Boosting the primary Zn-air battery oxygen reduction performance with mesopore-dominated semi-tubular doped-carbon nanostructures

Article abstract of DOI:10.1039/d0ta02741c

The low surface density of catalytic sites and undeveloped porosity have become a bottleneck for boosting the oxygen reduction reaction (ORR) activity of carbon catalysts. Herein, we propose a novel strategy for the synthesis of porous semi-tubular iron-nitrogen-doped-carbon nanostructuresviaa two-step calcination of a ferriporphyrin-based biomaterial by means of a natural tubulose nanoclay as a morphology-controlled template, followed by post Zn-activation and acid-leaching processes. The formation of mesopore-dominated semi-tubular carbons is beneficial for accelerating the ORR catalysis rate and improving the catalytic activity owing to the increased mass transport capacity of reactants to nitrogen-rich catalytic sites existing in the pores. The resultant doped-carbon catalyst not only displays excellent electrocatalytic behavior with an ORR onset potential of ~1.01 V and a half-wave potential of~0.85 V, but also exhibits a maximum power density of ~191 mW cm^-2, comparable to that of a Pt catalyst in a primary Zn-air battery, suggesting a very promising candidate for prevalent energy-conversion devices. It is probably due to the production of mesopore-dominated semi-tubular structures, more active-nitrogen species and dense surface active sites. This work can pave a new way for the original design of low-cost and high-performance doped-carbon catalysts by using natural biomaterials for widespread application in electrochemical energy devices.

Full text of DOI:10.1039/d0ta02741c

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10.1039/D0TA02741C.
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Boosting the primary Zn-air battery oxygen reduction performance
DOI: 10.1039/D0TA02741C
with mesopore-dominated semi-tubular doped-carbon nanostructures
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Xinyi Luoa,b†, Zhaoxu Lib†, Meifang Luoc†, Chaozhong Guoa*, Lingtao Suna, Shijian Lanb,
Ruyue Luob, Lan Huangb, Yuan Qina, Zhongli Luob*
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aChongqing Key Laboratory of Materials Surface & Interface Science, School of Materials
Science and Engineering, Chongqing University of Arts and Sciences, Chongqing 402160,
China.
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bCollege of Basic Medical Science, Chongqing Medical University, 400044, Chongqing, China.
cSchool of Resources and Civil Engineering, Wuhan Institute of Technology, Hubei 430070,
China.
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†These authors equally contributed to this work.
*Corresponding authors.
E-mail: guochaozhong1987@163.com (C.G.); zhongliluo@163.com (Z.L.)
Abstract: The low surface density of catalytic sites and undeveloped porosity become a
bottleneck for boosting the oxygen reduction reaction (ORR) activity of carbon catalysts.
Herein, we propose a novel strategy for synthesis of porous semi-tubular iron-nitrogen-doped-
carbon nanostructures via two-step calcination of ferriporphyrin-based biomaterial by means of
a natural tubulose nanoclay as a morphology-controlled template, followed by post Zn-
activating and acid-leaching processes. The formation of mesopore-dominated semi-tubular
carbons is beneficial for accelerating the ORR catalysis rate and improving the catalytic activity
owing to the increased mass transport capacity of reactants to nitrogen-rich catalytic sites
existing in the pores. The resultant doped-carbon catalyst not only displays excellent
electrocatalytic behavior with an ORR onset potential (~1.01 V) and a half-wave potential
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(~0.85 V), but also exhibits a maximum power density of ~191 mW cm-2 comparaDbOleI: 1t0o.10t3h9e/DP0TtA02741C
catalyst in primary Zn-air battery, suggesting a very promising candidate in prevalent energy-
conversion devices. It is probably due to the production of mesopore-dominated semi-tubular
structures, more active-nitrogen species and dense surface active-sites. This work can pave a
new way for original design of low-cost and high-performance doped-carbon catalysts by using
natural biomaterials for the wide application of electrochemical energy devices.
