  Hydrogen as a fuel plays a crucial role in driving the transition to net zero
greenhouse gas emissions. To realise its potential, obtaining a means of
efficient storage is paramount. One solution is using metal hydrides, owing to
their good thermodynamical absorption properties and effective hydrogen
storage. Although metal hydrides appear simple compared to many other energy
materials, understanding the electronic structure and chemical environment of
hydrogen within them remains a key challenge. This work presents a new
analytical pathway to explore these aspects in technologically relevant systems
using Hard X-ray Photoelectron Spectroscopy (HAXPES) on thin films of two
prototypical metal dihydrides: YH$_{2-\delta}$ and TiH$_{2-\delta}$. By taking
advantage of the tunability of synchrotron radiation, a non-destructive depth
profile of the chemical states is obtained using core level spectra. Combining
experimental valence band spectra collected at varying photon energies with
theoretical insights from density functional theory (DFT) calculations, a
description of the bonding nature and the role of d versus sp contributions to
states near the Fermi energy are provided. Moreover, a reliable determination
of the enthalpy of formation is proposed by using experimental values of the
energy position of metal s band features close to the Fermi energy in the
HAXPES valence band spectra.

Materials Science;Applied Physics