Microwave and NaCl driven synthesis of P-doped graphitic carbon at atmospheric pressure for long-life vanadium redox flow batteries†

Scalable and environmentally sustainable synthesis of phosphorus-doped graphitic carbon (SDG-PC) was developed utilizing a dual microwave-driven method that employs agricultural byproducts and sodium chloride (NaCl) as a microwave absorber. This process facilitated the formation of a highly graphitized structure enriched with phosphorus oxide (PO) and phosphorus–carbon (P–C) active sites. The distinctive chemical structure of SDG-PC, characterized by PO-dominated doping, significantly enhanced its catalytic activity for vanadium ion redox reactions (VIRR). Half-cell tests revealed a peak current density 2.93 times higher for SDG-PC compared to the control, demonstrating its superior electrochemical performance. Density functional theory (DFT) calculations confirmed that the PO sites in SDG-PC play a crucial role by providing strong adsorption for vanadium ions, reducing the deprotonation energy for VO²⁺, and enhancing reaction reversibility. The structural advantages were reflected in VRFB performance, where SDG-PC achieved a discharge capacity of 25.2 A h L⁻¹ at 400 mA cm⁻²—42.4% higher than commercial heat-treated graphite felt. Additionally, the SDG-PC electrodes exhibited exceptional durability, retaining 79.4% of their initial capacity after 1000 cycles under high current density (400 mA cm⁻²), which represents a significant improvement compared to previously published work using similar carbon structures.