Department of Mechanical Engineering, Clemson University, Clemson, South Carolina 29634-0921, USA.
ACS Nano. 2011 Nov 22;5(11):9044-51. doi: 10.1021/nn203260w. Epub 2011 Oct 31.
Recent experiments have shown that the capacitance of subnanometer pores increases anomalously as the pore width decreases, thereby opening a new avenue for developing supercapacitors with enhanced energy density. However, this behavior is still subject to some controversy since its physical origins are not well understood. Using atomistic simulations, we show that the capacitance of slit-shaped nanopores in contact with room-temperature ionic liquids exhibits a U-shaped scaling behavior in pores with widths from 0.75 to 1.26 nm. The left branch of the capacitance scaling curve directly corresponds to the anomalous capacitance increase and thus reproduces the experimental observations. The right branch of the curve indirectly agrees with experimental findings that so far have received little attention. The overall U-shaped scaling behavior provides insights on the origins of the difficulty in experimentally observing the pore-width-dependent capacitance. We establish a theoretical framework for understanding the capacitance of electrical double layers in nanopores and provide mechanistic details into the origins of the observed scaling behavior. The framework highlights the critical role of "ion solvation" in controlling pore capacitance and the importance of choosing anion/cation couples carefully for optimal energy storage in a given pore system.
最近的实验表明，随着孔径的减小，亚纳米孔的电容会异常增加，从而为开发具有更高能量密度的超级电容器开辟了新途径。然而，由于其物理起源尚不清楚，这种行为仍然存在一些争议。通过原子模拟，我们发现与室温离子液体接触的狭缝状纳米孔的电容在 0.75 到 1.26nm 的孔径范围内呈现出 U 形的标度行为。电容标度曲线的左分支直接对应于异常电容增加，从而再现了实验观察结果。曲线的右分支间接与实验结果一致，而这些结果迄今为止很少受到关注。整体 U 形标度行为为实验观察孔径依赖性电容的困难提供了一些启示。我们建立了一个理解纳米孔中双电层电容的理论框架，并提供了观察到的标度行为的起源的机制细节。该框架强调了“离子溶剂化”在控制孔电容方面的关键作用，以及在给定的孔系统中为实现最佳储能而谨慎选择阴离子/阳离子对的重要性。
