Limiting current density oxygen transport

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Figure 24 Schematic polarization data for oxygen reduction reaction (ORR) in neutral (pH 7.2) solution. Diffusion-limited current density (i L) is present due to mass transport limitations on dissolved oxygen.

Figure 16 shows the steady-state limiting current density, ilim, for the oxygen reduction reaction (ORR) on pure Al, pure Cu, and an intermetallic compound phase in Al alloy 2024-T3 whose stoichiometry is Al20Cu2(Mn,Fe)3 after exposure to a sulfate-chloride solution for 2 hours (43). The steady-state values for the Cu-bearing materials match the predictions of the Levich equation, while those for Al do not. Reactions that are controlled by mass transport in the solution phase should be independent of electrode material type. Clearly, this is not the case for Al, which suggests that some other process is rate controlling. 

The mass-transport-limited current density for oxygen reduction is independent of the kinetic parameters for this reaction rather it depends on factors such as the concentration and the diffusion coefficient of oxygen in the medium. It depends oh the rate of flow of the liquid in a pipe or around a sailing ship or a structure immersed in a river. 

If the cathodic reaction (oxygen reduction) takes place at the limiting current, then the resistance at the electrode-solution interface of the cathode, / pjj (Figure 7.4), behaves as a non-linear electrical element that limits the current (current limiter). Indeed, at the limiting current plateau one has = (dE/dT)n = oo. The anode corrosion rate Vj depends, in this case, only on the value of limiting current density at the cathode, t lji that is determined by prevailing mass transport conditions. Neglecting the currents 1 i and 4 n, we find ... 

On the other hand, if the transport rate of an oxidizing agent is rate limiting, equation (7.12) is valid. In the derivation of this equation, we assumed that the cathodic partial current at the less noble electrode is negligible, which is not always true. Let us assume that oxygen is reduced at both electrodes at the limiting current. In this case, we must take into account the cathodic partial current at each electrode. If the limiting current densities are identical i i = = 1 02 ). die corrosion rate of... 

If the oxygen mass transport rather than Butler-Volmer kinetics limits the rate of the cathodic reaction at fiprot, the protection current density corresponds to the limiting current density of oxygen i Q2 ... 

In this limit, the resistivity resulting from oxygen transport in the channel is zero and, hence, the last term in this equation is the resistivity because of the oxygen diffusion in the GDL. As it should be, this resistivity tends to infinity, as the cell current density J approaches the limiting current density... 

Resistive limiting current density (A cm ). Equation 4.221 Methanol-limiting current density (A cm ). Equation 5.222 Limiting current density due to oxygen transport in the GDL at the channel inlet (mol cm ). Equation 4.210 Liquid water flux density (Acm )... 

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