In situ coupling of Co0.85Se and N-doped carbon via one-step selenization of metal-organic frameworks as a trifunctional catalyst for overall water splitting and Zn-air batteries

Article abstract of DOI:10.1039/c7ta01453h

Developing efficient noble metal-free multifunctional electrocatalysts is highly effective to dramatically reduce the overall cost of electrochemical devices. In this work, we demonstrate for the first time a facile strategy for in situ coupling of ultrafine Co_0.85Se nanocrystals and N-doped carbon (Co_0.85Se@NC) by directly selenizing zeolitic imidazolate framework-67 (ZIF-67) polyhedra. Benefiting from the synergistic effect of the coupling between Co_0.85Se and NC, the Co-N-C structure, and the porous conductive carbon network, Co_0.85Se@NC affords excellent oxygen evolution reaction (OER) performance with a small overpotential, remarkable stability, and high faradaic efficiency. Furthermore, Co_0.85Se@NC can also efficiently catalyze hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), and we therefore investigated its applications as a trifunctional electrocatalyst for overall water splitting and Zn-air batteries. When used as both the anode and cathode for overall water splitting, a low cell voltage of 1.76 V is sufficient to reach the current density of 10 mA cm^?2; the obtained Zn-air batteries exhibit a very low discharge-charge voltage gap (0.80 V at 10 mA cm^?2) and long cycle life (up to 180 cycles). These results not only demonstrate a facile strategy for the synthesis of affordable Co_0.85Se@NC but also present its huge potential as a trifunctional electrocatalyst for clean energy systems.

Full text of DOI:10.1039/c7ta01453h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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DOI: 10.1039/C7TA01453H
Journal of Materials Chemistry A
ARTICLE
In Situ Coupling of Co0.85Se and N–Doped Carbon via One–Step
Selenizing of Metal–Organic Frameworks as Trifunctional
Catalysts for Overall Water Splitting and Zn–air Batteries
Tao Meng, Jinwen Qin, Shuguang Wang, Di Zhao, Baoguang Mao, Minhua Cao*
Received 00th January 20xx,
Accepted 00th January 20xx
Developing efficient noble metal–free multifunctional electrocatalysts is highly effective to dramatically reduce the overall
cost of the electrochemical devices. In this work, we for the first time demonstrate a facile strategy for in situ coupling of
ultrafine Co0.85Se nanocrystals and N–doped carbon (Co0.85Se@NC) by directly selenizing zeolitic imidazolate framework–
67 (ZIF–67) polyhedrons. Benefiting from the synergistic effect of the coupling between Co0.85Se and NC, the Co–N–C
structure, and the porous conductive carbon network, Co0.85Se@NC affords excellent oxygen evolution reaction (OER)
performance with small overpotential, remarkable stability, and high Faradaic efficiency. Furthermore, Co0.85Se@NC can
also efficiently catalyze hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), and we therefore
investigated its applications as trifunctional electrocatalysts for overall water splitting and Zn–air batteries. When used as
both the anode and cathode for overall water splitting, a low cell voltage of 1.76 V is required to reach the current density of
10 mA cm-2; the obtained Zn–air batteries exhibit a very low discharge–charge voltage gap (0.80 V at 10 mA cm-2) and long
cycle life (up to 180 cycles). These results not only demonstrate a facile strategy for the synthesis of affordable
Co0.85Se@NC but also present its huge potential as trifunctional electrocatalysts for clean energy systems.
DOI: 10.1039/x0xx00000x
www.rsc.org/
to pristine CoSe2.11,13 Zhang’s group achieved bifunctional overall
water splitting by anchoring CoO domains on CoSe2 nanobelts.14
Cui’s group constructed an efficient and stable HER electrocatalyst
Introduction
Growing global energy consumption and associated environmental
issues have motivated a considerable interest in clean energy
systems, such as, water splitting devices, rechargeable metal–air
batteries, and fuel cells. The electrocatalytic reactions involved in
these energy systems [e.g. hydrogen evolution reaction (HER),
oxygen reduction reaction (ORR), especially core oxygen evolution
reaction (OER)] all require high–performance electrocatalysts to
promote their commercial applications.1-3 Nowadays the state–of–
the–art electrocatalysts for OER, HER, and ORR are still noble
metals, such as Ir, Ru, Pt and so on.4-6 However, the prohibitive costs
and scarcity of these noble metals seriously hinder their large–scale
commercial use. Therefore, much effort has been devoted to
searching for noble metal–free electrocatalysts with performances
comparable to those in the noble metals. Recently, cobalt selenides,
as a type of highly active cobalt (Co)–based electrocatalysts,3,7-16
have attracted considerable attentions due to their low cost,
environmental friendliness, and high thermal and chemical stability.
And a variety of strategies have been developed to improve their
performance. For instance, Xie’s group designed and prepared
vacancy–rich ultrathin CoSe2 nanosheets with enhanced OER
activity.12 Yu’s group reported that CoSe2/Mn3O4 and CoSe2/CeO2
hybrids display significantly improved OER performances compared
by growing CoSe2 nanoparticles (NPs) on carbon fibre paper.15 And,
Co0.85Se/graphene hybrid was also used as a highly efficient catalyst
for ORR.16 In spite of these remarkable electrocatalytic behaviors in
OER or HER or ORR mentioned above, few cobalt selenide
electrocatalysts can function well toward all OER, HER and ORR in
a same electrolyte because of their unstable or inactive property in
unfavorable pH environment.14,17 Therefore, constructing cobalt
selenides with integrated performance for clean energy systems,
such as, overall water splitting and rechargeable zinc–air batteries is
still a challenging work.
Recently, a new family of metal–organic frameworks (MOFs)–
derived materials, including porous carbon and metal–
based/(heteroatom–doped) carbon hybrid nanocomposites have
drawn fast–growing interests, and they have been widely used in
electro–catalysis, electrochemical energy storage, and other
fields.2,3,18-21 The MOFs as both the precursor and template can
afford abundant channels to facilitate electron/ion transport or
provide (heteroatom–doped) carbon species to combine with metal
species by in situ carbonization to further increase the relevant
properties (i.e. active sites, conductivity, and adsorptivity).22-24
Besides, the huge families of MOFs, which can be obtained easily by
changing metal species and organic ligands, also enrich these
precursors and templates. On the other hand, the novel and high–
performance electrocatalysts like MoCx,2 especially those Co-based
groups, such as Co3O4–carbon hybrid,3 Co-C@Co9S8 double-shelled
nanocages,25 and CoxFe1-xP,26 have been successfully obtained by
Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key
Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of
Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing
100081, P. R. China. *E-mail: caomh@bit.edu.cn
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using corresponding MOFs. Nowadays, zeolitic imidazolate In a typical synthesis, 0.63 mmol of Co(NO3)2•6H2O was first
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framework–67 (ZIF–67), a type of common MOFs, has been used dissolved into 50 mL of ethanol under mDagOnI:e1t0ic.10s3ti9r/rCin7TgA.0T14h5e3nH,
as the precursor to prepare some functional materials, especially the 50 mL of ethanol containing 2.48 mmol of 2–methylimidazole
electrocatalysts like Co@Co3O4/NC,27 Co-C@Co9S8,25 CoP,28,29 and was dropwise added into the above solution under continuous
ZIF-Co0.85Se.30 However, these materials suffer from a time- stirring, and the resultant solution was further stirred at room
consuming, multi-step synthetic process, or cannot work well as temperature. After 24 h, the product (ZIF–67) was collected by
trifunctional electrocatalysts in OER, HER, and ORR. Also, cobalt centrifuging, washed with absolute ethanol, and then dried at 80
selenide derived from MOFs as an electrocatalyst has rarely reported ºC for 12 h.
so far.
