Selective Electrochemical Reduction of Carbon Dioxide to Ethanol on a Boron- and Nitrogen-Co-doped Nanodiamond

Article abstract of DOI:10.1002/anie.201706311

Electrochemical reduction of CO₂ to ethanol, a clean and renewable liquid fuel with high heating value, is an attractive strategy for global warming mitigation and resource utilization. However, converting CO₂ to ethanol remains great challenge due to the low activity, poor product selectivity and stability of electrocatalysts. Here, the B- and N-co-doped nanodiamond (BND) was reported as an efficient and stable electrode for selective reduction of CO₂ to ethanol. Good ethanol selectivity was achieved on the BND with high Faradaic efficiency of 93.2 % (?1.0 V vs. RHE), which overcame the limitation of low selectivity for multicarbon or high heating value fuels. Its superior performance was mainly originated from the synergistic effect of B and N co-doping, high N content and overpotential for hydrogen evolution. The possible pathway for CO₂ reduction revealed by DFT computation was CO₂→*COOH→*CO→*COCO→*COCH₂OH→*CH₂OCH₂OH→CH₃CH₂OH.

Full text of DOI:10.1002/anie.201706311

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Selective electrochemical reduction of CO2 to ethanol on B and N
codoped nanodiamond
Authors: Yanming Liu, Yujing Zhang, Kai Chen, Xie Quan, Xinfei Fan,
Yan Su, Shuo Chen, Huimin Zhao, Yaobin Zhang, Hongtao
Yu, and Michael R. Hoffmann
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201706311
Angew. Chem. 10.1002/ange.201706311
Link to VoR: http://dx.doi.org/10.1002/anie.201706311
http://dx.doi.org/10.1002/ange.201706311

10.1002/anie.201706311
Angewandte Chemie International Edition
COMMUNICATION
Selective electrochemical reduction of CO2 to ethanol on B and N
codoped nanodiamond
Yanming Liu,# Yujing Zhang,# Kai Cheng, Xie Quan,* Xinfei Fan, Yan Su, Shuo Chen, Huimin Zhao,
Yaobin Zhang, Hongtao Yu, and Michael R. Hoffmann
Abstract: Electrochemical reduction of CO2 to ethanol, a clean and
renewable liquid fuel with high heating value, is an attractive strategy
for global warming mitigation and resource utilization. However,
converting CO2 to ethanol remains great challenge due to the low
activity, poor product selectivity and stability of electrocatalysts. Here,
B and N codoped nanodiamond (BND) was reported as an efficient
and stable electrode for selective reduction of CO2 to ethanol. Good
ethanol selectivity was achieved on BND with high Faradaic efficiency
of 93.2% (-1.0 V vs RHE), which overcame the limitation of low
selectivity for multicarbon or high heating value fuels. Its superior
performance was mainly originated from the synergistic effect of B
and N codoping, high N content and overpotential for hydrogen
evolution. The possible pathway for CO2 reduction revealed by DFT
computation was CO2 → *COOH → *CO → *COCO → *COCH2OH
→ *CH2OCH2OH → CH3CH2OH.
morphology,[5]
exposed
surface
facet[6]
or
surface
functionalization[7] of metals. Although metals exhibit competent
activity, they often undergo easy deactivation or poor selectivity
for high energy density products. Transition metal oxides and
chalcogenides (e.g. Co3O4) have also been explored for
electrocatalytic CO2 reduction.[8] Their primary products are
limited to CO or HCOO- with maximum Faradic efficiency of 60.0-
85.1%. Recently, carbon based nanomaterials[9] such as B doped
diamond (BDD), N doped carbon nanotubes and graphene have
been proposed as promising electrocatalysts for CO2 reduction
due to their earth abundant component, good activity and
durability, which reduce CO2 to CO, HCHO or HCOO- with
maximum Faradic efficiency of 74.0-87.0%. Although notable
progress has been achieved, there remains a major challenge to
develop durable electrocatalysts that can efficiently and
selectively convert CO2 to high heating value fuels, especially that
with multicarbon such as CH3CH2OH.
