MXene supported CoxAy (A ¼ OH, P, Se), first OER-converted.docx

Thisjournal is© TheRoyal Societyof Chemistry 2019 J. Mater. Chem. A, 2019,7, 27383–27393| 27383
¼
PAPER
View Article Onli
View Journal | ViewIssue
ne
Journal of
Materials Chemistry A
Cite this: J. Mater. Chem. A, 2019, 7,
27383
Receiv ed 26th September 2019
Accepted 9th Nov ember 2019
DOI: 10.1039/c9ta10664b
rsc.li/materials-a
MXene supported CoxAy (A OH, P, Se)
electrocatalysts for overall water splitting:
unveiling the role of anions in intrinsic activity and
stability†
N. Clament Sagaya Selvam, a Jooyoung Lee, b Gwan H. Choi,
a
Min Jun Oh,
a
ShiyuXu,
a
Byungkwon Lim b and Pil J. Yoo *a
The development of effi
cient and stable bifunctional catalysts that outperform noble metal catalysts is
a crucial task and an ongoing challenge for sustainable w ater electrolysis. In this w ork, large-size-
exfoliated MXene sheets render a flat and flexible platform for the decoration of Co(OH)F, CoP and
Co7Se8, allow ing 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 low er overpotential (h ¼ 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 betw een 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.
1. Introduction
Electrocatalytic water splitting is a promising research area and
an ongoing challenge for sustainable production of H2 as
a future fuel.1
The water splitting process comprises the
hydrogen evolution reaction (HER) and the oxy gen evolution
reaction (OER), which conventionally have been performed
using the state-of-the-art catalysts Pt and Ir/Ru, respectively.1,2
However, the noble metals in these catalysts are exp ensive and
scarce, representing a hurdle to large-scale application. This has
provoked the develop ment of alternative HER/OER catalysts
such as transition metal oxides, sul des, selenides, carbides,
nitrides, phosphides, and heteroatom-doped carbons.3–9
In
general, HER and OER catalysts respectively perform well in
acidic and basic media, due to the facilitated availability of H+
and OH—
ions, respectively.10
However, this also imp lies the
limitation that simultaneously integrating HER and OER cata-
lysts in the same electrolyte would yield inferior activity for
overall water splitting. Likewise, overallwater splitting in acidic
media suff ers from incomp atibility between species in electro-
chemical stability and the mitigated electrocatalytic activity
arising from decomposition of the catalyst under strong acidic
conditions.11–13
For these reasons, the develop ment of non-
noble metal bifunctional catalysts that perform well in alka-
line medium is crucial for realizing ideal p erformance in overall
water sp litting. Accordingly, transition metal-based catalysts
have been develop ed as bifunctional catalysts that could work in
alkaline media; these catalysts include FeNi layered double
hydroxides,14,15
NiCoO4,16
MoO2,17
MoS2/Ni3S2,18
NiCo2S4,19
a
n
d
NiSe.20
In particular, transition metal phosphides,21–23
a
n
d
selenides24
have been investigated as bifunctional catalysts over
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
(SKKU), Suwon 16419, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ta10664b
a wide range of pH. However, the relatively low surface area and
poor morp hological features of most of these catalysts, arising
mainly from self-agglomeration, haverestricted their HER/OER
kinetics and thus limit their ability to be scaled to large-scale,
high-performance applications.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

27384 | J. Mater. Chem. A, 2019, 7, 27383–27393 Thisjournal is© TheRoyal Societyof Chemistry 2019
View Article Online
Paper
Journal of Materials Chemistry A
T T
MXenes (Ti3C2Tx; Tx ¼ –O, –OH, –F), having rich terminal
groups, sup erior hy drophilicity, higher electrical conductivity
(4600 1100 S cm—1
), and high charge carrier mobility (2.6
0.7 cm—2
V—1
s—1
), have been exp lored for various electro-
catalytic applications.25
Of note, MXene as a pure phase has
been exp lored as an HER catalyst,26–28
and it has also been
coupled with M oS2 for enhanced HER activity.29,30
MXenes have
also been incorp orated with CNTs,31
g-C3N4,32
FeNi-LDH,33
NiFeCo-LDH,34
and metal–organic frameworks35
to boost their
OER activity. However, very few studies have focused on the
develop ment of MXene-based hybrids as bifunctional catalysts
for overall water splitting applications.36–38
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 suff ered from reduced
catalytic activity dueto the limited electrolyte accessibility of the
active sites. Moreover, well-exfoliated thin layers of MXene
sheets have not yet been exp loited 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.
Therefore, in this work, to address the identi ed shortcom-
ings of the morphological features of catalysts, we present
strategic approaches to realize remarkable bifunctional activity
with effi cient HER/OER kinetics, by (i) hy bridizing the metal
phosphide and selenide with a novel two-dimensional (2D)
conductive support matrix of MXene to boost the charge
transfer kinetics and (ii) architecting the hierarchical
morphology to create more surface area with exp osed active
sites available for strong electrolyte contact. For example, the
resulting hybrid system, integrating highly porous CoP nano-
rods and conductive MXene sheets, shows remarkable HER and
OER p erformance, and is imp lemented as an outstanding
bifunctional hybrid catalyst for overall water sp litting. Speci -
cally, the bifunctional water splitting p erformance of CoP/
MXene even surp asses that of IrO2/C//Pt/C couples under alka-
line conditions.
More importantly, besides the development of effi cient
2. Experimental section
2.1. Materials
All chemicals used were purchased from Sigma-Aldrich and
directly used without further puri cation.