Preparation of Co0.85Se@NC
The materials with metal–nitrogen–carbon (Me–N–C) structure,
i.e. metal cations being coordinated with nitrogen functional groups
provided by organic ligands or N–doped carbon, have been
demonstrated to be high–performance electrocatalysts because of
their excellent electroactivity and strong durability.5
extensive works have been directed to those electrocatalysts with the
Me–N–C structure for OER, HER, and ORR. For instance, Co–N–C
species originating from N–doped Co9S8/graphene hybrid31 and Co
encapsulated in nitrogen–doped carbon nanotubes36 can boost both
OER and ORR performances; NiO combined with Fe(Co)–N–C5 and
To prepare Co0.85Se@NC, the obtained ZIF–67 and Se powder
with a mass ratio of 1:1 were put in two separate positions of a
porcelain boat. Then the porcelain boat was put into a furnace
with Se powder at the upstream side of the furnace and heated
at 600 ºC for 3 h with a heating speed of 2 ºC min-1 under H2/Ar
atmosphere (H2, 7 vol%). After being cooled to room
temperature, Co0.85Se@NC was obtained. For comparison,
another two Co0.85Se@NC samples were also prepared at 500
ºC and 700 ºC while keeping other reaction conditions constant.
,
31-36
Thus,
Ti–Nx–C32 constructed by the interaction between the carbon nitride Preparation of Co0.85Se
and the titanium carbide nanosheets can enhance the OER activity.
ZIF–67 was first calcined at 500 ºC for 3 h with a heating speed
However, these strategies all suffer from a multi-step, time-
consuming preparation process, or a high-temperature thermal
treatment. Therefore, above cases inspire us to seek for advanced yet
simple method to obtain the catalysts with the Me–N–C structure for
efficiently enhancing the electro–catalysis performance. Thus, a
of 2 ºC min-1 in air, leading to the formation of Co3O4. Then
Co3O4 and Se were reacted with the reaction parameters same
as those for the preparation of Co0.85Se@NC.
Preparation of N–doped carbon (named as NC)
promising fabrication strategy is produced by using MOFs with N– NC was prepared by etching the as-prepared Co0.85Se@NC
rich organic ligand followed by one-step carbonization to enure the with 1.0 M HCl aqueous solution for 24 h to remove Co0.85Se.
resultant electrocatalysts with the Me–N–C (e.g. Co–N–C) structure
Preparation of physically mixed Co0.85Se and NC
and controlled microstructure.
The as–prepared Co0.85Se was physically mixed with NC in a
mass ratio same as that for Co0.85Se@NC by grinding and
ultrasound treatment.
Herein, we, for the first time, demonstrated in situ coupling of
Co0.85Se nanocrystals and N–doped carbon (denoted as
Co0.85Se@NC) via one-step carbonization–selenylation procedure by
using zeolitic imidazolate framework–67 (ZIF–67) as both the
precursor and the template. The resultant Co0.85Se@NC can be used
as trifunctional electrocatalysts for overall water splitting and Zn–air
batteries Importantly, the carbonization of ZIF–67 not only can
provide N–doped carbon combining with Co0.85Se nanocrystals to
afford a synergistic effect, but also can construct the Co–N–C
structure to further enhance the multifunctional electrocatalysis
performances. Besides, the in situ carbonization of ZIF–67 can also
ensure the smaller size of Co0.85Se nanocrystals to increase the
catalytic sites, thus resulting in a high electrocatalytic activity.
Preparation of CoSe2@NC
CoSe2@NC was prepared with a mass ratio of ZIF–67 to Se
powder at 1:2 while keeping other reaction conditions same as
those of Co0.85Se@NC.
Preparation of Co@NC
ZIF–67 was directly heated at 600 ºC for 3 h with a heating
speed of 2 ºC min-1 under H2/Ar atmosphere (H2, 7 vol%).
Materials characterizations
Additionally, the resultant Co0.85Se@NC also inherits the porous Powder X–ray diffraction (XRD) analysis was performed on a
structure of the ZIF–67 template, which can increase the solid/liquid Bruker D8 Advance diffractometer with Cu–Kα radiation (λ ≈ 0.154
contact surface area and promote the efficient transfer of the nm) at 40 kV and 40 mA in a scanning range of 10°–80° (2θ).
electrolyte and the rapid diffusion of gas products. As
a
Raman spectra were collected on an Invia Raman spectrometer with
consequence, Co0.85Se@NC with the Co–N–C structure, the short a excitation laser wavelength of 633 nm. The thermogravimetric
diffusion pathways for electron/ion transport, and the synergistic analysis (TG) was performed on a Rigaku thermogravimetry (TG)
effect between the ultrafine Co0.85Se nanocrystals and NC, exhibits analyser from the room temperature to 800 °C with a heating rate of
excellent activity as trifunctional electrocatalysts for overall water 20 °C min-1 in air. X–ray spectroscopy (XPS) was performed on the
splitting and rechargeable Zn–air batteries.
ESCALAB 250 spectrometer (Perkin–Elmer). The X–ray absorption
near edge structure (XANES) measurements were undertaken at
Beamlines 1W1B at Beijing Synchrotron Radiation Facility (BSRF)
using transmission modes. The morphologies of as–obtained
products were observed on field emission scanning electron
microscopy (FE-SEM, HITACHI S–4800). Transmission electron
Experimental
Preparation of ZIF–67
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microscopy (TEM) and high–resolution TEM (HRTEM) were observed anodic current originates from the OER procVeiseswrAarttihcleerOtnhlianne
DOI: 10.1039/C7TA01453H
performed on a JEOL JEM-2010 microscopes. Energy dispersive other side reactions and meanwhile to further collect the Faradaic
spectroscopy (EDS) element mapping images were taken on efficiency. The ring potential of the RRDE was kept constantly at
scanning transmission electron microscope (STEM) (FEI Technai 0.4 V vs. RHE to reduce the O2 produced from catalyst on the disk
G2 F20). N2 adsorption-desorption isotherms were measured on a electrode in N2–saturated electrolyte. [ See Ma, T. Y.; et. al., J. Am.
Belsorp–max sorption analyzer at liquid nitrogen temperature (77 Chem. Soc. 2014, 136, 13925−13931, and Zhu, Y. P.; et. al., Adv.
K). Before measurement, the Co0.85Se@NC sample was degassed at Funct. Mater. 2015, 25, 7337−7347.] And, the Faradaic efficiency
200 °C for 3 h. Surface area was calculated by the multi-point (ε) was calculated based on the following equation: ε = Ir/(IdN),
Brunauer–Emmett–Teller (BET) method. The pore–size distribution where Id and Ir stand for the disk current and the ring current,
was calculated from the adsorption branch using the Barrett–Joyner– respectively, and N represents the current collection efficiency of
Halenda (BJH) model.
RRDE. Furthermore, IrO2 is a well-established benchmark OER
catalyst with almost 100% Faradaic efficiency because it is free of
probable carbon oxidation during the OER process, and therefore
IrO2 thin-film electrode is selected for the calibration of the
collection efficiency of the RRDE. [see T. Nakagawa, et. al., J. Am.
Chem. Soc. 2009, 131, 15578−15579] Thus, the N value is
determined to be 0.2. To collect the Faradaic efficiency, the disk
electrode current was held at a small constant current of 200 μA,
which is large enough to ensure an appreciable O2 production and
sufficiently small to minimize local saturation and bubble formation
at the disk electrode. [see C. C. L. McCrory, et. al., J. Am. Chem.
Soc. 2013, 135, 16977–16987]
The long–term stability test was performed by i–t curve at a
certain potential for 12 h using a rotating speed of 1500 r.p.m.. To
evaluate the electrochemical double-layer capacitance (ECSA),
cyclic voltammograms (CVs) were tested from 1.15 to 1.19 V (vs.
RHE, in 1.0 M KOH) with scan rates ranging from 20 to 60 mV s-1.
Electrochemical impedance spectroscopy (EIS) was performed at
potential of 1.6 V vs. RHE with frequencies from 500,000 to 1 Hz
with an amplitude of 5 mV. All the data presented were not
corrected for iR losses.
Electrode
preparation
and
electrochemical
characterizations
Preparation of electrocatalyst ink
The working electrodes for electrochemical measurements were
prepared by following processes. 4 mg of the as–prepared sample
was dispersed into 1 mL of solution containing 900 μL of ethanol
and 100 μL of 0.5 wt% Nafion solution and followed by ultrasonic
treatment for 30 min. Then, 20 μL of above slurry was coated onto a
glassy carbon electrode with a diameter of 5 mm and dried naturally
at room temperature.
