Nickel-Based Metal-Organic Frameworks: Synthesis, Properties, And Advanced Applications In Energy Storage And Catalysis

Molecular Composition And Structural Characteristics Of Nickel-Based Metal-Organic Frameworks

Nickel-based metal-organic frameworks are hybrid materials wherein Ni²⁺ or Ni³⁺ cations serve as nodes or secondary building units (SBUs), bridged by polytopic organic linkers such as carboxylates, phosphonates, or nitrogen-donor ligands 1,2,3. The coordination geometry around nickel typically adopts octahedral or square-planar configurations, dictated by the ligand denticity and synthesis conditions 4. For instance, Ni-NDC (naphthalene-2,6-dicarboxylic acid) frameworks synthesized hydrothermally on nickel foam substrates exhibit layered structures with interlayer distances ranging from 0.3 to 0.5 nm, facilitating ion diffusion in electrochemical applications 3. The choice of organic linker profoundly influences framework topology: terephthalate-based ligands yield pillared-layer motifs, whereas tripodal carboxylates such as trimesic acid generate cubic or hexagonal networks with pore apertures of 0.8–2.0 nm 2,4.

Key structural features include:

• High Specific Surface Area: Ni-MOFs commonly exhibit BET surface areas between 500 and 1800 m²/g, with micropore volumes of 0.2–0.6 cm³/g, enabling efficient guest molecule adsorption 2,10.
• Open Metal Sites: Partial desolvation or ligand removal exposes coordinatively unsaturated nickel centers, enhancing Lewis acidity and catalytic activity for oxygen evolution reactions (OER) and CO₂ reduction 3,4.
• Redox-Active Nodes: Nickel's ability to cycle between +2 and +3 oxidation states underpins pseudocapacitive charge storage, with theoretical capacitances exceeding 1000 F/g in aqueous electrolytes 1,13.
• Tunable Hydrophobicity: Incorporation of hydrophobic functional groups (e.g., alkyl chains, fluorinated moieties) onto organic linkers improves moisture stability, a critical requirement for atmospheric water harvesting and gas separation 9,14.

The crystallographic phase and particle morphology of Ni-MOFs are controlled via synthesis parameters such as metal-to-ligand molar ratio, solvent polarity, temperature (typically 80–180°C), and reaction time (6–72 hours) 2,7. Buffer-free solvothermal routes using nickel acetate and aromatic dicarboxylates in dimethylformamide (DMF) or water yield phase-pure products, whereas modulator-assisted synthesis with acetic acid or formic acid directs crystallite size and aspect ratio 7.

Synthesis Routes And Precursor Chemistry For Nickel-Based Metal-Organic Frameworks

Hydrothermal And Solvothermal Methods

The predominant synthesis strategy involves reacting nickel salts (Ni(NO₃)₂·6H₂O, Ni(OAc)₂·4H₂O, or NiCl₂) with organic linkers in polar solvents under autogenous pressure 2,3. A representative protocol for Ni-NDC/nickel foam composites comprises:

1. Precursor Preparation: Dissolve 0.5 mmol Ni(NO₃)₂·6H₂O and 0.5 mmol naphthalene-2,6-dicarboxylic acid in 30 mL deionized water with 2 mL ethanol as co-solvent 3.
2. Substrate Pretreatment: Clean nickel foam (1 cm × 2 cm) sequentially in 3 M HCl, ethanol, and water to remove surface oxides and enhance nucleation sites 3.
3. Hydrothermal Growth: Transfer the solution and substrate to a Teflon-lined autoclave, heat at 120°C for 12 hours, then cool naturally to room temperature 3.
4. Post-Synthesis Washing: Rinse the coated foam with DMF and ethanol, then activate under vacuum at 150°C for 6 hours to remove guest molecules 3.

This in-situ growth approach yields conformal Ni-MOF films (thickness 5–20 μm) with strong adhesion to conductive substrates, eliminating the need for polymeric binders in electrode fabrication 2,3.

Vapor-Phase Ligand Appending

An emerging technique involves post-synthetic modification of pre-formed Ni-MOFs via vapor-phase ligand exchange 6. For example, amine-functionalized ligands (e.g., 2-aminomethylpiperidine) are evaporated at 80°C and diffused into the framework pores, binding to open nickel sites without disrupting the parent structure 6. This method enables:

• Sequential Functionalization: Iterative cycles of ligand appending and detachment allow recycling of the MOF scaffold for different applications 6.
• Enhanced Selectivity: Amine-decorated Ni-MOFs exhibit 3–5× higher CO₂/N₂ selectivity compared to pristine frameworks, with uptake capacities of 4.2 mmol/g at 298 K and 1 bar 6.

