Guiding catalytic CO2 reduction to ethanol with copper grain boundaries

The grain boundaries (GBs) in copper (Cu) electrocatalysts have been suggested as active sites for CO2 electroreduction to ethanol. Nevertheless, the mechanisms are still elusive. Herein, we describe how GBs tune the activity and selectivity for ethanol on two representative Cu-GB models, namely Cu∑3/(111) GB and Cu∑5/(100) GB, using joint first-principles calculations and experiments. The unique geometric structures on the GBs facilitate the adsorption of bidentate intermediates, *COOH and *CHO, which are crucial for CO2 activation and CO protonation. The decreased CO–CHO coupling barriers on the GBs can be rationalized via kinetics analysis. Furthermore, when introducing GBs into Cu (100), the product is selectively switched from ethylene to ethanol, due to the stabilization effect for *CH3CHO and inapposite geometric structure for *O adsorption, which are validated by experimental trends. An overall 12.5 A current and a single-pass conversion of 5.18% for ethanol can be achieved over the synthesized Cu-GB catalyst by scaling up the electrode into a 25 cm2 membrane electrode assembly system.


Preparation of annealed GB-Cu electrodes
The annealed GB-Cu electrode was obtained by annealing GB-Cu electrode at 200°C for two hours under N2 atmosphere.

Electrochemical reduction of CO2 in a H-type cell
All electrochemical measurements were conducted in a custom gas-tight H-type electrochemical cell machined from PMMA. (manufactured by Gaossunion Co., Ltd.). The cell was sonicated in 20 wt. % nitric acid and thoroughly rinsed with the deionized water prior to all experimentation. The working and counter electrodes were parallel and separated by an anion-conducting membrane (FAA-3-50, FuMA-Tec). Gas dispersion frits were incorporated into both electrode chambers in order to provide ample electrolyte-gas mixing. The exposed geometric surface area of each electrode was 1 cm 2 and the electrolyte volume of each electrode chamber was 10 mL. The counter electrode was a glassy carbon plate that was also sonicated in 20 wt. % nitric acid prior to all experimentation. The working electrode potential was referenced against a Ag/AgCl electrode (saturated KCl electrolyte). A 0.05 M K2CO3 solution was used as the electrolyte. Metallic impurities in the as-prepared electrolyte were removed before electrolysis by chelating the solution with Chelex 100 (Na form, purchased from Sigma-Aladdin). Both electrode chambers were sparged with CO2 at a rate of 5 sccm for 30 min prior to and throughout the duration of all electrochemical measurements. Upon saturation with CO2 the pH of the electrolyte was 6.8. Electrochemistry was performed using a Autolab PGSTAT204 potentiostat. All electrochemical measurements were recorded versus the reference electrode and converted to the RHE scale. The electrocatalytic activity of each sample was assessed by conducting chronoamperometry for 70 min. Each electrode was tested at least three times in order to ensure the statistical relevance of the observed trends.

Electrochemical reduction of CO2 in the MEA system
The MEA cell (manufactured by Gaossunion Co., Ltd.) consists of a titanium a cathode bipolar plate with serpentine flow field, an anode bipolar plate with parallel flow field, associated nuts, bolts and insulating kit. The geometric area of each flow field is 4 and 25cm 2 . An AEM membrane (FAA-3-30, Fumatech) was activated in 0.1 M KOH for 24 hours, washed with the deionized water prior to use. The anode consisted of a IrRu alloy deposited on a 200 mesh Ni grid. A direct current power supply (UTP1300, UNI-T Group Co., Ltd) was used to apply current to the MEA. A Corrtest CS350M in a galvanostatic mode was used to measure the cell voltage. No iR compensation was applied. Aqueous KOH electrolyte (10 mM0.1 M) was used as the anolyte and was circulated using a peristaltic pump (EC200-01, Gaossunion Co., Ltd.). The electrolyte flow rate was kept at 20 mL min -1 . The flow rate of the CO2 gas flowing into the cathode flow field was kept at 50 sccm and 600 sccm by a mass flow controller (MC-Series, Alicat Scientific) for 4cm 2 and 25cm 2 , respectively. CO2 was flowed through a homemade humidifier (7/8 full of Milli-Q water, room temperature) prior to the MEA. The flow rate of the CO2 gas flowing out the cathode flow field was also measured by a flowmeter (M-Series, Alicat Scientific). The liquid products carried by CO2 gas are absorbed by low-temperature ultra-purity water obtained from an ice salt bath.

Analysis of CO2 reduction products
During electrolysis, gas products were quantified using an on-line gas chromatography system (GC7890B, Agilent Technologies, Inc.). The thermal conductivity detector (TCD) connected to a MolSieve 5A packed column (Agilent Technologies, Inc.) to detect H2, O2 and N2 and a back flame ionization detector (FID) connected to a Porapak Q packed column (Agilent Technologies, Inc.) to detect CO. A methanizer was installed to enable the back FID to detect CO with 1000 times higher sensitivity. A front FID connected to an HP-PLOT Al2O3 capillary column (Agilent Technologies, Inc.) to detect hydrocarbons (C1~C3). Ar was used as the carrier gas. After passing through the reactor, the gas was allowed to flow directly into the gas sampling loop of the gas chromatography for online gaseous product analysis.
In the performance test using H-type cell and the MEA system, the liquid products were collected from the cathode and anode chambers. The liquid products were analyzed by headspace gas chromatography (HS-GC) and 1H-NMR. HS-GC measurements were carried out using a BCHP HS-2 Headspace Sampler with GC2060 gas chromatography (Shanghai Ruimin Instrument Co., Ltd.). Typically, 10 mL vials were filled with 3 mL of the liquid sample and sealed. They were heated to 70 •C over 20 min in the headspace sampler and 1mL of the headspace gas composition was automatically injected into the GC. The sample loop (110 •C) and transfer line (110 •C) were both heated to avoid condensation. Ar was used as the carrier gas. An HP-INNOWax capillary column (Length: 60 m; ID: 0.32 mm; Film: 0.5 μm, Agilent) was used to separate the compounds in the sample. 1H-NMR was performed using AVANCE IIITM HD 400 MHz NanoBAY. The water suppression method was used. DMSO (10 mM) and phenol (50 mM) were added as internal standards. For CO2 reduction products showing multiple sets of NMR peaks, the set of peaks with the highest intensity were chosen for calibration and quantification.    Table S3. Summary of total current and conversion efficiency towards ethanol from different system.