Enhancing by nano-engineering: Hierarchical architectures as oxygen reduction/ evolution reactions for zinc-air batteries

Developing efficient electrocatalysts is extremely desirable to decrease the overall cost of electrochemical devices. It usually lies in interface modification for improving intrinsic activity of active sites, and structural optimization for facilitating mass transport. Herein, we rationally design a series of carbon-based hierarchical architectures as bifunctional electrocatalysts through control pyrolysis of metal-organic frameworks coated layered double hydroxide nanoplatelets. Three kinds of catalysts with hierarchical structures are produced, nanoparticles dispersed on carbonized metal oxide nanosheets, carbon nanotubes vertically grown on carbonized metal oxide nanosheets, and carbon nanosheets vertically grown on carbonized metal oxide nanosheets. Among them, the carbon nanotubes vertically grown on carbonized metal oxide nanosheets electrocatalyst possesses the highest electrochemical surface area, lowest charge-transfer resistance, and the fast mass transfer rate, owing to its unique hierarchical porous structure. These properties lead to excellent catalytic activities for oxygen reduction/evolution reactions in terms of half-wave potential (835 mV) and low overpotential (280 mV), which make it a significant electrocatalyst for zinc-air batteries with respectable discharge-charge performance, peak power density (235 mW cm(-2)), specific capacitance (875 mAh g(-1)) and better stability than precious metal catalysts.