Cathode Electrode Durability Under Combined Start-up/Shutdown and Load Cycles
Authors: Messing Marvin , Kjeang Erik 
Published: December 2020

Polymer electrolyte fuel cells (PEFCs) enable the use of hydrogen as an alternative to fossil fuels to power light and heavy-duty vehicles. The cathode electrode (CE) plays a crucial role in the overall durability of PEFCs and efforts are underway to further understand and model the degradation of the CE. Fuel cell models which can simulate durability under different conditions can reveal optimal fuel cell usage scenarios, however, extensive testing is required to improve modeling accuracy. Accelerated stress tests (ASTs) are commonly used to simulate various operating conditions and isolate different degradation mechanisms. Operating conditions such as load cycles and start-up/shutdown (SUSD) cycles primarily cause CE degradation. In load cycle ASTs, fuel cell voltage is cycled between a near open circuit voltage (OCV) and a fixed lower potential to cause rapid degradation [1], [2]. More realistic results can be obtained using standard driving schedules to simulate voltage profiles which occur during different driving scenarios [3]; however, long testing times are required. SUSD ASTs apply potentials above the OCV of the fuel cell [4]. In this case, more realistic results can be obtained, without increasing testing times, by switching fuel and oxidant gases to simulate fuel displacing oxidant during start-up (vehicle turned on), and oxidant displacing fuel during shutdown (vehicle turned off) [5]. To further develop the understanding of CE durability, this study (a) investigates the relationship between load cycle and SUSD cycle degradation mechanisms and (b) combines empirical results from both tests into an empirical CE durability model. The load cycle AST used in this work consists of square wave voltage cycles between 0.6 V and OCV. A method is developed to relate the AST results to drive cycles, which allows for rapid CE degradation while maintaining relevance to realistic usage scenarios. SUSD testing is done with gas-switching for increased accuracy. Load cycling and SUSD cycling are also combined to form a third, mixed degradation test protocol. Scanning electron microscopy images are obtained at different stages for each of the three scenarios. The results show linear degradation trends for load cycle and SUSD cycle tests, and polynomial models were developed for each case. Assuming no coupling exists between the two degradation mechanisms, the combined protocol degradation was simulated based on combining predictions from the load cycle and SUSD cycle models. Experimental results from the same combined protocol show good agreement with the predicted degradation (Figure 1). Therefore, under the experimental conditions of this study, coupling effects between load cycle degradation and SUSD cycle degradation are negligible. Furthermore, the number of equivalent AST cycles is estimated from drive cycles using an amplitude threshold. This threshold value is then used as an input for the load cycle model to obtain durability predictions based on different drive cycles, and compared to results from the US National Renewable Energy Laboratory (NREL) [6]. The predictions of the CE durability model developed in this study fall within expected overall durability of the fuel cell vehicle [7].

References

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Figure 1. Modeled and measured voltage degradation for combined load and start-up/shutdown cycling.

Figure 1