Solid-state NMR Study On Ion Adsorption And Charge Storage In Graphene Films

Thin film made of graphene stacks,also known as "graphene paper" or "graphene film",has been demonstrated as promising active material for electric double layer capacitors（EDLCs）,mainly due to its excellent mechanical flexibility and freestanding morphology.While extensive efforts have been devoted to the design and construction of high performance active materials,it is equally important to explore the underlying charge storage mechanism of EDLCs.Despite the motion of particles in porous carbon materials with well-defined and variable pore sizes has been well described at molecular/atomic scale with the molecular dynamic（MD）simulations and density functional theory（DFT）calculations,a direct observation on the ion distribution and ion population upon charging is highly needed.In recent years,nuclear magnetic resonance（NMR）has emerged as an element-selective,highly localized and quantitative technique to obtain the atomic-scale information on the local environments with direct observation.In this work,graphene-film based EDLCs are assembled in a two-electrode system with a layered structure and 1 M TEABF4/AN organic solution is used as electrolyte.The graphene-film electrodes prepared by chemically reduction of graphene oxide performed outstanding specific capacitance of 108 F g^-1,power density of 21 kW kg^-1 and energy density of 8.4 Wh kg^-1.The solid-state nuclear magnetic resonance（NMR）spectroscopy is firstly introduced in the investigation of distribution and variation pattern of electrolyte ions in graphene-film based EDLC electrodes.11B magic-angle spinning（MAS）Solid-state NMR is used to monitor the BF₄^-anions in graphene nano-channel.For neutral graphene films soaked with different amounts of electrolytes（1 M TEABF4/AN）,weakly and strongly adsorbed anions are identified based on the resonances at different 11B chemical shifts.Unlike other porous carbonaceous materials,the strongly adsorbed anions are found as the major electrolyte anions components in graphene films and play a predominant role on the charging process.Further measurements on the ion population upon charging are carried out with applying different charging voltages（0,0.5,1.0,1.5,2.0 and 2.5 V）on the graphene films.Results indicate that the charging process of graphene-film based EDLCs can be divided into two distinct charge storage stages（i.e.,ejection of co-ions and adsorption of counter-ions）for different voltages.During the charging process,the abnormal decrease or increase in the number of counter-ions or co-ions within graphene channels arise from the strongly ion-ion correlation between cations and anions.The as-obtained results will be useful for the design and fabrication of high performance graphene-film based EDLCs.