Electrochemical processes at negatively polarized electrical double layer capacitor (EDLC) electrode at different cell potentials have been studied using in situ synchrotron radiation excited X-ray photoelectron spectroscopy (XPS). 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) room-temperature ionic liquid (RTIL), as an electrolyte, and Mo2C derived carbon (C(Mo2C)) based micromesoporous electrodes, as an EDLC electrodes, were studied within very wide cell potential region (up to 4.0 V). To store more charge in a RTIL based EDLC, higher cell potentials have been applied leading to the cross-over of the limit of the ideal polarization of the capacitor electrodes and to the initiation of different faradaic processes. Therefore, parallel to XPS measurements, the cyclic voltammetry was used to obtain electrochemical data, correlated with previously calculated electrochemical (including electrochemical impedance spectroscopic) characteristics for C(Mo2C) | RTIL interface. In this paper we have, according to our knowledge, first polarized the RTIL based EDLC electrodes in real in situ vacuum conditions, measured and analyzed the supercapacitor two-electrode cell potential vs. XPS spectra relationship and discussed in the light of XPS data possible electrochemical reactions taking place at the negatively charged working electrode | RTIL interface at the different cell potential applied.

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The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF4] ionic liquid was recorded and compared to previously measured vapor phase UPS spectrum.

The electrochemical properties of the C(Mo 2 C) | EMImB(CN) 4 system have been analyzed using cyclic voltammetry, in situ X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy techniques. The cyclic voltammetry and electrochemical impedance measurements indicated that the tetracyanoborate anions started to electrooxidize and electropolymerize at E = 2.12 V (vs. Ag/AgCl in EMImBF 4 ). However, despite of the formation of compact dielectric polymer layer at the micro-mesoporous C(Mo 2 C) electrode surface very slow electrochemical polymerization of tetracyanoborate anions continued further at E > 2.12 V leading to the blocking of the C(Mo 2 C) electrode porous structure and to the reduction of the electrochemically active surface area necessary for the construction of the high energy density supercapacitors. Thus, the tetracyanoborate anion based ionic liquids can be used as the electrolytes for the initial formation of the passivating solid-electrolyte interface for the very wide cell potential supercapacitor positive electrodes, as an electrolyte additives for the solid-electrolyte interface formation or for the correction of the solid-electrolyte interface damages. Room temperature ionic liquids (RTILs) are remarkable electrolytes (salts) being in a liquid form at or near the room temperature. This means that it is possible to construct the electrical energy storage systems without the need for the solvent characterized usually with high vapor pressure. [1][2][3][4][5][6][7][8][9] The application of the RTILs is also very welcome in the energy technology due to their good electrical conductivity and high electrochemical stability, 10-13 low volatility [14][15][16][17] and relatively high thermal stability. [18][19][20] One class of the electrical energy storing systems, where RTILs can be used, is the electrical double layer capacitors (EDLCs). 10,[21][22][23][24][25][26][27] The EDLCs with very high power and the energy densities are also called as supercapacitors. [28][29][30] Some RTILs and the electrode materials have been investigated and described so far 12,13,22,[24][25][26][27][28][29][30][31][32][33] and the studies of the possible candidates for the supercapacitor materials and the development process of higher power and the energy density systems is still intensive. One way to increase the EDLCs energy density stored is to apply higher cell potentials while the amount of charge stored in a capacitor is proportional to the square of the applied cell potential. [28][29][30]34 Therefore it is of a great interest to study RTILs as possible electrolytes for supercapacitors at high cell potential (i.e. high operating voltage).26,34-44 However, higher cell potential can initiate the start of the faradaic processes at the supercapacitor electrodes, leading to the degradation of the materials or even to the burst of the cell body. 1,13,[45][46][47][48][49][50][51][52][53][54][55][56] One of the most electrochemically stable RTILs is 1-ethyl-3-methylimidazolium tetracyanoborate (EMImB(CN)...