Using a genetic algorithm search combined with hybrid density functional theory calculations, two groups of favorable structures for b-NiOOH were identified: (i) layered structures with alternate Ni(OH)2 and NiO2 layers, consistent with the doubling of the c-axis observed in TEM measurements; (ii) tunnel structures, which can provide a rationale for the mosaic textures observed in TEM.Two chemists at Princeton University have identified a promising new catalyst candidate for hydrogen fuel using a computational approach to gain insight into its structure. With hydrogen fuel being identified as a potential source of clean energy, this catalyst could significantly speed up the process of separating water into hydrogen and oxygen gas, a reaction that can be problematic and whose speed can be improved with a catalyst.
However, existing catalysts for hydrogen fuel are not efficient enough to allow the water splitting to be commercially viable. Although an accurate picture of its structure has been problematic due to it constantly changing during the reaction, one possible approach has identified the highly active compound iron-doped nickel oxide, β-NiOOH, as a promising catalyst to improve reaction times.
This new study, as reported in The Journal of Physical Chemistry Letters [Li, Y.-F., Selloni, A., J. Phys. Chem. Lett. (2014) DOI: 10.1021/jz502127g], used theoretical calculations involving a so-called genetic algorithm to offer a greater understanding of the atomic-scale structure of an active component of β-NiOOH under working conditions. The algorithm uses parameters that are inspired by evolution to develop repeated generations of structures to find the most suitable candidates.
“Understanding the structure is the basis for any further study of the material's properties. If you don't know the material's structure you can’t know what it’s doing.”Annabella Selloni
The findings from their algorithm search, combined with hybrid density functional theory calculations that estimate a molecule's electronic structure, allowed them to identify structures of nickel oxide that supported existing observations. As Annabella Selloni, who led the research, said “Understanding the structure is the basis for any further study of the material's properties. If you don't know the material's structure you can’t know what it’s doing.”
One of the observed features regarded the material’s mosaic-like texture, composed of small grain-like microstructures, which the team believes are stable tunnel structures that relieve any stress between layers. Another observation regarded the doubling of the distance between layers made of the same material, its c axis periodicity, which represents the alternating layers of Ni(OH)2 and NiO2 formed during the reaction.
Showing the complex structures for β-NiOOH is a completely new insight in this field, and could prove to be a starting point for understanding the water oxidation on NiO2 at an atomic level and help in the design of a commercial water oxidation catalyst. With their understanding of the material’s surface structure, the team is now keen to investigate the area of the mosaic texture using experimental techniques since it is related to the activity.