Density Functional Theory Study of Monoclinic FeNbO4: Bulk Properties and Water Dissociation at the (010), (011), (110), and (111) Surfaces

Monoclinic and orthorhombic FeNbO4-based materials have been developed for many applications, including hydrogen sensors and solid oxide electrolysis cell (SOEC) electrodes. Here, we have employed density functional theory (DFT) calculations to investigate the bulk and surface properties of the monoclinic FeNbO4 structure, as well as water adsorption and dissociation on its pristine surfaces. Our calculations show that the high-spin state Fe3+ cations have a relatively smaller Bader charge than the Nb5+ cations, which accounts for Nb–O bonds that are stronger than Fe–O bonds. The analysis of the density of states (DOS) shows that the O 2p orbital occupies most of the valence band, including its maximum (VBM), with negligible contributions from the 4d and 3d orbitals of Nb and Fe cations, respectively. We found that the 3d orbitals of Fe occupy the conduction band minimum (CBM), which explains that electrons are conducted via the Fe–O–Fe framework. The calculation of the elastic constants demonstrates that pure monoclinic FeNbO4 is mechanically stable. We have also considered the thermodynamic stability and structures of the seven low-Miller-index surfaces and found that the (010) facet has the lowest surface energy and expresses the largest area in the Wulff crystal shape of the particle. Finally, we have simulated the interaction of water with the Fe3+ and Nb5+ sites of the four most stable surfaces and found that the dissociative adsorption of water takes place only on the (110) surface, which has important implications for the use of this material as a SOEC electrode.