2. Precious metal-free layers in PEMFCs, which typically employ 60-100 µm M-N-C thick cathodes, may not u�lize the majority of the catalyst layer during O 2 reduc�on due to proton consump�on. Focus, therefore, should be made on decreasing the electrode thickness by further increasing the electrochemically accessible volumetric ac�ve site density of precious metal-free catalysts.