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Cite this: DOI: 10.1039/d1ee01696b
The product selectivity zones in gas diffusion
electrodes during the electrocatalytic reduction
of CO
2
†
Tim Mo
¨ller, Trung Ngo Thanh, Xingli Wang, Wen Ju, Zarko Jovanov and
Peter Strasser *
Here we report on the most prominent factors influencing the performance of a Cu-based CO
2
electrolyzer operating at high currents. Using a flow-electrolyzer design where CO
2
gas feed passes
directly through the electrode interacting with the Cu catalyst layer, we observed that the selectivity of
the electrochemical CO
2
reduction in (bulk) pH neutral media can greatly be influenced by adjusting the
structure of the electrode. In this, the variations in catalyst loading and ionomer content can profoundly
affect the selectivity of CO
2
RR. We explore the hypothesis that this originates from the overall mass
transport variations within the porous catalytic layer of the gas diffusion electrode. As further evidence
for this, apart from the CO
2
electrolysis results, we propose a special method to benchmark the reactant
mass transport in flow-cells using oxygen reduction reaction (ORR) limiting current measurements. Our
analysis suggests that a restriction of mass transport is highly desirable due to its connection to a local
alkalization and corresponding suppression of pH-dependent reaction products, given the absence of
local CO
2
concentration limitations. We further show how the electrode structure can be used to push
the observed catalytic CO
2
reduction selectivity either towards C
1
or C
2+
products, dependent on the
ionomer content and catalyst loading in a cathodic current range of 50 to 700 mA cm
2
. Measurements
at various KHCO
3
electrolyte concentrations agree with the notion of the local pH dictating the overall
selectivity and point towards the presence of pronounced concentration gradients within the system.
Overall, our work suggests that the differences in electrocatalytic CO
2
reduction selectivity at high
currents (in a range of pH neutral buffering electrolytes) largely originate from the local concentration
gradients defined by the initial catalyst ink formulation and architecture of the catalytic layer, both of
which represent a powerful tool for optimization in the production of selected value-added products.
Broader context
The electrocatalytic CO
2
reduction reaction (CO
2
RR) on Cu-based catalysts has the potential to enable the sustainable production of commodity chemicals and
fuels by upcycling waste-CO
2
into value-added compounds. Over the recent years, much progress has been made by fabrication of catalysts into so-called gas
diffusion electrodes (GDEs) to tackle the issue of low CO
2
solubility in aqueous electrolytes and to advance towards commercially viable current densities of
4300 mA cm
2
. Though the selectivity towards CO
2
RR could be largely improved over the competing HER using GDEs, the underlying factors for an efficient
production of the individual CO
2
RR products remain elusive. In the present study, we deploy a flow-electrolyzer system to investigate structure–selectivity-
interrelations of a Cu-based GDE in a buffering electrolyte. By systematic investigation of three parameters: (i) particle catalyst loading, (ii) ionomer to catalyst
ratio and (iii) buffer capacity, we set out to understand the structural properties that dictate the spatial variation of the selectivity across the catalyst layer at high
reaction rates. Thereby, we describe a sequence of zones of distinct selectivity within the catalyst layer (‘‘selectivity zones’’), which arise fromthe
inhomogeneous distribution of the reactants (CO
2
and pH) and control the overall selectivity of the system.
Introduction
The direct electrochemical CO
2
reduction reaction (CO
2
RR) has
been emerging as a potential technology for storage of renew-
able energy and sustainable production of carbon-based
chemicals.
1
In this area, the catalyst research has mainly been
The Electrochemical Energy, Catalysis, and Materials Science Laboratory,
Department of Chemistry, Chemical Engineering Division, Technical University Berlin,
†Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ee01696b
Received 3rd June 2021,
Accepted 30th September 2021
DOI: 10.1039/d1ee01696b
rsc.li/ees
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Science
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focused on Cu due to its capability to reduce CO
2
towards value-
added, post-CO products, such as hydrocarbons and alcohols.
2
The control over the electrocatalytic selectivity has been the
focal point of Cu-based catalyst research, due to parallel reaction
pathways and the resulting mixture of products.
3
Various modi-
fications of Cu have been investigated in order to steer the
selectivity towards distinct products, as for instance exposure of
specific facets through nanostructuring and altered catalyst
composition through oxidative treatments or addition of second
metals.
4–7
Apart from the catalyst itself, the reaction conditions
have been repeatedly reported to influence the selectivity of the
CO
2
RR. For instance, local pH value gradients establishing at
the electrode electrolyte interface, also described as alkalization,
were shown to have a decisive impact on reaction selectivity by
suppression of mechanistic pathways dependent on the proton
transfer as a rate-limiting step.
8–10
Studies performed in very
commonly used KHCO
3
electrolytes with neutral bulk pH, show
that the local pH gradients near electrode surface essentially arise
from a deficient mass transport unable to buffer the cathodic
production of OH
.
