Dynamically Unveiling Metal–Nitrogen Coordination during Thermal Activation to Design High-Efficient Atomically Dispersed CoN4 Active Sites

Article abstract of DOI:10.1002/anie.202017288

We elucidate the structural evolution of CoN₄ sites during thermal activation by developing a zeolitic imidazolate framework (ZIF)-8-derived carbon host as an ideal model for Co²⁺ ion adsorption. Subsequent in situ X-ray absorption spectroscopy analysis can dynamically track the conversion from inactive Co?OH and Co?O species into active CoN₄ sites. The critical transition occurs at 700 °C and becomes optimal at 900 °C, generating the highest intrinsic activity and four-electron selectivity for the oxygen reduction reaction (ORR). DFT calculations elucidate that the ORR is kinetically favored by the thermal-induced compressive strain of Co?N bonds in CoN₄ active sites formed at 900 °C. Further, we developed a two-step (i.e., Co ion doping and adsorption) Co-N-C catalyst with increased CoN₄ site density and optimized porosity for mass transport, and demonstrated its outstanding fuel cell performance and durability.

Full text of DOI:10.1002/anie.202017288

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 9516–9526
International Edition:
German Edition:
doi.org/10.1002/anie.202017288
doi.org/10.1002/ange.202017288
ORR Catalysts
Dynamically Unveiling Metal–Nitrogen Coordination during Thermal
Activation to Design High-Efficient Atomically Dispersed CoN Active
4
Sites
+
+
+
Yanghua He , Qiurong Shi , Weitao Shan , Xing Li, A. Jeremy Kropf, Evan C. Wegener,
Joshua Wright, Stavros Karakalos, Dong Su, David A. Cullen,* Guofeng Wang,*
Deborah J. Myers,* and Gang Wu*
2
+
Abstract: We elucidate the structural evolution of CoN sites
between Fe and H O are significant concerns due to the
2 2
4
[19–21]
during thermal activation by developing a zeolitic imidazolate
framework (ZIF)-8-derived carbon host as an ideal model for
generation of radical oxygen species (ROS).
and H O could attack the FeN active sites, carbon support,
organic ionomers, and polymer membranes, thereby accel-
erating performance degradation.
Both ROS
2
2
4
2
+
Co ion adsorption. Subsequent in situ X-ray absorption
[22–24]
spectroscopy analysis can dynamically track the conversion
An alternative Co-N-
from inactive CoÀOH and CoÀO species into active CoN sites.
C catalyst, which does not significantly promote Fenton
reactions, has recently garnered attention due to prominent
improvements in ORR activity and stability in acidic electro-
4
The critical transition occurs at 7008C and becomes optimal at
9
008C, generating the highest intrinsic activity and four-
[25–30]
electron selectivity for the oxygen reduction reaction (ORR).
DFT calculations elucidate that the ORR is kinetically favored
by the thermal-induced compressive strain of CoÀN bonds in
lytes (Table S1),
cell power density and performance durability.
and, most importantly, encouraging fuel
[
14,28,29,31–34]
Despite the significant progress in improving Co-N-C
CoN active sites formed at 9008C. Further, we developed
catalystsꢀ activity and stability via empirical engineering of
4
[25]
a two-step (i.e., Co ion doping and adsorption) Co-N-C
catalyst with increased CoN4 site density and optimized
porosity for mass transport, and demonstrated its outstanding
fuel cell performance and durability.
the composition and structure,
the fundamental under-
standing of the CoN4 site formation mechanism remains
unknown for decades. This is primarily due to the complexity
of the processes occurring during the critical high-temper-
ature pyrolysis step, involving the simultaneous carbonization
of carbon structure, nitrogen doping, and the CoN4 site
Introduction
generation. Unveiling the CoN active site evolution process
4
Atomically dispersed and nitrogen-coordinated MN sites
during the thermal activation is pivotal to illustrate the
4
(
M = Fe, Co, Ni, Cu, Cr, Ce, or Mn) in carbon (M-N-C) are
relationship between CoÀN atomic configuration and activity
the most promising platinum-group metal (PGM)-free cata-
lysts for oxygen reduction reaction (ORR),
tion,
and selectivity towards the four-electron ORR. The in-depth
understanding also offers valuable guidance toward the
efficient and targeted synthesis of high-performance Co-N-
[
1–4]
CO reduc-
Despite the
2
[
5–6]
[7–10]
and other relevant electrocatalysis.
most encouraging ORR activity achieved with Fe-N-C
C catalysts for various energy conversion technolo-
gies.
[
11–17]
[2,16,35–37]
catalysts,
their unsatisfied stability is the primary barrier
[
18]
for vital applications.
Even worse, the Fenton reactions
[
+]
[+]
[
*] Y. He, Dr. Q. Shi, Prof. G. Wu
E-mail: dmyers@anl.gov
Department of Chemical and Biological Engineering, University at
Buffalo, The State University of New York
Buffalo, NY 14260 (USA)
Prof. J. Wright
Illinois Institute of Technology
Chicago, IL 60616 (USA)
E-mail: gangwu@buffalo.edu
Prof. S. Karakalos
Department of Chemical Engineering, University of South Carolina
Columbia, SC 29201 (USA)
[
+]
W. Shan, Prof. G. Wang
Department of Mechanical Engineering and Materials Science
University of Pittsburgh
Pittsburgh, PA 15261 (USA)
Dr. D. A. Cullen
Materials Science and Technology Division
Oak Ridge National Laboratory
Oak Ridge, TN, 37831 (USA)
E-mail: cullenda@ornl.gov
E-mail: guw8@pitt.edu
Dr. X. Li, Dr. D. Su
Center for Functional Nanomaterials
Brookhaven National Laboratory
Upton, NY 11973 (USA)
+
[
] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202017288.
