Supplementary Materials for - Science...2 Supplementary Methods 1. CO 2 and CO reduction on carbon...

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Supplementary Materials for

CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface

Cao-Thang Dinh,* Thomas Burdyny,* Md Golam Kibria,* Ali Seifitokaldani,* Christine M. Gabardo, F. Pelayo García de Arquer, Amirreza Kiani,

Jonathan P. Edwards, Phil De Luna, Oleksandr S. Bushuyev, Chengqin Zou, Rafael Quintero-Bermudez, Yuanjie Pang, David Sinton, Edward H. Sargent†

*These authors contributed equally to this work.

†Corresponding author. Email: [email protected]

Published 18 May 2018, Science 360, 783 (2018) DOI: 10.1126/science.aas9100

This PDF file includes:

Materials and Methods Figs. S1 to S28 Tables S1 to S13 References

mailto:[email protected]

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Supplementary Methods

1. CO2 and CO reduction on carbon gas-diffusion layers:

Electroreduction for the main figure results were performed in a flow-cell configuration consisting

of a gas-diffusion layer, anion exchange membrane (Fumasep FAB-PK-130) and nickel foam (1.6

mm thickness, MTI Corporation) anode. All experiments in Fig. 1 and 2 were deposited on top of

the microporous side of a Freudenberg gas-diffusion layer. The combined catalyst and diffusion

layer, anion exchange membrane and nickel anode were then positioned and clamped together

using polytetrafluoroethylene (PTFE) spacers such that a liquid electrolyte could be introduced

into the chambers between the anode and membrane as well as the membrane and the cathode.

Gaseous CO2 could then be passed behind the gas-diffusion layer and diffuse into the liquid

electrolyte present at the catalyst. In the catholyte stream a port drilled into the PTFE spacer is

present for a Ag/AgCl reference electrode to be positioned a specific distance from the working

electrode.

All CO2 and CO reduction experiments were performed using an electrochemical workstation

(Autolab PGSTAT302N) with a Ag/AgCl reference (with 3 M KCl as the filling solution). iR

compensation losses between the working and reference electrodes were measured using

electrochemical impedance spectroscopy (EIS) as shown in Table S4. Electrode potentials after iR

compensation were rescaled to the reversible hydrogen electrode (RHE) reference by:

𝐸𝑅𝐻𝐸 = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.197 𝑉 + 0.591 𝑥 𝑝𝐻 (1)

where the pH in Eq. (1) is determined using a reaction-diffusion model as described in Section 6

of the Supplementary Materials and shown in Table S4 for 1, 5 and 10 M KOH. For all reported

data the pH value used in Eq. (1) is taken at a current of 0 mA/cm2 as this represents the worst-

case scenarios for our reported potentials. For completeness, the predicted electrode pH as a

3

function of current density up to 25 mA/cm2 is shown in Fig. S3. The geometric area is used for

reporting all the experimental data in the report, employing the same convention for reporting FE’s

and current densities as previous works. All potentials reported here were obtained by averaging

over a timespan of at least 150 s for each applied current.

The electrolytes (KOH solution of various concentrations, 100 mL) were circulated through the

electrochemical cell using peristaltic pumps with a silicone Shore A50 tubing. The electrolyte flow

was keep at 10 mL min-1. The CO2 (Linde, 99.99%) flow was kept constant at 50 mL min-1 using

a mass flow controller. The reactions were run for at least 150 s before the gas products were

collected for analysis. KOH concentrations of 15 M could not reliable be evaluated as salt

precipitated on the gas-diffusion electrode, preventing accurate measurement of onset potentials.

The gas products from CO2 reduction (CO, H2, CH4 and C2H4) and CO reduction (H2, CH4 and

C2H4) were analyzed using a gas chromatograph (PerkinElmer Clarus 680) coupled with a thermal

conductivity detector (TCD) and a flame ionization detector (FID). The gas chromatograph was

equipped with a Molecular Sieve 5A and Carboxen-1000 column packed columns. Argon (Linde,

99.999%) was used as the carrier gas. All Faradaic efficiencies reported were averaged from at

least three different runs.

The onset potentials for CO and C2H4 are the lowest potentials at which the Faradaic efficiency

for CO and C2H4 are respectively higher than 1 ±0.3%. To measure the onset potential, the flow

rate of CO2 and CO feedstock was reduced to 2 – 5 mL min-1 so that the GC detector signals of

the products are at least 5 times higher than the noise signal. 13C-labelling experiments were

performed in the similar conditions as those of 12CO2. The ethylene product was analyzed using a

mass spectrometer (PerkinElmer Clarus SQ 8C) coupled with a HP PLOT-Q capillary column

(Agilent). A typical chromatograph for ethylene analysis is shown in Fig. S24.

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The liquid products were quantified using Nuclear magnetic resonance spectroscopy (NMR). 1H

NMR spectra of freshly acquired samples were collected on Agilent DD2 500 spectrometer in 10%

D2O using water suppression mode, with Dimethyl sulfoxide (DMSO) as an internal standard.

Sixteen second relaxation time between the pulses was used to allow for complete proton

relaxation.

The CO2 reduction experiments using Cu catalysts (10 nm, 25 nm, 1000 nm, and 1000 g) were

also performed in an H-cell configuration as controls. Running in 1 M KOH produced only H2 as

the concentration of CO2 is less than 10-10 M. The electrolytes for both the cathode and anode in

were then run in 0.1 KHCO3 to compare with prior conditions where peak ethylene selectivities

were achieved. CO2 gas was bubbled through the catholyte for at least 30 min to saturate the

electrolyte with CO2. The CO2 flow was controlled at 30 mL min-1. The reactions were performed

under potentiostatic mode with all samples showing peak ethylene selectivities between -0.95 V

to -1.2 V vs. RHE (after iR correction). The gas and liquid products were analyzed after 1 hour of

reaction using gas chromatography and NMR as described above. All samples showed

substantially reduced ethylene FE’s in the range of 30-35%, confirming that the high C2 selectivity

and efficiency originates from the combination of reaction interface design and the strong alkaline

electrolyte; rather than the nature of the catalyst itself.

