Cobalt phosphide supported by two-dimensional molybdenum carbide (MXene) for the hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting

Article abstract of DOI:10.1039/d1ta05648d

Developing a low cost, high performance, and durable bifunctional catalyst to boost the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting is a critical yet challenging task. Transition metal phosphides have been identified as promising dual functional catalysts recently. Herein, we report a facile strategy to construct a heterostructure catalyst by integrating cobalt phosphide with molybdenum carbide (MXene). The CoP/Mo₂CT_x(T is the surface terminal group) catalyst exhibited good HER activity with an overpotential of 78 mV at a current density of 10 mA cm^?2, close to that of the Pt/C benchmark, and its OER performance is markedly better than that of the RuO₂benchmark, evidenced by a very small overpotential of 260 mV at 10 mA cm^?2in 1 M KOH. Impressively, when employed for overall water splitting, CoP/Mo₂CT_xalso outperformed the Pt/C + RuO₂combination with a voltage of 1.56 V @ 10 mA cm^?2. Density functional theory (DFT) calculations revealed that CoP/Mo₂CT_xhas appropriate water adsorption especially the optimal H* adsorption free energy (ΔG_H*), and the Mo₂C MXene support can significantly increase the total density of states and downshift the d-band center for the HER, while for the OER, multiple characterization techniques of CoP/Mo₂CT_xpost the OER test show that CoP in the catalyst can be transformed into Co-OOH during the electrocatalytic process. This study can provide a pathway for the design and fabrication of MXene-supported noble-metal-free bifunctional catalysts toward practical water splitting and energy conversion.

Full text of DOI:10.1039/d1ta05648d

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28 March 2018
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DOI: 10.1039/D1TA05648D
ARTICLE
Cobalt Phosphide Supported by Two-Dimensional Molybdenum
Carbide (MXene) for Hydrogen Evolution Reaction, Oxygen
Evolution Reaction, and Overall Water Splitting
Received 00th January 20xx,
Accepted 00th January 20xx
Shilong Liu,a Zongshan Lin,a Rendian Wan,a Yonggang Liu,b Zhe Liu,b Shuidong Zhang,c Xiaofeng
DOI: 10.1039/x0xx00000x
Zhang,d Zhenghua Tang*,b,c Xiaoxing Lu,*,a and Yong Tian*,a
Developing low cost, high performance, and durable bifunctional catalyst to boost the hydrogen evolution reaction (HER)
and oxygen evolution reaction (OER) for water splitting is a critical yet challenging task. Transition metal phosphides have
been identified as promising dual functional catalysts recently. Herein, we report a facile strategy to construct a
heterostructure catalyst by integrating cobalt phosphide with molybdenum carbide (MXene). The CoP/Mo2CTx (T is the
surface terminal group) catalyst exhibited good HER activity with an overpotential of 78 mV at the current density of 10
mA cm-2, close to the Pt/C benchmark, and its OER performance is markedly better than the RuO2 benchmark, evidenced
by a very small overpotential of 260 mV at 10 mA cm-2 in 1 M KOH. Impressively, when employed for overall water splitting,
CoP/Mo2CTx also outperformed the Pt/C+RuO2 combination with a voltage of 1.56 V @ 10 mA cm-2. Density functional
theory (DFT) calculations revealed that CoP/Mo2CTx has appropriate water adsorption especially the optimal H* adsorption
free energy (ΔGH*), and the Mo2C MXene support can significantly increase the total density of states and downshift the d-
band center for HER, while for OER, multiple characterizations of CoP/Mo2CTx post OER test show that, CoP in the catalyst
can be transformed into Co-OOH during the electrocatalytic process. This study can pave a pathway for design and
fabrication of MXene-supported noble-metal-free bifunctional catalysts toward practical water splitting and energy
conversion.
develop cost effective and high-performance bifunctional catalysts
to realize practical water splitting. So far, extensive research efforts
have been devoted to find alternative catalysts, such as transition
1. Introduction
The excessive dependence on fossil fuels has caused
ubiquitous environmental pollution and global energy crisis, and the
development and utilization of renewable energy hold great
metal sulfides, selenides, phosphides, nitrides, and heteroatom-
doped carbons.10-14 Among them, transition metal phosphide has
been attracting continuously increasing research attentions due to
their unique two-dimensional structure, increased numbers of
active sites, and promising electrocatalytic performance.15 For
instance, in 2015, Sun group found that the as-prepared Co-P films
exhibited remarkable catalytic performance for both HER and OER
in alkaline media, and 100% Faradaic efficiency for overall water
splitting, rivalling the integrated performance of Pt and IrO2.16 In a
recent report, Ji et al. documented the fabrication of CoP
nanoframes with hollow structure, which enabled a highly efficient
potentials to mitigate such issues.1-5 Hydrogen is
a clean,
environmentally friendly, renewable energy source, and water
splitting is an effective means to produce high-quality H2.6 Water
splitting includes two key electrochemical processes, hydrogen
evolution reaction (HER) and oxygen evolution reaction (OER), and
currently Pt/C and Ir/RuO2 are the best HER and OER catalysts.2, 7-9
However, the scarcity of metal resources and the associated
skyrocketing high price have significantly hindered the large-scale
applications of these catalysts. To that end, it is highly desirable to
water electrolyzer with a current density of 10 mA cm−2 driven by a
cell voltage of 1.65 V.17 Even so, the electrocatalytic performance of
CoP is still far from satisfactory mainly because of its poor
conductivity.18 However, it can be coupled with some conductive
substrate to enhance the conductivity and accelerate the electron
transfer hence improve the activity and stability.
