A new perspective on the initial hydrogenation of TiFe0.9M0.1 (M = V, Cr, Fe, Co, Ni) alloys gained from surface oxide analyses and nucleation energetics

• The activation kinetics of TiFe is enhanced by alloying with M = V, Cr, Co and Ni. • V and Cr enhance the activation kinetics more effectively than Co and Ni. • Thinner surface oxide layer is formed in TiFe 0.9 M 0.1 alloys than pure TiFe. • Slower activation kinetics is associated with larger nucleation barrier from DFT. Despite the promise of TiFe-based alloys as low-cost solid-state hydrogen storage materials with mild operating conditions and reasonable hydrogen capacity, their initial hydrogenation process is difficult, hindering broad utilization. The effect of alloying element on the initial hydrogenation kinetics of TiFe alloys, TiFe 0.9 M 0.1 (M = V, Cr, Fe, Co and Ni), was evaluated by analyzing changes to the passivating surface oxide layer that inhibits hydrogen permeation, as well as the ease of initial-stage hydrogen absorption into the underlying matrix. X-ray photoelectron spectroscopy and atom probe tomography revealed key variations in surface oxide compositions and thinning of the passivating oxide layer compared to pure TiFe, which suggests suppressed oxide growth by alloying-induced elemental redistribution. At the same time, density functional theory calculations predicted exothermic formation of hydride nuclei when alloying with V or Cr, as well as a reduced nucleation barrier when alloying with Co or Ni. Overall, these results are consistent with the observed experimental trend of the activation kinetics. We propose that improvements in activation kinetics of TiFe with alloying arises from the combined effect of reduced passivating oxide thickness and easier hydride nucleation, offering a starting point for alloy design strategies towards further improvement.