understanding the roles of sulfur dopants in carbonaceous...
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Very Important Paper
Understanding the Roles of Sulfur Dopants inCarbonaceous Electrocatalysts for the Oxygen ReductionReaction: The Relationship between Catalytic Activity andWork FunctionHeejong Shin+,[a, b] Narae Kang+,[a, b] Daye Kang,[c] Jin Soo Kang,[a, b] Ju Hong Ko,[c] DooHun Lee,[c] Subin Park,[a, b] Seung Uk Son,*[c] and Yung-Eun Sung*[a, b]
We prepared a series of hollow sulfur-doped carbons with
diverse S contents through the carbonization of microporous
organic networks (MONs), which were synthesized through the
Sonogashira coupling of thiophene moieties with different
numbers of S atoms as building blocks. This preparation
method enabled the doping level to be controlled without
inducing any notable differences in textural and morphological
characteristics, and these S-doped carbons did not show any
notable differences in the chemical properties of carbon,
regardless of the sulfur content. We used these well-controlled
MON-derived carbons as a model to elucidate the role of sulfur
dopants in the oxygen reduction reaction (ORR) and to
investigate the relationship between the activities and work
functions of carbonaceous catalysts. By excluding the effect of
electrical properties of the S-doped carbon catalysts using
conducting agents, we could successfully verify that increasing
the number of dopants led to an enhancement in the ORR
activities, and the high applicability of work function as the
activity descriptor was also demonstrated. We believe that our
experimental observations will provide a deeper understanding
of carbonaceous electrocatalysts with p-block dopants, and the
investigations performed in this study are also anticipated to
serve as a rational guideline in designing carbonaceous catalysts
for various electrochemical reactions.
1. Introduction
The oxygen reduction reaction (ORR) is critical for environ-
mental and sustainable energy devices such as fuel cells and
other energy conversion systems. Current studies on polymer
electrolyte membrane fuel cells (PEMFCs) have focused on
catalyzing the sluggish ORR kinetics at the cathode[1] and
developing alternative nonprecious metal[2] or metal-free[3]
catalysts to replace platinum materials. Since Dai and coworkers
reported the superiority of nitrogen-doped carbon nanotubes
to platinum for electrocatalysis of the ORR,[4] the field of metal-
free catalysts for ORR has experienced rapid development, and
several studies have concentrated on developing highly active
nitrogen-doped carbon materials.[5] It was found that nitrogen
doping can induce charge redistribution on the surface of
carbon and promote the chemisorption of oxygen molecules.
Various attempts to dope sulfur atoms in carbonaceous
materials have also been reported,[6] where it was revealed that
the sulfur dopants can induce significant spin polarization into
the carbon lattice based on a first-principle investigation
employing density functional theory methods. Moreover, co-
doping of carbon catalysts with different heteroatoms was
found to be an efficient way to further enhance the catalytic
activity for the ORR.[7] However, it is still challenging to improve
the ORR performance of carbon materials by doping non-metal
p-block atoms into the carbon lattice. Although various
heteroatom (for example, nitrogen, sulfur, boron or
phosphorus)-doped carbon materials with significantly en-
hanced ORR activity have been developed,[8] deeper under-
standing and rational principles are required for the judicious
design of electrocatalysts for ORR.
Numerous research groups have suggested various con-
cepts for explaining the effect of the dopant on the carbon
lattice. Some have insisted that a difference in the electro-
negativity of the dopants and carbon is important for inducing
charge redistribution on the carbon surface and distorting the
lattice structure.[9] On the other hand, Xia and co-workers
suggested that the electron affinity, which represents the
energy release from a neutral atom when an extra electron is
added to form a negative ion, could be the ability to transfer
electrons in the reaction.[10] They also proposed the combined
effect of the electron affinity and electronegativity of the
[a] H. Shin,+ Dr. N. Kang,+ Dr. J. S. Kang, S. Park, Prof. Dr. Y.-E. SungCenter for Nanoparticle ResearchInstitute for Basic Science (IBS)Seoul 08826, Korea
[b] H. Shin,+ Dr. N. Kang,+ Dr. J. S. Kang, S. Park, Prof. Dr. Y.-E. SungSchool of Chemical and Biological EngineeringSeoul National UniversitySeoul 08826, KoreaE-mail: [email protected]
[c] D. Kang, Dr. J. H. Ko, D. H. Lee, Prof. Dr. S. U. SonDepartment of ChemistrySungkyunkwan UniversitySuwon 16419, KoreaE-mail: [email protected]
[+] These authors contributed equally to this work
Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/celc.201800103An invited contribution to a Special Issue on Non-Precious-Metal OxygenReduction Reaction Electrocatalysis
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ArticlesDOI: 10.1002/celc.201800103
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dopants, as intrinsic parameters, for comparative evaluation of
the catalytic activity. Recently, some research groups reported a
correlation between the work function of the catalyst materials
and their kinetic activities during ORR.[11] They suggested that
doping heteroatoms into the carbon lattice or introducing
metal elements could increase the density-of-states near the
Fermi level and reduce the work function. However, the ability
to remove an electron to vacuum level might be different from
the reactivity to donate electrons to adsorbed reactants.
