Volume 1
Number 1
January 2023
Pages 1–96

EES Catalysis
rsc.li/EESCatalysis

ISSN 2753-801X

COMMUNICATION
Shuji Nakanishi, Kazuhide Kamiya et al.
Ultra-high-rate CO2 reduction reactions to multicarbon
products with a current density of 1.7 A cm−2 in neutralelectrolytes

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Cite this: EES Catal., 2023,
1, 9

Ultra-high-rate CO2 reduction reactions tomulticarbon products with a current density of
1.7 A cm 2 in neutral electrolytes†

Received 1st September 2022,
Accepted 1st November 2022

Asato Inoue,a Takashi Harada,ab

Shuji Nakanishi*ab and Kazuhide Kamiya*ab

DOI: 10.1039/d2ey00035k
rsc.li/eescatalysis

CO2 electrolysis to value-added products is a promising technology
to close the carbon cycle and sequester anthropogenic CO2 into
chemical feedstocks; an increase of the current density for multicarbon products is one of the requirements for practical implementation. We have successfully increased the partial current
density for gaseous CO2 reduction reactions to multicarbon products (C2+) over Cu nanoparticles on gas diﬀusion electrodes inneutral electrolytes to a record value of 1.7 A cm 2. The faradaic
eﬃciency for multicarbon products increased with the current
density below total current density of 2000 mA cm 2 and reached
76% at a total current density of 1600 mA cm 2. The turnover
frequency for the production of C2+ per Cu atoms exceeded 1.1 s 1.
Optimizing the standard components and their assembly as thecathode elicits the high-turnover frequency of oxide-derived Cu
catalysts, resulting in the record partial current density for C2+.
Especially, we demonstrated that the thickness of catalyst layers
was one highly sensitive factor in determining the maximum cur-

Broader context
The electrochemical reduction of carbon dioxide (CO2) has attracted
considerable attention to maintain the carbon cycle. An increase in thecurrent density for value-added multicarbon products (C2+), such as
ethylene and ethanol, is required for practical implementation of thistechnology. Herein, we successfully obtained the partial current density
for C2+ over cupric oxide (CuO) nanoparticles on gas diﬀusion electrodesin neutral electrolytes to a value of 1.7 A cm 2. Additionally, the faradaic
eﬃciency for multicarbon products increased with the current density
below Jtotal = 2000 mA cm 2 and reached up to 76%. Our ultra-high-rate
CO2 electrolysis was achieved by optimizing the ordinary components,
including oxide-derived Cu nanoparticles and carbon-based gas-diﬀusion
electrodes, and their assembly as the cathode. In the other word, thepresent study succeeded in eliciting the potential performance of an
ordinary combination. Although more detailed analyses are needed, ourelectrodes will teach us essential requirements for ultra-high-rate electrolysis. Additionally, the obtained knowledge through the detailed analysesof our electrodes would be widely utilized to develop novel materials, suchas catalysts and electrodes.

rent density for C2+.

Research Center for Solar Energy Chemistry, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Japan. E-mail: nakanishi.shuji.es@osaka-u.ac.jp,
kamiya.kazuhide.es@osaka-u.ac.jpbInnovative Catalysis Science Division, Institute for Open and Transdisciplinary
Research Initiatives (ICS-OTRI), Osaka University, Suita, Osaka 565-0871, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d2ey00035k

that the current density for the production of high value-added
products is correlated to the capital cost of the electrodes
employed.14 Therefore, improvement of the current density would
significantly affect the feasibility of this technology.
The use of gas diﬀusion electrodes (GDEs) that allow theCO2RR to occur at the solid-catalyst/liquid-electrolyte/gaseousCO2 interface, eﬀectively accelerate the CO2RR by solving the
problem of the mass transport limitation due to the inherently
low diﬀusion and solubility of CO2 in water.15,16 A catalyst layer(CL) composed of metal Cu particles and ionomers on carbonbased microporous layers has been widely studied as thestandard cathode for gaseous CO2RR to produce C2+ organics,
such as C2H4, C2H5OH, C3H7OH and CH3COOH.8,17–19
Although the partial current density for C2+ ( jC2+) has still
remained below 500 mA cm 2 in many studies,18,20–27 a few
studies have developed novel cathode assemblies or components toward an ultra-high-rate (UHR-) CO2RR with over jC2+ =1 A cm 2. Sargent et al. successfully achieved a jC2+ over
1.2 A cm 2 in 7 M KOH using an electrode fabricated by© 2023 The Author(s). Published by the Royal Society of Chemistry

