Elucidation of Critical Catalyst Layer Phenomena toward High Production Rates for the Electrochemical Conversion of CO to Ethylene

This work utilizes EIS to elucidate the impact of catalyst–ionomer interactions and cathode hydroxide ion transport resistance (RCL,OH–) on cell voltage and product selectivity for the electrochemical conversion of CO to ethylene. When using the same Cu catalyst and a Nafion ionomer, varying ink dispersion and electrode deposition methods results in a change of 2 orders of magnitude for RCL,OH– and ca. a 25% change in electrode porosity. Decreasing RCL,OH– results in improved ethylene Faradaic efficiency (FE), up to ∼57%, decrease in hydrogen FE, by ∼36%, and reduction in cell voltage by up to 1 V at 700 mA/cm2. Through the optimization of electrode fabrication conditions, we achieve a maximum of 48% ethylene with >90% FE for non-hydrogen products in a 25 cm2 membrane electrode assembly at 700 mA/cm2 and <3 V. Additionally, the implications of optimizing RCL,OH– is translated to other material requirements, such as anode porosity. We find that the best performing electrodes use ink dispersion and deposition techniques that project well into roll-to-roll processes, demonstrating the scalability of the optimized process.

tubing used in the station.Regardless, the current levels do not appear to poison the electrodes under the conditions in this work.

Mayer rod coating:
Mayer rod coating involves the deposition of a viscous ink to a substrate using a rod to draw down the ink in an even layer.Mayer rods are metal rods wrapped with wires of a certain diameter (typically 0.08-2 mm) and the resulting gap spacing controls the wet film thickness and therefore the catalyst loading.As such, Mayer rod coating requires a higher viscosity ink to prevent the ink from freely spreading on the substrate.In contrast, inks used for hand painting and spray coating typically have a lower solids content and a correspondingly low viscosity.To formulate an ink recipe suitable for Mayer rod coating, inks were prepared with 30 wt% Cu, processed using ball milling to disperse the catalyst and ionomer, and then subsequently diluted to 0.6-0.7 wt% for the hand painted and ultrasonic spray coated deposition techniques.These conditions helped to ensure similar particle size distributions were obtained initially prior to deposition.
The Cu cathode catalyst ink was prepared similarly to previously reported work. 3First, 3.0 g isopropanol (HPLC Plus Grade, 99.9% Sigma Aldrich) and 3.1 g water (18.2M-cm) were mixed in a 20 mL glass vial and then 0.9 g Nafion® D2020 (Fuel Cell Store) was added.While stirring this mixture with a magnetic stir bar, 3 g of Cu nanoparticles (U.S. Research Nanomaterials, 40 nm or American Elements, <100 nm) were added.After dispersion of the particles in the solvent (1-5 min), the stir bar was removed, and 55 g of 5 mm zirconium oxide beads (Glen Mills) were added.The vial was taped and placed on a Thermo Scientific digital bottle roller at 80 rpm or U.S. Stoneware jar mill roller at 20 speed units (~60 rpm) for 19-24 hours.After milling, the ink was coated at room temperature utilizing a ½" x 16" wire wound Mayer rod (RD Specialties-25 mil diameter) on a Qualtech automatic film applicator (QPI-AFA6800).The catalyst ink (2-3 mL) was deposited onto a ~5x8 cm Sigracet 39 BB carbon gas diffusion media (Fuel Cell Store) and rod coated at a fixed average speed of 55 mm/s.The catalyst coated gas diffusion media were transferred to an oven and dried at 80 °C.Loading for these electrodes, when Nafion ® was used, is consistently 2.7 mg/cm 2 , but increased to 3.7 mg/cm 2 when no Nafion ® was used.The loading for Mayer rod coated electrodes were measured from XRF, using a calibration curve of Cu electrodes with known loadings.

