science.sciencemag.org/content/367/6478/661/suppl/DC1

Supplementary Materials for

CO2 electrolysis to multicarbon products at activities greater than 1 A cm

F. Pelayo García de Arquer*, Cao-Thang Dinh*, Adnan Ozden*, Joshua Wicks*, Christopher McCallum, Ahmad R. Kirmani, Dae-Hyun Nam, Christine Gabardo, Ali Seifitokaldani, Xue Wang, Yuguang C. Li, Fengwang Li, Jonathan Edwards,

Lee J. Richter, Steven J. Thorpe, David Sinton†, Edward H. Sargent†

*These authors contributed equally to this work. †Corresponding author. Email: ted.sargent@utoronto.ca (E.H.S.); sinton@mie.utoronto.ca (D.S.)

Published 7 February 2020, Science 367, 661 (2020)

DOI: 10.1126/science.aay4217

This PDF file includes:

Materials and Methods Figs. S1 to S36 Tables S1 to S9 References

Supporting Information

Materials and Methods

Materials and chemicals

Precursors for electrolyte preparation (high grade KOH, KHCO3, K2SO3, H2SO4) were purchased

from Sigma Aldrich. Electrolyte solutions were prepared from stock solutions of higher

concentration in DI water, which were then diluted to the target molarity.

Sample fabrication and electroreduction reactions

The O2, CO2 and CO electroreduction characteristics of the cathode electrodes were investigated

using a potentiostat (Autolab PGSTAT302N), a custom-made flow cell with a fixed 1 cm2

electrode geometric area, a digital mass flow controller (Sierra, SmartTrack 100), a current booster

(Metrohm Autolab, 10 A), and two peristaltic pumps with silicone tubing.

Sample preparation:

Cathodic catalyst materials were deposited onto polytetrafluoroethylene (PTFE) gas

diffusion layers with a 450 nm mean pore size. Approximately ≈300 nm nominal thick Ag and Cu

films were sputtered onto the PTFE substrate using Ag and Cu targets (99.99%) at a sputtering

rate < 0.2 nm·min-1 in an Angstrom Nexdep sputtering tool at a base pressure of < 10-6 Torr.

Catalyst:ionomer planar heterojunctions (CIPH): The reference PTFE/metal electrodes

were modified by spray-coating an ionomer layer from a solution of 700 mg ionomer (Nafion

perfluorinated resin solution, product #527084-25 ml purchased from Sigma Aldrich) and 25 ml

methanol (99.8%, anhydrous, Sigma Aldrich) until the desired ionomer loading was achieved.

Samples were dried for at least 24 h at room temperature in a vacuum chamber before operation.

A single sample is typically 2 cm x 2 cm in size.

Catalyst:ionomer bulk heterojunctions (CIBH): CIBH samples were fabricated by spray

coating a mixture of Cu nanoparticles (25 nm diameter, Sigma Aldrich) and ionomer solution at

different ratios onto the CIPH electrodes. Samples were dried for at least 24 h at room temperature

in a vacuum chamber before operation.

2

Flow-cell components:

The flow cell is comprised of three chambers: anolyte, catholyte and gas. The anolyte

chamber (dimensions: 12 mm x 12 mm; 9 mm depth) contains the counter electrode (nickel foam;

1.6 mm thickness). The catholyte chamber (dimensions: 12 mm x 12 mm; 9 mm depth, square

through hole) contains the Ag/AgCl reference electrode (CH Instruments; filled with 3M KCl

solution) via a port drilled through the housing such that the frit of the reference electrode is in the

center of the chamber. The anolyte and catholyte chambers are separated by the anion exchange

membrane (Fumasep FAB-PK-130). The gas chamber (dimensions: 12 mm x 12 mm; 9 mm depth)

is used to supply the reactant gas. The gas and catholyte chambers are separated by the cathode.

The catalyst side of the cathode faces into the catholyte chamber, while the PTFE gas diffusion

layer faces the gas chamber. Silicone gaskets with a 1 cm2 window are placed between each layer

to achieve sufficient sealing. Each chamber has an inlet and outlet connection (1/8" OD; 1/16" ID)

to flow either electrolyte or gas.

Flow-cell assembly and operation:

The designed cathode and commercially available Ni foam anodes were mounted in their

respective chambers using Kapton tape for sealing and copper tape leads. Building up from the

anolyte chamber, the completed assembly is sealed with even compression from four equally

spaced bolts. The cathode is operated as the working electrode.

iR compensation losses between the reference and working electrodes were determined via

electrochemical impedance spectroscopy (EIS) analyses. The electrode potentials upon iR

compensation were scaled to the reversible hydrogen electrode (RHE) using the following

expression:

𝐸𝑅𝐻𝐸 = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.197 𝑉 + 0.059 × 𝑝𝐻 (26)

where ERHE is the potential of the reversible hydrogen electrode (RHE), EAg/AgCl is the applied

potential, and pH is the basicity of the catholyte. pH is calculated via a reaction-diffusion model

(12). Cell resistance was measured in the 1x1 cm2 flow cell at different pH conditions (table S1).

Cell resistances for reference and CIPH samples were measured to be within 10% at these

configurations.

