Electronic Structure, Chemical Bonding, and Electrocatalytic Activity of Ba(Fe_0.7Ta_0.3)O_3−δ Compounds

Ba(Fe0.7Ta0.3)O3-δ (BFTO) compounds were synthesized using a conventional, high-temperature solid-state ceramic reaction method by varying the sintering temperature (Ts = 1200–1350 °C). The crystal structure, electronic structure, and electrocatalytic activity of BFTO compounds were evaluated. The processing temperature induced phase transformations and structural quality influences the electronic structure and electrocatalytic activity of BFTO compounds. At Ts = 1200 °C, Ba(Fe0.7Ta0.3)O3−δ stabilizes as a mixture of the orthorhombic + rhombohedral phase (Amm2 + R3m). With increasing Ts (≥1250 °C), Ba(Fe0.7Ta0.3)O3−δ ceramics stabilize in tetragonal + rhombohedral [P4mm + R3m] mixed phase with a variation in the number of respective phases. High-resolution X-ray photoelectron spectroscopy (XPS) of constituent elements, namely, Ba 3d, Fe 2p, Ta 4f, and O 1s levels reveals the electronic structure changes due to changes in the chemical environment resulted from structural transformation. The XPS analyses indicate that the processing temperature significantly influences the chemical environment of Fe and Ta cations in BFTO. The Ba 3d5/2 core-level XPS spectra indicate that the perovskite phase gradually increases with increasing sintering temperature. The presence of absorption bands that are exclusively due to MO6 stretching vibration that is connected to Ba ion as well as stretching of M–O bonds in infrared spectroscopy data indicate the structural and chemical quality of the BFTO compounds. The electrocatalytic activity of BFTO was evaluated toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Though all of the samples demonstrated appreciable electrocatalytic properties, the best electrochemical catalytic activity was shown by BFTO samples sintered at 1350 °C. BFTO-1350 °C showed an onset potential of −0.690 V vs reversible hydrogen electrode (RHE) for HER and an onset potential of 0.73 V vs RHE for ORR indicating its significant electrocatalytic performance. A general increase in activity with sintering temperature is potentially due to the improved structural quality of the BFTO ceramics. In addition to offering the fundamental insights into perovskite materials based on co-doped BaTiO3 for electrocatalysis, the present work may contribute to the design and development of materials using co-doping of different chemical valence cations for other energy-related applications.