Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

Abstract

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Spontaneous Demetallation of Mn(II)Nx Catalytic Sites in Proton Exchange Membrane Fuel Cell Conditions

“…It means that a reaction with •OH and therefore the Fenton reaction cannot be the main cause of degradation either for Fe/N/C or for Mn/N/C. The same conclusion was also reached in a recent review about the degradation mechanism of metal–N–C catalysts for ORR, when the authors state that replacing Fe with other transition metals (such as Co, Mn, Sn) with lower production of H 2 O 2 and/or reactive oxygen species (ROS) does not seem to yield tremendously lower rates of degradation and decrease of the ORR activity during operation.…”

Screening potential anodic chemistry in lieu of the oxygen evolution reaction in electrolysis systems: the road to practical application

“…5b). 197–199 At the microscopic level, the poisoning of active sites by *CO, which is common in alcohol/aldehyde/carboxylic acid electrooxidation, 129,198,199 is one of the major reasons for the performance degradation for AAC. Polymerization of the aldehyde in alkaline electrolytes ( e.g.…”

Zn Single Atoms/Clusters/Nanoparticles Embedded in the Hybrid Carbon Aerogels for High-Performance ORR Electrocatalysis

“…Oxygen reduction reaction (ORR) plays an utmost important role in various advanced electrochemical energy conversion and storage technologies, especially in metal–air batteries and fuel cells. Nowadays, Pt-/Pd-based electrocatalysts have been recognized to improve the sluggish kinetics of ORR, but the scarcity, poor long-term stability, and methanol/carbon monoxide sensitivity of noble metal-based electrocatalysts would limit their widespread application . To look for desirable alternatives, various nonprecious-metal-based (such as Fe, Co, Ni, and Mn) electrocatalysts have been investigated. , Interestingly, atomically dispersed transition-metal–nitrogen–carbon (M–N–C) materials have achieved outstanding ORR performance and maximum atomic efficiency.…”