OER measurements
OER measurements were performed in a three–electrode system
using a rotating disk electrode (PINE Research Instrumentation, at a
rotation speed of 1500 r.p.m.) and the data were collected by a CHI
760E electrochemical workstation (CH instrument, Shaihai). A
Ag/AgCl (KCl, 4 M) electrode and a Pt foil (1*1 cm-2) were used as
the reference electrode and the counter electrode, respectively. All
current densities were normalized to the geometrical surface area
and the measured potential vs. Ag/AgCl was converted to the
potential vs. the reversible hydrogen electrode (RHE) according to
E(RHE) = E(Ag/AgCl) + (0.205 + 0.059 pH)V. Linear sweep
voltammetry (LSV) measurements were taken at a scan rate of 2 mV
s-1 to obtain the polarization curves, and 1.0 M KOH solution was
used as aqueous electrolyte with a continuous flow of O2 during the
test in order to ensure the O2/H2O equilibrium at 1.23 V vs. RHE.
The Tafel slope was calculated according to Tafel equation η =
b•log(j/j0) (η is the overpotential, b is Tafel slope, j is the current
density, and j0 is the exchange current density). Overpotential was
calculated by η = E (vs. RHE) – 1.23, considering O2/H2O
equilibrium at 1.23 V vs. RHE.
HER measurements
For HER measurements, the typical process is similar to that of
OER, in which a flow of N2 instead of a flow of O2 was maintained
over the electrolyte over the entire electrochemical tests to ensure the
H2/H2O equilibrium at 0 V vs. RHE.
Overall water splitting
To evaluate the full water electrolysis, the Co0.85Se@NC catalyst
coated onto nickel foam substrate was used as both anode and
cathode. The electrolyte was purged with N2 gas for 30 min under
ambient condition before the electrochemical tests.
ORR measurements
In order to investigate the electrocatalytic oxygen evolution
For ORR measurements, the typical strategy is similar to that of
OER. The electrochemical experiments were carried out in O2–
saturated 0.1 M KOH electrolyte for the ORR. The potential range is
cyclically scanned between 0.172 and 1.172 V vs. RHE with a scan
rate of 10 mV s-1. The CV and LSV were obtained at ambient
temperature after purging O2 or N2 gas for 30 min. The potential
cycling was repeated until the stable voltammogram curves were
obtained.
RDE measurements were carried out at rotating rates varying from
400 to 2025 r.p.m. at a scan rate of 10 mV s-1. The kinetic
parameters can be obtained on the basis of Koutecky-Levich
equations as follows:
mechanism,
the
rotating
ring–disk
electrode
(RRDE)
voltammograms were conducted on a RRDE configuration (Pine
Research Instrumentation, USA) composed of a disk electrode
(glassy carbon) and
a ring electrode (platinum). 20 μL of
electrocatalyst ink was coated onto the disk electrode, and a scan rate
of 2 mV s-1 was utilized for RRDE experiments with a rotation rate
of 1500 r.p.m.. In order to determine the reaction pathway for OER
by detecting the HO2- formation, the ring potential was held
constantly at 1.50 V vs. RHE to oxidize possible HO2- intermediate
in O2–saturated KOH aqueous solution during the OER process.
Next, a continuous OER (disk electrode) to ORR (ring electrode)
process occurring on the RRDE was used to confirm that the
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DOI: 10.1039/C7TA01453H
Scheme 1 Illustration of the fabrication procedure for Co0.85Se@NC.
1/J = 1/Jk + 1/(Bω1/2
B= 0.2nFCoDo2/3υ-1/6
Jk = nFkCo
)
Co0.85Se@NC. The phase composition of the resultant sample
was first characterized by X–ray diffraction (XRD). As shown
in Fig. 1a, the main diffraction peaks at 33.3o, 44.7o, 50.6o,
60.4o, 61.7o and 69.9o, can be assigned to (101), (102), (110),
(103), (112), and (202) planes of hexagonal Co0.85Se (JCPDS,
No. 52–1008), respectively, indicating that Co0.85Se alloy phase
has been successfully synthesized by the current strategy. That
broad peak in the 2θ range of 20o–30o is related to carbon
species, which is further confirmed by Raman spectroscopy.
From Fig. 1b, we can see that two prominent peaks at around
1341.5 and 1586.9 cm-1 are clearly observed, and they are
assigned to the D and G bands of carbon, respectively. The D
band is related to the structural defects or partially disordered
structures in carbon materials, while the G band is associated
with the degree of graphitization. The carbon species in
Co0.85Se@NC is determined to be around 31 wt% from
thermogravimetric analysis (TG) curve (Fig. 1c). Furthermore,
X–ray photoelectron spectroscopy (XPS) is performed to
further analyze the surface chemical information of
Co0.85Se@NC. The survey XPS spectrum in Fig. S2 confirms
Se, C, N, O, and Co elements existing in Co0.85Se@NC. The
presence of the O element is probably due to the absorbed
oxygen–species or the oxidized parts caused by being exposed
in air. Fig. 1d shows high resolution C 1s XPS spectrum, which
can be deconvoluted into three peaks centered at 284.6, 285.6,
and 288.7 eV, corresponding to C–C, C=N, and O–C=O bonds,
respectively. High resolution Co 2p XPS spectrum (Fig. 1e)
shows the peaks of Co 2P3/2 and Co 2P1/2 with binding energies
at 780.0 and 796.6 eV as well as two satellite peaks at 785.8
and 803.0 eV. Co 2P3/2 peak can be deconvoluted into four
components with binding energies at 777.8, 779.4, 780.3, and
781.6 eV, corresponding to Co0, Co2+, Co–Nx, and Co3+,
respectively.31,37-39 The Co2+ and Co3+ may result from the
surface oxidation under ambient air, consistent with the
reported result.27 Fig. 1f shows high resolution Se 3d XPS
spectrum, in which Se 3d5/2 and 3d3/2 peaks at 53.6 and 54.2 eV
with a spin-orbit splitting of 0.86 eV can be assigned to Se0,
while that wide and weak peak at about 59.7 eV discloses the
presence of SeOx.15,30 Furthermore, high resolution N 1s XPS
spectrum (Fig. 1g) can be deconvoluted into
Where J is the measured current density, Jk is the kinetic current
density, ω is the rotation speed (the constant 0.2 is used when the
rotation speed is expressed in rpm), n is the transferred electron
number, F is the Faraday constant (964 85 C mol-1), Co is the
saturated concentration of O2 in the electrolyte (1.21 * 10-3 mol L-1),
Do is the diffusion coefficient of O2 in the solution (1.9 *10-5 cm s-
1), υ is the kinetic viscosity of the electrolyte (0.01 cm2 s-1), and k is
the electron-transfer rate constant.
Zn–air batteries
The rechargeable Zn–air batteries were tested in home–made
electrochemical cells, and the prepared Co0.85Se@NC was coated on
carbon paper substrate as the air cathode, a polished Zn plate was
applied as the anode, and a 6 M KOH + 0.2 M Zn(Ac)2 aqueous
solution was utilized as the electrolyte. Battery tests were performed
at room temperature on a LAND CT2001A instrument. For the
cycling test, one cycle consists of one discharging step (10 mA cm-2
for 5 min) followed by one charging step of the same current density
and duration time.
Results and discussion
The overall fabrication process of Co0.85Se@NC is
schematically illustrated in Scheme 1 (details see Experiment
Section). First, ZIF–67 was prepared by a facile and scalable
method. Co(NO3)2•6H2O and 2–methyl–imidazole were mixed
in ethanol solvent under magnetic stirring at ambient
temperature and after 24 h, ZIF–67 was formed (Fig. S1a).
Subsequently, ZIF–67 and Se powder with an appreciate mass
ratio were put in two separate positions of a porcelain boat.