3

Table S1 – Surface pH and cell resistance as a function of KOH concentration for a

representative configuration (12)

KOH concentration (M) Surface pH Resistance (Ω)

1 12.4 5.1

5 14.5 2.0

7 14.7 1.6

Anode and cathode electrolytes of various concentrations were prepared as described above.

Electrolyte solutions were supplied to the cell at a constant flow rate of 10 ml·min-1 through

peristaltic pumps through silicone tubing (Shore A50). CO2 and CO (Linde, 99.99%) were

supplied to the gas chamber of the flow cell with a constant flow rate of 50 cm3/min, controlled by

a digital mass flow controller (Sierra). For the oxygen reduction reaction (ORR), air was circulated

into the gas chamber via peristaltic pumps.

For each applied potential, gas products from reduction reactions were collected in 1 mL

volumes using gas-tight syringes (Hamilton chromatography syringes) at least three times with the

time intervals of 200 s. This volume was injected into a gas chromatograph (PerkinElmer Clarus

680), equipped with a thermal conductivity detector (TCD), flame ionization detector (FID), and

packed columns (Molecular Sieve 5A and Carboxen-1000). Argon (Linde, 99.999%) was

employed as the carrier gas in the gas chromatograph.

The Faradaic Efficiencies (FEs) were determined as a function of operating current, gas

chromatography and flow rate at the outlet of the gas chamber as:

𝐹𝐸 =𝑛 ∙ 𝐹 ∙ 𝜃 ∙ 𝑓𝑚

𝐽 (6)

where n is the number of electrons for a given product; F is the Faradaic constant; θ is the volume

fraction of the gases; fm is the molar reacting gas flow rate; J is the current.

The combined cathodic energy efficiency (1/2) for C2 products was calculated as follows:

4

𝐸𝐸𝑐𝑎𝑡ℎ𝑜𝑑𝑖𝑐 = 𝛴𝐹𝐸𝐶2 × 𝐸𝐶2

1.23 − 𝑉 (7)

where FEC2 is the Faradaic Efficiency of C2 products (ethylene, ethanol, acetate); EC2 is the

thermodynamic cell potential for C2 products (i.e. EC2 = 1.17 V for ethylene); 1.23 V is the

thermodynamic potential for water oxidation in the anode side; and V is the applied potential vs.

RHE after iR compensation.

Liquid product analysis: liquid products were analyzed via nuclear magnetic resonance

spectroscopy (NMR) from respective catholyte solutions. A new cathode, catholyte, and anolyte

was used for the collection of a single liquid product distribution at a given applied potential. A

constant volume of 25 ml was recirculated through anode and cathode compartments using

peristaltic pumps. The flow cell was operated at the desired applied potential for at least 800 s.

Cathode electrolyte was collected from the flow cell and tubing, sealed and stored in a fridge until

NMR sample preparation. For NMR sample preparation, stored solutions were diluted 20 times in

DI water and mixed with an internal standard, dimethyl sulfoxide (DMSO), in NMR tubes.

1HNMR spectra were collected on Agilent DD2 500 spectrometer in D2O in water suppression

mode, and liquid product distributions were obtained by analyzing the resulting spectra in

MestReNova. The relaxation time between the peaks was selected as 16 s to ensure complete

proton relaxation. The detection threshold for liquid products was around 10 µM.

ECSA Methods: Cyclic Voltammetry (CV) scans were recorded at five scan rates with a minimum

of 3 cycles in the non-Faradaic region, specifically between -0.7 V vs. Ag/AgCl and -1.1 V vs.

Ag/AgCl. Scan rates of 20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s, 100 mV/s, and 200 mV/s were used.

The currents at a given potential, -0.8 V vs. Ag/AgCl, were recorded from the forward and reverse

scans of the third cycle. The difference between these currents was plotted against the scan rate to

obtain a straight line. The slope of this line corresponds to the capacitance of the catalyst’s electric

double layer in Farads. The electrochemical surface area (ECSA) can be determined by comparing

the electric double layer capacitance to that of a perfectly flat copper or silver material.

Measurements were conducted under constant CO2 flow and the recirculation of 5M KOH

electrolyte.

5

Table S2 – Double-layer capacitance determined by cyclic voltammetry

Sample Double Layer Capacitance (mF)

Copper on a PTFE GDL 2.5

Copper on a PTFE GDL with a 12.5 µl/cm2

ionomer overlayer 2.7

Silver on a PTFE GDL 0.98

Silver on a PTFE GDL with a 12.5 µl/cm2

ionomer overlayer 0.92

Partial pressure studies: Partial pressure studies were carried out using the same configuration.

The relative flows of CO2/N2 and CO/N2 gas mixtures were controlled using two mass-flow

controllers (Sierra), and the total flow maintained at ≈50 cm3/min.

H-cell experiments: Experiments in the h-cell were performed by using PTFE/metal samples as

working electrodes fully immersed in electrolyte solution. The area was masked using Kapton tape

to be ≈1 cm2. An anion exchange membrane was used together with a Pt foil counter electrode at

the anode.

Microscopies

Scanning Electron Microscopy: SEM images were acquired using a Hitachi SU-8230 apparatus at

5 keV and different magnifications. Cross-sectional elemental mapping was performed using a

Hitachi CFE-TEM HF3300, the Cu coated gas diffusion layer sample was prepared using Hitachi

Dual-beam FIB-SEM NB5000. Briefly, a slice (≈50-100 nm thick) of Cu coated gas diffusion layer

was sectioned from its back using Ga-beam and attached to a TEM stage with tungsten deposition

and lifted out for subsequent STEM-EDX analysis.

