Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochemical CO reduction reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we determined mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C₂₊ products are most likely limited by the dimerization of CO adsorbate. CH₄ production is limited by the CO hydrogenation step via a proton coupled electron transfer and a chemical hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alkaline electrolytes, respectively. Further, CH₄ and C₂₊ products are likely formed on distinct types of active sites.

Electrokinetic results are key to understanding the mechanisms in electrochemical CO reduction reaction. Here, the authors determine mass transport free kinetics using a gas-diffusion electrode and identified dependence of copper surface speciation on the electrolyte pH using in situ surface enhanced spectroscopies.

Bridging the Gap in the Mechanistic Understanding of Electrocatalysis via In Situ Characterizations

Electrocatalysis offers a promising strategy to take advantage of the increasingly available and affordable renewable energy for the sustainable production of fuels and chemicals. Attaining this promise requires a molecular level insight of the electrical interface that can be used to tailor the selectivity of electrocatalysts. Addressing this selectivity challenge remains one of the most important areas in modern electrocatalytic research. In this Perspective, we focus on the use of in situ techniques to bridge the gap in the fundamental understanding of electrocatalytic processes. We begin with a brief discussion of traditional electrochemical techniques, ex situ measurements and in silico analysis. Subsequently, we discuss the utility and limitations of in situ methodologies, with a focus on vibrational spectroscopies. We then end by looking ahead toward promising new areas for the application of in situ techniques and improvements to current methods.

Understanding the electric and nonelectric field components of the cation effect on the electrochemical CO reduction reaction

Electrolyte cations affect the activity of surface-mediated electrocatalytic reactions; however, understanding the modes of interaction between cations and reaction intermediates remains lacking. We show that larger alkali metal cations (excluding the thickness of the hydration shell) promote the electrochemical CO reduction reaction on polycrystalline Cu surfaces in alkaline electrolytes. Combined reactivity and in situ surface-enhanced spectroscopic investigations show that changes to the interfacial electric field strength cannot solely explain the reactivity trend with cation size, suggesting the presence of a nonelectric field strength component in the cation effect. Spectroscopic investigations with cation chelating agents and organic molecules show that the electric and nonelectric field components of the cation effect could be affected by both cation identity and composition of the electrochemical interface. The interdependent nature of interfacial species indicates that the cation effect should be considered an integral part of the broader effect of composition and structure of the electrochemical interface on electrode-mediated reactions.

Speciation of Cu Surfaces During the Electrochemical CO Reduction Reaction

Cu-catalyzed selective electrocatalytic upgrading of carbon dioxide/monoxide to valuable multicarbon oxygenates and hydrocarbons is an attractive strategy for combating climate change. Despite recent research on Cu-based catalysts for the CO₂ and CO reduction reactions, surface speciation of the various types of Cu surfaces under reaction conditions remains a topic of discussion. Herein, in situ surface-enhanced Raman spectroscopy (SERS) is employed to investigate the speciation of four commonly used Cu surfaces, i.e., Cu foil, Cu micro/nanoparticles, electrochemically deposited Cu film, and oxide-derived Cu, at potentials relevant to the CO reduction reaction in an alkaline electrolyte. Multiple oxide and hydroxide species exist on all Cu surfaces at negative potentials, however, the speciation on the Cu foil is distinct from that on micro/nanostructured Cu. The surface speciation is demonstrated to correlate with the initial degree of oxidation of the Cu surface prior to the exposure to negative potentials. Combining reactivity and spectroscopic results on these four types of Cu surfaces, we conclude that the oxygen containing surface species identified by Raman spectroscopy are unlikely to be active in facilitating the formation of C₂₊ oxygenates in the CO reduction reaction.

Overcoming immiscibility toward bimetallic catalyst library

Bimetallics are emerging as important materials that often exhibit distinct chemical properties from monometallics. However, there is limited access to homogeneously alloyed bimetallics because of the thermodynamic immiscibility of the constituent elements. Overcoming the inherent immiscibility in bimetallic systems would create a bimetallic library with unique properties. Here, we present a nonequilibrium synthesis strategy to address the immiscibility challenge in bimetallics. As a proof of concept, we synthesize a broad range of homogeneously alloyed Cu-based bimetallic nanoparticles regardless of the thermodynamic immiscibility. The nonequilibrated bimetallic nanoparticles are further investigated as electrocatalysts for carbon monoxide reduction at commercially relevant current densities (>100 mA cm^-2), in which Cu_0.9Ni_0.1 shows the highest multicarbon product Faradaic efficiency of ~76% with a current density of ~93 mA cm^-2. The ability to overcome thermodynamic immiscibility in multimetallic synthesis offers freedom to design and synthesize new functional nanomaterials with desired chemical compositions and catalytic properties.

Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

Highly alkaline electrolytes have been shown to improve the formation rate of C₂₊ products in the electrochemical reduction of carbon dioxide (CO₂ ) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH^- concentrations promote the C-C coupling chemistry. Herein, by systematically varying the concentration of Na⁺ and OH^- at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na⁺ ), rather than OH^- , exert the main promotional effect on the production of C₂₊ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na⁺ is chelated by a crown ether. Chelation of Na⁺ leads to drastic decrease in the formation rate of C₂₊ products. The promotional effect of OH^- determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.