In the past few years, great progress has been made in nonprecious metal catalysts, which hold the potential as alternative materials to replace platinum in proton exchange membrane fuel cells. One type of nonprecious metal catalyst, Fe-N-C, has displayed similar catalytic activity as platinum in rotating disk electrode tests; however, rapid degradation of Fe-N-C catalyst-based fuel cells is always observed, which limits its practical application. Although considerable research has been devoted to study the degradation of the catalyst itself, rather less attention has been paid to the membrane electrode assembly that makes the mechanism of fuel cell degradation remain unclear. In this work, a high-performance Fe-N-C catalyst-based membrane electrolyte assembly is prepared and used to study its degradation mechanism. The fuel cell performs with an initial peak power density as high as 1.1 W cm-2 but suffers a current loss of 52% at 0.4 V over 20 h only. The experimental and DFT calculation results indicate that Fe at active sites of catalysts is attacked by hydroxyl free radicals decomposed from H2O2, which is further leached out, causing an increase in activity loss. The ionomer of the catalyst layer and the membrane is further contaminated by the leached Fe ions, which results in an enlarged membrane resistance and cathode catalyst layer proton conduction resistance, greatly influencing the cell performance. In addition, it has been assumed in previous studies that the quick performance loss of Fe-N-C-based fuel cells is caused by water flooding within the catalyst layer, which is proved to be incorrect in our study through a dry-out experiment.
Keywords: degradation; electrochemical impedance spectroscopy; nonprecious metal catalysts; proton exchange membrane fuel cells.