Metal And Anion Exchange

Starting from a parent framework such as CFA-1 (a zinc-based MOF), partial substitution of Zn²⁺ with Ni²⁺ via soaking in nickel acetate solution yields mixed-metal Ni₃.₅Zn₀.₅(OAc)₃.₈Cl₀.₂(bibta)₃, where bibta = bis(1H-1,2,3-triazol-1-yl)acetic acid 14. This exchange modulates water sorption isotherms: the nickel-rich variant achieves 0.78 g H₂O/g MOF uptake across 27–70% relative humidity, with negligible hysteresis over 450 adsorption–desorption cycles 14. The synthesis involves:

1. Suspending 1 g CFA-1 in 50 mL 0.1 M Ni(OAc)₂ solution at 60°C for 48 hours 14.
2. Filtering, washing with methanol, and drying at 100°C under vacuum 14.
3. Characterizing metal content via inductively coupled plasma optical emission spectroscopy (ICP-OES), confirming 70 mol% nickel incorporation 14.

Physicochemical Properties And Performance Metrics Of Nickel-Based Metal-Organic Frameworks

Electrochemical Characteristics

Ni-MOFs demonstrate exceptional pseudocapacitive behavior due to reversible Faradaic reactions at the nickel centers (Ni²⁺ ↔ Ni³⁺ + e⁻) 1,13. A two-dimensional Ni-MOF/reduced graphene oxide (rGO) composite, synthesized by in-situ growth on rGO nanosheets, delivers:

• Specific Capacitance: 972 F/g at 5 A/g current density in 6 M KOH electrolyte, surpassing pure Ni-MOF powder (580 F/g) by 68% 13.
• Rate Capability: Retention of 78% capacitance at 20 A/g, attributed to rGO's conductive network reducing charge-transfer resistance (Rct = 0.8 Ω) 13.
• Cycling Stability: 91% capacitance retention after 10,000 galvanostatic charge–discharge cycles, with Coulombic efficiency >98% 13.

For energy storage applications, Ni-MOF-derived nickel sulfide (Ni₃S₂) encapsulated in carbon shells exhibits a discharge capacity of 680 mAh/g at 0.5 C in lithium-ion batteries, with <15% capacity fade over 500 cycles 1. The carbon shell (thickness 3–5 nm) mitigates volume expansion during lithiation and prevents electrolyte decomposition 1.

Catalytic Activity In Oxygen Evolution Reaction

Nickel-based MOFs serve as precursors or direct catalysts for OER in alkaline water electrolysis 3,4. A Ni-NDC/nickel foam electrode achieves:

• Overpotential: 290 mV at 10 mA/cm² current density in 1 M KOH, comparable to benchmark RuO₂ (270 mV) 3.
• Tafel Slope: 58 mV/dec, indicating favorable reaction kinetics via a four-electron pathway 3.
• Durability: Stable operation for 100 hours at constant current, with <5% increase in overpotential 3.

Mixed-metal Ni-Co-Cu MOFs exhibit synergistic effects, where cobalt enhances conductivity and copper provides additional active sites, yielding overpotentials as low as 250 mV without conductive additives 4. The electrocatalytic mechanism involves:

1. Adsorption of OH⁻ on nickel sites to form Ni–OH intermediates 4.
2. Oxidation to Ni–OOH species via proton-coupled electron transfer 4.
3. O–O bond formation and O₂ release, regenerating the active site 4.

Gas Adsorption And Separation

Ni-MOFs with imidazolate or pyrazolate linkers demonstrate selective adsorption of small molecules 5,12,15. For example, a nickel-pyrazolate framework (Ni-pz-MOF) exhibits:

• CO₂ Uptake: 3.8 mmol/g at 273 K and 1 bar, with isosteric heat of adsorption (Qst) = 28 kJ/mol, indicative of physisorption 12,15.
• CH₄/CO₂ Selectivity: 12:1 at 298 K, enabling natural gas purification 12.
• Moisture Stability: Retention of 95% crystallinity after exposure to 80% relative humidity for 30 days, attributed to hydrophobic methyl substituents on pyrazole rings 12,15.

Alkene capture via electrochemically controlled oxidation of Ni-MOFs (e.g., Ni₃HHTP₂, where HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) allows reversible ethylene binding with uptake capacities of 2.1 mmol/g at +0.6 V vs. Ag/AgCl 8. Reduction at −0.4 V releases ethylene with >90% recovery efficiency, enabling cyclic operation for olefin/paraffin separation 8.

