Performance of Mullite as Reference Electrode Membrane During the Electrochemical Reduction and Electrorefining Process

2019 ⦽ǎႊᔍᖒ⠱ʑྜྷ⦺⫭ ⇹ĥ⦺ᚁݡ⫭ םྙ᫵᧞Ḳ

399

Performance of Mullite as Reference Electrode Membrane During the Electrochemical

Reduction and Electrorefining Process

Seokjoo Yoon, Dokyu Kang, and Sungyeol Choi*

Korea Advanced Institute of Science and Technology, 291, Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea *

sungyeolchoi@kaist.ac.kr

1. Introduction

The purpose of this paper was to investigate the performance of mullite as the reference electrode membrane for the electrochemical reduction and electrorefining processes. The comparison of reproducibility and the potential stability was made against the pyrex[1] and quartz[2] membrane, which are widely used as the reference electrode membrane for high temperature electrolytes. Also, the mullite membrane was tested using different concentrations of reference electrode solutions, which is composed of AgCl in LiCl-KCl eutectic.

2. Experimental Setup

2.1 Reference Electrode Setup

The silver wire and the reference electrode solution enclosed in the membrane was utilized as the reference electrode. LiCl-KCl eutectic was chosen as an electrolyte, since it is widely used in the electrorefining processes. The concentrations of reference electrode solution were 1wt%, 1.6wt% and 1mol%(2.5wt%).

2.2 Experimental Setup

The experiments are carried under Argon (99.99%) environment with moisture and oxygen level kept

around 1ppm. The furnace which can go up to 900¶C and potentiostat which can provide voltage from -4 to 4V and current range up to 20A are utilized in this experiment to measure the electrochemical data.

2.3 Experiments

To test the reproducibility of the reference electrodes, the open circuit potential between the two reference electrodes with same membranes was measured. In this experiment, 1wt% of reference electrode solution was used. After this process, the potential difference of the mullite membrane with different concentrations of reference electrode solution was measured against the tungsten wire.

3. Results and Discussion

3.1 The reproducibility of the Reference Electrodes

The open circuit potential of the two reference electrodes with same membranes are shown in Table 1. The potential difference of the mullite membrane decreased with increasing temperature. In contrast, the pyrex membrane showed increasing potential values with increasing temperature, and the quartz membrane showed much higher potential difference than the other two membranes.

400

Mullite Pyrex Quartz

500C 103mV 49mV 283mV 650C 68mV 62mV 285mV 800C 26mV 71mV 97mV

3.2 Reference Electrodes With Different Concentrations of AgCl in LiCl-KCl Solution

The potential differences of the reference electrodes with different concentrations of AgCl in LiCl-KCl eutectic are given in Fig. 1. The potential difference between different concentrations of reference electrode solution was in the range of the results from Table 1, which shows that the effect of the concentration of AgCl can be considered negligible. 0 200 400 600 800 1000 1200 1400 1600 1800 -80 -60 -40 -20 0 20 40 Time (s) W-1.0 wt% of AgCl W-1.289 wt% of AgCl W-1.648 wt% of AgCl W-2.525 wt% of AgCl

Open circuit potential (mV)

Fig. 1. The Potential Difference of the Two Reference Electrodes with Different Concentrations of Reference

Electrode Solution.

4. Conclusion

The performance of the mullite as the reference electrode membrane was tested in different conditions for the electrochemical reduction and electrorefining processes. The potential difference of

the two reference electrodes with mullite membranes may imply that the mullite might be suitable as the reference electrode membrane at higher temperatures.

ACKNOWLEDGEMENT

This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (The grant number: NRF-2016M2B2B1945249).

REFERENCES

[1] Ling Yang, Robert G. Hudson, Some Investigations of the Ag/AgCl in LiCl-KCl Eutectic Reference Electrode, Journal of the Electrochemical Society, November 1959, 986-990.

[2] Pei Gao, Xianbo Jin, Dihua Wang, Xiaohong Hu, George Z. Chen, A quartz sealed Ag/AgCl reference electrode for CaCl2 based molten salts, Journal of Electroanalytical Chemistry, 579 (2005) 321-328.

[3] Han Wang, Nancy J. Siambun, Linpo Yu, George Z. Chen, A Robust Alumina Membrane Reference Electrode for High Temperature Molten Salts, Journal of The Electrochemical Society, 159 (2012) 740-746.