The world-wide utilization of fossil fuels deplete feasible fossil
fuel sources and produce large amount of CO2. Electrocatalytic
reduction of CO2 to fuels or other chemicals can reduce CO2
accumulation in atmosphere and alleviate energy crisis, which is
a promising method for carbon recycling if powered by renewable
energy sources. Electrocatalytic CO2 reduction can yield a variety
of products, including CO, HCOO-, HCHO, CH3COO-, CH3OH,
CH3CH2OH, CH4, etc. However, CH3CH2OH is preferred product
because it is a clean and renewable liquid fuel with high heating
value (-1366.8 kJ/mol, higher than -283.0~-890.4 kJ/mol for
others listed above). It is also an important raw material for
organic chemicals and disinfectants. Industrial production of
CH3CH2OH consumes large amount of agricultural feedstocks or
ethylene. Thus, there is a pressing need to find efficient
electrocatalysts that can selectively convert CO2 to CH3CH2OH.
Homogeneous metal-ligand complex and heterogeneous
electrocatalysts have been reported for CO2 reduction.[1] Metals[2]
and metal alloys[3] have been found to be active for
electrocatalytic reduction of CO2, where product selectivity and
efficiency could be tailored by controlling particle size,[4]
B and N codoped diamond (BND) is a potential electrocatalyst
with good stability. B and N doping regulate the electronic
structure of diamond,[10] which enable to form defect-induced
active sites for CO2 reduction. BND has a more negative H2
evolution potential than the electrocatalysts mentioned above,
which is more favorable for CO2 reduction with higher Faradic
efficiency. These properties make BND an attractive alternative
electrocatalyst for CO2 reduction. N doped diamond (NDD)
preferentially reduces CO2 to C2 product CH3COO-,[11] while the
main product reported for BDD is HCHO.[9b] Given the observed
synergistic effect of B and N codoping on electrocatalytic activity
of carbon nanomaterials,[10a,12] codoped BND may afford better
electrocatalytic performance than BDD and NDD, and convert
CO2 to highly reduced product. Moreover, CO2 reduction
performance of BND can be tuned by altering its chemical
composition. However, electrocatalytic CO2 reduction on BND
and the impact of codoping with B and N on CO2 reduction have
never been reported before. Herein, BND was explored for
electrocatalytic reduction of CO2. The effect of codoped B and N
on CO2 reduction performance was elucidated by experiments
combined with density function theory (DFT) calculation.
BND electrodes were prepared by depositing BND film on Si
substrate using hot filament chemical vapor deposition method
with a gas mixture of CH4/B2H6/N2/H2 that had 2.5% CH4. BNDs
with the same B2H6 content (12.5%) but different N2 levels (2.5%,
5.0% and 10.0%) were prepared, which are denoted as BND1,
BND2 and BND3, respectively. The surface morphology of
prepared BND electrodes was characterized by SEM images
(Figure 1a-c), which show BND films are composed of pyramidal
nanoparticles that cover the entire substrates. Well-faceted
diamond crystals with clear edges and corners are observed on
BND1. When N content of BND is increased, twin crystals appear
on the grain boundaries of BND2 and BND3.
[*] Dr. Y. M. Liu, Y. J. Zhang, Prof. X. Quan, Dr. X. F. Fan, Dr. S. Chen, Prof.
