Proton Transfer in Protic Ionic Liquids

不易燃的离子液体-在室温下为液体的盐-具有替代电解质的潜力，然而，大多数“常规”离子液体的高粘度使得电荷传输对于电化学应用来说太慢。这个缺点可以通过使用质子离子液体来规避，对于质子离子液体，电荷传输不仅源于大体积离子的运动，而且还源于小而轻的质子的传输。因此，电荷传输可以与质量传输解耦，并且电导率可以增加。然而，很难评估质子传输增强电解质电导率的程度（如果有的话）。到目前为止，这主要是从质子的平衡分布或从所有电荷载流子的长程输运来估计的。由于这些措施都不能隔离质子传导，这些研究导致了不满意和不一致的结果。在这里，我们提出了一个详细的调查质子离子液体中的电荷传输在所有相关的时间和长度尺度。这将通过（i）使用实验和模拟的组合来隔离质子转移对宽带介电谱的贡献来实现。我们将建立可逆的质子转移在极化MD模拟，从而实现协议与实验光谱从兆赫到远红外频率的各种纯甲基咪唑基离子液体和它们的混合物与极性溶剂乙腈。建立了这些模型系统的方法，我们的目标是（ii）了解质子离子液体中质子导电性的基本机制。为此，我们将使用氟化羧酸改变离子液体阴离子的质子供体强度。使用建立的方法，我们将阐明质子传导的阴离子和阳离子之间的质子平衡的效果。为了进一步提取长程质子迁移率，我们将使用光酸来触发过量质子的释放，并通过红外检测其到达接受基地来实时跟踪它们的运输。极化分子动力学模拟提供了一种新的策略来阐明和模拟质子离子液体中的电荷传输，其对未来电解质的设计具有影响。

Non-flammable ionic liquids – salts that are liquid at room temperature – have the potential to replace electrolytes, however, the high viscosity of most ‘conventional’ ionic liquids makes charge transport too slow for electrochemical applications. This drawback can be circumvented by using protic ionic liquids, for which charge transport originates not only from motion of bulky ions but also from transport of small and light protons. Charge transport can thus be decoupled from mass transport and the conductivity can be increased. It is however difficult to asses how much – if at all – proton transport enhances the electrolyte conductivity. So far this has been mainly estimated from the protons’ equilibrium distribution or from the long-ranged transport of all charge carriers. As none of these measures can isolate proton conduction, these studies have led to unsatisfactory and inconsistent results. Here we propose a detailed investigation of charge transport in protic ionic liquids at all relevant time- and length-scales. This will be achieved by (i) isolating the contribution of proton transfers to broadband dielectric spectra using a combination of experiments and simulations. We will establish reversible proton transfer in polarizable MD simulations such that agreement with experimental spectra from Megahertz to far-infrared frequencies for various pure methylimidazolium based ionic liquids and their mixtures with the polar solvent acetonitrile is achieved. Having established the methodology for these model systems, we aim at (ii) understanding the fundamental mechanisms of proton conductivity in protic ionic liquids. To this end, we will vary the proton donor strength of the ionic liquid anion using fluorinated carboxylic acids. Using the established methodology we will elucidate the effect of the proton equilibrium between the anion and the cation on proton conduction. To further extract long-ranged proton mobilities, we will use photoacids for a triggered release of excess protons and trace their transport in real-time via infrared detection of its arrival at an accepting base.We envision that the combination of experimental results and reactive, polarizable molecular dynamics simulations offers a novel strategy to elucidate and model charge transport in protic ionic liquids with impact on the design of future electrolytes.