Electrochemical Reduction of Carbon Dioxide to 1-Butanol on Oxide-Derived Copper

Article abstract of DOI:10.1002/anie.202008289

The electroreduction of carbon dioxide using renewable electricity is an appealing strategy for the sustainable synthesis of chemicals and fuels. Extensive research has focused on the production of ethylene, ethanol and n-propanol, but more complex C₄ molecules have been scarcely reported. Herein, we report the first direct electroreduction of CO₂ to 1-butanol in alkaline electrolyte on Cu gas diffusion electrodes (Faradaic efficiency=0.056 %, j_1-Butanol=?0.080 mA cm^?2 at ?0.48 V vs. RHE) and elucidate its formation mechanism. Electrolysis of possible molecular intermediates, coupled with density functional theory, led us to propose that CO₂ first electroreduces to acetaldehyde-a key C₂ intermediate to 1-butanol. Acetaldehyde then undergoes a base-catalyzed aldol condensation to give crotonaldehyde via electrochemical promotion by the catalyst surface. Crotonaldehyde is subsequently electroreduced to butanal, and then to 1-butanol. In a broad context, our results point to the relevance of coupling chemical and electrochemical processes for the synthesis of higher molecular weight products from CO₂.

Full text of DOI:10.1002/anie.202008289

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Electrochemical Reduction of Carbon Dioxide to 1-Butanol on
Oxide-Derived Copper
Authors: Louisa Rui Ling Ting, Rodrigo Garcia-Muelas, Antonio J
Martin, Florentine L. P. Veenstra, Stuart Tze-Jin Chen, Yujie
Peng, Edwin Yu Xuan Per, Sergio Pablo García, Nuria Lopez,
Javier Perez-Ramirez, and Boon Siang Yeo
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202008289
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10.1002/anie.202008289
Angewandte Chemie International Edition
RESEARCH ARTICLE
Electrochemical Reduction of Carbon Dioxide to 1-Butanol on
Oxide-Derived Copper
Louisa Rui Lin Ting,[a,b]† Rodrigo García-Muelas,[c]† Antonio J. Martín,[d]† Florentine L. P. Veenstra,[d]
Stuart Tze-Jin Chen,[a] Yujie Peng,[a] Edwin Yu Xuan Per,[a] Sergio Pablo-García,[c] Núria López,[c]
Javier Pérez-Ramírez,[d] and Boon Siang Yeo[a,b]*
[a]
L.R.L. Ting, S.T.-J. Chen, Y. Peng, E.Y.X. Per, Prof. Dr. B.S. Yeo
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543
E-mail: chmyeos@nus.edu.sg
[b]
[c]
[d]
L.R.L. Ting, Prof. Dr. B.S. Yeo
Solar Energy Research Institute of Singapore
National University of Singapore
7 Engineering Drive 1, Singapore 117574
Dr. R. García-Muelas, S. Pablo-García, Prof. Dr. N. López
Institute of Chemical Research of Catalonia
The Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona, Spain
Dr. A.J. Martín, F.L.P. Veenstra, Prof. Dr. J. Pérez-Ramírez
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences
ETH Zürich
Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland
† These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document. All the structures for computational results can be retrieved in ioChem-
BD. (Reviewer’s link: https://iochem-bd.iciq.es/browse/review-collection/100/23734/da1e9984881d741df6f45707)
Abstract: The electroreduction of carbon dioxide using renewable
electricity is an appealing strategy for the sustainable synthesis of
chemicals and fuels. Extensive research has focused on the
production of ethylene, ethanol and n-propanol, but more complex C4
molecules have been scarcely reported. Herein, we report the first
direct electroreduction of CO2 to 1-butanol in alkaline electrolyte on
(FE = 30-50%).[2,4] The main C3 product reported is n-propanol
(FE = 10-13%),[5,6] alongside small quantities of propionaldehyde,
allyl alcohol, acetone, propylene and propane (total FE < 3%).[4,5]
Reports on the formation of C4 molecules, many of which have
much higher commercial value, are scarce and have been limited
to hydrocarbons showing FE < 1%. [7] The drastic decrease in the
selectivity of a product as the number of carbon atoms in it
increases suggests that the coupling mechanism to form a multi-
carbon product follows a ‘polymerization’ scheme of *CO that
obeys the Flory–Schulz distribution.
