Selenium-Doped Hierarchically Porous Carbon Nanosheets as an Efficient Metal-Free Electrocatalyst for CO2 Reduction

Article abstract of DOI:10.1002/adfm.201906194

Electrochemical carbon dioxide reduction reaction (CO₂RR) provides a promising pathway for both decreasing atmospheric CO₂ concentration and producing valuable carbon-based fuels. To explore efficient and cost-effective catalysts for electrochemical CO₂RR is of great importance, but remains challenging. Se-doped carbon nanosheets (Se-CNs) with a micro-, meso-, and macroporous structure are proposed for electrochemical CO₂RR. Such an electrocatalyst combines the advantages of Se optimized active sites, hierarchical pores for facilitating reactant or ion penetration, transport and reaction, and large surface area for more accessible active sites. This Se-CNs electrocatalyst exhibits over 11-times enhanced partial current density of CO than the CNs without Se doping and high selectivity (90%) for CO₂ electroreduction to CO at a low potential of ?0.6 V versus the reversible hydrogen electrode (vs RHE). Density function theoretical calculations reveal that the Se introduction in CNs lowers the free energy barrier of CO₂RR and inhibits hydrogen evolution reaction effectively, thus improving the selectivity for CO₂ reduction to CO. This work presents a new member of the metal-free electrocatalyst family, which is easily prepared, low cost, adjustable, and highly efficient for CO₂RR.

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Selenium-Doped Hierarchically Porous Carbon Nanosheets
as an Efficient Metal-Free Electrocatalyst for CO Reduction
2
Bingxing Zhang, Jianling Zhang,* Fanyu Zhang, Lirong Zheng, Guang Mo, Buxing Han,
and Guanying Yang
their earth abundance and low cost. In
Electrochemical carbon dioxide reduction reaction (CO RR) provides a
2
addition, the high electrical conductivity,
large accessible surface area, multifunc-
tional catalytic activities and structural
tunability at the atomic and morphological
levels make them even more versatile than
promising pathway for both decreasing atmospheric CO concentration and
2
producing valuable carbon-based fuels. To explore efficient and cost-effective
catalysts for electrochemical CO RR is of great importance, but remains chal-
2
lenging. Se-doped carbon nanosheets (Se-CNs) with a micro-, meso-, and
[
11–13]
metal-based catalysts.
Two strategies
macroporous structure are proposed for electrochemical CO RR. Such an elec-
2
are generally adopted toward the reduction
of CO2 on carbon materials, i.e., doping
heteroatoms and constructing advanced
structures. The utilization of different
heteroatoms (e.g., N, P, Si, S, F, B) with
carbon can modulate the charge redistri-
bution among carbon atoms by charge-
transfer from heteroatoms or introduce
trocatalyst combines the advantages of Se optimized active sites, hierarchical
pores for facilitating reactant or ion penetration, transport and reaction, and
large surface area for more accessible active sites. This Se-CNs electrocatalyst
exhibits over 11-times enhanced partial current density of CO than the CNs
without Se doping and high selectivity (90%) for CO electroreduction to CO
2
at a low potential of −0.6 V versus the reversible hydrogen electrode (vs RHE).
Density function theoretical calculations reveal that the Se introduction in CNs
[
14–24]
additional active sites for catalysis.
The construction of carbon nanostruc-
lowers the free energy barrier of CO RR and inhibits hydrogen evolution reac-
2
[
25]
tures (e.g., 0D graphene quantum dot,
tion effectively, thus improving the selectivity for CO reduction to CO. This
2
[
14]
1
D
carbon nanowire,
2D carbon
work presents a new member of the metal-free electrocatalyst family, which is
[
26]
[17]
nanosheet
and membrane,
3D
easily prepared, low cost, adjustable, and highly efficient for CO RR.
[23,27]
2
mesoporous bulk carbon
) is helpful
for electrocatalysis due to the favorable
porosity, more accessible surface and
facile mass transport. Despite these strategies,[ the perfor-
28]
mance of the metal-free carbon electrocatalysts for CO RR
2
1
. Introduction
remain largely limited in view of the sluggish kinetics, high
overpotential, and low selectivity, especially compared with
Electrochemical carbon dioxide reduction reaction (CO RR)
into useful products offers one practical way to deal with the
2
[
12,13]
those of metal-based materials.