2. CO2 reduction on polymer-based gas-diffusion layers:

Electrochemical experiments performed on the polymer-based gas-diffusion layer in Fig. 3 and 4

were performed using the same potentiostatic equipment, system orientation, flow conditions and

product analysis procedures as the carbon gas-diffusion flow-cell experiments described above.

5

For the long-term stability tests the operating potential was fixed at -1.8V vs. Ag/AgCl. The

electrolyte used was 7 M KOH and was changed every 12 hours during the test to maintain similar

ion concentrations and conductivity in the catholyte and anolyte flow channels. The pH of the

catholyte slightly changed (

6

Vienna ab initio Simulation Package (VASP) (40) was used to perform all the plane wave density

functional theory (DFT) computations. The projected augmented wave (PAW) approach (41)

together with the generalized gradient approximation (GGA) parametrized by Perdew, Burke and

Ernzerhof (PBE) (44) are employed. Three different crystalline facets, (111), (100) and (110) of

the pristine copper are approximated by a 4×4×4 slab in a 20 Å vacuum. Due to the vacuum, dipole

corrections are implemented. To resemble the real bulk material and the surface, respectively, two

bottom layers are fixed in their positions while the two top layers are free to move due to interaction

with the adsorbates. A cut-off energy of 400 eV for the plane wave basis sets and a 4 × 4 × 1 Γ-

centered Monkhorst-Pack mesh for the k-points sampling in the first Brillouin zone, with a first

order Methfessel-Paxton smearing parameter σ of 0.1 eV ensured that the energy convergence

criteria is better than 1 meV. The k-points grid is doubled for charge density calculations. The self-

consistent field (SCF) convergence criterion is set to 1 × 10−4 eV for electronic iteration and the

ionic relaxation continued until the maximum force was less than 0.02 eV/Å. This was updated by

the conjugate gradient approach.

Almost all previous literature indicates that the CO-CO coupling is pH independent because no

proton participates in the coupling reaction. However, we explicitly investigate the effect of OH

on changing the electronic structure and, in turn, the coupling reaction energy. In this work, we

explicitly considered OH in our simulations to see how CO bonding and CO-CO coupling energies

are affected. For Cu(100) and Cu(111), in two cases, with OH ion and without OH ion, we

calculated the zero point energy (ZPE), heat capacity, and entropy of the adsorbate and molecular

gases based on the vibrational frequencies within quasi-harmonic approximation framework (42).

These values were added to the DFT ground state energy to obtain the room temperature Gibbs

7

free energies. Moreover, we considered the transition state to calculate the activation energy

barriers of the CO-CO coupling for these facets using the nudged elastic band (NEB) method (43).

In addition, to include the solvent effects, we considered three explicit water molecules with and

without OH ion. Previous studies show that three water molecules are enough to stabilize OH ion,

and we considered similar configurations in our analysis (45-47). We considered OH ion in two

forms: adsorbed on the surface and solvated within the explicit water molecules. The same

calculations for vibrational frequencies and transition states have been done to convert all energies

to the Gibbs free energy at room temperature and to calculate the activation energy barriers for

CO-CO coupling.

For the calculation without explicit water molecules, one and two adsorbed carbon monoxides are

simulated on all three surfaces with a varying number and proximity of hydroxides. In each case,

different rational possible configurations (overall more than 200 configurations) are considered to

find the global minima. The CO adsorption energy is calculated as:

𝐸𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 = 𝐸∗𝐶𝑂𝑛−𝑂𝐻 − (𝐸∗

𝑛−𝑂𝐻 + 𝐸𝐶𝑂) (2)

Where, 𝐸∗𝐶𝑂𝑛−𝑂𝐻 is the electronic structure energy of the adsorbed CO on the catalyst with n-OH

ions on the surface, 𝐸∗𝑛−𝑂𝐻 is the energy of the slab without the CO adsorbate but includes n-OH

ions, and 𝐸𝐶𝑂 is the CO energy in gas phase without the catalyst. The CO-CO coupling energy

barrier is calculated according to the following reaction and the corresponding reaction free

energy:

𝑂𝐶𝐶𝑂 ∗ = 𝐶𝑂

∗ + 𝐶𝑂 ∗ (3)

𝐸𝐶−𝐶 𝑐𝑜𝑢𝑝𝑙𝑖𝑛𝑔 𝑏𝑎𝑟𝑟𝑖𝑒𝑟𝑛−𝑂𝐻 = 𝐸 𝑂𝐶𝐶𝑂 ∗

𝑛−𝑂𝐻 − (𝐸 𝐶𝑂 ∗𝑛−𝑂𝐻 + 𝐸 𝐶𝑂 ∗

𝑛−𝑂𝐻) (4)

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In equation (4), the zero point energy (ZPE), heat capacity, and entropy are considered to convert

all energies to the Gibbs free energy. In addition, the transition state is considered via NEB method

to calculate the activation energy barrier.

In our simulations without explicit water molecules, 0, 1 and 2 OH are considered on a surface of

16 (4×4) copper atoms, corresponding to 0, 1/16 and 2/16 ML concentrations, respectively.

However, in configurations where OH is very close to the adsorbed CO, it is assumed that the OH

concentration is even higher and forced to be very close to the adsorbates, and the local minima is

calculated in this case. These cases are virtually described by 3/16 and 4/16 ML in Fig. S8.

Otherwise, in 0, 1/16 and 2/16 ML concentrations, the configuration with the global minima energy

is the reference for our calculations. This includes all possible configurations for OH ions adsorbed

on the surface or being present in the vacuum just above the surface. The DFT energy of the

optimized configurations with the global minimum energies are tabulated in Table S2. CO

adsorption energies and the CO-CO coupling energy barriers are also mentioned in Table S3.

