Polymer electrolyte membranes water balance

Water Flux in Polymer Electrolyte Membranes Water flux in the solid electrolyte membrane of the PEFC must be understood to grasp the concept of a local water balance in the fuel cell. From Chapter 5, we know that the ionic conductivity of perfluorosulfonic acid-based solid polymer electrolytes is a strong function of water content. Within the electrolyte, there are four basic modes of transport, as schematically illustrated in Figure 6.21 ... 

At present, polymer electrolyte membrane fuel cells and power plants based on such fuel cells are produced on commercial scale by a number of companies in many countries. As a rule, the standard battery version of the 1990s is used in these batteries, though in certain cases different ways of eliminating water and regulating the water balance (water management) have been adopted. 

The ionic transport numbers for alkaline PVA-based SPE are very important because the high anionic transport number of SPE can limit the carbonation problems [56]. The anionic transport number (f) was measured by the dynamic Hittorf s method [57]. A test cell, as shown in Fig. 11, with two Pt electrodes was made for electrolysis and the electrolysis current was imposed by a power supply. All PVA-based polymer electrolyte membranes were located and fixed at two separated compartments with the same 1 M KOH solution. The reaction occurred at the Pt-cathode, producing O2, H2 and OH while consuming water and OH . The balance of OH ions in each compartment led to OH transport number after a fixed amount of charge was passed through the polymer membranes. 

A key element in PEFC performance is the water balance. There is a complex relationship between moisture content and performance in PEFCs. For high ionic conductivity, the polymer electrolyte membrane must have high moisture content [8], However, as discussed in Chapter 5, liquid accumulation can restrict reactant availability at the electrode. 

The performance of a polymer electrolyte membrane (PEM) fuel cell is significantly affected by liquid water generated at the cathode catalyst layer (CCL) potentially causing water flooding of cathode while the ionic conductivity of PEM is directly proportional to its water content. Therefore, it is essential to maintain a delicate water balance, which requires a good understanding of the liquid water transport in the PEM fuel cells. 

This review has highlighted the important effects that should be modeled. These include two-phase flow of liquid water and gas in the fuel-cell sandwich, a robust membrane model that accounts for the different membrane transport modes, nonisothermal effects, especially in the directions perpendicular to the sandwich, and multidimensional effects such as changing gas composition along the channel, among others. For any model, a balance must be struck between the complexity required to describe the physical reality and the additional costs of such complexity. In other words, while more complex models more accurately describe the physics of the transport processes, they are more computationally costly and may have so many unknown parameters that their results are not as meaningful. Hopefully, this review has shown and broken down for the reader the vast complexities of transport within polymer-electrolyte fuel cells and the various ways they have been and can be modeled. 

Water management is one of the most critical and widely studied issues in PEFC. Water management is referred to as balancing membrane hydration with flooding avoidance. These are two conflicting needs to hydrate the polymer electrolyte and to avoid flooding in porous electrodes and GDL for reactant/ product transport. 

Solid Polymer Electrolyte The most common solid polymer electrolytes consist of a hydrophobic and inert polymer backbone which is sulfonated with hydrophilic acid clusters to provide adequate conductivity as discussed in Chapter 5. In order to ensure adequate performance, some membrane hydration is required. However, excess water in the electrodes can result in electrode flooding, so that a precarious balance must be achieved. Modern perflourosulfonated ionomer electrolytes for H2 PEFCs are 18-25 xm thick with a practical operating temperature limit of 120°C, although PEFC operation is rarely greater than 90°C due to excessive humidity requirements and operational low lifetimes. 

The membrane has two functions. First, it acts as the electrolyte that provides ionic conduction between the anode and the cathode but is an electronic insulator. Second, it serves as a separator for the two-reactant gases. Some sources claim that solid polymer membranes (e.g., sulfonated fluorocarbon acid polymer) used in PEMFC are simpler, more reliable, and easier to maintain than other membrane types. Since the only liquid is water, corrosion is minimal. Pressure balances are not critical. However, proper water management is crucial for efficient fuel cell performance [6]. The fuel cell must operate under conditions in which the by-product water does not evaporate faster than it is produced, because the membrane must be hydrated. Dehydration of the membrane reduces proton conductivity. On the other hand, excess of water can lead to flooding of the electrodes. 

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