Optimizing supercapacitor electrode density: achieving the energy of organic electrolytes with the power of aqueous electrolytes

Abstract

The value of electrode density is often overlooked in the pursuit of impressive supercapacitor metrics. Low-density electrodes deliver the best performance in terms of gravimetric energy and power densities when only the mass of the electrodes is considered. However, energy and power values with respect to the total system mass (electrode + electrolyte) or volume are more meaningful for practical application. Low-density electrodes are impractical due to both large mass contributions by the electrolyte and large system volumes. Here, we use highly compressible graphene aerogel electrodes (up to 87.5% volumetric compression) to systematically characterize the effects of electrode density on energy and power metrics. The results reveal that electrode density is similar to electrode thickness in that both parameters have a squared effect on power. Accounting for the aqueous electrolyte's mass lowered the gravimetric energy and power by almost an order of magnitude for 0.144 g cm⁻³ dense carbon electrodes but only by a factor of 1.5 when the electrode density was increased to 1.15 g cm⁻³ through compression. The high-density electrodes achieve 8 W h kg⁻¹, 70 000 W kg⁻¹, and 144 F cm⁻³ in a symmetric electrode setup after factoring in the aqueous electrolyte's mass. Therefore, in the pursuit of high energy per mass, it can be just as effective to lower the system's mass with smaller electrolyte fractions as it is to use electrolytes with larger voltage ranges. High electrode densities allow aqueous electrolyte supercapacitors to attain energy densities per the system mass comparable to those of commercially-available organic electrolyte supercapacitors while maintaining 10–100× greater power.