V2C Mxene synergatically coupling

1. Applied Catalysis B: Env ironmental 297 (2021) 12047 4
V2C MXene synergistically coupling FeNi LDH nanosheets for boosting
oXygen evolution reaction
Yafeng Chen a,b
, Heliang Yao b
, Fantao Kong b
, Han Tianb
, Ge Mengb
, Shuize Wanga
,
Xinping Mao a
, Xiangzhi Cui b,c,
**, Xinmei Hou a,
*, Jianlin Shi b
a
Beijing Advanced Innovation Center for Materials Genome Engineering, Collaborative Innovation Center of Steel Technology, University of Science and Technology
Beijing, Beijing, 100083, China
b
The State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai,
200050, China
c
School of Chemistry and Materials Science, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, 310021, China
A R T I C L E I N F O
Keywords:
MXene
Layered double hydroXide
Nanohybrids
OER
Synergistic effect
A B S T R A C T
OXy gen ev olution reaction (OER ) is a piv otal el ectrochemical reaction process for many renew able energy
technol ogies. Due to the sluggish OER kinetics, searching for efficient l ow -cost non-precious metal cataly sts is
one of the crucial but very challenging steps. Herein, V2C MXene sy nergistically coupled w ith hypophosphite-
intercalated FeNi (oXy )hy droXide (H2PO—
2 /FeNi- LD H - V 2C) el ectrocatalyst is sy nthesized. The H2PO—
2 /FeNi-
LDH-V2C ex hibits excellent OER performance with an overpotential of 250 mV (η10) and small Tafel sl ope of 46.5
mV dec—1
i n 1.0 M KOH el ectrolyte, and excellent rechargeabl e Zn-air battery performance with superior open
circuit potential (1.42 eV), pow er density (137 mW cm—2
) and w el l durability. The strong interaction and
el ectronic coupling with prominent charge-transfer between FeNi -LDHs and V2C MXene endow the composite
significant OER performance and structural stability , and the adsorption/desorption bal ance for the OER reaction
pathway , eventually promoting the intrinsic activity. This w ork demonstrates the great promise of MXene -based
nanohy brids as adv anced el ectrocatalysts for renewable energy applications.
1. Introduction
The oXygen evolution reaction (OER) is a pivotal electrochemical
reaction process for water splitting, fuel cells, andmetal-air batteries to
realize energy storage and conversion [1–3]. However, its sluggish ki-
netics caused by the complex multielectron reaction process requires
highly efficient electrocataly sts to overcome the large overpotential for
an enough current density of OER [4–6]. Up to now, the most widely
used oXygen electrocatalystsarenoble metal-based materials(i.e., RuO2
or IrO2) [7–9]. Nevertheless, the limited earth reserves, high cost and
inferior long-time stability hinder their large-scale applications [8,10].
Recently, earth-abundant transition metal-based compounds such as
oXides, spinel, sulfides, (oXy)hydroXidesandlayered doublehydroXides
have shown a great promise for efficient OER in alkaline electrolytes
thanks to their high catalytic activity and low cost [11–17]. Among
them, the layered double hydroXides(LDH) with highly adjustable
compositions and intercalated anions (e.g., CO3
2—
, NO—
3 , Cl—
) have been
considered as a promising oXygen electrocatalyst in alkaline electrolyte
due to their superior OER performance and lower costs [15,16,18–22].
However, the performance of LDHsis still far from satisfactory because
of its poor carriermobilities and aggregation during the process offilm
forming, which hinder charge separation and transfer [23–25]. There-
fore, some carbon supports such as carbon nanocubes, reduced graphene
oXides (rGO) and carbon paper have been combined with LDHs to
improve its electrical conductivity and agglomeration resistance [26–
29]. Unfortunately, these conductive materials usually need to be
chemical functionalization to enhance the surface hydrophilicity and
reactivity at the cost of electrical conductivity and structural integrity,
which limits the full exploitation of LDH-based catalysts for electro-
catalytic applications [30].
Recently, two-dimensional (2D) transition metal carbides/carboni-
trides, named MXene, have been w idely u sed in electrocatalytic
* Corresponding author.
** Corresponding author at:The State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese
Academy of Sciences, Shanghai, 200050, China.
E-mail addresses: cui Xz@mail.sic.ac.cn (X. Cui), houXinmeiustb@ustb.edu.cn (X. Hou).
https://doi.org/10.1016/j.apcatb.2021.12047 4
Receiv ed 28 March 2021; Receiv ed i n revised form 1 June 2021;Accepted 20 June 2021
Av ailable online 23 June 2021
0926-337 3/© 2021 El sev ier B.V. Al l rights reserved.
Contents lists av ailable at ScienceDirect
Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb

2. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
2
=
+
=
+
applications due to its high electrical conductivity, hydrophilicity, and
reducibility [31–33]. The MXene can be obtained by selectively etching
of A from the ternary carbides/carbonitrideswiththe general formulaof
Mn+1AXn, where M is an early transition metal, A isa group IIIA or ⅣA
element, X is C and/or N element and n 1 , 2, or 3 [34,35].The
resultantMXene with many surface functional groups ownwell com-
bination of hydrophilic surface, high metallic conductivity associated
with high electron density of states near the Fermi level, and rich surface
chemistries, whichcan fulfill the demand of optimal electronic proper-
ties and interfacial junction for electrochemical reactions [18,36,37].
Among the large family of MXene, V 2C is the lightest one and has
received broad attention because of itshigh electrical conductivity,
excellent mechanical properties and ample reactivity. In addition, the
V 2C MXene is also of great practical interest due to its synthesizability
and applicability for batteries [38]. Comparingto the Ti3C2 MXene, with
m ultiple oXidation states of V ion, the vanadium surface layers of V2C
cou ld potentially enable pseudocapacitive behavior [39], whichcan
promote charge transfer between the adsorbate and V2C support.
Nevertheless, the development of V2C-based synergistic hybrid system is
still need more attentions tomeetthe applicationsin electrochemistry.
