Hierarchical Copper with Inherent Hydrophobicity Mitigates Electrode Flooding for High-Rate CO2 Electroreduction to Multicarbon Products

Zhuang-Zhuang Niu, Fei-Yue Gao, Xiao-Long Zhang, Peng-Peng Yang, Ren Liu, Li-Ping Chi,

Article

Hierarchical Copper with Inherent Hydrophobicity Mitigates

Electrode Flooding for High-Rate CO Electroreduction to

Multicarbon Products

Zhi-Zheng Wu, Shuai Qin, Xingxing Yu, and Min-Rui Gao *

Cite This: https://doi.org/10.1021/jacs.1c01190

Read Online

ACCESS

Metrics & More

Article Recommendations

sı Supporting Information

ABSTRACT: Copper is currently the material with the most

promise as catalyst to drive carbon dioxide (CO ) electroreduction

to produce value-added multicarbon (C ) compounds. However,

a copper catalyst on a carbon-based gas diﬀusion layer electrode

often has poor stabilityespecially when performing at high

current densitiesowing to electrolyte ﬂooding caused by the

hydrophobicity decrease of the gas diﬀusion layer during operation.

Here, we report a bioinspired copper catalyst on a gas diﬀusion

layer that mimics the unique hierarchical structuring of Setaria’s

hydrophobic leaves. This hierarchical copper structure endows the

CO reduction electrode with suﬃcient hydrophobicity to build a

robust gas−liquid−solid triple-phase boundary, which can not only

trap more CO close to the active copper surface but also eﬀectively resist electrolyte ﬂooding even under high-rate operation. We

−

consequently achieved a high C₂₊production rate of 255 ± 5.7 mA cm with a 64 ± 1.4% faradaic eﬃciency, as well as outstanding

operational stability at 300 mA cm⁻over 45 h in a ﬂow reactor, largely outperforming its wettable copper counterparts.

INTRODUCTION

boundary where the CO RR occurs. Moreover, highly alkaline

electrolytes can be adopted to improve charge transfer kinetics

and suppress the competing HER, which give rise to marked

■

Electrochemical upgrading of greenhouse gases such as carbon

dioxide (CO ) into valuable carbon-based fuels and feedstocks,

improvements in FE, selectivity, and reaction rate toward C

ideally driven by electricity from renewable sources, oﬀers an

attractive pathway to reduce waste gas emissions, leading to a

closed loop for carbon footprint.

,16

products.

Although important advances have been made,

,8,17−19

−3

recent studies

have demonstrated that the stability of

Currently, copper (Cu) is

catalysts on the hydrophobic gas-diﬀusion layers (GDLs) was

poor in ﬂow reactors. One major reason for the instability is

due to the electrolyte ﬂooding caused by the reduced

perhaps the most paradigmatic catalyst capable of coupling C−

C bonds, yielding deeply reduced products such as ethylene,

ethanol, acetate, and n-propanol. The CO₂reduction

20,21

reaction (CO RR) on Cu-based catalysts has reached much

hydrophobicity of a GDL during the CO RR process.

increased faradaic eﬃciencies (FEs) and selectivity toward

GDL ﬂooding occurs, CO diﬀusion toward the catalyst layer is

energy-dense C products via strategies such as morphological

blocked and high CO RR rates can no longer be obtained.

10,11

control,

oxidation states.

in water (∼35 mM at 298 K and 1 atm pressure) and the

competing hydrogen evolution reaction (HER) considerably

^lⁱ^mⁱ^t^t^h^e^C²+ partial current densities from being practical. For

example, in a conventional H-shaped electrolyzer, the typical

introduction of dopants,

and regulation of

Various strategies have been employed to increase the

hydrophobicity of GDL electrodes, such as binding with

2,13

Nevertheless, the poor solubility of CO

22,23

hydrophobic polymers or waxy alkanethiols.

However,

chemical modiﬁcations often have limitations: they can

damage the intrinsic properties of catalytic electrodes, for

example, the electrical conductivity. Janaky’s group has

Cu catalyst known today can produce C fuels with an FE of

−

0% at a current density of a mere 40 mA cm , which stands

Received: February 9, 2021

far below the industrially relevant requirement (>200 mA

−

cm ).

In recent years, ﬂow-cell CO RR electrolyzers

,8,16

have

been engineered to circumvent the mass transport limitation,

in which the gas reactants diﬀuse directly to the electrode−

electrolyte interfaces, forming a gas−liquid−solid triple-phase

XXXX American Chemical Society

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 1. Nature-inspired hierarchical Cu electrode design. (a) Photograph of a water droplet on the Setaria leaf and the corresponding contact

angle test (inset). (b) SEM image of surface microstructures of the Setaria leaf. Scale bar, 40 μm. (c) Gas-trapping phenomenon of the Setaria leaf

under water. Available air is trapped in spaces in the sharp microstructures to form a cushion at the leaf−water interface that prevents the leaf from

being wetted. (d) Illustration of the microﬂuidic CO ﬂow cell (left) and the stable gas−liquid−solid triple-phase boundary enabled by the

hierarchical electrode design (right).

demonstrated a zero gap electrolyzer cell that mitigates

continuous operation. We attribute these results to the

hydrophobic Cu hierarchical structure, which not only enables

electrolyte ﬂooding during CO electrolysis. However, the

carbonate formation at the cathode becomes a major issue.

Thus far, achieving more commercially relevant current

densities, while preventing the ﬂooding issue encountered at

severe electrolysis, has remained a big challenge.

more CO

which increases the CO

a robust triple-phase interface that mitigates GDL ﬂooding

(Figure 1d).

trapped at the electrode−electrolyte interface,

RR rate and selectivity, but also oﬀers

In Nature, many plants such as Setaria have hydrophobic

leaves that allow water droplets to run oﬀ readily after morning

dew or rain. The leaf of the Setaria looks macroscopically

rough with a contact angle (CA) of 152.6° (Figure 1a and

inset), which is within the regime of superhydrophobicity.

Scanning electron microscopy (SEM; Figure 1b) uncovered

many oriented needles on the Setaria’s leaves, which form

unique micro/nanostructures. Consequently, available air can

be trapped near the sharp microstructures to create an air

cushion at the leaf−water interface (Figure 1c), leading to the

striking hydrophobicity. By contrast, the smooth banana leaf

contains rich wrinkles on the ﬂat surface with a CA of a mere

RESULTS AND DISCUSSION

■

Preparation and Characterizations of the Hierarch-

ical Cu Electrode. Metallic materials commonly possess

hydrophilic surfaces because of the high surface free

26,27

energy.

A popular approach to increase hydrophobicity

of metals is to coat a hydrophobic polymer layer, which,

unfortunately, often leads to reduced electrical conductiv-

24,28

ity.

