Article https://doi.org/10.1038/s41467-023-41278-7
ACO
2
electrolyzer tandem cell system for
CO
2
-CO co-feed valorization in a Ni-N-C/Cu-
catalyzed reaction cascade
Tim Möller
1,2
,MichaelFilippi
1,2
,SvenBrückner
1
,WenJu
1
& Peter Strasser
1
Coupled tandem electrolyzer concepts have been predicted to offer kinetic
benefits to sluggish catalytic reactions thanks to their flexibility of reaction
environments in each cell. Here we design, assemble, test, and analyze the first
complete low-temperature, neutral-pH, cathode precious metal-free tandem
CO
2
electrolyzer cell chain. The tandemsystemcouplesanAg-freeCO
2
-to-
CO
2
/CO electrolyzer (cell-1) to a CO
2
/CO-to-C
2+
product electrolyzer (cell-2).
Cell-1 and cell-2 incorporate selective Ni-N-C-based and Cu-based Gas Diffu-
sion Cathodes, respectively, and operate at sustainable neutral pH conditions.
Using our tandem cell system, we report strongly enhanced rates for the
production of ethylene (by 50%) and alcohols (by 100%) and a sharply
increased C
2+
energy efficiency (by 100%) at current densities of up to
700mAcm
−2compared to the single CO
2
-to-C
2+
electrolyzer cell system
approach. This study demonstrates that coupled tandem electrolyzer cell
systems can offer kinetic and practical energetic benefits over single-cell
designs for the production of value-added C
2+
chemicals and fuels directly
from CO
2
feeds without intermediate separation or purification.
In the light of intensified efforts to mitigate power and chemicals
production from fossil sources and their substitution by renewable
ones, electrocatalytic power-to-chemicals and power-to-fuels tech-
nologies are emerging as one of the future pillars of sustainable che-
micals industry1. Recent years have seen a drastic increase in attention
for the electrocatalytic CO
2
reduction reaction (CO
2
RR), advancing the
production of 2e−reduction products, such as CO and formic acid
,
towards commercialization2–4. In contrast, the electrocatalytic reaction
process pathway towards C
2+
hydrocarbons and oxygenates, in parti-
cular C
2
H
4
, Ethanol, Propanol, lacks similar technological advances5–10.
Following Hori’s pivotal work on CO
2
RR, establishing CO as key
intermediate in hydrocarbons and oxygenates production, studies
investigated CO as feedstock in what is referred to as the CO reduction
reaction (CORR). CORR achieves a higher faradaic efficiency (FE) for
the production of C
2+
compounds, in part because some common C
1
side-products of the CO
2
RR, such as CO or HCOO-,cannotbeformed,
but alsodue to more favorable kinetics and surface coverages of H*and
CO*.Specifically, the production of oxygenates has been shown to be
favoredbyCORRcomparedtoCO
2
RR, especially under alkaline con-
ditions that are only sustainable for CORR due to the invariable for-
mation of (bi)carbonate from CO
2
11–19.
The favorable kinetics of alkaline CORR compared to CO
2
RR for
production of C
2+
species has motivated researchers to tryto find ways
to separate off the CORR in a CO
2
RR electrolyzer. Tandem Cathode
Concepts follow the idea to spatially decouple the initial 2e-reduction
of CO
2
to CO and the subsequent reduction steps of CO towards C
2+
products. The tandem split can be realized at vastly different length
scales and distinct configurations: (i) at the atomic scale by two
neighboring active sites coupled by CO spillover, (ii) at the macro-
scopic electrode scale with macroscopically separated regions of dis-
tinct catalysts inside the same electrolyzer cell, and (iii) at the
macroscopic electrolyzer cell scale forming a coupled process
cascade20–35. While the idea of placing the active sites of CO source and
sink inside the same electrode is an intriguing one, it has been shown
Received: 11 July 2023
Accepted: 29 August 2023
Check for updates
1
The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division, Technical University
Berlin, Berlin, Germany.
2
These authors contributed equally: Tim Möller, Michael Filippi. e-mail: pstrasser@tu-berlin.de
Nature Communications | (2023) 14:5680 1
1234567890():,;
1234567890():,;
to be inherently sensitive to the relative spatial catalyst distribution
that is challenging to maintain over longer operation times consider-
ing the reported mobility of Cu catalysts under CO
2
RR36–41.
The present work sets out to explore the coupled tandem elec-
trolyzer cell approach, i.e., two individual electrolyzers in series, for a
significantly enhanced production of C
2+
compounds from CO
2
in a
catalyticreactioncascade.Wedeliberatelyavoidtheuseofhigh-
temperature technologies, harsh reaction conditions, and demon-
strate the advantages by avoiding CO
2
scrubbing steps to increase the
practicality of the overall process, and make use of kinetic benefits for
CO
2
/CO co-feeds agreeing with the presence of reactant- and product-
specific reaction sites on Cu catalysts20,42. Accordingly, we deploy a
noble metal-free single Ni atom site electrocatalyst (Ni-N-C) in a neu-
tral-pH, near-ambient temperature, zero-gap setup for selective con-
version of CO
2
to CO-rich CO
2
/CO mixed streams. The CO
2
/CO mixed
feed was then directly introduced to a second flow electrolyzer cell
equipped with a Cu-catalyst for conversion of CO
2
/CO mixed feeds
into C
2+
products. We report strongly enhanced rates for the pro-
duction of ethylene (~50%) and of alcohols (>= 100%) and a sharply
increased C
2+
energy efficiency (>=100%) in the tandem systems
compared to the conventional single CO
2
electrolyzer cell approach.
To validate our conclusions and provide kinetic insight, we also con-
ducted single-cell control experiments with controlled mixed CO
2
/CO
co-feeds to explore the origin of the observed tandem effect. These
experiments suggest that CO
2
is being selectively scrubbed from the
gas mixture by cathodically generated OH-in vicinity of the cathode
thus locally increasing the effective CO concentration and CO surface
coverage, mimicking the performance of CO-rich gas feeds.
Results and Discussion
Design of a low-temperature tandem electrolyzer cascade for
the production of C
2+
compounds
Figure 1a introduces the concept and the process design of the first
low-temperature coupled tandem CO
2
electrolyzer cell system devel-
oped and studied in this contribution.
The numbering of the cells refers to the order in which they were
deployed in the tandem system electrolyzer cascade and is used as a
denomination throughout this work. Cell-1, the first electrolyzer in the
reaction cascade, converts CO
2
to CO and is the point of entry for the
CO
2
feed. At the cathode Gas Diffusion Electrode (GDE) of cell-1, we use
a previously developed noble metal-free single Ni atom site electro-
catalyst (NiNC) deployed into a neutral anode-pH, zero-cathode-gap
electrode design43. Since the stoichiometric CO
2
ratio, λ
stoich
,remains
above one, the cathodic CO
2
to CO conversion in cell-1 invariably
results in mixed CO
2
/CO exit feeds, even if the cathode is operated at
100% faradaic CO efficiency. This mixed CO
2
/CO gas feed is directly
used as input feed for a Cu-based electrolyzer “cell-2”to efficiently
produce C
2+
products, as schematically depicted in Fig. 1a. Note, that
while within this work we refer to the commercial Cu catalysts as “Cu”
for simplicity, XRD analysis showed that the material is partially oxi-
dized and shows a presence of a Cu
2
O phase, see Supplementary Fig. 1.
