Theoretical Investigation of Water Oxidation on Fully Saturated Mn2O3 and Mn2O4 Complexes

Understanding the factors that affect efficiency of manganese oxides as water oxidation catalysts is an essential step toward developing commercially viable electrocatalysts. It is important to understand the performance of the smallest versions of these catalysts, which will in return be advantageous with bottom up catalytic design. Density functional theory calculations have been employed to investigate water oxidation processes on Mn2(μ-OH)(μ-O)(H2O)3(OH)5 (Mn2O4·6H2O), Mn2(μ-OH)2(H2O)3(OH)4 (Mn2O3·6H2O), and Mn2(μ-OH)2(H2O)4(OH)4 (Mn2O3·7H2O) complexes. The effect of different oxidation states of manganese is considered in this study. Thermodynamically, the lowest energy pathway for the fully saturated Mn2O4·6H2O complex occurs through a nucleophilic attack of a solvent water molecule to a Mn(IV1/2)O oxo moiety. The lowest energy pathway on the Mn2O3·6H2O complex proceeds with an attack of Mn(VI)O group to the surface hydroxo group on the same manganese atom; this pathway is related to the third lowest energy pathway on the Mn2O4·6H2O complex. The water oxidation process on the fully saturated Mn2O3·7H2O complex also involves a nucleophilic attack from a solvent water molecule to a Mn(V)O moiety. The formation of these manganese oxo groups can be used as a descriptor for selecting a manganese-based water splitting catalyst due to the high electrochemical potentials required for the generation of these groups.