Hydroxide Solvation and Transport in Anion Exchange Membranes
Hydroxide Solvation and Transport in Anion Exchange Membranes
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DOI：
10.1021/jacs.5b11951
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发表时间：
2016-01-27
影响因子：
15
通讯作者：
Voth, Gregory A.
中科院分区：
文献类型：
--
作者：
Chen, Chen;Tse, Ying-Lung Steve;Voth, Gregory A.
了解氢氧化物在阴离子交换膜（AEM）中的溶剂化和传输可以为这些新膜的设计原理提供重要的见解。为了准确地模拟氢氧化物的溶剂化和运输，我们开发了一个新的多尺度反应分子动力学模型的氢氧化物在水溶液中，然后随后修改的AEM材料。在此基础上，研究了氢氧化物在膜中的溶剂化结构和传输机理。我们发现，一个相对均匀的刚性侧链分离产生一个连续的重叠区域的氢氧化物运输，这是由第一水合壳的束缚阳离子基团。我们的研究结果表明，氢氧化物有一个显着的偏好，这个重叠区域，通过它和AEM侧链之间的车辆（标准扩散）和Grotthuss（质子跳跃）机制的实质性贡献运输。AEM与普通质子交换膜（PEM）的比较表明，在AEM中的过量电荷是离域比质子交换膜，这是与质子转移反应的更高的自由能垒。车辆机制也大大超过Grotthuss机制的氢氧化物运输在AEM，而我们以前的研究PEM系统显示了更大的贡献，从Grotthuss机制比车辆机制的质子运输。氢氧化物在AEM中扩散的活化能势垒大于质子在PEM中扩散的活化能势垒，这意味着在升高的温度下AEM中的离子传输的更显著的增强。
Understanding hydroxide solvation and transport in anion exchange membranes (AEMs) can provide important insight into the design principles of these new membranes. To accurately model hydroxide solvation and transport, we developed a new multiscale reactive molecular dynamics model for hydroxide in aqueous solution, which was then subsequently modified for an AEM material. With this model, we investigated the hydroxide solvation structure and transport mechanism in the membrane. We found that a relatively even separation of the rigid side chains produces a continuous overlapping region for hydroxide transport that is made up of the first hydration shell of the tethered cationic groups. Our results show that hydroxide has a significant preference for this overlapping region, transporting through it and between the AEM side chains with substantial contributions from both vehicular (standard diffusion) and Grotthuss (proton hopping) mechanisms. Comparison of the AEM with common proton exchange membranes (PEMs) showed that the excess charge is less delocalized in the AEM than the PEMs, which is correlated with a higher free energy barrier for proton transfer reactions. The vehicular mechanism also contributes considerably more than the Grotthuss mechanism for hydroxide transport in the AEM, while our previous studies of PEM systems showed a larger contribution from the Grotthuss mechanism than the vehicular mechanism for proton transport. The activation energy barrier for hydroxide diffusion in the AEM is greater than that for proton diffusion in PEMs, implying a more significant enhancement of ion transport in the AEM at elevated temperatures.