Detangling the factors influencing the capacitance and interfacial phenomena in rGO-based supercapacitors in acidic aqueous electrolytes

Reduced graphene oxide (rGO) has been widely studied as active material for supercapacitor electrodes. However, rGO samples derived from graphene oxide reduction have been shown to display varying electrochemical performances. The aim of this work is to complete the understanding of the correlations between rGO physico-chemical properties and electrochemical performance in aqueous acidic electrolyte. Therefore, different pristine rGOs with varying chemical, structural and morphological properties have been studied. The pivotal role of surface chemistry was further shown by demonstrating that carbonyl group content, notably drives interfacial charge storage mechanisms. The hydrazine-reduced rGO (rGO-Hz), although exhibiting the highest Specific Surface Area (SSA), reduction degree, and conductivity, had the lowest carbonyl content and capacitance. The highest capacitance was displayed by the rGO reduced hydrothermally (GHG), characterized by a low degree of reduction and the highest amount of carbonyl groups. This was explained by the redox reactivity of these moieties, giving rise to an increase of the pseudo-capacitance. The investigation of the storage mechanism using classical Electrochemical Quartz Crystal Microbalance (EQCM) and advanced ac -electrogravimetric analysis showed multi-species involvement (H + , SO 4 2- , HSO 4 - ions and free H 2 O molecules), and allowed their amount and flux dynamics to be extrapolated. The interfacial charge compensation mechanism is impacted by carbonyl content, as evidenced by significant H⁺ involvement and slower interfacial kinetics arising from additional faradaic processes. These analyses show that the surface redox reactions involve H⁺ charge adsorption/transfer and provide experimental evidence for the coexistence of faradaic processes and Electric Double-Layer (EDL) behavior, thereby complementing the cyclic voltammetry analysis in a valuable and comprehensive manner.