11–13
For H-type setups, effects of variations in
the structure of the catalytic layer have been repeatedly reported
to influence the mass transport, critically altering the reaction
environment and therefore determining the observed selectivity
of the system during electrochemical CO
2
RR. Here, the mass
transport can be divided into a transport of reactants directed
towards the surface and a transport of products leaving the
surface. On the reactant side, in the aqueous electrolyte dissolved,
CO
2
is transported towards the catalytic sites, where abundantly
available water molecules supply the protons to form reduced
carbon compounds under production of OH
. The generated
OH
canreadilyreactwiththecommonlyusedHCO
3
electrolyte
acting as a buffer. However, HCO
3
transport from the bulk
towards the surface is not sufficiently fast to keep up with the
OH
production rate, which locally shifts the pH and bicarbonate-
equilibrium near the reaction sites. This effect of local alkalization
during CO
2
RR was observed experimentally at already moderate
potentials by surface enhanced IR and Raman spectroscopy,
simulated based on bicarbonate equilibria and often suggested
to be the origin of a high selectivity for multi-carbon products
during electrochemical CO
2
RR.
12,14–18
On the products side,
carbon compounds need to be transported away from the surface
to allow for further reactants to fill in. However, the sustained
presence of reactive intermediates such as CO have also been
reported to show a beneficial effect on production of ethylene and
oxygenates, and was also suggested to show a suppression for the
competing HER from water reduction due to the occupation of
thereactivesites.
19–23
Generally, an increasing thickness of the
catalytic layer, together with the associated electrochemically
active surface area (ECSA), increases the mean path of transporta-
tion for the reactants and products. This has been proposed to
impede a homogeneous through-plane mass transport within the
catalytic layer, which results in more pronounced concentration
gradients throughout the system.
12,15,24
Accordingly, it has been
suggested for mesoporous Ag electrodes of various thicknesses
that the CO
2
RRselectivityissensitivetotheconcentrationgra-
dients and shows the strongest suppression of the competing
HERforthethickestelectrodeatthelowestpartofthecatalytic
layer, where the effect of alkalization was most pronounced.
25,26
Recently, the field of CO
2
RR has progressively moved
towards studies of the high current regime in pH neutral and
alkaline, gas-feed flow-electrolyzers to approach technological
rates.
27–29
As the cathodic production of OH
is a function of
the current, flow-electrolyzers in buffered electrolytes have been
suggested to be particularly affected by local alkalization due to
insufficient HCO
3
transport and the observed high selectivity
for C
2+
products has been, at least in part, associated to that
effect.
30–32
Additionally, the depletion of CO
2
near the catalyst
surface has also been reported to limit the accessible CO
2
RR
currents and greatly influence the observed catalytic selectivity.
33–35
Despite those indications for the crucial role of the electrode
structure for high-current CO
2
RR, correlations similar to the
impact of mass transport in low-current H-cells remain under-
explored for gas diffusion electrodes in flow-electrolyzers. It is
plausible that high rate CO
2
RR on GDEs in flow-cell electro-
lyzers shows the same fundamental correlations of mass trans-
port and catalytic selectivity as those reported for H-cell setups.
We propose that the catalytic selectivity of Cu during high rate
CO
2
RR in pH buffering KHCO
3
electrolyte is largely controlled
by the mass transport, which can be rationalized by a discussion
of CO
2
transport and transport of pH buffering HCO
3
anions.
Flow-cell electrolyzer setup
We employed a flow-cell electrolyzer with three distinct com-
partments to investigate CO
2
RR electrolysis at high currents
and catalytic rates (Fig. 1a). Two different solutions of 1 M
KHCO
3
were used separately as anolyte and catholyte and
continuously looped through the two liquid compartments
divided by an anion conducting membrane. Anodic and cathodic
reaction products were transported out of the respective compart-
ments with the flow of the electrolytes. A convective stream of CO
2
was fed from the third compartment through the backside of the
porous cathode towards the catalytic layer, which allowed a fast
supply of reactant to the cathodic reaction sites.
Cathode electrode design and architecture
The cathode catalyst layer was deposited by airbrushing an ink
containing a mixture of Nafion and cubic Cu
2
O nanoparticles
on a carbon-based Freudenberg H23C2 gas diffusion layer
serving as substrate. As anode, a commercially available dimen-
sionally stable electrode composed of a Ti sheet covered by an
Ir-MMO was routinely used. Throughout this study, we will
refer to the uncoated Freudenberg H23C2 as the gas diffusion
layer, ‘‘GDL’’, and to the particle-coated Freudenberg H23C2 as
gas diffusion electrode, ‘‘GDE’’. Fig. 1b shows the schematic
structure of a GDE prepared by spraycoating. In this system, the
different elements of the GDE are spatially separated and can
be assigned to different layers. The lowest layer consists of
carbon fibers and acts as mechanical and conductive backbone
of the entire structure. Next, the microporous layer (MPL)
functions as conductive and hydrophobic substrate, which
allows the electronic and CO
2
transport towards the adjoining
catalytic layer. The hydrophobicity, gas permeability and electrical
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conductivity of the GDL are important parameters that influence
the CO
2
RR performance and stability of the GDE. Especially, the
hydrophobicity has proven decisive to maintain a sufficient gas
(CO
2
) transport towards the catalyticallyactivesitesbypreventing
a‘‘flooding’’thatisanextensivewettingoftheporousGDE
structure. For carbon-based GDLs an instability of the wetting
characteristics has been observed under application of cathodic
potentials and correlated to the catalytic activity towards HER of
the carbon structure.