Very recently, Khan et al. created hierarchical

micro−nano pyramidal textures on silicon, which allows the

CO gas to be eﬃciently captured within the Si textures. Then,

they placed the obtained CO plastron close to the smooth Cu

5° (Figure S1 ), demonstrating its wetting nature.

wire catalyst for CO RR study in an H-cell reactor. In another

Inspired by Setaria’s leaves, we considered that a GDL

study, Wakerley et al. reported a bioinspired hydrophobicity

on Cu dendrites by uniform 1-octadecanethiol coating, which

enables a 74% selectivity for C₂₊production in an H-cell

reactor. However, this electrode allows only for current

electrode with appropriate sharp architectures may prevent it

from being wetted (Figure 1b). We thus engineered the GDL

by growing hierarchically structured Cu dendrites consisting of

sharp needles via a simple electrodeposition method. Using the

resultant electrode, we achieve a C production rate of 255 ±

−2

densities of <30 mA cm to be reached owing to the reduced

conductivity caused by 1-octadecanethiol modiﬁcation. More-

over, its long-term stability at current densities relevant to

commercial operation is questionable. Hence, their results,

although fundamentally interesting, did not provide a clear

−

.7 mA cm with an FE up to 64 ± 1.4%, far exceeding the

performance of its wettable counterparts. The electrode also

shows notable stability at 300 mA cm⁻after 45 h of

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 2. Physical characterization of Cu dendrites and particles. (a) Schematic diagram of a liquid droplet on a needle-like structure. P denotes

the Laplace pressure. φ and α denote the apex half-angle and tilt angle of the needle, respectively. (b) Relative P at various φ and α. (c) XRD

patterns of Cu dendrites (Cu-D) and Cu particles (Cu-P). (d, e) Optical microscope photograph (d) and SEM image (e) of hierarchical Cu

dendrites. Inset in (d) shows a water contact angle photograph of a Cu dendrite electrode. Scale bars, 20 μm (d) and 4 μm (e). (f) SEM image of a

typical sharp needle on Cu dendrites. Insets show HRTEM image and the corresponding FFT pattern on the edge of the needle. Scale bars, 400

and 1 nm (inset). (g, h) Optical microscope photograph (g) and SEM image (h) of Cu particles. Inset in (g) shows a water contact angle

photograph of a Cu particle electrode. Scale bars, 20 μm (g) and 1 μm (h). (i) HRTEM image and the corresponding FFT pattern (inset) of Cu

particles. Scale bar, 2 nm.

prospect on the potential of alkanethiol-modiﬁed Cu dendrites

maximum P value can be achieved at φ = 7° and α = 0°

(Figure 2b and Figure S3 ), revealing the marked hydro-

phobicity of the sharp structure.

for CO electrolysis at high current densities.

We thus turned to synthesize a Cu catalyst with a high

surface roughness via forming hierarchically sharp structures to

On the basis of the above considerations, we employed

electrodeposition as a scalable and simple method to prepare a

desired hierarchical Cu catalyst composed of sharp needles on

a GDL (see Experimental Section; Figure S4 ). For comparison,

we also synthesized Cu particles by a hydrothermal route and

deposited them on a GDL (see Experimental Section). The X-

ray diﬀraction (XRD) studies conﬁrm the obtained structures

to be metallic Cu (Figure 2c), whereas a slight surface

oxidation was detected by X-ray photoelectron spectroscopy

increase hydrophobicity. Inspired by the needle-shaped

microstructure on Setaria’s leaves, we prepared sharp Cu

needles that assemble into hierarchical architectures. Generally,

a hydrophobic surface has a positive Laplace pressure (P )

30,31

from gas existing in a nano/microscale texture,

determined by the following equation:

which is

P = γ πΩf sin φ cos α sinjθ − − φz

{

(1)

(

XPS; Figure S5 ) and Cu L-edge X-ray absorption spectros-

copy (XAS; Figure S6 ) measurements. After electrodeposition,

the electrode was inspected under an optical microscope,

which shows uniform coverage of Cu dendrites on the GDL

(Figure 2d). The contact angle test gives a high CA value of

154° (inset in Figure 2d), revealing the hydrophobicity of the

electrode where gas is expected to be trapped on the micro/

nanoscale. To verify that the observed hydrophobicity arises

where γ is the surface tension of the liquid, Ω is the density of

the needle, f is the adhesion fraction of the liquid−solid

interface, φ is the apex half-angle, θ is the static contact angle,

and α is the tilt angle of the needle (Figure 2a and Figure S2).

When the apex half-angle is 0° < φ < 20° and the tilt angle is

−

90 ° < α < +90°, P always has a positive value (Figure 2b and

Figure S3 ). The analysis of this formula suggests that the

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

−

Figure 3. Aerophilic behaviors of Cu electrodes. (a) Photograph of Cu-D in a CO -saturated KHCO solution. The CO gas ﬂow of 24 mL min

was constantly incident to the Cu electrode. (b) Photograph of the homemade platform that permits the operando observations. (c) Real-time

−

hydrophobicity monitoring of an electrolyte droplet on the Cu-D electrode at a current density of ∼10 mA cm . (d, e) CO bubble adhesion

experiments on Cu-D (d) and Cu-P (e) electrodes under a CO -saturated KHCO solution. An individual CO bubble (∼2.0 μL) was injected

from the syringe needle under solution to the Cu electrode surface.

from the hierarchical architectures, we studied the structuring

of the Cu catalyst by SEM. Figure 2e shows a representative

SEM image of the as-prepared Cu catalyst electrodeposited at

formation of a gas−liquid−solid triple-phase boundary that

permits improved CO mass transfer. However, the electrode

composed of Cu particles displays underwater hydrophilicity

−

0.7 V versus the Ag/AgCl reference electrode for 700 s. The

(Figure S13 ), which could cause poor CO transfer as well as

electrolyte ﬂooding.

We designed a homemade platform that enables the real-

time monitoring of hydrophobicity on the electrode surface

under electrochemical conditions (Figure S14 ). The hydro-

SEM image exhibits highly hierarchical dendrites constructed

by numerous sharp Cu needles (Figure S7). Close-up analysis

of these needles reveals an average φ value of 11° (Figure 2f

and Figure S8 ), which indicates a large P and thus outstanding

hydrophobicity. Control experiments exhibit that the hydro-

phobicity increases gradually as the electrodeposition time

increases, owing to the more hierarchically sharp structures of

the resultant electrodes (Figure S9). A high-resolution

transmission electron microscopy (HRTEM) image taken at

the needle tip uncovers a well-resolved lattice spacing of 0.21

nm, corresponding to the (111) planes of cubic Cu (inset in

Figure 2f). By contrast, uniform Cu particles deposited on the

GDL also expose (111) planes, which possess a wettable

surface with a CA value of 79° (Figure 2g−i and Figure S10 ).

Our electrochemical roughness factor measurements yield

values of 20.6 and 13.4 for Cu dendrites and particles,

respectively (Figure S11 ). Thus, we have achieved remarkable

hydrophobicity in the naked Cu catalyst on a GDL without

surface modiﬁcations via suitable control of the structural

hierarchy.

phobic surface permits a well-deﬁned aqueous KHCO droplet

on the top of the hierarchical Cu electrode (Figure 3b).

Applying a potential drives the oxygen evolution from the

droplet (Figure 3c). We found that the hydrophobicity does

not change appreciably after 10 min of electrochemical

operation. The subtle decrease in droplet size is likely a result

of oxygen formation that consumes water in the droplet

(Figure 3c). Because superhydrophobic surfaces in air are often

2−34

superaerophilic under water,

we surmise the high

aerophilicity of our designed hierarchical electrode. The CO₂

adhesion experiments show that the CO₂bubble can

immediately diﬀuse into the hierarchical Cu electrode once

reaching the electrode surface (Figure 3d), conﬁrming the

2,34

superaerophilic feature.

This result means that CO₂

molecules can quickly transfer and accumulate at the active

Cu surfaces in the electrolyte. By contrast, the CO bubble

32,35

We now study the surface behavior of the achieved

exhibits a “pinning state”

on the Cu particles, which is

electrodes in CO -saturated electrolytes. Considering that a

retained on the electrode even up to 30 s (Figure 3e),

single CO bubble reacts rapidly in 1 M KOH (Figure S12),

indicating that CO transfer across this surface is limited.

we used 1 M KHCO as a replacement for investigations,

which does not aﬀect the surface chemistry of Cu electrodes.

