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Encapsulated Co-Ni alloy boosts high-temperature CO[2]
electroreduction
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  * Open access
  * Published: 14 May 2025

Encapsulated Co-Ni alloy boosts high-temperature CO[2]
electroreduction

  * Wenchao Ma  ORCID: orcid.org/0000-0002-8352-0583^1,
  * Jordi Morales-Vidal  ORCID: orcid.org/0000-0002-8725-5051^2,
  * Jiaming Tian^1,
  * Meng-Ting Liu^3,
  * Seongmin Jin  ORCID: orcid.org/0000-0003-3407-4297^4,
  * Wenhao Ren^1,
  * Julian Taubmann^5,
  * Christodoulos Chatzichristodoulou  ORCID: orcid.org/
    0000-0001-6499-5914^5,
  * Jeremy Luterbacher  ORCID: orcid.org/0000-0002-0967-0583^4,
  * Hao Ming Chen  ORCID: orcid.org/0000-0002-7480-9940^3,
  * Nuria Lopez^2 &
  * ...
  * Xile Hu  ORCID: orcid.org/0000-0001-8335-1196^1 

Show authors

Nature volume 641, pages 1156-1161 (2025)Cite this article

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Subjects

  * Electrocatalysis
  * Electrochemistry
  * Materials chemistry

Abstract

Electrochemical CO[2] reduction into chemicals and fuels holds great
promise for renewable energy storage and carbon recycling^1,2,3.
Although high-temperature CO[2] electroreduction in solid oxide
electrolysis cells is industrially relevant, current catalysts have
modest energy efficiency and a limited lifetime at high current
densities, generally below 70% and 200 h, respectively, at 1 A cm^-^2
and temperatures of 800 degC or higher^4,5,6,7,8. Here we develop an
encapsulated Co-Ni alloy catalyst using Sm[2]O[3]-doped CeO[2] that
exhibits an energy efficiency of 90% and a lifetime of more than
2,000 h at 1 A cm^-^2 for high-temperature CO[2]-to-CO conversion at
800 degC. Its selectivity towards CO is about 100%, and its single-pass
yield reaches 90%. We show that the efficacy of our catalyst arises
from its unique encapsulated structure and optimized alloy
composition, which simultaneously enable enhanced CO[2] adsorption,
moderate CO adsorption and suppressed metal agglomeration. This work
provides an efficient strategy for the design of catalysts for
high-temperature reactions that overcomes the typical trade-off
between activity and stability and has potential industrial
applications.

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Main

Electrocatalytic CO[2] reduction to produce chemicals and fuels is a
potentially important pathway towards a net-zero-emission society^1,2
,3. Extensive research has been conducted on low-temperature CO[2]
electroreduction (below 100 degC), but such technology faces many
challenges, including low system-level energy efficiencies and
limited catalyst lifetimes^1,3. The energy efficiency and lifetime of
industrially relevant membrane electrode assembly (MEA) electrolysers
used for low-temperature CO[2] electroreduction are typically less
than 35% and 100 h, respectively, at current densities of 1 A cm^-^2
or higher^1,9,10,11. Furthermore, there is the intrinsic problem of
carbonate formation resulting from the reaction between CO[2] and OH^
-, which reduces the carbon efficiency and lifetime to unpractically
low values^12. In view of these problems, high-temperature CO[2]
electroreduction (600-1,000 degC) in solid oxide electrolysis cells
(SOEC) has emerged as an attractive approach for CO[2] utilization^4,
5,6,7,8. This approach uses pure CO[2] as the only reactant, without
inclusion of H[2]O or other hydrogen sources, thereby affording
complete selectivity for CO formation^4,7 (Fig. 1a). Moreover, an
energy efficiency of greater than 50% is achievable at 1 A cm^-^2 for
high-temperature CO[2] electroreduction^4,7.

Fig. 1: Schematic illustration of high-temperature CO[2]
electroreduction.
figure 1

a, SOEC configuration. b, Overview of cathode catalysts developed for
CO[2] SOEC.

Full size image

Current CO[2] SOEC catalysts, which consist of pure metals^13, simple
mixtures of metals and oxides^14,15,16, or composites of oxide
support and metal decoration^17,18,19, either have limited numbers of
active interfaces or are affected by severe particle agglomeration or
coke formation at evaluated temperatures, leading to activity/
stability trade-offs (Fig. 1b). Several strategies, including use of
bimetallic catalysis^13,14, exsolution^17,18, redox cycling^19 and
morphology engineering^20, have been explored to improve the
performance of these catalysts. However, despite these efforts, the
energy efficiencies and lifetimes achieved with catalysts based on
non-precious metals at industrially relevant current densities
(1 A cm^-^2 or greater) remain modest, typically below 70% and 200 h,
respectively^14,16,19,21. Although a lifetime of 1,000 h has been
reported for a precious Ru-Fe alloy catalyst, this was achieved at a
low current density of approximately 0.5 A cm^-^2, and it degraded by
about 60% thereafter^17.

Alloy engineering provides a strategy to modulate the surface
electronic and geometric structure of metals, thereby enhancing their
catalytic performances^17,22,23. We propose that encapsulating active
alloys within inert oxides could effectively prevent alloy
agglomeration while creating rich interfaces, breaking the activity/
stability trade-off (Fig. 1b). In this regard, we have designed a
non-precious Co-Ni alloy catalyst encapsulated with Sm[2]O[3]-doped
CeO[2] (SDC), which achieves both high activity and stability for
high-temperature CO[2]-to-CO conversion. The energy efficiency and
lifetime of this catalyst reach 90% and 2,000 h at 1 A cm^-^2 and
800 degC, respectively, surpassing those of state-of-the-art catalysts
under similar conditions. The unique combination of encapsulation as
the structural feature and efficient Co-Ni alloys contributes to
enhanced triple-phase interfaces and suppresses surface
reconstruction and coke formation, thereby enabling efficient and
stable high-temperature CO[2] electroreduction.

