Highly Efficient CO2 Electroreduction on ZnN4-based Single-Atom Catalyst

Article abstract of DOI:10.1002/anie.201805871

The electrochemical reduction reaction of carbon dioxide (CO2RR) to carbon monoxide (CO) is the basis for the further synthesis of more complex carbon-based fuels or attractive feedstock. Single-atom catalysts have unique electronic and geometric structures with respect to their bulk counterparts, thus exhibiting unexpected catalytic activities. A nitrogen-anchored Zn single-atom catalyst is presented for CO formation from CO2RR with high catalytic activity (onset overpotential down to 24 mV), high selectivity (Faradaic efficiency for CO (FE_CO) up to 95 % at ?0.43 V), remarkable durability (>75 h without decay of FE_CO), and large turnover frequency (TOF, up to 9969 h^?1). Further experimental and DFT results indicate that the four-nitrogen-anchored Zn single atom (Zn-N₄) is the main active site for CO2RR with low free energy barrier for the formation of *COOH as the rate-limiting step.

Full text of DOI:10.1002/anie.201805871

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Angewandte
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Accepted Article
Title: Highly Efficient CO2 Electroreduction on ZnN4-based Single-
Atom Catalyst
Authors: Fa Yang, Ping Song, Xiaozhi Liu, Bingbao Mei, Wei Xing,
Zheng Jiang, Lin Gu, and Weilin Xu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805871
Angew. Chem. 10.1002/ange.201805871
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http://dx.doi.org/10.1002/ange.201805871

10.1002/anie.201805871
Angewandte Chemie International Edition
COMMUNICATION
Highly Efficient CO2 Electroreduction on ZnN4-based Single-Atom
Catalyst
Fa Yang[a,b]†, Ping Song[a]†, Xiaozhi Liu[b,c] †, Bingbao Mei[d], Wei Xing[a], Zheng Jiang*[d], Lin Gu*[c],
Weilin Xu* [a]
Abstract: Electrochemical reduction reaction of carbon dioxide
(CO2RR) to carbon monoxide (CO) is the basis for the further
synthesis of more complex carbon-based fuels or attractive
feedstock. Single-atom catalysts have unique electronic and
geometric structures with respect to their bulk counterparts, thus
alone, such as Zn, are also the promising materials due to their
abundant reserves on earth.[8] As for the Zn, however, all the
reported Zn-based catalysts, such as traditional dendrite and
hexagonal Zn catalysts, for the CO production from CO2RR,
only show very limited performance (such as low activity,
selectivity and stability) with mechanisms unclear.[8b] Recently,
single-atom catalysts have shown great potential as effective
catalysts for some chemical transformations because of their
unique electronic and geometric structures.[9] Specially, many
metal (M=Fe, Co, Ni, Cu, Mn)-nitrogen (N)-doped carbon (C)
electrocatalysts for CO2RR have been reported with M-Nx as the
active sites.[6d,10] Nevertheless, the high efficient electroreduction
of CO2 catalyzed by single-atom catalysts has been rarely
reported.[6d, 7c, 9a]
exhibiting unexpected catalytic activities. Herein we report
a
nitrogen-anchored Zn single atom catalyst for CO formation from
CO2RR with high catalytic activity (onset overpotential down to 24
mV), high selectivity (Faradaic efficiency for CO (FECO) up to 95% at
-0.43 V), remarkable durability (>75 h without decay of FECO) and
large turnover frequency (TOF, up to 9969 h-1). Further experimental
and DFT results indicate that the four-nitrogen-anchored Zn single
atom (Zn-N4) is the main active site for CO2RR with low free energy
barrier for the formation of *COOHas the rate-limiting step.
Herein, we report a type of cost-effective carbon supported
nitrogen-anchored Zn single atom catalyst (ZnNx/C with Zn 0.10
wt% and N 0.61 wt %) for highly efficient CO2RR to produce CO
with a small onset overpotential of 24 mV and a large turnover
frequency (TOF) of 9969 h-1. At a small overpotential of 0.32 V, it
presents high Faradaic efficiency (95%) and durability (no
observable performance decay within 75 hrs) for CO formation.
Further experimental and DFT results indicate that the observed
higher performance for CO2RR could be mainly attributed to the
nitrogen-anchored individual Zn atoms (Zn-N4) with the
formation of *COOH as the rate-limiting step. The corresponding
CO2RR mechanism on such active site was also revealed.