H. M. Zhao, Prof. Y.B Zhang, Dr. H. T. Yu
Key Laboratory of Industrial Ecology and Environmental Engineering
(Ministry of Education), Faculty of Chemical, Environmental and
Biological Science and Technology
Dalian University of Technology, Dalian 116024, China
E-mail: quanxie@dlut.edu.cn
Dr. Y. Su, Prof. J. J. Zhao
School of Physics and Opto-Electronic Technology
Dalian University of Technology, Dalian 116024, China
Prof. M. R. Hoffmann
Linde+Robinson Laboratories
California Institute of Technology, California 91125, United States
[#] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
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10.1002/anie.201706311
Angewandte Chemie International Edition
COMMUNICATION
a
c
b
example, at -1.1 V, the measured production rate of CH3CH2OH
is 9.3 times, 17.5 times, 51.0 times as large as that of CH3OH,
and 11.4 times, 16.1 times, 12.9 times as large as that of HCOO-
for BND1, BND2 and BND3, respectively. Moreover, CH3CH2OH
production rate is significantly increased as potential negatively
shifted from -0.8 V to -1.1 V (decrease at -1.1 V due to competition
from H2 evolution), while CH3OH and HCOO- production rates
remain low in the range of potential applied. The CH3CH2OH
production rate of BND2 is higher than that of BND1 under any
potential applied, and lower than that of BND3 at -0.8~-1.0 V,
which indicates increasing N content of BND to some extent can
facilitate CH3CH2OH production. At -1.1~-1.2 V, BND2 has a
slightly higher CH3CH2OH production rate than BND3, which may
be attributed to more energy is consumed for H2 evolution on
BND3 than on BND2 (Figure S2).
500 nm
(111)
500 nm
500 nm
e
d
1220 cm-1
1320 cm-1
(220)
(311)
BND3
BND3
BND2
BND2
BND1
BND1
900
1200
1500
1800
20
40
60
2 theta (deg.)
80
100
Raman Shift (cm-1
)
Figure 1. SEM images of (a) BND1, (b) BND2 and (c) BND3 along with their
corresponding (d) XRD and (e) Raman spectra.
1
0
-1
Ar
The XRD spectra (Figure 1d) of BNDs display three
characteristic diffraction peaks at 43.9°, 75.3° and 91.5°, which
are assigned to (111), (220) and (311) facets of cubic diamond.
As revealed by peak intensity, the main facet is (111) for BND1,
while the main facet for BND2 and BND3 is (220), indicating the
increased N content alters the crystal growth direction of BND
under the conditions applied (details in supporting information). In
the Raman spectra (Figure 1e), peaks around 1220 cm-1 and
1315-1320 cm-1 appear on each of the BNDs. The peak located
at 1220 cm-1 can be attributed to polyacetylene generated at the
grain boundary of nanocrystalline diamond. The peak around
1315-1320 cm-1 corresponds to sp3 bonded carbon, which is the
characteristic peak of diamond. X-ray photoelectron spectroscopy
(XPS) analysis demonstrates B and N are doped into the diamond
lattice (Figure S1). The B contents of BND1, BND2 and BND3 are
similar (2.4 at.%, 2.4 at.%, 2.5 at.%), while their N contents
increase gradually (3.1 at.%, 3.6 at.%, 4.9 at.%).
Linear sweep voltammograms of BNDs were measured to
probe their electrocatalytic activity for CO2 reduction. As
presented in Figure 2, the current density in CO2 saturated
solution (jCO2) is greater than that in Ar saturated solution (jAr) for
each BND when potential is more negative than threshold value,
manifesting that the three BNDs are active for electrocatalytic
reduction of CO2. At the same applied potential, BND3 exhibits
the highest net current density for CO2 reduction (jCO2-jAr) at
potential from -0.6 V to -1.0 V, followed by BND2. The onset
potential for CO2 reduction is -0.6 V for BND3, much more positive
than those for BND2 (-0.7 V) and BND1 (-0.9 V). It is notable that
as the N content of BND increases, BND is more active for
electrocatalytic CO2 reduction in terms of both onset potential and
current density (at -0.6 V to -1.0 V). In addition, BNDs present
very negative potential for H2 evolution (-1.0~-1.1 V), on which
CO2 reduction can outcompete proton reduction even at potential
as negative as -1.1 V. Its high overpotential for H2 evolution is
favorable for improving CO2 reduction efficiency.
