Thickness-dependent β/γ-NiOOH transformation of Ni-MOFs in oxygen evolution reaction

[Display omitted] •The MOFs ligand is replaced by OH– along with electrooxidation of Ni ions in OER.•The ultrathin MOFs promote the transformation as β-NiOOH due to sufficient OH–.•The γ-NiOOH is generated on the thicken Ni-MOFs with insufficient OH– in OER.•The β-NiOOH displays the lower energy than γ-NiOOH for formation of O. In-situ generated hydroxides and oxyhydroxides have been gradually recognized as the real active sites of metal organic frameworks (MOFs) towards the oxygen evolution reaction (OER). Nevertheless, the fine phase evolution process is still less of concern and elusive. Herein, we successfully construct serious of Ni-MOFs with differing thickness ranging from 1 nm to 270 nm through a competitive coordination strategy. Utilizing the operando resonance spectroscopy, an accurate phase inversion behavior of the Ni-MOFs has been witnessed during the OER, and demonstrated as a thickness-dependent process. Ultrathin architecture is identified as a positive factor to promote the exchange between MOFs ligands and applied electrolyte during the electrochemical test. Hence, after removing most of the internal ligands, 2D MOFs favor to be transformed as β-NiOOH, which is more active than the generated γ-NiOOH in bulky system. The density function theory (DFT) calculation alongside with the in-situ electrochemistry tests further affirm the β/γ-NiOOH transformation behavior. This work offers a new understanding of the electrocatalytic mechanism from the view point of phase evolution in 2D MOFs.