Proton Conduction Properties of Intrinsically Sulfonated Covalent Organic Framework Composites

Abstract

The long–term stability of proton conductors is one of the most important factors in evaluating materials. Guest molecules can act as “bridges” for proton conduction channels and reside in the channels of covalent organic frameworks, but they are prone to leakage. Therefore, it is important to develop proton conductors with intrinsic proton conductivity. In this paper, we synthesized an intrinsically sulfonated covalent organic framework, TpPa–SO₃H, which has a more stable proton conducting performance than that of TpPa@H₂SO₄ by loading guest molecules. Meanwhile, the proton conductivity of TpPa–SO₃H was further improved by coating a superabsorbent polymer through an in situ reaction to obtain PANa@TpPa–SO₃H (PANa: sodium polyacrylate). As a result, the modified composite exhibits an ultrahigh proton conductivity of 2.33 × 10⁻¹ S cm⁻¹ at 80 °C under 95% relative humidity (RH). The stability of PANa@TpPa–SO₃H makes it an efficient proton transport platform with excellent proton conductivity and long–term durability.

1. Introduction

As economies grow and populations increase, various countries are experiencing increasing energy demand, leading to an increase in energy consumption [1]. The increasing energy consumption in countries around the world has put increasing pressure on the environment [2]. At present, fossil fuels (e.g., coal, oil, gas) are the main source of energy in the world and dominate the energy mix [3], while renewable energy sources such as solar and wind power still account for a small share of the total energy [4]. The combustion of fossil energy generates large amounts of pollutants and greenhouse gases, exacerbating global climate change and atmospheric pollution problems [5]. In order to solve these problems, scientists from various countries actively promote the development and utilization of sustainable new energy sources. New energy sources mainly include solar, wind, hydro, biomass, geothermal and hydrogen energy [6]. These energy sources have the advantages of being renewable, clean and low–pollution, which are important for promoting sustainable economic and social development and environmental protection [7].

Recently, the development of electrochemical energy technologies [8] has been increasingly accelerated due to its advantages of high efficiency, controllability and environmental protection. This technology is an important means to achieve sustainable energy development, and is mainly applied to energy storage and conversion, including lithium–ion batteries (LIB) [9], proton exchange membrane fuel cells (PEMFCs) [10], and electrochemical capacitors (EC) [11]. Among them, PEMFCs, as clean energy devices that generate electricity through electrochemical reactions, have good application prospects due to their mild working conditions and high energy conversion efficiency during operation [12]. As the core component of PEMFCs, the proton exchange membrane (PEM) directly affects the performance of the cells [13]. The ideal PEMs need to have low gas permeability, high chemical and thermal stability, and good film formation capability in addition to high proton conductivity. The most commonly used commercially available proton exchange membranes are Nafion–based materials [14], which have a proton conductivity of 10⁻¹ S cm⁻¹ under high humidity conditions. However, Nafion membranes are complicated to synthesize, expensive, and have a narrow operating temperature range, so new materials need to be developed to replace them [15]. Currently, the development of fuel cell technology is mainly directed toward the development of proton–conducting materials with excellent overall performance.

Porous framework materials including hydrogen–bonded organic frameworks (HOFs) [16], metal–organic frameworks (MOFs) [17] and covalent organic frameworks (COFs) [18] have attracted great interest as proton–conducting materials [19]. Compared to HOFs and MOFs with certain drawbacks in stability, COFs linked by covalent bonds not only have a high specific surface area, adjustable pore size, and customizable structure and function, but also have high thermochemical stability and permanent porosity [20]. These characteristics sow their great advantages as emerging proton–conducting membrane materials [21], and make them widely investigated in fields such as chemical sensing, separation and photoelectric catalysis [22]. The porous nature of COFs provides the opportunity to load various guest molecules such as water, phosphoric acid, phytic acid, polymetallic oxides, and organic heterocyclic compounds into their channels, resulting in increasing studies in the field of proton conduction [23]. In terms of proton conduction, guest molecules present in the channels of COFs can serve as “bridges” for proton transfer to enhance their proton conduction performance [24]. However, most of the COFs do not possess intrinsic proton conductivity, and the guest molecules loaded in the channels are very prone to leakage [25], so it is important to develop COFs with intrinsic proton conductivity to ensure more stable proton conducting performance.

