Tailoring the equilibrium hydrogen pressure of TiFe via vanadium substitution

We present that the equilibrium hydrogen pressure of titanium iron (TiFe) alloy, a room-temperature hydrogen storage material, can be tailored via vanadium alloying. While many 3d transition metal alloying elements (e.g., Mn, Cr, Co, and Ni) typically replace the Fe sublattice in TiFe, vanadium can replace both the Ti and Fe sublattices. Density functional theory calculation predicts that the substitution of Ti with V yields a unique effect: the equilibrium pressures of TiFe/TiFeH (P₁) and TiFeH/TiFeH₂ (P₂) are closer, resulting in a decreased P₂/P₁ ratio. Experimental pressure-composition isotherms confirm this theoretical prediction. The lower P₂/P₁ is beneficial because the two-step TiFe hydrogenation reactions can be contained within a narrow pressure range. In contrast, the substitution of V for Fe lowers both P₁ and P₂, but lowers P₁ more, resulting in a higher P₂/P₁ ratio. The contrasting effects contingent on the substitution site is a crucial factor in alloy design. It highlights the significance of vanadium as a versatile alloying element that modifies the hydrogen storage property of TiFe.