On The Interplay of Quantum Capacitance and Pseudocapacitance in rGO with Regulated C/O Ratios

Supercapacitors have evolved as mature energy storage devices, utilizing graphene as electrodes to store charge electrostatically. During the process, electric double layers are formed at electrode/electrolyte interface, offering electric double layer capacitance (EDLC). Although large surface area and high conductivity of graphene are favorable for enhanced EDLC, intrinsically low quantum capacitance of graphene limits the overall performance. In recent years, reduced graphene oxide (rGO) has been increasingly gaining favor as an alternative material based on graphene owing to its cost-effective means of production. Experimental characterizations have identified abundant epoxy and hydroxyl functional groups on its basal plane, while hosting relatively fewer carbonyl and carboxyl groups at its edges. Interestingly, these oxygen-containing species on the surface of rGO serve as electrochemically active sites for enhanced pseudocapacitance in aqueous EDLCs. While rGO electrodes have demonstrated superior performance compared to graphene electrodes, the reported capacitances have shown inconsistent responses to different C/O ratio. This highlights the necessity to comprehend the impact of the degree of reduction on the total capacitance and theoretical simulations can offer valuable insights to decouple the contributions to the total capacitance. Therefore, in this work, density functional theory (DFT) calculations have been employed for studying the capacitive behavior of rGO functionalized with epoxy and hydroxyl functional groups with different coverages on their basal planes. We have considered the cases for low-level, medium-level and high-level oxygen functionalization depending upon the C/O ratio. Under low-level oxygen functionalized graphene (LOFG), rGO with C/O ratios of 8, 6.4 and 5.33 are considered, while for medium-level oxygen functionalized graphene (MOFG) and high-level oxygen functionalized graphene (HOFG), rGO with 4.57, 4, 3.55 and 3.2, 2.9, 2.66 C/O ratios are considered, respectively. We have computed EDLC of rGO subjected to different degrees of reduction using implicit solvent model. The computation of EDLC from this method is validated by first calculating EDLC of graphene (116.52 Fg^-1) and compared with the experimental value (125-135 Fg^-1). Our calculations show that dispersed coverage of rGO with more of epoxy groups offer relatively higher EDLC than that with more of hydroxyl groups. However, in both the cases, EDLC decreases monotonically for decreasing C/O ratio. Quantum capacitances (C_Q) of LOFG, MOFG and HOFG are directly related to the changes in density of states (DOS) near the Fermi level. LOFG with more epoxy groups have zero bandgap, possessing higher C_Q while MOFG & HOFG with more hydroxyl groups have lower bandgap opening, possessing higher C_Q. As bandgap gets wider, maximum C_Q is obtained at higher voltages. OH-induced enhancement in quantum capacitance (C_Q) tends to be more dramatic with positive polarity because of relatively very low band gap than possessed by graphene with more epoxy functionals. Pseudocapacitance (C_pseudo) too exhibits the similar pattern, depicting decreased capacitance with decreasing C/O ratio. Such a behavior is explained by studying the variation in dipole moment and work function change. Maximum total capacitance is offered by rGO at C/O ratio of 5.33, well in accordance with an experimental study. Suppression of EDLC is mitigated by the enhancement in both C_pseudo and C_Q for LOFG while only increased C_Q compensates for MOFG. This study motivates future investigations into control and careful optimization of the oxygen content to balance the oxygen induced influences on electrical capacitance and quantum capacitance.