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SurfacE structure-Activity-Relationship in atomically-defined, ultrathin film perovskite Catalysts

Periodic Reporting for period 2 - SEARCh (SurfacE structure-Activity-Relationship in atomically-defined, ultrathin film perovskite Catalysts)

Reporting period: 2019-12-01 to 2020-11-30

Due to the intermittency of renewable electricity, conversion to chemical fuel is a necessity for the success of the transition to sustainable energy. A simple and attractive candidate for climate-neutral fuel is hydrogen, which can be produced directly through water splitting (so-called electrolysis). But substantial market penetration by commercial electrolysers (devices that perform water splitting using electricity) has not been achieved because it is not economically attractive yet. The reason for this lies in the need of expensive catalysts, which are necessary for the reaction. Today's catalysts are not stable enough, not efficient enough, and often used expensive and rare elements. To develop and exploit earth-abundant highly active catalyst materials, a detailed understanding of the underlying relationships between catalytic activity and atomic-level surface structure is required.
Therefore, we investigate Ni-Fe-based perovskite thin film catalysts that can be create with atomic precision to achieve the following objectives:
- Revalidate or replace activity trends found for less well-defined surfaces
- Derive an atomistic understanding of the catalysis reaction and degradation mechanisms
- Deduce design rules for beyond-state-of-the-art electrocatalyst materials and communicate them to the catalyst research and production communities for exploitation in “real-world” catalyst materials
The results of SEARCh will thus contribute to the goals of development and deployment of low-carbon technologies in line with the EU’s Strategic Energy Technology Plan.
1) We so far investigated the effect of surface composition of the electrocatalyst. We find that exposing the transition-metal site in perovskite oxide catalysts leads to enhanced catalytic activity and demonstrated how such termination-control can be achieved in epitaxial growth
2) We showed using operando spectroscopy that the reason for enhanced activity of the transition-metal site is related to a surface phase change: an active, oxyhydroxide type surface layer forms under reaction conditions. This layer can only form for the correct starting termination. So a single atomic layer determines whether an active pahse can form or not.
3) We showed that air exposure leads to alteration of the catalyst surface and a decrease in catalytic activity. Therefore, we developed an air-free transfer system to study the inherent activity of the electrocatalysts (quasi in situ electrochemical testing)
4) We showed that doping with Fe or Co leads to enhancement of the electocatalytic activity of LaNiO3 electrocatalysts.
5) We showed that the epitaxial strain has a comparably small effect on the electrocatalytic activity
6) We showed experimentally and using DFT that for LaNiO3, the (111) crystallographic orientation is the most active. Surprisingly, the (111) orientation is also the most stable surface. Typically, stability and activity show an inverse relationship.

Exploitation and dissemination:
The results listed above have been or will be communicated to the catalysis research com-munity with the goal of exploitation in commercial grade catalysts - based on the insights obtained with our model systems. In addition, each publication is accompanied by press releases and social media posts for communication to the interested public.

Accepted for publication: points 1 and 2 in Nature Materials (accepted for publication. No DOI yet)
To be submitted: point 3
Manuscript in preparation: point 6, more details for point 1 and 2
The importance of the surface composition of electrocatalysts for water electrolysis has not been recognized before. We therefore expect a paradigm shift in the derivation of so-called descriptors, i.e. materials properties that directly correlate with the catalytic activity. So far, they were mainly based on the bulk electronic structure of the catalyst. Our work shows that the surface properties may be dissimilar from the bulk properties, but that the surface is decisive for the catalytic activity. So derivation of descriptors based on surface properties will be more appropriate. Additionally, we highlight the importance of operando characterization because the surfaces transform under reaction conditions, and only by knowing the transformation pathways can we undertand and optimize electrocatalysts.
This realization will lead to general improvement in the optimization of electrocatalysts to make a transition to a hydrogen-based, sustainable industry and society more attainable.
We hope that our results and insights will contribute to a further and faster development of highly efficient electrocatalysts.
Important aspects for the optimization of electrocatalysts for water electrolysis