Transmission Electron Microscopy and elemental mapping: These maps and images were taken

on a FEI Titan 80-300 LB TEM, operated at 200kV. The instrument is equipped with a CEOS

image corrector and a Gatan Tridiem energy filter. EELS mapping reveals the presence of copper

nanoparticles and PFSA ionomer. These samples were prepared by a Zeiss NVision 40 FIB in

cross-section mode. Raw data for microscopies available at ref. (75).

6

WAXS measurements: WAXS measurements were carried out in transmission geometry at the

CMS beamline of the National Synchrotron Light Source II, a U.S. Department of Energy

(DOE) office of the Science User Facility operated for the DOE Office of Science by

Brookhaven National Laboratory. Samples were measured with an imaging detector at a

distance of 0.153 m using an X-ray wavelength of 0.729 Å. Nika software package was used to

sector average the 2D WAXS images (51). Data plotting was done in Igor Pro (Wavemetrics,

Inc., Lake Oswego, OR, USA). For grazing-incidence WAXS (GIWAXS), ionomer samples

were deposited by spray coating on cleaned Si substrates using a similar protocol to standard

samples.

Contact angle measurements: Contact angle measurements were performed using the sessile

drop method on a video-based contact angle measuring system (OCA 15EC). Briefly, a single

water droplet was placed on the sample and approximately 15 seconds was given before the

contact angles were measured by the computer software.

Raman measurements: In situ and ex situ Raman spectra were recorded with a Renishaw Raman

spectrometer using a 785 nm excitation laser and 1200 mm−1 grating. Spectra were collected in

the range of 200 - 3000 cm-1 over 10 acquisitions with an exposure time of 10 seconds for each

acquisition. These were averaged together and analyzed using WiRE 4.4 software. The laser

power was 200 µW and a 63x magnification immersion objective was used with a custom PTFE

flow cell.

The in situ flow cell had a liquid electrolyte reservoir in which the immersion objective

was dipped, and a gas diffusion electrode separated the electrolyte reservoir and the gas channel

that continuously delivered CO2 gas to the catalyst at a flow rate of 50 cm3/min. For ORR, air

was fed using peristaltic pumps. The area of the electrode in this configuration was 1 cm 2. The

counter electrode, a Pt wire, and the reference electrode, Ag/AgCl, were dipped in the

electrolyte reservoir ≈1 cm from the cathode.

XAS measurements: in situ XAS measurements was carried out at 9BM beamline of the

Advanced Photon Source (APS) in Argonne National Laboratory (Lemont, Illinois). Operando

XAS experiment for CO2RR proceeded by using in situ XAS flow cell (Applied potential: -2.0

V vs. Ag/AgCl (chronoamperometry), electrolyte: 5 M KOH, CO2 flow).

7

Local species concentration modeling. The system was modeled as a two-dimensional domain

shown in Figure S1 with a catalyst gas diffusion layer, ionomer layer, and electrolyte sub-domains,

building off of previously-well-established models (12, 27). The local concentrations of CO2,aq,

CO32-, HCO3

-, OH- , H+, and H2O in an electrolyte solution under CO2RR conditions were modeled

in COMSOL 5.4 (COMSOL Multiphysics, Stockholm, Se) using the Transport of Dilute Species

physics. This model, based on previous papers (12, 52), accounts for the acid-base carbonate

equilibria, as well as CO2 reduction via electrocatalysis in an electrolyte solution (e.g., KOH). A

time-dependent study was performed to simulate species evolution toward steady state.

Geometry

At the lower boundary, the gas-electrolyte interface, the CO2,aq concentration was specified

according to Henry’s Law and the Séchenov effect, with zero flux imposed for CO32-, HCO3

-, and

OH-. A symmetry condition was imposed at the right boundary to model a confined pore geometry,

and equilibrium concentration values were imposed at the top boundary. The left boundary

contained a thin catalyst region over which the CO2,aq was reduced and OH- was produced.

Figure S1: Schematic of CO2RR configuration.

8

CO2 solubility

The quantity of dissolved CO2 in solution is determined by the temperature, pressure, and solution

salinity. Assuming CO2 acts as an ideal gas, the dissolved amount is given by Henry's Law (53,

54):

[𝐶𝑂2]𝑎𝑞,0 = 𝐾0[𝐶𝑂2]𝑔, (1)

where,

ln 𝐾0 = 93.4517 (100

𝑇) − 60.2409 + 23.3585 ln (

𝑇

100), (2)

where T is the temperature of the solution in K. The solubility is further diminished due to high

concentration of ions in solutions according to the Séchenov Equation (55) as:

log ([𝐶𝑂2]𝑎𝑞,0

[𝐶𝑂2]𝑎𝑞) = 𝐾𝑠𝐶𝑠 , (3)

where,

𝐾𝑠 = ∑(ℎ𝑖𝑜𝑛 + ℎ𝐺) (4)

ℎ𝐺 = ℎ𝐺,0 + ℎ𝑇(𝑇 − 298.15), (5)

Table S3: Séchenov constants

Ion 𝒉𝒊𝒐𝒏

K+ 0.0922

HCO3- 0.0967

CO32- 0.1423

SO42- 0.1117

Other parameters

ℎ𝐺,0,𝐶𝑂2 -0.0172

ℎ𝑇 -0.000338

ℎ𝐺,0,𝑂2 0

ℎ𝐺,0,𝐶𝑂 0 (assumed)

9

Figure S2: Maximum (top) CO2, (middle) O2, and (bottom) CO solubility (mM) versus K(anion)

concentration.