CO2
-2
1
0
-1
Ar
CO2
-2
1
0
-1
-2
Ar
CO2
-1.2
-0.8
-0.4
0.0
Potential (V vs. RHE)
Figure 2. Linear sweep voltammograms of BND1, BND2 and BND3 in Ar or
CO2 saturated 0.1 M NaHCO3 solution (scan rate 100 mV s-1).
90
90
72
54
36
18
0
b
a
C2H5OH
CH3OH
HCOO-
C2H5OH
CH3OH
HCOO-
72
54
36
18
0
-1.2
-1.1
-1.0
-0.9
-0.8
-1.2
-1.1
-1.0
-0.9
-0.8
Potential (V vs RHE)
Potential (V vs RHE)
100
80
60
40
20
0
90
72
54
36
18
0
d
c
HCOO-
CH3OH
C2H5OH
CH3OH
HCOO-
C2H5OH
-1.2
-1.1
-1.0
-0.9
-0.8
-1.2
-1.1
-1.0
-0.9
-0.8
Potential (V vs RHE)
Potential (V vs RHE)
100
80
60
40
20
0
100
80
60
40
20
0
e
f
HCOO-
CH3OH
HCOO-
CH3OH
C2H5OH
C2H5OH
To confirm the occurrence of CO2 reduction on BNDs and
explore their reduction products, CO2 electrolysis was performed
on BNDs at applied potentials ranging from -0.8 to -1.2 V. The
detected products of CO2 reduction are CH3CH2OH, CH3OH and
HCOO- on BND1, BND2 and BND3 (Figure 3a-c). Interestingly,
all three BNDs preferentially convert CO2 to CH3CH2OH, for
-1.2
-1.1
-1.0
-0.9
-0.8
-1.2
-1.1
-1.0
-0.9
-0.8
Potential (V vs RHE)
Potential (V vs RHE)
Figure 3. Production rates of CH3CH2OH, CH3OH and HCOO- on (a) BND1, (b)
BND2 and (c) BND3 in CO2 saturated 0.1 NaHCO3 solution, and
corresponding Faradaic efficiencies on (d) BND1, (e) BND2 and (f) BND3.
M
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Achieving high Faradaic efficiency is still a great challenge for
electrocatalytic reduction of CO2, especially for producing high
heating value fuels such as CH3CH2OH. Here, Faradaic efficiency
for CO2 reduction on BNDs was determined to further evaluate
their performance (Figure 3d-f). As expected, the Faradaic
efficiency for CH3CH2OH is much higher than those for CH3OH
and HCOO- on BNDs under any potential applied. It increases
gradually as potential is shifted from -0.8 V to -1.0 V, and then
decreases at more negative potentials which may be due to
competition from H2 evolution reaction (Figure S1). The maximum
Faradaic efficiency for CH3CH2OH achieved on BND3 is 93.2% at
-1.0 V, which is 3107.7 times as great as that for CH3OH and 69.6
times as great as that for HCOO-. It illustrates BND presents very
high efficiency and selectivity for CH3CH2OH, which is the highest
among electrocatalysts reported to date.[5a,13] The Faradaic
morphology and dopant content, identical crystal structure and
dominated facet, the main factors contributing to the high activity
and selectivity for CH3CH2OH on BND could be the synergetic
effect of B and N codoping. Both NDD and BND preferentially
convert CO2 to C2 product, which suggests doping N into
diamond may be key for effective C-C coupling reaction. CO2
reduction on N doped graphitic carbon materials is reported to
yield either HCOOH or CO.[9c,9d] Thus, the high CH3CH2OH
selectivity of BND appears to be correlated to the intrinsic property
of codoped diamond.
