1D Composite Nanorods of Cobalt Phosphide-Cobalt Sulfide with Improved Electrocatalyst Performance

Article abstract of DOI:10.1002/cctc.201901386

Efficient electrocatalysts of cobalt phosphide-cobalt sulfide 1D composite nanorods for hydrogen evolution reaction (HER) are synthesized by heat-treatment of cobalt phosphide nanorod under CS₂ flow. The reaction of cobalt phosphide with CS₂ molecule leads to the formation of CoP?CoS composite with the maintenance of original 1D nanorod morphology. In comparison with the pristine CoP nanorod, the CoP?CoS composite nanorod shows much higher HER electrocatalytic activity with much lower overpotential, highlighting the beneficial effect of composite formation with metal sulfide on electrocatalyst performance of metal phosphide. The composite formation with CoS domain results in the remarkable enhancement of HER kinetics, electrochemical active surface area, and charge transfer kinetics, which is mainly responsible for the enhanced electrocatalytic activity of composite nanorod. The present study underscores that hybridization with metal sulfide can provide an efficient methodology to improve the electrocatalyst functionality of metal phosphide.

Full text of DOI:10.1002/cctc.201901386

Accepted Article
Title: 1D Composite Nanorods of Cobalt Phosphide-Cobalt Sulfide with
Improved Electrocatalyst Performance
Authors: Haslinda Binti Mohd Sidek, Xiaoyan Jin, Md. Shahinul Islam,
and Seong-Ju Hwang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201901386
Link to VoR: http://dx.doi.org/10.1002/cctc.201901386
A Journal of
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ChemCatChem
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1D Composite Nanorods of Cobalt PhosphideCobalt Sulfide with
Improved Electrocatalyst Performance
[
a]
[b]
[a]
[b],
Haslinda Binti Mohd Sidek, Xiaoyan Jin, Md. Shahinul Islam, and Seong-Ju Hwang *
Abstract: Efficient electrocatalysts of cobalt phosphidecobalt
them, cobalt phosphides have received prime attention as
sulfide 1D composite nanorods for hydrogen evolution reaction
efficient HER electrocatalysts because of their high redox
activity and excellent electrical conductivity.[
11-13]
Although
(HER) are synthesized by heat-treatment of cobalt phosphide
nanorod under CS flow. The reaction of cobalt phosphide with CS
2
2
several strategies such as nanostructure formation, chemical
doping, and coating with carbon have been exploited to
improve the electrocatalytic activity of metal phosphide,[14] it is
still demanded to develop new efficient methodology to
molecule leads to the formation of CoPCoS composite with the
maintenance of original 1D nanorod morphology. In comparison
with the pristine CoP nanorod, the CoPCoS composite nanorod
shows much higher HER electrocatalytic activity with much lower
overpotential, highlighting the beneficial effect of composite
formation with metal sulfide on electrocatalyst performance of metal
phosphide. The composite formation with CoS domain results in
the remarkable enhancement of HER kinetics, electrochemical
active surface area, and charge transfer kinetics, which is mainly
responsible for the enhanced electrocatalytic activity of composite
nanorod. The present study underscores that hybridization with
metal sulfide can provide an efficient methodology to improve the
electrocatalyst functionality of metal phosphide.
optimize its electrocatalyst performance.
The composite
formation with metal sulfide would provide alternative effective
way of improving the electrocatalytic activity of metal phosphide,
since the different band structures and dissimilar bonding
natures of metal phosphide and metal sulfide would be
advantageous in enhancing electrical conductivity and
facilitating the surface adsorption of reactant species via the
increase of surface bond polarity.[
15]
At the time of this
submission, we are unaware of any other reports about the
synthesis of 1D nanostructured metal phosphidemetal sulfide
nanocomposites and their electrocatalyst application.
In the present study, a novel methodology to optimize the
electrocatalytic activity of metal phosphide is developed via the
composite formation with metal sulfide. The evolution of the
electrocatalyst performance of CoP upon the composite
formation is investigated together with the accompanying
modifications of crystal structure, morphology, and chemical
bonding nature.