36
To circumvent this effect, PTFE mem-
branes have been investigated as GDL materials, but can suffer
from issues of electrical conductivity and mechanical instability.
29
In this study, we use a ‘‘flow-through’’ approach, depicted in
Fig. 1(b), to minimize flooding issues connected to the use of a
carbon-based substrate and focus exclusively on properties of
the catalytic layer. The catalytic layer represents the uppermost
element of the GDE and is the interface between gaseous
transports of CO
2
and liquid transport of electrolyte. Within
the catalytic layer, the electronic, ionic and reactant transport
intersect and define the reaction environment of the electro-
catalytic CO
2
RR and the reactivity of the system. The morpho-
logy of the catalytic layer obtained by spraycoating is not trivial
and showed a porous, 3-dimensional appearance with a rough
surface, visible in cross-section and top-view SEM images
of Fig. 1b.
In this paper, we use a previously reported nanocubic Cu
2
O
catalyst (see Fig. S1, ESI†) with an edge length of 35 nm to
systematically alter the structure of the catalytic layer of a gas
diffusion electrode (GDE) for tests in a flow-electrolyzer.
37
By changing macroscopic parameters, as the particle catalyst
loading and the ionomer to particle catalyst ratio (Nafion content),
we influence the accessibility of catalytically active sites and the
through-plane mass transport in the porous structure of the
catalytic layer. In this, we correlate the electrode structure with
restriction in mass transport, manifesting in local concentration
gradients and alkalization, which in turn influences the selectivity
of CO
2
RR during a high current density operation in a bulk
neutral bicarbonate electrolyte. We chose our previously reported
and well-characterized cubic Cu
2
O catalyst as a model catalyst
system to investigate the influence of concentration gradients
within a flow-cell electrolyzer by variation of three macroscopic
parameters:
Particle catalyst loading: first, we varied the amount of
deposited catalyst particles to obtain GDEs with various catalyst
loadings that show differences in layer thickness, roughness
and ECSA. In doing so, we affect the mean path of transporta-
tion from the bulk sources of CO
2
and KHCO
3
towards the
catalytic sites.
Ionomer to particle catalyst ratio: to introduce an impedi-
ment for the mass transport, we varied the Nafion content of
the ink formulation to obtain GDEs that show different iono-
mer to catalyst ratios for a constant absolute catalyst loading.
Nafion acts as a strong adhesive to introduce a mechanical
stability to the catalytic layer, however, the ionomer distribu-
tion is a known issue in fuel cell research due to its influence
on local reactant transportation and associated mass (oxygen)
transport resistances.
38,39
Likewise, the transport of reactants
in CO
2
RR towards the active sites should also be sensitive to the
local distribution of the ionomer, acting as a barrier for the CO
2
and HCO
3
transport.
Buffer capacity: to induce various buffer capacities, we
varied the concentration of the KHCO
3
electrolyte to alter the
buffer capacity within our system. A higher concentration
of HCO
3
offers more buffer capacity, in turn resulting in
decreased pH gradients at the interface and counteracting the
effect of alkalization.
The present work is not about setting new number records
in product selectivity, but to pinpoint controlling factors for the
product selectivity in a technological GDE under more realistic
reaction conditions. To characterize the mass transport pro-
perties of CO
2
RR GDEs, we employ oxygen reduction limiting
current measurements typically used in the hydrogen fuel cell
community as diagnostic tool in the field of CO
2
reduction,
the results of which reveal how closely GDE structure and the
resulting catalytic selectivity are connected. The conclusions
of this work are consistent with previous reports on the
importance of near electrode surface pH and CO
2
concen-
tration, but expand our understanding of the relation-
ships between catalyst layer structure of a GDE and reaction
selectivity during CO
2
RR.
12,29,31,33,40,41
The systematic investi-
gation of the effects within our work prompt us to propose
the notion of layered ‘‘selectivity zones’’, which has not been,
to the best of our knowledge, previously described for a
Cu-based GDE in a flow-electrolyzer. We believe that our
selectivity zone model will aid the future design of efficient
Cu-based GDEs for CO
2
electrolyzers in buffered electrolytes of
near-neutral pH.
Fig. 1 Schematic representation of a 3-compartment flow-cell electro-
lyzer with indications of the transport directions for reactants, products
and electrolyte (a). Schematic representation of carbon-based gas diffu-
sion electrode with cross-section (50 mm scalebar) and top view SEM
images (400 nm scalebar) of a GDE prepared by spray-coating a dispersion
of a cubic Cu
2
O catalyst with Nafion binder onto a carbon gas diffusion
layer (b).
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Results
Effect of particle catalyst loading on CO
2
electrolysis
In Fig. 2, we present the distributions of four important
products observed during CO
2
RR on Cu: C
2
H
4
,CH
4
,H
2
and
HCOO
, obtained at fixed applied current densities on electrodes
containing varying catalyst loadings. The influence of catalyst
loading on the product distribution has been tested at fixed
concentration of 1.0 M KHCO
3
and at fixed binder content
(10 wt% Nafion). Here, the FE of C
2
H
4
,CH
4
,H
2
and HCOO
is
displayed as a function of the applied current density for
catalyst mass loadings ranging from 0.3 mg cm
2
to 2.0 mg cm
2
.
