Gas diffusion electrodes (GDEs) with high electrochemically active surface areas (ECSAs) and triple-phase boundaries for efficient gas, electron, and ion transport offer a unique opportunity for high-rate electrochemical CO reduction (COR) in relative to traditional aqueous configurations. Cu-nanoparticle-based GDEs were fabricated by applying a mixture of carbon powders, copper acetate aqueous solution, and Teflon onto a Cu gauze substrate. The catalyst-coated substrate was air-dried, mechanically pressed, and subsequently annealed under forming gas to produce GDEs. Two distinctive types of GDE configurations, a flow-through configuration and a flow-by configuration, were constructed, characterized, and tested to quantitatively evaluate the effects of reactant gas transport on the activity and the selectivity of the GDE materials for COR. In the flow-through configuration, a high partial current density of 50.8 mA cm^(–2) for COR to C_2H_4 was achieved at −0.85 V vs RHE in 10 M KOH at −15 °C, while in the flow-by configuration with the same catalyst materials the partial current density for C_2H_4 generation was limited to <1 mA cm^(–2)

Han, Lihao

Xiang, Chengxiang

Zhou, Wuzong

Caltech Authors - Main

SUPPORTING INFORMATION FOR: 
High Rate Electrochemical Reduction of Carbon 
Monoxide to Ethylene Using Cu-Nanoparticle-
Based Gas Diffusion Electrodes 
Lihao Han,
1,2
 Wu Zhou,
1,2
 and Chengxiang Xiang*
1,2
 
1
 Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, CA 91125, 
USA 
2
 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 
91125, USA 
 
 
 
 
 
 
*To whom correspondence should be addressed: cxx@caltech.edu 
I. Fabrication 
To fabricate a GDE cell, a thin layer of Cu gauze (2 inch × 2 inch, Alfa Aesar, 100 mesh woven 
from 0.11 mm diameter wire) was used as the supporting substrate. Two layers of different 
slurries were applied onto the Cu gauze. The first slurry was consisted of 0.7 g C powders (Cabot 
Corporation, product number 1333-86-4) and 0.865 mL 60% Teflon (Fuelcell Store, product 
number 72500300, acting as adhesives) mixed in 6.48 mL DI water. It was transferred onto the 
Cu gauze and was pressed at 5000 psi for 2 min after drying in atmosphere. This procedure was 
repeated for two more times, providing a gas diffusion hydrophobic region once the final 
electrode was immersed in the electrolyte. The second slurry was consisted of 0.7 g C powders, 
0.73 g copper acetate monohydrate (Cu2(CH3COO)4(H2O)2) (98%, Steam Chemicals Inc., 
product number 93-2901), and 0.257 mL 60% Teflon mixed in 6.845 mL DI water. It was 
transferred onto the first slurry layer, and was pressed at 1000 psi for 2 min after drying in 
atmosphere. This procedure was repeated for two more times as well. Copper acetate reduction 
into Cu was realized by annealing the sample in forming gas (5% H2 and 95% N2, flow rate 0.5 
LPM) at 325 
o
C for 7 hours in an oven (10 
o
C min
-1
 ramping rate, Carbolite Gero Inc., CTF 
17/300), followed by a final pressing the assembly at 1000 psi for 30s. The sample was cut into 
small pieces, and pasted onto one end of a glass tube using epoxy (LOCTITE EA 9460, stable in 
high pH alkaline). The geometric active area of the GDE device was 6 mm in diameter. The 
sample was mounted with a thin flexible Cu wire through the glass tube in an H-shaped sealed 
reactor.  
In the flow-through GDE configuration, 20.0 sccm CO flew through the glass tube, Cu gauze, C 
powder and Cu particle layers successively in 10 M KOH electrolyte (98.60%, Amresco Inc., 
product number 0489, pH15), and the temperature of the electrolyte was controlled at around -15 
o
C in a cooling bath. In the flow-by GDE configuration, the gas inlet at the end of the glass tube 
in the previous configuration was sealed with epoxy, and the CO gas was injected from the side 
inlet along with rapid stirring to provide fresh CO-saturated electrolyte flowing by the GDE 
surface. 
A piece of nickel (Ni) coil was utilized as the counter electrode (CE) for oxygen evolution 
reaction (OER). The reference electrode (RE) is a conventional Ag/AgCl electrode (in 1M KCl, 
CH instruments Inc., CHI111_L), and a potential voltage was applied between the RE and GDE 
in the three-electrode mode using an SP-300 Potentiostat (Bio-Logic Scientific Instruments) with 
a 48 V voltage booster. A piece of AHO Selemion membrane (AGC Engineering CO. LTD.) was 
installed between the GDE/RE chamber for CORR and the CE chamber for OER in the H-
shaped cell. The injection CO gas (Airgas West Inc., research grade 4.0) was set at 20.0 sccm as 
an inlet flow rate into the cell by a controller (Alicat Scientific Inc., MC-100SCCM-D), and the 
outlet gas flow was washed and dried in two cells to get rid of the alkaline water vapor after the 
reaction. A flowmeter (Alicat Scientific Inc., M-100SCCM-D) connected to a computer was 
installed between the cell outlet and the GC inlet, and a flow between 19.8-20.1 sccm was 
monitored and recorded before injecting into the GC. 
II. Characterization 
Scanning Electron Microscope (SEM, FEI Inc., NOVA NanoSEM 450) with an integrated 
energy-dispersive X-ray (EDX) spectroscopy was introduced to analyze the morphology and the 
elements in the porous sample, respectively. Some particles were peeled off from the sample, 
distributed in acetone and the recollected using a Cu mesh for further morphology 
characterization by high-resolution transmission electron microscopy (HR-TEM, Tecnai G2, 
operating voltage 15kV). 
 
