Effect of Crosslinking on the Properties of QPVA/PDDA Anion Exchange Membranes for Fuel Cells Application

Poly (vinyl alcohol) (PVA, M_w = Mw 89,000–98,000 g mol⁻¹, 99% hydrolyzed) was obtained from Sigma-Aldrich. Poly(diallyldimethylammonium chloride) (PDDA, M_w = 400,000–500,000, 20 wt.% in H₂O) was purchased from Sigma-Aldrich. Glycidyltrimethylammonium chloride (GTMAC, ≥90%) was supplied by Sigma-Aldrich. Glutaraldehyde (GA, 25 wt.% in H₂O) was acquired from Sigma-Aldrich. Potassium hydroxide (KOH, Titrisol®) was acquired from Merck. Ultra-pure water with a resistivity of 18 MΩ-cm was produced with the Barnstead E-PURE 4-Module system.

QPVA was prepared according to Ref. 30. First, 5 g PVA was dissolved in 95 ml distilled water to obtain a homogeneous viscous solution. Then 10 ml of KOH solution (2 M) and 10 g GTMAC were added to the viscous solution under continuous stirring for 4 h at 65 °C for the quaternization of PVA chains. The resulting viscous mixture was washed with anhydrous ethanol to obtain the precipitate and dried at 40 °C for 24 h. Finally, the white QPVA particles were obtained. Figure 1 shows the quaternization reaction of PVA.

Zoom In Zoom Out Reset image size

AEMs were prepared using a solution casting method. QPVA was dissolved in ultra-pure water at 80 °C–90 °C while continuously stirring to acquire 10 wt.% QPVA. Then 10 wt.% PDDA was mixed with the QPVA, in a weight ratio of 1:0.5 with continuous stirring for 4 h. The resulting solution was cast using an Elcometer 4340 automatic film applicator with an opening casting knife of 50 μm and then dried for 24 h at 40 °C. Subsequently, the dried membranes were peeled from the glass substrate and stored in a sealed compartment. The membranes were annealed at 130 °C for one hour to initiate thermal crosslinking, followed by immersion in crosslinking solutions of varying concentrations to induce chemical crosslinking. Different concentrations of the crosslinker solutions were obtained by diluting commercial GA 25 wt.% (in H₂O) with acetone and adding a small quantity of HCl as a catalyst. To observe the change in membrane dimension due to the crosslinking treatment, the degree of swelling through-plane (SDt_CL) and in-plane (SDi_CL) after crosslinking was calculated using Eqs. 1 and 2, respectively.

$$\begin{eqnarray}&&{{\rm{SDt}}}_{CL}=\displaystyle \frac{{{\rm{T}}}_{2}-{{\rm{T}}}_{1}}{{T}_{1}}\times 100 \% \end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{SDi}}}_{CL}=\displaystyle \frac{{{\rm{A}}}_{2}-{{\rm{A}}}_{1}}{{{\rm{A}}}_{1}}\times 100 \% \end{eqnarray} \tag{ 2 }$$

where T₁ and A₁ are the thickness and surface area of the membrane before crosslinking. T₂ and A₂ are the thickness and surface area of the membrane after crosslinking. Table I shows the content of the GA-based crosslinker solution. Figure 2 illustrates the procedure of QPVA/PDDA AEMs preparation.

Zoom In Zoom Out Reset image size

The Fourier transform infrared spectroscopy (FTIR) analysis was performed with an IR-spectrometer (Bruker ALPHA, USA) to identify the functional structure of the AEMs. The IR-spectra were recorded in the wavenumber range 500−4000 cm⁻¹ with a resolution of 4 cm⁻¹. The surface morphology of membranes was imaged using an SEM measurement (Zeiss Supra 55VP) with a voltage of 15 kV. The thermal stability of AEMs was assessed by thermogravimetric analysis (TGA) using STA 449 C apparatus (Netsch). The TGA analysis was conducted at a heating rate of 10 °C min⁻¹ from 25 °C to 600 °C under an N₂ gas flow of 40 ml min⁻¹. The mechanical properties of the AEMs were measured using a ZwickRoell Z010 universal testing machine. The samples with a size of 10 × 20 mm were elongated at the strain rate of 5 mm min⁻¹ at ambient conditions.

