The historical development, current status and future prospects of chlor-alkali electrolysis with oxygen depolarized cathodes (ODCs) are summarized. Over the last decades, membrane chlor-alkali technology has been optimized to such an extent that no substantial reduction of the energy demand can be expected from further process modifications. However, replacement of the hydrogen evolving cathodes in the classical membrane cells by ODCs allows for reduction of the cell voltage and correspondingly the energy consumption of up to 30%. This replacement requires the development of appropriate cathode materials and novel electrolysis cell designs. Due to their superior long-term stability, ODCs based on silver catalysts are very promising for oxygen reduction in concentrated NaOH solutions. Finite-gap falling film cells appear to be the technically most mature design among the several ODC electrolysis cells that have been investigated.

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A mathematical model for a porous, silver-based electrode for the oxygen reduction in alkaline solutions, based on the thin film flooded agglomerate theory, was developed. These electrodes are employed in the energyefficient chlor-alkali electrolysis with oxygen depolarized cathodes. The model parameters were determined from overpotentials at different oxygen concentrations obtained in half-cell measurements. For the description of the reaction kinetics, it was necessary to introduce two Tafel equations, which might be explained by a change of the adsorption isotherm of the intermediate species during oxygen reduction. The model allows for a successful description of the overpotentials in the region of industrially relevant current densities. The analysis of the oxygen concentration distribution in the liquid electrolyte reveals that massive diffusion limitations occur although the calculated size of the agglomerates is only in the range of a few micrometers.Keywords Oxygen reduction Á Ag Á Chlor-alkali electrolysis Á Modeling List of symbolsA Constant describing the exchange current density (A m mol -1 ) c Molar concentration of oxygen dissolved in NaOH solution at boundary film-agglomerate (mol m -3 ) c* Molar concentration of oxygen dissolved in NaOH solution at boundary gas-film (mol m -3 ) c i Molar concentration of species i (mol m -3 ) D ij Binary Maxwell-Stefan diffusion coefficient for species i and j (m 2 s -1 ) D i,K Knudsen diffusion coefficient for species i (m 2 s -1 ) E°Equilibrium potential (V) E change Potential of change in Tafel slope (V) F Faraday constant (= 96485.3399(24) C mol -1 ) H Henry constant (Pa m 3 mol -1 ) j Current density (A m -2 ) k c (Chemical) reaction rate constant (s -1 ) L Characteristic length (m) m Molality of NaOH solution (mol kg -1 ) M i Molar mass of species i (kg mol -1 ) N iFlux of species i (mol m -2 s -1 ) P i Partial pressure of species i (Pa) P m Saturation partial pressure of water above NaOH solution (Pa) R Universal gas constant (= 8.314472(15) J mol -1 K -1 ) r Radius (m) r ag Radius of agglomerates (m) rReaction rate (mol m -3 s -1 ) S tf Specific surface area of thin film per total volume of reaction layer (m 2 m -3 )Length domain in reaction layer (m) z 0Length domain in gas diffusion layer (m) z 00Length domain in boundary layer (m)Greek letters d tf Thickness of thin film (m) DuPotential difference between solid electrode and electrolyte (V) e Porosity (dimensionless)

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