Figure S1. The original SEM image of Fig. 3(a) confirming the C powders were pressed densely 
onto the Cu gauze (a), and the zoomed-in part of the HR-TEM image in the Fig. 3(c) evidencing 
the lattice distance in the Cu material (b).  
 
The GDE material is assembled into the flow-through or flow-by configuration in the H-shaped 
cell, and the electrochemical performances were characterized. A scanning voltage between -0.8 
to -2.4 V vs. Ag/AgCl reference electrode was applied onto the GDE in the cyclic voltammetry 
mode, and the current for each voltage step was recorded. In the chronoamperometry mode for 
stability measurement, the potentiostat provided a constant voltage, and the current as a function 
of testing time was recorded. Fig. S2 shows the overtime cyclic voltammetry of both flow-
through and flow-by GDE configurations from -0.8 to -2.4 V vs. Ag/AgCl (0 to -0.9 V vs. RHE 
with IR-correction) at a scan rate of 40 mV s
-1
 in 10 M KOH. For both configurations, the CO 
flow rate was set to 20.0 sccm. The whole cell was immersed in a cooling bath and the 
temperature was controlled at around -15 
o
C. The initial (0 min) and final (210 min) curves for 
both configurations were plotted in Fig. 4c, and the current densities overtime were quite stable 
during the measurement despite of some increases as shown in Fig. S2.     
-2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8
-600
-500
-400
-300
-200
-100
0
100
j 
(m
A
 c
m
-2
)
V (V) vs. Ag/AgCl
Flow-through GDE
 0 min
 30 min
 60 min
 120 min
 150 min
 180 min
 210 min
-2.4 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8
-600
-500
-400
-300
-200
-100
0
100
j 
(m
A
 c
m
-2
)
V (V) vs. Ag/AgCl
Flow-by GDE
 0 min
 90 min
 210 min
 
Figure S2. The cyclic voltammetry evolution curves for the flow-through (a) and flow-by (b) 
GDE configurations operating in 10 M KOH at -15 
o
C (no IR-correction, no smoothen). 
 