Herein, we describe a simple strategy to obtain stable COF composites with high proton conductivity by introducing acid groups and enriching water molecules as proton sources and carriers. The two COFs (TpPa and TpPa–SO₃H) with a similar structure except for the presence or absence of substituted sulfonic groups were synthesized in accordance with the previous reports [26,27], and then sulfuric acid (H₂SO₄) was loaded into the channels of TpPa to obtain TpPa@H₂SO₄ and compare its proton conducting stability with TpPa–SO₃H. As expected, TpPa@H₂SO₄ has poor proton conducting stability because of the leakage of H₂SO₄ from the channels of TpPa under humid conditions, but TpPa–SO₃H exhibits highly stable intrinsic proton conductivity. Subsequently, the water–absorbing polymer was coated on the surface of TpPa–SO₃H to enrich the water molecules in its channels. This not only enhanced the water retention capacity, but also promoted the formation of smooth proton transfer pathways, thus greatly improving the proton conductivity of TpPa–SO₃H. At 80 °C and under 95% RH, the proton conductivity of PANa@TpPa–SO₃H reached 2.33 × 10⁻¹ S cm⁻¹, which can be compared with that of the excellent proton–conducting COF materials reported thus far; see Table S1.

2. Results and Discussion

TpPa and TpPa–SO₃H were synthesized using the solvothermal method via the reaction of 2,4,6–triformylphloroglucinol (Tp) with p–phenylenediamine (Pa) and 2,5–diaminobenzenesulfonic acid (Pa–SO₃H), respectively. TpPa@H₂SO₄ was prepared by grinding 85% H₂SO₄ into the channels, and PANa@TpPa–SO₃H was prepared by coating the water–absorbing polymer PANa on the surface of TpPa–SO₃H using an in situ reaction method. As shown in Figure S1a,b, Fourier transform infrared (FT–IR) spectra of TpPa and TpPa–SO₃H show the C=O stretching peak at 1643 cm⁻¹ corresponding to the Tp disappears, and that no new C=O and N–H characteristic peaks appeared, while the typical stretching peaks attributed to the C=C and C–N bonds appeared at 1582 and 1253 cm⁻¹, respectively, indicating that they were successfully prepared and bore a keto–enamine linkage structure; see Scheme S1. Meanwhile, the peaks at 1026 and 1083 cm⁻¹ attributed to symmetric and asymmetric O=S=O stretching bands in the IR spectrum of TpPa–SO₃H confirm the presence of sulfonic acid groups. The phase purity of TpPa and TpPa–SO₃H was evaluated using powder x–ray diffraction (PXRD) and elemental analyses as shown in the synthesis part. The PXRD patterns of TpPa and TpPa–SO₃H show that they all have symmetric structures with typical AA–layer stacks, which were simulated using Materials Studio 2018; see Tables S2 and S3. The unit cell parameters of TpPa–SO₃H were a = b = 23.3 Å, c = 3.5 Å, and the peaks corresponding to the (100), (110), (120) and (001) faces appeared at 4.6°, 8.2°, 11.6° and 26.3°; see Figure S1c. Accordingly, the unit cell parameters were a = b = 23.3 Å, c = 3.8 Å, and the strong peak attributed to the (100), (110) and (001) crystal face appeared at 4.6°, 8.1° and 26.3°, respectively; see Figure S1d. Both TpPa and TpPa–SO₃H have good crystallinity. It can be clearly seen from the SEM images that TpPa and TpPa–SO₃H had a similar and regular strip–shaped form; see Figure S2a,c. TGA curves display that the two COFs were thermally stable before 300 °C; see Figure S3.

