Copper-indium bimetallic catalysts for the selective electrochemical reduction of carbon dioxide

Recent Progress on Copper-Based Bimetallic Heterojunction Catalysts for CO2 Electrocatalysis: Unlocking the Mystery of Product Selectivity

Copper-based bimetallic heterojunction catalysts facilitate the deep electrochemical reduction of CO2 (eCO2RR) to produce high-value-added organic compounds, which hold significant promise. Understanding the influence of copper interactions with other metals on the adsorption strength of various intermediates is crucial as it directly impacts the reaction selectivity. In this review, an overview of the formation mechanism of various catalytic products in eCO2RR is provided and highlight the uniqueness of copper-based catalysts. By considering the different metals' adsorption tendencies toward various reaction intermediates, metals are classified, including copper, into four categories. The significance and advantages of constructing bimetallic heterojunction catalysts are then discussed and delve into the research findings and current development status of different types of copper-based bimetallic heterojunction catalysts. Finally, insights are offered into the design strategies for future high-performance electrocatalysts, aiming to contribute to the development of eCO2RR to multi-carbon fuels with high selectivity.

Regulation Strategy of Nanostructured Engineering on Indium-Based Materials for Electrocatalytic Conversion of CO2

Electrochemical carbon dioxide reduction (CO2 RR), as an emerging technology, can combine with sustainable energies to convert CO2 into high value-added products, providing an effective pathway to realize carbon neutrality. However, the high activation energy of CO2 , low mass transfer, and competitive hydrogen evolution reaction (HER) leads to the unsatisfied catalytic activity. Recently, Indium (In)-based materials have attracted significant attention in CO2 RR and a series of regulation strategies of nanostructured engineering are exploited to rationally design various advanced In-based electrocatalysts, which forces the necessary of a comprehensive and fundamental summary, but there is still a scarcity. Herein, this review provides a systematic discussion of the nanostructure engineering of In-based materials for the efficient electrocatalytic conversion of CO2 to fuels. These efficient regulation strategies including morphology, size, composition, defects, surface modification, interfacial structure, alloying, and single-atom structure, are summarized for exploring the internal relationship between the CO2 RR performance and the physicochemical properties of In-based catalysts. The correlation of electronic structure and adsorption behavior of reaction intermediates are highlighted to gain in-depth understanding of catalytic reaction kinetics for CO2 RR. Moreover, the challenges and opportunities of In-based materials are proposed, which is expected to inspire the development of other effective catalysts for CO2 RR.

Controlling the C1/C2+ product selectivity of electrochemical CO2 reduction upon tuning bimetallic CuIn electrocatalyst composition and operating conditions

Electrochemical carbon dioxide (CO2) reduction (eCO2R) over Cu-based bimetallic catalysts is a promising technique for converting CO2 into value-added multi-carbon products, such as fuels, chemicals, and materials. For improving the process efficiency, electrocatalyst development for the eCO2R must be integrated with tuning of operating conditions. For example, CuIn-based materials typically lead to preferential C1 product selectivity, which delivers the desired C2+ products upon varying the In/Cu ratio and operating conditions (i.e., in 0.1 M KHCO3 electrolytes using an H-type cell with a cation exchange membrane vs. in 1 M KOH electrolytes using a flow cell with an anion exchange membrane). At lower Cu-loading (i.e., InCu5Ox material), the maximum faradaic efficiency of HCOOH (FEHCOOH) of 70% was achieved at -1 V versus the reversible hydrogen electrode (vs. RHE) in an H-type cell. However, upon increasing the Cu loading, the preferential product selectivity could be altered: the InCu73Ox material led to a high CO selectivity (maximum FE of 51%) in the H-type cell at -0.8 V vs. RHE and delivered a current density of 100 mA cm-2 with a FEC2+ of up to 37% at -0.8 V vs. RHE in the flow cell configuration. Various characterization tools were also employed to probe the catalytic materials to rationalize the electrocatalytic performance.

Vertical Cu Nanoneedle Arrays Enhance the Local Electric Field Promoting C2 Hydrocarbons in the CO2 Electroreduction

Electrocatalytic reduction of CO₂ to multicarbon products is a potential strategy to solve the energy crisis while achieving carbon neutrality. To improve the efficiency of multicarbon products in Cu-based catalysts, optimizing the *CO adsorption and reducing the energy barrier for carbon-carbon (C-C) coupling are essential features. In this work, a strong local electric field is obtained by regulating the arrangement of Cu nanoneedle arrays (CuNNAs). CO₂ reduction performance tests indicate that an ordered nanoneedle array reaches a 59% Faraday efficiency for multicarbon products (FE_C2) at -1.2 V (vs RHE), compared to a FE_C2 of 20% for a disordered nanoneedle array (CuNNs). As such, the very high and local electric fields achieved by an ordered Cu nanoneedle array leads to the accumulation of K⁺ ions, which benefit both *CO adsorption and C-C coupling. Our results contribute to the design of highly efficient catalysts for multicarbon products.