Then the porcelain boat was put into a furnace with Se powder
at the upstream side of the furnace and heated at appropriate
temperature under H2/Ar atmosphere. During the heat treatment
process, the organic ligand of 2–methyl–imidazole in ZIF–67
was carbonized into N–doped carbon and simultaneously Co
existing in ZIF–67 was selenized into Co0.85Se surrounded by in
situ formed N–doped carbon, thus leading to the formation of
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DOI: 10.1039/C7TA01453H
100
80
60
40
20
0
(c)
(a)
(b)
G
D
~ 63 wt%
JCPDS, No. 52-1008
10 20 30 40 50 60 70 80
2(degree)
800
1200
1600
2000
0
200
400
600
800
Temperature (oC)
Raman shift (cm-1)
(f)
(e)
(d)
(g)
C 1s
Se 3d
Co 2p
N 1s
pyridinic N
Se 3d3/2
C-C
Co 2p3/2
SeOx
Co2+
Co0
Co3+
Co-Nx
Se 3d5/2
Co 2p1/2
sat.
pyrrolic N
C=N
sat.
graphitic N
oxidized N
O-C=O
Co-Nx
280 282 284 286 288 290 292
Binding energy (eV)
774 780 786 792 798 804 810
Binding energy (eV)
48 51 54 57 60 63 66
Binding energy (eV)
396 398 400 402 404
Binding energy (eV)
Fig. 1 (a) XRD pattern, (b) Raman spectrum and (c) TG curve of Co0.85Se@NC; High–resolution XPS spectra of (d) C 1s, (e) Co 2p, (f) Se 3d, and
(g) N 1s for Co0.85Se@NC.
five components with binding energies at 398.2, 403.7, 400.6,
401.8, and 403.7 eV, which can be assigned to pyridinic N, Co-
Nx, pyrrolic N, graphitic N, and oxidized N, respectively.39,40
And total nitrogen content in Co0.85Se@NC is determined to be
as high as 5.26 wt% by CHN element analysis. These XPS
results confirm the surface chemical states of all elements
involved in Co0.85Se@NC as well as the formation of Co–N–C.
If the original mass ratios of ZIF–67 to Se powder were 1:2 and
1:0, CoSe2@NC and Co@NC were obtained, respectively (Fig.
S3).
existence of the Co–Nx bond in Co0.85Se@NC, which is
constructed by the interaction between Co0.85Se and N–doped
carbon, in well agreement with above XPS results. It has been
well addressed that the Co–Nx bond is beneficial for improving
the electrocatalytic activity of the materials.32
Fig. 3a shows a representative field emission scanning
electron microscopy (FE-SEM) image of Co0.85Se@NC. It can
be clearly seen that the sample completely inherits the
polyhedral morphology of the ZIF–67 precursor (Fig. S1b,
Supporting information). Compared to ZIF–67 polyhedrons
with a fairly smooth surface, the Co0.85Se@NC polyhedrons are
much rougher in surface and have a length of 200 nm. During
the thermal treatment process, a full phase transformation
More information about the charge states and electronic
nature of Co0.85Se@NC was obtained from X–ray adsorption
near–edge structure (XANES) and extended X–ray absorption
fine structure (EXAFS) analysis. As shown in Fig. 2a, the pre–
edge of Co K–edge XANES spectrum of Co0.85Se@NC clearly
shifts towards the higher energy compared to that of pristine
Co0.85Se, indicating the existence of strong interaction between
Co0.85Se and N–doped carbon, which is caused by a negative
charge–transfer from Co to carbon.41,42 Furthermore, Fig. 2b
shows the Fourier transformed k3–weighted EXAFS data for
Co0.85Se@NC and Co0.85Se. Clearly, Co0.85Se@NCexhibits a
relatively strong peak at 1.56 Å, which can be assigned to
typical Co–N bond.41,43 This result further confirms the
occurs from ZIF–67 to Co0.85Se@NC, which is the reason for
the formation of the rougher surface. The microstructure of
Co0.85Se@NC is further investigated by transmission electron
microscopy (TEM). As depicted in Fig. 3b, the polyhedral
structure is also observed, same as above FE-SEM result. A
detailed examination for a single polyhedron (Fig. 3c) reveals
that the Co0.85Se@NC polyhedron is composed of numerous
small nanocrystals. Besides, a highly porous texture can be
clearly observed throughout the whole polyhedron, which may
result from the decomposition of organic species in ZIF–67
during the thermal treatment process. More details on the edge
of Co0.85Se@NC polyhedron can be obtained from Fig. 3d.
Numerous 10 nm–sized Co0.85Se nanocrystals (darker areas
indicated by red circles) are embedded in carbon matrix (as
verified by Raman, TG and XPS), which may play important
roles in prohibiting the further growth of the Co0.85Se
nanocrystals, stabilizing the polyhedral structure, and affecting
the electronic structure of Co. Furthermore, a representative
high–resolution TEM (HRTEM) image (Fig. 3e) clearly shows
that a carbon layer with a thickness of about 1–2 nm is formed
on the surface of a Co0.85Se nanocrystalline, which results from
in situ carbonizing of the organic ligand in ZIF–67. The
Co0.85Se@NC
(a)
Co K edge
1.2
0.8
0.4
0.0
(b)
Co0.85Se
Co-Se
Co0.85Se@NC
Co-N
Co0.85Se
Co foil
7700
7720
7740
7760
7780
0
1
2
3
4
5
6
Energy (eV)
R(Å)
Fig. 2 (a) Co K–edge XANES spectra for Co0.85Se@NC, Co0.85Se, and Co
foil. (b) Fourier transformed k3–weighted EXAFS spectra for Co0.85Se@NC
and Co0.85Se.
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DOI: 10.1039/C7TA01453H
(b)
(c)
(a)
200 nm
200 nm
50 nm
(f)
(d)
(e)
0.34 nm
Se
200 nm
C
Co
Co
N
Carbon
0.26 nm
2 nm
Se
Se
Co
10 nm
2 nm
0
5
10
Energy (KeV)
15
20
80
60
40
20
0
(g)
(i)
BJH model
(h)
Adsorption
Desorption
Co
N
Se
C
0.0 0.2 0.4 0.6 0.8 1.0
Rlative Pressure (P/Po)
0
10 20
Pore size (nm)
30
Fig. 3 (a) FE-SEM image, (b–d) TEM images, (e) HRTEM image and SAED pattern of Co0.85Se@NC. (f) EDS spectrum and scanning
transmission electron microscopy (STEM) image, and (g) corresponding element mappings of Co0.85Se@NC. (h) N2 adsorption–desorption
isotherms of Co0.85Se@NC and (i) corresponding pore size distribution curve from the BJH model.
resolved interplanar distances of the lattice fringes are 0.26 and with a mass loading of 0.4 mg cm-2 on a glassy carbon (GC)
0.34 nm, corresponding to the (101) planes of Co0.85Se16 and disk electrode (Fig. S6a), while the catalysts prepared with
the (001) planes of graphite, respectively. The selected–area lower or higher temperatures show slightly inferior OER
electron diffraction (SAED) pattern also confirms the performance (Fig. S6b), and therefore in the following discuss,
polycrystalline nature of the Co0.85Se nanocrystals. Additionally, we compare this optimal result with those of other samples.
in agreement with the results from XPS measurements, the First, as shown in Fig. S3b, the OER activity of Co0.85Se@NC
energy–dispersive spectroscopy (EDS) spectrum also confirms is obviously better than those of Co@NC and CoSe2@NC. As
that the Co0.85Se@NC samples is composed of Co, Se, C, and N we know, the Co-Se bond can lead to the Co in Co0.85Se@NC
elements (Fig. 3f), and that these elements are homogeneously with a high valence state,30,44 and this is able to boost the OER
distributed (Fig. 3g). Moreover, the N2 adsorption isotherms adsorption process, which can be further supported by its Tafel
recorded on Co0.85Se@NC belong to type IV with a hysteresis slop value (Fig. S3c) (75 vs. 101 mV dec-1 for Co0.85Se@NC vs.
