Highly Selective and Active PdCu Alloy Electrocatalyst for CO2 Reduction to CO

The escalating levels of atmospheric CO2, primarily due to the excessive consumption of fossil fuels, pose a significant threat to the global environment. Electrocatalytic CO2 reduction reaction (CO2RR) presents a promising pathway to mitigate this issue by converting CO2 into valuable chemicals. While electrocatalysis offers several advantages, including mild reaction conditions and high efficiency, challenges such as high overpotentials, low activity, and the competing hydrogen evolution reaction (HER) hinder its widespread adoption. The development of high-efficiency, low-cost catalysts is crucial for advancing CO2RR technology. Noble metals like Au, Ag, and Pd exhibit good selectivity for CO2RR products, but their high cost limits scalability. This research focuses on the synthesis and characterization of PdCu alloys as a cost-effective alternative to noble metal-only catalysts for CO2RR, aiming to improve efficiency and selectivity in the conversion of CO2 to CO.

Extensive research has explored various electrocatalysts for CO2RR, with noble metals showing significant promise due to their inherent catalytic activity and conductivity. However, the high cost and limited availability of these materials necessitate the exploration of more sustainable alternatives. Bimetallic catalysts, particularly those combining noble metals with earth-abundant elements, have emerged as a viable strategy to enhance catalytic performance while reducing cost. Several studies have reported the effectiveness of Pd-based bimetallic systems in CO2RR, showcasing improved activity and selectivity. The incorporation of a second metal can influence the electronic structure and surface properties of the catalyst, impacting the adsorption and desorption of reaction intermediates and ultimately affecting the overall catalytic activity and product selectivity. This work builds on these prior studies, specifically investigating the potential of PdCu alloys as highly active and selective CO2RR catalysts.

Super-branched PdCu alloys were synthesized using a modified solvothermal method. The precursor solution contained K2PdCl4, Cu(OAc)2·H2O, 1-methylimidazole, and ethylene glycol. The reaction was carried out at 160 °C for 6 h. Three different PdCu alloys (PdCu-1, PdCu-2, and PdCu-3) were prepared with varying Pd:Cu molar ratios. Pure Pd and pure Cu samples were also synthesized for comparison. The synthesized catalysts were characterized using various techniques including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), High-Resolution Transmission Electron Microscopy (HRTEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Powder Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) to determine the composition and structure. Electrochemical measurements were conducted using a three-electrode system in a CO2-saturated 0.5 M KHCO3 electrolyte. Linear sweep voltammetry (LSV) was performed to evaluate the electrocatalytic activity. Potentiostatic electrolysis experiments were conducted to determine the Faradaic efficiency (FE) of different products. Gas products (CO, CH4, C2H4, C2H6, and H2) were analyzed using gas chromatography, while liquid products were analyzed using ¹H NMR. CO2 adsorption experiments were performed to assess the adsorption capacity of the catalysts. Electrochemical impedance spectroscopy (EIS) was employed to analyze the charge transfer resistance. Time-resolved potential step (TRPS) measurements, commonly known as Time-resolved potential step (TPV), were conducted to evaluate the electron transfer rate. A stability test was performed to evaluate the long-term performance of the best-performing catalyst.

The PdCu-2 alloy, with a Pd:Cu mass ratio of approximately 60:40, exhibited the highest catalytic activity and selectivity for CO2RR to CO among all the synthesized catalysts. LSV curves showed significantly higher current density in CO2-saturated electrolyte compared to N2-saturated electrolyte, indicating significant CO2 reduction. PdCu-2 showed the highest current density and lowest onset potential for CO2RR among the PdCu alloys. The Faradaic efficiency (FE) for CO reached a maximum of 85% at -0.9 V (vs. RHE), exceeding the performance of several previously reported CO2RR electrocatalysts. XPS analysis revealed a negative shift in the binding energy of Pd 3d and a higher ratio of oxidized Pd to zero-valent Pd in the alloys, indicative of electronic interactions between Pd and Cu, potentially influencing the adsorption and desorption of intermediates. The time decay constant (τ) from the TPV experiment was largest for PdCu-2, indicating a slower electron transfer rate and a higher electron concentration around the active sites, which is beneficial for CO2 activation and reduction. The PdCu-2 catalyst demonstrated good stability, maintaining a current density of around -5 mA cm⁻² for over 17 h at -0.9 V (vs. RHE). CO2 adsorption experiments showed that PdCu-1 had the highest CO2 adsorption capacity. Electrochemical impedance spectroscopy (EIS) revealed that PdCu-2 exhibited the lowest charge transfer resistance among the PdCu alloys and pure metals. These findings collectively suggest a synergistic effect between Pd and Cu in the PdCu-2 alloy leading to the enhanced CO2RR performance.

The superior electrocatalytic performance of the PdCu-2 alloy for CO2 reduction to CO can be attributed to a combination of factors. The specific Pd:Cu ratio in PdCu-2 appears to optimize the balance between CO2 adsorption (favored by Pd) and electron transfer (facilitated by Cu), promoting the formation of CO and minimizing the competitive HER. The observed negative shift in Pd 3d binding energy and increased oxidized Pd content in XPS indicate a modification of the electronic structure due to alloying, enhancing the adsorption of *COOH and facilitating the desorption of *CO. The slower electron transfer rate, evidenced by the higher τ value in TPV, implies that more electrons accumulate on the catalyst surface, providing sufficient electrons for CO2 activation and reduction. The lower charge transfer resistance, as shown by EIS, further supports the enhanced electron transfer capability of the PdCu-2 alloy. The stability test demonstrates the robustness of the catalyst for practical applications. These observations support the proposed reaction mechanism illustrated in Figure 5, highlighting the synergistic roles of Pd and Cu in promoting CO2RR.

This study successfully demonstrated the synthesis of highly active and selective PdCu alloy electrocatalysts for CO2 reduction to CO. PdCu-2 displayed superior performance compared to pure Pd, pure Cu, and other PdCu alloys, achieving a high FE of 85% for CO production. The enhanced catalytic activity and selectivity are attributed to the synergistic effects of Pd and Cu, altering the electronic structure and enhancing electron transfer. This work provides valuable insights into the design of efficient and cost-effective electrocatalysts for CO2RR, promoting the development of sustainable energy technologies.

While the PdCu-2 alloy shows promising results, further investigations are needed to optimize the synthesis parameters for even better performance. The study primarily focused on the CO2-to-CO conversion; future research could explore the potential for producing other valuable products, such as methane or higher hydrocarbons, by tuning the catalyst composition and reaction conditions. Long-term stability tests under various operating conditions should be conducted to fully assess the durability of the catalyst.