2.2. Synthesis of Ti3C2Tx MXene sheets
Ti3C2Tx MXene nanosheets were synthesized according to
a previously reported procedure.42
Typically, 1 g of LiF was
added to 6 mol L—1
HCl (20 mL) and stirred for 10 min. One
gram of Ti3AlC2 powder was slowly added to this solution under
continuous stirring (500 rp m). Then, the temperature of the
reaction mixture was raised to 40 ○
C and maintained for 24 h.
The resulting reaction mixture containing Ti3C2Tx MXene (a er
comp lete removal of Al) was thoroughly washed with DI water
until it reached neutral pH ($6). The obtained M Xene was then
vacuum dried at 50 ○
C for 24 h. To exfoliate the obtained
multilay ered MXene into nanosheets, 0.2 g of M Xene was
disp ersed 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 rp m for 1 h)
and the supernatant containing monolayer or few -lay er MXene
nanosheets was collected for further use.
2.3. Synthesis of Co(OH)F/MXene and bare Co(OH)F
1 mmol of Co(NO3)2$6H2O, 8 mmol of NH4F, and 10 mmol
of CO(NH2)2 were dissolved in DI water (36 mL) and stirred
for 15 min. Colloids of MXene nanosheets were then added to
the mixture dropwise and gently stirred for 30 min. This
reaction mixture was poured into a Te on-lined stainless-steel
autoclave of 50 mL volume and reacted at 120 ○
C for 8 h. The
obtained product was thoroughly washed with DI water and
dried at 50 ○
C for 12 h in a vacuum oven. Bare Co(OH)F was
prepared using the same procedure except that no MXene was
added.
2.4. Synthesis of the CoP/MXene hybrid
The Co(OH)F/MXene precursor and NaH 2PO2 were taken in
a 1 : 5 mass ratio, kept separately in a porcelain boat, and
placed in a furnace. Ar gas was passed upstream from the
NaH PO and subsequently the temperature was increased to
bifunctional catalysts, we additionally investigate the structural 2 2
and comp ositional 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-
based catalysts to Co-oxyhydroxide under OER conditions,39–41
they only reported the transformation of the metal centre while
comp letely overlooking the role of oxidized anion species
during OER catalysis. Based on this understanding, here, we
have systematically investigated the oxidation reaction of anion
comp onents on the surface of catalysts a er OER catalysis. As
a result, we con rm the transformation of anion components (P
and Se) to their corresponding oxidized forms (POx and SeOx)
on the catalyst surface. We further verify the desirable role of
POx species in yet adverse eff ect of SeOx on the OER activity
and
stability.
300 ○
C at the rate of 5 ○
C min—1
and then maintained at this
temperature for 2 h. The resultant product was rinsed with DI
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 theprecursor.
The mass ratio of CoP to MXene in the hybrid was 0.9 : 0.1.
2.5. Synthesis of the Co7Se8/MXene hybrid
0.1 g of Co(OH)F/M Xene precursor and 0.3 g of Na2SeO3 were
dissolved in 50 mLof DI water. A er dropping 10 mLN2H4$H2O
into the solution under stirring, the mixture was transferred to
a Te on-lined stainless-steel autoclave of 100 mL volume and
heated at 180 ○
C for 8 h. The obtained product was thoroughly
washed with DI water and dried at 50 ○
C for 12 h in a vacuum
oven. Bare Co7Se8 was prepared using the same procedure using
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
Co(OH)F precursor. The mass ratio of Co7Se8 to M Xene in the
hybrid was 0.9 : 0.1.
2.6. Electrochemical measurements
The electrochemical performance of catalysts was evaluated in
1 mol L—1
KOH solution using a CH instrument (CHI600D). The
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
counter electrodes. Carbon bre paper (1 × 1 cm2
) was used asthe
working electrode. The catalyst, polyvinylidene uoride(binder),
and Ketjenblack (conductive agent) were mixed in a 70 : 20 :
10 weight ratio. N-Methyl-2-pyrrolidone was added to 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
a vacuum oven. The catalysts were deposited on CFP with
a loading of 1.5 mg cm—2
. Then, the OER and HER activities of
the as-prepared electrodes were measured in 1 mol L—1
KOH
solution using the LSV method with 95% iR compensation at
the scan rate of 5 mV s—1
. 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
splitting reaction in 1 mol L—1
KOH from 0.0 to 2.0 V at the scan
rate of 5 mV s—1
. 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
at various scan rates of 50, 100, 150, 200, and 250 mV s —1
.
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.
examined by means of scanning electron microscopy (FESEM-
JSM7600F, JEOL) and transmission electron microscopy
(HETEM-JEM2100F, JEOL). Phase formation was examined using
X-ray diﬀ raction (D8 Advance, Bruker) with Cu Ka radiation (l ¼
1.5406 Å). Surface chemistry and elemental composition were
analyzed by means of X-ray photoelectron spectroscopy (ESCA-
LAB250, Thermo XPS). N2 adsorption–desorption isotherms were
evaluated by means of the Brunauer–Emmett–Teller method
(Micrometrics ASAP 2020 instrument).
3. Results and discussion
3.1. Synthesis and characterization of catalysts
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-
native to noble metal catalysts.21,24
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 eﬃ cient water-splitting properties. Typically, a mild
etchant (LiF + HCl) was used to prepare large MXene sheets
from the Ti3AlC2 phase according to previously rep orted
procedures,42
whereby predominantly mono- or few-layered
MXene sheets were obtained. Atomic force microscopy (AFM)
showed that the MXene sheets thus obtained were at and
exible (Fig. 1a), indicative of the exfoliation of the Ti3AlC2
phase into high-quality 2D MXenesheets. Thesheets were of 5–
10 m m in average lateral size and 2–4 nm in thickness (Fig. 1a
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
h (V) ¼ ERHE — 1.23 V for the OER (1) physically exible.
