Boosted Oxygen Kinetics of Hierarchically Mesoporous Mo₂C/C for High-current-density Zn-air Battery

The high-current-density Zn-air battery shows big prospects in next-generation energy technologies, while sluggish O₂ reaction and diffusion kinetics barricade the applications. Herein, the sequential assembly is innovatively demonstrated for hierarchically mesoporous molybdenum carbides/carbon microspheres with a tunable thickness of mesoporous carbon layers (Meso-Mo₂C/C-x, where x represents the thickness). The optimum Meso-Mo₂C/C-14 composites (≈2 µm in diameter) are composed of mesoporous nanosheets (≈38 nm in thickness), which possess bilateral mesoporous carbon layers (≈14 nm in thickness), inner Mo₂C/C layers (≈8 nm in thickness) with orthorhombic Mo₂C nanoparticles (≈2 nm in diameter), a high surface area of ≈426 m² g^-1, and open mesopores (≈6.9 nm in size). Experiments and calculations corroborate the hierarchically mesoporous Mo₂C/C can enhance hydrophilicity for supplying sufficient O₂, accelerate oxygen reduction kinetics by highly-active Mo₂C and N-doped carbon sites, and facilitate O₂ diffusion kinetics over hierarchically mesopores. Therefore, Meso-Mo₂C/C-14 outputs a high half-wave potential (0.88 V vs RHE) with a low Tafel slope (51 mV dec^-1) for oxygen reduction. More significantly, the Zn-air battery delivers an ultrahigh power density (272 mW cm^-2), and an unprecedented 100 h stability at a high-current-density condition (100 mA cm^-2), which is one of the best performances.