Introduction
A crucial role of electrocatalysts in diverse renewable energy
2
technologies such as fuel cells, metalO batteries, and
electrolyzers evokes a great deal of research efforts for
exploration of efficient electrocatalyst materials active for
hydrogen evolution reaction (HER), oxygen evolution reaction
(
OER), and oxygen reduction reaction (ORR).[1] To dates, most
Results and Discussion
of high-performance electrocatalysts contain expensive noble
metal elements like Pt, Ir, and Ru, which is a serious obstacle
As efficient and economically-feasible electrocatalysts, the
cobalt phosphidecobalt sulfide composite 1D nanorods are
for
commercialization
of
these
emerging
energy
technologies.[
2,3]
To replace the precious noble metal-
synthesized by heat-treatment of 1D CoP nanorod under CS
flow, as depicted in Figure 1A and Figure S1. In this study,
several concentrations of CS (0.4, 0.6, 0.8, and 1.0 mL) are
2
containing electrocatalysts with economically-feasible ones,
intense research activities have been devoted to exploring non-
precious electrocatalyst materials such as 3d transition metal
oxides, hydroxides, and chalcogenides.[4] Recently transition
metal phosphides have attracted intense research interest
because of their excellent HER electrocatalyst functionality,
high electrical conductivity, and earth abundance.[5-8] Transition
metal phosphides have been widely used as industrial catalysts
for hydrodesulfurization and hydrodenitrification involving the
2
employed for the synthesis of CoPCoS nanorods using tube
furnace with the total volume of 458 mL[ to study the effect of
chemical composition on their properties and functionalities.
The obtained materials are denoted as CPCS4, CPCS6,
CPCS8, and CPCS10, respectively.
16]
The composite formation of cobalt phosphidecobalt sulfide is
verified with micro-Raman spectroscopy, since this vibrational
spectroscopy is quite sensitive for local atomic arrangement of
inorganic solid.[17] As shown in Figure 1B, all the CPCS
[9,10]
adsorption and desorption of hydrogen (H
phosphides possess high affinity for H , these materials are
supposed to be applicable for HER electrocatalyst. Among
2
).
Since metal
2
g
materials commonly exhibit distinct A phonon modes at ~370
1
cm corresponding to in-phase stretching of (CoS) bond as
well as phonon lines of CoP phase,[
18,19]
indicating the
[
[
a]
b]
H. B. M. Sidek, Dr. M. S. Islam
Center for Hybrid Interfacial Chemical Structure (CICS), Department
of Chemistry and Nanoscience
Ewha Womans University, Seoul 03760, Republic of Korea
Dr. X. Jin, Prof. S.-J. Hwang
incorporation of cobalt sulfide domain into cobalt phosphide
matrix. The treatment with CS causes a shift of (CoP)
2
vibration mode toward higher wavenumber side, indicating the
reinforcement of (CoP) bond upon sulfurization. As illustrated
in Figure 1C, this observation can be understood as a result of
bond competition, in which the incorporation of less covalent
(CoS) bond leads to the increased covalency of neighboring
Center for Hybrid Interfacial Chemical Structure (CICS), Department
of Materials Science and Engineering, Yonsei University
Seoul 03722, Republic of Korea
E-mail: hwangsju@yonsei.ac.kr
(
CoP) bond via the redistribution of electron density.
Supporting information for this article is given via a link at the end of
the document.
The crystal structures of the present CPCS materials are
examined with powder X-ray diffraction (XRD) patterns. As
1
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10.1002/cctc.201901386
FULL PAPER
presented in Figure 1D, all the CPCS materials under
investigation commonly display the Bragg reflections of
orthorhombic CoP phase (JCPDS No. 29-0497), indicating the
side, indicating the formation of divalent cobalt sulfide (CoS)
phase with an increase of Co2+ ion. Upon the CS
treatment,
2
weak pre-edge peak P related to the dipole-forbidden 1s → 3d
transition becomes enhanced, reflecting the frustration of the
octahedral symmetry of Co ion.[21] This spectral variation can
be interpreted as another evidence for the formation of CoS
species with the tetrahedral symmetry of Co ion. There are
several main-edge peaks A, B, and C assigned as the dipole-
allowed 1s → 4p transitions in all the present spectra.[22] The
maintenance of CoP lattice upon the reaction with CS
summarized in Table 1, the heat-treatment under CS
2
. As
flow
2
causes only negligible changes of the lattice parameters of
CoP. Taking into account significant difference in ionic sizes of
P3 and S ,

2 [20]
the observed insignificant alteration of lattice
provides strong evidence
parameter upon the reaction with CS
2
for no substitution of phosphide with sulfide and the formation
of intervened cobalt sulfide domain. The absence of cobalt
sulfide-related Bragg reflections in the present XRD patterns
would be ascribed to the low concentration and/or low
crystallinity of cobalt sulfide phase.