Further, each type of active site has dissimilar reaction kinetics
and a different reaction mechanism;[12] nevertheless, the carbon
materials used in previous studies contained various types of
heteroatoms, and even simultaneously combined iron metal
atoms with other non-metal p-block dopants. Thus, we suggest
that more systematic and controlled studies are required to
unravel the influence of the individual p-block dopant atoms
on the correlations between the work function and the ORR
kinetics.
Herein, we successfully prepare a set of catalysts containing
only one p-block sulfur dopants, sulfur, via the Sonogashira
coupling reaction. Recently, Cooper and others have reported
that coupling reactions of organic building blocks are very
efficient methods for the preparation of highly porous and
functional organic polymers.[13] Through Sonogashira coupling,
various MONs have been developed with high surface area and
catalytic activity. In this work, we suggested a strategy for
controlling the active sites of hollow sulfur-doped MONs
derived from thiophene moieties, and this doping approach led
to enhanced ORR performance. Using these well-controlled
series of catalysts, systematic investigations on the role of
dopants on work functions and ORR performances were carried
out. Moreover, relationship between the work function and the
catalytic activities were examined, and the applicability of work
function as a descriptor for the ORR activity was studied.
2. Result and Discussion
Hollow S-doped MONs (S-MON) which have different sulfur
contents in their carbon matrix were prepared by introducing
thiophene and bithiophene as building blocks. These porous
polymers were used as precursors to prepare sulfur-doped
carbon electrocatalysts for oxygen reduction. Hollow MON
structures were engineered based on our previous reported
method.[14] Figure 1 shows the synthetic scheme for the S-
MONs. Zeolite imidazolate framework-8 (ZIF-8) nanoparticles
were used as templates for hollow structure.[15] Sonogashira
coupling of tetrakis(4-ethynyl)phenylmethane and 2,5-diiodo-
thiophene (or 1,4-diiodobenzene for B-MON, 5,5’-diiodo-2,2’-bithiophene for SS-MON) was conducted on the surface of ZIF-
8 under palladium and copper catalysts. The resultant solids
were treated with acetic acid to remove ZIF-8 templates and to
form polyhedral hollow structure. The solid phase 13C NMR
analysis showed that the peaks of MONs were matched well as
expected (Figure S1, Supporting information). These materials
were annealed at 700, 850, and 1000 8C for 3 hours under inert
argon atmosphere, and then treated in aqua Regia solutions to
remove the residual palladium species (Table S1, Supporting
information). The more detail procedures are described in the
Experimental section. The resultant materials are denoted by
combining the annealing temperature and the type of initial
building block (for example, 700B indicates the sample
carbonized from MON reacted with 1,4-diiodobenzene, and
then annealed at 700 8C. The samples derived from the 2,5-
diiodothiophene unit are denoted as S and from 5,5’-diiodo-
2,2’-bithiophene as SS, indicative of the thiophene chain length
in the original forms).
The obtained carbonaceous materials were investigated by
scanning electron microscope (SEM) and transmission electron
microscopy (TEM) (Figure 2 and Figure S2 and S3, Supporting
information). All synthesized sulfur-doped hollow carbons had a
similar morphology resembling that of the mother ZIF-8
templates. As shown in the TEM images, the hollow inner void
space and dark contrasted shells were well distinguished. The
average size and thickness of the shell of the MON-derived
Figure 1. Synthetic route for hollow sulfur-doped MON-derived carbons using a ZIF-8 template.
