Overall Oxygen Electrocatalysis on Nitrogen-Modified Carbon Catalysts: Identification of Active Sites and In Situ Observation of Reactive Intermediates

Article abstract of DOI:10.1002/anie.202012615

The recent mechanistic understanding of active sites, adsorbed intermediate products, and rate-determining steps (RDS) of nitrogen (N)-modified carbon catalysts in electrocatalytic oxygen reduction (ORR) and oxygen evolution reaction (OER) are still rife with controversy because of the inevitable coexistence of diverse N configurations and the technical limitations for the observation of formed intermediates. Herein, seven kinds of aromatic molecules with designated single N species are used as model structures to investigate the explicit role of each common N group in both ORR and OER. Specifically, dynamic evolution of active sites and key adsorbed intermediate products including O₂ (ads), superoxide anion O₂^?*, and OOH* are monitored with in situ spectroscopy. We propose that the formation of *OOH species from O₂^?* (O₂^?*+H₂O→OOH*+OH^?) is a possible RDS during the ORR process, whereas the generation of O₂ from OOH* species is the most likely RDS during the OER process.

Full text of DOI:10.1002/anie.202012615

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Overall oxygen electro catalysis on nitrogen-modified carbon
catalysts: identification of active sites and in situ observation of
reactive intermediates
Authors: Yangming Lin, Zigeng Liu, Linhui Yu, Gui-Rong Zhang, Hao
Tan, Kuang-Hsu Wu, Feihong Song, Anna K Mechler, Peter
Philipp Maria Schleker, Qing Lu, Bingsen Zhang, and Saskia
Heumann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012615
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Overall oxygen electrocatalysis on nitrogen-modified carbon
catalysts: Identification of active sites and in situ observation of
reactive intermediates
[
a]
[a,b]
[a]
[c]
[d]
[e]
Yangming Lin* , Zigeng Liu , Linhui Yu , Gui-Rong Zhang , Hao Tan , Kuang-Hsu Wu , Feihong
[
a]
[a]
[a,b]
[f]
[g]
Song , Anna K. Mechler , Peter Philipp Maria Schleker , Qing Lu , Bingsen Zhang , and Saskia
[
a]
Heumann*
[
a]
Dr. Y. Lin, Dr. Z. Liu, Dr. L. Yu, F. Song, Dr. A. K. Mechler, Dr. P. P, Maria Schleker, Dr. S. Heumann
Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, Mülheim an der Ruhr, 45470, Germany.
E-mail: yang-ming.Lin@cec.mpg.de, saskia.heumann@cec.mpg.de
[
[
[
[
[
[
b]
c]
d]
e]
f]
Dr. Z. Liu, Dr. P. P, Maria Schleker
Institut für Energie und Klimaforschung (IEK-9), Forschungszentrum Jülich GmbH, 52425, Germany
Dr. G. Zhang
Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Technische Universität Darmstadt, Alarich-Weiss-Strasse 8, 64287 Darmstadt, Germany
Dr. H. Tan
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China
Dr. K. Wu
School of Chemical Engineering, University of New South Wales, Kensington, Sydney, NSW 2052, Australia
Dr. Q. Lu
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
g]
Prof. Dr. B. Zhang
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, P. R. China
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: The recent mechanistic understandings including active
tremendous attention over the last decade, owing to their
abundance and high sustainability compared with metal
catalysts.[4-5] Some impressive experiments and theoretical
predictions regarding the possible catalytic mechanisms and
active sites have been conducted through regulating the N
concentration in N-modified model materials or introducing the
simulation models of the active structures.[6-8] Although pyridinic
N and graphitic N are recently pointed out to be the most likely
species that could create active sites, the connection remains
sites, adsorbed intermediate products and rate-determining steps
(
RDS) of nitrogen (N)-modified carbon catalysts in electrocatalytic
oxygen reduction (ORR) and oxygen evolution reaction (OER),
however, are still rife with controversies due to the inevitable
coexistence of diverse N configurations and the technical limitation for
observing formed intermediates. In this work, seven kinds of aromatic
molecules with designated single N species are used as model
structures to investigate the explicit role of each common N group in
both ORR and OER. Specifically, dynamic evolution of active sites
controversial due to the inevitable mixing with other
N
and key adsorbed intermediate products including
superoxide anion -* and OOH* are monitored with in situ
spectroscopy. We propose that the formation of *OOH species from
-* (O -*+H O→OOH*+OH-) is a possible RDS during the ORR
process, whereas the generation of O from OOH* species is the most
O
2
(ads),
configurations (e.g. pyrrolic, amine or lactam) in doped carbon
catalysts synthesized by the recently deficient doping methods.
