Influence of B-site variation on the bifunctional performance of LaFexCo1-xO3 for Zn-air battery

Perovskites offer great promise as bifunctional catalyst in Zn-air batteries or reversible fuel cells. While there are optimization efforts for either oxygen reduction reaction (ORR) or oxygen evolution reaction (OER), there is a lack of information on the interplay between bifunctional performance and structural and electronic properties.

We investigated the influence of metal variation at the B-site in LaFe_xCo_1-xO₃ (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) on it's structural, magnetic and bifunctional electrocatalytic properties for sol-gel based materials. Characterization was carried out by X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), ⁵⁷Fe Mössbauer spectroscopy and superconducting quantum interference device (SQUID) magnetometry. Rotating ring disc electrode (RRDE) experiments and Zn-air battery measurements in single cells were made to explore the bifunctional catalytic characteristics. On the basis of the data, we were able to identify structural changes that could be linked with variations of the electronic properties. An average e_g orbital filling was determined and correlated to the ORR and OER performance data. The optimum e_g filling was reached at the transition point between orthorhombic and rhombohedral structure. Moreover, in the orthorhombic structure cobalt remained always in the low spin state, whereas its spin state changed much easier with B-site occupancy for the rhombohedral environment. LaFe_0.4Co_0.6O₃ was found to have the best bifunctional activity with a potential gap of 0.93 V, reaching a peak power density of 55 mW cm⁻² in a Zn-air battery single cell.

While these findings indicated that the electronic state might be more easily tuned in the rhombohedral structure types, for future application the synthesis route should be changed to enable significantly larger electrochemical active surface areas.