In this present work, the exfoliated few-layer V2C nanosheets and
FeNi-LDH nanosheetsare coupled by in-situ assembling through a sim-
ple hydrothermal method (denoted as H2PO—
2 /FeNi-LDH-V2 C), in which
the hypophosphite groups (H2PO—
2 ) w as introduced as intercalated an-
ions in the LDHs laminates in order to alter the surface electronic
structure and thus further enhance the OER activity of the composites
[19,40]. The strong interfacial interaction and electronic coupling be-
tw eenLDHs and V2C nanosheetsguarantee the fast charge transfer ki-
netics and stable structure of the hybrid material, leading to the
enhancement of OER. As a result, the H2PO—
2 /FeNi-LDH-V2 C catalyst
shows superior electrocatalytic activity and stability toward OER, witha
low overpotential of ~ 250mV at the current density of 10mA cm—2
and
a small Tafel slope of 46.5 mV dec—1
in 1.0 M KOH electrolyte. More-
over, the H2PO—
2 /FeNi-LDH-V2 C in rechargeable Zn-air battery reveals
su perior opencircuit potential (1.42 eV), power density (137mWcm—2
)
and durability to conventional Pt/C RuO2 air-cathode. This work
demonstrates the great promise of MXene-based nanohybrids in the
electrocatalytic application.
2. Experimental section
2.1. Materials
The V2AlC (MAX phase) powders were purchased from Shandong
Xiyan new material technology Co., Ltd. (99 wt%, 400 mesh). Hydro-
fluoric acid (~40 wt%), Iron nitrate nonahydrate (Fe(NO3 )3 9•
H2O, 98.5
wt%) and Nickel nitrate hexahydrate (Ni(NO3 )2 6
•
H2 O, 98.0 wt%) were
purchased from Sinopharm Group Chemical Reagent Co., Ltd. Sodium
hypophosphite (NaH2PO2, 99.0 %) and Ammonium fluoride (NH4F, 98
%) were purchased from Aladdin. Sodium hydroXide (NaOH, 90 %),
Tetrapropylammonium hydroXide (TPAOH, 40 wt%) were purchased
from Shanghai Titan Scientific Co., Ltd. Isopropanol were purchased
from Shanghai Lingfeng. Nafion D-520 dispersion (5 wt%) was pur-
chased from Dupont China Holding Co., Ltd. Commercial 20 wt% Pt/C
and the carbon black (XC-72) were purchased from Shanghai HEPHAS
Energy Equipment Co., Ltd. Allmaterialswere used asreceived without
fu rther purification.
2.2. Sample synthesis
2.2.1. Synthesis of V2C MXene nanosheets
First, 1.0 g V2AlC powderswere added gradually into 20 mL of HF
(~40 %) solution ina100mLTeflon-lined with stirring. The suspension
w as constantly stirred for 48 hat 40◦
C. Thenthe black suspension was
washed by Ar de-aerated distilled water for several timesuntil the pH of
the solution reached 6~7. After that, the precipitate was collected by
centrifugation and re-dispersed into 20 mL TPAOH aqueous solution for
24 h under stirring at room temperature. Subsequently, the as-
synthesized multilayer V2C nanosheets was collected and washed for
two times with oXygen-free water to remove the residual TPAOH, fol-
lowed by ultrasonic treatment in oXygen-free water for 2 h. Finally, the
dark green supernatant (few-layer V2C nanosheets) was collected after
the centrifugation for 1 h at 3500rpm, whichwasrestored at4 ◦
C in the
refrigerator before use.
2.2.2. Synthesis of H2PO—
2 /FeNi-LDH-V2C
Typically,thehypophosphite-intercalated FeNi-LDH combinedwith
few-layer V2C nanosheets were prepared via a hydrothermal method.
First, 0.6 mM Ni(NO3)2 6•
H2O and 0.2 mM Fe(NO3)3 9•
H2 O w ere dis-
solved in 10 mL DI water to form a homogeneous solution A (Ni: Fe = 3:
1). Meanwhile, 1.25 mmol NaOH and 0.4 mmol NaH2PO2 were dis-
solved in 10 mL DI water to form solution B (the DI water was boiled
ahead for 30min to remove the dissolved carbon dioXide and kept at40
◦
C for u se). Second, the solution A and B w ere drop wise added simul-
taneously into20mLV2C colloidal solution, followed with the pH ofthe
miXed solution adjusted to 14 by 0.2 M NaOH. And then4 mmol NH4F
w as added into the miXture to adjust the morphology of LDHs. After
stirring for 60min, the final solutionwasthen transferred intoa 50mL
Teflon-lined autoclave and hydrothermally heated at 120 ◦
C for 10 h.
After cooling to room temperature, the precipitates were harvested by
several centrifugation -rinsing cycles with deionized water followed by
freeze-drying.
2.2.3. Synthesis of CO2
3
—
/FeNi-LDH-V2C and H2PO—
2 /FeNi-LDH
According to the report of Feng et al. [41], the dissolved CO2
3
—
anions
in w ater have a high affinity to the LDH layers, thus the CO2
3
—
are
commonly acted asthe charge compensating anions in the LDH layers.
For comparison, the traditional CO2
3
—
/FeNi-LDH combined with
few-layer V2 C nanosheet (CO2
3
—
intercalated, denoted as
CO2
3
—
/FeNi-LDH-V2C) was also prepared with the same procedure as
H2PO—
2 /FeNi-LDH-V2 C without the addition of NaH2PO2 and using
common DI w ater. H2PO—
2 /FeNi-LDH and CO2
3
—
/FeNi-LDH w ere pre-
pared without the addition of V2C nanosheets. FeNi-LDH-V2C was pre-
paredwithout the addition of NaH2PO2 andusing DIwater boiled ahead
for 30 min.
2.3. Materials characterization
The powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku D/Max-2550 V X-raydiffractometer witha Cu Kαradiation (λ
0.154 nm, 40 kV, 40 mA) at a scan rate of 4◦
min—1
. SEM character-
ization was performed on a field scanning electron microscope (FEI
Magellan-400) with an accelerating voltage of 5 kV. Transmission
electron microscopy (TEM), high-resolution transmission electron mi-
croscopy (HRTEM), electron energy -loss spectroscopy (EELS), energy
dispersive X-ray spectrometer (EDS) and corresponding EDS-mapping
were performed using a JEM-2100F field emission transmission elec-
tron microscopy (200 kV). Atomic force microscopy (AFM) measure-
mentwasperformed by Veeco DI Nanoscope Multi Mode V system. X-
ray photoelectron spectroscopy (XPS) signals were measured on Thermo
Scientific K-Alpha with a monochromatic Al Kα source (15 kV, 15 mA)
and a charging correction with reference to C 1s at 284.8 eV.
