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International audience; Fundamental understanding of ion electroadsorption processes in porous electrodes on a molecular level provides important guidelines for next-generation energy storage devices like electric double layer capacitors (EDLCs). Porous carbons functionalized by heteroatoms show enhanced capacitive performance, but the underlying mechanism is still elusive, due to the lack of reliable tools to precisely identify multiple N species and establish clear structure property relations. Here, we use advanced analytical techniques such as lowtemperature solid-state NMR (ssNMR) and electrochemical quartz crystal microbalance (EQCM) to relate the complex nitrogen functionalities to the charging mechanisms and capacitive performance. For the first time, it is demonstrated at a molecular level that N-doping strongly influences the electroadsorption mechanism in EDLCs. Without N-doping, anion (SO 4 2−) adsorption−desorption dominates the charging mechanism, whereas after doping, Li + electroadsorption plays a key role. With the help of EQCM, it is demonstrated that SO 4 2− is strongly immobilized on the Ndoped surface, leaving Li + as the main charge carrier. The smaller size and higher concentration of Li + compared to SO 4 2− benefit a higher capacitance. Amine/amide N is responsible for high capacitance, but surprisingly the pyridinic, pyrrolic, and graphitic N groups have no significant influence. 2D 1 H-15 N NMR spectroscopy indicates that the conversion from pyridinium to pyrrolic N gives rise to a slightly decreased capacitance. This work not only demonstrates ssNMR as a powerful tool for surface chemistry characterization of electrode materials but also uncovers the related charging mechanism by EQCM, paving the way toward a comprehensive picture of EDLC chemistry.

Research into the design of electrolytes of ionic liquids (ILs) for supercapacitor applications has grown in popularity in recent years. Unlike Li batteries where the composition of the electrolyte and the stability must meet several requirements (formation of SEI, electrochemical kinetics), the use of ionic liquids in supercapacitors opens interesting perspectives, in particular thanks to their high electrochemical stability. [1]. In this communication, it will be presented the results of an experimental study conducted using quartz crystal microbalance (eQCM) on nanoporous carbon based electrodes in contact with different electrolytes : neat IL and solvated IL as well as electrolyte mixture. EQCM turns out to be an effective analytic tool to bring new insights regarding the charge compensation mechanisms occurring while polarizing the carbon based electrode. In particular, it will be shown that charge compensation can involve different levels of desolvation of the counter-ions and that co-ions can also be used when the carbon electrode is polarized [2,3,4]. Références : [1] P. Simon et Y. Gogotsi, Nature Materials, 7 (2008) 845-854. [2] W.-Y. Tsai et al. JACS 8722−8728 (2014) [3] J. Griffin et al., Nature Materials, 22 juin 2015 (doi: 10.1038 / nmat4318). [4] Y.-C. Wu, P.-L. Taberna, P. Simon, Communications en électrochimie. 93, 119-122 (2018).