Degradation Mechanisms of Supported Pt Nanocatalysts in Proton Exchange Membrane Fuel Cells: An Operando Study through Liquid Cell Transmission Electron Microscopy

Abstract

Nanocatalysts’ degradation is a limiting factor for the development of polymer electrolyte membrane for fuel cells (PEMFCs). In this work, a dedicated sample holder has been used to mimic the cathode of a PEMFC at the earliest stages of the aging process, i.e., under accelerated stress test after up to 500 cycles. The mechanisms of surface area loss of supported platinum nanoparticles (Pt NPs) have been monitored in real-time and operando conditions by coupling liquid cell transmission electron microscopy (LTEM) to energy-filtered transmission electron microscopy (EFTEM), while cyclic voltammograms (CVs) were simultaneously recorded. The study has been performed using an ink made of a commercial catalyst (Tanaka-TEC10V50E) containing Pt NPs intended for automotive applications (3.0 ± 0.4 nm). First, a protocol has been set up to mitigate the electron-beam-induced radiolysis effects to a level that related artifacts are hindered or at least not appreciably detected during the duration of the experiment. At the same time, the resolution limit of the microscope has been pushed below 1 nm. Afterward, several degradation pathways were first identified and categorized and then correlated to the evolution of both the electrochemical active surface area (ECSA) and the Pt oxide reduction peak. We analyze reported evidence that the electrochemical aging favors the dissolution of the smaller Pt NPs. Dissolved cations are then observed to redispose onto larger supported Pt particles (electrochemical Ostwald ripening) or to be transported within the electrolyte, being either the aqueous acidic solution or the ionomer where they can precipitate (dissolution/precipitation process).