Single-Atom Catalysts for the Revolutionizing of Oxygen Reduction Reactions

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Oxygen reduction reactions lie at the heart of many important energy-conversion processes, including fuel cells and hydrogen peroxide syntheses. However, the slow kinetics, in particular, under conventional catalysis, have always constituted an intransient challenge. It is here that the revolutionary concept of single-atom catalysts comes in. In the last couple of decades, SACs have attracted tremendous attention owing to their very good catalytic properties. Single-atom catalysts can maximize the utilization of catalytic sites by dispersing single metal atoms on a suitable support, thereby reducing the use and cost of precious metals. These catalysts have unprecedented control of the reaction pathway; hence, highly selective and efficient, especially on ORR. This review covers single-atom catalysts in revolutionizing the ORR wheel in the quest for more efficient, scalable, and greener energy solutions.

Basics behind Oxygen Reduction Reactions (ORR)

Oxygen reduction is a multi-electron process in which the molecular dioxygen, O₂, is reduced to either water, H₂O, or to hydrogen peroxide, H₂O₂, depending on the number of electrons. The 4-electron pathway results in the generation of water, which is a very good product in therapies from fuel cells because it is a source that closely boils with a high content of energy. On the other hand, when the 2-electron pathway is the case, hydrogen peroxide is produced, a source that can be utilized for various useful chemicals in industrial applications. However, such reactions have become a big challenge in terms of selectivity and efficiency. Conventional catalysts, such as platinum-based materials, often demonstrate high costs, slow kinetics, and tendencies toward degradation. Single-atom catalysts solve many of these problems because of their unique properties at the atomic scale, which enhance catalytic performance toward the ORR.

Single-Atom Catalysts for the New Era of Efficiency

Generally speaking, SACs consist of dispersed single metal atoms on the surface of any supporting material, usually carbon-based. Different from the traditional nanoparticle catalysts, every metal atom in SACs is exposed to maximize their utilization. Due to the exposure of each atom, there is much better atomic-level control over reaction intermediates and pathways. This means that, in particular, SACs can serve more effectively in the oxygen reduction reaction by facilitating it much more efficiently, leading to higher catalytic activity and selectivity.

Some works underline the key role that SACs have played in enhancing the performance of ORR. The catalysts have a very strong potential for tuning the electronic properties of the active sites toward an effect on the adsorption and activation of oxygen molecules. Atomic dispersion of metal atoms, like Co or Ni, forms a specific environment where OOH* and O*, intermediates of the ORR, are well stabilized, hence guaranteeing a higher catalytic activity.

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The Role of Coordination Spheres in ORR Selectivity

The tuning of the first and second coordination spheres around the metal center is one of the prime causes of success among the class of SACs in the ORR. The coordination sphere defines the structural arrangement of atoms or groups of atoms around the central metal atom. The catalytic activity and, most importantly, the selectivity of the SACs can be fine-tuned by changing this sphere. For example, tuning N and O coordination in SACs can switch the reaction pathway from a four-electron process to water into a two-electron process to hydrogen peroxide. Such tuning would enable SACs to exhibit remarkable selectivity, especially in the production of hydrogen peroxide, where high selectivity is of importance in industrial processes.

Recent work has demonstrated that the second coordination of the outside elements, the nearest neighbors of the active site, plays a crucial role in fine-tuning the activity of ORR. For example, the shift of the active site away from the metal atom has been induced by carbon-oxygen-carbon (C-O-C) groups in the second coordination sphere, aiming to properly tune the electronic properties for the best ORR activity. This active site configuration control is one of the most defining benefits of SACs in ORR.

Cobalt and Nickel-Based SACs, Leading the Charge

Among the various metals that have been tried in SACs, Co and Ni have been two of the most promising candidates for the ORR. Indubitably, fine catalysis has been observed for both metals when anchored to nitrogen-doped carbon supports. For example, Co-N4 and Ni-N4 SACs have shown remarkable ORR performance, predominantly in both acidic and alkaline media. These catalysts further possess very high stability and selectivity and have outperformed many of the state-of-the-art traditional catalysts.