It is urgent to develop
increasing fossil fuels consumption and CO overproduc-
the metal-free carbon materials with special heteroatom con-
figurations and advanced carbon structures for electrocatalytic
2
tion.[
1–4]
To date, metal catalysts such as Pd, Ag, Au, Cu and
their alloys remain the most efficient catalysts for electro-
CO RR, to achieve more accessible surface active sites and elec-
trolyte transport capabilities.
2
[5–10]
chemical CO RR.
Developing highly active, stable, and low
2
cost catalysts based on metal-free catalysts for electrochemical
In this work, we demonstrate for the first time the hierar-
chically micro-, meso-, and macroporous carbon nanosheets
(CNs) doped with selenium (Se) for efficient electrochemical
CO RR is more profound and highly rewarding in order to
2
realize their practical application. Metal-free carbon material
catalysts have practical advantages over metal catalysts due to
CO RR. As is known, Se is a nonmetallic chalcogen with a
2
B. Zhang, Prof. J. Zhang, F. Zhang, Prof. B. Han, G. Yang
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Colloid
Interface and Chemical Thermodynamics
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, P. R. China
E-mail: zhangjl@iccas.ac.cn
B. Zhang, Prof. J. Zhang, F. Zhang, Prof. B. Han
School of Chemical Science
University of Chinese Academy of Sciences
Beijing 100049, P. R. China
Prof. J. Zhang, Prof. B. Han, G. Yang
Physical Science Laboratory
Huairou National Comprehensive Science Center
Beijing 101400, P. R. China
Dr. L. Zheng, Prof. G. Mo
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201906194.
Beijng Synchrotron Radiation Facility (BSRF)
Institute of High Energy Physics
Chinese Academy of Sciences
Beijing 100049, P. R. China
DOI: 10.1002/adfm.201906194
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large atom size, high polarizability and rich d-electron, which
has been found to enhance electrical transport properties of
carbon framework. It is expected that Se would have prom-
ising potentials for electrocatalytic applications. A feasible
route of pyrolyzing cheap raw materials of selenium dioxide
Density function theoretical (DFT) calculations reveal that the
Se introduction in CNs lowers the free energy barrier of CO RR
2
[29]
and effectively inhibits hydrogen evolution reaction. The pro-
posed route for Se-CNs synthesis can be easily applied to the
fabrication of other chalcogen-doped CNs, such as Te-CNs and
(Se, Te)-CNs, which all exhibit high performance for electro-
(
SeO ), chitosan, and ammonium chloride (NH Cl) is proposed
2
4
for the synthesis of Se-CNs. Such an electrocatalyst possesses
chemical CO RR to CO.
2
Se optimized active sites toward the reduction of CO to CO,
2
hierarchical pores for facilitating reactant or ion penetration,
transport and reaction, and large surface area for easily acces-
sible active sites. Owing to these unique features, the Se-CNs
electrocatalyst exhibits over 11-times enhanced partial current
density of CO than the CNs without Se doping and high selec-
2
. Results and Discussion
2.1. Designed Synthesis and Characterizations of Se-CNs
tivity (90%) for CO electroreduction to CO at a low potential
The target product was prepared by pyrolyzing raw materials of
2
of −0.6 V versus the reversible hydrogen electrode (vs RHE).
SeO , chitosan and NH Cl. As an example, Figure 1 shows the
2
4
Figure 1. Structural characterizations of Se-CNs. a) SEM, b) STEM, c,d) TEM, e) HRTEM images, f) EDX mapping, g) HAADF-STEM image and
enlarged HAADF-STEM image (inset), and h) intensity profile along the line of 1 and 2 in the HAADF-STEM image. i) N2 adsorption–desorption
isotherm and pore size distribution (inset) of Se-CNs. Scale bars, 0.5 µm in (a), 250 nm in (b), 100 nm in (c), 40 nm in (d), 10 nm in (e), 400 nm in
(f), and 2.5 nm in (g).
