Department of Environmental Sciences and Engineering, Gillings School of Global Public Health , The University of North Carolina at Chapel Hill , Chapel Hill , North Carolina 27599 , United States.
ACS Appl Mater Interfaces. 2018 Nov 21;10(46):39745-39756. doi: 10.1021/acsami.8b14494. Epub 2018 Nov 8.
Ion-exchange membrane (IEM) performance in electrochemical processes such as fuel cells, redox flow batteries, or reverse electrodialysis (RED) is typically quantified through membrane selectivity and conductivity, which together determine the energy efficiency. However, water and co-ion transport (i.e., osmosis and salt diffusion/fuel crossover) also impact energy efficiency by allowing uncontrolled mixing of the electrolyte solutions to occur. For example, in RED with hypersaline water sources, uncontrolled mixing consumes 20-50% of the available mixing energy. Thus, in addition to high selectivity and high conductivity, it is desirable for IEMs to have low permeability to water and salt to minimize energy losses. Unfortunately, there is very little quantitative water and salt permeability information available for commercial IEMs, making it difficult to select the best membrane for a particular application. Accordingly, we measured the water and salt transport properties of 20 commercial IEMs and analyzed the relationships between permeability, diffusion, and partitioning according to the solution-diffusion model. We found that water and salt permeance vary over several orders of magnitude among commercial IEMs, making some membranes better suited than others to electrochemical processes that involve high salt concentrations and/or concentration gradients. Water and salt diffusion coefficients were found to be the principal factors contributing to the differences in permeance among commercial IEMs. We also observed that water and salt permeability were highly correlated to one another for all IEMs studied, regardless of polymer type or reinforcement. This finding suggests that transport of mobile salt in IEMs is governed by the microstructure of the membrane and provides clear evidence that mobile salt does not interact strongly with polymer chains in highly swollen IEMs.
在燃料电池、氧化还原液流电池或反向电渗析 (RED) 等电化学过程中，离子交换膜 (IEM) 的性能通常通过膜选择性和电导率来量化，这两者共同决定了能量效率。然而，水和共离子的传输（即渗透和盐扩散/燃料交叉）也会通过允许电解质溶液不受控制地混合来影响能量效率。例如，在使用高盐度水源的 RED 中，不受控制的混合会消耗 20-50%的可用混合能量。因此，除了高选择性和高电导率外，IEM 还希望具有低的水和盐渗透率，以最大限度地减少能量损失。不幸的是，对于商业 IEM，几乎没有可用的定量水和盐渗透率信息，因此很难为特定应用选择最佳的膜。因此，我们测量了 20 种商业 IEM 的水和盐传输特性，并根据溶液扩散模型分析了渗透率、扩散和分配之间的关系。我们发现，商业 IEM 之间的水和盐渗透率差异很大，在涉及高盐浓度和/或浓度梯度的电化学过程中，有些膜比其他膜更适合。发现水和盐扩散系数是导致商业 IEM 之间渗透率差异的主要因素。我们还观察到，对于所有研究的 IEM，水和盐渗透率彼此高度相关，无论聚合物类型或增强材料如何。这一发现表明，IEM 中可移动盐的传输受膜的微观结构控制，并提供了明确的证据，表明在高度溶胀的 IEM 中，可移动盐与聚合物链没有强烈相互作用。
