Cite this: DOI: 10.1039/c3ra41508b Impact on the ionic channels of sulfonated poly(ether ether ketone) due to the incorporation of polyphosphazene: a case study in direct methanol fuel cells Received 27th March 2013, Accepted 21st May 2013 DOI: 10.1039/c3ra41508b www.rsc.org/advances S. Gouse Peera, S. Meenakshi, K. Hari Gopi, Santoshkumar D. Bhat,* P. Sridhar and S. Pitchumani Blend membranes are fabricated from sulfonated poly(ether ether ketone) (SPEEK) and poly[bi- s(phenoxy)phosphazene] (POP). The effect of POP content on the distribution of ionic channels is investigated by atomic force microscopy (AFM). The water uptake and methanol permeability for the blend membranes are also investigated. The blend membranes are characterized in terms of their thermal and mechanical properties in conjunction with their ionic conductivity. The proton conductivity of the blend membranes slightly decreased with increasing POP content in comparison with the pristine SPEEK membrane. The hydrophobic nature of POP blocks the ionic channels in the SPEEK matrix, subsequently decreasing its water uptake and methanol permeability. The blend membranes showed higher power density compared to a pristine SPEEK membrane in direct methanol fuel cells (DMFCs). Introduction Direct methanol fuel cells are promising power sources for portable and stationary applications due to their advantages, including high efficiency, simple design and low emissions to the environment. The polymer electrolyte membrane (PEM) is an important component of DMFCs, it transfers protons from the anode to the cathode and also acts as a barrier to avoid the crossover of methanol. 1 Nafion, a perfluorosulfonic acid polymer is the state-of-the-art membrane material for DMFCs and shows high mechanical and chemical stability. 2 This polymeric membrane also has excellent ionic conductivity, long term durability and unique hydrophilic percolation. However, materials with permeable morphologies suffer from high methanol permeability and swelling in the presence of methanol during DMFC operation. Hence, for enhanced DMFC performance, membranes with low permeability to methanol, that can resist swelling are highly desirable. 3,4 Among the various sulfonated polymers studied in the literature, SPEEK has attracted considerable attention because of its good mechanical strength, high chemical stability, low cost and ease of preparation. 5 SPEEK is also resistant to methanol permeability, in comparison with Nafion in DMFCs, due to its narrow and more branched hydrophilic channels. 6 The proton conductivity, methanol permeability, water uptake and swelling ratio of SPEEK membranes are all dependent on the degree of sulfonation (DS). Although the SPEEK membrane with a low DS shows good performance in reducing perme- ability to methanol, there is an apparent reduction in ionic conductivity. It also becomes extremely difficult to process SPEEK with low DS due to its poor solubility in organic solvents. In contrast, a high DS results in excessive water uptake, increasing the swelling ratio and limiting the use of the SPEEK membrane. 7,8 The above factors may lead to a loss in the mechanical stability of the membrane making it unsuitable for long term operation in DMFCs. In order to address the above issues different routes for modification of the SPEEK polymer have been explored. One of the most important routes is to fabricate a blend of SPEEK with different hydrophobic polymers to control water uptake and methanol permeability. 9–14 Polyphosphazenes are polymers which possess a backbone of alternating phosphorus and nitrogen atoms. The properties of polyphosphazenes are mainly controlled by the choice of side groups. 15 Polyphosphazenes with various properties can be synthesized by careful choice of side groups. Polyphosphazenes have several advantages over established hydrocarbon based polymers. One of their prime attributes is the thermal and chemical stability of the polymer backbone, both phosphorus and nitrogen are in their highest oxidation states resulting in a high degree of thermo-oxidative stability. 16 The polar nature of the bonding along the polymeric backbone inhibits chemical attack by free radicals. 17 CSIR-Central Electrochemical Research Institute-Madras Unit, CSIR Madras Complex, Taramani, Chennai, 600 113, India. E-mail: sdbhatcecri@gmail.com; Fax: +91-44-22542456; Tel: +91-44-22542068 RSC Advances PAPER This journal is ß The Royal Society of Chemistry 2013 RSC Adv. Published on 23 May 2013. Downloaded by Daegu Gyeongbuk Institute of Science and Technolog on 11/07/2013 10:55:48. View Article Online View Journal