CO Electroreduction in a Flow Cell. To experimentally

probe the impact of the hierarchical Cu structure on CO RR

The hierarchical Cu electrode visibly traps CO molecules at

performance, we assessed the CO reduction activity of Cu

the electrolyte−electrode interface, leading to a shiny surface

dendrites and particles in a ﬂow-cell reactor with 1 M KOH

(

Figure 3a), analogous to the air trapping observed on a

(see Experimental Section). We fed the cathode with CO gas

−

Setaria’s leaf (Figure 1c). Such a layer of gas thus enables the

at a ﬂow rate of 24 mL min , and the outlet gas ﬂow rate for

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 4. CO RR performance in a ﬂow cell. (a) Linear sweep voltammetry curves on Cu-D and Cu-P electrodes. (b−d) C and C faradaic

eﬃciencies (b), partial current density (c), and Tafel lines (d) on Cu-D and Cu-P electrodes versus applied potentials. (e) Half-cell (cathodic)

power conversion eﬃciency of C₂₊products on Cu-D and Cu-P electrodes. (f) Comparison of C₂₊current densities and C₂₊FEs (see Table S4 for

details) for various Cu dendrite electrodes reported in the literature. (g) Stability test of the Cu-D and Cu-P electrodes at a total current density of

−

00 mA cm . The electrolyte was occasionally replaced by new 1 M KOH solution (red arrows) to recover the conductivity of the electrolyte and

to avoid the possible salt accumulation. All measurements were performed in CO -ﬂowed 1 M KOH electrolyte, and the reported potentials were

converted to the RHE reference scale with iR correction. Error bars are based on the standard deviation of three independent measurements.

each measurement was carefully tracked for gas product

analysis (Figure S15). The linear sweep voltammetry curves in

Figure 4a reveal that Cu dendrites give a more prominent

suggesting the promoted C−C coupling for the formation of

products. At −0.68 V versus RHE, however, the Cu

particles give a small C FE of only 25.3% (Figure 4b, Figure

reduction peak versus Cu particles, hinting at a faster CO RR

S20b , and Table S2 ), which reasonably agrees with results on

Cu particulate catalysts reported previously (Table S3 ). We

spotlight that the C :C selectivity on Cu dendrites over-

kinetics on Cu dendrites. Considering that charge transfer

resistance (R ) correlates with the electrons transferred from

the catalyst surface to the reactants, we conducted electro-

chemical impedance spectroscopy. At the applied potential of

whelmingly outperforms Cu particles at all potentials examined

(Figure S21 ). Remarkably, we obtained a maximum C :C

selectivity of 15.4 on Cu dendrites at −0.68 V versus RHE,

which is 19.3-fold higher than on Cu particles.

−

0.6 V versus RHE (reversible hydrogen electrode), our

measurements yield small R values of merely 5.5 and 5.8

ohms for Cu dendrites and particles (Figure S16), respectively.

Our measured geometric current densities for C₂₊and C₁

production on the two studied electrodes at various applied

potentials are plotted in Figure 4c. Intriguingly, the Cu

dendrites exhibits a maximum C partial current density of

CO RR electrolysis (Figures S17 −S19) displays that the

hierarchical Cu dendrites can generate C₂products at a range

of applied potentials between −0.53 and −0.68 V versus RHE

−

Figure S20a ). Figure 4b shows the evolution of C₂products

255 mA cm at −0.68 V versus RHE. This result indicates

that the hierarchical Cu electrode holds promise to operate at

industrial electrolyzer-relevant current densities. By contrast,

the Cu particles only oﬀer a C partial current density of 111

with a 30.1% FE at −0.53 V versus RHE on Cu dendrites. This

FE rapidly increases to 64% at a more negative potential of

−

0.68 V versus RHE, corresponding to 30% ethylene, 8.5%

acetate, 23.7% ethanol, and 1.6% n-propanol (Figure 4b, Figure

−

mA cm , even applying a more negative potential of −0.75 V

versus RHE (Figure 4c). Further, the electrochemically active

surface area (ECSA)-normalized current densities of Cu

dendrites largely exceed that of Cu particles at various applied

S20a, and Table S1). The FE of C products drops from 25.2%

to 4.2% when a negative potential from −0.53 to −0.68 V

versus RHE was applied (Figure 4b and Figure S20a),

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Figure 5. Mechanism of the enhanced C−C coupling. (a, b) Simulated surface K concentration near the Cu-D (a) and Cu-P (b) electrodes. Scale

bar, 300 nm. (c−e) Operando Raman spectra on Cu-D (c, e) and Cu-P (d) electrodes as a function of the applied cathodic current density. Insets in

(

c) and (d) depict diﬀerent CO* adsorption behavior on the Cu-D and Cu-P electrodes, respectively. Gray, carbon; red, oxygen.

potentials (Figure S22 ), indicating its superior intrinsic

The operating stability of the CO RR catalysts remains a

CO RR activity.

challenge to their application, partially owing to the ﬂooding

Tafel studies yield a similar slope for ethylene (119 mV

issue described above. To assess the long-term stability of the

−

−1

dec ) and ethanol (111 mV dec ) on the hierarchical Cu

electrode (Figure 4d), suggesting that the two products share

the same rate-determining step. The measured slopes for

ethylene and ethanol production agree reasonably with

hierarchical Cu electrode, we ﬁrst run continuous CO

electrolysis under a constant current density of 300 mA

−

cm for 10 h. The ratio of carbon-based products/(bi)-

carbonate during this process was consistently maintained at

−1

∼

20% (Figure S26). The electrode was found to be very stable

previous reports, which are close to 118 mV dec and

−

under the harsh conditions, with only a slight increase in

^o^v^e^r^p^o^t^eⁿ^tⁱ^a^l^aⁿ^d^s^m^a^l^l^l ^o ^s ^s ⁱ ⁿ ^C2+ selectivity in this time

interval (Figure 4g and Table S5). Our measurements show

that the initial pH of 13.64 in 1 M KOH electrolyte slightly

reduces to 12.84 after 10 h of electrolysis, resulting in a ∼1

ohm increase in solution resistance (Figure S27). Through

replacing the electrolyte occasionally, the overpotentials and

FE could be maintained over a 45-h-long electrolysis at 300

suggest a rate-determining 1e transfer involved in C−C bond

formation between adsorbed *CO intermediates

,36,37

(Figure

S23 ). By contrast, much higher Tafel slopes of 341 and 324

−

mV dec for ethylene and ethanol were obtained on a Cu

particulate electrode (Figure 4d), respectively, which imply

−

that the processes were not determined by 1e transfer.

Motivated by the high C FE (64%) and production rate

−

(

255 mA cm ) achieved on the hierarchical Cu electrode, we

−

mA cm (Figure 4g). By stark contrast, the Cu particle

electrode requires progressively increased overpotentials at this

turn to obtain the half-cell (cathodic) power conversion

eﬃciency (PCE; see Experimental Section). The results

Figure 4e) show that the Cu dendrites can deliver a cathodic

PCE for conversion to C products of 41.2% at a total current

operating current and H dominates the products rapidly

(

Figure 4g and Table S6 ). At a smaller current density of 120

−2

mA cm , the electrode can retain a C H FE of ∼40% over

−

density of 250 mA cm , far exceeding that of 16.5% observed

40 h of CO electrolysis (Figure S28 ). We attribute the

on Cu particles.

remarkable performance stability to the robust hydrophobicity

of the hierarchical Cu electrode that prevents ﬂooding. Indeed,

the GDL electrode after a long-term stability test was almost

not ﬂooded, in which no obvious carbonate crystallization can

be observed (Figure S29 ). Post-mortem characterizations

uncover that the dendritic morphology and metallic Cu

We also compared our CO RR performance with those

previously reported in terms of C₂₊FE and C₂₊partial current

density. The results obtained on our hierarchical Cu structure

represent the best performance reported to date as compared

12,24,39−43

to other Cu dendrites documented in the literature

for CO RR catalysis (Figure 4f and Table S4). Aside from the

phase (few Cu O originate from surface oxidation) are well

alkaline electrolyte, our hierarchical Cu electrode also yields

maintained (Figures S30 −S32 ). The capillary pressure e ﬀ ect

on the electrode structure is negligible (Figure S33 ).

