Structure, Activity, and Faradaic Efficiency of Nitrogen-Doped Porous Carbon Catalysts for Direct Electrochemical Hydrogen Peroxide Production

Article abstract of DOI:10.1002/cssc.201801583

Carbon materials doped with nitrogen are active catalysts for the electrochemical two-electron oxygen reduction reaction (ORR) to hydrogen peroxide. Insights into the individual role of the various chemical nitrogen functionalities in the H₂O₂ production, however, have remained scarce. Here, we explore a catalytically very active family of nitrogen-doped porous carbon materials, prepared by direct pyrolysis of ordered mesoporous carbon (CMK-3) with polyethylenimine (PEI). Voltammetric rotating ring-disk analysis in combination with chronoamperometric bulk electrolysis measurements in electrolysis cells demonstrate a pronounced effect of the applied potentials, current densities, and electrolyte pH on the H₂O₂ selectivity and absolute production rates. H₂O₂ selectivity up to 95.3 % was achieved in acidic environment, whereas the largest H₂O₂ production rate of 570.1 mmol g^?1_catalyst h^?1 was observed in neutral solution. X-ray photoemission spectroscopy (XPS) analysis suggests a key mechanistic role of pyridinic-N in the catalytic process in acid, whereas graphitic-N groups appear to be catalytically active moieties in neutral and alkaline conditions. Our results contribute to the understanding and aid the rational design of efficient carbon-based H₂O₂ production catalysts.

Full text of DOI:10.1002/cssc.201801583

Accepted Article
Title: Structure, activity, and faradaic efficiency of nitrogen-doped
porous carbon catalysts for direct electrochemical hydrogen
peroxide production
Authors: Yanyan Sun, Shuang Li, Zarko Jovanov, Denis Bernsmeier,
Huan Wang, Benjamin Paul, Xingli wang, Stefanie Kuehl, and
Peter Strasser
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201801583
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ChemSusChem
10.1002/cssc.201801583
COMMUNICATION
Structure, activity, and faradaic efficiency of nitrogen-doped
porous carbon catalysts for direct electrochemical hydrogen
peroxide production
[
a]
[a]
[a]
[a]
[a]
[a]
Yanyan Sun, Shuang Li, Zarko Jovanov, Denis Bernsmeier, Huan Wang, Benjamin Paul,
[
a]
[a]
[a]
Xingli Wang, Stefanie Kühl, and Peter Strasser*
Dedication ((optional))
Abstract: Carbon materials doped with nitrogen are active catalysts
process is coupled with water electrolyzers powered by
renewable power sources, chemical electricity storage is
intimately coupled with the production of useful chemicals.[
Based on the considerations above, it is of paramount
importance to design and synthesize the ORR catalysts with
for the electrochemical two-electron oxygen reduction reaction
13]
(ORR) to hydrogen peroxide. Insights into the individual role of the
various chemical nitrogen functionalities in the H production,
2 2
O
however, have remained scarce. Here, we explore a catalytically
very active family of nitrogen-doped porous carbon materials,
prepared by direct pyrolysis of ordered mesoporous carbon (CMK-3)
with polyethylenimine (PEI). Voltammetric rotating ring-disk analysis
in combination with chronoamperometric bulk electrolysis
measurements in electrolysis cells demonstrate a pronounced effect
of the applied potentials, current densities, and electrolyte pH on the
high catalytic activity and faradaic efficiency toward H
production.
2
O
2
To date, the proposed alternative ORR catalysts for H
2
O
2
production include noble-metal-based alloys,[
15, 17, 22-27]
transition-
metal oxides/complex,[
28-32]
transition-metal/C composites,
[33-35]
and metal-free carbon-based materials[
6,
13, 16, 18,
36-38]. Among
H
2
O
2
selectivity and absolute production rates. H
2
O
2
selectivity up to
them, metal-free carbon-based materials proved repeatedly to
2 2
be the most promising alternative ORR catalysts for H O
95.3% was achieved in acidic environments, whereas the largest
-
1
-1
H
2
O
2
production rate of 570.1 mmol g catalyst
h
was observed in
production, largely due to their low-cost, facile preparation,
favorable selectivity, and strong chemical stability. Considerable
efforts have been devoted to the design and screening of a
variety of nanostructured carbon-based materials with different
heteroatoms such as nitrogen or sulfur. More specifically,
nitrogen-doped porous carbon materials have received the most
attention recently, owing to their high specific surface area and
electronic conductivity, good catalytic activity and durability. To
date, reported synthetic strategies toward nitrogen-doped
porous carbon materials were twofold: (1) the direct synthesis in
neutral solution. X-ray photoemission spectroscopy (XPS) analysis
suggests a key mechanistic role of pyridinic-N in the catalytic
process in acid, while graphitic-N groups appear to be catalytically
active moieties in neutral and alkaline conditions. Our results
contribute to the understanding and aid the rational design of
2 2
efficient carbon-based H O production catalysts.
