Proton polymer-water interface

Proton Mobility near the Polymer-Water Interface.385... 

In this section, we describe the role of fhe specific membrane environment on proton transport. As we have already seen in previous sections, it is insufficient to consider the membrane as an inert container for water pathways. The membrane conductivity depends on the distribution of water and the coupled dynamics of wafer molecules and protons af multiple scales. In order to rationalize structural effects on proton conductivity, one needs to take into account explicit polymer-water interactions at molecular scale and phenomena at polymer-water interfaces and in wafer-filled pores at mesoscopic scale, as well as the statistical geometry and percolation effects of the phase-segregated random domains of polymer and wafer at the macroscopic scale. 

A major incentive of this article will be to stress the complicating traits of the membrane environment on effective proton transport and fuel cell performance. The polymer affects distribution and structme of water and dynamics of protons and water molecules at multiple scales. In order to describe the conductivity of the membrane, one needs to take into account explicit polymer-water interactions at molecular level, interfacial phenomena at polymer-water interfaces at mesoscopic scale and the statistical geometry and topology of randomly distributed aqueous and polymeric domains at macroscopic scale. 

Proton Transport Near the Polymer-Water Interface... 

Assuming that the size and shapes of water-filled domains are known, as well as the structure of polymer/water interfaces, proton distributions at the microscopic scale can be studied with molecular dynamics simulations (Feng and Voth, 2011 Kreuer et al., 2004 Petersen et al., 2005 Seeliger et al., 2005 Spohr, 2004 Spohr et al., 2002) or using the classical electrostatic theory of ions in electrolyte-filled pores with charged walls (Commer et al., 2002 Eikerling and Komyshev, 2001). An advanced understanding of spatial variations of proton mobility in pores warrants quantum mechanical simulations. 

At X > Xs, capillary effects control the equilibration of water with the polymer. In this regime, the values of molecular mobilities of protons and water approach the corresponding values in free bulk water, and hydrodynamic effects control transport phenomena. The PEM conductivity is described well by Equation 2.1. Highly functionalized polymer-water interfaces have a minor impact on transport mechanisms in this regime. An important consequence of this picture is that molecular-level studies of proton transport that account for details of ionomer structure are required strictly only for X <. At X >, it is sufficient to employ the well-established mechanism... 

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. 

In order to simulate proton transport in a realistic pore model that more closely resembles the structure of polymer-water interfaces, one has to resort to classical or... 

The structure of this interface determines fhe sfabilify of PEMs, the state of water, the strength of interactions in the polymer/water/ion system, the vibration modes of side chains, and the mobilities of wafer molecules and protons. The charged polymer side chains contribute elastic ("entropic") and electrostatic terms to the free energy. This complicated inferfacial region thereby largely contributes to differences in performance of membranes wifh different chemical architectures. Indeed, the picture of a "polyelectro-lyfe brush" could be more insighttul than the picture of a well-separated hydrophobic or hydrophilic domain structure in order to rationalize such differences. ... 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

The internal resistance of the polymer electrolyte membrane depends on the water content of the membrane. The water ionizes add moieties providing mobile protons, like protons in water [1-3]. Absorbed water also swells the membrane, which may affect the interface between the polymer electrolyte and the electrodes. Nafion, a Teflon/perfluorosulfonic acid copolymer, is the most popular polymer electrolyte because it is chemically robust to oxidation and strongly acidic. The electrodes are commonly Pt nanoparticles supported on a nanoporous carbon support and coated onto a microporous carbon cloth or paper. These structures provide high three-phase interface between the electrolyte/catalyst/reactant gas at both the anode and cathode. 

The photochemical and photophysical behavior of azobenzene adsorbed on polymer or silica films has been reported recently " azobenzene adsorbed in water-swollen Nafion-H+ (Nafion is a polymer consisting of a perfluorinated backbone and short pendant chains terminated by sulfonic acid groups) exhibited strong fluorescence. The irradiation resulted in the formation of benzo[c]cinnoline and benzidine in quantitative yield, while azobenzene incorporated into a methanol-swollen Nafion-H+ membrane did not emit fluorescence but merely underwent E,Z-isomerization. Thus, solvent-swollen Nafion is useful in the selective phototransformation of azobenzene. According to the authors, protonated azobenzene molecules are located at the fluorocarbon/water interface in water-swollen Nafion-H+ Z-Azobenzene formed on these membranes resulted in cychzation to give the observed products (Scheme 52). 

A photoinduced electron relay system at solid-liquid interface is constructed also by utilizing polymer pendant Ru(bpy)2 +. The irradiation of a mixture of EDTA and water-insoluble polymer complex (Ru(PSt-bpy)(bpy) +, prepared by Eq. (15)) deposited as solid phase in methanol containing MV2+ induced MV 7 formation in the liquid phase 9). The rate of MV formation was 4 pM min-1. As shown in Fig. 14, photoinduced electron transfer occurs from EDTA in the solid to MV2+ in the liquid via Ru(bpy)2 +. The protons and Pt catalyst in the liquid phase brought about H2 evolution. One hour s irradiation of the system gave 9.32 pi H2 after standing 12 h and the turnover number of the Ru complex was 7.6 under this condition. The apparent rate constant of the electron transfer from Ru(bpy)2+ in the solid phase to MV2 + in the liquid was estimated to be higher than that of the entire solution system. The photochemical reduction and oxidation products, i.e., H2 and EDTAox were thus formed separately in different phases. Photoinduced electron relay did not occur in the system where a film of polymer pendant Ru complex separates two aqueous phases of EDTA and MV2 9) (see Fig. 15c). 

---