Engineering nitrogen-doped porous carbon positive electrodes for high-performance sodium-ion capacitors: the critical role of porosity, structure and surface functionalities

Abstract

Sodium-ion capacitors are increasingly gaining momentum thanks to their high energy and power densities. However, there is still a lack of understanding of porous carbon positive electrode properties that affect their electrochemical performance. To address this challenge, carbon materials with controlled porosity, structure and surface functionalities are strongly required. Herein, we report the synthesis of nitrogen-doped porous carbons (NDPCs) by a combined soft-salt templating approach, that allows to achieve various nitrogen doping levels (up to 8 at%) via precursor amount modification. This results in materials with ultrahigh specific surface area (up to 2412 m² g⁻¹) and finely tuned pore size (up to 0.92 nm) matching the desolvated PF₆⁻ anion sorption requirement of 0.8 nm, along with controlled graphitization induced by the salt type. The materials exhibit specific capacities ranging from 83 to 159 mA h g⁻¹vs. Na/Na⁺, higher than that of commercial carbons. From positive linear correlations, it was identified that the improved capacity is driven by the large specific surface area, substantial microporous volume with appropriate pore size, and structural defects, which enhance ion adsorption and promote enhanced specific capacity. However, the capacity retention is improved by the mesoporous volume and graphitic domains. Moreover, the surface pseudocapacitive interactions involving Na⁺ and PF₆⁻ ions could be associated with specific oxygen-containing groups (phenol/ethers and anhydride) and nitrogen species (pyridinic-N/pyrrolic-N). The dual carbon full-cell configuration consisting of a hard carbon and N-doped carbon achieves a high energy density of 209 W h kg⁻¹ and a maximum power density of 5040 W kg⁻¹ with ∼100% coulombic efficiency.