Achieving electroreduction of CO2 to CH3OH with high selectivity using a pyrite-nickel sulfide nanocomposite

Article abstract of DOI:10.1039/c6ra26868d

Electrochemical reduction of carbon dioxide (CO₂) to methanol (CH₃OH) catalyzed by transition metals has been proved feasible and effective in aqueous electrolytes. In this work, we introduce a FeS₂/NiS nanocomposite electrocatalyst synthesized by traditional hydrothermal method, which selectively reduces CO₂ to CH₃OH with an unprecedented overpotential of 280 mV and a high faradaic efficiency up to 64% at the potential of ?0.6 V vs. reversible hydrogen electrode (RHE). The FeS₂/NiS nanocomposite electrocatalyst exhibits a stable current density of 3.1 mA cm^?2 over a 4 hour stability test. The high selectivity towards CO₂ electroreduction to CH₃OH may be attributed to the special ladder structure of the FeS₂/NiS nanocomposite. The active sites are located at the interface between FeS₂ and NiS which can effectively suppress the side reaction hydrogen evolution reaction and facilitate the CO₂ reduction reaction.

Full text of DOI:10.1039/c6ra26868d

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Achieving electroreduction of CO2 to CH3OH with
high selectivity using a pyrite–nickel sulfide
nanocomposite†
Cite this: RSC Adv., 2017, 7, 1376
*
*
Siqi Zhao, Sijie Guo, Cheng Zhu, Jin Gao, Hao Li, Hui Huang, Yang Liu
*
and Zhenhui Kang
Electrochemical reduction of carbon dioxide (CO2) to methanol (CH3OH) catalyzed by transition metals has
been proved feasible and effective in aqueous electrolytes. In this work, we introduce a FeS2/NiS
nanocomposite electrocatalyst synthesized by traditional hydrothermal method, which selectively
reduces CO2 to CH3OH with an unprecedented overpotential of 280 mV and a high faradaic efficiency
up to 64% at the potential of ꢀ0.6
V vs. reversible hydrogen electrode (RHE). The FeS2/NiS
nanocomposite electrocatalyst exhibits a stable current density of 3.1 mA cmꢀ2 over a 4 hour stability
test. The high selectivity towards CO2 electroreduction to CH3OH may be attributed to the special
ladder structure of the FeS2/NiS nanocomposite. The active sites are located at the interface between
FeS2 and NiS which can effectively suppress the side reaction hydrogen evolution reaction and facilitate
the CO2 reduction reaction.
Received 16th November 2016
Accepted 6th December 2016
DOI: 10.1039/c6ra26868d
www.rsc.org/advances
CH3OH, as an ideal chemical, is an important intermediate
of paint, plastics, and other common products.15 Besides,
Introduction
CH3OH with high energy density can be stored as liquid under
ambient conditions.16,17 The standard potential of CO2 electro-
reduction to CH3OH is only 0.016 V (vs. RHE). However, the 6 eꢀ
process of CO2 reduction to form CH3OH over the full reaction
is kinetically unfavorable. Signicant efforts towards selectively
converting CO2 into CH3OH have been made since early 1983
over semiconductor materials (p-GaP and p-GaAs) with a low
current density (<1 mA cmꢀ2).18 Lately, Frese et al. rstly
observed Teon-supported Ru electrodes could selectively
reduce CO2 to CH3OH with a low faradaic efficiency (FE) of
42%.19 Fe,20 Ni,21 Cu22 and their associated complexes23–26 have
been widely investigated in the yield of CO2RR as their rich
distribution and low cost. Among them, copper is demonstrated
as the effective catalyst for the electroreduction of CO2 to
hydrocarbon and alcohols.27,28 Le et al. reported that electro-
deposited cuprous oxide thin lms could directly reduce CO2 to
CH3OH with a rate of 43 mmol cmꢀ2 hꢀ1 and low FE of 38%.29
Therefore, it is urgent to seek for a highly active, selective and
effective catalyst towards electroreduction CO2 to CH3OH.
Herein, we fabricated a low-cost FeS2/NiS nanocomposite by
traditional hydrothermal method as an excellent electrocatalyst.