a. School of Pharmacy, Guangdong Pharmaceutical University, Guangzhou, 510006,
P. R. China. Emails:xxlu@gdpu.edu.cn, tian_yong_tian@163.com
b. Guangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy
Research Institute, School of Environment and Energy, South China University of
Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong
510006, China. Email: zhht@scut.edu.cn
c. Guangdong Provincial Key Laboratory of Technique and Equipment for
Macromolecular Advanced Manufacturing, South China University of Technology,
China.
MXene is an emerging two-dimensional material made of
metal carbides or nitrides (usually formulated as MnC2Tx, M
d. Institute of New Materials, Guangdong Academy of Science, Guangzhou, 510650,
China.
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represents an early transition metal), and its surface is rich in d-band center for HER, while CoP in the catalyst can transform into
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functional groups (Tx= -OH, -O, -F).19-22 Thanks to the merits Co-OOH during the OER process.
DOI: 10.1039/D1TA05648D
including high mechanical stability, excellent conductivity, and high
specific surface area, MXene has attracted widespread attentions in
the field of energy storage and conversion. The general precursor to
2. Experimental section
prepare MXene is Mn+1AXn, where A is IIIA or IVA group element
(e.g., Al or Ga), X represents the C or N element.23, 24 MXene can be
prepared by selectively etching the A layer from a large MAX phase
and substituted by -OH, -O, -F terminal groups, making MXene an
ideal substrate for anchoring various active substances.25-27
Consequently, MXene has been employed to anchor other
functional materials to form active electrocatalysts. For instance,
using Ti3C2Tx MXene as both the conductive support and the second
metal precursor, Li et al. prepared intermetallic compound of Pt3Ti
on MXene for HER, which exhibited 4.4 and 13 times of mass
activity and specific activity higher than Pt/Vulcan at an
overpotential of 70 mV.28 In another report, Ramalingam and
2.1 Materials
Cobalt nitrate hexahydrate (Co(No3)2·6H2O), ammonium
fluoride (NH4F), urea (CO(NH2)2) were supplied by Macklin bio-
chemical company, China. Hydrofluoric acid was purchased from
Damao Chemical Reagents, Tianjin, China. Ruthenium dioxide
(RuO2) and 20 wt.% Pt/C were obtained from Alfa Aesar. Deionized
water with a resistivity of 180.2 MΩ·cm-1 was prepared by the
Barnstead Nanopure water system. All chemical reagents were
directly used as received without further purification.
2.2 Synthesis of Mo2C MXene
Mo2Ga2C powders were synthesized from β-Mo2C and Ga
Varadhan groups constructed ruthenium single atoms supported by according to a previously reported method.36 The Mo2C MXene
Ti3C2 MXene, which displayed an overpotential of 76 mV at a nanosheets were produced by etching Mo2Ga2C powders in 14 M
current density of 10 mA cm-2 with a Tafel slope of 90 mV dec-1 in HF. Briefly, Mo2Ga2C powders (2.0 g) were added into the HF
0.5 M H2SO4 toward HER. Lim et al. fabricated a 2H-MoS2 on Mo2C solution (14 M, 20 mL) under magnetic stirring. Afterward, the
o
MXene nanohybrid, which showed exceptional durability, as less reaction temperature was raised to 55 C for 160 h. The resulting
than 30 mV overpotential degradation was observed after 10 product was thoroughly washed with deionized water until it
reached a pH approximately neutral. The product of MXene was
then vacuum dried at 60 oC for 12 h and sealed for preservation.
continuous days of electrolysis at a fixed -10 mA cm − 2 current
density or 100,000 successive cyclic voltammetric cycles.29 Muller
and coworkers documented the synthesis of a solid solution of
Mo2CTx:Co MXene phase with cobalt substitution on the
molybdenum sites with impressive HER performance, while DFT
2.3 Synthesis of Co(OH)F and Co(OH)F/Mo2CTx
First of all, Co(NO3)2·6H2O (291 mg), NH4F (296 mg), and urea
(600 mg) were co-dissolved in deionized water (30 mL) under
sonication, and kept stirring for 10 mins. Mo2C MXene powders
were then added into the solution and the mixture was kept stirring
for 20 mins. Subsequently, the solution was transferred into a
Teflon autoclave (45 mL) and heated at 120 oC for 10 h. The as-
obtained Co(OH)F/Mo2CTx was washed several times with deionized
calculations revealed that
upon cobalt substitution, the
thermodynamics of hydrogen binding on the MXene surface can be
significantly improved to enhance the HER kinetics.30 Despite the
above progress achieved, for MXene/transition metal hybrid
catalyst, there are still several critical questions remained to be
resolved. For instance, the HER activity and stability in alkaline
solution are not that desirable, the catalyst preparation process is
rather tedious with low yield, and more importantly, the catalytic
mechanism and related structure evolution of the catalyst during
the catalytic process are still puzzling. These questions form the
primary motivation and goal of our current study.
o
water and dried in a vacuum oven at 60 C. Co(OH)F was produced
by the same manner without adding MXene.
