Supporting Information

Efficient electrolyser for CO2 splitting in neutral water using earth abundant materials

Arnaud Tatin, Clément Comminges, Boniface Kokoh, Cyrille Costentin*, Marc Robert*, Jean-Michel Savéant*

Chemicals

Materials. KCl (99.5%), KHCO3 (99.5%) were purchased from Merk; KH2PO4 (97%) was purchased from Prolabo; K2HPO4 (99.5%) was purchased from Labosi; Toray® carbon paper (PTFE treated, TGP-H-60), Nafion® NRE-212 membrane (0.05 mm thick, ≥0.92 meq/g exchange capacity), and steel mesh (Stainless Steel gauze, 325 mesh woven from 0.0014 inch diameter wire, type 316) were purchased from Alfa Aesar; carbon powder (Vulcan XC 72R) was purchased from Cabot; ® Co(NO3)2.6H2O (98%), Fe(NH4)2(SO4)2.6H2O (99%), and Nafion perfluorinated resin solution (5 wt.

%, EW 1,100) were purchased from Sigma Aldrich; NH4PF6 (99.5%) was purchased from Acros Organics; free base 5,10,15,20-Tetra(4’-N,N,N-TriMethylAnilinium)Porphyrin Tetrachloride was purchased from Frontier Scientific. All chemicals were used as received without further purification.

Gases (Argon and CO2) were purchased from Air Liquide (Arcal™ Prime). Acetone (99.8%), methanol (99.8%), ethyl acetate (99.5%), dichloromethane (99%), 2-propanol (99.0%), and hydrochloric acid (38-38%) were purchased from VWR. Electrolyte solutions were prepared with ultrapure water (TKA MicroPure, 0.055 μS·cm−1). Synthesis and Characterization of Iron(III) 5,10,15,20-Tetra(4’-N,N,N-TriMethylAnilinium) Porphyrin Pentachloride WSCAT. Iron insertion into the commercial free base porphyrin was achieved by updating an existing procedure. (S1) A solution of 5,10,15,20-Tetra(4’-N,N,N- TriMethylAnilinium)Porphyrin Tetrachloride (102 mg, 9.5x10-5 mol) and Mohr’s salt -3 (Fe(NH4)2(SO4)2.6H2O, 324 mg, 1.5x10 mol) in ultra-pure water (41 mL) was degassed by Argon for 10 minutes; the mixture was stirred at 85 °C under inert atmosphere for 3 hours. Reaction product is -4 precipitated by adding 10 equivalents NH4PF6 (155 mg, 9.5x10 mol). The resulting suspension is centrifuged for 10 minutes at 10,000 rpm to allow a better separation of liquid and solid phases. The solid is collected and washed with 25 mL ultra-pure water and 10 equivalents NH4PF6 (155 mg, 9.5x10-4 mol). Traces of initial free-base porphyrin are removed by washing the solid with 15 mL acetone/CH2Cl2 (1:1), then 5 mL CH3OH, and eventually five times 5 mL acetone/CH2Cl2 (1:1). Then, the residue is treated by 22.5 mL acetone and 2.5 mL concentrated HCl (37 %) is added dropwise to exchange both iron ligand and anilinium counter-ions. The resulting suspension is sonicated for 10 minutes then centrifuged for 10 minutes at 10,000 rpm. After the supernatant is removed, the solid phase is collected and washed with 25 mL acetone. The residue is then dissolved in a minimum of methanol and precipitated with ethyl acetate. The solvent is eventually removed in vacuo to yield WSCAT as a dark red powder (49.9 mg, 44%). Catalyst characterization was assessed by Cyclic in N,N-Dimethylformamide and UV-Vis spectroscopy.

WSCAT

Methods and Instrumentation

Preparation of CO2-reduction . A previously reported procedure has been adapted to immobilize the WSCAT catalyst in a thin film onto the surface. (S2) Cathode material was porous carbon paper (Toray®) and total catalyst molar loading was between 3.7 – 7.4 × 10-7 mol cm-2. Electrodes were manufactured from a powder of WSCAT catalyst (0.40 mg cm-2), conductive carbon -2 -2 ® powder (0.94 mg cm ), and KHCO3 (0.10 mg cm ) suspended in a mixture of Nafion solution (25.4 µL cm-2) and 2-propanol (25.4 µL cm-2). After sonication for 10 minutes, the uniform suspension was sprayed onto the carbon paper (or a 3 mm diameter glassy carbon for experiments) then air-dried at room temperature. Electrodes were rinsed in ultra-pure water before use. Amount of catalyst leaching from the catalytic film into the rinsing solution was assessed by UV-vis spectroscopy to less than 1 % of total amount of catalyst immobilized onto the electrode. Then, no catalyst leaks were witnessed.

Preparation of H2O-oxidation electrodes. The CoPi oxygen evolution catalyst was electrodeposited onto a stainless steel mesh. The active surface area was estimated to be 2.2 times the geometric surface area. Depositions by controlled potential electrolysis were carried out at 0.827 V vs. SHE in a two- compartment cell; electrolyte was aqueous 0.1 M potassium phosphate at pH 7 with 0.5 mM Co2+ and the counter electrode was a platinum wire in a compartment containing a potassium phosphate solution separated by a glass frit. Deposition times vary accordingly to the target charge density (typically 50 mC/cm2). Electrochemical measurements. A Princeton Applied Research (PARSTAT 4000) and a Metrohm Autolab (PGSTAT 128N) interfaced with VersaStudio (2.42.3) and Nova (1.10.4) software respectively were used for all experiments. Cyclic voltammograms were obtained in a three- electrode cell with a platinum wire as counter electrode. All cyclic voltammetry experiments were carried out either under argon or carbon dioxide atmosphere at 21 °C, the double-wall jacketed cell being thermostated by circulation of water. was carried out at room temperature.