EES Catal., 2023, 1, 9–16 | 9

Introduction
The electrochemical carbon dioxide reduction reaction (CO2RR)
has attracted considerable attention as a promising strategy forthe conversion of anthropogenic CO2 into value-added
products.1–10 One advantage of this method is that this greenhouse gas can be reduced under mild conditions (close toambient temperature and pressure).11 However, the operative
efficiency, product selectivity and rate must be improved forpractical implementation.12,13 Among them, it has been reportedaView Article Online

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dropping Cu nanoparticles on a sputtered Cu film/porous
polytetrafluoroethylene (PTFE).28 Additionally, in this work,
the bulk heterojunction of catalysts and ionomers decoupled
the pathways for gases, electrons and ions, which resulted ina jC2+ over 1 A cm 2. Wang and co-authors reported that afluorine-modified copper catalyst deposited on GDEs exhibiteda total current density (Jtotal) of 1600 mA cm 2 with a faradaic
efficiency for C2+ (FEC2+) of over 80% (i.e., jC2+ = over 1.2 A cm 2)
in 0.75 M KOH.29 Very recently, Zheng et al. obtained jC2+ =0.91 A cm 2 using nitrogen-engineered Cu nanoparticles in 1 M
KOH.10In these reports, UHR-CO2RR has been achieved using highalkaline solutions to suppress H2 evolution, which competes
with the CO2RR. However, in addition to the toxicity andcorrosivity, alkaline solutions quickly absorb CO2 and convert
it to electrochemically inert (bi)carbonate.30–33 Based on thesedrawbacks, UHR electrolysis in neutral electrolytes is thus
required from a practical viewpoint. Furthermore, these previous
studies succeeded in achieving UHR-CO2RR by developing novelcomponents in the three-phase interfaces (i.e., catalysts orelectrodes). However, the obtainable maximum jC2+ value is still
unknown when optimizing the standard components; CuO
nanoparticle catalysts (CuONPs), which are known to serve asoxide-derived Cu catalysts for effective C2+ production,34–36 andcarbon-based gas diffusion layers. An electrochemical systemthat can elicit the maximum jC2+ using only standard components will become the primary platform for future studies todesign materials and interfaces for UHR-CO2RR. In other words,
the design guidelines obtained from the UHR-CO2RR with only
ordinary materials would be quite general, and thus, they are
expected to be widely applicable to novel materials and systems.
In the present work, we attempt to pursue a high partial
current density for C2+ ( jC2+) using neutral electrolytes by
optimizing the standard components and properly assemblingthem as the cathode. In particular, we achieved a record current
density of 1.7 A cm 2 for multicarbon products in neutralelectrolytes using synthetic or commercial CuONPs andcarbon-based gas diﬀusion layers. Firstly, we will explain thedetails in characterizations and CO2RR activity of our optimized electrode, and let us discuss the mechanism of UHRCO2RR. Then, the requirements for our UHR electrolysis will be
discussed with varying components and conditions.

EES Catalysis

Fig. 1 (a) Representative TEM image of the synthesized CuONPs. (b) Topview and (c and d) cross-sectional SEM micrographs of CuONPs-1.7/GDE.