2.4. Electrochemical measurements
The electrocatalytic performance ofthe catalystswasmeasured on a
CHI 760E (CH instruments, Inc., Shanghai) electrochemical workstation
with a standard three-electrode system in 1.0 M KOH solution. During
OER test, a glassy carbon (GC) electrode (Pine Instruments, 5 mm in
diameter) modified by the catalysts were served as the working elec-
trode. Ag/AgCl electrode saturated with KCl solution and graphite rod
w ere employed as reference electrode and counter electrode,

3. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
3
=
—
= +
+
=
× ×
×
—
respectively. The catalyst ink was prepared by ultrasonically dispersing
7 mg of as-prepared samples and 3 mg carbon black in the miXture of
isopropanol (900μL), deionized water (70μL) and Nafion (30 μL, 5 wt
%) toform ahomogeneous suspension. Apart ofthe catalyst ink (10μL)
w as then pipetted onto the surface of glassy carbon electrode (an
average mass loading of around 0.35 mg cm —2
) and dried under ambient
conditions before tests. All the potentialsmeasured against an Ag/AgCl
electrode in this work were converted to potential versus reversible
hydrogen electrode (RHE) according to the Nernst Equation (ERHE =
EAg/AgCl + 0.059 pH + 0.1989 V). The 1.0 M KOH electrolytes were
saturated with high-purity N2 for 30 min before test. A gas flow was
maintained over the electrolyte during the measurement to ensure
continuous gas saturation. Before the electrochemical data w ere ac-
quired, the working electrodes were electrochemically activated with
1 0–20 cyclic voltammetry (CV) cycle with a sweep rate of 50mV s—1
at
1600rpm. The linear sweepvoltammetry (LSV) curveswere obtained at
a scan rate of 5 mV s—1
with 100 % iR compensations. The Tafel slope
was obtained by fitting the linear part ofthe Tafel plots according tothe
Tafel equation (η a b log(j)) to evaluate the kinetic performance of as-
prepared catalysts. The electrochemical impedance spectroscopy (EIS)
measurements were carried out in the frequency range of 106
Hz to
1 0—2
Hz at 1.49 V vs. RHE with 5 mV amplitude. The double layer
capacitance (Cdl) of the electrodes were calculated from CV curves at
different scan rates of 5 30 mV s—1
in a non-Faraday area. The elec-
trochemically active surface areas (ECSAs) canbe calculated asECSA
Cdl / CS, where CS isthe specific capacitancevalue (60 μFcm—2
) for aflat
standard with1 cm—2
of real surface area. In addition, the LSV curves of
ORR were measured in 1.0 M O2-saturated KOH solution at a scan rate of
5 mV s—1
at a rotating speed of 1600 rpm.
2.5. Zinc-Air battery measurements
A homemade liquid Zn-air battery was assembled to estimate po-
tentiality of the prepared catalysts in the practical application. Typi-
cally, 8 mg catalyst and 2 mg carbon black were dispersed in 900 μL
isopropanol, 70 μL deionized water and 30 μL Nafion by ultrasonic to
form a uniform suspension. The as-prepared ink waspipetted onto the
composite substrate (porous carbon paper or nickel foam) as the air
—2
elementary steps:
OH—
+ * → *OH + e—
(1 )
*OH + OH—
→ *O + H2O + e—
(2 )
*O + OH—
→ *OOH + e—
(3 )
*OOH + OH—
→ * + O2 + H2O + e—
(4 )
where * denotes the active sites on the catalyst surface. Based on the
abovemechanism,the free energy ofthreeintermediatestates, *OH,*O,
and *OOH, are important to identify a given material ’s OER activity. The
computational hydrogen electrode (CHE) model [50] was used to
calculate the free energies ofOER, based onwhich the free energy ofan
adsorbed speciesis defined as:
ΔGads = ΔEads + ΔEZPE - TΔSads (5 )
where ΔEads is the electronic adsorption energy, ΔEZPE is the zero point
energy difference between adsorbed and gaseous species, and TΔSads is
the corresponding entropy difference between these two states. The
electronic binding energy is referenced as ½ H2 for each H atom, and
(H2O – H2) for each O atom, plus the energy of the clean slab. The
corrections of zero point energy and entropy of the OERintermediates
can be found inthe supportinginformation (Table S1).
3. Results and discussion
3.1. Catalyst synthesis and characterization
The synthesis of H2PO—
2 /FeNi-LDH-V2 C nanohybrids is achieved by
co-precipitation of Ni2+
and Fe3+
onto the few-layer V2C nanosheets
u nderhydrothermal conditions, as illustrated in Fig.1a. Firstly, multi-
layered V2C wasmade by selectively etching away the Al layers of bulk
V2AlC powders with 40 wt% HF. After intercalation by TPAOH and
exfoliation underultrasonic, few-layered V2C nanosheetswere obtained
with abundant surface functional groups (–OH, –F), which can facili-
tate the anchoring and nucleation of FeNi-LDH nanosheets on V2C sur-
face. In the following hydrothermal process, the hypophosphite-
intercalated FeNi-LDH nanosheets were grown on the flat surface of
cathode, witha loading of2 mg cm . A polished Zn foil was used as the V2C nanosheets to form H2PO—
2 /FeNi-LDH-V2C nanohybrids. The XRD
anode and 6.0 M KOH with 0.2 M Zn(Ac)2 miXed solution was applied as
the electrolyte. The polarization curves of Zn -air battery were recorded
using linear sweep voltammetry (LSV) and charge/discharge cycling
tests were performed on the LAND CT2001 instrument. The Pt/C RuO2
catalystwith themass ratio is1:1 asthe air cathode wasalso assembled
and tested underthe same conditions for comparison.
