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The working life of a stack is basically determined by the behavior of its weakest monomer; therefore, during a dynamic loading process, high-voltage uniformity plays a key role in stack durability. This work experimentally investigated dynamic responses and voltage uniformity with different loading strategies from an initial state (open-circuit state) to a target current at subzero temperatures. The results presented that the maximum voltage coefficient variation Cv values, representing the relative standard deviation of the monomer voltage, increased from 19.75 to 35.65 and the voltage uniformity sharply decreased with the increase in loading sizes at the stack temperature of −4 °C. It was possible that this could result in the instability of the single cell voltage output and a voltage uniformity fluctuation with a high step loading amplitude. The voltage uniformity became worse as the stack temperature decreased from −2 °C to −8 °C, the voltage uniformity between single cells of the stack continued to deteriorate under a step current density of 0.15 A/cm2 at a relatively low temperature of −8 °C. By comparing several constant loading rate strategies, it could be ascertained that reducing the loading rate was conducive to voltage uniformity in the whole investigated process. A comparison of the different loading rates indicated that the square increasing loading rate strategy is the best strategy among them and a Cv threshold of 5.5 can be obtained during the whole load process at a temperature of −4 °C, which might be a guideline and provide a reference in actual engineering applications for PEMFC dynamic loading responses. [ABSTRACT FROM AUTHOR]

The working life of a stack is basically determined by the behavior of its weakest monomer; therefore, during a dynamic loading process, high-voltage uniformity plays a key role in stack durability. This work experimentally investigated dynamic responses and voltage uniformity with different loading strategies from an initial state (open-circuit state) to a target current at subzero temperatures. The results presented that the maximum voltage coefficient variation Cv values, representing the relative standard deviation of the monomer voltage, increased from 19.75 to 35.65 and the voltage uniformity sharply decreased with the increase in loading sizes at the stack temperature of −4 °C. It was possible that this could result in the instability of the single cell voltage output and a voltage uniformity fluctuation with a high step loading amplitude. The voltage uniformity became worse as the stack temperature decreased from −2 °C to −8 °C, the voltage uniformity between single cells of the stack continued to deteriorate under a step current density of 0.15 A/cm2 at a relatively low temperature of −8 °C. By comparing several constant loading rate strategies, it could be ascertained that reducing the loading rate was conducive to voltage uniformity in the whole investigated process. A comparison of the different loading rates indicated that the square increasing loading rate strategy is the best strategy among them and a Cv threshold of 5.5 can be obtained during the whole load process at a temperature of −4 °C, which might be a guideline and provide a reference in actual engineering applications for PEMFC dynamic loading responses.

In a previous study, we numerically described the three distinct stages of ice evolution during the cold start of a polymer electrolyte fuel cell (PEFC), namely, the freezing, undersaturated, and melting stages. Based on our numerical observations, we propose an efficient start-up strategy for achieving a rapid cell temperature rise while simultaneously mitigating the rate of ice accumulation within a cell. The key to this cold-start strategy involves raising the operating current of the PEFC in the undersaturated stage, which accelerates the cell temperature rise without any further ice accumulation. Using a three-dimensional, transient cold-start model, we numerically demonstrate that raising the cell current in the undersaturated stage is very effective and significantly improves the cold-start behavior of a PEFC. In contrast, increasing the cell current in the freezing stage has a negative impact on the PEFC cold start in that the oxygen reduction reaction causes the water production rate to increase, leading to more rapid ice growth. This ultimately results in cold-start failure and the deterioration of the porous electrode structure. This study clearly illustrates that optimization of the cold-start operation is key to improving the cold-start performance, while also pointing to the importance of the roles played by PEFC cold-start modeling and the simulations used to search for an optimum cold-start strategy. [ABSTRACT FROM AUTHOR]