Capacitance-ohmic method

To increase fundamental knowledge about ionic resistance, it is important to develop a methodology to experimentally isolate the contributions of the various cell components. Electrochemical impedance spectroscopy has been widely used by Pickup s research group to study the capacitance and ion conductivity of fuel cell catalyst layers [24-27] they performed impedance experiments under a nitrogen atmosphere, which simplified the impedance response of the electrode. Saab et al. [28] also presented a method to extract ohmic resistance, CL electrolyte resistance, and double-layer capacitance from impedance spectra using both the H2/02 and H2/N2 feed gases. In this section, we will focus on the work by Pickup et al. on using EIS to obtain ionic conductivity information from operational catalyst layers. 

Under industrial conditions the moisture present in material must be determined by faster methods, such as by electrical methods of which three main varieties have become widespread moisture determination based on the change of the ohmic dc resistance, a measurement of the electrostatic capacitance (dielectric constant of the material), and a measurement of the loss in an ac field. Other quick methods are the chemical methods developed mainly for the most frequently occurring case, when the moisture is water, such as the Karl-Fischer analysis based on the chemical reaction of iodine in the presence of water [3], the distillation method, in which moisture is determined by distillation with toluene, and the extraction method, which is carried out with absolute ethanol. 

Capacitance measurements have become an important method in electrochemistry. Combinations of resistance and capacitance elements, the so-called equivalent circuits, describe the electrochemical properties of the double layer. In the case of an ideally polarizable electrode, the equivalent circuit is a linear combination of a double layer capacitance and an Ohmic electrolyte resistance (Figure 4.9a). The equivalent circuit of an... 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

The gavanostatic transient method can be used to measure the double layer capacity and the ohmic drop in the electrolyte between the working and reference electrodes. Figure 5.14 shows the initial variation of the potential resulting from a current step. During this period, diffusion is of little importance because its time constant is usually much larger. The electrode acts like a capacitor connected in parallel with a resistance, where the capacitor represents the double layer capacity and the parallel resistance corresponds to the transfer resistance of the electrode reaction (Section 3.5). The applied current I is the sum of the capacitive current Iq given by (5.94) and the faradaic current I-p described by the Butler-Volmer equation ... 

The spontaneous electric polarization Ps of a thermotropic ferroelectric liquid crystal is usually measured with the so-called triangular wave method [16]. For this method, a triangular voltage U is applied to the liquid crystal and the so induced current in the liquid crystal is measured. This current can be divided into several contributions. If measuring a thermotropic SmC liquid crystal, the main contributors to the total current /ei. are the ohmic current, caused by the resistance / ei. of the liquid crystal, the capacitive current, due to the capacity C of the cell, and the polarization reversal current, which originates from the spontaneous electric polarization Ps of the polar liquid crystal. 

Two experimental methods to characterize the catalyst layer are described in the literature [24] the H2/O2 method in which pure hydrogen and oxygen and nonzero DC current are used, and the H2/N2 method in which pure nitrogen is used on the cathode and zero DC current. These two methods have been chosen because the catalyst layer physics simplify to cases in which parameters such as ohmic resistance, CL electrolyte resistance, and double-layer capacitance can be readily extracted from impedance data [24]. 

Various correction methods have been tried—primarily adding parasitic coil capacitance values to the model. An intuitive, but the most efficient correction was a simple approach, where the coil was first measured in the air (without any metal plate nearby). The difference of the impedance, measured at every single measurement frequency, from the ideal coil impedance = jwL with a zero value real part (ohmic resistance), was found and fixed... 

Strictly speaking, the measuring result contains not only ohmic quantities, but also some undesired capacitive contribution. Certain traditional methods can... 

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