An online gas chromatography (GC) customized from SRI instruments 8610C was customized. 
In a traditional dual-detector GC, the injected sample gas flows through a thermal conductivity 
detector (TCD) and then a flame ionization detector (FID) in a tandem configuration. The signals 
from the same type of gas can be displayed in both chromatography channels simultaneously. An 
obvious disadvantage of this configuration is the long analyzing time of one measurement. In 
order to increase the time resolution in the online system, we modified the in-series configuration 
into in-parallel configuration: H2, O2, N2, CH4 and CO were detected by the TCD successively 
after flowing through a 3-inch MS 5A column, and CO, CH4, CO2, C2H4 and C2H6 were detected 
by the FID successively after flowing through a 6-inch HAYESEP-D column and a methanizer 
converting CO and CO2 into detectable CH4.  
(a) (b) 
Ar (Ultra High Purity 5.0 Grade, Airgas West Inc.) was used as the carrier gas in the online GC 
system, and its inlet pressure was controlled at an optimized pressure of 20 PSI. A higher flow 
pressure would result in a faster measurement, and a lower flow pressure would lead to clearer 
peak separations in the chromatography and more accurate peak area integrations. For a shorter 
measurement period, the oven temperature was set at 90 
o
C constantly in order to save heating up 
and cooling down time. With all these tricks, one measurement period was reduced from 15 min 
to 7.5 min in our customized configuration.  
Fig. S3 shows some of our calibrated gas concertation as a function of peak area detected by the 
online GC in an optimal flow configuration. The high (close to 1) coefficient of determination 
ensures the accuracy in quantitatively determining the concentration of each type of sample gas. 
0 2000 4000 6000 8000
0
500
1000
1500
R
2
=0.99978
P
e
a
k
 a
re
a
H
2
 concentration (ppm)
0 2000 4000 6000 8000
0
20
40
60
80
100
120
R
2
=0.99672
P
e
a
k
 a
re
a
CO concentration (ppm)
 
0 500 1000 1500 2000 2500 3000 3500
0
5000
10000
15000
20000
P
e
a
k
 a
re
a
CH
4
 concentration (ppm)
R
2
=0.99948
0 500 1000 1500 2000 2500 3000 3500
0
10000
20000
30000
40000
R
2
=0.9992
P
e
a
k
 a
re
a
C
2
H
4
 concentration (ppm)
 
0 500 1000 1500 2000 2500 3000 3500
0
10000
20000
30000
40000
R
2
=0.99964
P
e
a
k
 a
re
a
C
2
H
6
 concentration (ppm)
 
Figure S3. Integrated peak areas from customized GC flowing configuration as a function of the 
concentration of some important calibration gas (H2, CO, CH4, C2H4 and C2H6) with known ppm 
values.  
 
III. Calculation 
The Faraday efficiency (FE) was automatically calculated by a program according to the ratio of 
the amounts of reduced electrons and provided electrons. 2 and 8 electrons were reduced from 
CO when 1 mol of H2 and C2H4 were produced, respectively. The program recorded the 
integrated peak area of the sample gases, converted them into gas concentration using the 
calibration coefficients every 7.5 min. The averaged flow rate (sccm, temperature compensated) 
and current (mA) values from the previous 7.5-min measurement period were adopted as input, 
and the FE values of each species were determined until the errors in the consequent GC 
measurement were within the ±0.1% range. The partial current density of C2H4 was calculated by 
multiplying the averaged current density with its FE in each measurement period. 
Fig. S4 shows the FE for COR products as a function of the applied voltage bias vs. RHE when 
the GDEs were in the flow-through and flow-by configurations in 10 M KOH at -15 
o
C. The 
majority of the COR products was hydrogen for both configurations (detected using online GC 
described above, and the ethylene FE was plotted in Fig. 5a), and liquid products from COR, 
such as ethanol, methanol and acetaldehyde, were also detected using a Thermo Scientific 
TRACE 1300 offline GC. However, due to the flow-through configuration, quantitative FEs for 
those liquid products were challenging to obtain and were labeled as unknown reduction 
products. 
 