2.4 Synthesis of CoP and CoP/Mo2CTx
For preparing CoP/Mo2CTx, Co(OH)F/Mo2CTx (30 mg), and
NaH2PO2 (300 mg) were placed at the different sides of the tube
furnace. NaH2PO2 was loaded at the upstream side, while
Co(OH)F/MXene was at the other side. Subsequently, the
Herein, we developed a simple hydrothermal and calcination
approach that can grow CoP nanoarrays onto the surface of Mo2C
o
o
temperature was increased to 350 C at the rate of 2 C min-1 in Ar
gas and maintained for 2 h to acquire the final product of
CoP/Mo2CTx. CoP was prepared in a similar manner with the
precursor of Co(OH)F employed instead.
MXene to form
a heterostructure catalyst of CoP/Mo2CTx.
CoP/Mo2CTx exhibited comparable activity but superior stability to
the Pt/C catalyst for HER, and markedly outperformed activity and
long-term durability than the RuO2 benchmark for OER in alkaline
solution. Notably, it showed much lower voltage to achieve the
current density of 10 mA cm-2 than the Pt/C+RuO2 combined
benchmark catalyst for overall water splitting, also better than most
of the recently reported non-noble-metal based catalysts.31-35 DFT
calculations revealed that CoP/Mo2CTx has the optimal H*
adsorption free energy (ΔGH*), and the Mo2C MXene support can
significantly increase the total density of states and downshift the
2.5 Electrochemical measurements
Firstly, 450 μL deionized water, 5 wt.% Nafion solution (50 μL),
and 500 μL ethanol were mixed following by sonicating for 1 h to
form
a uniform suspension as the catalyst ink. The HER
measurements were performed in a glassy carbon electrode (GCE,
0.07 cm2). In a typical test, 20 μL of the catalyst ink (5 mg/mL) was
dropwise cast onto the GCE with a loading of 1.428 mg cm-2. Linear
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sweep voltammetry (LSV) was conducted between -1.6 V and -0.8 V method. The adsorption Gibbs free energy (∆G) wasViceawlcAurtilcaleteOdnlianes
with a scan rate of 5 mV s-1. In addition, the cyclic voltammetric (CV) follows:
DOI: 10.1039/D1TA05648D
measurements were conducted at a potential range of -1.2 V to -0.9
∆G= ∆E+∆ZPE-T∆S
V, and the scan rate was 20-200 mVs-1. The OER tests were
Where ∆E is the DFT-derived adsorption free energy, which is the
performed in a carbon cloth (0.25 cm2) with a catalyst loading of 0.4
energy difference of configuration after and before adsorbing a
mg cm- 2. LSV was performed within the potential range of 0 to 1 V
reaction species. ∆ZPE, T, and ∆S are the zero-point energy,
temperature, and the difference of entropy, respectively.
(vs. Ag/AgCl) at a scan rate of 10 mV s-1, while CV was conducted in
the potential range between 0 and 0.5 V at a scan rate of 10 mV s-1.
Water splitting measurements were performed using CoP/Mo2CTx
on carbon cloth with a loading of 0.4 mg cm-2 as both cathode and
anode catalysts in 1 M KOH. The potential was calculated via the
following equations. The electrochemical performance was carried
3 Results and discussion
3.1. Structural characterizations
out with an electrochemical workstation (CHI750E). HER and OER
The CoP/Mo2CTx composite was firstly prepared by following
tests were conducted by a conventional three-electrode cell in 1 M the previously reported method with some modifications.37 Scheme
KOH. In all electrochemical tests, graphite rod (in saturated KCl) was 1 presents the synthetic route for CoP/Mo2CTx. Firstly, HF as the
used as counter electrode. Hg/HgO was employed as reference selective etchant was employed to produce irregular MXene
electrode for HER, OER, and water splitting long-time i-t test, and nanosheets from the Mo2Ga2C phase according to a recently
besides that, Ag/AgCl (in saturated KCl) was used as reference reported procedure,38 whereas few-layered MXene sheets were
electrode for other electrochemical tests.
ERHE= EAg/AgCl + 0.197+0.0591*pH
HER potential η(V)= ERHE – 0 V
collected. Subsequently, cobaltous nitrate hexahydrate was added
into the above MXene suspension with NH4F and urea under
ultrasonication and mild stirring. As the surface of MXene is
negatively charged, Co2+ ions can be electrostatically assembled
onto the MXene. Upon hydrothermal treatment, the anchored Co2+
ions began to grow vertically onto the MXene. Finally,
Co(OH)F/Mo2CTx was subjected to phosphorization to form the
CoP/Mo2CTx composite.
