Sulfonated Poly(ether ether ketone) Membranes for Direct Methanol Fuel Cells

The SPEEK samples were obtained by sulfonating poly(ether ether ketone) (PEEK450 PF, Victrex) at room temperature.¹⁷ 15 g of PEEK were dissolved in 750 mL of concentrated sulfuric acid (95.9%, Fisher Scientific), vigorously stirred at room temperature for the desired amount of time (15-135 h), and gradually transferred into a large volume of ice-cold water under mechanical agitation. After settling for several hours, the polymer precipitate formed was filtered, washed thoroughly with distilled water until the pH was neutral, and dried at 100°C overnight. The degree of sulfonation was calculated from the ion exchange capacity (IEC) of SPEEK. The IEC was determined by suspending around 0.5 g of SPEEK in 30 mL of 2 M NaCl solution for 24 h to liberate the ions and then titrating with 0.1 M NaOH solution using phenolphthalein indicator. The membranes were obtained by casting onto a glass plate a solution of the SPEEK polymer w/v) and drying at 95°C overnight. The thickness of the membrane was controlled by changing the amount of SPEEK in the solution and all the membranes in this study had a thickness of

The percent liquid uptake was determined from the weight gain found on equilibrating the dry membrane (dried at 100°C for 24 h) in distilled water or methanol solution at different temperatures for 24 h followed by blotting carefully with a filter paper to remove the surface water droplets before weighing. Proton conductivity values were obtained from the impedance data collected with an HP 4192A LF impedance analyzer in the frequency range of 5 Hz to 13 MHz with an applied voltage of 10 mV after equilibrating the membranes with water vapor at each relative humidity (RH).

The electrodes for single-cell testing in DMFCs consisted of gas-diffusion and catalyst layers and the details of the electrode preparation are available elsewhere.²¹ The anode catalyst was a commercial 40% Pt-Ru (1:1)/Vulcan (E-TEK) while the cathode catalyst (40% Pt) was prepared in situ on high surface area Vulcan XC-72R support by reducing chloroplatinic acid with 0.5 M sodium formate at around 70°C. The electrodes thus prepared were impregnated with Nafion solution (5 wt % solution, DuPont Fluoro-products) by a spray technique and dried at 90°C under vacuum for 30 min. The loadings for cathode (Pt) and anode (Pt-Ru) were 3 and respectively, and the Nafion loading for both the anode and cathode catalysts was

The membrane-electrode assemblies (MEAs) were fabricated by uniaxially hot-pressing the anode and cathode onto a SPEEK membrane at 100°C for 2 min. For comparison, a MEA consisting of precleaned Nafion 115 as the electrolyte was also prepared by hot-pressing at 130°C for 2 min. The electrochemical performance of the MEAs in DMFCs was evaluated with a commercial fuel cell test system (Compucell GT, Electrochem. Inc.) using a single-cell fixture having an active area of and feeding a preheated (60°C) 2 M methanol solution into the anode at a flow rate of 2.5 mL/min by a peristaltic pump without back pressurization and humidified oxygen into the cathode at a flow rate of 600 mL/min with a back pressure of 40 psi.

Methanol crossover was evaluated by a qualitative method²² in which 2 M methanol solution was fed at a flow rate of 2.5 mL/min into the anode side of the MEA while the cathode side was kept in an inert humidified atmosphere. By applying a positive potential at the cathode side, the methanol permeation flux through the membrane could be calculated by measuring the transport-controlled limiting current of the methanol electro-oxidation process at the Pt/membrane interface.

Table I summarizes the IEC, degree of sulfonation (DS), proton conductivity (σ) at 80°C and 100% RH, and percent water uptake at 25 and 80°C for various sulfonation times. For comparison, the corresponding values for Nafion 115 membrane are also given in Table I. The IEC, DS, σ, and water uptake of the SPEEK membranes increase with increasing sulfonation time as expected. At higher degrees of sulfonation, the membranes swell too much and dissolve in water, which limits the level of sulfonation to around 50% for the membranes to be useful for fuel cell applications. Therefore, membranes with sulfonation levels of 44, 46, 54, and 58% were selected for further investigation and these membranes are designated hereafter as, respectively, SPEEK-44, SPEEK-46, SPEEK-54, and SPEEK-58.

Table I. 

