A key concept in classical electrokinetic theories is that the viscosity of interfacial fluids is much higher than that of bulk fluids, and this concept is indirectly supported by experimental evidence and molecular dynamics simulations. However, a universal mechanism that encompasses the breadth of experimental evidence is still lacking. Here we show, using molecular dynamics simulations, that the “apparent” thickening of interfacial fluids in electrokinetic transport near molecularly smooth surface originates mainly from the fact that ion-wall interactions are not accounted for in the hydrodynamic model of classical electrokinetic theories. Specifically, strong ion-wall interactions cause intermittent adsorption of ions on charged walls, and this in turn leads to loss of driving force for flow and screening of fluid flow by the adsorbed ions. Although not considered in the classical electrokinetic theories, these effects can significantly suppress electrokinetic transport. Consequently, when the classical theories are used to interpret the electrokinetic data, the viscosity of interfacial fluids appears to be greatly enhanced even if their material viscosity is similar to that of the bulk fluids. This mechanism for the apparent thickening of interfacial fluids is applicable to electrokinetic transport near any type of charged surface.

Engineering high-performance carbonaceous electrode materials from earth-abundant biomass has attracted substantial attention for its applicability in next-generation supercapacitors (SCs). However, these materials exhibit low specific energy due to the dominance of mesopores and a limited potential window. To overcome these shortcomings, herein, we synthesize Miscanthus sinensis (silver grass)-derived hierarchically-porous activated carbons (SHACs) via pyrolysis, carbonization, and KOH activation. We test the SHAC electrodes with different electrolytes, showing how an electrolyte–electrode pair can be tuned to boost energy and power densities. Owing to the synergetic effect of the size-balanced proportion of micropores matched with the size of electrolyte ions, in KOH electrolyte, the SHAC electrode produces a high specific capacitance (592 F g⁻¹) while, simultaneously, providing faster charging compared to Na₂SO₄ electrolyte. We rationalize these findings with molecular dynamics simulations, demonstrating the avoidance of power-density trade-off, typical for microporous SCs. Upon adding K₃Fe(CN)₆ redox species to KOH electrolyte (hybrid electrolyte), capacitance increases 2.53 fold (380 to 963 F g⁻¹ at 5 A g⁻¹) due to the synergy of capacitive and faradaic energy storage mechanisms. In the hybrid electrolyte, a SHAC electrode-embedded symmetric SC (SSC) offers a high cycling stability (97%) with 1.6 V wide operational voltage and permits energy storage and power density higher than those reported so far for aqueous electrolyte-based SSCs and asymmetric SCs. In addition, these SSCs provide long-lasting operational capabilities that are useful for driving various portable electronic devices. The obtained results demonstrate a feasible methodology to utilize the maximum available surface area of carbonaceous materials for electrochemical energy storage applications.