Dr. A. J. Kropf, Dr. E. C. Wegener, Dr. D. J. Myers
Chemical Sciences and Engineering Division
Argonne National Laboratory
Lemont, IL 60439 (USA)
9
516
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In this work, we firstly developed a nitrogen-doped Results and Discussion
carbon model host derived from zeolitic imidazolate frame-
work-8 (ZIF-8) for Co ion adsorption and then elucidated the
CoN4 active site evolution mechanism under controlled
thermal activation. Employing this carbon host could effec-
tively exclude the interference from the complex carbon-
ization and nitrogen doping processes, providing the benefit
of precise control of CoÀN coordination and local carbon
Design Catalyst Models via One-Step Adsorption
As illustrated in Figure 1a, we developed the ion adsorp-
tion method to elucidate the CoN4 active site formation
process by varying thermal activation temperatures. Firstly,
nitrogen-doped porous carbon (ZIF-NC) was prepared by
carbonizing ZIF-8 precursors at 11008C for one hour under
Ar flow. During the high-temperature pyrolysis, the organic
hydrocarbon linkers were directly carbonized to an N-doped
carbon skeleton. Due to a low boiling point of zinc at 9078C,
the simultaneous zinc evaporation yielded a high-surface-area
and porous carbon host. As verified by Brunauer-Emmett-
Teller (BET) surface area measurements (Figure S1a), pore
distribution plots (Figure 1b), and the nitrogen adsorption-
desorption isothermal curves (Figure S1b), the ZIF-NC host
structures. This well-defined and straightforward system is
desirable to exclusively determine the CoÀN bond structure
relevant to intrinsic activity and selectivity, which only
depends on thermal activation temperatures. Utilizing in situ
X-ray absorption spectroscopy (XAS) and atomic-resolution
electron microscopy, we found that thermal activation at
7
008C is critical to initiate active CoN sites, and the optimal
4
CoN sites are accomplished at 9008C. Furthermore, first-
4
principle density functional theory (DFT) calculations eluci-
2
À1
dated that thermal-induced CoÀN local strain in the CoN
retained the high surface area (920 m g ) and large micro-
2
+2
3
À1
and CoN sites is responsible for the experimentally-observed
pore volume (0.373 cm g ) from the ZIF-8 nanocrystal
4
2
+
ORR activity enhancement over 7008C. Motivated with this
precursor (Table S2). After the controlled Co ion adsorp-
understanding, we designed a two-step chemical Co doping
tion, the carbon hostꢀs N dopants can readily trap adsorbed
2
+
and adsorption strategy to increase the CoN site density and
Co ions, providing a stabilizing force against aggregation. X-
ray diffraction (XRD) patterns (Figure 1c), Raman spectra
(Figure 1d, S2, and Table S3), and X-ray photoelectron
spectroscopy (XPS) analysis (Figure S3 and Table S4–S6)
indicate that the carbon structure and nitrogen doping in the
carbon host kept unchanged up to 11008C. This further
evidences that the thermally stable carbon host is ideal for
4
optimize catalyst morphology for promoting mass transport.
This Co-N-C catalyst, therefore, exhibited much-enhanced
ORR activity and four-electron selectivity in challenging
acidic electrolytes. We further demonstrated the two-step Co-
N-C catalyst with promising performance and durability in
H /air proton-exchange membrane fuel cells (PEMFCs).
2
studying the CoN active sitesꢀ formation mechanism during
4
high-temperature treatments and further correlating the
CoÀN coordination structures with their catalytic properties.
Figure 1. a) The development of ZIF-8 precursor-derived N-doped nanocarbon (ZIF-NC) hosts through carbonizing at 11008C with defined N
dopants, surface areas, porosity, and graphitization degree. The ZIF-NC host used for Co ion adsorption followed by thermal activation at different
temperatures (ZIF-NC-Co-T) from room temperature to 11008C to elucidate CoN site formation mechanism. b) Pore size distributions, c) XRD
4
patterns, and d) Raman spectra of carbon of the ZIF-NC-Co-T samples.
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The ZIF-NC-Co-room temperature (RT) sample was
typically formed by adsorbing Co(NO ) into ZIF-NC nano-
ORR activity, with E1/2 reaching 0.73 V, indicating that this
temperature is critical for forming active CoN sites. The
3
2
4
particles followed by drying under vacuum. The ZIF-NC-Co-
RT was subsequently heat-treated at various temperatures
ORR activity was further improved with an increase of
activation temperature up to 9008C, reaching an E1/2 of 0.81 V.
However, a higher temperature of 11008C led to decreased
ORR activity.
