Cross-linking of Imidazole-grafted Sulfonated Poly(ether ether ketone) as Proton Exchange Membranes for Direct Methanol Fuel Cells†

doi:10.7503/cjcu20180084

Abstract

Abstract:

Sulfonated poly(ether ether ketone)(SPEEK) membrane is considered as a promising candidate to replace the Nafion membrane for the direct methanol fuel cell(DMFC) due to its high proton conductivity and excellent methanol resistance. However, the SPEEK always suffers the poor chemical stability. Herein, a novel imidazole functionalized cross-linked SPEEK membrane was synthesized via covalently cross-linked method. The results show that the modified SPEEK membrane exhibits greatly enhanced proton conductivity(0.14 S/cm at 25 ℃) in comparison with the Nafion 115 membrane(0.086 S/cm) by introducing the imidazole groups. Increasing the cross-linking degree of the membranes leads to a decrease in proton conductivity and a great reduction in methanol permeability due to the more compact network structure. When the cross-linking degree reaches 20%, the proton conductivity and methanol selectivity are 0.105 S/cm and 4.57×10⁵ S·s/cm³, respectively, demonstrating a good balance between proton conductor and methanol separator. Importantly, the oxidative and dimensional stabilities of cross-linked SPEEK membrane are significantly improved. A passive DMFC incorporated with the optimized Im-C-SPEEK-20 membrane delivers a peak power density of 29.7 mW/cm² at room temperature, close to that with Nafion 115(31.8 mW/cm²), suggesting a promising application potential in DMFCs.

Key words: Sulfonated poly(ether ether ketone), Proton exchange membrane, Imidazole grafting, Covalently cross-linking, Direct methanol fuel cell

CLC Number:

O631

TrendMD:

ZHU Xingye,QIAN Huidong,JIANG Jingjing,YUE Zhouying,XU Jianfeng,ZOU Zhiqing,YANG Hui. Cross-linking of Imidazole-grafted Sulfonated Poly(ether ether ketone) as Proton Exchange Membranes for Direct Methanol Fuel Cells^†[J]. Chem. J. Chinese Universities, 2018, 39(9): 2046.

Figures/Tables 7

Scheme 1 Synthetic procedure of Im-C-SPEEK-x

Fig.1 1H NMR spectrum of Im-SPEEK in DMSO-d6

Fig.2 X-Ray diffraction patterns of various polymers

Table 1 Mechanical properties and oxidative stability of different PEMs

Sample	Tensile strength/MPa	Young's modulus/MPa	Elongation at break(%)	Residual weight, R_w(%)
SPEEK78	16.1	510.4	56.0	30.1
Im-SPEEK	11.9	308.0	203.1	93.6
Im-C-SPEEK-10	14.7	582.7	87.6	99.0
Im-C-SPEEK-20	15.4	626.9	76.0	99.2
Im-C-SPEEK-30	26.1	774.4	68.7	99.5
Nafion 115	27.8	41.2	304.4	99.9

Fig.3 Water uptake(A) and swelling ratio(B) of various membranes at the temperature range from 25 ℃ to 80 ℃

Table 2 Methanol permeability, proton conductivity at 25 ℃ and the corresponding methanol selectivity of PEMs

Sample	Methanol permeability/(cm²·s^-1)	Proton conductivity/(S·cm^-1)	Selectivity/(S·s·cm^-3)
Im-SPEEK	6.01×10^-7	0.140	2.33×10⁵
Im-C-SPEEK-10	3.33×10^-7	0.116	3.48×10⁵
Im-C-SPEEK-20	2.30×10^-7	0.105	4.57×10⁵
Im-C-SPEEK-30	2.08×10^-7	0.076	3.65×10⁵
SPEEK-78	6.84×10^-7	0.059	8.62×10⁴
Nafion 115	1.80×10^-6	0.086	4.78×10⁴

Fig.4 Polarization and power density curves of the passive DMFCs with Nafion 115 and Im-C-SPEEK-20 membrane(A) and transient discharging curves of the passive DMFCs with Im-C-SPEEK-20 and Nafion 115 at a constant voltage of 350 mV fueled with 5 mL of 4 mol/L MeOH at 25 ℃(B)

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