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characterizations on the product obtained from the ternary mix-
ture with mass ration of 1:1:3. Scanning electron microscopy
can be assigned to the Se-C coordination at higher shells.[32]
The peaks from Se-O coordination (1.30 Å) of SeO and Se-Se
2
(
SEM) images reveal a geometrically sheet-like nanostructure in
thickness of ≈6 nm with abundant macropores of 80–300 nm
Figure 1a). Scanning transmission electron microscopy (STEM)
coordination (2.07 Å) of Se foil are absent for Se-CNs, which is
consistent with Se 3d XPS spectrum in Figure 2a and suggests
the atomically doped Se in carbon framework. The quantitative
coordination configuration of Se atom from the EXAFS fitting
shows that the Se-C coordination number in the first (Se-C1)
and second (Se-C2) coordination shells are 1.7 and 4.2, respec-
tively (Figure 2e,f, Table S2, Supporting Information). The cal-
culated Se-C1 mean distance is 1.89 Å and Se-C2 mean distance
is 2.86 Å in Se-CNs, which indicates that Se atom occupies the
edge on 6-membered ring of graphene edge. Such a configura-
tion is consistent with those of the reported Se-doping carbon
(
image reveals the formation of a cross-linked macroporous
structure (Figure 1b), which is consistent with the SEM image.
The macropores are surrounded by mesopores in 3–22 nm,
as evidenced by transmission electron microscopy (TEM)
(Figure 1c,d). The lattice surrounding around the mesopore
marked in high-resolution TEM (HRTEM) image (Figure 1e)
corresponds to (002) plane of graphitic carbon, which is con-
sistent with X-ray diffraction (XRD) result (Figure S1, Sup-
porting Information). No extra diffractions of SeO or NH Cl
[
32,33]
materials as revealed by experiment and theory calculation.
2
4
were observed, indicating the complete volatilization of SeO2
The wavelet transform (WT) results give solid support for the
−1
and NH Cl species. Energy dispersive X-ray (EDX) mapping
existence of SeC bonding (with a maximum at 6.1 Å ) in the
Se-CNs, as compared with Se foil (Figure 2g). Soft XANES was
further performed to investigate the local chemical configu-
ration and electronic structure of Se-CNs. N K-edge XANES
spectrum (Figure 2h) shows two N species of pyridinic N at
400.0 eV and graphitic N at 402.4 eV corresponding to π* reso-
nances, and one N species of C-N-C at 408 eV corresponding
4
(Figure 1f) demonstrates a homogeneous distribution of C, N,
and Se elements on the whole porous carbon nanosheet. The
aberration-corrected high-angle annular dark field (HAADF)
STEM images confirm the atomically dispersed Se on carbon
nanosheet (Figure 1g,h). N2 adsorption–desorption isotherm
exhibits a typical mode of type I mixed with type IV (Figure 1i),
which is related to micro- and meso-porous material. The
average micropore size is 1.2 nm, while the mesopore sizes dis-
tribute in the range of 3–23 nm (Figure 1i), which is in accord
with the TEM results. The Se-CNs catalyst exhibits a Brunauer–
[
34]
to σ* resonance. The C K-edge XANES spectrum (Figure 2i)
shows the excitations of π* resonances at 285.8 eV, at 288.8 eV,
and σ* at 291.0–294.0 eV, which are respectively attributed
to CC, CNC (or CSeC), and CC species. The soft
XANES results are consistent with the XPS and Se K-edge
EXAFS results. On the basis of the above results, the atomic
structure of Se-CNs is illustrated in Figure 2j, illustrating the
presence of -Se-C-N- sites on the edge of graphene.
2
−1
Emmett–Teller (BET) specific surface area of 645 m g with a
3
−1
total pore volume of 0.5 cm g (Figure 1i). Raman spectrum
reveals the characteristic D and G bands of Se-CNs, which are
2
associated with disordered carbon atoms and sp hybridized
graphitic carbon atoms,[
14,16,17,19]
respectively (Figure S2, Sup-
porting Information). The Se content is 1.31 wt% determined
by inductively coupled plasma mass spectrometry (ICP-MS).
2.3. Electrochemical Activities for CO Reduction
2
For comparison, two reference samples were synthesized
2
.2. Fine Structure of Se-CNs
by pyrolyzing a chitosan/NH Cl mixture and pure chitosan,
4
respectively. The experimental procedure was same with that
for preparing Se-CNs. The former produced CNs, while the
latter produced carbon block (CB) (see their characterizations
in Figures S5–S6, Supporting Information).