CO[2] electroreduction performance

M[x]Ni[1-][x] catalysts with a M/Ni molar ratio of x:(1 - x)
encapsulated by SDC (denoted M[x]Ni[1-][x]@SDC), where M represents a
second non-precious metal and x ranges from 0 to 1, were synthesized
using a sol-gel method. Nickel was chosen as the host metal owing to
its current efficiency as a non-precious metal catalyst for
high-temperature CO[2] electroreduction^4,8, whereas SDC was selected
as the oxide material owing to its high oxygen ion conductivity^24,25
,26. To evaluate the performance of the catalysts, we incorporated
them into electrolyte-supported cells that consisted of a M[x]Ni[1-][
x]@SDC cathode, an La[0.8]Sr[0.2]Ga[0.8]Mg[0.2]O[3-][d] (LSGM)
electrolyte, an La[0.6]Sr[0.4]Co[0.2]Fe[0.8]O[3-d] (LSCF) anode, and
two SDC buffer layers to prevent side reactions between the electrode
and electrolyte^4 (Supplementary Fig. 1 and Supplementary Table 1).
In a preliminary screening of various second non-precious metals, the
Co[x]Ni[1-][x]@SDC catalyst showed the highest current densities for
CO[2] electroreduction to CO at all investigated cell voltages at
800 degC, while maintaining a Faradaic efficiency of CO (FE[CO]) close
to 100% (Supplementary Fig. 2). Correlation analysis of the Co and Ni
molar ratios in the Co[x]Ni[1-][x]@SDC catalysts showed that the
current density increased with increasing Co content up to x = 0.5,
and further increase in Co content instead decreased the current
density (Supplementary Fig. 3). Consequently, Co[0.5]Ni[0.5]@SDC was
selected for use in subsequent experiments.

We further synthesized two reference samples: a composite catalyst of
Co[0.5]Ni[0.5] and SDC without an encapsulated structure, referred to
as Co[0.5]Ni[0.5]-SDC (with a composition similar to Co[0.5]Ni[0.5]
@SDC, as shown in Supplementary Table 2); and an encapsulated Ni sole
catalyst (Ni@SDC). For high-temperature CO[2] electroreduction, the
current densities followed the sequence SDC < Ni@SDC < Co[0.5]Ni[0.5]
-SDC < Co[0.5]Ni[0.5]@SDC at all investigated cell voltages (Fig. 2a
), and the FE[CO] remained around 100% (Supplementary Fig. 4a).
Notably, the current density over Co[0.5]Ni[0.5]@SDC reached 1.0 A cm
^-^2 at a cell voltage of only 1.1 V, approximately 1.5, 1.7 and 16.7
times higher than those over the Co[0.5]Ni[0.5]-SDC, Ni@SDC and SDC
catalysts, respectively (Fig. 2a). We further normalized the current
densities on the basis of electrochemical active surface areas
(ECSA), and the normalized activity followed a consistent trend
(Supplementary Figs. 4b and 5 and Supplementary Table 3). In
addition, a physical mixture of Ni@SDC and Co@SDC with segregated Ni
and Co phases (denoted Ni@SDC+Co@SDC) showed inferior activity
compared with Co[0.5]Ni[0.5]@SDC featuring a Co-Ni alloy phase
(Supplementary Figs. 6 and 7). These results demonstrate that both
the alloy composition and the encapsulated structure have crucial
roles in the high CO[2] electroreduction activity of Co[0.5]Ni[0.5]
@SDC.

Fig. 2: CO[2] electroreduction performance.
figure 2

a, Current densities at different cell voltages for different
catalysts. b, Energy efficiencies at different current densities for
different catalysts. c, Effects of CO[2] flow rate on CO single-pass
yield and selectivity at 1 A cm^-^2 over Co[0.5]Ni[0.5]@SDC. d,
Stability tests at a constant current density of 1.0 A cm^-^2. The
results are shown as the mean +- s.d. from three individual
experiments.

Source Data

Full size image

We further evaluated the energy efficiency and single-pass yield for
CO[2]-to-CO conversion in our system to demonstrate its practical
applicability. The energy efficiency followed a similar sequence of
SDC < Ni@SDC <Co[0.5]Ni[0.5]-SDC <Co[0.5]Ni[0.5]@SDC across all
examined current densities and reached 90% or greater at current
densities up to 1.0 A cm^-^2 over Co[0.5]Ni[0.5]@SDC (Fig. 2b). A
further increase in current density over Co[0.5]Ni[0.5]@SDC decreased
energy efficiency owing to an increase in cell voltage, but energy
efficiencies remained at 75% or greater at current densities up to
2.0 A cm^-^2 (Fig. 2b). The single-pass yield of CO over Co[0.5]Ni
[0.5]@SDC increased from about 22% to 90% as the flow rate of CO[2]
decreased from 50 to 12 ml min^-1 at 1 A cm^-^2, while a cell voltage
of approximately 1.1 V was maintained (Fig. 2c). The CO selectivity
on a molar carbon basis remained consistently close to 100% (Fig. 2c
).

Stability is a critical metric for assessment of the performance of
CO[2] electroreduction, particularly in high-rate systems with
industrial relevance. To evaluate the stability of our catalysts, we
conducted tests under a constant current density of 1.0 A cm^-^2. The
Co[0.5]Ni[0.5]@SDC catalyst with an encapsulated structure
demonstrated exceptional long-term stability, as evidenced by a minor
increase in cell voltage from 1.10 to 1.20 V after 2,000 h of
operation, accompanied by an FE[CO] that was consistently close to
100% throughout the operation (Fig. 2d). The degradation rate for
this catalyst was a mere 0.050 mV h^-1. By contrast, the Co[0.5]Ni
[0.5]-SDC catalyst lacking an encapsulated structure, despite
exhibiting high initial activity and approximately 100% FE[CO],
showed a significant increase in cell voltage from 1.21 to 1.60 V
after 800 h of operation (Fig. 2d), with a degradation rate of
0.49 mV h^-1. Similarly, the Ni@SDC catalyst demonstrated limited
long-term stability, with the cell voltage rising from 1.33 to 1.60 V
after 500 h of operation (Fig. 2d), corresponding to a degradation
rate of 0.54 mV h^-1. In addition, the FE[CO] gradually decreased
over time on the Ni@SDC catalyst (Fig. 2d), probably owing to coke
formation^26,27. The other Co[x]Ni[1-][x]@SDC catalysts (x = 0.2,
0.75 and 1.0) also showed inferior stability compared with Co[0.5]Ni
[0.5]@SDC (Supplementary Fig. 8). These findings suggest that both
the encapsulated structure and the Co-Ni alloy composition contribute
to the robustness of our Co[0.5]Ni[0.5]@SDC catalyst for
high-temperature CO[2] electroreduction.