The electrochemical oxygen reduction reaction (ORR) is a key
reaction for many renewable energy conversion and storage
technologies such as rechargeable fuel cells and metal-air
batteries.[1-6] Most catalyst discovery research is focused on
four-electron ORR catalysts in order to maximize the energy
conversion
conversion/storage devices.
two-electron ORR catalysts has long been overlooked, despite
the fact that it represents a promising and green route for in-situ
electrochemical H
the-art industrial anthraquinone process
potentially unsafe direct catalytic synthesis from hydrogen and
oxygen,[21] electrochemical H
production proceeding via two-
[11, 18,
the presence of carbon and nitrogen sources via pyrolysis,
3
9-40]
chemical vapor deposition,[41] or the newly developed
electrochemical doping;[42] (2) nanocasting strategy using
commercial SiO
2
as sacrificial hard template with subsequent
Although some promising results have been
efficiency
of
electrochemical
In contrast, the exploration of
energy
pyrolysis.[38,
43-44]
[
7-12]
achieved, there is plenty room for improvement in terms of
selectivity. On one hand, the activity and faradaic efficiency of
the
2 2
H O producing catalysts have remained too low for
2 2
O
production.[13-19] Compared to the state-of-
commercialization and need further improvement before
developing practically viable devices. On the other hand, there
are a number of important fundamental aspects about the H O
2 2
[20]
or the difficult and
2
O
2
formation process that have remained elusive: for instance, the
understanding of the catalytically active sites has remained
unclear, in part owing to the inevitable co-generation and co-
existence of chemically distinct nitrogen species (N-
functionalities) within the resultant nitrogen-doped carbon
materials and the lack of their direct experimental
observations.[1-2, 45] To remedy this, more controlled synthesis
methods are needed where only one type of N-functionality
forms in order to establish structure-selectivity relationships; or
else, analysis techniques are required that allow deconvolution
of the contributions of the chemically distinct N-functionalities
and their carbon environment in parallel. Once the catalytically
active site or chemical motif is identified, this would prompt
synthetic efforts to maximize their population (maximizing the
electron oxygen reduction possesses many advantages. These
involve simplified and in-situ small-scale production, reduced
energy input and waste generation, as well as improved process
safety compared to method using a direct explosive mixture of
hydrogen and oxygen. Moreover, if the two-electron ORR
[
a]
Yanyan Sun, Shuang Li, Dr. Zarko Jovanov, Dr. Denis Bernsmeier,
Dr. Huan Wang, Benjamin Paul, Xingli Wang, Dr. Stefanie Kühl,
Prof. Peter Strasser.
Department of Chemistry, Chemical Engineering Division,
Technical University of Berlin, 10623 Berlin, Germany
E-mail: pstrasser@tu-berlin.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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ChemSusChem
10.1002/cssc.201801583
COMMUNICATION
active site density), which would boost H
and faradaic efficiency.
In the present work, we have developed a facile synthetic
method for preparing porous nitrogen-doped carbon catalysts.
We investigated the effect of applied potentials and electrolyte
2
O
2
production activity
occurred,
forming
chemical
nitrogen-doped
carbon
functionalities of CMK-3. The porous properties and Brunauer-
Emmett-Teller (BET) surface areas were then evaluated by
nitrogen adsorption−desorption isotherms.
hysteresis loop at relative pressure range between 0.4 and 0.8
and a narrow pore size distribution centered around 5 nm for
both CMK-3 and PEI50CMK3_800T can be observed (Figure 1c
and d), confirming the mesoporous structures. In addition,
similar phenomenon was also observed for other nitrogen-doped
carbon materials prepared at different conditions (Figure S1).
