Figure 5: Energy storage mechanism of the BPOE at discharge process.

(a) p-undoping for the discharge process (at the potential above the initial OCV) of the BPOE. Electrons are extracted from the anode and are injected into the BPOE. Both anion (ClO₄⁻) and cation (Na⁺) will travel back to the electrolyte during the p-undoping discharge process to maintain electrical neutrality. (b) n-doping for the discharge process (at the potential below the initial OCV) of the BPOE. Electrons are extracted from the anode and are injected into the BPOE. Na⁺ will be doped into the BPOE during the n-doping discharge process to maintain electrical neutrality. (c) Summary of the discharge process of energy storage device using a BPOE, which is linearly, continuously connected p-dopable and n-dopable region. The total specific energy E_total (Wh kg⁻¹) can be calculated by summing the specific energy obtained during p-undoping E_p-undoping (Wh kg⁻¹), indicated by the red area, and the specific energy obtained during n-doping E_n-doping (Wh kg⁻¹), indicated by the blue area. E_total=E_p-undoping+E_n-doping can be calculated by: E_total= where Q(t) (mAh g⁻¹)=constant current density I₀(mA g⁻¹) × t(h), is the specific capacity, t(h) is the discharge time and V(t) is the potential.