CL polarization curve

Solution of Eq. (23.9) leads to the following general equation for the CL polarization curve [11] ... 

Functional models discussed below predict the distribution of local currents, reaction rate and species concentration in the CL. The distributions of local values are of great theoretical interest however, these distributions cannot be directly measured, at least with the present level of experimental techniques. Measurable quantities are the total current converted in the CL and the respective voltage loss (CL polarization curve). 

The primary goal of functional models is to derive the CL polarization curve. In some cases, this curve can be found without solving a problem for parameter distribution (Section 2.3). Comparison of theoretical and experimental polarization curves helps us to understand the regime of CL operation. 

The goal of DFT modeling is to understand the chain of elementary reaction events in the electrochemical conversion and to calculate the rate constants for these steps. The reaction mechanism and the rate constants, obtained from DFT, are then used to establish and parameterize time-dependent mass balance equations for the adsorbed/desorbed species. The steady-state solution of the surface coverage equations provides the conversion function, which can be used in the simplified current conservation equation in the CL model. The solution of the CL performance model yields the CL polarization curve, which can be used in the fuel cell or stack model. The chain of information transfer looks schematically like... 

Of foremost interest is the dependence of rjo on jo, the CL polarization curve. This dependence shows how much of the cell open-circuit potential should be spent for the electrochemical conversion of jo from ionic into electronic form, or vice versa. Huge efforts of CL designers and modelers have been directed toward lowering rjo. 

Thus, under the conditions of the section The Case of e < 1 and eJo < L the CL polarization curve is linear. Equation 4.113 in the dimensional form reads... 

The polarization curve, which takes into account the oxygen transport loss in the GDL, results from the substitution of Equation 5.41 into the CL polarization curves obtained in the previous sections. Consider first the simplest Tafel equation. Equation... 

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

Figure 26. Electrochemical performance in terms of polarization curve for the reconstructed CL structure.

CLs), resulting in a drastic drop in cell performance [17], Figure 3.13 also shows the difference between the theoretical cell potential (1.23 V) and the thermoneutral voltage (1.4 V), which represents the energy loss under reversible conditions (the reversible loss) [18], Very often, polarization curves are converted to power density versus current density plots by multiplying the cell voltage by the current density at each point of the curve. 

In the circuit, Rs is the electrolyte resistance, CPE indicates the double-layer capacitance, Rc, is the methanol oxidation charge-transfer resistance, while R1 and Cl are the mass transfer related resistance and capacitance (mainly due to methanol adsorption or CO coverage). The physical expression of these parameters can be deduced from the reaction kinetics. In the methanol oxidation reaction, the overall charge transfer rate is the sum of each charge-transfer step (rct). The Faradaic resistance (Rj) equals the inverse of the DC polarization curve slope ... 

Figure 11.25. The polarization curve. The corroding iron is used as an electrode, a variable voltage is applied (against an auxiliary electrode), and the current density is measured as cl function of the electrode potential. The measured current density is composed of the cathodic and anodic current density, i, = At the open circuit,...

Figure 2.3 Morse potential energy curves for the neutral and negative-ion states of CC14. The new quantity illustrated in this figure is photodetachment energy. It is larger than AEa and is the peak in the photodetachment spectmm. Thermal electron attachment is exothermic, that is, EDEA = a positive quantity. Two other states dissociating to Cl + CC13(—) and the polarization curve are not shown.

The analysis of polarization curves by the NOLI method [34] is helpful edso in determining the variations of the electrochemical parameters when R, is held constant and the corrosion current density is increased. This situation is of great practical importance because, for a large cl ss of electrochemical systems, the corrosion rate varies considerably, while the V2tlue of the resistance R, remains practically constant. 

Figure 6.13 Polarization curves of Inconel 690 in 0.5 M H2SO4+IOOO ppm Cl, in the absence (a) and in the presence (b) of 1 ppm dissolved H2S. Sweep rate is 10 mV/min. [8].

The total cell current is equal to the ionic current in the electrolyte, Jq. The voltage required to perform complete conversion of jo from ionic into electronic form is t]o (Figure 23.1) (see [3] for more details). The polarization curve of the CL is the equation r]o jo) and the task of modeling is to rationalize this dependence. 

The polarization curve of the CL with the optimal shape of catalyst loading is compared in Figure 2.10 to those curves for the CL with uniform... 

To understand the performance of the optimally loaded CL we note first that the cell current jo appears in the polarization curve (2.97) in the combination ejo = joija, where... 

Comparison of the cathode polarization curves between DNS predictions and experimental observations for the PEFC CL (Source Ref. 49, reproduced with permission from the Electrochemical Society.)... 

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