OER potential η(V)= ERHE – 1.23 V
2.6 Material characterization
The morphology and surface structure of the samples were
analyzed on field emission scanning electron microscopy (FE-SEM,
Merlin) and transmission electron microscopy (TEM, JEOL-2010). X-
ray diffraction (XRD, Empyrean) measurements were performed to
identify the crystal structural feature of the catalysts. X-ray
photoelectron spectroscopy (XPS) tests were conducted using Phi X-
tool instrument with an Al Kα radiation. The composition and
content of the elements in the catalysts were determined by
inductively coupled plasma optical emission spectrometry (ICP-
OES). Specific surface areas and pore diameter distributions of the
samples were estimated using Micromeritics ASAP2460 by
Brunauer-Emmett-Teller (BET).
Subsequently, the morphology and structure during the
preparation of MXene, CoP, and the CoP/Mo2CTx catalyst were
characterized by scanning electron microscopy. As shown in Figure
S1, stacked layers are clearly observed in the MAX precursor, where
some are clustered together. Fig. 1a shows the typical SEM image
of Mo2C MXene, which maintained the feature of the MAX
precursor. The SEM image of the Co(OH)F and Co(OH)F/Mo2CTx
precursors can be found in Figure S2. Numerous nanoarrays can be
observed in Co(OH)F, and such well-defined arrays can be clearly
visualized in CoP (Fig. 1b). It indicates that, the structure and
morphology can be well maintained upon phosphorization, and
such phenomenon also occurred in the Co(OH)F/Mo2CTx and
CoP/Mo2CTx composites. As shown in Fig. 1c, CoP nanoarrays grow
densely onto the MXene surface for CoP/Mo2CTx, inheriting the
pattern of Co(OH)F/Mo2CTx (Figure S2). Further TEM test (Figs. 1d,
1e) shows that, with CoP grown onto the MXene surface, the
heterostructure is clearly visible. As depicted in the high-resolution
TEM image in Fig. 1e, the legible lattice spacing of 0.282 nm can be
attributed to the (011) plane of crystalline CoP. The corresponding
fast fourier transform (FFT) pattern verified the presence of the
(011), (112), and (211) crystal planes of CoP in the heterostructure.
The HAADF-STEM image of CoP/Mo2CTx is presented in Fig. 1f, and
the corresponding energy dispersive X-ray (EDX) elemental mapping
(Figs. 1g-l) clearly shows that, all the Co, P, Mo, C, and O elements
are uniformly distributed in the sample. Then, the amount of each
element was quantitatively determined by ICP-OES, as summarized
in Table S1. It is worth noting that, the content of Co element is
2.7 Computational method
The DFT computations were performed using Vienna ab initio
simulation package (VASP). Projector augmented wave (PAW) and
the generalized gradient approximation (GGA) with the Perdew-
Burke-Ernzerhof (PBE) function were used to describe the ion-
electron interaction and the exchange-correlation potential. 400 eV
cutoff was employed while the convergence threshold for the self-
consistent field (SCF) and ion steps was set to be 1×10-5 eV and 0.01
eV Å-1. Van der Waals interaction was considered by the DFT-D3
method. The dipolar correction was included and the
symmetrisation was switched off. The vacuum space of all
investigated slab models was 15 Å. The Brillouin zones were
sampled by Monkhorst-Pack 3*3*1 for all slab models. The barrier
of water dissociation was estimated by nudged elastic band (NEB)
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much higher than the value obtained from EDX, probably due to
The HER performance of CoP/Mo2CTx was first evaluated
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DOI: 10.1039/D1TA05648D
that, the MXene surface is covered by CoP while EDX only detects through a standard three-electrode system in 1.0 M KOH. For
the surface atoms with a few nanometer depth.