2.6. DFT calculation details
All the density functional theory (DFT) calculationswere performed
by Vienna Ab-initio Simulation Package (VASP) [42,43], employing the
Projected Augmented Wave (PAW) method [44]. The revised
Perdew-Burke-Ernzerhof (RPBE) functional was used to describe the
exchange and correlation effects [45–47]. The GGA + U calculations are
performed using the model proposed by Dudarev et al. [48] withthe Ueff
(Ueff Coulomb U – exchange J) valuesof 6.4 eV and 4 eV for Ni and Fe,
respectively. The LDH/MXene nanohybrid structure wasmodeled by a
Fe-doped NiOOH monolayer (Fe: Ni = 1: 3) with exposed (001) surface
adsorbed on O-terminatedV2C MXene surface. The supercell consistsof
4 4 unit cells. For all the surface optimizations, the cutoff energy was
set to be 400 eV. The Monkhorst-Pack grids [49] were set to be 1 3 1
to carry out the surface calculations on all the models. At least 20 Å
vacuum layer wasapplied in z-direction of the slab models, preventing
the vertical interactions between slabs. The model structures were
optimized by ionic and electronic degrees of freedom using thresholds
for the total energy of10—4
eV and force of0.08 eV/Å.
In alkaline conditions, OER cou ld occur in the following four
patterns of bulk V2
AlC and few-layer V2C nanosheets are shown in
Fig. 1b. The sharp peaksat 13.5 and 41.3◦
agree well with the (002) and
(103) crystal planes of V2AlC (PDF#29—0101), respectively. For the
exfoliated few-layer V2C nanosheets, the peaks at 13.5◦
sand 41.3◦
are
complete disappeared, and the characteristic (002) peak is broadened
significantly and downshifted to 5.68◦
, confirming the completely
removal of Al atoms and an increase of c-lattice parameter of few-layer
V2C [33,51]. Moreover, the multilayered V2C exhibitsan accordion-like
morphology from theSEMimagein Fig.1c.Afterfurtherintercalation of
TPAOH,ultrasonic exfoliation and centrifugalization, the Tyndall effect
(right-down inset in Fig. 1d) can be observed in the supernatant meaning
the homogeneously dispersed few-layer V2C nanosheets. The TEM image
in Fig. 1d reveals the ultrathin flakes of few-layer V2C nanosheetswith
many defects edge, and the corresponding thickness is about 6 nm
(right-up inset in Fig. 1d)matching with the dimension of two or three
layers V2C nanosheets in thickness.
The prepared LDHs/MXenes exhibits the similar phase structure
(PDF#40 0215) shown in Fig. S1. The (002) peaks at 5.68◦
of MXene
are disappeared because of the suppressed restacking of MXene sheets
by the surface LDHs nanostructure and the high signals from LDHs.
H2PO—
2 /FeNi-LDH nanoplates w ith a polygon morphology w ere suc-
cessfully synthesized with the thickness of ~6 nm, and the highly ho-
m ogeneous dispersion of O, Fe, Ni, and P elementsverify the successful
insertion of hypophosphite (Fig. S2). For the H2PO—
2 /FeNi-LDH-V2 C
nanohybrids, the SEM and TEM (Fig. 1e and f) indicate the growth of
loosely flaky texture of FeNi-LDH nanosheets on the few-layer V2C

4. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
4
Fig. 1. (a) Sy nthesis of H2P O2
—
/Fe Ni- LDH - V 2C nanohybrids (b) XRD patter ns of bulk V2Al C and few -lay er V2C. SE M images of mul tilayered V2C (c) and H 2P O2
—
/Fe Ni-
LDH - V 2C nanohy brids (e). TE M images of few -l ayer V2C (d) w ith the corresponding Ty ndall effect (right-down inset) and A FM image (right-up inset) and H 2PO—
2 /
FeNi- L DH - V 2C nanohy brids (f), HRTEM images of H2PO—
2 /FeNi- LD H - V 2C nanohy brids (g) w ith the corresponding SAED pattern i n the inset, and the HAADF-ST E M
images i n (h).
surface. Moreover, such flaky morphology is highly desirable to promote
the mass diffusion and charge transfer during the electrochemical re-
actions [37]. High-resolution TEM (HRTEM) observation (Fig. 1g)
showstheinterplanar spacing of0.25 nm and 0.20nm correspondingto
the (100) plane of V2C and (018) plane of H2PO—
2 /FeNi-LDH, respec-
tively, confirming the formation of LDHs on the V2C surface. In addition,
the selected area electron diffraction (SAED) pattern inset in Fig. 1g
presents several diffraction ringsassignable to(110)and (100) planes of
V2 C and (018) plane of H2P O—
2 /FeNi-LDH, respectively, furtherverifying
the formation of H2PO—
2 /FeNi-LDH-V2 C nanohybrids. The uniform
distributions of O, Fe, Ni, V and P elements in H2PO—
2 /FeNi-LDH-V2 C
nanohybrids are shown by elemental mapping analysis in Fig. 1h.
Additionally, from the FTIR measurement (Fig. S3), the bands around
1180and 2358cm—1
canbe attributed to the P–O symmetric stretching
and P–H stretching v ibrational peak, respectively [40], indicating the
presence of the hypophosphite anions in the interlamellar space of the
as-prepared FeNi-LDH m aterials. For comparison, the TEM and
HAADF-STEM images of CO2
3
—
/FeNi-LDH-V2C (carbonate intercalated)
nanohybridsare shown in Fig. S4, with a similarflaky morphology and
u niform distribution of O, Fe, Ni and V elements.

5. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
5
3
3
X-ray photoelectron spectroscopy (XPS) wasmeasurement toinves- corresponding to M–O in oXide, M–OH bond in hydroXide and
tigate the surface compositions and binding structures of obtained cat-
alysts (Fig. 2 and Fig. S5). The XPS survey scan confirms the co-existence
of Fe, Ni, O, V , P and C elements in H2PO—
2 /FeNi-LDH-V2 C (Fig. S5a).In
the P 2p spectra (Fig. S5b), the peak at around 133.4 eV can be ascribed
to hypophosphite [40], consolidating the intercalation of hypophosphite
in the LDHs. The O 1s XPS (Fig. 2a) spectra were deconvoluted into three
component peaks at about 529.6 eV, 531.3 eV and 532.7 eV,
H–O–H in adsorbed w ater, respectively. Fig. 2 b shows the
high-resolution V 2p spectra,with twopeaks of V 2p3/2 at 516.8eV and
V 2 p1/2 at 524.3 eV, corresponding to the V4+
of V2C [52]. Thehigh-
resolution Ni 2p spectra (Fig. 2c) show two characteristic peaks
located at 855.9 eV and 873.5 eV for 2p3/2 and 2p1/2 doublet of Ni2+
,
and two shakeup satellites(defined asSat.), respectively [37]. The Fe 2p
spectra(Fig. 2d) also feature withtwo prominent peaks at 712.6 eV and
Fig. 2. High-resolution XPS spectra of (a) O 1s, (b) V 2p, (c) Ni 2p and (d) Fe 2p i n obtained H2PO—
2 /FeNi- LD H -V 2C, CO2—
/FeNi- LD H -V 2C and H2PO—
2 /FeNi- LD H . Ni
L23 edges (e) and Fe L23 edges (f) el ectron energy-loss spectroscopy (EELS) spectra of H2PO2
—
/F eN i- LD H - V 2C, CO2—
/FeNi- LD H- V 2C and H2PO—
2 /FeNi- LD H .

6. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
6
725.8 eV, corresponding to 2p3/2 and 2p1/2 orbits of Fe3+
, and two
satellite peaks, respectively. Comparing to H2PO—
2 /FeNi-LDH and
CO2
3
—
/FeNi-LDH (Fig. S6), the Ni 2p and Fe 2p peaks of
H2P O—
2 /FeNi-LDH-V2 C and CO2
3
—
/FeNi-LDH-V2C are positively shift
about 0.3—0.4 eV, respectively, the higher binding energies meaning the
higher oXidation states of Ni and Fe irons after combination of V2C.
Obviously, the charge transfer from LDHs to V2C resultsin thevalence
states increase of Ni and Fe irons, indicating the strong chemical inter-
action between the FeNi-LDH and the V2C matriX, which is known
beneficial to accelerate the redoX activity during the OER process [18,
53]. Additionally, according to the slightly higher binding energies of
H2P O—
2 /FeNi-LDH-V2 C than those of CO2
3
—
/FeNi-LDH-V2 C (Fig. 2c and
d), the intercalation of hypophosphite in the LDHs instead ofcarbonate
alsocan facilitate the charge transfer from LDHstoV2C MXene, leading
to the surface electronic reconstruction and higher catalytic activity of
the former. The chemical states of the Ni and Fe species were further
characterized by EELS (Fig. 2e and f), where the L3 and L2 white line
represent the electron excitations from 2p3/2 and 2p1/2 to 3d orbits,
respectively.As shown in Fig.2e and f, the positions ofNi L3,2 edgesand
Fe L3,2 edges shift to higher energy losses, indicating that both the
combination of V2C and the intercalation ofhypophosphite promote the
oXidation of Ni and Fe atoms [54]. Additionally, the total integral in-
tensity ratios (L3/L2) are correlated to the oXidation states of Fe. Ac-
cording to the calculation results, H2PO—
2 /FeNi-LDH-V2C has the highest
Fe L3 /L2 ratio (5.27) than those of CO2
3
—
/FeNi-LDH-V2 C (5.04) and
H2PO2
—
/FeNi-LDH (4 .72), manifesting its highest Fe valance [55,56].
Generally, the EELS analysisprovides evidence that with the combina-
tion of V2C, the charge will transfer from LDHs to V2C, leaving high
oXidation states of Ni and Fe ions.
3.2. Electrocatalytic OER performance
The OER electrocatalytic activity of the obtained catalysts was
evaluated in N2-saturated 1.0 M KOH electrolyte using athree-electrode
system. Fig. 3 a shows the activation process of H2PO—
2 /FeNi-LDH-V2 C
catalyst, demonstrating that after10 cycles ofcyclicvoltammetry scans,
the phase transformed from hydroXides to oXyhydroXides. The peak pair
in the potential range of1.25–1.55 V (vs. RHE) corresponds tothe redoX
cou ple of Ni(II)/Ni(III) [57]. The iR-correction linear sweep voltam-
metry (LSV) curves of all synthesized catalysts and references were
measured at a scan rate of 5 mV s—1
tominimize the capacitive current,
as shown in Fig. 3 b. The H2PO—
2 /FeNi-LDH-V2 C catalyst shows the
strongest redoX peaks at a potential of 1.46 V, indicating the signifi-
cantly accelerated Ni(II)/Ni(III) redo X process. Accordingly, it shows the
highest OERactivity, withalowest overpotential of250mV to achieve a
current density of 1 0 mA cm—2
(η10 = 250 mV) than that of
H2PO—
2 /FeNi-LDH (η10 = 2 70mV), CO2
3
—
/FeNi-LDH-V2 C (η10 = 286 mV)
and commercial RuO2 (η10 = 322 mV). In addition, the OER perfor-
mance of FeNi-LDH-V2 C also has been tested (Fig. S7). Comparing to the
OER performance of H2PO—
2 /FeNi-LDH-V2 C, the inferior catalytic ac-
tivity of FeNi-LDH-V2C (η10 =304 mV) indicated that the intercalated
H2PO—
2 anions in the LDH laminates play a significant role in enhancing
its OERperformance. It isworth noting that the V2C nanosheetsshows
negligible activity, confirming that the LDHs are the active phase for
OER. The combination of V2C nanosheetscan effectively accelerate the
electron transferfrom LDHs to V2C matriX subsequently enhancing the
Fig. 3. OER performance of synthesized catalysts i n 1.0 M KOH. (a) CV curv es of H2PO2
—
/ FeN i- LD H - V 2C at a scan rate of 50 mV s—1
. (b) LSV curves w ith iR-
correction. (c) The corresponding Tafel plots. (d) EIS and its fitting patterns. (e) doubl e-layer capacitance Cdl of sampl es. (f) Comparison of current densities
based on geometric areas and mass activities at 1.53 V v ersus RHE. (g) Faradaic efficiency of the H2PO—
2 /FeNi- LD H- V 2C catal yst i n 1 M KOH at 1600 rpm under N2
saturation. (h) Stability measure m en ts of H2PO—
2 /FeNi- L DH - V 2C and RuO2 at the constant ov erpotential of 0.25 V. (i) Comparison of η10 and Tafel sl ope between
H2PO—
2 /FeNi- LD H -V 2C i n this w ork and v arious Ni/Fe- b ase d and MXen e- base d catal ysts recently reported.

7. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
7
=
+
+
+
OER activity (Fig. 2c and 2d). Hence, it can be concluded that the high
activity of LDHs is associatingwith the hypophosphite-intercalated and
the higheroXidation statesof Ni and Fe ironscaused by the synergistic
electronic effects between LDHs and MXene. As shown in Fig. 3c, the
H2P O—
2 /FeNi-LDH-V2 C catalyst exhibits the lowest Tafel slope of 46.5
mV dec—1
with respect to H2 PO—
2 /FeNi-LDH (53.8 mV dec—1
),
CO2
3
—
/FeNi-LDH-V2C (72.9 mV dec—1
), FeNi-LDH-V2C (7 3.1 mV dec—1
,
Fig. S7b), commercial RuO2 (67.9 mV dec—1
) and V2C (161 mV dec—1
),
m anifestingthe superior OERkinetics.
The electrochemical impedance spectroscopy (EIS) was performed to
evaluated the electrochemical resistances on differentcatalystsat 1.498
V (vs. RHE) in the frequency range from 10—2
to 106
Hz. Fig. 3d dem-
onstrates elliptical semicircles of EIS plots, which is caused by the
roughened surface structure. Therefore, the constant phase element (Q)
is introduced to simulate the double-layered capacitor (inset in Fig. 3d)
[58], where Rs represent the solution resistance, Q1Rct1 and Q2Rct2
represent two charge-transfer processes corresponding to the trans-
formation ofthe active intermediatesand the OER process, respectively
[2 0]. According to the EIS plots in Fig. 3d and the fitting parameters
(Table S2), the H2PO—
2 /FeNi-LDH-V2 C catalyst exhibits the low est Rct
among the prepared catalysts, indicating a faster charge-transfer pro-
cess. The low er electrochemical resistance on H2PO—
2 /FeNi-LDH-V2 C
can be ascribed to the combination of V2C which can accelerate the
electron transfer consequently improving the electrical conductivity.
The electrochemically active surface areas(ECSAs) were also estimated
based on the electrochemical double-layer capacitance (Cdl) of the cat-
alystsrecorded inthe non-Faradaicregion (1.02–1.14 V vs.RHE) since it
had a linear relationship with ECSA in 1.0 M KOH (Fig. S8). As seen from
Fig. 3 e, the measured Cdl v alue of H2PO—
2 /FeNi-LDH-V2 C (6 .52 m F
cm—2
) is larger than those of H2PO—
2 /FeNi-LDH (5.99 m F cm—2
),
CO2
3
—
/FeNi-LDH-V2C (5 .59 m F cm—2
) and V2C (6.28 m F cm—2
).
Accordingly, the ECSA of H2PO—
2 /FeNi-LDH-V2 C is calculated to be
108.67 cm2
(Fig. S9a), which is the highest among the prepared cata-
lysts. Also, when the LSV curves are normalized against ECSA (Fig. S9b),
H2P O—
2 /FeNi-LDH-V2 C catalyst still exhibits much higher OER activity,
fu rtherindicatingthat the synergistic effects betweenLDHs and MXene
significantly improves its intrinsic activity. Additionally, the geometric
current densities and specific mass activities of as-prepared catalystsat
an overpotential of300mV are summarized in Fig.3fandTable S3.And
H2PO—
2 /FeNi-LDH-V2 C displays the highest geometric current density
(89.01 mA cm—2
) as well as relatively high mass activity (249.57 mA
m g—1
).
The turn over frequency (TOF) was calculated to estimate the
intrinsic catalytic activity of per active site (see SupportingInformation
for TOF calculation) [59], which is shown in Fig. S10 and S11. The
H2P O—
2 /FeNi-LDH-V2 C exhibits the highest TOF value of 0.82 s—1
at an
overpotential of 300 mV (Fig. S11), higher than that of H2PO—
2 /FeNi-LDH (0.091 s—1
) and CO2
3
—
/FeNi-LDH-V2 C (0.048 s—1
).
Furthermore, the Faradaic efficiency of H2PO—
2 /FeNi-LDH-V2C was
calculated by rotating ring-disk electrode (RRDE) method (Fig. S12),
where oXygen generated at the disk electrode was swept and reduced at
the Pt ring electrode (see the Supporting Information for the detail of
Faradaic efficiency calculation) [60,61]. Asshown in Fig. 3g, the highest
Faradaic efficiency can be obtained about 99.2 % at 1.48 V (vs RHE),
and then decreased to 67.3 % at 1.52 V (vs RHE). Thisdecrease may be
attributed to the large amounts of undissolved oXygen bubbles gener-
ated by the disk electrode. Thusan average Faradaic efficiency value of
9 6 % (three corresponding Faradaic efficiency values from 1.48to 1.50
V (vs RHE)) has been presented as the OER efficiency of the H2PO—
2 /FeNi-LDH-V2 C catalyst.