An obvious
2 2 2 2
pH on the H O selectivity and absolute H O production yields
by means of the rotating ring-disk electrode (RRDE) and
chronoamperometric bulk electrolysis tests at stationary planar
electrodes. In order to link the catalytic rates and efficiencies to
chemical and morphological structure of the catalysts in different
o
electrolytes,
a
detailed chemical speciation of nitrogen
An exception was the sample prepared at 900 C demonstrating
functionalities before and after the catalysis was conducted by x-
ray photoemission spectroscopy (XPS).
a certain degree of microporous structure. Moreover, all
nitrogen-doped carbon catalysts retained extremely high
Brunauer-Emmett-Teller (BET) surface areas from 726 to 1682
m2 g−1, which were beneficial for exposing more catalytically
active sites and improved mass transport during the ORR
process.[37, 44, 46]
The crystallinity of nitrogen-doped porous carbon catalysts was
investigated by powder X-ray diffraction (PXRD). For CMK-3,
o
o
there are two major diffraction peaks at 30.2 and 42.9 (Figure
S2a), which can be ascribed to be the (002) and (101) crystal
planes of graphite. Interestingly, after acid-treatment and
nitrogen doping, the characteristic interlayer (002) diffraction
o
peak shifted down to around 24 , indicating an expanded
interlayer distance.[13] All catalysts exhibited a broad diffraction
peak revealing characteristic features of amorphous carbon
(
Figure S2). Raman spectroscopy was carried out to assess the
graphitization degree and defect content in nitrogen-doped
carbon catalysts. To achieve this, we evaluated peaks at 1330
cm-1 and 1585 cm-1 for all catalysts (Figure 1e and Figure S3),
which are attributed to D and G band of carbon materials,
respectively. Generally, the intensity ratio of the D band to G
D G
band (I /I ) is a qualitative estimate of the defect content in the
doped carbon, since the D and G bands are related to
disordered and ordered crystalline sp2 carbon domains,
respectively.[ Pyrolytic nitrogen doping led to slightly increased
13]
D G
I /I values, implying an increase in structural disorder and
Figure 1. TEM images of (a) CMK-3 and (b) PEI50CMK3_800T; (c) N
2
defects. Besides, the slight downshifts of the G band after
pyrolytic nitrogen doping suggest that doping altered both the
crystallinity and the electronic structure of carbon nanostructures
likely due to a reduction in the mean sheet width of un-doped
graphene.[47-48]
adsorption isotherms, (d) pore size distribution and (e) Raman spectra of
CMK-3 and PEI50CMK3_800T with their characteristic D/G band intensity
ratios; (f) the deconvolution of the N 1s spectra of PEI50CMK3_800T with
different N species as shown.
X-ray photoelectron spectroscopy (XPS) was also performed to
analyze the surface composition and electronic states of the
nitrogen-doped carbon materials. Figure 1f shows the high-
resolution N 1s spectra of the nitrogen-doped carbon catalysts,
which can be further divided into five peaks assigned to different
nitrogen species including pyridinic-N (398.3 eV), pyrrolic-N
The nitrogen-doped porous carbon catalysts were prepared by
ultrasonically mixing ordered mesoporous carbons (CMK-3) with
calculated amounts of polyethylenimine (PEI) followed by
2
annealing in N atmosphere. Here PEI is chosen as nitrogen
precursor mainly due to its low-cost and high nitrogen content up
to 32.5%. A typical synthetic recipe involves mixing the CMK-3
carbon precursor with 50 µL PEI aqueous solution (30% w/v)
(
399.9 eV), quaternary-N (400.9 eV), graphitic-N (402.1 eV), and
[49-52]
N-oxide (404.1 eV).
The detailed content of carbon and
o
followed by annealing at 800 C (the resulting catalyst will
nitrogen, and different nitrogen species are summarized in
Table S1 and Table S2. The variations in the N content of the
resultant catalysts are consistent with that examined results from
element analysis (EA) (Table S3), indicating that the nitrogen
dopant amount can be tuned within the range from 0.07% to
henceforth
be denoted
as
PEI50CMK3_800T).
The
morphologies of CMK-3 and PEI50CMK3_800T were first
characterized by transmission electron microscopy (TEM), as
shown in Figure 1a-b. After nitrogen doping, the ordered porous
structure was well retained, thereby facilitating oxygen mass
2
.04%.
The electrochemical activity and selectivity of the nitrogen-doped
porous carbon catalysts toward ORR to H production was
evaluated using a three-electrode rotating ring-disk electrode
transport during the
2 2
H O production process. During the
formation process of nitrogen-doped carbon, PEI filled the pores
of CMK-3 owing to capillary siphons and then carbonization
2
O
2
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ChemSusChem
10.1002/cssc.201801583
COMMUNICATION
(
RRDE) techniques. Unlike pristine CMK-3, the N-modified
(89.8%) > KOH (83%) at +0.35 VRHE of disk potential. This trend
is rationalized by the establishment of the parallel 4-electron
pathway at smaller potentials.
