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

Abstract

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 (≈2µm in diameter) are composed of mesoporous nanosheets (≈38nm in thickness), which possess bilateral mesoporous carbon layers (≈14nm in thickness), inner Mo2C/C layers (≈8nm in thickness) with orthorhombic Mo2C nanoparticles (≈2nm in diameter), a high surface area of ≈426m2g-1, and open mesopores (≈6.9nm 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 (0.88V vs RHE) with a low Tafel slope (51mVdec-1) for oxygen reduction. More significantly, the Zn-air battery delivers an ultrahigh power density (272mWcm-2), and an unprecedented 100h stability at a high-current-density condition (100mAcm-2), which is one of the best performances.