Electrochimica Acta 362 (2020) 137143
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Conversion of maize straw into nitrogen-doped porous graphitized
carbon with ultra-high surface area as excellent oxygen reduction
electrocatalyst for flexible zinc–air batteries
Xiaoqiong Hao a,c,1, Weiheng Chen b,1, Zhongqing Jiang b,c,∗, Xiaoning Tian c, Xiaogang Hao a,
Thandavarayan Maiyalagan d, Zhong-Jie Jiang e
a
Department of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
Key Laboratory of Optical Field Manipulation of Zhejiang Province, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, PR China
c
Department of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo 315211, PR China
d
Electrochemical Energy Laboratory, Department of Chemistry, SRM Institute of Science and Technology, SRM Nagar, Kattankulathur 603203, India
e
Guangdong Engineering and Technology Research Center for Surface Chemistry of Energy Materials & Guangzhou Key Laboratory for Surface Chemistry of
Energy Materials, New Energy Research Institute, College of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China
b
a r t i c l e
i n f o
Article history:
Received 14 July 2020
Revised 30 August 2020
Accepted 17 September 2020
Available online 22 September 2020
Keywords:
Maize straw
Nitrogen-doped porous graphitized carbon
Oxygen reduction reaction
Flexible zinc–air batteries
Agricultural waste reuse
a b s t r a c t
Rational design and preparation of waste biomass derived carbon as air electrode catalysts are pivotal for
large-scale sustainable development of Zn–air battery, simultaneously promoting waste resource reuse.
Herein, a three-dimensional nitrogen-doped porous graphitized carbon (MS-NPC) is designed by employing maize straw as carbon precursors. The N-doping can cause increased active sites for oxygen
reduction reaction (ORR), and the FeCl3 activated porous graphitized structure provides an efficient O2
and electrolyte pathway toward easily access to active sites. The resultant N-doped porous graphitized
maize straw carbon (MS-NPC) surprisingly exhibits high specific surface area (1483 m2 g−1 ), high nitrogen content (4.70%), a rather positive onset potential (0.985 V vs. RHE) and large limiting current density
(5.8 mA cm−2 ), which is even better than most reported leading results based on biomass-derived carbon. Density-functional-theory computations confirm the synergetic effect between N-doping and FeCl3
activated porous graphitized structure are promising to accelerate ORR process which is the essential reason for high ORR catalytic performance of MS-NPC. Furthermore, a primary Zn–air battery (ZAB) designed
with MS-NPC electrode displays a maximum power density (127.9 mW cm−2 ), super specific-dischargecapacity (794 mAh gZn −1 at 300 mA cm−2 ), longtime stability, ZnO immunity and flexible properties.
Practical testing verifies that the home-made ZAB can easily power LEDs and electric fans, thereby presenting a promising strategy for the development of economical and highly active carbon as excellent
ORR electrocatalyst from waste biomass.
© 2020 Elsevier Ltd. All rights reserved.
1. Introduction
Owing to the declining fossil fuel reserves and environmental
pollution, the search for novel energy storage and conversion technologies has attracted wide attention. Among them, Zn–air batteries (ZABs) are viewed as the future promising energy sources due
to their high theoretical energy density (1084 Wh kg−1 ), safety and
environment friendly [1,2]. Unfortunately, the oxygen reduction reaction (ORR) with sluggish kinetics and low efficiency severely re-
∗
Corresponding author.
E-mail addresses: zhongqingjiang@zstu.edu.cn (Z. Jiang), xghao@tyut.edu.cn (X.
Hao), eszjiang@scut.edu.cn (Z.-J. Jiang).
1
These authors contributed equally.
https://doi.org/10.1016/j.electacta.2020.137143
0013-4686/© 2020 Elsevier Ltd. All rights reserved.
stricts the development of ZABs [3]. Consequently, it is extremely
vital to explore high-efficient, durable and low-cost electro-catalyst
to reduce the overpotential and thus promoting the ORR. Presently,
noble metal Pt-based catalysts have drawn extensive attention due
to their advantages of low overpotential and excellent catalytic activity. However, economic issue and unsustainable supply of these
catalysts hinder them from industrial implementation.
A substantial amount of research has been reported for the
development of new material to substitute precious metal Ptbased catalysts for ORR process, mainly including transition metal,
carbon-based materials and their composites [4,5]. Among these
materials, carbon-based materials, such as graphene [6], carbon
nanotubes [7], carbon quantum dots [8], etc., are regarded an
attractive option due to their high electrical conductivity, broad
X. Hao, W. Chen, Z. Jiang et al.
Electrochimica Acta 362 (2020) 137143
applicability, and unique features. Furthermore, several strategies
have been proposed to improve the ORR activities of carbonbased catalysts: (i) designing catalysts with large surface areas and
porous structure can improve pathways for products and reactants,
which play a key role in promoting mass transport, thus boosting
catalytic performance; (ii) doping heteroatoms (N, P, S and B) into
carbon materials to create more active sites, it has been reported
that especially by N-doping, the charge distribution of the adjacent
carbon nanostructures can be rearranged and can also serve as active sites for accelerating the ORR reaction, because of the inherent differences of the electronegativity between the nitrogen atoms
and the carbon atoms; [9] (iii) increasing the degree of graphitization of carbon catalysts could enhance their conductivity and
form graphitic-N, which are crucial for facilitating catalytic activities [10]. However, to date, few successful examples have been
reported the practical application of porous N-doped graphitized
carbon catalysts in energy devices, which largely result from expensive raw materials and complex synthetic process.
