Capacitance-Power-Hysteresis Trilemma in Nanoporous Supercapacitors

Abstract

Nanoporous supercapacitors are an important player in the field of energy storage that fill the gap between dielectric capacitors and batteries. The key challenge in the development of supercapacitors is the perceived trade-off between capacitance and power delivery. Current efforts to boost the capacitance of nanoporous supercapacitors focus on reducing the pore size so that they can only accommodate a single layer of ions. However, this tight packing compromises the charging dynamics and hence power density. We show via an analytical theory and Monte Carlo simulations that charging is sensitively dependent on the affinity of ions to the pores, and that high capacitances can be obtained for ionophobic pores of widths significantly larger than the ion diameter. Our theory also predicts that charging can be hysteretic with a significant energy loss per cycle for intermediate ionophilicities. We use these observations to explore the parameter regimes in which a capacitance-power-hysteresis trilemma may be avoided.

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Plain Language Summary

The global energy crisis necessitates the development of devices that can store energy efficiently. Nanoporous supercapacitors, which store energy by accumulating ions in nanosized pores, have become increasingly popular. However, a major stumbling block hindering the implementation of such supercapacitors is the trade-off between capacitance and power delivery: Capacitance is maximized only when the pore size approaches the ion size, and the rate of charging is slow for such close-fitting pores. In addition, theory and simulations reveal that charging can proceed via an abrupt adsorption or desorption transition, which may cause hysteresis and therefore increase the energy dissipation during each charge or discharge cycle. This hysteresis adds yet another hurdle to the capacitance-power dilemma already noted. Here, we study how the capacitance-power-hysteresis trilemma can be avoided.

Using analytical calculations together with Monte Carlo simulations, we develop a theoretical model for nanoconfined ions. Our model reveals that the key control parameter for supercapacitors is the ionophobicity or ionophilicity of pores: It tells us how favorable the pore is to ions and determines the pore occupation at no applied voltage. We find that the capacitance of ionophobic pores can be optimized at pore widths significantly larger than the ion diameter, which avoids the issue of slow charging. Moreover, we show that charging is hysteretic only for intermediate ionophilicities. Therefore, by tuning the ionophobicity, nanoporous supercapacitors can be simultaneously optimized to provide high power and energy densities without hysteretic losses.

We hope that our findings will provide a framework to assess and direct future efforts to experimentally realize an optimal supercapacitor and, in particular, to control the pore ionophobicity.