Interestingly, the direct production of 1-butanol
(CH3CH2CH2CH2OH) from electrochemical CO2 reduction has not
been reported. This oxygenate, which has a high volumetric
energy density of 29.2 MJ L−1 and is less hygroscopic and
corrosive than ethanol, has been suggested for direct use as a
fuel or in diesel-blends.[8] Schmid and co-workers have utilized
bacteria to convert CO (generated from CO2 electrolysis) to
1-butanol.[9] More recently, a mechanistic study of CO2 reduction
to n-propanol revealed that minor and yet-to-be-quantified
amounts of 1-butanol can be co-produced from the
electrochemical reduction of acetaldehyde and CO in 0.1 M KOH
on oxide-derived Cu electrodes.[10] Still, no strategy to
successfully electrosynthesize 1-butanol or any C4 oxygenates
from CO2 has been conceived. To tackle this challenge, it is
crucial to understand and map out the mechanism and kinetics for
its formation.
Cu gas diffusion electrodes (Faradaic efficiency = 0.056%, j1-butanol
=
−0.080 mA cm−2 at −0.48 V vs. RHE) and elucidate its formation
mechanism. Electrolyses of possible molecular intermediates,
coupled with density functional theory, led us to propose that CO2 first
electroreduces to acetaldehyde—a key C2 intermediate to 1-butanol.
Acetaldehyde then undergoes base-catalyzed aldol condensation to
give crotonaldehyde via electrochemical promotion from the catalyst
surface. Crotonaldehyde is subsequently electroreduced to butanal,
and then to 1-butanol. In a broad context, our results point to the
relevance of coupling chemical and electrochemical processes for the
synthesis of higher molecular weight products from CO2.
Introduction
The electrochemical reduction of carbon dioxide (CO2RR)
to fuels and chemicals, when powered by renewable electricity, is
a potentially sustainable way to alleviate our pressing global
energy demands and to avert climate change.[1] Copper-based
materials are the only family of catalysts that can reduce CO2 to
multi-carbon molecules with significant Faradaic efficiencies (FE)
and current densities (j).[2,3] Among the multi-carbon products, C2
molecules such as ethylene and ethanol can be facilely formed
Herein, we report and quantify for the first time the formation
of C4 oxygenates from alkaline electrolysis of CO2 using
CuO-derived Cu gas diffusion electrodes (GDE) in a flow cell. The
predominant C4 product was 1-butanol (FE = 0.056 %,
1
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10.1002/anie.202008289
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RESEARCH ARTICLE
j1-butanol = −0.080 mA cm−2) at −0.48 V vs. RHE (Reversible
Hydrogen Electrode; from here on, all potentials are referenced to
the RHE and all currents are normalized to the exposed geometric
surface area of the electrodes, unless otherwise stated). We then
elucidate the reaction mechanism by combining analyses of
reaction products from electrolyses of possible intermediates and
density functional theory (DFT) investigations. The formation of
the critical C4 intermediate, crotonaldehyde (CH3CH=CHCHO),
was traced to the aldol condensation (C-C bond formation) of two
acetaldehyde (CH3CHO) molecules generated from CO2
electroreduction (Figure 1a). The aldol reaction is promoted by
both OH− ions in the electrolyte and the electrocatalyst surface.
Crotonaldehyde then undergoes a two-step electroreduction to
1-butanol. We also unveil the critical role of pH at different stages
of the reaction mechanism, pointing towards new strategies for
increasing the performance of electro-assisted conversion of CO2
into 1-butanol.
Figure 1. (a) Simplified reaction scheme for CO2 reduction to 1-butanol. Further details are provided in Section S3. (b) Diagram of CO2 flow cell electrolysis set up:
characterization of CuO-derived Cu cathodes by (i) SEM, (ii) XRD and (iii) XPS (B.E. refers to binding energy). We note that the detection of Cu2O in the XRD and
CuO + Cu(OH)2 signals in the XPS data are likely due to surface oxidation of the electrode from its exposure to air (see Section S2.1 for a more detailed discussion);
(iv) large CO2 reduction current densities (in the order of −100 mA cm−2) which gave a sufficient rate of product formation to allow detection of minor products; (v)
sensitive analytical techniques like headspace GC can quantify minor products down to the μM-scale. (c) Faradaic efficiencies (FE, colored bars) and partial current
densities (j, ∎) of C2, C3 and C4 products from electrolysis of CO2 on CuO-derived Cu GDE in 1.0 M KOH. The major C2, C3 and C4 products are ethylene, n-propanol
and 1‐butanol respectively. Other detected products are shown in Table S1.
