润湿特性对超级电容器储能动力学的影响机理

润湿特性对超级电容器储能性能有着至关重要的影响。借助分子动力学模拟,本文研究了润湿特性对超级电容器储能动力学行为的影响。以石墨烯和晶体铜作为疏电解液和亲电解液电极材料。结果表明,在充电过程中,亲电解液铜电极呈现出非对称的U型微分电容曲线,负极电容是正极的～5.77倍,不同于经典双电层理论Gouy-Chapman-Stern(对称U型)和疏电解液型。该现象与离子自由能阻力分布密切相关,负极自由能阻力远小于正极(～2倍)和疏电解液电极,进而有利于强化双电层结构对电极电压的响应能力,导致更高微分电容。通过微分离子电荷密度,本文再现了微分电容演变规律,并发现改善润湿性会显著降低双电层厚度。最后,我们指出润湿性直接影响储能微观机理,将电荷储存机制从离子吸附和交换共同主导(疏电解液)转变到离子吸附主导(亲电解液)。本文所得结论揭示了润湿特性对储能动力学行为影响的原子层级机理,对超级电容器材料设计、构筑与润湿特性调控具有重要指导意义。

Influence of Wettability on the Charging Dynamics of Electric Double-Layer Capacitors

Electric double-layer capacitors (EDLCs) are advanced electrochemical devices that have attracted tremendous attention because of their high power density, ultra-fast charging/discharging rate, and superior lifespan. A major challenge is how to further improve their energy density. At present, a large number of research efforts are primarily focusing on engineering the morphology and microstructure of electrodes to achieve better performance, for example, enlarging the specific surface area and designing the pore size. More importantly, wettability plays a crucial role in maximizing the effective utilization and accessibility of electrode materials. However, its primary mechanisms/phenomena are still partially resolved. Here, we explore the effects of wettability on the charging dynamics of EDLCs using molecular dynamics (MD) simulations. Typically, hydrophobic graphene (GP) and hydrophilic copper (Cu) are employed as the electrode materials. Differential capacitances (C_D) as a function of electrode potentials (?) are computed by means of Poisson and Gaussian equation calculations. Simulation results show that during the charging process of EDLCs, the differential capacitances of hydrophobic GP are insensitive to the electrode potentials. However, superhydrophilic Cu electrode exhibits an asymmetric U-shaped C_D–? curve, in which the capacitance at the negative polarization can be ~5.77 times greater than that of the positive counterpart. Such an unusual behavior is obviously different with the conventional Gouy-Chapman-Stern theory (i.e., symmetric U-shaped), room temperature ionic liquids (i.e., camel-, or bell-shaped), and hydrophobic counterpart, which is closely correlated with the free energy barrier distributions. Compared with the positive polarization or hydrophobic case, the energy barriers near the negative hydrophilic electrodes are remarkably suppressed, which benefits ion populations at the interface and enables the convenient orientation or distribution of ions to shield the external electric fields from electrodes, thereby yielding higher differential capacitances. With differentiating the ion charge density, the as-obtained C_D–? curves are well resembled, quantitatively establishing the correlations between EDL microstructures and differential capacitances. Besides, we also point out that enhancing the wettability could significantly decrease the EDL thickness from ~1.0 nm (hydrophobic) to ~0.5 nm (hydrophilic). In the end, we demonstrate that wetting property also impacts a prominent role in the charge storage behavior of EDLCs, transforming the charging mechanism dominated by counter-ion adsorption and ion exchange (hydrophobic) to pure counter-ion adsorption (hydrophilic). The as-obtained insights highlight the significance of wettability in regulating charging dynamics and mechanisms, providing useful guidelines for precisely controlling the wetting property of electrode materials for advanced charge storage of EDLCs.