(
300 to 11008C) to prepare a series of samples (ZIF-NC-Co-
T) and track their CoÀN coordination evolution. Separating
the complicated carbonization and nitrogen doping processes
from the critical thermal activation makes the ZIF-NC model
The H O yield during the ORR was also measured using
2
2
a rotating ring-disk electrode (RRDE) to determine the
system desirable for exclusively studying CoN active sites.
catalystꢀs four-electron selectivity to H O (Figure 2b). The
4
2
Co-free ZIF-NC sample exhibited a high H O2 yield of
2
approximately 15%. In contrast, the ZIF-NC-Co-900 and
Investigation of CoN Site Evolution
À1100 catalysts demonstrated negligible H O yield (< 1%),
4
2
2
indicating the nearly complete reduction of O to H O instead
2
2
ORR activity of various ZIF-NC-Co-T (T= 300 to
of H O . This ultra-low H O yield is significantly different
2 2 2 2
1
1008C) catalysts was measured in O -saturated 0.5 M
from the previous Co-N-C catalyst derived from polyani-
2
[39,40]
H SO electrolyte to elucidate the role of the thermal
line,
suggesting a new type of Co active sites.
2
4
activation in forming active CoN sites. (Figure 2a). Without
Co doping, the ZIF-NC host showed poor ORR activity with
To determine that the ORR activity arose from CoN
4
4
+
moieties rather than CN species, ZIF-NC-Na -T samples
x
an onset potential (Eonset) of 0.80 V and a half-wave potential
obtained via adsorbing NaNO were prepared for compar-
3
(
E ) of 0.62 V under the defined testing conditions. This is
ison. Figure 2c reveals that CoN species were the dominant
1
/2
4
similar to typical ORR activity for “metal-free” CÀN active
active sites in these ZIF-NC-Co-T catalysts because ZIF-NC-
Na -T samples with identical thermal activation exhibited
x
[
38]
+
sites in acids. The E1 of the ZIF-NC-Co-RT even showed
/2
2
+
a negative shift after the adsorption of Co ions, probably
due to the blockage of micropores by the adsorbed Co
compounds, as evidenced in Figure 1b and Table S2. How-
ever, the micropore volumes recovered quickly following
a thermal activation, suggesting that Co ions/salts were first
located in the micropores and then gradually decomposed
into atomically-dispersed species. A relatively low temper-
ature of 7008C is sufficient to initiate a substantial increase in
vastly inferior ORR activity similar to “metal-free” CÀN
x
active sites.
The steady-state ORR polarization plots for ZIF-NC-
xCo-900 catalysts, derived from various adsorbed Co(NO3)2
contents, treated at the optimal 9008C, are shown in Fig-
ure 2d. These results provide evidence that the increased
number of CoN sites is responsible for the enhanced ORR
x
activity. The Co-free ZIF-NC-0Co-900 sample showed the
lowest catalytic activity, corresponding to the N-
doped carbon host activity. The ORR activity of
the ZIF-NC-xCo-900 samples initially showed
a positive correlation with Co content, following
the order of ZIF-NC-1Co < ZIF-NC-5Co < ZIF-
NC-10Co. It suggests that all Co ions can be
utilized entirely to form active CoN sites, rising
4
active site density. According to XPS analysis, the
Co content of the optimal ZIF-NC-10Co catalyst
is 0.5 at.% (Table S7). Excessive Co(NO ) ad-
3
2
sorption, however, led to decreased activity and
formation of inactive Co O and Co nanoparticles
3
4
during the thermal activation at 9008C, as verified
by XRD patterns (Figure S4), presumably due to
the insufficient number of anchoring N sites in the
carbon host. Therefore, the appropriate Co con-
tent is essential to maximize ORR activity. This
strategy demonstrated precise control of active
site density for Co-N-C catalysts without altering
the carbon structure and morphology (Fig-
ure S5,S6 and Table S8–S10).
Multiple characterization techniques were
employed to identify the Co species in various
ZIF-NC-Co-T samples from different heating
temperatures. Firstly, no significant Co-contain-
ing crystalline phases were detected in all ZIF-
NC-Co-T catalysts according to XRD patterns.
The valence state and evolution of the surface
composition during the thermal activation were
Figure 2. a) Steady-state ORR polarization plots and b) H O yield of ZIF-NC-Co-T
2
2
+
catalysts. c) The controlled ZIF-NC-Na -T samples treated at different temperatures
to verify that the adsorbed Co ions are exclusively responsible for enhanced ORR
activities. d) ORR polarization plots of the ZIF-NC-xCo-900 samples from adsorbing
various Co ion content (x represents the Co solution volume added in the solvents
2
+
for the Co ions adsorption). ORR testing conditions: O -saturated 0.5 M H SO
2
2
4
À2
electrolyte; rotating speed: 900 rpm; catalyst loading: 0.6 mgcm
.
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examined using XPS (Figure S3 and Table S4–S6). The ZIF-
ure S8). Fits to the EXAFS region of selected spectra are
NCꢀs N 1s spectrum exhibited two sharp peaks, ascribed to
pyridinic-N (398.4 eV) and graphitic-N (401.0 eV) (Fig-
ure S3b). After the adsorption of Co ions, a new peak
summarized in Table S11. The near edge region of the XAS
spectra of the catalyst precursor and several standard com-
pounds (i.e., Co, CoO, Co(NO3)2 6H O, and CoPc) are
2
(
399.6 eV) corresponding to CoÀN coordination appeared
compared in Figure 3a. The ZIF-NC-Co-RT and the CoO
referenceꢀs similar pre-edge energy indicates that Co in the
ZIF-NC-Co-RT is in the 2 + oxidation state. The EXAFS
spectrum (Figure 3b) and fits to the spectrum (Table S11) for
the ZIF-NC-Co-RT have characteristics similar to those
with a percentage of 13–16% (Table S6). The pyridinic-N
percentage is decreased from 30 at.% to 19 at.% after
2
+
adsorbing Co and slightly increased during the heat treat-
ment. This indicates that pyridinic-N initially bonded with Co
[41]
ions and formed unstable CoN species. During the thermal
observed for Co(OH) brucite with similar bond distances.
