Interface Engineering of Co‐LDH@MOF Heterojunction in Highly Stable and Efficient Oxygen Evolution Reaction

Abstract The electrochemical splitting of water into hydrogen and oxygen is considered one of the most promising approaches to generate clean and sustainable energy. However, the low efficiency of the oxygen evolution reaction (OER) acts as a bottleneck in the water splitting process. Herein, interface engineering heterojunctions between ZIF‐67 and layered double hydroxide (LDH) are designed to enhance the catalytic activity of the OER and the stability of Co‐LDH. The interface is built by the oxygen (O) of Co‐LDH and nitrogen (N) of the 2‐methylimidazole ligand in ZIF‐67, which modulates the local electronic structure of the catalytic active site. Density functional theory calculations demonstrate that the interfacial interaction can enhance the strength of the Co—Oout bond in Co‐LDH, which makes it easier to break the H‐Oout bond and results in a lower free energy change in the potential‐determining step at the heterointerface in the OER process. Therefore, the Co‐LDH@ZIF‐67 exhibits superior OER activity with a low overpotential of 187 mV at a current density of 10 mA cm−2 and long‐term electrochemical stability for more than 50 h. This finding provides a design direction for improving the catalytic activity of OER.


Preparation of ZIF-67
In a typical synthetic procedure, Co(NO 3 ) 2 (0.0515 mmol) and 2-Methylimidazole (3.37 mmol) with Hexadecyl trimethyl ammonium Bromide (CTAB) (0.00548 mmol) were dispersed into deionized water (8 mL), respectively; subsequently, Co(NO 3 ) 2 solution was poured into 2-Methylimidazole solution with stirring. The mixture was allowed to stir for 24 hours at 25 ℃. The final dark violet product was obtained through centrifuging and washing procedures (with deionized water for twice and ethanol for once). Then, it was dried in vacuum at 60 ℃ for 12 hours before further characterization.

Preparation of Co-LDH@ZIF-67
Co(NO 3 ) 2 (0.0515 mmol) and 2-Methylimidazole (3.37 mmol) with Hexadecyl trimethyl ammonium Bromide (CTAB) (0.00548 mmol) were dispersed into deionized water (8 mL), respectively; subsequently, Co(NO 3 ) 2 solution was poured into 2-Methylimidazole solution with stirring. The mixture was allowed to stir for 20 min at 25 ℃. The product was obtained through centrifuging and washing procedures (with deionized water for twice and ethanol for once). Then, it was dried in vacuum at 60 ℃for 12 hours before further characterization.

Preparation of Co-LDH
Co(NO 3 ) 2 (0.0515 mmol) and 2-Methylimidazole (3.37 mmol) with Hexadecyl trimethyl ammonium Bromide (CTAB) (0.00548 mmol) were dispersed into deionized water (8 mL), respectively; subsequently, Co(NO 3 ) 2 solution was poured into 2-Methylimidazole solution with stirring. The mixture was allowed to stir for 20 min at 25 ℃. The product was obtained through centrifuging, ultrasound (5 min each time) and washing procedures (with deionized water for three times and ethanol for once). Then, it was dried in vacuum at 60 ℃ for 12 hours before further characterization.
The product was obtained through centrifuging and washing procedures (with deionized water for twice and ethanol for once). Then, it was dried in vacuum at 60 ℃ for 12 hours before further characterization.

Characterization
The morphology of as-prepared samples was characterized by scanning electron microscopy (SEM, Hitachi SU8010) and transmission electron microscopy (TEM, JEM 2100 LaB6). Powder X-ray diffractometer (XRD) analysis was conducted on a Bruker D8 Advance instrument with a Cu Kα irradiation source at a scanning rate of 5° per min. Raman spectroscopy was performed on a HORIBA labRAM HR Evolution with a 633 nm excitation wavelength. Fourier transform infrared (FTIR) spectra were taken on a Bruker VERTEX 70 instrument. UV-Vis absorption spectra were investigated on a Hitachi U-3010 spectrometer.
X-ray photoelectron spectroscopy (XPS) was carried out on a PHI5000 Versaprobe using an Al Kα X-ray source, with all the binding energies calibrated to C 1s peak at 285.0 eV. The specific surface areas and pore size distribution of samples were evaluated on the ASAP2460 Surface Area and Porosity Analyzer (Micromeritics), calculated by N 2 sorption isotherms using a Brunauer-Emmett-Teller (BET) method. The atomic force microscopy (AFM) image was measured at a Bruker Scan-Dimension-Icon system. Thermal gravimetric analysis (TGA) was characterized on Mettler Toledo TGA/DSC 3+ thermogravimetric analyzer under argon or air atmosphere at a heating rate of 10 °C min −1 in the temperature range of 30-800 °C. ICP-MS analysis was conducted by an Agilent 7500cx instrument.

Electrochemical measurements
All the electrochemical experiments were performed in a CHI 660E potentiostat (Shanghai Chenhua Instrument Factory, China). A conventional three-electrode system was applied using the graphite rod and Ag/AgCl (3 M KCl) as the counter and reference electrode, respectively. The working electrode was prepared with the following procedures. Firstly, the well-dispersed catalyst ink was prepared by dispersing 2.0 mg of catalyst and 0.

Computational details.
Density functional theory (DFT) calculations were performed using Vienna ab initio Simulation Package (VASP) [1][2] with the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof (PBE) for the exchange correlation functional. [3] We used a two layer p (2 × 2) slab of Co(OH) 2 (001) slab and fixed the bottom layer to simulate Co-LDH. And we used a one layer p (2 × 2) slab of Co(OH) 2 (001) slab with two lower ZIF-67 ligands to simulate Co-LDH@ZIF-67. An energy cutoff of 400 eV is used for all calculations, and the k-point meshes of 3 × 2 × 1 were used for Brillouin zone integration. The atomic positions were relaxed until the force on each atom was less than 0.05 eV/Å, and the convergence tolerance of the energy was set to be 10 -5 eV.
A simple rotationally invariant DFT+U version was used to take into account the electronic correlation of Co 3d electrons. We used a value of U = 3.3 eV for Co atoms, which is the same as Deng et al. [4] The OER process is generally considered to be a four-step electron transfer mechanism: [5] OH -+ * → OH* + e -(1) O* + OH -→ OOH* + e - where * and i* represent the active site and the adsorbed intermediate on the surface, respectively.
The Gibbs free energy differences of these intermediates including zero point energy (ZPE) and entropy corrections (TΔS) can be calculated as where energy differences ΔE i is calculated as follows: The Gibbs free energy change for steps (5) (12) where U is the potential measured against normal hydrogen electrode (NHE) at standard conditions.
The catalytic performance was estimated by the free energy (G OER ) of potentialdetermining step (PDS) in the OER process: As commonly practiced, the standard chemical potential of H 2 O (l) is equivalent to the DFT total energy of H 2 O(g) together with corrections for the zero-point energy (ZPE) and entropy at 25°C and 0.035 atm. Through our tests, the ZPE difference of the same species adsorbed at Co-LDH and Co-LDH@ZIF-67 is less than 0.02 eV, so we use the following ZPE uniformly.