Characterization, Mitigation, and Modeling of Degradation in Platinum-Group Metal-Free Hydrogen Fuel Cells
Creators: name: Beltran, Diana
Publisher: name: Carnegie Mellon University
Year: 2023
Subjects: subject: Mechanical Engineering
subject: FOS: Mechanical engineering

description: Challenges to adopting proton exchange membrane fuel cells (PEMFCs) for fuel cell electric vehicles (FCEVs) include catalyst costs and durability. Groups are actively addressing this problem by designing new platinum-group metal (PGM)-free oxygen reduction reaction (ORR) catalysts that demonstrate significant catalytic activity advances. First, this thesis recommends a shift to focusing on mass activity, a normalized metric of activity per unit mass, for PGM-free catalysts. The purpose is to minimize bias towards PGM-free catalyst studies that use higher catalyst loading known to increase half-wave potentials, the most common method to compare catalysts. Next, it shows that PGM-free cathode fuel cell performance can be partially recovered by an in-situ electrochemical method by restoring a percentage of the original active sites or activating originally inactive sites. The same approach is also shown to increase the initial activity if applied to a pristine PGM-free cathode. This is achieved by applying low constant potential holds to the cathode in the absence of oxygen by either flowing nitrogen or blocking the delivery of air and removing the residual oxygen by the ORR. Our study further differentiates between two types of active sites with degradation mechanisms characterized by different populations and timescales. It is hypothesized that the recovery of sites is due to the reduction of Fe<sup>3+</sup> to Fe<sup>2+</sup>, restoring a percentage of the original active sites or activating initially inactive sites. This reduction has additionally been shown to generate the removal of oxygen adsorbates at the catalyst surface. Next, it studies the effects of contamination by acetonitrile (CH<sub>3</sub>CN) and sulfur dioxide (SO<sub>2</sub>) on operating platinum-group metal (PGM), and PGM-free cathode fuel cells were investigated. This was done by monitoring the changes in current density at a constant voltage hold before, during, and after sulfur dioxide and acetonitrile contamination. The overall contamination effects of each cell were characterized by air polarization curves, cyclic voltammetry, and electrochemical impedance spectroscopy. It is found that CH<sub>3</sub>CN at 20 ppm and SO<sub>2</sub> at 4.6 ppm do not result in any additional degradation in Fe-based cathode fuel cells during operation, while current density losses greater than 70% are seen in platinum (Pt)-based cathode fuel cells. This tolerance to contaminants is an often overlooked advantage of PGM-free catalyst cathode fuel cells which suffered no additional degradation due to the introduction of air contaminants in this study. Finally, this thesis studies the effects of operating conditions on the degradation rate of highly active Fe-based catalyst cathode polymer electrolyte fuel cells. This work explored designs and methods to mitigate degradation and expand models to predict PGM-free fuel cell degradation due to voltage cycling, specifically current density loss over time. Ultimately if the model is successful, diagnostics and degradation prediction can help researchers assume less risk when investing their efforts in advancing PGM-free cathode hydrogen fuel cells. The motivation behind investigating degradation mechanisms is that most recent advances have primarily focused on enhancing initial electrocatalytic activity and not on catalyst stability, where the advancements are less profound and are well below the level for commercialization. Based on our results, the most benign conditions for accelerated stress tests are at low relative humidity, low temperatures, and low cycling upper potential limits.