In Situ XPS and FTIR Studies of Electrochemically Polarized Carbide Derived Carbon and Other Carbon Electrodes in RTILs

Room temperature ionic liquids (RTIL) are interesting low volatility inflammable solvents applicable in modern electrochemical devices and materials processing (deposition/dissolution).¹ RTILs can be used as possible electrolytes in the electrical double layer (EDLC) and hybrid (HSC) supercapacitors, Li-ion and Na-ion batteries because of their relatively high conductivity and wide range of ideal polarizability.^2-4 It has been shown the possibility to  use the ethyl-methyl-imidazolium salts EMImBF₄ and EMImB(CN)₄ as an electrolyte and various amorphous carbon electrodes for constructing a supercapacitor with the range of ideal polarizability up to 3.5 V (T = 25 °C).⁴

The systematic attempts and novel solutions increasing the potential region of the ideal polarizability of EDLCs are very welcome in order to increase the energy and power densities. The need for more surface active and pure (i.e. electrochemically more stable) RTILs 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 production of supercapacitors in the industrial scale.

The specific adsorption and faradic processes take place mainly at the electrode | ionic liquid interface, hence, the behavior of this interface should be analyzed under real applied cell potential (under so-called in operando conditions) to establish the rate of adsorption/desorption and possible mechanism of the faradic processes occurring at the interface. Due to the very thin electrode | RTIL interface and small amount of substances, adsorbed or formed at the working electrode surface, new highly sensitive surface analysis methods have to be applied.

For this type of analysis, different spectroscopic methods like in situ infra-red, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and sometimes Raman and secondary ion mass-spectrometry are applied for identification of the surface compounds formed.^4-6

In this presentation, the processes taking place at the interfaces  between RTIL and negatively⁵ or positively charged carbide derived carbon and other amorphous carbon electrodes, as well as the elemental composition of the intermediates and products will be analyzed on the basis of the data collected by synchrotron radiation based electrochemical in situ XPS, in situ STM, in situ Fourier Transform Infrared and FIB-TOF-SIMS methods. 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, F 1s, J 1s, etc. peak intensities at the carbon electrode potentials E < ‑1.2 V and  E > 1,4 V (vs. Ag|AgCl in the same RTIL) are initiated by the faradic reduction and oxidation reactions. Strong adsorption of I^- with partial charge transfer will be discussed. 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. Very strong influence of the surface-active anion on the XPS, STM, etc. data have been established.

 

Acknowledgments: This work was supported by the Estonian target research project IUT20-13, the Estonian Centre of Excellence in Science Project TK117T "High-technology Materials for Sustainable Development". We are also grateful to the staff of Max-laboratory for the assistance and co-operation during the measurements, and Indrek Tallo for preparation of the carbide derived carbon electrode films.

References

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