The escalating global energy demand and reliance on fossil fuels have led to a surge in CO2 emissions, posing a significant threat to the environment. Electrochemical CO2 reduction (CO2RR) offers a promising solution by utilizing renewable electricity to convert CO2 into valuable chemicals and fuels. Carbon monoxide (CO) is a particularly attractive CO2RR product due to its high economic return and versatility as a building block for synthesizing various organic compounds and liquid fuels via Fischer-Tropsch synthesis. However, achieving high CO selectivity and activity remains challenging due to the competition from the hydrogen evolution reaction (HER). Noble metals like gold (Au) and silver (Ag) exhibit excellent CO production performance but are expensive and scarce, limiting their industrial applications. Furthermore, they often become inactive and prone to HER at high production rates. Therefore, the search for cost-effective catalysts that balance low overpotential, high current density, high selectivity, and long durability is crucial for economically viable CO2-to-CO conversion at scale. Copper (Cu), while capable of activating CO2 and producing various products, suffers from poor selectivity, especially for mono-carbon products like CO and formate. Recent advancements in Cu-based single-atom alloys (SAAs) have shown promise in improving selectivity for mono-carbon products by fine-tuning the electronic structure of the Cu base through alloying with single-atom metals. This approach optimizes the adsorption and desorption rates of reactants and intermediates, leading to the desired selectivity. However, even these binary SAAs can undergo reconstruction at high production rates due to strong bias and low dopant content, creating an activity-stability dilemma. To overcome this, the authors propose that introducing multiple single-atom metals into Cu could provide more control over the catalyst's properties and enhance stability by increasing the mixing entropy of the system, thus leading to a lower Gibbs free energy (ΔG).

The literature extensively documents the challenges and opportunities in electrochemical CO2 reduction. Numerous studies have explored various catalysts, including noble metals (Au, Ag, etc.) and transition metals (Cu, etc.), focusing on enhancing selectivity and activity towards specific products. Research on single-atom alloy catalysts has gained momentum, demonstrating their potential to improve selectivity by tuning the electronic properties of the base metal. However, the stability of these catalysts under high current densities remains a significant hurdle. Existing studies highlight the trade-off between activity and selectivity, especially for Cu-based catalysts, which often produce a mixture of products. Previous work by the authors and others has shown that doping Cu with single atoms can improve CO selectivity; however, maintaining this selectivity at high current densities remains a challenge. The concept of using multiple dopants to synergistically enhance both activity and stability has not been extensively explored in the context of CO2RR to CO, highlighting the novelty of this study's approach.

The researchers synthesized the trimetallic Cu92Sb5Pd3 catalyst using a co-reduction method in pure ethanol, avoiding the need for complexants and potential contaminants. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of Sb and Pd. X-ray diffraction (XRD) showed a pure Cu crystal structure, indicating the absence of Sb or Pd nanoparticles. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed the atomic dispersion of Pd and Sb atoms within the Cu matrix. Extended X-ray absorption fine structure (EXAFS) confirmed the single-atom dispersion of Sb and Pd, showing Sb-Cu and Pd-Cu bonds but no Sb-Sb or Pd-Pd bonds. Operando X-ray absorption spectroscopy (XAS) provided evidence of charge redistribution between the dopants and the Cu matrix under reaction conditions. CO2 electrolysis was performed in a standard three-electrode flow cell with 0.5 M KHCO3 electrolyte. Gas products were analyzed by gas chromatography (GC), while liquid products were quantified using ion chromatography (IC) and nuclear magnetic resonance (NMR) spectroscopy. Cyclic voltammetry (CV) investigated the hydrogen evolution reaction (HER). In situ differential electrochemical mass spectrometry (DEMS) was used to monitor the CO and C2H4 generation. Long-term stability testing was conducted in a membrane electrode assembly (MEA) at -100 mA cm⁻². In situ Raman spectroscopy, in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), and CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS) were used to investigate reaction intermediates and adsorption behavior. Synchrotron valence band spectra (SVBS) probed the electronic structure of the catalysts. Density functional theory (DFT) calculations were used to simulate the adsorption energies and reaction pathways of the CO2RR to CO, examining different Pd doping positions and analyzing charge redistribution.

