Electrocatalytic Coating Electrodes in the All-Vanadium Redox Flow Battery
Authors: Hernandez Sanchez Nicolas Kinich , Eguiluz Katlin I.B. , Ponce de León Carlos 
Published: February 2020

Redox flow batteries (RFB) are used to reversibly transform electrical energy into chemical energy that can be stored for long periods. They are adaptable and are used for a wide range of storage capacities depending on the electrode areas and the amount of electrolyte used. RFB system’s flexibility and modularity allows them to be easily transported to different environments (at any charge/discharge level) and increase or decrease their capacity to the desired level [1].
One of the most developed RFB systems is the all-vanadium redox flow battery (VRFB) which uses carbon electrodes. However, carbon degrades when used as the positive electrode during charging, which significantly decreases the lifetime of the battery and increases maintenance costs [2]. This is the main reason to test different electrode materials that have been successfully used in other electrochemical applications with similar extreme conditions of operation such as electrowinning and the chlor-alkali industry. In this sense, the so-called Dimensional Stable Anodes (DSA), introduced by Beer in the 1970’s [3], consists in a metallic substrate coated with different oxides which provide improved electrocatalytic properties and high resistance to corrosion.
The Pechini and ionic liquid [4] methods of preparation of DSA-type electrodes were chosen to manufacture the electrocatalytic coating electrodes with a composition of on a titanium plate substrate and tested in the positive electrolyte of the VRFB (0.01 mol dm-3 V2O5 in 5 mol dm-3 H2SO4). These methods have proven good morphology and electrochemical performance in the final coating in previous studies [5].
In this particular coating composition, iridium dioxide behaves as the electrocatalytic oxide for the V(IV)/V(V) redox reactions, tin oxide provides stability to the coating oxides solution and the antimony oxide helps to increase the electrical conductivity of the coating.
The morphology and physical characteristics of the coating are related to the lifetime of the electrode due to deactivation by detachment of the coating. This failure of the electrode could happen when oxygen passes through the coating to the titanium substrate and leads to the formation of titanium oxides/hydroxides between the substrate and the coating. In this study, the effect of the different preparation methods of electrodes on their morphology and composition were studied through physical characterization (SEM, EDS, XRD).
Figure 1 a) and b) show the characteristic mud-cracked morphology obtained by the Pechini method [5] where the width of the cracks are between 1 and 2 µm. Moreover, a semi-homogeneous distribution of the oxides is also seen. In Figure 1 c) and d), the surface of the electrocatalytic coating electrode prepared by the ionic liquid method is more homogeneous with the presence of cracks with a width below 1 µm.
The presence of more cracks in the electrode prepared by the Pechini method could lead to a shorter lifetime due to the easier passage of oxygen from the surface of the coating to the titanium substrate.
Cyclic voltammetry measurements were used to compare the electrocatalytic properties of the electrodes prepared by both methods towards the V(IV)/V(V) redox reaction.
The presentation includes the influence of the preparation method to manufacture the electrocatalytic layer in the electrochemical performance of the electrodes in the positive electrolyte of the VRFB.
[1] C. Ponce de León, A. Frías-Ferrer, J. González-García, D. A. Szánto, and F. C. Walsh, "Redox flow cells for energy conversion," Journal of Power Sources, vol. 160, pp. 716-732, 9/29/ 2006.
[2] M. Skyllas-Kazacos and F. Grossmith, "Efficient Vanadium Redox Flow Cell," Journal of The Electrochemical Society, vol. 134, pp. 2950-2953, December 1987 1987.
[3] H. B. Beer, "Electrode and coating therefor," United States of America Patent 3,632,498, 1972.
[4] D. T. Araújo, M. de A. Gomes, R. S. Silva, C. C. de Almeida, C. A. Martínez-Huitle, K. I. B. Eguiluz, et al., "Ternary dimensionally stable anodes composed of RuO2 and IrO2 with CeO2, SnO2, or Sb2O3 for efficient naphthalene and benzene electrochemical removal," Journal of Applied Electrochemistry, vol. 47, pp. 547-561, 2017.
[5] T. É. S. Santos, R. S. Silva, C. Carlesi Jara, K. I. B. Eguiluz, and G. R. Salazar-Banda, "The influence of the synthesis method of Ti/RuO2 electrodes on their stability and catalytic activity for electrochemical oxidation of the pesticide carbaryl," Materials Chemistry and Physics, vol. 148, pp. 39-47, 2014.

Figure 1