Porous carbon membrane embedded with high-loading NiCoP nanoparticles as bifunctional electrodes for supercapacitors and electrocatalytic water splitting

Abstract

Porous carbon membrane-based bifunctional electrodes (Ni_xCo_yP@CSA) are developed via direct carbonization, followed by vapor-phase phosphidation of Ni²⁺/Co²⁺-crosslinked starch aerogel. During the carbonization and phosphidation processes, the crosslinked starch framework is transformed into a hierarchically porous, conductive, and mechanically strong carbon membrane, while the Ni²⁺/Co²⁺ species are concurrently converted into embedded carbon-coated Ni_xCo_yP nanoparticles with a loading exceeding 40 wt%. Benefitting from its hierarchically porous structure, excellent electrolyte wettability, and abundant active sites, the best NiCo₂P@CSA electrode exhibits a high areal specific capacitance (C_a) of 9.41 F cm⁻² at 2 mA cm⁻², a volumetric capacitance (C_v) of 115.0 F cm⁻³ at 20 mA cm⁻³, and an NiCo₂P loading-based mass capacitance (C_m) of 1108.9 F g⁻¹ at 0.2 A g⁻¹ in a three-electrode system. In addition, an asymmetrical supercapacitor (SC) assembled by pairing NiCo₂P@CSA and an activated carbon/Ni foam (AC/NF) exhibits a high C_a of 2.46 F cm⁻² at 2 mA cm⁻², an energy density of 0.41 mWh cm⁻² at a power density of 1.1 mW cm⁻², and a capacitance retention of 75% with a unit coulombic efficiency after 10 000 cycles. In addition, NiCo₂P@CSA can be directly used as a binder-free bifunctional electrode for electrocatalytic water splitting, requiring low overpotentials of 192.4 and 418.5 mV to achieve a current density of 100 mA cm⁻² for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, and it exhibits superior durability in the HER (>100 h at 20 mA cm⁻²) and OER (>40 h at 20 mA cm⁻²).