Fig. 4. DFT computation results on energy, electron structure, and geometry analysis.

a Illustration of ORR process on CuN₂C₂ active site, (b) ORR free energy diagrams on Cu/CNT-8, Cu/CNT-4 and Cu/G, (c) side view and top view of the charge density difference for three models with O₂^*, where yellow and blue areas represent higher and lower charge density, respectively, and (d) geometric descriptor θ of CuN₂C₂ active site at various stages of ORR for the three models (inset: illustration of the model used for geometry analysis).