Figure 1b shows photographs of our experimental setup with
inserts of the individual electrolysis cells used during electrochemical
characterization.
The detailed tandem cell process scheme is given in Fig. 1c. Note
how cell-1 and cell-2 were connected in series by the CO
2
gas flow to
enable an enrichment with CO in the gas stream before entering cell-2.
A more detailed discussion on the geometry of the individual cells and
testing setup including operational parameters can be found in Sup-
plementary Fig. 2.
Electrochemical CO
2
/CO mixed gas feed reduction
Prior to coupled tandem electrolyzer cell experiments, we studied how
the presence of CO in the CO
2
reactant gas feed affects the product
production rates of a single Cu cathode-based CO
2
electrolyzer.
Figure 2displays the production rates of the four most prominent
reaction products, i.e., hydrogen, ethylene, ethanol, and n-propanol as
a function of CO mol% in the CO
2
/CO co-feed and of applied current
density. While the absolute volumetric flow of the gas feed was kept
constant at 50 mL min-1,wehavevarieditscompositionbyadjusting
the individual flow rates of CO
2
and CO that were mixed prior to
entering cell-2. Figure 2ashowsaprofoundeffectofCOintheco-feed
on the production of hydrogen. For CO-lean feeds, an increase in CO
raised H
2
production, as well, before H
2
production rates dropped for
CO-rich co-feeds. The molar CO concentration associated with the
maximum H
2
production depended on the applied current density.
While for low current densities such as 100 or 200 mA cm-2 the turning
point was at 66 mol%
CO
, larger current densities showed the transition
at 50 mol%
CO
(300 and 400 mA cm-2), or even at 34 mol%
CO
(500 to
700mAcm
-2). The single-maximum pattern of hydrogen production
appears surprising, as the total carbon content in the electrolyte
monotonically drops with more CO due to the vastly lower solubility of
CO versus CO
2
.So,CO
x
surface coverages should monotonically
decline, as well. For the production rate of ethylene, shown in Fig. 2b,
a distinctly different trend was apparent. Here, increasing CO mol%
improved the ethylene production rate. Note that this improvement
was most significant up to a co-feed composition of 50 mol%
CO
,
beyond which it plateaued. The production rates of ethanol and n-
propanol, Fig. 2c, d, showed unusually sharp enhancements with rising
molar CO mol% in the co-feed. In contrast to the trends observed for
ethylene, the beneficial effect on production rate was most pro-
nounced past 50 mol%
CO
approaching pure CO gas feed. Production
rates of other compounds suchas formate, acetate and ally alcohol can
be found in Supplementary Fig. 3. The trend for allyl alcohol generally
agrees with the behavior observed for other alcohols, whereas formate
production was suppressed by raising CO content in line with
expectations.
To better appreciate the effect of CO in the feed, let us consider
the kinetic enhancements of individual C
2+
production rates on a
relative scale to pure CO
2
feeds: CO in the feed has the most significant
effect on PrOH production, showing a 4x increase in rate, followed by a
2-3x increase of EtOH production and finally 2x increase in C
2
H
4
pro-
duction. This sharp enhancement of C
2+
oxygenate production under
CO-rich feed conditions is also reflected by the dramatically larger FE
values obtained for pure CO feed (CORR) compared to pure CO
2
feed
(CO
2
RR), Supplementary Fig. 4. These enhancements can be explained
mechanistically by enhanced cross-coupling CO dimerization rates on
the surface of the Cu catalyst20. The combined presence of CO
2
and CO
gas in the feed offers new C-C coupling pathways between two
adsorbed CO surface species derived from both CO
2
and CO on two
distinct, non-scrambling Cu surface sites, in line with the multi-site
hypothesis on Cu catalysts20,42.
Kinetic analysis of CO
2
RRandCORRofCO
2
/CO feeds
To gain a better understanding of the origin of the CO dependence of
the production rates, we tracked the applied cathode potentials as well
as the CO consumption during co-feed experiments. Figure 3ashows
IR-free cathode polarization curves obtained for experiments at vary-
ing CO feed concentrations. Evidently, cathode overpotentials
(E
cathode, IR free
) during electrolysis in pure CO
2
and in small molar CO
co-feeds of 10 mol% remained near −1.3 V
SHE
at a current density
approaching 700 mA cm–2. In contrast, the required cathode potentials
shifted to –1.4 V
SHE
for the case of a pure CO feed, possibly pointing to
concentration overpotentials due to lower CO solubility.
As our co-feeding experiments introduced two distinct reactants,
i.e., CO
2
and CO, we set out to quantify whether and how the shift in C
2+
production rates with CO mol% correlated with a shift from pre-
dominantly CO
2
-controlled electroreduction to a predominantly CO-
controlled one. For that purpose, we measured the CO outflux (area-
normalized molar outflow rates) out of the electrolyzer at various
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 2
co-feed compositions, Supplementary Fig. 5. Considering the large
stoichiometric excess of reactants in the input feed, CO conversion
was incomplete for all investigated co-feeds. Higher CO mol% in the
feed led to higher molar CO outflux, CO
out
, from the electrolyzer.
However, the decrease in CO
out
with larger current density was sig-
nificantly more pronounced for CO-rich co-feeds, suggesting a larger
relative consumption thereof. Taking the difference between the
molar CO influx, CO
in
, and the molar CO outflux during electrolysis, we
derived the net CO flux,defined as CO
net flux
=(CO
out
-CO
in
), as function
of applied current and co-feed composition, Fig. 3b. In this plot,
positive and negative values of CO
net flux
denote a catalytic net pro-
duction and net consumption of CO, respectively. Generally, both
higher CO mol% in the feed and larger current density led to increasing
CO consumption, i.e., more negative values of CO
net flux
.ForCOmol%
<= 50 in the feed, there appeared a critical current density, beyond
which net CO production turned into net CO consumption (10, 33 and
50 mol%
CO
at500,200and100mAcm
-2, respectively). For CO mol% >
50, only net CO consumption was apparent over the entire range of
current density. This data allowed us to estimate what fraction of the
produced C
2+
species can be accounted for by an exclusive CORR
pathway. To do that, we considered the apparent CO reduction reaction
selectivity,S
CORR
,defined as the ratio of the experimental CO con-
sumption rate, CO
consumption
=–CO
net flux
and the theoretical CO
demand that is required for the combined experimental C
2+
produc-
tion rate, according to:
SCORR =COconsumption
P
i
ð_
niνiÞ100% ð1Þ
where νiand _
nidenote the number of carbon atoms in species i and its
molar production rate, respectively.