In this work, firstly, based on the analytic tools of gas
chromatography (GC) and 1H nuclear magnetic resonance
(NMR) spectra, with cheap carbon black (BP 2,000 (BP)) as
support, urea and zinc acetate as N and Zn precursors,
respectively, an optimal catalyst of BP-15N-Zn1.5% (labeled as
ZnNx/C) for CO production from CO2RR was obtained by
pyrolyzing the mixture containing BP, urea and Zn 1.5 wt% with
mass ratio of 1/15 between BP and urea at 1000 oC (Fig. S1 and
Table S1-S2 in Supporting information, SI).[9b] Elemental analysis
(EA, for nitrogen) and inductively coupled plasma mass
spectrometry (ICP-MS, for Zn) (SI) indicate that the final
contents of dopants in ZnNx/C are N 0.61 wt % and Zn 0.10
wt % (Table S2), respectively. For comparison, the pure BP (C),
BP-N (N-C) and BP-Zn (Zn-C) were also prepared with the same
procedure as that for ZnNx/C (SI). Based on such optimal
ZnNx/C, it was found that the electrolyte of KHCO3 is better than
NaHCO3 for CO production from CO2RR (Fig. S2) due to the
fact that cation K+ can provide more available protons than
cation Na+ to buffer the pH increase near the cathode surface
and then make the concentration of molecularly dissolved CO2
higher.[5d, 11] So, all the following results presented were based
on 0.5 M KHCO3.
The consumption increasing of fossil fuels is depleting these
finite natural resources and leading to the overproduction of
carbon dioxide (CO2).[1] Electrochemical conversion of CO2 into
useful products offers one practical way to deal with this
challenge, and much effort has been devoted to searching for
catalysts with high efficiency and selectivity for the
electrochemical CO2RR.[2] In the present, besides the slow
kinetics with large overpotentials required to initiate CO2RR,[3]
the more kinetically preferred H2 evolution reaction (HER)
usually competes with the CO2RR and leads to low energetic
efficiencies and poor selectivity of target product.[4] Over the past
decades, noble metals have been the preferred groups of
heterogeneous electrocatalysts for the CO2RR.[5] For practical
applications, however, it is very desirable to develop highly
efficient electrocatalysts based on cost-effective materials for
CO2RR.[6] In that sense, based on lowcost nitrogen(N)-doped
carbon materials, some groups have reported catalysts with high
selectivity and stability for CO2RR, but with slow kinetics or
large overpotentials.[7] On the other hand, nonprecious metals
[a]
Fa Yang, Dr.Ping Song, Prof. Wei Xing, Prof. Weilin Xu
State Key Laboratory of Electroanalytical Chemistry, Jilin Province
Key Laboratory of Low Carbon Chemical Power
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences
5625 Renmin Street, Changchun, 130022, P. R. China
E-mail: weilinxu@ciac.ac.cn
ORCID 0000-0001-7140-8060
[b]
[c]
Fa Yang, Xiaozhi Liu
University of Chinese Academy of Sciences
Beijing, 100049, China
Xiaozhi Liu, Prof. Lin Gu
Institute of Physics
Chinese Academy of Sciences
Beijing 100190, China. E-mail: l.gu@iphy.ac.cn
ORCID 0000-0002-7504-031X
The distribution of trace amount of Zn (0.10 wt %) on optimal
ZnNx/C was characterized by high-resolution transmission
electron microscopy (HRTEM) and high-angle annular dark field-
scanning transmission electron microscopy (HAADF-STEM). As
shown in Fig. S3a, no Zn nanoparticles or clusters can be found
on ZnNx/C under HRTEM, while the individual Zn atoms can be
[d]
Bingbao Mei, Prof. Zheng Jiang
Shanghai Synchrotron Radiation Facility
Shanghai Institute of Applied Physics
Chinese Academy of Sciences
Shanghai 201204, China. E-mail: jiangzheng@sinap.ac.cn
These authors contributed equally to this work.
[†]
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10.1002/anie.201805871
Angewandte Chemie International Edition
COMMUNICATION
seen clearly on carbon support under HAADF-STEM (Fig.1a, Fig. 95% at a small overpotential of 0.32 V, which outperforms most
S4), indicating an atomic distribution of Zn on carbon.[5a, 9b, 12]
Furthermore, the homogeneous distributions of both Zn and N
on carbon were confirmed by the Energy Dispersive X-Ray
(EDX) analysis of the ZnNx/C (Fig. S3b). We further collected
the extended X-ray absorption fine structure (EXAFS) spectra
from Zn K-edge to ascertain quantitatively the structural
parameters for the atoms surroundings of the central Zn atoms.
