A Review on Cold Start of Proton Exchange Membrane Fuel Cells

In this paper, we present experimental results about the influence of a hydrophobic coating of gas diffusion layers (GDL) and the existence of a micro porous layer (MPL) on the cold start capability of Polymer Electrolyte Fuel Cells (PEFC). The experiments were performed on five different cell configurations with an active area of 1 cm 2 including GDLs of 0, 5, 20 wt% PTFE with and without MPL. Two different experiments were realized: First, a statistical analysis of more than 700 isothermal cold starts was performed to analyze the stochastic freezing behavior of supercooled water inside the fuel cells. Second, a certain number of cold starts were investigated with high-resolution neutron radiography in order to understand the implications of the water distribution on the freezing mechanism. Especially at low coating loads, it was found that cell-to-cell variations are more dominant than variations of the materials. In contrast, a significantly reduced variability between the individual cells as well as a general reduction of the probability of cell failure was observed at high coating loads. The observed effects can be mainly explained by changed morphology of the investigated materials. Regarding the existence of an MPL, only minor effects were observed which could be assigned to a different local distribution of water in the porous layers.

mentioning

confidence: 99%

Statistical Analysis of Isothermal Cold Starts of PEFCs: Impact of Gas Diffusion Layer Properties

Biesdorf

Forner‐Cuenca

Siegwart

et al. 2016

“…Concerning the aggregate state of water inside the electrode, contradictory findings have been published. Some studies 16,20,[26][27][28] claim that all product water produced during the start-up freezes inside the catalyst layer of the cathode. After filling a certain amount of the pore space with ice, the reactant gases are blocked, leading to a breakdown of the electrochemical reaction.…”

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confidence: 99%

When Size Matters: Active Area Dependence of PEFC Cold Start Capability

Biesdorf

Stahl

Siegwart

et al. 2015

Understanding the freezing mechanism of liquid water within polymer electrolyte fuel cells (PEFC) is crucial for its commercialization for mass market. Although various publications investigated the cold start capability of PEFCs, contradictory behaviors during cold starts at freezing temperatures have been published in literature. Interestingly, differences can be mainly identified between large and small size fuel cells: the latter have a significantly higher cold start capability than large size fuel cells. In order to understand the influence of different active areas, we present measurements of cold starts performed with cells sizes of 1 and 50 cm2 using the same materials, including a large count of experiments (>200) on the 1 cm2 cells to perform statistical analysis. The cold start capability is strongly reduced with an increasing size of the active area, and the operating times changes from stochastic values for small cells to reproducible values for large cells. Both effects can be explained by the higher probability of the aggregate state transition from super-cooled water to ice in large cells. Consequently this paper highlights the implications of downscaling PEFCs to draw conclusions for technical relevant cold-starts.

“…19 Since ice preferentially forms at the interfaces between the CL, membrane, 3 diffusion media, 20 and pores/channels, 21 it can lead to delamination of the membrane from the CL, resulting in increased contact resistance. 22 The charge transfer resistance measured from AC impedance during a cold start from −10…”

mentioning

confidence: 99%

Durability of Polymer Electrolyte Membrane Fuel Cells Operated at Subfreezing Temperatures

Macauley

Lujan

Spernjak

et al. 2016

The structure, composition, and interfaces of membrane electrode assemblies (MEA) and gas-diffusion layers (GDLs) have a significant effect on the performance of single-proton-exchange-membrane (PEM) fuel cells operated isothermally at subfreezing temperatures. During isothermal constant-current operation at subfreezing temperatures, water forming at the cathode initially hydrates the membrane, then forms ice in the catalyst layer and/or GDL. This ice formation results in a gradual decay in voltage. High-frequency resistance initially decreases due to an increase in membrane water content and then increases over time as the contact resistance increases. The water/ice holding capacity of a fuel cell decreases with decreasing subfreezing temperature (−10 • C vs. −20 • C vs. −30 • C) and increasing current density (0.02 A cm −2 vs. 0.04 A cm −2 ). Ice formation monitored using in-situ highresolution neutron radiography indicated that the ice was concentrated near the cathode catalyst layer at low operating temperatures (≈−20 • C) and high current densities (0.04 A cm −2 ). Significant ice formation was also observed in the GDLs at higher subfreezing temperatures (≈−10 • C) and lower current densities (0.02 A cm −2 ). These results are in good agreement with the long-term durability observations that show more severe degradation at lower temperatures (−20 • C and −30 • C).