92 The Polymer Society of Korea www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673 Macromolecular Research, Vol. 22, No. 1, pp 92-98 (2014) Sulfonation of PIM-1 - Towards Highly Oxygen Permeable Binders for Fuel Cell Application Byoung Gak Kim 1,2 , Dirk Henkensmeier* ,1 , Hyoung-Juhn Kim 1 , Jong Hyun Jang 1 , Suk Woo Nam 1 , and Tae-Hoon Lim 1 1 Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea 2 Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea Received July 8, 2013; Revised August 1, 2013; Accepted August 1, 2013 Abstract: The development of alternative, non-fluorinated membranes for polymer electrolyte membrane fuel cells necessitates the co-development of a non-fluorinated electrode catalyst binder to ensure compatibility between membrane and electrode. However, most hydrocarbon based polymers have lower gas permeability than perfluori- nated Nafion. In this work we tried to obtain a sulfonated, non-fluorinated binder based on PIM-1 (polymer of intrin- sic microporosity 1) which has up to 2000 times higher permeability than Nafion. However, sulfonation was not straightforward and often led to degradative side reactions. Sulfonated polymers were too brittle to give stable mem- branes and the highest experimental IEC was 1.03 meq/g, significantly lower than the theoretical IEC of 3.2 meq/g (2 sulfonic acid groups per repeat unit). Keywords: sulfonation, electrode binder, PIM, polymer of intrinsic microporosity, polymer electrolyte fuel cell (PEFC). Introduction The most important part of a fuel cell is the membrane electrode assembly (MEA). In polymer electrolyte fuel cells (PEFCs), the MEA consists of a polymeric electrolyte mem- brane, which is sandwiched between the anode and the cathode. The state-of-the-art membrane material is Nafion, a perflu- orinated sulfonic acid polymer. The electrode catalyst layer usually consists of a platinum/carbon catalyst, embedded in a Nafion binder. In order to get high performance, the metal catalyst should be in good electric contact, should be in con- tact with the proton conducting phase (via the electrolyte) and the reactant gas at the anode and oxygen at the cathode. While a high availability of protons can be obtained by fully covering the catalyst particle with the polyelectrolyte binder, it requires that the gases have to diffuse through this layer (Figure 1). Because of certain drawbacks of Nafion, there is signifi- cant research activity on the development of alternative membranes. 1 Some of the most promising materials are hydro- carbon based membranes, e.g. sulfonated polysulfones, sul- fonated poly(ether ether ketone) (SPEEK), or sulfonated polyimides. 2,3 However, only few publications deal with alternative binder materials. This is astonishing, since the chemical difference between perfluorinated Nafion as binder and hydrocarbon based membranes can lead to MEA delamina- tion and an increased charge transfer resistance. Solutions to this problem might be the chemical modification of the membrane surface by the introduction of perfluorinated graft chains 4 or direct fluorination. 5 The most elegant way would be the introduction of a novel binder material. Recently, there have been publications on the use of hydrocarbon based polymer electrolytes as binder material. However, the presented MEAs show a relatively poor performance and several authors concluded that the gas permeability of the binder was not sufficient. 6-9 To solve this problem, we were looking for a material showing a chemical similarity to hydrocarbon based mem- branes (good lamination), good proton conductivity (sulfonic acid groups) and good mechanical stability. We concluded that a sulfonated derivative of PIM could be an alternative to Nafion as the binder in pure hydrocarbon based MEAs. PIM-1 (polymer of intrinsic porosity 1) was presented by DOI 10.1007/s13233-014-2007-z *Corresponding Author. E-mail: henkensmeier@kist.re.kr Figure 1. Schematic of a PEFC cathode.