As shown in Fig. 1b and Table S3, it can be seen that only the
structure of Zn-N4 rather than Zn-N2 or Zn-Zn could produce a
satisfactory fit, indicating that Zn-N4 is the only main active form
of Zn in ZnNx/C catalyst. These results unambiguously indicate
that the Zn-based active sites on ZnNx/C are mainly in the form
of Zn-N4.
of the results reported before (Table S5).[5c, 6b, 10d, 13] In order to
further confirm the high activity of such Zn single atom catalyst
for CO2RR, we calculated the turnover of frequencies (TOFs)
per active site for CO production from CO2RR at different
potentials based on their partial CO current densities (SI).[9a]
Significantly, as shown in Fig. 2c, a large TOF of 9969 h-1 was
obtained at -0.73 V and higher than most of results reported
before for CO2RR (Table S6). The performance profile of
ZnNx/C during long-term operation was further examined by
collecting the current density and FE at different potentials (Fig.
S9). For example, at -0.43 V, significantly, as shown in Fig. 2d,
in such long time (75 h) window, the current density and the
corresponding FEs of CO maintained steadily at around -4.8 mA
cm-2 and 95%, respectively, unambiguously indicating
a
remarkable durability of ZnNx/C for CO2RR (Table S5). Such
remarkable durability was further confirmed by the minor
variation of ECSA after long-term CO2RR of ZnNx/C (Fig. S6c).
Figure 1. a) Typical HAADF-STEM image of the optimal ZnNx/C catalyst. The
circles indicate the individual Zn atoms. b) A typical fitting curve of the EXAFS
signal in R-space for the adsorbed ZnNx/C catalyst.
We further evaluated the catalytic activity of ZnNx/C for
CO2RR by linear sweep voltammetry (LSV) on a rotating ring
disk electrode (RRDE) in N2-saturated 0.5 M KH2PO4/K2HPO4
(pH 7.0) and CO2-saturated 0.5 M KHCO3 solution (pH=7.2). As
shown in Fig. 2a, the increases of the onset potential and
cathodic current in the presence of CO2 in the electrolyte
suggested that the CO2 can be efficiently reduced by ZnNx/C
catalyst; moreover, the ZnNx/C shows much larger response to
CO2RR than that on C, N-C or Zn-C. As shown in Fig. S5,
compared with the blank obtained in N2-saturated solution, the
catalysts of C, N-C and Zn-C show very low catalytic activity for
CO2RR apparently. Based on such fact, one can tell that the
large response of ZnNx/C to CO2RR could be mainly attributed
to the well-known synergistic effect between doped N and Zn
atoms, similar to that observed for oxygen reduction reactions
(ORR).[9b] Such synergistic effect can be further confirmed by the
tremendous increase of electrochemical activity surface area
(ECSA) after the codoping of Zn, N on carbon (Fig. S6a,b).
As for the possible products from CO2RR on these catalysts,
it was found that no liquid products could be detected from NMR
analysis (Fig. S7) while with H2 and CO as the main products
detected from GC analysis (Fig. S8). Such results indicate a
high selectivity for CO formation from CO2RR since H2 is from
electrolysis of water or HER. As shown in Fig. 2b, the catalyst
ZnNx/C presents a high onset potential of −0.13 V for CO
Figure 2. a) Up: pH-corrected LSV of N-C and ZnNx/C in N2-saturated
KH2PO4/K2HPO4 (pH 7.0) and CO2-saturated (pH 7.2) 0.5 M KHCO3 solution;
bottom: Comparison of LSV results for C, N-C, Zn-C, and ZnNx/C catalysts in
CO2-saturated 0.5
M KHCO3. b) FEs of CO and H2 at various applied
potentials on ZnNx/C catalyst. c) TOFs of ZnNx/C catalyst at different applied
potentials. d) Long-term stability of ZnNx/C at a potential load of −0.43 V and
the corresponding FEs of CO and H2.