We specifically focus on this set of products, as they have distinctly
different rate-limiting reaction steps, which show either a strong
pH-dependence (CH
4
,HCOO
and H
2
) or primarily independence
(C
2
H
4
) and can therefore provide insight in the local reaction
environment.
42,43
A first key observation is the clear suppression of
all pH-dependent products (CH
4
,HCOO
and H
2
)athighmass
loadings of the catalyst, which was invariably linked to a favored
production of ethylene. Note how the FE of CH
4
,whichisa
preferred product during CO
2
RR at high proton concentration,
plummetedtobelow1%,assoonascatalystloadingwasincreased
to 1.3 mg cm
2
and higher. In contrast, in experiments using very
low catalysts loadings of below 0.4 mg cm
2
the FE for CH
4
raised
to above 20%, see Fig. 2b. While this trend was qualitatively in line
with the production of H
2
and HCOO
(Fig. 2c and d) the change
in FE for C
2
H
4
showed the opposite behavior, see Fig. 2a. Here, the
decrease of catalyst loading below 0.4 mg cm
2
resulted in a FE for
C
2
H
4
of below 20%, whereas higher loadings allowed for a more
selective C
2
H
4
production and increased FE to around 30%. Due to
the well-known sensitivity of CO
2
RR selectivity to the electrode
potential, we plotted the product FE against IR-free RHE potentials
in Fig. S2 (ESI†). Note that as the shift in selectivity is also visible
on an IR-free potential scale, we thereby succeed in excluding
variations in electrode potential as the sole origin of the observed
effect. Additional information of FE for the remaining CO
2
RR
products (EtOH, PrOH and CO) are given in Fig. S3 (ESI†)and
agree with the observed favorable production C
2+
compounds at
high catalyst loadings. We also plotted the FE of gas products as a
function of time, see Fig. S4 (ESI†), which confirmed the applied
current density as cause for the observed change in selectivity over
the investigated testing time of 18 hours. Only for the low loading
samples of 0.3 mg cm
2
and 0.4 mg cm
2
a decline in CO
2
RR
selectivity could be observed at constant current, which suggests
a decreasing stability with decreasing particle catalyst loading.
Morphological investigation of the GDEs prepared by deposition
of various catalyst loadings showed a clear dependence on the
absolute catalyst loading. Here, top-view SEM images after CO
2
Fig. 2 Faradaic efficiency as a function of applied current density at varying catalyst mass loadings for C
2
H
4
(a), CH
4
(b), H
2
(c) and HCOO
(d). Reactions
were conducted under following conditions: 3 cm
2
of geometric surface area of cathode, 1 M KHCO
3
and 10 wt% of Nafion used as binder in catalyst ink.
Additional products (CO, EtOH, and PrOH) are given in Fig. S3 of the ESI.†Dashed lines are shown to guide the eye.
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electrolysisaregiveninFig.S5aandb(ESI†). We observed a
progressive roughening and introduction of pores to the surface
upon increase of particle loading. Such structural changes suggest
an increase in electrochemically accessible surface area (ECSA),
which we have investigated by quantification of the electrochemi-
cal charging of the double layer (see Fig. S6, ESI†). As a measure of
theECSA,wecalculatedthedouble-layer capacitance, shown in
Fig. S5c (ESI†), which showed an increase with particle loading
and agrees with our observation of a progressive surface rough-
ening in SEM images. In accordance with an increased ECSA, we
noticed a higher catalytic activity during CO
2
RR for the high
particle loadings visible in the polarization curves of Fig. S5d
(ESI†). Our observations suggest that the catalytic selectivity during
high-rate CO
2
RR is sensitive to the structure of the catalytic layer,
which can be affected by variation of the catalyst loading. We
suspect that the correlation between catalytic selectivity and
catalyst loading originates in changes of the mass transport
(e.g. transportofCO
2
and HCO
3
),whichinturnaffectthelocal
pH and reactant concentration near the electrode.
Effect of particle catalyst to ionomer ratio (Nafion content) on
CO
2
electrolysis
In order to further investigate the mass transport as potential
origin of shifts in catalytic selectivity, we varied the binder
content within the catalytic layer. Inspired by fuel cell research,
we suspect that increasing amounts of ionomer within the
catalytic layer represents a barrier for the mass transport of
reactive species as dissolved CO
2
and HCO
3
towards reaction
sites. Fig. 3 shows the effect of an increase in Nafion content
within the catalytic layer of a GDE on the selectivity during
CO
2
RR. A suppression of CH
4
and HCOO
is evident in samples
with high Nafion content, see Fig. 3b and d, similar to what we
have observed at high catalyst mass loadings. Interestingly,
samples with high Nafion content (30 and 50 wt%) show a
suppression of HER and increased C
2
H
4
FE only at a relatively
low current density of smaller than 300 mA cm
2
(Fig. 3a and c).
A further increase of current density, results in a strong rise of FE
for HER at the cost of total CO
2
RR FE. We suspect that this change
in selectivity at high currents is caused by an excessive content of
Nafion, which reduces the mass transport to a point, where the
reactant (CO
2
) transport becomes limiting for CO
2
RR and causes
theHERtodominateintheprocess.Whilesuchacorrelation
between the Nafion content and the limited reactant transport
is restricting for CO
2
RR selectivity for a current density larger
than 300 mA cm
2
, it shows an increase at lower rates.