Keywords: Primary Zn-air battery; Semi-tubular carbon; Mesopore-dominated; ORR
electrocatalysis; Hemin biomaterial
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10 1. Introduction
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The challenges of energy shortage and climate change have dramatically increased, resulting in
an urgent requirement to pursue green energy devices. Clean metal-air batteries (MABs) with
advantages of high specific-energy, low cost and stable-performance gradually became one of
the ideal and significant alternative energy devices in recent years.1-4 The rate and pathway
diversity of oxygen reduction reaction (ORR) at the cathode determines the general
performance of MABs.5 Therefore, the search for high-performance electrocatalyst to
accelerate the ORR performance is the key solution to solve those problems. Precious-metal Pt-
based catalysts, as we all know, were largely used as cathodic catalysts because of their
excellent ORR activity. However, it’s inadequate supply and high prices brought out the
bottleneck of the large-scale application of MABs. Thus, increasing great efforts have been
undertaken to develop non-precious metal catalysts (NPMCs) with characteristics of high
activity and rich sources, especially doped-carbon catalysts.6-10 And a large number of
researches have widely used renewable biomaterials as precursors of carbon catalysts instead
of chemical compounds or complexes 11-14, e.g., shrimp-shell15, enoki mushroom16, nori17, wine
mash18, coprinus comatus19, pig blood20, fish biowastes21 were previously reported to
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synthesize the doped-carbon catalysts for the ORR 22. Besides, two-step carbonization of
hemoglobin as a renewable, abundant and cheap biomaterial, firstly proposed by Maruyama et
al., served as a novel avenue to boost the ORR catalytic property of doped carbon catalysts 23.
Our group also developed blood protein with abundant protein/amino acids as the nitrogen
source of doped-carbon catalysts with good ORR activity and outstanding long-term stability
in terms of a similar strategy.20 However, the low surface density of catalytic sites and
undeveloped porosity on the majority of doped-carbon catalysts with complicated compositions
and structures might be the main reasons for lower catalytic activity compared with the Pt-based
catalysts.
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Hemin (iron (III) protoporphyrin (IX) chloride, molecular formula C34H32ClFeN4O4) as a
pivotal component of hemoglobin and a sort of natural transition-metal-based macrocyclic
compounds is considered as one of promising iron/nitrogen-containing precursors for designer
doped-carbon electrocatalysts because of its self-ability to support transition metal, nitrogen,
and carbon sources 24-26. To better enhance the ORR activity of doped-carbon catalysts, not only
the chemical components need to be designed, but also its intrinsic structure needs to be
regulated 27-28 As previously mentioned, the design of hollow or tubular porous structures using
a template-assisted strategy can effectively enhance the surface active-site density and promote
the pore development to boost the ORR activity of carbon catalysts. 29-33 Halloysite nanoclay
(HN) with a natural nanotube structure is composed of two layers in terms of the inner layer of
Al-OH groups and the outer layer of Si-O-Si groups), respectively.34 The excellent dispersibility
and strong surface adsorbability of nanoclay as an accessible and environmentally-friendly
template in water solution can help the contact ability and homogeneity of hemin in ammonia
solution 35.
Hence, we report the synthesis of iron-nitrogen-doped carbon nanomaterials (Hemin@HN-
800Zn) made by two-step annealing hemin by means of HN as a template and ZnCl2 as a post
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pore-former under nitrogen atmosphere. The role of ZnCl2 may largely facilitate to catalyze
DOI: 10.1039/D0TA02741C
further dehydroxylation and dehydration, and its vaporization at high-temperature to better
produce the porous structure. To the best of our knowledge, it is firstly reported that the
utilization of nanoclay template can induce the formation of semi-tubular nanostructure
dominated by mesoporous characteristics to facilitate the rapid transportation of the electrolyte,
reactants and products to accelerate the rate of oxygen reduction catalysis. The catalytic tests
indicate that Hemin@HN-800Zn exhibited comparable catalytic activity and long-term stability,
as well as similar ORR catalysis mechanism, compared to the top-ranking Pt catalyst. In
addition, the test of home-made Zn-air battery also suggests that the power density of
Hemin@HN-800Zn catalyst is can be up to ~191 mW cm-2, which approaches that of the Pt
catalyst but is higher than that of previously reported carbon-based or other non-Pt catalysts.
Such excellent ORR performance may be partially attributed to full exposure of more accessible
catalytic sites as well as a large concentration of nitrogen doping into high-porosity semi-
tubular carbon nanostructure. This study can provide a new idea and route for the original
design of various precious doped-carbon materials for a wide range of reactions in
electrochemical energy conversion devices.