Carbonate Equilibria

CO2, CO32-, HCO3

-, OH-, H+, and H2O are all in equilibrium in solution as given by (56–59):

𝐶𝑂2 + 𝐻2𝑂 ↔ 𝐻+ + 𝐻𝐶𝑂3 − (K1) (6)

𝐻𝐶𝑂3 − ↔ 𝐻+ + 𝐶𝑂3

2− (K2) (7)

10

𝐶𝑂2 + 𝑂𝐻− ↔ 𝐻𝐶𝑂3 − (K3) (8)

𝐻𝐶𝑂3 − + 𝑂𝐻− ↔ 𝐶𝑂3

2− + 𝐻2𝑂 (K4) (9)

𝐻2𝑂 ↔ 𝐻+ + 𝑂𝐻−, (Kw) (10)

where the rate constants are a function of temperature and salinity.

Species transport

Species transport in the various layers (including electrochemistry in porous electrodes near

polymer interfaces) is based on fundamentals presented by Newman and Thomas-Alyea (60) and

others (61, 62), and given by the Poisson-Nernst-Planck set of equations coupled with

electroreduction and acid-base equilibrium reactions:

𝜕𝑐𝑖

𝜕𝑡+

𝜕𝐽𝑖

𝜕𝑥= 𝑅𝑖, (11)

where Ji is the molar flux, given by:

𝐽𝑖 = −𝐷𝑖𝜕𝑐𝑖

𝜕𝑥, (12)

where Di, and is the diffusion coefficient species i (63):

Table S4: Infinite dilution diffusion constants

Species Diffusion coefficient (10-9 m2s-1)

CO2 1.91

CO32- 0.923

HCO3- 1.185

H+ 9.31

OH- 5.273

The reaction term Ri can be broken into carbonate equilibria (Equations 6-10) (12, 52):

11

𝑅𝐶𝑂2= (−[𝐶𝑂2][𝐻2𝑂]𝑘1𝑓 + [𝐻+][𝐻𝐶𝑂3

−]𝑘1𝑟) + (−[𝐶𝑂2][𝑂𝐻−]𝑘3𝑓 + [𝐻𝐶𝑂3−]𝑘3𝑟)

− 𝑅𝐶𝑂2𝑅𝑅

(13)

𝑅𝐶𝑂32− = ([𝐻𝐶𝑂3

−]𝑘2𝑓 − [𝐻+][𝐶𝑂32−]𝑘2𝑟) + ([𝐻𝐶𝑂3

−][𝑂𝐻−]𝑘4𝑓 − [𝐻2𝑂][𝐶𝑂32−]𝑘4𝑟)

(14)

𝑅𝐻𝐶𝑂3− = ([𝐶𝑂2][𝐻2𝑂]𝑘1𝑓 − [𝐻+][𝐻𝐶𝑂3

−]𝑘1𝑟) + (−[𝐻𝐶𝑂3−]𝑘2𝑓 + [𝐻+][𝐶𝑂3

2−]𝑘2𝑟)

+ (−[𝐶𝑂2][𝑂𝐻−]𝑘3𝑓 + [𝐻𝐶𝑂3−]𝑘3𝑟)

+ (−[𝐻𝐶𝑂3−][𝑂𝐻−]𝑘4𝑓 + [𝐻2𝑂][𝐶𝑂3

2−]𝑘4𝑟)

(15)

𝑅𝐻+ = ([𝐶𝑂2][𝐻2𝑂]𝑘1𝑓 − [𝐻+][𝐻𝐶𝑂3−]𝑘1𝑟) + ([𝐻𝐶𝑂3

−]𝑘2𝑓 − [𝐻+][𝐶𝑂32−]𝑘2𝑟)

+ ([𝐻2𝑂]𝑘𝑤𝑓 − [𝑂𝐻−][𝐻+]𝑘𝑤𝑟)

(16)

𝑅𝑂𝐻− = (−[𝐶𝑂2][𝑂𝐻−]𝑘3𝑓 + [𝐻𝐶𝑂3−]𝑘3𝑟)

+ (−[𝐻𝐶𝑂3−][𝑂𝐻−]𝑘4𝑓 + [𝐻2𝑂][𝐶𝑂3

2−]𝑘4𝑟)

+ ([𝐻2𝑂]𝑘𝑤𝑓 − [𝑂𝐻−][𝐻+]𝑘𝑤𝑟) + 𝑅𝑂𝐻𝐸𝑅

(17)

𝑅𝐻2𝑂 = (−[𝐶𝑂2][𝐻2𝑂]𝑘1𝑓 + [𝐻+][𝐻𝐶𝑂3−]𝑘1𝑟)

+ ([𝐻𝐶𝑂3−][𝑂𝐻−]𝑘4𝑓 − [𝐻2𝑂][𝐶𝑂3

2−]𝑘4𝑟)

