Electrolyte humidity

Ballard assembles and tests stacks, to confirm their characteristics and optimise such water management problems as electrolyte humidity control and cathode water removal, via wettable materials in the porous electrode, to the external air flow. Potential difficulties, exclusive to the water-producing PEFC, can be seen with 100% humidity in the tropics, and with freezing conditions in cool climates. High altitudes can be difficult for all fuel cell types, via low oxygen density. 

Water evaporates from or condenses onto an electrolyte solution depending on the relative humidity of the surrounding atmosphere. Electrolyte humidity sensors utilize this phenomenon. Lithium chloride is a typical material [7]. An electric hydrometer using lithium chioride was developed in 1939 by Dunmore [7] and is still widely used. In a typical lithium chloride humidity sensor the lithium chloride solution is impregnated into a plant pith substrate (10 mm x 4 mm x 0.2 mm) [8] and Pt electrodes are applied to both faces of the element. The plant pith possesses a fine reticulate structure and is therefore a suitable porous binder for the electrolyte. Lithium chloride solution trapped in such a porous binder is too stable to flow out even under very humid conditions. Since the humidity range covered by one such unit is about 30 7o r.h., a wider humidity range from 10 to 100% can only be measured by using a number of elements with different characteristics. 

The range of available ceramic sensors for measuring humidity in industrial process environments can be classified into four categories (1) ionic, (2) electronic, (3) solid electrolyte, and (4) rectifying-junction types. A solid electrolyte humidity sensor that uses doped strontium cerate has been reported... 

Although polyacetylene has served as an excellent prototype for understanding the chemistry and physics of electrical conductivity in organic polymers, its instabiUty in both the neutral and doped forms precludes any useful appHcation. In contrast to poly acetylene, both polyaniline and polypyrrole are significantly more stable as electrical conductors. When addressing polymer stabiUty it is necessary to know the environmental conditions to which it will be exposed these conditions can vary quite widely. For example, many of the electrode appHcations require long-term chemical and electrochemical stabihty at room temperature while the polymer is immersed in electrolyte. Aerospace appHcations, on the other hand, can have quite severe stabiHty restrictions with testing carried out at elevated temperatures and humidities. 

Rusting of iron consists of the formation of hydrated oxide, Fe(OH)3 or FeO(OH), and is evidently an electrochemical process which requires the presence of water, oxygen and an electrolyte — in the absence of any one of these rusting does not occur to any significant extent. In air, a relative humidity of over 50% provides the necessary amount of water. The mechanism is complex and will depend in detail on the prevailing conditions, but may be summarized as ... 

It follows from (1) that the more negative metal, at a bimetallic junction, can be subject to more aggravated attack because of this lowered cathodic polarisation. In part, this may result from the greater ease of replenishment of dissolved oxygen under conditions where the ratio of surface area to electrolyte volume is very high. Rosenfel d has also produced evidence to show that rapid convective mixing in the condensed layer, under conditions of lowered relative humidity which permit rapid evaporation, further hastens the arrival of dissolved oxygen at the cathode and results in an additional... 

Soluble corrosion products may increase corrosion rates in two ways. Firstly, they may increase the conductivity of the electrolyte solution and thereby decrease internal resistance of the corrosion cells. Secondly, they may act hygroscopically to form solutions at humidities at and above that in equilibrium with the saturated solution (Table 2.7). The fogging of nickel in SO2-containing atmospheres, due to the formation of hygroscopic nickel sulphate, exemplifies this type of behaviour. However, whether the corrosion products are soluble or insoluble, protective or non-protective, the... 

Water vapour is essential to the formation of an electrolyte solution which will support the electrochemical corrosion reactions, and its concentration in the atmosphere is usually expressed in terms of the relative humidity (r.h.). 

Rainfall, besides wetting the metal surface, can be beneficial in leaching otherwise deleterious soluble species and this can result in marked decreases in corrosion rate . A recent survey of rainfall analyses for Europe has shown that, with the exception of the UK, the acidity and sulphate content of rainfall markedly increased in the period 1956 to 1966, pH values having fallen by 0 05 to 0-10 units per ann. The exception of the UK may be due to anti-pollution measures introduced in this period. However, even in the UK a pH of 4 is not uncommon for rainfall in industrial areas. The significance of electrolyte solution pH will be discussed in the context of corrosion mechanisms. The remaining cases of electrolyte formation are those in which it exists in equilibrium with air at a relative humidity below 100%. 

