In the industrial production of chlorine, recently special electrodes have been introduced, which consume much less current than conventional systems. The method requires oxygen which is introduced into hot, highly concentrated sodium hydroxide solution, in which it is very poorly soluble. It is still unclear how industrial current densities can be achieved under these conditions.
“Electrodes have been used industrially for years, but we do not understand why they actually work,” explains Wolfgang Schuhmann, Head of the Department of Analytical Chemistry at the Center for Electrochemical Sciences (CES).
Wolfgang Schuhmann, Alexander Botz, Denis Öhl and other colleagues from the RUB and Clausthal University of Technology, have gained new insight into the processes at the electrodes. The team describes these findings in the journal Angewandte Chemie, published online on 3 August 2018.
Constantly changing reaction conditions
Near the electrode, where the chlorine production takes place, three phases meet: solid, liquid and gaseous. Thus far, researchers have mainly studied the concentration of reacting oxygen in the solid-phase environment, developing models that attribute the high current density to this parameter.
For the current study, the Bochum scientists analysed the liquid phase and stated: The concentration of water and hydroxide ions at the electrode surface shows intense fluctuation through the course of the reaction and is not uniform throughout.
“We have suspected for years that there must be significant local concentration fluctuations inside the electrode which could contribute to the high current densities,” explains Schuhmann. “These investigations are essential for the development of gas diffusion electrodes, which will be of great importance in the future for the binding of CO2 from the air and thus contribute to a reduction of the emission of greenhouse gases.”
“These drastic changes have not yet been considered in the models reflecting the reaction,” says Alexander Botz. “The results are tremendously important for the future optimisation of such electrodes.”
Original Publication: A. Botz, J. Clausmeyer, D. Öhl, T. Tarnev, D. Franzen, T. Turek, W. Schuhmann: Local activities of hydroxides and water determine the operation of Ag-based oxygen depolarized cathodes, in: Angewandte Chemie International Edition, 2018, DOI: 10.1002/anie.201807798