Bipolar membrane Bipolar membranes are a special type of layered ion exchange membrane. They consist of twopolymer layers carrying fixed charges; one is only permeable for the anions and the other onlyfor cations. Actually, unlike with membranes used for separation purposes, nothing should betransported from one side to the other. The desired function is a reaction in the bipolar junctionof the membrane where the anion and the cation permeable layers are in direct contact: water issplit into hydroxide ions and protons by a disproportionation reaction. The produced hydroxideion and proton are separated by migration in the respectie membrane layer out of themembrane. !nlike water splitting at electrodes during electrolysis, no gases are formed as a sideproduct to this reaction, nor are gases used up. "lectrodialysis with bipolar membranes #"$%B&'( can replace electrolysis with water splittingat the electrodes but has a wider ariety of applications. "$%B&' can be used to produce acidsand bases from a neutral salt as described in more detail below. )t is a membrane reactor processwhere a reaction and a separation occur in the same unit or een in the same membrane * watersplitting without gases inoled in the reaction is only possible when the reaction products areseparatedimmediately, otherwisethereersereaction, arecombinationtowater, cannot bepreented.Bipolar membrane functionThe function of a bipolar membrane can be explained by looking at the concentration profiles inthe membrane during operation,+igure . the bipolar membrane consists of two ion exchangelayers of opposite charge in intimate contact. )n the applied electric field, the hydroxide ions andthe protons produced in bipolar junction moe towards the respectie electrode in the electricfield. ,ater is replenished in the interface by diffusion through the gel%like membrane layers.Accordingtothe respectie interface e-uilibriumwiththe surroundingsolutions, not onlyhydroxideionsandprotonsbut alsotheacidanionsandthebasecationsarepresent inthemembrane phase. This leads to the undesired transport of these anions and cations across themembrane. The driing forces for the transport of these ions are both, the electrical potential andthe concentration gradients. The water splitting re-uires energy that is supplied by the appliedelectrical field.The separating feature of the membrane layers is mainly necessary to preent ions of the samecharge as the fixed charge #co%ions( from reaching the reactie bipolar junction and to allow theproduced ions of the opposite charge #counter%ions( to moe out of the membrane. This holdsalso for the extra membranes in the module.Schematic concentration profile in a bipolar membrane in contact with an acid and a base onits two sidesBipolar membranes are often modelled as homogeneous phases but most membranes are actuallyheterogeneous, similar to the simpler cation and anion permeable membranes . +or "$%membranes, only few transport descriptions start to be applied for transport in heterogeneousmembranes. +or modelling, pressure terms may be negligible in first approximations because thefluids are non%compressible. .oweer, the stagnant membrane phase can expand and thetransport properties of the membrane can change. /ne indication of that is the change of watercontent with changed solution concentrations or different counter ion for the fixed charges.+or the description of transport in bipolar membranes as well as other ion permeable membranes,two somehow different approaches are possible. 'ostly the phenomenological description withthe extended 0ernst%&lanck e-uations is used because it relates measurable physical propertiessuch as an electric potential difference or concentration gradients directly to ion fluxes. .oweer,it suffers from the fact that the main transport parameters, the ion diffusion coefficients, are notfixedalues but dependontheentityof theenironment theionencounters. )ntheotherdescription, basedonthe'axwell%1tefanapproach, binarydiffusioncoefficients or frictioncoefficients are used between all the pairs of the substances present in the system. The 'axwell%1tefan description has the adantage that the binary diffusion coefficients are constant oer widerconcentration ranges, howeer, they need to be determined for all the binary interactions betweenthe species present in the system. 1ome of these are not directly accessible, the determinationand interpretation is not straight%forward for all binary pairs .Limitations,iththefunctionofabipolar membraneintheelectrodialysismodule, it isclear that thelimitations of the bipolar membrane itself form limitations of the bipolar membraneelectrodialysis process. The main limitations are #2( the chemical stability, especially against the concentrated base on its anion permeable side,#3( the co%ion transport, and #4( the membrane layer resistance against ion transport. /ther factors influencing its behaiourarethecatalyst stability,thepossibilitytowithstandtheincreasedpressureinthemembranewhen hydroxide ions and protons recombine to water as soon as the current is switched off, andan eentual poisoning or scaling of the membrane by complex formation of the fixed charge withmultialent metal salts. Also a reduced water content is not faourable * at high current densities,the water used up in the water splitting reaction has to be replenished through diffusie transportacross the membrane layers. These limitations are subject of the chapters of this thesis. )n thefollowing, after short economical considerations, an oeriew is presented how these limitationscan be approached.