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Phosphorus-doped hierarchically porous carbon (HPC) is prepared with the assistance of freeze-drying using colloid silica and phytic acid dipotassium salt as a hard template and phosphorus source, respectively. Intensive material characterizations show that the freeze-drying process can effectively promote the porosity of HPC. The specific surface area and P content for HPC can reach up to 892 m² g⁻¹ and 2.78 at%, respectively. Electrochemical measurements in aqueous KOH and H₂SO₄ electrolytes reveal that K⁺ of a smaller size can more easily penetrate the inner pores compared with SO₄²⁻, while the developed microporosity in HPC is conducive to the penetration of SO₄²⁻. Moreover, P-doping leads to a high operation potential of 1.5 V for an HPC-based symmetric supercapacitor, resulting in an enhanced energy density of 16.4 Wh kg⁻¹. Our work provides a feasible strategy to prepare P-doped HPC with a low dosage of phosphorus source and a guide to construct a pore structure suitable for aqueous H₂SO₄ electrolyte.