Comparative studies were performed on undoped and Ti-doped NaAlH4 samples.

Main Results:

• The study detected hydrogen bonding involving a positive muon situated between hydrogen atoms of two independent alanate anions in NaAlH4.
• Ti doping was found to significantly reduce the kinetic barrier for the muon's transition from a bound state to a mobile interstitial state.
• This reduction in kinetic barrier by Ti doping implies an influence on the hydrogen atom mobility.

Conclusions:

• Hydrogen bonding between alanate anions acts as a primary bottleneck for hydrogen release and uptake in NaAlH4.
• The observed effect of Ti doping supports the hypothesis that hydrogen bonding dynamics are central to the hydrogen cycling limitations.
• These findings may be applicable to other complex hydrides, offering insights into improving their hydrogen storage capabilities.