In this first section, let us show the detailed characterizations
and electrochemical properties of our optimized electrode for
UHR-CO2RR, which is composed of the GDE (GDL 34BC) carrying our synthesized CuONPs. CuONP catalysts were synthesized
by wet-chemical reduction using NaBH4 as a reductant. Transmission electron microscopy (TEM) observations (Fig. 1a andFig. S1, ESI†) show that the particle morphology and size isspherical and 20 nm, respectively. X-ray diﬀraction (XRD)

patterns (Fig. S2, ESI†) indicate peaks at 35.51 and 38.71, which
are attributed to CuO(002) and CuO(111), respectively. This
oxidation state of the as-prepared sample was also confirmed
by narrow scan X-ray photoelectron spectroscopy (XPS) and X-ray
absorption near edge structure (XANES), as shown in Fig. S3 andS4 (ESI†), respectively. Note that the Cu(II) oxidation state was
reduced to Cu(I)/Cu(0) during electrolysis (Fig. S5 and S6, ESI†).
Morphological characterization of the CuONPs/GDEs (Fig. S7,
ESI†) was conducted using scanning electron microscopy (SEM).
Fig. 1b–d and Fig. S8 (ESI†) show top- and cross-sectional SEM
images of CuONPs-1.7/GDE (where the loading amount of
CuONPs is 1.7 mg cm 2 on GDL 34BC), respectively. CuONPs
completely covered the electrode surface based on the top-view
SEM images, and were deposited on the microporous layer(MPL) composed of carbon nanoparticles. The cross-sectional
SEM images reveal that the thickness of the MPL and the CL
were ca. 80 and 2 mm for CuONPs-1.7/GDE, respectively. Whenthe loading amount of CuONPs was varied to 0.34 mg cm 2 (Fig.
S9, ESI†) and 3.1 mg cm 2 (Fig. S10, ESI†), the CL thicknesseschanged to ca. 0.5 mm and 4 mm, respectively. The roughness
factors of the CL were obtained using a surface profilometer.
When increasing CL thickness, the surface becomes rougher(Fig. S11 and Table S1, ESI†).
We measured the CO2RR activity of the Cu-based GDE
catalysts. An in-house-built three-compartment electrochemicalcell was used for CO2RR evaluation (Fig. S12, ESI†).37,38 Jtotal vs.
potential (U) curves were obtained in 1 M KCl under CO2 andargon conditions, as shown in Fig. S13 (ESI†). The onset
potential of the cathodic current for bare and Cu-modified
GDEs were 1.6 and 1.2 V vs. Ag/AgCl under CO2, respectively.
When the Cu amount was increased from CuONPs-0.34/GDE to
CuONPs-3.1/GDE, the cathodic current under CO2 gradually
increased.
The CO2RR products in outlet gas streams from the gaschamber and electrolytes from the catholyte room were analysedusing gas chromatography (Fig. S14, ESI†) and 1H nuclear magnetic resonance (NMR) spectroscopy (Fig. S15, ESI†) after
constant-current electrolysis, respectively. Fig. 2a, b and Fig. S16(ESI†) show the FE and partial current density of CO2RR products
against the applied potential for CuONPs-1.7/GDE in 1 M KCl,
respectively. Under low current conditions (Jtotal = 200 mA cm 2)

10 | EES Catal., 2023, 1, 9–16

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Results and discussion
UHR-CO2RR over 1.7 A cm 2 for C2+ products obtained withCuONPsView Article Online

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Fig. 2 (a) FEs of the CO2RR products under various applied potentials on CuONPs-1.7/GDE in 1 M KCl. Reaction time: 30 min. (b) Partial current density
for C2+, C1 and H2 on CuONPs-1.7/GDE in 1 M KCl. (c) Comparison of FEC2+ and jC2+ for CuONPs-1.7/GDE with reported Cu-based catalysts in neutral
solution (for the detailed references, see Table S2, ESI†). (d) FEs for the CO2RR products under various applied potentials on CuONPs-1.7/GDE in 1 M
KOH. Reaction time: 30 min. (e) Comparison of FEC2+ in 1 M KOH and in 1 M KCl under various Jtotal over CuONPs-1.7/GDE. (f) Comparison of FEC2+ andjC2+ for CuONPs-1.7/GDE with reported Cu-based catalysts in alkaline solution (for the detailed references, see Table S5, ESI†).

