In concentration polarization

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

I. Rubinstein, E. Staude, and O. Kedem, Role of the membrane surface in concentration polarization at ion-exchange membrane, Desalination, 69 (1988), p. 101. 

The steady nonequilibrium space charge in concentration polarization at a permselective homogeneous interface [2]. It... 

A few remarks are due about this feature. The nonuniformity above is a formal expression of breakdown of the local electro-neutrality assumption in concentration polarization, described in the previous chapter. Essentially, this reflects the failure of a description based upon assuming the split of the physical region into a locally electro-neutral domain and an equilibrium double layer where all of the space charge is concentrated. The source of this failure, reflected in the nonuniformity of the corresponding matched asymptotic expansions, is that the local Debye length at the interface tends to infinity as the voltage increases. In parallel a whole new type of phenomena arises, which is not reflected in the simplistic picture above. The... 

Isaak Rubinstein, Israel Rubinstein, and E. Staude, High frequency rectification in concentration polarization, PCH Phys. Chem. Hydrodynamics, 6 (1985), pp. 789-802. 

It is to be noted that the Helmholtz double layer plays a significant role in concentration polarization since the concentration of the ions on the electrode surface, and the diffusion of ions from the bulk of the solution into the Helmholtz plane are contributing factors to the limiting current density. This situation may be visualized as shown below ... 

An important limit in concentration polarization is complete rejection R= 1, Cp = 0). For organics, this can lead to gel layer formation which controls the permeation rate (Vp). For this limiting case, the equation for concentration polarization becomes... 

M.E. Green and M. Yafuso, A study of the noise generated during ion transport across membranes, J. Phys. Chem., 1968, 72, 4072-4078 I. Rubinstein. Mechanism for an electrodiffusional instability in concentration polarization, J. Chem. Soc., Faraday Trans. 2. 1981, 77, 1595-1609 F. Maletzki, H.-W. Rosier and E. Staude, Ion transport across electrodialysis membranes in the overlimiting current density range Stationary voltage current characteristics and current noise power spectra under different conditions of free convection, J. Membr. Sci., 1992, 71, 105-115. 

Extended Space Charge in Concentration Polarization, Nonequilibrium Electroosmotic Slip, and Electroosmotic Instability... 

Although the flux decline is a typical aspect involved in concentration polarization and membrane fouling, there are some substantial differences between these phenomena. Concentration polarization is a reversible process which takes place over a few seconds. It can be easily controlled by decreasing the TMP, lowering the feed concentration or increasing the cross-flow velocity. In membrane fouling, the flux decline is irreversible and takes place over many minutes, hours or days. It is more difficult to describe and to control experimentally. In addition, a continuous flux decline can often be observed. 

Some battery manufacturers have included additives to the cell (e.g., Ti02 and BaS04) to assist in water management for high drain applications. Although the actual mechanisms of these additives are unknown, they are believed to assist in concentration polarization within the cell as evident in the improvement in performance made in these high drain applications. 

Agitation. By agitation, the thickness of the diffusion layer is decreased, and the rate of diffusion of ions increases. There is no build up of any concentration gradient between the corroding surface and bulk electrolyte. The end result is a decrease in concentration polarization and an increase in the rate of corrosion. As the rate of agitation is increased. 

The effect of feed concentration on flux and rejection is depicted in Fig. 4.17. As expected, a rise in the feed concentration resulted in a decrease in flux and percentage of impurity rejection. This is due to increase in concentration polarization and osmotic pressure at the membrane surface. Higher solute concentrations at the feed side induce an osmotic pressure gradient, which... 

Figures 18 to 20 show that the equilibrium cell voltage increases with the increase in fuel concentration. Although the cell performance increases initially but it does not increase proportionally with further increase in fuel concentration. This is because the increase in fuel concentration leads to the decrease in hydroxyl ion mobility. The hydrolysis reaction dominates with the increase in sodium borohydride concentration and thus the performance increases rather slowly. Further at higher concentration of NaBH4, viscosity of the fuel-electrolyte mixture increases leading to the rapid increase in concentration polarization at higher current densities and the performance decreases (Fig. 20). The maximum power density of 16.2 and 13.8 mW cm" were obtained for 3 M methanol and ethanol concentrations while 22.5 mW cm" for 2 M sodium borohydride. The fuel cell was operated at 25°C, 3 M KOH concentration and with 1 mg cm " of anode catalyst (Pt-black) loading catalyst and 3 mg cm" of cathode (Mn02) loading, respectively.

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