Hydrodynamics Change Tafel Slopes in Electrochemical CO2 Reduction on Copper

The hydrodynamics of electrochemical CO2 reduction (CO2R) systems is an insufficiently investigated area of research that has broad implications on catalyst activity and selectivity. While most previous reports are limited to laminar and CO2-sparged systems, herein we address a wide range of hydrodynamics via electrolyte recirculation systems. We find that increased hydrodynamics at the electrode surface results directly in changes to the ethylene and methane Tafel slopes, demonstrating that mass transport is on equal footing with catalyst active sites in determining reaction mechanisms and the ensuing product distribution. Mass transport is traditionally considered to be in the purview of systems-level engineering, yet the present work shows that CO2R mechanistic work must be considered in the context of the mass transport conditions. We extend our analysis to organic coatings, demonstrating that the films shield the active sites from variability in hydrodynamics and increase the residence time of CO so that it may be further reduced to desirable products.


Table of Contents
Materials and methods and synthetic procedures 2 Electrochemical measurements 3 COMSOL modeling information and LOWESS line fitting 4  Prior to each use, copper foil was mechanically polished to a mirror-like finish using nanodiamond suspension (first 3 μm then 0.1 μm, Buehler) followed by rinsing in water and drying under a stream of nitrogen gas.
The copper foil was then electropolished using a method similar to the one employed by Kuhl  Cu thin film electrocatalysts were fabricated using DC magnetron sputtering of a 2" Cu metal target at 50 W in 6 mTorr Ar onto a 100 mm-diameter Si wafer with an approximately 170 nm SiO2 diffusion barrier and 10 nm Ti adhesion layer, using a previously described sputter system with 10-5 Pa base pressure. 1 After deposition, the films were stored in a nitrogen purge box until the day of electrochemical testing, although no other catalyst treatment was performed prior to electrocatalyst screening. The Cu-X (X: Co, Zn, Mn, In) thin film electrodes were deposited under similar conditions from elemental metal targets with DC power adjusted to obtain designed composition in the wafer center. All the metal targets were pre-cleaned in the presence of 6 mTorr Ar for 10 min to remove any contaminants from the target surface. The non-confocal sputtering geometry provided a continuous composition gradient across the Si wafer with the composition variation within each 5 mm diameter electrode being less than 1% for the most Curich catalysts and about 2% for the most Cu-poor catalysts.
1H and 13C NMR spectra were in accordance with reported values. 2

Electrochemical Measurements
All H-cell electrochemical experiments were carried out using a Biologic VMP3 multichannel potentiostat with copper foil as the working electrode and a platinum foil as the counter electrode. The cathode compartment was separated from the anode compartment by a Selemion AMV anion-exchange membrane (AGC Engineering Co.). All potentials were measured versus a leakless Ag/AgCl reference electrode (Innovative Instruments) with an outer diameter of 5 mm that was inserted into the cathode compartment. The reference electrode was calibrated against H + /H2 on Pt in a 0.5 M sulfuric acid solution (0 V vs. standard hydrogen electrode).
Potentiostatic electrochemical impedance spectroscopy (PEIS) measurements were carried out prior to each electrolysis experiment to determine the Ohmic resistance of the flow cell. The impedance measurements were carried out at frequencies ranging from 100 Hz to 200 KHz to measure the solution resistance. A Nyquist plot was plotted and in the high-frequency part a linear fit was performed, and the axis intersection was identified, the value of which represents the Ohmic resistance of the cell. Typical values of the resistance range from 45 to 60 Ω.
All chronoamperometric experiments (unless stated otherwise) were performed for 30 min at 25 °C using CO2- Every 10 minutes, 1 mL of gas was sampled to determine the concentration of gaseous products. Liquid products were quantified by HPLC. Liquid products were only quantified in ANEC because the concentrations in the recirculation H-cells were too low due to the requisite volume for recirculation.

COMSOL Modeling Information
The Essentially, for each x-value in the dataset, a "smoothed" y-value is calculated by taking a weighted linear fit of the nearest n data points. This value of n is, by default in the Python implementation, ⅔ of the total data points. The weights for the linear fit are from a tricube function. Subsequent iterations of the refitting can be done with altered weights according to the residuals of the previous fitting. See Python's statsmodels.nonparametric.smoothers_lowess.lowess function for details on implementation and usage. In the case of Figure 3, in addition to a fitting, a qualitative metric for error bars on the data is shown. This error was calculated by taking a random sample of data points and then fitting the LOWESS to that sample. The average of 500 such samples is taken to represent the "error" qualitatively of the fit. The following Python code was used to generate the error from this fitting. Note that although this fitting procedure is "model-free", there are still parameters that we selected, including number of iterations, fraction of data points to fit, etc. Changing these parameters will quantitatively change the fit (primarily the error bars), but will not qualitatively change the results.
For Figure 5, the Faradaic efficiencies for the various gaseous products were fit using the LOWESS model as implemented in Python. The raw data points are summed up and plotted, i.e., first the FE toward methane is plotted, then the sum of FEs toward methane and ethylene is plotted as "ethylene", etc. A similar procedure is used to sum up the smoothed fits and label them on the plot.  b. Figure S3: The partial current density towards CO2 reduction products does not greatly change between a) cells at different convective ratios or b) with the addition of 1-Br2. The parallel and angled cells have slightly lower current densities as compared to ANEC or Kuhl, but are comparable to that of sparged H-cell Cu control data published previously in our own group. 2 This change may be attributable to the type of polycrystalline copper used in each experimentation. 8       Probability distributions for the slope and limiting current are shown with MAP drawn. Not shown is the probability distribution for the plateau at the bottom. This plateau is a simple "kink" in the system representing the lower detectability limit of the instruments.

Molecular Dynamics Discussion
All molecular dynamics calculations were performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software. 9 Valence (bond, angle, dihedral), electrostatic, and van-der-waals potentials were modeled by the Universal Force Field (UFF). 10 We began all simulations by a steepest descent minimization followed by a conjugate gradient minimization. The cell was then heated at constant volume (NVT ensemble) from 1 K to the desired temperature over the period of 10 ps via the Nose-Hoover thermostat. The cell was then maintained at the desired temperature (again NVT) for 2 ns to allow the system to reach equilibrium. After 2 ns of constant-temperature NVT dynamics, the 2-Phase Thermodynamics (2PT) method was used to calculate CO diffusion coefficients (DCO). 11 In essence, 2PT calculates the velocity autocorrelation function (VACF) and then integrates the VACF over time to yield DCO. The VACF was integrated over a period of 20 ps in order to achieve proper convergence. For all cases, DCO was averaged over 6 individual calculations to ensure adequate sampling.
Pure water systems included 282 water molecules and a single CO molecule. The volume was chosen to match the experimental density of pure water. Systems with additive featured 8 additive molecules, 200 waters, and a single CO molecule. Here the volume was kept the same as the system with no additive.