Proton-conductor sensors

Figure 16. Structure of proton conductor sensor. Reproduced with permission from Ref. 10. Copyright 1983 Kodansha Ltd. (Tokyo).

Amperometric Proton-Conductor Sensor for Detecting Hydrogen and Carbon Monoxide at Room Temperature... 

Fig. 36.1. Dependence of ejnX response of the potentiometric proton conductor sensor on H2 concentration in air and response transient of the sensor to 2000 p.pan. H2 in air at room temperature s (with permission, Kodansha Ltd).

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,),... 

Fig. 36.5. Structure of the thick-film type proton conductor sensor, (a) planar-type potentio-metric sensor, (b) laminated-type amperometric sensor (reprinted by permission of Elsevier Sequoia S.A.).

Most proton conductor sensors have aimed at sensing of Hj in inert gases or air, as mentioned above. However, other chemical components are also sensitive whenever they produce or consume protons through electrode reactions. Typical of such examples are and... 

J Kleperis, G Bajars, G Vaivars, A Kranevskis and A Lusis, gaseous sensors based on solid proton conductors. Sensors and Actuators A 32 (1992) 476-479... 

The coordination chemistry of the trichalcogenophosphonates is very undeveloped when compared to the analogous metal organophosphonates (RP032), which have been extensively studied owing to their potential and practical applications as ion exchangers, sorbents, sensors, proton conductors, nonlinear optical materials, photochemically active materials, catalysts and hosts for the intercalation of a broad spectrum of guests.145... 

The use of this approach can be illustrated by the perovskite structure proton conductor BaYo.2Zro.gO3 g- This material has been investigated for possible use in solid oxide fuel cells, hydrogen sensors and pumps, and as catalysts. It is similar to the BaPr03 oxide described above. The parent phase is Ba2+Zr4+03, and doping with... 

Dr. Hui has worked on various projects, including chemical sensors, solid oxide fuel cells, magnetic materials, gas separation membranes, nanostruc-tured materials, thin film fabrication, and protective coatings for metals. He has more than 80 research publications, one worldwide patent, and one U.S. patent (pending). He is currently leading and involved in several projects for the development of metal-supported solid oxide fuel cells (SOFCs), ceramic nanomaterials as catalyst supports for high-temperature PEM fuel cells, protective ceramic coatings on metallic substrates, ceramic electrode materials for batteries, and ceramic proton conductors. Dr. Hui is also an active member of the Electrochemical Society and the American Ceramic Society. 

Fitch, A. N. (1982) in Solid State Protonic Conductors (I) for Fuel Cells and Sensors, Eds. J. Jensen M. Kleitz, Odense University Press, Odense, p. 235. Geller, S. A. (1967) Science, 159, 310. 

E. Calvo, J. Drennan, B.C.H. Steele and W.J. Albery, in J.B. Goodenough, J. Jensen and M. Kleitz (Eds,), Solid State Protonic Conductors for Fuel Cells and Sensors II, Odense University Press, 1983, p. 289. 

Considering their possible applications in fuel cells, hydrogen sensors, electro-chromic displays, and other industrial devices, there has been an intensive search for proton conducting crystals. In principle, this type of conduction may be achieved in two ways a) by substituting protons for other positively charged mobile structure elements of a particular crystal and b) by growing crystals which contain a sufficient amount of protons as regular structure elements. Diffusional motion (e.g., by a vacancy mechanism) then leads to proton conduction. Both sorts of proton conductors are known [P. Colomban (1992)]. 

Solid-Electrolyte Hydrogen Sensor. Most of solid gas sensors so far developed need high temperature operation because of limited ionic conductivities when the electrolyte is near room temperature. If solid electrolytes with sufficiently large ionic conductivities are available, unique gas sensors operative near room temperature can be fabricated. An example is the following proton conductor hydrogen sensor proposed by our group (10, 11). 

The sensor construction is shown in Figure 16. As a proton conductor, Nafion (DU PONT, Nafion 117 H-type) membrane or disks of several inorganic ion exchangers were used. With sample gas and reference air passing over the respective electrodes, the electric... 

It has been reported (4,5) that solid electrolyte sensors using stabilized zirconia can detect reducible gases in ambient atmosphere by making use of an anomalous EMF which is unusually larger than is expected from the Nernst equation. However, these sensors should be operated in a temperature range above ca. 300°C mainly because the ionic conductivity of stabilized zirconia is too small at lower temperatures. On the other hand, solid state proton conductors such as antimonic acid (6,1), zirconium phosphate (8), and dodecamolybdo-phosphoric acid (9) are known to exhibit relatively high protonic conductivities at room temperature. We recently found that the electrochemical cell using these proton conductors could detect... 

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... 

Modification of the sensor structure. The above amperometric sensor has a rather complicated construction, because the sample gas (H2 + air) is separated from the reference air. So, we tried to simplify the sensor structure as shown in Figure 9. As proton conductor we used a thin antimonic acid membrane (mixed with Teflon powder) of 0.2 mm thickness. This membrane is thin and porous enough to allow a part of the sample gas to permeate. On the other hand, the counter Pt electrode was covered with Teflon and Epoxy resin in order to avoid a direct contact with the sample gas. 

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. 

A new type of amperometric sensor using a proton conductor could detect small amounts of H2 or CO in air at room temperature. 

Many - gas sensors based on - solid electrolytes operate under potentiometric conditions [iii]. The sensors for oxygen use oxide -> conductors, such as ZrC>2 -based ceramic, those for halogens use halide conductors (e.g., KAg s), while -> hydrogen sensors use protonic conductors. There are sensors for C02, N02, NH3, S03) H2S, HCN, HF, etc. (see -> lambda probe). 

J. Jensen and M. Kleitz eds, Solid State Protonic Conductors I-for Fuel Cells and Sensors , Odense University Press, Odense, 1982. 

---