Electrochemical reduction of CO2 at metal-free N-functionalized graphene oxide electrodes

Article abstract of DOI:10.1016/j.electacta.2018.06.107

Exploring metal-free catalysts for electroreduction of CO₂ into value-added fuels has been a crucial challenge in nowadays society. Herein we report some metal-free electrocatalysts for electrocatalytic reduction of CO₂ to liquid products, such as ethanol and acetone. We develop functionalized graphene oxide (GO) surfaces with five pyridine derivatives: pyridoxine, 4-hydroxypyridine, 4-aminopyridine, 8-hydroxyquionline, and 5-amino-1,10-phenanthroline. Remarkably, pyridoxine modified on GO sheets with the pyridinic N content of 2.32% shows the optimum catalytic ability for electrochemical reduction of CO₂, and the overall faradaic efficiency reaches up to 45.8%. Besides, other four pyridine derivatives functionalized GO exhibit various abilities for CO₂ reduction to ethanol and acetone. The experimental results show that N-doping can alter the electronic properties of GO for enhancing the catalytic active for CO₂ electroreduction, which is closely associated with the content of modified pyridinic N and the special structure of modified pyridine derivatives. Our work may provide inventive thought to design metal-free electrocatalysts for CO₂ reduction to useful chemical fuels.

Full text of DOI:10.1016/j.electacta.2018.06.107

Accepted Manuscript
Electrochemical reduction of CO at metal-free N-functionalized graphene oxide
2
electrodes
Jing Yuan, Wen-Ya Zhi, Li Liu, Man-Ping Yang, Huan Wang, Jia-Xing Lu
PII:
S0013-4686(18)31395-1
10.1016/j.electacta.2018.06.107
EA 32100
DOI:
Reference:
To appear in: Electrochimica Acta
Received Date: 27 April 2018
Revised Date: 6 June 2018
Accepted Date: 14 June 2018
Please cite this article as: J. Yuan, W.-Y. Zhi, L. Liu, M.-P. Yang, H. Wang, J.-X. Lu, Electrochemical
reduction of CO at metal-free N-functionalized graphene oxide electrodes, Electrochimica Acta (2018),
2
doi: 10.1016/j.electacta.2018.06.107.
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ACCEPTED MANUSCRIPT
Graphical abstract
In our work, we systematically and carefully synthesized a series of metal-free
electrocatalysts, i.e., pyridoxine functionalized graphene oxide (GO-VB ), which
6
present high catalytic active for electroreduction of CO to ethanol. Besides, other
2
four pyridine derivatives (4-hydroxypyridine, 4-aminopyridine, 8-hydroxyquionline,
and 5-amino-1,10-phenanthroline) functionalized GO were also successfully
synthesized and exhibit various catalytic abilities for CO reduction.
2

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Electrochemical reduction of CO at metal-free
2
N-functionalized graphene oxide electrodes
Jing Yuan, Wen-Ya Zhi, Li Liu and Man-Ping Yang, Huan Wang* and Jia-Xing Lu**
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai
2
*
*
00062, China.
Corresponding Author. Tel: +8621-52134935; E-mail: hwang@chem.ecnu.edu.cn.
*Corresponding Author. Tel: +8621-62233491; E-mail: jxlu@chem.ecnu.edu.cn
Abstract: Exploring metal-free catalysts for electroreduction of CO into value-added
2
fuels has been a crucial challenge in nowadays society. Herein we report some
metal-free electrocatalysts for electrocatalytic reduction of CO to liquid products,
2
such as ethanol and acetone. We develop functionalized graphene oxide (GO) surfaces
with five pyridine derivatives: pyridoxine, 4-hydroxypyridine, 4-aminopyridine,
8
-hydroxyquionline, and 5-amino-1,10-phenanthroline. Remarkably, pyridoxine
modified on GO sheets with the pyridinic N content of 2.32% shows the optimum
catalytic ability for electrochemical reduction of CO , and the overall faradaic
2
efficiency reaches up to 45.8%. Besides, other four pyridine derivatives
functionalized GO exhibit various abilities for CO reduction to ethanol and acetone.
2
The experimental results show that N-doping can alter the electronic properties of GO
for enhancing the catalytic active for CO electroreduction, which is closely
2
associated with the content of modified pyridinic N and the special structure of
modified pyridine derivatives. Our work may provide inventive thought to design
metal-free electrocatalysts for CO reduction to useful chemical fuels.
2
Keywords: Electrochemical reduction of CO , metal-free, N-functionalized graphene
2
oxide, pyridoxine, ethanol
1