Ultrasonic spraying and hand painting:
For the ultrasonic sprayed and hand painted electrodes made with ball-milled inks, the inks were diluted after ball-milling in 1:1 water/IPA solvent mix to 6 mg Cu/ mL solvent.For the ultrasonic sprayed electrodes made with sonicated inks-108 mg Nafion ® D2020 was added to a solvent mixture of 30 g of water and 30 g of IPA, this mixture was shaken and 360 mg Cu was added and horn-sonicated for 2 x 10 seconds at 1 J and then bath sonicated with ice for 30 minutes.
Inks were sprayed with a Sonotek ExactaCoat with Accumist spray nozzle (25 kHz) onto a vacuum hotplate at 80°C.Catalyst inks were loaded into a syringe equipped with a stir bar.The syringe dispensed inks according to a syringe pump and a stir plate was placed below so that the stir bar was constantly stirring at 200-300 rpm.The flow rate of the inks was controlled by a springe pump (Sonotek) at either a flow rate of 1.0 mL/min or 0.5 mL/min (fast flow rate and slow flow rate, respectively).
Hand painted electrodes were painted on a custom made vacuum hotplate heated to 90°C with a paint brush.Loading for hand painted and ultrasonic sprayed electrodes were calculated by mass and were within 2.7-3.0 mg/cm 2 .IrO 2 and Pt electrode deposition: IrO 2 inks were prepared by mixing 180 mg of 99.98% purity IrO 2 powder (Alpha Aesar Premion, 99.99% purity) first with 36 g of 18 M-cm distilled water.This mixture was then horn sonicated for 2 x 10 seconds at 1 J and then bath sonicated with ice for 20 minutes.In a separate 30 mL vial, 270 mg of a 10% perfluorinated anion exchange ionomer (PFAEM) 4 in IPA solution was combined with 24 g of n-propanol (nPA, HPLC Plus Grade, OmniSolv) and this solution was then shaken to combine.After the sonication of the IrO 2 /water mixture was completed, the PFAEM/nPA mixture was added and the vial shaken.The ink was ultrasonic spray coated (with a flow rate of 1.0 mL/min) onto 4 25 cm 2 Toray paper 5% wet-proofed (Fuel Cell Store) or titanium mesh (Fuel Cell Store) placed on a vacuum hotplate set to 80°C.This resulted in an IrO 2 loading of ~0.4 mg cm 2 .For the Pt and IrO 2 coated Ti mesh electrodes, the Pt was further coated onto the IrO 2 coated Ti mesh electrodes.For this ink, 180 mg of Pt black (Fuel Cell Store) was measured and 36 mg of water was added.After the water addition, 24 g nPA was added to the Pt mixture and finally 180 mg of Nafion ® D2020 was added.This ink was sonicated and deposited in the same way as the IrO 2 inks.
For the electrochemical impedance measurements, Pt/C electrodes were used.The catalyst ink was prepared by first combining Pt/HSC (Tanaka Kikinzoku Kyogo (TEC10E50E, 46.7 wt% Pt)) with 18.2 M water and then adding Nafion ® D2020, and finally nPA.This formulated recipe contained S-5 ratio of 1.5 (60 wt% H 2 O).The ink (60 g) was dispersed using the Ultra-Turrax ® high-shear rotorstator with (dispersing element 18G) at 10,000 rpm for 30 min.The vial was then placed on a Fisherbrand Digital Bottle Roller at 80 rpm for 1 h to remove any bubbles.Pt/HSC inks were coated at 22 C utilizing a ½" x 16" wire wound lab rod (RD Specialties-mil diameter = 30) on a Qualtech automatic film applicator (QPI-AFA6800).The catalyst ink (500 µL) was deposited on Sigracet 39 BB (Fuel Cell Store) and rod coated at an average speed of 55 mm/s.These electrodes were transferred to an oven and dried at 80 C.The average Pt loading was 0.2 mg/cm 2 .

Oxide content of the catalyst:
Early in this study, we observed inconsistent ionomer loading trends on ethylene FE% across different catalyst batches/suppliers.We found that the varying concentration of CuO, in the catalyst ink explained our results.Figure S1 shows the FE% of ethylene and hydrogen with oxidized Cu (60% CuO via XPS, Figure S1) as a function of ionomer loading.While the effect of Nafion ® percentage on hydrogen FE appears to be limited to a Nafion ® content of ≤1% on Cu NPs (32% CuO) in Figure 4, this is not true of electrodes made with more CuO content.As seen in Figure S1, the effect of Nafion® extends to ≤6%.The XPS for the electrodes used in the main manuscript can be seen in Figure S2 and S3.While there are reports of oxide-derived copper catalysts increasing C 2+ FE from CO reduction, we note that the catalyst, ink mixing and catalyst deposition techniques are different than reported here. 5,6cell operation: The H-cell results were taken in a custom two compartment cell with 1 M KOH as the catholyte and a 20 mL/min CO flow.

Pre-assembly of membrane electrode assembly:
To prepare for carbon monoxide electrolysis testing, the cathode controlled evaporative mixer was set (Bronkhorst) to 115 °C and the chiller was set to -10 °C.Two 1 L bottles of 1 M KOH were prepared for testing.