Biomass materials, due to the potential advantages of high carbon content, widely availability, renewability and low cost, have
become the most preferred precursor for production of carbon material. For example, a variety of potential biomass have been applied as electrocatalysts for ORR process as follows: Li et al. used
Mg5 (OH)2 (CO3 )4 /ZnCl2 as hard templates and 20 kinds of biomass
as carbon and heteroatom sources, among them, the N0.54 -Z4 /M1 900 present the highest surface areas of 2077 m2 g−1 and biggest
onset potential of 0.94 V (vs. RHE); [5] Zhao et al. fabricated N
doped porous carbon (NPC) using the soybean shell as carbon
source, showing the surface areas of 1036.2 m2 g−1 and onset
potential of 0.872 V (vs. RHE); [11] Zhang et al. reported their
work on nitrogen-doped carbon nanodots (N-CNDs) by hydrothermal treatment of natural biomass (fresh grass), and its surface areas and onset potential are 241 m2 g−1 and 0.91 V (vs. RHE), respectively; [12] Zheng et al. prepared the yolk-shell N/P/B ternarydoped biocarbon (NPBC-3) derived from yeast cells, whose surface
areas and onset potential are 574.7 m2 g−1 and 0.904 V (vs. RHE),
respectively; [13] Li et al. fabricated the fluffy porous carbon materials derived from 15 kinds of daily biomass (NMC-1) with the
surface areas of 1287.63 m2 g−1 and onset potential of 0.96 V (vs.
RHE), [4] and so on. We can see, for most reported biocarbon catalyst, it still has a bottleneck to simultaneously realize large specific surface areas and reach ideal onset potential. At this juncture,
in order to create superior structure and achieve competitive ORR
performance, how to choose suitable biomass precursors and adopt
the best modification method are greatly meaningful.
Maize straw is one of the most common agricultural residues
in China, and its annual production keeps increasing with rate of
1.4% [14]. Unfortunately, excess maize straw is often subject to be
burned directly or abandoned in fields and thus cause waste of resources and serious environmental problems [15]. Herein, we propose an innovative route using green maize straw as precursors
to obtain challenging biomass carbon for ORR electrocatalyst, involving simple chemical activation, high temperature calcination
and acid treatment. By virtue of the activation effect of FeCl3 and
the N-doping function of melamine, the maize straw biomass (MS)
underwent complete reconstruction and transformed into the Ndoped porous graphitized carbon (MS-NPC). More encouragingly, it
was strongly demonstrated by experiments and DFT computations
that this novel catalyst possesses the superior structure and outstanding performance, the surface areas reached to 1483 m2 g−1
and the onset potential was improved to 0.985 V (vs. RHE), as well
as the maximum power density and specific discharge capacity of
Zn–air battery were up to 127.9 mW cm−2 and 794 mAh gZn −1
(even at 300 mA cm−2 ), respectively, which are better than almost
all biomass carbon ORR catalysts reported previously. Furthermore,
for Zn–air battery, even after mechanically changing of depleted
zinc metal and electrolyte for four times, it still maintains constant
maximum power density and rate capacity, indicating its superb
stability and great ZnO immunity. As supplementary, the MS-NPC
based all solid-state Zn–air batteries also possess good flexibility,
verifying its application prospects in wearable electronic devices.
This study opens a new space for reuse of waste biomass and provides a novel strategy to prepare valuable N-doped porous graphitized carbon catalyst for energy conversion.
2. Experimental section
2.1. Preparation of MS-NPC, MS-NC, MS-PC and MS-C
The maize straw was collected from a farm of china. The collected maize straw was washed with deionized water to remove
impurities and crushed into 200 meshes. Firstly, the 5.0 g cleaned
maize straw (MS) powder was treated by hydrothermal reaction at
180 °C for 12 h in autoclave with 100 mL deionized (DI) water. After hydrothermal reaction, the collected product was washed with
water repeatedly and dried overnight. The 1 g dried sample was
mixed with 0.75 g melamine, 3 g FeCl3 and then being pyrolyzed
at 800 °C for 2 h under N2 atmosphere. Subsequently, the material was washed with HCl, distilled water to remove metal, and
then dried at 70°C to obtain N-doped porous graphitized carbon
(denoted as MS-NPC). For comparison, the N-doped carbon (MSNC) and porous carbon (MS-PC) were also prepared using the same
procedure as MS-NPC but without adding FeCl3 and melamine, respectively. Pure biomass carbon was also prepared without adding
both FeCl3 and melamine (MS-C). The metal element Fe in MSNPC, MS-NC, MS-PC and MS-C were measured by inductively coupled plasma-mass spectroscopy (ICP-MS), and the results showed
that the content of Fe in the MS-NPC, MS-NC, MS-PC and MS-C
was less than 0.1867 wt%, 0.1839 wt%, 0.5145 wt% and 0.0426 wt%,
respectively.
2.2. Characterization
The morphologies were observed by scanning electron microscopy (SEM, Model Quanta 650 FEG) at an operation voltage
of 20.0 kV. The transmission electron microscopy (TEM) were carried on a JEM-2100F transmission electron microscope at an accelerate voltage of 200 kV. The crystal structures were investigated with X-ray diffraction (XRD) using Cu Kα radiation source
(λ = 0.15405 nm) (operated at 40 kV and 100 mA). The specific
surface area and pore size distributions (PSD) were analyzed by
the surface areas analyzer (NOVA 20 0 0, Quantachrome) using N2
adsorption and desorption isotherms by applying the Brunauer–
Emmett–Teller (BET) method and the QSDFT model, respectively.
The Raman spectra were performed by employing a laser Raman
spectrometer (RENISHAW, IN VIA) with 532 nm laser in the scan
range of 50 0–40 0 0 cm−1 . The X-ray photoelectron spectroscopy
(XPS, Thermo Electron, U.K.) were conducted using an Al Kα as Xray source and the binding energy was calibrated with C 1s peak
at 284.6 eV.
