Controlled Synthesis of Non-Precious Mixed Metal Oxides Using Reverse Microemulsions for Energy Storage Systems.

The increasing global demand for energy necessitates the development of advanced and efficient energy storage systems. Alkali metal-oxygen (M-O₂) batteries, such as lithium-oxygen (Li-O₂), have emerged as a promising alternative to Li-ion batteries due to their exceptionally high theoretical specific energies, which stem from their unique redox chemistries. These batteries operate through the formation and breaking of chemical bonds between lithium and oxygen during discharge and charge cycles, respectively, offering the potential for significantly higher energy storage per unit mass than Li-ion batteries. However, their widespread use is hindered due to several challenges including high overpotentials, passivation of the cathode electrode, and undesirable side reactions leading to poor overall efficiency.^1,2 To address these issues, our research focuses on the strategic incorporation of electrocatalysts, which can significantly enhance the efficiency of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) – representing the electrochemical processes during discharge and charge cycles, respectively.²

We specifically investigate non-precious, mixed metal oxides with tunable crystal structures, focusing on Ruddlesden-Popper (A₂BO_4(±δ)) and perovskite (ABO_3(±δ)) oxides, where the A site metal represents La, Sr, Ca, and the B site metal represents Ni or Co. Previous work has shown that incorporating La₂NiO₄ (LNO) nanorods as cathode electrocatalysts synthesized using a reverse microemulsion technique leads to lower overpotentials and enhanced cyclability of Li-O₂ batteries.² In this study, we optimize the reverse microemulsion synthesis method by incorporating a dual microemulsion system, facilitating extrapolation to different crystal structures and compositions. This approach allows tuning the oxide catalyst surface properties through various parameters such as the pH of the reducing agent, controlling the intermicellar exchange rate, and adjusting the oxophilicity and doping concentrations of the metals, to sustain rod-like morphology which can potentially reduce overpotentials further and enhance the overall efficiency and stability of Li-O₂ batteries. We hypothesize that utilizing a weaker reducing agent enables the formation of a dual microemulsion system, which enables a vast operational space and control over parameters that govern rod-like growth of the catalysts, compared to a single microemulsion system. Our findings highlight that the dual microemulsion technique yields distinct differences in particle aspect ratios compared to the single microemulsion method. This approach introduces greater control over nanoparticle growth, potentially enabling more precise investigations into the dynamic processes governing catalyst morphology.

References:

[1] Carneiro, J.S.A, et. al., ACS Catalysis, 2020, 10, 1, 516-527

[2] Samji, S., et. al., Chem. Mat., 2019, 10, 1021