Journal of The Electrochemical Society, 2020 167 116503

Synthesis of Porous N-Rich Carbon/MXene from

MXene@Polypyrrole Hybrid Nanosheets as Oxygen Reduction
Reaction Electrocatalysts
Yanhua Lei,1,2,z Ning Tan,1 Yizhen Zhu,1 Da Huo,1 Shibin Sun,3 Yuliang Zhang,1,2 and
Guanhui Gao4
1
Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai, 201306, People’s
Republic of China
2
Shanghai Engineering Technology Research Centre of Deep Offshore Material, Shanghai, 201306, People’s Republic of
China
3
College of Logistics Engineering, Shanghai Maritime University, Shanghai 201306, People’s Republic of China
4
Material Science and Nano engineering Department, Rice University, Houston, Texas, United States of America

Noble Metal-free catalysts attracted much attention as promising candidates for Pt-based catalyst replacement to advance
applications related to oxygen reduction reaction (ORR), which is critical for large-scale renewable energy storage and conversion.
Herein, this work focused on a synthesis of noble metal-free ORR electrocatalysts consisting of porous N-rich carbon/MXene,
which was obtained using very conductive and reactive Ti3C2 MXene and polypyrrole (PPy) as a C and N source. The
electrocatalyst exhibited excellent electrocatalytic activity and stability with an onset and a half-wave potentials equal to 0.85 and
0.71 V, respectively. Results obtained in this work demonstrate how to design efficient noble metal-free ORR electrocatalysts
applicable to other chemical systems.
© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access
article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/
1945-7111/aba15b]

Manuscript submitted February 16, 2020; revised manuscript received June 16, 2020. Published July 8, 2020. This paper is part of
the JES Focus Issue on 2D Layered Materials: From Fundamental Science to Applications.
Supplementary material for this article is available online

Alternative energy solutions as a substitution of traditional fossil and is currently the most studied MXenes member. It was
fuels attracted enormous attention due to the increasing environ- synthesized by removing some Al atoms from Ti3AlC2 compounds,
mental concerns.1,2 Fuel cells can contribute to solving this problem which is a member of layered ternary carbide and nitride family
because they are very efficient and cause minimum pollution.3–5 called MAX. Wen et al.23 validated that the MXene-supported FeNC
However, their sluggish kinetics and the large overpotential during catalysts were excellent for ORR. Liu et al.25 used first principle
the cathodic oxygen reduction reaction (ORR) prevent their more calculations to understand whether O and F-terminated MXene ORR
extensive application and use.6,7 Materials containing platinum (Pt) catalyst are good substitution for Pt. Their analysis involved a
are considered the most effective ORR catalysts, but Pt is too microscopic assessment of the Tin+1CnTx surface structure as well as
expensive, scarce and often demonstrates poor long-term properties as ORR catalysts. Other literature reports mostly focused
durability.7,8 Therefore, more sustainable Pt alternatives are needed on MXenes stability as a noble or conventional metal support for
for ORR catalytic reaction for fuel cells to be advanced to larger ORR electrocatalysis.26 To date, MXenes could be as effective ORR
industrial scales. Several non-noble metal catalysts, including C- electrocatalysts because of the following advantages: (1) the
based materials (such as heteroatom doped,1,9 and N-doped C-based prominent metallic conductivity of 2D MXenes facilitates highly
materials10 as well as graphene11–13), are attractive alternatives efficient charge carrier transfer; (2) the terminal metal site on the
because of their efficiency. In particular, N-doped carbon materials surface (e.g., Ti, Nb, V or Ta) of 2D MXenes results in stronger
demonstrated good stability and excellent ORR electrochemical redox reactivity than that of other carbon-based materials; (3) 2D
activity. Therefore, N-doped C is currently considered to be the MXenes have high stability in aqueous media; (4) hydrophilic
most promising noble metal-free catalyst for ORR. Incorporation of surface of 2D MXenes leads to strong interaction with catalytic
N randomizes carbon defects, which facilitates the formation of targets or water molecules.27 However, catalytic properties of
active sites beneficial for ORR.14,15 Moreover, owing to their low MXenes by themselves were not reported to the best of our
coat, easy synthesis and varied morphology, polyaniline (PANI) and knowledge. Considering the above outstanding properties of
Polypyrrole (PPy) have been broadly employed to provide N and C MXenes and PPy, it is anticipated that coupled PPy and MXene
sources for fabrication of catalysts by pyrolysis. Additionally, due to Ti3C2 nanoparticles could be a promising electrocatalysts for oxygen
its high gas diffusion and conduction properties of PPy and PANI, reduction by taking advantage of the high electronic conductivity of
which offer a effectively route for metal-free catalyst with improved MXene Ti3C2 nanoparticles and better activity of N-doped carbon
oxygen transport through ionomers.16 from PPy.
MXenes belong to a new family of two-dimensional transition Therefore, this paper describes the synthesis and performance of
metal carbides/carbonitrides and possess high electro-conductivity a novel ORR catalyst containing N-doped C and MXene obtained
and active surface chemistries, and, as a result, show promise for using a simple two-step technique. First, the MXene was coated with
energy storage17–21 as well as NH3 sensors22 and ORR23–25 catalysts. PPy. The resulting composite was then pyrolyzed to obtain N-doped
MXenes general formula can be expressed as Mn+1XnTx (n = 1–3), C/MXene. Several materials with different N contents were prepared
where M is a transition metal, X is carbon and/or nitrogen, and Tx is by varying pyrolysis temperature, and catalytic ORR performances
a functional group (such as, O, F, and OH groups ). Currently, ∼20 of these materials were studied as a function of the N content.
different MXene compounds with various structures and chemical
compositions were synthesized. Ti3C2Tx was the first one discovered Experimental
Materials.—Ti3AlC2 (99%, 400 mesh) was purchased from 11
z technology Co. (China). LiF (99.5%), HCl (36%-38%), FeCl3∙6H2O
E-mail: yhlei@shmtu.edu.cn, ylzhang@shmtu.edu.cn
 Journal of The Electrochemical Society, 2020 167 116503