X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray
absorption spectroscopy were performed to characterize the
chemical states and bonding configurations of SeCNs. XPS
survey spectrum (Figure S3, Supporting Information) shows
The electrochemical reduction of CO2 on Se-CNs catalyst
was firstly determined by linear sweep voltammetry (LSV) in a
CO -saturated 0.1 m KHCO solution (Figure 3a). The Se-CNs
the presence of Se, C, N, and no NH Cl species in Se-CNs, con-
4
sisting with above results. Se 3d spectrum (Figure 2a) reveals
2
3
the presence of C-Se-C units at 55.5 eV.[
30,31]
The species of pyri-
catalyst exhibits a current density of 23.6 mA cm (normalized
−2
dinic (398.2 eV), graphitic (400.1 eV), and oxidized (401.7 eV) N
are present in high-resolution N 1s spectrum (Figure 2b, Sup-
porting Information).[ The results suggest the chemically
bonded Se and N in the carbon framework.
by the geometrical surface area) at potential of −0.8 V (vs RHE),
−
2
which is 5.1-fold and 2.8-fold relative to CB (4.6 mA cm ) and
15]
−2
CNs (8.3 mA cm ), respectively. In N -saturated KHCO solu-
2
3
tion, Se-CNs catalyst shows a much smaller current density
To get further information, the Se K-edge X-ray absorp-
tion near-edge structure (XANES) spectrum of Se-CNs as well
as the contrast samples with different oxidation states was
determined. As shown in Figure 2c, the XANES spectrum of
(Figure S7, Supporting Information) than that in CO -saturated
2
KHCO solution. It indicates that the high activity of Se-CNs
3
indeed derives from CO reduction. The electrolysis test shows
2
that CO and H were the only products in gas phase and no
2
Se-CNs lies in between Se foil and SeO , indicating that Se(IV)
liquid products were detected in the potential range from
−0.4 to −0.9 V (vs RHE) (Figure S8, Supporting Information).
The Se-CNs catalyst reaches a maximum CO faradaic effi-
ciency (FECO) of 90% at −0.6 V (vs RHE) and maintains high
FECO values (>80%) in the range of −0.5 to −0.8 V (vs RHE).
Then the FECO drops with more negative potential, which
2
in SeO was almost completely transformed into low valence
2
state (around +1.5) of Se by a reductive interaction during
pyrolysis (Figure 2c; Figure S4, Supporting Information). The
3
k -weighted extended X-ray absorption fine structure (EXAFS)
spectrum (Figure 2d) of Se-CNs exhibits Se-C scattering peak
at 1.48 Å in the first coordination shell. The peaks at 2.2–2.7 Å
can be attributed to the limitation of CO mass transport and
2
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Figure 2. Fine structure of Se-CNs. XPS spectra of a) Se 3d and b) N 1s. c) Se K-edge XANES spectra and fitted average oxidation states of Se in the
insert. d) Se K-edge EXAFS spectra. EXAFS fitting curves in e) R space and f) q space. g) Wavelet transform of Se-CNs (top) and Se-foil (bottom).
h) N K-edge and i) C K-edge XANES spectra. j) Atomic structure of Se-CNs. Se, red; N, blue; C, black.
competition from HER.[2] In a sharp contrast, the CB and CNs
catalysts exhibit FECO values lower than 8% and 44% in the
full potential range from to −0.4 to −0.9 V (vs RHE), respec-
tively (Figure 3b; Figure S9, Supporting Information). Further-
more, the CO partial current density (jCO) of Se-CNs shows a
maximum value of 17.6 mA cm− at −0.9 V, much higher than
those of the two reference electrocatalysts (<1.2 mA cm ,
Figure 3c). The BET surface area normalized turnover fre-
quency (TOF) based on N sites for CO generation of Se-CNs is
that the higher KHCO3 concentration lowers the selectivity
to CO, resulting from the more competitive HER in electro-
lyte with higher concentration. The CO2 reduction test for
19 h at −0.6 V (vs RHE) demonstrates the long-term stability of
Se-CNs (Figure 3d; Figure S12, Supporting Information). Com-
2
[15–27]
pared with the reported electrocatalysts for CO RR to CO,
2
−
2
the performance of the Se-CNs catalyst is outstanding, which
is superior to the transition metal based electrocatalysts and
even comparable to Au and Pt electrocatalysts (Table S 3, Sup-
porting Information).
