Tunable Selectivity on Cu-Sb Bimetallic Alloy via Electrodeposition for Electrochemical CO2 Reduction
Authors: Yoo Jungmin , Han Gyeong Ho , Ahn Sang Hyun 
Published: December 2024

For decades, global warming has posed a significant environmental challenge worldwide, CO2 generated by human activities is considered to be a major cause of its escalation. Among the methods to convert CO2 into high value-added chemicals, like carbon monoxide (CO), methane (CH4), ethylene (C2H4) and ethanol(C2H5OH), electrochemical CO2 reduction reaction (CO2RR) at a low temperature has gained considerable attention. Nevertheless, given the high reaction barrier, sluggish kinetics, and diverse pathways involved in CO2RR, well-designed electrocatalysts are crucial for facilitating reactions and adjusting selectivity for specific products. Copper (Cu) has garnered the most attention in recent years due to its ability to generate hydrocarbons beyond CO owing to its moderate *CO adsorption energy. However, Cu exhibits a low selectivity for specific products with wide range of hydrocarbons and undergoes the hydrogen evolution reaction which competes with CO2RR.

Incorporation of Cu-based bimetallic alloys can enhance selectivity by regulating the intrinsic active sites of the material to control the adsorption energy of reaction intermediates. Among many secondary metals, post-transition metals such as tin (Sn), indium (In), and bismuth (Bi) have been proven to be active in CO2RR alone, and studies have reported that combining them with Cu selectively produces CO and HCOO-. While antimony (Sb) possessed similar properties, it went unnoticed due to its lack of CO2RR activity alone. However, recent studies have revealed that the addition of a small amount of Sb to Cu results in high CO activity.

Electrocatalytic assessments are typically performed using a standard three-electrode H-type cell. Due to the limited accessibility of CO2 by the catalyst, constrained by the solubility of CO2 (approximately ~34 mM at ambient conditions), the highest achievable operating current density is capped at around ~20 mA cm−2. In a gas diffusion electrode system, gaseous CO2 is fed directly to the catalyst, enabling rapid mass transport of CO2. Among these methods, membrane electrode assembly (MEA) electrolyzer offers enhanced efficiencies owing to the reduced ohmic resistance inherent in their zero-gap design. In a MEA electrolyzer, the membrane is sandwiched between the cathode and anode catalyst layers on each side. During operation, the cathodic side receives humidified CO2 but no flowing electrolyte. The anode is typically fed an alkaline electrolyte and commonly consists of a carbon gas diffusion layer with deposited IrOx nanoparticles to facilitate the oxygen evolution reaction.

Here in, a series of bimetallic CuXSb10-X (X = 0, 2, 5, 8, 10) catalysts with adjustable compositions for CO2RR was prepared. CuXSb10-X catalysts were synthesized via direct co-electrodeposition onto a gas diffusion layer (MPL/CP) under the room temperature. Characterization reveals that CuXSb10-X catalysts have a typical morphology consisting of nano-sized particles with well-dispersed Cu and Sb. CO2RR tests were performed in a gas flow MEA single cell using humidified gaseous CO2 and anion exchange membrane (AEM). Unlike Pristine Cu, Cu5Sb5 and Cu8Sb2, which have major composition of Cu, show CO (72.4%) as a main product at 3.0 Vcell. Meanwhile, Cu2Sb8 exhibits enhanced selectivity for CH4 (49.7%) with a partial current density of 152.1 mA cm-2, at 3.6 Vcell despite pristine Sb having an inability to produce any CO2RR product. We assume that Cu serves as the main active site in this catalyst. At the same time, the CO2RR selectivity shifts towards CH4 when the site of Cu decreases and becomes isolated near Sb. Later, in situ Raman spectroscopy will be used to reason that the Cu-Sb interface in Cu weakens the binding energy of *CO. Also, Sb surrounding isolated Cu in Cu2Sb8 promotes the CO2RR to CH4 through enhancing the binding energy of *CHO which is key intermediate of CH4 reaction pathway.