MXene supported Co: XAy (A = OH, P, Se) electrocatalysts for overall water splitting: Unveiling the role of anions in intrinsic activity and stability

Article abstract of DOI:10.1039/c9ta10664b

The development of efficient and stable bifunctional catalysts that outperform noble metal catalysts is a crucial task and an ongoing challenge for sustainable water electrolysis. In this work, large-size-exfoliated MXene sheets render a flat and flexible platform for the decoration of Co(OH)F, CoP and Co₇Se₈, allowing them to exhibit high electrocatalytic performances; thanks to the maximized surface area and conductivity. CoP/MXene shows enhanced oxygen evolution reaction (OER) activity, with substantially lower overpotential (η = 230 mV) at 10 mA cm^-2 compared to those of IrO₂ (300 mV). Furthermore, a hybrid bifunctional electrode (CoP/MXene//CoP/MXene) exhibits highly stable and efficient overall water splitting performance (1.56 V?10 mA cm^-2) as compared to the benchmark electrode couple IrO₂/C//Pt/C (1.62 V?10 mA cm^-2) in alkaline solution. Furthermore, we elucidate the oxidation process of the anion components (P and Se) of the hybrid catalysts under OER conditions and verify their significant influence on the activity and stability. Notably, the surface oxidation of CoP/MXene results in a POx-enriched Co-OOH/CoP/MXene hybrid, which enables retention of consistent activity and stability. On the other hand, SeOx deposition on the Co-OOH/Co₇Se₈/MXene surface significantly deteriorates the activity and stability of the catalyst. These results not only highlight the insight on the correlation between oxidized anion species and the intrinsic activity of hybridized electrocatalysts but also impart the systematic synthetic design of MXene-supported catalysts with high water-splitting efficiency.

Full text of DOI:10.1039/c9ta10664b

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
MXene supported Co A (A ¼ OH, P, Se)
x y
electrocatalysts for overall water splitting:
Cite this: J. Mater. Chem. A, 2019, 7,
7383
2
unveiling the role of anions in intrinsic activity and
stability†
a
b
a
a
N. Clament Sagaya Selvam,
Shiyu Xu,
Jooyoung Lee,
Gwan H. Choi,
*
Min Jun Oh,
a
b
a
Byungkwon Lim
and Pil J. Yoo
The development of efficient and stable bifunctional catalysts that outperform noble metal catalysts is
a crucial task and an ongoing challenge for sustainable water electrolysis. In this work, large-size-
exfoliated MXene sheets render a flat and flexible platform for the decoration of Co(OH)F, CoP and
7 8
Co Se , allowing them to exhibit high electrocatalytic performances; thanks to the maximized surface
area and conductivity. CoP/MXene shows enhanced oxygen evolution reaction (OER) activity, with
ꢀ
2
substantially lower overpotential (h ¼ 230 mV) at 10 mA cm compared to those of IrO
2
(300 mV).
Furthermore, a hybrid bifunctional electrode (CoP/MXene//CoP/MXene) exhibits highly stable and
ꢀ2
efficient overall water splitting performance (1.56 V@10 mA cm ) as compared to the benchmark
ꢀ2
electrode couple IrO /C//Pt/C (1.62 V@10 mA cm ) in alkaline solution. Furthermore, we elucidate the
2
oxidation process of the anion components (P and Se) of the hybrid catalysts under OER conditions and
verify their significant influence on the activity and stability. Notably, the surface oxidation of CoP/MXene
results in a POx-enriched Co–OOH/CoP/MXene hybrid, which enables retention of consistent activity
7 8
and stability. On the other hand, SeOx deposition on the Co–OOH/Co Se /MXene surface significantly
Received 26th September 2019
Accepted 9th November 2019
deteriorates the activity and stability of the catalyst. These results not only highlight the insight on the
correlation between oxidized anion species and the intrinsic activity of hybridized electrocatalysts but
also impart the systematic synthetic design of MXene-supported catalysts with high water-splitting
efficiency.
DOI: 10.1039/c9ta10664b
rsc.li/materials-a
ꢀ
10
and OH ions, respectively. However, this also implies the
limitation that simultaneously integrating HER and OER cata-
1
. Introduction
Electrocatalytic water splitting is a promising research area and lysts in the same electrolyte would yield inferior activity for
an ongoing challenge for sustainable production of H as overall water splitting. Likewise, overall water splitting in acidic
2
1
a future fuel. The water splitting process comprises the media suffers from incompatibility between species in electro-
hydrogen evolution reaction (HER) and the oxygen evolution chemical stability and the mitigated electrocatalytic activity
reaction (OER), which conventionally have been performed arising from decomposition of the catalyst under strong acidic
1,2
11–13
using the state-of-the-art catalysts Pt and Ir/Ru, respectively.
conditions.
For these reasons, the development of non-
However, the noble metals in these catalysts are expensive and noble metal bifunctional catalysts that perform well in alka-
scarce, representing a hurdle to large-scale application. This has line medium is crucial for realizing ideal performance in overall
provoked the development of alternative HER/OER catalysts water splitting. Accordingly, transition metal-based catalysts
such as transition metal oxides, suldes, selenides, carbides, have been developed as bifunctional catalysts that could work in
3
–9
nitrides, phosphides, and heteroatom-doped carbons.
general, HER and OER catalysts respectively perform well in hydroxides,
In alkaline media; these catalysts include FeNi layered double
14,15
16
17
18
19
NiCoO
,
MoO
,
MoS
/Ni
S
,
NiCo
S
,
and
and
4
2
2
3
2
2
4
+
20
21–23
acidic and basic media, due to the facilitated availability of H
NiSe. In particular, transition metal phosphides,
24
selenides have been investigated as bifunctional catalysts over
a wide range of pH. However, the relatively low surface area and
poor morphological features of most of these catalysts, arising
a
School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419,
Republic of Korea. E-mail: pjyoo@skku.edu
b
School of Advanced Materials Science and Engineering, Sungkyunkwan University mainly from self-agglomeration, have restricted their HER/OER
(
SKKU), Suwon 16419, Republic of Korea
kinetics and thus limit their ability to be scaled to large-scale,
high-performance applications.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ta10664b
1
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 27383–27393 | 27383

View Article Online
Journal of Materials Chemistry A
Paper
MXenes (Ti
3
C
2
T
x
; T
x
¼ –O, –OH, –F), having rich terminal
2
. Experimental section
groups, superior hydrophilicity, higher electrical conductivity
ꢀ
1
2.1. Materials
(
0
4600 ꢁ 1100 S cm ), and high charge carrier mobility (2.6 ꢁ
ꢀ
2
ꢀ1 ꢀ1
.7 cm
V
s ), have been explored for various electro- All chemicals used were purchased from Sigma-Aldrich and
25
catalytic applications. Of note, MXene as a pure phase has directly used without further purication.
2
6–28
been explored as an HER catalyst,
coupled with MoS for enhanced HER activity.
also been incorporated with CNTs, g-C
and it has also been
29,30
2
MXenes have
FeNi-LDH,
2
.2. Synthesis of Ti
Ti MXene nanosheets were synthesized according to
a previously reported procedure. Typically, 1 g of LiF was
3 2 x
C T MXene sheets
31
32
33
3
N
4
,
3 2 x
C T
34
35
NiFeCo-LDH, and metal–organic frameworks to boost their
OER activity. However, very few studies have focused on the
development of MXene-based hybrids as bifunctional catalysts
42
ꢀ1
added to 6 mol L HCl (20 mL) and stirred for 10 min. One
gram of Ti AlC powder was slowly added to this solution under
continuous stirring (500 rpm). Then, the temperature of the
reaction mixture was raised to 40 C and maintained for 24 h.
