Proton transport in PEM

Structural diffusion is favored by conditions that enhance the stiffness of the hydrogen-bonded network between water molecules low temperatures and low acid concentration. The decrease in water content leads to an effective increase in the concentration of acid protons, which in turn suppresses the contribution of structural diffusion, as found in aqueous acidic solutions. This agrees with the finding of an enhanced contribution of vehicular transport in PEMs at low hydration. Such an observation is also supported by recent studies of molecular mechanisms of proton transport in PEMs at minimal hydration. ... 

We start with a physical theory of proton transport in PEMs as a function of... 

Studies of proton transport in PEMs or at interfaces, as well as studies of processes at the electrified interface, usually demand quantum mechanical simulations to incorporate electronic structure effects and hydrogen bond dynamics. Studies of structure formation and transport properties in heterogeneous media demand computationally efficient algorithms that enable simulations of sufficient length (>20 nm) and time... 

The value of the activation energy of proton transport in PEMs is low ( 0.1 eV), for water contents above X 3, and it is similar to the value in bulk water, as depicted in Figure 2.6. This similarity suggests that the widely studied relay-type mechanism of prototropic mobility in bulk water is relevant for PEMs above a critical water content. 

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. 

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

The value of the activation energy of proton transport in well-humidified PEMs, 0.12 suggests that the widely studied relay-type mechanism... 

Structure diffusion (i.e., the Grotthuss mechanism) of protons in bulk water requires formation and cleavage of hydrogen bonds of water molecules in the second hydration shell of the hydrated proton (see Section 3.1) therefore, any constraint to the dynamics of the water molecules will decrease the mobility of the protons. Thus, knowledge of the state or nature of the water in the membrane is critical to understanding the mechanisms of proton transfer and transport in PEMs. 

There are different ways to depict membrane operation based on proton transport in it. The oversimplified scenario is to consider the polymer as an inert porous container for the water domains, which form the active phase for proton transport. In this scenario, proton transport is primarily treated as a phenomenon in bulk water [1,8,90], perturbed to some degree by the presence of the charged pore walls, whose influence becomes increasingly important the narrower are the aqueous channels. At the moleciflar scale, transport of excess protons in liquid water is extensively studied. Expanding on this view of molecular mechanisms, straightforward geometric approaches, familiar from the theory of rigid porous media or composites [ 104,105], coifld be applied to relate the water distribution in membranes to its macroscopic transport properties. Relevant correlations between pore size distributions, pore space connectivity, pore space evolution upon water uptake and proton conductivities in PEMs were studied in [22,107]. Random network models and simpler models of the porous structure were employed. 

Kreuer etal. [21] provided an in-depth review of the basic mechanisms of transport in proton conductors. Transport of the proton can occur by two mechanisms structural diffusion and vehicular diSusion. Vehicular diffusion is the classical Einstein diffusive motion. The structural diffusion is associated with hopping of the proton along water molecules (the so-called Grotthuss mechanism). In the nanosized confined hydrophihc spaces within the membrane, both mechanisms are operative. What is important here is that the underlying mechanism of transport in PEMs changes as a function the level of hydration. Understanding the nature of these mechanisms and their dependence on the level of hydration and molecular structure is important in the development of advanced PEM materials that are more tolerant of higher temperatures and lower levels of saturation. 

In this chapter the scope of our discussion was restricted by the macrohomogeneous model of CL performance and its derivatives. The first numerical macrohomogeneous models of CCL for a PEM fuel cell were developed by Springer and Gottesfeld (1991) and by Bernard and Verbrugge (1991). These models included the diffusion equation for oxygen transport, the Tafel law for the rate of ORR and Ohm s law for the proton transport in the electrolyte phase. A similar approach was then used by Perry, Newman and Cairns (Perry et al., 1998) and by Eikerling and Kornyshev (1998) for combined numerical and analytical studies. 

The main contributions to irreversible heat loss, listed in the order of decreasing significance, are due to (i) kinetic losses in the ORR at the cathode (Qorr), including losses due to proton transport in the cathode catalyst layer, (ii) resistive losses due to proton transport in the PEM (Qpem)< (iii) losses due to mass transport by diffusion and convection in porous transport layers (Qmt), (iv) kinetic losses in the HOR at the anode (Qhor), and (v) resistive losses due to electron transport in electrode and metal wires (Qm)- Some of these losses are indicated in Figure 1.4. Energy (heat) loss terms are related to overpotentials by r)i = Qi/F, which will be discussed in the section Potentials. ... 

Recent models of proton and water transport in PEMs tend to support the notion of cylindrical pore networks. A qualitative distinction between superstructures will be made below, based on the analysis of water sorption data and evaluation of the implications of pore network reorganization upon water uptake. 

This section provides a systematic account of proton transport mechanisms in water-based PEMs, presenting studies of proton transport phenomena in systems of increasing complexity. The section on proton transport in water will explore the impact of molecular structure and dynamics of aqueous networks on the basic mechanism of proton transport. The section on proton transport at highly acid-functionalized interfaces elucidates the role of chemical structure, packing density, and fluctuational degrees of freedom of hydrated anionic surface groups on concerted mechanisms and dynamics of protons. The section on proton transport in random networks of water-filled nanopores focuses on the impact of pore geometry, the distinct roles of surface and bulk water, as well as percolation effects. 

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. ... 

It is worth mentioning that proton transport phenomena in UTCL pores resemble those in water-filled pores of PEM, discussed in the section Proton Transport in... 

Structure and water sorption characteristics of fuel cell media determine their transport properties. The dynamic properties of water determine microscopic transport mechanisms and diffusion rates of protons in PEM and CLs. Protons must be transported at sufficiently high rates, away from or toward the active Pt catalyst in anode and cathode catalyst layers, respectively. Effective rates of proton transport in nanoporous PEM and CLs result from a convolution of microscopic transport rates of protons with random network properties of aqueous pathways. Accounting for the geometry of these materials, namely, their external surface area and thickness, gives their resistances. 

Kulikovsky, A. A. 2011a. Polarization curve of a PEM fuel ceU with poor oxygen or proton transport in the cathode catalyst layer. 13, 1395-1399. 

Different polyelectrolyte multilayers have been used in the fabrication of membranes for different applications such as fuel cells or separation [16]. The multilayers have evidenced good performance in the separation of several species when they are used as separation membranes [62, 226]. The modification of separation membranes by PEMs for pervaporation or ultrafiltration applications has been widely developed to obtain a better performance of the existing media [62, 227]. Rmaile and Schlenoff [232] have designed chiral multilayers for separation of optical active compounds. Also the fabrication of membranes for selective ion separation have been developed using the LbL approach [233]. Daiko et al. [234] built PEMs optimized for proton transport in fuel cells. 

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