Fuel cell operating conditions pressure

To determine the fraction of expanded channels, T and the channel-size distribution must be known. The channel-size distribution gives the fully expanded channel radii and is taken to be the same for different operating conditions and the same as the distribution measured for a liquid-equilibrated membrane. The reasons that this distribution is assumed to be constant are that it should not vary significantly with pressure or temperature xmder typical fuel-cell operating conditions and is used only when there is a separate liquid-water phase. This assumption has been used and proved valid within error tolerances [13, 18, 57]. The pore-size distribution for Nafion has been measured by the method of standard contact porosimetry [29, 58, 59]. In those studies, the distribution included both the channels and the clusters. Since only the channel-size distribution is of interest, only that regime of data is fit using the log-normal distribution [39]. The average channel radius is around 1.5 nm as expected from the physical model and other studies [23, 60, 61). 

While comparing the in situ and ex situ postmortem investigations, it is evident that the contact pressure cycling has an enormous impact on the mechanical conditions of the MEAs and the defects seem to be only slightly increased by fuel cell operation conditions. In both cases, extensive defects like 400 pm wide cracks through all layers of the MEA could be detected (see Fig. 17.22) [21,23],... 

Fig. 14.14 Effect of membrane thickness and methanol feed concentration on the maximum power density from MEAs composed of Nafion 117 and 50/50 Nafion/FEP (50- and 100-p.m thickness). Fuel cell operating conditions 60°C, 2 ml min" methanol feed flow rate, ambient pressure air at 500 seem. (From [18], reproduced by permission of The Electrochemical Society, Inc.)...

Fig. 14.18 Effect of PBI content on the DMFC performance of Nafion-PBI membrane samples prepared from fully protonated Nafion powder, with 3, 5 and 7% PBI. T = 60°C, ambient pressure air at 500 seem, 1.0 M methanol feed. Crossover (denoted as cross.) is expressed as a relative fraction of the methanol flux observed for a Nafion 117 membrane at the fuel cell operation conditions. R denotes the membrane areal resistance. (Reprinted from [31], with permission from Elsevier.)...

Fig. 16 Performance curves of hydrogen/air m-PBI/PA fuel cells at different temperatures and 1 atm (absolute) pressure. Note that the fuel cell operating conditions are as follows constant flow rate, H2 at 400 SCCM, air at 1300 SCCM, no humidification, 44 cm active area, 1.0 mgcm" Pt catalyst loading, Pt - C 30% on each electrode...

Liu et al. synthesized fully aromatic poly(ether ketone)s based on the monomer p-biphenyl-hydroquinone. Then they prepared site-selective sulfonated biphenyl-ated poly(ether ether ketone)s (BiPh-SPEEKDKs) by mild and rapid sulfonation reactions. MEA fabrication and fuel cell operating conditions were conducted according to Los Alamos National Laboratory protocols. The cell was preconditioned under H2/air fuel cell operation at 0.7 V for 2 h. Pt-Ru black was used for anode and Pt black for cathode and 5% commercially available Nafion dispersion. The catalyst ink was painted onto the membrane until the catalyst loading reached 8 mg cm for anode and 6 mg cm for cathode. Active area is 5 cm. The cell was held at 80°C 1 and 2 M aqueous methanol solution was fed to the anode with a flow rate of 1.8 mL min- 90°C humidified air was fed at 500 seem without back pressure. The current density of BiPh-SPEEKDK at 0.5 V at 80°C in 1 and 2 M methanol reached... 

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

Under fuel cell operation, a finite proton current density, 0, and the associated electro-osmotic drag effect will further affect the distribution and fluxes of water in the PEM. After relaxation to steady-state operation, mechanical equilibrium prevails locally to fix the water distribution, while chemical equilibrium is rescinded by the finite flux of water across the membrane surfaces. External conditions defined by temperature, vapor pressures, total gas pressures, and proton current density are sufficient to determine the stationary distribution and the flux of water. 

The analysis of the conditions within a gas channel can also be assumed to be onedimensional given that the changes in properties in the direction transverse to the streamwise direction are relatively small in comparison to the changes in the stream-wise direction. In this section, we examine the transport in a fixed cross-sectional area gas channel. The principle conserved quantities needed in fuel cell performance modeling are energy and mass. A dynamic equation for the conservation of momentum is not often of interest given the relatively low pressure drops seen in fuel cell operation, and the relatively slow fluid dynamics employed. Hence, momentum, if of interest, is normally given by a quasi-steady model,... 

A numerical model to simulate the lattice expansion behavior of the doped lanthanum chromites under a cell operating condition has been proposed, and the deformation of the lanthanum chromite interconnectors has been calculated [33], In the model, the sample deformation is calculated from the profile of the oxygen vacancy concentration in the interconnector. Under a practical cell operation, the oxygen vacancy concentration in the interconnector distributes unevenly from the air side to the fuel side. The distribution of the oxygen vacancy concentration in the interconnector depends on both the temperature distribution in the interconnector and the profile of the oxygen partial pressure at the interconnector surface. Here, a numerical model calculation for the expansion behavior of the LaCrC>3 interconnector under a practical cell operation is carried out, and the uneven distribution of... 

The equilibrium of a hydrogen or carbon monoxide fuel cell operating at high temperature and pressure is defined using a flow sheet, which connects the cell to a fuel store at standard conditions, and to the environment, via combined isentropic and isothermal circulators and a Carnot cycle. 

The function of the polymeric membrane electrolyte is to permit the transfer of protons produced in anodic semi-reaction (3.11) from anode to cathode, where they react with reduced oxygen to give water. This process is of course essential for fuel cell operation, as it allows the electric circuit to be closed inside the cell. On the other hand, the membrane must also hinder the mixing between fuel and oxidant, and exhibit chemical and mechanical properties compatible with operative conditions of the fuel cell (temperature, pressures, and humidity). 

In the case of a fuel cell operating under reversible conditions, at constant pressure and temperature, the efficiency is ... 

The ability of the fuel cell to operate under a wide range of conditions with different system characteristics is described by the term fuel cell operational flexibility . Optimum fuel cell operational flexibility must take into account both specified conditions and an estimated amount of unexpected, or out-of-specification , conditions over the fuel cell target lifetime. Some of the conditions to consider include reactant composition and flow rates, operating and environmental temperature and pressure, humidification levels, peak load... 

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