(99.5%) and pyrrole (Py, 99.9%) were purchased from Aladdin Electrochemical Measurements
Reagent Co. 20 wt% Pt/C catalyst was acquired from Shanghai
Electrochemical analysis was performed using Gamry work-
Macklin Biochemical Co. Deionized (DI) water was used in all
station containing a three-electrode cell with Pt wire, Ag/AgCl, and a
experiments.
catalyst-modified glassy carbon as a counter, reference and working
electrodes, respectively. 5 mm (in diameter) rotating disk electrode
Ti3C2Tx MXene synthesis.—Ti3C2Tx MXene was synthesized
(RDE) acted as a working electrode substrate. It was first polished by
using mild etching conditions described elsewhere.28 Specifically,
aqueous alumina suspensions and then rinsed with DI water and
1.0 g of LiF was added to 10 ml of 12 M HCl and stirred for
ethanol. To prepare a catalyst ink, 2 mg of MXene@PPy was mixed
5–10 min. Then, 1 g of Ti3AlC2 was slowly added to prevent
with 400 μl of ethanol and 17.5 μl of 5 wt% Nafion solution. The
excessive heating due to the vigorous carbide decomposition. After
resulting MXene@PPy concentration in this homogeneous suspen-
Ti3AlC2 was added, the resulting mixture was stirred at 35 °C for
sion was 4.79 mg ml−1. 10 μl of this suspension was evenly applied
24 h, after which the solid residue was collected, rinsed with DI
onto RDE using a syringe and air-dried. The total catalyst loadings
water and centrifuged at 3500 rpm. This step was repeated until the
were 0.2 mg cm−2. Prior to the electrochemical tests, the electrolyte
pH of the supernatant was > 6. The powder was then ultrasonicated
(which was 0.1 M KOH) inside the cell was saturated by O2 for
in DI water for 30 min under the Ar atmosphere and centrifuged for
40 min.
1 h at 5200 rpm. The supernatant, which contained MXene suspen-
Cyclic voltammetry (CV) was performed using the same
sion, was collected. 5 ml of this liquid was filtered, and then vacuum
electrolyte at 50 mV s−1 scan rate. Linear sweep voltammograms
dried. The resulting solid was weighed, and the number as used to
(LSV) were recorded also using the same electrolyte at varying
calculate MXene concentration in the supernatant. This information
rotational speed equal to 100, 400, 900, 1600, 2000, 2025, and
was used to obtain 0.5 g l−1 MXene solution.
2500 rpm. Chronoamperometry (conducted for 8000 s in the same
electrolyte at 0.5 V vs RHE) was performed to estimate the long-
MXene@PPy synthesis.—0.5 g l−1 MXene solution was ultra-
term durability of the catalysts.
sonicated for 1 h and then transferred into a double-layer flask with
The ORR electron transfer number (n) was calculated by the
the circulating cooling pump. During the synthesis, the temperature
Koutechy-Levich equation23
in this flask was maintained at 2 °C. To perform in situ PPy
polymerization, 1 ml of the pyrrole monomer was injected into a 1 1 1 1 1
0.5 g l−1 MXene solution. This mixture was marked as solution A. In = + = + , [1]
j jL jK Bw1 / 2 jK
a separate flask, 3.89 g of FeCl3∙6H2O as oxidant was ultrasonicated
in 50 ml of DI water for 1 h. This orange solution was marked as
solution B. The mole ratio of the pyrrole and the oxidant was set to B = 0.62nFCo DO 2 / 3v-1 / 6 [2]
1:1. Solution B was added through a constant pressure drop funnel to
