Dehydration of the membrane

Concerning the membrane itself, phospholipid hydration was characterized in terms of the types of water, bound and bulk, and their exchange rates, as well as direct observation of the intermolecular contacts between the phosphate headgroup and bound water via HRMAS HOESY and between lipids in mixed membranes via HRMAS NOESY.112 Significantly, Zhou and co-workers found little dehydration of the membranes even when rotation rates as high as 9 kHz were used, providing some comfort that the centripetal forces of sample rotation are not changing the structure of the membrane. 

Balcom et al. developed a novel diagnostic technique suitable for a thin-film by MRI.42 The authors obtained one-dimensional water content profiles with nominal resolution of 6 p,m using a prototype resonator and double half k-space spin echo single-point imaging (SPI) technique.15 Even at room temperature, partial dehydration of the membrane in the anode side was captured clearly with high spatial resolution. 

The main disadvantage of Nafion is that the inverse micelles in the structure must contain water if the polymer is to conduct protons or sodium ions, and dehydration of the membrane at higher temperatures leads to failure. A second disadvantage, particularly serious in... 

For the following tests, the fuel utilization coefficient is set at the constant value of 0.90, the stoichiometric ratio ranges between 6 and 1.8, for a stack power from 700 W to 12 kW, With regard to the stack humidification, the inlet air is saturated at room temperature (290-300 K) to minimize the amount of energy required for the best membrane hydration and reduce the electric consumption of the humidifier and the purge frequency [4]. As a consequence, the stack temperature is controlled at about 315 K, to avoid any dehydration of the membranes. 

Another issue for PEMFCs is that of water management. Whereas, dehydration of the membrane leads to a reduction in proton conductivity, an excess of water may lead to the electrode being flooded. 

Strong electrolytes in Nation have been reported.Table 3 represents the diffusion coefficients of HCl electrolytes in Nafion. The diffusion of these electrolytes in Nafion is very rapid. For dilute electrolytes, it is one order of magnitude smaller than the selfdiffusion of water or HCl. The diffusion rate decreases with increasing concentration due to the dehydration of the membrane. 

Figure 18 shows the influence of caustic concentration upon conductivity of the membrane. The decrease of conductivity with the increase in caustic concentration is ascribed to the decrease in mobility of sodium ions caused by the dehydration of the membrane. The increase of apparent activation energy for ionic conductance along with caustic concentration as is given in Table IE reflects the existance of increasing interaction between sodium ion and the fixed ion in the membrane. 

Maggio et al. [34] studied the water transport in a fuel cell using a semi-empirical approach. They modeled the concentration overpotential effect using an empirical function between the cathode gas porosity and current density (since current density is related to water production). The effective gas porosity was assumed to decrease linearly with increasing current density. This is due to the increasing percentage of gas pores occupied by liquid water. Their results indicate that dehydration of the membrane is likely to occur on the anode side rather than the cathode side. 

The reasons for the deterioration of ceU performance can be distinguished in reversible and irreversible power loss. Inevitable irreversible performance loss is caused by carbon oxidation, platinum dissolution, and chemical attack of the membrane by radicals [7]. Reversible power loss can be caused by flooding of the cell, dehydration of the membrane electrode assembly (MEA), or change of the catalyst surface oxidation state [8]. If corrective actions are not started immediately, reversible effects lead to irreversible power loss that we define as degradation. In this chapter, we focus on the degradation of the catalyst layer due to undesired side reactions. 

For PEMFG, the water management is critical to the fuel cell performance Excess water at the positive electrode leads to flooding of the membrane dehydration of the membrane leads to the increase of ohmic resistance. In addition, the catalyst of the membrane is sensitive to carbon monoxide poisoning. In practice, pure hydrogen gas is not economical to mass produce. Thus, hydrogen gas is typically produced by steam reforming of hydrocarbons, which contains carbon monoxide. 

PEMFC functions twofold, it acts as the electrolyte that provides ionic communication between the anode and cathode, and also serves as a separator for the two reactant gases. Both optimized proton and water membrane transport properties and proper water management are crucial for efficient fuel cell operation. Dehydration of the membrane reduces proton conductivity, and excess water can flood the electrodes. Both conditions may result in poor cell performance. 

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. 

The main disadvantage of Nafion is that the inverse micelles in the structure must contain water if the polymer is to conduct protons or sodium ions, and dehydration of the membrane at higher temperatures leads to failure. A second disadvantage, particularly serious in direct methanol fuel cells, is that Nafion-type membranes tend to be permeable to the methanol, giving rise to major problems. Substantial effort has recently been put into fabricating higher temperature systems, especially for use in fuel cells. The main approaches have been to form a composite between the PFSA and... 

Severe dehydration of the membrane can result in significant high ohmic resistance. First, this is because the fuel cell ohmic over potential is mainly caused by ionic resistance in the electrolyte. Second, the conductivity of membrane, a critical factor of the ionic resistance, is a very strong function of its own water content. 

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