Applications Of Nickel-Based Metal-Organic Frameworks In Energy Storage Devices

Supercapacitors And Hybrid Capacitors

Ni-MOFs bridge the gap between electric double-layer capacitors (EDLCs) and batteries by combining high power density with moderate energy density 1,13. A symmetric supercapacitor assembled with Ni-MOF/rGO electrodes delivers:

• Energy Density: 42 Wh/kg at 800 W/kg power density in aqueous electrolyte 13.
• Voltage Window: 0–1.6 V, limited by water electrolysis 13.
• Self-Discharge: <10% voltage drop over 24 hours, indicating low leakage current 13.

Asymmetric configurations pairing Ni-MOF anodes with activated carbon cathodes extend the voltage window to 1.8 V in organic electrolytes (1 M LiPF₆ in ethylene carbonate/dimethyl carbonate), achieving 68 Wh/kg energy density 13.

Lithium-Ion And Lithium-Air Batteries

Sub-nanometric platinum particles (diameter 0.8–1.2 nm) embedded in multi-shell hollow Ni-MOFs function as bifunctional catalysts for lithium-air batteries 16. The composite cathode exhibits:

• Discharge Capacity: 12,500 mAh/g at 0.1 mA/cm² current density, corresponding to 85% utilization of theoretical capacity 16.
• Overpotential: Charge–discharge voltage gap of 0.9 V at 500 mAh/g capacity, reduced from 1.5 V for carbon-only cathodes 16.
• Cycle Life: 150 cycles at 1000 mAh/g limited capacity, with terminal voltage maintained above 2.5 V 16.

The hollow MOF architecture (shell thickness 15 nm, void diameter 80 nm) facilitates oxygen diffusion and accommodates Li₂O₂ discharge products, while platinum nanoparticles catalyze both oxygen reduction and evolution reactions 16.

Applications Of Nickel-Based Metal-Organic Frameworks In Catalysis And Environmental Remediation

Photocatalytic Degradation Of Volatile Organic Compounds

Ni-MOF thin films grown on nickel foam substrates serve as photocatalysts for VOC abatement under UV-visible irradiation (λ > 365 nm) 2. A Ni-MOF/NF composite achieves:

• Toluene Degradation: 92% conversion after 4 hours under 300 W Xe lamp illumination, with mineralization efficiency (CO₂ yield) of 78% 2.
• Quantum Efficiency: 4.2% at 400 nm, attributed to ligand-to-metal charge transfer (LMCT) transitions 2.
• Stability: No detectable leaching of nickel ions (<0.1 ppm in solution) and retention of 88% activity after 10 cycles 2.

The 3D crosslinked structure of nickel foam enhances light penetration and reduces mass-transfer limitations compared to powder catalysts, while the MOF's high surface area (1200 m²/g) promotes VOC adsorption 2. Mechanistic studies via electron paramagnetic resonance (EPR) reveal that superoxide radicals (O₂•⁻) and hydroxyl radicals (•OH) are the primary reactive oxygen species responsible for oxidative degradation 2.

Electrochemical Alkene Capture And Release

Conductive Ni-MOFs (e.g., Ni₃HHTP₂) enable voltage-controlled alkene separation from mixed gas streams 8. Upon oxidation at +0.6 V, nickel centers bind ethylene via π-backbonding, achieving:

• Ethylene Uptake: 2.1 mmol/g from a 10% C₂H₄/N₂ mixture at 298 K and 1 bar 8.
• Selectivity: >50:1 for ethylene over ethane, enabling olefin purification from cracking effluents 8.
• Release Kinetics: 95% desorption within 30 minutes at −0.4 V, with energy consumption of 0.8 kWh/kg ethylene 8.

This electrochemical swing adsorption (ESA) process offers advantages over thermal or pressure swing methods, including lower regeneration energy and ambient-temperature operation 8.

Agrochemical Applications: Nitrification Inhibition

Nickel-pyrazolate MOFs encapsulating nitrification inhibitors (e.g., 3,4-dimethylpyrazole phosphate) provide controlled release in soil, reducing nitrate leaching and N₂O emissions 12,15. Field trials demonstrate:

• Inhibitor Release Profile: Zero-order kinetics over 60 days, maintaining soil concentrations above 5 ppm (the threshold for >80% nitrification suppression) 12,15.
• Crop Yield: 12% increase in wheat grain yield compared to conventional urea fertilization, attributed to prolonged nitrogen availability 15.
• Environmental Impact: