Facile synthesis of CoX (X = S, P) as an efficient electrocatalyst for hydrogen evolution reaction

Article abstract of DOI:10.1039/c5ta03153b

Developing environmentally-friendly and earth-abundant electrocatalysts is desirable for an efficient hydrogen evolution reaction (HER). Herein, we report a facile and controllable synthesis of CoX (X = S, P) nanocatalysts by the chemical conversion of thin Co(OH)₂ nanoplates under mild conditions. Both catalysts delivered high catalytic activity for HER. Small onset potentials of 59 and 32 mV, along with low Tafel slopes of 56.2 and 54.8 mV dec^-1, were observed for CoS and CoP, respectively. Analyses suggest that the better HER performance of CoP nanocatalysts could be attributed to the intrinsically more positively charged nature of the Co metal center, the longer Co-P bond length, and more catalytic active sites due to the smaller size of the CoP nanocatalysts. High catalytic stability in acidic media was also observed for both CoS and CoP catalysts for a duration of 18 hours.

Full text of DOI:10.1039/c5ta03153b

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RSCPublishing
ARTICLE
Facile synthesis of CoX (X=S, P) as efficient
electrocatalysts for hydrogen evolution reaction
Cite this: DOI: 10.1039/x0xx00000x
Jiayuan Lia, Xuemei Zhoua, Zhaoming Xiaa, Zhiyun Zhanga, Jing Lia, Yuanyuan Ma*ab
Yongquan Qu*abc
,
Received 00th January 2012,
Accepted 00th January 2012
Developing the environmentꢀfriendly and earthꢀabundant electrocatalysts is desired for
efficient hydrogen evolution reaction (HER). Here we report a facile and controllable synthesis
of CoX (X=S, P) nanocatalysts by chemical conversion of thin Co(OH)2 nanoplates at the mild
conditions. Both catalysts delivered the high catalytic activity for HER. The small onset
potentials of 59 and 32 mV, along with the low Tafel slopes of 56.2 and 54.8 mV/dec, were
observed for CoS and CoP, respectively. Analyses suggest that the better HER performance of
CoP nanocatalysts could be attributed to their intrinsically more positive charged nature of the
metal center Co, longer Co−P bond length, and more catalytic active sites due to the smaller
size of the CoP nanocatalysts. The high catalytic stability in acidic media was also observed
for both CoS and CoP catalysts for a duration of 18 hours.
DOI: 10.1039/x0xx00000x
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nanostructure with more edge activity sites.16ꢀ20 Meanwhile, the
low chemical stability of MoS2 due to the oxidation in the
presence of oxygen further limits its application.16
Introduction
The increasing shortage of fossil fuels and the raising
deteriorated effect on the environmentals by extensive usage of
fossil fuels have already become obstacles to the sustainable
development of human society.1 A feasible solution is to search
for the sustainable, clean and highꢀefficient energy resources as
the alternatives.2 Hydrogen possesses a very high calorific
value which makes it as the promising alternative fuel in the
future.3 The water electrolysis is an important pathway to
obtain molecular hydrogen with high purity for future
applications. Therefore, the efficient and robust catalysts with
the low reaction overpotential are expected for the hydrogen
evolution reaction (HER) in the water splitting process.4
Platinum (Pt), as the most effective HER catalyst, is a rare
metal and expensive which limits its widespread use.5 The
earthꢀabundant nickelꢀbased alloys as the commercial HER
catalyst with the high performance exhibit a low catalytic
stability in the acidic electrolyte media.6ꢀ9 Recently, MoS2, as
an acidꢀstable HER catalyst, has already been considered as a
promising substitution for Pt owing to its relative abundant
reserve and high HER intrinsic catalytic activity.10ꢀ14 However,
the bulk MoS2 shows a poor HER performance.15 The superior
HER performance of MoS2 depends on the artificially designed
Hence, the great efforts have been focused on the earthꢀ
abundant transition metal (Co, Ni, Fe) sulfides/phosphides,21ꢀ25
which can deliver high catalytic activity toward HER and good
catalytic stability in acidic media. The previous works have
reported that the cobaltꢀbased sulfides/phosphides share the
same catalytic mechanism as the hydrogenase happened in
nature.21, 23 On the basis of the catalytic mechanism, the metal
center cobalt serves as the active sites.18 Specifically, the
function of cobalt is the hydrideꢀacceptor center and the
26
corresponding pendant base is the protonꢀacceptor.24,
Cooperatively, hydrogen can be generated. However, there
exists considerable differences in the HER performance of the
cobaltꢀbased sulfides and phosphides even though the
corresponding pendant base S and P have the nearby atomic
27
number in periodic table of elements.23,
CoP nanowires23
exhibited HER performance with an onset potential of 38 mV, a
Tafel slope of 51 mV/dec, and an overpotential of 100 mV to
reach the current density of 20 mA/cm2. Meanwhile, CoS2
nanowires27 delivered an onset potential of 75 mV and a Tafel
slope of 51.6 mV/dec for HER. To achieve the current density
of 20 mA/cm2, the overpotential of 160 mV was required. The
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performance of CoS2 film27 was also studied, in which the onset three times and then dried in vacuum at 60 ºC for 12 h.
