Selective electrochemical reduction of CO2 by a binder-free platinum/nitrogen-doped carbon nanofiber/copper foil catalyst with remarkable efficiency and reusability

Article abstract of DOI:10.1016/j.elecom.2018.06.018

In this report, Pt/NCNFs/Cu-foil, an efficient and stable catalyst, was prepared by the electrospinning method, which could be directly used for electrochemical reduction of CO₂. Formate with 93% Faradaic efficiency and about 46 mA cm^?2 partial current density could be obtained at ?0.6 V_RHE; alcohols with approximately 35% Faradaic efficiency and 14 mA cm^?2 partial current density were achieved at ?1.0 V_RHE using the same Pt/NCNFs/Cu-foil cathode. Moreover, Pt/NCNFs/Cu-foil could keep high efficiencies for at least 50 h durability tests.

Full text of DOI:10.1016/j.elecom.2018.06.018

Accepted Manuscript
Selective electrochemical reduction of CO2 by a binder-free
platinum/nitrogen-doped carbon nanofiber/copper foil catalyst
with remarkable efficiency and reusability
Heng-Pan Yang, Qing Lin, Han-Wen Zhang, Yu Wu, Liang-Dong
Fan, Xiao-Yan Chai, Qian-Ling Zhang, Jian-Hong Liu, Chuan-
Xin He
PII:
DOI:
Reference:
S1388-2481(18)30160-7
doi:10.1016/j.elecom.2018.06.018
ELECOM 6244
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
30 May 2018
26 June 2018
28 June 2018
Please cite this article as: Heng-Pan Yang, Qing Lin, Han-Wen Zhang, Yu Wu, Liang-
Dong Fan, Xiao-Yan Chai, Qian-Ling Zhang, Jian-Hong Liu, Chuan-Xin He , Selective
electrochemical reduction of CO2 by a binder-free platinum/nitrogen-doped carbon
nanofiber/copper foil catalyst with remarkable efficiency and reusability. Elecom (2018),
doi:10.1016/j.elecom.2018.06.018
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ACCEPTED MANUSCRIPT
Selective electrochemical reduction of CO2 by a binder-free
platinum/nitrogen-doped carbon nanofiber/copper foil
catalyst with remarkable efficiency and reusability
Heng-Pan Yang, Qing Lin, Han-Wen Zhang, Yu Wu, Liang-Dong Fan, Xiao-Yan Chai,
Qian-Ling Zhang, Jian-Hong Liu and Chuan-Xin He*
Shenzhen Key Laboratory for Functional Polymer, College of Chemistry and
Environmental Engineering, Shenzhen University, Shenzhen, Guangdong, 518060,
China
* Corresponding Author: Prof. Chuan-Xin He (E-mail:hecx@ustc.edu.cn).
Abstract:
In this report, Pt/NCNFs/Cu-foil, an efficient and stable catalyst, was prepared by the
electrospinning method, which could be directly used for electrochemical reduction of
CO2. Formate with 93% Faradaic efficiency and about 46 mA cm-2 partial current
density could be obtained at -0.6 VRHE; alcohols with approximately 35% Faradaic
efficiency and 14 mA cm-2 partial current density were achieved at -1.0 VRHE using
the same Pt/NCNFs/Cu-foil cathode. Moreover, Pt/NCNFs/Cu-foil could keep high
efficiencies for at least 50 h durability tests.
Keywords:
CO2 reduction; electrocatalysis; high catalyst productivity; excellent reusability;
electrospinning technology
1. Introduction
In recent years, electro-catalytic CO2 reduction reaction (CO2RR) has become a
cutting-edge research area [1-3]. However, challenges remain in CO2RR, especially
the difficulty in activating the relatively inert and stable CO2 molecule. Therefore,
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searching for suitable electrocatalysts for CO2RR is highly desirable in current
research.