Fig. 1b is a HR-TEM image of a triple-layered MXene sheet,
h (V) ¼ ERHE — 0 V for the HER (2)
The electrochemical surface area (ECSA) was calculated
using the following formula:
ECSA ¼ Cdl/Cs, (3)
taking Cs (sp eci c capacitance) equal to 0.040 mF cm—2
, as
adopted from a previous study on Co-based OER catalysts.43
The
roughness factor (RF) was calculated using the following
relationship.
RF ¼ ECSA/geometric area of the electrode (4)
2.7. Materials characterization
The size and thickness of the MXene sheets were measured by
meansof atomicforcemicroscopy (AFM ,Dimension3100, Veeco,
Plainview, NY). Surface morphology and structural details were
demonstrating that the sheets formed were indeed 2D thin
layered sheets. XRD patterns of theTi3AlC2 and Ti3C2Tx MXene
phases are shown in Fig. S1 (ESI†). The remarkably low angle
shi of the (002) plane at 7.2○
, comp ared to the 9.5○
of the
Ti3AlC2 phase, was due to the removal of Al from the Ti3AlC2
phase, resulting in the formation of a layered structure with
enlarged interlayer sp acing (consistent with TEM data) in the
Ti3C2Tx phase. Furthermore, the absence of the TiO 2 p eak
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
synthesizing hybrids with a CoP or Co7Se8 phase.
Accordingly, theschematic illustration given in Fig. 1cshows
a simp le and straightforward strategy to generate hierarchically
architectured CoP-arrays/MXene and Co7Se8-particulate/
MXene. First, a colloidal disp ersion of MXene sheets is added
dropwise to a Co2+
salt solution (with urea and NH4F) under
mild stirring. The positively charged Co2+
ions are immobilized
onto the negatively charged MXene sheets with terminal func-
tional groups on the basal plane. This electrostatic assembly
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
27386 | J. Mater. Chem. A, 2019,7, 27383–27393 Thisjournal is© TheRoyal Societyof Chemistry 2019
Fig. 1 (a) AFM images of MXene sheets (insets show the height proﬁle
CoP-array/MXene, having densely p opulated 1D CoP nanorods
on its upper side, produced a tensile stress and thus a bending
deformation (Fig. 2a). Basically, this deformation is exp ected to
be desirable for enhancing the electrochemical activity of the
CoP/M Xene by facilitating the exp osure 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
Co7Se8/MXene prepared from the Co(OH)F/MXene precursor via
a selenization process showed particulate morphology (Fig. 2d).
The TEM image revealed the heterojunction formation between
Co7Se8 and MXene (Fig. 2e) that was indeed bene cial for charge
transport across the interface. The corresponding SAED pattern
showed (101), (102), and (002) planes, evidencing the Co7Se8
phase (Fig. 2e, inset). The high-resolution TEM image showed
lattice fringes corresponding to a d-spacing value of
0.260 nm, corresponding to the (101) plane of crystalline Co Se
measured along the yellow dashed line), (b) HR-TEM image of MXene 7 8
sheet, show ing its triple-layer nature, and (c) schematic synthetic
procedure used to prepare Co(OH)F/MXene, CoP/MXene, and
Co7Se8/MXene hybrids.
process induces the nucleation of Co2+
ions on MXene sheets
and subsequent hydrothermal growth.
During the growth, the M Xene sheets with anchored Co2+
ions gradually sink to the bottom of the Te on reactor, resulting
in a planar placement. Then, the growth starts p erpendicularly
to form Co(OH)F rod arrays atop the upper surface of MXene
sheets. On the other hand, due to steric restrictions, the growth
of the Co(OH)F phase on the bottom side of MXene sheets
occurs horizontally along the lay ered direction, rendering
a Co(OH)F nanorod-covered side. Therefore, the Co(OH)F-
decorated MXene lay ers are highly asymmetric in shape. A
subsequent low-temp erature phosphidation process yields CoP/
MXene-arrays. Notably, the surface of CoP nanorods becomes
highly porously evolved owing to the dehydration of the
precursor Co(OH)F/MXene (i.e., the release of water and gas
molecules) during the annealing.44
On the other hand, the
selenization of the Co(OH)F/MXene precursor results in Co7Se8/
MXene with particulate morphology . The selenization reaction
in the reductive environment (N 2H4) led to obvious structural
collapse and deformation of the Co(OH)F rod morphology into
agglomerated Co7Se8 p articles that completely covered the
stacked MXene sheets.
The SEM image demonstrated a hierarchical structure con-
sisting of 1D CoP and 2D MXene that exhibited a bent
morphology, clearly showing that the asymmetric nature of
(Fig. 2f, inset). The SEM images for the Co(OH)F/MXene
precursor and other bare catalysts (Co(OH)F, CoP and Co7Se8)
are shown in Fig. S2 (ESI†) for morphological comparison.
Intriguingly, the N2 adsorption–desorption isotherms of
catalysts (Fig. S3, ESI†) suggest that the Brunauer–Emmett–
Teller (BET) surface areas of CoP-arrays/MXene (33.0 m2
g—1
)
and Co7Se8-particulate/MXene (26.7 m2
g—1
) were higher than
those of the bare CoP (16.9 m2
g—1
) and Co7Se8 (11.6 m2
g—1
),
indicating that the 2D MXene support imparted the high
surface for the hybrid catalysts. The Barrett–Joyner–Halenda
(BJH) pore-size distribution curves of the catalysts (Fig. S3, ESI†)
show few sharp peaks ranging from 4 to 10 nm and broad peaks
in the range of 20–70 nm, pertaining to mesopores and mac-
ropores of catalysts respectively.