The Cu92Sb5Pd3 catalyst demonstrated exceptional CO2RR performance. It achieved near-unity CO selectivity (100% (±1.5%)) at a high current density of -402 mA cm⁻² at -0.93 (±0.03) V vs. RHE. The catalyst exhibited high activity, exceeding -700 mA cm⁻² at -1.19 (±0.04) V vs. RHE while maintaining 90% (±2.8%) CO selectivity. Even at -1000 mA cm⁻², the FE for CO remained at 85% (±3.8%). The catalyst showed remarkable long-term stability, maintaining >95% FE for CO for 528 h (22 days) at -100 mA cm⁻². Operando XAS confirmed that the Cu matrix of Cu92Sb5Pd3 presented partially electron-deficient states throughout the reaction, indicating charge redistribution between the Sb/Pd additions and the Cu matrix. The Tafel slope of 138.7 mV dec⁻¹ for Cu92Sb5Pd3 suggested that the first electron transfer step of CO2 was the rate-determining step. In situ DEMS results verified the enhanced CO2 reduction rate on Cu92Sb5Pd3 compared to control samples. In situ Raman and ATR-SEIRAS spectra showed lower coverage of CO intermediates and easier desorption of CO* intermediates on Cu92Sb5Pd3 compared to control samples. CO-DRIFTS experiments revealed a faster desorption rate of CO* on Cu92Sb5Pd3. SVBS measurements confirmed a downward shift in the d-band center of Cu in Cu92Sb5Pd3, indicating a weaker binding strength of CO* intermediates. DFT calculations corroborated these findings, showing lower barriers for CO formation on Cu92Sb5Pd3 compared to control samples. Bader charge analysis further confirmed charge redistribution between the Sb/Pd atoms and the Cu matrix. The superior performance of Cu92Sb5Pd3 stemmed from the synergistic effects of Sb and Pd single atoms, which not only modulated Cu's electronic structure but also enhanced catalyst stability.

The results demonstrate a successful strategy for designing highly efficient and stable CO2RR catalysts. The synergistic effect of Sb and Pd atoms in modifying the electronic structure of the Cu matrix is crucial for achieving near-unity CO selectivity and high activity. The suppression of the HER is attributed to the modification of the electronic structure of Cu, leading to the absence of hydrogen desorption peaks in CV measurements. The enhanced stability is likely due to increased mixing entropy, which lowers the Gibbs free energy and prevents atom aggregation. This study challenges the reliance on noble metal catalysts for CO2RR, offering a cost-effective and highly efficient alternative. The findings provide valuable insights into catalyst design for CO2RR and can be extended to other element combinations and electrocatalytic reactions.

This research showcases a novel trimetallic single-atom alloy catalyst, Cu92Sb5Pd3, which significantly advances CO2-to-CO conversion. The catalyst exhibits exceptional performance in terms of selectivity, activity, and stability, surpassing many noble metal catalysts. The synergistic interplay between Sb and Pd dopants modifies the Cu electronic structure, promoting CO formation and suppressing the HER. The enhanced stability is attributed to increased mixing entropy. This work offers a promising pathway for developing cost-effective and efficient CO2 reduction catalysts for large-scale applications. Future research could explore other trimetallic or even multimetallic combinations to further optimize catalyst performance and investigate the scalability and long-term stability of the catalyst in industrial settings.

While the Cu92Sb5Pd3 catalyst demonstrates exceptional performance, the study primarily focuses on its activity and selectivity in a neutral electrolyte (0.5 M KHCO3). The long-term stability test was conducted at a relatively low current density (-100 mA cm⁻²). Further research is needed to evaluate the catalyst's performance under a wider range of conditions, including varying electrolytes, pH values, and higher current densities, to assess its robustness and scalability for industrial implementation. Additionally, a more comprehensive mechanistic understanding could be achieved through more advanced characterization techniques.