Fuel cell performance overpotential

Mass transport within the electrodes is of particular importance in determining the reflection of the porous media structure on the fuel cell performance. In fact, the main results of mass transport limitation is that the reactant concentrations (H2 and CO for the anode, and O2 for the cathode) at the reaction zone are lower than in the gas channel. When applying Equations (3.40) and (3.42), the result is that the lower the concentration of the reactants, the lower the calculated cell performance. The loss of voltage due to the mass transport of the gas within the electrodes is also referred to as concentration overpotential. Simplified approaches for determining concentration overpotential include the calculation of a limiting current, i.e. the maximum current obtainable due to mass transport limitation (cf. Appendix A3.2). 

From the above consideration it is clear that the exchange current density is the main factor affecting the activation overpotential, then the optimization of a PEM fuel cell performance requires the maximization of io- This can be obviously accomplished by increasing the catalyst activity, that means to raise the surface area, cell temperature, and reactant pressure (this last effect should also favor gas adsorption on catalyst sites). 

As an example of a cathode film the (Lag gSrg 2)o.9Mn03 (LSM) per-ovskite will be used for electrode preparation and evaluation. Composite symmetrical and asymmetrical (Lag gSr 2) MnOj-YSZ (LSM-YSZ) structures were prepared on dense YSZ substrate (0.4 mm thick) [21]. The symmetrical cell was used for composite LSM-YSZ and composite LSM-YSZAfSZ electrode overpotential evaluation, while asymmetrical for fuel cell performance and electrode overpotential measurement. The composite cathode was prepared similarly to the composite anode (Figure 3-18). The procedure follows ... 

Fuel cell performance of the composite LSM-YSZ/YSZ/Ni-YSZ cell was investigated using forming gas (10 vol% H2 in N2) as the fuel (Figure 3-25). The results showed that a maximum power density of about 0.26 W cm2 as obtained at a temperature of 850°C. The temperature dependence of the area specific resistances of the asymmetrical cell is shown in Figure 3-26. The electrode overpotential was estimated to 0.3 Q cm2 at 800°C, which is the total of anode and cathode overpotential. It appeared that about half of the overpotential originated from the anode, because the cathode overpotential determined from the symmetrical cell test was found to be about 0.14 Q cm2 at 800°C. The performance of the cell was mainly limited by the electrolyte resistance. The decrease in the electrolyte thickness would decrease electrolyte resistance. It can be concluded that the net shape technology can be successfully applied for the fabrication of cathode and anode electrodes. 

The exchange current density is the key property of catalyst layers. It determines the value of the overpotential needed to attain the targeted fuel cell current density. This property, thus, links fundamental electrode theory with practical aspects of fuel cell performance. The following parameterization distinguishes explicitly the effects of different structural characteristics,... 

The pores are for the transport of fuel cell reactants and product(s). Optimal porosity and pore size distribution can facilitate the mass transport process to minimize the fuel cell performance loss due to concentration overpotential. If some pores are more hydrophobic than others, what is the relative distribution Is the distribution of pore sizes and hydrophobicity within the allowable range ... 

A CFD model that describes a complete single cell was introduced by Lobato et al. [33]. The MEA was implemented as a single plane separating the anode and cathode. The model considers only the cathodic part of the overpotential from the Butler-Vohner equation and the simulation results are presented for operation with pure hydrogen and oxygen. The impact of three different flow field designs, serpentine-like, parallel, and pin-type, on the overall fuel-cell performance were investigated. The best performance was observed for serpentine-like and pin-type flow fields. It should be noted that the bad performance of the selected parallel... 

Reaction (3.15) is the adsorption of toluene on the Pt surface, which can increase the overpotential of the ORR by blocking Pt active sites, thus degrading fuel cell performance. Under PEM fuel cell operating conditions with excess oxygen, the adsorbed toluene may experience deep chemical oxidation through reaction (3.16) and produce CO2 [18], or electrochemical oxidation through reaction (3.17), thus releasing some of the Pt sites for the ORR. 

To evaluate the fuel cell performance in detail, the ohmic drop and the cathode overpotential (rj ) were determined and are shown in Fig. 16.8. The ohmic drop was measured using an in situ current interruption method during the H -O PEFC operation and rj was calculated using the equation ... 

The kinetics of the hydrogen oxidation reaction in acidic PEM fuel cells above room temperature are often so fast that they contribute a negligible voltage to the overall activation overpotential, which is therefore often assumed to be wholly attributable to the ORR [41]. This allows catalyst loadings on the anode to be as low as 0.05mgptcm without affecting overall fuel cell performance significantly [42] and means that catalyst development is mainly focused on the cathode. However, in alkaline media, the HOR on polycrystalline Pt has been... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

Fig. 23. (a) Experimental IR-free overpotentials in MCFC-based separator. Cell performance 0.25% C02 Feed. All curves calculated [32] (b) C02 production scheme using molten carbonate fuel cell stack. 

The presence of a small amount of water vapor (up to pH20/pH2 = -0.03) in fuel reduces anode overpotential. For anode-supported cells, the use of pore formers is important to tailor the shrinkage during cofiring and to create adequate porosity for better performance. The difference in cell power output could differ by as much as 100% for cells as porosity changes from -30 to -50%. 

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