The structural features of CoP/MXene and Co7Se8/MXene
were also examined using XRD, con rming the phase formation
as shown in Fig. S4 (ESI†). Thus, FE-SEM, TEM, and BET data
indicated the following meritorious structural features per-
taining to the successful synthesis of the CoP/MXene hybrid
compared to Co7Se8/MXene: (i) dense and uniform growth of 1D
CoP nanorods, which imparts high surface-area-to-volume ratio
and could expose more active surface sites for swi charge
transport; (ii) a highly enlarged surface area that is greatly
advantageous for ion adsorption; and (iii) mesopores generated
on the CoP nanorods, which are favourable for ion diﬀ usion
and O2/H2 gas release upon the OER/HER.
3.2. Electrocatalytic activity of designed catalysts
The electrocatalytic OER p erformance of the as -prepared cata-
lysts was examined by using a standard three-electrode system
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/202
0
4:32:05
AM.

View Article Online
Paper
¼
Fig. 2 (a) SEM image of CoP-array/MXene, (b) TEM image and (b, inset) corresponding SAEDpattern of CoP/MXene, (c) HR-TEM image of CoP/
MXene, displaying lattice fringes for CoP, (d) SEM image of Co7Se8-particulate/MXene, (e) TEM image of Co7Se8/MXene and (e, inset) corre-
sponding SAED pattern, and (f) HR-TEM image of Co7Se8/MXene, show ing lattice fringes for Co7Se8.
in 1 mol L—1
KOH solution. Linear sweep voltammetry (LSV)
curves (95% iR-compensated) recorded for the catalysts at the
scan rate of 5 mV s—1
are given in Fig. 3a. The CoP/MXene
exhibited remarkably lowered overpotential (h ¼ 230 mV)
comp ared to that of the state-of-the-art IrO2 (h 307 mV),
reaching the current density of 10 mA cm—2
(Fig. 3b), implying
Fig. 3 (a) OER polarization curves (iR-compensated) for various catalysts in 1 molL—1
KOH, CFP-carbon ﬁbre paper. (b) Overpotential plots at 10
mA cm—2
for various catalysts, (c) corresponding Tafel plots, and (d) TOF graph for various catalysts, (e) double-layer capacitance measurements
(plots of scan rate vs. current density) in 1 mol L—1
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 w as due to capacitive charging. (f) Chronoamperometry stability
tests at the steady state current density of 50 mA cm—2
for CoP/MXene (at 1.53 V vs. RHE) and Co7Se8/MXene (at 1.58 V vs. RHE) for 24 h.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
that the structural features of CoP and MXene presented in the
hybrid synergistically enhanced the OER activity. Meanwhile,
Co7Se8/MXene also exhibited relatively lowered overpotential (h
¼ 291 mV) compared to IrO2, indicative of the effi cacy of MXene
coupling in improving the OER performances.
However, the overpotentials required for bare CoP (h ¼ 280
mV),bareCo7Se8 (h ¼ 325 mV), and Co(OH)F/M Xeneprecursor
(h ¼ 320 mV) to achieve the current density of 10 mA cm—2
were
comp aratively higher than that for IrO2. The Tafel p lot is a vital
analysis in the evaluation of catalysts for water splitting reac-
tions; the Tafel slope generally re ects the kinetics of the elec-
trocatalytic reaction at the catalyst/solution interface. Tafel
plots recorded for the diff erent catalysts are shown in Fig. 3c.
CoP/M Xene (50 mV dec—1
) had thelowestslop evalue comp ared
to Co7Se8/MXene (81.5 mV dec—1
), CoP (56.5 mV dec—1
), Co7Se8
(97 mV dec—1
), and IrO2 (90.2 mV dec—1
), indicative of the
remarkably fast and enhanced OER kinetics of the CoP/MXene.
Notably, the CoP/MXene with lower overpotential and Tafel
slop e value outperformed IrO2 and would stand as the catalyst
showing the best OER performance under alkaline conditions
among the latest reported OER catalysts (shown in Table S1,
ESI†). The results summarized in Table S1† also show that
MXene serves as an outstanding support material to CoP
comp ared to all other carbonaceous supports (e.g. grap hene or
carbon nanotubes), exhibiting exceptional OER performance in
alkaline medium. The MXene-supported catalysts showed
enhanced mass activity comp ared to bare catalysts (Fig. S5†),
indicating the bene cial eff ect of MXene as a support material.
The intrinsic OER activity and reaction kinetics for the catalysts
were examined by calculating the turnover frequency (TOF),
a signi cant activity parameter in the water-splitting reaction.
TheTOF of CoP/MXene was calculated to be 0.005 s—1
, almost
three-fold that of bare CoP (0.0017 s—1
) at h ¼ 0.30 V (Fig. 3d).
Similarly,the Co7Se8/MXene(0.0013 s—1
) exhibited higher TOF
comp ared to bare Co7Se8 (0.00053 s—1
), demonstrating that the
intrinsic activity of surface Co atoms was remarkably improved
by the support of 2D M Xene sheets in the CoP/MXene hybrid.
Furthermore, the intrinsic electrocatalytic activity of the cata-
lysts for the observed OER p erformance can be interpreted
using calculations of electrochemically active surface area
(ECSA) and roughness factor (RF) (listed in TableS2, ESI†). The
cyclic voltammograms measured (for ECSA calculation) in the
non-faradaic region of the voltammogram at various scan rates
for the diff erent catalysts are shown in Fig. S6 (ESI†). The
ECSA,
proportional to the electrochemical double-layer capacitance
(Cdl), was investigated by p lotting current density vs. various
scan rates using the cy clic voltammetry method (Fig. 3e). The
CoP/M Xene exhibited the highest Cdl value of 11 mF cm—2
,
comp ared to 5.7 mF cm—2
for CoP. Similarly, the Cdl value of
Co7Se8/MXene (2.5 mF cm—2
) was higher than that of Co7Se8
(1.4 mF cm—2
). This increasing trend in Cdl demonstrated that
MXene hy bridization improved the Cdl of the bare catalysts,
re ected in the higher ECSA and RF as listed in Table S2 (ESI†).