much higher than that of CNs, while the TOF for H produc-
2
tion is lower than that of CNs (Figure S10, Supporting Infor-
mation). It indicates that Se atom constitutes the catalytically
active site for promoting the selectivity of CO production. In
2.4. Reaction Mechanism
0
.5 m KHCO solution, Se-CNs can achieve a total current den-
3
−
2
sity of 14.7 mA cm at −0.6 V (vs RHE) with FECO of 78%
Figure S11, Supporting Information). As compared with the
To uncover the CO RR kinetic origin of Se-CNs, the elec-
2
(
trochemical active surface area (ECSA) was measured.
According to the double-layer capacitance measurements, in
results in CO -saturated 0.1 m KHCO solution, it is evident
2
3
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Figure 3. CO RR performance on Se-CNs and control samples. a) LSV curves of different catalysts (in a CO -saturated 0.1 m KHCO solution, scanning
2
2
3
−
1
rate: 10 mV s ). b) FECO at various applied potentials. c) Potential-dependent CO partial current density. d) Durability test. e) Charging current density
differences plotted against scan rates. f) EIS spectra. g) Schematic illustration of the CO RR kinetic origin of Se-CNs.
2
which the slope could be a reference of ECSA[35] (Figure 3e;
Figure S13, Supporting Information), the ECSA for Se-CNs
is roughly ≈15 and ≈30 times higher than those of CNs and
CB, respectively. It clearly indicates that the Se-CNs cata-
lyst can afford much more catalytically active sites than the
other two catalysts. Electrochemical impedance spectroscopy
electrolysis, leading to remarkably enhanced activity for
CO reduction. This is consistent with the lower Tafel slope
2
(Figure S15, Supporting Information) of Se-CNs catalyst
−
1
−1
(79 mV dec ) than that of CNs (168 mV dec ), suggesting
the faster electron transfer for CO reduction on Se-CNs. The
2
high current density of Se-CNs can be attributed to the hier-
archical micro-, meso-, and macro-pores on carbon nanosheet
(Figure 3g), which expose much more active sites and act as
effective channels for reactant or ion penetration, transport,
(
EIS) demonstrates that Se-CNs electrocatalyst shows much
smaller interfacial charge-transfer resistance during CO2
reduction process (Figure 3f). It suggests that Se-CNs elec-
trocatalyst owns much faster charge-transfer process during
[
36]
and reaction.
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Figure 4. DFT calculation for CO reduction to CO. PDOS of a) Se-CNs and b) CNs. c) Energy profiles and intermediate configurations with the free
2
energy change pathways for CO reduction on Se-CNs (blue) and CNs (red). The bold curves indicate the lowest energy path for CO RR. Se, gold; N,
2
2
blue; C, gray; O, red; H, white.
To further gain insights into the CO RR selectivity
mechanism for Se-CNs electrocatalyst, DFT calcula-
tions were performed. Graphene slabs composed of edge
Se-C-N- configurations (Figure S14, Supporting Information)
were established according to above characteristic results.
Supporting Information). Compared with the doped N leading
2
[
11]
to an enhanced electronic/ionic conductivity of carbon, the
extra Se doping may adjust the active site with higher spin
density, edge strain, and charge delocalization due to its larger
atomic size and greater polarizability. The hydrogen adsorp-
-
The edge Se configuration was obtained by XPS and EXAFS
tion and CO adsorption as well as the intermediate process
2
analysis.[
30,31,33]
The pyridine N evidenced by XPS analysis
are all considered in CO RR process (Figures S16–S22 and
2
was adopted due to its primary activity both in HER and
Table S4, Supporting Information). The free energy profiles
[
11,20,37–40]
CO RR.
