Significant uptake and release of hydrogen in nonstoichiometric Fe-Al compounds

Highlights: • Hydrogen was electrochemically charged into Fe-poor nonstoichiometric Fe–Al alloys. • Release of taken-up hydrogen from Fe–Al alloys continued for 1 h. • 10⁶ times more hydrogen was released than the hydrogen solubility in pure α-iron. • Hydrogen trapping in Fe–Al alloys was attributed to constitutional vacancies. Hydrogen storage and permeation alloys have been studied based on their hydrogen affinities. The hydrogen uptake and release behavior of intermetallic compounds is important to understand the mechanism of trapping hydrogen atoms and for the advance to next-generation hydrogen energy materials. Here, we propose the site-controlled model by constitutional vacancies. We studied the uptake of hydrogen atoms in nonstoichiometric B2-type Fe–Al compounds by electrochemically charging hydrogen in an alkali water solution. For the Fe-poor nonstoichiometric alloys Fe₄₆Al₅₄ and Fe₄₈Al₅₂, the release of taken-up hydrogen began immediately after switching off the current and continued for 1 h, whereas no hydrogen was released from Fe₅₀Al₅₀ and Fe₅₂Al₄₈. The maximum amount of hydrogen released from Fe₄₆Al₅₄ charged at 10 V was about 6.30 cm³ per gram of sample, corresponding to an atomic ratio of H/M = 2.26 × 10⁻². This hydrogen concentration is 10⁶ times higher than the solubility of hydrogen in pure α-iron. Such a significant uptake of hydrogen is concluded to be peculiar to the Fe-poor nonstoichiometric compositions, which contain constitutional vacancies. This is the first report to adequately explain the high absorption capacity of Fe-poor Fe–Al intermetallic compounds.