SAXS Investigation of the Effect of Freeze/Thaw Cycles on the Nanostructure of Nafion® Membranes

Abstract

In this study, we performed small-angle X-ray scattering (SAXS) to investigate the structure of Nafion® membranes. The effect of freeze/thaw (F/T) cycles (from ambient temperature down to −40 °C) on the membrane nanostructure was considered for the first time. The SAXS measurements were taken for different samples: a commercial Nafion® 212 membrane swollen in water and methanol solution, and a water-swollen silica-modified membrane. The membrane structure parameters were obtained from the measured SAXS profiles using a model-dependent approach. It is shown that the average radius of water channels (Rwc) decreases during F/T cycles due to changes in the membrane structure as a result of ice formation in the pore volume after freezing. The use of water-methanol solution (methanol content of 20 vol.%) for the membrane soaking prevents changes in the membrane structure during F/T cycles compared to the water-swollen membrane. Modification of the membrane surface with silica (SiO2 content of 20 wt.%) led to a redistribution of water in the membrane volume and resulted in a decrease in Rwc. However, Rwc for the modified membrane did not decrease with the increasing number of F/T cycles due to the involvement of SiO2 in the sorption of membrane water and, therefore, the prevention of ice formation.

Cold start of proton exchange membrane fuel cell using build-in catalytic heater

“…Thus, analyze the described equations it can be finally conclude that with an increase in air flow increases the heating rate of the BP and decreases the heating efficiency BP (6). With an increase in the hydrogen concentration in the air, the heating rate decreases, which requires an increase in air flow (3) and causes an increase in pressure losses. Based on equations ( 4)-( 6) the requirements for high efficiency of BP heating (low air mixture flow rate) lead to an increase in the BP heating time.…”

“…at subzero temperature) there is a problem of water freezing in the volume of the device elements. Ice formation makes it difficult to successfully restart the system and causes permanent component damage and degradation of system performance [1][2][3]. To eliminate these negative effects, several cold start strategies and corresponding technical solutions are proposed.…”

“…To eliminate these negative effects, several cold start strategies and corresponding technical solutions are proposed. These scenarios include moisture removal by gas purging of watercontaining elements (for example, using dry nitrogen) [3][4][5][6][7][8][9][10][11][12][13][14][15][16], vacuum drying, and the use of antifreezes [11] (for example, an aqueous solution of methanol), heat exchangers, bifunctional microporous layers, the use of materials that prevent freezing or do not degrade when frozen, running the system at constant current or maximum power, etc. These methods ensure the device self-starts at −5 … −10 ℃.…”

Recent Advances in the Development of Nanocarbon-Based Electrocatalytic/Electrode Materials for Proton Exchange Membrane Fuel Cells: A Review

“…This phenomenon negatively affects the mass transfer of reagents and, over time, leads directly to degradation of the above FC components and deterioration of the device output characteristics [10][11][12]. When humidity is too low, the proton exchange membrane changes its geometry and dimensions [13], leading to mechanical failure of the CL being in direct contact with the membrane [14,15]. Deterioration in FC performance is also caused by various mechanisms of catalytic layer degradation, which occur under the influence of electrical potential, elevated temperature, and humidity [16,17].…”

“…This phenomenon negatively affects the mass transfer of reagents and, over time, leads directly to degradation of the above FC components and deterioration of the device output characteristics [10][11][12]. When humidity is too low, the proton exchange membrane changes its geometry and dimensions [13], leading to mechanical failure of the CL being in direct contact with the membrane [14,15].…”