2.1 Materials
Kraft lignin (KL) was obtained from Moorim pulp & Paper Co., Ltd. (South Korea). Methanol soluble lignin (ML) was isolated from KL as follows: KL was dissolved in excess methanol at room temperature and stirred overnight. Insoluble KL was removed by filtration, and the filtrate was evaporated under reduced pressure to obtain ML, which was used as a precursor for the synthesis of zwitterionic lignin (ZL). (3-aminopropyl)triethoxysilane (APTES, 99%) and 1,3-propane sultone (PS, 98%) were purchased from Sigma-Aldrich (USA). Anhydrous toluene (99.8%), chloroform (99.8%), N,N-dimethylformamide (DMF, 99.8%), sodium chloride (NaCl, 99.5%), lithium bromide (LiBr, 99.9%), and 0.1 M sodium hydroxide (NaOH) and hydrochloric acid (HCl) solutions were purchased from Daejung Chemicals & Metals Co., Ltd. (Korea). To fabricate the membranes, an alcohol dispersion of 5 wt% Nafion dispersion (D521) was purchased from DuPont (USA). All chemicals were used as received without any purification.
2.2 Synthesis of zwitterionic lignin (ZL)
ZL was synthesized via a modified two-step procedure based on our previous studies [24, 25]. In the first step, 2 g of ML was dissolved in 100 mL of anhydrous toluene, in a 250 mL round-flask connected to a reflux condenser (Findenser, Radleys, UK) with a magnetic stirrer, and dispersed by stirring for 30 min. Then, 16.17 mmol of APTES was added dropwise to the reaction mixture under a nitrogen atmosphere. Finally, the mixture was raised to 70°C under a nitrogen atmosphere and stirred for 24 h to induce condensation of the ML and APTES moieties to prepare APTES grafted lignin (AGL). After the reaction, the mixture was filtered using a filter paper (F2040, CHMLAB®, φ 90 mm) and washed several times with toluene to remove unreacted APTES. The resulting insoluble solids were dried in a vacuum oven at 40°C for 72 h to recover the AGL.
In the second step, 0.5 g of pre-synthesized AGL was placed in a round flask with 50 mL of chloroform and was sufficiently dispersed in an ultrasonic bath. Next, PS was added dropwise to the mixture at molar ratios of 4.10, 8.18, and 12.29 mmol, under a nitrogen atmosphere. Finally, the mixture was stirred and heated using a reflux condenser at 50°C for 24 h. After the reaction, the temperature of the round-flask was cooled, the mixture was filtered using a filter paper (F2040, CHMLAB®, φ 90 mm), and washed several times with chloroform. Thereafter, the insoluble solid of ZL was dried in an oven at 40°C for 72 h. The final products at PS dosages of 4.10, 8.18, 12.29 mmol were coded as ZL-1, ZL-2, and ZL-3, respectively.
2.3 Preparation of ZL/Nafion composite membrane
The ZL/Nafion membranes were fabricated using the casting method, as shown in Fig. 1. First, the Nafion dispersion was heated in oven at 50°C to completely evaporate the alcohol. Nafion was then dissolved in 20 g of DMF to obtain a homogenous Nafion solution. Next, different dosages of ZL-3 (0%, 0.3%, 0.5%, 1%, and 2% w/w, based on the Nafion weight) were added to the Nafion solution and sonicated for 30 min. The solution was stirred for 24 h at room temperature. Each mixture was poured into a culture dish (60 mm × 15 mm) and dried a vacuum oven at 60°C for 12 h. Each mixture was further dried at 80°C for 4 h. The composite membranes thus formed were peeled off from the culture dish and immersed in 0.1 M H2SO4 solution for 24 h. Finally, the membrane was washed with deionized water several times. The composite membranes are denoted as pure Nafion, ZL/Nafion 0.3%, ZL/Nafion 0.5%, ZL/Nafion 1%, and ZL/Nafion 2%, based on their ZL ratios.
2.4 Characterization of lignin samples
The chemical structures of the lignin samples were analyzed using a FT-IR spectrometer (Nicolet Summit, USA) equipped with attenuated total reflectance accessory. The FT-IR spectra were recorded in the range of 500–4000 cm− 1 at a resolution of 4.0 cm− 1 with 64 scans. XPS (K-Alpha+, Thermo Scientific, UK) data were collected to analyze the elemental distribution on the lignin surface. The XPS spectra were recorded with a dwell time of 30 ms and 30 scans. The elemental (C, N, and S) contents of the lignin samples were analyzed using an elemental analyzer (Thermo Scientific, FlashEA1112, UK) equipped with a thermal conductivity detector. The molecular weights of lignin samples were measured using a GPC (Shimadzu-20A, Japan) equipped with PL-gel columns (PL-gel 5-µm MIXED-C and -D and PL-gel 3-µm MIXED-E) (Agilent Technologies, USA) and a UV (ultraviolet) detector. The eluent system of DMF consisted of 0.1% LiBr, and the polystyrene standards were used to estimate the molecular weights in LiBr/DMF solvent at 70°C. The thermal properties of lignin samples were measured using TGA (SDT Q600, USA) in the temperature range of 25–700°C (heating rate = 10°C/min) under N2 atmosphere. To evaluate the surface charges of the lignin samples, their zeta potentials (Zetasizer Nano S; Malvern Instruments, UK) were measured using a previously reported method [24].