From Fig. S8 and the data in Table S3, related to the calculations without explicit water and without

considering the transition state, we see that CO-CO coupling energy barrier decreases with

increasing OH concentration. It is found that increasing the OH concentration on the slab surface

implicitly changes the charge density of the C atoms in adsorbed CO and OCCO, such that CO

bonding becomes weaker and CO-CO coupling becomes stronger and more favorable. On the

(100) surface, for instance, CO without any OH is adsorbed on the hollow site with four bonds to

the surrounding copper ions. However, increasing the OH concentration pushes CO to the bridge

site with two folds bonding and weaker binding energy. This bonding becomes even weaker as

OH is further added to the surface, such that OH in the vicinity pushes CO to the top site with one

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fold bonding. The same trend is observed on two other facets. Fig. S8 demonstrates this situation

on (100).

Table S.4 and S.5 show the calculated ZPE, heat capacity, entropy and the Gibbs free energies at

room temperature of the initial and final state of CO-CO coupling without OH, with 1 adsorbed

OH, and with 2 adsorbed OH on Cu(100) and Cu(111), respectively. The calculated

thermodynamic and activation energy barriers are mentioned in Table S6 and S7 for Cu(100) and

Cu(111), respectively. In few of the simulations, the transition state energy is less than the final

state energy, indicating a simple uphill energy barrier. In these cases, therefore, the final energy is

considered as the main energy barrier.

Including three explicit water molecules, we studied the CO-CO coupling energy barrier with and

without OH on both Cu(100) and Cu(111). OH is considered in two forms: adsorbed and solvated.

Surprisingly, when OH is solvated in the explicit water molecules, the activation energy barrier

for CO-CO coupling is higher than when OH is adsorbed on the surface still with presence of the

explicit water molecules. In general, we considered the lowest energy barrier between these two

situations as the activation energy barrier for CO-CO coupling with OH. Table S.8 and S.9 show

the detail thermodynamic energies of the initial and final states of CO-CO coupling on Cu(100)

and Cu(111) facets, respectively. The calculated activation energy barriers for these two facets are

mentioned in Table S10 and S11. Fig. S9 demonstrates these calculated activation energy barriers

with and without explicit waters on both Cu(100) and Cu(111) facet. Fig. S10 shows the initial

state (IS), transition state (TS) and the final state (FS) of the CO-CO coupling on Cu(100) with

explicit water molecules, without OH ion, with one OH ion adsorbed on the surface and one OH

ion solvated in explicit water molecules.

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In general, we conclude that at higher OH concentrations CO bonding is weaker and this could

lead to more CO in gaseous products and easier CO-CO coupling. Indeed, both are shown under

experimental conditions. We calculated the electronic charge density on each ion by Bader charge

analysis (51). As demonstrated in Fig. S8, we see that increasing the OH concentration, not only

decreases the CO-CO coupling energy barrier but also increases the charge difference between

two C atoms in coupled carbon monoxides. This increased charge difference, makes a stronger

intramolecular dipole and consequently more stable coupling due to the stronger ionic adsorption.

The electronic charge distribution and also charge difference between two C atoms in coupled

carbon monoxides, i.e. proportional to the dipole magnitude.

5. Material synthesis:

The carbon-based Freudenberg (Fuel cell Store) gas-diffusion layer (GDL) was used as substrate

for the experiments in Fig. 1 and 2. Cu (99.99%) was evaporated on the substrate using an

Angstrom Nexdep Evaporator. The deposition was performed in ~10-5-10-6 Torr at 1.5 Å/sec. The

thicknesses of the samples was confirmed via scanning electron microscopy (SEM). The 1000 µg

sample was prepared by drop-casting commercial Cu nanoparticles (NPs) (Sigma-Aldrich, particle

size

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fluorescence spectra were collected using a 4-element Si(Li) drift detector under ambient

atmosphere with a custom in-situ liquid cell fabricated from Teflon. CO2 gas was continuously

bubbled into the electrolyte to ensure saturation. The pre-edge region was scanned from 8879 eV

to 8974 eV at a rate of 5 eV/s, the near-edge region was scanned from 8974 eV to 9019 eV at a

rate of 0.45 eV/s and the post-edge region was canned from 9019 eV to 12 k with a rate of 0.05

k/s. The XAS data were analyzed using the software package Athena.

The electrochemically active surface areas of the catalysts were measured using a double layer

capacitance method in 0.1 M HClO4 electrolyte (49). Cyclic voltammetry scans were recorded in

the potential range from -0.05 to 0.2 V vs. RHE. The roughness factors of the catalysts were

compared to electrochemically polished Cu foil whose surface area is defined as 1.

7. Modeling of CO2 diffusion into the liquid electrolyte:

As described in Section 1 of the Supplementary Materials, a reaction-diffusion model is used to

predict the pH in the vicinity of the catalyst layer which can then be used to adjust the measured

potentials in Eq. (1). At 0 mA/cm2 this surface pH differs from the bulk KOH concentration due

to CO2 that diffuses across the gas-liquid interface and interacts with hydroxide, lowering the local

pH within the catalyst layer from that of the bulk KOH electrolyte. Additionally, as current is

applied to the catalyst layer, hydroxide will be generated causing the local pH to increase from this

minimum case and eventually surpass the bulk pH of the electrolyte. The reaction-diffusion model

takes into account interactions between the following species in the electrolyte (CO2, OH-, HCO3

-

and CO32-) as well as the consumption of CO2 and the production of OH

- in the catalyst layer. The

following equations governing these interactions are adopted from previous works (48, 50):

𝜕[𝐶𝑂2]

𝜕𝑡= 𝐷𝐶𝑂2

𝜕2[𝐶𝑂2]

𝜕𝑥2− [𝐶𝑂2][𝑂𝐻

−]𝑘1𝑓 + [𝐻𝐶𝑂3−]𝑘1𝑟 − 𝑅𝐶𝑂2 (5)

13

𝜕[𝐻𝐶𝑂3−]

𝜕𝑡= 𝐷𝐻𝐶𝑂3−

𝜕2[𝐻𝐶𝑂3−]