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1
. Introduction
Atmospheric CO accumulated, resulting in escalating environmental problems, has
2
been transformed into useful chemical fuels through viable methods relying on global
efforts.[1,2] Electrochemical reduction of CO with high efficiency and selectivity,
2
could potentially address the environment issues and energy crisis by utilizing
renewable source, such as solar, wind and tide energy.[3,4] Over the past few decades,
many researchers have been made numerous trials to explore catalysts with
remarkable efficiency for CO reduction reaction (CO RR), such as metals[5-7], metal
2
2
alloys[8,9] and metal complexes[10-12]. However, these metal-containing catalysts,
due to their characteristics including low abundance and high cost, have been
restricted their large-scale practical application.
Compared to metal-based catalysts, metal-free electrocatalysts, as renewable and
cost-effective materials, achieving comparable catalytic abilities to metal-containing
catalysts, have been attracted increasing concern for CO RR.[13-20] Recently,
2
heteroatom-doped carbon-based catalysts, owing to low-cost, superior stability and
high catalytic ability, have been expected to replace metal-containing catalysts for
CO RR.[17-20] For instance, Meyer and co-workers[17] demonstrated that N-doped
2
carbon nanotubes (CNTs) as functionalized catalysts for CO RR in KHCO aqueous
2
3
solution achieved a maximum faradaic efficiency (FE) for formate of 87%. Ajayan et
al.[18] revealed that CNTs, doped with N to form resident electron-rich defects, can
act as highly efficient, selective and stable catalysts for the conversion of CO to CO.
2
Sharma et al.[19] found that B-doped graphene exhibits high electrocatalytic activity
for the reduction of CO to HCOOH at a low overpotential. Han et al.[20] showed that
2
N-doped graphene-like materials (NGMs) are very efficient for CO RR to CH with
2
4
FE as high as 93.5% when using ionic liquids as the electrolytes.
Graphene, as unique two-dimensional carbon materials, play a very important role
in the performance of the electrocatalysts, as they actually enhance the conductivity of
catalysts and also provide a construct for electron capture and transport.[21-23]
Nevertheless, in the process of graphene functionalization, the fact of serious π-π
stacking interactions between graphene nanosheets could result in serious restacking
2

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and accumulation of graphene nanosheets, and ultimately increase the difficulty to
functionalize graphene. [24]
Although graphene oxide (GO), because of variable oxygen contents, displays
weak conductivity, it can tune electrical conductivity, which makes it apply to various
fields of electrochemistry. [25,26] Moreover, GO, containing a good deal of hydroxyl
and carboxyl, is considerably achieved easily to fabricate various structures, such as
N-doped materials. Pyridoxine (Vitamin B , VB ), as an important biologically active
6
6
compound, is a vital cofactor involving the metabolism of amino acids and lipids, also
the synthesis of neurotransmitters, histamine and hemoglobin, and other key biologic
processes. [27] Due to its low-cost, non-toxic and containing a pyridine ring, VB6
could be modified on all most carbon material surfaces including CNT, graphene and
carbon fiber. The high amounts of hydroxyl groups of VB serve as a bridge linked to
6
the carboxyl of GO in the process of catalyst functionalization. None reports have
shown such an intriguing structure applied for CO RR.
2
Herein, we synthesized a series of metal-free N-functionalized GO electrodes for
CO RR to produce liquid products, including ethanol and acetone. It was found that
2
VB functionalized GO (GO-VB ) materials showed high catalytic active for CO RR
6
6
2
and the efficiency observably relied on the pyridinic N content in the GO-VB6.
GO-VB with a pyridinic N content of around 2.32% has the best catalytic
6
performance among all considered VB modified GO. For the modification part, other
6
four pyridine derivatives, such as 4-hydroxypyridine (X-1), 4-aminopyridine (X-2),
8
-hydroxyquionline (X-3) and 5-amino-1,10-phenanthroline (X-4) were modified on
the GO sheets surface using the same modification procedure, displaying different
catalytic properties for CO RR, which is attributed to the special structure of grafted
2
pyridine derivatives.
2
. Experiment section
.1. Materials
2
Pyridoxine (purity> 98%) and 5-amino-1,10-phenanthroline (purity> 97%) were
3