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sulfur-doped carbon materials were 342�15 nm and 23�1.3 nm, respectively. To compare the surface area of all samples,
the Brunauer-Emmett-Teller (BET) N2 adsorption and desorption
isotherms analyses (Figure S4) were conducted. It showed
microporous characteristics and high surface areas of the
synthesized carbon materials and they had very similar surface
areas and pore distributions relative to each other. Moreover, X-
ray diffraction (XRD) revealed carbon-related peaks correspond-
ing only to the amorphous phase of the MON derived carbon
materials without any detectable crystalline species (Figure S5,
Supporting information). To compare the electrochemical sur-
face area of the samples, we measured the electrochemical
capacitance of the electrode-electrolyte interface in the double
layer regime of the cyclic voltammetry (CV) (Figure S6, Support-
ing information). The capacitance of all samples showed the
similar surface capacity, we could estimate that the surface
areas and pore distributions were comparable.
Elemental mapping using energy-filtered TEM (EF-TEM)
analysis of the 850S and 850SS carbons (Figure 3a and Fig-
ure S7) showed that sulfur was well and uniformly dispersed
over the MON-derived carbon structures, indicating successful
sulfur doping by the Sonogashira coupling reactions. To further
understand the changes due to the surface sulfur functionality
in the carbon matrix and the bonding configurations of sulfur
and carbon, near edge X-ray absorption fine structure (NEXAFS)
and X-ray photoelectron spectroscopic (XPS) analyses were
conducted. The NEXAFS carbon K-edge spectra of the series of
sulfur-doped carbons are shown in Figure 3b. The samples
displayed quite similar functionality, but the bonding config-
uration of the carbonaceous samples annealed at 700 8C were
somewhat different from those of the doped carbons treated at
850 and 1000 8C in the range 286.1–288.3 eV; moreover, the
former also had a dissimilar peak height ratio at 285.3 eV (1 s!
p* of sp2 hybridization) and 293 eV (1 s!s* of sp3 hybrid-
ization).[16] This result might indicate inadequate carbonization
of the MON polymers treated at 700 8C, while the carbonaceous
samples treated at higher temperatures (850 and 1000 8C) had
similar bonding configurations of sulfur and carbon. Moreover,
Raman spectroscopy (Figure S8) also provided direct proof of
the insufficient carbonization. The 1341 and 1589 cm�1 peaks
are recognized as D and G peaks, respectively.[17] There were no
differences between the Raman spectra of 850S and 1000S
(likewise, the profiles of 850SS and 1000SS were similar), but
that of 700S revealed a different ID/IG ratio. The origin of this
dissimilar ratio requires further study, but it seems that the
extent of carbonization was inadequate. For this reason, we
excluded the MON-derived carbon sample annealed at 700 8Cfrom the electrocatalysts used to investigate the actual role of a
single p-block dopant (sulfur) in the ORR because of its
dissimilar textural properties.
The nature of the synthesized hollow MON-derived carbons
was examined by XPS to estimate the surface concentrations of
sulfur, as an electrocatalytic active site, in the different sulfur-
doped carbons. The carbon and sulfur signals were clearly
observed, without signals from other metal or residues (Fig-
ure 3c,d and Figure S9). The XPS profiles of all the sulfur-doped
carbons carbonized in the range of 850–1000 8C were similar
bonding configurations (Figure S10, Supporting information).
The amount of sulfur was estimated by simply calculating the
ratio of the C 1s and S 2p peak areas and by considering the
sensitivity factors, as sulfur atoms were assumed to be doped
uniformly into the carbon matrix, as demonstrated in the
aforementioned EF-TEM images. The calculated sulfur content
is presented in Table 1 and reveals that the carbons developed
from the building component with 5,5’-diiodo-2,2’-bithiophene
Figure 2. HR-TEM images of MON-derived carbon materials: a) 700B, b) 700S,c) 700SS, d) 850B, e) 850S, f) 850SS, g) 1000B, h) 1000S, and i) 1000SS. Figure 3. a) Elemental mapping images using EF-TEM of 850S: carbon (red);
sulfur (green). b) Carbon K-edged NEXAFS spectrum of sulfur-doped carbons.XPS spectra of c) C 1s and d) S 2p of 850S, 850SS, 1000S, and 1000SS.
