Hydronium-Induced Switching between CO₂ Electroreduction Pathways

Abstract

Over a broad range of operating conditions, many CO electroreduction catalysts can maintain selectivity toward certain reduction products, leading to materials and surfaces being categorized according to their products; here we ask, is product selectivity truly a property of the catalyst? Silver is among the best electrocatalysts for CO in aqueous electrolytes, where it reaches near-unity selectivity. We consider the hydrogenations of the oxygen and carbon atoms via the two proton-coupled-electron-transfer processes as chief determinants of product selectivity; and find using density functional theory (DFT) that the hydronium (HO) intermediate plays a key role in the first oxygen hydrogenation step and lowers the activation energy barrier for CO formation. When this hydronium influence is removed, the activation energy barrier for oxygen hydrogenation increases significantly, and the barrier for carbon hydrogenation is reduced. These effects make the formate reaction pathway more favorable than CO. Experimentally, we then carry out CO reduction in highly concentrated potassium hydroxide (KOH), limiting the hydronium concentration in the aqueous electrolyte. The product selectivity of a silver catalyst switches from entirely CO under neutral conditions to over 50% formate in the alkaline environment. The simulated and experimentally observed selectivity shift provides new insights into the role of hydronium on CO electroreduction processes and the ability for electrolyte manipulation to directly influence transition state (TS) kinetics, altering favored CO reaction pathways. We argue that selectivity should be considered less of an intrinsic catalyst property, and rather a combined product of the catalyst and reaction environment.

System Design Rules for Intensifying the Electrochemical Reduction of CO2 to CO on Ag Nanoparticles

“…This suggests that in the low overpotential regime for Ag NPs, the CO 2 .− formation is indeed the RDS. This is in contrast to many computational studies performed on the eCO 2 RR to CO assuming a PCET RDS and in agreement with some computational studies assuming an ET RDS.…”

“…Anion effects are interpreted on the basis of specific adsorption and/or pH, whereas cation effects are explained using promoter effects, Frumkin effects, cation hydrolysis, and electrical double layer (EDL) stabilization . Studies have also examined the effects of electrolyte pH on eCO 2 RR . More specifically, for Ag catalysts, studies show that high pH suppresses the competing hydrogen evolution reaction (HER), gives higher CO partial current densities (j CO ), shows better FEs for CO production, decreases the onset potentials (V vs. RHE), and decreases the overpotential required for eCO 2 RR to CO .…”

Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction

“…The reactants on the electrode also interact with the liquid side of the reaction environment. Solvent composition (33)(34)(35)(36), the identity and concentration of the supporting electrolyte's anions (37)(38)(39) and cations (39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51), and the pH of the electrolyte (52)(53)(54)(55)(56)(57)(58) can greatly impact the product selectivity. For example, the rate of ethylene evolution during CO2 reduction was found to increase by a factor of ≈15 when switching from Li + -to Cs + -containing electrolyte (47).…”

Nanocrystal/Metal–Organic Framework Hybrids as Electrocatalytic Platforms for CO2 Conversion

“…As the MOF component of our hybrid catalyst, we selected the porphyrinic Al-PMOF, [29] which was previously studied and found to be stable under the conditions of the CO 2 RR. [14] As for the metal NC component, Ag is one of the most promising CO 2 RR electrocatalysts because of its high selectivity for CO. [30][31][32][33][34][35][36][37] Therefore,itisaperfect NC counterpart to study the synergistic interactions with the MOF.…”