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17 2. Experimental
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2.1 Synthesis of doped-carbon nanostructures
First, 20 mg of nanoclay and 4 mg of hemin, provided by Aladdin Chemical Co.Ltd., were
uniformly dispersed in 30 ml ammonia (50 wt.%) and vigorously stirred for 10 h at 25°C so
that hemin can be evenly and absolutely absorbed on the nanoclay surface. Subsequently, the
gained Hemin@HN precursor was freeze-dried for 12 h. After that, the precursor was annealed
at 800 C for 2 h under N2 protection to yield initial ORR catalysts (Hemin@HN-800). The
heating rate was kept at 10 °C min-1. After doing this, post Zn-activating process was further
performed, as follows: 4 mg of ZnCl2 was added into the initial Hemin@HN-800 catalyst,
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grounded up for 30 min and further annealed at corresponding temperatures for 1 h under N2
DOI: 10.1039/D0TA02741C
protection. For comparison, we synthesize Hemin@HN-700 and Hemin@HN-900 catalysts at
different temperatures under similar conditions. The diagrammatic synthesis of mesoporous
dominated-carbon semi-tubular nanostructures is indicated in Fig. 1. At last, all prepared
catalysts (e.g., Hemin@HN-700Zn, Hemin@HN-800Zn, Hemin@HN-900Zn) was
successively immersed in 0.2 mol l-1 HF, 0.1 mol l-1 HCl and deionized water for three times
(15 min every time) to drastically remove the residues of nanoclay, ZnCl2 and other metal
oxides.
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Fig. 1
2.2 Characterization
High-resolution scanning and transmission electron microscopy images were gained on Hitachi
UHR S4800 and FEI Tecnai-G2 F30 instruments, separately. X-ray photoelectron spectroscopy
(XPS) data were collected on a Kratos XSAM800 spectrometer. Nitrogen adsorption/desorption
isotherms were tested on a Micromeritics ASAP 2010 analyzer at 77 K. X-ray diffraction (XRD)
data were obtained on a Shimadzu XRD-6000 X-ray diffractometer (Cu Ka1 radiation, λ
=1.54178 Ǻ) at a scan rate of 4° min-1. Horiba HR800 Raman system was used to measure the
Raman spectroscopy with an excitation wavelength of 532 nm.
2.3 ORR Electrochemical tests
All electrochemical data were collected on a CHI760E Bipotentiostat (Shanghai Chenhua
Instruments Co.Ltd., China) with a three-electrode system, which is composed of a rotation ring
(Pt)-disk (glassy carbon, Φ=5 mm ) working electrode (RRDE, American Pine Instrument
Co.Ltd.), a saturated calomel reference electrode (SCE) and a graphite-rod counter electrode (1
cm2). The preparation of the catalyst-coated RRDE refers to the previous reports [18,23].
Generally, 10 µl of 10 mg ml−1 catalyst dispersion was dropped onto RRDE and naturally dried.
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The mass-loading of the catalyst was limited to be about 510 µg cm−2. All electrode potentials
versus the SCE in alkaline electrolytes were converted into the potentials versus the reversible
hydrogen electrode (RHE) on the basis of the Nernst equation. The scanning rate is 5 mV s-1
and the electrolyte is 0.1 mol l-1 KOH solution in all voltammetry tests. The transferred electron
number (n) and H2O2 yield (H2O2%) during the ORR were estimated using the following Eqs.
(1) and (2), respectively. The calculated equations are as follows:
2Ir / N
%HO2 100
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(1)
Id  (Ir / N)
Id
n  4
(2)
Id  Ir / N
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where Id is the Faradaic disk-current, Ir is the Faradaic ring-current, and N is the collection
efficiency of ring electrode (0.38). The Pt-ring potential was set at 1.5 V (vs. RHE) as reported
elsewhere.
12 3. Results and Discussions
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3.1 Electrocatalyst Characterization
The SEM image was used to observe the surface features of Hemin@HN-800Zn (Fig. 2a). It
can be clearly revealed that Hemin@HN-800Zn is composed of semi-tubular nanostructures
with abundant mesoporous textures. The formation of the structure may be owing to a
reasonable fact that the nanoclay was fully covered with hemin molecules during freeze-drying
and calcination processes, and the following acid-leaching behavior can effectively remove the
nanoclay residues to produce the hollow-structured semi-tubular morphology. The structure of
semi-tubular can help to facilitate the full exposure and utilization of a large number of nitrogen-
rich active sites on the doped-carbon catalyst surface. A large amount of mesopores on the semi-
tubular carbon surface can be also seen in corresponding TEM images (Fig. 2b and 2c). As
displayed in Fig. 2d and 2e, Hemin@HN-800Zn also suggests a disordered carbon structure
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with an interlayer spacing of ~0.35 nm, slightly higher than that of graphite layersD, OwI:h10ic.10h3V9mie/wDa0AyrTtiAcl0e2O7n4l1inCe
be mainly induced by the doping of high nitrogen content (6.53 at.%). X-ray diffraction pattern
of Hemin@HN-800Zn (Fig. 1S) further shows the amorphous graphite structure without other
crystallization phases, which is supported by high-resolution TEM and HAADF-TEM images
(Fig. 2f). Element mapping of Hemin@HN-800Zn displays the uniform distribution of four
main elements (C, N, O and Fe) on its surface, as shown in Fig. 2g. To our amusement, the size
of the bright dots implies that Fe elements are in the existence of a single-atom state
presumably.36 The metal Fe content was tested by inductively coupled plasma atomic emission
spectrometer (Agilent ICPOES730), which is only 2.8127 mg g-1 in the Hemin@HN-800Zn
catalyst.
