Graphene And Graphite Paper Are Used In Electrocatalytic Oxygen Reduction And Supercapacitor Applications

In recent years,due to the energy crisis and environmental pollution,the development of renewable energy is in urgent need.Supercapacitors and fuel cells are promising candidates for energy conversion and storage devices,but the low efficiency of oxygen reduction reaction?ORR?and the low energy density of supercapacitors are still the bottlenecks for their applications.In this work,we prepared the S/N-codoped graphene in an electrochemical method and used it to catalyze the oxygen reduction reaction.We also prepared electrochemically oxidized and moderately reduced graphite paper and used it as a flexible supercapacitor electrode.A simple and economic method was developed for synthesizing efficient S,N-codoped graphene based catalyst for oxygen reduction reaction via a one-step electrochemical exfoliation of graphite rod in melamine and?NH₄?₂SO₄ mixture solution.On one hand,melamine and?NH₄?₂SO₄ co-worked as electrolyte to enhance the exfoliation efficiency of graphite into graphene nanosheets.On the other hand,they also provided nitrogen and sulphur sources for in-situ codoping of the as-prepared graphene.As evidenced,the N and S elements were distributed uniformly throughout the graphene nanosheets which possessed larger interlayer spacing and higher defect density.Accordingly,the obtained sample showed efficient ORR catalytic activity with an onset potential of 0.88 V and a dominant four-electron transfer mechanism?n=3.98 at 0.5 V?.Moreover,the sample has long-time durability and excellent methanol tolerance,and should have great application prospect.In addition,a piece of graphite paper was electrochemically oxidized in the mixture of H₂SO₄ and HNO₃ first,followed by the mild reduction using hydroiodic acid to afford the partially oxidized and moderately reduced sample?noted as MOGP-4?.The as-prepared MOGP-4 not only had the abundant oxygenous groups,but also preserved the highly conductive network?sp² carbon?.Owing to the good conductivity?1.14×10³S/m?,MOGP-4 exhibited a high rate capability of 93%?from 18.3 mF/cm² to 17 mF/cm²?as the current density increases from 0.5 to 10 mA/cm² and predominant cycling performance.Furthermore,the flexible solid-state supercapacitors?SSCs?based on MOGP-4 with different sizes were assembled and delivered excellent durability,high flexibility and superior rate capability.The high electrochemical performances of the as-fabricated supercapacitors should envision promising applications for energy storage devices.