potential of 119 mV, the Tafel slope of 51.4 mV/dec and the
Synthesis of CoP nanocatalysts
overpotential of 206 mV for the catalytic current density of 20
mA/cm2 were observed. Obviously, the atomic configurations
and morphology characteristics of cobaltꢀbased sulfides and
phosphides play important roles in their HER performance.
However, the specific contribution of the atomic configurations
and morphology characteristics to their HER performance is
still unclear owing mainly to the lack of relevant experimental
investigations.
In this work, we reported a facile and controllable synthetic
approach by chemically converting the thin Co(OH)2 nanoplate
precursor into CoX (X=S, P) nanostructures. The powder Xꢀray
diffraction (XRD), Xꢀray photoelectron spectroscopy (XPS)
and transmission electron microscope (TEM) were used for
identifying the phase changes and morphology features of CoX
(X=S, P) while the electrochemical analyzer was utilized to
evaluate their HER performance. The electrochemical
measurements reveal the superior HER performance of both
CoS and CoP nanocatalysts with the small onset overpotential
of 59 and 32 mV and the low Tafel slopes of 56.2 and 54.8
mV/dec, respectively. The experimental results suggest that the
superior HER performance of CoP could be derived from its
intrinsically positive charged nature of the metal center Co, the
long bond length of Co−P and the abundant catalytic active
sites toward HER.
CoP nanocatalysts were synthesized through a solidꢀphase
reaction between βꢀCo(OH)2 nanoplates and NaH2PO2. For a
typical synthesis, 27.9 mg of βꢀCo(OH)2 nanoplate precursor
and 159 mg of NaH2PO2 were mixed together and ground into a
fine powder by using a mortar. Then, the mixture was calcined
at 300 ºC for 2h in a quartz tube with a ramping rate of 2
ºC/min under a flow of argon. The obtained product was
thoroughly washed with deionized water for the removal of the
surplus salts. The CoP nanocatalysts were dried in vacuum at
60 ºC for 12 h.
Characterization
The phase evolution of asꢀsynthesized nanostructures was
monitored by powder XRD. The XRD patterns with diffraction
intensity versus 2
θ were recorded in a Shimadzu Xꢀray
diffractometer (Model 6000) using Cu Kα radiation. XPS
spectra were acquired on a Thermo Electron Model KꢀAlpha
with Al Kα as the excitation source. TEM images and highꢀ
resolution TEM (HRTEM) images were taken with a Tecnai G2
F20 Sꢀtwin transmission electron microscope with an
accelerating voltage of 120 kV. The surface area was evaluated
by nitrogen physisorption (Micromeritics, ASAP 2020 HD88)
based on the BrunauerꢀEmmetꢀTeller (BET) method.
Experimental
Electrochemical measurements
All chemicals used in the experiments are analytical grade and
used as received without further purification.
Electrochemical measurements were performed with a CHI
660D electrochemical analyzer (CH Instruments, Inc.,
Shanghai). All the electrochemical measurements were
conducted by a typical threeꢀelectrode setup with an electrolyte
solution of 0.5 M H2SO4, a Pt counter electrode, and a saturated
calomel electrode (SCE) reference electrode. The preparation
method of the working electrodes containing HER catalysts was
described as follows. Briefly, the catalyst ink was prepared by
dispersing 4 mg of catalyst into 1 mL of water/ethanol
(v/v=768: 200) solvent containing 32 µL of 5 wt% Nafion.
Then 5 µL of this catalyst ink (containing 20 µg of catalyst)
was loaded onto a carbon fiber paper to achieve the sample
loading ~0.91 mg/cm2. The SCE reference electrode was
calibrated with respect to reversible hydrogen electrode (RHE)
by adding a value of (0.242 +0.059pH) V before the
Synthesis of thin βꢀCo(OH)2 nanoplates
βꢀCo(OH)2 nanoplates were synthesized through the same
process as demonstrated in our previous work28. Specifically,
NaOH (0.16 mol) was dissolved in 35 mL of MilliꢀQ water.