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The BET specific surface area and pore characteristics of Hemin@HN-800Zn are of great
significance in connection with its ORR catalytic behavior, which was measured by N2
adsorption-desorption isotherms. The tested results were shown in Fig. 2h. Hemin@HN-800Zn
has exhibited a Langmuir IV isotherm with a type H4 hysteresis loop. The hysteresis loop can
be evidently found at the relative pressure of 0.45 to 1.0, hinting that the increased porosity of
Hemin@HN-800Zn in the range of mesopore. The BJH pore-size distribution (Inset of Fig. 2h)
of Hemin@HN-800Zn shows large mesoporous structures with two maximum value (3.7 nm
and 20.8 nm) of pore size in Hemin@HN-800Zn, further proving the results of the above-
mentioned SEM and TEM analyses. The BET surface area is 473.75 m2 g-1 and the average
pore width is ~9.5 nm on Hemin@HN-800Zn. In addition, total pore volume (Vp) of
Hemin@HN-800Zn is ~0.66 cm3/g, but its t-Plot micropore volume (Vmicro) is ~0.13 cm3/g. The
resultant Vmicro/Vp ratio is only 19.7 %, meaning the mesopore dominates in pore distribution.
The occurrence of hierarchical micro-/meso-porous textures in Hemin@HN-800Zn can be
partially attributed to the post Zn-activating process and the thermal decomposition of ZnCl2
pore-former during the second-step annealing. Furthermore, the mesopore-dominated texture is
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beneficial for increasing ORRcatalytic activity, because it provides more nanochannels foVriefwaAsrtticle Online
DOI: 10.1039/D0TA02741C
transportation of the electrolyte, reactants and products and enhance the contact between
reactants and active sites to better decrease the oxygen-transportation resistance to the catalytic
sites of Hemin@HN-800Zn.37
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Fig. 2
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Raman spectroscopy was also used to study the defect and disordered structures of
Hemin@HN-800Zn. (Fig. 3a) The Raman spectrum of each sample is composed of two peaks:
“D” peak and “G” peak. The D band located at around 1355 cm-1 is associated with the lattice
defect of carbon atoms. The G band located at around 1586 cm-1 is due to the E2g mode for in-
phase stretching vibration of sp2-bonded carbon.38 The peak intensity ratio of the D band to the
G band ( ID/IG ) can correctly indicate the degree of structural defects. The ID/IG ratio (1.08) of
Hemin@HN-800Zn is larger than that of Hemin@HN-700Zn (1.05) and Hemin@HN-900Zn
(1.02), demonstrating that the Hemin@HN-800Zn exhibits more defect structures induced by
N-doping.39 Furthermore, FT-IR spectrum was tested to investigate the chemical structures of
all carbon catalysts (Fig. 3b). All catalysts show that the C-C bond, C-N/C-O bonds, and
C=N/C=O bonds are located at 928 cm-1, 1072 cm-1, and 1572 cm-1, respectively.40 Interestingly,
the peaks at ca. 560 cm-1 and 675 cm-1 can separately correspond to the existence of Fe-O and
Fe-N structures. The above results also suggest the successful doping of nitrogen into the carbon
matrix to produce C-N and Fe-N active structures in Hemin@HN-800Zn.
