Fuel cell performance activation polarization

A polarization curve, i.e., an 1-V curve of a fuel cell stack under certain operating conditions, is frequently used to describe fuel cell performance and determine performance decay with time. Polarization curve analysis can provide the most important kinetic parameters, such as the Tafel slope, exchange current density, cell resistance, limiting current density, and specific activity of Pt. A single fuel cell potential can be described by ... 

The performance portrait of the fuel cell is the polarization curve illustrating how much of the open-circuit potential has to be spent in order to generate a given load current. The product of the cell current density by the available cell potential gives the cell power density, that is, power generated by a unit cell active area. Understanding the contribution of every transport and kinetic process in the cell to the potential loss, is a key task of performance modeling. 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

Ultimately, the catalyst performance of a real fuel cell is of the greatest importance. The DEFC polarization curves for the two PtSn anode catalysts are tested and shown in Fig. 15.9. The characteristic data are summarized in Table 15.4. The PtSn-1 catalyst shows a strongly enhanced electron-oxidation reaction (EOR) activity and much better performance in both the activation-controlled region (low-current density region) and... 

Figure 2-1 shows that the reversible cell potential for a fuel cell consuming H2 and O2 decreases by 0.27 mV/°C under standard conditions where the reaction product is water vapor. However, as is the case in PAFC s, an increase in temperature improves cell performance because activation polarization, mass transfer polarization, and ohmic losses are reduced. 

Figure 3.3.7 Theoretical (dashed dotted) and real (solid) cell voltage (V) - current density (I) performance characteristics of a fuel cell. Overpotentials are responsible for the difference between theoretical and real performance and cause efficiency losses. They split into (i) activation polarization overpotentials at anode and cathode due to slow chemical kinetics, (ii) ohmic polarization overpotential due to ohmic voltage losses along the circuit, and (iii) concentration polarization overpotentials due to mass-transport limitations. The activation overpotentials of the cathode are typically the largest contribution to the total overvoltage.

More recently, Faubert et al. [129] studied in a more systematic fashion the effect of Tp on both the activity and the stability of FeTPP (presumably in its (t-oxo form) and CoTPP dispersed on XC-72 carbon incorporated into a gas diffusion electrode in actual fuel cells. Shown in Figure 3.72A and 3.72B are polarization curves recorded at 50 °C for FeTPP XC-72 and CoTPP XC-72, respectively, pyrolyzed at the specified temperatures, in a Nafion -based fuel cell configuration for which the performance of specimen in the range 700 < Tp < 900 °C was fairly comparable to that of 2% w/w Pt supported on high-area carbon. These same materials also displayed good stability up to about 10 h compared to samples treated at higher and lower Tp under the same conditions (see Figure 3.73A and 3.73B). Also shown for comparison are the results obtained for 2% w/w Pt dispersed in the same carbon. 

---