Phosphorus modification of cobalt-iron nanoparticles embedded in a nitrogen-doped carbon network for oxygen reduction reaction

Article abstract of DOI:10.1039/d0ra08768h

For the electrochemical reduction of oxygen the development of heteroatom-doped carbon-based transition metal catalysts has become a recognized strategy to replace traditional noble metal catalysts. In this work a catalyst consisting of CoFe nanoparticles encapsulated in N-doped carbon-based materials (NC) supported by carbon nanotubes (CNTs),i.e.Fe₃Co₁@NC/CNTs, was modifiedviatreatment with a phosphate salt to synthesize a P-Fe₃Co₁@NC/CNTs catalyst. The P-Fe₃Co₁@NC/CNTs exhibits with 5.29 mA cm^?2an enhanced current density which is comparable to a Pt/C catalyst. In addition, a stability and methanol resistance better than the Pt/C catalyst were observed which is ascribed to the carbon encapsulation and the synergies between the two transition metals. Finally, the reaction mechanism of P-doping was studied and discussed. These results provide possible directions for carbon-based catalysts and doping with heteroatoms for the improvement of catalytic activity. Moreover, the zinc-air battery assembled with P-Fe₃Co₁@NC/CNTs as the air-cathode exhibited a high-power density of 73 mW cm^?2, which is comparable to that of Pt/C (71 mW cm^?2) and a specific capacity of 763 mA h g^?1. The prepared catalyst could potentially serve to take the place of precious metal catalysts in rechargeable Zn-air batteries.

Full text of DOI:10.1039/d0ra08768h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Phosphorus modification of cobalt–iron
nanoparticles embedded in a nitrogen-doped
Cite this: RSC Adv., 2021, 11, 9450
carbon network for oxygen reduction reaction†
Rui Zhang, Zheng Wang, Lin Zhu, Weixin Lv * and Wei Wang*
For the electrochemical reduction of oxygen the development of heteroatom-doped carbon-based
transition metal catalysts has become a recognized strategy to replace traditional noble metal catalysts.
In this work a catalyst consisting of CoFe nanoparticles encapsulated in N-doped carbon-based
3 1
materials (NC) supported by carbon nanotubes (CNTs), i.e. Fe Co @NC/CNTs, was modified via
treatment with a phosphate salt to synthesize a P-Fe Co @NC/CNTs catalyst. The P-Fe
3
1
3
Co @NC/CNTs
1
ꢀ2
exhibits with 5.29 mA cm an enhanced current density which is comparable to a Pt/C catalyst. In
addition, a stability and methanol resistance better than the Pt/C catalyst were observed which is
ascribed to the carbon encapsulation and the synergies between the two transition metals. Finally, the
reaction mechanism of P-doping was studied and discussed. These results provide possible directions
for carbon-based catalysts and doping with heteroatoms for the improvement of catalytic activity.
Received 15th October 2020
Accepted 23rd February 2021
Moreover, the zinc–air battery assembled with P-Fe
3
Co
1
@NC/CNTs as the air-cathode exhibited a high-
ꢀ2
ꢀ2
power density of 73 mW cm , which is comparable to that of Pt/C (71 mW cm ) and a specific
DOI: 10.1039/d0ra08768h
ꢀ1
capacity of 763 mA h g . The prepared catalyst could potentially serve to take the place of precious
rsc.li/rsc-advances
metal catalysts in rechargeable Zn–air batteries.
appropriate dopant for tuning the surface reactivity of gra-
phene. Similarly, the introduction of phosphorus induces
1
. Introduction
11
Fuel cells (FCs) are widely regarded as one of the most prom- structural defects into the carbon matrix and increases the
ising energy conversion devices to mitigate energy and envi- electron delocalization due to the good electron donating
ronmental crises. Especially, a Zn–air battery is the most properties of phosphorus, which results in materials with
prominent energy storage device among the different metal–air enriched active sites and improved conductivity and charge
1
2,13
batteries because of its higher energy and power density, economic transfer.
Therefore, adding an appropriate amount of
1
viability, safety, and environmental friendliness. The electrochemical phosphorus into the material can improve its electrochemical
performance of Zn–air batteries depends greatly on the kinetics of the activity. Jing et al. prepared P-doped CNTs encapsulated nickel
2,3
oxygen reduction reaction (ORR) at the cathode. This reaction is hybrids via a one-step chemical vapor deposition followed by P-
a kinetically sluggish multi-electron transfer process; hence, highly doping treatment with NaH
2
PO
4
as the phosphorus source. The
4
active catalysts are required for fast progress of ORR. At present, ORR obtained catalyst showed excellent hydrogen evolution reaction
14
catalysts are still dependent on precious metal catalysts like Pt, but (HER) performance. Recently, some researchers have investi-
their high cost and poor stability seriously limit the large-scale gated the doping of carbon materials with both phosphorus and
5–7
15–17
application of fuel cells. Therefore, it is of great importance to nitrogen atoms to obtain a more active catalyst.
Hu et al.
develop low cost ORR catalysts with high activity and stability.
prepared FeP/N, P-doped carbon nanosheets, via a one-pot
N-Doped carbon-based materials can enhance the surface pyrolysis process by using 1,2-ethylenediphosphonic acid as
activity of carbon materials and have therefore been extensively the phosphorus source and melamine as the nitrogen source.