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Fig. 3
Fig. 4a displays the XPS analysis results of Hemin@HN-700Zn, Hemin@HN-800Zn,
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Hemin@HN-900Zn. The appearance of main peaks of nitrogen, carbon and oxygen can be
DOI: 10.1039/D0TA02741C
clearly observed, suggesting the nitrogen atoms are successfully included in the carbon matrix
of all carbon catalysts. However, the peak of Fe species is not found probably because ultralow
Fe content has gone beyond the XPS detection limit, being proved by the Fe element mapping
(Fig. 2g). The C1s region of Hemin@HN-800Zn (Fig. 4b) was first fitted into four peaks with
the binding energy of 284.7 eV, 285.4 eV, 286.29 eV and 290.0 eV, which are assigned to C=C,
C-C/C-N, C-N and O-C=O, respectively.41 The percentage of the C=N structure in
Hemin@HN-800Zn is ~24.5%, which higher than other carbon catalysts. It further supports the
successful doping state of nitrogen atoms into the carbon nanostructure. As shown in Fig. 4c,
the N1s spectrum of Hemin@HN-800Zn can be deconvoluted into four peaks, corresponding
to pyridinic-N, Fe-N, graphitic-N and pyridinic N+-O-, which are respectively located at 398.4
eV, 399.6 eV, 400.78 eV, 401.64 eV.42 According to a similar fitting strategy, we analyzed the
N1s spectra of other carbon catalysts (Fig. 4d and 4e), as summarized in Table 1S. The
graphitic-N dominates all nitrogen species in doped-carbon catalysts. With the rising of the
annealing temperature, the percentage of Fe-N has no obvious change, but the total N content
in catalysts has been gradually decreased and the percentage of pyridinic-N has been clearly
changed. When the temperature is 800 °C, the percentage of pyridinic-N reaches the maximum
(19.7%), but as the calcination temperature reaches 900 °C, its percentage reversely decreases
until to only 8.1%. And the catalytic activity of Hemin@HN-800Zn was the best, while that of
Hemin@HN-900Zn is obviously decreased. It suggests that the pyridinic-N species with a
planar structure should be active in ORR electrocatalysis. Besides, it is also found that inactive
pyridinic N+-O- accounts for the lowest proportion in total N content for Hemin@HN-800Zn,
implying that Fe-N and graphitic-N may be main components of catalytic sites, but pyridinic-
N may be directly related to the ORR activity in Hemin@HN-800Zn. The O1s region of
Hemin@HN-800Zn was shown in Fig. 4f. The O1s peak at 530.87, 531.81, 532.88 eV could
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be assigned to C-O-C, C=O and Fe-O.43 The above-mentioned results show that Fe atoms are
in the form of Fe-O and Fe-N.
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3.2 ORR electrocatalytic activity
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For one thing, the ORR electrocatalytic activities of Hemin@HN-800, Hemin@HN-700Zn,
Hemin@HN-800Zn and Hemin@HN-900Zn were tested in 0.1 mol l-1 O2-saturated KOH
solution by cyclic voltammetry (CV). There is a significant disparity in the ORR activity of
these catalysts, as shown in Fig. 5a and Fig. 2S, which is proved by the differences among these
CV curves. Under the same test conditions, Hemin@HN-800Zn with the metal-Fe loading of
0.28127 μg exhibits the best ORR catalytic activity among the tested samples with the most
positive Ep (0.90 V) and the highest Ip (0.92 mA cm-2) values. And the ORR activity of
Hemin@HN-800 has been improved obviously under the pore-forming effect of ZnCl2. Non-
featured CV curve can be also observed in the N2-saturated KOH solution, qualitatively
showing the selectivity of Hemin@HN-800Zn to the ORR electrocatalysis. The corresponding
ORR polarization curves of all doped-carbon catalysts were measured by the ring disk electrode
(RDE) combined with linear sweep voltammetry (LSV) technique, and the rotation rate was
fixed at 1600 rpm (Fig. 5b and Fig. 3S). All catalytic activity data were exhibited in Table 1S.
Notably, Hemin@HN-800Zn has exhibited the highest onset potential (EORR = 1.01 V) and half-
wave potential (E1/2 = 0.85 V), which can be comparable to other catalysts reported in the
previous references (Table 2S). Besides, the limited current density of Hemin@HN-800Zn at
0.3 V is 5.22 mA cm -2, apparently higher than that of other carbon-based catalysts prepared in
this work. These results suggest Hemin@HN-800Zn has superior ORR catalytic properties
among the tested samples.
