PEMFC limitations

For last few years, extensive studies have been carried out on proton conducting inorganic/organic hybrid membranes prepared by sol-gel process for PEMFC operating with either hydrogen or methanol as a fuel [23]. A major motivation for this intense interest on hybrid membranes is high cost, limitation in cell operation temperature, and methanol cross-... 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

For the stationary generation of heat and power the PEMFC is also in development. Fuel cell systems for combined heat and power generation mostly run on natural gas, and sometimes on biogas. Reformate is fed to the anode in these stationary systems. Only for backup power systems, which are designed for only a limited operating time, is pure hydrogen often used as fuel for the anode. 

High Temperature Operation of the PEMFC The first generation of commercial PEMFCs will use presently known components, consisting of a perfluorosulfonic acid membrane as electrolyte and catalyst compositions as cited above. The electrolyte determines that the fuel cell needs to be operated at fully humidified conditions and limits the operating temperature to 80-90 °C. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

Bipolar plates in PEMFCs were conventionally made of graphite with excellent corrosion resistance, chemical stability, and high thermal conductivity. However, graphite has a high cost, poor mechanical properties, and very little formability due to its microstructural nature. This limits its further applications as plate material and forces a search for alternative solutions. Nevertheless, the performance, durabilify, and cosf of fhe graphite plate (e.g., POCO graphite and graphite plates) have been taken as benchmark references to compare with those of alternative materials. 

The actually developed PEMFCs have a Nafion membrane, which partially fulfills these requirements, since its thermal stability is limited to 100 °C and its proton conductivity decreases strongly at higher temperatures because of its dehydration. On the other hand, it is not completely tight to liquid fuels (such as alcohols). This becomes more important as the membrane is thin (a few tens of micrometers). Furthermore, its actual cost is too high (more than 500 m ), so that its use in a PEMFC for an electric car is not cost competitive. 

Concentration overpotentials can be very high in low temperature fuel cells, where they often cause the occurrence of limiting currents (such as in PEMFCs, which operate at 80°C). However, they are less critical in SOFCs, due to the high operating temperatures (about 1000°C). 

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. 

The efficiency of fuel cells is largely limited by the kinetic barriers of the surface catalytic electrode reactions. In particular, the electroreduction of molecular oxygen at a PEMFC cathode severely limits high reaction rates and hence currents near the equilibrium cell voltage. 

Figure 6.36. Three-dimensional representation of impedance diagrams calculated for each cell of the fuel cell stack. The solid line links the low-frequency limits iRjj) of the diagrams. The large dots indicate the 100 Hz frequency for each diagram [37], (Reproduced by permission of ECS—The Electrochemical Society, and the authors, from Diard JP, Glandut N, Le-Gorrec B, Montella C. Impedance measurement of each cell of a 10 W PEMFC stack underload.)...

Electrocatalysts One of the positive features of the supported electrocatalyst is that stable particle sizes in PAFCs and PEMFCs of the order of 2-3 nm can be achieved. These particles are in contact with the electrolyte, and since mass transport of the reactants occurs by spherical diffusion of low concentrations of the fuel-cell reactants (hydrogen and oxygen) through the electrolyte to the ultrafine electrocatalyst particles, the problems connected with diffusional limiting currents are minimized. There has to be good contact between the electrocatalyst particles and the carbon support to minimize ohmic losses and between the supported electrocatalysts and the electrolyte for the proton transport to the electrocatalyst particles and for the subsequent oxygen reduction reaction. This electrolyte network, in contact with the supported electrocatalyst in the active layer of the electrodes, has to be continuous up to the interface of the active layer with the electrolyte layer to minimize ohmic losses. 

---