Fig. 6. Catalytic activity evaluated by DFT simulation. (a) Proposed reaction circle with geometry-optimized intermediates for OER over CoFe alloy and CoFe-CoxN@NC, respectively. (b) Gibbs free energy profiles at 1.23 V for OER over CoFe alloy, CoFe@C, CoFe@NC and CoFe-CoxN@NC, respectively. (c) Effect of interfacial space between CoFe alloy and carbon layers on the stability of heterojunction catalysts, where yellow and cyan regions indicate increased and decreased charge distributions, respectively in the inserted images. (d) 2D and (e) 3D display of charge density difference of CoFe@C doped with N element (CoFe@NC). (f) Charge density difference of intermediates for OER over CoFe-CoxN@NC