A Highly Porous Copper Electrocatalyst for Carbon Dioxide Reduction

Article abstract of DOI:10.1002/adma.201803111

Electrochemical reduction of carbon dioxide (CO₂) is an appealing approach toward tackling climate change associated with atmospheric CO₂ emissions. This approach uses CO₂ as the carbon feedstock to produce value-added chemicals, resulting in a carbon-neutral (or even carbon-negative) process for chemical production. Many efforts have been devoted to the development of CO₂ electrolysis devices that can be operated at industrially relevant rates; however, limited progress has been made, especially for valuable C₂₊ products. Herein, a nanoporous copper CO₂ reduction catalyst is synthesized and integrated into a microfluidic CO₂ flow cell electrolyzer. The CO₂ electrolyzer exhibits a current density of 653 mA cm^?2 with a C₂₊ product selectivity of ≈62% at an applied potential of ?0.67 V (vs reversible hydrogen electrode). The highly porous electrode structure facilitates rapid gas transport across the electrode–electrolyte interface at high current densities. Further investigations on electrolyte effects reveal that the surface pH value is substantially different from the pH of bulk electrolyte, especially for nonbuffering near-neutral electrolytes when operating at high currents.

Full text of DOI:10.1002/adma.201803111

CommuniCation
CO Electroreduction
www.advmat.de
2
A Highly Porous Copper Electrocatalyst for Carbon
Dioxide Reduction
Jing-Jing Lv, Matthew Jouny, Wesley Luc, Wenlei Zhu, Jun-Jie Zhu, and Feng Jiao*
fine control of production rates, wide scal-
Electrochemical reduction of carbon dioxide (CO ) is an appealing approach
2
ability of modular electrolyzer designs,
and the potential to produce a variety of
toward tackling climate change associated with atmospheric CO emissions.
2
[
7–9]
high-value products.
More importantly,
This approach uses CO as the carbon feedstock to produce value-added
2
this technology can be readily powered by
carbon-free energy sources such as wind,
solar, and nuclear, providing a zero-CO2
emission (or even negative) pathway for
commodity chemical production. A recent
scientific study on photovoltaic (PV)
clearly showed a decrease of PV electricity
price over time with a projected PV elec-
tricity price as low as $0.03 per kWh in the
chemicals, resulting in a carbon-neutral (or even carbon-negative) process for
chemical production. Many efforts have been devoted to the development of
CO electrolysis devices that can be operated at industrially relevant rates;
2
however, limited progress has been made, especially for valuable C2 prod-
+
ucts. Herein, a nanoporous copper CO reduction catalyst is synthesized and
2
integrated into a microfluidic CO flow cell electrolyzer. The CO electrolyzer
2
2
−2
exhibits a current density of 653 mA cm with a C2 product selectivity of
+
≈62% at an applied potential of −0.67 V (vs reversible hydrogen electrode).
[10]
near future. A similar trend also holds
The highly porous electrode structure facilitates rapid gas transport across
the electrode–electrolyte interface at high current densities. Further investiga-
tions on electrolyte effects reveal that the surface pH value is substantially
different from the pH of bulk electrolyte, especially for nonbuffering near-
neutral electrolytes when operating at high currents.
for wind energy with an electricity price
already being ≈$0.02 per kWh.[ Thus,
the low electricity price makes electro-
11]
chemically driven CO utilization technol-
2
ogies potentially profitable for commercial
[
12]
applications.
Recent studies in electrocatalytic CO2
reduction have been primarily focused
Current commodity chemicals are largely based on fossil fuel–
derived carbon sources such as coal and crude oil. Using carbon
on high-value multicarbon (C ) products, such as ethylene,
2+
[
13–16]
ethanol, and n-propanol.
Among all the catalysts, copper
dioxide (CO ) as an alternative carbon feedstock is an attractive
(Cu) is the most widely studied CO reduction catalyst with
2
2
[
17–21]
approach toward tackling CO2 emissions in the chemical
industry because it can drastically reduce or even result in nega-
tive carbon footprint,[ whereas traditional chemical processes
such as steam methane reforming and coal gasification are
a relatively high C2+ selectivity.