The most common and widely reported CO2RR catalysts are metals, their alloys
and oxides [4-7]; for example, Cu could produce multi-electron products such as
hydrocarbons and alcohols [4, 5]. Moreover, nitrogen heterocyclic molecules are
another choice for CO2RR; for example, pyridine and their derivatives with the
assistance of metal cathode could reduce CO2 into methanol [8-12]. Furthermore,
transition metal dichalcogenides (TMDs) [13, 14], metal-organic frameworks (MOFs)
[15, 16], and N-doped carbons could also be employed for CO2RR [17, 18]. However,
CO2RR catalysts are always restricted by some problems, including poor selectivity or
stability, low catalyst productivity, and difficulty of reclaiming powdered catalysts
[19].
Trying to solve these issues, we recently introduced the electrospinning technique,
which is an efficient technique for the production of N-doped carbon nanofibers
(NCNFs). This report is based on the direct electrospinning of NCNFs with platinum
nanoparticles (Pt NPs) onto copper foil (Cu foil). The as-synthesized composite could
be named as Pt/NCNFs/Cu-foil. Since pyridinic nitrogen, platinum and copper were
combined in one composite, Pt/NCNFs/Cu-foil might be efficient eletrocatalysts for
CO2RR.
2. Experimental section
2.1. Apparatus and reagents
All reagents were used as received.
Potentiostatic electrolysis and linear sweep voltammograms (LSV) were performed
using a CHI 760C electrochemical Station. Liquid phase products were analyzed by
1H-NMR spectra recorded on an Ascend 400. Microstructure, SAED and EDX spectra
were analyzed using FEI JEOL-7800F field emission scanning electron microscope
(FE-SEM). Transmission electron microscopy (TEM) was observed with JEM-2100F
field emission electron microscope. X-ray photoelectron spectroscopy (XPS)
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measurements were performed on a Perkin-Elmer PHI 5000C ESCA system.
2.2. General procedure
Synthesis of Pt/NCNFs/Cu-foil: Firstly, 1.6 g of polyacrylonitrile, 1.2 g ZnCl2 and
20 mg H2PtCl6·6H2O were dissolved into 20 mL of DMF. The precursor solution was
transferred into a syringe and electrospun directly onto the coated Cu foil substrate.
Then, the nanofibrous materials were carbonized under Ar gas flow.
CO2 reduction procedure: Electrolysis was performed with a three-electrode cell.
An Ag/AgCl was the reference electrode, and a Pt foil was the counter electrode. 20
mL 0.1 M KHCO3 was used as electrolyte, which was bubbled with 1 atm CO2
through the electrolysis. All potentials were converted to the reversible hydrogen
electrode (RHE) by Nernst equation: E(RHE) = E(Ag/AgCl) + 0.199 +0.059 × pH.
3. Results and discussion
The synthetic procedure of Pt/NCNFs/Cu-foil is schematically depicted in Fig. 1A.
In a typical procedure, H2PtCl6 and ZnCl2 were firstly embedded within PAN
nanofibers by electrospinning. After a heat treatment at 800 °C in Ar atmosphere, the
PAN molecules were converted into NCNFs. Meanwhile, The Zn species were
reduced and vaporized. BET tests show the specific surface area of Pt/NCNFs/Cu-foil
is 289 m2 g-1, revealing that ZnCl2 is an efficient pore-forming agent. The Pt4+ species
were reduced to metallic state Pt NPs.
According to FE-SEM, three-dimensional nanofiber web was formed on the surface
of Cu-foil (Fig. 1B), leading to an obvious change of the typical copper colour to dark
black (inset of Fig. 1B). The thickness of this nanofiber coating was estimated to 10
μm, which is similar to Cu-foil (Fig. 1C). The diameters of the nanofibers are in the
range of 300 to 500 nm (Fig. 1D). Importantly, we found that these nanofibers were
partially embedded in the Cu-foil, resulting in the stable immobilization of Pt/NCNFs
onto the Cu-foil surface (inset of Fig. 1C). As for TEM analysis, it can be observed
that more grown nanoparticles are embedded within the nanofibers. Pt NPs are evenly
distributed within the nanofibers, having an average diameter of ~6 nm (Fig. 1E, F).
