Nucleation and growth of ice in the catalyst layer of a proton-exchange-membrane fuel cell (PEMFC) are investigated using isothermal differential scanning calorimetry and isothermal galvanostatic cold-starts. Isothermal ice-crystallization rates and icenucleation rates are obtained from heat-flow and induction-time measurements at temperatures between 240 and 273 K for four commercial carbon-support materials with varying ionomer fraction and platinum loading. Measured induction times follow expected trends from classical nucleation theory and reveal that the carbon-support material and ionomer fraction strongly impact the onset of ice crystallization. Conversely, dispersed platinum particles play little role in ice crystallization. Following our previous approach, a nonlinear ice-crystallization rate expression is obtained from Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. A validated rate expression is now available for predicting ice crystallization within water-saturated catalyst layers. Using a simplified PEMFC isothermal cold-start continuum model, we compare cell-failure time predicted using the newly obtained rate expression to that predicted using a traditional thermodynamic-based approach. From this comparison, we identify conditions under which including ice-crystallization kinetics is critical and elucidate the impact of freezing kinetics on low-temperature PEMFC operation. The numerical model illustrates that cell-failure time increases with increasing temperature due to a longer required time for ice nucleation. Hence, ice-crystallization kinetics is critical when induction times are long (i.e., in the "nucleation-limited" regime for T > 263 K). Cell-failure times predicted using ice-freezing kinetics are in good agreement with the isothermal cold-starts, which also exhibit long and distributed cell-failure times for T > 263 K. These findings demonstrate a significant departure from cell-failure times predicted using the thermodynamic-based approach.Proton-exchange-membrane fuel cells (PEMFCs) show promise in automotive applications because of their high efficiency, high power density, and potentially low emissions. To be successful in automotive applications, PEMFCs must permit rapid startup with minimal energy from sub-freezing temperatures, known as cold-start. In a PEMFC, reduction of oxygen to water occurs in the cathode catalyst layer (cCL). Under subfreezing conditions, water solidifies and hinders access of gaseous oxygen to the catalytic sites in the cCL, severely inhibiting cell performance and potentially causing cell failure. 1-3 Elucidation of the mechanisms and kinetics of ice formation within the cCL is, therefore, critical to successful cell startup and high performance at low temperatures.Because of degradation and cell failure under subfreezing conditions, much attention has been given to understanding cold-start fundamentals. To date, experiments predominately focus on characterizing overall low-temperature cell performance.

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The effect of MEA preparation on the performance of single-PEM fuel cells operated at sub-freezing temperatures is presented. The cell performance and durability are dependent on the MEA and are probably influenced by the porosity of the catalyst layers. When a cell is operated isothermally at -10 o C in constant current mode, the voltage gradually decreases over time and eventually drops to zero. AC impedance analysis indicated that the rate of voltage loss is initially due to an increase in the charge transfer resistance and is gradual. After a period, the rate of decay accelerates rapidly due to mass transport limitations at the catalyst and/or gas diffusion layers. The high frequency resistance also increases over time during the isothermal operation at sub-freezing temperatures and was a function of the initial membrane water content. LANL prepared MEAs showed very little loss in the catalyst surface area with multiple sub-freezing operations, whereas the commercial MEAs exhibited significant loss in cathode surface area with the anode being unaffected. These results indicate that catalyst layer ice formation is influenced strongly by the MEA and is responsible for the long-term degradation of fuel cells operated at sub-freezing temperatures. This ice formation was monitored using neutron radiography and was found to be concentrated near cell edges at the flow field turns.

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The effect of MEA and GDL structure and composition on the performance of single-PEM fuel cells operated isothermally at subfreezing temperatures is presented. The cell performance and durability are not only dependent on the MEA/GDL materials used but also on their interfaces. When a cell is operated isothermally at sub-freezing temperatures in constant current mode, the water formation due to the current density initially hydrates the membrane/ ionomer and then forms ice in the catalyst layer/GDL. An increase in high frequency resistance was also observed in certain MEAs where there is a possibility of ice formation between the catalyst layer and GDL leading to a loss in contact area. The total water/ice holding capacity for any MEA was lower at lower temperatures and higher current densities. The durability of MEAs subjected to multiple isothermal starts was better for LANL prepared MEAs as compared to commercial MEAs, and cloth GDLs when compared to paper GDLs. The ice formation was monitored using high-resolution neutron radiography and was found to be concentrated near the cathode catalyst layer. However, there was significant ice formation in the GDLs especially at the higher temperature ( -10 o C) and lower current density (0.02 A/cm 2 ) operations. These results are consistent with the longerterm durability observations that show more severe degradation at the lower temperatures.