Catalyst characterizations

We carried out various characterizations to confirm the composition
and structure of the Co[0.5]Ni[0.5]@SDC (Fig. 3a), Co[0.5]Ni[0.5]-SDC
(Fig. 3b) and Ni@SDC (Fig. 3c) catalysts. X-ray diffraction
measurements showed that all catalysts consisted of face-centred
cubic metals and fluorite SDC (Supplementary Fig. 6). The diffraction
peaks corresponding to Co[x]Ni[1-][x] were located between those of
pure Ni and Co metals and underwent a negative shift with an increase
in Co content (Supplementary Fig. 6), which could be attributed to
the larger atomic radius of Co compared with that of Ni. This
indicates mutual alloying of Co and Ni in our catalysts^28.
Energy-dispersive X-ray spectroscopy mappings using scanning electron
microscopy and transmission electron microscopy (TEM) showed that all
Co[x]Ni[1-][x]@SDC had an encapsulated structure: Co and/or Ni
homogeneously formed a core, with the surface covered by Sm, Ce and O
(Fig. 3d,f and Supplementary Figs. 9-11). By contrast, Co[0.5]Ni[0.5]
-SDC had a random distribution of Co-Ni and Sm, Ce and O without an
encapsulated morphology (Fig. 3e and Supplementary Figs. 9b and 10b).
The homogeneous distribution of Co and Ni in Co[0.5]Ni[0.5]@SDC and
Co[0.5]Ni[0.5]-SDC further confirmed the alloying of Co and Ni in
both samples. Notably, the voids between the outer SDC particles
could ensure easy access of CO[2] to the inner active interfaces over
the Co[x]Ni[1-][x]@SDC catalysts (Fig. 3a,c). X-ray photoelectron
spectroscopy (XPS) measurements revealed surface Co and Ni contents
of approximately 15 mol.% for Co[0.5]Ni[0.5]@SDC and Ni@SDC, lower
than their respective bulk contents of approximately 80 mol.% as
determined by inductively coupled plasma mass spectrometry
(Supplementary Fig. 12). By contrast, the surface Co and Ni content
for Co[0.5]Ni[0.5]-SDC was about 60 mol.%, close to the bulk content
of approximately 80 mol.% (Supplementary Fig. 12). This result
further confirmed the encapsulated structures of Co[0.5]Ni[0.5]@SDC
and Ni@SDC and the composite structure of Co[0.5]Ni[0.5]-SDC.

Fig. 3: Characterizations.
figure 3

a-c, Schematic illustrations of the Co[0.5]Ni[0.5]@SDC (a), Co[0.5]Ni
[0.5]-SDC (b) and Ni@SDC (c) catalysts. d-f, Merged scanning electron
microscopy and energy-dispersive X-ray spectroscopy mappings for Co
[0.5]Ni[0.5]@SDC (d), Co[0.5]Ni[0.5]-SDC (e) and Ni@SDC (f)
catalysts. See Supplementary Fig. 9 for detailed distributions of
each element. g, Ni K-edge XANES spectra. h, Co K-edge XANES spectra.
Scale bars, 1 mm.

Source Data

Full size image

The quasi in situ XPS spectra (Supplementary Fig. 13) of Ni 2p and Co
2p showed reduced binding energies of metallic Ni in Ni@SDC and Co in
Co@SDC compared with their respective pure forms. The Ce 3d XPS
spectra also showed decreased Ce^3+ contents in both samples compared
with pure SDC. These trends suggest a charge transfer from Ce^3+ to
either Ni or Co in each sample. This phenomenon persisted upon
alloying, with the oxidation states of Co and Ni remaining below zero
and a slight increase in Ce^3+ content compared with their
monometallic counterparts. Such alterations are conducive to enhanced
oxygen ion transport^4. Moreover, Co[0.5]Ni[0.5]@SDC showed enhanced
charge transfer between Co-Ni and SDC compared with Co[0.5]Ni[0.5]
-SDC, suggesting an increased area of metal-oxide interfaces as a
result of encapsulation. Consistent with the XPS findings, X-ray
absorption near-edge structure (XANES) (Fig. 3g,h and Supplementary
Table 4) and operando Raman spectra (Supplementary Fig. 14)
corroborated the observed charge transfer from oxide to metal in all
samples, with Co[0.5]Ni[0.5]@SDC demonstrating more metal-oxide
interfaces than Co[0.5]Ni[0.5]-SDC. Extended X-ray absorption fine
structure spectra (Supplementary Fig. 15 and Supplementary Tables 5
and 6) showed similar bond lengths and slight variations in
coordination numbers for Ni(Co)-Co(Ni) among the samples, probably
owing to differences in particle sizes.

Postcatalytic characterizations showed that although there were no
noticeable changes in the elemental composition and crystalline phase
of any of the catalysts after the stability tests (Supplementary Fig.
16 and Supplementary Table 7), their morphologies showed distinct
trends (Supplementary Figs. 17-20). The average particle sizes of
metals in Ni@SDC and Co[0.5]Ni[0.5]-SDC increased notably by 60% and
100% following stability tests, respectively, whereas the increases
in other Co[x]Ni[1-][x]@SDC catalysts (x = 0.2, 0.5, 0.75 and 1.0)
featuring encapsulated structures were all below 15% (Supplementary
Fig. 19). Meanwhile, the increases in average particle sizes of SDC
in all catalysts remained below 10% (Supplementary Fig. 20). Raman
measurements and carbon elemental analysis revealed coke formation on
Ni@SDC after stability tests^26,27, whereas no carbon was detected in
other catalysts (Supplementary Fig. 21 and Supplementary Table 8).
Poststability XPS and XANES spectra revealed severe Co oxidation in
Co@SDC, in contrast to the modest changes in the oxidation states of
Co and Ni in Ni-containing catalysts (Supplementary Figs. 22 and 23);
this was probably due to the higher oxygen affinity of Co relative to
Ni^29. Moreover, catalysts with higher Co content underwent mass loss
(Supplementary Table 9). These results demonstrate that the
encapsulated structure can mitigate metal agglomeration, while the
optimal Co-Ni alloy can suppress coke formation, metal oxidation and
mass loss, thereby contributing synergistically to enhanced
stability. The encapsulated structure in Ni@SDC failed to inhibit Ni
agglomeration, possibly owing to coke formation on the Ni surface
disrupting the encapsulated structure.