3 2 x
The resulting reaction mixture containing Ti C T MXene (aer
complete removal of Al) was thoroughly washed with DI water
until it reached neutral pH ($6). The obtained MXene was then
vacuum dried at 50 C for 24 h. To exfoliate the obtained
multilayered MXene into nanosheets, 0.2 g of MXene was
dispersed in DI water (50 mL) and stirred for 10 min. Then, the
slurry was sonicated for 30 min with continuous Ar purging. The
obtained colloidal MXene was centrifuged (3500 rpm for 1 h)
and the supernatant containing monolayer or few-layer MXene
nanosheets was collected for further use.
36–38
3
2
for overall water splitting applications.
Most of these studies
have not yielded well-ordered hierarchical morphologies with
uniformly decorated electrochemical active phases on the
surface of the MXene. Instead, MXene-based hybrids with
agglomerated morphologies have also suffered from reduced
catalytic activity due to the limited electrolyte accessibility of the
active sites. Moreover, well-exfoliated thin layers of MXene
sheets have not yet been exploited for generating hybrids with
electrochemically active phases for HER/OER catalysis. Thus,
the combined benets of thin layered 2D MXene sheets and
a bifunctionally active cobalt phosphide and selenide phase
would be a motivation to hybridize them into well-dened
architectures while imparting more exposed active sites.
ꢂ
ꢂ
Therefore, in this work, to address the identied shortcom-
ings of the morphological features of catalysts, we present
2.3. Synthesis of Co(OH)F/MXene and bare Co(OH)F
strategic approaches to realize remarkable bifunctional activity 1 mmol of Co(NO ) $6H O, 8 mmol of NH F, and 10 mmol of
3
2
2
4
with efficient HER/OER kinetics, by (i) hybridizing the metal CO(NH ) were dissolved in DI water (36 mL) and stirred for
2
2
phosphide and selenide with a novel two-dimensional (2D) 15 min. Colloids of MXene nanosheets were then added to the
conductive support matrix of MXene to boost the charge mixture dropwise and gently stirred for 30 min. This reaction
transfer kinetics and (ii) architecting the hierarchical mixture was poured into a Teon-lined stainless-steel autoclave
ꢂ
morphology to create more surface area with exposed active of 50 mL volume and reacted at 120 C for 8 h. The obtained
ꢂ
sites available for strong electrolyte contact. For example, the product was thoroughly washed with DI water and dried at 50 C
resulting hybrid system, integrating highly porous CoP nano- for 12 h in a vacuum oven. Bare Co(OH)F was prepared using
rods and conductive MXene sheets, shows remarkable HER and the same procedure except that no MXene was added.
OER performance, and is implemented as an outstanding
bifunctional hybrid catalyst for overall water splitting. Speci-
2.4. Synthesis of the CoP/MXene hybrid
cally, the bifunctional water splitting performance of CoP/
The Co(OH)F/MXene precursor and NaH
a 1 : 5 mass ratio, kept separately in a porcelain boat, and
placed in a furnace. Ar gas was passed upstream from the
2
2
PO were taken in
MXene even surpasses that of IrO
line conditions.
2
/C//Pt/C couples under alka-
More importantly, besides the development of efficient
bifunctional catalysts, we additionally investigate the structural
and compositional transformation of the metal centre and
anion components of the catalysts aer OER catalysis to eluci-
date the time evolution stability of the catalysts. Although some
recent studies have demonstrated the surface oxidation of Co-
2 2
NaH PO and subsequently the temperature was increased to
ꢂ
ꢂ
ꢀ1
3
00 C at the rate of 5 C min and then maintained at this
temperature for 2 h. The resultant product was rinsed with DI
ꢂ
water and dried at 50 C for 12 h in a vacuum oven. Bare CoP
was prepared using the same procedure, except that pristine
Co(OH)F instead of Co(OH)F/MXene was used as the precursor.
The mass ratio of CoP to MXene in the hybrid was 0.9 : 0.1.
39–41
based catalysts to Co-oxyhydroxide under OER conditions,
they only reported the transformation of the metal centre while
completely overlooking the role of oxidized anion species
during OER catalysis. Based on this understanding, here, we
7 8
2.5. Synthesis of the Co Se /MXene hybrid
have systematically investigated the oxidation reaction of anion 0.1 g of Co(OH)F/MXene precursor and 0.3 g of Na SeO were
2
3
components on the surface of catalysts aer OER catalysis. As dissolved in 50 mL of DI water. Aer dropping 10 mL N H $H O
2
4
2
a result, we conrm the transformation of anion components (P into the solution under stirring, the mixture was transferred to
and Se) to their corresponding oxidized forms (PO
on the catalyst surface. We further verify the desirable role of heated at 180 C for 8 h. The obtained product was thoroughly
x
and SeO
x
)
a Teon-lined stainless-steel autoclave of 100 mL volume and
ꢂ
ꢂ
PO
x
species in yet adverse effect of SeO
x
on the OER activity and washed with DI water and dried at 50 C for 12 h in a vacuum
stability.
oven. Bare Co
7
Se
8
was prepared using the same procedure using
27384 | J. Mater. Chem. A, 2019, 7, 27383–27393
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
Co(OH)F precursor. The mass ratio of Co
hybrid was 0.9 : 0.1.
7
Se
8
to MXene in the examined by means of scanning electron microscopy (FESEM-
JSM7600F, JEOL) and transmission electron microscopy
(HETEM-JEM2100F, JEOL). Phase formation was examined using
X-ray diffraction (D8 Advance, Bruker) with Cu K radiation (l ¼
2
.6. Electrochemical measurements
a
˚
1
.5406 A). Surface chemistry and elemental composition were
The electrochemical performance of catalysts was evaluated in
1
electrocatalytic measurements were conducted by following
standard protocols (detailed in the ESI†). Hg/HgO saturated
with NaOH and Pt wire were respectively used as reference and
analyzed by means of X-ray photoelectron spectroscopy (ESCA-
LAB250, Thermo XPS). N adsorption–desorption isotherms were
evaluated by means of the Brunauer–Emmett–Teller method
Micrometrics ASAP 2020 instrument).
ꢀ
1
mol L KOH solution using a CH instrument (CHI600D). The
2
(
2
counter electrodes. Carbon bre paper (1 ꢃ 1 cm ) was used as
the working electrode. The catalyst, polyvinylidene uoride
3
. Results and discussion
(
binder), and Ketjenblack (conductive agent) were mixed in
a 70 : 20 : 10 weight ratio. N-Methyl-2-pyrrolidone was added to
3.1. Synthesis and characterization of catalysts
the mixture to make a slurry. The prepared slurry was uniformly
coated on carbon bre paper (CFP) and dried at 80 C for 12 h in
Exfoliated 2D MXene sheets exhibit physical exibility, elec-
trical conductivity, and hydrophilicity. These properties have
motivated material designs that integrate electrochemically
active phases for the engineering of bifunctional hybrid cata-
lysts. Meanwhile, cobalt phosphide/cobalt selenide, which has
excellent HER/OER activity, has emerged as a promising alter-
ꢂ
a vacuum oven. The catalysts were deposited on CFP with
ꢀ
2
a loading of 1.5 mg cm . Then, the OER and HER activities of
ꢀ
1
the as-prepared electrodes were measured in 1 mol L KOH
solution using the LSV method with 95% iR compensation at
ꢀ1
the scan rate of 5 mV s . A two-electrode system was made
using CoP/MXene as both anode and cathode (CoP/MXene//
CoP/MXene couple) and was used to conduct the overall water
21,24
native to noble metal catalysts.
In the present work, we
synergistically combined the unique properties of these two
phases in a well-dened architecture to create a hybrid having
highly efficient water-splitting properties. Typically, a mild
etchant (LiF + HCl) was used to prepare large MXene sheets
ꢀ1
splitting reaction in 1 mol L KOH from 0.0 to 2.0 V at the scan
ꢀ
1
rate of 5 mV s . The durability of the as-prepared electrodes
was measured using a chronoamperometric method on the
working electrode. The double-layer capacitances (Cdl) of the
catalysts were determined in the non-faradaic potential region
3 2
from the Ti AlC phase according to previously reported
42
procedures, whereby predominantly mono- or few-layered
MXene sheets were obtained. Atomic force microscopy (AFM)
showed that the MXene sheets thus obtained were at and
ꢀ1
at various scan rates of 50, 100, 150, 200, and 250 mV s
.
Electrochemical impedance spectroscopy (EIS) analyses were
carried out at 1.63 V over the frequency range of 0.1 Hz to 100
kHz. Potentials were expressed relative to the reversible
hydrogen electrode (RHE).
The overpotential (h) was calculated using the following
equation.

exible (Fig. 1a), indicative of the exfoliation of the Ti AlC2
3
phase into high-quality 2D MXene sheets. The sheets were of 5–
0 mm in average lateral size and 2–4 nm in thickness (Fig. 1a
1
inset). AFM images further evidenced the formation of a greater
number of sheets with the same thickness without any aggre-
gation; a few of them were folded, implying that they were
physically exible.
h (V) ¼ ERHE ꢀ 1.23 V for the OER
h (V) ¼ ERHE ꢀ 0 V for the HER
(1)
(2)
Fig. 1b is a HR-TEM image of a triple-layered MXene sheet,
demonstrating that the sheets formed were indeed 2D thin
layered sheets. XRD patterns of the Ti AlC and Ti C T MXene
3
2
3 2 x
The electrochemical surface area (ECSA) was calculated phases are shown in Fig. S1 (ESI†). The remarkably low angle
ꢂ
ꢂ
using the following formula:
shi of the (002) plane at 7.2 , compared to the 9.5 of the
Ti AlC phase, was due to the removal of Al from the Ti AlC
3
2
3
2
ECSA ¼ C /C ,
(3)
dl
s
phase, resulting in the formation of a layered structure with
enlarged interlayer spacing (consistent with TEM data) in the
ꢀ
2
s
taking C (specic capacitance) equal to 0.040 mF cm , as
Ti
3
C
2
T
x
phase. Furthermore, the absence of the TiO
2
peak
43
adopted from a previous study on Co-based OER catalysts. The
roughness factor (RF) was calculated using the following
relationship.
ꢂ
typically captured at 25 indicated that the preparation route
did not involve any oxidation. Thus, the results demonstrated
the formation of large, high-quality MXene sheets suitable for
7 8
synthesizing hybrids with a CoP or Co Se phase.
Accordingly, the schematic illustration given in Fig. 1c shows
a simple and straightforward strategy to generate hierarchically
RF ¼ ECSA/geometric area of the electrode
(4)
7 8
architectured CoP-arrays/MXene and Co Se -particulate/
MXene. First, a colloidal dispersion of MXene sheets is added
2.7. Materials characterization
2+
dropwise to a Co salt solution (with urea and NH F) under
4
2
+
The size and thickness of the MXene sheets were measured by mild stirring. The positively charged Co ions are immobilized
means of atomic force microscopy (AFM, Dimension 3100, Veeco, onto the negatively charged MXene sheets with terminal func-
Plainview, NY). Surface morphology and structural details were tional groups on the basal plane. This electrostatic assembly
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 27383–27393 | 27385

View Article Online
Journal of Materials Chemistry A
Paper
CoP-array/MXene, having densely populated 1D CoP nanorods
on its upper side, produced a tensile stress and thus a bending
deformation (Fig. 2a). Basically, this deformation is expected to
be desirable for enhancing the electrochemical activity of the
CoP/MXene by facilitating the exposure of the inner region of
CoP near the MXene support. The TEM image also showed the
hybrid structure of highly porously textured CoP nanorods on
an MXene sheet (Fig. 2b). The corresponding SAED pattern
showed intensied (011), (201), and (211) planes, correspond-
ing to the orthorhombic CoP phase (Fig. 2b inset). The high-
resolution TEM image showed distinct lattice fringes corre-
sponding to a d-space value of 0.281 nm, corresponding to the
(011) plane of crystalline CoP; this phase had obvious junctions
with the MXene sheet, revealing the formation of strong het-
erojunctions between hybridized phases (Fig. 2c). Hetero-
junction formation between CoP and MXene would be highly
benecial for facilitating electron transport. A SEM image of
Co Se /MXene prepared from the Co(OH)F/MXene precursor via
7
8
a selenization process showed particulate morphology (Fig. 2d).
The TEM image revealed the heterojunction formation between
Co Se and MXene (Fig. 2e) that was indeed benecial for
7 8
charge transport across the interface. The corresponding SAED
pattern showed (101), (102), and (002) planes, evidencing the
Co
showed lattice fringes corresponding to a d-spacing value of
.260 nm, corresponding to the (101) plane of crystalline Co Se8
7 8
Se phase (Fig. 2e, inset). The high-resolution TEM image
Fig. 1 (a) AFM images of MXene sheets (insets show the height profile
measured along the yellow dashed line), (b) HR-TEM image of MXene
sheet, showing its triple-layer nature, and (c) schematic synthetic
procedure used to prepare Co(OH)F/MXene, CoP/MXene, and
0
7
(Fig. 2f, inset). The SEM images for the Co(OH)F/MXene
precursor and other bare catalysts (Co(OH)F, CoP and Co Se )
7
8
Co
7
Se
8
/MXene hybrids.
are shown in Fig. S2 (ESI†) for morphological comparison.
Intriguingly, the N adsorption–desorption isotherms of
catalysts (Fig. S3, ESI†) suggest that the Brunauer–Emmett–
2
2
+
2
ꢀ1
process induces the nucleation of Co ions on MXene sheets Teller (BET) surface areas of CoP-arrays/MXene (33.0 m g
and subsequent hydrothermal growth.
