In Situ XPS Studies of Electrochemically Negatively Polarized Molybdenum Carbide Derived Carbon Double Layer Capacitor Electrode

Abstract

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.

In Situ XPS Studies of Electrochemically Positively Polarized Molybdenum Carbide Derived Carbon Double Layer Capacitor Electrode

“…43 At E = 1.00 V, one C 1s PE peak at BE = 286.25 eV (containing C 1s PE-s signal from C 1 -C 4 and C 6 ) and a shoulder at BE = 285.05 eV (containing C 1s PE-s signal from C 5 ) are visible (Fig. 3).…”

“…The XPS spectra, measured at different E, were thereafter corrected by the same BE shift (so called electrochemical binding energy shift). 39,43,50,51 The XPS signal reproducibility tests made for non-polarized WE show that the non-calibrated BE value for the C 1s PE peak, consisting of five C 1s photoelectron signals for C 1 -C 4 and C 6 (defined in Fig. 2) PE peaks is stabile having BE = 290.7±0.12 eV (n = 17, RSD = ±0.041%), and therefore we corrected the measured XPS data by applying the constant BE shift for the entire series of the XPS measurements.…”

“…The almost vertical position of the EDLC cell permitted us to use parallel to ground X-ray beam and to flow-down the exceeding amount of RTIL from the porous carbon electrode surface exited and investigated. 43 This makes our experimental cell unique compared to previously described quasi 38,39 and offered type in situ XPS electrochemical (EC) cells. [40][41][42] The EC cell ( Fig.…”

“…The I411 beamline and endstation parameters have already been described previously in detail elsewhere, 43,48 and only a brief summary is given here to readers for better understanding. The I411 beamline covers the excitation photon energy range from 40 eV to about 1500 eV, equipped with a modified SX-700 plane grating monochromator with 1220 mm −1 grating and a plane-elliptical focusing mirror and is well suited for high-resolution .…”

“…1) has 4 connector pins: two for polarization (for the connection of the counter electrode (CE) and working electrode (WE)) and two for the reservoir manipulation coil. 43 EMImBF 4 (Fluka Analytical, "For electrochemical purposes", purity ≥ 99.0% (determined by high pressure liquid chromatography (HPLC) technique), ≤ 200 ppm water) was used after additional long-lasting cleaning (drying) at T = 25 • C for 12 h under pressure 2 × 10 −7 mbar before the simultaneous in situ XPS and electrochemical investigations. Ethanol and acetone (Aldrich, with the HPLC analysis purity) were used to clean the electrochemical cell before experiments.…”