Importantly, we ﬁnd that the surface hydrophobic and

aerophilic features remain unchanged for the used electrode

(Figures S34 and S35 ). No appreciable change in the ECSA

was observed on our Cu dendrite electrode after diﬀerent

ⁿ^o^t^a^b^l^e^C²+ products in 1 M KHCO electrolyte with a C FE

−

of 52.2% and partial current density of 209 mA cm at −0.78

V versus RHE (Figure S24 ). Furthermore, our control

experiments using N gas feed instead of CO uncover that

no carbon-based products could be detected (Figure S25),

which excludes the possibility of KHCO as a reactant.

numbers of hours of CO electrolysis (Figure S36 ). These

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

results together conﬁrm the structural robustness of the new

comes from interaction between absorbed CO* intermediates

(inset in Figure 5c), which beneﬁts C₂₊production. The

disappearance of both Raman signals at high current density

CO RR cathode even working at high current densities,

comparing favorably with various GDL electrodes reported

previously (Table S7).

To further verify the importance of the hydrophobicity on

indicates that these intermediates react at very fast CO

reduction kinetics. However, Cu particles only show a single

−

the observed CO RR performances, we intentionally destroyed

mode at ∼2070 cm , pointing to a less favorable C−C

coupling on this electrode (Figure 5d and inset). Very

interestingly, we observe that the Raman peaks from both

electrodes experience a continuous negative shift as the applied

current density increases, which can be explained by the

electrochemical Stark eﬀect, a phenomenon where the local

electric ﬁeld can regulate the interaction between the electrode

and adsorbate, thus shifting the vibrational frequencies of

the hydrophobicity of the hierarchical Cu electrode by air

plasma bombardment and compared the resultant sample with

the untreated one. Results show that the hierarchical

morphology of the Cu catalyst is well retained after

bombarding for 5 min, whereas its surface turns highly

hydrophilic with a small water CA of 11° (Figure S37).

−

Holding at 200 mA cm , we obtained a low C FE of 39% on

the hydrophilic Cu electrode, accompanied with improved H₂

53,54

intermediates.

evolution (Figure S38 ). Moreover, long-term continuous CO

−1

At lower wavenumbers (400−640 cm ), our operando

electrolysis at 300 mA cm⁻revealed a quick decay of C₂₊

selectivity (Figure S39 ). The drop in C₂₊selectivity could

originate from the easy ﬂooding of the treated electrode that

Raman spectroscopy measurements on a hierarchical Cu

electrode reveal that peaks originated from surface CuO_x

disappeared immediately once negative currents were applied

hampers CO diﬀusion toward the catalyst.

(

Figure 5e), suggesting the reduction of surface oxide to

Finite-Element-Method Simulations and Operando

Raman Spectroscopy. Next, we turn to combine the

simulations and operando Raman spectroscopy to uncover

the underlying reasons for the exceptional C₂₊selectivity

observed on our hierarchical Cu dendrite electrode. Recent

studies have shown, both experimentally and computationally,

that the alkali-metal-cation eﬀect at the gas−liquid−solid

interface can not only create a dipole ﬁeld that stabilizes

metallic Cu. Meanwhile, several characteristic Raman peaks

−

emerge, which arise from the Cu−CO (∼360 cm ), CO

−

2−

−1

(

∼700, ∼1310, and ∼1550 cm ), and CO (∼1070 cm )

49,55

modes,

respectively. The noticeable Cu−CO band at a

−

large applied current density of 200 mA cm indicates stable

interaction between *CO and the Cu surface (Figure 5e),

which supports the perspective that the hierarchical Cu

electrode can stabilize the *CO intermediate and promote

C−C coupling. After removing the current, the electrode

intermediates with large dipole moments (e.g., *CO , *CO,

and *OCCO) but also greatly reduce the *CO dimerization

,44,45

barrier, promoting C−C coupling.

We thus applied ﬁnite-

to probe if the hierarchical

surface was quickly reoxidized to Cu O (Figure S45 ),

6−48

element-method simulations

consistent with our experimental observations above (Figures

S31 and S32).

Cu electrode could concentrate the electric ﬁeld and cation

concentration (see Experimental Section for details). We

created dendrites composed of multiple needles to represent

the Cu electrode immersed in an electrolyte. Our series of

simulated results reveal that Cu dendrites with more needles

and a smaller radius of curvature (ROC) exhibit a more

enhanced local electrostatic ﬁeld (Figures S40 −S43). By

contrast, Cu particles show an inferior local electrostatic ﬁeld

owing to the large ROC. Considering these factors, we thus

constructed a model that closely resembles the experimentally

observed structure (Figure 5a). Using the Gouy−Chapman−

Stern method, we can assess the impact of electric ﬁeld on the

Now we come to discuss the intrinsic factors that lead to the

marked eﬃciency and stability of our designed Cu electrode in

catalyzing CO RR to C fuels at commercially relevant

−

current densities (e.g., 300 mA cm ). We outline two

important scenarios for the performance enhancement. First,

the hierarchical Cu architecture that mimics Setaria’s leaves

induces remarkable water resistance and creates a robust

CO −electrolyte−Cu triple-phase boundary at the electrode,

which allows for omnidirectional CO mass transfer, giving rise

to a high local concentration of CO trapped near the active

CO RR surface. Moreover, the hydrophobicity of the electrode

adsorbed K concentration, because this model permits the

is rooted in its unique hierarchically sharp structure, which is

highly stable to prevent ﬂooding even at high-current

operation. Nevertheless, the wettable Cu particulate electrode

mapping of surface K density in the Helmholtz layer of the

electrical double layer near the catalyst surface. Intriguingly,

we observed a 10.5-fold increase in surface-adsorbed K⁺

concentration at the dendritic electrode (Figure 5a) as

compared to the particulate counterpart (Figure 5b). These

is readily ﬂooded during CO RR, and therefore the pathway of

CO transfer is largely blocked, causing low CO concentration

close to the catalyst for reaction. Second, we ﬁnd clear

evidence for the promoted C−C coupling on the hierarchical

Cu electrode from our operando Raman spectroscopy tests.

results therefore indicate highly concentrated K cations on the

surface of the hierarchical Cu electrode, which could promote

the formation of C−C bonds.

This is caused by the locally concentrated K ions at the highly

To monitor the adsorbed CO reduction intermediates, we

structured electrode, which renders an alkali-metal-cation

eﬀect that not only stabilizes intermediates but also reduces

the barrier of forming C−C bonds. As a consequence, we

surmise that the new hierarchical electrode design allows for a

performed operando Raman spectroscopy in CO -ﬂowed 1 M

KOH electrolyte at various applied current densities (Figure

S44 ). We speciﬁcally chose the excitation wavelength to be 785

nm because it can yield large Raman intensity of related

,49

robust gas−liquid−solid triple-phase boundary, suﬃcient CO

vibration modes.

With increasing current density, two clear

Raman peaks arise from the hierarchical Cu electrode (Figure

mass transfer, and the favorable alkali-metal-cation eﬀect,

which together lead to advances in selectivity for C2+

production at large current densities, in stark contrast to the

ordinary performance observed experimentally on the Cu

particulate counterpart.

−

c), where the peak at ∼2070 cm corresponds to the

stretching vibration mode of absorbed CO* at the top

site.