Tafel plots were evaluated from the experimental RRDE data in
different electrolytes to verify the influence of pH on ORR
mechanism (Figure 3c). Here the kinetic current densities (jdisk,
kin) can be calculated according to the Koutecky–Levich equation:
PEI50CMK3_800T catalyst exhibited relatively positive onset
potential, significantly increased disk currents and ring current at
the investigated range of applied potential (Figure 2a), which
was also observed for the other nitrogen-doped carbon catalysts
prepared at different conditions (Figure S4a and Figure S5a).
This demonstrates that nitrogen doping resulted in an enhanced
oxygen reduction activity coupled with improved H
2
O
2
selectivity.
1/푗푑푖푠푘 = 1/푗푑푖푠푘, 푘푖푛 + 1/푗푑푖푠푘, 푙푖푚
(1)
Quantitative selectivities (in %) and the number of
electrons transferred during the ORR process were evaluated,
2 2
H O
where jdisk and jdisk, lim are the measured current density and the
diffusion-limited current densitiy, respectively. Besides, jdisk, lim
[
35]
as well (Figure 2b, Figure S4b and Figure S5b). After nitrogen
can be obtained using the Levich equation:
ꢁ
⁄3 −ꢂ⁄ꢃ
휔ꢂ/ꢁ
doping, the H
7.4%, and the corresponding averaged number of electrons
transferred per oxygen molecule decreased from 2.46 to 2.05 at
0.3 VRHE, close to the theoretical value of 2 for H selectivity.
In particular, the PEI50CMK3_800T catalyst achieved the
highest H selectivity of 95.2%-98.5% within the potential
2
O
2
selectivity improved sharply from 77.2% to
푗푑푖푠푘, 푙푖푚 = 0.62ꢀ퐹퐷
푣
퐶
(2)
9
where C, F, D, n, ν, and ω refer to the bulk concentration of
oxygen (1.2 ×10-6 mol cm-3 for pH 5-13 and 1.1 ×10 mol cm-3
-6
-1
+
2
O
2
for pH 0.3), Faraday constant (96485 C mol ), the diffusion
coefficient of oxygen (1.9×10-5 cm s for pH 5-13 and 1.8×10
2
-1
-5
O
2
cm2 s-1 for pH 0.3), the number of electrons transferred, the
kinetic viscosity (0.01 cm2 s ) and the angular velocity of the
disk (ω =2πN, N is the linear rotation speed).[35, 55-57] The
corresponding Tafel slopes can be calculated to be 161.3 mV
dec-1 for pH 0.3, 95.7 mV dec-1 for pH 7.0, and 67.5 mV dec-1 for
pH 13.0, respectively, which are in accordance with the
2
-1
range from +0.1 to +0.4 VRHE, which outperformed previously
reported nitrogen-doped carbon[37-40] and noble-metal-based
alloys catalysts (Table S4).[
15, 17, 22-25, 53]
previously reported values for nitrogen-doped carbon-based
ORR catalysts.[
18]
These results demonstrate that the ORR
mechanism on the evaluated catalysts exhibits strong pH-
dependence, which affects the overall activity and selectivity for
peroxide production.
Figure 2. (a) Linear sweep voltammetry performed by a rotating ring-disk
electrode (RRDE) technique where the ring current is collected on the Pt ring
at a constant potential of 1.2 VRHE, and (b) calculated H O selectivity (%) and
2 2
the number of electron transferred (n) as a function of electrode potential for
two materials: CMK-3 and PEI50CMK3_800T. In all cases, measurements
-
1
were performed in O
2
-saturated 0.5 M H
2 4
SO at a scan rate of 5 mV s with
1
0
600 rpm at room temperature and the catalyst loading amount was set to
.05 mg cm .
-
2
Figure 3. Electrochemical measurement of the optimized sample
PEI50CMK3_800T in different electrolytes: (a) linear sweep voltammetry of
RRDE measurement with ring current collected on the Pt ring at a constant
In addition to the optimization of catalyst preparation, we studied
the effects of electrolyte pH on electrochemical H production
of the PEI50CMK3_800T catalyst (Figure 3). As shown in
Figure 3a, the onset potential (defined as potential at the current
2
O
2
potential of 1.2 VRHE
is 0.2475 cm ), (b)
,
and the disk geometric current density (geometric area
selectivity (%), and (c) Tafel plots. Note: the
-saturated 0.5 M H SO (pH=0.3), 0.1 M
(pH=7), and 0.1 M KOH (pH=13) at a scan rate of 5 mV s with 1600
2
2 2
H O
−
2
[54]
measurement was conducted in O
SO
rpm at room temperature and the catalyst loading amount is 0.05 mg cm .