The water uptake (WU), swelling degree through-plane (SDt) and the swelling degree in-plane (SDi) of the membranes were calculated by measuring the change in weight and volume of the AEMs before and after water immersion, respectively. The membrane samples were weighted (W_d). Furthermore, their thicknesses (T_d) and surface areas (A_d) were measured. The samples were then immersed in ultrapure water at ambient conditions for 24 h. The weight (W_w), thickness (T_w), and surface area (A_w) of the swelling membranes were measured immediately after the gentle elimination of the surface water. The WU, SDt, and SDi of the membranes were calculated using Eqs. 3–5, respectively.

$$\begin{eqnarray}&&{\rm{WU}}=\displaystyle \frac{{{\rm{W}}}_{{\rm{w}}}-{{\rm{W}}}_{{\rm{d}}}}{{{\rm{W}}}_{{\rm{d}}}}\times 100 \% \end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&{\rm{SDt}}=\displaystyle \frac{{{\rm{T}}}_{{\rm{w}}}-{{\rm{T}}}_{{\rm{d}}}}{{{\rm{T}}}_{{\rm{d}}}}\times 100 \% \end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\rm{SDi}}=\displaystyle \frac{{{\rm{A}}}_{{\rm{w}}}-{{\rm{A}}}_{{\rm{d}}}}{{{\rm{A}}}_{{\rm{d}}}}\times 100 \% \end{eqnarray} \tag{ 5 }$$

The IEC was evaluated using a back-titration method. First, membrane samples were weighted (W_d). Furthermore, an alkalization was performed by immersing the membrane samples in 1.0 M KOH solution for 24 h to convert the Cl⁻ into the OH⁻ form of the AEMs. The membrane was then further immersed in ultra-pure water for 24 h to wash out the remaining KOH residue. The membrane was then equilibrated with a 0.1 M HCl solution (C_HCl) for 24 h. The titration was performed with 0.1 M NaOH titrant solution. The consumed volumes of NaOH from the blank (V_b) and AEMs(V_m) samples were recorded. The IEC value was determined by Eq. 5:

$$\begin{eqnarray}&&{\rm{IEC}}=\displaystyle \frac{\left({{\rm{V}}}_{{\rm{b}}}-{{\rm{V}}}_{{\rm{m}}}\right).{{\rm{C}}}_{{\rm{HCl}}}}{{{\rm{w}}}_{{\rm{d}}}}\end{eqnarray} \tag{ 5 }$$

The hydroxide conductivity was determined by the method of electrochemical impedance spectroscopy. The AEMs samples were alkalized in a 1.0 M KOH solution for 24 h and then immersed in ultra-pure water for 24 h to remove excess KOH. The 2.5 × 1.0 cm samples were placed in the four-electrode configuration of the conductivity clamp (Bekktech BT110 lLC, Scribner Associates, USA) in ultra-pure water at room temperature. The impedance was measured using Gamry Reference 600 potentiostat (USA) over a frequency range of 0.1 Hz–10 kHz with a 50 mV amplitude. The membrane resistance (R_m) was obtained from the intercept of the Nyquist curve with the real axis Z_real. The OH⁻ conductivity was calculated according to Eq. 6.

$$\begin{eqnarray}&&\sigma =\displaystyle \frac{{\rm{d}}}{{{\rm{R}}}_{{\rm{m}}}.{\rm{T}}.{\rm{W}}}\end{eqnarray} \tag{ 6 }$$

Where d is the membrane length between electrodes, T is the thickness, and W is the width of the membrane.

Figure 3 depicts the image and surface morphologies of the QPVA/PDDA anion exchange membrane. The appearance of the transparent membrane can be seen in Fig. 3a. From the SEM images (Fig. 3b), it can be observed that the QPVA/PDDA AEMs shows a smooth, uniform and defect-free surface, thus indicating that QPVA and PDDA are compatible each other.

Zoom In Zoom Out Reset image size

FTIR analysis was performed to identify the most important functional groups in the structures of the AEMs. This analysis is beneficial to confirm the change in the structural composition of PVA after quaternization and crosslinking reaction of PVA polymer membranes. ⁴² Figure 4A depicts the typical FTIR spectra of pristine PVA and QPVA. The main peaks of PVA were observed at 3270 cm⁻¹, 2906 cm⁻¹, 1418 cm⁻¹, 1324 cm⁻¹, 1088 cm^−1, and 842 cm⁻¹. These peaks are recognized as the -OH stretching vibration, C-H stretching vibration, CH₂ stretching vibration, C-O carbonyl stretching vibration, another -OH stretching vibration, and C-H bending vibration. On the other hand, similar peaks were found for the QPVA polymer in addition to new peaks located at 1645 cm⁻¹ and 971 cm^−1, which refer to amine and typical C-N stretching vibration, respectively. ^43,44 In addition, physically, pristine PVA that dissolves in hot water becomes more stable after crosslinking with GA solution resulting in swelling properties less than 31% (Table II). This indicates that quaternary ammonium groups are successfully attached to the polymer chain of the PVA.