Moreover, the H2PO—
2 /FeNi-LDH-V2 C displays an outstanding sta-
bility without any obviousdecrement of the current density after30000
s (Fig. 3h) and even 50000 s (Fig. S13) OER tests, comparing to nearly 50
% decline for the commercial RuO2 catalyst. It’s worth noting that the
current density of H2PO—
2 /FeNi-LDH-V2C have a slight increase during
the OER at the beginning, which can be ascribed to the activation
process of H2PO—
2 /FeNi-LDH-V2C catalyst according to Fig. 3 a. Surpris-
ingly, the OER activity of H2PO—
2 /FeNi-LDH-V2C including the over-
potential and Tafel slope values is comparable and even su perior to
those of themost reported state-of-the-art OER electrocatalysts(Fig.3i
and Table S4). Also, aftera 50000 s OERtest, the XRD and TEM mea-
surements of H2PO—
2 /FeNi-LDH-V2 C w ere performed to further investi-
gate its stability. The XRD patterns in Fig. S14 shows the similar
diffraction peaks of H2PO—
2 /FeNi-LDH-V2C catalyst before and after re-
action. In addition, a hardly unchanged flaky morphology and elements
distribution have been reserved even after 50000 s OER test (Fig. S15),
indicating the excellent structural stability of H2PO—
2 /FeNi-LDH-V2 C
catalyst. Furthermore, the XPS of H2PO—
2 /FeNi-LDH-V2 C after 50000 s
reaction was measured (Fig. S16). It can be seen that the V 2p spectra
(Fig. S16a) still remain at 516.8 eV (V 2p3/2) and 524.3 eV (V 2p1/2),
corresponding to the V4+
state, revealing its favorable stability during
OER. The high-resolution Ni 2p spectra show new characteristic peaks
located at 857.6 eV and 875.5 eV corresponding to 2p3/2 and 2p1/2 or-
bits of Ni3 +
, which can be attributed to the oXidation of Ni2 +
during OER
(Fig. 3a). Also, a slightly positive shift of Fe 2p spectra (Fig. S16c) in-
dicates a slight rise in thevalance state of Fe after reaction. The partial
oXidation of the Ni and Fe species in H2PO—
2 /FeNi-LDH-V2 C after 50000
s reaction suggests its beneficial trend to accelerate the redoX activity
du ring the OER processagreeing withthe document reported [15,20].
3.3. Zinc–air battery performance
To further evaluate the electrocatalytic performance, the ORR
measurement was also performed in O2 saturated 1.0 M KOH solution by
using a standard three-electrode working system. The synthesized
H2PO—
2 /FeNi-LDH-V2 C shows considerable ORR catalytic properties
with an onset potential of0.89 V and a highhalf-wave potential of0.8V
vs. RHE (Fig. S17). The overall oXygen electrocatalysis property of the
catalyst was further evaluated through the potential gap (ΔE) between
the potential of OER at η10 and the half-wave potential (E1/2) of ORR.
And a smaller value of ΔE indicates a superior catalytic activity and
reversibility [62,63]. As illustrated in Fig. S18, the
H2PO—
2 /FeNi-LDH-V2 C electrode reveals a much lower ΔE v alue (0.673
V) than those of H2PO—
2 /FeNi-LDH (0.702 V), CO2
3
—
/FeNi-LDH-V2C
(0.714 V) and V2C (0.936 V), even a little smaller than that of a miXed
Pt/C + RuO2 commercial catalyst (0.682 V).Moreover, the sample
H2PO—
2 /FeNi-LDH-V2 C shows the electron transfer number of ~3 .8 ac-
cording to Fig. S19, illustrating the effective ORR catalytic activity.
Based on the excellent oXygen electrocatalysis properties, a liquid
rechargeable Zn-air battery (ZAB) employing H2PO—
2 /FeNi-LDH-V2C as
the air cathode was assembled to evaluate its feasibility in practical
energy devices. For comparison, the liquid ZAB using Pt/C RuO2
commercial catalyst (mass ratio 1: 1) as the air cathode was also
assembled. The schematic configuration ofa Zn-air battery assembly is
displayed in Fig. 4a. The liquid ZAB driven by the H2 PO—
2 /FeNi-LDH-V2 C
obtains a high open-circuit potential (OCV) of 1 .42 V (Fig. 4b),
exceeding that of driven by the Pt/C RuO2 (1.41 V). The discharge/
charge polarization curves and the corresponding power density curves
of the liquid ZABs are provided in Fig. 4c. The H2PO—
2 /FeNi-LDH-V2 C-
based ZAB exhibits a lower voltage gap between charge and discharge,
together with a maximum power density of 137 mW cm —2
, which is
higher than that of the Pt/C RuO2-based ZAB (43 mW cm—2
), indi-
cating a better rechargeability [64]. The cycle stability of ZABs were
performed under continuous galvanostatic discharge and charge at 5 mA
cm—2
with each cycle being 10 min. As observed in Fig. 4d, the H2PO—
2 /FeNi-LDH-V2 C-based ZAB shows relatively stable operation for 100
h, which is longer than the Pt/C + RuO2-based ZAB (~60 h). Moreover,
the v oltage gap of H2PO—
2 /FeNi-LDH-V2 C-based ZAB only
increased by 0.1 V aftercontinuous 60h (the voltage gap changes from
initial 0.73 V to0.83 V at the 360 th cycle), corresponding toonly 4.3 %
decrease of round-trip efficiency (discharging end voltage divided by
charging end voltage [65,66]) (Fig. 4 e). For a conventional Pt/C +

8. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
8
Fig. 4. Zn-air batteries w ith H2PO—
2 /FeNi- L D H- V 2C and Pt/C + RuO2 catal ysts as air cathodes. (a) Schematics of the primary configuration of a Zn–air battery. (b)
Open circuit potential curves. (c) The discharge/charge polarization curves and the corresponding pow er density curves. (d) Gal vanostatic charging/discharging
cy cling curves at a current density of 5 mA cm—2
. Long- term cy cling performance w ith H2PO2
—
/Fe Ni- L DH - V 2C (e) and Pt/C + RuO2 (f) catalysts as air cathodes. (g)
Photographs of different l ight-emitting diode w ith H2PO—
2 /FeNi- LD H - V 2C as air cathodes.
RuO2-based ZAB, itsvoltage gap increases from 0.77 V to 0.97 V, with
the round-trip efficiency decreases of 9.1 % from 61.3 % at the opening
stage to 52.2 % after 60 h (Fig. 4f). As an exemplification for practical
applications, the LED display (1.5 V) and different light-emittingdiode
(2 –2.2 V) were powered by two series of ZABs with theH2PO—
2 /FeNi-LDH-V2 C as air cathode (Fig. 4g), and the light-emitting
Fig. 5. (a) Side v iew of model structure of H2PO2
—
/F eNi- LD H -V 2C hy brid sy stem. (b) DOS of H2PO2
—
/ FeN i- LD H - V 2C and H2PO—
2 /FeNi- LD H . (c) PDOS of the Ni and Fe
3d orbitals from H2PO—
2 /FeNi- LD H- V 2C and H2PO2
—
/Fe Ni- L DH . The dashed l ines indicate the d-band center for each system. (d) The reaction pathway of OER in
al kaline sol ution. The free energy diagrams at an equilibrium potential of 0 V (e) and 1.23 V (f) for ov erall OER pathway.

