Proton conducting membranes based on photopolymerizable monomers

Abstract

The proton exchange barrier or Proton Exchange Membrane (PEM) is the critical part of a fuel cell. The basic function of the membrane is to enable proton transport, while being simultaneously impermeable for electrons and gas. Typically, membranes for the PEM fuel cells (PEMFC) are made of perfluorocarbon-sulfonic acid monomers. The best known material of this class is Nafion which has a unique interpenetrating structure of hydrophobic perfluorocarbon regions providing thermal and chemical resistance, mechanical strength and diffusional resistance combined with hydrophilic regions of water clusters surrounding charged sulfonic acid groups which allow selective proton transport. For these reasons, Nafion is still considered the benchmark against which most of the new materials are compared [1]. At the molecular level, proton transport may follow two principal mechanisms: (a) diffusion mechanism via H3O+ ion as a carrier and (b) proton hopping mechanism (Grotthuss transport) [2]. Contemporary PEMFCs are exclusively based on the vehicle mechanism. PEMFCs produce water as a by-product and H+ ions moving from the anode to the cathode pull water molecules by an electro-osmotic drag force. In addition, membrane suffers from evaporation of water at working temperatures of 60-90ºC. Nafion effectively conducts protons only when imbibed by water within a narrow range, which limits the operating temperature of PEM fuel cells to around 80oC. However an operating temperature above 100ºC is a highly desirable goal. PEM membranes are not dimensionally stable since the material significantly swells upon water absorption. Therefore the aim of our proton conducting membrane is a rigid polymer with perpendicular nano channels which are filled with a conducting sulfonic polymer where conductivity is mainly achieved by the Grotthuss mechanism. Several monomers and crosslinker in a broad range of concentrations in water and 1-Methyl-2-pyrrolidone (NMP) respectively were screened for their mechanical properties, water uptake and conductivity in porous membranes by photo polymerization with a polar photo initiator. As conductive polymer, primarily poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS) and poly(2-sulfoethyl methacrylate) (PSEM) respectively as well as polymers of phosphonic acid containing monomers or newly synthesized monomers were used. The conductive monomers were crosslinked with varying hydrophobic and hydrophilic multifunctional monomers like N,N'-methylene bisacrylamide (MBA), 2-Propenoic acid, 2-methyl-, 1,1'-(1,10-decanediyl) ester (D3MA) or polyethyleneglycol diacrylates with two varying chainlengths (PEG-DA700, PEG-DA330). The advantage of several different building blocks with known characteristics is the possibility to tune the polymer to special needs of an application. For example, some polymer compositions have good conductivity at lower temperatures whereas other polymers develop better properties at elevated temperatures. The research leading to these results has received funding from the European Community's FP7- NMP Programme, under the Project Acronym MultiPlat and with Grant Agreement: N 228943 and the Austrian Federal Ministry of Science and Research. We thank 3M for providing us with samples of the PP membrane.

1/Hamrock, S.J. and M.A. Yandrasits, Proton Exchange Membranes for Fuel Cell Applications. 2006. 46(3): p. 219 - 244. 2/ Hoogers, G., Membranes and Ionomers, in Fuel Cell Technology Handbook G. Hoogers, Editor. 2002, CRC Press. p.