(Invited) Non-PGM Electrocatalysts for PEM Fuel Cells: Origin of Their Instability

Abstract

Proton exchange membrane fuel cells (PEMFCs) have earned substantial commercial and government interests worldwide as highly promising renewable energy power sources for various applications (e.g., electric vehicles). However, a key contributor to their high cost remains the use of Platinum Group Metals (PGM) catalysts, especially at the cathode where the oxygen reduction reaction (ORR) is much more sluggish than the hydrogen oxidation reaction (HOR) occurring at the anode. The replacement of PGM catalysts by PGM-free catalysts at the cathode has so far been mainly hampered by the poor stability of the most active and performing PGM-free catalysts [1,2] that would otherwise become serious contenders to PGM catalysts. It is therefore important to determine the origin of the instability plaguing the PGM-free catalysts [3-8]. Based on our highly performing catalyst (Nature Commun., 2011, 2, 416) [2], INRS team has devoted, during recent years, much attention to the instability of this PGM-free catalyst [3-8]. This communication will present our very recent work on the catalyst stability which is summarized in two publications in Energy and Environmental Science [6, 8]. Our catalyst was obtained by ball milling ZIF-8, FeAc and 1,10 Phenanthroline, followed by two heat-treatments in Ar and in NH3. Via a systematic study of the instability behavior of the catalyst at different fuel cell potentials from 0.8 to 0.2 V at 80 °C and 25 °C, we discovered that the decay of the current density always involves the superposition of a fast and a slow exponential decay. With the combination of various characterization (e.g., BET, NAA and Mössbauer spectroscopy), we concluded that the fast exponential decay of the current density was the result of the specific demetalation of FeNx sites located in the micropores of the catalyst. Following this work, we explored the behavior of this catalyst before and after fluorination by F2 at room temperature. We discovered that all Fe-based catalytic sites were poisoned by reaction with F2, but not the ORR active CNx sites (and edge carbon sites) located at the surface of the catalyst carbonaceous support. Consequently, the instability behavior of fluorinated catalysts was found to be different from that of pristine catalyst, but similar to that of MOF_CNx_Ar+NH3 (a catalyst devoid of Fe, with only CNx sites and edge carbon sites). Further, the F2-poisoned catalysts can be partially reactivated under different heat-treatments. With a systematic study by fuel cell, BET, IR, TGA, XPS, XAS, as well as DFT and thermodynamic calculations, all the results demonstrate that the root-causes of instability of the FeN4 and CNx catalytic sites do not have the same origin. The two models (from INRS and Los Alamos) proposed so far to describe the instability curves of Fe-based catalysts will also be discussed.