at 1.36 V vs. Ag/AgCl electrode, CO was the main product (FECO
4 50%), and the FEC2H4 was below 20%. The FE for C2+ products
increased and reached 76% at 1.69 V vs. Ag/AgCl electrode(Jtotal = 1600 mA cm 2) when the applied potential was shifted to
the negative side. In particular, the FEs for C2H4 and C2H5OH
were 46% and 22% at 1.76 V vs. Ag/AgCl electrode, respectively.
The partial current density for C2+ products (C2H4, C2H5OH,
C3H7OH, and CH3COOH) was over jC2+ = 1.7 A cm 2 at 1.76 V
vs. Ag/AgCl electrode. On the contrary, the FEs for C1 products(CO, CH4 and HCOOH) and H2 were 8.8 and 14% at 1.76 V vs.
Ag/AgCl electrode, respectively. Fig. 2c and Table S2 (ESI†) show
a comparison of jC2+ and FEC2+ in neutral electrolytes among thereported values. The partial current density for C2+ production( jC2+ = over 1.7 A cm 2) is a record value, and the FEC2+ is also oneof the highest. Therefore, the present study succeeded in eliciting the potential performance of an ordinary combination for
UHR-CO2RR (i.e., CuO-derived Cu nanoparticles and carbonbased GDEs). Significantly, such a high jC2+ value for our assembly was remarkably reproducible even using diﬀerent batches ofelectrodes, as shown in Fig. S17 and Table S3 (ESI†). When theapplied potential was changed to 1.79 V vs. Ag/AgCl electrode(Jtotal = 2800 mA cm 2), the FE for C2+ products suddenly
decreased, and the FE for H2 increased. The total FEs at
1.79 V vs. Ag/AgCl electrode did not reach 100% (ca. 80%)
because the bubbles contained CO2 substrate, CO2RR products,
and H2 formed at the cathode surface and diﬀused into thecatholyte chamber. The applied potential was compensatedusing the current interrupt method as written in the experimental section39–41 (see Fig. S18 for the values versus reversiblehydrogen electrode, ESI†). However, as accurate potential corrections are diﬃcult under ultra-high-current conditions, we mustnote that the applied potentials are still referenced values.
Even when using neutral solutions, the local pH at thesurface of the catalyst increases significantly during UHRCO2RR due to the consumption of protons with an increasein the CO2RR current density. Our rough calculation andprevious reports showed that the local pH becomes almost
the same in neutral and alkaline electrolytes during highcurrent electrolysis (Fig. S19 and Table S4, ESI†). Therefore,
the cathode reaction proceeded under alkaline environmentsfor UHR-CO2RR even with neutral solutions; thus, the fabricated cathode was expected to be suitable for UHR-CO2RR evenin bulk alkaline solutions. Fig. 2d shows the FEs of CO2RR
products against the applied potential for CuONPs-1.7/GDE in
1 M KOH alkaline electrolyte, and Fig. 2e shows a comparisonof the FEs of C2+ products for CuONPs-1.7/GDE in 1 M KCl and
1 M KOH solutions. The FEs for CO2RR activity in alkaline andneutral solutions were almost the same in the range of Jtotal =200–2400 mA cm 2, which indicates that the local pH aroundthe CL in 1 M KOH and 1 M KCl solutions was not dependent
on the bulk pH during electrolysis at Jtotal = 200 mA cm 2. The
partial current density for C2+ products in alkaline solutions
reached jC2+ = 1.8 A cm 2, which is a record value in alkaline
solution, as compared and listed in Fig. 2f and Table S5 (ESI†)© 2023 The Author(s). Published by the Royal Society of Chemistry