Therefore, many efforts
have been devoted to exploring nanostructured Cu catalysts
1–4]
[14,22,23]
[24,25]
[26,27]
such as nanoparticles,
and nanopores
nanofoam,
nanowires,
[
28]
to further elucidate the structure-property
carbon intensive. CO reduction can be performed through bio-
correlation for CO2 reduction over Cu catalysts. However,
most of these investigations are typically performed in batch-
cell configurations which have some technical limitations.
For example, the low solubility of CO2 in electrolyte (usu-
2
logical, thermochemical, photochemical, and electrochemical
[3,5,6]
means, each of which has been widely studied.
Among
all potential approaches, electrochemical conversion of CO2
has attracted much attention because this technology has sev-
eral advantages over other competitive technologies, including
ally an aqueous solution) greatly limits the maximum CO
2
−
2
reduction current density to ≈40 mA cm , making it difficult
to examine catalytic behavior at more practical current densi-
−
2 [29,30]
ties (>200 mA cm ).
In addition, because batch-cell type
J.-J. Lv, M. Jouny, W. Luc, Dr. W. L. Zhu, Prof. F. Jiao
Center for Catalytic Science & Technology
Department of Chemical & Biomolecular Engineering
University of Delaware
Newark, DE 19716, USA
E-mail: jiao@udel.edu
studies often utilize CO2 saturated electrolytes, it is not possible
to use alkaline electrolytes because hydroxide ions strongly react
with dissolved CO to form carbonates. In order to overcome
2
these challenges associated with batch-cell type configurations,
[
31]
a flow cell configuration can be utilized,
which has been
recently demonstrated for silver catalyzed CO reduction to
J.-J. Lv, Prof. J.-J. Zhu
2
[
32,33]
State Key Laboratory of Analytical Chemistry for Life Science
School of Chemistry and Chemical Engineering
Nanjing University
CO.
Despite these efforts, there is currently no systematic
flow cell study on how nanostructuring and electrolyte can
affect CO reduction properties of Cu catalysts, especially in
2
Nanjing 210093, P. R. China
alkaline electrolytes.
In this work, we synthesized a nanoporous Cu catalyst with a
pore size of 100–200 nm and examined its catalytic properties for
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201803111.
DOI: 10.1002/adma.201803111
CO reduction in a microfluidic electrolysis cell. At an applied
2
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Figure 1. Characterization of nanoporous Cu catalyst. a) A typical SEM image, b) a typical HRTEM image (red dash line indicates the rough particle
surface), c) PXRD patterns (standards were also shown for comparison), and d) XPS spectra (CuO and Cu standards were added for comparison).
potential of −0.67 V versus reversible hydrogen electrode (RHE),
CuO (Figure S2f, Supporting Information). N2 adsorption–
desorption measurement showed a Brunauer–Emmett–Teller
surface area of 7.02 m g (Figure S3, Supporting Information).
After deposition of the CuO sample onto a gas diffusion layer
(GDL) and an in situ reduction treatment, a highly porous
network was formed (Figure 1a), an essential property that is
the catalyst exhibited a total Faradaic efficiency (FE) of 79% for
−
2
2
−1
CO reduction products at a current density of 653 mA cm .
2
An FE of 62% for C2+ products was achieved together with a C
partial current density exceeding 400 mA cm , which is among
the highest C2+ partial current densities that have ever been
2+
−2
obtained in CO electrolysis. Moreover, a variety of electrolytes
needed during high-rate CO electrolysis due to the substantial
2
2
(
KOH, KHCO , KCl, and K SO ) were also investigated, and the
generation of gaseous products at the gas diffusion electrode
interface.
3
2
4
results demonstrated that CO reduction in alkaline electrolyte
2
(
i.e., KOH) exhibited the highest C2+ FE. In the case of non-
The atomic structure of the Cu catalyst was further exam-
ined using high resolution transmission electron microscopy
(HRTEM) and PXRD techniques. The typical HRTEM images
are shown in Figure 1b and Figure S4 in the Supporting
Information where a polycrystalline nature of this catalyst
was observed. The image clearly shows both metallic Cu and
CuO domains, in which the particles were mostly metallic Cu
whereas the surface was dominated by CuO. Additionally, the
lattice fringes corresponding to Cu(111) were widely spread
out through the sample, indicating the high population of the
buffering electrolytes, the significant change in bulk pH due to
the generation of hydroxide ions during high-rate electrolysis
illustrates the uncertainty of the actual pH at the electrode–
electrolyte interface during operating conditions and demon-
strates the challenges associated with using the RHE scale. We
also emphasize the need for further modeling of the complex
triple-phase boundary at the electrode–electrolyte interface for a
CO flow cell electrolyzer.
