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.