loop of type H4 (Fig. 3h), which is usually observed for Co@NC). In addition, Co0.85Se@NC has a higher Co/Se atomic
mesoporous materials with slit-like pores, in good agreement ratio than CoSe2@NC, and this can enhance the adsorption
with above TEM observations. Correspondingly, the sizes of process in OER reaction as Co0.85Se@NC has a small Tafel
the mesopores are mainly in the range of 2–20 nm from the slop value (Fig. S3c), which is determined by its inherent phase
pore size distribution curve (Fig. 3i). Also, Co0.85Se@NC structure, commonly observed in other works.44,45 Moreover,
displays a high specific Brunauer–Emmett–Teller (BET) the NC and ZIF–67 show negligible current densities below the
surface area of 55 m2 g-1. The rich porous channel and high potential of 1.60 V vs. reversible hydrogen electrode (RHE),
surface area of Co0.85Se@NC may result from the porous while the pristine Co0.85Se presents the OER response with a
structure of the ZIF–67 template, and at the same time they are onset potential at 1.59 V (Fig. 4a), indicating that the catalytic
beneficial for improving its electrocatalytic performance.
sites come mainly from Co0.85Se. In contrast, the Co0.85Se@NC
The electrocatalytic OER activity of Co0.85Se@NC was first exhibits significantly enhanced OER catalytic activity.
evaluated by the polarization linear sweep voltammograms Specifically, the Co0.85Se@NC reveals a sharp onset of OER
(LSVs) recorded in O2–saturated 1.0 M KOH aqueous solution current at approximately 1.49 V, indicating that the synergistic
with a scan rate of 2 mV s-1 using a standard three–electrode effect between Co0.85Se and NC is responsible for the excellent
configuration (see Experimental section for details). For OER performance. Noticeably, although the IrO2/C affords a
comparison, CoSe2@NC, Co@NC, Co0.85Se (Fig. S4), NC (Fig. onset potential of approximately 1.47 V, slightly smaller than
S5), ZIF–67, Pt/C, and IrO2/C were also tested at the same that of Co0.85Se@NC, its OER current densities drop obviously
conditions. The Co0.85Se@NC exhibits the optimal performance
6 | J. Name., 2016, 00, 1-3
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View Article Online
0.8
DOI: 10.1039/C7TA01453H
Co0.85Se@NC
ZIF-67
Pt/C
Co0.85Se@NC
Co0.85Se
(b)
(a)
60
40
20
0
0.7
0.6
0.5
0.4
0.3
0.2
Co0.85Se
IrO2/C
NC
NC
ZIF-67
Pt/C
-1
-1
dec
mV
mV
dec
4
9
mV
7
9
0
-1
-1
IrO2/C
dec
8
10 mA cm-2
-1
mV
9
dec
mV
7
dec
-1
5
7
dec
mV
3
7
1.2
1.4
1.6
1.8
0.0
0.4
0.8
1.2
1.6
Log[J (mA cm-2)]
Potential (V vs. RHE)
10
8
10
0
(c)
(d)
Idisk = 0
-10
-20
-30
-40
-50
6
4e_
4OH_
4
2H O
+
2
O
+
2
O2-saturated 1.0 M KOH
2
I
disk = 200 μA
0
1.2
1.4
1.6
1.8
0
10 20 30 0 10 20 30 40 50 60
Time (s)
Potential (V vs. RHE)
120
80
40
0
(f)
1st cycle
1000th cycle
1.68
1.66
1.64
1.62
1.60
1.58
1.56
1.54
Co0.85Se@NC
Co0.85Se
0
2000 4000
Time (s)
1.2
1.4
1.6
1.8
Potential (V vs. RHE)
Fig. 4 (a) Polarization curves and (b) Tafel plots of Co0.85Se@NC, Co0.85Se, NC, ZIF–67, Pt/C, and IrO2/C in an O2–saturated 1.0 M KOH solution
(scan rate of 2 mV s−1). (c) The ring current of Co0.85Se@NC on a RRDE (1500 r.p.m.) in O2–saturated 1.0 M KOH solution (ring potential:1.50 V
vs. RHE). (d) The ring current of Co0.85Se@NC on a RRDE (1500 r.p.m.) in N2–saturated 1.0 M KOH solution (ring potential: 0.40 V vs. RHE). (e)
Chronoamperometric response at a constant potential (the potential at 10.0 mA cm-2), and the inset in (e) shows the chronopotentiometric response
of Co0.85Se@NC and Co0.85Se at 10.0 mA cm-2. (f) Polarization curves of Co0.85Se@NC before and after 1000 cycles.
below those of Co0.85Se@NC at high bases, indicating the in Fig. 4b. The Tafel slopes are found to be 75, 88, 90, 97, 94,
excellent OER catalytic activity of the Co0.85Se@NC. Moreover, and 73 mV dec-1 for Co0.85Se@NC, Co0.85Se, NC, ZIF–67, Pt/C,
we further compared the operating potentials required for all and IrO2/C, respectively. Clearly, the Tafel slope of
the catalysts to deliver the 10.0 mA cm-2 current density, the Co0.85Se@NC is close to that of the IrO2/C, but it is lower than
approximate current density expected for a 10% efficient solar those of the control samples and some of the previously
water-splitting device under 1 sun illumination.46,47 The reported OER catalysts (Table S1). This result suggests that the
operating potential of Co0.85Se@NC at the current density of 10 Co0.85Se@NC has
a
favorable OER reaction kinetics
mA cm-2 is 1.55 V, comparable to that of the state–of–the–art corresponding to its high catalytic activity.
IrO2/C catalyst (1.55 V), but much lower than those of Co0.85Se
To obtain further insights into the reaction mechanism, we
(1.65 V), NC (1.72 V), ZIF–67 (1.76 V), Pt/C (1.74 V), used the rotating ring–disk electrode (RRDE) technique with a
CoSe2@NC (1.64 V), Co@NC (1.69 V), and many other Pt ring electrode potential of 1.50 V to oxidize the peroxide
reported catalysts, for example, IrO2/C (1.60 V, 0.1 M KOH),48 intermediates formed on the Co0.85Se@NC surface during the
Pt/C (1.83 V, 0.1 M KOH),49 CoSe2 ultrathin nanosheets (1.55 OER process. As shown in Fig. 4c, it can be clearly seen that a
V, 0.1 M KOH),12 CoSe2/N–graphene (1.596 V, 0.1 M KOH),50 fairly low ring current of 6 μA is detected and this value is 3
Mn3O4/CoSe2 hybrids (1.68 V, 0.1 M KOH),11 CoS2/N,S-GO orders of magnitude lower than that of the disk current (mA
(1.61 V, 0.1 M KOH),51 and TCCN (1.65 V, 0.1 M KOH ).32
A
scale), which is so small that it can be negligible. So it can be
detailed comparison of various OER catalysts reported recently reasonably concluded that Co0.85Se@NC favors a desirable
is presented in Table S1, further confirming the outstanding four–electron pathway for water oxidation (4OH−→O2 + 2H2O
catalytic activity of the Co0.85Se@NC. The OER kinetics of the + 4e−). Moreover, in order to prove that the observed anodic
above catalysts were further examined by Tafel plots, as shown current results from the OER process rather than other side
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reactions and also to further collect the Faradaic efficiency, a these founds indicate the superior stability and excellent OER
View Article Online
DOI: 10.1039/C7TA01453H
RRDE with a ring electrode potential of 0.40 V (a value to catalytic activity of Co0.85Se@NC. Furthermore, from the
ensure that the ORR can be taken place) in N2-saturated chronopotentiometric curve tested at the current density of 10.0
medium was employed to reduce the generated O2 from the mA cm-2 (the inset in Fig. 4e), we can see that Co0.85Se@NC
OER process (Fig. 4d), thus guaranteeing a continuous OER holds a nearly constant operating potential at 1.55 V with a
(disk electrode) to ORR (ring electrode) process (the inset of trend of continuous potential decrease corresponding to its
Fig. 3d).3,44 When the disk current was fixed at 200 μA, a current increase in i–t curve in the first 3 h, which is owning to
constant ring current of about 39 μA (collection efficiency 0.2) the spread of the electrolyte, while the potential of Co0.85Se
can be detected, indicating that the anodic current catalyzed by increases about 13 mV from 1.650 to 1.663 V as a striking
Co0.85Se@NC is attributed to the OER process with a high contrast within 5000 s, again revealing the superior durability
Faradaic efficiency of 97.5%.10,51
of Co0.85Se@NC. Additionally, accelerated degradation studies
Another critical criterion for evaluating the OER are carried out by taking continuous cyclic voltammograms
performance of the electrocatalysts is the long–term durability, (CVs) at an accelerated sweep rate of 50 mV s-1 for 1000 cycles
which is significant for energy conversion and storage systems. to probe the stability of the catalyst, only 2.6% anodic current
As shown in Fig. 4e, the chronoamperometric response curve loss (4 mV increased) is observed for Co0.85Se@NC, further
for Co0.85Se@NC at a low potential of 1.55 V exhibits an confirming its highly stable performance (Fig. 4f).
