Fast charge transfer of single metal atom coordinating p-phenylenediamine covalently pillared graphene cathode for high-rate lithium-ion capacitors

Abstract

P-type redox-active organic molecules (ROMs) exhibit higher redox potentials due to deeper highest occupied molecular orbital (HOMO) energy level, but the inherent higher charge transfer energy barrier limits their rate performance, which cannot be overcome by simply hybridizing with carbon materials (CMs). Herein, Co single atoms (SAs) are introduced into p-phenylenediamine (pPD) pillared graphene (rGO-pPD-Co), aiming to leverage their catalytic effect on the pseudocapacitive charge transfer process. Experimental results and density functional theory (DFT) calculations reveal that Co SAs alter the local charge distribution, activating their redox activity toward ${\text{PF}}_6^{-}$ adsorption. Their electron-donating property disrupts the conjugation and elevates the Fermi level of rGO-pPD-Co, thus reducing the energy barrier for electron transfer. Additionally, the more positive electrostatic potential (ESP) of Co SAs coordination increases the affinity with ${\text{PF}}_6^{-}$ anion, lowering the total Gibbs free energy change for the redox reaction of rGO-pPD-Co. Although exhibiting slightly reduced redox potentials, the catalytically accelerated rGO-pPD-Co cathode achieves excellent rate performance (128 mAh g^-1 at 20 A g^-1). This work demonstrates the catalytic effect of coordinated single-atom metals (SAMs) on the charge transfer of p-type groups, providing a novel strategy to accelerate pseudocapacitive reaction kinetics and boost the rate capability of p-type ROMs.