Functioning mechanisms

To understand the variations in catalyst performance, we conducted
operando electrochemical impedance spectroscopy (EIS) measurements
and distribution of relaxation time (DRT) analyses under CO[2] and CO
atmospheres. The EIS results (Fig. 4a,b and Supplementary Fig. 24)
showed that with increasing Co content in Co[x]Ni[1-][x]@SDC, the
polarization resistances (R[p]) of the fuel electrode under CO[2]
first decreased (up to x = 0.5) and then increased, whereas they
continuously increased under CO. Compared with Co[0.5]Ni[0.5]-SDC, Co
[0.5]Ni[0.5]@SDC exhibited lower R[p] values for the fuel electrode
under both CO[2] and CO. All ohmic resistance (R[o]), R[p] of the air
electrode and double-layer capacitance values remained similar. In
the DRT analysis, the high-frequency, intermediate-frequency and
low-frequency peaks corresponded to oxygen ion migration in the
electrolyte, surface oxygen transfer on the air side, and
electrochemical adsorption and activation processes on the fuel side,
respectively^17,30,31. Increasing Co content in Co[x]Ni[1-][x]@SDC
led to low-frequency peaks under CO[2] first decreasing and then
increasing in intensity, whereas under CO they continuously
increased, with intermediate-frequency and high-frequency peaks
remaining consistent (Supplementary Fig. 25). Compared with Co[0.5]Ni
[0.5]-SDC, Co[0.5]Ni[0.5]@SDC exhibited smaller low-frequency peaks
under both CO[2] and CO (Supplementary Fig. 25). Collectively, these
results indicate that an optimal Co-Ni alloy enhances CO[2]
adsorption and activation while moderating CO activation compared
with monometallic Ni and Co. Compared with Co[0.5]Ni[0.5]-SDC, the
encapsulated Co[0.5]Ni[0.5]@SDC had more metal-oxide interfaces; this
promoted CO[2] adsorption and oxygen ion transport and led to higher
activity. Pure SDC exhibited weak activation of both CO[2] and CO
(Supplementary Fig. 26), consistent with its limited CO[2]
electroreduction activity (Fig. 2a). A correlation between CO[2]
electroreduction activity and CO[2] and CO adsorption and activation
abilities for different catalysts showed that strong CO[2] adsorption
and moderate CO adsorption were advantageous for CO[2]
electroreduction in our system (Fig. 4c,d). In long-term stability
tests, Co[0.5]Ni[0.5]@SDC showed modest changes in both EIS and DRT
spectra, whereas other catalysts showed greater increases in R[p] and
low-frequency peaks (Supplementary Figs. 27-29), reflecting the
durability of Co[0.5]Ni[0.5]@SDC. During CO[2] electroreduction, coke
formation proceeds by means of the Boudouard reaction: 2CO(g)
 - C + CO[2](g)^26. Weak CO binding can also help to suppress coke
formation, leading to the improved stability of Co[0.5]Ni[0.5]@SDC.

Fig. 4: Functioning mechanisms.
figure 4

a, Nyquist plots obtained from EIS at 1.0 V under CO[2] atmosphere in
cathode. b, Nyquist plots obtained from EIS at 0.9 V under CO
atmosphere in anode. c, Relation between CO[2] electroreduction
activity (current density at 1.1 V) and CO[2] adsorption and
activation abilities (low-frequency intensity from CO[2] DRT
analysis) for different catalysts. d, Relation between CO[2]
electroreduction activity (current density at 1.1 V) and CO
adsorption and activation abilities (low-frequency intensity from CO
DRT analysis) for different catalysts.

Source Data

Full size image

Density functional theory (DFT) simulations were conducted to
determine the adsorption energies of CO[2] and CO on different
models, as well as energy profiles for CO[2] electroreduction to CO.
We used face-centred cubic structures with (111) and (001)
terminations for the metals and a fluorite structure with a (111)
termination for SDC (Supplementary Figs. 30 and 31 and Supplementary
Table 10), reflecting the predominant facets observed in our
catalysts (Supplementary Figs. 6 and 11). A uniform distribution of
Ni and Co was assumed for the Co-Ni alloy, as no strong preference
for a particular surface termination was observed, even in the
presence of adsorbed CO (Supplementary Figs. 32-34). Bader charge
analysis indicated charge transfer from Co to Ni in all Co-Ni alloy
models (Supplementary Table 11). CO[2] adsorption was weak on
isolated metal or SDC surfaces, whether oxygen vacancies were present
in the SDC or not, with Gibbs free energies ranging from 1.47 to
0.70 eV (Supplementary Fig. 35). This result suggests that neither
the metal nor SDC alone could serve as an active site for CO[2]
adsorption. Instead, the metal-SDC interface, represented here by Ce
[3]SmO[7] clusters adsorbed on metal surfaces, enhanced adsorption of
CO[2] as carbonates, with Gibbs free energies ranging from -0.11 to
-0.39 eV (Supplementary Figs. 35 and 36) and showing no strong
correlation with the degree of charge transfer (Supplementary Fig. 37
). This indicates that the metal-SDC interface is the primary active
site for CO[2] adsorption. On the other hand, CO adsorption was more
favourable on the metal sites (Supplementary Fig. 38). The choices of
functional, thermal expansion effect^32 and CO coverage did not
affect the overall adsorption trend (Supplementary Fig. 39 and
Supplementary Table 12). Notably, Co-Ni-SDC demonstrated enhanced CO
[2] adsorption and weakened CO adsorption compared with Ni-SDC
(Supplementary Figs. 35c and 38c). On the basis of these findings, we
propose a dual-site reaction mechanism for our catalysts: CO[2] is
captured as a carbonate at the metal-SDC interface, followed by
reduction of CO[2] to CO at the adjacent metal sites. The Gibbs free
energy profiles for CO[2] electroreduction to CO further confirmed
that the reaction was energetically favoured on the dual sites rather
than on isolated metals or SDC, with similar reaction barriers on
Co-Ni-SDC and Ni-SDC and little influence of oxygen coverage
(Supplementary Figs. 40-46).

