Optimizing CO Coverage on Rough Copper Electrodes: Effect of the Partial Pressure of CO and Electrolyte Anions (pH) on Selectivity toward Ethylene

Abstract

The conversion of the initial intermediate CO in the electrochemical reduction reaction of CO 2 on the surface of oxide-derived Cu electrodes has been investigated as a function of partial pressure and pH, manipulated by the composition of the electrolyte. We show that in inert gas, an increase in partial pressure of CO results in a continuous increase in Faradaic efficiency (FE) for ethylene, at various potentials ranging from −0.7 to −1.1 V versus RHE, with the highest FE of ∼28% obtained using 1 bar CO at −0.8 V. When the partial pressure of CO is increased in a mixture of CO and CO 2 , an optimum in the ethylene FE was found for the partial pressure of CO in the range from 0.5 bar (at −1.1 V, FE is ∼45%) to 0.8 bar (at −0.9 V, FE is ∼35%). At lower negative potentials (−0.8 to −0.7 V), the presence of CO 2 has negligible influence, and similar data to reduction of CO in inert gas were obtained. Variation of the anion in solution (0.1 M concentration) shows that the optimized FE toward ethylene increases from 5.2% in KH 2 PO 4 to 43.2% in KOH. The observed differences in selectivity are attributed to anion buffering capacity and the associated local pH near the surface of the electrode. Using in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), it was determined that the CO coverage increases as a function of increasing pH, confirming that CO coverage and pH correlate. Collectively, the data herein outline the critical role of reactant partial pressures and the significant effect of anion composition (pH) on the surface coverage of CO and concomitant selectivity in electrochemical reduction of CO 2 to ethylene.

Product Distribution Control Guided by a Microkinetic Analysis for CO Reduction at High-Flux Electrocatalysis Using Gas-Diffusion Cu Electrodes

“…In addition to catalyst innovations, parameters associated with the electrolyte (ion identity, pH, ionic strength) and process conditions (gaseous reactant fugacity, temperature) change the catalytic performance. − The electrolyte cations likely perturb the interfacial electric field near the electrode and thereby energy levels of reaction intermediates. In some studies, larger alkali metal cations promoted the electrochemical CO–CO coupling and enhanced C 2+ formation on polycrystalline Cu (pc-Cu) surfaces via electrocatalytic CO (2) reduction. , In contrast, the influence of electrolyte anions on electrocatalytic performance remains controversial. ,,, For example, a study employing surface-enhanced IR absorption spectroscopy (SEIRAS) revealed that the identity of anions changed the CO chemisorption energy on Cu electrodes and in turn the electrocatalytic performance of CO (2) reduction, especially the specifically adsorbing anions (Cl – and PO 4 3– ) . On the contrary, another study demonstrated that product distributions of CO electroreduction over the pc-Cu were insensitive to identifying electrolyte anions in the presence of phosphate, carbonate, or perchlorate .…”

Promoting electrocatalytic CO2methanation using a molecular modifier on Cu surfaces

“…Previous studies have suggested that a lowered coverage of *CO facilitated *CO protonation for Cu-based catalysts. 19,38–42…”

“…Previous studies have suggested that a lowered coverage of *CO facilitated *CO protonation for Cu-based catalysts. 19,[38][39][40][41][42] We performed DFT calculations with the Cu (111) facet as the model slab to explore the impact of BDT on *CO coverage and CO 2 RR selectivity. 10 The BDT molecule rested on the Cu (111) slab in its energy-optimal form (Fig.…”

Rational Manipulation of Intermediates on Copper for CO2 Electroreduction Toward Multicarbon Products

“…The local pH near the electrode plays an important role in the *CO/*H coverage, which determines whether *CO subsequently undergoes dimerization or hydrogenation. At lower pH values, *CO prefers to form CH 4 in the presence of abundant *H. At higher pH, where *H is scarce and *CO coverage is high, ethylene formation dominates over ethane formation [158]. As the rate of OH − production depends on the applied potential, it is necessary to probe the local pH near the electrode under operational conditions.…”