Anion-Exchange Membranes pdf

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. 329 . 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. Anion-Exchange Membranes JR Varcoe, JP Kizewski, DM Halepoto, SD Poynton, RCT Slade, and F Zhao, University. 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.