2
x
activation, weak CoÀN bonding disappears, which explains
This observation indicates that hydrated nitrate decomposes
or is converted to the hydroxide species during the adsorption
process and/or subsequent drying step at 608C.
the phenomenon of CoÀN content in ZIF-NC-Co-RT is
higher than that in ZIF-NC-Co-T samples. We have done an
in situ Transmission electron microscope (TEM) test to
observe the change of morphology and elements. As shown in
Figure S7, we found that no apparent changes or particle
formation can be observed in HAADF-STEM images during
the heating treatment in a wide temperature range, indicating
that the aggregates Co species might be insignificant.
The in situ XAS data represented in the Fourier trans-
forms of the EXAFS (Figure 3b), the derivative of the
XANES region (Figure 3c), the XANES data (Figure S9),
and the fits to the EXAFS data (Table S11) show that the Co
undergoes a dramatic change in coordination during the
thermal treatment at temperatures between 5008C and
7008C. The scattering path between 2 and 3 Š in the Fourier
transform disappears, and the length of the scattering path
between 1 and 2 Š decreases. The length of the scattering
path in the Fourier transform of the XAS data taken at
Because the ex-situ XAS will involve changes in the
oxidation state due to air exposure during sample handling,
storage, and transportation, which are not accurate to
determine intermediate states during the active site forma-
tion. Therefore, we designed in situ XAS experiments for
dynamically tracking active sitesꢀ evolution during a continu-
ous heat treatment under vacuum. To determine the critical
CoÀN coordination during thermal activation, in situ XAS
ꢀ 7008C agrees well with that of the CoN phthalocyanine
4
(CoPc) standard. The short first shell bond lengths derived
from the EXAFS fits (e.g., 1.90 Š at 8008C, Table S11) agree
well with those expected for a square planar CoN complex
4
data were acquired at the Co K-edge for the ZIF-NC-Co-T
sample during a heat treatment up to 11008C. The near edge
region of the XAS spectra was recorded for the ZIF-NC-Co-T
at room temperature (Figure 3a) and various temperatures
during the thermal activation process (Figure 3b,c and Fig-
(CoPc: 1.92 Š, Table S11). Moreover, the first derivative of
the XANES pre-edge data, particularly the feature in the
7710 to 7715 eV energy range, clearly shows the signature
II
1s!4pz transition of in-plane Co N moieties emerge at
4
approximately 7008C (Figure 3c). Linear combination fitting
Figure 3. In situ a) Co K-edge X-ray absorption near edge structure (XANES) spectra, b) Fourier transforms of the Co K-edge extended X-ray
absorption fine structure (EXAFS), and c) derivative of the normalized XANES data for the ZIF-NC-Co acquired at the indicated temperatures
during real-time thermal activation. d) The dynamical structural evolution of adsorbed Co ions within a nitrogen-doped carbon host as a function
2
+
of thermal activation temperatures: Co hydroxide after Co adsorption at RT undergoes decomposition to an oxide-rich mixture up to 5238C, to
CoN with a minor Co metal contribution at ꢁ7008C, and entirely to atomically dispersed CoN active sites at >8008C.
4
4
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of the XANES region of the 7008C
spectrum (Figure S10) shows that
about 29% of the Co is in the
metallic state, most likely as Co
nanoparticles, and 71% is CoN . At
4
temperatures ꢀ 8008C, CoN is the
4
only Co species present within the
detection limit of the technique
(
Table S13, Figure S9). Besides, the
XANES spectrum of Co-N-C at the
activation temperature above 7008C
is similar to that observed by Zitolo
[
42]
et al.
Thus, it matches XANES
simulation for Co species in a micro-
pore-hosted porphyrinic coordina-
tion environment, i.e., CoN C12 (or
4
its defective derivatives) (Fig-
ure S11) according to the fitting with
the standard CoPc reference (Fig-
ure S12).
Figure 3d illuminates the dy-
namical structure changes of Co
species and the formation of atomi-
cally dispersed CoN4 sites within
a nitrogen-doped carbon host as
a function of thermal activation
II
temperature: conversion of the Co
hydroxide (room temperature) to an
oxide-rich mixture of phases up to
5
238C, to CoN4 with a minor Co
metal contribution ( ꢁ 7008C), and
CoÀN bond optimization and re-
moval of Co metal (> 7008C). At
a temperature over 7008C, the CoÀN
Figure 4. Morphology and atomic structure of a,b) the ZIF-NC-Co-RT and c–e) the ZIF-NC-Co-900C
catalysts observed by using medium-angle annular dark field-scanning transmission electron
microscope (MAADF-STEM) images, the STEM with energy-dispersive X-ray spectroscopy (STEM-
EDS) elemental maps, and aberration-corrected MAADF-STEM images and corresponding EEL
point spectra.