In order to further understand the kinetics of CO2RR on
ZnNx/C, Tafel analysis was performed on the CO partial current
density (Fig. 3a). As shown in Fig.3b, the Tafel slope for CO
formation on ZnNx/C yielded a value of 167 mV dec-1, much
smaller than the values obtained from N-C (279 mV dec-1) and
Zn-C (285 mV dec-1), indicating a faster electron transfer from
electrode of ZnNx/C to CO2 probably also due to the synergistic
effect between doped N and Zn in the form of Zn-N4. Such faster
electron transfer process was further confirmed by the Nyquist
plots as shown in Fig. 3c. It can be seen clearly that that the
ZnNx/C presents the smallest resistance for electron transfer
among these three catalysts. Moreover, the Tafel slope of 167
mV dec-1 on ZnNx/C is hugely different from those (~70 mV dec-
1) with fast protonation, indicating a faster electron transfer from
formation from CO2RR, corresponding to
a small onset
overpotential of 24 mV for CO formation (Table S4).[4] Moreover,
the catalyst ZnNx/C also presents high selectivity with FECO up to
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10.1002/anie.201805871
Angewandte Chemie International Edition
COMMUNICATION
electrode to CO2 than protonation.[4, 5d] In addition, a plot of log
(G-N, 16.5%). In this case, the high CO2RR performance of
ZnNx/C could be attributed to the Zn-N sites since that the N-C
and Zn-C only show limited activity for CO production from
CO2RR (Fig.3a and Fig. S14). In order to further confirm the
existence of Zn-Nx, Zn 2p spectrum for ZnNx/C catalyst was also
obtained (Fig.4b). It shows two peaks at 1021.3 and 1044.5 eV,
corresponding to the 2p3/2 and 2p1/2 electronic states of Zn2+,
respectively.[17] Moreover, the binding energy of 2p3/2 (1021.3
eV) is different from that for ZnO (1022 eV), convincing that only
the Zn-Nx rather than ZnO or metal Zn0 is the main active
content in ZnNx/C catalyst (Fig. S4). The only positive correlation
between the CO formation (FECO) and the content of Zn-N (Fig.
S15) also confirms that only the Zn-NX site is the main active site
for CO formation from CO2RR on ZnNx/C catalyst.[6d, 10a,10d,18]
Meanwhile, in order to understand the observed remarkable
-
(jCO) versus log ([HCO3 ]) for ZnNx/C shows a slope of 0.55 (Fig.
-
3d), indicating that the concentration of HCO3 can influence the
efficiency of CO2RR with reaction order of about 0.55. In this
-
case, the HCO3 not only can serve as the H+ donor during CO2
electroreduction but also can increase the CO2 concentration via
rapid equilibrium with bicarbonate.[5c, 9a]
In order to understand the origin of the observed high
CO2RR activity of ZnNx/C catalyst, we carried out some physical
characterizations for it. Firstly, it was found that the codoping of
Zn, N can increase the BET surface area (ZnNx/C: 1402.7 m2g-1
vs N-C: 1218.8 m2g-1 and Zn-C: 1272.5 m2 g-1) and form more
micropores and mesopores on carbon due to the evaporation of
Zn at high temperature (Fig. S10).[14] Furthermore, Fig. S11
shows that the codoping of Zn, N indeed can enlarge the active
surface area indicated by the enhanced CO2 adsorption capacity. durability of ZnNx/C for CO2RR as shown in Fig. 2d, we obtained
Due to the low Zn or N content on carbon supports, X-ray
diffraction (XRD) spectra for both N-C and ZnNx/C catalysts (Fig.
S12a) show similar peaks to amorphous carbon.[15] Fig. S12b
and Table S7 also show that the doping of Zn or N could
increase the defects on carbon indicated by the increase of ID/IG
ratio in Raman spectrum.
the XPS spectra of ZnNx/C after CO2RR for 30 h and 75 h,
respectively. As shown in Fig. S13b, c and Table S9, after 30 or
even 75 h electrolysis at -0.43 V, the content of the main active
component of Zn-Nx shows negligible variation, which is
consistent with the observed stable FECO (Fig. 2d). All these
results indicate that the observed remarkable durability of
ZnNx/C for CO2RR (Fig. 2d) could be due to the remarkable
durability of the active component of Zn-Nx on catalyst.
Figure 3. a) Partial current density (jCO) at various applied potentials for N-C,
Zn-C and ZnNx/C catalysts in CO2-saturated 0.5 M KHCO3. b) Tafel plots of
the partial CO current density for relevant catalysts at different applied
potentials. c) Nyquist plots for N-C, Zn-C and ZnNx/C. d) Partial CO current
density of ZnNx/C vs potassium bicarbonate concentration at constant
potential of -0.53 V.
