Short-circuited proton conductor

Figure 2 shows the structure of this sensor which is similar to that of the potentiometric sensor reported earlier (10). The only difference is that in this sensor a short circuit current between the sensing electrode and the counter electrode is measured with an ammeter. The proton conductor, antimonic acid (Sb205 2H20), was prepared from antimony trioxide and hydrogen peroxide according to a method described elsewhere (7,14). The sample powder was mixed with... 

Sensing mechanism of the modified sensor. The sensing mechanism in this modified sensor should be essentially the same as that of the unmodified one. It is noteworthy that a stationary short circuit current was obtained in spite of such sensor construction that the counter electrode was covered with Epoxy resin. Since the sensing electrode is placed in the same situation as the unmodified sensor, this fact indicates that the cathodic reaction is allowed to take place stationarily at the counter electrode. The proton conductor membrane is as thin as 0.2 mm, so that the reactant 02 and the produced H20 will permeate the membrane as shown in Figure 11. A part of H2 will naturally also permeate through the membrane, but the transfered H2 will be consumed by the reaction with 02 electro-chemically or catalytically at the counter electrode. 

Our approach is to separate the conduction paths for H+ ions and electrons through the incorporation of a ceramic second phase. This approach essentially eliminates the combined dependence of hydrogen flux on electronic and proton conductivities. The approach is to short-circuit the electron flow-paths so that the overall flux is limited only by the proton conductivity. A similar mixed conducting requirement exists for electrodes in high-temperature proton conducting fuel cells, and some work has been carried out to develop mixed conductors as electrodes [24]. 

The origin of the short-circuit current is the imbalance of electrode reactions (36.5) and (36.6). The protons produced by (36.5) cannot be consumed completely by (36.6) under the present conditions. Excess protons migrate through the proton conductor (AA) towards the counter electrode to be consumed there, accompanied by an equivalent electronic... 

R.H.-independent signal output has been achieved in thefour-probe type sensor shown in Fig. 36.4, where two additional Ag probes are inserted in the proton conductor bulk (AA) beneath the Pt electrodes. One of the Pt electrodes is covered by a layer of AA sheet, which acts as a sort of gas diffusion layer. The short-circuit current flowing between the two Pt electrodes is proportional to H2 concentration but dependent on R.H., just as in the previous amperometric sensor. On the other hand, the difference in potential between the two Ag probes (inner potential difference, AE g) with the outer Pt electrodes short-circuited is shown to be not only proportional to H2 concentration but also independent of R.H. as shown in Fig. 36.3b and Table 36.2. This mode of sensing has no precedence, and is noted as a new method to overcome the greatest difficulty in using proton conductor-based devices, i.e. their R.H. dependence. 

Fig. 36.3. (a) Dependence of short-circuit current (/) of the amperometric proton-conductor sensor on H2 concentration, and (b) dependence of (inner potential difference) of the four-probe type sensor on Hj concentration in air at different relative humidity (25 (reprinted by permission of The Electrochemical Society, Inc,),... 

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