O
2
Specifically, the involved intermediate products (e.g. adsorbed O
2
O
2
2
2
molecule, superoxide anion O -*, peroxide HO *) and rate-
2
2
2
determining steps (RDS) during the ORR and OER processes
have been still rarely studied by experimental approaches. The
precise structure-function relationship between active sites and
reactivities is still unclear. The inhomogeneities of N species
associated with the morphology of catalysts also hamper the
exploration for active sites. Determining the genuine active sites,
monitoring the important intermediate products and revealing the
possible reaction pathways are therefore highly important to
understand the catalytic nature and to optimize the design and
development of new carbon-based catalysts.
possible RDS during the OER process.
Introduction
Overall oxygen electrocatalysis, including oxygen reduction
reaction (ORR) and oxygen evolution reaction (OER), represents
the cornerstone for a wide range of renewable electrochemical
energy conversion and energy storage technologies.[1-3] Nitrogen
(
N)-modified graphitic carbon materials, as the most promising
To disclose the genuine role of each N species in oxygen
electrocatalysis, preparing N-containing graphitic carbon
metal-free catalysts in these two reactions, have received
1
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RESEARCH ARTICLE
materials with one single N species is ideal to provide direct
inference. But the inductive influence of surface oxygen species
and defect sites (heteroatom-free) of carbon materials on various
resultant N species is difficult to rule out. Alternatively, aromatic
organic molecules has been considered as graphene molecules
at nanoscale due to their similar π-conjugated structures and
properties.[9] Using functionalized aromatic organic molecules
(
Figure 1a) with isolated N species as model active matrices to
mimic the edge structure of N-doped carbon materials and to
identify the active sites are a promising strategy.[10-11] The
electronic and reactive properties of active components can be
tuned by extending the π-conjugated domains of these organic
molecules at a discrete molecular level.[10] Moreover, the intrinsic
low conductivities and dispersibilities of these organic molecules
can be improved by using nanocarbon materials as supports to
build electrically coupled catalytic systems through non-
destructive π-π interaction.[12,13]
In the present work, three nanocarbon supports, including
onion-like carbon (OLC), high temperature-treated carbon
nanofiber (HHT) and carbon nanotubes (CNTs), are chosen to
support seven different model aromatic organic molecules to form
model catalytic systems via a solvothermal self-assembly process
(
Figure 1b, here, OLC is used as an example in the preparation
process). The resulting model catalytic systems not only have a
high concentration of desired single N species, but also exhibit a
clear structure-activity relationship. The pyridinic N species are
demonstrated to be advantageous for both ORR and OER
processes over a wide pH range, where it could facilitate the ORR
by a four-electron-like dominated pathway associated with the
local conjugated structure of N species. The activities of catalysts
mainly originate from electrocatalytic reactions rather than from
carbon corrosion during the OER process. The possibly adsorbed
intermediate products and RDS are proposed for both ORR and
OER processes.
Figure 1. Structural, fabrication and elemental characterization of model
catalytic systems. (a) Structures of different aromatic organic molecules with
isolated N configurations (including pyrrolic, amine, lactam, and pyridinic N). (b)
The preparation of aromatic molecules supported on OLC using a solvothermal
method. Here, cyclohexane was used as the solvent. (c) XPS N1s spectra of
OLC and model catalytic systems. The FWHM of all N peaks is approximately
1
.6 eV.
