Metal-Free Multilayer Graphene Electrodes Activated by PECVD for Electrochemical Oxidation Enhancement

Multilayer graphene (MLG) was obtained by 3 h ultrasonication of thermally expanded graphite in n-propanol, producing 5–13-nm-thick flakes with 1–50 μm lateral size. The powder was dry-pressed into self-supported disk electrodes (MLGD) and surface-activated by brief MW-PECVD and RF-PECVD in Ar/CO2 and Ar/O2. PECVD raised Raman ID /IG from 0.07 to 0.26–0.28 (La ≈ 275 → 69–92 nm) and gave higher localized O for CO2 MW-PECVD (≈1.05 wt % vs ≈0.13 wt % for O2). Electrochemically, areal capacitance increased ≈11.9 → ≈16.5 mF·cm–2 (∼39%), and ethanol oxidation peak currents improved up to ≈ 40% (peak up to 2.94 mA·cm–2 for O2 RF-PECVD in 1.0 M NaOH + 2.0 M EtOH), with >95% capacitance retention after 15 CV cycles. These results highlight that PECVD post-treatment provides a rapid, reagent-free, and tunable surface functionalization of compacted MLG electrodes, enabling increased capacitance and enhanced ethanol oxidation with minimal bulk structural degradation.