Figure 3cplotsS
CORR
versus the co-feed compositions and the
applied current density. Both CO mol% in the feed and applied current
density control S
CORR
,withS
CORR
approaching 100% with larger
applied current density or with rising CO mol%. As expected, a pure CO
Fig. 1 | Illustration of the tandem electrolyzer cell system used for efficient CO
2
valorization. a Concept of coupled Tandem CO
2
Electrolyzer Cells highlighting the
flows of gas (red) and liquid (blue) reactants and products. Additional information
on the detailed reaction conditions can be found in Supplementary Fig. 2.
bPhotographs of the two electrolysis reactors used for coupled CO
2
-to-CO
electrolysis and CO
2
/CO-to-C
2+
electrolysis, with inserts showing an enlargement of
the cells used in each case. cDetailed Process scheme of tandem cell set up with
information on gas (red) and liquid (blue) flow directions and the most important
components of the investigated tandem system.
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 3
feed showed a S
CORR
of 100% at all currents, agreeing with the condi-
tions of CO being the only reactant. Importantly, sustained S
CORR
values near 100% were evident for 67 mol% CO feed and currents as
low as 200 mA cm−2. This observation suggests that under conditions
of 67 mol% CO and 200 mA cm-2 essentially all C
2+
product formation
can be accounted for by the reduction and dimerization of CO gas
(prevalent CORR). This kinetic effect arises because the catalytically
effective gas composition at the catalyst-electrolyte-interface differs
from the bulk gas feed composition. It is a consequence of the pre-
ferential acid-base reaction of CO
2
in CO
2
/CO co-feeds with catalyti-
cally generated OH−18,19. This acid-base reaction increases the effective
CO concentration for a given CO
2
/CO co-feed, and this will become
more severe at a larger current. Using S
CORR
, we split the catalytic co-
feed reduction kinetics into a CO-controlled regime (S
CORR
>50%)and
aCO
2
-controlled one (S
CORR
< 50%), see colored regions in Fig. 3c,
which deconvolutes the overall reduction reactivity.
Carbon Selectivity of C
2+
production under CO
2
/CO co-feeds
Rising CO concentration in CO
2
/CO co-feeds sharply enhances the
total production rates of C
2+
species (Fig. 2). In part, this influence
could be explained by the reduced number of electrons required to
produce C
2+
species from the CORR pathway, see Supplemental Dis-
cussion 1. However, charge stoichiometry falls short of the 2 to 4-fold
experimental increase shown in Fig. 2. This is manifested in the single
feed experiments CO
2
RR and CORR in Supplementary Fig. 4 that
showed drastically larger FE values for C
2+
species in CORR experi-
ments: In particular, the FE values of alcoholic oxygenates showed
sharp increase under CORR compared to CO
2
RR14,16.Wehypothesize
that the sharp rise in the C
2+
production must originate from a change
in surface catalytic selectivity associated with an enrichment of CO gas
in the feed inside the electrolyzer, and associated with a changed
surface coverage of CO at the Cu catalyst. Accurate calculation of
faradaic product selectivity requires knowledge of the electron trans-
fer numbers, which, however, are not known for co-feeding. This is
why, for the purpose of a mechanistic discussion, we calculated a
carbon selectivity of species i, CS
i
, (see Supplementary Fig. 6) defined as
the ratio between the carbon atom flux into an individual C
2+
species i
and the total carbon atom flux into all C
2+
products.
CSi=
_
niνi
P
i
ð_
niνiÞ100% ð2Þ
Here, _
niand νidenote the molar production rate of species i and the
number of carbon atoms in species i, respectively. CS
i
reveals changes
in mechanistic reaction pathways for different co-feeds and currents
and can be compared to kinetic modeling results. More detailed
information on CS
i
isgivenintheSupplementalDiscussion1.
In Supplementary Fig. 6, CS
ethylene
showed a maximum at 33 CO
mol% and decreased at higher CO mol% in the feed. Above 10 mol%
CO
,
CS
EtOH
rose gradually by ca. 10 percentage points, and so did CS
PrOH
.
Evidently, higher CO mol% favor formation of oxygenates. While the
applied current density showed a minor effect on CS
Ethylene
,thepro-
duction of either alcohol was profoundly affected: Higher currents
favor the production of EtOH over PrOH. Mechanistically, this can be
rationalized by a lower surface coverage of COthat makes formation of
C
3
products less likely, as concluded in prior experiments of pure CO
feeds at varying partial pressures12,13,44–46. A more detailed mechanistic
0 102030405060708090100
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Production Rate (PrOH) /PMcm
-2
s
-1
CO in Mixture /mol%
0 102030405060708090100
0
0.05
0.1
0.15
0.2
0.25
0.3
Production Rate (EtOH) /PMcm
-2
s
-1
CO in Mixture /mol%
0 102030405060708090100
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Production Rate (C
2
H
4
)/PMcm
-2
s
-1
CO in Mixture /mol%
0 102030405060708090100
0
0.2
0.4
0.6
0.8
150 100 200 300
400 500 600 700
Production Rate (H
2
)/PMcm
-2
s
-1
CO in Mixture /mol%
ab
cd
Fig. 2 | Influence of CO
2
/CO co-feed composition on production rates observed
for electrolysis in single-cell experiments of cell-2. Production rates of key
products using a single electrolyzer cell as a function of CO mol per cent (mol%) in
the CO
2
/CO co-feed, measured for various applied current densities in Cu-based
cell-2. Production rates are shown for ahydrogen, bethylene, cethanol and
dn-propanol. Electrolysis was conducted in single-cell experiments deploying only
the Cu-based electrolyzer denoted as cell-2. The total volumetric flow of co-feed
mixtures was kept constant at 50 mL min−1throughout all experiments, while the
CO concentration was gradually increased. Displayed values represent the average
and error bars the standard deviation of at least 2 independent measurements.
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 4
discussion of the observed trends in CS
i
values under CO
2
/CO co-feeds
is provided in the Supplemental Discussion 2.
Operation of a two-cell, tandem system for enhanced produc-
tion rates and energy efficiency of C
2+
compounds
After we had deconvoluted and rationalized the increase in overall C
2+
production rates under controlled CO
2
/CO co-feed electrolysis, we
moved to assemble, operate, and analyze the first complete low-tem-
perature, coupled CO
2
tandem electrolyzer cell system excluding
intermediate processing steps. Unlike tandem catalyst designs20,tan-
dem cell designs allow for individually optimized active sites and
electrolysis conditions for each step. We generated CO-rich co-feeds in
“Cell-1”using a neutral-pH, zero-gap electrolyzer with a Ni-N-C catalyst-
based Gas Diffusion Electrode (GDE)43. The use of the CO
2
-to-CO
electrolyzer as cell-1 allowed precise control over the resulting mixed
CO
2
/CO exit gas composition (equivalent down-stream to the CO
2
/CO
co-feed into cell−2) via two control parameters, that is the applied
current density and the CO
2
feed rate.
