(4iw) Revealing the Exceptional Capacitive Potential of High-Quality Bilayer Graphene Obtained through Electrochemical Exfoliation

Research Interests: Energy conversion & storage devices (fuel cells, batteries & supercapacitors), Electrochemical water splitting, Electrocatalysis, Material engineering for advanced functional materials, Intercalation

The liquid phase exfoliation methods, though scalable and economical, produce low-quality graphene, which suffers from layer number dispersion, small flake size, and oxygen functionalizations that render poor optoelectronic properties. We show the use of electrochemical exfoliation of highly oriented pyrolytic graphite (HOPG) in 98% w/w sulfuric acid for the synthesis of layer number-monodisperse bilayer graphene (BG) with high optical transmittance (∼94.87%) and conductivity (∼2.39 × 10⁶ S m^−1) comparable to the CVD-derived BG. The synthesis method was engineered using information from electrochemical characterizations to select the sulfuric acid concentration and electrode potentials that allow the synthesis of single-stage II graphite bisulfate (GB) electrode as the key intermediate whose controlled exfoliation provided single-stage II GB particles of small domain size with little oxidation or other defects. The GB particles were finally cleaved to BG in dispersion favorable N,N-dimethylformamide (DMF) solvent with 80.5% yield by suit-ably selecting the power density of ultrasonic agitation. The morphological, structural, and chemical analysis of BG showed large-size BG flakes (average area ∼ 93 μm²) with a high C/O ratio (30.89). Different graphene grades with varying oxygen contents were synthesized through exfoliation in 50–90% w/w sulfuric acid. BG, with the lowest (3.2%) oxygen content, exhibited the lowest specific capacitance (∼521.7 F g^−1) but the highest capacity retention (∼92%) when the discharge rate was increased by eightfold. GO, with the highest oxygen content (26.8%), exhibited the highest specific capacitance (∼2000 F g^−1) but the lowest capacity retention (∼71%) when the discharge rate was increased by eightfold.