In Situ XPS Studies of Electrochemically Polarized Molybdenum Carbide Derived Carbon Electrode

Room temperature ionic liquids (RTIL) are interesting electrolytes applicable as “green” low volatility inflammable ionic solvents in electrochemical devices and materials processing applications,¹ and as possible electrolytes in the electrical double layer capacitors (EDLC) because of their relatively high conductivity and wide range of ideal polarizability.^2-4 RTILs have been used in Li-ion batteries and fuel cells. Using ethyl-methyl-imidazolium tetrafluoroborate (EMImBF₄) as an electrolyte and carbide derived carbon electrodes it has been shown that it is possible to construct a supercapacitor, where the range of ideal polarizability is up to 3.5 V (T = 25 °C).⁴

Analyzing the electrochemical data obtained at higher temperatures (T ≥ 59 °C) and two-electrode cell potentials ΔE ≥ 3.5 V, it was shown that some faradic processes start to take place at the electrodes at ΔE above 3.0 V, despite of previous drying and cleaning of the RTIL used.^3,4

Taking into account that the energy- and power-density of a supercapacitor is proportional to ΔE², the systematic attempts and novel solutions increasing the potential region of the ideal polarizability of EDLCs are very welcome. The need for more pure and electrochemically more stable ionic liquids is obvious, however, there are some technological and financial limits for the cell completing conditions (e.g. cleanness of chemicals, stability of electrode materials, including current collectors, etc.) applicable and competitive for the preparation of supercapacitors for the consumers in the industrial scale.

Because the faradic processes take place mainly at the electrode | ionic liquid interface we have to analyze the chemical composition of this interface under real applied cell potential (under so-called in operando conditions) to establish the rate and possible mechanism of the chemical and/or electrochemical processes occurring at the interface. Due to the very thin electrode | RTIL interface and small amount of substances, formed at the working electrode surface, new highly sensitive surface analysis methods must be applied. For this type of analysis, different spectroscopic methods like in situ Raman, and infra-red, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, as well as secondary ion mass-spectrometry have been applied for identification of the surface compounds formed.

In this presentation, we will analyze the processes taking place at the negatively⁵ and positively charged carbide derived carbon electrode C(Mo₂C) | RTIL interface, applying synchrotron radiation based electrochemical in situ XPS method, analyzing the elemental composition of the intermediates and final compounds formed at the C(Mo₂C) electrode | RTIL interface. Synchrotron radiation (Maxlab II, I411 beamline, Lund) has been used as an X-ray energy source for the excitation of the 1 s electrons of the electrodes and electrolyte forming elements due to its high intensity and continuous spectra, allowing easy change and selection of the excitation energy.

Based on the XPS data we concluded that the changes in C 1s, N 1s, B 1s and F 1s peak intensities at ΔE > 3.0 V are initiated by the faradic reduction and oxidation reactions. Also the reduction of the carbon C1 in the EMIm⁺ ring is possible. At higher concentration of electrochemically formed radicals, they will recombine giving dimers through single sigma bond formed between two C₁ atoms of EMIm heterocyclic compound, discussed by Xiao and Johnson⁶ and confirmed later by semi empirical PM3 calculations.⁷ Continuing to increase the two-electrode cell potential over ΔE = 3.2 V, the carbon atoms at positions C₁ seem to be reduced and a double bond forms in EMIm dimer between carbons C₁.

At ΔE ≥ 3.2 V, the intensive start of a next reduction process (processes), leading to probable cleavage of N-CH₃ and/or N-C₂H₅ bonds, gives a variety of new carboneous compounds. As a result, nitrogen seems to exist in the reduction product(s) of the EMIm⁺ cation in two forms distinguishable by XPS measurements.⁵ However, it is a bit surprising, that the possible reduction of BF₄^- anions was not monitored in the B 1s binding energy (BE) spectra.

However, similar steps with comparable lengths in the 1s electrons binding energy shift (ΔBE) vs. ΔE graphs at the same cell potential for B 1s, C 1s, F 1s and N 1s photoelectrons indicate that the BE of the C 1s, N 1s, B 1s and F 1s electrons of the EMIm⁺ cation or EMImBF₄ ion-pair is influenced by significant changes in the chemical composition of EMImBF₄, caused by the intensive faradic processes starting at ΔE ≥ 2.7 V. Thus, only at ΔE < 2.7 V, the ideal capacitive behavior and parameters for EDLC can be established in the case of microporous-mesoporous C(Mo2C) | EMImBF₄ interface.

 

Acknowledgments: Authors thank Estonian Ministry of Education and Research for the Estonian Basic Research projects (SF0180002s08, SF0180046s07 and target-financed theme IUT2-25), Estonian Centre of Excellence: Materials for Sustainable Development, Estonian Energy Technology project, Estonian Science Foundation (grant 8737), ERDF projects (‘‘IRGLASS’’ 3.2.1101.12-0027), Graduate School on Functional Materials and Technologies (European Social Fund project 1.2.0401.09-0079), and European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement No 226716, for the financial support. We are also grateful to the staff of Max-laboratory for the assistance and co-operation during the measurements, Indrek Tallo for the preparation of the Mo2C derived carbon electrode films and Tavo Romann for the vacuum sputtering of Al current collectors onto C(Mo2C) electrode films.

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