As operating conditions of cell-1, we selected 200 mA cm−2and
near-100% FE
CO
including only minute amounts of H
2
to achieve a
controlled mixed of predominantly CO
2
/CO exit steam out of cell-1.
Supplementary Fig. 7 shows how the FE
CO
affects the absolute CO
productionrate as a function of the applied current density of cell-1. At
the chosen operating point, cell-1 generates about 7.6 ml
CO
min-1.
To ensure stable performance ofcell-1, we monitored the experimental
FE
CO
and cell potential over the electrolysis time of 12 hours at
200 mA cm−2, shown in Supplementary Fig. 8. We thereby convinced
ourselves that the FE
CO
value remained at near-unity without changes
in the cell potential. Since the CO
2
flow rate into cell-1 controls the
molar CO
2
/CO ratio of the exit steam, we have also investigated the
effect of CO
2
flow rate. We chose (i) a flow rate of 50 ml min−1corre-
sponding to the total flow rate of the co-feed tests above, and (ii) a
reduced CO
2
flow rate of 30 ml min−1to enrich the exit flow in CO
further. In cell-1 the CO
2
flow of 50 mL min−1was converted to a gas
mixture (co-feed) of CO
2
and CO with 18 mol% of CO and a total
volumetric flow rate of 42 mL min−1, whereas a CO
2
input of
30 mL min−1resulted in an exit flow of 22 mL min−1with 35 mol% of CO.
Figure 4compares the catalytic production rates of the tandem
system versus the reference single-cell CO
2
RR (using cell-2) as a
function of the applied current density of cell-2. The investigated CO
2
flow rates of 50 mL min−1and 30 mL min−1are denoted as Tandem-50
and Tandem-30, respectively, the reference is denoted as Single Cell.In
good agreement with our previous experiments, hydrogen production
slightly increased with more CO in the co-feed between cell-1 and cell-
2, Fig. 4a. Tandem-30 showed a larger H
2
production rate than Tan-
dem-50, consistent with its larger CO mol% in the exit of cell-1. While
Tandem-50 matched the H
2
production rate of the single-cell experi-
ment at 700 mA cm−2,H
2
rate of Tandem-30 increased rapidly at larger
current densities, most likely due to reactant transport limitation.
CO production rates of Tandem-50/30 at 50 mA cm−2,Fig.4b, are
-700 -600 -500 -400 -300 -200 -100 -50
0
20
40
60
80
100
120
Apparent Selectivty for CORR (S
CORR
)/%
j/mAcm
-2
100 mol%
CO
67 mol%
CO
50 mol%
CO
33 mol%
CO
10 mol%
CO
CO2
controlled
CO
controlled
-700 -600 -500 -400 -300 -200 -100 0
-2
-1.5
-1
-0.5
0
0.5
(COout -CO
in)/PMcm
-2 s-1
j/mAcm
-2
100 mol%
CO
67 mol%
CO
50 mol%
CO
33 mol%
CO
10 mol%
CO
00 mol%
CO
-700 -600 -500 -400 -300 -200 -100 0
-1.5
-1.4
-1.3
-1.2
-1.1
-1
-0.9
ECathode, IR-free /V
SHE
j/mAcm
-2
100 mol%
CO
67 mol%
CO
50 mol%
CO
33 mol%
CO
10 mol%
CO
00 mol%
CO
ab
c
Fig. 3 | Electrode potential and CO conversion under CO
2
/CO co-feed condi-
tions in single-cell experiments of cell-2. a IR-corrected cathode polarization
curves as a function of applied current density for various investigated co-feed
compositions. bThe net CO flux, CO
net flux
,defined as the difference in the molar
CO influx, CO
in
,andmolarCOoutflux, CO
out
, plotted versus the applied current
density and co-feed composition. cApparent selectivity for CORR, denoted S
CORR
,
as a function of applied current density and co-feed composition. S
CORR
was
defined as the ratio between the electrocatalytic CO consumption rate
(CO
consumption
=–CO
net flux
) and the theoretically required molar CO consumption
rate to produce all C
2+
species by CORR. Colored areas indicate regions controlled
by the respective primary reactant used in the production of C
2+
species, either CO
or CO
2
, dependent on the absolute value of S
CORR
. Electrolysis was conducted in
single-cell experiments deploying only the Cu-based electrolyzer denoted as cell-2.
The total volumetric flow of co-feed mixtures was kept constant at 50 mL min−1
throughout all experiments, while the CO concentration was gradually increased.
Displayed values represent the average and error bars the standard deviation of at
least 2 independent measurements.
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 5
controlled by CO production of cell-1. CO rates of Tandem-30 declined
steadily approaching the Single Cell, while CO rates of Tandem-50
showed a maximum at 200 mA cm−2before declining. Ethylene rates,
Fig. 4c, were significantly enhanced (up to 50% at 500 mA cm−2)for
the Tandem case over the entire current density range. A similar
behavior was seen in the EtOH production rates,Fig.4d, with space-
time yield enhancements of up to 100% at 700 mA cm−2. Likewise,
production rates of C
3
compounds, i.e., PrOH and AllylOH, Fig. 4e, f,
showed more than 100% enhanced production rates at 700 mA cm−2in
tandem configuration, with Tandem-30 suffering from mass transport
limitations.
The experimental performance of the tandem systems are in good
agreement with our CO
2
/CO co-feeds results. Tandems showed a sharp
increase in C
2+
production rates over the single-cell CO
2
RR experi-
ments. While C
2+
production rates of Tandem-30 were superior to
Tandem-50, consistent with the larger CO concentration in the co-feed
to cell-2, clear signs of mass transport limitation were apparent when
exceeding a current density of 500 mA cm−2,reflected in a rising H
2
rate and a declining C
2+
rate. Note that the extent of the decrease in C
2+
rates at large current density followed the order of PrOH/
AllylOH>EtOH>C
2
H
4
suggesting that C
3
compounds are more sus-
ceptible to the insufficient reactant transport. This behavior fits well
with our previous observation in single-cell experiments that showed
that C
3
production rates were particularly sensitive to CO depletion in
the gas feed.
In order to provide a direct comparison of product productions in
the tandem experiments (18 mol%
CO
for Tandem-50, and 35 mol%
CO
for Tandem-30) and the CO
2
/CO co-feed experiments, we overlaid
their production rates in Supplementary Fig. 9. This revealed some
unexpected, yet important kinetic discrepancies: In either tandem
experiment, the observed production rates suggested a higher CO
mol% in the co-feed to cell-2 than inferred from the overlaid co-feed
single-cell experiments. This trend is also reflected in the C
2+
product
carbon selectivity comparisons in Supplementary Fig. 10. With larger
current densities, this seeming CO-enrichment becomes progressively
more pronounced. To explain this observation, we recall the
lower actual tandem feed rates of 42 mL min−1and 22mLmin
−1for
Tandem-50 and Tandem-30, respectively, compared to 50 mL min−1
for the single cell case. This difference in available CO
2
becomes
clearer in a direct comparison of the molar reactant flow to cell-2 under
the various conditions, see Supplementary Fig. 11. As a result of this, at
agivenOH
-generation (given current density), the lower molar CO
2
flow into cell-2 results in a larger share of CO
2
being depleted. This
enhances the effective CO concentration near the electrode, and
results in a product pattern resembling a higher bulk mol%
CO
.