Through its great er ECSA, the CoP/MXene could imp art more
active sites for increased OER activity. Finally, the chro-
noamp erometry (CA) measurements demonstrated the greater
durability of CoP/MXene compared to Co7Se8/MXene during
a continuous 10 h p eriod of op eration in the OER (Fig. 3f).
Overall, theaboveOER electrocatalytic activity datarevealed the
exceptional performances of MXene-supported catalysts
comp ared to that of bare catalysts. Therefore, the role of MXene
(in the hybrid catalyst) during OER catalysis is investigated
using electrochemical imp edance spectroscopy (EIS) analysis
under identical OER experimental conditions. The Nyquist plot
(Fig. S7a, ESI†) tted by the two-time constant serial (2TS)
model (Fig. S7b†) provides the solution resistance (Rs) and
charge transfer resistance (Rc t) of the catalysts. In p articular, the
Rct value corresponds to the charge transfer resistance at the
electrode/electrolyte interface and is correlated with the OER
kinetics of the catalysts. Evidently, as given in Table S3,† the
CoP/M Xene (1.72 U) and Co7Se8/M Xene (3.35 U) loaded elec-
trodes exhibited smaller Rct values compared to bare CoP (4.31
U) and Co7Se8 (8.2 U) catalysts-modi ed electrodes, re ecting
that the electron transport for the OER is faster on the MXene-
supported catalysts. This result indicates that the MXene
support reduces the potential barrier for driving the charge
transport across the catalysts during OER catalysis comp ared to
that of bare catalysts. Similarly, recent reports revealed that the
MXene support increases the charge transfer kinetics (with
lower Rct values) of the host catalyst, facilitating the OER
catalysis.32,33,35,45
The HER p erformance was also evaluated to exp lore the
suitability of the catalysts for overall water splitting. Under
identical OER exp erimental conditions, 95% iR-compensated
HER p olarization curves were measured in 1 mol L—1
KOH
solution (Fig. 4a). It should be noted that the HER suff ered
from sluggish kinetics (due to insuffi cient availability of free H+
ions) under alkaline conditions compared to acidic conditions.
As exp ected, the HER performance of the catalysts was
relatively inferior to the benchmark system consisting of Pt/C,
and no considerable HER activity was observed for carbon
paper and bare MXene (Fig. 4a). On the other hand, as shown
in Fig. 4b, the HER p erformance of CoP/MXene was
signi cantly greater even while retaining the lowest
overpotential (h10 mA c m —2 ¼ 113 mV) than Co7Se8/MXene (h10
mA cm —2 ¼ 270 mV), bare CoP (h10 mA c m —2 ¼ 125 mV), and
bare Co7Se8 (h10 mA cm —2 ¼ 296 mV). Nevertheless, CoP/M Xene
had the lowest Tafel slop e (57 mV dec—1
) among the catalysts
tested (Fig. 4c), indicating the best HER kinetics. This Tafel
slop e value suggests that the HER proceeds through the
Volmer–Heyrovsky mechanism under the alkaline
conditions.46
In particular, the observed HER p erformance of CoP/MXene
comp ared favourably to the recently reported best performing
HER catalysts under alkaline conditions (Table S3, ESI†). The
stability observed in CA exp eriments (Fig. 4d) suggested that the
CoP/M Xene is stable under alkaline conditions, providing
excellent HER performance during a continuous run of 10 h.
The gradual increase of HER performance with time will be
exp lored in our forthcoming studies. However, as a result, the
observed eff ective HER/OER performances strongly suggested
that CoP/MXene can serve as a representative bifunctional
catalyst in a two-electrode system for overall water splitting.
Hence, CoP/MXene was employed as both the anode and
cathode material to implement a system that was tested for
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
— —
Fig. 4 (a) HER polarization curves (iR-compensated) for various
catalysts in 1 mol L—1
KOH, (b) overpotential plot at 10 mA cm—2
for
various catalysts, (c) corresponding Tafel plots, (d) chro-
noamperometry stability tests at the current density of —50 mA cm—2
for CoP/MXene ( 0.167 V vs. RHE) and Co7Se8/MXene ( 0.4 V vs. RHE)
for 24 h, (e) tw o-electrode polarization curves for overall w ater split-
ting in 1 mol L—1
KOH, scanned from 1.0 to 2.0 V at the rate of 5 mV s—1
without iR compensation, and (f) shows the stability of CoP/MXene as
a bifunctional catalyst at 1.69 V vs. RHEfor 24 h, (inset, f) photographof
the overall w ater splitting reaction performed using the bifunctional
CoP/MXene catalyst in 1 mol L—1
KOH.
overallwater splitting in 1 mol L—1
KOH solution. Fig. 4eshows
the LSV polarization curves representing the overall water
splitting performance of the catalyst coup les. The CoP/MXene//
CoP/MXene couple reached 10 mA cm—2
current density at
1.56 V applied potential, which represents better p erformance
than that of the IrO/C//Pt/C coup le (1.62 V@10 mA cm—2
). 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—2
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 theanode oxidizethe OH—
ions,
evolving O2. Eventually, the generated electrons shuttle through
the conductive MXene sheets and split water molecules at the
cathode, thereby evolving H2. The alkaline medium oﬀ ers more
OH—
ions at the anode, where the porous CoP having exposed
active sites accelerates the OER kinetics. Likewise, the negatively
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
adsorption energy,47
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 exp osed 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 diﬀ usion of
electrolyte ions and the subsequent release of O2 produced in the
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–Seshowed relatively lesser activity and stability
towards HER and bifunctional catalysis. A er 10 h of HER
catalysis, no considerable change in the surface chemistry of the
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
anions in the activity and stability of catalysts as discussed in the
following section.