8
18
studied as high performance ORR catalysts. The doping with The obtained catalyst showed excellent ORR performance.
nitrogen favors the adsorption of oxygen which facilitates the
In recent years, the combination of transition metal-based
9,10
19,20
21,22
23,24
ORR by lowering the electrochemical potential. As the closest materials (Fe,
Co,
Ni,
etc.) and a carbon matrix (e.g.
congener of nitrogen, phosphorus has also been found as an carbon nanotubes, graphene) was proposed as a prospective
strategy not only to improve the stability of metal-based mate-
rials but also to endow the hybrids with a favorable electro-
School of Chemistry and Chemical Engineering, Yancheng Institute of Technology,
25
catalytic activity for ORR. However, the exposed metal
nanoparticles are still vulnerable to the inuence of external
factors resulting in agglomeration similar to Pt/C catalyst and
Yancheng 224051, China. E-mail: lvweixin@ycit.edu.cn; wangw@ycit.edu.cn
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra08768h
1
9450 | RSC Adv., 2021, 11, 9450–9458
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
9
ꢁ ꢁ
2
calcined at 420 C for 2 h and at 750 C for 1 h in a N atmo-
consequently in a reduction of their electrochemical activity.
Based on this observation, it is important to conne the nano- sphere. Finally, the material was pickled with 0.5 M H SO at
2
4
ꢁ
particles into a limited space (encapsulate metal nanoparticles 80 C for 8 h to remove any unreacted transition metal in the
in an N-doped carbon layer structure), to maintain their excel- material to obtain the catalyst Fe Co @NC/CNTs. The P-doped
3
1
26,27
lent electrochemical activities.
3 1 3
Deng et al. synthesized catalyst P-Fe Co @NC/CNTs was obtained by heating Fe -
a FeCo alloy catalyst encapsulated in pod-like carbon nanotubes Co
and found through theoretical calculations that the carbon wall a N
1
@NC/CNTs with sodium dihydrogen phosphate powder in
ꢁ
2
atmosphere at 300 C for 30 min.
thickness of carbon-encapsulated metal catalysts signicantly
For further study and comparison, Fe
3 1 3
Co @NC and P-Fe -
affects the dissociative adsorption of O on the outermost Co @NC were synthesized using the same method mentioned
2
1
surface of CNTs. Reducing the carbon wall thickness around the above without the addition of functional CNTs.
metal would promote the ORR activity signicantly and intro-
ducing nitrogen atoms into the carbon lattice could further
enhance the dissociative adsorption of O . Hu et al. successfully
2
2.3 Characterization
28
The crystal structures of the samples were characterized via X-
ray diffraction (XRD, Bluker Rigaku D/MAX 2200 diffractom-
eter with Cu Ka). The morphologies and composition of the
samples were investigated via transmission electron microscopy
synthesized FeCo nanorods embedded in N-doped CNTs and
conducted electrochemical tests indicating a high stability and
electrocatalytic activity for oxygen evolution reaction (OER) and
ORR. The authors ascribed their observation to the presence of
a higher number of metal–NCNT heterointerfaces resulting in
(TEM, JEM2100F). X-ray photoelectron spectroscopy (XPS)
29
measurements were carried out using an ESCALAB 250Xi elec-
tron spectrometer.
more catalytically active sites for ORR and OER. However, the
mechanism by which ORR occurs when such nitrogenous carbon
layers bind to transition metals and the nature of the specic active
sites are still controversial. While some authors believe that the
transition metal particles are not part of the active site but can
2.4 Electrochemical test
All electrochemical measurements were made using a CHI 760E
electrochemistry workstation (CH Instrument, Shanghai,
China) in a standard three-electrode system. A platinum elec-
trode was used as the counter electrode, and a Hg/HgO (1 M
KOH) was used as the reference electrode. All the potentials
were initially measured against Hg/HgO electrode and then
converted to reversible hydrogen electrode (RHE) using the
relation: ERHE ¼ EHg/HgO + 0.098 + 0.059 ꢂ pH. 0.1 M KOH
solution was used as the electrolyte (170 mL). Before each
30–33
promote the formation of the catalytically active site,
others
reported that transition metals are indeed part of ORR active
34,35
sites.
In the light of these different points of view, further
research and exploration of ORR catalysts needs to be conducted.
In this paper, we synthesized ORR catalysts consisting of
transition metal particles encapsulated into N-doped carbon-
based materials (NC) and used a simple procedure to dope
the surface of the catalysts with phosphorus. The resulting
enhanced ORR performance and the catalytic kinetics of the P-
doped catalysts were investigated.
2
experiment, O gas (99.99%) was aerated into the electrolyte for
at least 40 min to reach oxygen saturation. The working elec-
trode was prepared by coating 20 mL of well dispersed catalyst
ink on the surface of the pre-cleaned rotating disk electrode
2. Experimental section
(RDE, 5 mm in diameter) and dried under ambient conditions.
2.1 Synthesis of functional CNTs
The catalyst ink was prepared with 4 mg of as-prepared catalyst
sample, 985 mL ethanol and 15 mL of Naon solution (DuPont,
D-520, 5 wt%) aer sonication for 1 h. For comparison, 20 wt%
Pt/C (Johnson Matthey) ink was formed like other catalyst inks
with a similar procedure. The linear sweep voltammetry (LSV)
CNTs were dispersed in 200 mL mixed solution containing 10%
HNO
3
and 30% H
2
O
2
with a volume ratio of 2 : 1 and stirred at
ꢁ
6
0 C for 5 h, then washed with deionized water, nally dried at
ꢁ
80 C for 24 h to synthesize functional CNTs.
As a result of the above reactions, CNTs were modied by
ꢀ1
curves were evaluated at a sweep rate of 5 mV s at different
a certain amount of carboxyl and hydroxyl groups, which acted
as the active centers to promote the next reaction with iron(II)
phthalocyanine (FePc), cobalt(II) phthalocyanine (CoPc) and
rotation rates from 400 to 2025 rpm. Long-time and methanol
crossover measurements were conducted in O -saturated solu-
2
tion under a rotating speed of 1600 rpm. For the anti-methanol
tests, the current was recorded when 1 M methanol was added
into the electrolyte at 400 s.