heat-treatment under CS
2
flow induces
a
significant
enhancement of peak A at around 7716 eV. Since the
reference CoS shows a large spectral weight for the main-edge
peak A in this energy region, the observed enhancement of this
feature confirms the incorporation of CoS domain via the
2
reaction with CS .
Table 1. Lattice parameters of the present CPCS materials.
Lattice
Å )
b = 3.2706
Parameter
Sample
CoP
Error ( 104)
(
a = 5.0654
a = 5.0676
a = 5.0755
c = 5.5951
c = 5.5878
c = 5.5987
1.8577
CPCS4
CPCS6
b = 3.2701
b = 3.2720
0.6775
0.7966
CPCS8
a = 5.0713
a = 5.0733
b = 3.2735
b = 3.2629
c = 5.5890
c = 5.5842
1.2882
3.1151
CPCS10
Figure 1. (A) Schematic synthesis route to CoPCoS composite 1D nanorod.
Figure 2. (A) Co K-edge XANES spectra (The inset figure represents an
(B) Micro-Raman spectra of CoP, CPCS4, CPCS6, CPCS8, and CPCS10.
expanded view for pre-edge spectra), (B) Co 2p3/2 XPS, (C) P 2p XPS, and
(C) Evolution of the chemical bonds of CoP lattice upon the incorporation of
(D) S 2p XPS data of CPCS8 and CPCS10.
CoS domain. (D) Powder XRD patterns of CoP, CPCS4, CPCS6, CPCS8,
and CPCS10.
According to Co 2p XPS analysis (Figure 2B), both the pristine
CoP and CPCS10 display doublet Co 2p3/2 peaks at ~778.6
The evolutions of the oxidation state and chemical bonding
and ~782.6 eV, which correspond to Co3+ and Co2+ species,
2
nature of CoP nanorod upon the reaction with CS are studied
respectively, together with the satellite shake-up peak (denoted
with Co K-edge X-ray absorption near-edge structure (XANES)
and X-ray photoelectron spectroscopic (XPS) analyses. As
plotted in Figure 2A, the sulfurization process for the pristine
CoP leads to the displacement of edge jump into lower energy
as “Sat.”) at ~785.6 eV. The peak convolution analysis clearly
demonstrates the increase of Co /Co ratio upon the reaction
2+
3+
with CS
2
, confirming the incorporation of CoS domain into the
[23]
CPCS nanocomposite, see Table S1. As shown in the P 2p
2
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XPS data of Figure 2C, the phosphide-related peaks are
discernible at ~129.8 and ~130.6 eV for both CoP and CPCS10.
Also, these materials demonstrate an additional P 2p peak at
~134.7 eV, which is attributable to the atmospheric oxidation of
surface phosphorus species. In Figure 2D, both of CPCS8 and
CPCS10 nanocomposites exhibit S 2p3/2 and S 2p1/2 peaks at
162.1 and 163.1 eV, which corresponds to the sulfide ion. Like
P 2p spectra, the sulfate-related peak appears at 169.7 eV,
which can be ascribed to the atmospheric oxidation of surface
sulfur species. Similar spectral features are discernible for the
other CPCS materials, confirming the formation of CoPCoS
nanocomposites for all the chemical compositions employed
here, see Figure S2.
The present XANES/XPS results
underscore the formation of CoPCoS nanocomposite via the
incorporation of CoS domain upon the heat-treatment of CoP
under CS flow. The micro-Raman and XPS analyses clearly
2
demonstrate that, despite a quite low CoS content, the
incorporation of small amount of CoS domains is quite effective
in tailoring the bonding nature of (CoP) bond and the oxidation
state of Co ion through bond competition effect, underscoring
the high efficacy of composite formation with CoS in modifying
the surface bond character of CoP. Since electrocatalysis
occurs on the surface of CoP catalyst material, the efficient
modification of surface bonding nature is mainly responsible for
the remarkable improvement of the HER activity of CoP upon
the incorporation of small amount of CoS domain.