𝜕𝑥2+ [𝐶𝑂2][𝑂𝐻

−]𝑘1𝑓 − [𝐻𝐶𝑂3−]𝑘1𝑟 − [𝐻𝐶𝑂3

−][𝑂𝐻−]𝑘2𝑓 + [𝐶𝑂32−]𝑘2𝑟 (6)

𝜕[𝐶𝑂32−]

𝜕𝑡= 𝐷𝐶𝑂32−

𝜕2[𝐶𝑂32−]

𝜕𝑥2+ [𝐻𝐶𝑂3

−][𝑂𝐻−]𝑘2𝑓 − [𝐶𝑂32−]𝑘2𝑟 (7)

𝜕[𝑂𝐻−]

𝜕𝑡= 𝐷𝑂𝐻−

𝜕2[𝑂𝐻−]

𝜕𝑥2− [𝐶𝑂2][𝑂𝐻

−]𝑘1𝑓 + [𝐻𝐶𝑂3−]𝑘1𝑟 − [𝐻𝐶𝑂3

−][𝑂𝐻−]𝑘2𝑓 + [𝐶𝑂32−]𝑘2𝑟 + 𝑅𝑂𝐻 (8)

where RCO2 and ROH account for the consumption of CO2 in the reduction reaction and the

production of OH-, respectively. As these reactions only occur within the catalyst layer the

following equations are spatially dependent and are assumed to occur homogeneously throughout

the entire catalyst layer:

𝑅𝐶𝑂2 = {

𝑗

𝐹(

𝐹𝐸𝐶𝑂

𝑛𝑒,𝐶𝑂+

𝐹𝐸𝐶2𝐻4

𝑛𝑒,𝐶2𝐻4+

𝐹𝐸𝐸𝑡𝑂𝐻

𝑛𝑒,𝐸𝑡𝑂𝐻)

𝜀

𝐿𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 , 0 ≤ 𝑥 ≤ 𝐿𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

0 , 𝑥 > 𝐿𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

(9)

𝑅𝑂𝐻 = {

𝑗

𝐹

𝜀

𝐿𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 , 0 ≤ 𝑥 ≤ 𝐿𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

0 , 𝑥 > 𝐿𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (10)

In calculating Eqs. (9) and (10) the geometric current density, j, and product selectivities, FECO,

FEC2H4, FEEtOH and FEH2, are imposed. A porosity of 60%, ε, and selectivity distribution of 10%

H2, 10% CO and 80% (EtOH+C2H4) is assumed. Panels (f), (g) and (h) in Fig. S3 assume a catalyst

thickness, Lcatalyst, of 100 nm, which matches the experimental catalyst used in Fig. 1.

As shown in Fig. S3e, a gas-liquid interface is assumed at x = 0 µm. The catalyst layer then extends

into the electrolyte a specified thickness from this boundary. A liquid diffusion thickness of x =

500 µm was assumed in our simulations as our cathode is recessed in a cavity of a similar depth

due to the compression of a sealing gasket. The fluid outside of this cavity is assumed to be

continuously mixed and replaced. A smaller diffusion thickness would slightly reduce the local

reaction pH by providing a shorter diffusion distance of hydroxide to the bulk, but the bounds of

this value depend on flow within the fluid channel. The maximum solubility of CO2 in KOH

14

electrolyte was modeled using Henry’s constant and the Sechenov equation at 1 atm and 298 K to

account for ‘salting out’ effects (Fig. S2b) (52).

The catalyst layer porosity of 60% is based on a loosely packed spheres model while some fuel

cell studies report porosities of 40% (53). The diffusion coefficient inside the simulated region was

adjusted for porosity and viscosity. The assumption of a homogeneous reaction occurring laterally

throughout the catalyst layer (Eq. (9)) is more accurate for lower current densities and thinner

catalyst layers.

The following boundary conditions were used to solve the coupled equations. The left boundary

condition of Eq. (5) was set as the solubility limit of CO2 in a specified bulk KOH concentration,

while a no-flux boundary condition was applied for CO2 at the right-hand boundary. For OH-,

HCO3- and CO3

2- no-flux boundary conditions were applied at the left boundary while the

concentrations at the right boundary were set to the equilibrium values in the specified bulk KOH

concentration. For all simulations we prescribe current density, selectivity, bulk KOH

concentration and catalyst layer thickness.

From the solved system of equations the concentration profile of CO2 and OH- in the electrolyte

can be found for a variety of inputs as shown in Fig. 1D and Fig. S3. As seen in Fig. 1D the KOH

media is particularly parasitic to CO2 and is neutralized by OH- well before the 500 µm liquid

diffusion boundary, with or without a catalyst layer or current density imposed. The largest

predicted correction from the bulk pH values occurs in 1 M KOH at 0 mA/cm2 where the potential

is adjusted using a pH of 12.42, instead of the bulk electrolyte pH (pH = 14), equivalent to a 93 mV

difference.

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Fig. S1. Structural characterization of the samples before and after CO2 reduction (a, b)

SEM images of the Cu deposited on a carbon gas-diffusion electrode with a thickness of 100 nm

before reaction. (c) Cross-sectional SEM image of the Cu sample simultaneously deposited on flat

silicon substrates for thickness measurement. (d, e, f) SEM images of the sample after the CO2

reduction reaction in 1, 5, and 10 M KOH electrolytes, respectively. The CO2 reduction tests were

performed at a current density of 50 mA/cm2 for 500 seconds.

17

Fig. S2. Validating the local surface pH by Cr catalyst. (a, b) SEM images of Cr catalyst

deposited on carbon gas diffusion layer. (c) Comparison of HER activity of Cr catalyst in 1 M, 5

M and 10 M KOH electrolyte. (d-f) HER activity of Cr catalyst in Ar and CO2 using 1 M, 5 M and

10 M KOH electrolytes, respectively.

18

Fig. S3. Modelling of the gas-liquid diffusion interface with and without a catalyst layer or

applied current. (a) Schematic of CO2 penetration across a gas-liquid interface. (b) Maximum

CO2 solubility in various bulk KOH concentrations due to ‘salting out’ effects. (c) CO2 diffusion

into various KOH concentrations without a catalyst layer or CO2 reduction (j = 0 mA cm-2). (d)

hydroxide concentration within the diffusion layer thickness (500 µm) at 0 mA/cm2 for various

KOH concentrations, (e) Schematic of CO2 penetration across a gas-liquid interface where CO2

reduction occurs on the red catalyst, (f) predicted electrode surface pH as a function current density

and KOH concentration for a 100 nm thick catalyst, (g) CO2 distribution throughout a 100 nm

thick catalyst layer at a current of 250 mA cm-2. (h) predicted hydroxide concentration inside the

catalyst layer over a broad current density and electrolyte concentration for a 100 nm thick catalyst.

