Mechanochemically engineered defect-rich Zn/Co-ZIF-8 solid solutions for enhanced electrochemical water splitting

Abstract

Zeolitic imidazolate frameworks (ZIFs), a subclass of porous MOFs, have attracted significant attention due to their high crystallinity and exceptional thermal and chemical stability. The synthetic protocols of Zn-ZIF-8 and Co-ZIF-8 strongly influence their thermodynamic and kinetic properties. Room-temperature (RT) aqueous synthesis yields less crystalline phases (pZn-ZIF-8-RT and pCo-ZIF-8-RT) compared to high-temperature (HT) solvothermal synthesis in DMF/DEF (pZn-ZIF-8-HT and pCo-ZIF-8-HT). Mechanical milling of equimolar RT-synthesized phases produces a highly disordered amorphous solid solution, a-Zn₅₀Co₅₀ZIF8-RT, which upon water/methanol vapor exposure at 25 °C transforms into a crystalline counterpart (c-Zn₅₀Co₅₀ZIF8-RT). Following the same protocol, milling of HT-synthesized precursors yields a more ordered amorphous phase, a-Zn₅₀Co₅₀ZIF8-HT. Comprehensive characterization using DFT, Raman spectroscopy, FE-SEM, and HR-TEM reveals that a-Zn₅₀Co₅₀ZIF8-RT possesses abundant active sites, enhanced electron density, and a less ordered structure, making it an efficient electrocatalyst. Consequently, it exhibits superior performance towards electrocatalytic water splitting with low overpotentials of 301 mV (OER) and 437 mV (HER) at 10 mA cm⁻², outperforming that of a-Zn₅₀Co₅₀ZIF8-HT, c-Zn₅₀Co₅₀ZIF8-RT and c-Zn₅₀Co₅₀ZIF8-HT.