Supporting Information

Identifying the role of Ni and Fe in Ni-Fe co-doped orthorhombic CoSe2 for

driving enhanced electrocatalytic activity for oxygen evolution reaction

Yongxiao Tuo a, Xueyuan Wang b, Chen Chen b, Xiang Feng b, Zhengqing Liu a, Yan Zhou a, b, , Jun

Zhang a, b, *

a School of Materials Science and Engineering, China University of Petroleum (East China),

Qingdao, 266580, China

b State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China),

Qingdao, 266580, China

Corresponding author: yanzhou@upc.edu.cn; zhangj@upc.edu.cn

Figure S1. a) SEM image of pure o-CoSe2 microspheres. b) HRTEM image of o-CoSe2 nanorods.

Figure S2. a) SEM image of Ni-Fe co-doped o-CoSe2 microspheres. Red circles point to the hollow

structure of microspheres. b) TEM image of Ni-Fe co-doped o-CoSe2 microsphere. The bright

region in the center of microsphere indicates a hollow structure.

Figure S3. XRD patterns of Ni0.2Co0.8Se2 and Fe0.2Co0.8Se2, no NiSe2 or FeSe2 phase was detected in

them.

Figure S4. EDX spectrum for the as-synthesized Ni0.04Fe0.16Co0.8Se2 sample and calculated element

percent in mole.

Table S1. The molar ratio of element in the samples determined by ICP-AES, normalizing the

atomic ratio of Se to 2.

Sample Ni Fe Co Se

Ni0.2Co0.8Se2 0.22 0 0.81 2

Fe0.2Co0.8Se2 0 0.16 0.85 2

Ni0.04Fe0.16Co0.8Se2 0.07 0.21 0.82 2

Figure S5. a) Ni 2p XPS spectra of Ni0.2Co0.8Se2 and Ni0.04Fe0.16Co0.8Se2. b) Fe 2p XPS spectra of

Fe0.2Co0.8Se2 and Ni0.04Fe0.16Co0.8Se2.

Figure S6. iR-corrected polarization curves for Ni0.04Fe0.16Co0.8Se2 samples prepared at different

temperatures. e.g. “200 oC-210 oC” means the first 30 min reaction was maintained at 200 oC, and

the later 45 min reaction was maintained at 210 oC.

Figure S7. The amount of theoretically calculated oxygen (green line) and experimentally

measured oxygen (orange dots) of Ni0.04Fe0.16Co0.8Se2 vs. time at 10 mA cm-2.

Table S2. Comparison of OER performance with recently reported metal selenide catalysts in 1.0 M

KOH condition.

CatalystsOverpotential at 10 mA/cm2

(mV)References

CoSe2 nanosheets 320J. Am. Chem. Soc.

2014, 136, 15670.

Co0.85Se 324Adv. Mater.

2016, 28, 77.

Ni0.88Co1.22Se4 320Chem. Mater.

2017, 29, 7032.

NiSe 290Adv. Energy Mater.

2018, 8, 1702704.

Fe-doped NiSe2 268Angew. Chem. Int. Ed.

2018, 57, 4020.

Fe0.09Co0.13-NiSe2 251Adv. Mater.

2018, 30, 1802121

CoFe0.7Se1.7 279Electrochim. Acta

2019, 297, 200.

CoSe2 UNMvac 284J. Mater. Chem. A

2019, 7, 2538.

CoSe2/FeSe2 nanocuboids 240Nanoscale

2019, 11, 10740.

Ni0.04Fe0.16Co0.8Se2 230 This work

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Figure S8. iR uncompensated LSV curves of Ni0.04Fe0.16Co0.8Se2 and commercial RuO2 for OER.

Figure S9. iR uncompensated LSV curve of Ni0.04Fe0.16Co0.8Se2 for OER in PH=7 electrolyte (1 M

PBS).

Figure S10. Cyclic voltammogram (CV) curves of o-CoSe2 and Ni, Fe doped o-CoSe2 samples with

different Ni and Fe contents at scan rates of 2-14 mV s-1 in 1.0 M KOH.