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Fig. S4. Kinetic of CO reduction to ethylene. (a) ethylene Faradaic efficiency at low applied

potential region showing similar ethylene onset potentials for 1 M and 10 M KOH electrolyte. (b)

Tafel plots for CO reduction to ethylene using 1 M and 10 M KOH electrolyte.

20

Fig. S5. In-situ X-ray absorption spectroscopy (XAS) study in 5 M KOH electrolyte. The Cu

K-edge spectra of (a) the 100 nm Cu sample under open circuit potential (OCP), at -0.16 V, and at

-0.96 V vs. RHE. (b) Reference standards of Cu metal, Cu2O, and CuO were taken to measure the

Cu0, Cu+, and Cu2+ oxidation states, respectively.

21

Fig. S6. Transmission electron microscopy (TEM) characterization of the samples before

and after CO2 reduction. (a, e) TEM images of the Cu deposited on a carbon gas-diffusion

electrode with a thickness of 100 nm before reaction. TEM images of the sample after CO2

reduction reaction in 1 M (b, f), 5 M (c, g), and 10 M (d, h) KOH electrolytes. The CO2 reduction

tests were performed at a current density of 50 mA/cm2 for 500 seconds.

22

Fig. S7. Effect of OH on the adsorbed OCCO intermediate. Oxidation states (charge density)

of the adsorbed OCCO elements and the adjacent copper as calculated by Bader charge analysis.

Oxidation states on (100), (110) and (111) facets of copper, without OH, with one and two OH

without including explicit water molecules are shown for the most stable configurations.

Cu (100)

No OH One OH Two OH

Cu (110)

Cu (111)

23

Fig. S8. Effect of OH ion on CO binding energy and CO-CO coupling on Cu (100), (110) and

(111) facets, without including explicit water molecules. (a) CO adsorption energy at different

OH concentrations. (b) Charge differences of two carbons in adsorbed OCCO at different OH

24

concentrations. (c, d) Electron charge density and the optimized configuration of the adsorbed CO

with no OH and one OH on Cu(100). (e, f) Electron charge density and the optimized configuration

of the adsorbed OCCO with no OH and one OH on Cu(100). The charge difference between two

carbons and the induced dipole in the presence of OH is shown in (f). (g) Density Functional

Theory (DFT) results for CO-CO coupling on Cu (100) surface with and without the presence of

OH. (h) DFT results showing the reduction of CO-CO coupling barrier in the presence of OH.

25

Fig. S9. Energies of the initial state (IS), transition state (TS) and the final state (FS) of CO-

CO coupling on both Cu(100) and Cu(111) facets with and without explicit water molecules,

with different number of OH ions.

26

Fig. S10. Initial state (IS), transition state (TS) and the final state (FS) of CO-CO coupling

on Cu(100) with three explicit water molecules. (left) CO-CO coupling without any OH ion,

(middle) CO-CO coupling with one OH ion adsorbed on the surface, and (right) CO-CO coupling

with one OH solvated in explicit water molecules. Brown and black spheres demonstrate copper

and carbon, respectively. Red spheres are for oxygen atoms in CO and CO-CO intermediates. Pink

spheres are oxygen in the explicit water. Purple sphere demonstrates oxygen in OH ion. White

spheres show hydrogen in explicit waters, and the light blue sphere shows hydrogen in OH ion.

27

Fig. S11. Morphology characterization of Cu deposited on carbon gas-diffusion layer. SEM

images of the samples with deposition thickness of (a) 10 nm; (b) 25 nm; (c) 1000 nm. (d) SEM

image of Cu nanoparticles deposited on gas-diffusion electrode with a catalyst loading of 1000

g/cm2 using drop-casting technique.

28

Fig. S12. Cross-section characterization of the 25 nm sample Structural and compositional

analysis technique using a focused ion beam (FIB) system and scanning transmission electron

microscope (STEM). (a) Cross-section STEM of sample 25 nm. The fill region shows the

roughness of the reaction interface where Cu is deposited. Elemental mapping for Carbon (c) and

Cu (d) and overlap of all elements (b) on the cross-section mode of 25 nm. Tungsten was used to

protect the surface during sample preparation using FIB.

29

Fig. S13. Chemical and physical characterization of Cu catalysts. (a) Cu 2p and (b) O 1s XPS

spectra of the samples showing the presence of metallic and oxide phases on the surface of all

thermal evaporated Cu samples and commercial Cu nanoparticles. (c) XRD diffraction patterns of

1000 nm samples showing the presence of metallic crystalline phase.

30

Fig. S14. Performance of Cu catalysts in an H-cell configuration. Faradaic efficiencies for gas

products in the potential range of -0.9 to -1.2 V vs. RHE (with iR correction) for 10 nm (a); 25 nm

(b); 1000 nm (c); and 1000 g (d). The electrolyte was 0.1 M KHCO3 for all runs.

31

Fig. S15. Electrochemical characterization of Cu catalysts. (a) Linear sweep voltammetry scan

with CO2 flow using 10 M KOH electrolyte on Cu samples of varying thickness. (b) Surface

roughness factors of the samples normalized to polished Cu foil. The 10 nm, 25 nm, and 1000 nm

samples are thermally deposited while the 1000 g sample uses drop-casted nanoparticles.

