Facile synthesis of various highly dispersive CoP nanocrystal embedded carbon matrices as efficient electrocatalysts for the hydrogen evolution reaction

Article abstract of DOI:10.1039/c4ta06630h

In order to promote the hydrogen evolution reaction (HER) catalytic efficiency of cobalt phosphide (CoP) and then construct efficient HER catalysts, a facile procedure has been adopted to prepare tiny CoP nanoparticle (NP) embedded carbon matrices without using any extreme conditions and harmful organic reagents or surfactants. Meanwhile, in order to explore the influence of structures of carbon matrices on preventing the free growth of CoP NPs and enhancing the HER catalytic efficiency of the CoP-carbon catalysts, imporous reduced graphene oxide (RGO), macroporous carbon (MPC), mesoporous carbon vesicles (MCVs) and ordered mesoporous carbon (OMC) were used for preparing CoP-carbon HER catalysts. SEM and TEM measurements show that size-controlled CoP NPs indeed grew more uniformly on the OMC frameworks than those on MCVs, MPC and RGO. As expected, the HER is catalyzed more efficiently on CoP-OMC accompanied by a small onset potential of -77.74 mV vs. RHE, a low Tafel slope of 56.67 mV dec^-1, a small over-potential value of 112.18 mV at 10 mA cm^-2, and the outstanding long-term stability. These results show that CoP-OMC inherently exhibits better HER catalytic activity than other CoP-based catalysts in acidic electrolytes. Such excellent performances are attributed to the synergistic effect of the highly catalytic sites provided by the uniformly dispersed and size-controlled CoP NPs embedded on OMC, excellent proton transport efficiency, and improved electron transport with a high electron transfer rate. Our results provide useful information that the mesoporous conductive matrices could be applied to greatly improve the HER catalytic efficiency of various HER catalytically active centers. This journal is

Full text of DOI:10.1039/c4ta06630h

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Facile synthesis of various highly dispersive CoP
nanocrystal embedded carbon matrices as efficient
electrocatalysts for the hydrogen evolution
reaction†
Cite this: J. Mater. Chem. A, 2015, 3,
4255
a
b
b
a
Mian Li, Xiaotian Liu, Yueping Xiong, Xiangjie Bo, Yufan Zhang,a Ce Hana
*
a
*
and Liping Guo
In order to promote the hydrogen evolution reaction (HER) catalytic efficiency of cobalt phosphide (CoP) and
then construct efficient HER catalysts, a facile procedure has been adopted to prepare tiny CoP nanoparticle
(NP) embedded carbon matrices without using any extreme conditions and harmful organic reagents or
surfactants. Meanwhile, in order to explore the influence of structures of carbon matrices on preventing
the free growth of CoP NPs and enhancing the HER catalytic efficiency of the CoP–carbon catalysts,
imporous reduced graphene oxide (RGO), macroporous carbon (MPC), mesoporous carbon vesicles
(MCVs) and ordered mesoporous carbon (OMC) were used for preparing CoP–carbon HER catalysts. SEM
and TEM measurements show that size-controlled CoP NPs indeed grew more uniformly on the OMC
frameworks than those on MCVs, MPC and RGO. As expected, the HER is catalyzed more efficiently on
CoP–OMC accompanied by a small onset potential of ꢀ77.74 mV vs. RHE, a low Tafel slope of 56.67 mV
decꢀ1, a small over-potential value of 112.18 mV at 10 mA cmꢀ2, and the outstanding long-term stability.
These results show that CoP–OMC inherently exhibits better HER catalytic activity than other CoP-based
catalysts in acidic electrolytes. Such excellent performances are attributed to the synergistic effect of the
highly catalytic sites provided by the uniformly dispersed and size-controlled CoP NPs embedded on
OMC, excellent proton transport efficiency, and improved electron transport with a high electron transfer
rate. Our results provide useful information that the mesoporous conductive matrices could be applied to
greatly improve the HER catalytic efficiency of various HER catalytically active centers.
