Unravel the mystery of NiFe layered double hydroxide for oxygen evolution

NiFe and CoFe layered double hydroxides are among the most active electrocatalysts for the alkaline oxygen evolution reaction. By combining operando experiments and rigorous DFT calculations, we unravel their active phase, the reaction center and the catalytic mechanism.

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Two friends reconnect after some years and found they share the same interest. I have met Dr. Zhenhua Zeng during my PhD in Denmark and there we discussed many times at the coffee machine about catalysis and oxide structures. After I moved to Berlin and Zhenhua to Purdue we have written each other on social media. There we realized that we were sharing the same scientific interest: figuring out the active phase of NiFe layered double hydroxide under oxygen evolution reaction! 

During our postdocs in Germany and US our research focuses on electrocatalysts for the water/oxygen cycle, i.e. water splitting and hydrogen fuel cells. Water splitting into hydrogen and oxygen driven by electricity from renewable energy sources is an important process for a transition to a greener society. However, the efficiencies of water electrolyzers is hampered in particular by the slow kinetics of the oxygen evolution reaction (OER) at the anode. For example, in alkaline electrolytes, even on the best catalysts, which consist in NiFe and CoFe based hydroxides, the OER is slower by over four orders of magnitude, in comparison to the hydrogen evolution reaction at the cathode.

A fundamental understanding of the active phase, the reaction center and the catalytic mechanism of NiFe and CoFe layered double hydroxides (LDHs) under in-situ conditions is crucial to guide the rational design of catalysts with enhanced performance.

Here, by combining operando X-ray based scattering, absorption spectroscopy and density functional theory calculations, we first identify the atomic-scale structure of the active phases. More specifically, we find that, in comparison with the as-prepared phase, the active phase is characterized by the contraction of the lattice spacing, in both the in-plane lattice constant and the interlayer distance, and switching of intercalated ions from anions to cations. 

Once the active phase is identified, we calculate the catalytic reaction mechanism comparing and contrasting factors such as the geometrical structure and electronic structure (oxidation state) of the active site, as well as non-covalent interactions originated from bulk crystal structure, the steady state of the surface configuration, and the electronic structure methods used in the calculations. Our results show that, [1] OER proceeds via a Mars van Krevelen mechanism; [2] the reaction is more favorable on the oxygen bridged Fe-M (M=Ni or Co) reaction center, i.e. Fe-O-M. The synergy of Fe with nearest-neighbor second metal M results in an optimized binding energy for the reaction intermediates that are more favorably stabilized than on other sites, such as single metal sites. 

Our paper shows that the performance of catalysts can be further improved, perhaps by introducing a more redox flexible metal than Fe, or significantly improved by breaking the OOH* vs. OH* scaling relationship. In general, the present study suggests that doping oxides with additional redox-flexible metals to form active reaction centers through the synergy with nearest neighbor metal sites constitutes a general design principle for the synthesis of new OER catalysts design with improved catalytic performance.

 

We are looking forward to hearing your opinion. The full article is available here: https://www.nature.com/article...

Fabio Dionigi

Postdoc, Technische Universität Berlin

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