How to Choose Suitable Reference Electrode and Aqueous Electrolyte to Avoid Error in Electrochemical Measurements.

Abstract

The correct method of using different reference electrodes and the stability after using for a long time were determined by experiments. We hope this work will help researchers correctly use reference electrodes to obtain more accurate results. some other reference electrodes are often used to calculate reversible hydrogen electrode (RHE) potential in place of SHE potential because of the cumbersome process of making and testing a SHE. Notably, even in the same electrolyte, using different reference electrodes results in different results. For example, Ji et al. and Jiang et al. used a one-step hydrothermal synthesis method to prepare high-performance supercapacitors composed of Ni(OH)2 and 3D graphene composite materials. In their work, a platinum plate counter electrode and a 6 M KOH electrolyte were used. While the former measured a specific capacitance of 166 F/g at a current density of 0.5 A/g using an Ag/AgCl reference electrode, the latter obtained a specific capacitance of 1346 F/g at 0.5 A/g using a Hg/HgO reference electrode. The specific capacitance obtained by the two experiments was clearly different. Obviously, the difference between the two is caused by the use of different reference electrodes. In addition to the selection of the reference electrode, electrochemical test results are also related to the stability of the reference electrode itself, which affects the reliability of results obtained. However, this issue has not attracted the attention of researchers. Three commonly used reference electrodes (Hg/HgO, SCE, Ag/AgCl) were applied to different water-based electrolytes. Then, a long-term test for electrode stability was conducted. Finally, experiments were conducted that the influence of different reference electrodes on the characterization performance of the same material. In this study, three commonly used reference electrodes (Hg/HgO, Hg/Hg2Cl2 (SCE) and Ag/AgCl) are evaluated in electrochemical experiments with different solutions. First, experimental potentials of these reference electrodes are compared with their theoretically calculated potentials. Understanding the deviations of these electrodes will help the most suitable reference electrode be chosen. Second, reference electrodes with a double salt bridge were used to elucidate the specific mechanism of the stable potential of reference electrodes. Furthermore, the stability of reference electrodes was tested over time to investigate the life of reference electrodes and to ensure the correctness of their potentials. Finally, different electrodes were applied to analysis of the same real-life material to compare the results obtained. (i) In an aqueous alkaline electrolyte, the result was more accurate using the Hg/HgO electrode with a double salt bridge and a 6 M KOH electrolyte. In the aqueous acidic electrolyte, the SCE with the double salt bridge was suitable, and the electrolyte could be at relatively low concentrations of H2SO4 (0.1 M or 0.5 M). In water-based neutral electrolytes, we used an Ag/AgCl electrode with a double salt bridge or the SCE, and the results were more accurate when the electrolyte was 1 M Na2SO4. (ii) By comparing the single salt bridge and the double salt bridge forms of the same electrode, it was found that using double salt bridges decreased the liquid junction potential and electrolyte contamination of the internal reference solution. Therefore, the results were more accurate. (iii) The Hg/HgO electrode in an alkaline electrolyte was stable for a long time. The SCE in an acid electrolyte and the Ag/AgCl electrode in a neutral electrolyte exhibited large potential fluctuations more quickly. Therefore, certain measures, such as calibration, should be taken before every use. (iv) The Hg/HgO electrode was within the required range for the first nine days, but then the potential became lower on the 10th day. Overall, the stability of the Hg/HgO electrode was the best. The potential of the SCE dropped sharply from the second day onward, and the deviation of the potential range was severe; thus, stability was lacking. The potential of the Ag/AgCl electrode increased sharply from the second day, which seriously deviated from the required potential range, thereby demonstrating poor stability. Therefore, it would be necessary to calibrate the electrode before using it as a reference electrode to ensure the validity of the data. (i) When the environment was alkaline water, the Hg/HgO electrode with a double salt bridge (saturated KCl as the external salt bridge solution) was the most suitable choice. Additionally, the solution in the salt bridge of the reference electrode was 1 M KOH, and the electrolyte was 6 M KOH. (ii) When the test environment was an acidic water system, it was most appropriate to use the SCE. Using a low concentration electrolyte (c (H2SO4) < 1 M) and a double salt bridge (KCl saturated as external salt bridge solution) exhibited a small deviation and the most stable performance. (iii) The SCE and Ag/AgCl electrodes (KCl saturated as internal salt bridge solutions) were equivalent in water-based neutral electrolytes. Moreover, the use of double salt bridges exhibited better performance (KCl saturated as external salt bridge solutions). (iv) SCE and Ag/AgCl electrodes could both be used in neutral electrolytes. The SCE and Hg/HgO electrodes in alkaline electrolytes had a more significant impact on the performance results of the material. When testing materials, we should adjust the voltage window with respect to the chosen reference electrodes to obtain more accurate results. The authors gratefully acknowledge the funding from “the General Programs of the National Natural Science Foundation of China” (Grant No. 51676058).