2
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product was 1-butanol (FE = 9.6 %, j1-butanol = −1.06 mA cm−2),
consistent with expectations from
a base-catalyzed aldol
Results and Discussion
condensation C-C coupling step. The remaining electrolysis
products were other C4 oxygenates such as butanal
(CH3CH2CH2CHO) and crotyl alcohol (CH3CH=CHCH2OH), as
well as ethanol. During the electrolysis, we observed some
coloration of the anion-exchange membrane due to its exposure
to the alkaline acetaldehyde-containing electrolyte, but control
experiments excluded its interference with our electrolysis results
(Figures S8-9).
We electroreduced CO2 at various potentials using CuO-
derived Cu GDE cathodes in a flow cell (Figure 1b, Sections S1-
S2). The catalyst was electrodeposited onto the GDE using a
previously-published procedure.[11] Aqueous 1.0 M KOH was
used as the electrolyte. The increased product yield from the flow
cell electrolysis (Figure 1b(iv)), which circumvents mass transport
limitations, combined with the use of highly sensitive headspace
gas chromatography (Figure 1b(v)) improves the detection and
quantification of liquid products produced with low FEs and
current densities. This allows us to detect CO2 reduction products
that have, to-date, never been observed.
The total FEs of carbonaceous products were ~70 % at
−0.48 and −0.58 V (Figure 1c, Table S1). The major multi-carbon
products are C2 molecules, namely ethylene and ethanol, which
are typically formed on oxide-derived copper catalysts.[2,12] The
highest FEC2 of 48 % was observed at −0.58 V, with a
corresponding jC2 of −210 mA cm−2. Minor C2 products
(FE ≤ 0.1 %) such as acetaldehyde and ethane were also
detected. In the case of acetaldehyde, the low FE is a result of the
chemical and/or electrochemical transformations it readily
undergoes, as we will discuss below. C3 species, mainly
n-propanol, were also detected, with a maximum FEC3 of 6.5 %
and jC3 of −18.5 mA cm−2 obtained at −0.48 and −0.58 V,
respectively. Overall, the catalytic activities toward C2 and C3
molecules from CO2 reduction are comparable or higher than
values previously reported for Cu catalysts loaded onto carbon
GDEs (Table S3). Interestingly, we detected C4 oxygenates such
as 1-butanol and butanal (maximum total FEC4 = 0.060 % at the
onset potential of −0.48 V), in contrast to previous studies which
only identified hydrocarbons for the C4 fraction.[7,13] The dominant
product was 1-butanol, which is in line with the fact that aldehydes
can be easily electroreduced to their corresponding alcohols, as
demonstrated in the cases of formaldehyde and acetaldehyde.[14]
No carbon-containing products were found in control experiments
performed without applied potentials.
As 1-butanol is the sole C4 alcohol product, it could not come
from C-C coupling of four individual C1 adsorbates such as *CO
in a Flory-Schulz distribution, since 2-butanol was not detected.
This led us to postulate that the formation of 1-butanol could occur
through a combination of electrochemical and chemical steps.
Specifically, the aldol condensation of two C2 intermediates, such
as acetaldehyde, gives rise to the C4 backbone of crotonaldehyde,
which is further reduced to C4 terminal oxygenates like butanal
and 1-butanol. While mechanisms for the formation of major C2
and C3 products, including acetaldehyde, have been widely
discussed in the literature,[6,10,15-18] pathways for producing C4
products are rarely mentioned. Herein, we focus on acetaldehyde
reactivity as it is much less explored mechanistically. Nonetheless,
we also present the two steps preceding acetaldehyde formation
which involve the key ethenyloxy intermediate, CH2CHO*, and
discuss the lateral pathways for CO2 reduction to C1-C3 products
in Section S3.
