Understanding the Roles of Sulfur Dopants in Carbonaceous …home.skku.edu/~sson/homepage/board/upload/sub3_1/up_sub3... · 2018-07-18 · Understanding the Roles of Sulfur Dopants

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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

1905ChemElectroChem 2018, 5, 1905 – 1913 � 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ArticlesDOI: 10.1002/celc.201800103

Wiley VCH Donnerstag, 12.07.20181814 / 108719 [S. 1905/1913] 1

http://orcid.org/0000-0002-1563-8328

http://orcid.org/0000-0002-1563-8328

https://doi.org/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.

1906ChemElectroChem 2018, 5, 1905 – 1913 www.chemelectrochem.org � 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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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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