Received 3rd December 2014
Accepted 7th January 2015
DOI: 10.1039/c4ta06630h
www.rsc.org/MaterialsA
of water electrolysis. As expected, platinum group metals are the
most efficient catalysts for the HER, capable of driving signicant
currents close to the thermodynamic potential.1,2 However, noble
Introduction
Hydrogen, a clean energy carrier, being the cleanest of all fuels,
has long been expected to play a pivotal role in our energy land-
scapes by mitigating (if not completely eliminating) our reliance
on fossil fuels. However, molecular hydrogen does not exist
naturally on earth. Although molecular hydrogen could be
obtained from water (the most abundant and renewable hydrogen
source) by the water-splitting reaction, it is inherently quite
difficult. The production of molecular hydrogen by the electro-
chemical reduction of water is an important component of several
developing clean-energy technologies. Similarly, it is also neces-
sary to use efficient hydrogen evolution reaction (HER) electro-
catalysts to increase the reaction rate and lower the over-potential
metals are unsuitable for large scale hydrogen production
because of their scarcity and high cost. It is therefore important to
explore efficient alternatives made entirely of earth-abundant
elements to noble metals for the HER. Abundant nickel-based
alloys are oen used commercially as HER catalysts functioning
in alkaline electrolytes.3,4 Unfortunately, these HER catalysts
cannot work under the strongly acidic conditions involved in the
proton-exchange membrane (PEM) technique and thus lack
compatibility with PEM-based electrolysis units.5 The develop-
ment of acid-stable non-noble-metal HER catalysts is still neces-
sary and huge progress has been made in the past years.
Molybdenum-based compounds have received great success for
catalyzing the HER in acidic electrolytes such as MoS2,6 MoSe2,7
Mo2C,8 etc. However, there still remains a big challenge to
improve their catalytic efficiency to meet the requirements of
practical application.9,10 Still one can see from these reported
results that the identication of novel high-performance HER
catalysts based on non-precious materials is very attractive.
aFaculty of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China.
E-mail: baoxj133@nenu.edu.cn; guolp078@nenu.edu.cn; Fax: +86-0431-85099762;
Tel: +86-0431-85099762
bSchool of Chemical Engineering and Technology, Harbin Institute of Technology,
Harbin 150001, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta06630h
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It is well-known that the hydrodesulfurization (HDS) and
HER work in the same way in that they both rely on the reversible
binding of catalysts and hydrogen, with hydrogen dissociation to
yield H2S in HDS and with protons bound to catalysts to promote
hydrogen formation in the HER. It is thus deduced that catalysts
for HDS may be active for the HER. More recently, transition-
metal phosphides (TMPs) have been an important class of
compounds with metalloid characteristics and are being inten-
sively studied for the application as catalysts for the HDS reac-
tion.11,12 Particularly, vast recent research has indeed proven
MoP,13 Ni2P,14 FeP,15,16 CoP17,18 and Cu3P19 to be efficient
hydrogen-evolving catalysts due to their attractive hydrogen
transfer properties. When used as non-noble metal HER cata-
lysts, they really show enhanced catalytic efficiency in acidic
electrolytes. However, the catalytic activity of these catalysts still
has not reached the level of Pt-based catalysts limited by the
density and reactivity of active sites. Furthermore, poor electron
and proton transport efficiency, low surface area, and instability
under operating conditions are common pitfalls of these HER
electrocatalysts.20,21 Generally, the exposed edges are affected by
the morphologies and structures of catalysts. Hence, designing
TMP-based HER electrocatalysts with more edge sites and better
electron and proton transport activity is one effective strategy to
enhance their HER catalytic activity. In this regard, forming
highly dispersed TMP nanoparticles (NPs) with the controllable
sizes (i.e., edge rich) on conducting matrices is an ideal protocol
to further enhance the HER catalytic efficiency of catalysts.22
The use of conductive carbon as a support material can not
only improve the conductivity of the hybrid catalyst but also
increase the dispersion of the active phases so that the catalyst
phases offer more active sites and prevent the rapid decrease of
activity sites as far as possible.23,24 Since they were synthesized
for the rst time, vast publications have paid intensive attention
on the electrochemical applications of ordered mesoporous
carbon (OMC),25 onion-like mesoporous carbon vesicles (MCVs)26
and macroporous carbon (MPC).27 Meanwhile, the research
results also demonstrated that the particular porous structures
and unique properties of these porous carbon materials really
make them very attractive materials for designing electrochemi-
cally catalytic platforms with abundant and accessible edge sites.