EES Catal., 2023, 1, 9–16 | 11

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respectively, i.e., this electrode can exhibit a record jC2+ in bothneutral and alkaline solutions. The mass activity, turnover
frequency (TOF) and energy efficiency values are also summarized in Fig. S20 and Table S6 (ESI†) as reference data.
The volcano-type relation between FEC2+and current density
was observed in the present UHR-CO2RR system. Especially, the
FEC2+ increased with increasing current density below Jtotal =2000 mA cm 2, and thus, a trade-on relationship between rateand selectivity was exhibited in this current region, as shown in
Fig. 2a. Let us discuss the mechanism for the increase of FEC2+
with the current density. It should be noted here that it isdiﬃcult to strictly distinguish whether the selectivity toward
the product depends on the current density (i.e., reaction rate)or on the applied potential because we cannot control eachparameter independently. We here explain the product selectivity from the viewpoint of current density: the eﬀect of (1) CO
partial pressure, (2) local pH,42 and (3) in situ formation of thethree-phase interface. The CO partial pressure in the CL
became high with an increase in the current density, whichfacilitated CO dimerization. Jun Li et al. controlled local COavailability at the catalyst–electrolyte interface and demonstrated that a high concentration of CO accelerated CO dimerization, and caused a higher current density with C2+
production.43 Jing Li and co-workers reported that a high CO
partial pressure suppresses H* adsorption, which leads tolowering of the hydrogen evolution reaction (HER).44 It should
also be noted here that the FE for C2H5OH against C2H4
increased with increasing Jtotal (Fig. S21, ESI†). It has beenreported that high CO pressure on a Cu surface favours
C2H5OH production over C2H4 because the *CHCHOH intermediate for C2H5OH becomes more stable than *CCH for C2H4
under high CO coverage.43,45 This is because CCH* and*CHCHOH are adsorbed on Cu sites with side-on and end-on
configurations, respectively, so that CCH* requires a larger
occupied molecular area than *CHCHOH. This change inselectivity between C2H4 and C2H5OH supports that the CO
local pressure depended on the current density. The increase in
the local pH at the surface of the CL due to proton consumption
by the CO2RR suppressed HER (vide supra). The pH at theelectrode surfaces is known to reach up to 11.9 under electrolysis at just Jtotal = 50 mA cm 2 in neutral electrolytes.15,46
Considering that the almost same product selectivity was
obtained in neutral (1 M KCl) and alkaline (1 M KOH) solutions
between Jtotal = 200–2400 mA cm 2, the local pH in KCl solution
became similar to that in KOH solution under electrolysis with
Jtotal of just Jtotal = 200 mA cm 2. Further increase in the current
density (Jtotal 4 1000 mA cm 2) would lead to a pH of over 15 at
the surface of the catalyst.7 Moreover, we propose that thein situ formation of the gas-transport pathway depending on
the current density occurs during electrolysis. Evolved gases,
such as H2, would migrate through gas-transport channels in
the CL; therefore, the area of the CO2RR active interface (threephase interfaces) could be expanded with an increase in thecurrent density (see Fig. S22, ESI†).
It is important to identify the origin of the decrease in the
CO2RR activity by application of Jtotal = 2800 mA cm 2 for

12 | EES Catal., 2023, 1, 9–16

EES Catalysis

Fig. 3 (a, c and e) Cross-sectional SEM images of CuONPs-1.7/GDE and(b, d and f) K+ distribution (purple) obtained by EDX after application of
Jtotal = (a and b) 1000 mA cm 2, (c and d) 2000 mA cm 2, and (e and f)
2800 mA cm 2 for 30 min under CO2RR conditions in 1 M KCl.

