A lithium iron phosphate reference electrode for ionic liquid electrolytes

Johannes Wandt,Junqiao Lee,Damien W.M Arrigan,Debbie S Silvester

doi:10.1016/j.elecom.2018.07.006

Abstract

We report on the development of a very simple and robust reference electrode suitable for use in room temperature ionic liquids, that can be employed with planar devices. The reference electrode is based on LiFePO4 (LFP), a common cathode material in Li-ion batteries, which is air and water stable. The reference electrode can be drop-cast onto a planar electrode device in a single step. We demonstrate that very low Li+-ion concentrations (millimolar or below) are sufficient to obtain a stable and reproducible LFP potential, thus making physical separation between the reference and working electrode unnecessary. Most importantly, the LFP potential is also entirely stable in the presence of oxygen and shows only a very small drift (<10 mV) in the presence of hydrogen gas, while the potential of a platinum pseudo-reference electrode is shifted >800 mV. This is the first time that such a stable reference electrode system for ionic liquids has been implemented in miniaturized, planar electrode devices.

Highlights

Room temperature ionic liquids (RTILs) are a very interesting class of solvent offering unique properties
The platinum pseudo-reference electrode potential hardly recovers in the 30 minute N2 periods following the O2 and H2 flushing, which is an indication of the formation of Pt-O and Pt-H surface species [27]
The random potential drift under pure N2 and the significant potential change in the presence of O2 and H2 clearly demonstrates that platinum pseudoreference electrodes are not suitable for use in electrochemical gas sensors where stable potentials are required

Summary

Introduction

Room temperature ionic liquids (RTILs) are a very interesting class of solvent offering unique properties. Ferrocene can be added at the end of an experiment for calibration purposes [3,8,9], but its relative low solubility [9], moderate volatility [10] and non-innocent nature in the presence of analyte species [8] must be considered. This “post-calibration” method may not be suitable for anything other than simple voltammetric experiments – for example, constant-potential methods, such as long-term chronoamperometry, where potential shifts can occur during the experiment

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