3.3. Catalytic mechanism: role of oxidized sp ecies of anions
in the OER activity and stability
S. Jin et al.48
reported that metal nitrides, phosphides, sul des
and selenides are oxidized to the corresponding metal oxides/
hydroxides, especially under the aqueous and strongly oxida-
tive conditions of OER catalysis. O. Mabayoje et al.49
reported
the formation of NiO from Ni3S2 under OER conditions.
Furthermore, recent reports39–41
demonstrated the surface
oxidation of Co-based catalysts to Co-oxy hydroxide 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 comp onents and their role in the activity and stability
with valid evidence. The structural and comp ositional features
of anion comp onents (P and Se) in the catalysts, before and a er
theOER catalysis, wereinvestigated using XPS(Fig. 5). TheXPS
analysis of the catalysts has con rmed the surface oxidation of
both CoP and Co7Se8 to Co-oxy hydroxide (Fig. 5a and d). The
fresh CoP/MXene sample showed Co–P peaks at 778.4 and
793.5 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
of Co–O.50
Similarly, the fresh Co7Se8/MXene sample exhibited
peaks for Co0
(778.2 & 793.2 eV), Co2+
(780.3 & 796.9 eV), and
Co3+
(784.8 & 801.5 eV), assigned to Co0.88–Se bonding features.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
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
(low er panels). (a–c) Narrow-scan XPS spectra of Co 2p, O1s, and P2p for CoP/MXene. (d–f) Narrow-scan XPS spectra of Co 2p, O1s, and Se 3d
for Co7Se8/MXene.
Importantly, the post-OER samp les demonstrated the disap-
pearance of Co–P and Co–Se p eaks with the emergence of new
peaks at 780.2, 781.5, 795.4, and 796.5 eV (Fig. 5a and d),
attributed to Co-oxyhy droxide phase formation.51
The O 1s
spectra of the fresh samp les (CoP/MXene and Co7Se8/MXene)
have shown p eaks at ~531.5 and ~533 eV, assigned to
hydroxide species and adsorbed oxy gen, respectively.39,52
Intriguingly, as shown in Fig. 5b and e, the post-OER samp les
retained the hydroxide sp ecies peak at ~531.5 eV with the
evolution of a new p eak at 529.3 eV, attributed to Co–O bond
formation. This result con rmed the formation of oxyhydroxide
on the surface of the catalyst under OER conditions.
By contrast, Fig. 5c and f demonstrate the drastic trans-
formation of anion comp onents on the surface of the catalysts
under OER conditions. The P 2p spectra of the fresh sample
(Fig. 5c) showed the phosphide peak (Co–P) at 129.1 and
129.9 eV with an additional peak p ertaining to the oxidized
species of P at 134 eV.41
Conversely, the post-OER sample
showed a single yet less intensep eak at 133.5 eV corresponding
to POx, indicative of the oxidation of phosp hide to POx that
partially depleted (migrated to the electrolyte solution). This
result revealed the presence of POx sp ecies on the surface of the
catalyst along with Co-oxyhy droxide a er the prolonged OER
catalysis. Similarly, the Se 3d sp ectra of the fresh sample
showed the selenide (Co–Se) p eak at 53.8 and 54.8 eV with an
additional peak due to the oxidized species of Se at 58.7 eV
(Fig. 5f).50
The Se 3d spectra of the post-OER sample showed
a single predominant peak at 60 eV, imp lying the comp lete
surface oxidation of Se anion to SeOx (Fig. 5f). It should be noted
in this regard that the SeOx phase strongly co-existed with Co-
oxyhy droxide on the surface of the catalyst with a slight deple-
tion during the prolonged OER catalysis.
The given schematic representation (Fig. 6) based on XPS
results demonstrates the surface oxidation of anion compo-
nents during OER catalysis and displays the subsequent
migration of oxidized sp ecies (POx and SeOx ) to the electrolyte
solution. This scheme illustrates the formation of a Co-
oxyhy droxide layer a er the OER catalysis on both samp les. It
further indicates the POx incorporation and SeOx layer deposi-
tion on the surface of Co–OOH/CoP/M Xene and Co–OOH/
Co7Se8/MXene catalysts, respectively. The HR-TEM images of
the post-OER samples (Fig. 7a and b) clearly manifested unique
lattice fringes corresponding to Co-oxy hydroxide on the surface
of the catalysts. It is worth mentioning here that the HR-TEM
observation explains the strong heterojunction formation
between the M Xene lay er and catalyst components even a er
the prolonged OER catalysis, which would bene cially impart
characteristics of swi electron transport and high conductivity
for the hybrids. Furthermore, the phase evolution involving the
Co-oxyhydroxide layer and the oxidized sp ecies of anions on the
surface of catalysts was veri ed with TEM /EDX analysis. The
comp ositions of the catalysts, before and a er OER catalysis,
are investigated using EDX spectra and the results are shown in
Fig. S8–S10.† Fig. 7cshows thattheP content ofthe CoP/MXene
(post-OER sample) decreased from 30.38 wt% to 2.30 wt%. The
Se content is reduced from 45.38 wt% to 4.50 wt% for Co7Se8/
MXene (post-OER samp le). This substantial decrease of anion
comp onents (P and Se) with the increase of oxy gen content
implies that the catalysts have been transformed/reconstructed
to oxide/oxyhydroxide-based catalysts during OER catalysis.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
Fig. 6 Schematic representation displaying the surface transformation of the metal centre and oxidation of anion components w ith 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
materials.53–55
