(600ac) Evaluating the Contribution of Direct Vs. Indirect Carbonate Production in Anion Exchange Membrane Fuel Cells

Evaluating the
Contribution of Direct vs. Indirect Carbonate Production in Anion Exchange
Membrane Fuel Cells

Michael Ignatowich,
Gregory Crettol, Molly Chhiv and William E. Mustain

Department of
Chemical, Materials and Biomolecular Engineering; University of Connecticut

Storrs, CT 06268

One of the most
widely discussed topics in the development of anion exchange membrane fuel
cells (AEMFC) is the formation of carbonate/bicarbonate anions at the AEMFC
anode.  For the hydroxide exchange membrane fuel cell, carbonate formation is
considered a poison that reduces electrolyte conductivity during operation,
mostly through the formation of bicarbonate.  In the carbonate exchange
membrane fuel cell, the high selectivity of carbonate is required to enable
high electrolyte stability while limiting the formation of bicarbonate anions. 

In both
devices, there are two primary routes through which carbonate anions can be
formed: direct electrochemical reduction of O₂ and CO₂,
and the indirect formation of carbonate through the reaction of hydroxide with
CO₂.  These two pathways are shown below. 

Direct Pathway:

O₂ +
2CO₂ + 4e^- ¦
2CO₃^-2                                          (1)

Indirect Pathway:

O₂ +
2H₂O + 4e^- ¦
4OH^-                                            (2)

OH^-
+ CO₂ ¦ HCO₃^-
                                                  (3)

HCO₃^-
+ OH^- ¦ CO₃^-2
+ H₂O                                     (4)

The context for
this talk will primarily be the evaluation of new catalysts for carbonate
exchange membrane fuel cells [1-2], though the analysis presented is directly
application to all AEMFCs.  Here, we have constructed an anion exchange
membrane fuel cell with the catalysts of interest at the cathode, a commercial
anion exchange membrane fuel cell and Pt/C at the anode.  A humidified mixture
of O₂ and CO₂ was fed to the cathode and humidified H₂
to the anode.  At the anode, hydrogen is efficiently oxidized by both CO₃^-2
and OH^- [3], which allows us to assume that all of the
charge-carrying carbonate anions are oxidized to CO₂ (Equation 5).            

          H₂ + CO₃^-2 ¦ CO₂ + H₂O + 2e^-                                (5)

The effluent of the fuel cell was
fed to a 0.1M Ca(OH)2 solution, which enable the precipitation of CaCO₃,
giving a direct measure of the amount of charge-carrying carbonate.  A
schematic for this cell is shown in Figure 1. 

In this work,
on-off AC polarography was used to precisely control the relative contribution
of the indirect chemical and direct electrochemical formation of carbonate on
each catalyst.  During ?device on? conditions, both the chemical and
electrochemical routes are active, while ?device off? conditions limit the cell
to the indirect pathway only.  Various ratios of on-off times act in a manner
similar to standard addition experiments in analytical chemistry, allowing for
amount of carbonate formed electrocatalytically to be quantified for the first
time. 

Figure
1 ? Improvement in anion exchange membrane stability in the carbonate/bicarbonate
form (b) compared to hydroxide (a). 

Preliminary
results show that Ca₂Ru₂O₇ has a significantly
improved direct pathway carbonate selectivity compared to Pt/C, making it a
very good candidate catalyst for carbonate exchange membrane fuel cells.  This
work also confirms that the primary route for carbonate formation on Pt-based
catalysts is through the indirect pathway.

References:

1.      
J. Vega, S. Shrestha, M. Ignatowich, W. Mustain. J. Electrochem. Soc.,
159 (2012) B12.

2.      
J. Vega, N. Spinner, M. Catanese, W. Mustain. J. Electrochem. Soc.,
159 (2012) B19.

3.      
J.A. Vega, S. Smith and W.E. Mustain, J. Electrochem. Soc., 158
(2011) B349