anodic current attenuation as small as 6.6% within 12 h,
The outstanding OER performance of Co0.85Se@NC
demonstrating a high stability. This insignificant activity originates from the synergistic effect between ultrafine Co0.85Se
decrease of Co0.85Se@NC may be caused by a small amount of nanocrystals and highly conductive N–doped carbon, the Co–
the Co0.85Se@NC catalyst peeling off the electrode during the N–C structure constructed by the interaction between the high
evolution of a large amount of O2 gas for the long process.45 In content N and Co0.85Se species, and unique porous architecture.
contrast, Co0.85Se displays not only 3.1 times larger current Firstly, the synergistic effect between the highly conductive N–
attenuation (20.7%) than Co0.85Se@NC, but also 100 mV larger doped carbon and ultrafine Co0.85Se plays an important role in
potential to afford the same current density response within 10 enhancing the OER performance of Co0.85Se@NC. As
h (Fig. S7), indicating the synergistic effect between Co0.85Se discussed above, the pre–edge of Co K–edge XANES spectrum
and NC. The TEM image of the Co0.85Se@NC electrocatalyst of Co0.85Se@NC (Fig. 2a) has shifted towards the higher energy
after a 12 h reaction shows a good polyhedron structure, almost compared to Co0.85Se due to negative charge transfer from Co
completely same as that of the fresh Co0.85Se@NC (Fig. S8a). to carbon,41,42 indicating strongly coupling effect between
Besides, the HRTEM image gives the interplanar distance of Co0.85Se nanocrystals and N–doped carbon species. And, the in
the lattice fringes to be 0.26 nm, which corresponds to the (101) situ incorporated N–doped carbon causes the impaired electron
planes for Co0.85Se (Fig. S8b), suggesting the fact that the density of Co atoms, which can make the electro–catalytically
Co0.85Se phase does not change during the OER process. All of active sites of Co more electrophilic,42,53
-200
Co0.85Se@NC
Co0.85Se
80
60
40
20
0
(b)
(a)
Co0.85Se@NC
Mixed Co0.85Se&NC
Co0.85Se
-100
0
0.24
0.16
0.08
1.2 1.3 1.4 1.5 1.6 1.7 1.8
0
100
Z' (ohm)
200
Potential (V vs. RHE)
100
Co0.85Se@NC
Co0.85Se
(c)
(d)
-2
50
120
cm
F
0
1.2
1.5
1.8
.2 m
3
=
Potential (V vs. RHE)
-2
C
C dl
N
80
40
cm
mF
15 mV s-1
2 mV s-1
Se@
85
=2.4
Se
85
C dl
0.
Co
0.
Co
1.6
1.7
1.8
1.9
20
40
60
Scan rates (mV s-1)
Potential (V vs. RHE)
Fig. 5 (a) Polarization curves of Co0.85Se@NC, physically mixed Co0.85Se with NC, and Co0.85Se. (b) EIS of Co0.85Se@NC and Co0.85Se tested at
1.60 V vs. RHE. (c) Polarization curves of Co0.85Se@NC and Co0.85Se at different scan rates. (d) Plots of the current density at 1.17 V (V vs. RHE)
vs. the scan rate of Co0.85Se@NC and Co0.85Se.
8 | J. Name., 2016, 00, 1-3
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Journal of Materials Chemistry A
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Journal Name ARTICLE
electro–catalytically active sites of Co more electrophilic,42,53 (Fig. 5b), revealing its smaller contact impedance and faster
View Article Online
thus facilitating the adsorption and reaction of OH− groups with charge transport along with favorable rDeOacI:t1io0n.103k9in/Ce7tTicAs0.14T53hHe
Co0.85Se@NC,54 resulting in outstanding OER performance. higher OER electrocatalytic activity and smaller charge transfer
Besides, the in situ uniformly anchoring of Co0.85Se on the N– impedance of Co0.85Se@NC both benefit from the strong
doped carbon, assured by the use of the ZIF–67 as the precursor, coupling between Co0.85Se and N–doped carbon. Additionally,
can greatly improve the conductivity and charge transfer N–doped carbon matrix might also play an important role in
capability of the Co0.85Se catalyst. The strong coupling effect prohibiting the growth of Co0.85Se nanocrystals, thus increasing
between them can take full advantage of the superior electric the catalytic sites and stabilizing the polyhedral structure of
conductivity of the N–doped carbon backbone, which can Co0.85Se@NC to contribute microstructure effect such as the
facilitate the charge transfer in the hybrid catalyst, thus porous structure.
resulting in an enhanced OER activity and stability. For
Secondly, the N content of Co0.85Se@NC can be achieved as
instance, compared to the physically mixed Co0.85Se and NC, high as 5.26 wt% determined by CHN elemental analysis. Such
and carbon–free Co0.85Se, Co0.85Se@NC has a smaller onset a high N content not only can directly act as OER active sites
potential with continually enhanced current density, suggesting but also can incorporate with metals (e.g. Co species) forming
its much higher OER activity (Fig. 5a). Moreover, Co–N–C structure (as proofed by XPS and XANES) to boost
Co0.85Se@NC has a smaller semicircular diameter in the the OER performance. Recently, Qiao32 reported that Ti–Nx
electrochemical impedance spectroscopy (EIS) than Co0.85Se motifs constructed by interacting between carbon nitride and
1.0
ZIF-67
Pt/C
Co0.85Se@NC
Co0.85Se
0
-10
-20
-30
-40
-50
(a)
(b)
0.8
0.6
0.4
0.2
0.0
Co0.85Se@NC
NC
-1
Co0.85Se
-1
dec
mV
5
2
2
dec
-1
mV
5
3
1
NC
ZIF-67
Pt/C
dec
mV
7
9
1
dec-1
mV
mV
5
2
1
dec-1
4
0
1
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
0.4
0.6
0.8
1.0
Log [J (mA cm-2)]
Potential (V vs. RHE)
0
(d)
-15
-10
-20
-30
-40
-50
Co0.85Se@NC
Co0.85Se
0
-10
-20
-10
-5
-30
Original
After 10 h
-40
0
0
5
10
15
-50
Z' (ohm)
20 mV s-1
-0.2
-0.5 -0.4 -0.3 -0.2 -0.1 0.0
Potential (V vs. HRE)
2
-0.8
-0.6
-0.4
0.0
Potential (V vs. RHE)
50
(e)
C
N
40
Se@
30
20
10
0
85
0.
Co
10 mA cm-2
m
foa
i
N
C
t/
P
1.2
1.4
1.6
Voltage (V)
1.8
2.0
Fig. 6 (a) Polarization curves and (b) Tafel plots of Co0.85Se@NC, Co0.85Se, NC, ZIF–67, and Pt/C for HER (N2–saturated 1.0 M KOH solution
with scan rate of 2 mV s-1). (c) Chronoamperometric response at a constant potential of –0.23 V vs. RHE, and inset in (c) shows polarization curves
at 2 mV s-1 before and after 10 h chronoamperometric tests. (d) Polarization curves of Co0.85Se@NC tested at scan rates ranging from 2 to 20 mV s-
1, and inset of (d) shows the EIS tested at potential of –0.25 V vs. RHE for Co0.85Se@NC and Co0.85Se. (e) Polarization curves of a two-electrode
alkaline electrolyzer using Co0.85Se@NC//Co0.85Se@NC, Pt/C//Pt/C and Ni foam//Ni foam as both cathode and anode at a scan rate of 2 mV s-1 in
1.0 M N2–saturated KOH. (f) Galvanostatic water electrolysis at a series current densities of 5, 10, and 20 mA cm-2 over the course of 35 h for
Co0.85Se@NC//Co0.85Se@NC.
