Sulfonic‐Pendent Vinylene‐Linked Covalent Organic Frameworks Enabling Benchmark Potential in Advanced Energy

Abstract Both proton exchange membrane fuel cells and uranium‐based nuclear techniques represent two green and advanced energies. However, both of them still face some intractable scientific and industrial problems. For the former, established proton‐conduction materials always suffer one or another defect such as low proton conductivity, high activation energy, bad durability, or just small‐scale product; while for the later, there still lacks available adsorbent to selectively recover of UO2 2+ from concentrated nitric acid (>1 M) during the spent fuel reprocessing due to the deactivation of the adsorption site or the decomposition of adsorbent under such rigorous conditions. It is found that the above two issues can be well solved by the construction of sulfonic‐pendent vinylene‐linked covalent organic frameworks (COFs), since these COFs contain abundant sulfonic units for both intrinsic proton conduction and UO2 2+ capture through strong coordination fixation and vinylene linkage that enhances the stability up to 12 M nitric acid (one of the best materials surviving in 12 M HNO3).


Materials and general methods
Reagents and solvents were commercially available (Alfa) and were used without further purification. X-ray powder diffraction were collected by a Bruker AXSD8 Discover powder diffractometer at 40 kV, 40 mA for Cu Kλ ( λ= 1.5406 Å). The simulated powder patterns were calculated by Mercury 1.4. Infrared Spectra (IR) were measured by a Bruker VERTEX70 spectrometer in the 500-4000 cm -1 region. The gas adsorption isotherms were collected on a Belsorp-max. Ultrahigh-purity-grade (>99.999%) N 2 gases were used during the adsorption measurement. SEM and EDS measurements were carried out using a Hitachi S-4800 microscope. The analyses of concentrations of metal ions in the solution was carried out by ThermoFisher iCap7600 ICP-OES instruments. X-ray photoelectron spectra (XPS) were collected by Thermo Scientific ESCALAB 250 Xi spectrometer. Solid-state NMR experiments were performed on Varian Infinityplus 300 solid-state NMR spectrometer (300MHz).

Impedance measurements
Proton conductivity of the COFs was measured by AC impedance using Ivium CompactStat potentiostat B31250 under controlled humidity and temperature. About 10 mg materials were cutted into quadrate pellets (4.7×2.3 mm in diameter, 1 mm in thickness) from the big block samples. The quadrate pellets were placed between two-electrode cell connected with Ivium CompactStat potentiostat B31250 by a conductive wire. The temperature dependence of proton conductivity was tested by EIS with a tuned frequency range from 1 Hz to 1MHz and an alternating potential of 100 mV in a humidity chamber maintained at 98% RH. The humidity dependence of proton conductivity was determined using different humidities controlled by saturated salt aqueous solutions in a constant temperature and humidity chamber. When changing temperature or humidity, the pellets were equilibrated for 6 h. Proton conductivity (σ, S cm −1 ) was calculated by the following equation: where σ is the proton conductivity (S cm −1 ), L is the thickness of the pellet (cm), A is the area of the pellet (cm 2 ), and R is the resistance (Ω) of the pellet corresponding to the real Z′ Nyquist plot.
Activation energy (E a ) of proton conductivity (σ) was extracted from the data measured at various temperatures (98% RH) by using the Arrhenius equation: where σ 0 , T and k are the pre-exponential factor, temperature and Boltzmann constant, respectively.

Synthesis
3.1 Synthesis of model compound of 2,4,6-tri(styryl)benzenesulfonic acid Model compound of 2,4,6-tri(styryl)benzenesulfonic acid was prepared as follows. A mixture of TBS (0.1 mmol), benzaldehyde (0.3 mmol), and benzoic anhydride (0.3 mmol) was mixed in the sealed tube and heated at 200°C for three days, then cooled to room temperature. The products were recrystallization from water to give the pure product as a colorless powder.
Mass spectrometry (HR-MS) was used to confirm the product and purity. EA (%): calc.
A mixture of DBS (0.1 mmol), 2-hydroxybenzaldehyde (0.2 mmol), and benzoic anhydride (0.3 mmol) was mixed in the sealed tube and heated at 200°C for three days, then cooled to room temperature. The products were recrystallization from water to give the pure product as a colorless powder. Mass spectrometry (HR-MS) was used to confirm the product and purity.

