Modulating Oxygen Evolution Reactivity in MnO2 through Polymorphic Engineering

Enhanced electrocatalytic activity of the oxygen evolution reaction (OER) can be achieved through modulation of the electronic structure of the electrocatalytic active sites. This modulation can also be achieved through stabilization of metastable or non-native polymorphs of the electrocatalyst. Non-native (NN) crystal structures differ in their discrete translational symmetry from the bulk native (N) crystal. The variable oxygen evolution reactivity in a basic medium of different polymorphs is demonstrated by synthesizing β/N-, γ/N-NN1-, r/NN1-, α/NN2-, and δ/NN3-MnO2 polymorphs of MnO2 which show different active site densities on the surface, XPS-derived oxidation state of Mn, and bulk electronic conductivity. The specific OER activity [activity per electrochemical surface area (ECSA)] of MnO2 is codependent on the oxidation state of Mn and electronic conductivity. A volcano-based relationship is observed for the specific OER activity of MnO2 polymorphs with the universal descriptor ΔGO* – ΔGHO*, computed from density functional theory (DFT). Both δ/NN3-MnO2 and α/NN2-MnO2 lie closer to the volcano peak but on opposite legs of the volcano. δ/NN3-MnO2 shows higher specific activity due to its low oxidation state (+3.5) of Mn, which is confirmed through the calculated average oxidation states (AOS) from XPS and Bader charge from DFT studies. α/NN2-MnO2 shows higher specific activity due to higher electronic conductivity, which is correlated with its low oxygen vacancy formation energy. Further, the electronic origin of high OER activity in two of the most non-native polymorphs (δ/NN3-MnO2 and α/NN2-MnO2) is ascribed to a shift in the valence Mn-d band closer to the Fermi level leading to stronger O adsorption. The present study demonstrates the efficacy of utilizing non-native polymorphs for OER and unfolds the correlation between OER and variable oxidation states and electronic conductivities, thereby providing directions toward the generalization of these effects to other polymorphic compounds.