Metal-organic frameworks (MOFs) have emerged as a new type of heterogeneous catalysts with rich composition, periodical arrangement and high porosity. They exhibit great activity in many reactions including electrocatalytic oxygen evolution reaction (OER).
So far, their structure-performance relationships, especially those regarding the reconstruction and role of active species, remain unexplored, which cast serious doubt on the catalytic activity origin of MOFs.
A research team led by Prof. TANG Zhiyong from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences and the collaborators discovered the high activity origin of OER in metal-organic frameworks. Accordingly, the designed Ni0.9Fe0.1-MOF displays an extremely low overpotential of 198 mV and 231 mV at 10 mA cm-2 and 20 mA cm-2, respectively. The finding was published in Nature Energy. In this study, by using operando X-ray absorption spectroscopy and high-resolution transmission electron microscopy imaging, the researchers revealed a full landscape of the biphasic structural transformation occurring at the nodes inside Ni and Co bimetallic MOF during OER.
Notably, this potential-induced two-step reconstruction is completely different from the reported one-step surface reconstruction in traditional catalysts.
They then used a series of operando advanced characterization techniques and computation calculations to determine the highly active species during OER.
The simultaneous correlation between the molecular structural revolution and the generated OER current discloses the origin of high oxygen evolution activity. The crystal orbital Hamilton population bonding analysis reveals that the in-situ formed oxygen-vacant oxyhydroxide from the hydroxide via Jahn-Teller distortion is the genuine origin. Particularly, the transition from hydroxide to oxyhydroxide at Ni and Co nodes can be synchronous by altering the metal ratio in NiCo-MOFs, leading to the best OER activity.
The same phenomenon can be observed in various MOF-based materials such as Ni-MOF, Fe-MOF, NiFe-MOF, Co-MOF and NiCo-MOF.
This study not only offer deep insight in designing highly active OER catalysts, but also lay the foundation to precisely elucidate the activity origin of MOFs and other catalysts.