FeS2/NiS nanocomposite displays incomparable operation in its
low overpotential of 280 mV and high selectivity with a CH3OH
FE up to 64% at the potential of ꢀ0.6 V (vs. RHE). The stability
test of FeS2/NiS nanocomposite was performed for 4 hours,
showing a stable current density of 3.1 mA cmꢀ2 at ꢀ0.6 V (vs.
RHE). There is no obvious degradation of the electrocatalyst
aer the long-time test. The following experiments reveal the
Electrochemical reduction of CO2 to carbon-containing fuels is
a feasible process which is conducive to solving the serious
environmental problems caused by excess CO2 such as iceberg
melting, sea level rise and coastal delta subsidence.1–3 However,
the main defects of electroreduction CO2 are the high over-
potential required to drive the reaction, the low selectivity
towards various products and the high cost of catalysts.4,5 The
high overpotential increases the consumption of energy and
makes it difficult to achieve the sustainable transformation of
CO2. As for the selectivity, in addition to the major side reaction
hydrogen evolution reaction (HER), the direct electroreduction of
CO2 in aqueous solution is able to generate diversiform carbon-
containing chemicals.5 The complex products increase the chal-
lenges towards generating target product. Transition metals are
commonly proposed in CO2 reduction reaction (CO2RR) because
of their vacant orbits and active d electrons.5 Among them, Au,6–9
Ag,1,10,11 and Pd12,13 have been explored comprehensively, while
the expensive price limits the substantial application of noble
metals in industry.14 Moreover, the main product from noble
metals is carbon monoxide (CO), which leads to the second
pollution at the cost of CO2 consumption.
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of
Functional Nano & So Materials (FUNSOM), Soochow University, 199 Ren'ai Road,
Suzhou, 215123, Jiangsu, PR China. E-mail: zhkang@suda.edu.cn; yangl@suda.edu.
cn; hhuang0618@suda.edu.cn
† Electronic supplementary information (ESI) available: The supplementary
gures are shown. See DOI: 10.1039/c6ra26868d
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insight mechanism of CO2RR catalyzed by FeS2/NiS nano-
composite. As a comparison, we synthesized the single FeS2 and
NiS nanocrystals and applied in CO2RR, respectively. The FeS2
nanocrystal shows a negative onset potential at ꢀ0.45 V (vs.
RHE) and a maximum current density of 4.2 mA cmꢀ2 at
ꢀ0.68 V (vs. RHE). The NiS nanocrystal shows a more negative
onset potential at ꢀ0.5 V (vs. RHE) and extremely low current
density of 1.0 mA cmꢀ2 at ꢀ0.68 V (vs. RHE). Therefore, it can
speculate that the active sites of the catalyst towards the process
locate at the interface of FeS2 and NiS. Besides, the FeS2/NiS
nanocomposite with an average diameter of 14 nm greatly
increases the specic surface areas and the number of active
sites.
FeS2 and NiS nanocrystals. The preparation of FeS2 nano-
crystal comes from the same hydrothermal method as FeS2/NiS
nanocomposite. 0.8 g Fe2O3 (5 mmol) and 1.6 g sulfur powder
(50 mmol) were dissolved in 30.0 ml 1-octylamine and 30.0 ml 1-
octanol at room temperature. Then, the mixture was transfer
into a 150 ml stainless steel autoclave. The autoclave was sealed
ꢁ
and heated to 260 C for 3 hours under nitrogen atmosphere.
The NiS nanocrystal was synthesized with the same hydro-
thermal method while changed the ratio of Ni(Ac)2 and sulfur
powder to 1 : 1.5.