Comparison with current SOECs and MEAs

We conducted a comprehensive comparison of our SOEC with
state-of-the-art high-temperature SOECs and low-temperature MEAs for
CO[2] electroreduction to CO. Our SOEC exhibited superior performance
across key parameters, including lifetime, energy efficiency, CO
single-pass yield and cell voltage at industrially relevant current
densities (Fig. 5 and Supplementary Tables 13 and 14). The typical
lifetime, energy efficiency, CO single-pass yield and cell voltage of
SOECs are less than 200 h, 70%, 60% and greater than 1.3 V
respectively, at current densities of 1 A cm^-^2 or higher, even for
electrode-supported cells^13,14,15,16,17,18,19,20,21 (Fig. 5 and
Supplementary Table 13). For low-temperature MEA systems, the
lifetime, energy efficiency, CO single-pass yield and cell voltage
were generally lower than 100 h, 35%, 50% and greater than 3.0 V,
respectively, at 1 A cm^-^2 or higher (Fig. 5 and Supplementary Table
14). Our SOEC achieved a lifetime, energy efficiency, CO single-pass
yield and cell voltage of 2,000 h, 90%, 90% and 1.1 V, respectively,
at 1 A cm^-^2 for high-temperature CO[2]-to-CO conversion (Fig. 5).
As a result, on the basis of a preliminary cost estimation^16
(Supplementary Note 1 and Supplementary Tables 15-17), our SOEC
demonstrated net cost reductions of approximately 60% and 80%
compared with state-of-the-art SOECs and MEAs, respectively (Fig. 5).
A lower electricity price (below US$0.038 per kWh), a longer
lifetime, and a higher current density (up to approximately 2 A cm^-^
2) further contribute to the economic benefit of our system
(Supplementary Fig. 47). These findings demonstrate the potential
applications of our system in efficient and economically viable CO[2]
conversion processes.

Fig. 5: Comparison.
figure 5

Comparison of our system and state-of-the-art low-temperature MEAs
and high-temperature SOECs for CO[2] electroreduction to CO.

Source Data

Full size image

Conclusions

In conclusion, we have successfully developed a cobalt-nickel alloy
catalyst encapsulated with relatively inert SDC for high-temperature
CO[2] electroreduction. The unique encapsulated structure, coupled
with an optimized alloy composition, synergistically improves CO[2]
adsorption, tempers CO adsorption and limits metal agglomeration,
leading to both high activity and stability. This work provides a
promising strategy for designing highly efficient and stable
high-temperature CO[2] electroreduction catalysts, with potential for
industrial applications.

Methods

Chemicals and materials

Nickel(II) nitrate hexahydrate (Ni(NO[3])[2]*6H[2]O, 99.9%, ABCR),
manganese(II) nitrate hexahydrate (Mn(NO[3])[2]*6H[2]O, 98%, ABCR),
iron(III) nitrate nonahydrate (Fe(NO[3])[3]*9H[2]O, 98%, ABCR),
cobalt(II) nitrate hexahydrate (Co(NO[3])[2]*6H[2]O, 98+%, ABCR),
copper(II) nitrate trihydrate (Cu(NO[3])[2]*3H[2]O, 98+%, Sigma),
ammonium heptamolybdate tetrahydrate ((NH[4])[6]Mo[7]O[24] *4H[2]O,
99%, Roth), indium(III) nitrate hydrate (In(NO[3])[3]*H[2]O, 99.9%,
Sigma), tin(II) chloride (SnCl[2], 98%, Sigma), citric acid (99.5%,
Acros), ethylene glycol (99+%, Brunschwig), cerium(III) nitrate
hexahydrate (Ce(NO[3])[3]*6H[2]O, 99.5%, ABCR), samarium(III) nitrate
hexahydrate (Sm(NO[3])[3]*6H[2]O, 99.9%, ABCR),
ethylenediaminetetraacetic acid (99%, IVALUA), ammonia solution (25%,
VWR), lanthanum(III) nitrate hexahydrate (La(NO[3])[3]*6H[2]O, 99.9%,
ABCR), strontium nitrate (Sr(NO[3])[2], 99+%, Fisher), LSGM (99%,
Sigma), copper(II) oxide (CuO, 99%, Sigma), dispersant (2%, Fiaxell
SOFC Technologies), binder (30%, Fiaxell SOFC Technologies) and
carbon dioxide (CO[2], 99.9%, Carbagas) were all used as received
without further purification.

Synthesis of M[x]Ni[1-x]@SDC

M[x]Ni[1-x]@SDC was synthesized using a sol-gel method. First,
20 mmol of Ni(NO[3])[2]*6H[2]O and 20 mmol of a second metal salt,
which included Mn(NO[3])[2]*6H[2]O, Fe(NO[3])[3]*9H[2]O, Co(NO[3])[2]
*6H[2]O, Cu(NO[3])[2]*3H[2]O, Zn(NO[3])[2]*6H[2]O, (NH[4])[6]Mo[7]O
[24]*4H[2]O, In(NO[3])[3]*H[2]O or SnCl[2], were dissolved in 50 ml
of H[2]O at 100 degC under vigorous stirring. Then, 100 mmol of citric
acid and 100 mmol ethylene glycol were added to the solution to form
a sol. Subsequently, 8 mmol of Ce(NO[3])[3]*6H[2]O and 2 mmol of Sm
(NO[3])[3]*6H[2]O were added. The reaction was kept at 100 degC for
about 3 h to evaporate H[2]O from the system. The remaining solvent
was removed at 300 degC overnight in an oven, followed by heat
treatment in air at 600 degC for 5 h with a ramping rate of 5 degC min^
-1. By changing the feeding molar ratio of Ni(NO[3])[2]*6H[2]O and Co
(NO[3])[2]*6H[2]O while keeping their total amounts to 40 mmol, we
obtained Co[x]Ni[1-][x]@SDC catalysts with different molar ratios of
Co and Ni. A pure SDC sample without addition of Ni(NO[3])[2]*6H[2]O
or a second metal salt was also prepared.

