Mitigating PEMFC durability limitations

Kurzfassung

A key requirement for PEM fuel cells for transport applications is longevity with a targeted lifetime of 5,000-6,000 h for light duty electric vehicles (LDV) and 25,000-30,000 h for heavy duty vehicles (HDV). In parallel, other KPIs such as high power density at certain voltage level (2024 target: 1.8 A cm-2 at 660 mV for LDV) and low Pt-loading (2024 target: 0.14 mg cm-2 total Pt loading for LDV) need to be achieved. This makes the durability targets even more challenging. Thereby, the lifetime of a PEMFC stack is determined by i) material properties and ii) by operation strategy. Hence, to increase PEMFC durability, investigations are pursued to propose improved components as well as to define appropriate operation conditions for different applications. The performance limitations of PEMFC become substantial particularly at low Pt loadings. Moreover, performance and material degradation is particularly critical at low Pt loadings as well; in this context the cathode catalyst layer contributes the most to performance decay. The dissolution of Pt nanoparticles and the degradation of the carbon support as well as ionomer are dominating processes. One approach to mitigate performance degradation is the development of stabilized catalysts. Low Pt loaded electrodes with the nano-cage stabilized Pt/C catalyst mitigate Pt dissolution, as will be shown in this presentation. The positive effect is clearly demonstrated by comparing the accelerated stress tests (ASTs) of a MEA with conventional Pt/C catalyst against a MEA with a stabilized Pt/C catalyst. In our tests, the conventional MEA exhibits dramatic ECSA loss, while it remains stable in case of the stabilized MEA as shown in panel (A) of the image. To assess durability of new MEA components, ASTs are necessarily required to avoid testing times of thousands of hours. Moreover, to provide realistic results, these ASTs need to simulate stressors occurring in the course of real operation conditions. To quantify the impact of specific operando stressors (such as high current density or high operation temperature) on individual MEA components (i.e. CCM and GDL), specific degradation tests have been carried out as part of the European ID-Fast project. Progress on these investigations will be shown in this presentation. In addition, special focus is dedicated to the analysis of reversible degradation losses and the corresponding recovery procedures. Within this framework, different recovery protocols proposed in the literature as well as by projects funded by the European Commission and the DOE are compared. The second approach focuses on increasing the lifetime by optimizing operation conditions of the PEMFC stack. This includes considering the power demand and dynamics of the subsystems and their balance of plant components in a hybrid system using a PEMFC and an energy storage system (i.e. battery or capacitor). Thereby the system power demand is derived from the velocity profiles of the target applications and has to be distributed between the subsystems. This approach allows mitigation of detrimental system operation, such as i) uncovered power demand, ii) discharged energy storage (no buffering of power peaks) and fully charged energy storage (no recuperation) and iii) excess of energy storage maximum charge current (partial buffering) as shown in panel (B) of the image. Moreover, faulty stack operation is mitigated thanks to using operando state-of-health monitoring.