The restricted reservoir of fossils fuels and the accelerating risks of climate change have motivated scientists to develop alternative technologies to synthesize eco-friendly fuels.
Green hydrogen produced through the electrolysis of water with renewable electricity is regarded as a next-generation renewable energy source. However, in reality, a great majority of hydrogen fuel is produced through the refining of fossils fuels as electrolysis is expensive.
At present, the efficiency of water electrolysis is very less and high cell voltage is mostly needed as a result of the lack of efficient electrocatalysts for hydrogen evolution reactions. Noble metals like platinum (Pt) are utilized as catalysts to enhance hydrogen generation in both acidic or alkaline media. However, such noble metal catalysts are very costly and exhibit poor stability under long-run operation.
In recent times, single-atom catalysts have exhibited outstanding activity than their nanomaterial-based counterparts. The reason is they are able to reach up to 100% atom utilization, whereas in nanoparticles, only the surface atoms are available for reaction. But as a result of the simplicity of the single-metal-atom center, carrying out additional alteration of the catalysts to execute complicated multistep reactions is rather hard.
The easiest method to alter the single atoms is by converting them into single-atom dimers, combining two different single atoms collectively. Tuning the active site of single-atom catalysts with dimers has the ability to enhance the reaction kinetics as a result of the synergistic effect occurring between two different atoms.
But while the synthesis and identification of the single-atom dimer structure have been well-known conceptually, its practical realization has been highly complicated.
This issue was addressed by a research group headed by Associate Director Hyoyoung Lee of the Center for Integrated Nanostructure Physics within the Institute for Basic Science (IBS) situated at Sungkyunkwan University. The IBS research group was successful in developing an atomically dispersed Ni-Co dimer structure stabilized on a nitrogen-doped carbon support. This was given the name NiCo-SAD-NC.
We synthesized Ni-Co single atom dimer structure on nitrogen (N)-doped carbon support via in-situ trapping of Ni/Co ions into the polydopamine sphere, followed by pyrolysis with precisely controlled N-coordination. We employed state-of-the-art transmission electron microscopy and x-ray absorption spectroscopy to successfully identify these NiCo-SAD sites with atomic precision.
Ashwani Kumar, Study First Author, Institute for Basic Science
The scientists discovered that tempering for around 2 hours at 800 °C in an argon atmosphere was the best condition for achieving the dimer structure. The other single atom dimers, like CoFe and CoMn, could also be synthesized with the help of the same method. This proves their plan’s generality.
The research group assessed the catalytic efficiency of this new system regarding the overpotential needed to push the hydrogen evolution reaction. The NiCo-SAD-NC electrocatalyst possessed a comparable level of overvoltage as commercial Pt-based catalysts in alkaline and acidic media.
Furthermore, NiCo-SAD-NC displayed eight times greater activity compared to the Ni/Co single-atom catalysts and heterogeneous NiCo nanoparticles present in alkaline media. Concurrently, it reached 17 and 11 times greater activity than Co and Ni single-atom catalysts, respectively, and 13 times higher compared to conventional Ni/Co nanoparticles in acidic media.
Besides, the scientists illustrated the long-term stability of the new catalyst, which was able to force the reaction for 50 hours without any alteration in structure. The NiCo-SAD displayed excellent water dissociation and optimal proton adsorption than other single-atom dimers and Ni/Co single-atom sites. This boosted the activity of the pH-universal catalyst depending on the density functional theory simulation.
Kumar, A., et al. (2021) Moving beyond bimetallic-alloy to single-atom dimer atomic-interface for all-pH hydrogen evolution. Nature Communications. doi.org/10.1038/s41467-021-27145-3.