2.3. Electrochemical measurement
For ORR, the performances of all prepared materials were analyzed by the CHI 760E electrochemical workstation (Chenhua Co.,
China) with a three-electrode cell at room temperature, where the
saturated calomel electrode (SCE) and a Pt foil were used as the
reference electrode and the counter electrode, respectively. The
testing potential values were converted to potential versus RHE according to: ERHE = ESCE + EƟ SCE + 0.059 pH. Firstly, 4 mg catalyst was dispersed in 652 μL water, 261 μL isopropanol and 87 μL
2
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Electrochimica Acta 362 (2020) 137143
5 wt% Nafion solutions by long-time sonication until forming a homogeneous ink. The working electrode was prepared by loading
10 μL catalyst slurry onto the polished rotation glassy carbon disk
electrode (RDE, 5 mm diameter), and the active catalyst mass loading was 0.2 mg cm−2 . The N2 or O2 -saturated 0.1 M KOH solution
was used as the electrolyte. The CV curves were tested at 10 mV
s−1 . The electrochemical impedance spectroscopy (EIS) were measured at 0.85 V (vs. RHE) at 5 mV in the frequency range of 0.01–
10 0,0 0 0 Hz. The kinetic current density for the ORR can be calculated by Koutecky–Levich plots using following equations:
1
1
1
=
+
j
jk
jD
straw, and melamine and ferric chloride with cheap prices, without using any toxic organic reagents. This preparation method is
not only conducive to large-scale production, but also promotes
the reuse of waste resources and environmental protection. On the
other hand, for the synthesis mechanism, iron source could induce the structure complete conversion from amorphous biomass
carbon to graphitized carbon, thereby greatly enhancing the electronic conductivity of the catalyst. N-doping mainly form more active sites and structure defect. Here, we combined the effect of
Fe-driving graphitization and N-doping together, more expected
curved defect-rich porous graphitic carbon in MS-NPC can be
formed. More specifically, during the pyrolysis process, the outmost carbon wall could further shrink under the effect of iron element, at the same time in-situ introduced nitrogen atoms could
cause structural distortion. And this expected special structure
would result in more exposed active sites and further promote the
ORR catalytic performance of biomass-derived carbon.
Fig. 1 shows the SEM and TEM images of the MS-based carbon
materials with different pyrolysis process. As shown in Fig.1a and
e, without adding any activation agent, the pristine maize straw
carbon (MS-C) mainly show quasi-spherical morphology and average diameter is 15 μm. However, after introduction of melamine,
the obtained MS-NC (Fig. 1b) present cross-linked network structure and smooth surface because the pristine spherical MS carbon
maybe covered and intertwined by pyrolysis product of melamine.
As same with SEM image, no obvious dispersed spherical structure can be found and it forms the large and thicker carbon
sheets (Fig. 1f). In contrast, after FeCl3 activation modification, the
SEM and TEM images (Fig. 1c and g) of MS-PC reveal the completely different skeleton compared with the MS-C and MS-NC.
Like cross-linked 3D graphene structure, it mainly shows highly
loosely-packed network structure with thin carbon layers and interconnected pores. Further, after the co-assistance of melamine
and FeCl3 , the SEM image of MS-NPC (Fig. 1d) also perfectly form
the 3D porous graphene-like structure, implying its great conductivity and large specific surface area. Above comparison results
suggest that pore-forming and graphitization mainly drive by iron
element during structural transformation process [16]. Besides, for
the TEM of MS-NPC (Fig.1h), the outline is slightly blurred owing
to the complicated pyrolytic reaction of Fe salt with melamine. And
the complexation of Fe atoms with N resource could cause better
Fe element dispersion and thereby promoting its catalytic effect
during pyrolysis [17]. Fig.1i–l shows the HR-TEM images, big difference in microstructure of carbon can be found among four samples
with and without Fe activation. As shown in Fig. 1i and j, the MS-C
and MS-NC mainly present the amorphous carbon structure, suggesting not been graphitized without Fe during pyrolysis process.
However, after assistance with Fe salt, the legible lattice fringes
and ordered graphitic layers can be clearly observed (Fig. 1k and
l), especially for the MS-NPC (Fig. 1l). The d-spacing of MS-NPC is
0.34 nm, corresponding to the (002) planes of graphite, suggesting
the high level of graphitization after Fe salt activation, in agreement with the XRD results.
More interestingly, the MS-NPC consists of the highly curved
graphitic layers, which are beneficial for ORR process as the following reasons: (i) the bending graphitic carbon could enhance localization of surface conjugated p electron, which probably play the
induced effect to create active sites, similar with N-doping; [18, 19]
(ii) it may result in the reactant (O2 ) to easily contact electrocatalytic active sites generated by N-doping [20,21]. Further observed,
many dislocation defects (as shown white circles) could be found
in graphite layers, originating from the structure distortions caused
by the doping of N atoms into graphite lattice, and these defective active sites and exposed edges could also lead to greatly improvement of ORR performances [22]. This phenomenon could further confirm the successful co-modification of N-doping and Fe salt
(1)
where j, jk and jD represent the tested current density, kinetic current density and diffusion limiting current density, respectively.
The RRDE polarization curves were conducted at 1600 rpm. The
number of electron transfer (n) and percentage proportion of peroxide yield (H2 O2 , %) can be calculated as the following equations:
n=
4ID
ID + (IR /N )
H2 O2 (% ) = 200
(2)
IR /N
ID + (IR /N )
(3)
where ID is the disk current, IR is the ring current, N is the collection efficiency of the Pt ring (0.4).
The ORR stability was conducted at −0.3 V (vs. SCE). And the
durability was analyzed after the introduction of methanol and CO.
