Cation doping and oxygen vacancies in the orthorhombic FeNbO₄ material for solid oxide fuel cell applications: A density functional theory study

The orthorhombic phase of FeNbO₄, a promising anode material for solid oxide fuel cells (SOFCs), exhibits good catalytic activity toward hydrogen oxidation. However, the low electronic conductivity of the material specifically in the pure structure without defects or dopants limits its practical applications as an SOFC anode. In this study, we have employed density functional theory (DFT + U) calculations to explore the bulk and electronic properties of two types of doped structures, Fe_0.9375A_0.0625NbO₄ and FeNb_0.9375B_0.0625O₄ (A, B = Ti, V, Cr, Mn, Co, Ni) and the oxygen-deficient structures Fe_0.9375A_0.0625NbO_3.9375 and FeNb_0.9375B_0.0625O_3.9375, where the dopant is positioned in the first nearest neighbor site to the oxygen vacancy. Our DFT simulations have revealed that doping in the Fe sites is energetically favorable compared to doping in the Nb site, resulting in significant volume expansion. The doping process generally requires less energy when the O-vacancy is surrounded by one Fe and two Nb ions. The simulated projected density of states of the oxygen-deficient structures indicates that doping in the Fe site, particularly with Ti and V, considerably narrows the bandgap to ∼0.5 eV, whereas doping with Co at the Nb sites generates acceptor levels close to 0 eV. Both doping schemes, therefore, enhance electron conduction during SOFC operation.