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had a much higher sulfur content than those derived from 2,5-
diiodothiophene, because the former building component
contains two thiophene units. As the annealing temperature
increased, the sulfur concentration declined. The sulfur content
in the MON-derived carbons followed the order: 1000S<
850S<1000SS<850SS. The 850SS carbon sample had the
highest content of sulfur of 5.50 % (Table 1). The bulk elemental
compositions were obtained by elemental analysis (EA) and are
listed in Table 1. The bulk analysis results correspond with XPS
data. Based on these results, it was confirmed that sulfur atoms
were successfully introduced into the carbon matrix via
Sonogashira coupling, and that sulfur doping can be achieved
using our method. The covalently bonded sulfur dopant can
induce both charge and spin densities on the carbon surface
and could serve as active sites, which may enhance the catalytic
performance.
The electrocatalytic activity of the sulfur-doped hollow
carbons for the ORR was evaluated using a rotating ring-disk
electrode (RRDE) in 0.1 M KOH solution (Figure 4a). The polar-
ization curve of the Pt/C catalyst is displayed with those of the
doped carbons. The catalytic performance of the sulfur-doped
MON-derived carbons was compared, where the 1000B carbon
sample was chosen as a control sample for which the sulfur
content was nearly zero, because there seemed to be little
difference between 850B and 1000B. The activity followed the
trend 1000B<1000S<850S<850SS<1000SS. The results re-
vealed in this work as well as in previous studies indicated that
heteroatom doping of the carbon matrix can provide enhanced
ORR activity.[8] For a more quantitative evaluation, kinetic
current densities were calculated at the kinetic region (i. e.
0.75 V) based on the Koutecky-Levich (K�L) equation. Although
the diffusion limited current densities of the synthesized sulfur-
doped carbons were relatively ill-defined, the limited current
density could be determined by using the Levich equation with
the total electron transfer number (n) experimentally calculated
from then RRDE system (Figure S11).[18] The kinetic current
densities of the carbon catalysts are presented in Figure 4b and
Table S2. The most active 1000SS carbon sample had an onset
potential of 0.84 V (vs. RHE), and its kinetic current density at
0.75 V was 3.04�0.12 mA cm�2. In a number of previous
Table 1. Physical parameters of sulfur-doped carbons: the concentration of sulfur atoms on carbon surface.
Carbonmaterials
Specific capacitance[F/g]
BET surfacearea [m2/g]
S content from XPS[%]
S content from EA[%]
Work function[eV]
Electricalconductivity[W�1 cm�1]
1000B 116.81 1287 0 0 5.22�0.015 0.412�0.0271000S 119.14 1089 1.65 2.17 5.17�0.008 0.417�0.023850S 110.82 1237 2.97 3.01 5.13�0.009 0.396�0.0191000SS 116.16 1121 4.04 4.34 5.10�0.013 0.364�0.024850SS 111.56 1208 5.50 5.87 5.07�0.014 0.338�0.034Vulcan XC-72 – – – – – 4.31�0.25
Figure 4. a) ORR polarization curves and b) kinetic current densities in 0.1 M KOH at a scan rate of 10 mV s�1 with 1600 rpm rotating. c) Secondary electroncutoff and d) valence band spectra of sulfur-doped carbons. e) The correlation of the content of sulfur dopants on the carbon surface with their work functionvalues.
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studies, improvement in the degree of graphitization by
thermal annealing at a higher temperature was suggested as a
reason for the enhancement in ORR activity.[19] However, similar
chemical states of carbons in 850S, 850SS, 1000S, and 1000SS
were confirmed by various physicochemical analyses, and the
differences in electrocatalytic performances were thereby
associated with the amount of sulfur contents in this study.
It has been proposed that the work function of the doped
carbon catalysts is correlated with their activity, and the former
is generally used as a descriptor for the ORR kinetics in earlier
studies. Earlier studies suggested that a catalyst with a lower
work function had a lower energetic barrier for donating
electrons to the adsorbed oxygen reactant. Thus, the work
function of the sulfur-doped MON-derived carbons was
evaluated herein using ultraviolet photoelectron spectroscopy
(UPS) (Figure 4c,d and Table 1) to understand the effect of the
heteroatom dopants on the ORR activity. The work function is
determined by measuring the width of the emitted electrons
from the onset of the secondary electrons up to the Fermi level
and subtracting the width from the energy of the incident light
energy.[20] The calculated work function values and other
physical parameters are listed in Table 1. In particular, the
results revealed that the content of sulfur in the doped carbons
was inversely proportional to the work functions of each
sample (Figure 4e). This indicated that the Fermi level of the
sulfur-doped carbons was elevated with increasing dopant
concentration, as the number of sites contributing electrons to
the carbon lattice increased. The trend in the work function of
the present carbon catalysts was in moderate accordance with
the ORR activity, but the trend in the activities of the 1000SS
and 850SS carbons showed some deviation. The previous
model proposed by other research groups, states that a better
catalyst for the ORR should have a smaller work function.[11] The
work function of doped carbon can thus play a key role as a
“descriptor” of the ORR activity. However, based on the present
experimental findings, we now propose that other factors must
be considered in applying the work function parameter, such as
the electrical conductivity of the carbon materials. The electrical
conductivity of the synthesized MON-derived carbons was
measured using the four-point probe method, and are listed in
Table 1. The data illustrate that a high content of sulfur species
can lead to a lower conductivity. In particular, the conductivity
of 850SS carbon was inferior (0.338�0.034 W�1 cm�1) to that of
the others, which would in turn have a deleterious effect on the
ORR kinetics.