2.4. DFT calculation
DFT calculations have been carried out by the Vienna an initio
Simulation Package (VASP). The parameters and methods are detailedly described in the supplementary material.
2.5. Zn–air battery and solid-state Zn–air battery
The performances of the Zn–air batteries were tested using
home-made electrochemical cells. Firstly, 40 mg catalyst was dispersed in 652 μL water, 261 μL isopropanol and 87 μL 5 wt% Nafion
solutions, and then, 50 μL catalyst slurry was loaded onto the gas
diffusion layer carbon paper (1.0 cm2 ), which was used as the air
cathode. The polished zinc foil was used as the anode.
For the assemble of solid-state Zn–air battery, firstly, the MSNPC air electrode was prepared by dropping the catalyst ink onto
carbon cloth with mass loading of ~1.5 mg cm−2 . The gel electrolyte was made as follows: 1 g polyvinyl alcohol (PVA) powder
was dissolved in 10 mL DI water at 90 °C for 1 h. Then, 1 mL of 18
M KOH with 0.2 M Zn(Ac)2 was added and the solution was stirred
at 90 °C for 0.5 h. The prepared solution was freezed at -5 °C for
3 h, and then thawed at room temperature. Finally, the solid-state
Zn–air battery was assembled with zinc foil and MS-NPC air electrode placed on the two sides of the above PVA gel.
3. Results and discussion
The preparation procedure of the MS-NPC material is presented
in Scheme 1. Simply, the grinded maize straw powder was firstly
subjected to hydrothermal treatment, followed by calcination with
melamine and FeCl3 at 800 °C for 2 h. And then the most Fe element was removed by HCl solution. For comparison, the MS-C,
MS-PC and MS-NC were synthesized without using both melamine
and FeCl3 or one of them. On the one hand, from the perspective of synthesis cost and large-scale production, the MS-NPC is
low-cost and environmentally friendly, and the source materials
used in the preparation process include waste biomass of maize
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Electrochimica Acta 362 (2020) 137143
Scheme 1. Illustration of the preparation process of the MS-NPC.
Fig. 1. SEM images of MS-C (a), MS-NC (b), MS-PC (c) and MS-NPC (d); TEM and HRTEM images of MS-C (e, i), MS-NC (f, j), MS-PC (g, k) and MS-NPC (h, l); the corresponding
elemental mapping analysis of MS-NPC (m).
activation during the preparation of MS-NPC. Meanwhile, the uniform distribution of C, N and O in the MS-NPC was shown by elemental mapping shown in Fig. 1m, which again verify that N atoms
were successfully doped into the MS-NPC carbon framework.
The X-ray diffraction (XRD) patterns of four samples are shown
in Fig. 2a. For the MS-C and MS-NC, the wide peaks located at
20° to 30° indicate the amorphous carbon structure. Interestingly,
with modification of FeCl3 , the MS-PC exhibits the narrow peak at
26.8°, reflecting the formation of graphitic carbon (002). Further
observed, the peak intensity of the MS-NPC is even higher than
that of the MS-PC, indicating the stronger degree of graphitization, which demonstrate that the simultaneously impregnating of
melamine and FeCl3 in the MS precursor obviously alter the carbonization process and improve the graphitization degree, which
also be beneficial to facilitate electron transfer during ORR pro-
cess [23]. Generally, the graphitization temperature of carbon reach
as high as 3500 °C without any catalysts, [24] but this designed
method greatly decreases to 800 °C. Besides, in all curves, no any
characteristic peaks of iron oxides or iron carbides can be observed, which suggest that the most Fe element has been removed
after acid treatment process. However, according to the results of
ICP-MS, there is still a very small amount of trace iron present as
shown in the Experimental Section, these irons may exist in the
form of iron monatomic, which is strongly stable to acid. These
trace irons may also contribute to the high oxygen reduction properties of biomass based carbon materials. As shown in Fig. 2b, the
specific surface areas (SSAs) were investigated by N2 adsorptiondesorption isotherms. The MS-C exhibits I-type isotherm and the
SSA is 527 m2 g−1 . Meanwhile, after introduction of N element, the
SSA of MS-NC (513 m2 g−1 ) is slightly smaller than that of MS-C
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Electrochimica Acta 362 (2020) 137143
Fig. 2. Structural characterization of MS-C, MS-NC, MS-PC and MS-NPC: XRD patterns (a); The N2 adsorption/desorption isotherm curves (b); pore size distribution (c); XPS
spectra of N 1s (d).