Excessive amounts of ionomer influence potentially the elec-
trical accessibility of the catalyst particles, which is why
we investigated the change in electrochemical double layer
Fig. 3 Faradaic efficiency as a function of applied current density with varying Nafion contents for C
2
H
4
(a), CH
4
(b), H
2
(c) and HCOO
(d). Conditions
were as follows: 3 cm
2
of geometric surface area, 0.7 mg cm
2
catalyst mass loading and 1 M KHCO
3
. Additional products (CO, EtOH, and PrOH) are
given in Fig. S11 of the ESI.†Dashed lines are shown to guide the eye.
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capacitance and catalytic activity. Here, we only observed minor
changes in double layer capacitance with varied Nafion loading,
but a lower catalytic activity of the 50 wt% Nafion sample
was apparent (see Fig. S7, ESI†). Again, to exclude differences
in electrode overpotential as cause of the observed trend, we
plotted IR-free RHE electrode potentials against the product FE
in Fig. S8 (ESI†), which still clearly shows the ionomer induced
shift in selectivity. In agreement with our previous observation,
the FE for gas products as a function of time showed only small
changes at constant current for the electrodes prepared with
different Nafion loadings, see Fig. S9 (ESI†), suggesting a stabi-
lity of the system over the investigated testing time. Furthermore,
we have also investigated the change in surface structure at
different Nafion contents by SEM, shown in Fig. S10 (ESI†). Here,
an intermediate content of Nafion (10 and 30 wt%) resulted in a
rough surface of the catalytic layer, whereas the extreme cases of
0 wt% and 50 wt% showed a relatively smooth surface. Note-
worthy is the pronounced coverage of the Cu
2
O cubes supposedly
by the ionomer in the 50 wt% Nafion sample, visible in images of
higher magnification, consistent with the idea of representing
a distinct barrier for the mass transport. Additional CO
2
RR
products (CO, EtOH and PrOH) are given in Fig. S11 (ESI†), which
showed a higher selectivity for EtOH with increased Nafion
content at moderate currents. Our observations on the effects of
increased Nafion content are qualitatively in line with the effect of
particle catalyst loading. In both cases, we generally observed a
higher selectivity for C
2+
products when either binder content
or particle loading was increased. It seems feasible that a similar
origin causes the observed selectivity shifts, given their high
qualitative agreement.
Effect of buffer capacity (KHCO
3
concentration) on CO
2
electrolysis
Fig. 4a–f shows the influence of the bulk KHCO
3
concentration
on the C
2+
(C
2
H
4
, EtOH and PrOH) and C
1
(CH
4
, HCOO
)
selectivity during CO
2
electrolysis in the flow-cell. Here, we
used different Nafion contents of 10 wt% to 50 wt% and a
constant catalyst loading of about 0.7 mg cm
2
. For the 50 wt%
Nafion sample we observed no significant change in catalytic
selectivity, when increasing electrolyte concentration from
0.1 M to 1.0 M KHCO
3
, see Fig. 4a and d. Both concentra-
tions showed the highest C
2+
selectivity of roughly 40% FE at
200 mA cm
2
, whereas a larger current density resulted in the
increase of the competing HER (Fig. S12a, ESI†). Further
increase of KHCO
3
electrolyte to 3 M lowered the C
2+
FE and
favored HER over the whole investigated current range.
In contrast, the FE for C
1
products seemed largely unaffected
by the change in KHCO
3
concentration. Next, the 30 wt%
Nafion sample showed a clear dependence of C
2+
and C
1
FE
on the KHCO
3
concentration during CO
2
RR, see Fig. 4b and e.
While we were able to achieve a combined C
2+
FE of almost 70%
at 600 mA cm
2
, the use of higher KHCO
3
concentrations of
1 and 3 M led to a decreased C
2+
FE to around 50% and 30%,
respectively. For the FE of C
1
products we observed an inverse
behavior, here 3 M KHCO
3
showed the highest combined C
1
FE
of roughly 20%, while the use of 1 M and 0.1 M KHCO
3
led to a
subsequent decrease. Our observations during CO
2
RR, using a
10 wt% Nafion sample were quite similar to the case of a
30 wt% Nafion sample, see Fig. 4c and f. Again, we achieved the
highest C
2+
selectivity at the lowest KHCO
3
concentration of 0.1 M,
Fig. 4 Effect of variations in KHCO
3
concentration on the CO
2
RR selectivity towards C
2+
products using 50 wt% (a), 30 wt% (b), and 10 wt% (c) of Nafion.
Effect of variations in KHCO
3
concentration on the CO
2
RR selectivity towards C
1
products using 50 wt% (d), 30 wt% (e), and 10 wt% (f) of Nafion. In all
cases Cu
2
O loading was const. at 0.7 mg cm
2
. Additional information on FE of H
2
and CO are given in Fig. S12 (ESI†).