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obtained from Macklin. 4-Pyridinal (purity> 98%), 4-aminopyridine (purity> 98%),
8
-hydroxyquionlinen (purity> 99.5%), sulfoxide chloride (SOCl , purity> 99%),
2
N,N-dimethylformamide (DMF, purity> 99.5%), glacial acetic acid (purity> 99.5%)
and ethyl acetate (purity> 99.5% ) were purchased from Sinopharm Chemical Reagent
Co.. The commercial graphene oxide was obtained from Beijing HWRK Chem Co.,
LTD. Nafion® 117 solution (5%) and Nafion 117 membrane were obtained from
Dupont. Carbon paper (CP, HCP010) was purchased from Shanghai Hesen Electrical
Co.. All chemicals were used as received without further purification.
2
.2. Materials characterization
Crystal-phase X-ray diffraction (XRD) patterns were recorded using an Ultima IV
X-ray powder diffractometer equipped with Cu Kα radiation (k = 1.5406 Å).
Microstructure was analyzed using Hitachi S-4800 field emission scanning electron
microscope (FE-SEM) equipped with an energy dispersive X-ray fluorescence
spectrometer. Transmission Electron Microscope (TEM) patterns were recorded by
2
TECNAI G F30 transmission electron microscope. Fourier transformed infrared
spectroscopy (FT-IR) was recorded by a Bruker IFS 66 V/S. The X-ray photoelectron
spectroscopy (XPS) analysis was performed on the Thermo Scientific ESCA Lab
2
50Xi using 200 W monochromatic Al Kα radiation. The 500 ꢀm X-ray spot was used.
-10
The base pressure in the analysis chamber was about 3×10 mbar. Typically, the
hydrocarbon C1s line at 284.8 eV from adventitious carbon was used for energy
referencing. The Raman spectra were obtained at room temperature in flame sealed
capillary on a FT Bruker RFS 106/S spectrometer, equipped with a 532 nm laser, in
-1
-1
the region from 3500 to 100 cm with a resolution of 2 cm . All the electrochemical
experiments were performed using a CHI 660C electrochemical station (Shanghai
Chenhua Instrument Co. Ltd).
2
.3. Materials synthesis
2
.3.1. Preparation of intermediate material: carbonyl chloride groups
functionalized GO (GO-COCl )
GO (15 mg) was placed in 15 mL of DMF solution, followed by the addition of SOCl2
4

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(3 mL), and then refluxed at 65ºC for 24 h. After filtration, the obtained solid was
washed with anhydrous DMF, and dried in a vacuum oven at 40ºC overnight, to give
GO-COCl intermediated material.
2
.3.2. Preparation of pyridine derivatives modified GO
The similar procedures were reported by other authors. [28] Different concentrations
of VB (2.96, 5.92, 11.8, 19.7, 29.6 and 39.4 mM) modification on the GO sheets
6
applied the same synthetic method, which were denoted as GO-VB -1, GO-VB -2,
6
6
GO-VB -3, GO-VB -4, GO-VB -5 and GO-VB -6, respectively. For simplicity,
6
6
6
6
taking GO-VB -4 as an example, the preparation steps are as follows. GO-COCl (15
6
mg) was added to DMF solution of VB (19.7 mM, 15 mL) in a round-bottomed flask,
6
and 2-3 drops of glacial acetic acid were added. Then, the mixture was refluxed at
6
5ºC for 24 h. After cooling to room temperature, the resulting suspension was
filtered and the product was obtained by washing with DMF and ethyl acetate, and
dried in a vacuum oven at 40ºC overnight. Similarly, other four pyridine derivatives
with the same concentration of 19.7 mM functionalized GO sheets (GO-X) adopted
the same modification procedure.
2
.4. Electrochemical study
To prepare the GO-VB and GO-X electrodes, 5 mg of the above prepared GO-VB or
6
6
GO-X was suspended in 40 ꢀL of deionized water with 20 ꢀL of Nafion® 117
dispersion (5 wt%) to form a homogeneous ink assisted by ultrasound. Then, the ink
was spread onto the CP surface by a micropipette and then dried under room
2
temperature. The loading of different GO-VB and GO-X catalysts was 2.5 mg/cm .
6
The electrolysis experiments were conducted at 25ºC in a gas diffusion electrode
cell, which consisted of a cathode (GO-VB /CP or GO-X/CP electrode), an anode (Pt
6
sheet electrode), and a saturated calomel electrode (SCE) as the reference electrode.
In the experiments, the cathodic and anodic compartments were separated through a
Nafion 117 proton exchange membrane. 0.1 M KHCO aqueous solution was used as
3
electrolyte. Under the continuous stirring, CO was bubbled through the catholyte for
2
3
0 min before electrolysis. Then, potentiostatic electrolysis of CO was carried out
2
with CO bubbling.
2
5