)
2
ꢀ1
7 8
and Co Se -particulate/MXene (26.7 m g ) were higher than
those of the bare CoP (16.9 m g ) and Co Se (11.6 m g ),
7 8
2
+
2
ꢀ1
2
ꢀ1
During the growth, the MXene sheets with anchored Co
ions gradually sink to the bottom of the Teon reactor, resulting indicating that the 2D MXene support imparted the high
in a planar placement. Then, the growth starts perpendicularly surface for the hybrid catalysts. The Barrett–Joyner–Halenda
to form Co(OH)F rod arrays atop the upper surface of MXene (BJH) pore-size distribution curves of the catalysts (Fig. S3, ESI†)
sheets. On the other hand, due to steric restrictions, the growth show few sharp peaks ranging from 4 to 10 nm and broad peaks
of the Co(OH)F phase on the bottom side of MXene sheets in the range of 20–70 nm, pertaining to mesopores and mac-
occurs horizontally along the layered direction, rendering ropores of catalysts respectively.
a Co(OH)F nanorod-covered side. Therefore, the Co(OH)F-
7 8
The structural features of CoP/MXene and Co Se /MXene
decorated MXene layers are highly asymmetric in shape. A were also examined using XRD, conrming the phase formation
subsequent low-temperature phosphidation process yields CoP/ as shown in Fig. S4 (ESI†). Thus, FE-SEM, TEM, and BET data
MXene-arrays. Notably, the surface of CoP nanorods becomes indicated the following meritorious structural features per-
highly porously evolved owing to the dehydration of the taining to the successful synthesis of the CoP/MXene hybrid
precursor Co(OH)F/MXene (i.e., the release of water and gas compared to Co
7
Se
molecules) during the annealing. On the other hand, the CoP nanorods, which imparts high surface-area-to-volume ratio
selenization of the Co(OH)F/MXene precursor results in Co Se
and could expose more active surface sites for swi charge
8
/MXene: (i) dense and uniform growth of 1D
44
7
8
/
MXene with particulate morphology. The selenization reaction transport; (ii) a highly enlarged surface area that is greatly
in the reductive environment (N H ) led to obvious structural advantageous for ion adsorption; and (iii) mesopores generated
2
4
collapse and deformation of the Co(OH)F rod morphology into on the CoP nanorods, which are favourable for ion diffusion
agglomerated Co Se
particles that completely covered the and O /H gas release upon the OER/HER.
stacked MXene sheets.
7
8
2
2
The SEM image demonstrated a hierarchical structure con-
sisting of 1D CoP and 2D MXene that exhibited a bent
3.2. Electrocatalytic activity of designed catalysts
morphology, clearly showing that the asymmetric nature of The electrocatalytic OER performance of the as-prepared cata-
lysts was examined by using a standard three-electrode system
27386 | J. Mater. Chem. A, 2019, 7, 27383–27393
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 2 (a) SEM image of CoP-array/MXene, (b) TEM image and (b, inset) corresponding SAED pattern of CoP/MXene, (c) HR-TEM image of CoP/
MXene, displaying lattice fringes for CoP, (d) SEM image of Co Se -particulate/MXene, (e) TEM image of Co Se /MXene and (e, inset) corre-
sponding SAED pattern, and (f) HR-TEM image of Co Se /MXene, showing lattice fringes for Co Se
7
8
7
8
7
8
7
8
.
ꢀ1
in 1 mol L KOH solution. Linear sweep voltammetry (LSV) exhibited remarkably lowered overpotential (h ¼ 230 mV)
curves (95% iR-compensated) recorded for the catalysts at the compared to that of the state-of-the-art IrO
(h ¼ 307 mV),
scan rate of 5 mV s are given in Fig. 3a. The CoP/MXene reaching the current density of 10 mA cm (Fig. 3b), implying
2
ꢀ
1
ꢀ2
ꢀ1
Fig. 3 (a) OER polarization curves (iR-compensated) for various catalysts in 1 mol L KOH, CFP-carbon fibre paper. (b) Overpotential plots at 10
ꢀ2
mA cm for various catalysts, (c) corresponding Tafel plots, and (d) TOF graph for various catalysts, (e) double-layer capacitance measurements
ꢀ
1
(
plots of scan rate vs. current density) in 1 mol L KOH used to determine ECSA for various catalysts. The cathodic and anodic charging currents
were measured at 0.1 V vs. RHE; current measured in the non-faradaic region was due to capacitive charging. (f) Chronoamperometry stability
ꢀ2
7 8
tests at the steady state current density of 50 mA cm for CoP/MXene (at 1.53 V vs. RHE) and Co Se /MXene (at 1.58 V vs. RHE) for 24 h.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 27383–27393 | 27387

View Article Online
Journal of Materials Chemistry A
Paper
that the structural features of CoP and MXene presented in the a continuous 10 h period of operation in the OER (Fig. 3f).
hybrid synergistically enhanced the OER activity. Meanwhile, Overall, the above OER electrocatalytic activity data revealed the
Co Se /MXene also exhibited relatively lowered overpotential (h exceptional performances of MXene-supported catalysts
7
8
¼
291 mV) compared to IrO , indicative of the efficacy of MXene compared to that of bare catalysts. Therefore, the role of MXene
2
coupling in improving the OER performances.
However, the overpotentials required for bare CoP (h ¼ 280 using electrochemical impedance spectroscopy (EIS) analysis
mV), bare Co Se
(h ¼ 325 mV), and Co(OH)F/MXene precursor under identical OER experimental conditions. The Nyquist plot
h ¼ 320 mV) to achieve the current density of 10 mA cm were (Fig. S7a, ESI†) tted by the two-time constant serial (2TS)
comparatively higher than that for IrO . The Tafel plot is a vital model (Fig. S7b†) provides the solution resistance (R ) and
(in the hybrid catalyst) during OER catalysis is investigated
7
8
ꢀ
2
(
2
s
analysis in the evaluation of catalysts for water splitting reac- charge transfer resistance (R ) of the catalysts. In particular, the
ct
tions; the Tafel slope generally reects the kinetics of the elec- Rct value corresponds to the charge transfer resistance at the
trocatalytic reaction at the catalyst/solution interface. Tafel electrode/electrolyte interface and is correlated with the OER
plots recorded for the different catalysts are shown in Fig. 3c. kinetics of the catalysts. Evidently, as given in Table S3,† the
ꢀ
1
CoP/MXene (50 mV dec ) had the lowest slope value compared CoP/MXene (1.72 U) and Co
7
Se
trodes exhibited smaller Rct values compared to bare CoP (4.31
(90.2 mV dec ), indicative of the U) and Co Se (8.2 U) catalysts-modied electrodes, reecting
8
/MXene (3.35 U) loaded elec-
ꢀ
1
ꢀ1
to Co
(
7 8 7 8
Se /MXene (81.5 mV dec ), CoP (56.5 mV dec ), Co Se
97 mV dec ), and IrO
ꢀ
1
ꢀ1
2
7
8
remarkably fast and enhanced OER kinetics of the CoP/MXene. that the electron transport for the OER is faster on the MXene-
Notably, the CoP/MXene with lower overpotential and Tafel supported catalysts. This result indicates that the MXene
slope value outperformed IrO and would stand as the catalyst support reduces the potential barrier for driving the charge
2
showing the best OER performance under alkaline conditions transport across the catalysts during OER catalysis compared to
among the latest reported OER catalysts (shown in Table S1, that of bare catalysts. Similarly, recent reports revealed that the
ESI†). The results summarized in Table S1† also show that MXene support increases the charge transfer kinetics (with
MXene serves as an outstanding support material to CoP lower Rct values) of the host catalyst, facilitating the OER
32,33,35,45
compared to all other carbonaceous supports (e.g. graphene or catalysis.