,49,50

According to Masuda and Hwang’s previous

1,52

−1

work,

the additional peak at lower energy (∼1980 cm )

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

^w^a^t^e^r^c^oⁿ^t^a^c^t^aⁿ^g^l^e^s^aⁿ^d^C^O2 bubble adhesion behaviors of

CONCLUSION

■

electrodes were performed after 10 h of CO electrolysis. Electrodes

In conclusion, the work reported here showcases that

hierarchically structuring Cu needles on a GDL can lead to

stable hydrophobicity of a CO RR electrode without any

hydrophobic coating. Besides preventing the GDL from being

ﬂooded, the obtained electrode can trap a gaseous layer near its

−2

after CO electrolysis at a total current density of 300 mA cm for 10

min were also studied by SEM, XRD, and XPS.

Preparation of Cathode Electrodes. The as-deposited Cu

dendrite gas-diﬀusion electrode (GDE) was directly used for

electrochemical measurements. The Cu particle GDE was prepared

by hand-painting the catalyst ink. In detail, 45 mg of catalyst powder

was mixed with 2 mL of isopropanol containing 50 μL of Naﬁon

ionomer solution (5 wt %), resulting in a homogeneous catalyst ink by

ultrasonic dispersion. The resulting ink was uniformly spread on the

surface, giving rise to a high local CO concentration that

improves the CO RR selectivity to C products. Conse-

quently, the electrode was very stable at a commercially

−

relevant current density (here 300 mA cm ) owing to the

robust electrode−electrolyte interface guaranteed by the

hydrophobicity. This bioinspired design thus suggests an

electrode structuring route of creating a stable gas−liquid−

solid triple-phase boundary that permits electrosynthesis of

multicarbon fuels at industry-scale current densities.

GDL of 4 × 4 cm area by using an airbrush, yielding the Cu particle

−2

electrode with a catalyst loading of ∼2.8 mg cm , which was the

same as that of the Cu dendrite electrode.

Electrochemical Measurements. All electrochemical measure-

ments were performed in a ﬂow cell with a VSP-300 potentiostat

(

^Bⁱ^o^-^L^o^gⁱ^c^,^F^r^aⁿ^c^e⁾^.^F^o^r^m^e^a^s^u^r^e^m^eⁿ^t^sⁱⁿ^a^ﬂ^o^w^c^e^l^l^,^g^a^s^e^o^u^s^C^O2

was passed through the gas chamber at the back side of the Cu GDE

(

1 × 1 cm ). The electrolyte (1 M KOH) was circulated through both

EXPERIMENTAL SECTION

■

the anode (nickel foam) and cathode chambers separated by the

anion exchange membrane (AEM, Fumasep FAB-3-PK-130). The

Material Synthesis. All chemicals were used as received without

further puriﬁcation. The GDL (SGL 29BC) used in this work was

purchased from Fuel Cell Store. The Cu dendrite catalyst was directly

electrodeposited on the GDL. To improve the repeatability of the

electrodeposition, the gas diﬀuse carbon paper was ﬁrst treated by air

plasma prior to catalyst deposition. The plating bath was a 0.1 M

CuSO ·5H O solution in deionized water (DIW) with pH = 2

CO (99.999%) and KOH electrolyte ﬂow was kept constant at 24

−

and 20 mL min controlled by a mass ﬂow controller (C100L,

Sierra) and a peristaltic pump (BT100-2J, Longer Pump),

respectively. The applied potentials were measured against a Ag/

AgCl reference electrode in saturated KCl and converted to the RHE

reference scale with iR correction by

adjusted by 2 M H SO . The Cu dendrite thin ﬁlm was deposited

E(vs RHE) = E(vs Ag/AgCl) + 0.205 + (0.0591 × pH) − iR_s

under a constant potential of −0.7 V (vs Ag/AgCl) for 700 s in a

three-electrode conﬁguration where the Ag/AgCl electrode (saturated

KCl) and Pt mesh were used as the reference electrode and counter

electrode, respectively. The as-deposited ﬁlm was rinsed with DIW

and ethanol and then dried in a vacuum oven at 60 °C for 4 h.

The Cu particle catalyst was prepared by a hydrothermal method.

In detail, 2 mmol of cupric(II) acetylacetonate (Cu(C H O ) ), 4 mL

(

where the solution resistance R was determined by electrochemical

impedance spectroscopy at frequencies ranging from 0.1 Hz to 100

kHz.

CO RR Product Analysis. To accurately quantify the gas products

during CO electrolysis, the gas outlet ﬂow rate from the reactor was

2 2

of propylamine (C H NH ), and 2 mL of 80% hydrazine hydrate

carefully monitored by a mass ﬂow meter (AST10-HLC, Asert

Instruments) at each current density (Figure S15 ). The cathodic

compartment was vented directly into the sampling loop of a gas

chromatograph (GC-2014, Shimadzu). The GC analysis was set up to

split the gas sample into two aliquots. One aliquot passed a thermal

(

N H ·H O) were mixed in 30 mL of N,N-dimethylformamide

2 4 2

DMF). The above solution was put into a 50 mL Teﬂon-lined

stainless steel autoclave and maintained at 200 °C for 8 h. Then the

autoclave was cooled to room temperature naturally. The as-

synthesized Cu particle catalyst was rinsed with DIW and ethanol

and then dried in a vacuum oven at 60 °C for 4 h. The Fourier-

transform infrared (FTIR) spectroscopy and baseline test were

performed to ensure that no ethanol remained on the electrode before

conductivity detector to quantify H concentration, and the other was

routed through a ﬂame ionization detector with a methanizer to

quantitatively analyze the content of CO and alkane species. The

faradaic eﬃciency for gas products was calculated by the following

formula:

the CO electrolysis (Figure S17).

Material Characterizations. Optical microscope images were

obtained with a polarizing microscope (Leica DM2700P, Germany)

equipped with a Leica MC190 HD camera. The morphology of the

samples was determined by SEM (Zersss Supra 40) and TEM (JEOL

nxFV

^jtotal

FE(%) = 100 ×

(3)

where n and x are the number of electrons transferred and the mole

fraction of a certain product, respectively, F is faradaic constant, V is

the total molar ﬂow rate of gas, and j is the measured total current.

010F(s)). HRTEM images were taken on a JEMARM 200F atomic

resolution analytical microscope with an acceleration voltage of 200

kV. ICP-AES data were obtained by an Optima 7300 DV instrument.

XRD was performed on a Japan Rigaku DMax-γA X-ray

diﬀractometer with Cu Kα radiation (λ = 1.541 78 Å). XAS was

carried out at the BL11U beamline of National Synchrotron Radiation

total

H NMR spectra measured with water suppression using a

presaturation method were collected on a Bruker 400 MHz

spectrometer to test liquid products. Typically, 500 μL of electrolyte

after CO RR electrolysis was mixed with 100 μL of D O containing

Laboratory in Hefei, China. Water contact angles and CO bubble

0 ppm (m/m) dimethyl sulfoxide as the internal standard. The same

adhesion behaviors were analyzed via an OCA20 machine (Data-

Physics, Germany) at room temperature. A water droplet with a ∼3

mm diameter was generated on a hydrophobic needle with a syringe

driven by an infusion and withdrawal pump at 0.05 mL/min. Then

water droplets were released to impact the surface of the electrodes.

Simultaneously, the side-view images of the droplet were captured.

For the measurements of CO₂bubble adhesion, the sample was

clamped ﬂat in a transparent quartz container and immersed in

KHCO solution, and an individual CO bubble (about 2.0 μL) was

spectral acquisition parameters were used for all measurements to

ensure complete relaxation and quantiﬁcation. The presented data

were the accumulated result of 32 scans.