Furthermore, the leaching of oxidised species o
f
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
solution resistance (Rs) 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 allthe catalysts at thehigh frequency region
have re ected the eff ective 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
samp le is obvious evidence for the non-detrimental eff ect of
the incorp orated POx sp ecies on the conductivity. This result
agrees wellwith the OER graph of thepost-OER samp leshowing
consistent activity (Fig. 7f) alongwith thestability (Fig. 3f).Thus,
the CoP/MXene catalyst surface enriched with POx and Co-
oxyhy droxide species could retain the catalytic activity even
a er the prolonged OER catalysis. On the other hand, the post-
OER sample of Co7Se8/MXene showed a drastically increased
Rct value (16.2 U) comp ared to the fresh sample (3.35 U), indic-
ative of the adverse eff ect of SeOx (smeared on the surface of
the catalyst, which imp arts passivation on the catalyst surface
due to the p oor conductive nature of the Se–O bond) in terms
of the conductivity. This high resistance of thepost-OER samp le
is thus
Fig. 7 Comparison between CoP/MXene and Co7Se8/MXene catalysts
after OER catalysis for 10 h. HR-TEM images of post-OERCoP/MXene
(a) and post-OER Co7Se8/MXene (b). (c) Pand Se contents of catalysts
before and after OERcatalysisfor 10 h (analyzed using EDX spectra).(d)
Dissolution concentration of Co, P and Se in the electrolyte after OER
catalysisfor 10 h (analyzed using ICP-OES). (e) Nyquist plotsforvarious
catalysts (measured at 1.63 V vs. RHE) in the frequency range from
0.1 Hz to 100 KHz. The 2TS model w as used for fitting the impedance
spectra of the catalysts (symbol-raw data; line-fitted curve). (f) OER
polarization curves (iR-compensated) for various catalysts in 1 mol L—1
KOH.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
re ected as observed poor OER activity (Fig. 7f) and stability
(Fig. 3f). Notethat thePOx-incorporated Co–OOH/CoP interface
(for the post-OER sample) has shown the best OER activity with
asmaller Rc t valueas comparedtotheCoOOH/Co(OH)F interface
(Fig. S11 and Table S6, ESI†), supporting the favourable role of
POx. However, the SeOx-deposited Co–OOH/Co7Se8 interface
rather exhibited poor OER activity with a remarkably higher Rct
value compared to all other samples, undoubtedly verifying the
detrimental eff ect of SeOx on retaining the activity of the
catalyst
surface.
4. Conclusions
In this study, as-prepared large-sized exfoliated 2D-M Xene
sheets, as a host matrix, facilitated the growth of 1D Co(OH)F
nanorods, and the subsequent p hosphidation process resulted
in the formation of a highly mesoporous CoP-arrays/M Xene
hybrid catalyst. Similarly, the selenization of the Co(OH)F/
MXene precursor resulted in the formation of Co7Se8/M Xene
with particulate morphology . The CoP/MXene hybrid imple-
mented with high ECSA and swi charge transfer kinetics
showed excellent bifunctional activity toward the out-
performing overall water splitting p erformances in alkaline
medium compared to other catalysts. The higher TOF of CoP/
MXene and Co7Se8/MXene comp ared to those of bare catalysts
con rmed that the intrinsic electrocatalytic activity of the
surface Co atoms was greatly enhanced by the heterojunction
formation with 2D MXene sheets. Most importantly, XPS anal-
yses on post-OER samples of catalysts revealed the oxidation
process of anion components on the catalyst surface with
subsequent depletion. Of note, the POx species desirably facil-
itated the activity and stability of catalysts, whereas the forma-
tion of SeOx sp ecies rather deteriorated their activity and
stability. Overall, this work successfully demonstrated a rational
design of a hierarchically structured and highly porous CoP/
MXene hybrid that would work as an outstanding bifunctional
electrocatalyst with greatly improved catalytic effi ciency and
consistent stability.
Conflicts of interest
There are no con icts to declare.
Acknowledgements
This work was supported by research grants of NRF
2017R1A2B2008132, 2018M3D1A1058624, and
2014M 3C1A3053035 funded by the National Research Founda-
tion under the Ministry of Science and ICT, Korea.
Notes and references
1 Z. W. She, J. Kibsgaard, C. F. Dickens, I. Chorkendorff ,
J. K. Nørskov and T. F. Jaramillo, Science, 2017, 355,
eaad4998.
2 Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev.,
2015, 44, 2060–2086.
3 H. F. Wang, C. Tang, B. Q. Li and Q. Zhang, Inorg. Chem.
Front., 2018, 5, 521–534.
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.
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.
9 H. Zhang, J. Nai, L. Yu and X. W. Lou, Joule, 2017, 1, 77–107.
10 Y. Yan, B. Y. Xia, B. Zhao and X. Wang, J. Mater. Chem. A,
2016, 4, 17587–17603.
11 N. Mahmood, Y. Yao, J. W. Zhang, L. Pan, X. Zhang and
J. J. Zou, Adv. Sci., 2018, 5, 1700464.
12 Y. Zheng, Y. Jiao, A. Vasileff and S. Z. Qiao, Angew. Chem., Int.
Ed., 2018, 57, 7568–7579.
13 L. Yang, G. Yu, X. Ai, W. Yan, H. Duan, W. Chen, X. Li,
T. Wang, C. Zhang, X. Huang, J. S. Chen and X. Zou, Nat.
Commun., 2018, 9, 1–9.
14 M. B. Stevens, L. J. Enman, A. S. Batchellor, M. R. Cosby,
A. E. Vise, C. D. M. Trang and S. W. Boettcher, Chem.
Mater., 2017, 29, 120–140.
15 Y. Jia, L. Zhang, G. Gao, H. Chen, B. Wang, J. Zhou, M. T. Soo,
M. Hong, X. Yan, G. Qian, J. Zou, A. Du and X. Yao, Adv.
Mater., 2017, 29, 1–8.