ZnSn nanocatalyst: Ultra-high formate selectivity from CO2 electrochemical reduction and the structure evolution effect

The introduction of tin (Sn) into Zn-based catalyst can change its intrinsic properties of the electrochemically reduction of CO₂ to CO, obtaining a high formate yield. The electron transfer from Zn to Sn lowers down the d-band center of Sn, leading to a more reliable surface adsorption of the *OCHO intermediate and high formate selectivity. The obtained ZnSn catalyst enables formate formation with a drastically boosted Faradaic efficiency (FE) up to 94%, which is 2.04 and 1.34 times of pure Zn and Sn foils, respectively, indicating a synergistic effect between Zn and Sn. During the electrochemical CO₂ reduction reaction (eCO₂RR) process, the morphology of the ZnSn catalyst evolved from nanoparticles to nanosheets, nanoneedles and collapsed structures, corresponding to the activation, stabilization and decay stages, respectively. This study provides a facile and controllable approach for the construction of novel bimetallic catalyst favoring formate selectivity based on the synergistic effect.

CuBi electrocatalysts modulated to grow on derived copper foam for efficient CO2-to-formate conversion

Electrochemical reduction of CO₂ to fuels and chemicals is an effective way to reduce greenhouse gas emissions and alleviate the energy crisis, but the highly active catalysts necessary for this reaction under mild conditions are still rare. In this work, we grew CuBi bimetallic catalysts on derived copper foam substrates by co-electrodeposition, and then investigated the correlation between co-electrodeposition potential and electrochemical performance in CO₂-to-formate conversion. Results showed that the bimetallic catalyst formed at a low potential of - 0.6 V vs. AgCl/Ag electrode achieved the highest formate Faradaic efficiency (FE_formate) of 94.4% and a current density of 38.5 mA/cm² at a low potential of - 0.97 V vs. reversible hydrogen electrode (RHE). Moreover, a continuous-flow membrane electrode assembly reactor also enabled the catalyst to show better performance (a FE_formate of 98.3% at 56.6 mA/cm²) than a traditional H-type reaction cell. This work highlights the vital impact of co-electrodeposition potential on catalyst performance and provides a basis for the modulated growth of bimetallic catalysts on substrates. It also shows the possibility of preparing Bi-based catalysts with no obvious decrease in catalytic activity that have been partially replaced with more economic copper.

Ultrasound-boosted selectivity of CO in CO2 electrochemical reduction

Among the possible products of CO₂ electrochemical reduction, CO plays a unique and vital role, which can be an ideal feedstock for further reduction to C₂₊ products, and also the important component of syngas that can be used as feedstock for value-added chemicals and fuels. However, it is still a challenge to tune the CO selectivity on Cu electrode. Here we newly construct an ultrasound-assisted electrochemical method for CO₂ reduction, which can tune the selectivity of CO₂ to CO from less than 10% to >80% at -1.18 V versus (vs.) reversible hydrogen electrode (RHE). The partial current density of CO production is significantly improved by 15 times. By in-situ Raman study, the dominating factor for the improved CO production is attributed to the accelerated desorption of *CO intermediate. This work provides a facile method to tune the product selectivity in CO₂ electrochemical reduction.

Engineering Bismuth-Tin Interface in Bimetallic Aerogel with a 3D Porous Structure for Highly Selective Electrocatalytic CO2 Reduction to HCOOH

Electrochemical reduction of CO₂ (CO₂ RR) into valuable hydrocarbons is appealing in alleviating the excessive CO₂ level. We present the very first utilization of metallic bismuth-tin (Bi-Sn) aerogel for CO₂ RR with selective HCOOH production. A non-precious bimetallic aerogel of Bi-Sn is readily prepared at ambient temperature, which exhibits 3D morphology with interconnected channels, abundant interfaces and a hydrophilic surface. Superior to Bi and Sn, the Bi-Sn aerogel exposes more active sites and it has favorable mass transfer properties, which endow it with a high FE_HCOOH of 93.9 %. Moreover, the Bi-Sn aerogel achieves a FE_HCOOH of ca. 90 % that was maintained for 10 h in a flow battery. In situ ATR-FTIR measurements confirmed that the formation of *HCOO is the rate-determining step toward formic acid generation. DFT demonstrated the coexistence of Bi and Sn optimized the energy barrier for the production of HCOOH, thereby improving the catalytic activity.