Hydrodynamics Determine Tafel Slopes in Electrochemical CO2 Reduction on Copper

Abstract

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.

Tafel Slope Plot as a Tool to Analyze Electrocatalytic Reactions

“…This indeed resulted in lower Tafel slope values. 48 However, note that CO 2 reduction is not limited by direct CO 2 transport limitations but can be limited by the homogeneous reaction between OH − and CO 2 that occurs near the surface due to the production of hydroxide in both HER and CO 2 RR or to interfacial pH and corresponding cation gradients in general. 49 Moreover, CO 2 RR is often further complicated by the possible production of a range of carbon products formed through multiple pathways with hydrogen as a byproduct, 12 which must be disentangled.…”

“…For CO 2 RR, this might be possible if high surface CO 2 concentrations are realized. 37 However, it is not a given that the requirements for Tafel analysis will be obtained for the investigated system. The application of the Tafel slope plot with different mass transport conditions and, if relevant, different loadings could be a powerful tool to show that such conditions are (not) obtained.…”

“…Watkins et al have used Bayesian analysis to study the effect of cell geometry on the observed Tafel slope during CO 2 RR on copper, also showing the importance of having the experimental (hydrodynamic, in their case) conditions right for obtaining (kinetically meaningful) Tafel slopes. 37 By comparison, the Tafel slope plot displays the change in the Tafel slope as a function of potential or current, which we believe provides an intuitive and easy way to see if a cardinal Tafel slope value is involved. Also, the potential dependence on the Tafel slope itself contains, in principle, meaningful information, which is not immediately evident from the Bayesian analysis.…”

Electrode Surface Heating with Organic Films Improves CO2 Reduction Kinetics on Copper

“…This consideration guided our design of combinatorial experimentation to characterize the transition in onset potential across temperatures and fit the resulting data to a temperaturedependent Tafel model coupled with a mass-transfer limiting current (Figure S13). 8,22 Using the data collected with organiccoated Cu at a range of temperatures and potentials, we established a Bayesian model for the posterior distributions for all model parameters (see the SI for discussion and derivation). The result is an apparent activation energy of 1.0 ± 0.2 eV for the reduction of CO 2 to C 2+ products (Figure 4a), which differs from previously reported values (ca.…”

“…28 The design of the cell varies slightly from our previous report on the effects of hydrodynamics on Tafel slopes to allow for a thermocouple to be placed inside the working compartment to monitor internal temperature. 22 We measured internal and outlet temperatures at five temperature points with surface heating (SH) to evaluate the degree of global heating of the system. At a surface temperature of 60 °C, we experimentally measure an internal temperature of 36 ± 1.1 °C and an outlet temperature of 26.8 ± 0.1 °C, which supports our goal of mitigating bulk electrolyte heating.…”