32

Fig. S16. Performance of Cu catalysts in 10 M KOH electrolyte. Faradaic efficiency for CO

(a); and CH4 (b) on 10 nm, 25 nm, 1000 nm, and 1000 g catalysts using 10 M KOH electrolyte

in the current range of 200 to 300 mA cm-2. (c) Faradaic efficiency for all products on the catalyst

at the optimum current density for ethylene: 10 nm (250 mA cm-2); 25 nm (275 mA cm-2); 1000

nm and 1000 g (225 mA cm-2). (d) C2H4 mass activity analysis showing exceptionally high mass

activity of abrupt reaction interface samples.

33

Fig. S17. Representative nuclear magnetic resonance (NMR) spectra of the liquid products.

NMR spectrum of the reaction products obtained upon CO2 reduction on 25 nm sample at current

density of 275 mA cm-2 using 10 M KOH electrolyte. (a) full spectrum (largest singlet at 2.71 ppm

corresponds to the reference DMSO signal, while the noisy part around 4.60 ppm is due to

suppressed water signal); (b) magnified portion of the spectrum demonstrating ethanol and acetate

product peaks; (c) magnified portion of the spectrum demonstrating the formation of a small

amount of formate.

34

Fig. S18. Effect of varying CO2 concentrations on overall CO2 reduction selectivity on the 25 nm

sample.

35

Fig. S19. CO2 reduction performance on an abrupt reaction interface sample. (a) Optimal C2

selectivity on the 25 nm sample using 10 M KOH (at 275 mA cm-2); 5.5 M KOH (at 500 mA cm-

2); 3.5 M KOH (at 750 mA/cm-2). (b) Applied potentials (iR corrected) at different current densities

on the 25 nm sample using an optimized electrolyte for high C2H4 selectivity: 10 M KOH (at 275

mA cm-2); 5.5 M KOH + 4 M KI (at 500 mA/cm-2); and 3.5 M KOH + 5 M KI (at 750 mA/cm-2).

36

Fig. S20. Stability test of the Cu catalyst deposited onto a traditional carbon gas-diffusion

electrode. Long-term performance test of CO2 reduction to ethylene in 7 M KOH on the 1000 nm

sample: (a) current density vs. time; and (b) Faradaic efficiency for gas products vs. time. The

sample was tested under a constant applied voltage of -0.6 V vs. RHE (at which the sample shows

the highest Faradaic efficiency for ethylene). Long-term performance test of CO2 reduction to

ethylene in 1 M KOH on the 1000 nm sample: (c) current density vs. time; and (d) Faradaic

37

efficiency for gas products vs. time. The sample was tested under a constant applied voltage of -0.7

V vs. RHE at which the current density is in the range of 100 mA/cm2. The samples were tested at

different KOH concentration electrolytes and current densities to show that the low stability of the

traditional gas-diffusion electrodes for CO2 reduction occurs at all testing conditions.

38

Fig. S21. Surface characterization of bare carbon gas-diffusion electrode under CO2

reduction conditions. Contact angle measurements on (a) the starting gas-diffusion electrode

and, (b) the electrode after applying a voltage of -0.8 V vs. RHE under CO2 reduction conditions

for 1 hour. The contact angle reduced from 145° (starting electrode) to 117° (after operation),

indicating that the hydrophobicity of the carbon electrode is reduced under CO2 reduction

conditions. (c) O 1s XP spectra of the carbon gas-diffusion electrode before and after being applied

an negative potential under CO2-RR conditions. The data show that the oxygen content in the

carbon gas-diffusion electrode was doubled after being applied negative potentials of -0.4 and -0.8

V vs. RHE, implying an increase in the surface OH, and COOH of the carbon electrode which

leads to a more hydrophilic surface.

39

Fig. S22. Characterization of Graphite/Carbon NPs/Cu/PTFE electrode. Top view SEM

image of Cu/PTFE after being coated with (a) carbon nanoparticles (NPs), and (b) by both carbon

NPs and graphite. Cu 2p (c) and O 1s (d) XPS spectra of Cu/PTFE before being coated by Carbon

and graphite showing a mixed oxidation state of Cu0 and Cu2+.

40

Fig. S23. Performance of the polymer-based gas-diffusion layer electrodes in 7 M KOH

electrolyte. (a) Dependence of current density on applied potential for Graphite/Carbon

NPs/Cu/PTFE and Graphite/Cu/PTFE electrodes. Faradaic efficiency for CO, H2 and CH4 on

Graphite/Carbon NPs/Cu/PTFE (b) and Graphite/Cu/PTFE (c) using 7 M KOH electrolyte in the

41

applied potential range of -0.33 to -0.6 V vs. RHE. (d) Faradaic efficiency for all products on the

Graphite/Carbon NPs/Cu/PTFE catalyst at the optimum potential for ethylene of -0.57 V vs. RHE.

42

Fig. S24. GC-MS analyses of ethylene produced from CO2-RR on Graphite/Carbon

NPs/Cu/PTFE using 12CO2 and 13CO2 as feedstock. The GC-MS analysis of ethylene obtained

from 12CO2 shows typical mass spectrum patterns of 12C-ethylene (m/s =28). When 13C-labeling

CO2 was used as the reactant, the mass spectrum of ethylene shows patterns of 13C-ethylene (m/z

=30), confirming that the ethylene is produced from CO2. The reaction was performed in 7 M KOH

electrolyte with a constant applied potential of – 0.55 V vs. RHE.

43

Fig. S25. Stability tests of polymer-based gas-diffusion electrodes. Long-term performance

tests of CO2 reduction to ethylene in 7 M KOH for Graphite/Carbon NPs/Cu/PTFE,

Graphite/Cu/PTFE and Cu/Carbon gas-diffusion layers: current density vs time (a) and Faradaic

efficiency for gas products vs. time on Graphite/Carbon NPs/Cu/PTFE (b); Graphite/Cu/PTFE (c);

44

and Cu on traditional Carbon gas-diffusion electrode (d). The samples were tested under a constant

applied voltage of -0.55 V vs. RHE.

45

Fig. S26. Characterization of Cu/PTFE and Graphite/Cu/PTFE before and after reaction.

