Increasing the lifespan of reference electrodes by increasing the diffusion length

Abstract

The lifetime and stability of reference electrodes is critical for the long-term reliability of electrochemical sensing technologies. In this work, we demonstrate that the operational lifespan of a reference electrode (RE) is heavily influenced by the length of the filling solution. A mathematical solution, based on 1D, Fickian diffusion between the RE filling solution and an infinite volume test solution, predicts that doubling the length of the filling solution channel will quadruple the lifespan of the RE. In comparison, halving the diffusion coefficient of the filling solution (e.g., by changing the material) only increases the RE lifetime by 20–25%. The quadratic dependence of the filling solution length on lifetime was experimentally observed by monitoring the open-circuit potentials of RE's composed of an Ag/AgCl couple and agar-gel filling solutions (agar + 1.0 M KCl) inside glass capillaries of different lengths. The 1 cm RE remained stable for 6 h, whereas a 10 cm RE was stable for approximately 18 days (430 h). Lastly, we demonstrate that 3D printed and CNC (computer numerical control) machined RE housings with long and narrow filling solution channels can be used to produce small REs with enhanced lifetimes. Reference electrodes prepared this way were low-cost, small, and more stable than a typical commercial reference electrode design. A 4 cm long, 3D printed RE housing with 3 mL of agar gel filling solution provided over 6 months of potential stability, outlasting a conventional RE design with double the amount of reference solution volume.