Maria Skyllas-Kazacos

New All‐Vanadium Redox Flow Cell

A laboratory-scale cell was constructed to test the performance of V(II)/V(III) and V(IV)/V(V) half-cells in an all-vanadium redox battery. Graphite plates were used as electrodes, and the membrane was manufactured from a sulfonated polyehylene anion-selective material. The average charging efficiency of the cell was over 90 percent. Stability tests on the reduced and oxidized electrolytes, measured over the temperature range of -5 C to 60 C, showed no accelerated decomposition at high temperatures and no crystallization at the lower temperatures. After prolonged usage, however, a slow deterioration of the positive electrode and the membrane was observed. 9 references.

A study of the V(II)/V(III) redox couple for redox flow cell applications

The electrochemical behaviour of the V(III)/V(II) redox couple has been investigated at glassy carbon electrodes using cyclic voltammetry. The oxidation/reduction is found to be electrochemically irreversible with a value ko = 1.2 × 10−4 at pH = 4. A slight trend in ko with pH is observed. Electrode poisoning effects are observed at the glassy carbon electrode, and surface preparation is found to be critical in determining observed behaviour.

Investigation of the V(V)/V(IV) system for use in the positive half-cell of a redox battery

Abstract The V(V)/V(IV) redox couple system has been studied at glassy carbon and gold electrodes in sulphuric acid solutions, using both cyclic voltammetry and

Modification of graphite electrode materials for vanadium redox flow battery application—I. Thermal treatment

Abstract The thermal activation of graphite felt was investigated at a range of temperatures and treatment times so as to enhance the electrochemical performance of this material for use in the vanadium redox cell. Graphite felt treated thermally at 400°C exhibited the greatest improvement in performance of the vanadium cell. Energy efficiencies of over 88% were obtained after this treatment compared with only 78% for the untreated felt. Results from XPS analysis showed that surface functional groups of CO and C  O increased dramatically compared with untreated samples, suggesting that these functional groups behave as active sites for the vanadium redox reactions.

Chemical modification of graphite electrode materials for vanadium redox flow battery application—part II. Acid treatments

The surface modification of graphite felt with concentrated sulphuric acid has led to dramatic improvement in the electroactivity of this material in the vanadium redox cell. A cell resistance of 2.50 Ω cm2 and energy efficiency of 91% were obtained in the vanadium cell, after the graphite felt electrode material was treated with boiling concentrated sulphuric acid for 5h. The electrochemical activity of the graphite felt was found to increase with increasing sulphuric acid concentration. Treatment of graphite felt with nitric acid and mixtures of sulphuric and nitric acids, however, led to higher cell resistance values compared with the sulphuric acid treatment. Surface analysis of treated and untreated felts using the XPS method has shown that the functional groups CO and CO increased dramatically compared with untreated samples. A sequence reaction mechanism was proposed for the VO2+ ⇌ VO+2 and V2+ ⇌ V3+ charge-discharge reactions occurring in the positive and negative 12-cells, respectively of the vanadium redox battery.

Efficient Vanadium Redox Flow Cell

Excellent performance characteristics have been obtained with the all‐vanadium redox flow cell employing V(II)/V(III) and V(IV)/V(V) redox couples for the negative and positive half‐cells, respectively. Using 1.5M vanadium solutions in , carbon felt electrodes and a polystyrene sulfonic acid cation selective membrane, the cell was charged at 40 mA/cm2 and discharged across various loads. The coulombic efficiency was 90%, while the voltage efficiency calculated over the range 10–90% state‐of‐charge was 81%. An overall energy efficiency for the cell of 73% which, together with the simplicity of the system, makes the vanadium redox cell one of the promising energy storage systems currently under development.

Characteristics of a new all-vanadium redox flow battery

Abstract The construction and performance of an all-vanadium redox flow system is described. The battery employs vanadyl sulphate in sulphuric acid solution as the electrolyte, carbon felt as the electrode material, and an ion-selective membrane as the separator. Working parameters, storage life, and a comparison of the characteristics with other battery systems are also presented. The cost of manufacture of a 1 kW battery of 5 kW h, 15 kW h, or 50 kW h capacity has been evaluated and the practical application of the system in large stationary installations and electric vehicles is also discussed.

Characteristics and performance of 1 kW UNSW vanadium redox battery

Abstract Energy efficiencies of up to 90% are reported for the 1 kW prototype vanadium redox battery being developed at the University of NSW. Solutions of 1.5–2 M vanadium sulphate in sulphuric acid are employed in both 1 2 -cells, and over 85% of theoretical capacity can be utilised at discharge currents ranging from 30 to 120 A. Energy losses of 2–3% are expected for pumping of electrolytes, so that overall energy efficiencies of 87–88% should be achieved. The vanadium battery thus continues to show great promise as one of the most efficient energy storage systems. The battery has already undergone over 100 charge—discharge cycles and further long-term testing is currently being undertaken.

Vanadium redox cell electrolyte optimization studies

The stability of the positive electrolyte of the vanadium redox cell has been studied at various temperatures and at different solution compositions and solution state-of-charge (SOC). It has been found that at elevated temperatures for extended periods, V(V) can slowly precipitate from solution, the extent and rate of which being dependent on temperature, vanadium and sulphuric acid concentration as well as the SOC of the electrolyte. A H2SO4 concentration of 3–4m has been found to be more suitable than 2m, not only from the point of view of increased stability, but also because of the higher electrolyte conductivity which leads to increased voltage efficiencies during battery cycling.

Membranes for redox flow battery applications.

The need for large scale energy storage has become a priority to integrate renewable energy sources into the electricity grid. Redox flow batteries are considered the best option to store electricity from medium to large scale applications. However, the current high cost of redox flow batteries impedes the wide spread adoption of this technology. The membrane is a critical component of redox flow batteries as it determines the performance as well as the economic viability of the batteries. The membrane acts as a separator to prevent cross-mixing of the positive and negative electrolytes, while still allowing the transport of ions to complete the circuit during the passage of current. An ideal membrane should have high ionic conductivity, low water intake and excellent chemical and thermal stability as well as good ionic exchange capacity. Developing a low cost, chemically stable membrane for redox flow cell batteries has been a major focus for many groups around the world in recent years. This paper reviews the research work on membranes for redox flow batteries, in particular for the all-vanadium redox flow battery which has received the most attention.