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Experimentally, there are several ways to determine the ohmic cell resistance. If the V-I curve has a substantial linear portion (in the center), the slope of this curve usually closely approximates the ASR of the cell. Only in such a linear portion of the V-I curve the ohmic resistance is dominant, and hence the determination of the ASR valid. Sometimes, a more accurate way to determine the ohmic resistance is from impedance spectroscopy. In an impedance spectrum of a fuel cell, the ohmic resistance is the real value of the impedance of the point for which the imaginary impedance is zero (Figure 2-5). As can be seen in the example, the ohmic resistance is invariant with gas concentration. The part of the impedance that is related to mass transport and kinetics, however, changes markedly with anode feed composition. Figure 2-5 Example of impedance spectrum of anode-supported SOFC operated at 850 °C (6). Rs is Ohmic resistance. Two measurements were with hydrogen/water vapor mixtures, and the other in diluted hydrogen. Finally, the electronic portions of the ohmic resistance could also be measured directly using a four-point probe or with a through-measurement. Given a certain cell design and operating temperature, the bulk material contributions to R (and hence the ASR) can also be calculated. Based on the detailed cell geometry, the length of both the ionic and electronic current paths and cross-sectional area for current conduction can be measured. Together with the resistivities of the materials used, they yield the bulk ASR. The contact resistance cannot be calculated from fundamental data, and is usually determined by difference between the measured total resistance and the computed bulk resistance. When using literature data for ASR, it is critical to verify the definition of ASR. Some researchers have defined “ASR”s to include the activation and concentration polarization as well as the ohmic polarization. Mass Transport-Related Losses: As a reactant is consumed at the electrode by electrochemical reaction, it is often diluted by the products, when finite mass transport rates limit the supply of fresh reactant and the evacuation of products. As a consequence, a concentration gradient is 2-14PDF Image | Fuel Cell Handbook (Seventh Edition)
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