carbon nanotubes), exhibiting exceptional OER performance in The HER performance was also evaluated to explore the
alkaline medium. The MXene-supported catalysts showed suitability of the catalysts for overall water splitting. Under
enhanced mass activity compared to bare catalysts (Fig. S5†), identical OER experimental conditions, 95% iR-compensated
ꢀ
1
indicating the benecial effect of MXene as a support material. HER polarization curves were measured in 1 mol L KOH
The intrinsic OER activity and reaction kinetics for the catalysts solution (Fig. 4a). It should be noted that the HER suffered from
+
were examined by calculating the turnover frequency (TOF), sluggish kinetics (due to insufficient availability of free H ions)
a signicant activity parameter in the water-splitting reaction. under alkaline conditions compared to acidic conditions. As
ꢀ1
The TOF of CoP/MXene was calculated to be 0.005 s , almost expected, the HER performance of the catalysts was relatively
ꢀ
1
three-fold that of bare CoP (0.0017 s ) at h ¼ 0.30 V (Fig. 3d). inferior to the benchmark system consisting of Pt/C, and no
ꢀ
1
Similarly, the Co Se /MXene (0.0013 s ) exhibited higher TOF considerable HER activity was observed for carbon paper and
7
8
ꢀ1
compared to bare Co Se (0.00053 s ), demonstrating that the bare MXene (Fig. 4a). On the other hand, as shown in Fig. 4b,
7
8
intrinsic activity of surface Co atoms was remarkably improved the HER performance of CoP/MXene was signicantly greater
by the support of 2D MXene sheets in the CoP/MXene hybrid. even while retaining the lowest overpotential (h10 mA cmꢀ2 ¼ 113
Furthermore, the intrinsic electrocatalytic activity of the cata- mV) than Co
lysts for the observed OER performance can be interpreted (h10 mA cmꢀ2 ¼ 125 mV), and bare Co
7
Se
8
/MXene (h10 mA cmꢀ2 ¼ 270 mV), bare CoP
7
Se
8
(h10 mA cmꢀ2 ¼ 296 mV).
using calculations of electrochemically active surface area Nevertheless, CoP/MXene had the lowest Tafel slope (57 mV
ECSA) and roughness factor (RF) (listed in Table S2, ESI†). The dec ) among the catalysts tested (Fig. 4c), indicating the best
ꢀ
1
(
cyclic voltammograms measured (for ECSA calculation) in the HER kinetics. This Tafel slope value suggests that the HER
non-faradaic region of the voltammogram at various scan rates proceeds through the Volmer–Heyrovsky mechanism under the
46
for the different catalysts are shown in Fig. S6 (ESI†). The ECSA, alkaline conditions.
proportional to the electrochemical double-layer capacitance In particular, the observed HER performance of CoP/MXene
dl), was investigated by plotting current density vs. various compared favourably to the recently reported best performing
scan rates using the cyclic voltammetry method (Fig. 3e). The HER catalysts under alkaline conditions (Table S3, ESI†). The
(C
ꢀ
2
CoP/MXene exhibited the highest Cdl value of 11 mF cm
,
stability observed in CA experiments (Fig. 4d) suggested that the
ꢀ
2
compared to 5.7 mF cm for CoP. Similarly, the Cdl value of CoP/MXene is stable under alkaline conditions, providing
Co Se /MXene (2.5 mF cm ) was higher than that of Co Se excellent HER performance during a continuous run of 10 h.
1.4 mF cm ). This increasing trend in Cdl demonstrated that The gradual increase of HER performance with time will be
ꢀ2
7
8
7
8
ꢀ
2
(
MXene hybridization improved the Cdl of the bare catalysts, explored in our forthcoming studies. However, as a result, the
reected in the higher ECSA and RF as listed in Table S2 (ESI†). observed effective HER/OER performances strongly suggested
Through its greater ECSA, the CoP/MXene could impart more that CoP/MXene can serve as a representative bifunctional
active sites for increased OER activity. Finally, the chro- catalyst in a two-electrode system for overall water splitting.
noamperometry (CA) measurements demonstrated the greater
Hence, CoP/MXene was employed as both the anode and
durability of CoP/MXene compared to Co
7
Se
8
/MXene during cathode material to implement a system that was tested for
27388 | J. Mater. Chem. A, 2019, 7, 27383–27393
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
charged MXene sheets enriched with functional groups increase
the proximal migration of water molecules toward the cathode,
facilitating the HER kinetics. Additionally, the electronegative P
+
atoms attract the H ions (H–OH) and reduce the hydrogen
47
adsorption energy, eventually facilitating the HER performance
at the cathode. Therefore, the integrated structural features of the
hybrid phases and the hierarchically developed morphology
enable CoP/MXene to serve as an excellent bifunctional catalyst
for overall water splitting in an alkaline medium. Overall, the
above electrocatalytic activity data revealed that the exceptional
performances of CoP/MXene could be attributed to the following
merits of the hybrid material: (i) the CoP nanorods grown on 2D
MXene sheets exposed more active sites for electrocatalysis; (ii)
MXene was an outstanding host matrix for the CoP, enhancing
the conductivity and electron transportation of the electrode; and
(iii) mesopores developed on the CoP facilitated the diffusion of
electrolyte ions and the subsequent release of O produced in the
2
OER. Furthermore, the electrocatalysis data also point out that
the anion components (P and Se) of the catalyst inuences the
activity, selectivity and stability. Notably, the Co–P is OER, HER
and bifunctionally active and stable, whereas Co–Se is selectively
OER active but showed inferior OER stability compared to Co–P.
Furthermore, Co–Se showed relatively lesser activity and stability
towards HER and bifunctional catalysis. Aer 10 h of HER
Fig. 4 (a) HER polarization curves (iR-compensated) for various catalysis, no considerable change in the surface chemistry of the
ꢀ
1
ꢀ2
catalysts in 1 mol L KOH, (b) overpotential plot at 10 mA cm for
various catalysts, (c) corresponding Tafel plots, (d) chro-
catalysts was observed, indicating that the catalysts are stable
during HER catalysis. The systematic investigation of catalysts
both before and aer the electrocatalysis had revealed the role of
ꢀ2
noamperometry stability tests at the current density of ꢀ50 mA cm
for CoP/MXene (ꢀ0.167 V vs. RHE) and Co Se /MXene (ꢀ0.4 V vs. RHE)
7
8
for 24 h, (e) two-electrode polarization curves for overall water split- anions in the activity and stability of catalysts as discussed in the
ꢀ1
ꢀ1
ting in 1 mol L KOH, scanned from 1.0 to 2.0 V at the rate of 5 mV s
following section.
without iR compensation, and (f) shows the stability of CoP/MXene as
a bifunctional catalyst at 1.69 V vs. RHE for 24 h, (inset, f) photograph of
the overall water splitting reaction performed using the bifunctional
3
.3. Catalytic mechanism: role of oxidized species of anions
ꢀ1
CoP/MXene catalyst in 1 mol L KOH.
in the OER activity and stability
48
S. Jin et al. reported that metal nitrides, phosphides, suldes
and selenides are oxidized to the corresponding metal oxides/
hydroxides, especially under the aqueous and strongly oxida-
ꢀ
1
overall water splitting in 1 mol L KOH solution. Fig. 4e shows
the LSV polarization curves representing the overall water
splitting performance of the catalyst couples. The CoP/MXene//
49
tive conditions of OER catalysis. O. Mabayoje et al. reported
the formation of NiO from Ni under OER conditions.