Both CH3CH2OH production rate and current efficiency of BNDs
are in the order of BND3> BND2> BND1 before H2 evolution
occurred (Figure 3). As the three BNDs have the same crystal
structure, similar morphology and B content, it can be deduced
that the enhanced CH3CH2OH production performance of BND
efficiencies for CH3CH2OH follows the order BND1<BND2<BND3, may be originated from its increased N content and/or altered
elucidating that higher N content of BND is favorable for
converting CO2 to CH3CH2OH.
main facet (during BND preparation, its main facet changes from
(111) to (220) with increased N content). BND3 is more active
than BND2 for CH3CH2OH production and both of their main facet
is (220), demonstrating the increased N content is responsible for
enhanced CH3CH2OH production performance of BND3. As high
N content will also promote H2 evolution, a balance between N
content and H2 evolution is crucial for achieving good CO2
reduction performance on BND. Meanwhile, DFT computation is
employed to probe the impact of (111) and (220) facets for the
preferential production of CH3CH2OH. As shown in Figure S7,
CO2 reduction is more favorable on (111) facet than (220) facet
with much lower reaction free energy, which suggests altered
facet may be not the main factor contributing to the superior
performance of BND for CH3CH2OH production.
Figure 5a shows the free energy diagram of possible pathways
for CO2 reduction on (111) facet of BND. The multi-electron
reduction of CO2 to CH3CH2OH proceeds as follows: CO2 →
*COOH → *CO → *COCO → *COCOH → *COCHOH →
*COCH2OH → *CHOCH2OH → *CH2OCH2OH → CH3CH2OH.
Interestingly, two neighboring CO* species can easily link
together (*CO → *COCO) with reaction free energy of -3.32 eV,
indicating C-C coupling reaction occurs easily on BND, which is
one of the reason that BND has much higher CH3CH2OH
selectivity over C1 product. In addition, the free energy for the
elementary reaction and overall process of CH3CH2OH
production is negative, which illustrates reducing CO2 to
CH3CH2OH is thermodynamically favorable on BND. Meanwhile,
the synergetic effect of B and N codoping for CH3CH2OH
production also can be revealed by DFT analysis. Figure 5b
shows the energetically favorable configuration associated with
elementary step of CO2 reduction at diamond (111) surface, which
was explored from possible structures. At first, the doped B atom
may enhance CO2 capture by bonding with one O atom of
absorbed CO2 (*CO2). In the subsequent *CO2 reduction process
which proceeds through addition of H, H preferentially adsorbs at
doped N atom before diffusion into *CO2. The doped N can
facilitate *H transfer in the elementary reactions involving
hydrogenation, taking the reaction *COCO → *COCOH for
example, the energy barrier is 2.74 eV when H is transferred from
the top C atom of diamond (without N doping), while it is 0.89 eV
for H transferred from N atom. Meanwhile, B atom also plays an
important role by forming B-O bond in the reactions *COOH →
The durability of BND3 for electrocatalytic reduction of CO2 was
evaluated by running 16 consecutive CO2 reduction experiments
with each run lasting for 3 h at -1.0 V. As shown in Figure 4a,
Faradaic efficiency for CH3CH2OH keeps around 93.2% during 16
sequential CO2 reduction experiments, which illustrates the high
durability of BND for electrocatalytic reduction of CO2.
a
b
HCOO-
HCHO
CH3OH
C2H5OH
100
80
60
40
20
0
100
80
60
40
20
0
CH3COOH
0
4
8
12
16
NDD BDD BND1 BND2 BND3
Cycle
Figure 4. (a) Faradaic efficiencies for CH3CH2OH during 16 consecutive runs
for CO2 reduction on BND3 at -1.0 V, (b) Faradaic efficiencies for CO2 reduction
on NDD, BDD, BND1, BND2 and BND3 at -1.0 V.
