高性能双电层电容器储能机理的实验与理论研究

The development of the electric double-layer capacitors (EDLCs) in the recent years has mainly focused on improving the device energy density, which is mainly determined by the electrode/electrolyte interface structure as well as the electrochemical stability of the electrolyte. For that reason, the present project will employ the room temperature ionic liquids (RTILs) with wide voltage window as the electrolyte and graphene as the solid substrate. By using surface forces apparatus technique and molecular dynamic (MD) simulation, we will systematically investigate the effects of certain key factors on the electric double layer (EDL) structure formed at the solid/liquid interface, which serves as the basic unit for charge storage in EDLCs. In detail, experimentally, we begin with exploring the adsorption mechanisms of the RTILs on graphene surface, in order to build the fundamental understanding of ion structuring at the specific interface. Then, we will investigate the effects of surface potential coupling with ionic properties and temperature, respectively, on the EDL structure, aiming to figure out the regulation mechanisms of applied electrode potential to interfacial capacitance. As the channel dimension is further reduced to a few nanometers, we will characterize the wetting behaviors of the highly confined RTILs between two graphene surfaces, which may reveal the charging mechanisms in nanoporous electrode materials. Theoretically, we will develop an MD model to describe the RTILs molecules structuring near the graphene surface. Comparing with the experimental results, the MD model could accurately predict the interface capacitance. The efforts made in the project will hopefully provide guidance in design and optimization of the high-performance EDLs.

现阶段双电层电容器的发展重点是提升能量密度，而能量密度主要取决于电极／电解质界面的双电层结构以及电解质的电化学稳定性。本项目选择具有宽电化学窗口的室温离子液体作为电解质，以其在石墨烯表面形成的双电层结构为研究对象，采用表面力仪测量为主、分子动力学模拟配合的手段，讨论关键因素对界面相互作用以及相应的离子分布结构的影响。实验方面，研究工作将以明确离子液体在石墨烯表面上的吸附机理为基础。通过探讨表面电势与离子特性、温度相互耦合对双电层结构的影响机制，总结电极电势调控界面电容的机理。本项目还将进一步研究当通道的空间尺寸减小到纳米量级时，受限离子液体在石墨烯表面的润湿行为，明确其迁移及吸附特性，借此阐释纳米多孔电极材料的储电机制。理论方面，依据实验建立并优化分子动力学模型，实现对真实工况中电极界面电容的准确预测。这些工作的开展将为设计和开发高性能双电层电容器提供基础理论方面的指导。

双电层电容器具有功率密度高、循环稳定性强、绿色环保和超低温特性稳定等优点，是未来理想的储能设备。该类电容器的发展重点是能量密度提升，这主要取决于电极／电解质界面的双电层结构以及电解质的电化学稳定性。为此，本项目选取离子液体在内的高浓度电解质为研究对象，采用表面力仪测量技术为主、分子动力学模拟为辅的技术手段，研究了关键参数对电解质在固/液界面的双电层结构的影响机制和内在机理。主要研究内容包括离子特异性及温度对离子液体固液界面结构的影响机理、水分子对离子液体固液界面结构的影响机理、高浓度电解质水溶液中固液界面的离子结构。所取得的研究成果总结如下：（1）明确了离子尺寸效应、工作温度分别对离子液体双电层结构和特征尺寸的影响机制，并从电解质微观结构以及与固体表面之间相互作用的角度给出了内在机理。（2）展示了离子液体界面结构随含水量不同的变化关系，并通过分析不同阶段水分子对电解质中离子间相互作用以及固体表面电荷的作用机理并解释了其中的原因。（3）证明了电解质水溶液在高浓度条件下的界面结构特征同样受到离子溶剂化效应的影响，并解释了离子水合和离子特异性在其中的作用机理。（4）通过改进表面力仪系统，实现了更高精度和分辨率的力探测技术。在项目的资助下，项目组共发表论文共计4篇，其中SCI收录论文3篇，EI收录论文1篇；培养及协助培养博士研究生1名，硕士研究生4名。