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