Overpotential local

Unsuitable position of the reference electrode resulting in inclusion of a high ohmic potential drop between reference and working electrode. Moreover, when extended surfaces are used over which the mass transfer boundary layer thickness depends on position, a suitable number of independent reference electrodes should be used to measure local overpotentials on electrically isolated segments of the working electrode. 

S0l., So2 and SHlo refer to the respective source terms owing to the ORR, e is the electrolyte phase potential, cGl is the oxygen concentration and cHlo is the water vapor concentration, Ke is the proton conductivity duly modified w.r.t. to the actual electrolyte volume fraction, Dsa is the oxygen diffusivity and is the vapor diffusivity. The details about the DNS model for pore-scale description of species and charge transport in the CL microstructure along with its capability of discerning the compositional influence on the CL performance as well as local overpotential and reaction current distributions are furnished in our work.25 27,67... 

As a result of the transferred species, loss mechanisms occur. In terms of the first law of thermodynamics these losses are well known as polarisation losses. Polarisation losses are sensitively influenced by numerous mechanisms, which are strongly non-linear with respect to a change of the operational parameters like the current density, electrical potentials, temperature, pressure, gas compositions and material properties. These parameters are assumed to be constant in case of a differential cell area. Thus, the loss mechanisms are summarised in a constant area specific resistance ASR [ 2cm2]. A change of the local overpotential (EN(Uf) — Vceii) at constant ASR complies with a proportional change in the local current density. 

Surface to volume fraction of catalyst atoms Total catalyst utilization Local overpotential Total overpotential of CCL Wetting angle of water on CCL pore walls Surface coverage of COacovered fraction of active sites... 

The peak of S at the electrolyte interface corresponds to the peak of local overpotential there (Figure 23.3). This suggests that the secondary electrochemical reactions, which may run in the CL, also peak at x = 0. This idea can be applied to model the dynamics of CL degradation, as discussed in the next section. 

The use of three-dimensional electrodes requires that the microkinetic polarization curve of the main reaction, sketched in Fig. 2, shows a potential range of width Arj within this the current density approaches the limiting current density. Thus, the optimal bed depth in the direction of current flow is introduced as a new important design parameter, for which the whole bed is working under limiting current conditions. This means that at each point the local overpotential lies within the Arj range, and the full limiting current density is realized. 

Figure 2.12 (a) Polarization curves of the anode catalyst layer for cja = 10 and indicated values of = Wa/w. Sohd lines Eq. (2.132), and dashed lines—numerical calculations for the general case. At fji > 0.1, analjdical and numerical curves are indistinguishable. Numerical curve for V = 10 is emphasized in red. (b) Numerical polarization curves of the catalyst layer for Wo = 0.1 and indicated values of Dashed hne analjdical solution (2.141) for 1. Local overpotential and proton current density in the points indicated by filled circles are shown in Figures 2.13 and 2.14. 

Once j" is obtained, the total local overpotential in the nth cell i) is calculated according to... 

The local overpotential t](x) is a measure of electrode departure from equilibrium in an REV at position x in the porous electrode. By definition, rj is given as... 

Absolute values of potentials in metal and electrolyte phases do not matter besides, they cannot be measured. For the determination of catalyst layer local overpotentials, it only matters by how much the local values of and deviate from their equilibrium values. 

The main function of a fuel cell electrode is to convert a chemical flux of reactants into fluxes of charged particles, or vice versa, at the electrochemical interface. Electrochemical kinetics relates the local interfacial current density j to the local interfacial potential drop between metal and electrolyte phases, illustrated in Figure 1.8. A deviation of the potential drop from equilibrium corresponds to a local overpotential q at the interface, which is the driving force for the interfacial reaction. The reaction rate depends on overpotential, concentrations of active species, and temperature. For the remainder of this section, it is assumed that the metal electrode material is an ideal catalyst, that is, it does not undergo chemical transformation and serves as a sink or source of electrons. The basic question of electrochemical kinetics is how does the rate of interfacial electron transfer depend on the metal phase potential ... 

If the current density is small, that is in the linear regime of the Butler-Volmer equation, the shape of the proton current and of the local overpotential is, in general. 

FIGURE 4.2 A schematic of a generic catalyst layer. The through-plane shapes of the ionic current density j, the electron current density y e, the feed molecules concentration c, and the local overpotential jj. 

The proton current density j decreases from jo at the membrane/CL interface, to zero at the CL/GDL interface. The electron current density je grows in this direction, from zero at x = 0 to jo at x = Icl- The electrochemical conversion is driven by the local overpotential r], which increases toward the membrane (Figure 4.2). Feed molecules (oxygen) are consumed in the reaction, and their molar concentration c decreases toward the membrane. Their flux is given by a transport equation, usually a diffusion equation. 

In Chapters 4 and 5, a positively defined ORR overpotential is assumed. Thus the local overpotential r] (jc) corresponds to the difference of electrolyte and metal phase potentials. This convention reverses the definition used in Chapters 1 and 3. Owing to the constancy of the metal phase potential, which is the default case, changes in r] (x) exactly match changes in the electrolyte phase potential, that is, A4> (x) = (x). This makes the redefinition of the overpotential a convenient choice in performance... 

The shape of the local overpotential follows from Equation 4.53 ... 

This equation is a conservation law, relating the local concentrations Cox and Cmt to the local overpotential fjox- Setting in Equation 4.213 x = 0 gives... 

Figure 4.33a shows the MOR rate through the ACL thickness for the 1M methanol concentration and the current densities of 100,200, and 300 mA cm . Figure 4.33b depicts the respective shapes of the proton current density and of the local overpotential r]. Parameters for these plots result from fitting of the polarization curves corresponding to 70°C (Table 4.4). 

FIGURE 5.19 (a) The impedance spectra of the CCL in the general case of mixed oxygen and proton transport losses. Indicated is the dimensionless cell current density Jq. The other parameters are listed in the last column of Table 5.6. (b) The oxygen concentration (solid lines) and the local overpotential (dashed lines) through the CCL for the same currents. 

DMFC cathodes is ignored (this effect will be considered in the section Impedance of DMFC Cathode ). Figure 5.19b depicts the respective shapes of the oxygen concentration and local overpotential through the CCL. 

Dimensionless parameter. Equation 4.72 Local overpotential in the catalyst layer (V)... 

To illustrate the detailed nature of results from such a model. Figure 11.9 shows the distribution of local overpotential in the pore of an internally... 

Figure 11.9 Distribution of local overpotential at the pore wall of an internally reforming anode, with fuel gas containing 33% CH4,66% H2O, balance CO and H2, at 0.2 Ajcm [51].

Then it is not difficult to show that substitution of equations (5.75) and (5.79) in the last formula gives again equation (5.83). This demonstrates the consistency of the theoretical expressions for the local concentration c(p), the local overpotential 7j(p) and the quasi-stationary nucleation rate / (/>) in the vicinity of a growing hemispherical cluster. 

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