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

Abstract

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.