Co(NO3)2. 6H2O aqueous solution (4.0 M) was slowly injected
into the NaOH aqueous solution. After stirring continuously for
30 min, the mixture was transferred into an electric oven at 100
°C for 12 hours. The βꢀCo(OH)2 nanoplates was centrifugated
off, thoroughly washed with copious amount of deionized water
and ethanol for several times, and then dried in vacuum at 60 ºC
overnight.
Synthesis of CoS nanocatalysts
measurements.
Linear
Sweep
Voltammetry
(LSV)
measurements were performed with a scan rate of 5 mV/s. The
onset potentials were determined based on the beginning of
linear regime of the Tafel plot. The time dependency of
catalytic currents during electrolysis for the catalyst was tested
Anion exchange reaction was utilized for synthesis of CoS
nanocatalysts. Briefly, asꢀprepared Co(OH)2 precursor (50 mg)
was well suspended in 12.5 mL of deionized water through
ultrasonication at room temperature. Sodium sulfide (0.5 g) was
slowly added into the brown βꢀCo(OH)2 nanoplate colloidal
solution sequentially. The solution was stirred at room
temperature for 10 min and then transferred into a 20 mL
Teflonꢀlined autoclave. The reaction was maintained at 80 ºC
for 12 h. The CoS nanocatalysts was obtained after
centrifugation and thorough washing with deionized water for
in 0.5
M
H2SO4 at η=121 mV after equilibrium.
Electrochemical impedance spectroscopy (EIS) measurements
were carried out on the AUTOLAB PGSTAT204
electrochemistry workstation in the frequency range from 100
kHz to 0.1 Hz at an open circuit potential, with 10 mV as the
amplitude potential.
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species33. The presence of the oxidized Co and P species can be
Results and discussion
βꢀCo(OH)2 nanoplates were synthesized by the coꢀprecipitation
method at 100 ºC for 12h.28 The phase and microstructures of
the asꢀsynthesized nanostructures were characterized by powder
XRD and electron microscopy, respectively. XRD pattern (Fig.
1a, black curve) indicates the as prepared nanostructures are βꢀ
Co(OH)2 phase (JCPDSꢀ45ꢀ0031).28 Similar to our previous
report, the nanostructures exhibit a plateꢀlike morphology (Fig.
1b).28 A wide distribution from tens of nanometers to hundreds
of nanometers was observed.
βꢀCo(OH)2 nanoplates were employed as the precursors and
converted into cobalt sulfides and phosphides through the anion
exchange method and the high temperature reaction approach,
respectively. It was found that the βꢀCo(OH)2 nanoplates were
completely converted into sulfides or phosphides, based on
their XRD patterns (Fig. 1a). The XRD patterns of asꢀobtained
nanostructures can be well indexed to the hexagonal CoS phase
(JCPDSꢀ75ꢀ0605, blue curve)29 and the orthorhombic CoP
phase (JCPDSꢀ29ꢀ0497, red curve).22 TEM images (Fig. 1c and
1d) reveal that the plateꢀlike morphology of the CoS products
was well preserved after the sulfidation reaction. The
interconnected or merged plates were also observed. The slight
transparent of the CoS plates under the electron beam indicates
the thin nature of asꢀprepared nanostructures. The calculated
lattice fringe of 0.291 nm from HRTEM (Fig. 1d) was
corresponded to the (100) crystal surface of the hexagonal CoS
phase, further indicating the formation of CoS nanoplates. After
phosphidation, a structural deformation for asꢀprepared CoP
was observed (Fig. 1e), in which the crystalline size of CoP
exhibits a dramatically decrease from tens of nanometers of the
βꢀCo(OH)2 nanoplate precursor to several nanometers. TEM
image (Fig. 1e) also reveals that the small crystalline of CoP
was integrated into a large porous structure. This morphological
transformation can be attributed to the effect of PH3 generated
by the decomposition of NaH2PO2 in the solidꢀphase reaction
and the structural collapse of the thin nanoplate precursor at
high temperatures.30, 31 The measured lattice spacing was 0.279
nm (HRTEM image, Fig. 1f), which corresponded to the (002)
plane of the orthorhombic CoP phase.
