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

Abstract

Electrochemical processes at positively polarized Mo 2 C derived carbon (C(Mo 2 C)) electrical double layer capacitor (EDLC) electrode in 1-Ethyl-3-Methylimidazolium tetrafluoroborate ionic liquid at different cell potentials have been studied using in situ synchrotron radiation excited X-ray photoelectron spectroscopy (in-situ XPS) method. Symmetric two-electrode cells were studied using in situ XPS method within very wide cell potential region up to 4.0 V. At cell potentials higher than 2.6 V the cross-over of the limit of the ideal polarizability of the capacitor electrodes has been observed and different faradaic processes have been verified by the cyclic voltammetry and in situ XPS methods. Thus, the supercapacitor cell potential vs. in-situ XPS signal relationship have been analysed identifying the possible electrochemical reactions taking place at the positively polarized working electrode | ionic liquid interface at the extreme EDLC cell potentials applied.Room-temperature ionic liquids (RTIL) give an interest due to use as "green" low volatility inflammable ionic solvents in organic chemistry, 1-4 electrochemical devices and materials processing applications. 5-7 RTIL-s as potential electrolytes for electrical double-layer capacitors (EDLC) [8][9][10][11][12][13][14][15][16] have been intensively tested because of their relatively high conductivity and wide range of ideal polarizability. [17][18][19][20] In addition, RTIL-s have been used in Liion batteries, 21-27 artificial photosynthesis reactors, 28 solar cells 29-31 and fuel cells. 32-34 Using 1-Ethyl-3-Methylimidazolium tetrafluoroborate (EMImBF 4 ) or 1-Ethyl-3-Methylimidazolium tetracyanoborate (EMImB(CN) 4 ) as an electrolyte and molybdenum and/or other dmetal carbide derived carbon (CDC) electrodes, the supercapacitors with the range of ideal polarizability up to cell potential 3.5 V (T = 25 • C) have been constructed and tested. 20,35 In our previous studies, where the cyclic voltammetry (CV) curves, series capacitance (C s ) and resistance (R s ) values, calculated from the electrochemical impedance data and measured at higher cell potentials ( E) and at various cell potential scan rates (v > 50 mV s −1 ) and at higher experiment temperatures (T ≥ 59 • C) have been analysed, it has been shown that some slow faradaic processes started at E above 3.0 V, despite of very careful previous drying and cleaning of RTIL used. 20 Increase in the EDLC charging time at T ≥ 59 • C and E ≥ 3.2 V, indicates reduction of the speed of mass-transfer of the ions inside the carbon electrode porous structure due to the blocking adsorption of the reaction intermediates or final products. 20,35,36 Taking into account that the energy density (E max ) and power density (P max ) of a supercapacitor is proportional to the square of the cell potential applied (E max ∼ E 2 ), the systematic attempts to use the novel technological solutions increasing E are very welcome. The need for more pure and electrochemically more stable ionic liquid as an electrolyte is obvious, h...

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X‐ray Photoelectron Spectroscopy

“…Comparing our XPS results under vacuum with these previous works, our experiments focused on a relatively smaller potential range (−0.4 to 0.4 V vs. Ag/AgCl QRE). This small range was intentionally used to avoid the potential for faradaic processes leading to IL degradation, as observed in a number of previous vacuum XPS IL studies applied to an electrochemical potential [36–40,43,54,74] . The absence of any evidence of chemical change via XPS (Figure 4) and the absence of faradaic current in the chronoamperometry data (Figures S1 and S2) both support the lack of IL degradation during these experiments.…”

“…It has been suggested that performing XPS experiments at IL/vacuum interfaces using soft X‐ray sources allow for probing WE/IL interfaces [35–40,49] . However, this is clearly not the case (see Figure 1b illustration of Au/IL junction) given the probing depth of XPS is on the order of a few nm for a lab‐based soft X‐ray source or soft X‐ray synchrotron source.…”

“…Facilitated by their high vacuum compatibility, over the past decade ILs have been fairly well characterized in‐vacuo by X‐ray photoelectron spectroscopy (XPS) as a function of electrochemical potential [34–58] . XPS allows for the determination of surface composition of an IL while also probing the binding energy shifts in the ions ( U XPS ; units of eV) as a function of an external electrochemical potential ( U ; units of V).…”

“…Previously reported values of the electrochemical shift ( S= ∂ U XPS /∂ U ) range from 0.5 to 1.0 eV/V depending on the experimental arrangement. Since the working electrode (WE) potential cannot be appropriately controlled in the absence of a constant potential internal reference, IL photoemission studies using two‐electrodes always obtained an electrochemical shift significantly different from 1.0 eV/V [36,37,41,42,44,45] . Using an additional quasi reference electrode (QRE) close to the WE is optimal, allowing for control over the WE potential ( U ).…”

Ionic Liquid for High Voltage Supercapacitor

“…Without destructive chemical interactions of the IL ion, it is electrochemically stable, but structural changes during cycling are still an important issue and should be carefully considered. To understand the aging or failure mechanism of ILs, electrochemical decomposition of ILs was investigated using in situ techniques (infrared and electrochemical spectroscopy methods and XPS) [45,46]. These studies were performed to understand the EDL structure of ILs, enhancing electrochemical properties by modifying positive or negative ions or adopting pseudoelectrode material.…”