Additionally, density functional theory calculations have theo-
retically indicated that the TMP surface, which has exposed metal
and phosphorus sites, exhibits an ensemble effect, whereby the
proton-acceptor and hydride-acceptor centers are both present to
facilitate the catalysis of the HER.28 As far as we are aware, there
was no attention given to the study of the electrocatalytic
application of OMC, MCV and MPC materials with different
pore characteristics for loading of TMP NPs and catalyzing the
HER. Thus, we wish to regard the CoP as an example and
structure the catalysts of CoP NPs supported on porous carbon
matrices. Meanwhile, we also expect to refrain CoP NPs from
growing freely in the preparation process in the presence of
Scheme 1 Illustration of the preparation procedures for the free CoP
NPs, CoP–RGO, CoP–MCV, CoP–MPC and CoP–OMC samples.
OMC) decorated with CoP nanocrystals (denoted as CoP–carbon)
by the low-temperature phosphorization of the corresponding
Co3O4–carbon precursors. Moreover, in order to prove the capital
importance of porous structures of carbon matrices in improving
the catalytic ability of HER catalytic centers, reduced graphene
oxide (RGO) nanosheets, without characteristic porous struc-
tures, have also been used as matrices for comparison. Wide-
angle X-ray diffraction (XRD), nitrogen adsorption–desorption
isotherms, scanning electron microscopy (SEM), transmission
electron microscopy (TEM), energy dispersive X-ray (EDX) spec-
troscopy, X-ray photoelectron spectroscopy (XPS), FT-IR spectra,
etc. were primarily used for analyzing the structures, morphol-
ogies and composition of as-prepared catalysts. Furthermore, the
HER catalytic activities were tested by using the cyclic voltam-
metry (CV) and linear sweep voltammetry (LSV) techniques.
Scheme 1 schematically shows the entire procedures for
preparing CoP–carbon catalysts.
Experimental
Reagents and apparatus
Reagents and apparatus used in this paper are described in the
ESI.†
Synthesis of the Co3O4–carbon and free Co3O4 NP samples
carbon matrices and then acquire more electrochemical active The Co3O4–carbon hybrids were synthesized using a hydro-
sites with more excellent charge transfer efficiency compared thermal method. In a typical synthesis, the as-prepared carbon
with the pure ones.
matrices (40 mg) were added into 48 mL ethanol and 2 mL
Herein, we present our recent efforts in developing nano- water, respectively. Aer the mixture was sonicated for several
hybrids that consist of porous carbon materials (MPC, MCV and minutes, 1.2 mL of 0.6 M Co(OAc)2 aqueous solution and 1 mL
4256 | J. Mater. Chem. A, 2015, 3, 4255–4265
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of NH4OH were added under stirring at room temperature. structures of MCV spheres are further shown in the TEM image
Then the suspension was kept at 80 ꢁC and stirred for 20 h. Aer (Fig. 1J). One can see that MCV spheres show multilayer vesicle
that, the mixture was transferred to an 80 mL Teon-lined mesostructures and the distances between two layers vary from
autoclave and maintained at 150 ꢁC for 3 h. Finally, the as- several nanometers to 25 nm, indicating the good structural
prepared products were collected by centrifugation and washed ordering of MCVs. The SEM image of OMC (Fig. 1G) demon-
with ethanol and deionized water. Free Co3O4 NPs were strates that OMC is made up of carbon nanorods. The typical
prepared using the same method without using any carbon TEM images viewed from [1, 0, 0] (Fig. 1K) and [0, 0, 1] (inset of
matrix.