A correct comparison of the electrochemical activities
would be possible only for the carbon samples with similar
textural and morphological characteristics and electrical con-
ductivity. To determine the severity of the effect of the electrical
conductivity of the samples on the ORR activity, the carbon
catalysts were mixed with an auxiliary conducting agent to
improve the poor conductivity of the sulfur-doped carbons,
especially for the 1000SS and 850SS carbons (details are
presented in the Experimental section). The added conducting
agent was Vulcan XC-72 carbon black,[21] which has a much
higher conductivity than the synthesized amorphous sulfur-
doped carbons (Table 1). The Vulcan XC-72 carbons comprised
nano-sized primary particles that were fused with the MON-
derived carbon to give an aggregate. The ORR polarization
curves of the sulfur-doped carbons are presented in Figure 5a.
The activity followed the trend: 1000B <1000S <850S
<1000SS <850SS. The addition of carbon as an assisting
conducting agent generally caused an improvement in the ORR
kinetics (Figure 5b and Table S2, Supporting information).
Notably, 850SS carbon showed the highest activity, surpassing
Figure 5. a) ORR polarization curves and b) kinetic current densities of sulfur-doped carbons after the carbon black was added as a conducting agent (opaquecolors) and before added (transparent colors). c) Nyquist plots of EIS results and d) real and imaginary capacitances plots for the complex capacitance analysisof 850SS carbon. e) The complex capacitance analyses of sulfur-doped MON derived carbon catalysts after the carbon black was added as a conducting agentinto the catalyst layer.
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that of 1000SS after the addition of a small amount of Vulcan
carbon.
To determine whether the Vulcan XC-72 carbon additive
could indeed alleviate the electrical conductivity problems in
the catalyst layers to maximize their utilization and decrease
polarization of the electrode, capacitive measurements were
carried out in a three-electrode electrolyte system. CV was
performed at different scan rates to investigate the properties
of the supercapacitors with or without auxiliary carbons. All
samples were evaluated over the potential range of 0.05–1.05 V
(vs. RHE) in a 6 M KOH solution. Signals other than capacitive
currents were hardly observable from the CV results, indicating
that the addition of carbon did not induce any other Faradaic
reactions (Figure S12, Supporting information). Due to the
excellent electric conductivity of Vulcan XC-72, the rectangular
CV curves of 850SS MON with the carbon additives were
retained even at the high scan rate of 1000 mV s�1, indicating
excellent capacitive behavior. However, in the case of 850SS
only, without auxiliary carbon additives, the specific capacitance
decreased with increasing scan sweep rates. Because the S-
doped 850SS MONs have huge void volumes in their hollow
structures that can provide micro-channels for the electrolyte,
the difference in the rate performances did not result from
differences in the mobility of the electrolyte ions. Instead, this
suggests that the inferior electrical conductivity of the 850SS
MON materials makes activation of electron transfer in the
catalyst layers difficult. This indicates that the electric con-
ductivity of the 850SS carbons was far inferior to that of the
Vulcan XC-72 carbon additives, as well as that of the developed
S-doped MON carbons; thus, the transportation of electrons in
the catalyst layer of the electrode was hampered. Furthermore,
this conclusion is supported by the impedance analysis
presented below.
A frequency response analysis was conducted using EIS
measurements to confirm the electrical and ionic conductivities.
Nyquist plots of the 850SS MONs in both cases are shown in
Figure 5c. The inset figure, which is a magnification of the
Nyquist plots in the high frequency region, shows a semicircular
profile for both samples. The semicircle may be associated with
the inter-granular electronic resistance between the active
carbonaceous particles or that from the gap between active
carbons and current collector interface. These resistances are
related to the intrinsic electrical conductivity of the 850SS
MON-derived catalysts. Therefore, the difference in the size of
the semicircle for both 850SS carbon samples illustrates that
Vulcan XC-72 carbon can function as a conducting agent in
these electrocatalyst systems. This is consistent with the
aforementioned CV data.
