(553d) Catalytic Advances and Electrolyte Stability for Carbonate Exchange Membrane Fuel Cells

Catalytic Advances
and Electrolyte Stability for Carbonate Exchange Membrane Fuel Cells

William E. Mustain

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

Storrs, CT 06268

Though
significant advances in have been made regarding the stability of anion
exchange membranes in highly alkaline environments, researchers are yet to find
a high stability, high conductivity electrolyte for the hydroxide exchange
membrane fuel cell (HEMFC).  To limit both chemical and mechanical degradation
of commercial and state-of-the-art membranes [1], several groups have begun
investigating a new type of anion exchange membrane fuel cell that operates on
the carbonate anion cycle.  Utilization of carbonate anions introduces a weaker
nucleophile to the electrochemical system that has been shown to significantly
reduce membrane degradation rates, Figure 1, though a modest sacrifice in ionic
conductivity is also realized compared to hydroxide [2].   

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

This led to two
very important questions.  First, can carbonate anions be used to oxidize
incoming fuels, most notably H₂ (Equation 1)? 

          H₂ + CO₃^-2 ¦ CO₂ + H₂O + 2e^-                                (1)

Second, can carbonate anions be
produced selectively through the direct electrochemical reduction of O₂
and CO₂ (Equation 2) under fully humidified conditions? 

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

In this talk, the methodology in the development and
performance of the first-known carbonate-selective electrocatalyst [3-4], Ca₂Ru2O₇,
will be discussed.  The performance and selectivity of the catalyst was
evaluated in both three electrode voltammetric and fuel cell experiments, while
surface chemistry and adsorption of reacting species was evaluated by
temperature programmed desorption.  The promises and limitations of this
first-generation electrocatalyst will be discussed in detail and candidate
materials for next-generation catalysts will be discussed. 

Gains in the stability and conductivity of low cost,
commercially available anion exchange membranes in carbonate and hydroxide
media will be compared.  Stability was evaluated by three primary metrics:
ionic conductivity, chemical changes (probed by ATR-FTIR) and mechanical
strength. 

Finally, the exchange current density of the hydrogen
oxidation reaction through the carbonate pathway [5] on carbon-supported
platinum will be evaluated and compared to hydrogen oxidation by hydroxide.

Each of the gains discussed above will be used to show
the promise of carbonate exchange membrane fuel cells as a high stability, high
performance alkaline polymer electrolyte fuel cell. 

References:

1.      
J.R. Varcoe and R. Slade, Fuel Cells 5 (2005) 187.

2.      
J.A. Vega, C. Chartier and W.E. Mustain, J. Power Sources, 195
(2010) 7176. 

3.      
J. Vega, S. Shrestha, M. Ignatowich, W. Mustain. J. Electrochem. Soc.,
159 (2012) B12.

4.      
J. Vega, N. Spinner, M. Catanese, W. Mustain. J. Electrochem. Soc.,
159 (2012) B19.

5.      
J.A. Vega, S. Smith and W.E. Mustain, J. Electrochem. Soc., 158
(2011) B349