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which enabled a combined FE of almost 60% over a broad range of
cathodic currents. The use of an increased KHCO
3
concentration
of 1 and 3 M resulted in a dramatically decreased C
2+
FE of 40%
and 5%, respectively. At the same time, the highest C
1
FE observed
on the 10 wt% Nafion sample increased from a maximum of 13%,
towards 17% and roughly 23% for respective KHCO
3
concentra-
tions of 0.1 M, 1.0 M and 3.0 M. In parallel to the CO
2
RR, the HER
also proved sensitive to KHCO
3
concentration and showed an
overall increase with KHCO
3
concentration, whereas the 50 wt%
sample seemed to be generally less sensitive to the buffer concen-
tration (Fig. S12a–c, ESI†). At all Nafion contents we observed a
higher selectivity for C
2+
products and generally lower HER and C
1
FE, when we employed KHCO
3
electrolytes of a lower concen-
tration during electrolysis, however, this effect seemed to be
dependent on Nafion content and was most pronounced for the
lowestNafioncontentof10wt%.
Implementation of the ORR as a diagnostic tool for
benchmarking the mass transport limitations within a complex
porous electrode system
As the effects of the local pH, the mass transport and the catalyst
kinetics are superimposed and, therefore, often difficult to be
unambiguously deconvoluted, especially due to parallel reac-
tion pathways of CO
2
RR on Cu and the competing HER, we
resorted to the oxygen reduction reaction (ORR) for further
discussion of the correlation between binder content and mass
transport. In doing so, we were able to exploit the more positive
standard reduction potential of the ORR, with respect to the
CO
2
RR, to fully avoid the HER. Hence, we obtained data that
we can interpret with overall less complexity, schematically
depicted in Fig. 5a. We take the view that such an approach
allows for a more direct investigation of the mass transport
limitations within our system. To probe the mass transport
towards the catalytically active centers of the Cu
2
O particles,
we varied the partial pressure of O
2
in an N
2
feed at a constant
electrode potential of 0.45 V
RHE
. We chose this potential
to achieve the highest possible rate of ORR, while avoiding
a region of considerable HER activity, indicated in Fig. 5b.
By tracing the change in ORR current as a function of O
2
partial
pressure at a fixed electrode potential, we can directly access
changes in mass transport caused by variations in binder
content. In Fig. 5c, we can see a rise in ORR current with
increase in partial pressure of O
2
until around 0.3 bar, where
the ORR current of the 10 wt% Nafion sample approaches a
Fig. 5 Schematic representation of CO
2
RR and ORR on the Cu surface of a GDE (a). Cyclic voltammetry under different ratios of O
2
/N
2
saturated gas
atmosphere in 2 M KHCO
3
for a 10 wt% of Nafion GDE. Different colors are indicating a potential regime of pure ORR or mixed regime of HER and ORR (b).
ORR current as a function of partial O
2
pressure from O
2
/N
2
mixtures in 2 M KHCO
3
for different Nafion contents in the catalytic layer at a constant electrode
potential of 0.45 V
RHE
with dashed lines to guide the eye. Different levels of shading indicate the primary limitation, either mass transport or reaction kinetics (c).
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plateau. This transition supposedly originates in a change from
a primarily mass transport dominated regime, towards one,
were activation resistances are denying higher reduction rates.
The regimes of mass transport and catalytic kinetics are indi-
cated by different levels of shading in Fig. 5c, but should be
rather understood as a visual orientation and not as a strict
border between the two. A comparison of three samples with
Nafion loadings ranging from 10 to 50 wt% shows an increased
ORR current in the mass transport domain of below 0.3 bar of
O
2
partial pressure for lower loadings of the ionomer, however,
this difference seemed to decrease progressively with a higher
partial O
2
pressure (Fig. S13, ESI†). The observed behavior
suggests that the mass transport remains limiting even at
higher O
2
partial pressure for samples of high Nafion content,
whereas samples of reduced binder content showed primarily
kinetic limitations at a similar partial pressure of O
2
and
constant electrode potential. Our results suggest that the
binder content of the catalytic layer of a GDE interferes with
the reactant transport. Here, a high binder content induces
larger resistances for the reactant transport and can lead to
significant mass transport limitations.
Discussion
Electrode structure determines concentration gradients during
CO
2
electrolysis
Combining our observations, we can schematically depict the
proposed influence of the catalytic layer structure on the
selectivity during high-rate CO
2
RR electrolysis in a pH-neutral
buffering media. Fig. 6a, shows a schematic GDE and illustrates
the relevant reactions, as well as the through-plane transport of
reactants and products in the catalytic layer during CO
2
RR.
While CO
2
is fed in a gaseous state to the GDE, we take the view
that CO
2
is highly likely to physically dissolve in the electrolyte
prior to its reduction on the catalyst surface due to the
presumable presence of a native solvent layer caused by the
hydrophilicity of charged electrodes. Accordingly, the transport
of reactive species (CO
2
and HCO
3
) and products has to occur
through a diffusion layer of finite thickness to reach or leave
the electrode surface. On the catalyst, OH
is being produced
during the reductive reaction of CO
2
and H
2
O. The OH
can
readily react with the present buffering HCO
3
anions that are
diffusively transported from the bulk electrolyte, which reduces
Fig. 6 Scheme qualitatively visualizing the flux of reactive species and products with indications of their respective transport directions throughout the
structure of a Cu-GDE during CO
2
RR in KHCO
3
(a). Schematic representation of the proposed influence of variations in the particle catalyst loading
(b and c) and the ionomer to catalyst ratio (b and d) on the concentration gradients of HCO
3
(grey), CO
2
(red) and OH
(blue) throughout the
catalytic layer.
