Influence of Water on Structure, Dynamics, and Electrostatics of Hydrophilic and Hydrophobic Ionic Liquids in Charged and Hydrophilic Confinement between Mica Surfaces
Influence of Water on Structure, Dynamics, and Electrostatics of Hydrophilic and Hydrophobic Ionic Liquids in Charged and Hydrophilic Confinement between Mica Surfaces
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DOI：
10.1021/acsami.9b10923
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发表时间：
2019-09-11
影响因子：
9.5
通讯作者：
Espinosa-Marzal, Rosa M.
中科院分区：
文献类型：
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作者：
Han, Mengwei;Espinosa-Marzal, Rosa M.
水在环境中无处不在，是超级电容器离子液体电解质操作限制的来源。在本研究中，水对亲水性（1-乙基-3-甲基咪唑硫酸乙酯，缩写[EMIM][EtSO4]）和疏水性(1-乙基-3-甲基咪唑三（五氟乙基）三氟磷酸盐和1-乙基-3-甲基咪唑二（三氟甲基磺酰基）亚胺，缩写[EMIM][FAP]和[EMIM][TFSI]的界面行为的影响，在由表面力装置精确调制的分离和控制相对湿度在0%和50% RH之间的条件下，分别研究了云母表面之间的离子液体。扩散实验表明，在一定的湿度阈值以上，水会自发地侵入纳米限制的疏水IL，在足够高的环境湿度下（类似于这里的45%），由于表面诱导的相分离，限制的疏水IL完全被水取代。这一行为预计也适用于其他不能与水完全混溶的离子，当它们被限制在几纳米宽的亲水纳米孔中时。通过动态力测量研究了环境湿度对界面结构、动力学和静电性能的影响。在干燥状态下，云母表面固定了几层离子，有效粘度增加了2个数量级，膜厚度从10 nm左右减少到3 nm左右。基于最近的工作，有人提出，纳米限制增强了高浓度电解质中的阴离子-阳离子结合，从而证明了il的流动性损失是合理的。当相分离被排除在外时，水被插入到三个il的层状结构中，导致层厚与干燥状态相比发生变化。此外，我们的研究结果表明，界面水阻止离子在表面固定，并通过降低亲水性和疏水性il的有效粘度来促进它们的流出，其顺序为[EMIM][FAP] < [EMIM][TFSI] < [EMIM][EtSO4]。通过筛选静电相互作用和不同程度的溶剂化，考虑水在促进离子解离中的作用，评估了潜在的机制。所讨论的实验结果支持了分子动力学模拟和中子散射研究的最新发现，即使用亲水性il与水作为共溶剂可以提高il基超级电容器的功率密度，因此，水（亲水性）il作为超级电容器的电解质是一个值得探索的方向。
Water is ubiquitous in the environment and is the origin for operational constraints in ionic-liquid based electrolytes for supercapacitors. In this study, the influence of water on the interfacial behavior of hydrophilic (1-ethyl-3-methylimidazolium ethylsulfate, abbr. [EMIM][EtSO4]) and hydrophobic (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, abbr. [EMIM][FAP] and [EMIM][TFSI], respectively) ionic liquids (ILs) confined between mica surfaces was investigated at separations precisely modulated by a surface force apparatus and at controlled relative humidity between 0% and 50% RH. Diffusion experiments revealed that water spontaneously invades the nanoconfined ILs above a certain humidity threshold and that the confined hydrophobic IL is completely replaced by water at sufficiently high environmental humidity (similar to 45% here) as a result of surface-induced phase separation. This behavior is expected to be universal for other ILs that are not fully miscible with water when they are confined in hydrophilic nanopores of a few nanometers in width. The effect of environmental humidity on interfacial structure, dynamics, and electrostatics was studied via dynamic force measurements. In the dry state, several layers of ions are immobilized on the mica surface, and the effective viscosity increases by up to 2 orders of magnitude with a decrease in film thickness from similar to 10 to similar to 3 nm. Based on recent work, it is proposed that nanoconfinement enhances the anion-cation association in highly concentrated electrolytes, thereby justifying the loss of fluidity of the ILs. When phase separation is excluded, water is intercalated in the layered structure of the three ILs, and it leads to a change of the layer thickness compared to the dry state. Furthermore, our results reveal that interfacial water prevents ions from being immobilized on the surface and facilitates the outflow of both hydrophilic and hydrophobic ILs by reducing their effective viscosity in the order [EMIM][FAP] < [EMIM][TFSI] < [EMIM][EtSO4]. The underlying mechanisms are evaluated by considering the roles of water in enhancing ion dissociation through screening of electrostatic interactions and solvation of the selected ILs to different extents. The discussed experimental observations support the recent discoveries made by molecular dynamic simulations and neutron scattering studies that using hydrophilic ILs coupled with water as cosolvent could lead to the enhanced power density of IL-based supercapacitors, and therefore, that water-in-(hydrophilic) ILs is a direction worth exploring as electrolytes for supercapacitors.