Model catalysts reveal ORR active sites
The ORR activity of these model catalytic systems in alkaline is
shown in Figure 2a. Only supported AD (AD+OLC) and PAD
Results and Discussion
(
PAD+OLC) model catalytic systems exhibit higher current
Determination of model catalytic system with single N group
densities and more positive onset potentials (Eonset) compared
with pure OLC and other supported model systems with
respective pyrrolic N (CB+OLC), O=C-NH (PTD+OLC) and -NH2
(PTA+OLC) groups, while unsupported model molecules do not
show the relevant activities (Figure S7), indicating pyridinic N
species are the possible active species in the ORR. To further
As shown in Figure 1c, the results of x-ray photoelectron
spectroscopy (XPS) confirm that main four N species: pyridinic N
groups (398.3 eV), lactam or amide N groups (399.2 eV) or
pyrrolic N groups (400.3 eV) exist in these model catalytic
systems.[14] Due to the good symmetry and the narrow full width
at half-maximum (FWHM, ~1.6 eV) of all N peaks, we can
conclude that each model catalytic system contains only one type
of N functional group. The content of N species on the model
systems is from 1.2 to 2.3 at.%. The structural features of these
model molecules on OLC support were further studied by
attenuated total reflectance infrared (ATR-IR) spectroscopy
investigate the role of pyridinic
N
species, two other
concentrations of AD molecules in AD+OLC are deliberately
regulated (Figure S6f). It can be found that the current density of
AD+OLC is declining with decreased concentrations of AD
molecules (Figure S8a and S8c), reflecting the important role of
pyridinic N species. Furthermore, the similar behavior of AD+OLC
and PAD+OLC catalysts indicates that the position of the pyridinic
N, noted as the edge zigzag or armchair position, negligibly affect
the catalytic performance. Anthracene (AT) was additionally used
to mimic the zigzag configuration of pure carbon materials (Figure
S9). The lower performance of supported AT (AT+OLC) relative
to AD+OLC further indicates the activity enhancement of pyridinic
N species as well as non-critical role of edge configuration in the
ORR process in alkaline.
(
Figure S1-S5). The main characteristic peaks of CB, PTD, PTA,
PAD and AD molecules can be observed clearly, meaning that
the structures of various model molecules maintain the same
during the preparation process. The net contents of these model
molecules on OLC support are quantified to 0.72-1.31 wt % by
using thermogravimetric (TG) measurements (Figure S6).
2
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To investigate the size influence of the π-conjugated
structure for the pyridinic N species on the ORR activity, two other
molecules, BAD and DBAD, were chosen. The net concentrations
of these two molecules are determined to be 1.18 wt% and 1.05
wt% (Figure S10). Their molecular structures exhibit good stability
as confirmed by ATR-IR (Figure S11-S12). In the C K-edge X-ray
absorption near-edge structure (XANES) spectra (Figure S13a),
compared with pristine OLC, an additional peak of supported
DBAD (DBAD+OLC), observed at 287.2 eV, can be assigned to
the σ* bond of C-N.[15] Another weak peak appeared at about
dominant N species on the surface of OLC is the expected
pyridinic N group. This is further supported by XPS results of
DBAD+OLC (Figure S16b), which reveals that only pyridinic N
species (at 398.4 eV, 1.8 at%) can be observed. The LSV results
displayed in Figure 2b and Figure S14a show that the current
densities are in direct proportion to the sizes of the π-conjugated
structures with pyridinic N species. Further electrocatalytic results
summarized in Figure 2c and Figure 2e demonstrate that the size
of π-conjugated system is a critical parameter to accelerate the
ORR process, showing as the gradually enhanced intrinsic
current densities and the gradually positive shift in onset potential
(Eonset), which might be ascribed to the enhanced electron
