Membrane unit operations fuel cell

Fig. 46 Arrhenius plot of the membrane lifetime in fuel cells obtained for the L3 and 2 meq/g SPI membranes with five repeat units in the ionic sequences and ODA as non-charged monomer (the operating times are expressed in seconds) [68]...

In Albany, NY, the state government started leasing Honda FCX hydrogen fuel cell cars on a cold November morning. Previous fuel cell vehicle demonstration programs have occurred in warmer areas to ensure that the fuel cell stacks would not freeze up. Subzero temperatures can change any liquid water present into expanding ice crystals that can puncture thin membranes or crack water lines. Honda has demonstrated that their fuel cell units can operate under winter conditions, this was an important achievement for practical fuel cell cars. 

Thousands of smaller stationary fuel cells of less than 10 kilowatts each have been built and operated to power homes and provide backup power. Polymer electrolyte membrane (PEM) fuel cells fueled with natural gas or hydrogen are the primary units for these smaller systems. 

One of the earliest proton exchange membranes was based on sulfonated polystyrene where divinylbenzene was used as a cross-linking unit for extra stability. Developed by General Electric, this membrane (21) was cheap and easy to manufacture, and it was used for fuel cells in the Gemini space pro-gram.i However, due to the sensitivity of the benzylic hydrogen to radical attack, lifetimes for these membranes under FC operating conditions were quite low. Thus, little work has been carried out on these systems since their inception. 

Under normal operation of an H2/O2 fuel cell, anodic oxidation of IT2 (or other hydrocarbons or alcoholic fuels)—that is, H2 —> 2H+ -1- 2e —produces protons that move through the polymer electrolyte membrane (PEM) to the cathode, where reduction of O2 (i.e., O2 -1- 2H+ -1- 2e —> H2O) produces water. The overall redox process is H2 -1-O2 —> H2O. The electronically insulating PEM forces electrons produced at the anode through an external electric circuit to the cathode to perform work in stationary power units, drive trains... 

Apart from hydrocarbons and gasoline, other possible fuels include hydrazine, ammonia, and methanol, to mention just a few. Fuel cells powered by direct conversion of liquid methanol have promise as a possible alternative to batteries for portable electronic devices (cf. below). These considerations already indicate that fuel cells are not stand-alone devices, but need many supporting accessories, which consume current produced by the cell and thus lower the overall electrical efficiencies. The schematic of the major components of a so-called fuel cell system is shown in Figure 22. Fuel cell systems require sophisticated control systems to provide accurate metering of the fuel and air and to exhaust the reaction products. Important operational factors include stoichiometry of the reactants, pressure balance across the separator membrane, and freedom from impurities that shorten life (i.e., poison the catalysts). Depending on the application, a power-conditioning unit may be added to convert the direct current from the fuel cell into alternating current. 

Galvanostatic discharge of a fuel cell (MRED method) provided information related to liquid water in a fuel cell in a minimally invasive manner.157 Stumper et al.158 showed that through a combination of this MRED method with a current mapping (segmented fuel cell similar to the one discussed in Stumper et al.135), it was possible to obtain the local membrane water content distribution across the cell area. The test cell was operated with a current collection plate segmented on the cathode along the reactant flow direction. In addition to the pure ohmic resistance, this experimental setup allowed the determination of the free gas volume of the unit cell (between the inlet and outlet valves). Furthermore, the total amount of liquid water presented in the anode or cathode compartment was obtained. 

A further example would be the silent watch auxiliary power unit developed by Hydrogenics for the Canadian Coyote armoured vehicle. This proton exchange membrane (PEM) fuel cell APU also uses hydrogen stored as a metal hydride. Hydrogen is produced using an on-board electrolyser powered during operation of the diesel engine. 

Springer and others were the first to use detailed, experimentally derived diffusion and electroosmotic drag coefficients of water in Nafion in a model for steady-state water profile and the resulting protonic conductivity in the membrane of an operating PEFC [87]. The distribution of water in a PEFC at steady, state (at constant current and reactant/water fluxes) was calculated in this model by considering water flow through five regions of unit cross-sectional area within the fuel cell two inlet... 

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