2
Nanoporous Cu catalyst was synthesized by precipitating a
Cu salt in an ammonia solution, followed by thermal annealing
and in situ electrochemical reduction. In the first precipita-
[
34,35]
Cu(111) facet.
These observations were in good agree-
ment with the PXRD analysis, where diffraction peaks for both
metallic Cu and CuO phases were identified (Figure 1c). For
the peaks corresponding to the Cu phase, the Cu(111) peak
was much higher than the Cu(200) peak, suggesting the pre-
tion step, interconnected Cu(OH) nanorods were synthesized
2
(Figure S1a, Supporting Information) to form a fiber-like net-
work (Figure S1b, Supporting Information). The high phase
purity of Cu(OH) was confirmed by powder X-ray diffraction
[
36,37]
dominant Cu(111) facet in the sample.
The oxidation state
2
(
PXRD) analysis (Figure S1c, Supporting Information). The
of Cu on the catalyst surface was also analyzed using X-ray
photoelectron spectroscopy (XPS) technique. The XPS results
(Figure 1d) showed a surface consisting of mostly metallic Cu
following thermal annealing step transformed the Cu(OH)2
nanorods into porous CuO with 3D interconnected pores of
1
2+
00–200 nm in diameter (Figure S2a–e, Supporting Information).
with the presence of Cu . It is important to note that because
The PXRD pattern confirmed the phase purity of monoclinic
nanosized Cu can be readily oxidized by O , it is possible that
2
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Figure 2. a) A diagram of the microfluidic electrolyzer with a zoomed-in schematic of the electrode–electrolyte interface illustrating how a highly porous
structure facilitates bubble management, b) a cross-sectional SEM image of the nanoporous copper catalyst on GDL, and c) the influence of catalyst
−
2
loading on the applied potential and FE at a constant current density of 200 mA cm .
−
2
the observed CuO phase was formed during sample handling
in air following in situ reduction. Future studies using surface
sensitive operando techniques are required to determine the
true surface nature of catalyst under reaction conditions.
The as-synthesized nanoporous Cu catalyst was investigated
in a microfluidic flow cell electrolyzer, which was composed of
two compartments for liquid electrolytes (catholyte and anolyte)
a catalyst loading of 0.6 mg cm , stable current profiles are
evident at a wide range of potentials (Figure 3a). The highest
−
2
current density (656 mA cm ) was achieved at an applied
potential of −0.66 V versus RHE (all the voltages are on the
RHE scale unless stated otherwise).
Figure 3b shows the correlation between C2+ FE, C2+ partial
current density, and applied potential, in which C2+ produc-
tion through CC coupling reaction is clearly favored at high
and one gas compartment for CO delivery and gas product
2
collection (Figure 2a). This configuration allowed for CO to
overpotentials. At −0.67 V, we have successfully obtained a C
2
2+
−2
be abundantly fed to the catalyst surface at the complex triple-
phase boundary of the electrode–electrolyte interface which
FE of 62% with a C2+ current density of 411 mA cm . More
importantly, a high CO reduction FE of 79% was also achieved
2
allowed for the investigation of CO electrolysis at high current
simultaneously with the high current density at −0.67 V.
2
densities. However, one major challenge is managing the trans-
port of gaseous products through the gas diffusion electrode
during high rates of electrolysis to prevent bubble accumu-
lation that would otherwise block catalytic sites. Therefore,
maintaining a degree of porosity in the gas diffusion electrode
is critical for allowing gaseous products to rapidly exit the
reactor while achieving high current densities.[ The porosity
of the electrode is demonstrated by the cross-sectional field
emission scanning electron microscopy (SEM) image shown in
Figure 2b. As such, we investigated the loading effect of nano-
Product analysis showed that the majority of CO reduction
2
products were ethylene (38.6%), ethanol (16.6%), CO (14.7%),
and n-propanol (4.5%) (Figure 3c). A small amount of acetate
and formate were also produced. At less negative potential
(vs RHE), the ethylene selectivity dropped significantly, whereas
both CO and H selectivities increased considerably. The trend
2
38]
suggests that CC coupling is promoted at high overpotentials
through the further conversion of surface bound CO, while
suppressing hydrogen evolution during CO electrolysis.
