Proton humidity

Typically, Nation ionomer is the predominant additive in the catalyst layer. However, other types of CLs with various hygroscopic or proton conductor additives have also been developed for fuel cells operafed xmder low relative humidity (RH) and/or at elevated temperatures. Many studies have reported the use of hygroscopic y-Al203 [52] and silica [53,54] in the CE to improve the water retention capacity and make such CEs viable for operation af lower relative humidity and/or elevated temperature. Alternatively, proton conducting materials such as ZrP [55] or heteropoly acid HEA [56] have also been added... 

Chao, W. K., Lee, C. M., Tsai, D. C., Chou, C. C., Hsueh, K. L., and Shieu, F. S. Improvement of the proton exchange membrane fuel cell (PFMFC) performance at low-humidity conditions by adding hygroscopic y-Al203 particles into the catalyst layer. Journal of Power Sources 2008 185 136-142. 

A. M. Kannan, L. Cindrella, and L. Munukutla. Functionally graded nanopo-rous gas diffusion layer for proton exchange membrane fuel cells under low relative humidity conditions. Electrochimica Acta 53 (2008) 2416-2422. 

The excellent prospects of PEFCs as well as the undesirable dependence of current PEMs on bulk-like water for proton conduction motivate the vast research in materials synthesis and experimental characterization of novel PEMs. A major incentive in this realm is the development of membranes that are suitable for operation at intermediate temperatures (120-200°C). Inevitably, aqueous-based PEMs for operation at higher temperatures (T > 90°C) and low relative humidity have to attain high rates of proton transport with a minimal amount of water that is tightly bound to a stable host polymer.33 37,40,42,43 yj-jg development of new PEMs thus warrants efforts in understanding of proton and water transport phenomena under such conditions. We will address this in Section 6.7.3. 

Alberti et al. investigated the influence of relative humidity on proton conductivity and the thermal stability of Nafion 117 and compared their results with data they obtained for sulfonated poly(ether ether ketone) membranes over the broad, high temperature range 80—160 °C and RHs from 35 to 100%. The authors constructed a special cell used in conjunction with an impedance analyzer for this purpose. Data were collected at high temperatures within the context of reducing Pt catalyst CO poison-... 

All acidic proton conductors discussed so far in this review have relied on the presence of large amounts of water (A = 10—30) as a mobile phase for the conduction of protons. Current targets for automotive use of hydrogen/air fuel cells are 120 °C and 50% or lower relative humidity. Under these conditions, the conductivity of the membrane decreases due to low water uptake at 50% relative humidity and thus creates large resistive losses in the cell. To meet the needs of advanced fuel cell systems, membranes will have to function without large amounts of absorbed water. Organic—inorganic composites are one preferred approach. ... 

Particularly, in the case of proton-conducting zirconium phosphate prepared via in situ growth within the preformed membrane,the proton conductivity of the highly dispersed filler may have some significance at high temperature and low humidity, where the conductivity of pure Nafion strongly decreases. 

Figure 21. Proton conductivity of PBPnH3P04 adducts, as a function of phosphoric acid concentration and relative humidity Data from another source (denoted by...

Figure 22. Proton conductivity of PBI nH3P04 adducts, as a function of temperature T and relative humidity RH for a given phosphoric acid concentration.

Upon comparison of eq 32 to 28, it is seen that the proton—water interaction is now taken into account. This interaction is usually not too significant, but it should be taken into account when there is a large gradient in the water (e.g., low humidity or high-current-density conditions). Upon comparison of eq 33 to 31, it is seen that the equations are basically identical where the concentration and diffusion coefficient of water have been substituted for the chemical potential and transport coefficient of water, respectively. Almost all of the models using the above equations make similar substitutions for these variables 15,24,61,62,128... 

Figure 13. Plot of cathode potential as a function of current density for a macrohomogeneous embedded model where the proton conductivity is assumed to be uniform (0.044 S/m), curve a, or varies with water production (changing humidity) across the catalyst layer, curve b. (Reproduced with permission from ref 98. Copyright 2002 The Electrochemical Society, Inc.)...

The standard electrode potential and its temperature coefficient are found in the literature.36 Kinetic parameter values were measured in-house for HOR,33 ORR,34 OER,35 and COR.12 22 Table 2 gives cell component materials and transport properties. The membrane and electrode proton conductivity in Table 2 are based on the measured membrane and electrode resistance,42,43 which is a strong function of relative humidity (RH). In what follows next, we will describe the... 

The conductivity data measured in situ in Fig. 1 are within the temperature range from room temperature (27°C) to -30°C. In contrast, Cleghorn et al. reported the proton conductivity for Gore-Select membranes in the temperature range of 40-100°C.23 Extrapolating the correlation of Cleghorn et al. to 27°C and at 100% relative humidity, the membrane conductivity is calculated to be 0.027 S/cm, which is in reasonable agreement with our in-situ measurement of 0.021 S/cm. 

A series of articles were published by Ennari et al. on MD simulation of transport processes in Poly(Ethylene Oxide) and sulfonic acid-based polymer electrolyte.136,137 The work was started by the determination of the parameters for the ions missing from the PCFF forcefield made by MSI (Molecular Simulations Inc.), to create a new forcefield, NJPCFF. In the models, the proton is represented as a hard ball with a positive charge. Zhou et al. used the similar approach to model Nation.138 The repeating unit of Nafion (Fig. 17) was optimized using ab initio VAMP scheme. The protons were modeled with hydronium ions. Three unit cell or molecular models were used for the MD simulation. The unit cell contains 5000 atoms 20 pendent side chains, and branched Nafion backbone created with the repeating unit. Their water uptakes or water contents were 3, 13, or 22 IEO/SO3, which correspond to the room temperature water uptakes at 50% relative humidity (RH), at 100% RH, and in liquid water respectively.18 The temperature was initially set at a value between 298.15 and 423.15 K under NVE ensemble with constant particle number, constant volume (1 bar), and constant energy. 

Very humid mortars and concretes, due to proton diffusion, exhibit no sharp carbonation, i.e., pH limit. 

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