delocalization effect from a larger π-conjugated system of model
molecules. In addition, the reasonable catalytic stability of
DBAD+OLC at two applied
2
86.6 eV for DBAD+OLC which might be attributed to π* (ring)
resonance of the DBAD molecule itself.[16] Other carbon-nitrogen
species could not be observed. In the N K-edge XANES spectra
(
Figure S13b), compared with pristine OLC, DBAD+OLC exhibits
an obvious peak at 398.3 eV which can be assigned to a π*
feature of pyridine-like
structure,[17] suggesting that the
N
2
Figure 2. Electrochemical measurements of model catalytic systems for the ORR in O -saturated electrolytes. (a) LSV curves of model catalytic systems and pure
OLC catalyst in 0.1 M KOH and (b) model catalytic systems bearing isolated pyridinic N species and extended π-conjugated structure at onset potential region. (c)
The intrinsic current densities of various model catalytic systems with isolated pyridinic N species. (d) Electron transfer (ET) numbers of supported catalysts with
isolated pyridinic N species and pristine OLC catalyst, respectively, based on rotating ring-disk electrode (RRDE) measurements. (e) Number of benzene units of
2
active components with single pyridinic N species as a function of ET number, HO - selectivity at 0.6 VRHE and onset potential (Eonset). Here, Eonset is defined by a
2
2 4
threshold current density of 1 μA/cm . (f) LSV curves measured in phosphate buffer solution (PH=7.0) and 0.1 M H SO media, respectively.
3
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potentials was demonstrated by not observing significant current
change for 2 h (Figure S16a). To further confirm the role of
pyridinic N species, DBAD was removed from DBAD+OLC
matrix accelerates the OER process. Moreover, similar Tafel
-1
slopes (77~85 mV dec ) are obtained for AD+OLC, BAD+OLC
and DBAD+OLC catalysts (Figure S20), implying the extended π-
conjugation does not change the OER reaction pathway. By using
the rotating ring-disk electrode (RRDE) technique (Figure S21),
the OER process occurring on DBAD+OLC is dominated by a
desirable four-electron pathway (99.9%) with negligible peroxide
intermediate formation.
(
Figure S15) by ultrasonication, the corresponding activity of the
residual OLC sample returns to the level of pristine OLC, which
evidences the advantageous role of pyridinic N species for the
ORR activity.
The introduction of pyridinc N species significantly increases
the electron transfer (ET) number from 2.4~2.7 of OLC to 2.8~3.0
for AD+OLC and can be further extended to 3.3~3.5 of
DBAD+OLC at 0.2~0.7 V vs. RHE (VRHE) (Figure 2d-e). The
2
corresponding selectivity in favor of HO - production gradually
decreases from 70~78% for OLC to 50~60% for AD+OLC, and
eventually reaches 19.0~38.0% for DBAD+OLC at 0.2~0.7 VRHE
(
Figure S14b). Moreover, as a comparison, AT+OLC sample with
single edge configuration (zigzag) and without any pyridinic N
species displays higher HO - selectivity than that of OLC and
2
AD+OLC (Figure S9b). All these findings indicate that (i) pyridinic
N species prone to contribute the ORR in a four-electron-like
manner (as clarified by increment of ET number and decrease of
2
HO - production); (ii) pyridinic N species could improve the Eonset
of ORR to more positive; (iii) the structure size of π-conjugated
pyridinic N species is a critical factor to facilitate the ORR process
and (iv) the edge structure in the absence of pyridinic N species
(
for example, only zigzag configuration) plays a negative effect in
accelerating four-electron pathway of ORR. Therefore, it can be
expected that carbon-based catalysts with a single pyridinic N
function group in
a
larger π-conjugated system (e.g.
nanographene) could show complete four-electron pathway.
Additionally, DBAD+OLC is selected as a representative
catalyst to study the role of pyridinic N in neutral (0.1 M phosphate
2 4
buffer solution, PH=7.0) and acidic (0.1 M H SO ) ORR processes.