+ (−[𝐻2𝑂]𝑘𝑤𝑓 + [𝑂𝐻−][𝐻+]𝑘𝑤𝑟)

(18)

and into CO2 reduction and OH- evolution according to the reactions (63, 64):

𝑅𝐶𝑂2𝑅𝑅 =𝑗

𝐹

𝜖

𝐿𝑐𝑎𝑡

∑𝐹𝐸𝐶𝑂2𝑅𝑅

𝑛𝑒𝐶𝑂2𝑅𝑅

[𝐶𝑂2]

[𝐶𝑂2,0], (19)

𝑅𝑂𝐻𝐸𝑅 =𝑗

𝐹

𝜖

𝐿𝑐𝑎𝑡, (20)

where j is the current density applied, F is Faraday’s constant, 𝜖 is the catalyst porosity (0.6), and

Lcat is the width of the catalyst layer, 𝐹𝐸𝐶𝑂2𝑅𝑅 is the Faradaic efficiency of a given product of CO2

reduction (based on experimental observations), 𝑛𝑒𝐶𝑂2𝑅𝑅 is the number of electrons required for

12

the reduction reaction. The rate of CO2 reduction depends on the local concentration, [CO2], which

is normalized by [𝐶𝑂2,0], defined as the maximum solubility concentration of CO2 based on the

electrolyte concentration, pressure, and temperature (Equations 1-5); all of which is ultimately

based on the Butler-Volmer relationship for concentration-dependent partial current (60, 66):

𝑗 = 𝑗0 [𝐶0(0,𝑡)

𝐶0∗ exp(−𝛼𝑓𝜂) −

𝐶𝑅(0,𝑡)

𝐶𝑅∗ exp((1 − 𝛼)𝑓𝜂) ], (21)

where j is the total current density, j0 is the exchange current density, C is the species concentration

(normalized by a reference concentration, C*), 𝛼 is the transfer coefficient, f = F/RT, and 𝜂 is the

overpotential.

The differential form for the diffusion-reaction equations and constants for carbonate species

production are found in previous works (12, 27, 52).

ORR

For the ORR simulations, the model geometry and boundaries were the same, except for the lower

boundary condition for which the saturation concentration of O2 in KOH was imposed for the

given O2 partial pressure. Furthermore, only diffusion and reduction of O2 were accounted for:

𝜕[𝑂2]𝑎𝑞

𝜕𝑡= 𝐷𝑂2,𝑎𝑞

𝜕2[𝑂2]

𝜕𝑥2 −[𝑂2]

[𝑂2,0]

𝑗

𝐹

𝜖

𝐿𝑐𝑎𝑡. (22)

Porous Domain Effective Diffusion

A porous domain with Bosanquet effective diffusivity (67) was employed for the Nafion layer,

which diminishes the effective gas diffusivity due to Knudsen diffusivity (i.e., frequent collisions

with the Nafion pore walls shown in Figure S3).

13

Figure S3: Knudsen diffusion schematic of gas interaction with hard cylindrical pore, where d is

the pore diameter, and l is the free path.

Here, the effective diffusivity is:

𝐷𝑒𝑓𝑓 = (1

𝐷𝑔+

3

√8𝑅𝑇

𝜋𝑀𝑑𝑝

)

−1

, (23)

where Dg is the bulk gas diffusivity, R is the gas constant, T is the temperature, M is the molecular

mass of CO2, dp is the mean pore diameter (2 nm for Nafion (68, 69)), yielding an overall

diffusivity of 2.5·10-7 m2 s-1. Although the effective diffusivity decreases substantially relative to

the gaseous diffusivity (1.6·10-5 m2 s-1), the effective diffusivity remains higher (by ≈400×) than

that of CO2 in KOH since the gas travels along the hydrophobic backbone. The CO2 penetration

depth into the Nafion (illustrated in Figure S4) is further enhanced due to the partition coefficient

(given by Henry’s Law above) between the gas in Nafion and the gas dissolved in electrolyte,

thereby increasing the total available CO2 for the Nafion case relative to the bare electrode case.

14

Fig. S4: Comparing (left two columns) bare electrode and (right two columns) Ionomer cases

for (top row) pH 14 and (bottom row) pH 15 for 0 and 1 A cm-2 current density. For the

ionomer cases, the ionomer is present within the left tall rectangle (0 to 10 nm), with electrolyte

outside this region.

15

S5 –ORR and CO2RR availability partial currents (from previous)

Fig. S5. Reacting gas concentration within the ionomer layer for different diffusion

coefficients (D) for (A) O2 and (B) CO2.

