Anion-Exchange Membranes pdf
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Anion-Exchange Membranes
JR Varcoe, JP Kizewski, DM Halepoto, SD Poynton, RCT Slade, and F Zhao, University of Surrey, Guildford,
UK
& 2009 Elsevier B.V. All rights reserved.
Introduction
Background
The vast majority of research into solid-state polymer
electrolytes for low-temperature (o200 1C) fuel cells has
focused on proton-exchange membrane (PEM) fuel cells
(PEMFCs). Recently, there has been interest in the ap-
plication of the analogous anion-exchange membranes
(AEMs), in alkaline forms, in low-temperature fuel cells
(Figure 1). These alkaline anion-exchange membranes
(AAEMs), which are at an early stage of development,
conduct hydroxide (OH
À
) anions (and/or (bi)carbonate
anions –HCO
3
À
/CO
3
2À
) rather than protons (H
þ
). This
article will discuss the current understanding on the
application of AEMs in chemical fuel cells containing
hydrogen, carbon-, boron-, and nitrogen-containing fuels
and also in microbial fuel cells (MFCs) utilizing bio-
logical energy generation.
The Driver for and Concerns with the Use of
Alkaline Anion-Exchange Membranes in Fuel
Cells
The cost of fuel cells still retards commercialization in
most markets. Alkaline fuel cells (AFCs), which tradi-
tionally utilize caustic aqueous potassium hydroxide as a
cheap electrolyte, are promising on a cost basis mainly
because cheap and relatively abundant non-platinum
group metals (non-PGM) are viable catalysts. Catalyst
electrokinetics (for fuel oxidation and oxygen reduction)
is also improved in alkaline, as opposed to acidic, con-
ditions (the acid-stability criterion precludes the use of
most non-PGM catalysts in PEMFCs). However, there
are concerns that carbon dioxide, which is a natural
component of air, will lead to performance losses due to
the formation in the aqueous alkaline electrolyte of less
ionically conductive, and less basic, bicarbonate
(HCO
3
À
) and carbonate (CO
3
2À
) anions (eqns [I] and
[II]). The pH of aqueous solutions at 25 1C increases
from 8–8.5 for sodium hydrogen carbonate (NaHCO
3
)to
10.5–12 for Na
2
CO
3
and to 13–14 for sodium hydroxide
(NaOH) (approximate pK
b
values ¼ 7.7, 3.7, and 0.2, re-
spectively):
CO
2
þ OH
À
"HCO
À
3
½I
HCO
À
3
þ OH
À
"CO
3
2À
þ H
2
O ½II
Metal CO
3
2À
=HCO
3
À
solid precipitates can also form
and these can not only block the pores of the AFC
electrodes, but also mechanically degrade the active
layers.
The replacement of the potassium hydroxide (aq)
electrolyte with an AAEM in AFCs retains the electro-
catalytic advantages but introduces carbon dioxide tol-
erance along with the additional advantage of being an all
solid-state fuel cell (as with PEMFCs – i.e., no seep-
ing out of aqueous potassium hydroxide). Addi-
tionally, thin (low electronic resistance) and easily
stamped (cheap) metal monopolar/bipolar plates can be
used with reduced corrosion-derived problems at high
pH (the cost of bipolar plates for PEMFCs can be rela-
tively high). A key requirement (see the section entitled
‘Alkaline Ionomer Developments’) is the development
of an alkaline ionomer (anionomer) to maximize ionic
contact between the catalyst reaction sites and the
3H
2
CH
3
OH + H
2
O
1½O
2
3H
2
O
CO
2
6e
–
6e
–
Catalyst
PEM
Anode
Cathode
6 H
+
H
2
O
3H
2
CH
3
OH
6H
2
O
CO
2
+ 5H
2
O
1½O
2
+ 3H
2
O
Load
6e
–
6e
–
Catalyst
Anode
Cathode
6 OH
–
H
2
O
Load
AAEM
Figure 1 Schematic comparison between hydrogen or methanol-fuelled alkaline anion-exchange membrane (AAEM) and proton-
exchange membrane (PEM) fuel cells.
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