The technological application of the tandem concept ultimately
requires an acceptable overall energy efficiency, henceforth denoted
EE. Estimation of EE values in a tandem system requires power-in
/power out consideration of both cells of the reaction cascade. Equa-
tion (3)defines the EE as the ratio between the rate of chemical energy
stored in products based on their higher heating values (HHV) and the
total electrical input power introduced to the electrolyzer system.
EEiHHVðÞ=
_
niHHVi
ICell1*ECell1+ICell2*ECell2
*100% ð3Þ
In Fig. 5, we compare the performance of the single-cell and tan-
dem system experiments with particular focus on the electrical power
consumption, energy efficiency, and system characteristics. Figure 5a
deconvolutes the electrical input power, P
in
, of the Tandem-50 system
Fig. 4 | Product production rate comparison of tandem electrolyzer cell cas-
cade (cell-1 and cell-2) versus a single electrolyzer cell. Production rate of
ahydrogen, bcarbon monoxide, cethylene, dethanol, en-propanol, and fallyl
alcohol as a function of current density applied to cell-2. Values given in the legend
correspond to the volumetric flow of CO
2
introduced to the tandem system. In the
tandem configuration, cell-1 has been continually operated at a current density of
200 mA cm−2while the current density applied to cell-2 has been varied. For both
cells the active geometric surface area is 5 cm2. The single-cell refers to CO
2
RR
experiments using only cell-2 of the reaction cascade at a CO
2
volume flow of
50 mL min−1. Displayed values represent the average and error bars the standard
deviation of at least 2 independent measurements.
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 6
in terms of cell-1 and cell−2inabsoluteandrelativeterms.Ascell-1was
operated at constant current and cell voltage, absolute P
in
of cell-1
remained constant, while P
in
of cell-2 strongly increased. This has
important implications for the relative P
in
balance between the two
cells: At low currents applied to cell-2, total P
in
is clearly dominated by
cell-1, later by cell−2 increasing to over 90% at 3.5 A. This is because of
the higher applied currents and cell resistances of the 1-gap design of
cell−2 compared to the zero-gap design of cell-1.
Figure 5b plots the EE of C
2
H
4
, EtOH and PrOH in Tandem-50 and
Tandem-30 versus the single-cell design. While at low currents the
single-cell system shows competitive C
2+
EE values, at larger currents >
1 A tandem designs sharply outperform the single-cell system. In par-
ticular, the Tandem-30 experiments shows a strongly enhanced total
absolute C
2+
EE of up to 13%. We then analyzed the individual EE of
C
2
H
4
, EtOH and PrOH as a function of total P
in
for each electrolyzer
design in Supplementary Fig. 12. The tandem systems showed relative
enhancements in C
2+
EE values of up to 100% over a broad operation
range, in particular Tandem-50 at high C
2+
productivities and large
power inputs. At low power input, the productivity of the tandem
system was dominated by cell-1 reflected in the high EE for CO and low
EE for H
2
in comparison to the single-cell system shown in Supple-
mentary Fig. 13. In addition to the advantages for EE, we also observed
a more efficient reactant utilization described by the single pass car-
bon efficiency, SPCE. When directly comparing the single-cell system
to the tandem-system, the SPCE nearly doubled from 17% to 30–35%
depending on the applied current, see Supplementary Fig. 14. The
increase in SPCE was larger than can be expected by a simple con-
sideration of the additional operation of cell-1 suggesting the inherent
benefit of tandem operation. A more detailed discussion can be found
in the Supplementary Information of this work.
Apart from the electrochemical performance differences, tandem
electrolyzer systems also differ from a single-cell system in terms of
some of their process characteristics. As the single-cell system deploys
only one single reactor, the total input power, the CO
2
consumption,
and product generation occur at the same physical location, schema-
tically depicted in Fig. 5d. This results in a single-stepped experimental
CO2
CO
C2+
E
Electrical Power
Single-Cell-System
Electrical Power
CO2
CO
C2+
Tandem-System
ab
cd
ef
Cell 1 Cell 2
Cell 2
Fig. 5 | Efficiency and system characteristics comparison between single-cell
and tandem experiments. a Total input Power, P
in
, as function of applied current
to cell-2 deconvoluted in absolute and relative terms into the individual contribu-
tions of cell-1 and cell−2 during tandem experiments with CO
2
input flow of
50 mL min−1.bEnergy efficiency for C
2
H
4
, EtOH and n-PrOH based on their higher
heating value (HHV) as a function of applied current for single-cell and tandem
experiments. Tandem experiments are labeled in the legend with the respective
input flow of CO
2
(30 or 50 mL min−1). c,dSchematic illustrations of c the tandem
electrolyzer cell system and d single-cell system with arrows indicating the flow of
products and reactants, CO
2
,COandC
2+
, as well as the distribution of electrical
power across the coupled electrochemical cells. e,fDistribution profiles of total
power input and concentrations of CO
2
,COandC
2
H
4
within the reaction cascade
for e tandem experiments with 50 mL min−1CO
2
feed and f single-cell experiments.
In the tandem configuration, cell-1 has been continually operated at a current
density of 200 mA cm−2while the current density applied to cell-2 has been varied.
For both cells the active geometric surface area is 5 cm2. Displayed error bars
represent the standard deviation of at least 2 independent measurements.
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 7
concentration and power profile, as given for 700 mA cm−2in Fig. 5f. By
contrast, in the tandem system the total input power splits into two
reactors and the conversion of CO
2
occurs along a reaction cascade,
schematically depicted in Fig. 5c. Consequentially, the experimental
concentration and power profile at 700 mA cm−2(cell-2), given in
Fig. 5e, reveals two distinct steps.
In summary, we have designed, assembled, and analyzed the first
full low-temperature tandem electrolyzer cell system designed for an
efficient electrochemical CO
2
reduction to C
2+
products, such as
Ethylene, EtOH, and PrOH. The tandem design consisted of two low-
temperature electrolyzer cells in series, which enabled a catalytic
reaction cascade of the reaction of CO
2
to mixed CO
2
/CO streams in
cell-1, coupled to the reaction of mixed CO
2
/CO feeds to C
2+
products
in cell-2. We demonstrated that the tandem system strongly enhanced
production rates of C
2+
compounds compared to the use of a con-
ventional singe-cell system. We also provided evidence that the addi-
tional power input required to operate the tandem system was
comperatively minor thanks to the drastically lower energetic demand
of the CO
2
-to-CO electrolyzer cell-1. Consequentially, the proposed
tandem system allowed for an increase in individual EE of C
2+
species
compared to the conventional single-cell system.