Figure 3. (A) TEM images of CPCS4, CPCS6, CPCS8, and CPCS10. (B)
HR-TEM images and (C) SAED pattern of CPCS10. (D) FE-SEM images of
CoP, CPCS4, CPCS6, CPCS8, and CPCS10. (E) EDSelemental maps for
CPCS10.
The effect of composite formation with CoS on the HER
electrocatalyst performance of CoP nanorod is investigated
with linear sweep voltammetry (LSV) with a scan rate of 5 mV
The effect of CoS incorporation on the crystal morphology of
CoP nanorod is probed with transmission electron microscopy
1
(
TEM) and field emission-scanning electron microscopic (FE-
s
2 4
and a rotating speed of 1600 rpm in 0.5 M H SO solution.
SEM) analyses. As illustrated in Figure 3A, all the CPCS
materials commonly exhibit 1D nanorod morphology with the
diameter of ~10 nm and the length of several micrometers,
indicating negligible morphological alteration upon heat-
As presented in Figure 4A and Table 2, all the CPCS materials
display much better HER activities with lower overpotentials
(10) than does the pristine CoP, highlighting the beneficial
effect of composite formation with CoS on the HER activity of
CoP nanorod. Among the materials under investigation, the
best electrocatalytic activity with the smallest overpotential (156
treatment with CS
2
.
As shown in Figure 3B, the HR-TEM
images of CPCS10 clearly demonstrate two different lattice
fringes corresponding to the (112) plane of CoP with the
interplanar distance of ~0.20 nm and the (110) plane of CoS
with the interplanar distance of ~0.17 nm, respectively. This
result underscores the coexistence of CoP and CoS phases in
2
mV at a current density of 10 mA cm ) occurs for CPCS8,
indicating the optimal CoS/CoP ratio of this material. Upto the
optimal CoS/CoP ratio of CPCS8, the incorporation of CoS
phase facilitates the surface adsorption of H+ via the increase
the present CPCS nanocomposites.
The formation of
of surface bond polarity,[15] which is responsible for the
intimately-coupled nanocomposites of CoS and CoP domains
is further confirmed by selected area electron diffraction (SAED)
analysis (Figure 3C), exhibiting the (101), (102), and (110)
planes of CoS domain and the (001), (111), and (112) planes of
CoP domain. Also, the FE-SEM analysis provides another
evidence for maintenance of 1D nanorod morphology upon the
improvement of HER activity upon the composite formation.
Beyond the optimal composition of CPCS8, a further increase
of CoS content has detrimental effect on the HER activity of
CPCS10 nanocomposite, which can be ascribed to the inferior
electrical conductivity of CoS than CoP. The depression of
charge transfer property upon the further incorporation of CoS
species is clearly evidenced by EIS data in Figure 4B. The
2
reaction with CS (Figure 3D). The homogenous incorporation
of cobalt sulfide domain into the cobalt phosphide matrix is
further evidenced by energy dispersive spectrometry
improvement of HER kinetics upon the CS
2
treatment is
evidenced by the lower Tafel slopes of CPCS nanocomposites
than that of CoP, see Figure 4C and Table 2. As plotted in
Figure 4D, the present CPCS8 nanocomposite shows only a
weak increase of overpotential by ~22 mV, indicating excellent
long-term durability of the HER activity of this material, which is
comparable to those of previously reported materials.[24,25] With
a reference to micro-Raman results in Figure 1B, the high
electrocatalyst cyclability of CPCS composite nanorod is
attributable to the reinforcement of (CoP) bond upon the
incorporation of (CoS) bond, resulting in the enhancement of
CoP lattice stability. In comparison with CPCS8 with lower
CoS content, the CPCS10 nanocomposite with higher CoS
content suffers from remarkable degradation of HER
performance with an increased overpotential by ~70 mV
(
EDS)elemental mapping analysis showing the uniform
distribution of Co, S, and elements, see Figure 3E.
