Thin-Film Ag/AgCl Structure and Operational Modes to Realize Long-Term Storage

A new thin-film Ag/AgCl structure and modes of operation are proposed.The entire silver layer, 0.3 m thick, was passivated with a polyimide layer leaving four square pinholes in the polyimide layer from which to grow AgCl layers.The dimensions of the pinholes were 50 50 m, 20 20 m, and 10 10 m.In one mode of operation, no AgCl layer is grown in the structure prior to the first use, and the AgCl layer is grown at a small constant current during operation.By using smaller pinholes and current, the potential settled at a stable level rapidly, and the polarization could be minimized.A lifetime of 300 h was achieved using the element with pinholes of 10 10 m at 10 nA.In another mode of operation, AgCl was grown under standby status.With AgCl layers grown at 1 A for 5 min, the element maintained the equilibrium potential of Ag/AgCl for longer than 24 h in saturated KCl solution.The AgCl layers could be grown repeatedly and the exhibited potential and the lifetime of the element were reproducible.The same experiments were conducted using a conventional Ag/AgCl structure.However, it was found that the novel modes of operation can never be applied to the conventional Ag/AgCl element.The new Ag/AgCl element was used in a microfabricated liquid-junction reference electrode.The variation of the potential was very similar to the one measured in a beaker, which showed that the performance of the Ag/AgCl element can be effectively maintained in combination with an appropriate liquid junction.