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Linear sweep voltammetry (LSV) measurements were carried out in a single
compartment cell with a three-electrode configuration, which consists of working
electrode (GO-VB or GO-X catalysts painted on the glassy carbon electrode), a Pt
6
sheet electrode, and an SCE reference electrode. The electrolyte was bubbled with N2
or CO for at least 30 min to form N - or CO -saturated solution. The LSV
2
2
2
measurements in gas-saturated electrolyte were conducted in 0.1 M KHCO solution
3
at a scan rate of 50 mV/s. All potentials were measured against the SCE electrode and
converted to the reversible hydrogen electrode (RHE) reference scale by following the
equation:
E (vs. RHE) = E (vs. SCE) + 0.059V × pH + 0.241V.
(1)
2
.5. Product analysis
1
Liquid phase products were analyzed by H-NMR spectra recorded on an Ascend 400
(
400 MHz, Bruker, Germany) spectrometer in D O with Me Si as an internal
2 4
standard.
To confirm that the evolved products originate from CO , isotopic labeling
2
experiments were performed under the same process mentioned above, except that
13
13
12
isotopic CO ( C 99%, Sigma-Aldrich) replaced CO as the carbon source, and
2
2
the liquid products collected were analyzed by gas chromatography-mass
spectrometry (GC-MS, Agilent TRACE 6890/5973N).
3
.
Results and discussion
3
.1 Characterization of catalysts
The schematic illustration for fabrication of GO-VB catalysts was shown in
6
Scheme S1. XPS spectrum of GO-VB -4 in Fig. S1C shows the presence of C1s, O1s
6
and N1s, demonstrating that the VB was successfully modified on the surface of GO
6
sheets, although N1s peak displays much less noticeable. Notably, an obvious peak at
~
399.1 eV in the high-resolution of N1s spectrum for GO-VB -4 catalyst was
6
observed from Fig. 1A, corresponding to the pyridinic N, while the bare GO sheets
6

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and GO-COCl intermediate material reveal no peak of N species (Fig. S1A, B). It can
be clearly seen from Fig. S2A that the C1s scan of bare GO sheets represents four
peaks at ~284.8, ~286.7, ~287.8 and ~289.0 eV, ascribed to the C-C or C=C, C-O-C,
C=O and COOH, respectively. Relatively, Fig. S2B shows an obvious peak at ~285.2
eV in the high-resolution C1s spectrum of GO-VB -4 catalyst, indicating the
6
existence of C=N, which suggests VB successfully modification on the GO sheets.
6
Besides, FT-IR also confirms VB was successfully grafted on the GO surface, which
6
-1
exhibits a small and sharp peak at 1565 cm , attributing to the stretching vibration of
C=N (Fig. 1B). Such a peak still exists for the GO-VB -4 catalyst after repeated
6
washing and steeping by ethyl acetate for 24 h, which reveals that the grafted group is
greatly stable in the harsh testing (Fig. S3). Raman spectra of GO, GO-COCl and
GO-VB -4 depicted in Fig. 1C also verify the grafting of VB on the GO sheets,
6
6
which shows that VB functionalization of the GO sheets surface results in a slight
6
increase in the intensity ratio (I /I ) of the D band (structural defects in the vicinity of
D
G
-
1
2
1
1
350 cm ) over the G band (in-plane vibrations of carbon sp bonds in the vicinity of
-1
580 cm ), [29] consistent with an increase of structural distortion relative to the GO
frameworks. As we know, the structural distortion can generate more defects or edges
introduced into GO frameworks, which can serve as catalytic sites. [30, 31] Therefore,
GO-VB sheets with larger I /I will express higher electrocatalytic activity.
6
D G
Furthermore, Fig. 1D gives the XRD patterns of GO, GO-COCl and GO-VB -4. The
6
diffraction peak of C for the present GO-VB catalyst displays more positive shift of
6
the diffraction position, compared with the common GO and intermediate material
GO-COCl, ascribed to the doping effect of electron-rich N, [32] which reconfirmed
VB successful modification on the GO sheets.
6
To obtain more information about the surface morphologies of the prepared
samples, SEM and TEM were used. As seen from Fig. 2A, GO displays a typical
wrinkled and crumpled surface. Fig. 2B shows surface morphology of GO after
treatment by SOCl , which reveals a flexibly aggregated, crumpled nanosheets
2
structure. Compared with GO, GO-VB , after grafting various concentrations of VB
6
6
(
Figure 2C and Figure S4), are sheets like and transparent, as reconfirmed by the
7

