Intrinsic exchange current density

To make a valid comparison, one should calculate the intrinsic exchange current density, which can be done using the following equation ... 

The intrinsic exchange current density is very important in fuel cells because the electroactive Pt surface area is significantly different from its geometric area, and the former changes with temperature. 

The intrinsic exchange current density,/ , is not a mere materials constant, but it depends on size distributions of catalyst nanoparticles, their surface structure, as well as surface composition in the case of alloy catalysts like PtRu. In this section, we discuss modeling approaches that highlight particle size effects and the role of surface heterogeneity in fuel cell electrocatalysis. 

Two electrochemical parameters define the electrochemical properties of an electrocatalyst material the intrinsic exchange current density fl and the Tafelparameter b. These parameters are determined from current-potential relationships measured by voltammetric methods of classical electrochemistry (Bard and Faulkner, 2000). One could write an expression for b of the form... 

Note that/ is the exchange current density per surface area of the porous electrode. It is related to the intrinsic exchange current density / of the catalyst as f = where pt is the specific catalyst surface in the electrode per cm of electrode area. 

TABLE 4.1 Measured and Simulated Exchange Current Densities for the HOR and ORR in a Nafion -112-Membrane-Based PEM Fuel Cell at 3.0 atm (Absolute), 100% RH, and Different Temperatures, in a Low Current Density Range ( 02/H20 Measured Apparent Exchange Current Density for the ORR, 1° is the Simulated Apparent Exchange Current Density for the HOR, and fo2/H20 H2/h+ Intrinsic Exchange Current Densities for the ORR and HOR, Respectively)[8]... 

The relationship between the intrinsic exchange current density and the apparent exchange current density (ioj/Hio) described using the... 

The exchange current density is a measure of the intrinsic reactivity of the electrode reaction, i.e., the rates of the anodic and cathodic partial reactions at the equilibrium potential. The empirical equation which often describes the exchange current density is... 

The main intrinsic parameter of the electrocatalytically active electrode material is the exchange current density,/. It tells us how rapid electron exchange can take place across the electrified interface under a dynamic equilibrium, with balanced rates of anodic and cathodic interfacial charge transfer, see the section Electrochemcial Kinetics. This parameter represents a reaction rate per unit of the metal surface real area. 

The effective exchange current density of the porous composite CL, given per unit of apparent (external) electrode surface area,y = J SecsaIcl = j Stotlci stat, is the key physical property of a CL. This parameterization reveals two main avenues for enhancing the value of f via improving the intrinsic electrocatalytic activity or via optimizing the structural CL design, embodied in Secsa and Icl-... 

Exchange current density per unit catalyst active surface area (intrinsic activity) (A cm )... 

Though the exchange current density is not an intrinsic property of the catalyst, it is strongly related to the catalyst type. Increasing the Jo of a catalyst/... 

In electrochemical reactions, the dominating kinetic parameter is the exchange current density io, which is a strong function of temperature and reactant concentration. The exchange current density is not an intrinsic parameter of a catalyst and is also a strong function of the morphology of the electrode structure ... 

The exchange current or current density is proportional to the standard rate constant of the redox reaction. Consequently, the slower the couple, the lower the exchange current is. It should be noted that the exchange current also depends on the concentration levels in the solution. This parameter is therefore not intrinsic to the redox couple in question, unlike the standard reaction rate constant. Numerical examples of jo and k° values for a few experimental situations are collected together in table 4.3. 

Fig. 8-16. Electron state density in a semiconductor electrode and in hjrdrated redox partides, rate constant of electron tunneling, and exchange redox current in equilibrium with a redox electron transfer reaction for which the Fermi level is close to the conduction band edge eF(sc) = Fermi level of intrinsic semiconductor at the flat band potential 1. 0 (tp.o) = exchange reaction current of electrons (holes) (hvp)) - tunneling rate constant of electrons (holes).

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