Advancements in bimetallic single-atom catalysts for water splitting and green hydrogen production

Water splitting, which involves the electrochemical breakdown of water into hydrogen and oxygen, is a cleaner method of producing hydrogen compared to traditional fossil fuel-based processes. However, efficient electrocatalysts are needed to drive water splitting at a competitive cost, and this has driven the development of single-atom catalysts (SACs) and bimetallic single-atom catalysts (bimSACs). The research team led by Zbořil discussed the advancements in the field of bimSACs and their role in water splitting for green hydrogen production, which is considered a crucial step in the transition to a sustainable energy future.

SACs have garnered significant attention due to their high catalytic activity, atom efficiency, and tunable electronic properties. SACs feature individual metal atoms dispersed on a support, which allows for nearly 100% metal atom utilization. They are especially effective in reactions that require efficient proton-electron exchange and multistep electron transfer, such as water splitting. While SACs have demonstrated outstanding performance in hydrogen evolution and oxygen evolution reactions (HER and OER), they suffer from limited active sites due to the isolation of single metal atoms. This limitation hinders their ability to catalyze reactions that require the co-adsorption of multiple reactants.

To address these challenges, bimSACs were introduced. BimSACs incorporate two different metal atoms into a single-atom structure, allowing for enhanced catalytic performance through the synergistic interactions between the metal centers. The presence of two metal atoms enables bimSACs to leverage metal-metal and metal-support cooperativity, which improves proton-electron exchange, substrate activation, and multistep electron transfer. This makes bimSACs particularly attractive for complex electrochemical processes like water splitting. The ability of bimSACs to fine-tune the electronic properties and coordination environment of each metal site enables higher selectivity, lower activation energy, and greater stability compared to traditional SACs.

The synthesis of bimSACs is a challenging endeavor due to the need for precise control over the atomic distribution of the metal atoms and their coordination environment. Several synthesis methods have been explored, including high-temperature pyrolysis, atomic layer deposition (ALD), and impregnation–adsorption. Pyrolysis is often used in combination with metal-organic frameworks (MOFs) or other precursors, which can be heated to high temperatures to disperse the metal atoms. However, the high temperatures used in pyrolysis may cause aggregation of metal atoms, leading to reduced catalytic activity. ALD, on the other hand, allows for precise control over the deposition of metal atoms on a substrate, enabling the creation of bimSACs with atomic precision. Impregnation–adsorption methods also show promise, as they involve adsorbing metal precursors onto supports before heating, allowing for the formation of isolated metal sites. Despite these advancements, the challenges of achieving uniform metal distribution, preventing aggregation, and maintaining the stability of bimSACs under reaction conditions remain significant.

Computational techniques, particularly density functional theory (DFT) and ab initio molecular dynamics (AIMD), are invaluable tools in the design and optimization of bimSACs. DFT calculations provide insights into the electronic structure, stability, and catalytic behavior of bimSACs by modeling the interaction between the metal atoms and the support. These calculations help identify optimal metal pairings and coordination environments that can maximize catalytic performance. AIMD simulations offer a more detailed view of the atomic dynamics, providing insights into how the catalysts behave under realistic reaction conditions. The combination of experimental and computational approaches has allowed researchers to predict the most effective catalyst designs and optimize their properties for specific reactions.

Advanced characterization techniques are essential for probing the structure and behavior of bimSACs at the atomic level. Techniques such as aberration-corrected high-resolution scanning transmission electron microscopy (STEM), X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) have been used to investigate the local coordination environment of metal atoms in bimSACs. These methods provide valuable information about the oxidation state, coordination geometry, and atomic interactions of metal centers. In particular, in situ XAS allows researchers to observe changes in the catalyst’s structure and electronic properties under reaction conditions, providing a deeper understanding of the catalyst’s behavior during electrochemical processes.

BimSACs hold significant promise not only for water splitting but also for other electrocatalytic processes, such as carbon dioxide reduction, nitrogen fixation, and hydrogen peroxide production. These reactions often involve complex multistep mechanisms that require precise control over the catalyst’s electronic structure. BimSACs can break traditional scaling relationships and decouple reaction sites, allowing them to effectively catalyze such complex reactions. In water splitting, bimSACs enhance both the HER and OER, which are critical for the hydrogen production process. The unique properties of bimSACs make them highly efficient in generating hydrogen from water, with the potential for large-scale applications in renewable energy systems.

The research team has concluded by emphasizing that while bimSACs represent a promising direction for improving electrocatalysis, there are still several challenges to overcome. Continued research into the synthesis of bimSACs, the understanding of their structure–property relationships, and the development of more efficient characterization techniques will be crucial for unlocking their full potential. The combination of experimental innovation, computational modeling, and advanced characterization techniques will drive the design of more effective bimSACs, pushing the boundaries of what is possible in clean energy production and other sustainable catalytic processes.

In summary, bimetallic single-atom catalysts have emerged as a powerful tool in catalysis, offering unique advantages over traditional SACs and nanoparticulate catalysts. Their ability to leverage synergistic interactions between two metal atoms allows for enhanced catalytic performance, stability, and selectivity, making them ideal for complex electrochemical reactions like water splitting. With ongoing advancements in synthesis, characterization, and computational modeling, bimSACs hold great potential for revolutionizing the field of catalysis and contributing to the development of sustainable energy technologies.