because the partial surface of the MS-C was covered by generated
C3 N4 layers, which is consistent with the SEM and TEM results
(Fig. 1a and b). In contrast, after FeCl3 activation, the MS-PC shows
IV-type isotherm and the SSA could reach to 1295 m2 g−1 . Gratifyingly, by the co-effect of FeCl3 activation and N-doping, the SSA of
MS-NPC further increase to 1483 m2 g−1 , which even better than
most of reported biomass carbon catalysts [11,12,25-28]. For the
pore size distributions (Fig.2c), obviously, the MS-PC and MS-NPC
mainly reflect the co-existence of micropores and mesopores, however, both the MS-C and MS-NC only present little porous structures [28]. Undoubtedly, the higher SSA and richer porosity feature of MS-NPC could lead to easier OH− /O2 transportation and
consequently guarantee foreseeable excellent electrocatalytic activity for ORR, [26,29] which further demonstrate that the designed
Fe salt activation and N-doping synergistic effect are necessary to
effectively modification method for biomass-based carbon, which
is consist with the TEM and XRD results. As shown in Raman
spectroscopy (Fig. S1), the ID /IG value of MS-NPC is smallest, indicating that simultaneously addition of FeCl3 and melamine intensively modify biomass skeletons and increase the graphitization
degree during preparation process. The survey XPS spectra show
the only existence of C, N and O characteristic peaks in Fig. S2a,
further confirming that major catalytic activity are affiliated to the
C and N atoms. The specific elements contents results are summarized in Table S1, among them, the O elements were mainly
from adsorbed moistures because the high surface areas and hierarchical porous structure of MS-based carbon materials are easily
to adsorb oxygen-containing molecules in air. The C 1s XPS spectra (Fig. S2b) of all samples mainly can be fitted into five peaks
attributed to C−C, C–OH, C–N&C=O, C=N&C–O and O–C=O. Obviously, the N content (Fig. 2d) was increased from 0.76% for MSC to 3.15% for MS-NC and 4.70% for MS-NPC after introduction of
melamine precursor, indicating the successful doping of N atoms
in the porous carbon skeletons during the pyrolysis process. The
N 1s spectra of the MS-NC and MS-NPC can be divided into four
feature peaks corresponding to pyridinic-N (398.4 eV), pyrrolic-N
(399.9 eV), graphitic-N (400.7 eV) and oxided-N (403.7 eV). As expected, pyridinic-N and graphitic-N are the dominant form of Ndoping. According to the fitted results, the MS-NPC possesses high
content (71.7%) of active dopants (graphitic-N and pyridinic-N),
suggesting its high ORR electrocatalytic activity. Whereas the low
ORR catalytically activity of pyrrolic-N only takes small portion of
N-doping for MS-NC (7.76%) and MS-NPC (3.25%). Here, pyridinic-N
could replace the carbon atom at the edge of carbon layer and also
offer a lone electron pair to adjacent carbon atoms, thus promote
the catalytic activity of carbon matrix and improve the onset potential. [30] Besides, graphitic-N could supersede the carbon atoms
in the graphite layer, further increase the limiting current density.
To explore the effect of N-doping and FeCl3 activation modification on the ORR electrocatalytic activities of the MS-based carbon, a series of electrocatalysts synthesized with different additive
were investigated. The results are shown in Figs. 3 and S3-6. In
Fig. 3a, the ORR polarization curves (LSVs) of all contrast catalysts
and commercial 20% Pt/C corrected by iR compensation in order
to eliminate the ohmic potential drop losses from the solution resistance (Fig. S3) were presented. The onset potentials of MS-C,
MS-NC, MS-PC and MS-NPC are 0.77, 0.82, 0.89 and 0.99 V (vs.
RHE), respectively. And the limiting current densities of these samples also present big differences, as following: MS-C (3.1 mA cm−2 )
< MS-NC (3.3 mA cm−2 ) < MS-PC (4.4 mA cm−2 ) < MS-NPC
(5.8 mA cm−2 ). Obviously, the ORR performances of these catalysts greatly increase as following order: MS-C < MS-NC < MS-PC
< MS-NPC, indicating the synergistic effect of FeCl3 activation and
N doping for the modification of MS-based carbon cause significantly positive effect. Besides, the onset potential and half-wave
potential of MS-NPC are 30 mV and 17 mV even higher than that
of commercial Pt/C, implying its prominent ORR catalytic activity.
Furthermore, the N-doped carbon (NC) was also prepared using
the same procedure as MS-NPC but without adding MS powder.
As shown in Fig. S4, the ORR performance of NC is worse than
MS-NPC, suggesting that the nitrogen-doped porous carbon (NPC)
formed by biomass MS plays an important role in improving the
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Electrochimica Acta 362 (2020) 137143
Fig. 3. ORR performances: (a) polarization curves; (b) CV curves (solid lines: in O2 -saturated 0.1 M KOH, dashed lines: in N2 -saturated 0.1 M KOH); (c) EIS Nyquist plots; (d)
RRDE polarization curves; (e) the electron transfer number and percentage proportion of peroxide from RRDE; (f) plots calculated from the electrochemically active surface
areas (ECSA) of MS-C, MS-NC, MS-PC and MS-NPC; (g) stability evaluation of MS-NPC and 20% Pt/C; (h) durability evaluation of MS-NPC and 20% Pt/C, the arrows indicate
the addition of methanol and CO, respectively; (i) comparison of the MS-NPC with other reports.
oxygen reduction performance. In this work, biomass MS could derive porous structure and melamine is mainly used to increase the
content of nitrogen doping.
Meanwhile, the ORR performances are further evaluated by CV
curves in Fig. 3b. For all samples, there are no any manifest peaks
can be observed in N2 -saturated 0.1 M KOH electrolyte. In contrast, the visible cathodic peaks emerge when the solutions are
saturated with O2 . Consistently, the MS-NPC shows the maximum
CV curve area and the most positive redox peak, which further
verifies that the porous structure and uniformly N-doping of the
MS-NPC play the pivotal role for the ORR activities. The electrochemical impedance spectroscopies (EIS) are tested to investigate
the ions diffusion dynamics and electrical conductivity of a series
of MS-based carbon materials (Fig. 3c) and the equivalent circuit
were also inserted. As shown in tested values (dots) and fitted full
lines, the simulated results are in good agreement with the experimental data. The specific results were shown in Table S2, where
the Rs and Rct represent solution resistance and charge transfer
resistance, respectively. The Rs values of four samples are similar,
however, the Rct values exhibit big difference. Among them, the
MS-NPC possesses the smallest Rct , suggesting its best conductivity, which can extremely enhance the reaction rate kinetics dur-
ing ORR process. The LSV curves at different rotation rates (from
400 rpm to 2025 rpm) are presented in Fig. S5. And the ORR kinetics of four samples were calculated and analyzed by KoutechyLevich equation. All the K–L curves present linearity and reflect
the first-order reaction kinetics, suggesting that the ORR reaction
rates are only related to the dissolved O2 concentration in KOH
electrolyte. [4] In order to clarify the ORR pathway, rotating ringdisk electrode (RRDE) tests were conducted. Based on the RRDE
data (Fig. 3d), the number of transferred electron (n) of MS-NPC is
3.81–3.90, which are higher than that of MS-C (2.99–3.63), MS-NC
(3.11–3.30) and MS-PC (3.14–3.35) and slightly lower than that of
Pt/C (3.79–3.96). And the H2 O2 yield of the MS-C, MS-NC and MSPC are 18.3–50.7%, 35.2–44.6% and 32.8–43.2%, respectively, which
are far higher than that of MS-NPC (5.4–9.5%) and commercial 20%
Pt/C (2.2–10.5%). Besides, the electrochemical active surface areas
(ECSA) of catalysts are proportional to the electrochemical doublelayer capacitance values (Cdl ), which can be calculated according
to CV curves (Fig. S6). As shown in Fig. 3f, the MS-NPC has the
highest Cdl (23.7 mF cm−2 ) than that of MS-C (0.54 mF cm−2 ), MSNC (0.81 mF cm−2 ) and MS-PC (21.3 mF cm−2 ), hence, the excellent ORR performance of MS-NPC partly originates from its largest
ECSA.
