Ohmic resistance/losses determination techniques

In Chapter 1, Figure 1.4 shows a typical polarization curve of a PEM fuel cell. The voltage loss of a cell is determined by its OCV, electrode kinetics, ohmic resistance (dominated by the membrane resistance), and mass transfer property. In experiments, the OCV can be measured directly. If the ohmic resistance (Rm). kinetic resistance (Rt, also known as charge transfer resistance), and mass transfer resistance (Rmt) are known, the fuel cell performance is easily simulated. As described in Chapter 3, electrochemical impedance spectroscopy (EIS) has been introduced as a powerfiil diagnostic technique to obtain these resistances. By using the equivalent circuit shown in Figure 3.3, Rm, Rt, and R t can be simulated based on EIS data. 

Ohmic losses consist of contact, ionic, and electronic resistance losses. There are many methods for quantifying the total ohmic losses in a cell (contact and current flow resistances). The two main methods are the current-interrupt and HFR methods. The current-interrupt method is the simplest and only requires an oscilloscope and fuel cell load, but can be difficult to achieve consistent measurements. The HFR technique is precise but requires an electrical impedance analyzer. To determine the contact resistance, several methods are available. Perhaps the simplest is to measure the total ohmic resistance between the two surfaces and compare to the resistance of the individual components. The contact resistance in fuel cells is generally a strong function of compression pressure or surface oxide formation, depending on the fuel cell type. 

High-Frequency Resistance A more sophisticated approach to measurement of the ohmic losses is the HFR measurement, as discussed in the previous section. In this approach, an AC signal is superposed on the DC from the fuel cell. At very high fiequencies, the AC will render the various electrochemical double-layer capacitances to zero, and only the purely ohmic resistance will be measured. Typical frequencies high enough to successfully use this technique are > 1 kHz for PEFCs. This approach only measures the path of least resistance however, and care should be taken to properly understand results. For example, in the electrodes, the HFR will normally measure the electrical resistance only, since it is generally much less than the ionic loss in the mixed conductivity stracture. Therefore, HFR could not generally be used to determine electrode ionomer performance. 

Interparticle contact is of critical importance to the behavior of lithium batteries. Most lithium-ion electrodes contain 2 to 15 wt% conductive filler, such as carbon black, in order to maintain contact among aU the particles of active material and in order to reduce ohmic losses in the electrodes. Presently, there are few models available for predicting contact resistance, and the effect of the weight fraction of conductive filler on the overall electronic conductivity of the composite electrode must be determined experimentally. Doyle et al. [35] demonstrate how the fuU-cell-sandwich model can be used to determine what minimum value of effective electronic conductivity is needed to make solid-phase ohmic resistance negligible. Then, one need only measure the effective conductivity of the composite electrode as a function of filler content, and one need not run separate experiments on complete cells to determine the optimum filler content. Modeling techniques for predicting effective electroitic conductivities of composite electrodes are under development, and hold promise to aid in optimizing filler shape and volume fraction [85]. 

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