Investigation of Li-rich manganese oxide spinel structures for electrochemical water oxidation catalysis

Abstract

The rapid development of efficient and cost-effective catalysts is essential for the oxygen evolution reaction. Herein, nanostructured spinels LiMn₂O₄, delithiated λ-MnO₂, and Li₄Mn₅O₁₂ have been synthesized at low temperatures and are investigated as electrocatalysts for alkaline water oxidation reactions. Among the nanostructured spinels, LiMn₂O₄, delithiated λ-MnO₂, and Li₄Mn₅O₁₂, the former spinel which is classical LiMn₂O₄ with 1/6th of the Mn replaced by Li outperforms for the OER that shows a current density of 5 mA cm⁻² at a lowest overpotential of 430 mV and Tafel slope of 74 mV per decade. Electrochemical impedance studies revealed the least value of charge transfer resistance of the Li₄Mn₅O₁₂ spinel and suggest fast reaction kinetics for the oxygen evolution reaction as compared to other spinels. The XPS and TEM of Li₄Mn₅O₁₂, recorded after a 12-hour stability test for oxygen evolution activity, confirm that the oxidation state of Mn and the morphology of Li₄Mn₅O₁₂ remain intact even after the electrocatalytic reaction, however, it undergoes amorphization. The higher activity of Li₄Mn₅O₁₂ synthesized in the present work is attributed to the low temperature synthesis resulting in the formation of a nanostructured Li rich spinel with a high surface area, along with an increased percentage of ionic bonding and the presence of 3D Li diffusion channels. The role of Li was further supported by XPS studies that revealed a shift in Li 1s binding energy as well as quantitative reduction relative to Mn for Li₄Mn₅O₁₂ after a long term test.