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The lattice distance as marked with the red arrows was measured as 2.3 Å,
corresponding to the (111) plane of Pt face centered cubic (fcc) crystal, in accordance
with selected area electron diffraction patterns (SAED, inset of Fig. 1F). Moreover,
Fig. 1G shows EDX mapping images of Pt/NCNFs, revealing the good match
between C, N and Pt elements, and verifies the homogeneous distribution of Pt NPs
within the nanofibers.
According to X-Ray Diffraction (XRD) patterns (Fig. 1I), no diffraction peak of Pt
crystal emerged in the XRD pattern of Pt/NCNFs/Cu-foil, suggesting that most of the
Pt-NPs are mainly embedded in the interior of nanofibers; a broad diffraction peaks
locating at 24.1 degree could be ascribed to (002) planes of carbon nanofibers. As for
copper composition, XRD pattern of pure Cu-foil show three intense peaks locating at
43.3, 50.4 and 74.1 degree, corresponding to (111), (200) and (220) plane of Cu fcc
crystal; besides diffraction peaks of pure Cu, Pt/NCNFs/Cu-foil also have extra peaks
belonging to CuO and Cu2O, revealing that Cu-foil was oxidized to some extent. The
N 1s XPS spectra (Fig. 1J) was separated into two peaks at 398.8 eV and 401.2 eV,
belonging to pyridinic N atoms and graphitic N atoms, respectively. The Pt 4f XPS
spectra (Fig. 1K) contains two distinct peaks at around 71.3 eV and 74.4 eV,
corresponding to Pt0.
According to former reports, Cu-based materials [4, 5], pyridinic nitrogen with the
assistance of Pt [8-12] are both efficient catalysts for CO2RR. Since their working
potential are much different, we scraped away the Pt/NCNFs nanofiber from
Pt/NCNFs/Cu-foil composite, and explored the electrochemical activity of Pt/NCNFs
and Cu-foil for CO2RR, respectively. LSV tests were firstly performed in CO2- or N2-
saturated 0.1 M KHCO3 solution (~0.5 mg cm-2 catalyst loading). As shown in Fig.
2A&B, obviously higher reductive currents are observed for both Pt/NCNFs and
Cu-foil electrodes under CO2 atmosphere than N2 atmosphere, demonstrating that
CO2 can be electrochemically reduced by Pt/NCNFs and Cu-foil catalysts,
respectively. The onset potential for CO2 reduction is -0.46 VRHE at Pt/NCNFs
electrode, much more positive than that of Cu-foil electrode (-0.85 VRHE).
LSV tests confirm that Pt/NCNFs and Cu-foil are active for electrochemical
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CO2RR at different potentials. Hence, controlled potential electrolysis using
Pt/NCNFs/Cu-foil catalyst was performed in the range of -0.4 to -1.1 VRHE cathode
potential, based on the onset potential for CO2RR at Pt/NCNFs and Cu-foil electrodes.
CO2 electrolysis were conducted in a divided glass cell and the cathodic compartment
was continuously purged with a constant CO2 flow. It is notable that
Pt/NCNFs/Cu-foil composite was cut into specific shapes and directly utilized as
working electrode for CO2RR without any binders (~5 mg cm-2 catalyst loading).
After electrolysis, the liquid products were detected by 1H NMR spectroscopy, which
revealed that almost pure formate was obtained around -0.6 VRHE; ethanol and
methanol were the major product around -1.0 VRHE. The Faradaic efficiencies of
products were summarized in Fig. 2C, which revealed that formate was the dominant
product before -0.6 VRHE, reaching the maximum Faradaic efficiency (93%) at -0.6
VRHE. At cathode potentials more negative than -0.8 VRHE, alcohols, including
methanol and ethanol, start to be produced; the maximum Faradaic efficiencies for
methanol and ethanol was 4% and 31%, respectively, at -1.0 VRHE
.