Considering that the competitive hydrogen
and corresponding optimized configurations of CO RR
2
2
evolution reaction (HER) is a major hurdle for selectivity of
catalyzed by Se-CNs and CNs are shown in Figure 4c. The
Se-CNs catalyst obviously presents stronger CO2 adsorption
rather than hydrogen adsorption, benefiting from above Se
optimized electronic structures of active site. In contrast, the
CNs catalyst shows much stronger hydrogen adsorption. The
lowest free energy change pathways for CO2 reduction show
that the Se-CNs catalyst presents smaller free energy barrier of
0.32 eV than that of CNs (0.63 eV), thus making more efficient
[
37–39]
CO RR,
the suitable electronic state of active site should
2
be more beneficial for bonding CO2 rather than hydrogen
adsorption. Previous works have shown that the edge of gra-
[
40]
phene often has strong hydrogen adsorption property, thus
the proper adjustment for its electronic structures is needed.
The partial density of states (PDOS) in Figure 4a,b show that
the Se introduction into CNs structure can partly reduce the
PDOS close to Fermi level, compared with the solely pyri-
dinic N site in the edge of CNs. It can also be supported by
the result of N K-edge XANES spectra that the pyridinic N
resonance shifts to higher energy for Se-CNs (Figure S15,
CO RR to CO on Se-CNs. Meanwhile, HER process is remark-
2
ably inhibited on Se-CNs. The protonation barrier of *CO to
2
*COOH is surmountable with 0.32 eV, while it is much higher
with 0.76 eV for *H formation, indicating high CO selectivity
2
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for Se-CNs over the HER in CO electroreduction. All of these
2
results show that Se-CNs catalyst has much better activity and
selectivity for the electrocatalytic CO RR to CO.
2
2
.5. Formation Mechanism of Se-CNs
The above results prove that the as-synthesized Se-CNs elec-
trocatalyst exhibits high-performance for CO RR by a simple
2
pyrolyzing route for raw materials of SeO , chitosan, and
2
NH Cl. To get information on the underlying mechanism for
4
the formation of the highly porous carbon nanosheets doped
with Se, a series of experiments were carried out. First, the other
Se-CNs samples were synthesized by varying the mass ratio of
SeO to chitosan and hierarchically porous carbon nanosheets
2
were also obtained (Figure S23, Supporting Information).
Interestingly, the samples show a void space enlargement with
increasing SeO2 amount according to the small-angle X-ray
scattering (SAXS) results[41–43] (Figure S24, Supporting Infor-
mation), implying SeO plays a key role for the hierarchically
2
porous structure formation. To verify this, a mixture of chitosan
and NH Cl with mass ratio of 1:3 (in absence of SeO ) was pyro-
4
2
lyzed at the experimental condition similar to that for Se-CNs
synthesis. The carbon nanosheets were produced, but are non-
porous (Figure S5, Supporting Information). It confirms that
the formation of pores in carbon nanosheet is dominated by a
self-templating effect of SeO , while the nanosheet structure is
2
dependent of NH Cl. It is also proved by a control experiment
4
that the pyrolysis of only chitosan derived carbon block with
an irregular morphology (Figure S6, Supporting Information).
Based on the above results, a mechanism for the formation of
Se-CNs is illustrated in Figure 5. By pyrolyzing, the SeO tem-
Figure 5. Schematic illustration for the formation of Se-CNs.
2
plating surrounded by carbon sublimates as the temperature
is higher than its volatility temperature (685 °C), resulting in
the formation of porous structure and simultaneous doping of
By combination of the results of Se-CNs, it is found that the
FECO decreases in order of Se-CNs > (Se, Te)-CNs > Te-CNs
under similar experimental conditions. Obviously, the more
electronegative the chalcogen, the higher selectivity of the
Se atoms into carbon framework. For NH Cl, since it decom-
4
poses into NH and HCl during pyrolysis process, the abundant
3
released gas with instantaneous high internal pressure favors
the exfoliation of carbon matrix to form carbon nanosheets.
Therefore, the pyrolysis of chitosan together with SeO2 and
CO RR toward CO. It can be explained by the more efficiency
2
[44]
of highly electronegative chalcogen in decreasing the PDOS
[
45]
close to Fermi level in CNs, which would increase the ability
to inhibit HER according to above DFT calculation results.
NH Cl produces the highly porous carbon nanosheet doped
4
with Se. The Se-CNs with different Se contents (Figure S23, Sup-
porting Information) were further tested for CO RR. The results
2
show that the FECO is increased with the increasing Se content
in Se-CNs (Figure 3b,c; Figure S25, Supporting Information).
