A-Site-Dependent Hydrogen Affinity in Ti3AlC2 Single Crystal and Ti3ZnC2-Oriented Platelet

Abstract MAX phases provide compositionally tunable layered structures for investigating hydrogen interactions, but the role of A-site chemistry remains insufficiently understood. Here, Ti3ZnC2-oriented platelets were prepared from Ti3AlC2 single-crystal precursors through Al-to-Zn substitution in a Lewis-acidic molten salt. Under the tested conditions, the temperature required for near-complete Al removal increased with the lateral size of the precursor platelets. Surface and cross-sectional EDS analyses were consistent with near-complete Al-to-Zn substitution in the examined regions. Density functional theory calculations of selected ideal-bulk configurations identified Ti–A layer interstitial sites as energetically favorable for H insertion in both phases. Replacing Al with Zn lowered the most favorable single-H formation energy from −0.22 to −0.61 eV and stabilized selected multi-H configurations. At 77 K and 72.2 bar, Ti3ZnC2 showed an H2 uptake of 0.303 wt %, compared with 0.145 wt % for Ti3AlC2. The Ti3ZnC2-oriented platelets reached 0.505 wt % at 100 bar, despite having slightly lower N2-accessible BET surface area and pore volume than the Ti3AlC2 precursors. These results reveal an A-site-dependent difference in ideal-bulk H-insertion energetics and total low-temperature H2 uptake. These findings identify A-site chemistry as an effective compositional variable for tuning hydrogen interactions in Ti-based MAX phases.