Remarkable hydrogen absorption/desorption behaviors and mechanism of sodium alanates in-situ doped with Ti-based 2D MXene

Abstract

MXene has proven to be an excellent catalyst for the energy storage materials. By doping MXene, the hydrogen absorption and desorption performances and cycling stability of NaAlH4 can be significantly improved. However, the preparation and doping of MXene still have a great influence on its catalytic performance. In this work, Ti3C2 with great morphology and high purity was prepared by MILD method (using HCl and fluoride salt instead of using HF). The as-synthesized MXene were in-situ doped into NaH/Al to prepare the Ti3C2-doped NaH/Al composite. At the temperature of 110 °C, the Ti3C2-doped NaH/Al composite uptakes 4.2 wt% H2 within 4.5 min. The initial dehydrogenation temperature of Ti3C2-doped NaH/Al composite is reduced to 76 °C, which is one of the lowest dehydrogenation temperatures of the reported Ti-doped sodium alanate. Ti3C2-doped NaH/Al composite possesses much lower active energy (92.5 kJ/mol for the first step and 58.1 kJ/mol for the second step), compared with the as-milled NaAlH4. Ti3C2-doped NaH/Al composite also has an extraordinary cycling hydrogen absorption and desorption properties, 99% of initial capacity can be maintained after 10 cycles. After microstructure analysis, It can be found that after ball milling, Ti3C2 decomposed into Ti and TiFx nanoclusters, which uniformly distributed on the surface of NaH/Al particles. These active titanium acts as a fast channel for hydrogen absorption and desorption. Moreover, Ti3C2-doped NaH/Al composite has a higher content of high-valence Ti compared with Ti3C2-doped NaAlH4, which may explain why its hydrogen storage properties are better than that of Ti3C2-doped NaAlH4.