Improvements in the Anion Exchange Membrane Transport of Carbonate and Bicarbonate for Low-Temperature CO2 Capture and Energy Conversion

In recent years, anion exchange membrane (AEM) devices have become a topic of significantly increased interest.  Unlike liquid electrolyte alkaline fuel cells, which precipitate potassium carbonate salts when exposed to carbon dioxide, the lack of mobile cations in anion exchange membranes prevents the rapid cell death that results from salting.   Though carbonate and bicarbonate transport in anion exchange membranes has typically been poor, recent improvements have facilitated better mobility and opened the door for many different types of carbonate based devices.¹  Electrochemical carbon dioxide separators^{2, 3}, room temperature carbonate fuel cells⁴, and methane activation reactors⁵ are all promising applications of carbonate based anion exchange membranes, but with typical current densities reported in the single mA/cm² range, improvements in carbonate transport are necessary to improve performance.

In a typical carbonate-based device, oxygen is reduced with carbon dioxide at the cathode, generally through an indirect pathway first to hydroxide, then (bi)carbonate.  This pathway results in a mixed anion composition throughout the AEM where three very different anions can carry the charge across the membrane.  Bicarbonate and carbonate (especially bicarbonate) have lower conductivities than hydroxide, but this cannot alone explain the 2 to 3 order of magnitude performance loss from a carbonate-free cell (typically 100s of mA/cm² to 1000s of mA/cm²).  In a carbon dioxide pump, the oxygen evolution reaction results in evolved carbon dioxide, while in a carbonate fuel cell hydrogen oxidation results in carbon dioxide without evolving oxygen.  The cell potential and resulting half reactions influence the mixed transport and the amount of carbonate, bicarbonate, and hydroxide anions transported.  In both carbon dioxide pumps and carbonate fuel cells a relationship is observed between the evolution of carbon dioxide at the anode and the flow of electrons in the cell.  This relationship between current and evolved CO₂ in both pump mode and fuel cell mode is indicative of the anion ratio and the exchange mechanisms occurring at the anode the cathode.

In this study, a peak current density of 400 mA/cm² was achieved using a carbonate-exchange membrane fuel cell (Figure 1a) and 100 mA/cm² was reached in CO₂ pumping mode (Figure 1b).  The difference in the current is a reflection of the difference in the dominating anion in the membrane as well as the difference in anode reactivity of hydrogen oxidation vs. carbonate electrolysis.  This performance was achieved with radiation grafted anion exchange membranes and ionomers.⁶  It was observed that performance was very sensitive to AEM humidification; that there is a delicate balance between maintaining a high water content in the membrane to facilitate anion transport and preventing catalyst and GDL flooding.  The order of magnitude increase in the carbonate/bicarbonate transport observed in this work significantly increases the viability of membrane based carbon dioxide pumps, fuel cells, and fuel conversion devices. However, our results also point to advances that are needed in both catalysts and membranes to increase the faradaic efficiency in high current density carbonate devices. 

References:

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