Composite Ag/AgCl/KCl Screen-Printed Reference Electrode

Abstract

The drive for automation in multiple industries has generated unmet demand for low-cost, disposable sensors with short response times for routine electrochemical sensing. Glass ion selective electrodes containing a liquid electrolyte have remained the standard for decades, despite their fragile, bulky nature and regular maintenance requirements restricting their suitability for long-term in-situ monitoring. Advances in printing technologies have enabled solid thick film electrodes to emerge as a possible solution to these challenges.Screen printing can be used to mass-produce units of simple, modifiable, and reproducible sensors using minimal materials, increasing portability and reducing fabrication costs. Traditional glass electrode materials are not suitable for printed fabrication, so new materials and designs are required to deliver reliable and robust printed sensors for a variety of industries. While electrochemical sensors can be used to monitor key aqueous environmental indicators, including pH, heavy metals, and oxidation-reduction potential, they often require a stable reference electrode to gain meaningful measurements. Many previous printed electrochemical sensors have relied on a pseudo-reference electrode containing no stabilising electrolyte, however any resultant potential variation can affect measurement accuracy in unknown solutions.In this study, a new screen-printed "true" reference electrode was developed based on Ag/AgCl and a solid composite KCl electrolyte. The electrode lifetime was tested in a 0.1 M K2SO4 solution, and open circuit potential (OCP) stability was tested in H2SO4, KOH, alkali cations (Li+, K+, Na+), halogen anions (F-, Cl-, Br-, I-), KMnO4, H2O2, ascorbic acid, KCl, and standard pH buffers (pH 4, 7, 10). The initial stabilisation and water permeation was monitored via electrochemical impedance spectroscopy (EIS).The observed electrode lifetime in 0.1 M K2SO4 was up to 39 days, with less than 5 mV deviation from the theoretical value (-45 mV vs SCE) and a shelf life of over 1 year. According to the EIS and OCP data, the electrode typically stabilised within 2 hours. In the stability tests, the electrode potential did not deviate more than 10 mV from the theoretical value in the presence of KOH, alkali cations, halogen anions, ascorbic acid, KCl, and standard pH buffers. The electrode was also stable in H2O2 at concentrations up to 0.1 M. On average, the electrode potential was 12 mV below the theoretical value in 10% H2SO4 and was significantly below the theoretical value in 0.1 M KMnO4.The results of this study suggest that this screen-printed reference electrode is suitable for use in electroanalytical systems in most tested chemical environments. The relatively long lifetime and stability of the electrode allows it to be used in a variety of applications, including environmental monitoring, laboratory measurements, and industrial process monitoring, as part of a printed sensor system.