Development of Mosaic-like δ-MnO ₂ –MnO _<i>x</i> Heterostructures with Arrested 2D Morphology for Oxygen Electrocatalysis

Manganese oxides are extensively studied for oxygen electrocatalysis, owing to their different crystal structures, morphologies, and Mn in variable oxidation states. However, design, synthesis, and electrocatalytic applications of their heterostructures have gained limited attention. Herein, we report δ-MnO2–Mn2O3 and δ-MnO2–Mn3O4 heterostructures with 2D flake morphology, in which the strain due to lattice mismatch leads to thinner δ-MnO2 flakes with higher interlayer spacing. Mn2O3 twin crystals embedded in the layered δ-MnO2 framework (MnO2–Ox-10) exhibit highly enhanced OER (Oxygen Evolution Reaction) activity, with an η200 (overpotential at 200 mA cm–2) of 570 mV, which is 30 mV lower than that of RuO2. The superior OER kinetics is corroborated by the low Tafel slope value of 71 mV dec–1, low charge-transfer resistance (Rct = 6.14 Ω), high turnover frequency of 0.6 s–1 (at 1.6 V), and high double-layer capacitance (Cdl = 14.2 mF cm–2). The constant potential plateau observed in the chronopotentiometry (CP) analysis at 50 mA cm–2 for over 50 h shows the high-performance stability of the heterostructure. The studies show improved ORR (oxygen reduction reaction) activity of MnO2–Ox-10 in terms of a high n value (3.93) and low peroxide yield (3.4%). The enhanced bifunctional activity of MnO2–Ox-10 is attributed to the higher concentration of lattice defects reflected from the adsorbed (Oads) to lattice (Olatt) oxygen ratio (0.7) in the O 1s XPS spectrum and the intense EPR spectrum with broad line width.