Acid anionic polymer electrolyte membrane

As is well known, and as discussed by Neyerlin and others, there is an increase in polarization losses with phosphoric acid-based fuel cells, in comparison to low-temperature PFSA-based fuel cells, and this effect is thought to be due to the presence of phosphoric acid and/or its anions that adsorb onto the surface of the catalyst [47]. Because of this, high-temperature stacks based upon phosphoric acid-doped polymer electrolyte membranes are larger in order to get the same power output. The overall question is whether the benefit in system simplification overcomes the need for larger stacks, so that there is an overall net system benefit. Reducing the effect of the adsorbed anion species would, of course, have significant benefit at the stack and system levels. 

Water was born to conduct protons (see Special Issue Is life possible without water [67]). The conductance of distilled water is miserable due to a negligible concentration of free protons (10 mol/liter), but the proton mobility in water is approximately five times higher than the mobility of an alkali cation (e.g. Na" ), an object of similar size as the hydronium (HaO ) ion [68]. So, donated protons can run fast through the aqueous phase. Excess protons result from dissociation of acidic molecules or molecular groups, e.g. in solutions of strong acids, hydrated polymer-electrolytes, or proteins. In acidic solutions both the protons and counter-anions are mobile. In polymer-electrolyte membranes and in proteins only protons are mobile in the connected aqueous phase while the counter anions are mostly a part of an immobile skeleton. 

Recent developments in AAEMs have opened up the possibiUty of an alkaline analog of the acidic solid polymer electrolyte fuel cell. This could utilize the benefits of the alkaline cathode kinetics and at the same time eradicate the disadvantages of using an aqueous electrolyte. As the AAEM is also a polymer electrolyte membrane (sometimes abbreviated as PEM), some clarity in abbreviations is required. In this chapter, PEM refers only to the proton exchange membrane fuel cells (acidic), AAEM refers to the anion exchange membrane H2/O2 fuel cells, and AFC exclusively refers to the aqueous electrolyte alkaline H2/O2 fuel cells. Anion exchange membranes are also employed in alkaline direct alcohol fuel cells, discussion of which will refer to them as ADMFC/ADEFC (methanol/ ethanol). 

Catalysts for anion-exchange membrane fuel cells are for the use in the alkaline medium. Thus, much more materials could be applicable as catalysts than those for polymer electrolyte fuel cells, which should be stable in acidic electrolyte. 

A more compact reactor design takes advantage of solid polymer electrolytes (SPEs). Hence, cast Nafion membrane and SPEEK were prepared, and used as cationic SPE whereas alkali-doped poly (vinyl alcohol), PVA and Amberlyst resin composites were made, and used as anionic SPE. These SPEs were combined with a cathode made of copper, electrodeposited onto porous carbon paper, and an anode consisting of Pt/C on the carbon paper. The main electroreduction products were formic acid, methanol, formaldehyde, carbon monoxide and methane, while undesired hydrogen gas by-product was also detected [120]. 

Some vinyl fluoride-based polymers with side chains of perfluorosulfonic acid (the Nation family) are important ion-exchange membrane materials used in practice for electrolysis of NaCl and in certain fuel cells. They show a proton conductivity of 0.01 S cm- at room temperature. However, such fast ionic transport occurs only when they are swollen with water. It is therefore not appropriate to call them solid electrolytes in the tme sense of the word. It was in 1970 that anionic conductivity, though not high, was reported for crown ether complexes such as dibenzo-18-crown-6 KSCN, in which cations are trapped by the ligand. " A few years later much higher cationic (instead of anionic) conduction was found in complexes of a chain-like polyether such as PEO or PPO with alkaline salts here, PEO stands for poly(ethyleneoxide), (CHjCHj-O), and PPO for poly(propyleneoxide)."2>"3 These were the flrst examples of tme polymer solid electrolytes and were followed by a great number of studies. Polymeric electrolytes are advantageous in practice because they are easily processed and formed into flexible Aims. 

AFC is another type of fuel cell with OH as current conduction medium. As the traditional liquid electrolyte AFC has many drawbacks, for example, CO2 sensitivity, AEM fuel cell (AEMFC) was more attractive to the researchers in recent years [92], Nafion, the reference material for proton-exchange membranes, was no longer suitable for AEMFC. Therefore, many studies about aliphatic membranes were conducted for AEMFC. Most of them were focused on PVA-based membrane. In these studies, PVA polymers were composited with other polymers like poly(12-acryloylaminododecanoic acid) (PAADA), PVP, and PS [15,16,63,118], For the membranes without anion like PVA/PAP and PVA/PS, they need to be immersed in alkali solution, for example, KOH solution, for anion conduction in fuel cell. While for the membrane with anion already, for example, PVA/PAADA, alkali solution is no longer required. In addition, to fix alkali in the manbrane, PVA pure or composite membranes were usually cross-linked before use [19,25] (Table 10.3). 

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