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Fig. 5
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To deeply study the origin of ORR activity difference on doped-carbon catalysts with post Zn-
activating process, the electrochemical active surface areas (ECSA) are evaluated by the CV
method to check the double-layer capacitances (Cdl) in a certain non-faradic region (1.05 to
1.15 V versus RHE). Fig. 5c, Fig. 4S and Fig. 5S shows CV curves of three carbon catalysts
tested at different scan rates (20 to 80 mV s-1). The double-layer capacitance was calculated in
Fig. 5d. There is a linear relationship between current density and the square root of the
scanning rate, presenting a typical diffusion-controlled electrochemical process. Hemin@HN-
800Zn shows a Cdl of 10.78 mF cm-2, being beyond that of Hemin@HN-700Zn (3.72 mF cm-2)
and Hemin@HN-900Zn (8.14 mF cm-2), demonstrating a correspondingly larger ECSA of
Hemin@HN-800Zn with more dense surface catalytic sites to enhance the ORR catalytic
activity. For this reason, the superior ORR catalytic property of Hemin@HN-800Zn can be
ascribed to four aspects: i) The formation of semi-tubular nanostructure can be effective
exposure and full utilization of nitrogen-rich catalytic sites; ii) The engendering of mesopore-
dominated characteristics can help to accelerate the transportation of the electrolyte, reactants
and products to facilitate the ORR; iii) The higher percentage of active nitrogen species (e.g.,
pyridinic-N, Fe-N and graphitic-N) can be produced in Hemin@HN-800Zn during calcination
process; iv) Hemin@HN-800Zn exhibits a higher surface active-site density, resulting from its
larger ECSA, to increase the inherent ORR activity.
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However, up to now, the function of the real ‘catalytically active sites’ remains controversial
because their contribution to the ORR activity is not well-defined. When the active sites in
carbon-doped ORR catalysts were studied, it can be found that active nitrogen species such as
pyridinic-N and graphitic-N in catalysts can also activate oxygen molecules and catalyze the
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ORR process,44 but their contributions to the ORR activity are unclear. SomDeOIi:m10.p10o3r9t/Da0nTtA02741C
researches pointed out that pyridinic-N and graphitic-N may be related to ORR onset potential
and limited current density, respectively.45 The selectivity of pyridinic-N to four-electron
reaction pathway is better than that of graphitic-N,37 while the former’s ORR activity is high
but the stability is poor.46 In addition, to better clarify the role of excessive metal-Fe atoms in
active sites neglected commonly, we have first designed an additional poisoning experiment by
using 10 mM thiocyanate ion (SCN-) as a poisoning electrochemical probe based on the strong
binding force of SCN- and Fe3+. We separately tested the LSV curves of Hemin@HN-800Zn
catalyst in O2-saturated KOH versus O2-saturated KOH containing 10 mM SCN-, as shown in
Fig. 6S in supporting information. We found that the half-wave potential for ORR was
negatively shifted by about 20 mV and the limited current density was obviously reduced after
testing in O2-saturated 0.1 mol l-1 KOH containing 10 mM SCN-. Additionally, the half-wave
potential for ORR has also decreased about 60 mV, while the Hemin@HN-800Zn catalyst was
leached in 0.5 mol l-1 H2SO4 solution for 24 h to potentially destroy the Fe-N coordinate
structures. The decrease on the ORR activity, caused by the acid-leaching effect and the
poisoning effect of 10 mM SCN-, has provided a direct evidence for the significance of the
Fe-N structure towards the improvement of the ORR catalytic activity. Thus, it probably
suggests that Hemin@HN-800Zn may have metal-centered active sites (Fe-N) for the ORR,
which was supported by many reported researches.47,48 Furthermore, based on the XPS results
and electrochemical data, there is relatively sufficient reason to judge that the graphitic-N may
be one of main components of active sites, but pyridinic-N may be also related to the promotion
of ORR activity in Hemin@HN-800Zn.
