Solid Electrolyte-Based Hydrogen Sensors

The majority of solid electrolyte sensors are based on proton conductors (Miura et al. 1989, Alberti and Casciola 2(X)1). Metal oxides that can potentially meet the requirements for application in solid electrolyte sensors are listed in Table 2.7. These proton condnctors typically do not have high porosity but rather can reach 96-99% of the theoretical density (Jacobs et al. 1993). Similar to oxygen sensors, solid-state electrochemical cells for hydrogen sensing are typically constructed by combining a membrane of solid electrolyte (proton conductor) with a pair of electrodes (electronic conductors) Most of the sensors that use solid electrolytes are operated potentiometrically. The voltage produced is from the concentration dependence of the chenucal potential, which at eqnihbrium is represented by the Nemst equation (Eq. 2.3). 

Metal oxide solid electrolytes Temp, of stability (°C) Type of gas sensor 

P potentiometric, A amperometric, C conductometric Source Data from Korotcenkov et al. (2009c) 

Proton conductivity of metal oxides can be obtained if protonic defects exist. In perovskite-type oxides with proton conductivity, proton defects can have concentrations of a few mole percent or more. There are two mechanisms for proton conduction (Sundmacher et al. 2005). The first is proton hopping, also called the Grotthus mechanism, in which the proton jumps between adjacent oxygen ions. 

A least-squares line is drawn through the data. (Reprinted with permission from Wakamura (2005), Copyright 2005 Elsevier) 

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Another interesting application of perovskite-based hydrogen sensors, which has now been commercialized, is for monitoring hydrogen in molten metal such as aluminium, zinc and copper (Yajima and Iwahara 1992). As cerates were not entirely suitable, the investigators used CaZr03 doped with indium oxide as the solid electrolyte (Iwahara 1996). 

Lu X, Wu S, Wang L, Su Z (2005) Solid-state amperometric hydrogen sensor based on polymer electrolyte membrane fuel ceU. Sens Actuators B 107 812-817... 

Ramesh, C., Velayutham, G., Murugesan, N., Ganesan, V., Dhathathreyan, K.S. and Perias-wami, G., An improved polymer electrolyte-based amperometric hydrogen sensor, Journal of Solid State Electrochemistry, 7(8), 511, 2003. 

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). 

Maffei, N. and Kuriakose, A.K. (1999) A hydrogen sensor based on a hydrogen ion conducting solid electrolyte. Sens. Actuators B, 56, 243-6. 

Narayanan, B.K., Akbar, S.A. and Dutta, P.K. (2002) A phosphate-based proton conducting solid electrolyte hydrogen gas sensor. Sens. Actuators B, 87, 480-6. 

FIGURE 2.14 Response/recovery time of the hydrogen sensor based on the (NH4)4Ta,oW03o solid electrolyte (Cj = 10 ppm Cj = 100,000 ppm). (From Zhuiykov, S., Hydrogen sensor based on a new type of proton conductive ceramic, Ira. J. Hydrogen Energy 21 (1996) 749-759. With permission.)... 

PANI/Nb Oj composites have rarely been investigated in the past, e.g., synthesis, characterization, and low-frequency AC conduction of PANI/ Nb Oj composites were reported by Ravikiran et al. [148]. It was recently demonstrated that PANI/Nb Oj-based materials can be used for highly sensitive solid electrolyte gas sensors (e.g., hydrogen, ethene, and propene) up to temperatures of 450°C because of significant thermal stabilization (Figure 2.11) of PANI with Nb O [149]. 

Fig. 44. Performance of a hydrogen sensor using a BaCeOs-based ceramic as a solid electrolyte. (Reprinted from Iwahara et al. 1991b by permission of the publisher, The Electrochemical Society Inc.)...

Nikolova V, Nikolov I, Andreev P, Najdenov V, Vitanov T (2000) Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures. J Appl Electrochem 30 705-710 Ogura K, Saino T, Nakayama M, Shiigi H (1997) The humidity dependence of the electrical conductivity of a soluble polyaniline-poly(vinyl alcohol) composite film. J Mater Chem 7 2363-2366 Opekar F (1992) An amperometric solid-state sensor for nitrogen dioxide based on a solid polymer electrolyte. Electroanalysis 4 133-138... 

Ruangchuay L, Sirivat A, Schwank J (2004) Electrical conductivity response of polypyrrole to acetone vapor effect of dopant anions and interaction mechanisms. Synth Met 140 15-21 Sakthivel M, Weppner W (2006a) Response behaviour of a hydrogen sensor based on ionic conducting polymer-metal interfaces prepared by the chemical reduction method. Sensors 6 284-297 Sakthivel M, Weppner W (2006b) Development of a hydrogen sensor based on solid polymer electrolyte membranes. Sens Actuators B 113 998-1004... 

Samec Z, Opekar F, Crijns GJEF (1995) Solid-state hydrogen sensor based on a solid-polymer electrolyte. Electroanalysis 7 1054-1058... 

Park CO, Akbar SA, Weppner W (2003) Ceramic electrolytes and electrochemical sensors. J Mater Sci 38 4639-4660 Park CO, Fergus JW, Miura N, Park J, Choi A (2009) Solid-state electrochemical gas sensors. Ionics 15 261-284 Phair JW, Badwal SPS (2006) Review of proton conductors for hydrogen separation. Ionics 12 103-115 Ponomareva VG, Lavrova GV, Hairetdinov EF (1997) Hydrogen sensor based on antimonium pentoxide-phosphoric acid sohd electrolyte. Sens Actuators B 40 95-98... 

In a humidity sensor, ytterbium doping into the rare-earth cerium base material is the key point to obtaining a suitable solid electrolyte for humidity detection. Cerium site substitution of the ytterbium trivalent cation produces holes in the cerate and it becomes an excellent protonic conductor in the presence of hydrogen or water vapor. Rare-earth doping in the cerate is essential to obtain a solid electrolyte for the humidity sensor. 

The same principle can also be applied to the construction of potentiometric sensors for other gases. Thus, cells with proton-conducting solid electrolytes (water-containing In-doped CaZrOa [540], cf. Section 5.6) are used to measure H activity in aluminium and hence to control the brittleness associated with hydrogen content. It is not necessary that the gas to be detected and the mobile ion refer to the same element. Let us consider a chlorine sensor based on AgCl. AgCl in contact with CI2 gas fixes a defined silver activity and cells of the type... 

Popular approaches to molecular self-assembly, which can give structures in the nanometer to millimeter range, are based on SAMs and LBL deposition of electrolytes. Self-assembly leads to equilibrium structures that are close to the thermodynamic minimum and result from multiple weak, reversible interactious betweeu subuuits which include hydrogen bonds, ionic bonds, and van der Waals forces. As information is already coded in the building blocks, this is a means to avoid defect formation in aggregate formation. SAMs are molecular assemblies of long chain alkanes that chemisorb on the patterned and unpat-temed surfaces of appropriate solid materials. The structures of SAMs, effectively 2D-crystals with controllable chemical functionality, make them a means to modify substrates to direct protein adsorption and cell attachment, surface passivation, ultrathin resists and masks and sensor development. 

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