Ice formation and distribution in the catalyst layer during freeze-start process – CRYO-SEM investigation

Li, Jing; Lee, Stephen; Roberts, Joy H.

doi:10.1016/j.electacta.2008.02.077

Cited by 56 publications

(38 citation statements)

References 10 publications

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“…• C. This is different from the result reported by Li et al 15 They reported that more than 70% of pores of the CL are filled with ice after the purge process without any subzero operation, and also pointed out that insufficient purge and water migration during the cooling process could be the reasons for the presence of this ice in their results. The result in Fig.…”

Section: Experimental Apparatus and Methodscontrasting

confidence: 56%

“…An alternative reason for the active ice formation near the MPL side could be the effects of the produced water transported to the membrane 12 and the heat generated by the reaction 15 as has been pointed out in the literature. However, the experimental results in Figs.…”

Section: Resultsmentioning

confidence: 99%

“…The movement of a more active ice formation region with vacant pores remaining near the membrane at the higher current density has not been reported in cryo-SEM investigations published so far. [13][14][15] This would be a critical process to consider when attempting to determine the ice storage capacity of the CL: the smaller amount of residual water after the membrane rehydration at 0.08 A cm −2 in Fig. 3 expresses a smaller amount of ice accumulated in the cathode CL.…”

Section: Resultsmentioning

confidence: 99%

“…15 At the midpoint of the operation they observed two distinct regions, a dense layer filled with ice close to the GDL and a porous layer containing less ice which was close to the membrane in the CL. Taken together, these observations appear to suggest different behaviors during the ice formation process in the CL, and further research is necessary to fully resolve and elucidate details of the effects of the current density and the cathode gas conditions on the freezing process.…”

mentioning

confidence: 99%

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Ice Formation Processes in PEM Fuel Cell Catalyst Layers during Cold Startup Analyzed by Cryo-SEM

Tabe

Yamada

Ichikawa

et al. 2016

J. Electrochem. Soc.

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For further improvements in the startup ability below freezing and the durability of polymer electrolyte fuel cells, understanding the ice formation mechanism during cold startup is particularly significant. This study observes cross-sectional ice distributions in a catalyst layer (CL) during isothermal galvanostatic operation at −20 • C using a cryo-scanning electron microscope. The effects of current density, cathode gas conditions, initial water content of the membrane, and cell temperature on the cold start characteristics and the ice formation process in the CL are evaluated. The observational results show that at higher current densities, the region with active ice formation moves from the membrane to the gas diffusion layer sides during the freezing period and vacant pores remain near the membrane even after cell shutdown, while the pores are completely filled with nearly-uniformly growing ice at lower current density operation. This is consistent with the experimental finding from the cold start characteristics that the estimated amount of ice accumulated in the cell until the shutdown decreases as the current density increases. Contrary to expectations, these changes are largely independent of cathode gas conditions, even with pure oxygen. Additional factors controlling the ice formation process are discussed based on the experimental results. The polymer electrolyte fuel cell (PEFC) is a potential candidate for automotive power sources and portable electricity generators because of its characteristics of high efficiency, high power density, low operating temperature, and other advantageous characteristics. In cold climates, however, ensuring startup and durability of PEFCs at subfreezing conditions is a critical issue for the practical use of PEFC. It has been reported that freezing of water produced by the cathode reaction induces shutdown (voltage failure) during startup at subfreezing temperatures, and this freezing causes various kinds of degradation of the cell performance.1-3 Therefore, a number of practicable strategies based on thermal management and control of the residual water present in the cell before cooling and starting have been developed. However, a basic understanding of details of the ice formation process at cold startup, which is particularly-significant for further improvements in the cold startup ability and durability, remains insufficiently addressed in the literature.Some modeling studies have been conducted to describe the heat and mass transfer, and ice formation during cold startup. [4][5][6][7] The research group of the authors has investigated the performance of a PEFC at temperatures below the freezing point by both simulation and experiments.4 That study identified the initial temperature at which self-starting becomes possible, and this is determined by the balance of the produced heat and water generated due to the reaction. With regard to freezing at temperatures closer to zero, like −10• C, it was reported that the produced water is present in a supercooled state, a...