Clearly, this catalyst delivered a higher current density than
pristine OLC in the neutral and acidic media (Figure 2f), which
suggests that the model catalyst containing pyridinic N species
can exhibit an elevated catalytic performance over a broad pH
range.
Model catalysts reveal OER active sites
In addition to identification of active sites for ORR, we also
unveiled the catalytic function of each N species for the OER
activity by evaluating the model catalytic systems in 0.1 M KOH
(
Figure 3 and Figure S17). In similarity to the ORR results,
AD+OLC and PAD+OLC catalysts show higher current densities
relative to other catalysts. This indicates that pyridinic N species
are also active for OER. On the contrary, pyrrolic, amine and
lactam species are inactive for OER process demonstrated by
negligibly improved performance of CB+OLC, PTA+OLC and
PTD+OLC relative to OLC (Figure 3a). Also, in line with the ORR
results, the OER process is insensitive to the edge configurations
as demonstrated by the similar current densities and Tafel slopes
of AD+OLC and PAD+OLC catalyst (Figure 3a and Figure S18).
By extending the π-conjugated system of the model molecules,
the catalysts show an increase in the OER activity (Figure 3b).
There is a direct proportional relationship between the number of
benzene units and theoretical TOF values, as shown in Figure
S19. Specifically, the value increases from 0.153 s-1 of AD+OLC
to 0.638 s-1 of DBAD+OLC at 1.60 VRHE. Both values are
significantly higher than that of the reported highly active Ni-/Fe-
based OER catalysts (0.028-0.075 s-1 at 1.63 VRHE).[18,19] Hence,
it can be concluded that the π-conjugated size of the carbon
Figure 3. Electrochemical measurements of model catalytic systems for the
OER in Ar-saturated 0.1 M KOH. (a) LSV curves of model catalytic systems and
pristine OLC catalyst. (b) LSV curves of model catalytic systems with isolated
pyridinic N species and extended π-conjugated structure catalysts. (c) Mass
2
spectra profiles of the produced CO (m/z 28) and O (m/z 32) with DBAD+OLC.
4
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RESEARCH ARTICLE
A high Faradaic efficiency (FE(O
2
)) of around 86 % at two low
In general, pyridinic N and graphitic N are considered to be
the most common species in oxygen electrocatalysis with N-
doped carbon as catalysts.[8,22,23] After confirming the positive role
of pyridinic N species with model molecules, four kinds of N-
doped OLC (NOLC) catalysts with enriched pyridinic N but without
any graphitic N species were synthesized and tested for both
ORR and OER processes (Figure S24). Among them, NOLC-4
possesses the highest N content (2.7 at%) and largest proportion
(63.1%) of pyridinic N species in all the N species. As shown in
Figure 4, through varying the contents of pyridinic N species in
different NOLC catalysts, the current densities and onset
potentials can be gradually manipulated for the ORR activity,
while only the current densities are impacted for the OER activity.
Some linear relationships between current densities or ET
numbers and the concentrations of pyridinic N species at different
applied potentials (Figure 4b, 4c and 4e) could be observed in the
ORR and OER processes, reflecting that pyridinic N species are
the catalytically active sites in ORR and OER processes.
Moreover, the Tafel slope values (Figure 4f and Figure S25,
potentials (e.g. 1.567 and 1.60 VRHE) was obtained, which are
comparable to reported metal-based catalysts to some
degree.[20,21] The loss of FE(O
) maybe partly inflicted by the
inefficient oxygen collection by the Pt ring electrode or
unavoidable carbon corrosion (e.g. CO or CO formation) in
alkaline medium. It should be emphasized that when acquiring
mass spectra, a very weak CO signal and an obvious O signal
2
2
2
were observed, as shown in Figure 3c, reflecting the current
density originates mainly from water oxidation rather than carbon
corrosion. DBAD+OLC shows a decent stability during 2 h at an
applied potential (Figure S16a). All the aforementioned results
suggest that pyridinic N species can efficiently accelerate both
ORR and OER processes, and their catalytic performance could
be regulated by the π-conjugated size of active components at
molecular level. To further verify the catalytic role of pyridinic N
species, two other kinds of carbon (CNT and HHT) were tested
as supports for DBAD. Indeed, dramatically enhanced catalytic
behavior can be observed with both DBAD+HHT and DBAD+CNT
catalysts for the ORR and OER processes (Figure S22 and 23),
which confirms the critical role of pyridinic N species in facilitating
the catalytic processes.