2
2
4
density of 0.01 mA cm )
the order of KOH (0.84 VRHE) > K
RHE) with the highest oxygen transport-limited disk current and
ring current obtained in 0.1 M KOH. In alkaline condition, the Pt
of oxygen reduction decreased in
-
1
K
2
4
2
SO (0.52 VRHE) > H SO (0.49
4
2
4
-2
V
ring current exhibited a maximum HO -
of disk potential, below which the HO
production at +0.5 VRHE
-
2
2 2
To evaluate the H O production of the N-modified carbons over
extended periods of time, bulk electrolysis tests were conducted
in a custom-made two-chamber H-Cell with different electrolytes
2
production decreased
gradually with negative shift of disk potentials. The effect of the
nature of the electrolyte on the H selectivity is shown in
Figure 3b. The highest H selectivity was observed in 0.5 M
SO , and decreased in the order of H SO (98.5%) > K SO
4
2
O
2
including 0.5 M H
2
SO
4
(pH=0.3), 0.1 M K
2 4
SO (pH=7), and 0.1 M
2
O
2
KOH (pH=13), whereby H
2
O
2
concentration was determined
H
2
4
2
4
2
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ChemSusChem
10.1002/cssc.201801583
COMMUNICATION
Figure 4. (a, c, e) H
PEI50CMK3_800T catalysts in O
2
O
2
production amount normalized by catalyst loading amount over the reaction time, and (b, d, f) faradaic H
-saturated 0.5 M H SO (pH=0.3) (a, b), 0.1 M K SO (pH=7) (c, d), and 0.1 M KOH (pH=13) (e, f).
2 2
O efficiency (FE) of
2
2
4
2
4
using UV-Vis absorption spectrometry. Figure 4a, c, e plot the
potentiostatic production normalized by the catalyst
loading over the reaction time at different applied potential of
0.3, +0.2, and +0.1 VRHE. As shown, the H production rates
neutral and alkaline solutions whereas no obvious
electrochemical reduction could be detected in acidic solution
(Figure S6b). In all, the PEI50CMK3_800T catalyst displayed
excellent electrochemical stability at different applied potentials
2 2
H O
+
2 2
O
increased with negative shift of applied potential in both acidic
and alkaline solutions. In neutral conditions, the optimal applied
over
a
wide range of pH values during the successive
production within 4 hours (Figure S7).
electrochemical H
2
O
2
electrode potential was +0.2 VRHE. The H
0.2 VRHE decreased in the order of K SO
2
O
2
production rate at
These results demonstrated that the PEI50CMK3_800T catalyst
is a very promising and efficient catalyst for the electrochemical
H O production.
2 2
In order to gain more insight into the chemical stability and
possibly related mechanistic roles of the different chemical
states of nitrogen during the 2-electron oxygen reduction
process, we investigated the quantitative changes in the
chemical nitrogen species before and after the catalysis using
+
-
2
4
(570.1 mmol g-1catalyst
1
-1
-1
(34.1 mmol g-
h ) > KOH (345.5 mmol g catalyst h ) > H
2
SO
catalyst h ), indicating that the pH has a significant effect on the
production. Figure 4b, d, f, report the faradaic H
efficiency: good faradaic H efficiencies of 40%-59% were
observed in 0.5 M H SO (pH=0.3), higher values of 37%-65%
were obtained in 0.1 M K SO (pH=7), while somewhat higher
4
1
-1
H
2
O
2
2 2
O
2
O
2
2
4
2
4
values of 51%-67% were measured 0.1 M KOH (pH=13). Clearly, XPS. Figure 5a-d illustrates the XPS peak analysis of the
the long-term faradaic H
2
O
2
efficiency differed significantly from
various N-functionalities before and after the catalysis at pH=0.3,
pH=7, and pH=13. The changes of N types after reaction are
found to be dependent on the electrolyte pH since significant
different changes in the pyridinic-N and graphitic-N contents
before and after the catalysis are observed in acidic, neutral,
and alkaline solutions (Table 1). In acidic condition, pyridinic-N
content decreased, while graphitic-N slightly increased. In
neutral and alkaline solutions, the opposite is observed: the
pyridinic-N content slightly increased, while the graphitic-N
content decreased. Furthermore, pyrrolic nitrogen is generally
the H selectivity obtained from voltammetric scans of the
2
O
2
RRDE method.