Optical photographs of Cu/PTFE before (a) and after CO2-RR in 7 M KOH electrolyte at -0.55 V

vs. RHE for 2 hours (b). Optical photographs of Graphite/Cu/PTFE before (c) and after CO2-RR

in 7 M KOH electrolyte at -0.55 V vs. RHE for 40 hours. The arrow mark in (b) shows the

disappearance of Cu layer which is due to the dissolution of non-electronically connected Cu area

on the PTFE substrate. The arrow mark in (d) shows the presence of Cu islands or exposed Cu

area which resulted from the detachment of the graphite layer from the Cu/PTFE surface. We

rationalize that the low stability of Graphite/Cu/PTFE catalyst is because of the weak interaction

between Cu surface and graphite which leads to the detachment of graphite from Cu surface. The

a b

c d

Disappearance of Cu

Exposed/re-deposited Cu

300 m 300 m

300 m300 m

46

Cu area without electrical contact would be dissolved in the electrolyte and redeposited on

graphite, forming a catalyst for hydrogen evolution.

47

Fig. S27. Scanning electron microscopy (SEM) images of NiFeOx deposited on Ni foam using

electrodeposition method. The sample was prepared as previously reported (39).

48

Fig. S28. (a) Oxygen evolution reaction (OER) performance of a NiFeO/Ni foam measured in a

3-electrode setup (with iR correction); (b) Full-cell potential performance (with iR correction); (c)

CO2-RR performance of the cathode potential vs. RHE as calculated from the OER (a) and full-

cell performance (b) data. The additional data point (black circle) was measured in a 3-electrode

setup and converted to a RHE scale using the predicted pH of the reaction-diffusion model. The

overlap between both the black and red circles confirm the surface pH modelling data; (d)

49

Chronoamperometry of full-cell operation using Graphite/Carbon NP/Cu/PTFE NiFeOx (39)

catalysts at a fixed cell-voltage of 2.4 V over 1 hour.

50

Table S1. Summary of CO2 reduction to C2H4 performance on different catalysts.

Catalyst

C2H4 onset

potential

(V vs.

RHE)

J(C2H4)

mA cm-2

C2H4

Faradaic

efficiency

(%)

C2H4

energy

conversion

(%)(*)

C2H4

mass

activity

(A mg-1)

Reference

Plasma-

Oxidized Cu

-0.6 12 60 33 - (13)

Plasma-Cu

Nanocubes

-0.6 16 40 25 - (54)

N-doped

graphene dots

-0.45 40 33 18 0.08 (11)

Cu

nanostructure

~ -0.3 100 40 23 0.33 (9)

Cu

nanodendrites

- 97 55 28 - (10)

Cu

nanoparticles

-0.36 150 36 23 0.15 (12)

Abrupt Cu

Interface

-0.165 473 66 40 17.8 This work

Abrupt Cu

Interface

-0.165 184 66 44 6.7 This work

(*) C2H4 energy efficiency is calculated for the half-cell (i.e. assuming the overpotential of the

oxygen evolution reaction is zero). C2H4 energy efficiency = (1.23 + (-EC2H4))*FE(C2H4)/(1.23

+ (-E)), where E is the applied potential vs. RHE; EC2H4 = 0.08 V is thermodynamic potential

(vs. RHE) of CO2 reduction to ethylene; FE(C2H4) is the measured C2H4 Faradaic efficiency in

percentage.

51

Table S2. Ground state energy of the adsorbates on three different facets of the copper,

without including explicit water.

Adsorbates

Ground state energy (eV)

111 100 110

CO -14.767 -14.767 -14.767

Cu -247.790 -243.406 -299.709

Cu-OH -258.531 -254.334 -310.672

Cu-2OH -269.182 -265.208 -321.635

Cu-CO -263.479 -259.149 -315.372

Cu-CO-OH -274.225 -270.036 -326.168

Cu-CO-2OH -284.884 -280.898 -336.956

Cu-2CO -279.154 -274.791 -331.114

Cu-2CO-OH -289.804 -285.540 -342.020

Cu-2CO-2OH -300.390 -296.602 -352.896

Cu-OCCO -277.486 -273.830 -329.684

Cu-OCCO-OH -288.273 -284.673 -340.667

Cu-OCCO-2OH -298.934 -295.757 -351.496

52

Table S3. CO adsorption energy and C-C coupling energy barrier on three different facets

of copper at different OH concentrations, without including explicit water molecules.

OH

Concentration

CO adsorption energy (eV) CO-CO coupling energy barrier

(eV)

111 100 110 111 100 110

0 -0.922 -0.976 -0.897 1.668 0.961 1.430

1/16 -0.927 -0.935 -0.730 1.530 0.867 1.353

2/16 -0.936 -0.924 -0.555 1.456 0.845 1.400

3/16 -0.686 -0.919 -0.726 1.371 0.822 1.304

4/16 -0.556 -0.823 -0.495 1.234 0.763 1.382

53

Table S4. Electronic structure energy (E), zero point energy (ZPE), heat capacity (Cv) and

entropy (S) at room temperature (T), and Gibbs free energy (G) all in eV, for CO-CO

coupling on Cu(100) without explicit water and with different number of OH ions. IS

mentions the initial state, i.e. 2 adsorbed CO on the surface, and FS mentions the final state,

i.e. OCCO intermediate.

OH Concentration State E ZPE Cv S.T G (eV)

0 IS -274.791 0.324 0.160 0.304 -274.611

FS -273.830 0.415 0.119 0.234 -273.530

1-OH (1/16 ML) IS -285.540 0.354 0.159 0.332 -285.359

FS -284.673 0.416 0.119 0.237 -284.375

2-OH (2/16ML)

IS -296.368 0.340 0.159 0.331 -296.200

FS -295.757 0.423 0.119 0.238 -295.453

54



coupling on Cu(111) without explicit water and with different number of OH ions. IS

mentions the initial state, i.e. 2 adsorbed CO on the surface, and FS mentions the final state,

i.e. OCCO intermediate.

