context: In addition, the galvanostatic discharge curves of the hybrid battery at different current densities are studied. As displayed in Fig. 6b, a capacity of 785 mA h gZn−1 is achieved at the current density of 2 mA cm−2 based on the mass of Zn consumed. The result is close to the theoretical value (820 mA h gZn−1). It demonstrates the high capacity of the hybrid battery, taking advantage of the Zn–air battery. When the active species in the cathode and the Zn consumed in the anode are both considered, a high gravimetric energy density of 950 W h kg−1 is achieved. Therefore, the results demonstrate the high capacity and energy density of the hybrid Zn battery. The stability of the hybrid battery was another crucial criterion for practical applications. As displayed in Fig. 6c, the hybrid battery exhibits good electrochemical durability with stable delivered capacity and charge/discharge profiles during cycling at 2 mA cm−2. After one thousand cycles, the voltage of the redox reaction and the ORR/OER process are still well retained (Fig. 6d), demonstrating the good long-term cycling stability of the hybrid zinc battery. In addition, high energy efficiency of 72% is achieved after one thousand cycles, which demonstrates the high efficiency of the hybrid battery.
answer: Zn