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the local pH increase near the surface and creates CO
32
in the
process. The change of the local pH is a function of the current
density that is a measure of the OH
production rate and the
molar flux of HCO
3
anions transported towards the electrode
surface in a diffusive manner. The generated CO
32
is being
transported towards the bulk of the electrolyte, where it can
equilibrate with excess CO
2
to regenerate the buffer by for-
mation of HCO
3
, therefore, preventing larger changes in bulk
pH. It is important to note that in GDEs the source of CO
2
and
buffering HCO
3
anions are on opposing sides as indicated by
opposed directions of arrows in Fig. 6a. This phase asymmetric
feed implies a largely decoupled transport of dissolved CO
2
and HCO
3
towards the electrode surface of GDEs in flow-
electrolyzers, which is in contrast to conventional electrodes
that are fully submerged in liquid electrolytes during CO
2
RR in
H-cell setups.
The proposed effect for variation of ionomer to catalyst ratio
(Nafion content) and particle catalyst loading is depicted in
Fig. 6b–d and is largely associated to the through-plane concen-
tration gradients of CO
2
and HCO
3
within the catalytic layer.
Here, the effect of increasing the particle catalyst loading can
be rationalized by a larger mean path of transportation for CO
2
and HCO
3
through the thicker catalytic layer. This longer
distance of transportation results in a further decrease along
the concentration gradients for both species, see Fig. 6b and c.
Thiswouldinturncauseregionsofhighalkalinityand
depletion of CO
2
, respectively. We suspect that the increase
in Nafion content causes a quite similar drop in reactant
concentration throughout the layer. Here, as we have shown
from our ORR measurements, an extensive content of Nafion in
the catalytic layer presents a barrier for the mass transport of
reactants, e.g. O
2
or CO
2
, towards the Cu sites. Such impedi-
ment of the mass transport can be rationalized by a slower
through-plane transport of reactants (CO
2
and HCO
3
) and
steeper associated concentration gradients perpendicular to
the catalytic layer, see Fig. 6b and d. Essentially, the outcome
is similar to the increased particle loading and results in
regions of high alkalinity and depletion of CO
2
at high
ionomer content. Additionally, both, the Nafion content and
particle catalyst loading influenced the apparent morphology
of the catalyst layer (see Fig. S5 and S10, ESI†), which has
often been reported in literature to change the catalytic
selectivity. However, given the comparatively small change
in double layer capacitance (see Fig. S6 and S7, ESI†), we take
view that such morphological changes did not critically
influence the overall performance. A more detailed discus-
sion on the change in morphology can be found in the ESI†of
this work.
Finally, the effect of an increased KHCO
3
bulk concentration
can also be rationalized by a discussion of the HCO
3
concen-
tration gradient. Here, the generally higher HCO
3
concen-
tration offers a higher buffer capacity and can in turn reduce
the pH gradient throughout the catalytic layer. An additional
effect could be caused by the sensitivity of the physical solubility
of CO
2
for the ionic strength, which could lead to a depletion of
CO
2
reactant at high KHCO
3
concentrations.
In line with the proposed emergence of concentration
gradients, as suggested within the present work and visualised
in Fig. 6, previous studies reported on considerable local
deviations from bulk reactant concentrations during CO
2
RR
based on calculations from transport models. Here, most
studies focused on the electrode to catholyte interface and
described a depletion in CO
2
concentration and an increase
in OH
and CO
32
concentration in this near-electrode surface
region. The magnitude of the emerging concentration gradients
was suggested to be heavily dependent on the diffusion layer
thickness, which is highly sensitive to convective interferences
through effects such as buoyancy.
12,14,31,44
Studies that addition-
ally included the thickness of an extended catalyst layer in their
transport models reported on an inhomogeneous reactant
concentration throughout the catalyst layer during high-rate
electrolysis, which is qualitatively in agreement with our propo-
sition depicted in Fig. 6.
26,32,45
Concentration gradients establish ‘‘selectivity zones’’
As a consequence of the through-plane concentration gradients
for CO
2
and pH within the catalytic layer, we propose zones of
distinctly different CO
2
RR selectivity as a function of distance
from the MPL substrate in direction of the catholyte, shown in
Fig. 7. Here, a schematic cross section of a GDE with indica-
tions for the suggested change in local reaction environment
and catalytic selectivity dependent on the spatial location
within the catalytic layer is depicted. The zone closest to the
MPL shows the highest proximity to the gaseous CO
2
feed and
furthest distance from the bulk KHCO
3
electrolyte, which
results in a region of high pH and CO
2
concentration. With
increasing distance from the MPL, those conditions reverse
and can result in a CO
2
-deficient zone with lower pH value.