coordination number and bond
length are similar to those for the
CoPc standard. The CoN structure
4
was maintained at 11008C even after
cooling down to room temperature,
indicating that the formed CoN active site over 7008C is very
stable (Figure S13). It is noteworthy that this process is quite
results (Table S4), these drifting atoms might be ascribed to
+
4
2
free Co ions, indicating the EELS beam breaks the weak
different from the formation process of FeN sites reported
CoÀN bonds. With an increase in heating temperatures above
4
[2]
previously. That work elucidated that ultrafine FeO par-
7008C, the co-existence of Co and N at the atomic level was
verified (Figure 4c–e). STEM and EEL point spectroscopy
analysis are consistent with the XAS analysis. This indicates
that the thermal activation can stabilize the CoÀN config-
x
ticles (< 2 nm) first formed at a low temperature (Figure S14)
and then transformed into atomically-dispersed FeN active
4
sites at a relatively lower temperature (4008C), which
becomes optimal active FeN coordination at 7008C.
uration because Co and N signals were simultaneously
detected when placing an electron probe on the isolated
bright spot.
4
Medium-angle annular dark-field (MAADF) images of
the ZIF-NC-Co-RT sample shown in Figure 4a indicate that
the single metal sites are uniformly dispersed in the carbon
phases. However, when the electron probe with 1 Š resolu-
tion was targeted on the bright metal spots, those metal sites
behaved significantly differently. A few single Co sites are
stable and are accompanied by N atoms observed in the EEL
point spectrum (Figure S15), implying that relatively strong
CoÀN coordination exists even after simple chemical adsorp-
Theoretical Studies of Thermal-Induced CoÀN Structures
The first principle DFT calculations were performed to
elucidate the origin of experimentally observed activity
enhancement with the structural evolution of CoN sites at
4
tion. In contrast, most single Co sites migrate under the beam,
and only a nitrogen signal was detected at the metal position,
as shown in Figure 4b and Figure S16. According to XPS
different temperatures. We applied a strained concept for the
CoÀN bonds in the CoN site based on the distorted active site
4
configuration and shorter CoÀN bond lengths discovered
9
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from experimental XAS re-
sults. Also, we introduced the
negative thermal expansion
coefficient (TEC) of gra-
[
43–46]
phene.
Therefore, the
established model is a distort-
ed nonplanar configuration
of CoN active sites formed
4
under elevated temperature.
In our previous study on
[
2]
strained FeN4 active site,
we found that some degrees
of compressive strain in the
FeN moiety would facilitate
4
the *OOH dissociation pro-
cess and hence kinetically
enhance the ORR. Here, we
calculated two possible sites:
a CoN4 site embedded in
a
graphene layer (Fig-
[
2,6,47–49]
ure 5a)
and a CoN2+2
active site bridging two gra-
phene
ure 5b).
edges
The 2% com-
(Fig-
[
48,50]
pressively strained CoN site
4
has a nonplanar configura-
tion with shortened CoÀN
bonds, while the 1.5% com-
pressively strained CoN2+2
active site still maintains the
planar configuration. Both
CoN and CoN2+2 sites with
4
compressive strain showed
more positive charge accu-
mulation on the central Co
atom (Figure S17), leading to
the stronger binding with O2
and intermediates.
Figure 5. Atomic structures of fully relaxed and compressively strained a) CoN active site embedded in
4
a compact graphene layer and b) CoN2+2 active site bridging two graphene edges. The gray, blue, and yellow
balls represent C, N, and Co atoms, respectively. Predicted limiting potentials of the ORR and activation
energy for *OOH dissociation on c) CoN and d) CoN active sites with varying local strain. e) Predicted
4
2+2
free energy evolution of the ORR on 2% compressed CoN active site through a four-electron pathway via
4
Moreover, we predicted
the optimized configurations
of *OOH, *O, and *OH
adsorbed on these strained
and unstrained CoN4 and
CoN2+2 sites (Figure S18,
*H O dissociation. f) Predicted free energy evolution of the ORR on CoN active sites with 0 and 1.5%
2
2
2+2
À
compressive strain through a 4e pathway via *OOH dissociation. g) Amperometric responses of the ZIF-
NC-Co-900 catalyst at a constant potential (0.2 V vs. RHE) with consecutive addition of 1.0 mmol H O in
2
2
an N -saturated 0.5 M H SO . h) Calibration curve for the current density response to H O concentration
2
2
4
2
2
from 0 to 5.0 mmol in an N -saturated 0.5 M H SO .
2
2
4
S19). The corresponding ORR limiting potentials were
calculated, which is the maximum applied electrode potential
to keep ORR thermodynamically favorable. The variation of
in our predicted ORR free energy evolution (Figure S21), the
applied 2% compressive strain lowered the activation energy
for *OOH dissociation from 1.15 eV on unstrained CoN4
active sites to 1.04 eV. However, all calculated *OOH
ORR limiting potential with the local strain of CoN and
4
CoN2+2 active sites is summarized in Figures 5c and d. A more
positive limiting potential indicates better ORR activity. At
dissociation activation energies on the CoN active sites are
still higher than 1.0 eV, implying a significant difficulty for
4
the same strained degree, the ORR on the CoN active sites
became more thermodynamically favorable relative to the
CoN2+2 sites.