solution A at a constant speed. The reaction was conducted at 2 °C where j, jK and jL are regular, kinetic and diffusion-limiting current
for 6 h under constant stirring. The resulting product was filtered and densities, respectively; w is the rotation rate of the electrode, F is the
rinsed several times with DI water followed by drying in a vacuum Faraday constant equal to 96485 C mol−1, Co is O2 saturation
at 60 °C. The resulting material was PPy-coated MXene named concentration (equal to 1.2 ´ 10-6 mol cm−1 for 0.1 M KOH), DO
throughout the paper as MXene@PPy. is the O2 diffusion coefficient (equal to 1.9 ´ 10-5 cm s−1), and v is
the kinetic viscosity of the 0.1 M KOH (equal to 0.01 cm2 s−1).
Pyrolysis of MXene@PPy.—MXene@PPy was annealed at 700 ° The four-electron selectivity of catalysts was evaluated by using
C, 800 °C, and 900 °C for 2 h under Ar atmosphere. The resulting the RRDE technique. The potential of the Pt ring electrode in the
samples were marked as MXene@PPy-T (T = 700, 800, and 900, RRDE system was set to 1.4 V vs RHE. The H2O2 yield can be
respectively). calculated from the following Eq. 429
IR / N
Material Characterizations H2 O2 % = 200 ´ , [ 3]
IR
Structural and morphological characterization was performed + ID
N
using transmission and scanning electron microscopies (TEM and
SEM, respectively) performed using JEOL 2100 and JEOL JSM- Then the electron transfer number (n) can be further calculated from
6510LA instruments, respectively. X-ray diffraction (XRD) was the following Eq. 4:
performed using UltimaIV instrument (Japan) in the 5°–90° 2θ range
at 40 mA and 40 kV. Raman spectroscopy performed using ID
n=4´ [4]
HORIBA Lab RAM HR Evolution instrument (France) was per- IR + ID
formed using 633 nm excitation wavelength. X-ray photoelectron
spectroscopy (XPS) was conducted by the ESCA Lab 250 Thermal Where ID and IR are the disk and ring currents, respectively, and N is
Scientific instrument. The Brunner–Emmet–Teller (BET) and the ring collection efficiency, equal to 0.37.
Horvath–Kawazoe (HK) methods were used to calculate specific All potential values measured relative to Ag/AgCl reference
surface area and porosity, respectively, using BELSORP-max electrode were transformed to the reversible hydrogen electrode
instrument. The thermal behaviors of the nanocomposites were (RHE) scale using the Nernst equation:
assessed using thermogravimetric analysis (TGA) performed by
E (RHE ) = E (Ag/AgCl ) + 0.0591pH + 0.197 [5]
the Perkin Elmer Diamond system. The TG–MS experiments were
performed using a thermogravi-metric analyzer (Netzsch STA
449 C/6/MFC/G Jupiter) coupled with an online mass spectrometer Results and Discussion
(Netzsch QMS 403) at a heating rate of 10 °C min−1 from room
temperature up to a final temperature of 900 °C under the helium Steps of the MXene@PPy nanocomposite synthesis are shown in
flow rate of 50 ml min−1. The derived products were swept into the Scheme 1. First, Ti3C2Tx MXene nanosheets were fabricated by
mass spectrometry through the hot capillary column connecting with etching and exfoliating the MAX phase using HCl and LiF solutions.
the TG analyzer. MXene@PPy was obtained after PPy was in situ polymerized
directly on MXene nanosheets followed by high-temperature heating
in the Ar atmosphere.
 Journal of The Electrochemical Society, 2020 167 116503