demonstrated the surface
ꢀ2
S
3 2
CoP/MXene couple reached 10 mA cm current density at
.56 V applied potential, which represents better performance
39–41
Furthermore, recent reports
1
ꢀ2
oxidation of Co-based catalysts to Co-oxyhydroxide under OER
conditions. Of note, these reports only showed the trans-
formation of the metal centre without investigating the role of
oxidized anion species during OER catalysis. Therefore, here,
we have systematically investigated the oxidation process of
anion components and their role in the activity and stability
with valid evidence. The structural and compositional features
of anion components (P and Se) in the catalysts, before and aer
the OER catalysis, were investigated using XPS (Fig. 5). The XPS
analysis of the catalysts has conrmed the surface oxidation of
than that of the IrO/C//Pt/C couple (1.62 V@10 mA cm ). This
remarkably lower cell voltage for overall water splitting can be
regarded as one of the best performing water splitting catalytic
performances and comparable to that of benchmark bifunc-
tional catalysts reported recently (Table S4, ESI†). CA analysis
evidenced the signicant stability of CoP/MXene as a bifunc-
tional catalyst at the steady-state current density of 50 mA cm
for 10 h (Fig. 4f inset). A photograph of the overall water split-
ting reaction is shown in Fig. 4f (inset).
As demonstrated in Fig. 4f, during the water splitting reaction,
Co atoms in the hybrid catalyst at the anode oxidize the OH ions,
2
evolving O . Eventually, the generated electrons shuttle through
the conductive MXene sheets and split water molecules at the
ꢀ2
ꢀ
both CoP and Co
fresh CoP/MXene sample showed Co–P peaks at 778.4 and
93.5 eV with an additional peak at 782.2 and 798.2 eV (due to
the inevitable surface oxidation of CoP), assigned to a thin layer
7 8
Se to Co-oxyhydroxide (Fig. 5a and d). The
7
cathode, thereby evolving H . The alkaline medium offers more
OH ions at the anode, where the porous CoP having exposed
active sites accelerates the OER kinetics. Likewise, the negatively
2
50
ꢀ
of Co–O. Similarly, the fresh Co Se /MXene sample exhibited
7 8
0
2+
peaks for Co (778.2 & 793.2 eV), Co (780.3 & 796.9 eV), and
3
+
Co (784.8 & 801.5 eV), assigned to Co0.88–Se bonding features.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 27383–27393 | 27389

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 5 Comparative XPS analyses for the fresh sample before OER catalysis (upper panels) and post-OER sample after 10 h of OER catalysis
lower panels). (a–c) Narrow-scan XPS spectra of Co 2p, O 1s, and P 2p for CoP/MXene. (d–f) Narrow-scan XPS spectra of Co 2p, O 1s, and Se 3d
for Co Se /MXene.
(
7
8
Importantly, the post-OER samples demonstrated the disap- oxyhydroxide on the surface of the catalyst with a slight deple-
pearance of Co–P and Co–Se peaks with the emergence of new tion during the prolonged OER catalysis.
peaks at 780.2, 781.5, 795.4, and 796.5 eV (Fig. 5a and d),
The given schematic representation (Fig. 6) based on XPS
51
attributed to Co-oxyhydroxide phase formation. The O 1s results demonstrates the surface oxidation of anion compo-
spectra of the fresh samples (CoP/MXene and Co Se /MXene) nents during OER catalysis and displays the subsequent
7
8
have shown peaks at ꢄ531.5 and ꢄ533 eV, assigned to migration of oxidized species (PO and SeO ) to the electrolyte
x
x
39,52
hydroxide species and adsorbed oxygen, respectively.
Intriguingly, as shown in Fig. 5b and e, the post-OER samples oxyhydroxide layer aer the OER catalysis on both samples. It
retained the hydroxide species peak at ꢄ531.5 eV with the further indicates the PO incorporation and SeO layer deposi-
evolution of a new peak at 529.3 eV, attributed to Co–O bond tion on the surface of Co–OOH/CoP/MXene and Co–OOH/
solution. This scheme illustrates the formation of a Co-
x
x
formation. This result conrmed the formation of oxyhydroxide Co
7
Se
8
/MXene catalysts, respectively. The HR-TEM images of
on the surface of the catalyst under OER conditions.
the post-OER samples (Fig. 7a and b) clearly manifested unique
By contrast, Fig. 5c and f demonstrate the drastic trans- lattice fringes corresponding to Co-oxyhydroxide on the surface
formation of anion components on the surface of the catalysts of the catalysts. It is worth mentioning here that the HR-TEM
under OER conditions. The P 2p spectra of the fresh sample observation explains the strong heterojunction formation
(Fig. 5c) showed the phosphide peak (Co–P) at 129.1 and between the MXene layer and catalyst components even aer
1
29.9 eV with an additional peak pertaining to the oxidized the prolonged OER catalysis, which would benecially impart
41
species of P at 134 eV. Conversely, the post-OER sample characteristics of swi electron transport and high conductivity
showed a single yet less intense peak at 133.5 eV corresponding for the hybrids. Furthermore, the phase evolution involving the
to PO , indicative of the oxidation of phosphide to PO that Co-oxyhydroxide layer and the oxidized species of anions on the
x
x
partially depleted (migrated to the electrolyte solution). This surface of catalysts was veried with TEM/EDX analysis. The
result revealed the presence of PO species on the surface of the compositions of the catalysts, before and aer OER catalysis,
x
catalyst along with Co-oxyhydroxide aer the prolonged OER are investigated using EDX spectra and the results are shown in
catalysis. Similarly, the Se 3d spectra of the fresh sample Fig. S8–S10.† Fig. 7c shows that the P content of the CoP/MXene
showed the selenide (Co–Se) peak at 53.8 and 54.8 eV with an (post-OER sample) decreased from 30.38 wt% to 2.30 wt%. The
additional peak due to the oxidized species of Se at 58.7 eV Se content is reduced from 45.38 wt% to 4.50 wt% for Co
7
Se
8
/
50
(Fig. 5f). The Se 3d spectra of the post-OER sample showed MXene (post-OER sample). This substantial decrease of anion
a single predominant peak at 60 eV, implying the complete components (P and Se) with the increase of oxygen content
surface oxidation of Se anion to SeO (Fig. 5f). It should be noted implies that the catalysts have been transformed/reconstructed
x
in this regard that the SeO phase strongly co-existed with Co- to oxide/oxyhydroxide-based catalysts during OER catalysis.
x
27390 | J. Mater. Chem. A, 2019, 7, 27383–27393
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 6 Schematic representation displaying the surface transformation of the metal centre and oxidation of anion components with subsequent
deposition.
Similarly, recent reports demonstrated the dissolution of P from
the metal phosphide catalysts (during OER/HER catalysis) with
the reconstruction of the surface to oxide/oxyhydroxide based
53–55
materials.
Furthermore, the leaching of oxidised species of
anion (P and Se) during OER catalysis is veried by ICP-OES
analysis. The contents of Co, P and Se present in the electro-
lyte (collected and analysed aer OER catalysis for 10 h) are
shown in Fig. 7d. The dissolution content of Co is relatively
lower than that of P and Se. Importantly, the substantial
amount of P and Se present in the electrolyte conrms the
dissolution during OER catalysis. The EDX and ICP-OES results
also conrmed the presence of P and Se (oxidised form) on the
catalyst surface aer the OER catalysis for 10 h.