OÀO bond-breaking via *OOH dissociation on these sites.
4
Alternatively, we propose a 2 + 2 electron pathway involving
the adsorption and dissociation of H O (Figure 5e). The
2
2
In contrast, the calculated activation energy for *OOH
H O molecule was predicted not to adsorb on the unstrained
2 2
dissociation on CoN and CoN2+2 active sites (Figure S20)
CoN sites with only 0.01 eVadsorption energy. However, the
4
4
shows the same trend as activation energy is reduced with
increasing compressive strain (Figure 5c,d). Thus, the ORR
activity could be kinetically enhanced on compressively
adsorption of H O2 is enhanced with adsorption energy
2
changed to À0.09, and À0.11 eV on 1% and 2% compres-
sively strained CoN active site, respectively. The activation
4
strained CoN and CoN2+2 active sites. In particular, as shown
energy for H O dissociation was predicted to be only 0.11 eV
4
2
2
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on 2% compressively strained CoN active sites. The ORR is
the ZIF-8 precursor during the carbonization to form a carbon
host for subsequent ion adsorption (Figure S23). The pore-
distribution plot, along with the nitrogen adsorption-desorp-
tion isothermal curve in Figure S24,S25 and the pore volume
values in Table S12, verified the role of pre-coped Co in
increasing mesopore volume. Besides, the pre-doped Co
slightly enhanced the graphitization degree of the carbon host
derived from ZIF-8 precursors (Figure S26), which is bene-
ficial for enhancing carbon corrosion resistance and catalyst
stability. Furthermore, the pre-Co-doping can significantly
reduce the residual Zn amount, providing more N-coordi-
nation sites for the subsequent Co ion adsorption. The
increased density of active sites in a Co-N-C catalyst via
a two-step synthesis (e.g., 1% pre-doping Co and Co
adsorption, thermally activated at 9008C) is verified by
higher N and Co content determined by using XPS analysis
(Figure S27, and Table S13–S15).
4
thermodynamically and kinetically favorable via *H O dis-
2
2
sociation on the 2% compressively strained CoN active site
4
(
Figure 5e). For the CoN2+2 active sites, the *OOH dissoci-
ation activation energy decreased from 0.59 eV on unstrained
sites to 0.43 eV on 1.5% compressively strained sites (Fig-
ure 5 f). However, the predicted limiting potential was also
reduced from 0.78 V to 0.55 V. Thus, a small degree of
compressive strain in CoN2+2 active sites could kinetically
facilitate the ORR process by enhancing *OOH dissociation
process. Still, the excessive strain would result in thermody-
namically unfavorable. This may explain that, instead of
[58]
1
1008C, 9008C is the optimal thermal activation temperature
with a balance between kinetics and limiting potentials for the
ORR on the strained CoN site. The thermal stability of CoN4
4
active sites was also investigated (Figure S22). The large
positive free energy change implies that CoN sites are very
4
stable, and only very high temperatures can initiate the
A series of two-step Co-N-C catalysts with various pre-
doped Co contents combined with optimal adsorption and
thermal activation at 9008C were prepared. Their ORR
activities were assessed using RDE measurements in an O2
dissociation of N from the CoN active sites.
4
To validate our computational predictions, we have
carried out an H O reduction experiment using the ZIF-
2
2
NC-Co-900C catalyst, which usually contains both CoN and
saturated 0.5 M H SO4 electrolyte (Figure S28 and Fig-
4
2
CoN2+2 sites. Figure 5g shows the amperometric responses of
H O reduction on the catalyst in N -saturated 0.5M H SO
ure 6a). The best-performing two-step Co-N-C catalyst from
1.0% Co pre-doping exhibited an Eonset of 0.97 V and an E
2
2
2
2
4
1/2
À2
with the addition of 1.0 mmol H O at the indicated time
of 0.83 V at a regular catalyst loading of 0.6 mgcm , which
2
2
intervals while holding the potential at 0.2 V. This catalyst
exhibited a fast amperometric response toward H O reduc-
tion and achieved steady-state current density. A remarkable
increase in current density response could be observed with
was ca. 30 mV positive of the one-step ZIF-NC-Co-900
without Co pre-doping. It is only ca. 40 mV negative to the
commercial Pt/C (e.g., 0.87 V) with Pt loading of 60 mgcm .
The ORR activity is exceptional when compared to other
reported Co-N-C catalysts (Figure S29). The enhanced ORR
activity measured with the two-step Co-N-C catalyst is
2
2
À2
continuously increasing H O2 concentration. The currents
2
scaled linearly with the bulk concentration of H O in the
2
2
region investigated (Figure 5h), which is characteristic of
a diffusion-limited electrochemical reaction. Therefore, the
combined computational and experimental studies suggest
that H O adsorption on CoN4 sites and the subsequent
contributed to the increased density of CoN active sites
4
through this pre-doping method according to the XPS
analysis (Figure S30). In particular, N and Co contents of
the two-step 1%Co-N-C + Co catalyst are increased to 3.9
and 0.6 at.%, respectively, when compared to the one-step
ZIF-NC-Co catalyst (3.2 and 0.4 at.%). The RDE tests of
these 1%Co-N-C + Co-T catalysts, treated at different tem-
peratures, verified the importance of thermal activation
temperature, consistent with the studies for one-step Co-N-
C catalysts (Figure S31). The best performing catalyst was
obtained at 9008C, corresponding to the highest intrinsic
activity.
