Cerium-Based Metal–Organic Framework Nanocrystals Interconnected by Carbon Nanotubes for Boosting Electrochemical Capacitor Performance

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Summary:
The authors fromDepartment of Chemical Engineering, National Cheng Kung University, Tainan City, Taiwan developedCe-MOF-808/CNT nanocomposites with tunable porosity, high electrical conductivity, and good water stability, achieving excellent capacitive performance in the application ofaqueous-based supercapacitors.

Background:
1. To address the problem oflow electrical conductivity of most metal-organic frameworks (MOFs) which limits their electrochemical applications, previous researchers either used MOFs as templates to synthesize MOF-derived materials (e.g., carbon, inorganic compounds) or designed MOF-based nanocomposites with nanocarbons. However, MOF-derived materials lose the intrinsic porous structure of MOFs, and existing MOF-carbon nanocomposites often require postsynthetic modification to introduce redox-active sites, leading to complex processes.
2. The authors in this study proposed aninnovative one-step solvothermal synthesis method to directly grow redox-active Ce-MOF-808 nanocrystals on carboxylic acid-functionalized carbon nanotubes (CNTs), avoiding postsynthetic modification. The resulting Ce-MOF-808/CNT nanocomposites combine the high porosity of Ce-MOF-808 and good conductivity of CNTs, showing enhanced supercapacitor performance.

Research Content:
1. Synthesis
The authors synthesized Ce-MOF-808 nanocrystals via a modified solvothermal method (ammonium cerium(IV) nitrate, trimesic acid (H₃BTC), formic acid, dimethylformamide (DMF) as precursors, 100 °C for 20 min). For Ce-MOF-808/CNT nanocomposites, 10, 20, or 30 mg of carboxylic acid-functionalized CNTs were added during Ce-MOF-808 synthesis, yielding Ce-MOF-808-10CNT, Ce-MOF-808-20CNT, and Ce-MOF-808-30CNT. A physical mixture (Ce-MOF-808 (34 wt%)/CNT) was also prepared for comparison.
2. Characterizations
1.BET and pore size distribution: Ce-MOF-808 had a BET surface area of 1420 m²/g and microporous structure (major pore size ~1.63 nm). With increasing CNT content, the BET surface area of nanocomposites decreased (960 m²/g for 10CNT, 580 m²/g for 20CNT, 320 m²/g for 30CNT), and microporosity reduced. CNT alone had a BET surface area of 280 m²/g.
2.SEM/TEM tests: Ce-MOF-808 had nanocrystals with an average particle size of 65 ± 19 nm; Scherrer equation calculated an average grain size of 26 nm. SEM/TEM images showed Ce-MOF-808 nanocrystals uniformly grown on CNT surfaces in nanocomposites, with smaller Ce-MOF-808 crystal sizes than the pristine MOF.
3.Other tests:
   -Electrical conductivity: Ce-MOF-808 had low conductivity (2.9×10⁻¹² S/cm); adding CNTs increased conductivity (3.6×10⁻⁴ S/cm for 10CNT).
   -XRD: Ce-MOF-808 and nanocomposites showed characteristic diffraction peaks (4.3°, 8.1°, 8.5°), confirming crystallinity; Ce-MOF-808 thin films retained crystallinity after 40-cycle CV in 0.1 M Na₂SO₄.
   -¹H NMR: Ce-MOF-808’s molecular formula was determined as Ce₆(μ₃-O)₄(μ₃-OH)₄(BTC)₂(HCOO)₃.₃₃(OH)₂.₆₇(H₂O)₂.₆₇, indicating unsaturated hexa-cerium nodes.
   -FTIR: Nanocomposites showed Ce-MOF-808’s characteristic peaks (1647, 1610, 1555, 1435, 1380 cm⁻¹), and CNT’s carboxylic peak disappeared, confirming MOF growth on CNTs.
3. Application
The materials were tested as active materials for aqueous supercapacitors (0.1 M Na₂SO₄ electrolyte):
- Ce-MOF-808-20CNT showed the highest area capacitance: 22.4 mF/cm² at 0.25 mA/cm² (vs. 9.9 mF/cm² for pristine CNT).
- Ce-MOF-808-20CNT retained 71% of initial capacitance after 2000 charge-discharge cycles (1 mA/cm²).
- Pristine Ce-MOF-808 had low capacitance; the physical mixture (34 wt% MOF/CNT) showed worse performance than Ce-MOF-808-20CNT.
4. Mechanism
-Redox hopping in Ce-MOF-808: CV curves of Ce-MOF-808 showed reversible redox peaks (0.70 V anodic, 0.42 V cathodic) from Ce(IV)/Ce(III) redox on unsaturated hexa-cerium nodes. Log(Jₚₐ) vs. log(v) and log(-Jₚc) vs. log(v) had slopes ~0.77, indicating redox-hopping charge transport (effective only near the electrode surface).
-Performance enhancement mechanism: CNTs in nanocomposites provided conductive pathways between Ce-MOF-808 nanocrystals, resolving the slow charge transport of pristine Ce-MOF-808. Ce-MOF-808’s porous structure offered high-density redox-active sites, synergistically boosting supercapacitor performance.

Outlook:
This study successfully synthesized Ce-MOF-808/CNT nanocomposites via one-step solvothermal synthesis, realizing the combination of MOF’s porosity and CNT’s conductivity without postsynthetic modification. The nanocomposites exhibit excellent aqueous supercapacitor performance, providing a new strategy for designing redox-active, water-stable MOF-based nanocomposites. This work also lays a foundation for Ce-MOF-based materials in other electrochemical applications (e.g., electrocatalysis) due to the unsaturated cerium nodes’ potential for further functionalization.

Cerium-Based Metal–Organic Framework Nanocrystals Interconnected by Carbon Nanotubes for Boosting Electrochemical Capacitor Performance
Authors: Cheng-Hui Shen, Cheng-Hsun Chuang, Yu-Juan Gu, Wei Huan Ho, Yi-Da Song, Yu-Chuan Chen, Yi-Ching Wang, Chung-Wei Kung
DOI: 10.1021/acsami.1c02038
Link: https://pubs.acs.org/doi/10.1021/acsami.1c02038

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