The Electrochemical Bipolar Membrane: A New Approach of Maintaining pH-Gradients When Coupling Electro-Organic Reactions at Anode and Cathode

• Applicant: Dr. Christoph Bondü
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540350221

It is of high interest to couple electrochemical aldehyde oxidation (at the anode) to the electrocatalytic hydrogenation of ketones (at the cathode). To this end it is necessary to maintain the pH gradient between an alkaline anolyte and an acidic catholyte, while allowing for an ion current between anode and cathode. Ionomer based bipolar membranes that are used for this purpose in water electrolysis, suffer from an increased electrolyte resistance in the presence of organic compounds, which poses a sever disadvantage during electrolysis. To overcome this issue, we develop in this project a conceptually novel “electrochemical bipolar membrane”, that utilizes the principles of bipolar electrochemistry and gas diffusion electrodes (GDEs). Here, we construct the electrochemical bipolar membrane by short circuiting two GDEs und place them such, that anolyte and catholyte are separated from one another. As every conducting object that is placed in the electric field between anode and cathode also the short circuited GDEs are polarized and become therefore a bipolar electrode. Once polarization becomes large enough the GDE exposed to the anolyte begins to split water into H2 and OH- and transfers the formed hydrogen into the gas phase. H2 diffuses to the GDE exposed to the catholyte, where it is oxidized and released as H+ into the catholyte. The liberated electrons are conducted back to the GDE in the anode compartment, where they are used for hydrogen evolution. In this way both an exchange of charge (via electrons) and of matter (via H2) between anode and cathode compartment can be achieved, leading to the selective release of H+ into the catholyte and of OH- into the anolyte. A well performing electrochemical bipolar membrane requires GDEs with high electric conductivity, low charge transfer resistance for hydrogen evolution and oxidation, and a low diffusion resistance for hydrogen. To realize this, we develop in this project procedures for the preparation of thin, porous metal films featuring high conductivity for electrons and hydrogen. Furthermore, we identify electrocatalysts that allow a small charge transfer resistance for hydrogen evolution and oxidation in the presence of organic compounds. In the following we use impedance spectroscopy to determine the conductivity of the electrochemical bipolar membrane prepared with the selected procedures and catalysts, and compare it to the conductivity of ionomer-based membranes. Furthermore, we are going to show that the electrochemical bipolar membrane is a viable alternative to ionomer-based membranes: To this end we develop an electrolysis cell that features the electrochemical bipolar membrane and conduct the simultaneous hydrogenation of ketones and oxidation of aldehydes with it.

• DFG Programme: Research Grants