comparison, the various catalysts of CoP, Co(OH)F/Mo2CTx, Mo2C
MXene and commercial Pt/C were also investigated under the same
conditions. Fig. 3a shows the HER polarization curves, where Pt/C
has excellent activity with the lowest overpotential of 23 mV to
reach the current density of 10 mA cm-2. Notably, CoP also exhibits
acceptable HER activity, however, CoP/Mo2CTx possesses superior
HER activity with an overpotential of 78 mV @ 10 mA cm-2, much
lower than that of CoP and the Co(OH)F/Mo2CTx precursor. It
indicates that, the phosphorization can significantly promote the
HER activity and there is synergistic catalytic effect between CoP
and the Mo2C MXene. The reaction kinetic and the rate determining
step in the electrocatalytic process can be revealed by Tafel plots,
and the Tafel slope can be calculated by linear fitting of the LSV
curves according to the Tafel equation: η= b*log j + a.41 As
illustrated in Fig. 3b, the Tafel slope of CoP/Mo2CTx is calculated as
66 mV dec-1, lower than that of CoP (95 mV dec-1), Co(OH)F/Mo2CTx
(158 mV dec-1), and Mo2C MXene (192 mV dec-1), slightly higher
than Pt/C (40 mV dec-1). It indicates CoP/Mo2CTx had an enhanced
HER kinetics, and the HER process followed the Volmer-Heyrovsky
mechanism. The close Tafel slope value with Pt/C suggests that,
they probably have the identical reaction pathway, that is, the rate
limiting step is the desorption of H2. The lower overpotential at 10
mA cm-2 and Tafel slope value of CoP/Mo2CTx for HER outperformed
the other samples in the series (Fig. 3c), also superior to the
recently reported top-level electrocatalysts (Table S2). The
exchange current density (j0) is also an important indicator to
evaluate the intrinsic HER activity of the electrocatalyst, and the j0
value can be obtained by extrapolating the Tafel plot to an
overpotential of 0 V (Figure S7). Besides Pt/C, CoP/Mo2CTx exhibits
the highest j0 value among these catalysts, indicating that
CoP/Mo2CTx has fast electron-transfer rate and displays favorable
HER reaction kinetics. The charge transfer resistance and electronic
conductivity can be used to unravel the HER mechanism. EIS
analysis was conducted at the same HER experimental condition
and the Nyquist plot was fitted via a simplified equivalent circuit
model (Fig. 3d). All the fitting parameters in the EIS spectra can be
found in Table S3. Note that, the Rct value represents the charge
transfer resistance at the electrocatalyst/electrolyte interface. As
expected, CoP/Mo2CTx exhibited the smallest Rct value and
semicircle than those of CoP, Co(OH)F/Mo2CTx, and Mo2C MXene,
reflecting a superior kinetic process and enhanced electronic
conductivity. Stability is another important parameter to evaluate
the performance of the electrocatalyst for long-term test. Fig. 3e
shows the chronoamperometric curves tested under the current
density of 10 mA cm-2 for CoP/Mo2CTx and Pt/C. According to the i-t
curve, 89% of the initial current was maintained after a continuous
operation of 50 h, while in sharp contrast, Pt/C could only retain
68% of the initial value. Moreover, in the accelerated durability test
(ADT, Fig. 3f), CoP/Mo2CTx exhibited an extremely small potential
shift (10 mV) after 3000 CV cycles of potential scan, superior than
that of Pt/C (Inset in Fig. 3f, 15 mV) at the same conditions,
implying remarkably excellent long-term durability for HER.
Next, the XRD patterns of the samples were probed. Compared
with the Mo2Ga2C precursor, the XRD pattern remained almost
unchanged for the Mo2C MXene, except a new broad peak
appeared at 7.3° after HF etching (Figure S3).36 It is probably due to
that, the removal of Ga caused the increase of the layer spacing in
the Mo2C MXene. As shown in Fig. 2a, for CoP/Mo2CTx, the peaks
with 2θ positioned at 39.4o, 42.8o match well with the Mo2C MXene
planes, while the peaks located at 31.5o, 36.4o, 46.2o, 48.0o, 52.3o,
and 56.9o can be attributed to the crystal planes of (011), (111),
(112), (211), (103), and (301) for CoP (Standard card: JCPDS No. 29-
0497), respectively.39 In addition, the XRD pattern of
Co(OH)F/Mo2CTx can be found in Figure S4, which is also in good
accord with the standard card. The composition and valence state
of CoP/Mo2CTx were then analyzed by XPS measurements. Fig. 2b
shows the high-resolution XPS spectrum of the Co element, where
the peaks with binding energy at 778.4 eV and 793.5 eV can be
assigned to Co-P, and the peaks at 782.2 eV and 798.2 eV are
attributed to Co(II)-O (Inevitable oxidation occurs on the surface of
CoP). As illustrated in Fig. 2c, the broad peak for the O1s electrons
can be deconvoluted into two peaks with binding energy at 531.5
and 533.0 eV, which are attributable to the hydroxide species and
adsorbed O, respectively. Fig. 2d presents the core-level XPS
spectrum of the P element, and the peaks with binding energy at
129.1 eV and 129.9 eV can be attributed to the Co-P and POx
species, respectively.40 Figure S5a shows the high-resolution Mo3d
XPS spectrum, which can be deconvoluted into six peaks. The peaks
with binding energy at 228.8 and 231.9 eV, 229.4 and 232.6 eV,
233.5 and 235.5 eV are attributed to Mo2+, Mo4+, and Mo6+,
respectively. The high-resolution C1s XPS spectrum (Figure S5b)
shows a broad peak which is deconvoluted into three peaks, where
the peak at 286 eV is attributable to C-O bond, the peak at 283.7 eV
relates to C-Mo bond, and the peak at 284. 7 eV is ascribed to C-C
bond.
Furthermore, as the surface area and porosity characteristics
are critical to the electrocatalytic performance, we also conducted
the Brunauer-Emmett-Teller (BET) test, in which the surface area
was estimated by N2 adsorption/desorption measurements and the
pore size distribution was determined by Barrett-Joyner-Halenda
method. As shown in Figure S6, the specific surface area of
CoP/Mo2CTx is calculated as 46.49 m2 g-1, much higher than CoP
(7.96 m2g-1), indicating that Mo2C MXene can significantly boost the
surface area for the heterostructure. In addition, the isotherm
curve exhibited type IV pattern, suggesting that there are both
mesopores and macropores in CoP/Mo2CTx. The inset graph of pore
size distribution in Figure S6 shows that, the average pore size of
CoP/Mo2CTx is 1.7-11.8 nm. Such high surface area and well-defined
pore structure might be beneficial for the ion and mass transport
during the electrochemical reaction.
