Fe-Nx Sites Research Articles

A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for High-Performance Polymer Electrolyte Fuel Cells

Non-precious-metal catalysts (NPMCs) for oxygen reduction reaction (ORR) have been of tremendous interests in energy conversion and storage devices. Among several classes of NPMCs, iron and nitrogen supported on carbon (Fe-N/C) have shown the most promising ORR activity. It has been widely suggested that an active site structure for Fe-N/C catalysts contains Fe-Nx coordination. However, the preparation of Fe-N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only Fe-Nx sites, but also a significant portion of less active large iron-based particles. This poses a great challenge to rational design of Fe-N/C catalysts with abundant Fe-Nx species. We developed “silica-protective-layer-assisted” synthetic approach that can preferentially generate the catalytically active Fe-Nx sites in Fe-N/C catalysts while suppressing the formation of large Fe-based particles. The catalyst preparation consisted of an adsorption of Fe porphyrinic precursor on carbon nanotubes (CNTs), silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. In situ X-ray absorption spectroscopy during the preparation of CNT/PC catalyst revealed that the interaction between the silica layer and Fe-N4 in a porphyrin precursor appears to protect the Fe-N4 site and to prevent the formation of large Fe-based particles. The CNT/PC catalyst showed very high ORR activity and remarkable stability in alkaline media. Importantly, an alkaline anion exchange membrane fuel cell (AEMFC) with a CNT/PC-based cathode exhibited record high current and power densities among NPMC-based AEMFCs. In addition, a CNT/PC-based cathode exhibited a high volumetric current density of 320 A cm-3 in acidic proton exchange membrane fuel cell, comparable with 2020 DOE target (300 A cm-3). We further demonstrated the generality of this synthetic strategy to other carbon supports including reduced graphene oxides and carbon blacks.

Tolerance of non-platinum group metals cathodes proton exchange membrane fuel cells to air contaminants

The effects of major airborne contaminants (SO2, NO2 and CO) on the spatial performance of Fe/N/C cathode membrane electrode assemblies were studied using a segmented cell system. The injection of 2–10 ppm SO2 in air stream did not cause any performance decrease and redistribution of local currents due to the lack of stably adsorbed SO2 molecules on Fe-Nx sites, as confirmed by density functional theory (DFT) calculations. The introduction of 5–20 ppm of CO into the air stream also did not affect fuel cell performance. The exposure of Fe/N/C cathodes to 2 and 10 ppm NO2 resulted in performance losses of 30 and 70–75 mV, respectively. DFT results showed that the adsorption energies of NO2 and NO were greater than that of O2, which accounted for the observed voltage decrease and slight current redistribution. The cell performance partially recovered when the NO2 injection was stopped. The long-term operation of the fuel cells resulted in cell performance degradation. XPS analyses of Fe/N/C electrodes revealed that the performance decrease was due to catalyst degradation and ionomer oxidation. The latter was accelerated in the presence of air contaminants. The details of the spatial performance and electrochemical impedance spectroscopy results are presented and discussed.

In Situ Self-Sacrificed Template Synthesis of Fe-N/G Catalysts for Enhanced Oxygen Reduction.

To facilely prepare high-performance Fe-N/G oxygen reduction catalysts via a simple and controllable route from available and low-cost materials is still a challenge. Herein, we develop an in situ self-sacrificed template strategy to synthesize Fe-N/G catalysts from melamine, glucose, and FeSO4·7H2O. Fe/Fe3C@graphitic carbon nanocapsules are uniformly formed on the NG surface to create a highly opened and stable mesoporous framework structure. Furthermore, effectively doped N sites and high active Fe-Nx sites are synchronously constructed on such structures, leading to an enhanced synergistic effect for ORR and promising the Fe-N/G catalyst a similar catalytic activity and four-electron selectivity, but superior stability to commercial 30 wt % Pt/C catalysts in 0.1 M KOH solution under the same loading.

Enhancement in Kinetics of the Oxygen Reduction Reaction on a Nitrogen-Doped Carbon Catalyst by Introduction of Iron via Electrochemical Methods.

The iron (Fe) electrodeposition-electrochemical dissolution has been employed on nitrogen-doped carbon material (P-PI) prepared via multi-step pyrolysis of a polyimide precursor to achieve the introduction of Fe species, and its influence on the oxygen reduction reaction (ORR) is investigated by cyclic and rotating ring-disk electrode voltammetry in 0.5 M H2SO4. After the electrochemical treatment, the overpotential and H2O2 production percentage of ORR on the P-PI are decreased and the number of electrons transferred is increased in the meanwhile. In combination with the results of X-ray absorption fine structure spectra, the presence of Fe-Nx sites (Fe ions coordinated by nitrogen) is believed to be responsible for the improved ORR performance. Further kinetic analysis indicates that a two-electron reduction of O2 is predominant on the untreated P-PI with coexistence of a direct four-electron transformation of O2 to H2O, while the introduction of Fe species leads to a larger increase in the rate constant for the four-electron reduction than that for the two-electron process, being in good agreement with the view that Fe-Nx sites are active for four-electron ORR.

Impact of iron precursors on the properties and activities of carbon-supported Fe-N oxygen reduction catalysts

Two iron precursors (iron(III) chloride and iron(II) ammonium sulfate) with a nitrogen-containing compound (ethylenediamine) were pyrolyzed on commercially available carbon blacks (Vulcan XC-72) under a nitrogen flow in this study. The properties of the resultant nonnoble electrocatalysts (Fe(Z)N/C-2 and Fe(Z)N/C-8) effected by using different iron precursors during the synthetic process were investigated by X-ray-based spectroscopies including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The electrocatalytic performance toward oxygen reduction reaction (ORR) of these electrocatalysts was also comparatively studied by rotating disk electrode and chronoamperometric techniques. The obtained results indicate that iron precursors play an essential role on the chemical microstructure, elemental states, and ORR performance of electrocatalysts. The electrocatalysts (Fe(III)N/C-8) prepared by using iron(III) chloride as starting precursors exhibit better electrochemical ORR activity and durability among all the synthesized catalysts. As evidenced by XPS and XAS studies, we conclude that this may be due to the formation of active FeNx sites, more surface Fe/C and N/C atomic ratios, and the coexistence of pyridinic-N and graphitic-N species in the Fe(III)N/C-8 electrocatalyst.

A density functional theory study of oxygen reduction reaction on non-PGM Fe-Nx-C electrocatalysts.

First-principles density functional theory (DFT) calculations were performed to explain the stability of catalytically active sites in Fe-Nx-C electrocatalysts, their ORR activity and ORR mechanism. The results show that the formation of graphitic in-plane Fe-N4 sites in a carbon matrix is energetically favorable over the formation of Fe-N2 sites. Chemisorption of ORR species O2, O, OH, OOH, and H2O and O-O bond breaking in peroxide occur on both Fe-N2 and Fe-N4 sites. In addition to the favorable interaction of ORR species, the computed free energy diagrams show that elementary ORR reaction steps on Fe-Nx sites are downhill. Thus, a complete ORR is predicted to occur via a single site 4e(-) mechanism on graphitic Fe-Nx (x = 2, 4) sites. Because of their higher stability and working potential for ORR, Fe-N4 sites are predicted to be prime candidate sites for ORR in pyrolyzed Fe-Nx-C electrocatalysts.