Proton transport properties

This oversimplified random network model proved to be rather useful for understanding water fluxes and proton transport properties of PEMs in fuel cells. - - - It helped rationalize the percolation transition in proton conductivity upon water uptake as a continuous reorganization of the cluster network due to swelling and merging of individual clusters and the emergence of new necks linking them. ... 

The general picture is such that the majority of excess protons are located in the central part of the hydrated hydrophilic nanochannels. In this region, the water is bulklike (for not too low degrees of hydration) with local proton transport properties similar to those described for water in Section 3.1.1.1.1. Therefore, the transport properties are indeed a function of the considered length and time scales,22 225 activation enthalpies of both... 

A.N. Khallan, LA4. Sanchez, C. Kodiweera, S.G. Greenbaum, Z. Bai, T.D. Dang, Water and proton transport properties of hexafluotinated sulfonated poly(arylenethioethersulfone) copolymers for applications to proton exchange membrane fuel cells, J. Power Sources 173 (2007) 853—859. 

The system, for which proton-transfer reactions are investigated best, is very simple and complex at the same time liquid water. Numerous theoretical studies - mainly based on different types of molecular-dynamics simulations - have been published in the last decades that try to reveal the secrets behind the proton-transport properties of water. Generally, these studies make use of an excess proton which might be solvated in two different ways either as a so-called Eigen ion (or Eigen complex) H9O/ or as a so-called Zundel ion (Zundel complex) H502i In the first, the excess proton is complexed by... 

Biochemical proton-transfer reactions are different from those in liquids, since they usually occur along an ordered, very well-defined path. This path - as we have seen - may lead along a number of arranged water molecules through a cell-membrane protein. The overall structure of the surrounding biomolecule is, therefore, enormously important for the proton-transport properties. Moreover, different transport-pathways can be used to proto-nate or deprotonate the cell, despite the fact that they may have different energy profiles. 

A rational analysis of filler effects on structural, proton transport properties and electrochemical characteristics of composite perfluorosulfonic membranes for Direct Methanol Fuel Cells (DMFCs) was reported [7]. It has been observed that a proper tailoring of the surface acid-base properties of the inorganic filler for application in composite Nafion membranes allows appropriate DMFC operation at high temperatures and with reduced pressures [7]. An increase in both strength and amount of acidic surface functional groups in the fillers would enhance the water retention inside the composite membranes through an electrostatic interaction, in the presence of humidification constraints, in the same way as for the adsorption of hydroxyl ions in solution [7]. 

In general, pores swell nonuniformly, as seen in the section Water Sorption and Swelling of PEMs. As a simplification, the random network was assumed to consist of two types of pores. Nonswollen or dry pores (referred to as red pores) permit only a small residual conductance resulting from tightly bound surface water. Swollen or wet pores (referred to as blue pores) contain extra water with high bulklike conductance. Water uptake corresponds to the swelling of wet pores and to the increase of their relative fraction. In this model, proton transport in the PEM is mapped as a percolation problem, wherein randomly distributed sites represent pores of variable size and conductance. The distinction of red and blue pores accounts for variations of proton transport properties due to different water environments at the microscopic scale, as discussed in the section Water in PEMs Classification Schemes. ... 

Basically, the nanoporous water-filled medium with chargeable metal walls works like a tunable proton conductor. It could be thought of as a nanoprotonic transistor. In such a device, a nanoporous metal foam is sandwiched between two PEM slabs, acting as proton source (emitter) or sink (collector). The bias potential applied to the metal phase controls proton concentration and proton transmissive properties of the nanoporous medium. The value of cp needed to create a certain proton flux depends on surface charging properties and porous structure of the medium. Moreover, coating pore walls with an electroactive material, for example, Pt, would transform it from a tunable proton conductor into a catalytic layer with proton sinks at the interface. Owing to the intrinsically small reaction rate of the ORR, it would not significantly affect the proton transport properties. 

The remainder of this section describes a CGMD methodology used to unravel self-organization phenomena in the CL and to analyze their impact on physicochemical properties (Malek et al., 2007 Marrink et al., 2007). In particular, the focus will be on structure and distribution of ionomer. Moreover, it will explore the implications of ionomer morphology and porous structure on water distribution (wettability), Pt utihzation, and proton transport properties. Validation of the emerging structural picture by experimental data on adsorption and transport properties will be discussed briefly. 

CGMD simulations have become a viable tool in studying self-organization processes in catalyst layers of PEFCs. Stmctural parameters of interest for such studies involve composition and size distributions of Pt/C agglomerates, pore space morphology, surface wettability, as well as the structure and distribution of ionomer. The latter aspect has important implications for electrochemically active area, proton transport properties, and net electrocatalytic activity of the CL. 

---