3.2. Electrocatalytic performance
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Notably, the feature of the Raman spectra before and after the HER of the catalysts, the mass activity of the samples forVHiewERArtaicnledOOnlEinRe
DOI: 10.1039/D1TA05648D
test for CoP/Mo2CTx remained almost unchanged (Figure S8), was calculated. As illustrated in Figure S14, CoP/Mo2CTx had much
further confirming the robust durability of CoP/Mo2CTx for higher mass activity than CoP, Mo2C MXene, and the
prolonged operation. The SEM and TEM images of CoP/Mo2CTx Co(OH)F/Mo2CTx precursor, also superior to Pt/C for HER and RuO2
after the HER stability test also reveal negligible morphology and for OER, indicating that Mo2C MXene as the conducive support can
structure change (Figure S9).
significantly promote both the HER and OER activity.
The OER performance was also examined in alkaline medium.
Inspired by the outstanding HER and OER properties, we
As shown in the polarization curves in Fig. 4a, the benchmark RuO2 constructed a two-electrode water splitting device in alkaline
catalyst demonstrated preferable OER activity, but it was still electrolyte using CoP/Mo2CTx as both anode and cathode catalysts,
inferior to that of CoP and CoP/Mo2CTx. To afford a current density which was compared with the benchmark combination of Pt/C
of 10 mA cm-2, CoP/Mo2CTx only needs an overpotential of 260 mV, deposited on carbon cloth cathode and RuO2 on anode. Fig. 5a
lower than that of CoP (290 mV) and RuO2 (310 mV). Meanwhile, shows the overall water splitting polarization curves of CoP/Mo2Tx
from Fig. 4b, it can be seen that, CoP/Mo2CTx also exhibits the and Pt/C+RuO2 in 1 M KOH solution with the reacting electrode
lowest Tafel slope (51 mV dec-1), in comparison with CoP (121 mV graph inserted. The bubbles on the two electrodes are H2 and O2
dec-1), RuO2 (137 mV dec-1), Co(OH)F/Mo2CTx (218 mV dec-1), and generated simultaneously. To reach the current density of 10 mA
Mo2C MXene (340 mV dec-1), indicating the favourable OER reaction cm-2, the cell voltage of CoP/Mo2CTx was 1.56 V, which is
kinetics. Note that, the smallest overpotential and Tafel slope of impressively lower than the precious metal-based catalyst
CoP/Mo2CTx for OER is lower than that of recently reported assembled by Pt/C and RuO2 (1.62 V) and most of the reported top-
43
transition
metal-based
high-performance
electrocatalysts level catalysts (Table S6).33,
In addition to excellent catalytic
(Summarized in Table S4). The turnover frequency (TOF) is an activity, CoP/Mo2CTx showed high stability during a 50 h test at 10
important intrinsic activity and kinetic parameter in OER, and the mA cm-2 in the i-t curve (Fig. 5b). CoP/Mo2CTx maintained 89% of
TOF value of various catalysts at different overpotentials was then the initial current, higher than that of the combined Pt/C and RuO2
caluclated.42 As demonstrated in Fig. 4c, the TOF value of catalyst (83%). Finally, in the ADT test in Fig. 5c, the LSV curve
CoP/Mo2CTx is higher than the other samples at the same shows that only a slight potential shift of 30 mV after continuous
overpotentials, suggesting remarkably enhanced OER activity. EIS operation of 10 h, indicating that CoP/Mo2CTx has advantageous
measurement was then conducted (Fig. 4d), and the fitted results durability and stability in alkaline medium. The above results
are compiled in Table S5. CoP/Mo2CTx exhibited the smallest demonstrate that CoP/Mo2CTx could be a potent bifunctional
semicircle, suggesting that the lowest charge transfer impedance. catalyst toward practical water splitting.
At last, the stability of CoP/Mo2CTx for OER was evaluated and
3.3. Investigation of catalytic mechanism
compared with the RuO2 benchmark catalyst. As shown in the
chronoamperometric test (Fig. 4e), after about 50 h’s
measurement, CoP/Mo2CTx can preserve much higher initial current
density than that of RuO2 (87% vs. 65%). Such markedly
outperformed long-term durability was further validated by the
ADT test, as CoP/Mo2CTx exhibited an overpotential degradation of
18 mV after 3000 cycles of potential scans, smaller than that of
RuO2 (25 mV) at the same conditions (Fig. 4f). In addition, SEM and
TEM images after the OER stability test (Figure S10) disclose that,
the structure and morphology of CoP/Mo2CTx remained almost
unchanged, further confirming the robust stability.
