Zeolite Electrolyte

Proton, that is, H+ ion, conductors are of importance as potential electrolytes in fuel cells. There are a number of hydroxides, zeolites, and other hydrated materials that conduct hydrogen ions, but these are not usually stable at moderate temperatures, when water or hydroxyl tends to be lost, and so have only limited applicability. 

The stoichiometric measurements were made using a similar dialysis technique. Exactly 50 mg zeolite ( 0.1-0.2 mg) was weighed into a dialysis membrane to which 5 ml 0.01 NaCl (or NaNOs) was pipetted. The mixture was equilibrated with 40 ml of a mixed electrolyte solution of 0.01 total normality, containing sodium and the bivalent cation (Ni, Zn, Co) in various proportions. Each combination requires two identical experiments, involving either a 22Na or a label of the transition element. Equilibrations were made in an end-over-end shaker (5 or 25°C) for one to two weeks. The ion distributions were calculated from the amounts of radioactivity of the initial and the equilibrium solutions for assays of duplicate 5 ml samples. For precision, the total number of counts collected in the 22Na-labeled samples was always about 10 . 

The reduction of Ti4+ to Ti3+ in TS-1 has also been observed with cyclic voltammetry using zeolite-modified carbon paste electrodes. With silicalite, neither anodic nor cathodic processes can be observed. However, TS-1 is electrochemically active, with a reduction process at +0.56 V versus a saturated calomel electrode (SCE) and an oxidation process at +0.65 versus SCE. These observations must be attributed to the redox system Ti4+/Ti3 +. The electrochemical process involves the Ti cations of the inner part of the zeolite crystals, provided that a suitable electrolyte cation can diffuse inside the channels to compensate for the electrical imbalance caused by the redox process in the solid ... 

Electric fields at field emitter tips, as discussed before, are typically on the order of 10 V ran-1. This is in the same range as the electrostatic fields that are present in zeolite cages (see below) and at the interface of electrode and electrolyte interface. Since these fields are all of the same order as the fields inside atoms and molecules, they are strong enough to induce the rearrangement of electronic orbitals of atoms and molecules. It is therefore expected that it should be feasible to stimulate chemistry with such electric fields. 

The meso/macroporous carbons have attracted much attention in their application as electrode materials in EDLCs, since the meso/macropores promote the formation of an effective doublelayer or the transfer of ions into the pores, resulting in the increases in the electrolyte wettability and the rate capability.67,68 In this regard, there has been considerable research targeted towards developing the synthetic methods of novel meso/macroporous carbons.17,36"55,69 72 Various types of such inorganic templates as silica materials and zeolites are widely used for the synthesis of the meso/macroporous carbons, since it was revealed17,36"55 that these inorganic templates contribute to the formation of the meso/macropores with various pore structures and broad PSD. 

FIGURE 3.25 Diffusion of electrolyte ions into the pores of a conventional activated carbon and the ordered microporous carbon prepared using the zeolite template. 

Ion exchange in aluminosilicate zeolites (or another ion exchanger) is a reaction where two particles take part the extra-framework cations present in the zeolite and the ions dissolved in the electrolytic solution. The ion-exchange reaction in aluminosilicate zeolites is represented by [21,24] ... 

The method for the evaluation of Kb uses the so-called Kielland plots, that is, a plot of In Kc versus XA, which is practically feasible, since aA and ab, the ion activities in the electrolytic solution, and XA and Xb, the equivalent fractions of exchangeable ions A and B in the zeolite, can be experimentally measured. On the contrary, aA and ab cannot be measured. 

It is possible that the rate-determining process in the kinetics of ion exchange is the film diffusion. Consider a spherical particle encircled by an aqueous solution sphere (see Figure 7.8), in which the zeolite is homoionic at t = 0, the electrolytic solution has a very high volume (i.e., C2A == constant), and the diffusion is stationary and one-dimensional in a direction perpendicular to the zeolite surface [44], Then, under the conditions discussed above, it is possible to calculate the exchange flux of cation B, that is,. 7 , as follows [23] ... 