The rate capability of the materials could also be estimated
from EIS measurement by calculating the complex capacitance
(Figure 5d). Complex capacitance analysis is a useful tool for
extracting frequency-related information. The complex capaci-
tance is defined as follows [Eqs. (1)–(4)]:
Z wð Þ ¼ 1
jwC wð Þ ð1Þ
C wð Þ ¼ C0
wð Þ � jC 0 0 wð Þ ð2Þ
C0
wð Þ ¼ �Z 0 0 wð Þw Z wð Þj j2
ð3Þ
C0 0
wð Þ ¼ Z 0 0 wð Þw Z wð Þj j2
ð4Þ
The value of C’(w), detected at low frequency, corresponds
to the static capacitance. C’’(w) is also correlated to the energy
dissipation of the capacitor by an irreversible process, for
instance, the irreversible faradaic charge transfer or IR drop.
From the peak frequency of C’’(w), the relaxation time constant
can be calculated, which reflects the kinetic performance of the
materials.[22] The C’’(w) plot of 850SS with auxiliary carbons (red
squares), as shown in Figure 5d, had a peak at 8.01 Hz in the
measured frequency range. The C’’(w) plot of 850SS MONs only
without carbon additives (red upper triangle) showed a peak at
lower frequency (1.18 Hz) than the former case. The higher
frequency peak indicates a shorter relaxation-time constant, as
well as faster kinetics. That is, the Vulcan XC-72 carbon additives
could offer direct electrical pathways for the electrons in the
MON-derived carbon particle layers on the electrode and
increase the electron transport rate, which may improve the
electrocatalytic performance. EIS analysis was also conducted
for all the other sulfur-doped MON carbons with Vulcan carbon
additives (Figure 5e and S13, Supporting information). To our
surprise, the MON-derived S-doped carbons manifested similar
peak frequencies for C’’(w) after the introduction of auxiliary
carbon. From this observation, we could verify that the carbon
additives, which increases the electrical conductivity of the
electrocatalysts effectively, result in the similar degree of
electron transport rates regardless of the intrinsic conductivities
of the S-doped carbon catalysts. This led to a decrease in the
polarization of the electrodes and better utilization of the active
catalyst materials. For this reason, we could conclude that our
S-doped carbon catalysts can be used as well-controlled model
catalysts to understand the relationship between the work
functions and activities with the presence of carbon black as
conducting agents.
Thus, we were able to compare the work function of the
sulfur-doped hollow carbons with their kinetic current densities
at 0.75 V (vs. RHE) without any physical limitations. The trend in
ORR activities from the newly acquired results was 1000B<
1000S<850S<1000SS<850SS as demonstrated in the afore-
mentioned ORR curves in Figure 5, which was in line with the
trends observed from S-contents and work functions. These
results indicated that the work function directly related the ORR
activities of carbonaceous catalysts with sulfur dopants, and it
can serve as a descriptor if other physical parameters such as
conductivity do not impose any additional limitations. Figur-
es 6a,b show the relationship between the work functions and
kinetic current densities of the carbonized MONs before (Fig-
ure 6a) and after (Figure 6b) the addition of conducting agents.
Without auxiliary carbon, a large deviation of activity from the
linear fit was apparent in the case of heavily doped carbons. In
contrast, a linear relationship between the catalytic perform-
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ance and work function was clearly observable after neglecting
the effect of electrical properties by adding the highly
conductive carbon black. In short, we could conclude that the
work functions can be used as the descriptor for the ORR
activities of S-doped carbons. Moreover, since the work function
and the doping level are closely related to each other, it could
also be perceived from our experimental investigations that
increasing the S contents leads to enhanced electrocatalytic
performances of S-doped carbon catalysts in ORR.