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Section: Experimental Apparatus and Methodscontrasting

confidence: 56%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

Ice Formation Processes in PEM Fuel Cell Catalyst Layers during Cold Startup Analyzed by Cryo-SEM

Tabe

Yamada

Ichikawa

et al. 2016

J. Electrochem. Soc.

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show abstract

“…Tajiri et al proposed a strict gas purge process before cold start operation, and estimated the ice distribution in the cathode catalyst layer at the end of cold starting at -30 °C at low and high current densities [5]. Recently, cryo-SEM observations were conducted to elucidate further details of the ice distribution in the catalyst layer at -25 °C or -20 °C; these observations were carried out with the examined components kept at very low temperature and in vacuum conditions to avoid thawing and covering by frost deposits [6][7][8][9]. At temperatures closer to zero, like -10 °C, it was reported that the produced water is present in a supercooled state, and that the freezing behavior in a PEFC can be visualized by infrared thermography and it was possible to capture the 2-dimensional propagation in the high temperature region caused by the release of heat due to freezing [10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Cold start characteristics and freezing mechanism dependence on start-up temperature in a polymer electrolyte membrane fuel cell

Tabe

Saito

Fukui

et al. 2012

Journal of Power Sources

131

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Cold start characteristics of a polymer electrolyte membrane fuel cell are investigated experimentally, and microscopic observations are conducted to clarify the freezing mechanism in the cell. The results show that the freezing mechanism can be classified into two types: freezing in the cathode catalyst layer at very low temperature like −20 °C, and freezing due to supercooled water at the interface between the catalyst layer and the gas diffusion layer near 0 °C like −10 °C. The amount of water produced during the cold start is related to the initial wetness condition of the polymer electrolyte membrane, because water absorption by the membrane due to back diffusion plays an important role to prevent the water from freezing. It is also shown that after the shutdown of the cold start the cell performance of a subsequent operation at 30 °C is temporarily deteriorated after the freezing at −10 °C, but not after the freezing at −20 °C. The ice formed at the interface between the catalyst layer and the gas diffusion layer is estimated to cause the temporary deterioration, and the function of a micro porous layer coating the gas diffusion layer for the ice formation is also discussed. Highlights• Two freezing types at cold start in and on the surface of a cathode catalyst layer • Direct observation of the ice formed on the catalyst layer surface • Temporary performance deterioration at 30 °C caused by ice on the surface

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Cold‐start durability of membrane‐electrode assemblies

Wang

Yang

Tabuchi

et al. 2010

Handbook of Fuel Cells

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Durability of membrane-electrode assemblies (MEAs) cycled under cold-start conditions is reviewed. It is shown that MEAs cycled under 100 mA/cm 2 from -30 o C show no degradation after 100 cycles, but exhibit mild degradation after 150 cycles under 300 mA/cm 2 from -30 o C, and furthermore suffer severe degradation after 110 cycles under 500 mA/cm 2 from -20 o C. Transmission electron microscopy (TEM) and X-ray diffraction using cross-sectional samples of the aged MEAs further revealed three primary degradation mechanisms: (i) interfacial delamination between the cathode catalyst layer (CL) and membrane, (ii) cathode CL pore collapse and densification upon melting of a fully ice-filled CL, and (iii) Pt particle coarsening and Pt dissolution in PFSA ionomer. The interfacial delamination and CL densification appear to be closely related to each other, and the key parameter to affect both is the ice volume fraction in the cathode CL after each cold-start step. Eliminating or minimizing these two degradation processes could improve the MEA cold-start durability by 280%. Mitigation strategies include improved gas purge prior to cold start, good MEA design, small startup current density, and low cell thermal mass.

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Ice formation and distribution in the catalyst layer during freeze-start process – CRYO-SEM investigation

Cited by 56 publications

References 10 publications

Ice Formation Processes in PEM Fuel Cell Catalyst Layers during Cold Startup Analyzed by Cryo-SEM

Ice Formation Processes in PEM Fuel Cell Catalyst Layers during Cold Startup Analyzed by Cryo-SEM

Cold start characteristics and freezing mechanism dependence on start-up temperature in a polymer electrolyte membrane fuel cell

Cold‐start durability of membrane‐electrode assemblies

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