Proposing a Model for Platinum Nanoparticles Dissolution in PEM Fuel Cells to Describe Unexplained Electrocatalyst Degradation Consequent to Low Cell Voltage Excursion
Authors: Colombo Elena , Casalegno Andrea , Jahnke Thomas , Baricci Andrea 
Published: February 2024

Polymer electrolyte membrane fuel cells (PEMFC) are required to achieve a durability target of more than 20’000 operating hours for application in the heavy-duty transport sector. To accelerate the wide-spread commercialization of this technology, specific attention is thus dedicated to the degradation mechanisms that affect the membrane and the cathode catalyst layer, the components mostly affected by ageing. A previous experimental analysis of durability [1] pointed out that degradation under real-world operating conditions for automotive applications induces a progressive decline in power density and efficiency, mainly associated to a loss in the electrocatalyst active surface area (ECSA). Further experimental tests confirmed that the cathode ECSA during the initial 1000 hours undergoes a stabilization of the nanoparticle distribution towards an average radius of 7 nm and this degradation is accelerated by excursions to low cell voltage values. Such events occur during regular operation while shutting-down the system for short time (the so called “short-stops”), when air is removed at the cathode side with hydrogen flowing at the anode to avoid air/hydrogen fronts. The aim of this work is to analyze this unexpected acceleration in the ECSA loss for oxygen reduction reaction by means of numerical modelling.
Dissolution of platinum nanoparticles in acidic media was extensively analyzed over the last decade and several models were proposed to simulate both cathodic [2] and anodic [3-5] dissolution. These models were applied to the experimental voltage profiles reported in Figure 1(a), which included a low voltage period that mimics a short stop. A systematic study of the low voltage values was carried out in the range between 0.7 V to 0 V. As also observed in Figure 1(b) state-of-the-art models from literature were not able to explain the observed difference between the cases. Aiming to provide a physical description of this aspect a new ageing mechanism was introduced in the model starting from [2] and different model formulations were considered. Formation of PtO2 was assumed to occur, supported by a specific feature that was observed in cathodic scan of the cyclic voltammetry within the potential range of the short-stop (0.1V - 0.3V vs DHE. The electrocatalyst surface coverage was thus estimated from voltammetry. An additional cathodic dissolution reaction is finally proposed that involves PtO2 reduction and a kinetic rate was included in the model, in analogy with the existing reaction mechanism taken from [2]. Good consistency was observed with the experimental results reported in Figure 1(c) and also over a wide dataset reporting cathode ECSA loss consequent to different voltage profiles. ECSA stabilization was attributed to the formation of PtO2 over the corners of the nanoparticle that could explain a mitigation of the dissolution mechanism for particles larger than 7 nm, in which corner sites lead to minor coverage.
The results of this work will allow to accelerate the development of mitigation strategies and innovative materials to improve the durability of State-of-the-Art electrocatalysts for PEMFC. This work received support from the Italian government (Progetto PERMANENT - BANDO MITE PNRR Missione 2 Investimento 3.5 A - RSH2A_0O0012).
References
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Figure 1