Structure of Active Sites of Fe-N-C Nano-Catalysts for Alkaline Exchange Membrane Fuel Cells

Abstract

Platinum group metal-free (PGM-free) catalysts based on transition metal-nitrogen-carbon nanomaterials have been studied by a combination of ex situ and in situ synchrotron X-ray spectroscopy techniques; high-resolution Transmission Electron Microscope (TEM); Mößbauer spectroscopy combined with electrochemical methods and Density Functional Theory (DFT) modeling/theoretical approaches. The main objective of this study was to correlate the HO2− generation with the chemical nature and surface availability of active sites in iron-nitrogen-carbon (Fe-N-C) catalysts derived by sacrificial support method (SSM). These nanomaterials present a carbonaceous matrix with nitrogen-doped sites and atomically dispersed and; in some cases; iron and nanoparticles embedded in the carbonaceous matrix. Fe-N-C oxygen reduction reaction electrocatalysts were synthesized by varying several synthetic parameters to obtain nanomaterials with different composition and morphology. Combining spectroscopy, microscopy and electrochemical reactivity allowed the building of structure-to-properties correlations which demonstrate the contributions of these moieties to the catalyst activity, and mechanistically assign the active sites to individual reaction steps. Associated with Fe-Nx motive and the presence of Fe metallic particles in the electrocatalysts showed the clear differences in the variation of composition; processing and treatment conditions of SSM. From the results of material characterization; catalytic activity and theoretical studies; Fe metallic particles (coated with carbon) are main contributors into the HO2− generation.

Vitamin Derived Nitrogen Doped Carbon Nanotubes for Efficient Oxygen Reduction Reaction and Arsenic Removal from Contaminated Water

“…Even more, our Fe@NCNT-rGO exhibits slightly lower, but comparable, current density of 5.7 mA cm −2 when compared to the 5.9 mA cm −2 observed in commercial Pt/C electrodes. A diffusion controlled and effective four-electron ORR pathway can be inferred from the broad current plateau [39] that was observed in the region of 0.2 to 0.65 V. The RDE polarization curves were measured at increasing rotation speeds from 400 to 2000 rpm (plotted in Figure 3c) and the corresponding Koutecky-Levich (K-L) plots of the Fe@NCNT-rGO electrodes that are exhibited in Figure 3d show parallel linearity in the range of 0.2 to 0.65 V, validating the nearly first-order kinetics of the ORR reaction [40,41]. The number of electrons (n) transferred per oxygen molecule, as calculated by K-L equation, and was found to be around 4.0, which is close to the theoretical value of 4.0 for commercial Pt/C, which also reiterates our contention that four electron pathway is followed in our newly developed Fe@NCNT-rGO electrodes.…”

Characterizing Complex Gas–Solid Interfaces with in Situ Spectroscopy: Oxygen Adsorption Behavior on Fe–N–C Catalysts

“…Both have been the subject of prior rigorous studies investigating ORR performance, and relating the performance to physicochemical properties. 10,21,32,61,[65][66][67] The other three samples are based on microporous NS particles produced by variations of a solvothermal synthesis method. 27,52,62,63 400 °C.…”

“…18,68 Indeed, many highly active Fe-N-C catalysts (including MOF-Fe 0.5 and SSM) are pyrolyzed at much higher temperatures, usually in excess of 900 °C. 10,21,32,61,65 Conversely, the NS samples were all pyrolyzed at lower temperatures of either 600 °C (NS-1) or 700 °C (NS-2 and NS-3). This difference in temperature is likely to impact the localized structure of the carbon surrounding a bimodal pore size distribution and the texturing can be explained by the presence of both mesoand micropores.…”

“…The highly active catalysts include an Fe−N−C material derived from a sacrificial Znbased imidazolate (ZIF-8) metal−organic-framework (MOF) and a templated material based on the sacrificial-supportmethod (SSM). 10,32,61 Model Fe−N−C structures based on nanospherical particles (herein referred to as NS-i, where i is 1, 2, or 3 depending upon the exact synthetic process, described in Section 2.1.1) on the order of 150−250 nm in diameter were produced by several variations of a previously reported hydrothermal synthesis. 27,52,62,63 This work provides evidence that (i) different NC and FeN x C y sites may show different strengths of interactions with oxygen and (ii) some sites are typically more accessible for adsorption than others (e.g., D1 vs D2).…”

“…The MOF-Fe 0.5 sample has the most open structure, with a high specific surface area mostly from micropores with some contribution from narrow mesopores 21. SSM has relatively smaller structural features, and is known to contain a bimodal pore size distribution due to the controlled size of poreformers 10,61. NS-3 shows the typical structure and morphology of the NS samples.…”