-1
75~78 mV dec ) of NOLC catalysts are analogous to the model
-1
catalytic systems (77~85 mV dec ), suggesting that both model
catalysts and doped catalysts have the similar kinetic reaction
process.
Doped catalysts for OER and ORR
Figure 4. Electrochemical measurements of various N-doped OLC (NOLC) catalysts with enriched pyridinic N and without any graphitic N species for ORR and
OER. (a) ORR LSV curves of NOLC catalysts. (b)-(c) Dependence of current densities and ET numbers at different potentials on the concentration of pyridinic N of
NOLC catalysts (based on XPS results). (d) OER LSV curves of NOLC catalysts. (e)-(f) Dependence of current densities at different potentials and Tafel slope on
the concentration of pyridinic N (based on XPS results).
Mechanistic Study of ORR and OER
ORR activity,[24-26] whereas OH*, O* and OOH* mainly dominate
the OER process.[27] It is significant to use effective experimental
measurements to elucidate the evolution processes of the surface
relevant species during electrochemical reactions. As shown in
Density functional theory calculations predicted that the
binding energy of some surface adsorbed intermediate oxygen
2 2
species, such as O (ads), O -*, OOH*, O* and OH*, governs the
5
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RESEARCH ARTICLE
-
1
the in situ ATR-IR spectra in Figure 5a, the peak located at 1400
cm-1 exhibits a quasi-reversible behavior with ORR potential
change, which is attributed to the O-O vibration of adsorbed
species (at 1018 cm ) is observed and no distinct other oxygen
species can be found in Figure 5d, implying that the RDS in the
OER is mainly associated with the OOH* species. Together with
the result of the primary isotope effect with the KIE values of
around 1.51-1.68 at 1.567-1.6 VRHE (Figure 5e), it allows us to
oxygen molecule O
processes monitored by two other peaks located at 1052 cm-1 and
019 cm-1 are observed (Figure 5b). Given that the infrared
vibration peaks of the O -* and OOH* typically are located in the
2
(ads) on the catalyst.[28] Moreover, evolution
1
conclude that the generation of O
possible RDS in the OER.
2
from OOH* species is the most
2
-
1 [28-33]
region 1100-980 cm ,
some control experiments were
-* and OOH*
To gain more insights into the active sites, C-N vibrational
band were monitored with in situ ATR-IR spectra. The red-shift of
the C-N peak at 1357 cm-1 (Figure 5f and Figure S27) with
potential-dependent tendency suggests that active sites should
originate from pyridinic N itself or adjacent carbon atoms during
the ORR and OER. To determine the real active sites, some post-
mortem measurements were further conducted after ORR and
OER stability tests. The XPS spectrum of DBAD+OLC after ORR
carefully introduced, to further differentiate O
2
species. The results of Figure 5c reveal that (i) no obvious peaks
at 1052 cm-1 and 1019 cm-1 positions can be observed in Ar-
saturated KOH, suggesting these two peaks corresponding to O-
containing species and (ii) an apparent red-shift behavior (Δ=~26
-1
-1
2
cm ) appears at position 1019 cm for O -saturated KOD
electrolyte, while the original vibration band at 1052 cm-1 is
unaffected (Figure 5c and Figure S26), indicating that the
vibration band at 1019 cm-1 is most likely to be OOH* rather than
(Figure S16b) shows the concentration of pyridinic
N is
decreasing and a significant new N1s peak at 400.1 eV appears,
which can be attributed to pyridonic N that transformed from
pyridinic N through a reaction involving the carbon atoms next to
pyridinic N and O-containing species (possibly OH or OOH).[6]
Therefore, it seems that the active sites are the adjacent carbon
atoms of the pyridinic N species rather than pyridinic N itself.