3. Conclusions
We designed a series of hollow sulfur-doped carbons for the
ORR by Sonogashira coupling, where the samples were
texturally and morphologically similar. The resultant hollow,
doped carbons can be used as efficient non-metal ORR electro-
catalysts. The actual influence of a single p-block dopant (sulfur)
in the doped carbon lattices on the ORR activity was
investigated by comparing the catalytic activity with the work
function value as a “descriptor” of the ORR activity, as asserted
in previous studies.[11] However, the trend in the activities of the
doped carbons seemed to deviate from the prior claims, and
based on observations, we propose that electric conductivity
may severely affect the ORR activity of the doped carbon
samples. The presence of defects that act as electron reservoirs
and traps on the amorphous carbon surface decreases the
electric conductivity. The work function is not directly corre-
lated with the electrical conductivity of the materials. By
excluding the effect of electrical properties of the sulfur-doped
carbonaceous catalysts using conducting agents, we could
successfully verify that increasing the number of dopants lead
to an enhancement in the ORR activities, and the high
applicability of work function as the activity descriptor was also
demonstrated. Consequentially, we effectively prepared that
sulfur-doped hollow carbons with various sulfur contents could
be synthesized by controlling the chain length of the initial
block units during Sonogashira coupling, and can be used as
model systems to understand the effects of dopants on the
ORR. This study could provide general guidelines for the further
development of non-metal electrocatalysts for the ORR.
Experimental Section
Synthetic Procedure for Hollow S-Doped Carbon
ZIF-8 nanoparticles were prepared according to the literature.[15]
For the preparation of S-MON, ZIF-8 nanoparticles (300 mg),Pd(PPh3)2Cl2 (10 mg, 14 mmol) and CuI (2.7 mg, 14 mmol) weredispersed in triethylamine (45 mL) in a flame-dried 100 mL Schlenkflask under Ar atmosphere. The mixture was sonicated for 1 hour atroom temperature. Then, tetrakis(4-ethynylphenyl)methane (60 mg,0.14 mmol) and 2,5-diiodothiophene (97 mg, 0.29 mmol) wereadded. The reaction mixture was heated at 90 8C for 24 hours. Theresultant materials were isolated by centrifugation, washed withacetone, dichloromethane and diethyl ether and dried undervacuum. ZIF-8@S-MON was added to acetic acid (15 mL) and stirredfor 1 hour. The resultant hollow S-MON was retrieved bycentrifugation, washed with water, methanol, dichloromethane anddiethyl ether and dried under vacuum. Metal catalysts were furtheretched by Aqua Regia (8 mL) for 2 h whilst stirring at 50 8C andwashed with excessive water and methanol. The retrieved powderwas dried under vacuum for 24 hours at 80 8C. For the preparationof B-MON and SS-MON, 1,4-diiodobenzne (95 mg, 0.29 mmol), 5,5’-diiodo-2,2’-bithiophene (120 mg, 0.29mmol) were used as buildingblock, respectively, instead of 2,5-diiodothiophene. For the carbon-ization process, the resultant B-MON, S-MON and SS-MON wereannealed at 700 8C, 850 8C and 1000 8C for 3 hours in Ar atmospherewith a heating rate of 5 8C/min.
Characterization
HR-TEM images were obtained using a JEOL 2100F unit operated at200 kV. XRD patterns were obtained using a Rigaku MAX-2200 andfiltered Cu-Ka radiation, ranging from 108 to 808 in 2q (generatorsettings were 40 kV and 200 mA). 13C-NMR data were acquired on400 MHz Solid state NMR spectrometer (AVANCE III HD, Bruker,Germany) at KBSI Western Seoul center. Specific surface areas andpore distributions were measured using a Micromeritics TriStar II3020. The atomic concentration of materials was determined bymeans of Element Analysis (EA, TruSpec Micro) and InductivelyCoupled Plasma Atomic Emission Spectroscopy (ICP-AES, OPTIMA8300, Perkin-Elmer). A four-point probe device (CMT-SP 2000N) wasused to measure the conductivity of the carbon materials whichwere sprayed as a film (about 2 mm) on slide glass with air brush(Iwata-Medea). The ink for spray was prepared as the synthesizedcarbons (10 mg) were dissolved in 700 mL of 2-propanol withoutany binders. Raman spectroscopy was measured using a HoribaLabRAM HV Evolution spectrometer with a 532 nm laser. XPS wasobtained to characterize the core level and the synchrotron X-raysource at the Pohang Light Source-II (PLS-II beamline 8A2, 3 GeV).UPS was obtained also at PLS-II 8A2 beamline, supplying synchro-tron radiation in 91.4 eV. In this process, 5 V bias was also appliedto separate the secondary electrons cut-off of sample from that ofthe detector. The Fermi level of the UPS system was determined byusing the Fermi edge of gold films. The total resolution was foundto be about 0.02 eV. In addition, NEXAFS of carbon K edge wasperformed using synchrotron X-rays at PLS-II beamline 4D.