P
According to the EDS analysis, the atomic ratio of Co:P:S is
calculated to be 46.50:53.49:0.01 for CPCS4, 45.60:54.37:0.03
for CPCS6, 47.96:51.99:0.05 for CPCS8, and 44.71:55.19:0.10
for CPCS10, respectively, see Table S2. Based on these
results, the CoS/CoP ratio can be determined as 0.02% for
CPCS4, 0.06% for CPCS6, 0.10% for CPCS8, and 0.18% for
CPCS10, respectively.
obviously show the presence of sulfur and a gradual increase
of sulfur content with increasing CS volume in the precursors,
highlighting the effective composite formation of CoP with CoS
via CS treatment.
All the CPCS nanocomposites
2
2
3
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ChemCatChem
10.1002/cctc.201901386
FULL PAPER
(
Figure S3). This result is attributable to more prominent
The effect of composite formation with CoS on the charge
transfer kinetics of CoP is also investigated with the
electrochemical impedance spectroscopy (EIS) measurement.
As plotted in Figure 4B, all the present CPCS composite
nanorods commonly display similar feature of EIS curves
consisting of semicircle at high-to-medium frequencies. Upon
the composite formation with CoS, the diameter of semicircle
becomes smaller, highlighting the improvement of charge
transfer kinetics. From the curve fitting analysis to the EIS data,
charge transfer resistance (Rct) is estimated as 495, 435, 262,
and 511 Ω for CPCS4, CPCS6, CPCS8, and CPCS10,
respectively, which are much smaller than that of the pristine
CoP (594 Ω), clearly demonstrating the remarkable
enhancement of charge transfer upon the composite formation
with metal sulfide. In addition to the improvement of Tafel
slope and ECSA, the decrease of Rct upon the sulfurization
makes significant contribution to the excellent electrocatalyst
performance of CPCS composite nanorods. Of noteworthy is
that CPCS10 with higher CoS content exhibits poorer charge
transfer kinetics than does CPCS8 with lower CoS content.
This observation can be ascribed to the lower conductivity of
CoS than CoP. The inferior charge transfer property of
CPCS10 overwhelms the positive effect of composite formation
between CoP and CoS, which is responsible for the poorer
HER activity of CPCS10 than that of CPCS8.
dissolution of CoS domain in acidic solution compared with
CoP having highly covalent (Co P ) bond.
3
+
3
Table 2. Overpotentials and Tafel slopes of CoP and CPCS nanorods in both
acidic and alkaline electrolytes.
Sample
0.5 M H
2
SO
4
1 M KOH
Tafel slope

10 (mV)
Tafel slope

10 (mV)
1
1
(
mV dec
)
(mV dec )
CoS
277
209
193
165
156
195
107
93
81
77
74
82
413
497
370
239
227
411
167
185
139
92
CoP
CPCS4
CPCS6
CPCS8
CPCS10
91
155
Conclusions
An efficient way of exploring high-performance metal
phosphide-based electrocatalyst is developed via the in-situ
incorporation of metal sulfide domain. The 1D composite
nanorods of CoPCoS can be synthesized by the heat-
2
treatment of CoP nanorod under CS flow. The obtained CPCS
composite nanorods possess improved HER electrocatalyst
functionalities with low overpotentials and excellent durability,
which can be ascribed to the improvement of HER kinetics,
ECSA, and charge transfer property by the introduction of
conductive CoS domains. As summarized in Table S3, the
HER performance of the present CPCS8 composite nanorods
is comparable to or superior to those of previously reported
metal phospide-based materials, verifying the usefulness of
composite formation with metal sulfide in improving the HER
activity of metal phosphide. The present strategy can provide
an efficient way of synthesizing promising metal phosphide-
based HER electrocatalyst materials. Judging from diverse
electrochemistry-based functionalities of metal phosphide such
as electrodes for supercapacitors and diverse batteries,[15] the
Figure 4. (A/E) LSV curves, (B) EIS data, and (C/F) Tafel plots in
acidic/alkaline conditions, (D) chronopotentiometry in acidic condition at a
constant current density of 10 mA cm2 of CoS, CoP, CPCS4, CPCS6,
CPCS8, and CPCS10.
heat-treatment under CS
explore novel functional
phosphidemetal sulfide. The current project of our group is
employing the present method for many other metal
phosphides with diverse performances such as electrodes and
photocatalysts.