Limiting current density

To determine the limiting partial current density (Figure S6), we first calculated the mean species

concentration in the catalyst layer since the concentration determines the overall reaction rate

(Equations 1,3). The partial current density is then given by:

𝑗𝑝𝑎𝑟𝑡𝑖𝑎𝑙 = 𝑗𝑎𝑝𝑝𝑙𝑖𝑒𝑑[𝐺]

[𝐺0], (23)

where the overbar denotes mean, [G] is either CO2 or O2, and [G0] is the maximum solubility

concentration based on Equations 1-5. This reference concentration is the same as that chosen for

Equation 19. The resulting partial current density versus applied current density was fit with a

saturation function:

𝑗𝑝𝑎𝑟𝑡𝑖𝑎𝑙 = 𝑗𝑙𝑖𝑚 tanh(𝑘 𝑗𝑎𝑝𝑝𝑙𝑖𝑒𝑑) , (24)

where jlim and k are fitting parameters as a guide to the eye. The parameter jlim is the saturated level

of the curve, thus providing the limiting current density for the given conditions modeled. Finally,

16

we determined the species diffusivity for specific, experimental conditions by fitting the limiting

current density versus diffusivity and interpolating based on the experimentally observed limiting

current densities.

Figure S6: Partial current density saturation highlighting the limiting current density for

increasing pH.

17

Temperature Effects

To model CO2 availability as temperature increases, we considered solubility and diffusion aspects

(Figure M6).

Figure S7: CO2 solubility versus temperature.

The diffusivity of CO2 will increase (69) according to:

𝐷 = 5019 ⋅ 10−9 exp (−19.51[

𝑘𝐽

𝑚𝑜𝑙]

𝑅𝑇 ), (25)

where R is the universal gas constant, and T is temperature. However, the rate constants toward

carbonate formation will also increase (57, 59, 71), meaning the overall CO2 availability will

effectively decrease due CO2 consumption by the electrolyte in the reference case.

18

S8 – Contact angle measurements

Fig. S8 - Contact angle measurement of reference and CIPH samples reveal a similar

hydrophobic/hydrophilic character after PFSA coating.

19

S9 – WAXS studies on reference and CIPH

Fig. S9. 2D WAXS patterns of (A) reference and (B) CIPH samples. The corresponding sector

cuts are shown in Figure 2. (111), (200), (220) and (311) lattice planes of Cu are clearly detected

in both samples. PFSA features are additional present in CIPH samples at around 2.0 A-1, and 1.2

A-1.

20

S10 – Raman studies for CIPH Ag catalysts

Fig. S10. Raman spectroscopies. (A) PTFE/Ag reference and CIPH Ag samples showing the

distinctive presence of sulfonate and –CF2 groups for CIPH. (B) Raman spectra of hydrated

samples. Samples were excited at 738 nm and the signal was collected through air or an immersion

63× objective lens.

21

Table S5 – Wavenumber for ionomer-relevant functional groups (42, 72)

Wavenumber (cm-1) Group

1980 H2O

1787 OH

1610 OH

1446 S=O

1386 v(C-C)

1300 CF3 a

1182 C-F a

1130 vs(SO3-)

1060 S-O CCO s

1005 vs(SO3-)

966 S-O C-S s

936 S-OH

893 v(C-S)

802 v(C-S)

733 CF CCC

640 CF2 rock

450 Metal-CO

388 δ(CF2)

22

S11 – In situ Raman studies for CIPH Cu catalysts

Fig. S11. In situ Raman spectra of Cu CIPH catalysts. The Raman spectrum of CIPH Cu

samples shows the distinctive presence of sulfonate and –CF2 groups, as well as a significant

amount of adsorbed CO at 280 and 350 cm-1 – and hence extended CO coverage – in the case of

CIPH samples. Samples were operated in a 5 M KOH electrolyte in a custom-made flow cell at

-1.6 and -2 V vs Ag/AgCl for reference and CIPH samples respectively. Beyond -1.6 V vs.

Ag/AgCl, reference samples led to substantial H2 generation. Samples were excited was 738 nm

and the signal collected through a 63× immersion objective.

23

S12 – Oxygen Reduction Reaction (ORR) optimization

Fig. S12. Oxygen reduction reaction and impact of PFSA loading. Cyclic voltammograms of

CIPH Ag samples for different PFSA loadings at 5 M KOH operation. The ORR limiting current

increases reaching a maximum for 12.5 µL/cm2 loading.

24

S13 – In situ (operando) Raman for ORR in Ag CIPH samples

Fig. S13. Raman spectrum of CIPH Ag and reference samples under ORR operation (1 M

KOH -1 V vs Ag/AgCl). Reference spectra has been scaled up. CIPH shifted 0.2 a.u. for clarity.

Peaks around 1150 cm-1 and 1550 cm-1 are ascribed to adsorbed O2 species (73–75), showing an

increase of O2 availability for Ag CIPH catalysts. Samples were excited was 738 nm with and the

signal collected through immersion 63× objective.

25

S14 – ORR/HER for Ag catalysts in 5 M KOH

Fig. S14. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under

(A) ORR and (B) HER operation in 5 M KOH electrolyte. N2 was purged during HER

operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the

PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR

current due to increased O2 availability.

26

S15 – ORR/HER for Ag catalysts in 1 M KHCO3

Fig. S15. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under

(A) ORR and (B) HER operation in 1 M KHCO3 electrolyte. N2 was purged during HER

operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the

PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR

current due to increased O2 availability.

27

S16 – ORR/HER for Ag catalysts for 0.5 M H2SO4

Fig. S16. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under

(A) ORR and (B) HER operation in 0.5 M H2SO4 electrolyte. N2 was purged during HER

operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the

PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR

current due to increased O2 availability.

