context: Determined by a continuous discharging test over 27 h, the recorded specific capacity of the battery with the CNT@CNP catalyst is 846.7 mA h gZn−1, and the relative energy density is 1059.92 mW h gZn−1 at an average voltage of 1.2 V, as shown in Fig. 7(d). These outcomes are better than those of most current Fe–N–C catalysts shown in Table 2. Moreover, the rechargeable performance of the battery, equipped with the CNT@CNP catalyst and commercial 20 wt% IrO2 on carbon paper as an air cathode, has been tested by us. As observed in Fig. 7(e), the novel battery shows remarkably improved ORR and OER properties, compared with the reference of the battery equipped with the commercial Pt/C and IrO2 counterpart. At a constant current density of 20 mA cm−2 for 10 h, the cycling of charge and discharge reaches 2 times with respect to the reference battery, as shown in Fig. 7(f). The battery almost maintains a stable discharge potential of 0.83 V and a recharge voltage of 2.25 V even after 60 cycles, which is much better than that of the reference battery shown in Fig. 7(f). So, the synthesized catalyst shows a very good power density in comparison with the corresponding results available in the literature shown in Table 2. Therefore, the optimal catalytic performance of the carbon interpenetrating networks may be attributed to the formation of abundant edge Fe–Nx moieties as active sites at the boundaries and interfaces in the CNT@CNP composites with a smart structure. If so, developing Fe–N–C with interpenetrating network structures may be major breakthroughs of non-PGM materials serving as next-generation catalysts for the ORR.
answer: air