Scheme 1. Fabrication of MXene catalysts

Composition and structure of the MXene@PPy catalysts.— Then, pyrolysis temperature on the Raman spectrum were investi-
XRD peaks of the MXene@PPy were weak (see Fig. 1a) probably gated. The results are given in Fig. S1is (available online at stacks.
because of the amorphous nature of the PPy chains, which became iop.org/JES/167/116503/mmedia). As shown in Fig. S1, ID/IG value
disordered during oxidative polymerization and heating. XRD increased as the increase of carbonization temperature, indicating
spectrum of MXene@PPy-800 showed a peak at 26.2°, which we more defects and disordered structures were generated.
attributed to the (002) plane of graphite. Thus, the graphitization SEM of the MXene@PPy demonstrated a homogeneous PPy
degree of carbon component increased after the pyrolysis. The distribution throughout the MXene surface (see Figs. 2a and 2b).
presence of graphitic materials is beneficial for charge transfer.30 Pyrolysis treatment did not change the layer structure of the
Raman spectrum of MXene@PPy showed bands at 1372 and catalysts. (see Fig. 2c). Both SEM and TEM demonstrated the
1583 cm−1 (see Fig. 1b), which were attributed to C–N, and C=C presence of MXene layers (see Fig. 2d), which indicates successive
stretching vibrations, respectively. The bands at 924 and 1078 cm−1 exfoliation of the bulk Ti3AlC2 structure. MXene maintained its
were interpreted as C–H bending vibrations. The peak at 988 cm−1 layered structure even after PPy surface polymerization occurred and
corresponded to the vibrations of the radical cation (polaron) consequent carbonization was conducted (see Figs. 2c, 2e–2h).
structure, while the band at 1260 cm−1 was characteristic for the However, the high angle annular dark-field (HAADF) micrographs
polaron presence. Raman spectrum of the MXene@PPy-800 showed and corresponding energy dispersive X-ray (EDX) mapping of an
no bands below 1369 cm−1. Two bands at 1369 and 1598 cm−1 were individual nanoplate of the MXene@PPy-800 sample confirmed
attributed to D- and G-bands typical for carbon-based materials, homogeneous distribution of C and N on the MXene nanoplate,
which correspond to disordered graphite and vibrations of sp2 presence of which was revealed by the Ti-rich islands (see Figs. 2i
hybridized carbon atoms, respectively. The ID/IG ratio increased –2l).
after the pyrolysis, which indicates the presence of more defects in High-resolution TEM demonstrated areas with lattice fringes,
the MXene@PPy-800 sample than in its precursor, MXene@PPy.31 which probably belong to MXene (see Fig. 3). These fringes,

Figure 1. (a) XRD patterns and (b) Raman spectra of MXene@PPy and MXene@PPy-P800 materials.
 Journal of The Electrochemical Society, 2020 167 116503

Figure 2. SEM micrographs of MXene@PPy (a), (b) and MXene@PPy-800 (c). TEM micrographs of pristine MXene (d), MXene@PPy (e), (f), and
MXene@PPy-800 (g), (h). HAADF-STEMs (g), (i) and C (j), N (k), and Ti (l) distribution obtained by the EDX analysis for the MXene@PPy-800 sample.