Zoom In Zoom Out Reset image size

Table II. Physicochemical parameters of QPVA/PDDA membranes.

Cross-linker	IEC (mmol·g−1)	WU (%)	SD-t (%)	SD-I (%)	TS (MPa)	Eb (%)	$\sigma$ (mS·cm−1)
—	—	—	—	—	26.56	269.40	—
GA 2.5	2.01	38.0	12.4	20.5	36.25	6.55	20.78
GA 5.0	2.19	42.9	14.8	20.7	39.10	6.60	28.47
GA 7.5	2.37	60.1	18.5	21.5	44.89	13.80	42.17
GA 10.0	2.13	72.7	27.5	21.7	45.57	23.00	50.77
GA 12.5	2.12	90.6	30.3	22.2	46.21	18.05	53.09

Figure 4B represents the FTIR spectra of the QPVAPDDA membrane without and with GA 7.5% treatment. The main peaks of non-cross-linked QPVAPDDA were detected at 3290 cm⁻¹, 2923 cm⁻¹, 1653 cm⁻¹, 1412 cm⁻¹, 1328 cm⁻¹, 1090 cm⁻¹, 989 cm⁻¹ and 842 cm⁻¹. These peaks are identified as the –OH stretching, C–H stretching, Amine, CH₂ stretching, C–O stretching, another –OH stretching, C–N stretching, and C-H bending vibration, respectively. In comparison to non-cross-linked QPVAPDDA, similar peaks were observed for the crosslinked QPVA polymer. The decreasing of –OH groups intensity at 3284 cm⁻¹ and the appearance of a small peak at 1134 cm⁻¹ in accordance with C–O–C groups indicate the chemical crosslinking reaction between the –H groups of PVA and the –CHO groups of GA was successfully formed. ^30,40,45

The thermal behavior of QPVA/PDDA AEMs with different concentrations of GA is described in Fig. 5. It can be observed that all thermogravigrams display a similar trend with three major weight loss stages. The first stage emerges between 25 °C and 65 °C, with the total mass loss about 5%, indicates the discharge of water molecules in the membranes and moisture absorbed from the atmosphere. The second stage occurs between 200 °C and 300 °C. The total mass loss at this stage in TGA was about 12%. This can be due potentially to the degradation of quaternary ammonium cationic groups, the cleavage of crosslinking bonds, and the breaking of some C–O and C–C bonds from QPVA and PDDA, respectively. ^42,44,46 The third stage arises between 310 °C and 470 °C with a total mass loss about 69% and is related to the degradation of QPVA and PDDA polymer backbones. ⁴⁶ It can be observed that the introduction of different concentrations of GA as crosslinking did not significantly affect the thermal stability of QPVA/PDDA AEMs. Furthermore, according to the thermogravimetric analysis results, QPVA/PDDA AEMs possess sufficient thermal stability for the application in low temperature fuel cells.

Zoom In Zoom Out Reset image size

An important feature that must be taken into account when preparing the membrane electrode assembly is the mechanical property of the AEMs. Sufficient mechanical strength is required for the membrane to withstand the pressing in the assembly process and the operation of fuel cells. The mechanical properties of QPVA/PDDA membranes with different concentrations of GA crosslinker have been evaluated with respect to tensile strength and elongation at break. The results are depicted in Fig. 6, and the data in Table II. The membrane without crosslinking showed a tensile strength of 26.56 MPa and an elongation at break of 269.40%. With the introduction of GA crosslinking from 2.5 wt.% to 12.5 wt.%, the tensile strength of QPVA/PDDA membranes improved from 36.25 MPa to 46.21 MPa. It was concluded that GA crosslinking could enhance the tensile strength of QPVA/PDDA membranes. Interestingly, with the addition of GA 2.5 wt.%, the elongation at break of membrane decreased sharply from 269.40% to 6.55%. Subsequently, on an increase of the GA from 2.5 wt.% to 12.5 wt.%, the elongation at break of membrane increases from 6.55% to 23%. With a further increase of the GA concentration to 12.5 wt.%, the elongation at break of the membrane decreased to 18.05%. The improvement in tensile strength and reduction in elongation at break was attributed to the crosslinked networks formed within QPVA, which restricted the mobility of the polymer chains in membranes. ³⁹

