The Capacitance-Power-Hysteresis Trilemma in Nanoporous Supercapacitors

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 tradeoff
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 the capacitance of
ionophobic pores can be optimized at pore 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 \emph{trilemma} may be avoided.