28

S17 – ORR/HER for Ag catalysts in K2SO4

Fig. S17. Current-voltage characteristics of Ag/PTFE reference and CIPH samples under

(A) ORR and (B) HER operation in 0.7 M K2SO4 electrolyte. N2 was purged during HER

operation. Reference and Ag CIPH samples exhibit a similar HER performance, revealing that the

PFSA ionomer layer does not modify water or ion transport. CIPH samples show enhanced ORR

current due to increased O2 availability.

29

S18 – Ag CIPH CO2RR product distribution

Fig. S18. Product distribution of (A) Ag/PTFE reference and (B) Ag CIPH samples under CO2

reduction operation at 5 M KOH electrolyte as a function of current density. CIPH samples sustain

efficient C1+ production at much higher productivities.

30

S19 –CO2RR in flow-cell (KHCO3 electrolyte)

Fig. S19. Characterization of Ag reference and CIPH samples in a flow-cell configuration

(1 M KHCO3). (A) Faradaic efficiency vs. current density and (B) current density vs. potential for

Ag control samples. (C) Faradaic efficiency vs. current density and (D) current density vs.

potential for Ag+ionomer (CIPH) samples. Reference samples show a CO2 to CO limiting current

density of 110 mA cm-2. No limiting current is observed for CIPH samples in this range.

31

S20 –CO2RR in flow-cell (K2SO4 electrolyte)

Fig. S20. Characterization of Ag reference and CIPH samples in a flow-cell configuration

(0.7 M K2SO4). (A) Faradaic efficiency vs. current density and (B) current density vs. potential

for Ag control samples. (C) Faradaic efficiency vs. current density and (D) current density vs.

potential for Ag+ionomer (CIPH) samples. CIPH samples evidence improved CO2 mass transport,

with JCO partial current density increasing from 50 to 110 mA cm-2.

32

S21 – Cu CIPH CO2RR product distribution

Fig. S21. Product distribution of (A) Cu/PTFE reference and (B) Cu CIPH samples under CO2

reduction operation in 5 M KOH electrolyte as a function of current density. CIPH samples sustain

efficient C2+ production at much higher productivities.

33

S22 – SEM post reaction of Cu CIPH samples

Fig S22. Scanning electron micrographs of Cu CIPH samples after CO2 reduction reaction

at different magnification. The samples were operated under 5 M KOH at -3 V vs Ag/AgCl for

50 min. The PFSA layer is evident at all magnifications. Cu surface has experienced surface

reconstruction leading to the formation of smaller grains.

34

S23 – Cu CIPH CORR product distribution

Fig. S23. Product distribution of (A) Cu/PTFE reference and (B) Cu CIPH samples under CO

reduction operation at 5 M KOH electrolyte as a function of current density. CIPH samples sustain

efficient C2+ production at much higher productivities.

35

S24 – Liquid-phase ORR in H-cell

Fig. S24. Characterization of Ag reference and CIPH samples in an h-cell configuration

(0.5 M H2SO4). Ambient air is bubbled in the catholyte. In this configuration, the ORR limiting

current density is determined by the gas solubility in the electrolyte. Both samples exhibit a similar

ORR limiting current, suggesting that the ionomer does not significantly modify local gas reactant

solubility.

36

S25 – Liquid-phase CO2RR in H-cell

Fig. S25. Characterization of Ag reference and CIPH samples in an H-cell configuration

(1 M KHCO3). CO2 gas is bubbled in the catholyte. In this configuration, the CO2R limiting

current density is determined by the gas solubility in the electrolyte. (A) Cyclic voltammogram

and Faradaic efficiency vs current density for (B) Ag reference and (C) Ag CIPH. Both samples

exhibit a similar CO2 limiting current, suggesting that the ionomer does not significantly modify

local gas reactant solubility.

37

S26 – In situ (operando) XAS experiments

Fig. S26. Operando XAS analysis of Cu reference and Cu/ionomer (CIPH) during CO2RR at -

2.0 V vs. Ag/AgCl in 5 M KOH electrolyte. (A) Cu K-edge XANES spectra of Cu reference. (B)

Fourier transformation of Cu K-edge EXAFS spectra. (C) Cu K-edge XANES spectra of CIPH.

(D) Fourier transformation of Cu K-edge EXAFS spectra of Cu CIPH.

To monitor possible changes in Cu atomic environment (oxidation state and coordination

number), we carried out operando X-ray absorption spectroscopy (XAS) (fluorescence mode) for

bare PTFE/Cu and PTFE/Cu/ionomer (CIPH) samples using a custom-made cell. 5 M KOH

electrolyte was circulated in both anode and cathode and CO2 gas was supplied from the backside

of cathode of the in-situ XAS flow cell.

38

Cu K-edge XANES spectra of both reference and CIPH samples revealed a similar

oxidation state of Cu (Cu0, metallic Cu), which was maintained during CO2R under the reducing

potential of -2.0 V vs. Ag/AgCl.

For reference samples, Cu-Cu coordination number (CN) obtained by EXAFS analysis

started to increase after CO2R (2 min), which was maintained during a 30 min initial study. CIPH

samples showed a similar trend for Cu coordination before and during reaction. This reveals that

the atomic local environment, coordination number and electronic structure of Cu active sites was

note affected by the presence of the ionomer gas channel.