XPS technique was employed to investigate the electronic
structures of the asꢀprepared CoS and CoP nanocatalysts. All
the spectra were referenced to the aliphatic carbon at a binding
energy (BE) of 284.5 eV. The XPS survey spectra of the asꢀ
prepared samples were shown in the Supporting Information
Fig. S1, which reveals the final products are CoS and CoP. Fig.
2a and 2b present the high resolution XPS spectra of CoP at Co
2p and P 2p regions. Two apparent peaks at 779.2 and 781.7 eV
in the energy level of Co 2p3/2 (Fig. 2a) can be assigned to the
positive charged Co in Co P32 and the oxidized Co species.33
As shown in XPS spectrum of P (Fig. 2b), three peaks at 129.7,
130.8 and 134.0 eV are observed. The peaks at 129.7 eV and
130.8 eV can be assigned to P 2p3/2 and P 2p1/2 in CoP.32 The
weak peak at 134.0 eV can be attributed to the oxidized P
Fig. 1 Structural characterizations of βꢀCo(OH)2 nanoplate, CoS and CoP
catalysts. (a) Powder XRD patterns of βꢀCo(OH)2 nanoplate precursor and asꢀ
prepared CoP and CoS. (b) TEM image of βꢀCo(OH)2 nanoplates. (c) TEM
image of CoS. (d) HRTEM image of CoS. (e) TEM image of CoP. (f)
HRTEM image of CoP.
attributed to the superficial oxidation of CoP sample in air,
similar to previous studies.33 Similarly, the peaks at 779.0 and
162.2 in the Co 2p3/2 and S 2p3/2 region corresponded to the
binding energy of Co and S in CoS.34 Meanwhile, the peaks at
781.4 eV of Co 2p3/2 and 168.0 eV of S 2p3/2 manifest the
superficial oxidation in CoS nanocatalysts upon the exposure to
air.34 Compared with the binding energy of Co metal (778.1–
778.2 eV) and elemental S or P (130.2 or 163.7 eV),35 the
energy level of Co 2p3/2 in both CoS and CoP shifts to high
energy, indicating the Co atom in both CoS and CoP with a
partial positive charge (δ+). In contrast, the energy levels of S
2p3/2 and P 2p3/2 in CoS and CoP exhibit a negative shift
compared to those of elemental S and P, suggesting the
partially negative charged S and P (δ－ ). The XPS spectra
manifest a charge transfer process from metal Co to P or S,32, 36
which is desired for high performance HER electrocatalysis.
To evaluate the HER activities of the CoS and CoP, the
catalysts were loaded onto and carbon fiber paper (CFP)
substrates with a density of 0.91 mg cmꢀ2 by drop casting. The
LSV curves of the bare CFP substrate, CoS and CoP electrodes
in 0.5 M H2SO4 electrolyte at a scan rate of 5 mV/s are shown
in Fig. 3a. Compared to the strong catalytic current densities of
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Fig. 3 HER catalytic activity of the CoS and CoP nanocatalysts. (a)
Polarization curves for Pt wire, CoS, CoP and blank carbon fiber paper (CFP)
at a scan rate of 5 mV/s. (b) The corresponding Tafel plots for Pt wire, CoS
and CoP. (c) The calculated turnover frequency curves for CoS and CoP
nanocatalysts in our experiment. (d) Timeꢀdependent current density.
Fig. 2 The XPS fine spectra of (a) Co 2p and (b) P 2p region for CoP sample.
The XPS fine spectra of (c) Co 2p and (d) S 2p region for CoS sample.
the CoS and CoP catalysts, the very weak current density (2.9
mA cmꢀ2) of the CFP substrate at 0.2 V applied potential (vs
RHE) indicates the inactive nature of CFP towards HER and a
background correction for the CFP substrate is unnecessary for
all catalytic electrodes.
A/cm2, which are close to the best reported results of the cobalt
phosphides/sulfides23, 27 and larger than most of the reported
HER catalysts.39 The exchange current density of CoP is
estimated to be 15 times larger than CoS. The larger j0 of CoP
can be attributed to its morphology characteristics that possess
the larger surface area and could provide more effective active
sites.42 More details will be discussed in the following.
As a comparison, the catalytic activity of the commercially
available Pt wire was also evaluated as shown in Fig. 3a. Pt
wire exhibits the expected catalytic activity with the near zero
onset overpotential and high HER catalytic activity at each bias.
Notably, both asꢀsynthesized CoS and CoP deliver good HER
catalytic activity as reflected by their small onset potentials and
large HER current densities at the specific applied voltages.