To obtain the comprehensive mechanistic insights on the
excellent catalytic performance, we first conducted DFT calculation
to disclose the HER mechanism. The H2O adsorption, adsorbed
water dissociation, and H* adsorption free energies (ΔGH*) were
performed by employing some simplified models (Figure S15). It is
well known that, for HER occurring in alkaline medium, either
Volmer-Heyrovsky or Volmer-Tafel mechanism is adopted.44
Nevertheless, the Volmer process takes place in the first step, that
is, water molecules are adsorbed onto the catalyst surface with Had
and OH- formed. For the Heyrovsky process: Had and adsorbed H2O
can react to generate H2, while for the Tafel process: two Had react
with each other to form H2. During the HER process, the adsorption
of water molecule, the dissociation of adsorbed water molecule,
the adsorption of Had and the desorption of the product are
involved. Notably, the water adsorption free energy, the
dissociation energy barrier of absorbed water, especially the Gibbs
free energy of Had are directly related to the HER activity of the
catalyst. Figs. 6a-c show the Gibbs free energy change of water
adsorption and the dissociation of adsorbed water on the surface of
CoP, Mo2C MXene, and CoP/Mo2CTx. It can be noted that,
CoP/Mo2CTx possesses a much lower Gibbs free energy for water
adsorption, indicating the heterostructure has a strong affinity to
To elucidate the physical origin of the excellent HER and OER
activity of CoP/Mo2CTx, the specific capacitance (Cdl) and
electrochemically active surface area values were calculated. As
presented in Figure S11 and Figure S12, from the cyclic
voltammograms of Mo2C MXene, CoP, and CoP/Mo2CTx in either
HER and OER, the current density as a function of scan rate showed
a rather linear regression relationship. In either HER or OER test,
CoP/Mo2CTx possessed much higher Cdl values than that of Mo2C
MXene and CoP. As the ECSA value holds a direct proportional
relationship with the Cdl value, consequently, CoP/Mo2CTx had
much higher ECSA values for both HER and OER than Mo2C MXene
and CoP, as compiled in Figure S13. To disclose the intrinsic activity
H2O molecule. Meanwhile, CoP/Mo2CTx has
a much smaller
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dissociation energy barrier (1.06 eV) than CoP (1.18 eV), suggesting further tested the catalyst after OER through XRD measurement. As
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o
o
DOI: 10.1039/D1TA05648D
that CoP/Mo2CTx is more favourable for the dissociation of the depicted in Figure S17, the peaks with 2θ positioned at 20 , 39 , and
adsorbed water. The Gibbs free energy of Had of the catalysts is 51o belong to the (003), (012), (015) crystal plane of β-CoOOH,
presented in Fig. 6d. It can be observed that, CoP/Mo2CTx has a verifying the formation of CoOOH. The element content of the
ΔGH* value of -0.037 eV, quite close to zero, and the absolute value catalyst before and after the OER test was studied by EDS mapping
is much lower than that of Mo2C MXene and CoP. The above (Figure S18 and Figure S19). It can be noted that, after the OER test
findings demonstrate that, CoP/Mo2CTx has enhanced water (Table S1), the content of P decreased from 25.02% to 4.48%, the
adsorption, lower water dissociation energy barrier, and especially content of oxygen increased from 17.45% to 30.04%, and the
the optimized H* adsorption free energy, which can drastically anionic component also decreased significantly, which are in good
promote the Volmer and Heyrovsky process hence boost the HER consistence with the above finding.
performance. These calculated results are also in good agreement
with the experimental findings.
4 Conclusions
To further unravel the promoting effect from the MXene
support, we also investigated the electron density of states and d-
band center of CoP/Mo2CTx, as compared with CoP. Previous study
In summary, a facile and straightforward method is developed to
grow CoP nanoarrays onto the surface of MXene and the obtained
has disclosed that the DOS intensity of Fermi level is intimately
CoP/Mo2CTx catalyst has excellent HER, OER, and overall water
connected with the electrical conductivity.45 Fig. 6e shows that, the
splitting properties, outperforming the Pt/C+RuO2 combined
total density of states for CoP/Mo2CTx increased significantly,
especially in the region near Fermi level. Such enhanced electrical
conductivity from CoP/Mo2CTx is conducive to rapid electron
transfer. Notably, the d-band center (εd) of CoP and CoP/Mo2CTx
relative to Ef was calculated as -1.09 eV and -1.23 eV, respectively
(Fig. 6f), manifesting that the d-band center of CoP/Mo2CTx is
shifted negatively upon the integration of the MXene support. Such
d-band center downshift can result in increased electron filling of
the antibonding states, leading to optimized H* adsorption
eventually.46
benchmark catalyst. For HER, DFT calculations disclosed that,
CoP/Mo2CTx has the appropriate water adsorption especially the
optimal H* adsorption free energy (ΔGH*). Particularly, the Mo2C
MXene support can significantly increase the surface area and
boost the electronic conductivity of the CoP/Mo2CTx catalyst hence
dramatically increase the total density of states and downshift the
d-band center. For OER, it was found that, CoP in the catalyst is
transformed into Co-OOH during the electrocatalytic process. This
study can pave a pathway for preparing MXene-supported noble-
metal-free bifunctional catalyst toward practical water splitting and
energy conversion.
