Development of an electrolyte with the addition of a proton conducting polymer for the anode and cathode electrolyte spaces of a vanadium flow battery

Duration of the work package:
from Month 1 to Month 18

Work package leader:
Assoc. Prof. Dr. Reneta Bukureștlieva

Participants in the implementation of the work package activities (when implemented in partnership):
Assoc. Prof. Dr. Plamen Nikolov, Assoc. Prof. Dr. Toma Stankulov, Assoc. Prof. Dr. Maria Matrakova, Senior Asst. Dr. Lyubomir Soserov, Senior Asst. Dr. Borislava Mladenova, Tech. Georgi Peev, Antonia Bakalova

Purpose of the work package:
Establishment of the type of proton conducting polymer additives and their optimal concentrations for the anode and cathode electrolyte space, ensuring optimal and improved electrical characteristics and long-term operation of the vanadium flow battery.

Within the framework of work package No. 1, it is planned to study and validate at the laboratory level the effect of adding different types of proton conducting polymer to the electrolyte in the cathode and anode electrode space on the electrical characteristics of the vanadium flow battery. The current-generating reactions of the two electrodes of the battery are different in their electrochemical nature and this determines the addition of different polymers and concentrations to the electrolyte space of the positive and negative electrodes, so as to optimize the electrical parameters of the battery.

Activity 1.1. Development of an “internal standard” of electrolyte corresponding to modern achievements and requirements for vanadium flow batteries.
The development of an internal standard aims to create a basic balanced electrolyte formulation in terms of the concentration of sulfuric acid forming the liquid matrix of the electrolyte and the concentration of active vanadium compounds, as well as other additional substances that are generally accepted to be added to the electrolyte formula. At the same time, the electrolyte formulation must provide optimal electrical characteristics of a test flow battery (cell).

Activity 1.2. Creation of a methodology for integrating proton conducting polymers into the basic electrolyte formula. For successful implementation of the proposed concept for introducing proton conducting polymers, it is necessary to select those that are water-soluble in the presence of sulfuric acid. This is determined primarily by the presence of ionogenic structural groups and the molecular weight of the polymer. In this aspect, polymeric substances having proton conductivity and solubility in an aqueous solution of sulfuric acid are planned to be studied in different concentrations: polyvinylpyrrolidone with a molecular weight of 10,000 to 360,000, copolymer of polyvinylpyrrolidone and styrene, polybenzimidazole, nafion, etc. In order for them to be successfully and effectively incorporated into the liquid sulfuric acid matrix, it is necessary to develop an appropriate methodology.

Activity 1.3. Study of the impact and effectiveness of polymeric substances on the electrochemical and chemical reactions occurring during battery operation.
Conducting laboratory tests and trials on a small test cell in order to establish the effect of proton conducting polymers on the electrochemical and chemical reactions occurring during battery operation in relation to the characteristics of the internal standard. The influence of additives on the kinetics of the main togen-generating reactions, as well as on undesirable side reactions, will be studied by applying different types of electrochemical polarization – voltammetry, direct current or voltage, impedance analysis, etc. Analytical methods and techniques for quantitative, qualitative and structural analysis (titrimetric, UV/VIS/NIR and Raman spectroscopy, dynamic light scattering, etc.) will also be applied in order to obtain information about the detailed impact of polymers on vanadium compounds formed as initial, intermediate and final products of the reactions. From a fundamental point of view, a correlation will be sought between various characteristics of the studied polymers, such as composition, structure, molecular weight, etc., and their influence on chemical and electrochemical reactions in vanadium flow batteries.

Activity 1.4. Development of algorithms for accelerated laboratory testing simulating conditions close to the operating environment and assessment of the effectiveness of the polymer additive.

The goal is to develop a methodology in a laboratory environment for assessing the impact of polymer additives on the electrical characteristics and parameters of the battery, such as discharge/charge power, discharge/charge energy, cyclic stability, efficiency, etc. These battery characteristics strongly depend on specific conditions, such as degree of charge, depth of discharge, temperature, etc. Existing standards and test protocols in the field will be taken into account, which will be adapted for a laboratory environment, so that they are as close as possible to the operational environment of the battery and provide reliable information about the possible degradation mechanisms. The test charge-discharge algorithms will be based on measurements at constant current in combination with constant voltage. At specifically selected operating points, fundamental electrochemical techniques and analytical methods for quantitative, qualitative and structural analysis will be applied, in order to extract information about degradation mechanisms and the impact of polymer additives.
The activities under points 1.3 and 1.4 will go through 2-3 iterative optimization cycles to achieve the best combination of parameters and maximize the positive impact of polymeric materials.