The CoP catalysts exhibited a higher HER catalytic activity
than the CoS catalysts under the identical conditions. The onset
potential of 32 mV for CoP catalysts was 27 mV smaller than
that (59 mV) for CoS in the acidic media (0.5 M H2SO4). An
overpotential of 107 mV of the CoP electordes realized a
catalytic HER current densities of 100 mA/cm2. In contrast, a
high overpotential of 196 mV for CoS was required to achieve
the catalytic current densities of 100 mA/cm2.
We also fitted the polarization curves of the Pt wire, CoP
and CoS shown in Fig. 3a to the Tafel equation. As presented in
Fig. 3b, the derived Tafel slope for the Pt wire is 30.5 mV/dec,
consistent with the previously reported values.22 The values of
Tafel slope for CoS and CoP nanocatalysts (Fig. 3b) are 56.2
and 54.8 mV/dec in the region of 5ꢀ60 mV, respectively. Those
values are comparable to or even better than those of cobaltꢀ
based sulfides/phosphides23, 27, 31, 37 and those of many acidꢀ
stable Mo, Fe and Ni based HER catalysts previously
reported.16, 24, 25, 38, 39 The values of Tafel slope also suggest the
VolmerꢀHeyrovsky reaction mechanism for CoS and CoP as the
HER catalysts.40, 41 Also, the exchange current density (j0) of
CoS and CoP are calculated to be about 1.62×10ꢀ5 and 2.4×10ꢀ4
The catalytic activity of each catalyst in terms of the values
of turnover frequency (TOF) at each active site can be derived
from the LSV curves shown in Fig. 3a. The number of the
active sites was estimated by scanning the cyclic voltammetrys
of the CoS and CoP electrodes in 1.0 M PBS (pH=7) with a
potential window from −0.2 to 0.6 V vs RHE and a scan rate of
50 mV/s (Fig. S2).43 The integrated charge within the scanning
potential window is roughly proportional to the total number of
the active sites of the electroactive materials. By assuming a
oneꢀelectron process for both reduction and oxidation, the
upper limit of the active site can be calculated. Details can be
found in the Supplementary Information. Fig. 3c shows the
calculated TOF curves of CoS and CoP in 0.5 M H2SO4 within
the range of applied potentials. On the basis of previous works,
Pt exhibits the best HER catalytic activity, which reaches a
TOF of 0.8 sꢀ1 at 0 V.18 Compared with Pt, the overpotentials of
133 and 82 mV were required to achieve the TOF of 0.8 sꢀ1 for
CoS and CoP, respectively. In contrast, an overpotentials of 300
mV was needed for MoS2 catalysts to realize a TOF of 0.725 sꢀ
1, further indicating the high catalytic activity of the CoS and
CoP catalysts.44 To reach a TOF of 4 sꢀ1, the overpotentials of
CoS and CoP are 253 and 246 mV, respectively, which are
smaller than those of MoS2ꢀbased nanowire catalyst (272 mV)45
and close to the best reported results of the cobalt
phosphides/sulfides (240 mV).23
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Besides the catalytic stability, the catalytic stability of the
CoS and CoP nanocatalytic was evaluated by running
chronoamperometric response (iꢀt) at an overpotential of 121
mV in the acidic environments (0.5 M H2SO4). During the
measurements, the working electrodes were continuously
stirred at 1600 rpm to remove the generated hydrogen bubbles.
The corresponding timeꢀdependent current density curves are
shown in Fig. 3d. Despite the fluctuation in the catalytic current
density at the initial several hours, the stabilized catalytic
current densities of CoS and CoP for HER were observed over
18 hours, indicating the high catalytic stability of both
electroactive materials in the strong acidic aqueous solution.
