Identification of Possible Degradation Mechanisms of PGM-Free Electrocatalysts during Fuel Cell Operation
Authors: Martinez Ulises , Komini Babu Siddharth , Holby Edward F. , Yin Xi , Manard Benjamin T. , Zelenay Piotr 
Published: February 2020

Significant advancement in the development of highly active platinum group metal-free (PGM-free) electrocatalysts is giving rise to the possibility of a low-cost replacement for Pt electrocatalysts in polymer electrolyte fuel cells. However, before the successful implementation of PGM-free electrocatalysts, the challenge of long-term stability under fuel cell operating conditions needs to be better understood. While several published articles have hypothesized various degradation mechanisms, namely demetalation, hydroperoxyl radical generation/attack, active site poisoning, carbon/nitrogen corrosion, and micropore flooding, no clear correlation has been demonstrated between these mechanisms and the severe performance losses observed during fuel cell operation. The analysis of these proposed degradation mechanisms suggests a general classification into two relevant length scales: (i) atomic-scale degradation of active sites, and (ii) macro/meso-scale degradation affecting the catalyst layer structure. Due to the complexity of PGM-free electrocatalysts and the mystery of the PGM-free active site, the combination of several physical/chemical/electrochemical characterization techniques and the development of novel PGM-free-specific characterization techniques is required to identify the source of PGM-free activity loss during various fuel cell operating conditions.

In this work, possible degradation mechanisms during fuel cell operation will be identified via the study of the aforementioned proposed degradation mechanisms at various conditions, i.e., voltage, relative humidity, oxygen partial pressure. Several characterization techniques, including imaging and analytical, will be combined to decouple the various possible degradation mechanisms, for example, CO2 generation caused by the high potential-driven corrosion mechanism in the presence of water distinguished CO2 generated from ionomer/membrane decomposition due to harmful hydroperoxyl radical formation. In particular, CO2 emissions measured using non-dispersive infrared spectroscopy (NDIR) coupled with F- and Fe emissions from ion chromatography and inductively coupled plasma measurements will be used to determine which degradation mechanism is most active and which materials (ionomer vs. catalyst) are degrading. Lastly, this study will also present kinetic models of PGM-free catalyst degradation, fitted to experimental data, combined with quantum chemical models, based on ab initio molecular dynamics, to corroborate the likely kinetic pathways for activity loss. Directly addressing the underlying mechanisms leading to loss of activity in PGM-free electrocatalysts during fuel cell operation will provide valuable insight necessary for the development of next-generation materials with improved durability capable of competing with Pt-based counterparts.

Acknowledgements

Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.