Direct Observation of Fe-N4 Species as Active Sites for the Electrocatalytic Oxygen Reduction

Platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) catalysts have been attracted progressive attention due to its great potential in large-scale commercialization of fuel cell-powered vehicles. Iron-nitrogen-carbon (Fe-N-C), as one type of promising PGM-free ORR catalyst, is characterized by theory and indirect experimental data that Fe-N4 structure is the proposed active site. Direct observation and confirmation of Fe-N4 as active site will build the landmark for the design of PGM-free catalyst. Here, Zelenay and his research group from Los Alamos National Laboratory reported a high efficient Fe-N-C ORR catalyst with direct observation of FeN4 as the active site, which was published in Science in 2017. Aniline was employed together with cyanamide (CM) as the N and C sources. Fe precursor was added at the same step of dissolution of aniline and CM. The necessity of CM lies on its decomposition at low temperature, inducing the formation of macroand micro-pores in carbon matrix. Commonly, micropores in carbon are believed to govern the ORR activity of PGM-free catalysts, while the effects of macropores are ignored, which prohibits the further possibility of enhancing catalytic activity. Macropores here were confirmed to provide greater accessibility to the catalytically active sites and to establish a more open framework for improving the ionomer distribution within the catalyst layers. This finding riches the understanding of roles of pore structures in the ORR catalytic activity and compensates the idea that only micropores contributes to the ORR catalytic activity. Fuel cell performances were characterized in H2-air and H2-O2 conditions, respectively. The maximum power density values of ~0.87 and 0.94 W cm of as-prepared Fe-N-C catalyst, obtained at pO2 of 1.0 and 2.0 bar, respectively, were the highest ever achieved with PGM-free ORR catalysts operating on oxygen. Under H2-air condition, the current density recorded in the kinetic range (> 0.75 V) of cathode operation, decreased with the increased thickness of catalyst-layer, illustrating the importance of catalyst-layer thickness on mass transport properties. Combined with decreased amount of Nafion and increased air flow, the current density reaches comparable value with that of commercial Pt/C. Atomic-resolution scanning transmission electron microscopy (STEM) images of as-prepared Fe-N-C showed single atoms dispersed across the carbon surface, which were confirmed to be primarily Fe by Electron energy-loss spectroscopy (EELS) (Figure 1). Except for highly dispersed single atoms, excess Fe also existed in larger, isolated particulate form as either iron or iron sulfide. However, these large particles are confirmed to be inert in ORR catalysis. EELS obtained directly around the Fe atom showed that N was associated with the Fe with the Fe-to-N ratio of 1:4, corresponding to the composition of FeN4. Previous active-site determinations were based on bulk material analyses with XPS, Mösabauer et al. It is the first time to direct microscopic evidence for the formation of individual FeN4. Theoretical calculations illustrated that the edge-hosted FeN4 instead of in basal spontaneously ligated by OH in the fuel cell environment, suggesting that the edge-hosted FeN4 sites are likely the major contributors to the overall high activity. A B