COFs have an excellent pore structure and large specific surface area, which can provide the directional assembly and orientation of guest molecules. The addition of guest molecules can modulate the chemical environment of COF channels to change proton density and proton conduction pathways and further improve its proton conduction efficiency. Therefore, their gas adsorption performance was also investigated using Brunauer–Emmett–Teller (BET) tests at 77 K; see Figure 1. The N₂ adsorption capacities of TpPa and TpPa–SO₃H were 213 and 145 cm³ g⁻¹, respectively, under 1 atm; see Figure 1a,b. It could be seen that the dense sulfonic acid groups made TpPa–SO₃H bear a smaller pore volume. Similarly, the calculated BET surface area of TpPa–SO₃H (78.9 m² g⁻¹) was also smaller than that of TpPa (160.3 m² g⁻¹). To verify the stability of the loaded guest molecules in the channels, TpPa@H₂SO₄ was obtained by introducing H₂SO₄ into the one–dimensional (1D) channels of TpPa. The gas adsorption test showed that the N₂ adsorption capacity of TpPa@H₂SO₄ decreased to 78 cm³ g⁻¹, and its pore size also decreased to 5.8 Å compared to the 15.5 Å pore size of TpPa; see Figure 1c. indicating that H₂SO₄ was successfully introduced into the channels of TpPa. This is also demonstrated by the IR tests; see Figure S4a. The O=S=O stretching bands at 1027, and 1081 cm⁻¹ appeared in the spectrum of TpPa@H₂SO₄. ICP tests show that the mass content of sulfur (S) was 13.2%, which is equivalent to the 39.6% H₂SO₄ content in TpPa@H₂SO₄. The SEM images display that compared to that of TpPa, there was no significant change in the morphology of TpPa@H₂SO₄ (Figure S2b) and that TpPa@H₂SO₄ had good crystallinity as shown in the PXRD pattern (Figure S1c), indicating that the structure of TpPa remained stable after loading sulfuric acid. Subsequently, the proton conductivity tests of TpPa and TpPa@H₂SO₄ were performed at 80 °C via the impedance technique using their pressed pellets; see Figure S5a,b. The results show that TpPa had almost no proton conductivity until 60% RH, and had a conductivity of only 1.56 × 10⁻⁷ S cm⁻¹ under 95% RH, while the corresponding value of TpPa@H₂SO₄ reached 1.33 × 10⁻¹ S cm⁻¹ under the same conditions; see Figure 2a,b and Figure S5a and Table S4. Compared to that of TpPa, the proton conductivity of TpPa@H₂SO₄ was improved by six orders of magnitude, which further confirms that H₂SO₄ was successfully introduced into the channels of TpPa.

However, the guest molecules loaded in the channels of COFs are usually unstable and prone to detachment, which will lead to the leakage of guest molecules and greatly reduce their proton conduction performance. Therefore, the stability of the proton conductivity of TpPa@H₂SO₄ was tested at 80 °C and 95% RH. The proton conductivity of TpPa@H₂SO₄ slightly decreased in the first three hours, but then decreased quickly; see Figure 2c. After testing for 12 h, the proton conductivity of TpPa@H₂SO₄ decreased to 82% of the original value. As demonstrated by the ICP test, the sulfur (S) content of TpPa@H₂SO₄ decreased to 10.7%, which is equivalent to 81% of the initial sulfur content. This result proves that there was a significant loss of H₂SO₄ molecules in the channels of COFs during the test, and the decrease in proton conductivity was proportional to the proportion of H₂SO₄ loss. Similarly, the proton conducting stability of TpPa–SO₃H was also tested at 80 °C; see Figure 2 and Table S4. Unlike TpPa, TpPa–SO₃H exhibited a moderate proton conductivity of 3.65 × 10⁻⁵ only under 50% RH. When the humidity increased to 95%, the proton conductivity increased to 1.62 × 10⁻² S cm⁻¹; see Figure 2a,b and Figure S5c. After testing 12 h, the value remained at 1.56 × 10⁻² S cm⁻¹ (Figure 2d) which suggests it was almost unchanged considering the error factor. The ICP tests also revealed that the sulfur (S) content in TpPa–SO₃H was 10.5% before and after testing for 12 h, which is consistent with the theoretical sulfur content of 10.5%. These results indicate the stable intrinsic proton conduction nature and obvious humidity dependence of TpPa–SO₃H.

