Novel sulfonated poly(ether ether ketone)/phosphonated polysulfone polymer blends for proton conducting membranes

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Shaikh A. Ali Center of Research Excellence in Renewable Energy and Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

S.M. Javaid Zaidi Center of Research Excellence in Renewable Energy and Chemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Khaled Mezghani Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (Received 12 January 2012; accepted 11 April 2012)

Phosphonated polysulfones in the acid form (PPSU-As) with degree of phosphonation (DP) 5 0.4, 0.75, and 0.96 were successfully synthesized and utilized for the preparation of polymer blend with sulfonated poly(ether ether ketone) (SPEEK) having a degree of sulfonation (DS) 5 75. The resulted blend membranes were characterized and investigated as new polyelectrolyte membrane for fuel cells applications. SPEEK/PPSU-A blend membranes formed ionic networks through hydrogen bonding bridges between the strong sulfonic acid groups and the amphoteric phosphonic acid groups. These ionic interactions resulted in enhanced membrane properties in terms of water swelling, methanol uptake, methanol permeability, mechanical strength, and thermal stability, without significant loss of proton conductivity. All the blend membranes were transparent to visible light with presence of microphases in the order of 10–20 nm. When compared to parent SPEEK membranes, the new SPEEK/PPSU-A blend membranes showed slightly lower methanol permeability compared to neat SPEEK membrane. Membranes with 30 wt% phosphonic acid content with DP 5 0.75 and 0.96, exhibited slightly higher proton conductivities at temperatures above 50 °C in comparison with Nafion membrane.

I. INTRODUCTION

Direct methanol fuel cell (DMFC) is an attracting technology that can provide power sources for micro and portable devices utilizing methanol as fuel without any fuel processing beforehand.1 The main technical barriers for commercialization of DMFC are slow methanol oxidation and methanol crossover through polymer electrolyte membrane (PEM). The latter results in significant decrease in cell potential and fuel efficiency due to oxidation of methanol at the cathode side.2–4 Sulfonated poly(ether ether ketone) (SPEEK) is one of the major potential PEMs for the DMFC,5–15 which due to its high thermal stability, high mechanical strength, low methanol permeability, and moderate proton conductivity. However, SPEEK membranes exhibit high swelling in a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.145 J. Mater. Res., 2012

water when reaching critical degree of sulfonation (DS) (.70%); or when reaching critical temperature (.60–80 °C), which leads to poor mechanical properties and low ion permselectivity. An extensive approach utilized for swelling reduction of SPEEK membranes is through blending with another swelling-tolerant materials such as inorganic additives7,10,16–21 and hydrophobic/hydrophilic polymers.

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