4. Electrochemical measurements
The electrocatalytic activity measurements were performed in
a standard three-electrode system. Saturated calomel electrode
(SCE) was acted as the reference electrode with a standard
electrode potential of 0.242 V (vs. RHE) and platinum wire was
acted as the counter electrode. The working electrode was the
catalyst modied glassy carbon disk electrode (GCE, 3.0 mm
diameter CH Instruments) and 0.5 M KHCO3 solution saturated
with CO2 was used as electrolyte. The cyclic voltammetry and
linear sweep voltammetry were performed at a range from 0.7 to
ꢀ0.7 V (vs. RHE) by a Model CHI 660C workstation under
ambient condition. All the potentials reported here were versus
Hg/HgCl2 and were converted to the RHE scale using the Nernst
equation:
Experiments
1. Materials
Ferric oxide (Fe2O3, 99%), nickel nitrate (Ni(Ac)2, 99%), sulfur
powder (99%), 1-octanol (99%), 1-octylamine (99%), ethanol
(99.5%), KHCO3 (99.7%) and Naon peruorinated resin solu-
tion (5 wt%) were purchased from Sigma-Aldrich and Adamas-
beta; Naon® 212 membrane was purchased from Dupont.
Deionized water (puried by a Milli-Q system) was used to
prepare all solutions and to rinse samples and glassware.
2. Instruments
ERHE ¼ EHg=HgCl þ EHq g=HgCl þ 0:0591 ꢂ pHelectrolyte
2
2
The crystal structure of the catalyst was measured by X-ray
diffraction (XRD) using an X'Pert-ProMPD (Holand) D/max-
gAX-ray diffractometer with Cu Ka radiation (l ¼ 0.154178 nm).
The high-resolution transmission electron microscopy
(HRTEM) and scanning transmission electron microscopy
(STEM) images were obtained using a FEI/Philips Tecnai G2 F20
TWIN transmission electron microscope. The Raman spectra
were acquired with an HR 800 Raman spectroscope (J Y, France)
equipped with a synapse CCD detector and a confocal Olympus
microscope. X-ray photoelectron spectroscopy (XPS) was
measured using a KRATOS Axis ultra-DLD X-ray photoelectron
spectrometer with a monochromatised Mg Ka X-ray source (hn
¼ 1283.3 eV). Scanning electron microscopy (SEM) images and
energy dispersive X-ray (EDX) spectroscopy were performed by
Carl Zeiss Supra 55 scanning electron microscope with an
acceleration voltage of 20 kV. The electro-catalysis activities
were measured by a Model CHI 660C workstation (CH Instru-
ments, Chenhua, China).
ERHE is the converted potential versus RHE. EHg/HgCl is the
2
external potential measured against the Hg/HgCl2 reference
electrode. EHq g=HgCl is the standard potential of Hg/HgCl2 at
2
ꢁ
25 C (0.242 V).
5. CO2 reduction measurements and products
quantication
The CO2 reduction measurements were performed in an airtight
electrochemical H-type cell under ambient temperature. The
cathode compartment was consist of working electrode and
reference electrode, while the anode compartment was
composed of platinum wire. Apply dots of FeS2/NiS nano-
composite catalyst (dissolved in 0.5% Naon) to modied
carbon ber paper electrode (0.7 cm ꢂ 0.7 cm), then connect
the carbon ber paper to the cathode electrode. Each of the
compartments loaded 75.0 ml 0.5 M KHCO3 saturated with CO2
and 40.0 ml carbon dioxide in the headspace. There is a slice of
proton exchange membrane (Naon® 212) at the connector to
separate the two compartments in case the electroreduction
3. Synthesis of catalyst
FeS2/NiS nanocomposite. In
a
typical hydrothermal products diffused to the anode. The electrolytic measurements
method,30 Fe2O3 and Ni(Ac)2 were introduced as iron and nickel were carried out under different potentials for 1 hour, respec-
sources, respectively. The ratio of nickel to iron is 1 : 1. Fe2O3 tively. The durability test was measured in an open electrolytic
(2.5 mmol), Ni(Ac)2 (2.5 mmol) and sulphur (50 mmol) were cell with 0.5 M KHCO3 solution (saturated with CO2) for 4 hours
dissolved in 30.0 ml 1-octylamine and 30.0 ml 1-octanol at room at ꢀ0.6 V. During the measurement, carbon dioxide was
temperature. Then, the mixture was transferred into a 150 ml bubbled into water continuously with a uniform velocity. The
stainless steel autoclave and heated to 260 ꢁC for 3 hours under carbon-contained gas products (CO, CH4, C2H4, and C2H6) and
nitrogen atmosphere. When cooled to room temperature, the major by-product (H2) were tested by a thermal conductivity
black precipitate was collected by centrifugation and thor- detector (TCD) with helium as the carrier gas. The liquid phase
oughly washed with ethanol for several times.
