Fabricating Stable Bipolar Membranes Using Nafion Cation Exchange Membranes

Overview

Bipolar membranes contain a cation exchange membrane bonded physically or chemically to an anion exchange membrane. These membranes conduct electricity through water splitting reaction at the junction between the two membranes (aka the bipolar junction) to produce H+ and OH- ions. When used in a H2/O2 fuel cell, these ions combine with the OH- and H+ ions at the cathode and anode respectively, to form water. Bipolar membranes allow for better pH control at fuel cell electrodes, which in turn reduces electrode corrosion. Also, bipolar membranes eliminate dehydration of the membranes through electroosmotic flow of water, thereby resolving a major challenge in a proton exchange membrane (PEM) fuel cell. Thus, using bipolar membranes allows for the use of inexpensive catalysts and eliminates the need for H2 gas humidification and special water handling systems at the cathode. Unfortunately, today’s bipolar membranes do not meet the electrical, chemical, and mechanical stability requirements of fuel cells mainly due to the instability of the anion exchange membranes. In this project, a new approach to fabricate stable bipolar membranes was developed. This was achieved this by creating the bipolar junction within the pores of the highly stable Nafion cation exchange membrane. Nafion is a registered trademark of E. I. Dupont De Nemours. This approach exploits large differences in the swelling properties of Nafion in methanol and water. The method was implemented by systematically choosing positively-charged self-assembling molecules that are: 1) small enough to freely enter Nafion pores that are swollen to twice their normal size in methanol and 2) large enough to remain trapped within the pores when the membrane pores are shrunk to their normal size by replacing methanol with water. The resulting membrane exhibits whole pore bipolarity when the trapped macromolecules do not exchange with other cations in solution. Results from my research indicate that self-assembling quaternary ammonium cations, such as Dodecyltrimethylammonium and Cetyltrimethylammonium, when incorporated into Nafion yield membranes with bipolarities on the order of 90 and 97% respectively. The bipolarities were determined by measuring sodium ion uptake by the membranes before and after conversion.

Competition history

• AJAS 2020 Category not listed

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