Synthesis of Co[0.5]Ni[0.5]-SDC

Co[0.5]Ni[0.5]-SDC without an encapsulated structure was synthesized
using a step-wise sol-gel method. First, pure SDC powder was first
synthesized as described. Second, 10 mmol of SDC powder was dispersed
in 50 ml of H[2]O at 100 degC under vigorous stirring, and 20 mmol of
Ni(NO[3])[2]*6H[2]O and 20 mmol of Co(NO[3])[2]*6H[2]O were then
added to the solution. Subsequently, 80 mmol of citric acid and
80 mmol ethylene glycol were added to form a sol. The reaction was
kept at 100 degC for about 3 h to evaporate H[2]O from the system. The
remaining solvent was removed at 300 degC overnight in an oven,
followed by heat treatment in air at 600 degC for 5 h with a ramping
rate of 5 degC min^-1.

Synthesis of LSCF

LSCF was synthesized using a modified sol-gel method^33. First,
40 mmol of ethylenediaminetetraacetic acid was added to 40 ml of a
25 wt.% aqueous ammonia solution. Subsequently, 12 mmol of La(NO[3])
[3]*6H[2]O, 8 mmol of Sr(NO[3])[2], 4 mmol of Co(NO[3])[2]*6H[2]O and
16 mmol of Fe(NO[3])[3]*9H[2]O were added. Next, 60 mmol of citric
acid was added, and the solution pH was adjusted to 8 by addition of
more aqueous ammonia solution. The resulting solution was heated at
100 degC under vigorous stirring for about 3 h to evaporate H[2]O from
the system. The remaining solvent was removed at 120 degC overnight,
followed by heat treatment in air at 950 degC for 5 h with a ramping
rate of 10 degC min^-1.

Cell preparation

An electrolyte-supported cell (M[x]Ni[1-x]@SDC || SDC || LSGM || SDC 
|| LSCF) was fabricated by a tape-casting process followed by several
screen-printing processes. To prepare tape-casting slurries of the
electrolyte, we mixed 20 g of LSGM powders, 14 g of dispersants and
20 g of binders by ball milling for 1 h. The resultant slurries were
tape-casted into well-defined tapes with an original thickness of
1.5 mm using a tape-casting machine (MTI Corporation), followed by
drying at room temperature for 48 h. The tapes were then punched into
shape and sintered at 1,400 degC for 10 h with a ramping rate of
2 degC min^-1 to form a dense electrolyte support. The screen-printing
slurries were prepared by mixing 2 g of powders, 2 g of dispersants
and 0.8 g of binders by ball milling for 1 h. A buffer SDC layer
(2 wt.% CuO was added to promote sintering^34) was first
screen-printed on to both sides of the electrolyte support using a
screen-printing machine (Fiaxell SOFC Technologies) to avoid any side
reaction between electrolyte and electrode. Then, the M[x]Ni[1-x]@SDC
cathode and LSCF anode (35 wt.% SDC was added to the LSCF to improve
O^2- conductivity) were screen-printed on to either side of the SDC.
The active areas of both cathode and anode were 1.5 cm^2. The printed
cells were sintered at 1,200 degC for 6 h with a ramping rate of
2 degC min^-1. The thickness and mass loading of each layer are shown
in Supplementary Table 1.

Electrochemical measurements

All electrochemical measurements were conducted using a SOEC set-up
from Fiaxell SOFC Technologies and were controlled by an Autolab
potentiostat (PGSTAT302N) equipped with a 20-A booster. The cell was
heated to 800 degC with a ramping rate of 5 degC min^-1 under air. Next,
20 vol.% H[2] diluted with N[2] was fed to the cathode compartment to
reduce the catalyst for 10 min. When the reduction process was
complete, CO[2] and air were fed to the cathode and anode
compartments, respectively, at a flow rate of 50 ml min^-1. The gas
outlet from the cathode compartment was connected to an online gas
chromatograph (SRI Instruments) for product analysis.

The Faradaic efficiency was calculated by equation (1):

$${\rm{FE}}=\frac{n\times F\times x\times {f}_{{\rm{out}}}}{i},$$
(1)

where FE is the Faradaic efficiency; n is the number of electrons
exchanged to produce CO from CO[2] (that is, 2); and F, x, f[out] and
i are the Faraday constant (96,485 C mol^-1), the molar concentration
of CO measured by gas chromatography (mol/mol), the flow rate of the
cathode gas outlet (mol s^-1) and the current (A), respectively.

The energy efficiency was estimated by equation (2):^5

$${\rm{E}}{\rm{E}}=\frac{\Delta H\times {\rm{FE}}}{({\rm
{electricity}}\;{\rm{energy}}+{\rm{heat}}\;{\rm{energy}})}=\frac{{E}_
{\Delta H}\times {\rm{FE}}}{({E}_{{\rm{app}}}+{E}_{\Delta S}/{\eta }_
{{\rm{h}}})},$$
(2)

where EE is the energy efficiency; and [?]H, E[[?]H], E[app], E[[?]S] and e
[h] are the enthalpy of formation, thermoneutral potential, applied
cell voltage, potential based on heat energy and heating efficiency,
respectively. For the reaction CO[2] - CO + 0.5 O[2] at 800 degC, E[[?]H]
and E[[?]S] are 1.467, and 0.481 V, respectively. The e[h] used for
industrial applications is usually at least 90% (refs. ^35,36); thus,
a value of 90% was used in our estimation.

The CO single-pass yield was calculated by equation (3):

$${\rm{C}}{\rm{O}}\;{\rm{single}}\;{\rm{pass}}\;{\rm{yield}}=x\times
{f}_{{\rm{out}}}/{f}_{{{\rm{CO}}}_{2}}\,,$$
(3)

where f[CO2] is the flow rate of inlet CO[2] gas (mol s^-1).

Operando EIS measurements were performed at 1.0 V for CO[2]
electroreduction and 0.9 V for CO electrooxidation at 800 degC. A
perturbation amplitude of 10 mV was applied with frequencies ranging
from 100 kHz to 0.01 Hz. The fitting of the EIS data and DRT analysis
were conducted using RelaxIS 3 software (rhd instruments).

