fig4

Figure 4. RFB performances. (A) Coulombic and energy efficiencies vs. cycle number of Ni₁/NC, NC, and GF at various current densities; (B) Voltage profiles vs. capacity of Ni₁/NC, NC, and GF at a current density of 80 mA cm^-2; (C) CE and EE profiles of Ni₁/NC, NC, and GF charged at 80 mA cm^-2 and discharged under high current densities ranging from 100 to 700 mA cm^-2; (D) Voltage and power density curves vs. current density of Ni₁/NC, NC, and GF; (E and F) Long-term cycling stability of Ni₁/NC, NC, and GF at 200 mA cm^-2. RFB: Redox flow battery; NC: nitrogen-doped carbon; GF: graphite felt; CE: coulombic efficiency; EE: energy efficiency.