3 6 6
melamine (C H N , >99%).
The apparent electron transfer number (n) for a typical ORR
is calculated from the Koutecky–Levich (K–L) equations:
2
.2 Synthesis of P-Fe
3
Co
Co
1
@NC/CNTs, Fe
@NC catalysts
3 1
Co @NC/CNTs,
Fe Co @NC and P-Fe
3
1
3
1
1
5
1
00 mg functional CNTs, 150 mg FePc (Aladdin, >97%) and
0 mg CoPc (Aladdin, >95%) were dispersed in an ultrasonic for
h in 75 mL N,N-dimethylformamide (DMF) (Aladdin, 99%).
1/2
1
/J ¼ 1/J
K
+ 1/J
L
¼ 1/J
K
+ 1/Bu
(1)
(2)
2
0
/3 ꢀ1/6
B ¼ 0.62nFC
0
D
n
The suspension was then stirred for 6 h. Aer adding melamine
6 g) the suspension was stirred for 1 h.
Aer ltration the obtained solid was rinsed three times with current density and diffusion limit current density, respectively;
(
where J, J and J are the measured current density, kinetic limit
K L
ꢁ
ꢀ1
DMF and dried in vacuo at 80 C for 24 h. The material was B is determined from the slope of the Koutecky–Levich plots (j
©
2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 9450–9458 | 9451

View Article Online
RSC Advances
Paper
ꢀ
1/2
vs. u
); u is the electrode rotation rate; F is the Faraday presence of alloys and elemental metals in Fe
3
Co
1
@NC/CNTs,
ꢀ
1
constant (96 485 C mol ); C is the volume concentration of O
which is still present in the spectrum of P-Fe Co @NC/CNTs
0
2
3
1
ꢀ3
ꢀ1
ꢀ
(
0
1.2 ꢂ 10 mol L ); D is the diffusion coefficient of O in indicating that the doping with phosphorus does not change
0
2
5
2
ꢀ1
.1 M KOH (1.9 ꢂ 10 cm s ); n is the kinematic viscosity of the existing form of transition metals in the material. It is
2
ꢀ1
ꢁ
notable that a smaller peak appears at 43.7 , which can be
electrolyte (0.01 cm s ).
For the rotating ring disk electrode (RRDE) tests, the ring attributed to the (111) plane of the transition metal nitrides
potential is set at 1.3 V constantly. The transferred electron Co5.47N and Fe
3
N. Since some studies have shown that Co
x
N
ꢀ
number (n) and the yield of HO
2
during the ORR process on the and Fe
x
N are also active sites for ORR, the presence of transition
base of RRDE can be calculated using the following equations:
metal nitride also provides an explanation for the catalytic ORR
activity of the synthesized Fe Co @NC/CNTs and P-Fe Co @-
NC/CNTs materials.
The TEM image of CNTs in Fig. 2a shows the smooth surface
and typical hollow structure of this material. The TEM images of
3
1
3
1
n ¼ 4I /(ID + I /N)
(3)
(4)
36,37
D
R
2ꢀ
R
D R
/N/(I + I /N)
HO (%) ¼ 200I
3 1
P-Fe Co @NC/CNTs in Fig. 2b and c depict that black particles
where I is the disk current, I is the ring current, and N is the
D
R
are encapsulated in the bamboo-like nitrogenous carbon-based
material with a thin tube wall (3 nm). This interpenetrated
conductivity network structure is favorable for electron trans-
collection coefficient of H O on the ring electrode (0.37).
2
2
26
2.5 Zn–air battery measurement
port within the CNTs. It is worth noting that the ԁ-spacing of
these black particles is about 2 nm, which corresponds to the
ԁ-spacing of the metallic elements Fe and Co and their alloys
Home-assembled Zn–air battery was fabricated by the as-
prepared catalysts or 20% Pt/C as cathode and a polished zinc
2
7 3
Co Fe and CoFe as calculated from the XRD pattern shown in
foil (2 cm ) as anode with 6 M KOH as electrolyte. Discharge
Fig. 1a. The results of the selected area electron diffraction
(SAED) pattern of P-Fe Co @NC/CNTs presented in Fig. 1d are
polarization curves were measured by linear sweep voltammetry
ꢀ
1
from 1.5 to 0.4 V at a scan rate of 5 mV s . Discharge current
and the corresponding power density were normalized to the
effective surface area of air-cathode electrode.
3
1
consistent with the XRD results and conrm the (002) crystal
plane belonging to C, and the respective planes to the metallic
elements Fe, Co and their alloys, Co Fe and CoFe, as well as the
7
3
nitrides of two transition metals, Fe N and Co5.47N.
3
3
. Results and discussions
It was preliminarily proved by TEM imaging that the used
preparation method could successfully encapsulate transition
metal particles in the NC which compound with CNTs. The
morphology NC is not homogeneous, showing bamboo-like
tubular, lamellar or spherical shape. Such NC have been shown
A comparison of the X-ray diffraction (XRD) patterns of Fe
Co @NC/CNTs and P-Fe Co @NC/CNTs (Fig. 1a) does not
reveal any change aer doping with phosphorus. A peak
centered at 26.2 corresponds to the (002) plane of the carbon
atom structure of CNTs. A relatively large peak observed at 44.7
can be assigned to the (111) and (002) plane of the Fe and Co
nanoparticles and the (110) plane of the metal compounds
3
-
1
3
1
ꢁ
38–40
to exhibit a higher activity compared to common CNTs.