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To investigate the ORR mechanism of Hemin@HN-800Zn, a series of polarization curves were
measured at different rotational speeds (600, 800, 1200, 1600, 2000, 2400 rpm) in O2-saturated
KOH solution, as displayed in Fig. 6a. The overall electron transfer number (n) can be
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calculated by the Koutecky-Levich (K-L) theory based on a fact that the ORR current deVnieswiAtyrticle Online
DOI: 10.1039/D0TA02741C
is proportional to the RDE rotation rate. The diffusion-limited current density (jd) on RDE was
estimated by utilizing the K-L equation.49
1/jd = 1/jk + 1/0.62nFCODO2/3υ-1/61/2
where jk, F, CO, DO , n and υ are the kinetic current density of the ORR, the Faradaic constant
(96,485 C mol-1), the O2 saturation concentration in the aqueous solution (1.210-6 mol cm-3),
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the O2 diffusion coefficient in 0.1 mol l-1 KOH electrolyte (1.910-5 m2s-1), the kinetic viscosity
of the solution (0.01 cm2s-1), the electrode rotation rate (rpm),respectively. Fig. 6b shows
corresponding K-L plots under different potentials (0.3, 0.4, 0.5, 0.6 and 0.7 V). It can be seen
that each line has an excellent linear relationship, and each line is parallel or almost overlapped
with each other. These results demonstrate that the calculated electron transfer numbers at
different potentials are similar and the concentration of dissolved O2 accords with the first-order
reaction kinetics. Then the calculated n value on Hemin@HN-800Zn was about 3.89. The ORR
mechanism and kinetic behavior were further studied by RRDE tests. As indicated in Fig. 6c,
Hemin@HN-800Zn has the highest disk current and lowest ring current. The average n value
and H2O2 yield rates of catalysts can be also calculated in Fig. 6d. Hemin@HN-800Zn shows
the best performance with an average n of 3.93 and an average H2O2 yield of 3.67 in the
potential range of 0.2 V to 0.8 V, being superior to other carbon catalysts as well as comparable
to those of the Pt catalyst, as displayed in Table 1S. For Hemin@HN-800Zn, the average n
value on RRDE test is almost identical to that on RDE test. The above results indicate that O2
is mainly converted into -OH with a pathway of four-electron reduction during the ORR. Thus,
it may be one kind of ideal catalysts for oxygen reduction in alkaline medium.
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The long-term stability for ORR electrocatalytic activity of Hemin@HN-800Zn iDsOoI:n10e.10o3f9/tDh0eTA02741C
important concerns because it is of great significance for the practical application of catalyst.
Therefore, it is necessary to test the catalytic stability of Hemin@HN-800Zn via using an
accelerated aging test (AAT), as indicated in Fig. 7. Compared with the initial peak current
density (0.92 mA cm-2), the current density (jd) has retained 89.1% (0.82 mA cm-2) after CV
test for 5000 cycles. The peak potentials (EP) have reduced from 1.04 V to 0.98 V. The onset
potentials (EORR) also have retained 96.0% (0.97 V) of the original onset potentials (1.01 V),
but the ORR current density has been almost unchanged. These results synthetically suggest
that Hemin@HN-800Zn performs well in the long-term stability test.
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Fig. 7
3.3 Primary Zinc-Air Battery Test
To further explore its potential application in practical energy devices, we constructed a primary
liquid-flowing Zn-air battery for evaluating the ORR general performance, as shown
schematically in Fig. 8a. The battery was a home-made electrochemical cell, in which
Hemin@HN-800Zn was loaded on carbon paper (1 cm2) as the cathode and Zn foil with a
contact area of 1 cm2 was used as the anode. 6 mol l-1 KOH electrolyte was filled into the cell
(10 ml volume). The open-circuit voltage of Zn-air battery using the Hemin@HN-800Zn
catalyst as a cathode is indicated in Fig. 7S. The polarization curves and corresponding power
density plots of the battery were shown in Fig. 8b, it can be found that the maximum power
density was ca. 191mW cm-2 at the current density of 267 mA cm-2, being slightly lower than
that of Pt/C catalyst (only ~25 mW cm-2). Moreover, Fig. 8c shows the power density
comparison between our sample and other previously reported catalysts.50-59 It is interesting
that Hemin@HN-800Zn prepared in this work has exhibited the highest power density among
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these catalysts. To further evaluate the long-term durability of the battery, we carried out the
DOI: 10.1039/D0TA02741C
battery continuous discharge at a current density of 5 mA cm-2 for 12 h. As indicated in Fig.
8d, the cell voltage using Hemin@HN-800Zn catalyst has been hardly decreased, supporting
the excellent durability of Hemin@HN-800Zn. These outstanding general performances of the
Zn-air battery imply that Hemin@HN-800Zn is a promising candidate in future applications of
air batteries or other electrochemical energy devices.
