On March 13, our groundbreaking research on molecularly engineering metal-organic frameworks for highly efficient electrochemical catalysis in water oxidation was published in the prestigious international materials science journal, Advanced Materials, with an impact factor of 32.086. This significant contribution, co-authored by Yizhe Liu, Xintong Li, and Shoufeng Zhang as co-first authors, represents a pioneering leap in the field.

A novel MOF material featuring a precisely structured thiol-nickel coordination chain has been designed and synthesized, exhibiting promising potential. The breakthrough cRED (Micro ED) dataset has successfully elucidated the intricate crystal structure of Ni(DMBD)-MOF. This innovative approach in organic linkage engineering not only establishes a robust foundation for screening water-splitting electrocatalysts within the MOF structure but also paves the way for an expansive exploration of this molecular engineering strategy. It holds immense promise in unlocking diverse advanced MOF catalysts, further amplifying their catalytic capabilities.

ReadCrystal Technology Co., Ltd. actively contributed to this research by leveraging Micro ED technology to analyze the microcrystalline structure inherent in MOF materials.

Abstract:

Metal-organic framework (MOF) solids are pivotal in advancing energy conversion technologies, yet the quest for electroactive and durable MOFs for electrocatalysis encounters persistent hurdles. Our innovation introduces a meticulously designed MOF system, leveraging molecular engineering to create a two-dimensional coordination network with thiol-metal bonds, exemplified by the Ni(DMBD)-MOF (Nickel Dithiomethylbenzene-dicarboxylate MOF). Employing continuous rotation electron diffraction (cRED) techniques, we successfully resolved the crystal structure from microcrystals. The computational analysis underscores the metallic electronic structure of Ni(DMBD)-MOF attributable to the Ni-S coordination, underscoring the efficacy of thiol ligands in enhancing conductivity. Notably, empirical and theoretical investigations demonstrate the superiority of (DMBD)-MOF over its non-thiol counterpart (e.g., 1,4-phthalic acid, BDC-MOF) in the electrocatalytic oxygen evolution reaction (OER). The energy barrier for the rate-limiting *O intermediate formation is significantly reduced in (DMBD)-MOF. Furthermore, the incorporation of iron into the NiFe (DMBD)-MOF yields remarkable outcomes, achieving a current density of 100 mA·cm at a mere overpotential of 280 mV. This breakthrough signifies the emergence of a novel MOF platform conducive to highly efficient OER catalysis.

Content quick overview

The DMBD molecule's thiol group, alongside the neighboring carboxyl group, creates a potent chelating framework for metal cations. Employing Continuous Rotating Electron Diffraction (cRED), also known as Micro ED, enabled the determination of the crystal structure of Ni(DMBD)-MOF. This MicroED analysis, vital for understanding the structure, received support from ReadCrystal Technology Co., Ltd. Despite the challenge posed by the minuscule size of the MOF crystals, measuring only about 200nm, ReadCrystal Technology managed to gather over 30 data sets. Out of these, 7 sets displayed superior quality and were amalgamated for comprehensive processing, culminating in a 4-day endeavor to complete the structural analysis.

Transmission electron microscopy (TEM) studies provided additional confirmation of the structure elucidated by cRED.

Exploratory analyses were conducted to examine both the coordination environment and electronic band structure. The DFT-optimized structures of Ni(BDC)-MOF and Ni(DMBD)-MOF revealed the presence of two octahedral-bonded Ni sites (depicted in Fig. 3c and 3d, respectively). Analysis of the density of states (DOS) exhibited distinct characteristics in Figures 3e and 3f. In particular, the non-zero DOS at the Fermi level within the Ni(DMBD)-MOF profile indicated a metallic electronic band structure (as shown in Figure 3f). Conversely, the Ni(BDC)-MOF phase displayed a comparatively narrow band, revealing a small gap of approximately 0.44 eV (illustrated in Fig. 3e).

The catalyst was synthesized on a nickel foam substrate, strategically chosen to enhance mass transfer efficiency during (electro)catalysis. To investigate the comparative electrocatalytic performance for Oxygen Evolution Reaction (OER), both Ni(DMBD)-MOF/NF and Ni(BDC)-MOF/NF were examined using a three-electrode setup in an O2-saturated, purified 1 M KOH electrolyte.

Conclusion:

The design and synthesis of well-defined MOF materials featuring thiol-nickel coordination chains enabled the successful resolution of the novel crystal structure of Ni(DMBD)-MOF, leveraging the cRED dataset. This structure was further affirmed through HRTEM studies. Our computational analysis revealed that Ni(DMBD)-MOF exhibits a metallic electronic structure, attributing its heightened conductivity to the presence of the Ni-S coordination chain, characterized by less electronegative sulfur donors. Notably, the electronic configuration of Ni(DMBD)-MOF correlated with its impressive electrocatalytic activity, as verified by both experimental observations and DFT calculations. Specifically, our findings demonstrated facile formation of Oads intermediates within Ni(DMBD)-MOFs, coupled with reduced activation energy barriers. Introducing Fe into the equation resulted in the formation of a 2-dimensional NiFe(DMBD)-MOF/NF composite, showcasing an overpotential of 280 mV at a current density of 100 mA·cm-2. Moreover, employing thiol-functionalized NiFe(DMBD)-MOF/NF as the anode electrode in an electrolytic cell for water splitting yielded promising outcomes, registering an applied 1.50 V@10 mA·cm-2 in an alkaline electrolyte, with Pt/C serving as the cathode. This strategic approach of organic linker engineering not only establishes a robust MOF structural platform for the screening of water-splitting electrocatalysts but also underscores the potential of this molecular engineering strategy in fostering a diverse range of advanced MOF catalysts.