The half-cell (cathodic) power conversion eﬃciency (PCE) of

C₂₊products was calculated using

(1.23 − E_C₂₊₍_p_r_o_d_u_c_t₎) × FE₍_C₂₊_p_r_o_d_u_c_t₎

PCE (%) =

∑

.23 − E

(4)

injected from the syringe needle under solution to the electrode

surface. In-situ Raman spectra were taken on a Raman microscope

where E is the applied potential vs RHE and E_C₂₊₍_p_r_o_d_u_c_t₎and

FE₍_C₂₊_p_r_o_d_u_c_t₎are thermodynamic potential vs RHE and measured

faradaic eﬃciency in percentage of a certain C₂₊product, respectively.

Finite-Element-Method Simulations. In our work, the electrical

double layer was modeled using the Gouy−Chapman−Stern model,

(

Renishaw) with a 785 nm excitation laser. The CO RR postreaction

electrode was washed by deionized water and stored in a vacuum oven

before measurements. Note that postreaction characterizations of

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

https://pubs.acs.org/10.1021/jacs.1c01190

Article

which consists of a Helmholtz layer and a diﬀusion layer. The former

consists of a monolayer of surface-adsorbed hydrated K cation on the

University of Science and Technology of China, Hefei

30026, China

catalyst surface. The latter consisted of cations and anions, which is

established as the result of a dynamic equilibrium between

electrostatic forces and diﬀusion. In a steady state, the Poisson−

Nerst−Planck equations could be solved to gain the electric ﬁeld and

Complete contact information is available at:

K concentration near the catalyst surface. A two-dimensional model

Author Contributions

Z.-Z.N., F.-Y.G., and X.-L.Z. contributed equally to this work.

was built to represent the catalyst in this work. The diﬀusion

−

2 −1

coeﬃcient D of the potassium ion was taken to be 2.14 × 10 m s

Notes

in water. The conductivity of the electrolyte (1 M KOH) and

−

The authors declare no competing ﬁnancial interest.

catalyst (metallic Cu) was taken to be 2.153 × 10 and 6.20 × 10 S

−

(ref 56), respectively. In this model, the electric potential applied

on the Cu catalyst was set to −1.0 V and the number of boundary

ACKNOWLEDGMENTS

■

layers on the catalyst surface was set to 8.

This work was supported by the National Basic Research

Program of China (Grant 2018YFA0702001), the funding

support from the National Natural Science Foundation of

China (Grants 21975237 and 51702312), the Anhui Provincial

Research and Development Program (Grant 202004a-

ASSOCIATED CONTENT

■

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/jacs.1c01190.

5020073), the USTC Research Funds of the Double First-

Additional SEM images, XRD and Raman patterns, XPS

spectra, and additional electrochemical data (PDF)

Class Initiative (YD2340002007), and the Recruitment

Program of Global Youth Experts.

REFERENCES

AUTHOR INFORMATION

Corresponding Author

Min-Rui Gao − Division of Nanomaterials & Chemistry, Hefei

National Laboratory for Physical Sciences at the Microscale,

University of Science and Technology of China, Hefei

■

1) Olah, G. A.; Prakash, G. K. S.; Goeppert, A. Anthropogenic

Chemical Carbon Cycle for a Sustainable Future. J. Am. Chem. Soc.

011, 133, 12881−12898.

2) Mistry, H.; Varela, A. S.; Kuhl, S.; Strasser, P.; Cuenya, B. R.

Nanostructured electrocatalysts with tunable activity and selectivity.

Nat. Rev. Mater. 2016, 1, 16009.

3) Montoya, J. H.; Seitz, L. C.; Chakthranont, P.; Vojvodic, A.;

30026, China; orcid.org/0000-0002-7805-803X;

Email: mgao@ustc.edu.cn

(

Jaramillo, T. F.; Nørskov, J. K. Materials for solar fuels and chemicals.

Authors

Nat. Mater. 2017, 16, 70−81.

Zhuang-Zhuang Niu − Division of Nanomaterials &

Chemistry, Hefei National Laboratory for Physical Sciences at

the Microscale, University of Science and Technology of

China, Hefei 230026, China

Fei-Yue Gao − Division of Nanomaterials & Chemistry, Hefei

National Laboratory for Physical Sciences at the Microscale,

University of Science and Technology of China, Hefei

(4) Dinh, C.-T.; Burdyny, T.; Kibria, M. G.; Seifitokaldani, A.;

Gabardo, C. M.; García de Arquer, F. P.; Kiani, A.; Edwards, J. P.; De

Luna, P.; Bushuyev, O. S.; Zou, C.; Quintero-Bermudez, R.; Pang, Y.;

Sinton, D.; Sargent, E. H. CO

hydroxide-mediated copper catalysis at an abrupt interface. Science

018, 360, 783−787.

5) Li, F.; Li, Y. C.; Wang, Z.; Li, J.; Nam, D.-H.; Lum, Y.; Luo, M.;

electroreduction to ethylene via

(

Wang, X.; Ozden, A.; Hung, S.-F.; Chen, B.; Wang, Y.; Wicks, J.; Xu,

Y.; Li, Y.; Gabardo, C. M.; Dinh, C.-T.; Wang, Y.; Zhuang, T.-T.;

30026, China

Xiao-Long Zhang − Division of Nanomaterials & Chemistry,

Hefei National Laboratory for Physical Sciences at the

Microscale, University of Science and Technology of China,

Hefei 230026, China

Peng-Peng Yang − Division of Nanomaterials & Chemistry,

Hefei National Laboratory for Physical Sciences at the

Microscale, University of Science and Technology of China,

Hefei 230026, China

Sinton, D.; Sargent, E. H. Cooperative CO -to-ethanol conversion via

enriched intermediates at molecule −metal catalyst interfaces. Nat.

Catal. 2020, 3, 75−82.

(6) Luc, W.; Fu, X.; Shi, J.; Lv, J.-J.; Jouny, M.; Ko, B. H.; Xu, Y.; Tu,

Q.; Hu, X.; Wu, J.; Yue, Q.; Liu, Y.; Jiao, F.; Kang, Y. Two-

dimensional copper nanosheets for electrochemical reduction of

carbon monoxide to acetate. Nat. Catal. 2019, 2 , 423 −430.

(7) Kim, D.; Kley, C. S.; Li, Y.; Yang, P. Copper nanoparticle

ensembles for selective electroreduction of CO to C −C products.

Ren Liu − Department of Materials Science and Engineering,

University of Science and Technology of China, Hefei

Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 10560−10565.

8) Lv, J. J.; Jouny, M.; Luc, W.; Zhu, W.; Zhu, J. J.; Jiao, F. A Highly

Nørskov, J. K.; Chan, K.; Wang, H. Metal ion cycling of Cu foil for

30026, China

Porous Copper Electrocatalyst for Carbon Dioxide Reduction. Adv.

Mater. 2018, 30, e1803111.

Li-Ping Chi − Division of Nanomaterials & Chemistry, Hefei

National Laboratory for Physical Sciences at the Microscale,

University of Science and Technology of China, Hefei

(9) Jiang, K.; Sandberg, R. B.; Akey, A. J.; Liu, X.; Bell, D. C.;

30026, China

selective C −C coupling in electrochemical CO reduction. Nat. Catal.

Zhi-Zheng Wu − Division of Nanomaterials & Chemistry,

Hefei National Laboratory for Physical Sciences at the

Microscale, University of Science and Technology of China,

Hefei 230026, China

Shuai Qin − Division of Nanomaterials & Chemistry, Hefei

National Laboratory for Physical Sciences at the Microscale,

University of Science and Technology of China, Hefei

2018, 1, 111−119.