products were qualied by a NMR (Bruker AVANCEAV III 400)
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spectroscopy, in which 0.5 ml electrolyte was mixed with 0.1 ml STEM image and corresponding chemical mappings of Fe–K,
D2O (deuterated water) and 0.05 ml dimethyl sulfoxide (DMSO, Fe–L, Ni–K, Ni–L and S–K for FeS2/NiS nanocomposite. A
Sigma, 99.99%) was added as an internal standard.
The calculation of faradaic efficiency:
For CH3OH,
homogeneous distribution of Fe, Ni, and S is reviewed in the
nanocomposite. Furthermore, the HRTEM image concludes
that the germination of FeS2/NiS nanocomposite arranges inti-
mately rather than the simple physical mixture of the two
compounds. Thus, we proposed that the active sites of CO2
electroreduction locate at the interface between FeS2 (200) and
NiS (100).
6F ꢂ nCH OH
3
FECH OH
¼
ꢂ 100%
3
I ꢂ t
For H2,
In order to identify the crystallinity and structure of the FeS2/
NiS nanocomposite, the XRD measurements were performed.
Fig. 2a shows the XRD patterns of FeS2/NiS nanocomposite
(brown trace), FeS2 standard card (blue trace) and NiS standard
card (red trace). The phanic peaks can be completely matched to
the FeS2 (JCPDS no. 42-1340) and NiS (JCPDS no. 02-1280)
standard diffraction peaks. In details, the predominant peaks at
46.0, 53.7, 30.2 and 34.7 degree correspond to the (100), (202),
(101) and (102) planes of NiS, respectively. The peaks at 33.0,
56.4, 37.1 and 47.5 degree are assigned to the (200), (311), (210)
and (211) planes of FeS2, respectively. There are no impurity
peaks from other crystal structures. The average size of FeS2/NiS
nanocomposite is ꢃ14 nm from the line width analysis of the
diffraction peak calculated by Scherer equation, which is
consistent with the SEM observation. FeS2/NiS nanocomposite
was further characterized with Raman spectrum and showed in
Fig. 2b. The peaks at 340 and 378 cmꢀ1 can be well attributed to
Raman vibrations of pyrite FeS2 and no other impurity peaks
from marcasite and troilite.30 The peaks at 222 and 285 cmꢀ1 are
matching with NiS completely.32 The peaks assigned to NiS at
335 and 376 cmꢀ1 are not emerged in the diagram obviously,
which is resulted by the overlap with the peaks of pyrite FeS2.
The XPS measurement was introduced to conrm the
element proportion and valence state of the FeS2/NiS nano-
composite. Besides the Fe, Ni and S elements from FeS2/NiS
nanocomposite, C and O elements are also detected in the full
spectrum (as shown in Fig. 3a). The elements of C and O may
attribute to the carbonization of the solvents. The high resolu-
tion spectra of C 1s and O 1s are shown in Fig. S1.† The binding
energy at 284.8, 286.4 and 288.8 eV of C 1s are consistent with
graphite carbon, C–OH and C]O, respectively.33 The O 1s peaks
at 531.9 and 532.9 eV from oxygen atoms are attributed to C]O
and O–C, respectively.34 The carbonized solvents with oxygenic
functional groups may not only improve the stability of the
2F ꢂ nH
2
FEH
¼
ꢂ 100%
2
I ꢂ t
where F is the Faraday constant, nCH OH is the moles of
produced CH3OH, and nH for the produced H2.
