Long-Term Stable Reference Electrodes with High-Pressure Tolerance and Salinity-Independence

Abstract

The reference electrode’s performance is essential for ensuring the accuracy of electrochemical sensors in marine environments. Yet, the many existing reference electrodes can exhibit sensitivity to salinity variations, potentially leading to inaccuracies in the measurement process. Herein, we have designed a reliable solid-state reference electrode by introducing SiO_x-stabilized 1-methyl-3-octylimidazolium bis(trifluoromethyl sulfonyl)imide ([C₈mim⁺] [Ntf₂^–]) into a P(VdF-co-HFP) matrix with a SPEEK/[C₈mim⁺] [Ntf₂^–] coated Ag/AgCl as substrate. The SPEEK/[C₈mim⁺] [Ntf₂^–] coating protects the AgCl substrate, and the incorporation of SiO_x improves the compatibility of the IL with the polymer matrix, thereby increasing the electrode’s resistance to interference and extending its long-term stability and lifespan. The developed reference electrode showed a stable and rapid response, with potential variations of less than 0.7 mV across various salinity solutions, including practical seawater, lake water, and their mixture samples. During extended periods of 18 days in deionized water and artificial seawater, the electrode demonstrated negligible potential drifts of 0.36 and 0.14 mV/d, respectively. Notably, the electrode could maintain a stable potential even after being stored in a preservative solution for 67 days. Furthermore, the electrode showed a stable response to withstand pressures of up to 100 MPa, covering the vast majority of the seafloor. This innovative reference electrode is capable of maintaining a stable reference potential across various salinities, ionic strength, and full ocean depth, making it versatile for use in diverse aquatic environments, underscoring its significant potential for advancing oceanographic research and enabling new insights into the unexplored depths of oceans.