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 mum thick, was passivated with a polyimide layer l eaving four square pinholes in the polyimide layer from which to grow AgCl layers. The dimensions of the pinholes were 50 x 50 mum, 20 x 20 mum, and 1 0 x 10 mum. In one mode of operation, no AgCl layer is grown in the structu re 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 potent ial settled at a stable level rapidly, and the polarization could be minimi zed. A lifetime of 300 h was achieved using the element with pinholes of 10 x 10 mm at 10 nA. In another mode of operation, AgCl was grown under stand by status. With AgCl layers grown at 1 muA for 5 min, the element maintaine d the equilibrium potential of Ag/AgCl for longer than 24 h in saturated KC l solution. The AgCl layers could be grown repeatedly and the exhibited pot ential and the lifetime of the element were reproducible. The same experime nts were conducted using a conventional Ag/AgCl structure. However, it was found that the novel modes of operation can never be applied to the convent ional Ag/AgCl element. The new Ag/AgCl element was used in a microfabricate d liquid-junction reference electrode. The variation of the potential was v ery similar to the one measured in a beaker, which showed that the performa nce of the Ag/AgCl element can be effectively maintained in combination wit h an appropriate liquid junction. (C) 2001 The Electrochemical Society.