Detection of crotonaldehyde (3.3 mM, equivalent to 13.2 %
acetaldehyde conversion) after electrolysis suggests that the C4
backbone of 1-butanol could be formed via the base-catalyzed
aldol condensation (Figure 2a, inset). We utilized the increase in
local pH close to the electrode under electroreduction conditions
to test this hypothesis and performed fixed-current electrolyses of
50 mM acetaldehyde in 0.01, 0.1 and 1.0 M potassium phosphate
buffer (pH 7, Figure 2b, Section S4.3). Higher buffer
concentrations mitigate the local pH increase during
electroreduction, thus lowering the overall local pH. Our results
reveal that electrolysis in 0.01 M potassium phosphate buffer
gave the highest selectivity toward C4 products (FE = 0.4 %; with
1-butanol as the main product) and percentage of acetaldehyde
converted to crotonaldehyde (1.5 mM, equivalent to 6.2 %
acetaldehyde conversion). In contrast, neither C4 products nor
crotonaldehyde were detected in 1.0 M buffer (Figure 2b inset).
These observations of an alkaline local pH promoting the
production of 1-butanol on copper, directly support its formation
via the aldol condensation of two acetaldehyde molecules and
suggest an enhanced reaction rate close to the catalytic surface
under electrolysis conditions.
We further investigated, using DFT, the base-catalyzed
aldol condensation mechanism to form the C4 backbone. High-
level methods including solvent and potential effects have been
employed to study electrochemical networks up to C2 including
the formation of acetaldehyde.[16] However, the multiple
conformations of C4 molecules and the complexity of the reaction
network with chemical (bulk solvent and interface) and
electrochemical steps limits us to the use of DFT coupled to the
Computational Hydrogen Electrode (CHE). The formation of the
C4 backbone comprises two steps: the C-C coupling between two
acetaldehyde
molecules
to
form
3-hydroxybutanal
(CH3CH(OH)CH2CHO), and the subsequent dehydration of the
latter to crotonaldehyde (Figure 2c). In solution, the aldol
condensation starts with a OH– stripping an α-hydrogen from
acetaldehyde to form an ethenyloxy anion (CH2CHO-, Figure
2c(i)). The α-carbon of CH2CHO– then attacks the carbonyl group
of a second acetaldehyde molecule, which is subsequently
protonated to 3-hydroxybutanal (Figure 2c(ii)). Then,
3-hydroxybutanal loses an α-hydrogen as a proton, to generate a
carbene species, (Figure 2c(iii)), which forms crotonaldehyde by
hydroxyl elimination. Consistent with findings from the
literature,[19] the latter is the rate-determining step in solution. We
further note that for the case of CO2 reduction, adsorbed
ethenyloxy species is formed as a precursor of acetaldehyde and
ethanol,[16,20] and thus can also readily react with an acetaldehyde
molecule in solution and be subsequently hydrogenated to form
3-hydroxybutanal (Section S3).
To test our hypothesis for 1-butanol formation via
acetaldehyde, we electroreduced 50 mM acetaldehyde in
0.1 M KOH on CuO-derived Cu electrodes (Section S4). The
product distribution at an optimized potential of −0.44 V is
summarized in Figure 2a (see also Table S5 for product
distributions at other applied potentials). The most selective
The amount of acetaldehyde converted to crotonaldehyde
during electrolysis (13.2 %) is larger than the 8.8 % conversion
(or 2.2 mM crotonaldehyde) when 50 mM acetaldehyde was aged
3
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Figure 2. Faradaic efficiencies of products from one-hour electrolysis of 50 mM acetaldehyde on CuO-derived Cu (a) in alkaline and (b) neutral buffer
(62 mol% K2HPO4 + 38 mol% KH2PO4) electrolyte. The insets of (a) and (b) show the percentage of acetaldehyde that was converted to crotonaldehyde in the
electrolyte in each case. (c) Potential energy diagram of acetaldehyde condensation to crotonaldehyde in solution (dark blue) and mediated by the surface (black).
The final OH− removal, which can be assisted by one electron donated from the surface, is promoted by reductive potentials (dark red). Water molecules were
omitted for clarity. (d) Faradaic efficiencies of products from alkaline electrolyses of 50 mM crotonaldehyde and 50 mM butanal on CuO-derived Cu. (e) Potential
energy diagram of crotonaldehyde reduction to 1-butanol via butanal (blue). Under negative potentials, butanal can be adsorbed via a one-electron transfer (dark
red), which promotes its further reaction instead of desorption. Dashed lines represent adsorptions/desorptions. Dotted lines represent proton-coupled electron
transfers (PCET). Additional (electro)chemical routes are shown in Sections S3 and S6.