In order to further probe CO2 reduction activity of BNDs, their
performance was compared to singly doped BDD and NDD. SEM
images show BDD film and NDD film are uniform with well-faceted
diamond crystals (Figure S3). Both BDD and NDD are cubic
diamond dominated by (111) facet (Figure S4). XPS shows BDD
has the same B content as BND1 and BND2 (2.4 at.%), and NDD
has the same N content as BND2 (3.6 at.%, slightly higher than
3.1 at.% of BND1). Their linear sweep voltammograms (Figure S5)
reveals both BDD and NDD are active for CO2 reduction and the
net current density of BND2 for CO2 reduction is much higher than
those of BDD and NDD, indicating co-doping significantly
enhances CO2 reduction activity. The main products for
electrocatalytic CO2 reduction are HCHO/HCOOH for BDD,
whereas they are CH3COOH/HCOOH for NDD (Figure S6).
Despite their generation of lower heating value products relative
to CH3CH2OH produced on BNDs, the Faradaic efficiency for
HCHO (53.9%)/HCOOH (26.1%) production on BDD, CH3COOH
(62.4%)/HCOOH (24.7%) production on NDD is much lower than
those of CH3CH2OH generation on BNDs, which are 83.3%, 88.8%
and 93.2% for BND1, BND2 and BND3, respectively (Figure 4b).
Considering that BDD, NDD and BND1 have the similar
*CO and *COCHOH
→ *COCH2OH → *CHOCH2OH →
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10.1002/anie.201706311
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COMMUNICATION
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*CH2OCH2OH, which can promote these elementary reactions.
Both experimental and DFT results imply that the synergetic effect
of B and N codoping is crucial for CH3CH2OH production on BND.
1
a
CO2
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0
*CO
*CH2OCH2OH
-1
CH3CH2OH
*COOH
*CHOCH2OH
-2
*COCH2OH
-3
*COCHOH
*COCOH
*COCO
-4
Reaction coordinate
b
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Figure 5. (a) Free energy diagrams for CO2 reduction on (111) facet of BND,
(b) Energetically favorable structures for elementary steps of CO2 reduction on
(111) facet of BND (gray = C, pink = B, blue = N, red = O, white = H).
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Efficient and selective electrochemical reduction of CO2 to
CH3CH2OH has been achieved on BND with CH3CH2OH
production rate one order of magnitude higher than those of minor
products. The Faradaic efficiency for CH3CH2OH was 93.2%,
significantly enhanced relative to values reported for other
electrocatalysts. Experimental analysis coupled with DFT
calculation indicated the high activity and selectivity of BND for
CH3CH2OH production mainly originated from the synergetic
effect of B and N codoping and fine balance between N content
and H2 evolution potential. The preferred pathway obtained from
DFT computation was CO2 →*COOH → *CO → *COCO
→*COCH2OH →*CH2OCH2OH → CH3CH2OH, where *COCO
formation occurred easily on BND. This work provides new insight
into converting CO2 to high heating value C2 products such as
CH3CH2OH by carbon based electrodes.
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Keywords: CO2 reduction • doped diamond • electrocatalysis •
ethanol • heterogeneous catalysis
This article is protected by copyright. All rights reserved.

10.1002/anie.201706311
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Efficient and selective electrochemical
reduction of CO2 to ethanol was
achieved on nonmetallic B and N
Y. M. Liu, Y. J. Zhang, K. Chen, X. Quan,*
X. F. Fan, Y. Su, S. Chen, H. M. Zhao, Y.
B. Zhang, H. T. Yu, M. R. Hoffmann
codoped
nanodiamond.
The
C2H5OH
C2H5OH
Page No. – Page No.
synergistic effect of B/N codoping, N
content and H2 evolution potential were
used as key factors for tailoring ethanol
Selective electrochemical reduction of
CO2 to ethanol on B and N codoped
nanodiamond
selectivity.
These
factors
and
2CO2 + 9H2O + 12e– → C2H5OH + 12OH−
uncovered CO2 reduction mechanism
is valuable for converting CO2 to
multicarbon or high heating value fuels.
This article is protected by copyright. All rights reserved.