Correlation between redox active sites and sodium storage behavior in dye/graphene nanohybrids

Sodium ion batteries (SIBs) have drawn attention for large-scale electrical energy storage owing to the abundant sodium resources and the low fabrication cost. However, the commercialization of SIBs is hindered by the sluggish diffusion and poor structural reversibility of electrode materials originating from the large ionic radius of sodium ions. In this study, organic dye (methylene blue, MB; methyl orange, MO)-based hybrid materials were fabricated via a simple hydrothermal reduction of dye-adsorbed graphene oxide (GO). The MB/rGO nanohybrid (MBG) electrodes showed a reversible capacity of 108 mAh g−1 at a high current density of 1000 mA g−1 after 1000 cycles, while the MO/rGO nanohybrid (MOG) electrodes showed a reversible capacity of 172 mAh g−1 at a mild current density of 50 mA g−1 after 200 cycles. By analyzing the electrochemical kinetics and DFT calculations, we investigated the sodium storage mechanisms of the dye/graphene nanohybrids based on the different redox-active sites. The introduction of N, S-containing heterocyclic moiety derived from the MB contributed surface induced capacitive reactions, while highly reactive azo moieties derived from the MO provided diffusion-controlled faradaic reactions. This work elucidates the correlation between the redox-active sites and the electrochemical characteristics in terms of the sodium storage mechanism, and provides novel insights for designing pseudocapacitive hybrid materials.