The oxygen reduction reaction (ORR) is a key process in energy generation technologies like air batteries and fuel cells, which are vital to a sustainable future. But the difficulty in breaking the strong O=O double bond requires an electrocatalyst. While metal catalysts are used in most industrial applications, researchers are looking for organic alternatives [Martínez-Fernández et al., Applied Materials Today 26 (2022) 101384, https://doi.org/10.1016/j.apmt.2022.101384 ].
“Most metal catalysts are based on precious metals whose worldwide reserves are very limited,” points out José L. Segura of Universidad Complutense de Madrid, who led the research with colleagues at Universidad Autónoma de Madrid, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), and Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia). “Over the last few years, carbon-based structures have emerged as a powerful tool in the development of catalysts.”
Transition-metal-based materials such as perovskite oxides, transition metal borides/nitrides/oxides, layered metal hydroxides, metal alloys and carbon-containing alternatives are particularly desirable because of their cheapness and abundance relative to widely used platinum (Pt)-based catalysts. Currently, however, the preparation of state-of-the-art carbon-based ORR electrocatalysts requires high-temperature transformation of organic material, which increases cost and limits control over structural features.
To get around this limitation, Segura and his colleagues have developed a family of metal-free electrocatalysts based on organic molecules with napthalenediimide (NDI) functional groups as the active centers for ORR. They synthesized a new type of polymer known as a covalent organic framework (COF), which has an organized porous structure, to form the basis of the electrocatalysts.
“In this way, by simple imide bond formation between naphthalene dianhydride and triamine compounds, a series of hexagonal NDI-based COFs are obtained,” explains Segura. “These COFs can be considered an organized pattern of metal-free electroactive NDI moieties, which are able to catalyze the ORR.”
Because the NDI-based COF electrocatalysts catalyze the ORR directly, a pyrolysis step is not required, which reduces costs compared with other carbon-based electrocatalysts. The use of different triamine linker molecules enables the distortion of the structure to be tuned, altering the accessibility for O2 molecules and, therefore, electrocatalytic activity. The NDI-COFs also show high O2 selectivity and stability.
“To the best of our knowledge, the enhancement of reactants’ accessibility to a heterogeneous active site is unexplored for COF-based electrocatalysts,” says Segura. “[But] by tuning the porous structure of these materials we were able to increase the accessibility of the O2 molecule to the catalytic centers and thus enhance the ORR response.”
The researchers believe that COFs offer a promising alternative to Pt catalysts because of their high surface area, inherent stability and durability, efficient charge transport, and affordability.
“Ultimately, our main goal is to incorporate these COFs into real functional devices such as zinc-air batteries or fuel cells for clean energy production,” adds Segura.
Combination of building blocks with good electrocatalytic activity and suitable linker moieties in covalent organic frameworks allows a strictly controlled arrangement of electroactive centers for electrocatalysis.