Fachbeirat 2022

In-depth Understanding of Cobalt Oxide based Electrocatalysts for Oxygen Evolution Reaction

Not scheduled

20m

Speaker

Felix T. Haase (Department of Interface Science, Fritz-Haber-Institute Berlin)

Description

Cobalt oxides are promising, non-precious catalysts for the anodic oxygen evolution reaction (OER) of alkaline water electrolysis. To promote a rational catalyst design, fundamental knowledge is required and topic of investigations. In this regard, it has been shown that as-prepared structurally-different CoO${}_x$(OH)${}_Y$ materials unify towards an octahedral CoO${}_6$ coordination with di µ-oxo-bridged Co ions under OER conditions.${}^1$ In the case of crystalline Co${}_3$O${}_4$ spinels, this transformation was found to happen reversibly during OER${}^{2,3}$ with the presence of (at least) two differently active sites.${}^4$ Regarding the catalytic activity, so-called X ray amorphous catalysts were found to excel over crystalline catalysts which can be further increased by the presence of Fe on the Cobalt oxide surface.${}^{5,6}$
To achieve an in-depth understanding of the Co-catalyzed alkaline OER, we applied operando techniques to selected catalysts and in particular, operando X ray absorption spectroscopy (XAS) acts as a powerful tool providing fundamental structural information on the Co redox electrochemistry. We furthermore complement our results from XAS with per se surface sensitive X-ray photoelectron spectroscopy (XPS) which we performed quasi in situ as well as depth-dependent. The two spectroscopic methods are then supported by electron microscopy methods as well DFT calculations on PBE level on the Co surface chemistry.
In this work, we present recent findings on Co-based OER catalysts studied by a variety of ex situ, in situ and operando techniques. In particular, we studied changes in the (electronic) structure of size-selected CoO${}_X$(OH)${}_Y$ NPs and followed the inherent reversible structural response during active state formation.${}^7$ Here, we benefited from an increased surface-to-volume ratio being sensitive to changes on catalyst surface sites with a bulk sensitive method. Furthermore, insights into the role of nanoscale inhomogeneities in Co${}_X$Fe${}_Y$O${}_4$ on their OER activity as well as their surface composition are presented.${}^8$ In the last part, the widely discussed beneficial role of Fe in the electrocatalysis was addressed in a case study on nanostructured Co${}_3$O${}_4$ with preferred (100) surface termination. All our studies benefited from the synergy of the different applied techniques and allows to identify crucial aspects in OER catalysis.

References
1. Bergmann, A. et al. Nat. Catal. 1, 711 (2018).
2. Bergmann, A. et al. Nat. Commun. 6, 8625 (2015).
3. Tung, C.-W. et al. Nat. Commun. 6, 8106 (2015).
4. Zhang, M., De Respinis, M. & Frei, H. Nat. Chem. 6, 362-367 (2014).
5. Kanan, M. W. & Nocera, D. G. Science 321, 1072-1075 (2008).
6. Burke, M. S., et al. J. Am. Chem. Soc. 137, 3638-3648 (2015).
7. Haase, F.T. Bergmann, A., Roldan Cuenya, B et al. Nat. Energy 7 765-773 (2022)
8. Haase, F.T. Bergmann, A., Roldan Cuenya, B et al. J. Am. Chem. Soc. 144, 12007–12019 (2022)