Modeling the State of the Water in Polymer Electrolyte Membranes

Abstract

The practical details for the construction and operation of polymer electrolyte membrane (PEM) fuel cells are now reasonably familiar topics which do not require further elaboration in this chapter. It should be evident; however, that one of the features of primary importance in the operation of such a cell is the efficient conduction of the protons through the membrane. Extensive research has gone into understanding the conduction of protons in a number of different media [1]. A typical hydrated ‘‘state-of-the-art’’ PEM is a system with two phases, one phase being the polymer backbone separated from a second phase composed of a random network of hydrated ion-conducting channels with strong internal electrical fields. Both, as a result of being present in a confined state and in an electrical field the water in the channels display properties that are strikingly different from those of bulk water. It is impossible for us to do justice to the abundance of literature available on the effects of confinement and electrical fields on water; however, in the following paragraphs we present a few examples that are particularly relevant to the subject of this chapter. Gompper, Hauser and Kornyshev [2] considered water that is restricted to the space between two hydrophobic surfaces. In the vicinity of the surfaces a drop in the coordination number around each water molecules (compared to bulk water) occurs resulting in layers with low coordination numbers. When the two walls are allowed to approach each other the layers overlap filling the space with entirely low-coordinated water. Bontha and Pintauro [3] developed a molecular level model of the partition coefficient in order to study the ability of membranes such as Nafion to absorb ionic species. Such a model requires knowledge of the difference between the electrochemical potentials of the relevant ions inside the pore and in the bulk medium, the former being strongly influenced by the electrical fields arising from the SO 3 groups which are tethered to the pore walls. The earlier work of Guzman-Garcia et al., [4] Verbrugge and Pintauro [5] andGur