Ohmic drop between , which is the cathode unless otherwise specified, and was compensated through the positive feedback compensation method implemented in the instrument. pH measurements were performed with Hanna pH210 and HI221 instruments and 6-mm microelectrodes (Fisher). Potentials were measured against a saturated calomel electrode (SCE) and converted to the standard hydrogen electrode (SHE) reference scale using: E(vs. SHE) = E(vs. SCE) + 0.244 V

CO2 splitting electrolysis. The gas-tight was made of two compartments (50 mL flasks) and a Nafion® proton-exchange membrane supplied the cathode with the protons produced at the . Electrolytes in both compartments had the same, close to neutral, pH (7.3) but they differ because no exogenous buffer shall be introduced in the cathodic chamber to achieve selective CO2 reduction to CO. Cathodic electrolyte was a CO2-saturated 0.1 M KCl + 0.5 M KHCO3 aqueous solution and anodic electrolyte was a 0.4 M potassium phosphate buffer degassed under Argon. Solutions were purged for 15 minutes before the start of electrolysis. The headspace of the cathodic chamber was either sealed with an expansion vessel or continuously purged with CO2. In both experiments, periodic manual injections in a gas chromatograph gave CO2 reduction products selectivity. The anodic chamber was originally filled with electrolyte; gas evolution was gauged over time and oxidation products were investigated by gas chromatography after electrolysis had run to completion. Cell voltage measurement. The potentiostats used did not allow for simultaneous measurement of both working electrode and counter vs. the reference electrode. Thus, cell voltage

(Ucell) was recorded and forwarded to an external input of the by means of a simple homemade differential amplifier (AD711 operational amplifiers with ±15 V power supply) with unity gain (identical 10 kΩ resistors) to measure:

Ucell (V) = Eanode - Ecathode Gas product analysis. Gas chromatography analyses of gas evolved in the headspace during electrolysis were performed with Agilent Technologies (7820A GC and 490 micro GC) systems equipped with a thermal conductivity detector. CO and H2 production was quantitatively detected using a CP-CarboPlot P7 capillary column (27.46 m in length and 25 μm internal diameter). Temperature was held at 150 °C for the detector and 34 °C for the oven. The carrier gas was argon flowing at 9.5 mL/min at constant pressure of 0.5 bars. Injection was performed via 100µL and 250-

μL gas-tight (Hamilton #1710N and #1725N) syringes previously degassed with CO2. Conditions allowed detection of H2, O2, N2, CO, and CO2. Calibration curves for each gas were determined separately by injecting known quantities of pure gas. Ionic product analysis. Both electrolytes were diluted 10-fold and 100-fold with ultra-pure water then analyzed using a Dionex DX100 ionic chromatograph containing the following elements: CD-20 conductometric detector, ASRS- 300 4-mm conductivity suppressor, and a Dionex IonPac AS10 ionic exchange column 4mmin diameter. The eluant was aqueous NaOH 50 mM flowing at 1 mL/min.

SEM ex-situ electrode analysis. The samples were dried under air. Scanning Electron Microscopy (SEM) images were collected on a Carl Zeiss AG-Supra 40 microscope operated at a voltage of 3.00 kV.

Supplementary Figures

10 Theoretical O2 evolution 9 Actual O2 production 8

7

6

5

4

3 Gas(mL) production 2

1

0 0 1 2 3 4 Elapsed time (hr) Fig S1. Representative Oxygen evolution at the CoPi anode. Bulk electrolysis was performed on a CoPi anode in a 0.4 M Phosphate buffer at pH 7.3 degassed under Argon. A current of 10 mA is applied during 4 hours. Faradic efficiency for O2 production is 99 % at the end of the electrolysis.

15

10 CO CO2 H2O O2

5

Electrode potential (V vs SHE) 0 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Current(mA) -5

-10

= 0.32 V = 0.50 V C  U=2.2 V -15 (including ohmic drop Ri = 50 mV)

After 24 hr electrolysis Before electrolysis

Fig. S2 Stability assessment. Catalyst WSCAT loading on the cathode was 0.40 mg/cm2 and the Cobalt film deposited on the anode was estimated to be 200 nm thick. Linear scans are recorded at a scan rate of 10 mV/s before and after 24 hr electrolysis at -0.96 V vs SHE. Cell potential increases by 15 mV. Theoretical oxidation and reduction potentials are E0  0.80 V vs SHE OHO22/ and E0 0.54 V vs SHE respectively. Dash lines represent anode potential corrected from CO2 /CO ohmic drop through the electrolysis cell.

-10.0 WSCAT -9.0 FeTPP

-8.0

-7.0

-6.0

-5.0

-4.0 Current (mA) Current

-3.0

-2.0

-1.0

0.0 0 15 30 45 60 75 90 105 120 Elapsed time (min) Fig. S3 Relative performance of WSCAT vs. FeTPP. Current over time for a bulk electrolysis at a cathodic potential of -0.96 V vs SHE in 0.1 M KCl + 0.5 M KHCO3 saturated with 1 atm. CO2.

References (S1). Costentin C, Robert M, Savéant JM & Tatin A (2015) Efficient and selective molecular catalyst for the CO2-to-CO electrochemical conversion in water. Proc. Natl. Acad. Sci. USA 112:6882-6886. (S2). He Q et al. (2012) Molecular catalysis of the oxygen reduction reaction by iron porphyrin catalysts tethered into Nafion layers: An electrochemical study in solution and a membrane-electrode- assembly study in fuel cells. J. Power Sources 216:67-65.