Therefore, the question of whether these oxidized species of
anions would inuence the OER catalysis or not, was attempted
to be resolved with EIS analysis. The Nyquist plot provides the
s
solution resistance (R ) and charge transfer resistance (Rct) of the
catalysts (Fig. 7e). As given in Table S6,† the observed small Rs
values (1.9–3.2 U) for all the catalysts at the high frequency region
have reected the effective contact between the electrode (current
collector) and catalysts. As shown in Fig. 7e and Table S6,† the
fresh and post-OER samples of CoP/MXene exhibited smaller Rct
values at 1.72 and 3.46 U, respectively, indicating the robustness
of the catalysts. Of note, the lower Rct value for the post-OER
sample is obvious evidence for the non-detrimental effect of
the incorporated PO species on the conductivity. This result
x
agrees well with the OER graph of the post-OER sample showing
consistent activity (Fig. 7f) along with the stability (Fig. 3f). Thus,
the CoP/MXene catalyst surface enriched with PO and Co-
x
Fig. 7 Comparison between CoP/MXene and Co
after OER catalysis for 10 h. HR-TEM images of post-OER CoP/MXene
a) and post-OER Co Se /MXene (b). (c) P and Se contents of catalysts
7 8
Se /MXene catalysts
(
7
8
oxyhydroxide species could retain the catalytic activity even before and after OER catalysis for 10 h (analyzed using EDX spectra). (d)
aer the prolonged OER catalysis. On the other hand, the post- Dissolution concentration of Co, P and Se in the electrolyte after OER
catalysis for 10 h (analyzed using ICP-OES). (e) Nyquist plots for various
7 8
OER sample of Co Se /MXene showed a drastically increased
catalysts (measured at 1.63 V vs. RHE) in the frequency range from
.1 Hz to 100 KHz. The 2TS model was used for fitting the impedance
spectra of the catalysts (symbol-raw data; line-fitted curve). (f) OER
Rct value (16.2 U) compared to the fresh sample (3.35 U), indic-
0
ative of the adverse effect of SeO (smeared on the surface of the
x
ꢀ1
catalyst, which imparts passivation on the catalyst surface due to polarization curves (iR-compensated) for various catalysts in 1 mol L
the poor conductive nature of the Se–O bond) in terms of the KOH.
conductivity. This high resistance of the post-OER sample is thus
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 27383–27393 | 27391

View Article Online
Journal of Materials Chemistry A
Paper
reected as observed poor OER activity (Fig. 7f) and stability
3 H. F. Wang, C. Tang, B. Q. Li and Q. Zhang, Inorg. Chem.
Front., 2018, 5, 521–534.
(
Fig. 3f). Note that the PO -incorporated Co–OOH/CoP interface
x
(
for the post-OER sample) has shown the best OER activity with
4 S. Anantharaj, S. R. Ede, K. Sakthikumar, K. Karthick,
S. Mishra and S. Kundu, ACS Catal., 2016, 6, 8069–8097.
5 M. S. Faber and S. Jin, Energy Environ. Sci., 2014, 7, 3519–
3542.
a smaller R value as compared to the CoOOH/Co(OH)F interface
ct
(Fig. S11 and Table S6, ESI†), supporting the favourable role of
PO . However, the SeO -deposited Co–OOH/Co Se interface
x
x
7
8
rather exhibited poor OER activity with a remarkably higher Rct
value compared to all other samples, undoubtedly verifying the
detrimental effect of SeO on retaining the activity of the catalyst
x
surface.
6 X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, 44, 5148–5180.
7 H. Zhang, Z. Ma, J. Duan, H. Liu, G. Liu, T. Wang, K. Chang,
M. Li, L. Shi, X. Meng, K. Wu and J. Ye, ACS Nano, 2016, 10,
684–694.
8
X. Li, H. Wang, Z. Cui, Y. Li, S. Xin, J. Zhou, Y. Long, C. Jin
and J. B. Goodenough, Sci. Adv., 2019, 5, eaav6262.
4
. Conclusions
9
H. Zhang, J. Nai, L. Yu and X. W. Lou, Joule, 2017, 1, 77–107.
In this study, as-prepared large-sized exfoliated 2D-MXene 10 Y. Yan, B. Y. Xia, B. Zhao and X. Wang, J. Mater. Chem. A,
sheets, as a host matrix, facilitated the growth of 1D Co(OH)F 2016, 4, 17587–17603.
nanorods, and the subsequent phosphidation process resulted 11 N. Mahmood, Y. Yao, J. W. Zhang, L. Pan, X. Zhang and
in the formation of a highly mesoporous CoP-arrays/MXene J. J. Zou, Adv. Sci., 2018, 5, 1700464.
hybrid catalyst. Similarly, the selenization of the Co(OH)F/ 12 Y. Zheng, Y. Jiao, A. Vasileff and S. Z. Qiao, Angew. Chem., Int.
MXene precursor resulted in the formation of Co Se /MXene Ed., 2018, 57, 7568–7579.
with particulate morphology. The CoP/MXene hybrid imple- 13 L. Yang, G. Yu, X. Ai, W. Yan, H. Duan, W. Chen, X. Li,
7
8
mented with high ECSA and swi charge transfer kinetics
showed excellent bifunctional activity toward the out-
T. Wang, C. Zhang, X. Huang, J. S. Chen and X. Zou, Nat.
Commun., 2018, 9, 1–9.
performing overall water splitting performances in alkaline 14 M. B. Stevens, L. J. Enman, A. S. Batchellor, M. R. Cosby,
medium compared to other catalysts. The higher TOF of CoP/
MXene and Co Se /MXene compared to those of bare catalysts
A. E. Vise, C. D. M. Trang and S. W. Boettcher, Chem.
Mater., 2017, 29, 120–140.
7
8
conrmed that the intrinsic electrocatalytic activity of the 15 Y. Jia, L. Zhang, G. Gao, H. Chen, B. Wang, J. Zhou, M. T. Soo,
surface Co atoms was greatly enhanced by the heterojunction
formation with 2D MXene sheets. Most importantly, XPS anal-
M. Hong, X. Yan, G. Qian, J. Zou, A. Du and X. Yao, Adv.
Mater., 2017, 29, 1–8.
yses on post-OER samples of catalysts revealed the oxidation 16 X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang
process of anion components on the catalyst surface with and Z. Lin, Angew. Chem., Int. Ed., 2016, 55, 6290–6294.
subsequent depletion. Of note, the PO species desirably facil- 17 Y. Jin, H. Wang, J. Li, X. Yue, Y. Han, P. K. Shen and Y. Cui,
itated the activity and stability of catalysts, whereas the forma- Adv. Mater., 2016, 28, 3785–3790.
tion of SeO species rather deteriorated their activity and 18 J. Zhang, T. Wang, D. Pohl, B. Rellinghaus, R. Dong, S. Liu,
x
x
stability. Overall, this work successfully demonstrated a rational
design of a hierarchically structured and highly porous CoP/
X. Zhuang and X. Feng, Angew. Chem., Int. Ed., 2016, 55,
6702–6707.
MXene hybrid that would work as an outstanding bifunctional 19 A. Sivanantham, P. Ganesan and S. Shanmugam, Adv. Funct.
electrocatalyst with greatly improved catalytic efficiency and
Mater., 2016, 26, 4661–4672.
consistent stability.
20 C. Tang, N. Cheng, Z. Pu, W. Xing and X. Sun, Angew. Chem.,
Int. Ed., 2015, 54, 9351–9355.