Stability tests
100 mg samples of these COFs were soaked in 30 mL 12M HNO 3 solution for three days.

Uranium uptake via batch experiments
U(VI) solution was prepared by dissolving uranyl nitrate (UO 2 (NO 3 ) 2 ·6H 2 O, analytical reagent) in deionized water. The pH value is adjusted by HNO 3 (1 M). Adsorption temperature is 298 K.
In kinetics experiments, the U solution with initial concentration of 100 ppm and pH=5 was used. The dose of adsorbent is 10 mg, while the U solution is 20 mL.
In isotherm experiments, the U solution with initial concentration of 10-800 ppm and pH=5 was used. The dose of adsorbent is 10 mg, while the U solution is 20 mL and the contact time is 8 h.
In determining affinity experiments, the U solution with initial concentration of 1 ppm and pH=5, 1 and 1-12 M HNO 3 was used. The dose of adsorbent is 10 mg, while the U solution is 20 mL.
In selective adsorption experiments, a 12-ions mixed solution (12M HNO 3 ) contains both U and other 11 metal ions with respectively initial concentration of 1 ppm was used. The dose of adsorbent is 10 mg, while the solution is 20 mL.

Some related calculation and fitting in this U adsorption experiments
The adsorption amount, Q e (mg/g), was calculated by the difference of the U(VI) equilibrium concentration before and after adsorption (see equation 3): ( ) where c 0 (mg/L) and c e (mg/L) are the initial concentration and equilibrium concentration of uranium in the solutions, respectively; V (mL) is the volume of testing solution and m (mg) is the amount of sorbent.
The adsorption kinetics was analyzed by simplified kinetic models such as the pseudofirst-order and pseudo-second-order, through the following two equations, Where Qe (mg/g) and Qt (mg/g) are the quantity of the adsorbed U(VI) at equilibrium and at t time, respectively, and K 1 (min -1 )/K 2 [g/(mg•min)] is the pseudo-first/second-order sorption rate constant that is deduced from the slope of the plot of t/Qt versus t.
The isotherm data was fitted by Langmuir and Freundlich models via the following two equations, Where Q e (mg/g) is the amount adsorbed at equilibrium and C e (mg/L) is the equilibrium concentration; Q m (mg/g) is the maximum adsorption amount; K L (L/mg) is an equilibrium constant related to the binding strength; n and K F (L/mg) are Freundlich constants which are indicators of the adsorption capacity and adsorption intensity, respectively.
The K d value and selectivity (S) is calculated from the following two equations, where the unit for K d value is mL/g.

Proton-conductivity Sensing of UO 2 2+
The response time was tested follows. Quadrate pellets of COF-TBS were soaked in 1 ppb UO 2 2+ solution with pH=5 for 0, 0.5 min, 1 min, 2 min, 3 min, 4 min, respectively, then the quadrate pellets were taken out of the solution and dried naturally for six hours. Next, the proton conductivity of these resultant quadrate pellets was analyzed by AC impedance spectroscopy at 298 K under RH= 98%.
Sensing at various UO 2 2+ concentration was tested follows. Quadrate pellets of COF-TBS were soaked in various UO 2 2+ solutions with pH=5 for 2 min, then the quadrate pellets were taken out of the solution and dried naturally for six hours. Next, the proton conductivity of these resultant quadrate pellets was analyzed by AC impedance spectroscopy at 298 K under RH= 98%.
Ion interference was tested follows. Quadrate pellets of COF-TBS were soaked in a 12ions mixed solution containing both U and other 11 metal ions with respectively initial concentration of 1 ppm and pH=5 for 2 min, then the quadrate pellets were taken out of the solution and dried naturally for six hours. Next, the proton conductivity of these resultant quadrate pellets was analyzed by AC impedance spectroscopy at 298 K under RH= 98%. Tables   400  420  440  460 Table S2. Fitting these data by the pseudo-first-order and pseudo-second-order models.