We next explored the OER mechanism, where the proposed
OER mechanism on the CoP/Mo2CTx surface is schematically
depicted in Fig. 7a. To scrutinize the compositional change of the
catalyst, Raman, XPS, IR, XRD, and EDX measurements were
conducted after the OER test. As shown in Fig. 7b, in the Raman
spectrum, the 508, 608 peaks of Co-O bond can be assigned to Co-
OOH.47 Meanwhile, spin-orbit splitting in Fig. 7c shows that, the Co-
P peak disappeared after oxidation, and two new peaks appeared,
which can be deconvoluted with four sub-peaks at 780.2, 781.5,
795.4, and 796.5 eV. It further confirms the presence of the Co-
OOH species after OER. As presented in Fig. 7d in the high-
resolution O1s XPS spectrum, the peaks with binding energy at
531.5 eV and 533 eV are from hydroxides and adsorbed oxygen,
and a new peak appeared at 529.3 eV, which is the characteristic
feature of Co-O. In contrast, the anionic components changed
dramatically, and the characteristic peak of Co-P (Fig. 7e)
disappeared after the OER test, whereas only the peak at 133.5 eV
attributed to P-Ox remained.40 The IR spectra of the CoP/Mo2CTx
sample after OER can be found in Figure S16, where the peaks at
3420, 3646, 1629, and 590 cm-1 are from -OH, O-H, Co-O, and Co-O2
in CoOOH, respectively.48 All the above results clearly show that Co
hydroxide is gradually formed during the prolonged OER process,
and POx is also formed on the surface of the catalyst. Notably,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This study is supported by the Opening Project of Guangdong
Provincial Key Laboratory of Technique and Equipment for
Macromolecular Advanced Manufacturing, South China
University of Technology, China. Z. T. acknowledges the
financial support from Guangzhou Science and Technology
Plan Projects (No. 201804010323). X. L. and Y. T thank the
Natural Science Foundation of Guangdong Province (No.
2020A1515010546).
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Scheme 1. The schematic illustration for preparing the CoP/Mo2CTx catalyst.
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Fig. 1. (a)-(c) Representative SEM images of Mo2C MXene, CoP, and CoP/Mo2CTx. (d) and (e) Representative TEM image of CoP/Mo2CTx with
different magnifications. (f) HR-TEM image of CoP and (f, inset) corresponding FFT pattern of CoP, displaying lattice fringes and diffraction
spots for CoP. (g) HAADF-STEM and (h)-(l) corresponding elemental mapping of Co, P, Mo, C, and O in CoP/Mo2CTx.
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Fig. 2. (a) XRD patterns of the Mo2C MXene, and CoP/Mo2CTx. (b)-(d) High resolution XPS spectra of the Co 2p, O 1s, and P 2p
electrons for CoP/Mo2CTx.
Fig. 3. (a) LSV curves, (b) Tafel plots, (c) The Tafel slope and overpotential @10 mA cm-2 for Pt/C, CoP/Mo2CTx, CoP, Co(OH)F/Mo2CTx, Mo2C
MXene in 1 M KOH. (d) The electrochemical impedance spectra (EIS), (e) Chronoamperometric curves of CoP/Mo2CTx and Pt/C at the
current density of 10 mA cm-2 for 50 h. (f) LSV curves of CoP/Mo2CTx catalyst before and after 3000 CV cycles and inset is the corresponding
LSV curves before and after 3000 CV cycles for Pt/C.
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Fig. 4. (a) LSV polarization curves and corresponding Tafel plots (b) of CoP/Mo2CTx, CoP, RuO2, Co(OH)F/Mo2CTx, and Mo2C
MXene in 1 M KOH. (c) The turnover frequency (TOF) per surface site for different catalysts in 1 M KOH at various overpotentials.
(d) The electrochemical impedance spectra (EIS), (e) Stability test at a current density of 10 mA cm-2 for 50 h. And (f) corresponding LSV
curves of CoP/Mo2CTx catalyst before and after 3000 CV cycles and inset is corresponding LSV curves before and after 3000 CV cycles for
RuO2.
Fig. 5. (a) LSV curves for water splitting cell consisting of CoP/Mo2CTx (+,-) and RuO2 (+)||Pt/C(-) in 1 M KOH, the inset is the
device photograph of the electrolyzer using CoP/Mo2CTx for both the anode and cathode catalyst. (b) Chronoamperometric
analysis for water electrolysis catalyzed by CoP/Mo2CTx and RuO2+Pt/C at 1.56 V. (c) LSV curves for CoP/Mo2CTx before and after
the electrolysis test.
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Fig. 6. The energy barriers for water dissociation on the surface of (a) CoP (011), (b) Mo2C MXene and (c) CoP/Mo2CTx, the inside
are the views of the structures of initial states (IS), transition states (TS), and final states (FS), (d) ΔGH* calculated at the
equilibrium potential of different models, (e) TDOS of CoP and CoP/Mo2CTx, (f) pDOS of d-state (the white bar shows d-band
center, εd) of CoP and CoP/Mo2CTx.
Fig. 7. (a) Formation of the active layer reconstruction on CoP/Mo2CTx during OER. (b) Raman spectra of CoP/Mo2CTx after OER
test. (c)-(e) High resolution XPS spectra of the Co 2p, O 1s, and P 2p electrons for CoP/Mo2CTx after the OER test.
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