Figure 4. XPS spectra for N 1s in a) ZnNx/C and b) Zn 2p in ZnNx/C,
respectively. c) The proposed reaction pathways for complete CO2RR on
carbon supported Zn-N4 active site and the free energy diagrams d) for this
process on ZnN4/C, N4/C and ZnC4.
High resolution X-ray photoelectron spectroscopy (XPS) was
performed to further clarify the activity–structure relationship of
the ZnNx/C catalyst by analyzing the chemical components
between Zn single atoms and doped N. For N-C catalyst as
shown in Fig.S13a and Table S8,[10d, 16] N 1s spectrum can be
divided into three peaks at pyridinic N (398.3, P-N, 58.4%),
pyrrolic N (400.4, Py-N, 19.7%) and graphitic N (401.4, G-N,
21.9%). While for the ZnNx/C, as shown in Fig.4a and Table S8,
with the co-doping of Zn and N on carbon, N 1s spectrum of
ZnNx/C shows four peaks at pyridinic N (P-N, 49.7%), Zn-Nx
(399.6, Zn-N, 19.3%),[17] pyrrolic N (Py-N,14.5%) and graphitic N
A profound understanding of CO2RR mechanisms for CO
production on Zn-N4-C, Zn-C and N4-C active sites was further
provided by DFT calculations.[5c, 19] From Fig.4c, one can see
that CO2 activation through *COOH formation is associated with
an uphill energy barrier for the first proton-coupled electron-
transfer step, which can be supplied in the form of overpotential
required to form reaction intermediates. The formation of
*COOH on Zn-N4 site presents the smallest free energy barrier
of 0.662 eV, followed by that on N4-C (1.754 eV), and Zn-C
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10.1002/anie.201805871
Angewandte Chemie International Edition
COMMUNICATION
(2.070 eV), indicating the highest CO2RR performance on Zn-N4
site with activity order of Zn-N4﹥N4-C﹥Zn-C or the synergistic
effect between doped Zn and N as observed on experiments
(Fig. 2a and Fig. S16). After the formation of adsorbed *COOH,
the free-energy pathway was thermodynamically downhill for the
second proton-coupled electron-transfer step to form adsorbed
*CO, except for Zn-N4 with little uphill barrier by 0.052 eV. That
the desorption of CO on Zn-N4 active site with more negative
free energy than the other two active sites indicates high CO
selectivity for Zn-N4. All these results show that the first proton-
coupled electron transfer step with *COOH formation was the
rate-determining step, and Zn-N4 showed high catalytic activity
for CO formation from CO2RR. Further DFT calculation (Fig.
S17) for CO2RR on Co-N4 site indicates that the Zn-N4-based
active site is indeed an ideal active site for high efficient CO2RR
to produce CO. [6d, 10b, 10e, 20] While the competition between HER
and CO2RR shown in Fig. 2b could be mainly attributed to the
different potential-dependence of electron transfer steps in these
two processes (Fig. S18).
Keywords: Single-atom catalyst • CO2 Electroreduction • ZnN4 •
Zinc
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Experimental Section
Preparation of ZnNx/C Catalyst: The optimal catalyst named as ZnNx/C
was obtained with conditions: 1000 °C for pyrolysis, mass ratio of BP:
urea = 1:15, Zn content of 1.5 wt% in the mixture before pyrolysis.
Catalyst characterization. The morphology and dimensions of as-
prepared samples were obtained using transmission electron microscopy
(TEM). Zn L3-edge absorption spectra (EXAFS) were performed on the
BL14W1 beamline, operated at 3.5 GeV with injection currents of 140-
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Acknowledgements
Work was funded by National Basic Research Program of China
(973 Program, 2014CB932700), National Natural Science
Foundation of China (U1601211, 21633008, 21733004,
21721003, and 21433003), K. C. Wong Education Foundation
and Science and Technology Innovation Foundation of Jilin
Province
for
Talents
Cultivation
(20160519005JH,
L20142200005) and Jilin Youth foundation (20160520137JH).
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Text for Table of Contents
Fa Yang†, Ping Song†, Xiaozhi Liu†,
Bingbao Mei, Wei Xing, Zheng
Jiang*, Lin Gu*, Weilin Xu*
Page 1 – Page 5
Highly Efficient CO2
Electroreduction on ZnN4-based
Single-Atom Catalyst
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