A comparative study of dual- and quaternary-doped reduced graphene oxide cathodes incorporating B, F, N, and S for lithium-ion capacitors

Reduced graphene oxide (rGO) offers a viable alternative, possessing many of graphene’s advantageous properties. Heteroatom doping is an effective strategy to modify the structural and electrochemical properties of rGO for Lithium Ion Capacitor (LIC) applications. While the use of one or two dopants in rGO is common and has been shown to improve performance, recent studies have explored the effects of four dopants in rGO synthesis. This study presents a quaternary-dopant strategy for rGO (BFNS–rGOs) that incorporates boron (B), fluorine (F), nitrogen (N), and sulfur (S) via boron trifluoride dihydrate and thiourea precursors in a hydrothermal method. The BFNS(1:1)–rGO and BFNS(1:3)–rGO were compared with pristine rGO, and rGO doped with dual-atoms (BF–rGO and NS–rGO). The performance of heteroatom-modified rGOs were investigated as electrode materials for LIC. BF-rGO exhibits the highest specific surface area (19.62 m² g⁻¹) and achieves the highest gravimetric capacitance, underscoring the importance of accessible surface area for charge storage. In addition, BFNS(1:3)–rGO, which has a lower B/F ratio than BFNS(1:1)–rGO, exhibits the highest crystallite stacking height (Lc = 1.89 nm), the largest number of layers (5.37), and the highest surface-area-normalized capacitance. These results indicate that the enhanced electrochemical activity of BFNS(1:3)–rGO is attributable to a favorable combination of structural ordering, defect sites, and heteroatom-induced electrochemical activity, rather than increased surface area alone. These findings suggest that precise control of heteroatom composition and dopant ratio is more critical than maximizing dopant concentration, providing a rational strategy for designing rGO-based electrodes with enhanced intrinsic electrochemical activity for LIC applications.