CuONPs-1.7/GDE (Fig. 2a and d) toward a further increase in
the production rate. It was assumed that the three-phase interface was broken over Jtotal = 2800 mA cm 2, which resulted in asevere decrease in the FE for C2+ products. Fig. 3 shows crosssectional SEM images after application of Jtotal = 1000, 2000,
and 2800 mA cm 2 (30 min) in 1 M KCl, respectively. The
particle size of the CuONPs became about 10% smaller after
electrolysis due to the reduction of CuONPs to Cu(I)/Cu(0)NPs(Fig. S23, ESI†). Cross-sectional elemental energy dispersive
X-ray spectroscopy (EDX) mapping show K+ derived from electrolytes entered the MPL layer. One-tenth and one-fourth of the MPL
layers were flooded after application of Jtotal = 1000 mA cm 2 and2000 mA cm 2, respectively. In contrast, after the degradation withapplication of Jtotal = 2800 mA cm 2 (30 min), the K+ distribution
was shown to reach half of the MPL layer (Fig. 3e). In addition,a residue of K+ crystals (KHCO3 and K2CO3, KCl and KOH)
was observed throughout the MPL after application of Jtotal =2800 mA cm 2 (Fig. S24, ESI†). Electrowetting eﬀects, which
decrease the solid–liquid interfacial tension and facilitate electrolyte penetration into the CL and MPL, are known to be emphasizedunder application of highly negative potentials.47–49 Although the
evolved gases may push out and expand the gas-transport pathways(as the in situ formation of the three-phase interface), flooding of
the MPL due to electrowetting may exceed the rate of pushing out
the electrolyte. The flooded CL (and MPL) produced only H2 due toa lack of CO2 transportation, and thus, H2 production should
become dominant at Jtotal = 2800 mA cm 2. This assumption is
also supported by the diffusion of evolved gases into the catholytechamber at Jtotal = 2800 mA cm 2 (vide supra).
UHR-CO2RR with varying components and conditions
Here, let us attempt to reveal which factors are sensitive for our
UHR-CO2RR by varying several components and conditions.
Fig. S25 and Table S7 (ESI†) show the FE of CO2RR products
under Jtotal = 2000 mA cm 2 when changing catalysts, thethickness of CL, electrolytes, and GDEs. One can notice thatseveral kinds of commercial GDEs are available for UHRCO2RR. Additionally, the high jC2+ of over 1.3 A cm 2 was
obtained using 1 M KHCO3, which has been widely used as aneutral electrolyte for CO2RR. The slight decrease of jC2+ in 1 M
KHCO3 is likely due to an acceleration of salt precipitation in
MPL in the bicarbonate-rich electrolytes.
Surprisingly, UHR-CO2RR with jC2+ of over 1.4 A cm 2 wasrealized even when using commercial CuONPs with the size of© 2023 The Author(s). Published by the Royal Society of ChemistryView Article Online

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50 nm. These results indicated that the GDE and catalyst usedin this study are not unique, and the optimized assembly elicits
their undermined potential. Further studies are currently inprogress to determine the detailed requirements for catalysts in
our laboratory.
Among these parameters, Fig. S25 (ESI†) reveals that the
thickness of CL is one high-sensitivity factor in determining
maximum jC2+. The optimal thickness of CL was required for
UHR-CO2RR, not too thin and not too thick. Thus, we investigated the detail of the dependence of CL thickness. The CO2RR
was investigated for CuONPs-3.1/GDE and CuONPs-0.34/GDE(detailed product selectivities are shown in Fig. S26 and S27,
ESI†) and the standard CuONPs-1.7/GDE (see the FEs in Fig. 2a)in 1 M KCl. The maximum jC2+ for CuONPs-3.1/GDE wasachieved at Jtotal = 2400 mA cm 2, which was the same as thatfor CuONPs-1.7/GDE. The FEs for C2+ over CuONPs-3.1/GDE
were 7% lower than those over CuONPs-1.7/GDE, although
there was a similar tendency of the potential dependence(Fig. 4a). The FEH2 over CuONPs-3.1/GDE was higher than thatover CuONPs-1.7/GDE for all current density regions (Fig. S28,
ESI†). These results indicated that a lack of Cu active sites did
not cause a decrease in FEC2+ over Jtotal = 2400 mA cm 2 for
CuONPs-1.7/GDE. It was presumed that the Cu sites immersedin the electrolyte cannot serve as CO2RR sites but only as HER
sites, and thus, selectivity toward H2 production increased withthe amount of Cu (Fig. 4d). Next, the Cu amount on the GDE wasdecreased to 0.34 mg cm 2. Although the FEC2+ over CuONPs0.34/GDE was greater than 80% at Jtotal = 1000 mA cm 2, it
decreased to below 60% at Jtotal = 1600 mA cm 2. In contrast,
when Jtotal was varied from 1000 mA cm 2 to 1600 mA cm 2, theFEs for CH4 and H2 increased from 1.6% to 14%, and from 5.4%

