Fig. 3.

Supercell (2$\times$2$\times$1) structures of (A) pure and (B) electrolyzed mackinawite computed by first-principles calculations. In B, the $\mathrm{FeS}-\mathrm{to}-{\mathrm{Fe}}^0$ electroreduction is expressed by the removal of one S atom from the interlayer surface. The resultant surface-exposed Fe was found to be the energetically most favorable site for the ${\mathrm{NH}}_3$ intercalation (see text).