4
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Figure 3. (a) 1H NMR spectra of 50 mM crotonaldehyde (δ refers to the chemical shift) in alkaline environment (0.1 M KOH, red line) and neutral (ultrapure deionized
water, blue line). Phenol (7.2 ppm) and dimethylsulfoxide (2.6 ppm) were dissolved in D2O (4.8 ppm) and added as internal standards. (b) Potential energy diagram
of crotonaldehyde hydration (I1 to I2) and subsequent hydrogenation. Dashed lines represent adsorption/desorption. (c) Faradaic efficiencies of crotonaldehyde
and butanal electrolysis products on CuO-derived Cu in neutral buffer electrolyte.
in 0.1 M KOH for one hour without applied potential (inset in
Figure 2a). This observation suggests that the Cu surface can
promote the aldol condensation at cathodic potentials. DFT
analysis reveals that this alternative pathway starts with 3-
hydroxybutanal adsorbing exothermically on Cu and losing an
α-hydrogen as an adsorbed H (dark red in Figure 2c). The
hydroxyl group is then eliminated, in parallel with an electron
transfer, to give crotonaldehyde. On the Cu surface, this step is
promoted by negative applied potentials. Overall, the DFT
investigation reveals that the alkaline electrolyte promotes the
initial C-C coupling step between two acetaldehyde molecules,
while the Cu surface promotes the subsequent dehydration step
to crotonaldehyde.
To elucidate the fate of C4 species after the aldol
condensation step, we electrolyzed 50 mM crotonaldehyde on
CuO-derived Cu in 0.1 M KOH at −0.44 V (Figure 2d, Section S5).
The major carbonaceous product was 1-butanol (FE = 14.8 %),
while small amounts of butanal (FE = 0.3 %) and crotyl alcohol
(FE = 1.1 %) were also detected. The Faradaic selectivity of
1-butanol (FE1-butanol normalized by the FE of all the C4 products)
from crotonaldehyde electrolysis was 91.4%, which is similar to
the case of acetaldehyde (94.7 %, Table S10). This reinforces the
role of crotonaldehyde as the main intermediate in the
electrosynthesis of acetaldehyde to C4 oxygenates. The presence
of ethanol (FE = 4.5 %) was attributed to the reduction of
acetaldehyde present due to the hydroxide-catalyzed retro-aldol
reaction of crotonaldehyde, which is known to occur at room
temperature.[21] This observation highlights the complexity of
crotonaldehyde chemistry under aqueous alkaline conditions, and
leads us to infer that the low total Faradaic efficiency of 72.2 % is
a consequence of undetected products from other side reactions
of crotonaldehyde in the alkaline electrolyte (Figure S10).
Butanal and crotyl alcohol are known intermediates in the
gas-phase hydrogenation of crotonaldehyde to 1-butanol,[22] and
their presence during crotonaldehyde electrolysis suggests that
they are potential electrochemically-active intermediates to
1-butanol. Therefore, we electrolyzed butanal and crotyl alcohol
under the same conditions (Figure 2d, Table S9). 1-Butanol was
the sole product from the electroreduction of butanal
(FE = 17.3 %; the balance product is H2). Only hydrogen was
detected during the electrolysis of crotyl alcohol, which indicates
that the latter is electrochemically inert, in good agreement with
theoretical calculations in Figure S11.
Theoretical analysis of crotonaldehyde reduction reveals
that butanal is formed by sequential hydrogenation of the β- and
α-carbons of crotonaldehyde (Figure 2e). Once formed, butanal
tends to desorb rather than further react. However, at potentials
more reductive than −1.02 V vs. SHE (standard hydrogen
electrode), butanal receives an electron from the cathode surface
to form the CH3CH2CH2C*HO– anion (I4 in Figure 2e). As this
adsorption does not involve proton transfers, it is independent of
the electrolyte pH in SHE scale. CH3CH2CH2C*HO– is
subsequently protonated to yield 1-hydroxybutyl (I5 in Figure 2e),
which is further hydrogenated in a chemical step (Ea = 0.39 eV) to
produce 1-butanol. These theoretical findings are corroborated by
our results from crotonaldehyde electrolysis performed at pH 7
5
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10.1002/anie.202008289
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RESEARCH ARTICLE
and pH 13 (Table S9). At −1.20 V vs. SHE, 1-butanol was the
most selective product in both electrolytes, consistent with the pH-
independent adsorption of butanal. However, at −0.90 V vs. SHE,
butanal was the most selective product, indicating that this
potential was insufficient for its further reduction to 1-butanol.