In
ꢁ
addition, the thinner wall can ease penetration of electrons from
the metal cores to the carbon surface. These features could be
responsible for the observed superior ORR performance.
Co Fe3 and CoFe. The occurrence of this peak indicates the
7
Fig. 1 (a) XRD patterns of Fe
3 1 3 1 3 1
Co @NC/CNTs and P-Fe Co @NC/CNTs. (b) TEM images of CNTs. (c and d) TEM images of P-Fe Co @NC/CNTs.
(
e) SAED of P-Fe Co @NC/CNTs.
3
1
9452 | RSC Adv., 2021, 11, 9450–9458
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Fig. 2 (a) XPS survey spectra of P-Fe
Co
3 1
3 1
@NC/CNTs and Fe Co @NC/CNTs. High-resolution XPS spectra of C 1s (b), N 1s (c), Fe 2p (d), Co 2p (e)
3 1
and P 2p (f) for P-Fe Co @NC/CNTs.
ꢀ
The element composition and chemical environment of P- of reactants (e.g., OH , O
2
), and promote the electron transfer
Fe Co @NC/CNTs and Fe Co @NC/CNTs have been studied by between the catalyst surface and reaction intermediates, thus
3
1
3
1
8
X-ray photoelectron spectroscopy (XPS). The spectra presented efficiently catalyzing ORR with favorable reaction kinetics. It is
in Fig. 2a indicate that P-Fe Co @NC/CNTs and Fe Co @NC/ generally believed that for ORR the catalytic activity decreases in
3
1
3
1
CNTs contain C, O, N, Fe, and Co elements. The quantitative the order pyridinic N > pyrrolic N > graphitic N > oxidized N.
analysis resulted in a nitrogen content in P-Fe Co @NC/CNTs Therefore, the higher pyridinic N content in P-Fe Co @NC/
and Fe Co @NC/CNTs of 10.28 at% and 8.87 at%, respec- CNTs can be responsible for its improved ORR activity.
tively. The higher nitrogen content in P-Fe Co @NC/CNTs The peaks in the Fe 2p XPS-spectrum (Fig. 2d) centred at
could result in a higher ORR performance of this material.
As illustrated in Fig. 2b, the C 1s spectra of P-Fe Co @NC/ Fe 2p3/2 states, respectively, whereas peaks centred at 721.8,
3
1
3
1
3
1
3
1
0
2+
707.1, 711.0, and 713.5 eV can be attributed to the Fe , Fe , and
3
+
3
1
0
2+
3+
CNTs and Fe Co @NC/CNTs can be deconvoluted with four 724.8, and 727.0 eV correspond to the Fe , Fe , and Fe 2p1/2
3
1
components corresponding to the graphitized C, C–N (C–P), states, respectively. Satellite peaks denoted as “sat.” are
9
44,45
C]O, and O–C]O/N–C]O atoms or functional groups. In the observed at 717.9 and 729.4 eV.
Similarly, in the Co 2p XPS-
two materials, graphitized C can be assigned to the majority of spectrum (Fig. 2e), the peaks at 779.0, 781.0, and 783.7 eV refer
0
2+
3+
peaks with total peak areas of 54.6% and 53.8%, respectively, in to the Co , Co , and Co 2p3/2 states and the peaks at 794.8,
0
2+
3+
line with the bonding type of CNTs. It is worth noting that the 796.3, and 798.5 eV to the Co , Co , and Co 2p1/2 states,
XPS spectra of P-Fe Co @NC/CNTs indicate C–N/C–P bonds at respectively. Two satellite peaks are also present at 788.6 and
85.73 eV due to the doping with phosphorus. The increase in 803.9 eV.
the peak area proves that phosphorus was successfully incor- that P–O (134.2 eV) and P–C (133.3 eV) are the two forms of
3
1
41
42,46
2
Finally, the P 2p XPS-spectrum (Fig. 2f) indicates
41
porated in the C structure to form C–P bonds.
doped phosphorus in P-Fe Co @NC/CNTs.
3 1
The deconvolution of the N 1s spectrum (Fig. 2c) results in
The XPS analysis reveals that the two transition metals
patterns corresponding to four kinds of nitrogen atoms: contained in the two materials before and aer phosphorus
pyridinic N (398.8 eV), pyrrolic N (400.1 eV), graphitic N (400.9 doping were present both as zero-valent metal elements and in
42,43
eV) and oxidized N (401.5 eV).
The content of pyridinic N is other oxidation states, which proves that this method could
the highest and increases from 37.0% of the total amount of successfully insert the metal elements and their alloys into NC.
nitrogen to 51.5% aer doping with phosphorus. As the widely Furthermore, the observation of the P–C bond peak proves the
accepted electroactive site, the electron-accepting pyridinic N specic existence of this form of phosphorus in the material.
species can impart a relatively high positive charge density on
the neighboring sp -bonded C atoms, facilitate the adsorption NC/CNTs and P-Fe
To evaluate the electrocatalytic ORR activities of Fe Co @-
3 1
2
3
Co
1
@NC/CNTs with different metal ratios,
©
2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 9450–9458 | 9453

View Article Online
RSC Advances
Paper
Fig. 3 LSV curves at various rotation rates for Fe
responding K–L plots.