Methods
Electrochemical cell and electrode preparation
Depending on the target product of electrolysis, electrochemical tests
were performed using two different cell geometries. In the case of CO
2
-
to-CO reduction, a “zero-gap”cell was used shown in Supplementary
Fig. 2a, referred to as cell-1. For the reduction of either CO
2
or CO, or
co-feed mixtures of the two to produce C
2+
species a “one-gap”cell was
used, shown in Supplementary Fig. 2b, referred to as cell-2.
Details on cell-1. The Ni-N-C catalyst used in this work was synthesized
through thermal carbonization of a Ni-imidazolate according to a pre-
viously established procedure. In short, to an aqueous solution of
Ni(NO
3
)
2
and imidazole, a NaOH solution was added dropwise. After
overnight stirring, the precipitate was filtered, washed, and freeze-dried.
This catalyst precursor material was then carbonized by thermal treat-
ment in N
2
atmosphere at 800 °C. The crude product was acid-washed
at 80 °C in H
2
SO
4
, followed by rinsing with water to pH-neutral and
freeze-drying to obtain the final catalyst powder43. For the preparation
of the cathode GDE, 55 mg as-prepared Ni-N-C catalyst, 195 mg Sustai-
nion solution (Dioxide Materials, 5 wt% Sustainion in ethanol solution),
100 μLDI-waterand2900μLisopropanolweremixedandsonificated
using a sonifer horn for 15 mins. Afterwards, the prepared ink was
sprayedcoatedat60°Conto5cm
2of the micro porous side of a
commercial gas diffusion layer provided by DeNora (GDL2). As elec-
trochemical cell, a membrane electrode assembly type electrolyzer
setup was deployed for the CO
2
to CO conversion. An exploded-view of
the electrolyzer system is given in Supplementary Fig. 2a. The catalyst-
coated cathode GDL, membrane (Sustainion membrane X37-50 RT,
Dioxide material), and the anode (5 cm2,commercialIrO
2
-GDE supplied
from Dioxide Materials) were assembled layer by layer including PTFE
gaskets (thickness: 200 microns; window size: 5c m2)toguaranteefor
leak-tightness. Finally, all layers were compressed between the electro-
lyzer endplates, which served as flow fields and current collectors. For
catalytic tests involving cell-1, an aqueous solution of 0.1 M KHCO
3
electrolyte was used on the anode side only and constantly recycled at a
volumetric flow rate of 20 mL min−1. On the cathode, a humidified
stream of CO
2
was introduced by a mass flow controller at a rate of
25 mL min−1. After the reaction in cell-1, the product stream, composed
mainly of CO and unreacted CO
2
, was introduced as reactant gas feed to
the cathode GDE of cell-2.
Details on cell-2. As anode material Ni foam (Fraunhofer IFAM,
thickness of 0.45 mm) was placed inside the flow field of the anode
Titanium endplate (Dioxide materials). For the preparation of the
cathode GDE a sintered PTFE membrane (Elringklinger, thickness of
0.5 mm) was used as substrate and coated by spray-painting with a
dispersion of commercial spherical Cu particles (Sigma-Aldrich,
40–60 nm) and PTFE particles (Sigma-Aldrich, 1 μm) in ethanol. The
relative content of PTFE particles was adjusted to achieve a final
loading of 20 wt.% in relation to the combined loading of Cu and PTFE
particles on the substrate. The absolute loading of Cu particles on the
PTFE substrate was controlled by the volume of the catalyst dispersion
used during the spray-painting process and fixed at 3.0 mg cm−2
throughout the whole study. When assembling the electrolysis cell, the
Cu-PTFE GDE is fixed to the cathode Titanium endplate with a con-
ductive Cu tape to enable electronic contacting of the catalyst layer.
Silicon gaskets of 0.5 mm in thickness and a cut-out window of 5 cm2
were used to expose 5 cm2of the electrode as geometric active area
and guarantee a leak-free operation during catalytic testing. An aqu-
eous solution of 1.0 M KHCO
3
with a volume of 500 mL each was
deployed as anolyte and catholyte solution. In the cathode compart-
ment, a custom-designed flow field composed of PEEK and a thickness
of 1.0 mm was deployed. The anode and cathode compartments of the
electrochemical cell were separated by an anion exchange membrane
(Selemion, AMV).
Electrochemical setup
For electrochemical testing the electrolysis cell was embedded in the
teststand depicted in Supplementary Fig. 2c that allowed careful
control over pressure levels, as well as gas and electrolyte flow rates.
Mass flow controllers (Bronkhorst) were used to accurately adjust the
flow of all gases (N
2
,CO,CO
2
). The reactant gas feeds, CO
2
and CO pure
feeds or gas mixtures of the two, were introduced at a total constant
rate of flow of 50 mL min−1. The anolyte and catholyte were constantly
recirculated through the respective compartments of the cell by
membrane pumps (KNF, SIMDOS 10) at a constant flow of 50 mL min−1.
Additionally, a dead volume filled with gas is induced on the inlets and
outlets of the membrane pumps to mitigate their inherent pulsation
during operation. The cell was operated with an overpressure on the
liquid side of the cathode GDE. Here, the differential pressure across
the cathode was set to 100 mbar by adjusting the absolute pressure of
the cathode gas side to 1.300 bar and the absolute pressure of the
catholyte reservoir headspace to 1.400 bar by use of back pressure
regulators (Bronkhorst).
Electrochemical measurement protocol
Electrochemical characterization for cell-2 was carried out according
to a preset measurement protocol comprising three successive mea-
surement techniques: Firstly, galvanostatic steps in current density,
secondly cyclovoltammetry (CV) and finally galvanostatic electro-
chemical impedance spectroscopy (GEIS). All electrochemical data
that show error bars represent the mean values and standard deviation
obtained from at least two independent samples.
Galvanostatic current density steps. Here, after an initial equilibra-
tion time of 20 min at open circuit potential (OCP), the cathodic cur-
rent density was increased stepwise as follows: 50, 100, 200, 300, 400,
500, 600 and finally 700 mA cm−2. After the 700 mA cm−2current
density step, the current was decreased again, retracing the previous
steps in the opposite direction towards a final current density of
50 mA cm−2. During the whole procedure, each of the set current
density steps was kept constant for a period of 20 min before moving
towards the next value.
Cyclovoltammetry (CV). After completion of the galvanostatic cur-
rent density steps, CVs were conducted in a potential window of
100 mV (−0.600 to −0.700 V
Ag/AgCl
). Within this potential window 5
CVs were conducted at a constant scan rate before increasing the scan
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 8
rate and performing 5 CVs again. This procedure was performed
sequentially for scan rates of 1, 2, 5, 10, 20, 50, 100, 200, 500, 700,
1000, 1500 and 2000 mV s−1.