Zoom In Zoom Out Reset image size

In order to achieve high OH⁻ conductivity, the presence of water in the AEMs is required. The increase in the conductivity of the AEMs is strongly influenced by the rise in WU. Water clusters can increase the ionic conductivity by forming transport passages for anions within the AEMs. ⁴⁰ In any case, exaggerated water uptake can cause intensive swelling of the membranes, which leads to a deterioration of the dimensional stability and possibly complicates the production of the membrane electrode assembly as the contact between the active layer, the electrodes and the AEM is reduced. ⁹ The excessive swelling might dilute the concentration of charge carriers to such an extent that the conductivity of the membrane decreases. ⁴⁷

Table III and Fig. 7 demonstrate the water uptake (WU), swelling degree through-plane (SD-t), and swelling degree in-plane of the QPVA/PDDA AEMs. The membrane with the addition GA 2.5 crosslinking showed a WU of 38%. The WU then increases slightly with the addition of GA 5.0 to 42.9%. Subsequently, through the addition of GA 7.5, GA 10.0, and GA 12.5, WU enhances quite dramatically to 60.1%, 72.7%, and 90.6%, respectively. The membrane with the addition of GA 2.5, exhibits the SDt and SDi of 12.4% and 20.5%, respectively. Eventually, SDt increases with the addition of GA 5.0, GA 7.5, GA 10.0, and GA 12.5, resulting in SDt of 14.8%, 18.5%, 27.5%, and 30.3%, respectively. On the other hand, the SDi increases only slightly to 20.7%, 21.5%, 21.7%, 22.2%, with the addition of GA 5.0, 7.5, GA 10.0, and GA 12.5, respectively. This means that swelling is more likely to occur vertically.

Zoom In Zoom Out Reset image size

The rise of WU and SD can also be explained by the change of membrane dimension during the crosslinking process (Fig. 8). After treatment with GA 2.5, the membrane has an SDt_CL and an SDi_CL of 19.2% and 2.3%, respectively. Subsequently, the SDt_CL decreases to 2.6%, −5.1%, −33.3%, −42.9% when treated with GA 5.0, GA 7.5, GA 10.0, and GA 12.5 respectively. By contrast, the SDi_CL increases to 15.1%, 15.6%, 41.7%, 66.5% when treated with GA 5.0, GA 7.5, GA 10.0, and GA 12.5, respectively. This implies that the membrane becomes wider and thinner and the surface area (bases) increases. The surface area growth (bases) induces an increased contact of the membrane with water and the rise in free volume between the polymer chains, which leads to an increased capacity of the water in the membrane. The comparison of WU and SD in this work and other previous work can be seen in Table III.

Zoom In Zoom Out Reset image size

Ion exchange capacity (IEC), an essential parameter of the AEMs, reveals information about the number of ion-exchangeable groups in the AEMs, which is relevant for the OH⁻ conductivity. ⁴¹ Table III and Fig. 9 illustrate the ion exchange capacity and OH⁻ conductivity of QPVA/PDDA AEMs at room temperature. The IEC values of AEMs are within the narrow range of 2.01–2.37 mmol·g⁻¹, which implies that the treatment with GA crosslinker has not significantly affected the IEC of the AEMs. The IEC values might be substantially influenced by the ion-conducting substance from QPVA and PDDA, which has a constant concentration and ratio for each membrane.

Zoom In Zoom Out Reset image size

The OH⁻ conductivity is one of the most critical properties of AEMs and corresponds to the performance of fuel cells. According to Fig. 9 and Table III, the membrane GA 2.5 showed the OH⁻ conductivity of 20.78 mS·cm⁻¹. Finally, the OH⁻ conductivity increases to 28.47, 42.17, 50.77, and 53.09 mS·cm⁻¹ when treated with GA 5.0, GA 7.5, GA 10.0, and GA 12.5, respectively. The increase of OH⁻ conductivity could be due to the increase in water content present in the membrane (water uptake). In accordance with the Grotthuss mechanism, water clusters are able to provide transport channels for anions within the AEMs, resulting in enhancement of the OH⁻ conductivity. ⁵¹

Figure 10 displays the temperature dependence of the OH⁻ conductivity of the QPVA/PDDA AEMs at different concentrations of GA crosslinking treatment. The OH⁻ conductivity of the AEMs reveals a positive increase with the elevation of temperature in the range of 30 °C–80 °C. The rise of temperature can also enhance the mobility of OH⁻ in the AEMs. Furthermore, the increase in temperature can also induce the expansion of free space in the membrane, which is beneficial for ion transport. ⁵² The highest OH⁻ conductivity of 82.9 mS·cm⁻¹ was achieved for the QPVA/PDDA membrane with GA 12.5 treatment at 80 °C.