Figure S11. Plots of the current density at 1.24 V vs. scan rate for o-CoSe2 and Ni, Fe doped o-

CoSe2 samples with different Ni and Fe content.

By using the computational hydrogen electrode (CHE) mode, the Gibbs free energy of

adsorbed species is defined as:

∆ G=∆ Eads+∆ EZPE−T ∆ Sads (S1)

where ∆Eads is the electronic adsorption energy, ∆EZPE is the zero point energy difference between

adsorbed and gaseous species, and T∆Sads is the corresponding entropy difference between these

two states (T was set to be 298 K). The calculation results of zero point energy and entropy of the

OER intermediates were listed in Table S2.

Table S3. The calculation results of zero point energy and entropy for the adsorbed and gaseous

species.

Species ZPE/eV TS/eV

*OH 0.37 -

*O 0.05 -

*OOH 0.48 -

H2O 0.57 0.67

H2 0.34 0.41

The Gibbs free energy barriers for OER reaction steps 1−4 can be expressed as:

∆ G1=∆ GOH−eU+∆ GH +¿(pH )¿ (S2)

∆ G2=∆ GO−∆ GOH−eU +∆ GH +¿(pH )¿ (S3)

∆ G3=∆ GOOH−∆ GO−eU+∆ GH +¿(pH )¿ (S4)

∆ G4=4.92−∆ GOOH−eU +∆ GH+¿( pH )¿ (S5)

where U is the potential measured against normal hydrogen electrode (NHE) at standard conditions.

The theoretical overpotential is then readily defined as:

η=max(¿ ∆ G1 , ∆ G2 , ∆ G3 , ∆G 4) /e−1.23 ¿ (S6)

Table S3 listed the energy barrier results of elementary steps for OER reaction on different surface

model.

Table S4. Calculated Gibbs free energy barrier results of elementary steps for OER reaction on

CoOOH(012), Ni-CoOOH(012), Fe-CoOOH(012) and NiFe-CoOOH(012).

CoOOH

(Co site)

Ni-CoOOH

(Co site)

Ni-CoOOH

(Ni site)

Fe-CoOOH

(Fe site)

Fe-CoOOH

(Co site)

NiFe-CoOOH

(Fe site)

∆ G1 1.14 1.23 1.50 0.84 1.05 0.86

∆ G2 1.90 1.82 2.04 1.07 1.86 1.12

∆ G3 1.17 1.10 0.45 1.87 1.29 1.81

∆ G4 0.72 0.77 0.93 1.14 0.72 1.13

Figure S12. Side view of the optimized geometries for a) CoOOH(012), b) Ni-CoOOH(012), c) Fe-

CoOOH(012) and d) NiFe-CoOOH(012) surface models. The Co, Ni, Fe, O and H atoms are

marked in violet, blue, green, red and white colors, respectively.

Figure S13. Optimized structures of OH, O and OOH intermediates on the Fe site of NiFe-

CoOOH(012). The Co, Ni, Fe, O and H atoms are marked in violet, blue, green, red and white

colors, respectively.

Figure S14. a) Optimized structures of CoOOH(012)/CoSe2(111) and OER intermediates adsorbed

CoOOH(012)/CoSe2(111). The Co, O, Se and H atoms are marked in violet, red, orange and white

colors, respectively. b) Comparison of Gibbs free energy diagrams for OER pathway between

CoOOH(012) and CoOOH(012)/CoSe2(111) at U=1.23 V.

Figure S15. a) LSV plots for Ni-doping dominated o-CoSe2 and b) the corresponding Tafel plots, c)

LSV plots for Fe-doping dominated o-CoSe2 and d) the corresponding Tafel plots.

Figure S16. LSV curves change for Fe-doping dominated o-CoSe2 by varying Ni content from 0 %

to 10 %.

Figure S17. HRTEM image of Ni0.04Fe0.16Co0.8Se2 after OER electrochemical stability test. Red lines

point to the thin layer formed on the catalyst surface.