Reaction Dependent Transport of Carbonate and Bicarbonate through Anion Exchange Membranes in Electrolysis and Fuel Cell Operations

Developments in alkaline anion exchange membranes have seen recent success for their use in new and exciting electrochemical applications.  Historically, the use of carbon dioxide in alkaline cells plagued performance by carbonating the electrolyte.  However, advances have allowed for use of polymer membranes that permit transport of carbonate and bicarbonate species ¹.  This results in a mixed anionic conductivity of these materials which presents new applications for their use in low temperature electrochemical reactions.  For example, carbon dioxide can be separated and compressed from a gas stream in electrolysis (pumping) mode ².  Furthermore, carbonate species could also be used for conversion of fuels in the anode ^3,4.  Therefore, exploring the transport properties of these materials under differing half-reactions can elucidate limitations and possibilities for future use.

A correlation can be made between the current applied in electrolyzer mode or drawn in fuel cell mode and the measured cell products, especially the evolution of carbon dioxide from the anode.  Carbon dioxide is first reduced along with oxygen in the cathode to form carbonate or bicarbonate anions.  This occurs through either indirect reactions which include a bicarbonate step or else they can be converted directly to carbonate.  In the fuel cell, hydrogen oxidation results in carbon dioxide without the oxygen evolution reaction by proceeding through a bicarbonate pathway.  Due to the contribution of different reactions, transport is typically mixed through the membrane.  Cell potential and resulting half-cell reactions will therefore determine the amounts of carbonate and bicarbonate in addition to hydroxide anions transported.  The measured ratio of electrons to moles of carbon dioxide is indicative of the overall exchange mechanisms occurring.  In this study, a careful evaluation of electrolysis and fuel cells is coupled with product analysis by spectroscopic and chromatographic methods of gaseous products from the anode to understand how the operating mode influences the carbonate/bicarbonate transport balance in the electrochemical anion exchange cells.

References:

1. J. R. Varcoe et al., Energy Environ. Sci., 7, 3135–3191 (2014)

2. J. Landon and J. R. Kitchin, J. Electrochem. Soc., 157, B1149 (2010)

3. W. E. Mustain, J. A. Vega, and N. S. Spinner,  Patent US2012/0193222 A1 (2012).

4. N. Spinner and W. E. Mustain, J. Electrochem. Soc., 160, F1275–F1281 (2013)