Polarization curves, SOFCs

FIG. 24-53 Polarization curves at different temperatures for 50-cm active length thin-wall SOFCs. 

Typical polarization curves for SOFCs are shown in Fig. 24-53. As discussed earlier, the open-circuit potential of SOFCs is less than 1 V because of the high temperature, but the reaction overpotentials are small, yielding ahnost linear curves with slopes corresponding to the resistance of the components. 

Experimental validation of SOFC models has been quite scarce. Khaleel and Selman °° presented a comparison of 1-D electrochemical model calculations with experimental polarization curves for a range of... 

Fig. 7.8 Polarization curves comparison between Tubular, HPD ad Delta9 SOFCs.

In the experimental session we have obtained also the polarization curves that describe the electrochemical behaviour of the various sectors of the SOFC generator. The curves have been obtained varying the values of the overall fuel consumption in two different sequential ranges of current ... 

At small s, the Tafel region disappears and the transition region directly links the linear and double-Tafel domains (Figure 23.2b). This situation is typical of a thick solid oxide fuel cell (SOFC) anode (anode-supported design). For such an anode, k is large and e is small, which corresponds to the polarization curve in Figure 23.2b. The well-known hnearity of a SOFC polarization curve means that the anode operates below the transition region. 

The polarization curve of a solid oxide fuel cell (SOFC) is close to linear (Mogensen and Hendriksen, 2003). This fact justifies the introduction of a... 

In spite of its simplicity (or rather because of it), Eq. (4.152) raises several questions. To a good approximation, the kinetics of electrochemical reactions on both sides of the cell follow the Butler-Volmer law, which establishes exponential dependence of cell current on the respective halfcell overpotential. Why then is the resulting polarization curve linear Does this mean that the resistive losses in SOFC dominate and the contribution of activation polarizations to the overall voltage loss is small ... 

The polarization curve of the SOFC anode is, hence, equivalent to the curve of the PEFC cathode with [Pg.163]

Equation (5.17) for the BP temperature contains voltage loss rj and local current j. These values are related by the cell polarization curve. In an SOFC, this curve is well approximated by a linear function... 

The polarization curve of SOFC is linear this means that rp and J are related through rf = RJ, where R is the cell resistivity. Thus, Eq. (5.71) transforms to... 

The polarization curve for an SOFC fed with pure hydrogen can thus be expressed in the following semi-empirical form ... 

The response of the fuel cell is determined by the electrochemical processes and associated kinetics at the electrode and electrode interface. The electrochemical processes depend on the mass and charge transfer between the bulk electrolyte solution and electrode surface. The rates at which these transfers occur are determined by the number of localized phenomena and largely depend on the materials involved. These processes are presented in this chapter and the relations between the fuel cell potential and current density are given in terms of BV and Tafel equations. The key losses in the fuel cell include the activation losses, ohmic losses, mass transport losses, and losses owing to reactant crossover and internal currents that are discussed in this chapter. The fuel cell polarization curve is presented and is discussed for low-temperature and high-temperature fuel cells such as PEMFC and SOFC, respectively. 

FIGURE 8.4 Polarization curve of an SOFC (1) the experimental open circuit potential is very close to the theoretical value of about 0.977 V (at 800°C) due to a fast charge transfer reaction, (2) at the moderate current density, the curve is fairly linear due to Ohm s law, and (3) the potential drops down due to transport limitation and approaching the limiting current. 

V for H2/O2 PEMFC and down to around 0.6 V for DMFC. The PEMFC (Figure 8.3) and SOFC (Figure 8.4) polarization curves at low current densities are quite different due to significant temperature dependence of the charge transfer reaction. At elevated temperatures, the electron transfer reaction is much faster, and therefore, the charge transfer resistance is much smaller. As a result, the experimental and theoretical (0.977 V) OCPs at 800°C are very close. 

Figure 4.19 Typical polarization curves for high-temperature SOFC in different gas environments at 1000° C. The high operating temperature enables the use of low-cost catalyst materials such as nickel (anode) and strontium-doped lanthanum manganite (cathode), with very low kinetic polarization losses. (Reproduced with permission from [5].)...

The final piece of the polarization curve to be modeled is the departure from the expected OCR given by the Nemst equation. For low-temperature PEFCs, the OCV is predicted to be around 1.2 V, but in practice, only about 1.0 V is observed. For a high-temperature SOFC, however, the actual OCV can be very close to the theoretical OCV. For the PEFC, the 0.2 V represents an incredibly significant efficiency loss before any useful current is even drawn. The departure from the theoretical OCV is typically a result of two phenomena ... 

Sketch a typical high-temperature fuel cell (SOFC, MCFC) polarization curve operating at temperature A. Then, sketch a polarization curve B with reduced temperature conditions. Be sure to think about all of the effects of this change. (Ignore the effects on mass transport region until Chapter 5.)... 

Example 5.3 Ohmic Losses in SOFC as a Function of Electrolyte Thickness Plot an ohmic-only polarization curve for a SOFC with (Zr02)o.92(Y203)o,o8 electrolyte at 1000°C for a 50-, 100-, and 300- xm-thick electrolyte and an OCV of0.997 V. That is, ignore kinetic and concentration polarization losses. Assume neat hydrogen and air at 1 atm back pressure is used. 

We reported the dotted and dash performance curves illustrated in Fig. 5 for an anode-supported Ni5Al (SDC) / SDC / LSM (SDC) cell running on humidified methane and air mixtures in mixed-gas mode at 700°C with 02 C ratio of 1.5 and 1.67, respectively [6]. In the previous paper, we suggested that the decrease of cell performance for mixed-gas SOFC results from an increase in the anode polarization and is due to the low catalytic activity of the... 

The poor resolution of Nyquist plots is problematic because the individual impedance-related processes of complex electrochemical systems, such as SOFC single cells, are numerous and their contributions to the impedance curve overlap. This problem is aggravated by the fact that not every process contributes in the same way to the total polarization loss. Therefore, it is very difficult to discover processes with a small contribution, because they are almost totally covered by the processes that show a large polarization loss. 

In continuum-scale electrochemistry, the materials and microstructure of the SOFC tri-layer are embedded in the parameters of the polarization losses. The electrochemistry of the SOFC is inherently dependent on the SOFC microstructure, including the surface area available for the electrochemical reactions, the porosity, tortuosity, and permeability of the porous media, and the material properties of the tri-layer. All these properties affect the rate of reactions in the electrodes and thus the overall voltage produced by the SOFC. Although continuum-scale electrochemistry does not resolve the explicit electrochemical reactions, they are able to accurately model the performance of the SOFC when using experimental data to estimate the parameters of the electrochemistry model. Often the parameters of the continuum-scale electrochemistry model, such as the pre-exponential factors, and activation energies and polarizations are used to fit the continuum-scale electrochemistry to experimental /-V curves. 

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