Fig. 1K) directions for OMC provide direct visualization of the
morphology and internal mesostructure of OMC, demon-
strating that OMC exhibits highly ordered 2D-hexagonal arrays
of carbon nanorods and well-developed mesopores between
adjacent carbon nanorods. The textural structure of the pristine
MPC is seen as a well-dened interconnected macroporous
nanostructure (Fig. 1H), meanwhile, the pores of MPC were
closely packed and well-interconnected with an average size of
about 100 nm (Fig. 1L).
Synthesis of the CoP–carbon and free CoP NP samples
20 mg of the Co3O4–OMC hybrid and 100 mg of sodium hypo-
phosphite were mixed together and grind to a ne powder by
using a mortar. Then, the mixture was calcined at 300 ꢁC for 2 h
with a heating speed of 2 C minꢀ1. The obtained product was
ꢁ
washed with deionized water and dried at 80 ꢁC overnight. The
same procedure was used to prepare free CoP NPs, CoP–RGO,
CoP–MCV and CoP–MPC samples.
The surface areas of RGO, MPC, MCV and OMC are deter-
mined by N2 adsorption–desorption isotherms. The N2 phys-
isorption isotherms of four samples all exhibit type IV shapes.
As shown in Fig. 1M–P, the remarkable hysteresis loops of MCV,
MPC and OMC indicate their porous nature. As recorded in
Table S1,† OMC has a BET specic surface area and a pore
volume of 905 m2 gꢀ1 and 1.58 cm3 gꢀ1, respectively, which are
larger than those of RGO (113 m2 gꢀ1 and 0.31 cm3 gꢀ1), MCVs
(559 m2 gꢀ1 and 1.15 cm3 gꢀ1), and MPC (449 m2 gꢀ1 and
0.92 cm3 gꢀ1). Additionally, the pore sizes of four carbon
matrices were obtained from desorption branches of their
isotherms by the Barrett–Joyner–Halenda method. One can see
that RGO has a pore size distribution mainly from 20 to 160 nm
(inset of Fig. 1M). However, MCVs and OMC contain the mean
pore size distributions of 6.7 (insets of Fig. 1N) and 4.8 nm
(inset of Fig. 1O), respectively, suggesting that the abundant
mesoporous structures are well-displayed for OMC and MCVs.
Distinctively, for MPC, the relatively large pore structures of
about 100 nm in size have been observed as the predominant
units (inset of Fig. 1P). These pore size distribution results
together with the SEM and TEM characterization results have
veried the structural characteristics of several carbon matrices
in detail.
The XRD analysis was performed to characterize the struc-
tures of carbon matrices, Co3O4–carbon nanohybrids and CoP–
carbon nanocomposites. As shown in Fig. 2A, the diffraction
peak located at about 11.8ꢁ indicates the crystalline structure of
GO (curve a). The broad peak located at about 25ꢁ in the XRD
pattern corresponds to the (002) reection of RGO (curve b).
Interestingly, the diffraction peak at 11.8ꢁ disappears for RGO,
suggesting that GO was thoroughly reduced by ethanol in the
preparation process. Broad peaks at about 25ꢁ also appear in
the XRD patterns of MPC (curve c), MCVs (curve d) and OMC
(curve e) matrices. In contrast, as shown in Fig. 2B, the free
Preparation of working electrodes
A rotating disk electrode (RDE, d ¼ 5 mm) was used for all
electrochemical experiments. Prior to be modied, it was pol-
ished carefully with 1.00, 0.30 and 0.05 mm alumina powders
and then cleaned with HNO3 (1 : 1), ethanol and deionized
water, respectively. Catalyst ink was prepared by mixing 5 mg of
the as-prepared catalyst powders with 1 mL of Naon solution
(0.1 wt%) within 45 min of ultrasonication. Then 11.1 mL of
catalyst ink (containing 55.5 mg of catalysts) was dropped onto a
cleanly washed GCE (loading ꢂ0.285 mg cmꢀ2) and dried under
an infrared lamp before electrochemical experiments.