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titanium carbide nanosheets can act as OER electroactive sites. which show the similar robust durability, indicating the super
View Article Online
Dai31 reported that N–doping into both Co9S8 and graphene stability of Co0.85Se@NC. We also studied the relationship
DOI: 10.1039/C7TA01453H
could efficiently enhance electrocatalytic performance for ORR between the HER current density and the scan rate. As shown
and OER by tuning the electronic properties of Co9S8 and in Fig. 6d, the HER current density for Co0.85Se@NC is not
graphene. And others also assumed that the metal–N–C susceptible to the scan rate increasing from 2 to 20 mV s-1. So,
structure can boost oxygen electrochemistry performance. the unique porous architecture of Co0.85Se@NC may contribute
Based on these facts, we conjecture that the Co–N–C structure to the enhanced HER performance. Besides, the synergistic
in our case also contributes to increasing the OER performance. effect between the highly conductive N–doped carbon and
Thirdly, the unique porous architecture of Co0.85Se@NC, on ultrafine Co0.85Se also contributes the enhanced HER activity
the one hand, can provide abundant and short diffusion by decreasing the charge transfer impedance (as evidenced by
pathways for electron/ion transport during the electrochemical EIS test in the inset of Fig. 6d). Encouraged by the excellent
reactions, and on the other hand, can afford a large active OER and HER performances, we accordingly assembled a
surface area for increasing catalytic sites. Noticeable, for the water electrolyzer with a two–electrode configuration in 1.0 M
OER polarization curves of Co0.85Se@NC, the current density KOH, in which the as-synthesized Co0.85Se@NC was coated on
is not susceptible to the scan rate increasing from 2 to 15 mV s- Ni foam as both the anode and the cathode (the inset in Fig. 6e).
1, while the Co0.85Se acts as a striking contrast (Fig. 5c). This As shown in Fig. 6e, a catalytic current is detected when the
result indicates that the high porosity of Co0.85Se@NC favors applied potential is larger than 1.42 V, and the current density
the efficient transfer of the electrolyte and the rapid diffusion of of 10 mA cm-2 can be gained at a cell voltage of 1.76 V,
the O2 product. Furthermore, the mesoporous Co0.85Se@NC representing a combined overpotential of 530 mV for full water
polyhedrons afford a large active surface area, which has been splitting. And, the overall water splitting performance of
confirmed by the electrochemical double-layer capacitance (Cdl) Co0.85Se@NC also outperforms those of the commercial Pt/C
and the calculated slope from the linear relationship of the and the Ni foam substrate. In addition, the stability test
current density against the scan rate (Fig. S9). Co0.85Se@NC indicates that Co0.85Se@NC possesses a high stability with
holds a larger Cdl than Co0.85Se (3.2 vs. 2.4 mF cm-2), indicating good rate performance as the current density increases from 5
an enlarged catalytic solid–liquid contact surface area for to 10, and 20 mA cm-2 during 35 h electrocatalysis process (Fig.
Co0.85Se@NC (Fig. 5d). Thus, the rich channel and large solid– 6f). The Co0.85Se@NC gives low overpotentials and superior
liquid contact surface area of Co0.85Se@NC provided by its stability, which demonstrates its promising practical application
unique porous architecture can obviously increase its OER for overall water splitting.
electrocatalytic performance.
Interestingly, apart from the excellent overall water splitting
The opposite reaction initiated on the oxygen evolution performance, Co0.85Se@NC also performs a remarkable ORR
electrode that can availably catalyze the corresponding cathode activity, making it a promising application for metal–air
hydrogen evolution process as well, is an import figure of merit batteries. Specifically, in cyclic voltammograms (CVs, Fig. 7a),
to pursue bifunctional water electrolysis electrode and simplify one characteristic ORR peak centered at 0.817 V is observed in
the installation. Hence, the catalytic capacity of Co0.85Se@NC 0.1 M O2–saturated KOH solution, showing the electrochemical
for HER is assessed in the identical electrolyte to OER (1.0 M reduction of oxygen initiated on Co0.85Se@NC, while, in sharp
KOH). As shown in Fig. 6a, Co0.85Se@NC holds a small onset comparison, no significant peak is obtained in N2–saturated
potential of around -140 mV at the current density of 2 mA cm- solution. And, this potential (0.817 V) is close to that of the
2
(Fig. S10), beyond which the cathodic current increases commercial Pt/C catalyst, but far more positive in comparison
rapidly at higher biases, indicating outstanding electrocatalytic to those for Co0.85Se (0.617 V), NC (0.73 V), ZIF–67 (0.732 V)
activity. The overpotential at the current density of 10 mA cm-2 (Fig. S11), and previously reported Co–based ORR catalysts
for Co0.85Se@NC is 230 mV, which is far smaller than those of (e.g. –0.3 V vs. Ag/AgCl for Co0.85Se/graphene hybrid
Co0.85Se (424 mV), NC (442 mV), ZIF–67 (599 mV), and some nanosheets,16 0.704 V vs. RHE for N–Co9S8/G,31 and –0.21 V
other reported non-noble metal HER electrocatalysts. vs. SCE for Co3O4/ON–CNW55), indicating an easier ORR
Co0.85Se@NC also possesses a favorable HER reaction kinetics process on Co0.85Se@NC. Accordingly, Co0.85Se@NC displays
with a small Tafel value of 125 mV dec-1, which is comparable a sharp onset potential at 0.912 V in the polarization LSVs
to that of commercial Pt/C (104 mV dec-1), but much smaller curves (Fig. 7b), and its limiting current density increases with
than those of Co0.85Se (197 mV dec-1), NC (224 mV dec-1), and increasing the rotation speed (e.g. from 400 to 2025 r.p.m.)
ZIF–67 (135 mV dec-1) (Fig. 6b). This result suggests that because of the increased diffusion rate at high speeds.56 The
Co0.85Se@NC has
a
favorable HER reaction kinetics corresponding Koutechy–Levich (K–L) plots of Co0.85Se@NC
corresponding to its high catalytic activity. As expected, are obtained from LSVs with potentials ranging from 0.272 to
Co0.85Se@NC performs excellent stability (tested by 0.572 V vs. RHE (Fig. 7c), and all plots show fairly good
chronoamperometric model) in basic solution within 10 h only linearity, suggesting first–order reaction kinetics toward
with a minor current drop at the beginning of the HER dissolved oxygen and similar electron–transfer number (n) for
measurements, which may be due to the loss of the the ORR process at different potentials. The n is calculated to
electrocatalyst from the electrode on rapid rotation (Fig. 6c). be in the range of 3.98 to 4.03 at the potentials ranging from
Furthermore, the polarization curves before and after operating 0.272 to 0.572 V (Fig. 7d), indicating that the Co0.85Se@NC
for 10 h in basic medium were also tested (the inset in Fig. 6c), electrode favors a four-electron oxygen reduction process. Thus,
10 | J. Name., 2016, 00, 1-3
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Journal Name ARTICLE
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DOI: 10.1039/C7TA01453H
4
2
0
(a)
(b)
O2-saturated
N2-saturated
-1
-2
-3
-4
400
600
900
1225
1600
2025
0
-2
-4
0.817 V
0.2 0.4 0.6 0.8 1.0 1.2
0.2
0.4
0.6
0.8
1.0
Potential (V vs. RHE)
Potential (V vs. RHE)
0.5
0.4
0.3
0.2
5
4
3
2
1
0
(d)
(c)
4.03
4.02
4.02
3.94
4.02
3.98
3.98
0.572 V
0.522 V
0.472 V
0.422 V
0.372 V
0.322 V
0.272 V
0.02 0.03 0.04 0.05 0.06
0.3
0.4
0.5
0.6
-1/2(rpm-1/2
)
Potential (V vs. RHE)
0
-2
-4
-6
(e)
ZIF-67
Pt/C
Co0.85Se@NC
Co0.85Se
NC
0.2
0.4
0.6
0.8
1.0
Potential (V vs. RHE)
Fig. 7 (a) CVs of the as–synthesized Co0.85Se@NC in an O2–saturated (black line) or N2–saturated (blue dot line) 0.1 M KOH solution (scan rate:
10 mV s-1). (b) Rotating–disk electrode polarization curves (LSVs) of Co0.85Se@NC in O2–saturated 0.1 M KOH at rotating rates ranging from 400
to 2025 r.p.m with a scan rate of 10 mV s-1. (c) K–L plots of Co0.85Se@NC obtained from the RDE data at 0.572, 0.522, 0.472, 0.422, 0.372, 0.322
and 0.272 V vs. RHE, and (d) the corresponded electron–transfer number. (f) Rotating-disk electrode LSVs of Co0.85Se@NC, Co0.85Se, NC, ZIF–67,
and Pt/C in O2–saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600 r.p.m. (f) The chronoamperometry curves of Co0.85Se@NC and
commercial Pt/C catalysts obtained at 0.672 V in O2–saturated 0.1 M KOH.
