The Study Of Anion Exchange Membrane Based On Polyphosphazenes For Fuel Cells

The civilization of humanbeing has gone through millions of years and reached unprecedented heights. However, the development of civilization is based on the consumption of fossil energy, which leading to serious environmental problems. Besides, the fossil energy is a non-renewable energy resource. This gives us impetus to research on the cleanenergy alternatives for satisfying growing energy demand. Fuel cell is regarded as a promising green energy conversion. Because of the small size, compact structure, high energy density and a quick start, the polymer electrolyte membrane fuel cell has been developing rapidly in recent years.Anion exchange membrane fuel cell (AEMFC) as one kind of polymer electrolyte membrane fuel cells combines the advantagessuch as high efficiency, high energy density, quiet operation, and environmental friendliness of proten exchange membrane fuel cells (PEMFCs) andtraditional alkaline fuel cells. They can also potentially address many of the shortcomings such as catalyst costand stability, and the negative effects of CO2of PEMFCs and traditional alkaline fuel cells. Hence, mucheffort has been devoted to developing novel anion exchange membranes (AEMs) materials that are as ionconductive and stable as proton exchange membranes (PEMs) as an alternative to PEMs.As one of the key components in AEMFCs, the anion exchange membrane (AEM) should have good thermal stability, mechanical strength, water uptake, and conductivity. In particular, conductivity is of great importance to the performance of AEMs in fuel cells. Increasing the ion exchange capacity (IEC) of ionomer is a traditional method to increase the conductivity of AEMs. However, any increase in IEC of the ionomer is generally accompanied by an undesired swelling ratio, which can reduce the mechanical properties and dimensional stability of the membranes. In our study, poly(aryloxyphosphazene)s were chosen as the polymer backbone of AEM, because its small repeating unit, and highly reactive of the side chains, the high IEC can be easily obtained. To solve the problems existing in the alkaline membrane, several solutions were applied in our study for obtaining AEM with high performance.1. Crosslinked Ion-Conductive PolyphosphazeAEMs were prepared by bromination of the methyl groups of poly(aryloxyphosphazene)s, followed by Menshutkin reaction with tertiary amine, N-methyl imidazole (NMI)and N,N,N’,N’-tetramethyl-1,6-hexanediamine(TMHDA) in spite of the ammonium and imidazolium are not the promising functional groups in alkali environment. For the synthesis, we carefully controlled the degree of bromination of the parent polymers, which directly determined the subsequent degree of quaternization. A balance between high conductivity and good physical properties was accomplished by careful adjustments of the crosslinker (TMHDA) contents. Also, the water affinity, ionic conductivity, mechanical properties, and long-term durability of AEMs were investigated.2. Polyphosphazenes with pendant quaternary phosphonium and ammoniumIn this chapter, we report a new class of hydroxide anion exchange membranes that consist of benzyltriphenylphosphonium cations appended to polyphosphazene. In consideration of the low basicity of triphenyl phosphine (TPP) and its quaternary phosphonium hydroxide, trimethylamine (TMA) was also introduced as aminatingreagent to enhance the ion exchange capacity and ionic conductivity of the AEMs because of its low sterichindrance, easy incorporation onto polymers, and the moderate conductivity of its quaternary ammoniumhydroxide. For the synthesis, we carefully controlled the degree of bromination of the parent macromoleculepolymer poly(aryloxyphosphazene) and the ratio of TPP and TMA. A balance between high conductivity andgood physical properties was accomplished by careful adjustments of the proportions of TPP and TMA. Also, thewater affinity, ionic conductivity, and long-term durability of the AEMs were investigated. 3. Layered double hydroxide/polyphosphazenes-based ionomer hybrid membranesAs one of the key components in AEMFCs, the pure polymer AEMs generally consist of a hydrophilic hydroxideconductive phase and a hydrophobic non-conductive phase, and these two phases are randomly distributed inhydrated AEMs. Transportation channels for hydroxide within the AEMs are formed by the connection of thehydrated hydrophilic phase, but the tortuous channels increase the ion transmission distance and reduceconduction efficiency. AEMs with oriented LDH nanoplatelets not only take full advantage of the bulkconductivity of LDH nanoplatelets, which produces short hydroxide transport distances but also induce theorientation in the trans-plane direction of the hydroxide transport channels formed by the imidazolium groups, which improves the utilization of the imidazolium groups. With the aim to improve the conductivity of anion exchange membranes (AEMs), the layered double hydroxide (LDH) was added to the polyphosphazene-based ionomer, and an external AC electric field was applied to theLDH-polyphosphazenes-based ionomer hybrid casting solution during solvent evaporation. The introduction ofLDH nanoplatelets in AEMs significantly improved the conductivity, ion exchange capacity, water uptake, andmechanical properties of hybrid AEMs, but sacrificed lower dimensional stability. And, most importantly, thepresent technique is potentially useful method for fabricating AEMs with oriented LDH nanoplatelets taking fulladvantage of the bulk conductivity of the LDH nanoplatelets and resulting in a39%increase in conductivity in thetrans-plane direction over that of normal methods. In addition, the novel polyphosphazene-based ionomer wasdesigned not only to balance the hydroxide ions but also to improve interfacial adhesion with LDH nanoplateletsfor improving the distribution of LDH nanoplatelets in hybrid AEMs.