Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: a progress report.
Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: a progress report.
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DOI：
10.1039/b618686f
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发表时间：
2007-04
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通讯作者：
H. Steininger;M. Schuster;K. Kreuer;A. Kaltbeitzel;Bahar Bingöl;W. Meyer;Siri Schauff;G. Brunklaus-G.-Brunk
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文献类型：
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作者：
H. Steininger;M. Schuster;K. Kreuer;A. Kaltbeitzel;Bahar Bingöl;W. Meyer;Siri Schauff;G. Brunklaus-G.-Brunk
研究了带膦酸基团的单官能化和双官能化直链烷烃的熔炼行为和输运性质与长度的关系。随着链长的增加，熔点温度升高，质子电导率降低，这可能是由于烷烃链段的排列顺序增加，限制了磷酸基的自由聚集。然而，质子迁移率比质子扩散系数降低的程度更大，这表明随着烷烃段长度的增加，质子传输的协同性增加。结果清楚地表明，在优化杂环基体系的质子电导率方面已被证明是成功的“间隔层概念”，在膦酸功能化聚合物的情况下是失败的。相反，高浓度的磷酸官能团形成“笨重的”氢键聚集体被认为是获得非常高的质子电导率所必需的。还建议聚集以减少通常在含膦酸体系中观察到的缩合反应。在此基础上，讨论了聚乙烯基膦酸和聚间苯膦酸的质子电导率。虽然这两种聚合物都表现出大量的磷酸基团，但聚集似乎受到了一定程度的限制，以至于在T = 150℃时，质子的固有电导率被限制在sigma = 10(-3) S cm（-1）以下的值。结果表明，与纯磷酸在准干燥条件下的极高的固有质子电导率相比，必须开发不同的固定概念，以尽量减少电导率的降低。然而，在高水活度的情况下（通常存在于PEM燃料电池中），膦酸功能化离聚物可能具有非常高的离子交换容量（IEC） （IEC bbb10 meq g(-1)），可能允许在中间温度范围内（约120 -160℃）具有高质子电导率。
The melting behaviour and transport properties of straight chain alkanes mono- and difunctionalized with phosphonic acid groups have been investigated as a function of their length. The increase of melting temperature and decrease of proton conductivity with increasing chain length is suggested to be the consequence of an increasing ordering of the alkane segments which constrains the free aggregation of the phosphonic acid groups. However, the proton mobility is reduced to a greater extent than the proton diffusion coefficient indicating an increasing cooperativity of proton transport with increasing length of the alkane segment. The results clearly indicate that the "spacer concept", which had been proven successful in the optimization of the proton conductivity of heterocycle based systems, fails in the case of phosphonic acid functionalized polymers. Instead, a very high concentration of phosphonic acid functional groups forming "bulky" hydrogen bonded aggregates is suggested to be essential for obtaining very high proton conductivity. Aggregation is also suggested to reduce condensation reactions generally observed in phosphonic acid containing systems. On the basis of this understanding, the proton conductivities of poly(vinyl phosphonic acid) and poly(meta-phenylene phosphonic acid) are discussed. Though both polymers exhibit a substantial concentration of phosphonic acid groups, aggregation seems to be constrained to such an extent that intrinsic proton conductivity is limited to values below sigma = 10(-3) S cm(-1) at T = 150 degrees C. The results suggest that different immobilization concepts have to be developed in order to minimize the conductivity reduction compared to the very high intrinsic proton conductivity of neat phosphonic acid under quasi dry conditions. In the presence of high water activities, however, (as usually present in PEM fuel cells) the very high ion exchange capacities (IEC) possible for phosphonic acid functionalized ionomers (IEC >10 meq g(-1)) may allow for high proton conductivities in the intermediate temperature range (T approximately 120 -160 degrees C).