3 1 3 1
Co @NC/CNTs (a), P-Fe Co @NC/CNTs (c). The insets in figure (b) and (d) refer to the cor-
linear sweep voltammetry (LSV) was conducted in O
.1 M KOH solution. Referring to the spectra shown in Fig. 3a the apparent transfer electron number in ORR.
and c for the undoped and P-doped materials, respectively, in The K–L curves of Fe Co @NC/CNTs (Fig. 3b) and P-Fe
the catalysis of ORR the onset potential and half-wave potential Co @NC/CNTs (Fig. 3d) can be approximated with a straight line
decreases at a rotation rate of 1600 rpm from Eonset ¼ 0.877 V with a good value for the goodness of t. The comparison shows
and E1/2 ¼ 0.814 V for Fe Co @NC/CNTs to E ¼ 0.860 V and that the apparent electron transfer number of Fe Co @NC/CNTs is
2
-saturated CNTs, K–L curves were tted for the two catalysts to calculate
0
3
1
3
-
1
3
1
onset
3
1
E1/2 ¼ 0.802 V for P-Fe Co @NC/CNTs. However, it is worth 3.7, which indicates that the reaction proceeds by a path domi-
3
1
ꢀ
2
noting that P-Fe Co @NC/CNTs shows with J ¼ 5.29 mA cm
nated by a four-electron transfer step but mixed with two and four
3
1
L
a higher limiting current density at 1600 rpm than Fe
3
Co
1
@NC/ electron steps in the ORR. However, P-Fe Co @NC/CNTs aer
3
1
ꢀ
2
CNTs (J
L
¼ 4.91 mA cm ) due to the doping with phosphorus. phosphorus doping exhibits a higher apparent transfer electron
To further study the catalytic mechanism of the two catalysts in number (n z 4) demonstrating that phosphorus doping improved
ORR before and aer phosphorus doping and the inuence of the four-electron step selectivity of the catalyst during ORR and
phosphorus doping on the reaction path of P-Fe
3
Co
1
@NC/ thereby further inducing the four-electron reaction path.
2ꢀ
Fig. 4 (a) LSV curves of different component catalysts at the rotation rate of 1600 rpm. (b) Corresponding Tafel plots. (c) HO yields (top panel)
and electron transfer numbers (bottom panel) of RRDE measurements.
9454 | RSC Adv., 2021, 11, 9450–9458
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
2 3 1
Fig. 5 (a) Durability evaluation at 0.6 V in an O -saturated 0.1 M KOH solution. (b) Chronoamperometric response of ORR for P-Fe Co @NC/
CNTs and Pt/C by introducing 1 M methanol solution after 400 s.
When comparing the LSV curves of the different materials and a lower peroxide formation can effectively prevent the
Fig. 4a) the P-doped P-Fe Co @NC/CNTs exhibits a higher limit catalyst from being eroded.
current density than Fe Co
tional commercial 20% Pt/C.
(
3
1
3
1
@NC/CNTs, CNTs and the tradi-
It is worth noting that the introduction of phosphorus voids
the electroneutrality leading to high positive spin density and
More information on the oxygen adsorption mechanism and atomic charge density for carbon atoms, which renders those
oxygen reduction kinetics of the catalysts can be deducted from carbon atoms adjacent to P as the catalytic sites for ORR.
the diffusion-corrected Tafel curve (Fig. 4b). In the low current Consequently, P-Fe Co @NC/CNTs exhibits
a
number of
3
1
density region, several catalysts, i.e., P-Fe Co @NC/CNTs transferred electrons very close to 4 similar to 20% Pt/C and
3
1
ꢀ
1
ꢀ1
(
3
69 mV dec ) and CNTs (62 mV dec ), exhibit lower Tafel a low hydrogen peroxide yield (<5%) indicating that P-Fe -
ꢀ
1
slopes than 20% Pt/C (76 mV dec ) and Fe
3
1 1
Co @NC/CNTs Co @NC/CNTs shows a good selectivity for the four-electron
ꢀ
1
(
78 mV dec ) in ORR. But in the high current density region, reaction pathway.
ꢀ1
P-Fe
3
Co
1
@NC/CNTs (122 mV dec ) has the lowest Tafel slope
The stability and methanol tolerance of the catalyst are also
among the catalysts above. A lower value of Tafel slope implies important indices for its electrochemical performance and its
47
faster kinetics indicating that the catalyst can quickly reach commercialization potential. As shown in Fig. 5a, aer 25 000 s of
a higher catalytic current density under a lower applied poten- the ORR, P-Fe Co @NC/CNTs still maintained a highly effective
3
1
tial. It can be concluded that P doping of Fe Co @NC/CNTs oxygen reduction activity with an activity loss of only 5.2%. However,
3
1
increases the limit current density and simultaneously the electrocatalytic activity of Pt/C catalyst decreased ꢃ30% aer
improves the ORR dynamics. This result is in good agreement 25 000 s of electrocatalytic reaction, which was mainly due to the
with the results of LSV curves presented in Fig. 4a.
In order to more accurately study the specic reaction paths particles from the carbon supports. The much slower decay in the
and reaction mechanisms of different catalysts during ORR, current density of P-Fe Co @NC/CNTs is a consequence of the
aggregation of Pt nanoparticles and the dissociation of Pt nano-
3
1
rotating ring-disk electrode (RRDE) experiments were conduct- synergistic contribution arising from the Fe–Co nanoparticles and
ed. One can conclude from the results depicted in Fig. 4c, that their coating materials of NC, which well avoids the structure vari-
the reaction path selectivity of the materials with encapsulated ation of the P-Fe Co @NC/CNTs and suppresses the agglomeration/
3
1
3 1
transition metal particles were signicantly improved under dissolution of the Fe Co nanoparticles during the ORR.