The water storage capacity of proton conductors is important for the formation of smooth proton conduction pathways. In the proton conduction process, water molecules can not only serve as carriers for proton conduction, but can also dissociate and conduct protons through the following equilibrium: H₂O = H⁺ + OH⁻. Therefore, to further improve the water absorption and retention of TpPa–SO₃H and enhance its proton conduction performance, the highly absorbent polymer PANa was coated on the TpPa–SO₃H surface using an in situ reaction strategy to synthesize PANa@TpPa–SO₃H. Among them, PANa accounted for 40% of the by weight percentage of PANa@TpPa–SO₃H. According to the SEM images, PANa was well–coated on the surface of TpPa–SO₃H; see Figure S2d. The gas adsorption tests showed that the N₂ adsorption capacity of PANa@TpPa–SO₃H was only 39 cm³ g⁻¹, which is much smaller than that of 145 cm³ g⁻¹ of TpPa–SO₃H; see Figure 1b. Meanwhile, it did not exhibit any pore size distribution; see Figure 1d. The results further indicate that the surface of TpPa–SO₃H was completely coated by PANa. To confirm this, the contact angle tests were performed on the JC2000 contact angle measuring instrument; see Figure 3. The results show that the angles of TpPa and TpPa–SO₃H were 78° and 21°, respectively. This indicates that the sulfonic acid groups existing in the channels of TpPa–SO₃H can effectively improve its hydrophilicity. However, PANa@TpPa–SO₃H exhibited complete wettability towards water, further showing that PaNa was successfully coated on the surface of TpPa–SO₃H, and that the hydrophilicity of TpPa–SO₃H was improved after coating. Correspondingly, the humidity–dependent proton conductivity of PANa@TpPa–SO₃H was tested at 80 °C in the humidity range of 50–95%; see Figure 2a,b and Figure S5c and Table S4. It can be seen that humidity has a significant effect on proton conduction. The proton conductivity of PANa@TpPa–SO₃H was 6.46 × 10⁻⁴ S cm⁻¹ under 50% RH. When the humidity increased to 95%, the proton conductivity reached 2.33 × 10⁻¹ S cm⁻¹, which is one order of magnitude higher than the corresponding value before coating, and close to the value of commercial Nafion membranes under the same conditions. Meanwhile, PANa@TpPa–SO₃H has excellent long–term durability as shown in Figure S8. This fully demonstrates the effectiveness of the coating strategy. Moreover, we also prepared PANa@TpPa and tested its proton conductivity, which only resulted in a proton conductivity of 8.9 × 10⁻² S cm⁻¹ at 80 °C and 95% RH, even though it also exhibited complete wettability towards water; see Figure 3c and Figure S9. This further proves that the proton source provided by –SO₃H has an important effect on the proton conductivity.

To further understand the proton conduction mechanism, the temperature–dependent proton conductivity of TpPa, TpPa–SO₃H, and PANa@TpPa–SO₃H was studied; see Figure 4, Figures S6 and S7. The activation energies (E_a) obtained by fitting the temperature–dependent proton conductivity data for TpPa, TpPa–SO₃H and PANa@TpPa–SO₃H are 0.41, 0.30, and 0.26 eV, respectively, indicating that the proton transport in TpPa may adopt the mixed Grotthuss and vehicle mechanism, while the proton transport in TpPa–SO₃H and TpPa@H₂SO₄ and PANa@TpPa–SO₃H mainly adopts the Grotthuss mechanism. This is because there are no active protons in the channels of TpPa, and its proton conduction depends on the adsorbed water molecules. However, the water molecules serve as both proton carriers and proton resources. When the concentration of water molecules is not sufficient, they cannot form a large number of effective hydrogen–bonding networks, so some protons can only be transported through vehicle mechanisms. For strongly hydrophilic TpPa–SO₃H and PANa@TpPa–SO₃H, their channels not only possess active protons but also can enrich more water molecules as shown in contact angle test experiments, thus forming effective hydrogen bonding networks for transferring protons.

Both water absorption and swelling rate have important effects on the performance and stability of proton conductors. High water absorption and swelling rates may lead to reduced stability, mechanical properties and proton conduction efficiency, and affect the durability of proton conductors. Therefore, the water absorption and swelling rates of different ratios of PANa/TpPa–SO₃H were also studied in detail via direct weighing measurements after adsorbing water for 6 h at 80 °C and 95% RH; see Figure 5. It can be clearly seen that the water absorption and swelling rate of PANa@TpPa–SO₃H gradually increase with an increase in PANa content. Among them, the swelling rate of PANa@TpPa–SO₃H shows a linear relationship with its PANa content. When the PANa content was 40%, the water absorption capacity of PANa@TpPa–SO₃H reached 33.8% of its own weight and the swelling rate was 11.5%, which are values that are smaller than the corresponding values for commercial Nafion–based membranes. This indicates that PANa@TpPa–SO₃H is expected to be an ideal candidate for future commercialized materials.