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to 17%, respectively. This is because the lack of catalytic Cu sites
would result in relative enhancement of the HER on carbon
particles that compose the MPL. In other words, the three-phaseinterface formed in CuONPs-0.34/GDE was thinner than that in
CuONPs-1.7/GDE, resulting that the current density with thehighest FEC2+ was smaller than the optimal for CuONPs-1.7/GDE(Fig. 4b and c). A significant increase in FE for CH4 with anincrease in current density was observed on CuONPS-0.34/GDE.
This tendency for CH4 was basically consistent with the reported
potential dependence of the selectivity of the CO2RR on Cubased catalysts.50–52 The same trends in the catalytic activity
under various amounts of catalyst loadings were observed in 1 M
KOH electrolyte (Fig. S29–S31, ESI†).
Fig. S32 (ESI†) shows the FEs depending on CL thickness at
500, 1000, 1600 and 2000 mA cm 2, to further demonstrate theimportance of CL thickness for maximum jC2+. The thin CL(CuONPs-0.34) showed the best FE for C2+, and FEC2+ monotonically decreased with increasing the thickness of CL at Jtotal =500 and 1000 mA cm 2. In contrast to FEC2+, the total FE for
CO2RR was almost constant (85–90%) at these low current
density regions, even when varying CL thickness. This is likely
because CO partial pressure was not enough to form C–C bonds
for thicker CL at the low current density regions. When increasing
current density to Jtotal = 1600 mA cm 2, the FEC2+ for CuONPs-1.7
became the best in these three electrodes. Thus, our results clearly
indicated that an optimal CL thickness for C2+ evolution depends
on Jtotal.
Although our study is not the first paper to optimize anddiscuss each parameter, including the size of catalysts, porosity, and thickness of CL, we think that these interdependent
parameters were optimized simultaneously, likely resulting in
the highest jC2+ in this work. However, we must note that somerequirements, especially for catalysts, toward UHR-CO2RR are
still veiled. This work is focused on demonstrating the potentialof ordinary catalysts; providing more detailed requirements willbe left for a future paper.
Long-term stability dependence on current density

Fig. 4 (a) FEC2+ against Jtotal for various amounts of Cu loading (CuONPs0.34, -1.7 and -3.1/GDE) in 1 M KCl. (b–d) Schematic image of the threephase interfaces at each position which are indicated in (a). The red andgreen arrows represent the thickness of three-phase interface and flooding catalyst layer, respectively.

Long-term stability might be a challenging issue in the practicalimplementation of UHR-CO2RR. For example, in ref. 29 theauthors reported that the FEC2+ started to decrease within one
hour at Jtotal = 1200 mA cm 2 ( jC2+ = 1 A cm 2) in 1 M KOH. In
this time scale, the decrease in the CO2RR activity on GDE is
not due to the chemical degradation of components, but due tothe flooding of the CL and MPL. However, there is no systematic knowledge about the long-term stability of UHR-CO2RR;
especially, a durability test of UHR-CO2RR in neutral solutions
has not been conducted at all. Therefore, we analysed thechange in the CO2RR activity for long-term electrolysis underultra-high-rate electrolysis conditions in neutral solutions.
Fig. 5b and Fig. S33 (ESI†) show the FEs for CO2RR products
at Jtotal = 1600 mA cm 2 in 1 M KCl as a function of theelectrolysis time. The change in the FEs for CO2RR products
under electrolysis at Jtotal = 1600 mA cm 2 was almost negligible
for over 2 h. Although the FE for C2+ and H2 then began todecrease and increase from 2 h, respectively, the FEC2+© 2023 The Author(s). Published by the Royal Society of Chemistry