2
2
reduction to H O was kinetically enhanced by the thermally-
2
induced compressive strain on the CoN and CoN2+2 active
4
sites, hosted in the atomically dispersed Co-N-C catalyst.
Two-Step Synthesis for High-Efficient Co-N-C Catalysts
The understanding of CoÀN coordination structural
evolution during the thermal activation is critical for ration-
ally designing highly active and efficient Co-N-C catalysts
Notably, we also performed ex-situ XAS for these two-
step 1%Co-N-C + Co-T catalysts (Figure S32–34), verifying
with increased CoN site density and optimal porosity for
the exclusive CoN site formation in catalysts. These two-step
4
4
mass transport in fuel cells. For example, we can accurately
control thermal activation temperatures to realize maximum
atomic dispersion of Co sites, and the highest intrinsic ORR
synthesized samples showed a similar Co speciation evolution
trend as observed for the one-step method, but required
higher temperature, 10008C vs. 8008C, to convert all Co to
À
activity and 4e selectivity on optimal CoÀN local strains.
CoN species completely. The different behaviors are due to
4
In the second part of this work, we developed a two-step
chemical doping and ion adsorption method to introduce
the nature of ex-situ and in situ XAS experiments. Also,
likely, the pre-doped CoN sites in the catalyst (in the first
4
more CoN sites and optimize the catalystꢀs pore structure.
Generally, increasing micropore volumes in carbon support
could afford additional defects for increased active sites.
step) inhibit the diffusion of adsorbed Co ions (in the second
step) from coordinating with N dopants, thus requiring
a higher temperature. It might be more valuable to study
4
Mesopores favor a faster O diffusion and expose more active
the CoN evolution of the two-step Co-N-C catalyst through
2
x
[
51–57]
sites to reactants.
In the first step, the pre-doped Co
in situ XAS experiments. However, it may generate several
confusions for illustrating the overall structural change. The
pre-doped Co atoms may cause the concurrent changing of
species in the ZIF-8 precursor is crucial in creating more
micropore and mesopores via facilitating Znꢀs removal from
9
522
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Figure 6. a) Steady-state ORR polarization plots for one- and two-step Co-N-C catalysts tested in 0.5 M H SO electrolytes and a Pt/C catalyst with
2
4
À2
6
0 mg cm tested in 0.1 M HClO . Stability studies, including potential cycling from 0.6 to 1.0 V (b) and potentiostatic tests at 0.85 V (c) for the
Pt 4
best performing two-step Co-N-C catalyst. d) Fuel cell MEA performance of the one- and two-step Co-N-C catalysts under H /air conditions. Fuel
2
cell stability and durability tests of the best-performing two-step Co-N-C catalyst in an H /air cell by using different protocols: e) voltage cycling
2
À2
between 0.6 to 0.9 V in H /air and f) constant voltage tests at 0.7 V. Cathode catalyst loading: 4.0 mgcm ; cell temperature: 808C; flow rates:
2
H /air 500/1000 sccm; 100% relative humidity. 1.0 bar H /air. Nafion 212.
2
2
nitrogen doping, graphitization, and local carbon structure
with increasing thermal activation temperatures (Figur-
es S24–S27). Therefore, it is challenging to precisely and
exclusively determine CoÀN bonds and their intrinsic effects
degree of graphitization that effectively improves catalyst
stability.
The two-step Co-N-C catalyst was further integrated into
membrane electrode assemblies (MEAs) for evaluating its
fuel cell performance (Table S16). The MEA was tested in an
H -O cell to evaluate catalyst performance with minimum
on ORR activity in the complicated two-step catalyst systems.
The two-step Co-N-C catalyst exhibited promising long-
term stability, only losing 35 mV of E1/2 (Figure 6b) after
a 30000-cycle accelerated stress test (AST) cycling from 0.6 to
2
2
mass-transport losses (Figure S37). The MEA exhibited an
open circuit voltage (OCV) of 0.94 V and a peak power
À2
1
.0 V. The current density after holding at 0.85 V for 50 hours
density of 0.73 Wcm , much higher than the one-step Co-N-
retained around 80% and only lost 25 mV in E , (Figure 6c
C
cathode catalyst (i.e., ZIF-NC + Co, 0.87 V and
1/2
À2
and S35). In contrast, the degradation of the one-step ZIF-
NC-Co-900 catalyst is 45 mV in E1/2 after 30000 potential
cycles (Figure S36). Thus, the pre-doping step can yield a high
0.50 Wcm ). The two-step Co-N-C catalyst cathode gener-
ated an increased current density of 60 mAcm at 0.8 V
(Figure 6d), which is six-times higher than that of the one-
step catalyst (i.e., 9.5 mAcm ), affirming the importance of
À2
À2
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pre-doping of Co during the two-step synthesis. Under more
heating temperature up to 9008C. Theoretical computations
practical H -air conditions, the peak power density is
demonstrate that the low H O yields determined in the Co-
2
2
2
À2
À2
0
.39 Wcm , much higher than 0.23 Wcm for the one-step
N-C catalysts are due to the enhanced activity of the strained
À
catalyst and other reported Co-N-C catalysts (Figure S38).
