Using pH Dependence to Understand Mechanisms in Electrochemical CO Reduction

Abstract

Electrochemical conversion of CO(2) into hydrocarbons and oxygenates is envisioned as a promising path toward closing the carbon cycle in modern technology. To date, however, the reaction mechanisms toward the plethora of products are disputed, complicating the search for alternative catalyst materials. To conclusively identify the rate-limiting steps in CO reduction on Cu, we analyzed the mechanisms on the basis of constant-potential density functional theory (DFT) kinetics and experiments at a wide range of pH values (3–13). We find that *CO dimerization is energetically favored as the rate-limiting step toward multicarbon products. This finding is consistent with our experiments, where the reaction rate is nearly unchanged on a standard hydrogen electrode (SHE) potential scale, even under acidic conditions. For methane, both theory and experiments indicate a change in the rate-limiting step with electrolyte pH from the first protonation step under acidic/neutral conditions to a later one under alkaline conditions. We also show, through a detailed analysis of the microkinetics, that a surface combination of *CO and *H is inconsistent with the measured current densities and Tafel slopes. Finally, we discuss the implications of our understanding for future mechanistic studies and catalyst design.

The mechanism for acetate formation in electrochemical CO(2)reduction on Cu: selectivity with potential, pH, and nanostructuring

“…We derive and validate this mechanism using density functional theory (DFT) calculations. In our derivation, we assume the same rate determining step (RDS) for all C 2 products based on previous experimental and theoretical studies, which has been suggested to either be a CO* dimerization 35–37 or the protonation of the OCCO* intermediate with water as the proton donor. 38 The same RDS is consistent with the range of similar Tafel slopes and the distinguishing lack of an inverse pH-activity relationship shared among the considered C 2 products (ethylene, ethanol, acetate) 11 (see Fig.…”

A unifying mechanism for cation effect modulating C1 and C2 productions from CO2 electroreduction

“…1d , red). Cu(100) surface, known to be active for C–C coupling reactions 46 , was chosen as the model catalyst surface for CH 4 and C 2 H 4 formation paths, as well as Ag(111) surface for CO formation path. At two different potentials of −0.5 V vs. standard hydrogen electrode (SHE) for the potential at point of zero charge ( E PZC ) and −1.0 V SHE for the interface charge of −18 μC cm −2 (Supplementary Fig.…”

“…Also, Monteiro et al showed a lack of CO 2 RR activity to CO without M + , which initiated an intensive discussion about the possibility of CCET 13 . In addition, Chan and coworkers investigated the kinetic importance of proton activity using pH control experiments 12 , 46 . They found that the pH variation significantly changes the CH 4 production rate 46 , while the CO and C 2 H 4 production rates are nearly unchanged on a SHE potential scale 12 , 46 .…”

“…In addition, Chan and coworkers investigated the kinetic importance of proton activity using pH control experiments 12 , 46 . They found that the pH variation significantly changes the CH 4 production rate 46 , while the CO and C 2 H 4 production rates are nearly unchanged on a SHE potential scale 12 , 46 . The reactions (1) and (3) infer a critical role of M + in the kinetics of CO and C 2 H 4 formation paths, and reaction (2) shows the kinetic importance of pH in the CH 4 formation path.…”

Selective CO2 Electromethanation on Surface-Modified Cu Catalyst by Local Microenvironment Modulation

“…Theoretical insight uncovered that the deprotonated carboxylate (−COO – ) showed a strong affinity for H 2 O binding, which could activate the H 2 O molecules and promote the protonation of intermediates. The study on the pH dependence of CO elelctroreduction also confirmed that anions with lower p K a ’s than water (like carboxyl) can act as proton donors, which facilitates the protonation of *CO and accelerates the rate-limiting step for methane production. These findings explained the facilitation of methane electroproduction by GSH ligands.…”