Transfer current density

The foremost objective is to obtain the highest reactivity or transfer current density of desired electrochemical reactions with a minimum amount of Pt-based catalysf (DoE target for 2010 0.29 g Pf per kilowatt). This demands a huge electrocatalytically active surface area small kinetic barriers to the transport of protons, electrons, and reactant gases and proper handling of product water and waste heat. 

The source term in the charge equation is used to describe the transfer current between the electronic and electrolyte phases inside of each anode and cathode catalyst layer. The transfer current densities are expressed as follows... 

The dependence of the charge transfer overpotential rjet on the charge transfer current density jct can be described in semilogarithmic form... 

Tafel plot — Figure 1. Calculated charge transfer current density j as a function of - overpotential r assuming a transfer coefficient a = 0.5 and an exchange current density jo = 0.1 A cm2, and, in simplified... 

Fig. 9 gives an example of how to determine the charge transfer current density. The plotted lines were evaluated according to the least squares method. The data were taken from the stationary potentiostatic current-potential curves at five different rotating electrode disk speeds. 

To investigate the effect of reactant transport on the electron-transfer current density, we can express Eqn (2.28) as Eqn (2.61) ... 

Faraday s constant, 96 487 C mol species diffusion/permeation flux, mol m current density, A cm superficial current density, A cm transfer current density, Acm" ... 

The ion-transfer rate is equal to the electrochemical reaction rate at the electrodes (which is the source term, or transfer current density in Eq. 2). According to Faraday s law, the flux or species i in phase k and rate of reaction h is related to the current as... 

From Fig. 1 and the above equations, one can see that the reaction distribution will depend on the various transport and reaction phenomena. For limiting cases, if one of the reactants is limiting in terms of transport or concentration, then the reaction or transfer current-density profile will be exponential towards the place with the incoming limiting reactant. If both are equally limiting, one obtains a parabolic reaction profile. If the reaction itself is limiting, then one obtains a uniform reaction profile. 

The solution of these equations gives the potential distribution in the electrodes and in the electrolyte. The reaction terms couple the electrolyte and electrode potentials through the reaction kinetics, which are described by Arrhenius expressions for both forward and backward reactions at one electrode surface for a one-electron charge transfer reaction. These terms become a Butler-Volmer expression by introducing the contribution of the electric potential difference at the electrode surface to the activation energy. This results in the following expression for the local charge transfer current density in the electrode [142] ... 

The charge transfer current density is related to the reaction term through Faraday s law ... 

The hydrogen evolution transfer current density is proportional to the oxygen evolution transfer current density. The electrode volume fraction filled with electrolyte can be evaluated as... 

In Equations 7.17a and 7.18a, charged transfer current densities 4 and are given on the basis of Butler-Volmer charge transfer kinetics described in Chapter 5 as follows ... 

Ramadass et al. [20] developed a capacity-fade model and represented the loss of active Hthium during charge-discharge cycling as due to a continuous SEI film formation over the surface of the negative electrode. No transport limitation for the solvent in the SEI was considered. A Butler-Volmer type kinetics was used to describe the side reaction kinetics. Briefly, for the negative electrode, an additional component, due to the side reaction for the SEI formation, was added to the anode transfer current density, ja. namely ja = Ja,electrode + Ja.side reaction. The contribution to the electrode overpotential is reflected by an additional ohmic drop due to the film formation, that is, [20] ... 

Linear sweep voltammetry (LSV) in combination with a rotating disk electrode (RDE) is a widely used technique to study electrode kinetics. Different methods exist to extract the values of the process parameters from polarization curves. The Koutecky-Levich graphical method is frequently used to determine the mass transfer parameters (Diard et al., 1996) the slope of a plot of the inverse of the limiting current versus the inverse of the square root of the rotation speed of the rotating disk electrode is proportional to the diffusion coefficient. If more than one diffusing species is present, this method provides the mean diffusion coefficient of all species. The charge transfer current density is determined from the inverse of the intercept. In practical situations, however, the experimental observation of a limiting current... 

The relationship between the surface overpotential and transfer current density is given by the Butler-Volmer equation. For the anode (from Equation 3-16) ... 

The velocity and pressure fields for the gas mixtures are solved first in the coupled gas channel-gas diffuser domains disregarding the changes in composition of the gas mixtures. This enables one to solve the flow and pressure fields for the gas mixtures first, and once these fields are found, the equations for the other dependent variables may be solved. The gas species concentrations are dependent on the transfer current densities therefore, the transport equations for the gas components are solved iteratively, together with the Butler-Volmer equations for anode and cathode catalyst layers. After convergence is achieved, one proceeds to solve for the transport equations related to the liquid water flow, for the membrane phase potential and current densities. Because the source terms of the energy equations are functions of the current density, a new level of iterations is needed, except for the velocity and pressure fields of the gas mixtures. 

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