Department of Electrochemical Methods

I. Fundamental research, development and application of Electrochemical Methods (Electrochemical Impedance Spectroscopy, Transient Technique and Spectral Analysis, Non-stationary Analysis, Electrochemical Noise Analysis, etc.) is carried out. The development and application of advanced techniques for investigation and testing of energy sources (heavy pulse testing, Non-stationary methods, original Differential Coulomerty Spectroscopy) is a durable practice. The Electrochemical Impedance Spectroscopy is under continuous developing - a set of new techniques for non-stationary investigations and data analysis is created. The advanced method of the Differential Impedance Analysis is carefully approbated, disseminated and finalized by the edition of an original monograph.

II. A new research priority is the development and implementation of e-Science instruments for efficient collaboration and virtual research, especially in the field of Electrochemical Impedance Spectroscopy.

MAIN ACTIVITIES

I. Electrochemical Methods

Differential Impedance Analysis

The Electrochemical Impedance Spectroscopy (EIS) is widely applied for the investigation of a large scale of electrochemical objects, due to its unique advantage to ensure clean separation of the involved in the investigated process kinetics of the different steps. The development of a precise and sufficiently cheap computerized measuring technique is an additional base for the quick expansion of the method in new scientific and industrial fields. Since the method does not provide for a direct measurement of a physical phenomenon, it calls for the construction of an impedance model, providing for experimental results' interpretation.

In the last years a new structural approach for the construction of the impedance model, based on a special technique, called Differential Impedance Analysis (DIA), has been developed. It increases the information capability and objectiveness of the impedance analysis because it extracts the impedance model from the experimental data. A sophisticated mathematical procedure AWACS and a catalogue with more than 20 basic impedance models, including their spectral presentation, support the new technique.

Advantages of the DIA:

· no need of a preliminary working hypothesis;

· high selectivity – recognition of strongly mixed time-constants;

· recognition of distributed parameters.

Applications: investigation of basic electrophysical properties and kinetic phenomena in a wide variety of electrochemical objects and processes, including solid state ionics and fuel cells.

Two steps reaction a) impedance diagram; b) spectral image obtained by DIA.
The two spectral peaks exhibit in an illustrative way the presence of two Time-constants, which are undistinguishable by the CNLS method.

Important ! Our team offers impedance analysis performance carried out by the new DIA technique.

International Projects with:

· Commission of The European Community

· Imperial College – London (UK)

· CNRS (France)

· IUPAC

· UNESCO

· CNR (Italy)

Some Publications:

1. Z. Stoynov, “Structural Spectral Analysis of Electrochemical Impedance” Electrochemica Acta, 34 (1989) 1189

2. Z. Stoynov, “Impedance Modeling and Data Processing: Structural and Parametrical Estimation”, Electrochemica Acta, 34 (1989) 1189

3. Z. Stoynov, Differential Impedance Analysis – an Insight into the Experimental Data, Polish J. Chem., 71 (1997) 1204

4. Z. Stoynov, “Advanced Impedance Techniques for Lithium Batteries Study, part IV: Differential Impedance Analysis”, in Materials for Lithium-Ion Batteries (Eds: C. Julien and Z. Stoynov), Vol. 3/85, p. 371, Kluwer Academic Publishers (2000).

5. D. Vladikova, P. Zoltowski, E. Makowska, Z. Stoynov, “Selectivity Study of the Differential Impedance Analysis - Comparison with the Complex Non-linear Least Squares Method”, Electrochimica Acta, 47 (2002) 2943.

6. D. Vladikova, Z. Stoynov, M. Viviani, “Application of the Differential Impedance Analysis for Investigation of Electroceramics”, J. Europ. Ceram. Soc. 24 (2004) 1121

7. D. Vladikova, Z. Stoynov, “Secondary Differential Impedance Analysis– a Tool for Recognition of CPE Behaviour”, J. Electroanal. Chem. 572 (2004) 377.

8. A. Barbucci, M. Viviani, P. Carpanese, D. Vladikova, Z. Stoynov, ^“Impedance Analysis of Oxygen Reduction in SOFC Composite Electrodes”, Electrochim Acta 51 (2006) 1641.

9. D. Vladikova, A. Kilner, S. J. Skinner, G. Raikova, Z. Stoynov, “Differential Impedance Analysis of Single Crystal and Polycrystalline Yttria Stabilized Zyrconia”. Electrochim Acta 51 (2006) 1611

10. Z. Stoynov, D. Vladikova, “Differential Impedance Analysis“, Marin Drinov Publ. House, Sofia, 2005

Nonstationary Analysis and Modeling of Batteries

A new method for battery performance modeling under nonstationary load is designed. Special powerful computer software for modelling, simulation and optimization of the power traction drive system of electric and hybrid electric vehicles is developed. A set of data for the dynamic and static characteristics of the vehicle corpus, tires, handing, motor, regulators and different types of batteries is introduced.

The new method determines the best fitting between the battery and the vehicle. Its application ensures the choice of the most suitable type and configuration of the battery for a given class of vehicles.

Some publications:

1. Z. Stoynov, B. Savova-Stoynov, T. Kossev, “Non-stationary Impedance Analysis of Lead/Acid Batteries", J.Power Sources, 30 (1990) 275.

2. B. Savova-Stoynov, Z. Stoynov, “Instanteous Impedance Analysis, Key Engineering Materials”, 59-60,(1991) 273.

3. Z. Stoynov, T. Nishev, V. Vacheva, N. Stamenova, “Non-stationary Analysis and Modelling of Battery Load Performance”, J. Power Sources, 64 (1997) 189.

Differential Coulometry Spectroscopy

The Differential Coulometry Spectroscopy is a new technique for extraction of valuable information from the testing of batteries. The spectral transform of the aperiodic testing voltage – time dependences provides for extraction and precise evaluation of the plateaus correspondent to the kinetic steps of the charge-discharge processes. In order to separate the kinetics from the domination of the transport limitations and to support the analysis, a very slow rate of testing is applied. The results obtained on Li-ion and Metal-hydride batteries are very promising. The technique opens a window towards a large unexplored area of investigations.

Fig. 1. Slow sweep voltage–time charge curve (h = 100 hours)

of 4 volts Li-ion sample with nominal capacity of 2200 mAh

Fig. 2. Capacity spectrum of the slow sweep voltage–time charge and discharge curves

(h = 100 hours) for the 4 volts Li-ion sample with nominal capacity of 2200 mAh

II. e-Science Instruments

Virtual Research Information Management System (VRIMS) for Impedance Spectroscopy

The development of the Information and Communication Technologies opens a niche for faster and easier application and dissemination of new techniques. They need virtual components, since the “Internet Highway” is extremely convenient and efficient for “transportation” of knowledge, raw data and final results.

The applied management system for virtual research (VRIMS) in the field of Impedance answers the demands for a “smooth” transition from traditional to new scientific instruments.

VRIMS offers:

· Unified data exchange formats, which overcome the difficulties coming from the increasing number of companies, producing measuring instruments of different generations and versions;

· Data Exchange Formats (DEF), which ensure efficiency in data banking (storage, sorting, archiving etc.) and high speed virtual (Internet) exchange of automatically produced large sets of data files;

· “Glossary” for symbolic description of impedance models, needed for DEF operation. The following conventions are introduced:

· Large structured files (LSF) for DEF description of series of experimental impedance data and images.

Our team is open for R&TD cooperation

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