Strongly Coupled MnO2 Nanosheets/Silver Nanoparticles Hierarchical Spheres for Efficient Oxygen Reduction Reaction Electrocatalysis

Manganese dioxide (MnO2) represents a promising oxygen reduction reaction (ORR) electrocatalyst, but its catalytic activity is largely limited by the essentially low electronic conductivity and large geometric size. Herein, hierarchical ε-MnO2 nanosheets (MnO2-H) strongly coupled with silver nanoparticle spheres (Ag NPs) were developed. The introduced Ag NPs can not only enhance the electronic transfer but also tune the electronic structure of MnO2 and act as active sites for ORR. Owing to the large electrochemically active surface area, good mass/electronic transfer, and a strong coupled interface between MnO2-H and Ag NPs, the optimal Ag-MnO2-H-1.53 showed much higher catalytic activity than MnO2-H, Ag microparticles, and commercial Pt/C catalyst for ORR. Moreover, Ag-MnO2-H-1.53 displayed robust catalytic stability with negligible shift in the half-wave potential after 5000 cycles. As a cathode catalyst, a homemade zinc–air battery (ZAB) based on Ag-MnO2-H-1.53 showed a high open circuit voltage (1.52 V), maximum peak power density (120.2 mW/cm2), and specific capacity (646 mAh/g) superior to those of Pt/C-based ZAB (1.48 V, 82.3 mW/cm2, and 612 mAh/g), along with a long lifetime. This work highlights the significance of strong interface coupling between MnO2-H and Ag NPs for boosting the ORR electrocatalysis, which will inspire more work for the rational design of efficient carbon-free transition metal electrocatalysts for energy conversion.