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TEM of GO-VB -4 (Figure 2D), which may create a large number of active sites for
6
CO conversion.
2
3
.2 Electrochemical performance of GO-VB -4 catalyst
6
To investigate the possibility of employing GO-VB -4 catalyst for CO RR, LSVs
6
2
were firstly measured in both N -saturated and CO -saturated 0.1 M KHCO aqueous
2
2
3
solutions (pH values 8.67 and 6.80, respectively) at a scan rate of 50 mV/s in the same
scanning region using an SCE reference electrode (Fig. 3A). In order to avoid the
effect of the solution of pH, we have normalized all potentials to the RHE reference
scale for comparison purposes by following the equation (1). As shown in Fig. 3A,
under N atmosphere, a sharply increased cathodic current on GO-VB -4 was likely
2
6
resulted from the hydrogen evolution reaction (HER), [33,34] while the higher current
in CO -saturated solution could be defined as the reduction of pyridinic N combined
2
+
with H and then reaction with CO through multiple electrons and protons transfer
2
processes. [35-37] To evaluate potential catalytic roles of pyridinic N heterocycles in
CO RR, LSV measurement was also carried out in a 0.1 M KCl solution, which
2
presents the similar catalytic activity of GO-VB -4 for CO RR in KHCO solution
6
2
3
(
Fig. S5). Interestingly, an obvious reduction peak was observed in CO atmosphere,
2
whereas lower reduction peak was displayed in Fig. S5 under N -saturated 0.1 M KCl
2
aqueous solution with HCl (pH values 5.14, which is the same pH value as the
CO -saturated 0.1 M KCl aqueous solution), demonstrating that an enhanced current
2
was observed when CO and pyridinic N are included together in the aqueous
2
solution.
To further explore the influence of electrolytic potentials, potentiostatic
electrolysis of CO on GO-VB -4 are represented in Fig. 3B. The liquid products are
2
6
1
ethanol and acetone, detected by H NMR (Fig. S6). As shown in Fig. 3B, the FEs of
ethanol and acetone ascend with the negative shift of applied potential, reaching the
maximum value for ethanol and acetone (36.4% and 8.9%, respectively) at -0.40 V vs.
RHE and subsequently decrease at more negative potentials due to an increase in
competing with HER. [33,34] Thus, -0.40 V vs. RHE as the best reaction potential,
was applied in the subsequent tests. The absolute value of current density (J),
8

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symbolizing the production rate, is further studied. In contrast to total JTotal, partial J of
ethanol (JEthanol) and acetone (JAcetone) are illustrated in Fig. 3B. The JTotal increases
with the negative shift of applied potential, however, JEthanol shows a volcanic shape
depending on the applied potentials, while that of acetone increases slowly. In order to
determine that evolved products originate from CO , multiple control tests were
2
carried out. When the electrolyte was purged with N , no liquid products were
2
1
3
detected at the same potential range (Figure S7). Moreover, a C-labeling isotopic
1
3
12
experiment was carried out, in which CO replaced CO as the carbon source. Fig.
2
2
1
2
S8 and S9 show the GC-MS spectrum after electrocatalytic conversion of CO and
2
1
3
CO respectively. The GC-MS full spectra of Fig. S8 and S9 display the existence of
2
1
3
H O solvent, ethanol and acetone. When CO was used as the reactant, the signals of
2
2
1
3
13
3
13
13
13
CH CH OH (m/z = 47, 48) and CH CO CH (m/z = 61) were detected, which
2
3
3
1
2
12
3
were significantly different from the signals for CH CH OH (m/z = 45, 46) and
2
1
2
12
3
12
12
CH CO CH (m/z = 58) when CO was used. This is solid evidence to prove
3
2
that the evolved product comes exclusively from CO and not from any other carbon
2
source.
To better illustrate the electrochemical reduction of CO on metal-free carbon
2
material, a graphite sheet electrode was replaced the Pt sheet as the counter electrode.
The electrolysis was carried out on GO-VB -4 at a variety of constant potentials from
6
-
0.2 to -0.6 V vs. RHE in CO -saturated aqueous 0.1 M KHCO solution. These
2
3
results were presented in Figure S10, which exhibits exactly the same electrocatalytic
ability obtained using the Pt counter electrode. This phenomenon suggests that the
anodic was just inducing water oxide whether using a Pt sheet or a graphite sheet as
the counter electrode.
3
.3 Influence of the VB amounts for CO RR on various GO-VB catalysts
6 2 6
To deeply explore the influence of modified amounts of VB on the GO sheets
6
surface, we have selected different concentrations of VB functionalized on the GO
6
sheets surface using the same synthetic method. The total N contents (N ) were
T
exhibited in Table 1, which were analyzed by elemental analysis, and the total input
concentrations of VB were also provided in this table. The N contents of the GO-VB
6
6
9

