Improvement of hydrogen uptake in iron and vanadium matrices by doping with 3d atomic impurities

Abstract

The insertion of hydrogen in V and Fe has been investigated by means of pseudopotential DFT calculations with localized basis sets. In Fe and V matrices we have replaced the central atom by a transition metal impurity X=Sc, Ti, Cr, Mn, Fe, Co and Ni to study the capacity of the environment to trap hydrogen. The dissolution energy and structural rearrangement upon H uptake at the different sites close to the doping impurity are calculated. Optimal electronic environments for H trapping are also determined through the calculation of the Fukui function. In the V matrix, the insertion of hydrogen is promoted by doping with the two impurities located at the left of V in the Periodical Table, that is, Ti and Sc. In the iron matrix, among the elements at its left in the Periodic Table, only Mn improves the H uptake, whereas doping with V and Ti worsen the capability of absorbing hydrogen. Finally, the H–H interaction is found to be strongly dependent upon the metal–hydrogen interaction. Elements like Mn or Fe which shorten the H–X distance, exhibit a strong 3d TM state–1s hydrogen state hybridization that seems to wash out the repulsive H–H Coulomb interaction below the 2.0Å limit. Addition of a small percentage of Fe or Mn in binary bcc alloys (V–Ti) is suggested to locally enhance the H storage capacity.