9. Y. Chen et al. Applied Catalysis B: Environmental 297 (2021) 120474
9
diode can be powered for 36 h without brightness decay. The results
corroborate the usability of H2PO—
2 /FeNi-LDH-V2 C in actual recharge-
able Zn-airbatteries, which demonstrates itspromisingapplication for
advanced energystorage and conversion technologies.
3.4. DFT calculation
To gain fundamental insight into the origin of oXygen electro-
catalytic activity for H2PO—
2 /FeNi-LDH-V2C, spin-polarized density
fu nctional theory (DFT) calculations with focus on the interaction be-
tw een FeNi-LDH and MXene and the catalytic pathways of oXygen
electrocatalytic reactionswere performed,as shown in Fig.5.According
to the activation process of H2P O—
2 /FeNi-LDH-V2 C in Fig. 3a and some
previous studies [15,20,67], the FeNi-LDHs usually undergo a phase
transformation from hydroXides to oXyhydroXides during the OERpro-
cess in alkaline solution, where the oXyhydroXidesare generally recog-
nized asthe active phase for OER. Therefore, the computation structure
of LDH/MXene is modeled as Fe-doped NiOOH monolayer (Ni: Fe = 3: 1)
su pported by O-terminated V 2C considering the alkaline conditions.
(Fig. 5a).
The Mulliken charge analysis indicates that an electron transfer of
0.42 e—
per unit cell from FeNi-LDH to V2C, leaving more positively
charged of Fe and Ni atoms than those in FeNi -LDH nanosheets, which is
well agreement with the XPS results (Fig. 2c and d). The density of states
(DOS) and partial density of states (PDOS) for H2PO—
2 /FeNi-LDH-V2C
and H2PO2
—
/FeNi-LDH are first calculated and exhibited in Fig. 5 b and
5c, respectively. As shown in Fig. 5b, the electron density around the
Fermi level of H2PO—
2 /FeNi-LDH-V2C exhibits enhanced DOS comparing
to that of H2PO—
2 /FeNi-LDH, w hich can promote the charge transfer and
facilitate the adsorption/desorption properties of reaction in-
termediates, thus increasing the conductivity of catalyst and reducing
the energy barriers for OER[53,68]. Fig.5c indicates thatthe PDOS (Fe
and Ni 3d orbital band) of H2PO—
2 /FeNi-LDH-V2 C are more down-shifted
than that of H2PO2
—
/FeNi-LDH, where the d-band center is calculated to
be —3.04 and —2.44 eV for H2PO—
2 /FeNi-LDH-V2 C and
H2P O—
2 /FeNi-LDH, respectively. The downshift of d-band center for
H2PO—
2 /FeNi-LDH-V2 C means the appropriate adsorption strength to-
w ard mediates, which canaccelerate the OERprocess according tothe
d-band center theory [50,69–71]. Thus, a balance between the d-band
center and intrinsic activity should be taken into consideration.
Fig. 5d shows the reaction pathway of OER. Accordingly, the free
energy diagrams at an equilibriumpotential of 0 V and1.23 V for overall
OER pathway are depicted in Fig.5e and 5f, respectively. Asillustrated
in Fig. 5e, for the H2P O—
2 /FeNi-LDH and H2PO—
2 /FeNi-LDH-V2 C, the OER
rate-determining step (RDS) is the formation of *OOH from *OHwith
the free energy barrier of 1.69 eV and 1.56 eV, respectively. The lower
free energy barrier of H2 PO—
2 /FeNi-LDH-V2 C, indicates that the forma-
tion energy of O–O band canbe reduced by the combination FeNi-LDH
withV2C. After1.23 V potentialhasbeen applied to the system (Fig.5e),
the ΔG*OH of H2PO—
2 /FeNi-LDH reveals a negative value, demonstrating
the strong adsorption with *HO, leading to a marked uphill formation
for the subsequent *O and *OOH intermediates. In contrast, relatively
low values of ΔG between intermediates are shown on H PO—
/FeNi-
alkaline electrolyte. The obtained H2 PO—
2 /FeNi-LDH-V2 C electrocatalyst
exhibits the remarkable OER performance with a low η10 of 250 mV, a
small Tafel slope of 46.5 mV dec—1
, and a long-time durability in1.0 M
KOH solution. Additionally, the H2PO—
2 /FeNi-LDH-V2 C in rechargeable
Zn-air battery reveals superior open circuit potential (1.42 eV), power
density (137 mW cm—2
) and well durability to conventional Pt/C +
RuO2 air-cathode. The remarkable oXygen electrocatalytic performance
can be attributed to the synergy and interplay of H2PO—
2 /FeNi-LDH and
conductive V 2C MXene. Strong electronic interaction between FeNi-
LDHs and V2C MXene guarantees the prominent charge transfer and
enhances the reaction kinetics for OER on the one hand, and on the other
hand, combination FeNi-LDHs to V2C MXene can reduce the O adsorp-
tion capacity by shifting down the d-band center of Fe/Ni atomsleading
to an appropriate balance between the adsorption of OH speciesand the
desorption of O2 and theneventually promoting the intrinsic activity.
CRediT authorship contribution statement
Yafeng Chen: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Visualization, Writing - original draft. Heliang
Yao: Investigation, Resources. Fantao Kong: Conceptualization,
Investigation, Resources. Han Tian: Investigation, Resources. Ge Meng:
Resources. Shuize Wang: Resources. Xinping Mao: Conceptualization,
Methodology. Xiangzhi Cui: Conceptualization, Methodology, Super-
vision, Investigation, Writing - review & editing. Xinmei Hou:
Conceptualization, Methodology, Supervision, Investigation, Writing -
review & editing. Jianlin Shi: Conceptualization, Methodology.
Declaration of Competing Interest
The authors report no declarationsof interest.
Acknowledgments
This work was supported by the National Science Fund for Distin-
guished Young Scholars (No. 52025041), the National Natural Science
Foundation of China (No.51974021), the Natural Science Foundation of
Shanghai (19ZR1479400), and the State Key Laboratory of Advanced
Technology for Materials Synthesis and Processing (Wuhan University of
Technology).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcatb.2021.120474.
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