(10) Liang, Z.-Q.; Zhuang, T.-T.; Seifitokaldani, A.; Li, J.; Huang,

C.-W.; Tan, C.-S.; Li, Y.; De Luna, P.; Dinh, C. T.; Hu, Y.; Xiao, Q.;

Hsieh, P.-L.; Wang, Y.; Li, F.; Quintero-Bermudez, R.; Zhou, Y.;

Chen, P.; Pang, Y.; Lo, S.-C.; Chen, L.-J.; Tan, H.; Xu, Z.; Zhao, S.;

Sinton, D.; Sargent, E. H. Copper-on-nitride enhances the stable

electrosynthesis of multi-carbon products from CO . Nat. Commun.

(

018, 9, 3828.

30026, China

11) Zhou, Y.; Che, F.; Liu, M.; Zou, C.; Liang, Z.; De Luna, P.;

Xingxing Yu − Division of Nanomaterials & Chemistry, Hefei

National Laboratory for Physical Sciences at the Microscale,

Yuan, H.; Li, J.; Wang, Z.; Xie, H.; Li, H.; Chen, P.; Bladt, E.;

Quintero-Bermudez, R.; Sham, T. K.; Bals, S.; Hofkens, J.; Sinton, D.;

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Chen, G.; Sargent, E. H. Dopant-induced electron localization drives

Article

(30) D ’Urso, B.; Simpson, J. T.; Kalyanaraman, M. Emergence of

superhydrophobic behavior on vertically aligned nanocone arrays.

Appl. Phys. Lett. 2007, 90, 044102.

CO reduction to C hydrocarbons. Nat. Chem. 2018, 10, 974−980.

(

12) De Luna, P.; Quintero-Bermudez, R.; Dinh, C.-T.; Ross, M. B.;

Bushuyev, O. S.; Todorovic,

Sargent, E. H. Catalyst electro-redeposition controls morphology and

P.; Regier, T.; Kelley, S. O.; Yang, P.;

(31) Yamauchi, Y.; Tenjimbayashi, M.; Samitsu, S.; Naito, M.

Durable and Flexible Superhydrophobic Materials: Abrasion/Scratch-

ing/Slicing/Droplet Impacting/Bending/Twisting-Tolerant Compo-

site with Porcupinefish-Like Structure. ACS Appl. Mater. Interfaces

oxidation state for selective carbon dioxide reduction. Nat. Catal.

(

018, 1, 103−110.

32) Lu, Z.; Xu, W.; Ma, J.; Li, Y.; Sun, X.; Jiang, L. Superaerophilic

019, 11, 32381−32389.

13) Yang, P.-P.; Zhang, X.-L.; Gao, F.-Y.; Zheng, Y.-R.; Niu, Z.-Z.;

Yu, X.; Liu, R.; Wu, Z.-Z.; Qin, S.; Chi, L.-P.; Duan, Y.; Ma, T.;

Zheng, X.-S.; Zhu, J.-F.; Wang, H.-J.; Gao, M.-R.; Yu, S.-H. Protecting

Copper Oxidation State via Intermediate Confinement for Selective

Carbon-Nanotube-Array Electrode for High-Performance Oxygen

Reduction Reaction. Adv. Mater. 2016, 28, 7155−7161.

(33) Wang, P.; Hayashi, T.; Meng, Q. a.; Wang, Q.; Liu, H.;

CO Electroreduction to C ₂ ₊Fuels. J. Am. Chem. Soc. 2020, 142,

Hashimoto, K.; Jiang, L. Highly Boosted Oxygen Reduction Reaction

Activity by Tuning the Underwater Wetting State of the Super-

hydrophobic Electrode. Small 2017, 13, 1601250−1601256.

14) Weiss, R. F. Carbon dioxide in water and seawater: the

400−6408.

solubility of a non-ideal gas. Mar. Chem. 1974, 2, 203−215.

15) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A

comprehensive review on PEM water electrolysis. Int. J. Hydrogen

Energy 2013, 38, 4901−4934.

16) Ma, S.; Sadakiyo, M.; Luo, R.; Heima, M.; Yamauchi, M.;

Kenis, P. J. A. One-step electrosynthesis of ethylene and ethanol from

CO in an alkaline electrolyzer. J. Power Sources 2016, 301, 219−228.

17) Verma, S.; Hamasaki, Y.; Kim, C.; Huang, W.; Lu, S.; Jhong,

H.-R. M.; Gewirth, A. A.; Fujigaya, T.; Nakashima, N.; Kenis, P. J. A.

Insights into the Low Overpotential Electroreduction of CO to CO

on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer. ACS

Energy Lett. 2018, 3, 193−198.

18) Jouny, M.; Luc, W.; Jiao, F. High-rate electroreduction of

carbon monoxide to multi-carbon products. Nat. Catal. 2018, 1, 748−

55.

19) Kim, B.; Seong, H.; Song, J. T.; Kwak, K.; Song, H.; Tan, Y. C.;

Park, G.; Lee, D.; Oh, J. Over a 15.9% Solar-to-CO Conversion from

Dilute CO Streams Catalyzed by Gold Nanoclusters Exhibiting a

(34) Chen, R.; Tian, W.; Jia, Y.; Xu, W.; Chen, F.; Duan, X.; Xie, Q.;

Hu, C.; Liu, W.; Zhao, Y.; Kuang, Y.; Zhang, Y.; Sun, X. Engineering

Interfacial Aerophilicity of Nickel-Embedded Nitrogen-Doped CNTs

for Electrochemical CO Reduction. ACS Appl. Energy Mater. 2019, 2,

(

991−3998.

35) Wang, J.; Zheng, Y.; Nie, F.-Q.; Zhai, J.; Jiang, L. Air Bubble

Bursting Effect of Lotus Leaf. Langmuir 2009, 25, 14129−14134.

36) Hanselman, S.; Koper, M. T. M.; Calle-Vallejo, F. Computa-

(

tional Comparison of Late Transition Metal (100) Surfaces for the

Electrocatalytic Reduction of CO to C Species. ACS Energy Lett.

(

018, 3, 1062−1067.

37) Kortlever, R.; Shen, J.; Schouten, K. J.; Calle-Vallejo, F.; Koper,

M. T. Catalysts and Reaction Pathways for the Electrochemical

Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073−82.

(38) Kibria, M. G.; Dinh, C. T.; Seifitokaldani, A.; De Luna, P.;

Burdyny, T.; Quintero-Bermudez, R.; Ross, M. B.; Bushuyev, O. S.;

Garcia de Arquer, F. P.; Yang, P.; Sinton, D.; Sargent, E. H. A Surface

Reconstruction Route to High Productivity and Selectivity in CO ₂

Electroreduction toward C ₂ ₊Hydrocarbons. Adv. Mater. 2018, 30,

1804867−1804873.

(

High CO Binding Affinity. ACS Energy Lett. 2020, 5, 749−757.

20) Yu, S.; Li, X.; Li, J.; Liu, S.; Lu, W.; Shao, Z.; Yi, B. Study on

hydrophobicity degradation of gas diffusion layer in proton exchange

P. Morphology Matters: Tuning the Product Distribution of CO

(39) Dutta, A.; Rahaman, M.; Luedi, N. C.; Mohos, M.; Broekmann,

membrane fuel cells. Energy Convers. Manage. 2013 , 76 , 301 −306.

21) Park, J.; Oh, H.; Ha, T.; Lee, Y. I.; Min, K. A review of the gas

Electroreduction on Oxide-Derived Cu Foam Catalysts. ACS Catal.

2016, 6, 3804−3814.

diffusion layer in proton exchange membrane fuel cells: Durability and

40) Rahaman, M.; Dutta, A.; Zanetti, A.; Broekmann, P.

degradation. Appl. Energy 2015, 155, 866−880.