OH

Concentration State E ZPE Cv S.T G (eV)

0-OH IS -279.154 0.359 0.156 0.297 -278.936

FS -277.486 0.398 0.115 0.234 -277.207

1-OH (1/16ML) IS -289.804 0.352 0.152 0.287 -289.587

FS -288.273 0.401 0.112 0.217 -287.977

2-OH (2/16ML)

IS -300.159 0.366 0.164 0.416 -300.045

FS -298.934 0.387 0.111 0.216 -298.652

55

Table S6. Thermodynamic Gibbs energy differences of the initial and final states, and also

the activation energy barrier for CO-CO coupling on Cu(100) without explicit water and

with different number of OH ions. IS mentions the initial state, i.e. 2 adsorbed CO on the

surface, TS mentions the transition state and FS mentions the final state, i.e. OCCO

intermediate.

OH

Concentration State G (eV)

Thermodynamic

Energy Barrier

(eV)

Activation

Energy Barrier

(eV)

0-OH

IS -274.611

TS -273.530 1.081 1.081

FS -273.530

1-OH (1/16ML)

IS -285.359

TS -284.375 0.984 0.984

FS -284.375

2-OH (2/16ML)

IS -296.200

TS -295.274 0.747 0.926

FS -295.453

56


the activation energy barrier for CO-CO coupling on Cu(111) without explicit water and

with different number of OH ions. IS mentions the initial state, i.e. 2 adsorbed CO on the

surface, TS mentions the transition state and FS mentions the final state, i.e. OCCO

intermediate.

OH


Thermodynamic

Energy Barrier (eV)

Activation

Energy Barrier

(eV)

0-OH

IS -278.936

TS -277.187 1.729 1.749

FS -277.207

1-OH

IS -289.587

TS -287.977 1.609 1.609

FS -287.977

2-OH

IS -300.045

TS -298.453 1.393 1.592

FS -298.652

57



coupling on Cu(100) with three explicit water and with one OH ion (i.e. 1/16ML) as adsorbed

or solvated. IS mentions the initial state, i.e. 2 adsorbed CO on the surface, and FS mentions

the final state, i.e. OCCO intermediate.

OH


0-OH IS -317.742 0.328 0.156 0.292 -317.550

FS -316.742 0.424 0.116 0.215 -316.417

1 adsorbed OH IS -328.754 0.349 0.160 0.370 -328.615

FS -327.809 0.425 0.115 0.213 -327.482

1 solvated OH IS -326.356 0.305 0.126 0.247 -326.172

FS -325.463 0.419 0.098 0.189 -325.135

58



coupling on Cu(111) with three explicit water and with one OH ion (i.e. 1/16ML) as adsorbed

or solvated. IS mentions the initial state, i.e. 2 adsorbed CO on the surface, and FS mentions


OH


0-OH

IS -321.991 0.351 0.152 0.284 -321.772

FS -320.306 0.392 0.132 0.267 -320.049

1 adsorbed OH

IS -332.896 0.353 0.149 0.278 -332.672

FS -331.235 0.392 0.132 0.271 -330.982

1 solvated OH

IS -330.715 0.344 0.121 0.218 -330.468

FS -328.678 0.387 0.121 0.249 -328.671

59


the activation energy barrier for CO-CO coupling on Cu(100) with three explicit water

molecules and with one OH ion (i.e. 1/16ML) as adsorbed or solvated. IS mentions the initial

state, i.e. 2 adsorbed CO on the surface, TS mentions the transition state and FS mentions


OH


Thermodynamic

Energy Barrier

(eV)

Activation

Energy Barrier

(eV)

0-OH

IS -317.550

TS -316.328 1.133 1.222

FS -316.417

1 adsorbed OH

IS -328.615

TS -327.474 1.132 1.140

FS -327.482

1 solvated OH

IS -326.172

TS -324.318 1.037 1.854

FS -325.135

60


the activation energy barrier for CO-CO coupling on Cu(111) with three explicit water

molecules and with one OH ion (i.e. 1/16ML) as adsorbed or solvated. IS mentions the initial

state, i.e. 2 adsorbed CO on the surface, TS mentions the transition state and FS mentions


OH


Thermodynamic

Energy Barrier

(eV)

Activation

Energy Barrier

(eV)

0-OH

IS -321.772

TS -319.887 1.724 1.886

FS -320.049

1 adsorbed OH

IS -332.672

TS -330.847 1.690 1.825

FS -330.982

1 solvated OH

IS -330.468

TS -328.171 1.797 2.297

FS -328.671

61

Table S12. Physical properties of KOH electrolyte and its effect on the C2H4 onset potential

and Tafel slope

KOH

concentration

(M) Resistance () (*) Surface pH (**)

C2H4 onset

potential (V vs.

RHE) (†)

C2H4 Tafel

slope (mV

dec-1) (‡)

1 2.58 12.42 -0.465 135

5 1.08 14.48 -0.285 97

10 0.98 14.94 -0.165 65

(*): Measured using electrochemical impedance spectroscopy (EIS); (**) Surface pH predicted

using a 1D reaction-diffusion model at 0 mA/cm2 accounting for the effect of CO2 gas in the

catalyst layer; (†) The potential at which the catalyst shows an ethylene faradaic efficiency of

1%; (‡) Calculated based on the experimental C2H4 partial current densities.

62

Table S13. Table of CO2 reduction to ethylene stabilities reported in Literature

Catalyst Reported

Duration (h) Stable C2H4

Selectivity

C2H4

Current

Density

(mA/cm2)

Source

Cu-Nanocubes 1 45 % 23 (54)

Cu-Nanoparticles 4 18 % 27 (12)

Plasma-Cu 5 58 % 7 (13)

Cu-Mesocrystals 6 23 % 4 (55)

CuZn-

Nanoparticles 8 ~30 % 14 (56)

Oxide-derived

CuZn 10 8 % 3 (58)

Cu-Nanocubes 10 32 % 7 (57)

Graphite/Cu/PTFE 24 55 % 55 This work

Graphite/Carbon

NP’s/Cu/PTFE 150 70 % 55-70 This work

63

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