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are closely connected with the concentrations of VB . Generally, the N contents
6
increase with increasing the concentrations of VB to 19.7 mM, however, following to
6
increase the concentrations of VB , the N contents acquire an obvious plateau, which
6
is around 2.32%. Hypothetically, the N contents can deeply affect the catalytic active
To verify the above guess, Fig. 4A reveals a dramatic difference in the amounts of
VB contents in the overall CO electrocatalytic reduction activity of GO-VB . For
6
2
6
GO catalyst, as shown in Fig. 4A and S11, much higher reduction current was
observed in CO -saturated solution than that in N -saturated solution, but none
2
2
reduction peak appeared. In sharp contrast, VB functionalized GO catalysts exhibit
6
obvious catalytic abilities for CO RR. Fig. 4A suggests that increasing the VB
2
6
contents in the GO-VB catalysts facilitates CO RR, leading to the enhanced activity
6
2
of CO reduction. The activities of GO-VB -1 and GO-VB -2 got worse performance
2
6
6
than all other catalysts in term of the reduction peak potentials and currents, which
resulted from the low contents of VB modified. Noteworthily, GO-VB -4, GO-VB -5
6
6
6
and GO-VB -6 showed similar performances, due to the almost same content of N
6
doping. The reduction peak potentials of GO-VB -3, GO-VB -4, GO-VB -5 and
6
6
6
GO-VB -6 displayed almost the same value, about -0.40 V vs. RHE, however, the
6
peak currents of GO-VB -4, GO-VB -5 and GO-VB -6 revealed the nearly identical
6
6
6
trend, which were higher than the one of GO-VB -3. The results clearly indicate that
6
an optimum content of VB in the GO-VB catalysts is required to achieve the
6
6
maximum activity for CO RR.
2
The electrocatalytic reduction of CO over different GO-VB sheets electrodes
2
6
employed a typical potential of -0.40 V vs. RHE were examined (Fig. 4B). As
expected, the raw material GO exhibited very poor catalytic property for CO RR
2
because of lack of the active center – pyridinic N. By contrast, after grafting VB on
6
the GO sheets surface, all of the prepared GO-VB samples showed superior
6
performance improvements for CO RR. From Fig. 4B, GO-VB -4, GO-VB -5 and
2
6
6
GO-VB -6, due to the similar contents of pyridinic N, showed almost no different
6
catalytic activities for CO RR, as consisted with the results of LSV. The increased FEs
2
of ethanol and acetone on the GO-VB sheets indicate that the enhanced catalytic
6
1
0

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activity is affected by an appropriate amount of VB modified on the GO surface. The
6
catalytic activities for CO RR on GO-VB -4, GO-VB -3, GO-VB -2 and GO-VB -1
2
6
6
6
6
gradually decline, ascribed to gradually decrement of pyridinic N contents. The
results indicate that low modification amounts of VB lead to not only the drop of N%
6
in GO-VB , but also clearly weaken catalytic abilities in the CO RR.
6
2
On account of the above results, we could assume that the pyridinic N species is the
main active species for electrocatalytic of CO to ethanol. The effect of the active
2
pyridinic N species on the catalytic active was exhibited in Fig. S12, which clearly
shows the relationship of the active N contents (calculated from Table 1) and the
products FEs. The FEethanol significantly rises from 7.5% to 36.4%, as increasing the
contents of active N species from 0.521% to 2.314%, whereas the FEacetone slightly
increases from 2.7% to 8.9%. Interestingly, the FETotal, FEEthanol and FEAcetone are all
positively related to the contents of pyridinic N for various GO-VB catalysts, which
6
suggests that the active pyridinic N species content plays a vital role for the high
catalytic ability of CO RR to ethanol and acetone.
2
3
.4 Material and electrochemical characterization of various pyridine derivatives
modified GO catalysts
For sake of investigate whether other pyridine derivatives modified GO also work
on the CO RR, a variety of pyridine derivatives including 4-pyridinal,
2
4
-aminopyridine, 8-hydroxyquionline and 5-amino-1,10-phenanthroline (the
structures of these pyridine derivatives are recorded in Table S1) with the same input
concentration of 19.7 Mm to modify GO, were explored. XPS characterized (Fig. S13)
and element analysis (Table S2) showed that these pyridine derivatives were
successfully grafted on the GO sheets surface. In addition, Raman spectra of GO-X-1,
GO-X-2, GO-X-3 and GO-X-4 were investigated, as seen from Fig. S14. The
intensities of ID/IG for GO-X distinctly exceed the one of GO (I /I =0.88),
D
G
demonstrated once again that the N heteroatoms were modified on the GO surface.
We also studied the electrochemical performance using the different GO-X electrodes
in 0.1 M KHCO solution and the results were provided in Fig. S15. GO-X catalysts
3
revealed different electrochemical activities towards CO RR in 0.1 M KHCO
2
3
1
1

