Designing Fluorographene with FeN4 and CoN4 Moieties for Oxygen Electrode Reaction: A Density Functional Theory Study

Abstract Development of the efficient bifunctional oxygen electrode is indispensable but challenging for the rechargeable metal–air batteries. The oxygen reduction reaction and oxygen evolution reaction of theTMN4 embedded graphene, graphane and fluorographene are systematically investigated by the density functional theory calculations (TM = Fe, Co and Ni). Our results show that the ORR/OER activity and the stability of the TMN4 moieties are dramatically changed by the graphene functionalization. According to the free energy analysis, the H/F decoration on the carbon skeleton improves the activities of FeN4 and CoN4 moieties, in comparison with the graphene counterpart. In detail, the FeN4-based electrodes are potential ORR ones where the overpotentials are reduced from 0.98 V of G/FeN4 to 0.46 V of GH/FeN4 and 0.38 V of GF/FeN4. Meanwhile, the CoN4-based electrodes possess good OER efficiency featured with the overpotentials of -0.50 V and -0.53 V for GH/CoN4 and GF/CoN4 with respect to -0.72 V for G/CoN4, respectively. On the other side, the high thermodynamic barrier of NiN4-based electrodes limits its application, regardless of the supports. Furthermore, the binding strengths between TM and its N coordination are substantially increased due to the presence of H/F attachments, indicating the enhanced TM capture, which ascribes to the corresponding wrinkle sp3 structure. Additionally, the structural integrity without any degradation in the molecular dynamic stimulation further supports the thermodynamic stability at the room temperature. The robustness of GH/TMN4 and GF/TMN4 illustrates the feasibility of the experimental synthesis. Considering the possible dehydrogenation of the graphane at the elevated temperature, the fluorographene with atomically dispersed FeN4 and CoN4 moieties is recommended as promising oxygen electrode. To shed light on the physical origination, the electronic structure analysis correlates the activity enhancement with the change of the TM d-orbital, being evidenced by the linearity between the OH affinity and the d band center. Therein, the influences of the graphene functionalization on the electrocatalysis provide new insights into the design of the bifunctional oxygen electrode.