Electronic Synergistic Effects on the Stability and Oxygen Evolution Reaction Efficiency of the Mesoporous LiMn_2–<i>x</i>M_<i>x</i>O₄ (M = Mn, Fe, Co, Ni, and Cu) Electrodes

Stable porous manganese oxide-based electrodes are essential for clean energy generation and storage because of their high natural abundance and health safety. This investigation focuses on mesoporous LiMn_2-<i>x</i>M_<i>x</i>O₄ (where M is Fe, Co, Ni, and Cu and <i>x</i> is 0, 0.1, 0.3, 0.5, and 0.67) electrodes and thin/thick films. The mesoporous electrodes and films are fabricated by coating clear and homogeneous ethanol solutions of the salts (LiNO₃, [Mn(OH₂)₄](NO₃)₂, and [M(OH₂)_<i>x</i>](NO₃)₂) and surfactants (P123 and CTAB) and calcining at elevated temperature (denoted as F-LiMn_2-<i>x</i>M_<i>x</i>O₄, G-LiMn_2-<i>x</i>M_<i>x</i>O₄, and <i>meso</i>-LiMn_2-<i>x</i>M_<i>x</i>O₄, respectively). The electrochemical properties, stability, and oxygen evolution reaction (OER) performance of the F/G-LiMn_2-<i>x</i>M_<i>x</i>O₄ electrodes are investigated in alkaline media using a three electrode setup. The F-LiMn_1.33M_0.67O₄ electrodes (where M is Mn, Fe, Co, and Ni) exhibit low Tafel slopes of 60, 43, 44, and 32 mV/dec, respectively. While all the Mn-rich and F-LiMn_2-<i>x</i>Fe_<i>x</i>O₄ electrodes degrade via Mn(VI) disproportionation reaction, the 33% Co electrode shows high stability during the OER. The nickel-based electrodes are stable with as little as 15% Ni and display excellent OER performance over 25% Ni, albeit undergoing a transformation that accumulates Ni(OH)₂ species on the electrode surface. Copper in the F-LiMn_2-<i>x</i>Cu_<i>x</i>O₄ electrodes is homogeneous at low Cu percentages but forms a CuO phase above 15% Cu, undergoes degradation, and displays a weak OER performance. In short, Co and Ni stabilize the F-LiMn_1.33Co_0.67O₄ and F-LiMn_1.7Ni_0.3O₄ electrodes, which display excellent OER performance.