Nanoscopic Supercapacitance Elucidations of the Graphene-Ionic Interface with Suspended/Supported Graphene in Different Ionic Solutions

Graphene-based supercapacitors have gained significant attention due to their exceptional energy storage capabilities. Despite numerous research efforts trying to improve the performance, the challenge of experimentally elucidating the nanoscale-interface molecular characteristics still needs to be tackled for device optimizations in commercial applications. To address this, we have conducted a series of experiments using substrate-free graphene field-effect transistors (SF-GFETs) and oxide-supported graphene field-effect transistors (OS-GFETs) to elucidate the graphene-electrolyte interfacial arrangement and corresponding capacitance under different surface potential states and ionic concentration environments. For SF-GFET, we observed that the hysteresis of the Dirac point changes from 0.32 to −0.06 V as the ionic concentration increases. Moreover, it results in the interfacial capacitance changing from 4 to 2 F/g. For OS-GFET, the hysteresis of the Dirac point remains negative (−0.15 to −0.07 V). Furthermore, the corresponding capacitance of OS-GFET decreases (53–16 F/g) as the ionic concentration increases. These suggest that the orderly oriented water structure at the graphene–water interface is gradually replaced by ionic hydration clusters and results in the difference of capacitance. The relationship between Dirac-point hysteresis value and ionic concentration can be modeled by using the first-order Hill equation to obtain the half occupation value (K = 1.0131 × 10^–4 for KCl solution and K = 6.6237 × 10^–5 for MgCl₂ solution). This also agrees with the variances of two minerals in ion hydration within the inner water layer at the interface. This work illustrates the influence of interfacial nanoscale arrangement on interface capacitance formation and layout implications for the development of supercapacitors.