3
. Conclusions
In summary, we demonstrate a strategy to construct the Se-
doped carbon nanosheets with a hierarchically micro-, meso-,
and macro-porous structure by using cheap raw materials of
SeO , chitosan and NH Cl. Such an electrocatalyst combines
2
.6. Other Chalcogen-Doped CN Electrocatalysts
2
4
The proposed route for synthesizing Se-CNs was expanded
to the fabrication of other chalcogen-doped CNs. The Te-CNs
catalyst with a hierarchically porous structure was obtained
the advantages of highly exposed active sites, facilitated ion
penetration and transport, and favorable electronic structure for
CO RR toward CO. The as-synthesized Se-CNs catalyst exhibits
2
(Figure S26, Supporting Information). It exhibits high activity
over 11-times enhanced partial current density of CO than CNs
and selectivity for electrocatalytic CO RR toward CO, e.g., FE
without Se doping and high selectivity (90%) for CO electrore-
2
CO
2
of 86% at −0.6 V (vs RHE) (Figure S27, Supporting Informa-
tion). Moreover, the route can be easily applied to the synthesis
of hierarchically porous CNs doped by mixed heteroatoms such
as (Se, Te)-CNs catalyst, of which the FECO value reaches 88%
at −0.6 V (vs RHE) (Figures S28–S29, Supporting Information).
duction at relatively low potential. The feasible route is versatile
in producing other chalcogen-doped CN electrocatalysts (e.g.,
Te-CNs and (Se, Te)-CNs) for CO RR. We anticipate that the
2
chalcogen-doped CN electrocatalysts are applicable not only for
CO RR but for other electrocatalytic reactions and battery field.
2
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with a scan rate of 10 mV s−1. ECSA referred the CV results under the
potential windows of 0.21–0.11 V (vs RHE). EIS measurements were
carried out with 5 mV amplitude in a frequency range from 100 KHz
to 100 mHz. For the faradaic efficiency analysis, gas products were
detected by gas chromatograph (GC, HP 4890D), which was equipped
with FID and TCD detectors using helium as the internal standard. The
4
. Experimental Section
Materials: Chitosan and NH Cl (99%) were purchased from
4
Sinopharm Chemical Reagent Co., Ltd. Potassium bicarbonate (99%),
SeO (99%), TeO (99.99%), sulfur powder (A. R. grade), Nafion D-521
dispersion (5% w/w in water and 1-propanol, ≥0.92 meg g exchange
capacity), Nafion N-117 membrane (0.180 mm thick, ≥0.90 meg g
2
2
−
1
−
1
1
liquid product was analyzed by H NMR on Bruker AVANCE AV III 400.
exchange capacity) and Toray Carbon Paper (CP, TGP-H-60, 19 × 19 cm)
were purchased from Alfa Aesar China Co., Ltd. Ethanol (A. R. grade),
sulphuric acid (98%), CO2 (>99.999%) and deionized water were
provided by Beijing Analysis Instrument Factory. All chemical reagents
were used as received without further purification.
Computational Details for Calculations: The structural optimization
and transition states were calculated using DMol3 code. The generalized
gradient approximation with the Perdew–Burke–Ernzerhof functional
and all-electron double numerical basis set with polarized function
were employed. The convergence tolerance of energy, maximum force
−
5
−3
−1
Preparation of Se-CNs: Typically, a mixture of chitosan, SeO , and
and maximum displacement are 1.0 × 10 Ha, 2.0 × 10 Ha Å , and
3
2
−
NH Cl with mass ration of 1:1:3 was first ground into a homogeneous
5.0 × 10 Å (1 Ha = 27.21 eV) for geometry optimization. Each atom
in the storage models is allowed to relax to the minimum in enthalpy
without any constraint. The vacuum space along the y and z direction
is set to be 15 Å, which is enough to avoid interaction between the two
neighboring images. The COOH, H, or CO groups were adsorbed on the
surface of catalyst. The reaction free energy for each species is calculated
according to the standard formula
4
powder, followed by a two-stage pyrolysis process (first stage: heating
−
1
from 20 to 300 °C at a ramping rate of 5 °C min and maintaining at
3
00 °C for 0.5 h; second stage: heating from 300 to 1000 °C at a ramping
−
1
rate of 5 °C min and maintaining at 1000 °C for 1 h) in a tubular
furnace under N flow with NaOH aqueous solution as tail gas absorber.