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Electrochimica Acta 362 (2020) 137143
Above results prove that synergistic effect between FeCl3
activation-modification and N-doping could greatly improve the
ORR catalytic activity of primary MS-derived carbon material due
to the following factors: (i) the Fe atoms directly induce the formation of graphitized carbon and porous structure, among them, the
graphitized carbon could completely improve the conductivity and
greatly increases the transfer of electrons; and the porous structure
could facilitate the mass diffusion of OH− and O2 , [31,32] therefore
further increase the ORR catalytic activity; (ii) the N-doping can
produce more nitrogen-rich defected structures in the graphitic
carbon matrix and more active sites, especially for pyridinic-N
and graphitic-N species, these N-doping can efficiently change the
charge densities of adjacent carbon atom and decrease the HOMOLUMO energy gap, [33, 34] (iii) the larger specific surface areas
could expose more active sites and promote the mass transfer of
electrolyte ions and enhance the adsorption of electrolyte inside
the catalyst in the ORR process [32].
As indicated in Fig. 3g, the stabilities were assessed by
chronoamperometric method at constant rotation speed with 1600
rpm. As a result, the MS-NPC still maintained 83.7% after 10,0 0 0 s,
whereas the 20% Pt/C showed the big decrease of approximately
30.3%, indicating that the MS-NPC have better durability over 20%
Pt/C. Besides, after longtime stability testing, the structure change
of MS-NPC was analyzed by XRD (Fig. S7), which also shows characteristic peak centered at 2θ = ~25°, indicating that the MS-NPC
has stable structure in ORR. Moreover, methanol or CO crossover
effect are also important factors for cathode materials because in
the practical application, especially for ORR process in fuel cells,
the commercial Pt/C catalysts are easily poisoned by methanol or
CO and result in drastically reduction in catalyst life [35]. As shown
in Fig. 3h, after inject of 0.3 M methanol or introduction of CO
in the electrolyte, the 20% Pt/C catalyst present rapidly attenuation. However, our MS-NPC still maintain almost unchanged current density at same conditions, indicating its excellent immunity
toward CO and methanol as a promising cathodic catalyst for fuel
cells. Most significantly, the comparison with the most recently reported ORR catalysts confirms that MS-NPC was the best catalysts
in terms of specific surface areas and the larger onset potential
as shown in Fig. 3i [4,5,10–13,16,19,21,22,25,27,28,35–39] and Table S3. All above results demonstrate that MS-NPC delivers superb
ORR catalytic activity, outstanding durability and better resistance
to methanol and CO poisoning than 20% Pt/C, verifying this preparation strategy is effective.
The above experimental results have verified that the MSNPC is an excellent cost-effective electrocatalyst for ORR. To further reveal the nature of electrocatalytic activities and underlying catalytic mechanisms of the MS-C, MS-NC, MS-PC and MSNPC samples, the density functional theory (DFT) computations
were also performed (Fig. 4). The free-energy diagrams at different potentials and the density of states were calculated to analyze the reaction kinetics and bandgap. As we all known, for the
ORR catalytic process, four steps are regarded as elementary reaction pathway as following: adsorbed O2 dissociation into ∗ OOH
(O2 (g)+H2 O(l)+e-→∗ OOH+OH− ); ∗ OOH decomposition into ∗ O
(∗ OOH+e− →∗ O+OH− ); ∗ O alkalization into ∗ OH (∗ O+ H2 O(l)+e→∗ OH+OH− ); ∗ OH detachment into OH− (∗ OH+e− →OH− ) [40]. To
examine the individually or synergistically effect of the type of N
doping, the basal active sites and the edge sites, we constructed
various types of structural models with different defects/edge, as
shown in Figs. S8 and S9. We have further calculated the free energy of intermediates (OOH∗ , O∗ , and OH∗ ) adsorbed on the basal
active sites (Fig. S8c–e) and the edge sites (Fig. S8b), including coordination unsaturated marginal C, pyridinic N and pyrrolic N. The
obtained free energy diagrams are compared with that adsorbed
on the basal active sites at U=1.23 V as shown in Fig. S8a, revealing
that the adsorption of OOH∗ on defective basal plane sites is more
favorable for ORR process with a more thermodynamically neutral
value of G. However, the adsorption of O∗ near edge pyridinic N
site delivers a more favorable value of G (−0.098 eV) for ORR
than on the basal active sites. In addition, the free energies of OH∗
adsorbed on the edge sites are all inferior to that on the defective
basal plane sites. We have also considered the adsorption of intermediates (OOH∗ , O∗ , and OH∗ ) on different N dopant species more
carefully. The results are displayed as shown in Fig. S9a, which indicate that the sites around the edge pyridinic N and the graphitic
N are the most active ones for the adsorption of O∗ and OH∗ , respectively. Besides, the OOH∗ on the defective basal plane sites is
still more beneficial for ORR process. Actually, according to the experimental data, the pyridinic N and graphitic N species dominate
all the N dopant content.