Electrochemical measurements
Electrochemical measurements were performed in a standard
three-compartment cell at 25 ꢁC. The modied RDE, Pt wire and
Ag/AgCl (in saturated KCl solution) electrodes acted as the
working, counter and reference electrodes, respectively. The
electrolyte used was 0.5 M H2SO4 aqueous solution. The CV and
LSV tests were carried out by using a Par 2273 Potentiostats
Electrochemistry Workstation. All potentials appeared in this
paper are referred to the reversible hydrogen electrode (the
potentials recorded and referred to the Ag/AgCl electrode in
each experiment were calculated by using the formula ERHE
¼
EAg/AgCl + 0.059pH + 0.198 V, where ERHE is the potential vs. the
reversible hydrogen electrode (RHE), EAg/AgCl is the potential vs.
the Ag/AgCl electrode, and pH is the pH value of the electro-
lyte).23 All current densities are the ratios of currents and
geometric areas of working electrodes.
Results and discussion
Fig. 1A–D show the schematic diagrams of the resulting carbon Co3O4 NPs (curve a), Co3O4–RGO (curve b), Co3O4–MPC (curve
matrices. Meanwhile, the corresponding SEM and TEM images c), Co3O4–MCV (curve d) and Co3O4–OMC (curve e) samples all
of these carbon materials are also shown in Fig. 1E–L. Fig. 1E show six additional peaks at about 31ꢁ, 37ꢁ, 45ꢁ, 56ꢁ, 59ꢁ and
and I show typical SEM and TEM images of RGO sheets, it is 65ꢁ, which are indexed to the (220), (311), (400), (422), (511) and
clear that RGO exhibits a representative laminated nanosheet (440) planes of Co3O4 nanocrystals (JCPDS 42-1467), respec-
structure. But MCV consists of irregular spheres containing the tively.29 Finally, aer the phosphorization procedure, only the
particle sizes in the range of 100–300 nm (Fig. 1F). The detailed diffraction peaks of the CoP phase are observed at 32ꢁ, 36ꢁ, 46ꢁ,
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Fig. 1 The schematic diagrams of the RGO (A), MCV (B), OMC (C) and MPC (D) matrices. SEM images of the RGO (E), MCV (F), OMC (G), and MPC
(H) matrices. TEM images of the RGO (I), MCV (J), OMC (K), and MPC (L) matrices. Nitrogen adsorption–desorption isotherms of the RGO (M),
MCV (N), OMC (O), and MPC (P) matrices, inset: the corresponding pore size distribution curves.
48ꢁ, 52ꢁ, and 57ꢁ for the free CoP NPs (curve a), CoP–RGO (curve with irregular morphology. Surprisingly, the particle size
b), CoP–MPC (curve c), CoP–MCV (curve d) and CoP–OMC distribution continues from 20 to a few hundred nanometers
(curve e) samples (Fig. 2C), which are indexed to the (011), (111), (Fig. S1B†). Fig. 3A–D show the general morphologies of CoP–
(112), (211), (103) and (301) planes of CoP nanocrystals (JCPDS- RGO, CoP–MPC, CoP–MCV and CoP–OMC by the typical SEM
29-0497), respectively.30 These observations suggested the images. CoP nanocrystals deposited on the surface of the RGO
successful chemical conversion of Co3O4 NPs into CoP NPs in matrix are well dispersed (Fig. 3A) and the particle size distri-
the phosphorization procedure.
bution is narrower compared with that for the free CoP NPs.
Fig. S1A† shows the general morphologies of free CoP NPs, it Meanwhile, the corresponding TEM image (Fig. 3E) and histo-
can be observed that the free CoP NPs have grown freely grams of particle size distribution (Fig. 3I) for CoP–RGO show
in solution during the preparation process. Meanwhile, the as- an average diameter of 7.58 nm (the particle size distribution is
prepared free CoP NPs would aggregate into large particles mainly from 5 to 11 nm). Especially, when the same preparation
4258 | J. Mater. Chem. A, 2015, 3, 4255–4265
This journal is © The Royal Society of Chemistry 2015