Low power density (operated current density) is one critical obstacle to the development of single flow zinc–nickel batteries (ZNBs). Three-dimensional porous nickel foams (NFs) are first introduced to ZNBs to improve power density. The relationship between utilized electrode area, potential distribution and operated current density has been studied. A high coulombic efficiency (97.3%) and energy efficiency (80.1%) are obtained at 80 mA cm−2 over 200 cycles, which is the highest value ever reported for ZNBs. The power density is improved nearly fourfold to 83 W kg−1. The results indicate that three-dimensional porous electrodes are more suitable as negative electrodes for ZNBs under high operated current densities. This provides an effective way to improve power density of ZNBs and will arouse a new revolution in the development of flow batteries.