The surface structure, stability, and catalytic performances toward O 2 reduction of CoP and FeCoP 2 .

The systematic atomistic level investigation of low-index surface structures, stabilities, and catalytic performances of CoP and FeCoP 2 towards the O 2 reduction reaction (ORR) is vital for their applications. Employing first-principles calculations, it is revealed that CoP and FeCoP 2 present the same surface stability in the order of (101) ≈ (011) > (111) > (001) > (110) > (010) > (100). They also possess a similar Wulff equilibrium crystal shape with (101) and (011) exposing the largest surface area. From the electronic view, FeCoP 2 presents improved electronic conductivity compared with CoP. From the energy view, whether FeCoP 2 delivers improved electrocatalytic activity toward the ORR with respect to CoP depends on the reactive surfaces and sites. Among the 4 surfaces considered, only CoP(101), FeCoP 2 (101) and FeCoP 2 (011) delivered ORR performances theoretically when the bridge metal-metal site acts as the reactive center, which makes CoP(011) the only exception. CoP(101)-b Co-Co and FeCoP 2 (011)-b Fe-Co exhibit a larger thermodynamic limiting potential than FeCoP 2 (101)-b Co-Co , suggesting their higher performances toward the ORR. The last step of HO* desorption as the rate-limiting step accounts for 3/4. The third step of transformation from O* to HO* as the most sluggish step accounts for 1/4. The work function, d-band center, Bader charge, and electronic localization function calculations are performed to reveal the HO adsorption nature. The present work provides fundamental insight into the effect of Fe doping into CoP, the determination of the catalyst surface and the key species adsorption nature to guide the rational design of high-performance materials.