As mentioned above, our Pt/NCNFs/Cu-foil catalyst could be directly used as the
working electrode for CO2RR without any binder or extra carriers. It is notable that
only ~30 and ~20 mins were needed for the electrolysis conducted at -0.6 and -0.9
VRHE, respectively, using Pt/NCNFs/Cu-foil. Furthermore, after previous electrolysis,
the Pt/NCNFs/Cu-foil cathode could be easily washed with water and then reused for
a next reaction run under the same conditions. The Faradaic efficiency of formate
could maintain around 90% for at least 10 times repeated electrolysis at -0.6 VRHE
(Fig. 2D). We also tested the long-term performance of Pt/NCNFs/Cu-foil at constant
cathode potential of -0.6 VRHE for 50 h. The partial current density kept a steady value
at around 46 mA cm-2 throughout the duration of the test (Fig. 2F). If
Pt/NCNFs/Cu-foil was tested at -1.0 VRHE, a modest drop of partial current density
from approximately 16 mA cm-2 to 14 mA cm-2 was observed after 50 h of
electrolysis (Fig. 2F); total Faradaic efficiencies of methanol and ethanol also
downgraded to about 23% after repeated tests at -1.0 VRHE (Fig. 2E). The
electrocatalytic activity of copper would decrease significantly with the deposition of
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poisoning species on the electrode surface especially in aqueous solutions [20, 21],
which might explain the above results.
According to the discussion above, we could find that Pt/NCNFs/Cu-foil was an
excellent catalyst for electrochemical CO2RR. To confirm the active sites for the
generation of corresponding products, a series of contrast experiments were
conducted under similar conditions, the results were summarized in Table 1.
Pt/NCNFs, NCNFs and Pt-NPs cathodes (Fig. 1H), which did not have copper
composition, could not generate any alcohols at -1.0 VRHE (Table 1, entry 1, 4, 5);
Cu-foil could produce methanol and ethanol with 3% and 30% Faradaic efficiencies,
respectively, at -1.0 VRHE. Pt/NCNFs produced formate with 91% Faradaic efficiency
at -0.6 VRHE and only a trace amount of formate was detected at -0.6 VRHE using Cu
foil cathode (Table 1, entry 1, 2). Former reports about CO2RR catalysed by Cu-based
materials reveal that single-electron products, such as CO and formate, predominate at
potentials less negative than -0.8 VRHE; CO2 could be further reduced to
multi-electron products, like hydrocarbons or alcohols, at potentials more negative
than -1.0 VRHE [20, 21]. Therefore, the formation of alcohols at -1.0 VRHE could be
attributed to Cu-foil; Pt/NCNFs are responsible for the majority of formate achieved
at -0.6VRHE. Furthermore, NCNFs/Cu-foil and NCNFs (Table 1, entry 3, 4) only
produced formate with 9% Faradaic efficiency at -0.6 VRHE, much lower than
Pt/NCNFs; no formate was obtained using pure Pt NPs (Table 1, entry 5) at -0.6 VRHE
.
These results indicated that Pt NPs and NCNFs were both necessary for the high
Faradaic efficiency of formate obtained at -0.6 VRHE. Based on these experimental
results and previous reports [19], we propose that the CO2 molecule was firstly
adsorbed to the carbon atom adjacent to the pyridinic N, combined with the H• radical
generated from the Pt surface to HCOO•, and then further reduced to HCOO− as the
product. The mechanism of copper catalysed CO2RR to alcohols (like ethanol), has
been widely reported, which always contains the adsorption and protonation of CO2
molecules onto the Cu surface, forming *CO, further dimerize and hydrogenate to
specific alcohol [2-5].
