Page S1

Supplementary Information

Thermally induced nanostructuring for the synthesis of core/shell-structured CoO/CoSx electrocatalyst

Min Soo Kima†, Muhammad Awais Abbasb†, Raju Thotab† and Jin Ho Bang*abc

aDepartment of Bionano Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan,

Gyeonggi-do 15588, Republic of Korea

bNanosensor Research Institute, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan,

Gyeonggi-do 15588, Republic of Korea

cDepartment of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-ro,

Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea

Author Contributions:

†These authors contributed equally to this work.

*Corresponding Author

E-mail: jbang@hanyang.ac.kr

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

Page S2

Fig. S1 XRD patterns of intermediates obtained during a hot H2 treatment of Co3O4 at various

temperatures for 1 h.

Fig. S2 SEM images of Co3O4 (a,b) before and (c,d) after a hot H2 treatment at 300 oC for 1 h.

Co3O4

220 °C, 1 h

240 °C, 1 hIn

tens

ity (a

.u.) 245 °C, 1 h

250 °C, 1 h

300 °C, 1 h

Co3O4

Co

20 30 40 50 60 70 802-Theta (degree)

CoO

Page S3

Fig. S3 The crystal structure of cobalt oxides and cobalt sulfides.

Scheme S1 Schematic representation of the grain fracture of Co3O4 and the formation of mesoporous

CoO with smaller crystallites from bulk Co3O4 under a reducing environment of hot NH3.

Co3O4

CoO

Co3O4 Co3O4/CoO CoO

Page S4

Fig. S4 Cyclic voltammograms of (a) CoO/CoSx and (b) Pt electrodes at various scan rates. (c) Tafel

polarization curves of CoO/CoSx and Pt counter electrodes. Equivalent circuits used for fitting the

Nyquist plots of (d) CoO/CoSx and (e) Pt counter electrodes. Rs is the equivalent series resistance, Rct is

the charge transfer resistance, ZN is the Nernst diffusion impedance, Rtrns is the electron transport

resistance in the carbon layer, and CPE2 and CPE3 are the constant phase elements associated with

resistances.

Table S1 Various resistance values extracted from fitting the Nyquist plots of CoO/CoSx and Pt

electrodes. EIS was carried out with symmetric dummy cells.

Counter Electrode Rs (Ω∙cm2)

Rtrns (Ω∙cm2)

Rct (Ω∙cm2)

ZN (Ω∙cm2)

CoO/CoSx 3.30 0.57 0.91 1.35

Pt 3.31 - 0.97 1.32

Page S5

Fig. S5 XRD pattern of CoO/CoSx and Rietveld refinement result that determines the composition of

each component (CoO: 86.2%, CoS: 11.9%, and Co3S4: 1.9%).

Observed Calculated Background Difference

Inte

nsity

(a.u

.)

CoO

CoS

20 30 40 50 60 70 802-Theta (degree)

Co3S4

Inte

nsity

(a.u

.)

CoSx

Co3S4

20 30 40 50 60 70 802-Theta (degree)

CoS

Page S6

Fig. S6 XRD pattern of CoSx obtained at 300 oC for 1 h. The compositional percentages of CoS and Co3S4

determined by Rietveld refinement were 76.5 and 23.5%, respectively.

Fig. S7 (a) N2 physisorption isotherms and (b) pore size distributions of CoSx. The BET surface area was

7.34 m2·g-1 and the total pore volume was 0.047 cm3·g-1.

Fig. S8 (a-c) TEM images and (d) FFT patterns of CoSx.

0 0.2 0.4 0.6 0.8 1.00

5

10

15

20

25

30

Volu

me

Ads

orbe

d (c

m3 g

-1)

Relative Pressure (P/P0)

Adsorption Desorption

1 10 1000

0.01

0.02

0.03

0.04

0.05

dV/d

logD

Pore Diameter (nm)

(a) (b)

0 0.2 0.4 0.6 0.80

5

10

15

20

Cur

rent

Den

sity

(mA

·cm

-2)

Voltage (V)

CoO/CoSx

Pt CoSx

· ·

Page S7

Fig. S9 J–V curve of DSSCs assembled with various CEs made of CoO/CoSx, Pt, and CoSx.

Table S2 J–V parameters of DSSCs fabricated with the above CEs.

CE Jsc (mA·cm2) Voc (V) FF PCE (%)

CoO/CoSx 15.5 ± 0.3 0.73 ± 0.01 0.66 ± 0.02 7.27 ± 0.06

Pt 15.9 ± 0.2 0.70 ± 0.01 0.65 ± 0.01 7.12 ± 0.05

CoSx 15.1 ± 0.5 0.71 ± 0.02 0.62 ± 0.03 6.73 ± 0.16

Fig. S10 (a) XRD patterns of CoO/CoSx obtained at various reaction conditions and (b) CoS and Co3S4

composition ratios determined by Rietveld refinement.

12.91%

87.09%

4.33%

95.67%CoSCo3S4

17.38%

82.62%

41.76%

58.24%

31.19%

68.81%

32.73%

67.27%

71.34%

28.66%

17.54%

82.46%

300 ºC, 10 min 400 ºC, 10 min

270 ºC, 1 h 400 ºC, 30 min

300 ºC, 30 min 300 ºC, 1 h

450 ºC, 10 min 400 ºC, 1 h

CoO

270 °C, 1 h

300 °C, 10 min

300 °C, 30 min

300 °C, 1 h

400 °C, 10 min

Inte

nsity

(a.u

.)