S27 – Partial pressure studies (CO2RR in 5 M KOH)

Fig. S27. Partial current densities as a function of CO2|N2 partial pressure for (A) CO current

Cu/PTFE reference (B) CO current Cu CIPH samples (C) C2H4 current Cu/PTFE reference (D)

C2H4 current Cu CIPH.

39

S28 – Partial pressure studies (CO2RR in 1 M KHCO3)

Fig. S28. Partial current densities as a function of CO2|N2 partial pressure for (A) CO current

Cu/PTFE reference (B) CO current Cu CIPH samples (C) C2H4 current Cu/PTFE reference (D)

C2H4 current Cu CIPH.

40

S29 – CIBH product distribution

Figure S29 - Product distribution for optimum CIBH catalyst at different currents in 7 M

KOH and with 50 cm3/min of CO2 flow. No other liquid products were detected for this

catalyst and these operating conditions.

41

Performance metric of CIBH catalysts in 7M KOH with 50 cm3/min of CO2 flow (data

corresponding to Fig. 4E).

Table S6. 1.67 mg/cm2, 4:3 Cu:PFSA

Table S7. 3.33 mg/cm2, 4:3 Cu:PFSA

Table S8. 5 mg/cm2, 4:3 Cu:PFSA

Table S9. Cu CIPH reference

42

S30 – C2+ current density vs EE1/2

Figure S30 – C2+ current density vs EE1/2. Partial current density towards C2 products versus

cathodic energy efficiency. Dashed lines represent isosurfaces with constant current × energy

efficiency. CIBH samples operated in a 7 M KOH electrolyte achieve more than a sixfold increase

in partial current density at cathodic energy efficiencies in the > 40% range compared to best stable

(> 1 h) reported catalyst.

43

S31 – CIBH performance statistics

Figure S31 – Performance statistics towards ethylene in 7 M KOH and with 50 cm3/min of

CO2 flow for optimized (catalyst:ionomer; nominal Cu thickness, and catalyst loading) CIBH

samples (A) Faradaic Efficiency and (B) partial current density over 8 different operations.

44

S32 – GC traces

Figure S32 – GC traces for best CIBH samples (Fig. 4F) (operation in a 7 M KOH electrolyte

and 50 cm3/min of CO2 flow).

45

S33 – Stability

Figure S33 – Stability of CIBH samples integrated into a membrane-electrode-assembly

(MEA) configuration. Current (top) and Faradaic Efficiency towards C2H4 (bottom) and of CIBH

samples operated continuously in a 0.1 M KHCO3 electrolyte at -3.9 V full cell potential.

46

S34 – Effect of temperature on CIPH samples

Figure S34 – CIPH catalyst operation at higher temperature operation. (A) FE towards

ethylene for flat reference and CIPH samples show a shift towards lower voltage for a given

selectivity as temperature increases from room temperature (RT) to 60°C. An opposite trend is

observed for reference samples. (B) The corresponding partial current density also increases at

similar potential, yielding a better C2H4 productivity at similar energy efficiency. Reference

samples show a moderate increase in partial current. iR compensation was applied based on EIS

measurements at RT (Rs = 1.62) and 60°C (Rs = 1.19) operating conditions and a 0.9 correction

factor. Dashed lines are a linear fit to serve as a guide to the eye. Samples operated in a 7 M KOH

electrolyte and 50 cm3/min of CO2 flow.

47

S35 – Effect of temperature on CIBH samples

Figure S35 – CIBH catalyst operation at higher temperature operation. (A) Potential vs.

applied current for CIBH samples at RT and 60°C. Improvements, up to 0.9 V are observed at

large currents. Ethylene FE (B) and partial current density (C) versus voltage: a similar FE is

sustained at lower potentials and higher partial current densities are obtained at lower potentials.

iR compensation was applied based on EIS measurements at RT (Rs = 1.55) and 60°C (Rs = 1.15)

operating conditions and a 0.9 correction factor. Samples operated in a 7 M KOH electrolyte and

50 cm3/min of CO2 flow.

48

S36 – Effect of temperature on CIBH samples

Figure S36 – CIBH catalyst operation at higher temperature operation (60°C). Faradaic

efficiency vs. current density for (A) room temperature and (B) high temperature cases. The

cathode chamber of the flow cell was minimized to 4 mm length and no reference electrode was

employed. Samples operated in a 7 M KOH electrolyte and 50 cm3/min of CO2 flow.

49

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Disclaimer: Certain commercial equipment, instruments, or materials are identified in this paper

in order to specify the experimental procedure adequately. Such identification is not intended to

imply recommendation or endorsement by the National Institute of Standards and Technology, nor

is it intended to imply that the materials or equipment identified are necessarily the best available

for the purpose.

Funding: This work was financially supported by the Ontario Research Foundation: Research

Excellence Program the Natural Sciences and Engineering Research Council (NSERC) of Canada,

the CIFAR Bio-Inspired Solar Energy program and TOTAL S.A. This research used resources of

the National Synchrotron Light Source II, which is a US DOE office of Science Facilities, operated

at Brookhaven National Laboratory under Contract No. DESC0012704. J.W. gratefully

acknowledges financial support from the Ontario Graduate Scholarship (OGS) program. A.S.

thanks Fonds de Recherche du Quebec - Nature et Technologies (FRQNT) for support in the form

of a postdoctoral fellowship award.

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