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4. Conclusions
In summary, Pt/NCNFs/Cu-foil composite, a highly efficient catalyst for CO2RR
was prepared by the electrospinning method, which could be produced with the size
of 20×25 cm at one time (determined by instruments). Pt/NCNFs/Cu-foil could be
directly used as cathode for electrochemical CO2RR without any binder under mild
conditions, formate with 93% or alcohols with ~35% Faradaic efficiencies could be
obtained at -0.6 VRHE and -1.0 VRHE cathode potential, respectively. Due to the
possibility of engineering their structure, high catalyst productivity, excellent
efficiency and reusability, electrospinning technology might have many more
applications in the CO2RR.
Acknowledgements
The financial support of the National Natural Science Foundation (NNSF) of China
(21574084 and 21571131), the Natural Science Foundation of Guangdong Province
(2015A030313554 and 2017A040405066), and Shenzhen Government’s Plan of
Science and Technology (JCYJ20160308104704791, JCYJ20170817095041212 and
JCYJ20170818091657056) are gratefully acknowledged.
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Fig. 1 Schematic illustration of the synthesis of Pt/NCNFs/Cu-foil (A); FE-SEM
patterns (B-D), TEM images (E, F), SAED (inset of F) and EDX mapping (G) of
Pt/NCNFs/Cu-foil; TEM images of NCNFs (H) and pure Pt NPs (inset of H); XRD
patterns of Pt/NCNFs/Cu-foil and pure Cu-foil (I); XPS spectra of N 1s and Pt 4f of
Pt/NCNFs (J, K).
Fig. 2 LSV recorded at a sweep rate of 0.1 V s-1 in 0.1 M KHCO3 solution at glassy
carbon electrode loaded with Pt/NCNFs (A) and Cu-foil (B); C: Faradaic efficiency
for products using Pt/NCNFs/Cu-foil electrode (2×2 cm) at potentials from -0.4 to
-1.1 VRHE, charge: 100 C, anode: Pt foil, 20 mL CO2-saturated 0.1 M KHCO3 solution;
Reuse test performed at -0.6 VRHE (D) and -1.0 VRHE (E) cathode potential using
Pt/NCNFs/Cu-foil as working electrode; Long-time test performed at -0.6 VRHE and
-1.0 VRHE, respectively (F).
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A
B
C
D
E
F
G
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I
H
J
K
Fig. 1
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A
B
D
C
F
E
Fig. 2
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Table 1. Electrochemical reduction of CO2 on different cathodes.[a]
Potential
/ VRHE
Formate
FE / %[b]
EtOH (MeOH)
FE / %[b]
Entry
1
Cathode
Pt/NCNFs
-0.6
-1.0
-0.6
-1.0
-0.6
-1.0
-0.6
-1.0
-0.6
-1.0
91
-
-
-
2
3
4
5
Cu-foil
NCNFs/Cu-foil
NCNFs
Trace
-
-
9
-
30 (3)
-
31 (3)
9
-
-
-
-
-
Pt NPs
-
-
[a] Faradaic efficiency of electrolysis products using different working electrode at -0.6 or -1.0
VRHE cathode potentials, charge: 100 C, anode: Pt foil, 20 mL CO2-saturated 0.1 M KHCO3
solution, temperature: 25 °C, 1 atm; [b] Products was detected by 1H NMR.
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Graphical abstract
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Highlights
 Pt/NCNFs/Cu-foil composite was prepared by electrospinning method.
 Pt/NCNFs/Cu-foil could be cut into specific shape and directly used as
cathode without any binder.
 Formate with 93% or alcohols with 35% Faradaic efficiency could be
achieved at the same cathode by just switching the cathode potential.
 Pt/NCNFs/Cu-foil cathode has excellent recyclability and reusability.
16

Graphics Abstract

Figure 1ag

Figure 1hk

Figure 2