400 °C, 30 min

400 °C, 1 h

450 °C, 10 min

CoO

CoS

20 30 40 50 60 70 802-Theta (degree)

Co3S4

(a) (b)

Page S8

Fig. S11 (a) SEM and (b) TEM images of commercially available IrO2.

Page S9

Fig. S12 Cyclic voltammograms of of (a) Bare GC, (b) Co3O4, (c) CoO, (d) CoO/CoSx, and (e) IrO2 electrodes

in 0.1 M KOH at various scan rates to determine the electrochemical double layer capacitance (Cdl). (f)

Linear scan voltammograms of CoO/CoSx as an electrocatalyst for OER before and after 1000 CV scans,

demonstrating excellent stability of CoO/CoSx.

0.84 0.87 0.90 0.93 0.96 0.99-0.010

-0.005

0.005

0.010

0.015

0

0.81 0.84 0.87 0.90 0.93 0.96 0.99-0.015-0.010-0.005

0.0050.0100.0150.020

0

0.78 0.81 0.84 0.87 0.90 0.93 0.96-0.06

-0.03

0.03

0.06

0.09

0.12

0

0.8 1.0 1.2 1.4 1.6 1.8

10

20

30

40

50

60

0

0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98-0.0045

-0.0030

-0.0015

0.0015

0.0030

0.0045

0

0.84 0.87 0.90 0.93 0.96-0.09

-0.06

-0.03

0.03

0.06

0.09

0.12

0

Cur

rent

Den

sity

(mA

⋅cm

-2)

Potential (V vs RHE)

2 mV·s-1 20 mV·s-1

5 mV·s-1 25 mV·s-1

10 mV·s-1 15 mV·s-1

2 mV⋅s-1

25 mV⋅s-1

(b)

Cur

rent

Den

sity

(mA

⋅cm

-2)

Potential (V vs RHE)

2 mV·s-1 15 mV·s-1

5 mV·s-1 20 mV·s-1

10 mV·s-1 25 mV·s-1

2 mV⋅s-1

25 mV⋅s-1

(c)C

urre

nt D

ensi

ty (m

A⋅c

m-2

)

Potential (V vs RHE)

2 mV·s-1 15 mV·s-1

5 mV·s-1 20 mV·s-1

10 mV·s-1 25 mV·s-1

(d)

Cur

rent

Den

sity

(mA

⋅cm

-2)

Potential (V vs RHE)

Initial After 1000 CV Cycles

(a)

Cur

rent

Den

sity

(mA

⋅cm

-2)

Potential (V vs RHE)

2 mV·s-1 20 mV·s-1

5 mV·s-1 25 mV·s-1

10 mV·s-1

15 mV·s-1

2 mV⋅s-1

(f)(e)

25 mV⋅s-1

25 mV⋅s-1

2 mV⋅s-1

2 mV⋅s-1

25 mV⋅s-1

Cur

rent

Den

sity

(mA

⋅cm

-2)

Potential (V vs RHE)

2 mV·s-1 20 mV·s-1

5 mV·s-1 25 mV·s-1

10 mV·s-1 15 mV·s-1

Page S10

Table S3 The ECSA calculations of each catalyst.

Sample Slope (mF∙cm-2) Cdl (mF∙cm-2) Rf ECSA (cm2)

Bare GC 0.154 0.077 - -

CO3O4 0.427 0.214 2.77 0.54

CoO 0.458 0.229 2.98 0.58

CoO/CoSx 3.7 1.85 23.99 4.70

IrO2 4.85 2.425 31.45 6.16

Fig. S13 Equivalent circuit used for fitting the impedance spectra of OER electrocatalysts. Rs represents

the solution resistance. R1 and C1 denote the diffusion/adsorption of reaction intermediates due to slow

diffusion through the reaction interface in the porous electrode. R2 and Cdl are ascribed to the charge

transfer resistance the capacitance associated with OER, respectively.

Table S4 Various resistance values extracted by fitting the equivalent circuit in Fig. S5 to the Nyquist

plots of various cobalt oxides and IrO2 when employed as OER electrocatalysts.

Catalyst Rs (Ω∙cm2) R1 (Ω∙cm2) Rct (Ω∙cm2)

Co3O4 2.30 111.82 952.36

CoO 1.19 0.97 185.24

CoO/CoSx 1.87 0.38 21.77

IrO2 2.11 1.77 40.68

Rs

C1

R1

Cdl

Rct

Page S11

Fig. S14 XRD patterns taken during the synthesis of (a) nanostructured MoN/MoS2 and (b)

nanostructured W0.62(N,O)/WS2.

Fig. S15 TEM images of taken during the synthesis of nanostructured MoN/MoS2: (a,b) MoO3, (c,d) MoN,

(e,f) MoN/MoS2, and (g) STEM-EDS mapping analysis results of MoN/MoS2.

WO3

NH3, 600°C, 1 h

Inte

nsity

(a.u

.)

H2S, 300°C, 1 h

W0.62(N,O)

20 30 40 50 60 70 802-Theta (degree)

WS2

MoO3

NH3, 600°C, 1 h

Inte

nsity

(a.u

.)

H2S, 300°C, 1 h

MoN

20 30 40 50 60 70 802-Theta (degree)

MoS2

(a) (b)

Page S12

Fig. S16 TEM images of taken during the synthesis of nanostructured W0.62(N,O)/WS2: (a,b) WO3, (c,d)

W0.62(N,O), (e,f) W0.62(N,O)/WS2, and (g) STEM-EDS mapping analysis results of W0.62(N,O)/WS2.