ORR conditions as evidenced by a higher number of transferred The chronoamperometric response of ORR to 1 M methanol
electrons and a lower peroxide yield compared to CNTs. A solution (Fig. 5b) indicates a dramatic decrease in the activity of
higher transfer electron number increases energy efficiency, the commercial Pt/C immediately aer the introduction of
2
Fig. 6 LSV curves in the O -saturated 0.1 M KOH solution at a rotation rate of 1600 rpm.
©
2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 9450–9458 | 9455

View Article Online
RSC Advances
Paper
Fig. 7 Schematic diagram of P element doping mechanism.
L
methanol, which oxidizes and blocks active sites on the catalyst. wave potential (Fig. 6a), and its limit current density (J )
ꢀ2
3 1
However, the effect of methanol on P-Fe Co @NC/CNTs was increased from 3.81 to 4.21 mA cm indicating that the ORR
negligible which demonstrates the higher durability of this performance of CNTs can be improved by P-doping. The LSV
catalyst to methanol in ORR. These results indicate the advan- curves shown in Fig. 6b do not reveal any obvious difference in
tage of metal particles being wrapped by carbon materials and the ORR performance of Fe Co @NC before and aer doping
3
1
the synergistic effects of both transition metals and of phos- with phosphorus. Therefore, we speculate that the reason for
phorus and carbon. The above series of electrochemical tests the excellent ORR activity of P-Fe Co @NC/CNTs is mainly
demonstrates the superior performance of P-Fe Co @NC/CNTs caused by the increased activity of CNTs aer P-doping. To
which is inexpensive and simple to synthesize thereby showing further prove this hypothesis, the ORR performance of the
a great potential for its use in the development of fuel cells. Fe Co @NC composite catalysts with different supports, CNTs
To elucidate the location of phosphorus doping and the and P-doped CNTs, were compared. Referring to Fig. 6c, Fe
mechanistic role of phosphorus in promoting the ORR of such Co @NC/P-CNTs did not show any obvious activity improve-
materials, CNTs and Fe Co @NC were doped with phosphorus ment compared to Fe Co @NC/CNTs, while both their ORR
3
1
3
1
3
1
3
-
1
3
1
3
1
by the same method and their ORR performance tested. The performance was lower than that of P-Fe Co @NC/CNTs. This
3
1
3 1
obtained P-CNTs exhibits a higher initial potential and half- result indicates that P-doping of Fe Co @NC/CNTs composites
Fig. 8 (a) Open circuit plots with inset showing the photograph of home-made Zn–air battery with an open circuit voltage of 1.47 V. (b)
Schematic representation of a rechargeable Zn–air battery. (c) Polarization curves and corresponding power density plots of the Zn–air batteries
with P-Fe
3 1 3 1
Co @NC/CNTs and Pt/C catalysts. (d) Discharge curves of the primary Zn–air battery with P-Fe Co @NC/CNTs catalyst until complete
ꢀ2
consumption of Zn at 10 mA cm current densities. Specific capacity was normalized to the mass of consumed Zn.
9456 | RSC Adv., 2021, 11, 9450–9458
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
have obvious advantages compared to treating the CNTs alone transport and rapid reaction kinetics during the catalytic
with phosphorus. process and which increases the dispersion of the material,
During the P-doping process, NaH PO produced phosphine consequently preventing P-Fe Co @NC/CNTs deactivation by
2
4
3
1
ꢁ
gas (PH ) during heating at 300 C, which reacted with the suppressing agglomeration in the long-term cycle test; (v) the
3
material. The possible reaction process is illustrated in Fig. 7. encapsulation of transition metal alloy particles into highly
PH
the vacancy of defective carbon layers through the depicted and diminished the inuence of external solution on the alloy
reactions, which is consistent with the XPS results of phos- particles thereby improving the stability of P-Fe Co @NC/CNTs;
3
molecules can be embedded into the skeleton of CNTs at active carbon-based material effectively prevented the erosion
3
1
phorus being mainly bound to the material by a P–C bond. For in addition, the thinner wall facilitates the transfer of electrons
the Fe Co @NC catalyst the ORR performance did not improve from the environment to the alloy particles through its surface;
3
1
signicantly aer P-doping. For both P-Fe Co @NC and P-CNTs (vi) P-doping improved the surface structure of carbon-based
3
1
the phosphorus content were about 0.8 at% as determined by material, thus increasing the number of surface active sites of
XPS. While these results indicate that phosphorus can be doped P-Fe Co @NC/CNTs and promoting its ORR activity. Moreover,
into Fe Co @NC and CNTs, as a new hypothesis it can be the Zn–air battery assembled with P-Fe Co @NC/CNTs catalyst
proposed that the defective Fe Co @NC and CNTs can react exhibits a high open circuit voltage and excellent long-term
with one PH molecule when two defect sites are adjacent to cycling performance. These highly efficient ORR catalysts with
each other. In this case, Fe Co @NC and CNTs can be joined encapsulated alloy particles in a conned space and with the
together by P atoms and attain favorable synergies. incorporation of heteroatoms, offer a new concept for future
3
1
3
1
3
1
3
1
3
3
1
Additionally, Zn–air battery performances were tested with P- clean energy catalyst and new opportunities for the further
Fe Co @NC/CNTs acting as the catalyst of the air cathode in development of fuel cell.
3
1
comparison with the coupled noble metal Pt/C catalyst. Fig. 8a
shows that the battery containing the P-Fe Co @NC/CNTs
affords an open-circuit voltage of 1.466 V. Multiple Zn–air
3
1
Conflicts of interest
batteries connected in series can thus be fabricated and inte- There are no conicts to declare.
grated into circuits to meet specic energy/power needs for
various applications. One proof-of-concept illustration is to
power a light emitting diode (LED, 2 V) with excellent operation
Acknowledgements
stability (Fig. 8b).
