Phosphorus-Functionalized Organic Linkers Promote Polysulfide Retention in MOF-Based Li–S Batteries

Metal–organic frameworks (MOFs) have been an area of intense research for their high porosity and synthetic tunability, which afford them controllable physical and chemical properties for various applications. In this study, we demonstrate that functionalized MOFs can be used to mitigate the so-called polysulfide shuttle effect in lithium–sulfur batteries, a promising next-generation energy storage device. UiO-66-OH, a zirconium-based MOF with 2-hydroxy­terephthalic acid, was functionalized with a phosphorus chloride species that was subsequently used to tether polysulfides. In addition, a molecular chlorophosphorane was synthesized as a model system to elucidate the chemical reactivity of the phosphorus moiety. The functionalized MOFs were then used as a cathode additive in coin cell batteries to inhibit the dissolution of polysulfides in solution. Through this work, we show that the functionalization of MOF with phosphorus enhances polysulfide redox and thereby capacity retention in Li–S batteries. While demonstrated here for polysulfide tethering in batteries, we envision this linker functionalization strategy could be more broadly utilized in separations, sensing, or catalysis applications

Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel <i>N</i>‑Heterocyclic Carbene–Isoquinoline Complex

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO₂ fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO₂ reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by <i>N</i>-heterocyclic carbene–amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO₂ to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni­(^Prbimiq1)]²⁺ (<b>1c</b>, where ^Prbimiq1 = bis­(3-(imidazolyl)­isoquinolinyl)­propane) emerges as a catalyst for electrochemical reduction of CO₂ with the lowest cathodic onset potential (<i>E</i>_cat = −1.2 V vs SCE). Using this earth-abundant catalyst with Ir­(ppy)₃ (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO₂ to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s^–1, respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO₂ to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion

Flexible 2D Boron Imidazolate Framework for Polysulfide Adsorption in Lithium–Sulfur Batteries

The “polysulfide shuttle,” a process initiated by the dissolution of polysulfides, is recognized to be one of the major failure mechanisms of lithium–sulfur (Li–S) batteries. Much research effort has been dedicated toward efficient cathode additives and host materials to suppress the leaching of polysulfide species. Herein, we report a new 2D metal–organic framework constituted by a tritopic ligand, boron imidazolate ([BH(Im)3]−, Im = imidazole), and Co2+ ions for lithium polysulfide adsorption. The cobalt imidazolate framework (CoN6-BIF) contains octahedrally coordinated Co centers that form two-dimensional layers in the a,b plane. Composite cathodes containing CoN6-BIF exhibited high sulfur utilization and capacity retention, resulting in improved specific capacity and cycle life compared to sulfur/carbon controls. Density functional theory (DFT) calculations suggest that CoN6-BIF linkers are rotationally flexible, allowing the framework to accommodate polysulfide in the expanded pores. This unusual property of BIFs opens up new avenues for exploring flexible metal–organic frameworks (MOFs) and their applications to energy storage

Flexible 2D Boron Imidazolate Framework for Polysulfide Adsorption in Lithium–Sulfur Batteries

The “polysulfide shuttle,” a process initiated by the dissolution of polysulfides, is recognized to be one of the major failure mechanisms of lithium–sulfur (Li–S) batteries. Much research effort has been dedicated toward efficient cathode additives and host materials to suppress the leaching of polysulfide species. Herein, we report a new 2D metal–organic framework constituted by a tritopic ligand, boron imidazolate ([BH(Im)3]−, Im = imidazole), and Co2+ ions for lithium polysulfide adsorption. The cobalt imidazolate framework (CoN6-BIF) contains octahedrally coordinated Co centers that form two-dimensional layers in the a,b plane. Composite cathodes containing CoN6-BIF exhibited high sulfur utilization and capacity retention, resulting in improved specific capacity and cycle life compared to sulfur/carbon controls. Density functional theory (DFT) calculations suggest that CoN6-BIF linkers are rotationally flexible, allowing the framework to accommodate polysulfide in the expanded pores. This unusual property of BIFs opens up new avenues for exploring flexible metal–organic frameworks (MOFs) and their applications to energy storage

Flexible 2D Boron Imidazolate Framework for Polysulfide Adsorption in Lithium–Sulfur Batteries

The “polysulfide shuttle,” a process initiated by the dissolution of polysulfides, is recognized to be one of the major failure mechanisms of lithium–sulfur (Li–S) batteries. Much research effort has been dedicated toward efficient cathode additives and host materials to suppress the leaching of polysulfide species. Herein, we report a new 2D metal–organic framework constituted by a tritopic ligand, boron imidazolate ([BH(Im)3]−, Im = imidazole), and Co2+ ions for lithium polysulfide adsorption. The cobalt imidazolate framework (CoN6-BIF) contains octahedrally coordinated Co centers that form two-dimensional layers in the a,b plane. Composite cathodes containing CoN6-BIF exhibited high sulfur utilization and capacity retention, resulting in improved specific capacity and cycle life compared to sulfur/carbon controls. Density functional theory (DFT) calculations suggest that CoN6-BIF linkers are rotationally flexible, allowing the framework to accommodate polysulfide in the expanded pores. This unusual property of BIFs opens up new avenues for exploring flexible metal–organic frameworks (MOFs) and their applications to energy storage

Computational and Experimental Study of the Mechanism of Hydrogen Generation from Water by a Molecular Molybdenum-Oxo Electrocatalyst

We investigate the mechanism for the electrocatalytic generation of hydrogen from water by the molecular molybdenum-oxo complex, [(PY5Me₂)­MoO]²⁺ (PY5Me₂ = 2,6-bis­(1,1-bis­(2-pyridyl)­ethyl)­pyridine). Computational and experimental evidence suggests that the electrocatalysis consists of three distinct electrochemical reductions, which precede the onset of catalysis. Cyclic voltammetry studies indicate that the first two reductions are accompanied by protonations to afford the Mo-aqua complex, [(PY5Me₂)­Mo­(OH₂)]⁺. Calculations support hydrogen evolution from this complex upon the third reduction, via the oxidative addition of a proton from the bound water to the metal center and finally an α-H abstraction to release hydrogen. Calculations further suggest that introducing electron-withdrawing substituents such as fluorides in the <i>para</i> positions of the pyridine rings can reduce the potential associated with the reductive steps, without substantially affecting the kinetics. After the third reduction, there are kinetic bottlenecks to the formation of the Mo-hydride and subsequent hydrogen release. Computational evidence also suggests an alternative to direct α-H abstraction as a mechanism for H₂ release which exhibits a lower barrier. The new mechanism is one in which a water acts as an intramolecular proton relay between the protons of the hydroxide and the hydride ligands. The calculated kinetics are in reasonable agreement with experimental measurements. Additionally, we propose a mechanism for the stoichiometric reaction of [(PY5Me₂)­Mo­(CF₃SO₃)]⁺ with water to yield hydrogen and [(PY₅Me₂)­MoO]²⁺ along with the implications for the viability of an alternate catalytic cycle involving just two reductions to generate the active catalyst