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

One bottleneck hampering the widespread use of fuel cellvehicles,in particular of proton exchange membrane fuel cells (PEMFCs), isthe high cost of the cathode where the oxygen reduction reaction (ORR)occurs, due to the current need of precious metals to catalyze thisreaction. Electrochemists tackle this issue in the short/medium termby developing catalysts with improved utilization or efficiency ofplatinum, and in the longer term, by developing catalysts based onEarth-abundant elements. Considerable progress has been achieved inthe initial performance of Metal-nitrogen-carbon (Metal-N-C) catalystsfor the ORR, especially with Fe-N-C materials. However, until now,this high performance cannot be maintained for a sufficiently longtime in an operating PEMFC. The identification and mitigation of thedegradation mechanisms of Metal-N-C electrocatalysts in the acidicenvironment of PEMFCs has therefore become an important research topic.Here, we review recent advances in the understanding of the degradationmechanisms of Metal-N-C electrocatalysts, including the recently identifiedimportance of combined oxygen and electrochemical potential. Resultsobtained 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 approachesthat the scientific community has hitherto investigated to overcomethe durability issues of Metal-N-C electrocatalysts.