The work was supported by the National Natural Science
Fig. 8c reproduces the power density curves for Zn–air Foundation of China (21906142 and 21876144).
batteries with P-Fe Co @NC/CNTs and Pt/C catalysts. The
3 1
maximum power density of the Zn–air battery using P-Fe
3
ꢀ
-
,
References
2
1
Co @NC/CNTs catalyst was calculated to be 73 mW cm
ꢀ
2
which is comparable to that of Pt/C catalyst (71 mW cm ). The
1 O. Z. Sharaf and M. F. Orhan, Renewable Sustainable Energy
Rev., 2014, 32, 810–853.
2 S. Yasuda, Y. Uchibori, M. Wakeshima, Y. Hinatsu,
H. Ogawa, M. Yano and H. Asaoka, RSC Adv., 2018, 8,
37600–37605.
ꢀ2
galvanostatic discharge curves at 10 mA cm are presented in
Fig. 8d. The battery with the P-Fe Co @NC/CNTs catalyst and
3
1
that with the Pt/C catalyst showed voltage plateaus both around
ꢀ
1
1
.20 V, with specic capacity of 763 and 732 mA h g , respec-
tively, when normalized to the weight of consumed zinc
electrode.
3 S. H. Tai and B. K. Chang, RSC Adv., 2019, 9, 6035–6041.
4 N. Yamada, D. Kowalski, A. Koyama, C. Zhu, Y. Aoki and
H. Habazaki, RSC Adv., 2019, 9, 3726–3733.
5
S. Y. Shan, J. Luo, J. F. Wu, N. Kang, W. Zhao, H. Cronk,
Y. G. Zhao, P. Joseph, V. Petkov and C. J. Zhong, RSC Adv.,
2014, 4, 42654–42669.
4
. Conclusions
In summary, using simple and inexpensive methods a series of
transition metal catalysts encapsulated in bamboo-like nitrog-
enous carbon-based material was prepared and the surface of
the materials successfully doped with phosphorus. Based on the
applied analytical methods and comparisons, it is proved that
phosphorus atoms are incorporated at the defect locations of
CNTs by means of P–C bonds. The superior electrocatalytic
activity of P-Fe
synergistic interaction of the two metals in P-Fe
promoting their ORR activity; (ii) the higher pyridinic-N
content; (iii) a small amount of metal nitride in P-Fe Co @-
6 T. Meng, A. Nsabimana, Z. Y. Liu, H. X. Jia, S. Y. An, H. Wang
and Y. F. Zhang, J. Colloid Interface Sci., 2020, 579, 12–20.
7 A. Zadick, L. Dubau, N. Sergent, G. Berthom ´e and
M. Chatenet, ACS Catal., 2015, 5, 4819–4824.
8 C. Y. Su, H. Cheng, W. Li, Z. Q. Liu, N. Li, Z. Hou, F. Q. Bai,
H. X. Zhang and T. Y. Ma, Adv. Energy Mater., 2017, 7,
1602420.
9 S. S. Li, W. H. Chen, H. Z. Pan, Y. W. Cao, Z. Q. Jiang,
X. N. Tian, X. G. Hao, T. Maiyalagan and Z. J. Jiang, ACS
Sustainable Chem. Eng., 2019, 7, 8530–8541.
3
Co
1
@NC/CNTs can be attributed to (i) the
3
Co @NC/CNTs
1
3
1
NC/CNTs providing more ORR active sites; (iv) the presence of 10 B. K. Barman and K. K. Nanda, ACS Sustainable Chem. Eng.,
CNTs facilitating the formation of the interpenetrated conduc- 2018, 6, 12736–12745.
tivity network structure which improves the electrical conduc- 11 N. Mohammadi-Rad, J. J. Sardroodi and M. D. Esrali, J. Mol.
tivity of the P-Fe Co @NC/CNTs to promote a fast electron Graphics Modell., 2020, 100, 107647.
3
1
©
2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 9450–9458 | 9457

View Article Online
RSC Advances
Paper
1
1
2 T. Y. Ma, J. R. Ran, S. Dai, M. Jaroniec and S. Z. Qiao, Angew. 30 A. L. Shen, Y. Q. Zou, Q. Wang, R. A. W. Dryfe, X. B. Huang,
Chem., Int. Ed., 2015, 54, 4646–4650.
S. Dou, L. M. Dai and S. Y. Wang, Angew. Chem., Int. Ed.,
2014, 53, 10804–10808.
3 W. Lei, Y. P. Deng, G. R. Li, Z. P. Cano, X. L. Wang, D. Luo,
Y. S. Liu, D. Wang and Z. W. Chen, ACS Catal., 2018, 8, 31 I. Y. Jeon, H. J. Choi, M. J. Ju, I. T. Choi, K. Lim, J. Ko,
2
464–2472.
H. K. Kim, J. C. Kim, J. J. Lee, D. Shin, S. M. Jung,
J. M. Seo, M. J. Kim, N. Park, L. Dai and J. B. Baek, Sci.
Rep., 2013, 3, 2260.
1
1
4 S. Y. Jing, D. R. Wang, S. B. Yin, J. J. Lu, P. K. Shen and
P. Tsiakaras, Electrochim. Acta, 2019, 298, 142–149.
5 Z. T. Li, W. N. Zhao, C. Z. Yin, L. Q. Wei, W. T. Wu, Z. P. Hu 32 T. Shari, G. Z. Hu, X. E. Jia and T. Wagberg, ACS Nano, 2012,
and M. B. Wu, ACS Appl. Mater. Interfaces, 2017, 9, 44519–
4528.
