Dual‐Phosphidation‐Engineered Ni/NiO Heterojunction Nanoflowers With Synergistic Surface PO ₄ ³⁻ Group and P Doping Sites for High‐Performance and Low Temperature (−40°C) Supercapacitors

Nickel oxide (NiO)‐based electrodes with high theoretical specific capacitance can effectively increase the energy density of supercapacitors—the key factor limiting their practical deployment. However, several issues still restrict the production of advanced NiO‐based electrodes. First, a facile strategy to concurrently enhance surface‐interface and bulk conductivity is still absent, limiting the overall electrode conductivity. Second, strategies that boost OH − affinity often increase its desorption barrier, impeding OH − migration. Herein, to address these issues, heterostructured Ni/NiO porous nanoflowers with P‐ doping sites and abundant surface PO 4 3− groups (P‐PO 4 ‐Ni/NiO) are prepared through simple phosphidation. In this material, (1) the heterojunction between Ni and P‐doped NiO increases the overall electrode conductivity; (2) surface PO 4 3− groups and P‐doping sites synergistically boost the affinity for OH − and improve their transfer kinetics; and (3) owing to their structure, porous nanoflowers show a large electrolyte contact area. As a result, the specific capacitance of P‐PO 4 ‐Ni/NiO is five times higher than that of pristine NiO. When assembled into asymmetric supercapacitors, the device exhibits an energy density of 33.1 Wh kg −1 at 750 W kg −1 . At −40°C, the device retains 62.0% of its room‐temperature capacitance and shows nearly no fade after 20,000 charge–discharge cycles. This work presents a robust route for the development of practical transition metal oxide‐based supercapacitors.