Lattice Strain Induced by Linker Scission in Metal–Organic Framework Nanosheets for Oxygen Evolution Reaction

Abstract

For electrochemical energy conversion, highly efficient and inexpensive electrocatalysts are required, which are principally designed and synthesized by virtue of structural regulations. Herein, we propose a rational linker scission approach to induce lattice strain in metal–organic framework (MOF) catalysts by partially replacing multicoordinating linkers with nonbridging ligands. Strained NiFe-MOFs with 6% lattice expansion exhibit a superior catalytic performance for the oxygen evolution reaction (OER) under alkaline conditions; the overpotential is reduced to 230 mV (86.6 mV dec–1) from 320 mV (164.9 mV dec–1) for the unstrained NiFe-MOFs at a current density of 10 mA cm–2. Operando studies by using synchrotron radiation X-ray absorption and infrared spectroscopy identified the emergence of a key *OOH intermediate on Ni3+/4+ sites during OER, providing strong evidence that the Ni3+/4+ sites are the active sites and the formation of *OOH is the rate-limiting step. The first-principles calculations were performed to reveal the strain-induced electronic structure changes of the NiFe-MOFs and the Gibbs free energy profile during OER. It is found that the optimized Ni 3d eg-orbital facilitates the formation of *OOH, thus enhancing the OER performance of the strained MOFs.

Surface‐Adsorbed Carboxylate Ligands on Layered Double Hydroxides/Metal–Organic Frameworks Promote the Electrocatalytic Oxygen Evolution Reaction

“…14,34 The result further demonstrates that the CLs can regulate the interatomic distances and induce the partial distorted structure, and meanwhile alter the electronic structures of active sites that could help to optimize the intrinsic activity. [35][36] Similarly, in Fig. 4g, FT-EXAFS curves at Fe K-edge shows the similar shifts of Fe-O and Fe-Ni/Fe bond lengths.…”

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

“…More recently, Ji et al proposed a facile linker scission strategy to induce lattice strain in MOF catalysts by partially replacing binary carboxylic acids with monocarboxylic acids. [182] The strained NiFe-MOFs with 6% lattice expansion showed excellent activity for the OER in an alkaline electrolyte with a low overpotential of 230 mV at a current density of 10 mA cm −2 and a small Tafel slope of 86.6 mV dec −1 .…”

“…For instance, incorporating nonbridging ligands into the MOF could significantly improve OER performance. [14,182] Converting bulk MOF crystals into 2D nanosheets could expose more active surface sites. [15,[183][184][185][186] Additionally, multimetallic MOFs present a significant improvement in catalytic activity due to the synergistic effect between different metal sites.…”

Designing High‐Valence Metal Sites for Electrochemical Water Splitting

“…prepared a lattice‐strained 3d transition metal MOF material as a highly active OER electrocatalyst by replacing some of the binary carboxylic acids with monocarboxylic acids as the linker scission. [ 189 ] Notably, the lattice‐strained NiFe MOFs with a 6% lattice expansion showed superior catalytic activity with a low overpotential of 230 mV to reach 10 mA cm −2 and a small Tafel slope of 86.6 mV dec −1 in an alkaline electrolyte ( Figure a,b). Operando XAS and operando Fourier transform infrared spectroscopy showed high‐valence Ni 3+ /Ni 4+ species during the OER process that were the main active sites for the OER in which the rate‐limiting step was the formation of *OOH (Equations (13)–(16) or (17)–(20)) (Figure 19c).…”

“…Reproduced with permission. [ 189 ] Copyright 2020, American Chemical Society. d) Comparison of activities and Tafel slopes of La 2 NiMnO 6 calcined at different temperatures.…”