Cation exchange membranes layers

FIGURE 15.8 Calculation results of ID dynamic porous electrode model for an electrode with a selective cation exchange membrane layer in front (located atx = 0) that is perfectly blocking for co-ions, (a) Macropore salt concentration profiles as function of time (direction of arrows) during application of -770 mV voltage to the electrode relative to the bulk (spacer channel) outside the membrane, and (b). After subsequent increase of the potential to +770 mV. 

Much more simply, the same result can be attained with bipolar membranes, membranes consisting of an anion- and cation-permeable (an anion- and cation-exchange) membrane laminated together. At such a membrane, when mounted between electrodes so that the cation-exchange layer faces the anode, water is split into and OH ions so that the acidic and alkaline solutions required for regeneration as above are produced at the respective surfaces of the bipolar membrane. When such membranes are suitably integrated into the sequence of membranes in the electrodialysis unit above, gas evolution at the electrodes is not needed the acid-base pair is produced with about half the power. 

Formulation. Consider two unity thick diffusion layers of a mixture of 1, 1-, and 1, z-valent3 electrolytes with a common anion, adjacent to a planar ideally permselective cation-exchange membrane. Direct the axis x normally to the membrane and let x = 0 coincide with the outer boundary of the diffusion layer. The diffusion layers will thus be located at 0 < x < 1 and 1 + Aelectro-diffusional transfer of ions across the membrane and the diffusion layers is... 

If an electrical current is passed across a cation-exchange membrane, salt depletion takes place in the surface layer facing the anode. Supply of salt occurs by diffusion across the unstirred film. Maximum diffusion flow occurs if the salt concentration near the membrane equals zero. 

Figure 5.4 Schematic drawing illustrating the concentration profiles of a salt in the laminar boundary layer on both sides of a cation-exchange membrane and the flux of ions in the solutions and the membrane.

Ion-exchange membrane coated with porous catalyst metal (Solid Polymer Electrolyte, SPE) as well as GDE can provide gas phase electrolysis of CO2. The first attempt was published by Ito et al., communicating thin porous Au layer electrode coated on a cation exchange membrane. The SPE could not enhance the cathodic current of CO2 reduction.2" DeWulf et al. applied an SPE with Cu as the catalyst layer on a cation exchange membrane (CEM) Na-fion 115. The SPE reduced CO2 to CH4 and C2H4 for a while, but the current density for CO2 reduction dropped below 1 mA cm after 70 min electrolysis. " ... 

Fan studied the transport properties of three ferrocene derivatives carrying different types of charge inside two gels of polyacrylamide and polyacrylate (55). The latter gel formed a passivation layer by electrophoresis, which behaved as a cation-exchange membrane. Chronoamperometry was applied while penetrating the film and revealed diffusion coefficients slightly below those inside the solution. From this, the author concluded that the transport occurs via water-filled domains. 

S STATIC BOUNDARY LAYERS C - CATION-EXCHANGE MEMBRANE A - ANION-EXCHANGE MEMBRANE FIGURE 21.2-5 Idealized representation of concentration gradients in electrodialysis. 

Static boundary-layers C = Cation-exchange membrane A = Anion-exchange membrane... 

Formation of a Thin Cationic Charged Layer on the Surface of the Cation Exchange Membrane... 

Figure 5.5 Transport properties of a cation exchange membrane having a cationic polyelectrolyte layer formed by electrodeposition. (A) PNaCa ( ) current efficiency (%) ( ) electrical resistance of the membrane during electrodialysis for 1 h. After solutions containing 0.0416N sodium chloride and poly(3-methylene-N, N-dimethylcyclohexylammonium chloride) of various concentrations had been electrodialyzed, for 60 min at a current density of 10 mA cm 2, as anolyte to electrodeposit the polyelectrolyte on the membrane surface (catholyte was 0.0416N sodium chloride), a 1 1 mixed solution of 0.208N calcium chloride and 0.208 N sodium chloride was electrodialyzed at a current density of 10 mA cmr1 for 60 min (cation exchange membrane NEOSEPTA CH-45T).

Figure 5.6 Change in transport number of calcium ions relative to sodium ions of a cation exchange membrane - with and without cationic charged layer - with concentration of the mixed salt solution. (X) NEOSEPTA CL-25T (A) NEOSEPTA CL-25T immersed in 100ppm polyethyleneimine solution for 60 min. 1 1 mixed solutions of calcium chloride and sodium chloride of different concentrations were electrodialyzed.

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