2.5 Characterization of ZL/Nafion composite membranes
The light transmittance of the membranes was measured using an ultraviolet-visible (UV-Vis) spectrophotometer (UV-2550, Shimadzu, Japan). The transmittance spectra of the membranes were recorded in the 200–800 nm range. The membrane was cut into 10 × 40 mm rectangular pieces prior to testing. The contact angle and thickness were determined using an L&W micrometer (code 250, Lorenzen & Wettre, Sweden) and a pocket goniometer (Contact Angle Analyzer, Sweden), respectively. Individual characterization was used to calculate the mean and standard deviation of eight samples of each membrane.
The water uptake and swelling ratio for all membranes were measured by immersing a piece of each membrane (2 × 2 cm) in deionized water at 20–80°C for 24 h, and then water uptake (WU) and swelling ratio (SR) values were calculated by using Eqs. (1) and (2), respectively:
$$\text{W}\text{U} \left(\text{%}\right)=\frac{{W}_{wet}-{W}_{dry}}{{W}_{dry}}\times 100$$
1
$$\text{S}\text{R} \left(\text{%}\right)=\frac{{L}_{wet}-{L}_{dry}}{{L}_{dry}}\times 100$$
2
Here, Wwet is the weight of the wet membrane, and Wdry is the weight of the dry membrane. Lwet is the length of the wet sample, and Ldry is the length of the dry sample.
The ion exchange capacities (IEC) of the membranes were calculated using an acid-base titration method [26]. First, the membrane samples were washed with deionized water, completely dried, weighed, and immersed in 1 M NaCl solution to exchange H+ ions with Na+ ions. Finally, the solution was titrated with 0.01 M of NaOH solution using a pH detector (HANNA HI 932, Hanna Instruments, USA), and the IEC (mmol/g) values of the membranes were calculated using Eq. (3):
$$\text{I}\text{E}\text{C} \left(\text{m}\text{m}\text{o}\text{l}/\text{g}\right)=\frac{{C}_{NaOH}-{V}_{NaOH}}{{W}_{dry}}$$
3
where CNaOH and VNaOH are the concentration and volume of the titrated NaOH solution, respectively, and Wdry is the weight of the dry membrane.
The (in-plane) proton conductivities of the membranes were measured in deionized water at various temperatures using a four-electrode membrane conductivity clamp (BT-110, Scribner, USA) equipped with a potentiostat (VersaSTAT 4, PAR) [27]. Before the measurement, the membranes were cut into 3 cm (length) × 1 cm (width) pieces and soaked in water for an hour to ensure sufficient wetting. The alternating current (AC) impedance was measured from 1 MHz to 0.1 Hz with a modulation amplitude of 5 mV, at a bias of 100 mV. The in-plane proton conductivity was calculated from Eq. (4):
$$\sigma \left(\text{S}/\text{c}\text{m}\right)= \frac{1}{\rho } =\frac{{L}_{V2-V1}}{R\times W\times T}$$
4
where σ (S/cm) is the proton conductivity of the membrane, ρ (Ω cm) is resistivity of the membrane, Lv2−v1 (cm) is the distance between inner voltage probes (0.425 cm), R (Ω) is the resistance of the membrane, W (cm) is the width of the membrane in cm, and T (cm) is the thickness of the membrane.
In addition, temperature-dependent measurements were performed to simultaneously calculate the activation energy using the proton conductivity measurements [28]. The activation energy of proton conductivity was calculated by fitting the Arrhenius equation (Eq. (5)).
$$\text{ln}\sigma =\text{ln}A- \frac{{E}_{a}}{RT}$$
5
Here, A is a pre-exponential factor, T is the temperature (K), and R is the ideal gas constant.
The oxidative stability of the ZL/Nafion membranes was evaluated using Fenton’s test [12]. The ZL/Nafion membranes were immersed in Fenton’s reagent and H2O2 solution of 3 wt% containing 4 ppm FeCl2. The tests were conducted at 80°C for 48 h, with Fenton’s reagent replaced with a fresh solution every 24 h. The weight loss of the membrane degraded using Fenton's reagent was measured every 24 h. The weight losses of the membranes were measured using Eq. (6).
$$Weight loss \left(\%\right)= \frac{{m}_{b}-{m}_{a}}{{m}_{b}}\times 100\%$$
6
Here, mb and ma are the dry weights of the membranes before and after treatment with Fenton’s solution, respectively.
The surface morphologies of the membranes before and after the oxidative stability tests were observed using field-emission scanning electron microscopy (SEM, JEM-2100F, Japan), and the elemental compositions of the membrane surfaces were determined using energy-dispersive X-ray spectroscopy (EDS).