AgCl layers

NHE (25 °C) or [Hg/HgO/ln NaOH in H2O] E = +140 mV versus NHE (25 °C) are, for example, interesting if chloride ions are not allowed. A problem of all mercury containing systems is the toxicity. A nontoxic system is [Ag/AgCl/sat. KCl in H2O] E = +197 mV versus NHE (25 °C). It is nearly temperature independent and insensitive against current flow. It can be easily prepared by anodic oxidation of a silver wire in KCl solution (coating with an AgCl layer, subsequently an excess of solid KCl is added to the electrolyte round the silver wire). But, diffusion of silver ions out of the partially soluble AgCl can be a problem. 

The photocatalytic oxidation of water to O2 on thin nanostructured AgCl layers has been reported.568 In subsequent work by the same group, AgCl photoanodes have been combined with either p GaMV69,570 or an amorphous silicon solar cell571 in the cathodic part of an electrochemical cell to split water. Modification of AgCl with gold colloids was found to enhance H2 and O2 production.571 572... 

Fig. 7. Energy level scheme for the photoassisted OER from water using AgCl layers after initial self-sensitization to generate Ag clusters.

Large DC current flows with the voltage supply adjusted to only one-tenth of a volt. No gas bubbles are initially seen on any of the electrodes. At the anode, the color stays the same, but the cathode loses the AgCl layer and a pure silver surface appears after some time. 

This is a much used electrode metal (silver covered by silver chloride) in biology and medicine for DC applications both because it is simple and because it has a well-defined DC potential not very dependent on DC current flow. It is therefore a nonpolarizable DC reference electrode. It usually consists of silver metal covered by an AgCl layer, often electrolytically deposited. Ag and AgCl are toxic and cannot be used in long-term living tissue contact. A salt bridge is often used to remove the electrode metal from direct tissue contact. 

Wet AgCl is a solid ionic conductor. With DC current flow, the AgCl layer will increase in thickness if it is deposited on silver metal anode. The polarization impedance will go through a minimum and thereafter increase as a function of AgCl-layer thickness (see the following section). As a cathode with DC current flow, the layer will diminish and eventually be stripped of. We are left with a pure silver surface with quite different properties (e.g., with much higher polarization impedance and a different equilibrium potential) (Figure 7.3). 

In another work [81] a silver thin-film pattern was covered with a polyimide protection layer with a 50 pm wide slit at the center of the pattern and the Ag AgCl layer was grown from there into the silver layer. A liquid junction was formed with a photocurable hydrophihc polymer. A silicone rubber passivation covered the entire area except for the pad and the end of the junction. The complete miniature liquid-junction reference electrode could maintain a stable level within 1 mV for time longer than 100 h with the aid of poly(vinyl pyrrolidone) matrix in the electrolyte layer. 

Fig. 11.3 (a) Cross section and (b) decomposed structiffe of an Ag/AgO planar electrode with a protective layer according to Suzuki [2]. 1 Silicon substrate, 2 adhesive layer, 3 gold layer, 4 sUvct layer, 5 AgCl layer, 6 upper polyimide layer, 7 lead, 8 pad... 

In an approach suggested by Nagy et al. [50], the reference signal will be produced by two separate electrodes consisting of Ag/AgCl layers covered with an anion and cation conducting membrane, respectively. The electrodes are connected in parallel via a resistor and intrinsically respond to monovalent cations... 

Most of those systems were using (thin film) Ag/AgCl elements, which are not free from problems in manufacturing (e.g. related to poor adhesion of silver to glass/silicone) leading to poor potential stability [8]. Improvements proposed [8] included application of intermediate polyimide layer instead of previously applied metal ones and special formation of AgCl layer on top of silver film. Thus obtained sensors were characterized with increased stability of potentials in KCl (up to 8 h) as well as in interferent (KI or K2S) solutions [8]. 

The applicability of screen-printed (on polyester foil) Ag/AgCl layer as internal reference, i.e., in the arrangement in which it was covered by hydrogel layer doped with electrolyte and then by ion-selective membrane to work as internal... 

Very few amperometric gas sensors have been studied so far. A protonic conductor has been used for a Hj amperometric sensor by Miura et al. One may also mention the chlorine sensor proposed by Liu and Weppner, " which is based on 3"-alumina as SIC with an AgCl layer. The electrode reaction is given by... 

The lifetime of this sensor could be limited by the formation of a covering AgCl layer at the electrode. 

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