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Proton-Conductive Cerium-Based Metal−Organic Frameworks
Summary:
The authors from National Cheng Kung University (Taiwan) and National Taiwan University (Taiwan) developed Ce-MOF-808, Zr-MOF-808, and bimetallic Ce/Zr-MOF-808 with high proton conductivity, good stability, and tunable Ce/Zr ratios, achieving excellent proton conduction performance in the application of proton conductor materials for energy-related fields.
Background:
1. To address the limitations of zirconium-based MOFs (Zr-MOFs) in proton conduction (e.g., high activation energyEa) and the lack of research on cerium-based MOFs (Ce-MOFs) for proton conduction, previous researchers modified Zr-MOFs (e.g., MOF-808) by incorporating imidazole or sulfamate to enhance proton conductivity, yet these modifications are complex and Ce-MOFs’ proton conduction potential remains unexploited.
2. The authors proposed an innovative method of synthesizing Ce-MOF-808, Zr-MOF-808, and bimetallic Ce/Zr-MOF-808 with varying Ce/Zr ratios via solvothermal reaction, obtaining materials with Ce-induced enhanced proton conductivity (σ) and reduced Ea, and revealed the mechanism through density functional theory (DFT) calculations.
Research Content:
-1. Synthesis
-Ce-MOF-808: 1.461 g ammonium cerium(IV) nitrate dissolved in 5 mL water (sonication); 67.2 mgH3BTC, 1.6 mL DMF, 1.2 mL Ce(IV) solution, and 4.12 mL formic acid added sequentially to a 20 mL vial; sealed, sonicated for 5 min, heated at 100 °C for 20 min; centrifuged (washed with DMF, acetone), vacuum-dried at 80 °C (nanocrystal size ~65 nm).
-Zr-MOF-808: 160 mg zirconium(IV) oxychloride octahydrate, 110 mgH3BTC, 20 mL formic acid, 20 mL DMF mixed; heated at 120 °C for 48 h; washed with DMF (plus HCl activation), acetone, vacuum-dried at 80 °C.
-Bimetallic Ce/Zr-MOF-808: 0.533 M Ce(IV) and Zr(IV) solutions mixed at ratios (Zr5Ce1: 1.0 mL Zr(IV)+0.2 mL Ce(IV); Zr3Ce3: 0.6 mL each; Zr1Ce5: 0.2 mL Zr(IV)+1.0 mL Ce(IV)); added to \(H_3BTC/DMF\) mixture, plus 4.12 mL formic acid; heated at 100 °C for 20 min; washed, dried as above.
-2. Characterizations
1)BET and pore size distribution:
-Zr-MOF-808: BET = 2120 m²/g, pore size 1.3–1.8 nm;
-Ce-MOF-808: BET = 1440 m²/g (pellet: 1115 m²/g), pore size 1.3–1.8 nm;
-Bimetallic MOFs: Zr5Ce1 (1600 m²/g), Zr3Ce3 (1380 m²/g), Zr1Ce5 (1500 m²/g), all pore size 1.3–1.8 nm.
2)SEM/TEM tests: Ce-MOF-808 is ~65 nm nanocrystals; Zr-MOF-808 has larger crystals; bimetallic MOFs show particle sizes between the two.
3)Other tests:
-XRD: All MOFs (and 23 MPa pellets) retain crystallinity;
-Electrical conductivity (\(\sigma_e\)): All MOFs are insulators (\(\sigma_e < 10^{-11}\) S/cm);
-TGA: Water uptake (99% RH): Zr-MOF-808 (61.8 wt%), Ce-MOF-808 (141.7 wt%).
-3. Application
Tested as proton conductors (99% RH, 25–60 °C) via EIS:
- Ce-MOF-808: 25 °C σ = 4.4×10⁻³ S/cm, \(E_a = 0.14\) eV; 60 °C σ = 6.7×10⁻³ S/cm;
- Zr-MOF-808: 25 °C σ = 8.9×10⁻⁵ S/cm, \(E_a = 0.33\) eV;
- Bimetallic MOFs: σ increases with Ce content (Zr5Ce1: 9.3×10⁻⁴ S/cm; Zr1Ce5: 4.0×10⁻³ S/cm at 25 °C);
- Long-term stability: Ce-MOF-808 retains σ over 28 days (99% RH); reversible σ when switching RH (58%→99%).
-4. Mechanism
-Proton conduction mechanism: Follows Grotthuss (proton-hopping) mechanism (σ in D2Ovapor ~1.4× lower than inH2Ovapor, matching theoretical \(\sqrt{2}\) ratio).
-DFT calculation: Ce reduces proton affinities (PAs) of MOF nodes: Zr-MOF-808 terminal -OH PA = 350.9 kcal/mol; Ce-MOF-808 = 320.8 kcal/mol, promoting proton release and hopping.
Outlook:
This research is the first to report Ce-MOF proton conduction, proving Ce-MOFs outperform Zr-MOFs as proton conductors. It provides a simple strategy to tune proton conductivity via Ce/Zr ratio, laying a foundation for Ce-MOF application in fuel cell proton membranes.
Proton-Conductive Cerium-Based Metal−Organic Frameworks
Authors: Wei Huan Ho, Shih-Cheng Li, Yi-Ching Wang, Tzu-En Chang, Yi-Ting Chiang, Yi-Pei Li, Chung-Wei Kung
DOI: 10.1021/acsami.1c17396
Link: https://pubs.acs.org/doi/10.1021/acsami.1c17396
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