Morphology-Driven Bifunctional Activity of Layered Birnessite-Based Materials toward Oxygen Electrocatalysis

Abstract

The chemical, structural, and morphological diversity of birnessite, a 2D layered MnO₂, has opened avenues for its application as an electrocatalyst toward both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Among pristine birnessites prepared by different methods, the freestanding flakes (primary structure) obtained from molten salt (MS-KMnO) showed remarkable bifunctional activity as compared to samples with thicker plates or a hierarchical honeycomb-like (type-I secondary structure) morphology. While the ORR onset potential (E_onset) and halfwave potential (E_1/2) for MS-KMnO were recorded at 0.89 and 0.81 V vs RHE, respectively, the OER overpotential (η) was found to be 300 mV. We demonstrated heat-induced secondary structure evolution by modification of the molten salt method, which led to a decrease in activity. In contrast to previous studies, the Co-doped birnessite (Co-KMnO) prepared in molten salt showed lower bifunctional activity (ORR, E_1/2 = 0.72 V; OER, η= 460 mV) as compared to MS-KMnO. Co-KMnO showed an interwoven wrinkled sheet-like (type-II secondary structure) morphology, with Co³⁺ present in both the in-layer and the interlayer. However, in Co-KMnO/360 prepared at a lower reaction temperature, the areal coverage of the type-II structure reduces, leading to an increase in ORR (E_1/2 = 0.76 V) and OER (η = 440 mV) activity. The chronopotentiometry for 100 h at a constant OER current of 50 mA cm^–2 showed an increase in potential from 1.62 to 1.89 V and the characterization of the sample post-treatment showed degradation of the layered structure in MS-KMnO. The samples obtained after 1000 CV cycles in both the ORR and the OER regions showed the formation of secondary structures with a substantial decrease in the Mn³⁺/Mn⁴⁺ ratio. This study demonstrates that morphology tuning within the 2D birnessite system has a marked effect on its bifunctional activity.