16 X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang
and Z. Lin, Angew. Chem., Int. Ed., 2016, 55, 6290–6294.
17 Y. Jin, H. Wang, J. Li, X. Yue, Y. Han, P. K. Shen and Y. Cui,
Adv. Mater., 2016, 28, 3785–3790.
18 J. Zhang, T. Wang, D. Pohl, B. Rellinghaus, R. Dong, S. Liu,
X. Zhuang and X. Feng, Angew. Chem., Int. Ed., 2016, 55,
6702–6707.
19 A. Sivanantham, P. Ganesan and S. Shanmugam, Adv. Funct.
Mater., 2016, 26, 4661–4672.
20 C. Tang, N. Cheng, Z. Pu, W. Xing and X. Sun, Angew. Chem.,
Int. Ed., 2015, 54, 9351–9355.
21 A. Dutta and N. Pradhan, J. Phys. Chem. Lett., 2017, 8, 144–
152.
22 P. Zhang, X. F. Lu, J. Nai, S. Zang and X. W. Lou, Adv. Sci.,
2019, 1900576, 1900576.
23 E. Hu, Y. Feng, J. Nai, D. Zhao, Y. Hu and X. W. Lou, Energy
Environ. Sci., 2018, 11, 872–880.
24 J. Masud, A. T. Swesi, W. P. R. Liyanage and M. Nath, ACS
Appl. Mater. Interfaces, 2016, 8, 17292–17302.
25 H. Wang, Y. Wu, X. Yuan, G. Zeng, J. Zhou, X. Wang and
J. W. Chew, Adv. Mater., 2018, 30, 1704561.
26 W. Yuan, L. Cheng, Y. An, H. Wu, N. Yao, X. Fan and X. Guo,
ACS Sustainable Chem. Eng., 2018, 6, 8976–8982.
27 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.
28 S. Li, P. Tuo, J. Xie, X. Zhang, J. Xu, J. Bao, B. Pan and Y. Xie,
Nano Energy, 2018, 47, 512–518.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

View Article Online
Paper
29 N. H. Attanayake, S. C. Abeyweera, A. C. Thenuwara,
B. Anasori, Y. Gogotsi, Y. Sun and D. R. Strongin, J. Mater.
Chem. A, 2018, 6, 16882–16889.
30 X. Wu, Z. Wang, M. Yu, L. Xiu and J. Qiu, Adv. Mater., 2017,
29, 1607017.
31 Y. Zhang, H. Jiang, Y. Lin, H. Liu, Q. He, C. Wu, T. Duan and
L. Song, Adv. Mater. Interfaces, 2018, 5, 1–9.
32 T. Y. Ma, J. L. Cao, M. Jaroniec and S. Z. Qiao, Angew. Chem.,
Int. Ed., 2016, 55, 1138–1142.
33 M. Yu, S. Zhou, Z. Wang, J. Zhao and J. Qiu, Nano Energy,
2018, 44, 181–190.
34 N. Hao, Y. Wei, J. Wang, Z. Wang, Z. Zhu, S. Zhao, M. Han
and X. Huang, RSC Adv., 2018, 8, 20576–20584.
35 L. Zhao, B. Dong, S. Li, L. Zhou, L. Lai, Z. Wang, S. Zhao,
M. Han, K. Gao, M. Lu, X. Xie, B. Chen, Z. Liu, X. Wang,
H. Zhang, H. Li, J. Liu, H. Zhang, X. Huang and W. Huang,
ACS Nano, 2017, 11, 5800–5807.
36 C. F. Du, K. N. Dinh, Q. Liang, Y. Zheng, Y. Luo, J. Zhang and
Q. Yan, Adv. Energy Mater., 2018, 8, 1–9.
37 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.
38 L. Xiu, Z. Wang, M. Yu, X. Wu and J. Qiu, ACS Nano, 2018, 12,
8017–8028.
39 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,
14271–14279.
40 J. Ryu, N. Jung, J. H. Jang, H. J. Kim and S. J. Yoo, ACS Catal.,
2015, 5, 4066–4074.
41 Y. P. Zhu, Y. P. Liu, T. Z. Ren and Z. Y. Yuan, Adv. Funct.
Mater., 2015, 25, 7337–7347.
42 A. Lipatov, M. Alhabeb, M. R. Lukatskaya, A. Boson,
Y. Gogotsi and A. Sinitskii, Adv. Electron. Mater., 2016, 2,
1600255.
43 T. Tang, W. J. Jiang, S. Niu, N. Liu, H. Luo, Q. Zhang,
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.
46 G. F. Chen, T. Y. Ma, Z. Q. Liu, N. Li, Y. Z. Su, K. Davey and
S. Z. Qiao, Adv. Funct. Mater., 2016, 26, 3314–3323.
47 Z. Fang, L. Peng, Y. Qian, X. Zhang, Y. Xie, J. J. Cha and G. Yu,
J. Am. Chem. Soc., 2018, 140, 5241–5247.
48 S. Jin, ACS Energy Lett., 2017, 2, 1937–1938.
49 O. Mabayoje, A. Shoola, B. R. Wygant and C. B. Mullins, ACS
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.
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.
Published
on
11
November
2019.
Downloaded
by
University
of
Zurich
on
1/3/2020
4:32:05
AM.

MXene supported CoxAy (A ¼ OH, P, Se), first OER-converted.docx

Recommended

Recommended

More Related Content

Similar to MXene supported CoxAy (A ¼ OH, P, Se), first OER-converted.docx

Similar to MXene supported CoxAy (A ¼ OH, P, Se), first OER-converted.docx (20)

Recently uploaded

Recently uploaded (20)

MXene supported CoxAy (A ¼ OH, P, Se), first OER-converted.docx