A DFT study on the fundamental mechanisms of quantum capacitance enhancement within the carbon-based electrodes through different classes of doped configurations from biomass-derived elements

Density functional calculations were performed on 15 functionalized graphene models to investigate the enhancement of quantum capacitance (CQ) by common dopant elements N, P, S, and O from biomaterials. Geometry optimizations and formation energy calculations demonstrated that the van der Waals radius and additional covalent bonds influenced the mechanical stress and formation energy, particularly due to the distortion of the graphene lattice caused by larger S or P atoms replacing carbon atoms. According to both the CQ and formation energy calculations, nitrogen emerged as the most promising doping element for enhancing CQ, followed by phosphorus, while sulfur showed a relatively lower contribution. Electron density profiles indicated that the improvement of CQ was facilitated by the lone pair electrons at the defects. The effects of dopants on electronic structures were further elucidated through CQ characteristics, resulting in the classification of functionalized graphene models into three types. The ‘graphitic’ type represented configurations that preserved most of the electronic structure of pristine graphene, while ‘p-type’ and ‘n-type’ represented those experienced the loss and gain of valence electrons, respectively. This electronic structure-based classification of doping provides valuable insights for future designs, enabling control over sintering and doping conditions in biomass-derived electrode materials for supercapacitors.