2
flow can offer useful opportunity to
nanocomposites of metal
Like the results in acidic condition (Figure 4A), the CPCS
nanorods also show higher HER electrocatalytic activities in
alkaline condition (1 M KOH) with lower overpotentials than that
of the pristine CoP, as presented in Figure 4E and Table 2.
The Tafel slopes of CPCS nanorods are much smaller than that
of the pristine CoP (Figure 4F), underscoring the positive effect
of composite formation on the HER kinetics of CoP in alkaline
condition. According to the determination of electrochemical
Experimental Section
2
active surface area (ECSA) (Figure S4), the CS treatment
makes larger the ECSA of CoP nanorod, highlighting the
beneficial effect of the composite formation with CoS on the
electrochemical activity of CoP nanorod.
Synthesis The precursor Co
3 4
O nanorod was prepared by dissolving
Co(NO ) .6H O (5 mmol), and CO(NH ) (25 mmol) in 50 mL of
3
2
2
2 2
deionized water under stirring. The obtained solution was transferred
into a Teflon-lined autoclave, which was followed by a hydrothermal
4
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ChemCatChem
10.1002/cctc.201901386
FULL PAPER
treatment at 150 °C for 2 h, as reported previously.[26] The CoP nanorod
[1]
[2]
[3]
[4]
[5]
[6]
[7]
J. Park, T. Kwon, J. Kim, H. Jin, H. Y. Kim, B. Kim, S. H. Joo, K.
Lee, Chem. Soc. Rev. 2018, 47, 8173.
was obtained by the phosphidation process of Co
O
3 4
nanorod with
NaH
5
2
PO
2
.H
2
O (1:10 mass ratio) at 350 °C for 2 h with a heating rate of
T.-H. Gu, D. A. Agyeman, S.-J. Shin, X. Jin, J. M. Lee, H. Kim, Y.-
M. Kang, S. -J. Hwang, Angew. Chem. Int. Ed. 2018, 57, 15984.
D. Jiang, W. Ma, Y. Zhou, Y. Xing, B. Quan, D. Li, J. Colloid
Interface Sci. 2019, 550, 10.
°C min under nitrogen flow.[27] Upon thermal decomposition of
1
NaH
phase transformation from Co
was heated at 350 C for 1 h under CS
2
PO
2
.H
2
O, phosphine (PH
3
) gas was generated, resulting in a
to CoP. The obtained CoP nanorod
diluted with Ar gas, resulting in
3 4
O
2
S. H. Yu, D. H. C. Chua, ACS Appl. Mater. Interfaces 2018, 10,
14777.
the composite formation with cobalt sulfide. To probe the effect of
chemical composition on the properties and functionality of CPCS
nanorod, several concentrations of CS (0.4, 0.6, 0.8, and 1.0 mL) were
2
employed for the tube furnace with the total volume of 458 mL.[17] The
experimental setup for the formation of CoPCoS nanocomposites is
depicted in Figure S1.
Y. Pei, Y. Cheng, J. Chen, W. Smith, P. Dong, P. M. Ajayan, M. Ye,
J. Shen, J. Mater. Chem. A 2018, 6, 23220.
H. Du, R. -M. Kong, X. Guo, F. Qu, J. Li, Nanoscale 2018, 10,
21617.
Y. Xue, M. Sun, Int. J. Hydrogen Energy 2019, 44, 16378.
[
8]
R. Boppella, J. Tan, W. Yang, J. Moon, Adv. Funct. Mater. 2018
807976.
,
1
Characterization The phonon modes and bonding natures of the
CPCS nanorods were examined using micro-Raman spectroscopy (JY
LabRam HR spectrometer, 632.8 nm of Ar+ ion laser). Powder XRD
[
9]
M. Sun, H. Liu, J. Qu, J. Li, Adv. Energy Mater. 2016, 6, 1600087.
M. Liu, J. Li, ACS Appl. Mater. Interfaces 2016, 8, 2158.
[10]
[
[
[
11] D. Jiang, Y. Xu, R. Yang, D. Li, S. Meng, M. Chen, ACS
Sustainable Chem. Eng. 2019, 7, 9309.
analysis was carried out to probe the crystal structures of the present
nanocomposites using Rigaku D/Max-2000/PC diffractometer (Cu Kα
radiation, 298 K). XPS data were obtained by using XPS instrument
12] Y. Ji, L. Yang, X. Ren, G. Cui, X. Xiong, X. Sun, ACS Sustainable
Chem. Eng. 2018, 6, 11186.
