Maneuvering the Physical Properties and Spin States To Enhance the Activity of La–Sr–Co–Fe–O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation

Perovskite oxides have attracted considerable attention as durable electrocatalysts for metal–air batteries and fuel cells due to their precedence in oxygen electrocatalysis in spite of the complexities involved with their crystal structure, spin states, and physical properties. Here we report optimization of the activity of a model perovskite system La 1– x Sr x Co 1– y Fe y O 3−δ (LSCF; x = 0.301, y = 0.298, and δ = 0.05–0.11) toward electrochemical water oxidation (OER) by altering the calcination temperature of the nonaqueous sol–gel synthesized nanoparticles (NPs). Our results show that improved OER activity is the result of a synergism between its morphology, surface area, electrical conductivity, and spin state of the active transition metal site. With an e g orbital occupancy of 1.26, the interconnected ∼90 nm LSCF NPs prepared at 975 °C (LSCF-975) outperforms the other distinguishable LSCF morphologies, requiring 440 mV overpotential to achieve 10 mA/cm 2, a performance comparable to the best-performing perovskite oxide electrocatalysts. While the interconnected NP morphology increases the propensity of electronic conduction across crystalline grain boundaries, the morphology-tuned high spin Co 3+ ions increases the probability of binding reaction intermediates at the available surface sites. Density functional theory based work function modeling further demonstrates that LSCF-975 is the most favorable OER catalyst among others in terms of a moderate work function and Fermi energy level facilitating the adsorption and desorption of reaction intermediates.