Conductors using protons

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). 

There is a class of nonporous materials called proton conductors which are made from mixed oxides and do not involve transport of molecular or ionic species (other than proton) through the membrane. Conduction of protons can enhance the reaction rate and selectivity of the reaction involved. Unlike oxygen conductors, proton conductors used in a fuel cell configuration have the advantage of avoiding dilution of the fuel with the reaction products [Iwahara ct al., 1986]. Furthermore, by eliminating direct contact of fuel with oxygen, safety concern is reduced and selectivity of the chemical products can be improved. The subject, however, will not be covered in this book. 

Finally, protonic conductors and protonic superionic conductors can be distinguished using their Oq and values as criteria. Fig. 3.1 and Table 3.3... 

In the following, we shall illustrate the application of various NMR techniques to understand the different aspects of local dynamics in FPCs. Where relevant, we shall also touch upon the long range motion briefly. We shall try to point out at appropriate places the problems associated with the interpretation of NMR results and attempts to resolve the problems. In Table 22.1, the information obtained on protonic conductors using various NMR techniques is presented. 

In this chapter, the distinctive features of proton conductors as an electrolyte for use in a fuel cell or in a steam electrolyser are discussed in comparison to those of oxide ion conductors. In addition, the possibility of using a proton conductor in a hydrogen gas separator is also described. As examples, the experiments on fuel-cells, steam electrolysers and hydrogen gas separators using proton conducting, perovskite-type oxides are described. 

Table 36.1. Solid-state gas sensors using proton conductors operative at ambient temperature...

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. 

In an ideal solid-state battery involving protonic reactions, the solid electrolyte must be only a protonic conductor the proton transference number should be equal to unity (t(H+> = 1). Then all the previous reactions cannot occur and it is theoretically possible to use electrode materials which were unstable in the presence of a liquid aqueous electrolyte. 

This present study covers only electrochromic devices using proton conductors. Indeed, since the proton is the smallest ion, proton conductors have been extensively studied and used in electrochromic cells either as the solid electrolyte or as the electrochromic material or counter electrode. 

Hamakawa S, Hibino T and Iwahara H (1993), Electrochemical methane coupling using protonic conductors , / Electrochem Soc, 140,459-462. 

Iwahara, H., Yajima, T., Hibino, T., and Ushida, H. (1993). Performance of solid oxide fuel cell using proton and oxide ion mixed conductors based on BaCei-jcSmjtOs-a Electrochem. Soc. 140 1687-1691. 

Nation sulfonated tetrafluoroethylene (tetrafluoroethylene and perfluorovinyl ether groups terminated with sulfonate groups) Nation is used as proton conductor for proton exchange membrane fuel cells. 

Miura N, Harada T, Shimizu Y, Yamazoe N (1990) Cordless solid-state hydrogen sensor using proton-conductor thick film. Sens Actuators B 1 125-129... 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Nevertheless there are some reactions which never change. Thus NO reduction on noble metals, a very important catalytic reaction, is in the vast majority of cases electrophilic, regardless of the type of solid electrolyte used (YSZ or P"-A1203). And practically all oxidations are electrophobic under fuel lean conditions, regardless of the type of solid electrolyte used (YSZ, p"-Al203, proton conductors, even alkaline aqueous solutions). 

M. Makri, A. Buekenhoudt, J. Luyten, and C.G. Vayenas, Non-Faradaic Electrochemical Modification of the Catalytic Activity of Pt using aCaZr09ln0 03.a Proton Conductor, Ionics 2, 282-288 (1996). 

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