^E ^l ^e ^c ^t ^r ^o ^c ^h ^e ^m ⁱ ^c ^a ^l ^R ^e ^d ^u ^c ^t ⁱ ^o ⁿ ^o ^f ^C ^O 2 into Multicarbon Alcohols on

Activated Cu Mesh Catalysts: An Identical Location (IL) Study. ACS

Catal. 2017, 7, 7946−7956.

22) Kim, B.; Hillman, F.; Ariyoshi, M.; Fujikawa, S.; Kenis, P. J. A.

Effects of composition of the micro porous layer and the substrate on

performance in the electrochemical reduction of CO to CO. J. Power

(41) Reller, C.; Krause, R.; Volkova, E.; Schmid, B.; Neubauer, S.;

Sources 2016, 312, 192−198.

^R ^u ^c ^k ⁱ ^,^A ^.^;^S ^c ^h ^u ^s ^t ^e ^r ^,^M ^.^;^S ^c ^h ^m ⁱ ^d ^,^G ^.^S ^e ^l ^e ^c ^t ⁱ ^v ^e ^E ^l ^e ^c ^t ^r ^o ^r ^e ^d ^u ^c ^t ⁱ ^o ⁿ ^o ^f ^C ^O 2

toward Ethylene on Nano Dendritic Copper Catalysts at High

Current Density. Adv. Energy Mater. 2017, 7, 1602114.

23) Wang, Q.; Dong, H.; Yu, H.; Yu, H. Enhanced performance of

gas diffusion electrode for electrochemical reduction of carbon

dioxide to formate by adding polytetrafluoroethylene into catalyst

layer. J. Power Sources 2015, 279, 1−5.

(42) Scholten, F.; Sinev, I.; Bernal, M.; Roldan Cuenya, B. Plasma-

Modified Dendritic Cu Catalyst for CO Electroreduction. ACS Catal.

(

24) Wakerley, D.; Lamaison, S.; Ozanam, F.; Menguy, N.; Mercier,

(

019, 9, 5496−5502.

D.; Marcus, P.; Fontecave, M.; Mougel, V. Bio-inspired hydro-

43) Dutta, A.; Montiel, I. Z.; Erni, R.; Kiran, K.; Rahaman, M.;

phobicity promotes CO reduction on a Cu surface. Nat. Mater. 2019,

Drnec, J.; Broekmann, P. Activation of bimetallic AgCu foam

electrocatalysts for ethanol formation from CO ₂ by selective Cu

oxidation/reduction. Nano Energy 2020, 68, 104331.

(

8, 1222−1227.

25) Endrodi, B.; Kecsenovity, E.; Samu, A.; Darvas, F.; Jones, R. V.;

Török, V.; Danyi, A.; Janáky, C. Multilayer Electrolyzer Stack

Converts Carbon Dioxide to Gas Products at High Pressure with

High Efficiency. ACS Energy Lett. 2019, 4, 1770−1777.

(44) Resasco, J.; Chen, L. D.; Clark, E.; Tsai, C.; Hahn, C.; Jaramillo,

T. F.; Chan, K.; Bell, A. T. Promoter Effects of Alkali Metal Cations

on the Electrochemical Reduction of Carbon Dioxide. J. Am. Chem.

Soc. 2017, 139, 11277−11287.

26) Meng, K.; Jiang, Y.; Jiang, Z.; Lian, J.; Jiang, Q. Cu surfaces with

controlled structures: From intrinsically hydrophilic to apparently

(45) Ringe, S.; Clark, E. L.; Resasco, J.; Walton, A.; Seger, B.; Bell, A.

superhydrophobic. Appl. Surf. Sci. 2014 , 290 , 320 −326.

T.; Chan, K. Understanding cation effects in electrochemical CO ₂

28) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.;

27) Aqra, F.; Ayyad, A. Surface energies of metals in both liquid and

reduction. Energy Environ. Sci. 2019, 12, 3001−3014.

solid states. Appl. Surf. Sci. 2011, 257, 6372−6379.

(46) Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.;

(

Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P.; Safaei, T.

S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.;

Li, X. Polyelectrolyte Multilayer as Matrix for Electrochemical

Deposition of Gold Clusters: Toward Super-Hydrophobic Surface.

J. Am. Chem. Soc. 2004, 126, 3064−3065.

Kelley, S. O.; Sargent, E. H. Enhanced electrocatalytic CO reduction

via field-induced reagent concentration. Nature 2016, 537, 382−386.

(47) Gao, F.-Y.; Hu, S.-J.; Zhang, X.-L.; Zheng, Y.-R.; Wang, H.-J.;

Niu, Z.-Z.; Yang, P.-P.; Bao, R.-C.; Ma, T.; Dang, Z.; Guan, Y.; Zheng,

X.-S.; Zheng, X.; Zhu, J.-F.; Gao, M.-R.; Yu, S.-H. High-Curvature

29) Khan, S.; Hwang, J.; Horn, Y.-S.; Varanasi, K. K. Catalyst-

proximal plastrons enhance activity and selectivity of carbon dioxide

electroreduction. Cell Reports Physical Science 2021, 2, 100318.

J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Transition-Metal Chalcogenide Nanostructures with a Pronounced

Article

Proximity Effect Enable Fast and Selective CO Electroreduction.

Angew. Chem., Int. Ed. 2020, 59, 8706−8712.

(

48) Saberi Safaei, T.; Mepham, A.; Zheng, X.; Pang, Y.; Dinh, C.-T.;

Liu, M.; Sinton, D.; Kelley, S. O.; Sargent, E. H. High-Density

Nanosharp Microstructures Enable Efficient CO Electroreduction.

Nano Lett. 2016, 16, 7224−7228.

(

49) Applied Energy Gao, J.; Zhang, H.; Guo, X.; Luo, J.;

Zakeeruddin, S. M.; Ren, D.; Grätzel, M. Selective C −C Coupling

in Carbon Dioxide Electroreduction via E ﬃ cient Spillover of

Intermediates As Supported by Operando Raman Spectroscopy J.

Am. Chem. Soc. 2019, 141, 18704−18714.

(

50) Ross, M. B.; Dinh, C. T.; Li, Y.; Kim, D.; De Luna, P.; Sargent,

E. H.; Yang, P. Tunable Cu Enrichment Enables Designer Syngas

Electrosynthesis from CO . J. Am. Chem. Soc. 2017, 139, 9359−9363.

(

51) Iijima, G.; Inomata, T.; Yamaguchi, H.; Ito, M.; Masuda, H.

Role of a Hydroxide Layer on Cu Electrodes in Electrochemical CO ₂

Reduction. ACS Catal. 2019, 9, 6305−6319.

(

52) Kim, Y.; Park, S.; Shin, S.-J.; Choi, W.; Min, B. K.; Kim, H.;

Kim, W.; Hwang, Y. J. Time-resolved observation of C −C coupling

intermediates on Cu electrodes for selective electrochemical CO2

reduction. Energy Environ. Sci. 2020, 13, 4301 −4311.

53) Mehandru, S. P.; Anderson, A. B. Potential-induced variations

in properties for carbon monoxide adsorbed on a platinum electrode.

J. Phys. Chem. 1989, 93, 2044 −2047.

54) Lambert, D. K. Vibrational Stark effect of adsorbates at

electrochemical interfaces. Electrochim. Acta 1996, 41, 623−630.

55) Chernyshova, I. V.; Somasundaran, P.; Ponnurangam, S. On the

origin of the elusive first intermediate of CO electroreduction. Proc.

Natl. Acad. Sci. U. S. A. 2018, 115, 9261−9270.

56) Gilliam, R. J.; Graydon, J. W.; Kirk, D. W.; Thorpe, S. J. A

review of specific conductivities of potassium hydroxide solutions for

various concentrations and temperatures. Int. J. Hydrogen Energy

2007, 32, 359−364.