ECSA was measured using a double-layer capacitance (C[dl]) method^37,
determined by means of EIS measurements using a symmetrical cell
configuration (M[x]Ni[1-x]@SDC || SDC || LSGM || SDC || M[x]Ni[1-x]
@SDC) at 800 degC under N[2] atmospheres on both sides. To establish a
baseline, a flat, pure electrolyte cell without catalysts (SDC ||
 LSGM || SDC) was fabricated. EIS data were recorded at 0 V with a
perturbation amplitude of 10 mV and frequencies ranging from 100 kHz
to 0.1 Hz. The electrode capacitance (C[dl]) was obtained as half the
capacitance derived from fitting the EIS using a simplified Randles
circuit (Supplementary Fig. 5). Consequently, the ECSA of an
electrode could be calculated by equation (4):

$${\rm{E}}{\rm{CSA}}={C}_{{\rm{dl}}}\times S/{C}_{{\rm{base}}}\,,$$
(4)

where C[dl] represents the capacitance of a specific electrode, C
[base] represents the capacitance of the baseline and S denotes the
geometric area of the electrode.

Characterizations

Metal elemental analysis was conducted on a NexION 350D inductively
coupled plasma mass spectrometer. X-ray diffraction patterns were
recorded on a PANalytical Aeris diffractometer using Cu K[a]
radiation (40 kV, 15 mA). Scanning electron microscopy images were
recorded on a Zeiss GeminiSEM 300, and the corresponding
energy-dispersive X-ray spectroscopy mapping was performed at 16 kV.
TEM measurements were carried out on an FEI Talos F200S at 200 kV.
Spherical-aberration-corrected TEM measurements were performed on an
FEI Titan Themis 60-300 instrument at 200 kV. Carbon elemental
analysis was performed in an UNICUBE (Elementar) microelemental
analyser by a combustion method.

XPS measurements were performed in an Omicron XPS system equipped
with a pretreatment chamber, using aluminium K[a] X-rays as the
excitation source at 15 kV and 300 W. For quasi in situ XPS
measurements of fresh samples, a reduction process was applied in
20 vol.% H[2] at 800 degC for 10 min in the pretreatment chamber.
Subsequently, samples were transferred to the vacuum chamber for XPS
measurements without exposure to air. After stability tests, samples
underwent no further treatment but were immediately sealed in an N[2]
-protected bag.

X-ray absorption spectroscopy (XAS) measurements were performed at
the 12B2 Taiwan beamline (SPring-8, Japan) of the National
Synchrotron Radiation Research Center, operated at an 8.0 GeV storage
ring with a constant current of approximately 99.5 mA. To prevent
oxidation, all samples were sealed in an N[2]-protected bag before
XAS measurements. Measurements at the Ni K-edge (8,333 eV) and Co
K-edge (7,709 eV) were performed in total fluorescence yield mode
using a Lytle detector. The scan ranges were 8,133-8,933 eV for the
Ni K-edge and 7,509-8,309 eV for the Co K-edge. XAS data were
processed using Athena software, with energy calibration performed on
the basis of the first inflection points in the absorption K-edges of
the Ni and Co foils, which were set to 8333.0 eV and 7709.0 eV,
respectively. Standard data processing procedures were applied,
including background subtraction and edge height normalization. The
average oxidation states of Ni and Co were determined by simulation
of the linear combinations of reference spectra from Ni and Co foils
and their oxides, NiO and CoO, respectively. Extended X-ray
absorption fine structure analysis was performed using Fourier
transforms on k^3-weighted oscillations in a k range from 3.0 to
11.0 A^-1.

Operando Raman spectroscopy measurements were performed using an
inVia confocal Raman microscope (Renishaw) equipped with a 532 nm
laser, coupled with a Linkam heat stage capable of controlling
temperature, gas atmosphere and cell voltage. The sample was first
reduced in 20 vol.% H[2]/N[2] at 800 degC for 10 min. Then, the
atmosphere was switched to CO[2], and Raman spectra were recorded at
cell voltages of 1.2, 1.4, 1.6 and 1.8 V at 800 degC.

DFT simulations

DFT simulations were conducted using the Vienna ab initio simulation
package^38,39. The Perdew-Becke-Ernzerhof^40 functional, supplemented
with the D3 dispersion correction^41, was used to account for the van
der Waals interactions. A Hubbard correction^42 was applied using the
Dudarev method^43 to describe the f-electrons of Ce and Sm in SDC
models, with a U[eff] of 4.5 and 4.0 eV for Ce and Sm, respectively^
31,44,45. Core electrons were represented using projector augmented
wave core potentials^46,47, whereas valence electrons were described
with plane waves at a kinetic cut-off energy between 450 and 700 eV.
The Monkhorst-Pack method^48 was used to generate a G-centred mesh
with a reciprocal grid finer than 0.036 A^-^1 for Brillouin zone
sampling.

Spin polarization was included in all simulations involving Co, Ni
and oxygen-defective SDC systems^45. A kinetic cut-off energy of
450 eV was used for most simulations, except for lattice parameter
optimizations, in which a higher cut-off of 700 eV was applied to
avoid Pulay stress. Dipole corrections^49 were included along the z
axis, and a vacuum region of 15 A was added between slabs. For Ni and
Co-Ni alloys, we modelled the (111) and (001) facets as p(4 x 4) and
p(2 x 2) slabs, respectively. Three different terminations were
considered for the Co-Ni(001) surface. Metal slabs contained four
atomic layers, with the two outermost layers allowed to relax,
whereas the two bottommost layers were fixed to their bulk positions.
For SDC, a p(3 x 3) slab of the CeO[2](111) was modelled,
incorporating Sm and oxygen vacancies. This slab consisted of nine
atomic layers (three O-Ce-O trilayers), with the five outermost
layers allowed to relax. The Ni-SDC and Co-Ni-SDC interfaces were
represented by Ce[3]SmO[7] clusters adsorbed on p(4 x 4) slabs of Ni
(001) and Co-Ni(001) surfaces, respectively. Six different Ce[3]SmO
[7] aggregates were generated on the basis of CeO[2](111) models, in
which a Ce atom was replaced by Sm, and an oxygen vacancy was
introduced. Various adsorption configurations of these clusters were
then explored on the metallic surfaces, resulting in 18 and 24
different structures on Ni(001) and Co-Ni(001), respectively.