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4. Conclusion
Herein, we propose a new strategy for facile synthesis of iron-nitrogen-doped porous carbon
nanocatalysts (Hemin@HN-800Zn) made by two-step annealing hemin biomaterial on the basis
of a template-assisted method under nitrogen atmosphere. The utilization of nanoclay template
can induce the formation of semi-tubular doped-carbon nanostructures dominated by mesopore-
dominated characteristics owing to the post ZnCl2-activating process. The synthesized catalyst
shows excellent ORR performance with an onset potential of 1.01 V and a half-wave potential
of 0.85 V and follows a direct four-electron transfer pathway in alkaline electrolyte. Moreover,
Hemin@HN-800Zn has exhibited an unexpectedly high-power density of 191mW cm-2 in
primary Zn-air battery, only 25 mW cm-2 lower than that of the Pt catalyst. After continuous
discharge of 5 mA cm-2 for 12 h, the cell voltage has been hardly decreased using the
Hemin@HN-800Zn catalyst. The excellent ORR catalytic performance may be attributed to the
formation of the semi-tubular nanostructure, the engendering of mesopore-dominated
characteristics, higher percentage of active nitrogen species, and higher surface active-site
density, in which can jointly facilitate the improvement of ORR catalytic activity. This study
can provide a new idea and route for the original design of various precious doped-carbon
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materials by using natural biomaterials for a wide range of reactions in electrochemical energy
DOI: 10.1039/D0TA02741C
conversion devices.
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Acknowledgements
4
This work was partially supported by the National Natural Science Foundation of China
(Project No: 21805024, 31771101), the Basic Research and Frontier Exploration Project of
Chongqing Municipality (Project No: cstc2018jcyjAX0461, cstc2015jcyjBX0072), the
Scientific and Technological Research Program of Chongqing Municipal Education
Commission (KJQN201901335, KJ1711289). We gratefully thank Zhongbin Li, Honglin Li,
Changguo Chen, Qin Xiang, Bingbing Hu, Guoqiang Li and Wei Sun for helpful discussions.
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Fig. 1. A schematic diagram of mesopore-dominated semi-tubular doped-carbon nanostructures
.
4
Fig. 2. (a) SEM, (b-e) TEM and (f-g) HAADF-TEM images of Hemin@HN-800Zn. (h)
Nitrogen adsorption-desorption isotherms; Inset is the corresponding BJH pore-size distribution
of Hemin@HN-800Zn.
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Fig. 3. (a) Raman and (b) FT-IR spectra of Hemin@HN-800Zn.
8
Fig. 4. (a) Survey and (c-e) N 1s XPS spectra of Hemin@HN-700Zn, Hemin@HN-800Zn, and
Hemin@HN-900Zn. (b) C1s and (f) O1s XPS spectra of Hemin@HN-800Zn.
Fig. 5. (a) CV curves of Hemin@HN-700Zn, Hemin@HN-800Zn, and Hemin@HN-900Zn in
N2 versus O2-saturated KOH. (b) LSV curves of Pt/C, Hemin@HN-700Zn, Hemin@HN-800Zn,
and Hemin@HN-900Zn in N2 versus O2-saturated KOH. (c) CV curves of Hemin@HN-800Zn
tested at different scan rates. (d) The double-layer capacitance of Hemin@HN-700Zn,
Hemin@HN-800Zn, and Hemin@HN-900Zn.
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Fig. 6. (a) ORR polarization curves of Hemin@HN-800Zn at different rotation speeds in O2-
saturated KOH. (b) The corresponding K-L plots under different potentials (0.3-0.7 V). (c) Disk
and ring currents obtained with LSV on RRDE for Pt-C, Hemin@HN-700Zn, Hemin@HN-
800Zn, and Hemin@HN-900Zn in O2-saturated KOH. (d) The corresponding electron transfer
numbers and H2O2 yields at the potential range of 0.2-0.8 V derived from (c).
Fig. 7. (a) CV and (b) LSV curves of Hemin@HN-800Zn and Pt-C before and after doing an
accelerated aging test (AAT).
Fig. 8. (a) A schematic diagram of primary Zn-air battery. (b) Polarization and power density
curves of primary Zn-air battery with Hemin@HN-800Zn and Pt/C. (c) Comparison of the
power density between this work and other results. (d) The ORR chronopotentiometric response
for a durability test at a constant current density of 5 mA cm-2 for 12 h.
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Figure 2
a
b
100 nm
500 nm
c
d
50 nm
500 nm
e
f
500
0.8
C
Fe
O
0.7
mesopore range
3.7nm
h
0.6
400
0.5
g
0.4
0.3
20.8nm
0.2
300
0.1
0.0
1
10
70
Pore Width (nm)
200
100
N
0
0.0
0.2
0.4
0.6
0.8
1.0
Reletive Pressure (p / p0)
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