-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3
0
20
40
60
80
100
Flow-through GDE
              C
2
H
4
                  H
2
 
      Unknown  
        Flow-by GDE
               C
2
H
4
 
                   H
2
 
        Unknown  
 
 
F
E
 (
%
)
V (V) vs. RHE
 
Figure S4. The FE for the reduction products, ethylene (red), hydrogen (pink) and some 
unknown products (gray) as a function of the applied voltage bias vs. RHE, when the GDEs were 
in the flow-through (solid) and flow-by (dash) configurations in 10 M KOH at -15 
o
C. 
 
The electrochemically active surface area (ECSA) of the GDE in each condition was estimated 
by determining the double-layer capacitance of the system from cyclic voltammetry 
measurement in the non-Faraday range (-1.2 to -1.0 V vs. Ag/AgCl) and varying the scan rate 
from 10, 20, 30, 40 to 50 mV s
-1
, as shown in Fig. S5(a). This non-Faradaic range is typically a 
0.1 V window around the open-circuit potential (OCP) value, and all measured current in this 
range is considered as a result of the double-layer charging.
1
 Based on this assumption, the 
charging current, ic, can be expressed as 
ic = νCDL (S1) 
where ν is the scan rate during the CV measurement, and CDL is the capacitance of 
electrochemical double layer. We took the flow-through GDE configuration at -2.0 V vs. 
Ag/AgCl at -15 
o
C condition as an example to show the ECSA calculation procedure. The anodic 
(red dot) and cathodic (blue square) charging currents measured at around -1.1 V vs. Ag/AgCl 
are shown in Fig. S5(b). The ECSA of the catalyst was calculated by dividing CDL by the specific 
capacitance of the sample 
ECSA = CDL / CS 
A general specific capacitance of 40 µF cm
−2
 in 10 M KOH.
1
 
-1.20 -1.16 -1.12 -1.08 -1.04 -1.00
-6
-4
-2
0
2
4
6
I 
(m
A
)
V (V)  vs. Ag/AgCl
 10 mV s
-1
 20 mV s
-1
 30 mV s
-1
 40 mV s
-1
 50 mV s
-1
(a)
0 10 20 30 40 50 60
-4
-2
0
2
4(b)
k
2
=-0.022±0.0034I 
(m
A
)
Scan rate (mV s
-1
)
k1=
0.05
38±0
.004
44
C
DL
 = (|k
1
|+|k
2
|)/2 = 0.0379 F
ECSA = C
DL
/ C
s
 = 948 cm
2
 
Figure S5. The cyclic voltammetry measurement of the flow-through GDE configuration poised 
at -2.0 V vs. Ag/AgCl in 10 M KOH at -15 
o
C using different scan rates (a), and the charging 
current as a function of CV scan rates (b). 
 
As plotted in Fig. S6, the ECSA of the flow-through GDE configuration, poised at -0.74 V vs. 
RHE in 10 M KOH at -15 
o
C, decreased gradually from 970 to 674 cm
2
 when the injection CO 
flow rate increased from 5 to 80 sccm. Under lower flow rate conditions, smaller gas pressure 
(S2) 
could not push the electrolyte down in the glass tube dramatically, resulting in larger triple-phase 
boundaries and bigger ECSA for the reaction. That’s why the ECSA of the flow-through GDE 
configuration could be a few thousands higher than its geometric surface area.  
0 20 40 60 80 100
600
700
800
900
1000
E
C
S
A
 (
c
m
2
)
CO injection flow rate (sccm)
Flow-though GDE
 
Figure S6. The electrochemical surface area as a function of CO injection flow rate in the flow-
through GDE configuration poised at -2.0 V vs. Ag/AgCl (-0.74 V vs. RHE) in 10 M KOH at -15 
o
C. 
 
REFERENCES 
(1)  McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. 
Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction 
Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347–
4357. 
 


High Rate Electrochemical Reduction of Carbon Monoxide to Ethylene using Cu-Nanoparticle-Based Gas Diffusion Electrodes

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High Rate Electrochemical Reduction of Carbon Monoxide to Ethylene using Cu-Nanoparticle-Based Gas Diffusion Electrodes

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