positioned 0.408 nm apart, were ascribed to the (001) plane of Ti2C3. Micropores in MXene@PPy-800 formed during carbonization,
The inverse the Fast Fourier Transform (FFT) image of the selective while meso- and macropores formed as spacings between the
area demonstrated disordered turbostratic carbon nanodomains of aggregated nanosheets.32,33 The large surface area of
MXene@PPy-800 (see in Fig. 3b), which caused the formation of MXene@PPy-800 provided abundant active sites, while its porosity
free volume and micro-porosity in the carbon skeleton. Figure 3e assisted in the reactants delivery. Carbonization temperature on the
shows our interpretation of the formed voids and pores. The N2 surface area and pore size distribution of the synthesized catalysts
adsorption-desorption isotherms and DFT pore size distribution were studied. The results of N2 adsorption-desorption isotherms and
(shown in Figs. 3f and 3g, respectively) revealed the hierarchical DFT pore size distribution were illustrated in Fig. S2, and the related
porosity and high surface area (equal to 390.5 m2 g−1) for data were summered in Table SI. As shown in Fig.S2 and Table SI,
MXene@PPy-800. The surface area MXene@PPy was just 28.0 the surface area of as-prepared catalysts improves with the increase
m2 g−1 that was almost 14 times lower than Mxene@PPy-800. of pyrolysis temperature from 700 °C to 900 °C, indicating the
 Journal of The Electrochemical Society, 2020 167 116503

Figure 3. (a) HR-TEM micrograph, (b) inverse FFT image of the selected area, (c) auto-correlation image of the inverse FFT image and (d) of the selected area
for the of the MXene@PPy-800 sample. (e) Structure and position of the pores and voids in N-doped C. (g) N2 adsorption-desorption isotherms and (g) pore size
distribution for MXene@PPy and MXene@PPy-800 materials.

pyrolysis temperature plays a key role in the synthesis of carbon- and 289.5 eV (see Fig. 4d), which were ascribed to C−Ti, C=C, C
based ORR electrocatalysts.34 −C, C−N bonds and to disordered C, respectively.1,23 We also used
XPS of MXene@PPy showed the presence of C, N, O, and Ti XPS to understand the differences in the samples pyrolyzed at
elements, confirming the chemical composition corresponding to the different temperatures. N 1 s XPS data for the MXene@PPy-700,
MXene@PPy composite (see Fig. 4a). The total N content was 7.9% MXene@PPy-800, and MXene@PPy-900 samples are shown in
(see Fig. 4b). High-resolution N 1 s spectrum of MXene@PPy was Fig. 5 and Table I. MXene@PPy-800 material exhibited the highest
deconvoluted into two peaks positioned at 399.8 and 401.9 eV, content of graphitic nitrogen contents.
which correspond to pyrrolic and positively charged (−N+−) The thermal stability of PPy and MXene@PPy was assessed
nitrogen,35 respectively (see Fig. 4b). N 1 s spectrum of the using TGA (see Fig. 6). TGA curve for the MXene@PPy was very
MXene@PPy-800 showed the presence of pyridinic, pyrrolic, similar to the PPy curve, which indicates its excellent thermal
graphitic nitrogen and oxidized N (see Fig. 4c). Contributions of stability. The weight loss up to 120°C was attributed to the
these groups to the total N content of the MXene@PPy-800 were desorption of the loosely bonded water, while weight loss after
equal to 17.62%, 11.19%, 66.33,and 4.86%, respectively (see 120 °C was due to the remove of doped anions and degradation of
Table I). These results indicate that high-temperature carbonization the PPy polymeric matrix.43 According to the results of TG-MS in
significantly affected the chemical state of nitrogen, especially Fig. S3, the fragment with m/z at 36 generated from around 150 °C
graphitic N content. N atoms in the pyridinic and graphitic was probably attributed to remove of doped Cl− anions in PPy. More
configuration participate in ORR as active sites.36–39 The nitrogen traces of compounds 1evolved when the temperature increased up to
atom in the pyridinic configuration possesses one lone pair of 600 °C, indicating the fracture of aromatic nucleus in PPy polymeric
electrons as well as electrons donated to the conjugated p-bond. matrix. The compound fragments includes CH3, CNH, C2H2, and the
Thus, its presence might enhance reductive O2 adsorption and non-hydrocarbon compounds include H2, H2O, and CO2.
exhibit improved catalytic properties.40 N atoms in graphitic
configuration can promote the electron transfer from the C bonding Electrochemical performance of the MXene@PPy and
orbitals to the O orbitals.41,42 The C 1 s XPS spectrum was MXene@PPy-T catalysts.—The ORR activity of the
deconvoluted into five peaks located at 284.3, 284.9, 286.1, 287.7, MXene@PPy-800, compared with that of MXene@PPy and 20 wt
 Journal of The Electrochemical Society, 2020 167 116503