Galvanostatic electrochemical impedance spectroscopy (GEIS).In
the final step, GEIS measurements were conducted at 1, 10, 20, 50,
100, 200, 300, 400, 500, 600, and 700 mA cm−2of absolute cathodic
current. The frequency was varied between 500 kHz and 100 mHz
and the amplitude of the interference signal was set to 10% of the
respective current density value at which the GEIS measurement was
carried out.
Quantification of products
All data that show error bars represent the mean values and standard
deviation obtained from at least two independent samples.
Gaseous products. After electrochemical reaction in the electrolyzer
the outgoing gas stream was mixed with a constant flow of N
2
as an
internal standard (16 mL min−1)andCO
2
(150 mL min−1) that has been
purged through the headspace of the catholyte compartment in order
to collect gaseous products that were released at the catholyte side
during operation. This mixture of gases was introduced into a gas
chromatograph (Shimadzu, GC 2014 series) in the following denoted
as GC. The GC was equipped with a thermal conductivity detector
(TCD) used in the quantification of H
2
and N
2
concentrations, as well as
aflame ionization detector (FID) that was used for quantification of
methane, ethylene and CO. For the detection of CO by the FID a
methanizer has been used for the thermal reduction of CO to CH
4
prior
to its detection. Determination of the absolute volumetric flow after
electrolysis has been conducted based on changes in the concentra-
tion of N
2
standard as detected by the TCD.
Liquid products. During electrolysis, aliquots of the catholyte have
been collected after 20 min of reaction time at constant current. These
aliquots were analyzed for their content of alcohols by liquid injection
gas chromatography (Shimadzu, GC 2010 series) equipped with an
FID. Furthermore, a high-performance liquid chromatograph (Agilent,
1200 series) equipped with a refractive index detector has been
deployed for quantification of carbonic acids and their respective salts.
Data availability
All data supporting the results of this study, are included in the pub-
lished article or the associated Supplementary Information. Additional
information will be made available upon reasonable request to the
corresponding author.
References
1. Gür, T. M. Review of electrical energy storage technologies, mate-
rials and systems: challenges and prospects for large-scale grid
storage. Energy Environ. Sci. 11,2696–2767 (2018).
2. De Luna, P. et al. What would it take for renewably powered elec-
trosynthesis to displace petrochemical processes? Science 364,
eaav3506 (2019).
3. Krause, R. et al. Industrial application aspects of the electro-
chemical reduction of CO
2
to CO in aqueous electrolyte. Chem. Ing.
Tech. 92,53–61 (2020).
4. Haas, T., Krause, R., Weber, R., Demler, M. & Schmid, G. Technical
photosynthesis involving CO
2
electrolysis and fermentation. Nat.
Catal. 1,32–39 (2018).
5. Masel, R. I. et al. An industrial perspective on catalysts for low-
temperature CO
2
electrolysis. Nat. Nanotechnol. 16,
118–128 (2021).
6. De Gregorio, G. L. et al. Facet-dependent selectivity of Cu catalysts
in electrochemical CO
2
reduction at commercially viable current
densities. ACS Catal. 10, 4854–4862 (2020).
7. Dinh, C.-T. et al. CO2 electroreduction to ethylene via hydroxide-
mediated copper catalysis at an abrupt interface. Science 360,
783 (2018).
8. Möller, T. et al. The product selectivity zones in gas diffusion elec-
trodes during the electrocatalytic reduction of CO
2
.Energy Environ.
Sci. 14, 5995–6006 (2021).
9. Zhong, M. et al. Accelerated discovery of CO
2
electrocatalysts
using active machine learning. Nature 581,178–183 (2020).
10. García de Arquer, F. P. et al. CO
2
electrolysis to multicarbon pro-
ducts at activities greater than 1 A cm−2. Science 367,
661–666 (2020).
11. Hori, Y., Murata, A., Takahashi, R. & Suzuki, S. Electroreduction of
carbon monoxide to methane and ethylene at a copper electrode in
aqueous solutions at ambient temperature and pressure. J. Am.
Chem. Soc. 109,5022–5023 (1987).
12. Li, J. et al. Constraining CO coverage on copper promotes high-
efficiency ethylene electroproduction. Nat. Catal. 2, 1124–1131 (2019).
13. Wang, L. et al. Electrochemical carbon monoxide reduction on
polycrystalline copper: effects of potential, pressure, and pH on
selectivity toward multicarbon and oxygenated products. ACS
Catal. 8,7445–7454 (2018).
14. Jouny, M., Luc, W. & Jiao, F. High-rate electroreduction of carbon
monoxide to multi-carbon products. Nat. Catal. 1,748–755 (2018).
15. Verdaguer-Casadevall, A. et al. Probing the active surface sites for
CO reduction on oxide-derived copper electrocatalysts. J. Am.
Chem. Soc. 137,9808–9811 (2015).
16. Li, C. W., Ciston, J. & Kanan, M. W. Electroreduction of carbon
monoxide to liquid fuel on oxide-derived nanocrystalline copper.
Nature 508,504–507 (2014).
17. Rabinowitz,J.A.,Ripatti,D.S.,Mariano,R.G.&Kanan,M.W.
Improving the energy efficiency of CO electrolysis by controlling
Cu domain size in gas diffusion electrodes. ACS Energy Lett.,
4098–4105 (2022).
18. Ma, M. et al. Insights into the carbon balance for CO
2
electro-
reduction on Cu using gas diffusion electrode reactor designs.
Energy Environ. Sci. 13,977–985 (2020).
19. Eriksson, B. et al. Mitigation of carbon crossover in CO
2
electrolysis
by use of bipolar membranes. J. Electrochem. Soc. 169,
034508 (2022).
20. Wang, X. et al. Mechanistic reaction pathways of enhanced ethylene
yields during electroreduction of CO
2
-CO co-feeds on Cu and Cu-
tandem electrocatalysts. Nat. Nanotechnol. 14,1063–1070 (2019).
21. Siahrostami, S., Bjorketun, M. E., Strasser, P., Greeley, J. & Ross-
meisl, J. Tandem cathode for proton exchange membrane fuel
cells. PCCP 15,9326–9334 (2013).
22. Lin, Y.-R. et al. Vapor-fed electrolyzers for carbon dioxide reduction
using tandem electrocatalysts: cuprous oxide coupled with nickel-
coordinated nitrogen-doped carbon. Adv. Funct. Mater. 32,
2113252 (2022).
23. Liu, Y., Qiu, H., Li, J., Guo, L. & Ager, J. W. Tandem electrocatalytic
CO
2
reduction with efficient intermediate conversion over pyramid-
textured Cu–Ag catalysts. ACS Appl. Mater. Interfaces 13,
40513–40521 (2021).
24. Gurudayal et al. Sequential cascade electrocatalytic conversion of
carbon dioxide to C–C coupled products. ACS Appl. Energy Mater.
2,4551–4559 (2019).
25. Lum,Y.&Ager,J.W.Sequentialcatalysis controls selectivity in
electrochemical CO
2
reduction on Cu. Energy Environ. Sci. 11,
2935–2944 (2018).
