Determination of Electrochemical Pretreatment Voltage Limits, Scan Rate and Cycle Number

Abstract

Electrochemical characterisation analyses are performed to determine the electrochemical behaviour of the electrode and electrolyte of vanadium redox battery components. A 3-electrode system which consisted of counter electrode, working electrode and reference electrode is used for electrochemical testing. It is important for the consistency of the proposed idea that the results obtained from the analyses are comparable and repeatable under the same conditions. In order to prepare the working electrode surface for electrochemical reactions, an electrochemical treatment procedure is applied in addition to the polishing procedure [1,2]. In this study, it is aimed to determine the optimum pretreatment conditions of the working electrode. For this purpose, a 3-electrode system consisting of a reference electrode (Saturated KCl, ALS RE-1CP), a working electrode (ALS GCE 3 mm) and a counter electrode (platinum wire) was established. All pretreatment procedure experiments were carried out using 2 M H2SO4. After each pretreatment step, cyclic voltammetry analyses were carried out with 1.6 M VOSO4 + 2 M H2SO4 solution. CV analyses were performed in the range of 0 V - 1.2 V at a scan rate of 20 mV/s. The upper voltage was chanced between 1.5 V and 2.3 V with 0.1 V steps at the constant lower voltage limit. In contrast, the establishment of the lower voltage limit encompassed incremental increases by 0.3 V steps from -0.7 V to 1.4 V at a constant upper voltage limit. Different scan rates (10 mV/s, 20 mV/s, 50 mV/s, 100 mV/s, and 200 mV/s) were explored to identify the optimal scan rate for the pretreatment process. Additionally, experiments were performed across varying cycle numbers (1, 2, 4, 6, 8, 10) under predefined pretreatment conditions.The results of the pretreatment procedure and CV analysis are graphically presented in Figure 1. The upper voltage limit determination and CV analysis results under a lower voltage limit are displayed in Figure 1.a and 1.b. Redox reactions did not occur when the upper voltage limit was 1.5 V-1.8 V. While redox reactions started to occur after 1.9 V, the highest anodic and cathodic peaks values were obtained in the 0-2 V range. Lower limit pretreatment results and CV analysis under constant upper voltage are shown in Figures 1.c and 1.d, respectively. These experiments indicated minimal distinctions between -0.7 V and 0 V CV results, leading to the selection of 0 V as the lower limit. In cases where the lower limit is greater than 0 V, redox reactions were negatively affected by the pretreatment process. There was no significant difference was observed between the CV results of 10 mV/s and 20 mV/s. However, redox reactions were negatively affected at scanning rates of 50 mV/s and higher. The scan rate of the pretreatment process was determined as 20 mV/s. Figures 1.g and 1.h show the results of pretreatment cycle number and CV analysis results at the 0 V-2 V voltage limits and 20 mV/s scan rate. Redox reactions did not occur when pretreatment was applied in 1 and 2 cycles while 4 and higher cycle numbers were obtained. The highest anodic and cathodic peaks were obtained when the cycle numbers were 6 and 8, and these results are very close to each other. The cycle number of the pretreatment process was determined as 6 cycle.It has been shown that the conditions of the pretreatment process have a significant influence on the reaction behaviour of the working electrode. It conclusively establishes the optimum pretreatment process conditions as a voltage range 0 V-2 V, scan rate 20 mV/s and number of cycles 6. Acknowledgements This work supported by the Scientific Research Projects Unit of Erciyes University under contract no: FDK-2020-10376. The first named author MT thanks to The Scientific and Technological Research Council of Turkey (TÜBİTAK) for their scholarships under “2211-C Priority Areas PhD Scholarship Program with the grant number 1649B032000390” and “2214-A International Research Fellowship Program for PhD Students with the grant number 1059B142200567”. References J. Noack et al., J. Energy Chem., 27, 1341–1352 (2018).N. Roznyatovskaya et al., Batteries, 5, 1–16 (2019). Figure 1