More deeply, contrasting the adsorption abilities of ORR intermediates (OOH∗ , O∗ , and OH∗ ) on different catalysts is the most
scientific method to evaluate their catalytic activities. Firstly, the
diagrams at U=0 V present big difference among four samples.
For the pristine MS-C, the adsorption of ∗ O is endothermic (first
step, ࢞G1 =0.492 eV), resulting in the most sluggish kinetic and
the worst ORR performance. Once doped by the N element or
introduced of the holes, all reduction processes become gradually downhill (࢞Gi < 0, i=1, 2, 3, 4), indicating their spontaneous exothermic processes of the MS-NC, MS-PC and MS-NPC,
and demonstrating the effective doping and modification process
are crucial to change the reaction pathway. However, after adopting the thermodynamic equilibrium potential of 1.23 V, the step 1
of all four samples are uphill, suggesting suppling energy is needed
to overcome the barrier of O2 protonation. According to the previous literature, this step (step 1) plays a pivotal role for the ORR
catalytic processes [41]. Similarly, after introduction of N-doping or
pores (at U=1.23 V), the initial step of MS-NC, MS-PC and MS-NPC
demand less energy compared with that of the pure MS-C, illustrating the positive effects caused by the structural changes. Especially for the MS-NPC, it only needs 0.115 eV in the first step, which
is far smaller than MS-C (1.722 eV), MS-NC (0.961 eV), and MS-NC
(0.811 eV), demonstrating its best performance. The reaction process with the highest endothermic value is the rate-determining
step (RDS), and the RDS determines the theoretical potential for
ORR, i.e., 1.722 eV (MS-C), 0.983 eV (MS-NC), 0.360 eV (MS-NPC)
and 0.811 eV (MS-PC), suggesting the ORR activities follows the order: MS-C < MS-NC < MS-PC < MS-NPC, which are agreement
with the LSV experimental trend. Fig. 4e shows the density of
states (DOS) of four samples, where the more overlapping areas
around Fermi level reflect better conductivities. Here, no obvious
bandgap can be found in the MS-NC and MS-NPC, while the MSC and MS-PC shows visible narrow bandgap, because the N atom
with high electronegativity easily form the higher electron density
areas [42]. As expected, the MS-NPC present the highest ORR catalytic activity and far exceed other three samples, strongly demonstrating the synergetic effect between N-doping and FeCl3 activated porous graphitized structure are promising to accelerate ORR
process. Overall, as shown in Fig. 4f, for the highly catalytic activity
of MS-NPC, the doped N atom could affect the electronic properties of adjacent carbon atom and redistribute the charges by conjugate electron, and finally cause more active sites for ORR [42].
As for porosity, the edge of the hole is apparently accompanied
by some defects, and the carbon atoms near these defects exhibit
higher binding affinity with some ORR intermediate species, effectively acting as active sites and contributing to the ORR process
[43]. In addition, the rich porosity can provide more channels for
the transport of ions and oxygen.
To verify the practicality of MS-NPC material, the primary zinc–
air batteries (ZABs) was assembled using the MS-NPC as the cathodic material, and the schematic illustration is shown in Fig. 5a.
As a comparison, the ZAB using commercial 20% Pt/C catalyst
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Electrochimica Acta 362 (2020) 137143
Fig. 4. Free-energy diagrams for the MS-C (a), MS-NC (b), MS-NPC (c) and MS-PC (d) at different potentials, insert images: structural models; (e) the calculated density of
states (DOS), where Fermi level was set at zero energy; (f) mechanism of oxygen reduction.
was also assembled. The polarization curve of ZAB using MS-NPC
present small voltage gap, indicating its satisfying discharge ability, with the peak power density of 127.9 mW cm−2 , which is even
better than 20 % Pt/C (118.6 mW cm−2 ) (Fig. 5b). The open-circuit
voltage could reach 1.451 V in Fig. 5c, without any attenuation
even after 45 h. As shown in insert photograph, the home-made
“ESIX” logo containing 43 LEDs could be lighted by the ZAB only
using 2 mg MS-NPC electrode material. Besides, as a more significant exemplification for practical applications, three ZABs assembled only with 3 mg MS-NPC even can power two fans to spin
quickly (Video S1). Fig. 5d depicts the discharge profiles of the ZAB
using MS-NPC, where the specific capacities were calculated to be
818, 818 and 794 mA h gZn −1 at big current density of 10 0, 20 0
and 300 mA cm−2 , respectively (normalized to the mass of consumed Zn plates). Gratifyingly, these values are outperforming than
most of recently reported ZABs, [25, 44, 45] reflecting the excellent
performance of MS-NPC electrode.
As our best knowledge, the easily deposition of zinc oxide on
the surface of cathode could severely hinders the long-term stability of the air electrode, because the ZnO could partially block
the pores of gas diffusion layer, thereby blocking the contact between air or the electrolyte and the catalyst, further resulting the
catalyst deactivated [46]. Hence, it is necessary to research the effect of ZnO poisoning [47]. As shown in Fig. 5e–h, after zinc deple-
tions, and mechanically changing of zinc and electrolyte for four
times, we repeatedly tested the voltage values (at 2 mA cm−2 ),
rate capability and polarization curves, and further study the structural changes of the MS-NPC material (coated on carbon paper
electrodes) before and after long-term testing by XRD patterns.