2
A comparison of the values in this work against literature
porous copper by conducting CO electrolysis under a constant
values is shown in Figure S6 and Table S1 in the Supporting
2
−2
[13,14,24,27,39–45]
current density (200 mA cm ) with different catalyst loadings
and the results are summarized in Figure 2c. An optimum
Information.
The nanoporous copper catalyst
performance is among the best that has ever been achieved
for CO electroreduction to C products. Attempts to operate
−2
C2+ FE was observed with a catalyst loading of 0.6 mg cm .
2
2+
−2
Furthermore, for a catalyst loading of 1.0 mg cm , the current
profile was unstable due to insufficient porosity causing
gaseous product bubbles to accumulate and block catalytic sites
the cell at higher overpotentials failed because the current pro-
file became highly unstable, which was predominantly caused
by electrolyte flooding and high-rate gas evolution at cathode.
(
0
Figure S5, Supporting Information). Therefore, we chose the
.6 mg cm catalyst loading for the rest of investigations. At
Additionally, the stability of CO electrolysis using nanoporous
Cu catalyst was investigated by conducting a continuous CO2
2
−
2
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Figure 3. a) Chronoamperometric curves at various potentials for CO reduction on nanoporous Cu in 1 m KOH electrolyte, b) C partial current
2
2+
density for electrolysis of nanoporous Cu in 1 m KOH electrolyte, c) corresponding FEs, and d) the voltage profile and C FEs at a constant current
2+
−
2
density of 200 mA cm .
−
2
−
electrolysis under constant current mode at 200 mA cm . The
results (Figure 3d) showed a very stable potential profile over
OH anions generated at cathode was negligible in comparison
−
to the total quantity of OH in the bulk electrolyte.
2
h without any significant voltage change. The product distri-
The CO electrolysis results are presented in Figure 4a,b,
2
butions were almost identical over the course of 2 h electrolysis
except the first 20 min, which could be due to initial activation
where we calculated the applied potential at each current den-
sity using the pH value of the bulk electrolyte after electrolysis
(Table S2, Supporting Information). Among all four electro-
lytes, the performance of KOH electrolyte was substantially
better than those of other electrolytes in terms of C2+ current
densities, overpotentials, and C2+ FE (Figure 4). A detailed
product analysis for each electrolyte is provided in Table S3
in the Supporting Information. We attempted to operate the
electrolyzer in KCl and K SO electrolytes at higher overpoten-
(Figure S7, Supporting Information). Both the morphology and
the composition of the nanoporous Cu electrode were main-
tained as indicated from postreaction SEM and XPS analysis
(Figure S8, Supporting Information).
The influence of electrolyte and pH has been previously mod-
eled and studied on Cu electrodes in batch-type electrochemical
configurations.[
46–49]
However, these studies were conducted at
2
4
relatively low current densities and are unable to utilize alkaline
tials (i.e., higher current densities); however, the electrolyzer
experienced high resistance, which quickly overloaded the
system. The high resistance is likely due to the poor ionic con-
ductivity of membrane in KCl and K SO electrolytes. We also
studied both anion exchange membrane and cation exchange
membrane, with neither of them being conductive enough
for high current densities. Future studies using a suitable
membrane or a membrane-less cell are necessary to explore
the CO2 electrolysis performances using near-neutral pH
electrolytes.
electrolytes that have been shown to enhance CO reduction on
2
[
32,50]
Ag and Au catalysts.
Herein, we investigated the catalytic
properties of nanoporous Cu in four different electrolytes, i.e.,
KOH, KHCO , KCl, and K SO . To isolate the anion effect, we
2
4
3
2
4
+
held the K concentration constant at 1 m for all the electrolytes.