Figure 4. (a) XPS survey and N 1 s spectra for the MXene@PPy. High-resolution N 1 s (c) and C 1 s (d) peaks for the MXene@PPy-800 material.

Table I. Contents (in wt%) of nitrogen in pyridinic, pyrrolic, and graphitic configurations for the MXene@PPy catalysts heated at 700 °C (sample
MXene@PPy-700), 800 °C (sample MXene@PPy-800) and 900 °C (sample MXene@PPy-900).

Sample Graphitic- N/Ntotal(%) Pyrrolic- N/Ntotal(%) Pyridini- N/Ntotal(%) Oxidized- N/Ntotal(%)

MXene@PPy-700 45.92% 14.59% 36.44% 3.05%

MXene@PPy-800 66.33% 11.19% 17.62% 4.86%
MXene@PPy-900 47.31% 16.28% 25.28% 11.13%

% Pt/C catalysts, was assessed using CV. MXene@PPy-800 sample further quantify the ORR pathway, a rotating ring-disk electrode
showed strong cathodic reduction peaks, which is indicative of the (RRDE) technique was used to detect peroxide species formed at the
excellent ORR activity (see Fig. 7a). LSV of MXene@PPy-800 disc electrode while the potential of Pt ring electrode was set to
showed an onset potential (Eoneset) equal to 0.85 V (see Fig. 7b), 1.4 V vs RHE, as shown in Fig. S4. As observed from Fig. S4, the
which is ∼0.12 V lower than Eoneset of 20 wt% Pt/C electrode (equal corresponding electron transfer number keeps at 3.89 ∼ 3.85 and the
to 0.97 V). Half-wave potential (E1/2) of MXene@PPy-800 was yield of H2O2 varies from 5.5% to 7.1% at a potential window of 0.2
equal to 0.71 V, which is 0.12 V lower than E1/2 of the 20 wt% Pt/C ∼ 0.7 V. This further indicates a good catalytic selectivity of
electrode (equal to 0.83 V). Thus, the commercial Pt/C catalyst MXene@PPy-800 approximately equal for a four-electron reduction
showed better performance than the MXene@PPy-800 catalyst very pathway.
likely because of Pt presence. Pyrolysis temperature is a vital parameter affecting final proper-
ORR kinetic parameters for MXene@PPy-800 were obtained ties of the synthesized C-based ORR catalysts.10,14,43 The graphiti-
using LSV recorded at different rotating speeds. The limiting current zation degree, which depends on the pyrolysis temperature, strongly
densities increased as rotating rates increased (see Fig. 7c). The correlates with the ORR activity of C-based catalysts.44 ORR
corresponding Koutecky-Levich (K-L) plots (see insert in Fig. 7d) performance parameters of the MXene@PPy-T catalysts (shown in
obtained from1 the LSV data curves disclosed the linear relationships Fig. 8) are listed in Table II. MXene@PPy-800 showed the best
between w- 2 and j-1，and exhibited the first-order reaction catalytic performance. Eonset values of MXene@PPy-900 (which
kinetics.10 The transferred electron numbers obtained from the K- was equal to 0.88 V) and of MXene@PPy-800 (which was equal to
L plots for the 0.2–0.5 V range were equal to 3.4–3.95 (Fig. 7d). 0.85) were more positive than of MXene@PPy-P700 (which was
Thus, it is very likely that a four-electron reduction pathway for the equal to 0.73 V). The half-wave potential of MXene@PPy-800
MXene@PPy-P800 catalyst existed during the ORR rection. To (which was equal to 0.71 V) was 170 and 80 mV more positive than
 Journal of The Electrochemical Society, 2020 167 116503

Figure 5. High-resolution N 1 s XPS data for (a) MXene@PPy-700, (b) MXene@PPy-800, and (c) MXene@PPy-900 samples.