As the results, after discharged at 2 mA cm−2 for total 80 h
(Fig. 5e), the MS-NPC based Zn–air battery show almost constant
voltage and excellent stability. For the rate capacity (Fig. 5f), in
the first cycle, with the increase of big current densities from 20
to 140 mA cm−2 , all the discharge voltages present highly stable
plateau. Afterward, the voltage even can return its original value
when the current density returns to 20 mA cm−2 , indicating its
outstanding rate capability. More importantly, the rate capability
still maintains almost constant values in subsequent three times
cycling. Meanwhile, the discharge voltage and power densities also
present negligible attenuation (Fig. 5g), and these above results
further strongly prove the super cycling performance of MS-NPC
electrocatalyst itself. As the important evidence, compared with
the fresh MS-NPC@CP (carbon paper) electrode, no any characteristic peaks of ZnO in XRD patterns can be detected after longtime cycling, which reflect its ultrahigh stability and indicate there
is no any inhibition effect of zincate ions to MS-NPC electrode,
which is extremely significant for the practical repeated application of zinc air batteries. As additionally, in recent years, the flexi8
X. Hao, W. Chen, Z. Jiang et al.
Electrochimica Acta 362 (2020) 137143
Fig. 5. (a) Schematic illustration of primary Zn–air battery (ZAB); (b) polarization and power density curves of ZAB using MS-NPC and 20% Pt/C catalysts; (c) Open circuit
plots with inset showing the working-LED lighted by the ZABs with 2 mg MS-NPC catalyst; (d) specific discharging capacity plots of the ZAB using MS-NPC at 10 0, 20 0
and 300 mA cm−2 ; (e) long-time durability, (f) rate capacities, (g) polarization and power density curves of the ZAB using MS-NPC, the numbers represent the cycles of
mechanical changing of zinc and electrolyte; (h) XRD patterns of MS-NPC catalysts (coated on carbon paper) before and after four times of changing of Zn and electrolyte;
(i) open-circuit voltage of the flexible solid-state ZAB using MS-NPC under bending at 0o , 30o , 90o and 180o , respectively.
(1483 m2 g−1 ), abundant nitrogen content (4.70%) and rich defects, which are beneficial properties as electrocatalysis. Especially
for the nitrogen atoms in the carbon lattice, it may be the most
contribution for the ORR activity. And the formed porous structure can be served as channel to enable the contact of OH− /O2
and electrolyte with the active site in the sample, and then be
quickly adsorbed and transported. Based on these advantages, the
experimental and DFT computational results confirm that the MSNPC present highly active toward ORR, including a positive onset potential (0.985 V vs. RHE), large limiting current density
(5.8 mA cm−2 ) with four electron transfer pathway, better durability and methanol or CO tolerance, high conductivity, and outperforms most recently reported biomass based electrocatalysts, even
better than 20% Pt/C benchmark. Furthermore, the assembled Zn–
air battery with MS-NPC as cathode catalyst, it shows the maximum power density (127.9 mW cm−2 ), super specific discharge capacity (794 mA h gZn −1 , 300 mA cm−2 ), outstanding cycling stability and even strong ZnO immunity. Practicably, three home-made
Zn–air battery assembled only with 3 mg MS-NPC even can power
two fans to spin quickly. The designed all solid-state battery with
MS-NPC also shows its potential in flexible electronic device appli-
ble electronic devices have attracted much interest. Here, flexible
solid-state zinc–air batteries were also fabricated using the MSNPC material (Fig. 5i). And its voltage maintains almost stable after bending with different angles, indicating that it has application
prospects in wearable electronic devices. In a word, these superior
Zn–air battery performances of MS-NPC over 20% Pt/C are mainly
attributed to the superb ORR catalytic activity originated from the
big surface areas, abundant porosity, more C–N active sites, defectrich and graphitized carbon structure of MS-NPC.
4. Conclusions
In summary, we have designed a facile, low-cost pyrolysis
strategy to turn waste to wealth. The three-dimensional N-doped
porous graphitized carbon (MS-NPC) was fabricated by using the
agricultural waste of maize straw, and melamine and ferric chloride with cheap prices as precursors. Driven by an iron source,
the original amorphous carbon can be converted into highly conductive graphitized carbon and form the porous structure. The
N-doped active sites were obtained by additive of melamine.
The resultant MS-NPC exhibit ultra-high specific surface area
9
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Electrochimica Acta 362 (2020) 137143
cations. This work provides a feasible, sustainable, economic and
promising technique to large-scale produce functional carbon electrocatalyst from agricultural waste biomass for energy conversion
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Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
CRediT authorship contribution statement
Xiaoqiong Hao: Investigation, Formal analysis, Data curation,
Writing - original draft. Weiheng Chen: Investigation, Formal analysis, Data curation. Zhongqing Jiang: Conceptualization, Methodology, Supervision, Writing - review & editing, Funding acquisition. Xiaoning Tian: Investigation, Formal analysis, Data curation.
Xiaogang Hao: Supervision, Validation. Thandavarayan Maiyalagan: Supervision, Validation. Zhong-Jie Jiang: Software, Writing review & editing, Funding acquisition.
Acknowledgments
Authors acknowledge the supported from the National Natural Science Foundation of China (No. 11975205), Natural Science
Foundation of Guangdong Province (No. 2017A030313092), Guangdong Innovative and Entrepreneurial Research Team Program (No.
2014ZT05N200), the Fundamental research funds for the central
university of South China University of Technology (No. 2018ZD25),
and the Science Foundation of Zhejiang Sci-Tech University (No.
18062245-Y).
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.electacta.2020.137143.
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