26. Zhang, J. et al. Tandem effect of Ag@C@Cu catalysts enhances
ethanol selectivity for electrochemical CO
2
reduction in flow
reactors. Cell Rep. Phys. Sci. 3, 100949 (2022).
27. Akter, T., Pan, H. & Barile, C. J. Tandem electrocatalytic CO
2
reduction inside a membrane with enhanced selectivity for Ethy-
lene. J. Phys. Chem. C. 126,10045–10052 (2022).
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 9
28. Ozden, A. et al. Cascade CO
2
electroreduction enables efficient
carbonate-free production of ethylene. Joule 5,706–719 (2021).
29. Zhang, T., Li, Z., Zhang, J. & Wu, J. Enhance CO
2
-to-C2+ products
yield through spatial management of CO transport in Cu/ZnO tan-
dem electrodes. J. Catal. 387,163–169 (2020).
30. She, X. et al. Tandem electrodes for carbon dioxide reduction into
C2+ products at simultaneously high production efficiency and
rate. Cell Rep. Phys. Sci. 1, 100051 (2020).
31. Han, H. et al. Selective electrochemical CO
2
conversion to multi-
carbon alcohols on highly efficient N-doped porous carbon-
supported Cu catalysts. Green. Chem. 22,71–84 (2020).
32. Chen, C. et al. Cu-Ag tandem catalysts for high-rate CO
2
electro-
lysis toward multicarbons. Joule 4,1688–1699 (2020).
33. Gao, J. et al. Selective C–Ccouplingincarbondioxideelectro-
reduction via efficient spillover of intermediates as supported by
Operando Raman Spectroscopy. J. Am. Chem. Soc. 141,
18704–18714 (2019).
34. Hoang, T. T. H. et al. Nanoporous Copper–Silver alloys by
additive-controlled electrodeposition for the selective elec-
troreduction of CO
2
to Ethylene and Ethanol. J. Am. Chem. Soc.
140,5791–5797 (2018).
35. Romero Cuellar, N. S. et al. Two-step electrochemical reduction of
CO
2
towards multi-carbon products at high current densities. J.
CO2 Util. 36,263–275 (2020).
36. Wang, M., Loiudice, A., Okatenko, V., Sharp, I. D. & Buonsanti, R. The
spatial distribution of cobalt phthalocyanine and copper nano-
cubes controls the selectivity towards C2 products in tandem
electrocatalytic CO2 reduction. Chem. Sci. 14,1097–1104 (2023).
37. Ma, Y. et al. Confined growth of Silver–Copper Janus nanos-
tructures with {100} facets for highly selective tandem electro-
catalytic carbon dioxide reduction. Adv. Mater. 34, 2110607 (2022).
38. Simon, G. H., Kley, C. S. & Roldan Cuenya, B. Potential-dependent
morphology of copper catalysts during CO
2
electroreduction
revealed by in situ atomic force microscopy. Angew.Chem.Int.Ed.
60,2561–2568 (2021).
39. Arán-Ais, R. M. et al. Imaging electrochemically synthesized Cu
2
O
cubes and their morphological evolution under conditions relevant
to CO
2
electroreduction. Nat. Commun. 11,3489(2020).
40. Vavra, J., Shen, T.-H., Stoian, D., Tileli, V. & Buonsanti, R. Real-time
monitoring reveals dissolution/redeposition mechanism in copper
nanocatalysts during the initial stages of the CO
2
reduction reac-
tion. Angew. Chem. Int. Ed. 60,1347–1354 (2021).
41. Ma, S. et al. Electroreduction of carbon dioxide to hydrocarbons
using bimetallic Cu–Pd catalysts with different mixing patterns. J.
Am. Chem. Soc. 139,47–50 (2017).
42. Lum, Y. & Ager, J. W. Evidence for product-specificactivesiteson
oxide-derived Cu catalysts for electrochemical CO
2
reduction. Nat.
Catal. 2,86–93 (2019).
43. Brückner, S. et al. Design and diagnosis of high-performance CO
2
-
to-CO electrolyzer cells. ChemRxiv (2023). https://doi.org/10.
26434/chemrxiv-2023-z6v6m.
44. Zhan, C. et al. Revealing the CO coverage-driven C–C coupling
mechanism for electrochemical CO
2
reduction on Cu
2
O
nanocubes via Operando Raman Spectroscopy. ACS Catal. 11,
7694–7701 (2021).
45. Schreier, M., Yoon, Y., Jackson, M. N. & Surendranath, Y. Competi-
tion between H and CO for active sites governs copper-mediated
electrosynthesis of hydrocarbon fuels. Angew. Chem. Int. Ed. 57,
10221–10225 (2018).
46. Huang, Y., Handoko, A. D., Hirunsit, P. & Yeo, B. S. Electrochemical
reduction of CO
2
using copper single-crystal surfaces: effects of
CO* coverage on the selective formation of ethylene. ACS Catal. 7,
1749–1756 (2017).
Acknowledgements
The research leading to these results has received funding from the
European Union’s Horizon 2020 research and innovation program
under grant agreement no. 851441, SELECTCO2 (W.J., S.B. and P.S.).
The research leading to these results has received funding from the
European Union’s Horizon 2020 research and innovation program
under grant agreement no. 101006701, ECOFUEL (T.M., M.F. and
P.S.). The work leading to this publication was supported by the
PRIMEprogramoftheGermanAcademic Exchange Service (DAAD)
with funds from the German Federal Ministry of Education and
Research (BMBF) T.M.
Author contributions
T.M. and M.F. designed the electrochemical experiments, analyzed the
results, and wrote the manuscript. S.B. and W.J. set up and conducted
electrochemical experiments using “cell−1”. P.S. co-designed and co-
wrote the manuscript. All authors participated in the discussion and
evaluation of results.
Funding
Open Access funding enabled and organized by Projekt DEAL.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains
supplementary material available at
https://doi.org/10.1038/s41467-023-41278-7.
Correspondence and requests for materials should be addressed to
Peter Strasser.
Peer review information Nature Communications thanks Fengwang Li
and the other anonymous reviewers for their contribution to the peer
review of this work. A peer review file is available.
Reprints and permissions information is available at
http://www.nature.com/reprints
Publisher’s note Springer Nature remains neutral with regard to jur-
isdictional claims in published maps and institutional affiliations.
Open Access This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as
long as you give appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons license, and indicate if
changes were made. The images or other third party material in this
article are included in the article’s Creative Commons license, unless
indicated otherwise in a credit line to the material. If material is not
included in the article’s Creative Commons license and your intended
use is not permitted by statutory regulation or exceeds the permitted
use, you will need to obtain permission directly from the copyright
holder. To view a copy of this license, visit http://creativecommons.org/
licenses/by/4.0/.
© The Author(s) 2023
Article https://doi.org/10.1038/s41467-023-41278-7
Nature Communications | (2023) 14:5680 10
