(340bn) Unraveling the Role of Fe and Oxygen Defects on CoOx nanoisland Structure and Water Splitting Activity Using Computational Approaches

In order to achieve a more sustainable future, improvements to our current methods of energy generation and storage are required. One process that can play a key role in advancing society’s sustainability is water splitting (2H₂O --> O₂ + 2H₂), which when paired with renewable energy sources can be a carbon free way to generate hydrogen fuel (H₂). However, for this process to be economically viable catalysts from earth abundant materials, such as Mn, Fe, Ni, and Co. The current best performing catalysts are made of Ir and Ru oxides materials so advancements in earth abundant catalyst earth catalyst design are needed. Prior work focusing on earth abundant catalyst design has found that some binary and tertiary metal oxides perform better than their unary counterparts, but an understanding of why is lacking [1]. Using Density Functional Theory (DFT) calculations in conjunction with experiments on well-defined model systems, the reason behind the increase in performance can be systematically investigated. Studies of well-defined cobalt oxide nanoislands have been shown to provide valuable insights into the properties of these oxide systems, for example, this approach has been able to elucidate the structure of Fe-doped CoO nanoislands [2-4]. Importantly, however, the key question about how the atomic structure of the CoO­_x nanoislands effects its activity is unanswered. Analysis of the role of defects, including Fe dopants, oxygen line defects and edges, can provide further insights into the reason for oxide catalyst activity.

CoO_x nanoislands supported on Au(111) have been previously studied through a combination of DFT and Scanning Tunneling Microscopy (STM) resulting in structural features of the nanoislands have been elucidated and discussed [2-7]. Edges of the nanoislands, which are defects that create undercoordinated sites, have been found to be important for the activity of CoO_x nanoislands [2,5]. However, other types of defects such as Fe dopants and oxygen line defects, and their role on structure and activity are less understood. It is known that reversible formation of oxygen line defects in bilayer CoO nanoislands (excess oxygen is incorporated into the structure) exist before a transition to the OER active CoO₂ phase takes place. DFT calculations have been done to investigate the formation of oxygen line defects to better understand how these defects effect the nanoparticle structure and activity. In addition to oxygen line defects, investigation of how these oxygen line defects effect the edges of the CoO_x nanoislands will be discussed as it has been shown these play a role in both the CoO to CoO₂ phase transition and OER activity. This work sheds light on the phase transformation of CoO_x which has applications beyond catalysis as well.

The presence of Fe and its role on the nanoisland structure and activity will also be presented. Fe has been shown to play an interesting role in CoO_x systems, sometimes causing increases in activity, sometimes causing activity to worsen. The reason for these observed trends is not full understood. Using a site-specific computational approach to study well-defined CoO_x nanoislands, we have found the role of Fe in our system and propose this type of analysis is what is truly needed to explain the unknown role of Fe across all CoO_x systems. Mapping out the impact of Fe and oxygen defects on the catalytic properties of the nanoisland will illustrate how we can use well-defined oxide systems to understand the beneficial effects binary oxides have over their unary counterparts. By having a truly atomic scale understanding of mixed Co/Fe oxides systems that have shown promise as OER catalysts, we hope to demonstrate key considerations for designing effective earth abundant catalysts.

Research Interests: catalysis, materials design, electrochemistry, density functional theory, oxides

References

1. Zhang B, et al. Science 352.6283 (2016):3330337
2. Fester J, et al. Nat Commun 2017;8:14169. (2017)
3. Rodríguez-Fernández J, et al. J Chem Phys. 150, 041731. (2019)
4. Curto A, et al. A. Nano Res. (2019) 12: 2364. (2019)
5. Zhaozong Sun, Anthony Curto, et al. The Effect of Fe Dopant Location in Co(Fe)O-OH on Au(111) for the Oxygen Evolution Reaction, in manuscript.
6. Fester, Jakob, et al. Angewandte Chemie International Edition57.37 (2018): 11893-11897.
7. Fester, Jakob, et al. The Journal of Physical Chemistry B122.2 (2018): 561-571.