Electrochemical Measurements
All electrochemical experiments were conducted with an Autolabpotentiostat (PGSTAT302N, Metrohm) and a conventional threeelectrode cell with Pt wire as counter electrode and a saturated Ag/AgCl as the reference electrode. All the potentials are relative tothe reversible hydrogen electrode (RHE), which was calibrated bythe determining the potential at which hydrogen evolution and
Figure 6. Correlation between ORR activity a) without and b) with theauxiliary carbon and their work function values.
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oxidation reaction occurred with Pt working electrode in a H2
saturated solution. To prepare the catalyst ink, the MON derivedcarbons (5 mg) were mixed with a 60 mL of Nafion resin (Nafionperfluorinated resin solution (5 wt%), Sigma-Aldrich) as the binderand dissolved in 740 mL of 2-propanol (Sigma-Aldrich). Thesecatalyst inks were deposited on the rotating ring-disk electrode(RRDE, Pine, E7R9 Series tips), at a catalyst loading of 0.2 mg cm�2.For addition of auxiliary carbon black (Vulcan XC-72, CabotCorporation) as a conducting agent to the catalyst ink, thesynthesized MON derived carbons (5 mg) and carbon black (1 mg)were mixed with binder and 2-propanol mentioned earlier. Beforetesting the electrocatalytic activity, 50 cycles of CV were conductedfrom 0.05 V to 1.05 V (vs. RHE) at a scan rate of 50 mV s�1 in anargon saturated 0.1 M KOH electrolyte to clean the catalyst surface.Subsequently, RRDE testing was performed at 1600 rpm at a scanrate of 5 mV s�1 in an O2 saturated 0.1 M KOH electrolyte. Duringthe measurements with RRDE system, the potential of ringelectrode was constant at 1.2 V (vs. RHE). To remove the non-Faradaic current from the RRDE measurement, the double layercapacitance obtained under same condition in an argon saturatedelectrolyte was subtracted.
The total number of electron transfer (n) of the ORR was calculatedin the catalyzed ORR using RRDE systems. Based on it, the kineticcurrent density was determined using the K-L equation [Eqs. (5)–(7)]:
n ¼ 4 id
id þ ir = Nð5Þ
1
id
¼ 1
ik
þ 1
il
¼ 1
ik
þ 1
Bw0:5ð6Þ
B ¼ 0:2 n F DO2
2=3 u�1=6 CO2ð7Þ
where id and ir are the measured disk current density and ringcurrent density on RRDE, respectively, and N is the RRDE collectionefficiency, which was determined to be 0.37 herein. Also, ik is kineticcurrent density at a certain potential, il is diffusion limited currentof RRDE system, and w is rotating speed in rpm. F, DO2, u, CO2
represent the Faraday constant (96485 C mol�1), diffusion coeffi-cient of O2 (1.9 · 10�5 cm2 s�1), the kinetic viscosity (1.1 · 10�2 cm2 s�1),and the bulk concentration of O2 (1.2 · 10�6 mol cm�3) in 0.1 M KOHsolution. B is the reciprocal of the slope of K-L plot, which can bedetermined using the total electron transfer number per oxygenmolecule calculated from RRDE. The constant 0.2 is adopted whenthe rotating rate is expressed in rpm unit.
Supercapacitor performance was measured by using the samethree electrode system and the working electrode was composedof the active materials deposited on a glassy carbon electrode witha disk diameter of 5 mm. The ink was prepared using the sameprocedure described in previous part. The performances wereobtained in 6 M KOH electrolytes with argon purging. The CVperformance was obtained with different scan rates from 50 to1000 mV s�1. The specific capacitance of the electrode was calcu-lated from the CV curves according to the following equation:
C ¼R
I dV
u �m � Vð8Þ
where C is the specific capacitance, I is the instantaneous current, uis the potential scan rate, m is the mass of individual sample, and Vis the potential window. The EIS was conducted under open circuitvoltage from 50 mHz to 100 kHZ with a 5 mV amplitude usingZAHNER PP211 potentiostat.
Acknowledgements
This work was supported by IBS-R006-A2.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: oxygen reduction reaction · sulfur-doped carbon ·work function · Sonogashira coupling · electrical conductivity
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Manuscript received: January 23, 2018Version of record online: April 6, 2018
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