In search of the optimal catalyst: Prof. Bettina Lotsch discusses the electronic processes that play a role in water splitting with Lars Grunenberg and Julia Kröger (from left). (Photo: Wolfgang Scheible for MPG)

Efficient catalysts are essential for the sustainable production of chemicals and fuels, such as green hydrogen. Covalent organic frameworks (COFs) are particularly promising in this field. Their advantage lies in the ability to fine-tune their properties through precise molecular design. “In addition, COFs can be functionalized with various metals, such as cobalt, iron, or zinc, and used as electrocatalysts,” explains Prof. Bettina Lotsch from the Max Planck Institute for Solid State Research in Stuttgart, whose work is supported by the e-conversion Cluster of Excellence. Together with colleagues from the Ruhr University Bochum and the Max Planck Institute for Sustainable Materials (MPI-SusMat), the materials chemist demonstrated in a recent publication that COFs are less stable under real reaction conditions than previously assumed but remain catalytically active. The findings were recently published in the journal Advanced Science.

Organic scaffolds as helpers for nanoparticles
“Our goal is to investigate the active centers of COFs in detail. From these insights, we aim to infer how reaction mechanisms can be made more efficient, ultimately enabling us to design better catalysts for green hydrogen production,” says Lotsch, whose research team was responsible for synthesizing and characterizing cobalt-functionalized COFs. Collaborating with the group led by Prof. Dr. Kristina Tschulik from Ruhr University Bochum, the researchers focused on the oxygen evolution reaction (OER). This partial reaction is critical in many industrially significant processes, such as water electrolysis for hydrogen production. However, it poses significant challenges for catalysts due to its harsh reaction conditions. Electrochemical studies of several cobalt-based COFs revealed that the cobalt ions dissolve from the framework directly after contact with the alkaline medium and transform into cobalt (oxy)hydroxide nanoparticles. These nanoparticles then act as the catalytically active species. This was confirmed by an elaborate material characterization carried out by electron microscopy experts at the MPI-SusMat led by Christina Scheu.

Strinking the right balance in binding strength
“As an electrochemist, I’ve always wondered a bit about how the catalytic activity of COFs actually comes about,” says Tschulik. “Now we know that they provide a suitable reaction environment and hold the nanoparticles in place. Normally, the particles tend to aggregate, which means that less of their catalytic surface is accessible, and they deactivate.” These nanoparticles would typically tend to aggregate, reducing the accessible catalytic surface area.” This highlights the critical role of the organic scaffold structures. Lotsch adds: “The COF framework, especially the binding strength between cobalt and the organic components, plays an essential role. The bond should be strong enough to guarantee the interaction between the nanoparticle and the COF but weak enough to enable the necessary transformation of cobalt ions into active nanoparticles.”

Organic precursors with potential
In their publication, the authors also propose strategies for designing COFs that remain stable and catalytically active under real reaction conditions. “Cobalt-functionalized COFs can be considered a kind of ‘pre-catalyst.’ The architecture of the organic scaffold component serves as an essential precursor that influences the ultimate performance of the catalyst,” explains Lotsch. Tschulik adds: “With the knowledge from this study, we can design future catalysts combining organic frameworks and nanoparticles that are far more efficient than COFs alone.”

Publication:
Shedding Light on the Active Species in a Cobalt-Based Covalent Organic Framework for the Electrochemical Oxygen Evolution Reaction
Pouya Hosseini, Andrés Rodríguez-Camargo, Yiqun Jiang, Siyuan Zhang, Christina Scheu, Liang Yao, Bettina V. Lotsch, and Kristina Tschulik
DOI: 10.1002/advs.202413555

Contact:
Prof. Bettina Lotsch
Nanochemistry Department
Max Planck Institute for Solid State Research
b.lotsch@fkf.mpg.de