2 2
The difference between long-term faradaic H O efficiency and
the RRDE-derived H
residence times of H
2
O
2
selectivity is likely due to the longer
2
2
O during bulk electrolysis, resulting in its
gradual chemical decomposition via disproportionation (H
O + ½ O ) or further electrochemical reduction (H + 2H +
e− O).[24] To validate this hypothesis, the chemical
2H
disproportionation and electrochemical reduction of H were
carried out and studied on the PEI50CMK3_800T catalyst in the
different electrolyte containing 2.5 mg L-1
and 1 mM H
respectively (Figure S6). As expected, disproportionation of
was observed in alkaline solution, whereas little to no H
2 2
O →
+
H
2
2
2 2
O
2
→
2
2
O
2
known to be more sensitive to oxidation than the pyridinic-N and
[45]
H
2
O
2
2
O ,
2
graphitic-N during the ORR process.
This is why generally
lower pyrrolic-N content is expected after the catalysis. However,
the experiments revealed that the pyrrolic-N content actually
increased after the catalysis in acidic, neutral, and alkaline
solutions.
H
2
O
2
2 2
O
chemical decomposition was observed in acidic and neutral
solutions (Figure S6a). Also, the PEI50CMK3_800T catalyst
2 2
exhibited the electrochemical activity toward H O reduction in
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Figure 5. (a-d) The relative content change of various nitrogen species of the PEI50CMK3_800T catalysts before (a) and after the catalysis in (b) pH=0.3, c)
pH=7, and d) pH=13.
Table 1. The nitrogen species change of PEI50CMK3_800T before and after
binding energy of pyrrolic-N.[1, 45] This is why we believe that the
the catalysis in different electrolytes including 0.5 M H
and 0.1 M KOH.
2 4 2 4
SO , 0.1 M K SO ,
decrease in pyridinic-N content coupled with the unexpected
increase in pyrrolic-N content after the catalysis in acidic solution
is rather due to the conversion of pristine pyridinic-N to
pyridonic-N than due to a rearrangement of the N atoms.[45] In
contrast, the attachment of –OH to the carbon atoms next to
graphitic-N was reported to result in the downshift of nitrogen
binding energy to around 400 eV, which again coincides with the
typically reported binding energy of pyrrolic-N.[58] This offers a
plausible explanation for the decrease in graphitic-N and
increase in pyrrolic-N in neutral and alkaline solutions. This
hypothesis further implies that pyridinic-N has a critical role as a
catalytically active site in acidic solution, whereas graphitic-N is
more likely the catalytically active site in neutral and alkaline
solutions. In other words, pyridinic-N does the catalysis in acidic
solution, whereas graphitic-N plays an important role in neutral
and alkaline solutions.
N species
Before (%)
Acid (%)
Neutral (%)
Alkaline (%)
Pyridinic-N
Pyrrolic-N
17.2
22.09
24.12
20.35
16.23
12.13
28.39
25.28
22.01
12.19
26.37
30.49
24.59
7.86
18.95
43.12
24.92
6.92
Quaternary-N
Graphitic-N
N-oxide
10.68
6.09
To rationalize the changes in the relative content of the N
species consistently, we look at the concomitant changes in their
chemical environment during the ORR process in more detail.
Recent work has revealed that during oxygen reduction
adsorbed oxygenated intermediates, such as Oads or OHads, may
also chemically modify the immediate chemical environment of
In conclusion, nitrogen-doped porous carbon catalysts were
prepared by direct pyrolysis of CMK-3 and PEI. RRDE results
and chronoamperometric bulk electrolysis measurements
demonstrated that the pH of the electrolytes and applied
[
1, 45,
the catalytically active site or structural motif of the catalysts.