3. Conclusions

In summary, we synthesized two COFs with and without sulfonate groups, demonstrating the instability of loaded guest molecules by introducing H₂SO₄ into the channels of TpPa. Meanwhile, the composite PANa@TpPa–SO₃H was constructed using an in situ reaction strategy. TpPa–SO₃H can provide orderly channels and bear active protons, and PANa can enrich water molecules into the channels of TpPa–SO₃H. After combining the advantages of both, PANa@TpPa–SO₃H possessed ultrahigh proton conductivity and a low swelling rate. This strategy provides a new approach for obtaining excellent COF–based proton–conducting materials with high stability and durability.

4. Materials and Methods

4.1. Materials

All the chemical reagents were commercially available and used without further purification. 2,4,6–Triformylphloroglucinol (99.9%), p–phenylenediamine (99.9%), and 2,5–diaminobenzenesulfonic acid (99.9%) were bought from Sigma Aldrich (shanghai, China). Ammonium persulphate (99%) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Acrylic acid (99%) and N,N’–methylene diacrylamide (99%) were supplied by Sahn Chemical Technology (Shanghai) Co., Ltd., Shanghai, China. Toluene, 1,4–dioxane, N,N–dimethylformamide, acetone, and ethanol were obtained from Tianjin Fuyu Fine Chemical Co., Ltd., Tianjin, China.

4.2. Synthesis Procedure

4.2.1. Preparation of TpPa

TpPa was synthesized in accordance with the method reported in the literature [26]. Briefly, 2,4,6–triformylphloroglucinol (21.0 mg, 0.1 mmol) and p–phenylenediamine (16 mg, 0.15 mmol) were dissolved in a 3 mL solution of mesitylene/1,4–dioxane (1/1) in a glass ampoule, to which 0.5 mL of 6 M acetic acid solution was added. The mixture was sonicated for 10 min, allowed to disperse uniformly, and then degassed in a liquid nitrogen bath using 3 freeze–pump–thaw cycles (vacuum < 50 mTorr). The tubes were sealed and then heated at 120 °C for 72 h to produce a light red solid at the bottom of the ampoule. After cooling to room temperature, the solvent was decanted and the solid was washed three times with N, N–dimethylformamide, acetone, and ethanol, respectively, and dried under dynamic vacuum at 120 °C for 10 h. The pale red powder was isolated in a 65% yield. The elemental analysis calculation for C₆ONH₄ revealed the following: C, 67.92; H, 3.80; N, 13.20; found: C, 67.98; H, 3.81; N, 13.19.

4.2.2. Preparation of TpPa@H₂SO₄

TpPa@H₂SO₄ was obtained by adding 85% of sulfuric acid (13 μL) into TpPa (10 mg), grinding it for half an hour. The sample was washed with a small amount of deionized water to remove the sulfuric acid on the surface of TpPa, and then dried under vacuum at 120 °C.

4.2.3. Preparation of TpPa–SO₃H

TpPa–SO₃H was synthesized in accordance with the method reported in the literature [27]. Briefly, 2,4,6–triformylphloroglucinol (21.0 mg, 0.1 mmol) and 2,5–diaminobenzenesulfonic acid (28 mg, 0.15 mmol) were dissolved in a 3 mL solution of mesitylene/1,4–dioxane (1/1) in a glass ampoule, to which 0.5 mL of 6 M acetic acid solution was added. The mixture was sonicated for 10 min, allowed to disperse uniformly, and then degassed in a liquid nitrogen bath using 3 freeze–pump–thaw cycles (vacuum < 50 mTorr). The tubes were sealed and then heated at 120 °C for 72 h to produce a red solid at the bottom of the ampoule. After cooling to room temperature, and the solid was washed three times with N, N–dimethylformamide, acetone, and ethanol, respectively, and dried under dynamic vacuum at 120 °C for 10 h. The red powder was isolated in 61% yield. for the elemental analysis calculation revaled the following. C₁₃O₅N₂H₈S: C, 49.31; H, 2.76; N, 9.58; found: C, 49.28; H, 2.71; N, 9.59.

4.2.4. Preparation of PANa@TpPa and PANa@TpPa–SO₃H

Acrylic acid, ammonium persulphate, and N, N’–methylene diacrylamide were utilized as a monomer, initiator, cross–linker, respectively. Specifically, 3.6 mL of acrylic acid was mixed with 5 mL of deionized water to form solution A. An amount of 2 g of NaOH was dissolved in 5 mL of deionized water to form solution B. Solution B was added into solution A dropwise with stirring and ice bath cooling. Then, 55 mg of ammonium persulphate was added into the mixed solution, followed by the addition of 2 mg of N, N′–methylene diacrylamide.