Zoom In Zoom Out Reset image size

Due to the contact of the membranes with ambient air during the measurement of the conductivity of AEMs, carbonation might partially occur. This process arises in which OH⁻ as a counter-anion of quaternary ammonium functional groups at QPVA and PDDA react with CO₂ present in the air resulting in CO₃ ²⁻ and HCO³⁻ based on reactions 1 and 2, respectively. ⁵³ The foremost outcome of this carbonation process is a significant decline in the effective OH⁻ conductivity in the AEM. ⁵⁴

$$\begin{eqnarray}&&{{\rm{OH}}}^{-}+{{\rm{CO}}}_{2}\rightleftharpoons {{\rm{HCO}}}^{{\rm{3}}-}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{HCO}}}^{3-}+{{\rm{OH}}}^{-}\rightleftharpoons {{{\rm{CO}}}_{3}}^{2-}+{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 2 }$$

The presence of CO₃ ²⁻ and HCO³⁻ have a negative effect by reducing the effective conductivity of the membrane. This phenomenon occurs due to the larger ionic radius of these anions than OH⁻, which leads to lower diffusion coefficients and mobility. ⁵³ This carbonation process reduces the OH⁻ conductivity, but comparing the OH⁻ conductivity with the published results of PVA-based AEMs (Table III), the conductivity is still relatively higher, indicating the potential of this AEM.

The issue in membrane stability, especially alkaline stability, is identified as the main obstacle affecting the performance of fuel cells. ⁵⁵ The primary cause for the poor stability of AEMFCs performance is the continuous chemical degradation that occurs in the anion conducting polymer due to the alkaline environment of the fuel cell during the operation. Under the harsh pH environment of AEMFCs, the molecular structure of the anion conducting polymers (both in the membranes and ionomers) breaks down due to high reactivity of the OH⁻ with the quaternary ammonium functional groups. This degradation, which leads to an unfavorable decrease in the ion exchange capacity of AEM, results in a reduction of OH⁻ conductivity, an increase in cell resistance and an abrupt decline in performance. ⁵⁶

In this work, the alkaline stability of the QPVA/PDDA AEMs with GA 7.5 crosslinking process was investigated by immersing the membranes in 5.0 KOH solutions at room temperature for a specific time. Finally, the OH− conductivity of the AEMs was measured after repeated washing with ultrapure water at 30 °C to completely remove the excess KOH on the surface. Figure 11 shows that the OH⁻ conductivity of the QPVA/PDDA membrane rises for the first 24 h, which might be caused by the increased ion-conducting units as a part of the KOH enters the membrane at high concentrations of KOH in an alkaline environment. ⁴⁴ Figure 11 shows that the OH⁻ conductivity of the QPVA/PDDA membrane rises for the first 24 h, which might be caused by the increased ion-conducting units as a part of the KOH enters the membrane at high concentrations of KOH in an alkaline environment. ⁴⁴ This could also be due to the fact that during activation with 1 M KOH before the initial conductivity test, not all of the counter ion Cl– in AEMs was exchanged for OH–. When immersed for 24 h with 5 M KOH, the remaining Cl– was exchanged with OH– due to the higher concentration than 1 M KOH solution, so that the conductivity increased. These results are also similar to those reported by Zhou et al. ⁴⁴

Zoom In Zoom Out Reset image size

The OH⁻ conductivity then declines for the next 48 h. Subsequently, the OH⁻ conductivity decreases slightly with time. The decreases of OH⁻ conductivity result from the well-known E2 Hoffman degradation and S_N2 nucleophilic substitution reaction, which lead to the decomposition of some functional quaternary ammonium groups. ^57,58 After treatment in the solution for 180 h, the OH⁻ conductivity of the AEMs remains at 49.51 mS·cm⁻¹, showing that QPVA/PDDA AEMs have good stability. The thermal and chemical crosslinking modification possibly accounts for the improvement of OH⁻ conductivity stability. ²⁷

However, it should be noted that at pH 5, the hydration level is still considered relatively high (λ = 11). ⁵⁹ When operating AEMFCs, low water content around the cathode causes the formation of hydroxides with low hydration levels. Lower hydration levels lead to increased OH⁻ nucleophilicity, which increases the kinetics of degradation, even for ionomers, which are considered stable highly alkaline solutions. ^56,60 Therefore, further studies are needed for the application of this membrane in fuel cells under real conditions.