5
8]
With the covalent attachment of an OH group to the carbon
atoms neighboring a pyridinic N, a pyridonic N species is formed
Figure S9), the N 1s binding energy of which was reported to
potentials have a pronounced effects on the H
production rate over the optimal nitrogen-doped porous carbon
catalysts. In acidic solution, the highest H selectivity of 95.3%
at 0.1 VRHE can be achieved whereas in neutral solution the
2 2
O selectivity and
(
2
O
2
upshift to 400.2 eV from the starting pyridinic N energy of 398.8
eV, which is actually very close to the conventionally reported
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ChemSusChem
10.1002/cssc.201801583
COMMUNICATION
highest H
2
O
2
production rate of 570.1 mmol g-1catalyst h-1 can be
homogenous catalysts ink can be firstly obtained through dispersing
catalysts powder into the mixture of water, isopropanol and Nafion
solution (5 wt%, Sigma-Aldrich) with the assistance of ultrasonic. Then
the resultant ink was dropped on the surface of RRDE and dried in the
oven at 60 °C for 10 min. The catalyst loading on the working electrode
obtained. Analyses of the chemical state trajectories of nitrogen
in the carbon catalysts suggest a significant role of pyridinic-N in
the catalytic process in acid, while graphitic-N groups appear to
be catalytically active moieties in neutral and alkaline conditions.
Our results contribute to our understanding of carbon-based
-
2
was fixed at 0.05 mg cm . All measurements were repeated at least
three times in order to ensure reproducibility.
2 2
H O production catalysts and also aid the rational design
thereof.
Prior to the oxygen reduction measurement, successive cycle
voltammetry (CV) was firstly performed to clean the Pt ring electrode in
N
+
2
-saturated 0.5 M H
0.05 to +1.2 V at a scan rate of 20 mV s until steady CVs were
obtained. Then the background current (capacitive current) was collected
2 4
SO electrolyte within the potential range from
-
1
Experimental Section
by measuring CV in N
from +1.1 to +0.05
2
-saturated electrolyte within the potential range
Chemicals
-
1
V
at a scan rate of 5 mV s . After that,
electrochemical impedance spectroscopy (EIS) was also carried out to
measure the uncompensated resistance (R) of the liquid electrolyte for
the subsequent iR-correction. iR correction values ranged from a few to a
maximum of 15 mV. For the H-cell bulk electrolysis measurements, the
uncompensated electrolyte resistance was corrected at the 85% level to
ensure the stability of the electrochemical cell experiment, using the
Manual IR compensation (MIR) technique (Biologic SAS EC Lab
software). The remaining fraction of the IR correction was neglected. For
all acid tests, the electrode potentials were converted to the reversible
hydrogen electrode (RHE) according to the following equation:
CMK-3 was purchased from East High Tech Limited. Polyethylenimine
(PEI) was obtained from Sigma-Aldrich. Hydrogen peroxide test kits were
acquired from Merck. All chemicals were used without further treatment.
Synthesis of nitrogen-doped mesoporous carbon
CMK-3 was firstly pre-treated in 2.4 M HNO
1
5
3
for 12 h, and dried at
00 °C. Then 30 mg acid-treated CMK-3 with different amount of PEI (25,
0, 100 µL) were dispersed in 2.5 mL of deionized water with the
assistance of grinding. The resultant mixture was annealed in a quarts
furnace under a continuous flow of nitrogen gas according to the
following procedure: firstly heated to 180 °C for 2 h, and then heated to
different annealing temperature (600, 700, 800 and 900 °C) for 4 h with
the rate of 5 °C min . For simplicity, the resultant N-doped CMK-3
samples were labelled as PEIXCMK3_YT (with X the PEI in microliters
and Y the annealing temperature in degrees Celsius).
퐸(푅퐻퐸) = 퐸(푀푀푆) + +0.704 ꢄ ꢅ푅 (푉)
(3)
After purging with O
was performed in O
2
at least 30 min, the linear sweep voltammetry (LSV)
-saturated electrolyte within the potential range from
-
1
2
1.1 to +0.05 V at a scan rate of 5 mV s-1 with the rotation speed of 1600
+
rpm. The faradaic currents on the disk electrode during the ORR process
are obtained by subtraction of the corresponding capacitive current. The
Structural and composition characterization
2
H O
2
amount produced at the disk electrode was evaluated by the ring
current at a fixed ring potential of +1.2 VRHE. The collection efficiency (N)
Powder X-ray diffraction (PXRD) patterns were performed on a Bruker
of the ring electrode was 37%. The H selectivity (expressed as
2
O
2
D8 Advance instrument with Cu K
α
radiation (λ = 1.54056 Å) in the range
H
2
O
2
%, also referred to as the fraction of H O produced) of the catalysts
2 2
from 10o to 90o. Brunauer-Emmett-Teller (BET) surface area was
and the number of electrons transferred (n) were calculated according to
the following equations[59]
measured with Quantachrome Autosorb-1-C equipment, and pore size
distribution can be obtained from the adsorption branches of the
isotherms according to cylindrical pores non-localized density functional
theory. The morphologies and structures of the catalysts were analyzed
:
퐻 푂 % = 200 × 퐼푟푖푛푔/((푁 × |퐼푑푖푠푘| + 퐼푟푖푛푔
)
(4)
(5)
ꢁ
ꢁ
2
ꢆꢇꢈꢉꢊ
ꢋ
by transmission electron microscopy (TEM, FEI Tecnai G 20 S-TWIN).
