(530a) 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. 

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