The bulk pH values were measured before and after electrolysis
at each applied potential and are presented in Table S2 in the
Supporting Information. For the KOH and KHCO electrolytes,
3
no significant pH changes were observed after electrolysis;
whereas both KCl and K SO electrolytes showed a substantial
As evidenced by the significant increase in measured bulk
2
4
−
increase in pH, a change up to 4 pH units at high current den-
pH, the OH concentration at the catalyst surface is likely
−
sities. This is likely due to large quantity of hydroxide (OH )
much higher than that of the bulk electrolyte under CO R
2
[
30,46,51,52]
anions produced at cathode during high-rate CO electrolysis
conditions.
Therefore, a better method is needed to
2
which caused a significant increase in bulk pH for nonbuff-
ering electrolytes (i.e., KCl and K SO ). In contrast, a smaller
estimate the applied potentials on the RHE scale that accurately
reflects the true pH at the electrode–electrolyte interface. At the
alkaline triple-phase boundary in the flow cell, OH hydrates
2
4
−
increase in bulk pH was observed in 1 m KHCO due to its buff-
3
ering capabilities. In the case of KOH electrolyte, the amount of
CO2 to form carbonates, leading to a complicated transport
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Figure 4. a) Total current density and b) C2+ FE for CO R of nanoporous Cu in different electrolytes. Note that the potentials (vs RHE) were estimated
2
using the pH value of electrolyte exiting the flow cell. c) Calculated surface pH versus current density for different electrolytes and d) total reduction
current density versus potential based on the calculated surface pH.
problem involving the diffusion of these reactive species
As we mentioned previously, operating a microfluidic CO
2
electrolyzer at current densities higher than 650 mA cm− is
2
between the CO /electrolyte interface near the catalyst surface
2
and the bulk electrolyte. We estimated the surface pH for
each electrolyte tested using a transport model similar to that
recently demonstrated by Dinh et al. (see Supporting Informa-
tion for calculation details).[ The relationship between the
applied current density and calculated surface pH for each elec-
trolyte tested is shown in Figure 4c. For 1 m KOH, the surface
pH is slightly reduced relative to the bulk pH due to significant
carbonate formation near the catalyst surface. In contrast, the
surface pH for the nonbuffering electrolytes (1 m KCl and 0.5 m
K SO ) is significantly higher than the bulk pH for all current
challenging, which is due to the fact that CO electrolysis at high
2
rates (i.e., high current densities) produces tremendous amount
of gaseous products that disturb and interrupt gas and ion diffu-
sion pathways to/away from the catalytic sites. As a result, large
quantity of liquid electrolyte is forced to the gas compartment
causing significant flooding and internal resistance fluctuation.
Although we have successfully achieved a stable operation at
45]
−2
200 mA cm for more than 2 h, it is considerably difficult to
maintain an efficient electrode–electrolyte interface at higher
rates to ensure that both gaseous CO and liquid electrolyte are
2
4
2
densities. Furthermore, there is a strong correlation between
the calculated surface pH and the production of C2+ products
in good contact with the solid catalyst.
Another technical challenge associated with the microflu-
(
Figure 4b), which is consistent with theoretical suggestions
idic CO electrolysis cell is salt accumulation at the electrode–
2
−
[45,49]
that near-surface OH facilitates CC coupling for CO R.
electrolyte interface, especially when a strong base (e.g., KOH)
is used as catholyte. The performance degradation due to salt
accumulation at the electrode–electrolyte interface has been
2
When the applied potential for CO R is referenced to RHE
2
using the calculated surface pH (Figure 4d), the overpotential
for KCl and K SO4 became less than that of KHCO . The
[50]
previously reported in the case of Au catalyst. In this work,
a highly porous Cu catalyst was employed to improve the acces-
sibility of catalyst surface (Figure 2a). However, the formation
of carbonate salt still slowly blocked the pores of the catalyst
and the GDL, causing substantial voltage losses and reactant/
product transport issues at the interface.
2
3
same set of results was also plotted using the potentials
versus Ag/AgCl for comparison (Figure S10, Supporting
Information). Interestingly, the trend in activity matches to
some degree with the trend in electrolyte conductivity with
KOH>KCl>K SO >KHCO (Figure S11, Supporting Informa-
2
4
3
tion). The conductivity may likely have a significant effect on
the total current density by increasing the active triple-phase
boundary area due to a lower resistance between thin electro-
Addressing these issues requires innovations in reactor engi-
neering and catalyst design. For example, tuning the hydro-
phobicity of the GDL support and/or catalyst surface could
potentially enhance the gas transport across the gas diffusion
[
30]
lyte coating within the catalyst layer and the bulk electrolyte.