For the adsorption energy of CO[2] and CO and the energy profiles for
CO[2] electroreduction, CO[2] and CO molecules in the gas phase were
used as thermodynamics references. Vibrational contributions to
enthalpy and entropy were included in the Gibbs free energies
calculations, along with rotational and translational contributions
for gas-phase CO[2] and CO. Gibbs free energies were computed at
800 degC and 1 atm of CO[2], in accordance with the experimental
reaction conditions. Transition states were located using the
climbing image nudged elastic band method^50. Numerical frequencies
calculations, using a step size of +-0.015 A, were performed to
confirm the nature of the transition states. Vibrational
contributions to enthalpy and entropy for the Gibbs free energies of
transition states were computed using standard approximations: the
ideal gas model, rigid rotor and harmonic oscillator. Only the
reactants were considered for metals, whereas for oxidic systems,
oxygen atoms were also included in the vibrational estimations owing
to their lower molecular weight.

Data availability

All data are available in the article and its Supplementary
Information. Data for all experiments and DFT calculations are
available at Zenodo (https://doi.org/10.5281/zenodo.14540651)^51 and
ioChem-BD (https://doi.org/10.19061/iochem-bd-1-314)^52,53,54,
respectively. The atomic positions of all DFT models are provided in
the Supplementary Data. Source data are provided with this paper.  

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Acknowledgements

W.M., J. Tian, W.R. and X.H acknowledge the financial support of
EPFL. J.M.-V. and N.L. acknowledge financial support from the Spanish
Ministry of Science and Innovation (PID2021-122516OB-I00 and Severo
Ochoa Grant MCIN/AEI/10.13039/501100011033 CEX2019-000925-S),
Generalitat de Catalunya and AGAUR (2023 CLIMA 00105). We thank
Barcelona Supercomputing Center-MareNostrum (BSC-RES) for providing
generous computational resources. H.M.C. acknowledges financial
support from the National Science and Technology Council, Taiwan
(contract no. NSTC NSTC 112-2123-M-002-008) and from National Taiwan
University (NTU-CC-113L893304). S.J. and J.L. acknowledge financial
support from the European Research Council (ERC) under the European
Union's Horizon 2020 research and innovation programme (grant:
CATACOAT, no 588251). J. Taubmann and C.C acknowledge financial
support from Innovation Fund Denmark (1150-00001B); further
information is available at www.MissionGreenFuels.dk. We thank J. Cai
and G. Yang from ShanghaiTech University for conducting some XPS
measurements using the BL02B01 of the Shanghai Synchrotron Radiation
Facility and SPECS ambient pressure XPS instrument with financial
support from the National Natural Science Foundation of China (no.
11227902); G. Wang, R. Li and Y. Shen from the Dalian Institute of
Chemical Physics for assistance with some characterizations; and B.
Victor from CIME, EPFL, for conducting aberration-corrected
high-angle annular dark-field scanning transmission electron
microscopy.

Funding

Open access funding provided by EPFL Lausanne.

Author information

Authors and Affiliations

 1. Laboratory of Inorganic Synthesis and Catalysis, Institute of
    Chemical Sciences and Engineering, Ecole Polytechnique Federale
    de Lausanne (EPFL), Lausanne, Switzerland

    Wenchao Ma, Jiaming Tian, Wenhao Ren & Xile Hu

 2. Institute of Chemical Research of Catalonia (ICIQ-CERCA), The
    Barcelona Institute of Science and Technology, Tarragona, Spain

    Jordi Morales-Vidal & Nuria Lopez

 3. Department of Chemistry and Center for Emerging Materials and
    Advanced Devices, National Taiwan University, Taipei, Taiwan

    Meng-Ting Liu & Hao Ming Chen

 4. Laboratory of Sustainable and Catalytic Processing, Institute of
    Chemical Sciences and Engineering, Ecole Polytechnique Federale
    de Lausanne (EPFL), Lausanne, Switzerland

    Seongmin Jin & Jeremy Luterbacher

 5. Department of Energy Conversion and Storage, Technical University
    of Denmark, Kongens Lyngby, Denmark

    Julian Taubmann & Christodoulos Chatzichristodoulou

Authors

 1. Wenchao Ma
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 4. Meng-Ting Liu
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 5. Seongmin Jin
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 6. Wenhao Ren
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 7. Julian Taubmann
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 8. Christodoulos Chatzichristodoulou
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 9. Jeremy Luterbacher
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10. Hao Ming Chen
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11. Nuria Lopez
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12. Xile Hu
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Contributions

W.M. designed and synthesized the catalysts, and performed most of
the electrochemical tests, characterizations and analyses; J.M.-V.
performed the DFT simulations and drafted the DFT parts of the paper
under the supervision of N.L.; J. Tian performed operando Raman
measurements under the supervision of J. Taubmann and C.C; M.-T.L.
performed the XAS measurements under the supervision of H.M.C.; S.J.,
W.R. and J.L. assisted with some measurements. W.M. and X.H. wrote
the paper, with input from all the other authors. X.H. directed the
research.

Corresponding author

Correspondence to Xile Hu.

Ethics declarations

Competing interests

W.M. and X.H. are inventors on a European patent application (PCT/
EP2024/069715) that includes the catalyst described in this work. The
other authors declare no competing interests.

Peer review

Peer review information

Nature thanks Min-Rui Gao, Yuan Wang and the other, anonymous,
reviewer(s) for their contribution to the peer review of this work.
Peer reviewer reports are available.

Additional information

Publisher's note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional
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Supplementary information

Supplementary Information

Supplementary Note 1, Figs. 1-47, Tables 1-17 and references.

Supplementary Data

Atomic positions for all DFT models.

Peer Review File

Source data

Source Data Fig. 2

Source Data Fig. 3

Source Data Fig. 4

Source Data Fig. 5

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Cite this article

Ma, W., Morales-Vidal, J., Tian, J. et al. Encapsulated Co-Ni alloy
boosts high-temperature CO[2] electroreduction. Nature 641, 1156-1161
(2025). https://doi.org/10.1038/s41586-025-08978-0

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  * Received: 14 August 2023

  * Accepted: 03 April 2025

  * Published: 14 May 2025

  * Issue Date: 29 May 2025

  * DOI: https://doi.org/10.1038/s41586-025-08978-0

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