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
Trang 1Anion-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
(AAEMs), which are at an early stage of development,
conduct hydroxide (OH) anions (and/or (bi)carbonate
anions –HCO3
/CO3
) rather than protons (Hþ) 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 (HCO3) and carbonate (CO3 ) anions (eqns [I] and [II]) The pH of aqueous solutions at 25 1C increases from 8–8.5 for sodium hydrogen carbonate (NaHCO3) to 10.5–12 for Na2CO3and to 13–14 for sodium hydroxide (NaOH) (approximate pKbvalues¼ 7.7, 3.7, and 0.2, re-spectively):
CO2þ OH"HCO
HCO3 þ OH"CO3þ H2O ½II
Metal CO3 =HCO3 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
3H2
3H2O
CO2
Catalyst
PEM
Anode Cathode
6 H +
H2O
3H2
CH3OH
6H2O
CO2+ 5H2O
1½O2 + 3H2O
Load
Catalyst
Anode Cathode
6 OH –
H2O
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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