Membrane Electrode Assembly:
For MEA testing, we utilized a custom 25 cm 2 cell described previously. 7To begin the cell assembly, the titanium cathode flow field was evenly sanded with 500 grit sandpaper and distilled water on a leveled block to strip away the titanium oxide layer and lower flow field resistance.Both the thicknesses of the cathode and anode gas diffusion electrodes (GDEs) were measured out.Using these measurements, the ideal gasket thickness was calculated to reach a 20% compression of the electrodes.Then 25 cm 2 active area PTFE gaskets with the thickness closest to the ideal were chosen.Using eight bolts and Belleville spring washers, the cell was tightened to 40 in-lbs.
From anode to cathode the cell assembly went as follows: anode end plate, PTFE coated fiberglass separator, gold plated current collector, platinum coated nickel serpentine anode flow field, anode GDE, PTFE gasket enabling 20% compression, rinsed (18 M-cm water) membrane, PTFE gasket enabling 20% compression for the cathode, cathode GDE, titanium serpentine cathode flow field, gold plated current collector, PTFE coated fiberglass separator and cathode end plate.

Cell Start-up:
For both galvanodynamic and galvanostatic experiments, the running conditions were kept constant.For start-up, the cell was heated to 60 °C at 95% RH with a flow of 1 SLPM of nitrogen through a stainless-steel line set to 70 °C.Care was taken to ensure that 95% cathode gas RH was maintained during the heat-up period.To keep a stable cell temperature at 60 °C, the 1 M KOH anolyte was heated to 55 °C and set to a flow rate of 50 mL min -1 (KNF Simdos 10 diaphragm pump).This was all operated at an absolute pressure of 101 kPa controlled by a back pressure regulator (Equilibar).A Gamry 3000 Potentiostat and 30k booster were used for all electrochemical testing.An additional hydration time of 1 hour was necessary for the electrodes with 10% Nafion content, which occurred after cell was brought to temperature, but before the break-in provided below.

Break-in:
Prior to the break-in procedure, 1 SLPM carbon monoxide was provided by a mass flow controller (Alicat MCV-series) and introduced into the cell.The break-in procedure consisted of two galvanodynamic scans from 1 to 500 mA cm -2 with the first at 1 mA cm-2 s -1 and then the second at 5 mA cm-2 s -1 .

S-8
The galvanostatic holds were run at 0.1, 0.3, 0.5, 0.7, and 1 A cm -2 for 5 minutes each.During these holds, liquid sampling of the anolyte was taken at 2.5 minutes and gas sampling was taken at 3 minutes.The liquid sampling utilized a 20 mL vial at the end of the anolyte outlet from the cell for 6 seconds and was conducted in the hood to maintain safe practices around carbon monoxide gas.A gas sampling bag was inserted to the gas outlet tubing for 20 seconds.At the end of the cell testing a liquid sample from the cathode condenser was taken.
After all 5 of the galvanostatic holds were conducted, a third galvanodynamic sweep is performed for end of test analysis.

Galvanostatic hold
• For short performance studies: 100, 300, 500, and 700 mA/cm 2 • For durability: 500 mA/cm 2 for 5-10 hours Product Analysis: Gas analysis was run on the Agilent 990 Micro Gas Chromatography System analyzing quantities of ethylene and hydrogen.Liquid analysis was run on the Agilent 1200 High Performance Liquid Chromatography System with anolyte samples being run without dilution and cathode condenser samples being run with a dilution to 25 mL with 18 MΩ water.Due to the demonstration from the H-cell with the full product analysis, we calculated the C 2+ product FE from the membrane electrode assembly (MEA) configuration by subtracting the measured hydrogen FE.This is due to product crossover and oxidation by the anode in the MEA configuration.

Electrochemical impedance spectroscopy (EIS):
As the details of this method are already published, we mention the essential aspects of the novel method in this section. 3The method is based on a MEA architecture with the following components-a Pt supported on high surface area carbon GDE typically used in proton exchange membrane (PEM) fuel cells, membrane of choice, and cathode GDEs (BM-SS, BM-RC, BM-HP, Son-SS etc.).Hydrogen was flowed through the Pt GDE while N 2 was flowed through the Cu GDE.Thus, the Pt GDE was a stable reference/counter electrode while the Cu GDE was used as the working electrode.In this configuration, we control the potential on the working electrode precisely and perform electrochemical impedance spectroscopy (EIS).We ensured capacitive charging at the cathode GDE by N 2 flow and minimized the complications arising due to Faradaic reactions through the chosen potential.Then, the EIS data for different cathode GDEs were fitted against the transmission line model of porous electrodes to extract capacitance (a qualitative measure of catalyst utilization) and ionic conductivity inside the electrode.
The different MEA designs were used to analyze-1) ionomer coverage (Setup A) and 2) ionic conductivity and capacitance inside the cathode GDE (Setup B, C).The experimental procedure is depicted in Figure S17A,B and C. The experimental protocols were the following:

Figure S7 .
Figure S7.Schematic of Mayer rod coating

Figure
Figure S18.A) Pore volume distribution versus pore radius, B) 3D rendering of solids and pores

Table S2 .
EIS parameters on capacitance from EIS setup B) from FigureS17