Dense binary Fe-Cu sites promoting CO2 utilization enable highly reversible hybrid Na-CO2 batteries

High-performance and low-cost catalysts are particularly desirable for the exploitation of practical low-overpotential Na-CO2 batteries with protracted cyclability. Herein, a well-defined morphology of nitrogen-rich graphitic carbon frameworks with dense bimetallic active sites (Fe-Cu-N-C) was facilely prepared by introducing Fe3+ and Cu2+ to regulate in situ grown carbon nanotubes as an advanced catalyst toward hybrid Na-CO2 batteries. Through metal content tuning and carbon architecture altering, Fe-Cu-N-C proved to be dramatically more effective than Cu-N-C and Fe-N-C. As the cathodic catalyst of a hybrid Na-CO2 battery, Fe-Cu-N-C can facilitate the fast evolution and degradation of flocculent discharge products and achieve an excellent long-term cyclability with up to 1550 cycles (over 600 h), which makes it one of the greatest catalysts for hybrid Na-CO2/air batteries that have been reported to date. The observed outstanding battery performance is attributable to the cross-linked conductive framework affording a "highway" for accelerated electron transport and Na+/CO2 diffusion. Besides, the synergistic effects among defect-rich interfaces, Fe/Fe3C nanocrystals, and Fe-N-x and Cu-N-x sites derived from nitrogen atom doping enhance the catalytic activity. In addition, the possible growth and decomposition mechanisms of NaHCO3 products with different morphologies on Fe-N-C, Cu-N-C, and Fe-Cu-N-C electrodes were presented and discussed.