An amount of 50 mg of ground TpPa–SO₃H was weighed on a glass slide, and the above mixed solution at 50 μL was added dropwise to TpPa–SO₃H and stirred well with a glass rod. Subsequently, a typical reaction of radical polymerization proceeded, and the mixture was placed in an oven at 65 °C for 2 h to allow PANa to grow in situ on TpPa–SO₃H. After the reaction was completed, the solid was washed with deionized water 3 times which to remove excess initiator, cross–linker and sodium hydroxide, then dried under dynamic vacuum at 65 °C for 2 h. The product was collected and weighed. PANa@TpPa was prepared with a similar procedure.

4.3. Proton Conductivity Measurement

The as–synthesized sample was placed in a mold and pressed into a pellet with a diameter of 3 mm (TpPa, TpPa@H₂SO₄, TpPa–SO₃H and PANa@TpPa–SO₃H) and a thickness range of 1–2 mm using a tableting machine. The pellet was placed in the center of the glass pellet and fixed horizontally with two 20 cm gold wires, two sides of the pellet were coated with silver glue, and we then waited for about 30 min for it to dry; see Figure S10. Impedance analysis was performed with 1260A Impedance/Gain–Phase Analyzer from 10 MHz to 0.1 Hz with an input voltage of 200 mV under constant temperature and humidity conditions, which were controlled using a BPS–50CL humidity control chamber. Each sample was pressed into at least three tablets, and repeated cyclic tests were performed on each tablet. Typically, the impedance at each temperature was measured after equilibration for a period of 6–10 h. The resistance values were obtained by fitting the impedance profile using zview software. The circuit equivalent used for fitting is as follows; see Figure 6:

R1 corresponds to the resistances of the wire and electrode, while R2 accounts for the bulk resistance of the pellet. The proton conductivity (σ, S cm⁻¹) of each sample was obtained via the following equation:

$$\sigma =\frac{l}{RS}$$

(1)

where l and S are the length (cm) and area (cm²) of the pellet, respectively, and R is the intrinsic resistance value (Ω) of the material fitted by the equivalent circuit of the first semicircle using zview software. The activation energy (E_a) of the material is estimated according to the following Arrhenius equation:

$$\sigma T={\sigma }_0\exp \left(-\frac{E_a}{k_{B}T}\right)$$

(2)

where σ₀ is the pre–exponential factor, T is the temperature, and k_B is the Boltzmann constant.

4.4. Water Uptake and Swelling Ratio of PANa@TpPa–SO₃H Pellets

The pellet’s water uptake was calculated from the weight difference between the dry pellet and wet pellet (Equation (3)), and their swelling ratios were tested by measuring the difference in length and thickness between the dry and wet pellets (Equation (4)). The dry and wet pellets were prepared with the same method as that used for proton conduction measurements.

$$\mathrm{Water} \mathrm{uptake}\left(\%\right)=100\times \frac{W_{wet}-W_{dry}}{W_{dry}}$$

(3)

$$\mathrm{Swelling} \mathrm{ratio}\left(\%\right)=100\times \frac{L_{wet}-L_{dry}}{L_{dry}}$$

(4)

where W_dry is the weight of the pellet dried in a vacuum until the weight is constant, and W_wet is the weight of the pellet placed under 95% RH and 80 °C for different durations.

4.5. Other Measurements

The date of powder X–ray diffraction (PXRD) was recorded on a Rigaku Ultima IV X–ray diffractometer with Cu–Kα radiation (l = 0.15418 nm). Diffraction patterns in the angle region (3–40°) were recorded at a rate of 2θ = 5°·min⁻¹. Thermal gravimetric analysis (TGA) was performed using the Mettler Toledo thermal analyzer with a heating rate of 10 °C min⁻¹ in the range of 40–900 °C under a N₂ atmosphere. The water contact angle tests were carried out on the JC2000 contact angle measuring instrument. Elemental analysis was conducted on a PerkinElmer 240C elemental analyzer for C, H, and N determination. N₂ adsorption and desorption tests were performed on a Tristar 2460 surface area analyzer. Field–emission scanning electron microscopy (FESEM) images were obtained on a Hitachi SU–8000 instrument. The inductively coupled plasma mass (ICP–MS) spectra were recorded using an Optima 8000 spectrometer.