Figure 3.

Illustration of the mechanism of the sodium storage in g‐C₃N₄ film activated Cu foil. a) The g‐C₃N₄ deposited on Cu foil (current collector) constitutes a metal–semiconductor heterojunction in which electrons will transfer from copper to g‐C₃N₄. b) In the charging process, Na⁺ ions permeate through thin g‐C₃N₄ films and deposit onto the Cu metal by underpotential deposition, thus forming there a Cu‐Na compound layer as sodium metal anode. c) The state after charging: sodium metal acts as the anode material and g‐C₃N₄ film works as SEI to avoid direct contact of the metallic sodium and liquid electrolyte.