Andero Kalju

Doctoral defence: Jinfeng Zhao “Electrochemical characteristics of Bi(hkl) and micro-mesoporous carbon electrodes in ionic liquid based electrolytes”

On 17. June at 12.15 Jinfeng Zhao will defend her doctoral thesis “Electrochemical characteristics of Bi(hkl) and micro-mesoporous carbon electrodes in ionic liquid based electrolytes”. Defence takes place at Chemicum, Ravila 14A-1020.


Prof. Enn Lust, University of Tartu, Estonia  
Research Fellow Ove Oll (Ph.D.), University of Tartu, Estonia 


PhD Olivier Crosnier, University of Nantes, France


Recently, the energy crisis is becoming more and more serious. Thus, it is important to accelerate energy upgrading and establish the growth of new energy economics. As one of the carriers of global energy transformation, the development of electric vehicles may contribute to establishing the electrification of society and the transformation of industrial structure. However, the rapid development of electric vehicles still represents several challenges and limitations, such as the charging capacity of power sources involving batteries and supercapacitors. With the continuous iteration in the technological progress of battery and supercapacitor materials, efforts have been made to develop high-performance electrochemical energy storage devices. The electrochemical energy storage process, in principle, occurs at the electrode-electrolyte interface. It is of great fundamental significance to study the electrochemical behavior at the interface. Therefore, the electrochemical behavior and energy storage characteristics of the electrode-electrolyte interfaces were studied in this work, based on the guidance of achieving high capacitance performance. 

In terms of electrolytes, this work focused on the electrochemical characteristics of the ionic liquid (EMImOTf, EMImTFSI, EMImBF4, EMImMeSO3, PMImI) and ionic liquid salt mixtures (EMImBF4 + X%EMImBr), as ionic liquids perform the high thermal, chemical and electrochemical stability. Regarding electrode materials, bismuth single crystal electrodes and micro- and mesoporous carbon electrodes were studied. As an excellent alternative electrode material for traditional mercury electrodes, bismuth single crystal electrode with low toxicity has been widely studied in the Department of Physical Chemistry of the University of Tartu. 

The capacitance-potential curves of pure ionic liquid and ionic liquid salt mixtures at the Bi(111) electrode were discussed mainly in this work conducted by the cyclic voltammetry and electrochemical impedance spectroscopy methods. The capacitance peaks of capacitance-potential curves suggest that the specifically adsorbed anions (MeSO3‾, I‾, Br‾) can increase the capacitance. Additionally, the presence of water as a common impurity for many ionic liquids and the influence of water concentration in an ionic liquid EMImOTf at bismuth single crystal electrodes were studied. The result showed that the interaction between water and OTf anion resulted in the anomalous capacitance peaks of the capacitance-potential curves. Interestingly, small amounts of adsorbed water in EMImOTf do not affect the electrochemical stability potential range of the base electrolyte but contribute to the increase of capacitance. According to the equivalent circuit modeling fit of experimental impedance data, the kinetics of surface processes involved mass transfer and faradaic characteristics were shown in the resistance-potential curves. In order to characterize the surface structure of the interface in a nanoscale, in situ scanning tunneling microscopy measurement was applied to image the surface structure of bismuth single crystal planes in ionic liquid within the potential range applied. It was found that the highly ordered structures on Bi(111) and Bi(01) planes were recorded relating to the strongly adsorbed anions consistent with electrochemical results. 

Since the specifically adsorbed halide anions (Br‾, I‾) at Bi(111) electrode can increase the capacitance at the interface, follow coming up with an assumption that such effect can help improve the capacitance performance in electrochemical energy storage devices, i.e., supercapacitors. In this thesis, the pure ionic liquid (EMImTFSI), a mixture of ionic liquid and its salts containing halide ions (EMImTFSI+x%EMIm-Cl/Br/I), and a mixture of ionic liquid and its salts containing alkali ions (EMImTFSI+x%Li/Na/K-TFSI) were applied to impregnate the micro- and mesoporous carbon electrodes of supercapacitors, filled with EMImTFSI electrolyte. Compared with the supercapacitor with the only EMImTFSI treated electrode, supercapacitors with the only halide salt mixtures treated electrode showed higher capacitance. In contrast, supercapacitors with the only alkali salt mixtures treated electrode did not improve capacitance performance. Moreover, supercapacitors with halide salt mixture treated positive electrode and alkali salt mixture treated negative electrode showed a consistently high capacitance value, suggesting an effective symmetrically doping methodology by stabilizing both electrodes. According to the equivalent circuit modeling fit of experimental impedance data, the capacitance enhancement is mainly attributed to the pseudocapacitive effect originating in the specifically adsorbed and redox-active halide ions within and near the carbon pores at the cell potentials applied.