(Thermo VG, UK, Al Kα). Co K-edge XANES spectra were measured at
13] J. Tian, Q. Lian, A. M. Asiri, X. Sun, J. Am. Chem. Soc. 2014, 136,
the beam line 10C of the Pohang Accelerator Laboratory (PAL) in Korea.
All the XANES measurements were carried out at room temperature in
a transmission mode using gas-ionization detectors. All the present
data were energy-calibrated by simultaneously measuring the reference
spectrum of Co metal. All the experimental spectra were analysed
using the standard procedure.[28] The crystal morphologies of the
7
587.
[
[
14] L. Peng, S. S. A. Shah, Z. Wei, Chin. J. Catal. 2018, 39, 575.
15] A. M. Elshahawy, C. Guan, X. Li, H. Zhang, Y. Hu, H. Wu, S. J.
Pennycook, J. Wang, Nano Energy 2017, 39, 162.
[16]
I. Y. Kim, J. Seo, S. M. Oh, S. B. Patil, S.-J. Hwang, Adv. Funct.
Mater. 2015, 25, 4948.
present materials were investigated with TEM/HR-TEM (Jeol JEM-
[
[
[
[
17] S. J. Parikh, K. W. Goyne, A. J. Margenot, F. N. D. Mukome, F. J.
2100F) with an accelerating voltage of 200 kV) and FE-SEM (Jeol JSM-
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6700F), respectively. The EDSelemental mapping analysis was
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performances of the present materials were tested using a standard
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calomel electrode (SCE), and a glassy carbon (GC) electrode were
utilized as the counter electrode, reference electrode, and working
electrode, respectively. The catalyst ink was obtained by dispersing 7
mg of active material, 3 mg of carbon black (Vulcan-XC72R), and 25 L
of a 5 wt% Nafion solution (Sigma-Aldrich) in an isopropanol/water (1/4,
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-saturated
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C.-Y. Su, G. Li, Dalton Trans. 2018, 47, 14679.
1
0.5 M H
2
SO
4
and 1 M KOH electrolytes with a scan rate of 5 mV s on
IVIUM analyzer. The potentials were normalized to the reversible
hydrogen electrode (RHE) unit using the following equation: E(RHE) =
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,
,
,
E(SCE) + 0.256 V (for 0.5 M H
2 4
SO solution) and E(RHE) = E(SCE) +
1
1.0464 V (for 1 M KOH solution). The ECSA results were evaluated
[
from cyclic voltammetry (CV) data with a potential window of 0.10.2 V
vs. RHE at different scan rates of 20, 40, 60, 80, 100, 120, 140, 160,
and 180 mV s1. The EIS data were collected in acidic media in the
1
[27]
5
frequency range of 0.1100000 Hz at 0.3 V vs. RHE.
[
[
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Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(
No. NRF-2017R1A2A1A17069463) and by the Korea
government (MSIT) (No. NRF-2017R1A5A1015365). The
experiments at PAL were supported in part by MOST and
POSTECH.
Keywords: Cobalt phosphide• Nanostructures • Composite
formation • Cobalt sulfide • Hydrogen evolution reaction
5
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ChemCatChem
10.1002/cctc.201901386
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Entry for the Table of Contents
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The CoPCoS composite nanorod
shows higher HER electrocatalytic
activity than does the pristine CoP,
which is attributable to the
Haslinda Binti Mohd Sidek, Xiaoyan Jin,
Md. Shahinul Islam, Seong-Ju Hwang*
Page No. – Page No.
improvement of charge transfer
kinetics upon the embedding of CoS
domain.
1
D Composite Nanorods of Cobalt
PhosphideCobalt Sulfide with
Improved Electrocatalyst
Performance
6
This article is protected by copyright. All rights reserved.