This concept may be invoked to account for electrolyte formation in microcracks in a metal surface or in the re-entrant angle formed by a dust particle and the metal surface. More importantly, it can also explain electrolyte formation in the pores of corrosion product and hence the secondary critical humidity discussed earlier. Ferric oxide gel is known to exhibit capillary condensation characteristic and pore sizes deduced from measurements of its adsorptive capacity are of the right order of magnitude to explain a secondary critical relative humidity as70 7o for rusted steel . 

In practice, however, the soluble components are often contained in a matrix of insoluble product and formation of electrolyte by both capillary and chemical condensation may occur in the same humidity range. 

With many electrolytes, AP is so large that the solid, when exposed to moist air, picks up water (deliquesces). This occurs with calcium chloride, whose saturated solution has a vapor pressure only 30% that of pure water. If dry CaCl2 is exposed to air in which the relative humidity is greater than 30%, it absorbs water and forms a saturated solution. Deliquescence continues until the vapor pressure of the solution becomes equal to that of the water in the air. 

Measurement of conductivity of a fibre. If a fibre is impregnated with an electrolyte, such as lithium chloride, its electrical resistance will be governed by its moisture content, which in turn depends on the humidity of the atmosphere in which it is situated, Iri a lithium chloride cell, a skein of very fine fibres is wound on a plastic frame carrying the electrodes and the current flowing at a constant applied voltage gives a direct measure of the relative humidity. 

The substitution of the two-sided tape with a film of an ionic conductor gives (Fig. 24) a triple-layered muscle working in air.114 The tape now acts as a solid electrolyte. Nevertheless, the system only works if the relative humidity in air surpasses 60%. Under these conditions, movements and rates similar to those shown by a triple layer working in aqueous solution were obtained. This device was developed in cooperation with Dr. M. A. De Paoli from the Campinnas University (Campinnas, Brazil). At the moment several groups are developing actuators, muscles, and electrochemomechanical devices based on bilayer or multilayer structures.115-125... 

FIGURE 39 Corrosion. Corrosion is the process of gradual deterioration of metals and alloys as a result of their interaction with the environment. The corrosion process is a reversal of metallurgical processes, whereby metals are recovered from the minerals in which they occur in nature (a). It is an electrolytic process, brought about by the passage of electric currents. Any metal or alloy contains sites in which there are slight local compositional differences. When such compositional differences are exposed to a humid or wet environment, extremely small electrolytic cells as the one shown in (b) are created in each cell, an electric current drives the otherwise nonspontaneous corrosion reactions. In a surface undergoing corrosion there are millions of electrolytic cells. 

Sucessful application of the air electrode requires solving some key problems the air electrode catalyst, the alkaline electrolyte carbonization, the oxygen reaction with anode materials, an influence of an air humidity on an electrode behavior. 

The air humidity influences the performance parameters of the air-zinc batteries. At high air humidity the electrode flooding is possible. On the other hand, at low air humidity a concentration of the electrolyte increases, the air electrodes get too dry. 

The conductivity ofthe film was calculated for 30 monolayers. The film was deposited onto a Ag microelectrode array with a 1-mm distance between fingers. The thickness ofthe monolayer was taken to be 2 x 10"7 cm. For an air humidity value of 60% the conductivity equals 1.3 x 10"6 (Q/cm)-1 The current through the film has an ionic character, and there is apparently layered solid electrolyte... 

As the film is conductive owing to electrolytic layers that contain water, the conductivity should depend on the quantity of water in these layers, and thus on the humidity in the air. This dependence for the Li+ form of LB film is shown in Figure 7.3. The conductivity increases within three orders ofmagnitude when the humidity rises from 0 to 100%. So, this film can be used in humidity sensors. 

For micro PS a decrease in the specific resistivity by two or three orders of magnitude is observed if the dry material is exposed to humid air [Ma8] or vapors of polar solvents, e.g. methanol [Be6]. This sensitivity of PS to polar vapors can be used to design PS-based gas sensors, as discussed in Section 10.4. This change in resistivity with pore surface condition becomes dramatic if the pores are filled with an electrolyte. From the strong EL observed under low anodic as well as low cathodic bias in an electrolyte it can be concluded that micro PS shows a conductivity comparable to that of the bulk substrate under wet conditions [Ge8]. Diffusion doping has been found to reduce the PS resistivity by more than five orders of magnitude, without affecting the PL intensity [Ell]. 

ISO 15106-1 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 1 Humidity detection sensor method ISO 15106-2 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 2 Infrared detection sensor method ISO 15106-3 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 3 Electrolytic detection sensor method... 

Bauer, F., Denneler, S. and Wilert-Porada, M. 2005. Influence of temperature and humidity on the mechanical properties of Nafion 117 polymer electrolyte membrane. Journal of Polymer Science Part B Polymer Physics 43 786-795. 

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