Among these, Co-based SACs have gained much attention since they can drive the ORR through both the two-electron and four-electron pathways. It has been seen that Co-N4 SACs can be rendered highly versatile depending on the applied potential during the reaction due to a change in selectivity. While hydrogen peroxide production is favored at lower potentials by Co-N4 SACs, higher potentials result in the formation of water. This tunable selectivity makes Co-based SACs ideal for applications ranging from fuel cells to hydrogen peroxide production.

To date, the nickel-based SACs have also shown good ORR activities, particularly along the two-electron path. The nickel atoms in these systems are coordinated to nitrogen-containing carbon support, leading to the formation of the Ni-N4 active sites. It has been confirmed that these sites are hydrogen peroxide production centers, with their Faradaic efficiencies exceeding 90%. Such high activity, selectivity, and stability make the Ni-N4 SAC a viable candidate for the large-scale production of hydrogen peroxide as a greener alternative to existing chemical methods.

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The Importance of Support Materials in Engineering the Perfect SAC

While the metal atom is the focus of SACs, the selection of support material is just as important. Carbon-based materials, including graphene, carbon nanotubes, and porous carbon, have been the most frequent supports in SACs owing to their high surface area, conductivity, and stability. These materials allow the dispersion of metal atoms on their platform and play an important role in enhancing the overall catalytic performance.

The metal atom interacts with the support material in a dramatically significant way, therefore strongly affecting the electronic structure of the active site. For example, nitrogen-doping of carbon supports can create strong bonds between the metal and nitrogen and hence stabilize the metal atom against agglomeration. Such stabilization of metal catalysts is critical for maintaining atomic dispersion so that every atom will contribute to the catalytic process.

Porous carbon supports are particularly favorable, thanks to the fact that they enhance both mass transport and electron transport efficiently, which is necessary for high-performance ORR. The hierarchical structure of these supports with macropores, mesopores, and micropores guarantees optimal accessibility to the active sites, hence making the catalysis more efficient. Besides, their electronic properties can easily be tailored via heteroatom doping, especially using nitrogen, thereby further enhancing the catalytic activities of SACs.

Challenges and Future Directions

Despite remarkable improvement in the sphere of SACs for ORR, several challenges remain among them, one of the most significant challenges is related to the stability of SACs under harsh reaction conditions. Although SACs exhibit commendable catalytic performance, it is tough to maintain the atomic dispersion of metal atoms over a long period. Agglomeration of metal atoms into nanoparticles reduces the efficiency of SACs due to a decline in catalytic activity.

Another challenge is the scalability of SACs for industrial applications. Although SACs are very successful in laboratory-scale experiments, upscaling for their practical use produces some of the most critical challenges. Precise control over metal atom dispersion in the synthesis of SACs will hardly be achieved in large-scale syntheses. In addition, support materials, especially high-quality, carbon-based supports, are quite expensive and may not economically allow large-scale applications.

This is future-oriented, and there are various ways in which these challenges are being tackled. Firstly considered in producing SACs with a high metal atom dispersion reliably on a large scale is the development of new synthesis methods. Atomic layer deposition and electrochemical synthesis can be scaled up for mass production. Thus, techniques are under scrutiny. Secondly, the use of new support material that is cost-effective and easy to synthesize is another direction being pursued.

The support towards the development of multi-metal SACs, in which various metal atoms are dispersed on one support, keeps on growing. Such catalysts can act synergistically to bring catalytic performance and stability to new highs. Some of the limitations with single-metal SACs may be overcome by finding a new horizon in ORR applications, exploiting the synergy of various metal combinations.

Conclusion

Single-atom catalysts are an advancement in the field of oxygen reduction reactions. Single-atom catalysts, with metal atoms maximally utilized and the reaction way to be under precise control, renew the way ORR is taken. Here, from the Co, Ni-based SAC to advanced coordination sphere engineering in use, these are the classes of catalysts that are the way forward for attaining efficient and sustainable energy solutions. Coming back to reality, the challenges that lay ahead are numerous, but the future of SACs in ORR appears to be truly amazing, with varied applications from fuel cells to hydrogen peroxide production. As research in the area continues to forge ahead, small atomic clusters are likely to form an integral part of our progress toward a cleaner and greener energy future.

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