The improved mass transport behavior also verified the
benefit of pore structure resulting from the Co pre-doping in
the first step.
Subsequently, the long-term durability test of the two-step
Co-N-C cathode in MEA was evaluated using multiple test
protocols. During the extensive ASTs by cycling voltage (0.6
CoN site for the H O reduction, thus promoting the 2 + 2e
4
2
2
pathway for the ORR.
Furthermore, the new understanding of CoÀN coordina-
tion structural changes motivated us to design effective
catalyst systems for fuel cells. We developed a two-step Co-
N-C catalyst by combining chemical Co-doping into ZIF-8
precursors and the subsequent Co ion adsorption to the ZIF-8
derived carbon host. Exceptional ORR activity and stability
in acidic media were achieved on the two-step Co-N-C
catalyst. Moreover, MEAs with the Co-N-C cathode catalyst
to 0.9 V) in H -air condition, the voltage drop at a current
2
À2
density of 0.8 Acm was only 10 mVafter 15000 cycles, likely
achieving the DOE target of < 30 mV loss (after 30000 cy-
cles), as shown in Figure 6e. The durability test was also
displayed encouraging H -air cell performance with a peak
power density of 0.39 Wcm , together with excellent stability
2
À2
evaluated by holding the H -air cell at a constant cell voltage
2
of 0.7 V. After the 27-hour test (Figure 6 f and S39), ꢁ 67% of
under various AST protocols. The remarkably enhanced
the current density was retained, and only a 20 mV voltage
performance can be attributed to the optimal local CoÀN
À2
loss at 0.8 Acm was noted in recorded polarization plots.
coordination, the increased density of CoN4 active sites,
optimized porosity with favorable mass transfer, and en-
hanced carbon corrosion resistance. Thus, in addition to
providing a fundamental understanding of CoÀN coordina-
The encouraging stability is due to the inherent CoÀN bond
stability at optimal thermal activation and the enhanced
carbon corrosion resistance. The mitigated carbon corrosion
is attributed to the eliminated Fenton reaction by using Fe-
free catalysts and increased graphitization degree of the
carbon host due to the pre-doped Co as evidenced by the
Raman analysis. Therefore, the two-step chemical-doping and
ion adsorption method effectively increases the density of
active sites, optimizes the mesopore volume for facilitating
the mass transfer, and enhances the carbon corrosion
resistance of the Co-N-C catalyst.
tion structural changes during the CoN site formation, we
4
also developed an effective approach to designing advanced
Co-N-C catalysts for fuel cells with enhanced performance
and durability. The general understanding of regulating
metal-nitrogen coordination structures through controlled
thermal activation can universally tune intrinsic activity and
selectivity of single metal sites catalysts, therefore benefiting
other electrocatalysis processes.
Conclusion
Acknowledgements
In summary, we elucidate the CoN active site formation
The authors acknowledge the U.S. Department of Energy,
Energy Efficiency and Renewable Energy, Hydrogen and
Fuel Cell Technologies Office (DOE-EERE-HFTO) through
the Electrocatalysis Consortium (ElectroCat) and the DOE
program managers, Dimitrios Papageorgopoulos and David
Peterson. Argonne National Laboratoryꢀs work is supported
by DOE-EERE-HFTO under contract DE-AC02-
06CH11357. The XAS experiments were performed at the
Advanced Photon Source (APS), a DOE Office of Science
User Facility operated for the DOE Office of Science by
Argonne National Laboratory, also under Contract No. DE-
AC02-06CH11357. The operation of MRCAT at the APS is
supported by the DOE the MRCAT member institutions. G.
Wu also thanks the support from National Science Founda-
tion (CBET-1604392, 1804326).
4
mechanism using ZIF-8 derived N-doped carbon as the ideal
host for Co ion adsorption, followed by thermal activation at
different temperatures. The structural evolution of the CoN4
site under controlled heating treatments was monitored via in
situ XAS characterization and atomic-level electron micros-
copy. In situ XAS analysis showed the evolution process of
the Co-containing species, which the Co hydroxide at room
temperature undergoes dehydration/decomposition to form
an oxide-rich mixture of phases up to 5238C, to CoN with
4
a minor Co metal contribution at ꢁ 7008C, and entirely to
atomically dispersed CoN active sites at > 8008C. While the
4
CoÀN bond could be formed at room temperature after
chemical adsorption, 7008C is the critical point to create
ORR active CoN sites, and > 8008C is the optimal temper-
4
ature for the complete transition. The formation of CoN sites
4
requires larger thermal energy, i.e., higher formation temper-
atures at 7008C than the FeN site formed at a relatively low Conflict of interest
4
[2]
temperature of 4008C. DFT calculations further elucidated
that the ORR was kinetically enhanced by the thermal-
The authors declare no conflict of interest.
induced compressive strain of CoÀN bonds in CoN and
4
CoN
active sites. However, the thermodynamics of the
+2
Keywords: Co-N-C · fuel cells · in situ XAS ·
2
ORR become less favorable on the compressively strained
oxygen reduction reaction · single metal site
CoN active sites. Therefore, the optimal strain is desirable for
4
maximum intrinsic activity. The strain concept for CoN sites
4
well explained the enhanced ORR activity with an increase of
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