Ultrafine Nanoclusters Unlocked 3d–4f Electronic Ladders for Efficient Electrocatalytic Water Oxidation

The vast extensional planes of two-dimensional (2D) nanomaterials are recognized as desirable ground for electrocatalytic reactions. However, they tend to exhibit catalytic inertia due to their surface-ordered coordination configurations. Herein, an in situ autoxidation strategy enables high-density grafting of ultrafine CeO2 nanoclusters on 2D Co(OH)2. Affluent active units were activated at the inert interface of Co(OH)2 via the formation of Co–O–Ce units. The optimized catalyst exhibits oxygen evolution reaction activity with an overpotential of 83 mV lower than that of Co(OH)2 at 10 mA cm–2. The cascade orbital coupling between Co (3d) and Ce (4f) in Co–O–Ce units drives electron transfer by unlocking a "d–f electron ladder". Meanwhile, the bond-order theorem analyses and the d-band center show that the occupancy of Co–3d-eg is optimized to balance the adsorption–desorption process of active sites to the key reaction intermediate *OOH, thereby making it easier to release oxygen. This work will drive the development of wider area electron modulation methods and provide guidance for the surface engineering of 2D nanomaterials.