The operando XAS experiment was performed at the ROCK beamline, PHYSICOCHEMISTRY OF LIQUIDS, where the quick-XAS monochromator enables a fast data collection SOFT MATTER, NANOCHEMISTRY compatible with high cycling rate. A in situ electrochemical cell was SYNCHROTRON SOLEIL HIGHLIGHTS 2020 specially designed for thin films configuration to collect the emitted fluorescence photons at V K-edge (Fig. 1a). The 1 µm-thick VN film was cycled between -0.4 V and -1.0 V vs Hg/HgO at 2 mVs in 0.5 M KOH-1 ROCK BEAMLINE electrolyte, using a Pt wire as the counter electrode. A clear shift of the absorption threshold was observed upon charge/ Associated publication discharge cycling, with a triangular shape of the energy shift as a function of the applied potential, as already reported for pseudocapacitive materials Novel insights into the charge storage [4] (Fig. 1b). To identify the vanadium species involved in the charge mechanism in pseudocapacitive storage mechanism, we plot the evolution of the absorption edge for VxOy vanadium nitride thick films for and VN reference compounds vs the valence of the vanadium element. high-performance on-chip micro- supercapacitors. We showed that VN is in fact not responsible for the pseudocapacitive process, but that an ultra-thin vanadium oxide layer is the key player in K. Robert, D. Stiévenard, D. Deresmes, the fast charge transfer occurring during the redox process, confirming D. Douard, A. Iadecola, D. Troadec, for the first time the hypothesis of Kumta et al. [5] The V average oxidation P. Simon, N. Nuns, M. Marinova, state was found to vary from 3.56 to 3.66 upon cycling (correlated with M. Huvé, P. Roussel, T. Brousse & XPS analyses), corresponding to an extrapolated capacitance close C. Lethien. to 88 mFcm. This capacitance is in a perfect agreement with that of-2 Energy & Environmental Science, 1 µm-thick VN film (/home/webapps/asp_fr/data/asp/publications/synchrotron-soleil/synchrotron-soleil-2020/soleil-highlights-2020-hd-ss-tc 90 mFcm). Once we had demonstrated that a-2 13(3): 949 (2020). mixed vanadium oxide VO/VO type was involved in the charge storage32 2 mechanism, the question arose about the location of VO within they References x VN film. To tackle this issue, EELS spectroscopy with a nanosized-spot [1] A. Raj & D. Steingart, was performed on 7 µm-thick sputtered VN film and we found a higher J. Electrochem. Soc., 165, oxidation state of V in the area between the VN columns, corresponding B3130 (2018). to the formation of VxOy. Sputtered VN films were also tested using [2] C. Lethien et al., Energy Environ. in situ AFM in KOH electrolyte and no contraction / expansion of the Sci., 12, 96 (2019). electrodes upon charge/discharge cycling (i.e. no structural changes) [3] O. Bondarchuk et al., J. Power was observed as expected for a pseudocapacitive material. Sources, 324, 439 (2016). [4] N. Goubard-Bretesché et al., Small, Based on these findings, we were finally able to fully unveil the charge 16, 2002855 (2020). storage process involved in sputtered VN films operating in aqueous [5] D. Choi et al., Adv. Mater., 18, 1178 electrolyte. We conclude that the sputtered VN film acts as a conductive (2006). scaffold for electron transport, while the amorphous mixed-valence Corresponding author vanadium oxides (+III/+IV), located at the edge of each column, are the redox-active material allowing fast charge transfer. Antonella Iadecola RS2E (FR3459) FIGURE 2 Réseau sur le Stockage Electrochimique de l'Energie (RS2E) Université de Picardie Jules Verne UMR7314 – LRCS 33 rue Saint-Leu 80039 Amiens Cedex 1, France Antonella.iadecola@ synchrotron-soleil.fr Captions FIGURE 1: a) Tilted view of the in situ electrochemical cell used for operando XAS on 1 µm-thick VN film in 0.5 M KOH electrolyte. b) Energy edge shift of V-edge XAS spectra upon charge/discharge. FIGURE 2: Graphical abstract summarizing the charge compensation occurring in VN thin film as micro- supercapacitors. 37