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electrode. Previous study has shown that the hydrophobicity
of GDL strongly affected the cell performance in the case of
Flow Cell Electrolysis: Electrochemical measurements were conducted
on an Autolab potentiostat (PG128N) in a three-electrode system. In this
study, the carbon paper with a microporous layer (Sigracet 29 BC, Fuel
Cell Store) was used as the gas diffusion layer for supporting catalyst,
collecting current and acting as a pathway for gas from flow channels
to the catalyst surface. The IrO2 coated GDL (loading: 0.5 mg cm )
was used as the counter electrode and Ag/AgCl (saturated KCl) as
the reference electrode. The nanoporous-Cu-coated GDL was applied
[53]
CO reduction to CO. Using the same strategy, the efficiency
2
of electrolyzer cell dedicated to C2+ products could be further
improved. Moreover, understanding the pH and electrolyte
effects at the electrode–electrolyte interface is also crucial to the
design of new generation flow cell electrolyzers with a more
robust interface. Strategies to minimize carbonate formation at
interface by controlling the interfacial pH will benefit long-term
operation of the microfluidic cell. Additionally, a more selective
catalyst than Cu could suppress salt product formation (e.g.,
formate and acetate) if they are not desired products.
−2
as the working electrode, which was prepared by hand-painting the
nanoporous Cu catalyst ink as in previously reported work.[
55,56]
The GDL
was weighed before and after deposition to record its actual catalyst
loading. To prepare the catalyst ink, 25 mg of catalyst was ultrasonically
dispersed in the mixture of 3 mL of n-propanol, 500 µL of carbon
nanotube (CNT) solution (5 mg multiwalled CNT (MWCNT) (>98%,
Sigma-Aldrich) dispersed in 5 mL tetrahydrofuran (THF)) and 20 µL of
(10 wt% aqueous solution, Fuel Cell Store). The MWCNTs were used to
facilitate oxide reduction and did not contribute to the electrocatalysis
(Figure S12, Supporting Information). Then, the mixture was sonicated
for 30 min.
In summary, we have synthesized a nanoporous Cu catalyst,
which exhibited high performance in a microfluidic CO elec-
2
trolyzer. At an applied potential of −0.67 V, we have achieved a
current density of 653 mA cm−2 with a C2+ selectivity of 62%,
which is among the best performances that have ever been
CO2 electrolysis was performed in a three-channel flow cell with
achieved for CO electroreduction to C products. The system-
3
2
2+
channels of dimension 2 × 0.5 × 0.15 cm . The electrode area was
1 cm2 and the electrode to membrane distance was 1.5 mm. An
external Ag/AgCl reference electrode (Pine Research) located ≈5 cm
from the cathode was used to measure the cathodic half-cell potential.
atic study of electrolyte effect showed that KOH exhibited much
better performance than bicarbonate and other nonbuffer near-
neutral electrolytes, suggesting that CC coupling reaction on
Cu are favored at high pH values. Further efforts are required
All potential measurements were converted to the RHE based on the
θ
following formula ERHE = EAg/AgCl + E
+ 0.059 × pH (in volts).
Ag/AgCl
to improve the interface robustness of the CO electrolyzer in
2
The pH values of bulk electrolyte after electrolysis were used for RHE
conversions unless stated otherwise. The device was fabricated from
order to achieve long-term operation at industrially relevant
rates.
acrylic and included the gas channel for flowing CO , the anode and
2
cathode channels for flowing electrolyte, the membrane for separating
the anode and cathode, and solid end pieces (Figure S13, Supporting
Information). Poly(tetrafluoroethylene) (PTFE) gaskets were placed
between each component for sealing and the device was tightened using
six bolts. The gas flow rate into the flow cell was set as 10 sccm via
a mass flow controller (Brooks GF40). The catholyte flow rates were
controlled via a peristaltic pump (Cole-Parmer), with the catholyte
Experimental Section
Synthesis of Nanoporous Copper: All chemicals were of analytical
grade and used as received without further purification unless otherwise
noted. The nanoporous copper were fabricated through the annealing of
−
1
flow rate ranging from 0.1 to 1 mL min depending on the current
density (lower flow rates were used at lower current densities to allow
for sufficient accumulation of liquid products). The anolyte flow rate
Cu(OH) nanorods and the electrochemical reduction of the nanoporous
2
CuO. Firstly, the Cu(OH) nanorods were prepared by a literature
2
[
54]
−1
method.
