Proton Conductivity at Low Temperature

There are a substantial number of studies reported on the characteristics of the membrane at low temperatures. This may be 

One study specifically designed for PEFC was reported by Thompson et al.8 They used a direct current to measure the proton conductivity at low temperature. In conjunction with the DSC data, they found the dependency of crossover temperature (temperature where the activation energy changes) on water content and hysteresis between freezing and melting. 

Most recently, Gallagher et al.21 measured the water uptake of Nafion membrane under subfreezing temperatures, which showed a significant reduction in the maximum water content corresponding to membrane full hydration. The Nafion membrane with 1,100 equivalent weight, for example, uptakes A 8 of water at -25°C when it equilibrates with vapor over ice because of the low vapor pressure of ice compared to supercooled liquid water. They also found the electro-osmotic drag coefficient to be 1 for Nafion membrane under sub freezing temperatures. 

The cool-down process of the cold-start experiment also provides an opportunity to obtain the membrane proton conductivity as a function of temperature at a known water content. Note that the temperature dependence of proton conductivity with low membrane water content is of particular interest here as PEFC cold start rarely involves fully hydrated membranes after gas purge. In addition, unlike PEFCs operated under normal temperatures, the membrane resistance under low water content and low temperature typical of cold start conditions is much greater than the contact resistance, making in-situ measurements of the membrane proton conductivity in a PEFC a simple but accurate method. 

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. 

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A point defect model has been proposed for the brownmillerite structure, based on a study of electrical conductivity and em/measurements of Ba2ln205 [236]. The experimental results matched well with modelling, while evidence was found for protonic conduction at low temperatures. In modelling, the unoccupied oxygen sites relative to the perovskite structure, below Tj, are regarded as structural units and therefore are potential interstitial sites for... 

The pressure of gas was 101.325 kPa. The ammonia synthesis rate was stable after passage of current for 2-6 min and this rate was at least three magnitudes higher than that of conventional catalytic reactor. The conversion of hydrogen was close to 100% which eliminated thermodynamic equilibrium limitation. The main problem of this method is that the conductivity of SCY ceramic is very poor at normal temperature. Even at 570°C, its conductivity is unsatisfied because the current density was smaller than 2 mA-cm and could not be further increased. This limited the efficiency of ammonia synthesis and theoretical research. The use of solid electrolyte with high proton conductivity at low temperature to replace the SCY ceramic may be favorable to decreasing the synthesis temperature and increasing the current density and production efficiency. 

The development of electrolytes that exhibit a higher conductivity at low temperature. Three candidates have emerged, namely doped ceria, doped lanthanum gallate, and doped barium zirconate. The first two of these are oxygen ion electrolytes, and the latter is a proton conductor. 

DTA studies revealed a rather strong interaction between water molecules in sulfonated hydrocarbon polymers and their sulfonic acid groups, which leads to high proton conductivities at high temperature and low hiunidity. 

Sulfonated poly(phenylene sulfide) and S-PPBP exhibit stable proton conductivities at elevated temperatures. For this reason, they are considered as prospective polymers for manufacture of proton-conducting electrolyte membranes operating at elevated temperatures and low humidity. 

On the other hand, water-free SOFCs operate at temperatures up to 1000 °C. Promising eandidates for a solid-oxide membranes are derivatives of CSHSO4, whereas CSHSO4 itself is not suitable as a separator material, because it is very soft, soluble in water, sensitive to reduction, and has a very low proton conductivity at room temperature. ... 

High proton conductivity at high temperature and low relative humidity can be achieved using acid-base polymer complexes between basic polymers and strong acids or polymeric acids [1,14]. The proton-accepting polymers include... 

Current state of the art for membranes used in fuel cells are PFSA-based membranes which are hmited to temperamres <80°C at fully humidified crmditions. Hence, the ideal membranes needed for the fuel cell operation must be stable at higher temperatures, should have high proton conductivity at low hunudity, be inexpensive, and durable. HPAs, which are super acids, have been used for H2/O2 and DMFCs in powdered as well as liquid form, but when used in their pure form have been found to give low performance and wash out easily with the water formed during the fuel cell operation. The HPA have been incorporated into various matrices both inert and ionomeric to be used at higher temperatures and drier conditions. Incorporation of HPAs into a membrane matrix can achieve aU these properties, and so the key issue that remains is that the HPAs should be effectively... 

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