3
2
Results and discussion
To explore the surface morphology of the FeS2/NiS nano-
composite, the SEM measurement was carried out as shown in
Fig. 1a. The acquired products are made up of uniform and even
nanoparticles with an average diameter of approximately
14 nm. The small and even size of the electrocatalyst conrms
high specic surface area and incremental active sites, which
effectively improve the catalysis activity. The HRTEM image of
FeS2/NiS nanocomposite is shown in Fig. 1b. The characteristic
lattice spacing of 0.27 nm is corresponding to the (200) plane of
FeS2 whose XRD peak is located at 2q ¼ 33.0.30 The lattice
spacing of 0.29 nm is indexed to the (100) plane of the NiS
which matches to the XRD peak at 2q ¼ 46.0.31 Fig. 1c shows the
Fig. 1 (a) The SEM image of FeS2/NiS nanocomposite. (b) The HRTEM
image of FeS2/NiS nanocomposite. (c) STEM image and corresponding Fig. 2 (a) The large-angle XRD patterns of FeS2/NiS nanocomposite
chemical elements mappings of Fe–K, Fe–L, Ni–K, Ni–L and S–K of (brown trace), FeS2 (blue trace) and NiS (red trace). (b) Raman spec-
FeS2/NiS nanocomposite.
trum of FeS2/NiS nanocomposite.
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and FeS2/NiS were measured in CO2-saturated 0.5 M KHCO3
solution. As showed in Fig. 4b, both the single FeS2 and NiS
nanocrystals show poor activities towards CO2RR compared
with FeS2/NiS nanocomposite, in which the FeS2 shows an onset
potential at ꢀ0.45 V for HER and the maximum current density
of 4.2 mA cmꢀ2 at ꢀ0.68 V. While the NiS nanocrystal performs
even worse, which shows an extremely low current density of 1
mA cmꢀ2 at ꢀ0.68 V (red trace). It is generally known that FeS2 is
feasible towards HER.38,39 For FeS2/NiS nanocomposite (black
trace), more positive onset potential at ꢀ0.3 V as well as higher
current density of 7.8 mA cmꢀ2 can be achieved towards CO2RR.
To further compare the electrocatalytic performance of FeS2
nanocrystal, NiS nanocrystal and FeS2/NiS nanocomposite for
CO2 reduction, the electrolytic reduction reaction of CO2 was
carried out in an electrochemical airtight H-type cell at the
potential range from ꢀ0.3 to ꢀ0.7 V. The gas products were
detected by gas chromatography (GC). The liquid-phase prod-
ucts were detected by 1H NMR and the DMSO was added as an
internal standard. As shown in Fig. S5,† the reduced products
are CH3OH aer CO2 electroreduction catalyzed by FeS2/NiS
Fig. 3 (a) The full XPS spectrum of FeS2/NiS nanocomposite and the
high-resolution XPS spectra of (b) S 2p, (c) Fe 2p and (d) Ni 2p.
catalyst but also act as a support.35 The high resolution scan nanocomposite for 2 hours and 4 fours. For FeS2 and NiS
spectra of the Fe 2p, Ni 2p and S 2p are presented in Fig. 3b–d. nanocrystals, H2 is the only product in gas phase and no
The atomic ratio of Fe to Ni is equal to 2 according to the XPS hydrocarbon products are detected in both gas and liquid
calculation. The result is consistent with the EDX spectroscopy phases. As shown in Fig. 4c, the FEs vs. the applied potentials
measurement (as shown in Fig. S2†). The binding energy at (ꢀ0.5, ꢀ0.6, ꢀ0.7 V) of H2 for FeS2 and NiS nanocrystals show
706.8 and 719.5 eV are conformed to the Fe 2p3/2 and Fe 2p1/2 a stable tendency at different potentials. For FeS2/NiS nano-
which are characterized of pyrite.36 The Ni 2p3/2 and Ni 2p1/2 composite, the FEs vs. the applied potentials (ꢀ0.3, ꢀ0.4, ꢀ0.5,
peaks are observed at 853.6 and 871.8 eV, exist two satellite ꢀ0.6, ꢀ0.7 V) for CH3OH (le axis) and H2 (right axis) are shown
peaks at 858.7 and 878.1 eV respectively,32 indicating that the in Fig. 4d. The CH3OH FEs show an overall growth tendency at
nickel existed in bivalent states. There are four peaks in the high the range from ꢀ0.3 V to ꢀ0.6 V but reach a plateau of 64%
resolution XPS spectra of S. The S 2p3/2 and S 2p1/2 peaks located approximately at ꢀ0.6 V. Aer that, it maintains a stable
at 161.9 and 162.8 eV are consistent with S2ꢀ
,
31 while the peaks tendency. We further compared the FEs for CH3OH in the
at 162.5 and 163.7 eV are identied with the S 2p3/2 and S 2p1/2 literatures and showed in Table S1.† To reach the same FE of
These results indicate that both S2 and S2ꢀ are
2ꢀ
2ꢀ
37
of S2
.