FIGURE 7.8 Liquid thin layer adhering to a zeolite (B-Z) spherical grain, where rQ, sphere radius Ar0, film thickness and ACB = Cf - C2B = constant in which Cf and C2 are the concentrations of B at the solid-thin layer and at the electrolyte solution, respectively. 

The limiting step in the kinetics of ion exchange in the zeolite is the interdiffusion of the electrolyte ions A zi and ions of the species B [24], In the case where the solid ion-exchanger particle is spherical (see Figure 7.9) and the particle diffusion control is the rate-determining process, then Fick s second law equation in spherical coordinates is [47]... 

Therefore, hydrated zeolites conduct through intra- and intercrystalline cationic conduction where the material acts as a solid electrolyte, with the help of the water adsorbed in the primary and secondary porosities of the zeolite [112,119], The conduction process is facilitated by oxidation-reduction reactions in the electrodes, initiated by OFF anions generated by an ion-exchange reaction between the protons generated by water dissociation and the Na+ extra-framework cations included in the zeolite. These reactions cause charge injection, which controls the conduction process [112],... 

The role of zeolites as solid electrolytes is linked to water adsorption in the zeolite secondary porosity (see Section 8.2.7), which can be evidenced in the use of natural or synthetic zeolites as solid electrolytes, since the cationic conduction of zeolites is highly enhanced by the adsorption of water in the secondary porosity [112],... 

Zeolite-based solid electrolytes have been used in the manufacture of Li secondary batteries with the zeolite acting as the host for Li ions [174,176], These materials were applied as hosts for Zn11, in Zn-Mn02 batteries [177], and in Pb11 in lead acid batteries [178]. 

Zeolites have been also used in the production of fuel cell electrodes, fundamentally as catalyst supports for metals such as Ag, Pd, Rh, Pt, Ru, and Ni [174], Besides, zeolite membranes have been used for the separation of the anode and the cathode sections of a fuel cell, because they allow the transfer of ions through it, for example, to carry an electric current in methanol fuel cells using a basic electrolyte such as carbonate, where the zeolite prevents the escape of methanol [174,179],... 

In order to undertake a large scale application of the obtained results, the zeolite MP was added to the electrolyte of Leclanche-type batteries, manufactured in the Yara Dry Cell Factory in Havana, Cuba [181]. The obtained batteries with the zeolite MP included in the electrolyte were tested under intermittent and continuous discharge procedures following the standard modus operandi of the Yara factory [181]. The results indicated that the batteries produced with the zeolite MP included in the electrolyte exhibited a better performance in comparison with the batteries produced following the standard technology [181]. 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

To diminish electrode pore flooding, McHenry and Winnick [106] studied new membrane formulations which included borosilicate glass and zeolites. These showed improved electrolyte retention and polarization behavior as compared with MgO-based membranes. The reduction in flooding allowed the cell to handle three times the current for the same applied overpotential. The same authors [107] also found that Lao,8Sro.2Co03 electrodes reacted with the molten pyrosulfate electrolyte. Lithium-doped nickel oxide replacement electrodes, however, were not degraded. 

There has been considerable debate about whether intrazeolitic species are elec-trochemically accessible and recent results on zeolite-encapsulated metal complexes are of interest in this regard. Before we discuss this topic, the role of ion exchange in electrochemical response is discussed. Electroactive transition metal ions can readily be ion exchanged into zeolites. Baker and co-workers reported that a Co +-zeolite Y-ZME resulted in reduction of cobalt ions if Li" was used as the supporting cation in the electrolyte solution, whereas the voltammetric response was absent in the presence of the larger tetrabutylammonium ion (TBA) [166]. Figure 32 compares the voltammetric data for Li+ and TBA. There are two ways to interpret these data ... 

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