2
1 A. Dutta and N. Pradhan, J. Phys. Chem. Lett., 2017, 8, 144–
52.
22 P. Zhang, X. F. Lu, J. Nai, S. Zang and X. W. Lou, Adv. Sci.,
019, 1900576, 1900576.
Conflicts of interest
1
There are no conicts to declare.
2
2
3 E. Hu, Y. Feng, J. Nai, D. Zhao, Y. Hu and X. W. Lou, Energy
Environ. Sci., 2018, 11, 872–880.
Acknowledgements
This work was supported by research grants of NRF 24 J. Masud, A. T. Swesi, W. P. R. Liyanage and M. Nath, ACS
2
2
017R1A2B2008132,
2018M3D1A1058624,
and
Appl. Mater. Interfaces, 2016, 8, 17292–17302.
014M3C1A3053035 funded by the National Research Founda- 25 H. Wang, Y. Wu, X. Yuan, G. Zeng, J. Zhou, X. Wang and
tion under the Ministry of Science and ICT, Korea.
J. W. Chew, Adv. Mater., 2018, 30, 1704561.
2
6 W. Yuan, L. Cheng, Y. An, H. Wu, N. Yao, X. Fan and X. Guo,
ACS Sustainable Chem. Eng., 2018, 6, 8976–8982.
7 A. D. Handoko, K. D. Fredrickson, B. Anasori, K. W. Convey,
L. R. Johnson, Y. Gogotsi, A. Vojvodic and Z. W. Seh, ACS
Appl. Energy Mater., 2018, 1, 173–180.
Notes and references
2
1
Z. W. She, J. Kibsgaard, C. F. Dickens, I. Chorkendorff,
J. K. Nørskov and T. F. Jaramillo, Science, 2017, 355,
eaad4998.
28 S. Li, P. Tuo, J. Xie, X. Zhang, J. Xu, J. Bao, B. Pan and Y. Xie,
2
Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev.,
Nano Energy, 2018, 47, 512–518.
2015, 44, 2060–2086.
27392 | J. Mater. Chem. A, 2019, 7, 27383–27393
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
Journal of Materials Chemistry A
29 N. H. Attanayake, S. C. Abeyweera, A. C. Thenuwara, 42 A. Lipatov, M. Alhabeb, M. R. Lukatskaya, A. Boson,
B. Anasori, Y. Gogotsi, Y. Sun and D. R. Strongin, J. Mater.
Chem. A, 2018, 6, 16882–16889.
Y. Gogotsi and A. Sinitskii, Adv. Electron. Mater., 2016, 2,
1600255.
3
3
3
3
3
3
0 X. Wu, Z. Wang, M. Yu, L. Xiu and J. Qiu, Adv. Mater., 2017, 43 T. Tang, W. J. Jiang, S. Niu, N. Liu, H. Luo, Q. Zhang,
2
9, 1607017.
W. Wen, Y. Y. Chen, L. B. Huang, F. Gao and J. S. Hu, Adv.
Funct. Mater., 2018, 28, 1–12.
44 Z. Pu, Q. Liu, P. Jiang, A. M. Asiri, A. Y. Obaid and X. Sun,
Chem. Mater., 2014, 26, 4326–4329.
45 C. Hao, Y. Wu, Y. An, B. Cui, J. Lin, X. Li, D. Wang, M. Jiang,
Z. Cheng and S. Hu, Materials Today Energy, 2019, 12, 453–
462.
1 Y. Zhang, H. Jiang, Y. Lin, H. Liu, Q. He, C. Wu, T. Duan and
L. Song, Adv. Mater. Interfaces, 2018, 5, 1–9.
2 T. Y. Ma, J. L. Cao, M. Jaroniec and S. Z. Qiao, Angew. Chem.,
Int. Ed., 2016, 55, 1138–1142.
3 M. Yu, S. Zhou, Z. Wang, J. Zhao and J. Qiu, Nano Energy,
2
018, 44, 181–190.
4 N. Hao, Y. Wei, J. Wang, Z. Wang, Z. Zhu, S. Zhao, M. Han 46 G. F. Chen, T. Y. Ma, Z. Q. Liu, N. Li, Y. Z. Su, K. Davey and
and X. Huang, RSC Adv., 2018, 8, 20576–20584. S. Z. Qiao, Adv. Funct. Mater., 2016, 26, 3314–3323.
5 L. Zhao, B. Dong, S. Li, L. Zhou, L. Lai, Z. Wang, S. Zhao, 47 Z. Fang, L. Peng, Y. Qian, X. Zhang, Y. Xie, J. J. Cha and G. Yu,
M. Han, K. Gao, M. Lu, X. Xie, B. Chen, Z. Liu, X. Wang, J. Am. Chem. Soc., 2018, 140, 5241–5247.
H. Zhang, H. Li, J. Liu, H. Zhang, X. Huang and W. Huang, 48 S. Jin, ACS Energy Lett., 2017, 2, 1937–1938.
ACS Nano, 2017, 11, 5800–5807.
49 O. Mabayoje, A. Shoola, B. R. Wygant and C. B. Mullins, ACS
3
3
3
3
6 C. F. Du, K. N. Dinh, Q. Liang, Y. Zheng, Y. Luo, J. Zhang and
Q. Yan, Adv. Energy Mater., 2018, 8, 1–9.
7 B. Cui, B. Hu, J. Liu, M. Wang, Y. Song, K. Tian, Z. Zhang and
L. He, ACS Appl. Mater. Interfaces, 2018, 10, 23858–23873.
8 L. Xiu, Z. Wang, M. Yu, X. Wu and J. Qiu, ACS Nano, 2018, 12,
Energy Lett., 2016, 1, 195–201.
50 T. Meng, J. Qin, S. Wang, D. Zhao, B. Mao and M. Cao, J.
Mater. Chem. A, 2017, 5, 7001–7014.
51 R. L. F. Jing Yang, H. Liu and W. N. Martens, J. Phys. Chem. C,
2010, 114, 111–119.
52 A. Sivanantham, P. Ganesan, L. Estevez, B. P. McGrail,
R. K. Motkuri and S. Shanmugam, Adv. Energy Mater.,
2018, 8, 1–13.
8017–8028.
9 L. Yan, B. Zhang, J. Zhu, S. Zhao, Y. Li, B. Zhang, J. Jiang,
X. Ji, H. Zhang and P. K. Shen, J. Mater. Chem. A, 2019, 7,
1
4271–14279.
0 J. Ryu, N. Jung, J. H. Jang, H. J. Kim and S. J. Yoo, ACS Catal.,
015, 5, 4066–4074.
53 H. Zhang, W. Zhou, J. Dong, X. F. Lu and X. W. Lou, Energy
Environ. Sci., 2019, 12, 3348–3355.
54 M. Miao, R. Hou, R. Qi, Y. Yan, L. Q. Gong, K. Qi, H. Liu and
B. Y. Xia, J. Mater. Chem. A, 2019, 7, 18925–18931.
55 L. Su, X. Cui, T. He, L. Zeng, H. Tian, Y. Song, K. Qi and
B. Y. Xia, Chem. Sci., 2019, 10, 2019–2024.
4
4
2
1 Y. P. Zhu, Y. P. Liu, T. Z. Ren and Z. Y. Yuan, Adv. Funct.
Mater., 2015, 25, 7337–7347.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. A, 2019, 7, 27383–27393 | 27393