Ion exchange capacity (IEC), degree of sulfonation (DS), proton conductivity (σ), and water uptake of SPEEK membranes obtained with different sulfonation reaction times.
Sulfonation time (h)	IEC(mequiv/g)	DS(%)	Water uptake (%)a		σ at 80°C and 100% RH(S/cm)
			25°C	80°C
15	0.98	31	-	-	-
25	1.23	39	-	-	-
35	1.36	44	1.4	8.6	0.0011
40	1.42	46	2.4	22.8	0.0018
45	1.62	54	5.1	140	0.0032
50	1.74	58	5.3	509.5	0.0070
65	1.92	65	19.9	Dissolved	-
85	1.95	67	17.4	Dissolved	-
115	2.06	71	18.3	Dissolved	-
135	2.09	72	25.1	Dissolved	-
Nafion 115b	0.91	-	18.0	26.7	0.014
a Membranes could not be prepared for low sulfonation levels due to the limited solubility of the membranes in the -dimethylacetamide solvent.
b For comparison, the data for Nafion are included.

Table II compares the percent liquid uptake at different temperatures and methanol concentrations for Nafion 115 and the SPEEK membranes with various levels of sulfonation. The liquid uptake and swelling generally increase with increasing temperature and methanol concentration, but the temperature effect is more profound. The membranes with high levels of sulfonation (SPEEK-54 and SPEEK-58) experience huge swelling at 80°C and, therefore, their use is limited to In contrast, the Nafion membranes experience huge swelling at much higher temperatures of around 140°C.¹³ The conductivities of the SPEEK membranes were significantly lower than that of Nafion at low RH, but increased rapidly with increasing RH. At 80°C and 100% RH, the conductivity of the SPEEK membranes increased with increasing sulfonation level, and the value of SPEEK-58 was half that of Nafion (Table I).

Figure 1 compares the performances of the SPEEK membranes and Nafion in DMFCs at 65 and 80°C. The polarization loss of the SPEEK membranes decreases with increasing degree of sulfonation, and SPEEK-54 exhibits better performance than Nafion 115 despite a lower proton conductivity. The better performance of SPEEK-54 is due to lower methanol crossover as indicated by a lower methanol crossover limiting current density in Fig. 2. Although the thickness of the SPEEK membranes is less than that of Nafion their limiting crossover current density is less than half that of Nafion, indicating a much lower methanol permeability. A stronger confinement of the water/methanol in the narrow channels of the aromatic polymers appears to result in a significantly lower water/methanol permeation.¹⁹

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Although all the SPEEK membranes given in Fig. 1 operated continuously for at least two days at 65°C, both SPEEK-46 and SPEEK-54 failed within a few hours on increasing the temperature to 80°C and a scission was observed at the line of overlap with the electrodes. This finding is similar to that reported for SPEEK membranes in PEMFCs at 100°C.¹⁵ However, no failure was observed for SPEEK-44 at 80°C for two days, but its performance was inferior due to a low level of sulfonation.

The oxidation of the crossed-over methanol at the cathode will produce protons and the migration of protons from the cathode to the anode via the membrane may carry some methanol back from the cathode to the anode (electro-osmotic drag of fluid by the protonic current). This leads to a lower measured value of the limiting current density at the cathode by the voltammetric method, and therefore, it must to be corrected to obtain the actual methanol crossover flux. According to the mathematical model developed by Ren et al.^{22 23} for Nafion membranes, the actual methanol crossover current density at open circuit is around 30% higher than the measured value at 65 or 80°C with 2 M methanol solution fed into the anode. The percent liquid uptake in 2 M methanol solution for SPEEK membranes at 65°C is lower or similar to that of Nafion (Table II), and under these conditions, the SPEEK membranes are known to have lower electro-osmotic drag coefficients than Nafion.^{13 20} Therefore, the difference between the measured and actual limiting crossover current density values should be smaller in SPEEK membranes compared to that with the Nafion membranes. This implies that the difference between the actual methanol crossover current densities of SPEEK and Nafion may be even larger than that seen in Fig. 2. However, the percent water uptake for SPEEK-54 at 80°C is so large (Table II), and it is possible that both the methanol concentration flux from the anode to cathode and the electro-osmotic drag flux in the opposite direction may be large, resulting in a smaller measured limiting current value in Fig. 2.

The electrochemical performances of SPEEK membranes with different degrees of sulfonation have been investigated in DMFCs. SPEEK membranes with a degree of sulfonation of around 50% exhibit performances comparable to or exceeding that of Nafion due to lower methanol crossover, but the operating temperature must to be limited to Membranes with high degrees of sulfonation experience huge swelling at higher temperatures (80°C) and undergo failure. The lower cost and methanol crossover compared to that of Nafion make the SPEEK membranes promising alternatives for DMFCs. However, further work is needed to fully assess the long-term stability in the DMFC environment. The performance of the SPEEK membranes could be improved further by blending with other polymers such as polysulfone²⁴ or by incorporating hydrated inorganic oxides such as ¹⁶ which may help to suppress the swelling behavior and increase the operating temperature.

This work was supported by the Welch Foundation Grant F-1254 and the Office of Naval Research through the Electric Ship Research and Development Consortium.

The University of Texas at Austin assisted in meeting the publication costs of this article.