Tailor-made metal-nitrogen-carbon bifunctional electrocatalysts for rechargeable Zn-air batteries via controllable MOF units

The majority of chemical syntheses involve the use of catalysts, which play a crucial role in the yield and conversion rates of chemical reactions. In view of the increasing demand for chemical commodities and specialties linked to the growth of the world's population and the living standards, highly efficient and low-cost catalysts are urgently required. The metal-nitrogen-carbon (M-N-C) catalysts family is one of the most promising candidates. In this work, a series of benzene-1,3,5-tricarboxylate linker based metal organic frameworks (MOFs) were used as self-sacrificial templates and tunable platform for designable preparation of M-N-C catalysts. Changing the pillars between the 2D layers and the nature of the metal ions in the pristine MOFs significantly influenced the structure, chemical composition and catalytic activity of the resulting M-N-C catalysts for the oxygen reduction reaction (ORR). Furthermore, the influence of the MOF units on the catalyst performance, the role of the metals in the M-N-C catalysts and the primary catalytically active sites for ORR were explored by a combination of density functional theory (DFT), in-depth structural and chemical/elemental characterizations, and electrochemical studies. Among the prepared catalysts, Co-BTC-bipy-700 exhibited the highest electrocatalytic activity for oxygen reduction reaction (ORR), which showed a larger limiting current density and similar half-wave potentials with less catalyst degradation and much higher methanol tolerance than the commercial Pt/C catalyst. Meanwhile, as a bifunctional electrocatalyst, Co-BTC-bipy-700 catalyst was also employed for oxygen evolution reaction (OER) and demonstrated a lower overpotential (lowered by 140 mV at a current density of 10 mA cm−2) and better durability than IrO2. Furthermore, in terms of device performance, the Zn-air battery enabled by Co-BTC-bipy-700 catalyst reached a maximum specific energy as high as 1009.8 Wh kg−1, which is 76.5% of the theoretical value (1320 Wh kg−1), and demonstrated higher discharge potential and lower charge potential than that based on the Pt/C catalyst. Importantly, the presented strategy for tailor-made M-N-C catalysts by controlling the synthesis of the pristine MOFs could offer a guide map for the future design of M-N-C catalysts family not only for electrochemical reactions but also beyond electrochemistry.