ꢀ
= 4|퐼푑푖푠푘|/(|퐼푑푖푠푘| +
)
TEM samples were prepared by dispersing the resultant PEIXCMK3_YT
into ethanol solution and then dropping onto the surface of Cu grid, finally
drying in the oven at 60 oC for several minutes. The element analysis of
the catalysts including C, H, and N contents were conducted with a CHN
elemental analyzer (Perkin−Elmer, Model 2400-II). X-ray photoelectron
spectra (XPS) were determined by an X-ray photoelectron spectrometer
where Iring is the positive ring current and Idisk is the negative disk current.
Note that equation (4) reports H selectivity, and not efficiencies,
because faradaic processes other than oxygen reduction are not taken
into consideration.
2 2
O
(
K-Alpha™, Thermo Scientific). Raman spectra measurement was
Bulk
H
2
O
2
production was further evaluated by means of
obtained on a Bruker SENTERRA Raman system.
chronoamperometry at different applied working electrode potentials
(
+0.1, +0.2, and +0.3 VRHE) in a custom-made two-chamber H-cell with a
Electrochemical characterization
pre-treated Nafion® membrane as separator in acidic solution,[ anion
exchange membrane (Selemion ASV, AGC Engineering Co., LTD) in
neutral solution, and Selemion AHO in alkaline solution. The influences
13]
The oxygen reduction performances of the resultant catalysts were
evaluated in home-made electrochemical cell with three-electrode
configuration including the rotating-ring disk electrode (RRDE, PINE
Research Instrumentation, 0.2475 cm2), Pt mesh, and
mercury/mercurous sulfate electrode (MMS, PAR AMETEK USA,
calibration against a reversible hydrogen electrode in 0.5 M H SO
resulted in a reference electrode potential of +0.704 VRHE) as working,
counter, and reference electrode, respectively. commercial
of different electrolytes (0.5 M H
production were also investigated. UV-Vis photometric method was
used to measure the produced H amount in regular time intervals.
2 4 2 4
SO , 0.1 M K SO , and 0.1 M KOH) on
2 2
H O
a
2 2
O
/
Sample solution aliquots were collected from the cathode compartment
of the H-Cell and analysed using a commercial hydrogen peroxide test
kits (Merck KGaA, Germany). The faradaic H
calculated from the H
passed:
2
4
O
2 2
efficiency (FE) was
A
2 2
O yield against the total quantity of charge
potentiostats/galvanostats (Biologic SAS, model VSP 140) was used. In
alkaline solutions, a calibrated commercial RHE (Hydroflex, Gaskatel)
was employed. The catalyst film electrode is prepared as follows. The
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ChemSusChem
10.1002/cssc.201801583
COMMUNICATION
퐻 푂 (퐹퐸, %) = 2퐶푉퐹⁄푄
(6)
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ꢁ
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-
1
where C is the H
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Acknowledgements
This work received funding by the German Federal Ministry of
Education and Research (Bundesministerium für Bildung und
Forschung, BMBF) under the grant 03SF0531B “HT-LINKED”.
ZELMI of Technical University Berlin is acknowledged for their
support with TEM measurements. Yanyan Sun thanks China
Scholarship Council (CSC) for the financial support.
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Keywords: nitrogen-doped porous carbon • oxygen reduction •
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COMMUNICATION
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COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Nitrogen-doped porous carbon
catalysts were prepared using CMK-3
and polyethylenimine (PEI), exhibiting
Yanyan Sun, Shuang Li, Zarko Jovanov,
Huan Wang, Stefanie Kühl, and Peter
Strasser*
2 2
high H O selectivity of 95.3% in acid,
whereas large production rate of
Page No. – Page No.
-1
-1
570.1 mmol g catalyst h in neutral
Structure, activity, and faradaic
efficiency of nitrogen-doped porous
carbon catalysts for direct
electrochemical hydrogen peroxide
production
solution. Ex-situ X-ray photoemission
spectroscopy analysis suggests a key
mechanistic role of pyridinic-N in acid,
while graphitic-N groups appear to be
catalytically active moieties in neutral
and alkaline conditions.
This article is protected by copyright. All rights reserved.