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solution, yet from Fig. 5, the reduction peak potential on GO-VB -4 was about -0.40
6
V vs. RHE, which was more positive than those on all GO-X catalysts. Among these
materials, the absolute value of current on GO-VB -4 catalyst (0.745 mA at -0.40 V vs.
6
RHE) was higher than those on GO-X-1 (0.350 mA), GO-X-2 (0.349 mA), GO-X-3
(
0.312 mA) and GO-X-4 (0.104 mA). Potentiostatic electrolysis of CO were then
2
performed using different GO-X as electrodes and KHCO solution as the electrolyte
3
at the applied potential of -0.40 V vs. RHE. The liquid products (ethanol and acetone)
1
were detected by H NMR. The FEs of the products were calculated and summarized
in Table 2, which are in consistent with the results of LSV for different GO-X
catalysts. GO-X-1 and GO-X-2 owing to the similar structures grafted on GO, get the
approximated pyridinic N% of 2.764% and 2.540% and lower total FEs of 17.9% and
1
6.4% than the one of GO-VB -4, respectively. Other pyridines modified GO with
6
large branches like X-3 and X-4 also demonstrate reduced total FEs of 14.3% and
3
.1%, respectively.
VB , as a low-cost, wide distribution and environment friendly pyridine derivative,
6
was grafted on the GO sheets surface, providing a relatively high efficiency towards
CO RR to ethanol. This molecule, containing some strong electron donating groups,
2
such as -OH and -CH , can lead to the increase for electronegativity of the pyridinic N,
3
and then easily absorb or react with CO . Although other pyridine derivatives, such as
2
X-1 and X-2, also have one strong electron-donating group (e.g. -OH or -NH ), these
2
groups disappeared in the process of material synthesis and translated into the groups
of -COO- or -CONH-, as reconfirmed by the XPS spectra of GO-X-1 and GO-X-2
(Fig. S13a, b). Similarly, the GO-X-3 and GO-X-4 catalysts are observed the above
finding. Unfortunately, both X-3 and X-4 pyridine derivatives have an extra benzene
ring that linked to the pyridinic N. The benzene rings in the X-3 and X-4 molecular
have ability to disperse the electrons, leading to further attenuation of
electronegativity for pyridinic N. Such complex structures for GO-X-3 and GO-X-4
catalysts can weaken the abilities of CO adsorption and then reduce CO reduction
2
2
abilities. In summary, the FEs are closely related to the pyridinic N contents and the
structure of grafted pyridine derivatives for different GO-VB and GO-X catalysts.
6
1
2

ACCEPTED MANUSCRIPT
A detailed comparison on heteroatom doped carbon materials (especially N-doped
carbons) that have been recently studied as catalysts for CO RR was summarized and
2
compared in Table S3. Numerous reports have been made to CO RR to produce gas
2
and C1 products, such as H [15], CO [18], CH [20] and HCOOH [17], on metal-free
2
4
N-doped carbon nanofibers, N-doped CNT, NGM and N-doped carbon nanomaterials.
To the best of our knowledge, few reports have been further investigated on CO RR
2
to ethanol using metal-free N-functionalized graphene material. [38,39] Our work was
the first finding to combine GO and VB forming metal-free carbon materials, which
6
can produce ethanol with reasonable FE.
3
.5 Mechanism analysis
According to previous reports based on density functional theory, the key initial
step for CO RR is the formation of COOH* or CO* intermediates through
2
proton-electron transfer on the electrodes surface, and CO* dimerization forming
-
*
C O intermediate is a rate-determining step for the reduction of CO2 to
2
2
ethanol.[39,40] N-doped carbon materials can facilitate the proton-electron transfer in
the CO RR.[41,42] The active pyridinic N sites uniformly distributed on the surface
2
of GO sheets, due to electron withdrawing effects, are more electronegative than
carbon atoms, subsequently inducing the adjacent carbon atoms to be positively
polarized. Such a special structure can efficiently supply the electrons for the
dimerization of the CO* intermediates to produce ethanol, [43] analogous to
previously reported catalysts, such as N-doped graphene quantum dot [38] and
N-doped mesoporous carbon [39]. Moreover, the pyridinic N retains a lone pair of
electrons, which could bind CO , and exhibits low reaction energy for CO* formation
2
[
44] that can be stabilized to some extent by the order layer structure with high
electron density, hindering the formation of CO as a product [39]. Wu et al [38] have
reported that the pyridinic N is believed to be active to induce C-C bond formation,
which leads to a higher yield of C2 and C3 products. Therefore, the pyridinic N in
N-doped GO materials are indicated as the active sites for electrocatalytic CO2
reduction to ethanol and acetone in our works, but it is a complex process. Numerous
experimental data and theoretical calculations are still needed to investigate deeply
1
3