existed. Thus, we integrate the peak areas respectively and
acquire the ratio of S22ꢀ to S2ꢀ is 2, which is also consistent with
the previously calculated ratio of iron to nickel.
In typical experiments, the electrolytic activities were
measured in a three-electrode system in 0.5 M KHCO3 aqueous
solution. The curves of Linear Sweep Voltammetry (LSV) for the
FeS2/NiS nanocomposite are shown in Fig. 3a. The black curve
shows the catalytic activity for HER under N2 atmosphere (pH ¼
8.5), which presents a high overpotential over 450 mV and a low
current density of 2.5 mA cmꢀ2 with the applied potential up to
ꢀ0.68 V. In comparison, an obvious enhancement of current
density of 7.8 mA cmꢀ2 is observed at the potential of ꢀ0.68 V
when the electrolyte is saturated with CO2 (pH ¼ 7.5). The onset
potential performs more positive at ꢀ0.30 V, indicating a low
overpotential less than 280 mV (conrmed by GC and 1H NMR).
The results reveal that the electrocatalyst can selectively reduce
CO2 and suppress the HER efficiently under CO2 atmosphere.
As
a
comparison, we investigated the electrocatalytic Fig. 4 (a) LSVs of FeS2/NiS nanocomposite in 0.5 M KHCO3 aqueous
solution under N2 (blue trace) and CO2 (red trace) atmosphere; (b)
performances of FeS2 nanocrystal and NiS nanocrystal for CO2
electroreduction, respectively. The detailed synthetic process is
shown in Experiment section. The specic characterizations
including XRD, SEM and TEM of the as-prepared FeS2 and NiS
LSVs of FeS2/NiS nanocomposite (black trace), NiS (red trace), and FeS2
(blue trace) in 0.5 M KHCO3 aqueous solution under CO2 atmosphere;
(c) faradaic efficiencies vs. potential of H2 for FeS2 and NiS nano-
crystals; (d) faradaic efficiencies vs. potential of CH3OH and H2 for
nanocrystals are shown in Fig. S3 and S4.† The LSVs of FeS2, NiS FeS2/NiS nanocomposite.
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CH3OH, FeS2/NiS nanocomposite shows a moderate potential
compared with those noble metal electrocatalysts. The H2 FEs
for FeS2/NiS nanocomposite show an opposite tendency
compared with CH3OH. The FEs of H2 continuously decreases
and decreases to a minimum of 17% at the applied potential of
ꢀ0.6 V. Moreover, the total FEs for the generation of CH3OH
and H2 are maintained at 81% over the whole process. These
results demonstrate that single FeS2 and NiS nanocrystals can
hardly reduce CO2 into hydrocarbon products, whereas the
FeS2/NiS nanocomposite can achieve an efficient and selective
electroreduction process for CO2. Thus, we propose the active
sites for CO2RR locate at the interface between FeS2 and NiS.
To further prove the stability of the FeS2/NiS nanocomposite,
the continuous tendency test was measured in a standard three-
electrode cell as shown in Fig. 5a. A stable current density of 3.1
mA cmꢀ2 is observed over the 4 hours electroreduction at the
potential of ꢀ0.6 V. Besides, the FE of the products (CH3OH and
H2) for the FeS2/NiS nanocomposite is maintained at about 81%
over the 4 hours electrolysis. It can observe no obvious deacti-
vation of FeS2/NiS nanocomposite for CO2 reduction throughout
the entire process. Then, the XRD measurement was performed
on the reacted FeS2/NiS nanocomposite as shown in Fig. 5b. The
XRD analysis indicates that there is no transformation taking
place upon the catalyst during the whole electroreduction. These
results fully conrm that FeS2/NiS nanocomposite is excellently
stable and efficient for CO2 electroreduction.