Our observed experimental catalytic CO
2
selectivities directly
correlate and support the zone model. Here, the region of high
pH and CO
2
concentration directly adjacent to the MPL gives
optimal conditions for selective production of pH independent
CO
2
RR (C
2+
) products, as ethylene. With further distance from
the MPL, the pH value decreases and the catalytic selectivity can
shift towards CO
2
RR products, which are preferred at higher
proton concentration (C
1
), e.g. CH
4
. Finally, in the outermost
region of the catalytic layer CO
2
concentration might become
deficient, which shifts the catalytic selectivity towards an
increased competition by HER. The proposed ‘‘selectivity zone’’
model is qualitatively in line with recent computational studies
that report on pronounced differences in spatial production
rates for individual CO
2
RR products throughout the extended
structure of the catalyst layer. Based on micro kinetic calculations,
the studies revealed pronounced differences in local reaction rates
that arise from a non-uniform reactant distribution. The reactant
concentration gradients have been described to establish in-plane
as well as through-plane of the catalyst layer, causing a complex
spatial distribution in CO
2
RR and HER activity.
26,32,45,46
Based on the proposed selectivity zones, a selective CO
2
RR
electrolyzer for production of C
2+
compounds in buffering
electrolytes would require the deliberate introduction of an
impediment for the transport of buffering anions. This could
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be realized by controlling the structure of the GDE through an
increased thickness and a higher density of the catalytic layer, if
the transport of CO
2
remains sufficient. Next to the discussed
parameters within this work, further structural control of the
catalytic layer could be achieved by variations in the deployed
catalyst particle size or introduction of conductive supporting
materials of a defined porosity. Additionally, operation condi-
tions could achieve a similar effect and contribute for further
increase in C
2+
selectivity. Here, lowering electrolyte flow-rates
and avoiding bubble formation at the electrode to electrolyte
interface could reduce the interfacial convection and in turn
lead to creation of an increased hydrodynamic boundary layer
thickness, further limiting the transport of buffering anions.
We hypothesize that the notion of spatial ‘‘selectivity zones’’
(selectivity zone model) has a broader validity; as a result of
this, the structure–selectivity-relationships described here
should be also applicable to other (non-Cu) CO
2
RR systems
or even to other reaction processes in GDEs. Similar interrela-
tions of catalyst layer thickness and CO
2
RR selectivity in
buffered electrolytes have been reported for CO evolving sys-
tems of Ag and Au catalysts, as well as for systems deploying Sn
particles for selective formation of HCOO
.
25,47–51
At its core,
the selectivity zones model describes a dependency of the HER
and CO
2
RR reaction rates on the spatial reactant concentration
due to concentration overpotentials and accordingly should
apply to various systems in the field of electrocatalytic CO
2
RR.
Conclusion
In this study, we have investigated relations between electrode
structure, more specifically, catalyst loading and Nafion content,
and the resulting product selectivity of Cu-based CO
2
RR electro-
lysisathighcurrentsinabulkpH-neutral flow-electrolyzer.
We found that with increasing catalyst loading, that is catalyst
layer thickness, or Nafion content the production of pH-sensitive
products (e.g. H
2
,CH
4
and HCOO
) could be suppressed and C
2+
species were produced more selectively. To explain this, we
showed that the Nafion content influences the mass transport
using an ORR limiting current analysis at varying oxygen partial
pressure as a diagnostic tool. We concluded that such changes in
mass transport define and control the local reaction environment
in form of pH and CO
2
concentration, and hence can be used to
deliberately tune the reaction selectivity. Here, concentration
gradients in through-plane direction of the porous catalytic layer
are more pronounced and shift the observed catalytic selectivity
during CO
2
RR electrolysis. Varying the KHCO
3
electrolyte concen-
tration showed that the selectivity of the system is highly sensitive
to the concentration of the buffering media and agreed with our
proposal that the local pH variations are crucial in determining
the CO
2
RR selectivity. Our study demonstrates how the structure
of the catalytic layer is a key parameter to influence local mass
transport and provides an effective way to tune the selectivity
during pH neutral (bulk) CO
2
RR electrolysis at high currents.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work received funding by the German Federal Ministry of
Education and Research (Bundesministerium fu
¨r Bildung und
Fig. 7 Schematic cross section of a Gas Diffusion Electrode (GDE) of a CO
2
electrolyzer comprising the Gas Diffusion Layer, GDL (including its
Microporous Layer, MPL) and Cu catalyst layer (not drawn to scale). Focus is on the cross section of the enlarged Cu catalyst layer illustrating the concept
of spatial ‘‘selectivity zones’’. Zones are denoted by their major reduction products (‘‘C
2+
’’ blue, ‘‘C
1
’’ aqua, and ‘‘H
2
’’ green). Origin of the zones are the
spatial through-plane variations in local pH (proton activity, red arrow) and local CO
2
concentration (CO
2
availability, blue arrow). The red and black
spatial distribution curves illustrate schematically the production rate of hydrogen and the production rate ratio of C
2+
/C
1
.
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Forschung, BMBF) under grant #033RC004E – (‘‘eEthylene’’).
The research leading to these results has received funding from
the European Union’s Horizon 2020 research and innovation
program under grant agreement no. 851441, SELECTCO2. The
research leading to these results has received funding from
the European Union’s Horizon 2020 research and innovation
program under grant agreement no. 101006701, EcoFuel.
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