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Fig. 5 FEC2+ stability tests with respect to the current density over
CuONPs-1.0/GDE in 1 M KCl under (a) Jtotal = 1000 mA cm 2,(b) 1600 mA cm 2 and (c) 2000 mA cm 2.

production remained over 75% for 2.5 h and 60% for over 6 h.
The electrode was analysed after the decrease of FEs for C2+ toclarify the origin of activity decrease. SEM observation (Fig. S34,
ESI†) after electrolysis for 6 h at 1600 mA cm 2 showed that themorphology of CLs did not significantly vary, and most of the
MPL was flooded in a similar manner to that described inprevious reports.53,54 The recycling test was also conducted. Fig.
S35 (ESI†) shows that the FEC2+ value was recovered by washing
the electrode with pure water after the decrease. This result also
indicates that the decrease in FEC2+ was due not to the degradation of catalysts but to the salt precipitation induced by flooded
MPL. The stability dependence on the applied current density
was investigated next (Fig. 5a–c and Fig. S33, ESI†). The FEC2+
of 70–80% almost unvaried for over 3 h and 1.5 h at Jtotal =1000 mA cm 2 and Jtotal = 2000 mA cm 2, respectively. These
results indicated a trade-off relationship between stability andcurrent density; however, the decrease in FEC2+ within a shorttimescale was due to flooding of the CL and MPLs. Therefore,
we can expect that the stability could be improved by theaddition of hydrophobic materials and modulation of theporosity in the CL and MPL. The present electrochemicalsystem and conditions will become one of the standard platforms toward these future studies.

Conclusions

EES Catalysis
analyses are needed, our electrodes will teach us essential requirements for UHR electrolysis. Furthermore, the obtained knowledge
through the detailed analyses of our electrodes would be widely
utilized to develop novel materials, such as catalysts and electrodes because our electrodes are composed of only standard
components. Systematic stability tests were also conducted atover jC2+ = 1 A cm 2. Although the stability under ultra-highcurrent conditions was still far from a practical level, SEM
observations revealed that flooding of the CL and MPL is aproblem that occurs within the shortest time scale. The electrochemical conditions and systems in this study can serve as aplatform for improvement of the stability of the UHR-CO2RR.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This research was based on the Integrated Electrochemical
Systems for Scalable CO2 Conversion to Chemical Feedstocks
project performed as part of the Moonshot Research and Development Program funded by the New Energy and Industrial
Technology Development Organization (grant no. 20001627-0).
This work was also supported by a JSPS KAKENHI Program(20H02568) and CREST (grant no. JPMJCR18R3) of the Japan
Science and Technology Agency (JST). TEM measurement was
carried out by using a facility in the Research Center for UltraHigh Voltage Electron Microscopy, Osaka University, and TEM
observation was supported by Dr Takao Sakata. Synchrotron
radiation experiments were performed using the BL01B1 beamline of SPring-8 with approval of the Japan Synchrotron Radiation Research Institute (Proposals 2020A1426, 2021A1294,
2021B1204, and 2022A1165). Mr Ryo Kurihara supported the
local pH calculation.

In this work, our electrode exhibited partial current densities
for CO2 reduction to C2+ of 1.7 A cm 2 in neutral solutions by
optimizing the standard components and their assembly.
When the total current density was increased, the FEs for C2+
increased monotonically below Jtotal = 2000 mA cm 2. Therefore,
the trade-on relationship between the reaction rate and selectivity
was shown in the present UHR-CO2RR under those Jtotal regions.
Although the cathode of this electrochemical system wascomposed of typical CuO-derived Cu nanoparticles, a carbonbased GDE, we successfully maximized the potential of thecatalytic activity of the Cu surface by constructing a CL with
optimal porosity and thickness. Especially, we revealed that the
thickness of CL was directly correlated to jC2+. The optimal
thickness of CL was required for UHR-CO2RR. It is noteworthy
that this electrode allowed us to achieve UHR-CO2RR even inneutral solutions, which is more practical than alkaline solutions.
The present system is expected to become one of the standards
when pursuing high-current density for the CO2RR by the designof novel components or assemblies. Although more detailed

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