Creating free standing covalent organic framework membranes by nanocrystal suturing in sol gel solutions

Abstract

The sol-gel synthesis represents a versatile platform to fabricate ceramic inorganic membranes. However, it is still a grand challenge to push the boundary of sol-gel chemistry towards high-quality organic membrane construction. Herein, a facile and controlled nanocrystal suturing strategy in sol-gel solutions is developed to afford highly crystalline and free-standing covalent organic framework membranes. The key chemistry design lies in deploying tiny threads (1 mol% dual-NH₂-tail linear polymer) to efficiently suture the highly charged covalent organic framework nanocrystals stabilized and confined in sol-gel solutions, creating a continuous and intact membrane surface. A subsequent treatment heals the sutured covalent organic framework nanocrystals, yielding a free-standing membrane with high crystallinity and ordered pores. The structure evolution and role of the thread linker are elucidated via operando spectroscopy and microscopy. The as-afforded covalent organic framework membranes demonstrate attractive proton transport performance in high temperature and anhydrous fuel cell applications.

Fig. 1. Schematic of free-standing COF membrane fabrication in a sol gel solution.

Illustration of constructing a thickness-tunable, flexible, and processable free-standing COF membrane via nanocrystal suturing with tiny threads, as demonstrated in this study.

Fig. 2. Preparation of the TpPa-COF-X% membrane.

a Synthetic scheme of TpPa-COF-X% membrane. b Photographs showing the transformation of the COF membrane precursor solution during the heating process. c Intensity-weighted particle size distributions of COF precursor sol-gel solutions with the addition of PPG-NH₂ subjected to 0-4 hours of heating, and d where the particle size in COF precursor solutions without the addition of PPG-NH₂ rapidly grew to the micrometer scale and exceeded the DLS measurement range after 1 hour of heating. e In-situ real-time monitoring of membrane fabrication and surface changes.

Fig. 3. Characterizations of the TpPa-COF-X% membrane.

a Experimental PXRD patterns of TpPa-COF-10% membrane, TpPa-COF-10% powder, and TpPa-COF powder. b N₂ sorption isotherms collected at 77 K for the aged TpPa-COF-10% membrane, TpPa-COF-10% membrane, TpPa-COF-10% powder, and TpPa-COF powder. c Summary of the BET surface areas calculated for the aged TpPa-COF-10% membrane, TpPa-COF-10% membrane, TpPa-COF-10% powder, and TpPa-COF powder. d ATR-IR spectra of TP (pink), PDA (light teal), PPG-NH₂ (sky blue), and TpPa-COF-10% membrane (green). e SS ¹³C NMR spectrum of TpPa-COF-10% membrane.

Fig. 4. Structure analysis of the TpPa-COF-10% membrane.

a Optical photographs of the TpPa-COF-10% membrane, with a quarter dollar coin (24.26 mm in diameter) as a reference. b SEM images of TpPa-COF-10% membrane. c AFM images of TpPa-COF-10% membrane.

Fig. 5. In-situ ¹H NMR investigation during TpPa-COF-10% membrane synthesis.

a In-situ ¹H NMR spectra of the reactions between TP and PPG-NH₂ monitored over the 4-hour heating period. b In-situ ¹H NMR spectra of the reactions between TP, PDA, and PPG-NH₂ monitored over the 6-hour heating period. c DFT-calculated reaction energies of forming the possible products and reaction routes during the sol-gel solution formation using 1-methoxy-2-propylamine as the alternative of PPG-NH₂.

Fig. 6. Evaluation of production feasibility of the nanocrystal suturing strategy to form free-standing COF membranes.

a Optical photographs of the free-standing TpPa-COF-5% and TpPa-COF-1% membranes. b Photographs showing the TpPa-COF-10% membrane, which has a diameter of up to ~9 cm, with a quarter dollar coin (24.26 mm in diameter) as a reference. c Photographs showing the TpPa-COF-10% membrane with a thickness of up to 1.01 mm. d Schematic illustration of creating TpBZ-COF-10% and TpPa-CO₂H-COF-10% membranes. e Experimental PXRD patterns of TpBZ-COF-10% and TpPa-CO₂H-COF-10% membranes. f N₂ sorption isotherms collected at 77 K for TpBZ-COF-10% and TpPa-CO₂H-COF-10% membranes, the BET surface areas were calculated as 821 and 135 m² g⁻¹, respectively.

Fig. 7. Performance investigation of the free-standing TpPa-COF-10% membrane.

a Rejection performance of TpPa-COF-10% membranes towards dyes with varying molecular sizes in water. b Nyquist plots of the H₃PO₄@TpPa-COF-10% membrane at different temperatures ranging from 100 to 160 °C. c Proton conductivity of the H₃PO₄@TpPa-COF-10% membrane at different temperatures ranging from 100 to 160 °C. d Arrhenius plots of the proton conductivity of the H₃PO₄@TpPa-COF-10% membrane. e Performance stability of the H₃PO₄@TpPa-COF-10% membrane after 48-h of continuous operation at 160 °C.