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

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Abstract
A new thin-film Ag/AgCl structure and modes of operation are proposed. The entire silver layer, 0.3 μm 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 μm,50×50 μm, 20×20 μm,20×20 μm, and 10×10 μm.10×10 μm. 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 μm10×10 μm at 10 nA. In another mode of operation, AgCl was grown under standby status. With AgCl layers grown at 1 μA 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. © 2001 The Electrochemical Society. All rights reserved.