6 H. L. Jiang, Y. H. Zhu, Q. Feng, Y. H. Su, X. L. Yang and
6, 8904–8912.
4
33 S. Chen, J. Y. Bi, Y. Zhao, L. J. Yang, C. Zhang, Y. W. Ma,
Q. Wu, X. Z. Wang and Z. Hu, Adv. Mater., 2012, 24, 5593–
5597, 5646.
1
1
C. Z. Li, Chem.–Eur. J., 2014, 20, 3106–3112.
7 J. Q. Chi, W. K. Gao, J. H. Lin, B. Dong, K. L. Yan, J. F. Qin, 34 H. T. Chung, J. H. Won and P. Zelenay, Nat. Commun., 2013,
B. Liu, Y. M. Chai and C. G. Liu, ChemSusChem, 2018, 11,
43–752.
8 K. Hu, Z. H. Xiao, Y. Cheng, D. F. Yan, R. Chen, J. Huo and
S. Y. Wang, Electrochim. Acta, 2017, 254, 280–286.
4, 1–5.
7
35 H. Yin, C. Z. Zhang, F. Liu and Y. L. Hou, Adv. Funct. Mater.,
2014, 24, 2930–2937.
36 Y. Y. Wen, C. Ma, Z. T. Wei, X. X. Zhu and Z. X. Li, RSC Adv.,
2019, 9, 13424–13430.
1
1
9 Y. Wang, W. Chen, Y. Chen, B. Wei, L. H. Chen, L. S. Peng,
R. Xiang, J. Li, Z. C. Wang and Z. D. Wei, J. Mater. Chem. A, 37 T. Thanh-Nhan, H. Y. Lee, J. D. Park, T. H. Kang, B. J. Lee and
018, 6, 8405–8412. J. S. Yu, ACS Appl. Energy Mater., 2020, 3, 6310–6322.
0 Y. C. Wang, Y. J. Lai, L. Song, Z. Y. Zhou, J. G. Liu, Q. Wang, 38 Y. J. Qian, Z. Y. Chen, S. F. Huang and D. X. Li, Integr.
2
2
X. D. Yang, C. Chen, W. Shi, Y. P. Zheng, M. Rauf and
S. G. Sun, Angew. Chem., Int. Ed., 2015, 54, 9907–9910.
1 C.-L. Zhang, B.-R. Lu, F.-H. Cao, Z.-Y. Wu, W. Zhang,
H.-P. Cong and S.-H. Yu, Nano Energy, 2019, 55, 226–233.
2 T. Sun, B. B. Tian, J. Lu and C. L. Su, J. Mater. Chem. A, 2017,
Ferroelectr., 2020, 209, 68–75.
39 M. T. Lu, Y. W. Li, Y. H. Wu, H. Xu, J. K. Gao and S. Q. Xu,
ChemNanoMat, 2020, 6, 1496–1501.
40 H. S. Liang, H. Xing, M. Qin and H. J. Wu, Composites, Part A,
2020, 135, 105959.
41 T. Dong, X. Zhang, P. Wang, H.-S. Chen and P. Yang, Carbon,
2019, 149, 222–233.
42 G. L. Wen, H. J. Niu, J. J. Feng, X. L. Luo, X. X. Weng and
A. J. Wang, J. Colloid Interface Sci., 2020, 569, 277–285.
43 L. J. Yang, S. Z. Feng, G. C. Xu, B. Wei and L. Zhang, ACS
Sustainable Chem. Eng., 2019, 7, 5462–5475.
44 Y. T. Xu, B. L. Chen, J. Nie and G. P. Ma, Nanoscale, 2018, 10,
17021–17029.
2
2
2
2
2
2
2
2
2
5
, 18933–18950.
3 M. Q. Wang, C. Ye, M. Wang, T. H. Li, Y.-N. Yu and S. J. Bao,
Energy Storage Mater., 2018, 11, 112–117.
4 J. L. Long, R. Li and X. L. Gou, Catal. Commun., 2017, 95, 31–
35.
5 H. J. Meng, X. H Chen, T. L. Gong, H. R. Liu, Y. M. Liu, H. Li
and Y. M. Zhang, ChemCatChem, 2019, 11, 6015–6021.
6 X. Liu, L. Wang, P. Yu, C. G. Tian, F. F. Sun, J. Y. Ma, W. Li
and H. G. Fu, Angew. Chem., Int. Ed., 2018, 57, 16166–16170. 45 Z. Liu, B. Tang, X. C. Gu, H. Liu and L. G. Feng, Chem. Eng. J.,
7 P. W. Cai, Y. Hong, S. Q. Ci and Z. H. Wen, Nanoscale, 2016,
, 20048–20055.
2020, 395.
8
46 W. Y. Gou, J. H. Bian, M. K. Zhang, Z. M. Xia, Y. X. Liu,
Y. D. Yang, Q. C. Dong, J. Y. Li and Y. Q. Qu, Carbon, 2019,
155, 545–552.
8 J. Deng, L. Yu, D. H. Deng, X. Q. Chen, F. Yang and X. H. Bao,
J. Mater. Chem. A, 2013, 1, 14868–14873.
9 H. Hu, Y. H. Xie, F. M. D. Kazim, K. G. Qu, M. Li, Z. K. Xu and 47 A. S. Arico, S. Srinivasan and V. Antonucci, Fuel Cells, 2001, 1,
Z. H. Yang, Sustainable Energy Fuels, 2020, 4, 5188–5194.
133–161.
9458 | RSC Adv., 2021, 11, 9450–9458
© 2021 The Author(s). Published by the Royal Society of Chemistry