According to the results of Raman spectrum in Fig.S1 and BET

in Fig. S2, although both the ID/IG value and surface area of
MXene@PPy-900 are larger than those of MXene@PPy-800, the
catalyst carbonized at 800 °C exhibits superior catalytic performance
than the catalysts treated at 900 °C. XPS results described above
showed that pyrrolic nitrogen content decreased as the pyrolysis
temperature increased, while that of graphitic N increased signifi-
cantly. Graphitic-N was considered to be more active sites for ORR,
mainly because the graphitic N in the carbon structure can promote
the transfer of electrons from the bonding orbital of carbon to the
back-orbiting of oxygen.41,42 Thus, the electrochemical efficiency of
our catalysts directly correlated with the graphitic nitrogen content,
which agrees with similar data previously reported in the literature.36
We also compared the long-term electrochemical stability of
MXene@PPy-800 and 20% Pt/C catalysts. An i-t response of
MXene@PPy-800 and Pt/C recorded for 8000 s at 200 rpm is shown
in Fig. 9. MXene@PPy-P800 catalyst exhibited better durability
than commercial Pt/C catalyst: only 13.8% of the activity was lost
after 8000 s of a continuous ORR. For comparison, commercial Pt/C
catalyst demonstrated 5% lower activity after 8000 s.
Figure 6. TGA curves for PPy and MXene@PPy. Conclusions

of MXene@PPy-700 and of MXene@PPy-900, respectively (which This work demonstrated the quick and practical synthesis of a
were equal to 0.54 and 0.63 V, respectively). As the pyrolysis composite material consisting of N-doped carbon and MXene, which
temperature was increased, the limiting current density of the was then used as an ORR catalyst. The best composite material was
corresponding catalysts first increased from 1.73 to 3.63 mA cm−2 obtained by sintering the precursor at 800 °C. The resulting catalyst,
and then decreased to −2.83 mA cm−2. MXene@PPy-800, contained 9.6% of homogeneously distributed
nitrogen. An onset and half-wave potentials of this catalyst during
 Journal of The Electrochemical Society, 2020 167 116503

Figure 7 (a) CV curves recorded at 50 mV s−1 scan rate and (b) LSV curves recorded at 1600 rpm for MXene@PPy-800 and 20% Pt/C. (c) LSV curves recorded
at various rotational speeds during ORR under the presence of the MXene@PPy catalyst. (d) K-L plots and the electron transfer numbers (shown in insert) of the
MXene@PPy-800 catalyst.

Figure 8. Curves showing results of (a) CV conducted at 50 mV s−1 and (b) LSV conducted at 1600 rpm for MXene@PPy-700, MXene@PPy-800, and
MXene@PPy-900 catalysts.

the ORR were 0.85 and 0.71 V, respectively. Such excellent catalytic better than that of a commercial Pt/C catalyst. Such excellent
performance was attributed to the high content of graphitic nitrogen, performance makes our novel catalyst a very good and sustainable
which was equal to 80.6%. A four-electron reduction pathway candidate for the next generation full cells and metal-air batteries.
during the ORR rection for the MXene@PPy-P800 catalyst were
concluded from the results of RDE and RRDE measurements. The
catalyst demonstrated long-term stability during ORR, which was
 Journal of The Electrochemical Society, 2020 167 116503

Table II. Onset and half-wave potentials as well as current density for 20%Pt/C and MXene@PPy-T catalysts prepared at T = 700, 800, 900 °C. All
potentials reported in here are vsRHE. Eoneset is the potential value corresponding to 5% of the value of the limiting current density.

Catalyst Eoneset, V E1/2, V Current density, mA cm−2 at 0.2 V

MXene@PPy-700 0.73 0.54 −1.73

MXene@PPy-800 0.85 0.71 −3.63
Mxene@PPy-900 0.88 0.63 −2.83
20wt.%Pt/C 0.97 0.83 −4.52

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