Relative humidity proton conductivity

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... 

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. ... 

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.

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. 

The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H2 and O2, and good mechanical properties under wet-dry and temperature cycles has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth. 

Regardless of the microscopic phenomena, protonic conductivity is critically sensitive to the water content inside crystals and on their surface. Intrinsically nonconductive materials may apparently exhibit proton transport in wet environments due to adsorbed and/or condensed water. Consequently, numerous reports on the conductivity of compacted powders at 90-100% relative humidity, when vapor condensation in pores cannot be avoided, are excluded from consideration. Heating or cooling may cause H2O loss or uptake from the atmosphere, thus altering the conditions for proton transport in crystals. In such situations, the apparent found... 

The baseline Nafion membrane is deficient in terms of ionic conductivity above 100°C and at low relative humidity required for atmospheric-pressure building applications. Such conditions tend to dry out the membrane, drastically reducing membrane proton conductivity. Furthermore, the loss of water causes membrane embrittlement, resulting in membrane cracking, reactant cross-leakage and poor electrode-membrane contact. Therefore, a cost-effective membrane, with proton conductivity that is less sensitive to change in water content, is needed. 

Phase 1 (a) Determine conditions under which nanoparticle sols of candidate oxides yield microporous gels (b) Measure proton conductivities of these gels as a function of temperature and relative humidity (c) Chemically adsorb anionic or cationic functional groups onto oxide particles contained in xerogels or membranes in order to enhance proton conductivity. 

Bulk protonic conductivity ohm cm Film, at 100% relative humidity 8 X 10-5 (3)... 

Fig. 19 Dependence of proton conductivity on lEC for SPSU and SPSU-f -PVDF membranes. 30 C, 95% relative humidity...

Proton conductivity of sulfonated poly(phenylene sulfide) is 10 Scm at room temperature and relative humidity of 30%. The conductivity exponentially grows with the increase in relative humidity and reaches a value of 2x10 cm at 94% humidity (Fig. 14). 

Fig. 14 Proton conductivity of the sulfonated polyphenylene sulfide (Scheme 2) (m = 2) at room temperature as a function of relative humidity [95]...

Experiments [7] on water absorption by S-PEEK and S-PPBP films showed that proton conductivities of the films containing equihbrium amounts of absorbed water depend on the relative hiunidity. Fig. 15 represents the dependency of the proton conductivities of S-PEEK and S-PPBP with different sulfonation levels as a function of relative humidity. 

It becomes clear that proton conductivities of the films increase with the relative humidity and water uptake and can become as high as 10 Scm (for S-PEEK). 

Fig. 15 Proton conductivity of S-PEEK (1) and S-PPBP (2-5) with different sulfonation levels as a function of relative humidity at room temperature [7]. Sulfonation level (mol%) 65 (1), 30 (2), 65 (3), 80 (4) and 85 (5)...

Fig. 16 Temperature dependences of proton conductivity as S-PPBP (1) and S-PEEK (2) with the same degrees of sulfonation (65 mol %) at a relative humidity of 120% [7]...

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