Typically, 1 g of Cu(NO3)2 is first dissolved in 100 mL
was 2 mL min . An anion exchange membrane (FAA-3, Fumatech)
distilled water. Then, 30 mL NH ·H O (0.15 m) solution was added to
was used in KOH and KHCO3 electrolyte, and a proton exchange
membrane (Nafion, Fumatech) was used in KCl and K2SO4 electrolyte.
The backpressure of the gas in the flow cell was tuned to atmospheric
pressure by adding vacuum as controlled by a backpressure controller
(Cole-Parmer). Each reported data point reflected the mean of at least
three measurements, where the error bars were the standard deviation.
Gas products were quantified on a Multigas #5 GC (SRI Instruments)
equipped with a HayeSep D and Molsieve 5A columns connected
to a thermal conductivity detector (TCD) and a HayeSep D column
connected to a flame ionization detector (FID). Hydrogen was quantified
using TCD, while ethylene, carbon monoxide (for CO2 electrolysis), and
methane were detected on both FID and TCD. The low concentration of
ethylene and CO was detected by FID after using a methanizer. The FE
for products was calculated using the following equation:
3
2
the Cu(NO3)2 solution under constant stirring at room temperature.
A blue precipitate of Cu(OH)2 was produced when dropwise added
−
1
1
0 mL of NaOH (1 m) solution (≈2 mL min ) to the above solution to
adjust the pH value to 9–10. After 30 min, the blue Cu(OH) precipitate
2
was filtered washed several times to obtain a solid product, and then
dried in the vacuum oven for overnight. Secondly, the nanoporous
CuO was prepared by annealing the Cu(OH)2 nanorods in the static
−
1
air atmosphere at 500 °C for 2 h with a heating rate of 32 °C min .
At last, the nanoporous copper was obtained by the electrochemical
reduction of nanoporous CuO on GDL at a constant current density of
−
2
1
0 mA cm in the same cell as CO reduction, and the corresponding
2
applied voltages of different electrolyte were listed in the Table S7 in the
Supporting Information.
Materials Characterization: The morphology and microstructure were
characterized by SEM (Auriga, 1.5 kV), transmission electron microscopy
nFxV
jTot
FE
(
%
)
=
×100
(1)
(TEM), and HRTEM using a JEM-3010 TEM operating at 200 kV. The
SEM images were taken directly by coating the sample on GDL (Fuel
Cell Store). The TEM sample was acquired by scraping off from the
nanoporous copper coated GDL. PXRD measurements were conducted
on a D8 ADVANCE X-ray diffractometer (Bruker Corporation, America).
To analyze the composition near the surface of the nanoporous Cu
catalyst, the commercial CuO powder (98%, Sigma-Aldrich) and Cu foil
where n = # of electrons transferred
F = Faraday’s constant
x = mole fraction of product
V = total molar flow rate of gas
j
= total current
Tot
(99.999%, Alfa-Aesar) were measured as control materials. A Thermo
Liquid products were quantified using a Bruker AVIII 600 MHz NMR
Scientific K-Alpha XPS System was used and XPS fitting was conducted
by CasaXPS software with the adventitious carbon peak being calibrated
to 284.8 eV. All peaks were fitted using a Gaussian/Lorentzian product
line shape and a Shirley background.
spectrometer. Typically, 500 µL of collected diluted catholyte was mixed
with 100 µL of internal standard solution (20 ppm (m/m) dimethyl
1
sulfoxide (≥99.9%, Alfa Aesar) in D O). The 1D H spectrum was
2
measured with water suppression using a presaturation method.
Adv. Mater. 2018, 1803111
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financial support by the Department of Energy under Award Number
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