Concentration polarization mass transfer limitation

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. 

Zone III the E vs ln(l — j/ji ) logarithmic curve corresponds to concentration polarization, which results from the limiting value ji of the mass transfer limiting current density for the reactive species and reaction products to and/or from the electrode active sites an increase inji from 1.4 to 2.2 Acm leads to a further... 

Under working conditions, with a current density j, the cell voltage E(J) decreases greatly as the result of three limiting factors the overvoltages r a and r c at both electrodes due to a rather low reaction rate of the electrochemical processes (activation polarization), the ohmic drop RJ in the electrolyte and interface resistance Re, and mass transfer limitations for reactants and products (concentration polarization). 

Feed concentration. As expected, the higher the feed concenyation is. the higher propensity of mass-transfer limiting phenomena such as concentration polarization, gel formation or fouling becomes. The resulting permeate flux is lower. A higher feed concentration also raises the viscosity. 

Stonehart and Wheeler17 and Popov et al.7 correlated the current densities on the microelectrodes, taking into account the change of concentration around them, with the current density on the macroelectrode. This is because the charge transfer occurs on the microelectrodes, while the mass-transfer limitations are related to the diffusion layer of the macroelectrode. In this chapter, the model of the surface of Popov et al.7 will be used to describe the polarization behavior of previously activated inert macroelectrode with active microelectrodes. 

Concentration polarization for Hquid film mass transfer can be coupled with the model for membrane transport (for example the solution-diffusion model Eq. (3)) [32, 38, 43, 44], to describe membrane transport in a mass transfer limited system. 

Micro-structured (membrane) reactors are quite interesting due to their (i) improved mass and heat transfer owing to the reduction of the scale length in the micro-channels (ii) removal of mass transfer limitations (concentration polarization) (iii) high degree of process intensification by integrating different process steps in a small-scale device. 

Additionally, because membranes are being produced with higher flux, another detrimental phenomena prevails in PB MRs, namely the increased bed-to-waU mass transfer limitation also known as concentration polarization. 

In most battery and fuel cell systems, part or all of the reactants are supplied from the electrode phase and part or all of the reaction products must diffuse or be transported away from the electrode surface. The cell should have adequate electrolyte transport to facilitate the mass transfer to avoid building up excessive concentration polarization. Proper porosity and pore size of the electrode, adequate thickness and structure of the separator, and sufficient concentration of the reactants in the electrolyte are very important to ensure functionality of the cell. Mass-transfer limitations should be avoided for normal operation of the cell. 

We still cannot fully investigate the concentration polarization without the tools of Chapter 5, which will allow us to predict the mass transfer limiting current density. 

In deriving eqn. (80), limitations due to mass transport at the interface were not considered. Strictly speaking, this is not realistic and as the reaction rate increases with overpotential in each direction a variation of the concentrations of reactant and product at the surface operates and concentration polarization becomes more important. Each exponential expression in eqn. (80) must be multiplied by the ratio of surface to bulk concentrations, ci s/ci b. The effect of mass transfer in electrode kinetics has been discussed in Sect. 2.4. 

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