Similar to the case of ORR, an impressive pyridonic N peak is
shown in XPS N1s peak after OER operation (Figure S16b),
which also implies the active sites are likely involved with the
adjacent carbon atoms of the pyridinic N species. Additionally, we
do not observe any characteristics of pyridine N-oxide species at
~402 eV in N1s peak of XPS and at ~1265 cm-1 in ATR-IR spectra
during the ORR and OER.[36] Thus, we propose that the
neighbored carbon atoms are strongly involved for accelerating
both ORR and OER processes.
O
2
-* species, thus the vibration band at 1052 cm-1 is ascribed to
the O -*. All the experiments demonstrate that O -* and OOH*
species as the important intermediates are involved in the ORR
process. Recently, the formation of O -* from O (ads) coupled
with the first electron transfer (that is O +e-→O -*) or the
generation of OOH* from O -* (O -*+H O→OOH*+OH- ) over
2
2
2
2
2
2
2
2
2
carbon catalysts in the ORR has been considered to be the most
common RDS.[34,35] To clarify the RDS, isotopic electrochemical
studies were further performed, as displayed in Figure 5e. Kinetic
isotope effect (KIE) values of around 1.45-1.61 at 0.6-0.7 VRHE are
obtained, suggesting that there is a potential primary isotope
effect and the RDS of the ORR process involves a cleavage of
the O-H bond of water molecules. Combined with the ATR-IR
results, assuming the following elementary step
+H O→OOH*+OH- is likely to be the potential RDS in the ORR.
In the case of the OER, a similar evolution behavior of OOH*
2
O -
*
2
6
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Figure 5. In situ ATR-IR spectra for monitoring dynamic evolution of the involved intermediate oxygen species, determining the rate-determining steps (RDS) and
identifying the active sites on DBAD. (a-c, f) ORR and (d) OER processes. The experiments including Ar-saturated KOH, O -saturated KOH and O -saturated KOD
were carried out at 0.6 VRHE. Here, O -*, OOH* and OOD* represent the adsorbed O -, HO and DO species, respectively. (e) Isotopic electrochemical studies of
2
2
2
2
2
2
representative DBAD+OLC catalyst in 0.1 M KOH and 0.1 M KOD.
Conclusion
species play a crucial role for the electrochemical ORR over a
wide pH range and alkaline OER. It can be concluded that
pyridinic N species prone to facilitate the ORR process by a four-
electron-like dominated pathway, and they also improve the
catalytic activity rather than carbon corrosion for the OER.
In summary, we have illustrated experimental insights into the
catalytic mechanisms at a molecular level by aromatic organic
molecules with designated N species as models. Pyridinic N
7
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Angewandte Chemie International Edition
10.1002/anie.202012615
RESEARCH ARTICLE
Furthermore, the location at edge zigzag or armchair position of
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The structure-function relationship of the active component can
be established by delicately controlling their longitudinal
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Keywords: metal-free carbon • active site • nitrogen doped • in
situ infrared spectroscopy • intermediate products
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Angewandte Chemie International Edition
10.1002/anie.202012615
RESEARCH ARTICLE
Entry for the Table of Contents
Dynamic evolution of active sites and key oxygen intermediate products during the ORR and OER on N-doped carbon catalysts are
monitored experimentally with in situ ATR-IR spectra. With the assistance of isotopic labeling technical, the formation of *OOH
-* (O -
*+H
O→OOH*+OH ) is suggested to be a possible RDS during the ORR process, whereas the generation of O
-
species from O
2
2
2
2
from OOH* species is the most possible RDS during the OER process.
9
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