Generally, it was found that the calculated TOFs of CoP
were generally larger than those of CoS at the specific
overpotentials (Fig. 3c), indicating the higher intrinsic catalytic
activity of CoP over CoS. On the other hand, the TOF value of
CoP is larger than CoS under the same overpotential, which can
reveal the better intrinsic catalytic activity of CoP than CoS.18
To understanding the native difference in the catalytic activity
of CoS and CoP, the catalytic mechanism was studied. The
superior HER performance of CoS and CoP nanocatalysts can
derive from their similar catalytic domain to the hydrogenases,
in which the active sites are characterized by the corresponding
pendant bases proximate to the metal centers.46 As shown in
XPS spectra (Fig. 2), the surfaces of CoS or CoP is also
characterized by the positively charged metal center Co (δ+)
with pendant base S or P (δ－). Thus, it is expected that CoS and
CoP share the similar HER catalytic mechanism to the
hydrogenases.23 On the basis of the hydrogenases catalytic
mechanism, the metal center Co (δ+) is the hydrideꢀacceptor
center and the pendant base S or P (δ－ ) is protonꢀacceptor
center, which can promote the HER and bring the high catalytic
activity.24, 26 The intrinsic catalytic activity of hydrogenasesꢀ
like catalysts is highly relevant to the intrinsically charged
nature of the metal center (δ+)—a proportionality relationship
between δ+ value and the catalytic activity.47ꢀ49 As revealed
from XPS spectra (Fig. 2), more positive shift of Co 2p3/2 for
CoP over CoS suggests the higher δ+ of Co metal active sites
for CoP, which can be use to explain the better intrinsic
catalytic activity of the CoP catalysts.
Fig. 4 Nyquist plots for CoS and CoP nanocatalysts.
transfer within the CoP electrode associated with a high HER
catalytic activity.49
Given that the CoS and CoP share the same catalytic
mechanism as hydrogenases, their morphological features also
play an important role in their HER performance. For CoS, the
plateꢀlike morphology is preserved and interconnection
between the plates is also observed. In contrast, the small
crystalline of CoP is presented, which can be attributed to the
evolution of PH3 and high temperature annealing.30,
31
The
small crystalline size can provide more active sites. As
evidenced from the N2 adsorptionꢀdesorption isotherms, the
BET surface area of CoP is 83.1 m2/g, which is larger than that
of CoS (28.1 m2/g). Besides, more edges are exposed, which
are generally considered as the catalytic sites with higher
activity due to their low coordination numbers for HER.
Therefore, with the same sample loading, the CoP with a small
crystalline size associated with more surface active sites and
more additional edge catalytic sites can qualitatively provide
more catalytic active sites for HER. Conclusively, the higher
catalytic activity of CoP can be rationalized as follows: (1)
more positively charged metal of CoP; (2) Longer bond length
of CoꢀP for improved electron delocalization during HER; (3)
Higher electron conductivity of CoP and subsequent better
charge transfer within the CoP electrode; and (4) More surface
active sites and edge catalytic sites with high HER activity for
CoP due to their morphological features.
In addition, the bond length between the metal center and
its pendant base can also function as an indicator to the intrinsic
catalytic activity.50 The increase of the bond length between the
metal active site and the corresponded pendant base favors the
better electron localization on the corresponding pendant base
and reduces the barrier to proton binding as a consequence.50
The bond length of Co−P in CoP (2.3~2.4 Å)51 is longer than
that of Co−S in CoS (2.1~2.2 Å),52 suggesting the high catalytic
activity of CoP.
Conclusions
In summary, CoX (X=S, P) nanocatalysts for HER was
synthesized through via an anion exchange reaction and a high
temperature phosphorizationm, in which thin βꢀCo(OH)2
nanoplates were used as the precursor. The superior catalytic
acitivity of both CoS and CoP were revealed by their small
onset potential of 59 and 32 mV and the low Tafel slope of 56.2
and 54.8 mV/dec for CoS and CoP, respectively. The good
catalytic stability was demonstrated by their stable HER
catalytic current densities for a duration of 18 hours in acidic
media. The results suggest the high catalytic activity of CoP
over CoS, which can be attributed to the intrinsically charged
nature of the metal center Co and the CoꢀX bond length, which
corresponds to the positive relationship between the intrinsic
53
Meanwhile, CoP is a good conductor for charges,32,
favoring the electron transfer of the CoP electrodes. As shown
in electron impedance spectroscopy (Fig. 4), the Nyquist plots
for both CoS and CoP electrodes show a semicircle, an
indication of the conductivity of the electrodes. The calculated
resistance of the CoP electrode (8.37 ꢁ) is much smaller than
that of the CoS electrode (28.78 ꢁ), indicating a better electron
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Acknowledgements
We acknowledge the financial support from a NSFC Grant
21201138 and 21401148. This work was also partially funded
by the Ministry of Science and Technology of China through a
973ꢀprogram under Grant 2012CB619401 and supported by the
Fundamental Research Funds for the Central Universities under
Grant xjj2013102 and xjj2013043. Technical supports for TEM
experiments from Frontier Institute of Science and Technology
and State Key Laboratory for Manufacturing Systems
Engineering, Xi’an Jiaotong University, are also acknowledged.
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