Abstract
Membrane water content is of vital importance to the freezing durability of proton exchange membrane fuel cells (PEMFCs). Excessive water freezing could cause irreversible degradation to the cell components and deteriorate the cell performance and lifetime. However, there are few studies on the critical membrane water content, a threshold beyond which freezing damage occurs, for cold storage of PEMFCs. In this work, we first proposed a method for measuring membrane water content using membrane resistance extracted from measured high frequency resistance (HFR) based on the finding that the non-membrane resistance part of the measured HFR is constant within the range of membrane water content of 2.98 to 14.0. Then, freeze/thaw cycles were performed from −50 °C to 30 °C with well controlled membrane water content. After 30 cycles, cells with a membrane water content of 8.2 and 7.7 exhibited no performance degradation, while those higher than 8.2 showed significant performance decay. Electrochemical tests revealed that electrochemical surface area (ECSA) reduction and charge transfer resistance increase are the main reasons for the degradation. These results indicate that the critical membrane water content for successful cold storage at −50 °C is 8.2.
Highlights
Proton exchange membrane fuel cell (PEMFC) is an energy conversion device that converts chemical energy directly into electricity, which has attracted great attention in recent years due to its high efficiency and environmentally friendly features
The results indicated that the cell purged with RH58.0% showed no performance loss after 20 freeze/thaw cycles, while the cell purged with RH64.9% became flooded at high current densities, which is attributed to the change in pore size distribution of the gas diffusion layer (GDL)
When the membrane water content is within the range of 8.2 to 14.0, continuous liquid water still remains in the catalyst layer but with lower saturation, as shown in Figure 6b, which eventually results in relative slight degradation after repeated freeze/thaw cycles
Summary
Proton exchange membrane fuel cell (PEMFC) is an energy conversion device that converts chemical energy directly into electricity, which has attracted great attention in recent years due to its high efficiency and environmentally friendly features. Sinha and Wang [27] developed a three-dimensional (3D) analytical model to describe the gas purge phenomena, and they classified the water removal process into four stages: through-plane drying, in-plane drying, vapor transport and equilibrium stage They elucidated the effect of purge conditions on purge effectiveness and indicated that purging with low relative humidity gas, high cell temperature, and high gas velocity is preferable. In order to make the residual water content more intuitive, high frequency resistance (HFR) was first introduced by Ge and Wang [28] as a direct indicator of purge effectiveness and membrane hydration. It is commonly used as a reflection of membrane hydration and as a calculation basis of membrane water content. The electrochemical and physical characterization were conducted to reveal the degradation mechanism caused by freezing
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