Exfoliated two-dimensional bimetallic Ni0.6Co0.4-metal organic framework nanosheets with mixed ligands for enhanced acid electrocatalytic water oxidation

Abstract

In the field of acidic water electrolysis for the oxygen evolution reaction (OER), metal–organic frameworks (MOFs) are often hampered by their inherent weak conductivity and high overpotential. Therefore, the development of MOF materials that are adept at facilitating the OER in acidic media represents a significant challenge. It has been found that the electrocatalytic properties of MOFs in an acidic environment are significantly affected by their structure and the metal ions within the structure. Here, different adaptable metal ion MOFs were synthesized with rigid 4,4′-bisimidazolylbiphenyl (BIBP) and flexible carboxylic acid H₂BEC (H₂BEC: 1,2-phenyldiacetic acid) as ligands, including Co-MOF [Co(BEC)₂(BIBP)₂]_n, Ni-MOF [Ni(BEC)₂(BIBP)₂(H₂O)]_n, and Ni_0.6Co_0.4-MOF [Ni_0.6Co_0.4(BEC)₂(BIBP)₂(H₂O)]_n. These MOFs exhibit a three-dimensional structure consisting of a stack of two-dimensional (2D) network nanosheets. In particular, they can be further exfoliated by an amended ultrasonic exfoliation to form a thin two-dimensional nanosheet structure. This strategy is capable of achieving thin nanosheets of different thicknesses by utilising adjustable power and reaction time. The oxygen evolution capacity of the three MOF nanosheets in acidic solution (50 mM H₂SO₄) was compared. Impressively, the best Ni_0.6Co_0.4-MOF nanosheet/carbon black composite not only exhibits a low overpotential (508.98 mV at 10 mA cm⁻²), good electrocatalytic stability and high durability, but also has good and far superior catalytic activity to MOF nanosheets, making it a potential candidate bimetallic MOF material for efficient OER catalysis in acidic environments.