Alternative chemical routes from butanal to 1-butanol are shown
in Figure S12.
the conversion of acetaldehyde to ethanol on copper is kinetically
facile and strongly competing (FEethanol = 36.5 %, while FE1-butanol
= 9.6 % at −0.44 V vs. RHE, Figure 2a).[17,26] The third factor is
that the formation of the C4 backbone and its subsequent
reduction to 1-butanol are promoted by conflicting experimental
conditions. While an alkaline environment facilitates aldol
condensation
between
acetaldehyde
molecules
to
Aldehydes can be hydrated to geminal diols in aqueous
alkaline solution. Signals belonging to hydrated crotonaldehyde
(CH3CH=CHCH(OH)2) were observed in nuclear magnetic
resonance (NMR) spectroscopic analyses of 50 mM
crotonaldehyde or acetaldehyde dissolved in 0.1 M KOH
(Figure 3a, Figure S13). We therefore considered the possibility
of hydrated crotonaldehyde as an intermediate to 1-butanol. DFT
suggests that hydrated crotonaldehyde cannot be further
electrochemically reduced due to a larger activation barrier
(+0.41 eV) compared to its desorption energy (+0.26 eV), as
shown in Figure 3b. This result was corroborated by the FE1-butanol
of ~46 % observed from the crotonaldehyde and butanal
electrolyses in 0.1 M potassium phosphate buffer (pH 7) at
−0.79 V (Figure 3c, Table S9), which was almost three times
more than the values from electrolyses in 0.1 M KOH. Figure 3a
shows that in neutral medium, only peaks corresponding to
crotonaldehyde were observed. The neutral buffer electrolyte
therefore likely suppressed the hydration process and increased
the availability of unhydrated crotonaldehyde for reduction.
To find a better electrocatalyst to enhance the FE of
1-butanol from acetaldehyde and crotonaldehyde reduction, we
proceeded to identify an activity descriptor. To this end, we
electrolyzed acetaldehyde in 0.1 M KOH and crotonaldehyde in
0.1 M potassium phosphate buffer on different metal discs
(Section S8). Since we have demonstrated that both the
formation of C4 oxygenates via the aldol condensation pathway,
and the reactivity of crotonaldehyde are affected by the (local) pH,
we employed a constant-current electrolysis at −10 mA cm−2 to
identify the different reactivities of the metals. For acetaldehyde
electrolysis, we found that the selectivity of Cu, Fe, Co, Ni, Ag,
and Au towards 1-butanol (Figure 4a) and all C4 products (Figure
S14a) can be correlated to the cathode metal-oxygen bond
strength, with Fe showing the highest selectivity towards
1-butanol (FE = 4.0 %). A similar trend was observed for the six
above-mentioned metals and also Pd and Pt for crotonaldehyde
reduction to 1-butanol (Figure 4b), with Fe also showing the
highest FE of 26.3 %. The linear-scaling relationships end sharply
in a selectivity cliff.[23] The origin of the discontinuity is likely due
crotonaldehyde, its hydration to an unreactive form is also
promoted.
Figure 4. Faradaic efficiencies of 1-butanol from −10 mA cm−2 constant-current
electrolysis of (a) acetaldehyde and (b) crotonaldehyde on selected metals as
a function of the DFT-computed adsorbed oxygen stability on these metals with
respect to water and hydrogen. Metals that are typically oxides at 0 V vs. RHE
at the pH of the supporting electrolyte are shown as hollow symbols. 1-Butanol
was not detected from acetaldehyde electrolysis on Ti, Cr, Mo, Pd, and Pt.
to
a phase transformation. According to their Pourbaix
diagrams,[24] Zn, Ti, Cr, and Mo may have surface oxide layers
under the working cathodic potentials and thus have poor yields
towards 1-butanol and other C4 products (Section S9).
Incidentally, these metals were more selective for reducing
acetaldehyde to crotyl alcohol (Table S11), and crotonaldehyde
to butanal (Table S12), probably because the dominating linear-
scaling relationships differ from those of the pure metals. Based
on these results, we put forward that the C4 product selectivity is
influenced by the affinity of the catalyst to oxygen.
