Surface Proton Conduction Why Bother

The simplest approximation to describe the operation of a PEM was discussed in the section PEM Conductivity Simply a Function of Composition It involves the following basic instructions  

As discussed, this approach works surprisingly well above a critical water content Xc. 

Efforts of polymer scientists and fuel cell developers alike are driven by one question What specific properties of the polymeric host material determine the transport properties of a PEM, especially proton conductivity The answer depends on the evaluated regime of the water content. At water content above kc, relevant structural properties are related to the porous PEM morphology, described by volumetric composition, pore size distribution and pore network connectivity. As seen in previous sections, effective parameters of interest are lEC, pKa, and the tensile modulus of polymer walls. In this regime, approaches familiar from the theory of porous media or composites (Kirkpatrick, 1973 Stauffer and Aharony, 1994), can be applied to relate the water distribution in membranes to its transport properties. Random network models and simpler models of the porous structure were employed in Eikerling et al. (1997, 2001) to study correlations between pore size distributions, pore space connectivity, pore space evolution upon water uptake, and proton conductivity, as will be discussed in the section Random Network Model of Membrane Conductivity.  

At a water content below kc, the specific molecular structure at polymer-water interfaces dictates the transport properties of PEMs. Relevant details of the molecular interfacial structure include chemical composition and length of ionomer sidechains, packing density of sidechains, and structure of the interfacial hydrogen-bonded network that forms between sulfonic acid head groups and interfacial water. At X Xc and low interfacial density of sidechains, referred to hereafter as SGs, protons will be trapped at interfaces and cannot generate a significant proton conductivity. 

However, intriguing phenomena arise if the SGs density at polymer-water interfaces is increased. In the regime of high SG density, proton transport in PEMs become similar to proton transport at acid-functionalized surfaces. Surface proton conduction phenomena are of importance to processes in biology. Yet, experimental findings of ultrafast proton transport at densely packed arrays of anionic SG have remained controversial. Theoretically, understanding of the underlying mechanisms is less advanced than for proton transport in bulk water. 

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