To understand the highly efficient electrocatalysis activity of
FeS2/NiS nanocomposite, we further compared the performances
of CO2RR catalysed by FeS2/NiS nanocomposite, pure FeS2 and
NiS nanocrystals. The single FeS2 and NiS nanocrystals show
poor activities towards CO2RR. The FeS2 nanocrystal shows
a negative onset potential at ꢀ0.45 V and the maximum current
density of 4.2 mA cmꢀ2 at ꢀ0.68 V. Then, the NiS nanocrystal
shows a more negative onset potential at ꢀ0.5 V and extremely
low current density of 1 mA cmꢀ2 at ꢀ0.68 V. Furthermore,
a physical mixture of FeS2 and NiS nanocrystals with the same
ratio as FeS2/NiS nanocomposite was fabricated. The electro-
catalytic activity for CO2 reduction was measured and showed in
Fig. S6.† The current density presents a slight increasement
under CO2 atmosphere compared with N2 atmosphere. Then, a 2
h-electrolysis in 0.5 M CO2-saturated KHCO3 for the physical
Fig. 6 The proposed reaction mechanism of electroreduction CO2 by
FeS2/NiS nanocomposite electrocatalyst.
mixture was performed at the potential of ꢀ0.6 V. Only H2 was
detected in the both gas phase and liquid phase products. The
increasement of current density may be ascribed to the effect of
pH. In contrast, the FeS2/NiS nanocomposite exhibits excellent
activity for CO2RR. It achieves an unprecedented overpotential of
280 mV and a stable current density of 3.1 mA cmꢀ2 at the
potential of ꢀ0.6 V. Comparing the electrocatalytic performances
of CO2RR in FeS2 nanocrystal, NiS nanocrystal, physical mixture
of FeS2 and NiS nanocrystals and FeS2/NiS nanocomposite, it can
speculate that the active sites of the catalyst locate at the interface
between FeS2 (200) and NiS (100) (as shown in Fig. 6). On the
other hand, many researches have evidenced that the electro-
reduction of CO2 to CH3OH is a complex process which includes
the transformation of 6 eꢀ and 6 H+.5,37–40 Still, more experimental
and theoretical calculations are needed to unravel the detailed
mechanism of this multistep reaction.
Conclusion
We synthesized the FeS2/NiS nanocomposite with a traditional
hydrothermal method. It was demonstrated that FeS2/NiS
nanocomposite can catalyze CO2RR at an unprecedented over-
potential of 280 mV and a high CH3OH faradaic efficiency up to
64% at the potential of ꢀ0.6 V. Moreover, the catalyst performs
a stable current density of 3.1 mA cmꢀ2 over the 4 hours stability
test and there is no obvious degradation aer electroreduction
from XRD observation. In experiments, the FeS2 nanocrystals,
NiS nanocrystals and the physical mixture of FeS2 and NiS
nanocrystals all show poor activity towards CO2RR, while the
FeS2/NiS nanocomposite exhibits excellent activity towards the
process. Thus, the high activity and selectivity towards CO2
electroreduction to CH3OH is probably attributed to the special
ladder structure of the FeS2/NiS nanocomposite. The active sites
may locate at the interface between FeS2 and NiS which could
effectively suppress the side reaction of HER and facilitate the
CO2RR. The low-cost FeS2/NiS nanocomposite is an efficient
alternative to expensive materials for the application of CO2
electroreduction in industry.
Fig. 5 (a) Stability of FeS2/NiS nanocomposite for CO2 reduction
operated at a potential of ꢀ0.6 V vs. RHE for 4 hours. Geometric
current density vs. time (left axis) and total FE for production (CH3OH
and H2) vs. time (right axis); (b) the large-angle XRD patterns of FeS2/
NiS nanocomposite before (red trace) and after (black trace) 4 hours
electroreduction.
Acknowledgements
This work is supported by the Collaborative Innovation Center
of Suzhou Nano Science and Technology, the National Natural
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