• The research team led by Prof. Sang Uck Lee from the School of Chemical Engineering, Sungkyunkwan University, in collaboration with Prof. Hyung-Sang Kim and Prof. Hyun-Sik Im from Dongguk University, has developed a groundbreaking strategy to significantly enhance oxygen evolution reaction (OER) catalyst performance. OER is a critical step in the water-splitting process to produce hydrogen and requires the highest energy input, particularly due to its high overpotentials. State-of-the-art commercial catalysts, such as IrO₂ and RuO₂, are noble-metal-based and exhibit excellent performance but are economically impractical for large-scale hydrogen production due to their scarcity. Hence, there is an urgent need to develop alternative catalysts using abundant metal-based materials for large-scale green hydrogen production.
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• Metal-organic frameworks (MOFs) have gained attention as effective and low-cost catalysts for OER, particularly due to their structural versatility and potential to expose active metal nodes. To maximize the catalytic performance of MOF-based catalysts, it is necessary to increase the exposure of active metal node sites and optimize the structural disorder within the framework. In this regard, the research team introduced a novel strategy by doping cerium (Ce) into MOFs, selectively inducing structural disorder and enhancing the exposure of catalytic sites, thus greatly improving OER electrochemical performance.

• The research successfully demonstrated the synthesis of a crystalline/amorphous heterostructure in nickel-based MOFs via Ce doping. X-ray diffraction (XRD) and electron microscopy revealed that while flat sheets retained a crystalline structure, Ce-rich regions formed amorphous spherical domains. The optimized NiCe-0.2 MOF/NF catalyst exhibited significantly lower overpotentials (η) of 205, 290, 410 and 450 mV to drive the OER under current densities of 10, 100, 1000 and 2000 mAcm⁻², respectively with a superior kinetic of 46.09 mVdec^{− 1} and a larger turnover frequency (TOF@η = 330 mV) of 0.36 s⁻¹, outperforming both Ni-MOF/NF and commercial IrO₂/NF catalysts in high-current-density applications. Additionally, it demonstrated outstanding stability, maintaining performance for over 146 hours at a current density of 1000 mA/cm².

• Prof. Sang Uck Lee and Ph.D Course Jun Ho Seok used density functional theory (DFT) calculations to elucidate the mechanism by which Ce³⁺ doping enhances OER catalytic performance. The DFT analysis revealed that Ce³⁺ ions reduce defect formation energy, facilitating the transformation of the MOF structure from crystalline to amorphous, exposing more active metal nodes. Ce³⁺ ions also promote electron transfer and enhance reactivity with OH^⁻ ions, accelerating the OER process. The study provided valuable insights into the geometric and electronic structure synergies that contribute to improved catalytic performance, offering a guideline for future MOF-based catalyst designs.

Title: Cerium guided site-selective crystal disorder engineering of MIL-88B(Ni) frameworks for electrocatalysis offering high-performance water oxidation

Author Information: 

• Nabeen K. Shrestha (First author, Dongguk University), Supriya A. Patil (Sejong University), Jun Ho Seok (Sungkyunkwan University), Amol S. Salunke (Dongguk University), Sangeun Cho (Dongguk University), Akbar I. Inamdar (Dongguk University), Youngsin Park (Dongguk University), The corresponding authors are Prof. Sang Uck Lee (Sungkyunkwan University), Prof. Hyungsang Kim (Dongguk University), and Prof. Hyunsik Im (Dongguk University). (A total of 10 authors participated.)

Journal: Materials Today Physics(IF: 10.0)

Paper Link: DOI: 10.1016/j.mtphys.2023.101252

Figure 1. Comparative electrocatalytic OER performances of various anodes. (A) Linear sweep polarization curves measured at 1 mVs^{− 1} in 1.0 M aqueous KOH solution and corresponding (B) OER overpotential vs. current density profiles, (C) Tafel slopes and (D) Nyquist plots exhibited by the anodes. The inset image in (d) is the equivalent circuit used to fit the impedance data.

Figure 2.  Theoretical evaluation on the oxygen evolution reaction (OER) catalytic activities of Ni-MOF and NiCe-0.2 MOF. (A) Differences in crystal structure metal-oxygen bond length and distortion index between Ni-MOF and NiCe-0.2 MOF, (B) Ce3+ ion induced charge transfer in NiCe-0.2 MOF, and (C) Proposed adsorbate evolution mechanism (AEM) pathway for the NiCe-0.2 MOF catalysts. Free energy diagrams (FEDs) of OER according to the active sites of (D) Ni-MOF and (E) NiCe-0.2 MOF at U = 0 V, U = 0.402 V, and U = 0.402 V+ η^OER in alkaline media. The pink shading indicates the potential determining step (PDS).