Conclusion
In summary, we detected for the first time C4 oxygenates from
CO2 electroreduction on CuO-derived Cu, with 1-butanol being
the most favored product amongst them (FE = 0.056 %,
j = −0.080 mA cm−2 at −0.48 V). The quantification was made
possible by a combination of the high-rate electrolysis, achieved
by using a GDE in a flow cell, and sensitive analytical techniques.
We ruled out the formation of 1-butanol from the C-C coupling of
four individual C1 adsorbates, such as *CO. Instead, the
combination of experimental and theoretical studies established a
rich reaction mechanism that combines chemical and
electrochemical steps, where the electrocatalytically generated
Collectively, our results gave the reasons for the low FE of
1-butanol during the electroreduction of CO2 on oxide-derived
copper. The first factor is the low activity for acetaldehyde
production
jCH3CHO ≤ −0.41 mA cm−2
jCH3CHO < 1 mA cm−2 in the literature[4,25]). The second factor is that
on
copper
materials
(FECH3CHO ≤ 0.1
%,
in
Table S1,
FECH3CHO < 2.1 %,
6
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[10]
X. Chang, A. Malkani, X. Yang, B. Xu, J. Am. Chem. Soc.
2020, 142, 2975-2983.
D. Ren, J. Fong, B. S. Yeo, Nat. Commun. 2018, 9, 925.
R. Kas, R. Kortlever, H. Yılmaz, M. T. M. Koper, G. Mul,
ChemElectroChem 2015, 2, 354-358.
acetaldehyde plays a prominent role. Its base-catalyzed aldol
condensation, promoted by high local pH and the catalyst surface,
produces crotonaldehyde, which is subsequently electroreduced
to 1-butanol.
[11]
[12]
[13]
R. Kortlever, I. Peters, C. Balemans, R. Kas, Y. Kwon, G.
Mul, M. T. M. Koper, Chem. Commun. 2016, 52, 10229-
10232.
This study further highlights the challenges associated with the
one-pot approach to converting a low molecular weight feedstock
like CO2 into complex functionalized molecules. We discover that
contrasting catalysts and conditions are required to maximize the
yield of each step. In addition to operating under highly alkaline
conditions, we also note that a single electrocatalytic surface can
hardly optimize all the required steps. For example, among the
metal discs tested, Fe was identified as the most selective catalyst
for acetaldehyde reduction to 1-butanol, but it is not active per se
for CO2 reduction. Therefore, designing a one-pot reactor for the
electrosynthesis of large molecules would inevitably be
associated with low performance. Instead, we propose that a
more viable synthetic strategy would be to deconvolute the multi-
step process into sequential operation units. Therein, chemical or
electrochemical reactions with different process conditions could
be independently optimized. The separate stepwise-optimized
reactors could then be placed in tandem for the efficient
conversion of each intermediate, leading to increased yield of the
desired product.
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Acknowledgements
We thank the National University of Singapore Flagship Green
Energy Program (R143-000-A64-114, R143-000-A55-733 and
R143-000-A55-646), Ministry of Education, Singapore (R143-
000-683-112), Spanish Ministry of Science and Innovation
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Keywords: 1-butanol • carbon dioxide reduction• density
functional calculations • electrochemistry • reaction mechanisms
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This article is protected by copyright. All rights reserved.

10.1002/anie.202008289
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Carbon dioxide was electroreduced to 1-butanol on copper surfaces, and the reaction mechanism was determined to proceed
through a combination of electrochemical and chemical steps, some of which require contrasting conditions. This example highlights
limitations in one-pot synthesis and provides a case for utilizing independently optimized sequential reactors in a tandem system to
build complex molecules from low molecular-weight feedstocks.
Institute and/or researcher Twitter usernames:
http://www.nus-lsec.com (Boon Siang Yeo)
https://twitter.com/catalysis_eth ; https://www.ace.ethz.ch/en/home (Javier Perez-Ramirez)
https://twitter.com/TheorHetCatICIQ ; http://www.iciq.org/research/research_group/prof-nuria-lopez/ (Nuria Lopez)
The authors declare no competing financial interest.
8
This article is protected by copyright. All rights reserved.