ACCEPTED MANUSCRIPT
and detailedly in the near future.
4
.
Conclusion
In summary, pyridine derivatives, such as 4-hydroxypyridine, 4-aminopyridine,
-hydroxyquionline, 5-amino-1,10-phenanthroline and pyridoxine, were successfully
8
grafted on GO sheets by adopting a simple and effective method, which show
remarkable enhanced catalytic abilities to reduce CO into ethanol. The metal-free
2
GO-VB catalysts demonstrated more superior activities for the electrochemical
6
reduction of CO to ethanol than other four pyridine derivatives modification on GO
2
sheets. The catalytic active of GO-VB was deeply affected by the pyridinic N
6
contents in the GO-VB catalysts. The experimental data reveal that pyridinic N is the
6
active site for CO RR. Besides, the catalytic activities of pyridine derivatives
2
modified GO exhibit distinct performances, which are not only closely related to the
pyridinic N contents of the GO-X, but also the structure of grafted the pyridine
derivatives. The optimal samples were observed for GO-VB with an N content of
6
around 2.32% on the surface of GO sheets, which produce the maximum FE ethanol
of approximately 37%. This work could instruct us to design large amounts of novel
materials with high catalytic activity, excellent selectivity and strong stability for CO2
electrochemical conversion.
Acknowledgements
Financial support from National Natural Science Foundation of China (21473060,
2
1673078, 21773071) is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary data related
to
this
article
can
be
found
at
https://doi.org/10.1016/j.electacta.2017.XX.XXX.
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Table 1. N % and C
of different GO-VB catalysts
T
VB6
6
a
b
Entry
GO
N (wt%)
CVB6 (mM)
T
0.189
0.521
0.972
1.562
2.314
2.317
2.321
0
GO-VB -1
2.96
5.92
11.8
19.7
29.6
39.4
6
GO-VB -2
6
GO-VB -3
6
GO-VB -4
6
GO-VB -5
6
GO-VB -6
6
a
N % represents the total N content.
T
b
CVB6 represents the total input concentration of VB6.
Table 2. Characteristics of the different GO-X catalysts with the FE for each product
at an applied potential of -0.40 V vs. RHE in 0.1 M KHCO3.
FE (%)
Material
Ethanol
14.2
Acetone
3.7
GO-X-1
GO-X-2
GO-X-3
GO-X-4
13.1
3.3
11.9
2.4
2.0
1.1
GO-VB -4
36.4
8.9
6
1

ACCEPTED MANUSCRIPT
6200
Raw
Background
Pyridinic N
GO-VB -4
6
B
A
6000
5800
-1
1
565 cm
νC=N
5
600
GO-COCl
GO
5400
390
395
400
405
410
4000
3500
3000
2500
2000
1500
1000
500
Bind energy (eV)
-1
Wavenumber(cm )
D band
G band
D
C
GO-VB -4
GO-VB -4
6
I /I =1.43
6
D
G
GO-COCl
GO-COCl
I /I =1.33
D
G
I /I =0.88
GO
D G
GO
70
1200
1500
1800
10
20
30
40
50
60
80
-1
Wavenumber(cm )
2Theta(θ)
Fig. 1. (A) High-resolution of N1s for GO-VB -4; FT-IR spectra (B), Raman spectra
6
(
C) and XRD patterns (D) of GO, GO-COCl and GO-VB -4.
6
A
B
C
D
Fig. 2. SEM images of (A) GO, (B) GO-COCl, (C) GO-VB -4; (D) TEM image of
6
GO-VB -4.
6
1

ACCEPTED MANUSCRIPT
5
4
3
2
0
0
0
0
0
.0
A
B
FEAcetone
FEEthanol
1.5
1.0
JTotal
JEthanol
JAcetone
-0.5
0
.5
.0
-1.0
10
0
GO-VB -4(N )
6
2
0
GO-VB -4(CO )
6
2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
-0.2
-0.3
-0.4
-0.5
-0.6
E(V vs. RHE)
E(V vs. RHE)
Fig. 3. (A) LSV patterns of GO-VB -4 catalyst on a glassy carbon electrode with
6
saturated CO (red line) and N (black line); (B) Absolute value of J and FEs for CO
2
2
2
reduction products on GO-VB -4 catalyst at various potentials in 0.1 M KHCO
6
3
solutions.
0
.0
40
FEethanol
FEacetone
A
B
-0.2
3
0
0
-0.4
GO
GO-VB -1
6
-0.6
2
GO-VB -2
6
-0.8
GO-VB -3
6
GO-VB -4
6
10
0
-1.0
GO-VB -5
6
GO-VB -6
6
-1.2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
GO
GO-VB6-1 GO-VB6-2 GO-VB6-3 GO-VB6-4 GO-VB6-5 GO-VB6-6
E(V vs. RHE)
Diffent GO-VB6
Fig. 4. (A) LSV patterns of GO and various GO-VB catalysts on a glassy carbon
6
electrode with saturated CO in 0.1 M KHCO aqueous solution as the electrolyte and
2
3
at a scan rate of 50 mV/s. (B) FEs of CO reduction on different GO-VB sheets
2
6
surface at -0.40 V vs. RHE in 0.1 M KHCO aqueous solution.
3
2

ACCEPTED MANUSCRIPT
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
GO-X-1
GO-X-2
GO-X-3
GO-X-4
GO-VB -4
6
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
E(V vs. RHE)
Fig. 5. LSV patterns of different GO-X catalysts and GO-VB -4 catalyst.
6
3