Co0.85Se@NC favors a desirable four-electron pathway for Co0.85Se@NC may be due to the Co–N–C structure, the porous
reversible OER and ORR process (4OH−↔O2 + 2H2O + 4e−), structure and the synergistic effect between the highly
which is of significant importance, especially for rechargeable conductive N–doped carbon and the ultrafine Co0.85Se
metal–air batteries and regenerated fuel cells involving these nanocrystals. These excellent ORR features of Co0.85Se@NC,
two reactions.
especially together with its outstanding OER performance, are
Moreover, we also evaluated the ORR performance of particularly desirable for the clean energy systems, such as,
Co0.85Se@NC, Co0.85Se, NC, ZIF–67, and commercial Pt/C rechargeable metal-air batteries and alkaline fuel cells.
catalyst using RDE technology at the rotating speed of 1600
Next, proof–to–concept experiments were conducted to
r.p.m. in 0.1 M O2–saturated KOH (Fig. 7e). Remarkably, demonstrate the potential application of Co0.85Se@NC in
Co0.85Se@NC displays a significant ORR performance, which rechargeable Zn–air batteries since OER and ORR are two
is not only better than Co0.85Se, NC, and ZIF–67 in both the fundamental steps in this application.
A
home–made
onset potential and the reaction current density, but also rechargeable Zn–air battery device was constructed by pairing
comparable to the commercial Pt/C, revealing the high ORR the zinc plate (the anode), the Co0.85Se@NC loaded on carbon
activity of Co0.85Se@NC. As expected, Co0.85Se@NC also fiber paper (the cathode), and O2–saturated KOH/Zn(Ac)2
shows
a
strong durability as revealed by the electrolyte (Fig. 8a). The battery has an open circuit voltage of
chronoamperometric response (Fig. 7f), retaining 85.2% of the ~1.4 V and a small internal resistance (Fig. S12). Clearly,
initial current even after 10 h, while the Pt/C catalyst loses 28% Co0.85Se@NC shows a current density of 186 mA cm-2 at a
of its initial current. The high-activity and good durability of
voltage of 1.0 V, which is higher than that of the commercial
Pt/C (162 mA cm-2) and other reported values (Table S2). The
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ARTICLE
Journal Name
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DOI: 10.1039/C7TA01453H
(a)
Co0.85Se@NC
(b)
1.6
400
300
200
100
0
Co0.85Se
Pt/C
1.2
0.8
0.4
0.0
0
200 400 600 800 1000
J (mA cm-2)
2.4
(c)
2.0
1.6
1.2
0.8
Co0.85Se@NC
Co0.85Se
Pt/C
0
10 20 30 40 50 60
J (mA cm-2)
Fig. 8 Electrochemical testing of Co0.85Se@NC as the cathode in a home–made Zn–air battery: (a) A schematic of the rechargeable Zn–air battery.
(b) A polarization curve (V~i) and corresponding power density plot of the battery using Co0.85Se@NC as the cathode catalyst compared with the
primary Zn–air battery using Co0.85Se and commercial Pt/C catalyst. (c) Charge and discharge polarization (V~i) curves of the rechargeable Zn–air
battery using Co0.85Se@NC compared with the one using Co0.85Se and Pt/C (red). (d) Cycling performance of the rechargeable Zn–air battery using
Co0.85Se@NC at 10 mA cm-2 and a 10-min cycle period compared with the one using Co0.85Se and commercial Pt/C.
peak power density for Co0.85Se@NC (Fig. 8b) is as high as feasible in practical metal–air batteries with enhanced cyclic
268 mW cm-2 at 0.64 V, which is also superior to those of stability.
Co0.85Se (139 mW cm-2), Pt/C (173 mW cm-2) and some
previously reported materials on Zn–air primary batteries
Conclusion
(Table S3). Fig. 8c shows the charge and discharge polarization
In summary, we have achieved in situ coupling of Co0.85Se
curves of the rechargeable Zn–air battery using the
nanocrystals and N–doped carbon via one–step selenizing of
galvanodynamic method with Co0.85Se@NC coated on carbon
metal–organic frameworks. The resultant Co0.85Se@NC was
fiber paper as both the ORR and OER electrodes. It can be
used as trifunctional electrocatalysts for overall water splitting
clearly observed that Co0.85Se@NC, Co0.85Se and commercial
and Zn–air batteries for the first time. When used as the
Pt/C all show excellent rate capability during the charging and
electrodes for overall water splitting in alkaline solution,
discharging process. Specifically, Co0.85Se@NC shows a
Co0.85Se@NC shows a low cell voltage of 1.76 V at the current
discharging potential superior to Co0.85Se, and comparable to
density of 10 mA cm-2 and good stability. At the same time the
the commercial Pt/C during the ORR process, while for the
obtained Zn–air batteries based on Co0.85Se@NC exhibit a very
charging process, Co0.85Se@NC obviously outperforms the
low discharge–charge voltage gap (0.80 V at 10 mA cm-2) and
Pt/C catalyst. And the charge–discharge voltage gap at 20 mA
long cycle life (up to 180 cycles). Its excellent electrocatalytic
cm-2 for Co0.85Se@NC is only 0.88 V, which is much smaller
performance can be attributed to the synergistic effect of the
than those of Co0.85Se (1.13 V) and Pt/C (0.96 V), and some
coupling between Co0.85Se and NC, the Co–N–C structure, and
results reported previously (Table S3). Importantly, when
the porous conductive carbon network. Our work establishes a
repeatedly charged or discharged at 10 mA cm-2 with a 10–min
facile strategy for the preparation of Co0.85Se@NC hybrid as
per cycle period (including discharging for 5 min, followed by
efficient and stable trifunctional electrocatalysts for overall
charging the same time) (Fig. 8d), Co0.85Se@NC as the air
water splitting and Zn–air batteries and this strategy can be
cathode affords a discharge voltage of 1.27 V and a charge
extended to fabricate other nanomaterials for energy fields.
voltage of 2.07 V with a small voltage gap of 0.80 V and a high
trip efficiency of 61.35% in the first cycle. At the same time, it
also shows a high cycling stability with no obvious potential
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21601014, 21471016 and 21271023) and
the 111 Project (B07012). The authors would like to thank the
change for over 180 charge–discharge cycles within 30 h,
whereas an apparent increase in both the charge and discharge
voltages is observed after 120 cycles in 20 h for both Co0.85Se
and Pt/C. Therefore, the bifunctional Co0.85Se@NC electrode is
12 | J. Name., 2016, 00, 1-3
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Journal of Materials Chemistry A
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA01453H
The table of contents entry
In Situ Coupling of Co0.85Se and N–Doped
Carbon via One–Step Selenizing of Metal–
Organic Frameworks as Trifunctional catalysts
for Overall Water Splitting and Zn–air Batteries
Tao Meng, Jinwen Qin, Shuguang Wang, Di Zhao, Baoguang Mao,
Minhua Cao*
The Co0.85Se@NC obtained by directly selenizing ZIF–67
can be used as trifunctional catalysts for water splitting and
Zn–air batteries.
100 nm