Partially etched Ti2AlN/Ti2N/TiN ternary composite as a synergistic catalyst for enhanced hydrogen storage kinetics and cycling stability of NaAlH4

This study presents the preparation of a partially etched Ti 2 AlN-derived composite with a combination of Ti 2 N, residual Ti 2 AlN, and in-situ TiN nanoparticles. The resultant two-dimensional ternary composite was used as a catalyst to enhance the hydrogen storage capacity of NaAlH 4 .Although NaAlH 4 possesses a high theoretical hydrogen storage capacity of 7.4 wt%, its practical application is plagued by several problems: high temperature (~180 °C) to release hydrogen, slow kinetics, and poor reversibility. Herein, Ti 2 N was prepared by etching the Ti 2 AlN MAX phase. The results show that the catalytic effect is optimal when the Ti 2 N content is 7.5 wt%, with the starting temperatures of the two-step hydrogen release from NaAlH₄ decreasing from 180 °C and 260 °C to 98.4 °C and 163.3 °C, respectively. Meanwhile, the apparent activation energies of the two-step reactions decrease from 128 kJ/mol and 160 kJ/mol to 66.0 kJ/mol and 91.6 kJ/mol, respectively, with the kinetic performance significantly improved. Importantly, Ti 2 N/NaAlH 4 composites retain more than 95% of their hydrogen storage capacity after 10 cycles. Detailed characterizations using XRD, XPS, and TEM disclosed that the coexistence of multi-phases in Ti 2 N/NaAlH 4 composites induces synergistic catalytic effects: stable Ti 2 AlN provides electrically conductive pathways, while the reversible phase transformation between TiAl and TiAlH x occurs at the interface and nano-sized TiN transformed into TiAl 3 , enabling improved hydrogen gas dissociation. Collectively, these results provide new fundamental insights into optimizing the properties of Ti 2 N MXenes as catalysts in complex hydride hydrogen storage materials and establish key design principles to inspire the development of various composite materials for efficient and practical solid-state hydrogen storage systems.