2
After cooling down to room temperature, the final product was obtained.
Preparation of Te-CNs and (Se, Te)-CNs: Te-CNs sample was
synthesized by pyrolyzing chitosan, TeO , and NH Cl with a mass ration
2
4
∆G = ∆E + ∆EZPE −T∆S
(1)
of 1:1:3. (Se, Te)-CNs sample was synthesized by pyrolyzing chitosan,
SeO , TeO , and NH Cl with a mass ration of 1:0.5:0.25:3. The other
where ∆E denotes the change of energy, ∆EZPE and ∆S are the changes
of zero-point energy and entropy, and the temperature T is set to 300 K.
2
2
4
experimental procedures are the same with that for synthesizing Te-CNs
except that the final product was leached with hydrochloric acid aqueous
solution.
Control Sample Synthesis: The CNs sample was synthesized by
Supporting Information
pyrolyzing chitosan/NH Cl. The CB sample was synthesized by
4
pyrolyzing pure chitosan. The experimental procedure was the same
with that for preparing Se-CNs.
Characterizations: The morphologies of the samples were observed
on field emission SEM (HITACHI S-4800), TEM (JEOL JEM-1011), field
emission TEM (JEOL JEM-2100F). The HAADF-STEM characterization
Supporting Information is available from the Wiley Online Library or
from the author.
was performed on a JEOL JEM-ARF200F TEM/STEM with a spherical Acknowledgements
aberration corrector. XRD was determined by a Rigaku D/max-2500
The authors thank the financial supports from National Natural Science
Foundation of China (21525316, 21673254), Ministry of Science and
Technology of China (2017YFA0403003), Chinese Academy of Sciences
diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) at 40 kV
and 200 mA. The porosities were determined by an Autosorb-iQ system
at 77 K using liquid nitrogen as coolant. The materials were degassed at
(QYZDY-SSW-SLH013), and Beijing Municipal Science & Technology
1
50 °C for 4 h before the measurements. Raman spectrum was
Commission (Z181100004218004).
obtained on laser confocal Raman spectroscopy (Labram-010,
a
Horiba-JY) employing the Nd: YAG laser wavelength of 633 nm). XAFS
measurement for Se K-edge was carried out from fluorescence mode
at room temperature at the 1W1B beamLine at Beijing Synchrotron
Radiation Facility (BSRF). Data of XAFS were processed using the Athena
and Artemis programs of the IFEFFIT package based on FEFF 6. Prior
to merging, the spectrum was aligned to the first and largest peak in
the smoothed first derivative of the absorption spectrum, background
Conflict of Interest
The authors declare no conflict of interest.
2
removed, and normalized. Data were processed with k -weighting and an
Keywords
Rbkg value of 1.0. Merged data sets were aligned to the largest peak in the
first derivative of the adsorption spectrum. Normalized µ(E) data were
obtained directly from the Athena program of the IFEFFIT package. SAXS
experiment was carried out at Beamline 1W2A at BSRF. The wavelength
was 1.38 Å and the distance of sample to detector was 2.10 m.
CO2 reduction, electrocatalysts, hierarchical pores, metal-free catalysts,
selenium
Received: July 29, 2019
Revised: October 7, 2019
Published online:
Electrochemical Test: The catalysts and 5 wt% Nafion solution were
ultrasonically mixed with 100 µL of ethanol to form inks. The work
electrode was obtained by dropping the catalyst ink on the surface of the
−
2
CP (catalyst loading of 1 mg cm ). Electrocatalytic CO RR was evaluated
2
in an H-type electrochemical cell with three electrode system in CO2-
saturated 0.1 m KHCO3 electrolyte. Pt gauze and Ag/AgCl (3.5 m KCl)
were used as the counter electrode and reference electrode, respectively.
The working and reference electrodes were placed in the cathode
chamber, while the counter electrode was placed in the anode chamber,
which was separated by a piece of Nafion 117 ionic exchange membrane
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1906194 (8 of 9)
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Adv. Funct. Mater. 2019, 1906194
1906194 (9 of 9)
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim